Getting the Blues
Listening to music in the car is a simple pleasure that many of us enjoy. However, especially on long journeys, it can often seem like a startlingly short amount of time before the CD ends and needs to be changed. The problem is that there is simply a limited amount of information that can be packed onto a CD. But now research being done by Professor Joe Trodahl and his colleagues at the MacDiarmid Institute may provide a solution to this frustrating problem.
Professor Trodahl’s research revolves around a family of materials essential in all aspects of the electronics industry, known as semiconductors. Semiconductors are interesting because their electrical properties can easily be manipulated, simply by varying factors such as temperature or the presence of impurity atoms. It is this versatility that makes them so useful in a variety of electronic components.
Silicon is the by far the most common element used in producing semiconductors. It is a convenient element to use, largely because it is so abundant. Pure silicon forms a lattice structure, much like the structure of diamond that is formed by carbon atoms. However, silicon has one severe disadvantage – it can not emit light. A variety of electronic components need to emit light, such as light emitting diodes (LEDs) and lasers, and therefore these cannot be made from silicon.
To make a semi conductor that emits light, it is instead necessary to use a substance that has the same structure as silicon, but is instead made up of an equal mixture of atoms from group three and group five on the periodic table, “Because they have the same number of electrons per pair of atoms as silicon does” said Professor Trodahl, “they end up with the same structure”. Depending on which group three and group five atoms are combined, different light emitting properties can be gained. For example Gallium Arsenide, a common semiconductor used in a variety of electronic components, emits light in the red to infrared range.
However, there is one colour that has proven extremely difficult to produce with traditional semiconductors – blue light. It is not possible to make a blue laser with Gallium Arsenide, for example. Although a variety of tricks have been developed that can be used to move towards the blue end of the colour spectrum, it is not possible to get very far with it.
“The only material that can do that with any efficiency at the moment is Gallium Nitride”, Professor Trodahl explained. Gallium Nitride (GaN) is part of the group of substances known as group three nitrides, and it is these compounds that are the subject of Professor Trodahl’s research. Specifically, the research involves developing new techniques for making thin films of group three nitrides, Gallium Nitride in particular. A thin film is a very thin layer of the substance, bonded to substrate such as silicon, and these are essential for use in electronic components.
Actually making a thin film of Gallium Nitride, however, is no easy matter. “It’s hard to make this material”, said Professor Trodahl, “because normally it is a gas”. Usually, thin films of semiconductors are created by melting the components together, but this is not possible for Gallium Nitride. “If you try to melt Nitrogen and Gallium together, the nitrogen gas leaks out” Professor Trodahl explained. “So you have to devise a new way of making it.”
Developing new methods for making thin films of Gallium Nitride and other related semiconductors is precisely what the team are working on. “We have devised a new way of making thin films of Gallium Nitride,” said Professor Trodahl. Basically, the process is as follows. To begin, the substrate is placed into a very good filtering system, to prevent any impurities. Then, they simultaneously deposit nitrogen and gallium ions onto the substrate. The result is the sought after thin film of Gallium Nitride.
Thin films of Gallium Nitride have several potential uses. For example, as mentioned in the beginning of this article, one interesting use could be to improve the technology used in CD players. “By using blue light instead of red light in the lasers that read CDs, – they are all read with red light at the moment – you can pack twice as many tracks on the CD, because the wavelength is shorter”, said Professor Trodahl. Other uses of the blue light emitted by thin films of Gallium Nitride include use in diodes for navigation equipment, and to complete the colour in full colour displays made from solid state materials, which are more efficient. In these situations, Gallium Nitride provides the missing blue colour that other materials can not provide.
Besides their usefulness in emitting blue light, there are a range of other applications that thin films of Gallium Nitride could be used for. For example, Professor Trodahl and his colleagues are currently trialling an application where Gallium Nitride is turned into a magnetic material. This would have applications in a field known as spintronics – electronics in which not only are the number of electrons controlled, but also a property known as their spin state. Control the spin state, Professor Trodahl says, and “You can do a lot more fancy stuff with it”.
The team also has a collaboration in France with researchers who are exploring whether or not Gallium Nitride, or any other nitride film can be used to store lithium. The interest there is it would make a very lightweight lithium battery. This would be useful anywhere small batteries would be an advantage, for example in hearing aids.
So far, Professor Trodahl has concentrated his efforts on thin films of Gallium Nitride, but there are also many other possibilities for working with other group three nitrides. “The other obvious candidates are Aluminium Nitride and Indium Nitride, or mixtures of those three, to make diodes and so on that emit various colours” he said. Another possibility is in making layers of combinations of group three nitrides. “There is a real advantage in making layers of Gallium Nitride followed by Aluminium Nitride, and other combinations” Professor Trodahl explained. “It’s a very unexplored area.”