Shuichi Nonomura

Characterization and role of hydrogen in nc-Si:H

Abstract The effects of hydrogen on the properties of polycrystaline (nanometer sized) SiH films (nc-SiH) prepared by plasma enhanced chemical vapor deposition from SiH 4 diluted with H 2 have been studied. In the gas effusion from a sample, three effusion peaks of H 2 are found near 400°C, 500°C and 600°C. The origins of these peaks are assumed to be hydrogen effused from Si–H 2 and Si–H bonds in grain boundaries, and Si–H bonds in amorphous regions, respectively. From a deconvolution of the gas effusion data, the hydrogen density in the grain boundaries is found to decrease with increasing crystalline volume fraction, X c , while that in the amorphous regions is independent of X c . Based on the dependence of the amplitudes Si–H vibrational modes on X c and annealing temperature, the modes near 840 and 2100 cm −1 are related to hydrogen–silicon bonds in grain boundaries. The properties and role of hydrogen in nc-Si:H are discussed.

Experimental evidence of photoinduced expansion in hydrogenated amorphous silicon using bending detected optical lever method

Photoinduced structural change in hydrogenated amorphous silicon (a-Si:H) has been studied by a sensitive bending detection method using an optical lever. We observed that a-Si:H films show not only thermal expansion due to a photothermal effect but also residual and persistent expansion after light soaking. The volume change is recovered by thermal annealing at 200 °C. A dehydrogenated sample annealed at 550 °C and a microcrystalline sample, in which photoinduced defects are not created, show little photoinduced expansion. The photoinduced expansion and photoinduced defect density show identical time evolution. These results suggest that the photoinduced expansion is related to the photoinduced defect creation. A quantitative evaluation of the photoinduced expansion indicates that the photoinduced structural change is spread over several molecular volumes around a photocreated defect.

Optical and electrical properties of amorphous and microcrystalline GaN films and their application to transparent TFT

Abstract Amorphous and microcrystalline GaN thin films have been made by reactive sputtering. The dark conductivity can be changed largely from 10−11 S/cm of amorphous GaN to 10−3 S/cm of microcrystalline GaN with a crystalline size of 700 A. Photoconductivity and persistent photoconductivity (PPC) are observed above room temperature in these films. And a thin film transistor (TFT) was made by using GaN film and observed the operation as a TFT.

Preparation and properties of reactive-sputtered amorphous CNx films

Abstract We prepared amorphous CNx films by a rf reactive magnetron sputtering using ArN2 mixed gas and a graphite target. The nitrogen-carbon ratio x = N C in films determined by the XPS increased from 0.48 to 0.62 as increasing the nitrogen gas partial pressure ratio r = p N 2 (p N 2 + p Ar ) . Optical energy gap E0 obtained by the Taucs plots increased from 1.4 to 1.6 eV with the nitrogen-carbon ratio x. Dangling bonds density of a-CNx was 1018 to 1019 cm−3 from ESR measurement. Dc conductivity of a-CNx is very small. As irradiated Xe lamp, photoconductivity was observed. The hydrogenation of a-CNx was tried by a hydrogen-plasma treatment.

Photoconductive a-GaN prepared by reactive sputtering

Abstract Amorphous and microcrystalline GaN thin films have been made by reactive sputtering. Transparent films of optical gap, E o 5 ∼ 3.95 eV, are obtained and these show semiconductor characteristics. The conductivity changes dramatically from ∼ 10 −11 S/cm to ∼ 10 −3 S/cm on microcrystallization. Photoconductivity is observed in a-GaN. The results of electron spin resonance and sub-gap absorption suggest a low mid-gap defect density of states. It is demonstrated that an amorphous semiconducting material with a large optical gap can be obtained.

Preparation and properties of photoconductive amorphous carbon nitride a-CNx films : the layer-by-layer method

Abstract To improve the photoconductivity of amorphous carbon nitride films a-CNx, hydrogenation was tried. Hydrogenation was not observed in a-CNx film but the improvement of the electronic properties was observed in a few layers of the films. Therefore, to get a better photoconductive a-CNx, the layer-by-layer method was tried with success. Photoconductivity to dark conductivity ratios of a-CNx films, obtained by this layer-by-layer method, were 5×106 on excitation with white light of 100 mW cm−2.

Preparation of silicon–carbon alloy films by hot-wire CVD and their properties

The capability of hot-wire chemical vapor deposition (HWCVD, Cat-CVD) has been studied for the preparation of silicon–carbon alloy (Si1−xCx) films. The changes in deposition rate, carbon content, optical gap and IR absorption of SiC and CHn are demonstrated for Si1−xCx alloy films deposited with the preparation conditions, gas ratio R=CH4/(SiH4+CH4+H2) from 0.3 to 0.6, and filament temperature from 1750 to 2100°C. A deposition rate over 0.9 nm/s is obtained for all samples. The samples have a heterogeneous structure, consisting of hydrogenated amorphous silicon–carbon alloy (a-Si1−yCy:H) and hydrogenated microcrystalline silicon (μc-Si:H). Impurity doping using B2H6 gas has been tried for the sample with a carbon content of ∼28%. The photoconductivity was measured, and the activation energy for the dark conductivity was 0.17 eV with the doping gas ratio, B2H6/(SiH4+CH4) of ∼0.054%.

Localized electronic states related to O2 intercalation and photoirradiation on C60 films and C70 films

Deep localized electronic states are created by O2 intercalation into C60 films and C70 films, which causes the Fermi level to shift down to the middle of gap. The states act as a trap level for charge carriers and as nonradiative recombination centers. It seems that prepared C60 films and C70 films have a shallow localized state. The shallow state is located at ∼0.2 eV under the conduction band and affects the electrical and optical properties. Furthermore, the photoirradiation of C60 films and C70 films causes polymerization of the O2-free sample and oxidization of the O2-intercalated sample. The quasistable electronic states at room temperature are created as a result of photo-oxidization of C60 films. C60 oxides create deep localized electronic states which cannot disappear under thermal annealing. The photoluminescence intensity of O2-free samples increases with photoirradiation for 1 h. It is found for the first time that this increase occurs along with a decrease of localized state density.

Photothermal deflection spectroscopy of chalcogenide glasses

Photothermal deflection spectroscopy has been applied to optical absorption measurements in As2S3, Se, and As–S glasses. The spectroscopic method is demonstrated to be suitable for evaluation of small absorption in thin samples, while its accuracy is affected by light scattering. For As2S3, as-evaporated films give higher residual absorption than those of the bulk. In the As–S system, As2S3 shows the most prominent weak absorption tail, and As17S83 exhibits a peculiar spectrum. The origins of these features are discussed.

Hydrogen-radical durability of TiO2 thin films for protecting transparent conducting oxide for Si thin film solar cells

Abstract Hydrogen-radical durability of TiO 2 thin films has been investigated under conditions for preparing Si thin film solar cells by catalytic chemical vapor deposition method. It is found that the composition and the optical transmittance of TiO 2 films are almost the same before and after hydrogen-radical exposures with a filament temperature at approximately 1700 °C and a H 2 pressure of approximately 133 Pa. The durability of TiO 2 film has also been observed even under the condition with higher hydrogen-radical density under a filament temperature at approximately 1900 °C, in which SnO 2 and ZnO are easily deoxidized. The application of TiO 2 film as a protecting material of transparent conducting oxide film for Si thin film solar cells are discussed by the hydrogen-radical durability and fundamental properties of TiO 2 thin film.