Properties Of Diamond Films Research Articles

Effect of Surface Hydrogen Coverage on Field Emission Properties of Diamond Films Investigated by High-Resolution Electron Energy Loss Spectroscopy

The influence of surface hydrogen coverage on the electron field emission of diamond films was investigated by high-resolution electron energy loss spectroscopy. It was found that hydrogen plasma treatment increased the surface hydrogen coverage while annealing caused hydrogen desorption and induced surface reconstruction. Field electron emission measurements manifested that increase of surface hydrogen coverage could improve the field emission properties, due to the decrease of electron affinity of the diamond surface by hydrogen adsorption.

Effects of substrate bias on the structural and field emission properties of diamond films

The effect of electron bombardment on modification of the structural property and the surface morphology of diamond films, and consequently the field emission properties have been investigated by introducing positive substrate bias. When increasing the bias voltage, the structural properties of diamond films are significantly deteriorated together with the increase of nondiamond carbon component and the surface morphologies of the films lost their unique facet shape. The reason for the increase of nondiamond carbon content is described in terms of both the increase of substrate temperature and the excessive generation of CHn radicals. The field emission properties of diamond films were substantially improved with increasing bias voltage. Enhancement of field emission property for the films prepared by increasing bias voltage and/or methane concentration has been discussed by correlating between the substrate–diamond interface property and the slope of the Fowler–Nordheim plot.

Defect structure and electron field-emission properties of boron-doped diamond films

The correlation between electron field-emission properties of diamond films prepared by the chemical vapor deposition (CVD) process and the defect structure induced by boron doping was examined. Secondary ion mass spectroscopic analysis indicates that the solubility limit of boron in diamond is (B3+)2=5×1021 cm−3, whereas the infrared absorption (IR) spectroscopic analysis reveals that the largest boron concentration that can be incorporated as substitutional dopants is only one tenth of the solubility limit, (B3+)d=5×1020 cm−3. Including boron species higher than this concentration induces large strain and atomic defects, which are inferred by the distorted Raman resonance peak, noisy IR spectra, and twinned microstructure for diamond. Presumably, the presence of atomic defects, which behave as electron traps, is the mechanism deteriorating the electron field-emission properties of CVD diamonds.

Field emission characteristic of diamond films grown by electron assisted chemical vapor deposition

By introducing positive substrate bias, ranging from 0 to 200 V, to the substrate during the growing procedure of diamond at 1, 2, and 3% CH4 concentration, we have investigated the effect of electron bombardment on modification of the structural property and the surface morphology of diamond films, and consequently the field emission properties. When increasing the bias voltage for each CH4 concentration, the structural properties of diamond films are significantly deteriorated together while the non-diamond carbon component increased and the surface morphologies of the films lost their unique facet shape. The reason for the deterioration of the structural property was attributed to both the increase of substrate temperature and the excessive generation of CHn radicals. Especially for the films deposited at 2% CH4 concentration under 100 V, it was observed that their morphological and structural characteristics approached those of graphitic carbon nature. The field emission properties of diamond films were substantially improved with increasing the CH4 concentration and with the application of bias voltage for each CH4 concentration. In order to investigate the correlation between the enhancement of field emission properties and the emission sites, we have examined the spatial distribution of the emission sites. From this result, a possible emission mechanism is discussed.

Structure and properties of diamond films deposited on porous silicon

Thick porous silicon (PS) layers, made by anodic etching of crystalline Si wafers, have been coated with diamond films using the hot-filament chemical vapor deposition (CVD) technique. Ethanol diluted in hydrogen was used as the carbon source for diamond deposition. We observed that diamond nucleation occurs predominantly on the top of the PS spikes, creating individual islands of growth. Nucleation is apparently homogeneous over the PS surface and the islands grow independently until the complete coalescence of the diamond film. Although the diamond film is polycrystalline, it does not have the usual columnar structure of diamond deposited on c-Si. A good adherence between diamond and PS was observed in diamond–PS and diamond–PS–diamond structures. The problems of the intrinsic and thermal stresses have been addressed. Samples were characterized by atomic force microscopy (AFM), scanning electron microscopy (SEM), Auger electron spectroscopy (AES), Raman and photoluminescence spectroscopy.

An investigation of the structural properties of diamond films deposited by pulsed bias enhanced hot filament CVD

The control of the surface roughness and morphology of diamond films is of critical importance for many applications such as for optical and electronic components. In order to produce uniform CVD diamond films with a controlled surface morphology over large areas, bias enhanced methods of nucleation offer better reproducibility than substrate pre-treatment methods such as mechanical abrasion. Hot filament chemical vapour deposition (HFCVD) with bias enhanced nucleation (BEN) has been used to increase the nucleation density of diamond films on Si{100} substrates. Optimal conditions of the nucleation step for subsequent growth of good quality diamond were a substrate temperature of 1100 K; methane/hydrogen ratio of 3%; pressure of 20 Torr; filament temperature of ∼2500 K; d.c. bias of −250 V and time of 30 min. We also demonstrate that re-nucleation of diamond is possible using short intervals of substrate biasing during normal diamond growth. The film morphology and surface roughness critically depend on the length of the bias cycle and may be controlled without reducing the diamond quality. The morphology and quality of the standard negatively biased and pulsed biased films were characterised by scanning electron microscopy and Raman spectroscopy. Atomic force microscopy was used to measure the surface roughness of the films.

