Effects of plasma treatments on the nitrogen incorporated nanocrystalline diamond films

The nitrogen incorporated nanocrystalline diamond (NCD) films were grown on n-silicon (100) substrates by microwave plasma enhanced chemical vapor deposition (MPECVD) using CH₄/Ar/N₂ gas chemistry. The effect of surface passivation on the properties of NCD films was investigated by hydrogen and nitrogen-plasma treatments. The crystallinity of the NCD films reduced due to the damage induced by the plasma treatments. From the crystallographic data, it was observed that the intensity of (111) peak of the diamond lattice reduced after the films were exposed to the nitrogen plasma. From Raman spectra, it was observed that the relative intensity of the features associated with the transpolyacetylene (TPA) states decreased after hydrogen-plasma treatment, while such change was not observed after nitrogen-plasma treatment. The hydrogen-plasma treatment has reduced the sp²/sp³ ratio due to preferential etching of the graphitic carbon, while this ratio remained same in both as-grown and nitrogen-plasma treated films. The electrical contacts of the as-grown films changed from ohmic to near Schottky after the plasma treatment. The electrical conductivity reduced from ~ 84 ohm^– 1 cm^− 1 (as-grown) to ~ 10 ohm^– 1 cm^− 1 after hydrogen-plasma treatment, while the change in the conductivity was insignificant after nitrogen-plasma treatment.     

Characterization of nanocrystalline diamond films implanted with nitrogen ions

Nanocrystalline diamond films, prepared by a microwave plasma-enhanced CVD, were implanted using 110-keV nitrogen ions under fluence ranging from 10¹⁶–10¹⁷ ions cm⁻². AFM, XRD, XPS and Raman spectroscopy were used to analyze the changes in surface structure and chemical state of the films before and after implantation. Results show that high-fluence nitrogen ions implanted in the nanocrystalline diamond film cause a decline in diamond crystallinity and a swelling of the crystal lattice; the cubic-shaped diamond grains in the film transform into similar roundish-shaped grains due to the sputtering effect of implanted nitrogen ions. Nitrogen-ion implantation changes the surface chemical state of the nanocrystalline diamond film. After high-fluence implantation, the surface of the film is completely covered by a layer of oxygen-containing groups. This phenomenon plays an importance role in the reduction of the adhesive friction between an Al₂O₃ ball and the nanocrystalline diamond film.     

Wettability of nanocrystalline diamond films

The wettability of nanocrystalline CVD diamond films grown in a microwave plasma using Ar/CH₄/H₂ mixtures with tin melt (250–850 °C) and water was studied by the sessile-drop method. The films showed the highest contact angles θ of 168 ± 3° for tin among all carbon materials. The surface hydrogenation and oxidation allow tailoring of the θ value for water from 106 ± 3° (comparable to polymers) to 5° in a much wider range compared to microcrystalline diamond films. Doping with nitrogen by adding N₂ in plasma strongly affects the wetting presumably due to an increase of sp²-carbon fraction in the films and formation of C–N radicals.     

Investigation of nitrogen addition on hydrogen incorporation in CVD diamond films from polycrystalline to nanocrystalline

The effect of nitrogen addition in the gas phase on hydrogen impurity incorporation into CVD diamond films was investigated. A series of thick diamond films of different morphology and quality ranging from large-grained polycrystalline to fine-grained nanocrystalline were deposited on silicon wafers using a 5 kW microwave plasma assisted CVD system. They were obtained only by changing the small amount of oxygen and nitrogen addition while keeping all other input parameters the same. Bonded hydrogen impurity in these diamond films was studied by using Fourier-transform infrared spectroscopy. It was found that with increasing the amount of nitrogen addition in the gas phase, the produced diamond films from large-grained polycrystalline gradually shift to fine-grained nanocrystalline and their crystalline quality is drastically degraded, while the amount of incorporated hydrogen impurity in the diamond films increases sharply. The role of nitrogen additive on diamond growth and hydrogen incorporation is discussed. These results shed light into the growth mechanism of CVD diamond films ranging from polycrystalline to nanocrystalline, and the incorporation mechanism of hydrogen impurity in CVD diamonds.     

