Single-crystal Boron-doped Diamond Research Articles

Single-crystal vs polycrystalline boron-doped diamond anodes: Comparing degradation efficiencies of carbamazepine in electrochemical water treatment

The ongoing challenge of water pollution by contaminants of emerging concern calls for more effective wastewater treatment to prevent harmful side effects to the environment and human health. To this end, this study explored for the first time the implementation of single-crystal boron-doped diamond (BDD) anodes in electrochemical wastewater treatment, which stand out from the conventional polycrystalline BDD morphologies widely reported in the literature. The single-crystal BDD presented a pure diamond (sp3) content, whereas the three other investigated polycrystalline BDD electrodes displayed various properties in terms of boron doping, sp3/sp2 content, microstructure, and roughness. The effects of other process conditions, such as applied current density and anolyte concentration, were simultaneously investigated using carbamazepine (CBZ) as a representative target pollutant. The Taguchi method was applied to elucidate the optimal operating conditions that maximised either (i) the CBZ degradation rate constant (enhanced through hydroxyl radicals (•OH)) or (ii) the proportion of sulfate radicals (SO4•−) with respect to •OH. The results showed that the single-crystal BDD significantly promoted •OH formation but also that the interactions between boron doping, current density and anolyte concentration determined the underlying degradation mechanisms. Therefore, this study demonstrated that characterising the BDD material and understanding its interactions with other process operating conditions prior to degradation experiments is a crucial step to attain the optimisation of any wastewater treatment application.

Synthesis of porous single-crystal diamond with boron doping under high temperature and high pressure

In this paper, porous single-crystal diamond with boron doping were synthesized by adding TiB2 to the Fe–Ni–C system at a pressure of 5.6 GPa and a temperature of 1360 °C. The boron and nitrogen impurities were investigated in depth along with the surface defects of the diamonds. The results indicate that when the TiB2 content was within the range of 0–10 wt%, an increase in the doping concentration of TiB2 can lead to a decrease in the nitrogen content and an increase in the boron content in the diamond, causing the crystal color to transition from yellow to pale yellow, pale yellow-blue, and black. When the TiB2 content was within the range of 20–40 wt%, several triangular concave porous structures appeared on the crystal surface. With an increase in the TiB2 addition, the side length of these triangular pits decreased from 2.85 μm to 1.09 μm, while their depth decreased from 590 nm to 50 nm. The roughness of the crystal surface measured by Rq demonstrated values up to 145.302 nm. The results indicate that the size, depth, and density of the triangular defects can be adjusted by the amount of additive TiB2. Our findings provide insights into the surface regulation and functionalization of boron-doped diamond single crystals.

Significant enhancement of electrical conductivity in boron-doped diamond through HPHT post-annealing treatment

Boron-doped diamond (BDD) is expected as a promising semiconductor candidate for potential application in next-generation high-power and high-voltage electronics. Unfortunately, a feasible strategy that focuses on the improvement of the semiconducting performance of BDD materials is still lacking and remains a fundamental challenge in material science. Thus, we aim to study the stability and adjustment behaviors of boron dopants in BDD single crystals through post-annealing treatment under high pressure and high temperature (HPHT) conditions. In this work, our study demonstrates that boron dopants show high structural stability, and no extra complex defects were observed during the HPHT annealing process, a superior advantage for the device application in extreme environments. Our work further reveals that the HPHT post-annealing process is effective to eliminate the intrinsic stress and local lattice strain that were inherently formed during the diamond growth process. This process is accompanied by the enhancement of ionization ratio of boron dopants. The carrier content of one 〈111〉-GS BDD increases by more than ten times after post-annealing at 1000 °C and 5.5 GPa, resulting in significant improvement of conductivity by a factor of 2. This result provides an effective strategy to improve the semiconducting performance of BDD materials through HPHT annealing approach and promote their practical device applications.

