Boron-doped Nanocrystalline Diamond Films Research Articles

Photochemical modification of a diamond surface using a pulsed UV laser as the energy source

The use of an excimer benchtop pulsed UV laser as the energy source for the photochemical modification of the surface of diamond films is reported. Boron-doped nanocrystalline diamond (NCD) films were deposited on Si wafers via plasma-enhanced chemical vapour deposition (PECVD). The as-deposited films were characterised by scanning electron microscopy (SEM), Raman spectroscopy, and X-ray diffraction (XRD). Following characterisation, the surfaces of the films were subjected to a photochemical reaction with 2,2,2-trifluoroethyl undec-10-enoate using two different sources of radiation: a benchtop UV lamp (254 nm) and a KrF excimer pulsed UV laser (248 nm). The photochemically treated films were characterised by contact angle measurements and X-ray photoelectric spectroscopy (XPS) using non-treated films as a control. The use of a pulsed UV laser source was shown to reduce the photochemical reaction time from several hours to a few minutes.

Inhomogeneities across boron-doped nanocrystalline diamond films

For large-scale device fabrication, information about film inhomogeneities is crucial for high fabrication yield. In this work, inhomogeneities in two-inch diameter heavily boron-doped nanocrystalline diamond (BNCD) films have been studied. Two BNCD films were grown using chemical vapour deposition (CVD) with different boron-to-carbon (B/C) ratios. Their superconducting properties were measured as a function of distance from the centre of the film. The critical temperature (Tc) and critical magnetic field (Hc2) decreased radially outwards from the centre for both films. Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and scanning electron microscopy (SEM) were done on the samples to pinpoint the underlying explanation for the observed behaviour. Raman spectroscopy suggested a reduction in boron concentration and diamond purity over both films while moving radially outwards from the centre. XPS data from both films, however, did not show similar behaviours to that observed from the Raman data for the B/C ratios or diamond content. The AFM scans and SEM analysis showed a decreasing grain size further away from the film centre irrespective of the B/C ratio. This is due to the film being thinner at the edges when compared with the centre of the film. Raman analysis showed that the film with the higher B/C ratio had a higher diamond purity across the film. As expected, the film with a higher B/C ratio showed a more robust superconducting behaviour. The observed reductions in boron concentration, diamond purity, film thickness and decreased grain sizes are responsible for the diminishing superconductivity at the edge of the films.

Superconducting boron doped nanocrystalline diamond microwave coplanar resonator

A coplanar waveguide resonator (CPR) is presented for kinetic inductance (Lk) and penetration depth (λL) measurements of superconducting boron doped nanocrystalline diamond (B-NCD) at microwave frequencies of 0.4 to 1.2 GHz and at temperatures below 3 K. Using finite element modelling and experimental measurements, this work demonstrates that thin granular B-NCD films (thickness d≈ 500nm) on Si have a large penetration depth (λL≈3.8μm), and therefore an associated high kinetic inductance per square (Lk,□≈ 670 to 690 pH/□). These values are much larger than those typically obtained for films on single crystal diamond, which is likely due to the high granularity of the nanocrystalline films. Based on the measured Q factors of the structure, the calculated surface resistance is found to be around ≈ 1 to 6μΩ at T<2 K in the 0.4 to 1.2 GHz range, demonstrating the potential for granular B-NCD for high quality factor superconducting microwave resonators and highly sensitive kinetic inductance detectors.

Microstructure and Anisotropic Order Parameter of Boron-Doped Nanocrystalline Diamond Films

Unconventional superconductivity in heavily boron-doped nanocrystalline diamond films (HBDDF) produced a significant amount of interest. However, the exact pairing mechanism has not been understood due to a lack of understanding of crystal symmetry, which is broken at the grain boundaries. The superconducting order parameter (Δ) of HBDDF is believed to be anisotropic since boron atoms form a complex structure with carbon and introduce spin-orbit coupling to the diamond system. From ultra-high resolution transmission electron microscopy, the internal symmetry of the grain boundary structure of HBDDF is revealed, which can explain these films’ unconventional superconducting transport features. Here, we show the signature of the anisotropic Δ in HBDDF by breaking the structural symmetry in a layered microstructure, enabling a Rashba-type spin-orbit coupling. The superlattice-like structure in diamond describes a modulation that explains strong insulator peak features observed in temperature-dependent resistance, a transition of the magnetic field-dependent resistance, and their oscillatory, as well as angle-dependent, features. Overall, the interface states of the diamond films can be explained by the well-known Shockley model describing the layers connected by vortex-like structures, hence forming a topologically protected system.

