Field emission from diamond and diamond-like carbon films

Field emission from diamond and diamond-like carbon thin films deposited on silicon substrates has been studied. The diamond films were synthesized using hot filament chemical vapor deposition technique. The diamond-like carbon films were deposited using the radio frequency chemical vapor deposition method. Field emission studies were carried out using a sphere-to-plane electrode configuration. The results of field emission were analyzed using the Fowler-Nordheim model. It was found that the diamond nucleation density affected the field emission properties. The films were characterized using standard scanning electron microscopy, Raman spectroscopy, and electron spin resonance techniques. Raman spectra of both diamond and diamond-like films exhibit spectral features characteristic of these structures. Raman spectrum for diamond films exhibit a well-defined peak at 1333cm^?1. Asymmetric broad peak formed in diamond-like carbon films consists of D-band and G-band around 1550 cm^?1 showing the existence of both diamond (sp³ phase) and graphite (sp² phase) in diamond-like carbon films.     

Study of the moderate-temperature growth process of optical quality synthetic diamond films on quartz substrates

Growth of undoped and boron-doped diamond films on quartz substrates at moderate temperature of 500 °C by microwave plasma chemical vapor deposition method was studied in terms of growth rate, surface roughness and optical transmittance. Similar density of diamond seed particles on quartz surfaces seeded mechanically before the deposition process and diamond grains within diamond films grown on those substrates is observed. The growth rate is found similar to that reported for diamond deposited on silicon substrates in the same plasma deposition system, although with substantially higher activation energy. Furthermore, increased level of dopant concentration in the gas mixture resulted in a decrease of the growth rate, while a gradual reduction of the surface roughness occurred at high dopant levels. Overall, the highest measured regular optical transmittance of the undoped diamond film on quartz was 45% at 1100 nm (including quartz absorption), whereas that of boron-doped diamond peaked 5% at 700 nm (tail absorption of boron centers).     

Characterization of hypervelocity impact craters on chemical vapour-deposited diamond and diamond-like carbon films

Microwave plasma chemical vapour-deposited (CVD) process has been used to grow polycrystalline diamond films over silicon substrates. Diamond-like carbon (DLC) thin films were grown over silicon substrates using a microwave plasma disc reactor. Reactant gases of CH4 and H2 were used in both CVD processes. Some preliminary feasibility tests were performed on the possible applicability of diamond and diamond-like carbon thin films for space-protective applications against artificially simulated electrically actuated plasma drag hypervelocity impact of olivine particles. As-deposited films were analysed by Raman for their chemical nature. The morphology and dimensions of hypervelocity impact craters in diamond and DLC films was also studied by scanning electron microscopy (SEM) and optical microscopy. The velocity of debris particles was determined by high-speed photography using a streak camera. The size of the impact particles was determined by measuring the size of the holes formed in the mylar sheet mounted just above the target diamond and DLC film/silicon and coordinates of the impact sites were determined using the same apparatus. This revised version was published online in November 2006 with corrections to the Cover Date.     

Influence of co-catalyst on growth of carbon nanotubes using alcohol catalytic CVD method

This paper describes the influence of a co-catalyst on growth of carbon nanotubes (CNTs) by alcohol catalytic chemical vapour deposition (ACCVD) method. Silicon wafers covered with thermal oxide or polycrystalline diamond thin film were used as substrates. Ni thin film supported with Al, Cu or Ti was used as a catalyst. The films were deposited by pulsed laser deposition technique. Comparison of the various types of the co-catalyst (Al, Cu, Ti) leads to the conclusion that Cu co-catalyst is suitable for producing very thin single wall carbon nanotubes (SWCNTs) and combination of Al and Ni provide a good condition to the catalytic growth of CNTs. In addition, we observed also the influence of the various diffusion barriers (thermal oxide and polycrystalline diamond) on growth of CNTs. Prepared samples were analysed by Raman spectroscopy (RS) and scanning electron microscopy (SEM).     

Effects of low temperatures, low pressures and seeding over the crystalline quality, yield and stress of diamond films grown by ECR-assisted chemical vapor deposition

We have studied diamond films grown by electron cyclotron resonance (ECR)-assisted chemical vapor deposition (CVD) on Si (100) substrates seeded with diamond, boron nitride and unseeded. Relatively low temperatures (550–710°C) and low pressures (1–2 Torr) were employed. Raman spectroscopy, scanning electron microscopy (SEM), and X-ray diffraction (XRD) were used to characterize the crystalline quality, diamond yield, and stresses developed in these films. Most of the diamond films exhibit a Raman blue-shift with respect to natural diamond, indicating that the net stress is compressive. However, this net stress is significantly more compressive than the one estimated by taking into account the thermal interfacial stress and the stress developed at the grain boundaries. In addition, this net stress exhibits an inverse correlation with diamond yield, and a direct correlation with crystalline quality. These results were interpreted in terms of the critical interplay between the supply of precursor species to the growing surface and the surface mobility of adsorbed species. The excess (or intrinsic) compressive stress shows an inverse correlation with diamond crystalline quality, indicating that the creation of point defects serves as a stress-relieving mechanism. Seeding effects, in general, are deleterious to diamond quality, in this temperature and pressure regime studied. Seeding with boron nitride had the effect of reversing the net stress from compressive into tensile, but this effect was rapidly lost as the diamond yield increased.     

