High-Selectivity Growth of GaN Nanorod Arrays by Liquid-Target Magnetron Sputter Epitaxy

Selective-area grown, catalyst-free GaN nanorod (NR) arrays grown on Si substrates have been realized using liquid-target reactive magnetron sputter epitaxy (MSE). Focused ion beam lithography (FIBL) was applied to pattern Si substrates with TiN(x)masks. A liquid Ga target was sputtered in a mixture gas of Ar and N-2, ranging the N(2)partial pressure (P-N) ratio from 100% to 50%. The growth of NRs shows a strong correlation withP(N)on the selectivity, coalescence, and growth rate of NRs in both radial and axial directions. The growth rate of NRs formed inside the nanoholes increases monotonically withP(N). TheP(N)ratio between 80% and 90% was found to render both a high growth rate and high selectivity. When theP(N)ratio was below 80%, multiple NRs were formed in the nanoholes. For aP(N)ratio higher than 90%, parasitic NRs were grown on the mask. An observed dependence of growth behavior upon theP(N)ratio is attributed to a change in the effective Ga/N ratio on the substrate surface, as an effect of impinging reactive species, surface diffusivity, and residence time of adatoms. The mechanism of NR growth control was further investigated by studying the effect of nanoholes array pitch and growth temperature. The surface diffusion and the direct impingement of adatoms were found to be the dominant factors affecting the lateral and axial growth rates of NR, respectively, which were well elucidated by the collection area model.