Symposium Organizers
Dhananjay Kumar, North Carolina Agricultural and Technical State University
Ningzhong Bao, Nanjing Tech University
Sergio D'Addato, Università di Modena e Reggio Emilia
Arunava Gupta, University of Alabama
NT2.3: Piezoelectrics
Session Chairs
Wednesday AM, March 30, 2016
PCC North, 100 Level, Room 132 AB
9:00 AM - NT2.3.01
Phase Transition and Domain Evolution of PLZST Antiferroelectric Single Crystal with MPB Composition
Jinghan Gao 1,Qiang Li 1,Yiling Zhang 2
1 Department of Chemistry, Tsinghua University Beijing China,2 State Key Laboratory of New Ceramics and Fine Processing, Tsinghua University Beijing China
Show AbstractThe Pb (Zr, Sn, Ti)O3 family of perovskite antiferroelectrics, modified with La, has attracted much attention in recent years because of its high longitudinal strain and short switching time at the depolarization temperature (Td) in the vicinity of the morphotropic phase boundary (MPB). However, its electrical properties and structure evolution while depolarized from induced ferroelectric phase (FEin) to antiferroelectric phase (AFE) still need further investigation. In order to understand the influence of domain structure on the phase transition and depolarization behaviors, (Pb, La)(Zr, Sn, Ti)O3 single crystals with MPB composition have been grown by flux method. The virgin sample was firstly induced to a metastable ferroelectric state by applying a sufficiently large electric field. Thereafter, the temperature induced phase transition of [100]-orientated PLZST single crystal was explored by electric measurements and in situ polarized light microscope (PLM). Td is proved to be around 115oC by exploring temperature-dependent dielectric constants of a poled sample. A temperature induced phase transition from FEin to AFE state could be proved by its electric measurement: typical ferroelectric behavior below Td with single polarization versus electric field hysteresis loop (P-E loop) and current versus electric field curve (I-E curve) with only two peaks; when the temperature is above Td, AFE phase is confirmed by double loops in P-E measurements and I-E curves with four peaks. By observing the domain evolution before and after the phase transition, it was confirmed that the structure changed from rhombohedral to tetragonal state by their extinction angles and the orientation of domain walls. A large strain (0.77%) was obtained at a low electric field of 2.5 kV/mm. The remarkable strain accompany with induced phase transition makes the PLZST single crystal a promising candidate for switch and actuator application.
9:15 AM - NT2.3.02
High Aspect Ratio KNbO3 Nanofibers – Synthesis, Characterization and Applications
Rajasekaran Ganeshkumar 1,Kostiantyn Sopiha 1,Ping Wu 1,Chin Wei Cheah 1,Rong Zhao 1
1 Singapore University of Technology and Design Singapore Singapore,
Show AbstractPotassium niobate (KNbO3), a lead-free ferroelectric compound with perovskite-type structure, is considered a promising nanomaterial for numerous applications due to their superior optical and ferroelectric properties. Despite progress in 1D perovskite materials, preparing high aspect ratio KNbO3 nanostructures is still a challenge.
Here, we report a simple and effective method for synthesizing ultra-long KNbO3 nanofibers with different morphologies (solid and porous) using sol-gel assisted far-field electrospinning process. XRD data and Raman spectra confirmed that KNbO3 nanofibers with pure perovskite – orthorhombic phase were obtained. SEM images showed the morphological feature and a large quantity of KNbO3 nanofibers were centimetres long, with the diameter of 100 nm. AFM images reveal that uniform grains of ~40 nm are closely stacked along the direction of nanofiber axis. Due to its bio-eco-compatibility, chemical stability and large surface-to-volume ratio, KNbO3 nanofibers are utilized as an active material for environmental applications.
The as-synthesized KNbO3 nanofibers were collected on a SiO2/Si substrates with interdigitated Ta electrodes to fabricate a fast and highly sensitive humidity sensor. The sensor displays a logarithmic-linear dependence behaviour of the conductance with relative humidity (RH). The resistivity decreases from 7x109 Ω to 5x105 Ω (> 4 orders of magnitude) when relative humidity varies from 11% to 95% RH at room temperature. In addition, the sensor exhibits an ultra-fast response (2 s) and recovery (< 10 s) time, retaining its linearity and reproducibility.
Furthermore, photocatalytic activity of the as-synthesized KNbO3 nanofibers has been evaluated by photo degradation of Methylene Blue (MB) under direct sunlight irradiation and UV LEDs (370 nm). KNbO3 nanofibers have large surface area providing enough reactive sites to facilitate redox reactions. The results show MB dye removal percentage of ~80% after 120 min at a degradation rate constant of 0.013 min-1. KNbO3, being a ferroelectric material with an indirect bandgap of 3.35 eV, the changes in dipole and internal field should promote photo-generated charge carriers under UV irradiation. Effect of spontaneous polarization of ferroelectric KNbO3 nanofibers in humidity sensing and photocatalytic efficiency is studied.
9:30 AM - *NT2.3.03
Challenges to Measuring Piezoelectricity in Thin Films Using Scanning Probe Microscopy#xD;
Arthur Baddorf 1,Petro Maksymovych 1,Stephen Jesse 1,Qian Li 1,Sergei Kalinin 1,Nina Balke 1
1 Oak Ridge National Laboratory Oak Ridge United States,
Show AbstractPiezoelectric and ferroelectric properties form the basis for many information and energy technologies by converting mechanical to electrical energy or vice versa. Local characterization of electromechanical response is needed to measure nanoscale responses, particularly for structure-function relationships of inhomogeneous materials. Unfortunately, existing approaches fail to provide quantitative values and pose significant ambiguity with respect to the origin of electromechanical response, for example in distinguishing between the effects of charge injection and piezoelectricity.
Piezoelectric Force Microscopy (PFM) has become a very successful methodology to probe piezoelectric and ferroelectric materials, by detecting surface deformation in the electric field of a biased microscope tip. While PFM is an easily accessible characterization technique for thin films, quantification of the piezoelectric properties and the role of measurement artifacts are still poorly understood. Hysteresis of electromechanical response can originate from electrostatic interactions between the tip and sample involving slow surface charging or ionic mechanisms that mimic ferroelectric responses. Similarly, charge writing and strong electrostatic tip-sample interactions can appear as persistent domains in PFM. Distinguishing intrinsic and extrinsic contributions to the electromechanical sample response in oxide and chalcogenide-based films has been widely discussed, however, clear guidelines on how to differentiate charge injection from ferroelectric response is missing.
We present a unified approach to electromechanical measurements that that enables effective differentiation of ionic displacement, piezoelectric deformation, and electrostatic interactions in a nanoscale contact geometry. Furthermore, by modeling cantilever dynamics, the surface displacements can be quantitatively extracted from the measurements, providing a systematic way to quantify piezoelectric response. These studies establish new opportunities for scanning probe microscopies to evaluate electroactive materials outside of the limited family of strongly polarized ferroelectric materials, now including ionic conductors, biological materials, and electroactive polymers.
Support was provided by the U.S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division through the Office of Science Early Career Research Program (NB) and US DOE MSED (SVK). Experiments were performed at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility.
10:00 AM - NT2.3.04
Solution-Processing of Complementary Resistive Switching Arrays Based on ZrO2 and Printed Contacts
Jeremy Smith 1,Seungjun Chung 1,Jaewon Jang 1,Vivek Subramanian 1
1 University of California-Berkeley Berkeley United States,
Show AbstractSolution-processed metal oxide materials have been studied extensively in the past several years for their use in a variety of thin-film electronic applications such as display backplanes, solar cells, smart windows, gas sensors, and resistive RAM. They exhibit a wide range of properties that spans the spectrum of dielectrics, semiconductors and metals, and can be deposited from solution using sol-gel chemistries and techniques such as printing, spray coating, dip coating, or spin coating. The field of resistive switching for non-volatile memory applications has also grown significantly due to its potential for low power consumption and high density of devices. Transition metal oxides such as Ta2O5, NiO, HfO2 and ZrO2 are commonly employed as the switching material in such systems. The mechanism of operation usually involves the reversible formation of conductive filaments within the oxide that can bridge between metallic contacts. These elements can then be incorporated into arrays with a selector element to allow reliable memory addressing. Recently, complementary resistive switching (CRS) using two back-to-back memory elements has been demonstrated as a route to eliminate the selector [Linn, Nat. Mater. 9, 403-406]. Typically these materials are vapor deposited to achieve high device density and uniformity, however, the demonstration of solution-processed memory arrays is less well studied and of great interest from a low-cost, large-area manufacturing perspective.
In this work we have fabricated arrays of Au-ZrO2-Ag back-to-back memory elements using inkjet printed metal contacts and sol-gel ZrO2 deposited by spin coating on glass substrates. The gold acts as the inert contact with silver filament formation occurring in the oxide. Complementary resistive switching is observed at voltages less than 5V with good cycle endurance (104 cycles) and retention times (105 sec). Careful control of printing to obtain reliable contact overlap, thus defining the device area, is found to be critical for high yields. Additionally, the sol-gel ZrO2 can be tuned to give symmetric SET and RESET voltages, which is needed for optimal CRS behavior. Combining these factors allows us to design simple device arrays that operate with negligible crosstalk during memory addressing and offer the potential for novel memory applications.
10:15 AM - NT2.3.05
Measurement Methodology and Properties of Interdigitated Transducers with Lead Zirconate Titanate Thin Films
Robin Nigon 1,Trygve Raeder 1,Nachiappan Chidambaram 1,Paul Muralt 1
1 EPFL Lausanne Switzerland,
Show AbstractInterdigitated electrode (IDE) transducers with lead zirconate titanate (PZT) thin films have interesting properties for MEMS applications. Apart of a higher voltage response for harvesting devices, they also display excellent polarization stability, lower dielectric losses and lower leakage currents. However, the derivation of material properties is quite difficult because the electric field shows a complex line pattern and is not homogeneous. The electric field has to be rescaled to obtain correct polarization (PV) and capacitance (CV) loops, and dielectric constants. Since the direction of the electric field lies mostly in the film plane, it is expected that the grain boundaries of columnar, polycrystalline PZT thin films play a different role in the electric and ferroelectric properties of the film, as compared to the case of parallel plate geometry. For use with a Si substrate, the film has to be grown on an insulating buffer layer for proper device operation. This insulating layer influences as well the electric field distribution.
In this work, we grew (100)-textured PZT thin films by a sol-gel route on an electrically insulating buffer layer of MgO/SiO2 on silicon substrates. The process was well reproducible. All films obtained in this way show a dense, columnar microstructure with a smooth surface. In PZT thin films much thinner than the IDE gap, the electric field between the fingers is nearly constant and always lower than the simple expression of voltage divided by finger distance. In the case of high permittivity substrates, such as strontium titanate (STO) as used for epitaxial growth of PZT films, the electric field becomes less homogeneous and is even more reduced inside PZT. Both kinds of substrate contribute capacitively to the film properties and this should be taken into account for determining the real PV and CV loops of the ferroelectric film. We can extract the dielectric constant of the film (about 1100 at zero bias) from the CV loop by means of a mathematical model for IDE capacitance computation, and those results are line with finite element simulations. Sputter deposition of the IDE is found to introduce some damage near the electrode/film interface, which causes the PV loops to become slanted. These damages can be partially cured by annealing at high temperatures under oxygen flow. Finally, IDE on PZT thin films show a strong tendency to age, i. e., the PV loops of poled samples display a voltage offset building up with time. Quicker aging is observed for Ti-richer compositions. We believe that grain boundaries act as dielectric layers where interface charges accumulate, driven by the electric field generated by the bound charges of the polarization of the neighbouring grains. The accumulating charges are originating from charge detrapping within the grain boundary. The aging contributes to the stabilization of the poled state, which is beneficial for small field operations, and in particular for energy harvesting.
10:30 AM - *NT2.3.06
Science and Technology of Interface-Engineered BiFeO3 /SrTiO3/ BiFeO3 Nanolaminates with High Piezoelectricity and Low Leakage for Multifunctional and Biomedical MEMS/NEMS Devices
Geunhee Lee 1,Erika Fuentes-Fernandez 1,Guoda Lian 1,Ram Katiyar 3,Orlando Auciello 2
1 Department of Materials Science and Engineering University of Texas at Dallas Richardson United States,3 Institute for Functional Nanomaterials University of Puerto Rico San Juan United States1 Department of Materials Science and Engineering University of Texas at Dallas Richardson United States,2 Department of Bioengineering University of Texas at Dallas Richardson United States
Show AbstractLead-free ferroelectric/piezoelectric BiFeO3 (BFO) thin films can provide superior piezoelectric properties in both epitaxial and polycrystalline thin films for potential applications to multifunctional MEMS/NEMS devices (actuators and sensors), particularly for medical applications, for which BFO may provide biocompatibility as opposed to Pb(ZrxTi1-x)O3, which contains non-biocompatible lead. The remnant polarization Pr and out-of-plane converse piezoelectric coefficient d33 of BFO films are comparable to those of tetragonal Ti-rich PZT films. However, BFO films exhibit large coercive fields and large leakage current, which might limit the applicability of BFO in devices. Prior work focused on reducing the leakage current of BFO films via insertion of dopants in the material, but still the reduction in leakage is limited. This talk will focus on reviewing the R&D performed during the last three years on a novel approach to reduce the leakage current and increase the piezoelectric response of BFO film-based MEMS/NEMS devices by producing structured nanolaminates involving BFO films via insertion of an insulating layer like SrTiO3 between two BFO layers, all with nanometer-scale thickness. The BFO/STO/BFO nanolaminates provide the means to achieve a strain-engineered structure at the BFO/STO interfaces, which result in very high piezoelectric deflection with low leakage current (~ 10-7 - 10-8 A/cm2 at 1 V). This review will include a discussion of the mechanism responsible for the observed high piezoelectricity and low leakage current behaviors of the BFO/STO/BFO nanolaminates, as revealed by systematic high-resolution TEM and PFM based studies. The BFO/STO/BFO NLs properties provide the bases for excellent potential application of the BSB-NLs into lead-free piezoelectrically actuated MEMS/NEMS devices, especially for biomedical applications (e.g., biosensors and drug delivery systems) based on the potential demonstrable biocompatibility of BFO components.
11:00 AM - NT2.3.07
Giant Piezoelectricity and Strain Induced Curie Temperature Enhancement of Epitaxial Films
Yanxi Li 1,Yaodong Yang 2,Zhiguang Wang 1,Jianjun Yao 1,Jiefang Li 1,Dwight Viehland 1
1 Materials Science and Engineering Virginia Tech Blacksburg United States,2 Multi-Disciplinary Materials Research Center, Frontier Institute of Science and Technology Xi’an Jiaotong University Xi’an China
Show AbstractBarium titanate (BTO) is one classic perovskite ferroelectric oxide which has attracted lots of research interest due to its high-dielectric constant and large piezoelectric coefficient. In recent years, lots of works have been done for the doped BTO systems to further improve its properties, such as ferroelectricity and piezoelectricity. However, the researchers need to face the problem of sharp decreasing of Curie temperature (Cp) after the doping.
In our work, utilizing pulsed laser deposition (PLD) technique, high quality epitaxial Sn-doped BTO thin films have been successfully grown on various substrates with different lattice coefficient. The observation of much better ferroelectric properties have been found from the Sn-doped BTO thin film compared with the bulk material of the same chemical composition. Moreover, with the demonstration of successfully epitaxial growth, The Cp of those films has been enhanced due to the effect of epitaxial constraint which prevent external dimension changes of thin film. Furthermore, giant piezoelectric coefficient has been obtained from that Sn-doped BTO thin film grown on certain substrate, which would provide particular strain to the film.
The microstructure of those thin films has been examined by the state-of-the-art electron microscopy for the mechanism of the origin of such giant electro-mechanical property of epitaxial thin film under that certain strain compared to other ones. Those improvements of doped BTO thin films would provide lots of potential application possibilities in multi-functional materials areas.
NT2.4: Ferroelectrics/Multiferroics
Session Chairs
Wednesday PM, March 30, 2016
PCC North, 100 Level, Room 132 AB
11:30 AM - *NT2.4.01
Epitaxial BaTiO3 on Silicon and Silicon Germanium: Nanoscale Characterization, Ferroelectricity and Integration into TiN-Gated Devices
Lucie Mazet 1,Sangmo Yang 2,Martin Frank 3,Eduard Cartier 3,Hiroyuki Miyazoe 3,John Bruley 3,Thibaud Denneulin 4,Robin Cours 4,Martin Hytch 4,Jordan Bouaziz 1,Claude Botella 1,Cesar Magen 5,Roger Guzmán 5,Sergei Kalinin 2,Vijay Narayanan 3,Sylvie Schamm-Chardon 4,Catherine Dubourdieu 1
1 CNRS-INL Ecully France,2 Oak Ridge National Laboratory, CNMS Oak Ridge United States3 IBM T.J. Watson Research Center Yorktown Heights United States4 CEMES-CNRS, Université de Toulouse Toulouse France5 LMA-INA, Universidad de Zaragoza and TALEM, CNRS-INA Zaragoza Spain
Show AbstractFerroelectric material integration on semiconductors offers great potential for nanoelectronic and integrated-photonic applications. In particular, introducing a ferroelectric as a gate oxide could decrease the sub-threshold slope below the thermodynamic limit of 60 mV/dec in field-effect transistors (FETs) operating at room temperature. There are still major challenges with growing a ferroelectric oxide on a semiconductor and progress are still needed in obtaining a high quality interface (structurally and electrically), controlling the crystalline structure and orientation, controlling the cationic stoichiometry and oxygen content, and controlling the ferroelectric polarization and domain structure. Beyond film growth, also major progress is needed for the fabrication of integrated devices using the current Si-based nanoelectronic technologies.
In this talk, we will review our work on the molecular beam epitaxy of BaTiO3 on silicon and silicon germanium surfaces. A detailed study of the impact of oxygen partial pressure during the growth of BaTiO3 films on SrTiO3-buffered Si(001) will be presented. The elemental profiles throughout the stacks were determined from electron energy loss spectroscopy (EELS) analyses. A quantitative analysis of atomic structure images (HRTEM or STEM-HAADF) using the geometric phase analysis (GPA) was carried out in order to map the strain in the stacks. We will show that local composition and crystalline parameters are strongly connected.
Si1-xGex is of interest as a channel for future FETs and, via its Ge content, has the potential to be a lever for strain engineering. Passivation of the starting surface of Si1-xGex with Sr or Ba fluxes has been studied by in situ X-ray photoelectron spectroscopy; only partial passivation with Sr is evidenced when using our standard recipe for Si surface. The direct growth of BaTiO3 on Si1-xGex will be discussed.
We will discuss the ferroelectricity of the films grown on Si and Si1-xGex, as a function of BaTiO3 – and if relevant - SrTiO3 buffer thicknesses. Piezoresponse force microscopy imaging and hysteresis loops as well as Kelvin probe microscopy were carried out in various conditions.
Finally, we will describe i) the fabrication of Metal Oxide Semiconductor (MOS) capacitors using pre-patterned Si substrates with oxide isolation and using TiN as a metal gate. Ii) the fabrication of TiN-gated BaTiO3 gate transistors based on a low temperature route starting from damascene pre-patterned structures. Technical challenges and achievements will be discussed.
12:00 PM - NT2.4.02
Switchable Ferroelectric/Piezoelectric and Energy Storage Properties in Morphotropic Phase Boundary 0.5BaZr0.2Ti0.8O3-0.5Ba0.7Ca0.3TiO3 (BZT-BCT) Lead-Free Polycrystalline Thin Films
Venkata Puli 3,Dhiren Pradhan 3,Shiva Adireddy 2,Alexei Gruverman 4,Ram Katiyar 3,Douglas Chrisey 2
1 Univ of Texas El Paso United States,2 Physics and Engineering Physics Tulane University New Orleans United States,3 Physics University of Puerto Rico San Juan United States,3 Physics University of Puerto Rico San Juan United States2 Physics and Engineering Physics Tulane University New Orleans United States4 Physics University of Nebraska Lincoln United States
Show AbstractPolycrystalline morphotropic-phase-boundary lead (Pb) free 0.5BaZr0.2Ti0.8O3-0.5Ba0.7Ca0.3TiO3 (BZT-BCT) thin films were deposited on Pt(111)/TiO2/SiO2/Si substrates by pulsed laser deposition (PLD). X-ray diffraction confirmed a polycrystalline growth of the as-deposited films with single-phase perovskite crystallographic structure with body centered tetragonal symmetry. The optimal ferroelectric response with a high spontaneous polarization Ps~110 μC/cm2, remnant polarization of 32.5 μC/cm2 with a coercive field of 0.18 MV/cm is observed from P-E hysteresis loops with a frequency of 1 kHz at room temperature. A giant recoverable energy-storage density of 33.39 J/cm3 at 3.58 MV/cm. The optimized BZT–BCT thin film exhibited a high dielectric constant of with a low dielectric loss. Distinct polarization contrast with a complex mosaic-like domain structure was observed in the out-of-plane mode of piezoresponse force microscopy (PFM). Switchable ferroelectric PFM hysteresis loops were obtained. Ferroelectric switching is not detected from PFM phase, amplitude images, after applying ±15V which exceeding the coercive bias voltage (from the obtained hysteresis loops). This may be due to rapid polarization relaxation.
12:15 PM - *NT2.4.03
Antiferromagnetic Coupling at the Interface of Two Ferromagnetic Layers: La0.7Sr0.3MnO3 and SrRuO3
A. Solignac 1,Georg Kurij 2,Ruben Guerrero 2,Philippe Gogol 2,Thomas Maroutian 2,Frederic Ott 3,Ludovic Largeau 4,Philippe Lecoeur 2,Claude Fermon 1,Myriam Pannetier-Lecoeur 1
1 SPEC, CEA, CNRS Université Paris-Saclay, CEA Saclay Gif sur Yvette France,2 Université Paris Sud, CNRS, UMR8622 Inst. Elect. Fondamentale Orsay France3 LLB, CEA, CNRS Université Paris-Saclay, CEA Saclay Gif sur Yvette Cedex France4 CNRS-Laboratoire de Photonique et de Nanostructures, route de Nozay Marcoussis France
Show AbstractLa0.7Sr0.3MnO3 (LSMO) and SrRuO3 (SRO) are two ferromagnetic materials with Curie temperatures of respectively 350K and 150K. A positive exchange bias has been observed in bilayers and superlattices of LSMO/SRO [1,2,3] which originates from an antiferromagnetic coupling at the interface between these two materials. This phenomenon is of particular interest to obtain a hard layer for magnetic oxide junctions where the stabilization of a reference layer is a crucial point.
LSMO/SRO bilayers have been grown epitaxially by pulsed laser deposition on SrTiO3 (STO) substrate. By controlling the substrate properties (orientation and surface properties) and the layer growth mode, we are able to control the SRO crystallographic variants present in LSMO/SRO bilayers. X-Ray diffraction and AFM characterizations show the good crystalline quality of the deposited layers but also reveal the SRO crystallographic variants.
In this paper, we will show that the SRO crystallographic variants seem to strongly affect the magnetic behavior of the bilayer measured by magnetometry. If several crystallographic variants are present in the SRO, an asymmetry in the LSMO hysteresis loop is observed [4]. We attributed it to the presence of two different exchange coupling strengths at the interface between LSMO and SRO by combining magnetometry, Polarized Neutrons Reflectivity (PNR) measurements and simulations based on an extended Stoner-Wohlfarth model. When the SRO has one crystallographic variant, the hysteresis cycles of the LSMO, coupled to SRO, are also asymmetric but in an unexpected way. PNR measurements show that the LSMO magnetization reversal is uniform and that only one exchange coupling strength is, this time, present at the SRO/LSMO interface. Simulations and complementary PNR experiments are currently performed to understand the origin of the hysteresis cycle asymmetry. One possibility is the SRO uniaxial anisotropy which possesses an out-of-plane component.
[1] X. Ke, M. S. Rzchowski, L. J. Belenky and C. B. Eom, Appl. Phys. Lett. 84, 5458 (2004)
[2] P. Padhan, W. Prellier and R. C. Budhani, Appl. Phys. Lett. 88, 192509 (2006)
[3]M. Ziese, I. Vrejoiu, E. Pippel, P. Esquinazi, D. Hesse, C. Etz, J. Henk, A. Ernst, I.V. Maznichenko, W. Hergert and I. Mertig , Phys. Rev. Lett. 104, 167203 (2010)
[4] A. Solignac, R. Guerrero, P. Gogol, T. Maroutian, F. Ott, L. Largeau, Ph. Lecoeur, M. Pannetier-Lecoeur, Dual antiferromagnetic coupling at La0.67Sr0.33MnO3/SrRuO3 interfaces, Phys. Rev. Lett. 109, 027201 (2012).
12:45 PM - NT2.4.04
Nature of Magnetoelectric Coupling in Multiferroic Composites
Dhiren Pradhan 1,Venkata Puli 2,Shalini Kumari 1,Kallol Pradhan 1,Ram Katiyar 1
1 Univ of Puerto Rico San Juan United States,2 Department of Mechanical Engineering, College of Engineering University of Texas El Paso United States
Show AbstractMultiferroic magnetoelectric materials – those which exhibit simultaneous ferroelectric and ferromagnetic behaviors and permit control and switching of the magnetic order parameter, magnetization M via electric field E and, vice versa, polarization P with magnetic field H, have drawn significant interest in recent years because of their intriguing physical origin and great potential for multifunctional applications. Understanding the coupling of the ferroic (electric and magnetic) order parameters in multiferroics is a long-standing scientific challenge. We report studies of the ferroelectric and magnetic phase transition of Pb(Fe0.5Nb0.5)O3-Co0.65Zn0.35Fe2O4 (x = 0.2) composite with emphasis upon the nature of magnetoelectric coupling at room temperature. The presence of all cationic elements with their required stoichiometry has been confirmed by SEM and XPS studies. The composite shows well-saturated ferroelectric and ferromagnetic (multiferroic) behavior at room temperature. A ferroelectric-paraelectric phase transition around 428 K has been confirmed from the temperature dependent dielectric spectra along with DSC and Raman spectroscopic studies. Existences of different magnetic orderings have been observed in this composite. The nature of magnetoelectric coupling is found to be biquadratic (P2M2) from magnetic and magnetocapacitance measurements. The mechanism responsible for the magnetoelectric effect in this composite takes place indirectly via ferroelastic strain rather than a direct coupling between the magnetic and electric order parameters. This composite shows strong bulk (not interfacial) biquadratic magnetoelectric coupling at room temperature, which can be useful for potential multifunctional device applications.
NT2.5: 2D Materials/Topological Insulators
Session Chairs
Ningzhong Bao
Ashutosh Tiwari
Wednesday PM, March 30, 2016
PCC North, 100 Level, Room 132 AB
2:30 PM - NT2.5.01
TiO2 as a Catalyst for Topological Insulator Nanowire Growth
Piet Schoenherr 1,Fengyu Zhang 2,Thorsten Hesjedal 1
1 University of Oxford Oxford United Kingdom,1 University of Oxford Oxford United Kingdom,2 University of Science and Technology of China Hefei China
Show AbstractTopological insulators (TIs) are materials in which spin-orbit interaction and time-reversal symmetry cause exciting quantum properties such as electronic spin-momentum interlocking and dissipationless current flow. These phenomena emerge at the interface of the TI with an insulator, e.g. vacuum. Bismuth chalcogenide TIs have a high n-type bulk conductivity mostly due to vacancies and defects which masks electronic transport through surface electrons. Hence, TI nanowires are a natural choice to enhance the surface effects and suppress the relative contribution of bulk carriers.
Typically, TI nanowires, such as Bi2Te3, are grown using wet-chemical synthesis and chemical vapor deposition (CVD). CVD is more suited to produce clean samples with pristine surfaces. Catalyzed nanowire growth usually employs gold nanoparticles as seeds. We found three shortcomings following this approach: (1) The nanowires are contaminated with Au. (2) The crystal quality is low. (3) Undesired, differently shaped nanostructures are grown alongside the nanowires.
Therefore, we explored more exotic catalysts such as SiC and TiO2, as well as self-catalyzed nanowire growth for comparison [1]. A special mixture of TiO2 nanoparticles, P-25, delivered excellent results and performed much better than the conventional Au catalyst. P-25 catalyzed Bi2Te3 nanowires are of high crystalline quality, free of catalyst contaminations, and they grow almost purely as nanowires without the undesired formation of ribbons or clusters.
P-25 also supports the integration of Sn into the crystal structure of the TI nanowires. Sn atoms act as p-type dopants to counter the intrinsic, defect-related n-type doping of the material so that the bulk becomes truly insulating as desired for applications.
