Symposium EQ06—Surfaces and Interfaces in Electronics and Photonics

For electronic circuit fabrication, deposition and etching are commonly performed as individual processes in separate reactor systems. Several vapor/surface exchange and conversion reaction mechanisms are known where material deposition simultaneously liberates another volatile species at the surface deposition site. In contrast, surface reactions that achieve simultaneous delocalized deposition and etching in neighboring regions on a patterned surface are not well known. We have recently discovered that at low temperature (<400°C) using a single set of vapor phase reactants, it is possible to achieve deposition in a desired growth region while simultaneously etching a neighboring region on a patterned film surface. As a primary example, we find that at 220°C, atomic layer deposition (ALD) of tungsten can proceed simultaneously with chemical vapor etching (CVE) of TiO₂. For a range of materials and process conditions, thermodynamic modeling confirms that deposition and etching are both energetically favorable. Akin to high temperature selective epitaxy reactions, the resulting net deposition is inherently self-aligned with the pre-patterned starting surface because the etching reaction locally consumes the deposition reactant, thereby avoiding unwanted nuclei. Using simultaneous deposition and etching, we show area-selective deposition of tungsten on nanopatterned surfaces with 200 nm half-pitch. Thermodynamic modeling and initial experimental results show that the concept extends to a range of other material systems, indicating that simultaneous deposition and etching provides opportunities for low temperature bottom-up self-aligned patterning for electronic and other nanoscale systems.

Area-selective deposition (ASD)-driven processes gained recently a lot of attention from the microelectronics industry as a potential solution for the issues associated with top-down pattern formation at the nanoscale. However, the defectivity of typical ASD processes relying on delayed nucleation on a non-growth area is one of the key concerns for the adoption of this technology. Among the strategies addressing the defectivity issue, repetitive refreshment of the non-growth surface and selective etching of undesired nuclei are particularly promising.

This work is focused on the ASD of TiO₂ and TiN deposited by atomic layer deposition (ALD). In this respect, as an ALD inhibition route, we have been investigating both halogen plasma surface modification of amorphous carbon materials (non-growth area) and methyl-terminated monomolecular organic films on SiO₂ (non-growth area) substrates. On one side, on a-C non-growth area, we examined the possibility to combine the two defect-reduction strategies by employing low power Cl₂ or CF₄ plasmas for both surface functionalization/re-functionalization and for removal of nuclei. We employed prototypical water-based metal oxide process such as ALD TiO_x (TiCl₄/H₂O) to demonstrate area-selective deposition on top of silicon oxide using both blanket films and 45-60 nm half-pitch patterns of a-C. At first, the plasma parameters were optimized to minimize etching of a-C and silicon oxide. Then the selectivity of the ALD processes under study was checked on the plasma halogenated a-C films using a standard ALD sequence and a sequence interrupted by the appropriate plasma treatment steps. On the other side, self-assembled monolayers (SAMs) derived from octadecyltrimethoxysilanes allow ASD of >10nm of TiN (TiCl₄/NH₃) at temperatures as high as 325^○C. Patterning of SAMs has been deployed to demonstrate successful selectivity and ASD surface dependency.

Special attention in this work is paid to the defectivity analysis of patterned a-C, which typically has surface composition deviating from that of the original coating as a result of employed dry etching. The efficiency of the proposed defect elimination strategy and its impact on resulting pattern quality are assessed using advanced wafer-level metrology including scatterometry and XPS.

Atomic layer deposition (ALD) is an essential tool in semiconductor device fabrication that allows the growth of ultrathin and conformal films to precisely form heterostructures and tune interface properties. The self-limiting nature of the chemical reactions during ALD provides excellent control over the layer thickness and film properties. However, in contrast to idealized growth models, it is experimentally challenging to create continuous monolayers by ALD because surface inhomogeneities and precursor steric interactions result in island growth during film nucleation. Thus, the ability to create closed monolayers by ALD would offer new opportunities for controlling interfacial charge and mass transport in semiconductor devices, as well as for tailoring surface chemistry. Here, we report full encapsulation of c-plane gallium nitride (GaN) with an ultimately thin (~3 Å) aluminum oxide (AlO_x) monolayer, which is enabled by the transformation of the GaN surface oxide into AlO_x using a combination of trimethylaluminum (TMA) deposition and hydrogen plasma exposure.[1] The introduction of ultrathin AlO_x significantly modifies the physical and chemical properties of the surface, decreasing the work function and introducing new chemical reactivity. The AlO_x-terminated surface strongly reacts with phosphonic acids under standard conditions, which we exploited to create functional self-assembled monolayers with densities approaching the theoretical limit. Moreover, we show that surface band bending of n-type GaN is reduced by over 0.4 eV following the transformation of the gallium oxide layer into AlO_x. As a consequence, we achieved a low contact resistance and Ohmic behavior before annealing of titanium-contacted n-doped GaN with interfacial AlO_x. Given the high reactivity of TMA with surface oxides, the presented monolayer AlO_x deposition scheme likely can be extended to other dielectrics and III-V-based semiconductors, with significant relevance for applications in optoelectronics, chemical sensing, and (photo)electrocatalysis.
[1] Henning, A.; Bartl, J. D.; Zeidler, A.; Qian, S.; Bienek, O.; Jiang, C.-M.; Paulus, C.; Rieger, B.; Stutzmann, M.; Sharp, I. D. Adv. Funct. Mater. 2021, 31, (33), 2101441.

The passivation of Ge and SiGe surfaces has been a long-standing challenge. Currently this challenge has become extra relevant due to the application of (Si)Ge as channel materials for the newest gate-all-around field-effect transistors^[1], and also regarding the interest in Ge and SiGe as light emitting and absorbing media for silicon photonics^[2]. These emerging applications put various demands on the materials, thicknesses and process conditions that can be used to provide the required level of surface passivation to enable satisfying device performance. It is therefore relevant to develop an extensive toolbox of passivation materials and techniques to meet these demands.
In this work we discuss our latest findings about several effective passivation schemes for Ge including atomic layer deposited (ALD) Al₂O₃, plasma enhanced chemical vapor deposited (PECVD) a-SiN_x:H, and a-Si:H/Al₂O₃ stacks. We have determined the surface recombination velocity (S_eff), fixed charge density (Q_f), and in some cases also the interface defect density (D_it) and an estimation of the capture cross-sections of electrons and holes. With these parameters we were able to determine the passivation potential of each of these schemes and the type of application for which they are suited best. Moreover, we have visualized several of the investigated interfaces by cross-sectional TEM and we have linked our findings on Ge to earlier findings on Si surfaces. Key achievements of our work include a surface recombination velocity on Ge as low as 2.7 cm/s and a negative fixed charge density as high as 9x10¹² cm^{-2 [3]}.
[1]. P. Ye, T. Ernst, and V. M. Khare: The Last Silicon Transistor: Nanosheet devices could be the final evolutionary step for Moore’s Law. IEEE Spectr. 56(8), 30 (2019).
[2]. E. M. T. Fadaly, A. Dijkstra, J. R. Suckert, D. Ziss, M. A. J. Van Tilburg, C. Mao, Y. Ren, V. T. Van Lange, K. Korzun, S. Kölling, M. A. Verheijen, D. Busse, C. Rödl, J. Furthmüller, F. Bechstedt, J. Stangl, J. J. Finley, S. Botti, J. E. M. Haverkort, and E. P. A. M. Bakkers: Direct-bandgap emission from hexagonal Ge and SiGe alloys. Nature 580, 205 (2020).
[3]. W. J. H. Berghuis, J. Melskens, B. Macco, R. J. Theeuwes, L. E. Black, M. A. Verheijen, and W. M. M. (Erwin) Kessels: Excellent surface passivation of germanium by a-Si:H/Al₂O₃ stacks. J. Appl. Phys. 130(13), 135303 (2021).

To enable p-type metal oxide semiconductors that are competitive with their n-type counterparts, numerous material systems and synthesis strategies have been explored. Cuprous oxide (Cu₂O) has been identified as a promising p-type oxide material for functional devices due to its high mobility (> 100 cm²/Vs^-1)¹ and moderate direct bandgap (2.1-2.6 eV).^1,2 However, Cu₂O often suffers from interfacial defects in solar cells, high contact resistance in thin film transistors, and poor stability in photoelectrochemical cells. Therefore, new synthesis strategies for high-quality p-type Cu₂O requires atomically-precise control of film composition, structure, and morphology. Among the various deposition methods, atomic layer deposition (ALD) facilitates conformal deposition with sub-nm precision in thickness and composition, allowing for atomically-precise engineering of surface and interfacial properties.

We have previously introduced a new process for plasma-enhanced ALD (PE-ALD) of CuO_x films with tunable phase, oxidation state, and morphology.³ This is facilitated by alternating exposures of oxygen and hydrogen plasma, which allows for sequential oxidation and reduction half-reactions, respectively. The resulting p-type Cu₂O films were incorporated into bottom-gate TFTs, and on/off current ratios of ∼10⁵ were achieved for the films with the largest Cu(I) fraction and largest grain size.

One way to tune the structural, optical and electronic properties of semiconductors is through alloying.⁴ Here, we introduce a solution anion exchange step after the ALD process, which facilitates substitutional incorporation of sulfur into the oxygen sublattice. This novel approach to fabricate oxysulfide films allows for tunable stoichiometry and phase, while circumventing the need for flammable and toxic H₂S gas in the ALD process. By varying the solution molarity and time, the sulfur incorporation in the CuO_x films were tracked using both grazing-incidence XRD and XPS depth profiling. To describe this ion exchange process, a diffusion model was developed, which enables the prediction and control of the sulfur profile within the film. In addition to compositional control, the film morphology was also observed to evolve as a function of solution concentration, time, and temperature, which was quantified using AFM.

To demonstrate the power of this integrated PE-ALD + sulfur anion exchange process, conformal coating of 3-D nanowire architectures was demonstrated, generating core-shell structures with tunable shell thickness, composition, and structure. Furthermore, area-selective anion exchange was demonstrated, illustrating the potential of this simple and scalable process to pattern surface and interfacial chemistry in layered devices. Finally, electrical measurements after sulfur anion exchange chemistry indicate a reduction in the sheet resistance of the starting Cu₂O phase by five orders-of-magnitude, signifying the potential of this integrated approach to tune interfacial resistance in electronic devices.

Works Cited:

1. Z. Wang, P.K. Nayak, J.A. Caraveo-Frescas, and H.N. Alshareef, Advanced Materials 28, 3831 (2016).
2. Y. Wang, P. Miska, D. Pilloud, D. Horwat, F. Mücklich, and J.F. Pierson, Journal of Applied Physics 115, 073505 (2014).
3. Lenef, Julia D., Jaesung Jo, Orlando Trejo, David J. Mandia, Rebecca L. Peterson, and Neil P. Dasgupta. The Journal of Physical Chemistry C 125, 9383 (2021).
4. Stevanović, Vladan, Andriy Zakutayev, and Stephan Lany. Physical Review Applied 2, 044005 (2014).

The low-temperature (< 400 ^○C) deposition of polycrystalline AlN and GaN films on insulators such as SiO₂, SiN, and SiC is demonstrated by atomic layer annealing (ALA) using tris(dimethylamido) aluminum (TDMAA) or tris(dimethylamido) gallium (TDMAG) and anhydrous N₂H₄ with a rare gas plasma treatment utilizing a RF bias to tune the ion energy. ALA is a variant of ALD in which after the TDMAA/TDMAG and N₂H₄ are dosed, a pulse of low energy inert ions is used to bombard the growth surface to induce crystallinity in each ALD cycle. While on conducting substrates a simple DC bias can be employed, this is not effective or can result in dielectric breakdown on insulating substrates. In addition, for deposition of AlN, the DC bias is less efficient for thicker film growth. To overcome these issues, RF bias was employed which allows high quality polycrystalline AlN and GaN deposition on insulators at low temperature. Using TDMAA and N₂H₄ + Ar⁺ or Kr⁺ high-quality AlN films are deposited with large grain size and low oxygen/carbon contamination which can be used as a templating layer for further high speed AlN film growth via sputtering. While ALD of GaN has been reported in the literature, the most common deposition techniques are MOCVD and MBE at high temperatures >700 ○C. Using ALA, the deposition of polycrystalline gallium nitride at a reduced temperature, 250 ^○C, was demonstrated. For optimal RF bias, the films displayed strong (002) crystallinity, an average crystallite size of 11.4 nm, and near bulk density. Application for low temperature AlN ALA deposition on insulators include templating of AlN heat spreaders

We report on the development of conformal and functional nanoparticle (NP)-based coatings, with flexible chemical modification, applied to the surface of mid-infrared (MIR) waveguide (WG)-based sensors, to simultaneously achieve sensitivity and selectivity enhancement. In the past decade, increased focus has been placed on miniaturization of the conventional bench top MIR spectroscopic instrumentation into on-chip WG-based sensing systems. However, achieving high sensitivity in small form factor portable devices remained a challenge. One way to enhance detection sensitivity is to use surface coatings to concentrate gas molecules within the evanescent field (EF) of a WG. Nevertheless, commonly designed coatings cannot be seamlessly integrated within the WGs due to their tendency to aggregate, which can form micron thick coatings in the case of traditional inorganic porous materials. In addition, these commonly designed coatings have limited ability for hosting analyte molecules close to the EF due to the lack of inherent porosity and induced spectral overlap due to similar functional groups to those in volatile organic compounds (VOCs) in the case of organic polymeric coatings. Here, we use conformal coatings on the WGs, which can be deposited at nanometer-scale controlled thicknesses, have high surface area readily available for adsorption and concentration of analytes in the vicinity of a WG for improved sensitivity, as well as variable surface chemistry for improved selectivity. The coatings were composed of inorganic NPs, which were deposited on the WGs using our newly developed hybrid deposition technique incorporating a slow and controlled substrate withdrawal speed within the layer-by-layer (LbL) method for controlled NP deposition. Two types of LbL systems were explored: NP/NP and polymer/NP. The NP/NP system consisted of spherical ZnO₂ and SiO₂ NPs with diameters of 4.2 nm and 20 nm, respectively. The polymer/NP system was composed of branched polyethylenimine and mesoporous silica NPs of ~150 nm in diameter and was followed by calcination. We investigated the effect of deposition conditions, such as withdrawal speed and solution pH, on the thickness, porosity and morphology of the coatings. The substrate withdrawal speed was a critical factor, where homogeneous and uniform coatings were only achieved in the convective regime (≤ 0.001 cm/sec). In addition to enhanced detection sensitivity, these coatings also improved the selectivity of the WGs due to the preferential interaction of polar gas molecules, such as acetone and ethanol, with the polar surfaces of the metal oxide NP-based coatings. Moreover, silane functionalization of such inorganic coatings was successfully achieved to enhance the selectivity of gas molecules with different polarities. Using LbL-coated MIR WGs, we show selective and reversible sensing of mixtures of methane, acetone and ethanol using the C-H stretching vibrational bands of these analytes. Such a spectroscopic analyte identification platform can facilitate the development of sensitive, selective and low-cost on-chip sensor technologies for a number of applications including health diagnostics, environmental monitoring and process control.

In this work, we investigate different combinations of Tin-doped Indium Oxide (ITO) and Indium Gallium Zinc Oxide (IGZO) as ultrathin (channel thickness less than 5 nm) channel layer for high performance short channel thin-film transistor (TFTs). The TFTs have been fabricated at low-temperature (<350°C) combining sputter deposition (channel layer) and atomic layer deposition (gate insulator HfO₂), making our fabrication approach compatible with low-thermal budget Cu interconnects for back-end-of-line. Our approach results in significant improvement in TFT performance above our earlier published record results for ~ 30 nm IGZO (channel) and ~ 10 nm HfO₂ (gate insulator). In this study, various ultrathin combinations have been explored, specifically ITO/IGZO heterostructure and superlattice. We have achieved a highest field-effect mobility of ~ 100 cm²V^-1s^-1 for ultra-thin ITO/IGZO channel with thickness of 4 nm. The introduction of a thin-layer ITO significantly suppresses the hysteresis in the device due to compensation of the interfacial/bulk traps. Moreover, the use of ultrathin combination compensates for the expected negative shift in V_th resulting in a V_th ~ 0V. Short channel immunity is observed with significantly low off-state current (<100 fA/µm), low subthreshold swing (SS) of <100 mV/decade and high ON/OFF ratio >10⁸, for 50 nm channel length devices. We also see a significant decrease in SS for the super lattice device. Furthermore, we have also studied ultrathin films of ITO and IGZO in isolation. We have successfully fabricated TFTs with ~ 2 nm ITO films as channel layer with a decent on/off ratio of 10⁶. There is a noticeable degradation in the ON state current as the thickness of IGZO is scaled below 6 nm. For the pure ITO situation, as the thickness is increased, we identify an increase in the ON state current but the Vth become progressively negative with poor SS. However, when we combined them as heterostructure and superlattice, the performance of our ultra-thin hybrid channel layer is comparable to emerging two-dimensional materials and superior to that of existing metal oxides. Moreover, unlike Si, there is no significant degradation in long channel transistor performance as the active layer thickness changes from 33 nm to 4 nm. Our approach combines the high carrier concentration in thin ITO and controlled electron flow through IGZO due to energy barrier formation at interface of these layers, resulting in improved device performance. This unique layered structure offers prospects for future sub 5 nm regime devices for BEOL compatible, advanced CMOS application.

This talk presents current advances in atomic scale processing of function accelerated complex materials and the key challenges and opportunities in the future that can enable novel integrated circuits for commercial and defense applications. Amongst all enablers, plasma processing science, due to its intrinsic nonequilibrium nature, is at the heart of many key technologies and provides breakthrough solutions in the information processing system hierarchy, translating physics to a physically functioning embodiment with enhanced device performance. To realize the full potential of these materials, plasma surface science becomes increasing more important in material synthesis, material processing, interfacial engineering and device embodiment, all requiring atomic level control and precision. In this talk, plasma enhanced atomic layer deposition and etching will be showcased as examples that leverage the fundamental knowledge base in plasma physics and chemistry to address future technological grand challenges in interface engineering.

In situ transport measurements on epitaxial and polycrystalline Rh and Ir layers with thickness d = 5–261 nm are used to quantify the resistivity size effect and evaluate the potential of Rh and Ir as a replacement for Cu in highly-scaled interconnects. Epitaxial Rh(001) and Ir(001) layers grown on MgO(001) have negligible resistance contributions from roughness and grain boundary scattering, allowing direct quantification of electron surface scattering. The measured resistivity ρ vs d data is well-described by the classical Fuchs–Sondheimer model and indicates effective mean free paths λ_eff = 9.5 ± 0.8 and 7.4 ± 1.2 nm for Rh and Ir, respectively. This corresponds to products of the bulk resistivity ρ_o times the mean free path ρ_oλ = (4.5 ± 0.4)×10⁻¹⁶ and (3.8 ± 0.6)×10⁻¹⁶ Ωm² which are 33% and 43% smaller than for Cu, indicating a considerably higher predicted conductivity for Rh and Ir in the limit of narrow wires. Deposition of a Ti liner with thickness d_Ti = 0.1–13.0 monolayers on 10-nm-thick Rh(001) results in a small 5% increase in sheet resistance R_s, considerably smaller than the corresponding 40% and 10% increase for Cu(001) and Co(0001) layers. Similarly, exposure to 0.8 Torr O₂ causes a <5%, 26%, and 22% increase in R_s for Rh, Cu, and Co, respectively, suggesting a negligible effect for Rh but indicating that adatoms on Cu and Co disturb the surface potential, leading to diffuse electron scattering and a resulting resistance increase. A similarly small Δρ/ρ < 7% is measured during air exposure of 10-nm-thick epitaxial layers of electronegative metals including Ru, Rh, Ir, W, and Mo, while Δρ/ρ increases to 11–36% for more electropositive metals including Cu, Ag, Co, Ni, and Nb. These results suggest that the higher charge transfer at surfaces of electropositive metals perturbates the surface potential and causes diffuse electron scattering while specular scattering is retained at surfaces of electronegative metals like Rh and Ir. The resistivity contributions from grain boundary scattering is quantified using 111-textured polycrystalline Rh and Ir layers deposited on SiO₂/Si(001) and Al₂O₃(0001) substrates. The average grain size is varied by changing the deposition temperature and is measured as a function of d by electron backscatter diffraction. Applying a combined Fuchs–Sondheimer and Mayadas–Shatzkes model to the measured ρ vs d yields grain boundary reflection coefficients of 0.41 ± 0.05 and 0.51 ± 0.02 for Rh and Ir. The overall results indicate an excellent resistivity scaling for Rh and Ir and thereby confirm the great promise of Rh and Ir as alternate metals for narrow high conductivity interconnects.

Random strain fields can be formed by interfacing various ceramics with silicon. In this work we report enhanced bandgap emission properties stemming from interfacing CZ-Si with spin-coated silica doped with rare-earth metals. Optical properties was found to be depend strongly on the annealing parameters of the deposited films. In addition, the source of erbium can alter the critical temperature required for optimal light emission. Two fold light emission enhancement was obtained at the bandgap was achieved. In this talk we will present silica coatings synthesized by various catalysts including mixture of acid and base, different erbium doping concentrations, annealing temperatures, and in some samples effect of pumping power on the observed luminescence. A working hypothesis focuses on the inhomogeneous interfacial stresses and strains role on bandgap modulations in a way that favors the radiative recombination of formed carrier pairs. The talk will also share the results of several characterization techniques.

The role of interfacial effects on the properties of material composites cannot be overstated. Atomic patterning of an interface offers potentially new roads forward to customising both traditional material and metamaterial properties. For applications in energy generation and storage, electronic devices, photonics, and thermal management control of these interfacial properties are particularly exciting.

In many of these applications the desired properties for a material are somewhat contradictory. For example an ideal thermoelectric material would have a high electrical conductivity with a low thermal conductivity. However, this combination happens very rarely in nature and most efforts to boost one property have an adverse effect on the other[1]. Combinations of materials into composite structures is a common approach to overcoming these kinds of problems, forming heterostructures or more complex metamaterials which exploit the combination of their constituent material properties or the material geometry itself to overcome limitations[2][3]. While this methodology is reaching maturity, until recently the interface geometry has often been overlooked as a further opportunity to tune properties; effectively, in addition to the choice of materials A and B, the interface geometry brings in a third “material” that can be used to customise properties[4].

