Pressure Induced Deep Gap State of Oxygen in GaN

O and Si donors in GaN are studied by Raman spectroscopy under hydrostatic pressure p. The ground state of O is found to transfer from a shallow level to a deep gap state at p>20GPa reminiscent of DX centers in GaAs. Transferred to AlₓGa_1-xN we predict that O induces a deep gap state for $x>0.40$. In GaN:Si no such state is induced up to the highest pressure obtained (p0ex0ex=0ex0ex25GPa) equivalent to x0ex0ex=0ex0ex0.56 in AlₓGa_1-xN and possibly higher. We attribute this distinction to the lattice sites of the dopants. O substituting for N is found to be the origin of high free electron concentration in bulk GaN crystals.