Naoteru Shigekawa | Osaka City University (original) (raw)
Papers by Naoteru Shigekawa
Journal of Applied Physics, Feb 21, 2022
We studied electrical properties of n-Si/n-Ga2O3 heterojunctions fabricated by surface-activated ... more We studied electrical properties of n-Si/n-Ga2O3 heterojunctions fabricated by surface-activated bonding. The energy barrier heights (qϕb) at the Si/Ga2O3 interface for different reverse voltages (Vrev) were derived from temperature-dependent current density–voltage (J–V–T) characteristics. With shifting Vrev to the negative direction, qϕb gradually decreased and reached a constant value due to negatively charged interface states. The conduction band offset at the heterointerface was estimated to be 0.18 eV from the Vrev dependence of qϕb. The qϕb calculated from a capacitance of the heterojunction at thermal equilibrium was larger than those derived from the J–V–T characteristics, attributing to spatially inhomogeneous qϕb caused by the non-uniform distribution of the charged interface states. The density of shallow interface states was also extracted from the reverse J–V–T characteristics, which was estimated to be about 6 × 1012 cm−2 eV−1.
Solid-state Electronics, Sep 1, 1989
The deviation of electron scattering probability in semiconductor superlattices from its bulk val... more The deviation of electron scattering probability in semiconductor superlattices from its bulk value is proposed as an index of 3-D properties of conduction electrons in semiconductor superlattices. The relative deviation of scattering probability is numerically calculated for acoustic-phonon scattering, ionized-impurity scattering, and polar-optical phonon scattering in GaAs/A1045Ga0.55As superlattices. By comparing the three scattering processes, it is found that the probability of acoustic-phonon scattering in the superlattices deviates to the largest extent, and that the difference in the relative deviations of the scattering probabilities is due to the wave-vector dependences of the respective scattering processes.
Semiconductor Science and Technology, Mar 1, 1992
The effects of disorder on the electronic conduction properties at subsidiary energy minima (uppe... more The effects of disorder on the electronic conduction properties at subsidiary energy minima (upper valleys) of In0.53Ga0.47As alloys are evaluated using a novel Monte Carlo supercell approach with both the bond-length and bond-angle fluctuations considered. It is found that the width of the spectral function obtained in the present approach is much larger than that obtained using the coherent-potential approximation (CPA). This result indicates that the bond-length and the bond-angle fluctuations-which are not sufficiently considered in the CPA framework-induce strong effects on the electronic properties of the upper valleys.
Applied Physics Letters, Sep 28, 1992
We measured the electron velocity versus electric field (v-E) relationships for GaAs/AlGaAs multi... more We measured the electron velocity versus electric field (v-E) relationships for GaAs/AlGaAs multiple quantum wells with different well-layer thicknesses (50, 100, and 200 Å). No clear dependences on the well thickness have been observed in the measured v-E’s, which indicate that the confinement of electrons in well layers does not induce strong effects on the momentum relaxation and the energy loss rate. The results of the measurement were also compared with those derived by the Monte Carlo calculation, and a remarkable discrepancy between measured and calculated results was found in the well thickness dependence of the electron peak velocity.
Applied Physics Letters, Oct 13, 2014
Journal of Applied Physics, 2011
The valence band offsets, ⌬E V , of In 0.17 Al 0.83 N / GaN, In 0.25 Al 0.75 N / GaN, and In 0.30... more The valence band offsets, ⌬E V , of In 0.17 Al 0.83 N / GaN, In 0.25 Al 0.75 N / GaN, and In 0.30 Al 0.70 N / GaN heterostructures grown by metal-organic vapor phase epitaxy were evaluated by using x-ray photoelectron spectroscopy ͑XPS͒. The dependence of the energy position and the full width at half maximum of the Al 2p spectrum on the exit angle indicated that there was sharp band bending caused by the polarization-induced electric field combined with surface Fermi-level pinning in each ultrathin InAlN layer. The ⌬E V values evaluated without taking into account band bending indicated large discrepancies from the theoretical estimates for all samples. Erroneous results due to band bending were corrected by applying numerical calculations, which led to acceptable results. The evaluated ⌬E V values were 0.2Ϯ 0.2 eV for In 0.17 Al 0.83 N / GaN, 0.1Ϯ 0.2 eV for In 0.25 Al 0.75 N / GaN, and 0.0Ϯ 0.2 eV for In 0.30 Al 0.70 N / GaN. Despite the large decrease of around 1.0 eV in the band gap of InAlN layers according to the increase in the In molar fraction, the decrease in ⌬E V was as small as 0.2 eV. Therefore, the change in band-gap discontinuity was mainly distributed to that in conduction band offset.
