It was recalled that free electron concentrations of 1019 to 1020/cm3 in nominally undoped material had been attributed to intrinsic defects because no chemical impurity had been found at such high concentrations. According to theoretical models, a N vacancy could act as a donor. However, its formation energy was very large in n-type material. The nature of this unidentified donor was investigated using high hydrostatic pressures. The results which were obtained by using infra-red reflection and Raman scattering methods indicated strong evidence for the localization of free carriers by large pressures. The carrier density decreased by 2 orders of magnitude between 20 and 30GPa.

C.Wetzel, W.Walukiewicz, E.E.Haller, J.W.Ager, A.Chen, S.Fischer, P.Y.Yu, R.Jeanloz, I.Grzegory, S.Porowski, T.Suski, H.Amano, I.Akasaki: Materials Science Forum, 1995, 196-201, 31-6