Diamond for electronic devices III

Resume : Diamond based unipolar devices are very attractive for power electronics applications. Indeed, diamond shows the highest theoretical figures of merit. In particular, diamond is considered to be the ultimate material for high temperature and low loss, high frequency applications. Tremendous efforts were done to obtain high quality substrates and devices, showing the strong potential for power electronic applications [1]. Several diamond Schottky barrier diodes (SBD) were already reported in the literature [2-6] presenting good performances in reverse operation mode (blocking voltage BV > 1 kV, leakage currents < 10-8 A/cm2 at 1 kV ) as well as in forward bias (>103 A.cm-2 at 6 V). However, in order to increase the performances of diamond SBD, one needs to design and fabricate efficient edge terminations (ET) for decreasing the electric field crowding at the main electrode edge. In this way, 1D electrical breakdown of the device is achievable and physical parameters such as the avalanche impact ionization coefficients could be extracted. In this work, we designed and fabricated diamond pseudo-vertical SBD with floating metal rings (FMR). 2.5D TCAD simulations were performed to optimize the device structure, i.e. identifying the number of FMR, their spacing and width . The optimized simulated structure shows a 35 % decrease of the peak electric field at a reverse bias voltage of 250 V as compared to the one without any ET. Static current-voltage analysis and local electric field observation by electron beam induced current (EBIC) will be shown and discussed. The results show that efficient ET techniques are required to take advantage of the full potential of diamond in terms of breakdown capabilities. [1] H. Umezawa, MAT SCI SEMICON PROC, vol. 78, p. 147 156, mai 2018. [2] P.-N. Volpe et al., APL, vol. 97, no. 22, pp. 223501–223501–3, Nov. 2010. [3] A. Traoré et al., APL, vol. 104, no. 5, p. 052105, Feb. 2014. [4] V. D. Blank et al., DRM, vol. 57, no. Supplement C, pp. 32–36, Aug. 2015. [5] V. S. Bormashov et al., DRM, vol. 75, pp. 78–84, May 2017. [6] H. Umezawa et al., APEX, vol. 6, no. 1, p. 011302, 2013.

Resume : Time-resolved differential absorption (DA) and light induced transient grating (LITG) optical techniques were applied for the investigation of nonequilibrium exciton-carrier recombination and diffusion processes in CVD and HPHT diamond. Carrier density range from 1015 to 1020 cm-3 was achieved by combining two-photon and one photon excitations. Temperature was varied in the 10 – 800 K range. The LITG diffusion coefficient peaked at room temperature under low excitation conditions. At lower temperatures it transferred to much lower exciton diffusion coefficient. A strong decrease of diffusion coefficient under higher excitations was explained by exciton formation with a low diffusion coefficient and many body effets, as polyexciton, electron-hole droplet formation. High temperature phonon-limited diffusion coefficient was weakly dependent on carrier density. Low excitation carrier lifetime was ~ 700 ns above 200 K. At lower temperatures however, the decay time reduced by two orders of magnitude, which was explained by the formation of biexcitons, while at higher carrier densities increase of the carrier recombination rate was attributed to Auger recombination of polyexcitons and electron-hole droplets. At high temperatures, the increase of the recombination rate was attributed to phonon assisted excitonic recombination process. The combination of DA and LITG measurements provided excitation and temperature dependent effective diffusion lengths in a 0.3-50 um range.

Resume : Diamond is considered as an attractive semiconductor to open up next-generation electronics based on its unique physical and electrical properties. For instance, the highest breakdown voltage among wide-band-gap semiconductors with high thermal conductivity and high saturation velocity potentially lead to a higher attainable power density. In addition, the excellent spin characteristics of nitrogen-vacancy complexes including long coherence time and optical initialization and read-out properties lead to a potential candidate for single photon source, magnetic sensors, and quantum applications. High quality single crystal diamond growth and impurity doping underlay the design of virtually all these applications. n-type Fermi controlling is still challenging target, since diamond generally prefers to be p-type semiconductor even in an intrinsic diamond. Carbon vacancies including impurity-vacancy complexes are all acceptor-type defects that capture electrons. From the viewpoint of n-type semiconductor diamond, the suppression of these vacancies is one of the most important factors of Fermi controlling as well as suppression of boron acceptors which become compensating donors. On the other hand, from the viewpoint of defect engineering, n-type donors can supply electrons to vacancy defects, leading the stabilization of their charge states. Nitrogen-vacancy complexes have three charge states, and only NV- centers are required for quantum spin applications. In this talk, we would like to discuss the engineering significance of n-type Fermi control from the viewpoints of following two targets. 1) Inversion channel control in diamond/Al2O3 configuration 2) Phosphorus doping for charge-stage control of nitrogen-vacancy complexes Acknowledgements: This work was supported by JSPS KAKENHI Grant Number JP17916364 and JP16776363 and Japan Science and Technology Agency (JST) CREST Grant Number JPMJCR1773, Japan.

