We report on the lattice location of ion-implanted Ca and Sr in thin films of single-crystalline wurtzite GaN. Using the emission channeling technique the angular distributions of β~- particles emitted by the radioactive isotopes ~(45)Ca (t_(1/2)=163.8 d) and ~(89)Sr (t_(1/2)=50.53 d) were monitored with a position-sensitive detector following 60 keV room-temperature implantation. Our experiments give direct evidence that ～90% of Ca and > 60% of Sr atoms were occupying substitutional Ga sites with root mean square displacements of the order of 0.15-0.30 A, i.e., larger than the expected thermal vibration amplitude of 0.074 A. Annealing the Ca implanted samples at 1100-1350℃ in high-pressure N_2 atmosphere resulted in a better incorporation into the substitutional Ga site. The Sr implanted sample showed a small decrease in rms displacements for vacuum annealing up to 900℃, while the substitutional fraction remained nearly constant. The annealing behavior of the rms displacements can explain why annealing temperatures above 1100℃ are needed to achieve electrical and optical activations, despite the fact that the majority of the acceptors are already located on Ga sites immediately after ion implantation.
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