Symposium EQ19—Diamond and Diamond Heterojunctions—From Growth to Applications

Despite the superior Baliga’s figure of merit of diamond with respect to other wide band gap materials [1], it is still not extensively used for electronic devices, in general, or power electronic devices, in particular. Aspects such as size of diamond substrates, extreme mechanical hardness, capability of getting high quality n-type doped layers or reproducibility of high quality growings are some of the reasons that make difficult to implement diamond in power electronics market and constitute a set of challenges to overcome in the manufacture of devices. But not only this, manufacture also involves deposition of other materials on diamond grown layers, as oxides gates for MOS transistors, metals for ohmic or Schottky contacts, which not always are well controlled. In addition, diamond surface termination is also determinant in how all these materials are finally bounded to diamond. Indeed, electrical characteristics depend directly on the latter. Therefore, electrical performances directly result from the structural way in which other materials, as metals or oxides, are bound to diamond.
Transmission electron microscopy (TEM) is one of the most powerful tools to directly analyze interfaces. Information about roughness of the interface, atomic arrangement, chemical environment, crystalline nature of the oxide onto diamond, orientation relations between diamond and crystals forming the deposited layer, energy gap of the oxide, chemical composition variations or formation of interface specimens can be obtained through TEM related techniques as HREM, HR-STEM, EDX and (V)EELS. XPS or DFT-periodic calculations are complementary to understand the TEM related results and can give relationships to electrical characteristics.
This contribution will present an overview about diamond interfaces characterization carried out at the University of Cádiz on different electronic devices studied by TEM related techniques. In particular, several interfaces as metal/diamond, oxides/diamond and carbides/diamond by means of their atomic interface configuration, i.e., atomic bondings and grain configuration, are investigated and related to their electrical characteristics. In addition, TEM spectroscopies complemented by XPS measurements are shown to be of first importance to understand band setting at such interfaces. Role of interface states and their relation with crystallinity of oxide in MOS stacks will be discussed for Al₂O₃ and ZrO₂ MOSCAPs and MOSFETs. Al₂O₃ is the most common oxide for these purposes and it is well characterized [2], although ZrO₂ is also a promising candidate due to its high dielectric constant [3], large band gap and high breakdown field. Performance of diamond based-Schottky diodes is also greatly dependent on diamond surface terminations [4] and, in the case of tungsten carbide contacts, some results on the WC/O-diamond are presented identifying crystallinity and stoichiometry of WC phases. Oxygen termination has also an important role in Zr/diamond Schottky interfaces.

[1] B.J. Baliga. IEEE Electron Devices Lett. 10 (1989) 455-457
[2] J. Cañas et al. Appl. Surf. Sci. 461 (2018) 93-97
[3] D. Vanderbilt et al. Thin Solid Films 486 (2005) 125–128
[4] N. Donato et al. J. Phys. D: Appl. Phys. 53 (2020) 093001

One of the bottlenecks in the fabrication of diamond-based electronic devices is related to the diamond interface state passivation. The surface termination quality and the fact of not having a native oxide hinders high performance and reliability of the device. The improvement of the oxide/diamond interface quality leads to a minimization of the interface states density in the diamond gap which strongly modify the electrical response. The starting point of this study is the interesting characteristics of the ZrO₂ such as the high dielectric constant (of the order of 19.7), the high band gap (6eV characterized by Electron energy loss spectroscopy), its band setting providing a barrier for holes (of the order of 2eV) or the high breakdown field (of the order of 2MVcm^-1). In the present contribution, different zirconia layers have been deposited by two methods: Atomic layer deposition and radio frequency Sputtering. The crystallinity of the layers has been studied by High resolution electron microscopy and its related Fast Fourier transform and by X-ray diffraction. The band alignment will be established by X-ray photoelectron spectroscopy. Also, by Energy dispersive X-ray spectroscopy is possible to analyse the stoichiometry throughout the dielectric comparing the oxygen content, and thus the possibility of finding electron-trapping mechanisms, from the two methods of deposition. Finally, the analysis of the electrical characteristics (I-V, C-V, C-f) for zirconia deposited under several conditions allows an understanding of how microstructure influences electrical performance.

The spatially resolved and non-destructive measurement of impurities and defects in diamond is key for the deployment of diamond in electronics. Techniques used so far such as the X-ray Topography using synchrotron radiation for monitoring defects in substrates exhibits a low scalability. In the large-scale production picture, the lack of high throughput characterization technique could rapidly become a bottleneck. We propose that Cathodoluminescence Spectroscopy (CL) is a useful alternative, because of its sensitivity to defects. In addition to this, the excitonic spectrum of diamond has been useful to measure dopant concentrations in diamond for many years, but relies on prior knowledge of the setup performances, i.e. a diamond standard of known doping level for the calibration of the setup.
Going beyond this state-of-the-art, we study dopants in diamond using Time-Resolved CL (TR-CL) with a picosecond time resolution, and CL spectral imaging. TR-CL evidences that the excitation dynamics of a CL experiment are significantly different from their PL counterparts, because many excitons are generated in a very small interaction volume even if few electrons impact the sample. This novelty provides direct, quantitative access to the electrically active Boron concentration in a sample, regardless of compensation or non-incorporated impurities, with sub-micron spatial resolution and attractive measurement throughput. We demonstrate how this can be applied to map doping variations in a Boron-doped High-Pressure-High-Temperature diamond substrate across growth sectors, which shows doping concentration varying over 1 order of magnitude, in the 1.5e16 – 4.0e17 [cm-3] range. The measurements are performed at a spatial scale sufficient to resolve crystalline defects, and study their impact on the spectral signal and Boron incorporation.

Electrochemiluminescence (ECL) is a technique where chemical species generated at the surface of an electrode interact; producing an excited state, which then emits photons. The intensity of the emitted light is proportional to the concentration of the target molecule and so is quantifiable. Exploiting features of both electrochemical analysis and chemiluminescence, the use of ECL has become very popular in many areas due to the wide range of advantages it displays over more conventional fluorescence techniques: the ability to control the exact time and position of the photon emission, the absence of background optical signal (high signal/noise ratio) and good selectivity.
ECL generally occurs where the oxidized and reduced forms of the target analyte are produced by sweeping from cathodic to anodic potential, or vice versa. The recombination of these two species is what produces the excited state. However, in aqueous solutions this becomes difficult as the relatively high and low potentials needed can cause the electrolysis of water, disrupting the ECL process. This is usually overcome with the use of a co-reacting species, where the luminescent species are oxidized at the electrode together with the co-reactant, giving a strong reducing agent. However, these additional compounds can often be expensive and significantly reduce the selectivity of the technique.
Boron Doped Diamond electrodes have several advantages over other electrode materials. The most important for ECL assays is that it possesses a wide potential window in water, meaning we have the ability to mitigate the splitting of water without the need for the addition of co-reacting species. Another is BDD ability to withstand very high and low potentials (over 1.5V and under -1.5V), this has already been implemented for the in-situ electrochemical cleaning of the electrode for both electrochemical and ECL analysis. It also means that we have the ability to apply these very high potentials for the rapid production radicals (hydroxyl and super oxide) at the surface of the electrode. Many ECL assays depend on the interaction of the target analytes with these radicals, like in the case of indolic compounds, to produce the exited state that give the ECL signal – but are often performed in organic solvents or with co-reactants. The robustness of diamond gives the opportunity to create ECL reactions and analyses of these compounds in aqueous solution. Here, we investigate the potential of using the unique properties of diamond electrodes to produce novel ECL assays.

Ultrawide bandgap semiconductor technologies offer potentially revolutionary advances in the rapidly developing areas of quantum communication, short wavelength optics, smart energy conversion and biomedical interfaces. These strongly demanding technologies can be partly constructed using conventional devices but new hybrid architectures are needed to overpass current performances and add functionalities. Here, we propose a new concept based on the specific properties of a diamond pn junction combined with both an electric and optical control of the depletion region. Using this junction as a gate in a junction field effect transistor, we report a proof of concept of a non volatile diamond photo-switch. A diamond pn junction made with nitrogen deep donors in the n−side is demonstrated to be optically activated thanks to visible light. The n-type diamond gate is almost devoid of free carriers in the dark and thus insulating. Illuminating the device renders the standard electrical gate control of the transistor efficient. Without illumination, the device is frozen, keeping a permanent memory of the current state. This new way of operating the device opens numerous possibilities to store and transfer information or energy with applications in the field of electrical aircraft or aerospace electronics, power electronics, bio-electronics and quantum communication.

Diamond has excellent physical properties as a semiconductor material, such as a high breakdown electric field (~10 MV/cm), the highest thermal conductivity among solid materials (~22 Wcm^-1K^-1), and a low dielectric constant (~5.7), and is expected to be applied to high-power, high-frequency p-FETs. To realize high-power and high-speed switching complementary inverters, there is an urgent need to develop vertical p-channel power FETs comparable with n-type FETs and suitable for integration. We have reported vertical-type diamond MOSFET using two-dimensional hole gas (2DHG) which is induced at any C-H diamond surface independent from crystal orientation covered by high temperature ALD-Al₂O₃[1][2][3]. However, the drift layer by lightly B doped layer (p^- layer) has not been implemented because the controllability of lightly doping of boron into diamond is very difficult. In this work, we fabricated (001) vertical-type 2DHG diamond MOSFET with a p^--drift layer and achieved a breakdown voltage of V_B: -580 V, which is the highest among the vertical diamond MOSFETs currently reported.
Fabrication process was as follows. First, a p^--drift layer ([N]: 1.0 × 10¹⁶ cm^-3, [B]: 8.0 × 10¹⁷ cm^-3) of 4 μm was deposited on a p⁺-type diamond substrate using the microwave plasma chemical vapor deposition (MPCVD) method and nitrogen-doped layer of 2 μm were deposited as a vertical current blocking layer on a p^--drift layer by the MPCVD method. Second, trench with a depth (D_T) of 3 μm, a width (W_T) of 6 μm are formed by inductively coupled plasma reactive ion etching using O₂ plasma. After trench formation, a 300 nm regrown undoped layer was deposited by the MPCVD method to induce the 2DHG and secure the drift layer on the trench sidewall. Third, source electrodes (Ti/Pt/Au) were formed and the 200-nm-thick Al₂O₃ was deposited as the gate insulator through the high-temperature atomic layer deposition (ALD) method. Finally, drain electrodes (Ti/Au) was formed on the back surface of the substrate and gate electrode (Al) was formed to complete the vertical-type device. The source-gate length (L_SG), gate-trench (L_GT), gate-drain (L_GD), source-drain (L_SD) and gate length (L_G) for this device are 3 μm, 4 μm, 10 μm, 17 μm, 4 μm, respectively. The area size of an active region for this device (L_SS×W_G= 7.5 × 10^-6 cm²) is defined by multiplying the distance between the trench-side edges of two sources (L_SS: 30 μm) and the gate width (W_G: 25 μm).
From the I_D-V_DS characteristics, the maximum drain current density (I_D,max) normalized by the gate width is 210 mA/mm at V_DS: -50 V and V_GS: -20 V. In the active region, the I_D,max is 1400 A/cm² and the on-resistance (R_on) is 23 mΩcm². The drain current density obtained in this study is comparable to that of a vertical device of the same device size without a p^--drift layer [1]. No gate leakage current is observed at room temperature (300 K) and on/off ratio is about 6 orders of magnitude. A high breakdown voltage of -580 V is achieved, this value is more than 60 % (200 V) higher than the breakdown voltage (V_B: -359 V) [1] of a device of the same size without a p^--drift layer, and is the highest value reported for vertical diamond FETs. From the above, it can be confirmed that the p-drift layer does not significantly reduce the current value and increase the breakdown voltage. In the near future, a p-type power trench diamond MOSFET with high voltage and low ON-resistance will be realized by increasing the thickness of the p^--drift layer (currently 4 μm) to about 10 μm, implementing a trench gate structure [3] and a high-density boron-doped layer [4] under the source electrode in this device structure.
[1] N. Oi, H Kawarada et al., Sci. Rep., vol. 8, Art. No. 10660, (2018).
[2] J. Tsunoda, H. Kawarada et al., Carbon, 176, 349-357, (2021).
[3] J. Tsunoda, H. Kawarada et al., IEEE TED, accepted, (2021).
[4] S. Imanishi, H. Kawarada, et al., IEEE EDL, 42, 2, 204-207, (2021).

Diamond transistors are promising for high temperature, power and frequency applications in harsh environments, including those with high radiation levels. This is due to the unique properties of diamond as a wide band-gap (5.5eV) semiconductor. The majority of Diamond field-effect transistors (FETs) fabricated to-date use a 2D hole gas that emerges when an H-terminated diamond surface comes into contact with a range of adsorbates and/or selected metal oxides. However, this unconventional manner of creating a p-type region in a semiconductor suffers from instability and reproducibility problems and low operational mobility issues.
Several lateral O-terminated diamond FET concepts and architectures have been recently fabricated while for vertical FETs, traditionally favored for high power devices, fabrication is still challenging despite the recent efforts of the diamond device community. The reported lateral devices display ‘normally-on’ characteristics, which are not ideal for high power applications. Further, reported breakdown voltages are typically poor in comparison to diamond’s full potential mainly due to limitations in substrate and epilayer quality alongside existing processing capabilities. In this work, these problems are addressed. A novel and a normally-off lateral Schottky-drain reverse blocking (RB) MESFET based on molybdenum Schottky contacts on O-terminated boron doped diamond has been fabricated. The device display a current level of 70µA/mm and a transconductance of 147µS mm^-1 at 425 K. Importantly, the blocking voltage capabilities of the devices at this temperature are >1 kV with no leakage current and reproducible characteristics after the high voltage stress test.

