Deep Levels and Drift Mobility Measurements in Hydrogenated Amorphous Silicon
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DEEP LEVELS AND DRIFT MOBILITY MEASUREMENTS IN HYDROGENATED AMORPHOUS SILICON E. A. SCHIFF, M. A. PARKER, AND K. A. CONRAD Department of Physics, Syracuse University, Syracuse, NY
13244-1130
USA
ABSTRACT Drift-mobility measurements in undoped amorphous hydrogenated silicon (a-Si:H) are reviewed with emphasis on the effects of deep levels (principally the D or dangling bond defect) on the electron drift mobility. An outline of several techniques for measuring drift mobilities is also given to establish their relationships to the transient drift-mobility function .L(t). Three aspects of the electron p(t) in undoped a-Si:H are described in detail: (i) anisotropy at long times, requiring a distinction between axial electric fields (parallel to the growth axis) and planar fields normal to it, (ii) the D center deep-trapping cutoff observed in the axial u(t), and (iii) D center multiple-trapping at long times in the planar drift-mobility. Microstructure effects which might account for the electron drift-mobility are discussed. INTRODUCTION This article reviews the present understanding of the role of deep-levels (ie. defects with energies near the middle of the density-of-states gap) in determining the drift-mobility in undoped hydrogenated amorphous silicon (a-Si:H). There is of course no need to emphasize the importance of obtaining an understanding of carrier dynamics in this material. The topic is of fundamental scientific interest because of the poorly understood nature of electrical transport in amorphous materials, and it is of technological interest because most devices depend on transport processes. The importance of drift-mobilities, however, may not be clear. Especially in amorphous semiconductors, where traditional techniques of crystalline semiconductor characterization such as the Hall effect prove - at least to date - uninterpretable,' drift-mobilities are the simplest information available about electrical transport processes. The first test for a theory of carrier transport should thus be whether it accounts for drift-mobility measurements, or more specifically a carrier's transient drift mobility p(t) at a delay t past its generation (usually by interband photoexcitation). In a-Si:H we find fairly widespread agreement as to the success and, to a lesser extent, the parameters of a simple multiple-trapping model in describing the drift-mobility at short times, where drift-mobilities 2 appear to be dominated by the conduction and valence bandtail states. -3 At longer times, which are crucial for most devices and where deep levels are involved, there is as yet no widely accepted model for the drift mobility in a-Si:H. This fact is surprising, because the deep levels themselves are generally understood in a-Si:H using a standard model involving only a single type of defect 4 - the D-center observed in electron spin resonance - in three charge states. We shall argue here that the difficulty lies not so much with the standard model for the deep levels but instead in our understanding of transport itself. In
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