Aluminium nitride thin films directly grown on conducting boron-doped nanocrystalline diamond films without using buffer layer for high frequency applications

Aluminium Nitride (AlN)-diamond hetero-structures are promising candidates for high frequency acoustic wave resonator applications. This paper reports the electrical, piezoelectric and mechanical properties of AlN films directly grown on boron-doped nanocrystalline diamond (B-NCD) films. X-ray diffraction (XRD) and Raman spectroscopy were used to determine the crystallinity of B-NCD and AlN films. The resistivity (∼8 ×107Ω.m) and dielectric constant (∼14) were obtained from the electrical characterisation of the fabricated Al/AlN/B-NCD capacitors. Piezoelectric coefficient (d33, 4.3 pmV−1) of AlN films was measured by piezo force microscopy (PFM) technique. Nanoindentation technique was used to determine the hardness (∼14 GPa) and elastic modulus (∼202 GPa) of AlN films. These different properties of AlN/B-NCD/Si (100) hetero-structure show that it will provide a high-performance platform to realize high frequency resonators for 5G applications.

Effects of Rashba-spin–orbit coupling on superconducting boron-doped nanocrystalline diamond films: evidence of interfacial triplet superconductivity

Among the many remarkable properties of diamond, the ability to superconduct when heavily doped with boron has attracted much interest in the carbon community. When considering the nanocrystalline boron doped system, the reduced dimensionality and confinement effects have led to several intriguing observations most notably, signatures of a mixed superconducting phase. Here we present ultra-high-resolution transmission electron microscopy imaging of the grain boundary and demonstrate how the complex microstructure leads to enhanced carrier correlations. We observe hallmark features of spin–orbit coupling (SOC) manifested as the weak anti-localization effect. The enhanced SOC is believed to result from a combination of inversion symmetry breaking at the grain boundary interfaces along with antisymmetric confinement potential between grains, inducing a Rashba-type SOC. From a pronounced zero bias peak in the differential conductance, we demonstrate signatures of a triplet component believed to result from spin mixing caused by tunneling of singlet Cooper pairs through such Rashba-SOC grain boundary junctions.

Electrical and optical characteristics of boron doped nanocrystalline diamond films

Boron doped diamond is a prospective material which can be used as a conductive and optically transparent thin-film electrode in a variety of optoelectronic applications. In this work, we present the results of the temperature resolved electrical conductivity, optical reflection, transmission and absorption of thin boron doped nanocrystalline diamond films grown by a microwave plasma enhanced chemical vapor deposition. Optical transmittance, reflectance and absorptance properties of layers were studied by a photo-thermal deflection spectroscopy analysis. Raman spectroscopy with various excitation wavelengths was employed for the analysis of nanocrystalline diamond layers. The measured position, shift and broadening of the characteristic boron doped diamond Raman lines were used for the determination of the boron concentration. Correlation between the results of the atomic boron concentration estimated via the Raman analysis and measured electrical conductivity values is presented.

Charging effects and anomalous resistive features of superconducting boron doped diamond films

Anomalous resistive peaks below the superconducting transition temperature in heavily boron doped nanocrystalline diamond films could have potential application in switching devices, however it’s exact origin is still under study. We establish a temperature dependence of this resistive phase similar to what has been reported for in Josephson junction arrays and other granular superconductors where the charge duel of the Berezinskii-Kosterlitz-Thouless (BKT) transition has been observed. Non-linear magnetic field dependence of the resistance with a temperature dependent peak feature below the critical field are also presented. Pronounced temperature dependent hysteresis in the current voltage sweeps at temperatures below the determined BKT critical point are related to pinning of charge defects. It is shown that these collective features allude to a Charge-BKT transition between charge and anti-charge analogues.

Finite bias evolution of bosonic insulating phase and zero bias conductance in boron-doped diamond: A charge-Kondo effect

We report novel transport features in heavily boron-doped nanocrystalline diamond films, in particular an anomalous resistance peak near to the superconducting transition temperature and a strong zero bias conductance peak in the differential current-voltage spectra. The shape of the resistance-temperature curves near the critical temperature is seen to be strongly influenced by both magnetic field and bias current. As the bias current is lowered, the resistance peak becomes more pronounced, whereas when the magnetic field is varied the peak shifts towards lower temperatures. The resistance upturn shows a quadratic temperature dependence as expected for a Kondo transition. We find that a number of transport features such as resistance peak height, zero bias conduction peak height and width scale according to a power law dependence. We interpret these features as a result of a charge-Kondo effect where hole dopants act as degenerate Kondo impurities by opening additional pseudo-spin scattering channels.

