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. By inserting a GeOx interlayer directly under Ge and selecting substrates with different coefficients of thermal expansion, we modulated the strain in the polycrystalline Ge layer, ranging from approximately 0.6% (tensile) to - 0.8% (compressive). Compressive strain enlarged the grain size to 12 µm, but decreased the electron mobility. The temperature dependence of the electron mobility clarified that changes in the potential barrier height of the grain boundary caused this behavior. Furthermore, we revealed that the behavior of the grain boundary barrier height with respect to strain is opposite for the n- and p-types. This result strongly suggests that this phenomenon is due to the piezoelectric effect. These discoveries will provide guidelines for improving the performance of Ge devices and useful physical knowledge of various polycrystalline semiconductor thin films.

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, Ge_1-x-ySi_xSn_y. The solid-phase crystallization of the highly densified amorphous precursors allowed the formation of high-quality polycrystalline Ge_1-x-ySi_xSn_y layers on an insulating substrate. The small compositions of Si and Sn in Ge_1-x-ySi_xSn_y (x < 0.15 and y < 0.05) lowered the thermal conductivity (3.1 W m^-1 K^-1) owing to the alloy scattering of phonons, while maintaining a high carrier mobility (approximately 200 cm² V^-1 s^-1). The solid-phase diffusion of Ga and P allowed us to control the carrier concentration to the order of 10¹⁹ cm^-3 for holes and 10¹⁸ cm^-3 for electrons. For both p- and n-type Ge_1-x-ySi_xSn_y, the power factor peaked at x = 0.06 and y = 0.02, reaching 1160 μW m^-1 K^-2 for p-type and 2040 μW m^-1 K^-2 for n-type. The resulting dimensionless figure of merits (0.12 for p-type and 0.20 for n-type) are higher than those of most environmentally friendly thermoelectric thin films. These results indicate that group IV alloys are promising candidates for high-performance, reliable thin-film thermoelectric generators.

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. The LE technique enabled the formation of a uniform MLG with a wide range of thicknesses (25-500 nm) on Ta foil. The charge/discharge characterization using coin-type cells revealed that the total capacity, which corresponded to Li intercalation into the MLG interlayer, increased with increasing MLG thickness. In contrast, cross-sectional transmission electron microscopy showed a metal oxide formed at the MLG/Ta interface during annealing, which had small Li capacity. MLG with sufficient thickness (500 nm) exhibited an excellent Coulombic efficiency and capacity retention compared to bulk graphite formed at high temperatures. These results have led to the development of inexpensive and reliable rechargeable thin-film batteries.

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. Our proposed linear regression analysis method, which is based on deriving the acceptor levels and their densities from the temperature dependence of the hole concentration, revealed the presence of two different acceptor levels. A systematic analysis of the effects of grain size and post annealing on the hole concentration suggests that deep acceptor levels (53-103 meV) could be attributed to dangling bonds located at grain boundaries, whereas shallow acceptor levels (< 15 meV) could be attributed to vacancies in grains. Thus, this study proposed a machine learning-based simulation method that can be widely applied in the analysis of physical properties, and can provide insights into the understanding and control of acceptor defects in polycrystalline Ge thin films.

Si1-xGex 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 Si_1-xGe_x (x = 0-1) layers on plastic films using Al-induced LE. The resulting SiGe layers exhibited high electrical conductivity (up to 1200 S cm^-1), reflecting the self-organized doping effect of LE. Moreover, the Si_1-xGe_x layer synthesized by the same process was adopted as the anode for the lithium-ion battery. All Si_1-xGe_x anodes showed clear charge/discharge operation and high coulombic efficiency (≥ 97%) after 100 cycles. While the discharge capacities almost reflected the theoretical values at each x at 0.1 C, the capacity degradation with increasing current rate strongly depended on x. Si-rich samples exhibited high initial capacity and low capacity retention, while Ge-rich samples showed contrasting characteristics. In particular, the Si_1-xGe_x layers with x ≥ 0.8 showed excellent current rate performance owing to their high electrical conductivity and low volume expansion, maintaining a high capacity (> 500 mAh g^-1) even at a high current rate (10 C). Thus, we revealed the relationship between SiGe composition and anode characteristics for the SiGe layers formed by LE at low temperatures. These results will pave the way for the next generation of flexible batteries based on SiGe anodes.

