Hydrogen in Crystalline and Amorphous Silicon

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HYDROGEN IN CRYSTALLINE AND AMORPHOUS SILICON Guido L. Chiarotti(a), F. Buda(a), R. Car(a), M. Parrinello(ab) (a) - InternationalSchool for Advanced Studies, Trieste, Italy (b) - IBM Research Division, Zurich Research Laboratory,Riishlikon, Switzerland Abstract We investigate static and dynamic properties of hydrogen in crystalline and amorphous silicon by means of ab initio molecular dynamics simulations. In the crystalline case we focus mainly on the diffusion process of an isolated positively charged hydrogen impurity at high temperature. In the amorphous case we analyze the local order and the dynamical properties corresponding to an atomic hydrogen concentration of - 11%, typical of a device quality material. In both crystalline and amorphous cases, our results are in good agreement with available experimental data and give unique insight into the microscopic details of hydrogen incorporation in silicon.

INTRODUCTION Hydrogen (H) can be incorporated in crystalline silicon (c-Si) up to equilibrium atomic concentrations of - 10-'. Much larger concentrations are possible in amorphous silicon (a-Si) due to the presence of a disordered network. In this case the typical H concentration in the so called device quality materials is in the range - 2 + 15%. Despite the large difference in concentrations, H plays an equally important role in both c- and a-Si, due to its ability to passivate impurity-related states in c-Si [1], and to eliminate gap-states, thus making doping practical, in a-Si [2]. It is therefore no surprise that H incorporation in Si has been the object of extensive investigations in the last decade. In spite of that, our understanding of the phenomena associated to the presence of H in Si is far from being complete, particularly at the microscopic level. Some of the crucial questions that one would like to answer are the following: (i) what are the equilibrium locations of H, (ii) how do they correlate with the modifications induced by H in optical and vibrational spectra, and (iii) how can H diffusion be characterized microscopically. We have investigated the above questions with ab initio molecular dynamics (MD) simulations (3]. In this method one computes numerically the atomic trajectories resulting from interatomic forces derived directly from the instantaneous electronic ground-state, which is treated with accurate density-functional techniques. The scheme is parameter-free and is particularly suited to describe systems, like covalent semiconductors, where chemical bonds may break and form as a consequence of the atomic motion. As usual in MD simulations, trajectories appropriate to different temperatures can be generated by changing the initial conditions for particle motion. In this way one can simulate thermal treatments, such as heating and cooling, or one can investigate processes occurring at finite temperature, such as atomic diffusion. In this paper we report on recent progress made in simulations of H in both c- and a-Si. In the case of c-Si we have studied an isolated positively charge