Symposium EL05—Diamond and Diamond Heterojunctions—From Growth and Technology to Applications

Diamond is an interesting substrate for the growth of GaN nanowires and other nanostructures such as nano-fins for a number of reasons. It is chemically and mechanically very stable, has a high thermal conductivity, a much larger electrochemical window than GaN, is optically transparent up to a photon energy of about 5.3 eV, and can be very effectively doped p-type by controlled substitutional boron incorporation, e. g. during CVD growth. Therefore, heteroepitaxial structures of AlGaN thin films on diamond substrates have been investigated with increasing emphasis since more than a decade, mainly for electronic and optoelectronic applications. In comparison, much less work has been devoted the growth of AlGaN nanowires on diamond, which is the main topic of the work described here.

We are interested in GaN nanowires on single crystalline diamond substrates for electrical and optical addressing and read-out of nitrogen-vacancy (NV) centers as the most sophisticated solid-state qubits available so far. The target of this work is to develop a technology for the integration of surface-near NV-centers in the intrinsic layer of p/i-diamond substrates with self-aligned heteroepitaxial n-type GaN nanowires. Self-alignment is achieved by low-energy implantation of N atoms through a nano-patterned mask which later on also serves as the mask for the selective area growth of GaN nanowires. In addition, GaN/AlGaN nano-fins and nanogrids on diamond are investigated for photo-electrochemical electrodes and applications in high power electronics.

In this presentation we will first summarize the results of our investigation of GaN nanowire nucleation and growth on (111) diamond substrates. It was observed that the surface termination of the diamond substrate (by either hydrogen, oxygen or nitrogen) has a pronounced influence on the nucleation and the polarity of the GaN nanowires. Numerical simulations show that the polarity has a strong influence on the electronic level scheme at the p-diamond/n-GaN hetero-interface. Complete p/i-diamond/n-GaN nanowire hetero-diodes with varying concentrations of implanted NV-centers were then fabricated and characterized with respect to their optical and electronic properties. In particular, it was observed that the efficiency of the optical read-out of the NV photoluminescence is increased by an order of magnitude for diamond covered with an optimized periodic GaN nanowire array compared to the bare diamond substrate. Fourier imaging of the NV photoluminescence allows the determination of the crystallographic orientation of single NV centers underneath a GaN nanowire. Furthermore, efficient electroluminescence of NV centers could be achieved by forward current injection. Finally, an outlook will be given to the future implementation of GaN nano-fins and -grids on different substrates including diamond.

The key to the efficient transmission and conversion of low-carbon electrical energy is the improvement of power electronic devices. Diamond is considered to be the ultimate wide bandgap semiconductor material for applications in high power electronics due to its exceptional thermal and electronic properties. Two recent developments - the emergence of commercially available electronic grade single crystals and a scientific breakthrough in creating a MOS channel in diamond technology, have now opened new opportunities for the fabrication and commercialisation of diamond power transistors.
These will result in substantial improvements in the performance of power electronic systems by offering higher blocking voltages, improved efficiency and reliability, as well as reduced thermal requirements thus opening the door to more efficient green electronic systems.
In the literature, several diamond-based field-effect-transistors (FETs) have already revealed good on state performance and high blocking voltage capability (~2kV) in a wide range of operating temperatures. The possibility of generating an inversion regime in diamond metal-oxide-semiconductor FET (MOSFET), and the new Deep Depletion regime (D2MOSFET) specific to wide bandgap semiconductors pave the way for a new generation of power devices. The critical part of the transistor is the gate oxide, with electrical charge traps located within the oxide or at its interface with the semiconductor. These traps can screen the gate potential and shift the threshold voltage, making the devices unusable.
The current research carried out mainly in Japan and Europe will be presented, with the various device architectures explored, including MOSFET, MESFET, D2MOSFET and rectifiers. Results obtained in the framework of the first European research collaboration on diamond devices, aiming at fabricating the first HVDC diamond based converter will also be presented.

Nitrogen-vacancy(NV) centers in diamond have superior physical properties at room temperature for quantum sensing of the magnetic field, electronic field, temperature, and pressure enabling scalable applications from atomic-scale to macroscopic range. We would like to introduce our recent progress on materials and devices.
-Homoepitaxial growth: Higher generation yield of selectively-aligned NV ensemble formed with precise depth control for scalable applications [1,2]. - Heteroepitaxial growth: Diamond film with aligned NV ensemble on Si (111) substrates via the 3C-SiC layer by original antenna-edge type microwave plasma CVD [3]. - Devices: Charge state control of NV centers by pn junctions [4]. As applications, we would like to introduce the bio-cell imaging with the magnetic marker[5], internal device sensing [6] , and probe-type prototype system.
This study was supported in part by MEXT Q-LEAP and KAKENHI (17H01262 and 18H01472).

[1] H. Ozawa, et al., APEX 10, 045501 (2017).
[2] H. Ishiwata, et al., APL 111, 043103 (2017).
[3] T. Suto, et al., APL 110, 062102 (2017).
[4] M. Shimizu, et al., APEX 11, 033004 (2018).
[5] Y. Hatano, et al., Phys. Status Solidi A, 215, 1800254 (2018).
[6] T. Iwasaki, et al., ACS Nano, 11, 1238 (2017).

In this presentation, our latest work in the development of an ultra-sensitive biosensor based on diamond waveguides will be highlighted. We have during the last years been working on the design and fabrication of mid-infrared (MIR) diamond waveguides. The diamond MIR waveguides together with a broadband tunable quantum cascade laser (QCL), emitting in the so called molecular fingerprint region, are the two key elements in our newly developed label free MIR biosensor. In our approach MIR light is coupled into microfabricated diamond waveguide structures, which are purposefully designed to sustain single or multimode standing waves. The associated evanescent waves that are created at the waveguide surface will extend into the surrounding media and interact with the analyte thus provide the sensing volume. In existing commercial technologies, attenuated total reflection IR (ATR-IR) spectroscopy, IR light from a thermal emitter is coupled through a millimeter sized diamond crystal, with up to 10 internal reflections in the diamond crystal. This is a powerful technology and is today used for i.e. quality control in medical and food industry, protein analysis, forensics, etc. However, the relatively low sensitivity of this technique hinders its use in many applications. Our biosensor achieves unsurpassed sensitivity due to the large number of standing waves nodes created in our waveguides which roughly can be translated to the number of internal reflections per length unit is maximized. Moreover, the use of a bright QCL laser as a light source further increases the sensitivity of our biosensor – therefore an ultra-sensitive sensor is realized. Finally, the diamond waveguide material facilitates further functionalization by means of organic chemistry thus extending the capabilities of traditional silicon or gold based biosensor technologies, and we demonstrate here one possible route to fabricate selective affinity layers for protein fishing on nanocrystalline diamond overlayers.
Several designs of diamond waveguides are discussed, ranging from thin film diamond slab waveguides, rib waveguides, to microfabricated free hanging diamond waveguides. Different dimensions of microfabricated polycrystalline diamond waveguides have successfully been fabricated with thicknesses ranging between 5 to 15 µm. Focused ion beam etching has been applied to study the effect of controlled surface roughness of the in- and out coupling area of the waveguides, and to establish limits of attainable throughput of the waveguides. The complete setup of the biosensor is presented.
To demonstrate the strength of using a bright QCL as a light source, we show how a multi-reflection diamond crystal (450 µm thick) can be used for non-invasive glucose monitoring in human saliva. Furthermore, we present measurements on different types of analytes at low concentrations using our waveguide designs to demonstrate the sensitivity of the sensor.
Currently, our main activity is on analyzing different forms of the protein alpha-synuclein, which is relevant in understanding the mechanism behind Parkinson’s disease. We have previously shown that ATR-IR spectroscopy can be used to analyze the secondary structure of different alpha-synuclein aggregates. Moreover, it is also possible to see the difference in the IR-spectra between the native state and the neurotoxic misfolded state of the protein. However, the ATR element is impractical to functionalize and the sensitivity of the ATR-IR spectroscopy is too low to be used on relevant samples from patients. In contrast, our diamond waveguide biosensor exhibits the desired key properties to analyze the secondary structure of alpha-synuclein at biologically relevant concentrations. Future work includes the functionalization of the diamond waveguide sensor surface to selectively adsorb alpha-synuclein from cerebrospinal fluid, with the ultimate goal to detect Parkinson’s disease at an early stage.

Central to quantum information processing is the entanglement generation rate, which can be Purcell enhanced by coupling a quantum emitter to a cavity. One-dimensional hole-y photonic crystal cavities in diamond have already demonstrated >10,000 quality factors and small mode volumes [1], two necessary characteristics for achieving high cooperativity. However, color centers such as the nitrogen vacancy center typically exhibit severe spectral diffusion near surfaces, resulting in reduced coupling rate under the presence of holes. Inspired by Hood et. al.’s atomic platform [2], we propose using the alligator photonic crystal cavity that removes the airgaps and thus pushes the emitters farther away from surface charge states. Here, we report the simulation results and the characterization of alligator cavities in bulk diamond. Realization of this nanophotonic structure allows for a more noise-resilient cavity QED system essential for constructing a scalable quantum network.
Acknowledgements
K. C. Chen acknowledges funding support by the National Science Foundation Graduate Research Fellowship Program (GRFP) and the Army Research Laboratory Center for Distributed Quantum Information (CDQI).
References
[1] S. Mouradian, N. H. Wan, T. Schroeder, and D. Englund, Appl. Phys. Lett. 111, 021103 (2017)
[2] J. D. Hood, A. Goban, A. Asenjo-Garcia, M. Lu, S.-P. Yu, D. E. Chang, and H. J. Kimble, PNAS 113, 38 (2016)

The ability of pigeons to sense the magnetic field of the Earth has long been established through behavioral studies, however, the underlying biophysical mechanisms are yet to be determined. Magnetoreceptors in the beak¹, retina² and inner ear hairs³ have been proposed. Markers for neuronal activation have previously been used to link the vestibular brain-stem region to magnetic stimulation⁴. This activation is contingent on the lagena being intact, providing strong evidence that magnetoreceptors are present in inner ear hairs and support the ferrimagnetic particle receptor hypothesis for magnetoreception. These results warrant further investigation into iron containing cells within this part of the pigeon anatomy. The discovery of quasi-spherical structures in the cuticular plate of the inner ear hair of pigeons was subsequently made, with these structures being termed ‘cuticulosomes’³. These objects are of potential relevance to the problem of magnetoreception due to their position in the pigeon anatomy and because they contain ferritin granules. Cuticulosomes have been imaged using TEM and characterized using elemental analysis. Statistics surrounding the occurrence of these spherical structures having been developed through Prussian Blue staining experiments, however, we have limited information on their magnetic properties³.
Quantum magnetic microscopy techniques^5–7 provide an opportunity to remedy that situation. The nitrogen-vacancy centre in diamond has a sensitivity to magnetic fields which allows for the mapping of both fluctuating magnetic moments and of static magnetic fields from samples affixed to the surface of the diamond⁸. Here we apply diamond-based quantum magnetic microscopy to probe the fluctuating magnetic fields emanating from individual cuticulosomes. The magnetic images are correlated back to the anatomical location providing critical in-situ magnetic information. The stray magnetic field from the cuticulosomes was also mapped using our quantum imaging microscopes under the application of a bias magnetic field. The magnitude of the stray magnetic field increases with the size of the bias field applied with the dependency of the stray fields on the externally applied field providing additional insight into the magnetic properties of cuticulosomes.
The quantum imaging technique applied here provides new physical insights into the magnetic properties, i.e. magnetic susceptibility, of individual cuticulosomes. The information gleaned will aid in the effort to understand the functionality of cuticulosomes, and crucially, if they have any bearing on the ability of pigeons to sense magnetic fields.

1. Hanzlik, M. et al. Ultrastructural Analysis of a Putative Magnetoreceptor in the Beak of Homing Pigeons. 360, 350–360 (2003).
2. Schierwater, B. Retinal cryptochrome in a migratory passerine bird : a possible transducer for the avian magnetic compass. 585–588 (2004). doi:10.1007/s00114-004-0578-9
3. Lauwers, M. et al. An iron-rich organelle in the cuticular plate of avian hair cells. Curr. Biol. 23, 924–929 (2013).
4. Le-Qing Wu & Dickman, J. D. Neural Correlates of a Magnetic Sense. Sci. Rep. 336, 1054–1058 (2012).
5. Tetienne, J.-P. et al. Quantum imaging of current flow in graphene. Sci. Adv. 3, e1602429 (2017).
6. Tetienne, J. et al. PHYSICAL REVIEW B 97 , 085402 ( 2018 ) Spin properties of dense near-surface ensembles of nitrogen-vacancy centers in diamond. 085402, 1–11 (2018).
7. Schirhagl, R., Chang, K., Loretz, M. & Degen, C. L. Nitrogen-Vacancy Centers in Diamond: Nanoscale Sensors for Physics and Biology. Annu. Rev. Phys. Chem. 65, 83–105 (2014).
8. Mccoey, J. M. et al. Rapid , High-Resolution Magnetic Microscopy of Single Magnetic Microbeads. 1805159, 1–9 (2019).

For laser materials processing like cutting and welding, multi-kw continues wave fiber and disc lasers are used. State-of-the-art high power laser optics used for these applications are mainly based on fused silica as an optical material. High purity synthetic fused silica offers high transmission, but due to the limited thermal conductivity, significant radial thermal gradients occur inside the optical components caused by the residual absorption of the substrate’s and coating’s material. As the lenses have to be cooled at the outer non-irradiated section of the aperture, these gradients cannot be avoided by design. The inhomogeneous temperature distribution of the lenses causes three effects: firstly, the shape of the lens changes due to thermal expansion. Secondly, the index of refraction varies, and thirdly, mechanical tension is caused by inhomogeneous heating of the lens, and thereby stress birefringence can be observed. Therefore, first and higher order optical properties of optical systems for high-power lasers depend on the temperature, leading to focal shift and thermally induced aberrations commonly referred to as thermal lensing. In addition, high thermal loads of the optical coatings and the bulk material limit the minimum diameter of the optical setup.
Due to its outstanding optical, thermal and mechanical properties, monocrystalline synthetic diamond is a very promising material to overcome these limitations: the extremely high thermal conductivity which is approximately three orders of magnitude higher compared to fused silica results in a nearly uniform temperature profile and provide efficient cooling of the optical coatings on the diamond-air interfaces. In addition, the diamond’s high Young’s modulus and index of refraction allow for thinner lenses as the radius of curvature is nearly three times larger for a diamond lens compared to a fused silica lens of the same power, and the coating design also benefits from the high index of refraction.
In a cooperation of the three Fraunhofer Institutes ILT, IAF and IPT the complete process chain for setting up small and lightweight laser processing heads has been developed. This process chain includes optimized CVD based growth of synthetic diamond, grinding, centering as well as characterization of diamond lenses, optical design of all-diamond optical systems for materials processing, high performance optical thin film coatings for diamond lenses, and mechanical design as well as testing of laser processing heads. To characterize the diamond lenses, we have measured surface deviation, surface roughness, local birefringence and the intensity profile of a laser beam focused by diamond lenses designed and manufactured by the Fraunhofer consortium. The coating consists of a single layer of a low-absorption material.
We have realized two compact material processing heads based on diamond lenses. The first demonstrator is designed for thin metal sheet cutting. One of the main goals of the optical and mechanical design process was to minimize the laser processing head’s weight and size. Therefore, no conventional fiber mount and collimating optics was foreseen. Instead the optical fiber is directly connected to the laser processing head. The lens mount is made of copper and cooled by water. To adjust the nozzle’s outlet to the laser’s focal plane, the nozzle can be axially moved by a threaded cylinder. The realized laser processing head weights approx. 200 g. For the cutting experiments, a 1 kW single mode fiber laser was used, and the sheet metal was attached to a motorized x-y-table. The second demonstrator is used for laser material deposition and is based on a similar optical and mechanical design, but generates a larger laser spot. This demonstrator was also successfully tested in first material processing experiments.

An inductively coupled toroidal plasma is used as an energy source alternative to microwave plasma for chemical vapor deposition (CVD) of diamond material. With up to 12 kW output power, the patented Plasmability platform leverages a powerful, scalable, and efficient 400 kHz RF power supply.

The Plasmability system has been designed specifically for the needs of diamond deposition: high flux atomic hydrogen (power density ~800 W/cm3) distributed uniformly as a line source. As currently manifested, the plasma shape is ideal for the growth of single crystal diamonds in arrays up to 15mm wide by 35mm long. Plasma-to-sample distance is dynamically controlled during the entire growth process over a range of several millimeters, keeping the growing surfaces at all times at an optimum distance relative to the plasma.

Recent average growth rates of homoepitaxial single crystal diamond exceed 160 um/hr with optical grade clarity. Raman spectroscopy, XRD rocking curve, and birefringence of the material verify the growth of high-quality diamond. Leveraging the high growth rates available, Plasmability is pursuing large area single crystals (beyond 1 inch).

This work is carried out in part under support from Army Research Laboratory SBIR Contract W911QX-18-C-0017.

Progresses in CVD allowed high quality diamond films to be deposited on large area for emerging applications. 4-inch diameter diamond films can be produced by large and low energy-efficiency hot-filament CVD and 915MHz plasma-assisted CVD systems. At the opposite, small chamber and energy-efficient 2.45GHz plasma-assisted CVD systems appear less attractive for large area deposition due to the shorter wavelength of the microwave source. De facto, three resonance modes of the 2.45GHz microwave cavity must be necessarily superposed in order to obtain a plasma large enough to cover a 4-inch area¹. Recent encouraging simulations indicated that superposed resonance modes² of a 2.45GHz microwave cavity could be stable in a clamshell-type MPACVD system. Simulations aside, stable microwave resonance modes must be experimentally identified in presence of plasma and feed gas mixture to perform CVD process. Indeed, the presence of plasma and gas mixture with non-uniform dielectric constants distribution in the microwave cavity modified the resonance modes from simulation results.
In this study, a 2.45GHz MPACVD has been employed to grow 4-inch diamond films on Silicon wafers. Effects of plasma conditions and substrate temperature on the deposition rate and thickness uniformity of diamond films have been studied. It suggested that substrate temperature distribution and diamond film quality are affected by gas convection and IR light emissions from the plasma. Furthermore, film thickness uniformity has been improved by a continuous plasma boundary layer on the diamond film. Preliminary results showed variations in diamond film thickness <1% and <5% for 3-inch and 4-inch areas respectively, with growth rates in the order of 0.8 µm/h using high quality process conditions (low methane, no nitrogen). Values above 3 µm/h are conceivable at high methane content or beyond with nitrogen addition.

References:
1 - A. Vikharev et al., Diamond Relat. Mater. 83, pp 8-14 (2018)
2 - J. Weng et al., Vacuum 147, pp 134-142 (2018)

Thanks to its intrinsic outstanding optical and semi-conducting properties, diamond is one of the most promising candidates for a large number of high added-value applications[1]. Among them power electronics devices as well as quantum sensors based on nitrogen-vacancy or silicon-vacancy color centers offer great perspectives. However, the development of such applications is still hampered by the low availability of large and thick diamond single crystals with very well controlled of point and extended defect densities.

