The role of O as a shallow donor and a DX-state in GaN was elucidated by recent Raman experiments under hydrostatic pressure and the findings of first principles calculations. A pressure induced transfer of electrons from a shallow donor state to a deep DX-like state of the same donor can be correlated with vibrational gap modes by monitoring the freeze-out dynamics. Both features were unique to O doped GaN and could not be observed in Si doped material. The gap modes can be well explained by a linear chain model of impurity vibrations of substitutional O on the N site. A mode variation, and switching steps in its pressure behavior, which occurred in parallel to the carrier freeze-out were proposed to reflect three different charge states of the strongly localized states of O. This DX-type behavior as well as the experimental threshold pressure values were in excellent agreement with the theory results.

DX-Like Behavior of Oxygen in GaN. C.Wetzel, H.Amano, I.Akasaki, J.W.Ager, I.Grzegory, B.K.Meyer: Physica B, 2001, 302-303, 23-38