Photoreflectance studies of N- and Ga-face AlGaN/GaN heterostructures confining a polarisation induced 2DEG

Photoreflectance measurements have been carried out in order to determine the electric field strength F within the topmost layers of Ga-face polarity Al_xGa_1-x/GaN and N-face polarity GaN/Al_xGa _1-x/GaN heterostructures containing high-mobile polarisation induced 2DEGs. For both types of samples F decreased from 400 kV/cm at room temperature up to 200 kV/cm when cooling down the structure to T = 5 K. Our results strongly emphasise the existence of surface donor and surface acceptor states of the Ga- and N-face samples, respectively. The temperature dependence of F is explained by the change of the piezoelectric and spontaneous polarization. From self-consistent conduction band calculations the bare surface potential was obtained.