Symposium F.EL01—Diamond and Diamond Heterojunctions—From Growth to Applications

In photoconductive semiconductor switches (PCSS), the conductance, measured between the two electrodes placed on the semiconductor, is modulated by the absorption of optical radiation in the gap between them. This switching device can be applied to pulsed power technology for high speed and jitter free switching. These may include radar, particle acceleration, and pulsed high-power lasers. However, typical materials studied for photoconductive switching, such as Si and GaAs suffer from thermal runaway effects at high power [1]. Since diamond is an ideal material for high 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 (~1.1μm/mK) [2], we chose to study its photoconductive properties and potential as a high power optoelectronic switch. Past studies have shown photoconductivity in intrinsic diamond by UV-band excimers in high electric fields up to 2 MV/cm [3].

In this work, however, we utilized boron-doped diamond substrates to excite carriers by sub-bandgap photon energies while maintaining the desirable properties of diamond that would facilitate higher breakdown field and better thermal management. Boron is the only known p-type dopant occupying substitutional sites and has an activation energy of 0.3-0.37eV above the valence band [4].
In designing extrinsic PCSS, on/off current ratios should be maximized for high performance switching. In the off state, the material conductance primarily depends on the number of ionized dopants, which decreases with lower doping levels or compensation. In the on state, the conduction is increased due to the photogeneration of free carriers through dopants and traps, which increases with doping concentration. However, on conductance is also limited by the number of excitable carriers based on the surface area of the substrate and absorption depth at the excitation wavelength. Thus, by analyzing the conduction as a function of doping concentration between the on and off state, a doping level of 10¹⁸ cm^-3 was chosen in efforts to maximize the on/off current ratio in fabricated devices.

In this work, we have fabricated lateral electrodes on the surface of boron doped single crystalline diamond substrates and analyzed the photoresponse. The 1μm doped epilayer was grown by MPCVD on a diamond HPHT seed. Then, electron-beam evaporation was used to deposit ohmic Ti/Pt/Au contacts on its surface to form planar devices. Testing of the extrinsic diamond substrate was performed by exciting with a frequency doubled (532nm) Nd:YAG pulsed laser with an output energy of 20mJ and ~10ns duration. Our experiments with these devices have shown a pulsed photoresponse with a full width half maximum of 11.5 ns. Furthermore, sharper rise times and slower fall times were observed, which could be explained by the presence of traps. Aided by residual heating from the source, its captured carriers could still be thermally excited leading to a current tail. Through this work, photoconductivity was successfully shown in extrinsic homoepitaxial diamond using a lower energy, below bandgap Vis-NIR excitation source.

This work was supported by Precourt Institute, Stanford under the Bits and Watts initiative. Fabrication was done in the Stanford Nanofabrication Facility.

[1] S. Feng, et al., IEEE Transactions on Electron Devices, 37, 12, 2511-2516, (1990).
[2] Tsao, J. Y., et al., Adv. Electron. Mater., 4, 1600501 (2018).
[3] Yoneda, H., et al., Appl. Opt. 40, 6733-6736 (2001)
[4] M. Dutta, et al., IEEE Electron Device Letters, 37, 9, 1170-1173 (2016).

Although a Raman spectroscopy presents a basis for an effective assessment of the atomic boron concentration in the boron-doped diamond, determination of further physical properties of diamond using non-destructive Raman approach requires advancement in the Raman analysis. In this work, we analyze Raman spectra of heavily boron-doped epitaxial (113) diamond layers measured at temperatures up to 600 °C based on the interaction between phonon and electronic Raman scattering effects. Raman spectra are modelled using a combination of multiple decoupled Fano-functions and the established “dirty-metal” electronic Raman scattering cross section model. The presented analysis demonstrates the imperative contribution of the electronic Raman scattering to the Raman spectrum and opens a new practical route towards determination of electronic transport properties in boron-doped diamond.

Lab-grown diamonds are candidate materials for several emerging technologies, including next-generation high-power electronics. These diamonds can be grown using high-pressure high-temperature (HPHT) synthesis or chemical vapor deposition (CVD). However, these methods produce defects in the diamond crystal structure, impairing the diamond’s performance. To better understand defect formation in diamonds, the Fraunhofer USA Center for Coatings and Diamond Technologies (CCD) is developing a quantitative birefringence setup capable of imaging these defects. Because a defect-free diamond has an isotropic crystal structure, it transmits all polarizations of light identically. However, when defects are present, the resulting strain produces birefringence, in which the polarization of transmitted light is distorted. Diamond samples are measured in an imaging polarimeter. By analyzing the raw data with Mueller calculus, the amount and orientation of birefringence are mapped across the diamond, revealing crystal defects.
Not only can defects, such as threading edge dislocations, be visualized, the relative strain produced by defects can be quantified. Furthermore, statistical analysis on defect type and strength within the diamond can be performed, enabling distributions, concentrations, and types of defects to be identified and analyzed relative to reactor conditions. This analysis is part of a larger effort to develop data pipelines for diamond research. These pipelines will be integrated into an overall diamond database to enable cross-correlative analysis across different measurement techniques such as x-ray rocking curve or Raman mapping on diamond samples grown using specific processes. Cross-comparisons using statistical analysis (data science) on these data sets are predicted to yield deeper insights into process-structure relationships of diamond growth.

Nanomembranes (NMs) are very thin (≤ 500 nm) and have very high aspect ratios of thickness to lateral dimension (≥ 10⁴). This makes them extremely flexible due to the linear decrease of bending strain with thickness. At the extreme thinness, materials fold much easier [1]. When nanocrystalline diamond (NCD) thin films are grown on substrates, which have a higher thermal expansion coefficient than diamond, unavoidably, compressive stress is generated. When a NCD NM is released from such substrate a pattern of wrinkles and wrinklons evolves. The out-of-plane deformation is associated with the onset of an elastic instability, where the total energy is best minimized by the film bending rather than straining in-plane. These deformations strongly affect the electrical properties of B-doped NCD NMs [2]. Therefore, one cannot disregard the influence of wrinkles when dealing with any type of device that is based on NMs.

In this work, we present finite element simulations of compressive stress relaxation under gravity in a diamond NM as it is released from a supporting substrate. As NCD consists of hard diamond grains and soft grain boundaries, the Young's modulus and Poisson's ratio are not expected to be the same as in bulk diamond [3]. To study the impact of elastic constants on wrinkling, the calculations were made with different Young's moduli (E = 300, 600, 900, and 1200 GPa) and Poisson's ratios (ν = 0, 0.07, 0.14, and 0.21). The initial compressive stress of the NM was varied in the range from 0 Pa up to −6 GPa. NCD thin film, which is 150 nm thick and has a compressive stress of up to −10⁴ Pa, relaxes without forming wrinkles when is released from the substrate as the NCD membrane. Symmetrical wrinkles form when the membrane relaxes from −10⁵ Pa and higher compression. At absolute stresses larger than −1 GPa the asymmetric wrinkling is dominant. Young’s modulus and Poisson’s ratio does not have notable impact on the relaxed membrane shape as stress does. However, the Young’s modulus has higher impact on the relaxed membrane area and out-of-plane deformation range than Poisson’s ratio. The higher the Young’s modulus (or Poisson’s ratio) the smaller the area (and out-of-plane deformation range) of the NM is obtain after the compressive stress relaxation. Finally, the obtained results on wrinkling phenomena are explain through the system’s energy minimization when streching, bending and twisting events are competing.

References
[1] J.A. Rogers, et al., Nature 477, 45 (2011).
[2] S. Drijkoningen, et al., Scientific Reports 6, 35667 (2016).
[3] C.A. Klein and G.F. Cardinale, Diamond and Related Materials 2, 918 (1993).

NV centers in diamond are point like defects that have attracted a lot of attention as one of the best candidates for quantum technologies particularly for sensing and imaging nanoscale magnetic fields. For this application, the use of an ensemble of NV centers with a high concentration in a high quality diamond layer is of prime interest. In previous works, it has been demonstrated that N₂O doping rather than N₂, allows improving diamond quality by limiting the formation of macroscopic defects and NV’s photobleaching [1]. More recently, the growth of N₂O doped samples thicker than 1.5 mm has been achieved with substitutional nitrogen (Ns) concentration as high as a few tens of ppm which is very difficult to obtain by a CVD process using a conventional N₂ precursor. After irradiation by a 10 MeV electron beam, a conversion ratio of 20% has been obtained leading to a CVD grown layer with 4 ppm of NV^-. A decrease of the longitudinal spin relaxation time T₁ observed when the magnetic field is aligned along the [100] direction (i.e. when all NV centers classes are equally degenerate) indicates that NV-NV dipolar coupling becomes a dominant mechanism in such highly doped samples.
In this work, we will more particularly focus on the optimization of N₂O concentration in the gas phase in order to limit NV^--NV^- interaction while maximizing density and spin properties. In the same time, samples grown in the presence of N₂ + O₂ and N₂ only will be compared in order to investigate the benefit of the use of N₂O. We aim at evaluating the maximum sensitivity achievable in a magnetic quantum sensor using such defect ensembles.

[1] A. Tallaire, L. Mayer, O. Brinza, M.A. Pinault-Thaury, T. Debuisschert, J. Achard, Highly photostable NV centre ensembles in CVD diamond produced by using N₂O as the doping gas, Appl. Phys. Lett., 111 (2017) 143101.

The recently discovered negatively charged tin-vacancy center (SnV-) in diamond combines high brightness single photon emission [1,2], large Debye-Waller factors and Fourier-limited linewidths [3,4]. Furthermore, the ground state fine structure splitting exceeds 800 GHz which potentially suppresses phonon mediated decoherence of the lowest ground state’s spin doublet, which is the predominant decoherence mechanism for other negatively charged group-IV – vacancy centers (Silicon-vacancy center (SiV-) and germanium-vacancy center (GeV-)). Therefore, the SnV- center supposedly combines the advantages of the most established colour centers, namely the long spin coherence time of the nitrogen vacancy center and the large Debye-Waller factor and insensitivity to first order stark shifts of the silicon vacancy center. The exploitation of these very promising features for quantum information processing is up to now limited by the charge instability of the center under resonant optical excitation [4]. This instability results in termination of the fluorescence and therefore inhibits the use of coherent manipulation protocols.
We here present studies on stabilizing the SnV- charge state based on the recently discovered Coulomb-driven single defect engineering by co-implantation of charge donors into diamond [5]. We co-implant oxygen, sulfur and boron together with tin and find a suppression of bleaching (reduction of fluorescence intensity under continuous illumination) and fluorescence termination together with increased photo-luminescence intensity for sulfur co-doping. These results pave the way for application of the SnV center in coherent optical spin control experiments.
[1] Iwasaki T. et al., Phys. Rev. Lett. 119, 253601 (2017)
[2] Tchernij S. D. et al., ACS Photonics 4, 25802586 (2017)
[3] Trusheim M. E. et al., Phys. Rev. Lett. 124, 023602 (2020)
[4] Görlitz J. et al., New J. Phys. 22, 013048 (2019)
[5] T. Lühmann et al., Nat. Commun. 10, 4956 (2019)

Ensembles of negatively-charged nitrogen-vacancy (NV^-) centres in diamond constitute a promising platform for magnetometry applications. However, the sensitivity of present NV-ensemble devices and the need for diamond material with reproducible properties have hindered progress toward many envisioned applications. The work presented here addresses these challenges by optimising a chemical vapor deposition (CVD) growth process to produce material at scale with improved NV-ensemble properties.

Through development of the CVD process it is shown that nitrogen incorporation can be increased without an increase in the density of unwanted vacancy-related defects which act as charge acceptors, reducing the [N_s⁰]/[N_S] and [NV^-]/[NV] ratios, and limiting the dephasing time, T₂*. The process of irradiation and annealing is optimised and subsequent measurements of the NV concentration, charge ratio and contrast suggest that these grown-in defects have a detrimental impact on the NV-ensemble properties. Strain is also demonstrated to be a critical parameter to control.

This work also investigated the potential to produce material with reproducible NV concentration, NV charge ratio, strain homogeneity, and T2* by characterising a batch of samples containing approx. 4 ppm NV. The measured NV^- and NV⁰ concentrations in the batch of samples varied by less than 7% (1 standard deviation) and exhibited well-controlled strain inhomogeneity. 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 material reported in this work enables immediate sensitivity improvements for present quantum-sensing devices and the correlations observed between material properties at different stages of processing provide efficient metrics for future work targeting other NV densities or sample geometries such as micron-scale, NV-rich layers for magnetic imaging.

Color centers in diamond are promising candidates for emerging quantum sensing and quantum information processing applications due to their exceptional properties. Coupling diamond color centers with metal plasmonic nanostructures is reported to enhance the optical properties of color centers effectively, but the plasmonic metal structures of previous studies are vulnerable to the environment, making the diamond-metal system unstable. Here, we demonstrate a novel and highly-tunable structure of Ag nanospheres coated with a layer of diamond containing SiV centers. The Ag was grown by thermal vapor deposition and then plasma treated to form uniform nanospheres, which were protected by the stable diamond shell grown by plasma-enhanced chemical vapor deposition (PECVD) method. This enhances the emission rate and brightness of SiV in diamond drastically. The structure also shows great stability to extreme corrosive and high-temperature conditions. Our finding is promising for developing ultrabright quantum sensing, high-efficiency imaging, and surface plasmon-quantum emitter waveguide.

The NV^- center in diamond is a notable example of a system that has open-shell electronic structure. The challenge in describing open-shell systems for computational methods is that the wavefunctions for the ground-state and excited-states are composed of two or more electronic configurations. Wavefunction-based techniques to describe these multi-configurational systems, then, will necessarily be sums of multiple Slater determinants, called “multi-reference wavefunctions.” Their calculation is quite expensive for all but the smallest molecular systems. The Spin-Flip ansatz, however, allows for electronic structure methods based on an initial single-reference calculation that can capture contributions from the multiple configurations for open-shell systems. This method was originally applied to the Configuration Interaction and Equation-of-Motion Coupled-Cluster approaches, and then later applied to density-functional techniques, called Spin-Flip Time Dependent Density Functional Theory (or SF-TDDFT). In this work, we consider the Spin-Flip method being adapted for electronic structure calculations from the framework of the GW/BSE approach, which gives the Spin-Flip Bethe-Salpeter Equation, a method with lower scaling than Coupled Cluster Singles-Doubles and other similar wavefunction-based methods. SF-BSE is also ab initio and is accurate in solids. The SF-BSE Hamiltonian resembles that of SF-TDDFT but with an ab initio kernel. We present results for the open-shell test systems of ethylene under torsion and the NV^- center defect in diamond. The excitation energies calculated for the NV^- center in diamond compare reasonably well with both experiment and Configuration Interaction-based theoretical methods.

