Photoreflectance Studies of AlGaN/GaN Heterostructures Containing a Polarisation Induced 2DEG

We have carried out photoreflectance measurements in order to determine the electric field strength FB within the unintentionally doped barriers of Ga-face polarity AlxGa1—xN/GaN heterostructures containing high-mobile polarisation induced 2DEGs. For the sample with both the lowest alloy composition (x = 0.06) and lowest sheet carrier density (ns = 1 × 1012 cm—2) the analysis of the FKO yields FB = 335 kV/cm at RT. FB decreases by 130 kV/cm when cooling down the structure to T = 5 K. In comparison, samples with larger x and ns values show lower field strength at RT but a similar temperature dependence. The results can be explained by a change of piezoelectric polarisation.