Diamonds being used to develop next-gen transistors

Diamonds may traditionally be a girl’s best friend but these little gems could soon become the electronics engineer’s best friend too.

They are not only the hardest material known, they are a good conductor of heat and, when suitably doped, become an excellent conductor of electricity.

Laurens H Willems van Beveren, a senior postdoctoral research fellow in solid state physics at Melbourne University, is researching these and other properties of diamonds to perhaps develop the next generation of transistors.

“We’re investigating the fundamental behaviour of charge carriers in synthetic diamonds - that is how electrons and holes behave in a diamond,” he says. Future transistors will ideally operate at much higher speeds than today’s devices and their working might not be based on electrical charge but rather on the ‘magnetic moment’ (a tiny magnetic field) of individual charge carriers.

This magnetic moment is referred to as ‘spin’ by physicists and can display quantum mechanical behaviour. The emerging field of technology known as ‘spintronics’ is focused on building electronic circuits based on this physical quality.

It has been predicted and recently shown by several groups that a superconducting state can exist in diamond films that are doped with a very high concentration of boron atoms.

“We are trying to observe the same phenomenon in nitrogen-doped ultra nanocrystalline diamond films (UNCD) for which the charge carriers are electrons rather than holes. This work could result in a new class of diamond-based, high-temperature superconductors with sensing and device applications.

“For our experiments we used a recently commissioned dilution refrigerator to subject our samples to a broad range of temperatures. This can cool to a minimum temperature of -273°C - more than 100 times colder than the average temperature in space.”

In solid materials, atoms are typically arranged in a periodic fashion called a lattice. By stacking layers of lattice planes together, a crystal can be formed. A solid typically comprises many crystals but can also be made up of a single crystal only.

It is very difficult to synthetically grow a large piece of high-quality, single crystal diamond as it must be grown from a single seed under very precise growth conditions.

Generally the atoms in a crystal are not static. They vibrate back and forth and at room temperature there is enough thermal energy to shake the lattice continuously. These thermally activated vibrations are called phonons that play a major role in determining the physical properties of solids such as thermal and electrical conductivity because of their interaction with mobile charge carriers.

At dilution refrigerator temperatures there is not enough thermal energy to shake the atoms and the presence of phonons is drastically suppressed. This implies that mobile charge carriers in solids can become free of interacting (scattering) phonons and their true quantum mechanical nature can be revealed.

Research is currently focused on two different diamond devices. The first is made from nitrogen-doped (20%) UNCD and is arranged as a Hall bar - a standard shape used in solid state physics that can be used to determine the density and mobility of charge carriers in the device. This is done by applying a magnetic field perpendicular to the sample surface.

The nitrogen-doping provides an excess electron charge which is otherwise an electrical insulator. The excess electrons are mobile and responsible for the flow of electricity in this material. The UNCD films are polycrystalline materials consisting of ultrasmall diamond grains ranging from 2 to 5 nm to 1 micron. Due to the small size of these grains there is a lot of surface that interconnects these grains as opposed to a single crystal of diamond. It is these boundaries between the grains where dopants are most likely to gather and contribute to electrical flow.

The researchers expect that not only the doping concentration but also the size of the crystals and the surfaces between the grains of the polycrystalline film play in important role in the conductivity of the material. The second device is a Hall bar transistor, fabricated from a synthetic single crystal diamond which contains very little nitrogen. To create excess charge carriers in these high-quality single crystal diamonds, a technique called hydrogen (H)-termination is used. The crystals are exposed to a microwave plasma containing hydrogen which, in combination with a thin water layer, introduces a hole-type surface conductivity.

In these hydrogen crystals the majority charge carriers are holes rather than electrons and the current flows on the diamond surface only, rather than in the bulk of the crystal. To determine the fundamental properties of the charge carriers in the diamond samples, the Hall effect was used where the electrical current passing through the sample depends on the intensity of a magnetic field applied to the sample. This relationship allows a determination of the fundamental properties of the charge carriers such as density and mobility. This is than repeated for different temperatures to see if the material behaves as an insulator or carrier.

From the preliminary low-temperature measurements it is seen that the UNCD samples behave as insulators. That is, the electrical current disappears when the temperature is dropped below a certain point - somewhere between 1 and 10 K, depending on the doping concentration. Other research groups have demonstrated otherwise, says Beveren, and more work is needed to reconcile the Melbourne data with other published results. The single crystal diamond samples, however, conduct electricity all the way down to the lowest possible temperatures the dilution refrigerator can reach and display only moderate temperature dependence.

The Melbourne research is fundamental and not application driven. However, as more and more scientific knowledge of charge carriers’ behaviour becomes available, this research might be applied to the design of future transistors and/or sensors. In the near future, the researchers want to create nanometre-scale electronic devices in diamonds using the electron-beam lithography facility at the Melbourne Centre for Nanofabrication. Such ultrasmall devices will lead to novel biocompatible sensors such as those developed for use in the bionic eye and diamond transistors whose operation will depend on the presence of a single hole charge. Importantly, the group has passivated the H-terminated diamond surfaces with an alumina capping layer. This means that the technology for implementing surface-conducting diamonds into device applications is now more robust and might become a reality.