Electricity seems very simple - you just flick a switch and there it is. But how
often have you thought about what is happening to the electrons which make up
that electricity? The electrons' behaviour can change depending on what material
they are in.
Materials can be divided into three categories according to their ability to
conduct electrons. In conductors, such as metals, electric
charge can easily flow. In insulators, such as wood, charge does not flow at all.
The third set of materials is semiconductors, where the material's ability to
conduct charge can be controlled so that it can behave like a metal or an insulator
or somewhere between the two. We study how these materials behave under different
conditions so that we can learn more about the physics behind them and how they
can be used to make devices for the modern world. You have already come across
semiconductors in electronic devices. Every processor chip inside a computer uses
a semiconductor - in this case silicon. This makes semiconductors an important
area of research for industry.
Making semiconductors
The properties of the semiconductors we study are very dependent on the exact layout
of the atoms inside the material. That is why we have machines that use a technique
known as Molecular Beam Epitaxy (MBE), where we can grow semiconductors one atomic
layer at a time - that is a precision of about 10-10 m. This gives us a great level
of control over the materials we study.
A Molecular Beam Epitaxy machine installed at the Cavendish Laboratory for
growing semiconductors with atomic precision.
A semiconductor sandwich
The most important use of machines like the one shown is to grow different types of
layered structures. One of common way of growing semiconductors is to sandwich
a very thin layer of one material between two layers of a different material. For
example, an extremely thin layer of gallium arsenide can be sandwiched between two
layers of aluminium gallium arsenide.
Low-dimensional semiconductors
These two different materials have different electronic properties, and so electrons
introduced into this semiconductor sandwich can be trapped inside the sandwich
filling (the gallium arsenide layer), unable to move up and down. They have been
trapped into only two dimensions. When we confine the electrons like this, their
properties can change dramatically. Such layered structures can be used as a
starting point for many different types of electronic device.
We can also confine the electrons so that they can only move forwards or backwards
in straight line. In this case they are trapped in only one dimension. We can do
this either by using physical walls or electrical forces. If we confine the electrons
to one spot, the electrons have zero dimensions for movement. These are called
quantum dots.
We can also confine the electrons so that they can only move forwards or backwards
in straight line. In this case they are trapped in only one dimension. We can do
this either by using physical walls or electrical forces. If we confine the electrons
to one spot, the electrons have zero dimensions for movement. These are called quantum dots.
An Atomic Force Microscope image of quantum dots
Lasers and imaging
Careful design and planning of the semiconductor growth allows us to create
materials where the energy of the electrons inside the semiconductor can be used
to produce light at specific wavelengths. This is useful as we can develop lasers
for wavelengths that cannot easily be produced by conventional methods. Some of
these wavelengths could be very useful for imaging people either for security or
medical purposes and, unlike x-rays, are not harmful.
