Interference Effect on the Phase of Franz-Keldysh Oscillations in GaAs/AlGaAs Heterostructures

We have studied photoreflectance spectra of GaAs/Al0.3Ga 0.7As heterostructures from the viewpoint of the phase of Franz-Keldysh (FK) oscillations. The phase of the FK oscillations originating from the GaAs buffer layer shifts with the change of the Al0.3Ga 0.7As-layer thickness, while the period does not vary. The FK-oscillation phase does not depend on the pump power, which suggests that the phase shift is not caused by a difference in the magnitude of modulation. We propose a calculation model for FK oscillations that includes the interference of probe light. Photoreflectance spectra calculated on the basis of this model are compared with the measured spectra. We conclude that mixing of the real and imaginary parts of a modulated dielectric function, which is caused by the probe-light interference, gives rise to the phase shift of the FK oscillations. We also derive a novel analysis method for a linear plot of the extremum positions of FK oscillations based on the above line-shape model. The present method remarkably reduces ambiguity in the estimation of band-gap energy that is considerable in a conventional analysis.