Diamond is the next generation wide bandgap semiconductor material, which can realize high performance power devices. The H-terminated (C-H) diamond has low activation energy, high carrier mobility¹ and high hole density. We have ever developed C-H diamond MOSFETs reported high drain current density² and high breakdown voltage³. However, C-H diamond has a weak oxidation resistance, there is a problem that it is difficult to apply it to commercialization. Therefore, we proposed C-Si bonded (C-Si) diamond structure using SiO₂ to achieve further diamond device development. The SiO₂ is advantageous for power devices in terms of chemical stability and wide bandgap, so it can be used as the gate insulator in the diamond power device applications. In previous study, we fabricated C-Si diamond MOSFETs with the boron-doped diamond selective growth by using the SiO₂ as the mask for the formation of source-drain electrode and the gate insulator^4,5. The C-Si diamond device had C-Si bonds as the p-channel. We consider C-O-Si on the diamond-SiO₂ interface can reduce the interface state and improve the field-effect mobility. In addition, the C-Si diamond MOSFET achieved normally-off operation⁴and wide temperature stability (~673 K)⁵. In this study, we have further improved the characteristics by thinning the SiO2 gate insulating film.
The fabrication process is as follows. The 2 mm undoped layer was grown on nitrogen-doped (001) diamond substrate by microwave plasma chemical vapor deposition (MPCVD). The 100 nm SiO₂ layer was formed on the substrate by utilizing tetra ethoxy silane chemical vapor deposition (TEOS-CVD). The source-drain patterns were formed by the inductive coupled plasma reactive ion etching. Then, boron-doped diamond ([B]:~10²¹ cm^-3). was selectively grown by MPCVD on the selectively etched undoped diamond layer by 50 nm with the SiO₂ mask patterns with. The growth temperature and microwave output power of MPCVD were 1200 K and 360 W, respectively. The C-Si diamond interface has been formed between the undoped diamond and the SiO₂ mask during selective growth of boron-doped diamond layer. Source-drain electrode were deposited on the surface of the selected boron-doped diamond. Ohmic contacts were formed by annealing (773 K). The SiO₂ film was removed by ICP-RIE, apart from the channel region. The 100 nm Al₂O₃ was deposited as a gate insulating on the diamond substrate using the high temperature ALD method (723 K). Finally, gate electrode (Al) was deposited so as to overlap the source-drain electrode by 2 mm. The gate width, gate length and channel length were 25, 14-22 and 4-12 mm, respectively.
The maximum drain current density of the C-Si diamond MOSFET normalized by gate width is 108 mA/mm at V_DS of -30 V and V_GS -40 V, and the maximum transconductance is 3.1 S/mm at the V_DS of -30 V. The field effect mobility (m_EF) is 122 cm²V^-1s^-1. This value is comparable or better than that of C-H diamond MOSFETs. In addition, the C-Si diamond MOSFET have the normally-off operation with threshold voltage V_th of -0.2 V. This result exhibits that the C-Si bond contributes to normally-off operation of diamond MOSFET indicating a new Vth control mechanism. Furthermore, this device has a thin gate insulating film, which results in low subthreshold characteristics and a steep on /off ratio. In addition, the maximum drain current density increases due to the increased capacity of the oxide film. The drain current on/off ratio is about 7 orders of magnitude at 300 K. In future, C-Si diamond MOSFET be expected to vertical-type diamond devices, and it will contribute significantly to the diamond p-FET for CMOS circuits combined with GaN or SiC n-FETs.
¹Y.Sasama, T.Yamaguchi, et al.,APL Mater. 6(2018)
²K.Hirama, M.Kasu, et al., Jpn. J. Appl. Phys. 51, no.9.(2012)
³Y.Kitabayashi, H.Kawarada, et al. IEEE Electron Device Lett.38(2017)363.
⁴W.Fei, H.Kawarada et al., Appl. Phys. Lett.116,212103(2020)
⁵T. Bi, H.Kawarada et al., Carbon,175,525-533(2021)

We have investigated the characteristics of electric signal propagation in seawater by utilizing diamond solution gate FET (SGFET) [1]. The channel surface of a SGFET and a gate electrode are directly immersed in solutions and the drain current (I_DS) can be remotely modulated by electric signals (V_GS) applied on the gate electrode via solution. Due to these properties, we proposed a new electric seawater communication method utilizing the gate electrode as a transmitter, seawater as propagation medium and SGFET as a receiver [2].
Seawater wireless communication has been required in a wide range of fields such as environmental research, scuba diving, and autonomous underwater vehicle. In seawater, radio waves have short transmission distances, due to the high electric conductivity of seawater. Present seawater wireless communication mainly employs the use of acoustic waves. Recently optical waves are considered or. Optical waves face some drawbacks due to turbidity absorbing scattering centers such as sands, planktons in seawater. While acoustics waves exhibit relatively low absorption, they have low frequency band (~kHz) [3].
To address these issues, we have proposed a new seawater wireless communication method utilizing a diamond SGFET that can operate in electrolyte solutions as the receiver. Diamond SGFET can respond to AC electric signals applied on the electrically independent gate electrode, which indicates successful wireless communications. SGFET output signals were observed as differential pulses responding to the input square potential change. In addition, we have reported that increasing the ion concentration in the electrolyte solution favors signal propagation [2]. The result suggests that the signals are not propagated by radio waves, but that sodium and chloride ions in the solution play an important role in propagation. However, the mechanism for signal propagation has not been clarified yet. In this work, we evaluate the amplitude (V_peak) of the differential pulses and investigate the effect of the transmission distance (gate-SGFET channel distance) up to 25 m on the output characteristics in order to consider the mechanism of signal propagation.
This communication system composes of three components: a gate electrode as the transmitter, an SGFET as the receiver, and an electrolyte solution as the propagation medium. 3.5 % NaCl solution was filled into PVC tubes with a length of 1 to 25 m (diameter = 2.5 cm). The transmission distance was changed by inserting the gate electrode and SGFET at both ends of each tube. The gate electrode was applied up to 10 MHz and 1 V amplitude square waves electric signal (v_G). The drain electrode of SGFET was connected to a load resistance (R_L) and was applied with a supply voltage (V_DD). Then, drain-source voltage (v_DS) and load resistance voltage (v_RL) were measured as outputs.
The output characteristics show that SGFET respond to v_G at least 200 kHz frequency at the distance of 25 m. The amplitude (V_peak) at 50 kHz drastically decreases from 1 m (V_peak > 80 mV) to 10 m (V_peak < 10 mV). On the other hand, V_peak at 50 kHz gradually decreases from 10 m to 25 m, and we obverse that V_peak is about 5 mV at 25 m. However, the response above 500 kHz is not observed at 25 m even though diamond SGFET can respond at 10 MHz input voltage v_G at 1 m. This is because the propagation of differential pulses caused by the derivative of input voltage v_G is delayed and the upper limit of frequency response becomes lower as the distance increases.
We explain these results by introducing two types of propagation models; ion conduction current due to solution resistance R_s, and ion plasma wave model that is propagation as longitudinal waves of sodium and chloride ions.

[1] H. Kawarada, et al, Phys. Stat. Sol. (a) 185 (2001) 79.
[2] K. Tadenuma, H. Kawarada et al., MRS Fall 2019.
[3] H. Kaushal and G. Kaddoum, IEEE access 4 (2016) 1518.

Diamond has a wide band gap (5.5 eV), high thermal conductivity (22 W/cm K), and high breakdown field. Therefore, Diamond is expected to apply RF amplifier. We fabricated high-voltage Diamond MOSFETs with Al₂O₃ (200 nm) as gate insulator formed by high temperature ALD process. We achieved the highest output power density P_out = 3.8 W/mm [1] in p-type FETs. The high output has been obtained by reducing on-resistance, increasing operating voltage V_{DS, Q} and current density I_D,Q, and increasing actual current by extending the gate width W_G. The device with extended W_G may reduce current density because of self-heating, and influence characteristics of devices caused by increase of gate resistance.
In this work, we fabricated ALD-Al₂O₃ MOSFETs with a boron-doped layer on a pure (111) diamond and evaluated DC and RF performance as well as contact resistance in detail. Devices were fabricated on a pure (111) diamond. We etched selectively by ICP-RIE through the evaporated metal mask to form space for heavily boron-doped epitaxial layer (150 nm of p⁺⁺ layer) for source and drain. Source and Drain electrodes are deposited Ti/Pt/Au (20 nm/30 nm/90 nm). The thickness of Al₂O₃ gate insulator is 200 nm. And we adopted double gate finger structure for the reduction of gate resistance.
From I_DS-V_DS characteristics, the maximum drain current density I_Dmax of 1.03 A/mm, R_on of 23 Ω mm was obtained at V_GS = -28 V and V_DS = -40 V. To obtain even higher current density, it is valid to shorten the distance between the source and drain, for example. It is expected that the drain currency density achieves 1.5 A/mm when L_SD is designated 0.5 µm. This small L_SD device has been manufactured by using electron beam lithography when we coat resist of metal mask for p⁺⁺ layer. We measured TLM pattern on the substrate which are (a) 2DHG and p⁺⁺ layer and (b) only 2DHG without p⁺⁺ layer. As for (a), contact resistance R_contact was 1.77 Ω mm and sheet resistance R_sheet was 6.7 kΩ/sq. As for (b), R_contact was 27.5 Ω mm and R_sheet was 16.8 kΩ/sq. These results suggest that p⁺⁺ layer under the source and drain area reduce both contact resistance and sheet resistance. (c)In order to evaluate the contact resistance between boron-doped layer and 2DHG, we measured contact resistance to p⁺⁺layer and sheet resistance R_sheet of p⁺⁺ layer to be 0.49 Ω and 64 Ω/sq, respectively. From these results, each element of resistance can be calculated. R_TiC-Boron was 0.49 Ω mm, R_Boron was 0.32 Ω mm, R_Boron-2DHG was 0.96 Ω mm. This result suggests that the barrier between boron-doped layer and 2DHG is not a serious problem because the contact resistance (=0.96 Ω mm) is very small. And the large signal performance of a device was evaluated by using a load pull system at 1 GHz. As for the power output density measured by the load pull system, 3.6 W/mm was obtained on the device of W_G = 200 µm. This value is almost equal to the maximum value of Diamond MOSFETs (= 3.8 W/mm), which was obtained at W_G = 100 µm; that is, actual power in this new device is almost twice as that of former maximum value.
In summary, we fabricated boron-doped MOSFETs on a pure (111) diamond substrate and evaluated DC and RF performance as well as contact resistance between boron-doped layer and 2DHG. As a result, over 1 A/mm of drain current density was obtained. To obtain even higher current density, it is valid to shorten the L_SD. As for the TLM measurement, boron-doped layer reduced both contact resistance and sheet resistance. Then we found that the barrier between the boron-doped layer and 2DHG is not a serious problem. As for the large signal performance, power output density 3.6 W/mm was obtained. This is to be equal to the maximum value of Diamond MOSFETs, but actual output power is twice than that of maximum value.
This work is supported in part by MEXT Project on Design & Engineering by Joint Inverse Innovation for Materials Architecture.
[1] S.Imanishi, H. Kawarada et al: IEEE EDL.40 (2019) 279

Nanodiamonds science and technology has been extensively explored in the past decade with prospects and examples of applications from biomedicine to quantum technologies. Ample reviews and seminal articles are available [Mochalin, Nature Nanotechn. 7 (2012) 11; Arnault (Ed.), Nanodiamonds, Elsevier, 2017]. Yet many scientific and application related problems remain to be addressed. Driving motivation is to better understand, control, and make use of nanodiamonds unique properties for photocatalysis, energy harvesting, sensing, health and safety. In this contribution, we review our recent progress in technology to better control nanodiamond properties and we analyze interactions (structural, chemical, electronic, optical) of nanodiamonds with molecules and inorganic materials by correlating microscopic, spectroscopic, and computational methods. We focus on two representative types of nanodiamonds (NDs) produced by HPHT or detonation synthesis that in some aspects exhibit distinctively different features.
We present a technology of controllable and high-yield purification and size reduction of both types of NDs down to 2 nm (mode size) by using annealing in air at optimized temperatures of 450-520 °C for 10-50 min. The effect differs on HPHT and detonation NDs. Size and nitrogen inhomogeneity in the primary DND particles is revealed via salt-assisted deaggregation. We characterize the effect of size on multiple methods, from DLS and AFM to SAXS and Raman. We observe distinct size-related effects on surface chemistry and Raman spectral features (incl. breathing modes) in sub-5nm range on HPHT and detonation NDs. We show that even at the molecular size the nanodiamond surfaces can be reversibly modified from oxidized to hydrogenated by thermal processes.
We demonstrate use of such controlled nanodiamonds on various examples. For instance, 2 nm H-DNDs enable formation of extremely dense (10^13 cm^-2), thin (2 nm), and smooth (RMS < 0.8 nm) nucleation layers and then highly uniform sub-6 nm CVD diamond films. Photoluminescence of SiV centers in the films < 10 nm is fully switched between on and off states by O/H surface terminations. Nanodiamonds themselves inherently assume electrical charge on substrates (up to 0.4 V) that affects both electron emission and photoluminescence. surface potential of nanodiamonds is affected also by adsorbed water that forms a thicker layer on hydrogenated and binds more tightly on oxidized NDs as shown by TGA and FTIR. Specific interaction with biological solutions leads to formation of different protein corona on 2 nm O/H DNDs and thereby to different interaction with cell membrane and cytotoxicity. On bacteria, the most pronounced long-term inhibition of E. coli growth (45%) is provided by hydrogenated NDs, via oxidative stress predominantly.
We show that composites of nanodiamonds with other materials represent a promising future direction. TiO2/nanodiamond composites exhibit more efficient photocatalytic degradation of soman via enhanced hole transfer. Ag/nanodiamond composites exhibit better stability and photoactive effects than Ag nanoparticles alone. Based on AFM and FTIR analysis, hydrogenated or oxidized DNDs in composites with Au nanoparticles provide control of drug molecule adhesion and assembly (upright or layered) both on nanodiamond and Au and thereby control also impedance biosensor response. Nanodiamond surface chemistry controls also assembly of polypyrrole-DND complexes and thereby generation of photovoltage. Based on the experiments and theoretical DTF computing, polyfunctional DNDs with both oxygen and hydrogen surface groups may be the best nanodiamonds for PV, serving as effective inorganic electron acceptor. We demonstrate prototype spin-coated DND-PPy solar cell incl. cross-sectional SEM analysis.
We gratefully acknowledged the long-term support from the Czech Science Foundation, Ministry of Education Youth and Sport, and the project CZ.02.1.01/0.0/0.0/15_003/0000464 (CAP).

Doped semiconductors exhibit interesting optical and optoelectronic properties such as localized surface plasmon resonances^1,2 and carrier-phonon coupling³. Boron-doped diamond is a wide band-gap, p-type semiconductor which has been previously found to produce interesting phenomena such as electron-phonon coupling that leads to Fano resonances⁴ and superconductivity⁵. Here, we characterize boron-doped nanodiamond powders by electron-energy loss spectroscopy (EELS) using a scanning transmission electron microscope to reveal potentially new optical transitions. The nanodiamond samples were synthesized commercially by high-pressure-high-temperature methods and obtained from Adamas Nanotechnologies. The particles were imaged using high-resolution transmission electron microscopy to confirm the crystalline structure and measure their size which was on the order of 0.5-1 microns. The boron concentration was estimated to be ca. 800 pm by comparing the Fano line shape of the main peak in the Raman spectrum to results in the literature⁴. For EELS, we focused on the valence-band where a clear shoulder was observed on the zero-loss peak (ZLP). Through data processing, the ZLP was either deconvoluted or subtracted from the inelastic scattering spectra using the reflected-tail method and fitted using Voigt peaks to identify multiple peaks in the range of 0.1-0.2 eV. We note that the feature was completely absent in a control undoped nanodiamond sample synthesized under the same conditions. We present theoretical simulations to explain the origin of this signal, which suggest that the new resonances are driven by low-lying electronic excitations in the valence band. We also investigate the role of resonant electron-phonon coupling, which was previously shown to lead to observable changes in the electronic and vibrational properties in boron-doped diamond⁶.

