Diamond for electronic devices

Resume : P- and n-type doped diamonds underlay the design of virtually all electronic and optoelectronic applications. To achieve such valency control, comprehensive knowledge of the fundamental processes that control impurity doping is required. For example, heavily doped layer over 1020 cm-3 can reduce the series resistance including n-type Ohmic contact issue, and lightly doped layer can improve the carrier lifetime in the active region of several junction devices, indicating wider doping levels are important for diamond electronic applications. Recently, we have achieved n-type control of (111)-oriented diamond films with ultra-lightly (~1015 cm-3) and -heavily (~1020 cm-3) phosphorus doping applying a newly optimized doping conditions based on microwave plasma-enhanced chemical vapor deposition. The phosphorus concentration, estimated by secondary-ion mass spectrometry, can be reproducibly controlled by a standard mass flow controller between 1×1015 cm-3 and 3×1020 cm-3. Electrical properties were characterized by Hall-effect measurements as a function of wide temperature range between 220 K and 900 K. The n-type conductivity with thermal activation from phosphorus donor lever of 0.57eV was clearly obtained even in the ultra-lightly doping with phosphorus concentration of 1015 cm-3 orders. When phosphorus concentration was 2×1015 cm-3, the electron mobility was obtained to be 1060 cm2/Vs at 300 K and 1500 cm2/Vs at 225 K. Detailed aspect of impurity doping, electrical properties, and device applications will be discussed.

Resume : Diamond is an exceptional semiconductor material due to its wide bandgap, superlative thermal conductivity, and high electron and hole mobilities. The growth of electronic grade diamond is necessary for the fabrication of high power and high frequency diamond devices. The realisation of such devices requires controllable levels of n-type dopants such as phosphorus, while maintaining extremely high crystalline quality. Consequentially, improving the phosphorus doping process remains an area of significant current research interest [1]. The temperature of the substrate during the growth of boron doped single crystal diamond (SCD) has been shown to affect the surface morphology and dopant concentrations [2], and in phosphorus doped SCD the effect is even more dramatic, with doping only occurring within a narrow band of substrate growth temperatures [3]. The optimisation of the growth of phosphorus doped SCD is investigated in this work, through the characterisation of films grown in a controlled series to determine the optimised conditions for achieving n-type diamond suitable for high-power diamond device applications. Diamond was deposited homoepitaxially in an ASTeX-type microwave plasma enhanced chemical vapour deposition reactor, on type Ib high-pressure high-temperature (111) oriented SCD substrates, with feedgas mixtures including hydrogen and methane with varying ratios of phosphine. The samples were characterised by photocurrent measurements, low temperature Fourier transformed infrared spectroscopy, Hall Effect, Raman spectroscopy, and optical and electron microscopy, to analyse the effect of pretreatment steps, phosphine concentration, and substrate temperature on defect morphology, electrical properties and phosphorus incorporation. This work will summarise the current state of the art in phosphorus doping, and present strategies for improving the deposition of n-type diamond. This work was performed within the H2020 Research and Innovation Action Project "GreenDiamond" (www.greendiamond-project.eu) under grant agreement N°640947. [1] R. Ohtani, T. Yamamoto, S.D. Janssens, S. Yamasaki, and S. Koizumi. Applied Physics Letters, 105, (2014) 232106 [2] S. Nicley Demlow, R. Rechenberg, and T.A. Grotjohn. Diam. Relat. Mater. 49, (2014) 19. [3] S. Koizumi, T. Teraji, and H. Kanda., Diam. and Relat. Mater. 9, (2000) 935.

Resume : Traditionally, homoepitaxial CVD films are deposited on commercially available high-pressure high-temperature (HPHT) substrates. HPHT synthesis leads to material that heterogeneous in terms of structural defects and impurity concentrations. In addition, when preparing flat substrates, surface polishing creates additional defects in the subsurface. A common strategy to remove this damage is to introduce a plasma etching step on the top layers of a substrate prior to CVD diamond growth, in an attempt to reduce detrimental effects on quality of the (doped) CVD film grown on top, Here the influence of varying nitrogen concentrations in HPHT (111) substrates, due to so-called ?zoning?, and surface pre-treatments by O2/H2 and H2 plasmas on the structural and electrical quality of phosphorous-doped CVD diamond layers grown upon them is investigated. Surface and bulk defects of the substrates were visualized by synchrotron X-ray Bragg diffraction imaging, while confocal µ-Raman/photoluminescence (PL) was used to analyse the relative variation of nitrogen impurities in the substrates. The lifetimes of charge carriers in CVD diamond layers were determined by an optical pump-probe technique, which is based on a differential transmission/reflection of a laser probe-beam (1064 nm) under optical excitation of an epilayer at 213 nm. For a deeper insight, measurements were done at various excitation fluencies and temperatures. The obtained results were compared in detail to the µ-Raman/PL data. P-doped diamond layers, ~4 µm thick, grown on nitrogen-rich substrate sectors pre-treatment for 5 min with a O2/H2 plasma clearly showed short lifetimes of charge carriers (40 ps), while layers on nitrogen-poor sectors of the same substrate exhibited longer lifetimes (2 ns). Surprisingly, while a longer pre-treatment (? 30 min) resulted in 7-fold prolonged lifetimes on N-rich substrate sectors, no improvement for layers deposited on N-poor sectors could be detected. The recombination modelling in the CVD layers over the N-poor substrate sectors yielded 5 ns lifetime in the bulk with a surface recombination velocity of 105 cm/s. Higher P-doping concentrations did not affect the lifetimes for layers on N-rich sectors, which tentatively leads to the suggestion that dislocations can be the major cause of the observed short carrier lifetimes. This work was performed within the H2020 Research and Innovation Action Project "GreenDiamond" (www.greendiamond-project.eu) under grant agreement N°640947.

