Dr Hoe Tan prepares a wafer ready for insertion into the MOCVD reactor.
MOCVD stands for Metal-Organic Chemical Vapour Deposition. This is a technique for depositing thin layers of atoms onto a semiconductor wafer. Using MOCVD you can build up many layers, each of a precisely controlled thickness, to create a material which has specific optical and electrical properties. Using this technique it's possible to build a range of semiconductor photodetectors and lasers, the devices that lie at the heart of the information revolution.
one of the most sophisticated Metal-Organic Chemical Vapour Deposition
(MOCVD) laboratories in the world, and thereby gives Australia the capacity
to keep in touch with the latest developments relating to the industrial
manufacture of optoelectronic devices.
Who runs it: The MOCVD reactor is housed in a purpose-built clean room in the Department of Electronic Materials Engineering (Research School of Physical Sciences and Engineering). It was installed in 1991 and has undergone constant refinements and upgrades since then.
The MOCVD approach was chosen over other methods because of its flexibility in growing precision controlled layers for special applications as well as it's ability to be scaled up to industrial-scale production with relative ease.
How does it work:The principle of MOCVD is quite simple. Atoms that you would like to be in your crystal are combined with complex organic gas molecules and passed over a hot semiconductor wafer. The heat breaks up the molecules and deposits the desired atoms on the surface, layer by layer. By varying the composition of the gas, you can change the properties of the crystal at an almost atomic scale. It can grow high quality semiconductor layers (as thin as a millionth of a millimetre) and the crystal structure of these layers is perfectly aligned with that of the substrate.
The first project using this technology investigated the growth and characterisation of gallium arsenide (GaAs) and aluminium gallium arsenide (AlGaAs) crystal layers (epigrowths) suitable for a range of applications in advanced communications systems.
Prof Jagadish has led the MOCVD group since early 1992. During this time there have been several research highlights, including world records for the highest concentration and most abrupt atomic layer doping, so-called delta doping, for carbon, silicon and zinc in GaAs and AlGaAs layers.
The group has also grown narrow multi layers, so called quantum well structures, with very interesting optical and phonon (vibrational) properties.
More recently, the group has taken giant steps towards the fabrication of practical devices. For example, the group has fabricated laser sources, light reflectors and modulators of the types used in CD players to record and read digital information stored on the disk. Novel operating modulators at wavelengths of 550-630nm have been produced.
Semiconducting lasers are able to convert electrical impulses to light, and they have many uses in advanced optical communications. Indeed, EME researchers have made a range of so called GRINSCH (Graded Index Confinement Heterostructure) lasers which have about 40 different layers of varying thickness, compositions and dopings.
Some of the quantum well layers are only seven nanometers thick. 'We have developed novel growth and processing methods to tune the wavelength of the lasers by tuning width or composition of the quantum well', said Prof Jagadish.
MOCVD reactor has recently been augmented with a new, state-of-the-art
reactor. Costing around $3 million to install, the new reactor can process
more wafers in a given time, and deposit layers with a higher precision
than the original reactor. Indeed, even after years of fine tuning the
original reactor could only lay down layers to within 10% of the desired
thickness. While this was a remarkable achievement when you consider how
thin these layers are (measured in atoms), it still wasn't good enough
for the devices they are attempting to build. The new reactor is able
to deposit layers to within 1% of the desired thickness.