In the last twenty years, the microwave Plasma Assisted Chemical Vapour Deposition (PACVD) technology has witnessed important progresses allowing the control of crystal quality, purity and doping[2]. Nevertheless, synthesis of high quality single crystalline diamond layer still remains limited in size due to the very low availability of substrates with dimensions higher than typically 5x5 mm² at a reasonable cost [3]. Moreover, the ability of the process for growing diamond layers thicker than a few mm without any growth interruption is not yet clearly demonstrated.

In this presentation we will focus on this particular topic where growth strategies for diamond single crystals fabrication by MWPACVD will be presented. The control of crystal morphology as a function of growth conditions will be discussed as well as the feasibility of fabrication of 4 mm thick large area (8X8 mm²) diamond substrates without growth interruption. Attention will be also paid to the control of doping level in order to provide diamond material suitable for power and quantum sensing related applications.

References
[1] Bucciolini, M., C. De Angelis, and C. Talamonti, 8.15 - Diamond Detectors for Dosimetry, in Comprehensive Biomedical Physics, A. Brahme, Editor 2014, Elsevier: Oxford. p. 229-248.
[2] R. Sussmann, CVD diamond for electronic devices and sensors, Materials for electronic and optoelectronic applications, Wiley, 2009.
[3] Tallaire, A., et al., Growth of large size diamond single crystals by plasma assisted chemical vapour deposition: Recent achievements and remaining challenges. Comptes Rendus Physique, 2013. 14(2–3): p. 169-184.

The electrical, mechanical, and thermal properties of diamond make it a promising material for new generation electronic devices. To accelerate the continuing progress of this technology, researchers require access to high quality, large-area substrates. One approach to producing large area substrates is to use Microwave Plasma-Assisted Chemical Vapor Deposition to grow a continuous layer of single crystal diamond across an array of individual diamond plates in a process known as the mosaic technique. In this technique multiple individual diamond substrates are cut and polished to be lattice matched and then a continuous diamond layer is grown across all the individual substrates forming a larger single crystal piece. Our effort has been to minimize the misorientation of the neighboring substrate tiles, reducing stress in the overgrown layer, and demonstrating low misorientation (≤ 0.25°) across the mosaic crystal for the three orthogonal crystal directions. In this report, we especially focus on the capacity of the diamond lattice to accommodate and relieve the stress due to the remaining misorientation along selected crystal directions as the crystal growths thicker. High resolution x-ray diffraction is used to map the orientation of the grown layer across the surface. Subsequent growths allow us to chart the evolution of the misorientation as the layer thickness is increased. Results show how the overall reduction rate in relative misalignments are dependent on the initial interface configuration and top surface off-cut orientations. Mosaic samples composed of up to 4 tiles have been grown up to 11mm x 11mm using this technique.

This material is based upon work supported by the US Army Research Laboratory (ARL) under Contract No. W911QX-18-C-0005.

Interest in the diamond material system for electronic applications has rapidly increased in recent years, becoming a global scale area of interest. With its ultrawide band-gap of 5.47 eV, extremely high thermal conductivity of > 20 Wcm^-1K^-1 and intrinsically high breakdown field of 10 MV/cm, diamond is a promising candidate in achieving next generation high-power electronic devices ^[1-4]. Progress in this area has been typically hindered by the lack of matured doping techniques and on-going development of novel fabrication strategies to overcome the challenges in working with diamond ^[5]. The U.S. Army Research Laboratory is investing in the development of surface transfer doped diamond field effect transistors for RF power applications ^[6-8]. Surface transfer doping offers an alternative to substitutional doping that alleviates the challenges of introducing impurity dopants into diamond’s tightly packed carbon lattice. Historically, spontaneous accumulation of volatile atmospheric adsorbents on the hydrogen terminated diamond surface when exposed to air has provided the necessary surface acceptor states for transfer doping ^{[1, 9]}. However, this method of transfer doping when exposed to ambient air is highly sensitive to environmental conditions such as temperature, humidity and molecular composition ^[9]. More recent results demonstrate enhanced surface transfer doping utilising high electron affinity transition metal oxides, such as MoO₃. By further encapsulating these materials with thick dielectric, we observe excellent long-term and thermal stability. This approach of encapsulated metal oxide has been incorporated into our diamond MESFET designs, improving output current density and stability of devices in both atmosphere and with elevated temperatures. Through a novel approach to gate lithography, gate-source spacing has been reduced to < 100 nm for a 400 nm gate length. Comparing a range of gate-source and gate-drain spacings showed improved peak output current and transconductance, while also significantly improving device breakdown. These results show great scope for improvement of diamond power devices through adoption of tailored design features, such as asymmetric gates, and incorporation of robust, encapsulated, transition metal oxides for superior doping.

[1] Wort, C. J. H., Balmer, R. S. “Diamond as an Electronic Material”, Materials Today, 11, 22-28, (2008)
[2] Hirama, K., Takayanagi, H., Yamauchi, S., Jingu, Y., Umezawa, H., Kawarada, H. “High-Performance p-Channel Diamond MOSFETs with Alumina Gate Insulator”, IEDM Technical Digest, 873, (2007)
[3] Wang, J. J., He, Z. Z., Yu, C., Song, X. B., Wang, H. X., Lin, F., Feng, Z. H. “Comparison of Field-Effect Transistors on Polycrystalline and Single-Crystal Diamonds”, Diamond & Related Materials, 70, 114-117, (2016)
[4] Kasu, M., Oishi, T. “Recent Progress of Diamond Devices for RF Applications”, IEEE Compound Semiconductor Integrated Circuit Symposium, (2016)
[5] Mainwood, A. “Theoretical modelling of dopants in diamond”, Journal of Materials Science: Materials in Electronics, 6, (2006)
[6] J. Weil, P.B. Shah, A.G. Birdwell, T.G. Ivanov, “Fabrication of Sub-100nm, Hydrogen-Terminated Diamond FETs”, GOMACTech Proceedings, (2018)
[7] Shah, P., Weil, J., Birdwell, A., & Ivanov, T. “Charge Trapping Analysis of High Speed Diamond FETs.” MRS Advances, 2(41), (2017)
[8] K. G. Crawford, D. Qi, J. McGlynn, T. G. Ivanov, P. B. Shah, J. Weil, A. Tallaire, A. Y. Ganin, and D. A. J. Moran, “Thermally stable, high performance transfer doping of diamond using transition metal oxides,” Scientific Reports, 8, (2018)
[9] Maier, F., Riedel, M., Mantel, B., Ristein, J., Ley, L. “Origin of Surface Conductivity in Diamond”, Physical Review Letters, 85, 3472, (2000)

The technological efforts to access geothermal energy or natural resources, such as fossil fuels, are steadily increasing and motivate the industry to drill much deeper wells. In order to drill wells efficiently, data monitoring from the downhole drilling process such as, pressure, temperature, inclination, and azimuth are all important to the drilling equipment’s condition. However, during the downhole drilling process, temperature variations, pressure amplitude fluctuations, and mechanical rig vibrations have the potential to cause high noise levels in the sensor circuits. That can significantly decrease the signal-to-noise ratio of the sensor circuits, which can ultimately result in a loss of the monitoring signal. To acquire data from a high noise environment, high-temperature stable low-noise sensor circuits are required. The base unit of those circuits is a durable low-noise field effect transistor (FET). Current commercially available Si-based sensor circuits can operate at a maximum temperature of 300 °C for wells just as deep as 12,000 ft. A great challenge of downhole drilling beyond 12,000 ft is the steadily increasing temperature by a rate of 25 per 3,280 ft. Furthermore, the geothermal energy industry targets highly efficient geothermal plants with wells requiring temperatures in excess of 450 °C.

In order to match the temperature requirements, the silicon-based circuits need to be replaced. Materials with wider bandgaps than silicon provide that distinctive advantage of high-temperature operation stability. The wider bandgaps offer higher operating temperatures before the thermally activated intrinsic carrier current causes latch-up in the sensor circuits. Diamond is chosen for its ultra-wide bandgap with 5.45 eV compared to silicon with a bandgap of just 1.1 eV. However, all doping impurities in diamond possess a high thermal activation energy. To engineer a diamond circuit able to operate over a wide temperature range between 0 °C and 600 °C, a diamond delta-channel FET concept is proposed. This concept is based on a sandwich structure that consists of a highly boron doped film in between of two intrinsic diamond layers. A full activation of the boron acceptors can be achieved by high doping concentrations of 10²⁰ cm^-3. The total sheet charge concentration of the FET profile should not exceed 4×10¹³ cm^-2 profile due to breakdown limitations, requiring a maximum thickness of the delta channel of only 4 nm. Because of an out-diffusion of free charge carriers into the intrinsic diamond material adjacent to the highly doped delta-channel, the charge carriers can reach higher hole mobilities within the delta-channel design. The high charge carrier density combined with the high speed of its charges lead to a high current drive, and correspondingly results in a high transconductance, which perfectly fits to a low-noise sensor circuit design.

In this work, a low noise diamond delta-channel FET is proposed for electronic circuits operating up to of 450 °C. This delta-channel FET structure is designed in TCAD Sentaurus, and simulation results of its performance will be presented. The noise performance versus temperature will also be reported on based on the simulations.

Seawater wireless communication has been required for scuba diver’s communication, seawater drone control, development of seabed resources, etc. The conventional communication methods are acoustic wave, visible light wave and electromagnetic wave. On the other hand, electric seawater communication can take advantage of the high electrical conductivity of seawater. This is more suitable for short distance communication than the conventional methods [1].
We have studied diamond solution gate FETs (diamond SGFETs) which can operate in solutions [2]. Diamond SGFETs are directly immersed in solutions and the drain current (I_DS) is remotely controlled by electric signals (V_GS) from a gate electrode. We proposed the new electric seawater communication method utilizing the gate electrode as a transmitter and SGFET as a receiver [3]. However, the previous work was not considered entirely wireless because a common source potential line was connecting the gate electrode with SGFET. In this work, we removed the common potential line to realize an electric wireless communication and investigated propagation distance in the seawater environment.
In this communication system, the gate electrode was applied with an input voltage v_G from 100kHz to 10MHz. SGFET was applied with an output voltage V_DD. A load resistance R_D was connected to the drain electrode of SGFET. Then, the electric signals (v_G) were transmitted from the gate electrode to SGFET in NaCl 35g/L solutions which had the same concentration as seawater. Distance between the gate electrode and SGFET was gradually increased from 0.1m to 5m using a cylindrical tube (diameter=2cm). In each distance, the switching characteristics, v_DS of SGFET, were measured.
The switching characteristics show that SGFET responds to v_G at least 10MHz in each distance. The v_DS of SGFET is observed with differential pulses responding to the v_G potential change. We assume that differential pulses mainly occur due to series CR (high-pass filter) circuit with the electric double layer capacitance C_EDL of a SGFET channel and the load resistance R_D. As a result, propagation distances of electric signals are at least 5m. In contrast, Vpeak of differential pulses decreases as the distance increases from 0.1m to 5m. This is because the solution resistance R_Sol increases as the electric signals are transmitted in a longer path. R_Sol is defined as R_Sol=ρl /A where ρ is resistivity of solutions, l is distance and A is cross sectional area. In this case, ρ=0.2Ωm, A=π×10^-4m², then, the value of R_Sol is calculated as (0.2/π)l×10⁴Ω. However, R_Sol might become much smaller in a large cross-sectional area such as seawater in open space. In conclusion, this method can be applied to wireless communication in seawater with the propagation distances of at least 5m.

[1] J. Joe and S. Toh: OCEANS (2007) 1.
[2] H. Kawarada, Y. Araki, T. Sakai, T. Ogawa and H. Umezawa: Phys. Stat. Sol. (a) 185 (2001) 79.
[3] Y. Iyama, H. Kawarada et al., MRS Fall 2018.

The H-terminated surface of diamond when exposed to NO₂, O₃ or just laboratory air forms a 2D hole conductive layer in the diamond surface with a surface resistance 1.5 k ohm sq^-1. Field effect transistors, FETs, made in this conductive layer have exhibited impressive currents, 1.3 A mm^-1 [1], and high operational frequencies, f_T = 70 GHz [2]. However, this surface layer is not stable and FET performance degrades over time on the scale of hours to days [3]. Overcoating the diamond surface with Al₂O₃ generates a higher surface resistance, 5 k ohm but the diamond’s surface resistance is stable in time. However, FETs using Al₂O₃ degrade when electrically stressed [4]. This presentation describes using an atomic layer deposition technique, ALD, to deposit an oxide alloy of Al₂O₃ and SiO₂ with resistances between 1.8 to 3.5 k ohm sq^-1, which is stable at room temperature in laboratory air for > ½ yr and to 600 °C in N₂. FETs using this alloy exhibit no degradation when electrically stressed for >120 hr in laboratory air.

This 2D hole conductive layer is generated by a non-diamond absorbed or deposited layer that removes electrons for diamond’s valance band, generating mobile positive charges, holes, in the diamond. Compounds that have such electron gettering properties are called Lewis Acids. Oxide alloys of Al₂O₃ and SiO₂ are industrially used as Lewis acid catalysis in the petroleum industry. Neither Al₂O₃ nor SiO₂ alone exhibit significant acid properties. However, the surface of the oxide alloy contains a very deep electron acceptor with a Hammett acidity, H_o,max, of -11.99 [5] at a surface density of 1.6x10¹⁴ cm^-2 [6]. From the H_o,max value the acceptor trap is estimated to be 0.7 eV below the valence band of H-terminated diamond.

We found that ALD deposited Al₂O₃/SiO₂ did not decrease the surface resistance over that of just Al₂O₃. However, two procedures were found to give lower resistances. One was to precoat the ALD system with SiO₂ and then deposit ALD Al₂O₃ on the H-terminated diamond to the desired thickness. The other approach was to co-deposit SiO₂ and Al₂O₃ using H₂O, Al(CH₃)₃, and (OC(CH₃)₃)₃SiOH, which resulted in a diamond surface resistance equivalent to that obtained with Al₂O₃ alone. However, by rapid thermal annealing to 600°C for 1 minute the lower resistance was obtained. Typical surface mobilities with both ALD procedures were from 50 to 70 cm² V^-1 s^-1 with carrier densities of 3x10¹³ to 6x10¹³ cm^-2. We speculate that carrier densities >10¹⁴ and lower surface resistances are possible with appropriate control of this alloy at the diamond surface.

References
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[4] M. Syamsul, Y. Kitabayashi, D. Matsumura, T. Saito, Y. Shintani, H. Kawarada, Appl. Phys. Lett. 109, 203504 (2016) DOI.org/10.1063/1.4967999
[5] A. Satsuma, Y. Westi, Y. Kamiya, T. Hattori, Y. Murakami, Bull Chem. Soc. Jpn. 70, 1311-1317 (1991).
[6] K. R. P. Sabu, K. V. C. Rao, C. G. R. Nair, Bull Chem. Soc. Jpn. 64, 1920-1925 (1991).

DISTRIBUTION STATEMENT A: Approved for public release: distribution unlimited. This material is based upon work supported by the Under Secretary of Defense for Research and Engineering under Air Force Contract No. FA8702-15-D-0001. Any opinions, findings, conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the Under Secretary of Defense for Research and Engineering.

Two-dimensional hole gas (2DHG) diamond FETs have demonstrated excellent radio-frequency (RF) performances such as cutoff frequency (f_T) = 70 GHz [1], maximum oscillating frequency (f_max) = 120 GHz [2] and output power density (P_out) > 2.1 W/mm at 1 GHz [3, 4]. These achievements are results of miniaturizing gate length and access region using self-aligned-gate (SAG) process. However, diamond FETs fabricated by SAG process is low breakdown voltage (about ~20 V). So, the carrier velocity (v) caluculated from f_T = v/2πL_G is low (< 0.6×10⁷ cm/s) and RF voltage swing at large-signal operation is small. To solve these issues, diamond FETs need to be fabricated a high-voltage resistant structure. Recently, we have fabricated high voltage diamond FETs exceeding 1500V [5] with 200 nm atomic-layer-deposition (ALD)-Al₂O₃ film. The channel mobility of 2DHG diamond FETs are as low as around 100 cm²/Vs, Above 0.4 MV/cm (40 V/μm), however, the hole velocity reaches the saturation velocity of 1.4×10⁷ cm/s and the low mobility does not so much affect FET performancein the high electritlic field.
In this study, ALD-Al₂O₃ 2DHG diamond MOSFETs with 100 [6] and 200 nm Al₂O₃ were fabricated on a II-a polycrystalline diamond substrate which has high crystallinity, (110) preferential orientation and grain size of approximately 300 μm. The dimension of devices at 200 nm thick Al₂O₃ diamond MOSFETs are as follows. The source-gate length (L_SG) and the gate length (L_G) were fixed 0.5 μm and 1 μm, respectively, while the L_GD ranged from 1.5 to 4.5 μm.
As a result, the carrier velocity exceeded 1.0×10⁷ cm/s at 100 nm thick Al₂O₃ diamond MOSFET and reached to 1.4 ×10⁷ cm/s at 200 nm thick Al₂O₃ diamond MOSFET. The latter devices maximum drain current density was −740 mA/mm at V_GS = −28 V and V_DS = −40 V, the lowest on-resistance of 25 Ω mm and the transconductance of 13 mS/mm at V_DS = −40 V with 9 V V_GS 16 V were obtained. Extrinsic f_T was 22 GHz at V_GS = 16 V and V_DS = −40 V for L_GD = 1.5 μm. Hence, using f_T = v/2πL_G, the carrier verocity achieved 1.4×10⁷ cm/s by f_T of 22 GHz at L_G of 1 μm. It is the first diamond MOSFET operation at the saturation velocity and is comparable to GaN based HEMTs. RF large-signal performance was evaluated using load pull system at 1 GHz in class-A operation. At 100 nm thick Al₂O₃ diamond MOSFETs, the maximum P_out of 3.8 W/mm was obtained at high V_DS =50 V. It is the highest in diamond [6]. At 200 nm thick Al₂O₃ diamond MOSFET, the P_out of 2.5 W/mm and maximum power gain of 8.0 dB were obtained at V_GS = 16 V andV_DS = −70 V for L_GD = 4.5 μm. The large-signal RF performance was eveluated at V_DS = −70 V for the first time in diamond.

[1] X. Yu et al: IEEE EDL. 39 (2018) 1373.
[2] K. Ueda, M. Kasu, et al: IEEE EDL. 27 (2006) 570.
[3] M. Kasu, T. Makimoto et al: Electron. Lett. 41 (2005) 1249.
[4] K.Hirama, H. Kawarada et al: IEDM (2007) 873.
[5] H. Kawarada et al. Sci. Rep. (2017) 42368.
[6] S. Imanishi, H. Kawarada et al: IEEE EDL. 40 (2019) 279.

Diamond is a fascinating semiconductor with exceptional physical properties such as a wide band gap, a high breakdown electric field (10 MV/cm), an outstanding thermal conductivity (20 W/cm/K) and high carrier mobilities. These exceptional properties, or more precisely, the combination of some of these properties makes diamond an ideal semiconductor for high power and/or high frequency electronics which should surpass other materials like silicon, silicon carbide or gallium nitride. Numerous diamond field effect transistors are under investigation: H-terminated accumulation FET, O-terminated inversion channel FET, metal-semiconductor FET and junction FET.