Resume : Doping of diamond for electrical applications has been hindered by the large activation energies required for the known dopants, boron >325 meV and phosphorous > 600 meV. The result is that to achieve meaningful carrier concentrations, one must heavily dope the material, significantly reducing the high mobilities of intrinsic diamond. A well known strategy for solving this problem, at least in a two dimensional sense, is ?delta doping?. Single crystal diamond epitaxial layers with buried boron ?delta doped? layers as thin as 1.8 nm have been grown in a custom chemical vapor deposition reactor designed specifically for ?delta doped? layers. High resolution transmission electron micrographs demonstrate defect free epitaxial growth on a laboratory grown (high pressure, high temperature) (100) face of a diamond single crystal substrate. Initial electrical measurements on various sample demonstrate Hall mobilities of 10 to 200 cm2/Vsec with carrier concentrations between 1011 and 1013 cm-2. Further SIMS compositional and electrical characterization with Hall bar structures will be presented.

Resume : Growth of heavily boron-doped (p+) diamond substrates is an important and challenging topic for fabrication of vertical-type high-output power devices. Several critical issues on this topic are (a) low doping efficiency of boron from CVD gas phase to diamond, (b) anomalous lattice strain caused by heavily boron-doping, since covalent radius of boron (0.088 nm) is larger than that of carbon (0.077 nm), (c) relatively high resistivity which restrict the high-output power operation. Assuming the 100-A current operation, total parasitic resistance must be less than 5E-5 ohmcm2: the substrate resistivity of < 10 mohmcm with a thickness of 50 micron is necessary. In this study, we have investigated the growth of p+ diamond by hot-filament (HF) CVD. High doping efficiency of nearly 100% was realized. Boron concentration could be well controlled from 1E19 to 1E21 cm-3, which corresponding the levels of the hopping conduction to the metallic transport. The p+ diamond was coherently grown on (100) substrates, i.e. pseudomorphic growth, which might be a key to suppress emerging dislocations at interface. Resistivity was monotonically decreased to 1.2 mohmcm by increasing boron concentration. These results indicate a large potential for p+ diamond. By comparing the growth by microwave plasma-enhanced (MP) CVD, the important strategy to fabricate high-quality p+ diamond will be presented in the conference.

Resume : Doping of diamond with Si is commonly used to form silicon-vacancy» color centers (SiV), interesting for realization of optical biomarkers and single photons sources in quantum optics [1]. Higher concentrations of silicon impurity allow co-formation of a new phase ?silicon carbide (SiC), which has excellent electronic and thermal properties important for high-power and high-temperature electronic applications. The composite «SiC?Diamond» would be a promising material for optical and electronic devices [2]. In the present work, we synthesized a layered composite structure using two-step microwave plasma chemical vapor deposition (ARDIS-100 system, 2.45 GHz, 5 kW) in H2-CH4-SiH4 gaseous environment to form consequently SiC and polycrystalline diamond layers on single-crystal Si substrate by variation the silane content in the reactor. After the growth process, the substrate may be chemically removed to obtain ?SiC?Diamond? composite free-standing film. The characterisation of both phases was performed using scanning electron microscopy, Raman spectroscopy, and photoluminescence studies. This work was supported by the Russian Science Foundation, grant No. 14-22-00243. [1] Sedov, V. S., et al. "Growth of Si-Doped Polycrystalline Diamond Films on AlN Substrates by Microwave Plasma Chemical Vapor Deposition." Journal of Coating Science and Technology 2.2 (2015): 38-45. [2] J. Rabkowski, D. Peftitsis, H.P. Nee. "Silicon carbide power transistors: A new era in power electronics is initiated."Industrial Electronics Magazine, IEEE 6.2 (2012): 17-26.

Resume : Heteroepitaxial growth is a promising method to fabricate large-size diamond wafer but it is necessary to improve the quality of diamond in order to apply to semiconductive devices. From our previous study, we found out that patterned nucleation growth is an effective method to improve mosiaicity and to control bowing of heteroepitaxial diamond films. Studies on dot, stripe and grid patterns have been done and it became clear that the quality of the diamond films depends on both pitch and orientation of the patterns. But the difference in mosiaicity and film bowing against pattern shape is still not clarified because studies on each pattern were carried out independently. In this study, diamond films grown from different pattern shapes with same pitch and orientation arranged on same Ir/MgO substrate were compared. From these results, ideal nucleation pattern to realize high crystallinity without any bow will be discussed.

Resume : Reactive Ion Etching (RIE) has emerged as a preferred method for diamond substrate surface treatment and device patterning. This process, especially in the case of surface smoothing and sub-surface polishing damage removal is crucial to achieve the fabrication of devices fully exploiting the exceptional properties of diamond. Polishing-induced dislocations are an important hindrance to high quality diamond growth as the dislocations propagate from the substrate through the epitaxial layers and strongly affect device performance, especially in electronic applications. Building on work optimizing an O2/CF4 Inductive Coupled Plasma RIE etch for the removal of sub-surface damage, surfaces of unchanged or reduced roughness were achieved and characterized. Amongst other methods, grazing incidence angle x-ray diffraction was performed at ESRF to determine the depth and effect of processing on the surface defects, to be presented in this paper.

Resume : We report on charge multiplication in 3.25 µm thick free standing intrinsic scCVD diamond membrane under high electric field conditions. Using an O-16 ion (18 MeV kinetic energy) beam, the excess charge carriers upset events were created homogenously along the diamond film by means of the ion micro-beam. Employing calibrated charge sensitive electronics chain, pulse-height spectra of single events were recorded for increasing applied electric field. After initial rise to 100% of the charge collection efficiency (CCE) plateau, a charge multiplication phenomenon is observed for electric fields exceeding 30 V/micron (0.3 MV/cm), with up to x 2.6 increase in amount of collected charge at a maximum applied field of 250 V/micron (2.5 MV/cm). To confirm the impact ionization origin of the charge multiplication, transient current technique (TCT) is applied to record fast induced current signals. With a present limitations of the experimental setup the time constant of the charge avalanching was estimated to be < 170 ps at maximum, thus excluding the photoconductive gain or erratic currents origin of observed multiplication. Furthermore, a first practical application of the avalanching process will be presented in the diamond radiation detection technology: a complete recovery of CCE of previously radiation damaged detector while preserving spectroscopic character of the recorded pulse-height spectra.

