9:00 PM - EP4.11.02
Probing the Surface Passivation and Selectivity of Contacts in Solar Cells
Mathieu Boccard 1,Zachary Holman 1
1 Arizona State University Tempe United States,
Show AbstractWe present here a methodology to identify whether a candidate material intended for use as a contact in a silicon solar cell is selective towards holes or electrons. This is done by building two test devices and comparing the measured external open-circuit voltage (e.g. with a Suns-Voc setup) to the measured “implied” open-circuit voltage (e.g. with a Sinton lifetime tester or from calibrated PL), which reflects the quasi-Fermi-level splitting in the bulk of the absorber. For each device, one of the contacts is the material to test and the other contact is a nominally perfect hole contact or electron contact. (Perfect means that it provides excellent passivation and that the potential at the contact corresponds to the quasi-Fermi-level position of the corresponding carriers in the bulk). Though applicable to any technology, we use crystalline silicon wafer as the absorber here. Intrinsic/doped amorphous silicon (a-Si:H) layer stacks capped with indium tin oxide are used as nominally perfect contacts since they provide excellent passivation and selectivity: when both contacts are a-Si:H, we measure iVoc > 730 mV for 180-µm-thick wafers and Voc > iVoc -10 mV). As the material to test is expected to be limiting, the iVoc indicates the passivation capability of this new materials whereas the Voc/iVoc ratios indicate its carrier selectivity: A good hole selective contact candidate will have Voc = iVoc when using an intrinsic/n-type a-Si:H stack on the other side of the wafer, but Voc close to 0 V when using an intrinsic/p-type a-Si:H.
When testing evaporated MoO3 with this methodology, we measured reasonable passivation (iVoc = 650 mV for both test structures, i.e. both when combined with intrinsic/p-type and intrinsic/n-type a-Si:H) and strong hole-selectivity (Voc/iVoc > 0.9 when using an n-type a-Si:H contact on the other side, but Voc/iVoc
9:00 PM - EP4.11.03
High-Mobility Hydrogenated Indium Oxide without Introducing Water during Sputtering
Mathieu Boccard 1,Alec Jackson 1,Michael Bernstein 1,Zachary Holman 1
1 ASU Tempe United States,
Show AbstractHydrogenated indium oxide (IO:H) is a very attractive transparent conductive oxide material since it can achieve mobility values above 100 cm2/Vs. With a carrier concentration in the 1020 cm-3 range, this makes very transparent and conductive electrodes for silicon heterojunction solar cells. These layers are typically fabricated by sputtering at room temperature from an indium oxide target in an atmosphere of argon, oxygen and water vapor, and subsequently annealing at ~200 °C to induce solid-phase crystallization. The water partial pressure during sputtering was shown by Koida et al. to be crucial to obtain high mobility-films, and should be comprised between 5.10-7 mbar and 1.10-5 mbar based on his conditions. Such tiny partial pressure requires a very small flow which is delicate to control, especially for water vapor which tends to be more delicate to regulate than most other gases.
We investigate two approaches to fabricate high-mobility IO:H circumventing the introduction of water vapor: The first one relies on water vapor from ambient air, the second one uses hydrogen instead of water. A sputtering tool equipped with a residual gas analyzer allows us to monitor the partial pressure of H2, O2 and H2O in the system, and to link the gas composition to the properties of the deposited films. When not introducing intentionally any source of hydrogen, we varied the pumping time after opening the chamber before starting the deposition to have different base pressures (1. 10-7 mbar to 3. 10-7 mbar), which are mostly composed of residual water. An optimum base pressure around 3. 10-6 mbar was found to yield highest post-deposition-annealed mobility values, the corresponding H2O partial pressure will be investigated in more details, as well as the possibility to replace the time-consuming opening of the system with introduction of small amount of ambient air; this alternative approach is particularly relevant for systems equipped with a load-lock. High mobility films could also be obtained after several hours of pumping by introducing a small flow of hydrogen (partial pressure of 2 .10-5 mbar) together with argon and oxygen, with a mobility as high as 100 cm2/Vs for a carrier density of 2.3 .1020 cm-3 after annealing.
9:00 PM - EP4.11.04
Nature of Amorphous Silicon Carbide / Crystalline Silicon Interface Recombination
Mathieu Boccard 1,Alec Jackson 1
1 Arizona State University Tempe United States,
Show AbstractAmorphous silicon films enable the fabrication of high-efficiency crystalline-silicon-based heterojunction solar cells due to the excellent surface passivation of the crystalline silicon surface and the films’ permeability to electrical charges. However, one of the limitations of amorphous silicon is that the passivation it provides degrades upon high-temperature processes, limiting post-deposition fabrication possibilities. Amorphous silicon carbide is an excellent candidate to improve the temperature stability of the passivating layers, as carbon integration into an amorphous silicon film increases hydrogen incorporation, decreases the diffusion coefficient of hydrogen and increases the temperature of both hydrogen effusion peaks. While the properties of bulk amorphous silicon carbide have been thoroughly investigated, the passivation of thin films and device-relevant stacks of amorphous silicon carbide have yet to be fully examined. We investigate the potential use of thin amorphous silicon carbide passivating layers to mitigate this susceptibility to high-temperature processes. The passivation obtained using stacks of intrinsic amorphous silicon carbide are evaluated and their stability upon exposure to high-temperature processes is assessed, amorphous silicon carbide being shown to surpass amorphous silicon for temperatures above 300 °C. Stacks of amorphous silicon carbide and amorphous silicon display the most temperature-stable passivation and it is demonstrated that the lifetimes increase as annealing temperatures approach 300 °C, and then rapidly decrease after this threshold. We hypothesize that this temperature dependence is a function of hydrogen diffusion and redistribution between the two amorphous layers. In order to elucidate the cause of these lifetime behaviors, we plan to fabricate different stacks and assess the temperature dependence of the optical bandgap, stack composition and hydrogen effusion from these layers. The bandgap and optical properties of the layers after subsequent annealing steps from 250 °C to 450 °C will be verified using spectrometric ellipsometry. Hydrogen effusion data will be collected for the different stacks and compared to the temperature-dependent lifetime measurements. FTIR analysis of the passivating stacks will also be performed to monitor their hydrogen content and bonding configurations. Potential correlations with the hydrogen effusion and lifetime may suggest that the decline in passivating qualities of thin film amorphous silicon carbide at high temperatures is due to the formation of dangling bonds via hydrogen effusion.
9:00 PM - EP4.11.06
Visualizing the Path of Light inside a Textured Silicon Solar Cell
Salman Manzoor 1,Miha Filipic 2,Marko Topic 2,Zachary Holman 1
1 Arizona State University Tempe United States,2 University of Ljubljana Ljubljana Slovenia
Show AbstractCrystalline silicon is a poor absorber of light with energies near its (indirect) bandgap energy, but it has nevertheless succeeded as a photovoltaic absorber material. This is in part because the path length of weakly absorbed light can be elongated by etching silicon in an alkaline bath, resulting in a random pyramidal texture on the silicon surface that scatters and “traps” light. When modeled, these pyramids are most often assumed to be randomly placed and to have a distribution of heights, but to all have the same, ideal base angle of 54.7o [1][2]. However, recent studies have shown that real random pyramids are not ideal and instead have a distribution of base angles with a mean angle lower than 54.7o [3]. Therefore, in this work we seek to (1) accurately measure the surface profile of real random pyramids, and (2) determine the effect on light trapping of real versus ideal random pyramids.
Height maps of silicon wafers with random pyramid textures were recorded using atomic force microscopy (AFM). This is challenging as the features can be as tall as 7 μm (beyond the limit of most AFMs), the minimum scan area has to be sufficiently large to produce the same optical response as the actual wafer, and the AFM tip needs to reach in to the deep valleys between pyramids. Thus, the accuracy of the AFM scans were judged by comparing the total reflectance and angular-resolved reflectance generated with geometrical ray tracing of the height maps to the measured reflectance obtained from a spectrophotometer. The deviation between the ray-traced and measured reflectance is considerably smaller with the AFM height map than with an assumed ideal-pyramid height map.
The AFM height maps were next used to do further ray tracing of the evolution of (weakly or non-absorbed) light inside the wafer. In particular, the mean path length enhancement averaged over all angles of incidence approaches the 4n2 random-scattering limit for the AFM height map, but not ideal pyramids. The angular distribution function (ADF) of light inside the wafer reveals why: The ADF for real random pyramids quickly becomes near Lambertian as the light bounces off the textured front and rear surfaces of the wafer, but it remains far from Lambertian for many more bounces for ideal random pyramids. That is, the non-ideal surface texture obtained during alkaline etching enables the excellent light trapping of silicon solar cells.
[1] P. Campbell and M. A. Green, "Light trapping properties of pyramidally textured surfaces," J. Appl. Phys., vol. 62, pp. 243-249, Jul 1987.
[2] P. Campbell, "LIGHT TRAPPING IN TEXTURED SOLAR-CELLS," Solar Energy Materials, vol. 21, pp. 165-172, Dec 1990.
[3] S. C. Baker-Finch and K. R. McIntosh, "Reflection distributions of textured monocrystalline silicon: implications for silicon solar cells," Progress in Photovoltaics: Research and Applications, pp. n/a-n/a, 2012.
9:00 PM - EP4.11.07
Large-Wavevector Phonon Population Anisotropy in Silicon Nanomembranes
Kyle McElhinny 1,Gokul Gopalakrishnan 1,Martin Holt 3,Dave Czaplewski 3,Paul Evans 1
1 Univ of Wisconsin-Madison Madison United States,2 University of Wisconsin - Platteville Platteville United States,1 Univ of Wisconsin-Madison Madison United States3 Center for Nanoscale Materials Argonne National Lab Argonne United States
Show AbstractPhonon engineering via the creation of surfaces and interfaces in nanomaterials provides an increasingly important degree of control over the properties of materials. In silicon-based nanomaterials the dispersion and scattering rates of phonons determine the key parameters of phonon-mediated thermal transport and have a crucial role in determining the electron mobility via electron-phonon scattering. The fabrication of nanostructures creates surfaces and interfaces with the large elastic discontinuities leading to boundary scattering and to the spatial confinement of phonons. Phonons with large wavevectors, with magnitudes approaching the span of a Brillouin zone are particularly important in Si nanomaterials, playing an important role in both thermal and electronic transport. New modes with distinct vibrational displacements appear in nanomaterials, for which description in terms of longitudinal and transverse polarizations is not strictly correct. The frequencies and atomic displacements associated with these normal vibrational modes depend on the direction of the wavevector due to the symmetry of the crystal lattice and the geometry imposed by the formation of interfaces. Crystallographic directions that are equivalent in the bulk (e.g. [001], [100], and [010] in a cubic lattice) are no longer equivalent in nanostructures, such that in-plane and out-of-plane directions in nanomembranes can exhibit differences in the phonon dispersion.
