Comparison of Light-Induced Degradation in Low and High-Rate Deposited VHF-GD a-Si:H: Effect of Film Inhomogeneities
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COMPARISON OF LIGHT-INDUCED DEGRADATION IN LOW AND HIGH-RATE DEPOSITED VHF-GD a-Si:H: EFFECT OF FILM INHOMOGENEITIES H.CURTINS, M.FAVRE, Y.ZIEGLER, N.WYRSCH and A.V.SHAH Institut de Microtechnique, Breguet 2, CH-2000-Neuchitel, Switzerland (+41 38) 24 60 00
ABSTRACT We have grown two series, 'SLOW' and 'FAST', of high quality a-Si:H films by the VHF-GD Yery-IIgh-Erequency Glow-Discharge) technique, the first at a low deposition rate of R-3.6 A/sec and the other at a high rate of R-17 A/sec. Both series show the same bulk deep defect density Nb--4xlO05/cm 3 in the annealed state 'A' as measured by PDS. From lightinduced degradation experiments (Staebler-Wronski effect, SWE) the following interesting features are observed : a) The SWE is a bulk effect. To demonstrate this result we propose the use of PDS in combination with variable film thicknesses as a powerful and unambiguous method, b) The increase of the deep bulk defect density ANb up to the surated state of the SWE (state 'B') is the same for series 'SLOW and 'FAST', c) However, the kinetics of the SWE differ considerably. These results are interpreted with the help of experimentally observed continuous variation of the Urbach energy E{0 of the valence bandtail accross the film. The E0 profile is, in turn, related to the deposition rate R. The presence of this inhomofgenei yis also discussed in context with.the recently developed equilibration theories relating deep and shallow (bandtail) states. Preliminary results in this study seem also to support the model involving conversion of weak Si-Si bonds into dangling bond upon light soaking. Further the influence of internal film stress as well as the effect of impurities on the degradation behaviour are addressed. 1. INTRODUCTION
Extensive research on the light-induced creation of metastable defects (Staebler-Wronski effect, SWE) in amorphous hydrogenated silicon (a-Si:H) carried out over the last few years could not yet lead to a consensus as to the details of the mechanisms involved. There are still a number of major discrepancies with regard to the influence of various film parameters on the kinetics and on the magnitude of the SWE. Based on experimental data obtained from ESR and photoconductivity measurements, Stutzmann [1] concluded that there is a surface layer (thickness -=0.5 gtm) as well as a bulk contribution to the SWE (inhomogeneities in the films); Kakinuma et al. [2] attribute the changes of dark and photoconductivity primarily to a bulk SWE. Stutzmann [1] (ESR method) and Kocka [3] (subbandgap absorption via CPM) find correlation between stress and metastable defects; Guha [4] and Kurtz [5] find no correlation based on light induced changes of the photoconductivity. The effect of impurities (O,C,N..) at low concentration levels 250"C asfunction of reciprocalthickness dfor series irreversible changes, i.e. increases in 'FAST' (ty) and series 'SLOW' (o). (o,tj ) annealed state 'A', (*,a): saturatedstate 'B' E 0 and ND, were observed. The after 14 hours, 500 mW/cm 2 ELH white light changes were at
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