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Industrial-scale production and narrow size distribution make detonation nanodiamonds (DNDs) a convenient precursor for onion-like carbon (OLC) formation. Apart from frequently used high-temperature annealing of DNDs in an inert environment, their laser irradiation in liquid can be effectively used for the OLC formation. Due to processing in chemically reactive environment, the produced OLCs can carry specific surface functional groups that mediate their solubility in various solvents¹. Moreover, the transformation was shown as a reversible process, i.e. nanodiamonds (NDs) were back-transformed after the OLC laser irradiation².
In this work, we focused on this process more in detail by monitoring the surface chemistry, zeta potential, and structure of DNDs as a function of laser irradiation duration. Firstly, we used fully deagregated hydrogenated DNDs (H-DNDs) dispersed in ethanol which were irradiated by a 532 nm NdYAG laser with energy 150 mJ in a pulse (5 J/cm²) at 10 ns pulse duration and 10 Hz repetition rate. The irradiation lasted for 1, 2, 3, 4, 5, 10, 20, 40, and 60 minutes giving a rise to nine samples. With increasing irradiation time, the brown colloid of the pristine H-DND gradually changed to black and finally turned to a transparent yellow colloid. FTIR measurements revealed a monotonical decrease in the C-H_x bands intensities and an increase of the C-O and C=O features. This was accompanied by a zeta potential decrease from 54 mV for the pristine sample down to 34 mV for the 10 min sample. The 20 and 40 min samples were already colloidally unstable. Gradual photo-oxidation of the H-DNDs was confirmed by XPS which revealed an increase of the oxygen content with increasing laser irradiation time. The formation of OLC-like structure was proven by transmission electron microscopy (TEM). In addition, TEM also showed a gradual loss of nanoparticle character with increasing the irradiation time. The 40 min sample no longer consisted of ~5 nm particles but an interconnected chain-like mesostructure. Two fundamental aspects were investigated by Raman spectroscopy. First, we focused on the time-dependent H-DND transformation into OLC structures. In agreement with XPS and TEM, Raman measurements confirmed the gradual increase of the sp² C with increasing laser irradiation time. However, no OLC-to-ND back transformation was detected. Second, we investigated the laser-treated samples after their purification from sp² carbon by air annealing treatment. Suprisingly, for all samples irradiated up to 40 min, the typical and unchanged DND Raman spectrum was successfully recovered after the annealing in air at 450°C for 300 min. This finding indicates the inhomogeneous sp³ to sp² transformation during the laser irradiation process as well as the insensitivity of the DND Raman spectra to the surface chemistry, size, and transient structural changes. Our results demonstrate the potential and limits of the laser irradiation technique as an effective tool for the controllable formation of water-soluble OLCs and bring deeper understanding to relations between sp³ and sp² nanocarbons.

(1) Jang, D. M.; Im, H. S.; Back, S. H.; Park, K.; Lim, Y. R.; Jung, C. S.; Park, J.; Lee, M. Laser-Induced Graphitization of Colloidal Nanodiamonds for Excellent Oxygen Reduction Reaction. Phys Chem Chem Phys 2014, 16 (6), 2411–2416. https://doi.org/10.1039/C3CP54039A.
(2) Xiao, J.; Ouyang, G.; Liu, P.; Wang, C. X.; Yang, G. W. Reversible Nanodiamond-Carbon Onion Phase Transformations. Nano Lett. 2014, 14 (6), 3645–3652. https://doi.org/10.1021/nl5014234.

Dislocations —one dimensional topological defects—in electronic materials, are considered to have a negative influence on electronic properties; their presence is thought to cause non-radiative recombination of charge carriers which lead to overheating of the crystal and malfunction of the device. Therefore, great effort has been made to produce dislocation-free crystals. However, recent studies show that dislocations can also be turned into useful functional features. For instance, it is reported that magnetic properties of dislocations differ from the lattice due to broken symmetry of the magnetic order and local non-stoichiometry of the dislocation core [1][2]. In another study, it is shown that inherent screw dislocations in certain elemental and compound semiconductors generate coherent spin currents through spin orbit coupling effect [3]. Thus, dislocations, with unique and localized magnetic and electronic properties, as well as their interactions in attracting point defects could be engineered to designing next generation technological devices and there is need for a thorough theoretical understanding of their influence on electronic and magnetic properties by means of atomic scale modelling which provide a predictive way to model strongly distorted dislocation core region.
We present a systematic study of the core structure and electronic and magnetic properties of various dislocations in diamond using density functional theory (DFT). DFT calculations are performed with the Vienna ab initio Simulation Package (VASP), using norm-conserving pseudopotentials. Exchange and correlation contributions are treated within both the generalized gradient approximation and hybrid HSE06 (Heyd-Scuseria-Ernzerhof) functionals. To be able to satisfy periodic boundary conditions and to minimize long range elastic field interactions, supercells with two dislocations having opposite sign Burgers vectors (.i.e. dislocation dipole) are used, which result in an infinite quadrupolar array of dislocations. The analysis of the electronic band structure along the dislocation line together with band decomposed charge density distributions reveal mid-gap energy states induced by broken atomic bonds in the dislocation core. Deep levels associated with dislocations are found to be donor type (n-type) charged dislocations. In addition, we compute the interaction of dislocations with nitrogen-vacancy centers. These results have a strong potential for tailoring the functionality of dislocations in various elemental and compound semiconductors for future electronic devices.
[1]Shimada, T., Xu, T., Araki, Y., Wang, J., & Kitamura, T. (2017). Multiferroic dislocations in ferroelectric PbTiO3. Nano letters, 17(4), 2674-2680.
[2]Sugiyama, I., Shibata, N., Wang, Z., Kobayashi, S., Yamamoto, T., & Ikuhara, Y. (2013). Ferromagnetic dislocations in antiferromagnetic NiO. Nature nanotechnology, 8(4), 266.
[3] L. Hu, H. Huang, Z. Wang, W. Jiang, X. Ni, Y. Zhou, V. Zielasek, M. G. Lagally, B. Huang, F. Liu, “Ubiquitous Spin-Orbit Coupling in a Screw Dislocation with High Spin Coherency” Phys. Rev. Lett. 121, 066401 (2018).

Reflectivity has always been a problem for a variety of processes involving interfaces and electromagnetic waves. For most substrates, the reflectivity can be reduced by introducing thin anti-reflective (AR) coatings. However, the conventional coating technology reaches its limit when dealing with diamond: due to the high refractive index of diamond it is nearly impossible to find suitable coating materials.
The bioinspired approach to achieve anti-reflective surfaces leads to so called moth-eye nanostructures, pillar-like structures with sizes in the nano- and micrometer-regime on the surface of the substrate. In nature, the structures have proven to effectively reduce the reflectivity. Copying these structures has shown that artificial moth-eye structures are also capable of reducing the reflectivity to a minimum. Here, the AR effect is not based on destructive interference created by defined layers of an AR coating and therefore does not dependent on the fulfillment of interference conditions. In the case of moth-eye nanostructures, a density effect leads to continuous refractive index matching. Due to this, the AR properties are not limited to certain wavelengths or angles of incidence but are achieved for a broad band of incoming wavelengths and angles of incidence. This already poses a great advantage compared to the limited possibilities of conventional coating methods. Additionally, the size and shape of the pillar like structures have an impact on the enhanced optical properties, making it possible to tailor the structures to achieve the desired optical performance. Furthermore, by changing the optical properties in a surface-based and intrinsic process, the challenge of finding suitable coating materials can be neglected and the unique thermal, mechanical and electronic properties of diamond are not falsified by a comparably fragile coating.
So far, conventional lithographic methods have been used to generate such structures. These methods are comparably laborious, especially for large areas. We propose a new method to produces artificial moth-eye structures in polycrystalline diamond: using the self-assembly based block copolymer micellar lithography (BCML) and reactive ion etching (RIE), we found a possibility to implement moth-eye nanostructures in different materials. Originally developed for glassy silicon dioxide (SiO₂), the process has been refined for a variety of different substrates, ranging from simple glasses like SiO₂ to semiconductors like silicon up to high-performance materials like sapphire and diamond. In comparison to the conventional lithographic methods widely used to produce these moth-eye structures, the self-assembly based BCML method is comparably fast but still as reliable as conventional methods. For diamond, the structuring process has increased the maximum transmission over 15 % compared to the untreated substrate, also reducing the reflectance severely. These advantages show the significance of the presented method, especially for high performance materials with atypical properties such as diamond.

The development of applications in quantum sensing require dedicated solid-state material platforms with unprecedented control over quality, purity and doping. Indeed, Quantum Technologies (QT) explore our ability to coherently control the peculiar properties of nanoscale quantum systems and harness their potential in a wide range of fields including health, communications, security and environment. In particular, sensors and devices having a disrupting performance as compared to classical systems are foreseen in a relatively short-term [1].
Atomic-scale defects in solid-state materials are among the most promising candidates and several platforms are being considered such as donors in silicon, color centers in semiconductors (NVs in diamond and SiC), rare-earth ions in oxides, quantum dots etc. One of the inherent drawbacks of the extreme sensitivity of such spin systems to the field they are supposed to measure, is the necessity to carefully control their close crystalline environment in order to maximize coherence properties. This pushes the fabrication technologies to their limits and requires that tailored materials are produced on purpose. In this presentation, we will review the main challenges in the synthesis of suitable “quantum-grade” materials with a special emphasis to color centers in diamond and more specifically NV centers. Issues related to controlling their density and maximizing their coherence properties through precise engineering will be discussed. (i) Atomic scale defects need to be selectively doped on demand within the matrix. High amounts are usually preferred in most sensing schemes but limits exist in the ability to control growth at high doping levels as well as to avoid interaction between nearby spins. (ii) Nuclear spins within the matrix itself might cause unwanted decoherence and growth of isotopically enriched materials allows considerably extending T₂ times or investigating specific coupling schemes with nearby spins. This has serious implications on the precursors used for material synthesis. (iii) In the same vein, magnetic sensitivity of a sensor based on NV ensembles can be considerably increased by producing diamond with NV centers having preferential orientation which can be obtained by using diamond substrates with specific crystallographic orientation such as (111) or (113) (iv) Controlling the spatial localization of color centers is crucial to the sensing device since their interaction strongly depends on the distance to the field that is to be sensed. Positioning them close to the surface with ion implantation is an efficient and widely followed approach but which comes along with the generation of residual defects that can seriously impact the spin properties.

[1] Degen, C. et al. Reviews of Modern Physics 2017, 89 (3), 035002.

Engineering coherent systems is a central goal of quantum science and quantum information processing. Point defects in diamond known as color centers are a promising physical platform. As atom-like systems, they can exhibit excellent spin coherence and can be manipulated with light. As solid-state defects, they can be produced at high densities and incorporated into scalable devices. Diamond is a uniquely excellent host: it has a large band gap, can be synthesized with sub-ppb impurity concentrations, and can be isotopically purified to eliminate magnetic noise from nuclear spins. Currently-known color centers in diamond either exhibit long spin coherence times or efficient, coherent optical transitions, but not both. We have developed new methods to control the diamond Fermi level in order to stabilize a new color center, the neutral charge state of the silicon vacancy (SiV) center. This center exhibits both the excellent optical properties of the negatively charged SiV center and the long spin coherence times of the NV center, making it a promising candidate for applications as a single atom quantum memory for long distance quantum communication. We have recently discovered bound exciton transitions associated with SiV0, which enable efficient optical spin polarization and optically detected magnetic resonance. Finally, I will describe our efforts to integrate SiV0 centers in nanophotonic devices, specifically in heterogeneously integrated III-V/diamond nanophotonic platforms designed to enhance the atom-photon interaction and achieve quantum frequency conversion to the telecom band.

Despite the technological maturity of nitrogen-vacancy (NV) centers in diamond as a platform for quantum technology, their performance in most applications is hindered by imperfect state initialization and readout [1]. This is especially true for quantum sensing, where the NV center’s charge and spin stability are typically degraded by proximity to surfaces and impurities. This talk will present a framework for optimizing the performance of NV-center quantum sensors using all-optical techniques that improve the signal-to-noise ratio through spin-to-charge conversion [2] and deterministic charge initialization [3]. I will also discuss recent efforts to create nanodiamond assemblies and to characterize their material and quantum properties, towards the realization of new sensing modalities.
D. A. Hopper, H. J. Shulevitz, and L. C. Bassett, Micromachines 9, 437 (2018)
D. A. Hopper et al., ACS Nano 12, 4678 (2018).
D. A. Hopper et al., Phys. Rev. Applied, 13, 024016 (2020).

This work was supported by the NSF through a CAREER award (ECCS-1553511) and the University of Pennsylvania Materials Research Science and Engineering Center (MRSEC) (DMR-1720530).

