Metal-insulator transition in GaN crystals

By means of high-pressure Raman spectroscopy of coupled LO phonon-plasmon modes, a qualitatively new model of the metal-insulator transition observed in bulk GaN crystals at pressure around 20 GPa is proposed. This transition was interpreted recently as caused by the emergence of the localized donor state into the energy gap of GaN. Our new experimental evidence suggests that two types of donors can supply electrons to the bulk crystal: the first of a localized character and the second of a shallow character with contributions to the electron concentration of 5 x 10(19) and 3 x 10(18) cm(-3), respectively. The localized state carl be associated with the nitrogen vacancy, while the shallow state can be tentatively attributed to impurities such as oxygen. A number of interesting phenomena take place beyond the metal-insulator transition in bulk GaN and an attempt to explain them will be made in the present work.