[1] Schönherr, P. et al. Comparison of Au and TiO2 based catalysts for the synthesis of chalcogenide nanowires. Applied Physics Letters 104, 253103 (2014).
2:45 PM - *NT2.5.02
Pulsed Laser Synthesis of Oxide and Chalcogenide-Based 2D van der Walls Materials
Ashutosh Tiwari 1
1 University of Utah Salt Lake City United States,
Show AbstractDuring the last few years, two-dimensional (2D) inorganic materials have emerged as a premiere class of materials for fabricating modern electronic devices. The interest in 2D layered Transition Metal Chalcogenides and Oxides is particularly high. These materials are being heavily researched due to their novel functionalities and suitability for a wide range of electronic and optoelectronic device applications. In this talk, I will present the latest work going on in this area in my lab at the university of Utah. Major focus will be on the synthesis and the layer dependent evolution of crystal, phonon, and electronic structure of these materials. A pulsed laser assisted deposition method capable of producing layer-by-layer growth will be introduced. Detailed characterization results using atomic force microscopy (AFM), Raman spectroscopy, UV-Vis spectroscopy, and photoluminescence measurements will be presented.
3:15 PM - NT2.5.03
First Thin Films and the Correlation of Crystal Quality to Titanic MR in Weyl Semimetal, WTe2
Mazhar Ali 1
1 IBM/MPI-Halle San Jose United States,
Show AbstractRecently the layered transition metal dichalcogenide WTe2 was reported to have an extremely large, non-saturating magnetoresistance (XMR), reaching values of 13 million % at 0.53 kelvin (K) and 60 tesla (T). Larger than the giant MR (GMR) or colossal MR (CMR) effects previously seen, this makes WTe2 of interest due to both its fundamental materials physics and its potential for low-temperature applications in magnetic sensors or computing-related devices. Following this work, additional studies on WTe2 have been conducted with findings including pressure-induced superconductivity, orbital spin texturing, a large and linear Nernst effect, predictions of a quantum spin Hall insulating state under strain, and the realization of one of the first Weyl Semimetals and, in particular, the first example of a Type II Weyl Semimetal. Several other materials such as Cd3As2, NbSb2, TaAs, NbAs, and NbP, ZrSiS, have also recently been reported to show a similarly large MR effect.
Here we present a correlation of RRR, MR ratio and average carrier mobility in the crystal growth of WTe2. A preferred method of single crystal synthesis is presented, consisting of a Te flux followed by a cleaning step involving self-vapor transport. The method is reproducible and yields consistently higherquality single crystals than are typically obtained via halide-assisted vapor transport methods. Magnetoresistance (MR) values at 9 tesla and 2 kelvin as high as 1.75 million %, nearly an order of magnitude higher than previously reported for this material, were obtained on crystals with residual resistivity ratio (RRR) of approximately 1250. The MR follows a near B2 law (B = 1.95(1)) and, assuming a semiclassical model, the average carrier mobility for the highest-quality crystal was found to be 167, 000 cm2/Vs at 2 K. Also, we will present the first successful creation of crystalline thin films of WTe2 using a simple and scalable technique (8” wafer processing is demonstrated). These are the first thin films of any Weyl Semimetal; film characterization and transport properties will be discussed.
3:30 PM - NT2.5.04
Electrical and Electromechanical Properties of WS2 Nanotubes
Roi Levi 1,Jonathan Garel 1,David Teich 2,Gotthard Seifert 2,Reshef Tenne 1,Ernesto Joselevich 1
1 Materials and Interfaces Department Weizmann Institute of Science Rehovot Israel,2 Theoretische Chemie Technische Universität Dresden Dresden Germany
Show AbstractThe use of various nanostructures such as nanotubes and 2D sheets in electrical and electromechanical devices is the subject of intensive research in recent years. In particular, the electronic properties of inorganic compounds such as the dichlcogenides sparked the research of their incorporation into nano-electro-mechanical systems (NEMS). WS2 nanotubes (INT-WS2) have been shown to exhibit superior mechanical properties and interesting stick-slip mechanical phenomena1 and thus are a natural candidate for electro-mechanical devices.
We show here that INT-WS2 possess significant field-effect mobility and surprisingly high current carrying capacity2. We further present the first demonstration of a significant electro-mechanical response in pure inorganic nanotubes3. The INT-WS2 exhibited a highly repeatable increase of the conductivity in response to strain and/or torsion. These results are in qualitative agreement with the theoretical calculations presented here for torsion and previous theoretical predictions for strain. The large sensitivity to torsion and tension suggests INT-WS2 as promising in NEMS such as nano-gyroscopes and accelerometers.
References
(1) Nagapriya, K. S.; Goldbart, O.; Kaplan-Ashiri, I.; Seifert, G.; Tenne, R.; Joselevich, E. Physical Review Letters 2008, 101, 195501.
(2) Levi, R.; Bitton, O.; Leitus, G.; Tenne, R.; Joselevich, E. Nano Letters 2013, 13, 3736-3741.
(3) Levi, R.; Garel, J.;Teich, D.; Seifert, G.; Tenne, R.; Joselevich, E. ACS Nano 2015, Article ASAP.
3:45 PM - NT2.5.05
Atomically Thin Ohmic Contacts to Two Dimensional Semiconducting Films
Hui Gao 1,Marcos Guimaraes 1,Kibum Kang 1,Yimo Han 1,David Muller 1,Jiwoong Park 1
1 Cornell University Ithaca United States,
Show AbstractTwo dimension materials, such as graphene and transition metal dichalcogenides (TMDs), could enable atomically thin devices and circuits with novel electrical, optical, and mechanical properties, including the valley Hall Effect observed in MoS2. However, the integration of these materials into functional devices is challenging due to the large contact resistance between them and the conventional metal electrodes (e.g. Ti, Au) with the typical values greater than 50 kΩ.µm. The large Schottky barrier height at the metal/TMD junction limits the injection of electrons to the TMDs and thus degrade the performance of the device, an effect even more significant when the dimensions are scaled down to sub-micron level where the contact resistance becomes dominant.
Here we demonstrate the use of CVD graphene as a universal contact electrode to various TMDs in which the junction is atomically sharp and shows low resistance ohmic behavior. To form the lateral contact, CVD graphene is transferred and patterned on a SiO2/Si substrate, after which TMDs are grown using MOCVD. The resulting graphene and TMD lateral heterostructures are mechanically strong and form atomically sharp junction in which the properties of each individual materials are well preserved. The low temperature measurements, down to 77K, shows the independence between contact resistance and temperature which implies a very small Shottky barrier height. At the same time, we have shown that graphene can be used as a universal contact electrode to TMDs by demonstrating its similarly low contact resistance (~20 kΩ.µm) to MoS2 and WS2 with excellent spatial controllability and scalability.
4:30 PM - *NT2.5.06
Novel Two-Dimensional Multifunctional Materials and Direct Conversion of Carbon into Diamond (Discovery of Q-Carbon)
Jagdish Narayan 1
1 North Carolina State Univ Raleigh United States,
Show AbstractThis lecture covers novel two-dimensional materials, including graphene and graphene oxide, topological insulators, and discovery of new phase of carbon (Q-carbon). Independent control of crystal structure and chemistry can lead to transformative new functional materials and devices. These materials are grown by laser annealing or by templating the structure of the underlying growth substrate. By laser annealing, near surface regions are modified by introducing defects and /or dopants which lead to desirable 2D electrical and magnetic properties. Templating leads to 2D metamaterials that assume the structure of the growth substrate and exhibit novel properties not otherwise achievable. We have created novel two-dimensional metamaterials such as bcc Ni, NiO, ZnO (wurtzite and zinc blende) and novel SrSnO3 and BiTeSe2 TIs. High-power pulsed laser annealing will be used to introduce defects in the top few layers of ZnO and NiO and modify the electrical, optical and magnetic properties in a controlled way.
The second part focuses on the discovery of new phase (distinct structure, bonding and entropy) of carbon, named as Q-carbon. The properties of Q-carbon are as follows: (1) The Q-carbon is amorphous and it can be semiconductor or metallic, and it has robust ferromagnetism with Curie temperature over 500K; (2) The Q-carbon has very high electron emission (negative electron affinity), needed for a variety of display devices; (3) Q-carbon can be harder than diamond as average bond length in Q-carbon is shorter C-C length in diamond; (4) Q-carbon can be converted into diamond rather inexpensively in the form of nanodots, microdiamonds, nanoneedles and microneedles, and large-area single-crystal films; and (5) diamond can be doped both n-type and p-type, creating a new frontier in high-power devices and high-temperature transistors. We have obtained recently parallel results from hBN, Q-BN and cBN.
5:00 PM - NT2.5.07
Formation and Electronic Characterization of Large Area MoS2 Single-Layer Films
Michael Gomez 1,Joseph Martinez 1,Mike Valentin 1,Daniel Lu 1,Edwin Preciado 1,Velveth Klee 1,Ariana Nguyen 1,Ludwig Bartels 1
1 Materials Science and Engineering UC Riverside Riverside United States,
Show AbstractUsing a novel high vacuum CVD process we synthesize large area monolayer MoS2 films. Our process involves exposure of a hot Mo filament to organic chalcogen which then precipitate on a thermally-controlled substrate. The resultant films are photoluminescent at the 1.87 eV as expected for monolayer material; their Raman modes are indistinguishable from exfoliated material. Metal contact formation to these films was investigated under UHV conditions utilizing X-Ray Photoelectron Spectroscopy. These measurements permit us to follow the formation of a Schottky Barrier with increasing metal film thickness on the Angstrom scale. We utilize core level spectroscopy to indicate the evolution of the MoS2 valence band under metal deposition. Our measurements provide direct indication on the charge transfer direction at metal contacts and the ensuing band-bending in two-terminal devices.
5:15 PM - NT2.5.08
Tuning the Properties of MoS2 through Covalent Surface Functionalization
Elizabeth Keenan 1,Linyou Cao 2,Wei You 1
1 Univ of North Carolina-Chapel Hill Chapel Hill United States,2 North Carolina State University Raleigh United States
Show AbstractMolybdenum disulfide (MoS2) is a transition metal chalcogenide that has unique properties on a two-dimensional level. MoS2 has potential to be used in different electronic applications, but more needs to be known about doping and controlling the properties of few layer films. This study presents a way to chemically modify the surface of MoS2, which leads to doping the film without disruption of the crystal structure. There are examples of doping MoS2 with an adsorbed molecule, but few where the dopant is covalently bound to the surface. By taking advantage of the sulfur defects on the surface, this process is able to create a covalent bond between MoS2 and an organic molecule.
First, MoS2 was grown through a chemical vapor deposition (CVD) process to produce large area films with uniform composition. An organic molecule, 3-aminopropyltrimethoxysilane (APTMS) was used to functionalize the surface through a CVD process. The presence of APTMS on the surface was confirmed using X-ray photoelectron spectroscopy (XPS) by the appearance of a Si peak. The O 1s XPS peaks show the appearance of an Mo-O peak, which suggests that the APTMS bonds to the MoS2 through an Mo-O bond. The Mo 3d and S 2p XPS peaks show no change, which supports the fact that APTMS functionalization offers a stable, yet non-destructive, method to dope MoS2.
The APTMS has a p-doping effect on the MoS2, which is seen through XPS and photoluminescence (PL) spectroscopy. First, the APTMS functionalization causes a downshift in binding energy of both the Mo and S XPS peaks. This indicates a lowering of the Fermi level, making the MoS2 more p-type. A Fermi level change is also suggested through valence band XPS, which shows a change in the valence band energy of MoS2 after functionalization. Second, the PL peak shows a large increase in intensity after functionalization. It has been previously reported that p-doping of MoS2 will cause an increase in PL intensity. This result agrees with the XPS that the APTMS does p-dope the MoS2.
Lastly, this experiment was run using a different organic molecule, octyltrichlorosilane (OTS). This molecule is believed to bond to the MoS2 surface in the same way, through an Mo-O bond. The OTS molecule causes very similar changes in the MoS2 properties and also points toward p-type doping of the material. Even though the two molecules have different end groups, they give a similar effect. This is evidence that the main influence on the doping of the MoS2 is the silane at the interface and the covalent bond and not the rest of the molecule. This gives insight into how changes at the interface of MoS2 are important to the underlying properties.
This procedure provides a way to tune the optical properties and the doping of MoS2, which in turn may affect the electronic properties. This technique can be expanded to include more organic molecules and has potential to be used for different chalcogenide semiconductors, leading to new ways to dope different materials.
5:30 PM - NT2.5.09
Two-Dimensional Molybdenum Disulfide from ALD Molybdenum Oxide
Brent Keller 2,Adam Bertuch 1,Nicola Ferralis 2,J Provine 3,Jeffrey Grossman 2,Ganesh Sundaram 1
2 Materials Science and Engineering Massachusetts Institute of Technology Cambridge United States,1 Ultratech-CNT Inc Waltham United States3 3Department of Electrical Engineering and Center for Integrated Systems Stanford Stanford United States
Show AbstractRecent advances in the field of two-dimensional (2D) dichalcogenide materials have indicated that the use of vapor deposition techniques such as Chemical Vapor Deposition (CVD)1 or Atomic Layer Deposition (ALD)2 could be instrumental to the production of large area two-dimensional materials and simultaneously offer superior alternatives to exfoliating methods. In this work we consider the transition metal dichalcogenide MoS2 – specifically its growth starting from an ALD route comprised of (tBuN)2(NMe2)2Mo and O3 to produce amorphous MoO3,3 followed by subsequent sulfurization steps to produce MoS2. The chemical conversion to MoO2-xSx and finally 2H MoS2 through the multistep sulfurization process is studied. Data from a variety of direct measurement techniques for determining the existence and quality of monolayer MoS2, including atomic force microscopy, x-ray photoelectron spectroscopy, Raman spectroscopy, and photoluminescence spectroscopy will be presented. A method of thinning oxide films from the post nucleation growth regime by plasma etching to produce the desired thickness of MoS2 is demonstrated. Finally, the effects of temperature, incorporation of plasma, precursor exposure, and surface cleaning – both wet and plasma – on ALD nucleation kinetics are explored and shown to be critical to the production of high quality films in the monolayer and few layer regime without an intermediate etching step.
References
1. Shi et. al. Chem. Soc. Rev. 2014; DOI: 10.1039/c4cs00256c
2. Song et. al. ACS Nano, 2014, 7 (12), pp 11333-11340; DOI: 10.1021/nn405194e
3. Bertuch. et.al. J. Vac Sci Tech. A 32, 01A119, 2014; DOI: 10.1116/1.4843595
NT2.6: Poster Session II: Energy
Session Chairs
Thursday AM, March 31, 2016
Sheraton, Third Level, Phoenix Ballroom
9:00 PM - NT2.6.01
A New Member of the Family of Topological Insulators in the Chalcogenide Group: (Bi0.6Sb0.4)2Se3
Piet Schoenherr 1,Shilei Zhang 1,Yuanqian Liu 2,Patryk Kusch 3,Stephanie Reich 3,Terence Giles 4,Dominik Daisenberger 4,Dharmalingam Prabhakaran 1,Yulin Chen 1,Thorsten Hesjedal 1
1 University of Oxford Oxford United Kingdom,1 University of Oxford Oxford United Kingdom,2 University of California Berkeley United States3 Freie Universität Berlin Berlin Germany4 Diamond Light Source Didcot United Kingdom
Show AbstractTopological insulators (TIs) showcase fascinating quantum effects that have attracted more and more interest during the last decade, as well as offering a promising platform for applications in spintronics. The signature of a TI is its topologically protected surface state. The 3D TI Bi2Se3 was among the first that were experimentally realized. It is part of the chalcogenide family of TIs which includes the layered, binary compounds formed by Bi, Sb, Se, and Te among which only Sb2Se3 is not layered and a trivial insulator. The Dirac point of Bi2Se3 and Bi2Te3 is located under the valence band maximum due to n-type doping caused by vacancies and defects. Counterdoping can reduce the intrinsic carriers and align the Fermi level with the Dirac point. Another approach is simply the reduction of the surface-to-volume ratio by using nanomaterials, where the contribution of surface electrons to the electronic transport is increased.
We explored the growth of ternary chalcogenide nanowires using chemical vapor deposition (CVD) with a special catalyst, TiO2, that increased the yield of nanowires. We found a new TI with a non-layered structure [1]. The material, (Bi0.6Sb0.4)2Se3, has an orthorhombic crystal structure characterized by conducting quasi one-dimensional atomic chains. Its electronic band structure displays a single Dirac cone and the Fermi energy is aligned with the bandgap as measured by nano angle-resolved photoemission spectroscopy (nanoARPES). The new material resembles Sb2Se3, which has been shown to be a TI at high pressures as well.
[1] Schönherr, P. et al. A new topological insulator built from quasi one-dimensional atomic ribbons. Phys. Status Solidi RRL 9, 130–135 (2015).
9:00 PM - NT2.6.02
Recombination Mechanisms in Cu(In1-xGax)Se2 Photovoltaics at Single-Grain Resolution
Ahsan Ashraf 4,D. M. Nanditha Dissanayake 2,Wei Zhang 1,Dan Dwyer 3,Kim Kisslinger 1,Feng Wang 1,Harry Efstathiadis 3,Matthew Eisaman 4
1 Brookhaven National Laboratory Upton United States,4 Stony Brook University Stony Brook United States,1 Brookhaven National Laboratory Upton United States,2 Voxtel Inc. Eugene United States1 Brookhaven National Laboratory Upton United States3 SUNY Polytechnic Institute Albany United States
Show AbstractChalcopyrite semiconductors such as Cu(In1-xGax)Se2 (CIGS) are promising materials for high-efficiency thin-film photovoltaics; however, due to significant lateral spatial inhomogeneity in composition (and consequently structure), the recombination mechanisms that limit power conversion efficiency (PCE) can vary at a single-grain level. Although the PCE of CIGS solar cells is believed to be largely limited by the radiative and non-radiative recombination processes in the bulk (space-charge region and the quasi-neutral region), these measurements have been performed on large area devices, thereby averaging out the effects of spatial inhomogeneities. To understand the effects of these inhomogeneties on the recombination mechanism, we carry out electrical characterization on specific grains as a function of temperature to investigate the nanoscale recombination dynamics of CIGS photovoltaic devices with an optimum gallium composition of ~0.3 and overall copper composition of ~0.9. We correlate these nanoscale electrical measurements with energy dispersive X-ray spectroscopy to determine composition, confocal Raman spectroscopy for phase, and aberration corrected transmission electron microscopy with selected area electron diffraction to ascertain structure at the specific CIGS region to identify the source of the recombination. We find that regions with a higher fraction of copper-rich grains in the absorber material show interfacial (as opposed to bulk) recombination, as revealed using temperature-dependent current-voltage measurements, even though the overall copper composition of the CIGS is sub-stoichiometric. These results provide additional insight into the complexity of recombination dynamics in chalcopyrite materials, and can prove beneficial in identifying strategies for lowering recombination rates in an effort to develop high-efficiency thin-film solar cells.
9:00 PM - NT2.6.03
WS2/WO3 Nanostructured Films for Solar Hydrogen Production: Morphology–Photoactivity Relationship
Aya Mohamed 1,Nageh Allam 1
1 American Univ in Cairo New Cairo Egypt,
Show AbstractNanoporous and nanoflake tungsten oxide (WO3) films were produced on metallic W foil via one-step potentiostatic anodization method. The effect of annealing temperature on the morphological, structural and optical properties of the material was studied. The nanoflake films conserved their topology under different annealing temperatures (up to 500 oC) with particle sizes in the range of 37 to 46 nm. However, the nanoporous films showed a gradual deterioration in the porous structure upon annealing up to 500 oC. X-ray diffraction and Raman spectroscopy measurements were done to investigate the structural properties of the materials. The photoelectrochemical activity tests reveal that nanoflakes annealed at 500oC exhibited a superior photocatalytic performance among all other samples, achieving a photocurrent density of 0.98 mA cm-2 at 1.0 VSCE under AM 1.5 illumination. The UV-Vis absorption and incident photon-to-current efficiency measurements are used to explain the activity-morphology relationship in such nanostructured films. Also, the stability of the films was investigated under light on/off conditions.
9:00 PM - NT2.6.04
On the Properties of Cu2ZnSnS4 Films Prepared by Atmospheric Sulfurization of CuS-SnS-ZnS Precursors Using Ditert-butylsulfide
Ho Ching Ni 2,Chu-Hsien Lin 1,Hsin-Yueh Lin 3,Sheng Wen Wen 1,Shih-Hung Yang 1,Chien-Ming Su 1,Bo-Hao Huang 1,Jyh-Rong Gong 1
1 Department of Physics National Chung Hsing University Taichung Taiwan,2 Department of Physics National Yuanlin Senior High School Yuanlin Township, Changhua County Taiwan,1 Department of Physics National Chung Hsing University Taichung Taiwan1 Department of Physics National Chung Hsing University Taichung Taiwan,3 Department of Computer Science National Taichung Industrial High School Taichung Taiwan
Show AbstractCu2ZnSnS4 (CZTS) films were prepared by atmospheric sulfurization of chemical bath deposited (CBD) CuS-ZnS-SnS precursors using ditert-butylsulfide [(C4H9)2S: DTBS] to serve as an alternative sulfur source for CZTS sulfurization owing to the merits of its improved environmental friendliness and reduced bond dissociation energy of DTBS comparing to that of H2S. In this study, CuS-ZnS-SnS precursors were prepared by immersing glass substrates in diluted solutions containing copper, zinc, tin and sulfur species. The CBD-deposited precursors were, then, sulfurized by using DTBS in an atmospheric pressure quartz reactor at a constant temperature ranging from 400 to 600°C for 10 (or 20) min, under a constant DTBS flow rate of 2 (or 4) μmol/min carried by purified nitrogen gas. The characteristics of DTBS-sulfurized CZTS films were investigated by x-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), absorption spectroscopy, and x-ray photoelectron spectroscopy (XPS). It was found that increments of sulfurization time interval and DTBS admittance at high sulfurization temperatures tended to enable the formation of CZTS structure with improved optical quality and stoichiometry. Sufficient DTBS replenishment and thermal energy are essential to complete the reactions of sulfides through sequential pathways so that high quality DTBS-sulfurized CZTS films can be achieved.
9:00 PM - NT2.6.05
CdS/PbS Thin-Films Deposited by Chemical Bath Deposition for Photovoltaic Applications
Diego Muro 1,Manuel Quevedo 2,Amanda Carillo 1,Maria Mota 1
1 Universidad Autonoma de Ciudad Juarez Ciudad Juarez Mexico,2 Universidad de Sonora Sonora Mexico
Show AbstractThe developed of photovoltaic devices based on CdS / PbS semiconductors offer an alternative to silicon, which require to apply high temperatures to synthesize, generating a high cost in the production to apply this material in the electronic. On the contrary the CdS and PbS offer low costs and it is not necessary to use expensive synthesizing methods. Using chemical bath, large-scale photovoltaic devices can be produced, it is not necessary high-tech tools neither working to elevated temperatures reducing the production costs. The aim of the present work is shows results from the development of photovoltaic device based on chalcogenide semiconductor materials deposited by chemical bath technique.
9:00 PM - NT2.6.06
Studies of Nano-Engineered Fluorine-Doped SnO2 Thin Films with Tunable Haze Factor for Photovoltaic Application
Shanting Zhang 2,Gael Giusti 1,Stephane Brochen 1,Lukas Schmidt-Mende 3,Matthieu Jouvert 1,Jean-Luc Deschanvres 1,Carmen Jiménez 1,David Munoz-Rojas 1,Vincent Consonni 1,Daniel Bellet 1
1 Grenoble Institute of Technology Grenoble France,2 Technical University of Darmstadt Darmstadt Germany,1 Grenoble Institute of Technology Grenoble France3 University of Konstanz Konstanz Germany
Show AbstractIn the field of photovoltaics, especially the third generation, light trapping structure has been widely developed and utilized to improve device performances. A common approach is to texture the surface of the front electrode usually made of transparent conductive oxides (TCOs). One of the important TCOs is polycrystalline fluorine-doped SnO2 (FTO) thin film. Differently textured FTO thin films have been studied and were shown to improve the quantum efficiency of microcrystalline Si (µc-Si) thin film solar cells [1].
However, the texturing of FTO surface is usually realized by specific surface morphology development during film growth, which involves complicated studies thus is time-consuming and not at all straightforward. In this study, we report a cost-effective and easy-to-reproduce routine to fabricate high haze FTO electrodes by introducing the use of nanoparticles. In the first studies, ZnO and sulfur doped TiO2 (S:TiO2) nanoparticles have been used to make ZnO-FTO and S:TiO2-FTO nanocomposites. The structural, electrical and optical properties of such nanocomposites are presented and discussed. By varying the nanoparticle surface density, the haze factors of the nanocomposites can be varied from 0.4% up to 64% in the visible range without degrading total optical transmittance (~90%), while maintaining good electrical properties (i.e. sheet resistance 10-15 ohm/square) [2][3]. With such high haze factors, the nano-composite FTOs are very valuable in integration in solar cells.
[1] T. Oyama et al. MRS online Proc. Libr. (2008) 1101
[2] G. Giusti et al. ACS Appl. Mater. and Interf. 6 (2014) 14096
[3] S.T. Zhang et al. Phys. Status Solidi A (2015), submitted.
9:00 PM - NT2.6.07
Facile Electrochemical Synthesis of p-Type AgSbTe2 Thin Films and Their Thermoelectric Characterization
Jiwon Kim 1,Joo-Youl Lee 2,Jae-Hong Lim 2,Nosang Myung 1,Tingjun Wu 1
1 Chemical and environmental engineering Univ of California-Riverside Riverside United States,2 Electrochemistry Department Korea Institute of Materials Science Changwon Korea (the Republic of)
Show AbstractSilver antimony telluride (AgSbTe2) thin films for the thermoelectric (TE) applications near room temperature were electrochemically synthesized for the first time. This study allows to investigate the key factor that optimizes the TE power of the ternary system as well as to correlate the crystallinity and transport properties with respect to composition. To investigate the effects of crystallinity and composition on electrical and TE properties, amorphous AgxSbyTez solid solutions with various compositions were first electrodeposited by tailoring the Ag+ concentration in the electrolytes, followed by a thermally driven solid-state amorphous-to-nanocrystalline phase transition. Due to the relatively lowered carrier concentration in the nanocrystalline phases, almost two-times higher Seebeck coefficients compared to the values observed in the bulks and higher Hall mobility led to the enhanced power factor of 90 ~ 553 μW/mK2. The optimum TE property of 553 μW/mK2 was obtained from the nanocrystalline AgSbTe2 thin films at room temperature, which is highly comparable to the literature data of the electrodeposited p-type V-VI compounds. This improvement of the TE properties of electrodeposited AgSbTe2 thin films shows the possibility to be utilized as p-type legs for the room temperature operation using the simple and cost-effective electrodeposition method.
9:00 PM - NT2.6.08
Influence of the Critical Thickness on the Optical and Electrical Properties of Transparent Composite Electrodes
Aritra Dhar 1,Zhao Zhao 1,Terry Alford 1
2 Intel Corporation Hillsboro United States,1 Arizona State Univ Tempe United States,1 Arizona State Univ Tempe United States
Show AbstractThe recent increase of interest in indium-free transparent conducting oxides (TCOs) warrants the development of alternatives to replace indium tin oxide (ITO). A valid candidate is the transparent composite electrode (TCE) that has a layered structure of transparent metal oxide (TMO)/metal/TMO. As potential TCE candidate, structures of TiO2/Au/TiO2 (i.e., with various Au thicknesses) are fabricated on flexible substrate by physical vapor deposition at room temperature. The influence of Au thicknesses on optical, electrical properties is also addressed by UV-VIS, four-point probe and Hall measurement. Our results show that the optimal transmittance and sheet resistance are achieved when the Au layer reaches a critical thickness. For this thickness, the optical transmittance at 550 nm is as high as 86% and a figure of merit corresponds to a value of ~ 18 Χ10-3 Ω-1 (which is one of the highest values reported to date).
