Photoluminescence studies of In$_{0.7}$Al$_{0.3}$As/In$_{0.75}$Ga$_{0.25}$As/In$_{0.7}$Al$_{0.3}$As metamorphic heterostructures on GaAs substrates

The influence of the design of the metamorphic buffer of In$_{0.7}$Al$_{0.3}$As/In$_{0.75}$Ga$_{0.25}$As metamorphic nanoheterostructures for high-electron-mobility transistors (HEMTs) on their electrical parameters and photoluminescence properties is studied experimentally. The heterostructures are grown by molecular-beam epitaxy on GaAs (100) substrates with linear or step-graded In$_x$Al$_{1-x}$As metamorphic buffers. For the samples with a linear metamorphic buffer, strain-compensated superlattices or inverse steps are incorporated into the buffer. At photon energies $\hbar\omega$ in the range 0.6–0.8 eV, the photoluminescence spectra of all of the samples are identical and correspond to transitions from the first and second electron subbands to the heavy-hole band in the In$_{0.75}$Ga$_{0.25}$As/In$_{0.7}$Al$_{0.3}$As quantum well. It is found that the full width at half-maximum of the corresponding peak is proportional to the two-dimensional electron concentration and the luminescence intensity increases with increasing Hall mobility in the heterostructures. At photon energies $\hbar\omega$ in the range 0.8–1.3 eV corresponding to the recombination of charge carriers in the InAlAs barrier region, some features are observed in the photoluminescence spectra. These features are due to the difference between the indium profiles in the smoothing and lower barrier layers of the samples. In turn, the difference arises from the different designs of the metamorphic buffer.