Diamond films control for tribological applications

Diamond coatings have been regarded for many years as a promising material for wear protection and general tribological applications. The tribological behaviour of diamond coatings is strongly influenced by the physico-chemical properties of the surfaces in contact and by the texture of the films, which depend on the preparation processes and on the deposition parameters. This paper presents the tribological behaviour of diamond films obtained by combustion flame method. Their friction coefficients μ in ambient atmosphere are in the range 0.1–1.1 and are characterised by two stages. In the initial stage, the friction coefficient is very high, in the range 0.8 to 1.1, and depends on film morphology and roughness. In the second stage, the friction coefficient is reduced to μ=0.3–0.1. This wide difference between the two stage properties can be damaging for tribological applications. The objective of this paper is to study tribological properties of diamond films, and to propose a surface treatment technique to reduce the friction coefficient and to restore diamond films suitable for a variety of applications; we discuss then the influence of the surface modifications on their tribological behaviour.

Effect of nitrogen addition on the microstructure and mechanical properties of diamond films grown using high-methane concentrations

We report on the microstructure and mechanical properties of diamond films grown using varying nitrogen additions to a plasma with a high-CH4 fraction of 15% (in hydrogen) and an operating pressure of 125 Torr. Films were grown at N2/CH4 ratios ranging from 0 to 0.30 by fixing the CH4 flow rate and changing only the N2 flow rate. With increasing nitrogen addition, we observe an increase in intensity and a decrease in the full width at half maximum (FWHM) of the Raman band at 1550 cm−1, while the crystalline diamond peak at 1332 cm−1 decreases in intensity and increases in the FWHM. X-ray diffraction confirms that the film crystallinity and diamond grain size decrease rapidly with increasing nitrogen additions up to a N2/CH4 ratio of 0.10, but then do not change significantly above this ratio. A similar trend is observed for film surface roughness. In addition, we find from indentation testing that all films exhibit high hardness values ranging from 70 to 90 GPa and that the toughness of the films improves with increasing nitrogen addition. Optical emission spectroscopy reveals that an increase in CN species relative to C2 in the plasma is responsible for the formation of tetrahedral amorphous carbon (indicated by the Raman band at 1550 cm−1).

Laser method for the determination of the optical and thermal characteristics of diamond at high temperatures

The temporal and temperature dependences of the reflectivity, transmissivity, and absorptivity and of the heat capacity of thermally thin diamond plates were obtained in the course of heating up to 2500 °C at a rate of 103 — 104 K s-1 by laser radiation with λ = 1.064 μm. The temperature range in which significant irreversible changes take place in the optical properties of diamond films was determined. These changes are associated with the formation of graphite microstructures in a thin surface layer about 100 nm thick. The formation of clusters of aromatic carbon atom groups with the sp2 bonds is described. It is shown that the extinction coefficient of the modified layer is three orders of magnitude higher than that of the unmodified layer.

Determination of the thermal conductivity of polycrystalline diamond films by means of the photoacoustic effect

A new method of determining the heat-conducting properties of diamond films is proposed, based on the photoacoustic effect. This method is used to study diamond polycrystalline films grown on silicon by chemical vapor deposition in a microwave discharge plasma. The thermal conductivity obtained was approximately half that for single-crystal diamond.

Studies of field emission from bias-grown diamond thin films

We have investigated the field emission properties of diamond films grown under substrate bias conditions in a microwave plasma chemical vapor deposition system, with a substrate temperature of 800 °C, microwave power of 600 W, and a total pressure of 11 Torr. One group of films was grown with a substrate bias of −100 V in gas mixtures of 1% N2 and 1%–20% CH4 in H2, while a second group of films was grown with a substrate bias ranging from +100 to −150 V in a gas mixture of 1%N2–10%CH4–89%H2. The field emission performance in terms of threshold field and emission current improved considerably as a function of increasing CH4 concentration and negative bias voltage. Ultraviolet Raman analysis showed that the field emission enhancement resulting from an increase in CH4 concentration from 1% to 5% correlates with a decrease in the sp3 bonding character in the diamond film. The dependence of field emission on negative bias voltage appears to be correlated with ion bombardment-induced damage in the film during growth. The scanning electron microscopy image of the film grown with −150 V bias showed: smaller surface topographic features as compared to films grown under 0 and +100 V bias. The film grown with a bias of −150 V showed the lowest threshold field (∼2.0 V/μm) corresponding to an emission current density of 12.7 μA/cm2. J vs E0 measurements across a length of 40 mm over the film showed a uniform threshold field (2.0±0.55 V/μm). The film grown with a positive bias (+100 V) showed a relatively poor field emission performance.