We investigate the effect of nanoscale interface patterns in an example silicon/germanium heterostructure. Using density functional theory we demonstrate the effectiveness of three different interface geometries in manipulating the electronic, photonic and thermal properties of the heterostructure. We show how subtle changes in the interface geometry can have dramatic effects on properties such as electron effective mass and optical absorption. A clear example of this is the modification of the optical gap by 31.8% of the abrupt interface value when introducing a chequerboard interface geometry (0.43eV to 0.57eV). This level of control expands the already expansive potential for engineered properties in composite materials, giving potential for improved performance in commonly available materials for a wide range of application.

[1]Advanced Functional Materials, 1903841, 30(8), 2020

[2]Nature volume 499, pages419–425 (2013)

[3]Nature Reviews Physics volume 1, pages198–210 (2019)

[4]Nature volume 427, pages423–426 (2004)

Molecule-based devices may govern the advancement of logic and memory devices for next-generation computers and may be suitable for quantum computation. Molecules are unparalleled nanostructures to serve as device elements because chemists can mass-produce a variety of molecules with unique optical, magnetic, and transport characteristics. However, the biggest challenge is to connect two or three metal electrodes to the Molecule (s) and develop a robust and versatile device fabrication technology that can be adopted for commercial-scale mass production. Utilizing tunnel junction as a testbed for making molecular devices solve many critical problems [1]. We focused on producing magnetic tunnel junction-based molecular devices (MTJMSD). This talk will show that an MTJMSD evolves when molecules are bridged along the exposed side edges of a tunnel junction. With the MTJMSD approach, many unprecedented advantages become available to devise researchers. MTJMSD enables the connection between ferromagnetic electrodes and a variety of molecules with the spin state. For the first time, the MTJMSD approach enabled magnetic measurements and conventional transport studies[1]. Magnetic studies showed that molecules could transform the magnetic[2] and transport properties of the MTJs[3] at room temperature. Molecules' strong impact on ferromagnetic electrodes produced several orders resistance changes at room temperature. An MTJMSD approach is a high yield method and can be mass-produced with the conventional microfabrication tools present in typical labs. Our MTJMSD approach also allows numerous control experiments to provide a deep understanding of device mechanisms. This talk will demonstrate two types of control experiments to prove that we successfully made a molecular device. The first control experiment is the reversible impact of making and braking molecular channels on the overall charge transport. We also performed ferromagnetic resonance before and after damaging the tunnel barrier and damaging the molecular channels by using the plasma. In addition, this talk will discuss how the MTJMSD approach is also suitable for making light sensors, biochemical sensors, as molecules are present in the open region.
1. P. Tyagi, "Multilayer edge molecular electronics devices: a review," J. Mater. Chem., vol. 21, pp. 4733-4742, 2011.
2. P. Tyagi, C. Baker, and C. D'Angelo, "Paramagnetic Molecule Induced Strong Antiferromagnetic Exchange Coupling on a Magnetic Tunnel Junction Based Molecular Spintronics Device," Nanotechnology, vol. 26, p. 305602, 2015.
3. P. Tyagi and E. Friebe, "Large Resistance Change on Magnetic Tunnel Junction based Molecular Spintronics Devices," J. Mag. Mag. Mat., vol. 453, pp. 186-192, 2018.

The active perovskite layer plays a crucial role in influencing the solar cell's efficiency. Device lifetime, charge carrier density, and the existence of trap states are strongly affected by the morphology of the perovskite active layer. Although the active layer is an essential component of the solar cell device, achieving a crack-free perovskite layer is still challenging. Recently triple cation perovskites (TCP) have gained much attention owing to their excellent photovoltaic performance and stability. Antisolvent assisted crystallization method is considered a meaningful way to achieve high-quality TCP films.
In this project, we considered a TCP with Formamidinium, Methylammonium, and Cesium (FAMACs) cations assisted by antisolvent crystallization. Different antisolvents, Isopropyl alcohol, Chlorobenzene, Ethyl acetate (EA), and Toluene were considered to fabricate the perovskite layers. Devices fabricated with EA antisolvent in the volume ratio of 1:4, perovskite to EA, showed high-quality crack-free perovskite films. This ratio also showed an improvement in device power conversion efficiency of 15.45% compared to other devices. Furthermore, we have considered an inverted structure device with PEDOT: PSS as the hole transport layers, which is not a standard structure reported for TCP devices.

Stretchable electronics have great potential in wide-reaching areas including wearables, personal health monitoring, and soft robotics. Many recent advances in stretchable electronics leverage liquid metals, particularly eutectic gallium-indium (eGaIn). A wide range of electromechanical (resistance vs strain) behaviors have been reported, ranging from bulk conductor responses all the way to effectively strain-insensitive responses. However, numerous measurement techniques have been used throughout the literature, making it difficult to directly compare the various proposed formulations. In this talk, we will review the relative change in resistance responses of eGaIn found in the literature, then investigate the electromechanical properties of pure eGaIn, which is supposedly a bulk conductor, using multiple electrical resistance measurement techniques. Our results indicate significant differences between different measurement methods, which can largely be accounted for by correcting for a fixed offset corresponding to the resistances of various parts of the measurement circuits. Even accounting for sources of experimental error, we find the average relative change in resistance of pure eGaIn as a function of strain is lower than predicted by commonly used bulk conductor assumptions. We will also discuss our recent biphasic (solid-liquid) gallium-indium mixture (bGaIn) and its relative strain-insensitivity against neat eGaIn. Collectively, our results point to the importance of experimental design and suggest that there is much to be learned about the electromechanical behaviors of liquid metal-derived stretchable circuits.

HAT-CN organic small molecules is used to shift threshold voltages of both n-MoS₂ field-effect transistor (FET) and p-MoTe₂ FET toward 0 V. It is attributed to the charge transfer between HAT-CN organic molecules layer and TMD channel. It is remarkable result because TMD based FETs have unwanted large threshold voltage. To demonstrate the applicable advantage of such effect, a complementary metal oxide semiconductor (CMOS) is built, and HAT-CN is deposited on the channels. Because V_TH of both n-FET and p-FET in the CMOS are shifted towards 0 V, the performance of CMOS is greatly improved in aspect of voltage gain and power consumption

Optoelectronic devices can improve the quality of life by providing optical communication and the transport of information. In particular, photodetectors (PDs) refer to optoelectronic devices that detect a light signal by converting photons into a precise electrical signal. Recently, several studies have been conducted on flexible PDs to apply the new applications, such as oximeters, flexible cameras, and e-eyes. For mechanical stability, various materials, such as ZnO quantum dots and GaS nanosheets have been applied as flexible PDs. However, those materials have a slower response time and require a complex fabrication process. To overcome these problems, we proposed a controllable method to fabricate ultra-thin copper (Cu)-based metal-semiconductor-metal (MSM) PDs using laser-induced oxidation. Particularly, Cu is low price, excellent electrical conductivity, and its flexibility on flexible substrates, so it is a material suitable for the metal part of the MSM structure. Moreover, Cu can simply be converted to Cu oxide compounds (Cu_xO) using a laser oxidation process, which is possible to fabricate a site-selective oxidation. The Cu_xO is a promising p-type semiconductor with high stability and good photovoltaic characteristics. The structure of Cu-Cu_xO-Cu can improve its rectifying characteristics by providing a wider space-charge region.
However, despite research on various Cu-based PDs, the amount of photocurrent changes according to channel scaling and charge-transport mechanism is still a lack of understanding. In recent years, scanning photocurrent microscopy (SPCM) has been verified as measurement technique to investigate the charge-transport mechanism in semiconductors. Therefore, we utilized to verify how the amount of photocurrent changes according to channel scaling. We confirmed that the photocurrent declined farther from the interface and large number of photo-excited carriers were created near the interface. Also, the photocurrent increases as the length of the Cu₂O decreases. Because the long channel-length devices exhibit more gradual band slopes. In addition, the amount of photocurrent increases as the width of the Cu₂O decreases was confirmed. Because the quasi-fermi level of photo-excited electrons shifts toward the conduction band. Furthermore, bending tests were performed to prove the flexibility of our devices. The bending radius is ~ 4 mm and it was tested at room temperature. Despite various numbers of bending cycles (up to 10,000 times), the resistance of our devices does not change. These results mean that Cu-based MSM PDs have substantial flexibility. Our findings could contribute an important guideline for understanding the mechanisms of other flexible PDs.
Acknowledgments
This work was supported by the National Research Foundation of Korea (NRF) grants funded by the MSIT (2019R1F1A1061883 and 2019M3C1B8090840), by the DGIST R&D Program of the Ministry of Science and ICT (19-CoE-BT-03), and by the Samsung Electronics University R&D program. J.S. and K.K acknowledge the support of the National Research Foundation of Korea (NRF) grant funded by the Korea government (MIST) (2020R1C1C1011219) as well as the research fund (1.190128.01 and 1.210035.01) of UNIST (Ulsan National Institute of Science and Technology). This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

Emerging gallium-based liquid metal (LM) particles are attractive candidates for stimuli-responsive electronics and therapeutics. These core-shell materials consisting of a solid oxide skin and a deformable core can be ruptured to form conductive pathways under an external applied force. While there are many advantages in utilizing the oxide skin for LM processing, the intrinsic properties of the gallium oxide impose limitations on their long-term stability. For instance, the oxide thickness gradually increases when aged under ambient conditions. When exposed to water, gallium oxide hydroxide crystallites form on the LM oxide skin, affecting its mechanical properties while further consuming gallium in the process.¹ Highly acidic or basic environments etch the oxide, causing the LM particles to coalesce into a bulk droplet. The accompanying changes in the LM particle properties are directly correlated to changes in their native oxide, which can either positively or negatively influence the overall material performance. Thus, expanding the capabilities of LM particles beyond the properties of the native gallium oxide shell would offer better stability and tunability of these materials. In this work, we discuss strategies by which to modify the surface properties of LM particles through various non-native coatings. Specifically, we investigate coating these particles with inorganic materials such as silica or 2D graphene oxide, which improve the surface properties of LM particles. These coatings serve as a protective barrier against undesirable environmental conditions (i.e., pH, reactive oxygen species, etc.) while providing additional mechanical stability to LM particles.² Currently, other 2D materials are being explored as surface modifiers for improving the optical response of LM particles. We have also investigated the use of phosphonic acid organic ligands in functionalizing the surface of the LM oxide which can lead to polymerized LM networks or hydrophobic surface properties, depending on the terminal groups chosen.^3,4 By purposefully selecting the materials that mediate the surface interactions of LM particles, we can expand the palette of non-native coatings that enable previously unattainable properties for LM-based responsive, multifunctional electronics.
References:
1. Creighton, Megan A., et al. "Oxidation of Gallium-based Liquid Metal Alloys by Water." Langmuir 36.43 (2020): 12933-12941.
2. Creighton, Megan A., et al. "Graphene-based encapsulation of liquid metal particles." Nanoscale 12.47 (2020): 23995-24005.
3. Farrell, Zachary J., et al. "Route to universally tailorable room-temperature liquid metal colloids via phosphonic acid functionalization." The Journal of Physical Chemistry C 122.46 (2018): 26393-26400.
4. Thrasher, Carl J., et al. "Mechanoresponsive polymerized liquid metal networks." Advanced Materials 31.40 (2019): 1903864.

The very high charge density induced by an electric double layer formed at the solid-liquid interface has recently been used to induced or “gate” exotic phase transitions, therefore electronic ground states of multifunctional oxides in the interfacial region, via the subtle interplay between electrostatic doping (electronic phenomena) and chemical redox effects (field-driven ionic motion) depending on field polarity and defect instability. It is highly expected that leveraging ionic electrolyte gating would be fertile ground for exploration in a broad range of functional oxides.

In this talk, I will present two developing frontiers of ionic electrolyte gating within two contrasting mechanistic frameworks by illustrating most recent in-situ and real-time X-ray studies to deliver fundamental understanding of structural and chemical basis and their inherent links during gating on representative functional oxide heterostructures. In one end, we drive forward the limits of electrochemically emergent transformations by manipulating ionic defects (e.g. vacancy formation and distribution) during gating. For example, a combination of electronic and ionic doping processes across the interface of perovskite nickelate heterostructure (e.g. NdNiO3) by switching between positive and negative ionic gating voltages can be utilized in realizing electrochemical transistors. Moreover, ionic gating process can induce dynamically manipulating oxygen octahedra-controlled properties in the complex oxides (e.g. WO3) for the design of highly responsive multifunctional materials (e.g. MIT and electrochromic behaviors). In the other end, we create a new paradigm of highly efficient ionic gating toward sub-voltage operation regime (e.g. enhancing carrier doping but without defect generation and perturbation across the interface) by designing redox actuatable poly-ionic-liquids.

Printing with organic materials is a versatile approach that can lead to devices that are economical and easily reconfigurable. Here we present the design and additive processing of polymeric materials to achieve robots that harvest light energy for locomotion, and subsequently we also incorporated redox materials in structural materials for energy storage. First, the goal for our robots is to demonstrate self-sustained movement with high load-carrying capacity. We use a biomimetic strategy inspired by anthropods that combine soft actuators with an exoskeleton. The soft actuation components are based on liquid crystal elastomers formed into spring structures by extrusion printing (Advanced Intelligent Systems, 2021, 2100085; Advanced Materials, 2021, 33, 2002541). The exoskeleton structures are printed from polycarbonate with high strength to increase the load-carrying capacity. Leveraging the strength of the exoskeleton and the high stress of the actuator, the robot transports a load 22 times its body weight, by harvesting constant sunlight radiation for continual power.

In the second part of this talk, we examined the integration of redox polymers into supercapacitor structures to serve as energy supplies that will be useful for untethered robots and general wireless electronics. A new Faradaic electrode material comprised of a narrow bandgap donor−acceptor conjugated enhanced charge delocalization and thus improved stability dramatically. The supercapacitors were patterned by electrodeposition and showed a high areal power density of 227 mW/cm², allowing rapid charging and high power output. The capacitance retention was 84% after 11,000 full redox cycles, offering the critical benefit of long cycle life. This work demonstrated the application of a new class of stable redox-active materials suitable to meet the energy storage needs for short-range wireless electronics.

Conventionally seen as detrimental to thin-film transistor high frequency performance, contact effects are increasingly being exploited for superior low-power analog functionality [1,2]. Our previous study [3] demonstrated a convenient route to realizing a contact energy barrier via deposition of a nanometre-scale Al₂O₃ layer between the semiconductor and contact metal. The result is an effective means of inducing a contact barrier, so that source-gated transistor (SGT) operation [4–6] is consistently achieved, even in fabrication processes in which a Schottky contact is difficult to obtain uniformly [1].
Here, we further investigate the operation of such tunnel-contact devices by electrical measurement with varying temperature and TCAD simulation using Silvaco Atlas.
Fabricated devices [3] using IGZO as the active layer, Ni source and drain contacts and a 3 nm Al₂O₃ tunneling layer deposited by atomic layer deposition (ALD) show: low saturation voltage with low gate voltage dependence (~0.14 V/V); tolerance to large variations in channel length (L); and drain current proportional to the source injection area for source lengths in the few-micron range. Hence, the source-gate overlap (S), rather than L, is the design parameter for SGTs [7]. The drain current, controlled by the interplay between the semiconductor, contact metal and nanoscale tunnelling layer, is in the order of nAµm^-1 device width. Such features contribute to very low power dissipation when operating the device in saturation, e.g. as a current source or voltage amplifier. Moreover, the change in current with temperature is found to be 0.044 nAµm^-1K^-1and 0.145 nAµm^-1K^-1 for devices with source S = 9 µm and 45 µm, respectively (V_G = -8.4 V, V_D = 5 V). This is much lower, and with distinctly different characteristics, than in Schottky SGTs, in which shorter S typically produces higher temperature dependence [8].
TCAD simulations confirm tunnel contact device operation as SGTs. Specifically: drain current independence on L; and a relatively large range of S over which drain current depends proportionally on source injection area. This allows for versatile design using S rather than L as a design parameter, promoting increased current uniformity. In Schottky-contact SGTs, the drain current depends profoundly on temperature, especially for small feature sizes [8]. Different source metals were investigated to confirm that operation mainly relies on the tunnelling layer. As the source contact work function reduces from 4.5 eV to 4.03 eV, drain current increases by only one order, significantly less than expected if the Schottky process would dominate. Critically, these tunnel-contact devices show significantly lower temperature dependence, a feature which ultimately simplifies circuit design [8].
With advances in rapid and low-cost fabrication processes for precisely controlled nanoscale films, such as ambient or spatial ALD [9,10], this device architecture shows significant promise for low-power thin-film circuits. This interfacial layer technique may provide increased uniformity and control over the critical effective energy barrier height at the source, in a manner superior to Schottky contacts and further benefiting large-scale manufacturability.

[1] J. Zhang et al., Proc. Natl. Acad. Sci. U. S. A., 116, 4843–4848 (2019).
[2] G. Wang et al., Adv. Sci., 2101473, 1–23 (2021).
[3] R.A. Sporea et al., Adv. Mater., 31, (2019).
[4] J.M. Shannon and E.G. Gerstner, IEEE Electron Device Lett., 24, 405–407 (2003).
[5] J.M. Shannon et al., IEEE Trans. Electron Devices, 60, 2444–2449 (2013).
[6] A. Valletta et al., J. Appl. Phys., 114, 064501 (2013).
[7] R.A. Sporea and S.R.P. Silva, in: Proc. Int. Semicond. Conf. CAS, (2017), pp. 155–158.
[8] R.A. Sporea, M. Overy, J.M. Shannon, and S.R.P. Silva, J. Appl. Phys., 117, (2015).
[9] J. Sheng et al., ACS Appl. Mater. Interfaces, 11, 40300–40309 (2019).
[10] Y.Y. Lin et al., ACS Appl. Mater. Interfaces, 7, 22610–22617 (2015).

Magnetoresistance sensor applications such as automotive, magnetic communication and point-of-care diagnostics require high detectivty in ultra low magnetic fields. Improving the detection limits of magnetic sensors requires high sensitivity as well as reduced noise levels. In this study, a self-balancing bridge PHMR sensor with integrated resistance compensator was devised for low-frequency noise reduction in the frequency range of 0.5 Hz to 200 Hz. The structure of the PHMR sensor used in the self-balancing bridge sensor is a NiFe (10 nm)/IrMn (10 nm) bilayer structure. The proposed resistance compensator integrated with bridge sensor offers a low-cost alternative to offset reduction for marketable MEMS and significantly improves the noise level by adjusting the offset voltage at the wafer level. In addition, the noise components of electronic and magnetoresistive sensors were identified and analyzed by measuring the sensor noise spectral density as a function of temperature and operating power. The lowest achievable noise in the devised structure was estimated to be 3.34 nV /√Hz at 100 Hz.

Biosensors based on electrolyte-gated organic field-effect transistors (EGOFETs) can transform biological signals into electrical signals with a strong amplification [1]. This amplification allows us to detect small variations of voltages produced by (I) electrically excitable cells or (II) the binding of biomolecules [2]. In the second case, the transistor is then functionalized by recognition layers, such as self-assembled monolayers (SAM) and/or antibodies to detect the presence of antigens in the electrolyte [3]. In both situations, the response of the EGOFET is strongly determined by the processes and modifications taking place at the electrolyte/gate and the electrolyte/semiconductor interfaces, as these determine the behaviour of the electrical output. Functionalized layers at these interfaces result in changes in their distributed capacitance and fixed charges. These modify the conductivity of the semiconductor, which leads to a change in the electrical signal (source-drain current) [4]. Even in non-functionalized EGOFETs, the presence of Stern layers at these interfaces can profoundly affect the EGOFET characteristics. To investigate these effects, numerical simulations constitute an excellent tool since they allow isolating the specific effect of the different physical parameters (a charge, specific capacitance, ionic concentration solution).
In the present work, we present the finite-element numerical modelling of EGOFETs in the Nernst-Planck-Poisson and drift-diffusion frameworks, focusing on revealing the role played by interfacial layers on their performance. We predict the transfer and output current-voltage curves of the EGOFETs for different material properties of the semiconducting material, the geometry of the system, the ionic concentration of the electrolyte, but overall, on the properties of the interfacial layers (charge and specific capacitance). Additionally, we also derive the source-gate capacitance-voltage characteristics. For the different situations, we analyze the distribution of charges and potential across and alongside the EGOFET. We examine the formation of the diffusive layers in the electrolyte and its relevance to form the accumulation layer of charge carries in the semiconductor surface. With these studies, we provide further insight into the physics of these devices, including the formation of the pinch-off. The comparison of the results with existing experimental results or theoretical predictions is performed.
[1] Macchia, E., Picca, R.A., Manoli, K., Di Franco, C., Blasi, D., Sarcina, L., Ditaranto, N., Cioffi, N., Österbacka, R., Scamarcio, G. and Torricelli, F., 2020. About the amplification factors in organic bioelectronic sensors. Materials Horizons, 7(4), pp.999-1013.
[2] Kyndiah, A., Leonardi, F., Tarantino, C., Cramer, T., Millan-Solsona, R., Garreta, E., Montserrat, N., Mas-Torrent, M. and Gomila, G., 2020. Bioelectronic recordings of cardiomyocytes with accumulation mode electrolyte gated organic field-effect transistors. Biosensors and Bioelectronics, 150, pp.111844.
[3] Torricelli, F., Adrahtas, D.Z., Bao, Z., Berggren, M., Biscarini, F., Bonfiglio, A., Bortolotti, C.A., Frisbie, C.D., Macchia, E., Malliaras, G.G. and McCulloch, I., 2021. Electrolyte-gated transistors for enhanced performance bioelectronics. Nature Reviews Methods Primers, 1(1), pp.1-24.
[4] Kyndiah, A., Checa, M., Leonardi, F., Millan-Solsona, R., Di Muzio, M., Tanwar, S., Fumagalli, L., Mas-Torrent, M. and Gomila, G., 2021. Nanoscale Mapping of the Conductivity and Interfacial Capacitance of an Electrolyte-Gated Organic Field-Effect Transistor under Operation. Advanced Functional Materials, 31(5), p.2008032.

