Role of Spontaneous and Piezoelectric Polarization Induced Effects in Group-III Nitride Based Heterostructures and Devices

The wurzite group-III nitrides InN, GaN, and AlN are tetrahedrally coordinated 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–nitrogen bond, a large spontaneous polarization oriented along the hexagonal c-axis is predicted. 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 properties of GaN based heterostructures to a certain extent, due to the huge polarization constants which are one of the most fascinating aspects of the nitrides. 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.