404
14 Nanostructures
[001]
cleavage
plane
QW
QW
50nm
Fig. 14.5 Principle of CEO quantum wires and 2-fold CEO quantum dots. Part (a) depicts a layered structure (quantum
wells or superlattice, blue), (b) describes the growth on the cleaved facet used for fabrication of quantum wires. In (c) a
second cleave and growth on top of the plane allows the fabrication of quantum dots. From [1344]. (d) Cross-sectional
TEM image of CEO GaAs/AlGaAs quantum wires. Two quantum wells (QW) and the QWR at their junction are labeled.
The first epitaxy was from left to right. The second epitaxy step was on top of the cleavage plane (dashed line) in the
upward direction. Adapted from [1345], reprinted with permission, ©1997 APS
is continued on the cleaved facet. At the junctures of the {110} layer and the original quantum wells
QWRs form. Due to their cross-sectional form they are also called T-shaped QWRs. A second cleave
and another growth step allow fabrication of CEO quantum dots [1344, 1345] (Fig. 14.5c).
14.2.3 Nanowhiskers
Whiskers are primarily known as thin metal spikes and have been investigated in detail [1346]. Semiconductor whiskers can be considered as (fairly short) quantum wires. They have been reported for
a number of materials, such as Si, GaAs, InP and ZnO [1347]. A field of ZnO whiskers is shown in
Fig. 14.6. If heterostructures are incorporated along the whisker axis [1348], quantum dots or tunneling
barriers can be created (Fig. 14.7a). The growth mode relies often a VLS (vapor-liquid-solid) mechanism in which the wire materials are first incorporated into a liquid catalyzer (most often gold) drop
at the tip and then used to build up the nanocrystal. In [1349], the layer-by-layer growth of a GaAs
nanowire via this mechanism has been observed in-situ by TEM (Fig. 14.7b) and is also available as
an impressive video. Another nanowire growth mechanism is the VSS (vapor-solid-solid) mechanism
that works without liquid drop on top of the wire.
14 Nanostructures
[001]
cleavage
plane
QW
QW
50nm
Fig. 14.5 Principle of CEO quantum wires and 2-fold CEO quantum dots. Part (a) depicts a layered structure (quantum
wells or superlattice, blue), (b) describes the growth on the cleaved facet used for fabrication of quantum wires. In (c) a
second cleave and growth on top of the plane allows the fabrication of quantum dots. From [1344]. (d) Cross-sectional
TEM image of CEO GaAs/AlGaAs quantum wires. Two quantum wells (QW) and the QWR at their junction are labeled.
The first epitaxy was from left to right. The second epitaxy step was on top of the cleavage plane (dashed line) in the
upward direction. Adapted from [1345], reprinted with permission, ©1997 APS
is continued on the cleaved facet. At the junctures of the {110} layer and the original quantum wells
QWRs form. Due to their cross-sectional form they are also called T-shaped QWRs. A second cleave
and another growth step allow fabrication of CEO quantum dots [1344, 1345] (Fig. 14.5c).
14.2.3 Nanowhiskers
Whiskers are primarily known as thin metal spikes and have been investigated in detail [1346]. Semiconductor whiskers can be considered as (fairly short) quantum wires. They have been reported for
a number of materials, such as Si, GaAs, InP and ZnO [1347]. A field of ZnO whiskers is shown in
Fig. 14.6. If heterostructures are incorporated along the whisker axis [1348], quantum dots or tunneling
barriers can be created (Fig. 14.7a). The growth mode relies often a VLS (vapor-liquid-solid) mechanism in which the wire materials are first incorporated into a liquid catalyzer (most often gold) drop
at the tip and then used to build up the nanocrystal. In [1349], the layer-by-layer growth of a GaAs
nanowire via this mechanism has been observed in-situ by TEM (Fig. 14.7b) and is also available as
an impressive video. Another nanowire growth mechanism is the VSS (vapor-solid-solid) mechanism
that works without liquid drop on top of the wire.