Alternative Mechanism for Defect Generation in a-Si:H
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ABSTRACT Recombination-driven mechanisms for defect formation do not account well for several experimental facts, and indeed conflict with some facts. To overcome these problems, an alternative mechanism is proposed in which latent defect centers are converted to metastable defects (dangling bonds) without a recombination event. The transitions are driven simply by electron capture at the center with no energy barrier. A new configuration-coordinate diagram expressing this mechanism includes essential configuration-induced changes in the energies of band edges, and accounts for several other previously problematic observations. This carrier-induced mechanism (CIM) is consistent with defect formation in forward bias and defect inhibition in reverse bias, light-enhanced annealing, as well as the usual light-induced and beam-induced formation. For low hwo the process starts by ionization of an existing defect.
INTRODUCTION Despite 18 years of extensive efforts to elucidate the metastability of a-Si:H, the current level of its understanding is quite limited. General agreement on its cause hardly goes beyond localized defects with a metastable state having a dangling Si bond capable of three possible charges. Neither the nature of these defects nor their origin is beyond controversy, and there is still not full agreement as to whether they have intrinsic or extrinsic origin. Even phenomenological descriptions of the kinetics of transitions be-
tween the ground and metastable states are not settled. Many analogies with the prop-
erties of better-understood defects in crystalline semiconductors have been explored, but with only partial successes. Furthermore, there are several experimental results that are either not well explained by the most common theories, or that conflict with them. For these reasons it seems worthwhile to reexamine our assumptions and interpretations and to consider other possible descriptions that may have some special properties so far overlooked. In that spirit, this paper proposes a mechanism for defect formation in a-Si:H that has several new features and one special attribute in order to explain observed phenomena. Some of the salient observations and their problems for existing models are summarized here. For comparison with conventional mechanisms, we first review the usual two-level configuration-coordinate description of an otherwise unidentified metastable center that is generally used to represent the dangling-bond center. Shown in Fig. 1, this has an inert ground state and a metastable state of slightly different energies that are separated by an energy barrier that stabilizes the metastable state. Annealing places most of the centers in their ground state, and excitation provides energy that causes transitions over the barrier into the metastable state with its observable defect properties; these transitions are called defect formation. Two of the key properties of this metastability that were noted early by Staebler and coworkers in a p-i-n structure of a-Si:H are (1) light-induced
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