Ensuring a bright future for diamond electronics

A recent study by scientists at the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) and Princeton University explored how to grow diamond at lower temperatures than current methods allow. Diamond’s unique crystal lattice structure makes it appealing to the semiconductor industry due to its ability to withstand high electrical voltages and efficiently dissipate heat.

PPPL Principal Research Physicist Igor Kaganovich explained: “This work is part of PPPL’s broader efforts to advance microelectronics by providing critical research into the materials and processes that could prove essential to ensuring a continued competitive advantage for the United States in this high-tech field.”

Producing diamond in labs typically requires high temperatures that would damage computer chips. Scientists, therefore, have been searching for low-temperature methods that maintain diamond’s quality. “If we want to implement diamond into silicon-based manufacturing, then we need to find a method of lower-temperature diamond growth,” said Yuri Barsukov, a computational research associate at PPPL and lead author of the study. “This could open a door for the silicon microelectronics industry.”

Finding the optimal temperature

Diamond is often produced using plasma-enhanced chemical vapour deposition (PECVD), where acetylene is known to aid diamond growth but also produce soot, which hinders optical and electronic performance. The study identified the factors determining whether acetylene would form diamond or soot. Barsukov noted:

“Now we have an answer. Like water to ice, there is a critical temperature for the transition of one phase to another. Above this critical temperature, acetylene contributes mostly to diamond growth. Below this critical temperature, it contributes mostly to soot growth.”

Published in Diamond & Related Materials, the study also found that the critical temperature is influenced by acetylene concentration and atomic hydrogen near the diamond’s surface. “Hydrogen atoms don’t fuel diamond growth directly,” said Alexander Khrabry, a Princeton University Research Scholar and co-author of the paper. “But hydrogen dissociation is crucial for transforming methane into acetylene and transporting atomic hydrogen to the diamond growth surface.”

Advancing quantum diamond research

Producing high-quality diamond at lower temperatures is only one part of the process for diamond electronics. Some applications require specialised ‘quantum diamonds’, in which certain carbon atoms are replaced with nitrogen, creating nitrogen-vacancy (NV) centres that are valuable for quantum applications.

Another study, published in Advanced Materials Interfaces by researchers from PPPL, Princeton University, and the Royal Melbourne Institute of Technology, investigated methods to protect the surfaces of quantum diamonds while preserving NV centres. Alastair Stacey, a principal research physicist at PPPL and co-author, explained:“The electrons in this material don’t behave according to the laws of classical physics as heavier particles do. Instead, they follow quantum physics. The advantage of qubits is that they can hold much more information than regular bits can,” he said. “This means they can also give us much more information about their environment, making them extremely valuable as sensors.”

Hydrogenation techniques for quantum diamond

Hydrogen atoms, essential for both microelectronics and quantum sensors, help prepare diamond surfaces for further chemical modifications. The challenge lies in evenly distributing a single layer of hydrogen atoms across the diamond’s surface without disturbing its underlying structure. Nathalie de Leon, an associate professor of electrical and computer engineering at Princeton University and a co-author, explained: “People have been trying to control diamond surfaces for a very long time. Diamond is a very tight lattice, so it’s hard to get things in there.”

The research team assessed traditional hydrogenation methods and explored alternatives, such as forming gas annealing, which uses a mixture of hydrogen and nitrogen, and cold plasma termination, which avoids directly heating the diamond with plasma. Both techniques achieved electrically conductive hydrogenated diamond surfaces, with notable trade-offs. For example, forming gas annealing required extreme precision to avoid oxygen contamination, which would compromise the results. “You have to get that oxygen off the diamond,” explained Daniel McCloskey, the study’s first author, noting that temperatures of 900°C were sometimes necessary.

Cold plasma termination created a hydrogen layer without damaging the NV centres, though the layer’s quality was lower than with heated approaches. Using photoluminescence spectroscopy, the team found that neither alternative technique compromised NV centre fluorescence. However, the traditional method irreversibly reduced fluorescence by nearly half.

“This highlights the trade-off between surface quality and NV properties that will have to be balanced in future applications,” said McCloskey. In biomolecular sensing, for example, it’s crucial that NV centres remain intact near the surface.

Charting the path forward

Further research is needed to optimise the methods for producing hydrogenated diamond surfaces with preserved NV centres. PPPL’s Quantum Diamond Laboratory, opened in March 2024, is positioned to drive this research forward. While a uniform hydrogen layer may be the goal for some applications, for others, it could be the foundation for additional surface modifications.

By developing a ‘recipe book’ of hydrogenation techniques, the team aims to standardise diamond preparation for quantum applications. “We’re writing a recipe book and characterising different ways of properly hydrogenating diamond surfaces so that we understand how to do this better for a number of applications,” said McCloskey.

Main image: A peek into a diamond reactor at PPPL. The pink glow is a plasma used in the chemical vapor deposition process. (Photo credit: Michael Livingston / PPPL Communications Department)