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.     

Optoelectronic properties of nitrogenated amorphous carbon films synthesized by microwave surface wave plasma chemical vapor deposition system

The n-type nitrogen doped amorphous carbon (a-C:N) thin films have been grown by microwave (MW) surface wave plasma (SWP) chemical vapor deposition (CVD) system on silicon, quartz and ITO substrates at different nitrogen flow rates (1 to 4 sccm). The effects of nitrogen doping on chemical, optical, structural and electrical properties were studied through X-ray photoelectron spectroscopy, Nanopics 2100/NPX200 surface profiler, UV/VIS/NIR spectroscopy, Raman spectroscopy and solar simulator measurements. Argon, acetylene and nitrogen are used as plasma sources. Optical band gap decreased and nitrogen atomic concentration (%) increased with increasing nitrogen flow rate as a dopant. The a-C:N/p-Si based device exhibits photovoltaic behavior under illumination (AM 1.5, 100 mW/cm²), with a maximum open-circuit voltage (V_oc), short-circuit current (J_sc) and fill factor of 4.2 mV, 7.4 μA/cm² and 0.25 respectively.     

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.     

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.     

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.     

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.     

Optical properties of ultrananocrystalline diamond/amorphous carbon composite films prepared by pulsed laser deposition

Ultrananocrystalline diamond (UNCD)/amorphous carbon (a-C) composite thin films were grown in ambient hydrogen by pulsed laser deposition using a graphite target, and their optical properties were determined by optical absorption spectroscopy and Raman scattering spectroscopy. Three optical bandgaps exist. Two bandgaps are indirect and their values were estimated to be 1.0 eV and 5.4 eV; these bandgaps correspond to the a-C surrounding the UNCDs and the UNCDs respectively. The third bandgap is direct and has a value of 2.2 eV, which significantly contributes to a large absorption coefficient, (10⁶ cm^− 1 at 3.0 eV). Possible origins of the third bandgaps are the grain boundaries (GBs) between the UNCDs and the a-C since they are specific to the UNCD/a-C composite films. The infrared absorption spectrum and the Raman scattering spectrum revealed the incorporation of hydrogen in the GBs. The hydrogen incorporated in the GBs might also have some influence on the appearance of the direct bandgap and its value.     

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.     

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.     

Effects of iodine doping on optoelectronic properties of diamond-like carbon thin films deposited by microwave surface wave plasma CVD

We report the effects of iodine (I) doping on the electrical and optical properties of diamond-like carbon (DLC) thin films grown on silicon and quartz substrates by microwave surface wave plasma chemical vapor deposition at low temperature (<100 °C). For film deposition, we used argon gas with methane or camphor dissolved with ethyl alcohol composition as plasma source. The optical gap and photoconductivity measurements of the samples were carried out before and after the iodine doping. The results show that optical gap dropped from 3.4 to 0.9 eV corresponding to nondoping to iodine-doping conditions, respectively. The photovoltaic measurements show that the open-circuit voltage (V_oc) and short-circuit current density (J_sc) of I-doped DLC film deposited on n-type silicon substrate under light illumination (AM1.5, 100 mW/cm²) were approximately 177 mV and 1.15 μA, respectively, and the fill factor was found to be 0.217.     

Fourier transform infrared spectroscopic study of nitrogen-doped ultrananocrystalline diamond/hydrogenated amorphous carbon composite films prepared by pulsed laser deposition

Nitrogen-doped ultrananocrystalline diamond (UNCD)/hydrogenated amorphous carbon (a-C:H) composite films, which possess n-type conduction with enhanced electrical conductivities, were prepared by pulsed laser deposition and they were structurally studied by Fourier transform infrared (FTIR) spectroscopy. The film with a nitrogen content of 7.9 at.% possessed n-type condition with an electrical conductivity of 18 S/cm at 300 K. The FTIR spectra revealed peaks due to nitrogen impurities, C = N, C-N, and CH_n (n = 1, 2, 3) bands. The sp²-CH_n/(sp²-CH_n + sp³-CH_n), estimated from the area-integration of decomposed peaks, were 24.5 and 19.4% for undoped and 7.9 at.% doped films, respectively. The nitrogen-doping not only form the chemical bonds between carbon and nitrogen atoms such as C = N and C-N bonds but also facilitate the formation of both sp² and sp³ bonds, in particular, the sp³-CH_n bond is preferentially formed. From the analysis of the FTIR spectra, it was found that the hydrogen content in the film is increased with an increase in the nitrogen content. The increased hydrogen content might be owing to the enhanced volume of grain boundaries (GBs) between UNCD grains, and those between UNCD grains and an a-C:H matrix, which is caused by a reduction in the UNCD grain size. The CH_n peaks predominantly come from an a-C:H matrix and GBs. Since the nitrogen-doping for a-C:H has been known to be hardly effective, the n-type conduction with the enhanced electrical conductivities might be attributed to the sp²-CH_n formation at the GBs.     

