Influence of substrate temperature on formation of ultrananocrystalline diamond films deposited by HFCVD argon-rich gas mixture

The influence of the substrate temperature on the formation of ultrananocrystalline diamond (UNCD) thin films, prepared by an argon-based hot filament chemical vapor deposition (HFCVD), is discussed in this work. The gas mixture used for diamond growth was 1 vol.% methane, 9 vol.% hydrogen and 90 vol.% argon at a total flow rate of 200 sccm and at a total pressure of 30 Torr. The substrate temperature range was from 550 to 850 °C at deposition time of 8 h. Mass growth rate was determined at different deposition temperatures. The activation energy for UNCD growth, determined from the Arrhenius plot, was lower (5.7 kcal/mol) than the values found for standard diamond deposition (around 11 kcal/mol). In this work, we suggest that the activation energy was lower because the growth of these films occurs at conditions that there is a high growth competition between diamond phase and sp² phases. To support this hypothesis, systematic characterization studies based on Raman scattering spectroscopy, high-resolution X-ray diffractometry and high-resolution scanning electron microscopy were performed.     

Annealing effects on the mechanical properties of near-frictionless carbon thin films

In this work, the near-frictionless carbon (NFC) thin films developed at Argonne National Laboratory were annealed at 100 °C, 150 °C, 200 °C, 400 °C, and 600 °C in nitrogen atmosphere. The changes of the NFC mechanical properties were measured with both static and dynamic nanoindentation methods. It was found that the Young's modulus and hardness decreased with increasing annealing temperatures. Raman spectroscopy, atomic force microscopy (AFM), and scanning electron microscopy (SEM) were used to characterize the film's structural change before nanoindentation testing. Raman characterization indicated that the G peak shifted upwards as the annealing temperature was increased above 150 °C, which indicated decreasing sp³ content. The intensity of the D peak was shown to increase with annealing temperature indicating that the NFC film became more graphite-like. AFM analysis showed an increase of sp² clustering with annealing temperature, which resulted in an increase in surface roughness. SEM characterization indicated that as the films were annealed large cracks and numerous pinholes were generated. The characterization results were in good agreement with the measured mechanical properties.     

Preparation of tetrahedral amorphous carbon films by filtered cathodic vacuum arc deposition

Tetrahedrally bonded amorphous carbon (ta-C) and nitrogen doped (ta-C:N) films were obtained at room temperature in a filtered cathodic vacuum arc (FCVA) system incorporating an off-plane double bend (S-bend) magnetic filter. The influence of the negative bias voltage applied to substrates (from −20 to −350 V) and the nitrogen background pressure (up to 10⁻³ Torr) on film properties was studied by scanning electron microscopy (SEM), electron energy loss spectroscopy (EELS), Raman spectroscopy, X-ray photoemission spectroscopy (XPS), secondary ion mass spectroscopy (SIMS) and X-ray reflectivity (XRR). The ta-C films showed sp³ fractions between 84% and 88%, and mass densities around 3.2 g/cm³ in the wide range of bias voltage studied. In contrast, the compressive stress showed a maximum value of 11 GPa for bias voltages around −90 V, whereas for lower and higher bias voltages the stress decreased to 6 GPa. As for the ta-C:N films grown at bias voltages below −200 V and with N contents up to 7%, it has been found that the N atoms were preferentially sp³ bonded to the carbon network with a reduction in stress below 8 GPa. Further increase in bias voltage or N content increased the sp² fraction, leading to a reduction in film density to 2.7 g/cm³.     

Hydrogen detection and quantification at polymer surfaces investigated by elastic peak electron spectroscopy (EPES)

The elastic peak electron spectroscopy (EPES) combined with the electron single scattering model calculations is applied for detecting and quantifying hydrogen at a surface region of selected polymers: polyethylenes (low, high densities and ultra high molecular weight - PELD, PEHD, PEUHMW), polypropylene - PP, polystyrene - PS, and poly[methyl(phenyl)silylene] - PMPSi. The physics under the procedure is based on an electron quasi-elastic scattering on a target atom (a recoil effect). The photoelectron C 1s and Auger electron C KLL spectra induced by Mg Kα radiation (1253.6 eV) are used for characterization of the polymer surfaces' damage produced by incident electron irradiation when applying the EPES method. The polymer surfaces irradiated with low electron dose revealed hydrogen content and C sp²/sp³ ratio close to the nominal values. Ongoing electron irradiation led to the hydrogen content decrease and the C sp² percentage growth. The effect of surface charging on the recoil effect is also discussed.     

