On the other hand, we used TEM to investigate the crystalline quality of the UV-LEDs grown on the FS-GaN and the sapphire substrates. In other words, the two-beam TEM images will be used for quantifying the nature and density of defects. Heying et al.18 pointed out that the pure edge and the mixed defects can be visible under a two-beam condition; the pure screw and the mixed defects can be visible under a two-beam condition. In our case, Figs. 5(a) and 5(b) show bright field scanning TEM images for UV-LEDs grown on sapphire and FS-GaN substrates, respectively. In addition, the two-beam TEM images were taken, as shown in Figs. 5(c) and 5(d) for the UV-LEDs on the sapphire, and in Figs. 5(e) and 5(f) for the UV-LEDs on the FS-GaN. When UV-LEDs were grown on sapphire, we clearly find that the epilayer of the UV-LEDs grown on the sapphire exhibited many edges, screws, and mixed defects. In addition, the defects can be seen radiating vertically from the interface between the GaN-based epilayer and sapphire into the InGaN/InAlGaN MQWs region and the p-AlGaN layer, as shown in Figs. 5(c) and 5(d). Therefore, quite a large number of defects were presented in the whole film on the sapphire substrate, as shown in Fig. 5(a). From Fig. 5(a), the edges, screws, and mixed defects were estimated to be , , and , respectively, which led to a total defect density of about . However, when the substrate for the epitaxy of the UV-LEDs was changed from sapphire to FS-GaN, it can be clearly seen that the crystallography of the UV-LEDs epilayer was drastically different from that of UV-LEDs epilayer on a sapphire substrate. No edges, screws, and mixed defects were observed throughout the observed area. As shown in Figs. 5(b), 5(e), and 5(f), the total defect density including edges, screws, and mixed types was considered to be or less, which agrees well with our HRDCXD rocking curve data, further proving that homo-epitaxial is an effective measure to improve the crystal quality of UV-LEDs.