This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, CTC and CPIMS Programs, under Award DE-SC0019053.

Maskless lithography has been combined with the microwave plasma chemical vapor deposition (CVD) of diamond to fabricate a variety of micro-sensors that can be deposited on substrates and encapsulated in a protective CVD diamond layer to survive extreme conditions of high-pressure, high- and low-temperatures, corrosive, and high radiation environments. These hardened sensors provide electrical conductivity, magnetic susceptibility, and stress data on a variety of metallic and superconducting materials under extreme conditions of pressures and temperatures in diamond anvil cell devices. In addition, boron-doping of CVD diamond also enables the fabrication of micro-heaters in diamond anvil cell devices. Microfabrication techniques have also been utilized to selectively grow localized nanodiamond structures on single crystalline diamond substrates. Since nanodiamond structures lack the cleavage planes representative of single crystal diamond; ultrahigh pressures of over 500 GPa can be generated in this nanodiamond/single crystal diamond structures. The studies on materials under extreme conditions using these diamond-based sensors and nanodiamond structures are carried out at the Advanced Photon Source, Argonne National Laboratory. Future research directions in vertical integration of multiple sensors, wireless communications, and controlling the sp-3 bonded content in nanodiamond microstructures will be presented.

Current state of the art scintillating neutron detectors are typically of large form factor, or detection node, on the order of cubic metres to achieve efficiencies of ~70%. This results in spatial resolutions of cm², which relatively low for detecting certain particle species. However, detectors with a lower form factor (mm²) built into an array are expected to provide an order of magnitude increase in spatial resolution. Moreover, placing such detectors in series will enable higher resolution in for energy spectrum as well as tracking of scattered particles. Diamond is a highly favorable material for sensing radiation in very harsh environments, such as the beamlines of high energy particle colliders and space environments. This is primarily due to the strong covalent carbon-carbon bonding in the sp³ hybridized orbital structure of diamond. Arranged in a periodic lattice this molecular structure also give rise to high electron mobilities and thermal transport properties which have the net effect of simplifying radiation sensor device design while maintaining a fast response rate and low dark current. A design consisting of a high cross-section neutron activation layer sandwiched by layers intrinsic CVD diamond and Boron doped semi-conducting diamond has been proposed.
The “sandwich” design takes advantage of the molecular density of diamond, allowing for detection in either the intrinsic or semiconducting layers. The activation layer material can be chosen to optimize detection at a desired energy. This device will be built and commissioned for use with the Facility for Rare Isotope Beams. To facilitate nuclear experiments at the FRIB’s capacity of 400MeV per nucleon, we plan to provide high efficiency detection of neutrons up to 250 MeV, in a form factor allowing for modular addition of neutron detection to experiments which otherwise would not have the capability. Due to the radiation hardness of diamond, the device could also effectively be employed as a permanent beamline monitoring or veto detector. The development of the device requires several stages of work, beginning with design optimization through computer modeling.
The current stage of work develops Monte Carlo simulations of atomic and nuclear processes inside of diamond crystal using several simulation frameworks. We simulate irradiation of diamond of neutrons and a variety of heavy ions. Using the GEANT 4 framework we are able to model the crystal lattice structure and monitor structure damage through the simulation. Through judicious selection of GEANT 4 physics lists we are able to model non-ionizing energy loss, both elastic and inelastic scattering, and nuclear interactions during irradiation. Secondary and tertiary effects and interactions can also be monitored. This allows us to identify points of lattice damage; creation of vacancies and interstitial ions. Throughout development, simulations were benchmarked against SRIM 2013. These benchmarks, discrepancies, and improvements are discussed.

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

In recent years, wide band gap semiconductors have been recognized as very promising sensitive materials for detection of toxic pollutants and hazardous gases. A majority of research work has been carried out on metal oxide (MO) semiconductors by simple change of resistance measurements. However, the main issues for MO chemiresistor sensors are high operating temperature (above 300°C) and cross-sensitivity/selectivity. These drawbacks can be partially solved by utilizing the combination of two or more types of semiconductor materials (p-n heterostructures). The use of the combination of n-type and p-type semiconductor materials is a relatively recent field of research within chemiresistors. Such bilayer structures offer a variety of advantageous characteristics, including a large surface area, improved crystallinity, defect-free interface, and enhanced electrical properties.
In this work we report on the development of heterostructures based on n-type Pd-doped SnO₂ and p-type boron doped diamond (BDD) for room-temperature NO₂ detection. Here, three different gas sensor devices were developed based on (i) BDD, (ii) Pd-SnO₂, and (iii) Pd-SnO₂/BDD heterjunction structures using a combination of ink-jet printing, microwave linear antenna plasma enhanced chemical vapour deposition, and reactive laser induced forward transfer (rLIFT) techniques.
The gas sensing properties of fabricated sensor devices were tested in oxidizing/reducing gases (NO₂, CO, NH₃) at various analyte concentrations (5-20 ppm) in the dark and under UV light illumination. It was found that all sensor structures measured under UV light illumination demonstrate significant enhancement of gas sensor response in comparison to the measurements performed in dark. Moreover, the sensors demonstrate high sensitivity to NO₂ gas and showed dependence on the intensity of UV light, fast response, and reversible character. The Pd-SnO₂/BDD heterojunction structure exhibited a remarkable NO₂ gas sensor response in comparison to Pd-SnO₂ based structure (i.e., 280% for Pd-SnO₂/BDD and 12% for Pd-SnO₂). Finally, the Pd-SnO₂/BDD structures possess the highest response to 10 ppm NO₂, complete reversibility, and reproducible character under UV light illumination.

This work was supported by the Czech Science Foundation (GAČR) contract no. 19-09784Y.

Due to its excellent properties in comparison with other carbon-based materials, boron doped diamond (BDD) has become a popular electrode material especially for electrooxidation. These include low background current, wide potential window in the region of positive potentials, mechanical robustness and low proclivity to electrode passivation. Cresols are classified by the US Environmental Protection Agency as a toxic and persistent chemical, exhibiting chronic effects at 12 mg.L⁻¹, however they are commonly used for wood impregnation, production of polymers, disinfection and herbicides. Wastewater from these industries along with coal conversion contains a high concentration of cresols and is therefore a problematic environmental contaminant, which requires monitoring.

In this work, the influence of, 1- boron-doping levels (B/C ratio in the gas phase (ppm): 500, 1000, 2000, 4000, and 8000), 2- surface termination (H- vs. O-) and 3- ex-situ polishing with alumina, of planar BDD on its electrochemical properties and voltammetric behaviour of cresols was thoroughly investigated. In addition, we compare planar BDD with porous BDD samples for changes in their electrochemical properties. We show that all examined factors have a significant impact on cresol electrooxidation, and therefore should be carefully considered when designing BDD electrodes for electrochemical sensing of these environmental contaminants.

BDD deposition methods will be described together with BDD structural and composition characterisation, and the electrochemical methods used.

Betavoltaic cells directly convert the energy of beta particles into electricity.[1] The operation principle of a betavoltaic cell is similar to that of a solar cell, except that the energy source is beta-emitting isotopes. In betavoltaic cells, semiconductor junctions play the role of energy conversion. These cells have the advantage of longevity, which is determined by the half-life (T_1/2) of the beta-emitting radioisotopes. The cells can operate for several years to a decade. Betavoltaic cells are expected to be utilized for battery applications in remote locations such as outer space and underground.

Diamond is advantageous material for betavoltaic cells because the wide bandgap could offer high open circuit voltage due to large built-in potential of the junction, which improves the conversion efficiency of the cells. From theoretical perspective, ultrawidebandgap materials such as diamond and BN have potential to realize extreme conversion efficiency of the cells.[2] Moreover, diamond has durability against radiation and high temperature. Diamond betavoltaic cells could be utilized to application under extreme condition. In this study, we show the highest semiconductor conversion efficiency among the betavoltaic cells reported thus far by utilizing the large built-in potential of a diamond pn junction.
We formed pseudo-vertical diamond pn diodes. Homoepitaxial p⁺, p, n^-, and n⁺ diamond layers were grown on a {111} high-pressure high-temperature (HPHT) type-Ib diamond substrate by using microwave plasma-assisted chemical vapor deposition (MPCVD). Each B- (p-type) and P- doped (n-type) diamond layer were grown with the use of individual MPCVD systems. The thickness of the p⁺, p, n^-, and n⁺ layers were, respectively, 2, 0.1, 1.8, and <0.1 μm. The impurity concentrations confirmed by secondary ion mass spectrometry (SIMS) were [B]: 3 × 10¹⁹ cm^-3 for the p⁺ layer, [B]: 2 × 10¹⁸ cm^-3 for the p layer, [P]: 1 × 10¹⁶ cm^-3 for the n^- layer, and [P]: 1 × 10²⁰ cm^-3 for the n⁺ layer. Highly P- doped diamond was grown using homemade CVD reactor to enhance P incorporation into diamond to reduce series resistance. After the growth, mesa structures were formed by induction coupling plasma etching with oxygen gas. Subsequently, Ohmic contacts were formed by electron beam deposition of Ti/Mo/Au both on the mesa structure of the n⁺-layer and on the p⁺-layer. The diameters of the mesa and electrodes were, respectively, 240 and 150 μm.

Then we evaluated power – voltage (P-V) and current-voltage (I-V) characteristics under electron-beam irradiation. For electron-beam irradiation, a scanning electron microscope was used as the electron source. Electron beams at accelerating voltages of 15 kV were irradiated on top of the n⁺ layer. The open-circuit voltage (V_oc) under the electron-beam irradiation was 4.26 V, which is the highest among the betavoltaic cells reported thus far. The output power at the maximum point (P_MP) was 10.7 nW. Fill factor (FF) and semiconductor conversion efficiency (η_s) calculated as following eqs (1) and (2)
FF = P_MP/I_scV_oc , …(1)
η_s = VocFF/ε , …(2)
where ε is the electron–hole pair creation energy. Here, we substitute ε = 13 eV in eq. (2). The FF and the semiconductor conversion efficiency η_s were respectively, 0.85 and 28%. Among the semiconductors, the diamond pn junction achieved the highest η_s , which enables to be utilized to energy efficient and long-life cells for remote sensing applications. In the talk, we will also present charge collection efficiency and total conversion efficiency of diamond pn diode against different energy of electrons.

References
[1] P. Rappaport, The electron-voltaic effect in p−n junctions induced by beta-particle bombardment, Phys. Rev. 93, 246 (1954).
[2] S. Maximenko, J. Moore, C. Affouda, P. Jenkins, Optimal semiconductors for ³H and ⁶³Ni betavoltaics, Sci. Rep. 9, 10892 (2019).

The introduction of wide-bandgap semiconductors such as GaN and SiC have revolutionized the semiconductor electronics industry, especially in the areas of power switching and high frequency RF power amplifiers. While these materials have provided orders of magnitude improvements relative to silicon devices, today we are beginning to see the limitations of these wide-bandgap materials.
Consequently, electronic devices based on diamond, an ultra-wide-bandgap semiconductor, are increasingly explored due to diamonds’ extraordinary material properties. A number of promising lateral [1],[2] and vertical diamond transistors [3] have been demonstrated; however, so far all diamond devices rely solely on surface conduction, either through a hydrogen terminated surface or a surface accumulation layer. The use of these transistors for power and RF applications may result in a number of issues, such as current collapse induced by surface trapping and poor reliability caused by non-uniform heat generation. A fin-based vertical transistor based in part on bulk conduction would allow us to take advantage of the superior material properties of diamond while mitigating these issues.
In this work, we use TCAD simulations in Silvaco to explore the potential of vertical diamond FinFETs [4] and compare it to an equivalent vertical GaN FinFET device. In our TCAD framework we incorporate several effects unique to diamond, including incomplete ionization, the metal-insulator transition of boron in diamond [5], and variable range hopping conduction [6].
For power applications, a vertical diamond FinFET of fin width 0.2 µm, fin pitch 0.9 µm, and fin height 0.8 µm was simulated along with an optimized GaN structure. Assuming a critical electric field of 10 MV/cm for diamond and 3.3MV/cm for GaN, we find that to achieve a breakdown voltage of 1200 V, a drift region thicknesses of 1.4 µm and 4.7 µm are needed for diamond and GaN devices, respectively. For such relatively large drift regions, the incomplete ionization of boron in diamond in the on-state leads to a higher specific on-resistance for diamond (1.21 mΩcm^-2) than GaN (0.35 mΩcm^-2). We note, however, that if the dopants were completely ionized for diamond, the specific on-resistance would drop to 0.063 mΩcm^-2, approximately 5x lower than GaN. This could potentially be done by raising the device nominal temperature, by optical activation of boron acceptors, or by high electric fields.
In addition, 200 V breakdown (60 V DC bias operation) devices for high frequency RF applications were simulated with dimensions of fin width 0.1 µm, fin pitch 0.6 µm, and fin height 0.2 µm. In this case, the optimized drift region thicknesses were 0.25 µm and 0.85 µm for diamond and GaN, respectively. Compared with the GaN device, the diamond device exhibited a higher J_maz (640 vs. 240 kA/cm²), lower V_knee (25 vs. 30 V), and higher f_T (53 vs. 42 GHz). As a result, the calculated linear power density of diamond was 7.36MW/cm² (44.2 W/mm per fin), which is about 2.7x higher than the GaN calculated linear power density at 2.7 MW/cm² (16.2W/mm per fin). In the case of the RF diamond device, as the drift region was relatively thin, the large number of holes injected from the channel lead to a significant carrier density in the drift region, which made up for the lack of carriers from unionized acceptors.
In summary, we have simulated a novel vertical fin transistor in diamond along with a reference GaN device, one set for power switching applications and one set for high power RF applications. We identify key priorities and initial results show great promise for vertical diamond FinFETs
Acknowledgement.- This work was partially funded by DARPA through a program monitored by Dr. Young-Kai Chen.
References
Huang et al. Scientific Reports 2018
Imanishi et al. EDL 2019
Iwataki et al. EDL 2020
Zhang et al. EDL 2019
Thonke et al. Semicond. Sci. Technol. 2003
Hathwar et al. JAP 2016.

