Gareth M. Fuge

Pulsed laser ablation and deposition of thin films

Pulsed laser ablation is a simple, but versatile, experimental method that finds use as a means of patterning a very diverse range of materials, and in wide areas of thin film deposition and multi-layer research. Superficially, at least, the technique is conceptually simple also, but this apparent simplicity hides a wealth of fascinating, and still incompletely understood, chemical physics. This overview traces our current physico-chemical understanding of the evolution of material from target ablation through to the deposited film, addressing the initial laser-target interactions by which solid material enters the gas phase, the processing and propagation of material in the plume of ejected material, and the eventual accommodation of gas phase species onto the substrate that is to be coated. It is intended that this Review be of interest both to materials scientists interested in thin film growth, and to chemical physicists whose primary interest is with more fundamental aspects of the processes of pulsed laser ablation and deposition.

Structural characterisation of CNx thin films deposited by pulsed laser ablation

Abstract Carbon nitride (CN x ) film growth by 193 nm pulsed laser ablation of graphite in a low pressure of N 2 has been investigated both by studying optical emission from the plume and by analyses of the composition, structure and bonding of material deposited at a range of substrate temperatures. Spectral analysis of the emission reveals the presence of C + ions, C atoms, C 2 and CN radicals and N 2 + molecular ions within the ablation plume travelling towards the substrate. Films deposited at low substrate temperature ( T sub ) are amorphous, with an N/C ratio of ∼20 at.%. Raman analysis shows CN x films grown at higher T sub to be increasingly nanocrystalline, but thinner, and suggests that N inclusion encourages nanocrystallite formation. X-ray photoelectron spectroscopy reveals that CN x films grown at higher T sub also have a reduced overall N content. The observations have been rationalised by assuming an increased propensity for sputtering or desorption of more labile CN species from the growing film surface at higher T sub , resulting in a higher fraction of CC bonding—most probably in the form of graphitic nanocrystallites embedded in an amorphous matrix.

In situ plasma diagnostics of the chemistry behind sulfur doping of CVD diamond films

Microwave plasma enhanced chemical vapour deposition (CVD) has been used to grow sulfur doped diamond films using a 1% CH yH gas mixture with various levels of H S addition (100–5000 ppm), upon undoped Si substrates.X-Ray photoelectron 42 2 spectroscopy has shown that S is incorporated into the diamond at number densities (F0.2%) that are directly proportional to the H S concentration in the gas phase.Four-point probe measurements showed the resistivity of these S-doped films to be a 2 factor of three lower than undoped diamond grown under similar conditions.Sulfur containing diamond film was also obtained using a 0.5% CS yH gas mixture, although the high resistivity of the sample indicated that the sulfur had been incorporated into 22 the diamond lattice in a different manner compared with the H S grown samples.Molecular beam mass spectrometry has been 2 2 plasma region as a result of gas phase reactions.Additional measurements from a 1% CS yH plasma gave similar species mole 22 fractions except that no CS was detected.These results suggest that CS may be the first step toward C–S bond formation in the film and thereby a pathway allowing S incorporation into diamond.Optical emission spectroscopy has shown the presence of S 2 in both gas mixtures, consistent with the observed deposition of sulfur on the cool chamber walls. 2002 Elsevier Science B.V. All rights reserved.

Sulfur doping of diamond films: Spectroscopic, electronic, and gas-phase studies

Chemical vapor deposition (CVD) has been used to grow sulfur doped diamond films on undoped Si and single crystal HPHT diamond as substrates, using a 1% CH4/H2 gas mixture with various levels of H2S addition (100–5000 ppm), using both microwave (MW) plasma enhanced CVD and hot filament (HF) CVD. The two deposition techniques yield very different results. HFCVD produces diamond films containing only trace amounts of S (as analyzed by x-ray photoelectron spectroscopy), the film crystallinity is virtually unaffected by gas phase H2S concentration, and the films remain highly resistive. In contrast, MWCVD produces diamond films with S incorporated at levels of up to 0.2%, and the amount of S incorporation is directly proportional to the H2S concentration in the gas phase. Secondary electron microscopy observations show that the crystal quality of these films reduces with increasing S incorporation. Four point probe measurements gave the room temperature resistivities of these S-doped and MW grown films as ∼20...

