Nanoscale indium variation along InGaN nanopillars grown on (1 1 1) Si substrates

The nanoscale structural properties of InGaN nanopillars grown on (1 1 1) Si substrates, by radio frequency molecular beam epitaxy, were explored using electron microscopy techniques. Single phase, single crystalline initial nanostructures merged to form final nanopillars. Given that tuning of the band gap is controlled by the InN mole fraction, the compositional homogeneity along the c-axis of the nanopillars was examined and correlated with their luminescence properties. High growth temperature and desorption phenomena led to a negligible indium incorporation in the initial stages of growth. Here, among other possible explanations, indium accumulation at the tip area of nanopillars is interpreted by the arrangement of In clusters at steps of the top surface of merged nanopillars and the formation of a thin In-adlayer. The limited mobility of In atoms in the absence of appropriate migration paths restrained their diffusion along the c-axis, resulting in a 20% In-content at the tips of nanopillars, and hence high luminescence intensity at the true blue region. Narrow, non-coalesced nanopillars with fine reconstructed top surface exhibited a higher growth rate and were found In-free.