For some of them, the conducting channel of the ON-state is composed by a 2D carrier gas at the diamond/oxide interface: accumulation and inversion channel FET. However, the 2D hole channel mobility values reported in the literature, ranging between 3 and 300 cm²/Vs, are systematically lower than the maximum hole mobility reported in bulk material (2000 cm²/Vs)¹.

Recently, Li et al² reported a description of the 2D hole gas mobility in H-terminated diamond. Using the triangular potential approximation, they discussed about the importance of the different scattering modes to the mobility values. In this work, we will extend the previous work to the general of diamond/oxide interface (H- and O-terminated) and introduce a full description of the valence bands and hole scattering mechanisms (light holes, heavy holes, spin-orbit and sub-bands due to quantum confinement). The maximum expected mobility achievable in a diamond 2D hole gas will be discussed and compared to recent experimental results.

References
¹ J. Pernot “Carrier mobility in diamond: from material to devices”, published in « Power Electronics Device Applications of Diamond Semiconductors - 1st Edition », edited by S. Koizumi, H. Umezawa, J. Pernot, M. Suzuki, p. 174, 2018, Woodhead Publishing.

² Y. Li, J.-F. Zhang, G.-P. Liu, Z.-Y. Ren, J.-C. Zhang, and Y. Hao, Physica Status Solidi (RRL) – Rapid Research Letters 12, 1700401 (2018).

We demonstrated the first operation for an inversion channel MOSFET using diamond semiconductor [1]. Here, we precisely controlled the MOS interface using an atomic layer deposition of Al₂O₃ film on OH-terminated diamond (111) surface and pn-junctions using selective growth of heavily B doped on n-type diamond layer for the formation of source/drain. The field-effect mobility μ_FE of the p-channel diamond MOSFET was 8 cm²/Vs at the highest, which is much lower than the hole mobility of 6,300 cm²/Vs at RT [2]. Recently, we investigated the interface state density D_it dependence of μ_FE in inversion channel diamond MOSFETs [3]. In this study, we will report the key techniques for the demonstration of the inversion channel diamond MOSFET and its recent progress.

Acknowledgements
This work was partially supported by Kanazawa University SAKIGAKE Project 2018 and JSPS KAKENHI Grant Numbers JP17H02786, 17H03248, 18KK0383, and 19K15042.

References
[1] T. Matsumoto, N. Tokuda et al., Sci. Rep. 6 (2016) 31585.
[2] I. Akimoto, N. Naka, N. Tokuda, Diamond. Diamond Relat. Mater. 63 (2016) 38.
[3] T. Matsumoto, N. Tokuda et al., Appl. Phys. Lett, in press.

A prior study from our group has achieved an ohmic contact with thin PEALD Al₂O₃ on top of a hydrogenated diamond surface establishing a sheet resistance ~16 kOhms/square. In this study, we extend the application of the PEALD interfacial layer as a protective passivation layer to avoid degradation of the hydrogen terminated conducting surface during during device fabrication process. from XPS characterization, the valence band offset confirms that the electrons from the valence band of diamond can be transferred to states of this very thin Al₂O₃ interfacial layer. As a first step to fabricating MOSFET devices, different PEALD Al₂O₃ thicknesses are used to establish the source and drain contact. We used pure Au as the contact metal, and Al / Au for the gate contact. A circular FET design was used to reduce the gate leakage current and a 4 nm thermal ALD Al₂O₃layer was used for the gate dielectric. A threshold voltage of 0.8 V is shown in the device with a highest drain current of ~40 mA/mm. The gate length of these devices is 4 or 6 microns with an asymmetrical distance of L_GS of 4 microns and L_GD of 16 microns. The results show a decreasing trend of the contact resistance of the PEALD Al₂O₃ interfacial layer with reduction of the thickness of this tunneling layer. Our device results have indicated that the interfacial Al₂O₃ layer, may reduce the contact resistance for a H-diamond MOSFET.
This research is supported by the NSF through grant DMR-1710551 and a grant from MIT-Lincoln Laboratory.

In power devices, vertical structure is suitable for low on-resistance and large current operation which are required for power electronics. We have ever reported (001) vertical-type diamond MOSFETs [1] using two-dimensional hole gas (2DHG) which is induced irrelevantly of their crystal orientation by C-H termination structure and high temperature atomic layer deposition (ALD)-Al₂O₃ [2]. In this paper, we fabricated vertical-type 2DHG diamond MOSFETs with hexagonal trench structures using the (111) diamond for the first time. The (111) diamond has advantages for high carrier density owing to their C-H dipole structure and low sheet resistance [3] compared to the (001) diamond. As the results, the maximum current density is 3 times higher than that of the previous works at the same drain voltage (V_DS) [1]. And, the minimum specific on-resistance R_on of 9.2 mΩcm² was obtained the device with large hexagonal trench of 12 µm on a side. Fabrication process was as follows. First, the undoped and nitrogen-doped layer were epitaxially grown on the (111) p+ diamond substrate by microwave plasma chemical vapor deposition (MPCVD). The nitrogen-doped layer (~1×10¹⁹ cm^-3) worked as a current blocking layer. Second, hexagonal trench structures were formed by inductive coupled plasma reactive ion etching. Length on each side of the trench (W_T) was 10-20 µm, and depth (D_T) was fixed at 3.0 µm. Third, 200 nm regrowth undoped layer was regrown by MPCVD to induce the 2DHG. Finally, source and drain electrode were formed and 200 nm Al₂O₃ was deposited by ALD method. The gate length was fixed to 6 µm, while 3 μm (half) of the gate overlapped with the source electrode. The drain current density (I_DS) normalized by gate width of 72 µm (6W_T) at V_DS of -20 V and -50 V are -250 mA/mm and -680 mA/mm, respectively. Those values are almost 3 times higher than those of vertical-type (001) diamond FETs at the same V_DS [1]. The main reason is the conduction path of the former (001) vertical FET are composed of {001} surface only, but those of the present (111) vertical FETs are higher (111) surface and {110} or {211} side walls, where the sheet conductance is higher than that of {001}. The drain current on/off ratio is about 9 orders of magnitude at room temperature. For this result, the film thickness and concentration of the nitrogen-doped layer are sufficient. The maximum drain current density and lowest specific on-resistance normalized by device active area are -5400 A/cm² and 9.2 mΩcm² for V_DS of -50 V and V_GS of -20 V. The specific on-resistance is lowest ever reported for the (111) diamond FETs. These performances will be improved by the device structure miniaturization and drift layer resistance reduction. If the trench size shrinks to as small as 2 µm, the R_on will be below 1 mΩcm².
[1] N. Oi, H. Kawarada et al., Sci.Rep. 8, 10.1038 (2018)
[2] H. Kawarada et al., Appl. Phys Lett. 105, 013510/1-4 (2014)
[3] H. Kawarada, et al., Jpn. J. Appl. Phys. 51, 090111/1-6 (2012)

Surface transfer doping of hydrogen-terminated diamond (Diamond:H) leading to a two-dimensional hole gas (2DHG) represents a significant recent advance towards high-performance diamond transistors. Recently, Transition-Metal Oxides (TMO), such as MoO₃, WO₃, V₂O₅ and ReO₃,^1-4 have been shown to yield a far more efficient and stable surface transfer doping process. This is of great significance for future diamond electronics.
In this talk, we present recent progress in Diamond:H FETs by engineering the gate stack from a typical metal-oxides-semiconductor FET (MOSFET) towards a metal-semiconductor FET (MESFET). This is seen to yield a drastic improvement in key features such on/off ratio (8 orders of magnitudes), a very low subthreshold swing (67 mV/dec) and low gate leakage.
A comparative Diamond:H/Transition-Metal Oxide FET^5-7 study will be also presented and will show how the different Transition-Metal Oxides properties influence FET performance. A proposed oxide approach is suggested and demonstrated that enhances hole drift mobility by a factor of two, while keeping a high value of carrier concentration.
Based on these findings, a path towards improvement of Diamond:H/TMO transistor performance and stability will be outlined.
References:
1. Russell et.al. Appl. Phys. Lett., vol. 103, no. 20, pp. 2–6, 2013.
2. Tordjman et.al. Adv. Mater. Interfaces, vol. 1, no. 3, pp. 1–6, 2014.
3. Crawford et.al. Appl. Phys. Lett., vol. 108, no. 4, pp. 1–5, 2016.
4. Tordjman et.al. Appl. Phys. Lett., pp. 1–13, 2017.
5. Vardi et.al. IEEE Electron Dev. Lett. 35, 12, 2014.
6. Yin & Tordjman et.al. IEEE Electron Dev. Lett. 39, 4, 2018.
7. Yin & Tordjman et.al. Science Advances ,4, eaau0480, 2018.

The p-type conduction of hydrogen terminated (C-H) diamond is attractive for developing high preferment of high power or high frequency devices such as high-power field effect transistor (FET) [1]. In this work, the interface charge effect with nitrogen doping on C-H diamond FET is investigated. So far, we have reproduced the FETs characteristic of C-H diamond FET by 2-dimensional (2D) acceptor model [2] or 2D negative charge sheet model [1] confirmed 2D hole gas (HG) close to surface. However, these acceptors or negatively charged sites are scattering centers for carrier (holes) transport near the C-H surface. The device exhibits good DC operation and achieved both normally off and normally on characteristics depending on nitrogen concentration [3]. Before that the normally off mode achieved with oxygen termination in diamond full or partially close to C-H diamond FET. Partially SiO2 in C-H diamond also exhibited normally off operation achieved with -3ev threshold voltage and maximum current density IDMAX=-44mA/mm[4].
In this work, the normally off (enhancement mode) C-H diamond Al gate overlapped field effect transistor (FET) with donor investigated. We simulated nitrogen doping (donor model) in concentration of 3e16 cm-2 corresponding to near surface positive charge of 5e11cm-2. All the device parameters are similar to the former simulation [1] where source and drain contact resistance is very low because of very low Schottky barrier height (SBH) [5]. As gate is negatively biased, an upward bending of valence band is observed. It indicates that there is a hole accumulation layer (2DHG) under valence band maximum at the diamond side. The MOSFET capacitor characteristic shows the fixed fermi level position as a function of capacitance voltage at 1.7eV from the conduction band due to nitrogen as donor. The maximum current density of the Al2O3/C-H diamond device is -70 mA/mm at the gate voltage of -26V with-voltage step of 4V between +26V~-26V. Also, we observed the gate threshold voltage Vth= -3V with negative gate bias normally off operation. The reason is that no intersection between valence band and fermi level caused by the positive charge cancelling the negative charge at Al2O3/C-H diamond interface. So, the negative charge effect at the interface becomes very week leading to normally off.
Reference:
1) H. Kawarada et al.; Scientific Reports. 7, 42368/1-8, (2017)
2) K. Tsugawa, H. Kawarada et al., Diam. Relat. Mater., 8, 927 (1999). 3) N. Oi, H. Kawarada et al. Elec. Dev. Lett. 40, 239 (2019).
4) T. Yabe, H. Kawarada et al. MRS Fall Meeting (2018)
5) K. Tsugawa, H. Kawarada et al Phys. Rev. B, 81, 045303/1-11 (2010)

Electrochemiluminescence (ECL) is a phenomenon in which photons are emitted during electrochemical reactions. It already proved to be a very promising analytical tool for sensing applications where both high sensitivity and selectivity are needed. Indeed, it merges the advantages of chemiluminescent analysis with the absence of background optical signal, with wide possibilities for reaction control using diverse electrochemical protocols. ECL can be observed in organic solvents where both oxidized and reduced forms of luminescent species are produced, e.g. by sweeping electrode potential between anodic and cathodic. Excitation energy is then obtained from recombination of oxidized and/or reduced species. As an example, superoxide ions may be involved through electrochemical reduction of dissolved oxygen. In aqueous environments, simultaneous oxidation and reduction of luminescent species is difficult to achieve due to electrochemical splitting of water, implying the use of a co-reactant. In this case luminescent species are oxidized at the electrode together with the co-reactant, which gives a strong reducing agent after some chemical transformations. In this context, Boron Doped Diamond electrodes offer numerous advantages that have been little explored for ECL analytical applications. They display high electrochemical stability together with resilience to fouling, and good optical transparency. Furthermore, in aqueous media electro-generated hydroxyl radicals play important roles in ECL, implying that a co-reactant family can be expanded by taking advantage of the strong oxidation ability of electrogenerated hydroxyl radicals at boron doped diamond electrodes. The potential of diamond electrodes for ECL analytical applications will be discussed and illustrated through a comparative study using different electrode materials. An example of application for foodstuff analysis will be presented.

Some properties of diamond are very interesting for the development of implantable medical devices. Especially they offer a way to achieve stable implants. This is due to the chemical stability of the material, along with its high density, which prevents any chemical species to penetrate into the bulk of any diamond layer. Furthermore diamond can be doped with boron to achieve outstanding electrode materials for the stimulation of biological tissues.
A major issue associated with chronical implant for neuronal interfacing is that structure is not hermetic. This is due to the fact that metal tracks are generally embedded in polymer layer to isolate them from the tissues. But following long term implantation, the polymers are likely to swell and water molecule will migrate inside the polymer thus modifying its dielectric properties. To address this issue, we propose to develop a full diamond implant by combining intrinsic material and BDD electrodes. By assembling the same material both for stimulation electrodes and passivation layer we are aiming to prevent current leakage at the interface.

To validate this new structure we developed strips that combine boron doped and intrinsic diamond materials. These structures are placed on an accelerated aging set up for several days. The BDD electrodes are stimulated by biphasic pulses in Phosphate Bovine Solution (PBS) and the stability of the electrodes in these conditions is monitored by electrochemical impedance recording. Strips with conventional materials have also been fabricated (organic or inorganic materials) to compare their behavior in the same experimental conditions with our diamond based structures. The preliminary trials demonstrate that diamond structures are stable at least over 45 days.

Furthermore, in order to achieve a cortical implant, it is necessary to fabricate soft implantable devices. It is well known that diamond is the hardest material with a high Young modulus close to 1000 GPa. To demonstrate the feasibility of a soft full diamond implant structure, a novel implant containing metal track in contact with BDD electrodes and encapsulated in thin intrinsic diamond was fabricated. Here intrinsic diamond is locally open above the BDD electrodes to allow interfacing with neurons. The mechanical properties (flexibility, etc.) of such an implant are currently being investigated and in-vitro and in-vivo testing are being prepared.

Redox flow batteries (RFBs)are considered by many to be the most logical approach to addressing grid-scale energy needs. A RFB operates via an exchange of energy through the simultaneous electrochemical reduction and oxidation of redox couples in half-cells. A redox couple consists of an oxidizing species and its corresponding reducing species. The fundamental RFB consists of two electrolytes (containing the redox couples), two electrodes, and an ion-exchange membrane. The electrolytes are stored in external tanks and are cycled through the electrochemical cell via pumps or gravity. As the electrolytes flow, redox species are brought to the electrode surface where they either give or receive an electron from an external circuit. During this process the cell is either charged or discharged and the converted species flow back to the storage tanks.
RFB technology offers several advantages over current state of the art energy storage technologies including: 1) Scalability – capacities ranging from 1kW to 100+MW with minimal design changes; 2) High efficiencies – expected round-trip efficiency of ~85%; and 3) Separation of power and energy – capacity can be increased by simply increasing the volume of stored electrolyte. Implementation of RFBs have been hindered by availability of redox couples. Vanadium is the most predominant redox couple as it possesses multiple oxidation states, allowing it to be used in both sides of the battery (Positive electrode: V⁵⁺↔V⁴⁺, Negative Electrode: V²⁺↔V³⁺). While other redox couples are such as Fe, Cr, and Br are frequently reported, there are very few that are compatible with the graphite electrodes used in the standard RFB configuration. Recent results have demonstrated boron-doped diamond (BDD) to be an exceptional alternative to the standard graphite electrode by enabling a wide range of redox couples that are more cost effective, environmentally friendly, and sustainable while also offering improved efficiencies and energy densities than current state-of-the-art.
Diamond has proven itself to be a superior material for a variety of applications and its utility in electrochemistry has become increasingly apparent over the past decade. In addition to diamond’s ability to resist fouling and corrosion in extreme environments, it’s most critical advantage for flow batteries is it’s extremely high overpotential for both hydrogen and oxygen evolution (-1V to 2.5V, respectively). This large window enables the use redox couples that would occur in either the hydrogen or oxygen evolution reaction regime with common electrodes such as graphite or platinum. Despite these advantages, it has not been until now that BDD has been considered for use in RFB technology and has led to the identification of several additional relevant redox couples that were previously out of reach. This research represents a monumental advancement of redox flow battery technology and could be the solution to grid-scale energy challenges.