Electrical properties of thick boron and nitrogen contained CVD diamond films

The acceptor and donor defects of thick (approx. 0.4 mm) free-standing boron and nitrogen containing microwave plasma CVD polycrystalline diamond films were investigated. Charge-based deep level transient spectroscopy (Q-DLTS) was applied to study impurity-induced defects, their density and energy distribution in the energy range of 0.01 eV≤E−E_v≤1.1 eV above the valence band. It was shown, that differential capacitance–voltage, and Hall effect measurements combined with DLTS data can be used to determine the degree of compensation, and the concentration of compensating donors (mostly the positively charged single-substitutional nitrogen (N⁺)) in p-type CVD polycrystalline diamond films. It was found, that incorporated boron atoms induce three levels of electrically active defects. Two of them with concentration (2–3)×10¹⁶ cm⁻³ each have activation energies of 0.36 and 0.25 eV with capture cross-sections of 1.3×10⁻¹³ and 4.5×10⁻¹⁹ cm², respectively. The third type of defect has an activation energy of 0.02 eV, capture cross-section 3×10⁻²⁰ cm² and concentration 10¹⁵ cm⁻³, this shallow trap being a probable general caterer of holes in low-doped films. The total concentration of electrically active uncompensated acceptors in all p-type diamond samples was approximately 2×10¹⁷ cm⁻³ with hole concentration of approximately 1.5×10¹⁴ cm⁻³ and hole mobility in the range of 30–40 cm² V⁻¹ s⁻¹ at room temperature. If assumed that compensating donors are mostly nitrogen, the films contained no less than 3×10¹⁶ cm⁻³ of N⁺.     

Gas sensing properties of nanocrystalline diamond films

Toxic gas sensing device with metal electrodes built into nanocrystalline diamond (NCD) is investigated. The NCD morphology is controlled via seeding and/or deposition time. The surface properties and morphology of NCD are studied using scanning electron microscopy (SEM) and atomic force microscopy (AFM). AFM measurements reveal increase in NCD surface area by up to 13%. Gas sensing properties of H-terminated NCD device show high sensitivity towards oxidizing species where the surface conductivity is increased by an order of magnitude for humid air and by three orders of magnitude for COCl₂. The surface conductivity exhibits a small decrease to reducing spices (CO₂, NH₃).     

Thermal analysis of submicron nanocrystalline diamond films

The thermal properties of sub-μm nanocrystalline diamond films in the range of 0.37–1.1 μm grown by hot filament CVD, initiated by bias enhanced nucleation on a nm-thin Si-nucleation layer on various substrates, have been characterized by scanning thermal microscopy. After coalescence, the films have been outgrown with a columnar grain structure. The results indicate that even in the sub-μm range, the average thermal conductivity of these NCD films approaches 400 W m^− 1 K^− 1. By patterning the films into membranes and step-like mesas, the lateral component and the vertical component of the thermal conductivity, k_lateral and k_vertical, have been isolated showing an anisotropy between vertical conduction along the columns, with k_vertical ≈ 1000 W m^− 1 K^− 1, and a weaker lateral conduction across the columns, with k_lateral ≈ 300 W m^− 1 K^− 1.     

AC behavior of intrinsic nanocrystalline diamond films

Nanocrystalline diamond was prepared by hot filament assisted chemical vapor deposition technique. The nanometer sized dimension of diamond grains was determined by X-ray line broadening. AC electrical response of deposits, constituted by well formed diamond grains, was studied by admittance spectroscopy at different temperatures. Grain boundary and grain surface were considered different regions able to influence differently the frequency dependent AC response. Observed variations in admittance spectra were attributed to a modification of the grain surface response as frequency and temperature rise. A semiconductor to metal-like transition was evidenced in admittance spectra increasing the frequency of applied signal at lower temperatures.     

Subgap photoluminescence spectroscopy of nanocrystalline diamond films

We report on the effect of ambient conditions and UV irradiation on the subgap photoluminescence of nanocrystalline diamond prepared by microwave plasma enhanced chemical vapour deposition. We measured the photoluminescence of self-supporting membranes of thickness about 290 nm with the grain size up to 40 nm under variable ambient conditions – pressure, temperature, air, nitrogen and helium atmospheres. We have found that intensity of photoluminescence of samples kept under low pressure increases during the time. The photoluminescence intensity of samples under low pressure depends on sample temperature with maximum at about 260 K. The photoluminescence increase can be enhanced substantially by UV irradiation (325 nm) of the sample under certain conditions: temperature greater than ~ 280 K, low pressure of ambient atmosphere. We interpret the experimental results in terms of desorption of water molecules and their interaction with the of individual diamond nanocrystals in the membrane.     

Characterization of crystalline diamond incorporated diamond-like carbon films

The purpose of this paper is to show the production and characterization of diamond-like carbon (DLC) films with incorporated crystalline diamond (CD), produced by plasma enhanced chemical vapor deposition. CD-DLC films were characterized by scanning electron microscopy, X-ray diffraction, atomic force microscopy and Raman scattering spectroscopy. Wetting contact angle, stress and friction coefficient were also evaluated. Our results demonstrated CD-DLC films are more hydrogenated and hydrophobic, with higher fiction coefficient. The stress values kept almost constantly.     

