402
14 Nanostructures
Fig. 14.1 Schematic geometry and density of states for 3D, 2D, 1D and 0D electronic systems
Fig. 14.2 Schematic cross
section of a GaAs/AlGaAs
heterostructure grown on a
channeled substrate,
illustrating the concept of
self-ordered quantum-wire
fabrication. Adapted from
[1341], reprinted with
permission, ©1992,
Elsevier Ltd
of 10 nm. The side facets of the groove are {111}A. Subsequent deposition of GaAs leads to a larger
upper radius ρ u > ρ l of the heterostructure. The GaAs QWR formed in the bottom of the groove is
thus crescent-shaped as shown in Fig. 14.3a. A thin GaAs layer also forms on the side facets (sidewall
quantum well) and on the top of the ridges. Subsequent growth of AlGaAs leads to a resharpening
of the V-groove to the initial, self-limited value ρ l . The complete resharpening after a sufficiently
thick AlGaAs layer allows vertical stacking of crescent-shaped QWRs of virtually identical size and
shape, as shown in Fig. 14.3b. In this sense, the self-limiting reduction of the radius of curvature and its
recovery during barrier-layer growth leads to self-ordering of QWR arrays whose structural parameters
are determined solely by growth parameters. The lateral pitch of such wires can be down to 240 nm.
To directly visualize the lateral modulation of the band gap, a lateral cathodoluminescence (CL)
linescan perpendicular across the wire is displayed in Fig. 14.4. In Fig. 14.4a, the secondary electron
(SE) image of the sample from Fig. 14.3a is shown in plan view. The top ridge is visible in the upper
and lower parts of the figure, while in the middle the sidewalls with the QWR in the center are apparent.
In Fig. 14.4b, the CL spectrum along a linescan perpendicular to the wire (as indicated by the white
line in Fig. 14.4a) is displayed. The x-axis is now the emission wavelength, while the y-axis is the
lateral position along the linescan. The CL intensity is given on a logarithmic scale to display the full
dynamic range. The top QW shows almost no variation in band gap energy (λ = 725 nm); only directly
at the edge close to the sidewall does a second peak at lower energy (λ = 745 nm) appear, indicating
a thicker region there. The sidewall QW exhibits a recombination wavelength of 700 nm at the edge
to the top QW, which gradually increases to about 730 nm at the center of the V-groove. This directly
14 Nanostructures
Fig. 14.1 Schematic geometry and density of states for 3D, 2D, 1D and 0D electronic systems
Fig. 14.2 Schematic cross
section of a GaAs/AlGaAs
heterostructure grown on a
channeled substrate,
illustrating the concept of
self-ordered quantum-wire
fabrication. Adapted from
[1341], reprinted with
permission, ©1992,
Elsevier Ltd
of 10 nm. The side facets of the groove are {111}A. Subsequent deposition of GaAs leads to a larger
upper radius ρ u > ρ l of the heterostructure. The GaAs QWR formed in the bottom of the groove is
thus crescent-shaped as shown in Fig. 14.3a. A thin GaAs layer also forms on the side facets (sidewall
quantum well) and on the top of the ridges. Subsequent growth of AlGaAs leads to a resharpening
of the V-groove to the initial, self-limited value ρ l . The complete resharpening after a sufficiently
thick AlGaAs layer allows vertical stacking of crescent-shaped QWRs of virtually identical size and
shape, as shown in Fig. 14.3b. In this sense, the self-limiting reduction of the radius of curvature and its
recovery during barrier-layer growth leads to self-ordering of QWR arrays whose structural parameters
are determined solely by growth parameters. The lateral pitch of such wires can be down to 240 nm.
To directly visualize the lateral modulation of the band gap, a lateral cathodoluminescence (CL)
linescan perpendicular across the wire is displayed in Fig. 14.4. In Fig. 14.4a, the secondary electron
(SE) image of the sample from Fig. 14.3a is shown in plan view. The top ridge is visible in the upper
and lower parts of the figure, while in the middle the sidewalls with the QWR in the center are apparent.
In Fig. 14.4b, the CL spectrum along a linescan perpendicular to the wire (as indicated by the white
line in Fig. 14.4a) is displayed. The x-axis is now the emission wavelength, while the y-axis is the
lateral position along the linescan. The CL intensity is given on a logarithmic scale to display the full
dynamic range. The top QW shows almost no variation in band gap energy (λ = 725 nm); only directly
at the edge close to the sidewall does a second peak at lower energy (λ = 745 nm) appear, indicating
a thicker region there. The sidewall QW exhibits a recombination wavelength of 700 nm at the edge
to the top QW, which gradually increases to about 730 nm at the center of the V-groove. This directly