9:00 PM - NT2.6.09
Development of Planar Peltier Devices Using Chalcogenide Thin Films with Overall Sputtering Process
Min-woo Jeong 1,Yong-Jin Park 1,Ho Yong 2,Hoo-Jeong Lee 2,Young-Chang Joo 1
1 Materials Science amp; Engineering Seoul National University Seoul Korea (the Republic of),2 Advanced Materials Science and Engineering Sungkyunkwan University Suwon Korea (the Republic of)
Show AbstractNowadays, over-heat-induced performance degradation of application processor (AP) is a hot topic in the mobile device industry. It is expected that application of Peltier devices, cooling system based on thermoelectric (TE) materials such as chalcogenides, will be the best solution for that problem. Peltier devices have been developed based on bulk TE materials. Because heat problems occur at focused area and space for cooling system is limited on AP, planar (thin-film) and radial type will be the best solution for mobile device rather than bulk type. However, as dimension of TE materials shrinks from bulk- to thin-films, unprecedented problems raised, which are degradation of TE properties, poor adhesion with substrate, and self-heating due to contact resistance. In this study, we design the planar type radial peltier devices for focusing at the heating area, with considering the problems of TE thin-films. We improved TE material properties with controlling defects in TE thin-film, adhesion property, and selected electrode metal satisfying ohmic contact between metal and TE materials for carrier transport with low resistance.
TE films were deposited on SiO2/Si wafer by RF magnetron sputtering. TE films were co-deposited with Bi2Te3 and Te for N-type film and Bi2Te3 and Sb2Te3 for P-type film. Annealing, N doping, and Ar implantation was proceeded for improvement of TE properties. Ge2Sb2Te5 (GST) film was selected for adhesion layer. Electrode was deposited with DC magnetron sputtering.
For improving TE properties, Seebeck coefficient (α) and electrical conductivity (σ) of the films were increased with annealing temperature below 300 °C. N doping, Ar implantation changed native defects of TE thin film. Carrier concentration and mobility were increased simultaneously at Ar implanted thin film. After all, TE thin films were optimized similarly with other process for deposition TE thin film, which has about 1300 S/cm and -110 μW/K2cm for N-type and 1800 S/cm 130 μW/K2cm for P-type, α and σ respectively. For enhancement of adhesion, using GST under layer improved adhesion property. Both GST and TE materials have the similar hexagonal unit cell structure which can reduce the lattice mismatch. Furthermore, the low thermal conductivity of GST prevent heat spreading into substrate which causes power wasting of device. For lowering contact resistance, the work function of metal has to lower than N-type and higher than P-type for ohmic contact. Electrode is deposited with various metal, Ti, Cu, Au. Among them, Au satisfied above conditions and showed lowest contact resistance. Based on the studies of each of TE type film deposition above, planar type radial peltier device can be fabricated with overall sputtering process, which cool down the small and focused heat area. N-type and P-type TE materials are connected in series with low contact resistance metal. The performance of planar radial peltier devices will be also discussed using thermos-graphic camera.
9:00 PM - NT2.6.10
Fabrication, Characterization and Integration of Ultrathin High Voltage and High-Density Power Capacitors with High Frequency Stability and Low ESR
Parthasarathi Chakraborti 1,Himani Sharma 1,Markondeya Raj Pulugurtha 1,Rao Tummala 1
1 Georgia Inst of Technology Atlanta United States,
Show AbstractThis work demonstrates the fabrication, characterization and integration of thin-film high density capacitors (HDCAP) based on tantalum in a power module for low power applications. Existing discrete tantalum capacitor technology meets the HDCAP requirements. However, they have incompatibility issues with organic/silicon packages bringing in several packaging challenges. Similarly, silicon trench capacitors employ ALD process rendering it expensive and non-scalable.
Demand for higher volumetric efficiency capacitors with electronic package compatibility requires high surface area architectures, low temperature, low cost and scalable processes and a more elegant approach to fabricate HDCAPs. In this work, thin-film capacitors were fabricated by printing and subsequent sintering of Ta nanoparticles to form high surface area anode followed by conformal dielectric oxide growth through the process of anodization and dispensing of cathode fluid inside the porous electrode structure. The pre-fabricated capacitor foils were laminated to the silicon substrate with a thin, ultra-low loss adhesive using a vacuum laminator. A planarization layer was laminated onto the tantalum films. Multiple blind vias were drilled through the Tantalum and polymer layers. Some of the vias were used to access the copper pads on the Si and the others to access the capacitor cathodes and anodes. As a final step, semi-additive copper process was used to form via connections and also the top assembly and test-pads.
The anodization process leads to growth of thin films with higher capacitance densities and higher yield at low temperatures. The self-limiting nature leads to uniform oxide coating on porous foils. The low temperature dielectric synthesis could enable their integration in ICs and packages. This will assist system miniaturization marred by a host of process incompatibility /manufacturability issues. Such integrated HDCAPs could be used in (a) high speed digital applications for noise suppression in power supply, (b) bio-electronic systems (c) miniaturized RF components, (d) storage cells and gate dielectrics for semiconductor devices. The capacitors demonstrated stable volumetric capacitance densities of more than 4 µF/mm3 upto 10 MHz with leakage of less than 0.1 µA/µF at 3 V. Compositional (EDS), Structural (SEM/XRD) and electrical studies were done to elucidate tantalum-pentoxide films.
9:00 PM - NT2.6.11
Towards Lead-Doped ZnS Nanocrystals for Solar Hydrogen Generation
Allison Durr 1,Armanda McFadden 1,Dylan Perraut 1,Judith Jenkins 1
1 Chemistry Eastern Kentucky University Richmond United States,
Show AbstractEfficient and affordable energy conversion and energy storage technologies are required to meet society’s increasing demands. Semiconductor nanocrystals are particularly attractive materials for solar energy conversion applications, as their tunable optoelectronic properties can be manipulated to both optimize the absorbance of solar photons and to afford desirable electronic properties for various solar electricity, photocatalytic, or thermoelectric applications. Further tunability of binary systems can be realized through substitutional doping, where a small fraction of the cations or anions in the nanocrystal lattice are replaced by ions of other elements, thereby introducing new electronic states into the nanocrystal. However, doping can be difficult, as the dopants can cause significant lattice strain in the host crystals. Thus, increased synthetic understanding is necessary to afford new functional material systems. Lead-doped ZnS nanocrystals are one promising material for the conversion of solar photons into storable fuels such as hydrogen gas. The ZnS conduction band is sufficiently high in energy to reduce protons, and the lead dopants are hypothesized to add filled states in the ZnS band gap, thereby extending the absorbance of the crystals into the visible region. This work details progress towards controllable doping of ZnS nanocrystals with lead cations using modified hot injection procedures. Preliminary results suggest that the temperature of the ZnS reaction matrix upon lead precursor injection can be used to afford various doped products. Spectroscopic data will be presented to demonstrate doping-dependent optical and electronic properties in combination with high resolution transmission electron micrographs for structural information. The early results of this work suggest that functional materials for hydrogen gas generation may be readily synthesized from earth-abundant precursors, representing a viable option for future solar energy conversion platforms.
9:00 PM - NT2.6.12
CuS Thin Films Obtained by Chemical Bath Deposition for Transistor Devices
Oliver Garcia Saucedo 1,Daniela Ortiz Ramos 2,Luis Gonzalez 3,Carlos Muniz Valdez 1
1 Facultad de Ingenieria de la Universidad Autonoma de Coahuila Arteaga Mexico,2 Nanotecnologia Centro de Investigacion y Estudios Avanzados del Instituto Politecnico Nacional Ramos Arizpe Mexico3 Ingeniería Ceramica Centro de Investigacion y Estudios Avanzados del Instituto Politecnico Nacional Saltillo Mexico
Show AbstractNowadays, many applications of optoelectronics require materials with tailored properties, specifically, those referred to get a desired electrical conductivity and optical transparency. Most of the investigations in the literature correspond to the development of n-type transparent metallic oxide semiconductors, which are characterized by having a wide bandgap energy value. However, optoelectronics applications also require p-type semiconductors. Recently, besides of metallic oxides, chalcogenide metallic materials have become an important research field in order to get p-type semiconductors. Specifically, CuS is a promising material for device applications such as transistor devices. Here, we show preliminary results on CuS films obtained by a chemical bath deposition method (CBD). For the reaction solutions we used cupric Nitrate (Cu (NO3)2) as source of Copper (Cu) ions, Thiourea as source of Sulfur (S) ions and triethanolamine (TEA) as complexing agent. Potassium hydroxide (KOH) was used to adjust the solution to pH 9. Clean glass substrates were vertically immersed into the solution, contained in a beaker at a temperature of 60oC, and were taken out after 15 minutes. For all the cases, the results from X-ray diffraction shows that CuS thin films are amorphous. However, the Raman spectroscopy showed the presence of CuS. The morphology of the CuS thin films was a homogeneous deposition of rice shaped particles. The size of the particles was approximately 50 nm. The XEDS[L1] analyses showed a stoichiometric deposition of Cu and S. The optical transmittance obtained for the CuS thin films was approximately 75% and the results of the energy band gap was 2.38 eV. The obtained thin films were used to fabricate thin film transistors (TFTs) which have shown good output curves. A study on the electrical response of the TFTs to luminous sources is also shown.
[L1] X-ray energy dispersive spectroscopy.
9:00 PM - NT2.6.13
Characterization of PbS Thin Films Obtained by Chemical Bath at Low Temperature Using Sodium Citrate as Complexing Agent
David Ramirez Ceja 1,Luis Gonzalez 1,Jose Escorcia-Garcia 1,Arturo Martinez Enriquez 1
1 CINVESTAV-IPN Ramos Arizpe Mexico,
Show AbstractThe lead sulphide (PbS) is one of the semiconductor materials with low band gap suitable to absorb the near-infrarred region of solar spectrum, in which lies nearly half of the energy from the sun. The PbS is abundant and inexpensive with a band gap Eg that can be tuned from 0.4 to 2.3 eV by changing its crystallite grain size. Furthermore, the PbS is soluble in water which allows its deposition by aqueous chemical methods. In the present work is presented the synthesis and characterization of PbS thin films obtained by chemical bath deposition for their application in thin film solar cells. The chemical solution was composed of lead acetate [Pb(C2H3O2)2 3H2O] and thiourea [CH4N2S] as the precursors for the metal and sulfur ion sources. Sodium citrate [Na3C3H5O(COO)3 2H2O] was used as complexing agent for metal ions. The PbS thin films were deposited at bath temperature of 40°C during 30 and 45 min. The pH of the solution was kept at 12.5 by adding potassium hydroxide [KOH]. The preliminary results show that the surface morphology of the as-deposited PbS thin films are constituted of particulates of 250-300 nm in size, distributed homogeneously over the surface without the presence of voids or cracks. The XRD patterns of the films, obtained at glazing incidence, resemble the cubic structure Galena, indicating that they are crystalline with a crystallite size of 13 nm. The estimated Eg from the Tauc plots of the as-deposited PbS films is of 1.55 and 1.23 eV for the deposition times of 30 and 45 min. On the other hand, the electrical characterization indicates that the PbS films are photo-sensitive to the light; the photo-response of the film is better for that obtained at 30 min while the conductivity is higher for that obtained at 45 min. The hot-probe measurement of the PbS films gives p-type conductivity. An improvement of the optical, electrical and structural properties of the PbS films will be done by heating them at different temperatures under argon/sulfur atmospheres. The resulting PbS films will be tested in a solar cell using a CdS thin film partner as window into the cell structure.
9:00 PM - NT2.6.14
Effect of Ammonium Acetate Concentration on the Structural and Optical Properties of CdS Thin Film Grown by Chemical Bath Deposition Technique
Hamda Al-Thani 1,Abeer Al Yafeai 1,Falah Hasoon 1
1 National Energy and Water Research Center Abu Dhabi United Arab Emirates,
Show AbstractThis study focuses on understanding the influence of incorporating Ammonium Acetate into the chemical bath used for the deposition of CdS thin films, on the final films’ optical, morphology, and microstructural properties. Thus, CdS thin films were deposited on 1” × 2” microscopic glass substrates using chemical bath deposition (CBD) method. The deposition process was carried out in a double jacket beaker with fixed chemical bath temperature of 90°C for a deposition time of 40min. The chemical bath solution consisted of fixed concentrations of Cadmium Acetate, Thiourea, and Ammonium Hydroxide; as such the concentrations values are of 4.8×10-4M; 0.97×10-4M; and 0.2M, respectively. However, in order to study the impact of the ammonium acetate incorporation into the chemical bath and to induce variations in the morphology, structural, and optical properties of the final deposited CdS films. Accordingly, Ammonium Acetate was incorporated into the chemical bath with concentrations that were varied from 0.003M to 0.0122M. Meanwhile, for comparison purposes and to indicate the initial conditions and properties of the CdS films, additional CdS films were deposited in the same chemical bath, but without adding Ammonium Acetate. The pH of the chemical bath was measured during the deposition process. The films’ morphology and the chemical composition was examined by Field Emission Scanning Electron Microscopy (FE-SEM), and the Energy Dispersive spectrometer (EDS), respectively. The X-Ray Diffraction (XRD) was applied to study the structure of the films, including the lattice parameters. Atomic Force Microscopy (AFM) was used to measure the Root-Mean-Square (RMS) surface roughness of the films. Dektak Profilometer was used to determine the CdS films’ thickness, where the films’ optical properties were measured using UV-Vis-NIR spectrometer. Optical energy band gap (Eg), and absorption coefficient (α) were calculated from the transmission spectral data. Preliminary results showed a decrease in the PH of the solution with increasing the concentration of the Ammonium Acetate; leading to increase in the growth rate and consequently increase of the final CdS film thickness. The PH value of the bath was decreasing (from 9.2 down to 8.2) with increasing the concentration of the Ammonium Acetate incorporated into the bath. The chemical composition of the films revealed slight increase in the Cd:S ratio beyond 1 with increasing the Ammonium Acetate concentration in the deposition bath. Whereas, the CdS films exhibited hexagonal crystal structure with (002) preferred orientation. The calculated lattice parameters using Cohen’s method, showed reduction in the crystal size (c:a ratio of the lattice parameters) with increasing Ammonium Acetate concentration in the deposition bath. However, further data and interpretation of the results of this research work will be presented in the detailed paper.
9:00 PM - NT2.6.15
Photoluminescence Imaging as a Tool to Study Catalytic Activity of MoS2
Koichi Yamaguchi 1,Sahar Naghibi 2,Ariana Nguyen 1,Gretel Son 2,Joseph Martinez 1,Emily Li 2,Miguel Isarraraz 3,Michael Valentin 1,Cindy Merida 1,Ludwig Bartels 2
1 Material Science and Engineering University of California Riverside United States,2 Department of Chemistry University of California Riverside United States3 Electrical and Computer Engineering University of California Riverside United States
Show AbstractMoS2 is a key industrial catalyst for hydrodesulfurization (HDS) of crude oil. Atomistic investigation of the HDS reaction is hampered by the elevated pressures and temperatures required for HDS. With regards to the chemical/geometric setup of the catalyst particle, though, there is close correlation between the actual catalyst material and model single-layer MoS2 flakes that can be prepared in a lab. We developed a new experimental approach that takes relies on the strong photoluminescence (PL) characteristics of monolayer MoS2 (peak @ 670-690 nm). The PL intensity of MoS2 inversely correlates with its defect density and also with the adsorption of reactive species on its surface. We present data that correlated PL mapping under and immediately following reactive conditions to the reactions product distribution as measured by mass spectrometry. The use of wide field imaging drastically reduces the data acquisition time. Our work is based on recent findings, that sputtering of MoS2 allows removal of sulfur atoms from its basal plane resulting in dimming of its photoresponse. Exposure to an organic sulfur source such as benzenethiol then permits recovery of photoluminescence as a direct indicator of ongoing hydrodesulfurization at basal plane defects. In addition to presenting data on the sputter-induced formation of basal plane defects, we also show how MoS2 photoluminescence responds to hydrogen exposure at elevated temperatures resembling the surface conditions during hydrodesulfurization.
9:00 PM - NT2.6.16
The Effects of Alkali Elements on Charge Carrier Collection at Grain Boundaries in Cu(In,Ga)Se2 Thin Film Solar Cells
Bradley West 1,Michael Stuckelberger 1,Harvey Guthrey 2,Robert Lovelett 3,Lei Chen 3,Barry Lai 4,Joerg Maser 4,Volker Rose 5,William Shafarman 3,Mowafak Al-Jassim 2,Mariana Bertoni 1
1 ECEE Arizona State University Tempe United States,2 National Renewable Energy Laboratory Golden United States3 Institute of Energy Conversion University of Delaware Newark United States4 Advanced Photon Source Argonne National Laboratory Lemont United States4 Advanced Photon Source Argonne National Laboratory Lemont United States,5 Center for Nanoscale Materials Argonne National Laboratory Lemont United States
Show AbstractCu(In,Ga)Se2 (CIGS) thin film solar cells have reached record efficiencies greater than 21% on a laboratory scale. A key stepping stone to achieving these efficiencies was the discovery of the beneficial effects of alkali elements such as Na and K. It is known that Na increases the majority carrier concentration, when added through a post deposition treatment (PDT) or by diffusion from the soda lime glass (SLG) substrate during CIGS growth. The distribution of the alkali elements in the CIGS film and the position and role in the crystal lattice are not well understood; however, it is argued that Na could be located at the grain boundaries, aiding in chemical passivation[1].The KF PDT has been correlated to an increase in Voc and decrease in J0 [2]. It is argued that KF treatment improves the p-n junction quality by allowing Cd to diffuse into the CIGS layer creating a buried junction [3]. While it is understood that alkali elements are beneficial to device performance, the mechanisms that provides the enhancement is still under discussion.
Synchrotron based x-ray fluorescence (XRF) and x-ray beam induced current (XBIC) are excellent techniques to investigate the correlation between composition and charge carrier collection with nanometer scale spatial resolution. These techniques are non-destructive and are capable of quickly sampling areas > 100 μm2 to achieve statistically meaningful results over many grains and grain boundaries[4]. We have employed XRF and XBIC to investigate the impact of alkali PDTs and diffusion from the SLG on grain boundaries in CIGS solar cells. Two sets of samples with KF and NaF PDT are compared with identically grown films that did not receive a PDT, as well as samples with varied Na addition from the substrate. Results from this study will determine the impact of alkali elements and PDTs on the composition distribution of grain boundaries and if a corresponding increase in the distribution of the charge carrier collection is observed. We believe that studying the impact of alkali elements and PDTs in a statistically meaningful way will help shed light on the position of alkali elements in the film and their impact on recombination centers. This will provide a path forward to further increasing the efficiency of these devices.
[1] F. Pianezzi et al., Phys. Chem. Chem. Phys. 2014, 16, 8843.
[2] P. Jackson et al., Phys. status solidi - Rapid Res. Lett. 2015, 9, 28.
[3] A. Chirilă et al., Nat. Mater. 2013, 12, 1107.
[4] B. West et al., Proceedings of the 42nd IEEE Photovoltaic Specialist Conference, 2015, New Orleans, LA.
9:00 PM - NT2.6.17
Selenization of Mechanically Alloyed CuIn0.7Ga0.3Se2 Nanoparticle Based Thin Films
Rohini Neendoor Mohan 1,Pablo Reyes- Figueroa 2,Fabian Andres Pulgarin Agudelo 3,Latha Marasamy 4,Aruna Devi Rasu Chettiar 4,Velumani Subramaniam 1,Osvaldo Vigil Galan 3
1 Electrical Engineering CINVESTAV -IPN Mexico City Mexico,2 Electrical Engineering CINVESTAV-IPN Mexico City Mexico3 Escuela Superior de Fìsica y Matemàticas Instituto Politécnico Nacional Mexico City Mexico4 Program on Nanoscience and Nanotechnology CINVESTAV-IPN Mexico City Mexico
Show AbstractCopper indium gallium diselenide (CIGSe) thin films are regarded as a promising chalcogenide material to be used as an absorber layer in solar cells. The photovoltaic conversion efficiency of CIGSe solar cells reached up to 21.7% using vacuum method (co-evaporation). Since vacuum based processes have higher production cost and possess difficulty in scaling up, non-vacuum based methods are more encouraging due to efficient use of materials, high throughput and low cost. In the present work, CIGSe thin films were deposited using non-vacuum mechanically alloyed nanoparticles and the impact of annealing and selenization treatments on thin film properties is reported. Nanoparticles were synthesised by mechanical alloying of elemental Cu, In, Ga and Se using ball to powder ratio of 25:1 at milling speed of 400 rpm for 2 h. The structural and compositional properties of synthesised nanoparticles were studied by X-ray diffraction (XRD) and Energy Dispersive Analysis of X-rays (EDAX), which confirmed the formation of single phase chalcopyrite CIGSe nanoparticles with desired composition (Cu/In+Ga=1, Ga/In+Ga=0.3). The ink was formulated by dispersing the nanoparticles in absolute ethanol with the aid of organic binders, ethyl cellulose and terpineol. CIGSe thin films were deposited on glass substrate by paste coating of the ink. The films were air annealed at 300 and 400 °C for 1 h in order to remove the carbon contents. The film annealed at 300 °C is referred as film A and film annealed at 400 °C is referred as film B. The air annealed films were selenised in two steps, first at 400 °C for 30 min and then at 550 °C for 15 min. The structural properties of as-prepared, air annealed and selenised films were characterised using XRD analysis. Compared to the XRD patterns of the as-prepared and air annealed films, the selenised films showed intense and sharp peaks. Film A retained the single phase of CIGSe while film B decomposed into binary phases. After selenization, XRD pattern of film B showed peaks related to CuSe phase along with chalcopyrite phase of CIGSe. In addition, film B showed peak splitting along with a small shift in peak position towards lower 2 theta values whilst film A exhibited well defined peaks corresponding to chalcopyrite CIGSe phase. EDAX data showed increase in Se content from 46.55 to 49.43 at% and from 28.46 to 40.51 at% in the film A and film B respectively, subsequent to selenization. Field Emission Scanning Electron Microscopic images of both the selenized films showed dense surface morphology. As selenised film A showed superior structural and compositional properties than film B, film A was chosen for further studies. The current- voltage (I-V) curve of CIGSe film showed ohmic nature of contact resistance. The current of CIGSe film at 10 V showed 1.2 fold increases in the presence of light, which proves its applicability as absorber layer in solar cells.
9:00 PM - NT2.6.18
Enhanced Grain Growth of CuIn1-xGxSe2 Films Using Mechanically Alloyed Off-Stoichiometric Nanopowders
Rohini Neendoor Mohan 1,Pablo Reyes- Figueroa 1,Fabian Andres Pulgarin Agudelo 2,Latha Marasamy 3,Aruna Devi Rasu Chettiar 3,Velumani Subramaniam 1,Osvaldo Vigil Galan 2
1 Electrical Engineering CINVESTAV-IPN Mexico city Mexico,2 Escuela Superior de Fìsica y matemàticas Instituto politécnico nacional Mexico city Mexico3 Program on Nanoscience and Nanotechnology CINVESTAV-IPN Mexico city Mexico
Show AbstractEnhanced grain growth and re-crystallization for CuIn0.7Ga0.3Se2 (CIGSe) thin films deposited using particle based approach can be achieved by selenization of off-stoichiometric precursor films. Stoichiometric (CuIn0.7Ga0.3Se2) and off-stoichiometric (CuIn0.7Ga0.3Se0.5 and Cu0.85In0.7Ga0.3Se0.5) nanopowders were synthesised by mechanical alloying of elemental Cu, In, Ga and Se. X-ray diffraction analysis (XRD) of synthesised nanopowders showed existence of secondary phases such as Cu11In9, In2Se3, Ga2Se3, Cu2-xSe in both the off-stoichiometric nanopowders while stoichiometric nanopowders showed only single phase of chalcopyrite CIGSe. Field emission scanning electron microscopy (FESEM) results revealed that both stoichiometric and off-stoichiometric nanopowders consist of highly agglomerated nanoparticles. Energy dispersive analysis of X-rays (EDAX) exhibited homogeneous distribution of the elements Cu, In, Ga and Se in the stoichiometric nanopowders whereas inhomogeneous distribution of elements was observed in the off-stoichiometric nanopowders. The nanopowders ink was formulated with the inclusion of organic additives in order to deposit thin films by knife-coating. Afterwards, the films were annealed at 300 °C for 1 h in order to eliminate the organic additives and selenized at 550 °C for 15 minutes to obtain dense and continuous films. The structural, compositional and morphological properties of films before and after selenization were studied using XRD, EDAX and FESEM analyses. The XRD pattern revealed that no phase evolution was occurred in the stoichiometric films after selenization whilst the binary phases were vanished and single phase CIGSe was formed during selenization of off-stoichiometric films. The EDAX results showed that both the stoichiometric and off-stoichiometric films had CIGSe phase composition subsequent to selenization indicating the reaction of Cu-In-Ga-Se phases in the off- stoichiometric films with the Se which leads to the formation of single phase CIGSe. The selenized CuIn0.7Ga0.3Se0.5 films were slightly Cu-rich (Cu/Se= 0.6) compared to selenized CuIn0.7Ga0.3Se2 (Cu/Se=0.49) and Cu0.85In0.7Ga0.3Se0.5 (Cu/Se=0.45) films. Planar and Cross-sectional FESEM images proclaimed improved grain growth and recrystallization in the selenized off-stoichiometric films than selenized stoichiometric films. Hence, the strategy of CIGSe film preparation using off-stoichiometric nanopowders could be a possible way to improve the performance of particle based thin film solar cells.
9:00 PM - NT2.6.19
Combinatorial Synthesis of Novel High-Efficiency Selenide-Based Oxygen Evolution Catalysts
Xi Cao 1,Manashi Nath 1
1 Department of Chemistry Missouri Samp;T Rolla United States,
Show AbstractNowadays, it is crucial to discover highly active electrocatalysts composed of earth-abundant materials for the oxygen evolution reaction (OER). Combinatorial methods has been reported to provide an efficient way to screen and discover mixed metal oxides as promising OER electrocatalysts. Here, we have tried to explore Ni-based selenides doped with other transition metals as potential OER electrocatalysts using the Ni-Co-Fe ternary phase diagram in a combinatorial approach. In our work, we synthesized transition metal selenide films containing a mixed metal composition of Ni-Fe-Co by electrodeposition on different substrates including Au-coated glass and glassy carbon (GC). We systematically investigated how the doping with Fe and Co affects the electrocatalytic activities of Ni-selenide for water oxidation. The structure and morphology of these electrodeposited films were characterized with power X-ray diffraction, X-ray photoelectron spectroscopy, Scanning and Transmission electron microscopy and Raman spectroscopy. The catalytic activities were studied through electrochemical studies in alkaline conditions using linear sweep voltammetry, while stability of the catalyst was investigated by chromoamperometric studies at constant potential. In the presentation we will show some novel catalyst compositions containing NixFeyCozSen, NixCoySe4, and NixFeySe4 which show low onset potentials for O2 evolution along with extended stability. Some of these catalysts were found to be active for H2 evolution as well, essentially functioning as full water splitting catalyst. A systematic exploration of binary and ternary Ni:Co:Fe selenides along with electrochemical studies for catalytic efficiency will be presented in the poster. Such studies provide the foundation to investigate other metal selenide combinations.
9:00 PM - NT2.6.20
Microstructure and Physical Properties of CdS Thin Films Deposited By Chemical Bath Technique
Liangmin Zhang 1,William Cheung 1,David Bishel 1,Salvador Montes 1
1 California State Univ Turlock United States,
Show AbstractCadmium sulfide (CdS) is a wide band gap semiconductor that has emerged as an important material due to its applications in photovoltaic cells, optical filters, multilayer light emitting diodes, photodetectors, thin film field effect transistors, and gas sensors. CdS is naturally an n-type material of a band gap between 2.1 - 2.4 eV. The deposition of CdS film has been explored by various techniques, such as thermal evaporation, sputtering, molecular beam epitaxy, spray pyrolysis, and chemical bath deposition. Among of these techniques, Chemical bath deposition (CBD) is known to be a simple, easy, and inexpensive large-area deposition technique to grow CdS thin films under low temperature with high quality.
We use the CBD technique to synthesize CdS thin films. Unlike most reported work, the reagents we use are cadmium sulfate (CdSO4), ammonia water (NH4OH) and thiourea (CS(NH2)2). From our results, comparing with the method of using cadmium acetate, cadmium sulfate is easier to synthesize high quality CdS thin films under lower temperatures to prevent the quick evaporation of solution at high temperatures. All of reagents are of analytical grade and used without further purification. The glass substrates were well cleaned with 5% HF solution, left for 20 minutes under ultrasonic duty in isopropyl alcohol, washed with distilled water and finally dried in air. The typical procedures for the film deposition is described as follows. Drop by 25% NH4OH into a 100 ml beaker containing 25 ml of 1M CdSO4 solution until the initially formed white precipitate is completely dissolved. The clean substrates are mounted vertically in the bath beaker. 25 ml of 1M CS(NH2)2 is then poured into the mixtures. The distilled water is gradually added to make the volume up to 100 ml. The deposition is made at 50 – 80oC under magnetic stirring for all samples. In order to vary the composition of the films, different concentrations of CdSO4 and thiourea are used.
