Structural Equilibration in Pure and Hydrogenated Amorphous Silicon
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STRUCTURAL EQUILIBRATION IN PURE AND HYDROGENATED AMORPHOUS SILICON GERHARD MOLLER AND GERHARD KROTZ Deutsche Aerospace AG, Postfach 80 11 09, 8000 Munchen 80, Germany.
ABSTRACT Chemically pure (a-Si) and hydrogenated amorphous silicon (a-Si:H) are metastable materials which are thermodynamically unstable with respect to crystalline silicon (c-Si). In both materials, however, partial thermal equilibria can be established between certain structural, configurational and electronic degrees of freedom. The present paper discusses experiments on both amorphous (a-) materials showing that two kinds of structural change can take place within random Si networks: structural relaxation and configurational equilibration. The first process can be observed in both materials indicating that it is supported by intrinsic degrees of freedom of the random Si networks. During these changes partial thermal equilibria between distorted and broken bonds are established via irreversible and relatively long-range relaxation processes. The second kind of change can only be observed in a-Si:H, indicating that it is H-related. The H-related degrees of freedom support reversible valence alternation reactions in which the local bonding configuration of the dopant and defect sites is changed and in which their charge states are altered. These latter interactions establish a strong coupling between the electronic system and the configurational degrees of freedom of the random Si networks. Formally, these latter changes bear strong similarity to the electrochemical processes that take place in liquid electrolytes. 1. INTRODUCTION Bias-annealing experiments have convincingly shown that, at elevated temperature, a-Si:H tends to establish an overall equilibrium between the electronic system and the lattice with its dopant and defect sites 1 ,2,3,4. Sub-bandgap optical absorption 5 and drift mobility 6 measurements, on the other hand, indicate that, during these changes, the exponential distributions of the valence and conduction band tail states do not change. These observations are explained in terms of a H-glass model which assumes an essentially rigid Si matrix which, in turn, supports a sub-matrix of hydrogenated material 7 . The metastable structural changes, induced by the bias-annealing treatments, are thought to take place within this latter submatrix by means of diffusive re-arrangements of the bonded hydrogen. In addition to these reversible metastable changes, also irreversible changes in the Urbach tail width and in the density of "stable" dangling bond defects are observed when deposition conditions are changed or when post-deposition annealing treatments are performed 8. In the present paper experiments on a-Si and a-Si:H are discussed in parallel. In doing so, it is intended to separate intrinsic from H-related structural degrees of freedom and to provide an overall view of the relaxation and equilibration phenomena in device-grade a-Si:H. In the first part of the paper attention is drawn to the phenomenon of irreversible structural
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