The effect of negative bias pulse on the bonding configurations and properties of DLC films prepared by PBII with acetylene

The effect of negative bias pulse applied to substrate on the bonding configurations and properties of diamond-like carbon (DLC) films prepared by plasma-based ion implantation (PBII) with acetylene were investigated. The research results show that as the plasma density is 10⁹ cm⁻³ and the negative bias pulses applied to substrate decrease from 50 to 10 kV, the Raman spectra of the carbon films all possess the most dominant characterizations of typical a-C:H [J. Robertson, Mater. Sci. Eng., R 37 (2002) 129-281.], the positions and FWHM for G and D peaks vary no distinguished, but the ratio ID/IG decreases monotonically, as the negative bias pulse decreasing to 5 kV, the Raman spectrum possesses rather strong photoluminescence characterized the polymer-like phase. The variation of Raman spectra for plasma density 10⁸ cm⁻³ is analogous to that of the plasma density 10⁹ cm⁻³. The binding energies of XPS C1s peak decrease from the side of diamond peak to the side of graphite peak with the increasing of negative bias pulse from 10 to 50 kV monotonously, the sp³ content in the films increases with the decreasing in the negative bias pulse. With the increasing of negative bias pulse from 0 to 50 kV, the surface electric resistance of the films decreases monotonously, but the surface nanohardness at first increases with the increasing of the negative bias pulse from 0 to 10 kV, then decreases with the increasing of the negative bias pulse from 10 to 50 kV monotonously. These properties of the films are corresponding to the bonding configurations of the films. The reason for the highest sp³ fractions of the a-C:H films formed at higher effective ion energy per C atom in PBII is discussed in this paper.     

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.     

Substrate bias effect on structure of tetrahedral amorphous carbon films by Raman spectroscopy

Diamond-like carbon (DLC) films form a critical protective layer on magnetic hard disks and their reading heads. Now tetrahedral amorphous carbon films (ta–C) thickness of 2 nm are becoming the preferred means due to the highly sp³ content. In this paper, Raman spectra at visible and ultraviolet excitation of ta–C films have been studied as a function of substrate bias voltage. The spectra show that the sp³ content of 70 nm thick DLC films increases with higher substrate bias, while sp³ content of 2 nm ultra-thin films falls almost linearly with bias increment. And this is also consistent with the hardness measurement of 70 nm thick films. We proposed that substrate bias enhances mixing between the carbon films and either the Si films or Al₂O₃TiC substrate such that thin films contain less sp³ fraction. These mixing bonds are longer than C–C bonds, which inducing the hardness decreasing of ultra-thin DLC films with bias. But for 70 nm DLC, the effect of mixing layer can be negligible by compared to bias effect with higher carbon ion energy. So sp³ content will increase for thick films with substrate bias.     

Chemical analysis and vibrational properties of boronated tetrahedral amorphous carbon films

Boronated tetrahedral amorphous carbon (ta-C:B) films were prepared by filtered cathodic vacuum arc technique using boron mixed graphite targets. The effect of boron content on the chemical bonding and vibrational properties of these films has been investigated by X-ray photoelectron spectroscopy, Raman spectroscopy and Fourier transform infrared spectroscopy. It has been found that boron atoms are predominantly configured in a graphitic network, while the carbon atoms in the ta-C:B films are mainly in sp³ hybridization which tend to decrease as boron content increases. The Raman and infrared spectra of ta-C:B films both show prominent features in the regions of 1100–1900 cm^− 1 and 900–1600 cm^− 1 respectively. It was identified that the Raman parameters are strongly correlated with the boron content which is due to the clustering of sp² domains induced by B introduction. The activation of infrared spectrum of ta-C:B film is a consequence of heteroatomic (C–B) vibration combined with changes in the sp² carbon configuration. And the enhanced infrared absorption of ta-C:B with increased boron incorporation results from the increased effective charges in the delocalized sp² carbon phase.     