The CdS thin films deposited on the substrates using this technique are optically transparent, adherent, homogeneous and yellowish in color without powdered precipitation. After deposition, the deposited CdS films are cleaned thoroughly in distilled water and dried in the air at room temperature. In order to better understand the effect of growth conditions on film quality, the films were characterized by X-ray diffraction, scanning electron microscope, and UV-Vis absorption spectra. The measurements show that as deposited CdS films are polycrystalline and the crystallization of CdS particles is improved with increasing deposition time.
9:00 PM - NT2.6.21
Thermoelectric Properties of Metal Chalcogenide Thin Films Deposited with a Diamine-Dithiol Solvent Mixture
Yuanyu Ma 1,Prajwal Nagaraj 2,Robert Wang 2
1 Material Science Arizona State University Tempe United States,2 Mechanical Engineering Arizona State University Tempe United States
Show AbstractSolution-phase processing of semiconductor thin films is an attractive approach for fabricating low cost devices for optoelectronics and thermoelectrics. While most metal chalcogenide semiconductors are generally insoluble in regular solvents, a number of them can be prepared using hydrazine.[1] Unfortunately the highly hazardous nature of hydrazine poses a problem for industrial application. More recently, the use of less hazardous diamine-dithiol solvent mixtures have demonstrated that these too can enable solution-phase processing of many metal chalcogenides.[2] This diamine-dithiol solvent approach has been successfully used to deposit SnSe and SnS.[3, 4] However, the corresponding thermoelectric properties have not been reported.
In this poster presentation, we present our measurements on the thermoelectric properties (Seebeck coefficient, electrical conductivity, and thermal conductivity) of SnS, SnSe, and SnSxSe1-x films made by solution-processing with a diamine-dithiol solvent mixture. We also explore the incorporation of dopants into the SnSxSe1-x films and report the corresponding effects on the thermoelectric properties. The incorporation of these dopants will be achieved by mixing in appropriate amounts of Ag2S, CdS, CdSe, ZnS, and ZnSe, all of which can also be dissolved in diamine-dithiol solvents.
[1] Mitzi et al., Nature, 428, 299 (2004)
[2] Webber and Brutchey, JACS, 135, 15722 (2013)
[3] Antunez et al., Chem. Mat, 26, 5444 (2014)
[4] Lin et al., ACS Nano, 9, 4398 (2015)
9:00 PM - NT2.6.22
Cu2ZnSnSe4 (CZTSe) Solar Cell Absorbers Spray-Coated from Solution of Dissolved Elemental Cu, Zn, Sn, S and Se Powders
Cheik Sana 1,Shaimum Shahriar 1,Donato Kava 1,Jose Galindo 1,Vanessa Castaneda 1,Manuel Martinez 1,Robert Cotta 2,Edison Castro 2,Eva Deemer 3,Tahmina Akter 2,Geoffrey Saupe 2,Russell Chianelli 3,Luis Echegoyen 2,Deidra Hodges 1
1 Department of Electrical and Computer Engineering University of Texas at El Paso El Paso United States,2 Department of Chemistry University of Texas at El Paso El Paso United States3 Department of Metallurgical, Materials and Biomedical Engineering University of Texas at El Paso El Paso United States
Show AbstractSolution processed Cu2ZnSnSxSe4–x (CZTSSe) has attracted considerable interest as a material capable of driving economical and terawatt capacity photovoltaic module production. It has already shown promising results with record solar cells efficiency 12.6%. The interest to use CZTSSe as an absorber layer for thin film solar cells is due to its large absorption coefficient of over 104 cm-1 in the visible range, its tunable optical band gap of 1~1.5 eV, the low cost and earth abundance of its constituents copper (Cu), zinc (Zn) and tin (Sn) and sulfur(S). In this paper, non-vacuum solution based method to deposit Cu2ZnSnSxSe4–x (CZTSSe) was investigated. In this approach, a precursor solution of CZTSSe is formed by reacting elemental Cu, Zn, Sn S, and Se in a mix solution of thioglycolic acid, ethanolamine and 2-methoxyethanol. The precursor solution was then spray coated onto a heated solid lime glass substrate, followed by annealing at different temperatures. This process allowed for the growth of homogenous and large grain crystalline layers of CZTSSe. Optical, structural, and chemical composition characterizations of thin films were performed using scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), X-ray diffractometer (XRD), spectrometer and UV-Vis spectrophotometer. Electrical properties of the surface of CZTSSe thin films were investigated with conductive atomic force microscopy (AFM) and Scanning Kelvin probe force microscopy (SKPM). Film thickness and surface roughness were measured using KLA Tencor D-120 surface profilometer. SEM images show the formation of a continuous and homogenous film of large grains with size ranging from 0.5µm to 3µm. The Se/(S+Se) ratio ranges from 30 to 70% as measured by EDS performed in top view. The formation of the desirable Cu-poor and Zn-rich composition of the synthetized CZTSSe was confirmed by EDS also. X-Ray diffractograms show different shifts of the kesterite/stannite (112) peak, which indicate the presence of CZTSSe. Three major peaks corresponding to (112), (220), and (312) planes were respectively located near 28°, 47.5° and 56°. The shift of the peaks depends on the ratios of S/Se in the synthetized material. The lattice constants decrease linearly with increasing S contents in the precursor solution. Raman spectroscopy confirmed the formation of CZTSSe with traces of quaternary CZTS, CZTSe. A higher positive surface potential bending and a higher current flow are observed in the vicinity of the grain boundaries.
9:00 PM - NT2.6.23
Synthesis of Stoichiometric Sb2S3 Nanorods of High Purity by Microwave Heating
Claudia Martinez-Alonso 1,Eliot Olivos-Peralta 2,Merida Sotelo-Lerma 4,S. A Mayen-Hernandez 1,Hailin Hu 3
1 Facultad de Química Universidad Autonoma de Queretaro Queretaro Mexico,3 Instituto de Energías Renovables UNAM Temixco Mexico,2 Instituto de Investigaciones en Materiales UNAM Mexico Mexico4 Departamento de Investigación en Polímeros y Materiales Universidad de Sonora Hermosillo Mexico3 Instituto de Energías Renovables UNAM Temixco Mexico
Show AbstractAntimony sulfide (Sb2S3) is a semiconductor material of group V-VI with band gap of 1.78 to 2.50 eV, covering the visible and near infrared spectrum. It has a high photosensitivity and good photoconductivity and can be applied as absorbing material in solar cells.There are several methods of synthesis for Sb2S3, such as thermal decomposition, vacuum evaporation, hydrothermal reaction, solvothermal reaction, microwaves, among others. Evaporation methods require high temperature and are difficult to obtain exact stoichiometric compositions. The solvothermal and hydrothermal methods usually employ ligands during the synthesis which are sources of impurities in the final Sb2S3 products. The formation of Sb2S3 under microwave heating is fast and the obtained products show nanostructures and high purity. In this work, Sb2S3 nanorods were synthesized by microwave method without any ligands. Sodium thiosulfate (ST), thiourea (TU) and thioacetamide(TA) were used as sources of sulfur, and antimony chloride as source of antimony. Ethylene glycol (EG) and dimethylformamide (DMF) were used as solvent. The temperature of synthesis was varied from 140 °C to 170 °C, and the reaction time, for 30 min. It is found that EG was a very reactive solvent during the synthesis of Sb2S3 by microwave; with TA no such product was formed and with TU or ST, a high percentage of oxygen in the final products was obtained. With DMF as solvent, nanorods of crystalline (stibnite) was formed in all the Sb2S3 products with a band gap of around 1.7eV. Among the three sulfur sources, ST led to a S:Sb ratio of 7.77:1 in the final product, out of stoichiometry of Sb2S3, and the final product contained a high percentage of oxygen. In the case of Sb2S3 synthesized from TU and TA, the atomic ratio of S:Sb was close to 1.5:1 by EDS analysis. The SEM images of those products show the formation of microrods with average diameter and length of 0.95 and 13.42 um, respectively. XPS spectra of those samples show the characteristic signals of the Sb2S3 (Sb 3d5 / 2 = 528.5 eV, Sb 3d3 / 2 = 538 eV in the Sb-S bond and S-2 = 161.1-162 eV). A very weak signal around 131 eV was observed that could indicate the presence of traces of oxygen less than 3% in the final products in both cases. It is concluded that Sb2S3 microrods of high purity can be obtained by microwave heating using DMF as solvent and TU or TA as sulfur source. The very long nanorod microstructure of the semiconductor products is quite attractive for hybrid solar cell applications.
9:00 PM - NT2.6.24
Reducing Backscattering Electron Effect in EBL with Bi-Layer PMMA on Cadmium Sulfide
Masoud Mollaee 2,Abram Young 2,Warren Beck 2,Delmar Barker 3
1 Material Science University of Arizona Tucson United States,2 Physics University of Arizona Tucson United States,2 Physics University of Arizona Tucson United States3 Raytheon Missile Systems Tucson United States
Show AbstractCadmium Sulfide (CdS)--a chalcogenide semiconductor--is of major interest for electronic and optoelectronic applications. Electron Beam Lithography (EBL) is a common method for defining electrode patterns, however, low-angle backscattered electrons (BSE) from the high-atomic number Cd-rich substrate can significantly degrade lithographic resolution using EBL.
In this study, we demonstrate the nature of this back scattered electron problem using CASINO 2.42 Monte Carlo electron trajectory simulations. Using these simulations as a guide, we show how lithographic resolution can be optimized via judicious selection of electron beam energy and dose, and through use of a bi-layer resist comprised of both 495 PMMA and 950 PMMA resists.
9:00 PM - NT2.6.25
Unipolar Resistive Switching Behaviors and Enhancement Memory Endurance in a Forming Gas Annealed Pt/HfO2/Pt Resistors
Yong Chan Jung 1,Sejong Seong 1,Taehoon Lee 1,In-Sung Park 1,Jinho Ahn 1
1 Hanyang University Seoul Korea (the Republic of),
Show AbstractResistive random access memory (RRAM) has attracted as one of the promising future non-volatile memory devices because of its low power consumption, high operation speed, and probability of high density integration. Resistive switching occurs in insulating films that are sandwiched between two metal electrodes as follows metal-insulator-metal (MIM) structure. Resistive switching has been studied in various oxides, such as TiO2, ZrO2, Ta2O5, and HfO2. Recently, there is a great interest of HfO2 for resistive switching insulator films, since HfO2 is already used as a high-k dielectric in complementary-metal-oxide-silicon (CMOS) technology. Many mechanisms postulated to explain the resistive switching phenomenon in RRAM, the prospective one is oxygen vacancy-oxygen ion based mechanism.[1]
In this study, the resistive switching characteristics of Pt/HfO2/Pt resistors have been investigated. To generate more oxygen vacancies (Vo) from the HfO2 insulator, forming gas anneling (FGA) was performed at temperature ranges of 300-500 °C for 60 min. in 5%H2/N2 ambient. FGA is a conventional process to remove oxygen in the high-k dielectric by reduction annealing processes.[2] Typical unipolar resistive switching characteristics is observed in Pt/HfO2/Pt resistors, which displayed similar resistive switching feature for positive bias polarity. Nevertheless, the endurance is prolonged twice, and the reset current is lowered about 104 times for resistors with FGA at 300 and 400 °C. In order to clarify the change of oxygen bonding states of annealed HfO2 films, the X-ray photoelectron spectroscopy (XPS) measurements were carried out. As a result, it has great potential for low power consumption and good endurance for non-volatile memory applications.
[1] H.-S.P. Wong, H.-Y. Lee, S. Yu, Y.-S. Chen, Y. Wu, P.-S. Chen, B. Lee, F.T. Chen and M.-J. Tsai, Proc. IEEE. 100(6), 1951 (2012).
[2] J.K. Schaeffer, L.R.C. Fonseca, S.B. Samavedam, Y. Liang, Appl. Phys. Lett. 85, 1826 (2004).
9:00 PM - NT2.6.26
Chemical Phase-Separation in Ferroelectric Layered Transition Metal Thiophosphates
Michael Susner 1,Alexei Belianinov 1,Albina Borisevich 1,Qian He 1,Panchapakesan Ganesh 1,Hakan Demir 2,David Sholl 2,Douglas Abernathy 1,Michael McGuire 1,Petro Maksymovych 1,Anton Ievlev 1
1 Oak Ridge National Laboratory Oak Ridge United States,2 Georgia Institute of Technology Atlanta United States
Show AbstractMany transition metal thio- and selenophosphates (TMTP) form layered materials, within individual layers comprising a honeycomb sub-lattice of metal cations immersed into a cage of thiophosphate (P2S6)4- or selenophosphate (P2Se6)4- anions. Owing to ionic bonding in this class of materials, the electronic bandgaps range from 1.6-3.5 eV. Furthermore, in small subset of the known TMTP compositions, mostly involving Cu+1 and Ag+1, the ions spontaneously order to develop ferroelectric, antiferroelectric and mutliferroic ground states. Therefore, TMTP possess distinct and unique properties among layered materials, that can greatly benefit the development of future electronic devices derived from layered and 2D conductors and semiconductors, in the form of encapsulated and/or hybrid heterostructures.
Motivated by finite ionic conductivity in these materials, we have investigated them experimentally and theoretically from the view point of solid solutions. Specifically, we have investigated deviations from a perfect stoichiometry in a ferroelectric CuInP2S6 compound, using a combination of growth, imaging techniques and first principles theoretical modeling. Our key finding is that depleting CuInP2S6 of electroactive Cu+1 ions creates a unique layered structure – which has all the signatures of coherent spinodal decomposition into stoichiometric CuInP2S6 and In2P3S9 phases. When probed on the nanoscale, the materials reveal a rich and self-organized texture of ferroelectric and non-ferroelectric domains with nearly atomically-sharp chemical boundaries. Spinodal decomposition occurs without any significant perturbation of the basic layered structure – producing cleavable and air-stable surfaces and quasi-2D sheets. Perhaps most strikingly, we found that chemical phase-separation increases the Curie temperature for ferroelectric ordering in CuInP2S6 phase by up to 40K – which we assign to the chemical pressure effect. In combination with hydrostatic pressure dependent data, we have inferred the entropy of ferroelectric ordering of ~2.5kb which is at least 10 times larger than in conventional ferroelectrics, such as PbTiO3 and BaTiO3. This enables large modulation of Tc even by moderate pressures on ~100 MPa and suggests further design criteria for controlling Tc. We are currently exploring the stability of 2D and quasi 2D sheets of these materials as a stepping stone to novel van-der-Waals heterostructures with electronic and semiconducting 2D materials.
This research was sponsored by the Laboratory Directed Research and Development fund at the Oak Ridge National Laboratory.
M. A. Susner et al, “High Tc layered ferrielectric crystals by coherent spinodal decomposition”, ACS Nano in review (2015).
Symposium Organizers
Dhananjay Kumar, North Carolina Agricultural and Technical State University
Ningzhong Bao, Nanjing Tech University
Sergio D'Addato, Università di Modena e Reggio Emilia
Arunava Gupta, University of Alabama
NT2.7: Batteries and Fuel Cells
Session Chairs
Thursday AM, March 31, 2016
PCC North, 100 Level, Room 121 B
9:30 AM - NT2.7.01
Iron Pyrite FeS2 Thin Films as High Capacity Cathodes for All-Solid-State Lithium Batteries
Brigitte Pecquenard 1,Florian Flamary 2,Vincent Pele 2,Frederic Le Cras 2
1 ICMCB Pessac France,1 ICMCB Pessac France,2 LETI CEA Grenoble France2 LETI CEA Grenoble France
Show AbstractThe steady miniaturization of electronic devices and the emerging need for self-powered miniaturized systems (stand-alone sensors, medical implants, MEMS,…) boost the development of miniaturized energy storage solutions such as all-solid-state thin film batteries (i.e. microbatteries). Up to now, most studies on cathodes focused on intercalation compounds (LiCoO2, LiMn2O4, V2O5,…), despite their limited capacity (650–1150 mAh.cm-3) [1,2]. Nevertheless, other materials reacting with lithium according to a conversion reaction described by the general formula MaXb + (b.n) Li↔aM + bLinX (M = metal; X = O, N, F, S, P, H) are promising [3]. Among them, pyrite (FeS2 or Fe2+S22-) which may insert up to 4 Li+ per atom of iron enabling one of the highest theoretical capacity among transition metal sulfides is an ideal candidate: FeS2 + 4Li+ + 4e- → Fe + 2Li2S leading to theoretical capacities of 894 mAh.g-1 and 435 µAh.cm-2.µm-1. Moreover, this reaction that occurs at around 1.5 V vs Li+/Li meets the requirement for lower supply voltages in various microelectronics systems. Nevertheless, the use of pyrite in commercial systems is still confined to lithium primary cells and thermal batteries. Indeed, similarly to lithium–sulfur cells, the main phenomenon hindering the cycle life is the formation of polysulfide species which are soluble in the liquid electrolyte [4], whereas the practical delivered capacity is limited by the size of the pyrite particles.
Here, striking results obtained in electrochemical cells combining stacked thin film geometry and the use of a solid state electrolyte will be presented. Lithium microbatteries which comprise a FeS2 positive electrode and a vitreous electrolyte (LiPON) deposited by Radio-Frequency magnetron sputtering, and a lithium negative electrode, allowed demonstrating the efficiency of such a cell design. Physico-chemical characterizations carried out on the deposited pristine materials (EPMA, SEM, Raman and Mössbauer spectroscopies, GIXRD) confirmed the pyrite structure. Besides, ex situ transmission microscopy (HRTEM) analyses at different states of charge couple with EELS, revealed the lithiated pyrite compounds obtained during first lithium insertion/de-insertion. Full Li/LiPON/FeS2 microbatteries delivered excellent capacities of 300 µAh.cm-2.µm-1 for more than 800 cycles. Thorough electrochemical characterization suggested that the reversibility of the conversion reaction was affected by a continuous cycling in the low voltage region, indicating a progressive evolution of the phase distribution inside the electrode material [5].
References:
[1] J. B. Bates et al., J. Power Sources, 54 (1995) 58–62,
[2] B.Wang et al., J. Electrochem. Soc., 143 (1996) 3203–3213,
[3] J. Cabana et al., Adv. Mater., 22 (2010) E170–E192,
[4] T. A. Yersak et al, Adv. Energy Mater., 31 (2012) 120–127,
[5] V. Pelé, F. Flamary, L. Bourgeois, B. Pecquenard, F. Le Cras, Electrochem. Comm., 51 (2015) 81-84
9:45 AM - NT2.7.02
The Role of Water in a Nanostructured V2O5 Cathode for Rechargeable Mg Batteries
Sanja Tepavcevic 1,Justin Connell 1,Pietro Lopes 1,Baris Key 2,Nenad Markovic 1
1 Material Science Division Argonne National Laboratory Lemont United States,2 Chemical Science and Engineering Argonne National laboratory Lemont United States
Show AbstractIntroduction The current quest for new cathode materials for rechargeable Mg batteries is focused on intercalation compounds exhibiting high working potential and capacity1. Vanadium oxides are an attractive alternative, as vanadium is known to exist in a wide range of oxidation states and our recent results2 show that it is possible to extend the reversible ion-insertion chemistry of V2O5 to Mg2+.
Results and Discussion V2O5 was used as a model system to develop an understanding of the critical parameters that govern intercalation and de-intercalation of Mg2+ ions in layered oxide materials, focusing in particular on the role of water and defects. The strong polarity and active electrochemistry of water make it a likely player in many battery phenomena including cation solvation, SEI formation and redox behavior. Molecular dynamics simulations show that a significant amount of water remains in the structure solvating inserted Mg ions and diminishing the strength of the interaction with apical oxygens. Thermogravimetric analysis of as-prepared bilayered V2O5 shows that near 120 oC, free (adsorbed) and structural (hydrogen-bonded) water are removed from the structure, while strongly bonded hydroxyl groups persist until temperatures of ~ 300 oC. Removal of these structural OH groups results in poor electrochemical performance similar to highly crystalline orthorhombic V2O5. We confirm the presence of these unique structural hydroxyl groups with FTIR, focusing on the stretching vibrations of OH groups in the region 3000-4000 cm-1. These results, together with preliminary NMR, XPS and atom probe tomography data confirming the presence of hydroxyl groups in our structure, will be discussed. We believe that that these strongly bonded hydroxyl groups play two important roles in reversible Mg cycling: 1) they maintain a sufficient interlayer spacing to allow for the diffusion of Mg cations, and 2) they reduce the symmetry of the V2O5, which contributes to electron transfer and Mg intercalation.
Conclusions We successfully used V2O5 as a model system to develop understanding of the role of the water in the intercalation and de-intercalation of Mg2+ ions into layered oxide materials. We have shown that the presence of water and the hydration of Mg ions are important for the reversible cycling of Mg. In the future, we plan to investigate intercaltion into amorphous V2O5 thin films.
Acknowledgement. This work was supported by the U. S. Department of Energy, US DOE-BES, under Contract No. DE-AC02-06CH11357.
References
(1). On the Way to Rechargeable Mg Batteries: The Challenge of New Cathode Materials E. Levi, Y. Gofer, and D. Aurbach, Chem. Mater. 2010, 22, 860–868.
(2). Nanoarchitectured Layered Cathode for Rechargeable Magnesium Batteries, Sanja Tepavcevic, Yuzi Liu, Dehua Zhou, Barry Lai, Jorg Maser, Xiaobing Zuo, Henry Chan, Petr Král, Christopher S. Johnson, Vojislav Stamenkovic , Nenad M. Markovic and Tijana Rajh, ACS Nano, 2015, 9 (8), pp 8194–8205.
10:00 AM - *NT2.7.03
High Performance Hybrid Electrodes for Advanced Energy Storage: Lithium Ion Batteries
Yanglong Hou 1
1 Department of Materials Science and Engineering, College of Engineering Peking University Beijing China,
Show AbstractPresently, safe energy storage is one of the most demanding technologies by the developing society. In this regard, lithium ion batteries (LIBs) have got tremendous attention due to their high energy and power densities; have been considered as promising power source for future electric vehicles (EVs). Here, we have developed different hybrid structures of metal oxides, nitrides, sulfides, hydroxides and metal alloys with doped graphene to control above mentioned problems and to achieve the goals set by USABC. All these composites possess extraordinary performances as electrodes of LIBs with long cyclic stability and excellent rate capability. The high performance of the composites based on the synergistic effect of several components in the nanodesign. These strategies to combine the different property enhancing factors in one composite with engineered structures will bring the realization of these devices in road market.
References
Li, Q.; Mahmood, N.; Zhu, J.; Hou, Y.; Sun, S. Nano Today 2014, 9, 668–683.
Zhang, C.; Mahmood, N.; Yin, H.; Liu, F.; Hou, Y., Adv Mater 2013, 25, 4932–4937.
Mahmood, N.; Zhang, C.; Hou, Y., Small 2013, 9, 1321–1328.
Mahmood, N.; Zhang, C.; Liu, F.; Jinghan, Z.; Hou, Y., ACS Nano 2013, 11, 10307–10318.
Mahmood, N.; Zhang, C.; Yin, H.; Hou, Y., J. Mater. Chem. A 2014, 2, 15–32.
Mahmood, N.; Tahirb, M.; Mahmood, A.; Zhu, J.; Hou, Y., Nano Energy, 2015, 11, 267–276.
Yin, H.;Zhang, C.;Liu, F. ; Hou, Y., Adv. Funct. Mater. 2014, 24, 2930–2937.
Mahmood, Hou, Y., Adv. Sci., 2014, 1, 1400012.
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10:30 AM - NT2.7.04
New Lithiated Titanium Oxysulfide Cathodes for All-Solid-State Lithium-Ion Thin Film Batteries: Syntheses and Electrochemical Performance
Frederic Le Cras 3,Brigitte Pecquenard 1,Vincent Dubois 2
3 LETI CEA Grenoble France,1 ICMCB Pessac France1 ICMCB Pessac France,2 STMicroelectronics Tours France
Show AbstractThe significant growth of both the number and the kinds of portable electronic systems, generally battery-powered has triggered the race for the development of high-performance microprocessors and systems on chips using low power consumption integrated circuits. As a consequence, the energy supply of such optimized components can also be ensured today by miniaturized power sources such as Li microbatteries. The latter have the advantage of being manufactured by vacuum deposition process also widely used in the microelectronics industry [1]. Nevertheless to be considered as an electronic components, the microbattery need to sustain temperature higher than 230°C for the reflow soldering to attach the component to the PCB . As Li metal anode melts at 181°C, Li microbatteries are not tolerant to such high temperature. For this reason we have manufactured Li-ion all-solid-state thin films batteries based on Si at the negative electrode and a new lithiated titanium oxysulfide as a cathode [2]. This new thin film materials was prepared by sputtering from a home-made LiTiS2 target. The resulting films are quite dense, amorphous with a typical composition close to Li1.2TiIVO0.5SII2.1. A full electrochemical delithiation of this electrode corresponding to a capacity of 64 µAh.cm-2.µm-1 was achieved in all-solid-state cells. As Ti4+ cannot be involved in the oxidation process, only sulfide species are oxidized during charge up to 2.9 V/Li+/Li forming S22- disulfide anions. The subsequent lithiation and delithiation curves showing no capacity loss, demonstrate the perfect reversibility of the electrochemical processes. Actually, when cycled in the 3.2-1.5 V voltage window versus a lithium anode, extra lithium can be inserted in the material below 2V and titanium is partly reduced. The increase of the current density from 2 to 130 µA.cm-2 (i.e from C/50 to 2C rates) induces only a slight capacity decrease, mainly due to an increase of the polarization near the end of charge. Besides, the capacity retention is found to be excellent during extended cycling with a capacity fading lower than -0.01 % per cycle and a mean coulombic efficiency close to 100%. Excellent electrochemical performances in terms of capacity value and cycling stability were also obtained in all-solid state Li-ion cells with Si as a negative electrode. After three successive solder-reflow processes (thermal treatment at 260°C), the capacity, the cycle life as well as the voltage profile remained unchanged. Finally, Li1.2TiO0.5S2.1 is confirmed as a new interesting lithiated positive material for thin film lithium and lithium-ion microbatteries. Moreover, as its synthesis does not require any thermal treatment (contrary to the well-known LiCoO2), is well-adapted to thermally sensitive substrates such as flexible polymers foils
References:
[1] B. Pecquenard, F. Le Cras et al., J. Power Sources, 196 (2011) 10289
[2] F. Le Cras, B. Pecquenard et al., Adv. Energy Mater., 5 (2015) 1501061
11:15 AM - NT2.7.05
Probing Ionic Transport Mechanisms in Y-Doped Barium Zirconate
Jilai Ding 1,Janakiraman Balachandran 2,Evgheni Strelcov 2,Gabriel Veith 5,Chris Rouleau 2,Craig Bridges 3,Sergei Kalinin 2,Nazanin Bassiri-Gharb 1,Panchapakesan Ganesh 2,Raymond Unocic 2
2 Center for Nanophase Materials Sciences Oak Ridge National Laboratory Oak Ridge United States,1 School of Materials Science amp; Engineering Georgia Institute of Technology Atlanta United States,2 Center for Nanophase Materials Sciences Oak Ridge National Laboratory Oak Ridge United States5 Materials Science and Technology Division Oak Ridge National Laboratory Oak Ridge United States3 Chemical Sciences Division Oak Ridge National Laboratory Oak Ridge United States4 G. W. Woodruff School of Mechanical Engineering Georgia Institute of Technology Atlanta United States,1 School of Materials Science amp; Engineering Georgia Institute of Technology Atlanta United States
Show AbstractIonic transport dynamics underpin the functionality of most of the energy storage and conversion technologies. Barium zirconate (BZO) has been widely studied for use in proton-conducting solid oxide fuel cells (PT-SOFCs), due to its high proton conductivity and excellent chemical stability at intermediate temperatures (400-700°C). Literature reports focusing on approaches to enhance conductivity in BZO are mostly based on developing new processing methods as well as new dopant and sintering additive systems, with macroscopic characterization such as X-ray diffraction and impedance spectroscopy. In order to identify proton transport mechanisms at the nanoscale, it is important to investigate the influence of atomic scale defects on ion transport mechanisms and kinetics as a function of dopant concentration.
Here we explore the ionic transport mechanisms in pure and Y doped BZO (Y-BZO), using energy discovery platforms, a synergy of nanofabricated device and in-situ characterization methods under controlled external stimuli with time resolved Kelvin probe force microscopy (tr-KPFM). A series of Y-BZO films (Y = 0 to 20 %) were prepared by pulsed laser deposition (PLD). Aberration corrected STEM was used to directly image defects and dopant clusters, which provide localized lattice strain and help facilitate proton conduction.