Inversion symmetry makes the color centers insensitive to stray electric fields in diamond which enables to create indistinguishable solid state single photon sources. The most studied and employed representative is the negatively charged silicon-vacancy defect [SiV(-)] in diamond. However, the fine electronic structure of SiV(-) limits the coherence times of the electron spin, thus it is not practical. One possible route is to use isovalent tin or lead which are predicted to possess longer coherence times [1]. Alternatively, SiV(0) and their akin isovalent variants [2] may be used where the ground state spin has a long coherence time [3]. We show that the excited state of SiV(0) is highly complex in which the Rydberg excited states can be used to optically readout the spin state which is a crucial step towards coherent manipulation of single SiV(0) centers. We identify further inversion symmetric color centers in diamond such as the negatively charged magnesium-vacancy [MgV(-)] defect. We find that the electronic structure of MgV(-) permits the coexistence of two loosely separated spin-states, where both can emerge as a ground state and be interconverted depending on the temperature and external strain. These results demonstrate a route to control the magneto-optical response of a qubit by modulating the operational conditions [4]. We also identified the negatively charged nickel-vacancy center in diamond which has favorable spin coherence time at cryogenic temperature which can be coherently controlled by all-optical means. This work was supported by the Ministry of Innovation and Technology and the National Research, Development and Innovation Office of Hungary (NKFIH) within the Quantum Information National Laboratory of Hungary, the National Quantum Technology Program (NKFIH Grant no. 2017-1.2.1-NKP-2017-00001). A.G. acknowledges the National Excellence Program (NKFIH Grant no. KKP129866), the EU QuantERA project “Q_magine” (NKFIH Grant no. 127889) as well as the support of the European Commission within the Quantum Technology Flagship Project ASTERIQS (Grant no. 820394).
[1] G. Thiering and A. Gali, Phys. Rev. X 8, 021063 (2018)
[2] G. Thiering and A. Gali, npj Comp. Mat. 5, 18 (2019)
[3] Zi-Huai Zhang, Paul Stevenson, Gergo Thiering, Brendon C. Rose, Ding Huang, Andrew M. Edmonds, Matthew L. Markham, Stephen A. Lyon, Adam Gali, and Nathalie P. de Leon, Phys. Rev. Lett. 125, 237402 (2020)
[4] Anton Pershin, Gergely Barcza, Örs Legeza, and Adam Gali, npj Quantum Information 7, 99 (2021)

Long ago, Chen and Lang¹ demonstrated spin dependent deep level transient spectroscopy in metal-oxide-semiconductor (MOS) capacitors, combining the advantages of both electron paramagnetic resonance (EPR) and deep level transient spectroscopy (DLTS). EPR and the closely related electrically detected magnetic resonance (EDMR)² techniques are able to identify the chemical and physical nature of paramagnetic point defects while DLTS has the capability to determine point defect energy levels. We report on a simplification of the Chen and Lang method which we term spin dependent transient spectroscopy (SDTS) and demonstrate the magnetic field and frequency independence of this technique. Additionally, we show that it can be extended to utilize near zero field magnetoresistance (NZFMR)³ spectroscopy. Our SDTS measurements were performed on negative-bias-temperature-stressed⁴ Ti-Al/SiO₂/p-Si capacitors.

In our SDTS measurements we somewhat modified the spin dependent DLTS technique of Chen and Lang, utilizing a single boxcar averager for time-resolved data acquisition. In addition, we show that this modified technique has essentially the same sensitivity at X-band (≈9.4GHz) and quite low RF (≈500MHz) frequencies. The physics of this method is quite simple. Applying a voltage pulse to the metal gate brings the interface Fermi level from near the valence band to near the conduction band. This creates a current transient due in part to charge carriers filling defect levels at the insulator-semiconductor interface. Since the capture process can be spin dependent, the recombination component of the transient can be affected by magnetic resonance. These observed spin dependent events allow for the identification of the involved interface states.

We have made these measurements as functions of the pulse voltage (V_min), sustained base voltage (V_max), pulse time, and acquisition window. Each of these parameters has an effect on the amplitude of the spin dependent response. We focus on the pulse and base voltages of our waveform in order to determine energy levels in the bandgap. Additionally, we show that we can perform SDTS at high and low fields/frequencies as well as in the NZFMR regime. In conclusion, we show that SDTS is a potentially powerful new tool with comparable sensitivity to other EDMR techniques with the advantage of observing interface states in simple metal-insulator-semiconductor structures.

¹ M.C. Chen and D. V. Lang, Phys. Rev. Lett. 51, 427 (1983).
² D. Kaplan, I. Solomon, and N.F. Mott, J. Phys. Lettres 39, 51 (1978).
³ N.J. Harmon, S.R. McMillan, J.P. Ashton, P.M. Lenahan, and M.E. Flatte, IEEE Trans. Nucl. Sci. 67, 1669 (2020).
⁴ D.K. Schroder and J.A. Babcock, J. Appl. Phys. 94, 1 (2003).

The structure elucidation of macromolecules and molecular complexes is an important topic in molecular biology and biochemistry. The nitrogen vacancy center (NV center)is seen as a promising platform for scaling nuclear magnetic resonance down to the single-molecule level. However, the sensing capability of near-surface NVs – a necessary prerequisite for single-molecule NMR - is severely limited by charge state instabilities and decoherence due to surface traps and electronic and magnetic surface noise, respectively. Here, we report on strategies to improve the stability of NV centers and their coherence properties by controlled termination of the diamond surface, and to functionalize the diamond surface to bind molecules of interest.
In this talk, we discuss the creation of 3 to 7 nm deep NVs in nano-structured diamond surfaces that show T₂ coherence times of 50 to 300 us The creation of photonic structures boosts the collection efficiency and allow us to follow single NV centers as the diamond undergoes surface termination and functionalization steps . We investigate the effects of oxygen- and nitrogen-termination, and find that the former improves the electron spin coherence properties if annealing is performed below the graphitization-threshold. Further, we chemically functionalize the diamond surface with fluorinated silanes and show that, after silanization, a ¹⁹F NMR signal can be detected. The successful functionalization is further confirmed by complementary X-ray Photoelectron Spectroscopy (XPS) and fluorescence measurements. We give an outlook on the steps needed to perform nuclear magnetic resonance (NMR) measurements on the single-molecule level.

Carbon fibers reinforce polymers (CFRP) is the principal structural material used in the aerospace industry due to its high strength-low weight ratio as well as its high stiffness. In contrast, CFRP is a bad electrical and heat conductor. This limits heat dissipation which causes high damages when lightning impacts occur. In addition, the aircraft structure should act as a Faraday cage to protect electronic devices which is not possible with standard CFRP. In this work it is evidenced that the use carbon fiber coating made of highly doped boron polycrystalline diamond (HDBPD), increases the electrical conductivity of carbon fiber (CF) for the manufacture of aeronautical structures.

HDBPD was deposited by microwave plasma enhanced chemical vapor deposition (MW PE CVD) on one commercial carbon fiber used in the aerospace industry, specifically HexTow AS7 from Hexcel, using a clamshell ASTeX 6500 series reactor. The growth conditions chosen to avoid fiber damage due to plasma were: low temperature (700 °C) and pressure (35 Torr), 1% methane and 40 Sccm of trimethylborane (TMB). Once the polycrystalline diamond was deposited on a commercial tow, different individual fibers were taken to prepare samples for transmission electron microscopy (TEM) and atomic force microscopy (AFM). Both the traditional polishing as Focus Ion Beam (FIB) methods were used to check to check the suitability of both methods in this type of CF/diamond material to avid sample preparation artifacts.

Electron beam techniques such as high-resolution electron microscopy (HREM), electron energy loss spectroscopy (EELS) and scanning electron microscopy (SEM), were used to determine their CF/interface/diamond nanostructure. HREM showed that an intermediate layer grows on the fiber surface first, on which the polycrystalline diamond is subsequently formed. The analysis of this layer by EELS revealed its strong sp² character which is in accordance with the bibliography where this layer is described as Carbon nanowalls. The subsequent polycrystalline diamond layer was ∼1 micrometer thick and contained spherical grains averaging 70 nanometers.

The electrical properties of the HDBPD-carbon fiber were measured using conductive and scanning microwave impedance microscopy atomic force microscopy (C-AFM and sMIM-AFM) and Kelvin method. The improvement of the electrical conductivity of the CF is evidenced after HDBPD deposition.

CVD Diamond is one of the most promising semiconductor materials for high power applications, because of its exceptional mechanical, electronic and thermal properties, such as wide band-gap, high breakdown electric field, high mobility and high thermal conductivity. Diamond bipolar devices are promising for ultra-high voltage applications (>10kV), but diamond PN junctions provides limitations due to (1) a high turn-on voltage (~5V) giving a significant on-state voltage drop and (2) n-type diamond having higher resistivity and poor ohmic contacts. In order to address this problem, the implementation of an alternative n-type UWBG semiconductors with shallow donor dopants should be considered. Beta-Gallium Oxide (β-Ga₂O₃) is a semiconductor that has gained significant attention due to its attractive properties like its wide bandgap (4.85eV) and high breakdown field in the range of 8 MV/cm. Also, commercially available larger area high quality β-Ga₂O₃ wafers will lead to potentially more affordable high performance power devices. Diamond’s outstanding thermal properties can serve as a heat dissipater at high power operations, compensating the poor thermal conductivity of β-Ga₂O₃. As an experimental demonstration of a diamond/ Ga₂O₃ pn junction device structure a photodiode [1] has been demonstrated in the literature using a nanomembrane of Ga₂O₃ on diamond.
In this study we report the device modeling using TCAD Sentaurus and analysis of n-type β-Ga₂O₃/p-type diamond heterojunction PIN diodes for future applications in power electronic devices. Material parameters input entered into the TCAD model will be presented. The heterojunction modeled had a band discontinuity of ΔEc=2.7eV at the conduction band and ΔEv=2.1eV at the valence band. It was observed that at voltages >5V, the on-state shows conductivity modulation in the drift region in the forward regime. The current density, carrier concentration and carrier recombination spatial distribution for the diamond intrinsic region with a thickness of 1um as a function of temperature and carrier lifetime will be examined.

[1] H. Kim et al., “Ultrawide-Bandgap p-n Heterojunction of Diamond/β-Ga2O3 for a Solar-Blind Photodiode,” ECS J. Solid State Sci. Technol., vol. 9, no. 4, p. 045004, Apr. 2020, doi: 10.1149/2162-8777/ab89b8.

Recently, inversion type p-channel diamond MOSFETs with normally off characteristics have been demonstrated for the first time [1]. However, the improvement of the channel mobility is one of the major issues towards the practical application of the diamond based power devices [2]. The previous studies have suggested that high interface defect density (D_it) at the Al₂O₃/diamond interface significantly reduce the channel mobility [2].

Although such macroscopic evaluation method employed in the previous studies is capable of quantitatively evaluating D_it with good energy depth resolution, the lateral resolution is quite limited. On the other hand, recently there have been reports suggesting that interface traps have an inhomogeneous spatial distribution, which makes characterizing interface traps with high lateral resolution quite interesting and important [3].

In the present study, in order to give microscopic insights on the physical origin of the high D_it, we investigate the Al₂O₃/diamond interfaces using scanning nonlinear dielectric microscopy (SNDM) [4, 5].

SNDM permits nanoscale evaluation of local MOS capacitance [5] which can perform local differential capacitance (dC/dV) imaging, local CV profiling [6], and local deep level transient spectroscopy (DLTS) using a time-resolved SNDM (tr-SNDM) setup [7].

The samples were fabricated from highly B-doped p⁺- diamond layer grown on the back side of a high-pressure and high-temperature p-type (111) substrate for ohmic contact formation and had a 50 nm thickness atomic layer deposited Al₂O₃ top layer[1, 2]. We measured three different samples, whose diamond surfaces were terminated by H, O, and OH-groups, respectively.

At first, local CV profiles were here obtained. The H-terminated sample exhibited strongly negative shift reflecting the formation of surface accumulation layer, while the O- and OH-terminated samples showed depletion at 0 V. The three profiles also showed different behaviors in the transition from depletion to accumulation, suggesting D_it is highest in the O-terminated samples followed in order by the OH- and H-terminated samples. We could find that these local CV profiles also showed microscopic spatial fluctuations, which might be correlated with D_it [8].

Second, we measured dC/dV images of two different areas with atomic flatness and high roughness in the OH-terminated sample. In both images, there are non-uniformly distributed dark, or low dC/dV, spots indicating the low slope of local CV profiles. Since dC/dV becomes lower due to higher D_it broadening the local CV profile, the observed images possibly visualize the high D_it spots non-uniform distributing at the Al₂O₃/OH-terminated diamond interface and the difference of the interface by the flatness of the diamond surfaces.

Finally, we also performed local DLTS on the same sample. We found the non-uniform D_it distribution, again. In addition, the line-shaped features of D_it distribution could be observed especially in the atomic flat area. The line-shaped features might be related to the variation of D_it at the atomic steps in the buried interface. In fact, the previous study by conductive atomic force microscopy and Kelvin probe force microscopy also showed the steps seem to generate D_it [8].

[1]T. Matsumoto et al., Sci. Rep. 6, 31585 (2016).
[2]T. Matsumoto et al., Appl. Phys. Lett., 114, 242101 (2019).
[3] N. Chinone, A. Nayak, R. Kosugi, Y. Tanaka, S. Harada, H. Okumura, and Y. Cho, Appl. Phys. Lett., 111, 061602 (2017).
[4]Y. Cho, A. Kirihara, and T. Saeki, Rev. Sci. Instrum, 67, 2297 (1996).
[5]Y. Cho, “Scanning Nonlinear Dielectric Microscopy: Investigation of Ferroelectric, Dielectric, and Semiconductor Materials and Devices”, Elsevier, ISBN 9780128172469 (2020).
[6]K. Suzuki et al., in 2019 IEEE International Integrated Reliability Workshop (IIRW) (2019).
[7]Y. Yamagishi and Y. Cho, Appl. Phys. Lett. 111, 163103 (2017).
[8]E. H. Nicollian and A. Goetzberger, Bell Labs Tech. J. 46, 1055 (1967).

Diamond is an excellent host for spin-based qubits, and the spin in diamond has excellent properties. Nitrogen-vacancy (NV) center in diamond is one of the most promising candidates for quantum sensing. The energy levels of NV centers are sensitive to magnetic fields, electric fields, strain, and temperature, enabling scalable applications from the atomic to the macroscopic range [1].
Core technologies on material, sensor systems, and applications for energy electronics and life science are introduced.
Materials
Selectively aligned NV ensemble formed by distinctive CVD-growth for scalable applications.
The heteroepitaxial growth of NV-contained diamond on Si substrate for large area and on-chip integration.
Multi-scale and multi-modal sensor systems
- Direct sensing of nano-scale internal electric-field in the power device [2]
- EV battery monitoring [3]
- Magnetocardiography (MCG) of mammalian animals [4]

This work was supported by MEXT QLEAP Grant Number JPMXS0118067395.
The author would like to thank Lab and Q-LEAP members for their contributions and helpful discussion.
[1] L. Doherty et al., Phys. Rep. 528, 1 (2013).
[2] Bang Yanget et al., Physical Review Applied 14, 044049 (2020).
[3] Y. Hatano et al., APL 118, 034001 (2021).
[4] K. A rai et al., arXiv2105.11676 (2021).

Nitrogen vacancy (NV) center in diamond has much attention as a quantum material that can realize spin-based sensing due to its excellent characteristics such as long coherence time, fast manipulation rates, optical initialization and readout, etc. Precise control of material-related parameters including NV concentrations, axis alignment, volume, charge-state, are essential for higher sensitivity and are important to bring the NV centers closer to quantum application in practical use. Plasma-enhanced chemical vapor deposition (CVD) with microwave is one of the major approaches to introduce well-defined NV centers in diamond, as well as electron beam irradiation and ion implantation techniques. So far, we have already proposed following ideas based on semiconductor engineering; the charge-state stabilization by n-type Fermi control [1,2], longer coherent time by phosphorus and carbon isotopic control [3], perfect alignment of NV axis [4], device applications with PIN junction [5-7]. All these achievements are based on our comprehensive progresses of CVD diamond growth, impurity doping, junction management, and device fabrication processes. Details will be discussed focusing on technical aspects peculiar to plasma CVD engineering of diamond NV centers.

