Field emission studies of CVD diamond thin films: effect of acid treatment

Field emission from CVD diamond thin films deposited on silicon substrate has been studied. The diamond films were synthesized using hot filament chemical vapor deposition technique. Field emission studies of as-deposited and acid-treated films were carried out using ‘diode’ configuration in an all metal UHV chamber. Upon acid treatment, the field emission current is found to decrease by two orders of magnitude with increase in the turn-on voltage by 30%. This has been attributed to the removal of sp² content present in the film due to acid etching. Raman spectra of both the as-deposited and acid-treated films exhibit identical spectral features, a well-defined peak at 1333 cm⁻¹ and a broad hump around 1550 cm⁻¹, signatures of diamond (sp³ phase) and graphite (sp² phase), respectively. However upon acid treatment, the ratio (I_d/I_g) is observed to decrease which supports the speculation of removal of sp² content from the film. The surface roughness was studied using atomic force microscopy (AFM). The AFM images indicate increase in the number of protrusions with slight enhancement in overall surface roughness after acid etching. The degradation of field emission current despite an increase in film surface roughness upon acid treatment implies that the sp² content plays significant role in field emission characteristics of CVD diamond films.     

Growth and characterization of diamond particles,diamond films,and CNT-diamond composite films deposited simultaneously by hot filament CVD

It is important to understand the growth of CNT-diamond composite films in order to improve the inter-link between two carbon allotropes, and, in turn, their physical properties for field emission and other applications. Isolated diamond particles, continuous diamond thin films, and thin films of carbon nanotubes (CNTs) having non-uniformly distributed diamond particles (CNT-diamond composite films) were simultaneously grown on unseeded, seeded, and catalyst pre-treated substrates, respectively, using a large-area multi-wafer-scale hot filament chemical vapor deposition. Films were deposited for four different growth durations at a given deposition condition. The changes in surface morphology and growth behavior of diamond particles with growth duration were investigated ex situ using field emission scanning electron microscopy and 2D confocal Raman depth spectral imaging, respectively. A surface morphological transition from faceted microcrystalline nature to nanocrystalline nature was observed as a function of growth duration in the case of isolated diamond particles grown on both unseeded and catalyst pre-treated substrates. However, such a morphological transition was not observed on the simultaneously grown continuous diamond thin films on seeded substrates. 2D confocal Raman depth spectral imaging of diamond particles showed that the local growth of CNTs did not affect the growth behavior of neighboring diamond particles on catalyst pre-treated substrates. These observations emphasize the importance of surface chemical reactions at the growth site in deciding sp² or sp³ carbon growth and the final grain size of the diamond films.     

Field emission properties of composite nano-Au/DLC films prepared by CVD technique

Nanocrystalline gold incorporated diamond-like carbon (nano-Au/DLC) films were deposited by capacitively coupled plasma (CCP) r.f. chemical vapour deposition (CVD) technique. Gold content in the DLC matrix was controlled by the amount of argon in the argon + methane mixture in the plasma. Field emission properties of these films were studied critically. Bonding environment (sp²/sp³ ratio) in these films was obtained from Raman measurements. Modification of the surface with the incorporation of gold nanocrystallites and associated modulation of sp²/sp³ ratio in the films culminated in improved field emission properties. Fowler-Nordheim model was used to ascertain the work function (?) which varied between 19 and 64 meV. The field factor (β) varied between 172 and 1050.     

Synthesis of carbon films containing diamond particles by electrolysis of methanol

Carbon films containing diamond particles were deposited onto a Si (100) substrate by electrolysis of methanol under a direct current potential of 1200 V, with a current density of about 52 mA/cm², at atmospheric pressure and in the temperature range of 50-55 °C. The surface morphology, microstructure and crystalline structure of the deposited films were characterized by scanning electron microscopy (SEM), Fourier transformation infrared (FTIR) spectroscopy, Raman spectroscopy and transmission electron microscopy (TEM) respectively. The SEM images show that the films are formed by particle clusters and a surrounding glassy phase. The Raman spectra of the films indicate that the particle clusters are composed of diamond and that the glassy phase is composed of amorphous carbon. The FTIR measurements suggest the existence of hydrogen which is mainly bonded to the sp³ carbon in the films. The transmission electron diffraction patterns further indicate that the particles in the films consist of single-crystalline diamond. Both TEM and Raman measurements have confirmed unambiguously the formation of diamond crystals in the deposit, although the particles are not uniformly distributed on the entire surface.     

