Cyclotron-resonance study of nonparabolicity and screening in GaAs-${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{Al}}_{\mathrm{x}}$As heterojunctions

Cyclotron resonance has been used to study the frequency, electron-concentration, and temperature dependences of the effective mass in GaAs-${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Al}}_{\mathrm{x}}$As heterojunctions. In samples of low carrier concentration the energy dependence of the low-temperature mass has been measured in the quantum limit, and the variation of the mass with Fermi energy at low fields has been studied in samples of various electron concentrations and by the use of persistent photoconductivity. The results give an energy dependence of the effective mass which is ∼20% lower than that measured in bulk GaAs and which is accounted for by five-band k⋅p theory only, consistent with a dramatic reduction in the polaron coupling due to screening. Further evidence of the importance of screening comes from the temperature dependence of the effective mass, which shows a rapid increase of order 2% as the temperature is raised to ∼100 K. This is attributed to the reappearance of the polaron coupling as the screening is reduced by thermal broadening of the carrier distribution.