B-doped Nanocrystalline Diamond Research Articles

Large area deposition of boron doped nano-crystalline diamond films at low temperatures using microwave plasma enhanced chemical vapour deposition with linear antenna delivery

We report on the preparation and characterisation of boron (B) doped nano-crystalline diamond (B-NCD) layers grown over large areas (up to 50 cm × 30 cm) and at low substrate temperatures (< 650 °C) using microwave plasma enhanced linear antenna chemical vapour deposition apparatus (MW-LA-PECVD). B-NCD layers were grown in H2/CH4/CO2 and H2/CH4 gas mixtures with added trimethylboron (TMB). Layers with thicknesses of 150 nm to 1 μm have been prepared with B/C ratios up to 15000 ppm over a range of CO2/CH4 ratios to study the effect of oxygen (O) on the incorporation rate of B into the solid phase and the effect on the quality of the B-NCD with respect to sp3/sp2 ratio. Experimental results show the reduction of boron acceptor concentration with increasing CO2 concentration. Higher sp3/sp2 ratios were measured by Raman spectroscopy with increasing TMB concentration in the gas phase without CO2. Incorporation of high concentrations of B (up to 1.75 × 1021 cm3) in the solid is demonstrated as measured by neutron depth profiling, Hall effect and spectroscopic ellipsometry.

Enhanced p-type conduction of B-doped nanocrystalline diamond films by high temperature annealing

We report the enhanced p-type conduction with Hall mobility of 53.3 cm2 V−1 s−1 in B-doped nanocrystalline diamond (NCD) films by 1000 °C annealing. High resolution transmission electronic microscopy, uv, and visible Raman spectroscopy measurements show that a part of amorphous carbon grain boundaries (GBs) transforms to diamond phase, which increases the opportunity of boron atoms located at the GBs to enter into the nano-diamond grains. This phase transition doping is confirmed by the secondary ion mass spectrum depth profile results that the concentration of B atoms in nano-diamond grains increases after 1000 °C annealing. It is also observed that 1000 °C annealing improves the lattice perfection, reduces the internal stress, decreases the amount of trans-polyacetylene, and increases the number or size of aromatic rings in the sp2-bonded carbon cluster in B-doped NCD films. These give the contributions to improve the electrical properties of 1000 °C annealed B-doped NCD films.

Effects of annealing temperature on the microstructure and p-type conduction of B-doped nanocrystalline diamond films

Annealing of different temperatures was performed on boron-doped nanocrystalline diamond (BDND) films synthesized by hot filament chemical vapor deposition (HFCVD). Effects of annealing temperature on the microstructural and electrical properties of BDND films were systematically investigated. The Hall-effect results show that smaller resistivity and Hall mobility values as well as higher carrier concentration exist in the 5000 ppm boron-doped nanocrystalline diamond film (NHB) as compared with those in 500 ppm boron-doped nanocrystalline diamond film (NLB). After 1000 ℃ annealing, the Hall mobility of NLB and NHB samples were 53.3 and 39.3 cm2·V-1·s-1, respectively, indicating that annealing increases the Hall mobility and decreases the resistivity of the films. HRTEM, UV, and visible Raman spectroscopic results show that the content of diamond phase in NLB samples is larger than that in NHB samples because higher B-doping concentration results in a greater lattice distortion. After 1000 ℃ annealing, the amount of nano-diamond phase of NLB and NHB samples both increase, indicating that a part of the amorphous carbon transforms into the diamond phase. This provides an opportunity for boron atoms located at the grain boundaries to diffuse into the nano-diamond grains, which increases the concentration of boron in the nano-diamond grains and improves the conductivity of nanocrystalline diamond grains. It is observed that 1000 ℃ annealing treatment is beneficial for lattice perfection of BDND films and reduction of internal stress caused by doping, so that the electrical properties of BDND films are improved. Visible Raman spectra show that the trans-polyacetylene (TPA) peak (1140 cm-1) disappears after 1000 ℃ annealing, which improves the electrical properties of BDND films. It is suggested that the larger the diamond phase content, the better lattice perfection and the less the TPA amount in the annealed BDND samples that prefer to improve the electrical properties of BDND films.

