Epitaxial growth of chalcopyrite CuInS2 films on GaAs (001) substrates by evaporation method with elemental sources

Full text : Ternary chalcopyrite semiconductor CuInS2 is one of the potential candidates for absorber layers in high-efficiency thin film solar cells due to its direct bandgap Eg of 1.5 eV, which matches with solar spectrum. However, CuInS2 solar cells face the problem of lower solar conversion efficiency compared with Cu(InGa)Se2 solar cells. Investigation of fundamental properties of CuInS2 films is necessary to understand key issues for solar cell performance. Although in bulk CuInS2 is known to crystallize into chalcopyrite (CH) structure, in thin film other structures such as Cu-Au (CA) and sphalerite (SP) structures may coexist. It was reported epitaxial growth of slightly Cu-rich CuInS2 films with c-axis orientated CA only and/or with a mixture of a- and c-axes orientated CH structures on GaP (001) at substrate temperature of 500 degrees using the conventional evaporation method with three elemental sources. Successful growth of epitaxial CH structured CuInS2 were observed for films grown on GaP at 570 degrees with slightly Cu-rich composition. In this paper, CuInS2 films with various [Cu]/[In] ratios are grown on GaAs(001) substrates, and the composition range in terms of the [Cu]/[In] ratio where epitaxial films with CH structure grow and the structural qualities of the films are discussed in comparison with those on GaP substrates. Films with various ratios of [Cu]/[In]=0.8 ≤1.9 are grown at 500 degrees and 570 degrees using the evaporation system described in our previous reports. Regardless of the substrate temperature, noticeable X-ray diffraction (XRD) peaks of CH structured CuInS2 phase are observed in slightly Cu-rich films. However, reflection high energy electron diffraction (RHEED) patterns of the slightly Cu-rich films grown at 570 degrees exhibit noticeable spots not only due to the CH structure but also due to the CA structure. The amount of the CA structure is considered to be small because of the absence of the XRD peaks of the CA structure. Taking account of the previous report that the slightly Cu-rich films grown on GaP (001) at 570 degrees had only the c-axis oriented CH structure patterns even in RHEED, the slight appearance of the Cu-Au structure is possibly related the larger lattice mismatch for GaAs