Diamond research: highlights from 2023

Abstract

The year 2023 stands as a pivotal chapter in the chronicles of diamond research, marked by a series of exciting progress that have illuminated the multifaceted potential of this extraordinary material. The editors of Functional Diamond selected some key research achievements of 2023, including electronics, quantum science, MEMS, heat dissipation, electrochemistry, diamond growth, and optics. As we navigate through the highlights encapsulated in this retrospective, it becomes evident that the research progress has not only broadened the horizons of scientific understanding but has also laid the foundation for unprecedented possibilities in diverse fields.

Keywords:

• Electronics
• quantum science
• MEMS
• heat dissipation
• electrochemistry
• diamond growth

1. Oxidized-silicon-terminated (C–Si–O) diamond p-MOSFETs: ideal platform for diamond industrial applications

Diamond is a promising semiconductor with superior properties to other commercially available semiconductor materials (Si, SiC, GaN). The hydrogen-terminated (C–H) surface shows diamond’s potential as the best p-FETs for RF and power device applications but does not meet many industrial requirements such as durability, stability and compatibility to device fabrication process. This is because C–H bonds terminate the diamond surface without chemical bond extension, which cannot satisfy the adhesion necessary for long-term use [1].

To solve this problem, Kawarada et al. at Waseda University reported that oxidized-silicon-terminated (C–Si–O) diamond was able to improve the MOS quality of diamond FETs [2], which made it an ideal platform for the industrial application of diamond, as shown in Figure 1. In this work, two methods were proposed to obtain C–Si–O diamond: the “SiO₂ reduction” method and the “Si direct deposition” method [3]. As a result, C–Si–O diamond p-MOSFETs showed the hole channel mobility of exceeding 150 cm²V⁻¹s⁻¹, which was higher than the electron mobility of SiC n-MOSFETs. The threshold voltage of diamond p-MOSFET was negatively large enough (V_TH < −3 V) for normally-off operation at high voltage circuits. Maximum drain current densities were >300 mAmm⁻¹ in lateral FETs [4] and >200 mAmm⁻¹ in vertical FETs [5]. They were the highest in normally-off diamond p-FETs. These results indicate that diamond is a leading candidate for p-FET to realize complementary power circuits with SiC, GaN or Ga₂O₃ n-FETs in the near future.

Figure 1. (a) Atomic arrangement of Al₂O₃/SiO₂/diamond (001) with C-Si-O bonds at the interface, (b) HAADF image of Al/Al₂O₃/SiO₂/diamond, (c) EDS (energy dispersive X-ray spectrometer) image of Si at Al₂O₃/SiO₂/diamond, (d) schematic image of diamond MOSFET with Al₂O₃ gate insulator and C-Si-O interfacial layer by Si supply, (e) vertical diamond MOSFET with a C-Si-O channel, (f) I_D,max and V_TH benchmarks of the reported normally-off C-H diamond FETs and the proposed C-Si-O FETs. Reproduced with permission from Y. Fu et al. [4], IEEE trans. on electron devices, 69, 8, 4144-4152 (2022). Copyright 2022 IEEE.

2. Quantum sensing: nanodiamonds EPR sensors, spin-mechanical chips, and diamond quantum magnetometry

In a confluence of pioneering research, scientists explored the potential of nanodiamonds for quantum, spin-mechanical quantum chips, and diamond quantum magnetometry.

Quantum sensing, as a burgeoning detection method, has shown immense potential in high-precision measurements and quantum computing. Researchers at the University of Science and Technology of China have made significant strides in quantum precision measurement, focusing on utilizing nitrogen-vacancy (NV) centers within individual nanodiamonds (NDs) for quantum sensing [6]. They placed the NDs on a coverslip by spin coating (Figure 2). Overcoming the challenge of random tumbling of the ND, the team successfully employed NV centers in NDs to perform quantum sensing. The inherent difficulties in detecting the electron magnetic resonance spectrum of paramagnetic ions in solution under in situ conditions were overcome in this research. By exploiting the unique quantum properties of NDs, they achieved precise in situ detection of the magnetic resonance spectrum of paramagnetic ions in solution. This research opens new avenues for utilizing flexible NDs as electron paramagnetic resonance (EPR) sensors to enable in situ and even in vivo EPR measurements. Beyond research, the application prospects include structural and functional studies of biomolecules in biomedicine, environmental monitoring, and energy-related fields.