Zn1-xGexOy Passivating Interlayers for BaSi2 Thin-Film Solar Cells

BaSi₂ 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 BaSi₂. In this study, we investigated amorphous Zn_1-xGe_xO_y films as interlayers for BaSi₂. The Zn/Ge atomic ratio in the Zn_1-xGe_xO_y film and the optical band gap depend on the substrate temperature during sputtering deposition. A suitable i-Zn_1-xGe_xO_y/BaSi₂ heterointerface with spike-type conduction band offset was achieved when Zn_1-xGe_xO_y was deposited on BaSi₂ at 50 °C. Furthermore, photoresponsivity measurements revealed that Zn_1-xGe_xO_y has an excellent surface passivation effect on BaSi₂. When the thickness of Zn_1-xGe_xO_y was 2 nm, a high photoresponsivity of 0.9 A/W was obtained for a 500 nm thick BaSi₂ layer at a wavelength of 780 nm under an applied bias voltage of 0.5 V between the front and rear electrodes, where the photoresponsivity in the short-wavelength region was significantly improved compared with that of BaSi₂ capped with an amorphous Si layer. X-ray photoelectron spectroscopy revealed that the Zn_1-xGe_xO_y films suppressed the oxidation of the BaSi₂ surface, decreasing the carrier recombination rate. This is the first demonstration of passivation interlayers for BaSi₂ with suitable band alignment for carrier transport and surface passivation effects.

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⁻¹ K⁻² for p-type Si_0.4Ge_0.6 and 390 µW m⁻¹ K⁻² for n-type Si_0.85Ge_0.15, owing to self-organized doping in LE. Furthermore, the power factors indicated stable behavior with changing measurement temperature, an advantage of SiGe as an inorganic material. An in-plane π-type TEG based on these SiGe layers showed an output power of 0.45 µW cm⁻² at near room temperature for a 30 K temperature gradient. This achievement will enable the development of environmentally friendly and highly reliable flexible TEGs for operating micro-energy devices in the future Internet of Things.

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 3dtransition metal. Fe₄N and Co₄N are ferromagnetic at room temperature (RT), and the minority spins play a dominant role in the electrical transport properties. However, Mn₄N is ferrimagnetic at RT and exhibits a perpendicular magnetic anisotropy caused by tensile strain. Around the magnetic compensation in Mn₄N induced by impurity doping, researchers have demonstrated ultrafast current-induced domain wall motion reaching 3000 m s^-1at RT, making switching energies lower and switching speed higher compared with Mn₄N. In this review article, we start with individual magnetic nitrides-such as Fe₄N, Co₄N, Ni₄N, and Mn₄N; describe the nitrides' features; and then discuss compounds such as Fe_4-xA_xN (A = Co, Ni, and Mn) and Mn_4-xB_xN (B = Ni, Co, and Fe) to evaluate nitride properties from the standpoint of spintronics applications. We pay particular attention to preferential sites of A and B atoms in these compounds, based on x-ray absorption spectroscopy and x-ray magnetic circular dichroism.

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 0.5% (tensile) to -0.5% (compressive), which reflects both thermal expansion difference between Ge and substrate and phase transition of Ge from amorphous to crystalline. For both tensile and compressive strains, a large strain provides large crystal grains with sizes of approximately 10 μm owing to growth promotion. The potential barrier height of the grain boundary strongly depends on the strain and its direction. It is increased by tensile strain and decreased by compressive strain. These findings will be useful for the design of Ge-based thin-film devices on various materials for Internet-of-things technologies.

Current-Driven Domain Wall Dynamics in Ferrimagnetic Nickel-Doped Mn4N 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 (Mn_4-xNi_xN) films, in which magnetization and angular momentum compensation can be obtained by the fine adjustment of the Ni content. Large domain wall velocities, approaching 3000 m/s, are measured for Ni compositions close to the angular momentum compensation point. The reversal of the DW motion direction, observed when the compensation composition is crossed, is related to the change of direction of the angular momentum with respect to that of the spin polarization. This is confirmed by the results of ab initio band structure calculations.

Impact of the carbon membrane inserted below Ni in the layer exchange of multilayer graphene

High-quality multilayer graphene on glass is achieved at a low temperature (400 °C).

Large Current Driven Domain Wall Mobility and Gate Tuning of Coercivity in Ferrimagnetic Mn4N 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 Mn₄N, a rare-earth free ferrimagnet made of abundant elements, is an exciting candidate for the development of sustainable spintronics devices. Mn₄N thin films grown epitaxially on SrTiO₃ substrates possess remarkable properties, such as a perpendicular magnetization, a very high extraordinary Hall angle (2%) and smooth domain walls at the millimeter scale. Moreover, domain walls can be moved at record speeds by spin-polarized currents, in absence of spin-orbit torques. This can be explained by the large efficiency of the adiabatic spin transfer torque, due to the conjunction of a reduced magnetization and a large spin polarization. Finally, we show that the application of gate voltages through the SrTiO₃ substrates allows modulating the Mn₄N coercive field with a large efficiency.