Studies on optical transmittance of boron-doped nanocrystalline diamond films

Thickness is one of the most important parameters in many applications using thin layers. This article describes determination of the thickness of boron-doped nanocrystalline diamond (NCD) grown on fused silica glass. The spectroscopic measurement system has been used. A high refractive index (2.3 @ 550 nm) was achieved for NCD films. The thickness of the NCD samples has been determined from transmission spectrum. Full Text: PDF ReferencesZ. Li, S. Butun, K. Aydin, 'Large-area, lithography-free super absorbers and color filters at visible frequencies using ultrathin metallic films', ACS Photonics, vol. 2.2, pp. 183-188 2015. CrossRef L. Yu, D.D. Tune, C.J. Shearer, J.G. Shapter, 'Implementation of antireflection layers for improved efficiency of carbon nanotube–silicon heterojunction solar cells', Solar Energy, vol. 118, pp. 592-599, 2015. CrossRef D. Majchrowicz, Daria, et al., Nitrogen-Doped Diamond Film for Optical Investigation of Hemoglobin Concentration, Materials, vol. 11.1, pp. 109, 2018. CrossRef K.L. Konnerth, F.H. Dill, 'In-situ measurement of dielectric thickness during etching or developing processes', IEEE Transactions on Electron Devices, vol. 22.7, pp. 452-456, 1975. CrossRef Z.G. Hu, P. Prunici, P. Hess, K.H. Chen, 'Optical properties of nanocrystalline diamond films from mid-infrared to ultraviolet using reflectometry and ellipsometry', Journal of Materials Science: Materials in Electronics, vol. 18.1, pp. 37-41, 2007. CrossRef J. Adamczewska, et al., 'Procesy technologiczne w elektronice półprzewodnikowej', WNT, Warsaw, 1980.J.P. Dilger, L. R. Fisher, D. A. Haydon, 'A critical comparison of electrical and optical methods for bilayer thickness determination', Chemistry and Physics of Lipids, vol. 30.2-3, pp. 159-176, 1982. CrossRef M, Ficek, et al, 'Optical and electrical properties of boron doped diamond thin conductive films deposited on fused silica glass substrates', Applied Surface Science, vol. 387, pp. 846-856, 2016. CrossRef R. Bogdanowicz, et al, 'Opto-Electrochemical Sensing Device Based on Long-Period Grating Coated with Boron-Doped Diamond Thin Film', J. Opt. Soc. Korea, vol. 19, pp. 705-710, 2015. CrossRef M. Sobaszek, et al., 'Optical and electrical properties of ultrathin transparent nanocrystalline boron-doped diamond electrodes', Optical Materials, vol.42, pp. 24-34, 2015. CrossRef R. Bogdanowicz, et al, 'Improved surface coverage of an optical fibre with nanocrystalline diamond by the application of dip-coating seeding', Diamond and Related Materials, vol. 55, pp. 52-63, 2015. CrossRef Z.G. Hu, P. Hess, 'Optical constants and thermo-optic coefficients of nanocrystalline diamond films at 30–500oC', Applied physics letters, vol. 89.8: 081906, 2006. CrossRef

Effect of structural disorder and Coulomb interactions in the superconductor-insulator transition applied to boron doped diamond

The effect of disorder, both structural (non-diagonal) and on-site (diagonal), is studied through the inhomogeneous Bogoliubov-de Gennes (BdG) theory in narrow-band disordered superconductors with a view towards understanding superconductivity in boron doped diamond (BDD) and boron-doped nanocrystalline diamond (BNCD) films. We employ the attractive Hubbard model within the mean field approximation, including the short-range Coulomb interaction between holes in the narrow acceptor band. We study substitutional boron incorporation in a triangular lattice, with disorder in the form of random potential fluctuations at the boron sites. The role of structural disorder was studied through non-uniform variation of the tight-binding coupling parameter where, following experimental findings, we incorporate the concurrent increase in structural disorder with increasing boron concentration. We illustrate stark differences between the effects of structural and on-site disorder and show that structural disorder has a much greater effect on the density of states, mean pairing amplitude and superfluid density than on-site potential disorder. We show that structural disorder can increase the mean pairing amplitude while the spectral gap in the density of states decreases with states eventually appearing within the spectral gap for high levels of disorder. This study illustrates how the effects of structural disorder can explain some of the features found in superconducting BDD and BNCD films such as a tendency towards saturation of the Tc with boron doping and deviations from the expected BCS theory in the temperature dependence of the pairing amplitude and spectral gap.

