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 $p>\mathrm{}20\mathrm{}\mathrm{GPa}$ reminiscent of DX centers in GaAs. Transferred to ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}$ we predict that O induces a deep gap state for $x>\mathrm{}0.40\mathrm{}$. In GaN:Si no such state is induced up to the highest pressure obtained $(p=25\mathrm{}\mathrm{GPa})$ equivalent to $x=0.56\mathrm{}$ in ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}$ 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.