Yasutake Toyoshima

Vacuum ultraviolet absorption cross sections of SiH4, GeH4, Si2H6, and Si3H8

The vacuum ultraviolet absorption cross sections of SiH4, GeH4, Si2H6, and Si3H8 are reported for the wavelength region 107–220 nm using synchrotron radiation as a light source. Absorption maxima of these compounds were found at the exciting wavelengths of 115–119 nm. Broad peaks observed were mostly assigned as primarily Rydberg transitions of the σSiH and σSiSi bonding electrons to the 4s, 4p, and 4d orbitals. The absorption features of germane resemble those of monosilane. In the photoexcitation of monosilane, the emission of the SiH(A 2Δ→X 2Π) transition was observed and its onset was found to be 132±2nm. The absorption spectrum of disilane showed five peaks. They were mostly assigned as 2a1g→4s, 2a1g→np(n=4−6) transitions and the strongest band was overlapped by 1eg→4d and 1eu→4p Rydberg transitions. In trisilane molecules three very weak and broad peaks were recognized and assigned as 3b2→4s, 4p and 4a1→4s, 4d Rydberg transitions. The strongest band was tentatively assigned as the superposition of...

Substrate dependence of initial growth of microcrystalline silicon in plasma‐enhanced chemical vapor deposition

Very thin microcrystalline silicon films have been deposited by plasma‐enhanced chemical vapor deposition (CVD) from hydrogen diluted SiH4 gas in order to study its initial growth mechanism. The dependence of the crystallinity and the Si‐H bonding configuration on electrical and chemical properties of substrates have been studied using a high sensitivity Raman spectrometer. It is found that conductive and oxygen free substrates provide good crystallinity in the initial stage. The origin of the substrate dependence is explained in terms of the ion bombardment from the plasma and the chemical nature of the substrate surface.

Real time in situ observation of the film growth of hydrogenated amorphous silicon by infrared reflection absorption spectroscopy

The growth of hydrogenated amorphous silicon films on Al substrates in a flow reactor was studied using infrared reflection absorption spectroscopy. All three hydride species (SiHx , x=1–3) in the growing films were detected as stretching and bending absorption bands in P polarization spectra. The dominant absorption band, initially originating from higher hydrides, was shifted to lower wave numbers with an increase of film thickness. A steep rise in absorption intensity at the initial stage and a time delay in SiH emergence are discussed in terms of the enhancement of detection sensitivity in a hydrogen‐rich layer and the time needed for the formation of a bulk‐network structure, respectively.

In situ characterization of the growing a-Si:H surface by IR spectroscopy

The surface hydrogen on the growing a-Si:H film in a glow discharge reactor is investigated by using real time in-situ infrared reflection absorption spectroscopy (IRRAS). The temperature dependence of the dominant bonding configuration for the surface hydrides is determined in the vibrational spectra of stretching modes. The integrated absorption intensities at temperatures below 380° C remain almost constant and correspond to the monolayer coverage of the growing surface by hydrogen.

Real‐time detection of higher hydrides on the growing surface of hydrogenated amorphous silicon by infrared reflection absorption spectroscopy

Real‐time in situ observations of the growth of a‐Si:H films have been carried out in a rf glow discharge plasma reactor by use of infrared reflection absorption spectroscopy. Deuterium substitution of an interface layer is employed so as to differentiate the higher hydride species on the growing surface from those located at the film interface on the substrate. A three‐layer model is presented to give a quantitative discussion on the absorption signal intensity in the reflection spectroscopy with respect to the normal incident transmission spectroscopy.

