Electrical properties and deep traps spectra of N-polar and Ga-polar AlGaN films grown by molecular beam epitaxy in a wide composition range

Abstract

The polarity type, surface morphology, electrical properties, and deep trap spectra were studied for undoped AlxGa1−xN films (x=0–0.6) grown by molecular beam epitaxy on on-axis (0001) sapphire using composite buffers consisting of low temperature (LT) AlN nucleation layer, AlN, and AlN/AlGaN superlattice. It is shown that the films grow with N-polarity if the LT AlN layer is deposited under N-rich conditions and with Ga-polarity for the LT AlN layers deposited under Al-rich conditions. For both polarities the film morphology was acceptable for fabrication of typical GaN-based devices. It is demonstrated that the Ga-polar AlGaN films are heavily compensated p-type, with the dominant acceptors believed to be due to C. N-polar films are n type, with the residual donors pinning the Fermi level being most likely due to Si. N-polar films show a high concentration of deep electron traps.