Creation of Staebler-Wronski Defects at Low Temperatures
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CREATION OF STAEBLER-WRONSKI DEFECTS AT LOW TEMPERATURES
PAUL STRADINS AND H. FR1TZSCHE University of Chicago, The James Franck Institute, Chicago, IL, U.S.A.
ABSTRACT We extended the kinetics study of light-induced creation of metastable defects in undoped a-Si:H down to T = 4.2K. Using band gap light with photocarrier generation rate Go = 3.8x10 2 1 cm-3s- 1 we find that the change in metastable defect concentration AND depends on exposure time te as t,' with m = 0.35 ± 0.02 at 4.2K, 80K and 300K. The dependence of AND on generation rate is GO.44 at 4.2K. The relation between ND and photoconductivity at 300K is nonlinear and not single-valued.
INTRODUCTION The kinetic models proposed by Stutzmann et al.1 and Redfield et al. 2 have been quite successful in explaining many observations of the light-induced creation of metastable defects in a-Si:H. Yet given our present understanding it is difficult to predict whether the rate of defect creation would increase or vanish as one approaches very low temperatures. According to the model of Stutzmann et al. the rate is proportional to the product np of electron and hole photocarriers in the conduction and valence band tail states. This product increases significantly with decreasing T. On the other hand, the configuration changes that stabilize the new defects may be thermally activated. If so, one expects the creation rate to vanish at T = 0. In Redfield et al.'s formalism, the stretch parameter fP or other rate coefficients might vanish at low T. Earlier work 3,4 indicates that the creation efficiency of metastable defects decreases by about a factor 3 between 300K and 78K. We extend these measurements to 4.2K and study the effect of the metastable defects on % (T), the temperature dependence of the steady state photoconductivity.
EXPERIMENTAL DETAILS 15 The undoped a-Si:H was 1 m thick and contained after annealingND = 6.7x10 cm-3 defects according to photothermal deflection (PDS) measurements. The dark conductivity was (6±0.5)xl0-11Q•-cm-1 at 300K with an activation energy Ea =0.8±0.02eV. Photo-induced defects were created by hv = 2.1 ± 0.1 eV light at a photocarrier generation rate Go = 3.8xl021cm- s-1. We used G = 5xl0l7cm-3s-1 or less to measure oap. Changes in ND were monitored by measuring the changes in subgap absorption Aa below
hv
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