Effect of titanium metal in the prenucleation of ultrananocrystalline diamond film growth at low substrate temperature

Ultrananocrystalline diamond (UNCD) film is usually grown in methane–argon plasma unlike methane–hydrogen plasma conventionally used to deposit microcrystalline diamond film. The prenucleation and growth mechanism of these two types of diamond films are different as well. The present study introduces titanium metal powder during ultrasonication of silicon substrate to enhance the nucleation density of UNCD. A titanium thin film was also used at the interface to find the effect of metal on the growth of diamond film. The nucleation density of as-grown film was estimated from the FE-SEM images. After 20 min of growth, nucleation density reaches to 10¹¹/cm² on a surface pretreated by titanium mixed nanodiamond powder. Raman study was carried out for qualitative analysis of different carbon phase present in the UNCD films. X-ray photoelectron spectroscopy (XPS) was used to understand the growth mechanism by detecting the formation of carbon phase and metal carbide formation at the surface after stopping the growth at different time intervals.     

Temperature influence on the interlayer and surface morphology of diamond coating on 3D porous titanium substrates

Nanocrystalline (NCD) and/or microcrystalline (MCD) diamond films grown on three-dimensional porous titanium (Ti) substrate were obtained by hot filament chemical vapor deposition (HFCVD) technique. The morphology variation of diamond films grown on porous three-dimensional titanium substrate was studied at four different deposition temperatures to investigate their influence on nucleation density. Scanning electron microscopy images depicted the continuous change from microcrystalline diamond grains with a random crystallographic orientation, at 500 °C and 600 °C, to a cauliflower-like structure for deposits at 700 °C and 800 °C. Visible Raman spectroscopy confirmed the good quality of diamond films and revealed that the amount of amorphous carbon increased associated to the film morphology changes from MCD to NCD. X-ray diffraction analyses, performed both through θ–2θ scans and at grazing incidence angle, allowed the investigation of the crystallographic properties and structural evolution of the different film/substrate interface phases, such as TiC(111), TiC(200) and TiH₂. The results revealed that the temperature enhanced the nucleation sites for diamond growth.     

Low-Temperature Synthesis of Diamond-like Carbon Films on Steel Substrates by Electron-Cyclotron-Resonance Plasma-Enhanced Chemical Vapor Deposition

Using microwave electron-cyclotron-resonance plasma-enhanced chemical vapor deposition, diamond-like carbon films were directly grown at low temperatures (lessthan equal to400°C) on Fe-based alloy substrates without diamond seeding or use of a template layer. A single, broad line in the Raman spectra was observed in the region of 1328-1335 cm^-1 for films grown in gas mixtures with a ratio of CH₄:H₂ greaterthan equal to 2%. In contrast, disordered carbon and graphite phases appeared in the spectra for film grown with a concentration of 20% CH₄ in hydrogen. Diamond nucleation with an amorphous carbon layer was observed in the initial growth stage, while many diamond particles with irregular morphological features were observed on the surface of thicker films. These growth features are a consequence of the catalytic nature of the Fe-based substrate.     

Structural and optical properties of diamond and nano-diamond films grown by microwave plasma chemical vapor deposition

We compare structural and optical properties of microcrystalline and nanocrystalline diamond (MCD and NCD, respectively) films grown on mirror polished Si(100) substrates by microwave plasma chemical vapor deposition. The films were characterized by SEM, Raman spectroscopy, XRD, and AFM. Optical properties were obtained from transmittance and reflectance measurements of the samples in the wavelength range of 200–2000 nm. Raman spectrum of the MCD film exhibits a strong and sharp peak near 1335 cm⁻¹, an unambiguous signature of cubic crystalline diamond with weak non-diamond carbon bands. Along with broad non-diamond carbon bands, Raman spectra of NCD films show features near 1140 cm⁻¹, the intensity of which is significantly higher in the film grown at 600°C compared to the NCD film grown at higher temperature. The Raman feature near 1140 cm⁻¹ is related to the calculated phonon density of states of diamond and has been assigned to nanocrystalline or amorphous phase of diamond. XRD patterns of the MCD film show sharp peaks and NCD films show broad features, corresponding to cubic diamond. The rms surface roughness of the films was observed to be approximately 60 nm for MCD film that reduced substantially to 17 and 34 nm in the NCD films grown at 600 and 700°C, respectively. Tauc's optical gap for the diamond film is found to be approximately 5.5 eV. NCD grown at 700°C has a high optical absorption coefficient in the whole spectral region and the NCD film grown at 600°C shows very high transmittance (∼78%) in the near IR region, which is close to that of diamond. This indicates that the NCD film grown at 600°C has the potential for applications as optical windows since its surface roughness is significantly low as compared to the MCD film.     