Through tr-KPFM, the surface potential mapping in both the space and time domains are obtained, enabling analysis of local proton transport dynamics on the 10-2 to 102 s time scale as a function of temperature and dopant concentration. Experimentally measured activation energy increases from 0.49 eV to 0.61 eV with increasing dopant concentration, which is in good agreement with existing ac conductivity measurements [1]. The enhanced conductivity is explained by local lattice distortions from the dopant clustering and confirmed via density functional theory. The electrochemical and transport processes are also simulated through finite element method, resulting in creation of a physical model consistent with observed phenomena, and deconvolution of physical parameters such as proton injection rate and diffusivity.
The combination of results from theory and modeling suggests that in proximity of an optimal doping of ~20%, dopants begin to cluster and essentially trap protons. This is the reason for reduced conductivity at higher dopant concentrations in BZO and possibly all known oxide materials that require acceptor dopants. The latter is supported by computations. We will also present ways to mitigate such clustering effects using a combined modeling/experimental approach.
The research was sponsored by the Laboratory Directed Research and Development fund at the Oak Ridge National Laboratory. Computations were performed at NERSC.
[1] D.Pergolesi et.al, High proton conduction in grain-boundary-free yttrium-doped barium zirconate films grown by pulsed laser deposition, Nature Materials 9, 846–852 (2010).
11:30 AM - NT2.7.06
Synthesis of Electrocatalytically Active Epitaxial (PrxBa1-x)CoO3-δ Thin Films
Felix Gunkel 2,Daniel Bick 1,Marco Moors 2,Ilia Valov 2,Regina Dittmann 2,Rainer Waser 1
1 RWTH Aachen University Juelich Germany,2 Peter Grünberg Institute 7 FZ Jülich Juelich Germany,1 RWTH Aachen University Juelich Germany2 Peter Grünberg Institute 7 FZ Jülich Juelich Germany
Show AbstractCobaltate perovskites are promising materials for implementation as cathodes in solid oxide fuel cells (SOFCs) as well as for application as catalysts in the oxygen evolution reaction (OER) taking place during electrochemical water splitting. Although great efforts have been made to understand both oxygen incorporation in SOFC cathodes as well as the OER in electrolysers, details of these processes are still under debate. One particular issue is the lack of model systems to systematically study the effects of various material parameters involved.
One way to generate model systems is to grow epitaxial thin films. In terms of surface area these systems cannot compete with poly-crystalline thin films or ceramics. However, the variation of material parameters (composition, oxygen deficiency, etc.) and thin film parameters (strain, thickness, etc.) can be tuned in a systematic manner.
Recently, the compounds (Prx,Ba1-x)CoO3-δ (PBCO) has attracted a signifficant amount of attention [1,2]. However, due to its complex composition, multiple material parameters such as oxygen deficiency (δ), valence state of Co (2+/3+) and Pr (3+/4+) may be involved in its catalytic activity. Moreover, additional effects such as phase separation and formation of Co-oxides or even metallic Co may also affect the catalyst's properties.
In this study, we discuss the synthesis of epitaxial PBCO thin films using RHEED-controlled pulsed laser deposition. In terms of epitaxial growth, this compound is particularly challenging due to the large difference in the ionic radii on the A-site compounds Pr and Ba, that causes cation ordering for x=0.5 [3] and a significant change of the lattice constant with Pr/Ba ratio. Moreover, the lattice constant is strongly affected by the oxygen load as revealed by X-ray diffraction measurements. As a result, epitaxial strain and thus the optimum choice of the substrate material varies with both composition and oxygen pressure during growth.
We deposited PBCO thin films on various perovskite substrates including LaAlO3, SrTiO3 and DyScO3. In oxidizing conditions, we find single phase PBCO films that can be manufactured either in a strained or relaxed perovskite structure depending on lattice mismatch. Independent of epitaxial strain, we observe the formation of CoO secondary phases at low growth pressures.
While low-pressure samples show electrically insulating behavior, well-oxidized single phase PBCO thin films show sufficient electrical conduction at room temperature being essential for catalytic performance. We present first results on electrocatalytic properties of these epitaxial samples and discuss their dependence on strain and growth conditions.
[1] D. Bick et al., submitted (2015)
[2] Choi et al., Sci. Rep. 3, 2426 (2013)
[3] Grimaud et al., Nat. Comm. 4, 2439 (2013)
NT2.8: Thin-Film Solar Cells
Session Chairs
Dhananjay Kumar
Srinivasa Rao
Thursday PM, March 31, 2016
PCC North, 100 Level, Room 121 B
11:45 AM - *NT2.8.01
New Family of Earth-Abundant Materials for Solar Energy Conversion Applications
Karthik Ramasamy 1,Hunter Sims 2,Sergei Ivanov 1
1 Los Alamos National Laboratory Albuquerque United States,2 German School of Simulation Sciences Julich Germany
Show AbstractThe relentless demand in energy generation using non-fossil fuels inspires the scientific community to develop stable and better performing materials that are composed of abundant, non-toxic, and cost-effective elements. As compared to conventional silicon-based solar cells, thin film solar cell panels are lightweight and flexible and are thus preferable for a number of applications. In objective of identifying materials composed of sustainable and non-toxic elements for thin film solar cells, we have been investigating number of chalcogenide containing compounds. Here we report a detailed solution-based synthesis, optical studies and electronic structure calculations of nanocrystalline Cu-Sb-S and Cu-A-B-S (A = In, Ga, Al; B=Zn, Sn) systems. The study suggests that all these materials are p-type semiconductors with an optical band gap between 0.8 eV and 1.6 eV and large absorption coefficient values over 104 cm-1. Band structure calculations on these materials predict most of them are direct band gap materials and the energy gap values are in consistent with experimental results. The details of the syntheses methods, structural and optical characterizations and band structure calculations will be presented and discussed
12:15 PM - NT2.8.02
Molecular-Ink Route to 13.0% Efficient Low-Bandgap CuIn(S,Se)2 and 14.7% Efficient Cu(In,Ga)(S,Se)2 Solar Cells
Alexander Uhl 1,John Katahara 1,Hugh Hillhouse 1
1 University of Washington Seattle United States,
Show AbstractWith a band gap of 0.98 eV, CuInSe2 (CIS) is an important candidate for the bottom cell of tandem devices, as it exhibits excellent current matching to some of the best performing hybrid perovskite solar cells [1,2]. In this work we show a new solution processing route to deposit CuInSe2-based absorber layers that yield 13.0% efficiency devices [3]. To our knowledge, this is the highest reported conversion efficiency for a CIS (or 1.00 eV bandgap) solar cell from any non-vacuum method, exceeding routes with hydrazine solvent [4] or nanocrystals [5,6]. Devices with gallium-containing absorbers from the same process (CIGS) and bandgap of 1.15 eV show up to 14.7% conversion efficiency. The excellent material properties of absorber layers is further elucidated by time-resolved photoluminescence measurements, which confirm minority carrier lifetimes exceeding 20 ns.
To minimize the processing cost of these layers, this novel molecular-ink route omits sophisticated nanoparticle fabrication and stabilization as well as toxic and explosive solvents and gases, such as hydrazine, H2, H2S, or H2Se. Single crystal and powder XRD, as well as Raman spectroscopy confirm the chemical coordination of metal cations in both solid and liquid state. The formation of metal-organic complexes was found critical to control oxidation states and loss of metals during processing and tailor the final composition of the absorber. These findings may pave the way for all-printed tandem solar modules and dramatically reduced cost of solar electricity.
References
[1] Zhou et al., Science 2014, 345, 542.
[2] Meillaud et al., Sol. Energ. Mat. Sol. C. 2006, 90, 2952.
[3] Uhl et al., Energy Environ. Sci., 2016, 9, 130.
[4] Liu et al., Chem. Mater. 2010, 22 (3), 1010-1014.
[5] Norsworthy et al., Sol. Energ. Mat. Sol. C. 2000, 60, 127-134.
[6] Uhl et al., Prog. Photovolt: Res. Appl. 2015, 23, 1110.
12:30 PM - NT2.8.03
Controlling Morphology of Polycrystalline Semiconductors by Phase Transformation of Assembled Metastable Nanocrystals
Ajay Singh 2,Delia Milliron 2
1 Lawrence Berkeley National Lab Berkeley United States,2 The McKetta Department of Chemical Engineering University of Texas at Austin Austin United States,2 The McKetta Department of Chemical Engineering University of Texas at Austin Austin United States
Show AbstractEarth abundant compound semiconductors are promising candidates for the fabrication of high-quality thin film solar cell absorber. In particular, these compound (I2-II-IV-VI4) semiconductor material possess high optical absorption coefficients (105 cm-1), high power conversion efficiencies, offer good photostability against long-term radiation, and cause less environmental problems due to their relatively low toxicity. However, device efficiencies for these materials are significantly lower than the best efficiencies achieved for chalcopyrite based solar cells. Low device efficiency for earth abundant semconductors have been attributed to difficulties in achieving large grained, single phase thin films with controlled stoichiometry using vacuum-based deposition or precursor annealing techniques. These techniques can also result in the unintentional formation of a large number of secondary phases in the Cu-Zn-Sn-S system that significantly reduces the device performance. Nanocrystal based methods for the fabrication of thin film photovoltaics are an attractive alternative to vacuum deposition processes due to their simplicity and cost effectiveness. Here, we develop a simple and reliable solution approach to process large grained, single phased kesterite Cu2ZnSnS4 films via thermal treatment of assembled nanocrystal-precursor films consisting of wurtzite Cu2ZnSnS4 nanorods. The crystal structure, composition and electrical properties of the final film were investigated with SEM, XRD, Ramna, PL and Hall effect measurements.
1) M. F. A. M. van Hest, D. S. Ginley, Solution Processing of Inorganic Materials, Wiley-VCH, Hoboken, NJ, 2008.
2) H. Katagiri, Thin Solid Films 2005, 480, 426.
3) Q. Guo, G. M. Ford, W. C. Yang, C. J. Hages, H. W. Hillhouse, R. Agrawal, Sol. Energy Mater. Sol. Cells, 2012, 105, 132.
4) T. K .Todorov, J. Tang, S. Bag, O. Gunawan, T. Gokmen, Y. Zhu, D. B. Mitzi, Adv. Energy Mater., 2013, 3, 34.
12:45 PM - NT2.8.04
Synchrotron-Based In Situ Characterization of CuInxGa1-xSe2 Solar Cells during Selenization Process
Michael Stuckelberger 1,Bradley West 1,Simone Bernardini 1,Rupak Chakraborty 2,Robert Lovelett 3,Lei Chen 3,Barry Lai 4,Joerg Maser 4,Tonio Buonassisi 2,Mariana Bertoni 1
1 School of Electrical, Computer and Energy Engineering Arizona State University Tempe United States,2 Photovoltaics Research Laboratory Massachusetts Institute of Technology Cambridge United States3 Department of Chemical and Biomolecular Engineering University of Delaware Newark United States4 Advanced Photon Source Argonne National Laboratory Argonne United States
Show AbstractSynchrotron-based characterization techniques—first of all x-ray fluorescence (XRF) and x-ray beam induced current (XBIC)—have been shown to be very powerful tools for the in-situ investigation of diffusion and growth processes of solar cells [1]. Thanks to the excellent spatial resolution (< 100 nm) and sensitivity of state-of-the-art synchrotron beamlines, we are able to map multi-grain areas of completed CuInxGa1-xSe2 (CIGS) solar cells, correlating point-by-point the charge collection efficiency from XBIC with the elemental distribution from XRF measurements. This points out the beneficial and detrimental effects of the distribution of Se, Cu, In, and Ga on device performance, which is an important step towards active engineering of high-efficiency devices rather than improvements by experimental optimization [2-4].
The holy grail of engineering high-efficiency solar cells goes even a step further, investigating the distribution of the elements in-situ during the growth of CIGS solar cells. For that purpose, we have built a heating stage for the specific requirements of XRF and XBIC measurements under conditions that are close to those of solar cell manufacturing or processing, covering a temperature range up to 600°C and working under corrosive atmospheres such as H2Se and H2S [5]. Furthermore, the mechanical concept of the stage provides excellent stability, allowing to compensate thermal drift and finally tracking sample features in nanoscale over the full temperature range.
Using that heating stage and the expertise in XRF analysis of multi-layered multi-compositional thin films, we investigate in the present work CIGS solar cells during the selenization process. We have deposited the layer stack of CIGS solar cells including the Mo back contact on glass and a layer of metallic Cu/In/Ga precursors with a Se capping layer, as described in [6]. We follow the selenization procedure described therein using a heating phase in an inert atmosphere that is followed by a sulfurization step in helium-diluted H2S. During these steps, when the Se diffuses into the Cu/In/Ga precursor and forms CIGS crystallites, we measure the distribution of the elements Cu, Ga, In, and Ga in situ. This shows how CIGS grains nucleate and grow to the final state in solar cells, including segregation of elements throughout the grain cores or at grain boundaries. Finally, the correlation of these growth observations with XBIC measured for cells grown under varied conditions gives direct access to the significance of growth parameters for further performance improvements of CIGS solar cells.
[1] M. I. Bertoni, et al, Energy Environ. Sci. (2011)
[2] B. West, et al., IEEE PVSC proc., New Orleans (2015)
[3] B. West, et al., submitted for publication (2015)
[4] M. Stuckelberger, et al., IEEE PVSC proc., New Orleans (2015)
[5] R. Chakraborty, et al., submitted for publication (2015)
[6] D. M. Berg, et al., IEEE PVSC proc., Denver (2014)
NT2.9: Magnetic and Multifunctional Oxides
Session Chairs
Thursday PM, March 31, 2016
PCC North, 100 Level, Room 121 B
2:30 PM - *NT2.9.01
Tunability of Exchange Bias in Thin-Film Assemblies of Ni@NiO and Ni@CoO Core-Shell Nanoparticles
Alessandro Ponti 1,Elena Capetti 1,Anna Ferretti 1
1 Istituto di Scienze e Tecnologie Molecolari Consiglio Nazionale delle Ricerche Milano Italy,
Show AbstractIn the last years, synthetic methods for magnetic core-shell nanoparticles (NPs) have been developed. Such NPs may show exchange bias (EB) when they comprise an exchange-coupled ferromagnetic-antiferromagnetic (FM-AFM) interface that induces a unidirectional magnetic anisotropy. The interest in NPs displaying EB thus renewed and research focused on the goal of “beating the superparamagnetic limit” for successful application of NPs in high-density magnetic recording.
We prepared thin-film assemblies of Ni@NiO and Ni@CoO core-shell NPs by sequential layer deposition of (i) a thin MO layer (1 nm), (ii) Ni nanoparticles (12 nm), and (iii) a variable-thickness MO layer (0-6 nm for M = Ni; 1-3.5 nm for M = Co). The thickness of the bottom MO layer and the Ni NP size and areal density were kept fixed to study the effect of the thickness of the top MO layer on the magnetic behavior. The NPs were characterized by XPS, SEM, HRTEM, and STEM. The magnetic properties of the NP assemblies were investigated by analyzing the field-cooled (FC) hysteresis loops, the thermal magnetization in FC and ZFC mode, and the thermoremanence.
The Ni@NiO NPs displayed negligible EB when tNiO ≤ 1.6 nm, but for thicker shells, the EB field Hb steeply increased without sign of saturation up to Hb = 0.57 kOe. Coercivity Hc showed a similar trend and reached Hc = 0.55 kOe for the thickest NiO shell. The microcoercivity spectrum displayed a thickness-related shift towards larger values in the direction opposite to the cooling field. This behavior is related to the morphology of the top NiO layer. At low deposited dose, NiO islands form and enlarge on the Ni cores, achieving a complete shell at t = 1.6 nm, which then grows and finally forms a discontinuous NiO matrix embedding the Ni cores.
Ni@CoO and Ni@NiO@CoO NPs were also investigated. Comparing shells with similar t, one finds that CoO is more effective than NiO in creating EB and increasing coercivity. The maximum Hb = 2.15 kOe and Hc = 2.76 kOe at nominal tCoO = 3.5 nm. Since the Ni/CoO interface is expected to be (at best) of similar quality as the Ni/NiO interface, the larger EB can be attributed to the larger anisotropy of CoO (KCoO/KNiO ≈ 105) or to a larger AFM/FM exchange coupling. The CoO blocking temperature TB is about 240 K, ca. 50 K lower than TN of bulk CoO, and does not depend on the CoO layer thickness. Ni@NiO@CoO NPs have larger Hb and Hc than both Ni@NiO and Ni@CoO NPs with similar t. This could be attributed to the synergy between the better quality of the Ni/NiO interface and the larger anisotropy of the CoO coating.
We thus demonstrated the viability of the sequential layer deposition method to both systematically investigate the effect of morphology on exchange-coupled core-shell NPs and to tune the EB by the fine regulation of a single morphological parameter. Our results are a first step in the direction of the rational design and synthesis of NPs able to beat the superparamagnetic limit.
3:00 PM - NT2.9.02
Electronic, Optical and Magnetic Properties of Iron Doped Nickel Oxide
Luisa Scolfaro 1,John Petersen 1,Fidele Twagirayezu 1,Gabriel Leitao 1,Pablo Borges 2,Wilhelmus Geerts 1
1 Texas State Univ San Marcos United States,2 Universidade de Vicosa Rio Paranaiba Brazil
Show AbstractThe transition metal oxides (TMOs), including NiO and FeO, are currently being considered to be used in Resistive Random Access Memory devices (ReRAM). ReRAM is based on the reversible switching of a thin TMO layer between a low and high resistance state using the mechanism of soft breakdown. ReRAM’s high integration density, its high endurance and good retention, its low energy use, and its high speed make it an interesting technology to possibly replace Flash in non-volatile memory applications in the future. NiO has been well-studied in this regard, but PyO has never been studied by first principles, and experimental publications on the material are scarce. The addition of other elements in NiO is known to influence the morphology and crystal structure of polycrystalline thin films in particularly important for ReRAM as the switching between the high and low resistance state is inhomogeneous and nano-filaments are formed at the grain boundaries in the thin oxide film. In this paper we consider the effect iron has on the electronic structure of NiO.
The VASP code based on the density functional theory and the spin-local density approximation was used to study Fe-doped NiO in the rock salt structural phase. For the exchange-correlation potential, we adopted the Generalized Gradient approximation (GGA) and the GGA+U methods, with U set to 3.0 eV for both Fe and Ni, and 0.0 eV for O. Calculations were done on a 32-atom super cell consisting of 16 nickel and 16 oxygen atoms. Preliminary calculations show that NiO is an anti-ferromagnetic semiconductor which is in agreement with experimental results. The calculation with the GGA method yielded a direct bandgap of 0.6 eV, while the calculation with GGA+U method yielded a direct bandgap of 2.6 eV, in better agreement with experimental findings (~ 3.7 eV). In order to further improve the band gap description, the Hartree Fock theory using a hybrid calculation (the HSE06 approach) was incorporated in the VASP calculation.
Iron-doped NiO was obtained by replacing one Ni atom with one Fe atom and maintaining the AFM order, i.e. a rhombohedral symmetry in the [111] direction. This corresponds to a Fe concentration of approximately 6.3 % in our supercell calculations. The results obtained with the GGA+U method showed that Fe-doped NiO has a net magnetic moment as the moment on the iron atom differs from the moment on the nickel atoms so they no longer cancel out in the AFM state. We found that Fe introduces a localized state in the band gap of NiO, and the effects of Iron in the optical properties of NiO are analyzed.
We acknowledge financial support from Texas State University (Research Enhancement grant) and from DOD (HBCU/MI grant W911NF-15-1-0394).
3:15 PM - NT2.9.03
Electrical and Magnetic Properties of Cobalt Telluride Nanostructures
Bishnu Dahal 2,Rajendra Dulal 2,Ian Pegg 2,John Philip 2
1 The Catholic University of America Washington United States,2 Department of Physics Vitreous State Laboratory, The Catholic University of America Washington United States,
Show AbstractWe have grown CoTe nanostructures using a wet-chemical synthesis. The CoTe nanostructures exhibit NiAs hexagonal crystal structure and show semiconducting behavior. We have fabricated nanoscale devices of CoTe nanostructures using electron beam lithography. The gate-dependent I-V characteristics show that the CoTe nanostructures are p-type. They are ferrimagnetic with a saturation magnetization of 0.25 µB/Co and having a large Curie temperature. They display magnetoresistance effect, which is observed only below 50 K. The maximum magnetoresistance is observed at 10 K, which is around 54 %.
This work was supported by the National Science Foundation under ECCS-0845501 and NSF-MRI, DMR-0922997
3:45 PM - NT2.9.05
Evolution of Crystal and Electronic Structure of SrCoOx Probed In Situ during Electrochemically Driven Topotactic Phase Transition
Qiyang Lu 2,Yan Chen 3,Hendrik Bluhm 4,Bilge Yildiz 3
1 Laboratory of Electrochemical Interfaces Massachusetts Institute of Technology Cambridge United States,2 Department of Materials Science and Engineering Massachusetts Institute of Technology Cambridge United States,1 Laboratory of Electrochemical Interfaces Massachusetts Institute of Technology Cambridge United States,3 Department of Nuclear Science and Engineering Massachusetts Institute of Technology Cambridge United States4 Advanced Light Source and Chemical Sciences Division Lawrence Berkeley National Laboratory Berkeley United States1 Laboratory of Electrochemical Interfaces Massachusetts Institute of Technology Cambridge United States,2 Department of Materials Science and Engineering Massachusetts Institute of Technology Cambridge United States,3 Department of Nuclear Science and Engineering Massachusetts Institute of Technology Cambridge United States
Show AbstractTopotactic phase transitions of functional oxides induced by changes in oxygen non-stoichiometry can largely alter multiple important physical and chemical properties, including electrical conductivity, magnetic state, oxygen diffusivity and oxygen reduction/evolution reactivity. This enables conveniently tuning functional oxide properties and switching between distinct states, which is of pivotal importance for applications such as memristors or electro-catalysis (e.g. solid oxide fuel cell cathodes). Here we demonstrate a novel and feasible means to trigger topotactic phase transitions by precisely controlling oxygen non-stoichiometry in a functional oxide. The method involves electrochemical control of oxygen non-stoichiometry as previously demonstrated by Chen et al1. We use electrochemical potential to change the non-stoichiometry and thereby induce phase transition in strontium cobaltite (SrCoOx, denoted as SCO) between the Brownmillerite phase SrCoO2.5 (BM-SCO) and the perovskite phase SrCoO3- δ (P-SCO). In situ high-temperature X-ray diffraction (HTXRD) measurements revealed that the BM-SCO to P-SCO phase transition can be readily induced by applying electrochemical bias at a temperature of 300°C. Moreover, the transition was shown to be fully reversible. We used in situ X-ray spectroscopy methods to probe the evolution of electronic structure during the phase transition. In situ ambient pressure X-ray photoelectron spectroscopy revealed a binding energy shift towards lower energy side upon an anodic bias, consistent with increasing hole concentration and downwards shift of Fermi level in SCO. In situ X-ray absorption spectra (XAS) showed that the unoccupied states had evident O2p characteristics, indicating strong hybridization between O2p and Co3d states. Resonant photoelectron spectroscopy performed on Co-resonance photon energy clearly showed that the Co 3d states are populated near the top of valence band during the BM-SCO to P-SCO phase transition. More importantly, in situ XAS was utilized to assess the kinetics of phase transition and identify the rate-limiting steps. A revised nucleation-growth model, which includes an additional oxygen incorporation term, was used to describe the phase transition process, which matched very well with the experimental data. This study provides a complete picture on the electronic structure evolution of SCO that is important for assessing the mechanisms involved in a red-ox based resistive switch and electro-catalyst applications of this material.
1 Chen, D. & Tuller, H. L. Voltage-Controlled Nonstoichiometry in Oxide Thin Films: Pr0.1Ce0.9O2−δ Case Study. Adv. Funct. Mater. 24, 7638–7644 (2014).
NT2.10: LEDS, OLEDS and Flat Panel Display
Session Chairs
Thursday PM, March 31, 2016
PCC North, 100 Level, Room 121 B
4:45 PM - NT2.10.02
Band Gap Bowing of NixMg1-xO (0 ≤ x ≤ 1) Thin Films
Sneha Rhode 1,Christian Niedermeier 1,Bin Zou 1,Suman-Lata Sahonta 2,Neil Alford 1,Michelle Moram 2
1 Imperial College London London United Kingdom,2 Materials Science and Metallurgy University of Cambridge Cambridge United Kingdom1 Imperial College London London United Kingdom,2 Materials Science and Metallurgy University of Cambridge Cambridge United Kingdom
Show AbstractNiO (Eg = 3.6 eV) and MgO (Eg = 7.8 eV) are both direct band gap materials which crystallize in the cubic rock-salt structure. As the maximum lattice parameter mismatch between the two materials is less than 0.8 % [1], NixMg1-xO thin films on MgO single crystals can be grown over the entire composition range with high crystalline quality. This epitaxial growth of NixMg1-xO thus offers enormous flexibility for tailoring the NixMg1-xO (0 ≤ x ≤ 1) band gap in the deep UV region within energies of 3.7 eV to 7.8 eV.
In contrast with theoretical measurements which predict that the band gap of a binary alloy semiconductor should vary linearly according to Vegard’s law, our optical measurements observe that the band gap increases only slightly from 3.7 eV for NiO to about 4.8 eV for Ni0.1Mg0.9O despite the large band gap of MgO (7.8 eV). This asymmetry or non-parabolic bowing has previously been attributed to the presence of Mg rich clusters [2] and oxygen vacancies [3] in the films. This minor band gap increase is surprising and so far very little attention has been given to the exceptional NixMg1-xO band gap trend as a function of composition. Moreover no nano-scale structural investigations have been carried out to study the reason for this band gap bowing. Therefore our work investigates the nano-scale structural and compositional homogeneity of epitaxially grown NixMg1-xO films grown on MgO (100) substrates by pulsed laser deposition over the entire composition range of the NiO-MgO solid solution system and its effect on the band gap of the alloy using high-resolution X-ray diffraction analysis employing synchrotron radiation, high-resolution transmission electron microscopy (HRTEM) and aberration corrected scanning transmission electron microscopy (HRSTEM) using the high angle annular dark field detector (HAADF), energy dispersive X-ray spectroscopy (EDXS) and electron energy loss spectroscopy (EELS).
Our HRSTEM results and HRSTEM-EDXS results on NixMg1-xO (x = 0.1 and 0.2) films show homogenous films with the absence of both Mg and Ni clustering. Moreover both HRTEM and aberration-corrected HRSTEM-HAADF studies showed epitaxially grown low defect density containing films. Oxygen vacancy studies on the NixMg1-xO films using STEM-EELS and resistivity data also showed the absence of high concentration of point defects around the low Ni content region where a high degree of band gap bowing is observed. These results confirm that Mg clustering and oxygen vacancies do not cause band gap asymmetry and bowing in NixMg1-xO films as predicted previously, and the band gap asymmetry could be related to other effects such as the presence of localized deep states in the band gap.
[1] A. Kuzmin and N. Mironova, Journal of Physics: Condensed Matter 10, 7937 (1998).
[2] Z.-G. Yang, L.-P. Zhu, Y.-M. Guo, Z.-Z. Ye and B.-H. Zhao, Thin Solid Films 519, 5174 (2011).
[3] R. Boutwell, M. Wei, A. Scheurer, J. Mares and W. Schoenfeld, Thin Solid Films 520, 4302 (2012).
