Characteristics of homoepitaxial heavily boron-doped diamond films from their Raman spectra

As the boron incorporation level and the wavelength of the exciting laser were varied, we observed systematic modifications of the Raman spectra of homoepitaxial diamond films. A pronounced change in the lineshape of the zone-center optical Raman peak as well as a wide and structured signal at lower wavenumbers appeared simultaneously when the boron incorporation was increased above an abrupt threshold around 3×10²⁰ cm⁻¹. This threshold was found to depend on the excitation laser wavelength. Possible origins for the wide peaks at 500 and 1225 cm⁻¹ are also discussed.     

EPR study of hydrogen-related defects in boron-doped p-type CVD homoepitaxial diamond films

Boron-doped p-type single crystalline chemical vapor deposition (CVD) homoepitaxial diamond films were investigated by electron paramagnetic resonance (EPR). Carbon dangling bond defects, which were accompanied by a nearby hydrogen atom, were observed in boron-doped p-type CVD diamond films on a IIa substrate similar to those observed in undoped diamond. This result suggested that the energy level position of the defects is located below the Fermi energy of boron-doped diamond, at around 0.3 eV above the valence-band top. The reason why the Fermi energy could be changed by the incorporation of boron atoms at low density (10¹⁶–10¹⁷/cm³) in the film in spite of the existence of the large defect density of EPR centers (10¹⁸/cm³) is thought to be that the singly occupied electron states of defects are located near the band edge. As for the thermal annealing effect of the defects, it was revealed that the concentration of the defects and the mobility of the p-type film did not change after annealing up to 1200 °C which is much higher than the temperature of boron–hydrogen pair dissociation.     

Surface morphology and electrical properties of boron-doped diamond films synthesized by microwave-assisted chemical vapor deposition using trimethylboron on diamond (100) substrate

Prime novelty: The smoothness of the synthesized boron-doped diamond was improved by the pre-treatment of a hydrogen plasma. Moreover, the Hall mobility also increased with this pre-treatment.Surface morphology and electrical properties, such as electrical conductivity, hole concentration and Hall mobility, were investigated for boron-doped diamond films, which were synthesized by microwave-assisted chemical vapor deposition (MPCVD) on a (100) diamond substrate. Trimethylboron (TMB) was used as a dopant source and methane (CH₄) was used as a carbon source. The morphology of the synthesized diamond surface depended on the MPCVD conditions such as TMB and CH₄ concentrations in the gas phase, and lower concentrations of TMB and CH₄ lead to a smoother surface. When the substrate was treated in a hydrogen plasma, the electrical properties of the boron-doped diamond films, as well as the smoothness of the surface, were improved. After optimizing the synthesis conditions, Hall mobility reached to 2020 cm² V⁻¹ s⁻¹ at 243 K for a diamond film with a hole concentration of 5×10¹² cm⁻³.     

Doping-induced anisotropic lattice strain in homoepitaxial heavily boron-doped diamond

As a result of the larger covalent radius of boron (r_B = 0.88 Å) when compared to that of carbon (r_C = 0.77 Å), the introduction of substitutional boron into diamond leads to an expansion δa/a of the lattice parameter. This has been found previously to follow a linear interpolation (Vegard law) as long as the boron content is lower than about 0.5 at.% in MPCVD epilayers or 1.5 at.% in HPHT bulk crystals.Above those concentrations, the expansion is less pronounced than predicted by Vegard. In order to explain this effect, we have performed ab initio calculations on C:B substitutional alloys. The results show that the presence of interstitial boron and of boron clusters is not necessary to explain the experimental data available in the literature. Moreover, quantitative estimates are proposed for the deformation potential of the valence band maximum and for the steric effect associated to boron pairing. We then apply these conclusions to discuss the different variations of δa/a vs boron contents observed by high resolution XRD experiments performed on “insulating” and metallic (and superconducting) p⁺⁺ diamond epilayers grown by MPCVD on (100)- and (111)-oriented type Ib substrates, for which boron concentration profiles have been determined by Secondary Ion Mass Spectroscopy.     