Diamond is a promising semiconductor material for high temperature and high power electronics due to its following properties: ultra-wide bandgap (5.45 eV), high electric breakdown field, and high thermal conductivity. Additionally, doped semiconducting diamond has a unique inverse temperature versus resistance correlation. Unlike many other semiconductors, the resistivity of diamond decreases with rising temperature until 150^oC and remains low through 400^oC . As a result, diamond device has lower theoretical "ON-resistance" and power loss than similar SiC and GaN device at temperatures higher than 150^oC.
Diamond field effect transistors (FETs) can operate through surface or bulk conduction. Surface conduction is achieved via a two-dimensional hole gas (2DHG) formed on hydrogen-terminated diamond surface. Bulk conduction is typically achieved via holes generated in p-type boron-doped diamond. Surface conduction diamond FETs, also referred to as hydrogen-terminated diamond FETs, have attracted the most interest due to its current capability being close to commercial SiC and GaN MOSFETs of similar size. However, maintaining stability of the 2DHG at elevated temperatures can be challenging. Generally, hydrogen-terminated diamond FETs are not recommended to operate at temperatures higher than 400^oC even with surface passivation.
In this study, our primary interest is to design and optimize bulk conduction diamond FETs with superior thermal stability and competitive power electronics capability. Three types of boron-doped conduction FETs were fabricated: lateral metal-semiconductor FET (MESFET), lateral metal-oxide-semiconductor FET (MOSFET), and vertical junction FET (JFET). The performance of the MESFET was demonstrated at elevated temperatures up to 430^oC. It operated as a normally-on device at 0 V gate voltage, V_GS and became pinched-off at V_GS = 6V. The drain current density, I_DS increased rapidly with the increasing temperature until ~150^oC. An I_DS value had approximately a 100-time increase from room temperature to 150^oC and is attributed to the increase in the activated boron concentration because the boron acceptor level is deep at a value of 0.36 eV. The I_DS value peaked at 150^oC and remained high until 430^oC. Despite the high thermal stability, the metal-semiconductor junction limits the applicable gate forward bias to its Schottky turn-on voltage, which is around -1 V. This also limits the maximum drain current at a given drain voltage. To improve the current capability, a similar MOSFET structure was fabricated. With the introduction of an insulating Al₂O₃ layer between the gate metal and diamond, a higher gate forward bias could be applied and a higher maximum drain current could be obtained. For instance, at 150^oC, the maximum drain current of the MOSFET at -10 V_D, achieved at -14 V_GS, was more than two times of that obtained from the MESFET. However, the thermal stability of Al₂O₃ deteriorated with rising temperature. Its electric breakdown field decreased from above 5 MV/cm at room temperature to 1 MV/cm at 250^oC. The lateral transistors had breakdown voltages demonstrated to above 500 V.
Both lateral MESFET and MOSFET exhibited relatively low current capability due to high channel resistance. To significantly reduce the channel resistance while maintain the high thermal stability and high breakdown voltage, a vertical JFET structure without oxide was designed. A TCAD simulation estimated the "ON-resistance" to be less than 1 mΩ*cm² with a targeted 1200 V breakdown voltage. A key fabrication step, selective n-type phosphor doped layer of diamond, was successfully conducted through diamond etching and subsequent re-growth. Preliminary results for the vertical JFET will be presented.

The properties of cubic boron nitride (c-BN) and diamond make them ideal materials for applications in a variety of extreme conditions, such as thermal or corrosive protection, ionizing radiation detection, and high-power high frequency electronics. One of the persistent challenges preventing many electronic applications of epitaxial c-BN on diamond is an interfacial hexagonal boron nitride (h-BN). While local epitaxy has been observed previously, a uniform interface for electronic grade applications has not been realized. Understanding the nucleation and subsequent growth of c-BN films on an atomic level at the growth interface and the properties across that interface is fundamentally critical to fully realize the materials’ electronic properties.
Research at Arizona State University has studied the effects of hydrogen concentration during the growth of c-BN on diamond via electron cyclotron resonance plasma enhance chemical vapor deposition, and have found that, by forcing the hydrogen to be the limiting reactant in the growth environment, the reaction favors c-BN while suppressing the formation of h-BN impurities. In-Situ X-ray Photoelectron Spectroscopy (XPS) was used to characterize the interface, and Ultraviolet Photoelectron Spectroscopy (UPS) was used to determine the valence band offset of the pure interface. In this presentation we show that minimizing the hydrogen concentration during the deposition maximizes the cubic boron nitride phase purity at the diamond interface.
This research is supported by The National Science Foundations (NSF) and Northrop Grumman.

The surface temperature of Venus is ~460 C, with a corrosive atmosphere including CO₂ and sulfuric acid at a pressure of 92 bar. A number of missions to Venus are being considered or planned that would include surface or atmosphere probes, and high temperature electronics would be required for power control, mechanical system control, and sensor electronics. While it is long been recognized that the wide bandgap of diamond would result in a negligible increase in the intrinsic carrier concentration, at high temperature the devices based on Schottky barriers would show a great reduction in the forward-reverse current rectification ratio. This research presents results of diamond diodes with a thin n-type layer in a Schottky-n-i-p structure, which achieves high current density, high breakdown field and a designed on-off rectification ration. The metal-n-i-p diamond diodes were prepared on single crystal p-type, boron doped (100) or (111) substrates, with undoped intrinsic epitaxial layers and phosphorus doped epitaxial layers prepared by microwave plasma CVD. The electrical results and simulations show regimes with low drift layer resistance where the contact resistance limits the current. The diodes have been confined in a high pressure CO₂ atmosphere for over 100 hrs. After removal, inspection of the surfaces shows dark regions thought to be carbon containing layers, which did not result in evident changes of the current-voltage characteristics. More extensive Venus atmosphere testing planned at the NASA GEER facility will include active electrical measurements. The results establish that with control of p- and n-type doping optimal device designs can be achieved for diamond electronics at high temperature.
This research supported by NASA through grant NNX17AG45G as part of the HOTTech program.

For decades, diamond has shown superior properties that make it desirable for novel electronic materials. However, fabricating large size high quality single crystal diamond wafers faces multiple challenges. Single crystal diamond (SCD) is a promising wide bandgap semiconductor material. Epitaxial lateral outgrowth (ELO) using microwave plasma assisted chemical vapor deposition (MPACVD) has shown its advantages in synthesizing large size high quality SCD. This research details experiments focused on the impact of physical boundary conditions on the epitaxial lateral growth of SCD using MPACVD. Growth dynamics, such as lateral growth rates, and as grown diamond crystal structural properties, such as X-ray rocking curves (XRC), relative to pocket holder boundaries is discussed. The boundary conditions of ELO are established relative to growth rate, and internal structure (planar alignment and mosaicity of the lattice) of outgrown areas are discussed. Results indicate the time varying effects of holder boundaries on growth rate can be parameterized into an exponential decay function based on the kinetics of phase change to predict the total lateral outgrowth possible for a given holder design. As expected, these results, in turn, will enable the design next generation reactors to accommodate continuous larger area single crystal diamond growth. Additionally, the structural characteristics of the internal lattice are discussed relative to reactor conditions of holder geometry, temperature, and gas pressure.
Among diamond growth methods, MPACVD has distinct advantages compared to high pressure high temperature (HPHT) growth, but primarily its ability to synthesize diamond at relatively low pressure and temperature makes it attractive as a scale-up method for diamond growth. While a plethora of literature on the chemical and macroscale mechanisms of diamond growth exists, synthesizing large size high quality SCD remains a significant challenge due to the emergence of heterogeneous structure and morphology from uncontrolled nucleation based on reactor design. This research reveals how the pocket holder and the boundary conditions it creates influence the morphology that a CVD diamond (type IIa) growth on HPHT substrate (type Ib) assume. Crystal structure characteristics reflected from XRC mapping on both peak position and FWHM for both (400) diamond crystal planes shows the importance of the geometry of the sample holder. Further growth results of SCD using multiple types sample holders and their crystal structural measurement and analysis will be shown at the conference. This will give feedbacks on growth processing and lay the foundation for the real large size high quality SCD growth in the near future.

Atomically thin diamond has various industrial applications because of their extreme hardness and excellent thermal conductivity. In this study, we have investigated the possibility of a novel synthesis technique for diamane and diamond thin films from multilayer graphene at pressures far below the graphite → diamond transformation pressure using the atomistic modeling approach. We have used the Molecular Dynamics technique with reactive force fields to study the formation mechanism of diamane. Our results demonstrate a drastic reduction (by a factor of two) in the graphite → diamond transformation stress upon using a combined shear and axial compression. The shear deformation in the multilayer graphene lowers the phase transformation energy barrier and plays the role of thermal fluctuations, which itself promotes the formation of diamond. We revealed weak temperature dependence of the transformation strain and stresses. The transformation stress vs. strain curve for the bulk graphite drops exponentially for finite temperatures. Therefore, our results support the proposition that shear strain plays a vital role in graphite → diamond phase transformation.

Diamond has many unique physical properties. Due to these unique properties diamond has high potential for applications; electrical devises, quantum devices, etc. Depending on the target application we need to develop the growth and fabrication technique, In this talk, we introduce the electronic applications and the quantum application using nitrogen-vacancy centers from the viewpoint of diamond growth and fabrication technique.

Using a simplified kinetic Monte Carlo (KMC) model of CVD diamond growth, the diamond and computational chemistry groups at the University of Bristol have previously reported simulating many atomic layers of diamond growth at relatively low computational cost. These simulations have been found to reproduce many of the growth features seen experimentally, including growth rates, while keeping the computational time required on the order of hours.¹ However, one limitation of such a simplified model is the use of a cubic representation of the diamond atomic lattice. Although this allows low calculation times and simplified methods, the geometries of specific adsorbates or surface defects thought to be important in the diamond growth mechanism cannot be correctly modelled.

Transitioning the model from a simple cubic lattice to a true tetrahedral diamond structure allows a more realistic representation for the diamond lattice, allowing implementation and testing of more complicated CVD growth reaction schemes, with application of modern software development strategies to maintain computational efficiency. Visualisation of the diamond surface as CVD growth progresses in true tetrahedral co-ordination is also now possible, allowing robust and powerful analysis of implemented reaction schemes and their effect on surface morphology development as a function of time.
Experimental observations of (100) CVD diamond growth have suggested the presence of species incorporation and etching mechanisms whose interplay results in step-flow growth² and aligned dimer row domains,³ implying the presence of an anisotropic etching mechanism.⁴ Previous theoretical models have included etching mechanisms which facilitate the removal of isolated surface species,^5–7 but there has yet to be theoretical modelling studying each mechanism operates as growth conditions and mechanism energetics are varied.

Using our kMC model for (100) CVD diamond growth, we aim to investigate the various etching mechanisms and their associated energetics reported in the literature within a consistent growth model to assess how experimental growth rates can best be reproduced while observing commonly seen experimental surface features.

References
1. W. J. Rodgers, P. W. May, N. L. Allan, and J. N. Harvey: Three-dimensional kinetic Monte Carlo simulations of diamond chemical vapor deposition. J. Chem. Phys. 142(21), 214707 (2015).
2. K. Hayashi, S. Yamanaka, H. Okushi, and K. Kajimura: Stepped growth and etching of (001) diamond. Diam. Relat. Mater. 5(9), 1002 (1996).
3. R. E. Stallcup and J. M. Perez: Scanning Tunneling Microscopy Studies of Temperature-Dependent Etching of Diamond (100) by Atomic Hydrogen. Phys. Rev. Lett. 86(15), 3368 (2001).
4. C. L. Cheng, H. C. Chang, J. C. Lin, K. J. Song, and J. K. Wang: Direct observation of hydrogen etching anisotropy on diamond single crystal surfaces. Phys. Rev. Lett. 78(19), 3713 (1997).
5. A. Netto and M. Frenklach: Kinetic Monte Carlo simulations of CVD diamond growth—Interlay among growth, etching, and migration. Diam. Relat. Mater. 14(10), 1630 (2005).
6. M. Grujicic and S. G. Lai: Atomistic simulation of chemical vapor deposition of ( 111 ) -oriented diamond film using a kinetic Monte Carlo method. J. Mater. Sci. 34(111), 7 (1999).
7. C. C. Battaile, D. J. Srolovitz, I. I. Oleinik, D. G. Pettifor, A. P. Sutton, S. J. Harris, and J. E. Butler: Etching effects during the chemical vapor deposition of (100) diamond. J. Chem. Phys. 111(9), 4291 (1999).

The deposition of closed nanocrystalline diamond (NCD) thin films on non-diamond substrates requires diamond nucleation sites. An established technique for creating nucleation sites is a surface treatment with a water-based colloidal solution of ultra-dispersed nanodiamond (ND) particles.¹ In ND seeding the most important factors are the surface charge of the substrate and the zeta-potential of the used colloid. Together, they determine the nucleation density, which can lead to an extreme difference in the observed seeding density.^2–6 However, surface contamination by hydrocarbons, which is an unavoidable and quick process even in a cleanroom environment,⁷ is often not taken into consideration. Such contamination can alter the surface wetting properties⁸ and surface charge⁹, and thus the eventual seeding density. In this work we investigate and model the impact such surface contamination has on the ND seeding efficiency.

Tantalum is a metal well known for its resistivity to corrosion, bioinertness, biocompatibility and excellent osteoconductivity.¹⁰ After sputtering a thin film of Ta on silicon substrates, these stacks were exposed to the ambient atmosphere for different times, allowing to achieve various degrees of surface contamination. Then, the Ta surface wettability was measured, samples were seeded with NDs, and the subsequent seeding density evaluated. As surface coverage by hydrocarbons increased, the wetting properties gradually turned from hydrophilic to hydrophobic. The ND seeding density first decreased and then increased again. To understand this correlation, we propose an analytical model that describes the dynamics of surface contamination and hydrocarbon contaminant interaction with the surface, and the resulting ND seeding density.

