  We report epitaxial growth and characterization of GaN thin films on sapphire
(0001) substrates using low temperature GaN intermediate layer by plasma
assisted molecular beam epitaxy (PA-MBE) technique. As grown and annealed GaN
thin films were studied by high- resolution X-ray diffraction (HRXRD), atomic
force microscopy (AFM), Hall Effect and photoluminescence (PL). It has been
found that there is a significant improvement in the quality of the GaN films
after annealing at 725 \degree C in terms of electron mobility, full width at
half maximum (FWHM) of omega ({\omega}) scan around (0002) XRD peak of GaN
films. Screw dislocation density obtained from the FWHM of GaN (0002) \omega
scan and etch pitch density calculated from AFM image are 6.4 \times 10^8
cm^{-2} and 5.1\times 10^8 cm^{-2} respectively. In PL measurement, FWHM of
near band edge (NBE) peak of GaN films has been found to be 30 meV.