Surface modification of nanocrystalline diamond/amorphous carbon composite films

The surfaces of nanocrystalline diamond/amorphous carbon (NCD/a-C) nanocomposite films deposited from a 17% CH₄/N₂ mixture have been subjected to a variety of plasma and chemical treatments, namely H₂ and O₂ microwave plasmas, a CHF₃ 13.56 MHz plasma, and a chemical treatment with aqua regia (HCl:HNO₃ 3:1). The resulting surfaces have been studied with respect to their chemical nature by X-ray photoelectron spectroscopy (XPS) and time of flight secondary ion mass spectrometry (TOF-SIMS), concerning their morphology with atomic force microscopy, and by contact angle measurements to study their hydrophobicity and their stability. As-grown surfaces are hydrogen terminated, but the number of C–H bonds can slightly be increased by a H₂ microwave plasma, while treatment with aqua regia considerably lowers the number of C–H bonds at the surface. O₂ and CHF₃ plasmas, on the other hand, lead to a replacement of the terminating C–H bonds by C–O or C–OH and C–F_x groups, respectively. Finally, by contact angle measurements over a period of 150 days it could be shown that the H-terminated surface is very stable whereas the contact angle of the O-treated surface changed considerably with time, probably due to the adsorption of contaminants.     

A consideration of ternary C-H-O diagram for diamond deposition using microwave in-liquid and gas phase plasma

The purpose of this study is to clarify the mechanism of diamond synthesis using an in-liquid plasma chemical vapor deposition (CVD) method. We investigated the chemical reactions from a liquid mixture of methanol and ethanol (in-liquid plasma CVD) and a gas mixture of methane and hydrogen (gas-phase CVD). Carbon monoxide (CO) is firstly synthesized and then a chemical reaction using the remaining carbon (C) and hydrogen (H) is induced to synthesize a carbon substance. Residual H radicals act as an etchant removing the incompletely binding carbon atom that hinders diamond crystal growth. From spectroscopic measurements, CO peaks were clearly observed when the oxygen component is contained in the raw materials. From the experimental results of carbon deposits using various liquid and gas mixtures as the raw materials, we found that the region of the remaining H and C after CO synthesis satisfying H/C > 20 is necessary to synthesize diamonds using in-liquid plasma CVD method. The region of H/C > 20 in the Bachmann C-H-O diagram nearly agrees with the experimental results of synthesizing diamonds.     

Diamond-like carbon thin films by microwave surface-wave plasma CVD aimed for the application of photovoltaic solar cells

The nitrogen doped diamond-like carbon (DLC) thin films were deposited on quartz and silicon substrates by a newly developed microwave surface-wave plasma chemical vapor deposition, aiming the application of the films for photovoltaic solar cells. For film deposition, we used argon as carrier gas, nitrogen as dopant and hydrocarbon source gases, such as camphor (C₁₀H₁₆O) dissolved with ethyl alcohol (C₂H₅OH), methane (CH₄), ethylene (C₂H₄) and acetylene (C₂H₂). The optical and electrical properties of the films were studied using X-ray photoelectron spectroscopy, Nanopics 2100/NPX200 surface profiler, UV/VIS/NIR spectroscopy, atomic force microscope, electrical conductivity and solar simulator measurements. The optical band gap of the films has been lowered from 3.1 to 2.4 eV by nitrogen doping, and from 2.65 to 1.9 eV by experimenting with different hydrocarbon source gases. The nitrogen doped (flow rate: 5 sccm; atomic fraction: 5.16%) film shows semiconducting properties in dark (i.e. 8.1 × 10^{− 4} Ω^{− 1} cm^{− 1}) and under the light illumination (i.e. 9.9 × 10^{− 4} Ω^{− 1} cm^{− 1}). The surface morphology of the both undoped and nitrogen doped films are found to be very smooth (RMS roughness ≤ 0.5 nm). The preliminary investigation on photovoltaic properties of DLC (nitrogen doped)/p-Si structure show that open-circuit voltage of 223 mV and short-circuit current density of 8.3 × 10^{− 3} mA/cm². The power conversion efficiency and fill factor of this structure were found to be 3.6 × 10^{− 4}% and 17.9%, respectively. The use of DLC in photovoltaic solar cells is still in its infancy due to the complicated microstructure of carbon bondings, high defect density, low photoconductivity and difficulties in controlling conduction type. Our research work is in progress to realize cheap, reasonably high efficiency and environmental friendly DLC-based photovoltaic solar cells in the future.     

