Strain relaxation correlated with the transport properties of AlN/GaN heterostructure grown by plasma-assisted molecular-beam epitaxy

We report on the transport properties of two-dimensional electron gases confined at the AlN/GaN heterointerface grown by plasma-assisted molecular-beam epitaxy on (0001) sapphire substrates. Two-dimensional (2D) sheet carrier density has been found to vary with the AlN barrier layer thickness due to the existence of strain-induced piezoelectric polarization field at the heterointerface. The highest 2D sheet carrier density achieved was 2.3x10(13) cm(-2) for a 35-Angstrom-thick AlN barrier layer. Further, with the increase of the AlN barrier width about 50similar to60 Angstrom, a drop in 2D sheet carrier density was noticed due to the annihilation of piezoelectric polarization, caused by the partial tensile-strain relaxation of the AlN barrier layer. Inconsistently, the 2D electron mobility was high: about 853 cm(2)/V s. The interface roughness of the heterostructure was estimated to be 6 Angstrom rms for a 50-Angstrom-thick AlN barrier layer, using grazing incidence x-ray reflectivity studies. (C) 2003 American Institute of Physics.