Characterization of free-standing GaN with thermal treatment

We have grown free-standing GaN by hydride vapor phase epitaxy. The free-standing GaN samples were annealed by rapid thermal annealing for 3 minutes at 600, 650, 700, 750, and 800$^{\circ}$C in vacuum. The crystal structure and quality of GaN samples were characterized by X-ray diffraction. We found that the full width half maximum of the (0002) and (10-12) of GaN samples are decreased significantly after thermal annealing. The (0002) rocking curve of GaN sample was 218.8 arc-sec without thermal annealing, and it became 49.5 arc-sec after annealing at 800$^{\circ}$C. The FWHM of the (10-12) rocking curve was 113.5 arc-sec without thermal annealing, and it reduced to 58.3 arc-sec after annealing at 800$^{\circ}$C. According to the observation of atomic force microscopy, we found that the dislocation density of annealed GaN samples (7.04x10$^{6}$/cm$^{2})$ is smaller than that of pre-annealed GaN samples (1.02x10$^{7}$/cm$^{2})$. The optical properties of the samples by photoluminescence measurement which showed that the sample annealed at 650$^{\circ}$C had the best quality due to its narrowest FWHM at 3.4eV. According to these analytic results, we found that thermal annealing treatment could improve the quality of free-standing GaN.