When a green laser shines on a special kind of diamond with atomic-scale defects, it can cause it to fluoresce red. This process is depicted in this photo of a quantum diamond sample as it was struck by the light of a green laser pointer, with a red-light filter added to half of the sample to reveal the red fluorescence. Credit: Michael Livingston/ PPPL Communications Department. CC BY.

Researchers are developing new ideas about the best ways to make lab-grown diamonds while minimising other forms of carbon, such as soot.

These diamonds aren’t destined for rings and necklaces, though; these are the kinds that are needed for the computers, optics and sensors of the future. 

One new study, conducted by researchers at the US Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) and Princeton University, investigated ways to reliably grow diamond at lower temperatures than those currently used.

Diamond has properties that make it attractive to the semiconductor industry. With its particular crystal lattice structure, diamond can withstand high electrical voltages. It’s also very good at dissipating heat. 

PPPL Principal Research Physicist Igor Kaganovich, a co-author on the paper, said: “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.”

Growing diamond in a laboratory typically involves high heat beyond what computer chips can handle; therefore, scientists have long been searching for ways to reduce the heat without sacrificing diamond quality. 

Lead author Yuri Barsukov, a computational research associate at PPPL, said: “If we want to implement diamond into silicon-based manufacturing, then we need to find a method of lower-temperature diamond growth.

“This could open a door for the silicon microelectronics industry.”

A close-up of one of the quantum diamond reactors at PPPL’s Quantum Diamond Lab. The glow inside the device comes from the plasma used to make quantum diamond using a process known as chemical vapour deposition. Credit: Michael Livingston/ PPPL Communications Department. CC BY.

Finding the critical temperature

Past experiments into a process for making diamond, known as plasma-enhanced chemical vapour deposition, showed that acetylene can contribute to diamond growth. However, acetylene was also known to contribute to the growth of soot, which can grow on top of diamond and inhibits performance for optics, sensors and chips.

The factors that determined whether the acetylene became diamond or soot were unclear.

Barsukov said: “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.”

According to the study the critical temperature depends on several factors, including the concentration of acetylene and atomic hydrogen near the surface of the diamond. 

Princeton University research scholar Alexander Khrabry, an author of the paper, said: “Hydrogen atoms don’t fuel diamond growth directly, but hydrogen dissociation, or breakdown, is crucial for transforming methane into acetylene and transporting atomic hydrogen to the diamond growth surface.

“These are both important for diamond growth.”

With more hydrogen near the surface, more diamond can form, even at lower temperatures.

This model of quantum diamond shows the carbon atoms in black. The purple ball represents the nitrogen, while the blue ball represents an empty spot in the lattice. Together, the nitrogen and empty space create the nitrogen-vacancy (NV) centre used in quantum applications. Credit: Michael Livingston/ PPPL Communications Department. CC BY.

Protecting quantum diamond

Perfecting the process for growing quality diamond at lower temperatures is just one piece of the puzzle to reliably making diamond for electronics. Some applications require a more complicated form of diamond in which some carbon atoms are removed, and a neighbouring atom is replaced with nitrogen. This creates what scientists call nitrogen-vacancy centres or NV centres.

A separate study related to NV centres was published in the journal Advanced Materials Interfaces by researchers from PPPL, Princeton University and the Royal Melbourne Institute of Technology. This study looked at ways to protect the surface of this special material, which is known as quantum diamond, while keeping the NV centres intact.

Co-author Alastair Stacey, a managing principal research physicist and head of quantum materials and devices at PPPL, said: “The electrons in this material don’t behave according to the laws of classical physics as heavier particles do.

“Instead, like all electrons, they behave according to the laws of quantum physics.”

One way researchers in which hope to harness these quantum behaviours is by making special bits called qubits.

Stacey added: “The advantage of qubits is that they can hold much more information than regular bits can.

“This means that they can also give us much more information about their environment, making them extremely valuable as sensors, for example.”

Adding an even layer of hydrogen atoms

Attaching hydrogen to the diamond surface has implications both for microelectronics and quantum sensors.

Hydrogen atoms can interact with diamond surfaces and lead them to conduct electricity, and, at the same time, they are needed as a starting point before attaching other, more complex molecules.

The challenge is creating a single layer of hydrogen atoms evenly distributed on the surface of the quantum diamond without changing what lies beneath. 

Co-author Nathalie de Leon, an associate professor of electrical and computer engineering at Princeton University, associated faculty at PPPL, said: “People have been trying to control diamond surfaces for a very long time.

“It’s an interesting fundamental science question because diamond is sort of a weird solid. You have a material that is exactly the same everywhere, and then at the very surface, it has to somehow bond to something else.

“But diamond is very inert, meaning it doesn’t want to react with things. It’s a very tight lattice, so it’s hard to get things in there. It’s also the hardest material in the world, so it’s very hard to polish and prepare in various ways.”

The study explores more reliable and less damaging techniques for adding that single layer of hydrogen atoms to the surface of the diamond so that it is ideal for certain quantum applications.

The way atoms bond in diamond makes the material well-suited for quantum applications, including quantum computing, secure communications, and highly accurate measurements of temperature and magnetic fields.

Barsukov said: “We need to precisely control the chemistry on the surface of the diamond using plasma, but plasma-surface interactions are not very well understood.

“People usually use a method of trial and error. So, we’re trying to shed light on some of the processes on the surface, just to make the picture a little bit clearer.”

Typically, this hydrogen layer is added by exposing the diamond to a hydrogen plasma under high heat. But, much like silicon in standard computer chips, the NV centres can’t handle this environment. 

Creating a recipe book for quantum diamond

The research team sought to find better methods for making hydrogenated quantum diamond with the NV centres intact.

First author Daniel McCloskey, of the School of Physics at the University of Melbourne, said: “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.”

The international research team investigated the traditional approach as well as the following two alternative hydrogenation methods:

• Forming gas annealing, which uses a mixture of hydrogen molecules and nitrogen gas (rather than a plasma made only of hydrogen).
• Cold plasma termination, which uses a hydrogen plasma but avoids direct heating of the diamond with the plasma.

Both alternative techniques produced hydrogenated diamond that could conduct electricity, but there are key differences and trade-offs.

Specifically, the team found that the quality of the hydrogen layer made with forming gas annealing was highly dependent on the temperature used and the purity of the gas mixture.

While no oxygen should be present during the experiment, some can leak in, and even that relatively small amount made a big difference. 

McCloskey explained: “You have to get that oxygen off the diamond.” That required temperatures in excess of 900°C.

McCloskey said developing ways to reduce and eliminate oxygen entering the reaction chamber is another important area of research that needs to be explored, adding that they had to go above and beyond the standard protocols to make it work.

The cold plasma termination method also created a hydrogen layer on the quantum diamond without damaging the NV centres.

However, the trade-off is that the hydrogen layer made with cold plasma termination was of lower quality than the traditional heated approach.

Assessing the damage to NV centres

To investigate the impact of the hydrogenation methods on NV centres, the team used a technique called photoluminescence spectroscopy.

Stacey said: “This is a way to peek in at a diamond sample that is full of NV centres by exciting them with green light and making them fluoresce.”

Neither of the two new hydrogenation methods affected fluorescence, even when the processes were repeated.

However, the traditional heated plasma treatment resulted in an irreversible loss of nearly half of the NV centre fluorescence.

McCloskey said: “This highlights the trade-off between surface quality and NV properties that will have to be balanced in future applications. For instance, in biomolecular sensing projects, it is absolutely crucial that NVs be preserved close to surfaces.”

The research is published in Diamond and Related Materials.