Cold plasma functionalization of nanodiamond particles

Functionalization of three different types of detonation nanodiamonds was performed using a cold plasma discharge generated with fluorine containing gas. The chemical bonds formed between the reactive species generated in the plasma and nanodiamond particle surfaces were confirmed by FTIR and XPS analysis. From results of FTIR and XPS characterization, it was concluded that initial type of nanodiamond has little overall effect on the degree of fluorination. Fluorination of nanodiamonds allowed for the previously unavailable colloidal suspension in anisole.     

砷化镓上生长金刚石薄膜的研究

以CH4和H2为气源,用微波辅助等离子体装置,在10.0 mm×7.0 mm的砷化镓基底上沉积了CVD金刚石薄膜,用扫描电子显微镜观察沉积效果,拉曼光谱表征沉积质量,分析薄膜附着力与砷化镓材料性能的关系。结果表明,当基体温度为600℃,气压为5 kPa,甲烷浓度为2.0%时,在砷化镓片表面上沉积出了CVD金刚石薄膜,晶粒尺寸均匀,晶形完整、规则,晶界非常清晰。     

Mechanical properties of nanocrystalline diamond/amorphous carbon composite films prepared by microwave plasma chemical vapour deposition

Nanocrystalline diamond films (NCD) have been deposited by microwave plasma chemical vapour deposition from CH₄/N₂ mixtures with varying methane content. They consist of diamond nanocrystallites with sizes of 3–5 nm embedded in an amorphous matrix with grain boundary widths of 1–1.5 nm. The CH₄ content in the gas phase has almost no influence on the microscopic structure but a strong effect on the macroscopic structure and morphology. The mechanical and tribological properties of these films have been investigated by nanoindentation, nano tribo tests, and nano scratch tests. The hardness of a 4-μm-thick film deposited with 17% methane was about 40 GPa, the indentation modulus 387 GPa, and the elastic recovery 75%. Ball-on-disk tests against an Al₂O₃ ball revealed, after initially higher values, a friction coefficient of ≤0.1. Tribo tests and scratch tests proved a strong adhesion and a protective effect on silicon substrates. Finally, the correlations between the macroscopic structure of the films and their mechanical and tribological properties are discussed.     

UV Schottky photodiode on boron-doped CVD diamond films

We report on experimental study of photosensitivity and Q-DLTS spectra of polycrystalline CVD diamond UV photodetectors. The measured characteristics of Schottky photodiode on boron-doped diamond films are compared with those obtained for planar photoconductive structures (photoresistor type) based on undoped CVD diamond. The Schottky photodiode exhibited a sharp cut-off in photoresponse with spectral discrimination ratio (between wavelengths of 190 nm and 700 nm) as high as 5 · 10⁵ at zero bias voltage (at zero dark current). The photodiode showed the maximum of photoresponse at wavelength < 190 nm, and a low density of trapping and recombination centers as evaluated with the Q-DLTS technique. The devices demonstrated the photoresponsivity at 190 nm from 0.03 to 0.1 A/W with quantum yield of 0.20 to 0.67 in closed circuit, while the photovoltage ≥ 1.6 V was measured in open circuit regime. Another type of UV detector, the planar photoconductive structures with interdigitizing ohmic electrodes fabricated on undoped diamond film and operated under a bias voltage, revealed a higher density of (surface) defect centers and the maximum photoresponse at  210 nm wavelength. A strong influence of UV light illumination on the Q-DLTS spectra of the planar photoconductive structures was observed. This effect can be used for development of new UV detectors and dosimeters based on the Q-DLTS signal measurements.     

Structural investigation of nanocrystalline diamond/amorphous carbon composite films

Nanocrystalline diamond/amorphous carbon (NCD/a-C) composite films have been prepared by microwave plasma chemical vapor deposition (MWCVD) from methane/nitrogen mixtures. The complex nature of the coatings required the application of a variety of complementary analytical techniques in order to elucidate their structure. The crystallinity of the samples was studied by selected-area electron diffraction (SAED). The diffraction patterns revealed the presence of diamond crystallites within the films. From the images taken by transmission electron microscopy (TEM) the crystallite size was determined to be on the order of 3–5 nm. The results were confirmed by X-ray diffraction (XRD) measurements exhibiting broad (111) and (220) peaks of diamond from which the average size of the crystallites was calculated. The grain boundary width is 1–1.5 nm as observed by TEM images which corresponds to a matrix volume fraction of about 40–50%. This correlates very well with the crystalline phase content of about 50% in the films estimated from their density (2.75 g/cm³ as determined by X-ray reflectivity). The bonding structure of the composite films was studied by electron energy loss spectroscopy (EELS) in the region of carbon core level. The spectra were dominated by a peak at 292 eV indicating the diamond nature of the investigated films. In addition, the spectra of NCD/a-C films possessed a shoulder at 284 eV due to the presence of a small sp² bonded fraction. This phase was identified also by X-ray photoelectron spectroscopy (XPS). The sp²/sp³ ratio was on the order of 10% as determined by deconvolution of the C1s XPS peak.     