Effect of substrate bias voltage on the properties of diamond-like carbon thin films deposited by microwave surface wave plasma CVD

Diamond-like carbon (DLC) thin films were deposited on silicon and ITO substrates with applying different negative bias voltage by microwave surface wave plasma chemical vapor deposition (MW SWP-CVD) system. The influence of negative bias voltage on optical and structural properties of the DLC film were investigated using X-ray photoelectron spectroscopy, UV/VIS/NIR spectroscopy, Fourier transform infrared spectroscopy and Raman spectroscopy. Optical band gap of the films decreased from 2.4 to 1.7 with increasing negative bias voltage (0 to − 200 V). The absorption peaks of sp³ CH and sp² CH bonding structure were observed in FT-IR spectra, showing that the sp²/sp³ ration increases with increasing negative bias voltage. The analysis of Raman spectra corresponds that the films were DLC in nature.     

Effect of deposition voltage on the microstructure of electrochemically deposited hydrogenated amorphous carbon films

Hydrogenated amorphous carbon (a-C:H) films were deposited on Si substrates by electrolysis in a methanol solution at ambient pressure and a low temperature (50 °C), using various deposition voltages. The influence of deposition voltage on the microstructure of the resulting films was analyzed by visible Raman spectroscopy at 514.5 nm and X-ray photoelectron spectroscopy (XPS). The contents of sp³ bonded carbon in the various films were obtained by the curve fitting technique to the C1s peak in the XPS spectra. The hardness and Young’s modulus of the a-C:H films were determined using a nanoindenter. The Raman characteristics suggest an increase of the ratio of sp³/sp² bonded carbon with increasing deposition voltage. The percentage of sp³-bonded carbon is determined as 33–55% obtained from XPS. Corresponding to the increase of sp³/sp², the hardness and Young’s modulus of the films both increase as the deposition voltage increases from 800 V to 1600 V.     

Study of the mechanical properties of tetrahedral amorphous carbon films by nanoindentation and nanowear measurements

Nanoindentation and nanowear measurements, along with the associated analysis suitable for the mechanical characterization of tetrahedral amorphous carbon (ta-C) films are discussed in this paper. Films of approximately 100-nm thick were deposited on silicon substrates at room temperature in a filtered cathodic vacuum arc evaporation system with an improved S-bend filter that yields films with high values of mass density (3.2 g/cm³) and sp³ content (84–88%) when operating in a broad bias voltage range (−20 V to −350 V). Nanoindentation measurements were carried out on the films with a Berkovich diamond indenter applying loads in the 100 μN–2 mN range, leading to maximum penetration depths between 10 and 60 nm. In this measurement range, the ta-C thin-films present a basically elastic behavior with high hardness (45 GPa) and high Young's modulus (340 GPa) values. Due to the low thickness of the films and the shallow penetration depths involved in the measurement, the substrate influence must be taken into account and the area function of the indenter should be accurately calibrated for determination of both hardness and Young's modulus. Moreover, nanowear measurements were performed on the films with a sharp diamond tip using multiple scans over an area of 3 μm², producing a progressive wear crater with well-defined depth which shows an increasing linear dependence with the number of scans. The wear resistance at nanometric scale is found to be a function of the film hardness.     

Thermally stable solar-blind diamond UV photodetector

Thermally stable, deep-ultraviolet (DUV) photodetectors are developed by fabricating a semi-transparent tungsten carbide (WC) or hafnium nitride (HfN) Schottky contact and an annealed Ti/WC Ohmic contact on a boron-doped homoepitaxial p-type diamond layer. Thermal annealing at 500 °C improves the rectifying current–voltage characteristics of the photodiode, resulting in a dramatic enhancement (by a factor of 10³) of DUV responsivity at 220 nm. The discrimination ratio between DUV and visible light is measured to be as large as 10⁶ at a reverse bias voltage as small as 2 V, and it remains almost constant after annealing at 500 °C for 5 h. The short-circuit photocurrent of the HfN Schottky photodiode gradually decreases as the annealing temperature increases, which is well explained by the dependence of the depletion layer width beneath the contact interface on the annealing temperature. In contrast, the short-circuit photocurrent of the WC photodiode is rapidly reduced as the temperature increases. The B doping is found to affect the time response property and reducing the B concentration significantly reduces the response time. Metal carbide and nitride contacts for diamond are thus useful for developing a thermally stable diamond UV photodetector.     

