ScienceWise - Summer 2010


Article Illustration
Principle of an MOCVD reactor. The heat around the substrate wafer causes the metal organic gas molecules to dissociate depositing their cargo atoms onto the wafer surface where they settle to form a perfect extension of the underlying crystal lattice.
Article Illustration
When the temperature is lower the substrate wafer is no longer hot enough to cause the gas to dissociate. However, the liquid gold eutectic drop is able to absorb the cargo atoms from the gas and then deposit them on the top of the growing nanowire.

A Recipe for Amazing Devices

Nanotechnology is a hot topic in science at the moment, but what is it that’s so special about making things very small? Nanoscale materials have a number of properties that make them behave in a fundamentally different way from the same material on a large scale.

Firstly, when a given quantity of material is broken up into pieces that are only a few nanometres across vastly more of the atoms lie on the surface than they would in a single lump with the same mass. This increases the surface area for reactions but it also changes the way the surface atoms are bonded. A combination of these effects makes many materials that would be inert in bulk behave in a very reactive manner when engineered into nanoparticles.

When semiconductor devices are created on the nano scale, these surface effects generate additional benefits. One of the performance limitations of devices like transistors and lasers is the presence of imperfections within the crystal lattice. These are simply a product of thermodynamics and are very difficult to totally eliminate in large-scale materials. However, when a structure is only a few atoms across the thermodynamics and internal stresses become quite different. It often takes more energy to create defects than to have a perfect lattice, leading to far better crystal growth. The small scale also means that you can stack materials that have different atomic spacing on top of each other in layers without the major disruptions to the lattice that would occur in large-scale devices, which is important in creating devices.

The second major change that happens as devices approach the nano scale is that quantum mechanics begins to play an increasingly important role in their behaviour.

By combining these nanoscale effects it has become possible to create semiconductor devices that would have been pure science fiction twenty years ago.

Professor Chennupati Jagadish leads a research group at the Australian National University focusing on the development of nano scale semiconductor devices.

“It’s the combination of surface and quantum effects that make nanotechnology such a unique and interesting area to work in.” Professor Jagadish says. “What we’re doing is essentially engineering but at the atomic and molecular scale”

One device Professor Jagadish’s group are currently developing is a nanowire laser. These structures are just a few atoms across and are quantum engineered to achieve an amazingly efficient conversion of electricity into light. In the centre of each wire is a quantum dot – a region of material only a few atoms in each dimension. This incredibly small space only permits electrons to have very specific energies, defined by the quantum rules. This in turn means that the light emitted is of very specific wavelengths. At either end of this central active region of the nanowire are multi layer reflective structures known as Bragg mirrors that complete the laser. One exciting application of nanowire lasers is in generating the single photons of light that are required for secure quantum communications.

But how do you create something like a nanowire laser in the first place?  The answer lies with a technique known as Metal Organic Chemical Vapour Phase Deposition (MOCVD). Essentially, a stream of gas is passed over a heated semiconductor wafer known as the substrate. The gas contains organic molecules in complexes with the component atoms of the desired semiconductor. One of the most common semiconductors used is gallium arsenide (GaAs).

The process begins with a super clean and highly polished GaAs substrate wafer being loaded into the reactor. The complex gases are then passed over the surface. At room temperature nothing much happens, but once the wafer is heated to about 600°C the gas begins to dissociate, depositing gallium and arsenic atoms on the surface of the wafer. The high temperature gives these atoms lots of kinetic energy to move around and the slow deposition rate allows them plenty of time to shuffle around until they create a perfect extension of the underlying lattice of the substrate wafer. In effect, the substrate wafer becomes a thicker, but retains its perfect crystal structure. Of course if this were the end of the matter, it would be rather pointless because you’d just be making thicker wafers. But what the gas stream technique also allows you to do is change the composition of the additional material as it grows by changing the gas mix.

In this way you can build up layers with different compositions and different impurities giving each layer unique electrical and optical properties. The thickness of the layers in this sandwich can be controlled by the growth time, the longer you pass a particular gas combination over the substrate, the thicker that particular layer becomes.

At the end of the growth, you have a wafer that is typically 50mm in diameter with a multi layer sandwich on top. Because the wafer has perfect crystal structure, it can be easily cleaved along the atomic planes to create thousands of tiny individual devices each one of which is roughly 1mm square.  But whilst that’s fine for devices live DVD player lasers, it’s still gigantic by nano engineering standards!

In order to create nano versions of these lasers you use the same basic method but there are a few extra tricks you need. Before loading the wafer in the reactor it has to be coated with a colloidal solution of ultra fine gold particles, each a few nanometres across. This creates millions of tiny dots on the surface of the wafer.

Gold itself has a very high melting point (1064°C) as does GaAs (1238°C) but when heated together, they form a eutectic – a compound with a very much lower melting point than its constituent parts.

If one were to heat the wafer to over 600°C as before and pass the gases over, an epitaxial surface layer of new crystal would form right across the surface and the gold eutectic wouldn’t have much of a role. But if the temperature is kept down to around 400°C, the gases don’t dissociate on the surface. However their metal cargo atoms do become incorporated into the liquid gold eutectic. This results in a super saturated solution of gallium, arsenic and whatever other compounds you have in the gas, forming in the nanoscale eutectic drop.

This is where something quite extraordinary begins to happen. The supersaturated eutectic begins to deposit these elements onto the wafer in the form of perfect crystal lifting the gold dot upwards as it goes. As the gases flow, these minuscule needles of crystal begin to rise from the surface with the gold eutectic cap on the top.

Just as with the large area growth, if you change the composition of the gases you change the type of crystal that’s laid down under the gold eutectic drops. This makes it possible to build up different slices along the nanowire as it grows. In this way it’s possible to create long fine nanowires with multilayer mirrors at the ends and an active laser region in the centre.

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