Molecular beam epitaxy growth and doping of III-nitrides on Si(1 1 1): layer morphology and doping efficiency

Wurtzite GaN layers grown by plasma-assisted molecular beam epitaxy on Si(1 1 1) reveal strong morphology changes as a function of the III/V ratio. For nominally N-rich conditions, GaN nanocolumns are reproducibly grown with diameters ranging from 600 to 1500 Å. These nanocolumns are fully relaxed from lattice and thermal strain, having a very good crystal quality characterized by strong and narrow (2 meV) photoluminescence excitonic lines at 3.472–3.478 eV. The nanocolumns generate from a reduced Ga adatoms diffusion due to the excess nitrogen (Ga-balling). Si-doping yields 2×1019 and 8×1019 electrons cm−3 in compact GaN and AlGaN (up to 45%) layers, respectively. In addition, Si-doping decreases the threading dislocation density while enhancing the layer biaxial tensile strain. P-type doping with Be, Mg and C is analyzed and compared. Carbon shows a low solubility according to theoretical predictions. Mg-doping is efficient leading to hole densities in the mid 1017 cm−3 range. Be is the shallowest acceptor level (90–100 meV), but its efficiency is hampered by the generation of deep Be-related traps, most probably VGa–Bei complexes, and by self-compensation by Bei. Positron annihilation spectroscopy results; the effect of Mg when codoping with Mg+Be, and the presence of a strong Be-related yellow luminescence back these assumptions.