Measurement of Two Deep H Bonding Levels in Device Quality Glow Discharge a-SI:H
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Mat. Res. Soc. Symp. Proc. Vol. 377 01995 Materials Research Society
first to the rate equation for loss of Si-H bonds and then to the diffusion equation for H in bulk material. In our previous (isochronal) work [10], we obtained effusion parameters similar to literature values obtained by both H evolution and SIMS profiling, including a 1.4 eV Ea. In that study, we also observed the presence of a more tightly bonded H component, but we were unable to reliably determine its effusion kinetics. In this paper we present studies on a device quality glow discharge (GD) a-Si:H film using the isothermal approach. We obtain rate coefficient(s) for the loss of Si-H bonds at each anneal temperature, and we show that the loss of Si-H bonds is activated. We demonstrate that there are two distinct rate processes for loss of Si-H bonds, whose fractional magnitudes are comparable to the NMR fractions for clustered and isolated H. The activation energy and the diffusion prefactor for the majority (clustered) H component are again consistent with literature values. Our results for the minority component show that isolated Si-H bonds are bonded more tightly than their clustered counterpart, and exhibit a diffusion limited effusion behavior as well. We discuss these results in the context of other experimental data presently available for H bonding in a-Si:H. EXPERIMENTAL DETAILS The a-Si:H sample used in this study was a 6000 A thick GD film deposited at 250'C on a crystal silicon substrate and containing about 10% H. No discernible IR absorption in the Si-H stretch mode at 2090 cm-1 was observed, and an H evolution spectra showed only the usual high temperature evolution peak (with a higher temperature shoulder) typical of device quality material [11]. Thus, the influence of (SiH2)n bonding can be neglected in the present study. Solar cell efficiencies obtained utilizing this material as the intrinsic layer are routinely in the 9% -10 % range. Individual pieces of this film were annealed isothermally in air in a box furnace at temperatures of 425', 450', 4750 and 500'C. After each anneal, a room temperature IR spectrum was taken with a Nicolet 710 Fourier Transform IR spectrometer, and the relative decrease in the amount of Si-H absorption was determined by measuring the area under the absorption curves. Since we always measured the same portion of each sample, we used the same baseline for all spectra taken at each individual annealing temperature. In our previous isochronal study using a different GD sample [10], the decrease in Si-H absorption upon annealing was carefully studied for both the Si-H stretch (2000 cm- 1 ) and wag (630 cm- 1 ) modes. We observed that, for a reduction in Si-H absorption down to
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