Abstract A group of several methods for detailed investigation of electrically active defects (EA... more Abstract A group of several methods for detailed investigation of electrically active defects (EAD) and their properties in insulator-semiconductor (IS) and metal-insulator-semiconductor (MIS) structures has been developed. It consists of different ways of analyzing EAD including precision measurements of local electrophysical properties of IS structures, mapping of defects in IS structures by means of nematic liquid crystals (NLC), application of scanning electron microscopy (SEM) in electron beam induced current (EBIC) mode for localization of EAD in the semiconductor of MIS structures. Local electrophysical measurements are carried out by using dynamic unsteady-state current–voltage characteristics (DUCVC) and their modifications which have higher sensitivity to detection of generation processes in MIS structures as compared with ordinary capacitance–voltage characteristics. Depth distributions of bulk generation lifetime of minority carriers in semiconductors, obtained by the DUCVC method, reflect normalized EAD profiles near the IS interface. The NLC method is nondestructive and makes possible to analyze lateral distribution of defects with high local conductivity in insulating films. It was shown that in the case of thin SiO2 films on silicon EAD at semiconductor surface are also revealed by the NLC method. As a result determination of EAD coordinates opens possibility for further detailed investigation of separate EAD properties by the DUCVC method using local mercury probe 5 μm in size. Special MIS gated p–n junctions have been used for mapping of EAD with high recombination velocity in semiconductor under the metal or polysilicon gates. Combination of the described methods was applied for investigation of local EAD properties of Si-based structures, which are used for fabrication of integrated circuits. The influence of gettering on EAD concentration in Si and SiO2 has been investigated.
Abstract Group-IV alloy semiconductors have garnered increasing attention as advanced thin-film m... more Abstract Group-IV alloy semiconductors have garnered increasing attention as advanced thin-film materials for next-generation electronics. We have demonstrated polycrystalline Ge thin films with the highest recorded crystallinity and carrier mobility using a multistep heating process in solid-phase crystallization. In this study, we apply these recent findings in Ge to Si1-xGex (x: 0–1) and Ge1-ySny (y: 0–0.04) alloys and investigate their crystal and electrical properties. For all compositions, controlling the temperature in each stage increases the grain size to the micrometer order, improving the carrier mobility and reducing the number of defect-induced acceptors. Sb doping further enlarges the grain size (up to 10 µm) in addition to n-type conduction control, whereas the electron concentration varies with the composition. Both hole and electron mobilities significantly depend on the composition owing to the effects of carrier effective mass, grain size, and carrier concentration: the hole and electron mobilities peak at 350 and 150 cm2 V−1s−1, respectively. The relationship between the composition and various physical properties revealed in this study will contribute to the better understanding, control, and device application of polycrystalline thin films based on group-IV alloy semiconductors.
Barium disilicide (BaSi2) shows great promise as a new material for thin film solar cells (Fig. 1... more Barium disilicide (BaSi2) shows great promise as a new material for thin film solar cells (Fig. 1) [1]. It has a suitable bandgap of 1.3 eV, a large optical absorption coefficient of 3×104 cm−1 for a photon energy of 1.5 eV, and a large minority-carrier diffusion length of about 10 μm. Furthermore, it is composed of only earth abundant elements and highly stable. Therefore, BaSi2 can be used for future terawatt-class power generation. We have achieved the operation of BaSi2 homojunction solar cells (Fig. 2) [2]. In this structure, an open-circuit voltage V OC beyond 0.8 V and a conversion efficiency (η) beyond 25% are expected [3]. However, the achieved η was very small. In this study, three-dimensional (3D) optical simulation using a 3D pyramid texture that is effective in preventing reflection was performed on a BaSi2 solar cells in order to increase the photocurrent. Futhermore, we suggested a new device structure using Al-doped ZnO (AZO) electron transfer layer (ETL). In this st...
