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 ∼3.37eV 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 ∼50meV 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 ∼210kV∕cm has been confirmed by the observation of GaN-related Franz-Keldysh oscillations in the PR spectra. Obtained results show that donorlike states located ∼50meV below the conduction band are one of the sources of high concentration of the two dimensional electron gas in undoped AlGaN∕GaN transistor structures.