Thermionic electron emission from nitrogen-doped homoepitaxial diamond

Nitrogen-doped homoepitaxial diamond films were synthesized for application as low-temperature thermionic electron emitters. Thermionic electron emission measurements were conducted where the emission current was recorded as a function of emitter temperature. At a temperature < 600 °C an emission current was detected which increased with temperature, and the emission current density was about 1.2 mA/cm² at 740 °C. The electron emission was imaged with photoelectron emission microscopy (PEEM) and thermionic field-emission electron microscopy (T-FEEM). The image displayed uniform electron emission over the whole surface area. Thermionic emission and ultraviolet photoemission spectroscopy were employed to determine the temperature dependent electron emission energy distribution from the nitrogen-doped homoepitaxial diamond films. The photoemission spectra indicated an effective work function of 2.4 eV at 550 °C. These values indicate reduced band bending and establish the potential for efficient electron emission devices based on nitrogen-doped homoepitaxial diamond.     

Measurement of field emission from nitrogen-doped diamond films

This study explores issues related to the measurement of the field emission properties of nitrogen-doped diamond grown by microwave plasma chemical vapor deposition (CVD). Growth conditions have been optimized to produce films with a low concentration of sp²-bonded carbon which results in high electrical resistance. Field emission characteristics were measured in an ultrahigh vacuum with a variable distance anode technique. For samples grown with gas phase [N]/[C] ratios less than 10, damage from micro-arcs occurred during the field emission measurements. Samples grown at higher [N]/[C] content could be measured prior to an arcing event. The occurrence of a micro-arc is related to the film properties. The measurements indicate relatively high threshold fields (>100 V μm⁻¹) for electron emission.     

Structure and electrical conductivity of nitrogen-doped carbon nanofibers

The effect of nitrogen concentration in carbon nanofibers (CNFs) on the structural and electrical properties of the carbon material was studied. CNFs with nitrogen concentration varied from 0 to 8.2 wt.% (N-CNFs) with “herringbone” structure were prepared by decomposition of ethylene and ethylene mixture with ammonia over 65Ni-25Cu-10Al₂O₃ (wt.%) catalyst at 823 K. Detailed investigation of the CNFs and N-CNFs by XPS, FTIR and Raman spectroscopy showed that the nitrogen introduction in carbon material distorts the graphite-like lattice and increases the structure defectiveness. Both effects become more significant as the nitrogen concentration in N-CNFs grows.The electrical conductivity of N-CNFs with different nitrogen concentrations is caused by the competition of the nanofiber graphite-like structure disordering after introduction of nitrogen atoms and doping of an additional electron into the delocalized π-system of the graphite-like material. As a result, the maximum electrical conductivity among the samples studied was observed at nitrogen concentration in N-CNFs equal to 3.1 wt.%.     

Electrochemical characteristics of boron-doped,undoped and nitrogen-doped diamond films

Boron-doped, undoped and nitrogen-doped diamond films were synthesized by microwave plasma assisted chemical vapor deposition (MP-CVD). Raman spectroscopy, XPS, EPMA and UV–Vis were used to characterize the properties of the synthesized films. Electrochemical characteristics for several redox systems on the three kinds of diamond films were examined. For Li⁺/Li (E⁰=−3.05 V) and H⁺/H₂ (E⁰=0.00 V) redox couples, the marked differences in cyclic voltammetric (CV) behaviors were observed on the nitrogen-doped diamond films, whereas for Fe(CN)³⁻/⁴⁻ (E⁰=0.36), Au/AuCl₄⁻ (E⁰=1.00) and O₂/H₂O (E⁰=1.23 V) couples, the CV behaviors on the nitrogen-doped films were similar to those on the boron-doped or undoped diamond films. The significant differences of CV behaviors could be explained by hypothesizing that the electron transfers of the redox species in the solution on diamond electrodes happened at the top of valence band together with the surface doping model suggested by F. Maier and colleagues [F. Maier, M. Riedel, B. Mantel, J. Ristein, L. Ley, Phys. Rev. Lett. 85 (2000) 3472].     

Thermal conductivity measurements on CVD diamond

Data are presented on chemical vapor deposited (CVD) diamond films with thicknesses between 0.1 and 0.8 mm. Infrared and Raman measurements are compared with the through-thickness and in-plane thermal conductivities measured using thermal flash and heated bar techniques, respectively. We have presented and discussed, in terms of a simple model, a measured correlation between the thermal conductivity and the integrated absorption in the infrared region of the spectrum between 2760 and 3030 cm⁻¹.     

