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

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The top-down approach limits the dimensions of
devices to what is technically achievable using
lithography. This is the means by which patterns can
be drawn, either in stone as the Vikings did when
they carved messages into granite, or into Si as the
electronics industry does today to build integrated
circuits. Lithographic techniques can create device
features as narrow as 130 nm and the industry sees
the road ahead pretty well drawn up for line-widths
down to ~50 nm. This continued progress does not
come without a price; the cost of new fabs is growing
extremely fast, at a pace that may limit continued
progress, simply because devices and circuits become
too expensive to be economically viable.
So what is new? The new technique allows us to mimic
nature and self-assemble nanowire materials and devices at
extremely downscaled dimensions, making it as easy to
fabricate a 5 nm as a 200 nm nanoelectronic device. As an
example of how ultra-small devices are traditionally made
using top-down fabrication, I will use one of the most
important and interesting device families, the so-called
tunneling devices. These are of great interest for high-speed
electronics, as well as photonics applications, and are
examples of quantum devices, since tunneling is
fundamentally a quantum mechanical phenomenon
1
.
What is a heterostructure quantum
device?
First, I will give a brief introduction to quantum devices in
general and tunnel devices in particular, since these may not
by Lars Samuelson
Self-forming
nanoscale devices
Lund University, 
Solid State Physics/the Nanometer Consortium,
Box 118, S-221 00 Lund, 
Sweden
E-mail: [e-mail]
Image shows nanowires with heterostructure interfaces.
(Courtesy of Reine Wallenberg, Lund University.)
October 2003
2 2
ISSN:1369 7021 © Elsevier Ltd 2003
Devices that have been beyond the reach of
engineers can now be fabricated in new ways. The
crucial factor has been the development of a
technique by which extremely narrow rods, or
nanowires, of a semiconductor can be formed. The
bottom-up, self-assembly process enables accurate
control of dimension, location, composition, and
other properties. The materials are the same
semiconductors, like Si and GaAs, that we have, for
the last forty or so years, been shaping into devices
and circuits. But this process has relied on top-down
fabrication techniques.

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REVIEW
FEATURE
be familiar to general materials researchers. An emphasis will
be put on the materials science approaches for building
quantum structures and quantum devices. 
In quantum physics, properties are dominated by the
wave-nature of electrons (or holes), related to the quantum
mechanical wavefunction we use to describe every state in
which an electron can appear. The allowed energy states and
corresponding wavefunctions are defined by the solution to
the Schrödinger Equation (SE). A well known case is that of
the solutions to the SE for an electron bound by the
attractive potential of a proton, describing the bound states
(ground state plus excited states) of the hydrogen atom. This
classical illustration of quantum physics is often compared to
the particle-in-a-box problem, which describes the properties
of electrons (and holes) bound in a square-well potential
formed by a quantum well (QW) structure. QWs, consisting
of ultra-thin layers of a small band gap semiconductor
sandwiched between larger gap semiconductor materials,
effectively form an attractive potential in which charge
carriers can be trapped. Fig. 1 illustrates, in a simplified way,
how epitaxial growth of combinations of different
semiconductor materials, such as GaAs and AlGaAs, may
create such particle-in-a-box structures. Fig. 2 compares the
way electrons bind to a (spherical) atomic potential and a
square potential formed by the heterostructure interfaces
between different semiconductors. This comparison gave rise
to the term ‘artificial atoms’ for quantum structures formed
by three-dimensional square well potentials, so-called
quantum dots, in which the energy structure is completely
quantized like in an atom. 
If the sequence in which the different semiconductors are
layered is altered, such that a large band gap material is
placed in between low band gap semiconductors, the motion
of electrons through the structure is hindered by a potential
barrier. This effectively separates the two low band gap
semiconductors from each other. Provided that the barrier is
sufficiently thin, quantum mechanics allows another
counterintuitive phenomenon to occur, namely that the