Boron-doped {113}, {115} and {118}-oriented single-crystal diamond electrodes: Effect of surface pre-treatment

In this study, the effect of vicinal crystal orientation ({113}, {115}, and {118}), boron doping level, and surface termination induced by various pre-treatments on electrical and electrochemical properties of single crystal boron-doped diamond (SC-BDD) electrodes was carefully investigated. van de Pauw measurements confirmed Hall concentration to increase with increasing vicinal angle, {118} < {115} < {113}. As-deposited H-terminated surfaces were converted to predominantly O-terminated surfaces (confirmed by X-ray photoelectron spectroscopy) using hot acid, ozone and anodic pre-treatments, or by spontaneous oxidation in air. This conversion was recognized by inhibited heterogeneous electron transfer kinetics of inner-sphere redox markers ([Fe(CN)6]3−/4− and dopamine), by an increase in double layer capacitance and charge transfer resistance values. The most pronounced surface oxidation was recognized for electrodes with the lowest boron content as a result of blocked boron atoms by oxygen functionalities and loss of superficial hydrogen on the surface. Reverse conversion from oxidized to H-terminated surfaces using cathodic pre-treatment led mainly to restoration of electrochemical parameters. Moreover, spontaneous oxidation of electrode surfaces in ambient atmosphere was confirmed, with the fastest being within the first five days. The results show the key role of boron content, and hence the crystal orientation and superficial hydrogen in affecting conductivity and performance of SC-BDD electrodes, which is demonstrated by changes in heterogenous electron transfer kinetics of surface sensitive redox systems.

Characterization of boron-doped single-crystal diamond by electrophysical methods (review)

A critical analysis of the existing methods of controlling the concentration of impurity and majority charge carriers in wide bandgap semiconductors and the issues of improvement of modern diagnostics of the main electrophysical properties of single-crystal diamond are considered based on the results of our studies and the works of other authors. It was found that independent assessment of impurity concentration and concentration of free charge carriers is of fundamental importance for semiconductor diamond due to very low (less than 1%) degree of ionization of the introduced impurity. The advantages and prospects of admittance spectroscopy as a diagnostic method for ultrawide bandgap semiconductors are shown and solutions aimed at the correct interpretation of the experimental data are proposed. The high ionization energy of boron impurity in diamond (370 meV) results in a strong frequency dispersion of the measured barrier capacitance. It is shown that under disturbance of quasi-static conditions in capacitance-voltage measurements, low frequencies and high temperatures should be used for correct assessment of the charge carrier concentration. The results of electrophysical studies are compared with traditional measurements of impurity concentration in diamond by optical methods. A decrease of hole activation energy from the boron impurity level from 325 to 100 meV was found upon increasing the boron concentration NA from 2·1016 to 4·1019 cm-3. The transition to the hopping mechanism of conductivity within the impurity (acceptor) band with thermal activation energy of 10-20 meV was registered for NA≥5·1018 cm-3 at temperatures of 120-150 K. Keywords: single-crystal diamond, boron impurity, charge carrier concentration, activation energy, admittance spectroscopy, capacitance-voltage measurements.

Heavily boron-doped diamond grown on scalable heteroepitaxial quasi-substrates: A promising single crystal material for electrochemical sensing applications

In this work, three distinct heteroepitaxial single-crystal boron-doped diamond (SC-BDD) electrodes were fabricated and subjected to detailed surface analysis and electrochemical characterization. Specifically, the heteroepitaxy approach allowed to synthesize large-area (1 cm2) and heavily-doped (100)-oriented SC-BDD electrodes. Their single-crystal nature and crystal orientation were confirmed by X-ray diffraction, while scanning electron and atomic force microscopies revealed marked variations in surface morphology resulting from their growth on respective on-axis and off-axis substrates. Further, absence of sp2 impurities along with heavy boron doping (>1021 cm−3) was demonstrated by Raman spectroscopy and Mott-Schottky analysis, respectively. Cyclic voltammetry (CV) in a 0.1 M KNO3 solution revealed wide potential windows (∼3.3 V) and low double-layer capacitance (<4 μF cm−2) of the SC-BDD electrodes. Their highly conductive, ‘metal-like’ nature was confirmed by CV with [Ru(NH3)6]3+/2+ probe manifesting near-reversible redox response with ΔEp approaching 0.059 V. The same probe was used to record scanning electrochemical micrographs, which clearly demonstrated homogeneously distributed electrochemical activity of the heteroepitaxial SC-BDD electrodes. Minor differences in their electrochemical performance, presumably resulting from the somewhat different morphological features, were only unveiled during CV with surface sensitive compounds [Fe(CN)6]3−/4− and dopamine. The latter was also used to show the possibility of applying herein developed heteroepitaxial SC-BDD electrodes for electrochemical sensing, whereas experiments with anthraquinone-2,6-disulfonate revealed their enhanced resistance to fouling. All in all, heteroepitaxial SC-BDD represents a highly attractive electrode material which can, owing to the fabrication strategy, easily overcome size limitation, currently preventing broader use of single crystal diamond electrodes in electrochemical applications.

