Direct observation of the Cu2−xSe phase of Cu-rich epitaxial CuInSe2 grown on GaAs (001)

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Abstract
Cu 2−x Se is an important impurity phase of the ternary chalcopyrite semiconductor CuInSe 2 associated with Cu/In composition ratios greater than unity. We have observed directly in a prototypical epitaxial system the formation of Cu 2−x Se on Cu-rich CuInSe 2 thin films epitaxially grown on GaAs (001). Atomic force microscopymeasurements of the surfacetopology of as-grown films clearly show faceted rectangular crystallites with dimensions on the order of 100 nm. Cross-sectional transmission electron microscopymeasurements of the Cu-rich CuInSe 2 showed rectangular protrusions on the surface as well as wedge shaped facets in the CuInSe 2 film. Two-dimensional reciprocal space x-ray mapping of the as-grown Cu-rich CuInSe 2 showed the in-plane lattice constant of the Cu 2−x Se phase to be partially strained to the CuInSe 2 layer. The presence of the β phase of Cu 2−x Se is also presented as an alternative explanation for Cu–Pt ordering reports in CuInSe 2 that have appeared in the literature. Strain-related surface undulations observed only in Cu-rich CuInSe 2 are also linked to the presence of this strained Cu 2−x Se layer.