The properties of free-standing diamond films after plasma high temperature treatment of the rapid heating

The plasma treatment of rapid heating was introduced for increasing fracture strength of free-standing diamond films. The effects of plasma high temperature annealing treatment on surface morphology, internal stress, vacancy defects, impurities and fracture strength of free-standing diamond films were investigated by scanning electron microscopy (SEM), Raman, positron annihilation technique (PAT) and mechanical property testing. It showed that the fracture strength of the diamond films increases up to 70% for lower fracture diamond films with treating temperature (1500-1600 °C). The graphitization in surface and interior of diamond films would be produced by high temperature treatment more than 1700 °C. Fracture strengths of diamond films could be enhanced after high temperature treatment and the main factor of that was compressive stress state in diamond films induced by graphitization. The impurity of N was segregated and integrated with vacancy cluster to become [N-V]⁰ and [N-V]⁻.     

Free-standing diamond films deposited by DC arc plasma jet on graphite substrates with a destroyable Ti interlayer

A destroyable Ti interlayer on graphite substrate was used for fabrication of crack-free free-standing diamond films by high-power DC Arc Plasma Jet. Ti interlayer was arc ion plated on the polycrystalline graphite substrate. The thickness, morphology and composite phase of the Ti interlayer were examined by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Titanium carbide (TiC) was detected in both sides of the interlayer, which played an important role with respect to reasonable adhesion and diamond nucleation. Semi-translucent and crack-free diamond films were obtained and characterized by SEM and Raman spectroscopy. It is shown that the diamond films so obtained have excellent Raman signature. The overall results suggest that plating Ti interlayer on graphite substrate is an effective way to obtain potentially crack-free free-standing diamond films.     

Deposition of stress-free diamond films on Si by diamond/β-SiC nanocomposite intermediate layers

Stress-free diamond films have been synthesized by using microwave plasma enhanced chemical vapour deposition (MWCVD) technique. Nanocrystalline diamond/β-SiC gradient composite film system was used as an interlayer for the diamond top layers. As a result, Raman phonon line shifts (obtained from diamond top layers) corresponding to diamond are recorded very close to the stress-free value of 1332 cm^− 1. This implies an extraordinarily low biaxial compressive stress state in the diamond films. The interlayer accommodates to a considerable extent, the stress that occurs due to the difference in the thermal expansion coefficient of the diamond film and the underlying substrate.     

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.     

Grazing angle reflectance spectroscopy of organic monolayers on nanocrystalline diamond films

The nanocrystalline diamond (NCD) layers were grown by the large area (linear plasma) MWCVD on polished silicon substrates with and without intermediate mirror-like metallic coatings. The optical reflectance and Raman spectroscopy in the ultraviolet, visible and near infrared region (UV-VIS-NIR) reveals the thickness and the optical quality of NCD layers. The modified grazing angle reflectance (GAR) spectroscopy is applied in the mid infrared region 800-4000/cm to detect the molecular vibrations (functional groups) at the functionalized NCD surface. The optical absorbance of functionalized NCD surface is evaluated from p-polarized reflectance spectra measured at Brewster angle of incidence (BAR) to eliminate the interference fringes. We report a significant enhancement of sensitivity of BAR using NCD growth on metal mirrors.     

Ex-situ spectroscopic ellipsometry studies of micron thick CVD diamond films

Micron thick diamond films have been studied by spectroscopic ellipsometry (SE). The films were grown, on previously prepared Si(100) substrates, by the plasma enhanced chemical vapor deposition (PECVD) technique. Ex situ SE measurements were carried out on samples grown under different conditions, such as substrate temperature and methane fraction in the gas mixture. An optical model consisting of five layers was constructed in order to explain the SE spectra and to provide the optical and structural parameters of the films. This model was deduced from results of various measurements performed by other characterization techniques (Raman spectroscopy, scanning electron microscopy, atomic force microscopy and positron annihilation spectroscopy) which have revealed the optical and structural parameters of the samples. Its sensitivity to the surface and interface roughness as well as to the absorption of the nondiamond phase of the film is demonstrated. Several values of the percentage of the nondiamond phase can be obtained, with the same fit quality, however, depending on the amorphous carbon reference used in the model. These references were obtained by performing SE measurements on various amorphous carbon films. Finally, our SE analysis has allowed us to monitor the lateral homogeneity of the thickness, surface and interface roughness and nondiamond phase concentration over the diamond film.     