The role of pressure on thin amorphous carbon films deposited using microwave surface wave plasma CVD

We report the effects of gas composition pressure (GCP) on the optical, structural and electrical properties of thin amorphous carbon (a-C) films grown on p-type silicon and quartz substrates by microwave surface wave plasma chemical vapor deposition (MW SWP CVD). The films, deposited at various GCPs ranging from 50 to 110 Pa, were studied by UV/VIS/NIR spectroscopy, atomic force microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy and current–voltage characteristics. The optical band gap of the a-C film was tailored to a relatively high range, 2.3–2.6 eV by manipulating GCPs from 50 to 110 Pa. Also, spin density strongly depended on the band gap of the a-C films. Raman spectra showed qualitative structured changes due to sp³/sp² carbon bonding network. The surfaces of the films are found to be very smooth and uniform (RMS roughness < 0.5 nm). The photovoltaic measurements under light illumination (AM 1.5, 100 mW/cm²) show that short-circuit current density, open-circuit voltage, fill factor and photo-conversion efficiency of the film deposited at 50 Pa were 6.4 μA/cm², 126 mV, 0.164 and 1.4 × 10^− 4% respectively.     

Field penetration and its contribution to field enhanced thermionic electron emission from nanocrystalline diamond films

Field emission from sulfur doped nanocrystalline diamond films is characterized by intense emission sites with nm scale diameters. Field emission measurements were obtained at room temperature and analyzed in terms of the Fowler–Nordheim expression where electron emission is due to tunneling through a diminished barrier. The electron emission versus temperature was also recorded at a series of applied fields from 0.5 to 0.8 V/μm. These results were analyzed in terms of a modified Richardson–Dushman relation which describes field dependent thermionic emission. It was found that both sets of data could be fit with a work function of 2.0 eV and a field enhancement factor of  1750. The large field enhancement could not be correlated with specific structures on the relatively flat surfaces. The field and thermionic-field emission from the sulfur doped nanocrystalline diamond films is evaluated by a model which includes barrier lowering as a result of field penetration effects.     

Vibrational properties of nitrogen-doped ultrananocrystalline diamond films grown by microwave plasma CVD

Ultrananocrystalline diamond (UNCD) films grown in an argon-rich Ar/CH₄/H₂ microwave plasma with nitrogen gas added in amounts of 0%–20% were studied by Raman spectroscopy with multiple excitation wavelengths in the range of 244–647 nm and by optical absorption in UV–visible. The Raman spectra have demonstrated the presence of diamond, amorphous carbon and polyacetylene in the UNCD films. Analysis of vibrational and optical properties of amorphous carbon phase proves that nitrogen stimulates the transition from amorphous carbon into an ordered graphite-like structure with narrowed optical band gap, which is supposed to be responsible for the high electrical conductivity of the N-doped UNCD.     

Bonding defects and optical band gaps of DLC films deposited by microwave surface-wave plasma CVD

The effects of CH₄ / C₂H₄ flow ratio and annealing temperature on the defect states and optical properties of diamond-like carbon (DLC) films deposited by novel microwave surface-wave plasma chemical vapour deposition (MW SWP CVD) are studied through UV/VIS/NIR measurements, atomic force microscopy, Raman spectroscopy and electron spin resonance analysis. The optical band gap of DLC has been tailored between a relatively narrow range, 2.65–2.5 eV by manipulating CH₄ / C₂H₄ flow ratio and a wide range, 2.5–0.95 by thermal annealing. The ESR spin density varied between 10¹⁹ to 10¹⁷ spins/cm³ depending on the CH₄ / C₂H₄ flow ratio (1 : 3 to 3 : 1). The defect density increased with increasing annealing temperature. Also, there is a strong dependence of spin density on the optical band gap of the annealed-DLC films, and this dependency has been qualitatively understood from Raman spectra of the films as a result of structural changes due to sp³/sp² carbon bonding network. The surfaces of the films are found to be very smooth and uniform (RMS roughness < 0.5 nm).     

Mechanical properties of DLC films prepared inside of micro-holes by pulse plasma CVD

Diamond-like carbon (DLC) films are metastable amorphous carbon materials with superior tribological characteristics. In order to improve wear resistance of micro-extrusion dies with numerous imperceptible holes, DLC films were deposited on the inner wall surface of model dies with holes of 2 and 0.9 mm in diameter, and 20 mm in depth by using pulse plasma CVD method. This paper will discuss how argon gas, deposition pressure and time affect the characteristics of films deposited on the inner wall surface of dies. This micro-coating method can be applied widely for inner wall surface treatment of components with thin holes.     

Control of abnormal grain inclusions in the nanocrystalline diamond film deposited by hot filament CVD

Formation of abnormal grain inclusions in nanocrystalline diamond films deposited by hot filament CVD (HFCVD) was investigated. The phenomenon was attributed to two different origins: an intrinsic and an extrinsic one. The inclusions due to the intrinsic origin could be either avoided or weakened by controlling chamber pressure, CH₄/N₂ concentrations in H₂, and by positive substrate bias. The extrinsic origin for the abnormal grains was found to be the contamination from the alumina insulation tubes for the thermocouple placed near the substrate, which were degraded by the extended exposure to the high temperature and strongly reducing atmosphere.