Optical Properties and Band Alignments of III-V Heterostructures

Abstract

Photoluminescence (PL) and photoluminescence excitation (PLE) studies have been made of both short period, m=n≤8, (GaAs)_m−(A1As)_n superlattices (SL) and pseudomorphic InGaAs-GaAs structures. Systematic investigations of the electronic properties, band alignments and superlattice effects highlight important differences between these systems arising from the nature of the lowest conduction band (CB) state and the resulting subband dispersion in thin samples. In (GaAs)_m (AlAs)_n SLs PL and PLE measurements reveal that for m≥n4 the lowest CB state is a folded X_Z minimum. PLE spectroscopy also provides information about the higher energy direct Γ-Γ transitions. The observed energy gaps are compared with a simple Kronig-Penney description of the electronic states and the limits of the model as the layer thicknesses are decreased are explored. Optical data is also reported on both a 25Å In_xGa_l−xAs-400Å GaAs quantum well (QW) structure and on a 25Å In_xGa_l−xAs-100Å GaAs SL structure (x=0.12). Low temperature PL spectra exhibit sharp peaks and in the PLE spectrum of the QW sample the n=1 heavy-hole exciton peak is clearly resolved from the onset of the excited states and the continuum edge. These observations provide the first direct measurement of the exciton binding energy in a strained sample and for a thickness at which we expect the binding energy (as a function of QW width) to have gone through a maximum. In the 5 well SL sample, there is significant overlap of the electron and hole wavefunctions of adjacent wells, leading to a mixing and a splitting of the ground state into 5 discrete levels. The predicted splittings are observed in the PLE spectrum of this sample.