Fig. 1. c-plane，a-plane，m-plane and r-plane of GaN crystal structure.

Fig. 2. （a_i）~（c_i）Planar and（a_ii）~（c_ii）cross-sectional view SEM images of the NW samples a₁，a₂，and a₃，where the MQS were grown at 730 ℃‍，740 ℃，and 750 ℃，respectively.（a_iii）~（c_iii）Corresponding CL mapping of the samples；（d）Survey-scanned CL spectra of the NW samples^［46］.

Fig. 3. SEM images of silica nanospheres deposited on a GaN/sapphire template with different deposition rates（a）0.15 mm/min，（b）0.3 mm/min，（c）0.6 mm/min. Plan-view SEM and roomtemperature panchromatic CL images of fully coalesced samples with（d，g）multilayer，（e，h）densely packed ML，and（f，i）loosely packed ML of silica nanospheres^［47］.

Fig. 4. 2-D kinetic Wulff plots mapped onto a-planes for（a）undoped and（b）Ge-doped GaN. Cross-sectional SEM images of（c）undoped and（d）Ge-doped（2021）GaN grown on a patterned（2243）sapphire substrate^［48］.

Fig. 5. Nomarski optical microscope images of（2021）GaN grown on the patterned sapphire substrate.（a）Before and（b）after the CMP process；（c）Photo of a 2 in（1 in=2.54 cm）（2021）GaN/sapphire after the CMP process；（d）AFM image with a scanning area of 10 μm×10 μm^［54］.

Fig. 6. Schematic diagrams showing the growth evolution of nonpolar a-GaN films on r-PSS，based on microstructural characterization by TEM^［55］.

Fig. 7. Schematic diagram of a-GaN layer growth.（a）a-GaN layer grown on r-plane sapphire substrate with SiO₂ nanopillar mask；（b）Fabrication of SiO₂ nanopillar mask on a-GaN layer；（c）a-GaN layer etching process；（d）Etched a-GaN；（e）Fully coalescent a-GaN layer^［56］.

Fig. 8. （a）3D schematic diagram and crystal orientation of semi-polar Micro-LEDs samples on pattern sapphire substrates；（b）SEM image of semi-polar Micro-LEDs array，Inset：Photo of luminescence under the injection current of 0.1 mA^［61］.

Fig. 9. Process flow for the fabrication of a full-color RGB pixel array.（a）Micro-LED array process；（b）Black PR matrices and p-electrode metal lines；（c）Red，green，and blue（transparent）pixel lithography process；（d）Color pixel bonding^［62］.

Fig. 10. Schematic diagram of the MHPSS and NHPSS preparation process^［65］

Fig. 11. GaN-based multishell nanotube heterostructure LED arrays.（a）Schematic diagrams of the nanotube LED arrays and a single nanotube LED；（b）Tilted view FESEM image of the nanotube LED arrays；（c）EDX intensity line profile of the L-characteristic rays of gallium（empty squares）and indium（solid circles）for nanotube MQWs grown at temperatures of 670，720，760 ℃（from left to right）. The profiles were obtained along the radial direction from cross-section samples^［66］.