Effects of Boron Doping on the Bulk and Surface Acoustic Phonons in Single-Crystal Diamond.

We report the results of the investigation of bulk and surface acoustic phonons in the undoped and boron-doped single-crystal diamond films using the Brillouin-Mandelstam light scattering spectroscopy. The evolution of the optical phonons in the same set of samples was monitored with Raman spectroscopy. It was found that the frequency and the group velocity of acoustic phonons decrease nonmonotonically with the increasing boron doping concentration, revealing pronounced phonon softening. The change in the velocity of the shear-horizontal and the high-frequency pseudo-longitudinal acoustic phonons in the degenerately doped diamond, as compared to that in the undoped diamond, was as large as ∼15% and ∼12%, respectively. As a result of boron doping, the velocity of the bulk longitudinal and transverse acoustic phonons decreased correspondingly. The frequency of the optical phonons was unaffected at low boron concentration but experienced a strong decrease at the high doping level. The density-functional-theory calculations of the phonon band structure for the pristine and highly doped samples confirm the phonon softening as a result of boron doping in diamond. The obtained results have important implications for thermal transport in heavily doped diamond, which is a promising material for ultra-wide-band-gap electronics.

Morphology, defects and electrical properties of boron-doped single crystal diamond under various oxygen concentration

In this work, high quality boron-oxygen co-doped homoepitaxial diamond has been proposed. The morphology of diamond was smooth and flat with the O2/H2 ratios less than 1%. The X-ray diffraction, Photoluminescence and Raman spectroscopy illustrated that the substitution of oxygen into diamond lattice could inhibit nitrogen related defects and improve the crystallinity. The Raman mapping showed that tensile stress were formed from film surface to substrate. Under the O2/H2 ratio of 1%, the mobility of 892 cm2/V·s at boron concentration of 4.7 × 1017 cm−3 were obtained, which showed decent electrical properties.

Correlated Kelvin-probe force microscopy, micro-FTIR and micro-Raman analysis of doping anisotropy in multisectorial boron-doped HPHT diamonds

Boron-doped diamond single crystals of cubo-octahedral habit have been obtained by temperature gradient method at high pressures and high temperatures (HPHT) in Fe-Al-B-C system, and multisectoral plates of different orientations have been produced. Spatial distribution of uncompensated boron impurity and surface potential through the interfaces of different growth sectors of diamond plates have been studied by scanning Fourier transform infrared (FTIR) and Raman micro-spectroscopy, as well as scanning spreading resistance microscopy (SSRM) and Kelvin-probe force microscopy (KPFM). FTIR mapping showed strongly non-uniform growth-sector dependent distribution of uncompensated boron impurity gradually decreasing in a sequence {111} → {110} → {113} → {001} from ~7 · 1018 cm−3 down to 2 · 1017 cm−3. Micro-Raman mapping is used to reveal the submicron distribution of structural quality and boron content through the interfaces of different growth sectors. The inter-sectoral jumps of the surface potential in the axial and radial directions of the growth sectors were studied by KPFM, their theoretical estimates are obtained, and the features of the band structure of the inter-sector interfaces are revealed. A high level of structural perfection of inter-sectoral boundaries without electrically active defects is demonstrated.