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. A 600 °C-annealed sample in which 15 layers of 4-nm-thick a-C and 0.5-nm-thick Ni were laminated recorded an electrical conductivity of 1430 S/cm. This value is close to that of highly oriented pyrolytic graphite synthesized at approximately 3000 °C. This improvement is likely related to the bond weakening in a-C due to the screening effect of Ni. We expect that these results will contribute to low-temperature synthesis of MLG using a solid-phase reaction with metals.

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. The domain size and surface coverage of MLG were determined by the supplied amount of C atoms and the thickness of the metal layer associated with the solid solution amount of C. An average domain size of 2.5 μm and surface coverage of approximately 50% were obtained for a 1 μm thick Ni layer. Transmission electron microscopy demonstrated the high crystalline quality of the MLG layer despite the low processing temperature. Therefore, this simple sputtering technique has great potential for integrating graphene-based devices on various platforms.

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 Al₂O₃ interlayer between the C and Ni layers. Hall effect measurements reveal the carrier mobility for t = 50 nm is 550 cm²/Vs, which is the highest Hall mobility in MLG directly formed on an insulator. The electrical conductivity (2700 S/cm) also exceeds a highly oriented pyrolytic graphite synthesized at 3000 °C or higher. Synthesis technology of MLG with a wide range of thicknesses will enable exploration of extensive device applications of carbon materials.

Nanoscale measurement of giant saturation magnetization in α″-Fe16N2 by electron energy-loss magnetic chiral dichroism

Metastable α″-Fe₁₆N₂ thin films were reported to have a giant saturation magnetization of above 2200 emu/cm³ 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″-Fe₁₆N₂ phase, as it is often mixed with the parent α'-Fe₈N phase in thin films. Here, we use electron energy-loss magnetic chiral dichroism (EMCD), aberration-corrected transmission electron microscopy, X-ray diffraction and macroscopic magnetic measurements to study α″-Fe₁₆N₂ (containing ordered N atoms) and α'-Fe₈N (containing disordered N atoms). The ratio of saturation magnetization in α″-Fe₁₆N₂ to that in α'-Fe₈N is determined to be 1.31 ± 0.10 from quantitative EMCD measurements and dynamical diffraction calculations, confirming the giant saturation magnetization of α″-Fe₁₆N₂. Crystallographic information is also obtained about the two phases, which are mixed on the nanoscale.

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). Some layer-exchange metals allowed for low-temperature MLG synthesis at 600 °C, whereas others allowed for high-quality MLG with a Raman G/D peak ratio of up to 8.3. Based on the periodic table, we constructed metal selection guidelines for growing MLG on an insulator, opening the door for applications that combine advanced electronic devices with carbon materials.

Fabrication of SrGe2 thin films on Ge (100), (110), and (111) substrates

Semiconductor strontium digermanide (SrGe₂) 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 SrGe₂ thin films via a reactive deposition epitaxy on Ge substrates. The growth morphology of SrGe₂ dramatically changed depending on the growth temperature (300-700 °C) and the crystal orientation of the Ge substrate. We succeeded in obtaining single-oriented SrGe₂ using a Ge (110) substrate at 500 °C. Development on Si or glass substrates will lead to the application of SrGe₂ to high-efficiency thin-film solar cells.

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_d, ranged from 50 to 200 °C. According to the atomic density of the precursor and the T_d dependent SPC properties, the precursor conditions were determined by three regimes: the low-density regime (T_d < 100 °C), high-density regime (100 ≤ T_d ≤ 125 °C), and nucleation regime (T_d > 125 °C). The use of the precursor in the narrow high-density regime enabled us to form SPC-Ge with a hole mobility of 340 cm²/Vs, the highest value among semiconductor thin films grown on insulators at low temperature (<900 °C). The origins of the high hole mobility were determined to be both a large grain size (5 µm) and a low energy barrier height (6.4 meV) for the grain boundary. The findings from and knowledge gained in this study, that is, the influence of the precursor conditions on subsequent crystal growth, will be universal and applicable to various materials.

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., the density, uniformity, and crystal quality of Ge NWs. The metal-induced layer exchange yielded a (111)-oriented Ge seed layer at 325 °C, which allowed for the VLS growth of vertically aligned Ge NWs. The Ge NW array had almost the same quality as that formed on a bulk Ge(111) substrate. Transmission electron microscopy demonstrated that the Ge NWs were defect-free single crystals. The present investigation paves the way for advanced electronic optical devices integrated on a low-cost flexible substrate.

Self-organization of Ge(111)/Al/glass structures through layer exchange in metal-induced crystallization

Al-induced crystallization enabled the self-organization of high-quality Ge/Al/glass structures through layer exchange at a low temperature of 325 °C.