AlN epilayers are grown directly on sapphire （0001） substrates each of which has a low temperature AlN nucleation layer. The effects of pretreatments of sapphire snbstrates, including exposures to NH3/H2 and to H2 only ambients at different temperatures, before the growth of AlN epilayers is investigated. In-plane misoriented crystals occur in N-polar AlN epilayers each with pretreatment in a H2 only ambient, and are characterized by six 60°-apart peaks with splits in each peak in （1012） phi scan and two sets of hexagonal diffraction patterns taken along the [0001] zone axis in electron diffraction. These misoriented crystals can be eliminated in AlN epilayers by the pretreatment of sapphire substrates in the NH3/H2 ambient. AlN epilayers by the pretreatment of sapphire substrates in the NH3/H2 ambient are Al-polar. Our results show the pretreatments and the nucleation layers are responsible for the polarities of the AlN epilayers. We ascribe these results to the different strain relaxation mechanisms induced by the lattice mismatch of AlN and sapphire.
The effects of polarization and related structural parameters on the intersubband transitions of A1GaN/GaN multi- quantum wells （MQWs） have been investigated by solving the Schr6dinger and the Poisson equations self-consistently. The results show that the intersubband absorption coefficient increases with increasing polarization while the transition wavelength decreases, which is not identical to the case of the interband transitions. Moreover, it suggests that the well width has a greater effect on the intersubband transitions than the barrier thickness, and the intersubband transition wavelength of the structure when doped in the barrier is shorter than that when doped in the well. It is found that the influences of the structural parameters differ for different electron subbands. The mechanisms responsible for these effects have been investigated in detail.
