Molecular Beam Epitaxial Growth and Correlation Between Electrical and Optical Properties of Modulation - Quantum Wells

For this work, several lattice-matched AlGaAs/GaAs and pseudomorphic AlGaAs/InGaAs/GaAs MDQWs were grown by molecular beam epitaxy (MBE) under different growth conditins. The best pseudomorphic Al_{0.2}Ga _{0.8}As/In_ {0.2}Ga_{0.8} As/GaAs MDQWs with a 125 A well thickness had 77 K mobilities > 28,000 cm^2 /V-s, and the best AlGaAs/GaAs double-interface MDQWs with a well of thickness of 105 A had 77 K mobilities > 92,000 cm^2/V -s. These figures are comparable to the best figures reported in the literature for similar structures. The MBE growth conditions (mainly the As/Ga incorporation ratio and the growth temperatures) as well as the undoped spacer thickness were systematically varied to yield sets of samples with different carrier densities and mobilities in both material systems. The correlations between the optical and electrical properties of MDQWs in both material systems are quite similar. At low temperatures (4.2 K), free carrier induced broadening of the PL spectrum is influenced by the material quality of the structure. The PL linewidths depend on both the sheet carrier density and the material quality of the structure. At 77 K, differences in the material quality do not affect the PL linewidth, and the PL linewidth is an excellent measure of the sheet carrier density. The ratio of 77 K to 4.2 K PL linewidths provides a good measure of the crystalline quality. A simple theoretical model was developed to account for these correlations between phototluminescence spectra and the sheet carrier density and mobililty obtained from Hall measurements. The photoluminescence peak energy exhibited a dependence on the free-carrier density. There was a smooth reduction in the PL transition energy with increasing carrier density in both material systems. This is a consequence of bandgap renormalization due to many-body effects in these degenerate 2-dimensional electron gas systems. Rapid thermal annealing of these MDQW structures produced interesting effects. As a consequence of Si auto-compensation in the doped AlGaAs region, the free carrier density, and consequently, the PL linewidth and transition energies are changed. The linewidth decreases and the transition energy increases with increasing anneal time. This behavior is very unlike undoped quantum wells in which changes in transition energy and linewidth imply layer mixing.