We assume that when a hydrocarbon (HC) molecule lands on the Ta surface it attracts negative surface charge towards it, while creating a positive surface charge around it. Due to said dipole formation, the top part of the HC molecule is charging negatively. Solving differential equations for the dynamics of the normalized surface coverage number density of the HC molecules, n, and surface charge density, σ, we derive σ (n,η) = n + (1 – n)^{(1 + η)}, where η is a ratio between an area of the depleted charge around the HC molecule and the area of the HC molecule itself. The assumption that the contact angle, θ, is linearly proportional to the surface coverage by HC, n = θ /θ_sat, makes wettability measurements a valid method for determining the degree of surface contamination. If the sum of electrostatic double-layer and Van der Waals interaction energies becomes positive, a potential energy barrier for a ND particle approaching a surface exists. ND particles must surmount a positive potential peak for a deposition onto the surface to occur. This occurs with a probability exponentially proportional to the potential barrier. By fitting the experimental data, we find that the ND seeding density is exponentially proportional to the calculated positive surface charge number density, which forms around HC molecules. This constitutes 57% of the HC molecule area, while the created barrier height is estimated to be ~ 0.18 eV.

References
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[10] H. Matsuno, et al., Biomaterials 22, 1253 (2001).

Wide bandgap electronics made from GaN and Ga₂O₃ are currently under development due to their potential to create advanced RF and power electronics. However, the performance and reliability of these devices are often controlled by their electrothermal response during device operation. A key feature which limits thermal control of the peak temperature is the thermal resistance that is encountered intrinsically from the material as well as across material interfaces with these devices. One method that has shown promise for mitigating the high thermal resistance of these devices is through their heterogenous integration with other materials which assist in heat dissipation and aid thermal management. For this, thermal transport across interfaces and novel integration methods are necessary.

In this work we will present modeling efforts which guide the impact of heterogenous integration on the thermal performance of UWBG devices. Thermal metrology methods used to measure interfacial thermal conductance will be discussed and new insights into the control of thermal boundary conductance given. Techniques such as surface activated bonding will be presented to demonstrate the integration of GaN and Ga₂O₃ with high thermal conductivity substrates such as SiC and diamond. The use of other high thermal conductivity dielectrics for improved heat dissipation from the top side and bottom side of the devices will be discussed. Finally, the prospects for the future development of thermal management via heterogenous integration will be presented.

Diamond is a promising candidate for a base material of high power electronic devices, as well as for a superior heat spreading substrate, due to its excellent physical properties such as the highest electrical breakdown field strength, the highest thermal conductivity, and a high RF power capability that is 3 times higher in comparison with SiC. For the practical application toward power devices, we need diamond/semiconductor heterointerfaces with high thermal stability and can withstand the temperature rise of power devices during operating. Recently, Liang has demonstrated that diamond/Si [1] and diamond/GaAs [2] heterointerfaces with high thermal stability can be fabricated by surface activated bonding (SAB) at room temperature (RT). In this work, we have examined the bonding mechanism using cross-sectional scanning transmission electron microscopy (STEM) combined with electron energy loss spectroscopy (EELS) and energy dispersive x-ray spectroscopy (EDX).
In the SAB process, surfaces of diamond and Si (or GaAs) wafers are activated at RT before bonding by creating dangling bonds via the irradiation of high-energy Ar atoms in a high vacuum, and the surfaces are then bonded by the contact most of the time under pressure to form strong chemical bonds even for imperfect surfaces. For diamond/Si heterointerfaces, atomic diffusion at the interfaces, presumably due to the transient enhanced diffusion assisted by the point defects introduced in the surface activation process [3], is confirmed during the bonding process. The crystallinity at the diffusing region is rather low, and it is recovered by 1000 ^oC annealing via the formation of SiC layers, that would play a pivotal role in the relaxation process of residual stress due to the misfit of thermal expansion coefficient between diamond and Si [1]. Therefore, the defect-assisted atomic diffusion at the interfaces would be a key concept for the formation of high thermal-stability diamond/Si heterointerfaces. Similar atomic diffusion is also confirmed at diamond/GaAs heterointerfaces during the SAB process.
[1] J. Liang, S. Masuya, M. Kasu, and N. Shigekawa, Appl. Phys. Lett. 110 (2017)111603.
[2] Y. Nakamura, Y. Shimizu, Y. Ohno, K. Shirasaki, Y. Nagai, M. Kasu, N. Shigekawa, and J. Liang, 13th Topical Workshop on Heterostructure Microelectronics (Toyama, Japan, 2019) 11-7.
[3] Y. Ohno, H. Yoshida, N. Kamiuchi, R. Aso, S. Takeda, Y. Shimizu, Y. Nagai, J. Liang, and N. Shigekawa, Jpn. J. Appl. Phys. 59 (2020) SBBB05.

N-polar gallium nitride (GaN) high electron mobility transistors (HEMTs) have shown superior performances when scaling for W-band frequencies over Ga-polar GaN HEMTs [1]. However, due to the shorter channel and closer gate fingers its ability to dissipate the heat from the channel to the heat-sink is limited. Polycrystalline (PC) diamond due to its very large thermal conductivity (TC) is one of the solutions for self-heating problem in high-power high-frequency devices. PC Diamond can be grown in a closed proximity to high temperature spot in the channel and spread the heat efficiently [2]. The standard Si₃N₄ thickness as a gate dielectric for W-band N-polar HEMTs is 5 nm, which can be used as thermal interface material to reduce thermal boundary resistance (TBR<2 m²K/GW) between diamond and GaN as well [3]. As diamond growth takes place in a high temperature hydrogen plasma environment using MPCVD, it can decompose both Si₃N₄ and GaN layers by reacting with silicon, nitrogen, and gallium and making volatile gases (SiH₄, NH₃, and GaH, respectively) [4]. Any damage to GaN channel may result in a degradation of device performance after the growth. In this work, we investigated the impact of diamond growth nucleation and main stages on 2-dimentional electron gas (2DEG) properties such as electron mobility, sheet carrier concentration, and sheet resistivity (before growth: μ_n=1200 cm²/V.s, n_s=1.34E13 cm^-2, and ρ_s=388 Ω/sq).
The main goal of the growth nucleation stage is to guarantee no damage to the 2DEG by covering the substrate (Si₃N₄/GaN) with thinnest possible nano-crystalline diamond (NCD) in shortest possible time as NCD has a poor TC (<100 W/m.K). The 2DEG has been characterized using Hall mobility measurement, and the thickness of nucleation layer has been measured by high resolution TEM. The nucleation layer can prevent diffusion of hydrogen radicals into the substrate which is highly dependent on the microwave power and gas pressure. To increase the diamond growth rate and decrease substrate etching rate the sample holder was redesigned to provide higher temperature (~700 C) with minimum power (600 W) and pressure (20 Torr). The CH₄% and time were changed from 3 to 10% and 10 to 15 min, respectively for nucleation stage followed by the same main growth for all samples (2% CH₄, 1600 W, 60 Torr, 45 min). We observed no degradation in 2DEG by using >5% CH₄ and 15 min growth, while by lowering the time to 10 min we started to see about 20-30% reduction in µ_n and 50-150 Ω/sq increase in ρ_s. This degradation varies with different cap layers on top of the channel; no cap layer, 1-2 nm AlN, or 1-2 nm 46% AlGaN.
To study the impact of main growth on 2DEG, we have chosen 5% CH₄ and 15 min for nucleation stage and changed main growth parameters including CH₄% (0.5-2%), power (600-1600 W), pressure (20-90 Torr), and sample holder design. CH₄% showed the highest impact in determination of damage level to 2DEG and pressure had the lowest impact. With the sample holder same as nucleation stage, 1% CH₄ is the minimum required for no damage. For the case with original sample holder, 1.5% CH₄ is the lowest acceptable value for the first 45 min of the growth.
In this work, we were able to achieve a smooth interface between diamond and GaN (including 5 nm Si₃N₄ interfacial layer) without using argon or nitrogen in the gas mixture by redesigning the sample holder and modifying growth parameters for lower etch rate of the substrate. This recipe is compatible with W-band N-polar GaN HEMTs as maintains device performance after the growth.
This work was supported by the Semiconductor Research Corporation (SRC) and DARPA under the JUMP program.
[1] B. Romanczyk et al., IEEE Electron Device Lett., vol. 41, no. 3, pp. 349–352, 2020.
[2] M. Malakoutian et al., Crystals, vol. 9, no. 10, pp. 1–14, 2019.
[3] J. Cho et al., J. Appl. Phys., vol. 121, no. 5, 2017.
[4] Y. S. Zou et al., Cryst. Growth Des., vol. 8, no. 5, pp. 1770–1773, 2008.

Compared to classical resonant cavity CVD, linear antenna microwave plasma enhanced CVD (LA MW CVD) is a relatively new route of depositing diamond films onto different vulnerable substrates [1]. It has been reported earlier that GaN surfaces are prone to etching under conventional CVD diamond deposition parameters. So, it was envisaged that due to the low pressure and low temperature regimes of LA MW CVD, it might be possible to deposit diamond onto otherwise reactive GaN surfaces. Although it is a regular practice to put an intermediate SiN buffer layer for successful deposition of diamond films on GaN [2], there are only few reports of such deposition onto bare GaN substrates [3], and not by a LA MW CVD reactor. which could potentially solve the problem associated with an additional SiN barrier layer, i.e. an extra thermal barrier. In the present work, the effect of different LA MW CVD processing parameters on the optimal deposition of the diamond films has been systematically studied. First of all, the distances between the quartz tube and the samples were varied, which has direct impact on the substrate temperature. Thereafter, the microwave input power was altered to attain varied substrate temperatures. There is an upper limit of the substrate temperature beyond which the microwave plasma alone is insufficient to increase the temperature further. Hence, an additional heater was used under the stage for attaining various substrate temperatures. Moreover, the microwave power input was also used in pulsed mode for further optimization of the film properties. The microstructure of the deposited films was observed in the scanning electron microscope (SEM) and the elemental analysis of the film surfaces was done by electron diffraction spectroscopy (EDX). Moreover the diamond film quality and crystallinity was evaluated by Raman spectroscopy. It will be shown that the said parameters have a significant impact on the grown films, both in terms of their quality and microstructure. It was found that the optimum film quality without GaN surface etching was obtained under the pulsed microwave power regime.

References
[1] S. Drijkoningen, P. Pobedinskas, S. Korneychuk, A. Momot, Y. Balasubramaniam, M.K. Van Bael, S. Turner, J. Verbeeck, M. Nesládek, K. Haenen, Crystal Growth & Design, 17, 4306 (2017).
[2] Y. Zhou, R. Ramaneti, J. Anaya, S. Korneychuk, J. Derluyn, H. Sun, J. Pomeroy, J. Verbeeck, K.Haenen, M. Kuball, Applied Physics Letters, 111, 041901 (2017).
[3] S. Mandal, Evan L. H. Thomas, C. Middleton, L. Gines, J. T. Griffiths, M. J. Kappers, R. A.Oliver, D. J. Wallis, L. E. Goff, S. A. Lynch, M. Kuball, O. A. Williams, ACS Omega, 2, 7275 (2017).

In the present work, doped diamond films have been grown on GaN substrates. The CVD diamond processing is not straight forward, as GaN is susceptible to H₂ plasma etching, with formation of elemental Ga in the resulting diamond film, which can thereby enhance the electron emission properties of the stack. However, the electrical resistivity across the CVD grown film surface was found to become very high (~MΩ) after depositing diamond on GaN substrates. So, in order to induce electrical conduction on the top surface film, the diamond films were doped with nitrogen inside a 2.45 GHz resonant cavity reactor [1], and with phosphorous [2] inside a linear antenna CVD chamber [3]. 1%, 3% and 5% N₂ gas was added to the conventional H₂/CH₄ gas mixture. On the other hand, 0.1% PH₃ gas diluted in H₂ was used as precursor gas for phosphorous doping. Different types of microstructures were produced while doping with two different dopants. Nitrogen gas changes the diamond film microstructure from faceted to spherical grains with signs of GaN substrate etching, whereas, the phosphorous doping produced much smaller nanocrystalline grains with strong Ga peaks in their EDX spectra. Nitrogen incorporation in the diamond lattice can happen easily at lower substrate temperatures of 500-600^oC, as found from their photoluminescence data, but phosphorus doping was only possible at a high substrate temperature of 900^oC, as evidenced from its XPS signal at 132 eV. The porous nanocrystalline grains in the doped diamond could possibly enhance the field emission characteristics in comparison to the bare GaN substrate. The electrical conductivities of the coated and uncoated GaN substrates will also be presented.

References
1. J. A. Cuenca; K. J. Sankaran ; P. Pobedinskas ; K. Panda, I. N. Lin, A. Porch, K. Haenen, O. A. Williams, Carbon, 145 (2019) 740-750
2. W. Janssen, S. Turner, G. Sakr, F. Jomard, J. Barjon, G. Degutis, Y-G. Lu, J. D'Haen, A. Hardy, M. Van Bael, J. Verbeeck, G. Van Tendeloo,; K. Haenen, Physica Status Solidi-Rapid Research Letters, 8 (2014) 705-709..
3. S. Drijkoningen, P. Pobedinskas, S. Korneychuk, A. Momot, Y. Balasubramaniam, M.K. Van Bael, S. Turner, J. Verbeeck, M. Nesládek, K. Haenen, Crystal Growth & Design, 17 (2017) 4306.

Diamond nanoparticles are complicated materials, presenting a diverse range of sizes, shapes, speciation and defects that cannot easily be controlled during synthesis. Each of these features contribute to structure/property relationships, but in different ways. Attempts to purify samples to tune the properties often fail due to lack of specificity, and polydispersivity usually persists. An alternative approach is to separate classes of nanodiamonds based on their properties, rather than seeking monodispersed samples based on their structure. To investigate if this approach has merit we have used machine learning to classify nanodiamonds and identify class-dependent properties and characteristics. Using a fully reconstructed data set that spans the experimentally observed size range from 1 nm to 4.5 nm we find that there are multiple classes of particles, based on their similarity in the high dimensional feature space. By using an interpretable classifier, with excellent accuracy, precision and recall, we also identify possible purification pathways based on the important features ranked by the AI.