The characteristic phonon wavevector for an arbitrary location within the Si Brillouin zone is on the order of 1 Å-1, which poses a challenge for phonon characterization techniques. The small-wavevector regime of the phonon dispersion in nanomaterials, in which wavevectors are on the order of 1 µm-1, has been extensively probed by Raman and Brillouin scattering. The momentum transfer available through Raman scattering with visible and or UV photons, however, is too small to probe more than the 1% of phonon modes lying near the zone center. The vast majority of phonon modes in Si nanomaterials are thus uninvestigated by these techniques.
In this work, we probe an extremely wide range of acoustic phonon wavevectors, extending from near the zone center to the Brillouin zone boundary, through the use of synchrotron x-ray thermal diffuse scattering (TDS). TDS probes the population of large-wavevector phonons in single-crystals with uniquely small volumes, on the order of 10 μm3. Synchrotron x-ray TDS experiments used to sample phonon populations across the entire Brillouin zone of a silicon nanomembrane show the onset of phonon anisotropy at thicknesses of a few tens of nanometers. The TDS intensity profiles extracted along a series of crystallographic directions demonstrate the breaking of the directional degeneracy of the phonon dispersion in nanomembranes due to the loss of symmetry introduced by closely separated surfaces.
9:00 PM - EP4.11.08
Improvement of PEDOT:PSS/Crystalline Silicon Hybrid Solar Cell by Passivating Amorphous Silicon Thin Layer
Somnath Mahato 2,Luis Gerling 1,Cristobal Voz 1,Ramon Alcubilla 1,Joaquim Puigdollers 1
3 Dept. Enginyeria Electronica Universitat Politecnica de Catalunya Barcelona Spain,1 Centre de Recerca en Nanoenginyeria (CrNE) Barcelona Spain,2 Department of Applied Physics Indian School of Mines Dhanbad India,3 Dept. Enginyeria Electronica Universitat Politecnica de Catalunya Barcelona Spain,1 Centre de Recerca en Nanoenginyeria (CrNE) Barcelona Spain
Show AbstractAbstract: Hybrid organic/inorganic n-type crystalline silicon (c-Si) based solar cells using poly-(3,4-ethlenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) as an alternative hole-selective layer were fabricated and characterized [1]. The PEDOT:PSS thin layers (~60 nm) were deposited by spin coating with the incorporation of dimethyl sulfoxide (DMSO) as co-solvent and capstone FS-31 as surfactant [2]. This allowed to improve the conductivity and wettability of PEDOT:PSS, obtaining highly transparent and conductive films. Additionally, an ultra-thin (~5nm) intrinsic amorphous silicon (a-Si:H) interlayer was deposited by Plasma-Enhanced Chemical Vapour Deposition to improve the surface passivation of the crystalline silicon substrate and increase the minority carrier lifetime.
The current density-voltage response of the solar cells (100 cm2 area) was characterized under standard AM1.5g illumination at 25 degree Celcius, measuring an increase in the open-circuit voltage from 560 to 600 mV for the device with a-Si:H interlayer. This result can be explained by a reduced surface recombination velocity at the interface. In a similar manner, the fill factor improved from 49.7 to 54.4%. Overall, a maximum power conversion efficiency of 10.15% was achieved for the device with the passivating interlayer, as compared to 8.5% without it.
In parallel, different concentrations of DMSO [3] were used to increase the conductivity of PEDOT:PSS layer. Transfer Length Method (TLM) measurements revealed that the sheet resistance of PEDOT:PSS with 5% DMSO has a minimum value (122 Ω/sq). This value is comparable to the sheet resistance of Indium-Tin-Oxide (ITO), allowing its use as a hole-selective layer in an ITO-free solar cell.
[1] K. A. Nagamatsu, S. Avasthi, J. Jhaveri, and J. C. Sturm 12% Efficient Silicon/PEDOT:PSS Heterojunction Solar Cell Fabricated at < 100 C IEEE Journal of Photovoltaics. 2014, 4, 260- 264.
[2] D. Chi, B. Qi, J. Wang, S. Qu, and Z. Wang High-performance hybrid organic-inorganic solar cell based on planar n-type silicon, Applied Physics letters. 2014,104, 193903-193907.
[3] C.S. Pathak, J.P. Singh and R. Singh Effect of dimethyl sulfoxide on the electrical properties of PEDOT:PSS/ n-Si heterojunction diodes Current Applied Physics 2015,15, 528-534.
9:00 PM - EP4.11.09
Calcination Condition Dependence of the Passivation Quality of Spin-Coated Alumina Passivation Films for Silicon Solar Cells
Ryosuke Watanabe 1,Mizuho Kawashima 1,Yoji Saito 1
1 Seikei University Tokyo Japan,
Show AbstractReducing surface recombination loss of the silicon substrates is a key factor for improving the efficiency of silicon solar cells. Since 2006, alumina passivation films have been widely investigated as a new type of passivation films for silicon solar cells because of good passivation quality and large amount of negative fixed charge density in the films. Usually the alumina passivation layers have been prepared by atomic layer deposition (ALD) or plasma-enhanced chemical vapor deposition (PECVD) methods; however, these methods need high vacuum system and are high cost.
We have evaluated the passivation properties of alumina thin films prepared by low cost sol-gel wet process. Our recent research indicated that minority carrier lifetime of the prepared samples was extended moderately. We also found that the large interface state density of the samples prepared by sol-gel process causes to degrade passivation quality of the films. Thus, reducing the interface state density is required for the sol-gel process.
In this presentation, we evaluate minority carrier lifetime of silicon substrates with sol-gel alumina passivation layers that were prepared in a variety of calcination conditions. Here, we compared the passivation properties of the samples calcined in O2, N2, H2, and air environment. Also, the influence of mixing of water vapor in a calcination process was considered. Passivation properties of the samples were evaluated by minority carrier lifetime, capacitance-voltage (C-V), and X-ray photoelectron spectroscopy (XPS) methods.
The used sol-gel solution was consist of aluminum acetylacetonate (Al(acac)3) with 2-methoxyethanol or aluminum isopropoxide with benzene. The alumina films were spin-coated onto (100) oriented 12-18 Ωcm p-type single-crystalline silicon substrates. After spin-coating, the samples were calcined at 300°C and 600°C for 1 hour in an electric furnace.
The sample calcined at 500°C in a water vapor condition with oxygen carrier gas indicates extended carrier lifetime up to 350 μsec from 150 μsec (without water vapor). Extended carrier lifetime was also obtained by some of other calcination conditions with water vapor.
Sol-gel wet process is easy for preparing alumina passivation films, and it is appropriate for low-cost industrial silicon solar cells. We confirmed that it is effective to change the calcination condition for improving the passivation quality of sol-gel deposited alumina passivation films.
9:00 PM - EP4.11.10
Development towards an Integrated Combination of Thin-Film Silicon Multi-Junction Solar Cell and Lithium Ion Battery in Photo-electrochemical Application
Solomon Agbo 1,Tsvetelina Merdzhanova 1,Shicheng Yu 2,Hermann Tempel 2,Hans Kungl 2,Rudiger-A Eichel 2,Uwe Rau 1,Astakhov Oleksandr 1
1 Institute of energy and climate research (IEK-5)-Photovoltaics, Forschungszentrum Julich Julich Germany,2 Forschungszentrum Institute of Energy and Climate Research (IEK-9)-Fundamental Electrochemistry Julich Germany
Show AbstractRapidly growing use of modern portable electronic devices and new developments of distributed electronic systems like “cyber physical systems” are closely related to the questions of an off-grid power supply. Extension of battery life or ideally fully autonomous operation of a portable device is very relevant problem especially taking into account large populated areas with unstable power grid coverage.
From the point of view of scalability, light-weight and portability, thin-film silicon solar cells can make a good match with battery for a monolithic integrated autonomous energy solution for portable electronic devices. When developed as multi-junction device based on amorphous (a-Si:H) and microcrystalline silicon (µc-Si:H) thin-film silicon solar cells have the ability to deliver high voltage (~1.3 V in the tandem device to around 3 V in the quadruple junction) that can be directly utilized to charge storage batteries in an integrated photovoltaic/battery cell.
In this work, we have developed thin-film silicon triple-junction solar cells based on a-Si:H and µc-Si:H and used them to charge lithium ion battery. The battery was directly connected to the solar cell without any additional electronics as a proof of concept on the possibility of a monolithic integration of the two devices. The solar cells were characterized under a range of illumination intensities including AM1.5 standard for further power gain analysis. The battery was developed with Lithium iron phosphate (LFP) cathode and Lithium titanate (LTO) anode. These materials for the battery were chosen because of their stabilities against over-charging and their commercial availability. The charge voltage of the battery cell is 1.93 V and matches perfectly the voltage at maximum power point of the triple-junction solar cells. The battery was cycled several times by connecting the solar cell directly to the battery in series. Our results show that triple cell based on top and middle a-Si:H and a bottom µc-Si:H solar cells provide current-voltage characteristics over a wide range of illumination intensities that satisfies the charging requirement of lithium ion battery based on LFP cathode and LTO anode to reach full state-of-charge. In our report we will present recent advances on the development of thin-film photovoltaic and battery cells towards an integrated power supply.