5:00 PM - NT2.10.03
From Combinatorial Screening to an Indium-Free Anode for Large-Area Flexible OLEDs
Monica Morales-Masis 1,Fabien Dauzou 1,Quentin Jeangros 1,Ali Dabirian 1,Aicha Hessler-Wyser 1,Rainald Gierth 2,Manfred Ruske 3,Date Moet 4,Christophe Ballif 1
1 Ecole Polytechnique Fédérale de Lausanne (EPFL) Neuchatel Switzerland,2 Philips GmbH, Innovative Technologies, Research Laboratories Aachen Germany3 Philips GmbH, Business Center OLED Lighting Aachen Germany4 TNO/Holst Centre Eindhoven Netherlands
Show AbstractWe present the development of an indium-free anode from a combinatorial material study up to its application in large-area flexible white organic light-emitting diodes (OLEDs). The material developed is a void-free transparent and conductive zinc tin oxide (ZTO) amorphous compound. Following a combinatorial screening of a range of aluminum doped zinc oxide (ZnO:Al) and tin oxide (SnO2) mixtures by co-sputtering deposition, we determined the optimized ratio of Sn/Zn to achieve the highest conductivity. Extensive chemical, electrical and microstructural characterization of the distinct material phases formed in the combinatorial development was performed to determine the influence of the deposition parameters on the films properties. The optoelectronic properties of the material were subsequently optimized by adjusting the parameters of the sputtering process from a single target manufactured with the optimized Sn/Zn ratio. Transmission electron microscopy (TEM) analysis indicates that this material as-deposited offers a defect-free microstructure which allows achieving high electron mobilities of up to 21 cm2/Vs. The ZTO anodes also present an absorptance of less than 5% in the visible range of the spectra and surface roughness of less than 0.2nm, following the requirements for application as anode for small molecule (sm)-OLEDs. The ZTO films are also extremely stable under thermal and acid treatments. In addition, the optimized void-free amorphous ZTO anodes are fabricated by sputtering deposition, which is a common industrial process that will facilitate upscaling and ITO replacement at the industrial level. Large-area (41 cm2) white sm-OLEDs were fabricated on these ZTO electrodes in combination with a metal grid (ZTO/grid), and compared to devices fabricated with ITO/grid anodes. We show that large-area white sm-OLEDs fabricated with the ZTO/grid anode show better performance than those with ITO/grid anodes, confirming that ZTO can already substitute ITO for large-area OLED lighting applications.
5:15 PM - NT2.10.04
Facile Preparation of High Mobility La:BaSnO3 Thin Films Co-Doped with Interstitial Hydrogen
Christian Niedermeier 2,Sneha Rhode 1,Keisuke Ide 2,Michelle Moram 1,Hidenori Hiramatsu 2,Hideo Hosono 2,Toshio Kamiya 2
1 Materials Imperial College London London United Kingdom,2 Materials and Structures Laboratory Tokyo Institute of Technology Yokohama Japan,1 Materials Imperial College London London United Kingdom2 Materials and Structures Laboratory Tokyo Institute of Technology Yokohama Japan
Show AbstractTransparent oxide La:BaSnO3 demonstrates great potential as a high-mobility electron transport layer for applications in flat panel displays, solar cells, light-emitting diodes, and multi-functional perovskite-based electronic devices. While an extraordinary high carrier mobility of 320 cm2/Vs has been reported for La:BaSnO3 single crystals[1], epitaxial La:BaSnO3 thin films grown on SrTiO3 substrates suffer from the presence of dislocations and domain boundaries that cause electron scattering. Up to present, optimum electron transport properties have been presented for vapour phase grown epitaxial thin films and by La substitutional doping well beyond the equilibrium solubility limit in BaSnO3.
This work presents the solid-phase epitaxial growth of high mobility La:BaSnO3 thin films by crystallization through thermal annealing of pulsed laser-deposited amorphous/nanocrystalline films at room temperature. The solid phase epitaxy excels in its remarkably facile preparation and single crystal-like quality thin film growth as confirmed by 200 out-of-plane rocking curves as narrow as 0.10o FHWM. Hall mobilities up to 30 cm2/Vs are obtained, which is the highest among those reported with carrier concentrations as low as 4x1019 cm-3. The La concentrations in the thin films are accurately determined by SIMS depth profiling to assess the actual solubility after the solid-phase epitaxial growth and to evaluate the donor activation efficiency in La:BaSnO3.
This work further presents the first experimental report of H doping to BaSnO3 thin films, as implemented by H plasma treatment and high pressure H2 annealing. Carrier concentrations as high as 1x1020 cm-3 are obtained, and the noticeably improved electronic properties support the prediction of first-principles calculations suggesting that interstitial H acts as a shallow donor in high electron affinity oxides[2] and BaSnO3 in particular[3]. The (La,H):BaSnO3 thin films show enhanced free carrier absorption, and the optical mobility and the electron effective mass are analysed from FT-IR spectra. Carrier scattering mechanism will be discussed based on these results and temperature-dependent electronic transport properties.
[1] H. J. Kim et. al. Appl. Phys. Express 5, 061102 (2012).
[2] Ç. Kiliç and A. Zunger. Appl. Phys. Lett. 81, 73 (2002).
[3] D. O. Scanlon. Phys. Rev. B 87, 161201 (2013).
5:30 PM - NT2.10.05
Site-Selective Defect Passivation in NiO Solar Photocathodes by Targeted Atomic Deposition
Cory Flynn 1,Shannon McCullough 1,EunBi Oh 1,Lesheng Li 1,Candy Mercado 2,Byron Farnum 1,Wentao Li 1,Carrie Donley 1,Wei You 1,Arthur Nozik 3,James McBride 4,Thomas Meyer 1,Yosuke Kanai 1,James Cahoon 1
1 Chemistry Univ of North Carolina at Chapel Hill Chapel Hill United States,2 Dept of Chemistry Renewable and Sustainable Energy Institute Boulder United States3 Dept of Chemistry and Biochemistry University of Colorado at Boulder Boulder United States4 Dept of Chemistry Vanderbilt University Nashville United States
Show AbstractAs the size of semiconductor materials continues to shrink, the surface-to-volume ratio increases and surface defects often dominate the performance of the materials, particularly in solar-energy applications. Surface defect sites can alter the optical and electronic characteristics of materials by changing the Fermi level, charge-carrier mobility, optical absorption, and surface reactivity. Here, we present a new vapor-phase deposition method to selectively and completely passivate defect sites in semiconductor nanomaterials termed targeted atomic deposition (TAD). We demonstrated the capabilities of this selective passivation method by applying a TAD of alumina to the widely used cathode material, Nickel Oxide. By exploiting a temperature-dependent deposition process, we were able to selectively passivate the most highly reactive sites in NiO: the oxygen dangling bonds and Ni vacancies. The new TAD treatment resulted in the complete passivation of all measurable surface defects, optical bleaching of the material, and significant improvements in all photovoltaic performance metrics in dye-sensitized solar cells. The technique was proven to be generic to numerous forms of NiO, and these results suggest that TAD could be generic for many 0D, 1D, and 2D nanomaterials with a variety of highly-reactive vapor-phase precursors.
NT2.11: Poster Session III: Oxide and Chalecogenide Thin Film and Nanomaterials
Session Chairs
Friday AM, April 01, 2016
Sheraton, Third Level, Phoenix Ballroom
9:00 PM - NT2.11.01
High-Mobility CdS Thin-Film Transistors by Sol-Gel Method
SungMin Kwon 1,Jaehyun Kim 1,HanLim Kang 2,Myung-Gil Kim 1,Sung Kyu Park 1
1 Chung-Ang University Seoul Korea (the Republic of),2 Korea University Seoul Korea (the Republic of)
Show AbstractRecently, metal chalcogenides materials have been intensively researched due to their diverse optical and electronic properties, such as large band gap tunability, high infrared transparency, high conductivity, and high carrier mobility. The metal chalcogenides have been successfully implemented for various applications, such as photovoltaics, photodetector, non-linear optics, telecommunication, transistor, and thermoelectric generator. All of these successful industrial demonstrations relied on the thin-film or bulk materials. Especially the thin film metal chalcogenide expand their usage from the conventional electronic and optoelectronic applications to next generation energy and sensor applications. The cost effective fabrication of metal chalcogenide thin-film become increasingly important especially for large area applications, such as thin-film solar cell, large area display, and sensor arrays. Although the solution processing of metal chalcogenide enable the cheap deposition of large area thin-film, the lack of soluble metal chalcogenide precursor hinder the facile low cost deposition of thin-film.
In this reports, we investigated novel sol-gel type chalcogenide precursor systems on CdS. In order to achieve soluble precursor, cadmium salt and thiourea are used. We can observe cadmium sulfide precursors are dissolved in organic solvent. Then, the precursor solution is spin-coated onto thermally oxidized silicon substrate, with the subsequent thermal annealing at 400'C. After the heat treatment, the carbon impurities in active layer are removed. The continuous cadmium sulfide films with thickness ranging 20-100Å have been successfully achieved. X-ray diffraction confirms that the films are CdS, with the cubic structure. The source and drain electrodes are thermally evaporated onto the CdS film. The n-channel cadmium sulfide shows a saturation field-effect mobility and on/off ratio of 11.8 cm2/V-s and >106, respectively.
9:00 PM - NT2.11.02
Large-Area Precise Printing of Sol-Gel Oxide Dielectrics via Bar-Coating Method with Selective Direct-Patterning for Solution-Processed Metal-Oxide Transistor Arrays
Won-June Lee 1,Sungjun Park 3,Sujin Sung 3,Yong-Young Noh 2,Myung-Han Yoon 3
1 Materials Science and Engineering Gwangju Institute of Science and Technology Gwangju Korea (the Republic of),1 Materials Science and Engineering Gwangju Institute of Science and Technology Gwangju Korea (the Republic of),3 Research Institute for Solar and Sustainable Energies Gwangju Institute of Science and Technology Gwangju Korea (the Republic of)2 Energy and Materials Engineering Dongguk University Seoul Korea (the Republic of)
Show AbstractIn this work, we demonstrate the fabrication of thin and densified sol–gel metal oxide films for gate dielectric layers by a simple bar-printing method in large-area printed metal oxide thin-film transistor arrays. The thickness of single-coated aluminum oxide and hafnium oxide dielectric films can be accurately controlled with a nanometer resolution (10 – 40 nm) by adjustable rheological factors (various bar-shearing speeds and the precursor concentrations), whereas thick metal oxide dielectric films (~100 nm) are accomplished by repeated sol–gel oxide coatings. The fabricated high-quality oxide dielectric layers show remarkable results of very smooth surface morphology with excellent insulating properties, performing very low leakage current density in the range of 10–8 A cm-2 at 2 MV cm-1 and very high areal capacitance of over 400 nF cm-2, in company with very uniform large-area coverage up to 4-inch silicon wafer. Lastly, in combination with direct-oxide patterning process via selective surface wettability, solution-bar-coated metal oxide transistor arrays were achieved using a self-patterned sol-gel indium–gallium–zinc oxide semiconductor on the aluminum oxide-coated dielectric layer, which obtain sufficient transistor performance along with electron mobilities of >5 cm2 V-1 s-1, high current on–off ratios of 105, and minimal gate leakage current in the stable range of 10–9 A cm-2 at a very low operation voltage below 2 V.
9:00 PM - NT2.11.03
The Cu(In,Ga)Se2 Solar Cells Inserted Se Interlayer with Na Incorporation
Jae-kwan Sim 1,Ji-Hyeon Park 1,San Kang 1,Seung kyu Lee 1,Daeyoung Um 1,Da som Lee 1,Taek-Soo Jang 1,Cheul-Ro Lee 1
1 Chonbuk National Univ. Jeonju Korea (the Republic of),
Show AbstractA Cu(In, Ga)Se2 (CIGS) solar cell is the most promising material owing to its excellent light trapping ability, broadband light absorption, and environment-friendly manufacturing processes. The CIGS solar cell has a crucial advantage of using various flexible substrates such as stainless steel (STS), polyimide. The preparation of CIGS solar cells on STS substrate, which shows higher temperature stability than polyimide during the selenization process, requires Na incorporation into CIGS absorber. Na incorporation is well kwon to improve the efficiency of CIGS solar cells. However, Ga grading in the CIGS absorber occurs when sodium diffuses into the CIGS absorber. It is also reported that sodium influences In and Ga inter-diffusion. In this study, Mo-Na layer and Se interlayer are used for incorporating Na and uniform Ga distribution in the CIGS absorber along the depth.
Mo-Na layer is deposited by sputtering using Mo-Na alloy (90:10) target on STS substrate. A CIGS absorber is grown by co-sputtering system and selenization process followed by deposition of Mo back contact on the Mo-Na layer. The content of Na diffused into the CIGS absorber is controlled by variation of growth pressure for a Mo-Na layer, as is confirmed by SIMS depth profile. Also, in order to confirm Ga inter-diffusion, two CIGS absorbers are prepared by converting CuInGa (CIG) precursors without and with Se interlayer. These CIG precusors consisted of CuGa/Se/CuIn/In/Mo/STS and CuGa/CuIn/In/Mo/STS stacks are selenized at 500 °C for an hour. Through the SIMS depth profile and XRD, there is a change between prepared CIGS absorbers that the amount of Ga elements decreases from the top to bottom layer in the CIGS absorber gradually. We will discuss the effect of Se interlayer in the CIGS absorber and its effect on solar cells.
9:00 PM - NT2.11.04
Effects of Cathode Materials on Thermally Annealed Cu-SiO2 CBRAM
Wenhao Chen 1,Mehmet Balaban 1,Hugh Barnaby 1,Michael Kozicki 1
1 Arizona State University Tempe United States,
Show AbstractConductive-bridge random access memory (CBRAM) is a promising alternative to flash for non-volatile data storage. CBRAM is composed of an electrochemically active anode (copper or silver) and an electrochemically inert cathode, a layer of ion conductor (chalcogenide glass or oxide) is sandwiched between the two electrodes. Resistance change between a high-resistance state (HRS) and a range of low-resistance states (LRS) originates from creation or dissolution of conducting metallic filament in ion conducting layer. Similar to Ag-chalcogenide based CBRAM, CBRAM consisting of a Cu anode and a SiO2 ion conductor possesses merits such as low power, scalability, and fast speed, and it has much better compatibility with BEOL CMOS processing. Considering these benefits, this CBRAM variant is gaining significant attention from industry.
Doping the ion conductor with anodic metal atoms is a critical step to achieve stable resistive switching and better LRS retention. Unlike chalcogenide glass, SiO2 can be hardly doped with Cu through photo-doping process. Doping Cu into SiO2 is achievable through thermal diffusion which requires high temperature process. Therefore, the stability of cathode at high temperature is crucial to achieve reliable resistive switching performance. Generally, it’s ideal to have an absolute inert cathode that does not react and generate unwanted residues, such as metal oxides and/or metal ions, during thermal annealing. Platinum (Pt) are frequently used as CBRAM cathode in researches because of its excellent high temperature stability, however, the high cost and CMOS-incompatibility of Pt hinders its application in industry. Cathode using CMOS compatible materials are needed to replace Pt for foundry adaptability.
In this study, we studied the thermal annealing effects on Cu-SiO2 CBRAM with tungsten (W) and nickel (Ni) cathode. The devices are fabricated in a 32x32 crosspoint array. The arrays are sealed with SiO2 and are annealed at 450°C to 550°C with continuous nitrogen gas flow to prevent anodic and cathodic oxidization. The resistive switching performance is characterized before and after thermal annealing. It is found that electroforming voltage of the thermally annealed devices with Ni cathode is much larger than devices with W cathode. The influence of high temperature on W and Ni cathode and cathode/SiO2/Cu interface are investigated through materials analysis and electrical characterization. The full results of this study will be presented in the paper.
9:00 PM - NT2.11.05
Efficiency Enhancement of Photovoltaic Devices by Reducing the Charge Recombination through Plasma Treatment of FTO Glass Substrate
Van-Duong Dao 1,Liudmila Larina 1,Ho Suk Choi 1
1 Chungnam National University Daejeon Korea (the Republic of),
Show AbstractWe present a novel strategy for increasing the efficiency of photovoltaic (PV) devices, including dye-sensitized solar cell (DSC) and perovskite solar cell (PSC), through modifying the surface of FTO glass by atmospheric pressure plasma treatment. This strategy allows the suppression of charge recombination at the interface between FTO substrate and electrolyte as well as hole transfer material. HRSEM, water contact angle, and XPS measurements confirmed the change of the FTO surface properties. The plasma treatment (150 W power, 5 lpm Ar gas flow rate, and 1 min treatment time) decreased the water contact angle of FTO surface down to zero, which refers to super-hydrophilicity. This property allows the formation of uniform and thin TiO2 BL on FTO surface without any aggregation. First, we applied the developed electrode in working electrode of DSCs. DSC employing plasma-treated FTO substrate showed higher efficiency of 8.61% than 8.10% of the device fabricated without plasma treatment. The IVMS data suggest that the improvement of device PV parameters induced by plasma treatment is because of high charge transfer resistance at the interface between treated FTO and BL, resulting in low interface recombination rate. The increase of BL specific surface area led to the improvement of dye loading on BL. Furthermore, the transmittance of the plasma-treated FTO/BL interface is higher than that of plasma-untreated interface. This is because the plasma-treated interface reduces the reflection of light. These effects lead to the decrease in the recombination loss through the junction and the enhancement of light harvesting, in turn to an increase in short-circuit current density. We believe that this work provides the possibility of improving the efficiency of perovskite solar cells through plasma treatment of charge collection layer. Indeed, PSCs based on the newly developed electrode had 39% higher efficiency than reference devices. The obtained results provide direct evidence in favor of the developed strategy.
9:00 PM - NT2.11.06
Antimony Selenide (Sb2Se3) Thin Film Solar Cells Fabricated by Electrodeposition
Young Bin Kim 1,Sung Woon Cho 1,Seung Ki Baek 1,Hyung Koun Cho 1
1 Sungkyunkwan University Gyeonggi-do Korea (the Republic of),
Show AbstractThin film having high absorption coefficient is considered as an alternative absorber since it allows the low cost solar cells with a high efficiency. Representatively, Cu(In, Ga)Se2 showed the impressive solar energy conversion efficiency of 20.4 %. However, scarcity of the In, and Ga restricted the terawatt scale deployment. Thus, Cu2(Zn, Sn)S4 only containing the earth-abundant materials has been focused and achieved the 11.1 % energy conversion efficiency. Nevertheless, formation of the secondary phases, such as CuxSnySz and SnS is competitive and limits the improving efficiency.
Sb2Se3, simple binary compound having high absorption coefficient(>105 cm-1), 1.0-1.2 eV of band-gap and high mobility (> 42 cm2/Vs) is appropriate for the thin film solar cells. Unlike Cu2(Zn, Sn)S4, it is easy to prepare the uniform composition and large grains since it only has fixed composition and low-melting temperature (~885 K). Recently, J. Tang group employed the thermal evaporation method for Sb2Se3 layer and more than 5 % of energy conversion efficiency. In our study, Sb-Se precursors were prepared by electrodeposition and conveniently annealed at 573 K to form the Sb2Se3 compounds. After that, to manufacture the photovoltaic device, CdS, ZnO and ITO layers were sequently deposited on the Sb2Se3 by chemical bath deposition and sputtering method. As a result, we simply achieved thin film solar cells having the energy conversion of > 1 %.
9:00 PM - NT2.11.07
Suitable Yellow-Green and Red Emitting Nanophosphors Blend for Producing Efficient Warm White-LEDs
Savvi Mishra 1,Divi Haranath 1
1 National Physical Lab Delhi India,
Show AbstractThe artificial lighting system has passed through three phases; incandescent lamp, neon light, and Compact Fluorescent Lamp (CFL), and has been advancing towards the fourth phase-semiconductor lighting, especially white Light emitting diode (w-LEDs). White LEDs are believed to be attractive green lighting source of the 21stcentury and are gaining importance due to their superior features and wide applications. Other light sources are considered very inefficient, owing to inevitable large losses of energy during thermal excitation. Present commercial LEDs exhibit efficiencies less than 50%, are limited by materials quality and device-structure imperfections. Therefore, the LED is considered to have potential to become the next generation lighting. One of the famous and widely used approaches for creating white light by means of LEDs is, using a blue LED with a phosphor having broad yellow-green emission. Indeed, Y3Al5O12:Ce (YAG:Ce)/blue LED system has a poor color rendering index (CRI) because of the red spectral deficiency of YAG:Ce phosphor, which hinders the development of w-LEDs with high CRI and low correlated color temperature (CCT). Y2O3:Eu and Y2O2S:Eu, the traditionally used phosphors to resolve the above-mentioned issue either have negligible (
9:00 PM - NT2.11.08
Effects of Negatively Charged Energy-Beam-Irradiation on the Sputtered Indium-Based Oxide Thin Films
Young Joon Yoon 1
1 Korea Institute of Ceramic Engineering and Technology Gyeongsangnam-do Korea (the Republic of),
Show AbstractThe negatively charged energy beam (NCEB) was employed for the surface treatments indium-based oxide thin film grown by RF sputtering process. NCEB was mainly composed of electron and oxygen negative ion generated from induction plasma and it could be accelerated with the energy from 0.1 ~ 5 kV. NCEB gun with the diameter of 60mm was attached on the RF sputtering system and it could be operated with sputtering gun simulataneously. Through the additional electron supply into the plasma during sputtering process, the working pressure could be kept below 5x10-4 Torr. Therefore, both the mean free path and the mobility of sputtered atoms were increased and the well ordered and the highly dense microstructure could be obtained compared to those of conventional sputtering condition. Sputtering process is widely used in Si-based semiconductor industry and it is also an ideal method to deposit transparent oxide materials for thin-film transistors (TFTs) due to the low substrate temperature. The oxide films grown at low temperature by conventional RF sputtering process are typically amorphous state including a large number of defects such as oxygen vacancies. Those play a crucial role in the electron conduction in transparent electrode, while those are the origin of instability of semiconducting channel in oxide TFTs due to electron trapping. Therefore, post treatments such as high temperature annealing process are commonly progressed to obtain high conductivity and good stability.
In this work, the physical properties of transparent oxide films such as conducting indium tin oxide and semiconducting indium gallium zinc oxide films treated by NCEB process will be discussed in detail. Those films showed the high conductivity and the high mobility without additional post annealing process.
9:00 PM - NT2.11.09
Effects of Annealing Temperature on the Performance of W-SiO2-Cu PMC Devices
Mehmet Balaban 1,Wenhao Chen 1,Hugh Barnaby 1,Michael Kozicki 1
1 Arizona State University Tempe United States,
Show AbstractProgrammable metallization cells (PMCs) are one of the top candidates for storage elements in future non-volatile resistive memory systems as they operate at low power and voltage, possess multi-level storage capability, and have excellent scaling potential. They are also relatively simple to integrate with CMOS and this is especially true of structures that use Cu as the top electrode and a low-density SiO2 as the switching layer as these materials are already being used in back-end-of-line (BEOL) processes. In these devices, the on state resistance is determined by the formation of a Cu filament in the relatively porous oxide. We have investigated the effects on device performance of different annealing temperatures from 400 oC to 550 oC on W-SiO2-Cu PMC devices in a 32x32 crosspoint structure. Several device parameters such as retention time, forming voltage, and initial resistance have been assessed. The density of the e-beam evaporated SiO2 has been characterized using X-Ray Reflectivity (XRR). Cross-section images and copper depth profile inside the switching layer and inter-electrode insulator layer have been gathered using Transmission Electron Microscopy (TEM). We have found that increasing annealing temperature up to 500 oC leads to an increase in forming voltage and a decrease in initial resistance. At 550 oC, the forming voltage decreases dramatically, while the initial resistance increases relative to the 500 oC case. The formation of Cu islands on the Cu/SiO2 interface at 550 oC appears to be the main cause of the reduction in the forming voltage. Furthermore, increasing the annealing temperature leads to higher concentration of copper inside the switching layer which improves the retention. This is due to the fact that more copper in the switching reduces diffusion and dissolution of the conductive filament after it is formed. The full results of this study will be presented in the paper.
9:00 PM - NT2.11.10
Synthesis and Micro-Raman Study of Phonon Softening in Sharp Vertical ZnO Nanowires
Seungho Ahn 1,Hongbin Yu 1
1 Arizona State University Tempe United States,
Show AbstractOne dimensional nanostructure materials have been extensively synthesized and studied in the past decades. Among them, the semiconducting zinc oxide (ZnO) is a particularly attractive inorganic material. ZnO nanostructures show the interesting and various characteristics for research because of its direct wide band gap of 3.37 eV, large excitonic binding energy of 60 meV, as well as potential piezoelectric and optical properties. Recently, ZnO vertical nanowires have drawn significant attention because of their proposed applications in the low voltage and short-wavelength (368 nm) electro-optical devices, transparent ultraviolet (UV) protection films, solar-blind UV detectors [1], gas sensors [2], and even spintronic devices. Despite of the practical importance, the current knowledge of vibrational (phonon) properties of ZnO vertical nano-pillars is rather constrained. Understanding the details of both optical and acoustic phonon spectra of ZnO nanostructures, such as nano-pillars, by the micro-Raman investigation can assist in development of ZnO based optoelectronic devices.
Raman scattering has been used as an ideal sensitive, non-destructive characterization tool. Its possibility to obtain information about sample quality for analysis of specific aspects of lattice dynamics such as isotopic effects and phonon lifetime as well as position of the doping ions in the host lattice and probe the presence of impurities which are undetectable by X-ray analysis. With these characteristics, Raman spectroscopy has been essential method of choice for many studies of vibration properties of ZnO, especially for bulk, thin films, and nanostructured samples, pure and doped. On this way in ZnO has been investigated multi phonon process, electron-phonon coupling, dopant incorporation, local atomic arraignment, changes in samples with annealing process, temperature dependence of Raman modes and many others.
In this paper, we present how ZnO vertical nanowires are synthesized and determined how certain growth mechanism governs those ZnO vertical nanowire structures. Especially, we discuss some intriguing results that brought up the ideas how and why the Raman peak shift ranged between 388 cm-1 and 403 cm-1 with the ZnO nanowires grown on the first three high temperatures of 1000°C, 970°C, and 940°C. Furthermore, based on these Raman Spectroscopy results, we can clarify the origin of the peak shift for ZnO nanowires due to their size effect, which no other papers have been discussed before. [1]H. Yu, E. A. Azhar, T. Belagodu, S. Lim, and S. Dey, “ZnO nanowire based visible-transparent ultraviolet detectors on polymer substrates,” Journal of Applied Physics, vol. 111, no. 10, p. 102806, May 2012. [2]T. Belagodu, E. A. Azhar, and H. Yu, “Modulation of charge conduction in ZnO nanowires through selective surface molecular functionalization,” Nanoscale, vol. 4, no. 23, pp. 7330–7333, Nov. 2012.
9:00 PM - NT2.11.11
Growth and Properties of Tin Oxynitride Thin Films
Hyojin Gwon 2,Seok-Jin Yoon 1,Ji-Won Choi 1,Chong-Yun Kang 1,Sahn Nahm 2,Jin-Sang Kim 1,Seung-Hyub Baek 3
1 KIST Seoul Korea (the Republic of),2 Korea University Seoul Korea (the Republic of),1 KIST Seoul Korea (the Republic of)2 Korea University Seoul Korea (the Republic of)1 KIST Seoul Korea (the Republic of),3 Korea University of Science and Technology Seoul Korea (the Republic of)
Show AbstractOxynitride, where both oxygen and nitrogen ions are bonded with metal cations, is gaining more attention for its potential benefits of merging physical properties of oxides and nitrides. Controlling oxygen/nitrogen ratio can provide a tool to engineer the bandgap, the electrical conductivity, and the crystal structure. Moreover, the different ionicity/covalency of oxygen and nitrogen may play a unique role to determine the physical properties of oxynitrides. Here, recent results will be reviewed on the epitaxial growth of tin oxynitride thin films using reactive sputtering technique and the characterization of their physical properties. Our results will provide new opportunities to design novel properties of materials for next-generation electronics as well as to explore physics on aliovalent anion substitution.
9:00 PM - NT2.11.12
High Performance Gas Sensors by Combination of Micro/Nanostructured Porous Thin Film and MEMS-Based Chip
Weiping Cai 1
1 Institute of Solid State Physics, Chinese Academy of Sciences Hefei China,
Show AbstractThe conductometric thin film gas sensors, with ultra-fast response, high sensitivity and low power-consumption, have been urgently expected and in challenge. In this talk, a flexible strategy is presented for such gas sensors based on combination of structure-controllable metal oxide porous thin films and MEMS-based chip. Typically, SnO2 is taken as the example to demonstrate the validity of this strategy. By solution-dipping and transferring the organic-colloidal template, the structure-controllable SnO2 micro/nanostructured ordered porous thin film is in-situ fabricated on a sandwich chip, which contains micro-spaced interdigital electrodes and microheater, to form a new gas sensor. Such conductometric thin film gas sensor has shown the second-level response time and 10ppb-detectable sensitivity to ethanol and acetone, good stability and low power-consumption with mW-level, exhibiting the high performances. Importantly, the fabrication strategy is suitable for mass production of such gas sensors in batches (kilo-sensors in each batch) and facile for regulating sensing films with different microstructures and stoichiometries. This work provides a realizable way for design and fabrication of the conductometric thin film gas sensors with high performances.
