Investigations of AlGaN/GaN field-effect transistor structures by photoreflectance spectroscopy

Undoped AlGaN/GaN heterostructures with different content of AlGaN layer have been investigated by photoreflectance (PR) spectroscopy. PR resonances associated with the absorption in both GaN and AlGaN layers have been observed. The observed AlGaN-related transition has been identified as a band-to-band absorption in the layer being in under a strong built-in electric field. The electric field has been determined from the period of Franz-Keldysh Oscillations (FKOs) typical of layers being in a built-in electric field. It has been shown that the internal electric field in AlGaN layer increases by 21% (from ∼240 to 290 kV/cm) with the rise of Al mole fraction from 20 to 27%. This phenomenon has been attributed to an increase in the surface potential for AlGaN layer due to the rise of Al mole fraction. In addition, PR signal related to GaN has been analyzed. It has been shown that the GaN-related part of PR spectrum exhibits FKOs, besides, sharp PR features which could be associated with an excitonic absorption far to the AlGaN/GaN, i.e. in the part of GaN layer being in weak internal electric field. The GaN-related FKOs have been attributed the band-to-band absorption in the GaN layer being in strong internal electric field, i.e. in the part of GaN layer close to the AlGaN/GaN interface. It has been determined that this field is in the range of 220-230 kV/cm.