Non-perturbing sensing techniques applicable to biological systems currently are of central interest in the biosciences. Nanodiamond is a highly biocompatible nanomaterial for construction of nanosensors, which can accommodate various photoluminescent crystal defects. Nitrogen-vacancy (NV) centers show high photostability and unique electronic sensitivity to magnetic field. Their spin properties can be read by optical means, which enables construction of various probes based on quantum mechanical interactions. If embedded in nanodiamond particles, NV centers can be exposed to biological environment and report sensitively on the spin-related processes occurring in a close vicinity of the particle. Molecular transducers designed for NV centers transposing the presence of particular analytes to a selective and unambiguous readout will be presented. Different types of surface architectures will be shown [1, 2]. Optical nanosensors designed for selective measurements of pH, redox potential, and ascorbate under physiological conditions will be discussed.

References
Rendler T., Neburkova J., Zemek O., Kotek J., Zappe A., Chu Z., Cigler P., Wrachtrup J. (2017). Optical imaging of localized chemical events using programmable diamond quantum nanosensors. Nat. Commun., 8, 14701.
Vavra J., Rehor I., Rendler T., Jani M., Bednar J., Baksh M. M., Zappe A., Wrachtrup J., Cigler P. (2018). Supported Lipid Bilayers on Fluorescent Nanodiamonds: A Structurally Defined and Versatile Coating for Bioapplications. Adv. Funct. Mater. 28, 1803406.

Milled fluorescent nanodiamonds containing nitrogen-vacancy (NV) centers provide an excellent platform for sensing applications due to their quantum sensitivity to a variety of external fields at nanometer length scales. Currently, the irregularity in crystal morphology and the difficulty of functionalizing the nanodiamond surface impede efforts to realize scalable assemblies of nanodiamonds, or to create heteroassemblies with other materials. Poor control over the nanodiamond composition and surface chemistry also creates significant heterogeneity in their optical and quantum characteristics. We will present scalable strategies to form ordered arrays of nanodiamonds using template-directed capillary assembly. This method enables the precise spatial arrangement of primary nanodiamonds with diameters below 50 nm, and it is compatible with the assembly of other compositions of colloidal nanoparticles. Large-scale arrays facilitate automated, repeatable measurements of the nanodiamonds’ optical, structural, and quantum characteristics, enabling improved statistical understanding of these key properties and their heterogeneity. The flexible assembly method will allow for detailed studies of chemical treatments, integration of nanodiamonds within photonic and electronic devices, and the realization of quantum heterostructure devices composed from complementary nanomaterials.

For many applications the surface chemistry of nanodiamond (ND) is a key factor for the control of properties such as the wetting behavior, biocompatibility and functionality[1]. By attaching appropriate moieties, the interaction with physiological environments can be controlled.
An important point for biomedical applications is the inhibition of a nonspecific protein corona. This shell of e.g. serum proteins masks the desired functions and renders the physiological properties of the nanoparticle uncontrollable[2].
Here we report on the grafting of zwitterionic moieties enabling the formation of NDs without protein corona [3] that enable the specific grafting of antibodies, targeting peptides and CO releasing drug molecules using visible light. The resulting constructs have been characterized using IR, Raman, x-ray spectroscopies as well as particle analysis, TGA and electron microscopy.
The ND conjugates have been tested in vitro as well as in cell assays and demonstrated their functionality in the cellular environment. Additionally, these NDs have been functionalized with pyrene moieties for fluorescence sensing in solution with strong ion background and demonstrated colloidal stability, sensitivity and specificity for the detection of heavy metal ions against a background of ions present in physiological environments.
This research has received funding from Volkswagenstiftung under Grant Agreement no. 88393.

References
[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.

Here we show how the diamond crystal orientation influences the interaction with biological systems[1]. More specifically we are investigating the interaction between the <100>, <110> and <111> surfaces of single crystal diamond with biomolecules, cell culture medium, mammalian cells and bacteria. We show that the crystal orientation significantly alters these biological interactions. Most surprising is the two orders of magnitude difference in the number of bacteria adhering on <111> surface compared to <100> surface when both the surfaces were maintained under the same conditions. We also observe differences in how small biomolecules attach to the surfaces. Neurons or HeLa cells on the other hand do not have clear preferences for either of the surfaces. To explain the observed differences, we theoretically estimated the surface charge for these three low index diamond surfaces and followed by the surface composition analysis using x-ray photoelectron spectroscopy (XPS). We conclude that the differences in negative surface charge, atomic composition and functional groups of the different surface orientations lead to significant variations in how the single crystal diamond surface interacts with the studied biological entities.
[1] Damle, V., Wu, K., De Luca, O., Ortí-Casañ, N., Norouzi, N., Morita, A., de Vries, J., Kaper, H., Zuhorn, I.S., Eisel, U. and Vanpoucke, D.E., 2020. Influence of diamond crystal orientation on the interaction with biological matter. Carbon, 162, pp.1-12.

Diamond electrodes terminated with surface layers can impart significant changes to the electronic properties of a diamond interface and we have previously reported on studies of the tunability of surface work function by substituting hydrogen with oxygen under controlled conditions [1]. Engineering a functionalised diamond surface that is stable and reproducible is of technological importance to future diamond electronic devices such as Schottky junctions and electron-emitting electrodes for field emission, thermionic energy converters, secondary electron multipliers.
O’Donnell and his group have demonstrated the formation of a stable Li-O termination of diamond by physical vapour deposition of a monolayer of Li on the diamond surface which develops and exhibits Negative electron affinity (NEA) and hence lowers the work function, even at an elevated temperature of 800 °C [2]. In this work we are seeking an alternative method, a novel and easy chemical means of introducing and coordinating lithium with oxygen on the diamond surface to realize a more controlled approach of creating a stable –O-Li dipole. with uniformity over the diamond surface which also serves as a means to study and visualize for the first time the effect of temperature on the concentration of Li atoms in the diamond surface and on the Li-O species on the surface of diamond sample.
Also, Tin monoxide has been considered as potential material for battery anodes, coating, gas sensing, catalysis, etc. The application area of SnO extends over photovoltaics, displays, information storage, etc. as it has a large optical bandgap of 4.12 eV, high transparency (~90%) non-toxic and abundant [3]. SnO has been found to form a layered structure in a crystallographic direction of [001] with a monolayer thickness of 0.48 nm. Sn 5s form a lone pair of electrons which point towards the interlayer spacing and result in dipole-dipole interaction leading to a Van der Waals gap of 2.52 A between the layers. Various groups have demonstrated the growth of SnO mono and multiple layers by various methods but many of these result in an upper SnO₂ layer that forms as soon as the sample is taken into the oxygen environment as SnO is metastable and hence rapidly converts into SnO₂.
We are doping oxygen terminated Boron doped Polycrystalline diamond and Nitrogen doped HPHT diamond (111) with Li atoms using microwave plasma technique. The oxygen termination was induced by acid washing. We analyze the sample with X- Ray Photoemission Spectroscopy (XPS) and Raman spectroscopy. The main aim of this study is to analyze the change in concentration of Li atoms in the bulk and on the surface of diamond with annealing temperature. We have produced sub ML SnO on conductive, p-type single crystal diamond under ultra-high vacuum (UHV) conditions by e-beam evaporation. The samples will be characterised in-situ by Spot Profile Analysis Low-Energy electron diffraction (SPA-LEED) and Energy Filtered Photo Emission Electron Microscopy (EF-PEEM) and X-ray Photoemission Spectroscopy (XPS). We quantify the change in work function between mono and few layer SnO and its effect upon the barrier height of a p-type diamond Schottky junction.

References

[1] Wan, G., Cattelan, M. & Fox, N. A. Electronic structure tunability of diamonds by surface functionalization. J. Phys. Chem. C (2019). doi:10.1021/acs.jpcc.8b11232
[2] K. M. O’Donnell et al., “Diamond Surfaces with Air-Stable Negative Electron Affinity and Giant Electron Yield Enhancement,” Adv. Funct. Mater., vol. 23, no. 45, pp. 5608–5614, Dec. 2013.

[3] Pei, Y., Liu, W., Shi, J., Chen, Z. & Wang, G. Fabrication and Characterization of p-Type SnO Thin Film with High c-Axis Preferred Orientation. 45,

When the surface of diamond is H-terminated, a p-type surface conductive layer is formed. Atmospheric impurities, NO₂, O₃, fluoropolymer, transition metal oxides and Al₂O₃ adhere to the diamond surface and activate diamond’s surface conductance by generating and trapping negative charges on the diamond surface. These charges increase the positive p-type carrier densities in the diamond surface and reduce its resistance. High drain current field effect transistors (FETs) of 1.2 to 1.3 A mm^-1, high frequency f_T of 75 GHz, and high voltage, >2000 V, devices have been demonstrated with these activation surface chemistries. However, these devices degraded in a few days or when stressed with high voltage.
Research to stabilize diamond’s surface conductance has met with some success. H-terminated diamonds overcoated using atomic layer deposition (ALD) Al₂O₃ have the best demonstrated stability, but the surface resistance is high (5 to 12 kW sq^-1). Diamonds coated with transition metal oxides such as WO₂, MoO₃, and V₂O₅ have lower resistances of 2.5 to 3.5 kW sq^-1 [1,2]. Their stability has been demonstrated although these oxides are conductive, have several oxidations states, and may electrochemically degrade under the high electric fields associated with FETs. This presentation will discuss the use of a mixed-oxide (Al₂O₃-SiO₂) ALD film to generate resistances between 1.5 and 3.5 kW sq^-1 with demonstrated stability in excess of half a year.
The low surface resistance obtained with this mixed oxide is a result of a change in the structure of Al₂O₃ when mixed with SiO₂. The electron trap surface density of Al₂O₃ is ~7x10¹² cm^-2 while the mixed oxide surface density is 3x10¹³ to 8x10¹³ cm^-2 [3,4] with an estimated trap depth of 5.4 eV. This mixed oxide is just one of a group of oxides [5] with deep, > 5 eV, high-density electron traps.
Increased stability of the diamond surface is obtained by terminating the surface with deuterium atoms instead of hydrogen atoms. This additional stability is beneficial during device fabrication. This presentation will discuss diamond’s surface conduction with Al₂O₃-SiO₂ combined with deuterium atom termination.
References
[1] C. Verona, F. Arciprete, M. Foffi, E. Limiti, et. al. Appl. Phys. Lett. 112 (2018), 181602.
[2] K. G. Crawford, D. Qi, J. McGlynn, T. G. Ivanov, et. al. Sci. Rep. 8 (2018), 3342.
[3]. K. Raghavan, P. Sabu, K. Vijayachandra, C. Rao C. G. Ramachandran. Bull Chem. Soc. Jpn. 64 (1991), 1920.
[4] A. Satsuma, Y. Westi, Y. Kamiya, T. Hattori, Y. Murakami. Bull Chem. Soc. Jpn. 70 (1997), 1311.
[5] A. Corma. Chem. Rev. 95 (1995), 559.
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Surface acoustic waves (SAWs) and bulk acoustic waves are two of MEMS technologies of industrial relevance and can be found in a myriad of devices. Acoustic RF filters, for instance, are integral parts of wireless communication systems. SAW devices have been adapted as sensors which can be configured to operate both passively and wirelessly. For example, SAWs find applications in life sciences, in structural health monitoring, etc. In particular, SAW devices can also be operated in extremely confined environments such as implantable sensors to monitor or to provide real-time treatment. In this case, it would make sense to protect the sensor surface and leave only the functionalized area visible for detection. This is true for implantable sensors where space is a major constraint and therefore requires the development of an intrinsically protected design. Also, this is true for biosensors where the surface is continuously exposed to the analysed biofluid.

In this work, we propose the use of a shear surface wave guided between the semi-infinite substrate and high acoustic velocity thin film materials. The upper high acoustic velocity layer works as an acoustically isolating layer, protecting the guided surface shear wave from undesired mechanical damping and eliminate the need for packaging. With very high acoustic velocities, diamond and silicon carbide (SiC) are excellent candidates for packageless SAW sensors applications. In previous studies, we have already demonstrated the possibility of diamond coating of SAW devices using Microwave Plasma Enhanced Chemical Vapor Deposition with Linear Antennas (MW-LA-PECVD) at low temperature [1, 2].

Here we investigate, theoretically and experimentally, the use of diamond and SiC as a passivation layer for SAW devices. Theoretical calculations were carried out using COMSOL Multiphysics FEM simulation software. Two different piezoelectric materials were investigated, ST-cut quartz and 36°YX LiTaO₃ as well as two common guiding layer materials – zinc oxide (ZnO) and silicone oxide (SiO₂). Theoretical calculations show, that 2 μm thick diamond or SiC layers are sufficient to confine the acoustic wave in the guiding layer at a wavelength close to 10 μm. From the displacement curve we could see, that for 36°YX LiTaO₃ substrates the acoustic wave is very well confined for both guiding layer materials. On the contrary, shear wave disperses within the ST-cut quartz bulk with the addition of a passivation layer.

Based on theoretical results, SAW devices were fabricated on 36°YX LiTaO₃ substrates with 10 nm/190 nm Ti/Au electrodes patterned by RF sputtering, photolithography and chemical wet etching techniques. The SiO₂ layer was deposited using low temperature plasma enhanced chemical vapor deposition with a resulting normalized thickness equal to h_SiO2/λ = 0.2. Depositions of nano-crystalline diamond (NCD) and SiC layers were carried out by MW-LA-PECVD with resulting normalized thicknesses of h/λ = 0.2. All depositions were carried out at a temperature below 500 °C to preserve the piezoelectric properties of substrates.

Our results are promising for waveguide protection applications as well as opening other directions, such as the introduction of the local sensitive zone in the diamond layer, which can be interesting for bio or electrochemical sensing applications.

This work has been supported by the Grant Agency of the Czech Republic (GACR) contract 19-09784Y, Czech Science Foundation (project 20-03187S) and by the Czech Technical University in Prague project SGS20/084/OHK4/1T/17.

