Quantum dots, nano-scale islands of one semiconductor material embedded in a sea of another, are often described as artificial atoms; in experiments they share many properties exhibited by those of simple atomic systems. This is somewhat amazing as they are, in fact, comprised of anywhere up to hundreds of thousands of atoms. Their scale, and the materials from which they are composed, allow them to be easily integrated into existing semiconductor processing technology. This leads many to predict a very exciting future for optoelectronic devices incorporating quantum dots. In the near future lasers incorporating quantum dot technology will outperform existing technology and, in the long term, a second revolution of electronics and optoelectronics has been foreseen, with a new class of device that is able to exploit quantum mechanics at the level of individual quanta. Such technology would open the door to applications including light sources that enable physically-private communication to take place on public networks, a memory element which is capable of storing individual photons and releasing them at whim and, with time, in components to form the building blocks of an optical quantum computer.
My Royal Society funded research group at Lancaster University aims to make trivial the process of reading, writing and manipulating quantum information.
Molecular Beam Epitaxy
Semiconductor structures containing quantum dots are grown, literally one layer of atoms at a time, at Lancaster by a process known as molecular beam epitaxy. The machine itself is quite grand:
(Molecular Beam Epitaxy machine-A at Lancaster
The growth process takes place at temperatures typically between 400 and 800°C, in an ultra-high vacuum to avoid unintentional incorporation of stray atoms. To help create such a good vacuum, the chamber in which the growth takes place is surrounded by liquid nitrogen at a chilly -200°C.
The process of growing quantum dots is remarkably simple, and similar in many ways to the in mechanism causing water to bunch-up and form droplets when spilled on a sheet of glass. We deposit one material (normally Gallium Antimonide, GaSb, or Indium Arsenide, InAs) on a second (Gallium Arsenide, GaAs, say). The atomic spacing for these two materials is different, this results in a build-up of strain as the second material is desposted and, ultimately, for the top layer to break up into droplets, or dots, to minimise this strain.
Alongside quantum dots we can also grow nano-scale rings by a slightly modified process. The picture below shows a cross-section of one such ring, bright dots in the picture represent individual atoms of the element Antimony. The major-radius of the torus is around 20nm, about ten thousand times smaller than the width of a human hair.
(xSTM image of a nano-ring, courtesy of TU/e)
With these rings we hope to observe the Aharonov–Bohm effect, a strange phenomena unique to quantum mechanics, in which an electronic charge can interact with a magnetic potential, without the presence of a field. The diagram below illustrates the arrangement:
In this experiment consider the coil in the centre to be very long, a current flowing generates a magnetic field in the coil but, crucially, not outside it. The Aharonov-Bohm effect leads to electric charge circulating such a coil being affected by the magnetic field within it, despite the two not meeting... a surprising result.
The dot-containing samples are characterised optically through a process known as photoluminescence. A laser excites and frees electrons that would otherwise be bound to the crystal lattice of the sample. These free electrons migrate into quantum dots and, after some time, relax emitting light. It is this light from our samples that we measure and analyse. During this process we can cool our structures down to -271°C or heat them to +100°C and expose them to huge magnetic fields with a super-conducting magnet. Experiments take place in the blue cylinder below...
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