Correlation between Powder in the Plasma and Stability of High Rate Deposited a-Si:H
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A11.3.1
Correlation between Powder in the Plasma and Stability of High Rate Deposited a-Si:H Guozhen Yue, Gautam Ganguly, Baojie Yan, Jeffrey Yang, and Subhendu Guha United Solar Ovonic Corporation, 1100 West Maple Road, Troy, Michigan 48084 ABSTRACT Hydrogenated amorphous silicon (a-Si:H) solar cells incorporating high deposition rate (810Å/s) intrinsic layers were deposited using modified very high frequency (MVHF) plasma. We have monitored the light scattered from powder generated in the plasma using an Ar-laser and a silicon photodiode. This simple, non-invasive technique allows us to make measurements on the same reactor used to make the solar cells. First, we have varied the total flow rate and observed a maximum in the scattered light intensity from powder in the plasma during the deposition of the intrinsic layer, and correlated this with the degradation, as well as the stabilized performance of the solar cells. Then, we have studied the effects of varying the deposition temperature and/or the addition of germane to the gas mixture on the scattered light intensity due to powder in the plasma.
INTRODUCTION Improvement of the stability of hydrogenated amorphous silicon (a-Si:H) alloys deposited at high rates is perhaps the most important task for cost reduction of solar cells based on this material. One of the factors that are generally believed to enhance degradation is ‘higher silane radicals’[1]. The success of the hydrogen dilution technique is considered to be evidence of the reduction of the contribution of ‘higher silane radicals’ to growth. On one hand, correlations between higher silane density (from mass spectroscopy), the concentration of silicon di-hydride bonding in the film and the light soaked (‘stabilized’) solar cell fill factor have suggested that ‘higher silane radicals’ arriving at the growth surface are responsible for the development of nonoptimum bonding configurations, which are potential sites for formation of metastable defects [1]. On the other hand, it has been shown that solar cell intrinsic layers incorporating clusters produced in the plasma are more stable than ‘standard’ a-Si:H [2]. Here we report our efforts to ascertain which of these observations is valid in a reactor used to fabricate high efficiency single junction solar cells incorporating high deposition rate (8-10Å/s) intrinsic layers using modified very high frequency (MVHF) plasma [3]. Correlations have been demonstrated for the ‘higher silane density’ measured using mass spectroscopy and ‘solar cell stability’ [1]. However, there are problems inherent in the mass spectrometer in that the relative sensitivity of the instrument to different gases (e.g. silane to hydrogen) varies with time when the filament is turned ‘on’ to ionize fragments of different mass to charge (m/e) [4]. Scattering of light from ‘powder’ particulates has been studied as a function of growth time, gas flow rates, frequency of plasma discharge, gas composition etc. by various groups [5,6]. Since there are significant differences between nominal
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