9:00 PM - EP4.11.11
Laser Annealing of Hydrogenated Amorphous Silicon Below Crystallization Temperature
W. Beyer 2,J. Bergmann 3,U. Breuer 4,Friedhelm Finger 2,S. Haas 2,A. Lambertz 2,N.H. Nickel 1,T. Schmidt 3,U. Zastrow 2
1 Institut für Silizium-Photovoltaik Helmholtz-Zentrum Berlin für Materialien und Energie Berlin Germany,2 IEK5-Photovoltaik Forschungszentrum Jülich GmbH Jülich Germany,3 Photovoltaische Systeme Leibniz-Institut für Photonische Technologien Jena Germany4 ZEA-3 Forschungszentrum Jülich GmbH Jülich Germany2 IEK5-Photovoltaik Forschungszentrum Jülich GmbH Jülich Germany1 Institut für Silizium-Photovoltaik Helmholtz-Zentrum Berlin für Materialien und Energie Berlin Germany
Show AbstractLaser annealing of hydrogenated amorphous silicon (a-Si:H) below crystallization temperature is of interest for both thin-film silicon and silicon heterojunction solar cell technologies, since by hydrogen diffusion defects may get deleted. For characterization of the annealed state and thus for controlled annealing, knowledges of both H diffusion length L and of temperature T during laser treatment are highly desirable. Recently we reported that by SIMS measurements of deuterium and hydrogen interdiffusion in layered structures of deuterated and hydrogenated a-Si material both latter parameters can be determined [1]. SIMS measurements yield directly the H diffusion length from which, using the laser treatment time, the H diffusion coefficient during laser treatment is obtained. Literature data of the Arrhenius-dependence of H diffusion then yield the laser treatment temperature. Here we report results of laser treatment using a 532 nm (green) continuous wave laser. Data on L and T are presented as a function of scanning speed v, residence time t and laser power P for films of 0.3 to 0.8 µm thickness deposited on c-Si wafers and on glass substrates. Significant differences are found for the different substrate materials. For films deposited on c-Si we find for fixed laser power a square root time dependence of L. Accordingly, the H diffusion coefficient and the temperature T are rather independent of scanning speed [1]. In case of glass substrates, both H diffusion coefficient and temperature T increase considerably with rising residence time. The temperature T corresponds largely to data obtained from analysis of melting of the a-Si films at high laser power. Our data show that roughly a factor of 100 higher laser power density is required for c-Si compared to glass substrates to reach equal H diffusion lengths at fixed residence time. Reason for this is likely the very different heat conductance of the substrates.
[1] W.Beyer, J. Bergmann, U. Breuer, F. Finger, A. Lambertz, T. Merdzhanova, N.H. Nickel, F. Pennartz, T. Schmidt, U. Zastrow, Mat. Res. Soc. Proc. Vol. 1770 (2015), DOI: 10.1557/opl.2015.431
9:00 PM - EP4.11.12
Full-Visible Emission from Silicon Quantum Dots in Oxide Matrix: Role of Quantum-Dot Size
Ateet Dutt 2,Yasuhiro Matsumoto 1,Guillermo Santana Rodriguez 2,Jaime Santoyo Salazar 3,Srinivas Godavarthi 4
2 Instituto de Investigaciones en Materiales UNAM Coyoacán Mexico,1 Electrical Engineering Department Centro de Investigación y de Estudios Avanzados del IPN Mexico City Mexico3 Departamento de Física Centro de Investigación y de Estudios Avanzados del IPN Mexico City Mexico4 Instituto de Ciencias Físicas UNAM Cuernavaca Mexico
Show AbstractOver the past few decades, obtaining visible emission from silicon quantum dot’s (QD’s) and hence to explain the mechanism of emission has been one of the vital tasks for most of the research groups. In this work, a visible, and even white intense luminescence has been observed in silicon quantum dots embedded in amorphous silicon oxide. Depositions have been made using hot wire chemical vapor deposition (HW-CVD) at a low substrate temperature of about 200°C. Furthermore, the mechanism of emission is studied in depth using various morphological analysis and luminescence experiments. It is observed that the size variation of nano-particles creates various surface states, which is responsible for the different emission wavelengths with an average extreme brightness. Using atomic force microscopy (AFM), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) structural and morphological studies have been made. Using SEM and AFM various size distributions of particles could be observed whereas using FTIR bonding states related to silicon, hydrogen and oxygen has been observed. XRD results have shown the polycrystalline nature of the films.Post-deposition heat treatments have been performed to find out the possible mechanism of emission. The white photoluminescence has been correlated with the size of nc-Si particles and/or the defects present in the thin film.The current study could lead to the development of light emitting devices (LED) or lasers based on silicon technology in future.
9:00 PM - EP4.11.14
Conductance Tomography of Filamentation in Next-Generation Silicon Suboxide Intrinsic Resistive RAM Memories Using Conductive Atomic Force Microscopy
Mark Buckwell 1,Luca Montesi 1,Steve Hudziak 1,Adnan Mehonic 1,Anthony Kenyon 1
1 Univ College London London United Kingdom,
Show AbstractResistive RAM is a next-generation technology offering data storage density and efficiency advancements over current memories such as flash. Such devices are simple, comprising a stack of a thin dielectric layer sandwiched between a pair of conductive electrodes. Sub-breakdown electrical stress is used to reversibly switch the dielectric between states of resistance with a contrast of up to several orders of magnitude, a phenomenon known as resistance switching. For dielectrics such as silicon suboxide, SiOx, whose intrinsic behaviour is not reliant on metallic dopants, various models exist to describe the switching mechanism. These are generally based on the formation of conductive bridges through the dielectric layer, between the electrodes. For SiOx these bridges, known as filaments, are thought to be chains of electron-trapping oxygen vacancies. However, a full understanding of these sub-micron features is yet to be developed. We used conductive atomic force microscopy to perform tomography on SiOx films following the creation of filaments, a process referred to as electroforming. By imaging with the scanning tip pressed into the sample we were able gradually remove material and profile through the dielectric while collecting conductivity data. Three-dimensional rendering of the resulting current map images enabled the first clear visualisation of filaments in an intrinsic switching material, allowing us to study their structure. Our results confirm that filamentation is responsible for switching and show that the conductive pathways conform to the intrinsic structure of the layer. However, they also indicate that this may be a powerful technique three-dimensional conductivity mapping in other solid films.
9:00 PM - EP4.11.15
Induction-Based Local Annealing of Metals for low Resistance Contacts
Jacob Clenney 1,April Jeffries 1,Mariana Bertoni 1
1 Arizona State University Tempe United States,
Show AbstractHigh efficiency amorphous/crystalline silicon heterojunction (SHJ) solar cells provide excellent promise for future solar energy production but are at present not achieving their theoretical efficiency. One of the most important limitation resides in the series resistance losses. Standard solar cell manufacturing process for forming low-resistance ohmic contacts using silver paste printing involves temperatures up to 800o C. At these temperatures, degradation of the intrinsic hydrogenated amorphous silicon (a-Si:H) layer occurs, whose properties are essential to SHJ’s high open-circuit values. Therefore, a lot of effort has been put into curing the silver contacts at lower temperatures. Consequently, and despite development of silver pastes especially designed for this purpose, the contact resistance obtained so far is not yet sufficiently low and results in additional series resistance in the device and loss in the fill factor.
In view of this we propose an alternative to the use of conventional curing of contacts at high temperatures, using an induction furnace instead of a radiative heating belt or a muffle furnace. This allows targeted, localized heating of the contacts with limited heat transfer to the rest of the cell structure, thus avoiding thermal degradation of the a-Si:H layer.
In the work herein, we present the preliminary evaluation of the induction method to locally heat metal contacts, placed inside an induction coil. Initial experiments show that indium-tin oxide layers and silicon do not respond to induction annealing, under any of the applied power and frequency conditions. We show, however, that we can achieve localized annealing of silver printed contacts on full SHJ cells and we also observe that the heating is originating from the metal and only minimally transfers to the surrounding materials. Our results show that after induction furnace annealing for 2.5 seconds, the sheet resistance of the silver was in the order of 5 mΩ/sq. which is approximately one-third of the sheet resistance of the silver contacts annealed at 200°C for 20 minutes in a muffle furnace. This proves that the bulk quality of the silver contacts can be improved by the use of induction furnace curing compared to standard furnace annealing.
9:00 PM - EP4.11.17
Tailoring a-Si Nanopillar Antenna Resonances for Angle-Insensitive Color Filters
Katherine Fountaine 2,Mikinori Ito 3,Harry Atwater 2
1 Northrop Grumman Aerospace Systems Redondo Beach United States,2 California Institute of Technology Pasadena United States,2 California Institute of Technology Pasadena United States,3 Sony Electronics, Japan Japan2 California Institute of Technology Pasadena United States
Show AbstractSpectrally-selective nanophotonic and plasmonic structures enjoy widespread interest for application as color filters in imaging devices, due to their potential to improve performance as replacements for traditional organic dyes and pigments. Organic dyes are straightforward to implement with predictable optical performance, but suffer from inherent optical cross-talk and stability (UV, thermal, humidity) issues. Nanophotonic and plasmonic color filters are more robust, but often have polarization and/or angle-dependent optical response. Here we report on design and fabrication of polarization- and angle-insensitive color filters based on a-Si nanopillar arrays. Specifically, we address subtractive CYM filters via uniform arrays and additive RGB filters via multi-radii arrays.
Herein, we present results on sparse a-Si nanopillar arrays as color filters, supported by experiment, simulation, and analytic theory. The a-Si nanopillar array dimensions were optimized via analytic waveguide and Mie theory in combination with FDTD electromagnetic simulations. Subsequently, the designed a-Si nanopillar arrays were fabricated using e-beam lithography and reactive ion etching. The angle-dependent transmission of the resulting arrays was measured and good agreement with simulation was achieved. In particular, a fabricated hexagonal-packed array (a=720 nm) of a-Si nanopillars (height=320 nm, diameter=120 nm) demonstrated cyan filter properties (red subtractive), achieving ~60% absorption within the red portion of the spectrum (600-700 nm) and ~90% transmission outside of this band for unpolarized light. This performance was maintained for angles of incidence upwards of 30°. Similar results were obtained with uniform arrays for the other subtractive filters (yellow, magenta). Additive filters consisted of two sub-lattices of different radii nanopillars to realize resonant absorption over two color regions.
The color filtering mechanism in these arrays is a product of the scattering and absorption properties of individual nanopillars. Each nanopillar strongly scatters and resonantly absorbs at resonant mode frequencies, thereby filtering out a certain portion via absorption and transmitting the remainder of the spectrum. The dominant resonant antenna mode of the a-Si nanopillars strongly resembles the b11 Mie mode and HE11 waveguide mode of an infinite cylinder, and its resonant frequency is well-approximated from these theoretical frameworks, although a slight-blue shift is observed in practice due to longitudinal confinement. Consequently, analytic waveguide theory was used to predict the approximate nanopillar radius, and electromagnetic simulations were used to optimize nanopillar radius and height to achieve the desired optical response. Weak coupling in sparse arrays and minimal dispersion of the resonant modes results in the observed polarization and angle-independent optical response.
