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NATURE OF CHARGED METASTABLE DEFECTS IN NETWORK REBONDING MODEL R. BISWAS* AND B. C. PAN*§ *Department of Physics and Astronomy, Microelectronics Research Center and Ames Laboratory-USDOE, Iowa State University, Ames, Iowa 50011 §Department of Physics, University of Science and Technology of China, Hefei 230026, People’s Republic of China ABSTRACT We recently developed an atomistic model of metastability of a-Si:H, where defect creation is driven by the breaking of weak silicon bonds. The kinetics of degradation in this model is simulated with coupled rate equations that show t1/3 kinetics of defect creation and saturation behavior similar to experiment. Saturated defect densities of neutral dangling bonds are accompanied by a much smaller density of negatively charged floating bonds and positively charged dangling bonds (D+). We propose a two-step annealing mechanism where the positively charged D+ dangling bonds are annealed at low temperature and the D0 at higher temperature – which accounts for hysteresis in mobility lifetime products.

INTRODUCTION The study of metastability (Staebler-Wronski effect[1]) and the nature of light-induced defects is one of the most actively pursued areas in amorphous semiconductor research. Light-soaking of hydrogenated amorphous silicon produces metastable dangling bonds with mid-gap states and properties indistinguishable from native dangling bonds. The defects can be annealed at temperatures between 180-200 C. [1-2] The primary electron-spin resonance (ESR) signature is from the neutral dangling bonds (D0). A number of studies have indicated the presence of more than one species of metastable defects. These studies have concentrated on measurements of mobility-lifetime (µτ) products of the electrons which dominate the photoconductivity. Measurements of degradation kinetics and the lack of correlation between µτ and of optical absorption (α) at different light-intensities and temperatures by Wronski and coworkers [3,4] have suggested the presence of charged (non-D0) defects. Annealing studies of µτ products and α find two annealing regimes[3,5-7]. At low temperatures (T130 C) the annealing causes only a small decrease of (µτ)-1 but a large decrease in α, indicating the presence of at least two types of defects in the light-soaked state. The nonD0 defects have a large effect on (µτ) products[4-7]. In this paper we propose a model for this behavior utilizing our recent network-rebonding model of metastability[8]. We describe the kinetics and steady-state defect densities- and compare our microscopic model to experiment. THEORETICAL MODEL We have recently developed a very promising model for the atomistic mechanisms underlying metastability in amorphous hydrogen silicon (a-Si:H), that provides a new platform for A11.3.1 Downloaded from https://www.cambridge.org/core. University of Arizona, on 27 Jul 2018 at 06:54:59, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1557/PROC-715-A11.3

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