Photoreflectance study of pseudomorphic high electron mobility transistors

Room temperature photoreflectance (PR) measurements have been carried out on a series of pseudomorphic AlGaAs/InGaAs/AlGaAs high electron mobility transistor (HEMT) structures with different doping profiles grown by molecular beam epitaxy. The GaAs and AlGaAs features in the PR spectrum were studied and their origins were determined by a sequential etching method. It was found that the GaAs buffer layer and AlGaAs barrier layer dominate the GaAs and AlGaAs features in the PR spectrum, respectively. The electric fields for Franz-Keldish oscillations (FKOs) associated with the E0 transition of both GaAs and AlGaAs are affected by the doping on both sides of the InGaAs channel, but are not directly related to the channel carrier concentration. The aluminum composition in the AlxGa1−xAs layers was determined and compared with the results of FKO energy and the critical-point energy methods.