Changes in the Density of States in a-Si:H with the Generation and Annealing of Light-Induced Defects
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Changes in the Density of States in a-Si:H with the Generation and Annealing of Light-Induced Defects* K. Shepard a), Z E. Smith b), S. Aljishi c), and S. Wagner, Princeton University, Princeton, NJ 08544 a) Solid State Electronics Laboratc-y, Stanford University, Stanford, CA 94305 b) Xerox Palo Alto Research Center, 3333 Coyote Hill Road, Palo Alto, CA 94304 c) Max-Planck-Institut fuer Festkoerperforschung, Heisenbergstrasse 1, 7000 Stuttgart, West Germany * This work is supported by the Electric Power Research Institute. Abstract: Through sub-bandgap absorption spectra and photoconductivity measurements of high-quality hydrogenated amorphous silicon (a-Si:H), changes in the electronic density of states with light-soaking and subsequent annealing are qualitatively described. Included is a more complete understanding of the role of deep defects as recombination centers. In this way, the simple power laws of Stutzmann, Jackson, and Tsai for the light-induced degradation of defect density and photoconductivity are. retained in the modified forms 2
N(0
1
G 25 2+1 25+1 N~)G t anda oc•N,.-A
Evidence is also presented that the states closest to midgap, those states that act as effective recombination centers, are the first to anneal. Introduction: The most comprehensive study to date on the problem of light-induced dangling-bond defect generation and annealing [1] in amorphous silicon (a-Si:H) was that of Stutzmann, Jackson, and Tsai [2]. These authors proposed a model which relied only on the breaking of bonds during light-soaking through nonradiative tail-to-tail transitions. For defect densities sufficiently high or intensities sufficiently low so that monomolecular recombination dominates, 2 3 a Ns(t) - G /3tl/ relation was derived and confirmed with electron spin resonance (ESR) measurements on material 3 with initial defect densities exceeding 1016 cm- . In this relation, N. is the defect (spin) density determined from the ESR signal with a g-value of 2.0055, G is the volume generation rate for free carriers which is proportional to the intensity of illumination, and t is the illumination time. The relationship between photoconductivity and defect density, aph - N,-1, was also derived and demonstrated. Recent work has called into question the validity of these two key relations derived by Stutzmann, Jackson, and Tsai particularly for material with low defect densities. Lee et al. [31 found that for a-Si:H films with 3 as-deposited ESR spin densities of less than 1016 cm- , the defect density as a function of illumination time does not exhibit unique power law behavior and is at all times less than tl/ 3 . In addition, the relationship between photoconductivity and spin density was found to deviate significantly from an inverse relation when Ns was varied by annealing from an electron irradiated state. The accuracy of electron spin resonance (ESR) for determining defect density has been drawn into question. The issue has been raised that during light-soaking and annealing the density of paramagnetic defects may
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