9:00 PM - NT2.11.13
Native Oxide Growth during Plasma Enhanced Atomic Layer Deposition of High-κ Gate Dielectrics
Kaveh Ahadi 1,Kenneth Cadien 1
1 Univ of Alberta Edmonton Canada,
Show AbstractState-of-the-art complementary metal oxide semiconductor (CMOS) technology currently utilizes gate dielectric layers with higher dielectric constant (high-κ) than SiO2 or SiON. Due to high energy gap and compatibility with Si, both HfO2 and ZrO2 have been considered in CMOS technology. Silicon substrate is commonly HF etched for native oxide removal prior to plasma enhanced ALD deposition of high-κ dielectric. After introduction of first precursor pulse the first oxygen plasma pulse is introduced while the Si surface is barely protected. Plasma oxygen can grow even thicker native oxide compared to molecule one. Presence of low κ silicon oxide causes degradation of capacitance. Moreover, it might build up near-interface oxide traps. On the other hand, thermal ALD reveals higher bulk oxide defects compared to plasma enhanced one. Subsequently, starting with few thermal cycles and then switching to plasma enhanced ALD might be a good trade off and might provide the optimal electrical properties.
Sixty cycles of zirconia films were grown with atomic layer deposition (Kurt J. Lesker 150LX). In-situ spectroscopic ellipsometry (J.A. Wollam M2000DI) was utilized to investigate optical properties of the thin films during growth. P-type Si (100) substrates were buffer oxide etched prior to deposition and substrate was maintained at 100 °C for all depositions. MOSCAP devices were fabricated using Cr as gate metal. Anomalous growth per cycle was observed during the first cycles of plasma enhanced atomic layer deposition of high-κ dielectrics using in-situ ellipsometry, while thermal atomic layer deposition of these oxides has steady growth per cycle during the initial stages of deposition. The anomalous growth per cycle was attributed to oxidation of the substrate by plasma oxygen. Thermal grown films have lower capacitance density and higher leakage current but lower density of interfacial traps compared to plasma enhanced grown films. For plasma enhanced grown films the leakage current is dominated by direct tunnelling while trap assisted tunnelling seems to be dominant in thermal grown films. Initiating the oxide growth with thermal atomic layer deposition and then switching to the plasma enhanced process protects the substrate surface from plasma oxygen and lowers the Dit. Starting with ten cycles of thermal atomic layer deposition of ZrO2 enhances the capacitance density while decreasing the Dit. The lowest value of Dit was obtained with twenty cycles of thermal atomic layer deposition (1.8x1010 cm-2 eV-1). The mid-gap Dit reduces systematically with increasing number of thermal ALD cycles. Furthermore, the frequency dispersion in accumulation shrinks dramatically with increasing number of thermal ALD cycles up to twenty.
9:00 PM - NT2.11.14
Structural Relaxation-Driven Changes of Electronic State of Oxygen Vacancies in Non-Stoichiometric Amorphous Oxide Semiconductors
Gi Baek Lee 1,Han-Wool Yeon 1,Young-Chang Joo 1
1 Materials Science amp; Engineering Seoul National Univ Seoul Korea (the Republic of),
Show AbstractOxygen vacancies (VOs) are known to be major electron donors in amorphous oxide semiconductors (AOSs) and doping concentration (ND) in AOSs can be easily modulated by the degree of oxygen non-stoichiometry. However, remarkable thing is that not all VOs act as electron donors and they also can act as deep donors or electron traps depending on the local atomic conditions. We previously reported that structural relaxation (SR) of amorphous phase can increase ND in amorphous In-Ga-Zn-O (a-IGZO) without modulating the degree of oxygen non-stoichiometry. Free volume reduction during SR changes VOs in deep donor or electron trap states to shallow-donor state, which is called SR-driven doping effect. Thus, SR as well as the degree of oxygen non-stoichiometry is the important factor to determine ND in AOSs. However, the influence of oxygen non-stoichiometry in AOSs on SR-driven doping effect still remains to be unknown. Understanding the relationship between non-stoichiometry and SR-driven doping effect is crucial to utilize a-IGZO into the future electronics and estimate the device reliability against thermal stress.
In this study we investigated the SR-driven doping effect on electrical properties of a-IGZO with respect to the degree of oxygen non-stoichiometry using vertically structured metal/a-IGZO/metal devices. As a-IGZO thin-films were covered with Al/Ta/W multi-layer metal electrodes, we could solely observe SR-driven doping effect on electrical properties of a-IGZO during annealing without environmental (e.g., ambient atmospheres) effects. The degree of oxygen non-stoichiometry in a-IGZO thin-films was modulated by oxygen partial pressure (PO2,Dep) during the film deposition using radio frequency sputtering. After annealing at 300-450 °C for 1-16 h, current-voltage (I-V) characteristics of the devices were measured using Agilent 4156C semiconductor parameter analyzer.
In the as-fabricated state, electrical conductivity of the devices increases as PO2,Dep decreases from 10×10-3 Pa (the 10 devices) and 3.3×10-3 Pa (the 3.3 devices) to 0 Pa (the 0 devices). ND was derived by analyzing the conducting mechanism of the devices, and ND in the 10 devices, the 3.3 devices, and the 0 devices yielded to 1014, 1016, and 1018 cm-3, respectively. After annealing at 450 °C for 9 h, ND in the 10 devices and the 3.3 devices increased to 1019 cm-3. Interestingly, ND in the 0 devices is nearly unchanged after annealing. These results imply that SR-driven doping occurred in the 10 and the 3.3 devices, while it did not occur at highly oxygen non-stoichiometric a-IGZO in the 0 devices. We suggest that as the degree of oxygen non-stoichiometry increases, the amount of free volume changes induced by SR decreases, results in diminish of SR-driven doping effect. In addition to I-V analysis, the changes of a-IGZO film thickness after post-fabrication annealing, which is direct evidence of SR, will also be discussed using transmission electron microscope (TEM) analysis.
9:00 PM - NT2.11.15
A Novel Low-Voltage Variable Capacitor Based on Dendritic Filaments
Weijie Yu 1,Runchen Fang 1,Hugh Barnaby 1,Michael Kozicki 1
1 Arizona State University Tempe United States,
Show AbstractExisting voltage controlled variable capacitors are mainly based on micro- electromechanical systems (MEMS) in which capacitance is altered by varying the gap between the two electrodes. With the scaling of systems containing such devices, MEMS based variable capacitors will eventually encounter physical limitations. We have designed a new scalable solid state variable capacitor which comprises two coplanar lateral electrodes with one or more finger electrodes between them and beneath a dielectric layer. By growing a dendritic metallic filament between the upper coplanar electrodes, we can tune the capacitance between the filament and the underlying electrodes. Devices with different top electrode gaps (tg), different finger widths (fw), and different finger gaps (fg), containing a lateral structure of Ni/GeSe:Ag/Ag for dendritic filament growth and Ni lower electrodes were fabricated. Electrical characterization of these devices was performed, including DC analysis and capacitance vs frequency measurements. The results showed that all devices with no filaments in place exhibit an ultra-low capacitance. The growth of Ag based dendritic filaments between the top electrodes was initiated by applying a bias to the electrodes and device capacitance assessed for different filament lengths. Silvaco simulation was performed for various condition, including one, two and four finger electrodes. The results show that the unit capacitance increase rapidly from several afs to hundred fFs once the filament tip grew pass the finger electrode. For multiple finger electrodes, the capacitance increases every time passing the finger electrodes performing as a capacitance ladder during the growth of the filaments. The results matched quite well with both our experimental data and theoretical analysis which disclosed the relationships of capacitance with tg, fw and fg.
9:00 PM - NT2.11.17
Dual Wavelength (Ultra-Violet and Green) Photodetection Using Solution Processed ZnO Nanoparticles
Mohammed Ibrahem 1,Emanuele Verrelli 2,Ali Adawi 2,Jean-Sebastien Bouillard 4,Mary ONeill 2,Fei Cheng 3,Fahad Alharthi 3,Stephen Kelly 3
2 Department of Physics and Mathematics University of Hull Hull United Kingdom,1 Laser Physics Branch, Applied Sciences Department The University of Technology Baghdad Iraq,2 Department of Physics and Mathematics University of Hull Hull United Kingdom2 Department of Physics and Mathematics University of Hull Hull United Kingdom,4 Department of Physics King's College London London United Kingdom3 Department of Chemistry University of Hull Hull United Kingdom
Show AbstractZinc oxide nanoparticles have attracted widespread attention due to their wide band gap of 3.37 eV and high exciton binding energy of 60 meV, making them the material of choice for ultra-violet (UV) photo-detection and other photonic applications. ZnO is well known as an excellent host for many different surface defects such as oxygen vacancies, zinc vacancies, oxygen interstitials, antisite oxygen as well as zinc interstitials. Those defects play a major role in its optical properties, for example, giving rise to luminescence over a broad range of the visible spectrum. Hence, there are new opportunities to exploit the defect chemistry and surface processing of ZnO for optoelectronic applications in the visible spectral region, particularly given the many different nanostructure shapes available. Here we report for the first time a dual wavelength (UV and green) photodetector based on solution processed ZnO nanoparticles spin coated on ITO interdigitated electrodes. The particles have a diameter of 12 nm and are functionalized with octylamine organic ligands for uniform film deposition. In addition to a UV induced photocurrent, a photocurrent is observed with excitation in a narrow spectral region about 550 nm. The responsivity of the green photodetection can be controlled by surface treatment and the device storage environment. For example, the responsivity in the visible is 21 mA/W, 9.6 mA/W and 3.3 mA/W for devices stored in air, dry air and nitrogen respectively. The green photoconductivity is temporally stable for more than two months. Its response time is significantly different to the UV photoconductivity, suggesting a different origin. The relationship between the photocurrent and surface defects will be discussed.
9:00 PM - NT2.11.18
High Stable Double Layered-ZITO/ Er-Doped ZITO TFT Fabricated on Polyimide Substrate
Yun-been Na 1,Ji-Woong Yang 1,Chan Hwa Hong 1,Woo-Hyung Seo 1,Woo-Seok Cheong 1
1 Nanointerface device Electronics and Telecommunications Research Institute Daejoen Korea (the Republic of),
Show AbstractTransparent flexible displays will attract interest of applications to the next-generation displays using organic light emitting diode(OLED), liquid crystal display(LCD) and so on. It is important that oxide channel based thin film transistors(TFTs) should have strong stability as well as high mobility for driving transistors of flexible displays. For the purpose, we fabricated the top-gate oxide-TFT based on ZITO/Er-doped ZITO as double channel layers and Al2O3(150nm) as barrier layer on the polyimide substrate. After annealing process at 300 degree Celsius, Vth of -1.74V, SS of 0.43V/decade and mobility(uFE) of 14 cm2/V s were achieved. At the PBS(positive bias stress) test, the oxide-TFT exhibited low △Vth of 0.54V for 1hour, which is considered to be due to strong binding energy with oxygen of Er. From this study, we could obtain very stable ZITO/Er-doped ZITO TFT on polyimide substrate.
9:00 PM - NT2.11.19
n-CdS/n-CdO Core/Shell Nanowires on FTO Glass for High-Efficiency Z-Scheme Hydrogen Generation
Ki-Hyun Cho 1,Joo-Won Lee 1,Yun-Mo Sung 1
1 Korea Univ Seoul Korea (the Republic of),
Show AbstractThe photoelectrochemical activity of CdS/CdO heterostructure was demonstrated by forming core/shell nanowires on FTO glass substrates. In contrast to the reported CdS nanowire photoelectrode system, high-density CdS nanowires were directly grown on a conducting substrate via the solution-liquid-solid (SLS) approach without using ZnO nanorod templates. CdS buffer layer coated on an FTO glass by sputtering enabled the vertical growth of nanowires having the average diameter and length of ~20 nm and ~1 mm, respectively. Subsequently CdO shell with a thickness of ~10 nm was formed on CdS nanowires to construct a Z-scheme water splitting system. Formation of n-CdS and n-CdO heterojunction through the core/shell structure leads to rearrangement of band structures due to Fermi level alignment. The band edges of CdO shift up to negative potential, while those of CdS shift down to positive potential. Therefore, depletion layers induced near the interface can separate the carriers, so that electron-hole recombination could be suppressed. Enhanced charge separation from the Z-scheme structure could effectively suppress photoluminescence of CdS nanowires. Also, photoelectrochemical performance of CdS/CdO core/shell nanowires was improved compared to bare CdS nanowires at an external potential of 0.0 V (vs. Ag/AgCl) under light illumination. The onset potential for photocurrent density of CdS/CdO nanowires shifts to negative, which implies that the Fermi level of CdS/CdO nanowire system is lower than that of CdS nanowires. From these results, we conclude that the formation of CdS/CdO heterostructured nanowires can effectively improve the photoelectrochemical performance compared to CdS thin films and CdS nanowires, which originates from the large surface area of vertically grown nanowires and the Z-scheme formation.
9:00 PM - NT2.11.20
Controlling the Exciton Dissociation Rates in Semiconductor Nanocrystal Films
Natalia Kholmicheva 1,Mikhail Zamkov 1
1 Bowling Green State University Bowling Green United States,
Show AbstractColloidal nanocrystal solids represent an emerging class of functional materials that hold strong promise for technological applications. The macroscopic properties of these disordered assemblies are determined by complex trajectories of exciton diffusion processes, which are still poorly understood. With the lacking theoretical insight, experimental strategies for probing the exciton dynamics in quantum dot solids are in great demand. Here, we develop an experimental technique for mapping the motion of excitons in semiconductor nanocrystal films with a sub-diffraction spatial sensitivity and a picosecond temporal resolution. This goal was accomplished by doping metal chalcogenide nanocrystal solids with metal nanoparticles that force the exciton dissociation at well-defined distances from their birth. The optical signature of the exciton motion was then inferred from the changes in the emission lifetime, which was mapped to the location of exciton quenching sites. By correlating the metal-metal interparticle distance in the film with corresponding changes in the emission lifetime, we could obtain important transport characteristics, including the exciton diffusion length, the number of pre-dissociation hops, the rate of interparticle energy transfer, and the exciton diffusion mobility. The benefits of this approach to device applications were demonstrated through the use of two representative film morphologies featuring weak and strong interparticle coupling.
9:00 PM - NT2.11.21
Growth of Large Area PbZrxTi1-xO3 Thin Films for Hydrophone Applications
Martando Rath 1,Ramachandra Rao 1
1 Indian Institute of Technology, Madras Chennai India,
Show AbstractLead Zirconate Titanate (PbZrxTi1-xO3) (PZT) is an inorganic ceramic processed at high temperature, which possesses the perovskite cubic structure in its paraelectric phase where Pb2+ ions occupying the cube corners, Zr4+/Ti4+ ions are at the body center and O2- ions are at the face center positions. Below the Curie temperature, depending on the compositional ratio, Zr/Ti and crystallographic structure, it deforms to rhombohedral or tetragonal structure. Therefore, below the transition temperature, PZT shows two ferroelectric phases i.e. tetragonal and rhombohedral structures due to the movement of the body center ions along any polar axis i.e. along c-axis and along the body diagonal respectively. It has been found that the saturation polarization and piezoelectric coefficient of PZT ceramics are around 40 mC/cm2 and 500 pC/N respectively near the morphtrophic phase boundary (MPB) which separates the rhombohedral (Zr rich) and tetragonal (Ti rich) symmetry in the PZT phase diagram. PZT ferroelectrics are considered for most applications involving ferroelectric oxide materials, from piezoelectric transducers to dynamic nonvolatile random access memories1.
In the present work, ferroelectric large area (4 sq. in) PZT thin films were grown on (111) oriented Pt coated Si (001) substrate using off-axis pulsed laser deposition (PLD) with a beam-rastering mechanism2. In this mechanism, a focused laser beam sweeps across the rotating target which covers the entire 1 inch target size. The phase purity of the film was confirmed from X-ray diffraction study. Root means square (RMS) surface roughness of PLD grown PZT thin film were determined to be ~ 5.0 nm using atomic force microscopy. The electrical characterization of the PZT thin films was carried out using ferroelectric loop tracer and impedance analyser with gold (Au) as top electrode. The polarization hysteresis loop has been recorded at different positions on the large area sample. Results will be presented and discussed in detail.
References:
Ionela Vrejoiu et.al., Adv. Mater., 18, 1657 (2006)
S. R. Foltyn,et.al., APL 59 11 (1991)
9:00 PM - NT2.11.22
Structural and Electrical Characteristics of Hafnium Silicate MOS Capacitors
Ramazan Lok 1,Ercan Yilmaz 1,Huseyin Karacali 1,Senol Kaya 1,Aysegul Kahraman 1
1 Abant Izzet Baysal Univ Bolu Turkey,
Show AbstractMicroelectronic technology should be renovate itself for compensate the needs of the future. So, many researchers try to find alternative gate material instead of SiO2. Hafnium silicates may be alternative gate material with high dielectric constant, excellent thermal stability, adequate bandgaps, and compatibility for future MOS- based devices. Hence, Hafnium silicates were deposited by RF sputtering system onto p-type (100) Si substrate and than annealed at 1000 0C in nitrogen environment for 30 minutes. Structural examinations were studied X-ray diffraction and atomic force microscopy. Electrical characteristics of the capacitors were determined by C–V and G/ω–V measurements for several frequencies from 50 kHz to 1 MHz. The XRD results illustrate that the polycrystalline structure were obtained, and AFM confirms the granular phase formation after annealing. On the other hand, it is observed that the C–V and G/ω –V characteristics of the devices are sensitive to applied voltage frequency due to time dependent surface states. In addition the interface state densities of the fabricated device were calculated to be 1.2x1011 cm-2,1.0x1011 cm-2 8.4x1010 cm-2 for 50 kHz, 750 kHz, 1 MHz respectively. These obtained results demonstrate that calculated parameters are important for future MOS technology
9:00 PM - NT2.11.23
Strucutral and Optical Characterization of Metal Chalcogenide Nanowires
Malik Ko 1,Marvin Wu 1
1 North Carolina Central Univ Durham United States,
Show AbstractMetal chalcogenide nanowires exhibit advantageous optoelectronic properties that make them excellent candidates for inclusion into advanced devices such as photovoltaics and photodetectors. We report here studies of crystalline structure and carrier dynamics, which critically affect possible future device performance, in individual metal chalcogenide nanowires. CdSxSe1-x and In2Se3 nanowires produced through the vapor – liquid – solid mechanism using physical vapor deposition were characterized using electron microscopy and optical spectroscopy. Crystalline orientation was mapped along individual nanowires using transmission Kikuchi diffraction (TKD), and carrier dynamics were mapped using sub – micron resolution ultrafast transient absorption (TA) and photoluminescence (PL) lifetime microscopy. Significant variations were observed in the optical data along individual nanowires, and between nanowires taken from the same growth substrate. PL mapping showed variations in intensities of band edge and below bandgap emission attributed to defects. The lifetime of below bandgap emission also varied with position along individual nanowires. TA microscopy data confirms this, as bleach lifetimes and lifetimes of features at the PL defect emission energies are well- correlated with PL data. TKD measurements show that nanowires grow in the 0001 direction, and exhibit some “twisting” of the orientation perpendicular to the growth direction. These observations suggest that carrier trapping at inhomogenoeusly distributed surface sites are the source of observed variations in carrier dynamics.
9:00 PM - NT2.11.24
The Influence of UV Laser Irradiation on the Semiconducting Nature of Reactive Sputtered Nickel Oxide Thin Films
Srikanth Itapu 1,Kamruzzaman Khan 1,Daniel G. Georgiev 1
1 University of Toledo Toledo United States,
Show AbstractNickel oxide, a p-type semiconductor with a wide bandgap of 3.6-4.0eV and a small electron affinity of 1.4-1.8eV, is a potential material for UV LEDs, resistive switching memory devices and electron blocking layer in solar cells. Furthermore, NiO can be easily deposited using electron beam evaporation, pulse laser deposition, RF sputtering, reactive sputtering etc. Because of relative simplicity and good control over stiochiometry and uniformity in thickness, RF and reactive sputtering are often preferred deposition techniques for feasibility studies and related device fabrication. In this work, we study the conversion of p-NiO to n-NiO by ultra-violet (UV) laser irradiation. Point defects can be introduced using pulsed lasers, which provide a controlled source of electronic excitations. Laser irradiation induces rapid local heating resulting in reorientation of chemical bonds with the removal of certain ionic species.
NiO thin films of thickness 200nm were deposited by reactive sputtering of Ni target (99.99% pure, 3-in diameter) under various Ar:O2 gas mixture ratios at room temperature, 300oC and 400oC. Subsequently, laser irradiation of the films was performed using the 4th harmonic wavelength of Nd:YAG laser (λ =266nm) with laser fluence of 1mJ for different number of pulses. Physical examination of the films shows no damage to the films, which suggests the laser fluence was below any melting or ablation limit. The crystalline structure of the NiO films was characterized by X-ray diffraction (XRD) showing temperature dependency of the crystallinity. All NiO films were polycrystalline and for those deposited at 300oC, (111) and (200) peaks are observed at 2θ=37.5o and 42.5o respectively. For films deposited at 400oC, the (200) peak intensity increases and the (111) peak intensity decreases, revealing that temperature increase results in crystal aligning along (200) plane. The peak intensities decrease considerably for laser irradiated NiO films.
Four probe resistivity and Hall-effect measurements were performed to determine any change in the electrical properties of the as-deposited and laser irradiated NiO films. As-deposited NiO films, obtained at 300oC show p-type conduction with resistivity of 12 Ω-cm and carrier concentration of 3.95x1017 /cm3. The NiO films were laser irradiated for various numbers of pulses with laser fluence of 150mJ/cm2. For 500 laser pulses, there was more than one order magnitude decrease in the resistivity (6x10-1 Ω-cm) resulting in the conversion from p-type to n-type NiO with carrier concentration of 4.22x1020/cm3. Hence, we present a possibility of a p-n junction device based on transition metal oxide based electronics.
Symposium Organizers
Dhananjay Kumar, North Carolina Agricultural and Technical State University
Ningzhong Bao, Nanjing Tech University
Sergio D'Addato, Università di Modena e Reggio Emilia
Arunava Gupta, University of Alabama
NT2.12: Zinc Oxide
Session Chairs
Friday AM, April 01, 2016
PCC West, 100 Level, Room 106 A
9:00 AM - *NT2.12.01
Investigations of Radiation Effects in Amorphous Transparent and Conductive Oxides
Valentin Craciun 1
1 National Institute for Lasers, Plasma and Radiation Physics Magurele Romania,
Show AbstractMost of the studies investigating the effects of radiation on the structure and properties of various materials have been performed on single crystals or sintered pellets having large grain sizes. However, more and more devices are incorporating transparent and conductive amorphous oxides. Recent investigations of the effects of radiation on such films found differences with respect to the results obtained for polycrystalline films. The absence of long distance order allows for short diffusion distances of the created defects before encountering sites in the network that act as sinks. Therefore, the optical and electrical properties are not strongly affected by exposure to radiation. To investigate in detail the radiation effects we used the Pulsed Laser Deposition (PLD) technique, which is very suitable to grow nanocrystalline or amorphous thin films of almost any materials starting from inexpensive targets. By changing the deposition parameters, films possessing different chemical compositions and/or structures could be easily obtained. The surface morphology of the deposited films is very smooth, allowing for the use of characterization techniques such as X-ray reflectivity, grazing incidence X-ray diffraction, X-ray photoelectron spectroscopy, or nanoindentation that all possess depth resolutions of the order of few nm. The effect of ions, gamma and X-ray irradiation on the structure, chemical composition, mechanical, optical and electrical properties of the typical TCO films, IZO and IGZO with various In, Zn and Ga compositions were investigated and compared with those obtained on polycrystalline films.
9:30 AM - NT2.12.02
Tunable Nanoporous Superhydrophobic Coatings to Promote Dropwise Condensation and Enhance Heat Transfer
Lance Brockway 2,Hayden Taylor 2
1 University of California Berkeley Berkeley United States,2 BEARS Singapore Singapore,
Show AbstractDropwise condensation has been predicted and shown to enhance heat-transfer in air-side heat exchangers and can be achieved with superhydrophobic coatings. Typical coatings consist of spray-on polymers or photolithographically templated surfaces. Polymer coatings are thermally resistive and prone to abrasive and thermal degradation, while template-based methods are costly and time-consuming.
Here we present a technique for the bottom-up synthesis of tunable nanoporous coatings of various transition-metal oxides on aluminum substrates from equimolar solutions of metal nitrates and hexamine. The coating thickness and pore size can be tuned to give optimal water-shedding properties while maintaining the high thermal conductivity required for heat-transfer surfaces. Using porous ZnO coatings, sessile drop contact angles greater than 177° with a hysteresis less than 3° have been observed.
The heat-transfer performance of these coatings has been characterized in a prototype finned-tube heat exchanger. By shedding sub-200 µm-diameter droplets from the fin surfaces under the shear stresses of flowing air, the thermal conductance of the fins can be enhanced relative to hydrophilic surfaces, and the air can thus be cooled more rapidly. This increase in conductance potentially allows the cooling water temperature to be raised, increasing system efficiency. These new coatings may also limit inter-fin water bridging, enabling reduced fin spacing and hence more rapid air cooling.
9:45 AM - *NT2.12.03
Growth and Characterization of Aligned Hexagonal ZnO Nanostructures on Cubic MgO (001) Substrates by Pulsed Laser Deposition without Any Catalyst
Ramanjaneyulu Mannam 1,Nandita DasGupta 2,Ramachandra Rao 1
1 Department of Physics and Nano Functional Materials Technology Centre Indian Institute of Technology Madras Chennai India,2 Department of Electrical Engineering, Microelectronics and MEMS Laboratory Indian Institute of Technology Madras Chennai India
Show AbstractZnO is a wide band gap (~ 3.37 eV at RT) semiconducting material, with its piezo-electric, optical, electrical and tunable bandgap, ability to grow in different directions, makes it suitable for many applications such as piezoelectric nano-energy generators, opto-electronic devices, sensors, and transparent conducting electrodes. Growth of ZnO hexagonal nanostructures on non-polar cubic MgO (001) substrates is very interesting because of its different crystal structures as ZnO has hexagonal wurtzite structure and MgO crystallizes in cubic structure. In addition to this, the polarization of ZnO is parallel to c-axis of ZnO and so the growth of a polar face on a neutral plane (MgO (100)) will result in different properties compared to the ZnO.
In the present work, undoped ZnO (UZ) and 1 mol % of P doped ZnO (P: ZnO) nanostructured thin films were grown on a non-polar MgO (001) substrates using pulsed laser deposition (PLD). Structural properties of ZnO nanostructures are studied using X-ray diffraction and Raman spectroscopy. X-ray diffraction patterns reveal that both UZ and P: ZnO nanostructures are highly textured along the c-axis of the hexagonal wurtzite structure. Raman spectra of both UZ and P: ZnO nanostructures exhibited intense modes corresponding to E2(high) and E2(low) of hexagonal wurtzite structure of ZnO confirming the wurtzite structure of the nanostructures. Morphology of the nanostructures is analyzed using field emission scanning electron microscopy (FESEM). FESEM micrographs which revealed nanorod formation in the case of UZ and nanocone formation in the case of P doped ZnO. Energy dispersive X-ray spectroscopy (EDS) shows the presence of P in the ZnO thin films. Cathodoluminescence spectroscopy studies showed a high intense UV emission in both UZ and P: ZnO nanostructures films. All the results will be presented and discussed in detail.
Key words: ZnO nano rods, doping, XRD, CL, SEM.
References:
[1]. Ü. Özgür et.al., J. Appl. Phys. 98, 041301 (2005)
[2]. J.G Lu et.al., Appl. Phys. A 88 222114 (2009)
10:15 AM - NT2.12.04
Atomic Layer Deposited Thermoelectric Nanocomposite
Mikko Ruoho 1,Taneli Juntunen 1,Ilkka Tittonen 1
1 Department of Micro and Nanosciences Aalto University Espoo Finland,
Show AbstractThermoelectric materials can convert heat into electricity by the Seebeck effect in a solid state. For this reason, thermoelectric materials are expected to play a role in waste heat energy harvesting to reduce carbon emission with more efficient use of energy. The performance of thermoelectric material is quantified by a dimensionless figure of merit ZT and depends on thermal conductivity, electrical conductivity, and thermopower. High thermopower and electrical conductivity are needed, whereas thermal conductivity should be minimized. Nanostructuring of materials has proven to be effective means to lower the thermal conductivity of a material from its bulk value. Atomic layer deposition method allows large scale fabrication of complex geometries, making it of particular interest for the production of thermoelectric nanostructures.
We have fabricated high aspect ratio nanostructures and studied their thermoelectric and structural properties. We also discuss briefly how these conformally deposited of the film of a porous substrates can be used to determine the thermal conductivity of the deposited film [1].