Japanese Journal of Applied Physics, 2022
Chemical composition around diamond/silicon heterointerfaces fabricated by surface activated bond... more Chemical composition around diamond/silicon heterointerfaces fabricated by surface activated bonding (SAB) at room temperature is examined by energy-dispersive X-ray spectroscopy under scanning transmission electron microscopy. Iron impurities segregate just on the bonding interfaces, while oxygen impurities segregate off the bonding interfaces in the silicon side by 3–4 nm. Oxygen atoms would segregate so as to avoid the amorphous compound with silicon and carbon atoms, self-organized at the bonding interfaces in the SAB process. When the bonding interfaces are annealed at 1000 °C, the amorphous compound converts into cubic silicon carbide (c-SiC), and nano-voids 5–15 nm in size are formed at the region between silicon and c-SiC, at which the oxygen density is high before annealing. The nano-voids can act as the gettering sites in which metal impurities are preferentially agglomerated, and the impurity gettering would help to improve the electronic properties of the bonding interfa...
The Japan Society of Applied Physics, 2016
Direct bonding of single-crystal diamond and Si substrate 梁 剣波 、桝谷 聡士 、嘉数 誠 、重川 直輝 1 Osaka City U... more Direct bonding of single-crystal diamond and Si substrate 梁 剣波 、桝谷 聡士 、嘉数 誠 、重川 直輝 1 Osaka City Univ. , Saga Univ., Jianbo Liang, Satoshi Masuya, Makoto Kasu, Naoteru Shigekawa E-mail: liang@elec.eng.osaka-cu.ac.jp 【はじめに】ダイヤモンド半導体は、SiC と GaN と比べても、熱伝導率、耐熱性、絶縁破 壊強度、耐放射線性などといった物理特性が最 も優れていることから、次々世代のパワー半導 体として大いに期待されている。しかし、デバ イス応用に必要不可欠なダイヤモンド単結晶 の市販品の寸法は 4ミリ角程度にすぎない。そ こで、我々は、大面積ウェハーの入手が可能な Si 単結晶基板にダイヤモンド単結晶を埋め込 めれば、ダイヤモンドのパワー素子を Si LSI のプロセスラインで作製可能にできるだけで なく、同一基板上でダイヤモンド素子と様々機 能をもつ Si LSI を組み合わせるなど、エレク トロニクス応用へ発展する可能性があると考 えて研究を行った。本研究では、Si(100)基板 とダイヤモンド単結晶の直接接合を目的とし、 表面活性化接合法[1]という手法を用いて Si/ ダイヤモンド異種接合の作製を行った。 【実験方法】4ミリサイズのダイヤモンド(100) 単結晶を表面活性化接合法により Si(100)単結 晶基板と接合し、ダイヤモンド/Si 構造の異種 接合を作製した。接合後の断面構造が電界放出 形走査電子顕微鏡(FE-SEM)で観察を行った。 【結果】単結晶ダイヤモンドと Si 基板の接合 した試料写真を図 1 に示す。左上部の黄色はダ イヤモンド単結晶中の窒素不純物による光吸 収に由来するが、接合が不完全であるため干渉 縞が見えるが、ダイヤモンド単結晶の右下部の 約40%の領域で干渉縞が見えず、ダイヤモン ドと Si の接合面が得られていることが確認で きる。図 2 にダイヤモンドと Si 接合面の断面 FE-SEM 像を示す。接合界面の全体で、空洞や 機械的な破損が観察されず、室温で単結晶ダイ ヤモンド/Si 直接接合を確認した。 【謝...
2015 IEEE International Meeting for Future of Electron Devices, Kansai (IMFEDK), 2015
The electrical properties of n<sup>+</sup>-Si/n-GaN junctions by room-temperature bon... more The electrical properties of n<sup>+</sup>-Si/n-GaN junctions by room-temperature bonding were investigated. The n<sup>+</sup>-Si/n-GaN junctions exhibited linear current-voltage characteristics.