Enhanced field emission from ZnO nanoneedles on chemical vapour deposited diamond films

ZnO nanoneedles were coated on hot filament chemical vapour deposited diamond thin films to enhance the field emission properties of ZnO nanoneedles. The virgin diamond films and ZnO nanoneedles on diamond films were characterized using scanning electron microscopy, X-ray photoelectron spectroscopy and Raman spectroscopy. The field emission studies reveal that the ZnO nanoneedles coated on diamond film exhibit better emission characteristics, with minimum threshold field (required to draw a current density ~ 1 μA/cm²) as compared to ZnO needles on silicon and virgin diamond films. The better emission characteristic of ZnO nanoneedles on diamond film is attributed to the high field-enhancement factor resulting due to the combined effect of the ZnO nanoneedles and diamond film.     

Effects of applied power on hydrogenated amorphous carbon (a-C:H) film deposition by low frequency (60 Hz) plasma-enhanced chemical vapor deposition (PECVD)

Hydrogenated amorphous carbon (a-C:H) films were deposited onto glass substrates using low frequency (60 Hz) plasma-enhanced chemical vapor deposition and the effects of the applied power on a-C:H films deposition were investigated. During deposition, the electron temperature and the density of CH₄-H₂ plasma were 2.4-3.1 eV and about 10⁸ cm⁻³, respectively. The main optical emission peak of the carbon species observed in the CH₄-H₂ plasma is shown to be excited carbon CH* at 431 nm. The sp³/sp² ratio, band gap, hydrogen content, and refractive index of a-C:H films gradually increased up to a power of 25 W and then saturated at higher power. This tendency is similar to the variation of plasma parameters with varying applied power, thereby indicating that a strong relationship exists between the properties of the films and the plasma discharge.     

Effect of rapid thermal annealing treatment on the field-emission characteristics of nanocrystalline diamonds grown on various metal/silicon substrates

In this study, nanocrystalline diamond (NCD) films were deposited on various metal/silicon substrates using a microwave plasma chemical vapor deposition system. Metal layers used are chromium, titanium, aluminum and were used as the electron source for field emitters. These NCD/metal/silicon structures were subsequently annealed at 500 °C in a rapid thermal annealing (RTA) furnace. After RTA treatment, the surface of NCD films becomes flat and the grain boundaries can no longer be clearly seen. The intensity of graphitic peak is substantially decreased and the sp³ content of NCD films is increased. The chemical composition of NCD film remains unchanged after RTA treatment, but the sp³/sp² ratio in C 1s has been increased. It is found that the field-emission characteristics of diamond emitter not only can be effectively controlled by the metal used in the metal/NCD/Si structure, but also can be further enhanced by the improved microstructure of the NCD film obtained after RTA treatment.     

Plasma-enhanced chemical vapor deposition of nanocrystalline diamond

Nanocrystalline diamond films have attracted considerable attention because they have a low coefficient of friction and a low electron emission threshold voltage. In this paper, the author reviews the plasma-enhanced chemical vapor deposition (PE-CVD) of nanocrystalline diamond and mainly focuses on the growth of nanocrystalline diamond by low-pressure PE-CVD. Nanocrystalline diamond particles of 200–700 nm diameter have been prepared in a 13.56 MHz low-pressure inductively coupled CH₄/CO/H₂ plasma. The bonding state of carbon atoms was investigated by ultraviolet-excited Raman spectroscopy. Electron energy loss spectroscopy identified sp²-bonded carbons around the 20–50 nm subgrains of nanocrystalline diamond particles. Plasma diagnostics using a Langmuir probe and the comparison with plasma simulation are also reviewed. The electron energy distribution functions are discussed by considering different inelastic interaction channels between electrons and heavy particles in a molecular CH₄/H₂ plasma.     

Tem characterization of nanodiamond thin films

The microstructure of thin films grown by microwave plasma-enhanced chemical vapor deposition (MPCVD) from fullerene C₆₀ precursors has been characterized by scanning electron microscopy (SEM), selected-area electron diffraction (SAED), bright-field electron microscopy, high-resolution electron microscopy (HREM), and parallel electron energyloss spectroscopy (PEELS). The films are composed of nanosize crystallites of diamond, and no graphitic or amorphous phases were observed. The diamond crystallite size measured from lattice images shows that most grains range between 3–5 nm, reflecting a gamma distribution. SAED gave no evidence of either sp²-bonded glassy carbon or sp³-bonded diamondlike amorphous carbon. The sp²-bonded configuration found in PEELS was attributed to grain boundary carbon atoms, which constitute 5–10% of the total. Occasionally observed larger diamond grains tend to be highly faulted.     

Low-temperature plasma-enhanced chemical vapor deposition of hard carbon films

Carbon films were prepared by the plasma-enhanced chemical vapor deposition method from methane gas under different regimes of a growing film surface ion bombardment. Ion energy and ion flux was measured for different deposition regimes and were controllably and independently varied for deposition of different film samples. The structure of the films deposited was studied by Raman scattering spectroscopy. It was found that ion bombardment of the growing film surface is a key factor in hard carbon films deposition. A correlation between the sp³ hybridized carbon fraction content with both ion energy and ion flux was found. The maximum sp³ hybridized carbon fraction content was achieved for films deposited under maximum ion energy and ion flux conditions. Additionally, a dependence of the film properties on the substrate material and on the final film thickness was observed.     

