In situ optical reflectance traces of growths for samples D (a) and E (b). The controlled growth interruptions are labeled in (a) for samples A–D. The reflectance in the second segment (shaded in green) in (a) corresponds to the first-step growth under a high V/III and a high pressure.
Nomarski optical microscopy images of samples A–D, showing the evolution of the surface morphology during the two-step growth process.
SEM surface images of samples A–D, showing the evolution of the surface morphology during the two-step growth process. The circles (in red) in (a) marked out regions showing lateral growth of big mesas along over the neighboring small islands.
Kinetic Wulff plots (-plots) for the conditions used for the first-step [(a)–(c)] and the second-step [(d)–(f)] growth processes. (a) and (d) are the -plots for the prism planes mapped onto the basal -plane, (b) and (e) are the -plots for the orientations within the -plane, and (c) and (f) are the -plots for the orientations within the -plane. As compared to our previous work (Ref. 21), the additional , , and are the fast-growing facets (Refs. 21 and 29) corresponding to the saddle points (Ref. 29) in the 3D -plots (to be published elsewhere).
(a) In situ optical reflectance trace of the three-step growth for sample F; (b) Nomarski optical microscopy image of sample F. The reflectance trace in the second segment (shaded in blue) in (a) corresponds to the third-step growth under a high pressure.
SEM image of an SAG mesa with both convex and concave growth fronts (grown out of an annular ring opening) for the illustration of XRC configurations. For and 90°, the x-ray rocking direction is parallel and perpendicular to the -axis , respectively. The incident and diffracted x-ray beams (arrows in red) rock within a plane (shaded in green) for each diffraction plane with its index and orientation labeled in the vicinity. The on-axis and off-axis planes , such as , , and , were measured in symmetric and skew symmetric geometries, respectively.
The FWHM of the on-axis (a) and off-axis (b) XRCs of as a function of the azimuthal angle .
The FWHM of the XRCs as a function of the plane inclination angle for the two sets of planes at (a) and 90° (b).
(a) The FWHMs of the 30°-inclined -plane (, 2, and 3); (b) the modified WH plots. The straight lines in (b) are the linear fits to the and data points of each sample.
Plan-view TEM images of samples E [(a) and (b)] and D [(c) and (d)]. The vectors were for (a) and (c), and 0002 for (b) and (d), to reveal the SFs and PDs, respectively. The Frank–Shockley PDs and Shockley PDs were indicated by the downward (in blue) and the upward (in red) arrows, respectively. Two PSFs observed on the surface of sample D were circled in (c).
Low-magnification cross-sectional TEM images of samples E [(a) and (b)] and D [(c) and (d)]. (a) and (c) were taken near the  axis with . (b) and (d) were taken near the axis with .
High-magnification cross-sectional TEM images of the GaN/AlN/sapphire interface regions of samples E [(a) and (b)] and D [(c) and (d)]. (a) and (c) were taken near the  axis with . (b) and (d) were taken near the axis with . The dashed lines (in white) in (c) sketched out a possible island near the interface, and the dotted arrows (in yellow and cyan) were drawn to trace the turning paths of some PDs due to the -axis lateral overgrowth of a big tall mesa from the left. In (d) the dark regions near the interface marked out by the dashed rectangles (in red) were probably some small defective islands that were fully/partially buried by the -axis lateral overgrowth of big tall mesas during the second-step growth, and hence the overgrown regions contained fewer defects. The bright spot in the bottom-left corner of (c) was probably a void formed during the -axis lateral overgrowth of the facets.
Schematic of the BSF blocking and PD bending in two-step growth viewed along the -axis (a) and the -axis (b). The dashed lines (in blue) represent the growth fronts, and the solid lines (in red) are the PDs. BSFs are shown as the shaded area in (a) and the straight black lines in (b). The vertical black lines overlap with the lines (in red) in (b), indicating that most of the BSFs are bounded by PDs.
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