ELECTROREFLECTANCE STUDIES OF THIN GAAS ALGAAS QUANTUM-WELL STRUCTURES WITH TUNABLE 2DEG DENSITIES/

We report the results of low-temperature electroreflectance (ER) and p hotoluminescence (PL) investigations on thin n-type modulation-doped G aAs/AlGaAs single quantum well structures. The density of the two-dime nsional electron gas (2DEG) is varied continuously from zero to N(s) a pproximately 610(11) cm-2, using a Schottky gate, and is detected opt ically via the Stokes shift between the ER and PL subband transitions. For the first time, the splitting between transitions involving the f irst electron subband and the first heavy and light hole subbands, res pectively, is studied as a function of the 2DEG density using ER spect roscopy. The results are discussed in comparison with the calculated i nplane dispersion of the hole subbands. Clear evidence is given for th e influence of exciton screening effects.