We have investigated growth mechanisms of GaN on Si (1 1 1) by pulsed laser deposition (PLD) using reflection high energy electron diffraction (RHEED) and X-ray photoelectron spectroscopy (XPS). We have found that the use of the AlN buffer layer dramatically improves the crystal quality of GaN. RHEED observations have shown that the epitaxial growth of the buffer layer starts with a two-dimensional mode followed by island formation. XPS measurements have revealed that the AlN/Si hetero-interface has no SixNy interfacial layer. We have also found that the Si atoms in N sites are segregated at the surface of the AlN buffer layer.