Resume : The production of diamond nanoparticles containing colour centres has attracted increasingly interest in the last years for quantum computation applications. Particularly, the SiV colour centre has highlighted as a promising photon source among other colour centres in diamond, due to its interesting optical properties at room temperature. SiV colour centres acting as single photon emitters can be created through different approaches in in-situ CVD growth by introducing silicon as a solid source or in a gas phase. In this work diamond nanoparticles with custom colour centres were produced from bulk material via milling strategies. First a diamond film containing NV/SiV centres was grown onto a passive substrate, followed by sacrificial etching of the substrate. The resulting diamond film was milled using a planetary mill based on either steel or SiN. The milled material was purified by acid reflux. To measure particle sizes, different slurries containing the diamond nanoparticles obtained were prepared and characterized by using Dynamic Light Scattering (DLS) and Nanoparticle Tracking Analysis (NTA). XPS was also used to qualify the milling process contamination.

Resume : Elementary particles of detonation nanodiamond (EPDND) were first revealed by DFT calculations, consisting of some patches of sp2, a mantle of sp2+x, and a core of sp3 carbons [1] and their existence was indicated by the anisotropy of the whiskers from well-dispersed solutions of detonation nanodiamonds (NanoAmando@, NanoCarbon Research Institute Ltd.) [2][3]. A substantial part of the nanodiamonds must be similar both in size and shape and otherwise the linear tapes of the whiskers cannot be explained well. We demonstrate herein that a Langmuir monolayer from arachidic acid (C19H39COOH) induced crystalline precipitates of ultrathin (26 nm) linear tapes from the solutions. The change in surface pressure by an addition of the solution to water sub-phase suggested that an adsorption took place on the monolayer. After opening the trough of the monolayer, we found innumerable ultra thin rectangular tape pieces on the water surface. We deposited them on glass or silicon plates through a horizontal method by using a natural evaporation of water for a few days. After the air dry, we observed their uniform birefringence and the Raman spectra of nanodiamonds from the tapes. They have never been found without the acids, thus showing that the acids work as a nucleating agent or a flocculant. [1] A. S. Barnard, E. Osawa; Nanoscale 6, 1188 (2014). [2] E. Osawa; Diamond & Related Materials. 16, 2018–2022 (2007). [3] T.Tanaka, et al, Abstract of the 54th FNTG Symposium, 28(2018).

Resume : Schottky-barrier diodes are one of the cornerstones of modern high-power switching electronics, along with the diamond transistors. The combination of the two in terms of diamond technology would enable the handling of large currents with high efficiency, compared semiconductor devices formed from other wide band-gap materials. It is notoriously difficult to achieve the low boron doping levels required for effective device operation by purposeful doping by the use of boron-containing gases during CVD growth. This paper addresses a novel, and new, method for making diamond devices through the use of ‘doped’ nanodiamonds (NDs) overgrown with an intrinsic diamond layer. This approach offers the prospect of an inexpensive, safe, method for diamond device fabrication for high power applications. We have previously published evidence for how B-doped detonation nanodiamonds (B-DNDs) can display effective semiconducting properties [1]. In the current study this form of diamond has been used along with chemically synthesized B-doped NDs and so-called ‘crushed’ B-doped diamond nanodiamonds for the formation of highly effective Schottky Barrier diamond diodes (SBDs). All three types of SBD devices exhibit slightly different properties; however, they all confirm the possibility of purposeful low-level doping of diamond films using previously doped nanodiamond ‘seeds’. This eliminates the need for using diborane or other boron gas sources during CVD growth. Whilst initial results were based on a single ‘doped’ layer through the use of B-ND seeds prior to intrinsic growth, preliminary results suggest that multi-layers of B-ND seeds/i-Diamond/B-ND seeds/i-Diamond etc.., can enable highly effective rectification and high current switching capabilities. This is an entirely new way of making inexpensive high-power diamond diodes, and ultimately transistors, and the prospects for this technology will be addressed in detail. [1] Nanodiamonds for device applications: An investigation of the properties of boron-doped detonation nanodiamonds Abdulkareem Afandi, Ashley Howkins, Ian W. Boyd & Richard B. Jackman Scientific Reports 8, 3270(2018), doi:10.1038/s41598-018-21670-w