2016 IEEE International Meeting for Future of Electron Devices, Kansai (IMFEDK), 2016
n+-Ge/n-4H-SiC heterojunctions were formed by surface activated bonding. The electrical character... more n+-Ge/n-4H-SiC heterojunctions were formed by surface activated bonding. The electrical characteristics of the heterojunctions were experimentally investigated by measuring their current-voltage (I-V) and capacitance-voltage (C-V) measurements. The band diagram of heterojunctions was estimated based on the C-V characteristics.
Extended Abstracts of the 2017 International Conference on Solid State Devices and Materials, 2017
We successfully fabricate coplanar waveguides (CPWs) composed of 17 µm Al foils on sapphire subst... more We successfully fabricate coplanar waveguides (CPWs) composed of 17 µm Al foils on sapphire substrates by surface activated bonding (SAB). We demonstrate that transmission and line loss are improved by using Al foils in comparison with CPWs composed of evaporated Al films. The obtained results imply that CPWs with better performances are realized by using SAB.
Japanese Journal of Applied Physics
We investigate the effects of several-hundred-micron thick luminescence down-shifting (LDS) films... more We investigate the effects of several-hundred-micron thick luminescence down-shifting (LDS) films composed of sol–gel glass with Zn-based nanoparticles (NPs) dispersed on the characteristics of Si solar cells. Their internal quantum efficiencies (IQEs) are successfully measured by separating the contributions of downshifted photons in measuring reflectance for 300–400 nm, wavelengths of incident photons absorbed by the NPs. We find that IQEs for this wavelength range are more enhanced by employing thicker LDS films, i.e. LDS films with higher optical densities. We also discuss the relationship between the number density of NPs in LDS films, their optical properties, and the IQEs of cells. We observe a discrepancy between the measured and calculated IQEs and note that this is the result of downshifted photons escaping across the sides of the LDS films.
ECS Meeting Abstracts, 2020
Ⅲ-Ⅴ on Si multijunction (MJ) solar cells are promising as next-generation solar cells since they ... more Ⅲ-Ⅴ on Si multijunction (MJ) solar cells are promising as next-generation solar cells since they can provide high efficiency with low cost in comparison with conventional Si and Ⅲ-Ⅴ MJ cells. In fabricating such III-V/Si MJ cells, Ⅲ-Ⅴ subcells are placed on Si bottom cell by hybrid approaches such as surface-activated bonding (SAB). The bonding interfaces with lower interface resistances are strongly required so as to achieve better performance of hybrid MJ cells. It was found that the resistance across the directly-bonded III-V/Si interfaces in MJ cells was higher than the resistance in junctions made of heavily-doped substrates because of the thin heavily-doped bonding layers in actual subcell structures. It was reported that indium tin oxide (ITO) films as intermediate layers between III-V and Si subcells played a role of lowering the series resistance of MJs [1]. It was found, however, that the resistance of GaAs//ITO/Si junctions increased by annealing them [2]. Furthermore, th...
2017 IEEE 44th Photovoltaic Specialist Conference (PVSC), 2017
Using surface-activated bonding technologies we fabricate InGaP/GaAs/ITO/Si hybrid triple-junctio... more Using surface-activated bonding technologies we fabricate InGaP/GaAs/ITO/Si hybrid triple-junction (3J) cells with p+-GaAs/ITO and those with n+-GaAs/ITO bonding interfaces. ITO films deposited on the emitter of Si bottom cells work as intermediate layers between III-V and Si sub cells. The samples are not heated during the bonding process. The photovoltaic characteristics of the fabricated 3J cells are compared with characteristics of conventional 3J cells without intermediate layers. The InGaP/GaAs/ITO/Si 3J cells with nnn + -GaAs/ITO bonding interfaces reveal the highest conversion efficiency and the lowest differential resistance among the investigated 3J cells, which implies the potential of ITO-based intermediate layers for achieving more excellent performances of hybrid multi-junction cells.
The Japan Society of Applied Physics, 2020
The Japan Society of Applied Physics, 2019
2019 6th International Workshop on Low Temperature Bonding for 3D Integration (LTB-3D), 2019
Elemental distribution around GaAs homointerfaces fabricated by surface-activated bonding (SAB) i... more Elemental distribution around GaAs homointerfaces fabricated by surface-activated bonding (SAB) is examined by atom probe tomography (APT). Our APT detected small amount of contaminants accidentally introduced on the SAB interfaces. We confirmed that As-deficient intermediate layer at the as-bonded interfaces was recovered by post-bonding annealing.
