Mitsuo Nakatani

Excimer laser initiated chemical vapor deposition of tungsten films on silicon dioxide

Photochemical vapor deposition technique using an ArF excimer laser has been employed to deposit W films on SiO2 and Si from a WF6 and H2 system. Adhesion characteristics of the film to SiO2 are found to depend both on substrate temperature and on H2/WF6 gas flow ratio: good adhesion is obtained with an increase in the temperature or the ratio. Film formation has reaction orders of 1, 1/2 , and 1 with respect to deposition time, and WF6 and H2 partial pressures, respectively. An activation energy of 0.36 eV is estimated for this film formation on both SiO2 and Si; this energy is plausibly due to H atom diffusion on the W surface. These findings are different from conventional thermal chemical vapor deposition. Film resistivities as low as about 2× the value of bulk W have been observed in the substrate temperature range 250–500 °C. The crystalline structure of the film deposited in this temperature range is uniquely of the α phase. The crystal orientation of the film depends both on substrate temperature ...

Gap States in a-SiGe:H Examined by the Constant Photocurrent Method

Gap states in a-SiGe:H alloys were examined by the deconvolution analysis of subgap absorption spectra obtained by the constant photocurrent method. A broad distribution of defect states was found in a-SiGe:H alloys by the analysis postulating a Gaussian distribution of defect states. Analysis postulating two Gaussian peaks suggested that defects in a-SiGe:H can be divided into Si- related and Ge-related types. The energy positions of two defect peaks relative to the conduction band edge were almost constant irrespective of the optical gap of alloy films, and the position of one peak agreed with the defect peak in a-Si:H. The other peak which locates around 0.75 eV below the conduction band edge appeared when Ge was introduced into a-Si:H. The ratio of these two peaks exhibited a fairly good correlation to the Ge/Si ratio of alloys.

Chemical Vapor Deposition of a-SiGe:H Films Utilizing a Microwave-Excited Plasma

Hydrogenated amorphous silicon-germanium films were prepared by a chemical vapor deposition method utilizing a microwave-excited plasma produced at low gas pressures (~10-3 Torr). Deposition methods in which SiH4 and GeH4 were decomposed by a plasma stream of Ar or hydrogen or by direct excitation were examined at 200°C and at the pressures optimized for the preparation of hydrogenerated amorphous silicon films. Highly photoconductive narrow gap films (an optical gap of 1.5 eV) with photoconductivity higher than 10-5 S/cm and photosensitivity of about 104 were obtained by these methods, keeping a high deposition rate.

Microwave-Excited Plasma CVD of a-Si:H Films Utilizing a Hydrogen Plasma Stream or by Direct Excitation of Silane

The microwave-excited plasma CVD of a-Si:H films, in which SiH4 was decomposed by a plasma stream of hydrogen or by direct excitation at low gas pressures (10-4 to 10-3 Torr), was examined and compared with a previously studied Ar plasma stream method. It was clarified that an Ar plasma stream was not necessary to obtain highly photoconductive a-Si:H films. However, the deposition characteristics in these deposition methods were different from the Ar plasma stream method. On the other hand, the influence of the hydrogen plasma stream was not apparent when the SiH4 flow rate was high, suggesting an ionization cross-section dependence of the plasma discharge.

Chemical Vapor Deposition of a-Si:H Films Utilizing a Microwave Excited Ar Plasma Stream

A Microwave-excited Ar-plasma stream produced at low gas pressures (10-4 to 10-2 Torr) was applied to a preparation of hydrogenated amorphous silicon films made from SiH4. The electric and optical properties of the films were investigated as functions of the deposition conditions. The film properties greatly depended upon the substrate temperature and the deposition pressure and slightly upon the microwave power. The conductivity of the films prepared at 200°C and at pressures ranging between 0.6 and 0.8 mTorr were about 5×10-10 Scm-1 in the dark and 4×10-4 Scm-1 under AM 1.5 illumination. The films prepared at the lower pressures were porous and contain a large amount of gases; the films prepared at higher pressures were deteriorated in the same way as those prepared by an RF glow discharge from Ar-diluted SiH4.

Influence of deposition conditions on properties of a-SiGe:H prepared by microwave-excited plasma CVD

The influence of the deposition conditions on properties of a-SiGe:H prepared by the microwave-excited plasma CVD was examined. Highly photoconductive a-SiGe:H alloys were obtained at the pressures lower than 0.8 mTorr when the substrate was heated to 200degC. The photoconductive property of a-SiGe:H deteriorated when the microwave power was decreased. Similar deterioration was observed when the deposition pressure was increased. It was suggested that the deterioration of the photoconductive property was caused by the depression of SiH4 decomposition. The photoconductivity of a-SiGe:H alloys increased with the decrease of the deposition pressure even though the amount of the isolated SiH2 bonds in the film also increased with the decrease of the deposition pressure. The deposition mechanism inducing this deposition pressure dependence was discussed.

The transition state of disilane plasma

Abstract The initial transition state in the plasma decomposition of disilane was investigated by measuring the electron energy in a disilane plasma, and by studying both optical emission spectroscopy and film properties. At the beginning of the discharge, the emission of SiH ∗ was not observed, and the electron energy in the plasma was about 10 times lower than in the steady state. These phenomena were caused by an abrupt increase of the pressure just after the ignition, followed by disilane decomposition. Under insufficient energy conditions, production of silylene is highly favoured by the disilane decomposition which gives porous hydrogenated amorphous silicon (a-Si:H) films. We shall prove the importance to shorten the initial transition period in which the plasma energy is insufficient for disilane decomposition. Keeping the gas pressure constant at the beginning of the reaction enables to avoid the depression of the plasma energy and to obtain a-Si:H films with good optoelectronic properties.

A correlation between crystalline qualities and growth parameters in GaAs on Si grown by molecular‐beam epitaxy

The effects of the As4/Ga flux ratio and the growth temperature on the crystalline qualities are demonstrated in the heteroepitaxy of GaAs on Si grown by molecular‐beam epitaxy. To improve surface morphology, either the As4/Ga flux ratio must be optimized to 8 when grown at 560 °C or the growth temperature must be increased. In electron‐beam‐induced current images, a lot of complicated dark regions have been observed. The dark regions decreased and photoluminescence intensity became strong with the increase in the growth temperature.

Light-induced change in photoconductivity of a-SiGe-H alloys

The Staebler-Wronski effect (SWE) of a-SiGe:H for solar cells has been examined for highly photoconductive alloys prepared by the microwave-excited plasma CVD (chemical vapor deposition) method. It was found that the light-induced change in photoconductivity depends upon the optical gap of films. i.e. the Ge content in alloys, rather than upon the initial value of the photoconductivity. The light-induced change in photoconductivity decreased with the decrease in optical gap and disappeared for the alloys with an optical gap around 1.4 eV, irrespective of the initial value of the photoconductivity. This dependence of the SWE upon the optical gap was not continuous to a-Si:H, and the light-induced change in photoconductivity of the alloys with small amounts of Ge was larger than that of a-Si:H. The photosensitivity slightly decreased under light soaking, which suggests a structural change during illumination. The changes in infrared spectra and electron-spin-resonance spin densities of a-SiGe:H alloys caused by light soaking were examined. The Si-H and Ge-H stretching bands (centered at 2000 and 1880 cm/sup -1/) decreased by the same amount without any shift in wavenumber.<<ETX>>