Observation of high junction electric fields in modulation-doped GaAs/AlGaAs heterostructures by room temperature photoreflectance spectroscopy

Room temperature photoreflectance (PR) was utilized to determine the junction electric field in molecular-beam epitaxy (MBE)-grown modulation-doped GaAs/AlGaAs heterostructures (MDH) with varied spacer layer thickness and doping concentration. The PR spectra exhibited Franz–Keldysh oscillations (FKOs) at energies above the GaAs band gap. Linear regression from the energy plots of the FKO extrema allowed for the calculation of the junction electric field strengths. High junction electric fields of magnitude 270–430 kV/cm were obtained from the samples. These measured fields were almost a factor of two higher than what can be accounted for by the measured carrier concentration. The presence of impurities introduced during the growth process, which could account for the added field strength, was checked by photoluminescence (PL) spectroscopy at 10 K. The similar PL spectra of the samples with different electric field strength ruled out carbon as the source of added field observed. Deep level transient spectroscopy (DLTS) measurement performed on MBE-grown Si-doped GaAs revealed electron traps corresponding to M4 and EL2. M4 is an impurity-related trap involving As vacancies while EL2 is an As-antisite trap. Although DLTS was not performed directly on MDH samples, results in the doped GaAs indicate As-related electron traps as the possible cause of the added field observed by PR.