Synthesis and electron field emission properties of nanodiamond films

Effects of CH₄/H₂ ratio and bias voltage of the microwave plasma-enhanced chemical vapor deposition (MPE-CVD) process on the nucleation behavior and associated characteristics of nanodiamonds were investigated. While the scanning electron microscopy (SEM) microstructure and Raman crystal structure of the films insignificantly vary with CH₄/H₂ ratio and bias voltage, electron field emission properties of the materials markedly change with these deposition parameters. The predominating factor modifying the electron field emission properties of the nanodiamond films is presumed to be the increase in the proportion of sp²-bonded grain boundaries when the grain size of the nanodiamond films decreases. Between these two major factors, the bias voltage shows more prominent effects on modifying the granular structure of the nanodiamonds than the CH₄/H₂ ratio does. The best electron field emission properties attainable are J_e=500 μA/cm² at 20 V/μm and E₀=8.5 V/μm.     

Biased enhanced growth of nanocrystalline diamond films by microwave plasma chemical vapor deposition

Smooth nanocrystalline diamond thin films with rms surface roughness of ∼17 nm were grown on silicon substrates at 600°C using biased enhanced growth (BEG) in microwave plasma chemical vapor deposition (MPCVD). The evidence of nanocrystallinity, smoothness and purity was obtained by characterizing the samples with a combination of Raman spectroscopy, X-ray diffraction (XRD), atomic force microscopy and Auger electron spectroscopy. The Raman spectra of the films exhibit an intense band near 1150 cm⁻¹ along with graphitic bands. The former Raman band indicates the presence of nanocrystalline diamond. XRD patterns of the films show broad peaks corresponding to inter-planar spacing of (111) and (220) planes of cubic diamond supporting the Raman results. Auger line shapes closely match with the line shape of diamond suggesting high concentration of sp³ carbon on the surfaces of the films. The growth of dominantly sp³ carbon by BEG in the MPCVD system at the conditions used in the present work can be explained by the subsurface implantation mechanism while considering some additional effects from the high concentration of atomic hydrogen in the system.     

Growth of nanocrystalline diamond films by biased enhanced microwave plasma chemical vapor deposition

Nanocrystalline diamond (NCD) films were grown using biased enhanced growth (BEG) in microwave plasma chemical vapor deposition on mirror polished silicon substrates at temperatures in the range from 400 to 700°C. The films were characterized by Raman spectroscopy, X-ray diffraction (XRD), Auger electron spectroscopy and atomic force microscopy (AFM). Hardness of the films was measured by nano-indentor. Apart from graphitic D and G bands in the films, the Raman spectra exhibit NCD features near 1140 cm⁻¹. The relative intensity of the NCD to graphitic G band in the Raman spectra of the films is negligible in the films grown at 400°C. It increases with temperature and attains a maximum at 600°C following a sharp decrease in the films grown at higher temperatures. XRD results also indicate a maximum concentration of NCD in the film grown at 600°C. Average hardness of the films increases with temperature from ∼5 GPa to ∼40 GPa up to 600°C followed by a decrease (∼24 GPa) in the film grown at 700°C. Substrate temperature seems to play a crucial role in the growth of NCD in BEG processes. An increase in growth temperature may be responsible for evolving bonded hydrogen and increasing mobility of carbon atoms. Both factors help in developing NCD in the films grown at 500 and 600°C with a combination of subplantation mechanism, due to biasing, and a high concentration of H atoms in the gas-phase, typical of CVD diamond process. At 700°C the implanted carbon atoms may be migrating back to the surface resulting in domination of surface processes in the growth, which in turn should result in increase in graphitic content of the films at such a high methane concentration and continuous biasing used in the present study.     

The effect of hydrogen plasma chemical annealing on electron field emission of amorphous carbon films

Layer-by-layer deposition method, in which nanometer-thick film deposition and hydrogen plasma annealing processes were alternatively repeated, was applied to fabricate hydrogenated amorphous carbon films in our present work. It was found that the hydrogen plasma treatment changed the sp²/sp³ ratio due to chemical etching. Consequently, a stable vacuum electron emission with a low threshold field was achieved compared with that from conventionally deposited a-C films. The threshold electric field is as low as 2 V/μm. The influence of the hydrogen plasma chemical annealing on the field emission behavior was systematically investigated. The improvement of field emission characteristics can be attributed to the large field enhancement effect due to the inhomogeneous distribution of nanometer scale sp² clusters.     

