“`html
To broaden the application of diamond in semiconductor and quantum technologies, scientists are working on enhanced methods for producing diamonds at lower temperatures that won’t harm the silicon in computer chips. These improvements involve discovering how to create protective hydrogen layers on quantum diamonds while maintaining essential attributes such as nitrogen-vacancy centers.
Researchers are exploring innovative approaches to producing lab-grown diamonds while minimizing other carbon forms like soot. These diamonds, however, are not meant for jewelry; they are essential for the advanced computers, optics, and sensors of tomorrow.
A recent study performed by researchers at the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) and Princeton University examined methods for consistently growing diamond at lower temperatures than those typically used. Diamond possesses characteristics that make it appealing for the semiconductor sector. Its distinctive crystal lattice structure allows it to endure high electrical voltages and efficiently dissipate heat.
“This research contributes to PPPL’s extensive efforts to advance microelectronics by providing vital insights into materials and processes that could ensure the United States maintains its competitive edge in this high-tech arena,” explained Igor Kaganovich, Principal Research Physicist at PPPL and a co-author of the study.
Creating diamonds in a lab normally requires extreme heat that exceeds the tolerance of computer chips, prompting scientists to seek ways to lower the temperature without compromising diamond quality.
“If we aim to integrate diamond into silicon-based manufacturing, we must discover a technique for growing diamonds at lower temperatures,” remarked Yuri Barsukov, a computational research associate at PPPL and lead author of the study. “This could pave the way for advancements in the silicon microelectronics industry.”
A detailed view of one of the quantum diamond reactors at PPPL’s Quantum Diamond Lab. The glow within the apparatus is produced by the plasma utilized in the diamond-making process known as chemical vapor deposition. (Photo credit: Michael Livingston / PPPL Communications Department)
Identifying the Critical Temperature
Previous investigations into diamond production via a method called plasma-enhanced chemical vapor deposition revealed that acetylene plays a role in diamond growth. However, it was also recognized for contributing to soot accumulation, which can hinder the functionality of optics, sensors, and chips. The variables determining whether acetylene became diamond or soot remained elusive.
“We now have clarity on this issue,” Barsukov stated. “Similar to how water transforms into ice, a critical temperature governs the phase transition. Above this temperature, acetylene mainly promotes diamond growth, while below it, soot formation predominates.”
The study, published in the journal Diamond & Related Materials, indicates that the critical temperature relies on several factors, including the concentration of acetylene and atomic hydrogen near the diamond’s surface.
“While hydrogen atoms do not directly fuel diamond growth, their dissociation is essential for converting methane into acetylene and delivering atomic hydrogen to the growth surface, both crucial for diamond formation,” stated Alexander Khrabry, a Research Scholar at Princeton University and co-author of the paper. An increased presence of hydrogen at the surface allows for greater diamond formation, even at lower temperatures.
Safeguarding Quantum Diamond
Refining the method for growing high-quality diamonds at reduced temperatures is only part of the challenge in reliably producing diamonds for electronic applications. Certain functions require a more intricate version of diamond, where specific carbon atoms are removed, and a nitrogen atom replaces a neighboring atom, creating nitrogen-vacancy centers, or NV centers.
A different study focusing on NV centers was published in the journal Advanced Materials Interfaces by researchers from PPPL, Princeton University, and the Royal Melbourne Institute of Technology. This research explored ways to safeguard the surface of this specialized material, known as quantum diamond, while ensuring the integrity of the NV centers.
This model of quantum diamond depicts carbon atoms in black, with a purple ball symbolizing nitrogen and a blue ball representing a vacancy in the lattice. Together, these form the nitrogen-vacancy (NV) center, which is pivotal for quantum applications. (Image credit: Michael Livingston / PPPL Communications Department)
“Electrons in this material do not conform to classical physics like more massive particles. Instead, they follow the principles of quantum physics,” explained Alastair Stacey, managing principal research physicist and head of quantum materials and devices at PPPL, who co-authored the study. Researchers aim to leverage these quantum behaviors by developing special bits called qubits. “Qubits can encompass significantly more information than traditional bits, allowing them to offer much richer environmental data, making them especially valuable for sensor applications,” added Stacey.
Applying a Uniform Layer of Hydrogen Atoms
Introducing hydrogen to the diamond surface has significant importance for both microelectronics and quantum sensors. Hydrogen atoms can interact with diamond surfaces to facilitate electrical conductivity, and they must also be present as a foundation before attaching more complex molecules. The challenge lies in achieving an even monolayer of hydrogen atoms on the quantum diamond surface while preserving the underlying structure.
“Many have sought to control diamond surfaces for quite some time,” said Nathalie de Leon, an associate professor of electrical and computer engineering at Princeton University, associated faculty at PPPL, and co-author of the paper. “It’s a fascinating question of fundamental science because diamond is quite unique. It is consistent throughout, and yet at the surface, it must bond with something else. Diamond’s inert nature makes it resistant to reactions, presenting difficulties in introducing other materials. Additionally, its extreme hardness poses challenges in polishing and preparing the surface.”