Resume : The impurities of diamond are promising candidates as building blocks for quantum computing. In particular, the high-spin negatively charged nitrogen-vacancy defect (NV) in diamond has become a leading contender in solid-state quantum information processing. The initialization and readout of the spin is based on the spin-selective decay of the photo-excited electron to the ground state which is mediated by spin-orbit coupling between excited states states and phonons. While there are models describing [1,2] the decay from the dynamic Jahn-Teller [3] (JT) 3E triplet, there is little known [4] about the decay from the 1E singlet. This is a critical step toward understanding spin polarization and readout of this quantum bit. Here we show a theoretical model for the intersystem crossing (ISC) of the 1E singlet state of NV. We find that the singlet states are coupled with pseudo-JT interaction [1] modulated by diamond phonons. Our model suggests that this coupling governs the ISC rate from the 1E singlet to the 3A2 triplet ground state. We also suggest that the pseudo-JT produces the anomalous difference in absorption and emission sideband of the 1.19-eV optical line between the two aforementioned singlets. We yield quantitatively good parameters for the model via ab initio supercell density functional theory (DFT) calculations. We determined the strength of pseudo-JT interaction up to 10 phonon limit in the Born-Oppenheimer basis. Our theoretical model [5] is verified by the good agreement with the theoretical and observed absorption and luminescence spectrum of the singlets. Finally, we study the spin-selective and ISC rates (Γz and Γ±) from the 1E state towards the ms=±1 or ms=0 substate of 3A2 triplet by our ab-initio data, respectively. By taking our ab-initio data and the observed 1.19 eV energy gap between the singlets, we were able to reproduce the 2-3 MHz ISC rate [6,7], in addition to the temperature dependence of the singlet lifetime. In summary, we provide deep insight in understanding the optical spin-polarization process of NV center in diamond. 1. M. L. Goldman, et al. Phys. Rev. Lett. 114, 145502 (2015). 2. G. Thiering, A. Gali Phys. Rev. B 96, 081115 (2017). 3. I. Bersuker, The Jahn-Teller effect (Cambridge University Press, 2006). 4. M. W. Doherty, Physics Reports 528, 1 (2013). 5. G. Thiering, A. Gali Phys. Arxiv: 1803.02561 (2018) 6. V. M. Acosta, et al. Phys. Rev. B 82, 201202(R) (2013). 7. L. Robledo, et al. New Journal of Physics 13, 025013 (2011)