Incorporation of hydrogen in diamond thin films

In this investigation, diamond thin films with grain size ranging from 50 nm to 1 µm deposited using hot filament chemical vapor deposition (HFCVD) have been analyzed by elastic recoil detection analysis (ERDA) for determining hydrogen concentration. Hydrogen concentration in diamond thin films increases with decreasing grain size. Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) results showed that part of this hydrogen is bonded to carbon forming C–H bonding. Raman spectra also indicated the increase of non diamond phase with the decrease in crystallite size. Incorporation of hydrogen in the samples and increase of hydrogen content in nanocrystalline sample are discussed. Large separation between filament and substrate used for the synthesis of nanocrystalline film helped to understand the large incorporation of hydrogen in nanocrystalline diamond films during growth. The study addresses the hydrogen trapping in different samples and higher hydrogen concentration in nanocrystallites by considering the synthesis conditions, growth mechanisms for different grain sized diamond films and from the quality of CVD diamond films.     

Secondary electron emission from CVD diamond films

Secondary electron emission from boron doped diamond polycrystalline membranes (hole concentration 5×10¹⁸ cm⁻³), prepared by microwave plasma assisted CVD, was investigated in both the reflection and transmission configurations. The model of secondary electrons behavior taking into account the distribution and diffusion mechanism of secondary electrons is proposed to explain the yield dependencies on primary electron energy in both configurations. The model predicts the SEE yield K=19 at the primary electron energy E₀ close to 1 keV for reflection configuration and K=3–7 at E₀=15–30 keV for transmission configuration for polycrystalline films used in the study. Experimental measurements of the SEE yield vs. primary electron energy (18 at E₀=950 eV for the reflection scheme and 3.5–4 at E₀=25 keV for the transmission one) are found to accord well with the theoretical results. Estimations, which were made using the model, show that SEE yield in transmission configuration can be increased up to 60 for the primary electron energy of about 10 keV. Since such high yields in transmission scheme may be obtained in monocrystalline membrane, another approach using porous polycrystalline diamond membranes is considered. Porous diamond membranes having SEE yield in transmission scheme of more than 10 at the primary electron energy E₀=1 keV were fabricated.     

Adhesion of CVD diamond films on silicon substrates of different crystallographic orientations

In order to gain insight into the adhesion mechanisms of diamond films, we examine Si substrates with three different crystallographic orientations at the various stages of the deposition process. This allows one to distinguish the surface phenomena involved in diamond deposition from those due to gaseous plasma processes. We find that the initial ultrasonic scratching treatment produces partial graphitization of the diamond powder, and it controls the crystallite size through the carbon residues. On the other hand, an increased surface roughness due to H-atom etching correlates with increased adhesion. The deposited film adhesion is found to increase in the sequence Si(111)相似文献   

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.     

Determination of the optical constants of diamond films with a rough growth surface

The determination of the optical constants n and k for polycrystalline diamond films with typical growth surface roughness σ of the order of tens to hundreds of nanometers is both an important and a difficult procedure. A method for determining n, k and σ for rough diamond films is presented here. It is based on theoretical expressions for the optical reflectance R and transmittance T for a free-standing thin film incorporating the effects of surface scattering at one surface. The usefulness and validity of this approach are illustrated using experimental R and T data for diamond films. Simplified expressions for R and T are also presented which are valid when scattering or absorption are sufficiently strong to cause interference effects in the films to be averaged out.

Prime novelty: Simple expressions for n, k and σ are presented which will enable researchers to extract this important information about the optical constants and surface roughness of their CVD diamond films.     

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.     

CVD diamond for spintronics

The ability to minimise, control and manipulate defects in CVD diamond has grown rapidly over the last ten years. The application which best illustrates this is probably that of quantum information processing (QIP) or ‘diamond spintronics’. QIP is a rapidly growing area of research, covering diverse activities from computing and code breaking to encrypted communication. All these applications need ‘quantum bits’ or qubits where the quantum information can be maintained and controlled. Controlled defects in an otherwise high perfection diamond lattice are rapidly becoming a leading contender for qubits, and offer many advantages over alternative solutions. The most promising defect is the NV⁻ defect whose unique properties allow the state of its electron spin to be optically written to and read from. Substantial developments in the synthesis of CVD diamond have produced diamond lattices with a high degree of perfection, such that the electron spin of this centre exhibits very long room temperature decoherence times (T₂) in excess of 1 ms. This paper gives a brief review of the advantages and challenges of using CVD diamond as a qubit host. Lastly the various qubit applications being considered for diamond are discussed, highlighting the current state of development including the recent development of high sensitivity magnetometers.     

Xenon difluoride plasma fluorination of polymer surfaces

Xenon difluoride (XeF₂) plasma treatment of a series of polymers containing different repeat units gives rise to varying levels of surface fluorination. Alkene and aromatic C-H bonds appear to be more susceptible towards reaction compared to their sp³ counterparts. The extent of fluorine incorporation can be accounted for in terms of a structure-behaviour relationship derived from extended Huckel molecular orbital calculations. Comparison with CF₄ plasma modification shows that XeF₂ electrical discharges are more effective at fluorinating polymer surfaces.