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(a) SEM plan view of the dodecagonal pinhole in the surface of the AlGaN film. Schematics of the side view (left) and plan view (right) illustrating the formation of the pinhole in GaN basal layers (b) and in AlGaN film (c).
(a) Bright field TEM image of the pinhole, where region I is the AlGaN layer and region II corresponds to the interfacial layers of represents the Al mole fraction determined by EDX. (b) Schematic diagram of the geometry for the pinhole in (a). (c) Selected area electron diffraction pattern from region I for  orientation. Besides the hexagonal pattern an extra set of spots can be observed. (d) Extra pattern extracted from (c), which is a ZB  pattern of AlN.
Structural transition of AlGaN thin film from WZ (a) to ZB (c) on GaN basal layers. (b) and (d) show the top views of configurations (a) and (c), respectively. The dotted and dash lines exhibit the symmetry change during the structural transition.
Average transition energy of thin film as a function of compressive strain . Curve (a) for system with and (b) for . Negative represents the energetically preference of ZB phase.
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