Pressure Induced Deep Gap State of Oxygen in GaN

Abstract

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 $p>20\mathrm{GPa}$ reminiscent of DX centers in GaAs. Transferred to ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}$ 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 $(p\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}25\mathrm{GPa})$ equivalent to $x\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.56$ in ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}$ 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.