Selective area heteroepitaxy of nano-AlGaN ultraviolet excitation sources for biofluorescence application

Abstract

We report on the selective area heteroepitaxy and facet evolution of AlGaN nanostructures on GaN/sapphire substrate using various mask materials. We also report on the challenges associated with selection of an appropriate mask material for selective area heteroepitaxy of AlGaN with varying Al composition. The shape and the growth rate of the nanostructures are observed to be greatly affected by the mask material. The evolution of the AlGaN nanostructures and Al incorporation were studied exhaustively as a function of growth parameters including temperature, pressure, NH 3 flow, total alkyl flow, and TMAl/(TMAl+TMGa) ratio. The growth rate of nanostructures was reduced drastically when higher Al percentage AlGaN nanostructures were grown. The growth rates were increased for higher Al percentage AlGaN using a surfactant, which resulted in a high-quality pyramidal structure. As indicated by high-resolution x-ray diffraction and cathodoluminescence spectroscopy, the composition of Al in the AlGaN nanostructure is significantly different from that of a thin film grown under the same growth conditions.

Selective area growth of AlGaN nanopyramids by conventional and pulsed MOVPE

“…The surface between them becomes rougher also and the thickness of the deposited layer on the mask is enhanced for the case of the sample B with higher AlN molar fraction (figure 1(b)). Indeed, Al adatoms have a high sticking coefficient and a low surface diffusion [10]. Thus, the higher the Al/Ga ratio, the more the deposition on the mask increases, leading to smaller truncated pyramids.…”

“…This can be explained by the advantages of such structures. Indeed, the hexagonal pyramids grown on patterned templates can notably improve the material quality by the filtering of dislocations through the mask layer, limit the internal electric field in the quantum wells grown on semi polar planes, thus providing a better optical confinement, and improving the light extraction through the semi-polar facets [10][11][12]. High quality GaN pyramids have been successfully obtained by SAG on patterned templates using a metal organic vapor phase epitaxy (MOVPE) system [13].…”

Selective area growth of AlGaN nanopyramid arrays on graphene by metal-organic vapor phase epitaxy

“…There is hardly any Al in the pyramid, except a small amount at the very top, and significant amount at the bottom. We attribute this low Al incorporation in the pyramid to the parasitic reaction 36 especially due the use of a horizontal flow MOVPE and a longer diffusion length of Ga species as compared to Al. 37 The longer diffusion length of Ga makes Ga supply more effective as compared to Al at the bottom part of the pyramid, resulting in a lower Al composition.…”