Figure 2. Sketch of the experimental setup. The NDs are dispersed and fixed on a coverslip, which is placed in a confocal microscope. Yellow wire indicates the coplaner waveguide to radiate microwave. Figure by Z. Qin, et al./CC by 4.0 [6].

Parallelly, Du’s group at University of Science and Technology of China, has successfully engineered a spin-mechanical quantum chip designed to explore exotic interactions [7]. Dark matter exploration stands as one of the foremost challenges in fundamental science, necessitating innovative approaches to uncover the mysteries of the universe. The research team aimed to shed light on this enigma by proposing a spin-mechanical quantum chip capable of probing exotic interactions beyond the standard model. This chip integrating a mechanical resonator and a diamond with single nitrogen vacancy at the microscale, demonstrated a two-order-of-magnitude improvement in constraints related to spin-velocity-dependent interactions. Notably, within the force range below 100 nm, no evidence of new bosons was observed, i.e., in the rest-mass window of 2–10 electronvolts. The spin-mechanical quantum chip represents a promising tool for exploring exotic interactions, offering scalability for on-chip detectors. This research provides a path for significantly enhancing the sensitivity of experiments, crucial for dark matter exploration. The chip’s low-cost and high-yield nature positions it as a catalyst for advancing fundamental physics experiments on-chip.

Researchers at Cambridge University achieved a quantum sensing breakthrough using Nitrogen-vacancy Centers (NVCs) in diamond fragments [8]. Diamond quantum magnetometry, the key technique, promises highly sensitive vectorial magnetic field sensing with minimal backaction. Shifting focus to haematite, an antiferromagnet, unveiled intricate antiferromagnetic spin textures. Employing diamond quantum magnetometry, the study revealed emergent monopolar, dipolar, and quadrupolar magnetic charge distributions in haematite. The direct vorticity read-out, capturing rotational motion, established a crucial link to magnetic charge via a duality relation.

This breakthrough not only unveils unprecedented insights into antiferromagnetic landscapes but also underscores diamond quantum magnetometry’s transformative role in exploring emergent phenomena in quantum materials. Signifying a paradigm shift in understanding two-dimensional monopolar physics within magnetic systems, this discovery enriches fundamental knowledge and holds promise for innovative applications in quantum technologies, marking a significant advancement in quantum sensing and materials exploration.

3. Integrated on-chip high-reliability diamond MEMS magnetic sensor

Achieving electrically integrable, highly sensitive, and reliable magnetic sensors at elevated temperatures remains a difficulty for the conventional magnetic sensors, attributed to the issues such as low sensitivity, limited thermal stability, and constrained operational temperature ranges.

Single-crystal diamond (SCD) emerges as a highly promising material for micro-electromechanical system (MEMS) techniques due to its exceptional physical, chemical, mechanical, and electrical properties. Zhang et al. at NIMS demonstrated an on-chip SCD MEMS magnetic sensor that integrated SCD with a huge magnetostrictive FeGa film [9]. The FeGa film was multifunctionalized to actuate the resonator, self-sense the external magnetic field, and electrically read the resonance signal. Figure 3(a) depicts the measurement setup of an SCD-based resonator magnetic transducer featuring the on-chip self-sensing and actuation configuration. The electric field distribution of an SCD-based resonator was simulated using COMSOL software (Figure 3(b)), illustrating the confined electric field around the SCD resonator. This simulation suggested that the SCD-based resonator effectively achieved actuation through this configuration. Figure 3(c) illustrates the resonance frequency shifts in response to varying magnetic fields tuned via temperature. The resonance frequency shift demonstrated a positive correlation with the applied magnetic field. The magnetic sensor exhibits a sensitivity of 3.2 Hz/mT across a wide temperature range from room temperature to 500 °C. Additionally, it maintains a low noise level of 9.45 nT/Hz^1/2 up to 300 °C. Notably, the minimum fluctuation of the resonance frequency is measured at 1.9 × 10⁻⁶ at room temperature and 2.3 × 10⁻⁶ at 300 °C. Furthermore, they have achieved an SCD MEMS resonator array with parallel electric readout, laying the groundwork for the development of magnetic image sensors.

