Hole-state reversal and the role of residual strain in (In,Ga)As-GaAs superlattices

We use photoluminesence excitation (PLE) spectroscopy to investigate the electronic properties of ${\mathrm{In}}_{0.10}$ ${\mathrm{Ga}}_{0.90}$As-GaAs superlattice (SL) structures in which compressive and tensile strain are incorporated simultaneously in alternate layers. This is achieved by growing the SL on thick ${\mathrm{In}}_{0.05}$ ${\mathrm{Ga}}_{0.95}$As buffer layers, deposited on GaAs substrates. By varying the thickness of the alloy well layers, we demonstrate the predicted crossover of the lowest-energy emission from a direct, electron-to-heavy-hole transition to a spatially indirect (type-II) electron-to-light-hole transition. In addition, we use PLE spectroscopy, to characterize the strain relaxation in the ${\mathrm{In}}_{0.05}$ ${\mathrm{Ga}}_{0.95}$As buffer layers. The compressive strain in the alloy buffer layer produces a splitting between the light- and heavy-hole exciton peaks observed in the PLE spectra, and hence we measure directly the residual strain as a function of ${\mathrm{In}}_{0.05}$ ${\mathrm{Ga}}_{0.95}$As thickness. Even in layers that are several micrometers thick, we still see evidence for the presence of strain, suggesting that, at small strain energies, dislocations are an inefficient means of relieving strain on a macroscopic scale. Our PLE results are confirmed by x-ray-diffraction measurements on the same samples. The degree of residual strain determined by mapping the x-ray intensity is in excellent agreement with our PLE values. Residual compressive strain in the buffer layer produces significant modifications of the electronic properties of the overgrown SL layers, and must be taken properly into account in order to explain our experimental results.