Network Rebonding Model for Metastability in Amorphous Silicon
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NETWORK REBONDING MODEL FOR METASTABILITY IN AMORPHOUS SILICON R. BISWAS*, B.C. PAN*, ** and Y. YE*,† *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 † Center of Analysis and Testing, Wuhan University, Wuhan, People’s Republic of China ABSTRACT We propose a new network rebonding model to describe light-induced metastability in hydrogenated amorphous silicon. This model involves global changes in both the silicon and hydrogen bonding sites. The first step in the process is the breaking of a weak silicon bond generating a dangling bond and floating bond pair. The mobile floating bond diffuses away leaving behind an isolated dangling bond in agreement with ESR measurements. Tight-binding molecular dynamics simulations show clear evidence for each of these processes. Floating bonds are an intermediate transient species that do not remain in the light-soaked steady state. Floating bonds annihilate by reacting with SiH bonds and locally displacing hydrogen atoms. This model provides a new explanation for several major experimental features of the Staebler-Wronski effect, including the t1/3 kinetics for the growth of defects, and the anticorrelation of dangling bonds with H. This new model provides a new platform for understanding the atomistic origins of light-induced degradation. INTRODUCTION It has been more than two decades since the first discovery of the light-induced metastability[1] or the Staebler-Wronski effect in hydrogenated amorphous silicon (a-Si:H). Yet the study and characterization of metastability in a-Si:H remains one of the most active areas or research. Illumination of a-Si:H causes an increase of midgap electronic states associated with neutral silicon dangling bonds (D0) which saturate for long-time illumination at densities of 1016–1017 cm-3 [1,2]. There is an accompanying decrease in the photoconductivity and transport properties of the film. Electron-spin resonance studies suggest that the metastable dangling bonds are at least 40 Å away from each other and that they are >4 Å away from H atoms i.e. dangling bonds appear to be formed in H-depleted regions [3,4]. Heating the a-Si:H films at temperatures of 180-200 C anneals away the metastable defects. At the annealing temperatures H diffusion starts to be significant and many researchers have found a strong link between defect annealing and H-motion. The understanding of the atomic origins of metastability and the reduction of light-induced degradation will have a fundamental impact on thin film solar cell technology. Among the first contributions to the understanding of the metastability was the StutzmannJackson-Tsai (SJT) model [2] of the kinetics of dangling bond formation- which naturally explained the t1/3 kinetics of defect formation at room temperature. Recently Branz [5] proposed the H-collision model, where defect formation is caused by bre
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