Figure 3. (a) Schematic diagram of the measurement setup for the single-crystal diamond (SCD)-based resonator magnetic transducer with the on-chip self-sensing and actuation configuration. (b) Simulation of the electric field distribution of an SCD-based resonator with on-chip actuation. (c) Resonance frequency shifts of the magnetic transducer vs. the measurement temperature at different magnetic fields. Reproduced with permission from Z. Zhang et al. [9], IEEE. Adv. Funct. Mater. 33, 2300805 (2023). Copyright 2023 Wiley‐VCH GmbH.

This work opens the avenue for the development of highly integrated on-chip MEMS resonator transducers, showcasing both high performance and thermal stability. The successful implementation of the SCD-based magnetic transducer array for magnetic sensing represents a crucial step towards the creation of robust magnetic image sensors with excellent sensitivity and spatial resolution, particularly for harsh environments in the future.

4. Diamond as heat dissipation material in GaN power devices

Thermal management is a critical consideration in GaN power devices, and the integration of diamonds with GaN stands out as a highly promising solution to enhance heat dissipation. Liang et al. at Osaka Metropolitan University, has achieved a marked progress by fabricating GaN High Electron Mobility Transistors (HEMTs) using diamond as a substrate [10]. This involved the transfer of AlGaN/GaN/3C-SiC layers grown on silicon to a large-size diamond substrate, followed by the fabrication of GaN HEMTs directly on the diamond, as shown in Figure 4. This innovative technology exhibits more than twice the heat dissipation performance of HEMTs with the same configuration but fabricated on a silicon carbide (SiC) substrate. To fully leverage the high thermal conductivity of diamonds, the researchers incorporated a 3 C-SiC layer, a cubic polytype of silicon carbide, between the GaN and diamond layers. This technique not only significantly reduces the thermal resistance at the interface, thereby enhancing heat dissipation, but also demonstrates resilience to high-temperature annealing processes. Overcoming the challenges associated with mass production is a noteworthy aspect of this technological advancement. This new technology has the potential to significantly produce GaN-on-diamond devices by integrating them into the standard device fabrication process, making practical applications feasible.

Figure 4. (a) AlGaN/GaN/3C-SiC/diamond bonded sample, (b) optical microscope image of GaN HEMTs fabricated on the diamond, (c) cross-sectional TEM image of 3 C-SiC/diamond bonded interface, (d) comparison of heat dissipation properties of GaN HEMTs fabricated on Si, SiC, and diamond substrates. Reproduced with permission from R. Kagawa et al. [10], Small, 2305574 (2023). Copyright 2023 Wiley‐VCH GmbH.

5. Codoped diamond for high-efficiency of electrochemical CO₂ reduction

Reducing the CO₂ emissions arouse great attentions in the present century due to the challenge from global warming and climate change. Born-doped diamond (BDD) holds good potentials as a cathodic electrode for the electrochemical reduction of CO₂ based on the fascinating characteristics, including high durability, environmental friendliness and effective hinderance of the cathodic hydrogen evolution reaction. However, BDD still encounters the hardship of high overpotentials and low efficiency for electrochemical CO₂ reduction.

Recently, Einaga et al. at Kei University synthesized various boron and nitrogen-coped diamond (BNDD) films and investigated the impact of boron and nitrogen concentration on the electrochemical CO₂ reduction [11]. The BNDD demonstrated superior faradaic efficiency of the formic acid formation than BDD, achieving to as high as 94.9%. Notably, the faradaic efficiency remained higher even at a less negative potential of −2.0 V vs Ag/AgCl. It was revealed that the nitrogen site of the BNDD facilitates the hydrogen addition reaction. which reacts with CO₂ to transform into intermediates of [*COOH] or [*OCHO] (Figure 5), eventually, greatly accelerating the formic acid formation. This work provides a new methodology to construct diamond electrodes for high-efficiency electrochemical CO₂ reduction reaction.

Figure 5. Proposed reaction pathways of CO₂RR at low overpotentials. Figure by Y. Miyake, et al./CC by 4.0 [11].

6. 2-inch heteroepitaxial single-crystal diamond self-supporting substrates

The applications of diamond for electronic and sensing devices have been limited by large-size single crystal diamond substrates. In the year 2021, Adamant Namiki Precision Jewel Co., Ltd., in collaboration with Kasu et al. at Saga University succeeded in realizing 2-inch diamond wafer on a sapphire substrate based on a step-glow growth mode. [12]. The threading dislocation density was determined to be 2.6 × 10⁷ cm⁻². The diamond grown on a sapphire substrate with 7 degree misorientation toward the [1$\overline{1}$00] direction, the widths of the (004) and (311) XRC were 98.35 and 175.3 arcsec, respectively.

