Depth profile of the lattice constant of the Cu-poor surface layer in (Cu2Se)1−x(In2Se3)x evidenced by grazing incidence X-ray diffraction Original Research Article

The surface of CuInSe2 or related compounds is an essential part of heterojunctions for photovoltaic applications. It is generally accepted that the reconstruction of the surface of slightly Cu-deficient film results in a Cu-depleted defect layer. Since this surface layer generally exhibits a CuIn3Se5 stoichiometry the existence of a defect chalcopyrite phase CuIn3Se5 has also been proposed. Although efforts have been made to identify this phase structurally by means of grazing incidence X-ray-diffraction (GIXRD), it has not been detected so far. However, the existence of a Cu-depleted surface layer fits well in the existing theory, which models the functioning of photovoltaic devices. In this contribution we present for the first time evidence for a Cu-depleted surface layer measured by X-ray diffraction. A careful analysis of GIXRD-data at incident angles from 0.3 to 10° by comparing the measured shape and width with simulated spectra confirms structural changes in the surface layer. The thickness of this layer varies from 5 to 60 nm with the integral Cu-content of the film. Hence GIXRD provides a unique means for non-destructive depth profiling.