Zero-biased and visible-blind UV photodetectors based on nitrogen-doped ultrananocrystalline diamond nanowires

Taking advantage of the superflat surface of ultrananocrystalline diamond (UNCD), highly precise UNCD nanowire (NW) arrays were fabricated to develop high-performance UV photodetectors. The large surface-to-volume ratio of a nanowire significantly increases the number of surface trap states, and the reduced dimensionality effectively confines the active area of the charge carrier and shortens the transit time, which results in an enhanced photoconductivity and response speed. In this paper, the zero-biased UV photodetectors based on nitrogen-incorporated ultrananocrystalline diamond nanowire arrays have been demonstrated and characterized. The estimated responsivity was 2.0 A/W for 350 nm incident light when the device operated at room temperature. The UVA and UVB photocurrent signals from this visible blind photodetector were well defined with a rise and decay time of less than 1 s.     

Sustainable nitrogen-doped carbon latexes with high electrical and thermal conductivity

Nitrogen-doped carbon particles were produced using the hydrothermal carbonization of a nitrogen-containing carbohydrate, namely glucosamine, under mild temperature conditions (180 °C) followed by further calcination under a stream of inert gas at 750 °C. The resulting materials contain significant amounts of nitrogen doping within their structure, mainly as quaternary N involved in an aromatized/graphitized carbon structure according to X-ray photoelectron spectroscopy. This nitrogen-doped material was dispersed with nanolatexes having a high affinity for carbon. The resulting hybrid dispersions could be conveniently cast into dense and stable films for thermal and electrical conductivity measurements. The conductivities were commensurate with technical carbon nanotube latex-based films. A morphological analysis of the dispersing mechanism suggests that the potential for high performance materials realized in this contribution is very competitive, but still far from being fully exploited.     

Vibrational properties of nitrogen-doped ultrananocrystalline diamond films grown by microwave plasma CVD

Ultrananocrystalline diamond (UNCD) films grown in an argon-rich Ar/CH₄/H₂ microwave plasma with nitrogen gas added in amounts of 0%–20% were studied by Raman spectroscopy with multiple excitation wavelengths in the range of 244–647 nm and by optical absorption in UV–visible. The Raman spectra have demonstrated the presence of diamond, amorphous carbon and polyacetylene in the UNCD films. Analysis of vibrational and optical properties of amorphous carbon phase proves that nitrogen stimulates the transition from amorphous carbon into an ordered graphite-like structure with narrowed optical band gap, which is supposed to be responsible for the high electrical conductivity of the N-doped UNCD.     

Raman and conductivity studies of boron-doped microcrystalline diamond,facetted nanocrystalline diamond and cauliflower diamond films

We present a large amount of data showing how the electrical conductivity and Raman spectra of boron-doped CVD diamond films vary as a function of both B content and film type — in particular, diamond crystallite size. Three types of film have been investigated: microcrystalline diamond (MCD), faceted nanocrystalline diamond (f-NCD) and ‘cauliflower’ diamond (c-NCD). For the same B content (measured by SIMS), the conductance of MCD films was much higher than those for the two types of smaller grained films. Multi-wavelength laser Raman spectroscopy showed that Fano interference effects were much reduced for the smaller grain-sized material. The position of the Lorentzian contribution to the 500 cm^− 1 Raman feature was used to estimate the B content in each type of film, and compared to the value measured using SIMS. We found that the Raman method overestimated the concentration of B by a factor of ~ 5 for the f-NCD and c-NCD films, although it remains reasonably accurate for MCD films. The shortfall may be explained if only a small fraction of the B found in the small-grained films is being incorporated into substitutional sites. We conclude that in diamond films with a high concentration of grain boundaries, the majority of the B (80% in some cases) must be present at sites that do not contribute to the continuum of electronic states that give rise to metallic conductivity and the Fano effects. Such sites may include (a) interstitials, (b) the surface of the crystallites, or (c) bonded within the non-diamond carbon impurities present at the grain boundaries. This suggests that heavy doping of nanograined diamond films will give rise to a material with many different conducting regions, and possibly different conducting pathways and mechanisms.     

