The nitrides of Al, Ga and In are III-V compound semiconductors with properties more closely akin to those of the II-VI system and applications problems of similar type. All three have wide, direct band gaps and relatively light, therefore mobile, electrons. Less encouragingly, native point defects appear to play a significant role in both optical and electronic properties.
Both experiment and theory point to a triplet of donor states associated with the nitrogen vacancy, with deep compensating centres deriving from antisite defects. The ionic radius of the metal then seems to determine the conductivity of as-grown material, indium is reluctant to occupy nitrogen sites while aluminium does so readily and gallium is equivocal. Thus the upper donor level of InN is not depleted and n-type behaviour is always observed, the equivalent level in AIN is always overcompensated and the remaining donor levels are too deep to contribute free electrons at normal temperatures so that the material is consistently insulating. GaN may be n-type or semi-insulating since compensation ratios either side of unity appear to be possible, depending on the method of growth.
In this paper we review the evidence, both optical and electrical, for the existence, nature and energetic location of the four basic point defects in each nitride, noting in particular that all four broad luminescence bands in GaN:Zn can be accounted for by the presence of nitrogen on gallium sites and of nitrogen vacancies.