Ruthenium with a high melting point and a low electrical resistivity is of great potential to replace copper for use as the next-generation interconnect. However, with scaling, the impact of grain-boundary scattering and interface scattering on its resistivity needs to be minimized. Important parameters including grain-boundary reflectivity, r, and interface permeability, p, also need to be carefully examined. Hence in this study, thin ruthenium films with different thicknesses (from 3 to 200 nm) were prepared on two types of substrates (oxide and tantalum nitride) by physical and chemical vapor depositions, followed by thermal annealing at 400°C. The microstructure, composition, electrical resistivity, interfacial bonding configuration and interfacial adhesion strength of the films were characterized. The size effect on the electrical resistivity with scaling, i.e. the prediction of reflectivity and permeability, was investigated by using the FS-MS model. Experimental results indicated that the grain-boundary reflectivity and interface permeability were strongly influenced by the structure and impurity segregation at the grain boundaries and the bonding and adhesion strength of the interfaces, respectively. The physically vapor deposited low-purity ruthenium films with oxygen segregation at the grain boundaries generated a high reflectivity of 0.8, while the chemically vapor deposited high-purity films yielded a low reflectivity of only 0.4. After annealing for defect elimination and structure recovery, the reflectivity decreased to 0.6-0.7 and 0.3, respectively. Moreover, strong chemical bonding between ruthenium and tantalum nitride was found to cause serious interface diffuse scattering (resulted from the complex Fermi surface of tantalum) and thus a low permeability of 0.2 (in comparison, a high permeability of 0.9 for weak bonding between ruthenium and oxide). After annealing to enhance interfacial binding energy, the interface scattering became more serious, and the permeability further slightly decreased to 0.1 and 0.8 for the tantalum nitride and oxide substrates, respectively. The interfacial adhesion strength measured by nanoscratching tests further suggested its inverse proportion to the permeability.

Metal oxide nanoparticles are suitable fillers in low-density polyethylene (LDPE) composites, yielding new insulating materials for high voltage direct current (HVDC) cables. These are envisioned in the underwater transmission of energy from Sahara’s solar cell parks to European cities. These composites have shown a resistivity two orders of magnitude higher than that of ultra-clean LDPE used as insulation in HVDC cables and rated up to 640 kV. Ensuring high insulation resistivity and high breakdown strength, and control of space charge accumulation is one of the main tasks in the future development of HVDC cables. The particular interest in nanoparticles stems from their ability to create a large particle–polymer interface at low particle fractions, which has been emphasized as the key to improved insulation properties. The effects of particle surface lattice termination by zinc or oxygen were in this study evaluated for composites containing micro-sized ZnO particles with rod shapes containing 17% oxygen terminations or ball shapes having 67% oxygen terminations. Both composites containing the micro-sized particles showed a conductivity almost two orders of magnitude lower than that of the LDPE reference material (1.2 × 10⁻¹⁴ Sm⁻¹), although specific differences in the conductive properties were observed. When a silica coating was applied to the particles, the encapsulation eliminated the difference. It also resulted in both cases in an increase in conductivity by one order of magnitude to ca. 2 × 10⁻¹⁵ Sm⁻¹. The 25 nm ZnO nanoparticles, on the contrary, showed a record low conductivity of 1 × 10⁻¹⁷ Sm⁻¹ for 3 wt% nanoparticles with aliphatic hydrocarbon tails. This low conductivity would, in an applied environment (if realized), permit higher operation voltages than any currently employed HVDC cable systems (> 1 MV) and provides, with further development, possibilities to tailor the future dielectric design of cable components.

The electronic structure properties of interfaces are of central importance to photovoltaics. Computational approaches can provide information complementary to experimental characterization. In particular, first principles methods can establish an unambiguous link between atomic structures features and their electronic consequences. This strength is also a challenge, because realistic atomic structure models must be available for such calculations. Only for highly idealized situations, such as the epitaxial interface between two reasonably lattice matched materials with the same crystal structure (coherent interface), can the atomic interface structure be trivially constructed. The situation becomes much more complex for a general interface problem, including highly lattice mismatched systems or materials with different stoichiometries or crystal structures.
We adopted the kinetically limited minimization (KLM) approach, previously developed for unconstrained bulk crystal structure prediction [J Chem Phys 154, 234706 (2021)], to the substrate/thin-film interface problem in slab geometry with vacuum separation. Using density functional theory (DFT) total energy calculations, we sample atomic structures for SnO2/CdTe interfaces. In CdTe photovoltaics, CdCl2 treatment is a necessary process step for acceptable device efficiencies. Therefore, we predict the interface structure both without and in the presence of CdCl2. The simulations treat SnO2 as a substrate with a given crystal and surface structure, whereas for the film (CdTe and CdCl2) there are no initial assumptions about the crystal structure other than choosing suitable coincidence site lattices that can accommodate the CdTe zinc-blende structure with acceptable strain. Besides the unknown interface structure, the well-established CdTe bulk and back-surface structures are correctly reproduced by the algorithm. The addition of CdCl2 results in improved bonding and periodicity, strongly reduces the total interface energy. Electronically, the CdCl2 treatment results in an almost perfect conduction band alignment and dramatically reduces the defect density of states in the CdTe band gap, highlighting its beneficial effects for solar cell performance.

We recently reported carrier-resolved photo Hall (CRPH) effect technique¹ that enables us to extract remarkable amount of information of majority and minority carrier in semiconductors under light illumination such as their mobilities, photo-carrier density, recombination lifetime and diffusion lengths. This new technique extends the capability of the original Hall effect that only reveals majority carrier information (type, density and mobility). The CRPH technique also presents a more unified understanding in extracting charge carrier information from electronics materials combining electric field, magnetic field and light excitation. This technique has been applied to high performance perovskite and kesterite solar materials. The technique is enabled by two new advances: a new formula that yields mobility difference from conductivity and Hall coefficient curve with varying light intensity and the development of AC Hall technique using parallel dipole line Hall system. We will review both development in more details and present several interesting CRPH studies in various materials. [1] O. Gunawan et. al., Nature 575, 151 (2019).

Porphyrin aggregates are promising materials for mixing with TiO₂ owing to its high light adsorptivity and charge separation efficiency. Porphyrin aggregate has been made either by the self-assembly of porphyrin molecules or by the preparation of porphyrin-based framework particles. However, the conventional method is difficult to control the size of porphyrin aggregation to optimize light absorption efficiency and diffusivity for high catalytic activity. Moreover, the stability is very poor when the composite is made by simple adsorption of a molecule or porphyrin-based framework particles. Here, we show that controlled porphyrin aggregates can be uniformly coated on TiO₂ using a porphyrin-based network nanogel. The porphyrin-based network is made by mixing tetrakis(4-aminophenyl)porphyrin and hexamethylene diisocyanate in dimethyl sulfoxide solvent which is known as the organic sol-gel method. The porphyrin-based urea network nanogel dispersed solution was obtained after a certain reaction time. The nanogel was applied to the TiO₂ surface by using spin coating. The nanogel was uniformly coated in several nanometer thicknesses on TiO₂ nanoparticles. The porphyrin-based network had high chemical stability. UV-Vis analysis confirmed that porphyrins form J-aggregates in the network layer. The porphyrin-based network layer has a short photoluminescence lifetime thanks to the porphyrin J-aggregate, which means that it has high charge separation efficiency. The porphyrin aggregate thin layer coated TiO₂ composite shows high photocatalytic performance.

For the future integrated electronic and photonic devices, single-crystalline III-V materials have been experimentally proposed to reach the theoretically predicted performance. The conventional epitaxy including homoepitaxy and heteroepitaxy performs a basic function to provide high-quality nanomaterials using the well-oriented substrate. However, a specific method must be provided to exfoliate and transfer the grown films from the bulk substrate. Although the previously presented approaches such as chemical etching, mechanical spalling, and thermally driven exfoliation suggest a way to separate large-area films from substrates, both surfaces separated from the interface between the epilayer and the substrate may receive physical and chemical damage during the exfoliation process. In addition, various defects caused by lattice mismatch and atomic diffusion at the interface are critical issues for applying grown epilayers to heterointegrated devices.
Here, we clearly demonstrate the crystal structure and interface properties of homoepitaxy and heteroepitaxy III-V system with graphene-coated substrates to reveal the role of interfaces including dry-transferring graphene on atomic interaction.[1] Compared to dry-transferring graphene, the unstable interface composed of wet-transferring CVD graphene from Cu foils and the substrate with the oxidized surface due to residual oxygen groups from graphene weakens the atomic interaction and inhibits epitaxial growth, forming polycrystalline films confirmed by cross-sectional scanning transmission electron microscope and electron energy loss spectroscopy.[2] The average distance of the interface region from the atom below dry-transferring graphene and the atom above is measured as 0.631 nm. On the other hand, the interatomic distances are more scattered in the case of epitaxy on wet transferred graphene, with an average value of 0.802 nm, which is significantly larger than the case of dry-transferred graphene.
In addition, various defects and second phase of GaP in lattice-mismatched conventional heteroepitaxy are originated from the fluctuated interface formed by atomic interdiffusion between InP epilayer and GaAs substrate. Since the conventional heteroepitaxy with a large lattice mismatch creates an unstable energy state on the surface, a non-uniform interface is formed by the second phase with low formation energy and acts as a source of defect growth. However, the epilayers grown on dry-transferring graphene show the effectively controlled interface and transferable single-crystalline thin films with relaxed strain due to the physical barrier of monolayer graphene, providing a novel approach to obtain freestanding III-V materials for advanced electronic and photonic devices.[3]
[1] Impact of 2D-3D heterointerface on remote epitaxial interaction through graphene, ACS Nano 15, 10587 (2021)
[2] Role of transferred graphene on atomic interaction of GaAs for remote epitaxy, Journal of Applied Physics 130 (2021)
[3] This work was supported by the Basic Science Research Program through NRF funded by the Ministry of Education (NRF Award Nos. NRF-2021R1C1C1008949 and NRF-2020R1A6A1A03043435).

For transparent and deformable electronics, ultrathin metal film has been considered as a promising candidate due to its transparency and flexibility. But oxidation and corrosion problems of metal films inevitably cause degradation in conductivity and device performance. To overcome these problems, many studies have applied passivation layers on metal films as a barrier between air/water and metal. This study presents that the top copper (Cu) layer can have a significant anti-corrosion effect despite the N-doped amorphous carbon (a-C) film being underneath the metal layer. In addition, the ultrathin Cu layer grown on the a-C film shows an enhanced conductivity compared to the bare Cu layer. The main cause of corrosion protection and conductivity enhancement of Cu on the a-C film is nitrogen in the a-C film, which interacts with Cu atom electrically. Also, the Cu layer on the a-C film shows high transmittance over 80%. We systemically analyze the mechanism of the underlying a-C film to reduce the resistance of the top Cu film and prevent long-term corrosion.

Despite many efforts in structuring surfaces using mechanical instabilities, the practical application of these structures to advanced devices remains a challenging task due to the limited capability to control the local morphology. Here, we propose a novel approach to generate pixelated wrinkles that freely change their orientation at a small area via controlling the alignment of mechanically anisotropic, reactive mesogens (RMs). A custom-built optical setup employing a spatial light modulator (SLM) illuminates polarized ultraviolet (UV) light on a particular area of photo-alignment layers. RMs are in contact with these photo-alignment layers, can reorient to the respective direction of the incident polarization. Wrinkles that are produced during the plasma-assisted polymerization of RMs can accordingly be aligned. Oxygen plasma treatment promotes the polymerization of RM layers. Due to the low penetration of plasma energy, the skin layer of RM is locally polymerized while the under layer remains unpolymerized. These two layers have different thermal expansion coefficients, and thus when the RM layers are heated from the plasma environment, the polymerized RMs turn into wrinkles by buckling instability. Since typical rod-like RMs have a higher elastic modulus along with the director (i.e., extraordinary axis) than those perpendicular to the director (i.e., ordinary axes), these wrinkles are self-aligned without controlling the stress distribution. The optics based on a spatial light modulator assembles wrinkle pixels of a notably small dimension over a large area at fast fabrication speed. Furthermore, these pixelated wrinkles can be formed on curved geometries. The pixelated wrinkles can record images, which are naturally invisible by mapping the gray level to the orientation of wrinkles. They can retrieve those images using the patterned optical phase retardation generated under the crossed polarizers. As a result, it is shown that pixelated wrinkles enable new applications in optics such as image storage, informative labeling, and anti-counterfeiting. We demonstrate the anti-counterfeiting concept based on pixelated wrinkling technology. A quick response (QR) code was constructed using pixelated wrinkles of θ = 0° and 45° and implemented on a banknote. This QR code is not observed under the usual circumstance without the polarization because there are no effective phase differences across the pixels. This image is retrieved only when appropriate optical filters (i.e., the crossed polarizers) are used. The principle of using optical phase retardation can apply to ultraviolet or infrared light sources, which are currently used to discriminate against counterfeits. This image is also readable when placed on the opaque surface by the reflection mode with one polarizer. We note that other configurations of the optical filter can be employed. The capability of producing pixelated wrinkles with a high pixel resolution and density affords their use in informative labels and security tags to prevent the forgery of products.

Here, we designed and developed an organic field-effect transistor (OFET)-based gas sensor by applying solvatochromic dye (Nile red, NR) with twisted intramolecular charge-transfer (TICT) behavior depending on the polarity of the surrounding molecules, as an auxiliary NR sensing medium (aNR-SM). As a polar molecule approaches, intra-charge transfers from the donor diethylamine group to the ketone group occur in the NR molecule, resulting in the twisting of the donor functional group and thereby increasing its dipole moment. Using this characteristic, the NR was applied as an auxiliary sensing medium to the OFET for detecting ammonia (NH₃), a representative toxic gas. The Top-NR case, where the aNR-SM covers only the top of the organic semiconductor layer, showed the best gas sensing performance, and its response and recovery rate were improved by 46 and 94 %, respectively, compared to the pristine case. More importantly, a sensitivity of 0.87 ± 0.045 ppm^-1 % was measured, having almost perfect linearity (0.999) over the range of measured NH₃ concentrations, which is the result of solving the saturation problem in the sensing characteristics of the OFET-based gas sensor. Our result not only improved the sensing performance of the OFET-based sensor but also made an important advance in that the reliability of the sensing performance was easily secured by applying solvatochromic and TICT behavior of an auxiliary sensing medium.

Passive daytime cooling, which transfers the heat from terrestrial objects to outer space without external energy input, has attracted increasing interest.¹ As cooling accounts for 15% of our global energy consumption, it provides a promising pathway to alleviate the global energy demand. With minimizing the adsorption in the solar range (0.3 - 2.5 µm), on the one hand, and maximizing the emissivity (adsorption) in the “sky window” range (8 - 13 µm), on the other hand, materials with designed optical properties could cool down itself below ambient temperature, even under the direct sunlight. Dielectric materials, for instance, silica micro-/nanoparticles, have been widely utilized in passive cooling devices to adjust the optical properties. Remarkable passive cooling performance was theoretically and experimentally demonstrated.² However, tuning of the absorption properties within the sky window range is quite limited with dielectric structures.
In this work, we have investigated two types of metal-dielectric-metal (MDM) approaches to tune the mid-infrared (MIR) emission properties in sky window range, namely ZnS-Au-Si substrate supported Au disc arrays³ and SiO₂-indium tin oxide (ITO) based plasmonic-photonic structure. In the former, we fabricated a 2D periodic array of Au discs on the top of a ZnS-Au-Si substrate via photolithography. We observed that MIR absorptance induced by magnetic polariton resonance of the first order strongly depends on the individual disc diameter. The absorptance peak shifts linearly to longer wavelengths with increasing diameter. In addition, the induced magnetic polariton resonances were observed to be independent of the neighboring disc resonances. Based on that, a mixed resonator with a broadband absorptance that spans the entire sky window range was achieved by constructing multiple discs within one unit cell. In the latter, a non-close packed silica monolayer was self-assembled on the top of Au substrate via polymer-assisted drop-casting technique. By partially covering the SiO₂ sphere with an ITO shell, the MIR absorptance in the sky window range is substantially enhanced, arising from the interaction between the localized surface plasmon (LSP) and the magnetic dipole Mie resonance.
1. Hossain, M. M.; Gu, M., Radiative Cooling: Principles, Progress, and Potentials. Adv. Sci. 2016, 3 (7), 1500360.
2. Zhai, Y.; Ma, Y.; David, S. N.; Zhao, D.; Lou, R.; Tan, G.; Yang, R.; Yin, X., Scalable-manufactured randomized glass-polymer hybrid metamaterial for daytime radiative cooling. Science 2017, 355 (6329), 1062-1066.
3. Pech-May, N. W.; Tobias, L.; Retsch, M., Design of Multimodal Absorption in the Mid-IR: A Metal Dielectric Metal Approach. ACS Appl. Mater. Interfaces 2021, 13 (1), 1921-1929.

In strongly correlated electronic materials (SCEMs), the strong correlations between lattice, charge, orbital, and spin can lead to the emergence of fascinating physical phenomena. Vanadium dioxide (VO₂), one of the typical SCEMs, exhibits a first-order metal-insulator transition (MIT) accompanied by a structural phase transition from rutile (R) to monoclinic (M1) phases near room temperature (~341 K). One of the most important issues with VO₂ has been manipulating electronic and optical properties through applied stress or strain. Stress engineering has regarded as a method controlling the reversibility and tunability of domain structures and phase transition properties in VO₂-based films and nanowires. However, the substrate-induced inhomogeneous stress in such planar and nanocrystal systems leads to complications induced by phase competitions or the creation of intermediate phases. Importantly, a core-shell architecture can provide a methodological approach for preventing the inhomogeneous stress.
In this work, we demonstrate the MIT properties of VO₂ nanobeams via core-shell heterostructures-enabled stress engineering, providing a strategy for designing the phase transition pathway and properties. We apply an amorphous Al₂O₃ (a-Al₂O₃) shell to encapsulate VO₂ nanobeams using atomic layer deposition (ALD). To understand the influence of shell formation on the VO₂ surface, we experimentally investigate the temperature-dependent evolution of electrical and structural phases, combining with the finite element (FE) analyses of stress states in core-shell heterostructured VO₂ (CS-VO₂) and pristine VO₂ nanobeams. Interestingly, CS-VO₂ nanobeams exhibit a simple and direct M1–R phase transition pathway at lower temperature without appearance of metastable intermediate phases, which is distinctly different from pristine VO₂ nanobeams with an M1–M2–R transition pathway. These results can be explained in terms of different thermal behaviors coupled to the elastic modulus between the VO₂ core and a-Al₂O₃ shell.

Electrons act as anions in the cavities of a positively charged framework in electrides, which are ionic crystals. Electrides have unique features such as low work functions and resistance anisotropy. Two-dimensional electron gas (2DEG) on the surface of dicalcium nitride (Ca₂N), which is one of the two-dimensional (2D) electrides, attracts attention, even for the monolayer. However, because electrides are readily degraded by air and moisture due to their high reactivity, it is expected that their unstable surface's electrical properties of electrides will be easily altered. Here, we introduce the electronic properties of the bilayer Ca₂N with point defects by using ab initio calculations based on density functional theory. The 2DEG on the surface is spin-polarized for the outermost Ca atom vacancy (V_out-Ca), whereas the other models are not spin-polarized. The formation energy is 1.89 eV for V_out-Ca, and the formation energy for the N atom vacancy (V_N) is 2.78 eV, which is the highest case. We calculated that the work function for V_out-Ca is 3.19 eV lower than pristine bilayer Ca₂N (3.38 eV), whereas the other cases have larger work functions (3.46 and 3.45 eV) than pristine bilayer Ca₂N. Finally, we consider the substitutional H or N impurity atom for V_out-Ca. In these cases, we can confirm that the electrical characteristics of 2DEG on the surface have changed.

Bottom-up strategies for precise allocation of ideal materials are critical for applications, including microchips and microfluidic bioanalytical lab-on-a-chip technologies. However, there is a problem with current bottom-up nanotechnology platforms for microchip manufacturing that features are often restricted based on engineering resolution, and are the result of templating or ablative strategies in which the features are cut or formed from larger features. To this extent, area selective deposition (ASD) is an appropriate strategy for overcoming this problem. In this work, it is shown that functionalized poly-p-xylylene species are capable of depositing selectively onto different substrates (Si, Cu) via Chemical Vapor Deposition (CVD) and that, through the use of surface behavior measurements, model previously reserved for PPX can be expanded to all functionalized PPX deposition on both good and poor substrates. We found that the area-selective property of functionalized PPX can be changed by deposition conditions, such as deposition temperature and working pressure, which confirmed by the deposition model. To assess these finding in an industrially relevant environment, area selective deposition is observed and characterized for template substrates expressing regions of both Si and Cu.

We present the strain-induced atomic-scale modification of magnetic canting in antiferromagnetic insulator LaFeO₃, which was investigated using high-precision imaging and diffraction in scanning transmission electron microscopy (STEM). Antiferromagnetic insulators have gained attention due to their low loss, fast switching, and potential for the next generation of spintronics applications. To advance this class of materials, we aim to acquire a precise understanding and control of how magnetic properties change with external stimuli, such as epitaxial strain. High-quality LaFeO₃ thin films were grown on [001] SrTiO₃ under in-plane compressive strain with ultrahigh vacuum off-axis sputtering. Superconducting quantum inference device magnetometry revealed increased net moment with decreasing film thickness, with the first few layers of the film having a larger magnetic moment (0.23 µB/Fe) relative to bulk (0.01 µB/Fe). We trace the structural origin of this change in magnetism to the changes in the lattice distortion at the interface. Atomic-scale STEM images show that the rotation of Fe-O octahedra changes within the first ~ 5 orthorhombic unit cells, with both the in-plane and out-of-plane rotations, progressively decreasing near the interface. Cation (La) positions are also affected by the strain at the interface, showing less distortion due to the connection to the cubic SrTiO₃. Nanoscale structural domains were also observed, and they are connected to the formation of magnetic domains near the interface, which we directly image using Lorentz TEM. Based on the experimental results, density functional theory calculation is performed to help elucidate the exact mechanism of the observed structure-property relationship. Our current study provides an atomic-level understanding of fast, efficient tuning of the magnetic states that is essential to advancing next-generation spin-electronics.

