Formation of High-Photoresponsivity BaSi Films by Radio-Frequency Sputtering and Evaluation of a BaSi/TiN/Glass Heterojunction by Transmission Electron Microscopy.

Barium disilicide (BaSi) is a thin-film solar cell material composed of abundant elements, and its application potential is further enhanced by its formation on inexpensive substrates, such as glass. The effect of the substrate temperature on the co-sputtering of BaSi and Ba targets to form BaSi films on Si(111) and TiN/glass substrates was investigated. Contrary to expectations, the photoresponsivity reached maximum values exceeding 5 and 2 A W, respectively, the highest value ever reported for as-deposited samples formed at 750 °C, more than 100 °C higher than those reported previously.

Semantic segmentation in crystal growth process using fake micrograph machine learning.

Microscopic evaluation is one of the most effective methods in materials research. High-quality images are essential to analyze microscopic images using artificial intelligence. To overcome this challenge, we propose the machine learning of "fake micrographs" in this study.

Strain-dependent grain boundary properties of n-type germanium layers.

Polycrystalline Ge thin films have attracted considerable attention as potential materials for use in various electronic and optical devices. We recently developed a low-temperature solid-phase crystallization technology for a doped Ge layer and achieved the highest electron mobility in a polycrystalline Ge thin film. In this study, we investigated the effects of strain on the crystalline and electrical properties of n-type polycrystalline Ge layers.

High Thermoelectric Performance in Polycrystalline GeSiSn Ternary Alloy Thin Films.

Group IV materials are promising candidates for highly reliable and human-friendly thin-film thermoelectric generators, used for micro-energy harvesting. In this study, we investigated the synthesis and thermoelectric applications of a Ge-based ternary alloy thin film, GeSiSn. The solid-phase crystallization of the highly densified amorphous precursors allowed the formation of high-quality polycrystalline GeSiSn layers on an insulating substrate.

Thickness Dependency of Battery Anode Properties in Multilayer Graphene.

With the development of practical thin-film batteries, multilayer graphene (MLG) is being actively investigated as an anode material. Therefore, research on determining a technique to fabricate thick MLG on arbitrary substrates at low temperatures is essential. In this study, we formed an MLG with controlled thickness at low temperatures using a layer exchange (LE) technique and evaluated its anode properties.

Acceptor defects in polycrystalline Ge layers evaluated using linear regression analysis.

Polycrystalline Ge thin films have recently attracted renewed attention as a material for various electronic and optical devices. However, the difficulty in the Fermi level control of polycrystalline Ge films owing to their high density of defect-induced acceptors has limited their application in the aforementioned devices. Here, we experimentally estimated the origin of acceptor defects by significantly modulating the crystallinity and electrical properties of polycrystalline Ge layers and investigating their correlation.

SiGe anode synthesis on plastic films for flexible rechargeable batteries.

SiGe is a promising anode material for replacing graphite in next generation thin-film batteries owing to its high theoretical charge/discharge capacity. Metal-induced layer exchange (LE) is a unique technique used for the low-temperature synthesis of SiGe layers on arbitrary substrates. Here, we demonstrate the synthesis of SiGe (x = 0-1) layers on plastic films using Al-induced LE.

ZnGeO Passivating Interlayers for BaSi Thin-Film Solar Cells.

BaSi is a promising absorber material for next-generation thin-film solar cells (TFSCs). For high-efficiency TFSCs, a suitable interlayer should be found for every light absorber. However, such an interlayer has not been studied for BaSi.

Flexible Thermoelectric Generator Based on Polycrystalline SiGe Thin Films.

Flexible and reliable thermoelectric generators (TEGs) will be essential for future energy harvesting sensors. In this study, we synthesized p- and n-type SiGe layers on a high heat-resistant polyimide film using metal-induced layer exchange (LE) and demonstrated TEG operation. Despite the low process temperature (<500 °C), the polycrystalline SiGe layers showed high power factors of 560 µW m K for p-type SiGe and 390 µW m K for n-type SiGe, owing to self-organized doping in LE.