Chemical bond analysis of amorphous carbon films

Hydrogen-free carbon films with the various sp³ bond fractions between 83% and 40% were prepared by mass-separated ion beam deposition (MSIBD). These sp³ bond fractions were obtained by electron energy loss spectroscopy (EELS). Chemical bond analysis of these carbon films was performed by x-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and Raman spectroscopy, and the comparison of these methods was examined. XPS C1s spectra of carbon films show two contributions at the energies of 284.5 and 285.5 eV, which are originated from sp²-bonds and sp³ bonds, respectively. The sp³ bond fractions obtained by XPS are in good agreement with the values given by EELS. The fine structure of AES spectra at the kinetic energy region between 245 and 265 eV reflects the sp³ bond fraction. AES spectra are changed from the diamond-like feature to the graphite-like one with decreasing the sp³ bond fraction. Raman spectra show two broad peaks of G band and D band, the ratio of two peak intensities is independent on the sp³ bond fraction of films. The shift of G peak position has a correlation with the sp³ bond fraction in the sp³ bond rich region.     

Plasma-enhanced chemical vapor deposition of nanocrystalline diamond

Nanocrystalline diamond films have attracted considerable attention because they have a low coefficient of friction and a low electron emission threshold voltage. In this paper, the author reviews the plasma-enhanced chemical vapor deposition (PE-CVD) of nanocrystalline diamond and mainly focuses on the growth of nanocrystalline diamond by low-pressure PE-CVD. Nanocrystalline diamond particles of 200–700 nm diameter have been prepared in a 13.56 MHz low-pressure inductively coupled CH₄/CO/H₂ plasma. The bonding state of carbon atoms was investigated by ultraviolet-excited Raman spectroscopy. Electron energy loss spectroscopy identified sp²-bonded carbons around the 20–50 nm subgrains of nanocrystalline diamond particles. Plasma diagnostics using a Langmuir probe and the comparison with plasma simulation are also reviewed. The electron energy distribution functions are discussed by considering different inelastic interaction channels between electrons and heavy particles in a molecular CH₄/H₂ plasma.     

Effects of SiNx interlayer on characterisation of amorphous diamond-like carbon films

The magnetron sputtering amorphous diamond-like carbon film is successfully deposited by SiN_x interlayer approach. The scanning electron microscopy study reveals the creation of high uniform surface micrograph diamond-like carbon films with SiN_x interlayer. For comparison, diamond-like carbon films with different interlayers are also grown. The Raman spectra are analyzed in order to characterize the stressed induce peak shifts of the films. The interactions of C atom with Si(100) and SiN_x surface are studied by density functional theory simulation. The effects of interlayers on the films deposition and the considering deposition mechanism are discussed. It is suggested that the diamond-like carbon and SiN_x bilayer structure can help to render applications in protective coatings and high quality silicon on diamond related radiation tolerance devices.     

Synthesis of functional diamond-like carbon nanocomposite films containing titanium dioxide nanoparticles

Synthesis of diamond-like carbon (DLC) films with UV-induced-hydrophilicity function was studied by inductively-coupled plasma (ICP) chemical vapor deposition. Titanium tetraisopropoxide (TTIP) and oxygen gases were employed as the precursors to deposit diamond-like nanocomposite films containing titanium dioxide (TiO₂) nanoparticles. X-ray diffraction and high-resolution transmission electron microscopy revealed that TiO₂ nanocrystallites were formed in the DLC films when oxygen concentration was higher than TTIP concentration during deposition. The DLC nanocomposite film was hydrophobic without ultraviolet (UV) irradiation, and became highly hydrophilic under UV irradiation, exhibiting the self-cleaning effect. A very broad peak centered at 1580 cm^− 1 was observed in the Raman spectra confirming the formation of DLC films. The hardness of the film was about 8 GPa with a stress of 3 GPa. ICP was essential in forming the photocatalytic TiO₂ nanoparticles in the DLC matrix.     

Field emission characteristics of fast grown nanocrystalline diamond/amorphous carbon composite films by microwave plasma-enhanced chemical deposition method

This study synthesized the nanocrystalline diamond/amorphous carbon (NCD/a-C) composite films by the microwave plasma-enhanced chemical vapor deposition (MPCVD) system with Ar/CH₄/N₂ mixtures. A localized rectangular-type jet-electrode with high density plasma was used to enhance the formation of NCD/a-C films, and a maximum growth rate of 105.6 µm/h was achieved. The content variations of sp² and sp³ phases via varying nitrogen gas flow rates were investigated by using Raman spectroscopy. The NCD/a-C film which synthesized with 6% nitrogen concentration and no hydrogen plasma etching treatment possessed a low turn-on electric field of 3.1 V/µm at the emission current of 0.01 µA.     