Effect of ion beam nitriding on diamond nucleation and growth onto steel substrates

Ion beam nitriding was successfully employed to overcome the difficulty of diamond growth on ferrous base substrates. Commercial steels were pretreated by an ion beam method in an ambient environment of nitrogen gas, diamond was then deposited by hot filament chemical vapor deposition (CVD). The deposited films were characterized by scanning electron microscopy, X-ray diffraction and Raman spectroscopy. Continuous diamond films with a sharp characteristic Raman peak of 1337.7 cm⁻¹ were grown and adhered well on the nitrided region of the steel substrates. On the other hand, a mixture of diamond crystallites, amorphous carbon and graphitic carbon was loosely deposited on the unnitrided region. A thin layer of iron and chromium nitrides, formed on the steel surface by ion beam nitriding, enabled the subsequent nucleation and growth of high-quality CVD diamond.     

Dependence of diamond nucleation and growth through graphite etching at different temperatures

Diamond films have been grown on silicon substrate from graphite etching as a carbon source with atomic hydrogen instead of using conventional hydrocarbon in the feed gas. A graphite plate was used as sample support in a hot filament chemical vapor deposition reactor. Graphite temperature demonstrated to have a strong dependence with the diamond nucleation and growth rate. Scanning electron microscopy (SEM) images of graphite targets associated with their Raman spectra were used to analyze their graphite morphology and structural properties before and after etching process for each graphite temperature studied. The results showed that chemical erosion intrinsically induces graphite surface changes that influence Raman spectra. The disorder behavior from the I_D/I_G ratio presented a maximum value at 900 °C, for 60 min of etching time, when compared with graphite surface at room temperature before atomic hydrogen etching. SEM diamond images were also used to analyze the nucleation rate. Diamonds grown during 15 min at 600 and 700 °C presented the higher nucleation rate. For growth time of 30 min the diamonds are continuous, covering the entire Si substrate surface, with submicrometer grain size. Raman spectra showed good quality diamond coating. Diamond content or diamond purity values, evaluated for growth time of 15, 30 and 60 min increases with the increase of the graphite temperature confirming the high carbon content in the first stage of diamond growth.     

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.     

Thermal oxidation of CVD diamond

A thermal oxidation process of diamond films grown by chemical vapor deposition (CVD) has been studied. The oxidation was realized via heating of the CVD films in air. Pristine and oxidized CVD diamond films were analyzed with Raman spectroscopy and scanning electron microscopy (SEM) techniques. Raman spectroscopy revealed substantial changes in the polycrystalline diamond film composition induced by oxidation. A selective oxidation of disordered carbon and small size diamond crystallites was obtained at appropriate temperatures. A model explaining the formation and oxidation of the CVD diamond films containing the micrometer single diamond cores surrounded by the nanocrystalline diamond and disordered carbon has been proposed on the basis of the obtained results.     

Evolution and properties of adherent diamond films with ultra high nucleation density deposited onto alumina

