EVIDENCE OF PHASE-SEPARATION IN AMORPHOUS FEXSI1-X FILMS

Anomalous small-angle X-ray scattering studies of magnetron sputtered FexSi1-x alloys (0.044 less than or equal to x less than or equal to 0 .54) have been performed. For films with compositions between 4.4 and 23 at.% Fe, there is a strong, anisotropic small-angle scattering maxi mum that is dependent on the X-ray energy near the Fe K-edge. Based on the similarity of these results with those reported for amorphous M-G e (M = Fe, Mo, W) alloys and on the observations reported in the prese nt paper, the Si atoms are most likely distributed homogeneously. The scattering results from an inhomogeneous distribution of Fe atoms. Den sity calculations indicate phase separation into regions of a-Si and a n intermetallic close in composition to a-FeSi2, consistent with the d isappearance of this scattering maximum for x greater than or equal to 0.34. Analysis of the scattering in terms of correlation functions in dicates that the phase-separated regions are well correlated in the fi lm plane and of the order of 0.6 nm. In the growth direction, however, the regions are poorly correlated and typically 1 nm in size. The met al/insulator transition, which occurs in a composition range in which phase separation is observed, thus apparently results from the percola tion of metallic a-FeS2 in a-Si and not from a continuous, homogeneous structural transition from an open network to a dense intermetallic s tructure.