Resume : High-temperature solar cells are possible by exploiting Photon-enhanced Thermionic Emission (PETE), which represent a novel and very attractive concept for the harversting of solar radiation, especially if concentrated, and characterized by promisingly high conversion efficiency. PETE converters rely on the concept that engineered semiconductor structures can provide an electron emission, induced by hot-electrons produced by photons with sufficient energy, combined to a thermionic emission, sustained by the high temperatures induced by every other thermalization process. Chemical-vapour-deposited (CVD) diamond represents a suitable and promising material for PETE applications. Surface nanotexturing combined to surface-hydrogenation, aimed at achieving negative electron affinity conditions and a work function as low as 1.7 eV with an emitting-layer nitrogen-doping, are here proposed as a radically new and potentially efficient PETE cathode completely based on CVD diamond, able to ensure an efficient thermionic emission at temperatures up to 780 °C. CVD diamond is transparent to solar radiation due to its wide bandgap, consequently advanced and novel techniques are needed for implementing a defect-engineering strategy and thus processing diamond to become an efficient sunlight absorbing material (i.e. black diamond). Surface texturing by fs-laser, boron-implantation, buried and distributed graphitic structures and other technological steps allow for the fabrication of an innovative defect engineered diamond cathode to be efficiently exploited for the conversion of concentrated solar radiation. A diamond-based anode with an even lower work-function can be combined to the black diamond cathode for completing a potentially efficient energy converter.

Resume : P-type conduction of the Hydrogenated diamond surface is attractive for the development of high power, high frequency field-effect transistors (FETs). The study of stable materials for surface transfer doping in diamond is an important task in order to improve device performance and robustness of operation i.e. thermal and over-time stability. The aim of this work is to investigate the properties of different acceptor insulators featured by high electron affinity, such as Nb2O5, WO3, V2O5 and MoO3. The low electron affinity Al2O3 has been also studied for comparison. Hole transport properties were evaluated in the passivated hydrogenated diamond films by Hall effect measurements, and compared to un-passivated diamond films (air-induced doping). A drastic improvement has been observed in passivated samples in terms of conductivity, stability with time and stability in air and resistance to temperatures up to 300 °C. These surface acceptor materials generate a higher hole sheet concentration, up to 6.5×10^13 cm^-2, and a lower sheet resistance, down to 2.6 k?/sq, in comparison to the atmosphere-induced values of about 1×10^13 cm^-2 and 10 k?/sq respectively. On the other hand, hole mobilities were reduced by using high electron affinity insulator dopants. Hole mobility as a function of hole concentration in hydrogenated diamond layer was also investigated, showing a well-defined monotonically decreasing trend. The present findings may help in improving the existing applications relying on H-terminated diamond and can be used as a guideline for the realization of superior electronic devices based on diamond surface conductivity.

Resume : Diamonds band gap of 5.5eV makes it ideal as an optically blind UV photodetector. Type II diamond starts to show photoconductive behaviour from below about 236nm, which is slightly above the band gap wavelength of 225nm. In this work polycrystalline diamond is grown using MWPECVD onto thin silicon substrates and interdigitated electrodes are patterned onto the diamond surface using standard lithography techniques. Repeated methane and air anneals are used to reduce the detectors response in the visible and also to reduce surface conductivity that is caused by hydrogen terminated carbon atoms on the surface. There are numerous problems however in using polycrystalline diamond as a UV photodetector. Inhomogeneities across the surface caused by varying grain sizes and the presence of grain boundaries make polycrystalline diamond unsuitable for imaging purposes. The surface roughness also increases the difficulty of patterning the interdigitated electrodes. The surface can be smoothed using polishing methods, but this can induce subsurface damage that produces trapping centres. These trapping centres can cause the response of the detector to react slowly to a change in the UV illumination. To this end, methods to reduce surface roughness without the need for polishing are explored and the resulting photoconductivity measured.

Resume : Diamond is considered to be a very promising material for electronics, due to its intrinsic parameters superior to semiconductors as carriers mobility, fast response, very low noise etc. In addition, its high radiation hardness makes the diamond appealing for radiation detection and particle counting. Here we report on our attempt to prove the usefulness of single-crystal diamond detectors produced by Chemical Vapour Deposition (sc-CVD) for accelerated proton beam diagnostics by using single proton counting approach. At the Institute of Nuclear Physics PAS in Krakow, Poland, there are two cyclotrons in operation to accelerate protons for hadrontherapy and nuclear physics experiments. These accelerators provide beams of different characteristics (time structure, energy range, intensity etc.) which are relevant to be monitored and verified on-line. We have measured the in-beam response of a thin (50?m) sc-CVD diamond detector. Rapid signals from the detector provide intrinsically a good time resolution, however a wideband, ultra-low noise electronics with high amplification is required. The results on time structure and intensity of the proton beams for the both INP cyclotrons obtained for different wideband electronics have been compared. The measurement results of distances between micropulses (9.4 ns, 38.1 ns) or typical beam current density (1567±37 p/ms/mm^2) are in good agreement with system specifications. Linear response of the system to different beam currents was also proven with 70 MeV protons. Our results show that proton counting with the sc-CVD diamond detector we used, allowed to measure beam intensities with extremely high sensitivity (single particle level) and dynamic range >10^6. Our fidings might be of special importance for qualification of single spot profiles of pencil beams, recently introduced in hadrontherapy.