In this work, we report on adherent diamond films with thickness of up to 4.5 μm grown on polycrystalline alumina substrates. Prior to deposition, alumina substrates were ultrasonically abraded with mixed poly-disperse slurry that allows high nucleation density of values up to ∼5×10¹⁰ particles/cm². It was estimated that the minimal film thickness achieved for continuous films was ∼320 nm, obtained after a deposition time of 15 min with diamond particles density (DPD) of ∼4×10⁹ particles/cm². Continuous adherent diamond films with high DPD (∼10⁹ particles/cm²) were obtained also on sapphire surface after abrasion with mixed slurry and 15 min of deposition. However, after longer deposition time, diamond films peeled off from the substrates during cooling.The poor adhesion between the diamond and sapphire is attributed to the weak interface interaction between the film and the substrate and to difference in coefficient of thermal expansion. On the other hand, it is suggested that the reason for good adhesion between diamond film and alumina substrate is that high carbon diffusivity onto alumina grain boundaries allows strong touch-points at the grooves of alumina grains, and this prevents the delamination of diamond film. This adhesion mechanism, promoted by sub-micron diamond grain-size, is allowed by initial high nucleation density.The surface properties, phase composition and microstructure of the diamond films deposited onto alumina were examined by electron energy loss spectroscopy (EELS), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and high-resolution scanning electron microscopy (HR-SEM). The residual stress in the diamond films was evaluated by diamond Raman peak position and compared to a theoretical model with good agreement. Due to the sub-micron grain-size, the intrinsic tensile stress is high enough to partially compensate the thermal compressive stress, especially in diamond films with thickness lower than 1 μm.     

Effects of cobalt and cobalt oxide buffer layers on nucleation and growth of hot filament chemical vapor deposition diamond films on silicon (100)

An initial study on the nucleation and growth of diamond, using hot filament chemical vapor deposition (HFCVD) technique, was carried out on Co and CoO thin buffer layers on non-carbon substrates (Si (100)), and the results were compared with conventional scratching method. The substrate temperature during the growth was maintained at 750±50 °C. A mixture of CH₄ and H₂ (1: 100 volume %) was used for deposition. The total pressure during the two hour deposition was 30±2 Torr. X-ray photoelectron spectroscopy (XPS) study showed the diamond nucleation at different time periods on the Co and CoO seed layers. It is observed that Co helps in nucleation of diamond even though it is known to degrade the quality of diamond film on W-C substrate. The reason for improvement in our study is attributed to (i) the low content of Co (~0.01%) compared to W-C substrate (~5–6%), (ii) formation of CoSi₂ phase at elevated temperature, which might work as nucleation sites for diamond. SEM analysis reveals a change in the morphology of diamond film grown on cobalt oxide and a significant reduction in the size of densely packed crystallites. Raman spectroscopic analysis further suggests an improvement in the quality of the film grown on CoO buffer layer.     

Photoelectron spectroscopy of CVD diamond

We report a study of the electronic structure of thin diamond films by core state (X-ray photoelectron spectroscopy (XPS)) and valence band (UV photoelectron spectroscopy (UPS)) photoelectron spectroscopy. These techniques were used to investigate the different phases in the initial growth of polycrystalline diamond films on Si(100) substrates. The films were deposited by a standard microwave technique as well as by bias-enhanced microwave plasma chemical vapour deposition in a dilute mixture of methane in hydrogen. The influence of sample preparation (such as prebiasing or prescratching of the silicon surface with different sized diamond powder prior to deposition) on nucleation density and electronic structure was also investigated by scanning electron microscopy and photoelectron spectroscopy (PES) respectively. The surface composition was probed as a function of deposition time. The XPS data reveal the formation of an SiC phase at the early stage on nucleation, preceding the gradual growth of diamond. At intermediate stages a combination of different carbon phases was observed. The atomic structure of the interface phases is discussed and a growth model is proposed. Valence band spectra of the different samples show the extreme sensitivity of PES to impurities and to surface properties such as reconstruction. The obtained data were compared to valence band measurements of natural diamond and other forms of carbon and to some extent to data obtained with Raman spectroscopy.     

Early Stages of Diamond-Film Formation on Cobalt-Cemented Tungsten Carbide

The surface composition of cemented tungsten carbide (WC-5.8 wt% Co) was studied by X-ray photoelectron spectroscopy (XPS), during the early stages of diamond-film deposition, by hot-filament chemical vapor deposition (HFCVD). The nucleated diamond films were analyzed by scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and automatic image analysis (AIA). The evolution of the surface composition of cemented tungsten carbide during the early stages of diamond-film deposition was strongly dependent on the substrate temperature. At relatively low temperature (750°C), cobalt-rich particles started to segregate at the substrate surface after a few minutes of diamond deposition. The conspicuous segregation of the binder partly inhibited the formation of stable diamond nuclei, through intense carbon dissolution or carbon segregation at the binder surface, but did not affect nucleic growth. At higher temperatures (940°C), no cobalt-rich particles formed at the substrate surface, even after 2 h of deposition. However, XPS results demonstrated the presence of cobalt in a surface layer, although in a lower amount than at 750°C. Nevertheless, the nucleation density of diamond at 940°C was much lower than at 750°C. Gaps between WC grains formed within 10 mins. Therefore, intergranular cobalt was removed at 940°C, a finding attributed to the etching performed by monohydrogen, rather than to binder evaporation. The time evolution of the substrate area fraction covered by diamond islands, S ( t ), was well described by Avrami kinetics for two-dimensional phase transformations, suggesting that diamond formation took place via a heterogeneous nucleation process. The S ( t ) functions exhibited a similar trend at 750° and 940°C, because the higher growth rate of diamond crystallites at higher temperature counteracted the slower nucleation rate at the higher temperature.     

