Defect Relaxation Dynamics in Amorphous Silicon

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DEFECT RELAXATION DYNAMICS IN AMORPHOUS SILICON J. DAVID COHEN, THOMAS M. LEEN, FAN ZHONG, AND R.J. RASMUSSEN Department of Physics and Materials Science Institute, University of Oregon, Eugene, OR 97403 ABSTRACT Using transient capacitance and transient spin techniques, we have the determined the manner in which the mobility gap energy of the D defect is altered following a change in its charge state. This relaxation process gives rise to a power law rather an exponential thermal release of defect electrons with time and also causes the charge emission and spin transients to obey a scaling law. We also deduce that the Do/D+ transition rate depends on the tenure of the preceeding D-/DO transition. This last aspect of the D defect emission behavior implies that it must be treated as a non-Markovian process. Such relaxation dynamics have profound consequences for the steady state distribution of D defect energies. Using the relaxation parameters determined by the transient measurements we have been able to solve a set of coupled differential equations under steady-state conditions to provide the energy distributions of both the Do and D- defect sub-bands. The results of these calculations agree remarkably well with the experimental distributions determined by modulated photocurrent and steady-state capacitance measurements. This implies that the statistical variations in the occupation history of the defect may be the dominant factor determining both distributions. INTRODUCTION Tremendous effort has been made to characterized the energy distribution and trapping dynamics associated with the dominant mobility gap deep defect (D) in hydrogenated amorphous silicon (a-Si:H). This center is well known to be responsible for the bands of thermal 1 and optical2 transitions observed near midgap. Important aspects of the structure of this defect have remained unresolved; for example, why defect bands revealed at differing energies within the mobility gap are attributed to the same D'/Do transition of the defect. 3 These inconsistencies suggest that the D defect forms at different energies in intrinsic vs. doped samples. Attempts have thus been made to account for the variation in deep defect distributions using the "defect pool model" in which the Fermi level governs the energies at which D defects will form. 4 ,5 An alternative approach has also been taken: to explain these differing energies by a defect relaxation mechanism. 6,7 In this paper we review our recent experimental evidence in favor of applying the second approach using a combination of charge transients and spin transient measurements. 8 We also present new results in which we employ these relaxation dynamics in a detailed calculation that also takes into account carrier trapping and emission from the defects. We show that such a calculation can accurately reproduce the experimental steady-state distributions of both the Do and the Ddefect sub-bands as determined from modulated photocurrent and junction capacitance measurements, respectively. SAMPLES AND SAMPLE P