Resume : Thermionic energy conversion, which can directly convert heat energy into electricity, is one of promising technologies for waste heat recovery applications. Heavily phosphorus-doped NCD films were grown on Mo substrate by plasma-enhanced chemical vapor deposition for its thermionic emission cathode. The phosphorus atoms of 5×1020 cm-3 were successfully incorporated into films when the gas flow ratio of phosphine and methane was 1. Thermionic emission properties were measured in a vacuum as a function of temperature from 300 to 600 oC. The saturation current density was recorded as ~8 mA/cm2 at 600 oC, which is a value comparable to that of the reported nitrogen-doped NCD films. According to the Richardson-Dushman equation, the work function and Richardson constant of heavily phosphorus-doped diamond are estimated to be ~2.3eV and ~15 A/cm2/K2, respectively. The surface morphology was observed by differential microscopy with a Nomarski prism and atomic force microscope using contact mode with SiN cantilever. The structural characterization was performed by Raman spectroscopy with a green laser of 532 nm at room temperature, transmission electron microscope, and electron energy-less spectroscopy. The grown phosphorus-doped diamond films have a typical NCD structure with a combination of sp3 diamond gains and sp2 graphitic grain boundaries. Detailed aspect of impurity doping and structural characterization will be discussed. Acknowledgements: This work is partially supported by a-step program "Adaptable and seamless technology transfer program through target driven R&D" of Japan science and technology agency.

Resume : New diamond materials with unique physical, mechanical and electrical properties are promising for use as functional elements of power and computer electronics. Boron-doped diamonds are extremely rare in nature and are attractive material for electronic application. To produce boron-doped p-type semiconducting diamond at P=5.8?6.3 GPa and T=1400?1500 ºC Fe-Al-C or Co-Fe-Ti/Al-C system with the addition of boron or boron-containing compounds are usually used. The process for semiconducting diamond single crystals growth in Mg?C system at P=7.2?8.5 GPa and T=1800?2000 ºC was developed. Grown diamonds have a characteristic infrared boron-related absorption consisting of a series of peaks at 2460, 2810 cm-1. The results indicate that in the Mg?C system it?s possible to obtain semiconducting diamond crystals with uncompensated acceptors concentration 2?7 ppm without the addition of boron or boron-containing compounds to the growth system. The acceptor ionization energy is found to be Ea=0.36 eV, which is close to ionization energy of the acceptor level of boron. It was established that Mg?C crystallization system provides very favorable conditions for selective capture of boron at crystallization front, even thought B is present in the initial carbon source (graphite) in trace amounts about 0.001 wt%. This is in contrast with many conventional transition metal solvent catalysts, for which deliberate doping with B is necessary to produce boron-doped diamonds.

Resume : Color centers in diamond, such as nitrogen-vacancy (NV), silicon-vacancy (SiV), germanium-vacancy (GeV), chromium- or nickel related centers are of increasing interest for photonics, quantum information technologies, bright biocompatible fluorescent biomarkers, and scintillators for X-ray beam monitoring [1]. However, the ?impurity atom-vacancy? centers allow a limited range of wavelengths with its own shortcomings for each center. A novel approach to preparing optical centers in diamond was proposed by Magyar et al. [2] with the incorporation of optically active atoms of rare-earth (RE) element (europium oxide) into the diamond lattice, which resulted in bright photoluminescence at the wavelength of 612 nm. Here we propose another much simpler way to integrate rare-earth elements in the form of fluoride micro- and nanoparticles into the diamond because fluorides exhibit bigger quantum efficiency than oxides. The main focus of the work was on the incorporation of EuF3 nanoparticles into microcrystalline and single-crystal CVD diamond films. Such composite material showed local bright photoluminescence at the wavelength of 612 nm. Incorporation of other RE fluorides will also be reported. The proposed approach may be used to obtain composite materials with unique optical and magnetic properties, including bright up-conversional and X-ray luminescence. This study was supported by RFBR (grant No. 16-29-11784_ofi_m) and the grant of the President of the Russian Federation (No. SP-2575.2015.5). [1] Yang, Nianjun, ed. Novel Aspects of Diamond: From Growth to Applications. Vol. 121. Springer, 2014. [2] Magyar, Andrew, et al. "Synthesis of luminescent europium defects in diamond." Nature communications 5 (2014).

Resume : It is well documented in literature that chemical vapor deposition (CVD) diamond films are good materials for UV photocathodes (PCs) and they are stabler than CsI ones. Very recently, PCs based on nanodiamond (ND) particles, developed by the authors, exhibit high efficiency and stability. In this contribution, the authors present a detailed investigation on two types of diamond powders with similar particle sizes of about 250 nm and having different sp2 (graphite phase) and sp3 (diamond phase) carbon contents. For this, the powders are classified as rich-diamond and rich-graphite nanodiamonds. The active layers of PCs are made of as received (untreated) and hydrogen-treated ND particles deposited on p-doped silicon substrates by the pulsed spray technique at low temperatures (120 °C). The sprayed ND layers are characterized by Raman and FTIR spectroscopies, TEM microscopy and photoemission measurements. The latter measurements allow to assess the quantum efficiency (QE) of the PCs in the UV spectral range of 146-210 nm. The sp2 content affects strongly the quantum efficiency of the PCs. Specifically, the rich-graphite ND layers show a QE of 22 % higher than that of rich-diamond ones (11 %) at 146 nm, although the hydrogenation of rich-diamond is more effective than rich-graphite. Moreover, the PCs exhibit a good stability over time upon exposure to air. The QE value of rich-graphite PC is the highest recorded in the state of international art.

Resume : Diamond, containing boron, is very attractive both as valuable and rare gemstone and as a technological material for new semiconductor electronic devices. This study presents comprehensive investigation of electronic spectrum for set of natural type IIb and CVD grown B doped diamonds. A non-destructive experimental technique ? a complex of temperature and frequency admittance spectroscopy ? together with appropriate simulations was employed to obtain energy characteristics for boron deep center (activation energies and capture cross sections, corresponding emission rates in 15 ? 470 K). The majority carrier (hole) concentration in the samples and its distribution into depth were obtained. FTIR measurements of uncompensated B concentrations showed good coincidence with C-V results. The results for blue mined diamonds hole concentrations cover a narrow range (2 ? 4.5) x 10^16 cm^-3, and thermal activation energies are between 315 and 320 meV (capture cross sections were about 10^-13 cm^2). In contrast, for CVD diamonds with increasing boron concentration (10^16 ? 10^19 cm^-3) we saw gradual decreasing of activation energy down to 20 meV. Even at low impurity densities the highest registered activation energy (320 meV) is less than optical boron ionization energy (370 meV). The reason for the noticed difference between thermal and optical activation energies will be discussed.