Transition metal nitrides and their mixed crystals for spintronics.

Anti-perovskite transition metal nitrides exhibit a variety of magnetic properties-such as ferromagnetic, ferrimagnetic, and paramagnetic-depending on the 3transition metal. FeN and CoN are ferromagnetic at room temperature (RT), and the minority spins play a dominant role in the electrical transport properties. However, MnN is ferrimagnetic at RT and exhibits a perpendicular magnetic anisotropy caused by tensile strain.

Grain size dependent photoresponsivity in GaAs films formed on glass with Ge seed layers.

The strong correlation between grain size and photoresponsivity in polycrystalline GaAs films on glass was experimentally demonstrated using Ge seed layers with a wide range of grain sizes (1‒330 μm). The crystal evaluations using Raman spectroscopy, scanning electron microscopy, electron backscatter diffraction, and transmission electron microscopy revealed that 500-nm-thick GaAs films epitaxially grown from the Ge seed layers at 550 °C inherited the grain boundaries and crystal orientations in Ge. With increasing grain size, the photoresponsivity corresponding to GaAs increased from 0.

Strain effects on polycrystalline germanium thin films.

Polycrystalline Ge thin films have attracted increasing attention because their hole mobilities exceed those of single-crystal Si wafers, while the process temperature is low. In this study, we investigate the strain effects on the crystal and electrical properties of polycrystalline Ge layers formed by solid-phase crystallization at 375 °C by modulating the substrate material. The strain of the Ge layers is in the range of approximately 0.

Current-Driven Domain Wall Dynamics in Ferrimagnetic Nickel-Doped MnN Films: Very Large Domain Wall Velocities and Reversal of Motion Direction across the Magnetic Compensation Point.

Spin-transfer torque (STT) and spin-orbit torque (SOT) are spintronic phenomena allowing magnetization manipulation using electrical currents. Beyond their fundamental interest, they allow developing new classes of magnetic memories and logic devices, in particular based on domain wall (DW) motion. In this work, we report the study of STT-driven DW motion in ferrimagnetic manganese nickel nitride (MnNiN) films, in which magnetization and angular momentum compensation can be obtained by the fine adjustment of the Ni content.

High-electron-mobility (370 cm/Vs) polycrystalline Ge on an insulator formed by As-doped solid-phase crystallization.

High-electron-mobility polycrystalline Ge (poly-Ge) thin films are difficult to form because of their poor crystallinity, defect-induced acceptors and low solid solubility of n-type dopants. Here, we found that As doping into amorphous Ge significantly influenced the subsequent solid-phase crystallization. Although excessive As doping degraded the crystallinity of the poly-Ge, the appropriate amount of As (~10 cm) promoted lateral growth and increased the Ge grain size to approximately 20 μm at a growth temperature of 375 °C.

Large Current Driven Domain Wall Mobility and Gate Tuning of Coercivity in Ferrimagnetic MnN Thin Films.

Spintronics, which is the basis of a low-power, beyond-CMOS technology for computational and memory devices, remains up to now entirely based on critical materials such as Co, heavy metals and rare-earths. Here, we show that MnN, a rare-earth free ferrimagnet made of abundant elements, is an exciting candidate for the development of sustainable spintronics devices. MnN thin films grown epitaxially on SrTiO substrates possess remarkable properties, such as a perpendicular magnetization, a very high extraordinary Hall angle (2%) and smooth domain walls at the millimeter scale.

Impact of Amorphous-C/Ni Multilayers on Ni-Induced Layer Exchange for Multilayer Graphene on Insulators.

Layer exchange growth of amorphous carbon (a-C) is a unique technique for fabricating high-quality multilayer graphene (MLG) on insulators at low temperatures. We investigated the effects of the a-C/Ni multilayer structure on the quality of MLG formed by Ni-induced layer exchange. The crystal quality and electrical conductivity of MLG improved dramatically as the number of a-C/Ni multilayers increased.

Low-Temperature (400 °C) Synthesis of Multilayer Graphene by Metal-Assisted Sputtering Deposition.