Correlation between the OES plasma composition and the diamond film properties during microwave PA-CVD with nitrogen addition

The mechanisms of nitrogen incorporation in diamond are still an unsolved riddle. This is mainly due to the complexity of the processes involved as they not only depend on empirical parameters (e.g. vessel pressure, substrate temperature, the gas phase composition, type and concentration of the nitrogen containing compound used), but also on the plasma chemistry and the surface chemical reactions. In this study, small quantities (ppm range) of diatomic nitrogen are added to a conventional hydrogen-methane feed gas mixture in order to investigate the effect of nitrogen incorporation in diamond films prepared by microwave plasma assisted chemical vapour deposition (CVD). Optical emission spectroscopy (OES) is used to survey the plasma composition during deposition. The intensities of the CN, CH and C 2 emitting radicals and the Balmer atomic hydrogen emission lines are correlated to the Raman film quality and to the nitrogen content in the film measured by secondary ion mass spectrometry (SIMS).

Processing–structure–adhesion relationship in CVD diamond films on titanium substrates

The hardness and adhesion properties of diamond films deposited on pure Ti and Ti–6Al–4V alloy are related to the structural characteristics of the films and of the intermediate layers formed at the film/substrate interface. Deposition experiments were performed by hot-filament CVD systematically varying the deposition temperature (650–850 °C) and time (60–360 min). The morphology and structure of the coatings and interfaces were investigated by optical and electron microscopy (SEM) and by the combined use of reflection diffraction (RHEED) and X-ray powder diffraction (XRPD), used also in the GID (Grazing Incidence Diffraction) mode. Hardness measurements (HV) of the deposits were made at a constant loading of 25 g on specifically prepared transversal sections of the specimen. Only small differences concerning the mechanical properties have been found for films grown on pure Ti and Ti–6Al–4V substrates. Adhesion of diamond on the substrate has been determined by the scratch test. The analysis of the micrographs of the indenter scratches and of the fracture-correlated acoustic emissions clearly indicates the occurrence of a multi-step detachment process, leading progressively to effective delamination of the diamond coatings.

Submicrocrystalline diamond film deposited by CVD of Freon 22: fabrication of pressure sensing devices

Au/diamond/Si Schottky diodes were fabricated with submicron diamond films deposited on Si substrate by chemical vapour deposition of Freon 22 (CHF 2 Cl≤10 vol.%) and hydrogen at a substrate temperature of ∼725 K. The diamond films were ∼0.7 μm thick with a grain size of 500–600 nm. Characterization of the devices by J – V analysis ( J being the current density) showed that an increase in stress (internal or external) makes the forward-biased J – V plot steeper. Films with a low intrinsic stress were used for the fabrication of pressure-sensing devices, which showed a reversible behaviour with an external load (130–900 g) applied perpendicularly to the diode structure. The force sensitivity factors (0.04–0.20 V N −1 ) of the devices (with an area of ∼0.04 cm 2 ) were obtained for different forward-biased current conditions (0.15–0.40 mA cm −1 ). The observed pressure dependence of the electrical properties of the MIS Schottky devices was due to changes in the properties of diamond film under stress.

High Rates of Diamond Deposition with Various Crystals Sizes by Combustion Flame Method

A combustion-flame method is used to nucleate and grow various forms of polycrystalline diamond under ambient atmosphere. A two-colour pyrometer gives information to the system which monitors the stabilisation of the substrate temperature in the deposition zone. The deposition parameters are significantly controlled, leading to high quality and reproducibility of the diamond growth. This study has shown that the crystal growth, the texture and the physical properties of diamond films are strongly dependent on the processing conditions. At normal growth conditions (deposition under laminar flow) and in the first step of deposition, the growth rate and the diamond crystals size increase with the substrate temperature. At high substrate temperature deposition (Ts=900°C), the average crystals size reaches 30 μm and the growth rate is about 500 μm/h. The growth rate decreases with the times deposition but the crystal size continues to increase with the thickness of the deposited film. When the total gases flow is increased to the turbulent state, the deposition leads to very fine crystals (about 1 μm) with good quality. In that case, the growth rate reaches 700 μm/h. A model of crystal growth is proposed to explain this phenomenon.