We demonstrated the marked photoresponsivity enhancement of BaSi2 epitaxial films by 5 min post-a... more We demonstrated the marked photoresponsivity enhancement of BaSi2 epitaxial films by 5 min post-annealing at 850 °C–1000 °C in contrast to those at 600 °C–800 °C. Post-annealing at 1000 °C increased the photoresponsivity up to 9.0 A W−1 at a wavelength of around 800 nm under a bias voltage of 0.5 V applied between the top and bottom electrodes. The hole concentration decreased monotonously with annealing temperature from 8.3 × 1016 to 5.4 × 1015 cm–3, and the mobility exceeded 1000 cm2 V–1 s–1. The a-axis orientation of the BaSi2 films was significantly deteriorated at temperatures higher than 800 °C.
We fabricated a small-area metal (CoSi2)/insulator (CaF2) hot electron transistor using electron-... more We fabricated a small-area metal (CoSi2)/insulator (CaF2) hot electron transistor using electron-beam lithography. The transistor is composed of a CoSi2/CaF2 (1.9 nm)/CoSi2 (1.9 nm) tunnel emitter and a CaF2 (5 nm) collector barrier on an n-Si(111) substrate. The emitter mesa area is 0.9 × 0.9 µm2. Although the measured characteristics show, for the first time, clear transistor action with a curve similar to those of semiconductor HETs, the collector current increases without saturation due to leakage current through the SiO2 film under the external electrode pads. The intrinsic device characteristics (zero leakage current) exhibited saturation, and a current gain β ≥ 36 was obtained at 77 K.
The antiperovskite ferrimagnet Mn4N has perpendicular magnetic anisotropy and small spontaneous m... more The antiperovskite ferrimagnet Mn4N has perpendicular magnetic anisotropy and small spontaneous magnetization, both of which are favorable properties for current induced domain wall motion. Previously we have investigated the magnetic structure of 3d-element-doped Mn4N thin films and demonstrated ultrafast domain wall velocities reaching 3000 m s−1 in the vicinity of the magnetic compensation composition of Ni-doped Mn4N at the current density of j = 1.2 × 1012 A m−2 at room temperature (RT). In this study, we investigate the effect of Au doping on the magnetic structure of Mn4N films, and present a composition ratio-dependent sign reversal of the anomalous Hall effect at RT. X-ray magnetic circular dichroism measurement revealed that the magnetic moment of the face-centered Mn atoms of Mn4− xAu xN reversed between x = 0.1 and 0.2, and became parallel to that of the corner-site Mn atoms for x = 0.2 and 0.3. This result suggests that the ferrimagnetic-ferromagnetic phase transition o...
We focus on rare-earth-free anti-perovskite Mn4− xNi xN epitaxial films, which can be used for ul... more We focus on rare-earth-free anti-perovskite Mn4− xNi xN epitaxial films, which can be used for ultrafast current-induced domain wall motion (CIDWM) in magnetic strips. The magneto-transport properties of these materials are very important for a deep understanding of CIDWM. In this study, we investigated the magneto-transport properties of Mn4− xNi xN epitaxial films grown on SrTiO3(001) and MgO(001) substrates through anisotropic magnetoresistance (AMR) measurements at temperatures between 2 and 300 K. In samples with a small Ni composition such as x = 0.05−0.1, the AMR ratio of Mn4− xNi xN drastically decreased with increasing temperature. We also analyzed the twofold and fourfold symmetries in the AMR curves. Fourfold symmetry is caused by tetragonal crystal fields and is unique to anti-perovskite 3 d-metal nitrides such as Mn4N and Fe4N. Only slight fourfold symmetry was observed in Mn4− xNi xN. We also performed first-principles calculations with the Vienna ab initio simulation ...
2018 IEEE 7th World Conference on Photovoltaic Energy Conversion (WCPEC) (A Joint Conference of 45th IEEE PVSC, 28th PVSEC & 34th EU PVSEC), 2018
In this study, we investigated defect levels and their densities by deep-level transient spectros... more In this study, we investigated defect levels and their densities by deep-level transient spectroscopy (DLTS). In BaSi 2 epitaxial films, there is a hole trap with a density of N T = 1013 cm−3 at 0.27 eV from the valence band edge. This is interpreted to originate from Si vacancies. We also discuss the minimum lifetime of BaSi2 from the viewpoint of the Shockley-Read-Hall recombination model.
