Crystal morphology and optical emissions of GaN layers grown on Si(111) substrates by molecular beam epitaxy

Crystal morphology of GaN layers grown on Si(lll) evolves from whisker-like microcrystals to compact films as a function of the III/V ratio. Small changes in the III/V ratio (from Ga-rich to N-rich) during the growth of a compact layer result in the appearance of microcrystals on the top of the layer, indicating a sharp transition between the two growth regimes. Four different morphologies are obtained by increasing the III/V ratio: a) completely columnar whisker-like samples exhibiting a pair of intense excitonic emissions at 3.471-3.478 eV; b) a mixture of compact regions with columnar microcrystals showing two pairs of excitonic emissionsnbsp c) compact layers with very small microcrystals on the top surface with a weaker dominant transition at 3.415 eV (+/- 5 meV) and, d) full compact and smooth layers with a single dominant excitonic emission at 3.466 eV. A combination of PL measurements with SEM photographs and CL imaging reveals that both pairs of emissions in samples b) come from the columnar microcrystals. The high energy pair (3.471-3.478 eV) is attributed to the free-exciton A and a donor-bound exciton while the low energy pair (3.452-3.458 eV) is assigned to acceptor-bound excitons associated to valence bands Gamma(9v) and Gamma(7uv). Power and temperature dependence together with time-resolved data show that the dominant peak at 3.415 eV (+/- 5 meV) present in samples c) correspond to a donor-acceptor transition, CL, measurements as a function of electron beam energy (depth) also indicate that this emission is more intense towards the interface between the layer and the sample. Finally, the excitonic emission in samples d) is shifted to lower energies due to residual biaxial tensile strain of thermal origin.