Compressively strained InxAl1 xN (15 nm)/Al0:22Ga0:78N (3 nm)/GaN heterostructure field effect tr... more Compressively strained InxAl1 xN (15 nm)/Al0:22Ga0:78N (3 nm)/GaN heterostructure field effect transistors (FETs) with regrown AlGaN contact layers were fabricated. The increase in compressive strain in InAlN reduced the polarization charge, i.e., the density of two-dimensional electron gas decreased from 6:5 10 to 1:3 10 cm 2 as the In content of InAlN increased from 0.245 to 0.325. With the insertion of the AlGaN layer, electron mobility of as high as 1570 cm V 1 s 1 was achieved at the In content of 0.245. Selectively regrown AlGaN contact layers reduced the sheet resistance from 17,000 to 584 /sq. at the access layer for In0:325Al0:675N/Al0:22Ga0:78N/GaN. We fabricated FETs with this structure. The FETs without the regrown AlGaN contact layer did not operate at all owing to the high resistance. In contrast, the devices with the regrown AlGaN did. The maximum transconductance is 60mS/mm, and the drain current is 0.11A/mm. The threshold voltage becomes shallower, changing from 3:2...
2014 4th IEEE International Workshop on Low Temperature Bonding for 3D Integration (LTB-3D), 2014
The effects of annealing process on the electrical properties of n+-Si/n-SiC junctions fabricated... more The effects of annealing process on the electrical properties of n+-Si/n-SiC junctions fabricated by using surface-activated bonding are investigated. It is found by measuring their current-voltage (I-V) characteristics that the reverse-bias current decreases and activation energy increases with increasing annealing temperature to 700 °C.
2019 6th International Workshop on Low Temperature Bonding for 3D Integration (LTB-3D), 2019
Arsenic (As) vacancies are introduced beneath the GaAs surfaces irradiated by Ar atoms for surfac... more Arsenic (As) vacancies are introduced beneath the GaAs surfaces irradiated by Ar atoms for surface activation, while As interstitials are co-introduced at deeper regions. After 673 K annealing, As vacancies disappear by the recombination with As interstitials, via the migration of As interstitials.
Journal of Applied Physics, Feb 21, 2022
We studied electrical properties of n-Si/n-Ga2O3 heterojunctions fabricated by surface-activated ... more We studied electrical properties of n-Si/n-Ga2O3 heterojunctions fabricated by surface-activated bonding. The energy barrier heights (qϕb) at the Si/Ga2O3 interface for different reverse voltages (Vrev) were derived from temperature-dependent current density–voltage (J–V–T) characteristics. With shifting Vrev to the negative direction, qϕb gradually decreased and reached a constant value due to negatively charged interface states. The conduction band offset at the heterointerface was estimated to be 0.18 eV from the Vrev dependence of qϕb. The qϕb calculated from a capacitance of the heterojunction at thermal equilibrium was larger than those derived from the J–V–T characteristics, attributing to spatially inhomogeneous qϕb caused by the non-uniform distribution of the charged interface states. The density of shallow interface states was also extracted from the reverse J–V–T characteristics, which was estimated to be about 6 × 1012 cm−2 eV−1.
Solid-state Electronics, Sep 1, 1989
The deviation of electron scattering probability in semiconductor superlattices from its bulk val... more The deviation of electron scattering probability in semiconductor superlattices from its bulk value is proposed as an index of 3-D properties of conduction electrons in semiconductor superlattices. The relative deviation of scattering probability is numerically calculated for acoustic-phonon scattering, ionized-impurity scattering, and polar-optical phonon scattering in GaAs/A1045Ga0.55As superlattices. By comparing the three scattering processes, it is found that the probability of acoustic-phonon scattering in the superlattices deviates to the largest extent, and that the difference in the relative deviations of the scattering probabilities is due to the wave-vector dependences of the respective scattering processes.
Semiconductor Science and Technology, Mar 1, 1992
The effects of disorder on the electronic conduction properties at subsidiary energy minima (uppe... more The effects of disorder on the electronic conduction properties at subsidiary energy minima (upper valleys) of In0.53Ga0.47As alloys are evaluated using a novel Monte Carlo supercell approach with both the bond-length and bond-angle fluctuations considered. It is found that the width of the spectral function obtained in the present approach is much larger than that obtained using the coherent-potential approximation (CPA). This result indicates that the bond-length and the bond-angle fluctuations-which are not sufficiently considered in the CPA framework-induce strong effects on the electronic properties of the upper valleys.