Determination of band alignment between GaO x and boron doped diamond for a selective-area-doped termination structure

An n-GaO x thin film is deposited on a single-crystal boron-doped diamond by RF magnetron sputtering to form the pn heterojunction. The n-GaO x thin film presents a small surface roughness and a large optical band gap of 4.85 eV. In addition, the band alignment is measured using x-ray photoelectron spectroscopy to evaluate the heterojunction properties. The GaO x /diamond heterojunction shows a type-II staggered band configuration, where the valence and conduction band offsets are 1.28 eV and 1.93 eV, respectively. These results confirm the feasibility of the use of n-GaO x as a termination structure for diamond power devices.

Focused ion beam-based microfabrication of boron-doped diamond single-crystal tip cantilevers for electrical and mechanical scanning probe microscopy

In this paper, the fabrication process and electromechanical properties of novel atomic force microscopy probes utilising single-crystal boron-doped diamond are presented. The developed probes integrate scanning tips made of chemical vapour deposition-grown, freestanding diamond foil. The fabrication procedure was performed using nanomanipulation techniques combined with scanning electron microscopy and focused ion beam technologies. The mechanical properties of the cantilever were monitored by the measurement of thermally induced vibration of the cantilever after every fabrication step, allowing the mass changes in range of ng to be estimated. The endurance of the developed probes was tested during hundreds of topography measurements, which corresponds to a scanning length equal to 13.6 m, performed on a test sample in contact and lateral force microscopy modes. Analysis of the roughness parameters confirmed the extremely high wear resistance of the fabricated probes. The linear current voltage response on a highly-oriented pyrolytic graphite sample was recorded.

Growth-sector dependence of morphological, structural and optical features in boron-doped HPHT diamond crystals

Semiconducting boron-doped diamond single crystals of cubo-octahedral habit with prevalent development of octahedron {111} faces and insignificant area of cube {001}, rhombo-dodecahedron {110} and tetragon-trioctahedron {311} faces were obtained using solution-melt crystallization at high pressure 6.5 GPa and temperatures 1380…1420 °C. Using the Fe-Al solvent, which allows controlled incorporation of boron dopant between 2·10–4…10–2 at.% made it possible to vary the electro-physical properties of the crystals. Methods of micro-photogrammetry, atomic force microscopy, and micro-Raman spectroscopy were applied to reveal sectorial inhomogeneity of impurity composition and morphology of different crystal faces. The obtained crystals were shown to have high structural perfection and boron concentration ranging approximately from 1·1017 up to 7·1018 cm–3. An increase in boron concentration increases the area of {111} faces relatively to the total crystal area. Nanoscale morphological features like growth terraces, step-bunching, dendrite-like nanostructures, columnar substructures, negative growth pyramids on different crystal faces are shown to reflect peculiarities of carbon dissolution at high pressures and temperatures. The changes in the crystals’ habit and surface morphology are discussed in relation to inhomogeneous variation of thermodynamic conditions of crystal growth and dissolution at different boron concentrations.

Role of sp2 carbon in non-enzymatic electrochemical sensing of glucose using boron-doped diamond electrodes

Boron-doped diamond (BDD) is of increasing interest for applications in electrochemical sensing. It is well known that the sp2carbon content in BDD influences its electrochemical properties as electrode material. In this work, evidence is provided that the surface sp2 carbon content plays a crucial role in the electrochemical sensitivity of BDD towards glucose. Single-crystal BDD, freestanding polycrystalline BDD and glassy carbon (sp2 carbon reference material) were examined by voltammetry. Neither single-crystal BDD, which is free of sp2 carbon, nor pure sp2 glassy carbon could detect glucose in the range of 0.2–1.0 V. On the other hand, glucose oxidation was observed on polycrystalline BDD, and with increasing intensity with increase of sp2 carbon content. Thus, an optimum amount of (B-doped) sp2 carbon in the BDD electrode is needed for best sensing performance. Understanding this, and being able to control the composition of BDD, are not only important to glucose detection but to any electrochemical sensing application involving BDD.

