Microstructure and the Urbach Edge in Glow Discharge Deposited a-SiC:H
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MICROSTRUCTURE AND THE URBACH EDGE IN GLOW DISCHARGE DEPOSITED a-SiC:H A. H. Mahan, A. Mascarenhas, D. L. Williamson,* and R. S. Crandall Sol~r Energy Research Institute, Golden, CO 80401 Colorado School of Mines, Golden, CO 80401
ABSTRACT infrared, and Raman measurePhotothermal deflection spectroscopy, ments are presented for a series of a-SiC:H samples containing different amounts of microstructure. The Urbach edge is seen to widen systematically as the film microstructure increases, but the width of the Raman transWe argue that the increase in verse optical mode remains unchanged. Urbach edge width with increasing C content is due entirely to the increase in microstructure. In addition, from the invariance of the Raman measurements with increasing microstructure and the assumed sensitivity of the Raman technique to surface states on the interior surfaces of these microstructural features, we are able to place lower limits on the size of these features. Such sizes are corroborated by preliminary small angle xray scattering measurements on identically prepared samples.
INTRODUCTION The hydrogenated amorphous silicon carbon alloy (a-SiC:H) has recently received much attention in the thin film solar cell community because of the possibility of varying the material band gap with C incorporation [II. In addition to its utilization as a window layer in an amorphous silicon (a-Si:H) p-i-n solar cell [2], the wide band gap of this material also suggests its incorporation as an intrinsic layer into a Although progress has recently multijunction thin film solar cell [31. been made in this regard [4,51, the material properties of intrinsic a-SiC:H still remain sufficiently inferior (in comparison to device quality a-Si:H) that progress in multijunction cells utilizing a-SiC:H is much slower than is desirable. Several recent publications have attempted to link film microstructure to the a-SiC:H material quality [6-81. In the present paper, we report photothermal deflection spectroscopy (PDS) and Raman measurements of a series of a-SiC:H films, and also a series of a-Si:H films, whose infrared and photoconductivity properties have already been measured when plotted against a parameter indicative of increasing [7,8]; microstructure, the Urbach edge values for all samples follow the same trend [9]. Raman measurements on the same samples indicate no increase in the full width at half maximum (FWHM) of the Si-Si transverse optical (TO) mode. From these two sets of measurements we advance our interpretation of the origin of the wider Urbach tails observed in our a-SiC:H samples, and at the same time obtain a lower size limit on the microstructural features that we believe are responsible for these effects, assuming a void-substance composite. We support this last interpretation by small on identically prepared angle X-ray scattering (SAXS) measurements samples.
SAMPLE PREPARATION Deposition of the a-SiC:H and a-Si:H films was carried out in a capacitively coupled, parallel plate, radio frequency (rf) glow discharge apparatus [1
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