Cross-sectional images of the ELOG structures. [(a)–(c)] Taken with contact-mode AFM in parallel to the SCM data collection. [(d)–(f)] Taken with SEM on different areas of the same sample. (a) and (d) show sample A. (b) and (e) show sample B. (c) and (f) show sample C.
SCM phase data for (a) sample A, (b) sample B, and (c) sample C. Black areas are n-type, white areas are p-type, and noisy areas have no detectable conductivity.
SCM amplitude data for (a) sample A, (b) sample B, and (c) sample C. For regions which the SCM phase data show to be doped, SCM amplitude data show brighter contrast for areas with a lower doping concentration. Note that p- and n-type materials of the same gray shade may have different carrier concentrations as p- and n-type GaN respond differently to the SCM bias.
SCM data were taken for sample D. (a) shows the raw SCM amplitude signal. The region of the dopant staircase contained in the gray box in (a) was used to make the calibration curve shown in (b). [Please note that the SIMS data in (b) are plotted on a log scale.] By using the calibration curve in (b), it was possible to obtain estimates of the n-type carrier concentrations marked by crosses in (a).
A schematic representation of the ELOG growth mechanism.
Schematic representations of GaN coalescence without magnesium. (a) ELOG: here the GaN grown through the mask (dark) forms perfect triangular cross sections. (b) ELOG: here the GaN grown through the mask (dark) still exhibits (0001) facets at the start of the coalescence step. During coalescence, material grown on inclined facets (medium gray) exhibits greater dopant incorporation than the GaN grown on the (0001) facets (light gray). (c) A similar orientation-dependent dopant incorporation mechanism could be responsible for the roughness of the uid interface layer found in some GaN samples adjacent to the sapphire substrate. Note that here the dark gray represents the 3D islands produced in the early stages of growth.
A SCM phase image of a sapphire:GaN interface at which n-type conductivity can be observed. Note that the n- to undoped-GaN interface is rough.
Estimated carrier concentrations for the regions marked with white crosses in Fig. 4(a). The errors on these carrier concentrations are estimated to be .
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