On the thermionic emission from nitrogen-doped diamond films with respect to energy conversion

Thermionic energy converters utilize thermal energy and efficiently transform it into more useful electrical energy. A key aspect in thermionic energy conversion is the emission of electrons at elevated temperatures, where the electron emitter is separated from the collector by a vacuum gap and a voltage is generated due to the temperature difference between the emitter and collector. In this study, nitrogen-doped diamond films with a negative electron affinity surface have been synthesized with plasma-assisted chemical vapor deposition, and the electron emission has been imaged using high-resolution electron emission microscopy. This study reports the measurement of a thermovoltage and current, i.e. energy conversion, at temperatures considerably less than 1000 °C.     

Effective thermal conductivity of a diamond coated heat spreader

Thin diamond coatings are often suggested to enhance thermal conductivity of some substrate. We measured the effective thermal conductivity of varying thicknesses of diamond on tape cast, polycrystalline silicon carbide. The effective thermal conductivity of 30 μm diamond on tape cast silicon carbide is 1.7 W/(cm K). The effective thermal conductivity can be increased to 2.2 W/(cm K) by increasing the diamond thickness to approximately 70 μm. With the measured effective thermal conductivity, the thicknesses of the diamond film and substrate, and knowledge of the thermal conductivity of the substrate material, the thermal conductivity of the diamond layer can be calculated from a simple formula. The thermal conductivity of the 30-μm and 69-μm diamond layers were found to be 3.9 W/(cm K) and 5.8 W/(cm K), respectively.     

Thermal conductivity of diamond composites sintered under high pressures

Thermal conductivity at room temperature of diamond composites of two types: with a diamond skeleton and with diamond grains imbedded in a non-diamond matrix was evaluated in dependence of the diamond grain size (d) varied from a ten of microns to 500 μm. The thermal conductivity of the compacts with diamond skeleton obtained in the Cu–diamond system at high pressure of 8 GPa strongly increases with diamond particles size approaching the maximum value of 9 W/cm K at d ≈ 200 μm. The compacts sintered in the Cu–Ti–diamond, Al–Si–diamond and Si–diamond systems at lower pressure (2 GPa) are formed predominantly owing to the presence of the binder. It was found for these conditions that the thermal conductivity is less sensitive to the diamond grain size, reaching the value of 6 W/cm K for the composites with SiC–Si matrix.     

Strong photoluminescence from N-V and Si-V in nitrogen-doped ultrananocrystalline diamond film using plasma treatment

Raman, photoluminescence, and transport properties of nitrogen-doped ultrananocrystal diamond (UNCD) films were investigated following treatment with low energy microwave plasma at room temperature. The conductivity of nitrogen-doped UNCD films treated by microwave plasma was found to decrease slightly due to the reduced grain boundaries. We speculate that the plasma generated vacancies in UNCD films and provided heat for further mobilizing the vacancies to combine with the impurities, which led to the formation of the silicon-vacancy (Si-V) and nitrogen-vacancy (N-V) defect centers. The generated color centers were found to be distributed uniformly in the samples using a PL mapping technique. The PL emitted by the plasma treated nitrogen-doped UNCD film was strongly enhanced in comparison with the untreated films.     

Low-temperature thermionic emission from nitrogen-doped nanocrystalline diamond films on n-type Si grown by MPCVD

Temperature-dependent emission current–voltage measurements were carried out for nitrogen (N)-doped nanocrystalline diamond (NCD) films grown on n-type Si substrates by microwave plasma-assisted chemical vapor deposition (MP-CVD). Low threshold temperature (~ 260 °C) and low threshold electric field (~ 5 × 10^− 5 V/µm) were observed. Both the temperature dependence and the electric field dependence have shown that the obtained emission current was based on electron thermionic emission from N-doped NCD films. We have also studied the relation between nitrogen concentration and the saturation emission current. The saturation current obtained was as high as 1.4 mA at 5.6 × 10^− 3 V/µm at 670 °C when the nitrogen concentration was 2.4 × 10²⁰ cm^− 3. Low value of effective work function (1.99 eV) and relatively high value of Richardson constant (~ 70) were estimated by well fitting to Richardson–Dushman equation. The results of smaller φ and larger A′ suggest that N-doped NCD has great possibility of being a highly efficient thermionic emitter material.     