Abstract A group of several methods for detailed investigation of electrically active defects (EA... more Abstract A group of several methods for detailed investigation of electrically active defects (EAD) and their properties in insulator-semiconductor (IS) and metal-insulator-semiconductor (MIS) structures has been developed. It consists of different ways of analyzing EAD including precision measurements of local electrophysical properties of IS structures, mapping of defects in IS structures by means of nematic liquid crystals (NLC), application of scanning electron microscopy (SEM) in electron beam induced current (EBIC) mode for localization of EAD in the semiconductor of MIS structures. Local electrophysical measurements are carried out by using dynamic unsteady-state current–voltage characteristics (DUCVC) and their modifications which have higher sensitivity to detection of generation processes in MIS structures as compared with ordinary capacitance–voltage characteristics. Depth distributions of bulk generation lifetime of minority carriers in semiconductors, obtained by the DUCVC method, reflect normalized EAD profiles near the IS interface. The NLC method is nondestructive and makes possible to analyze lateral distribution of defects with high local conductivity in insulating films. It was shown that in the case of thin SiO2 films on silicon EAD at semiconductor surface are also revealed by the NLC method. As a result determination of EAD coordinates opens possibility for further detailed investigation of separate EAD properties by the DUCVC method using local mercury probe 5 μm in size. Special MIS gated p–n junctions have been used for mapping of EAD with high recombination velocity in semiconductor under the metal or polysilicon gates. Combination of the described methods was applied for investigation of local EAD properties of Si-based structures, which are used for fabrication of integrated circuits. The influence of gettering on EAD concentration in Si and SiO2 has been investigated.
Abstract Group-IV alloy semiconductors have garnered increasing attention as advanced thin-film m... more Abstract Group-IV alloy semiconductors have garnered increasing attention as advanced thin-film materials for next-generation electronics. We have demonstrated polycrystalline Ge thin films with the highest recorded crystallinity and carrier mobility using a multistep heating process in solid-phase crystallization. In this study, we apply these recent findings in Ge to Si1-xGex (x: 0–1) and Ge1-ySny (y: 0–0.04) alloys and investigate their crystal and electrical properties. For all compositions, controlling the temperature in each stage increases the grain size to the micrometer order, improving the carrier mobility and reducing the number of defect-induced acceptors. Sb doping further enlarges the grain size (up to 10 µm) in addition to n-type conduction control, whereas the electron concentration varies with the composition. Both hole and electron mobilities significantly depend on the composition owing to the effects of carrier effective mass, grain size, and carrier concentration: the hole and electron mobilities peak at 350 and 150 cm2 V−1s−1, respectively. The relationship between the composition and various physical properties revealed in this study will contribute to the better understanding, control, and device application of polycrystalline thin films based on group-IV alloy semiconductors.
Barium disilicide (BaSi2) shows great promise as a new material for thin film solar cells (Fig. 1... more Barium disilicide (BaSi2) shows great promise as a new material for thin film solar cells (Fig. 1) [1]. It has a suitable bandgap of 1.3 eV, a large optical absorption coefficient of 3×104 cm−1 for a photon energy of 1.5 eV, and a large minority-carrier diffusion length of about 10 μm. Furthermore, it is composed of only earth abundant elements and highly stable. Therefore, BaSi2 can be used for future terawatt-class power generation. We have achieved the operation of BaSi2 homojunction solar cells (Fig. 2) [2]. In this structure, an open-circuit voltage V OC beyond 0.8 V and a conversion efficiency (η) beyond 25% are expected [3]. However, the achieved η was very small. In this study, three-dimensional (3D) optical simulation using a 3D pyramid texture that is effective in preventing reflection was performed on a BaSi2 solar cells in order to increase the photocurrent. Futhermore, we suggested a new device structure using Al-doped ZnO (AZO) electron transfer layer (ETL). In this st...