Passivation effects of deuterium exposure on boron-doped CVD homoepitaxial diamond

The deuterium (hydrogen) passivation effect on acceptors in boron-doped CVD homoepitaxial diamond was studied by electrical (Hall-effect) and secondary ion mass spectroscopy (SIMS) measurements. Deuterium was incorporated into the samples using microwave (MW) deuterium plasma at 673 K for 2–24 h. We observed the progress of acceptor passivation with p-type conduction, which finally resulted in a highly resistive state.     

New directions in structuring and electrochemical applications of boron-doped diamond thin films

Conductive boron-doped diamond electrodes have been shown to be highly suitable as electrochemical detectors in flow injection analysis and high performance liquid chromatographic analysis, achieving high sensitivity and stability for certain species that cannot be detected at other electrodes due to electrode deactivation or high electrochemical oxidation potential. The use of this electrode material for the detection of chlorophenols and theophylline is demonstrated. Apart from the electrochemistry of diamond, various methods have been developed to fabricate well-aligned nanocylindrical diamond films and periodic bulb-like structures, which may be useful for sensors and electronic devices such as field emission displays.     

Characterization and electrochemical properties of CF4 plasma-treated boron-doped diamond surfaces

The effect of CF₄ plasma etching on diamond surfaces, with respect to treatment time, was investigated using scanning electron microscopy (SEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and electrochemical measurements. SEM observations and Raman spectra indicated an increase in surface roughening on a scale of 10–20 nm, and an increase in crystal defect density was apparent with treatment time in the range of 10 s to 30 min. In contrast, alteration of the diamond surface terminations from oxygen to fluorine was found to be rather rapid, with saturation of the F/C atomic ratio estimated from XPS analysis after treatment durations of 1 min and more. The redox kinetics of Fe(CN)₆^3−/4− was also found to be significantly modified after 10 s of CF₄ plasma treatment. This behavior shows that C–F terminations predominantly affect the redox kinetics compared to the effect on the surface roughness and crystal defects. The double-layer capacitance (C_dl) of the electrolyte/CF₄ plasma-treated boron-doped diamond interface was found to show a minimum value at 1 min of treatment. These results indicate that a short-duration CF₄ plasma treatment is effective for the fabrication of fluorine-terminated diamond surfaces without undesirable surface damage.     

The role of boron atoms in heavily boron-doped semiconducting homoepitaxial diamond growth — Study of surface morphology

Surface properties of CVD diamond films grown with high admixture of B₂H₆ to H₂ and CH₄ are characterized in this paper. Results show that high contents of diborane in the gas phase cause a reduction of imperfections such as unepitaxial crystallites and pyramidal hillocks, which is characterized by optical microscopy. These results suggest that the boron atoms play an important role at the beginning of nucleation and during the CVD growth of diamond.     

Surface electronic properties on boron doped (111) CVD homoepitaxial diamond films after oxidation treatments

Surface electronic properties on oxidized boron (B) doped (111) homoepitaxial diamond films are investigated by Hall effect measurements and Schottky junction characterizations. Surface electronic properties on (111) diamond strongly depend on annealing treatments after wet-chemical oxidation, whereas for those on (001) diamond no change due to annealing can be detected. Hall effect results show that a p-type surface conductive layer (SCL) exists on (111) diamond surface in air after wet-chemical oxidation followed by annealing in Ar atmosphere (WO–AN) above 300 °C, but does not if only wet-chemical oxidation or air-oxidation is applied. This SCL disappears at annealing temperature above 350 °C in air. Schottky junction characteristics suggest that the Fermi level is unpinned at the (111) surface after WO–AN. Surface electronic characteristics on (111) diamond after WO–AN are similar to those generated by hydrogen termination.     

