The effect of the III/V ratio and substrate temperature on the morphology and properties of GaN- and AlN-layers grown by molecular beam epitaxy on Si(1 1 1)

Abstract

We have studied the effect of the III/V ratio and substrate temperature on the growth of GaN and A1N films on Si(1 1 1) substrates by molecular beam epitaxy, where active nitrogen was generated by a radio frequency plasma source. In the case of GaN, two distinct regimes of growth (Ga-rich and N-rich conditions) lead to different crystal morphologies and luminescence properties. Scanning electron micrographs of the cleaved edges of films grown under highly N-rich conditions reveal columnar features, while growth under Ga-excess results in compact layers. The lowtemperature photoluminescence associated with the N-rich films is dominated by intense and narrow exciton lines, with peaks having full-width at half-maximum of less than 2 meV, whereas the Ga-rich films exhibit weaker and broader emissions. For increasing substrate temperatures above 700°C, stoichiometry is reached at higher Ga/N ratios, pointing to an enhancement of Ga desorption characterized by an activation energy of 2.5 eV. A similar study of A1N films shows that the desorption of A1 in terms of growth rate is not relevant for the substrate temperature range studied (850–920°C). III/V ratios close to the stoichiometric value and substrate temperatures above 900°C lead to high-quality A1N layers on Si(l 1 1) substrates. Complete relaxation is reached, for both GaN and A1N, in films with thicknesses well below 1 μm.