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.     

Effect of nitrogen and oxygen addition on morphology and texture of diamond films (from polycrystalline to nanocrystalline)

We investigate the effect of simultaneous nitrogen and oxygen addition into conventional methane–hydrogen plasma on morphology and texture of diamond films produced in a high power high pressure 5 kW microwave plasma chemical vapor deposition (MPCVD) reactor. Diverse diamond films ranging from large-grained polycrystalline to nanocrystalline can be achieved by simply changing the amount of nitrogen and oxygen addition, while keeping other parameters the same. This opens a new operating window for tailoring the growth of diamond for different applications. Our study shows that: (1) a small amount of oxygen addition leads to a {111}facet dominated large-grained polycrystalline diamond film of mixed <211> and <110> texture; (2) a small amount of nitrogen addition favours the formation of a <110> textured nanocrystalline diamond film; whereas (3) the coupled effect of simultaneous nitrogen and oxygen addition is in between them. The versatility of nitrogen and oxygen addition on tailoring diamond growth ranging from polycrystalline to nanocrystalline diamonds with different microstructures, morphologies and textures is clearly demonstrated.     

Bioproperties of nanocrystalline diamond/amorphous carbon composite films

Nanocrystalline diamond/amorphous carbon (NCD/a-C) nanocomposite films have been deposited by microwave plasma chemical vapour deposition (MWCVD) from CH₄/N₂ mixtures. The films have been thoroughly characterized by a variety of methods with respect to their composition, morphology, structure and bonding environment. Thereafter, the bioproperties of these films have been investigated. Tests with osteoblast-like cells and pneumocytes proved that the NCD/a-C films are not cytotoxic. In addition, exposure of the films to a simulated body fluid revealed that they are bioinert. Further experiments addressed the question whether biomolecules such as RNA or proteins bind unspecifically on the surfaces of NCD/a-C films. By means of atomic force microscopy (AFM) and scanning force spectroscopy measurements it was established that, in contrast to control experiments with mica and glass, no interaction between the nanocrystalline diamond and either RNA or protein molecules took place. The results of these experiments concerning the biologically relevant properties of NCD/a-C films are discussed in view of possible future applications, e.g. as a material for the immobilization of biomolecules and their characterization by AFM measurements and related techniques.     

Synthesis of nanocrystalline diamond films using microwave plasma CVD

Diamond is one of the most valuable materials for the industrial applications because of its excellent properties including high hardness, with good electrical insulation and thermal conductivity. Mechanical polishing processes of diamond are difficult and very costly. To limit those costs, it is reasonable to think that the surface roughness of the as-grown diamond film should be as small as possible. In this study, a nanocrystalline diamond film was synthesized on a 4-inch Si wafer at 923 K and methane concentration of 10 vol.%, (H₂/CH₄=100/10 sccm) using a microwave plasma CVD system. In order to increase the nucleation density, the substrate was pretreated by dry scratch method with diamond powder of two sizes (250 nm and 5 nm). The nucleation density was approximately 1×10¹¹ cm⁻². The grown diamond films were analyzed by Raman spectroscopy and X-ray diffraction (XRD). The grain size was observed to be approximately 10 nm by FE-SEM observation. Surface roughness was measured as Rms=8.4 nm by atomic force microscope (AFM). The as-grown properties of those nanocrystalline diamond films were almost efficient for tribological and the optical applications.     

Nitrogenated nanocrystalline diamond films: Thermal and optical properties

Ultrananocrystalline diamond films have been grown by microwave plasma CVD using CH₄/H₂/Ar mixtures with N₂ added in plasma in amounts up to 25%. The films were characterized with AFM, Raman, XRD, and UV–IR optical absorption spectroscopy mainly focusing on optical and thermal properties. In comparison with polycrystalline CVD diamond the UNCD are very smooth (R_a < 10 nm), have low thermal conductivity ( 0.10 W/cm K), high optical absorption ( 10³ cm^− 1 at 500 nm) and high concentration of bonded hydrogen ( 9 at.%). The nitrogen presence in the plasma has a profound impact on UNCD structure and properties, particularly leading to a decrease in resistivity (by 12 orders of magnitude), thermal conductivity, Tauc band gap, optical transmission and H content. The UNCD demonstrated rather good thermal stability in vacuum: the diamond phase still was present in the films subjected to annealing to 1400 °C.     

