Filling factor dependent cyclotron mass in space-charge layers on GaAs.

We report frequency domain studies of cyclotron resonance in electron space-charge layers on GaAs. We observe variations of the cyclotron mass correlated with the filling factor. This is discussed in terms of conduction band nonparabolicity, spin- splitting of the Landau-levels and filling factor dependent screening of the electron optical phonon interaction. In samples with mobilities lower than 5x10 sup 5 cm sup 2 /Vs there are line shape anomalies with features of anti-level crossing that do not scale with the filling factor but depend on the electron density itself.