In 2023, Xi’an Jiaotong University also achieved high-quality, 2-inch heteroepitaxial single crystal diamond self-supporting substrates (as shown in Figure 6) using microwave plasma chemical vapor deposition (MPCVD) [13]. By effectively controlling the uniformity of film formation, temperature field, and flow field, the yield of heteroepitaxial single crystal diamond has been improved. The half maximum width (FWHM) of the XRD (004) and (311) rocking curves was less than 91 arcsec and 111 arcsec, respectively.

Figure 6. 2-Inch heteroepitaxial single crystal diamond self-supporting substrate [13].

7. Diamond laser-towards narrow-linewidth and multi-wavelength

Narrow-linewidth lasers, distinguished by their exceptional monochromaticity and spatial mode output, are indispensable in a variety of fields such as coherent measurement, optical storage, spectral analysis, and quantum optics. The primary challenge, however, is the simultaneous achievement of high power, narrow linewidth, and multi-wavelength output, which is hindered by the limitations of the existing laser gain media. Diamond crystals, renowned for their superior nonlinear optical gain capabilities, especially in Brillouin and Raman lasing, present a promising solution to reconcile the issues of linewidth and wavelength inherent in conventional inversion lasers.

Recently, researchers at Hebei University of Technology have developed a continuous single-frequency diamond Brillouin laser with a power output exceeding 20 W, achieving up to four times linewidth compression. This development marks the first instance where the feasibility of linewidth narrowing using a free-space Brillouin laser has been theoretically and experimentally verified [14, 15]. Concurrently, a dual-wavelength laser output with peak power in the hundreds of kilowatts was realized through cascading intracavity diamond Raman conversion, as illustrated in Figure 7. The output power is 1–2 orders of magnitude higher than that of previously reported intracavity and extra-cavity diamond Raman lasers [16]. Moreover, by integrating Brillouin and Raman processes, a multi-band emission laser system employing diamond as the nonlinear medium was demonstrated, achieving cascaded Brillouin frequencies at the two Raman bands of 1.2 and 1.5 μm, respectively [17].

Figure 7. Schematic diagram of the intracavity cascaded diamond Raman laser. Figure by H. Chen, et al./CC by 4.0 [16].

Leveraging the exceptional optical and thermal properties of diamond, in conjunction with the adaptability provided by free-space cavity structures, the above studies establish a robust foundation for generating high-power, narrow-linewidth lasers with designated wavelengths.

8. Self-healing for enhanced ceramic material durability in nanotwinned diamond composite

Diamond, renowned for its hardness, transparency, and thermal conductivity, grapples with inherent brittleness, posing challenges to the longevity of devices featuring diamond components. Overcoming the dilemma of bolstering diamond’s fracture toughness while preserving its extreme hardness stands as a global imperative.

In a collaborative effort between Beihang University and Yanshan University, researchers have tackled the brittleness issue by previously introducing a Nano-Twinned Diamond Composite (ntDC) with significantly improved fracture toughness. Despite this advancement, concerns lingered regarding catastrophic failures due to microcracks. The latest study targets the achievement of self-healing capabilities in diamond to counteract the impact of microcracks, revealing ntDC’s extraordinary self-repairing abilities [18]. Through in-situ scanning electron microscopy and transmission electron microscopy mechanical experiments, researchers showcased remarkable crack healing in fractured ntDC at room temperature, with an impressive healing efficiency of approximately 34%.

This research presents a groundbreaking strategy for designing robust and fracture-resistant ceramic materials, especially those with strong covalent bonds. The exploration of diamond’s self-healing behavior promises to guide the development of materials with strong covalent bonds, with significant implications for applications like room-temperature wafer bonding.

Acknowledgements

This highlight is the outcome of the collaborative efforts of the editors of Functional Diamond. Special thanks are extended to Meiyong Liao from the National Institute for Materials Sciences, Haitao Ye from University of Leicester, Yu Fu from Xidian University, Zilong Zhang from the National Institute for Materials Sciences, Jianbo Liang from Osaka City University, Zhaofeng Zhai from the Institute of Metal Research, Chinese Academy of Sciences, Wei Qiang and Wei Wang from Xi’an Jiaotong University, Zhenxu Bai from Hebei University of Technology, and Lifang Shen from the editorial office for their invaluable contributions.

Disclosure statement

No potential competing interest was reported by the authors.