Small-Angle X-Ray Scattering Studies of Glow-Discharge-Produced a-SiGe:H Alloys

Abstract

Electron density fluctuations associated with microstructural features on a scale from about 1 to 25 nm in glow-discharge-deposited a-SilxGex:H films were studied by the technique of small-angle x-ray scattering (SAXS). Films prepared in four different deposition systems (in different laboratories) have been characterized and a general increase in the SAXS signal with increasing x is observed. Density deficiencies determined from film flotation measurements lead to the correlation of the increased scattering intensities with increases in the volume fractions ofmicrovoids. Modeling ofthe data yields void size distributions that demonstrate significantly more of the larger voids (2 to 6 nm) than found at x--0 (around I nm). For the alloys with x>0.4, the scattering at the smallest angles was observed to decrease substantially upon tilting of the sample relative to the x-ray beam. This result contrasts with the small or no changes in SAXS upon tilting device-quality x--0 films. This anisotropic scattering associated with the tilting experiments has been modeled with distributions of ellipsoidal microvoids that are preferentially oriented with their major axes normal to the film plane. This latter result is consistent with a columnar-like microstructure. However, one film with x=0.37 shows no evidence for such microstructure.