Raman and conductivity studies of boron-doped microcrystalline diamond,facetted nanocrystalline diamond and cauliflower diamond films

We present a large amount of data showing how the electrical conductivity and Raman spectra of boron-doped CVD diamond films vary as a function of both B content and film type — in particular, diamond crystallite size. Three types of film have been investigated: microcrystalline diamond (MCD), faceted nanocrystalline diamond (f-NCD) and ‘cauliflower’ diamond (c-NCD). For the same B content (measured by SIMS), the conductance of MCD films was much higher than those for the two types of smaller grained films. Multi-wavelength laser Raman spectroscopy showed that Fano interference effects were much reduced for the smaller grain-sized material. The position of the Lorentzian contribution to the 500 cm^− 1 Raman feature was used to estimate the B content in each type of film, and compared to the value measured using SIMS. We found that the Raman method overestimated the concentration of B by a factor of ~ 5 for the f-NCD and c-NCD films, although it remains reasonably accurate for MCD films. The shortfall may be explained if only a small fraction of the B found in the small-grained films is being incorporated into substitutional sites. We conclude that in diamond films with a high concentration of grain boundaries, the majority of the B (80% in some cases) must be present at sites that do not contribute to the continuum of electronic states that give rise to metallic conductivity and the Fano effects. Such sites may include (a) interstitials, (b) the surface of the crystallites, or (c) bonded within the non-diamond carbon impurities present at the grain boundaries. This suggests that heavy doping of nanograined diamond films will give rise to a material with many different conducting regions, and possibly different conducting pathways and mechanisms.     

Doping of vanadium to nanocrystalline diamond films by hot filament chemical vapor deposition

Doping an impure element with a larger atomic volume into crystalline structure of buck crystals is normally blocked because the rigid crystalline structure could not tolerate a larger distortion. However, this difficulty may be weakened for nanocrystalline structures. Diamonds, as well as many semiconductors, have a difficulty in effective doping. Theoretical calculations carried out by DFT indicate that vanadium (V) is a dopant element for the n-type diamond semiconductor, and their several donor state levels are distributed between the conduction band and middle bandgap position in the V-doped band structure of diamond. Experimental investigation of doping vanadium into nanocrystalline diamond films (NDFs) was first attempted by hot filament chemical vapor deposition technique. Acetone/H₂ gas mixtures and vanadium oxytripropoxide (VO(OCH₂CH₂CH₃)₃) solutions of acetone with V and C elemental ratios of 1:5,000, 1:2,000, and 1:1,000 were used as carbon and vanadium sources, respectively. The resistivity of the V-doped NDFs decreased two orders with the increasing V/C ratios.     

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.     

Spatially resolved near-infrared excited Raman spectroscopy of nanocrystalline diamond films

Scanning Raman spectroscopic measurements were performed on nanocrystalline diamond thin films with 0.5 μm lateral steps and excitation spot limited to 1 μm in diameter using 488 nm and 785 nm excitation wavelengths. The comparison of the spectra measured with different excitation energies showed that in contrast to the well-known five bands in the 488 nm excited Raman spectra of nanocrystalline diamond a number of narrow peaks appears in the spectra when using near-infrared excitation. The intensity and position of the latter vary when moving the excitation spot along the sample. The detailed analysis of the sequences showed that the 785 nm excited Raman spectroscopy allows the detection and identification of the Raman peaks arising from individual diamond crystallites of the nanocrystalline diamond films.     

Ultrafast photoluminescence spectroscopy of H- and O-terminated nanocrystalline diamond films

We use femtosecond photoluminescence spectroscopy to study the light-induced changes in the sub-gap energy states of nanocrystalline diamond samples (thickness ~ 500 and 1000 nm) prepared on a spectral-grade fused silica substrate by microwave plasma enhanced chemical vapour deposition technique. The decay of photoluminescence in the visible spectral interval excited by blue femtosecond light pulses (405 nm,70 fs) shows that photoexcited charge carrier dynamics depend strongly on the ambient air pressure and on the light irradiation by the laser pulses. Specifically, at lower ambient air pressure (0.5-300 Pa) the irradiation leads to the peak photoluminescence intensity increase and to its faster decay. At higher air pressures (> 600 Pa) the photoluminescence intensity slightly decreases with no change in decay rates. O- and H-termination of nanocrystalline diamond films had negligible effect on their photoluminescence dynamics. The photoluminescence decay curves are described very well by the power-law decay reflecting the importance of the carrier trapping in the dynamics. Based on our results we propose a model of surface and sub-surface structure of nanodiamond films.