Acknowledgments
This work was partially supported by MEXT Q-LEAP (JPMXS0118067395), JST CREST (JPMJCR1773), MIC R&D (JPMI00316), and JST Moonshot R&D (JPMJMS2062).

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Negatively charged nitrogen vacancy (NV) center is expected to be a platform for magnetic sensors and quantum communications because of its optically detectable spin properties and long coherence time [1]. One of the most leading application of NV center is magnetic sensor with high sensitivity even at room temperature. Especially, the sensitivity of a magnetic sensing scales with the square root of its quantum coherence time and the total number of NV centers [1]. Thus, enhanced NV concentration while maintaining long coherence times is desired to improve magnetic sensitivity.
Ion implantation is frequently used to increase NV concentration [2]. However, this method results in unwanted paramagnetic defects or charge traps due to knock-on-atoms [3]. Delta-doping with CVD diamond growth is also used to create high density NV ensemble [4]. Yet, this technology has the problem of low N-to-NV conversion efficiency. Therefore, it is an effective approach to maximize N-to-NV conversion efficiency while suppressing damage to the substrate by electron irradiation.
Single-ended accelerator [5] or TEM [6] is used to create vacancies on the substrate with electron irradiation. TEM can adjust the electron irradiation amount per unit time by narrowing the beam diameter. As a result, it is possible to irradiate a high dose of electrons that cannot be achieved by the accelerator. TEM also has the advantage of being easily accessible compared to the accelerator. In fact, several groups have reported using a 200-keV TEM for (100) diamonds, while none have reported for (111) diamonds. Since (111) diamond can incorporate more dense nitrogen atoms than (100) diamond [7], electron irradiation of (111) diamond is promising for the realization of high density NV ensemble. Hence, by irradiating (111) diamond with electrons, we investigated the change in NV density and spin properties in the irradiated region and estimated the magnetic sensitivity.
We irradiated nitrogen-rich ([N]=4×10¹⁹ [cm^-3]) HPHT (111) diamond with electrons using 300-keV TEM (JEOL JEM-3010) because the minimum energy of electrons required to form a defect in (111) diamond is 220-keV [8]. Next, a custom-built laser scanning confocal fluorescence microscope (CFM) was used to estimate the concentration of NV center. We also measured the coherence time of the NV ensemble. We then implemented photoluminescence measurements (RENISHAW inVia Basis) to confirm the presence of vacancy as GR1 center in PL spectrum.
As a result, the PL spectra in the electron-irradiated region showed that the peak intensity at 638 nm, which is the ZPL of , intensified with increasing electron dose. The volume density of the NV center in the NV ensemble increased with increasing electron fluence from 10¹⁶ to 10²² [cm^-2], The NV volume density was calculated by comparing the luminescence intensity of the CFM image in the NV ensemble with that of the Single NV center. At a dose of 1.0×10²² [cm^-2], the NV volume density reached to 8×10¹⁸ [cm^-3], which is one of the highest NV density. The coherence time remained on the order of even when the electron fluence was increased, which is expected to improve the magnetic sensitivity.

This work was supported by This work was supported by MEXT Quantum Leap Flagship Program (MEXT Q-LEAP) Grant Number JPMXS0118067395.
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The nitrogen-vacancy (NV) center in a nanodiamond (ND) crystal is one of the best candidate materials for quantum sensing and metrology at elevated environmental conditions. The NV center can be used to achieve an unprecedented level of sensitivity for sensing applications at high temperatures and pressures. In addition, the NV center is proven to be useful for magnetic field sensing. This opens avenues for applications in the sensing of valuable minerals such as rare earth elements (REEs) in crude and waste subsurface materials which carry free electronic spins locally. In this work, we present our computational and experimental studies on electronic and optical properties of bulk diamond with N impurity and N with a carbon (C) vacancy defect. Experimentally, we encapsulate NDs in MOF/polymer materials. We measure the transverse spin relaxation time of NV center in bare and encapsulated NDs. In order to study the surfaces with shallow NV center that are useful for the quantum sensing applications, we will introduce different doping elements on the surfaces and present surface electronic properties.

The investigation of magnetic order in 2D materials requires dedicated probes. While conventional probes of magnetism with nanoscale resolution, like Lorenz microscopy or MFM fail for few layer- or monolayer samples, STM requires dedicated sample preparation. NV-based magnetic probes on the other hand are very well suited to provide quantitative data with a few ten nm spatial resolution and sufficient sensitivity, even for monolayer samples. In the talk I will describe experiments on CrBr₃ which show the domain structure of the material [1]. Upon imaging material with different number of layers we gained insight into interlayer coupling and it impact on magnetic order. We also measure magnetic order over a wide range of temperatures and derive information on the physics of the phase transition of CrBr₃. We also measured CrI₃ samples of different thickness and relative order and find signatures of periodic magnetic order in twisted multilayers. [1] Qi-Chao Sun et al. Magnetic domains and domain wall pinning in atomically this CrBr₃ revealed by nanoscale imaging, Nature Comm. 12, 1989 (2021)

In order to profit from diamond's unique material properties for demanding device applications, high quality single crystals are needed. For their synthesis by chemical vapor deposition (CVD) two alternative strategies are currently being explored with great effort. The first is based on homoepitaxy using low-dislocation-density crystals from the high pressure high temperature (HPHT) method as growth seeds. While the obtained crystals excel in terms of highest structural quality, the concept still suffers from limitations in size. In the alternative heteroepitaxy concept, diamond is nucleated and grown on a single crystal surface of a foreign material. Among all potential crystalline materials, iridium has been identified as unique. A major advantage of heteroepitaxy consists in its better scalability which has been demonstrated in 2017 by the realization of a diamond/Ir/YSZ/Si(001) wafer with a diameter of of 92 mm [1]. However, the minimum values in dislocation density (7×10⁶cm^-2) and mosaic spread (FWHM: polar 0.03° and azimuthal 0.05°) [2], are still significantly higher than for homoepitaxially grown crystals.
After reviewing the current state of the art, recent and new attempts to reduce the dislocation densities are described. These comprise different variants of epitaxial lateral overgrowth (ELO) and metal assisted termination (MAT).
Next, the properties of threading dislocations in heteroepitaxial diamond as traps for holes and electrons have been investigated. The experimentally derived lifetime values facilitate estimations of the corresponding capture cross sections and extrapolations on critical defect densities for potential applications [2].
Progress in dislocation density reduction and increase in lateral dimensions call for efficient techniques to duplicate the wafers so that the excellent structural properties once achieved on large area substrates can be transferred to new samples. In this context, a new smart-cut process with easy scalability has been developed [3].
In the final part, gain formation as an old and fundamental problem in the application of diamond as solid state ionization chamber for dosimetry of high energy radiation is discussed. A model is presented that can quantitatively explain the formation of gain and its huge variation ranging from less than 1 to more than 10⁶ [4].
[1] M. Schreck, S. Gsell, R. Brescia, M. Fischer: Ion bombardment induced buried lateral growth: the key mechanism for the synthesis of single crystal diamond wafers, Sci. Rep. 7, 44462 (2017).
[2] M. Schreck, P. Ščajev, M. Träger, M. Mayr, T. Grünwald, M. Fischer, S. Gsell: Charge carrier trapping by dislocations in single crystal diamond. J. Appl. Phys. 127, 125102 (2020).
[3] M. Schreck, M. Mayr, M. Weinl, M. Fischer, S. Gsell: Liftoff of single crystal diamond by epitaxial lateral overgrowth using SiO₂ masks, Diamond Relat. Mater. 101, 107606 (2020).
[4] T. Grünwald and M. Schreck: Photoconductive gain in single crystal diamond detectors, J. Appl. Phys. 129, 124502 (2021).

Quantum technologies is attracting significant investment due to the range of potential applications, but behind any new technology are enabling materials. Diamond is one such material and ensembles of negatively-charged nitrogen-vacancy (NV^-) centers constitutes a promising platform for sensing applications utilizing the quantum properties of this defect. However, the sensitivity of present NV^-ensemble devices and the need for diamond material with reproducible properties has the potential to hinder progress toward many envisioned commercial-scale applications.

The work covered in this presentation will address the material-related aspects of these challenges by discussing the progress towards the development of chemical vapor deposition (CVD) growth processes incorporating high substitutional-nitrogen (N_S) concentrations (> 8 parts-per-million carbon atoms), specifically targeted for magnetic-field sensing. Through this optimisation process the effects of N-doping during CVD growth have also been investigated by characterization of the as-grown sample properties, including concentrations of [N_S⁰] and [N_S⁺], as well as color.

Through detailed study of varying CVD processes for high-[N_S] diamond it is shown that increased nitrogen incorporation does not necessarily result in increased densities of unwanted vacancy-related defects which act as a source of broadband absorption in as-grown CVD diamond (commonly resulting in a brown coloration). Previously suggested correlations between [N_S⁺] and the level of broadband absorption, as well as the bands at 360 nm and 520 nm are also investigated and strong links have been established in this study. Measurements of the NV concentration, charge ratio and contrast suggest that these grown-in defects have a detrimental impact on the NV-ensemble properties observed after irradiation and annealing.

This work also investigated the potential to produce material with reproducible as-grown [N_S] at a level of ~16 ppm, to demonstrate increased scale production of plates with improved NV-ensemble properties. NV concentration, NV charge ratio, strain homogeneity, and T₂* after irradiation and annealing were investigated in such material by characterizing a batch of samples treated to create 3.8 ppm of NV centres. The measured NV^- and NV⁰ concentrations in these samples varied by less than 7% (1 standard deviation) and exhibited well-controlled strain inhomogeneity, as characterized by NV-strain mapping and (as a proxy) birefringence. The resulting T₂* = 1 µs in material enriched with >99.99% ¹²C was as expected for the concentration of N, establishing that other sources of decoherence have been controlled. The resulting samples also have a high measured fraction of [NV^-]/[NV] in UV-Vis, which ensures a high optically-detected-magnetic-resonance (ODMR) contrast (>10%), which results in enhanced device sensitivity.

The material reported in this work enables immediate sensitivity improvements for present quantum-sensing devices utilizing bulk samples. Additionally, the progress towards µm-scale thickness layers of equivalent material grown on high-purity substrates for magnetic-field imaging applications will be presented.

Nitrogen-vacancy (NV) centres in single crystal diamond (SCD) have been extensively investigated as promising platforms for photonics, sensing, imaging, and other applications¹. However, the constraints with such systems include inferior optical properties of NV centres and limitations in the SCD upscaling process. A promising alternative to the NV centre are the Group IV element (Si, Ge, Sn, Pb) defects in diamond². As nanocrystalline diamond (NCD) has wafer scalability, it is interesting to investigate the colour centre formation in NCD layers. In addition, recent studies have demonstrated GeV as a potential temperature sensor³.
This study reports in-situ germanium-vacancy (GeV) fabrication in NCD on Ge substrates using the microwave plasma-enhanced chemical vapour deposition technique. Detonation nanodiamond seeded Ge substrates are exposed to 1% CH₄ in H₂ plasma at 3000 W and 45 Torr process conditions. We discuss the challenges in such an approach, such as the proximity of deposition temperature to Ge’s melting point (937°C) and the absence of a carbide phase at the NCD / substrate interface, potentially limiting adhesion. Nevertheless, the gas-phase plasma etching of the substrate, followed by Ge-vacancy formation in diamond, ensures that the Ge substrate can act as a solid dopant source.
Electron energy loss spectroscopy analysis confirms that carbide phases are absent; instead, an amorphous carbon layer of 2 nm forms at the diamond/substrate interface. Hence, growing thicker NCD films (~ 9 µm) self-separating freestanding GeV incorporated diamond films with dimensions up to 1 × 1 cm² can be fabricated. Room temperature photoluminescence (PL) measurements reveal a ~ 602 nm peak characteristic of GeV formation in diamond^4,5. We also present time-of-flight secondary ion mass spectroscopy (ToF-SIMS) results that confirm Ge incorporation in NCD and discuss the non-uniformities in Ge incorporation in the film with PL mapping supported by the high-resolution ToF-SIMS elemental imaging technique. In addition, we also present studies to achieve uniformly distributed GeV in nanodiamonds via spontaneous nucleation as well as overgrowth GeV formation studies in polycrystalline diamond substrates.

References
1. Aharonovich, I., Greentree, A. D. & Prawer, S. Diamond photonics. Nat. Photonics 5, 397–405 (2011).
2. Bradac, C., Gao, W., Forneris, J., Trusheim, M. E. & Aharonovich, I. Quantum nanophotonics with group IV defects in diamond. Nat. Commun. 10, 1–13 (2019).
3. J. W. Fan. et al., Germanium-Vacancy Color Center in Diamond as a Temperature Sensor, ACS Photonics, 5, 765−770 (2018)
4. Iwasaki, T. et al. Germanium-Vacancy Single Color Centers in Diamond. Sci. Rep. 5, 1–7 (2015).
5. Ralchenko, V. G. et al. Observation of the Ge-Vacancy Color Center in Microcrystalline Diamond Films. Bull. Lebedev Phys. Inst. 42, 157–164 (2015).

Heterojunctions comprising of p-type diamond substrates and thin exfoliated n-type β-Ga₂O₃ layer were fabricated using direct-bonding technique at a low temperature. Due to using low temperature for the fabrication, formation of p-type diamond/n-type β-Ga₂O₃ heterostructure was, therefore, possible. In this work, p⁺-diamond/n-Ga₂O₃ and p^--diamond/n-Ga₂O₃ heterostructures were fabricated using direct- bonding technique. From electrical investigations, p⁺-diamond/n-Ga₂O₃ heterostructure exhibited ohmic behavior, resulting from p⁺-diamond substrate behaving as a metallic layer. In the case of p^--diamond/n-Ga₂O₃ heterostructure, it showed a rectifying action as a conventional pn heterojunctions with turn-on voltage ~ 4.5 V, rectifying ratio >10⁹ at ±10 V and leakage current < 10^-12 A. Ideality factor and barrier height of p^--diamond/n-Ga₂O₃ heterojunctions were 2.7 and 1.51 eV, respectively. Electrical comparison between p^--diamond/n-Ga₂O₃ heterojunctions and β- Ga₂O₃ Schottky junctions confirmed that p-type diamond/n-type β-Ga₂O₃ heterojunctions were truly formed using direct-bonding technique. This confirmation is suggesting that lightly doped diamond and β-Ga₂O₃ are preferable to realize the formation of diamond/β-Ga₂O₃ heterojunctions. Energy band diagram of the fabricated pn heterojunctions was provided. This work can serve as an important reference for fabrication of diamond/β-Ga₂O₃ heterojunctions for application in future electronic devices operating under extreme conditions such as high-power, high temperature and strong radiation.