Resume : Nowadays the detection of pathogens is an inherent part of environmental or food industry safety. In spite of good selectivity of conventional methods, they are time consuming and labor intensive. Biosensors are good candidates for real-time monitoring and fast detection of pathogenic agents ‎[1]. Acoustic devices are in the focus of researchers for bio-sensing applications using appropriate surface functionalization ‎[2]. Love wave surface acoustic wave (LW-SAW) sensors possess high sensitivity in liquid ‎[3]. Acoustic energy is confined in the guiding layer close to the sensitive surface, and the energy is not radiated in the liquid due to using pure shear wave with displacement parallel to the surface ‎[4]. Integration of diamond layer brings many favorable properties such as biocompatibility, various biomolecules attachment and prolonged stability of attached biomolecules ‎[5]‎[6]‎[7]. For these reasons, we theoretically investigated properties of layered structures Diamond/SiO2/36°YX LiNbO3 and Diamond/SiO2/ST-cut quartz for potential biosensor applications. Theoretical calculations were carried out with normalized thickness hSiO2/λ in the range of (0.01 – 1) and different thicknesses of the diamond coating. Legendre and Laguerre polynomial approach of wave propagation in layered structures ‎[8] was used for determination of the phase velocity vp and electromechanical coupling coefficient K2 dispersion curves. We also investigated the sensitivity of Diamond/SiO2/ST-cut quartz and Diamond/SiO2/36°YX LiNbO3 structures by using 100 nm thick PMMA film surface loading. The optimal sensitivity of the Diamond/SiO2/36°YX LiNbO3 is obtained for silicon dioxide normalized thickness hSiO2/λ between 0.3 and 0.6 for all tested diamond thicknesses. In this range of normalized thicknesses hSiO2/λ, the electromechanical coupling coefficient is steeply decreasing from 15 % to 5 %. The diamond/SiO2/ST-cut quartz structure simulation results show similar behavior. The highest sensitivity and K2 of this structure are obtained for normalized thickness hSiO2/λ between 0.2 and 0.6. Results of this theoretical study show it is possible to fabricate LW-SAW devices with a very thin diamond coating without significant loss of the sensitivity. Properties of LW-SAW devices fabricated on 36° YX LiNbO3 substrate will be presented and compared with theoretical results and previous studies on LW-SAW devices fabricated on ST-cut quartz substrate [9]. [1] SINGH, A. et al., Biosensors and Bioelectronics, 2009, 24(12), 3645-3651. [2] LÄNGE, Kerstin et al. Analytical and Bioanalytical Chemistry 2008, 391(5), 1509-1519 [3] GRONEWOLD, Thomas M.A., Analytica Chimica Acta, 2007, 603(2), 119-128. [4] RABUS, D., et al., Journal of Applied Physics, 2015, 118(11), 114505 [5] KRUEGER, Anke a Daniel LANG., Advanced Functional Materials, 22(5), 890-906 [6] Vadym N. Mochalin, et al., Nature Nanotechnology, 7(1):11–23. [7] VAIJAYANTHIMALA, V., et al., Biomaterials, 33(31), 7794-7802 [8] BOU MATAR, Olivier, et al., The Journal of the Acoustical Society of America. 2013, 133(3), 1415-1424 [9] Drbohlavová, L., et al., Proceedings 2017, 1, 540.

Resume : Raman spectroscopy is a simple and non-destructive characterization technique that can be used to estimate the boron concentration in highly boron-doped diamond with metallic electrical conductivity [1,2]. While the Raman spectrum of boron-doped diamond is well described with three main bands at 500 cm-1 (B1), 1200 cm-1 (B2) and the Fano shaped diamond zone-center phonon line (B3), there is no accurate consensual description of the origin of the B1 and B2 bands. In this work, we study the Raman spectrum of epitaxial boron-doped diamond layers with various boron concentrations, excitation wavelengths, and temperatures to further understand the mechanism at the origin of this characteristic Raman spectrum. We will present a simple analysis that accurately describes measured Raman spectra of boron-doped diamond layers based on electronic Raman scattering and Fano effect models. We will demonstrate the Raman spectra result of the interaction of the diamond phonon density of states and the electronic Raman scattering. Based on this model, we will present and discuss the evolution of the electronic Raman scattering, the characteristic bands of the boron doped diamond Raman spectrum and the Fano asymmetric parameters as a function of the excitation wavelength, the temperature, and the boron concentration. Acknowledgment: This work was financially supported by project 17-05259S of Czech Science Foundation and the J.E. Purkyně fellowship awarded to V. Mortet by the Czech Academy of Sciences. [1] V. Mortet, Z. Vlčková Živcová, A. Taylor, O. Frank, P. Hubík, D. Trémouilles, F. Jomard, J. Barjon, L. Kavan, Insight into boron-doped diamond Raman spectra characteristic features, Carbon. 115 (2017) 279–284. doi:10.1016/j.carbon.2017.01.022. [2] M. Bernard, A. Deneuville, P. Muret, Non-destructive determination of the boron concentration of heavily doped metallic diamond thin films from Raman spectroscopy, Diamond and Related Materials. 13 (2004) 282–286. doi:10.1016/j.diamond.2003.10.051.

