Determination of Drift, Extended State Mobility and Recombination Lifetime in Compensated a-Si:H by Photomixing
- PDF / 375,926 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 35 Downloads / 176 Views
DETERMINATION OF DRIFT, EXTENDED STATE MOBILITY AND RECOMBINATION LIFETIME IN COMPENSATED a-Si:H BY PHOTOMIXING YI TANG*, R. BRAUNSTEIN*, BOLKO VON ROEDERN**, F. R. SHAPIRO*** *Department Physics, University of California, Los Angeles, CA 90024 "**NationalRenewable Energy Laboratory, 1617 Cole Boulevard, Golden, CO 80401-3393 ***Department of Electrical and Computer Engineering, Drexel University, Phila., PA 19104
ABSTRACT Mobilities of a series of compensated a-Si:H samples, measured earlier by the time-of-flight technique [1], were determined by the technique of photomixing. We have found that both the extended state and the drift mobilities decrease as the compensation increases. Modelling these transport processes in the context of the photomixing technique, it is shown that long-range potential fluctuations can account for the decrease in the extended state mobility in compensated samples. INTRODUCTION Charge transport in amorphous materials is determined not only by scattering mechanisms of charge carriers (extended state transport), as is in crystalline materials, but also by trapping and thermal emission of charge carriers into and out of the band tail states. The Time-Of-Flight (TOF) technique only measures drift mobilities which are convolutions of the extended state transport and the trapping effects. Similarly, steady state photoconductivity measurements, in both amorphous and crystalline materials, merely yield products of recombination lifetime and extended state mobility. It is of great importance to separate extended state transport from trapping and recombination kinetics in order to study the transport dynamics, which is very important for the understanding of, e.g., the long-range potential fluctuations [2-6] and the Staebler-Wronski effect [7], that are not yet well understood. We have developed a photomixing technique to measure simultaneously drift mobility and lifetime. The extended state mobility, the characteristic energy and the capture rate for the band tail, and the recombination lifetime can be determined through data analysis. Evidence for the long-range potential fluctuations in compensated a-Si:H is provided through the decrease of the extended state mobility with increasing compensation. THEORY OF PHOTOMIXING The photomixing technique [8, 91 has previously been applied to determine the transport characteristics of crystalline compensated silicon [8], intrinsic a-Si:H with different preparations [91 and a-As 2Te3 [10]. The photomixing and the frequency dependent amplitude photomodulation techniques [ 11, 12] are related, however the high frequencies attainable by the photomixing allow determinations of drift and extended state mobilities and shallow gap states. Charge transport in amorphous materials is mitigated by trapping and thermal emission of charge carriers into and out of localized states below the band edges as well as recombinations. In intrinsic and compensated a-Si:H samples the charge transport is dominated by electrons. The rate equation for electrons in the conduction
Data Loading...