Germanium-tin (GeSn) is a promising silicon-compatible group IV semiconductor that can achieve a direct band gap and has been the subject of increasing research in recent years for both electronic and photonic applications. However, the compositions that are reported to produce a direct band gap in unstrained GeSn, ~10 at% tin, are metastable. The bulk solubility limit of Sn in solid solution with Ge is ~1 at%. ¹ Metastability of GeSn at high Sn contents limits the thermal processing window for growth and annealing of epitaxial layers.^{2, 3} Detrimental Sn phase separation has been previously reported after annealing of strained GeSn epitaxial layers. We report X-ray photoelectron spectroscopy (XPS) and correlated electron microscopy studies of surface morphology after annealing of random core/shell Ge/Ge_1-xSn_x nanowire assemblies both ex-situ in the XPS chamber and in-situ in the reduced pressure chemical vapor deposition growth reactor. We have previously demonstrated the elastic compliance of small diameter Ge core nanowires in the core/shell structure to enable largely strain-free epitaxial Ge_1-xSn_x shells.⁴ Utilizing the strain minimization of this geometry, we investigate the annealing characteristics of the GeSn surface and native oxide for Sn contents in the range of 2 at%–11 at%, unaffected by substrate-induced misfit strain. Samples that form a native oxide through post-growth air exposure show the presence of a Sn-rich oxide that exhibits a composition dependent thermal decomposition temperature measured via ex-situ annealing in the XPS chamber. The surface composition evolution of the as-grown GeSn surface was also examined via samples annealed in-situ without exposure to the atmosphere. We show a dramatic difference in the surface chemical and morphological stability between H₂ and rough vacuum annealing with H₂ annealed samples exhibiting surface Sn segregation at temperatures more than 100 °C lower than vacuum annealed samples.

¹M. Seifner, A. Dijkstra, J. Bernardi, A. Steiger-Thirsfield, M. Sistani, A. Lugstein, J. Haverkort, S. Barth, “Epitaxial Ge_0.81Sn_0.19 Nanowires for Nanoscale Mid-Infrared Emitters,” ACS Nano 13, 7 (2019), 8047-8054.
²S. Wu, L. Zhang, B. Son, Q. Chen, H. Zhou, C. Tan, “Insights into the Origins of Guided Microtrenches and Microholes/rings from Sn Segregation in Germanium-Tin Epilayers,” J. Phys. Chem. C 124 (2020), 20035-20045.
³W. Wang, L. Li, Q. Zhou, J. Pan, Z. Zhang, E. Tok, Y. Yeo, “Tin surface segregation, desorption, and island formation during post-growth annealing of strained epitaxial Ge_1-xSn_x layer on Ge(001) substrate,” Applied Surface Science 321 (2014), 240-244.

Chemical intuition, first-principles calculations, and experiments suggest that sulfides and selenides in the perovskite and related crystal structures - chalcogenide perovskites - are promising semiconductors for optoelectronics and energy conversion applications [1]. Chalcogenide perovskites feature the large dielectric response familiar in oxide perovskites, but also have band gap in the VIS-IR and strong light absorption with direct band gap. Chalcogenide perovskites feature excellent excited-state charge transport properties (e.g., long diffusion lengths), while also being thermally-stable and comprised of abundant and non-toxic elements. Nearly all experimental results on chalcogenide perovskites have been obtained on powders and microscopic crystals; thin film synthesis is in its infancy. Realizing the promise of a new family of semiconductor alloys requires developing methods for epitaxial thin film synthesis on-par with established semiconductor materials (Si-Ge, III-Vs, etc.) and complex oxides.
We report the first epitaxial synthesis of chalcogenide perovskite thin films by molecular beam epitaxy (MBE): BaZrS₃ films on (001)-oriented LaAlO₃ substrates. The films are stoichiometric and oxygen-free, as confirmed by X-ray fluorescence and electron energy loss spectroscopy. The perovskite phase is confirmed by X-ray diffraction (XRD), scanning transmission electron microscopy (STEM), and Raman spectroscopy. The films are atomically-smooth over large areas, and STEM data show an atomically-abrupt substrate/film interface with no chemical interdiffusion.
The sulfide perovskite film has a pseudo-cubic lattice constant more than 30% larger than the oxide perovskite substrate. Strain considerations predict rotated-cube-on-cube growth, wherein the cube diagonals of the substrate align with the cube edges of the film, reducing the lattice constant mismatch. Remarkably, we find that growth is instead dominated by the formation of a fully-strain-relaxed film with cube edges aligned with the substrate, enabled by a self-assembled buffer layer. Direct, rotated-cube-on-cube epitaxy is observed only at substrate step edges, and constitutes a minority of the film. The propensity for buffered epitaxy can be controlled by the H₂S gas flow during growth. We will report detailed studies of the role of H₂S in passivating the substrate, leading to the formation of an atomically sharp buffer layer with van der Waals characteristics, supporting the growth of fully-relaxed, epitaxial films of a sulfide perovskite on an oxide perovskite (hetero-chalco-epitaxy) despite the giant lattice constant mismatch. We will report on the generality of this phenomenon for different oxide substrates, and on the use of substrates with higher step-edge density to enhance the propensity for direct epitaxy. We will present detailed STEM and high-resolution XRD studies that investigate the self-assembled buffer layer, and provide clues to its formation. If time allows, we will also present on the use of BaZrS₃ films to stabilize BaZr(S,Se)₃ alloys in the perovskite structure, using H₂S and H₂Se sources, providing a path towards tuning the direct band gap in the range 1.3 – 1.9 eV.
This work sets the stage for developing chalcogenide perovskites as a family of semiconductor alloys with properties that can be tuned with strain and composition in high-quality epitaxial thin films. Our methods also represent a revival of gas-source chalcogenide MBE, with potential for impact on research on other sulfur- and selenium-containing compounds.
[1] Jaramillo, R. & Ravichandran, J. In praise and in search of highly-polarizable semiconductors: Technological promise and discovery strategies. APL Materials 7, 100902 (2019).
[2] Sadeghi, I., Ye, K., Xu, M., Li, Y., LeBeau, J. M. & Jaramillo, R. Making BaZrS₃ Chalcogenide Perovskite Thin Films by Molecular Beam Epitaxy. Advanced Functional Materials n/a, 2105563 (2021); DOI 10.1002/adfm.202105563.

The high tunneling electro resistance and non-linear current-voltage characteristics are essential for high density resistive random access memory devices that are compatible with cross point array structure. Recently, high performance of ferroelectric tunnel junction have been achieved by introducing metal-ferroelectric-semiconductor structure and barrier modulation by dead layer. However, ferroelectric tunnel junction has limitation due to its linear tunneling current and symmetric property. In this work, we proposed a new ferroelectric tunnel junction enabling selector less resistive readout memory with large TER of 10⁵. Non-linear electrical characteristics were exhibited by the migration of additional oxygen vacancies in post annealed Bi_0.9Ca_0.1FeO₃ ultra thin film. Tunneling barrier modulation via oxygen vacancy migration was simulated by tunneling current model. These results could be useful for the design of novel and multi-functional memory cells.

Imperfections associated with the surface of a solid are a fundamental limitation to the performance of various technologies such as superconducting resonators used in qubits and single-photon detectors. Although the role of the surface is well known, experimental approaches to mitigate these imperfections are lacking. Here, we report an atomic layer etching/deposition (ALE/ALD) method to remedy the surface imperfections on aluminum (Al) films. Our approach enables the removal of the surface metal oxide (Al₂O₃) and deposition of an ultrathin layer of AlF₃ with monolayer precision. Atomic force microscopy (AFM) analysis reveals a reduction in surface roughness of around 50% compared to the original samples. Characterization of the etched sample using x-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, and energy-dispersive spectroscopy confirms the removal of alumina and deposition of ultrathin AlF₃ on the Al layer. We characterize the dielectric loss of the resulting film by measuring the quality factor and resonator noise of Al superconducting microwave resonators with the surface treated with the ALE/ALD process. We anticipate that surface engineering using ALE/ALD may be applicable to other technologies that are limited by surface and interface imperfections.

One-dimensional nanomaterials – materials with one dimension outside the nanoscale, further noted as 1D NMs – represent a class of very important nanomaterials with continuously increasing importance. Due to their intrinsic features, unique properties and diversity of functionalities, they count among the most widely studied materials nowadays. While considerable research efforts have been spent to synthesize various 1D NMs (e.g. nanopores, nanotubes or nanofibers), limited efforts have been devoted to surface modification and property tailoring of these materials, in particular in various energy devices, such as batteries, solar cells, but also sensors.
However, it is their surface that comes into direct contact with various media (air, gases, liquids, solids) and influences the reactivity, stability and biocompatibility of these materials. The surface and aspect ratio (defined as their diameter to length ratio) influence the performance of these materials in various applications. Considering these facts, it is more relevant to tailor the surface of these materials and to be able to influence their properties and reactivity at the nanoscale, rather than to deal with tailoring their own bulk material composition.
The focus of this presentation is on the modification of two types of 1D nanomaterials – nanotubes and nanofibers. Numerous techniques can be utilized for this purpose, such as for example wet chemical or physical deposition techniques. However, it is only the Atomic Layer Deposition (ALD) that is capable of really uniform and homogenous coating of these 1D nanomaterials, in particular those of very high-aspect ratio.
The presentation will be mainly focused on modification of TiO₂ nanotube layers and various nanofibers of different aspect ratios via ALD that are applicable in energy conversion and storage devices.
Experimental details and some very recent application examples [1-9] of surface modification of these high surface area materials, including their structural characterizations will be discussed.


1) H. Sopha et al (2017), Appl. Mater. Today, 9, 104.
2) H. Sopha et al. (2018), Electrochem. Commun., 97, 91.
3) S. Ng et al. (2017), Adv. Mater. Interfaces, 1701146.
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6) M. Motola et al. (2019), Nanoscale 11, 23126.
7) S. Ng et al. (2020), ACS Appl. Mater. Interfaces, 12, 33386.
8) M. Motola et al. (2020) ACS Appl. Bio Mater. 3, 6447.
9) M. Rihova & M. Knez & J. M. Macak et al. (2021), Nanoscale Adv., 3, 4589.

Superhydrophobic surfaces provide an exotic opportunity for engineering diverse interfacial phenomena through their extreme liquid repellence.¹ One smart strategy for rendering superhydrophobicity is to simply roughen hydrophobic surfaces, which is often coined as the lotus effect, producing a static contact angle (θ_c) of >150° and a contact angle hysteresis (Δθ ) of <10° for water droplets. The θ_c of flat hydrophobic surfaces is typically 100–120° because of their low surface chemical energy, whereas superhydrophobicity (θ_c > 150° and Δθ < 10°) can be achieved when the surfaces are structured below or above wavelength scale.² Superhydrophobic surfaces can trap air beneath water droplets (i.e. Cassie state), which offers a means to promptly eliminate contaminants and to avoid icing and fouling in various environmentally benign applications.³
Although superhydrophobic coatings have been investigated for over twenty years, there are still space for improvement in terms of scale up, anti-reflection function, and vision resolution, which restricts their usage in low-maintenance glass-based applications. Particularly, to impart an anti-reflection function to superhydrophobic coatings, their thickness and refractive index must be precisely adjusted to the wavelength of incident light and must be uniform over the entire coated area. In addition, such coated surfaces must possess an optical flatness (i.e. root-mean-square surface roughness < one-tenth of a reference wavelength) to exhibit glass-like transparency.² To address these issues, in this study, we develop a scalable, high-precision-thickness superhydrophobic coating that reduces surface reflectance without degrading visual clarity. The rationale for this technology originates from hierarchically designed surfaces in which each three-dimensional array of colloidal or fumed silica nanoparticles acts independently to provide anti-reflection and self-cleaning functions, respectively. This design strategy is a key breakthrough because there is a fundamental trade-off between the optical and wetting properties; a roughened surface is desirable for rendering superhydrophobicity, which adversely affects visual clarity. The developed superhydrophobic surfaces exhibit outstanding droplet mobility such that their self-cleaning capability is fast and effective for various sizes of contaminants compared with uncoated glass and even commercial coatings. The optical properties of high transmittance and high clarity are valid over a broad range of wavelengths and incident angles, which enable high-performance solar energy and display applications. As a proof-of-concept, we conducted experiments on photovoltaic devices in a dusty environment and observed improved power conversion efficiency and a complete recovery of device performance using the hierarchically designed surfaces developed herein.
References
[1] Lafuma, A. & Quéré, D. Superhydrophobic states. Nat. Mater. 2, 457–460 (2003).
[2] Wang, D. et al. Design of robust superhydrophobic surfaces. Nature 582, 55–59 (2020).
[3] Geyer, F. et al. When and how self-cleaning of superhydrophobic surfaces works. Sci. Adv. 6, eaaw9727 (2020).

Sensors play an increasingly important role as life becomes more automated and interconnected. Heterostructures composed of semiconducting metal oxides (SMOX) are fundamental for the development of efficient gas sensors. In spite of their importance in real applications, however, the understanding of the transduction mechanisms, whether related to the heterojunction or simply to the core and shell materials, is lacking. The engineering of heterojunctions with well-defined core and shell layers is required to better understand the sensing response of heterostructured nanomaterials. Here, a comprehensive understanding of the role of semiconductor heterojunctions and the sensing response of core-shell heterostructures is achieved by synthesizing a series of logically designed, well-defined, and well-controlled heterostructures with different thicknesses of the core and shell layers. Prototypes _n,pSMOX-CNT and _p,nSMOX-_n,pSMOX-CNT hierarchical coaxial core-shell heterostructures are therefore proposed to achieve this objective. The designed heterostructures exhibit sensing response related to the _n,pSMOX-shell layer, or in some cases to the heterojunctions between _nSMOX and _pSMOX. A comparison of the sensing response in order to understand the transduction mechanism across the interfaces in atomic layer deposition (ALD) grown heterojunctions will be presented.

Interfacial resistance built at the junction between TCO(Transparent conductive oxide) nano-particle is a massive obstacle for utilizing the colloidal ink for the solution-based fabrication process. In this study, we present a method for modifying surface/interfacial properties of TCO by constructing the core-shell structured TCO-metal composite to produce photoelectric devices. The optical/electrical performance of surface-engineered TCO-metal composites and transparent conductive electrode harnessing colloidal ink-based fabrication process is inspected by transmittance and van der Pauw analysis, which showed 93% transmittance in the visible range and significantly improved electrical conductivity/carrier density/mobility. The ascription of the enhanced optoelectrical response of surface-modified TCO composite is because of electrical junction built-in TCO-metal and metal-metal interface modulating charge-transfer mechanism. Thereby, this work expands the conductive and functional materials scheme for solution-based electrode deposition and encourages the use of a cost-effective solution-based fabrication process for photonic and electronic applications.

Atomic layer deposition (ALD) is the leading technique for nanoscale control of film growth and patterning, but it is challenging to characterize the growing surface and understand the conditions where ALD breaks down. ALD is usually achieved by pulsing a precursor chemical that reacts in a self-limiting fashion with the surface, alternating with a co-reagent pulse that reactivates the surface. ALD of gold in this fashion has been recently demonstrated using a trimethylphosphinotrimethylgold(III) (Me₃AuPMe₃) precursor and H₂-plasma as co-reagent [1]. Interestingly, ALD-style deposition is also possible by pulsing the precursor alone, albeit crossing over to chemical vapor deposition (CVD) at high temperature. In this work, we explain this single-source deposition using first-principles modelling of the deposition chemistry and examining the effect of temperature and pressure. We performed density functional theory (DFT) calculations of total energies and free energies of adsorption for the precursor and associated decomposition products on the Au (111) surface using the Quantum ESPRESSO and Jaguar DFT codes within the Schrödinger Materials Science Suite. Based on these calculations, we observe that the most thermodynamically-favorable ALD and CVD reactions are in competition at temperatures close to that where the crossover from ALD to CVD is observed in experiment. We are thus able to identify the surface intermediate that is responsible for self-limiting ALD, namely surface-Au^[0]PMe₃, which has a C:P ratio consistent with the experimental x-ray photoelectron spectroscopy (XPS) results [1].
[1] Van Daele, Michiel, et al. "Reaction mechanism of the Me₃AuPMe₃–H₂ plasma-enhanced ALD process." Physical Chemistry Chemical Physics 22.21 (2020): 11903-11914.

New materials with versatile applications that can meet growing global energy demands while simultaneously aligning to the global call to shift away from carbon-intensive energy systems are highly desirable. Finely controlling stoichiometry in multinary materials will afford frameworks with tunable electro/photoelectrochemical properties (e.g. interfacial charge transfer kinetics, adsorption thermodynamics, small-molecule conversion selectivity), tunable charge storage and transfer properties (e.g. charge transfer resistance, electrochemical capacitance, conductivity), and tunable semiconductor properties (e.g. carrier concentration/lifetime, mid-gap state formation/population, conduction/valence band positions).
Chevrel-phase (CP) M_xMo₆T₈ (M=alkali/transition metal; x=0-4; T=S, Se, or Te) materials have shown promise as intercalant hosts for multivalent transition metals, owing to their unique intercalation capacity which in turn affects their structural and electronic properties.
Furthermore, powder X-ray diffraction (PXRD), energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectroscopy (XAS) can be used to further understand crystal structure, electron localization, structural disorder, and elucidate electron transfer within the CP’s interesting cluster framework. I have focused efforts in understanding these effects by evaluating stoichiometric control of Cu intercalation into the binary CP sulfide-phase (Mo₆S₈) host framework via electrochemical methods in aqueous environment. To date, I have successfully identified the electrochemical potentials that correspond to the intercalation/deintercalation of stoichiometric amounts of Cu in the CP host (C_xMo₆S₈; x=1-3). Additionally, PXRD, EDS, XPS, and XAS confirm successful incorporation and subsequent structural and electronic changes to the CP-sulfide host as a function of Cu intercalation.
In this work CP cluster frameworks comprised of a transition metal (Mo) octahedron enclosed in a chalcogen (S) cage are used as a host material. These individual cluster units are aligned in an extended structure via the sharing of terminal chalcogen and as a result, produce cavities that are ideal for the hosting of cationic transition metal (CTM) species via intercalation/deintercalation. By controlling the stoichiometric amounts of these CTM’s (in this case Cu) intercalated into the structure via the varying of experimental parameters (e.g., electrochemical potentials, scan rates, electrolyte concentrations, etc.), we can take advantage of electronic and structural changes in the host materials.
To identify the electrochemical potentials at which Cu ions were intercalated/deintercalated into the CP host, cyclic voltammetry was used. These potential extremes that defined our cyclic voltammograms’ potential windows were based on pourbaix diagrams of Cu to 1) operate within a potential window with an ideal oxidation state (i.e. Cu^2+/1+ vs Cu) and 2) to avoid competing reactions, especially metal plating and hydrogen evolution reactions (HER). Next linear sweep voltammetry was used to control the stoichiometric amounts of Cu incorporated into the CP cavities by varying potential and scan rates.
Confirmation of the presence and stoichiometric amount of the metal in the CP host was achieved firstly via bulk analysis methods such as PXRD and EDS. After confirmation of C_xMo₆S₈; x=1-3 via bulk analysis, more sensitive characterization (i.e. XPS and XAS) was used to further understand crystal structure, electron localization, and structural disorder of the intercalated CP host. The XAS data, specifically the X-ray absorption near edge structure (XANES), was especially helpful in monitoring host changes as it allowed for the observation of the effect and amount of intercalated Cu in the CP-sulfide host, due to increased electron density donation as a function of Cu content and subsequent depression of the S-k pre-edge feature.

Motivated by the interest in bottom-up and self-aligned fabrication, area-selective deposition (ASD) processes are currently being considered for a variety of applications in nanomanufacturing. The main approach for obtaining selectivity during atomic layer deposition (ALD) is to functionalize the surfaces on which no growth is desired with inhibitor molecules. Besides the extensive research on self-assembled monolayers (SAMs) as inhibition layer, in recent years there is increasing interest in using small molecule inhibitors (SMI),¹ because of their compatibility with industrial processing flows.
In this contribution, the mechanisms of precursor blocking using SMIs will be discussed based on a combination of simulation and experimental studies. The focus will be on using diketones and related molecules for functionalization of oxides and aromatic molecules for functionalization of metals. In general it is more challenging to block ALD growth using SMIs as compared to SAMs because of their small size and relatively low density on the surface. The packing and density of the SMIs on the surface was studied using random sequential adsorption (RSA) simulations, using bonding configurations from density functional theory (DFT) simulations and in-situ Fourier transform infrared spectroscopy (FTIR) as input. The requirements for the SMI and precursor molecules for achieving a high selectivity will be discussed.
1. Mackus, A. J. M.; Merkx, M. J. M.; Kessels, W. M. M. From the Bottom-Up: Toward Area-Selective Atomic Layer Deposition with High Selectivity. Chem. Mater. 2019, 31, 2–12.

HfO₂ and HfO₂-based ferroelectric materials have received increasing attention for their potential to be components of next-generation memory and transistors in electronics. Although researchers have explored the influence of various variables (e.g. dopants, strain, and applied electric fields) on the stability of HfO₂’s polar orthorhombic phase compared to its nonpolar tetragonal and monoclinic phases [1-3], the effect of the surface composition has not been investigated fully. We therefore used density functional theory to first assess the effect of ferroelectric polarization on the stabilities of an array of reconstructed surfaces of polar orthorhombic HfO₂(001). We determined that while a stoichiometric slab with symmetrically terminated oxygen surfaces is unstable due to its inability to screen surface polarization charges, a nonstoichiometric slab with a relatively oxygen-rich, positively polarized surface can adequately screen such charges. We then studied the reciprocal interaction and determined the influence of the surface composition on the stability of the ferroelectric polarization for free-standing orthorhombic slabs. At thicknesses of five to 11 HfO₂ layers, the symmetric slabs depolarize to a monoclinic-like phase. In contrast, asymmetric slabs can maintain a stable orthorhombic phase with polarizations (all above the bulk value) increasing with decreasing thickness. However, for the ultrathin case of three-HfO₂-layer thickness, the symmetric slab can sustain a strong polarization without an explicit dipole screening mechanism whereas the asymmetric slab undergoes a transition to a polar rhombohedral (R3)-like phase. Our findings can account for the recently observed absence of a ferroelectric critical thickness in HfO₂-based materials and are consistent with polarization enhancement at reduced thickness even at the ultrathin limit [4].