Deposition of diamond-like carbon films by using liquid phase electrodeposition technique and its electron emission properties

We have deposited diamond-like carbon (DLC) films by electrodeposition technique in methanol liquid. XPS showed the films mainly contain carbon. IR spectrum indicated that as-deposited films are hydrogenated carbon films, with the hydrogen mainly bonded to sp³ carbon. Raman measurement suggested that the films consisted of sp³ and sp² carbon. The field emission (FE) property of DLC coated on Si has been measured. The field emission of DLC films started at an applied voltage of 160 V, compared with silicon tip arrays at 600 V, and an emission current of DLC films up to 55 A at 360 V was achieved.     

Synthesis of fluorinated diamond films using a laser technique

A laser chemical vapour deposition process for growing fluorinated diamond thin films on two bearing materials, SiC and 440 C stainless steel, is described. The type of laser, carbon feedstock, laser-precursor gas interactions, and deposition conditions have been established. Raman spectroscopy analysis revealed that the films deposited on SiC consisted of a mixture of diamond and graphite, while the films on 440 C steel were composed of diamond, diamond-like carbon and graphite. The feasibility of diamond formation using laser light-gas interactions is explained.     

Nanoelectromechanical device fabrications by 3-D nanotechnology using focused-ion beams

Nanoelectromechanical devices, which can be used as nanotools in nanofactories, were fabricated by focused ion beam chemical vapor deposition (FIB-CVD). The devices are made of diamond-like carbon (DLC), deposited on a Si substrate using gasified phenanthrene (C₁₄H₁₀) as a carbon source. The Young modulus and density of the deposited DLC were measured as 190 GPa and 3.8 g cm⁻³, respectively. The work function was smaller for DLC (2.9 eV) than for W (4.7 eV) and Fe (5.2 eV) deposited by FIB-CVD. A nanomanipulator was manufactured by FIB-CVD and used for actual manipulations. A glass capillary based local field emitter was developed and produced as a tool for spot deposition, and its electron field emission was confirmed. FIB-CVD is proven as an efficient fabrication technology of novel nanoelectromechanical devices.     

Pressure dependent structural and optical properties of silicon carbide thin films deposited by hot wire chemical vapor deposition from pure silane and methane gases

Silicon carbide (SiC) thin films were deposited using hot wire chemical vapor deposition technique from silane (SiH₄) and methane (CH₄) gas precursors. The effect of deposition pressure on structural and optical properties of SiC films was investigated. Various spectroscopic methods including Fourier transform infrared spectroscopy, Raman scattering spectroscopy, Auger electron spectroscopy, and UV–Vis–NIR spectroscopy were used to study these properties. Films deposited at low deposition pressure were Si-rich, and were embedded with nano-crystals of silicon. These films showed strong absorption in the visible region and had low energy band gaps. Near stoichiometric SiC film, were formed at intermediate deposition pressure and these films were transparent in the visible region and exhibited a wide optical band gap. High deposition pressures caused inhomogeneity in the film as reflected by the increase in disorder parameter and low refractive index of the films. This was shown to be due to formation of sp ² carbon clusters in the film structure.     

Effects of argon and oxygen addition to the CH4-H2 feed gas on diamond synthesis by microwave plasma enhanced chemical vapor deposition

In order to investigate the effects of argon and oxygen on diamond synthesis, the behaviors of diamond deposition using microwave plasma chemical vapor deposition method have been studied by varying the concentrations of argon and oxygen in the methane-hydrogen gas mixture. Diamond films were deposited on silicon wafer under the conditions of substrate temperatures: 1073 1173 K, total reaction pressure: 5333 Pa (40 Torr), methane concentrations: 0.5 5.0%, and they were characterized by scanning electron microscopy, Raman spectroscopy and optical emission spectroscopy. The deposition rates of diamond films were enhanced by adding argon into the methane-hydrogen system, but nondiamond carbon phases in the films also increased. It resulted from the increase of hydrocarbon radicals in the plasma. As oxygen was added, the quality of deposited diamond films was improved due to the decrease of C₂ radicals and increase of OH radicals in the plasma. Simultaneous addition of 0.3% oxygen and 20% argon has been able to effectively suppress the formation of nondiamond carbon components and increase the deposition rate of diamond films. It appears that the ionized argon (Ar⁺) and excited argon atoms (Ar*) may activate the various chemical species and promote the reactions between the gas phase species and oxygen in the plasma.