Group III-nitride based hetero and quantum structures

The present paper attempts an overview of a presently very active research field: the III-nitrides and their interesting possibilities for a range of device applications employing heterostructures and low-dimensional quantum structures. The family of materials containing AlN, GaN, InN and the alloys between them span a range of direct bandgaps between 6.2 and 1.9 eV, with very large band offsets in type I heterojunctions, which is very favourable for a number of interesting device concepts. A very important feature of these materials is the dominant influence of strong polarisation fields (spontaneous as well as piezo-electric) on the physical properties of multilayer structures, as well as on devices. Exciton binding energies are large, and excitonic effects are therefore important at room temperature. Many alloy systems, in particular InGaN, have a high miscibility gap, leading to a strong tendency for phase separation and consequently to many novel physical properties which yet have to be explored in detail. Localization effects for carriers and excitons are very important in quantum structures based on these alloys. Devices based on III-N heterostructures cover a wide range, from optical devices (violet lasers, LEDs covering a range from UV to red, white LEDs, photodetectors, UV cameras) to high-frequency power devices, both unipolar transistors (AlGaN/GaN HEMTs) and bipolar HBTs.