EpitaxialCuIn1−xGaxS2on Si(111)(0⩽x⩽1): Lattice match and metastability

Epitaxial thin films of $\mathrm{Cu}{\mathrm{In}}_{1\ensuremath{-}x}{\mathrm{Ga}}_{x}{\mathrm{S}}_{2}$ $(0\ensuremath{\leqslant}x\ensuremath{\leqslant}1)$ were grown on single-crystalline silicon substrates of (111) orientation by means of molecular-beam epitaxy from elemental sources. Employing x-ray diffraction in transmission and reflection geometries, as well as Rutherford backscattering spectroscopy, the lattice parameters and compositions of these films were determined, respectively. A linear dependence of the lattice constants $a$ and $c$ on the gallium content $x$ was found. Best lattice match with the cubic silicon substrate was observed for $x=0.56$. Furthermore, the coexistence of the metastable CuAu-type ordering with the ground-state chalcopyrite structure was found to depend on the gallium content. This coexistence of the stable ground-state structure with a metastable ordering is accompanied by a nonlinear dependence of the tetragonal distortion on the gallium content of the thin films.