Analysis Of Photoreflectance Spectra Research Articles

Photoreflectance of CuAlxIn1-xSe2 Alloys

Photoreflectance (PR) measurements have been carried out at 77 K on single crystals of CuAlxIn1-xSe2 grown by the chemical vapor transport method. Energies of excitons associated with uppermost valence bands have been determined as a function of composition based on the analysis of PR spectra. Spin-orbit and crystal field parameters have been obtained.

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.

Direct and indirect transition in (GaAs)n/(AlAs)nsuperlattices withn=1–15

Photoreflectance and photoluminescence measurements are carried out in (GaAs${)}_{\mathit{n}}$/(AlAs${)}_{\mathit{n}}$ superlattices with n ranged from 1 to 15 at room temperature. Analysis of photoreflectance spectra gives several critical-point energies with a high accuracy which coincide with the photoluminescence peak energies in superlattices with n\ensuremath{\ge}10. In superlattices with n10 the lowest transition energy determined from photoreflectance analysis is found to be larger than the lowest peak energy of photoluminescence, whereas the higher-energy peak of photoluminescence agrees well with the transition energy obtained from the photoreflectance analysis. The present results strongly suggest an existence of crossover of direct and indirect transition in the superlattices around n=10. Energy-band structures and momentum-matrix elements are calculated by tight-binding method based on ${\mathit{sp}}^{3}$${\mathit{s}}^{\mathrm{*}}$ with including the second-nearest-neighbor interactions, which show a crossover of direct and indirect transition around n=8. The lowest peak energies of photoluminescence data agree with the lowest calculated indirect band gaps in superlattices with n10 and with the lowest direct band gaps for n\ensuremath{\ge}10. The lowest transition energies determined from the photoreflectance analysis exhibit a feature similar to the layer-number dependence of the calculated direct band gaps in the whole range of n, but a disagreement in the energy values between the two exists in the range of n from 5 to 10. A disagreement between the present and previous data for n=5 is explained in terms of layer-number fluctuation, which is supported by the energy-band calculation of a long-period superlattice such as (GaAs${)}_{\mathit{n}}$/(AlAs${)}_{\mathit{n}}$/(GaAs${)}_{\mathit{n}+1}$/(AlAs${)}_{\mathit{n}}$.