A FRANZ-KELDYSH MODEL FOR PHOTOREFLECTANCE FROM GAAS/GAALAS HETEROJUNCTION STRUCTURES

A model is described for the simulation of photoreflectance spectra from GaAs/GaAlAs heterojunctions. The Franz-Keldysh theory is used to obtain the field-induced modulation of optical properties within the structure, and a transfer matrix method is applied to calculate the modulation of reflectances in the presence of heterojunctions and spatially varying electric fields. The photoreflectance results of Sydor et al. [J. Appl. Phys. 67, 7423 (1990)] on structures of GaAlAs layers grown on GaAs and capped with GaAs were interpreted with this approach. It is shown that in the presence of a 3-mu-m GaAlAs layer, rapid interference oscillations are both predicted and observed in the spectrum from the lower GaAs layer, preventing easy characterization of internal electric fields. However, comparison of simulated spectra with experimental results does allow estimates of the dc electric fields in the two layers to be made, and also enables the strength of the ac modulation in the two layers to be compared. Photoreflectance from a high electron mobility transistor structure has also been modeled. Calculations reveal, in addition, that electro- and photoreflectance spectra from multilayer structures can be very sensitive to the angle of incidence used in measurements, the presence or absence of cap layers, and the optical properties of the component semiconductors.