A method and apparatus is provided for preparing a substrate for forming electronic devices incorporating III/V compound semiconductors. Elemental halogen gases, hydrogen halide gases, or other halogen or halide gases, are contacted with liquid or solid group III metals to form precursors which are reacted with nitrogen sources to deposit a nitride buffer layer on the substrate. The buffer layer, which may be a transition layer, may incorporate more than one group III metal, and may be deposited with amorphous or crystalline morphology. An amorphous layer may be partially or fully recrystallized by thermal treatment. Instead of a layer, a plurality of discrete nucleation sites may be formed, whose size, density, and distribution may be controlled. The nitrogen source may include reactive nitrogen compounds as well as active nitrogen from a remote plasma source. The composition of the buffer or transition layer may also vary with depth according to a desired profile.