Great details remain unclear to overcome the challenges in diamond growth using Microwave Plasma Assisted Chemical Vapor Deposition (MPACVD). This research will report the single crystal diamond (SCD) grown in several angled holders designed to achieve better epitaxial lateral outgrowth (ELO) and to maintain an optimized lateral growth rate. Multiple depositions are carried out using angled holders from larger angled pockets to smaller angled pockets. These results in larger size ELO with possible PCD growth, intermediate smooth ELO growth, and inward lateral growth.

All as-grown samples are measured with the X-ray rocking curve (XRC) mapping technique to reveal the crystallographic structural properties, and compared to that of the original substrates. Diamond 400 crystal plane curvature/flatness and morphology, XRC FWHM of 400/113/111 diamond peaks are plotted using self-made analytical software to compare the quality revolution before and after the growth. Quantitative birefringence maps are also taken to present the internal crystal structural defects within the CVD diamond. Other than these, the growth behavior within such kinds of adjusted constrained systems will be analyzed to reveal the preference of atomic level diamond growth.

Mapping results show that CVD SCD grown using wider angled pocket, though with PCD rims, has better flatness (small curvature) and higher average structural quality (small FWHM); Diamond grown with intermediate pocket, with pure SCD growth, also has not only a good lateral growth behavior, but also intermediate crystal morphology and intermediate structural quality; Those are using smaller pocket results area shrink, but with larger crystal plane curvature, indicating the CVD SCD is compressed due to the smaller size of the pocket.

Cross polarized birefringence and quantitative birefringence images of all samples care taken to understand the internal structure of the CVD SCDs.

Thus, an intermediate choice would be the best way for iterative SCD growth to maintain the lateral growth rate and the crystal quality at the same time.

Diamond is an attractive material for high power electronic devices because of its unique properties such as wide band-gap and high thermal conductivity. Heteroepitaxial growth of diamond offers the possibility of making devices over a large surface area. In order to characterize electrical properties of heteroepitaxial diamond and address its suitability for electronic application, a diamond device is fabricated from a nominally undoped heteroepitaxial diamond film. The diamond film was grown using plasma enhanced chemical vapor deposition (PECVD). Here the temperature-dependent DC electrical conductivity s (T) of heteroepitaxial diamond is reported. Owing to its extremely low conductivity at room temperature, measurements of current-voltage characteristics were made between 300 °C and 550 °C.
A particular technique was used to measure electrical conductivity of heteroepitaxial diamond at high temperature, which provided for a precise measurement circuits enabling thermal control with sufficient stability for electrical measurements of very small currents (<10^-13 A). Ohmic contacts with diamond and procedures for preparing diamond surface will be addressed. In this study, a single activation energy of 1.40 eV is found for the diamond device. The heteroepitaxial diamond conductivity profile closely matches that of previously studied natural Type IIa diamond. A thermally activated electrical conductivity s (T), with single activation energy 1.40 eV, does not match natural diamond, but does match previously studied homoepitaxial diamond, suggesting the presence of electronic states comparable with synthetic diamond.

Porous DDB morphology for the electrochemical oxidation process has been a focus of researches in recent years due to the unique DDB properties as well as the possibility to allow contact between the electrolyte and the material, raising the electrochemical area and enhancing the electrode efficiency. In this context, the film porosity may be controlled from nano to macroporous according to the substrate porosity or using some specific diamond surface post-growth treatment. Regardless of the material porous size or its applicability, it is normal to use a seeding pre-treatment with diamond powder to enhance the diamond growth by chemical vapor deposition (CVD) process. Nonetheless, we are presenting diamond films got without substrate seeding. This approach was obtained with success using titanium dioxide nanotube substrate (DTNT) produced on a titanium grid. This process allowed greater surface control and homogeneity. DTNT is a singular substrate because the formed hydrocarbon radicals originated from the methane/hydrogen mixture are used to grow diamond. From this system, the TiO₂ -TiC conversion enables and enhances diamond nucleation, which is also increased by the substrate porosity. Besides, carbide formation avoids the usual hydrogen attack observed on titanium substrate also preventing the titanium hydride evolutions. In this context, this work aiming at the ultranocrystalline boron-doped diamond film growth without seeding on DTNT electrochemically anodized on 3D Ti grid (B-UNCD_WS/DTNT/Ti_GRID). The samples were analyzed physically and morphologically by scanning electron microscopy (SEM), micro-Raman scattering spectroscopy, and X-ray diffraction. Electrochemical characterizations were performed regarding the B acceptor concentration evaluations from Mott-Shottky curves while from cyclic voltammetry the reversibility study in redox pair was performed. The Ti grid (25x25x1 mm) was cleaned in acetone and polished in acid solution. Subsequently, the sample was anodized in a 40oC glycerol solution containing fluoride for 3h under 60 V. The counter electrode was a Pt network as an open cylinder that was able to evolve the Ti grid for both sides assuring their simultaneous anodization. After anodization, the sample was put in a hot filament CVD reactor to grow B-UNCD without the diamond powder seeding process. Diamond growth was also provided on the sample backside following a similar procedure. SEM images confirmed a continuous and uniform 3D film following the substrate morphology for both sample sides. The pore bottoms and walls were covered by ultrananocrystalline diamond in addition to the CVD ballas diamond clusters with their density increase with increasing the film growth time. Also, Raman spectra, as well as XRD diffractograms, confirmed the B-UNCD_WS/DTNT/Ti_GRID quality and crystallinity. Their good electrochemical response with doping level from 10¹⁸ to 10²⁰ B.cm^-3 and satisfactory reversibility in a redox pair confirmed them as promising new electrode material, mainly for anodes in the electrooxidation process.

Nanodiamond (ND) synthesis by nanosecond laser irradiation has sparked a tremendous scientific and technological interest. Herein we have demonstrated the conversion of PTFE (low thermal conductivity polymer) into NDs by the melting route. Notably, UV laser irradiation of PTFE at low energy densities (0.6-0.8 Jcm^-2) was effective for amorphization and subsequent melting of amorphous PTFE to obtain NDs. High energy densities (>1 Jcm^-2) generated ablation with minimal control over amorphization and melting. COMSOL simulations yielded a melt regrowth velocity of 5.6 m/s. We also noted that the graphitic content in these NDs may be controlled by altering the number of shots. Finally, this ND film was utilized as a seed layer to selectively generate adherent and dense CVD microdiamond film.

Nanoscale diamond particles, generally known as nanodiamonds (NDs), have several outstanding material qualities, offering a wide range of potential for basic science and industrial applications. In particular, a number of optically addressable impurity defects, such as nitrogen-vacancy (NV) centers residing in the diamond lattice, have been deployed for next-generation quantum technologies due to their unique spin properties. Currently, there are mainly two methods for the large-scale fabrication of NDs: “bottom-up” detonation NDs and “top-down” milling of bulk high-pressure high-temperature (HPHT) diamond. It is well known that the raw ND powders (i.e., detonation or HPHT) contain a considerable amount of unwanted impurities [e.g., ultra-small (<10 nm) sized NDs, disordered carbons, metal, and metal oxides], which are naturally introduced during synthesis and processing. And previous reports demonstrated that the non-diamond phases present at the surface of NDs are detrimental to the properties of embedded quantum defects (e.g., NV centers). Therefore, the removal of those impurities becomes a critical step before the ultimate applications of the NDs. Several purification methods, like the air oxidation (at 400 - 600 °C) and wet chemical (e.g., HNO₃/H₂SO₄/HClO₄, molten KNO₃) treatments, have been adopted to remove the adsorbed impurities on NDs. Despite the considerable effort devoted to overcoming these challenges, it is still difficult to obtain NDs with a well-defined surface, especially when examined at the individual particle level.
Here, we developed a simple, reliable and reproducible purification method, namely the salt-assisted air oxidation (SAAO) treatment, requiring only one additional pre-step, i.e., mixing NDs with a proper amount of salt crystals (e.g., sodium chloride) prior to conventional air oxidation method. Our inspiration for this method came from the cooking procedure used in the preparation of a well-loved Chinese snack, fried chestnuts with stones. The role of the salt crystals added in NDs powder is closely analogous to that of the stones added in the Chinese snack, and ensures that the NDs (the “chestnuts”) are evenly and thoroughly etched by SSAO. This method rounds the original shard-like shape of the NDs with clean surface, and can be scaled up to permit their manufacture in large quantities. We also investigated the underlying mechanism of the SSAO process, i.e., the “salt-assisted etching atmosphere” would easily etch away non-diamond impurities and significantly accelerate the oxidation of NDs as well. These findings will significantly enhance the scope of these little gemstones in diverse scientific and industrial fields, particularly in demanding areas such as biomedical and quantum sensing requiring stable and sound surface functionalities.

The long spin coherence time of NV center electrons make it particularly attractive for bio-sensing applications. The NV centers are particularly sensitive to the magnetic field projected along the NV axis, so that orientation change of a diamond sample (containing NVs) would cause detectable modulation in its optically detected magnetic resonance spectrum, a method known as the vector magnetometry. This method has excellent spatial resolution and measurement sensitivity on the orientation change of NVs in the diamond sample. In the present work, we will discuss a newly developed scheme to investigate the mechanical properties of soft materials using diamond nanoparticles as the quantum sensors. The feasibility of the method is firstly demonstrated using polydimethylsiloxane (PDMS) film and gelatin microparticle. The excellent sensitivity and spatial resolution associated with such a technique enable the disclosure of heterostructured nature of the former, and effect of surface tension in the latter. We then study the mechanical properties of fixed Hela cells using the nanodiamond-sensing-enabled non-local deformation measurement, enabling observation of the competition between elasticity and capillarity upon indentations on cells. The works are carried out in collaboration with Renbao Liu, Yue Cui, Wenghang Leong, Kangwei Xia, and Chufeng Liu. We acknowledge funding from GRF of RGC (Project No. 14300720), CRF of RGC (Project No. C4007-19G); and ANR/RGC (project No. A-CUHK404/18).

Individual, luminescent point defects in diamond termed color centers are stable, atomically small quantum systems. Such individual quantum systems are extremely sensitive sensors, e.g., for magnetic fields. Their strength is to harness quantum mechanics for ultimate sensitivity and versatility while simultaneously boosting spatial resolution in imaging.
To fully exploit the potential of this novel class of sensors, dedicated diamond materials as well as nanostructures are needed. We here present our work on using large area, heteroepitaxial diamond for sensing applications. We also discuss recent results on integrating diamond with two-dimensional, luminescent transition metal dichalcogenides to establish a hybrid sensor material. The talk will furthermore summarize recent advances in fabricating diamond nanostructures for sensing. In addition to material and process optimization, we employ optimal control algorithms to shape the laser pulses as well as microwave pulses that adress the color center spins.

Nitrogen-vacancy (NV) center spins in diamond are good quantum sensors for their long quantum coherence times. In particular, quantum coherence of NV centers can be manipulated even at high temperature. Therefore, we can use NV centers to detect magnetic transitions and fluctuations of nanoparticles for a broad range of materials. Here we will present examples of controlling quantum coherence of NV centers at high temperature, detection of magnetic transitions of single nanoparticles, measurement of magnetic fluctuations near Curier temperatures of magnetic nanoparticles, and methods for improving magnetometry sensitivity near zero magnetic fields (which is important for non-invasive study of magnetic properties of materials).

This work was supported by Hong Kong RGC Collaborative Research Fund Project C4006-17G and was done in collaboration with Q. Li, G.-Q. Liu, C. F. Liu, N. Wang, K. Xia, W.-H. Leong, T. Zhang, X. Feng, M. H. Kwok, H. Zeng, S. H. Li, F. Dolde, A. Finkler, A. Denisenko, S. Yang, J. Wrachtrup, X. D. Cui, and H. Fedder.

References:
1. C. F. Liu et al, National Science Review 8, nwaa194 (2021).
2. G. Q. Liu et al, Nature Communications 10, 1344 (2019).
3. T. Zhang et al, Nature Communications 9, 3188 (2018).
4. N. Wang et al, Physical Review X 8, 011042 (2018).

Diamond is expected to be the semiconductor material of the next-generation high-power and high-frequency devices as well as quantum technologies because of its superior physical and electronic properties. Etching is one of the most important techniques for semiconductor wafer and device fabrication processes. The etching techniques for SiC and GaN as next generation power semiconductor materials have been already established. However, the diamond etching technique has not yet reached the required quality for semiconductor wafer and device processing because of the inherent difficulties arising by its extremely high physical hardness and chemical stability.
Recently, we have reported a novel diamond etching process based on the carbon (C) solid solution into Ni [1-5]. The etching is a solid-solid reaction, which occurs at Ni/diamond interfaces. Therefore, the process can selectively etch diamond surfaces in contact with Ni. We also demonstrated diamond imprint lithography with Ni mold [2] and surface planarization with flat Ni plates [3] based on this novel diamond etching process. Additionally, the diamond etching shows a high etching rate (~10 μm/min) and strong anisotropy [4,5], which is similar to the anisotropic Si etching using KOH solution for device fabrication. This novel diamond etching processes will be introduced in detail.

Acknowledgements
This work was partially supported by Adaptable and Seamless Technology transfer Program through Target-driven R&D (A-STEP) from Japan Science and Technology Agency (JST), Kanazawa University SAKIGAKE Project 2020 and JSPS KAKENHI Grant Number JP18KK0383.

References
[1] K. Nakanishi, N. Tokuda et al., Dia. Relat. Mater. 68 (2016) 127.
[2] T. Tabakoya, N. Tokuda, Dia. Relat. Mater. 114 (20121 108294.
[3] K. Sakauchi, N. Tokuda et al., Dia. Relat. Mater. 116 (2021) 108390.
[4] M. Nagai, N. Tokuda et al., Sci. Rep. 8 (2018) 6687.
[5] M. Nagai, N. Tokuda et al., Dia. Relat. Mater. 103 (2020) 107713.

