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Thin film GaN growth on c-plane sapphire (0001) substrate using two-step method by nitrogen plasma assisted MBE is reported. After growth optimization, a low temperature buffer layer was grown followed by the main GaN thin film. The Optical and structural properties of the film were investigated using XRD, Raman spectroscopy, Photospectrometry and in-Situ RHEED. The polycrystalline nature of the grown film containing both wurtzite as well as cubic structures was obtained through XRD. The observed diffused hallo rings in the RHEED pattern confirms polycrystalline structures of the film. Optical band-gap was calculated using spectrophotometry transmission curves. Whereas, the existence of h-GaN and c-GaN phase's were validated by employing high resolution Raman spectroscopy. These results shows that the LO and TO active vibration modes of GaN were due to wurtzite and cubic phases into p-region.