AC impedance studies of anodically treated polycrystalline and homoepitaxial boron-doped diamond electrodes

The electrochemical properties of several types of diamond electrodes, including polycrystalline and homoepitaxial films, that underwent anodic treatment were examined with the electrochemical impedance spectroscopic (EIS) technique, as well as with capacitance-potential measurements. From an analysis of the impedance behavior, it was found that an additional capacitance element, which is apparent in the relatively high-frequency range (100-1000 Hz), was generated on the polycrystalline and (1 0 0) homoepitaxial diamond electrodes after anodic treatment. This capacitive element can be characterized as being non-Faradaic, because it has negligible dependence on the applied potential. Acceptor densities and depth profiles were calculated from the Mott-Schottky plots, and the acceptor densities in the near-surface region of the anodically treated surfaces were found to be extremely low. These results indicate that passive layers were generated on the diamond surfaces by the anodic treatment. The capacitance-potential behavior was also consistent with a model consisting of a semiconductor with a passive surface film. The passive film is proposed to arise as a result of the removal of hydrogen acting as an acceptor in the subsurface region, leaving hydrogen that is paired essentially quantitatively with the boron dopant, effectively neutralizing it.     

Selective electrochemical behavior of highly conductive boron-doped diamond electrodes for fluvastatin sodium oxidation

Boron-doped diamond electrodes have received much attention for electrochemical determination due to their attractive electrochemical properties over other electrodes. The electrooxidation of fluvastatin sodium at boron-doped diamond electrode was investigated using cyclic, differential pulse and square wave voltammetry. The possible mechanism of oxidation was discussed with model compounds that have indole oxidation. The dependence of the peak current and potentials on pH, concentration, scan rate, and the nature of the buffer were investigated. The oxidation of fluvastatin was irreversible and exhibited a diffusion-controlled fashion. The slope of the log i_p–log v linear plot was 0.44 indicating the diffusion control for pH 10.00 Britton–Robinson buffer solution. The linear response was obtained in the ranges of 1 × 10^− 6 M–6 × 10^− 4 M in pH 10.00 BR buffer solution. The detection limit of the standard solution is estimated to be 1.37 × 10^− 7 M for DPV, 1.44 × 10^− 7 M for SWV. The repeatability of the methods was found as 0.66 and 0.15 RSD % for peak currents for differential pulse and square wave voltammetry, respectively. The practical analytical value of the method is demonstrated by quantitative determination of fluvastatin in pharmaceutical formulation and human serum, without the need for separation or complex sample preparation, since there was no interference from the excipients and endogenous substances. Selectivity, reproducibility and accuracy of the developed methods were demonstrated by recovery studies.     

Superconducting properties of homoepitaxial CVD diamond

Superconductivity was achieved above 10 K in heavily boron-doped diamond thin films deposited by the microwave plasma-assisted chemical vapor deposition (CVD) method. Advantages of the CVD method are the controllability of boron concentration in a wide range, and a high boron concentration, compared to those obtained using the high-pressure high-temperature method. The superconducting transition temperatures of homoepitaxial (111) films are determined to be 11.4 K for T_C onset and 8.4 K for zero resistance from transport measurements. In contrast, the superconducting transition temperatures of (100) films T_C onset = 6.3 K and T_C zero = 3.2 K were significantly suppressed.     

Light penetration depth dependence of photocarrier life time and the Hall effect in phosphorous-doped and boron-doped homoepitaxial CVD diamond films

Photocurrent in phosphorous-doped CVD diamond film of the bandgap of 5.5 eV with the density of 2 × 10¹⁸ cm^− 3 decreases with increasing photon energy in the energy range higher than 5.8 eV at room temperature (RT). The photocarrier life time is 0.3 ms at the excitation energy of 5.8 eV and decreases with increasing excitation energy. These show that the photocarriers, ascertained to be electrons by the Hall effect of the photocurrent, are trapped near the surface. The life time of photo-excited holes in Boron-doped CVD diamond film with the density of 9 × 10¹⁷ cm^− 3 is 35 ms at RT and decreases with decreasing Boron density, which is explained from the relation between the Fermi energy and the density.     

Photoelectron emission from heavily B-doped homoepitaxial diamond films

We discuss the energy band structure near the valence band maximum based on photoemission yield spectroscopy experiments using a hydrogen-terminated heavily boron-doped homoepitaxial diamond film with concentration of 3 × 10²⁰ cm^− 3. The experimental results showed a metallic photoemission behavior with a negative electron affinity surface. Based on the fitting as metallic photoemission behavior with a Fowler plot, the Fermi level should be at 5.35 eV below the conduction band minimum, which means that the Fermi level lies at 0.12 eV (5.47–5.35 eV) above the valence band maximum. Thus the film shows metallic conduction by the Mott transition, but not as degenerate semiconductor.     

