AlGaN/GaN and InGaN/GaN Heterojunction Configurations
Theoretical Calculations of Polarization Effects
Semiconductors such as GaN, AlN, and InN are polar materials since they induce net polarization at the heterojunction interface due to the shift in the cation and anion sublattices. In unstrained zinc-blende structures, the cation and anion sublattices are arranged in such a way that there is no net polarization in the material. However, in the wurtzite crystal, spontaneous polarization is produced in the crystal due to the asymmetric lattice structure and ionicity of the bonds. Strain can cause a relative shift between the cation and anion sublattices and hence create net polarization at the heterointerface which is called piezoelectric polarization.
These high polarization and their resulting electric fields produce high interface charge density at group-III-nitride interfaces and spatial separation of the electron and hole wave functions in GaN-based quantum well structures. If the polarization-induced sheet charge density is positive, free electrons will tend to compensate the polarization-induced charge. These electrons will form a 2DEG of a certain sheet carrier concentration, assuming that the band offset in the group-III-nitride interfaces is reasonably high and that the interface roughness is low. A negative sheet charge density will at the same time cause the hole accumulation in other interfaces due to charge neutralization.