Transient Infrared Stimulated Photoconductivity in a-SI:H at Low Temperatures
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ABSTRACT Dual beam photoconductivity with bandgap primary light and hv = 0.4- 0.6eV infrared light steps was measured with Ims time resolution in hydrogenated amorphous silicon (a-Si:H) at 4.2K. The results can be described by assuming that the photocurrent transients are due to energy-loss hopping of photocarriers and that the infrared light promotes recombination by reexciting photocarriers thereby enhancing the probability of tunneling recombination.
INTRODUCTION Infrared light-induced transients of photoluminescence (PL) and photoconductivity (PC) provide detailed information about the recombination and relaxation of photocarriers in semiconductors. At normal temperatures these processes are complicated because of thermal reexcitations. The situation is simpler at low temperatures where only energy-loss processes need be considered [1]. Nonetheless, several unexpected transient phenomena have been reported which indicate that we fail to understand crucial elements of the underlying physics, for example, PC transients in a-Si:H were found to occur more than lOims later than the PL transients at low temperatures [2]. Understanding the carrier recombination and relaxation processes is important because they determine the kinetics of light-induced defect creation in a-Si:H and related alloys which is one of the major problems in these materials. Low temperature transients in a-Si:H have been studied by Street et al. [3], Boulitrop [4], Vollmar et al. [2] and Tran et al. [5]. It was found that the photocarrier concentration n in the tail states accumulate slowly up to a steady state value while PL appears essentially without delay. After the light is turned off carriers decay very slowly over periods of hours but they recombine within seconds giving rise to a transient photocurrent when reexcited by infrared light. This paper studies the PC transients in dual beam experiments with improved time resolution. With a simple model of energy-loss hopping we obtain values for the residual photocarrier concentrations and recombination volumes from these experiments.
EXPERIMENTAL DETAILS About [gm thick a-Si:H samples grown in our standard PECVD reactor at 230'C and containing about N=4xl01 5 cm- 3 defects were cooled to Helium temperatures. They were contacted by predeposited interdigited Mo electrodes of 5cm total length and 0.01cm spacing. Bandgap light, hv = 2eV, from light-emitting diodes cooled to 78K provided carrier generation rates G of different pulse length and intensity. A specially designed spring-loaded mirror allowed infrared (IR) illumination from a Xe-arc lamp filtered by crystalline Ge to be turned on with a rise time of about 0.5ms. A preamplifier with similar rise time located on top of the cryostat was connected to the sample with a triaxial cable. The PC transient currents were between 10-13 and 10-1 0 A. After amplification they were digitized, noise averaged on a logarithmic time scale and smoothened by a Fourier transform procedure. This method leave's oscillations on the data traces corresponding
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