Preparation of diamond whiskers using Ar/O2 plasma etching

Radio frequency (RF) plasma etching of chemical vapor deposition (CVD) diamond film has been investigated in Ar/O₂ plasmas, with an emphasis to elucidate the effects of reacting gas on the fabrication of diamond whiskers. Diamond whiskers were formed on diamond films pre-coated with Al. It was found that diamond whiskers preferentially formed at the diamond grain boundaries. The densities of diamond whiskers increased with O₂ / Ar ratio. Whiskers obtained in pure O₂ plasma etching were 50 nm in diameter and 1 μm in height. The etching rate was increased by mixing Ar with appropriate volume of O₂. Al coated on the diamond surface reacted with O₂ to form Al₂O₃, serving as mask to restrain the etching underneath. Raman spectroscopy measurement confirmed that the whiskers kept sp³ diamond bonding structure after RF plasma etching. The field emission characteristics of the whiskers were also inspected.     

Study the effect of O2 addition on hydrogen incorporation in CVD diamond

The effect of a small amount of O₂ addition on film quality and hydrogen incorporation in chemical vapour deposition (CVD) diamond films was investigated and the films were grown using a 5-kW microwave plasma CVD reactor. Film quality and bonded hydrogen were characterized using micro-Raman and Fourier transform infrared (FTIR) spectroscopy, respectively. It was found that in general for films grown using CH₄/H₂ plasma both without and with O₂ addition, the hydrogen incorporation increases with increasing substrate temperature, while a small amount of O₂ addition (O₂/CH₄=0.1) into CH₄/H₂ (4%) plasma strongly suppresses the incorporation of hydrogen into the film. Raman spectra show that the added oxygen improved film quality by etching and suppressing the amorphous carbon component formed in the film. The above effect of oxygen addition on hydrogen incorporation and film quality is discussed according to the growth mechanism of CVD diamond. The CVD diamond specific hydrogen related IR vibration at 2828 cm⁻¹ appears as a sharp and strong peak only in the FTIR spectra of poor quality films grown at high temperature both without and with O₂ addition, but it appears much stronger in the film grown without O₂ addition. This result experimentally excludes the assignment of the 2828 cm⁻¹ peak arises from hydrogen bonded to oxygen related defect in the literature.     

Cathodoluminescence and electron field emission of boron-doped a-C:N films

The optical and electrical properties of so-called carbon nitride films (a-C:N) and boron doped so-called carbon nitride films (a-C:N:B) are studied with cathodoluminescence (CL) spectroscopy and electron field emission measurement. The a-C:N films were first deposited on Si by a filtered cathodic arc plasma system, and then boron ions (∼1 × 10¹⁶ cm⁻²) were implanted into the a-C:N films to form a-C:N:B films by a medium current implanter. The structural and morphological properties of a-C:N and a-C:N:B films were then analyzed using secondary ion mass spectrometer, X-ray photoelectron spectroscopy, FT-IR spectra, Raman spectroscopy and atomic force microscopy. The a-C:N film exhibits luminescence of blue light (∼2.67 eV) and red light (∼1.91 eV), and the a-C:N:B film displays luminescence of blue light (∼2.67 eV) in CL spectra measured at 300 K. Furthermore, the incorporated boron atoms change the electron field emission property, which shows a higher turn on field for the a-C:N:B film (3.6 V/μm) than that for the a-C:N film (2.8 V/μm).     

Nanodiamond planar lateral field emission diode

We report the fabrication and field emission characteristics of the nanodiamond planar lateral field emission diode. Nanodiamond films with grain size as small as 5–10 nm have been realized through the process of CH₄/H₂/N₂ microwave plasma enhanced chemical vapor deposition (MPECVD) by employing an effective growth rate reduction technique. Well-controlled processes have been developed; including reactive ion etch (RIE) to pattern the nanodiamond films to fabricate lateral field emission devices with planar lateral fingers. An anode–cathode spacing of 2 μm between the nanodiamond anode and cathode has been achieved. A nanodiamond lateral diode equipped with 6 fingers and an inter-electrode separation of 3 μm exhibits a turn-on voltage of 5.9 V (threshold electric field of 1.9 V/μm), one of the lowest reported for lateral field emission devices, and a high emission current of 1.1 mA ( 183 μA current per finger) at an anode voltage of 100 V ( 30 V/μm). The emission current is found to be stable with 4% fluctuation at 1 mA over 10 h. The nanodiamond lateral device is very promising for applications in vacuum nanoelectronics, sensors, and nanoelectromechanical systems.     

