Characterization of GaN/AIN films with different polarities grown by molecular beam epitaxy on sapphire substrates

Wurtzite Ill-nitrides crystallize uniaxially and non-center-symmetrically. They exhibit large spontaneous and strain-induced polarization effects [1]. The large amount of polarization charge appearing at their heterointerfaces and the strong electric fields associated with it is unique to Ill-nitride heterostructures and has a dramatic effect on their optical and electrical properties. When a GaN film is grown on the c-plane of sapphire substrates by molecular beam epitaxy (MBE), it does not share the same atomic stacking order with sapphire. Consequently, the crystal direction [0001] of a GaN film can be either parallel or anti-parallel to the growth direction, leading to epilayers with two different polarities, Ga-and N-polar films. Investigations have shown that these two polar films have vastly differing growth, surface, and other properties [2]. Therefore, proper control of the film polarity during growth, as well as characterization of the materials with different polarities, are very important considerations in the application of these materials.