Superconducting quantum interference devices (SQUIDs) are widely used as high-sensitive magnetometers in several fields. However, there is still a problem with material robustness in advanced SQUID applications such as scanning SQUID microscope. Superconducting boron-doped diamond is one of the materials that can solve the above problems due to its excellent robustness and characteristics. Recently, we have demonstrated a first single crystalline diamond SQUID [1], whose Josephson junctions (JJs) were formed by step-edge structure. However, it was operated at 2.6 K because the structure included (001) sector superconducting transition temperature T_c of 4.0 K [2]. Operating temperature of SQUID is significant for constructing a complex SQUID system with a simple cooling system. Thus, in this work, we have demonstrated diamond SQUIDs operating above liquid helium temperature 4.2 K using only (111) growth layer with T_c=10 K [3].
JJs were fabricated by epitaxial growth on shallow trench as below: The boron-doped diamond layer was epitaxially grown across the 30 nm deep trench, which was formed via focused ion beam method. The weak link was formed in the boundary of discontinuous (111) sector on the trench. The effective loop area (A_eff) was 68×68 µm² and strip width was 36 µm. It showed two-step superconducting transition at 10.5 K and 8.0 K. The I-V characteristics from 1.6 K to 10 K showed clear DC Josephson effect. There was no hysteresis above 4.0 K. The critical current I_c was 0.42 mA and the I_cR_n product was 0.21 mV at 4.2 K, where R_n is the normal resistance. In the magnetic field dependence of voltage, SQUID oscillation was observed up to 10 K. The oscillation interval (B_ext) was 0.52 µT. This value agreed with calculated B_ext obtained by Φ₀/A_eff where Φ₀ is flux quantum. We also fabricated another sized SQUID, which had smaller A_eff (=21×56.5 µm²) and narrower strip width (=15 µm) than the former. It operated up to 8.0 K. For this sample, B_ext was 1.70 µT, which agreed with calculated B_ext. Therefore, the operations of SQUIDs were demonstrated up to 10 K for former larger sample, and 8.0 K for latter smaller sample. These operating temperatures are dramatically higher than those of SQUIDs of both step-edge structure [1] and nanocrystalline diamond [4]. For the larger sample, V_p-p was 2.0 µV and magnetic field sensitivity V_p-p/B_ext was 3.8 µV/µT at 8.0 K. For the smaller sample, V_p-p and V_p-p/B_ext were 4.0 µV and 2.4 µV/µT, respectively at 8.0 K. These values are higher than that of diamond SQUID with step-edge structure, 1.6 µV/µT.
In conclusion, we have demonstrated two different sized diamond SQUIDs composed of only (111) boron-doped growth layer operating above liquid helium temperature.
[1] T. Kageura, H. Kawarada et al., (Sci. Rep. 2019 in press)
[2] A. Kawano, H. Kawarada et al., Phys. Rev. B 82 (2010) 085318
[3] T. Kageura, H. Kawarada et al., DRM 90 (2018) 181-187
[4] S. Mandal et al., ACS nano 5.9 (2011): 7144-7148

Particle therapy is an innovative mode of radiotherapy (RT) for cancer treatment. The determination of the relative biological effectiveness (RBE) of the clinical particle beams deployed in such RT is of great importance for both the estimation of the therapeutics effects in tumour and the right input parameters for the treatment planning system with the aim of sparing the healthy tissue around the tumour. The physical microdosimetric parameters, such as lineal energy, are used as input parameters for the MK (microdosimetric kinetic) model to calculate the RBE of a given ion beam (protons and ¹²C ions). However, there is a lack of practical experimental tools to characterize this quantity, which is a challenging task. In this context, diamond detectors have shown dosimetric advantages, such as outstanding spatial resolution, good sensitivity to high beam intensities, small energy dependence, radiation hardness and tissue equivalence. For more than ten years, CEA-LIST has been developing diamond-based dosimeters for RT applications. Currently, based on scCVD diamond membranes¹, new prototypes of microdosimeters for particle therapy are being developed at the CEA Diamond Sensors Laboratory (LCD). Materials and Methods: Two novel diamond-based dosimeter prototypes have been developed based on a super-thin self-standing scCVD diamond membranes obtained by deep Ar/O₂ plasma etching process. The processing of the first prototype, DIAµDOS p^+, consists additionally of a p⁺-i junction which was created by a CVD growth of thin boron-doped scCVD layer on top of an intrinsic 4 μm thick diamond membrane. Multiple microdosimetric sensitive volumes (μSVs) were formed on the membrane surface using projection photolithography, followed by local plasma etching of the p⁺ diamond layer. The characteristic behaviour of the micro p⁺-i junction was determined by a 1.8 V built-in potential which resulted in full depletion of the device at 0V-bias². In the processing of the second prototype, DIAμDOS-guard-ring, only single crystal CVD diamond membrane was involved. A thin layer of Al for electrical contacts has been deposited on both sides of the membrane. On the front side, using laser photolithography, μSVs in different sizes have been shaped. These μSVs are interconnected by thin bridges and surrounded by an isolating guard ring in a distance of 2 - 10 μm from the active region. Applying different bias to the back electrode of the device and collecting the signal at the μSV's electrode, a full CCE for different particles can be achieved. Results: The characterization of the diamond microdosimeters has been made possible by particles microbeams at IBIC facilities in Australia, France and Croatia. The active μSVs of the microdosimeter were irradiated using a raster scanning method and the charge transport properties of the device determined with sub-micron precision by measuring the charge collection efficiency, radiation hardness, μSVs 3D spatial definition and the pulse-height spectra. By combining all measurements a large range of experimental measured lineal energy was explored. Using MC radiation transport and TCAD simulations, the obtained measurements with the diamond microdosimeter were compared with theoretically predicted results. The radiation response, as well as the design of the sensors, were optimized. The final prototype of the diamond-based device was integrated with universal sponsors carrier and a suitable multi-channel electronics. The performance of the diamond device was evaluated using clinical particle beams at Ion Therapy Centers in France and Japan. Finally, the response of the diamond microdosimeter in conjunction with MKM was used for RBE determination and demonstrates the great potential of diamond microdosimeter and its ability to measure microdosimetric quantities in clinical ion beams. ¹M.Pomorski et al., Appl. Phys. Lett. 103, 112106, 2013, ²I. A. Zahradnik et al., Phys. Status Solidi A, 1800383, 2018.

The properties of diamond that contribute to its value in radiation detection include large bandgap, high electron and hole mobilities, high breakdown field, and high displacement damage threshold. These properties combine to enable radiation hard, low background, and efficient particle detectors. Research at Arizona State University has developed and demonstrated diamond p-i-n particle detectors based on epitaxial growth of n-type, phosphorus doped diamond, p-type, boron doped diamond, and high purity undoped (intrinsic) diamond. The detector i-layer thickness has been adjusted to match the penetration depth of designated alpha particles, which minimizes background due to gamma radiation and high energy particles. Moreover, the detectors operate in pulse mode with a bias of only a few volts. The unique properties of the p-i-n diode led to the development of a new method to mitigate the effects due to the accumulation of trapped charge in the diamond. Recognizing these advantages led us to propose the development of an efficient thermal neutron detector through the integration of a boron nitride neutron absorption layer and an optimized diamond p-i-n detector. Like diamond, boron nitride is a hard, high temperature material that is expected to survive in extreme environments. A number of BN/diamond pin neutron detectors have been fabricated at Arizona State University and tested at the Ohio State University Nuclear Reactor Laboratory. The tests include pulse counting measurements at a thermal neutron beamline with a flux of 4*10⁶ n/cm²/s, and irradiation of the detectors in the core of the reactor for a fluence of 10¹⁵ n/cm². The thermal beam results show performance as expected with no significant background above threshold and with a reasonable expectation of pulse counting capability at fluxes up to 10¹⁰ n/cm²/s. The irradiation results indicate the onset of a new capability in advanced sensor instrumentation based on diamond diodes.

Quantum bits based on soli-state spins are promising and potentially scalable systems for the implementation of quantum technologies ranging from quantum information processing to quantum-enhanced sensing and metrology. Ideally, they combine individually addressable spins with very long coherence times, optical emission spectra with narrow homogeneous and inhomogeneous broadenings and bright single-photon emission. In this respect, impurity-vacancy color centers in diamond based on group-IV elements (SiV, GeV, SnV, PbV) have emerged as interesting systems promising to combine all desired favorable properties.

Both the SiV and the GeV center feature superior spectral properties, i.e. at liquid helium temperatures (4K), they exhibit a narrow zero phonon line (ZPL) with a four-line fine structure and close to lifetime-limited linewidths [1,2]. Furthermore, both allow for fast all-optical addressing and control of their spin states [3,4]. However, at temperatures around 4K both color centers exhibit spin coherence times (T₂^*) of only a few ten nanoseconds due to phonon-induced decoherence processes [3,4] and the SiV reaches millisecond spin coherence times only at millikelvin temperatures and in pure samples [3,5]. A potential resort are vacancy defects with a heavier group-IV impurity atom, such as SnV and PbV centers, featuring a larger ground state splitting and thus less susceptibility to phonon-induced decoherence.

Here, we report on spectroscopy of SnV centers [6,7,8] where we find two charge states, i.e. SnV(-) and SnV(0), which both show promising optical properties. The SnV(-) features a ground and excited state splitting considerably larger (850 GHz and 3000 GHz, respectively) as compared to the SiV center, potentially enabling long spin coherence times even at liquid He temperatures. This defect is a bright single photon emitter, showing a narrow inhomogeneous distribution of zero phonon lines in a high-temperature annealed sample [6] and truly lifetime-limited transition linewidths down to 20 MHz. Furthermore, we determine the charge transition from the negative to the neutral charge state as a function of the excitation wavelength and find it to coincide well with theoretical predictions [9]. For the SnV(0) center we find emission lines that again agree well with theoretical calculations [10]. The neutral charge state is particularly interesting due to its potentially long electron spin coherence times.

[1] L. J. Rogers et al., Nat. Commun. 5, 4739 (2014).
[2] M.K. Bhaskar et al., Phys. Rev. Lett. 118, 223603 (2017).
[3] J. N. Becker et al., Phys. Rev. Lett. 120, 053603 (2018).
[4] P. Siyushev et al., Phys. Rev. B 96, 081201 (2017).
[5] D. D. Sukachev et al., Phys. Rev. Lett. 119, 223602 (2017).
[6] T. Iwasaki et al., Phys. Rev. Lett. 119, 253601 (2017).
[7] S. D. Tchernij et al., ACS Photonics 4, 2580 (2017).
[8] M.E. Trusheim et al., arXiv:1811.07777
[9] G. Thiering et al., Phys. Rev. X 8, 021063 (2018).
[10] G. Thiering et al., NPJ CompMat.5, 18 (2019).

The unique properties of the negatively charged nitrogen vacancy color center (NV^-) in single crystalline diamond have been explored for developing new types of devices for photonics and scanning probe magnetometers with high sensitivity and spatial resolution over more than a decade. This growing field is in need of thin, free-standing ultra-clean diamond devices with complex geometries. The fabrication of such structures in large numbers will be the topic of this presentation.
Free-standing diamond devices as photonic crystals or NV-scanning-probe heads¹ are typically manufactured from diamond membranes of below 30µm thickness. The latter are expensive and available only in small numbers which makes a scaling of the production very challenging. In addition to this, the homogeneity in thickness below micron level is crucial for many applications and remains practically unachievable via polishing on large surfaces.

An alternative method based on so-called faraday-cage-angled-etch (FCAE) pioneered at the group of prof. Loncar at Harvard university² is based on the capability to create free-standing diamond devices via dry chemical underetching. We have employed FCAE to create mechanical and optical components and will report on the characteristics of the devices investigated via SEM as well as optical and mechanical metrology.

References
[1] C.J. Widmann, et al., Diam. Relat. Mater., 54(2015), pp. 2-8
[2] Burek, Michael J.; Leon, Nathalie P. de; Shields, Brendan J.; Hausmann, Birgit J. M.; Chu, Yiwen; Quan, Qimin et al. (2012), Nano letters 12 (12), S. 6084–6089.

Magnetic microparticles are used in a variety of research applications including cell sorting¹, targeted drug delivery² and optical force traction microscopy³. The magnetic properties of such particles can be customized for specific applications with the uniformity of individual magnetic microparticles having a significant bearing on their function. Prior to the study discussed here, most magnetic characterization techniques have quantified the magnetic properties from large bead ensembles⁴. As such, there is a significant demand for magnetic imaging techniques to evaluate and visualize the magnetic fields from single beads. New insights into the magnetic uniformity, anisotropy, and alignment of magnetic domains can be found through measurements of the magnetic properties of single beads.
Here, magnetic microscopy based on the nitrogen-vacancy centre in diamond is applied to image and characterize individual magnetic beads with varying magnetic and structural properties: ferromagnetic and superparamagnetic/paramagnetic, shell (coated with magnetic material), and solid (magnetic material dispersed in matrix). The magnetic microscopy described here probes both the fluctuating moments of the beads studied as well as their static magnetic moments. The single-bead magnetic images identify irregularities in the magnetic profiles from individual bead populations. Magnetic simulations account for the varying magnetic profiles and allow to infer the magnetization of individual beads⁵. Additionally, this work shows that the imaging technique can be adapted to achieve illumination-free tracking of magnetic beads, opening the possibility of tracking cell movements and mechanics in photosensitive contexts.

1. Reverte, L., Garibo, D., Flores, C. & Caixach, J. Magnetic Particle-Based Enzyme Assays and Immunoassays for Microcystins: From Colorimetric to Electrochemical Detection. (2013). doi:10.1021/es304234n
2. Veiseh, O., Gunn, J. W. & Zhang, M. Design and fabrication of magnetic nanoparticles for targeted drug delivery and imaging *. Adv. Drug Deliv. Rev. 62, 284–304 (2010).
3. Yoshie, H. et al. Article Traction Force Screening Enabled by Compliant PDMS Elastomers. Biophysj 114, 2194–2199 (2018).
4. Science, S. High temperature superconductor dc SQUID micro-susceptometer for. (2004). doi:10.1088/0953-2048/17/5/046
5. Tetienne, J. et al. Proximity-Induced Artefacts in Magnetic Imaging with Nitrogen-Vacancy Ensembles in Diamond. 13–16 doi:10.3390/s18041290

The recent discovery of long-range magnetic order in atomically thin ``van der Waals’’ (vdW) crystals has attracted significant attention due to their fundamental and technological interest, including predictions of exotic magnetic phases and unique opportunities to control magnetism at the atomic scale. I will present recent experiments, where we employ a single-spin-based quantum sensing technology for quantitative, nanoscale probing of atomically thin vdW magnets. Specifically, I will describe experiments, where we employ a diamondbased, scanning probe quantum sensor to address magnetism in the prototypical vdW magnet CrI3, down to the level of atomic monolayers. Our approach enabled nanoscale imaging of magnetic domains, quantitative determination of CrI3‘s layerdependent magnetization, and revealed a delicate interplay between magnetic and crystalline order in CrI3. Next to addressing fundamental open questions in the nanomagnetism of atomically
thin CrI3, our results yield attractive perspectives for future probing of dynamical properties of two-dimensional spin systems using single spin quantum sensors. Amongst other things, these could yield experimental evidence for the still elusive quantum spin liquid, which is believed to occur in monolayers of certain vdW compounds.

Antiferromagnetic materials are promising candidates for new memory devices with fast electrical writing and readout capabilities. Imaging the magnetic state of antiferromagnets on the sub-micrometer scale, however, is challenging because these materials do not exhibit a macroscopic magnetization.

In this talk, we will discuss the application of scanning diamond magentometry for investigating the microscopic domain structure of in-plane layered antiferromagnets, like tetragonal CuMnAs. After introducing the basics of the technique, we will discuss models and concepts for analyzing the magnetic stray field emanating from the antiferromagnetic domains. We will further show that current pulses lead to changes in the domain pattern and analyze these changes as a function of the current amplitude and direction.

The negative nitrogen-vacancy (NV^-) defect in diamond has quantum spin properties observable from cryogenic temperatures to ~1000 K. Parallel to this, SiC based divacancy (VV) complexes photoluminesce in the telecom infrared range, extending their technological applicability. Both of these spin-based quantum sensors are known to be responsive to changes in local strain, electric, magnetic, and thermal fields. Building on this foundation, we discuss recent advances of defect 3D localization in the context of improving and controlling their spatial resolution and crystalline environment. This progress is fueled by nanoimplantation and nanoscale strain-sensitive X-ray imaging techniques. The insights provided serve to better understand the defect’s lattice surroundings, guide future synthesis efforts, improve the creation efficiency, and advance the goal maintaining coherence times of the spin states.

Defect centers in diamond are promising candidates for applications such as quantum sensing, networking and computation. Previous work has experimentally identified several defects such as the NV^– and group IV related defects. However none of these color centers have shown the ideal combination of optical transitions, coherence times, and ease of fabrication and integration, motivating an ab initio search of new color centers in diamond. Towards this goal, we present first principles calculations of a new class of emitters, the group III-vacancies, which are theoretically shown to have a promising ground state structure for quantum applications, a noise insensitive optical transition, and are thermodynamically favored in intrinsic diamond. The ground state fine structure, as well as the excited electronic structure will be discussed in this talk. Further, we will show the electron-phonon coupling parameter calculations to determine the Jahn-Teller distortion, as well as the phonon sideband of the optical emission. Finally, we will discuss how the ab initio results compare with experimental quantities such as the ground state spin structure and optical emission properties.

The nitrogen vacancy (NV) center defect in diamond is emerging as a powerful quantum-enabled technology, in particular in the realm of sensing and imaging with ultra-high spatial resolution. The performance of these solid-state quantum sensors is highly dependent on their quantum coherence and charge-state stability, which are sensitive to their local environment. Surfaces are an important part of the defect's environment, in particular when targeting high spatial resolution sensing, which necessitates close proximity between the sensor and target. To identify and mitigate the deleterious environmental effects, I discuss several materials-based and quantum-control approaches. Specifically, I discuss the formation of highly-coherent NV centers via a gentle, bottom-up method of nitrogen delta-doping during chemical vapor deposition growth of diamond thin films followed by low energy (~ 150 keV) electron irradiation. I present measurements of the density and coherence properties of the NV centers formed in this way as a function of growth and irradiation parameters [1]. In the second part of the talk, I discuss the spin and charge state properties of shallow NV centers, and introduce techniques to mitigate decoherence due to paramagnetic surface spins [2] as well as charge state instabilities near surfaces [3]. These approaches to improved sensors will ultimately enable truly nanoscale spatial resolution imaging of magnetic, electric, and thermal fields in a variety of condensed matter and biological systems.

[1] “Optimizing the formation of depth-confined nitrogen vacancy center spin ensembles in diamond for quantum sensing”, T. R. Eichhorn, C. A. McLellan, A. C. Bleszynski Jayich, https://arxiv.org/abs/1901.11519

[2] "Extending the Quantum Coherence of a Near-Surface Qubit by Coherently Driving the Paramagnetic Surface Environment", Dolev Bluvstein, Zhiran Zhang, Claire A. McLellan, Nicolas R. Williams, Ania C. Bleszynski Jayich, https://arxiv.org/abs/ 1905.06405

[3] "Identifying and mitigating charge instabilities in shallow diamond nitrogen-vacancy centers”, D. Bluvstein, Z. Zhang, A. C. Bleszynski Jayich, Phys. Rev. Lett. 122, 076101 (2019)

Diamond is a very interesting material for many applications due to its excellent properties. The advantage of nano-crystalline diamond (NCD) facing single crystal diamond is the possibility to cover large areas. However, the granular nature of NCD leads to properties based on the interplay between grains and grain boundaries.^[1] Grain boundaries are then fundamental in NCD layers. In this direction, the addition of nitrogen during the growth process has been reported to affect to the grain size. Appropriate N₂ contents can increase the sp² bonding structure in NCD films, promote the smoothness of diamond surface, and efficiently raise the conductivity of electrons, thus making the films possessing n-type conducting property.^[2] However, these properties are associated to a modification of the microstructure of the diamond films which, in turn, altered the electrical properties of the films. An alternative for n-type doping is the use of phosphorus atoms. Substitutional phosphorus incorporation in NCD films with a concentration of about 10¹⁹ cm^-3 has been already reported.^[3]
In this work, NCD layers co-doped with nitrogen and phosphorous are studied. With the aim to obtain the advantages of both dopants, i.e. good conductivity and smoothness surface, a 200 nm thickness N-P co-doped NCD layer has been grown by microwave plasma enhanced chemical vapor deposition (MW PE CVD) over 100-oriented Si substrates. The phosphorous content as well as the methane concentration were kept constant while N₂ was varied from 0.2% to 2% for each sample. Optical emission spectroscopy was carried during the depositions revealing the main species in the plasma for the different gas mixtures. Scanning electron microscopy (SEM) observations show an increase of the surface roughness for samples growth at low N₂ ratio. This is confirmed by transmission electron microscopy (TEM). TEM micrographs show a drastic change in the film morphology with the N₂ increasing. Grain size is reduced with the increasing of the N₂ during growth. Scanning mode TEM (STEM) - electron energy loss spectroscopy (EELS) shows the increase of the sp²-sp³ ratio with the N₂ addition. Phosphorus incorporation in the layers is also revealed.