Field emission from tetrahedral amorphous carbon films with various surface morphologies

Field emission properties of tetrahedral amorphous carbon films prepared by filtered cathodic vacuum arc technique have been compared with different surface morphologies. With fewer cycles of conditioning, field emission from relatively rough granular ta-C films on nickel-coated silicon substrates was routinely improved, due to a local field enhancement resulting from both a ‘protrusion-on-protrusion’ geometry and a relatively high sp² content in the film. A 2-MeV ion implantation machine was also employed to intentionally produce local graphitic channels in smooth ta-C films with a high fraction of sp³ content on bare silicon. A relatively low threshold field was obtained from the ta-C film implanted at a dose of 10¹² cm⁻², which still remained an extremely smooth surface. However, for the highly graphitic sample implanted with a higher dose of over 4×10¹³ cm⁻², no electron field emission was observed, even under a very high electric field of 40 V μm⁻¹. Therefore, a suitable sp² content in an sp³ matrix, resulting in graphitic conductive channels in amorphous carbon films to produce a local field enhancement, may be the main factor in obtaining low threshold fields. Furthermore, protrusive structures could further increase the field enhancement factor, due to a ‘protrusion-on-protrusion’ geometry.     

Nitrogen transformations in a flooded soil in the presence and absence of rice plants: 2. Denitrification

Denitrification rates (d) in a flooded alkaline clay were measured following addition of either to the floodwater, by collecting evolved N₂ + N₂O in an enclosure in the absence or presence of rice plants. Similar estimates of d were obtained in the treatment when the isotopic composition of the enclosed atmosphere was determined using arc redistribution or direct mass spectrometric analysis. Approximately 90% of the gaseous products of denitrification were physically trapped in the soil five days after addition. Mechanical shaking of the soil-water system was an effective method for releasing entrapped gas. Denitrification showed a marked diurnal variation in both and treatments planted to rice, with higher rates during the day than at night. Measured rates of denitrification were higher in planted than in unplanted pots for both and treatments for normal gas sampling. However, evidence was obtained that this was not a real effect, but was due to release of entrapped gas. Denitrification losses corrected for gas entrapment were estimated at <5% of applied . The ¹⁵N mass balance indicated that a much larger amount of applied ammonium (15–25%) was lost by NH₃ volatilisation. The rate of denitrification corrected for gas entrapment was similar to the rate of nitrification estimated by inhibition of ammonium oxidation. Although the inhibitors 2-ethynylpyridine and acetylene prevented denitrification by effectively inhibiting nitrification of , the total recovery of ¹⁵N in the soil-plant system did not increase. The total recovery of was 7–9% higher in the presence than in the absence of rice.     

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.     

Evolution of compressive stress and electrical conductivity of tetrahedral amorphous carbon films with phosphorus incorporation

Successful modification of stress and conductivity for tetrahedral amorphous carbon (ta-C) films is realized by phosphorus incorporation via filtered cathodic vacuum arc technique with PH₃ as the impurity source. By establishing the structure as a function of phosphorus content, it is found that phosphorus fraction in phosphorus incorporated ta-C (ta-C:P) films increases with varying levels of PH₃ from 3 to 30 sccm, and that all samples retain their amorphous structures without remarkable changes, just exhibiting the clustering of sp² sites and the evolution of structural ordering. Furthermore, the addition of phosphorus causes the compressive stress relaxation in terms of the rearrangement in atomic bonding structures. The increased number of localized electronic π and π⁎ states as hopping sites after phosphorus incorporation results in several orders of magnitude increase in the conductivity, and the films represent the hopping conduction in band tail states in the temperature range of 293–463 K. However, more H induced by excessive PH₃ may saturate some defects and compensate the hopping sites, leading to a slight drop in the conductivity. The nature of ta-C:P films as n-type semiconductors is proved from the features of rectifying current–voltage cures.     

Formation and characteristics of undoped and doped tetrahedral amorphous carbon films

Synthesis of undoped and doped tetrahedral amorphous carbon (ta-C) films has been achieved using magnetic field filtered plasma stream system in an ambient gas of pure Ar and Ar with N₂, respectively. The optical and electrical properties of these films as a function of the substrate bias voltages (V_b) or nitrogen partial pressures (P_N) have been studied using UV-visible optical absorption spectroscopy, Fourier-transform infra-red spectroscopy (FTIR) and measurements of electrical conductivity. The results show that ta-C films with a high sp³ fraction were formed when the V_b was in the range of −10 to −50 V. The optical band gap of such ta-C films was found to be larger than 3 eV. The incorporation of nitrogen into the ta-C films deposited at low P_N (P_N<25%), results in a slight drop in activation energy, which indicates that there is evidently some doping effect of nitrogen. The configurations of N atoms in ta-C network are identified and discussed.     