Signatures of two-dimensional transport in superconducting nanocrystalline boron-doped diamond films

The possibility of a two-dimensional (2D) phase is investigated in superconducting boron-doped nanocrystalline diamond films. The fluctuation spectroscopy schemes such as the Azlamazov-Larkin (AL) and Lerner-Varlamov-Vinokur (LVV) ones are used to determine the dimensionality crossovers occurring near the superconducting phase transition. It is found that a distinct 2D phase in the fluctuation regime occurs. One of the consequences of the 2D transport is the manifestation of the Berezinskii-Kosterlitz-Thouless (BKT) transition which is verified through both current voltage power law scaling as well as the Halperin-Nelson fitting to the temperature-dependent resistance. The suppression of the BKT transition with applied field is found to be correlated with the decrease of Josephson coupling between grains. The identification of a 2D phase accompanied by the BKT transition highlights the subtle effects of dimensionality in this unique system.

Raman scattering in boron doped nanocrystalline diamond films: Manifestation of Fano interference and phonon confinement effect

Heavily boron doped nanocrystalline diamond films grown on glass substrates by the method of plasma-chemical deposition, were investigated using Raman spectroscopy. Analysis of the spectra showed both the phonon confinement effect in nanocrystalline grains and Fano interference effect due to the contribution of electron Raman scattering in heavily doped p-type diamond films. The increase of boron concentration led to decrease of the size of crystalline diamond grains. The films are semitransparent and have good conductivity, so that it can be used as transparent electrodes in giant-scale electronics and optoelectronics.

Emission properties of undoped and boron-doped nanocrystalline diamond films coated silicon carbide field emitter arrays

A new process for the fabrication of two-tier silicon carbide field emission array (FEA) of nanoscale tips coated with thin nanocrystalline undoped and highly boron-doped diamond films has been developed to improve the electron emission characteristics of the pure silicon carbide tips. The effects of boron-doped film on field emission properties have been studied in comparison with undoped ones, using a diode configuration. The FEA covered with highly boron-doped film demonstrated the lower turn on electric field and higher emission current due to lower work function, 1.5 times lower than for undoped one. Emission current at various values of the electrode gap from 10 to 500 μm has been studied. It was shown that field enhancement factor of two-tier FEA decreases at small interelectode gap (&amp;lt;100 μm) due to perturbations of electric field and mutual screening of microtips, which leads to significant increase in an applied electric field required for maintaining the preset emission current. Both fabricated FEAs demonstrated high current stability with fluctuations not exceeding 5% at relatively low vacuum (10−5–10−6 Torr).

Possible observation of the Berezinskii-Kosterlitz-Thouless transition in boron-doped diamond films

The occurrence of the Berezinskii-Kosterlitz-Thouless (BKT) transition is investigated in heavily boron-doped nanocrystalline diamond films through a combination of current-voltage and resistance measurements. We observe transport features suggesting a robust BKT transition along with transport features related to vortex pinning in nanocrystalline diamond films with smaller grain size. The vortex core energy determined through analysis of the resistance temperature curves was found to be anti-correlated to the BKT transition temperatures. It is also observed that the higher BKT temperature is related to an increased vortex-antivortex binding energy derived from the activated transport regions. Further, the magnetic field induced superconductor insulator transition shows the possibility of the charge glass state. The consequences of granularity such as localization and vortex pinning can lead to tuneable BKT temperatures and strongly affects the field induced insulating state.