9:00 PM - EP4.11.18
Extending Electrical Scanning Probe Microscopy Measurements of Semiconductor Devices Using Microwave Impedance Microscopy
Stuart Friedman 1,Yongliang Yang 1,Oskar Amster 1,Fred Stanke 1
1 PrimeNano Inc Palo Alto United States,
Show AbstractAdvances in semiconductor manufacturing technology continue to drive development and refinement of the analytical techniques used for failure analysis. Electrical characterization measurements using Atomic force microscopy (AFM), such as scanning capacitance microscopy (SCM), which has existed for more than three decades, have shown value in many circumstances, but suffer from significant limitations. In contrast, Scanning Microwave Impedance Microscopy (sMIM) directly measures the real (called sMIM-R) and imaginary parts (sMIM-C) of the impedance of the interface between the AFM tip and sample, and thereby adds significant electrical measurement capability over existing techniques while simultaneously producing the same dC/dV measurements as SCM. For example, sMIM’s direct measure of sMIM-C and sMIM-R provide (1) a more systematic dependence on doping concentrations over a larger range of doping levels and (2) information on and contrast from linear regions of the sample, such as oxides, metals and polysilicon structures. Application of a localized DC bias sweep produces nano-scale capacitance vs voltage (C-V) curves that characterize the non-linear properties of nanoscale doped regions.
In addition to the more easily interpreted information from a broad range of doping levels, sMIM can provide physical information from non-doped materials such as insulators, semiconductors and metallic regions of the sample. For example, in sMIM-C images from image sensors, features such as poly gates, storage diffusion, photocathodes and shallow trench isolation (STI) are all differentiated due to their different capacitances. For the non-linear regions, the dC/dV phase image provides valuable information about carrier type, which is available in conjunction with the new information only available from the sMIM-C channel.
In this presentation, we will discuss the performance of scanning microwave impedance microscopy when applied to a CMOS image sensor for dopant profiling and electrical imaging of other features not typically accessible in SCM. We will show that we are able to reveal features using sMIM that are not seen in simultaneously acquired dC/dV data. We will also show individual point capacitance vs voltage curves directly using the sMIM-C output. The C-V curves for regions of different doping levels provide information on carrier type and doping concentration variation. The combination of dC/dV and sMIM-C can be powerful when considering the ability to acquire data from a wide range of materials as well as ascertain the dopant type in a subset of those materials.
9:00 PM - EP4.11.19
Highly Efficient and Economical Flexible Thin Film Transistors Based on High Mobility Single-Crystalline-Like Si by PECVD
Ying Gao 2,Mojtaba Asadirad 1,Pavel Dutta 2,Sicong Sun 1,Yao Yao 1,Monika Rathi 2,Yongkuan Li 1,Jae-Hyun Ryou 2,Venkat Selvamanickam 2
1 Materials Engineering Univ of Houston Houston United States,2 Texas Center for Superconductivity at the University of Houston University of Houston Houston United States,1 Materials Engineering Univ of Houston Houston United States2 Texas Center for Superconductivity at the University of Houston University of Houston Houston United States
Show AbstractSilicon (Si) is by far the most widely-used semiconductor material for thin film transistors (TFTs) since it is abundant in nature, inexpensive and environmentally benign. However, the performance of TFTs is limited by the low mobility and poor stability for amorphous Si (a-Si), and nonuniformity of field-effect mobility and threshold voltage of polycrystalline Si (p-Si). With an objective of fabrication of high performance TFTs, we have developed high-textured, single-crystalline-like Si (c-Si) film on inexpensive, flexible and polycrystalline substrates using inductively-coupled plasma enhanced chemical vapor deposition (PECVD) by epitaxial growth on templates made by ion beam assisted deposition (IBAD). The c-Si film was highly oriented along (004) direction and strongly biaxially-textured with narrow spread in in-plane (1.7°) and out-of-plane (1.2°) texture. Raman spectroscopy revealed fully crystallized of Si films with sharp Si transverse optical (TO) mode peak (5.2 cm-1), nearly comparable to Si wafer (4.6 cm-1). Controllable PH3 doping was achieved in n-type Si films with high electron mobility of 230 cm2/V-s. Then high performance TFTs were fabricated based on the single-crystalline-like n-Si channel on flexible metal substrates. The resulting devices exhibited exceptional performance with on/off current of ~106, a threshold voltage of -2 V and a field-effect mobility of ~200 cm2/V-s. These devices with superior saturation current of ~1.6 mA opens up a new era toward the realization of TFTs for next-generation flexible electronics.
9:00 PM - EP4.11.20
Low Temperature Spalling for Low Cost Silicon Wafers
Pablo Guimera Coll 1,Tine Naerland 1,Mariana Bertoni 1
1 Arizona State University Tempe United States,
Show Abstract
High costs in solar cells manufacturing limit global adoption of solar energy where currently 35% of the cost comes from the wafer sawing process. Thermal spalling has been proposed to substitute the conventional wafering technique, eliminating slurry and wires while doubling the yield of a silicon ingot. This technology is based on the propagation of a planar fracture parallel to the silicon surface to cleave the wafer. The stress required to propagate the fracture is produced when cooling by the mismatch of the Coefficient of Thermal Expansion (CTE) between a metal layer and the silicon block. One of the main problems to propel this technology to industrial scale, however, is the significant losses in bulk and surface quality. The reason is that these state-of-the-art techniques rely on a heating cycle in the presence of a metallic stressing layer causing in-diffusion of charge carrier lifetime killing impurities that has been shown to be detrimental even down to ppb level.
The work presented herein is a study on the effect of thermal spalling at low temepratures in order to minimize the unwanted decrease in charge carrier lifetime. Our results show that at low temperatures choosing a material with the right CTE mismatch to the silicon, provides the right plane-strain conditions to spontaneously propagate a crack through the silicon. In addition, we show that by controlling the temperature, staying just above the onset of spontaneous spalling, and applying a mechanical force perpendicular to the surface, the crack propagation can be controlled so a smooth wafer surface is achieved.
9:00 PM - EP4.11.21
Multiscale Self-Assembly of Quantum Dots into an Anisotropic Three-Dimensional Random Network
Serim Ilday 1,Fatih Ilday 1,Rene Huebner 2,Ty Prosa 3,Isabelle Martin 3,Gizem Nogay 4,Ismail Kabacelik 5,Zoltan Mics 6,Dmitry Turchinovich 6,Hande Ustunel 4,Daniele Toffoli 4,David Friedrich 2,Bernd Schmidt 2,Karl-Heinz Heinig 2,Rasit Turan 4
1 Bilkent University Ankara Turkey,2 Helmholtz-Zentrum Dresden-Rossendorf Dresden Germany3 CAMECA Instruments Inc. Madison United States4 Middle East Technical University Ankara Turkey5 Akdeniz University Antalya Turkey6 Max Planck Institute for Polymer Research Mainz Germany
Show AbstractOne of the well-known challenges in design of nanomaterials is to simultaneously achieve material properties pertaining to few-atom scale and bulk properties through which the material connects to other materials or interacts with devices. This is difficult because properties arising from physics at different scales are often mutually exclusive. An important example is the 30 year-old problem of realizing a connected-but-confined Si nanostructure embedded in a dielectric matrix (e.g., SiO2) that simultaneously brings together quantum-dot (QD)-like optical properties and good electrical conduction. Here, we solve this problem through creation of a hierarchically self-assembled anisotropic random network of Si QDs: At the atomic scale, QDs are formed, which sparsely interconnect without inflating their diameters to form an isotropic random network, and larger scales, this network becomes anisotropic, preferentially growing in the vertical direction to form nanowire-like structures. We report simultaneous achievement of good electrical conductivity (~0.1 S/cm) and a bandgap tuneable over the visible light range (from 1.8 to 2.7 eV).
In order to determine how to self-assemble such a topology without using advanced control over dynamical details of the system, we developed a toy model of the stochastic deposition process, from which we related the intended topology to parameters governing stochastic growth and determined the experimental conditions that can give rise to it. Monte Carlo and Molecular Dynamics simulations are performed to guide our methodology and fabrication was done using magnetron sputter deposition. The two leverages that we used for multiscale self-assembly were as follows: (i) We keep the substrate “cold” and adjust how “hot” the deposited particles are. This way, we create spatio-temporal temperature gradients on the surface and thereby, we control surface diffusion and promote vertical growth in the microscale resembling nanowires. (ii) We fine-tune the thin-film stoichiometry in order to control the phase-separation. This way, we control the nominally unstable medium that QDs are embedded in and limit further inflation of their diameters in the atomic scale. This way we show that self-assembly under nonequilibrium conditions and nonlinear dynamics sweeps aside a large number of factors that influence the details of thin-film growth, but provides a couple of simple “rules” (with clearly identifiable corresponding experimental conditions) to determine the final morphology. The generality and material-independence of this methodology is strongly suggestive of possibility to apply it to solve a variety of other nanomaterial problems, which also pertain to multiple scales.
9:00 PM - EP4.11.22
Silicon Mirco-Origami
Hanqing Jiang 1,Cheng Lv 1,Zeming Song 1
1 Arizona State Univ Tempe United States,
Show AbstractOrigami, i.e., paper folding, is inspiring an innovative way to make 3D structures from 2D thin film. However, when the scale of the structure is extremely small (~100 μm), conventional fabrication methods will fail. Here, we use compression-induced instability to achieve 3D microscale thin-film patterns. By the addition of patterned PDMS vertical walls on a uniform PDMS substrate the contact area between thin film and substrate can be minimized. After carefully design of the contact area, shape of final products can be readily controlled. 300nm silicon film was transfer-printed on PDMS walls with 20% pre-strain. After releasing the pre-strain, desired patterns will be generated. Finite element analysis was adopted as theoretical approach to study the mechanical characteristic of the design, which can be used as guidance for manufacture.
