O. Ambacher et al., Role of spontaneous and piezoelectric polarization induced effects in group-III nitride based heterostructures and devices, PHYS ST S-B, 216(1), 1999, pp. 381-389
The wurzite group-III nitrides InN, GaN, and AlN are tetrahedrally coordina
ted direct band gap semiconductors having a hexagonal Bravais lattice with
four atoms per unit cell. As a consequence of the noncentrosymmetry of the
wurzite structure and the large ionicity factor of the covalent metal-nitro
gen bond, a large spontaneous polarization oriented along the hexagonal c-a
xis is pre dieted. In addition, group-III nitrides are highly piezoelectric
. The strain induced piezoelectric as well as the spontaneous polarizations
are expected to be present and to govern the optical and electrical proper
ties of GaN based heterostructures to a certain extent, due to the huge pol
arization constants which are one of the most fascinating aspects of the ni
trides. In this paper we will present theoretical and experimental results
demonstrating how polarization induced electric fields and bound interface
charges in AlGaN/GaN, InGaN/GaN and AlInN/GaN heterostructures lead to the
formation of two-dimensional carrier gases suitable for the fabrication of
high power microwave frequency transistors.