Studies of the plume emission during the femtosecond and nanosecond ablation of graphite in nitrogen

Comparative studies of the pulsed laser ablation of graphite in 20mTorr of N2 using both 15ns and 450fs pulses at a wavelength of 248nm are reported. Emissions from the resulting ablation plumes, and from collisions with ablated material and the background N2 gas molecules, have been investigated by wavelength-, space-, and time-resolved optical emission spectroscopy (OES), and the observations correlated with the results of the analyses of films formed when such material is incident on a silicon substrate. Wavelength-dispersed spectra of the plume arising in nanosecond ablation reveal CI, CII, and C2 emissions—concentrated close to the target—and, at greater distances, strong CN and weak N2+ emissions. N2+(B–X) emission dominates in the case of femtosecond ablation. Time-gated imaging studies have allowed estimation of propagation velocities for these various emissions. Possible production routes for secondary emitters such as CN and N2+ are discussed, and arguments presented to show that measurements of...

Comparing the short and ultrashort pulsed laser ablation of LiF

Pulsed laser ablation of LiF was studied using both nanosecond (ns) and femtosecond (fs) pulses at 248nm. Optical emission from electronically excited Li and F atoms in the plume of ejected material was investigated by wavelength, time and spatially resolved imaging methods. Careful analysis of images of species selected optical emission yielded estimates of the mean velocities of the Li+ ions arising in both excitation schemes (∼11 and ∼13km∕s, respectively), and highlighted the dramatic effects of radiation trapping, most notably by the reabsorption of Li(2p→2s) emission by ground state Li atoms in the ns ablation studies. Plumes formed by fs excitation are found to contain a higher fraction of energetic∕electrically excited components, including excited F atoms and ions, indicative of an explosive boiling mechanism, whereas the ablation plume resulting from ns ablation is deduced to arise primarily from thermal evaporation of the transiently heated target surface. The amount of target material removed ...

Simulation of H-C-S containing gas mixtures relevant to diamond chemical vapour deposition

Mole fractions of 24 species present within x%H Sy1%CH yH (xs0–1%) and 1%CS yH gas mixtures have been calculated 24 2 2 2 using the CHEMKIN computer package in conjunction with a mechanism based on the composite conversion: CH q 4 2H S|CS q4H .Arrhenius parameters for each elementary reaction involving S-containing species are presented, along with 22 2 associated thermodynamic properties for each species.Molecular beam mass spectrometric measurements of species mole fractions in microwave activated x%H Sy1%CH yH (xs0–1%) mixtures agree well with the model calculations, if we assume a 24 2 (reasonable) gas temperature of 1630 K.The agreement between similar measurements of both 0. 5%H S y1%CH yH and 24 2 1%CS yH hot filament activated gas mixtures is less good, but the calculations succeed in reproducing many of the observed 22 trends in species mole fraction with change in filament temperature.

Electrospray Deposition of Diamond Nanoparticle Nucleation Layers for Subsequent CVD Diamond Growth

Nucleation is the rate-determining step in the initial stages of most chemical vapour deposition processes. In order to achieve uniform deposition of diamond thin films it is necessary to seed non-diamond substrates. Here we discuss a simple electrospray deposition technique for application of 5 nm diamond seed particles onto substrates of various sizes. The influence of selected parameters, such as experimental spatial arrangement and colloidal properties, are analysed in optimizing the method by optical and electron microscopy, both before and after nanocrystalline diamond deposition on the seed layer. The advantages and limitations of the electrospray method are highlighted in relation to other commonly exploited nucleation techniques.

Field emission observed from metal-diamond junctions revealed by atomic force microscopy

A noncontact atomic force microscopy technique has been developed that enables sources of field emission to be detected and mapped in an air ambient. Areas as large as 900μm2 have been mapped. This new technique enables determination of the location and extent of the emission area on an individual emitting particle. Emission from nanodiamond particles is shown to occur not at the tip of the diamond, but from near the base where it forms a triple junction with the metal substrate. The reported observations should assist exploration of novel methods of controlling electron emission from devices constructed using diamond particles.

Radiation trapping in LiF ablation plumes

This work highlights a potential pitfall associated with using optical emission to estimate the expansion velocity of material in laser ablation plumes, speci.cally when the monitored emission involves transition to a state that is present in high number density. Comparisons of time-gated, spatially resolved, images of the Li(3d. 2p) and Li(2p. 2s) emissions arising in the nanosecond 248 nm pulsed laser ablation of LiF enabled estimation of the distribution of ground state Li atoms in the plume. Analysis of these images revealed that the density of Li(2p) atoms in the early stages of the plume expansion was su.ciently high that even the Li(3d → 2p) emission profiles show evidence of radiation trapping.