MICROVOID, SI, H, AND AL DYNAMICS IN ALPHA-SI-H/AL2O3/AL STRUCTURES - A SMALL-ANGLE X-RAY-SCATTERING AND INFRARED-ABSORPTION STUDY

A small-angle x-ray-scattering (SAXS) and infrared-absorption study of microvoid dynamics and hydrogen evolution in a-Si:H is described. In films deposited on Al foil by rf sputtering, annealing at 350 degrees C and above sharply increased the integrated SAXS intensity e, although the overall film density remained nearly constant. Incipient formation of microcrystalline Si domains by Si diffusing into the Al substrate is insufficient to account for the increased SAXS intensity. Rather, the close correlation between Q and the Si-bonded H content CH suggests that it more likely results from migration of Si-bonded H to deep traps at the microvoid surfaces and its accumulation as H-2 in the isolated voids. The density of the Si network then increases as its H content decreases, and the void sizes increase as hydrogen accumulates in them. These processes are discussed in relation to well-known characteristics of hydrogen diffusion in a-Si:H. In contrast to the noncolumnar samples, the values of Q and CH Of the columnarlike film did not change during prolonged annealing (beyond 6 h) at 430 degrees C. This behavior is believed to result from migration of most of the Si-bonded H to the internal surfaces of that film, recombination to molecular H-2, and its escape through the largely interconnected voids. Most of the remaining hydrogen was then probably bonded to the void surfaces in isolated sites which are not adjacent to a neighboring H atom. Annealing for 6 h at 430 degrees C also resulted in some crystallization sufficient to be detected by x-ray diffraction (XRD) and Raman scattering. Annealing for 18 h at that temperature sharply reduced the SAXS tilt-angle dependence of the columnarlike film. Finally, following 36 h at that temperature, the Raman and XRD spectra indicated that similar to 50 vol. % of the film was still amorphous. Auger profiles yielded an Al concentration which increased from similar to 2.2 at. % near the surface to similar to 3.1 at. % near the substrate of that film. However, the sharp reduction of the tilt-angle dependence is probably not due to filling of the voids by the diffusing Al but rather to the relaxation and reduction of the columnarlike morphology.