Temperature Dependence of the Decay of Optically Excited Charge Carriers in Amorphous Silicon
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TEMPERATURE DEPENDENCE OF THE DECAY OF OPTICALLY EXCITED CHARGE CARRIERS IN AMORPHOUS SILICON J. Whitaker and P. C. Taylor Department of Physics, University of Utah, 115 South 1400 East Room 201, Salt Lake City UT 84112-0830 ABSTRACT We report the temperature dependence of the growth and decay of the optically induced electron spin resonance (LESR) on short and long time scales (10-3 s < t < 2500 s). This range of times spans the region between previously published photoluminescence and the LESR data. In addition, we examine the steady-state density of optically excited charge carriers as a function of temperature. These measurements lead to a better understanding of the band tail structure of amorphous silicon as well as the kinetics of the excitation and recombination processes. INTRODUCTION Modeling of the eventual recombination of optically excited electrons and holes in amorphous silicon at low temperatures (T < 40 K) involves two distinct processes, the hopping down in energy via tunneling of each individual carrier (diffusion) and recombination of electrons and holes via tunneling [1]. At short times the hopping down process dominates, and at long times the recombination process dominates. On intermediate time scales (10-3 < t < 1 s) and at low temperatures the two processes compete: tunneling by a single carrier to a lower energy state, and recombination of an electron and a hole via tunneling. At finite temperature where excitations that increase the energy are possible, a third mechanism, namely variable range hopping of the charge carriers, must be considered. The optically induced electron spin resonance (LESR) signal of amorphous silicon at low temperatures has been well studied for time scales greater than about 1 s [2,3]. The photoluminescence (PL) signal, which is related to the LESR, in amorphous silicon has also been well studied for time scales less than about 1 ms [4]. In this paper we report measurements that extend the LESR results down to time scales approaching those probed in the PL experiments. We also examine the temperature dependence of the decay of optically excited carriers over a wide range of times from approximately 10-3 s to 103 s. In particular, using LESR we examine the recombination kinetics of both short-lived and long-lived optically excited carriers in aSi:H at finite temperatures using LESR. The discovery in 1989 by Shklovskii et al. [1] that the simultaneous diffusion and recombination of electron-hole pairs in amorphous semiconductors is a universal property that does not depend on the densities of localized band-tail states has prompted renewed interest in the low temperature recombination processes in hydrogenated amorphous silicon (a-Si:H). At short times and low temperatures, hopping of the carriers downward in energy plays a major role in recombination, and at long times carriers become effectively trapped and recombination via tunneling is the only important process. At
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