Photoreflectance investigations of a donor-related transition in AlGaN/GaN transistor structures

Below-bandgap photoreflectance (PR) features observed for GaN layers and undoped AlGaN/GaN transistor structures have been analyzed in this letter. In addition to PR signal associated with the interference oscillations a strong PR feature at similar to 3.37 eV has been resolved for some AlGaN/GaN structures. This feature has been attributed to an electron transition between the valence band and a donorlike state located similar to 50 meV below the conduction band. An absorptiontype experiment, such as PR spectroscopy, makes it possible to observe such a transition because this donorlike state is ionized by the strong internal electric field existing in the GaN layer at the AlGaN/GaN interface. The existence of this electric field with a magnitude of similar to 210 kV/cm has been confirmed by the observation of GaN-related Franz-Keldysh oscillations in the PR spectra. Obtained results show that donorlike states located similar to 50 meV below the conduction band are one of the sources of high concentration of the two dimensional electron gas in undoped AlGaN/GaN transistor structures. (C) 2005 American Institute of Physics.