Direct observation of the Cu_2-xSe phase of Cu-rich epitaxial CuInSe₂ grown on GaAs (001)

Cu_2-xSe is an important impurity phase of the ternary chalcopyrite semiconductor CuInSe₂ associated with Cu/In composition 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 faceted rectangular crystallites with dimensions on the order of 100 nm. Cross-sectional transmission electron microscopy measurements of the Cu-rich CuInSe₂ showed rectangular protrusions on the surface as well as wedge shaped facets in the CuInSe₂ film. Two-dimensional reciprocal space x-ray mapping of the as-grown Cu-rich CuInSe₂ showed the in-plane lattice constant of the Cu_2-xSe phase to be partially strained to the CuInSe₂ layer. The presence of the β phase of Cu_2-xSe is also presented as an alternative explanation for Cu-Pt ordering reports in CuInSe₂ that have appeared in the literature. Strain-related surface undulations observed only in Cu-rich CuInSe₂ are also linked to the presence of this strained Cu_2-xSe layer.