New experiments on the relationship between light-induced defects and photoconductivity degradation
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New experiments on the relationship between light-induced defects and photoconductivity degradation Stephan Heck and Howard M. Branz National Renewable Energy Laboratory, Golden, CO, 80401 U.S.A. ABSTRACT We report experimental results that help settle apparent inconsistencies in earlier work on photoconductivity and light-induced defects in hydrogenated amorphous silicon (a-Si:H) and point toward a new understanding of this subject. After observing that light-induced photoconductivity degradation anneals out at much lower T than the light-induced increase in deep defect density, Han and Fritzsche[1] suggested that two kinds of defects are created during illumination of a-Si:H. In this view, one kind of defect degrades the photoconductivity and the other increases defect sub-bandgap optical absorption. However, the light-induced degradation model of Stutzmann et al.[2] assumes that photoconductivity is inversely proportional to the dangling-bond defect density. We observe two kinds of defects that are distinguished by their annealing activation energies, but because their densities remain in strict linear proportion during their creation, the two kinds of defects cannot be completely independent. In our measurements of photoconductivity and defect absorption (constant photocurrent method) during 25Û&OLJKWVRDNLQJDQGGXULQJDVHULHVRILVRFKURQDODQQHDOVEHWZHHQ 25 < T < 190°C, we find that the absorption measured with E H9ILUVWLQFUHDVHVGXULQJ annealing, then exhibits the usual absorption decrease found for deeper defects. The maximum in this absorption at E H9RFFXUVVLPXOWDQHRXVO\ZLWKDWUDQVLWLRQIURPIDVWWRVORZUHFRYHU\ of photoconductivity. The absorption for E H9VKRZVtwo distinct annealing activation energies: the signal rises with about 0.87 eV and falls with about 1.15 eV. The 0.87 eV activation energy roughly equals the activation energy for the dominant, fast, recovery of photoconductivity. The 1.15 eV activation energy roughly equals the single activation energy for annealing of the light-induced dangling bond absorption.
INTRODUCTION Several decades of measurements have revealed a great amount of information on how defects influence photoconductivity (σ) in hydrogenated amorphous silicon (a-Si:H). It is therefore surprising that we still do not yet fully understand σ in a-Si:H, as demonstrated by the fact that crucial results are still interpreted within a wide range of models[3]. The difficulties of understanding σ in a-Si:H become especially apparent in the StaeblerWronski Effect (SWE). Already in 1977, Staebler and Wronski[4] discovered the decrease ("degradation") of σ with illumination and its recovery by annealing at moderate temperatures (> 150Û& 7KLVOLJKW-induced degradation has prompted considerable experimental study, because it limits the use of a-Si:H in solar cells and other applications. After the discovery that the electron spin resonance (ESR) signal of neutral three-fold-silicon dangling-bond defects (D0) increases with illumination[5, 6], Stutzmann et a
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