Resume : Acquiring the crystals of preset type is important for the development of electronics. Dislocations are very resistant defects which have impact on crystals properties, that is why studying and observation them with the selective etching method enables gaining important information concerning the structure of grown diamond. The diamond single crystals of the Ib, IIa, IIb types grown by the temperature gradient method and the diamonds grown by the spontaneous crystallization have been used for the research. The selective etching has been held in the mixture of potassium oxide and potassium saltpeter in the muffle furnace with the temperature 550?660?? and atmospheric pressure; the time of etching has varied from 10 to 30 min. After etching crystals have been thoroughly washed with the distilled water and ethanol. For the crystals grown at temperature 1420?1500 º? and growth rate of 1-2 mg/h the dislocations density ND=0,8?3?102 cm-2. For crystals grown at 1280-1450 º? with the growth rate 3?5 mg/h ND=1,1?2,2?103 cm-2. Dislocations density of the spontaneous crystals which were grown at 1280?1350 º? with growth rate of 20?25 mg/h equals to1,06÷1,35?106 cm-2. As a result of the research such conclusions have been drawn: with the increasing of the growth temperature for different diamond single crystal types the density of dislocations decreases, and with the increasing of the growth rate from 1-2 to 20-25 mg/h it increases from 0,8÷3?102 cm-2 to 1,06÷1,35?106 cm-2.

Resume : We report for the first time on the DC and RF performance of Metal-Insulator-Semiconductor Field-Effect Transistors (MISFETs) realized by Vanadium Pentoxide (V2O5) as insulating material. As opposed to the typical oxide materials (such as Al2O3), the high electron affinity of the proposed oxide allows for the p-type charge transfer doping of the H-terminated diamond substrate, leading to increase hole concentration. Moreover, the V2O5 oxide demonstrated to be stable in atmosphere and thermally robust up to 250 °C. The active devices were characterized in terms of static I-V characteristics, static transconductance as well as of S-parameters for the evaluation of the maximum cutoff frequency and the maximum oscillation frequency. The DC characteristics of the diamond MISFETs showed a maximum drain current density of about 300 mA/mm, a maximum transconductance of 80 mS/mm and the threshold voltage, about +1.5 V, was found. Time stability of the drain current was evaluated overnight observing a maximum fluctuation of 7%. The small signal S-parameters of MISFET with 2 × 100 ?m gate width and 1.5 ?m gate length were also measured. The maximum reported oscillation frequency is 5.5 GHz, while the maximum cut-off frequency is about 2.1 GHz. Investigations on temperature dependence of diamond MISFETs performance were also performed up to 150 °C and compared with the better established diamond MESFET technology.

Resume : High quality single crystal diamond with thin ?-shaped boron-doped epilayers have been thought to offer a viable approach towards high speed, high power and high temperature applications. ?- 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 (? 1.5 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. Acknowledgements: We acknowledge Johan Adell for the support at the beamline I4. Phil King, Thomas Frederiksen and Ion Errea for the valuable discussions about the data acquisitions and their understanding. The authors gratefully acknowledge Diamond Microwave Ltd (www.diamondmw.com) for access to the 'delta-doped' diamond samples grown under contract by Element Six Ltd (www.e6.com) and for essential help from Dr Richard Lang and Dr Richard Balmer respectively.  Likewise the ?bulk? doped samples were grown at the Cardiff Diamond Foundry, under the supervision of Prof Oliver A Williams (www.astro.cardiff.ac.uk) assisted by one of the authors (APT).  Diamond Microwave and the UKs Engineering and Physical Sciences Research Council (EPSRC) are also thanked for the award of a PhD ?CASE? award to APK, and EPSRC for an award to one of the applicants (RBJ) for financial support of the work (EP/H020055/1)

Resume : Nanodiamonds are an important class of nanocarbons for many applications, with NDs produced by a detonation process being unique in offering access to the sub-5nm particle size. The inclusion of boron within nanodiamonds to create semiconducting properties would create a new class of applications in the field of nanodiamond electronics. Theoretical studies have differed in their conclusions as to whether sub-5nm NDs would support a stable substitutional boron state, offering semiconducting properties, or whether such a state would be unstable, with boron instead aggregating or attaching to edge structures. In the present study detonation-derived NDs with purposefully added boron during the detonation process have been studied with a wide range of experimental techniques. The individual DNDs are of ~4nm in size, and have been studied with CL, PL, Raman and IR spectroscopies, AFM and HR-TEM as well as electrically measured through the use of impedance spectroscopy. When the results from these differing techniques are combined it is apparent that the B-DNDs studied here do indeed support substitutional boron species and hence will be acting as semiconducting diamond nanoparticles. Evidence for moderate doping levels in some particles (~1017 B cm-3), is found alongside the observation that some particles are heavily doped (~1020 B cm-3) and likely to be quasi-metallic in character. The current study has therefore shown that substitutional boron doping in sub-nm NDs is in fact possible, opening up the path to a whole host of new applications for this interesting class of nano-particles.

Resume : The realization of high quality doped diamond is a pre-requisite for the acceptance of this material into the development space for high performance electronics. Whilst heavily boron doped diamond, displaying quasi-metallic properties, is a readily available material, lightly died material, as required for active electronic devices is much less so. Type IIa, nominally undated, HPHT material has recently become available, along with lightly boron doped, type IIb single crystal diamond. This paper reports the first electrical measurements on this martial, revealing Hall mobilities greater than 1500 cm2/Vs. Devices formed on this material will be discussed and the prospects for active devices presented.