9:00 PM - EP4.11.25
Enhancement of Reliability Characteristics by Optimizing Active Air Gap Process for Sub-20 nm 2D NAND Flash Memory Devices
Minchul Lee 1,Minho Jeong 1,Byoungjun Park 1,Sungpyo Lee 1,Kwanghyun Yang 1,Kangjae Lee 1,Daehwan Yun 1,Byungduck Jo 1,Seungwan Seo 1,Ilhong Min 1,Seongjo Park 1,Myoungkwan Cho 1,Heehyun Jang 1,Jinwoong Kim 1,Changjin Sunwoo 1
1 SK Hynix Cheongju-si Korea (the Republic of),
Show AbstractAs 2D-NAND Flash memory cells have been scaling down into sub-20nm, the distance between cells has been decreased both X- and Y- direction. Therefore, reliability characteristics are degraded due to widening of distribution of cell’s threshold voltage (Vth) which is originated from a cell interference as NAND Flash memory cells become smaller and closer. At first, to overcome these obstacles, Word Line (WL) air-gap technique was introduced in 2D NAND flash memory process in order to decrease WL to WL interference. Also, active air-gap technique has been adopted for reduction of bit line (BL) to bit line interference. This technique leads less interferences between neighboring BLs, however program/erase cycling characteristics will be degraded due to side effects. This research is focused on an improvement in reliability characteristics of both interference and cycling stress by optimizing an active air-gap shape and portion. To observe air-gap shape and portion, transmission electron microscope is used. Electrical characteristics such as gate mobility, cell leakage, Vth distribution, endurance and data retention have also been measured for investigating NAND properties in sub-20 nm Flash devices.
9:00 PM - EP4.11.26
Charge Transport in Germanium Nanoparticle Thin-Films for Solution-processed Electronics
Zeynep Meric 1,Christian Mehringer 1,Michael Jank 2,Wolfgang Peukert 1,Lothar Frey 1
1 Univ of Erlangen Erlangen Germany,2 Fraunhofer IISB Erlangen Germany2 Fraunhofer IISB Erlangen Germany,1 Univ of Erlangen Erlangen Germany
Show AbstractSolution-processing offers a low-cost alternative to conventional thin-film electronics. A natural possibility for facile solution-processing of functional thin-films is the application of nanoparticles (NPs) as building blocks. In this context, the surface chemistry of NPs is particularly important for device application. Moreover, the interface of NPs with the contact material may determine the device characteristics in applications like thin-film transistors (TFTs) and the enhancement of device performance is possible by engineering of injection barriers between contacts and NPs. Among other group-IV NPs, germanium (Ge) combines high bulk charge carrier mobilities [1] with superior thermodynamic properties over Si, i.e. enabling processing at lower temperatures. This work investigates Ge NPs as building blocks for NP based TFTs.
Previously established experimental approaches [2] were applied to Ge-NP systems in order to investigate surface termination effects. From NP synthesis [3] to inverter integration, the whole value chain is covered. The conduction mechanisms in the layers are evaluated using TFT-like interdigitated. Annealing reveals n-type transfer characteristics of the quasi-TFT devices. Permanent encapsulation of the layers with Poly(methyl methacrylate) (PMMA) further improves the current levels. Due to more efficient passivation of trap states on the particle surfaces, more charge carriers can be contributed to transport within the layer. Encapsulation by atomic layer deposition (ALD) of Al2O3 onto the layers confirms reduction of surface traps and alters the layer conduction mechanism from n- to p-type. This could be attributed to a large fixed negative charge concentration in the ALD layer, which overcompensates surface defects and introduces inversion charges in the NP layer. To further investigate this effect, the passivated devices are submitted to additional annealing steps where ambipolar behavior is observed. As an outlook, an inverter set-up with two ambipolar TFTs is demonstrated [4].
Besides, charge injection over metal/NP interfaces plays a dominant role in the definition of device characteristics. Based on the optimized layer, the influence of channel length and width variation as well as different contact metals, i.e. Al, Au and ITO was evaluated. Whereas with short channel lengths device characteristics are governed by contact effects, longer channels reveal the basic film properties. However, with increasing channel length the current decreases stronger than linear which can be attributed to percolation effects. Comparison of three different electrode materials indicates electrode barriers could be better overcome by gold and ITO contacts than by aluminum.
[1] Kim et al., Nanoscale, 2014, 6(17), p. 10156 [2] Weis et al., Small, 2011, 7, p. 2853 [3] Mehringer et al., Nanoscale, 2015, 12, p. 5186 [4] Meric et al., PCCP, 2015, 17, p. 22106
9:00 PM - EP4.11.27
A Multiscale Modeling Approach to Study Transport in Silicon Heterojunction Solar Cells
Pradyumna Muralidharan 1,Stuart Bowden 1,Stephen Goodnick 1,Dragica Vasileska 1
1 Electrical, Computer and Energy Engineering Arizona State University Tempe United States,
Show AbstractThe device performance of an amorphous silicon (a-Si)/crystalline silicon (c-Si) solar cell depends strongly on the interfacial transport properties of the device. The energy of the photogenerated carriers at the barrier strongly depends on the strength of the inversion at the heterointerface and their collection requires interaction with the defects present in the intrinsic amorphous silicon buffer layer. In this work we present a multiscale model which can bridge the gap in time scales between different microscopic processes to study the transport through the interface by coupling an ensemble Mone Carlo (EMC) and a kinetic Monte Carlo (KMC). The EMC studies carrier properties such as the energy distribution fucntion (EDF) at the heterointerface whereas the KMC method allows us to simulate the interaction of discrete carriers with discrete defects. This method allows us to study defect transport which takes place on a time scale which is too long for traditional ensemble Monte Carlo's to analyze. We analyze the injection and extraction of carriers via defects by calculating transition rates for different processes. By using the principles of SRH recombination, this method can also be extended to study recombination processes at the interface and in the amorphous bulk which are crucial parameters for solar cell performance. Therefore, by using the multiscale approach all important processes can be studied rigorously to evaluate device performace.
9:00 PM - EP4.11.28
Impact of Morphology on Charge Transport in In2O3:H
Alexander Niebroski 1,Srikanth Gangam 1,Sebastian Husein 1,Michael Stuckelberger 1,Laura Ding 1,Mariana Bertoni 1
1 Arizona State University Tempe United States,
Show AbstractSolar cells with highly resistive absorber or contact layers require the introduction of a transparent conducting oxide (TCO) as contacting layer. In these devices, the TCO serves as a lateral carrier transport medium, and its contribution to solar cell series resistance is proportional to its sheet resistance, proportional itself to the layer thickness. As thickness is usually fixed (e.g., because the TCO simultaneously serves as an anti-reflection coating), and unwanted free-carrier absorption is proportional to carrier density, high mobility is the only route to higher efficiencies. We estimate that an absolute efficiency gain of 1% is possible for heterojunction solar cells when increasing the mobility from 20 cm2/(Vs) (a typical value for the industry standard tin-doped Indium Oxide, ITO) to 100 cm2/(Vs). However, greater electron mobilities in transparent conducting oxide thin films can not only impact solar cell performance but any optoelectronic device that relies on a TCO.
Recently, hydrogen-doped indium oxide (IO:H) has been introduced as a new TCO with an electron mobility over 100 cm2/(Vs). A small partial pressure of water of 6 µTorr during the deposition, as well as a post-deposition annealing promotes crystallization and improves electron mobility. It is believed that hydrogen from water acts as a donor in the In2O3 system. Initially, the sputter process produces an amorphous structure of the In2O3:H system, which transforms into a polycrystalline material with grain sizes in the order of 100 nm during the post-deposition annealing.
Temperature-dependence measurements are crucial for the understanding of transport properties as different mechanisms have distinctive temperature-dependence signatures. Therefore, we present in this work the impact of grain boundary scattering on the electrical properties through temperature-dependent impedance-spectroscopy measurements. We analyze the electron transport in-plane and transversally in IO:H films with different hydrogen contents. The electrical results will be complemented with X-ray diffraction and scanning-electron microscopy structural analysis.
9:00 PM - EP4.11.29
An Amorphous-to-Crystalline Phase Transition within Thin Silicon Films Grown by Ultra-High-Vacuum Evaporation and Its Impact on the Optical Response
Farida Orapunt 2,Li-Lin Tay 3,David Lockwood 3,Jean-Marc Baribeau 3,Mario Noel 3,Joanne Zwinkels 3,Stephen O'Leary 1
2 University of Regina Regina Canada,3 National Research Council of Canada Ottawa Canada1 Univ of British Columbia Kelowna Canada
Show AbstractA number of thin silicon films are deposited on crystalline silicon, native oxide on crystalline silicon, and optical quality fused quartz substrates through the use of ultra-high-vacuum evaporation at growth temperatures ranging from 98 to 572 oC. An analysis of their grazing incidence X-ray diffraction and Raman spectra indicate that a phase transition, from amorphous-to-crystalline, occurs as the growth temperature is increased. Through a peak decomposition process, applied to the Raman spectroscopy results, the crystalline volume fraction associated with these samples are plotted as a function of the growth temperature for the different substrates considered. It is noted that the samples grown on the crystalline silicon substrates have the lowest crystalline phase transition onset temperature, the samples grown on the oxidized crystalline silicon substrates having a greater onset temperature, those grown on the optical quality fused quartz substrates having the highest onset temperature. These resultant dependencies on the growth temperature provide further evidence in support of the presence of an amorphous-to-crystal phase transition within these silicon films. It is noted that the thin silicon film grown at 572 oC, possessing an 83 % crystalline volume fraction, exhibits an optical absorption spectrum which is quite distinct from that associated with the other thin silicon films. We suggest that this is due to the onset of sufficient long range order in the film for wave-vector conservation to apply, at least partially. Finally, we use a semiclassical optical absorption analysis to study how this phase transition, from amorphous-to-crystalline, impacts upon the spectral dependence of the optical absorption coefficient.
