Figure 1 Schematic showing the evolution of temperature and carrier gas flux with time during GaN barrier growth when H2/N2 is used (full line) or only N2 (dotted line).

Figure 2 10um x 10um AFM scans of samples All N2 (a), All H2/N2 (b), and Half H2/N2 (c). To highlight the variations in prominence of the trench defects across the three samples, a common Z-scale of 15 nm is employed.

Figure 3 HAADF-STEM image of the QW stack in sample Half H2/N2, observed along the <11-20> zone-axis.

Figure 4 (a) HAADF-STEM image of a typical trench defect in sample Half H2/N2 highlighting the highly disturbed QW stack in the enclosed area, observed along the <11-20> zone-axis. (b) Bright field TEM image of the same region, using g = (1-100) showing the presence of a BSF at the bottom of the trench defects.

Figure 5 (a) Weak beam dark field TEM image using g = (1\bar{1}00) showing the presence of a BSF at the bottom half of the QW stack but not connected to a V-shaped ditch in sample Half_H2/N2, observed along the <11\bar{2}0> zone-axis. (b) HAADF-STEM image of the same area showing the undisturbed QW stack grown on top of the BSF.