(a) Bright-field CTEM image along the zone axis and (b) Z-contrast STEM image along of the overall GaN QD heterostructure of the 10 ML sample grown on semipolar AlN template. The heterostructure is perturbed by TDs, and depressions are observed at the GaN/AlN interface.
Plan-view Z-contrast STEM images along , obtained from the 5 ML sample, showing GaN QDs with rectangular and trapezoidal-like projected shapes. The orientation of the in-plane axis is indicated, and dotted lines have been drawn around some QDs to aid the eye regarding shape determination.
RHEED patterns with the electron beam (a) along the and (b) azimuths. Extra spots in the patterns correspond to the QD lateral facets, i.e., in (a), and the crystallographically equivalent facets, in (b).
Cross sectional HRTEM image of GaN QDs in the 10 ML sample, observed along . The facets indicated by solid lines are of type.
Cross sectional HRTEM image of GaN QDs in the 10 ML sample, observed along . The side facets are indicated by solid lines. The dashed lines indicate facets of average orientation or .
Schematic illustration, along three perpendicular projection directions, of the two delimiting QD morphologies of the -nucleated QDs. The orientation of the glide mirror plane is indicated.
(a) Z-contrast STEM image showing QDs nucleated at facet junctions. (b) High resolution STEM image of one of the QDs depicted in (a). The fringes corresponding to the basal planes are seen to be vertical to the facets indicated by white solid lines.
(a) Cross-sectional weak-beam CTEM image of the 10 ML sample obtained off the zone axis using 0002. Inclined TDs are discernible as well as a row of QDs stacked along the basal plane and indicated by an arrow. Other arrows indicate TDs that appear to be associated with this row of QDs. (b) Cross sectional HRTEM image of the 5 ML sample, obtained along , and showing a region with at least two ascending TDs, indicated by dotted lines. The region is seen to comprise facets where QDs have nucleated. The orientation of the planes is indicated by solid white lines. The WL between the QDs is also well-resolved, as indicated by arrows.
(a) Cross sectional HRTEM image of a -nucleated QD observed along . (b) Corresponding lattice strain map along the growth direction obtained with 0.71 nm spatial resolution ( mask size). (c) FFT of the HRTEM image of (a). The spatial frequencies employed for the GPA strain analysis are indicated. (d) Lattice strain profile along the growth direction. The profile was obtained using 6 nm integration width from the area indicated in (b). The spatial resolution for the particular profile was 0.95 nm ( mask size), and the accuracy of strain determination was 0.4%.
In (a), the FE mesh is shown superimposed on the proposed QD geometry. The capping layer has been removed for illustrative purposes only. In (b) and (c), the elastic strain distribution is presented in 2D projected contour plots of a QD cross-section through the - and -planes, respectively. In (d) and (e), the corresponding distributions of the electrostatic potential (in volt) are presented in 2D projected contour plots.
Quantification of the results of the FE analysis for a cross section through the center of the QD on the -plane [i.e., ]. In (a) and (b), the elastic and lattice normal strain components are given as a function of position along the growth -axis. In (c), the variation in the electrostatic potential with position along the -axis is given.
Schematic illustration of the relaxed simulated and interfaces projected along . Shading denotes distinct levels along the projection direction. .
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