Diamond is perhaps the most attractive candidate for ultra-high voltage (multi-kV) power electronics due to its large bandgap (5.47 eV), high breakdown electric field (10 MV/cm), high carrier mobility, high thermal conductivity (10-20 W/cm.K), and low coefficient of thermal expansion (~1.1um/mK) [1]. Thus, the development of diamond Schottky barrier diodes and PIN diodes may lead to much more efficient power electronics. The diamond diodes fabricated in this work have shown superlinear current-voltage curves in the forward bias region following the Mott-Gurney law, allowing for higher on-current densities at lower voltages.
In this work, intrinsic layers of varying thicknesses (~0.5-7um) and n-type top layers of 100nm with up to 5e19 /cm³ phosphorous doping were grown on (111) and (100)-oriented diamond p+ (type IIb diamond) substrates by Microwave Plasma Enhanced Chemical Vapor Deposition. To fabricate the diode, Ti/Pt/Au was evaporated on the backside of the p+ diamond plate to form an ohmic contact, and depending on the crystal orientation, the circular contacts made to the n-layer on top were either ohmic or Schottky. Because phosphorous incorporation is significantly improved on (111) oriented surfaces as opposed to (100) substrates, the diodes on the (111) substrate have shown PIN diode behavior, whereas the diodes based on the (100) substrates exhibit Schottky diode behavior [3].
In our Schottky devices, we have observed that the turn on characteristics can be analyzed through the thermionic emission equation J_F=A^*T² exp(-qΦ_B/(kT))[exp(-qV_F/(nkT)-1)], where A^* is the Richardson constant, Φ_B is the Schottky barrier height, and n is the ideality factor [2][3]. However, at higher voltages after turn-on, the current exhibits Mott-Gurney behavior or space charge limited current. Because of the low doping densities in the intrinsic layers of the devices, the total on current at higher bias regions are instead dominated by charge carriers injected from the highly doped adjacent regions as opposed to the intrinsic carriers present in the drift region. As a result, the current can be described by the equation J_MG=(9/8)ε₀ε_rμV²/d³, where ε_r is the relative permittivity of diamond, μ is the carrier mobility, V is the bias voltage, and d is the drift region thickness [2]. This behavior, a characteristic notable in vacuum diodes, allows for greater current densities at lower forward biases in comparison to traditional diode behavior which typically exhibits linear behavior based in Ohm’s law. Diode current increasing as the square of the voltage applied across it should be viewed favorably in power electronics as it may allow for lower voltage operation at equivalent current output, thus leading to lower power consumption and reduction of thermal runaway effects. This observation of Mott-Gurney’s law is an indication of highly developed diamond growth techniques producing high quality material, as well as optimized diode fabrication. Additionally, it was observed that as the intrinsic layer thicknesses were increased, the turn on voltage and onset of Mott-Gurney effect was shifted to higher voltages. Also, by elevating the temperature (up to 400^oC) the current was increased due to increased ionized carriers and a lower turn-on voltage was observed. As a result, the onset of Mott-Gurney effect was shifted to lower voltages as well.

This work was supported by ULTRA, an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE).

[1] Tsao, J. Y., et al., Adv. Electron. Mater., 4, 1600501 (2018).
[2] Brezeanu, M, et al,. Diamond and Related Materials, Volume 17, Issues 4–5, 2008, 736-740.
[3] Dutta, M, et al., IEEE Electron Device Letters, Vol. 35, Num. 5, 2017

Diamond based electronics could exploit the unique materials properties (carrier mobilities, breakdown voltage, thermal characteristics) that would enable superior devices in particular for high-power, high frequency applications. Practical solid state devices necessitate n-type and p-type material with controlled properties specifically the doping concentration and its profile, both, in the bulk and at interfaces. Interface or near surface doping properties will also be important for heterostructures. While abrupt doping profiles should provide preferred performance, these abrupt interfaces are not always observed and cannot always be reliably prepared. As a result, graded junctions would then define device characteristics like current flow, switching speed and breakdown voltage. We present results for phosphorus doped homoepitaxial diamond on (111) substrates. The epitaxial layers were prepared by microwave-plasma assisted chemical vapor deposition utilizing a 200ppm trimethylphosphine (TMP) in hydrogen mixture as dopant source in addition to hydrogen and methane to achieve a phosphorus concentration > 1x10¹⁹ cm^-3. With a dual-wavelength pyrometer and optical emission spectroscopy, the in-situ growth characterization was related to results from secondary ion-mass spectroscopy (SIMS), which was utilized to determine the doping profile of the epitaxial diamond layer. Phosphorus doped homoepitaxial diamond layers were grown on intrinsic CVD substrates in various plasma focusing geometries and a relation to the doping profile was established. With enhanced plasma focusing a shift toward a more abrupt doping profile was observed. Optical emission spectroscopy of the phosphorus related signal (930nm) during growth indicated a relation to the doping profile. With dual-wavelength pyrometer temperature measurements a small change (5°C orabout 0.5%) in the growth temperature resulted in a change of about 7% in the doping concentration. These observations indicated the sensitivity of the phosphorus incorporation on the growth temperature and availability of donor species in the reactor. We will further elaborate on the effects of the substrate surface (miscut angle, polishing) on the phosphorus incorporation.

This work was supported as part of ULTRA, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under award DE-SC0021230.

Diamond outstanding properties make it a promising candidate for its application on high-power rectifying electronic devices. The intrinsic nature of diamond is related to the diamond electronic devices strong dependence on doping, and surface/interface phenomena. The understanding and control of such phenomena is key for the development of reliable electronic devices that can compete with current silicon-based solutions.
The position of the Fermi level is a fundamental parameter that has served for the prediction of the ideal band diagram of electronic devices and is also required for the extraction of some of its fundamental operational parameters. Among others, the Schottky barrier height of metal-diamond junctions can be estimated based on Capacitance-Voltage (C-V) measurements by which the resulting built-in potential is added to the Fermi level to diamond valence band energy distance. For Metal-oxide-diamond structures the ideal flat band voltage is obtained as the metal-diamond Fermi level distance, which allows to identify and estimate the oxide charge net density by comparison to the experimental C-V value. Other techniques such as Kelvin Probe Atomic Force Microscopy or X-ray photoelectron spectroscopy also allows the determination of the Fermi level position and requires theoretical values for the characterization of the diamond electronic phenomena.
Besides its wide application, the calculation for the diamond Fermi level position is commonly made based on numerical calculations. In the present contribution, an analytical expression for the Fermi level on diamond is deduced in order to simplify diamond-based electronic device design and analysis procedures. The limits of this expression, i.e. the validity of the Boltzmann approximation, are discussed and its application to some real cases shown.

Diamond semiconductor can be used to develop high-power devices owing to its wide bandgap of 5.47 eV and superior properties compared to SiC and GaN. Till date, we have demonstrated field-effect transistors (FETs) with NO₂ p-type doping on hydrogen-terminated diamond (H-diamond) with an output power density of 345 MW/cm² [1]. However, conventionally doped diamond FET structures have several problems, such as lower hole mobility (µ) (~100 cm²/V s) in the channel than in the bulk (>1200 cm²/V s) and degradation of hole channel during lengthy operations. To overcome these limitations, we propose modulation or selective doping in diamond. We separated the NO₂ acceptor impurities from the hole channel on H-diamond using an Al₂O₃ gate insulator layer in the FET structure.
First, we investigated the modulation-doping effect by employing Hall effect measurements of a modulation-doped sample with the structure Al₂O₃/NO₂/Al₂O₃ spacer layer/H-diamond by changing the thickness of Al₂O₃ spacer layer. Kenzan Diamonds^®, high-quality (001) heteroepitaxial diamonds [2]. The results revealed that the sample with an Al₂O₃ spacer layer thickness of 8 nm exhibited maximum mobility by maintaining high hole sheet concentration. Thus, we observed the effects of modulation doping unambiguously.
Second, we performed temperature-variable Hall effect measurements of the modulation-doped sample and observed that the high mobility and hole sheet concentration were retained until at least 500 K. Thus, the thermal stability was improved. The low temperature dependence of mobility exhibited the characteristics of modulation doping.
Next, we fabricated modulation-doped diamond FETs. The NO₂ p-type doping was performed on the H-diamond surface to form the access channels and on the 8-nm-thick first Al₂O₃ gate insulator layer. The diamond FETs exhibited a maximum drain current density of −627 mA/mm and maximum transconductance of 131 mS/mm. The effective mobility increased to 2465 cm²/V s near the gate threshold voltage, which was a factor of 3.7 greater than that of the conventionally doped FETs, and was near the highest reported value (3800 cm²/V s) in the purest form of diamond. The off-drain voltage was –703 V and the on-state resistance was 37.55 Ω mm. Consequently, Baliga’s figure-of-merit, an available output power density of the modulation-doped FET, was obtained experimentally as 179 MW/cm².
[1] N. C. Saha et al., IEEE Electron Device Lett., 41, 1066 (2020).
[2] S.-W. Kim et al., Appl. Phys. Lett., 117, 202102 (2020).

High-quality nitrogen-doped diamond is expected to be applied for power devices and high-sensitive quantum sensors at room temperature [1,2]. For example, for n-channel MOSFETs, to reduce the contact resistance, it is important to use diamond films with high nitrogen concentration ([N]). In addition, for high-sensitive quantum sensors, to improve the sensor sensitivity, it is important to increase [N] involving high [NV^-] with high film quality. However, previous studies have reported that high [N] doping causes nucleation and deterioration of film quality, but the details about nucleation mechanism are not yet known [3,4]. Thus, in this study, we investigated the effect of N-doped growth on nucleation and used lateral growth modes to eliminate nucleation factors other than N-doping [5,6]. Furthermore, we analyzed the nucleation mechanism using first-principles calculations. As a result, it was found that the carbon atoms in the plasma are more easily adsorbed by the nitrogen atoms than the carbon atoms on the substrate. Moreover, by optimizing the N-doped lateral growth conditions, we succeeded in developing a growth technique that suppresses 2D nucleation as much as possible.
Atomically flat diamond surfaces were grown by homoepitaxial lateral growth on HPHT Ib type diamond (111) mesas in order to prevent 2D nucleation and hillocks due to dislocations in the diamond substrate; then homoepitaxial N-doped growth was carried out under N₂/CH₄ = 0 ~ 2000 % for 10 min. The surface morphology was evaluated with a laser microscope and an atomic force microscope. The nucleation mechanism was examined by the potential energy surface (PES) calculation using Octopus as a simulator.

Acknowledgements: This work supported by Kanazawa University SAKIGAKE Project 2020 and Q-LEAP.

[1] T. Matsumoto et al., Sci. Rep. 6, 31585 (2016).
[3] H. Ozawa et al., Phys. Status Solidi A 215, 1800342 (2018).
[4] E. Boettger et al., J. Appl. Phys. 77, 6332 (1995).
[5] A. T. Sowers et al., J. Appl. Phys. 86, 3973 (1999)
[6] N. Tokuda et al., Jpn. J. Appl. Phys. 51, 9R (2012).
[7] H. Ozawa et al., Appl. Phys. Express 10, 045501 (2017).

Inverse design, which predicts the required structure based on a desired property, is a necessary part of industrial design. Property/structure relationships of materials are far more difficult that the forward structure/property relationships, however, since there could be numerous combinations of materials characteristics that present the same properties and conventional predictions do not distinguish between them. They are even more challenge in nanomaterial design, where the design space is even greater, and materials are prized for their multi-functionality. Optimization based on hypothetical databases predicted using machine learning has shown promise in recent years, but suffers from a high computational cost, lack of specificity, and no guarantees that the optimal nanomaterial has been found. There are simply too many unknown variables (structural characteristics) and not enough known variables (properties) for optimization to be reliable. In this presentation we will describe an alternative approach to inverse design that overcomes each of these limitations. By drawing on the multi-functionality of nanomaterials and using multi-target machine learning methods, we develop a workflow that is fast, easy to use, and predicts the characteristics of a single nanoparticle that simultaneously meets a set of performance criteria, with a fault tolerance. The method focusses the outcome on the most important characteristics in an entirely data-driven way, and with comparable accuracy and generalizability as traditional forward structure/property machine learning predictions. We have demonstrated the new inverse design workflow on diamond nanoparticles, an important platform with significant potential for a range of industrial applications.

Diamond’s unique geometric and electronic properties have gathered interest, for it can undergo a surface transfer doping mechanism which can then be used to build high-field, high-frequency device applications [1]. Recent experimental evidence suggests that adsorbing NxOy functional groups on hydrogen-terminated diamond surfaces could play an important role in improving device parameters such as current density and threshold voltage [2, 3]. In this research, we performed a computational study on the adsorption of the NxOy functional groups on diamond (100) surfaces using Density Functional Theory (DFT). The adsorption energies were calculated to predict the most favorable structure. Band structures and density of states will be reported to explain the electronic properties of the most favorable system for device design. Nudged Elastic Band calculations will also be reported for the adsorption pathways to form these most stable adsorbate configurations. This research will present several models of NxOy functional groups on diamond (100) surfaces and will provide critical insight into surface electronic properties of the diamond surfaces for surface doped diamond devices.

Acknowledgement: ORAU-Journeyman Fellowship sponsored by CCDC US Army Research Laboratory, Adelphi, MD

1. K. G. Crawford and et. al, IEEE Transactions on Electron Devices, 67, 6 (2020).
2. M. Kasu, Jpn. J. Appl. Phys. 56, 01AA01 (2017).
3. N. C. Saha et al., IEEE Electron Device Letters, 42 (6), 903-906 (2021).

Boron-doped diamond (BDD) electrodes are very attractive material, because of their wide potential window, low background current, chemical inertness, and mechanical durability.[1] In these years, we have reported several examples for electrochemical sensor applications.[2] Here, novel microsensing systems for in vivo real time detection of local drug kinetics are reported. Furthermore, applications for electrochemical organic synthesis[3] including CO₂ reduction[4], and electrochemiluminescence (ECL) systems[5] are also reported.
Microsensing system for in vivo real time detection of local drug kinetics[2b]
We have developed a microsensing system for in vivo real time detection of local drug kinetics and its physiological relevance. The system consists of two different sensors of both a micro-sensor composed of BDD microelectrodes with tip diameter ~40 μm and a glass microelectrode. By using the system, we have first tested bumetanide, a diuretic that is ototoxic but applicable to epilepsy treatment.
CO₂ reduction[4]
We investigated the electrochemical reduction of CO₂ to HCOOH in a flow cell using BDD electrodes. The faradaic efficiency (FE) for the production of HCOOH was as high as 94.7%. The selectivity for the production of HCOOH was more than 99%. Furthermore, recently, by optimizing certain parameters and conditions used in the electrochemical process with BDD electrodes, such as the electrolyte, the boron concentration of the BDD electrode, and the applied potential, we were able to control the selectivity and efficiency with which carbon monoxide is produced.[4c] Recently, an intermittent flow cell system was presented. A stop-start motion of the flow conditions in the intermittent cell was created using a piston pump, and this considerably increases the rate of electrochemical conversion of CO₂ to HCOOH compared to a continuous flow system.[4d]
Electrochemiluminescence (ECL)[5]
A novel coreactant-free electrogenerated chemiluminescence (ECL) system is developed where Ru(bpy)₃²⁺ emission was obtained on BDD electrodes. The method exploits the unique ability of BDD to operate at very high oxidation potential in aqueous solutions and to promote the conversion of inert SO₄²⁻ into the reactive coreactant S₂O₈²⁻. This novel procedure was rather straightforward, not requiring any particular electrode geometry, and, since the coreactant was only generated in situ the interference with biological samples was minimized. The underlying mechanism was similar to that of the Ru(bpy)₃²⁺/S₂O₈²⁻ system. Furthermore, recently, another ECL system was presented. The system took advantage of the unique properties of BDD to promote oxidation of carbonate (CO₃²⁻) into peroxydicarbonate (C₂O₆²⁻), which further reacted with water to form hydrogen peroxide (H₂O₂), which acted as a coreactant for Ru(bpy)₃²⁺ ECL.