References:
[1] TALBI, A., A. SOLTANI, A. RUMEAU, et al., Physica status solidi (a), 2015, 212(11), 2606-2610 DOI: 10.1002/pssa.201532188. ISSN 18626300.
[2] DRBOHLAVOVÁ, L., L. FEKETE, V. BOVTUN, et al., Sensors and Actuators A: Physical, 2019, 298, DOI: 10.1016/j.sna.2019.111584. ISSN 09244247.

Phononic metamaterials (PnMs), analogue to photonic and plasmonic in optics, have attracted a great deal of attention in the last two decades. Based on phononic band-gap and local resonance mechanisms, these artificial materials enabled advanced control of elastic waves in solid condensed matter including wave-guiding, trapping, multiplexing, demultiplexing, etc. They offer the possibility to confine and focus an acoustic wave in an ultra-small region of sub-wavelength dimensions and to extend mechanical resonance lifetimes (high quality factor (Q)). For biosensing applications, Shear surface wave (Sh-SAW) and Antisymmetric Lamb waves are the most interesting, as they are compatible with liquid environments. Diamond is an attractive material for biosensing applications thanks to its chemical inertness, biocompatibility and prolonged stability of covalently attached biomolecules. Therefore, combining diamond PnMs and shear surface waveguides is of great interest to enhance the performance of biosensor.

In this work, we investigated the properties of Sh-SAW sensors with diamond pillar PnMs. Band structures and sensitivity were calculated with the finite element method (FEM, COMSOL Multiphysics). The sensitivity of studied structures was investigated by adding 200 nm thick polymethyl methacrylate layer on top of the structure. The studied structure consists of 60 µm high and 10 µm wide block of 90ST-cut quartz substrate, which supports shear wave propagation. For comparison of sensitivity either a 2 µm thick continuous SiO₂ guiding layer or the diamond pillars with radius = 1 µm and height = 1 or 2 µm were introduced on top of the quartz substrate. For the diamond PnMs, the slowest shear wave mode is completely localized in the pillar for both heights. The sensitivity is significantly increased for diamond pillar phononic crystals (4.4 % for 2 μm height pillar) in comparison with plain ST-cut quartz (0.9 %) and also ST-cut quartz + 2 µm SiO₂ guiding layer (2.5 %). These results are very promising for the fabrication of highly sensitive LW-SAW sensor for biosensing applications.

This work has been supported by the Grant Agency of the Czech Republic (GACR) contract 19-09784Y, Czech Science Foundation (project 20-03187S) and by the Czech Technical University in Prague project SGS20/084/OHK4/1T/17.

To develop practical applications for diamond-based electronics it will be important to have calibrated device models for use in circuit simulators such as LTSpice and Cadence. These Spice models consist of lumped-element components with values extracted by DC and RF measurements for bias conditions that represent typical operating conditions. Of particular importance for PN junction diodes are the on-resistance, R_on, and off-state capacitance, C_off. The product R_onC_off is a useful figure of merit that is minimized by careful optimization of the device design. While C_off is controlled largely by the thickness of the intrinsic region of the diode, there are several factors that contribute to the on-resistance, including the contact resistance present at the interfaces between the metal contacts and the doped semiconductor. To minimize the contact resistance, high doping concentration is required in the n- and p-type diamond regions. To further minimize on-resistance, smaller i-layer thickness is required. We have fabricated vertical diamond P-I-N diodes by plasma enhanced chemical vapor deposition of intrinsic and heavily phosphorus doped diamond films on boron doped diamond <111> substrates. Secondary ion mass spectroscopy (SIMS) confirms that the boron doping in the substrate exceeds 10²⁰ cm^-3, the unintentional p-type doping in the i-layer is ~10¹⁶ cm^-3, while the peak concentration at the surface of the n-layer is ~ 10¹⁹ cm^-3. The thickness of the intrinsic layer was 300 nm and that of the heavily doped n-layer was 50 nm. Despite the heavy doping of the n-layer it is thin enough to be fully depleted by the metal contact resulting in a Schottky PIN diode.

The diodes were fabricated with ground-signal-ground (GSG) contacts on the top surface allowing for DC and RF probing. To electrically isolate the devices a partial mesa etch was performed using O₂/SF₆ reactive ion-etching to define the diode area and top anode contact. A second RIE step etched through the i-layer to expose the p-type substrate on the top surface. Electron-beam evaporation of Ti/Ni/Au (50 nm/50 nm/300 nm) metals form the top cathode and anode contacts to the n and p-layer diamond respectively.

A combination of DC, and S-parameter measurements were used to extract the small-signal parameters for a diode Spice model. At a forward bias of 15 V the diode supports a current density in excess of 36 kA/cm², with an on-resistance of 9x10^-5 Ohm-cm². The S-parameter measurements were converted to Z-parameters from which the diode off-capacitance was found to be 17.5 nF/cm². The FOM is 1.6 ps corresponding to a frequency of 101 GHz. The full diode model provides excellent agreement with the measured current-voltage characteristics up to a forward bias of 15 V/3 A. It also reproduces the one-port s-parameter, S11, up to 25 GHz over all DC bias conditions.

This research is supported, in part, by NSF contract ECCS-1542160.

Hydrogen-terminated surfaces of diamond are known to exhibit p-type conductivity, and power devices are being considered as an application that will take advantage of this property. Recently, poly-crystalline diamond devices have been attracting attention because it was reported that high breakdown voltage (>1 kV) MOSFETs can be fabricated^[1] despite its lower cost compared to single-crystalline diamond. This property originates from non-doped hydrogen-terminated poly-crystalline diamond. However, the electrical characteristics variability depends on the crystal orientation. In this study, the correlation between the local electrical characteristics obtained by scanning probe microscopy (SPM) and the crystal orientation obtained by electron backscattering diffraction (EBSD) was evaluated in order to clarify the electrical defect state of each crystal orientation.
Poly-crystalline diamond substrates (size: 3 mm × 3 mm, thickness: 0.33 mm) manufactured by Diamond Materials GmbH were used in this experiment. After cleaning the substrate in the order of aqua regia (80 °C, 5 min.), piranha solution (120 °C, 10 min.) and HNO₃ and H₂SO₄ mixed acid (240 °C, 30 min.), the substrate was subjected to microwave plasma CVD with 199.5 sccm of CH₄ and 0.5 sccm of H₂ to grow diamond films. The hydrogen termination was processed by the same CVD equipment. Afterwards, the substrate surfaces were measured by current-AFM (c-AFM), and the same area was measured by EBSD. We used SPM-9600 (Shimadzu Corp.) for c-AFM measurement, using PtIr-coated conductive cantilever to measure surface morphology and c-AFM current images at bias of -0.05 V.
As a result of the measurements using c-AFM, it was confirmed that there were spots with different current flow patterns. These current spots were concentrated at the grain boundaries, and they were also confirmed in some parts of the grain. In addition, variation in the current spots density was confirmed. Next, we measured the same area as the c-AFM site with SEM/EBSD to investigate the correlation between the current spots in the grains and the crystal orientation. The surface morphology images and current images obtained from c-AFM measurements were compared with the inverse pole figure (IPF) map obtained from SEM/EBSD measurements. These results suggest that there is a correlation between the current spot density distribution and crystal orientation, ex, (111) oriented crystal shows lower current spots density, and (101) oriented crystal shows high current spots density. The results suggest that this is caused by the difference in the etching and/or growing condition of each crystal orientation. This study can reveal the suitable crystal orientation for electrical device fabrication that requires low electrical defect density.

[Reference]
[1]M. Syamsul et al., IEEE Electron Device Lett. 38, 5, 607-610 (2017)

[Acknowledgements]
This work is supported by JSPS KAKENHI JP18K13804 and the NAIST Special Fund

Nanodiamonds (NDs) have excited a lot of interest in the field of biomedical application as they can be used as spin probes for optical magnetic sensing and biological environments. Due to their optical and biological properties, NDs are expected to be employed as multitasking markers for the accurate visualization of biophysical parameters such as and magnetic fields and temperature. However, the engineered NDs to intracellular structures of interest within a complicated biological environment remains a challenge.
In this work, we investigated each property of the 4 types of nanodiamonds like SEM, PL spectra and then the NDs were transfected into human skin cells (WS1) using silicon nanopillar. For the development of nanodiamonds as multifunctional biosensor, researchers are still engaged in the in-depth study of NDs and their effect on life interfaces.

Non-perturbing sensing techniques applicable to biological systems currently are of central interest in the biosciences. Nanodiamond is a highly biocompatible nanomaterial for construction of nanosensors, which can accommodate various photoluminescent crystal defects. Nitrogen-vacancy (NV) centers show high photostability and unique electronic sensitivity to magnetic field. Their spin properties can be read by optical means, which enables construction of various probes based on quantum mechanical interactions. If embedded in nanodiamond particles, NV centers can be exposed to biological environment and report sensitively on the spin-related processes occurring in a close vicinity of the particle. Molecular transducers designed for NV centers transposing the presence of particular analytes to a selective and unambiguous readout will be presented. Different types of surface architectures will be shown [1, 2]. Optical nanosensors designed for selective measurements of pH, redox potential, and ascorbate under physiological conditions will be discussed.

References
Rendler T., Neburkova J., Zemek O., Kotek J., Zappe A., Chu Z., Cigler P., Wrachtrup J. (2017). Optical imaging of localized chemical events using programmable diamond quantum nanosensors. Nat. Commun., 8, 14701.
Vavra J., Rehor I., Rendler T., Jani M., Bednar J., Baksh M. M., Zappe A., Wrachtrup J., Cigler P. (2018). Supported Lipid Bilayers on Fluorescent Nanodiamonds: A Structurally Defined and Versatile Coating for Bioapplications. Adv. Funct. Mater. 28, 1803406.

NV centers in diamond are point like defects that have attracted a lot of attention as one of the best candidates for quantum technologies particularly for sensing and imaging nanoscale magnetic fields. For this application, the use of an ensemble of NV centers with a high concentration in a high quality diamond layer is of prime interest. In previous works, it has been demonstrated that N₂O doping rather than N₂, allows improving diamond quality by limiting the formation of macroscopic defects and NV’s photobleaching [1]. More recently, the growth of N₂O doped samples thicker than 1.5 mm has been achieved with substitutional nitrogen (Ns) concentration as high as a few tens of ppm which is very difficult to obtain by a CVD process using a conventional N₂ precursor. After irradiation by a 10 MeV electron beam, a conversion ratio of 20% has been obtained leading to a CVD grown layer with 4 ppm of NV^-. A decrease of the longitudinal spin relaxation time T₁ observed when the magnetic field is aligned along the [100] direction (i.e. when all NV centers classes are equally degenerate) indicates that NV-NV dipolar coupling becomes a dominant mechanism in such highly doped samples.
In this work, we will more particularly focus on the optimization of N₂O concentration in the gas phase in order to limit NV^--NV^- interaction while maximizing density and spin properties. In the same time, samples grown in the presence of N₂ + O₂ and N₂ only will be compared in order to investigate the benefit of the use of N₂O. We aim at evaluating the maximum sensitivity achievable in a magnetic quantum sensor using such defect ensembles.

[1] A. Tallaire, L. Mayer, O. Brinza, M.A. Pinault-Thaury, T. Debuisschert, J. Achard, Highly photostable NV centre ensembles in CVD diamond produced by using N₂O as the doping gas, Appl. Phys. Lett., 111 (2017) 143101.

The recently discovered negatively charged tin-vacancy center (SnV-) in diamond combines high brightness single photon emission [1,2], large Debye-Waller factors and Fourier-limited linewidths [3,4]. Furthermore, the ground state fine structure splitting exceeds 800 GHz which potentially suppresses phonon mediated decoherence of the lowest ground state’s spin doublet, which is the predominant decoherence mechanism for other negatively charged group-IV – vacancy centers (Silicon-vacancy center (SiV-) and germanium-vacancy center (GeV-)). Therefore, the SnV- center supposedly combines the advantages of the most established colour centers, namely the long spin coherence time of the nitrogen vacancy center and the large Debye-Waller factor and insensitivity to first order stark shifts of the silicon vacancy center. The exploitation of these very promising features for quantum information processing is up to now limited by the charge instability of the center under resonant optical excitation [4]. This instability results in termination of the fluorescence and therefore inhibits the use of coherent manipulation protocols.
We here present studies on stabilizing the SnV- charge state based on the recently discovered Coulomb-driven single defect engineering by co-implantation of charge donors into diamond [5]. We co-implant oxygen, sulfur and boron together with tin and find a suppression of bleaching (reduction of fluorescence intensity under continuous illumination) and fluorescence termination together with increased photo-luminescence intensity for sulfur co-doping. These results pave the way for application of the SnV center in coherent optical spin control experiments.
[1] Iwasaki T. et al., Phys. Rev. Lett. 119, 253601 (2017)
[2] Tchernij S. D. et al., ACS Photonics 4, 25802586 (2017)
[3] Trusheim M. E. et al., Phys. Rev. Lett. 124, 023602 (2020)
[4] Görlitz J. et al., New J. Phys. 22, 013048 (2019)
[5] T. Lühmann et al., Nat. Commun. 10, 4956 (2019)

Ensembles of negatively-charged nitrogen-vacancy (NV^-) centres in diamond constitute a promising platform for magnetometry applications. However, the sensitivity of present NV-ensemble devices and the need for diamond material with reproducible properties have hindered progress toward many envisioned applications. The work presented here addresses these challenges by optimising a chemical vapor deposition (CVD) growth process to produce material at scale with improved NV-ensemble properties.

Through development of the CVD process it is shown that nitrogen incorporation can be increased without an increase in the density of unwanted vacancy-related defects which act as charge acceptors, reducing the [N_s⁰]/[N_S] and [NV^-]/[NV] ratios, and limiting the dephasing time, T₂*. The process of irradiation and annealing is optimised and subsequent measurements of the NV concentration, charge ratio and contrast suggest that these grown-in defects have a detrimental impact on the NV-ensemble properties. Strain is also demonstrated to be a critical parameter to control.