Applied Physics Letters, Sep 28, 1992
We measured the electron velocity versus electric field (v-E) relationships for GaAs/AlGaAs multi... more We measured the electron velocity versus electric field (v-E) relationships for GaAs/AlGaAs multiple quantum wells with different well-layer thicknesses (50, 100, and 200 Å). No clear dependences on the well thickness have been observed in the measured v-E’s, which indicate that the confinement of electrons in well layers does not induce strong effects on the momentum relaxation and the energy loss rate. The results of the measurement were also compared with those derived by the Monte Carlo calculation, and a remarkable discrepancy between measured and calculated results was found in the well thickness dependence of the electron peak velocity.
Applied Physics Letters, Oct 13, 2014
Journal of Applied Physics, 2011
The valence band offsets, ⌬E V , of In 0.17 Al 0.83 N / GaN, In 0.25 Al 0.75 N / GaN, and In 0.30... more The valence band offsets, ⌬E V , of In 0.17 Al 0.83 N / GaN, In 0.25 Al 0.75 N / GaN, and In 0.30 Al 0.70 N / GaN heterostructures grown by metal-organic vapor phase epitaxy were evaluated by using x-ray photoelectron spectroscopy ͑XPS͒. The dependence of the energy position and the full width at half maximum of the Al 2p spectrum on the exit angle indicated that there was sharp band bending caused by the polarization-induced electric field combined with surface Fermi-level pinning in each ultrathin InAlN layer. The ⌬E V values evaluated without taking into account band bending indicated large discrepancies from the theoretical estimates for all samples. Erroneous results due to band bending were corrected by applying numerical calculations, which led to acceptable results. The evaluated ⌬E V values were 0.2Ϯ 0.2 eV for In 0.17 Al 0.83 N / GaN, 0.1Ϯ 0.2 eV for In 0.25 Al 0.75 N / GaN, and 0.0Ϯ 0.2 eV for In 0.30 Al 0.70 N / GaN. Despite the large decrease of around 1.0 eV in the band gap of InAlN layers according to the increase in the In molar fraction, the decrease in ⌬E V was as small as 0.2 eV. Therefore, the change in band-gap discontinuity was mainly distributed to that in conduction band offset.
Japanese Journal of Applied Physics, 2022
Chemical composition around diamond/silicon heterointerfaces fabricated by surface activated bond... more Chemical composition around diamond/silicon heterointerfaces fabricated by surface activated bonding (SAB) at room temperature is examined by energy-dispersive X-ray spectroscopy under scanning transmission electron microscopy. Iron impurities segregate just on the bonding interfaces, while oxygen impurities segregate off the bonding interfaces in the silicon side by 3–4 nm. Oxygen atoms would segregate so as to avoid the amorphous compound with silicon and carbon atoms, self-organized at the bonding interfaces in the SAB process. When the bonding interfaces are annealed at 1000 °C, the amorphous compound converts into cubic silicon carbide (c-SiC), and nano-voids 5–15 nm in size are formed at the region between silicon and c-SiC, at which the oxygen density is high before annealing. The nano-voids can act as the gettering sites in which metal impurities are preferentially agglomerated, and the impurity gettering would help to improve the electronic properties of the bonding interfa...