Ohmic and Schottky contacts of hydrogenated and oxygenated boron-doped single-crystal diamond with hill-like polycrystalline grains* *Project supported by the Key-Area Research and Development Program of Guangdong Province (Grant No. 2020B0101690001) and the National Natural Science Foundation of China (NSFC) (Grant No. 51972135).

Hill-like polycrystalline diamond grains (HPDGs) randomly emerged on a heavy boron-doped p+ single-crystal diamond (SCD) film by prolonging the growth duration of the chemical vapor deposition process. The Raman spectral results confirm that a relatively higher boron concentration (∼ 1.1 × 1021 cm−3) is detected on the HPDG with respect to the SCD region (∼ 5.4 × 1020 cm−3). It demonstrates that the Au/SCD interface can be modulated from ohmic to Schottky contact by varying the surface from hydrogen to oxygen termination. The current–voltage curve between two HPDGs is nearly linear with either oxygen or hydrogen termination, which means that the HPDGs provide a leakage path to form an ohmic contact. There are obvious rectification characteristics between oxygen-terminated HPDGs and SCD based on the difference in boron doping levels in those regions. The results reveal that the highly boron-doped HPDGs grown in SCD can be adopted as ohmic electrodes for Hall measurement and electronic devices.

Nanoscale Reactivity Mapping of a Single-Crystal Boron-Doped Diamond Particle.

Boron-doped diamond (BDD) is most often grown by chemical vapor deposition (CVD) in polycrystalline form, where the electrochemical response is averaged over the whole surface. Deconvoluting the impact of crystal orientation, surface termination, and boron-doped concentration on the electrochemical response is extremely challenging. To tackle this problem, we use CVD to grow isolated single-crystal microparticles of BDD with the crystal facets (100, square-shaped) and (111, triangle-shaped) exposed and combine with hopping mode scanning electrochemical cell microscopy (HM-SECCM) for electrochemical interrogation of the individual crystal faces (planar and nonplanar). Measurements are made on both hydrogen- (H-) and oxygen (O-)-terminated single-crystal facets with two different redox mediators, [Ru(NH3)6]3+/2+ and Fe(CN)64-/3-. Extraction of the half-wave potential from linear sweep and cyclic voltammetric experiments at all measurement (pixel) points shows unequivocally that electron transfer is faster at the H-terminated (111) surface than at the H-terminated (100) face, attributed to boron dopant differences. The most dramatic differences were seen for [Ru(NH3)6]3+/2+ when comparing the O-terminated (100) surface to the H-terminated (100) face. Removal of the H-surface conductivity layer and a potential-dependent density of states were thought to be responsible for the behavior observed. Finally, a bimodal distribution in the electrochemical activity on the as-grown H-terminated polycrystalline BDD electrode is attributed to the dominance of differently doped (100) and (111) facets in the material.

Impact of methane concentration on surface morphology and boron incorporation of heavily boron-doped single crystal diamond layers

The methane concentration dependence of the plasma gas phase on surface morphology and boron incorporation in single crystal, boron-doped diamond deposition is experimentally and computationally investigated. Starting at 1%, an increase of the methane concentration results in an observable increase of the B-doping level up to 1.7×1021 cm−3, while the hole Hall carrier mobility decreases to 0.7±0.2 cm2 V−1 s−1. For B-doped SCD films grown at 1%, 2%, and 3% [CH4]/[H2], the electrical conductivity and mobility show no temperature-dependent behavior due to the metallic-like conduction mechanism occurring beyond the Mott transition. First principles calculations are used to investigate the origin of the increased boron incorporation. While the increased formation of growth centers directly related to the methane concentration does not significantly change the adsorption energy of boron at nearby sites, they dramatically increase the formation of missing H defects acting as preferential boron incorporation sites, indirectly increasing the boron incorporation. This not only indicates that the optimized methane concentration possesses a large potential for controlling the boron concentration levels in the diamond, but also enables optimization of the growth morphology. The calculations provide a route to understand impurity incorporation in diamond on a general level, of great importance for color center formation.