Local boron environment in B-doped nanocrystalline diamond films

Thin films of heavily B-doped nanocrystalline diamond (B:NCD) have been investigated by a combination of high resolution annular dark field scanning transmission electron microscopy and spatially resolved electron energy-loss spectroscopy performed on a state-of-the-art aberration corrected instrument to determine the B concentration, distribution and the local B environment. Concentrations of ~1 to 3 at.% of boron are found to be embedded within individual grains. Even though most NCD grains are surrounded by a thin amorphous shell, elemental mapping of the B and C signal shows no preferential embedding of B in these amorphous shells or in grain boundaries between the NCD grains, in contrast with earlier work on more macroscopic superconducting polycrystalline B-doped diamond films. Detailed inspection of the fine structure of the boron K-edge and comparison with density functional theory calculated fine structure energy-loss near-edge structure signatures confirms that the B atoms present in the diamond grains are substitutional atoms embedded tetrahedrally into the diamond lattice.

Effects of annealing time on the microstructural and electrochemical properties of B-doped nanocrystalline diamond films

The effects of annealing time under 1000 ℃ on the microstructural and the electrochemical properties of boron-doped nanocrystalline diamond (BDND) films are investigated by HRTEM, UV and visible Raman spectroscopy, and cyclic voltammetry measurements. The results show that the size of nano-diamond grain in the film decreases with annealing time increasing. When the annealing time is 0.5 h, the grain size decreases from about 15 nm in the unannealed sample to about 8 nm and the content of diamond phase increases. When the annealing time increases to 2.0 h, the diamond grain size decreases to 2-3 nm, and the content of diamond phase decreases with the grain boundary increasing. In the case of annealing time of 2.5 h, the grain size of nano-diamond and the content of diamond phase increase slightly. The variations of nano-diamond grain size and the content of diamond phase indicate that the transformation between the diamond phase and the amorphous carbon occurs under the annealing with different times. The visible Raman spectra show that the G-peak position and the ID/IG value exhibit similar variations with annealing time increasing, revealing that the ordering of the amorphous graphite phase is improved when sp2 carbon cluster increases in number or size. The reactions on the electrode surface are quasi-reversible when the annealing times are 0.5, 1.0, 1.5 and 2.0 h. On the contrary, the reactions are irreversible when the sample is unannealed or annealed for 2.5 h. It is observed that the annealing treatment is beneficial to the improvement of the electrode mass transfer efficiency of BDND film. When the annealing time is 0.5 h, the electrode mass transfer efficiency as well as the ability of catalytic oxidation of BDND film is best. The results suggest that the smaller size of nano-diamond grain, the higher content of diamond phase and the uniform distribution of the nanocrystalline diamond grains are conducible to the improvement of the reaction reversibility on the electrode surface and the ability of catalytic oxidation of BDND films.

On the high curvature coefficient rectifying behavior of nanocrystalline diamond heterojunctions to 4H-SiC

Heterojunctions of p+ B-doped nanocrystalline diamond (NCD) to n− 4H-SiC were studied by electrical and cathodoluminescence (CL) methods. Current rectification at 30 °C had a curvature coefficient γ0 of 42.1 V−1 at zero bias, γmax of 105.35 V−1 at 0.2 V, and a reverse current of &amp;lt;10 nA/cm2. The NCD sheet resistance decreased from 4.1×1011 to 403.56 Ω/sq. as the carrier density Ns was increased from 3.5×105 to 1.5×1016 cm−2 by B2H6 doping. The 348 cm2/V-s mobility of the B-free NCD films was comparable to that of single crystal diamond. CL data revealed traps 0.6–0.8 eV from the NCD EV edge.

Influence of annealing on the microstructure and electrochemical properties of B-doped nanocrystalline diamond films

The annealing under different temperatures was performed on boron-doped nanocrystalline diamond films synthesized by hot filament chemical vapor deposition (HFCVD). The effects of annealing on the microstructure and electrochemical properties of films were systematically investigated. The results show that there are four peaks at 1157,1346,1470 and 1555 cm-1 in Raman spectra of the unannealed sample. When the films were annealed at temperatures above 800 ℃, there are only two peaks of D and G band, indicating that the hydrogen in grain boundaries significantly decreased. The area-integrated intensity ratio of D band to G band (ID/IG) reaches minimum value, revealing that the cluster number or cluster size of sp2 phase was reduced. The G peak position shifts to lower wave number, indicating an decrease in the ordering of graphitic component. The electrode exhibits the widest potential window and the highest oxygen evolution potential, and the quasi-reversible reaction occurs on the surface of the samples. The D peak is quite sharp and its intensity increases when the sample was annealed at 1000 ℃. The ID/IG value attains to the maximum value and the G peak position clearly shifts to higher value. The electrode exhibits the narrowest potential window and the lowest oxygen evolution potential, and the reversible electrochemical reaction occurs in the surface of the sample. The above results reveal that the cluster number or cluster size of sp2 phase, the amounts of trans-polyacetylene related to hydrogen in the grain boundaries, the disordering of graphitic components and the boron diffusion in the nanocrystalline diamond films give contributions to the complex change in electrochemical properties of the films with the annealing temperature increasing.