Catalyst-free growth of nanographene films on various substrates

We have developed a new method to grow uniform graphene films directly on various substrates, such as insulators, semiconductors, and even metals, without using any catalyst. The growth was carried out using a remote plasma enhancement chemical vapor deposition (r-PECVD) system at relatively low temperatures, enabling the deposition of graphene films up to 4-inch wafer scale. Scanning tunneling microscopy (STM) confirmed that the films are made up of nanocrystalline graphene particles of tens of nanometers in lateral size. The growth mechanism for the nanographene is analogous to that for diamond grown by PECVD methods, in spite of sp2 carbon atoms being formed in the case of graphene rather than sp3 carbon atoms as in diamond. This growth approach is simple, low-cost, and scalable, and might have potential applications in fields such as thin film resistors, gas sensors, electrode materials, and transparent conductive films.     

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.     

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.     

Structure of diamond and diamond like carbon thin films grown by hot-filament chemical vapour deposition technique

Micro-crystalline diamond (MCD) and diamond like carbon (DLC) thin films were deposited on silicon (100) substrates by hot-filament CVD process using a mixture of CH₄ and H₂ gases at substrate temperature between 400–800°C. The microstructure of the films were studied by X-ray diffraction and scanning electron microscopy. The low temperature deposited films were found to have a mixture of amorphous and crystalline phases. At high temperatures (> 750°C) only crystalline diamond phase was obtained. Scanning electron micrographs showed faceted microcrystals of sizes up to 2μm with fairly uniform size distribution. The structure of DLC films was studied by spectroscopic ellipsometry technique. An estimate of the amount of carbon bonds existing insp ² andsp ³ form was obtained by a specially developed modelling technique. The typical values ofsp ³/sp ² ratio in our films are between 1·88–8·02. Paper presented at the poster session of MRSI AGM VI, Kharagpur, 1995     

Hydrogen induced microstructural variation in diamond and diamond like carbon thin films

Hydrogen plays a crucial role in the growth of micro-crystalline diamond (MCD) and diamond like carbon (DLC) thin films grown by plasma assisted chemical vapour deposition (PACVD) processes. It selectively etches graphite phase and helps in stabilizing the diamond phase. The presence of various hydrocarbon species in the plasma and their reaction with atomic, excited or molecular hydrogen on the substrate surface decide the mechanism of diamond nucleation and growth. Several mechanisms have been proposed but the process is still not well understood. Control of hydrogen and other deposition parameters in the PACVD process leads to deposition of yet another class of materials called diamond like carbon. By varying the concentration of hydrogen it is possible to produce purely amorphous carbon films on the one hand and amorphous hydrogenated carbon films (with as high as 60% hydrogen) on the other. Very hard, optically transparent and electrically insulating films characterize the diamond like behaviour. The proportion of hydrogen and its bonding with carbon or hydrogen in the film can be varied to obtain very hard to very soft films which could be optically transparent or opaque. The microstructure of these films have been investigated by a large number of techniques. The results show interesting situations. This paper reviews the work on the role of hydrogen on the growth, structure and properties of MCD and DLC thin films.     

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.     

Growth of diamond films on SiC, WC and cubic BN substrates

The growth morphology of diamond films grown on single crystals of SiC and on sintered WC and cubic BN (CBN) substrates by hot filament assisted chemical vapour deposition was examined using transmission electron microscopy and scanning electron microscopy. Diamond was found to have the form of particles on the substrates of SiC and WC in the initial stage of film growth. Both an amorphous layer and a directly bonded area were seen at the interface. Several orientation relationships, different from the cube/cube relation, were observed in these systems. On the other hand, in the case of diamond films on CBN substrates, the growth morphology of diamond was affected by the surface condition of the substrates. When CBN substrates were polished with a diamond paste before deposition, diamond grew in the form of particles. The growth morphology was changed by ion sputtering of the surface of the substrate from particle growth to uniform film growth. These results are discussed on the basis of lattice mismatch at the interface. This revised version was published online in November 2006 with corrections to the Cover Date.     

Correlation between diamond grain size and hydrogen incorporation in nanocrystalline diamond thin films

Hydrogen-incorporated nanocrystalline diamond thin films have been deposited in microwave plasma enhanced chemical vapour deposition (CVD) system with various hydrogen concentrations in the Ar/CH₄ gas mixture. The bonding environment of carbon atoms was detected by Raman spectroscopy and the hydrogen concentration was determined by elastic recoil detection analysis. Incorporation of H₂ species into Ar-rich plasma was observed to markedly alter the microstructure of diamond films. Raman spectroscopy results showed that part of the hydrogen is bonded to carbon atoms. Raman spectra also indicated the increase of non-diamond phase with the decrease in crystallite size. The study addresses the effects of hydrogen trapping in the samples when hydrogen concentration in the plasma increased during diamond growth and its relation with defective grain boundary region.     