Thick and widened high quality heavily boron doped diamond single crystals synthetized with high oxygen flow under high microwave power regime

The aim of this paper is to optimize the growth conditions of thick boron doped diamond single crystals, which requires the use of high microware power density to have high growth rate, in order to allow enlarging CVD deposited layers keeping both high boron doping level and high crystal quality. It is shown that the use of a high amount of oxygen (1 to 2%) in the gas phase leads to 1 mm lateral growth after the growth of 500 μm-thick CVD diamond layer. Thus, the surface is increased from 10 mm2 to 18 mm2. The obtained films exhibit high crystal quality confirmed by Raman spectroscopy and confocal microscopy. Depending on the gas composition introduced in the reactor, the boron concentration varied between 5 × 1019 at/cm3 and 3 × 1020 at/cm3 as measured by SIMS.

Electrical Properties of High-Quality Synthetic Boron-Doped Diamond Single Crystals and Schottky Barrier Diodes on Their Basis

The temperature dependences of the specific resistance and Hall coefficient of high-quality synthetic boron-doped diamond single crystals grown via a high-pressure high-temperature method are studied. The concentration of acceptors in the (001) cut plates was varied in a range of 2 × 1015–3 × 1017 cm–3 by varying the concentration of boron in the growth mixture (0.0004–0.04 at %). Thin rectangular plates with the uniform concentration of boron and free from extended structural defects are cut out by a laser after the X-ray topography and mapping of UV luminescence. The concentrations of donors and acceptors in the samples are calculated from the data of the Hall effect and capacitance–voltage characteristics. The obtained results correlate with the concentration of boron in the growth mixture. The minimum compensation ratio of acceptors with donors (below 1%) is observed in the crystals grown with the concentration of boron in the growth mixture of 0.002 at %. The ratio increases when the amount of boron is increased or decreased. The samples grown at such a concentration of boron have the maximum mobility of charge carriers (2200 cm2/(V s) at T = 300 K and 7200 cm2/(V s) at T = 180 K). The phonon scattering of holes dominates throughout the range of temperatures (180–800 K), while the scattering by point defects (neutral and ionized atoms of the impurity) is insignificant. The diamond crystals which are grown from a mixture containing 0.0005–0.002 at % boron and have perfect quality and a lattice mechanism of scattering can be considered as a reference semiconductor.

Slow Electron–Phonon Cooling in Superconducting Diamond Films

We have measured the electron–phonon energy-relaxation time, ${\boldsymbol{\tau }_{{{\rm eph}}}}$ , in superconducting boron-doped diamond films grown on silicon substrate by chemical vapor deposition. The observed electron–phonon cooling times vary from 160 ns at 2.70 K to 410 ns at 1.8 K following a ${\boldsymbol{T}^{ - 2}}$ -dependence. The data are consistent with the values of ${\boldsymbol{\tau }_{{{\rm eph}}}}$ previously reported for single-crystal boron-doped diamond films epitaxially grown on diamond substrate. Such a noticeable slow electron–phonon relaxation in boron-doped diamond, in combination with a high normal-state resistivity, confirms a potential of superconducting diamond for ultrasensitive superconducting bolometers.

Elastic and mechanical softening in boron-doped diamond

Alternative approaches to evaluating the hardness and elastic properties of materials exhibiting physical properties comparable to pure diamond have recently become necessary. The classic linear relationship between shear modulus (G) and Vickers hardness (HV), along with more recent non-linear formulations based on Pugh’s modulus extending into the superhard region (HV > 40 GPa) have guided synthesis and identification of novel superabrasives. These schemes rely on accurately quantifying HV of diamond-like materials approaching or potentially exceeding the hardness of the diamond indenter, leading to debate about methodology and the very definition of hardness. Elasticity measurements on such materials are equally challenging. Here we used a high-precision, GHz-ultrasonic interferometer in conjunction with a newly developed optical contact micrometer and 3D optical microscopy of indentations to evaluate elasticity-hardness relations in the ultrahard range (HV > 80 GPa) by examining single-crystal boron-doped diamond (BDD) with boron contents ranging from 50–3000 ppm. We observe a drastic elastic-mechanical softening in highly doped BDD relative to the trends observed for superhard materials, providing insight into elasticity-hardness relations for ultrahard materials.