Kenji Kohno

Visible Photoluminescence from Si Microcrystals Embedded in SiO2 Glass Films

We report the observation of an efficient visible photoluminescence (PL) from Si microcrystals embedded in SiO2 glass films. The Si microcrystals were formed by the rf magnetron sputtering technique. The peak position of the PL spectra at 77 K shifts to a lower-energy side as the average diameter of the Si microcrystals increases.

Preparation and Properties of Ge Microcrystals Embedded in SiO2 Glass Films

Ge microcrystals were successfully doped into SiO2 glass films by the rf magnetron sputtering technique. The dependence of the average size of Ge microcrystals on substrate temperature and annealing time was discussed. The optical absorption spectra of Ge microcrystals clearly exhibited the blue shift compared with a bulk Ge crystal, which seems to be due to a quantum size effect.

Photoluminescence of Si Microcrystals Embedded in SiO2 Glass Films

Si microcrystals were doped into SiO2 glass film by the rf magnetron sputtering technique. The average diameter of Si microcrystals was estimated by transmission electron microscopy observation. The dependence of photoluminescence of Si microcrystals on the average diameter is discussed in detail. The absorption of Si microcrystals and the photoluminescence for the larger diameter (≥ 3 nm) seem to be determined by the bulk property of a quantum dot. For the smaller diameter (≤ 3 nm), surface effects of a quantum dot seem to play a role in the photoluminescence. The experimental line shape of the PL spectrum was explained theoretically by the inhomogeneous broadening of the size distribution function of a quantum dot for the larger diameter (> 3 nm).

Preparation and Properties of Si Microcrystals Embedded in SiO2 Glass Films

Si microcrystals were successfully doped into SiO2 glass films by the rf magnetron sputtering technique. The dependence of the average size of Si microcrystals on substrate temperature was discussed with reference to transmission electron microscopy observations. The optical absorption spectra of Si microcrystals clearly exhibited the blue shift compared with a bulk Si crystal, which seems to be due to a quantum size effect.

Photoluminescence from Si Network in SiO2-Doped Si Films

We report the photoluminescence from a Si network in SiO2-doped Si films. The peak position of the photoluminescence spectrum was at the photon energy of 1.08 eV and the full width at half-maximum (FWHM) was about 0.38 eV. It was speculated that this photoluminescence is due to multiple bound states induced by SiO2 islands, not to Si microcrystals with average diameter of 50 nm.

Defect Compensation by Bonded Hydrogen in Undoped a-Ge:H Films with Mono- and Dihydride Bonding

Hydrogenated amorphous Ge(a-Ge:H) thin films are prepared by the capacitive-coupled plasma chemical vapor deposition method using GeH4 as a reactive gas. Two kinds of films, one which involves only monohydride bonding and the other, which involves mainly dihydride bonding, are deposited in different deposition conditions. Although no impurity atoms, such as P or B atoms, are doped, both n- and p-type films are formed, depending on the deposition conditions. Infrared absorption spectra, electronic properties including photo- and dark conductivities, and optical properties of a-Ge:H have been studied. The effect of mono- and dihydride bonding on defect compensation is confirmed from the measurements, and it is concluded that the monohydride-bonding hydrogen atoms terminate the gap states mainly below midgap, and that the dihydride-bonding atoms chiefly decrease the number of gap states above midgap.

Electronic properties of post-hydrogenated lightly-boron-doped CVD amorphous silicon

Lightly-boron-doped CVD amorphous Si with a doping ratio between 3.3×10-7 and 2.5×10-5 was hydrogenated by hydrogen plasma annealing, and the electronic properties including photoconductivity and photoluminescence were investigated. At a doping ratio of 4×10-6 amorphous Si with the intrinsic property is produced, of which the photoconductivity at AM1 (100 mW/cm2) is 3×10-5 Ω-1cm-1, the localized state density in the mobility gap is lowest, and the photoluminescence intensity is highest. It is suggested that some boron atoms become inactive, forming B-H bonds with penetrating hydrogen atoms at doping ratios above 4×10-6.

Dependence of electronic properties of hydrogenated amorphous Ge on deposition condition

Hydrogenated amorphous Ge(a-Ge:H) thin films are prepared by the capacitive-coupled plasma chemical vapor deposition method using GeH4 as a reactive gas. The deposition temperature and radio frequency (rf) power density are changed from 100°C to 260°C and from 0.2 W/cm2 to 2 W/cm2, respectively. The dependence of the optical and electronic properties including the infrared absorption, photo- and dark conductivities on both the deposition temperature and rf power density are examined. For a substrate temperature of 175°C, the photo- and dark conductivities exhibit maxima and the bimolecular recombination process in the photoconduction becomes dominant compared to the monomolecular recombination process. It is also found that the value of the energy gap is proportional to the hydrogen content in the region below 2 at.%. When the rf power density is increased from 0.2 to 2.0 W/cm2 under the constant substrate temperature of 160°C, the films are improved although the hydrogen content is unchanged.

Preparation of B–Si–Ge Alloys by Sputter-Assisted-Plasma CVD

Microcrystalline and amorphous films of B–Si–Ge alloy have been deposited by the sputtering of a Ge target in atmospheres of SiH4 and B2H6. A microcrystal is mainly composed of Si and Ge in spite of large amounts of boron included in the film (up to 50 at.%). Probably most of the boron is located in the grain boundary. A lattice vibration mode due to a B–B configuration was observed as a sharp Raman scattering line at 1121 ± 1 cm-1.

Physical Properties of SiO2-doped Si Films and Electroluminescence in Metal/SiO2-doped Si/p-Si Diodes

The doping of SiO2 islands in thin Si films was carried out using rf magnetron sputtering. These films are referred to SiO2-doped Si films. These films display efficient optical absorption in the optical energy region of 1.4–2.0 eV and the absorption is comparable to that of a-Si:H films. Efficient photoluminesecence from Si network in these films was observed. Also, efficient electroluminescence from metal/SiO2-doped Si/p-Si diodes was observed. It is concluded that this luminescence originates from the optical transition between electron- bound states in films and injected holes from p-Si. These optical characteristics suggest that the SiO2-doped Si films are useful for photovoltatic applications.