PRESSURE-INDUCED DEEP GAP-STATE OF OXYGEN IN GAN

O and Si donors in GaN are studied by Raman spectroscopy under hydrost atic pressure p. The ground state of O is found to transfer from a sha llow level to a deep gap state at p >20 GPa reminiscent of DX centers in GaAs. Transferred to AlxGa1-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 highe st pressure obtained (p=25 GPa) equivalent to x=0.56 in AlxGa1-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 f ree electron concentration in bulk GaN crystals.