AlGaN/GaN heterostructures were grown by metalorganic vapour phase epitaxy (MOVPE) on the c-plane sapphire substrate. The sample consists of low temperature GaN buffer layer, high temperature 1.5 μm GaN layer, and 30 nm thick Al_xGa_1-xN layer. All these layers are nominally undoped. For the set of samples the Al content changes from 20 % to 35 %. The electron concentration in the structure was determined by the impedance spectroscopy method. We have found that for investigated structures the two dimensional electron gas (2DEG) concentration at the interface was in the 2.0-3.0 10¹² cm^-2 range. Optical properties of AlGaN/GaN heterostructures and a presence of the 2DEG have been investigated by both absorption-like (photoreflectance PR) and emission-like (photoluminescence PL) experimental techniques. In photoreflectance, spectral features associated with AlGaN barrier, the 2DEG at the interface, and bulk GaN were observed. PL spectra were measured at two combinations: first was excitation through AlGaN layer, and second was excitation through GaN layer. In these two combinations different PL spectra have been observed. In the first case PL spectrum was dominated by transitions from AlGaN layer, and in the second case features associated with GaN layer and 2DEG are observed. All PL and PR features change with the increase of Al content. The nature of absorption and emission transitions and their temperature dependence was studied and discussed in this work.

Corresponding author: Fax: +48-71-3283696; E-mail address: robert.kudrawiec@pwr.wroc.pl (R. Kudrawiec).