Pulsed laser ablation provides a means of depositing thin coatings, of a wide range of target materials, on a wide range of substrates, at room temperature. Despite its versatility and wide applicability, however, many aspects of the detailed chemical physics underlying the ablation process are still far from completely understood. The process is often envisaged as a sequence of steps, initiated by the laser radiation interacting with the solid target, absorption of energy and localised heating of the surface, and subsequent material evaporation. The properties and composition of the resulting ablation plume may evolve, both as a result of collisions between particles in the plume and through plume-laser radiation interactions. Finally the plume impinges on the substrate to be coated; incident material may be accommodated, rebound back into the gas phase, or induce surface modification (via sputtering, compaction, sub-implantation, etc.). Such a separation has conceptual appeal but, inevitably, is somewhat over-simplistic. Furthermore, the laser-target interactions will be sensitively dependent both on the nature and condition of the target material, and on the laser pulse parameters (wavelength, intensity, fluence, pulse duration, etc.). Subsequent laser-plume interactions will also be dependent on the properties of the laser radiation, while the evolution and propagation of the plume will also be sensitive to collisions and thus to the quality of the vacuum under which the ablation is conducted and/or the presence of any background gas. Obviously, the ultimate composition and velocity distribution (or distributions, in the case of a multi-component ablation plume) of the ejected material is likely to be reflected in the detailed characteristics of any deposited film.
We use excimer laser radiation to ablate a range of prototypical target materials e.g. elemental materials like graphite, CVD diamond, Cu and Al, two component systems like ZnO and LiF, and various polymeric materials, under vacuum and in the presence of lower pressures of background gas (He, Ar, H2, N2, O2).
Figure 1. Schematic diagram of an apparatus for PLA of a solid target with deposition on an on-axis mounted substrate.
Current activities are focussed on two aspects of pulsed laser ablation and deposition. Fundamentals of the ablation process are investigated by wavelength, spatially and temporally resolved optical emission spectroscopy, by ion probe measurements and through use of a purpose designed and built quadrupole mass spectrometer designed to allow measurement of the nascent kinetic energy distributions of mass selected neutrals and charged particles within the plume of ablated material. The second strand of current research focuses on determining characteristics of the deposited film, influencing film composition, morphology and/or crystallinity by appropriate choice of growth parameters, and the possibility of using such films as a template for subsequent oriented material growth. Techniques available for detailed analysis of the resulting films (which are deposited on a range of substrate materials, at substrate temperatures ranging from room temperature to ~700°C) include laser Raman and IR transmission spectroscopy, ellipsometry, SEM and TEM, X-ray and electron diffraction and, when appropriate, XPS and SIMS. The experimental programme is complemented by modelling, both of the laser- target interaction, and of the subsequent evolution of the ablation plume. Examples of recent work are illustrated below and can be found in our publications list.
Figure 2. SEM images of an MgB2 target ablated at 193 nm at a fluence of 12 J cm-2. (a) pre-ablation, (b) after ablation with one pulse and (c) after ablation with 10 pulses.
Figure 3. A selection of i-CCD images of the emitting species arising in the 248 nm nanosecond pulsed laser ablation of a graphite target in a vacuum: (a) C+ ions (b) C neutrals (c)-(e) particulates. (a), (b) and (d) are accumulated images of 200 laser pulses, while (c) and (e) are single pulse events.
Figure 4. (a) TEM dark field image of a thin film of ZnO grown onto a NaCl substrate at Tsub = 573 K. Selected areas of this substrate surface have been masked with a thin layer of amorphous carbon, prior to ZnO film deposition, thus allowing investigation of epitaxial growth on NaCl. (b) and (c) show SAED patterns from ZnO deposited on areas pre-coated with a carbon film, and on the bare NaCl substrate, respectively.
Figure 5. a) SEM image (viewing at a tilt angle of ~ 40º) of a nanorod array grown by PLD onto a Si wafer at Tsub = 600ºC for t = 45 mins. (b) TEM image of a selection of rods broken from this sample. (c) A tilt-view SEM image of a nanorod array grown by PLD for T600 = 44 mins on top of a ZnO buffer layer deposited for T300 = 1 min. (d) TEM image of selected nanorods from this sample.
Figure 6. ZnO nanotube samples grown by hydrothermal methods on a Si wafer that had been pre-coated with a thin ZnO film: (c) cross-sectional SEM images of samples grown for t = 10 hrs. (d) TEM image illustrating the tubular 'syringe-like' morphology of these ZnO nanostructures.