A Comparative Study of Defect States in Light-Soaked and High-Temperature-Annealed a-Si:H
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A COMPARATIVE STUDY OF DEFECT STATES IN LIGHT-SOAKED AND HIGH-TEMPERATURE-ANNEALED a-Si:H Z. M. SALEH,* H. TARUI, S. TSUDA, S. NAKANO AND Y. KUWANO Functional Materials Research Center, SANYO Electric Co., Ltd. 1-18-13 Hashiridani, Hirakata, Osaka 573 (Japan). *Present Address: Department of Physics, University of Utah, Salt Lake City, UT 84112 (USA) ABSTRACT Our previous results of light-induced electron spin resonance (LESR) indicate that, in hydrogenated amorphous silicon (a-Si:H), light-induced defects differ from those formed during deposition or high-temperature annealing. A plausible interpretation, in which light-induced defects occupy higher-energy states, was proposed to explain these differences. In this study, the constant photocurrent method (CPM), dark conductivity and steady-state (SS) LESR are used to supply new evidence for the difference and conduct two important tests on our hypothesis. In striking agreement with our predictions, we find that the light-induced changes in the SS-LESR lineshape (a decrease in the narrow component relative to the broad one upon light exposure) become indeed more dramatic as the demarcation energies move closer to the midgap by increasing temperature or decreasing bias-light intensity for SS-LESR.
INTRODUCTION An important feature of hydrogenated amorphous silicon (a-Si:H), and amorphous materials in general, is a continuous distribution of states in the forbidden gap. The absence of long-range order in these materials leads to band-tailing (tail states), while defects such as dangling bonds and impurities lead to states localized deeper in the gap (defect states). The most commonlyaccepted defect states arise from silicon dangling bonds (DO). Defect properties are particularly important when a-Si:H is used for photovoltaic applications such as solar cells. It has been widely assumed that light-exposure increases the density of DO which degenerate the optoelectronic properties by acting as recombination centers [1,2]. Recently, however, there has been mounting evidence that DO states alone cannot explain all observations and that additional effects (or defects) may be involved [3-51. Several studies are beginning to identify important differences between light-induced and as-grown defects [3-5]. Constant photocurrent method (CPM) is a useful probe for defect states in the gap. CPM spectra are characterized by a tail-to-tail absorption region (Urbach tail) and a defect-absorption region (excitations of electrons from defect states to the conduction band). The change in illumination intensity needed to maintain a constant photocurrent as the photon energy is varied, is taken to be proportional to the absorption coefficient. The excitations of electrons from the valence band to the defect states are usually ignored in CPM analysis because their contribution to the photocurrent is negligible in a-Si:H. We believe this to be misleading because this process may change the absorption coefficient significantly without affecting our only indicator, the photocurrent. The comb
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