This work also investigated the potential to produce material with reproducible NV concentration, NV charge ratio, strain homogeneity, and T2* by characterising a batch of samples containing approx. 4 ppm NV. The measured NV^- and NV⁰ concentrations in the batch of samples varied by less than 7% (1 standard deviation) and exhibited well-controlled strain inhomogeneity. 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 material reported in this work enables immediate sensitivity improvements for present quantum-sensing devices and the correlations observed between material properties at different stages of processing provide efficient metrics for future work targeting other NV densities or sample geometries such as micron-scale, NV-rich layers for magnetic imaging.

Color centers in diamond are promising candidates for emerging quantum sensing and quantum information processing applications due to their exceptional properties. Coupling diamond color centers with metal plasmonic nanostructures is reported to enhance the optical properties of color centers effectively, but the plasmonic metal structures of previous studies are vulnerable to the environment, making the diamond-metal system unstable. Here, we demonstrate a novel and highly-tunable structure of Ag nanospheres coated with a layer of diamond containing SiV centers. The Ag was grown by thermal vapor deposition and then plasma treated to form uniform nanospheres, which were protected by the stable diamond shell grown by plasma-enhanced chemical vapor deposition (PECVD) method. This enhances the emission rate and brightness of SiV in diamond drastically. The structure also shows great stability to extreme corrosive and high-temperature conditions. Our finding is promising for developing ultrabright quantum sensing, high-efficiency imaging, and surface plasmon-quantum emitter waveguide.

Here we show how the diamond crystal orientation influences the interaction with biological systems[1]. More specifically we are investigating the interaction between the <100>, <110> and <111> surfaces of single crystal diamond with biomolecules, cell culture medium, mammalian cells and bacteria. We show that the crystal orientation significantly alters these biological interactions. Most surprising is the two orders of magnitude difference in the number of bacteria adhering on <111> surface compared to <100> surface when both the surfaces were maintained under the same conditions. We also observe differences in how small biomolecules attach to the surfaces. Neurons or HeLa cells on the other hand do not have clear preferences for either of the surfaces. To explain the observed differences, we theoretically estimated the surface charge for these three low index diamond surfaces and followed by the surface composition analysis using x-ray photoelectron spectroscopy (XPS). We conclude that the differences in negative surface charge, atomic composition and functional groups of the different surface orientations lead to significant variations in how the single crystal diamond surface interacts with the studied biological entities.
[1] Damle, V., Wu, K., De Luca, O., Ortí-Casañ, N., Norouzi, N., Morita, A., de Vries, J., Kaper, H., Zuhorn, I.S., Eisel, U. and Vanpoucke, D.E., 2020. Influence of diamond crystal orientation on the interaction with biological matter. Carbon, 162, pp.1-12.

Diamond nanoparticles are complicated materials, presenting a diverse range of sizes, shapes, speciation and defects that cannot easily be controlled during synthesis. Each of these features contribute to structure/property relationships, but in different ways. Attempts to purify samples to tune the properties often fail due to lack of specificity, and polydispersivity usually persists. An alternative approach is to separate classes of nanodiamonds based on their properties, rather than seeking monodispersed samples based on their structure. To investigate if this approach has merit we have used machine learning to classify nanodiamonds and identify class-dependent properties and characteristics. Using a fully reconstructed data set that spans the experimentally observed size range from 1 nm to 4.5 nm we find that there are multiple classes of particles, based on their similarity in the high dimensional feature space. By using an interpretable classifier, with excellent accuracy, precision and recall, we also identify possible purification pathways based on the important features ranked by the AI.

For many applications the surface chemistry of nanodiamond (ND) is a key factor for the control of properties such as the wetting behavior, biocompatibility and functionality[1]. By attaching appropriate moieties, the interaction with physiological environments can be controlled.
An important point for biomedical applications is the inhibition of a nonspecific protein corona. This shell of e.g. serum proteins masks the desired functions and renders the physiological properties of the nanoparticle uncontrollable[2].
Here we report on the grafting of zwitterionic moieties enabling the formation of NDs without protein corona [3] that enable the specific grafting of antibodies, targeting peptides and CO releasing drug molecules using visible light. The resulting constructs have been characterized using IR, Raman, x-ray spectroscopies as well as particle analysis, TGA and electron microscopy.
The ND conjugates have been tested in vitro as well as in cell assays and demonstrated their functionality in the cellular environment. Additionally, these NDs have been functionalized with pyrene moieties for fluorescence sensing in solution with strong ion background and demonstrated colloidal stability, sensitivity and specificity for the detection of heavy metal ions against a background of ions present in physiological environments.
This research has received funding from Volkswagenstiftung under Grant Agreement no. 88393.

References
[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.

Milled fluorescent nanodiamonds containing nitrogen-vacancy (NV) centers provide an excellent platform for sensing applications due to their quantum sensitivity to a variety of external fields at nanometer length scales. Currently, the irregularity in crystal morphology and the difficulty of functionalizing the nanodiamond surface impede efforts to realize scalable assemblies of nanodiamonds, or to create heteroassemblies with other materials. Poor control over the nanodiamond composition and surface chemistry also creates significant heterogeneity in their optical and quantum characteristics. We will present scalable strategies to form ordered arrays of nanodiamonds using template-directed capillary assembly. This method enables the precise spatial arrangement of primary nanodiamonds with diameters below 50 nm, and it is compatible with the assembly of other compositions of colloidal nanoparticles. Large-scale arrays facilitate automated, repeatable measurements of the nanodiamonds’ optical, structural, and quantum characteristics, enabling improved statistical understanding of these key properties and their heterogeneity. The flexible assembly method will allow for detailed studies of chemical treatments, integration of nanodiamonds within photonic and electronic devices, and the realization of quantum heterostructure devices composed from complementary nanomaterials.

The deposition of closed nanocrystalline diamond (NCD) thin films on non-diamond substrates requires diamond nucleation sites. An established technique for creating nucleation sites is a surface treatment with a water-based colloidal solution of ultra-dispersed nanodiamond (ND) particles.¹ In ND seeding the most important factors are the surface charge of the substrate and the zeta-potential of the used colloid. Together, they determine the nucleation density, which can lead to an extreme difference in the observed seeding density.^2–6 However, surface contamination by hydrocarbons, which is an unavoidable and quick process even in a cleanroom environment,⁷ is often not taken into consideration. Such contamination can alter the surface wetting properties⁸ and surface charge⁹, and thus the eventual seeding density. In this work we investigate and model the impact such surface contamination has on the ND seeding efficiency.

Tantalum is a metal well known for its resistivity to corrosion, bioinertness, biocompatibility and excellent osteoconductivity.¹⁰ After sputtering a thin film of Ta on silicon substrates, these stacks were exposed to the ambient atmosphere for different times, allowing to achieve various degrees of surface contamination. Then, the Ta surface wettability was measured, samples were seeded with NDs, and the subsequent seeding density evaluated. As surface coverage by hydrocarbons increased, the wetting properties gradually turned from hydrophilic to hydrophobic. The ND seeding density first decreased and then increased again. To understand this correlation, we propose an analytical model that describes the dynamics of surface contamination and hydrocarbon contaminant interaction with the surface, and the resulting ND seeding density.

We assume that when a hydrocarbon (HC) molecule lands on the Ta surface it attracts negative surface charge towards it, while creating a positive surface charge around it. Due to said dipole formation, the top part of the HC molecule is charging negatively. Solving differential equations for the dynamics of the normalized surface coverage number density of the HC molecules, n, and surface charge density, σ, we derive σ (n,η) = n + (1 – n)^{(1 + η)}, where η is a ratio between an area of the depleted charge around the HC molecule and the area of the HC molecule itself. The assumption that the contact angle, θ, is linearly proportional to the surface coverage by HC, n = θ /θ_sat, makes wettability measurements a valid method for determining the degree of surface contamination. If the sum of electrostatic double-layer and Van der Waals interaction energies becomes positive, a potential energy barrier for a ND particle approaching a surface exists. ND particles must surmount a positive potential peak for a deposition onto the surface to occur. This occurs with a probability exponentially proportional to the potential barrier. By fitting the experimental data, we find that the ND seeding density is exponentially proportional to the calculated positive surface charge number density, which forms around HC molecules. This constitutes 57% of the HC molecule area, while the created barrier height is estimated to be ~ 0.18 eV.

References
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Wide bandgap electronics made from GaN and Ga₂O₃ are currently under development due to their potential to create advanced RF and power electronics. However, the performance and reliability of these devices are often controlled by their electrothermal response during device operation. A key feature which limits thermal control of the peak temperature is the thermal resistance that is encountered intrinsically from the material as well as across material interfaces with these devices. One method that has shown promise for mitigating the high thermal resistance of these devices is through their heterogenous integration with other materials which assist in heat dissipation and aid thermal management. For this, thermal transport across interfaces and novel integration methods are necessary.

In this work we will present modeling efforts which guide the impact of heterogenous integration on the thermal performance of UWBG devices. Thermal metrology methods used to measure interfacial thermal conductance will be discussed and new insights into the control of thermal boundary conductance given. Techniques such as surface activated bonding will be presented to demonstrate the integration of GaN and Ga₂O₃ with high thermal conductivity substrates such as SiC and diamond. The use of other high thermal conductivity dielectrics for improved heat dissipation from the top side and bottom side of the devices will be discussed. Finally, the prospects for the future development of thermal management via heterogenous integration will be presented.

Diamond is a promising candidate for a base material of high power electronic devices, as well as for a superior heat spreading substrate, due to its excellent physical properties such as the highest electrical breakdown field strength, the highest thermal conductivity, and a high RF power capability that is 3 times higher in comparison with SiC. For the practical application toward power devices, we need diamond/semiconductor heterointerfaces with high thermal stability and can withstand the temperature rise of power devices during operating. Recently, Liang has demonstrated that diamond/Si [1] and diamond/GaAs [2] heterointerfaces with high thermal stability can be fabricated by surface activated bonding (SAB) at room temperature (RT). In this work, we have examined the bonding mechanism using cross-sectional scanning transmission electron microscopy (STEM) combined with electron energy loss spectroscopy (EELS) and energy dispersive x-ray spectroscopy (EDX).
In the SAB process, surfaces of diamond and Si (or GaAs) wafers are activated at RT before bonding by creating dangling bonds via the irradiation of high-energy Ar atoms in a high vacuum, and the surfaces are then bonded by the contact most of the time under pressure to form strong chemical bonds even for imperfect surfaces. For diamond/Si heterointerfaces, atomic diffusion at the interfaces, presumably due to the transient enhanced diffusion assisted by the point defects introduced in the surface activation process [3], is confirmed during the bonding process. The crystallinity at the diffusing region is rather low, and it is recovered by 1000 ^oC annealing via the formation of SiC layers, that would play a pivotal role in the relaxation process of residual stress due to the misfit of thermal expansion coefficient between diamond and Si [1]. Therefore, the defect-assisted atomic diffusion at the interfaces would be a key concept for the formation of high thermal-stability diamond/Si heterointerfaces. Similar atomic diffusion is also confirmed at diamond/GaAs heterointerfaces during the SAB process.
[1] J. Liang, S. Masuya, M. Kasu, and N. Shigekawa, Appl. Phys. Lett. 110 (2017)111603.
[2] Y. Nakamura, Y. Shimizu, Y. Ohno, K. Shirasaki, Y. Nagai, M. Kasu, N. Shigekawa, and J. Liang, 13th Topical Workshop on Heterostructure Microelectronics (Toyama, Japan, 2019) 11-7.
[3] Y. Ohno, H. Yoshida, N. Kamiuchi, R. Aso, S. Takeda, Y. Shimizu, Y. Nagai, J. Liang, and N. Shigekawa, Jpn. J. Appl. Phys. 59 (2020) SBBB05.

N-polar gallium nitride (GaN) high electron mobility transistors (HEMTs) have shown superior performances when scaling for W-band frequencies over Ga-polar GaN HEMTs [1]. However, due to the shorter channel and closer gate fingers its ability to dissipate the heat from the channel to the heat-sink is limited. Polycrystalline (PC) diamond due to its very large thermal conductivity (TC) is one of the solutions for self-heating problem in high-power high-frequency devices. PC Diamond can be grown in a closed proximity to high temperature spot in the channel and spread the heat efficiently [2]. The standard Si₃N₄ thickness as a gate dielectric for W-band N-polar HEMTs is 5 nm, which can be used as thermal interface material to reduce thermal boundary resistance (TBR<2 m²K/GW) between diamond and GaN as well [3]. As diamond growth takes place in a high temperature hydrogen plasma environment using MPCVD, it can decompose both Si₃N₄ and GaN layers by reacting with silicon, nitrogen, and gallium and making volatile gases (SiH₄, NH₃, and GaH, respectively) [4]. Any damage to GaN channel may result in a degradation of device performance after the growth. In this work, we investigated the impact of diamond growth nucleation and main stages on 2-dimentional electron gas (2DEG) properties such as electron mobility, sheet carrier concentration, and sheet resistivity (before growth: μ_n=1200 cm²/V.s, n_s=1.34E13 cm^-2, and ρ_s=388 Ω/sq).
The main goal of the growth nucleation stage is to guarantee no damage to the 2DEG by covering the substrate (Si₃N₄/GaN) with thinnest possible nano-crystalline diamond (NCD) in shortest possible time as NCD has a poor TC (<100 W/m.K). The 2DEG has been characterized using Hall mobility measurement, and the thickness of nucleation layer has been measured by high resolution TEM. The nucleation layer can prevent diffusion of hydrogen radicals into the substrate which is highly dependent on the microwave power and gas pressure. To increase the diamond growth rate and decrease substrate etching rate the sample holder was redesigned to provide higher temperature (~700 C) with minimum power (600 W) and pressure (20 Torr). The CH₄% and time were changed from 3 to 10% and 10 to 15 min, respectively for nucleation stage followed by the same main growth for all samples (2% CH₄, 1600 W, 60 Torr, 45 min). We observed no degradation in 2DEG by using >5% CH₄ and 15 min growth, while by lowering the time to 10 min we started to see about 20-30% reduction in µ_n and 50-150 Ω/sq increase in ρ_s. This degradation varies with different cap layers on top of the channel; no cap layer, 1-2 nm AlN, or 1-2 nm 46% AlGaN.
To study the impact of main growth on 2DEG, we have chosen 5% CH₄ and 15 min for nucleation stage and changed main growth parameters including CH₄% (0.5-2%), power (600-1600 W), pressure (20-90 Torr), and sample holder design. CH₄% showed the highest impact in determination of damage level to 2DEG and pressure had the lowest impact. With the sample holder same as nucleation stage, 1% CH₄ is the minimum required for no damage. For the case with original sample holder, 1.5% CH₄ is the lowest acceptable value for the first 45 min of the growth.
In this work, we were able to achieve a smooth interface between diamond and GaN (including 5 nm Si₃N₄ interfacial layer) without using argon or nitrogen in the gas mixture by redesigning the sample holder and modifying growth parameters for lower etch rate of the substrate. This recipe is compatible with W-band N-polar GaN HEMTs as maintains device performance after the growth.
This work was supported by the Semiconductor Research Corporation (SRC) and DARPA under the JUMP program.
[1] B. Romanczyk et al., IEEE Electron Device Lett., vol. 41, no. 3, pp. 349–352, 2020.
[2] M. Malakoutian et al., Crystals, vol. 9, no. 10, pp. 1–14, 2019.
[3] J. Cho et al., J. Appl. Phys., vol. 121, no. 5, 2017.
[4] Y. S. Zou et al., Cryst. Growth Des., vol. 8, no. 5, pp. 1770–1773, 2008.