Surface stress distribution in diamond crystals in diamond–silicon carbide composites

Diamond–silicon carbide composites were sintered at high temperature, up to 2273 K, and high pressure, up to 10 GPa. Raman microscopy was used to map stress distribution in diamond crystals on surfaces of the composites. Splitting of the triple degenerate band of diamond and frequency shifts of its components were used to calculate the magnitudes of stress. Those magnitudes varied with location and reached maximum values near crystals boundaries. Stress depended on the sintering temperature, pressure, and the crystal size and was attributed to differences in thermal expansion coefficients and bulk moduluses of diamond and silicon carbide.     

Influence of ambient humidity on the surface conductivity of hydrogenated diamond

The enhancement of the electrical conductivity of hydrogen-terminated diamond surfaces due to the adsorption of various chemical species represents a challenging problem and has a high application potential as well. In the transfer doping model of Maier et al. the presence of a water layer that forms spontaneously in moist atmospheres on any surface plays a crucial role. Hence, in the present contribution we have focused our attention on the influence of the relative humidity (RH) on the surface conductivity. In order to perform conductivity measurements in a wet atmosphere an original technique eliminating leakage effects due to adsorbed water was developed. The conductivity was measured at room temperature for RH between 2% and 100%. Despite a rather weak dependence on RH, three well distinguished regions can be identified in the conductivity as a function of RH. Not pretending to offer a first-principles account for this behaviour, a phenomenological explanation of the data is given.     

Surface properties of differently prepared ultrananocrystalline diamond surfaces

Ultrananocrystalline diamond/amorphous carbon composite films have been deposited by microwave plasma chemical vapour deposition from 17% CH₄/N₂ mixtures at 600 °C. Thereafter the films were subjected to various treatments (plasma processes, UV/O₃ exposure) to obtain hydrogen, oxygen, and fluorine terminated surfaces, which then have been characterized with respect to their composition, roughness, wettability, and other properties. Among others, it will be shown that H- and F-terminated surfaces are very stable even if exposed for long time to air, while O-terminated ones are prone to contaminations. H- and O-termination can be patterned by applying the UV/O₃ treatment through a mask. Finally, it will be shown that a non-fouling poly(ethylene glycol) layer can be grafted directly on oxygen terminated surfaces by an atom transfer radical polymerization process using α-bromoisobutyryl bromide as an initiator.     

On the local conductivity of individual diamond seeds and their impact on the interfacial resistance of boron-doped diamond films

In many electroanalytical and bio-electrochemical applications conductive diamond films act as contact layers. These films are grown starting from a Si-surface seeded with undoped diamond particles. In this study, the impact of the seeds and their electrical properties on the interfacial resistance through the diamond film − substrate is determined on the nanometer-scale by probing the nucleation side of the conductive diamond films using scanning spreading resistance microscopy. We evidence that, although the diamond film is grown in a B-rich ambient, no significant B incorporation occurs into the particles and they remain non-conductive after growth. We demonstrate that they impact strongly on the interfacial resistance, increasing it by more than one order of magnitude depending on the seed layer coverage. We further establish a model linking the seed size and density to this interfacial resistance, with excellent agreement to our experimental results. Based on this model, we predict that it is necessary to limit the undoped particle density to less than 5 × 1010 cm−2, for 20 nm particle size, in order to eliminate the contribution of the undoped seeds to the interfacial resistance. Our model also indicates that the fundamental solution to this problem lies in the use of B-doped seeds.     

Hydrogenated diamond crystal C(100) conductivity studied by STM

The topographic and spectroscopic capabilities of the scanning tunnelling microscope (STM) have been used to explore the conductivity of hydrogenated diamond C(100)-(2 × 1) surfaces. It has been shown that the surface conductivity is determined by the interplay between various factors: the adsorption of atmospheric species on the surface, the doping concentration of the sample, the presence of sub-surface species and the presence of the top layer of hydrogen.     

Transport properties of CVD diamond elucidated by DC and AC conductivity measurements

Detailed comprehensive study was carried out on diamond thin films to understand the charge carrier transport with respect to temperature and frequency domain of AC conductivity. Metal-Diamond-Metal structures were used to study the bulk transport and surface conductivity of freestanding films. Large differences were observed between the surface and bulk admittance values of as grown films. Bulk transport resulted in thermally activated behavior both in AC and DC measurements with activation energies of 0.65 eV for thermionic emission of trapped charge. Normalized Cole–Cole plots can be explained on the basis of a superposition of four Voigt elements representing an effective medium constituted by a distribution of grains, elongated in the growth direction, and temperature dependent leakage paths. A simple model of the grain boundary is introduced to discuss and justify the observed results.