Carrier Localization in Gallium Nitride

In wide bandgap GaN a large number of interesting and important scientific questions remain to be answered. For example, the large free electron concentration reaching 1019 to 1020 cm-3 in nominally undoped material are ascribed to intrinsic defects because no chemical impurity has been found at such high concentrations. According to theoretical models a nitrogen vacancy acts as a donor but its formation energy is very large in n-type materials making this suggestion controversial. We have investigated the nature of this yet unidentified donor at large hydrostatic pressure. Results obtained by infrared reflection and Raman scattering indicate strong evidence for localization of free carriers by large pressures. The carrier density is drastically decreased by two orders of magnitude between 20 and 30 GPa. Using several techniques we provide independent evidence for results contained in earlier reports and present the first quantitative analysis. A possible interpretation of this effect in terms of the resonant donor level is presented.