Quantum Hall effect devices based on AlGaAs/GaAs structures studied by photoreflectance spectroscopy

The optical properties of MBE-grown quantum Hall effect devices have been studied by photoreflectance spectroscopy (PR) at different temperatures. The room temperature PR spectra show two typical signals around 1.42 and 1.85 eV attributed to the energy band edges of GaAs and AlGaAs, respectively. Between the former band edge transitions broad oscillations (BO) are observed. 15 K PR spectra from samples and photoluminescence spectroscopy reveal that a component of the BO is originated from the surface quantum well (SQW) created by the AlGaAs–GaAs–vacuum discontinuity. This signal experience a blue shift when the GaAs is gradually etched-off in good agreement with the PR spectra behavior from SQWs previously reported. Besides, PR measurements were performed at low temperatures. In this process, the near-surface band bending of the structures is virtually flattened during the cooling procedure.We observed a signal whose intensity decreased as the temperature is lowered. Therefore, the former results suggest that BO consist in two superimposed signals coming from the SQW and the electric field cap region.