Kazuhiro Shirakawa

Photoelectric Properties of Oxygen-Doped a-Si:H Prepared by rf Sputtering

It was observed that the incorporation of a small amount of oxygen into sputtered a-Si:H films considerably enhances the photoconductivity at room temperature in comparison with that in undoped films. The mechanism of the enhancement was investigated in terms of the dangling-bond density, electron mobility and Fermi-level location. The electron lifetime was found to be roughly inversely proportional to the Si dangling-bond density. Time-of-flight experiments revealed that the electron mobility at room temperature in slightly oxygen-doped films is higher by about one order of magnitude than that in undoped films. The greatest part of the photoconductivity enhancement could be understood on the basis of an increase in the electron mobility and a reduction in the dangling-bond density.

Hole Transport in Glow-Discharged a-Si:H Films Compensated with Boron

In lightly boron-doped a-Si: H films electrons become highly dispersive in time-of-flight measurement while holes clearly show a non-dispersive transport, in drastic contrast with observations in undoped or lightly phosphorus-doped films. Mechanisms responsible for the drastic change in the electronic transport characteristic between undoped or phosphorus-doped slightly n-type and lightly boron-doped films are briefly discussed.

Dangling Bond Creation in Hydrogenated Amorphous Silicon by Light-Soaking

It has been observed in glow-discharged undoped amorphous hydrogenated silicon (a–Si:H) films that the Fermi level position moves downwards and is pinned near the midgap by a low dose irradiation with gamma-rays. Films irradiated lightly with gamma-rays offer an opportunity for investigating Staebler-Wronski effect without any ambiguity due to Fermi level shift. Under the condition of the pinned Fermi level, we have confirmed a decrease in the electron lifetime and in parallel an increase in the ESR spin density relating to neutral Si dangling bonds after light-soaking.

Electron transport and photoelectric properties in glow discharge a-Si:H films prepared from disilane

Abstract The electron transport and photoelectric properties in a-Si:H films prepared from disilane by an rf glow-discharge decomposition process have been measured as a function of substrate temperature and have been compared with those prepared from monosilane. At substrate temperatures below 200°C, the electron transport properties of disilane films are inferior to those monosilane films because of the preferential formation of highly disordered polysilane structures in disilane films. Substrate temperatures higher than 200°C yield disilane and monosilane films nearly the same electron mobility and photoconductivity at room temperature. The temperature dependence of the electron mobility reveals a significant difference in the tail state distribution below the conduction band between disilane and monosilane films. This suggests that local atomic structures of a-Si: H films depend critically on source gas species, even if the room-temperature properties are apparently the same.

Comparative study on electron transport in glow discharge a-Si:H films prepared from mono- and disilane gases

Abstract Electron transport of glow discharge a -Si:H films prepared from disilane has been studied by means of time-of-flight measurement and compared with those of monosilane films. The room-temperature electron mobility in disilane films deposited at substrate temperatures above 190°C is comparable to that for monosilane films. However, the temperature dependence of the electron mobility reveals a significant difference in the tail state distribution between these films. This suggests that atomistic local structures of a -Si:H films depend critically on source gas species. The difference in the local structures of these films is also demonstrated by the substrate temperature dependence of the electron mobility; the room-temperature electron mobility for disilane films decreases more rapidly with decreasing substrate temperature below 180°C than the case for monosilane films.

Time-of-flight measurement of hole transport in lightly boron-doped a-Si:H films

Effects of slight boron-doping on the mobility and the mobility-lifetime ( μτ ) product of holes in glow-discharged hydrogenated amorphous silicon films have been investigated by means of time-of-flight measurement. When a small amount of boron in the range of the flow rate ratio B 2 H 6 /SiH 4 = 17–50 ppm is doped, hole transport at room temperature becomes non-dispersive, in contrast with a dispersive nature in undoped films. The hole mobility and the μτ product at room temperature have large values at these flow rate ratios. However, when the flow rate ratio B 2 H 6 /SiH 4 is further increased, holes show again dispersive transport accompanying a reduction in mobility and μτ product. The activation energy of the hole mobility is nearly 0.4 eV, independent of boron-doping level. These results are tentatively interpreted in terms of an introduction of localized states associated with boron or on the basis of a two-phase model in which a-Si:H films are considered to be composed of an entangled network of paramicrocrystalline zones and highly disordered Si:H alloy regions.

Hole transport in boron-compensated a-Si:H films

Abstract It is confirmed that in boron-compensated glow-discharged a-Si:H films electrons are highly dispersive while holes show non-dispersive nature in clear contrast with those observed in undoped or lightly phosphorus-doped films. These peculiar features of boron-compensated films are discussed on the basis of two-phase (quasimicrocrystalline and amorphous regions) structure.

Photoelectric characterization of interface between a-Si:H and a-Sinx:H

Abstract Interface properties between glow-discharge a-Si:H and a-Si:H have been studied by photocurrent spectroscopy in parallel to the heterointerface. In samples with neatly flat band condition, it is found that geminate recombination is necessary to be taken into account in order to get a good agreement between theory and experiment. Interface recombination velocity is estimated as a function of optical gap of a-SiN x :H layer. However, the concept of recombination velocity becomes apparent when band bending exists.