Nanoscale Patterned Growth Assisted by Surface Out-Diffusion of Adatoms from Amorphous Mask Films in Molecular Beam Epitaxy

Nanoscale patterned growth (NPG) of GaAs requires the suppression of nucleation on the amorphous SiO2 mask film, defined on a substrate by patterning. It is determined by the Ga adatom kinetics on SiO2, leading to desorption and surface out diffusion (SOD) to the area beyond the mask. Their relative contributions to NPG are examined both theoretically and experimentally. From the rate equation of thin-film growth, a relationship between incident Ga flux and growth temperature for NPG is analytically derived as a function of the lateral dimension of the SiO2 mask film, L-M. In the NPG by molecular beam epitaxy, L-M is varied in the range of Ga adatom migration length. From comparison with the model, the activation energy of a Ga adatom for desorption, E-des, is found to be comparable to that for surface diffusion, E-diff, on the SiO2 mask. They are both in the range of 2.7-2.9 eV, lower than the Ga desorption energy from the GaAs substrate, confirming the validity of SOD and, as a result, NPG. This also implies that they are not clearly distinguishable on the amorphous surface, in contrast to crystalline surfaces, where E-diff < E-des, which is attributed to the random fluctuations in the potential lacking long-range order. SOD can induce an actual growth rate significantly enhanced from the nominal rate calibrated on an unpatterned wide area by the additional adatom diffusion flux across the substrate-mask boundary. Its role in controlling the shape and size of the nanostructures selectively grown on the adjacent substrate surface is addressed.