Carrier-concentation-dependent electron<i>–</i>LO-phonon coupling observed in GaAs-(Ga,Al)As heterojunctions by resonant-polaron cyclotron resonance

Abstract

The cyclotron resonance (CR) of the two-dimensional electron gas (2D EG) in GaAs-(Ga,Al)As heterojunctions has been studied in the resonant-polaron regime for 2D carrier densities ${N}_{s}$ in the range (0.8--5.4)\ifmmode\times\else\texttimes\fi{}${10}^{11}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$. A reflectivity technique has allowed the CR to be recorded at energies up to 35.63 meV, within the GaAs reststrahlen band, and a calculation of the dielectric response of the complete heterostructure has enabled the effective masses to be reliably evaluated from the line shapes and positions of the resonances. The results indicate that the 2D electrons are coupling to the LO phonon (36.7 meV), in agreement with theoretical predictions. At low carrier densities, the resonant-polaron contribution to the effective mass becomes apparent at cyclotron energies above 25 meV, and increases in size as the LO-phonon energy is approached: however, this mass enhancement is removed rapidly as ${N}_{s}$ is increased, indicating the importance of Landau-level occupancy and screening in the 2D EG. Close to the LO-phonon energy, large shifts in the resonance position, which are several times the linewidth in size, are produced by varying ${N}_{s}$. This large ${N}_{s}$ dependence explains previous conflicting reports of ``enhanced'' or ``reduced'' polaron effects in the 2D electron gas. A comparison of the experimental results with existing memory-function calculations of the polaron contribution to the effective mass indicates that the greater part of the ${N}_{s}$ dependence can be ascribed to Landau-level occupancy effects.