Opto-electronic studies of semiconductor tunnelling structures and quantum wells

This thesis describes optical and electrical studies of two groups of low dimensional semiconductor structures. The majority of the work concerns an optical study of electron populations (n1,n2) in the ground (E1) and first excited (E2) states of the quantum well (QW) regions of various GaAs-AlGaAs double and triple barrier resonant tunnelling structures (DBRTS and TBRTS respectively). The relative populations of the E2 and E1 confined states are determined from analysis of the photoluminescence (PL) intensities. When the structures are biased for tunnelling into the E2 state, rate equation analysis shows that n2/n1 is approximately equal to τi/τ1, where τi is the E2-E1 intersubband scattering time and τ1 is the tunnelling-out time from E1. The relative electron populations are thus expected to be a sensitive function of the QW and collector barrier widths. The second group of structures studied in this thesis consists of a series of shallow GaAs-AlyGa1-yAs multi-quantum well structures, with aluminium concentrations of 1 to 4.5%. The 1s-2s exciton energy separation is measured from PL excitation (PLE) experiments and is combined with a small correction for the 2s binding energy to give an accurate determination of the exciton binding energy (Ex). It is found that even in these very shallow QW structures there is a marked enhancement in the value of Ex over that in 3D GaAs