Improved Ti-mask selective-area growth (SAG) by rf-plasma-assisted molecular beam epitaxy demonstrating extremely uniform GaN nanocolumn arrays

The Ti-mask selective-area growth (SAG) of GaN nanocolumns was performed at the growth temperature of 900 °C, while decreasing the supplied nitrogen flow rate ( Q N2) from 3.5 to 0.5 sccm. Highly uniform arrays of GaN nanocolumns were demonstrated. At low Q N2, both the desorption and diffusion of Ga from/on the nitrided Ti mask were accelerated, which sufficiently suppressed the crystal nucleation on the Ti-mask surface, and hence the SAG of the GaN nanocolumns was achieved even when the spacing between the nanocolumns was several hundred nm. The enhancement of Ga desorption with decreasing Q N2 brought about a reduction in the growth rate of GaN nanocolumns from 1.05 to 0.15 μm/h. The lateral growth rate of the GaN nanocolumns rapidly increased above the critical Q N2 value of 1.5 sccm and became 45 nm/h at Q N2 of 3.5 sccm. For low Q N2 values less than 1.5 sccm, the lateral growth rate became sufficiently low, approximately 8 nm/h; this contributes to well-controlled SAG of GaN, where the underlying nanomask patterns are well traced.