The Japan Society of Applied Physics, 2016
Direct bonding of single-crystal diamond and Si substrate 梁 剣波 、桝谷 聡士 、嘉数 誠 、重川 直輝 1 Osaka City U... more Direct bonding of single-crystal diamond and Si substrate 梁 剣波 、桝谷 聡士 、嘉数 誠 、重川 直輝 1 Osaka City Univ. , Saga Univ., Jianbo Liang, Satoshi Masuya, Makoto Kasu, Naoteru Shigekawa E-mail: liang@elec.eng.osaka-cu.ac.jp 【はじめに】ダイヤモンド半導体は、SiC と GaN と比べても、熱伝導率、耐熱性、絶縁破 壊強度、耐放射線性などといった物理特性が最 も優れていることから、次々世代のパワー半導 体として大いに期待されている。しかし、デバ イス応用に必要不可欠なダイヤモンド単結晶 の市販品の寸法は 4ミリ角程度にすぎない。そ こで、我々は、大面積ウェハーの入手が可能な Si 単結晶基板にダイヤモンド単結晶を埋め込 めれば、ダイヤモンドのパワー素子を Si LSI のプロセスラインで作製可能にできるだけで なく、同一基板上でダイヤモンド素子と様々機 能をもつ Si LSI を組み合わせるなど、エレク トロニクス応用へ発展する可能性があると考 えて研究を行った。本研究では、Si(100)基板 とダイヤモンド単結晶の直接接合を目的とし、 表面活性化接合法[1]という手法を用いて Si/ ダイヤモンド異種接合の作製を行った。 【実験方法】4ミリサイズのダイヤモンド(100) 単結晶を表面活性化接合法により Si(100)単結 晶基板と接合し、ダイヤモンド/Si 構造の異種 接合を作製した。接合後の断面構造が電界放出 形走査電子顕微鏡(FE-SEM)で観察を行った。 【結果】単結晶ダイヤモンドと Si 基板の接合 した試料写真を図 1 に示す。左上部の黄色はダ イヤモンド単結晶中の窒素不純物による光吸 収に由来するが、接合が不完全であるため干渉 縞が見えるが、ダイヤモンド単結晶の右下部の 約40%の領域で干渉縞が見えず、ダイヤモン ドと Si の接合面が得られていることが確認で きる。図 2 にダイヤモンドと Si 接合面の断面 FE-SEM 像を示す。接合界面の全体で、空洞や 機械的な破損が観察されず、室温で単結晶ダイ ヤモンド/Si 直接接合を確認した。 【謝...
2015 IEEE International Meeting for Future of Electron Devices, Kansai (IMFEDK), 2015
The electrical properties of n<sup>+</sup>-Si/n-GaN junctions by room-temperature bon... more The electrical properties of n<sup>+</sup>-Si/n-GaN junctions by room-temperature bonding were investigated. The n<sup>+</sup>-Si/n-GaN junctions exhibited linear current-voltage characteristics.
2016 IEEE International Meeting for Future of Electron Devices, Kansai (IMFEDK), 2016
n+-Ge/n-4H-SiC heterojunctions were formed by surface activated bonding. The electrical character... more n+-Ge/n-4H-SiC heterojunctions were formed by surface activated bonding. The electrical characteristics of the heterojunctions were experimentally investigated by measuring their current-voltage (I-V) and capacitance-voltage (C-V) measurements. The band diagram of heterojunctions was estimated based on the C-V characteristics.
Extended Abstracts of the 2017 International Conference on Solid State Devices and Materials, 2017
We successfully fabricate coplanar waveguides (CPWs) composed of 17 µm Al foils on sapphire subst... more We successfully fabricate coplanar waveguides (CPWs) composed of 17 µm Al foils on sapphire substrates by surface activated bonding (SAB). We demonstrate that transmission and line loss are improved by using Al foils in comparison with CPWs composed of evaporated Al films. The obtained results imply that CPWs with better performances are realized by using SAB.
Japanese Journal of Applied Physics
We investigate the effects of several-hundred-micron thick luminescence down-shifting (LDS) films... more We investigate the effects of several-hundred-micron thick luminescence down-shifting (LDS) films composed of sol–gel glass with Zn-based nanoparticles (NPs) dispersed on the characteristics of Si solar cells. Their internal quantum efficiencies (IQEs) are successfully measured by separating the contributions of downshifted photons in measuring reflectance for 300–400 nm, wavelengths of incident photons absorbed by the NPs. We find that IQEs for this wavelength range are more enhanced by employing thicker LDS films, i.e. LDS films with higher optical densities. We also discuss the relationship between the number density of NPs in LDS films, their optical properties, and the IQEs of cells. We observe a discrepancy between the measured and calculated IQEs and note that this is the result of downshifted photons escaping across the sides of the LDS films.