[1] Phys. Rev. B 102, 214108 (2020)
[2] Phys. Rev. Lett. 125, 257603 (2020)
[3] Phys. Rev. Lett. 127, 087602 (2021)
[4] Nature 580, 478 (2020)

Hafnium oxide (HfO₂) has been shown to be an effective protective layer against oxidisation, copper corrosion, and general weathering^1–3. In this work we demonstrate the robustness of thin (~10 nm) HfO₂ films in protecting other sensitive dielectric films and silicon surfaces when subject to aggressive wet chemical processing typically used in the integrated circuit and photovoltaic industries.
Atomic layer deposition (ALD) has revolutionised thin film dielectric deposition over the past decade and is routinely used in the integrated circuit and photovoltaic industries. It is now relatively straightforward to deposit nm-scale HfO₂ films on the surfaces of electronic materials. We deposit HfO₂ films at 200^oC using plasma enhanced atomic layer deposition (ALD) which provides excellent deposition conformality, uniformity and control over film thickness. Additionally, plasma enhancement allows for relatively low deposition temperatures compared to previous techniques (e.g. chemical vapour deposition – CVD)^4,5.
As it is so commonly used in silicon device fabrication, our main focus has been on optimising the chemical resistance to hydrofluoric acid (HF). We have found that there are three distinct stages to the etch resistance of the HfO₂ films in HF. Films annealed at lower temperatures (<300^oC) will typically etch fully within the first few minutes while those annealed above 350^oC will show no clear sign of degradation after multiple hours in a 5% HF solution. In between (300-325^oC), there is an intermediate stage in which the films will etch slowly over the course of several hours. When compared to grazing incidence X-ray diffraction (XRD) results it becomes clear that these stages align with film crystallinity; fully crystalline films are very etch resistant, amorphous films etch very quickly, and mixed phase films etch more slowly than their amorphous counterparts.
In contrast, when subjecting the HfO₂ films to other wet chemical processing solutions (e.g. tetramethylammonium hydroxide – TMAH) we found no indication of the films etching after several hours, in either an amorphous or crystalline state. Additionally, we noted no change in the results of HfO₂ films deposited on top of another dielectric layer (i.e. Al₂O₃/HfO₂ stack) compared to films deposited directly on silicon. This implies that the characteristics of the HfO₂ films, before and after annealing, do not change whether deposited on dielectric or silicon surfaces.
From our research we have determined that ALD grown HfO₂ makes a highly controllable coating material, with the flexibility to be incorporated into stacks without losing effectiveness, and can be deposited and manipulated using low and room temperature methods. As such, we have utilised HfO₂ to protect patterned dielectric structures on silicon surfaces during chemical processing, in order to form an optimised photomodulator, whereby Terahertz radiation can be efficiently modulated and detected⁶. With further understanding of the relationship between annealing temperature, film crystallinity and wet chemical etch resistance, we expect thin (<10nm) HfO₂ films to serve multiple applications in any one device, including passivation, chemical resistance, and insulation.
1 K. Mergia, V. Liedtke, T. Speliotis, G. Apostolopoulos and S. Messoloras, Adv. Mater. Res., 2008, 59, 87–91.
2 J. S. Daubert, G. T. Hill, H. N. Gotsch, A. P. Gremaud, J. S. Ovental, P. S. Williams, C. J. Oldham and G. N. Parsons, ACS Appl. Mater. Interfaces, 2017, 9, 4192–4201.
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With the growing demands on nanostructure fabrication for advanced electronics, area selective deposition is gaining attention as an important method to achieve pattern features at the sub-10 nm length scale. Area selective deposition is a bottom-up patterning process in which material is deposited only where desired, and the process holds great potential for a variety of applications. This talk will describe how control over the substrate surface chemistry can be used to achieve area selective atomic layer deposition (AS-ALD). The technique of ALD relies on self-saturating, layer-by-layer, gas-surface reactions to deposit conformal thin film materials and is an excellent choice for selective deposition because its chemical specificity provides a means to achieve selectivity on a spatially patterned substrate. A deep understanding of the gas-surface chemistry behind ALD can be critical toward developing improved ALD and AS-ALD processes. In this talk, examples of tuning surface chemistry to both enhance and inhibit ALD nucleation will be presented, and mechanistic insights developed through in situ and ex situ measurements will be shared. The development of inhibitory layers such as self-assembled monolayers (SAMs) to alter the native surface reactivity will be described. We will show that the inhibition process provides good selectivity in the deposition of thin films on a variety of substrate materials, including dielectrics and metals such as Cu, Co, W and Ru. The talk will conclude by showing how improved selectivity in AS-ALD can be achieved by combining both nucleation enhancement and inhibition strategies.

Surface states significantly affect various device properties. This is especially true for GaN and similar polar semiconductors. Broad luminescence bands at sub-band gap energies are commonly observed in wide gap polar semiconductors. In GaN, the most known emission feature is yellow-luminescence (YL) peaking around 2.2 eV. The origin of this luminescence remains unclear despite the numerous studies. Proposed explanations roughly divide between bulk and surface states. The former is based on ab-initio studies suggesting that Ga-vacancies compounded with O or O-C complexes in bulk are responsible for this emission. However, most of these first principle studies did not take into account surface states at all, and some suggested that the energetic configuration of defects on the surface may be same as in bulk.[1] In contrast, the latter is based mostly on various spectroscopic measurements, in which the surface origin was clearly evident. Several of these studies have further pointed out a correlation to surface adsorption. Experiments have shown that in air, UV illumination promotes adsorption, while in vacuum, the same illumination induces desorption of O₂.[2] In both cases, both surface photovoltage spectroscopy (SPS) and photoluminescence (PL) spectra of the yellow band were affected, which may be an evidence for its surface origin. Apparently, the most likely reason for the lack of conclusive identification of the molecules responsible for the YL lies in limitations of existing characterization methods. Most methods strongly perturb the surface equilibrium during measurement, thus affecting the measured features. For example, in PL, powerful UV laser beam is utilized for excitation. However, in addition to the desired effect this beam simultaneously changes the measured state. Moreover, PL only shows the total distribution of states, and cannot characterize the charge trapped inside them. Here, we propose a new method for characterization of charge trapped in surface states, based on SPS. We illuminate the sample by constantly increasing below-gap photon energies and measure the change in the surface band bending. At this photon energy range, surface band bending is mostly affected by two processes: excitation of surface charge over the surface barrier, and excitation of deep level charge within the depletion region. In most cases, surface charge excitation is the dominant process due to the high density of surface defects, compared with bulk states.
We first applied our method to GaN HEMT. We obtained quantitative charge distribution and the total charge density at the AlGaN barrier surface, as well as the surface Fermi level position. Operating the transistor at different gate voltages, we were also able to characterize the dynamics of this surface charge.[3] On bare wafers, we were able to obtain qualitative surface charge distributions. After 450 K annealing and cooling back to room temperature in vacuum, we observed a major decrease in the YL-related feature. Although prior studies suggested that desorption of O₂ may underlie this effect, we suspected water. To test this hypothesis, we cooled the sample to 77 K in vacuum and gradually heated it up to achieve surface dehydration by sublimation. This was followed by 450 K anneal and cooling down to 300K while still in vacuum. Now, the YL band in the observed spectrum was practically diminished, while the 2.9 eV feature remained unchanged. These findings suggest that surface adsorbed water, likely in complex with other air elements, is the origin of the YL in GaN.
References: [1] C. G. Van de Walle, D. Segev, J. Appl. Phys. 101, 081704 (2007); [2] M. Foussekis et. al, Appl. Phys. Lett. 94, 162166 (2009); [3] Y. Turkulets, I. Shalish, Appl. Phys. Lett. 115, 023502 (2019)
Acknowledgement: Financial support from the Office of Naval Research Global through a NICOP Research Grant (No. N62909‐18‐1‐2152) is gratefully acknowledged. Approved for public release (DCN# 43-9231-22).

For (opto)electronic devices, it is generally key to reduce the recombination of charge carriers at the semiconductor surfaces and interfaces. This is becoming more and more challenging due to the ongoing diversification in semiconductor materials to be used in the devices (extending from Si to SiGe and Ge and from group-IV to III-V semiconductors) but also due to the increasing surface-to-volume ratio of the nanostructures employed in electronics and photonics. Surface passivation can be achieved by employing ultrathin films of semiconductor or dielectric materials which often serve other functionalities in the devices too. The underlying mechanism of the surface passivation can be the reduction of surface defect states (i.e. so-called chemical passivation) and/or band bending due to so-called field-effect passivation effect (e.g. due to fixed charges in the film).
In this contribution the surface passivation of semiconductors by some innovative nanolayer approaches prepared by atomic layer deposition (ALD) and chemical vapor deposition (CVD) will be presented. These nanolayers include materials such as Al₂O₃, PO_x, a-Si:H and (doped) ZnO as well as stacks thereof. The application of these nanolayers on semiconductors such as Si, SiGe, Ge and InP will be discussed, both for wafers and nanowires. Special attention will be given to the underlying mechanism of the surface passivation achieved as well as its relation to the properties of the nanolayers employed.

With continuous scaling down towards the sub-3 nm node technology and beyond, the back-end of line interconnects process has confronted several challenges. To circumvent issues, area-selective atomic layer deposition (AS-ALD) is one of the promising techniques with reducing the number of process steps and by alleviating key challenges associated with lithography and layer alignment at the sub-5 nm node.^1–3 However, despite the significant scientific efforts in recent years, lack of fundamental understanding of surface impede the development of AS-ALD. Specifically, in-situcharacterization of the metal surface during cleaning and passivation with self-assembled monolayers (SAMs) has been rarely reported.
Herein, post-CMP Cu substrates were cleaned with glacial acetic acid (CH₃COOH), followed by passivation using a 1 mM of octadecanethiols (ODTs) solution for 20 h. An in-situ reflectance absorption infrared spectroscopy (RAIRS) system equipped with an ALD chamber were employed to elucidate the surface chemistry, stability of ODTs during ALD of AlO_x process using TMA and H₂O at 120 ^oC. During surface cleaning with CH₃COOH, adventitious surface contaminants (e.g., –CH_x, –CO₃, and –OH) were removed, and most importantly, the surface oxide (Cu₂O) were reduced to metallic copper by forming copper acetate. In the ex-situ XPS and RAIRS, the SAMs on the CH₃COOH-treated Cu sample exhibits poor selectivity of ALD-AlO_x compared to the SAMs on the as-is Cu, suggesting that the residual copper acetate on the surface can affect the chemisorption of ODTs during passivation. On the other hand, subsequent UHV annealing treatment (~10^-8 Torr at 75 ^oC) after CH₃COOH cleaning of the Cu sample can effectively reduce the copper acetate and residual contaminants on the surface, which can improve not only ODTs quality in the passivation process but also the increase of nucleation delay during the consecutive ALD process. With additional UHV annealing process, nucleation delay of ALD-AlO_x was improved from 2 to 15 ALD cycles. The detailed experimental results will be presented.
The authors acknowledge Lam Research Foundation for the partial financial support and Rasic Inc. for providing the Brute N₂H₄ as well as their partial support.
¹ N.F.W. Thissen et al., 2D Mater. 4, (2017)
² L.F. Pena et al., ACS Appl. Mater. Interfaces 10, 38610 (2018)
³ M. He et al., J. Electrochem. Soc. 160, D3040 (2013)

Sputtered polycrystalline titanium ditelluride (TiTe₂) has recently been introduced as a thermal barrier layer in phase change heterostructures (PCHs) to realize energy-efficient and neuro-inspired phase change memory (PCM) [1-3]. PCHs such as TiTe₂/Sb₂Te₃ rely on the high melting temperature and predicted low thermal conductivity of ultrathin (few-nanometer) polycrystalline TiTe₂ layers [1,2]. The thermal properties of such TiTe₂ thermal barrier layers play a vital role within the thermally-driven phase change in PCH. Thus, the characterization of thermal transport across sputtered thin TiTe₂ films is critical for the adoption and optimization of PCH in PCM technology.
In this work, we use time-domain thermoreflectance to uncover the thermal transport properties of sputtered TiTe_2. We explore the temperature (T) and the thickness dependence of sputtered TiTe₂ thin films deposited at varying temperatures. Additionally, our T-dependent Raman and X-ray diffraction (XRD) measurements shed further insight into the micro-structural evolution of TiTe₂ films with temperature.
We sputter TiTe₂ thin films of 10 nm, 20 nm, 50 nm, and 120 nm in thickness at room temperature (RT) and at 230^oC on a silicon substrate. All films were capped in situ with 10 nm TiN to protect the TiTe₂ from surface oxidation. XRD spectra confirm both the amorphous and polycrystalline states of the TiTe₂ films deposited at RT and 230^oC, respectively. Our measured intrinsic thermal conductivity (k) of the amorphous TiTe₂ is ~0.1 W/m/K at RT, ~5x lower than the k of commonly used phase change material (Sb₂Te₃) in PCH. The RT thermal conductivities of amorphous TiTe₂ do not show a thickness dependence and hold constant at ~0.1 W/m/K, even for a 120 nm thick amorphous film. However, we measure a significantly higher k of ~1.95 W/m/K for TiTe₂ (polycrystalline) films deposited at 230^oC, regardless of thickness. Such an increase in k for the polycrystalline sputtered TiTe₂ contrasts with earlier predictions in literature [5] and thus warrants careful consideration for PCH-based PCM design.
We also measure the T-dependent k of the amorphous 120 nm thick TiTe₂ film. We observed ~15x increase in the thermal conductivity upon in-situ annealing at 180^oC from its RT value, pointing to the possibility of a phase transition from an amorphous state. The k was then found to decrease to ~10x its RT value at 220^oC and continued unchanged to 250^oC. Interestingly, there is a 3x increase in k with polycrystalline films deposited at 230^oC compared to amorphous films deposited at RT and annealed to the same T. We expect -dependent in-plane and cross-plane electrical resistivity measurements to provide further insight into such a trend in the measured k.
The unique thermal transport and phase transitions within our sputtered TiTe₂ films are further elucidated by the dynamical analysis of the crystal structure. Our -dependent Raman spectroscopy on the amorphous 120 nm films reveal the formation of in-plane/cross-plane Te bonds beginning at 125^oC, with a minimum of cross-plane Raman peak intensity near 230^oC [6]. We suspect a possible dynamic disparity in Te cross- and in-plane bond arrangement facilitated by the evolution of van der Waal gaps between adjacent sheets of Te and Ti [7].
In summary, our detailed thermal measurements and material analysis of sputtered TiTe₂ thin films reveal an unexplored phase transition that has a dramatic effect on the thermal transport. These results are expected to enable accurate modeling and optimization of thermal barrier PCH based on TiTe₂.
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The resistivity size effect is an important limitation in device interconnect technology. Specifically, as the dimensions of interconnect wires approach ~10nm scales, increased resistivity becomes a fundamental limitation to device performance. While it has been understood that electron scattering from surfaces and grain-boundaries are responsible for this effect, the dominant mechanisms remain poorly understood. Computational approaches can play an important role in elucidating the resistivity size effect and potentially point towards improved design of films and wires.
In this contribution, an approach based on realistic tight-binding models fit carefully to Density-Functional Theory (DFT) results is coupled with an efficient numerical approach based on the Kernel-Polynomial Method (KPM) is used to compute resistivity in Ruthenium thin films with the addition of steps and static phonon disorder. It is shown that single-crystalline films, which have been realized experimentally, can exhibit resistivity values comparable to bulk values, and that surface steps have a minimal impact on resistivity. In addition to scattering from static surface defects, the role of electron-phonon scattering is also presented. It is demonstrated that electron-phonon coupling is different at surfaces where atom coordination is lower. Finally, we will describe work to further develop methodology towards understanding transport at interfaces including with a substrate layer.

Metal-insulator-metal (MIM) devices find application as high speed tunnel diodes for rectenna based energy harvesting and sensing of IR radiation, hot-electron transistors, single electron transistors, resistive random access memory (RRAM), and back end of line (BEOL) capacitors. Precise knowledge of band-offsets / energy barriers, φ_Bn, at metal/insulator interfaces is critical for predicting and understanding charge transport and optimizing MIM device operation.
The Schottky-Mott rule, which neglects charge transfer across the interface, states that the difference between the Fermi level of metal, E_F, and the conduction band of the insulator should vary directly with the metal vacuum work function, Φ_M,vac, and the insulator electron affinity, χ_i so that φ_Bn=Φ_M,vac-χ_i. In induced gap state theory, charge transfer at intrinsic interface traps creates an interfacial dipole that drives the E_F towards the charge neutral level of the insulator, E_CNL,i, the energy at which the dominant character of the interface states switches from donor-like to acceptor-like. A metal on an insulator will thus behave as if it has an effective work function, Φ_M-eff, where Φ_M,eff=E_CNL,i+S(Φ_M,vac-E_CNL,i), where S=d_φBn/d_ΦM,vac.¹ Empirically, S=1/(1+0.1(ε_opt-1)²), so that as the optical dielectric constant of the insulator, ε_hf, increases, S decreases from an ideal 1 towards 0 and the insulator more effectively "pins" E_FM at E_CNL-i.² Despite some success of this theory, it is difficult to calculate or determine E_CNL for a given material and it is observed that actual φ_Bn's often deviate substantially from predictions. In practice, φ_Bn depends on material processing, extrinsic effects such as interfacial and near-interfacial trapped charge arising from point defects, dipoles due to interfacial chemical reactions, and scavenging of oxygen from the insulator. It is therefore necessary to directly measure φ_Bn for the specific metal/insulator combination in use. The only technique capable of measuring in-situ barrier heights in working device structures is internal photoemission (IPE) spectroscopy, an electro-optical technique that allows φ_Bn measurement at both metal interfaces in an MIM device.^3-5 To date, relatively little work has been reported on IPE of MIM structures.^6-11
In this invited talk, we describe IPE spectroscopy measurements of φ_Bn's in MIM structures between various insulators (including Al₂O₃, HfO₂, SiO₂, NiO, CoO_x, and ferroelectric HfZrO_x) deposited via atomic layer deposition (ALD) and various metals including ALD Ru, sputtered thin film glassy metals (ZrCuAlNi, TaWSi, and TaNiSi), and benchmark metals (TaN, Al, and Au). Comparisons made with electrical measurements on the same devices are found to be qualitatively consistent with the IPE determined φ_Bn, even when inconsistent with Schottky model predictions, demonstrating that IPE is a powerful tool for understanding and optimizing MIM device operation.

1. Y.C. Yeo, T.J. King, and C. Hu, J. Appl. Phys. 92, 7266 (2002).
2. W. Mönch, Phys. Rev. Lett. 58, 1260 (1987).
3. V.K. Adamchuk and V.V. Afanas’ev, Prog. Surf. Sci. 41, 111 (1992).
4. V.V. Afanas’ev and A. Stesmans, J. Appl. Phys. 102, 081301 (2007).
5. N.V. Nguyen, O. A. Kirillov, and J.S. Suehle, Thin Solid Films 519, 2811 (2011).
6. V. Afanas’ev, N. Kolomiiets, M. Houssa, and A. Stesmans, Phys. Status Solidi A 215, 1700865 (2017).
7. V.V. Afanas’ev et al., Appl. Phys. Lett. 98, 132901 (2011).
8. M.A. Jenkins, T. Klarr, D.Z. Austin, W. Li, N.V. Nguyen, and J.F. Conley, Jr., Phys. Status Solidi RRL – Rapid Res. Lett. 12, 1700437 (2018).
9. M.A. Jenkins, J.M. McGlone, J.F. Wager, and J.F. Conley, Jr., J. Appl. Phys. 125, 055301 (2019).
10. M. A. Jenkins, T. Klarr, J.M. McGlone, J.F. Wager, and J.F. Conley Jr., ECS Trans. 85, 729 (2018).
11. M. A. Jenkins, K.E.K. Holden, S.W. Smith, M.T. Brumbach, M.D. Henry, C. Weiland, J.C. Woicik, S.T. Jaszewski, J.F. Ihlefeld, and J.F. Conley. ACS AMI 13, 14634 (2021).

Since the electrochemical/photochemical reactions involve charge transfer between matters, the energetics of charge transfer between catalyst surface and reactants should be taken into accounts of reaction kinetics predictions. In general, (photo)catalytic reaction activities of materials are predicted or analyzed using normal density functional theory (DFT) calculations, which can give theoretical overpotentials based on the computed adsorption free energies. However, in reality wide gap semiconductor materials can have varying Fermi level. Therefore, adsorption energy of charge acceptor or charge donor on a semiconductor surface must be expressed as a function Fermi level. In addition, in some catalytic reactions, the potential barriers in Helmholtz layer also must be considered in the computations of reaction energetics. In this talk, I will present recent theory-guided experimental works which demonstrate the Fermi level dependency of anion adsorption energies on p-type or n-type semiconductors, and the theoretical framework which enables accurate prediction of H₂O₂ formation kinetics using wide band gap semiconductors.

Precise dry etching of transition metals is one of the major challenges in, for example, magnetoresistive random-access memory (MRAM) device fabrication. Atomic layer etching of iron, cobalt and FeCoB alloy surfaces with halogen and an organic molecule will be described. We successfully performed etching of Fe and Co thin films by forming volatile metal complexes at low temperature after cyclic sequential reactions of Cl₂ and acetylacetone (acac) or hexafluoroacetylacetone (Hfac). The extent of the Cl₂ reaction determines the etching rate of the metal. In-operando X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) simulation shows that the reaction of Hfac with the chlorinated Co surfaces is likely following a complex reaction pathway instead of simple diketone substitution for the metal chloride. Furthermore, significant surface smoothing was demonstrated using AFM surface topographic imaging. Our preliminary results on the FeCoB alloy show the plausibility of the ALE reaction using cyclic sequential dosing of Cl₂/Hfac. Initial investigations of MgO as an etch stop were positive, as no obvious changes were observed in the thickness of MgO or itsoxidation state. Sequential dosing of Cl₂/acac has also been shown to successfully etch Cu in early experiments. The dosing conditions of Cu are much milder than those of our previous experimental procedures.