Nanocrystalline diamond films as UV-semitransparent Schottky contacts to 4H-SiC

A heterojunction between thin films of nanocrystalline diamond (NCD) and 4H-SiC has been developed. Undoped and B-doped NCDs were deposited on both n− and p− SiC epilayers. I-V measurements on p+ NCD∕n− SiC indicated Schottky rectifying behavior with a turn-on voltage of around 0.2V. The current increased over eight orders of magnitude with an ideality factor of 1.17 at 30°C. Ideal energy-band diagrams suggested a possible conduction mechanism for electron transport from the SiC conduction band to either the valence band or acceptor level of the NCD film. Applications as an UV semitransparent electrical contact to 4H-SiC are discussed.

Infrared optical properties of heavily B‐doped nanocrystalline diamond films on low alkaline glass substrates

AbstractWe measured the optical reflectance and transmittance spectra of the heavily B‐doped nanocrystalline diamond thin films deposited on low alkali barium alumino‐borosilicate glasses in a broad spectral range from ultraviolet to infrared light. We succeed to calculate separately the dielectric functions of the semitransparent B‐doped nanodiamond film and the glass substrate. The infrared dielectric function of a glass substrate is approximated by the classical Lorentz sum model of bounded oscillators whereas the infrared dielectric function of the heavily B‐doped nanodiamond film by the Drude model of free carriers. The free carrier concentration calculated from the plasma frequency only roughly corresponds to the free carrier concentration measured in Hall effect. Moreover, from the Drude model follows the high damping of free carrier oscillations in B‐NCD that correlates to their low Hall mobility. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Superconductive B-doped nanocrystalline diamond thin films: Electrical transport and Raman spectra

Electrical transport properties of thin boron doped nanocrystalline diamond films with thicknesses of 60–500nm have been studied. The Raman spectra measured exhibit Fano resonances, characteristic for B concentrations close to the metal-to-insulator transition. Upon increasing the B concentration, the sp2 carbon related Raman resonances vanish. In such boron-doped nanocrystalline diamond films, a positive magnetoresistance could be observed at liquid helium temperatures. The boron doped diamond films show conductivity similar to that of B-doped epitaxial diamond without any significant contribution of the grain boundary transport, leading to the superconductive transition in nanocrystalline diamond at ∼1.66K.

Superconductivity and low temperature electrical transport in B-doped CVD nanocrystalline diamond

In this work, we report on superconductivity (SC) found in thin B-doped nanocrystalline diamond films, prepared by the PE-CVD technique. The thickness of the films varies from about 100 to 400 nm, the films are grown on low-alkaline glass at substrate temperatures of about 500–700 °C. The SIMS measurements show that films can be heavily doped with boron in concentrations in the range of 3 × 1021 cm−3. The Raman spectra show Fano resonances, confirming the substitutional B-incorporation. The low temperature magnetotransport measurements reveal apositive magnetoresistance. The SC transition is observed at about Tc = 1.66 K. A simple theory exploiting the concept of weak localization accounting for this transition is proposed.

Electrical Transport in Heavily B-Doped Epitaxial Diamond and NCD

ABSTRACTThis paper deals with a LT transport study in B-doped diamond. To understand the electrical transport in heavily B-doped diamond and the influence of disorder onto the electrical transport, we have prepared nanocrystalline and epitaxial B-doped diamond films at CEA Saclay. The transport properties of these layers have been studied at the Institute of Physics Czech Academy of Sciences in Prague. It has been found that our B-doped nanocrystalline diamond exhibits also the SC transition, similarly as the original Russian work done on HPHT polycrystals or single crystal diamond. Additionally, the properties of (111) epitaxial films are discussed.