General Explanation of Bias-Anneal Effects in a-Si:H
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GENERAL EXPLANATION OF BIAS-ANNEAL EFFECTS IN a-Si:H DAVID REDFIELD Department of Materials Science and Engineering Stanford University - Stanford, CA 94305-2205 ABSTRACT A new explanationis offeredfor the metastable changes caused by combined electric field and elevated temperature inpropertiesof a-Si:H such as doping efficiency, solar-cell efficiency, and "structuralrelaxation." This explanation is basedon propertiesof the localized centers thatare associatedwith the light-inducedStaebler-Wronskieffect and solar-cell degradation. The newfeature is recognition that the metastablestate of these localized centers is more polarizablethan the groundstate, so that afield increasesthe energy difference between the two states. Then, when the temperature is high enough to permit transitions,the density of metastable defects decreases,producing the observed effects. This explanation is general in the sense thatit does not depend on any atomic model of the centers involved.
The simultaneous application to a-Si:H of a high electric field (-105 V/cm) and elevated temperature (=200' C), followed by cooling with the field applied, has been found to produce several significant, but metastable, changes in the material properties: (a) The doping efficiency (i.e., the fraction of electrically active dopant atoms) increases [1]. (b) The efficiency of p-i-n solar cells made of a-Si:H increases [2]. Since the properties of these solar cells are dominated by the i-layer, this is evidence that bias-anneal effects occur in undoped, as well as doped material. (c) The so-called "structural relaxation" inferred from carrier-sweep-out measurements of doped layers changes quantitatively [3]. After removal of the field, all of these effects anneal out rapidly at about 1500 C, and more slowly at lower temperatures, with the pre-treatment properties eventually being recovered. The purpose of the present paper is to offer a single new explanation of all of these effects using only general properties of localized centers that can form metastable defects (MSDs) in the material. These are the same defects that are responsible for the Staebler-Wronski effect [4] and the associated degradation of solar cells by bright light. Therefore, this explanation links all of these effects, showing that the bias-anneal effects are just manifestations of the existence of MSDs. The approach taken here is to represent the localized centers that can become MSDs by a general configuration-coordinate diagram that is independent of details of the nature of the centers. This approach has recently been successful in relating a number of observations involving the kinetics and energetics of MSDs [5]. For the present purposes, that description needs to be extended a bit further, while maintaining its essential features, that the ground state of the center (i.e., a latent defect) has no observable properties, specifically no spin, while the metastable state possesses the observable defect properties. From these features, we infer that all the covalent bonds associated with
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