Below bandgap transitions in an AlGaN/GaN transistor heterostructure observed by photoreflectance spectroscopy

摘要

Photoreflectance (PR) spectroscopy has been applied to study the optical transitions in an AlGaN/GaN transistor heterostructure at room temperature. A strong PR feature at ～3.37 eV has been observed besides PR features related to the band-to-band absorption in GaN and AlGaN layers. This feature has been attributed to an electron transition between the valence band and a donor-like state located ～50 meV below the conduction band. Such a transition is possible to observe in absorption-type experiment (such as PR spectroscopy) at room temperature because the donor-like state is ionized by the strong electric field existing in GaN layer. The existence of the electric field with a magnitude of ～233 kV/cm has been confirmed by the observation of GaN-related Franz-Keldysh oscillations in the PR spectra.