HFCVD diamond grown with added nitrogen: film characterization and gas-phase composition studies

Recently there has been considerable interest in the addition of nitrogen to precursor gas mixtures and its influence on the properties of the resulting diamond films. Careful control of the nitrogen concentration in the CVD process results in changes to both the film growth rate and morphology. In addition to the well-defined and precisely controlled deposition conditions, thorough characterization of the vapour-grown material is indispensable in order to tailor diamond film properties according to specific applications. High quality diamond films have been produced via hot filament CVD (HFCVD) using a precursor gas mixture of 1% methane in hydrogen with N 2 additions varying from 50 to 5000 ppm (N/C 0.01–1.0). We report the results from both the structural and compositional characterization of the as-deposited HFCVD diamond films by scanning electron microscopy, Raman spectroscopy and X-ray diffraction. Both the diamond phase purity and growth rate are maximized with 200 ppm N 2 in the gas mixture with further additions resulting in reduced growth rates and poorer film quality. We explain these findings in terms of the concentrations of gas phase species obtained from chemical equilibrium calculations and in particular the importance of the CN radical to the diamond growth process.

Photoconductivity and electrical properties of diamond films grown by MPCVD

Abstract The electrical and photoconductive features of as-grown microwave-plasma-assisted chemical-vapour deposition (MPCVD) diamond films are studied in correlation with magnetic results obtained from electron paramagnetic resonance (EPR). Also, the morphology is analysed by atomic force microscopy (AFM) showing 〈111〉 crystals with a good uniformity of the deposit. The photoresponse as well the current-voltage features observed show an efficient photogeneration of carriers while the optoelectronic characteristics of the metal-diamond junction have an ideality factor of 1.6 together with a rectification ratio of about 10 4 at ±2.5 V. The nature of the mechanisms responsible for the conduction is discussed.

Laser ablation doping process for the synthesis of conductive diamond thin film

Basic to the understanding of the electrical properties of diamond films is the achievement of the synthesis of doped films through specific doping processes. We report a novel laser ablation process for doping diamond thin films. Boron doped diamond thin films were synthesized by the activated beam deposition process with H 2/C 2H 4 gaseous mixtures, and the simultaneous irradiation of a solid boron target with a CO 2 laser beam. The presence of boron in the film was confirmed by SIMS analysis. The electrical characteristics of the boron doped films were ascertained by resistivity, carrier concentration and Hall mobility measurements. Raman spectroscopy and SEM micrographs confirmed the quality and crystallinity of the films. Conductivity dependence on temperature was measured between a temperature range of 303 K and 873 K, and the possibility of the application of the process to synthesize alternative layers of insulating and conducting films was demonstrated.

On nitrogen incorporation during PE-CVD of diamond films

Abstract Nitrogen is one of the most widely studied impurities in diamond films. The reason is that nitrogen takes part in a number of defects and therefore has a deep influence on the optical and electrical properties of diamond films. The mechanism of nitrogen incorporation in diamond is still an open question. This is mainly due to the complexity of the N-incorporation processes, depending not only on experimental parameters (e.g. type and concentration of the N-containing gas, substrate temperature etc.) but also on the surface termination and on the orientation of the chemical vapor deposition (CVD) diamond film. The aim of this paper is to compare nitrogen incorporation in CVD diamond films by addition of small amounts (ppm range) of two different N-containing precursors—nitrogen and vaporised nitromethane—to a conventional CH4/H2 gas mixture during microwave PE-CVD. Optical emission spectroscopy (OES) was used to survey the plasma chemistry during deposition. The relative intensities of the Hβ atomic hydrogen emission line and of the CN and C2 emitting radicals were recorded as a function of the nitrogen fraction added to the plasma. The influence of nitrogen incorporation on the diamond film properties was investigated by a number of complementary techniques such as scanning electron microscopy (SEM), X-ray diffraction (XRD), photothermal deflection spectroscopy (PDS-optical absorption) and Raman spectroscopy.

Effect of SP3/(SP2+SP3) Carbon Fraction on the Photoelectric Threshold and Electron Affinity of Diamond Films

AbstractWe report a significant decrease in the photoelectric threshold of chemical vapor deposition grown diamond films as the fraction of sp3 carbon to sp2 plus sp3 carbon in the films decreases. Raman spectroscopy and x-ray photoelectron spectroscopy are used to characterize the different forms of carbon in the films and the sp3/(sp2 + sp3) carbon fraction at the surface. We observe a decrease in the photoelectric threshold from 4.5 eV to 3.9 eV as the sp3/(sp2 + sp3) carbon fraction at the surface decreases from 71% to 55%. Ultraviolet photoelectron spectroscopy of the films shows that they have a negative electron affinity surface. Therefore, the work function of the films decreases from 4.5 eV to 3.9 eV. We propose that the decrease in photoelectric threshold is due to a decrease in the band gap of sp2-sp3 carbon networks at the grain boundaries. The observed decrease in photoelectric threshold can be used to tailor the electronic properties of diamond films for specific applications.