Test of the thermal equilibration model for persistent photoconductivity in doping-modulated amorphous silicon.
The persistent photoconductivity effect in doping modulated amorphous silicon is compared to the light-induced conductivity changes in compensated amorphous silicon. The recent proposal that the defect structure of doped amorphous silicon is in metastable thermal equilibrium is able to account for the metastable excess conductivity in both systems, and for the several orders of magnitude difference in exposure times needed.
Photoinduced metastable surface effects in boron‐doped hydrogenated amorphous silicon films
We observed metastable light‐induced conductance changes in p‐type hydrogenated amorphous silicon films which are different from those discovered by Staebler and Wronski. In contrast to the large metastable decrease in the dark conductance after prolonged exposure to band‐gap light (Staebler–Wronski effect) we observed an increase in the dark conductance by more than a factor of 30 at room temperature. This light‐induced increase can be reversed by annealing. The anomalous conductance increase is associated with the presence of a surface oxide layer because it disappears after etching the sample. We find that the enhanced conductance is due to an increase of a hole accumulation layer in the sample caused by a light‐induced metastable change in the surface oxide.
Characterized of glow-discharge deposited a-Si:H
Abstract The study of hydrogenated amorphous silicon, a-Si:H has become an active and large subfield of growing interest in noncrystalline semiconductors. This paper reviews recent successes as well as difficulties encountered in interpreting some fundamental physical properties of a-Si:H prepared by glow-discharge deposition. This material appears to contain structural and compositional heterogeneities which depend on the preparation conditions and affect its mechanical and electronic properties. These are related to the concentration and bonding configurations of hydrogen. Determinations of the density and distribution of localized gap states by means of field effect and capacitance measurements are discussed as well as evidence for and against the presence of surface states. Optical absorption near the absorption edge is described as are conductivity measurements. The latter reveal many interesting puzzles. Staebler and Wronski discovered that the dark conductivity and the photoconductivity can be changed significantly by strong exposure to light. Moreover, space charge layers adjacent to the substrate interface and near the free surface can strongly influence the electronic properties of a-Si:H films, especially those having low densities of gap states. The pre-exponential factor of the conductivity increases with its activation energy in accordance with the Meyer-Neldel rule regardless of whether the changes in dark conductivity are produced by doping, light exposure or space charge layers. Attempts are made to identify causes for the scatter of data on some film properties reported by various laboratories. Our present understanding of the recombination processes involved in photoluminescence and photoconductivity is presented. We conclude that a-Si:H films of superior quality can be expected when the preparation conditions become more fully understood.