The nanocomposites are realized by conformal deposition of ZnO films of different thicknesses onto a 20 µm thick ion track etched polycarbonate membrane. In this manner we may control very precisely the combination of the electrical and thermal properties of polycarbonate and ZnO. As a result, the nanocomposites exhibit very low thermal conductivity while being thermoelectric character and electrical conductivity. Accordingly, the thermal properties of the nanocomposites resemble that of the polycarbonate substrate whereas the electrical those of the deposited ZnO thin film.
The thermal conductivity of ZnO thin films was analyzed by the laser flash technique. By using this procedure we could determine the thermal conductivity of the deposited thin film from the total thermal diffusivity of the nanocomposite structures. The method has been used to obtain the in-plane thermal conductivity of the deposited ZnO layers within the thickness range of less than 100 nm.
[1] M. Ruoho, K.Valset, T. Finstad and Ilkka Tittonen. Measurement of thin film thermal conductivity using the laser flash method. Nanotechnology 26 195706 (2015) DOI: 10.1088/0957-4484/26/19/195706
10:30 AM - NT2.12.05
Flexible, Transparent Nanogenerators with Exceptionally High Power Output Based on Ultrathin ZnO Nanoflakes
Dae Joon Kang 1,Ngoc Huynh Van 1
1 Sungkyunkwan Univ Suwon Korea (the Republic of),
Show AbstractRecent years has witnessed a growing Interest in nanogenerators (NGs) that harvest mechanical energy through piezoelectric or triboelectric processes. We herein, report a novel and simple synthetic route to obtain ultrathin ZnO nanoflakes (NFs) that can be grown on arbitrary substrates using an aqueous precipitation method at room temperature. Microscopic study revealed that the NFs had plate-like morphologies with thickness below 10 nm, which is expected to play a vital role in power generation and overall device performance enhancement. This novel structure was exploited to achieve high-power-output NGs. The as-fabricated NGs, exhibited a maximum output peak open-circuit voltage density of 110 V/cm2, which is 2.9 times higher than the past record of 37 V and a maximum output peak short-circuit current density of 57 μA/cm2, which is 4.7 times of the recent highest value of 12 μA/cm2 of ZnO nanowire-based NGs (NWNGs).1 We believe that, the high surface area and especially the strong domination of surface piezoelectricity originated from unique morphology of ultrathin ZnO nanoflakes, contributed to their exceptional performance.2 The as-fabricated NGs were further investigated with different vibration sources such as periodic pounding of a heavy object and a speaker sound. Different interface mechanism to minimize the screening effects such as thin insulating layers, p-n junction and hybrization of two piezoelectric materials were reviewed and verified. The best results were obtained when using the hybrization of two piezoelectric materials of organic piezoelectric poly(vinylidene fluoride-trifluoroethylene)-P(VDF-TrFE) coated ZnO NFs. The maximum peak open-circuit voltage of 127 V and short-circuit current density of 80 μA/cm2, 3.4 times the output voltage and 6.6 times the output current density, much higher than those of NWNGs and 3.4 times output current density higher than that of ultralong PZT NWs array based NGs were demonstrated.3 With the devices made up of 1 x 1 cm2 area NFs nanogenerators, we showed that the output electricity can power up to 20 commercial green light-emitting diodes (LED), while the device using 4 x 4 cm2 device area NFs, can power up to 60 commercial LED, without the energy storage process. Both the facile synthetic route for ZnO nanoflakes and the straightforward device fabrication process present great scaling potential suitable to power mobile and personal electronics used in smart wearable systems, transparent and flexible devices, implantable telemetric energy receivers, electronic emergency equipment, and other self-powered nano/micro devices.
References
Zhu et al. Nano Lett. 12, 3086-3090, 2012.
Xu et al. Smart Mater. Struct. 23, 035020, 2014.
Gu et al. Nano Lett. 13, 91-94, 2013.
10:45 AM - NT2.12.06
Performance Boosting of Flexible ZnO UV Sensors with Rational Designed Absorbing Antireflection Layer and Humectant Encapsulation
Heng Zhang 1,Youfan Hu 1,Zongpeng Wang 1,Zheyu Fang 1,Lian-Mao Peng 1
1 Peking Univ Beijing China,
Show AbstractFlexible ZnO thin film UV sensors with three orders of magnitude improvement in sensitivity and two orders of magnitude acceleration in speed are realized via light absorption efficiency enhancement and surface encapsulation. Devices are constructed on polyethylene substrate incorporating morphology controlled ZnO nanorod arrays (NRAs) as absorbing antireflection layers. By adjusting the morphology of ZnO NRAs, the light absorptance exceeds 99% through effectively trapping incident photons. As a result, the sensitivity of the UV sensor reaches 109000. Moreover, a mechanism of competitive chemisorption between O2 and H2O at oxygen vacancy sites is proposed to explain the phenomenon of the speed acceleration in moist environment. A new approach of humectant encapsulation is used to make H2O participant rapid processes dominant for speed acceleration. Two orders of magnitude speed enhancement in reset time is achieved by polyethylene glycol encapsulation. After a total 3000 cycles bending test, the decay in the responsivity of UV sensor is within 20%, indicating good mechanical stability. All these results not only demonstrate a simple, effective and scalable approach to fabricate high sensitive and fast response flexible ZnO UV sensors, but also provide meaningful references for performance boosting of photoelectronic devices based on other oxide semiconductors.
11:00 AM - NT2.12.07
Recognition of Acceptors and Compensated Intrinsic Donors in Nitrogen Doped ZnO
Shulin Gu 1
1 Nanjing Univ Nanjing China,
Show AbstractAs a wide-bandgap semiconductor with a large exciton binding energy of 60meV, ZnO is considered to be a potential candidate for low-cost, high efficiency light-emitting diodes and lasers with near-UV emission. However, such device applications has encountered great difficulties when stable and controllable p-type doping is hard to achieve in ZnO. Almost all possible candidate dopants have been calculated and tried to realize p-type doping in ZnO. Among them, nitrogen substitution on the O site (NO) is considered as the promising dopant to obtain p-type ZnO, even though the deep acceptor fact of NO in ZnO have been widely revealed and accepted by the more accurate calculation based on HSE recently. More recently, theoretical calculation shows that almost all N-related shallow acceptors as assigned previously are actually deep acceptors, but NH4 molecule occupied on Zn site is suggested to be a possible shallow acceptor candidate due to its low formation energy and high stability in ZnO. However, no direct experiment evidence has been provided on the existence of NH4 on Zn site up to now. In order to elucidate the veiled facts about the acceptors and donors in N-doped ZnO, high quality nitrogen-doped ZnO (ZnO:N) films were grown on the ZnO template substrate by metal-organic chemical vapor deposition (MOCVD) with NH3 used as N doping source. Rapid thermal annealing at different temperature was employed to activate the possible acceptors. Optical and electronic properties have been measured to get the knowledge about the evolution of the intrinsic interstitials, vacancies and related complexes during the annealing process. The behavior of intrinsic zinc defect such as Zni and VZn during the post-growth annealing has been carefully presented and the possible evidences of existence of NH4 on Zn site in N-doped ZnO has been provided. This study indicates that NH4 on Zn site could be a shallow acceptor, which is responsible for the observed p-type characteristics and acceptor bound excitons observed in N-doped p-type ZnO.
NT2.13: Titanium and Other Oxides
Session Chairs
Friday PM, April 01, 2016
PCC West, 100 Level, Room 106 A
11:30 AM - *NT2.13.01
Synthesis and Hierarchical Assembly of Ultrasmall Nanoparticle “Building Blocks” for Photoresponsive Oxide and Metal Chalcogenide Nanostructures
David Geohegan 1,Masoud Mahjouri-Samani 1,Kai Wang 1,Mengkun Tian 2,Gerd Duscher 2,Gyula Eres 3,Alexander Puretzky 1,Chris Rouleau 1,Kai Xiao 1,Xufan Li 1,Mina Yoon 1,Raymond Unocic 1,Miaofang Chi 1
1 Center for Nanophase Materials Sciences Oak Ridge National Laboratory Oak Ridge United States,2 Department of Materials Science and Engineering University of Tennessee Knoxville United States3 Materials Science and Technology Division Oak Ridge National Laboratory Oak Ridge United States
Show AbstractThe nonequilibrium synthesis and evolution of ultrasmall nanoparticle (UNP, ~ 3 nm) “building blocks” into photoresponsive nanostructures and hierarchical architectures is described. Through the use of time-resolved, in situ diagnostics we describe the general synthesis of stoichiometric ‘amorphous’ UNPs by pulsed laser ablation of solid targets into low-pressure background gases, and characterize their propagation[1] to understand the formation of pure nanoparticle architectures by pulsed laser deposition (PLD). The UNPs collected at room temperature are characterized by nano-beam electron diffraction, EELS, and atomic-resolution electron microscopy. The amorphous UNP assemblies crystallize into hierarchical mesoporous architectures at elevated temperatures and evolve into functional phases. We describe the formation of a new form of “black TiO2” by the synthesis of amorphous TiO2 UNPs and annealing in Ar atmospheres. Understanding the structure of black TiO2 is the key to developing robust high efficiency photocatalysts for solar hydrogen production. We reveal that the black TiO2 NPs are core-shell structures consisting of a 1-2 nm thick defective layer of Ti2O3 in a strict crystallographic relationship with a perfectly crystalline rutile core.[2] When amorphous TiO2 UNPs are PLD-deposited at high substrate temperatures, we show that they serve as tunable building blocks for the catalyst-free formation of crystalline nanowires, nanosheets, or vertically-oriented crystalline nanorods with metastable phases, such as TiO2(B), which are useful for multiple energy-related applications. Similarly, when amorphous metal chalcogenide (GaSe, MoSe2) UNPs are deposited at higher temperatures, we show that they directly transform into highly photoresponsive networks of 2D nanosheets.[3] Alternatively, we show that the UNPs can serve as stoichiometric precursors for the digital transfer growth of large 2D crystals in prepatterned locations.[4] Combining the PLD of oxide and metal chalcogenide UNPs as “building blocks” will be discussed as a promising and versatile method to integrate both materials for unique functional nanostructured architectures.
Research sponsored by the U.S. Dept. of Energy, Office of Science, Basic Energy Sciences, Materials Science and Engineering Div. (synthesis science) and Scientific User Facilities Div. (characterization science).
References
1. C. M. Rouleau, et al., “Slowing of femtosecond laser-generated nanoparticles in a background gas”, Appl. Phys. Lett. 105(21), 213108 (2014).
2. M. Tian et al., “Structure and Formation Mechanism of Black TiO2 Nanoparticles”, ACS Nano ASAP DOI: 10.1021/acsnano.5b04712 (2015).
3. M. Mahjouri-Samani et al., "Pulsed Laser Deposition of Photoresponsive Two-Dimensional GaSe Nanosheet Networks" Adv. Funct. Mat. 24, 6365 (2014).
4. M. Mahjouri-Samani et al.,“Digital Transfer Growth of Patterned 2D Metal Chalcogenides by Confined Nanoparticle Evaporation” ACS Nano 8(11), 11567 (2014).
12:00 PM - NT2.13.02
High Electron Mobility in Epitaxial SnO2-x in Semiconducting Regime
Hyosik Mun 1,Hyeonseok Yang 1,Jisung Park 1,Chanjong Ju 1,Kookrin Char 1
1 Seoul National Univ Seoul Korea (the Republic of),
Show AbstractWe investigated the electronic transport properties of epitaxial SnO2-x thin films on r-plane sapphire substrates. The films were grown by pulsed laser deposition technique and its epitaxial growth direction was [101] and the in-plane alignment was of SnO2-x [010] // Al2O3 . When the SnO2-x films were grown in the oxygen pressure of 30 mTorr, we have found the electron mobility of the 30 nm thick SnO2-x thin films strongly dependent on the thicknesses of the fully oxidized insulating SnO2 buffer layer. When the buffer layer thickness increased from 100 nm to 700 nm, the electron mobility values increased from 23 cm2 V-1 s-1 to 106 cm2 V-1 s-1 and the carrier density increased from 9×1017 cm-3 to 3×1018 cm-3, which we attribute to reduction of large density of dislocations as the buffer layer thickness increases. In addition, we studied the doping dependence of the electron mobility of SnO2-x thin films grown on top of 500 nm thick insulating SnO2 buffer layers. The oxygen vacancy doping level was controlled by the oxygen pressure during deposition. As the oxygen pressure increased to 47.5 mTorr, the carrier density were found to decrease to 9.1×1016 cm-3 and the electron mobility values to 13 cm2 V-1 s-1, which is consistent with the dislocation limited transport properties. We also checked the conductance change of the SnO2-x during thermal annealing cycles, demonstrating unusual stability of its oxygen. The correlation between the electronic transport properties and microstructural defects investigated by the transmission electron microscopy were drawn. The excellent oxygen stability and high electron mobility of low carrier density SnO2-x films demonstrates its potential as a transparent oxide semiconductor.
12:15 PM - NT2.13.03
Exploring New Pathways for the Synthesis of Nanostructured Suboxide Magnéli Phases of Titanium with Advanced Functionality
Elham Baktash 1,Clement Sanchez 1,Sophie Carenco 1,David Portehault 1
1 Sorbonne Universités, UPMC Univ Paris 06, CNRS, Collège de France, Laboratoire de Chimie de la Matière Condensée de Paris Paris France,
Show AbstractMagnéli phases of titanium have been known since 1956.[1] These oxides, comprise a series of distinct compounds having the generic formula TinO2n-1, where n is an integer between 3 and 10. Numerous applications have been considered for these materials that are robust in aggressive media and exhibit high conductivity. Despite the interesting properties of these oxides, advances in their applications for energy conversion, data storage or catalysis were hindered by synthetic and processing difficulties. First, the high temperatures required for manufacturing Magnéli phase materials yield mixtures of stoichiometries. Second, harsh synthesis conditions yield large grain growth, so that nanostructured Magnéli phases are elusive. Consequently, Magnéli phases have never been explored in heterogeneous catalysis, which relies on a high surface-to-volume ratio typical of nanostructures. In addition, nanostructuration can strongly impact transport properties, especially by reducing the thermal conductivity via phonon scattering at nanoscale interfaces.[2,3] These rich scientific avenues have been mostly left aside, as nanostructured Magnéli phases were unattainable. Therefore we at the Laboratory of Chimie de la Matière Condensée de Paris are aiming at exploring new pathways for the synthesis of nanostructured suboxide Magnéli phases. The sol-gel process was combined with spark plasma sintering (SPS) to yield percolated nanocomposites based on metal oxide nanoparticles embedded in a carbon matrix with low electrical resistivity and reduced thermal conductivity with respect to bulk phases.[4] Moreover the combination of sol-gel chemistry and the electrospinning process have been tried and appeared as a highly relevant approach to shape nanostructured Magnéli phase materials with efficient control over composition, texture and nanostructure.[5] Templating approach has been employed to prepare silica coated nanoparticles with Magnéli phases. Via this protocol severe sintering of the nanoparticles at elevated temperature was avoided. Controlled reduction of silica coated titanium nanoparticles resulted in separate Magnéli phases. We believe that the design of corresponding nanostructures could therefore lead to important changes or enhancement of existing properties, emergence of new behaviours and novel processing possibilities.
[1] S. Andersson, B. Collen, U. Kuylenstierna, A. Magnéli, Acta Chem. Scand. 1957, 11, 1641.
[2] K. C. See, J. P. Feser, C. E. Chen, A. Majumdar, J. J. Urban, R. A. Segalman, Nano Lett. 2010, 10, 4664.
[3] S. Harada, K. Tanaka, H. Inui, J. Appl. Phys. 2010, 108, 083703.
[4] D. Portehault, V. Maneeratana, C. Candolfi, N. Oeschler, I. Veremchuk, Y. Grin, C. Sanchez, M. Antonietti, ACS Nano 2011, 5, 9052.
[5] V. Maneeratana, D. Portehault, J. Chaste, D. Mailly, M. Antonietti, C. Sanchez, Adv. Mater. 2014, 26, 2654.
12:30 PM - NT2.13.04
Epitaxial Integration of TiO2 with Si(100) through a Novel Approach of Oxidation of TiN/Si(100) Epitaxial Heterostructure
Adele Moatti 1,Reza Bayati 2,Jagdish Narayan 1,Srinivasa Rao 1
1 Materials Engineering North Carolina State University Raleigh United States,2 Intel Corporation Hillsboro United States
Show AbstractIn this study, we provide a novel approach to the epitaxial integration of TiO2 with Si(100). Epitaxial TiO2 thin films were grown on a TiN/Si(100) epitaxial heterostructure through oxidation of TiN where a single crystalline rutile-TiO2 (r-TiO2) with a [110] out-of-plane orientation was obtained1. The epitaxial relationship is determined to be TiO2(1-10)||TiN(100) and TiO2(110)||TiN(110). We rationalized this epitaxy using the domain matching epitaxy paradigm. First TiN is grown epitaxially on Si(100), where a cube-on-cube epitaxy is achieved despite over 22% lattice misfit. In the TiN/Si(100) epitaxy 4/3 and 5/4 domains alternate by matching integral multiple of lattice planes across the TiN/Si interface. Subsequently, TiN/Si(100) samples are oxidized to create r-TiO2/TiN/Si(100) epitaxial heterostructures. Considering the epitaxial alignment at the TiO2/TiN interface, (110) planes of TiO2 match with (110) planes of TiN. The inter-planar spacing of TiO2(110) and TiN(110) equal to 3.25 and 2.99 Å, respectively. This results in a 8.69% compressive misfit strain in TiO2 lattice which relaxes through 11/12 and 12/13 alteration domains with a frequency factor of 0.5, based on the domain matching epitaxy paradigm. Along the perpendicular direction in the film plane, TiO2(001) planes match with TiN(-110) planes. This results in a 1.34% tensile misfit strain in the titania lattice. These strains are extremely small and remain unrelaxed in the system. The details of the mechanism behind the oxidation of single crystalline TiN to TiO2 was investigated using atomic scale high resolution electron microscopy techniques. Below the r-TiO2 epitaxial layer, we observed cuboids, which are mostly voids. In our model, Ti out diffusion during oxidation leads to collapse of the nitrogen octahedron. This collapse makes neighboring Ti bonds weaker, promoting these Ti atoms to diffuse out next. As a result, the number of Ti vacancies increases, forming the cuboids with their length and width lies onto low energy surfaces of TiN and TiO2. Cuboids filled with atomic nitrogen are formed, which then form N2 gas. The N2 pressure in these cuboids was estimated to be as high as 359 MPa, assuming all N2 is retained in the cuboids. This pressure can exceed the fracture stress of TiO2 and leads to rupture of thin TiO2 surface, which has been observed under certain conditions. The driving force for N2 migration to the surface is aided by the high-pressure generated by N2. The nitrogen out diffusion will reduce the fracture stress needed for rupture. We present structure property correlations and its impact on the next generation solid state devices.
1: A. Moatti, et al., Epitaxial growth of rutile TiO2 thin films by oxidation of TiN/Si{100} heterostructure, Acta Materialia (2015), http://dx.doi.org/10.1016/j.actamat.2015.10.022
12:45 PM - NT2.13.05
Nanowire WOx Sensors from Oxidation of WC Thin Films
Jun Jiang 1,Xiaofei Guan 1,Masaru Tsuchiya 2,Shriram Ramanathan 1
1 Harvard University Cambridge United States,2 SiEnergy Systems LLC Cambridge United States
Show AbstractTungsten oxides are of great interest for understanding fundamental properties of transition metal oxides in low symmetry lattices and applications in sensors and chromic devices. In this presentation, tungsten oxide nanostructures synthesis from controlled oxidation of WC thin films will be presented. Both W18O49 nanowires and WO3 nanoparticles were synthesized by tailoring the annealing temperature and oxygen partial pressure. The growth kinetics of these nanowires reminiscent of classical whisker growth will be discussed in depth. In addition, the WOx nanowires have excellent gas sensing properties with zero noble metal loading and we will present these results in the context of low power sensors for hydrogen and natural gas detection.
NT2.14: Dielectrics/Capacitors/Memristors
Session Chairs
Friday PM, April 01, 2016
PCC West, 100 Level, Room 106 A
2:30 PM - NT2.14.01
Thermal Transport of Tantalum Oxide Films for Memristors
Thomas Beechem 1,Colin Landon 3,Rudeger Wilke 1,Michael Brumbach 1,Geoff Brennecka 2,Mia Blea-Kirby 1,Jon Ihlefeld 1,Matthew Marinella 1
1 Sandia National Labs Albuquerque United States,3 Hillsboro United States2 Colorado School of Mines Golden United States
Show AbstractDepleting the oxygen content in transition metal oxides like HfO2, TiO2, and Ta2O5 leads to charged states that lower electrical resistance. The mobility of these charged states in response to an electrical field and the concomitant heating creates nonvolatile changes of resistivity that are the basis of memristive technology. While it is accepted that the thermal environment during switching contributes to the concentration and distribution of oxygen—and hence device functionality—prediction of the temperature field remains elusive owing to incomplete understanding of the basic thermal properties of these oxides.
In response, the thermal conductivity of tantalum oxide films synthesized with varying oxygen content is examined. Homogeneous films are fabricated to span the stoichiometry most pertinent for the memristive switching process as shown with electrical resistivity and X-ray photoelectron spectroscopy (XPS) measurements. Employing time domain thermoreflectance (TDTR), the thermal conductivity is found to have a lower bound of ∼ 0.7 W/mK for large oxygen content that grows to ∼ 5 W/mK as the concentration of oxygen decreases. Similarly, the thermal boundary conductance (TBC) between TaOx and platinum is found to increase by >5X with decreasing oxygen. Variations in thermal conductivity are well described by a two-part model where an electrical contribution is quantified via the Wiedemann-Franz relation and the vibrational contribution by the minimum thermal conductivity limit for amorphous solids. Likewise, variations in TBC are attributed to the increasing amount of charge available to efficiently cross the interface. Taken together, this framework—extendable to other memristive material systems—highlights that the dominant thermal carrier in TaOx switches from vibrational in origin to electronic. Such switching can be controlled either by specification of the oxygen content during deposition, or dynamically by field-induced charge state migration thus hinting at the possibility of a material class possessing tunable thermal conductivity.
Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.
2:45 PM - *NT2.14.02
Multifunctional Epitaxial Oxide Heterostructures on Semiconductors
Srinivasa Rao Singamaneni 1,John Prater 1,Jagdish Narayan 1
1 North Carolina State Univ Raleigh United States,
Show AbstractMultifunctional oxide heterostructures exhibit a wide range of functional properties, including colossal magneto-resistance, magnetocaloric effects, and multiferroic behavior, and some interesting physical phenomena including spin, charge, and orbital ordering. However, putting this functionality to work remains a challenge. To date, most of the previous works reported in the literature have dealt with heterostructures deposited on closely lattice matched (using lattice matching epitaxy-LME) insulating substrates such as DyScO3 (DSO), NdGaO3 (NGO), MgO, SrTiO3 (STO) and molecular beam epitaxy (MBE)-grown STO buffered Si(100). However, the strain (1-2%) in the resulting heterostructures grown by LME is not completely relaxed and the layers can contain detrimental defects such as threading dislocations in the active region which degrade the physical properties and adversely affect device performance. Most of the above substrates are incompatible with existing CMOS-based technology, where Si (100) substrates dominate. This presentation discusses the major advances in the integration of multifunctional oxide materials onto ubiquitous silicon semiconductor platform reported1-6in the recent past using a novel thin film growth approach, called ‘domain matching epitaxy’(DME), which minimizes the strain and nucleation of unwanted defects in large lattice mismatch systems. This approach allows integration of multifunctional materials on a silicon chip, enabling sensing, manipulation and rapid response functions to be combined for next generation ‘smart’ devices. In general, pulsed laser deposition (PLD) has been used to grow these materials epitaxially on silicon substrates, but other techniques can be used to extend the concepts developed here. The DME paradigm has been used across the large misfit scale (7-25%), a chief advantage of DME. Of particular interest, this presentation focuses on the resulting properties of several important thin film heterostructures including two-phase multiferroics such as BiFeO3(BFO)/La0.7Sr0.3MnO3 (LSMO), BaTiO3(BTO)/LSMO, and heterostructures of two-ferromagnetic oxides such as LSMO/SrRuO3(SRO). These significant materials advancements may herald a flurry of exciting new advances in CMOS-compatible multifunctional devices.
3:15 PM - NT2.14.03
Two-Port Tunable Interdigital Capacitors Fabricated on Low-Loss Ba0.29Sr0.71TiO3
Cedric Meyers 1,Christopher Freeze 2,Susanne Stemmer 2,Robert York 1
1 ECE Univ of California-Santa Barbara Santa Barbara United States,2 Materials Univ of California - Santa Barbara Santa Barbara United States
Show AbstractTwo-port, tunable interdigital capacitors (IDC) were fabricated on perovskite oxide thin films. The devices utilize electric-field tunable BaxSr(1-x)TiO3 (BST) thin films grown by hybrid molecular beam epitaxy (MBE) on LaAlO3 (LAO) substrates. In this growth technique, barium and strontium were evaporated from solid-source effusion cells while titanium was introduced as a metal-organic vapor source, titanium tetraiospropoxide (TTIP), which also supplied the necessary oxygen. This hybrid approach allowed for a broad temperature growth window for a high degree of stoichiometric control. For this work, 273 nm of 29% Ba composition BST was used to ensure sufficiently large tuning range while allowing for high quality factor. The BST was compressively strained on the LAO substrate to ensure that no cracking occurred in the BST thin film. A high-quality electrode interface was achieved by in-situ annealing in oxygen followed immediately by hot-sputtering epitaxial platinum electrodes. The devices were passivated with reactively sputtered SiO2 and metallized with 1µm of gold to lower the series resistance. The devices were measured from 100 MHz to 40 GHz and the results fitted to a frequency-dependent RLC equivalent circuit model. The electric-field-dependent series capacitance was fitted to closed-form equations described elsewhere and demonstrated an exceptionally close fit when an extra non-tunable fringing capacitance term was included in the equation. The IDCs demonstrate high quality factors (200) in the S- and L-bands combined with 2:1 tunability and a self-resonant frequency of 50 GHz. The quality factor was suppressed at frequencies above 5 GHz by the finite self-resonant frequency of the devices. The resultant roll-off in quality factor occurs when the IDC’s parasitic inductance begins to dominate the reactance of the device. The IDCs were also evaluated using a commutation quality factor, defined as the difference-squared in reactance across the range of tuning voltages divided by the product of the resistance across the same range. The devices demonstrated a commutation quality factor averaging 6,000 across the L band which surpasses any existing results in this band. Future work will strive to increase the self-resonant frequency of the IDCs and increase the Q-factor over the entire band in an effort to increase the CQF to higher values and to lower the DC leakage current to enable higher power handling and larger tuning fields.
3:30 PM - NT2.14.04
Rapid Formation of Very Low-Temperature Processed High-Performance Solution-Based Metal-Oxide Dielectrics via Novel Synthesis of Aluminum Nanocluster Precursors
Jeong-Wan Jo 1,Kyungtae Kim 1,Jingu Kang 1,SungMin Kwon 1,Myung-Gil Kim 1,Sung Kyu Park 1
1 Chung-Ang Univ Seoul Korea (the Republic of),
Show AbstractSolution-based metal oxide alumina gate dielectrics have been actively studied to realize next generation flexible and low cost printed electronics. They have been used for high-performance electronic devices such as thin-film transistors (TFTs) and other optoelectronic devices, and have exhibited impressive performance comparable to those of vacuum deposited devices. Despite these several advantages of the solution-processed of AlOx gate dielectric, a high process temperature and repeated coating process for favorable characteristics as gate dielectric hinder the wide applications of the materials in soft and printed electronics. So far, most of solution-processed AlOx are based on aluminum acetate, nitrate, and chloride precursors (conventional sol-gel precursors), which generally require relatively high annealing temperature to achieve decent performance of gate dielectric layer because of their relatively complicated reaction steps. Metal oxide films made by aforementioned conventional sol-gel precursor system typically suffer from non-uniform property, substantial organic residues, and non-stoichiometry, which are possibly due to burnout of the organic residues and insufficient activation energy of the systems.
In this work, we investigate alternative aluminum nanocluster precursors which are more suitable for low-temperature photochemical reaction rather than conventional high-temperature thermal annealing process by adjusting the structure of general AlOx sol-gel precursors. Using the aluminum nanocluster precursors, we have achieved high-performance and quite uniform AlOx gate dielectrics via a single step coating with very low process temperature (~60°C), having an areal capacitance of ~89 nF/cm2 and a gate leakage current density of