Fig. 1. (Color online) (a) Growth flow chart of GaN inverted pyramid surface structures on PSS. Low temperature buffer layer is grown as nucleation layer at 525 °C. High temperature buffer layer is grown at ~1000 °C to level GaN surface. Finally, GaN inverted pyramid structures are grown due to competition between crystal planes. (b) Cross-sectional SEM image of GaN inverted pyramids.

Fig. 2. (Color online) SEM images of the surface morphology of 3D GaN with inverted pyramids grown under (a) 940 °C, (b) 950 °C, (c) 965 °C, and (d) 985 °C.

Fig. 3. (Color online) Schematic diagram of crystal growth competition based on Wulff construction. (a) When the angle between crystal planes is greater than 180°. (b) When the angle between crystal planes is less than 180°. (c) Schematic diagram of the relationship between temperature and growth rate of different facets.

Fig. 4. (Color online) SEM images of samples with different Ⅴ/Ⅲ ratios at the same growth temperature (985 °C) and their corresponding growth competition models. NH₃ flowrate: (a, b) 4000 sccm, (c, d) 3000 sccm, (e, f) 2000 sccm, (g, h) 1000 sccm.

Fig. 5. (Color online) (a, b) SEM images of sample H and its corresponding growth competition models. (c, d) SEM images of sample H based on rotated PSS and its corresponding growth competition models. The inserts show the arrangement of patterns on PSS/rotated PSS.