This study investigates more dependable and less invasive methods for applying a single layer of hydrogen atoms to the diamond surface, making it suitable for specific quantum purposes. It forms part of a broader research initiative at the lab that aims to prepare diamond surfaces for quantum computing and sensing applications. The opening of the Quantum Diamond Laboratory at PPPL in March 2024 solidifies its position as a key player in this research.
The atomic bonding in diamond positions the material ideally for quantum applications, such as quantum computing, secure communications, and precise measurements of temperature and magnetic fields.
“Careful control of the surface chemistry of the diamond is essential using plasma technology; however, the interactions between plasma and surfaces remain poorly understood,” remarked Barsukov, who also contributed to the second study. “The typical approach involves trial and error, so we are working to illuminate some of these surface processes to enhance our understanding.”
Generally, this hydrogen layer
“““html
This is achieved by exposing diamonds to hydrogen plasma at high temperatures. However, similar to silicon used in common computer chips, the NV centers cannot withstand this environment.
Developing a Guide for Quantum Diamond
The research team aimed to discover improved techniques for producing hydrogenated quantum diamonds that retain their NV centers. “We are compiling a guide and examining various methods to effectively hydrogenate diamond surfaces, enabling us to enhance our understanding for multiple applications,” commented Daniel McCloskey, the lead author of the study and a researcher at the School of Physics at the University of Melbourne.
The international team explored both the conventional approach and two alternative hydrogenation techniques:
- Forming gas annealing, which employs a combination of hydrogen molecules and nitrogen gas instead of a pure hydrogen plasma.
- Cold plasma termination, utilizing hydrogen plasma without directly heating the diamond.
Both alternative methods successfully created hydrogenated diamonds that can conduct electricity, yet there are significant differences and compromises involved. The research indicated that the quality of the hydrogen layer formed via forming gas annealing was significantly influenced by the temperature and the purity of the gas mixture used. Although oxygen should ideally be eliminated during the process, even minor leaks of oxygen can lead to major impacts.
“It is essential to remove any oxygen from the diamond,” McCloskey noted, which necessitates temperatures exceeding 900°C. He emphasized that advancing techniques to minimize and prevent oxygen contamination in the reaction chamber is critical and remarked that they had to extend beyond customary procedures to achieve successful outcomes.
The cold plasma termination technique also generated a hydrogen layer on the quantum diamond without harming the NV centers. However, the downside is that this method produced a lower quality hydrogen layer compared to the traditional heated technique.
Evaluating Damage to NV Centers
To assess the effects of the hydrogenation techniques on NV centers, the team employed a method known as photoluminescence spectroscopy. “This technique allows us to examine a diamond sample rich in NV centers by exciting them with green light and observing them fluoresce,” Stacey explained. Neither of the newly introduced hydrogenation techniques impacted the fluorescence, even after multiple uses. Conversely, the standard heated plasma method caused an irreversible reduction of nearly 50% in NV center fluorescence.
“This underscores the compromise between surface quality and NV characteristics that needs to be addressed in future applications. For example, in biomolecular sensing initiatives, it is crucial that NVs remain intact near surfaces,” McCloskey remarked.
More investigative work is necessary to refine the new techniques for consistently achieving high-quality hydrogenated diamond surfaces with optimal NV centers. There are also numerous additional opportunities for PPPL and its colleagues to explore. While achieving a uniform layer of hydrogen atoms may be a primary goal for certain uses, for others, it may just be an initial step toward crafting a bespoke surface with additional elements.
The first study, titled “Quantum Chemistry Model of Surface Reactions and Kinetic Model of Diamond Growth: Effects of CH3 Radicals and C2H2 Molecules at Low-temperatures CVD,” received backing from the DOE under the “microelectronics co-design” research program of national laboratories and utilized computing resources from Princeton University’s Adroit and Stellar clusters. In the second study, “Methods for Color Center Preserving Hydrogen-Termination of Diamond,” the authors acknowledge support from the Australian Research Council (ARC) through grants DP200103712, CE170100012, and FL130100119. Additional funding was provided by the University of Melbourne’s proof-of-concept grant and the ARC Centre of Excellence in Quantum Biotechnology through project number CE230100021. The U.S. National Defense Science and Engineering Graduate Fellowship also contributed support. The in situ annealing and spectroscopy studies at Princeton were primarily funded by the DOE’s Office of Science and Office of Basic Energy Sciences under award number DESC0018978, with instrumentation development supported by the National Science Foundation’s CAREER program grant number DMR1752047. This research is based on work supported by the DOE’s Office of Science, Office of Fusion Energy Sciences, and Office of Basic Energy Sciences under award number LAB 21-2491.
“`
