HETEROEPITAXY AND CHARACTERIZATION OF CUGASE2 LAYERS GROWN BY LOW-PRESSURE METALORGANIC CHEMICAL-VAPOR-DEPOSITION

CuGaSe2 chalcopyrite compounds were grown heteroepitaxially on both GaAs and GaP substrates by means of the low-pressure metalorganic chemical-vapor deposition method. Optical and structural properties were characterized comprehensively by photoreflectance (PR), photoluminescence (PL), x-ray diffraction, transmission electron microscopy, transmission electron diffraction, and electron-probe microanalysis. The CuGaSe2 epilayers had c(001) surface on GaAs(001) substrates and a(100) surface on GaP(001) substrates, respectively, the results being similar to the case of CuAlSe2. Energies of A, B, and C excitons associated with uppermost valence bands were determined from analysis of PR spectra, and the energies of good-quality epilayers are close to those of the bulk crystal. The slight increase of the crystal-field splitting in the valence bands were discussed in terms of the lattice strain in the epilayer caused by the lattice mismatch. Low-temperature PL spectra exhibited an intense peak at 1.71 eV, the energy being in good agreement with the A-exciton energy. A weak peak due to a free-to-acceptor transition was also observed at 1.66 eV. A broad PL peak at 1.76 eV was observed together with the intense band-edge PL at 1.67 eV, and the peak was assigned to relate to the B-exciton transition.