Fundamental studies of pulsed laser ablation

The pulsed ultraviolet (#lambda# = 193, 248 nm) laser ablation of graphite, polycrystalline diamond and ZnO targets has been investigated. Characteristics of the resulting plumes of ablated material have been deduced using optical emission spectroscopy (wavelength dispersed spectra, together with spatially and temporally resolved measurements) and electrostatic probes. Deposited films were analysed using a variety of techniques, including scanning electron microscopy, UV-visible absorption and Raman spectroscopy, and X-Ray diffraction. The study of graphite ablation focused mainly on the identification and characterisation of laser-plume interactions. Such processes are very important in the creation of the highly excited ablation plasma. Laser-plume interactions were probed directly, by monitoring the absorption of the laser light by the ablation plasma, and indirectly, using optical emission spectroscopy (OES) and a Faraday Cup. Laser-plume interactions are shown to be responsible for the observed asymmetry (with respect to the surface normal) of the emission from C"+ species in the plume. Pulse duration effects were probed also, by comparing characteristics of the ablation process induced using short (#approx#picosecond) and the more traditional (nanosecond) laser pulses. Raman spectroscopy on the deposited diamond-like-carbon films indicated that sp"3 fractions >60% are achievable. Investigations on the ablation of ZnO targets (with and without added Al and Ga dopant) have resulted in a coherent view of the resulting plume, which exhibits a multi-component structure correlated with different regimes of ablation, which are attributed to ejection from ZnO and ablation from a Zn melt. OES measurements show that the emitting Zn component within the plume accelerates during expansion in vacuum - an observation attributable to the presence of hot, fast electrons in the plume. The same acceleration behaviour is observed in the case of Al atomic emissions resulting from ablation of an Al target in vacuum. Deposition conditions, substrate temperature and background gas pressure were all varied in a quest for optimally aligned, high quality ZnO thin films. Initial ab initio calculations were performed also, to aid in understanding the stability of these c-axis aligned films. (author)Available from British Library Document Supply Centre-DSC:DXN051204 / BLDSC - British Library Document Supply CentreSIGLEGBUnited Kingdo