Medium-Range Order in a-Si:H Below and Above the Onset of Microcrystallinity
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2
- Wi2 ) 1/ 2, where W. is the measured width from the
sample. Since the absorption length of this 0.154 nm radiation is about 70 gm in Si, most of the scattering signal originates from the SS substrate. However, this signal is strongly localized in the bcc/fcc diffraction peaks and some secondary phase peaks, none of which interfere with the FSP (located near 20 = 27.60) from a-Si:H nor with the (111), (220), or (311) c-Si diffraction peaks. Long counting times were used to achieve good signal-to-noise and the bare SS substrate signal was subtracted from each XRD pattern (with appropriate absorption corrections). Since the SS apparently has different degrees of polycrystalline texture for the various samples, this would not typically remove the strongest bcc peaks at 20 = 440 and 650. These were then removed by a smoothing procedure in order to examine more carefully the a-Si:H and gc-Si:H peaks. The full-width-at-half-maximum intensity (Win) of the FSP of a-Si:H, as well as the fractional area (integrated intensity) of the tc-Si:H (111)+(220)+(311) peaks compared to that of the total area from 150 to 650 were extracted via a least-square fitting routine based on a superposition of Voigt line-shape functions. This function is symmetric but has two width parameters used to adjust the shape. Typically the second a-Si:H peak centered near 20 = 510 was included in the fit due to the slight overlap of its low-angle tail with the FSP. To account for some slight asymmetry in this second peak, a third peak was included at a fixed angle near 450 in all fits of fully amorphous films. An example of the data and this type of fit is shown in Fig. 1 for a fully amorphous film. Also shown is a fit to a partially microcrystalline film where the sharper (111), (220), and (311) peaks of the gc phase are indicated. The residual amorphous phase FSP remains clear enough to track its width in the partially microcrystalline material. 253
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Two-theta (degrees) FIG. 1. Example least-square fits of X-ray diffraction patterns from the two type of films studied here, completely amorphous-Si:H (with the first scattering peak, FSP, indicated) and mixed amorphous/microcrystalline. Superpositions of Voight line-shape functions and a linear background were used as indicated by the dashed curves to generate the solid line fit through the data. The stick diagram for randomly-oriented c-Si is shown with the lowest 3 Bragg peaks. RESULTS AND DISCUSSION PECVD Material Figure 2 demonstrates the substrate sensitivity of the high-H 2 -diluted PECVD films to the formation of partially g.c material. These films were all made under nominally identical conditions and grown to a common thickness of 0.5 pgm. One can see that the presence of the thin intermediate a-n or ptc-n layer on the SS prevents the partial microcrystallinity, at least to the sensitivity level o
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