GaAs layers grown on (001) InP surfaces by low-pressure metalorganic chemical vapor deposition were investigated with photoluminescence spectroscopy, x-ray diffraction, Raman scattering, and transmission electron microscopy. Correlations between the optoelectronic properties, the strain relaxation, and the structural defects were established for layer thickness D ranging between 0.1 and 3.0 μm. A comparison with the case of InP layers grown on GaAs substrates is presented. Radiative recombinations to split light- and heavy-hole valence bands near the zone center are seen at 12 K in the photoluminescence spectra. The splitting is due to a biaxial tensile strain. With increasing temperature, the heavy-hole transitions gain intensity and at around 140 K they are the only features in the spectra. In the 12-50 K temperature range the intensity ratio between the heavy- and light-hole transitions also depends on laser power. The hole activation energy determined from the temperature dependence of the intensity ratio above 50 K agrees with the valence-band splitting. The strain for D≳0.3 μm arises from differences in linear thermal expansion and has contributions from the lattice mismatch in thinner layers. The strain values yielded by x-ray diffraction are smaller than those obtained from the valence-band splitting measured with photoluminescence. The difference is attributed to a temperature dependence of the linear thermal expansion, which was corroborated by the shifts of the longitudinal optical phonon frequencies measured with Raman spectroscopy at 300 and 12 K. A comparison is made of the absolute magnitude of the strain and the x-ray diffraction linewidths for heteroepitaxial GaAs and InP layers on InP and GaAs substrates, respectively. The contribution to the strain from the lattice mismatch relaxes in GaAs faster than in InP and the GaAs x-ray linewidths are narrower for D<1.0 μm. These results are understood in terms of the growth habit and the behavior of threading and interfacial dislocations.
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