High Quality a-Si:H Films Grown at High Deposition Rates
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ABSTRACT Intrinsic a-Si:H samples were grown with and without hydrogen (H2) dilution of silane at different growth rates. We find that the dilution leads to a considerable reduction in the defect density, in particular at high growth rates. The defect density is particularly low for samples grown using H2 dilution conditions at growth rates as high as 10 A/sec. Using transient photocapacitance measurements we find evidence for a small concentration of microcrystallitesembedded in the amorphous films. An increase in the microcrystalline fraction correlates with a decrease in the defect density. INTRODUCTION Considerable effort has been devoted to obtain hydrogenated amorphous silicon (aSi:H) films that are grown at high deposition rates but contain a low defect density (before and after light soaking) [1]. Usually, cells deposited by plasma decomposition of pure silane are grown at rates of 3-5 A/sec. It has been known for some time that hydrogen dilution of silane results in films with better stability [2,3]. A variety of deposition techniques have been utilized to increase the growth rate while maintaining the stabilized cell efficiency [4,5]. In this paper we report measurements of two series of intrinsic a-Si:H films that were
deposited with and without hydrogen dilution. The growth rate was changed by altering the radio frequency (rf) power. We show that samples grown from H2 diluted silane at a relatively high growth rate exhibit a remarkably low defect density. Subband gap absorption spectra reveal that these amorphous materials contain a small fraction of microcrystallites embedded in the amorphous matrix. The correlation between the superior electronic properties and the microcrystalline fraction agrees with recent studies [6-9], and suggests that the most suitable materials for solar cells applications are those that are grown near the amorphous / microcrystalline boundary. EXPERIMENTAL Intrinsic a-Si:H films were grown using rf plasma decomposition of silane at a substrate temperature of 250'C. One set of samples was deposited from pure silane at a pressure of 20 mTorr with a gas flow of 30 sccm. The rf power was varied (in the range of 10 to 100 Watts), thus affecting the growth rate. Another set of samples was deposited from hydrogen diluted silane in 4:1 ratio. The silane flow was kept the same as in the first
139 Mat. Res. Soc. Symp. Proc. Vol. 557 ©1999 Materials Research Society
set, so that the total pressure in the chamber was 100 mTorr. In this case the H 2 dilution results in only small decrease of growth rate. For comparison we also grew a non-diluted sample at the higher pressure. The growth conditions are summarized in Table I. More details on the growth system and conditions and more information about these samples are given in ref. 10. In all cases the samples were co-deposited on p÷ c-Si and quartz, for capacitance and electron spin resonance (ESR) studies, respectively. In addition, a matching set of films, 7000 to 8000A thick, were grown on n÷ c-Si, and coated with semitransparent (5
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