Iron Oxide Nanoparticles Employed as Seeds for the Induction of Microcrystalline Diamond Synthesis

Iron nanoparticles were employed to induce the synthesis of diamond on molybdenum, silicon, and quartz substrates. Diamond films were grown using conventional conditions for diamond synthesis by hot filament chemical vapor deposition, except that dispersed iron oxide nanoparticles replaced the seeding. X-ray diffraction, visible, and ultraviolet Raman Spectroscopy, energy-filtered transmission electron microscopy , electron energy-loss spectroscopy, and X-ray photoelectron spectroscopy (XPS) were employed to study the carbon bonding nature of the films and to analyze the carbon clustering around the seed nanoparticles leading to diamond synthesis. The results indicate that iron oxide nanoparticles lose the O atoms, becoming thus active C traps that induce the formation of a dense region of trigonally and tetrahedrally bonded carbon around them with the ensuing precipitation of diamond-type bonds that develop into microcrystalline diamond films under chemical vapor deposition conditions. This approach to diamond induction can be combined with dip pen nanolithography for the selective deposition of diamond and diamond patterning while avoiding surface damage associated to diamond-seeding methods.     

Raman scattering characterization of CVD graphite films

Raman spectroscopic study has been performed for thin graphite films grown on nickel substrates by chemical vapor deposition from a mixture of hydrogen and methane activated by a direct current discharge. Depending on the growth conditions, the CVD films are composed of graphene layers parallel to the substrate surface or of plate-like crystallites with the predominant orientation of their graphene layers perpendicular to the substrate surface. A comparison of the Raman spectra for the CVD films and for the highly oriented pyrolytic graphite has been performed. The mechanisms governing the Raman scattering process in the films are discussed. An important role of a double resonance mechanism in the Raman spectra of these graphite-based materials has been revealed. The Raman band positions and intensities and their dependence on excitation wavelength confirm a high degree of the structural order in the CVD graphite films.     

生长时间对纳米金刚石薄膜微结构的影响

文章研究了不同沉积时间下制备的不同厚度纳米金刚石薄膜的微观结构和相组成。采用热丝化学气相沉积法分别制备了沉积时间为52、67、97和127min的纳米金刚石薄膜。采用扫描电子显微镜和拉曼光谱表征薄膜的微观结构和相组成。结果表明,纳米金刚石薄膜表面颗粒尺寸大小无明显变化,约为50nm。随着生长时间增加,金刚石相含量保持稳定没有明显的增加或减小趋势,石墨相有序度以及石墨团簇尺寸随着生长时间增加而增加。     

Nucleation and Growth of Diamond Films on Ni-Cemented Tungsten Carbide: Effects of Substrate Pretreatments

The nucleation and growth of diamond films on Nicemented carbide is investigated. Substrates made of WC with 6 wt% of Ni were submitted to grinding, and then to different pretreatments (scratching, etching, and/or decarburization) before diamond deposition. Diamond synthesis was carried out by hot-filament chemical vapor deposition (HFCVD) using a mixture of CH₄ (1% v/v) and H₂. Depositions were performed for different lengths of time with the substrates at various temperatures. The specimens were analyzed before and after deposition by scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and X-ray diffractometry (XRD). Raman spectra showed that the phase purity of the diamond films was not affected by the presence of nickel on the substrate surface. After wet etching pretreatments, the nucleation of diamond was enhanced, mainly at the WC grain boundaries. Continuous films were obtained on scratched and etched substrates. The decarburizing treatment led to the formation of metallic tungsten and of brittle nicke–tungsten carbide phases. These phases reacted in the early stages of diamond film formation with gaseous carbon species with a parallel process which competes with stable diamond nucleus formation. The diamond film formed after long-term deposition on these samples was not continuous.     