Resume : The ability to form an efficient interface between material and neural cells is a crucial aspect for construction of neuroelectrodes. Diamond offers material characteristics that could, to a large extent, improve the performance of neuroelectrodes. The greatest advantage of diamond is a large variety of material and surface properties such as electrical conductivity, surface morphology, and surface chemistry. Such a variety of material characteristics can lead to various cellular responses. Here we compare survival, adhesion, and neurite formation of primary neurons on diamond thin films of various morphologies and treatments with several types of polymers commonly used to enhance cell adhesion. We found that the variation of surface roughness of nanocrystalline diamond film when coated with polymer does not have a major influence on neuron survival or adhesion. The adhesion of neurons can be influenced by the selected type of polymer coating. High molecular weight of polyethylenimine resulted in lower viability, adhesion and neurite formation. The addition of laminin to treated films did not lead to significant improvements in neuron adhesion and neurite development. Our findings emphasize the importance of the correct polymer treatment over morphological properties of diamond thin films as a material for forming interfaces with primary neurons. This work has been accepted for publication in Advanced Engineering Materials.

Resume : The realisation of solid state devices that offer the ability to sense a range of chemicals / bio-chemicals within the marine environment with high sensitivity is a key enabling technology for many applications. If these devices can be enabled for remote communication even more potential applications are encountered. In terms of maritime vehicles, devices that monitor chemicals / bio-chemicals that encounter the vessel would be of significant interest in terms of bio-fouling and corrosion studies, and devices that could detect marine bound chemicals as local signature of recent marine activity, either with devices on the vessel or trialled behind the vessel could enhance detection and surveillance capabilities. Using the communication technologies often described as the ‘internet of things’ remotely deployed solid state (miniature) sensors could enhance surveillance capability in the wider marine environment. Limitations that prevent current technologies being deployed in such a way include the lack of resilience of the sensor materials to the marine environment, a lack of sensitivity and selectivity when in sea-water and a lack of miniaturised technology, whereby driver and read-out electronics is integrated with the sensing platform. Diamond a wide band gap (5.5eV) semiconductor material with extreme properties electronic, optical, thermal, hardness and chemical properties. Not only does this make a sensor platform the most suitable in terms of resilience in a marine environment, but diamond has also been shown to resist bio-fouling more than other materials [Electrochimica Acta 55 (2010) 6586–6595]. Diamond is now a commercially available Engineering material, with high purity diamond (grown by CVD methods) being offered a number of suppliers at modest cost [see for example, E6cvd.com]. Diamond can be doped with boron to reveal p-type semiconducting properties; at high boron concentrations diamond becomes a quasi-metal enabling it to be used as an electrode material [phys. stat. sol. (a) 154, 423 (1996)]. Diamond displays the highest ‘electrochemical window’ of any electrode material in water (the potential at which the redox breakdown of water occurs) meaning that diamond devices can operate at higher voltages than those made conventional materials. Any solid state detector that aims to look at organic or bio-organic materials immediately encounters the issue that they all contain carbon, rendering differences seen between species difficult to determine. One spectroscopy that overcomes this problem is Raman. Raman spectroscopy is a technique for looking at molecular vibrations, and hence the identification of the types of chemical bonds present. This information can then be used to identify the chemical composition of the material being examined. A brief overview of the technique will be offered and recent results presented shoiwing that diamond is perhaps unrivalled as a semiconductor for this application. Acknowledgments BAE Systems Maritime and the UKs Engineering and Physical Sciences Research Council (EPSRC) are gratefully acknowledged for the award of a ‘CASE’ PhD studentship to MR. BAE Systems are also thanked for financial support for the project.

