High-pressure studies of semiconductors in the far-infrared: Donor states in quasi-2D

We review recent experimental advances by the Buffalo group in performing f ar-infrared magnetospectroscopy under fine tuning of applied high hydrostat ic pressure. Experiments are reported for the effects of pressure on Si don ors in modulation doped GaAs/AlGaAs quantum wells. We clearly observe press ure-mediated competition between free (i.e., Landau level) and bound electr on states - the latter arising from both neutral (D-o) and charged (D-) don or species. With increasing pressure, there is a progression of the observe d spectra from being dominated by cyclotron resonance and the D- singlet (o r singlet-like bound magnetoplasmon) transitions, to showing the D-o 1s --> 2p(+) line. The main reason for this evolution is the decrease in electron s due to the crossover of the Si levels associated with the Gamma (well) an d X (barrier) conduction minima. Indeed, for pressures above 30 kbar the Ga mma (well)-X(barrier) crossover quenches all the transitions. However, we f ind strong evidence that electrons are independently lost to a trap, which becomes active several kbar below this crossing. A possible candidate for t his trap is residual Se impurities in the barriers. We present the results of detailed numerical calculation which are found to agree very well with t he measured field dependencies of the cyclotron resonance, D-o and D- trans ition energies. In the sample with the highest doping, a new transition is observed for fields and pressures above 7.5 T and 5 kbar. Reasons for this apparent anomaly are discussed.