Introduction

The presence of metastable defects in device materials can have a variety of effects, some of them slight and some very important; but because the emphasis in this book is material phenomena, device effects are only summarized here. There are cases in which optically (or electronically) induced changes in defects degrade device performance, and some other cases in which similar processes speed the annealing of existing degradation. Either way, the inherent properties of metastability make its effects more prominent at lower temperatures, with the metastable-barrier energy determining the maximum temperature at which significant effects are observed. Since the energy gap of the host material limits the size of this barrier, Si devices operating at room temperature (Eg= 1.1 eV) are not much affected; GaAs (Eg = 1.4 eV) devices are affected, generally at lower temperatures; and compounds with larger gaps show the greatest effects. By far the greatest device impact is in photovoltaic cells of a-Si:H, whose degradation by bright light has been discussed at length in Chapters 4 and 5 and is the motivation for a great deal of the research reviewed there. There are effects in devices using crystalline III–V materials, but their impacts are less pervasive, and in other devices of a-Si:H they are not critical. For stable deep-level defects, the most prominent effect is as lifetime “killers” in minority-carrier devices, and indeed that is also the dominant effect of metastable defects in a-Si:H. In majority-carrier devices or semiconductor lasers other effects are more important.