Hydrogen Distributions and Model Lineshapes of ESR Signals of Dangling Bonds in a-Si:H
- PDF / 467,766 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 48 Downloads / 167 Views
Hydrogen Distributions and Model Lineshapes of ESR Signals of Dangling Bonds in a-Si:H P. A. Fedders Department of Physics, Washington University, St. Louis, Mo 63130 ABSTRACT For many years it has been assumed widely that hydrogen is involved in some way in the formation of light induced defects. However recently some doubt has been cast on this because of experimental evidence that there is little H near light induced dangling bonds. In this paper we present a number of model calculations comparing ESR spectra of dangling bonds with and without correlations with H positions. The above models include different distributions of H and correlations or anti correlations of nearby H. In all cases the spectra are compared to those with no correlations or clustering. Our results coupled with published experimental data strongly suggest that dangling bonds are not correlated positively with the presence of nearby H and, in fact, it appears that light induced dangling bonds are negatively correlated with nearby H.
INTRODUCTION One of the most intellectually stimulating, technologically important, and vexing questions in the study of a-Si:H is the mechanism of light induced defects, viz. the Staebler-Wronski effect. Over the years most (but not all) models of the effect invoke mobile hydrogen which necessitates the presence of hydrogen near the induced dangling bonds [1]. To our knowledge, there is no direct experimental evidence for this nearby hydrogen and, in fact, several ESR experiments have failed to detect a positive correlation between dangling bonds (original or light induced) with nearby H. Some of these experiments are pulsed spin echo experiments [2,3] and some are continuous wave (CW) [4,5]. The pulsed spin echo experiments suggest that new or light induced dangling bonds are anti-correlated with nearby hydrogen. However, to our knowledge, there have been no detailed calculations on this subject. In this paper we introduce a number of calculable models for the correlations of dangling bonds with nearby H. Although the results are explicitly calculated for CW experiments, the methods can easily be applied to pulsed experiments. Our calculations include spatial correlations (positive or negative) of dangling bonds with nearby H. We also consider the effects of H within the ‘‘radius" of the dangling bond. Because of broadening due to a distribution of gfactors, it is difficult to make a direct comparison between theory and experiment. However, one can make very accurate theoretical and experimental determinations of the ESR lineshapes before and after the formation light induced defects. Thus most of our results are presented as figures comparing the results with and without correlations which might correspond to before and after light soaking. As far as we know, there are three significant contributions to the ESR linewidth of a singly charged dangling bond. First, there is broadening due to a distribution of g-factors that obtains simply because various dangling bonds have slightly different bond angles and othe
Data Loading...