P-type metal oxide thin film transistors can be practically applied for CMOS logic circuit with n-type metal oxide TFTs, which are actively used in the display industry. Especially, p-type TFTs have attracted much attention due to the possibility of reducing power consumption by lowering off current. Among various materials, cuprous oxide (Cu₂O) is a promising p-type semiconductor candidate due to its high hole mobility and low cost. However, Cu₂O TFTs have poor on/off current ratio characteristics due to high off current, so it is not suitable for CMOS circuits until now. In the previous study, gallium or nitrogen was doped Cu₂O thin film during Cu₂O thin film deposition to lower the high off current, or new passivation layers were added to reduce defect sites formed in the back channel. In this case, additional materials or layers are required, which result field effect mobility degradation. Here, we have demonstrated Cu₂O TFTs with improved low off current by controlling deposition condition and annealing in N₂ atmosphere. First, copper oxide was deposited on P++ Si/SiO₂ substrate by RF sputtering in argon and oxygen gases. After that, the deposited copper oxide was annealed at 300~800 °C for 1 hour in tube furnace under nitrogen flow. Finally, source-drain electrodes were deposited on Cu₂O film with a shadow mask to measure the electrical properties of TFTs. To find out the characteristics depending on the annealing temperature, we measured the electrical characteristics of TFTs with different temperature, from 300 to 800 °C. As the annealing temperature increased, the TFTs characteristics changed from N-type to insulator, and finally P-type semiconductors with the highest temperature. The characteristics of P-type semiconductor were shown when the device was annealed over 700 °C, and the best TFT characteristics were shown at 800 °C. These TFTs achieved outstanding electrical properties. The field effect mobility value was 1.5 cm²/Vs, which was similar to those of the existing Cu₂O TFTs, but the on/off current ratio was improved to 8.2 × 10⁴ (~1 nA at off state). In addition, subthreshold swing value was 3.3 Vdec^-1, indicating that defects inside the TFTs were greatly reduced. Also, according to XRD analysis of thin film annealed at 800 °C, we confirmed that the intensity of the main peaks of Cu₂O increased, and crystallinity was also improved. Nitrogen plays a role in promoting Cu₂O phase formation while it prevents CuO phase formation during the annealing process. In addition, oxygen added during sputtering increased the M-O bonding and improved crystallinity after annealing. By applying this process, a high-quality Cu₂O thin film could be fabricated. Such proposed method is expected for application in a wide range of display industries in the future.

Excessive use of oxidants during the chemical mechanical planarization (CMP) process causes galvanic corrosion between the copper (Cu) and ruthenium (Ru) films, resulting in defects that reduce device performance. Cu film becomes the anode and Ru film becomes the cathode in Cu interconnects due to the electromotive force differential between Cu and Ru films in a Ru barrier structure. The open circuit potential difference between Cu and Ru films is 0.49V, according to electrochemical analysis in a 0.05 M KIO₄ solution with 3 percent H₂O₂ at pH 10. This result suggests that it is a significant driving force for Cu and Ru galvanic corrosion. As a result, reducing the potential difference to near 0V, or as close as feasible, is critical for suppressing galvanic corrosion of Cu and Ru in the Ru barrier CMP process.
By controlling the affinity between the inhibitor and the metal films, we have investigated a unique inhibitor, nicotinic acid, for reducing galvanic corrosion of Cu. According to bonding orbital theory, the electron configuration of each metal determines its affinity for the pyridine functional group. The development of a thick protective layer by nicotinic acid reduces anodic reactivity in Cu films. Ru, on the other hand, forms just a sparse protective layer due to its low affinity. Interfacial interaction analysis and electrochemical interaction evaluation were used to demonstrate the mechanism. The CMP slurry performance was evaluated based on the researched analysis, which showed that the selectivity between Cu and Ru was 1: 1.

Controlling the depth and shape of nanopatterns down to sub-10nm precision has been realized by utilizing photolithography and nanoimprint lithography in many tactful ways. Accordingly, ultra-precision metallic nanopatterns are playing pivotal role in developing many diverse photonic components, sensor devices, and so forth. In particular, the plasmonic nanoarchitecture with orderly metal nanopatterns generating localized surface plasmon resonance (LSPR) is highlighted as the next-generation eco-friendly sensor platform because it enables precise detection of target materials only by using visible light. While metallic nanostructures made of such as copper, gold, and silver have led the majority of plasmonic device research because of their strong LSPR resonances, the fabrication complexity and low throughput may be further improved towards commercially-feasible manufacturing level by integrating continuous and scalable processing principles.
In this work, we present a high-throughput nanomanufacturing methodology for flexible and scalable plasmonic sensors by adopting roll-to-roll nanoimprint lithography (R2R NIL) process and verify the femtogram level sensitivity of the fabricated sensors [1,2]. Briefly, a UV curable resin can be uniformly coated to a large-area flexible substrate suitable for R2R NIL, by using the facile airbrush process [3,4]. Then by conducting the R2R NIL process on a resin-coated substrate, nanopatterns are continuously produced. Gold is deposited onto the nanopatterns at a certain oblique angle, which allows the scalable fabrication of double-bent Au strip (DAS) LSPR structures without resort to additional etching. Modulation of the Au deposition angle can tune the LSPR characteristics of resulting DAS structure, which can be optimized by 2D finite difference time domain (FDTD) simulation. Finally, the performance of the DAS structure as a plasmonic sensor is evaluated with amyloid beta analytes, one of the pathological biomarkers for Alzheimer's disease. The LSPR peak shifts as the concentration of amyloid beta applied to the DAS structure changes, with the sensitivity as precise as 20 femtogram levels. As a follow-up study, the size-dependent detection sensitivity is evaluated by depositing polysterene (PS) beads having different sizes on the DAS structure [5,6]. The R2R NIL-based DAS fabrication protocol may provide a reliable and practical solution for creating scalable and flexible LSPR sensors with high sensitivity and productivity.

Acknowledgment
This work was supported by the National Research Foundation of Korea (NRF) grants (No. 2020R1F1A1073760, No. 2021M3H4A3A02099204 (Ministry of Science and ICT), and No. 2019R1A6A1A03032119 (Ministry of Education)), funded by the Korean Government.

References
1. J. S. Wi, S. Lee, S. H. Lee, D. K. Oh, K. T. Lee, I. Park, M. K. Kwak, and J. G. Ok, Nanoscale, 9, 1398 (2017).
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6. L. Chen*, A. Panday*, J. Park, M. Kim, D. K. Oh, J. G. Ok, and L. J. Guo (*equal contributions), ACS Nano 15 (9), 14185-14192 (2021).

Fabrication of nanopatterns typically relies on the complicated, costly, and mask/mold-based processes such as nanoimprint lithography and photolithography. Achieving the improved productivity, reduced cost, and extended applicability, we present the piezo actuated one-dimensional patterning (POP) principle in this work [1]. In POP, the rigid mold having specific shapes (usually sharply cleaved flat edge) vertically vibrating by the high-precision actuation of a piezo stack, periodically indents the pattern on the horizontally fed polymer substrate. Compared to the previously demonstrated vibrational indentation techniques typically based on 3-axis vibration [2], POP tactfully utilizes one-dimensional vibration by employing the uniaxially-actuating piezo stack, thereby enabling much higher precision nanopatterning. The control of nanopattern period, depth, and shape can also be conducted by modulating the piezo vibration frequency via Pulse Width Modulation (PWM) module, and by adjusting the tool-substrate gap and tool mounting angle, and so forth.
The POP system consists of four main modules. The first is a piezo actuator module that applies direct vibration, the second is a mold mounting arm that can hold the mold at an appropriate angle, the third is a force sensor module, and finally the fourth is the substrate feeding stage module. The system design drawn by a 3D CAD software and the developed prototype system can be presented in the conference. POP can continuously create the nanograting patterns with diverse periods without resort to specific mold or mask prefabrication, readily by controlling the vibration frequency and substrate feeding speed. The nanopattern period (λ) is simply determined by the ratio of the substrate feed rate (ν) and the tool vibration frequency (f) (λ = ν / f). Also, the nature of POP principle which is purely based on the mechanical machining, allows a very wide choice of substrate materials; any material softer than the tool material (i.e., typically Si or SiO2) can be patterned by POP. A real-time control of vibration frequency further enables the single-stroke fabrication of a period-tunable nanopatterning, namely, chirped grating pattern.
In summary, POP realizes period-tunable nanopatterning on various polymer substrates in a continuous and high-speed fashion. POP creatively adopts the high-precision mechanical machining principle where the rigid tool with high-frequency vertical vibration periodically indents the well-defined nanoscale period patterns on a horizontally fed substrate. The nanopattern periods and depths can be readily controlled by the modulation of tool vibration and substrate feeding, and choice of tool shape and substrate material. POP does not necessitate time-taking and costly prefabrication of molds or masks, but can achieve period-tunable nanopatterning at high throughput with simple setup. Many applications may thus benefit from POP, in the fields such as polymer photonics, flexible electronics, chemical and biosensors, and energy devices.

Acknowledgment
This work was supported by the National Research Foundation of Korea (NRF) grants (No. 2020R1F1A1073760, No. 2021M3H4A3A02099204 (Ministry of Science and ICT), and No. 2019R1A6A1A03032119 (Ministry of Education)), funded by the Korean Government.

References
1. S. Lee*, N. Lee*, G. Yeon*, J. Park, H. Choi, S. Koo, D. K. Oh, and J. G. Ok (*equal contributions), ACS Nano 15 (2), 3070-3078 (2021).
2. S. H. Ahn*, J. G. Ok*, M. K. Kwak, K.-T. Lee, J. Y. Lee, and L. J. Guo (*equal contributions), Advanced Functional Materials 23 (37), 4739-4744 (2013).

For giant enhancement of the sensing property. We investigated the defect-rich SnO2 heterojunction nanofiber device, which can be fabricated by the electrospinning system and annealing process between 300 degrees to 800 degrees. According to defect engineering and grain size effect, the width of depletion not only can be created by defect structure but also served as a heterojunction gate to control the current generation rate. Furthermore, we also obtain the photo current with the multiple wavelengths (365 nm, 520 nm) LED light illuminated and gas sensing (NO, CO)by using the electrical measurement system. These results indicated that defect-rich SnO2 nano-heterojunction sensing devices can be a good gas sensor in our daily lives.

The prediction and control of penetration of water and ions through encapsulation coating is of critical importance in bioelectronic technology. Beside the materials challenges of defining and manufacturing thin-film hermetic encapsulation, the topology of bioelectronic systems that usually include openings in form of vias or electrodes with direct exposure of active materials to the biological medium, introduces demanding use conditions.
In this work, we focus on eventual side permeation of thin film encapsulation i.e., the diffusion of water and ions through the interfaces and/or sidewalls defined within the TFE. We report on an experimental set-up and protocol to systematically quantify side permeation. Magnesium (Mg) small patterns (1mm² squares and 0.5mm radius circles) are thermally evaporated on glass substrates and coated with selected TFE barriers including organic and inorganic films with overlaying but wider patterns. Mg etches readily in water. Using image tracking, corrosion of the patterned Mg areas is monitored over time under standard temperature and humidity conditions (57°C/100%RH). Next an equivalent Sidewall Water Vapor Transmission Rate (SWVTR) values of the TFE is calculated. The impact of the width (50 μm, 100 μm and 150 μm) of the barrier film surrounding the Mg patterns is also quantified. Interfacial contribution to water side permeation is evaluated by comparing the corrosion of the Mg patterns encapsulated with plain versus multilayered TFEs. We found wide barrier overlay and strong interfacial adhesion are beneficial to delay and reduce water side permeation. Next, we will explore strategies such as surface treatments and adhesion promoters to further minimize side permeation.
These experiments inform on design guidelines for the layout of “through-TFE” openings in bioelectronic devices that are compatible with reliable hermeticity.

According to the unsubsidized Levelized Cost of Energy Comparison published by Lazard, solar energy has decreased from the most expensive source of electricity in 2009 to the least expensive source of energy generation technology in 2020. Perovskite solar cells are particularly attractive due to economic feasibility and relatively simple manufacturing. Perovskites possess a numerous attractive electrical and optical properties and solid-state phenomena that have applications that range from electrical insulation to semiconducting and superconducting characteristics. This class of materials is used in various applications, such as solar cells, LED lights, display screens, memory devices, lasers, and photodetectors. Due to the wide array of applications, mechanical and electrical performance of these materials is of great interest.
Enhancing adhesion between perovskite crystals and the substrates they are synthesized on is a key factor that increases mechanical integrity and overall stability of the photovoltaic device. Furthermore, increased adhesion can lead to performance enhancements of the devices in certain areas, particularly for the electronic properties. Therefore, it is in the interest of manufacturers and researchers to optimize the adhesion of perovskites on their respective substrates. However, reliable determination of perovskite adhesion can prove difficult as the size scale of the devices becomes increasingly smaller. The motivation of this work is to investigate perovskite adhesion on thermally evaporated metallic thin films on rigid substrates. Measurement of perovskite adhesion on substrates could provide useful methodologies and materials substrate-perovskite selection and pairing as well as device optimization.
For this work, various metals and metal composite stacks were deposited onto silicon wafer substrates. Examples include chromium-nickel, chromium-silver, chromium, aluminum, and silver. Thickness of the deposited thin films was controlled and validated through use of a profilometer. Perovskites were synthesized using a solution-based spin coating method. After depositing the perovskites, the morphology of the crystals was analyzed by scanning electron microscopy. Preliminary results show perovskites with lateral dimensions of 1-4 µm were formed on the substrates. Following the morphological analysis of the perovskite crystals, this research will use a modified nanoindentation method to determine the energy required to dislodge the perovskite crystals from the substrate surfaces to characterize the adhesion energy at the perovskite-substrate interface.

A large class of toxic compounds, including organophosphates, react with water. Although detection of such hydrolysable toxic compounds are important, gas sensors that utilize hydrolysis as a detection mechanism have not been developed, mainly because water quickly evaporates and thus cannot be employed in the sensor that operates in the open-air environment. In this work, we demonstrate that carbon nanotube (CNT)-based chemiresistor array coated with aqueous solution of hygroscopic salts selectively detects water-soluble and hydrolysable toxic compounds. The aqueous solutions of LiCl, LiBr, and H₃PO₄ in this study did not dry out and remained as liquid film indefinitely, owing to their extremely low (<15%) deliquescence relative humidity (DRH) above which the salts become aqueous solution by absorbing moisture from the air. Upon exposure to diphenyl chlorophosphate (DPCP) and 2-chloroethyl phenyl sulfide (CEPS), both of which react with water, the resistance of the sensors showed a large decrease, and the amount of resistance decrease was much greater than that of dry CNT chemiresistors without the aqueous film. We also tested the sensor response to compounds that are not hydrolyzed and confirmed that our sensors coated with aqueous film show high sensitivity and selectivity toward hydrolysable compounds. The amount and direction of the resistance change varied with both the type of salts dissolved and the analytes, suggesting that our sensors potentially apply to discrimination of a wide range of water soluble and/or hydrolysable chemical compounds

The self-stratifying coating is a one-step process enabling the formation of a complex multi-layer coating structure at the surface of the polar substrate, combining optimized surface and adhesion properties. Self-stratifying coatings offer three major advantages. First, there are the economic benefits of applying two coats in one operation. Second, it is eco-friendly than common multi-layer coating methods through reducing VOC emissions contributing to the air pollution problem. Third, bonding strength at the interface is much stronger than the one between separately applied coats, thus less possibility of inter-layer adhesion failure is expected. In this study, we developed a self-stratified two-phase coating system based on different surface tension. The stratification behavior was evaluated by the difference in interfacial energy. The research systematically characterized the correlation between the interfacial energy difference and the layer separation behavior. We have selected a mixture consisting of epoxy and acrylic copolymer since epoxy can provide excellent adhesion to the polar substrate, while acrylic copolymer can provide better durability, flexibility, sunlight resistance. To systemize the self-stratifying behavior based on the surface tension differences, we synthesized various acrylate copolymers. It has not only lower fluorine content than conventional fluorinated chemicals but also has low surface energy. Therefore, fluorinated copolymers are not only more eco-friendly than conventional fluorinated chemicals but also provide the antifouling and self-cleaning effect. To reach a perfect stratification, the influence of various parameters also including the polarity of the resins and the volatility of the solvent was studied. In conclusion, the research indicated that a homogeneous mixture of two polymers with a different surface tension of more than 15 mN/m can be stratified. The results of the research show potential commercialization in various industries such as multi-layered coatings for future automobiles, and electronic devices.

Silicon carbide (SiC) is a remarkable semiconductor material for high-temperature and high-voltage applications. It is also used as a material for a focus ring in etching process of silicon wafers, due to its high plasma resistance. SiC is commonly fabricated via chemical vapor deposition (CVD) or physical vapor transport (PVT) methods, and CVD-SiC is widely used as a material for the focus ring to protect the electrostatic chuck and create a uniform plasma condition during the plasma etching process of a silicon wafer.

Currently, CVD-SiC is widely used because it is favorable for manufacturing SiC with a low micropipe density. However, CVD-SiC has an issue in that the unit cost is high. PVT-SiC, on the other hand, offers the advantage of a cheap production cost despite the difficulty of reducing micropipe density. While it has been reported that micropipes reduce electron mobility in SiC, no specific influence on plasma resistivity has been identified.
In this study, we investigated the etching rate of CVD-SiC with small grain size and PVT-SiC with large grain size when SiC crystals grown by PVT and CVD methods are etched by reactive ion etching (RIE) under severe etching circumstances.

As a result, the etching behavior in the PVT-SiC was revealed to differ from CVD-SiC. CVD-SiC with a small grain size has a problem in that SiC particles are peeled off grain by grain during the etching process, affecting the Si wafer quality. In the case of PVT SiC, however, the etching resistance was found to be higher due to lower grain boundary density. We also observed that, despite its relatively high micropipe density, PVT-SiC could be used as a focus ring material due to its high plasma resistance. The lower unit cost of the SiC focus ring, which is required as a consumable in the semiconductor process, is expected to increase production efficiency.

In this research work, defect-rich poly-crystalline SnO₂ nanofiber, which has plenty of oxygen defects by the electrospinning system and high-temperature annealing process. Besides, according to defect engineering, plenty of depletion areas can be created at the nano interface, which became a nano junction gate to well control the bend structure and electrical transport property between the grain-boundary. Furthermore, the oxygen defect structure also can be a reaction center to binding with water molecules and detecting multiple wavelength lights in different humidity environments. We also obtain the Raman spectrum of oxygen vacancies with different humidity environments by using the in-situ Raman spectrum system. Hence, a defect-rich SnO₂ nanofiber photocatalytic sensing device can be achieved. This device provided many advantages. First, the device can be operated in a low and high humidity environment (3%-75%) to enhance the sensing property. Second, the high photocatalytic property with multiple wavelength light ( 365 nm, 465 nm, and 520 nm ). Defect-rich SnO₂ nanofiber photocatalytic sensing device has good potential for commercialization and use as toxic gas and photon sensor in the high humidity environment.

Interest regarding flexible photonic layers of polymeric materials, especially one-dimensional photonic crystals, is growing rapidly as they can find applications to create selectively reflective surface coatings and waveguides. These can be tuned to different wavelengths while maintaining efficiency and flexibility. Distributed Bragg Reflectors (DBR) are a class of one-dimensional photonic crystals that feature alternating high and low refractive index materials at particular thicknesses to selectively reflect certain portions of the electromagnetic spectrum. Although DBRs are widely researched in applications where a single wavelength is targeted, mathematical simulations indicate that they may be fabricated to target multiple wavelengths when certain constraints are applied. Herein, we explore the conditions and variables that can be manipulated to maximize peak efficiencies of DBRs of unique geometries. Through our experimentation, we demonstrate high levels of efficiency and a wider array of targeted wavelengths. By carefully selecting polymers and nanoparticles that possess orthogonal solvent profiles (such as Cellulose Acetate and Polyvinyl Carbazole) we have demonstrated a high degree of control over targeted wavelength and band thickness, particularly in the wavelength of lower infrared regions (~1.5 microns). Oxide-based nanoparticles were used in this study, with surface-based functionalization. Furthermore, this study will discuss concepts of interfacial roughness, film thickness, film flexural properties, and angle-dependent light scattering.

The conversion of solar energy into chemical energy by photocatalytic water splitting is considered a promising technique to generate pollution-free and sustainable energy. Herein, using first-principles density functional theory and time-dependent nonadiabatic molecular dynamics calculations, we propose C₃N₃/C₃N₄ van der Waals heterostructure as a possible Z-scheme photocatalyst for water splitting satisfying both oxidation and reduction potentials. Additionally, using GW and Bethe-Salpeter equation approximations, we identify the presence of long-lived interlayer excitons that enables the separation of electron and hole in different layers. It facilitates the oxygen evolution reaction and hydrogen evolution reaction on C₃N₃ and C₃N₄ layers, respectively. The free energy analysis using the computational hydrogen electrode model for both reactions suggests facile reaction pathways upon light illumination. Further, red shifting of optical spectrum in the heterostructure with respect to the monolayers leads to significant enhancement of the optical absorption in the visible range, suitable for practical photocatalytic applications. The high charge carrier mobilities also augment the efficient utilization in the photocatalysis process before recombination. Moreover, the overall solar-to-hydrogen efficiency for the Z-scheme heterostructure is found to be three times higher than the type-II heterostructure. The present study formulates several underlying physical and chemical principles of efficient photocatalysis, which would further lead to the identification of new metal-free carbon nitride-based Z-scheme heterostructures.

