Study of the Microstructure of Amorphous Germanium Alloys Using AFM and SAXS

Abstract

Many researchers have found the photoresponse of germanium-based films to be directly related to the homogeneity of the film’s microstructure, with more homogeneous films having better photoresponse. Transmission electron microscopy (TEM) gives the resolution needed for studying microstructure, but is typically limited to imaging films with a thickness of less than 200 nm. Surface measurement by atomic force microscopy (AFM) gives a three-dimensional representation of the film surface and can be used for measuring any thickness film. A more complete idea of the microstructure can be gained by combining AFM with small-angle x-ray scattering (SAXS) measurements. SAXS measurements yield the size of internal voids or less dense regions and indicate whether the inhomogeneities have a preferred orientation, such as might be associated with columnar growth. Films of amorphous germanium and germanium carbon alloy were grown by the PECVD method from SiH₄, GeF₄, H₂ and CH₄ source gases. The films were then characterized using AFM, SAXS, and FTIR. The effect that various growth conditions had on the microstructure and photoresponse will be reported.