Maskless lithography has been combined with the microwave plasma chemical vapor deposition (CVD) of diamond to fabricate a variety of micro-sensors that can be deposited on substrates and encapsulated in a protective CVD diamond layer to survive extreme conditions of high-pressure, high- and low-temperatures, corrosive, and high radiation environments. These hardened sensors provide electrical conductivity, magnetic susceptibility, and stress data on a variety of metallic and superconducting materials under extreme conditions of pressures and temperatures in diamond anvil cell devices. In addition, boron-doping of CVD diamond also enables the fabrication of micro-heaters in diamond anvil cell devices. Microfabrication techniques have also been utilized to selectively grow localized nanodiamond structures on single crystalline diamond substrates. Since nanodiamond structures lack the cleavage planes representative of single crystal diamond; ultrahigh pressures of over 500 GPa can be generated in this nanodiamond/single crystal diamond structures. The studies on materials under extreme conditions using these diamond-based sensors and nanodiamond structures are carried out at the Advanced Photon Source, Argonne National Laboratory. Future research directions in vertical integration of multiple sensors, wireless communications, and controlling the sp-3 bonded content in nanodiamond microstructures will be presented.

The surface temperature of Venus is ~460 C, with a corrosive atmosphere including CO₂ and sulfuric acid at a pressure of 92 bar. A number of missions to Venus are being considered or planned that would include surface or atmosphere probes, and high temperature electronics would be required for power control, mechanical system control, and sensor electronics. While it is long been recognized that the wide bandgap of diamond would result in a negligible increase in the intrinsic carrier concentration, at high temperature the devices based on Schottky barriers would show a great reduction in the forward-reverse current rectification ratio. This research presents results of diamond diodes with a thin n-type layer in a Schottky-n-i-p structure, which achieves high current density, high breakdown field and a designed on-off rectification ration. The metal-n-i-p diamond diodes were prepared on single crystal p-type, boron doped (100) or (111) substrates, with undoped intrinsic epitaxial layers and phosphorus doped epitaxial layers prepared by microwave plasma CVD. The electrical results and simulations show regimes with low drift layer resistance where the contact resistance limits the current. The diodes have been confined in a high pressure CO₂ atmosphere for over 100 hrs. After removal, inspection of the surfaces shows dark regions thought to be carbon containing layers, which did not result in evident changes of the current-voltage characteristics. More extensive Venus atmosphere testing planned at the NASA GEER facility will include active electrical measurements. The results establish that with control of p- and n-type doping optimal device designs can be achieved for diamond electronics at high temperature.
This research supported by NASA through grant NNX17AG45G as part of the HOTTech program.

Diamond has many unique physical properties. Due to these unique properties diamond has high potential for applications; electrical devises, quantum devices, etc. Depending on the target application we need to develop the growth and fabrication technique, In this talk, we introduce the electronic applications and the quantum application using nitrogen-vacancy centers from the viewpoint of diamond growth and fabrication technique.

Betavoltaic cells directly convert the energy of beta particles into electricity.[1] The operation principle of a betavoltaic cell is similar to that of a solar cell, except that the energy source is beta-emitting isotopes. In betavoltaic cells, semiconductor junctions play the role of energy conversion. These cells have the advantage of longevity, which is determined by the half-life (T_1/2) of the beta-emitting radioisotopes. The cells can operate for several years to a decade. Betavoltaic cells are expected to be utilized for battery applications in remote locations such as outer space and underground.

Diamond is advantageous material for betavoltaic cells because the wide bandgap could offer high open circuit voltage due to large built-in potential of the junction, which improves the conversion efficiency of the cells. From theoretical perspective, ultrawidebandgap materials such as diamond and BN have potential to realize extreme conversion efficiency of the cells.[2] Moreover, diamond has durability against radiation and high temperature. Diamond betavoltaic cells could be utilized to application under extreme condition. In this study, we show the highest semiconductor conversion efficiency among the betavoltaic cells reported thus far by utilizing the large built-in potential of a diamond pn junction.
We formed pseudo-vertical diamond pn diodes. Homoepitaxial p⁺, p, n^-, and n⁺ diamond layers were grown on a {111} high-pressure high-temperature (HPHT) type-Ib diamond substrate by using microwave plasma-assisted chemical vapor deposition (MPCVD). Each B- (p-type) and P- doped (n-type) diamond layer were grown with the use of individual MPCVD systems. The thickness of the p⁺, p, n^-, and n⁺ layers were, respectively, 2, 0.1, 1.8, and <0.1 μm. The impurity concentrations confirmed by secondary ion mass spectrometry (SIMS) were [B]: 3 × 10¹⁹ cm^-3 for the p⁺ layer, [B]: 2 × 10¹⁸ cm^-3 for the p layer, [P]: 1 × 10¹⁶ cm^-3 for the n^- layer, and [P]: 1 × 10²⁰ cm^-3 for the n⁺ layer. Highly P- doped diamond was grown using homemade CVD reactor to enhance P incorporation into diamond to reduce series resistance. After the growth, mesa structures were formed by induction coupling plasma etching with oxygen gas. Subsequently, Ohmic contacts were formed by electron beam deposition of Ti/Mo/Au both on the mesa structure of the n⁺-layer and on the p⁺-layer. The diameters of the mesa and electrodes were, respectively, 240 and 150 μm.

Then we evaluated power – voltage (P-V) and current-voltage (I-V) characteristics under electron-beam irradiation. For electron-beam irradiation, a scanning electron microscope was used as the electron source. Electron beams at accelerating voltages of 15 kV were irradiated on top of the n⁺ layer. The open-circuit voltage (V_oc) under the electron-beam irradiation was 4.26 V, which is the highest among the betavoltaic cells reported thus far. The output power at the maximum point (P_MP) was 10.7 nW. Fill factor (FF) and semiconductor conversion efficiency (η_s) calculated as following eqs (1) and (2)
FF = P_MP/I_scV_oc , …(1)
η_s = VocFF/ε , …(2)
where ε is the electron–hole pair creation energy. Here, we substitute ε = 13 eV in eq. (2). The FF and the semiconductor conversion efficiency η_s were respectively, 0.85 and 28%. Among the semiconductors, the diamond pn junction achieved the highest η_s , which enables to be utilized to energy efficient and long-life cells for remote sensing applications. In the talk, we will also present charge collection efficiency and total conversion efficiency of diamond pn diode against different energy of electrons.

References
[1] P. Rappaport, The electron-voltaic effect in p−n junctions induced by beta-particle bombardment, Phys. Rev. 93, 246 (1954).
[2] S. Maximenko, J. Moore, C. Affouda, P. Jenkins, Optimal semiconductors for ³H and ⁶³Ni betavoltaics, Sci. Rep. 9, 10892 (2019).

When the surface of diamond is H-terminated, a p-type surface conductive layer is formed. Atmospheric impurities, NO₂, O₃, fluoropolymer, transition metal oxides and Al₂O₃ adhere to the diamond surface and activate diamond’s surface conductance by generating and trapping negative charges on the diamond surface. These charges increase the positive p-type carrier densities in the diamond surface and reduce its resistance. High drain current field effect transistors (FETs) of 1.2 to 1.3 A mm^-1, high frequency f_T of 75 GHz, and high voltage, >2000 V, devices have been demonstrated with these activation surface chemistries. However, these devices degraded in a few days or when stressed with high voltage.
Research to stabilize diamond’s surface conductance has met with some success. H-terminated diamonds overcoated using atomic layer deposition (ALD) Al₂O₃ have the best demonstrated stability, but the surface resistance is high (5 to 12 kW sq^-1). Diamonds coated with transition metal oxides such as WO₂, MoO₃, and V₂O₅ have lower resistances of 2.5 to 3.5 kW sq^-1 [1,2]. Their stability has been demonstrated although these oxides are conductive, have several oxidations states, and may electrochemically degrade under the high electric fields associated with FETs. This presentation will discuss the use of a mixed-oxide (Al₂O₃-SiO₂) ALD film to generate resistances between 1.5 and 3.5 kW sq^-1 with demonstrated stability in excess of half a year.
The low surface resistance obtained with this mixed oxide is a result of a change in the structure of Al₂O₃ when mixed with SiO₂. The electron trap surface density of Al₂O₃ is ~7x10¹² cm^-2 while the mixed oxide surface density is 3x10¹³ to 8x10¹³ cm^-2 [3,4] with an estimated trap depth of 5.4 eV. This mixed oxide is just one of a group of oxides [5] with deep, > 5 eV, high-density electron traps.
Increased stability of the diamond surface is obtained by terminating the surface with deuterium atoms instead of hydrogen atoms. This additional stability is beneficial during device fabrication. This presentation will discuss diamond’s surface conduction with Al₂O₃-SiO₂ combined with deuterium atom termination.
References
[1] C. Verona, F. Arciprete, M. Foffi, E. Limiti, et. al. Appl. Phys. Lett. 112 (2018), 181602.
[2] K. G. Crawford, D. Qi, J. McGlynn, T. G. Ivanov, et. al. Sci. Rep. 8 (2018), 3342.
[3]. K. Raghavan, P. Sabu, K. Vijayachandra, C. Rao C. G. Ramachandran. Bull Chem. Soc. Jpn. 64 (1991), 1920.
[4] A. Satsuma, Y. Westi, Y. Kamiya, T. Hattori, Y. Murakami. Bull Chem. Soc. Jpn. 70 (1997), 1311.
[5] A. Corma. Chem. Rev. 95 (1995), 559.
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Diamond electrodes terminated with surface layers can impart significant changes to the electronic properties of a diamond interface and we have previously reported on studies of the tunability of surface work function by substituting hydrogen with oxygen under controlled conditions [1]. Engineering a functionalised diamond surface that is stable and reproducible is of technological importance to future diamond electronic devices such as Schottky junctions and electron-emitting electrodes for field emission, thermionic energy converters, secondary electron multipliers.
O’Donnell and his group have demonstrated the formation of a stable Li-O termination of diamond by physical vapour deposition of a monolayer of Li on the diamond surface which develops and exhibits Negative electron affinity (NEA) and hence lowers the work function, even at an elevated temperature of 800 °C [2]. In this work we are seeking an alternative method, a novel and easy chemical means of introducing and coordinating lithium with oxygen on the diamond surface to realize a more controlled approach of creating a stable –O-Li dipole. with uniformity over the diamond surface which also serves as a means to study and visualize for the first time the effect of temperature on the concentration of Li atoms in the diamond surface and on the Li-O species on the surface of diamond sample.
Also, Tin monoxide has been considered as potential material for battery anodes, coating, gas sensing, catalysis, etc. The application area of SnO extends over photovoltaics, displays, information storage, etc. as it has a large optical bandgap of 4.12 eV, high transparency (~90%) non-toxic and abundant [3]. SnO has been found to form a layered structure in a crystallographic direction of [001] with a monolayer thickness of 0.48 nm. Sn 5s form a lone pair of electrons which point towards the interlayer spacing and result in dipole-dipole interaction leading to a Van der Waals gap of 2.52 A between the layers. Various groups have demonstrated the growth of SnO mono and multiple layers by various methods but many of these result in an upper SnO₂ layer that forms as soon as the sample is taken into the oxygen environment as SnO is metastable and hence rapidly converts into SnO₂.
We are doping oxygen terminated Boron doped Polycrystalline diamond and Nitrogen doped HPHT diamond (111) with Li atoms using microwave plasma technique. The oxygen termination was induced by acid washing. We analyze the sample with X- Ray Photoemission Spectroscopy (XPS) and Raman spectroscopy. The main aim of this study is to analyze the change in concentration of Li atoms in the bulk and on the surface of diamond with annealing temperature. We have produced sub ML SnO on conductive, p-type single crystal diamond under ultra-high vacuum (UHV) conditions by e-beam evaporation. The samples will be characterised in-situ by Spot Profile Analysis Low-Energy electron diffraction (SPA-LEED) and Energy Filtered Photo Emission Electron Microscopy (EF-PEEM) and X-ray Photoemission Spectroscopy (XPS). We quantify the change in work function between mono and few layer SnO and its effect upon the barrier height of a p-type diamond Schottky junction.

References

[1] Wan, G., Cattelan, M. & Fox, N. A. Electronic structure tunability of diamonds by surface functionalization. J. Phys. Chem. C (2019). doi:10.1021/acs.jpcc.8b11232
[2] K. M. O’Donnell et al., “Diamond Surfaces with Air-Stable Negative Electron Affinity and Giant Electron Yield Enhancement,” Adv. Funct. Mater., vol. 23, no. 45, pp. 5608–5614, Dec. 2013.

[3] Pei, Y., Liu, W., Shi, J., Chen, Z. & Wang, G. Fabrication and Characterization of p-Type SnO Thin Film with High c-Axis Preferred Orientation. 45,