From metal contacts to solar cells and batteries, interfaces dominate the physics of devices. Their desired features can often stem from properties unique to the interface, not displayed in either constituent material. Whilst modelling of the individual constituents of such systems is well understood, the modelling of interfaces is limited due to their structure being relatively unknown. Prediction of these interfaces currently boils down to human intuition, which is fraught with problems. We have developed ARTEMIS [1], an interface structural prediction tool to aid users in characterising the electronic, optical, and phononic properties of interface systems using common approaches within the community.
ARTEMIS is a tool undergoing constant development to aids in the exploration of interface structures with the goal of eliminating human bias and reducing computational cost. The package explores the energy space to find the lowest energy configuration. The physics at the heart of the interface problem involves the balancing of mismatch strain and compensating dangling bonds, which can be broken into two key challenges that need to be overcome; 1) identify the most energetically favourable alignment between the two constituents, and 2) model reconstruction that occurs across the interface. Using a combination of matching and predictive methods, ARTEMIS reduces the time and computational cost of tackling these key challenges.
Using a range of metal contact interfaces, we show how ARTEMIS can produce interface structures for complex systems in order to find the lowest energy interface. For the first challenge, we present a set of metal-semiconductor interfaces, such as those between transition metal dichalcogenides and metal contacts, to highlight ARTEMIS’s ability to identify the lowest energy separation between two materials. For the second challenge, we present MgO/graphite interfaces to show how it can be used to predict new material phases, highlighting its capabilities, strengths and current limitations compared to other approaches. Finally, we demonstrate how the predicted interfaces can aid in the identification of unusual dielectric and electronic properties.
References
[1] Ned Thaddeus Taylor et al. “ARTEMIS: Ab initio restructuring tool enabling the modelling of interface structures”. Comput. Phys. Commun. 257 (2020), p. 107515.

In-Situ interface preparation allows us to fabricate low turn-on (knee) voltage (~0.3 V) Schottky-diodes of a four-layer (4L) MoS₂ / GaN stack. The provess involves ultra-high vacuum cleaning and nitridation of the GaN substrate followed by growth of the MoS₂ overlayer using metallic molybdenum and hydrogen sulfide gas as precursors. The process leads to a clean nitrogen-terminated GaN surface that bonds well to the MoS₂ film. A 2x2 reconstruction at the interface/first MoS₂ layer is observed in low-energy electron diffraction (LEED). Atomic force microscopy and X-ray photoelectron spectroscopy provide clear images of the GaN terraces through the MoS₂ overlayer confirming close adhesion and absence of oxygen and other contaminants. Density functional theory calculations predict the formation of the 2x2 superstructure at a clean interface. Transport measurements show diode behavior at an on/off ratio of ~10⁵ for ± 1V with a forward direction for positive voltage applied to the MoS₂ layer. Combining transport and photoelectrons spectroscopy measurements with theory, we deduce a Fermi-level position in the MoS₂ gap consistent with interface charge transfer from the MoS₂ to the substrate. The high performance of the MoS₂/Gan diode highlights the technological potential of devices based on GaN/MoS₂ interfaces.
See also:
Hae In Yang, Daniel J. Coyle, Michelle Wurch, Prachi R. Yadav, Michael D. Valentin, Mahesh R. Neupane, Kortney Almeida, and Ludwig Bartels, Epitaxial Molybdenum Disulfide/Gallium Nitride Junctions: Low-Knee-Voltage Schottky-Diode Behavior at Optimized Interfaces, ACS Appl. Mater. Interfaces 2021, 13, 29, 35105–35112

1. Introduction Technology for building high-speed thin-film transistors (TFTs) at low temperatures is the key to fabricating high-performance flexible devices. Ge has been expected to be a channel material for TFTs owing to its high carrier mobility and relatively low crystallization temperature. To date, low-temperature syntheses of Ge layers have been achieved using various techniques; however, most polycrystalline (poly-) Ge has been poorly crystalline.
We have developed an advanced solid-phase crystallization (SPC) technique using a densified amorphous (a-) Ge precursor [1] and updated the Hall carrier mobility of poly-Ge to 620 cm² V⁻¹ s⁻¹ for holes [2] and to 370 cm² V⁻¹ s⁻¹ for electrons [3]. The high-mobility Ge layers have been also developed on a plastic substrate [4]. The performance of the accumulation-mode p-channel TFTs exceeded the most of TFTs based on poly-Ge layers [5]; however, the TFTs characteristics varied for each device within the same chip.
In this study, we clarified that the characteristic variation of the Ge-TFTs was caused by the variation of grain boundaries and crystal orientation. Additionally, to suppress the variation, we investigated metal-induced lateral crystallization (MILC) using the densified a-Ge precursors that enabled large-grained SPC-Ge. We demonstrate three-dimensionally orientation-controlled Ge rods formed at 325 °C.
2. Experimental method We deposited 100-nm-thick a-Ge layers on SiO₂ glass substrates at 125 °C using a molecular beam deposition system. The samples were annealed at T_anneal = 450 °C for 5 h in N₂ to induce SPC. The detailed flow for the TFT fabrication is described elsewhere [5]. For MILC, 5-nm-thick metal (Ag, Au, Bi, Co, Fe, Ni, Pd, and Pt) strips with a width of 5 µm were formed on the a-Ge layer. The samples were then annealed in an N₂ atmosphere at T_anneal = 325–350 °C for 0–100 h. The samples were characterized using optical microscopy, Raman scattering spectroscopy, electron backscatter diffraction (EBSD) analysis, and transmission electron microscopy (TEM) analyses with an energy dispersive X-ray (EDX).
3. Results and Discussion The EBSD analyses indicated that the channel region of the TFT was composed of randomly oriented grains with a few μm. The TFT characteristics showed a large variation with field-effect mobility of 10-190 cm² V⁻¹ s⁻¹ and on/off ratio of 10¹-10³. This variation became more prominent when the channel length was narrower, suggesting that the variation of the TFT characteristics was affected by the randomness inherent in polycrystalline channels.
To control the grain boundaries and the orientation, we investigated the MILC using 8 different metals. According to the optical microscopy and Raman scattering spectroscopy, the lateral growth of crystalline Ge was observed over several μm in the vicinity of each metal pattern at T_anneal = 350 °C. EBSD measurements revealed that the grain size in the MILC region was a few μm. This is the first time that μm-order grains were detected in MILC-Ge, indicating that densification of a-Ge precursors was effective in MILC as well as SPC [1].
Among the metals, here we focused on Ni. Lowering T_anneal to 325 °C enlarged the MILC region, which consisted of (110)-oriented rod-shaped crystals (15 μm length, 2 μm width). Furthermore, the longitudinal orientation of the growth direction was <110>. The amount of Ni in Ge was below the detection limit of EDX (< 1%). The TEM measurement revealed that the high-crystal quality of the three-dimensionally orientation-controlled Ge rods, which will show single-crystal-like properties. We are now fabricating the flexible TFTs based on the Ge layers.
[1] K. Toko et al., Sci. Rep. 7, 16981 (2016).
[2] T. Imajo et al., Appl. Phys. Express 12, 015508 (2019).
[3] M. Saito et al., Sci. Rep. 9, 16558 (2019).
[4] T. Imajo et al., Sci. Rep. 11, 8333 (2021).
[5] K. Moto et al., Appl. Phys. Lett. 114, 212107 (2019).

In this presentation the significance, role, attributes and challenges associated with the use of analytical (“mathematical”) modeling in electronics and photonics materials science are addressed, as well as the interaction of such modeling with computer simulations and accelerated testing. The emphasis is on some critical and even paradoxical, i.e., a-priori non-obvious, problems and situations. The general concepts are illustrated by practical numerical examples. Our objective is to show materials scientists and chemical engineers not involved in electronics and photonics materials science and engineering what kind of value they could bring to this important area of “high-tech” field. It is concluded that the era of creating new materials by "heating, beating and praying" has gone, that predictive modeling in this area is critical, that analytical modeling should always complement computer simulations, such as, e.g., finite element analyses (FEA), and should always precede experimental investigations and reliability testing: analytical modeling and computer simulations are based, as a rule, on different assumptions and employ different calculation techniques, and if the computed data obtained using these two major modeling tools are in agreement, then there is a good reason to believe that these data are accurate and trustworthy.

Silicides are widely used as contact materials in complementary metal oxide semiconductor (CMOS) technology to reduce contact resistance. In this work, a unique structure of a NiSi2 crystal embedded in a silicon nanowire was fabricated and the electrical properties were discussed. Although NiSi2 has higher electrical resistivity than NiSi, it has a smaller lattice mismatch with silicon. The similar lattice structure and parameters of NiSi2 and silicon direct epitaxial growth of the NiSi2 in nanowire by controlling vapor-liquid-solid (VLS) growth. The sharp interface of NiSi2 nanocrystal and silicon was investigated by scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS). Furthermore, The structure and the electrical properties were simulated in TCAD. We found that the NiSi2 nanocrystal could be evaluated as a transistor channel in the nanowires where the NiSi2 reduced the channel resistance. The results from experimental measurements were compared to the TCAD simulations.

The structure of the interface in metal to n-type 3C-SiC ohmic contacts has been important in determining the specific contact resistance and hence the functionality of devices. Recently, the use of Cr/Ni metallization on n-type 3C-SiC has shown an equivalent specific contact resistance to standard Ni contacts with but with an improved surface morphology and reduced interdiffusion [1]. In this paper, we examine for the first time the interfacial characteristics of metallizations containing layers of Ni and Cr (Au/Ni/Cr/ and Au/Cr/Ni) on n-type 3C-SiC. A study of these layered structures both as-deposited and annealed at 750-1000°C has been performed using depth profiling with X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). Elemental mapping by AES in conjunction with SEM imaging have also been used to examine the distribution of elements and the surface morphology after annealing. For the Au/Ni/Cr/n-SiC structure, the analysis has shown a wide-scale interdiffusion of the layers after annealing with the formation of surface globules which were enriched in Ni and Si. In comparison, the Au/Cr/Ni/n-SiC metallizations have shown a limited indiffusion of Ni and outdiffusion of C and Si with the formation of relatively smooth surfaces. These results have indicated that the intermediate layer of Cr has acted as a diffusion barrier for the Ni. The Au/Cr/Ni/n-SiC contacts annealed at 1000 °C have correlated with a minimum in specific contact resistance as measured using circular transmission line model (CTLM) test patterns. [1] P.W. Leech, M.H. Kibel, A. Barlow, G.K. Reeves, A.S. Holland, P. Tanner, Microelectronic Engineering, 215:111016 (2019).

Metal to insulator transition (MIT) in VO₂ is a notable phenomenon which makes it a versatile material to be utilized for infrared detection, electronic and resistive switches, smart windows, etc [1]. Temperature is one stimulus which triggers the MIT, while electric field is also other stimuli which can trigger the transition but at a much lower temperature [2]. However, at room temperature alone VO₂ requires higher electric field for transition which restricts its application in low power electronic devices. TiO₂ in rutile phase plays a major role in interfacial strain engineering for VO₂/TiO₂ heterostructures and facilitates the modulation of transition electric field, transition temperature, magnitude of resistance change and potentially lowers the required electric field of transition [3]. In this work, attempt has been made to reduce the MIT electric field using TiO₂ as an interfacial layer. Studies were conducted on two different systems, namely VO₂/Al₂O₃ and VO₂/TiO₂/Al₂O₃. Here, the VO₂ and TiO₂ were deposited on sapphire substrate (0001) using the pulse laser deposition (PLD) technique [4]. X-ray diffraction (XRD) was performed in order to observe the phase formation and growth direction in both systems. Additionally, temperature dependent in situ XRD was performed to analyse the structural phase transition from monoclinic (M1) phase (at room temperature) to tetragonal (T) phase above 68^oC [5] . Further electrical characterization was also carried out for both the samples in two different configurations i.e. with in-plane and out of plane contacts. Temperature dependent resistance measurement was done to observe the order of change in resistance at MIT for both the systems. Out of all four configurations, the VO₂/TiO₂/Al₂O₃ in plane contact configuration showed the highest change in magnitude of resistance. Subsequently, the effect of TiO₂ was observed on transition voltage from I-V measurements at temperatures ranged between 20^oC to 80^oC. Further, the VO₂/TiO₂/Al₂O₃ in-plane contact configuration showed transition at the lowest electric field i.e. 1.4 MV/m. This indicates that VO₂/TiO₂/Al₂O₃ in plane contact configuration is superior to the VO₂/TiO₂/Al₂O₃ out of plane configuration and VO₂/Al₂O₃ system in terms electric field and magnitude of resistance change. Lowering in electric field may be attributed to Poole–Frenkel effect or the joule heating at the VO₂-TiO₂ junction. Therefore, the VO₂ deposited on TiO₂ interface of sapphire can be used in resistive memory devices and other low powered sensors.
References:
[1] S. A. Bukhari, S. Kumar, P. Kumar, S.P. Gumfekar, H.J. Chung, T. Thundat, A. Goswami, “The effect of oxygen flow rate on metal–insulator transition (MIT) characteristics of vanadium dioxide (VO₂) thin films by pulsed laser deposition (PLD),” Appl. Surf. Sci., vol. 529, no. April, p. 146995, 2020.
[2] S. Jessadaluk, N. Khemasiri, P. Rattanawarinchai, S. Rahong, A. Rangkasikorn, N. Kayunkid, S. Wirunchit, A. Klamchuen, J. Nukeaw, “A tunable thermal switching device based on Joule heating-induced metal-insulator transition in VO₂ thin films via an external electric field,” Jpn. J. Appl. Phys., vol. 58, no. SD, 2019.
[3] E. Breckenfeld, H. Kim, K. Burgess, N. Charipar, S.F. Cheng, R. Stroud, A. Pique, “Strain effects in epitaxial VO2 thin films on columnar buffer-layer TiO2/Al2O3 virtual substrates,” ACS Appl. Mater. Interfaces, vol. 9, no. 2, pp. 1577–1584, 2017.
[4] D. Bhardwaj, A. Goswami, and A. M. Umarji, “Synthesis of phase pure vanadium dioxide (VO2) thin film by reactive pulsed laser deposition,” J. Appl. Phys., vol. 124, no. 13, 2018.
[5] R. McGee, A. Goswami, S. Pal, K. Schofield, S. A. M. Bukhari, and T. Thundat, “Sharpness and intensity modulation of the metal-insulator transition in ultrathin VO2 films by interfacial structure manipulation,” Phys. Rev. Mater., vol. 2, no. 3, pp. 1–10, 2018.

Granular metals (GMs) consist of metal islands dispersed within an insulating matrix. Depending on the metal concentration, GMs exhibit either metallic or insulating behavior. The metal concentration at which this transition occurs is the percolation limit and depends on the dimensionality of the sample and the geometry of the metal islands. Below the percolation limit, GM conduction is due to thermally activated tunneling and capacitive transport. Tunneling is strongly dependent on both island size and the tunneling barrier height. Consequently, GMs have been a focus of fundamental research on the conductivity in discontinuous films. Despite being studied for over 50 years, there remain significant gaps within the GM literature. For example, studies on GMs with high melting point metals and/or non-oxide insulators and studies on the interfacial metal-insulator interactions are almost entirely lacking. We synthesize thin GM films by co-deposition of Co or Mo with YSZ or SiN_x using radio frequency co-sputtering. The structural and electronic properties are studied as a function of volumetric metal concentrations, from 20% to 80%. The island morphologies of both as-deposited and post-annealed samples are examined using transmission electron microscopy (TEM). X-ray photoemission spectroscopy (XPS) is used to probe the metal-insulator interfaces. We confirmed that, for metal fractions below the percolation limit, conductivity was dominated by thermally activated charge carriers, by low-temperature (~77K) conductivity measurements. These low-temperature measurements showed a several orders-of-magnitude decrease in conductivity versus the room-temperature measurements due to the decreased tunneling decay lengths at low temperatures. High-temperature annealing offers a second means to control the tunneling probability since annealing results in increased island separation distances. Additionally, the YSZ-based granular metals show metal-induced oxygen vacancies as well as downward band bending at the metal-insulator interface which serve to increase the conductivity. For the SiN_x-based GMs, nitrogen vacancies result in strong metal-Si interactions which are dependent on the chosen metal. These metal-insulator interactions have been largely neglected in the literature and along with controlling the island separations via annealing provide a path for tuning the electrical transport in future granular metals.
Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy National Nuclear Security Administration under contract DE-NA0003525.

The nanoscale self-assembling systems obtained by intercalation of 3d-elements (e.g., Ni) into a matrix of InSe semiconductor layered crystal are interesting in terms of creating materials with paramagnetic properties for use in spintronic devices. The metal nanostructures in the interlayer gaps become surface ones after cleavage, and thus could be studied by low electron energy diffraction (LEED) and scanning tunneling microscopy / spectroscopy (STM/STS) methods and X-ray photoelectron spectroscopy (XPS).
Nickel intercalated InSe crystals have been grown by the Bridgman method from the pre-synthesized components. Synthesis was carried out by a special procedure to prevent the formation of nickel selenides. Further thermo treatment of grown crystals in evacuated quartz ampoules, that was carried out for 60 h at 870 K, led to Ni intercalation. LEED data were analyzed using the SPECS SAFIRE Diffraction Image Acquisition and Processing System. The STM/STS data were acquired using the Omicron NanoTechnology STM/AFM System.
The clear LEED spots of InSe hexagonal structure are observed for InSe (Ni). The calculated values of the two-dimensional (0001) InSe (Ni) lattice constants a = b = 4.0 Å, correspond to the ones for bulk InSe crystal. Therefore, it was confirmed that Ni was introduced in the interlayer gap. The additional LEED spots corresponding to metallic Ni phase are determined at high (~ 190-200 eV) primary electron beam energies.
STM suggests the unreconstructed structure of (0001) surface covered by intercalate, which is similar for pure InSe. However, some clusters are observed on the surface in the form of local blurring of hexagonal structure. 2d FFT filtering of these clusters revealed the structure with 3.5 Å period which is characteristic for metallic nickel. The analysis of density of states by STS confirms the "metallic nature" of local areas on the (0001) InSe surface. XPS elemental and phase analysis of Ni/(0001) InSe nanosystem shows nickel quantity on the (0001)InSe surface less than 4%.

In semiconductor manufacturing, various low-yield wafers are produced inevitably. Determining the root cause of low yield is an important procedure for preventing production of low-yield wafers as well as providing feedback of process modifications. However, the procedures are complicated due to broad data to investigate and an insufficient number of faulty wafers to compare. In addition, failure analysis includes various investigating fields, such as process step information, measurement data, inspection maps, delay time, equipment error, source change, etc. Therefore the strategic examination of possible causes is required to increase the efficiency of quality control.
Failure analysis of anomalies starts with defining the group of faulty wafers. Afterwards, process modifications and operating equipment assignment are reviewed generally. In some cases, causes of anomalies are not revealed by general analyzing procedures and some features of issues indicate that each process in equipment of individual wafer is needed to be screened. In this study, we propose the addition of comparing trace sensor data across the whole manufacturing processes for detecting the faulty process. Pre-processing includes labeling, process sequencing, and signal adjusting. Besides, the sensor data are enormous even for a few wafers because they pass hundreds steps during the fabrication. Therefore the comparison between faulty wafers and normal wafers requires prioritizing technique with proper scoring.
As an empirical case, we analyzed an example, with a few intermittent faulty wafers having a specific fail mode, by the proposed method. The priority of sensor signal comparison was high since only a few wafers were defective and no changed process were matched with the course of the wafers group. Moreover, the measurement and inspection data of good and bad wafers were not sufficient in the early stage. Based on binary groups, we compared sensor signals with the capable computing tools combining pre-processing, comparing and prioritizing. Accordingly, the root cause process was identified by distinctive signal differences at a specific step. Consequently, the unstable point of the process was clarified with signal information and subsequently repaired, using domain knowledge.
In conclusion, the addition of the sensor data comparison for failure analysis provides not only possible causes but also clues to process feedback, since sensor signals of equipment contain process information. This method is particularly advantageous when the problems are equipment related and the trace sensor data are recorded. Furthermore, the sensor signals are commonly available for every wafer or lot in consideration of recent progress of equipment sensors in the semiconductor manufacturing industry. Therefore, in the early stage of issue when only a few abnormal wafers are detected, this method is possibly used as a powerful tool to discover the root cause of excursion.

Nickel oxide (NiO) with rock-salt type structure is a well-known p-type wide-bandgap semiconductor, which have attracted interests for device application in optoelectronics such as photodetectors, electrochromic layers, photovoltaics and so on ^[1,2]. It is important to modify conductivity, morphology, and structure of surface and interface of epitaxial thin films for utilization in such functional devices. Aliovalent ions such as Li⁺ have been doped to increase carrier density and enhance p-type conduction of NiO thin films. Meanwhile, modification of p-type conduction in NiO without impurity doping would advance development of multilayered optoelectronic devices with controlled interfacial properties. There have been a few reports about altered electronic states and properties of polycrystalline NiO films after UV-ozone treatment using mercury lamps ^[3,4]. Further research on properties and structures of epitaxial NiO thin films irradiated with UV light depending on planar ionic arrangement would help us understand the photo-induced phenomena. In addition, application of vacuum-ultraviolet (VUV) light with larger photon energy and efficient production of active oxygen species would also contribute to modify the properties. In this study, noticeable conductivity improvement of epitaxial NiO thin film surface was obtained by excimer VUV-light irradiation, and the effect of epitaxial crystal orientation on the modified properties and morphology was investigated by changing the substrate planes.
The NiO epitaxial thin films with various orientations such as (111), (110) and (100) were grown on mirror polished SrTiO₃ single crystal substrates by pulsed laser deposition (PLD) technique using a KrF excimer laser (λ=248 nm, E~1.2 J/cm²) and a pure NiO sintered target. The thin film growth took place at room-temperature (not-heated intentionally) in an oxygen pressure of 1×10^–3 Pa. The NiO thin films were subsequently introduced to VUV-light treatment using a Xe₂ excimer lamp (λ=172 nm, E~65 mW/cm² at lamp surface). The VUV-light was irradiated to thin film surfaces in air at the distance between lamp and sample surface of 0.5 mm. The resistivity of the PLD-grown 40 nm-thick epitaxial NiO thin films were ~1×10⁴ Ωcm or higher, although VUV-light irradiation drastically reduced the value of NiO (111) and (110) films to ~6×10^–1 Ωcm and ~2×10^–1 Ωcm, respectively after the treatment for 120 minutes. The resistivity of NiO (100) film was remarkably reduced films by four-digits or more, that ~8×10^–2 Ωcm was obtained after the irradiation. The epitaxy of rock-salt type NiO remained even after VUV-light irradiation, while slight decrease of plane spacings suggested redox reaction near the surface. Thickness dependent structural and morphological change according to the VUV-light irradiation would also be discussed in detail.
[1] M. R. Hasan et al., APL. Mater. 3 (2015) 106101.
[2] S. G. Danjumma et al., Int. J. Eng. Res. Technol. 8 (2019) 461–461.
[3] G. H. Aydogdu et al., J. Appl. Phys. 108 (2010) 113702.
[4] M. T. Greiner et al., J. Phys. Chem. C 114 (2010) 19777–19781.