Stimulation of the diamond nucleation on silicon substrates with a layer of a polymeric precursor in deposition of diamond films by microwave plasma

It has been shown that layers of a certain type of polymers applied to silicon substrates at the thermal destruction form (in parallel with sp² coordinated carbon) diamond nanoparticles, which act as nucleation centers of crystallites in the subsequent chemical vapor deposition of diamond. Microcrystalline diamond films have been synthesized from methane-hydrogen mixtures by microwave plasma using poly(naphthylhydrocarbyn) and poly(hydrocarbyn) preceramic polymers.     

Broad band photoluminescence studies of diamond layers grown by hot-filament CVD

Photoluminescence and Raman spectroscopy were employed to investigate the broad band luminescence in thin diamond films grown on a silicon substrate by the HF CVD technique. The broad band luminescence with a maximum emission at 1.8–2 eV observed for CVD diamonds is characteristic for amorphous carbon with sp²-hybridized carbon bonds. As was shown by the Raman spectroscopy our diamond layer contained certain amounts of amorphous carbon phase and diamond nanocrystals which were the source of an additional energy state within the diamond energy gap. The experimental results precluded the possibility of broad band luminescence being due to the electron–lattice interaction. The amorphous carbon and diamond nanocrystals admixture in polycrystalline diamond layer introduced a defect state in the energy gap not in the form of point defects but rather in the form of a line or extended defects. In consequence these extended defects were responsible for the broad PL spectrum in the CVD diamond films.     

A comparative study of p-type diamond films using Raman and transport measurements

The influence of deposition temperature in the properties of synthetic diamond films grown by two different chemical vapor deposition (CVD) techniques, hot-filament- and microwave-plasma-assisted, was investigated. These samples were obtained using the optimal growth conditions previously achieved in this work. Raman spectroscopy was employed in order to investigate the diamond film quality as a function of the deposition temperature. It was found that the nondiamond carbon bands decrease as the deposition temperature increases for both the deposition methods, leading to higher-quality diamond films. The micro- and macro-Raman spectra showed that the nondiamond band is already present in a single diamond grain. Both techniques provided well homogeneous diamond films and with equivalently good quality. Boron-doped diamond films with different carrier concentration levels were also studied. In order to get details about the electrical properties of the films, resistivity as a function of the boron concentration—in association with Raman spectra—and temperature-dependent transport measurements were employed. The results showed that the boron doping is the main responsible for the conductivity and that the variable range hopping (VRH) mechanism dominates the transport in these doped diamond films.     

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.     

Diamond and diamond-like carbon films

Polycrystalline diamond films are grown from low pressure gas mixtures, the deposition techniques are Microwave Plasma Chemical Vapour Deposition and Hot filament Chemical Vapour Deposition, in both techniques the deposition temperature is close to 900°C. The film growth process is strongly dominated by the initial nucleation stage, after this stage, the film grows at a rate of one micron per hour. The carbon atoms in the diamond film are fully fourfold (sp³) co-ordinated and the film properties are close to those of single crystalline diamond: extremely hard, resistant and transparent from UV to IR.Diamond-like carbon (DLC) films are amorphous and contain a variable amount of hydrogen in their structure, the carbon atoms are partially threefold (sp²) co-ordinated. Films are obtained at temperatures below 250°C and deposited on almost any substrate. Film composition, structure and functional properties are strongly dependent on the level of ionic bombardment of the film during growth. DLC films are very hard, have a low friction coefficient and good wear resistance, are chemically inert and are transparent in the IR.     

Growth and characterization of bamboo-shaped carbon nanotubes using nanocluster-assembled ZnO:Co thin films as catalyst

Bamboo-shaped carbon nanotubes (CNTs) had been successfully fabricated by a plasma enhanced chemical vapor deposition method, in which nanocluster-assembled ZnO:Co thin film was used as catalyst. It was found that bamboo-shaped CNTs were generally grown in a direction perpendicularly to the substrate surface with the tops of CNTs dominated by the droplet-like catalyst covered by the carbon layer. The diameter of CNTs was ranged from 20-50 nm. High resolution of TEM image showed that the typical CNT had a multi-walled structure with an inner core presented. The ordered graphite layers were inclined to an axis of CNT about 18 degrees and the interlayer space of a CNT was about 0.35 nm. Two peaks in Raman spectrum at 1586 cm(-1) and 1372 cm(-1) were identified as G-band and D-band for graphite, respectively. The results showed that catalyst based on ZnO:Co thin films could be used for the growth of CNTs with bamboo-shaped structure.     

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.