Shozo Imao

Saturation of optical degradation in a-Si:H films with different morphologies

The saturation effect of the optical degradation in hydrogenated amorphous silicon films (a-Si:H) deposited at various temperatures Ts has been studied by means of photoconductivity measurements. It was confirmed that the saturation is mainly related to the simultaneous annealing at a higher light-soaking temperature T1 (?20?C) as well as the depletion of the weak Si-Si bonds at lower T1 (?0?C). The weak bond density (1016?1018/cm3) estimated from the low-temperature saturation effect decreases with increasing Ts corresponding to the growth of Si clusters.

AC Conductivity of Undoped a-Si:H and µc-Si:H in Connection with Morphology and Optical Degradation

Ac conductivity of a-Si:H and µc-Si:H films is studied experimentally with theoretical consideration. By separating ac conductivity into band, variable range hopping (VRH), band tail multiple hopping and intimate pair hopping conductivities, the effects of the morphology and optical degradation in each type of the conductivities were investigated to elucidate the nature of electronic conduction. Experimental facts on the pair-hopping conductivity seem to be explained in terms of a two-electron correlated barrier hopping (CBH) model between oppositely charged dangling bond pairs located at low-density Si–H alloy regions (voids). Long-range transport (band, multiple hopping, VRH) seems to be related to the percolation through Si clusters and microcrystalline grains, being sensitive to the morphology and Fermi level position.

Preparation of a-Si:H Films Resistive to the Staebler-Wronski Effect

Undoped a-Si:H films with much reduced sensitivity to the optical degradation (Staebler-Wronski effect) were prepared by the combination of conventional plasma CVD starting from pure silane and subsequent lamp annealing at 450~500°C for 10~15 min in nitrogen. It was observed that both the photoconductivity and the ratio of photoconductivity to dark conductivity was not appreciably degraded by the lamp annealing, indicating the absence of the unintentional doping which often appears in a-Si:H prepared by conventional plasma CVD with high substrate temperature and in µc-Si:H from highly diluted SiH4 with H2.

High-Field Electron Transport and Hot Electron Phenomena in Hydrogenated Amorphous Silicon Films

Electron transport under high electric field in hydrogenated amorphous silicon (a-Si:H) films has been investigated by means of time-of-flight (TOF) measurements. The drift mobility µ increased with increasing applied electric field E accompanied by a simultaneous increase in the dispersion parameter α resulting in nondispersive transport. The electron thermal runaway breakdown field E* with optical carrier injection decreased with increasing ambient temperature T a in accordance with the hot electron theory in amorphous solids [H. Frohlich: Proc. R. Soc. London A 118 (1947) 521]. The field dependencies of µ and α are also explained by taking account of the increase in the effective electron temperature (hot electron phenomenon) at high field in the theory of dispersive transport.

High-Field Electron Transport in a-Si:H

High-electric-field electron transport in a-Si:H films has been investigated by means of time-of-flight (TOF) measurements. The drift mobility increased with increasing applied field at lower temperature (≤300 K), accompanied by a simultaneous increase in the dispersion parameter resulting in nondispersive transport. The mobility was much less field-dependent at higher temperature where the nondispersive transport was attained. The experimental results are consistently explained by taking account of the increase in electron temperature (hot electron) at higher field into the theory of dispersive transport.

Effect of Phosphorus Doping and Deposition Temperature on the Deep-Level Transient Spectra in a-Si:H

Both voltage- and photoinjected deep-level transient spectroscopy measurements were performed on phosphorus-doped a-Si:H films to clarify the nature of a hole trap state located at Ev+0.6 eV, which was extinguished by light soaking. The density of trapped holes at the state in as-deposited films was weakly dependent on the phosphorus doping concentration. However, it decreased with increasing deposition temperature of the films. These results suggest that the hole trap state is related to weak silicon bonds which act as a precursor to optical degradation.

Optical Degradation of a-Si:H films with Different Morphology

The optical degradation (Staebler-Wronski (SW) effect) of a-Si:H films prepared by plasma CVD has been studied in connection with the morphological inhomogeneity of the films. The relative degradation rate of cubic photoconductivity α widely varied with deposition substrate temperature Ts and had its maximum at Ts=200~250°C. In the films of both the lower and higher Ts, α decreased due to the increase of stable dangling bonds in the former and the decrease of weak bonds in the latter. The SW effect was hardly observed in µc-Si:H deposited from highly diluted silane with hydrogen.

Staebler-wronski effect in a-Si:H films with different morphology

Abstract Optical degradation (Staebler-Wronski effect) of a-Si:H and annealing of optically induced defects has been studied in connection with the morphological inhomogeneity of the films. The relative degradation rate α widely varied with deposition substrate temperature Ts and has its maximum at Ts=200–250°C. In the films of both the lower and higher Ts, α decreased due to the increase of stable dangling bonds in the former and the decrease of weak bonds in the latter. Thermal and optical annealing of degradation were also studied. It was found from AC conductivity measurements that band conductivity was strongly affected by the degradation but that hopping conductivity was not. A degradation-resistant film was obtained by lamp annealing and subsequent exposure to a hydrogen plasma.

Internal Stress and Optical Degradation in a-Si:H Deposited on Glass Substrates

Internal stress in a-Si:H films prepared by plasma chemical vapor deposition was studied in connection with optical degradation or Staebler-Wronski effect (SWE). It was found that the stress increased with increasing discharge power density (0.03 ∼ 1.5 W/cm 2 ) of silane, accompanied by increase in the introduction rate of SWE. These results imply that highly decomposed radical species due to high discharge power density dominate the surface reaction. Consequently, larger stress and larger introduction rate of SWE are brought about in the films. The stress also slightly increased at lower power density ( 2 ), suggesting that neutral silane molecules take part in the reaction at these power density.

Deep Level Transient Spectroscopy Study of Staebler-Wronski Effect in a-Si:H

Both voltage- and photoinjected DLTS study of the Staebler-Wronski effect in a-Si:H using isothermal high-speed capacitance-time (C-t) measurements (10-3~10 s) were done to elucidate the processes of defect creation and annealing. It was found that the densities of hole traps at Ev+0.6 eV and electron traps at Ec-0.4 eV were decreased by light soaking, accompanied by a simultaneous increase of electron traps at Ec-0.9 eV and hole traps at Ev+0.8 eV due to the dangling bonds. This fact suggests that the shallower traps may relate to the weak bonds, which convert to the metastable dangling bonds by light soaking.