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(a) Cross-sectional TEM bright-field image of sample HT taken under two-beam conditions off the zone axis with . The sapphire substrate is out of contrast. Areas of moiré fringes indicate the presence of misoriented grains. (b) Cross-sectional HRTEM image of sample LT taken along . The different-orientation grains are outlined and their corresponding zone axes of TEM observation are indicated ( is the zone axis for the -plane GaN regions). A twin boundary is visible by the abutting basal plane fringes of the two variants of misoriented crystallites, viewed along the and zone axes, respectively.
SAED patterns corresponding to the zone axes: (a) , and (b) . The SAED pattern of (a) is given for sample LT, and that of (b) is given for sample HT. The dark solid lines indicate the orientation of sapphire and the white solid lines indicate -plane GaN. The dotted lines indicate crystallographically equivalent variants of the misoriented crystallites. Reflections are indexed in a distinguishing manner using the three indices system for sapphire, subscript (1) for -plane GaN (OR1), and subscript (2) for the misoriented GaN variants (OR2). We note that the basal plane reflections of the OR2 variants have been indexed here as 0002 but polarity determination has not been performed.
(a) HRTEM image of the interfacial area between GaN epilayer and sapphire substrate viewed along . A misoriented crystallite is seen associated with a sapphire step’s facet (indicated by a dotted line). The crystallite exhibits both hexagonal (wurtzite) GaN viewed along and cubic (sphalerite) GaN viewed along . (b) Corresponding SAED pattern of the sapphire and the -plane GaN orientation. (c) Schematic illustration of the sapphire substrate showing the two variants of the step facets and the observation directions, corresponding to the HRTEM image of (a). (d) Schematic illustration of an atomic model of the interface between the misoriented GaN grain and the step facet of the -plane sapphire surface. The low misfit and planes are indicated.
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