Past exam of the MPhil in micro- and nanotechnology enterprise course of the University of Cambridge / 2018 / Paper 2 / Section B / 18 / ii

Two suitable epitaxial techniques are metalorganic vapour-phase epitaxy and molecular beam epitaxy.

MOVPE supplies metalorganic group-III precursors and a reactive nitrogen source in a flowing gas. Surface reactions, mass transport and selective nucleation control growth; it is well suited to conformal radial growth and relatively high throughput. molecular beam epitaxy supplies molecular/atomic beams in high vacuum, with beam fluxes and shutters controlling delivery. It offers precise flux switching and in-situ surface monitoring, but flux, geometry and nitrogen activation can limit growth rate and conformality. Neither method automatically gives identical growth on all wire crystal facets.

For an overall target near $40\mathrm{nm}\mathrm{mi}{\mathrm{n}}^{-1}=2.4\mu \mathrm{m}{\mathrm{h}}^{-1}$, MOVPE is a natural choice. This is a practical selection, not a theorem that molecular beam epitaxy cannot attain that rate.

Prepare the substrate/buffer and patterned SiN growth mask; grow donor-doped gallium nitride cores with controlled axial morphology; change conditions to favor radial deposition and incorporate indium for the thin indium gallium nitride active shell; then overgrow acceptor-doped p-GaN without degrading the active layer. Finally expose/contact the core and shell appropriately. Monitor facet-dependent thickness and indium content, and characterize the interfaces. If the active-layer normal rate itself were $40\mathrm{nm}\mathrm{mi}{\mathrm{n}}^{-1}$, 5 nm would take 7.5 s; the stated overall rate does not establish that the alloy shell actually grows at that same rate.