Thermal oxidation induced high electrochemical activity of boron-doped nanocrystalline diamond electrodes

Thermal oxidation treatment was executed on boron-doped nano-crystalline diamond (B-NCD) films to change both the microstructure and the terminal groups of grains and grain boundaries. Their contributions to the electrochemical activity and electrical properties were investigated. The grain boundaries become narrowing, with ordered graphite phase firstly, and then they become disordered for long time oxidized treatment. This causes that the mobility of the B-NCD electrodes increases to 84.97cm2/Vs after 30min oxidation and then it decreases obviously with oxidation time further increasing. Nanocrystalline diamond grains with high carrier concentration are growing larger and naked on the B-NCD electrode’s surface after oxidization. Also, part of the terminal CH groups on the grain phases turn to CO groups with positive electron affinity, which is prone to attract electrons from nearby anions. These results lead to larger electrochemical activity area with quick charge transfer rate in the surface of B-NCD electrode. The silane is used to shield the OH sites from electrochemical activity. The covered area become less with the oxidized time, further confirming that the electrochemical reaction mainly occurs on the grain phases and the OH groups is mainly growing in grain boundaries.

Fluctuation spectroscopy as a probe of granular superconducting diamond films

We present resistance versus temperature data for a series of boron-doped nanocrystalline diamond films whose grain size is varied by changing the film thickness. Upon extracting the fluctuation conductivity near to the critical temperature we observe three distinct scaling regions -- 3D intragrain, quasi-0D, and 3D intergrain -- in confirmation of the prediction of Lerner, Varlamov and Vinokur. The location of the dimensional crossovers between these scaling regions allows us to determine the tunnelling energy and the Thouless energy for each film. This is a demonstration of the use of \emph{fluctuation spectroscopy} to determine the properties of a superconducting granular system.

Investigations of the co-doping of boron and lithium into CVD diamond thin films

Lithium has been incorporated into heavily boron-doped single-crystal (SCD), microcrystalline (MCD) and nanocrystalline diamond (NCD) films at concentrations up to ~2×1020cm−3 using Li3N as a solid-state Li source for in-diffusion and diborane as the B source. The quality, morphology, electrical resistance and concentration of B and Li dopants present in a range of B+Li co-doped SCD, MCD and NCD films have been studied. Analysis of the SIMS depth profiles for Li enabled the diffusion constants, D, to be measured (in units of cm2s−1) as: 2.5×10−15, 1.3×10−14 and 7.0×10−14 for SCD, MCD and NCD, respectively, at 1100K. The value for D for SCD agrees closely with that in the literature, while the much larger values for the polycrystalline films provide direct evidence that Li can diffuse rapidly along or through diamond grain boundaries at elevated temperatures. If prolonged diffusion allows the Li to reach the Si substrate, the Si acts as a sink for Li absorbing large quantities and reducing its concentration in the diamond film.

Optically transparent composite diamond/Ti electrodes

It is important for electro-optical applications of nanocrystalline diamond (NCD) films to achieve high optical transparency with low electrical resistivity. However, currently fabrication of highly transparent NCD films with reasonable electrical conductivity remains a challenge – any increase in conductivity simultaneously reduces transparency. Here, we report on fabrication of highly conductive and yet highly transparent electrodes based on boron-doped nanocrystalline diamond (B-NCD) films and Ti grids. We studied B-NCD films with variable boron content and thickness to determine preferable electrical and optical characteristics. Composite electrodes with a sheet resistance of 200 Ω/sq and optical transparency of 80% have been obtained by the integration of a thin nanostructured Ti-grid, sandwiched between the glass substrate and B-NCD layer.

Consecutive deposition of amorphous SiO2 interlayer and diamond film on graphite by chemical vapor deposition

Chemical vapor deposition (CVD) diamond films can be used as effective protective coatings on graphite components, while the film-substrate adhesion is poor probably due to the graphite etching caused by the atomic hydrogen. It is supposed as a useful way to pre-deposit an amorphous SiO2 (a-SiO2) layer for solving this problem. In this study, by interposing a-SiO2 interlayers, four types of diamond films are deposited on common commercial graphite substrates, and noteworthy is that such the a-SiO2 layer and CVD diamond film are consecutively synthesized in the same hot filament CVD apparatus. It is proved that as-deposited a-SiO2 layer has typical amorphous features, provides sufficient protections on the substrate, and avoids the natural removal of the diamond film when growing. Micro-crystalline diamond, nano-crystalline diamond, boron-doped micro-crystalline diamond (BDMCD) and boron-doped nano-crystalline diamond films deposited on a-SiO2 coated graphite substrates present typical features in consistent with micro- or nano-sized diamond films on other substrates. Among the four diamond films, mainly facing to some application requirements for the electrical conductivity, the BDMCD film shows relatively better comprehensive performance, including high diamond purity, high nano hardness (74.21 GPa) and elastic modulus (890.93 GPa), favorable adhesion and low electrical resistivity (29 μΩ m).