Identification of the Cu2-xSe Surface Phase in Epitaxial Cu-rich CuInSe2 Grown on GaAs (001)

Cu_2−xSe is an important impurity phase of the ternary chalcopyrite semiconductor CuInSej associated with Cu/In ratios greater than unity. We have observed directly in a prototypical epitaxial system the formation of Cu_2−xSe on Cu-rich CuInSe₂ thin films epitaxially grown on GaAs (001). Atomic force microscopy measurements of the surface topology of as-grown films clearly show facetted rectangular crystallites with dimensions on the order of 100 nm. Two dimensional reciprocal space x-ray mapping of the as-grown Cu-rich CuInS₂ showed the in-plane lattice constant of the Cu_2−xSe phase to be partially strained to the CuInS₂ layer with the out-of-plane lattice constant assuming a value of 0.5722 nm and the in-plane lattice constant 0.5770 nm. Triple axis measurements of the lattice constant of Cu_2−xSe/GaAs grown under identical growth conditions yielded an in-plane lattice constant of 0.5764 nm and an out-of-plane lattice constant of 0.5756 nm, indicating that the Cu_2−xSe was largely relaxed. The presence of the β-phase of Cu_2−xSe, is also presented as an alternative explanation for CuPt ordering reports in CuInS₂ that have appeared in the literature.