The inherent quasi-3D structure and the concentration effect of the hydrophobic paper led to a significant enhancement in the Raman scattering.^[1] On the other hand, size-tunable aluminum nanoparticles (AlNPs) have been suggested as a remarkably low-cost alternative plasmonic material due to its adjustable localized surface plasmon resonance band from UV-visible to near-infrared (NIR) regions.^[2]
Herein, we combine the merits of the AlNPs and hydrophobic paper to introduce a sensitive but cost-effective Al paper-based SERS approach. Initially, AlNPs in different diameters were synthesized. Then, the 3D finite-difference time-domain (FDTD) optical simulation was applied to investigate the optimal size and density of the AlNPs, which contribute to the strongest electric field intensity and the desired electric field distribution. Next, a 785 nm portable Raman spectrometer was chosen to test out the possibility of the Al paper-based SERS approach as an on-site detection platform. Namely, the Al paper-based SERS approach was utilized to identify samples made of edible and industrial dyes and a candy blended with an illegal color additive. Apparently, the AlNPs paper-based SERS approach has proven its effectiveness as an on-site detection tool by rapidly distinguishing different kinds of colorants in all samples.
In a nutshell, through decorating the AlNPs on hydrophobic paper, we established a novel on-site SERS sensing platform with excellent sensitivity and notably low cost, which is hard to achieve in other conventional materials.
Reference:
1. Lee, M., et al., Subnanomolar Sensitivity of Filter Paper-Based SERS Sensor for Pesticide Detection by Hydrophobicity Change of Paper Surface. ACS Sensors, 2018. 3(1): p. 151-159.
2. Renard, D., et al., Polydopamine-Stabilized Aluminum Nanocrystals: Aqueous Stability and Benzo[a]pyrene Detection. ACS Nano, 2019. 13(3): p. 3117-3124.

Chemical mechanical planarization is an enabler for the packaging level integration schemes in addition to barrier level applications. The yield of chip packaging operations is a function of the surface finish that is solderable and wire-bondable. Palladium (Pd) is a material that is known for its ability to improve processing cost and reliability at the packaging level with lead frame in addition to its potential use as a sacrificial layer for copper (Cu) integration to protect the copper from oxidation. CMP selectivity becomes critical where a competitive removal rate is desired between the Cu, Cu-barrier (TaN) and Ni films against palladium. This paper focuses on removal rate selectivity of Pd in commercial silica-based slurries tuned for Cu CMP with detailed electrochemical analyses and incorporating the effect of temperature and its effects on surface energy as well as CMP selectivity. Pd being a novel metal shows strong passivation in the presence of an oxidizer, that can be tuned by slurry chemistries. Furthermore, lowering the slurry temperature promotes its CMP removal rate selectivity against Ni, Cu and TaN by enhancing slurry viscosity with no detrimental effect observed on surface defectivity.

Hydrophobic and water-repellent surfaces can be used in various applications including microfluidics and self-cleaning solar cell surfaces. Numerous studies reported enhanced hydrophobicity by creating micro-/nano-structures and/or spin-/spray-coating of hydrophobic materials. Conventionally, those approaches require complex fabrication processes such as photolithography, growth, etching, chemical synthesis, electrospraying, and so on. On the other hand, the aerogel is a porous nanostructure which is mostly filled with a gas. Due to its surface roughness, silica aerogels are one of attractive materials for hydrophobicity. Here, we report a simple method to create hydrophobic and water-repellent surface on various elastomers.
We implemented hydrophobic surfaces on three different elastomers: Polydimethylsiloxane (PDMS; Dow Corning, USA), Dragon Skin 10 (Hyup Shin, Seoul, South Korea), and EcoFlex 5 (Hyup Shin). Those precursors were mixed with their corresponding curing agent in ratios recommended by each manufacturer (10:1, 1:1, and 1:1 for PDMS, Dragon Skin, and EcoFlex, respectively). Next, we spread a commercial silica aerogel powder (AP-1000, Gelhouse, Daejeon, South Korea) on the surface of each elastomer to cover the whole area. This simple step formed a thin layer that introduces hydrophobicity and water repellency. After this, samples were cured for 2 hours at 80°C in a convection oven. Finally, the aerogel powder which was not fixed onto the surface of the elastomers was removed by a blower.
Structure and composition of the silica aerogel layer were analyzed by scanning electron microscope as well as energy-dispersive X-ray spectroscopy (EDX; Teneo VS, FEI). The silica particle size was measured to be ~20 µm in average. The EDX analysis showed Si and O are only components of aerogel. Next, to characterize their hydrophobicity, we placed a 20 µL of deionized (DI) water droplet on each elastomer surface. Contact angles (CAs) of PDMS, Dragon Skin, and EcoFlex were enhanced from 107°, 110°, and 99° to 134°, 132°, and 138° without and with silica aerogel co-curing, respectively; they were 25.2, 20.0, and 39.4% improvements in CAs. In addition to the hydrophobicity, the silica layer substantially boosted water repellency of each polymer surface. All samples demonstrated tilting angles < ~5° which were remarkably reduced from their initial tilting angles (58°, 38°, and 36° for PDMS, Dragon Skin, Ecoflelx, respectively). To test the reliability of the fabricated hydrophobic layers, we immersed our samples in an ultrasonic bath (CPXH 3800, Branson Bransonic) at 110 W and 40 kHz for 5 min. The CAs of the samples were changed by <10% from the original values.
In this study, we introduced a simple and effective way to create a hydrophobic layer on the surface of various elastomers by co-curing silica aerogel powder. Both hydrophobicity and water-repellency of the three polymers were considerably increased. We expect our proposed method can be applicable to other kinds of elastomer to enhance their hydrophobicity for diverse applications.

* Hyeonhee Roh and Hyunsun Song both contributed equally to this work.

Third-generation photovoltaic devices, Antimony Chalcogenide, Sb₂(S,Se)₃ thin-film solar cells have received growing attention due to their favored properties with <9.2 % efficiency. In particular, they use earth-abundant and non-toxic absorber materials with tunable bandgap (1.0 eV – 1.7 eV), stable upon exposure to sunlight under ambient conditions. Sb₂(S,Se)₃ has a high absorption coefficient at visible light (>10⁵ cm^-1), good carrier mobility (<15 cm²/Vs), long carrier lifetime (<67 ns), and a simple binary compound with high vapor pressure and low melting point (550 C). Most common Sb₂(S,Se)₃ film growth techniques include vapor transport deposition (VTD), closed-space sublimation (CSS), magnetron sputtering, rapid thermal evaporation (RTE), and atomic layer deposition (ALD).
Sb₂Se_x alloy films with different doping elements have been studied in various applications, including optical-recording applications, phase-change data storage applications, and infrared-transmitting lenses. In particular, Ge-doped Sb₂Se_x thin-films have been studied with various compositions and doping concentrations, showing different crystallization, surface, and optical characteristics. Sb₂Se₃ thin-films are crystalline as deposited and on heating with orthorhombic structures. As a few molar percent of Ge doped into Sb₂Se₃ (<15 %) films (GeSbSe), polycrystalline films are formed upon annealing above 200 - 250 C with orthorhombic structures, demonstrating no significant dependence of lattice constant on the Ge doping level. As the concentration of Ge doping increases above 15 %, the crystallization temperature increases up to around 450 C. However, the majority of GeSbSe studies are focused on amorphous Sb₂Se₃ films doped with higher Ge concentration (> 15 %).
In this study, we have fabricated and investigated the surface and morphological properties of polycrystalline GeSbSe films (<15 %) for the application to the photovoltaic absorber. We studied critical optical properties such as absorption coefficient and optical bandgap at different film growth temperatures by exploring the optical responses to applied light stimuli with UV-Vis spectrometer. Furthermore, scanning electron microscopy (SEM) was used for surface morphological characterization of GeSbSe thin-films grown by VTD at different film growth temperatures. The optimum optical characteristics of thin-film absorber materials for photovoltaic applications depend on film surface microstructure, which in turn affects the overall optical behaviors of GeSbSe films.
Polycrystalline Ge-doped Sb₂Se₃ thin-films (<15 %) show thickness around 1 um) grown by Vapor Transport Deposition in vacuum at two different temperatures such as 500 C and 520 C). As the deposition temperature increases from 500 C to 520 C, uniform grains of approximately 0.9 um at 500 C become mixed grains of larger (~6 um) and smaller grains (~0.9 um), revealed by Scanning Electron Microscopy characterization. Moreover, the surface morphology becomes smooth (500 C) to irregularly rougher (520 C). For the characterization of optical properties, UV-Vis spectroscopy was used to study the absorption characteristics and optical bandgap of Ge-doped Sb₂Se₃ thin-films. Using the widely used Tauc's relation, the optical bandgap of Ge-doped Sb₂Se₃ thin-film absorbers is extracted as 1.15 eV and 1.23 eV for samples grown at 500 C and 520 C, respectively. All in one, polycrystalline Ge-doped Sb₂Se₃ thin-films with a thickness of approximately 1 um were fabricated with the smooth surface structure and 1.25 eV bandgap at 500 C. Once the deposition temperature increases to 520 C, the surface morphology becomes rougher and irregular with the mixed grains with 1.15 eV bandgap. The absorption coefficient of 500 C is approximately 3 x 10⁵ cm^-1 at 600 nm.

Interfaces govern the behaviour of all electronic devices. Herbert Kroemer coined the famous phrase “the interface is the device” in his 2000 Nobel Prize lecture, and we are still applying tremendous effort to understand interfaces in new material generations, with wide-bandgap materials being no exception. If anything, wide bandgap materials are more vulnerable to defect states purely due to their larger bandgap. Understanding gained for the bulk behaviour of semiconductors can often not be extended to the behaviour of materials in structured film stacks were interfaces play a vital role. SiC/SiO₂ is a prototypical wide-bandgap semiconductor/dielectric interface, which represents the challenges faced by many such material systems. A multitude of different defects leads to unacceptably large defect densities exceeding 10¹³ cm^-2 eV^-1 in the vicinity of the conduction band of 4H-SiC. The management of interfacial defects remains a topic of lively discussion and current interest.
This work presents a systematic study of the 4H-SiC/SiO₂ interface in industrially manufactured samples with a particular focus on interface defects and their passivation through nitridation in N₂, NO, NH₃ and NO+NH₃ atmospheres. Using a combination of energy-dependent and angular-dependent hard X-ray photoelectron spectroscopy (HAXPES) a depth profile of both the elemental distribution as well as chemical environments across the interface is obtained. The extracted information on the local chemistry at the interface after the different nitridation processes can be directly correlated to changes in electrical behaviour. This study also showcases a successful combination of both synchrotron-based HAXPES with data from a new generation of laboratory-based spectrometers. Overall, HAXPES is able to deliver a complete picture of this important technological interface and provides a benchmark for the capabilities of this spectroscopic technique for the exploration of buried interfaces in device heterostructures in general.

Current packaging technologies rely on macroscopic hermetic metallic, ceramic and glass materials that are not suitable for encapsulating microfabricated and/or large-area devices and systems, let alone mechanically compliant ones. No solution currently exists for thin and scalable hermetic coatings. The design of hermetic encapsulation and high diffusion barrier materials is paramount to an increasing number of applications including implantable medical devices, biopackaging or new generations of solar cells. The diversity in substrates for these devices and systems calls for low temperature and scalable deposition of inert barrier coatings. The latter also need to satisfy the requirements for challenging environmental and biological conditions and lifetime projection spanning from months to decades. Accurate and precise measurements of moisture permeability are also essential to assess the performance of the desired packaging and encapsulation materials.
We report on the design, development, optimization and characterization of multilayered thin-film encapsulation based on alternating organic and inorganic materials that can be grown at low temperature (< 100 °C) and on a variety of substrates commonly used in the above-mentioned applications. Alternating multilayers mitigates the challenges of growing single-layer barrier coating with low permeability and low defect density thereby promising high barrier performance. The creation of a tortuous pathway for diffusion within the layered structure leads to a major increase of the lag time before water molecules may reach the device functional layers. We explore the use of bioresorbable thin-film devices integrated within the encapsulated microfabricated bioelectronic systems as real-time markers of hermeticity. We further demonstrate the sub-micron thick coating provides reliable encapsulation even for stretchable micro-LED arrays designed for implantable neurotechnologies.

Photophysical detection, identification and characterization of nanoparticles, quantum dots (QDs) and single emitter are essential to enhance the efficiency of preparation methods as well as their electronic and optical properties. Different parameters define these properties, e.g., particles size, number of surfaces defects and trap states, interface interactions, energy and electron transfer behavior and, of course, absorption properties and response to photon stimulation.
Characterization of such materials is usually done by Raman and photoluminescence spectroscopy as well as by electron microscopy, which can gather information about chemical structure, homogeneity and some optical properties. But there still is a lack of information about important parameters that are necessary for understanding and optimizing the preparation and efficiencies of these materials. Especially the identification of single emitter is not possible with the previously mentioned characterization methods.
We demonstrate here how combining time-resolved laser scanning microscopy – offering a broad range of techniques – with a spectrometer results in a valuable and powerful toolbox. This combination of microscopic (e.g., FLIM, PLIM, fast switch between widefield and confocal resolution and antibunching) and spectroscopic methods (such as time-resolved photoluminescence or wavelength dependent emission scanning) allows investigating the photophysical properties of nanoparticles, QDs, nanostructures and single emitter on a whole new level. This additional information enables gaining a deeper understanding which will help for the optimization of properties and efficiencies in practical applications.

Metal thin-film on semiconductor surfaces takes an important role in condensed matter physics in terms of exploring low-dimensional metallic properties. For example, thin-film of In on Si substrates has been attracting attention because of its 2-dimensional superconductivity. It was considered that In one-atom-thick on Si(111)√7×√3 surface shows relatively high Tc (3.18 K), which is quite analogous to Tc of bulk In (3.41 K).^[1] After that, it was revealed that the In layer on Si(111)√7×√3 is not one-atom-thick but double layers by a theoretical study.^[2] Despite a lot of worthful studies, it is not well understood that whether the superconductivity-related electronic properties of the In double layers basically come from an ultrathin limit of bulk In or interface effects from its substrate. Thus, it is useful to study by changing the Si(111)√7×√3 substrates to Si(111)√3×√3-B surface of In layers to identify the interfacial effects.
In this study, we suggest the feasible interfacial structures of In layers on Si(111)√3×√3-B and address its electronic properties using density functional theory, within the generalized gradient approximation of exchange-correlation functional. Since the periodicity of In layers and Si substrates are different as √7×√3 and √3×√3, we introduced 3√7×√3 supercells. It is known that the B addition on the Si(111) surface induces the surface reconstruction to a √3×√3 periodicity that contains B at the subsurface and a Si adatom on each B.^[3] Interestingly, we identified that In layers can be adsorbed only on the surface that has no Si adatom, despite the presence of Si adatom in the clean surface. We also ascertained that the thickness of In layers should be 2 mono-layer from adsorption energy calculations, which coincides with the result of In/Si(111)√7×√3.^[2]
To clarify the interfacial effects on the electronic structure of In double layers, we compared the band dispersion of In/Si(111)√3×√3-B and In/Si(111)√7×√3. Band unfolding method was utilized to expand the narrow first-Brillouin zone due to the supercell approach. The features of the unfolded band structure were similar among the two substrates. In addition, we calculated a band dispersion of free-standing In double layers. Detailed band dispersions and electronic structures will be discussed in the presentation.
[1] T. Zhang et al., Nat. Phys. 6, 104-108 (2010).
[2] J. Park and M. Kang, Phys. Rev. Lett. 109, 166102 (2012).
[3] R. L. Headrick, I. K. Robinson, E. Vlieg, and L. C. Feldman, Phys. Rev. Lett. 63, 1253-1256 (1989).

Vapor-phase infiltration (VPI), a material hybridization method derived from atomic layer deposition (ALD), infuses gaseous inorganic precursors into the matrix of organic templates in a sequential and cyclic manner, generating organic-inorganic hybrids with unique material properties. A following, selective removal of organic matrix also leads to a generation of inorganic nanostructures inheriting the morphology and positional registry of starting organic templates. Combined with structured organic templates such as lithographically patterned photoresists or self-assembled block copolymer (BCP) thin films, VPI can be used as an inorganic nanopatterning technique. Especially, the block-selective VPI in self-assembled BCP films features an analogy to area-selective ALD in term of self-aligned material deposition within a specific reactive polymer domain. In this talk, I will showcase our recent progresses on utilizing VPI for microelectronics applications, including patterning and wafer-scale integration of metal oxide nanowire array photodetectors, hybrid resistive memory devices with controlled switching stochastics, hybrid nanopillars with ultrahigh elastic energy storage capacity for potential applications as micro/nanoelectromechanical system (MEMS/NEMS) components, and hybrid extreme ultraviolet (EUV) photoresists for next-generation semiconductor manufacturing.

The progressive miniaturization of feature sizes in microelectronic devices has increased the risk of exposing the system to severe service conditions, such as higher power densities and consequently higher local temperatures. These devices consist of a complex multi-metallic-layered architecture and when subjected to such conditions, detrimental interdiffusion phenomena can occur between adjacent layers, compromising the integrity, functionality and reliability of these components.
The problem of interdiffusion is particularly persistent in power semiconductor devices that employ a copper (Cu) metallisation scheme and a titanium-tungsten (TiW) diffusion barrier. The TiW barrier is required to isolate the copper from the silicon substructure but high thermal events can induce a segregation of titanium out of the barrier layer and diffusion into the overlaid copper layer, leading to the degradation and failure of the barrier. Additionally, the TiW/Cu interface can act as a sink for the accumulation of oxygen due to the strong gettering ability of titanium, which further adds an additional risk to the system and often promotes delamination of the layers.
Here, un-patterned, Si/SiO₂/TiW and Si/SiO₂/TiW/Cu thin film stacks annealed for varying durations at 400°C under an inert atmosphere were characterized using a combination of soft and hard X-ray photoelectron spectroscopy (SXPS and HAXPES). Combining the two techniques provided the opportunity to non-destructively study both the titanium diffusion mechanism, and the oxidation behaviour of TiW at multiple depths. The findings were able to systematically showcase the dependence of the titanium surface enrichment and oxidation behaviour on the annealing duration, and provided a detailed account of the degradation and failure mechanisms associated with these TiW/Cu heterostructures. Overall the combinatorial XPS characterisation approach worked well as the respective techniques complemented each other’s shortcoming and therefore the approach holds promise for the characterisation of other multi-metallic systems where similar problems exist.

The study of spin pumping in a ferromagnetic/nonmagnetic (FM/NM) system has gained increasing interest in the last decade for the fundamental understanding of spin-orbit coupling (SOC) materials and their promising applications in spin-orbit torque based magnetic random-access memories. The angular momentum transfer through the FM/NM interface is susceptible to the quality of the interface between the particular materials involved and their properties. Several attempts have been made to increase the efficiency of SOC materials by alloying of the two elements, but this approach suffers from the drawback related to the presence of undesirable extrinsic scattering contributions [1].
In this work, the spin pumping in ion-beam sputtered β-W/Co₂FeAl(CFA)/Al heterostructure grown on Si(100) substrate is systematically investigated. In the heterostructure, the thickness of the CFA and capping Al layer is fixed at 8 nm and 3.5 nm, respectively, but the thickness of the β-W layer is varied from 0.5 nm to 10 nm. While the α_eff for bare CFA(8 nm)/Al(3.5 nm) was 0.0061, after the introduction of 6 nm of W, α_eff becomes ~0.0087 i.e., almost an enhancement of nearly ~47% in α_eff. From the variation of effective damping constant (α_eff) with the β-W thickness and considering ballistic transport model [2], the real part of spin mixing conductance (g_eff^↑↓) of the interface and spin diffusion length (λ_S) of the β-W are found to be 11.76±0.34 nm^-2 and 2.17±0.27 nm, respectively.
Further to understand the impact of the different modified interface on the spin pumping, different interlayers (ILs) with different SOC strengths and different thicknesses have been introduced in between the β-W and CFA layers in β-W(6 nm)/CFA(8 nm)/Al(3.5 nm). Four different ILs were chosen, two of them (Al and Mg) are weak-SOC materials and the other two (Ta and Mo) are high SOC materials with different SOC strengths (SOC strength of Ta is reported to be higher than that in Mo).
In the case of the two weak-SOC ILs, the spin pumping from the CFA layer to the β-W layer is blocked with the α_eff ~0.007. This is understandable since the low SOC strength of the ILs, it does not permit the transfer of spin angular momentum from CFA to β-W. On the other hand, in the case of the two high SOC ILs, although the variation in α_eff is similar, however, it is completely different from that observed in the case of weak-SOC ILs. For Ta (Mo), up to 2 nm (2 nm), α_eff increases to 0.0106 (0.0098) and then reduces up to 4 nm (4 nm) and finally saturates with an α_eff value of ~0.0085 (0.0079). This can be explained in terms of the λ_S of the IL and transparency of the IL - β-W interface. The initial increase in the α_eff up to 2nm of IL probably due to combined effect of the IL and the β-W layer as the spin angular momenta can pass through the IL and to β-W for IL thickness lesser than the λ_S of the ILs (which varies 2-4 nm depending upon the interface for both the ILs). However, for higher IL thickness (> λ_S), the spin angular momentum will experience damping within the IL only and will not be able to transfer into the β-W layer. Hence, the only contribution of the IL will be observed, as if high SOC β-W layer is not there at all. The α_eff value will be saturated depending on the SOC strength of that particular IL. Here, as the SOC strength of Ta (Mo) is lesser than that of β-W, the α_eff saturates at a lower value than that of β-W(6 nm)/CFA(8 nm)/Al(3.5 nm). Now among the two high SOC ILs, Ta has higher SOC strength compared to Mo and hence α_eff saturates at a higher value.
Thus, in this way, one can tune the transfer of spin angular momentum by choosing the specific interlayer of appropriate thickness without necessitating the alloying of the layers. Our study provides the fundamental understanding of how the different SOC layers altogether can increase spin pumping, which is important for future spintronics applications.
References:
[1] doi: 10.1103/PhysRevB.96.140405.
[2] doi: 10.1063/1.1853131.