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.