Films and Devices Deposited by Hwcvd at Ultra High Deposition Rates
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FILMS AND DEVICES DEPOSITED BY HWCVD AT ULTRA HIGH DEPOSITION RATES A. H. Mahan, Y. Xu, E. Iwaniczko, D. L. Williamson*, W. Beyer**, J. D. Perkins, M. Vanecek***, L. M. Gedvilas, and B. P. Nelson National Renewable Energy Laboratory (NREL), Golden, CO 80401, USA * Physics Department, Colorado School of Mines, Golden, CO 80401, USA ** Forchungszentrum Julich, Julich, Germany *** Inst. Of Physics, Czech Academy of Sciences, Prague 6, Czech Republic Abstract The structure of a-Si:H, deposited at rates in excess of 100Å/s by the hot wire chemical vapor deposition (HWCVD) technique, has been examined by x-ray diffraction (XRD), Raman spectroscopy, H evolution, and small-angle x-ray scattering (SAXS). As the film deposition rate (Rd) is increased, we find that the XRD, Raman and the H evolution peak curves are invariant with Rd, and exhibit structure consistent with state-of-the-art, compact a-Si:H films deposited at low Rd. The only exception is the SAXS signal, which increases by a factor of ~100 over that for our best low Rd films. We relate changes in the film electronic structure (Urbach edge) to the increase in the SAXS signals. We also note the invariance of the saturated defect density versus Rd, and discuss possible reasons why the increase in the SAXS does not play a role in the Staebler-Wronski Effect for this type of material. Finally, device results are presented. INTRODUCTION An intense effort has recently been initiated to raise the deposition rate (Rd) of hydrogenated amorphous silicon (a-Si:H) films without sacrificing material quality . The a-Si:H grown for industrial purposes is typically grown at Rd‘s from 1-3Å/s, and most of the viable manufacturing processes still grow a-Si:H at rates below 10Å/s (1). To date, little information is available on the electronic and structural properties of high Rd films. In addition, when high Rd deposited films have been incorporated into solar cells, the result has typically been that not only does the initial performance become inferior, but also the Staebler-Wronski Effect (SWE) increases (2). It is thus an open question whether a significant increase in Rd can be accompanied by a preservation of electronic and structural properties as well as device performance. In previous studies of a-Si:H deposited by hot wire chemical vapor deposition (HWCVD), a 9.8% device efficiency for a laboratory device was reported with the i-layer deposited at 16.5Å/s (3); this remains a record initial efficiency for a device whose i-layer was deposited at this (high) rate. More recently, the first results were presented on the electronic properties of HWCVD films deposited at much higher rates (up to 150Å/s) using multiple W filaments (4). In this paper, we describe the first detailed results on the structural properties of these HWCVD films. We show that the structural ordering, as probed by x-ray diffraction (XRD), Raman, and H evolution, remain unchanged with increasing Rd, and approach those of the best HWCVD material grown at 5-8Å/s. The only exception to this behavior is a sizeab
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