Spontaneous Formation of Indium-Rich Nanostructures on InGaN(0001) Surfaces

InGaN(0001) surfaces prepared by molecular beam epitaxy have been studied using scanning tunneling microscopy and first principles total energy calculations. Nanometer-size surface structures are observed consisting of either vacancy islands or ordered vacancy rows. The spontaneous formation of these structures is driven by significant strain in the surface layers, and by the relative weakness of the In-N bond compared to Ga-N. Theory indicates that In will preferentially bind at the edges and interior of the structures, thereby giving rise to an inhomogeneous In distribution at the surface. The vacancy islands typically form a nearly uniform array with a separation of about 5 nm, which is close to the observed length scale of compositional fluctuations in InGaN alloys.