Diamond is well known for its appeal as a gemstone. Perhaps less well known are some of its extreme material properties.
As well as being the hardest material in nature, diamond is very good at conducting heat (at elevated temperatures) making it an ideal material for heat-management applications.
My colleagues and I at the University of Melbourne and La Trobe University are harnessing some of these less-well-known material properties of diamond in order to, hopefully, develop the next generation of transistors. Transistors form the building blocks of computer chips which can be found in devices such as laptops and smartphones.
We’re investigating the fundamental behaviour of charge carriers in synthetic diamond – that is, how electrons and holes – a theoretical lack of an electron and therefore a positive charge – behave in diamond.
Future transistors will ideally operate at much higher speeds than conventional transistors and their working might not be based on electrical charge, but rather on the “magnetic moment” (imagine 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 (a flow of electric current without any resistance) can exist in diamond films that are doped to a very high concentration with boron atoms.
We are trying to observe the same phenomenon in nitrogen-doped ultra nanocrystalline diamond (UNCD) films, 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.
The operation of this particular fridge does not rely on liquid helium (an expensive and finite resource) but it can cool down to a minimum temperature of roughly 10 milli-Kelvin (-273.14℃) – more than 100 times colder than the average temperature in space.
Solids and phonons
In solid materials, atoms are typically arranged in a periodic fashion, which scientists refer to as a lattice. By stacking layers of these lattice planes together, we can eventually form a crystal. A solid is typically comprised of many crystals (polycrystalline material), 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. This is one of many reasons gemstones, such as diamond, can be so expensive.
Generally, the atoms in a crystal are not static – they vibrate back and forth. At room temperature, there is enough thermal energy to shake the lattice continuously.
These thermally activated vibrations are called phonons. Phonons play a major role in determining the physical properties of solids, such as the thermal and electrical conductivity, because of their interaction with mobile charge carriers.
At dilution refrigerator temperatures, however, there is not enough thermal energy available 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 with) with phonons, and their true quantum mechanical nature can be revealed.
Two diamond devices
As part of our research, we’re currently investigating two different types of diamond devices.
The first device type is made from nitrogen-doped (20%) UNCD and is arranged as a Hall bar – a standard shape used in solid-state physics (see image below) which can be used to determine the density and mobility of charge carriers in the device. We do this by applying a magnetic field perpendicular to the sample surface.
The nitrogen-doping provides excess electron charge to the diamond, which is otherwise electrically insulating. These excess electrons are mobile and are therefore responsible for the flow of electricity in this material.
The UNCD films we are investigating are polycrystalline materials consisting of ultra-small diamond grains ranging from 2-5 nanometres up to 1 micron in size. Due to the small size of these grains there is a lot of surface area that interconnects these grains, as opposed to a single crystal of diamond.
It is these boundaries between the grains where impurities (“dopants”) are most likely to migrate to and contribute to electrical transport.
We therefore expect that not only the doping concentration, but also the size of the crystals – and therefore the surface area in between the grains of the poly-crystalline film – play an important role in the conductivity of the material.
Single crystal diamond:
In contrast, the second device type is a Hall bar transistor fabricated out of synthetic (i.e. lab-made) single-crystal diamond, which contains very little nitrogen.
To create excess charge carriers in these high-quality single crystal diamonds we use a technique called hydrogen (H)-termination. Here, we expose our crystals to a microwave plasma containing hydrogen gas, which in combination with a thin water layer, introduces a hole-type surface conductivity.
So, in these hydrogen (H)-terminated 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.
In order to determine the fundamental properties of the charge carriers in our diamond samples, we used a standard method in physics, usually referred to as the Hall effect. Here, the electrical current through the sample (in this case the diamond devices) depends on the intensity of the magnetic field applied to the sample.
This relationship allows us to determine fundamental properties of the charge carriers, such as density and mobility. This is then repeated for different temperatures to see if the material behaves as an insulator or metal.
From the preliminary low-temperature measurements we have performed so far, we have learnt that the UNCD samples behave as insulators. That is, the electrical current disappears when you drop the temperature below a certain point – somewhere between 1 and 10 Kelvin – depending on the doping concentration.
Other research groups have demonstrated otherwise and therefore more work is needed to reconcile our data with their published results.
The single-crystal diamond samples, however, conduct electricity all the way down to the lowest possible temperatures the dilution refrigerator is capable of reaching (10 milli-Kelvin) and display only moderate temperature dependence.
So, what does all this mean?
First of all, it’s worth noting that our research is fundamental in nature and is, as such, not application driven. That said, as more and more scientific knowledge of charge carriers’ behaviour becomes available, our research might be applied to the design of future transistors and/or sensors.
In the near future we want to create nanometre-scale electronic devices in diamond using the electron-beam lithography facility at the Melbourne Centre for Nanofabrication (MCN). Such ultra-small devices will lead to novel bio-compatible sensors – such as has already been used in the Bionic Eye – and diamond transistors whose operation will depend on the presence of a single hole charge.
Importantly, we have recently succeeded in passivating (protecting by overgrowth) the H-terminated diamond surfaces with an alumina capping layer. This means that the technology for implementing surface conducting diamond into device applications is now more robust and might become a reality.
So, the next time you look at that diamond engagement ring on your finger, stop and think for a moment. There’s a lot of fundamental physics living below that shiny surface.