Determination of Composition and Phase Depth-Profiles in Multilayer and Gradient Solid Solution Photovoltaic Films Using Grazing Incidence X-ray Diffraction

A method for using grazing incidence x-ray diffraction (GIXD) for profiling composition changes with depth of photovoltaic quality thin films is presented. The average thickness of the first layer in a multi-layer film of CuIn2Se3.5/CuInSe2/Mo and the variation in solid solution composition of a Cu(In1-xGax)Se2 (CIGS) film with depth are solved using this method. The phase volume fraction and thephase composition profiles are developed from peak intensity and d-spacing measurements respectively at a series of fixed incident angles corresponding to a set of increasing 1/e penetration depths tau. Inverse Laplace and numerical methods are applied to the tau profiles converting them to true depth profiles. Vegard's law is applied to the d-spacing vs z-profile to obtain x in the formulaCu(In1-xGax)Se2. The results show that an approximately 1 mu m thick layer of CuIn2Se3.5 is present on the surface of the multi-layer film and that the CIGS film consists of a Ga rich surface layer approximately 2000 Angstrom thick followed by a gradual decrease in Ga content with increasing depth. This gradient appears to be desirable for producing photovoltaic quality CIGS films.