Low-temperature synthesis of multilayer graphene (MLG) is essential for combining advanced electronic devices with carbon materials. We investigated the vapor-phase synthesis of MLG by sputtering deposition of C atoms on metal-coated insulators. Ni, Co, and Fe catalysts, which have high C solid solubility, enabled us to form MLG at 400 °C.

Nanoscale measurement of giant saturation magnetization in α″-FeN by electron energy-loss magnetic chiral dichroism.

Metastable α″-FeN thin films were reported to have a giant saturation magnetization of above 2200 emu/cm in 1972 and have been considered as candidates for next-generation rare-earth-free permanent magnetic materials. However, their magnetic properties have not been confirmed unequivocally. As a result of the limited spatial resolution of most magnetic characterization techniques, it is challenging to measure the saturation magnetization of the α″-FeN phase, as it is often mixed with the parent α'-FeN phase in thin films.

High-Electrical-Conductivity Multilayer Graphene Formed by Layer Exchange with Controlled Thickness and Interlayer.

The layer exchange technique enables high-quality multilayer graphene (MLG) on arbitrary substrates, which is a key to combining advanced electronic devices with carbon materials. We synthesize uniform MLG layers of various thicknesses, t, ranging from 5 nm to 200 nm using Ni-induced layer exchange at 800 °C. Raman and transmission electron microscopy studies show the crystal quality of MLG is relatively low for t ≤ 20 nm and dramatically improves for t ≥ 50 nm when we prepare a diffusion controlling AlO interlayer between the C and Ni layers.

Metal Catalysts for Layer-Exchange Growth of Multilayer Graphene.

Metal-induced layer-exchange growth of amorphous carbon (a-C) is a unique technique for fabricating high-quality, uniform multilayer graphene (MLG) directly on an insulating material. Here, we investigated the effect of transition-metal species on the interaction between metals and a-C in the temperature range of 600-1000 °C. As a result, metals were classified into four groups: (1) layer exchange (Co, Ni, Cr, Mn, Fe, Ru, Ir, and Pt), (2) carbonization (Ti, Mo, and W), (3) local MLG formation (Pd), and (4) no graphitization (Cu, Ag, and Au).

Improving carrier mobility of polycrystalline Ge by Sn doping.

To improve the performance of electronic devices, extensive research efforts have recently focused on the effect of incorporating Sn into Ge. In the present work, we investigate how Sn composition x (0 ≤ x ≤ 0.12) and deposition temperature T (50 ≤ T ≤ 200 °C) of the GeSn precursor affect subsequent solid-phase crystallization.

Fabrication of SrGe thin films on Ge (100), (110), and (111) substrates.

Semiconductor strontium digermanide (SrGe) has a large absorption coefficient in the near-infrared light region and is expected to be useful for multijunction solar cells. This study firstly demonstrates the formation of SrGe thin films via a reactive deposition epitaxy on Ge substrates. The growth morphology of SrGe dramatically changed depending on the growth temperature (300-700 °C) and the crystal orientation of the Ge substrate.

High-hole mobility polycrystalline Ge on an insulator formed by controlling precursor atomic density for solid-phase crystallization.

High-carrier mobility semiconductors on insulators are essential for fabricating advanced thin-film transistors, allowing for three-dimensional integrated circuits or high-performance mobile terminals. We investigate the low-temperature (375-450 °C) solid-phase crystallization (SPC) of Ge on a glass substrate, focusing on the precursor conditions. The substrate temperature during the precursor deposition, T , ranged from 50 to 200 °C.

Vertically Aligned Ge Nanowires on Flexible Plastic Films Synthesized by (111)-Oriented Ge Seeded Vapor-Liquid-Solid Growth.

Transfer-free fabrication of vertical Ge nanowires (NWs) on a plastic substrate is demonstrated using a vapor-liquid-solid (VLS) method. The crystal quality of Ge seed layers (50 nm thickness) prepared on plastic substrates strongly influenced the VLS growth morphology, i.e.