Nanostructure study of the coalescence growth of GaN columns with molecular beam epitaxy

We investigate the structural properties of molecular-beam-epitaxy coalescence overgrowth of GaN columns at the nanoscale with transmission electron microscopy and other characterization techniques. Two samples grown over nanocolumns of different widths and spatial densities (columns/area) are compared. It is found that columns with a larger cross section (~500 nm) and correspondingly lower spatial density normally lead to un-coalesced overgrown domains ranging 5-8 μm in size. On the other hand, the overgrowth on the columns of a smaller cross section (~100 nm) and correspondingly higher density results in coalesced domains ranging from 1 to 5 μm in size. It is believed that among the smaller, more closely spaced columns the strain distribution resulting from overgrowth is more effective in leading to the uniformity of crystalline orientation, and hence successful coalescence. The optical characterization leads to the conclusion that the defect density in the sample grown on smaller columns is lower when compared with that grown on larger columns.