9:00 PM - EP4.11.30
Self-Organization of Metal Silicide Epilayers at Grain Boundaries in Silicon
Yutaka Ohno 1,Kaihei Inoue 1,Kentaro Kutsukake 1,Momoko Deura 1,Takayuki Ohsawa 1,Ichiro Yonenaga 1,Hideto Yoshida 2,Seiji Takeda 2,Ryo Taniguchi 3,Hideki Otubo 3,Shigeto Nishitani 3,Naoki Ebisawa 4,Yasuo Shimizu 4,Koji Inoue 4,Yasuyoshi Nagai 4
1 IMR, Tohoku Univ Sendai Japan,2 ISIR, Osaka Univ Osaka Japan3 Kwansei Gakuin Univ Sanda Japan4 The Oarai Center, IMR, Tohoku Univ Oarai Japan
Show AbstractCrystalline materials with grain boundaries (GBs), involving excess free energy because of their structural imperfection, can reduce their energy by the nanoscopic structural changes of the GBs via impurity segregation. Those local changes can stabilize nonequilibrium nanostructures, resulting in the drastic change in the macroscopic physical properties. In the present work, we found coherent precipitation of copper (Cu) along small-angle tilt boundaries (SATBs) in silicon (Si), and clarified the nanoscopic precipitation mechanism using scanning transmission electron microscopy (STEM) and atom probe tomography (APT) combined with ab-initio calculations [1]. Our finding might provide a guidance to control metal precipitates at GBs in Si, which would help to fabricate cost-effective functional metal silicides such as ferromagnetic epilayers for generating pure spin current and spin transport at room temperature in Si, as well as to establish metal-semiconductor interface nanotechnologies.
The Cu precipitation process is explained in terms of the balance of the precipitation and interface energies for the precipitates as well as the strain energy due to lattice mismatch and edge dislocations (EDs) composing SATBs. At the initial stage of the precipitation, Cu atoms agglomerate along SATBs forming coherent layers of Cu3Si with a body-centered cubic structure in a metastable state (a = 0.285 nm). As the layers thicken, they become semi-coherent layers with misfit dislocations (MDs) on the interphase boundaries so as to reduce coherency strains. However, as Cu atoms agglomerate further, the metastable layers converted into incoherent polyhedrons of orthorhombic η”-Cu3Si in the equilibrium state, forming interphase boundaries off the SATBs. The precipitation process at SATBs differs from that at localized nucleation sites such as isolated dislocations and point defects; incoherent η”-Cu3Si precipitates accompanied with dislocation loops are formed. Based on the coherent precipitation mechanism, other metallic impurities such as iron and nickel might precipitate at SATBs, forming cubic M3Si-type epilayers, at the initial precipitation stage.
[1] Y. Ohno, K. Inoue, K. Kutsukake, M. Deura, T. Ohsawa, I. Yonenaga, H. Yoshida, S. Takeda, R. Taniguchi, H. Otsubo, S. R. Nishitani, N. Ebisawa, Y. Shimizu, H. Takamizawa, K. Inoue, Y. Nagai, Phys. Rev. B 91, 235315 (2015).
9:00 PM - EP4.11.31
Mechanical Manipulation of Flexible a-Si:H Nanowire Solar Cells
Minoli Pathirane 1,Pranav Gavirneni 1,William Wong 1
1 University of Waterloo Waterloo Canada,
Show AbstractThree-dimensional (3-D) nanowire structures have shown improved light trapping and high minority carrier diffusion lengths to enhance the device performance of photovoltaic devices. These structures are particularly beneficial for hydrogenated amorphous Si (a-Si:H) thin-film devices whose disordered structure limit carrier lifetimes. Reports have shown that the ordering, aspect ratio, and density of the nanowire arrays affect the optical response of these 3-D structures. The heterogeneous integration of nanowire solar cells on flexible platforms could enable new opportunities for enhancing optical absorption through mechanical manipulation of the nanowires and possibly improve the electrical performance of nanowire solar cells. To enable the direct integration of nanowires on flexible substrates, ZnO nanowires were grown in a hydrothermal solution directly onto polyethylene naphthalate (PEN) substrates using a ZnO nanoparticle seed layer. A p-i-n a-Si:H thin-film was deposited over the disordered ZnO nanowire array at < 200C using plasma-enhanced chemical vapor deposition (PE-CVD) to fabricate 3-D radial-junction solar cells on flexible substrates.
Mechanical manipulation of the disordered 3-D a-Si:H nanowire solar cells was used to change the effective spacing between the radial junction devices, increasing the effective density and aspect ratio of the nanowire array. A 9% increase in short-circuit current density (JSC) was achieved by mechanically bending the substrate in a concave up orientation at a radius of curvature of 38 mm. As the radius of curvature was decreased to 6 mm, the JSC further increased from an initial value of 13.5 mA/cm2 to 15.7 mA/cm2. Optical measurements of the nanowire array showed suppressed reflectance when the devices were bent into a concave up orientation. Geometric analysis indicates the change in JSC is due mainly to an effective increase in the nanowire density due to the mechanical manipulation of the nanowires, resulting in enhanced optical absorption throughout the visible spectrum by 10%. When the flexible nanowire devices were bent in a concave down orientation, the JSC and corresponding optical absorption decreased by approximately 8%, due to an effective reduction in the effective nanowire density. In contrast, flexible planar a-Si:H p-i-n solar cells did not display any change in JSC or optical absorption enhancements under mechanical bending. Additional analysis of the effect of mechanical strain on the solar cell performance will also be presented.
9:00 PM - EP4.11.32
Electron-Blocking Properties of Crystalline-Silicon/Cu2O Heterojunctions for Photovoltaics
Pramod Ravindra 1,Sushobhan Avasthi 1
1 Centre for Nanoscience and Engineering Indian Institute of Science Bangalore India,
Show AbstractElectron/hole-blocking layers are known to improve carrier collection in solar cells. Although there are various reports of hetero-junctions of oxides on crystalline-silicon, very few of these have been employed as electron-blocking layers. This is mainly due to non-availability of oxides with small valence band offset and appreciable conduction band offset with silicon. Copper(I) Oxide (Cu2O) is a material that is widely studied as a potential candidate as a solar absorber. It is one of the few known p-type oxides which has a bandgap of 2.25 eV. Reported values for the valence band maximum suggest that there is a very small valence band offset with silicon, while there is a conduction band offset of 1.36 eV. This offset can selectively block electrons while allowing hole transport from Si to Cu2O thereby enhancing carrier collection and open circuit voltage. In this paper, we demonstrate that Cu2O can act as an effective electron-blocking layer for silicon solar cells. Thin films of Cu2O were deposited on n-Si at room temperature by reactive sputtering of copper in presence of oxygen. Phase-pure films of Cu2O were obtained when oxygen concentration was maintained at a small value of 2.53% which is evident from X-ray diffraction patterns. Characteristic Cu+1 peaks were also observed in X-ray Photoelectron Spectroscopy with the absence of satellite peaks which further confirms the formation of Cu2O without any Cu+2 impurities. In this study we present the device characteristics of hetero-junction solar cells fabricated by depositing Cu2O on n-Si. The device structure consists of gold as top electrode and a blanket layer of Aluminium as a bottom electrode to form a Au/Cu2O/n-Si/Al stack. Electrical characterization shows that the device exhibits rectifying behaviour. The diode shows classic photovoltaic properties under AM 1.5 sunlight. Efficient electron-blocking allows the solar cell to achieve an open-circuit voltage of 328 mV. In conclusion, we show that a hetero-junction of Cu2O on n-Si acts as a barrier for electron transport, and that a thin-film of Cu2O can be used as an effective electron-blocking layer in silicon solar cells to improve carrier collection and the open circuit voltage.
9:00 PM - EP4.11.33
A New Dimension in Silicon: Functional Elements Buried in Silicon
Onur Tokel 1,Ahmet Turnali 1,Ihor Pavlov 1,Fatih Ilday 1
1 Bilkent University Ankara Turkey,
Show AbstractFunctional optical and electrical elements fabricated on silicon (Si) constitute fundamental building blocks of Si-photonics and electronics. Available techniques fabricate the optical and electronic elements on the top thin layer of the silicon-on-insulator platform. In spite of the successes of the available lithography techniques, functional elements embedded inside silicon simply does not exist. Here, we present a maskless, one-step laser writing technique for positioning buried functional elements inside Si wafers.
This new phenomenon is separated from the previous work, in that the surface of Si is not modified. By exploiting nonlinear absorption within the focal volume of a tightly focused infrared laser, permanent refractive index changes were induced inside Si through amorphization. The imprinted high-index contrast was then used to demonstrate various functional elements and capabilities inside Si [1]. For instance, we demonstrate the first functional optical element inside Si, i.e., a lens. In addition, we present the first information-storage capability inside Si, creation of high-resolution subsurface holograms that can compete with commercial spatial light modulators, and the creation of complex 3D architectures. We note that using silicon as the active material for these capabilities is fully CMOS compatible for electronics industry.
The advantage of the silicon industry has always been being able to cram more complexity on to the wafer. We have expanded these capabilities by developing controlled multilevel architectures embedded in Si. This new approach takes advantage of the real estate under the Si surface and therefore can pave the way for creating entirely new electronic devices through electronic-photonic integration. In particular, this new capability can potentially be used for the realisation of multilayered Si chips for electronics, silicon-photonics and microfludics/optofluidics experiments. We envisage the presented capabilities as building blocks of a 3D fabrication toolbox for deep silicon engineering.
[1] Tokel et. al., Arxiv.org/abs/1409.2827
9:00 PM - EP4.11.34
A Simple Approach to Grow BaSi2 Thin Film on Foreign Substrates as an Absorber for High-Performance Thin Film Solar Cell
Noritaka Usami 4,Kosuke Hara 4,Yoshihiko Nakagawa 1,Cham Thi Trinh 4,Takamichi Suhara 1,Jefferson Adrian Wibowo 1,Isao Takahashi 1,Takashi Suemasu 4
1 Nagoya University Nagoya Japan,4 CREST, Japan Science and Technology Tokyo Japan,2 University of Yamanashi Kofu Japan,4 CREST, Japan Science and Technology Tokyo Japan1 Nagoya University Nagoya Japan3 University of Tsukuba Tsukuba Japan,4 CREST, Japan Science and Technology Tokyo Japan
Show AbstractOrthorhombic barium disilicide (BaSi2) has been proposed as an altenative absorber for thin film solar cells due to the high absorption coefficient and suitable bandgap (1.3eV) for single junction solar cells. In addition, all the constituent elements are earth-abundant, which could be of great benefits for global deployment. By utilizing a-axis oriented BaSi2 thin film on Si(111) grown by molecular beam epitaxy, attractive properties such as conductivity control by doping, long minority carrier lifetime/diffusion length, electrically inactive domain boundaries, photocurrent under illumination, and so on, have been revealed. As a next step, it is necessary to develop a growth method of high-quality BaSi2 thin film for large-area and low-cost foreign substrates for solar cell applications. The method should be compatible with large-scale manufacturing technologies.
