Fig. 1. Plan-view SEM image of the NPSS.

Fig. 2. (Color online) Schematic diagrams of two samples with different structures.

Fig. 3. (a–c) Plan-view SEM images of surface morphology of sample I at end of the three growth stages. (d–f) The corresponding cross-sectional SEM images for (a), (b) and (c). The black dashed line in (a) indicates direction of the cross-sectional view as (d), (e) and (f). All images use the same scale bar as (a).

Fig. 4. Plan-view SEM images of surface morphology of Sample II.

Fig. 5. (a) 25° tilted-view SEM image of surface morphology of for as-grown MT-AlN. The inset shows cross-sectional view with the direction indicated by the black dashed line. (b) Schematic diagram of AlN growth keeping the 3D morphology. (c) and (d) Schematic of the AlN atomic structure.

Fig. 6. (Color online) (a) Cross-sectional SEM image and (b) AFM image (2 × 2 μm²) of the Sample II after 2D growth.

Fig. 7. (Color online) Schematic diagram of the facet evolution of both samples.