Single Heterostructures

In bulk material, neutralization of the enormous electric fields produced by the spontaneous polarization can be achieved by a relatively modest adjustment of the population of surface states. This is no longer possible in the case of inhomogeneous material exhibiting inhomogeneous spontaneous polarization, such as the AlGaN/GaN heterostructure. A layer of AlGaN grown epitaxially on GaN has not only a spontaneous polarization larger than that of GaN but it has, in addition, a piezoelectric polarization associated with the elastic strain inevitably present as a consequence of the lattice mismatch. No straightforward adjustment of the population of the surface states can eliminate the resultant fields. As a result, nitride heterostructures differ uniquely from heterostructures of cubic semiconductors in exhibiting large, built-in electric fields, and these result in the spontaneous formation of a quasi-2D electron gas at the AlGaN/GaN interface. In the cubic case such a gas must be produced by doping, thereby, ineluctably, introducing charged-impurity scattering even when the system is modulation doped. Doping is unnecessary in nitride structures so, in principle, enormous lowtemperature mobilities are possible and will, no doubt, be achieved as growth techniques evolve.

An estimate of the density of the polarization-induced electron population at the AlGaN/GaN interface can be obtained assuming the existence of a common branch-point Fermi energy and simple electrostatics.