A study of the initial stages of diamond deposition on ferrous substrates coated by a nitrided chromium interlayer and on nitrided polycrystalline chromium substrates

The use of a nitrided chromium interlayer has been found to improve the interfacial properties of diamond films deposited on ferrous substrates. This is achieved by hindering diffusion process of carbon and iron, good adhesion of the interlayer to the steel substrate, and very stable mechanical and chemical bonding between the interlayer and the diamond film. In the present study the initial stages of diamond deposition on steel substrates coated by a nitrided chromium interlayer and on nitrided polycrystalline chromium substrates are reported. Nitridation of chromium films deposited by electrochemical methods and polycrystalline chromium substrates resulted in the formation of two chromium nitrides phases, CrN and Cr₂N, and a rough surface morphology. The initial stages of diamond deposition were found to be accompanied by carburization of the substrates surface resulting in chromium carbide formation. The incubation time, diamond particle density and growth rate at the very initial stages of the deposition process were found to differ for these two substrates. It is suggested that these differences originate from different carburization rates of the two substrates. Phase transformation, recrystallization and diffusion processes in the near surface regions of both substrates resulted in very stable chemical bonding and good adhesion of the diamond film to the substrates. Raman spectra of the deposited films, on both substrates, show shift of the diamond peak position to higher wave numbers and split of the peak. These effects are associated with compressive stresses in the diamond film. Residual stresses in the deposited films were calculated from the shift of the diamond Raman peak. The residual stresses, as calculated from the Raman spectra, were found to increase with deposition time reaching values of 8.4 and 6.9 GPa for continuous diamond films on steel substrate coated with the nitrided chromium film and on nitrided chromium substrates, respectively. Based on a simple model it was estimated that thermal stress, arising from mismatch between the thermal expansion coefficient of diamond and the underlying substrates, is the major component of the compressive stress in the diamond films.     

Boron-doped hydrogenated amorphous carbon films grown by surface-wave mode microwave plasma chemical vapor deposition

We report the effects of boron (B) doping on optical and structural properties of the hydrogenated amorphous carbon thin films grown by surface-wave mode microwave plasma (SW-MWP) chemical vapor deposition (CVD) on n-type silicon and quartz substrates at room temperature. Argon and acetylene were used as a carrier and carbon source gases respectively. Analytical methods such as X-ray photoelectron spectroscopy (XPS), Nanopics 2100/NPX200 surface profiler, JASCO V-570 UV/VIS/NIR spectroscopy, Fourier transform infrared spectroscopy (FT-IR) and Raman spectroscopy were employed to investigate the properties of the films. Low atomic concentration of B (0.08 at.%) was found in the doped film. The optical band gap of the undoped film was 2.6 eV and it decreased to 1.9 eV for the B-doped film. Structural property shows the crystalline structure of the film and it has changed after incorporating B as a dopant. The structural modifications of the films leading to being more graphite in nature were confirmed by the Raman and FT-IR characterization.     

Hardness of nanocomposite a-C:Si films deposited by magnetron sputtering

Hydrogen-free a-C:Si films with Si concentration from 3 to 70 at.% were prepared by magnetron co-sputtering of pure graphite and silicon at room temperature. Mechanical properties (hardness, intrinsic stress), film composition (EPMA and XPS) and film structure (electron diffraction, Raman spectra) were investigated in dependence on Si concentration, substrate bias and deposition temperature. The film hardness was maximal for ∼ 45 at.% of Si and deposition temperatures 600 and 800 °C. Reflection electron diffraction indicated an amorphous structure of all the films. Raman spectra showed that the films in the range of 35–70 at.% of Si always contain three bands corresponding to the Si, SiC and C clusters. Photoelectron spectra showed dependency of Si–C bond formation on preparation conditions. In the films close to the stoichiometric SiC composition, the surface and sub-surface carbon atoms exhibited dominantly sp³ bonds. Thus, the maximal hardness was observed in nanocomposite a-C:Si films with a small excess of carbon atoms.