^[1] P. Achatz, J. A. Garrido, M. Stutzmann, O. A. Williams, D. M. Gruen, A. Kromka, and D. Steinmüller, Appl. Phys. Lett. 88, 101908 (2006).
^[2] J. Birrell, J. A. Carlisle, O. Auciello, D. M. Gruen, and J. M. Gibson, Appl. Phys. Lett. 81, 2235 (2002).
^[3] W. Janssen, S. Turner, G. Sakr, F. Jomard, J. Barjon, G. Degutis, Y-G. Lu, J. D’Haen1, A. Hardy, M. Van Bael1, J. Verbeeck, G. Van Tendeloo, and K. Haenen, Phys. Status Solidi RRL, 1–5 (2014).

CVD diamond is one of the most promising semiconductor materials for RF applications due to its exceptional mechanical, electronic, and thermal properties, such as wide bandgap, high breakdown electric field, high mobility, and high thermal conductivity. Therefore, CVD diamond constitutes a material ideal for high-power, high-frequency, and fast switching applications. The Schottky potential barrier at a metal-semiconductor interface is good for fast switching and it has a low voltage drop in the forward biased regime of a Schottky Barrier Diode (SBD). SBDs are fast because the dynamic response of the switching operation is dominated by the extraction and injection of the majority carrier into the depletion region of the Schottky/diamond junction, with no minority carrier recombination being involved. This study fabricated and tested diamond SBDs for high frequency, high power applications.
Schottky Barrier Diodes were fabricated on layered lightly/highly boron doped (p-/p+ respectively) epilayers on High-Pressure, High-Temperature (HPHT) diamond substrates. High-quality diamond epitaxial layers were grown at MSU by Microwave Plasma-Assisted Chemical Vapor Deposition (MPACVD) using 2.45 GHz microwave resonant cavity systems. The large-signal SBD impedances were obtained using Active Load/Source-Pull (L/S-P) by injecting 20 dBm at 10 GHz to the diode with -4 V applied, giving a value of _out = 40.2 - j169.3 Ω and _in = 57.74 - j293.8 Ω. After conjugately matching the diode input and output, an output power of ~32 dBm = 1.58 W was attained for an injected power of ~34 dBm, giving an RF power density of W/mm**2 and a loss of approximately 2 dB. The SBD s-parameters were measured. In order to extract the loss due to impedance mismatch during the s-parameters measurement, impedance matching at 10 GHz was performed using the Advanced Design System (ADS) software using stub input/output matching networks. A loss of ~1.75 dB was obtained in the post-matching simulations. It should be underscored that the small-signal SBD impedances extracted from the s-parameters vary from the large-signal impedances obtained via Active L/S-P. This is expected since the SBD dynamic resistance changes with RF power.

The ability to modify and functionalise the surface of nanodiamonds make them suitable and useful for a vast variety of applications from drug delivery to nanocomposites^1,2, however, the detonation synthesis of nanodiamonds produces shells of graphitic carbon around the bulk diamond-like core³. The graphitic shell is desired in applications such as drug delivery and biological composites as it enhances surface reactivity⁴. However, in the majority of applications, this shell is considered an impurity as it prohibits the properties of the nanodiamond surface, which can be terminated and treated in ways which are beneficial.

Whilst sp² carbon surface layers can be generated at high temperatures in ultra-high vacuum, the surface layers of sp² carbon on diamond and nanodiamond are removed by annealing in either air or oxygen atmospheres⁵ at ambient pressures. Previous studies have reported that the temperature at which sp³ carbon at the bulk diamond surface is converted to sp² carbon can be significantly reduced in the presence of a metal catalyst⁶. The conversion at the bulk diamond-metal interface can be controlled to form single or multi-layer graphene as diamond can provide the source of carbon and is an ideal lattice-match substrate for the growth of graphene⁷.

A similar process of heating in the presence of a metal catalyst was applied to nanodiamonds where a reduction in the conversion temperature between sp³ carbon to disordered sp² carbon was observed. Using surface sensitive photoelectron-based techniques, the removal and generation of sp² carbon on diamond and nanodiamond surfaces has been studied for a selection of substrate materials and surface preparation.

References
1. Mochalin, V. N., Shenderova, O., Ho, D. & Gogotsi, Y. The properties and applications of nanodiamonds. Nat. Nanotechnol. 7, 11–23 (2012).
2. Moore, L. et al. Comprehensive interrogation of the cellular response to fluorescent, detonation and functionalized nanodiamonds. Nanoscale 6, 11712–11721 (2014).
3. Shenderova, O. A. & Gruen, D. M. Ultrananocrystalline diamond : synthesis, properties, and applications. (William Andrew Pub, 2006).
4. Zhang, J. et al. Surface Chemistry and Catalytic Reactivity of a Nanodiamond in the Steam-Free Dehydrogenation of Ethylbenzene. Angew. Chemie Int. Ed. 49, 8640–8644 (2010).
5. Osswald, S., Yushin, G., Mochalin, V., Kucheyev, S. O. & Gogotsi, Y. Control of sp2/sp3 Carbon Ratio and Surface Chemistry of Nanodiamond Powders by Selective Oxidation in Air. J. Am. Chem. Soc. 128, 11635–11642 (2006).
6. Cooil, S. P. et al. Iron-mediated growth of epitaxial graphene on SiC and diamond. Carbon N. Y. 50, 5099–5105 (2012).
7. Cooil, S. P. et al. Controlling the growth of epitaxial graphene on metalized diamond (111) surface. Appl. Phys. Lett. 107, 181603 (2015).

The optical and magneto properties of the negatively charged nitrogen-vacancy (NV) center in diamond has been extensively investigated because of its application to quantum sensing, such as magnetic and electric fields and to quantum information technologies. In most cases, near-infrared luminescence upon the excitation by green laser can be controlled by the irradiation by microwave, which tunes the optical transition path between the excited and ground states. To extend the optical property of diamond over the current generation of linear optical regime, one requires ultrashort laser pulses, which enables one to induce nonlinear optical effect, such as the nonlinear refraction and nonlinear absorption. Almeida et al. recently reported on nonlinear optical spectra in high-purity diamond using femtosecond laser pulses with photon energy from 0.83 to 4.77 eV and measured coefficients for the nonlinear refraction and nonlinear absorption [1]. Because of the absence of defect-related bands below the band-gap (E_g= 5.5 eV), the two nonlinear optical effects were enhanced for E = E_g/2. Although there are several investigations on nonlinear optical effects in diamond and nano-diamonds [2,3], the effects of the NV center on the nonlinear optical phenomena have not yet been examined. This issue is, however, important to further study new functionality of diamond photonics to advance nonlinear quantum sensing. Here we explore the NV center induced nonlinear optical effects in pure diamond, specifically optical Kerr effect and two-photon absorption in transparent region using 800 nm light, at which NV related bands will be sensitive. It is found that both the nonlinear optical effects enhanced by the introduction of NV centers.

[1] J. M. P. Almeida et al., Sci. Rep.Vol. 7, 14320 (2017).
[2] J. I. Dadap et al.,Opt. Lett. Vol. 16, 499–501 (1991).
[3] F. Trojanek et al.,Opt. Exp.Vol. 18, 1349 (2010).

The fabrication of high power diamond electronic devices requires an improvement of the crystalline quality of the doped layers. This is one of the limiting factors of the structral and electrical properties of chemical vapor deposited (CVD) layers. Although recent advances in CVD synthesis have enabled the growth of free-standing, single crystal, intrinsic, and boron doped diamond, defects–such as dislocations–remains an important point of attention. Several research groups^1,2 have reported on the significant influence of the [B]/[C] ratio in the plasma feedgas on the electrical properties of B-doped single crystal diamond (SCD) films. In this study, the impact of the methane concentration on the crystalline quality and electrical properties of lightly B-doped SCD layers is investigated. The samples were grown in a homemade CVD reactor on 3 Χ 3 mm² Ib (100)-oriented high pressure high temperature (HPHT) substrates. Growth was performed in a conventional [CH₄]/[H₂] plasma (1%, 2%, and 3%) with residual trimethylboron as the boron source, used for all of the samples. The p^- layers were characterized by Hall effect in a wide temperature range, Fourier transformed infrared (FTIR) spectroscopy, X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), optical microscopy, and transmission electron microscopy (TEM). The TEM study of the sample grown at 1% methane concentration showed a high-quality p^- SCD film without any observed dislocations in a thickness of 1 μm. In the XAS measurements of the sample grown at 3% methane, small absorption peaks occurred around 282 eV to 285 eV in the C K-edge region which are assigned as acceptor levels in semiconducting B-doped diamonds.³ These peaks were not observed in the samples grown at lower methane concentrations (1% and 2%). XPS measurements indicated that the intensity of the B 1s peak is increased by increasing the methane concentration which showed a higher B-doping level. In FTIR spectroscopy, the 2800 cm^-1 peak has been used by research groups^4,5 to estimate the vacant acceptor concentrations in the range of 10¹⁶ to 10¹⁸ cm^-3. The boron concentrations estimated by FTIR were compared to the result of the Hall effect which confirmed the same [CH₄]/[H₂] dependence as observed by X-ray spectroscopy, with a sample grown at 3% methane showing a higher boron incorporation than those grown at 1% and 2% methane concentrations in the plasma feedgas. These results were corroborated by first principle density functional theory (DFT) calculations. The formation of more growth centres, due to higher methane concentrations, are found not to significantly change the adsorption energy of the boron impurity at nearby sites. Instead, these growth centres are shown to dramatically increase the formation of missing H defects, which are premium boron incorporation sites, thus indirectly increasing the boron concentration.

References:
1- J. Achard, et al., Phys. Status Solidi A 209(2012), 1651-1658.
2- J. Achard, et al., Diam. Relat. Mater. 20(2011), 145-152.
3- Y. Muramatsu, et al., X-Ray spectrom, 36(2007), 162-166.
4- E. Gheeraert, et al., Diam. Relat. Mater. 7(1998), 1509-1512.
5- R. Issaoui, et al., Appl. Phys. Lett. 100(2012), 122109-4.

For decades, diamond has shown superior properties that make it desirable for novel electronic materials. However, fabricating large size high quality single crystal diamond wafers faces multiple challenges. Lateral outgrowth of CVD diamond with designated pocket holders makes it highly possible for large size diamond wafers to be realized. A mixed analysis of crystallographic structure of laterally grown CVD diamond and influence of pocket holder boundary condition is key to understanding how to eventually fabricate large size high quality single crystal diamond wafers by microwave plasma assisted chemical vapor deposition (MPACVD).

In this work, single crystal diamonds were grown laterally on High Pressure High Temperature (HPHT) substrates with designated pocket holders by Microwave Plasma Assisted Chemical Vapor Deposition (MPACVD) and characterized by a micro X-ray diffraction (μ-XRD) mapping technique with a 300 μm beam size to show the evolution of diamond crystal structure information during growth and provide feedback on growth strategies. By understanding how crystal structure evolves during growth we can understand the mechanisms linking processing conditions the structure and properties of grown material.

A series of CVD diamond is grown on standard HPHT (3.5mm x 3.5 mm) with various thicknesses (600 μm – 1400 μm) using designated sample holders. High-resolution x-ray rocking curve (HRXRC) and full width at half max (FWHM) μ-mapping of the (400) diamond peaks illustrate the evolution of crystallographic changing after each processing. Relatively low values for misorientation (typically within 0.05°+/-0.003°) and FWHM (typically around 0.025°+/-0.003°) for CVD diamond are compared after a series of growth. Evolution mapping of growth and regrowth suggest an upper limit in FWHM exists for a single CVD growth, which is promising for CVD diamond to have a high quality single crystal sample. Thus, a large size single crystal CVD diamond with uniform crystal orientation (low bending in the crystal structure) and low mosaicity (small FWHM) can be achieved.

Moreover, preliminary results demonstrate single crystal CVD diamond prefers to grow laterally in an exponentially decay way towards the pocket boundary, and smooth single crystal diamond (SCD) edges are formed after growth. The growth trajectory of the diamond profile satisfies an exponential decay relationship as it gets closer to the pocket boundary, which suggests a moderate plasma density region makes the main factor for a diamond sample to grow as a single crystal. Based on the growth rate according to specific diamond crystal orientation, lateral outgrowth using series of designated pocket holders shows the effective diamond area can be enlarged from the 3.5mm x 3.5mm standard HPHT to around 5mm x 5mm CVD diamond in a single run. Regrowth on CVD plate using iteration method with wider iteration holders is promising for SCD samples to keep growing even larger.

Diamond film collects all the outstanding properties of mechanics, electronics, heat, and optics etc. together making it have potential applications in the fields of wide range optical transparent window material, super-hard coating tolls, especially, in the field of high power electron devices which can work with high frequency and in high temperature environment Recently, researchers have extensively investigated hydrogen-terminated diamond (H-diamond) field effect transistors (FETs)^1,2. H-diamond surface can form a p-type conduction layer with a sheet hole density and hole mobility of 10¹³ cm^-2 and 50-150 cm²V^-1s^-1,repectively. However, this conduction layer is thermally and chemically instable and vulnerable to the ambient environment such as oxidation of diamond surface or higher annealing even in vacuum. All these would cause the missing of p-type conduction layer and the degradation of FET performance. This problem could be solved by fabricating dielectric layers on diamond surface to maintain the p-type conduction channel, which also acts as a insulators. Various dielectric layers have been used such as SiO₂, Al₂O₃, HfO₂, Ta₂O₅, and ZrO₂ on H-diamond surface. However, there are few reports on using dielectric of HfSiO₄, which has excellent thermal stability, adequate band gaps (from 15 to 25 depending on hafnium content).

In this work, investigation of H-diamond FETs with ALD-Al₂O₃/ SD-HfSiO₄ dielectric layers have been carried out. Firstly, gold electrodes were patterned on H-diamond by electron beam evaporation technique. Secondly, UV-ozone was used to insulate p-type conduction channels between source and drain electrodes. The length of channels were 20mm. Then, 4nm ALD-Al₂O₃/ 30nm SD-HfSiO₄ dielectric layers were fabricated on p-type conduction channels by ALD and SD technique. Finally, electrical properties of this diamond FETs were investigated. The detail will be presented in the conference.

Endogenous and exogenous, reactive oxygen species (ROS) have a significant, damaging effect on cell metabolism, including in neural systems. Peroxynitrite (PON) has emerged as a secondary ROS that is linked with oxidation and/or nitration of biomolecules, and is the product of the diffusion-limited reaction of superoxide and nitric oxide radicals. Its short lifespan and fast reactivity increase the challenge of measuring its concentration under physiologic conditions.
Several methods exist for peroxynitrite detection in aqueous environments, yet the problem of real-time quantification remains. In this paper, the electrochemical behavior of peroxynitrite has been studied using boron-doped diamond (BDD) microelectrodes. For more than a decade, diamond-based sensors have been successfully applied to electrochemical detection of various analytes in biological and environmental systems, due to its wide potential window, and stable, low background current. In addition, the diamond has more predictable surface to fine-tune for optimized detection of peroxynitrite.
In this research study, the direct, real-time electrochemical oxidation of peroxynitrite on BDD electrodes was investigated. A wide range of physicochemical characterizations was conducted to demonstrate the catalytic oxidation of PON on surface-modified diamond microelectrodes. Initial attempts were made to determine whether PON is generated during simulated, neural stimulation pulses, in a nitric oxide containing environment. Developing BDD microsensors that can monitor the electrochemical signature of peroxynitrite will be an important step forward towards improving the safety of electrical stimulation of cells as well as general monitoring of PON levels as an indicator of tissue deterioration and age-associated diseases.

Overheating cripples the efficiency of electrical devices. Diamond heat sinks can facilitate heat dissipation and directly lower operating temperatures. The advent of nanocrystalline diamond (NCD) deposition techniques provide the ability to work with diamond on nanometer length scales, which cannot be done with traditional machining. Thermal optimization of metal/NCD junctions, used to adhere NCD to the device, is required to maximize the efficiency of the heat sink.
We examine thermal transport across Au/contact/NCD junctions, for the contacts Cr, Ni, and Ti. For each contact, the thermal boundary conductance (TBC) is measured using frequency domain thermoreflectance for contact thicknesses between 0 and 10 nm. Cr and Ti are predicted to chemically react with NCD to form enthalpically favorable carbides. Grown carbides add thermal resistance, lowering the TBC of the Au/contact/NCD junction. During deposition of the contact, NCD grain boundaries may act as low-resistance highways for carbon diffusion, promoting carbide growth. Thus, fine-grained materials like NCD may be at risk of forming low TBC junctions with metals, hindering the ability of future NCD heat sinks. To mitigate reactions, we deposited 0 to 10 nm thick Ni diffusion barriers between Cr or Ti and the NCD. The measured TBC of the Au/(Ti or Cr)/Ni/NCD junctions is used to quantify the improvement from reaction mitigation.

Diamond electrodes terminated with surface layers can impart significant changes to the electronic properties of a diamond interface and we have previously reported on studies of the tunability of surface work function by substituting hydrogen with oxygen under controlled conditions ¹. Engineering a functionalised diamond surface that is stable and reproducible is of technological importance to future diamond electronic devices such as Schottky junctions and electron-emitting electrodes for field emission, thermionic energy converters, secondary electron multipliers.
Tin monoxide has been considered as potential material for battery anodes, coating, gas sensing, catalysis, etc. The application area of SnO extends over photovoltaics, displays, information storage, etc. as it has a large optical bandgap of 4.12 eV, high transparency (~90%) non-toxic and abundant ². SnO has been found to form a layered structure in a crystallographic direction of [001] with a monolayer thickness of 0.48 nm. Sn 5s form a lone pair of electrons which point towards the interlayer spacing and result in dipole-dipole interaction leading to a Van der Waals gap of 2.52 A between the layers. Various groups have demonstrated the growth of SnO mono and multiple layers by various methods but many of these result in an upper SnO₂ layer that forms as soon as the sample is taken into the oxygen environment as SnO is metastable and hence rapidly converts into SnO₂.
We have produced mono and multiple layer SnO on conductive, p-type single-crystal diamond under ultra-high vacuum (UHV) conditions by e-beam evaporation and characterised in-situ by Spot Profile Analysis Low-Energy electron diffraction (SPA-LEED)and Energy Filtered Photo Emission Electron Microscopy (EF-PEEM) and X-ray Photoemission Spectroscopy (XPS). We quantify the change in work function between mono and few-layer SnO and its effect upon the barrier height of a p-type diamond Schottky junction.

