The top-down approach limits the dimensions of devices to what is technically achievable using

alignment with the quantized level in the well, resulting in a peak in the current-voltage characteristics and a region with negative differential resistance. 

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REVIEW
FEATURE
Forming low-dimension nanostructures
Let me now give a brief overview of the approaches taken to
form nanowires and QDs. The use of lithography for top-
down fabrication of dots and wires is illustrated in Fig. 5a.
Fig. 5b and 5c give two examples of bottom-up self-assembly
of QDs. Fig. 5b illustrates the principle of patterned growth
of an island structure by which a QD structure can be formed
at the apex of the formed pyramid. The single most
important method of fabrication for QDs is based on a strain-
induced rearrangement of a thin, compressively-strained
layer into small islands, often formed on a wetting layer a
few atoms thick
5
. This method, called Stranski-Krastanow
(SK) growth and shown in Fig. 5c, has been used extensively
for basic investigation of the optical properties of single QDs,
as well as for active zero-dimensional structures in tunneling
devices and semiconductor laser structures. If incorporation
of QDs in or on a semiconductor wafer is not required,
colloidal or aerosol methods may be used to form highly
ideal QDs such as core-shell structures. These types of
nanoparticle QDs are finding important application as
fluorescent probes in biomedical research. 
Various techniques have been used to realize one-
dimensional nanowires, such as the accumulation of multiple
atomic steps during growth on vicinal substrates. This
effectively allows a nanowire to form at multiple-step
locations
6
. Another approach uses the fact that a thin film
ideally forms as a monolayer (ML) during growth. For a
vicinally cut wafer, it may be possible to grow a fraction of a
ML of the low and high band gap material to form a laterally-
defined vertical wire, either directly or via the formation of a
serpentine superlattice structure
7
. The third example is
growth on a pre-etched and undulated substrate, in which
nanowires are formed in the bottom of V-grooves that
develop during the selective growth on certain crystalline
facets
8,9
. It should be noted that none of these self-assembly
techniques allow modifications of nanowire properties, e.g. in
terms of doping modulation or the formation of
heterostructures along the nanowires. This limits their use for
quantum devices.
Catalytically active nanoparticles
The rest of this article will describe a method based on the
use of catalytic nanoparticles to induce the growth of a
homogeneous rod of a semiconductor, with the diameter of
the wire determined by that of the nanoparticle. I stress that
these crystals are rod-like, to avoid confusion with nanotube
structures consisting of one (or more) cylindrical layers of
atoms wrapped up in a coaxial fashion. The method I will
concentrate on, shown in Fig. 6 and often called the vapor-
liquid-solid (VLS) growth mode, was first described in the late
1960s by Wagner
10
. At that time, however, the technique
October 2003
2 5
Fig. 4 A top-down approach to making one-dimensional quantum devices, like resonant tunneling via QDs. Method pioneered by Randall and Reed at Texas Instruments in the late 1980s
4
.
The resulting device properties were limited by fabrication-induced damage and imperfect lateral control. (© American Physical Society 1988.)
Fig. 5 Technologies to form QD structures. (a) illustrates top-down fabrication via
lithographic patterning, etching, and overgrowth. (b) shows an example of pattern-