Journal of Applied Physics vol:100 issue:2 pages:1-6
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 beta(-) particles emitted by the radioactive isotopes Ca-45 (t(1/2)=163.8 d) and Sr-89 (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 similar to 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 degrees C 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 degrees C, while the substitutional fraction remained nearly constant. The annealing behavior of the rms displacements can explain why annealing temperatures above 1100 degrees C 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. (c) 2006 American Institute of Physics.