In this contribution, we show that simple vacuum evaporation using BaSi2 granule as a source material could permit to realize BaSi2 thin film on various foreign substrates under appropriate conditions. Careful design of experiments is of crucial importance due to relatively complex elementary process. For example, vapor composition would be Ba-rich especially at the initial stage of evaporation. Therefore, to realize stoichiometric BaSi2, depleting Si must be somehow supplied. When the growth is carried out on Si, Si could be spontaneously supplied from the substrate at sufficiently high grow temperature. As for other substrates, pre-deposition of Si plays an important role in realization of stoichiometric BaSi2. In fact, we have successfully obtained BaSi2 thin film on Si, Ge, SUS304, titanium, SiO2, and so on. In addition, we disclosed that epitaxial growth of BaSi2 by vacuum evaporation is possible on Si(111) by close control of vapor composition and growth temperature. This was achieved by two-step process starting with sublimation of Ba from BaSi2 followed by melting BaSi2 source to provide large vapor flux of Ba-Si. The first step is intended for solid-state reaction of Ba and Si substrate for formation of a-axis oriented BaSi2 template. The second step is designed for epitaxial growth on the template. By applying this process to (111)-oriented polyscystalline Si thin film on SiO2, realization of a-axis oriented BaSi2 thin film on SiO2 is also expected. Our research has shown that BaSi2 with high absorptivity and appropriate band gap can be simply fabricated on various substrates to confirm that BaSi2 is very promising as an alternative absorber for high-performance thin film solar cell.
9:00 PM - EP4.11.35
BaSi2: An Alternative to Silicon for High-Performance Thin-Film Solar Cell
Robin Vismara 1,Olindo Isabella 1,Miroslav Zeman 1
1 Delft University of Technology Delft Netherlands,
Show AbstractReducing material and production costs of silicon solar cells triggered the development of thin-film silicon (TFSi) solar cells. These devices, however, have not been able to compete in performance and in costs with (thin) wafer-based crystalline silicon (c-Si) architectures. The investigation of better performing materials with low manufacturing costs typical of TFSi solar cells is therefore of great interest. In this respect, orthorhombic barium di-silicide (BaSi2) has appealing opto-electrical properties and is an abundant and inexpensive material.
In this contribution we show how the optical performance of thin-film devices with a BaSi2 absorber compares to equivalent architectures based on nano-crystalline silicon (nc-Si:H). The analysis is carried out by means of 3-D optical modelling. Simulations were based on spectroscopic ellipsometry and reflectance-transmittance characterization of BaSi2 layers sputtered on c-Si substrate carried out at University of Tsukuba (Japan). Combining our measurement results with previously reported findings, we could obtain the complex refractive index and the band-gap energy (Eg = 1.25 eV) of BaSi2. The absorption coefficient (α) of BaSi2 was found to be several times higher than the one of nc-Si:H (e.g. at E – Eg = 0.5 eV, αBaSi2 = 4.6 x 104 cm-1 and αnc-Si:H = 0.1 x 104 cm-1 for BaSi2 and nc-Si:H, respectively). This is in accord with prediction by Kumar et al.
In our previous work, we showed that decoupled front-back periodic textures applied to nc-Si:H single-junction solar cells can exhibit an optical performance exceeding the well-known 4n2 absorption limit. This light trapping scheme consists of sharp pyramids at the front side of the device for light in-coupling and shallow pyramids at the back side for light scattering of (near) infra-red photons. When applied to solar cells endowed with 2-μm thick nc-Si:H absorber, this scheme could deliver a record potential implied photocurrent density (Jph) of 36.0 mA/cm2. The same cell configuration was used in this work but substituting the nc-Si:H absorber with BaSi2. Our results determined a Jph of 41.1 mA/cm2. This value is very close to the 4n2 limit calculated for a 2-μm thick BaSi2 layer (41.5 mA/cm2) and significantly higher than the Jph found in the nc-Si:H case. In addition, we simulated other BaSi2 devices with the same configuration (layers and textures), but different absorber thicknesses and for several angles of incidence. Jph’s equal to 38.8 mA/cm2 and 40.5 mA/cm2 were found for 330-nm and 1-μm thick absorbers, respectively, and for perpendicular incidence. These values are either very close or higher than the short-circuit current density achieved by state-of-the-art c-Si device realised in hetero-junction configuration (39.5 mA/cm2) but employing a much thicker absorber (~100 μm). For its high absorptivity, BaSi2 can constitute an attractive absorber for high-performing thin-film and multi-junction solar cells.
9:00 PM - EP4.11.36
Lower Temperature Thermal Silicon Nitride ALD on Si0.5Ge0.5(110)
Steven Wolf 1,Mary Edmonds 1,Tyler Kent 1,Daniel Alvarez 2,Andrew Kummel 1
1 University of California San Diego La Jolla United States,2 Rasirc, Inc San Diego United States
Show AbstractSilicon nitride deposition on semiconductor surfaces can serve as an etch stop layer, a double patterning layer, or function as a diffusion barrier or channel passivation layer prior to dielectric deposition. Previous work demonstrated stoichiometric Si3N4 ALD growth on Si(100) by long half cycle reactions of N2H4 and Si2Cl6 at temperatures in excess of 350°C with solid ammonium chloride by-product formation1. These films contained oxygen due to the hydrazine source containing residual water. This study focuses on developing a low temperature fast silicon nitride ALD process free of solid by-product formation and film contamination. The ALD process produces an HCl(g) desorption byproduct, but other solid by-products, are avoided by heating the chamber to 125°C to eliminate precursor wall condensation and reactions. Anhydrous hydrazine was employed for ALD growth in order to create high quality, surface passivating SiNx films free of oxygen contamination. STM, STS, and XPS measurements are employed to characterize the ALD process at 275°C and 350°C.
A p-type Si0.5Ge0.5(110) surface was cleaned ex-situ via a wet organic clean consisting of sequential sonications in acetone, IPA, and water before dipping into a 2% HF/water solution. The sample was placed in toluene before being rapidly loaded into the UHV chamber where an 1800 Langmuir atomic hydrogen clean at a substrate temperature of 330°C was performed to remove carbon from the surface. A 400 MegaLangmuir anhydrous hydrazine dose was completed with subsequent XPS showing N-Hx surface termination. Twenty silicon nitride ALD cycles were performed at 275°C consisting of 13.5 MegaLangmuir Si2Cl6 followed by 20 MegaLangmuir hydrazine. XPS confirmed uncontaminated silicon nitride film growth with an increase in the Si 2p and N1s signals, and no increase in C 1s and O 1s peaks. Saturation hydrazine and Si2Cl6 pulses for ALD growth on SiGe(110) were performed at 275°C and 350°C in order to optimize the ALD growth processing parameters necessary for producing smooth, uniform, and electrically passivating films. STM measurements show 20 cycles of SiNx growth at 275°C on SiGe(110) produces uniform surface coverage. STS measurements show 20 cycles of SiNx growth at 275°C on SiGe(110) leaves the surface Fermi level unpinned and near the valence band edge consistent with the surface being electrically passivated.
1. S. Morishita et. al., Appl. Surf. Sci., 112, p:198-204 (1997).
9:00 PM - EP4.11.37
Record Low Reflectivity for Silicon in High Lifetime Tapered Microwire Arrays for High Efficiency Photovoltaics
Sisir Yalamanchili 1,Hal Emmer 1,Christopher Chen 1,Nathan Lewis 1,Harry Atwater 1
1 California Institute of Technology Pasadena United States,
Show AbstractWe demonstrate the lowest reported reflectance yet achieved for high quality Si structures in tapered microwires with microsecond minority carrier lifetimes. We show that inductively coupled plasma reactive ion etching (ICPRIE) followed by damage removal etching by KOH provides a way to fabricate microstructures that form tapered high aspect ratio Si structures. When coated with 200nm thick silicon nitride, these structures show an angle averaged reflectivity as low as 0.97% over wavelengths from 400-1100 nm and from 0 to 50 degree incidence angle. We show that these structures can be embedded in a polymer, peeled off the substrate, and contacted on the rear with a reflecting film to realize flexible, free standing, highly absorbing films. The effective thickness of silicon in these films is less than 20 microns, and normal incidence absorption is as high as 86.4% under AM 1.5 solar spectrum. We also show that tapered microwire structures etched in a silicon wafer with >1ms minority carrier lifetime not only absorb light strongly but also show minority carrier lifetimes as high as 1.9 microseconds (diffusion lengths ~75 micron), as measured by microwave detected photoconductivity decay. We report carrier lifetimes in tapered microwires under various liquid and solid surface passivation techniques including solution passivation methods such as concentrated Hydrochloric acid, dilute and concentrated Hydrofluoric acid, Iodine in ethanol, and benzoquinone in methanol; and solid passivation methods such as intrinsic amorphous silicon (i-a-Si) deposited by plasma enhanced chemical vapor deposition (PECVD), gallium phosphide deposited by metal organic chemical vapor deposition (MOCVD), silicon nitride deposited by PECVD, and aluminum oxide deposited by atomic layer deposition with surface prepared using damage removal by varying concentrations of potassium hydroxide and the etching times. We find that concentrated hydrochloric acid exhibited the best surface passivation in solution, whereas i-a-Si deposited by PECVD exhibits superior stable passivation under ambient conditions. We discuss the role of surface recombination center passivation in achieving high lifetimes and long diffusion lengths, along with projected efficiency estimates for solar cells fabricated using tapered Si microwires.
9:00 PM - EP4.11.38
Si Radial p-i-n Heterojunction Nanowires for Photovoltaic Applications
Jinkyoung Yoo 1,J. Baldwin 1,Paul Schuele 2,David Evans 2
1 Los Alamos National Laboratory Los Alamos United States,2 Sharp Laboratories of America Camas United States
Show AbstractSemiconductor nanowires have not utilized their full potential for photovoltaics despite of enhanced light absorption due to diffuse scattering and efficient photogenerated carrier collection through ultrafast carrier separation. In terms of the key metrics of photovoltaic performance semiconductor nanowire photovoltaics has been hindered by low open-circuit voltage which could be resulted from dominant surface recombination of photogenerated carriers. Surface passivation has been considered as a decisive solution though passivation of nanowire surfaces has not been thoroughly studied.
