Changes in hydrogenated amorphous silicon upon extensive light-soaking at elevated temperature
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Changes in hydrogenated amorphous silicon upon extensive light-soaking at elevated temperature N. Hata 1, C. M. Fortmann 2, and A. Matsuda 1 1 National Institute of Advanced Industrial Science and Technology, AIST Central 2, Tsukuba, Ibaraki 305-8568, Japan 2 Dept. Applied Mathematics, State University of New York at Stony Brook, Stony Brook, NY 11794-3600 USA ABSTRACT Previously a through the substrate ellipsomtery technique was used to study the high temperature dynamics of light induced reversible changes in amorphous silicon thin films [1]. Since this technique was based on above gap optical changes it is sensitive to the structural aspects of the light induced effects, differently from the below-gap absorption techniques which detect dangling-bond defect states [2-3]. It was found that high intensity light soaking at an elevated temperature causes surprising large, reversible, changes [1]. By comparing these optical changes with the changes in dangling bond concentrations probed by electronic and below gap methods, a fuller picture of temperature dependent light-induced defect creation and annealing dynamics emerges. A high temperature high intensity light soaking method is developed which reduces saturation times, decreases the saturated dangling bond density, as well as decreases the annealing activation energy. These results are discussed in terms of the coupling between network disorder and its relaxation with respect to defect concentrations at high temperature. INTRODUCTION Since the discovery of light-induced degradation of optoelectronic properties in hydrogenated amorphous silicon (a-Si:H) over twenty years ago [4] numerous studies have been undertaken. These studies have resulted in significantly improved a-Si:H solar cells and other electronic devices because better device designs have emerged and because better materials have been identified. For example, improved-stability material identification has been facilitated by basic studies relating the mobility lifetime products (µτ), subgap absorption (αsubgap), as well as device performance to deposition parameters such as temperature, source gas material, growth A12.6.1
rate, etc. Also, post-deposition techniques such as annealing at higher temperature than that used for deposition have been tried. Other investigations have focused on the electronic spin of the dangling bond as a function of deposition condition, measurement temperature, and light intensity.
These studies have produced much knowledge about the dangling bond relationships
to deposition conditions, measurement condition, history of light exposure and thermal annealing. Relatively little is known about the structural changes that may accompany dangling bond production even though the lattice terms may play a significant role in dangling bond production [5]. Light-induced structural or network disorder change has been investigated extensively [6-7]. These techniques suffer drawbacks including difficult analysis, sensitivity to atmosphere (even at typical vacuum pressures of ~ 10-8 Tor
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