Resume : High quality single crystal diamond with thin δ-shaped boron-doped epilayers have been thought to offer a viable approach towards high speed, high power and high temperature applications. δ- doping diamond has been conjectured to achieve high mobilities and carrier concentrations, proper- ties of real interest for electronic applications. Taking advantage of diamond’s thermal and electronic properties, thin films can be incorporated into realistic nanoscale devices more easily than the parent bulk system. Using angle-resolved-photoemission spectroscopy (ARPES), we uncover the electronic structure of bulk and thin films (≈ 2 nm) of boron-doped diamond. Surprisingly, the ARPES measurements do not reveal any significant differences for these systems, irrespective of their physical dimensionality. This suggests that it is possible to grow nearly atomic-scale structures whilst still preserving the properties of bulk diamond facilitating the use of thin films diamond for devices which necessitate nearly atomic-scale components. ARPES measurement collected at photon energy 170 eV for a thick (> 1μm) boron-doped diamond film show- ing the binding energy (EB) dispersion as function of ⃗k||; the measurement corresponds to the band structure of boron- doped diamond acquired along the green line shown in the inset representing the BZ of diamond. Three bands (labeled as A, B and C) disperse close to the Fermi level (EF ), with ‘A’ crossing EF , hence contributing to the metallic character of the sample. Additional horizontal bands (H1 and H2, see horizontal guide depicted by the dashed lines) are detected: we believe that these bands are an artifact coming from the edges of the Brillouin zone, where the electronic states have exceptionally high intensity and therefore such intensity can scatter across to the Brillouin zone center.

Resume : Mercury naturally exists in crude oil in quantities ranging from 0.1 to 20,000 µg/kg and is released into the atmosphere and wastewater streams during extraction and processing[1]. Even in trace amounts mercury poses a severe threat to both human health and the environment because it is highly toxic and tends to form complexes with ligands of biological matter, leading to an accumulation in the food chain[2]. The World Health Organisation guideline for the maximum safe quantity of mercury in drinking water is 6µgl-1 ca. 30 nM[3]. It is imperative that trace contamination of potable water by oil processing wastewater streams is identified in situ. This can be achieved through electrochemical techniques, which also have the advantage of relative low cost and simplicity compared to optical methods. Boron doped diamond (BDD) electrodes are particularly well suited for the trace detection of mercury due to the wide electrochemical window, low background currents, resistance to fouling and stability at extreme temperature, pressure and pH associated with the material[4]. In this work square wave anodic stripping voltammetry (SWASV) was used with a BDD working electrode in a nitric acid electrolyte (0.1 M). This system is considered to be one of the most sensitive electro-analytical techniques for trace analysis, with detection of mercury recorded here in the pico-molar range. To improve the sensitivity of mercury detection the BDD electrode surface was decorated with gold nanoparticles, which act catalytically during the pre-concentration step of the SWASV measurements. A comparison study was made between drop-casting the nanoparticles and depositing thin films of gold which were then de-wet into nanoparticles by annealing in an inert atmosphere. Through the annealing method a range of nanoparticle sizes and dispersions on the electrode surface were investigated, to identify the optimum gold nanoparticles for catalytic action in sensing mercury by SWASV. We have successfully detected mercury at pico-molar concentrations and initial results show increased sensitivity with smaller gold nanoparticles (<15 nm) on the BDD surface. [1] Lang, D. et al. “Mercury arising from oil and gas production in the United Kingdom and UK continental shelf” IKIMP (2012)

Resume : Carbon-based materials are strongly expected to be applied to electron emitter cathodes. CNT and graphene with low-dimensional nano-materials composed of sp2 bonds can be possible with higher emission current, while it is vulnerable to heat and shock and long-term stability is one of the remained issues. Diamond has attracted attention as a material of an electron source because of their potentially low or negative electron affinity. Diamond possesses the highest hardness of the material and is composed of strong C-C bonds sp3 structure, which the material stability and reliability are very high. On the other hand, conductivity control with impurity doping has still difficulties than other materials. Nano crystal diamond (NCD) is a material composed of a mixed structure of sp2 and sp3 bonds and attracts attention as one of materials that can exploit both these features. Doping of phosphorus impurities as well as boron and nitrogen has become possible in recent years, and technology for lowering resistance has also been established. The resistivity of NCD can be reduced by almost 2 orders of magnitude lower than that of single crystal diamond. In addition, NCD can be formed on various substrates such as silicon substrate, metal substrate, glass, etc., leading the high controllability of structure and area-size enlargement. Many researches on electron emission characteristics and thermionic emission characteristics have reported with attractive properties, i.e., low work function, high thermionic emission properties, low threshold voltage, etc. In this paper, we focused on phosphorus-doped NCD films and discuss their structural characterization from cross-sectional TEM and electron energy loss spectroscopy study. Acknowledgements: This work was supported by JSPS KAKENHI Grant Number JP16776363.