We present formation of Si radial p-i-n amorphous/crystalline heterojunction nanowires and their physical characteristics. Si radial p-i-n junctions consisted of core Si nanowires and Si shells. Core Si nanowiress were prepared by Au-catalyzed vapor-liquid-solid growth via chemical vapor deposition (CVD). Dimensions and electrical doping profiles of Si shells were controlled by low-pressure CVD growth. According to our study radial shell formation in sub-micrometer scale shows different behaviors from conventional planar thin film growth. Optical absorptance of Si radial p-i-n heterojunction wires and minority carrier transport in a single radial heterojunction were investigated by reflectance measurements, finite domain time difference calculation, and electron beam induced current microscopy.
We demonstrate Si nanowire solar cells prepared on stainless steel substrate. Moreover, a solution to prevent Si nanowire solar cells from unintentional contamination through interdiffusion of metal entitites from stainless steel to nanowires will be discussed.
9:00 PM - EP4.11.39
Improving the Performance of Graphene/Bulk-Silicon Schottky-Junction Photodectors by using Silicon Quantum Dots
Ting Yu 1,Feng Wang 1,Lingling Ma 1,Deren Yang 1,Yang Xu 1,Xiaodong Pi 1
1 Zhejiang University Hangzhou China,
Show AbstractGraphene/bulk-Si Schottky-junction photodetectors have offered a broad spectral photoresponse in the UV-NIR range. But their key figures of merit such as responsivity, detectivity and response time are relatively mediocre.1 In our work we develop novel hybrid silicon-quantum-dot/graphene/bulk-silicon (Si-QD/Gr/bulk-Si) Schottky-junction photodectors. Si QDs are initially hydrosilylated with 1-dodecene.2 They are then dispersed in solvent. The resulting Si-QD ink is subsequently cast onto graphene to form porous Si-QD films. We find that porous Si-QD films with anti-reflection effect increases the light absorption of photodetectors. Charge transfer between Si QDs and graphene raises the Schottky–barrier height. Therefore, the Si-QD/Gr/bulk-Si photodetectors show excellent responsivity and detectivity in the wide spectral range of 300 - 1000 nm. It is found that the Si-QD/Gr/bulk-Si photodetectors may work at zero bias because of their photovoltaic effect. Their response time may be rather short (e. g., 25 ns).
9:00 PM - EP4.11.40
GaP Passivation for Silicon Photovoltaic Devices
Chaomin Zhang 1,Jongwon Lee 1,Stanislau Herasimenka 1,Nikolai Faleev 1,Christiana Honsberg 1
2 Arizona State University Tempe United States,1 Solar Power Lab Tempe United States,
Show AbstractCarrier selective contact (CSC) on Silicon substrate has been scrutinized to develop high efficiency heterojunction Silicon solar cells. Gallium Phosphide (GaP) is an excellent candidate as CSC for Silicon [1]. However, directly grown GaP layer on silicon has difficulty to offer good passivation impacts, as a result it shows low minority carrier lifetime and low implied open circuit voltage (iVoc), which indicates that there are high interface traps or defects in the GaP/Si interface. An approach to achieve good passivation effect for GaP/Si structure is utilizing the intermediate layers on silicon surface before GaP deposition such as chemically grown ultra-thin (1~2nm) wet-oxide. The wafers demonstrated with structure a-Si/wet-oxide/c-Si/wet-oxide/a-Si are annealed in tube-furnace at 850 oC in the nitrogen atmosphere and it shows good passivation (1.8ms lifetime). After that, GaP is grown on the doped a-Si and it shows good electrical properties. Instead of developing a-Si, in this work, GaP is symmetrically deposited on wet-oxide layers and then annealed in the furnace. 50nm GaP was deposited via molecular beam epitaxy (MBE) on both sides of the Silicon (n-type, p-type and high resistance) wafers coated by wet-oxide, forming symmetrical GaP/wet-oxide/c-Si/wet-oxide/GaP structure. Much improvement in passivation effect is observed after thermal treatment in the tube furnace. The passivation effect from different doping levels and varied growth rates of GaP has been also investigated. In the MBE chamber, by increasing the P flux exposure time on Silicon surface before GaP deposition, the passivation result shows getting better, and it suggests that P from GaP layers or interface may play a critical role in this passivation impact. Furthermore, increasing annealing temperature (850 oC to 950 oC) can also boost minority carrier lifetime. Promising results that 650mV implied Voc from this structure with p-Si substrate have been achieved. The passivation performance could be further improved by optimizing both GaP deposition and furnace annealing conditions. In addition, TEM and SIMS will be applied to investigate the passivation mechanisms from this structure.
[1] Y. Liu, et al., Phys Chem Chem Phys 16 (29), 15400-15410 (2014).
9:00 PM - EP4.11.41
Amorphization and Nanocrystallization of Silicon under Shock Compression
Shiteng Zhao 1,Bimal Kad 1,Eric Hahn 1,Marc Meyers 1
1 University of California, San Diego La Jolla United States,
Show AbstractHigh-power, short-duration, laser-driven, shock compression and recovery experiments on [001] silicon unveiled remarkable structural changes above a pressure threshold. Two distinct amorphous regions were identified by high resolution transmission electron microscopy: (a) a bulk amorphous layer near the surface and (b) directional amorphous bands initially aligned with {111} slip planes. Further increase of the laser energy leads to the re-crystallization of amorphous silicon into nanocrystals with high concentration of nano-twins. This amorphization is produced by the combined effect of high magnitude hydrostatic and shear stresses under dynamic shock compression. Shock-induced defects play a very important role in the onset of amorphization. Molecular dynamics simulation corroborates the amorphization, showing that it is initiated by the nucleation and propagation of partial dislocations. This methodology can be potentially used to fabricate amorphous/nanocrystalline silicon based devices.
9:00 PM - EP4.11.42
Strong Sub-Band Gap Absorption from Gold Hyperdoped Silicon beyond Cellular Breakdown
Wenjie Yang 1,Lachlan Smillie 1,Austin Akey 2,Michael Aziz 2,Jim Williams 1
1 Department of Electronic Materials Engineering, Research School of Physics and Engineering Australian National University Acton Australia,2 John A. Paulson School of Engineering and Applied Sciences Harvard University Cambridge United States
Show AbstractA Si-based photodetector that exhibits strong sub-band gap optical absorption in the mid infrared was recently demonstrated using Au hyperdoped Si synthesized by ion implantation and pulsed laser melting (PLM) [1]. The absorption increases monotonically with Au dose but further improvement was thought to be impeded by cellular breakdown at high Au concentrations- a phenomenon in which the resolidified Si shows characteristic impurity-rich ‘cell-walls’ that are expected to contain precipitated Au [2].
In this study, high resolution Rutherford backscattering spectrometry combined with ion channeling (RBS/C) has been used to analyse the atom location of supersaturated Au in the Si lattice as a function of Au dose following PLM. At Au concentrations below those producing cellular breakdown, the Au retained below the Si surface is more than 80% substitutional in the Si lattice. Scanning electron microscopy (SEM), cross-sectional transmission electron microscopy (XTEM) and energy dispersive spectroscopy (EDS) have been used to examine the Au-Si morphology. Although characteristic cell-like structures are observed on the surface there is no evidence for Au precipitation after cellular breakdown, and the Au appears to be ‘dissolved’ in the Si at local concentrations as high as 10 atomic %. This Au-Si behaviour following PLM appears to be analogous to the behaviour of a high Co content Si:Co system studied by atom probe tomography [3]. Beyond cellular breakdown, we have observed strong infrared optical absorption (up to 15% at 1300 nm)- a result that is encouraging for the fabrication of an efficient Si-based mid-infrared photodiode.
1. Mailoa, J.P., et al., Room-temperature sub-band gap optoelectronic response of hyperdoped silicon. Nature Communications, 2014. 5: p. 3011.
2. Recht, D., et al., Supersaturating silicon with transition metals by ion implantation and pulsed laser melting. Journal of Applied Physics, 2013. 114(12): p. 124903
3. Akey, A.J., et al., Single-Phase Filamentary Cellular Breakdown Via Laser-Induced Solute Segregation. Advanced Functional Materials, 2015. 25(29): p. 4642-4649.
9:00 PM - EP4.11.43
Investigation of Molybdenum Oxysulfides Potential as Contacts for Silicon Solar Cells
Laura Ding 1,Steven Harvey 2,Glenn Teeter 2,Mariana Bertoni 1
1 Arizona State University Tempe United States,2 National Renewable Energy Laboratory (NREL) Golden United States
Show AbstractTwo principal sources of efficiency loss in amorphous silicon/crystalline silicon heterojunction solar cells are parasitic light absorption and carrier recombination, both related to the properties of the contact materials interfacing directly crystalline silicon. Therefore, one approach in boosting silicon cell performance tackles the implementation of novel wide-bandgap selective carrier-transport contact. Recently, oxygen-deficient molybdenum oxide (MoOx, xInterestingly, MoOx can be used as precursor in the synthesis of molybdenum disulfide (MoS2) [2]. Metal sulfides exhibit an interesting layered crystalline structure that enables enhanced transport properties, and some particular metal sulfides (e.g. CdS) are widely integrated as buffer material in many thin-film PV devices. While CdS has never been used in silicon cells, the passivating properties of sulfur have been documented [3]. The transition from MoOx to MoS2 by sulfidation can be tuned to the various degree of sulfur incorporation, but little is known on the opto-electrical properties of these mixed oxysulfides (MoOySz), the possibility for band gap engineering and the passivation benefits of S incorporation.
In this contribution, we investigate MoOySz films of various stoichiometry obtained by sulfidation of the oxide from solid sulfur source. We utilize a combination of X-ray photoelectrons—dark and operando— spectroscopy, optical and compositional analytical techniques to investigate the relationship between composition-band structure and band alignment on silicon. The direct silicon/metal oxide interface is further analyzed by quasi-steady state photoconductance decay and surface photovoltage spectroscopy to assess the quality of surface passivation. Finally, we construct full device structures for the most promising composition.
[1] J. Geissbuhler et al., Applied Physics Letters 107, 081601 (2015).
[2] T. Weber, et al., J. Phys. Chem.,100, 14144-14150 (1996).
[3] Y. M. Ali, et al., Journal of applied physics, 101(10), 103708-103708 (2007).