Comparative Study of Hydrogen Diffusion in Hot-Wire and Glow-Discharge-Deposited a-Si:H
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4 23 J. SHINAR,' R. SHINAR, ' K. E. JUNGE,' E. IWANICZKO, A. H. MAHAN, 4 4 R. S. CRANDALL, AND H. M. BRANZ 'Ames Laboratory - USDOE and Department of Physics and Astronomy, 2Iowa State University, Ames, IA 50011 Microelectronics Research Center, Iowa State University, Ames, IA 50011 3 Microanalytical Instrumentation Center, Iowa State University, Ames, IA 50011 4National Renewable Energy Laboratory, Golden, CO 80401
ABSTRACT Long-range atomic H motion in hot-wire deposited (HW) a-Si:H is compared directly to that in glow-discharge deposited (GD) a-Si:H by monitoring the deuterium secondary ion mass spectrometry (DSLMS) profiles in [GD a-Si:H]/[GD a-Si:(H,D)]/[HW a-Si:H] multilayers vs annealing temperature and time. While the profiles in the GD layer are in excellent agreement with complementary error-function behavior and previous studies, the profiles in the HW layer suggest that the multiple-trapping motion of the H and D atoms is much slower, possibly due to an interface layer of defects. However, an exponential "tail" of D atoms extends deep into the HW layer, probably due to a long diffusion length of mobile D atoms, consistent with the established release times of H and D from the GD layer and H loss typical during growth of HW films. The results are also discussed in terms of the H exchange model and compared to previous NMIR studies of HW a-Si:H, which suggest that most of the hydrogen in the HW layer is concentrated in H-rich clusters dispersed in a network of very low H content. INTRODUCTION Various studies of HW a-Si:H [1 - 3] indicate that it is a promising photovoltaic material which is quite different from a-Si:H prepared by any other method. Specifically, its H content of -2 to -3 at.% is much lower than that of device-quality GD films [1 - 3], its 'H NMR spectrum suggests that most of the H atoms are concentrated in clusters [4], and it exhibits an internal friction which is anomalously low for amorphous materials [5]. These properties suggest a material in which the isolated H-rich regions are surrounded by a Si network which is significantly more ordered than that of GD films. The close relation between the metastable defect dynamics (i.e., the Staebler-Wronski effect) and H dynamics in a-Si:H suggested by numerous studies [6 - 9] warrant a study of hydrogen motion in this material. This paper describes a preliminary deuterium secondary ion mass spectrometry (DSIMS) study of longrange H diffusion in this material. Previous DSIMS studies of H diffusion in a-Si:H showed that the H diffusion constant is generally time-dependent [10 - 15], usually decreasing with time t as D(t) = Doo(cta)"awhere the dispersion parameter 0 • a • 1. D(t) is determined experimentally by fitting the DSIMS profiles of annealed [a-Si:H]/[a-Si:(H,D]/[a-Si:H] multilayers to a complementary error function c(x,t) = noerfc{x/[24((r 2 (t))] } noerfc{z} where
(r2(t)) ---Jo'D(-r)dr=[Doo(dXt)l'a]/[ o(l-a)] --rolt1-a
299 Mat. Res. Soc. Symp. Proc. Vol. 557 ©1999 Materials Research Society
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