The paper focuses on growth of GaN thin films via plasma assisted molecular beam epitaxy on SiC/Si(111) substrate formed by atom-by-atom substitution method. A comparison with GaN film grown in the same process directly on Si(111) substrate is performed. The films were studied by ellipsometry, electron diffraction and Raman spectroscopy. The results of these measurements are used to compare polarity of the films, crystalline properties, quality and residual elastic stresses in GaN films grown on Si and SiC/Si substrates. It is shown that SiC buffer layer has a positive effect on the structural quality of growing GaN thin films. It was found that use of SiC/Si(111) substrate leads to formation of N-polar film, whereas Ga-polar film is formed on Si(111).