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Scanning electron microscopy images of (a) -plane GaN QDs and (b) -plane GaN QWires. The AlN buffer layer deposited on -plane SiC is 300 nm thick for QDs and 50 nm thick for QWires, respectively.
GI-MAD of -plane GaN QDs. (a) Scan along the direction on the reflection and (b) scan along the  direction on the (0004) reflection. On each figure is presented the amplitude of the scattered intensity (open circles), and the extracted amplitude for the Ga (straight line) and atoms (open squares). r.l.u. are relative to the SiC substrate and have been corrected for grazing-incidence effects.
EDAFS oscillations (EXAFS-like oscillations) for QD (upper curve) and QWire (lower curve) samples, the two spectra are shifted along the axis for sake of clarity. The best fit curves are shown as solid curves.
Scheme of GaN wurzite structure. The -plane and the most relevant nearest neighbor distances reported in the text and Table II are represented. Ga (resp. N) atoms are represented by black (resp. white) spheres.
(a) HRTEM image taken along the zone axis showing two stacking faults (indicated by arrows), and misfit dislocations (surrounded by circles) corresponding to a Burger vector . Inset: low magnification image showing the systematic association of stacking faults to QDs. (b) Variation of the lattice parameter from the geometrical phase analysis of (a).
(a) HRTEM image of a QWire taken along the  zone axis showing a misfit dislocations (surrounded by a circle) corresponding to a Burger vector . (b) HREM image taken along the  zone axis and cut in two parts to show a longer part of the GaN wire (the black dot corresponds to the same point). Two dislocations are surrounded by black circles and indicated by arrows. They are separated by 45 nm.
In-plane parameters of the 300 nm (50 nm) thick AlN buffer layer used for GaN QD (QWire) growth. Values for bulk AlN and SiC are given for comparison.
Experimental values of in-plane and out-of-plane lattice parameters for GaN QDs and QWires extracted from GI-MAD, DAFS, and HRTEM experiments. The residual strain state of GaN with respect to relaxed GaN has been calculated along and along . For both samples, the in-plane mismatch values between fully relaxed GaN and the measured AlN buffer layer calculated using data in Table I are shown in brackets. Ga–N and Ga–Ga distances deduced from the fit of DAFS oscillations for QDs and QWires are also reported.
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