ECS Meeting Abstracts, 2020
Ⅲ-Ⅴ on Si multijunction (MJ) solar cells are promising as next-generation solar cells since they ... more Ⅲ-Ⅴ on Si multijunction (MJ) solar cells are promising as next-generation solar cells since they can provide high efficiency with low cost in comparison with conventional Si and Ⅲ-Ⅴ MJ cells. In fabricating such III-V/Si MJ cells, Ⅲ-Ⅴ subcells are placed on Si bottom cell by hybrid approaches such as surface-activated bonding (SAB). The bonding interfaces with lower interface resistances are strongly required so as to achieve better performance of hybrid MJ cells. It was found that the resistance across the directly-bonded III-V/Si interfaces in MJ cells was higher than the resistance in junctions made of heavily-doped substrates because of the thin heavily-doped bonding layers in actual subcell structures. It was reported that indium tin oxide (ITO) films as intermediate layers between III-V and Si subcells played a role of lowering the series resistance of MJs [1]. It was found, however, that the resistance of GaAs//ITO/Si junctions increased by annealing them [2]. Furthermore, th...
2017 IEEE 44th Photovoltaic Specialist Conference (PVSC), 2017
Using surface-activated bonding technologies we fabricate InGaP/GaAs/ITO/Si hybrid triple-junctio... more Using surface-activated bonding technologies we fabricate InGaP/GaAs/ITO/Si hybrid triple-junction (3J) cells with p+-GaAs/ITO and those with n+-GaAs/ITO bonding interfaces. ITO films deposited on the emitter of Si bottom cells work as intermediate layers between III-V and Si sub cells. The samples are not heated during the bonding process. The photovoltaic characteristics of the fabricated 3J cells are compared with characteristics of conventional 3J cells without intermediate layers. The InGaP/GaAs/ITO/Si 3J cells with nnn + -GaAs/ITO bonding interfaces reveal the highest conversion efficiency and the lowest differential resistance among the investigated 3J cells, which implies the potential of ITO-based intermediate layers for achieving more excellent performances of hybrid multi-junction cells.
The Japan Society of Applied Physics, 2020
The Japan Society of Applied Physics, 2019
2019 6th International Workshop on Low Temperature Bonding for 3D Integration (LTB-3D), 2019
Elemental distribution around GaAs homointerfaces fabricated by surface-activated bonding (SAB) i... more Elemental distribution around GaAs homointerfaces fabricated by surface-activated bonding (SAB) is examined by atom probe tomography (APT). Our APT detected small amount of contaminants accidentally introduced on the SAB interfaces. We confirmed that As-deficient intermediate layer at the as-bonded interfaces was recovered by post-bonding annealing.
Compressively strained InxAl1 xN (15 nm)/Al0:22Ga0:78N (3 nm)/GaN heterostructure field effect tr... more Compressively strained InxAl1 xN (15 nm)/Al0:22Ga0:78N (3 nm)/GaN heterostructure field effect transistors (FETs) with regrown AlGaN contact layers were fabricated. The increase in compressive strain in InAlN reduced the polarization charge, i.e., the density of two-dimensional electron gas decreased from 6:5 10 to 1:3 10 cm 2 as the In content of InAlN increased from 0.245 to 0.325. With the insertion of the AlGaN layer, electron mobility of as high as 1570 cm V 1 s 1 was achieved at the In content of 0.245. Selectively regrown AlGaN contact layers reduced the sheet resistance from 17,000 to 584 /sq. at the access layer for In0:325Al0:675N/Al0:22Ga0:78N/GaN. We fabricated FETs with this structure. The FETs without the regrown AlGaN contact layer did not operate at all owing to the high resistance. In contrast, the devices with the regrown AlGaN did. The maximum transconductance is 60mS/mm, and the drain current is 0.11A/mm. The threshold voltage becomes shallower, changing from 3:2...
2014 4th IEEE International Workshop on Low Temperature Bonding for 3D Integration (LTB-3D), 2014
The effects of annealing process on the electrical properties of n+-Si/n-SiC junctions fabricated... more The effects of annealing process on the electrical properties of n+-Si/n-SiC junctions fabricated by using surface-activated bonding are investigated. It is found by measuring their current-voltage (I-V) characteristics that the reverse-bias current decreases and activation energy increases with increasing annealing temperature to 700 °C.
2019 6th International Workshop on Low Temperature Bonding for 3D Integration (LTB-3D), 2019
Arsenic (As) vacancies are introduced beneath the GaAs surfaces irradiated by Ar atoms for surfac... more Arsenic (As) vacancies are introduced beneath the GaAs surfaces irradiated by Ar atoms for surface activation, while As interstitials are co-introduced at deeper regions. After 673 K annealing, As vacancies disappear by the recombination with As interstitials, via the migration of As interstitials.