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Faster radial strain relaxation in InAs–GaAs core–shell heterowires
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Image of FIG. 1.
FIG. 1.

Secondary electron micrographs of (a) sample 1 and (b) sample 2 viewed perpendicular to the surface (sample tilt zero). The edge of each image is aligned with the ⟨110⟩ wafer edge. The circled wires are ZB nanowires (square cross-section) that have grown closest to perpendicular (001).

Image of FIG. 2.
FIG. 2.

Bright field TEM image and selected area diffraction pattern (1120) of a WZ wire showing relaxation. Multiple spots associated with any particular (hkil) is explained by double diffraction.

Image of FIG. 3.
FIG. 3.

Lattice image of a WZ wire (60 nm diameter; left shell thickness 5 nm) showing dislocations (circled) at the shell-core interface. The insets show a lower magnification bright field image of the heterowire with its corresponding selected area diffraction pattern magnified below indicating a (1120) image plane. Radial relaxation has not occurred in this wire. Axial relaxation is 4.7% with an average dislocation spacing of 7.2 nm.

Image of FIG. 4.
FIG. 4.

Bright-field TEM image of a ZB InAs–GaAs (001) core–shell nanowire. The image plane is (110). The InAs core radius tapers from bottom to top (from 27 to 14 nm), whereas the average shell thickness is asymmetric, 10 nm at the right and 30 nm on the left side, but remains relatively constant.

Image of FIG. 5.
FIG. 5.

Plot of percentage strain relaxation vs the ratio of the shell thickness to core radius for individual ZB heterowires in radial (○) and axial (•) wire directions.

Image of FIG. 6.
FIG. 6.

Dark-field TEM images of the same area of a ZB wire tilted toward (a) {220} and (b) {111} diffraction planes. The arrows point in the direction of the diffraction vector (normal to the diffracting planes in each case).

Image of FIG. 7.
FIG. 7.

Dark field TEM images of the same area of another ZB wire tilted about axes parallel and perpendicular to the wire length toward (a) {220} and (b){004} diffraction planes. The arrows point in the direction of the diffraction vector (normal to the diffracting planes in each case).

Image of FIG. 8.
FIG. 8.

High-magnification lattice image of a sidewall region of a ZB nanowire. The two arrows indicate the position of a (110) sidewall interface. The circled area identifies the location of two misfit dislocations diagrammed to the left. The line defects running parallel to {111} planes are multiple {111} stacking faults.

Image of FIG. 9.
FIG. 9.

Dark-field TEM images of a WZ heterowire showing (a) (0002) and (b) (1010) tilt orientations from the same region. Two dislocations and associated stacking faults are highlighted in the magnified images.

Image of FIG. 10.
FIG. 10.

(Color online) (a) Bright-field TEM image and (b) selected area diffraction pattern of a ZB InAs–GaAs, core–shell heterowire viewed close to its [001] growth direction. Three interfaces are visibly relaxed due to dislocations. (c) Scanning TEM image and energy dispersive x-ray spectroscopy elemental maps of a similar heterowire.

Image of FIG. 11.
FIG. 11.

(Color online) Schematic diagrams of feasible dislocations showing possible glide planes (shaded) for radial (dark lines) and axial (dotted lines) relaxation in (a) ZB and (b) WZ core–shell nanowires. In (a) a dislocation (dotted line) shown nucleating as a loop in the top right corner expands to the interfaces via glide on a {111} plane relaxing axial strain on the right sidewall (perpendicular component of b) and generating tilt and twist on the top facet with no strain relaxation. The pure edge dislocations (dark lines) relax radial strain forming perhaps via the reaction of two dislocations on complementary glide planes one is depicted. These could nucleate as loops at the surfaces (one set diagrammed) and then glide to the {110} interface but necessarily on more energetic planes (e.g., {100}) compared to {111}.

Image of FIG. 12.
FIG. 12.

(Color online) Shear strain ɛxy in a square InAs core (5 nm) surrounded by a GaAs shell (2.5 nm).


Generic image for table
Table I.

Summary of the MBE growth details for the InAs–GaAs core–shell nanowires of this study, including sample number, No., Growth temperature, T, Effective planar thickness, d, Growth rate, d/t, and Radii thickness, s. Average wire lengths were 0.1–1 μm (ZB) and up to 5 μm (WZ).


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Faster radial strain relaxation in InAs–GaAs core–shell heterowires