Resume : Heterostructure Graphene on hydrogen terminated monolayer nanodiamond (Gr-H-ND) has demonstrated a new way to improve the carrier transport characteristics in this electronic system, offering up to 60% improvement when compared with graphene on SiO2/Si substrates. These devices offer excellent device current-carrying characteristics for the graphene system whilst offering the prospect of low cost large area production, given graphene can be a large area material as can be the similarly low cost large area compatible nanodiamond layers. The C-H bonds present on H-terminated NDs, strongly influence the heterostructure; the hydrogen links both ND and graphene to create charge transfer within this system. Field effect transistors (FETs) fabricated on this novel type of herterostructure are shown to have excellent characteristics. In this paper, a cost effective method to create a monolayer ND capable of tuning the properties of graphene for the fabrication of Field-Effect Transistors (FETs) is demonstrated. Compared to pristine graphene transferred onto SiO2/Si substrates, the mobility increased by 60% on Gr-H-ND. The detailed material properties of graphene on ND surface with and without hydrogen termination treatment have been investigated using Fourier transform infrared spectroscopy (FTIR), and Raman spectroscopy. It shows that the hydrogen termination treatment not only removed surface contamination from monolayer ND, but also provided a suitable linkage between ND and graphene to form a conductive path, as demonstrated in electrochemical impedance spectroscopy (EIS) measurements. The carrier mobility of graphene on hydrogen terminated ND (GrHND) is comparable that of graphene on hydrogen terminated SCD (GrHSCD). In addition, GrHND demonstrated a stable Hall mobility with temperature. High-k dielectric top-gate graphene transistors with gate length of 200 nm and 500 nm were fabricated using focused ion beam (FIB) using Tungsten carbides (WC) contacts to improve the energy transfer between diamond and metal. This research offers a new approach for commercial graphene transistor applications.

Resume : Negative electron affinity materials are an important class of materials, particularly when small numbers of free electrons need to be reliably amplified - as is the case with image intensifiers. Here, highlights from several years of study into improving secondary electron emission (SEE) characteristics of nanocrystalline diamond (NCD) layers is presented for the first time. The effect of growth conditions on nanocrystalline diamond growth morphology, composition and SEE has been investigated - using typical diamond growth temperatures down to extremely low growth temperatures in order to successfully integrate NCD into current image intensifier systems. Interestingly, it was noticed that the design of the cooling stage within the CVD chamber is of critical importance for a large secondary electron yield (SEY). The integration of diamond layers within multi-channel plate (MCP) based image intensifiers for night vision applications will be discussed.

Resume : Nanodiamond produced by a detonation process (DNDs) have become an important class of materials, both in terms of CVD diamond growth, where they can act as effective seeds, and in a range of applications for the DNDs themselves. A number of these applications would benefit if the DNDs were electrically conductive and material that has had boron incorporated in the detonation process has become available. However, ?bulk? electrical measurements, whilst indicating greater conductivity then is the case for DNDs created without the presence of boron, do not show the bahaviour expected for homogeniously doped nanodiamond particles. Here we have used an AFM equipped with a current imaging capability to ?map? the conductivity of DNDs with their topography. So-called Peak Force Tunneling AFM (PF-TUNA) has been used to indicate that in the DNDs studied some NDs are highly conductive (indicative of boron uptake) whilst others are not. The basis of this technique will be presented along with TUNA results for a range of boron-incorporated and non-boron incorporated DNDs.

Resume : Diamond combines a multitude of properties that make it an interesting candidate for electrical and electromechanical devices that are sought to withstand harsh environments. However, doping of diamond is still challenging, especially for the case of n-type doping. An alternative can be found in ultrananocrystalline diamond (UNCD) films since they can exhibit an n-type electrical conductivity at room temperature. We present electrically conductive UNCD films that were grown by hot filament chemical vapor deposition. The electrically conductive UNCD films were grown on 3 inch silicon wafers, using a hot filament chemical vapor deposition system, employing a gas mixture of CH4/H2/NH3. Films with different electrical conductivity were grown by using different NH3 concentrations. All grown films show similar grain sizes around 6 nm. The presented samples have electrical conductivities ranging from 16 µS/cm up to 3.7 S/cm. Using nanoindentation, we measured the microhardness and elasticity of the UNCD films and compare it with the electrical conductivity of the films. The measured data reveals that the softer samples exhibit a higher electrical conductivity. This can be explained by the different structure of the grain boundaries. As confirmed by Raman measurements, the ordering of the sp2 bonded carbon atoms in the grain boundaries is higher for the samples with higher conductivity and smaller hardness.

Resume : The key challenge in designing high performance electronic devices based on diamond is to identify shallow acceptor and donor dopants and to create an effective doping strategy enabling their implementation into the diamond lattice. In the midst of searching for an interesting elements manifesting shallow acceptor and donor states, we carried out a systematic study of Al, Be, Br, Co, Cr, Li, Na, N, Mg, S, Se and Ti substitutional centers in diamond using spin-polarized density functional theory approach. The Perdew-Burke-Ernzerhof (PBE) functional was applied for the total energy calculation. In order to avoid elastic interaction between periodic images of dopants a large supercell of N=512 atoms was considered. For each element the equilibrium geometry, formation energy as a function of charge state, nett spin and defect charge transition levels were determined. On the basis of formation energies vs Fermi level diagrams, relative stability of different charge states were predicted and the elements with shallow accetros/donors levels were selected. Depend on intrinsic electronic structure of dopants and their atomic radius variety of low symmetry configurations were found with some exhibiting high magnetic ground states. Our theoretical results provide a valuable information on a wide range of dopands in diamond, in terms of their electronic structures and may be useful in designing new high performance electronic devices.