References

1. Wan, G., Cattelan, M. & Fox, N. A. Electronic structure tunability of diamonds by surface functionalization. J. Phys. Chem. C (2019). doi:10.1021/acs.jpcc.8b11232
2. Pei, Y., Liu, W., Shi, J., Chen, Z. & Wang, G. Fabrication and Characterization of p-Type SnO Thin Film with High c-Axis Preferred Orientation. 45,

High pressure and high temperature (HPHT) technique is a process enabling diamond synthesis and the control of diamond properties. The HPHT-processed diamonds show characteristic properties which cannot be achievable by the other technique. E. g., there is a report for optical emissions from Sn-vacancy (Sn-V) center in HPHT-treated diamond reported by Iwasaki et al.[PRL 119, 253601, 2017.], which shows the prospect for its application to a light source of quantum emitters. Nanoscale structural design trying to obtain new functions of existing materials is a key issue for the realization of future devices and applications. To explore the functions of diamond, HPHT technique would have potential to activate its unknown properties as in the case of impurity activations of the above reports. We consequently investigate the effect of HPHT treatment on P and Sn implanted diamond on its surface.
For the investigations, we employed commercially available single crystalline diamond (001) substrate synthesized by chemical vapor deposition. We introduced P and Sn by ion implantation at 1×10²⁰cm^-3 uniformly from the surface to the depth 300 nm and 50 nm, respectively. Subsequently, we applied HPHT treatment at 15 GPa and 2300°C for 20 min. The treatment is expected to induce recrystallization and heal the damage of the diamond surface. The sample surface after HPHT treatment showed square holes aligned to specific crystal orientation, suggesting crystallographic rearrangement of constituent elements. Secondary ion mass spectrometry showed the profiles of P and Sn after HPHT with the density similar to the ion implantation. We also observed atmospheric impurities, H, O, and N in the diamond. The amount of those elements varies by the conditions of ion implantation and HPHT treatment. Hence, the proper ion implantation and HPHT treatment can preserve the implanted elements in the host matrix and suppress the contaminations of atmospheric elements. The Sn-doped sample showed sharp photoluminescence emission at 1.999eV and 2.003eV at low temperatures after HPHT as reported by Iwasaki. These results show great potential of HPHT as a material engineering tool for diamond.

Electronically active low dimensional structures have been made in a variety of material systems. Nanowires in particular are of interest in fields as varied as quantum computing, sensing and power electronics to name but a few.
By utilising a mix of additive and subtractive processing a novel lateral diamond nanowire is fabricated and characterised. The substrate and foundation for this nanowire is a highly boron doped diamond epilayer, grown on high purity insulating single crystal diamond. The dopants are confined spatially in a 1-2nm region, this delta function like distribution can be shown to have similar electronic properties to a bulk doped. The ∂-doped epi-layer is then patterned and etched to leave wires with dimensions on the order 10-20nm and arbitrary controllable lengths. In this talk we will discuss the transport mechanisms and phenomena observed, as well as showing a novel field effect transistor utilising a number of these wires to achieve high current handling and demonstrate the scalability of this unique technology.

Single crystals of diamond have high mechanical strength and are very resilient to high-intensity X-ray radiation. Diamonds are thus irreplaceable in applications across X-ray optics and high-pressure physics, serving as windows, monochromators, and anvils in high pressure cells. The physics underlying diamond’s unusual properties stems from its strong sp³ covalent bonds and simple symmetry. As diamonds eventually damage under ultra-high intensity X-ray radiation, the tetrahedrally bound sp³ carbon atoms are known to graphitize to trigonal bipyramidal sp² hybridized carbon atoms. This change in local symmetry mechanically destabilizes the material by adding low-energy pathways along which the crystal may deform. Studies of the mechanism by which high-energy X-rays graphitize diamond and the resulting changes to the mechanical properties have been limited. Few characterization techniques can measure the lattice defects and microstructure of a crystal deep beneath its surface—especially as the material responds to external stimuli.

We use ultrafast dark-field X-ray microscopy to directly image changes to the phase, strain and lattice tilt with ultra-high sensitivity as diamond responds to the high intensity X-ray radiation at an X-ray free electron laser. With these results, we demonstrate how minor changes to the microstructure of diamond from the incident radiation cascade into large-scale deformations. This work demonstrates how this novel high-sensitivity technique can give us a new view of the link between radiation and mechanical damage in diamond, informing applications across X-ray science, high-pressure science and astrophysics.

This work was performed in part under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

Diamond is a wide band gap semiconductor with high thermal conductivity, carrier mobility and breakdown field, which make it particularly suitable for the fabrication of high power electronic devices. However, the development of diamond devices is limited by the high ionization energy of its dopants, which leads to only a small fraction of them being ionized at room temperature. When stressed by a high electric field, a current multiplication has been observed in boron-doped diamond (BDD) attributed to Impurity Impact Ionization (III) and the ionization of the neutral acceptors¹. In this work, we analyse the current multiplication in BDD including the combined effects of Self-Heating (S-H) and III.
Epitaxial BDD layers with different acceptor concentrations in 3.1×10¹⁸—6.1×10¹⁹ cm^-3 range have been grown in an AX5010 microwave plasma enhanced chemical vapour deposition system. Quasi-static current-voltage (IV) characteristics were measured using a Transmission-Line Pulse setup² with 100 ns pulse duration on concentric titanium/gold ring-disk electrodes structures patterned by standard lithography and wet etching methods. S-H was analysed using Transient Interferometric Mapping (TIM)³ of the thermal energy distribution between electrodes in ns time scale. The experimental IV characteristics were modelled by taking into account S-H effect and III using finite element method. The simulated IV characteristic, in particular the negative differential resistance (NDR) region appearing due to S-H effect, is in good agreement with the experimental data. This work was financially supported by the project 17-05259 S of Czech Science Foundation and the SGS Student Grant “Electric Discharges II: experimental research, modeling and applications” ID: SGS19/167/OHK3/3T/13.

¹ V. Mortet and A. Soltani, Applied Physics Letters 99, 202105 (2011).
² V. Mortet, L. Drbohlavova, N. Lambert, A. Taylor, P. Ashcheulov, M. Davydova, J. Lorincik, M. Aleshin, and P. Hubik, Diamond and Related Materials 98, 107476 (2019).
³ D. Pogany, S. Bychikhin, C. Furbock, M. Litzenberger, E. Gornik, G. Groos, K. Esmark, and M. Stecher, IEEE Transactions on Electron Devices 49, 2070 (2002).

Semiconducting diamond is touted for its outstanding intrinsic and thermal properties that promise tremendous devices performances advantages for power control and conversion applications. Over the last few decades, basic works in diamond growth and doping have allowed the extension of known devices technologies to diamond as well as the development of new devices concepts. However, to achieve high performance diamond devices that can outperform the existing and emerging technologies, much remains to be understood in many areas such as the diamond devices physics, the effects of defects (intrinsic and extrinsic) on diamond devices transport properties, the diamond surface passivation, and the diamond devices design (edges termination issues).

This presentation will be focused on the electrical transport in diamond bipolar devices such as diamond p-i-n diode. Bipolar diamond devices are very promising and attractive devices because of the conductivity modulation phenomenon that allows to counterbalance the effects of high ionization energies of dopants in diamond (0.58 eV and 0.38 eV for Phosphorus donor and Boron acceptor respectively).
In our work, diamond p-i-n diodes have been used to study carrier recombination mechanisms, charge carrier dynamics, and impact ionization in diamond. Our pioneering works on the electrical characterization of the diamond p-i-n diode in dynamic and the optimization of diamond p-i-n devices design to suppress the electric field crowding at devices edges for an accurate measurements of impact ionization coefficients by electron beam induced current method (EBIC), will be introduced and discussed.

The wide bandgap, high carrier mobilities, high breakdown field and highest thermal conductivity identify diamond as the ideal power semiconductor material. A particular advantage of diamond over all other semiconductors is operation in high temperature, high radiation and extreme environments. Diamond PIN diodes or PIN-Schottky diodes will be crucial components in power conversion and control systems that can operate in extreme environments. In this study the transport characteristics of high purity diamond PIN and PIN-Schottky diodes are measured and simulated for temperatures ranging from room temperature to 500°C. While the forward current density increases due to dopant activation and thermionic emission, the reverse current increases by many orders of magnitude consistent with thermionic emission theory. The diode forward/reverse rectification ratio consequently decreases by many orders of magnitude. Attempts to engineer the barrier using different metals (with different work functions) show very limited success. This study presents an alternative approach where the diode barrier is engineered by increasing the phosphorus doping in the n-type layer. Experimental results and simulations establish that the barrier can be varied from about 1 eV to greater than 3 eV which restores the high temperature (500°C) rectification ratio to nearly 10⁴ . The simulations also consider the forward current properties and they establish how the specific diode characteristics can be tuned for operation at elevated temperature up to and exceeding 500°C. This tunable approach appears may provide an advantage for diamond diodes (over other power semiconductors) for high temperature operation.
Research supported by NASA through the HOTTech program, and the NSF through grant IIP–1747133. Authors acknowledge the support of the ASU NanoFab supported by NSF grant NNCI – 1542160.

Electronic properties of doped-diamond are of great technological interest due to the possibility of diamond-based electronic devices for high power electronics and for the fabrication of QUBITs. With a bandgap of 5.5 eV at room temperature (RT), diamond is an electrical insulator however when it comes to substitutional incorporation of impurities in its lattice via doping, it becomes extremely selective unlike Si and Ge and its conductivity has been seen to change drastically with doping. Doping of diamond lattice by boron to make it p-type has been the most successful so far by incorporating boron atoms in the gaseous phase during the growth process and this has shown enhanced electrical conduction making it metallic and even superconducting [1,2] signaling an insulator-to-metal transition (IMT) at low temperatures provided the dopant concentration is above a critical value ~ 10²¹ cm^-3. Nitrogen and phosphorus are the famous candidates for the n-type doping in diamond which contributes an extra electron in the system. Nitrogen doping has not yet brought much success in terms of electrical conductivity during the past one decade as it fails to form a shallow donor level with its donor level at E_c – 1.7 eV because of dramatic localization of the donor electron on a neighboring carbon atom with a subsequent formation of a lone-pair on the nitrogen atom. Phosphorus, on the other hand, being a large impurity does not relax in an analogous fashion unlike nitrogen, and thus leads to a substantially shallower donor level at E_c − 0.6 eV, which is still rather deep for room-temperature ionization.
Over the last two decades, much work has been done on making diamond n-type with increased conductivity but so far, the highest achieved conductivity is 143 (Ω-cm)^-1 [3,4] in nitrogen doped (20% N₂) ultrananocrystalline diamond (UNCD). For the first time, we report the formation of n-type diamond by nitrogen doping in ultrananocrystalline diamond deposited on silicon substrate with resistivities similar to copper (ρ_copper ~1.68 x 10^-8 Ω-m at 20 °C) with metallicity being observed throughout the temperature regime for few doping concentrations. Developing nitrogen doped UNCD thin films was carried out by depositing an ultrananocrystalline diamond thin film on Si in hot filament chemical vapor deposition reactor followed by ion implantation by N⁺ ions with different energy and fluences and subsequent annealing to make the vacancies and dopants mobile so that substitutional doping can be ensured inside the lattice. These implanted films were characterized by Raman spectroscopy for their UNCD nature before and after annealing post-implantation.
References:
1. Ekimov, E. A., et al. Nature 428.6982 (2004): 542.
2. Abdel-Hafiez, M., Kumar, D., Thiyagarajan, R., Zhang, Q., Howie, R. T., Sethupathi, K., ... & M. S. R Rao. (2017). Physical Review B, 95(17), 174519.
3. Bhattacharyya, S., et al. Applied Physics Letters 79.10 (2001): 1441-1443.
4. Bhattacharyya, Somnath. Physical Review B 70.12 (2004): 125412.

The metallic behavior of highly boron doped diamond (BDD) is an asset for the development of the future diamond-based electronic technology. However, boron concentration quantification still remains expensive and, in some cases, difficult. Secondary ion mass spectroscopy (SIMS) is the most accurate technique but requires special care when steep profiles are involved ^[1]. It is also a very expensive tool. Alternatively, boron doping can also be quite precisely evaluated by cathodoluminescence (CL). This technique collects the photons generated by the interaction of the incident electrons with the electrons of the sample. The consecutive radiative recombination signal depends on the electronic properties of the sample. In particular, at low temperature, excitons bound to boron impurities have specific spectral signatures that make it possible to determine the boron levels. However, one of the main restrictions to the use of this technique is the presence of dislocations. The strain generated around the dislocation bend the valence and conductions bands trapping all the carriers. Their mobility is then strongly reduced and electrons and holes recombine mostly through non-radiative channels, while a relatively weak broad band (A-band) may result from the remaining radiative carrier recombination at the dislocation. In regions where the density of defects is high enough to close all radiative channels, the evaluation of the doping levels by CL is no more possible. In this case, the quantification of boron becomes a challenge. We propose here to use the interaction between boron atoms and dislocations specific to these regions in order to estimate the doping level by transmission electron microscopy.
Substitutional boron incorporation within BDD can be seen as allowing elements into the crystalline lattice forming a solid solution. The non-uniformity generated by boron atoms makes plastic deformation more difficult by impeding dislocation motion, thus creating an increase in the yield stress of the material also called strengthening. Upon lateral growth, dislocations are pushed forward, along a specific course ^[2]. Their trajectory varies when they pass through a doped layer because of the solid solution strengthening. An increase in the dislocation length is observed, necessary to accommodate locally the additional tension generated by the boron impurities in the doped layer. In this work, boron doped/undoped superlattices have been laterally overgrown over patterned substrates. Two 001-oriented HPHT substrates were etched away by inductively coupled plasma reactive ion etching (ICP-RIE) leaving a pattern array of 5 µm diameter and 1µm height disks. Over these substrates, p⁺⁺/p^- bilayers were grown with different [B]/[C] ratios for each sample. The presence of dislocations was ensured by the roughness substrate surface after etching. The dislocation length was measured by high resolution TEM for both samples. Boron content was then estimated by the above proposed method, and compared to that reported in the literature for similar growth conditions.

^[1] A. Fiori, F. Jomard, T. Teraji, G. Chicot and E. Bustarret, Thin Solid Films 557, 222 (2014)
^[2] F. Lloret, M. Gutierrez, D. Araujo, D. Eon and E. Bustarret, Phys. Status Solidi A, 214, 1700242 (2017)

Diamond has been identified as an attractive material for high power devices because of its physical and electronic properties. In comparison to other wide-bandgap semiconductors diamond exhibits high carrier mobilities and thermal conductivity. The electronic properties of diamond are defined by the dopants that are present during growth or introduced in diamond to achieve p- and n-type conductivity by ion implantation of boron or phosphorous respectively. This process of introducing dopants also creates defects such as vacancies and interstitials along with the presence of dopant atoms. Among the implanted dopants, the number of activated dopants is found to be only a few percent. Thus, it is interesting to study the geometry of dopants and other factors which result in such low levels of active dopants.

In this work we have used first principles density functional theory based calculations within the supercell approximation to identify the structure and electronic properties of complexes formed from carbon vacancy, boron, boron-hydrogen and interstitials in diamond. Total energy calculations were performed to identify ground state structures for various defects and formation energies were used to predict the probability of presence of that defect. Ionization energies for different defects are evaluated using the marker method. Formation energy of substitutional boron is found to be lower in presence of hydrogen. As seen by others, we also find that a single vacancy relaxes into a tetragonal distorted structure of the nearest carbon atoms from the original tetrahedral structure. This structure is lower by 0.22 eV compared to the original tetrahedral structure. Activation energy of vacancy diffusion is found to be 2.8 eV using the nudged elastic band method. Spin-polarized calculation of single vacancy shows that there is small magnetic moment on all 4 carbon atoms surrounding the vacancy and the total energy of such a calculation is lower than a paramagnetic calculation. Other results which include spin-polarized calculations on diamond will also be shown.

Nitrogen vacancy (NV) center in diamond is expected as a magnetic sensor with high sensitivity at room temperature and single NV center succeeded in detecting ¹H spins out of the substrate [1]. For improvement magnetic sensitivity, both shallow and aligned NV ensemble is required since the magnetic sensitivity is inversely proportional to the root of number of NV centers contributing to the measurement [2, 3]. Delta-doping of N was tried by diamond growth on N-terminated (001) diamond and estimated 2D concentration of NV centers was 1 x 10¹¹ cm^-2 [4]. This method can control the depth of NV ensemble precisely unlike ion implantation or N-doped layer which are popular methods to fabricate NV ensemble. However, NV centers in (001) delta doped layer would be not aligned because possibility of each four sites adjacent to N becoming a vacancy are equivalent. In the case of (111) delta doping layer, NV centers will be aligned because a top site is more preferable to become a vacancy than other three equivalent sites. Recently, we established the method to fabricate N-terminated diamond with high coverage causing no damage on the surface [5, 6]. We partially succeeded to make aligned two-dimensional NV ensemble from N-terminated (111) diamond with half mono layer coverage embedded by high purity (111) diamond film.
N-terminated (111) surface was fabricated by nitrogen radical exposure method [5, 6]. After that, thin layer (thickness is about 10 nm) of high purity (111) diamond film was epitaxially grown on the N-terminated surface using microwave plasma enhanced chemical vapor deposition method. We confirmed that N density in the high purity diamond film was below 1 x 10¹⁶ cm^-3 by secondary ion mass spectroscopy (SIMS). We also confirmed the formation of NV ensemble with 2D concentration of about 1 x 10¹¹ cm^-2 using the confocal PL-intensity mapping. Optically detected magnetic resonance (ODMR) with static magnetic field applied along the [111] axis was used to determine whether the NV ensemble was highly aligned or not [7]. The ODMR spectrum of the NV ensemble had only two dips, which might be derived from the NV center whose orientations were aligned along the [111] axis. In conclusion, NV ensemble was fabricated by diamond growth on N-terminated (111) diamond and was aligned along the [111] axis.

[1] T. Staudacher, J. Wrachtrup, et al., Science 339, 561-563 (2013).
[2] J. Michl, J. Wrachtrup, et al., Appl. Phys. Lett. 104, 102407 (2014).
[3] L. Pham, R. Walsworth, et al., Phys. Rev. B 86, 121202(R) (2012).
[4] M. Chandran, A. Hoffman, et al., Appl. Phys. Lett. 109, 221602 (2016).
[5] T. Kageura, H. Kawarada, et al., Appl. Phys. Exp. 10, 055503 (2017).
[6] S. Kawai, H. Kawarada, et al., J. Phys. Chem. C 123, 3594-3604 (2019).
[7] T. Fukui, N. Mizuochi, et al., Appl. Phys. Exp. 7, 055201 (2014).

Acknowledgements
We thank Dr. Liam P. McGuinness and Prof. Fedor Jelezko for their help in building CFM setup.

The recent development of novel super-resolution imaging techniques coincides with the efforts to synthesize optically bright and stable biomarkers. It is known that the electrical resistivity of nanodiamonds is quite high with a bandgap of 5.5 eV and at room temperature they are dielectrics. Fluorescent nanodiamonds (FNDs) can be produced using a variety of methods with a wide range of sizes ranging from several nanometers to tens of micrometer, which defines their penetration and adhesion properties. Nano-diamonds (NDs) synthesized by microwave plasma chemical vapor deposition (MPCVD) techniques are an interesting class of diamond materials that can be used for a plethora of applications.
There has been low usage of FNDs as a biomarker due to their weak quantum efficiency compared to all other biomarkers, nanoparticle, and fluorescent dyes. Thus, the main challenge is to enhance the emission intensity from the FNDs. The NDs are subjected to nitrogen ion-implantation at an implantation energy 20keV with various implantation fluences. Implantation depth concentration and crystalline distortion level profiles have been calculated using Low Energy Approximation. The NV defects fabricated through implantation are optically active color centers which will enhance the photoluminescence properties by energy transitions between the defect states and carbon states. Here we report the comparison between the unimplanted and nitrogen ion-implanted diamond nanoparticles on the basis of photoluminescence and Raman spectroscopies. In addition, we will be investigating the concentration of nitrogen atoms and crystal disorder influence on the optical emission dynamics of diamond nanoparticles.
The present study will provide guidance for the future synthesis of fluorescent nano-diamond markers for biosensing and bioimaging applications in multidisciplinary fields. For example, we can use the differently doped nanodiamond fluorophores mixed with milk powders as biomarkers to identify the various milk constituents.

