Annealing the Defects in a-Si:H Under Illumination
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ANNEALING THE DEFECTS IN a-Si:H UNDER ILLUMINATION HELENA GLESKOVA*, P.A. MORIN** AND S. WAGNER
Princeton University, Department of Electrical Engineering, Princeton, NJ 08544
ABSTRACT
The results of a study of the kinetics of the light-induced annealing of the deep-level defects in hydrogenated amorphous silicon (a-Si:H) are presented. They show that at elevated temperatures illumination increases the rate of annealing compared to annealing in the dark. We also detected light-induced annealing at room temperature. On the basis of a model in which the defects are generated by electron-hole recombination and annealing occurs through the action of a single carrier, we found values of 0.86 eV for the activation energy of the light-induced generation coefficient Y-1, and 1.23 eV for the light-induced annealing prefactor X. INTRODUCTION
Light-induced degradation of the properties of amorphous silicon (a-Si:H) has been studied extensively since 1977 [1]. During the past few years high-intensity light-soaking of a-Si:H has been used as a tool for the rapid evaluation of its stability [2,3]. The saturation of the defect density close to room temperature under high-intensity illumination and the dependence of the saturated defect density on temperature during this light-soaking have been described [37]. Recently, it was shown that for elevated temperatures illumination increases the rate of defect annealing compared to annealing in the dark [8,9,10]. Models have been proposed for the time dependence and the saturation of the defect density [10-18]. Comprehensive rate laws for metastable defects [10,19,20] contain four terms. Redfield [19] and Bube and Redfield [20] suggested the following rate law for the metastable defects: dN = C1 R (Nt -N) - C2 R N +Vl (N, - N) - v 2 N (1) dt The terms on the right side denote light-induced generation of defects, light-induced annealing, thermal annealing and thermal generation, respectively. N, is the maximum density of the sites in the material that can be converted to metastable defects and N is the density of defects at time t. C1R, C2 R, v, and V2 are rate coefficients. The rate R has been formulated in terms of the mechanisms proposed for light-induced generation and annealing, as G or G2/N2, where G is the volume generation rate of electron-hole pairs. R. Meaudre and M. Meaudre [10] suggest replacing equation (1) by: dN - G_22 -X G N +vl (Nt - N) - v 2 N dt y N2
(2)
The four terms have the same meaning as in Eq. (1). For annealing in the dark both Eqs.(l) and (2) can be simplified to the form: dN = v1 (Nt - N) -v 2 N dt
(3)
The kinetics of the removal of the defects under illumination are not well known, and are the subject of our work. We carried out annealing experiments under illumination and in the dark at four temperatures ranging from 92*C to 152*C, i.e., in a temperature range where lightinduced defect annealing proceeds quickly. We also measured light-induced annealing at 30°C, where the process is very slow.
Mat. Res. Soc. Symp. Proc. Vol. 297. 1,1993 Materials R
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