Synthesis of boron-doped homoepitaxial single crystal diamond by microwave plasma chemical vapor deposition

The deposition of boron-doped homoepitaxial single crystal diamond is investigated using a microwave plasma-assisted chemical vapor deposition system. The objective is to deposit high-quality boron-doped single crystal diamond and establish the relationships between the deposition conditions and the diamond growth rate and quality. Experiments are performed using type Ib HPHT diamond seeds as substrates and growing diamond with varying amounts of diborane in a methane–hydrogen gas mixture. The deposition system utilized is a 2.45 GHz microwave plasma-assisted CVD system operating at 135–160 Torr. Experiments are performed with methane concentrations of 4–6% and diborane concentrations of 5–50 ppm in the feedgas. Diamond is deposited with growth rates of 2 to 11 µm/h in this study. The deposited diamond is measured to determine its electrical conductivity and optical absorption versus wavelength in the UV, visible and IR portions of the spectrum. Data is presented that relates the growth rate and diamond properties to the deposition conditions including substrate temperature and feedgas composition.     

A study of the crystalline growth of highly boron-doped CVD diamond: preparation of graded-morphology diamond thin films

An investigation was made of graded-morphology diamond thin films deposited on Si substrates by use of the microwave plasma chemical vapor deposition (CVD) technique. The preparation of graded diamond thin films not only offers new perspectives into functional materials, but it also gives clues to the growth mechanism of CVD diamond. Although it is clear that the substrate temperature and the deposition time affect the grain size in particular, the difference depending on the boron concentration in the growing process was studied in the present work. As a result, in highly boron-doped (10⁴ ppm) diamond films, the Raman peak which was ascribed to the boron-doping was not observed at the very initial growth stage. However, the crystal growth process was almost irrelevant for the boron-doping quantity. Furthermore, we showed the morphology dependence of electrochemical properties as one example of the excellent functions of the graded-morphology highly boron-doped diamond film.     

Growth and characterization of hillock-free high quality homoepitaxial diamond films

A suitable pre-treatment process to significantly suppress growth hillocks on homoepitaxial diamond surfaces has been successfully developed. Nanometer-scale morphologies obtained for the homoepitaxial diamond films by means of an atomic force microscope show that the mean roughness (root mean square) of the homoepitaxial diamond films with thickness of 1 μm was approximately 5 nm in the scanning region of 2×2 μm². The results from cathodoluminescence (CL) measurements indicate that the developed pre-treatment and deposition by a high power ASTeX microwave plasma chemical vapor deposition (MWP-CVD) apparatus led to growth of homoepitaxial diamond films with high crystalline quality yielding only band-edge emissions as the main peak in CL spectra at low temperatures (80 K).     

Surface roughening of diamond (001) films during homoepitaxial growth in heavy boron doping

A systematic investigation of heavily boron-doped diamond (001) surfaces was performed by atomic force microscopy. Heavily boron-doped diamond surfaces produced by chemical vapor deposition with boron/carbon (B/C) ratios of more than 1600 ppm in the gas phase were rough, whereas an unintentionally doped diamond surface was atomically flat. These results show that the surface roughening is due to heavy doping by boron during homoepitaxial diamond growth.     

High-pressure and high-temperature annealing effects on CVD homoepitaxial diamond films

High-pressure and high-temperature (HPHT) annealing effects on the chemical vapor-deposited (CVD) homoepitaxial diamond films were investigated. By the HPHT annealing, the intensity of free-exciton (FE)-related emission was increased by  2 times and the luminescence bands from 270 to 320 nm, which originate from 5RL and 2BD bands, were almost completely eliminated in the cathodoluminescence (CL) spectrum. The CL intensity of band-A emission, which is related to crystal defects in diamond, was also decreased. The hole mobility at room temperature was increased from 826 to 1030 cm²/Vs by HPHT annealing. These results suggest that HPHT annealing decreases the crystalline defects and improves the optical and electronic properties of homoepitaxial diamond films.