Laser leap

A project to develop laser diodes with the world’s shortest wavelength is due to start next month at Sheffield and Bristol universities. If successful, the technology could be used in a cheap, portable system to continuously monitor for biowarfare pathogens. It could also be employed to increase the storage capacity of DVDs.

The scientists are fabricating the first 337nm laser diode based on gallium nitride/aluminium gallium nitride (GaN/AlGaN) to replace the existing nitrogen gas-based laser for use in the detection of biological particles. The researchers say the shortest laser diode so far reported measures 343nm, whereas most biological molecules show strong absorption in the ultraviolet spectral region ranging from 280nm to 340nm.

‘The project would mainly focus on developing a light source that works in the ultraviolet spectral range, which is needed to excite the biological molecules,’ said Prof Martin Kuball, an expert in thermography technologies at Bristol.

Dr Tao Wang, a specialist in III-nitride materials and devices at Sheffield, added: ‘In order to detect any biological molecules we have to use a very short wavelength, such as the 337nm. The challenge to achieve the shorter wavelength is crystal quality.’

Semiconductor materials are usually grown using a process known as metal-organic chemical vapour deposition, whereby the materials are grown through the surface reaction of metalorganics containing the required chemical elements on a substrate. It is normally a homoepitaxial process, which means the semiconductor grown on the substrate is the same as the substrate material.

‘Homoepitaxy means that, for example, in the ordinary semiconductor the gallium arsenide is grown on the gallium arsenide substrate. For the shorter wavelength we have to use GaN, which we would normally grow on the GaN substrate. Unfortunately this is not available to us at the moment, so we have to use another substrate — sapphire,’ said Wang.

Sapphire is a popular substrate for III-nitrides, such as gallium nitride, due to a similarity in crystal structure. However, the lattice mismatch between sapphire and III-nitride is large.

‘In the deeper ultraviolet emission range, we have to use AlGaN, which is even more difficult to grow than GaN because it exhibits a larger bandgap,’ said Wang. ‘Gallium nitride’s bandgap is around 3.43eV, equivalent to 362nm, but if we want to move to the shorter wavelength and achieve a 337nm laser diode we have to use aluminium gallium nitride materials. This material must be grown on a high-quality aluminium nitride buffer layer, which is extremely difficult to achieve.’

He said the Sheffield team has developed a new technology that can significantly improve the crystal quality of the device structure on the buffer layer and this will be employed for the project. ‘Some companies have reported they can provide the substrates, but the price is extremely high. For example, a gallium arsenide substrate costs £100, but a gallium nitride or aluminium nitride substrate costs up to £5,000.’

According to Bristol’s Kuball, the shorter laser diode will decrease the size of the detector required for biomolecule detection. ‘The problem is that a small light source that works in the ultraviolet range does not really exist. There are very large light sources but you need a large power supply for them and they are bulky. Laser diodes, on the other hand, are very, very tiny,’ he said.

Wang added that the new diode would also significantly increase the quality of biological images which, at present, are taken using LEDs. ‘If you use a laser, the resolution will be significantly improved because the intensity of the power is very high, whereas the power of the LED is low, and also the beam diameter is very narrow, which can improve image quality,’ he said.

Another application would be in the area of information storage. ‘The current red colour DVD means that the laser used on the DVD is normally 635nm, and if you move to the shorter blue, which is in the nitride area, we would significantly improve the capacity of the information storage,’ said Wang.

He believes halving the wavelength would increase the DVD capacity by a factor of four. ‘At 635nm, the red DVD has a capacity of one gigabyte and the double-sided Blu-ray DVD, with an approximate 400nm wavelength, can have an increased capacity of 49 gigabytes.

‘I estimate that if the wavelength decreases by a factor of two, the 337nm DVD, using both sides, would increase the capacity to 80 gigabytes,’ he said.