We demonstrated the marked photoresponsivity enhancement of BaSi2 epitaxial films by 5 min post-a... more We demonstrated the marked photoresponsivity enhancement of BaSi2 epitaxial films by 5 min post-annealing at 850 °C–1000 °C in contrast to those at 600 °C–800 °C. Post-annealing at 1000 °C increased the photoresponsivity up to 9.0 A W−1 at a wavelength of around 800 nm under a bias voltage of 0.5 V applied between the top and bottom electrodes. The hole concentration decreased monotonously with annealing temperature from 8.3 × 1016 to 5.4 × 1015 cm–3, and the mobility exceeded 1000 cm2 V–1 s–1. The a-axis orientation of the BaSi2 films was significantly deteriorated at temperatures higher than 800 °C.
We fabricated a small-area metal (CoSi2)/insulator (CaF2) hot electron transistor using electron-... more We fabricated a small-area metal (CoSi2)/insulator (CaF2) hot electron transistor using electron-beam lithography. The transistor is composed of a CoSi2/CaF2 (1.9 nm)/CoSi2 (1.9 nm) tunnel emitter and a CaF2 (5 nm) collector barrier on an n-Si(111) substrate. The emitter mesa area is 0.9 × 0.9 µm2. Although the measured characteristics show, for the first time, clear transistor action with a curve similar to those of semiconductor HETs, the collector current increases without saturation due to leakage current through the SiO2 film under the external electrode pads. The intrinsic device characteristics (zero leakage current) exhibited saturation, and a current gain β ≥ 36 was obtained at 77 K.
The antiperovskite ferrimagnet Mn4N has perpendicular magnetic anisotropy and small spontaneous m... more The antiperovskite ferrimagnet Mn4N has perpendicular magnetic anisotropy and small spontaneous magnetization, both of which are favorable properties for current induced domain wall motion. Previously we have investigated the magnetic structure of 3d-element-doped Mn4N thin films and demonstrated ultrafast domain wall velocities reaching 3000 m s−1 in the vicinity of the magnetic compensation composition of Ni-doped Mn4N at the current density of j = 1.2 × 1012 A m−2 at room temperature (RT). In this study, we investigate the effect of Au doping on the magnetic structure of Mn4N films, and present a composition ratio-dependent sign reversal of the anomalous Hall effect at RT. X-ray magnetic circular dichroism measurement revealed that the magnetic moment of the face-centered Mn atoms of Mn4− xAu xN reversed between x = 0.1 and 0.2, and became parallel to that of the corner-site Mn atoms for x = 0.2 and 0.3. This result suggests that the ferrimagnetic-ferromagnetic phase transition o...
We focus on rare-earth-free anti-perovskite Mn4− xNi xN epitaxial films, which can be used for ul... more We focus on rare-earth-free anti-perovskite Mn4− xNi xN epitaxial films, which can be used for ultrafast current-induced domain wall motion (CIDWM) in magnetic strips. The magneto-transport properties of these materials are very important for a deep understanding of CIDWM. In this study, we investigated the magneto-transport properties of Mn4− xNi xN epitaxial films grown on SrTiO3(001) and MgO(001) substrates through anisotropic magnetoresistance (AMR) measurements at temperatures between 2 and 300 K. In samples with a small Ni composition such as x = 0.05−0.1, the AMR ratio of Mn4− xNi xN drastically decreased with increasing temperature. We also analyzed the twofold and fourfold symmetries in the AMR curves. Fourfold symmetry is caused by tetragonal crystal fields and is unique to anti-perovskite 3 d-metal nitrides such as Mn4N and Fe4N. Only slight fourfold symmetry was observed in Mn4− xNi xN. We also performed first-principles calculations with the Vienna ab initio simulation ...
2018 IEEE 7th World Conference on Photovoltaic Energy Conversion (WCPEC) (A Joint Conference of 45th IEEE PVSC, 28th PVSEC & 34th EU PVSEC), 2018
In this study, we investigated defect levels and their densities by deep-level transient spectros... more In this study, we investigated defect levels and their densities by deep-level transient spectroscopy (DLTS). In BaSi 2 epitaxial films, there is a hole trap with a density of N T = 1013 cm−3 at 0.27 eV from the valence band edge. This is interpreted to originate from Si vacancies. We also discuss the minimum lifetime of BaSi2 from the viewpoint of the Shockley-Read-Hall recombination model.
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Papers by Takashi Suemasu