Characterization of diamond fluorinated by glow discharge plasma treatment

The surface fluorination of diamond by treatment in glow discharge plasmas of CF₄ for different times has been investigated. High quality diamond films were deposited onto silicon substrates using hot filament chemical vapor deposition (HFCVD). Subsequently, the films were exposed to a radiofrequency glow discharge plasma of CF₄ for times ranging from 5 min to 1 h. The effects of the plasma treatment on the surface morphology, diamond quality and elemental composition were investigated using atomic force microscopy (AFM), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS), respectively. Differences in film roughness caused by the plasma treatment were detected by AFM and confirmed by scanning electron microscopy (SEM). Raman spectroscopic analyses showed that the original diamond was of high quality and that the bulk of each film was unchanged by the plasma treatment. Analyses using XPS revealed increased surface fluorination of the films at longer treatment times. In addition, the density of free radicals in the films was probed using electron paramagnetic resonance spectroscopy (EPRS), revealing that untreated diamond possesses an appreciable density of free radicals (6×10¹² g⁻¹) which initially falls with treatment time in the CF₄ plasma but increases for long treatment times.     

Tribological properties of DLC films with different hydrogen contents in water environment

Diamond-like carbon (DLC) films were deposited on silicon wafers by thermal electron excited chemical vapor deposition (CVD). To change the hydrogen content in film, we used three types of carbon source gas (C₇H₈, CH₄, and a CH₄+H₂) and two substrate bias voltages. The hydrogen content in DLC films was analyzed using elastic recoil detection analysis (ERDA). Tribological tests were conducted using a ball-on-plate reciprocating friction tester. The friction surface morphology of DLC films and mating balls was observed using optical microscopy and laser Raman spectroscopy.Hydrogen content in DLC films ranged from 25 to 45 at.%. In a water environment, the friction coefficient and specific wear rate of DLC films were 0.07 and in the range of 10⁻⁸–10⁻⁹ mm³/Nm, respectively. The friction coefficient and specific wear rate of DLC film in water were hardly affected by hydrogen content. The specific wear rate of DLC film with higher hardness was lower than that of film with low hardness. Mating ball wear was negligible and the friction surface features on the mating ball differed clearly between water and air environments, i.e., the friction surface on mating balls in water was covered with more transferred material than that in air.     

The effect of argon on the electron field emission properties of α-C:N thin films

Amorphous carbon nitride (α-C:N) thin films were synthesized on silicon as electron emitters by the electron cyclotron resonance chemical vapor deposition (ECR-CVD) system in which a negative dc bias was applied to the graphite substrate holder and a mixture of C₂H₂ and N₂ was used as precursors. The addition of Ar combined with the application of a negative dc bias can increase nitrogen content (N/C) measured by X-ray photoelectron spectroscopy (XPS), eliminate the dangling bonds in the film determined by Fourier transform infrared (FTIR) spectroscopy, decrease the film thickness measured by field emission scanning electron microscope (FE-SEM), increase the film roughness measured by atomic force microscope (AFM) and raise the graphitic content examined by Raman spectroscopy. The result shows that the onset emission field of α-C:N with Ar addition to the precursors can be as low as 4.5 V μm⁻¹ compared with 9.5 V μm⁻¹ of the film without the addition of Ar.     

Enhancement of the field emission properties of low-temperature-growth multi-wall carbon nanotubes by KrF excimer laser irradiation post-treatment

Multi-wall carbon nanotube (CNT) films were fabricated by microwave plasma chemical vapor deposition at low temperatures ( 500 °C). The films when properly post-treated by laser irradiation exhibited a factor of 2–3 enhancement in the emission current, while the turn-on field (E_on) was reduced from 4.89–5.22 to 2.88–3.15 V/μm. The introduction of excessive oxygen during laser irradiation, however, degrades the performance of field emission properties drastically. Raman spectroscopy measurements revealed the intimate correlation between the parameter I_D/I_G (intensity ratio between the two representative Raman peaks seen in carbon nanotubes) and the field emission performance. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses showed that the irradiation-induced modification of the tube morphology and crystallinity might be responsible for the observations.     

Deposition of amorphous carbon films from C60 fullerene sublimated in electron beam excited plasma

C₆₀ fullerene clusters are used as a carbon source for amorphous carbon films deposition in an electron beam excited plasma. C₆₀ clusters are sublimated by heating a ceramic crucible containing the C₆₀ powders up to 850 °C, which is located in a highly vacuumed process chamber. The sublimated fullerene powders are injected to the electron beam excited argon plasma and dissociated to be active species that are propelled toward the substrates. Consequently, the carbon species condense as a thin film onto the negatively biased substrates that are immersed in the plasma. Deposition rates of approximately 1.0 μm/h and the average surface roughness of 0.2 nm over an area of 400 μm² are achieved. Decomposition of the C₆₀ fullerene after injecting into the plasma is confirmed by optical emission spectroscopy that shows existence of small carbon species such as C₂ in the plasma. X-ray diffraction pattern reveals that the microstructure of the film is amorphous, while fullerene films deposited without the plasma show crystalline structure. Raman spectroscopic analysis shows that the films deposited in the plasma are one of the types of diamond-like carbon films. Different negative bias voltages have been applied to the substrate holder to examine the effect of the bias voltage to the properties of the films. The nano-indentation technique is used for hardness measurement of the films and results in hardness up to about 28 GPa. In addition, the films are droplet-free and show superior lubricity.     

