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Schematic representation of the fabrication process of GaN core-shell structure: (a) as-grown layer structure, (b) pillar formation by ICP etching, and (c) after p-GaN shell overgrowth.
SEM images of (a) array of as-etched GaN pillars with 0.5 μm diameter and 2.5 μm pitch; (b) circle-, square-, and diamond-shaped pillars before and after p-shell growth (plan-view); (c) and (d) p-shells grown for 10 min at 1020 °C and HCl/NH3 flow rates of 6/200 (sccm) on (c) 0.5 μm and (d) 0.2 μm pillars; (e) p-shells grown for 5 min at 1040 °C and HCl/NH3 flow rates of 20/1500 (sccm) on square pillars with 10 μm width and 20 μm pitch. Tilted SEM images were taken at 70°. The scale bars are 2 μm for (a)–(d) and 20 μm for (e).
Dark-field cross-sectional TEM images of two different core-shell structures with original square core with diameter 0.5 μm in (a) and (b). The cross-sections were prepared using FIB instrument. The arrow in (a) points to the bending of a dislocation as it crosses into the shell. Dashed lines in (b) outline the shape of the initial GaN core. The scale bar is 0.2 μm in both the images.
Room-temperature PL spectra of bulk GaN (black), unetched thin film (red), etched pillars (green), after p-shell growth (blue), and after the activation anneal (magenta). The dashed line indicates the peak position of room-temperature NBE of strain-free GaN. The etched sample showed significant reduction in the intensity. The PL intensity of activated p-shell increased by a factor of 10, when compared to the as-grown shell.
Room-temperature Raman scattering spectra from bulk GaN (black), unetched thin film (red), etched pillars (green), after p-shell growth (blue), and after the Mg activation anneal (magenta). The position of E2 H is given in parenthesis.
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