Detection of Interstitial Ga in GaN

We report the direct detection of interstitial Ga by optical detection of electron paramagnetic resonance (ODEPR) in the photoluminescence of $n$-type GaN after irradiation in situ at 4.2 K with 2.5 MeV electrons. It is stable upon annealing until room temperature, where it becomes mobile and trapped to form a new defect which is observed to emerge as the interstitial disappears. The time constant of the process at room temperature is $\sim 200\min$. The emergence of another ODEPR center beginning at $\sim 135\mathrm{K}$ suggests even easier migration of one of the other intrinsic defects in the GaN lattice.