Effect of microwave power on diamond-like carbon films deposited using electron cyclotron resonance chemical vapor deposition

Diamond-like carbon (DLC) films have been deposited using electron cyclotron resonance chemical vapor deposition (ECR-CVD) under various microwave power conditions. Langmuir probe measurement and optical emission spectroscopy (OES) were used to characterize the ECR plasma, while the films were characterized using Raman and infrared (IR) spectroscopies, hardness and optical gap measurements. It was found that the ion density and all signal peaks in the optical emission (OE) spectra increased monotonously following the increase in microwave power. Raman spectra and optical gap measurements indicate that the films become more graphitic with lower content of sp³-hybridized carbon atoms as the microwave power was increased. IR and hardness measurements indicate a reduction in hydrogen content and decrease in hardness for the film produced at relatively high microwave powers. A deposition mechanism is described which involved the ion bombardment of film surfaces and hydrogen–surface interactions. The deposition rate of DLC film is correlated to the ion density and CH₃ density.     

Electron field emission from defective diamond films deposited on chrome electrode

Deposition of the good electron-emitting diamond films on a chrome electrode, which is essential for the development of the actual display device, was successfully carried out. Emission current densities of 1 μA/cm² and 1 mA/cm² were measured at the electric field of 6.6 and 12.3 V/μm, respectively. The emission images revealed that the emission site density was ∼10⁴ sites/cm². Both Raman spectroscopy and scanning electron microscopy showed that these were defective diamond films, similar to those deposited on silicon substrates under similar deposition conditions. Comparing the emission characteristics of the films deposited on silicon and on chrome, we conclude that the interface between the back contact and the film is not the current-limiting factor. Moreover, we discuss the importance of the inclusion of sp²-bonded carbons for good electron emission.     

Field emission of polycrystalline diamond films grown by microwave plasma chemical vapor deposition. II. Effect of p-type doping in diamond

The effects of boron (B) doping on the field emission (FE) of diamond films grown by a microwave plasma chemical vapor deposition technique were studied. Raman scattering spectroscopic analysis revealed that B-doping significantly suppressed formation of non-diamond components in the diamond film. The B-doped p-type diamond films had low resistivity, ranging from 0.07 to 20 Ω cm, and various volume fractions of non-diamond components in the diamond films. The turn-on electric field, F_T, was independent of the resistivity, the film thickness, and the volume fraction of the non-diamond components. The lowest F_T value of 8 V μm⁻¹ and the highest emission current of 3×10⁻² A cm⁻² were obtained in the B-doped diamond films. The high efficiency of the electron emission in the B-doped diamond films was believed to be due to the increase in volume fraction of the conductive regions in the film and the high density of emission sites on the film surface.     

Field emission from patterned carbon nanotube emitters produced by microwave plasma chemical vapor deposition

Large area carbon nanotube patterns were fabricated by microwave plasma chemical vapor deposition. The carbon nanotubes were grown on pre-patterned catalyst films. Scanning electron microscopy and Raman spectroscopy were used to characterize the structure of the carbon nanotubes. The carbon nanotubes were very uniform and approximately 100 nm in diameter. The Raman spectrum shows a good graphitization for the carbon nanotubes. Aligned growth was found on the pattern line area. Field emission characteristics of the patterns were characterized. A threshold field of 2.0 V/μm and emission current density of 1.1 mA/cm² at 3.6 V/μm were achieved. A clear and stable image showing the patterns were obtained.     

Low-temperature synthesis of graphene on Cu using plasma-assisted thermal chemical vapor deposition

Plasma-assisted thermal chemical vapor deposition (CVD) was carried out to synthesize high-quality graphene film at a low temperature of 600°C. Monolayer graphene films were thus synthesized on Cu foil using various ratios of hydrogen and methane in a gaseous mixture. The in situ plasma emission spectrum was measured to elucidate the mechanism of graphene growth in a plasma-assisted thermal CVD system. According to this process, a distance must be maintained between the plasma initial stage and the deposition stage to allow the plasma to diffuse to the substrate. Raman spectra revealed that a higher hydrogen concentration promoted the synthesis of a high-quality graphene film. The results demonstrate that plasma-assisted thermal CVD is a low-cost and effective way to synthesis high-quality graphene films at low temperature for graphene-based applications.     

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.