A simple guide to Low Dimensional Semiconductor Physics, written specifically for the non-physicist reader.
Many people ask me what I do and what is the latest news from my research field, and expect a concise single sentence answer. Physics is obviously not a new science: Rather a lot has already been done, and a lot of things are well understood. New research inevitably delves somewhat deeper, and the cornerstones of physics upon which it is based are not always completely familiar to the general public. The concise single sentence answer is thus not feasible as an explanation to everyone whom I might meet, and it is not without some regret that I admit this.
With that in mind, I devote this page to explaining some of the underlying physics here where there is room for a little expansion. If you are not a physicist then please read on as this page is written for your benefit — I promise to include nothing technical that is not explained beforehand, but expect a little common knowledge about the world in which we live that I hope everyone already possesses. Each paragraph deals with a concept and it is necessary to know one concept before proceeding to the next, although skip further on if you know the concept already…
Motivation: Things in everyday use
The field of low dimensional semiconductor physics is a very active one - much of the technology in common use relies upon it. Take for example the computer you are using to read this page. The processor that runs everything on your computer is in actuality a complex circuit board made from a semiconductor material, silicon, measuring only a few millimetres across. As you probably know, the CD (or DVD) drive found in most computers relies on a laser to read the data from the disc. The laser uses low dimensional semiconductor technologies devised and researched by physicists many years before the first CD drive appeared in a computer. With the ever increasing popularity of the internet comes the demand to be able to retrieve the data faster, to the point where the speed of electrical signals down wires is no longer fast enough. There is a whole field of low dimensional semiconductor research into the technologies to make it possible (as is currently used today) to send signals with pulses of light down ‘wires’ known as optical fibres. Amongst others, my research is based on such technologies as these.
Its a 3D world
The world in which we live is based on three dimensions of space (3D). For example if you hold in your hand a (small) ball, you can move the ball up, down, left, right, forwards and backwards (just hold it, don’t throw it). Each of those motions are in one of the three spatial dimensions and any motion of the ball can be broken down into components of the three dimensions. Now place the ball on a tray, and roll it around just by tilting the tray a little. Notice how forwards, backwards, left and right are possible relative to the surface of the tray, but up and down is not. Now put the ball in a closed box the same size as the ball and try to move it without moving the box. Hmmm… doesn’t move does it? Alright, so this is nothing amazing but just remember the ball, the tray and the box because we’ll come to them later.
Smaller, smaller, smaller!
Consider the ball itself now, and what is it made of? If you were to chop it into very small pieces it would be still made up of the same stuff, right? What about if the pieces were chopped so small you couldn’t actually see them? What are they now? It is common knowledge in the world today that everything is made of ‘atoms’, which for a long time were regarded as the smallest (indivisible) elements of everything. There are plenty of elementary articles out there that discuss atoms, so I won’t dwell too much here. Atoms are made from even smaller particles known as electrons, protons and neutrons. Atoms that contain different combinations of these three subatomic particles are different elements such as Gold, Carbon, Helium or Uranium.
Wires and drinking straws
Having discussed dimensions of space and the atoms that we’re made of, we are now ready to understand semiconductor materials. Let us define a semiconductor by consideration of two common objects: a wire (i.e. copper), and a drinking straw (i.e. plastic). Both are long and thin, but only one will allow you to use a battery to light a bulb. The copper is a conductor; the plastic is an insulator. These concepts are in fact the two extremes of a single system. Electrons can travel quite freely in copper, but not in plastic, which is why the bulb lights up. A semiconductor is a material in which the electrons can move more easily than in the drinking straw but less easily than in the wire. Semiconductors thus take some of their properties from each extreme case, but are not completely like either case. This is one of the reasons why they are of much interest to physicists: their properties can be modified in the right conditions.
The realm of the quantum
So what do we mean by Low Dimensional semiconductors? This is one such modification of the properties of the ‘normal’ semiconductor. This is the conversion from the ball moving freely to placing it on the tray (as discussed above). The ball on the tray only has two dimensions in which it can freely move; it is confined in the other dimension. Replace the ball with an electron and the world with some semiconductor material, and we now have the distinction between a bulk semiconductor and a quantum well. In a quantum well system the electron is confined in one of the spatial dimensions, and represents the simplest example of a low dimensional semiconductor.
An electron in a quantum dot is analogous to the ball in the box: Confinement in all three spatial dimensions. Much of my research studies quantum dots and the properties of their electrons that result from this ‘full’ confinement. You can’t see them because they are only about 10 nanometres (10 millionths of a millimetre) across, but when doing an experiment such as shining light through the quantum dot material and comparing the results with a bulk semiconductor material it is obvious they are there. The behaviour of the electrons in quantum dots is of most interest to my research, especially when the quantum dots are placed in a high magnetic field or when extremely short laser pulses are shone on them; it is these experiments that allow us to study some of the most fundamental interactions in physics.