Photoreflectance spectra from GaAs HEMT structures reinvestigated: solution of an old controversy

An old controversy concerning the origin of the short-period oscillations i n the photoreflectance (PR) spectra of GaAs high electron mobility transist or (HEMT) structures is solved. Combining experimental and theoretical PR s tudies, it is demonstrated that these oscillations are not generated in the channel region, but are due to an ionized acceptor plane at the buffer/sub strate interface which causes a weak homogeneous electric field in the buff er layer. The interface sheet charge density is obtained from the PR oscill ation period. The utilization of PR spectroscopy for measuring charge densi ties at interfaces is of interest beyond the scope of this work.