The inclusion of Silicon Vacancy (SiV) centers into synthetic diamond has already proven to be a valuable tool for the production of optoelectronic devices such as p-i-n diodes, photonic crystals or single-photon emitters[1].
In our recently published work we have shown the preparation of synthetic diamond multiply twinned particles (MTPs) in icosahedron shape[2]. Here we present the expansion of this work regarding MTPs having the shape of both completed dodecahedrons and icosahedrons which were grown by plasma-assisted CVD on Si(111) surfaces. This process ensures the reliable inclusion of SiV centers thus eliminating the necessity of any post-CVD implantation methods. Also, the resulting lms provide a sufficient conductivity for further applications.
The growth mechanism was found to consist of several differentiable steps: first, octahedral, single crystalline particles grow up to edge lengths around ca. 300 nm. At this stage, the formation of penetration twin structures sets on, resulting in the controlled growth of particles with dodecahedral and later icosahedral symmetries. The SiV centers form predominantly by redeposition of sputtered Si atoms at the initial growth stages and by bulk diffusion along grain boundaries for thicker films.
Their characteristic photoluminescence emission at 738 nm was found to have a FWHM in the range 3-4 nm. Closed MTP layers were used to fabricate diodes and μ-LEDs based on vertical diamond/Ti Schottky contacts by sputtering 500 μm thick Ti electrodes atop the grown films. Already with this simple approach a long-time photostability with constant spectral features could be achieved.
References:
[1] I. Aharonovich, S. Castelletto, D. A. Simpson, C.-H. Su, A. D. Greentree, S. Prawer, Rep. Prog. Phys. 2011, 74 (7), 076501.
[2] V. Lebedev, T. Yoshikawa, C. Giese, L. Kirste, A. Zukauskaite, A. Graff, F. Meyer, F. Burmeister, O. Ambacher, J. Appl. Phys. 2019, 125 (7), 075305.

Single-Crystal diamond is a promising material of interest for next-generation radiation sensors in extreme conditions, such as the LHC or FRIB. A major property of interest is the resistance of diamond to radiation damage.

Radiation damage in diamond is confined almost exclusively to the creation of point-like defects. Due to its radiation hardness, the lattice is not amorphized as in Silicon, with interstitial-vacancy pairs being the primary damage caused. At temperatures above 100C, carbon interstitials become mobile and can recombine with lattice vacancies, healing the lattice. At temperatures >400C, vacancies mobilize, further

Defect structures are studied by the use of Raman and Photoluminescence (PL) spectroscopy, expanding on previous work that used X-Ray diffraction (XRD). XRD imaging comparing irradiated to non-irradiated regions of diamond show no clear connection between diffraction profile and radiation damage. Radiation damage at doses of 10¹⁵ n_eq/cm² does not appear to cause amorphization, but instead a distribution of point defects. Point defects in diamond are optically active, accessible with PL, and Raman can measure structural irregularities that create additional vibrational modes.


This study presents spatially resolved imaging of point defects, correlating residual damage directly to the intensity of incident radiation. The effects of different annealing procedures on the types of defects (GR1, NV0) present in the diamond are investigated. Additionally, concentrations and the spatial correlations between different types of defects are studied.

Radiation damage causes decreases in detector efficacy and charge collection efficiency. Removing these point defects by annealing increases the overall lifespan of the detector, reducing operational costs. This research evaluates and determines the optimal annealing procedure for single-crystal diamond damaged by GeV heavy ions, and possible applications to other radiation damage.

Nanodiamonds are a promising nanomaterial being considered for an increasing number of applications as they possess many of the exceptional bulk diamond properties, such as hardness and non-toxicity, but at sub-micron length scale, making them more bio-compatible¹. The termination, functionalisation or modification of the nanodiamond surface is an important factor when considering its potential applications; for example, functionalising the nanodiamond to attach specific bio-molecules². This is due to its high surface to bulk ratio and surface reactivity. There is further interest in nanodiamonds as fluorescent biomarkers due to the well-explored defect in diamond known as the nitrogen-vacancy (NV^-) centre. Incorporation of the NV^- centre, which is sensitive to external magnetic fields and can be detected optically³, is very successful in bulk diamond and in larger nanodiamonds⁴, but is still challenging for smaller nanodiamonds.

There is an interest in amine-terminated nanodiamonds as it would allow a variety of functional molecules, such as DNA, antibodies, and polymer building blocks, to be more easily and straightforwardly bonded to the nanodiamond⁵. Understanding the surface chemistry and properties is important in order to optimise the attachment of different molecules to the nanodiamonds and deliverance into the body.

In this study, the surface of amine-terminated nanodiamonds has been measured and characterised using a variety of spectroscopic techniques. Surface-sensitive photoelectron spectroscopy techniques (XPS, UPS, and NEXAFS) under ultra-high vacuum and in near ambient pressures (NAP-XPS) have been used to anneal and measure the surface of the nanodiamonds. Fourier Transform Infrared Spectroscopy (FTIR), a non-destructive widely used technique used to identify organic materials, was used to provide more detail on the chemical structure of the nanodiamond. The FTIR spectra were recorded at the AU-IR beamline at ASTRID2, Denmark, where the synchrotron radiation based FTIR provides enhanced spatial resolution as a result of increased brightness.

This paper presents a detailed spectroscopic study of amine-terminated nanodiamonds with the aim of better understanding the surface for potential biophysics applications.

References
1. Mochalin, V. N., Shenderova, O., Ho, D. & Gogotsi, Y. The properties and applications of nanodiamonds. Nat. Nanotechnol. 7, 11–23 (2012).
2. Chipaux, M. et al. Nanodiamonds and Their Applications in Cells. Small 14, 1704263 (2018).
3. Stursa, J. et al. Mass production of fluorescent nanodiamonds with a narrow emission intensity distribution. Carbon N. Y. 96, 812–818 (2016).
4. Smith, B. R., Gruber, D. & Plakhotnik, T. The effects of surface oxidation on luminescence of nanodiamonds. Diam. Relat. Mater. 19, 314–318 (2010).
5. Krueger, A. & Lang, D. Functionality is Key: Recent Progress in the Surface Modification of Nanodiamond. Adv. Funct. Mater. 22, 890–906 (2012).

Fluorouracil (5FU) is a highly asymmetrical, polar molecule commonly used in a range of cancer drug treatments including bowel, breast and skin cancer. Previous studies show that 5FU suffers a catalytic degradation on silver substrates, but is inert on graphene [1,2].
Nanodiamonds are enabling the exploitation of diamond’s unique properties at ever decreasing length scales in applications such as quantum optics, biosensors and drug delivery [3,4]. With their high surface: bulk atomic ratio, the surface termination of nanodiamonds is a crucial design parameter in all of these applications; for example in controlling their attachment to specific bio-molecules or aggregation in specific biological sites based on their zeta potentials [5]. The Nitrogen-Vacancy (NV-) centre, which can be commonly found in nanodiamonds, allows for in-situ biosensing.

To investigate changes in the surface termination of nanodiamonds exposed to gases and liquids at high temperature, in-situ photoelectron spectroscopy has been applied under ultra-high vacuum and at near-ambient pressures using laboratory and synchrotron radiation excitation. This has been done to monitor changes in chemical composition and electronic properties of the nanodiamonds and to observe, in real time, the evolution of oxygen species to discover the optimal treatment controls for obtaining specific bonding types for attaching individual drug molecules.

Using these modified surfaces, 5FU has been attached to various diamond and nanodiamond surfaces. The adsorption was found to be strongly dependent on the surface termination, where the ratio of 5FU to the diamond surface can be controlled, and this effect can be seen for all sizes of nanodiamonds.

[1]. Silver catalyzed Fluorouracil degradation; a promising new role for graphene, F. Mazzola et al, 2D Mater. 2 025004 (2015).
[2]. Degradation of the chemotherapy drug 5-fluorouracil on medical-grade silver surfaces ' Risinggård , H K , et al. 2018 ,
[3]. The properties and applications of nanodiamonds. V. Mochalin, O. Shenderova, D. Ho, Y. Gogotsi, Nature Nanotechnology, volume 7, pp 11–23 (2012).
[4]. Nanodiamonds as multi-purpose labels for microscopy. S. R. Hemelaar, P. de Boer, M. Chipaux, et al. Scientific Reports, volume 7, 720, (2017).
[5]. Bactericidal Activity of Partially Oxidized Nanodiamonds. J. Wehling, R. Dringen, R. N. Zare, M. Maas, and K. Rezwan. ACS Nano, 8 (6), pp 6475–6483 (2014).

In this study, we report on the direct conversion of sp² rich carbon nanotube (CNT) and amorphous carbon structures into nanostructured diamond and Q-carbon by nanosecond pulsed laser annealing for electron field emission (FE) applications. Upon irradiating the CNTs with nanosecond pulsed laser at a suitable energy density, the tips and bends of the CNTs are melted in a highly undercooled state (due to the geometry constraint of the temperature distribution and 1D heat flow) to form the ideal platform for efficient FE devices. The highly undercooled state of carbon is a metastable phase and the nucleation of nanodiamonds occurs from this state. The whole process (rapid melting, solidification, and growth) takes place in less than 200-250 ns. The characteristic Raman peak at 1332 cm⁻¹ downshifts to 1324 cm⁻¹ for diamond nanostructures due to the phonon confinement effect. The high-resolution electron-energy-loss spectroscopy of the laser irradiated CNTs shows a characteristic peak at 292 eV for σ* for sp³ bonding of diamond. The diamond crystal structure and the local crystalline perfection and orientation at the laser irradiated regions are confirmed by electron-back-scatter-diffraction in high-resolution scanning electron microscopy. The laser irradiated CNT structures with nanodiamond seeds were used to grow large diamond crystals in hot-wire chemical vapor deposition. Furthermore, nanosecond pulsed laser annealing technique was used to convert the amorphous carbon film (grown by pulsed laser deposition) into large area quenched carbon (Q-carbon) film for field emission applications. The Q-carbon field emitter shows excellent field emission properties, such as- a low turn-on electric field of ~2.40 V/μm, the high emission current density of ~32 ��A/cm² and a critical field of ~2.45 V/μm for transition from a linear region to the saturation region in the F-N plot. Moreover, the Q-carbon field emitters illustrate enhanced field emission properties and good field emission stability throughout the temperature range from room temperature to 500K. Overall, this work shows that the pulsed laser annealing technique can be used efficiently for the controlled fabrication of nanodiamond and Q-carbon based high-performance field emitters on different substrates for versatile applications.

Storage of blood, even in blood bank conditions, generates the deoxygenation of red blood cells (RBC) and morphological changes that alter the erythrocyte membrane, changing its normal shape and functional reliability. These disturbing phenomena impair the capacity of RBC to transport oxygen and diminish the quality of blood. In this work, we investigate how the RBC changes evolve in time (from 0 to 18 days) with a combined scanning laser technique based on AFM-Raman to evaluate roughness, osmotic fragility, and oxygenation characteristics. To enhance the Raman signal and distinguish the oxygen markers, we added nanodiamonds (ND). The ND showed an additional feature; they helped to reoxygenate the blood and interrupted the generation of reactive oxygen species (ROS). The analysis of the deoxygenation process revealed that morphological alterations do not generate biochemical changes in the hemoglobin Raman fingerprint, even though RBC became more fragile and prone to breakage. This effect is noteworthy because after a transfusion RBC have to pass through tiny capillary vessels, where good elastic properties of RBC are needed. The results also indicate that blood has to be used within the first five days after collection to reduce storage lesions and assure quality.

Nanodiamonds (ND) have a significant impact on medicine and biology because of their biocompatibility properties and unaltered performance in physiologic environments. Furthermore, their low toxicity allows working very close to cells, even at the intracellular level. The metabolic processes related to the life of the cell generates or consumes thermal energy. The disturbance of those vital processes by pathologies and stressing factors induce changes in the cell temperature, while reparation or programmed death are triggered. The measurement of temperature improve the knowledge of biologic events, and it is a useful resource for monitoring treatment or therapy; it also provides information of particular interest in the study of the pathology of cancer. A nanothermometer based on ND is an excellent option because it possesses magnetic, electric, and optical properties that exhibit a dependence on temperature in the biologic range centered around 37.5^oC. In this work, we present experimental insights and data from the emission spectra of fluorescent nanodiamonds (FNDs), and we followed its behavior as the temperature changes through a phenomenological model. For a more realistic approach, we used FNDs with different N-V centers concentrations in solution. We fit the fluorescence curves to remove background and other spurious luminescent processes associated mostly with impurities or contamination. We also performed calculations based on changes of intensity, semi-width, and position shifts related to the Zero-Phonon Lines of N-V centers to obtain thermometric scales with low errors and precision around 0.01^oC. Also, we incubated FND HeLa cells and evaluated the time stability of the fluorescent signal at RT. The results allow establishing FND as thermal probes for reading temperature both in situ and in real time, based in their luminescence signal with the use of simple instrumentation.

Hip implants are often made from titanium or titanium-based alloys. However, wear during the operation inside the human body is a key source of implant failure and adverse health effects. We propose new insight on the lubrication of titanium components. Addition of small amounts (less than 0.2 wt.%) of nanodiamonds (NDs) to simulated body fluid promotes a substantial improvement in friction (3 times reduction) and wear (up to 2 orders of magnitude wear reduction) behavior of the titanium surfaces. Interestingly, the amount of NDs needed for improvement of friction and wear characteristics is critically dependent on the applied loads. With higher contact loads, larger concentrations of NDs are needed for better friction and wear reduction. Analysis of the wear track formed during sliding indicates the formation of a carbon-rich tribolayer which improves tribological properties of the contacting surfaces. Our results suggest that the carbon layer is formed from the nanodiamonds embedding in the top layer of titanium.

The diamond material possesses very attractive properties, such as superior electronic properties (including high carrier mobility), large electrochemical potential window, and a controllable surface termination. Boron-doped diamond surfaces, with attached Pt nanoparticles as the catalytic surface, are nowadays working as a new class of electrode materials. The boron-doped diamond electrode is a semiconducting material with very promising properties like i) a wider potential window in aqueous solution, ii) low background current, and iii) corrosion stability in aggressive environments. The phenomena of diamond surface termination have experimentally been observed to significantly influence the broad-band infrared reflectivity and conductivity. H-terminated diamond surfaces have been found to be hydrophobic, and to show unique p-type surface electronic conductivity. On the other hand, oxygen-terminated diamond surfaces generally show hydrophilic properties, but no electronic conductivity.

The surface reactivity of diamond is expected to affect both chemical processes at the surface and properties related to the surface electronic structure. Examples of factors with the capability to influence the surface reactivity are i) type of plane, ii) surface termination, and iii) doping. Theoretical modeling based predominantly on Density Functional Theory (DFT) has during the last decade proven to become highly valuable in the explanation and prediction of experimental results. The simulation and theoretical analysis of especially surface reactivities has been shown to aid important information.

At this presentation, the combined effect of surface termination and doping on the electronic properties of diamond surfaces, will be especially highlighted. In addition, the effect by a water adlayer will also be considered. All results are based on high-level calculations using DFT calculations.

Diamond surface XPS contributions is still having a big controversy in the literature and most of the measurements are based on surface normal photoelectron emission. In this sense, angle-resolved XPS experiments allow to obtain different sensitivity depth by varying the electron take-off angle, what is useful to clarify surface contributions, although it is rarely found in diamond.

Recently, a downward band bending in the very near surface has been reported by using this technique. However, the common interpretation of a band bending in X-ray photoelectron spectroscopy, based on peak energy shifts referenced to a known fixed energy level, can be wrong in some cases. Besides XPS peak shifts could exist due to band bending, the correct estimation in the case of a large band bending with a short width near the surface must be done considering the band bending region as a new XPS peak. The shape of this peak depends on the band bending characteristics and XPS experimental parameters.

In this work, an approximation to the shape of a surface band bending is described theoretically and applied to the deconvolution of ARXPS spectra of H-terminated and O-terminated diamond surfaces. A downward and an upward surface band bending are found, respectively.

REFERENCES:
[1] S. Kono et al., «Band diagram for chemical vapor deposition diamond surface conductive layer: Presence of downward band bending due to shallow acceptors», Surf. Sci., vol. 604, n.^o 13-14, pp. 1148-1165, 2010.
[2] S. Kono et al., «Carbon 1s X-ray photoelectron spectra of realistic samples of hydrogen-terminated and oxygen-terminated CVD diamond (111) and (001)», Diam. Relat. Mater., vol. 93, n.^o February, pp. 105-130, 2019.

Although catalytic metals play an essential role in the production of industrial high temperature and high pressure (HTHP) diamonds, their role has not been clearly understood. This study provides a microscopic interpretation on the role of the Ni catalytic metal through density functional theory (DFT) calculations. In addition to the metal solvent-catalytic effect, it has been found that Ni removes the strong π-bond (Pandey chain) on the diamond (111) surface, which is an obstacle to diamond growth. When Ni atoms were attached to the diamond (111) surface, the diamond bulk-like surface was more stable than the Pandey chain surface structure, indicating that de-reconstruction could occur by Ni. The results of projected density of state (PDOS), charge distribution, and Bader charge analysis show directly that the surface C-C π-bond is weakened by the charge redistribution caused by covalent bonding of Ni and C. Also, the nudged elastic band (NEB) calculation shows that the Ni surfactant-mediated growth is kinetically possible. Therefore, applying these results to other elements will provide a better understanding of the role of catalytic metals in diamond growth.

For many applications the surface chemistry of diamond is a key factor for the control of properties such as the wetting behavior, the electron affinity of the surface[1] or the control of charge states of lattice defects[2]. By attaching either electron withdrawing or donating surface groups the charge state e.g. of shallow NV centers can controlled.
Additionally, surface termination also plays a role in the density of states and the band structure of the diamond material. Therefore, the homogeneous and stable surface functionalization with electrically active atoms or groups is a key step in the fabrication of diamond based devices for e.g. photocatalytic or quantum applications.
Here we report on the fluorine termination of diamond surfaces using different wet-chemical fluorination methods. These treatments lead to highly functionalized materials with different surface species. Characterization using e.g. solid-state NMR techniques, XPS and IR methods have been used to characterize the fluorinated materials qualitatively and quantitatively.
This research has received funding from the European Union’s Horizon 2020 Program (Grant Agreement no. 665085, DIACAT, www.diacat.eu) and Deutsche Forschungsgemeinschaft under grant KR3316/6-2.

References
Zhu, D.; Zhang, L.; Ruther, R.; Hamers, R., Nature Materials 2013, 12, 836-841.
Cui, S.; Hu, E. L., Appl. Phys. Lett. 2013, 103, 051603.