Resume : Paramagnetic point defects in diamond are candidates for quantum bit and quantum information applications. The N2V defect is formed by an aggregation of neighbor substitutional nitrogen atoms trapping a mobile vacancy [1]. The defect in its neutral charge state was assigned to H3 color center [2] and W26 ESR center [3]. As the H3 center exhibits a singlet ground state and an optically accessible metastable triplet state we wished to explore the properties of the N2V defect for quantum memory applications. We have characterized this defect in diamond by means of ab initio methods relying on density functional theory (DFT) calculated parameters of a Hubbard model Hamiltonian. It is shown that this approach appropriately describes the energy levels of correlated excited states of this defect using only DFT wave function and energies. With magneto-optical parameters determined by means of DFT calculations that go beyond the conventional Kohn-Sham DFT methods, we demonstrate that optical spin polarization of the triplet state and spin polarization transfer to the existing nuclear spins is principally feasible. Thisway a long-living quantum memory may be realized with N2V defect. [1] A. Mainwood, Phys. Rev. B 49, 7934 (1994). [2] R. Jones, P. R. Briddon, S. Öberg, and V. J. Torres, in Defects in Semiconductors 17, Materials Science Forum Vol. 143 (Trans Tech Publications, Zürich, 1993), pp. 45–50. [3] J. A. van Wyk and G. S. Woods, J. Phys.: Condens. Matter 7, 5901 (1995).

Resume : Diamond is an attractive material for optoelectronics and power devices due to its outstanding properties such as high critical field, high hole mobility, high thermal conductivity and low dielectric constant. Diamond Schottky barrier diodes with blocking capabilities up to 10 kV have been already reported [1], and diamond Field Effect Transistors through MOSFETs [2] and MESFETs [3] have been recently investigated. Among them, diamond MESFETs are well suited for high frequency and high power devices. In this work, diamond MESFETs have been fabricated and characterized. Heavily boron doped layer was grown selectively to reduce source and drain contact resistance. The gate length LG varies from 10 μm to 30 μm and the gate to drain length LGD from 30 μm to 50 μm. The drain current was 1.4 μA.mm-1 @0V with a back-gate biased at -40V to reduce the depletion region at the substrate interface. The measurements were done under vacuum and light. In the dark, there is no current flowing corresponding to an off-state. A breakdown voltage over 4 kV is expected for this device. The presence of trap in the diamond bulk or at the interface could be one reason to this activation by light. Indeed, a shift of the threshold voltage was observed when performing current-voltage measurement between two consecutive sources. This phenomenon has already been reported for gallium nitride GaN and silicon carbide SiC transistors [4]. Wavelength dependency will be discussed in the presentation. [1] P. Volpe et al., Phys. Status Solidi. 207 (2010) 2088–2092. [2] Y. Kitabayashi et al., IEEE Electron Device Lett. 38 (2017) 363–366. [3] H. Umezawa et al., IEEE Electron Device Lett. 35 (2014) 1112–1114. [4] S.C. Binari et al., IEEE MTT-S Int. Microw. Symp. Dig. 3 (2002) 1823–1826.

Resume : With the superlative physical properties of a high breakdown field strength, high carrier mobility, and high thermal conductivity, single-crystal diamond has attracted much attention as a material for next-generation low-loss power devices. High switching capabilities have been demonstrated for both unipolar and bipolar devices, Schottky rectifiers, p-i-n diodes , and field-effect transistors (FETs). However, most of these characteristics were reported using insulating substrates. To enhance the device performance further, the development of low-resistivity diamond substrates is required. Vertical architectures can withstand very high currents with large electrodes on the front and back sides of substrates. In addition, the potential distributions within vertical structures are more favorable than those in lateral architectures to support high voltages. In this study, heavily B-doped p+ diamond (100) films were grown by hot-filament CVD. Low resistivity (<0.003 Ohmcm) was successfully realized with high-rate growth conditions (1–4 micron/h). The as-grown film surface exhibited step-bunching faces without non-epitaxial grains; lateral growth was maintained over thick films (>100 micron). Freestanding crack-free films were prepared by removing the seed substrates via laser-cut and mechanical polishing processes. The X-ray diffraction rocking curve (004) spectrum showed the full-width at half maximum value of 25 arcsec, equivalent to that of the seed substrates employed.

