Cyclotron-resonance study of polarons in GaAs-AlxGa1−xAs heterostructures

Cyclotron-resonance transmission and emission experiments were performed in the two-dimensional electron system of an ${\mathrm{Al}}_{\mathrm{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As-GaAs heterojunction using a nonresonant three-level excitation technique. It is found that in comparison to bulk GaAs, the polaron effect is reduced in the investigated energy range. The reduction of the polaron effect in the two-dimensional electron gas can be explained by occupation number effects. In addition, we determined the polaron effect in a hot-electron situation. At low electric fields the mass increase with energy is caused mainly by nonparabolicity. At electron temperatures above 100 K the polaron contribution to the effective mass reaches values observed in high-purity bulk GaAs.