Diamond-like carbon films: electron spin resonance (ESR) and Raman spectroscopy

Tetrahedral diamond-like carbon (ta-C) films and hydrogenated a-C:H films were deposited onto Si substrates using filtered cathodic vacuum arc (FCVA) process and direct ion beam deposition from CH₄/C₂H₄ plasma, respectively. Stress of deposited films was varied in the range 2.8–8.5 GPa depending on deposition conditions. Stationary and pulse electron spin resonance (ESR), and Raman spectroscopy techniques were used to analyze sp² related defects in pseudo-gap of undoped as deposited and annealed 20–100 nm thick films.¹ High density of ESR active paramagnetic centers (PC) N_s=(1.0–4.5)×10²¹ cm⁻³ at g=2.0025 was observed in the films. The dependence of ESR line width and line shape vs. deposition conditions and internal film stress were investigated. The several actual mechanisms for ESR line width broadening were considered: spin–spin dipole–dipole and exchange interactions, super-hyperfine interaction (SHFI) with ¹H (for a-C:H), averaging of SHFI due to electron jumps between PC positions with different SHFI values, and broadening due to Mott's electron hopping process. Three types of samples were revealed depending on relative contribution of these mechanisms. Effects of annealing on mechanical and paramagnetic properties of films were studied. An electrical resistance anisotropy at room temperature for ta-C films and g-value anisotropy at low temperature (T<77 K) for both ta-C and a-C:H films were found for the first time. Nature and distribution details of paramagnetic defects in DLC films, anisotropy effects and Raman spectroscopy data are discussed.     

Structure and properties of ta-C films prepared by vacuum cathodic arc with an unbalanced external electromagnetic field

ta-C thin films were deposited by vacuum cathodic arc technology with an unbalanced external electromagnetic field. The intensity of the magnetic field was adjusted by changing coil current and its effect on structure and properties of ta-C films was studied. FEM simulation results showed that as coil currents increased, the transverse, normal magnetic field and the magnetic intensity from arc cathode to substrates holder gradually increased. The UBM system increases the magnetic field intensity of the entire vacuum chamber by mainly improving the normal magnetic flux density near the arc cathode. The surface morphology, microstructure, tribo-mechanical properties of ta-C films were analyzed by SEM, Raman spectrometer, XPS, 3D surface profiler, Nano-indentation and ball-on-disk tribometer. Results showed that as the coil current increased, the average target voltage increased from 21.14 V to 25.97 V and target power rose from 845 W to 1043 W. Changes of magnetic field affected the plasma in vacuum chamber and the number of ions reaching the substrates, which had significant influences on film structure and properties. With a proper magnetic field (at coil current of 4A), ta-C films perform the highest hardness of 58.8 GPa, and the highest sp³ content of about 71% with the lowest ratio of I_D/I_G (0.38). During the wear process, the transfer layer was formed by oxidation and graphitization of contact surfaces. High hardness provided good support for transfer film, which leads to a low wear rate.     

Microstructure effect on field emission from tetrahedral amorphous carbon films annealed in nitrogen and acetylene ambient

Tetrahedral amorphous carbon (ta-C) films have been deposited by filtered cathodic vacuum arc technique. The samples were then annealed at various temperatures in nitrogen and acetylene ambient. The surface morphologies and microstructure of the films were characterized using atomic force microscopy, scanning electron microscopy, visible and ultraviolet Raman spectroscopy. A thin layer of amorphous carbon was deposited on the surface of the ta-C films after annealed at 700 and 800 °C while submicro crystalline pyrolytic graphite was formed on the surface of the ta-C film annealed at 900 °C. The surface layer was found to enhance the sp² clustering of the underlying ta-C layer. Field emission results reveal that the sp² cluster size plays an important role in electron field emission properties. The threshold field decreases as the sp² cluster size increases. For the film annealed at 800 °C, the lowest threshold field and the largest cluster size concurred.