References
Bull. Chem. Soc. Jpn., 91, 1752 (2018)., Chem. Soc. Rev., 48, 157 (2018).
(a) Proc. Natl. Acad. Sci., 113, 8981 (2016). (b) Nat. Biomed. Eng., 1, 654 (2017). (c) Science, 359, 1287 (2018). (d) Anal. Chem., 92, 13742 (2020).
Angew. Chem. Int. Ed., 51, 5443 (2012).
Angew. Chem. Int. Ed., 53, 871 (2014). (b) Angew. Chem. Int. Ed., 57, 2639 (2018). (c) J. Am. Chem. Soc., 141, 7414 (2019). (d) ACS Sustainable Chem. Eng., 9, 5298 (2021).
(a) J. Am. Chem. Soc., 138, 15639 (2016). (b) J. Am. Chem. Soc., 142, 1518 (2020).

Boron Nitride is an ultra-wide bandgap material with significant potential as an active or dielectric layer in high-power switches, high-frequency transistors, diodes, detectors, and other applications. The cubic phase (c-BN) is a semiconductor with a 6.4 eV bandgap and a near lattice match to diamond. The layered hexagonal phase (h-BN) may perform as a dielectric in hydrogen terminated-diamond field effect transistors. High-quality BN / diamond heterostructures (whether as heteroepitaxial semiconductor or as a gate dielectric) requires a smooth interface. At Arizona State University, we have grown BN thin films on B-doped, p-type diamond substrates using electron cyclotron resonance microwave plasma chemical vapor deposition (ECR MPCVD). By varying the growth conditions, the BN layer can have predominant c-BN or h-BN structure. Results show that a smooth, flat interface can be achieved for layers that are predominantly h-BN, while some c-BN layers show island growth and relatively rough interface structures. The morphology of the interface and growth surface is characterized with optical phase contrast microscopy and cross section TEM. The electrical properties of the mixed phase c/h-BN are electrically characterized using lithographically patterned contacts. Results correlate the growth parameters with the morphology and electrical properties of the c/h-BN – diamond heterostructure.
This work was supported as part of ULTRA, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under award DE-SC0021230.

Owing to its superior mechanical and tribological properties, diamond is an attractive candidate used in various applications, including micro / nanoelectromechanical systems (MEMS / NEMS) [1]. Although single crystal diamond (SCD) has been successfully integrated and used for such MEMS / NEMS based devices, high fabrication costs and limited upscaling hinders its wide industrial usage. Such bottlenecks can be mitigated using nanocrystalline diamond (NCD) and (U)NCD as an alternative. Furthermore, with the availability of large-area diamond deposition systems, such as the linear antenna microwave CVD (LA MW CVD), such (U)NCD based devices can potentially be fabricated on an industrial scale [2]. However, very few studies have reported stress studies, an essential parameter for device integration, in NCD films grown in such systems [3].

In this work, we present residual stress studies in NCD films (~ 250 nm thick) deposited on silicon substrates in the LA MW CVD system. We compare intrinsic and doped films grown in a H₂/CH₄/CO₂ plasma and with varying phosphine (PH₃) gas concentrations up to a maximum [P]/[C] ratio of 8090 ppm and substrate temperatures between 400°C and 900°C while maintaining working pressure at 0.23 mbar and MW power at 2800 W process conditions.

Scanning electron microscopy results reveal that under increasing [P]/[C] ratios, the film morphology transitions from NCD with randomly facetted grains to ultra-NCD-like with dendrite features. Similar morphology changes also occur at higher substrate temperatures above 800°C. Raman analysis verifies diamond formation, and the corresponding sp³/sp² bonded carbon ratios correlate well with the associated changes in film morphology. Substrate curvature measurements reveal compressive stress in all the samples in this study, with residual stress decreasing under increasing [P]/[C] concentrations in the plasma and higher substate temperatures. The lowest value of -0.25 GPa is observed for the NCD films grown with a [P]/[C] ratio of 8090 ppm and 900°C substrate temperature. Furthermore, transmission electron microscopy results indicate that additional impurities, such as silicon and oxygen, are present in the deposited films.

Based on the above results, a qualitative relationship between the sp³ content and projected mean grain size is proposed and the origin and the role of impurities in residual stress evolution in LA MW CVD diamond films is discussed. Finally, X-ray photoelectron spectroscopy results that indicate successful phosphorus incorporation into the carbon matrix for the layers deposited at 900°C are presented.

References
[1] O. Auciello and D. M. Aslam, Journal of Materials Science 56, 7171-7230 (2021).
[2] S. Drijkoningen et al., Scientific Reports 6, 35667 (2016).
[3] A. Taylor et al., Diamond & Related Materials 69, 13-18 (2016).

Nitrogen is the most common impurity in diamond and forms different types of color centers, particularly via combination of one or more N atoms with vacancies. Traditional production of fluorescent diamond particles focuses primarily on the introduction vacancies via irradiation with high energy electrons and subsequent thermal annealing to promote vacancy diffusion and formation of stable optical centers with N. The N content and state are inherited from diamond synthesis and N rearrangement via diffusion is typically not considered for modifications during traditional post-synthesis processing. As a result, the fluorescent color palette is typically restricted to particles emitting only in red or green, either from NV centers produced in type Ib synthetic diamond containing single N atoms, or NVN centers produced in type Ia natural diamond containing N pairs, respectively. Recently, we demonstrated that ultra-high temperature annealing (>1500 ^oC) facilitating N diffusion in the lattice, generates conditions for the controlled formation of one-, two- and three-atom nitrogen complexes with vacancies in electron irradiated type Ib synthetic diamond, providing vibrant luminescence in the red (NV), green (NVN) and blue (N3 centers) spectral ranges [1]. Remarkably, this treatment also enhanced the optically induced ¹³C hyperpolarization [2], which promotes a new paradigm in dual-mode fluorescence/magnetic resonance imaging enabled by multicolor fluorescent nanodiamonds [3]. Ultra-high temperature annealing also significantly impacts the quantum properties of NV centers, leading to increased spin-lattice (T1) and spin-spin (T2, T2*) coherence times, increased ODMR contrast, and magnetically induced fluorescence contrast. This opens routes for multiplexed labeling in combination with sensing when NV centers coexist with NVN or N3 centers within individual particles. We discuss opportunities for nano- and microparticulate diamond in a number of imaging and sensing applications in biological and industrial fields based on controlled and reproducible formation of specific color centers previously not possible in type Ib synthetic diamond.
[1] L. Dei Cas, S. Zeldin, N. Nunn, et al., From Fancy Blue to Red: Controlled Production of a Vibrant Color Spectrum of Fluorescent Diamond Particles, Advanced Functional Materials 2019, 1808362.
[2] M. Gierth, V. Krespach, Al. I. Shames, et al., Enhanced Optical 13C Hyperpolarization in Diamond Treated by High-Temperature Rapid Thermal Annealing, Adv. Quantum Technol, 2000050 (2020).
[3] X. Lv, J. Walton, E. Druga, et al., Background-free dual-mode optical and 13C magnetic resonance imaging in diamond particles, PNAS, 2021 118 (21) e2023579118.

The chemical, physical and biological properties of a material are strongly related to its surface. For nanodiamond (ND), this holds true as well. And its surface can be modified easily using its broad variety of surface groups.[1] Thus, ND is attractive for applications in biological environments as it also possesss high biocompatibility and low toxicity. Further, it is available in large quantities and additional characteristics, such as luminescence from lattice defects make them candidates for applications such as imaging, drug delivery etc.

However, when nanoparticles are dispersed in typicyl cell culture media or body fluids, proteins from these media are adsorbed and a so-called “protein corona” forms around the particles. This corona can mask the actual functional moieties of the surface and is rather strongly attached by non-covalent interactions. Both, the colloidal stability and the physiological properties are not controllable anymore.[2]
Here, we report nanodiamond functionalized with a multifunctional surface termination,[3] consisting of zwitterionic moieties as headgroups in combination with oligoethylene glycol chains attached to hydrophobic chains to improve the colloidal dispersion in buffered cell culture media and the prevention of protein corona formation. Further, the attachment and effect of other charged surface groups such as small peptides will be discussed.
This project has received funding from the Volkswagenstiftung under Grant Agreement no. 88393
[1] A. Krueger, D. Lang, Adv. Funct. Mater. 22 (2012), 890.
[2] W. C. W. Chan, C. D. Walkey, J. Am. Chem. Soc. 134 (2012), 2139.
[3] V. Merz, J. Lenhart, Y. Vonhausen, M. E. Ortiz Soto, J. Seibel, A. Krueger, Small (2019) 1901551.

Detonation nanodiamonds (DNDs) are diamond nanoparticles synthesized through detonation process with a size-range of 3-5nm. Their small size, non-toxicity and biocompatibility has drawn huge interests, particularly in the biomedical field. To capitalize the large total surface area and flexible surface chemistry afforded by the unique particle size and surface structure, understanding the DND dispersion behaviour has been one of the endeavours to improve their application potential in the biomedical field.
We have recently shown that purified DNDs dispersed in water, without any additional surface modification, exhibit a striking lacey network-like morphology, formed dynamically by self-assembled chain-like structures. This discovery was carried out using direct cryo-TEM imaging of the DND dispersions. Moreover, using cryo-TEM electron tomography, we found that there is a preferential orientation of the DND aggregates that results in a 2-D planar superstructure. These new findings have significant impact on the performance of DNDs, particularly for biomedical applications. For example, in drug delivery applications, the monodispersed DNDs offer higher surface area that can promote drug loading whereas the formation of aggregate can slow down the release.
In light of the importance of the DND dispersion and their aggregate morphology distributions to the biomedical applications, we have conducted a systematic study of purified and surface functionalized DNDs dispersed in biomedically relevant media with varying pH and saline content. The large datasets generated in our studies using cryo-TEM was analyzed using our recently developed machine learning (ML) based approach. The ML analysis from cryo-TEM datasets on DND assembly will offer direct and quantitative information regarding the aggregation behavior as well as its size and morphology distribution in biomedical relevant media which is a crucial step towards a better design of DNDs for biomedical and further applications.

The significantly high thermal conductivity of diamond up to 2200 W/m.K makes it an ideal choice for heat spreading applications by integration to high-frequency high-power devices such as GaN high-electron-mobility-transistors (HEMTs). One of the approaches for diamond integration to GaN HEMTs is chemical vapor deposition (CVD) of polycrystalline (PC) diamond on top of the device [1]. PC diamond has a lower thermal conductivity (100-1000 W/m.K) compared with single crystal diamond depending on its average grain size [2]. However, this lower thermal conductivity is still higher than any other competing materials such as Si₃N₄, GaN, AlGaN, and SiC. Besides thermal conductivity, the thermal boundary resistance (TBR) between the diamond heat spreading layer and GaN device is a key property which determines diamond cooling capabilities, as it can change the whole GaN channel thermal resistance. The smoothness/abruptness of the interface between diamond and GaN along with the nature of diamond nucleation during the CVD growth defines the TBR value.
Due to the presence of high temperature H₂-plasma in the CVD chamber during diamond growth, it is challenging to grow diamond without any protection layer on top of the GaN. This protection layer (MOCVD-grown Si₃N₄) not only protects the GaN channel from exposure to H₂-plasma and decomposition but also enhances diamond adhesion to the substrate by promoting covalent bonding. Carbon diffusion inside the Si₃N₄ replaces some of the nitrogen atoms (2N + 3H₂ ↔ 2NH₃) and transforms a portion of this layer to SiC which forms covalent bonds with diamond [3]. However, it is essential to identify which carbon state is nucleating on SiC in the first 1-2 nm of the diamond nucleation layer. It can be amorphous carbon (sp² a-C) or sp³ diamond depending on the nucleation recipe before the main growth. The ideal case is to nucleate sp³ carbon states on SiC as it yields a lower thermal resistance at the interface.
In this work, we identify the local carbon state by using scanning tunneling electron microscopy (STEM) to perform electron energy loss spectroscopy (EELS), a technique which studies the excitations from the 1s orbital of carbon to the states above the Fermi level (K-edge) [4]. This study showed that by using a low-density CH₄/H₂-plasma and high temperature (> 700 ^oC) for diamond nucleation, a monolayer of sp² a-C is first formed on the surface and then the carbon state gradually transforms to sp³ diamond. With this nucleation method, carbon only diffused ~1 nm inside the Si₃N₄ protection layer. On the other hand, when using a high density CH₄/H₂-plasma at lower temperatures (~ 600 ^oC) no a-C peak was observed on the K-edge intensity mapping spectra and sp³ carbon immediately nucleated on SiC layer. In this sample carbon diffused into the Si₃N₄ layer ~ 5 nm due to the highly energized CH₃⁺ radicals in the chamber. According to the Si₃N₄ thickness before the diamond growth and the target use of the remaining portion of the Si₃N₄ we can select accurate plasma parameters (microwave power, chamber pressure, CH₄concentration, and sample surface temperature) for the nucleation stage of the diamond growth. Our nucleation recipe achieved both strong adhesion of the diamond to the substrate and low thermal resistance interface formation enabling high efficiency channel cooling capabilities at higher power levels.

This work was partially supported by the Semiconductor Research Corporation (SRC) and DARPA under the JUMP program.

[1] M. Malakoutian et al., Crystals, vol. 9, num. 10, p. 498, 2019.
[2] J. Anaya et al., Acta Materialia, vol. 103, pp. 141–152, 2016.
[3] M. Malakoutian et al., Cryst. Growth Des., vol. 21, num. 5, pp. 2624–2632, 2021.
[4] D. Muller et al., Nature, vol. 366, pp. 725–727, 1993.