Resume : Electrical field enhancement at the edge of the Schottky electrode on diamond under reverse bias condition was characterized by electron beam induced current (EBIC) measurement. The increase of the EBIC signal was clearly observed at the edge of the electrode when the electrical field was increased more than 0.9 MV/cm. The high EBIC signal area corresponds to the laterally expanded depletion layer (LDL). We also confirmed that the EBIC signal was not uniformly distributed in LDL but very bright spots, such as hot spots, existed. The leakage current of the diodes corresponded to the bright spots at the edge of the electrode. The position of the hot spots correspond to that of the roughness at the periphery of the Schottky electrode which have caused during the lift-off process [1]. We extracted carrier multiplication factor by analyzing EBIC intensity and applied electrical field on the devices. The value is lower than that of the reported up to now. Reference: [1] H. Umezawa, H. Gima, K. Driche, Y. Kato, T. Yoshitake, Y. Mokuno, and E. Gheeraert, "Defect and field-enhancement characterization through electron-beam-induced current analysis," Applied Physics Letters, 110, 182103 (2017).

Resume : High degree hyperpolarization of arbitrary samples and nuclear magnetic resonance (NMR) agents is of great importance for sensitivity enhanced NMR sensing and imaging measurements for decades. Protocols use only optical illumination and operate even at room temperature are of highest importance. Here, we report on the combined ab initio and model spin Hamiltonian theoretical model for a recently reported hyperpolarization measurement on a diamond sample consisting N3V and P1 centers. [1] Our theory explains the experimental observations and show how the initial static electron spin polarization is dynamically distributed between paramagnetic defects and nuclear spins. [1] B. L. Green et.al, Phys. Rev. B 96, 054101 (2017)

Resume : Previous works devoted to preparation of [100] and [111] textured diamond films with (100) and (111) faceted microcrystals as well as to search for the specific technological regimes allowing the relatively stable growth of diamond single crystals (grains) with the mentioned faceting have been analyzed. General features responsible for the growth of freestanding diamond grains of certain faceting without additional nucleation on the faces have been discussed. The PE CVD method with magnetic field discharge stabilization was applied for the growth of arrays of freestanding diamond grains (island films) as well as continuous films on Mo and Si substrates, respectively. Raman, SEM, XRD, PL and SIMS methods were used for search of the difference in the defect states embedded into (100) and (111) faceted grains. Keywords: Diamond microcrystals; PE CVD technique; Raman scattering; Fourier-transform infrared spectroscopy; (FTIR); Photoluminescence; Scanning electron microscopy (SEM).

Resume : Reactive Ion Etching (RIE) has emerged as a preferred method for diamond substrate surface treatment and device patterning. This process is crucial to achieve the fabrication of high power devices fully exploiting the exceptional properties of diamond. The material properties of diamond however pose challenges to achieving smooth etched surfaces, especially for etch depths beyond 2 microns. Following work optimising Inductive Coupled Plasma RIE etching for surface smoothing and removal of sub-surface damage, an investigation into the effects of etch process parameters on diamond patterning is pursued. The aim is the achievement of deep etching with controlled wall angle, minimized micro-masking and etched surface damage. More specifically, the impact of gas ratio and ICP power settings on the physical and chemical properties of the etch is studied with relation to mesa quality. Results highlight the importance of high platen/ICP power ratio, increased O2/Ar gas ratio and intermittent Ar/Cl2 plasma to reduce damage and micromasking on the etched surface, producing a high quality smooth 8 micron deep etch. Acknowledgements The EU Horizon 2020 project ?GreenDiamond? is gratefully acknowledged for financial support, as is the UKs Engineering and Physical Sciences Research Council (EPSRC) for the award of a PhD studentship to M-LH.