Diamond-Based Optoelectronic Device Reveals Unexpected Phenomenon Reminiscent of Slow-Motion Lightning

Diamond is in many ways the ultimate material. In addition to its enduring aesthetic value, diamond is also an extremely versatile industrial material. Although its claim to be the hardest substance known to science has been usurped by ultra-rare minerals and recently developed synthetic materials, it still tops many rankings of material properties.

Diamond’s ability to conduct heat (its “thermal conductivity”) is unmatched, and it also provides an ideal host environment for the quantum bits (qubits) that are currently revolutionizing magnetic field sensors and could one day make room-temperature quantum computing possible.

Perhaps less well known is that diamond is also the ideal material for high-power electronics: the essential circuit elements in power plants, electrical distribution substations, electric cars and other emerging technologies.

At least theoretically.

Diamond is hard

None of these technologies currently use diamond, and the vast majority use silicon, which must be kept cool and has limited operating voltages.

About 10% of the electrical energy produced is wasted due to the limitations of silicon, and diamond could reduce these losses by 75%. So why aren’t we driving electric cars with diamond power electronics?

Well, because diamond is hard. It’s hard to manufacture, hard to bond to metals, hard to produce in large quantities, and hard to engineer with the right impurities to tailor its electrical properties – all of which are necessary for the scalable production of electronic components.

We do not yet fully understand how charges flow inside diamond and how unavoidable impurities and defects affect these electrical properties.

Looking at a diamond is electric

In a recent study published in Advanced materialsConducted with colleagues from the University of Melbourne, RMIT University and the City College of New York, we sought to combine electrical measurements of a diamond optoelectronic device with 3D optical microscopy.

Our motivation was to understand several curious findings that others have reported when these two techniques – electrical measurements and optical microscopy – were used independently.

By combining them, we were able to see for the first time, in stunning three-dimensional detail, what happens when charges enter and move through a diamond electronic device.

To do this, we used impurities in the diamond crystal lattice that are formed from nitrogen atoms located next to a space in the lattice, called nitrogen vacancy centers or NV centers.

These NV centers are well known as sensors and can act as qubits in quantum computing.

NV centers can be either neutral or negatively charged, and we can detect the charges flowing in the diamond by monitoring the electrical charge of these defects.

In our microscope, neutral NVs appear slightly more orange than negative NV centers, allowing us to create an image of where a current has flowed.

Our experiment consists of a diamond with many NV centers covered with two metal electrodes inside an optical microscope.

We use a green laser to generate an electric current in the diamond (similar to the process that turns light into electricity in solar cells), then observe and record where that current flows.

Our technique enables mapping of the complete 3D structure of charge generation and flow over time.

What we saw was unexpected.

A miniature lightning strike in slow motion

Current appears to flow from one side of the device to the other in thin, streamer-like filaments that initiate (nucleate) from specific points along the metal electrodes.

It feels like love at first sight.

You may have heard that lightning comes from the ground, not from the clouds. In reality, just before lightning strikes, an invisible channel of ionized gas called a “step tracer” descends from the cloud to the ground.

This leader is attracted to things on the ground, such as tall trees or lightning rods on buildings.

The channel this leader leaves in the air presents a highly conductive path for the return stroke of visible lightning which then bursts from the ground, creating a brilliant flash of bifurcating and zigzagging light.

Real lightning consists of thousands of amps of current flowing in microseconds (millionths of a second), but similar principles underlie the process occurring in our diamond, except it is measured in picoamps (a billionth of an amp) and over whole seconds.

In our diamond circuit, electrons (the invisible “leader”) are attracted to specific features of the metal-diamond connection (the “ground”).

As the electrons flow, they create behind them filamentary channels of increased conductivity along which the positive charge flows in the “return motion” that we capture through our microscope.

Why do electrons flow in filaments? We don’t know yet.

Designing better diamond-based electronics and quantum computers

We believe that these specific “ground” features on the electrode – analogous to trees and skyscrapers that attract lightning – are actually points where electrical contact with the diamond is better than other points.

We can use our technique to precisely locate these focal points, making it a powerful diagnostic tool that can shed light on the problem of creating good metal-diamond connections in electronic devices.

Through this study, we also showed that we can modify the charge states of the NV centers in diamond to change the way current flows.

We essentially draw patterns inside the diamond with lasers, “charging” the NV centers and other impurities, and creating a kind of circuit. This is a potential building block for creating optically reconfigurable diamond electronic devices.

Our work opens a whole new field of research around the control of charge transport in diamonds and the imaging of the results. This is important both for emerging power electronics and for quantum technology.

We can also apply our technique to other materials that are much more advanced in their electronic applications, such as silicon carbide, which already powers new generations of electric vehicles.

We hope this will lead to improvements in interfacing electronics with quantum materials and in building room-temperature quantum computers using diamond.