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(111)

We have studied the effect of the III/V ratio and substrate temperature on the growth of GaN and AIN films on Si(111) substrates by molecular beam epitaxy, where active nitro 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 low-temperature 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 era-rich films exhibit weaker and broader emissions. For increasing substrate temperatures above 700 degrees 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 AIN films shows that the desorption of Al in terms of growth rate is not relevant for the substrate temperature range studied (850-920 degrees C). III/V ratios close to the stoichiometric value and substrate temperatures above 900 degrees C lead to high-quality AIN layers on Si(111) substrates. Complete relaxation is reached, for both GaN and AlN, in films with thicknesses well below 1 mu m.