The heteroepitaxial growth of (0001) GaN thin films directly on (111) Si by laser-molecular beam epitaxy without the formation of a SiNx interlayer at the GaN/Si interface is reported. We also find that SiNx can be formed subsequently as a result of nitrogen diffusion to the GaN/Si(111) interface. The orientation relationship of GaN on Si(111) was determined using x-ray diffraction and selected area electron diffraction. The atomic structure of the interfaces was studied by high resolution transmission electron microscopy. A Fourier filtered image of the cross-sectional GaN/Si(111) interface demonstrated domain matching epitaxy of 6:5. Distributions of N and Ga concentrations near the GaN/Si interface were determined using electron energy loss spectroscopy.