LOW-TEMPERATURE GROWTH OF CRYSTALLINE SILICON ON A CHLORINE-TERMINATED SURFACE

We report on the role of surface termination during growth of crystalline silicon at low temperatures. Microcrystalline silicon was fabricated using plasma-enhanced chemical-vapor deposition with a hydrogen-diluted dichlorosilane (SiH2Cl2)/monosilane (SiH4) mixture to study the role of hydrogen and chlorine in crystal formation. When varying the fraction of SiH2Cl2, x=[SiH2Cl2]/([SiH4]+[SiH2Cl2]), good crystallinity was obtained for x=0 and 1, whereas the crystallinity was markedly deteriorated for intermediate x. Optical emission spectroscopy of the plasma suggests that film precursors different from SiHx fragments and atomic chlorine are generated for x≠0 and that atomic hydrogen is generated in all cases. Infrared reflection absorption spectroscopy indicates that the surface coverage is hydrogen for x=0, chlorine for x=1, and a hydrogen–chlorine mixture for intermediate x. These results imply that low-temperature growth of crystalline silicon is facilitated on a chlorine-terminated surface as well as o...

Hydrogenated amorphous silicon prepared by ArF and F2 excimer laser‐induced photochemical vapor deposition

Hydrogenated amorphous silicon (a‐Si:H) films were prepared by laser‐induced photochemical vapor deposition using ArF and F2 excimer lasers. Disilane (Si2H6) and trisilane (Si3H8) diluted with helium were used as the reaction gases for the ArF laser and monosilane (SiH4), disilane, and trisilane for the F2 laser. A good linear correlation was found between the estimate of deposition rate derived from gas phase photoabsorption and the observed deposition rate, indicating that one‐photon process governs the film deposition rate. Density of excited molecules in the gas phase was found to be an important factor in controlling the film properties. Laser irradiation onto the growing surface also affects the film properties through a decrease in the hydrogen incorporated in the film.

Photochemical vapor deposition of hydrogenated amorphous silicon films from disilane and trisilane using a low pressure mercury lamp

The photolysis of disilane (Si2H6) and trisilane (Si3H8) under direct excitation by light from a low pressure mercury lamp was carried out to prepare hydrogenated amorphous silicon films (a‐Si:H). The electronic and optical properties of the films were investigated as functions of preparation conditions such as partial pressure and substrate temperature. The conductivity of the films prepared from Si3H8 at 300 °C was 10−10 S cm−1 in the dark and 10−5 S cm−1 under the illumination of a He‐Ne laser with a photon flux of 1015 cm−2 s−1. The high photoconductivity was attained when the silane gas was blown over the substrate from a slit‐type nozzle placed beside the substrate plate at a pressure of less than 20 Torr.

Absorption spectra of SiCl4, Si2Cl6, SiF3CH3 and GeF4 in the VUV region

Abstract In the range 110–200 nm the absorption features of Si2Cl6 closely resemble those of SiCl4 and the peaks observed are tentatively assigned to the Rydberg transitions of a Cl lone-pair electron. Two diffuse bands in the SiF3CH3 absorption are also assigned to Rydberg excitations. The spectrum of GeF4 shows a broad band considered to be a valence excitation of the outermost orbital. The emission of the SiCl2 (A1B1→X1A1) transition was found in the photoexcitation of Si2Cl6.

Structural study of initial layer for μc-Si:H growth using real time in situ spectroscopic ellipsometry and infrared spectroscopy

Abstract Real time observations by spectroscopic ellipsometry (SE) and infrared attenuated total reflection spectroscopy (ATR) have been performed to characterize initial layers from which μc-Si:H nucleates in rf plasma-enhanced chemical vapor deposition. The SE analysis showed an enhanced surface smoothening of a-Si:H layers with increasing hydrogen dilution ratio, R (=[H 2 ]/[SiH 4 ]), owing to surface diffusion of precursors. No change was found for the absorbance intensities of SiH and SiH 2 bonds in the a-Si:H bulk layers with variation of R , while μc-Si:H formation reduces the incorporation of SiH bonds. In contrast, sub-surface analyses performed by Ar plasma treatment of the deposited a-Si:H layers showed larger sub-surface hydrogen contents in the a-Si:H incubation layers deposited at larger R than those in a-Si:H layers deposited without hydrogen dilution. We suggest that the a-Si:H network formation process in the sub-surface differs in a-Si:H deposition at larger R compared with the smaller R case and this causes the μc-Si:H nucleation.