Hexagonal GaN layers were grown on (111)B GaAs substrates by gas-source molecular beam epitaxy using hydrazine as a source of nitrogen. A smooth and abrupt AlN/GaAs interface was prepared by nitridation of an AlAs buffer layer grown on clean GaAs(lll)B surface. This buffer layer is stable at growth temperatures above 700 degrees C. The AlN layer prepared on this buffer exhibits two-dimensional growth. The subsequent GaN and GaInN layers show a quasi-two-dimensional growth. The photo-and cathodoluminescence spectra of these samples show a narrow (similar to 160 meV) band-edge emission and the absence of the "yellow" defect band. Further narrowing (similar to 20 meV) of the edge emission in unintentionally In-doped GaN is ascribed to the presence of confined domains of Ga1-xInxN.