The effects of the diamond surface can influence its use in electronic, optical and quantum applications and it is particularly important for nanodiamonds, due to their large surface to bulk ratio. The usual preparation methods for single crystal diamond involve mechanical polishing and etching with the aim of producing flat, defect-free O-terminated or H-terminated surfaces. Nanodiamonds are prepared using similar etching procedures. The surface structure and the terminating species influence properties such as wettability, electron emission and adsorption and the control of surface properties requires precise control of the preparation methods. Here we compare the oxidation and reduction of single crystal diamond and nanodiamonds using acid etching, in-vacuo plasma exposure and annealing in near ambient pressure oxygen and hydrogen. The surface sensitive methods of ultraviolet and x-ray photoelectron spectroscopy (PES) using laboratory and synchrotron sources were used to monitor changes in the physical, chemical and electronic structure during surface oxidation and reduction. Near-Edge X-ray Absorption Spectroscopy (NEXAFS) was applied to determine the unoccupied density of states and the orientation of surface bonding and this synchrotron method is particularly sensitive to non-diamond carbon species at the surface. In-vacuo annealing removes and modifies the surface oxygen as revealed by real-time monitoring of the O1s and C1s core level spectra from room temperature to 1000 C. The PES results are compared with DFT calculations that suggest that ether (C-O-C) groups are the preferred bonding arrangement with alcohol (C-OH) groups preferentially desorbed during the annealing process. Changes in composition were further correlated with changes in electrical conductivity and work function. DFT was also applied to model the desorption of oxygen species at specific temperatures and showed a deformation in the surface/sub-surface structure in situations where only some types of oxygen groups are present. Oxidation and reduction of diamond and nanodiamond surfaces at higher gas pressures were studied using near ambient pressure PES to reveal surfaces of comparable and, in some cases, superior quality to those produced by in-vacuo annealing and plasma exposure. At 1 mbar of O₂gas, non-diamond carbon species were removed and an additional oxygen species was observed that was not present on the vacuum-prepared samples, but was predicted by the DFT calculations.

Diamond color centers are attractive candidates for integrated quantum photonics circuitry. Over last decade, much focus has been put to study the NV center in diamond. However, its broad emission and low proportion of zero phonon line photons, makes it challenging to scale up and efficient collection of coherent photons.
In this talk I will describe our recent efforts into studying a new class of defects, associated with group IV elements such as Si and Ge. These defects exhibit narrow band emission and form a symmetric complex with two vacancies, therefore less susceptible to strain. In my talk I will focus on two aspects:
First, I will describe the progress of fabricating nanoscale diamond membranes, that we utilize for electrical excitation and for device fabrication. An example of a rink resonator and a microdisk cavity is shown in figure 1. Second, I will describe emerging experiments involving anti stokes excitation of single quantum emitters, that provide a new avenue for optical sensing with unprecedented sensitivity. I will summarize with an outlook into new potentially interesting color centers.

Point defects in diamond are being intensively investigated, both for their applications in processing and communication of quantum information, as well as for metrology. So far, the negatively charged nitrogen-vacancy center (NV⁻) has been the most studied defect [1]. Thanks to its efficient optical spin polarization and spin-state dependent fluorescence, it is being exploited, for example, in the context of high-sensitivity magnetometers [2]. More recently, owing to their superior optical properties, the group-IV-vacancy centers (SiV⁻ [3, 4], GeV⁻ [5] and SnV⁻ [6]) have emerged as the leading type of point defects for quantum computing and networking applications. Whereas it is generally accepted that the N atom in the NV⁻ center occupies a substitutional C site, the group-IV atoms in group-IV-vacancy centers are expected to occupy bond-centered sites, in a split-vacancy configuration. In this configuration, rather than being accompanied by a neighboring vacancy, the group-IV atom is located at the center of two neighboring lattice positions, while the vacancy is “split” over the two lattice sites. However, experimentally, these structural configurations had so far been only indirectly determined.

In this contribution, we present a detailed, direct characterization of the structure of the SnV⁻ defect. To this end, we relied on the electron emission channeling technique to determine the lattice site of implanted radioactive ¹²¹Sn ions in diamond with very high precision (experiments performed at the ISOLDE facility at CERN, Geneva). The anisotropic patterns of the charged β^- particles emitted during the decay of the radioactive isotopes are characteristic of the lattice site of the emitting Sn atoms. From fitting the experimental data to theoretically calculated patterns [7] using a two-dimensional fit procedure [8], the Sn lattice location can be accurately determined. Using a combination of electron emission channeling from ¹²¹Sn and photoluminescence measurements, we have unambiguously identified the SnV⁻ defect in the split-vacancy configuration, with the Sn atom located exactly at the bond-center position (with a precision better than 0.04 Å). While in the as-implanted state the PL spectrum from the sample was dominated by radiation damage-induced features, following annealing at 915°C for 1 hour the signature of the SnV^- center (in the form of a strong 621 nm line) was clearly visible. Establishing a detailed understanding of the SnV⁻ structure is particularly important, since the superior properties of the group-IV-vacancy centers are to a large extent a consequence of the inversion symmetry of the split-vacancy configuration.

[1] Doherty M.W. et al., Physics Reports, 528 (2013) 1–45
[2] Rondin L. et al., Reports on Progress in Physics, 77 (2014) 056503
[3] Sipahigil A. et al., Science, 354(2016) 847–850
[4] Sukachev D.D. et al., Physical Review Letters, 119 (2017) 223602
[5] Bhaskar M.K. et al., Physical Review Letters, 118 (2017) 223603
[6] Iwasaki T. et al., Physical Review Letters, 119 (2017) 253601
[7] H. Hofsäss and G. Lindner, Phys. Rep., 201 (1991) 201
[8] U. Wahl et al., Nucl. Instr. Meth., B 136-138 (1998) 744

Most of the mined diamonds are facing ethical, eco-responsibility and traceability issues. In addition to that, a strong decline in the extraction is expected (minus 50 % is predicted within 10 years) whereas the demand for jewelry increases in particular in the US and Asia. The only solution, the only ethical alternative to these problems relies in lab-grown diamonds.
Microwave assisted plasma vapour deposition reactors have been widely used to elaborate carbon-based materials in particular diamond. The performance of the process is strongly influenced by the interaction between microwave and the plasma which in fact depends on the precursor gas composition, pressure and operating conditions. Growing diamond for jewellery implies growing rough diamond of 4 to 6 mm in thickness and managing the colour of the diamond after they are cut. Many attempts with different gas composition have been achieved around the world leading to different colours of diamonds as well as different growth rates. We propose here an overview of the different processes used to grow these thick diamonds.
Also we propose to show how the local conditions at the plasma / surface interface may strongly change as a function of the conditions in the case of the most simple gas composition, i. e. a mixture of hydrogen and methane. A self-consistent model has been constructed incorporating the stiff methane-hydrogen chemistry. We will discuss some of the results obtained using this self-consistent model and their comparison with the experiments.

GaN-based high electron mobility transistors (GaN HEMTs) have been expected for high output power density applications such as radars and long distance wireless communication. Although the GaN has superior electric characteristics, a low thermal conductivity of GaN limits the output power density. In order to improve thermal dissipation, the GaN HEMTs on high-thermal-conductivity diamond has been reported. In addition to the backside of GaN HEMTs, diamond on GaN is expected to improve thermal dissipation. However, there were few reports on the deposition of diamond layer on GaN HEMT and the heat-spreading effect by diamond deposition has not been investigated in detail. In this study, we carried out growth of diamond on GaN HEMTs and investigated the heat-spreading effect by Raman thermography.
Polycrystalline diamond films were directly grown by hot-filament chemical vapor deposition (HF-CVD) on AlGaN/GaN MIS HEMTs. The HEMT devices with SiN_x insulator showed low gate leakage current of <10^-2 A/mm even though the diamond growth was carried out under high temperature. In addition, the HEMTs with overgrown polycrystalline diamond films showed higher transconductance of 154 mS/mm than that of non-diamond grown HEMTs of 134 mS/mm. In order to evaluate effect of surface diamond heat-spreader, we performed Raman thermography and clearly observed the temperature reduction of around 10 degrees even under relatively low power operation of ~5 W/mm. This temperature reduction inhibits the degradation of electron mobility caused by increased optical phonon scattering, resulting in increase in transconductance as we experimentally observed. Therefore, direct growth of diamond by HF-CVD on AlGaN/GaN MIS HEMTs enables to temperature reduction and improve its performance.
[Acknowledgements]: This work was partially supported by Innovative Science and Technology Initiative for Security, ATLA, Japan.

With improvements to epitaxial growth and device fabrication, GaN device performance is now limited by near-junction heat removal¹. A possible solution to this problem is to deposit a layer of diamond onto the GaN to act as a heat spreader². Diamond has excellent thermal conductivity, making it well-suited for use as a heat spreader. However, its use in GaN devices is hindered by the significant differences in thermal expansion and lattice constants between diamond and GaN^3,4,5.
Work has been centred on developing a low temperature diamond CVD process to overcome the significant lattice mismatch and thermal expansion differences between the diamond and GaN layers, which has previously resulted in delamination. A novel seeding process involving 2-step seeding with micro- and nano-diamond particles has also been developed to increase thermal conductivity across the diamond-GaN interface. The deposited layers have been non-destructively examined utilising X-Ray Tomography (XRT), allowing an assessment of the quality of the interface in the middle of the wafer without inducing any stresses and potential delamination onto the substrate. With the optimisation of growth procedures, diamond layers 50 µm thick can now be reliably grown directly onto GaN substrates with no barrier layer, however the interface remains too mechanically weak for most device applications.
To improve adhesion, diamond films were also grown on GaN substrates with barrier layers of either 200 nm of AlN or 20 nm of Al metal. The Al layer is thin enough that we believe it completely converts to aluminium carbide during the CVD process. We shall present results from these growth studies and evaluate the thermal and mechanical performance of these barrier layers, as well as report on progress towards developing a large-area CVD reactor suitable for 6”-diameter GaN wafers.

References
1. T. J. Flack, B. N. Pushpakaran and S. B. Bayne, J. Electron. Mater., 45 (2016) 2673.
2. A. Hassan, Y. Savaria and M. Sawan, IEEE Access, 6 (2018) 78790.
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4. P. W. May, H. Y. Tsai, W. N. Wang and J. A. Smith, Diam. Relat. Mater., 12 (2006), 526.
5. S. Mandal, et al. ACS Omega 2 (2017) 7275.

Gallium Nitride (GaN) as a wide bandgap material, is an attractive choice for high-power and high-frequency semiconductor devices. However, self-heating effect limits GaN-based high electron mobility transistor (HEMT) capabilities due the lack of effective heat spreading through the device [1]. Polycrystalline (PC) diamond, owing to its high thermal conductivity (10-2000 W/m.K), can be used as a capping layer for spreading the heat [2]. For effective cooling, diamond must be grown as close as possible to the source of heat, which is the channel in a HEMT. In this abstract we describe our approach of growing PC diamond on GaN layer to allow maximum heat transfer from the GaN channel to the diamond layer. Achieving a high-quality diamond-GaN interface is necessary to accomplish effective heat transfer. As hydrogen plasma is one of the main species in diamond growth using microwave plasma chemical vapor deposition system, it damages the GaN and changes the properties of the 2-dimensional electron gas (2DEG). A 20 nm Si₃N₄ has been used as a protection layer on top of the N-polar GaN on sapphire wafer, which also reduces delamination of the diamond by making a SiC interlayer. As the coefficient of thermal expansion (CTE) for diamond (~1.1 µm/m.K) and GaN/sapphire (~4.1/5.3 µm/m.K) are different, the possibility of diamond delamination due to large residual stress is high. In another aspect, to reach the thermal properties of bulk diamond, it is vital to increase the quality of the grown layer. In this work, we have used Raman spectroscopy and scanning electron microscopy (SEM) to investigate the effect of growth plasma temperature (600-850 C, with 50 C steps) on both the stress and quality factor of PC diamond.

The grain size of the diamond layer increases from 113 nm at 600 C to 419 nm at 850 C and the thickness changes from 130 to 696 nm when the temperature changes from 600 to 850 C. We have observed a higher growth rate in vertical axis than lateral axis, which is due to the high nanoparticle seeding density (>10¹² cm^-2) and growth using hydrogen plasma. The quality factor can be determined by sp³/sp² carbon bonding ratio of their Raman peaks. Diamond quality factor shows a parabolic behavior and increases with temperature (90.10% at 600 C to 98.57% at 850 C), as the thickness and grain size of the film increase.

The residual stress (thermal and intrinsic) of the diamond film plays a critical role in the delamination of diamond from the substrate. Thermal stress is caused by the difference in the CTE between diamond and substrate during the cooling cycle and intrinsic stress is due to the incorporation of impurities and defect formation. The residual stress in the diamond film can be calculated using the shift in the sp³ diamond Raman peak from 1332 cm^-1 [3]. The diamond layer stress shows a compressive nature and increases with temperature by a parabolic behavior (-4.2 GPa at 600 °C to -6.3 GPa at 850 C). Increase in the thickness and grain size is the main reason for larger compressive stress. In this study, up to 850 C we observed no delamination of the diamond film from the substrate. However, the absolute value of the stress is high, which can be explained by the large compressive stress in sapphire during the cooling process.

Increasing thickness and grain size of diamond results in a higher quality factor (close to bulk diamond), while increases the compressive stress and the possibility of delamination. So, in order to prevent delamination and have a high-quality diamond on GaN/sapphire, it is needed to decrease the compressive stress by relaxing PC diamond layer. The relaxation of diamond layer can be done by adding a thin ultra-nano-crystalline-diamond (UNCD) layer during the growth.

[1] X. D. Wang, et al., IEEE Trans. Elec. Dev. 59, 5, (2012).
[2] Y. Zhou, et al., Appl. Phys. Lett. 111, 041901 (2017).
[3] R. Tu, et al., RSC Adv. 8, 16061 (2018).

This research is supported through the SRC JUMP program.

With the development of long-distance transmission, offshore transmission and renewable energy technology, AC and DC systems have become the popular trend for future power systems. The AC/DC converter is the key part to make it happen. In modern power systems, converter modules like half-bridge structures and full-bridge structures of paralleled insulated-gate bipolar transistor (IGBT) and Schottky barrier diodes are common in the industry. However, the current of a single Schottky barrier diode at the same voltage level lacks greatly behind the IGBT performance. Thus, multiple Schottky barrier diodes may need to be paralleled with IGBTs in order to match the current performance of the IGBTs operating at the same voltage level. Furthermore, due to inherent small variances in the manufacturing process of the Schottky barrier diodes, devices will slightly differ from each other and current asymmetries and an imbalanced voltage distribution potentially occurs in the power modules, leading to higher harmonic waves and severe power losses. Therefore, there are urgent needs of improved Schottky barrier diodes for efficient AC/DC converters in the market.

Diamond is considered as an ultimate semiconductor for high power applications due to its high charge carrier mobility and saturation velocity and excellent thermal conductivity. The extremely high breakdown field up to 10⁷ V/cm is very promising for medium and high voltage rectifiers and converters. To address the highly visible diode performance shortcomings, we designed and investigated a high-power vertical diamond Schottky barrier diode. The newly designed high-power diodes possess a current drive higher than 50 A at a forward bias of 10 kV in the temperature range between room temperature (300 K), optimal diode operation temperature (450 K) and the temperature at full carrier activation of the boron acceptors (800 K). However, the diode performance will be affected by several parasitic elements originated by the choice of the used materials and the device design, which can lead to an electrical field concentrating edge effect. In order to decrease the electric field within the high-power diode and to improve its performance, this work investigates edge effect termination strategies by introducing passivation layer structures, n⁺-guard rings, and floating n⁺-islands. Several high-power diode designs were in-silico tested at different temperatures (300 K, 450 K and 800 K). According to the TCAD simulations, the on-resistance of the power-diode at 450 K is the lowest and a high current density of ~2500 A/cm² can be achieved at 10 kV for the basic diamond Schottky diode design. This performance is also investigated versus variations in the expected electric field breakdown strength of diamond. The implementation of passivation layer structures, guard rings, and floating islands is studied for performance improvements. The here developed design shall be technologically realized and implemented in a highly efficient AC/DC power converter, later on.

Despite recent advances in the synthesis of chemical vapor deposition (CVD) single crystal diamond (SCD), the device performance of applications based on diamond still remains far below the theoretical capacities.¹ This is mostly associated with the presence of dislocations which leads to an increase of the leakage current and a reduction of the boron concentration in the heavily B-doped layer.² Therefore, the optimization of the growth conditions of B-doped layers has a crucial role in the improvement of diamond based electronic applications such as Schottky barrier diodes. We have grown a series of B-doped SCD films which were characterized to identify the optimal conditions. The B-doped SCD films were grown by microwave plasma enhanced CVD in a home-built reactor on 3 Χ 3 mm² Ib (100)-oriented high pressure high temperature (HPHT) substrates. Two sets of samples, B-doped and un-doped, were grown with different amount of methane concentration (0.5%-3%) in the plasma. For heavily B-doped SCD depositions, trimethylboron (TMB) was used as the boron source with a constant 997 ppm [B]/[C] ratio. For un-doped samples, the residual TMB in the chamber was used.The grown layers were characterized by Hall effect in a wide temperature range, Fourier transformed infrared (FTIR) spectroscopy, Raman spectroscopy, X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), optical microscopy, and transmission electron microscopy (TEM). By means of TEM, the high quality of the p⁺⁺ and p^- layers was shown, except for the sample grown with 0.5% methane concentration, on which defect features similar to etch-pits were observed and confirmed by AFM. This sample also showed a very low boron incorporation into the p⁺⁺ layer due to the plasma gas chemistry which seems to create etch-pits, as seen in the morphology. FTIR,³ Raman spectroscopy,⁴ and Hall effect measurements indicate a higher B-doping level when the methane concentration is increased in both heavily and lightly B-doped SCD films. The electrical conductivity and mobility of the p⁺⁺ samples grown at 1%, 2%, and 3% methane concentration had almost no temperature dependence due to the metallic-like conduction properties in these heavily B-doped films.A careful investigation of the XPS spectra of lightly B-doped SCD layers at various methane concentrations indicated that the intensity of B 1s peak became stronger by increasing the methane concentration. The C K-edge XAS spectra of these layers were recorded in surface sensitive total electron yield (TEY) mode under different incident angle, which demonstrated that the diamond lattice structure can be distorted by increasing the methane concentration resulting in a higher B-doping level.The increase of the B-doping level with increasing methane was corroborated by first principle density functional theory (DFT) calculations. These showed that the presence of CH₂ sites has no significant impact on the binding energy of an adsorbed B atom. However, the presence of CH₂ sites increase the chance of H-defect site formation, providing additional B binding sites. The effect of using various methane concentrations in heavily B-doped SCD layers were further studied in their influence on the performance of Schottky barrier diodes.
References:
1- H. Umezawa, et al., Diam. Relat. Mater. 24 (2012), 201-205.
2- J. Achard, et al., Diam. Relat. Mater. 20(2011), 145-152.
3- J. Achard, et al., Phys. Status Solidi A 209 9(2012), 1651-1658.
4- M. Bernard, et al., Diam. Relat. Mater. 13(2004), 282-286.