Resume : Photocatalytic conversion of CO2 to hydrocarbons has gained significant attention within the last decades regarding its impact on global climate change as CO2 emissions are considered to be one of the primary factors for global warming. CO2 storage and conversion is therefore the ultimate goal to reduce CO2 and to use the conversion products as fuels. In this regard, the identification of reaction products generated from CO2 plays an important role [1]. Recently, the reduction of CO2 by solvated electrons has been presented [2, 3]. Here we present a study investigating the conversion of CO2 in imidazolium-based room temperature ionic liquids (RTILs) using in-situ IR- attenuated total reflection (ATR) spectroscopy. RTILs Based on the vibrational signatures, reduction products may be directly identified using IR spectroscopy. For the conversion, a hydrogenterminated diamond ATR crystal was illuminated with pulsed UV laser light. The Illumination generates photo-excited electrons from H-terminated diamond that are emitted into the RTIL solution containing the dissolved CO2. In this perspective Hterminated diamond has attracted great interest in terms of its negative electron affinity. To our best knowledge, this is the first report to identify in-situ reaction products of CO2 in RTILs during illumination of H-terminated diamond using IR spectroscopy. An IR-ATR cell was customized with a lid containing a 1 mm thick UV transparent fused silica window in order to control the CO2 content in the adjacent medium, which was saturated with CO2. CO2 is a strong IR absorber and has distinct asymmetric stretching mode at 2343 cm-1 . The decrease of CO2 content in the RTIL was monitored investigating the change in the asymmetric stretching mode. A pulsed N2 laser with a repetition rate of 10 Hz was chosen as UV light source (𝜆 = 337 nm) to pump a dye placed in a cuvette leading to a final emitting wavelength of 212 nm. Experimental conditions such as illumination time and water content of the RTIL have been investigated. Monitoring the appearance of bands in the IR spectrum will allow identifying different possible pathways of CO2 reduction under the given conditions. Besides the identification and quantification of the final products, conversion rates may also be determined. Results indicate a conversion of CO2 to oxalate (C2O4 2-). [1] X. Chang, T. Wang, J. Gong, Energy Environ. Sci., 2016, 9, 2177-2196. [3] L. Zhang, D. Zhu, G. Nathanson, R. Hamers, Angew. Chem. Int. Ed., 2014, 53, 9746 –9750. [4] R. J. Hamers, J. A. Bandy, D. Zhu, L. Zhang, Faraday Discuss., 2014, 172, 397- 411.

Resume : Low-resistivity substrates are base component materials used to fabricate vertical-type high-output power devices. For diamond, low-resistivity can be reproducibly obtained by heavily boron-doping.However, the commercially available p+ substrates possess relatively high resistivity in the order of 0.1 cm. Assuming the 100-A operation, the total parasitic resistance must be less than 5E5 cm2which constitutes the 5% of on-resistance. In this point of view, substrate resistivity of < 10 mcm with a thickness of 50 m is necessary. Recently, we have demonstrated some advantageous of p+ diamond growth by utilizing hot-filament CVD.[1] High doping efficiency (~100%) was realized for {100} growth. Resistivity was monotonically decreased to be 1.2 mcmwith increasing boron concentration. In this study, to further investigate the incorporation mechanism, p+ diamond was grown on misoriented substrates possessing vicinal (100) planes from 0 to 5.Change in the surface morphology, incorporation efficiency, and growth rate was discussed. Details will be presented in the conference. [1] S. Ohmagari, K. Srimongkon, H. Yamada, H. Umezawa, N. Tsubouchi, A. Chayahara, S. Shikata, and Y. Mokuno, Diam. Relat. Mater.58 (2015) 110.

Resume : DIADEMS is an EC funded project that aims at exploiting the unique physical properties of NV color centres in ultrapure single-crystal CVD-grown diamond to develop innovative devices with unprecedented performances. The atom-like structure of the NV exhibits spin dependent optical transitions, which makes optics-based magnetometry possible. The objectives of DIADEMS are to develop wide-field magnetic imagers with 1 nT sensivity, scanning probe magnetometer with sensitivity 10 nT and spatial resolution 10 nm, sensor heads with sensitivity 1 pT. To reach such performances, DIADEMS is using new theoretical protocols for sensing, developing ultrahigh purity diamond material with controlled single nitrogen implantation with a precision better than 5 nm, processing scanning probe tips with diameter in the 20 nm range, improving the emission properties of NV by coupling them with photonic cavities. DIADEMS targets new applications such as calibration and optimization of write/read magnetic heads for future high capacity storage disk required for intense computing, imaging of electron-spin in graphene and carbon nanotubes for next generation of electronic components based on spintronics, non-invasive investigation of living neuronal networks to understand brain function, demonstration of magnetic resonance imaging of single spins allowing single protein imaging for medical research. The presentation will give an overview of the results achieved so far in the project.

Resume : Ways of finely tuning the properties of 1D triangular nanobeam diamond based photonic devices, with the aim of enhancing the photon collection efficiency at predetermined wavelengths at ambient temperature are described. Delicate control on the physical dimensions of the device which are required to obtain these are achieved by deposition of HfO2 monolayers (thickening) and by H2 plasma etching (thinning). These, result in shifting the optical properties of the thus treated photonic crystal to the red and to the blue respectively. The observed shifts demonstrate an enhancement of the nitrogen-vacancy (NV) center's zero-phonon line fluorescence by a factor of 40 at ambient temperature. The methods described here demonstrate a practical and straight-forward way to fine-tune the diamond photonic nanocavity into resonance with the zero-phonon line of the NV centers at room temperature. This approach may find valuable improvements in diamond-based quantum computing and communications.

Resume : The nano-scale sensing using nitrogen-vacancy (NV) centers in diamond has already attracted much interest, however, there is a room for improvements of diamond materials. We can reduce photon shot noise of NV luminescence by using high density NV center ensemble, while the use of the ensemble can degrade the contrast of optically detected magnetic resonance (ODMR). Selectively-aligned NV ensemble created by chemical vapor deposition (CVD) growth can be a remedy for this problem. We present CVD-growth, characterization method, and sensing application of such NV-containing diamond sample. Selectively-aligned high density NV ensemble is formed by CVD-growth on (111)-oriented diamond substrate. We perform intensive nitrogen gas doping to gain high density NV centers. The key to create selectively-aligned NV centers is finding the critical condition to achieve step-flow growth. Quantification of the alignment ratio is not a straightforward problem for dense ensemble of the NV centers. We estimate the alignment ratio of ensemble NV centers along the [111] direction in (111) diamond by ODMR measurements. Finally we demonstrate the magnetometry application of the selectively-aligned high density NV ensembles. Although the spin coherence time of high density NV centers is not as good as that of low density NV centers, selective alignment and high density result in good magnetometer performance.

Resume : The negatively charged nitrogen-vacancy center (NV-) in diamond shows outstanding optical and spin properties and thus is one of the most promising candidates to fabricate a full scalable quantum computer or a sensor for measuring external magnetic fields, even at room temperature. For this and other applications in quantum physics, near-surface NV centers (1-10 nm below the surface) are required. But in this case, the charge state of near-surface NV centers switch in an uncontrolled way between NV- to NV0 or even to NV+ [1]. Therefore, an active control of its charge state is very important. In this presentation, we demonstrate active and fast switching of the charge state of a single and near-surface NV center by using an in-plane Schottky diode geometry with aluminum on a hydrogen terminated diamond surface [2,3]. A switching between NV+, NV0 and NV- on a timescale of 10 to 100 ns can be achieved by applying a potential to the Al-gate. A detailed discussion of the results from simulations of the 2D Al-Schottky-Diode with single NV-centers will be also given. With the software ATLAS from Silvaco Inc., we simulated the in-plane band structure of the Schottky junction at different bias voltages and estimated the charge carrier capture cross section of a single NV center. From simulating the time-resolved switching of the charge state, we could establish a model based on Shockley-Read-Hall mechanism. Furthermore, we estimated the range of depth, at which it is still possible to manipulate a single NV center near the Al-gate. The latter finding will be crucial to evaluate the feasibility of the technology for applications such as quantum computing. For such application, a long spin coherence time of the NV- spin is required, which increases with increasing distance from the H-terminated diamond surface. [1] L. Rondin et al., Phys. Rev. B. 82 (2010) 115449. doi:10.1103/PhysRevB.82.115449. [2] C. Schreyvogel et al., Active charge state control of single NV centres in diamond by in-plane Al-Schottky junctions., Sci. Rep. 5 (2015) 12160. [3] J.A. Garrido et al. , Capacitance?voltage studies of Al-Schottky contacts on hydrogen-terminated diamond, Appl. Phys. Lett. 81 (2002) 637.

Resume : The negatively charged nitrogen-vacancy (NV) centre in diamond is a remarkable defect which allows monitoring of its magnetic sublevel state through its fluorescence intensity. The induced change in emission intensity of NV fluorescent nanodiamond (FND) has been proposed as basis for background-free biological microscopy[1,2]. The FNDs emission can be discriminated by ON/OFF switching of resonant microwaves (RMW) and/or a static magnetic field (SMF). It is now important to understand the origin of the contrast in each case, and the optimal experimental parameters. Using a compact spectrometer, NV fluorescence spectra are measured with RMW, with SMF, or without any of them, to determine the relative changes in integrated intensity. This way, both RMW and SMF contrasts are accurately obtained in a wide range of optical intensities, in a diamond single crystal as well as in FNDs. SMF-induced contrast is found to be higher in general, and monotonuously increasing with excitation level, in contrast to the RMW-induced contrast that is vanishing under intense excitation. These results are well described within a 5-level model that includes radiative and nonradiative decay, intersystem crossing, and ground state spin relaxation. The advantage of the field-induced contrast approach is illustrated for FNDs in neuronal cultures in several standard biological microscope set-ups. [1] R. Igarashi, et al, Nano Lett. 2012, 12, 5726 ; [2] R. Chapman, T. Plakkhoitnik, Opt. Lett. 2013, 38, 1847

Resume : Defect induced magnetism, which can be controllably generated by ion or neutron irradiation, is attracting intensive research interest. It not only challenges the traditional opinions about magnetism, but also has some potential applications in spin-electronics. SiC is a new candidate for the investigation of defect-induced ferromagnetism after graphitic materials and oxides due to its high material purity and crystalline quality [1, 2]. In this contribution, we made a comprehensive investigation on the structural and magnetic properties of ion implanted and neutron irradiated SiC samples. In combination with X-ray absorption spectroscopy, high-resolution transmission electron microscopy and first-principles calculations, we try to understand the mechanism in a microscopic picture. For neon or xenon ion implanted SiC, we identify a multi-magnetic-phase nature [3]. The magnetization of SiC can be decomposed into paramagnetic, superparamagnetic and ferromagnetic contributions. The ferromagnetic contribution persists well above room temperature and exhibits a pronounced magnetic anisotropy. By combining X-ray magnetic circular dichroism and first-principles calculations, we clarify that p-electrons of the nearest-neighbor carbon atoms around divacancies are mainly responsible for the long-range ferromagnetic coupling [4]. Thus, we provide a correlation between the collective magnetic phenomena and the specific electrons/orbitals. With the aim to verify if a sample containing defects through its bulk volume can persist ferromagnetic coupling, we applied neutron irradiation to introduce defects into SiC [5]. Besides a weak ferromagnetic contribution, we observe a strong paramagnetism, scaling up with the neutron fluence. The ferromagnetic contribution only occurs in a narrow fluence window or after annealing. First-principles calculations hint towards a mutually exclusive role of the concentration of defects: Defects favor spin polarization at the expense of magnetic interaction. Although both Raman scattering and X-ray diffraction reveal essential structure damage to SiC due to irradiation, high-resolution transmission electron microscopy does not detect significant structural variation even upon the largest neutron fluence. [1] L. Li, et al., Appl. Phys. Lett. 98, 222508 (2011). [2] Y. Wang, et al., Phys. Rev. B 90, 214435 (2014). [3] Y. Wang, et al., Phys. Rev. B 89, 014417 (2014). [4] Y. Wang, et al., Scientific Reports, 5, 8999 (2015). [5] Y. Wang, et al., Phys. Rev. B 92, 174409 (2015).