Ablation of Single Crystal MgO by UV Excimer Irradiation
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ABLATION OF SINGLE CRYSTAL MgO BY UV EXCIMER IRRADIATION R. L. WEBB, L. C. JENSEN, S. C. LANGFORD, AND J. T. DICKINSON Washington State University, Pullman, WA 99164-2814
ABSTRACT The ablation of single crystal MgO irradiated with 248 nm excimer laser light is studied by means of time resolved spectroscopy and quadrupole mass spectrometry. Luminescence spectra and SEM observations indicate that repeated laser bombardment gradually increases the density of potentially absorbing defects. In polished samples, this progressive growth is preceded by an initial clean-up (reduction) of surface damage. Unlike many wide band gap materials, defect production in MgO by electronic mechanisms is not likely. Chemical etch techniques indicate the presence of high dislocation densities in regions etched by the laser, suggesting that point defect production by dislocation motion is important. The ablation plume is composed of charged particles, including cluster ions, as well as a high density of excited neutrals. The growth of the plume with repeated bombardment correlates with defect formation as indicated by luminescence intensities.
INTRODUCTION The effect of intense laser irradiation on wide band gap materials is strongly enhanced by the presence of absorbing lattice defects. The defect density can increase dramatically during laser irradiation, producing electron traps which can be ionized by the laser. Laser heating of the resulting free carriers eventually melts the surface at fluences below the single-pulse damage threshold. 1-41,2,3,4 Such defects are often created by the relaxation of electronic excitations, e.g. F centers in the alkali halides. 5 However, in MgO the energy available from sub-band gap electronic excitations is insufficient for the production of lattice defects. Electron beam damage of MgO typically requires hundreds of keV, sufficient for the knock-on displacement of lattice ions. 6 Thus different mechanisms for defect production are required in MgO. A detailed understanding of the processes involved in laser desorption and ablation is important for the control and optimization of laser applications in material removal and deposition technologies. In laser ablation deposition, the control of the density and stoichiometry of species in the plume and their kinetic and internal energies is especially important. Adverse side effects, such as fracture of the target material, must also be avoided. Novel techniques to modify the laser-material interaction may facilitate the production of ablation plumes with desirable properties for thin film deposition and for chemical analysis applications. EXPERIMENT Experimental details are given in Ref. 1. All experiments were conducted in a diffusion pumped, liquid nitrogen trapped vacuum chamber at a pressure of 10-5 Pa. The influence of surface contamination (e.g., H2 0 sorption) was probed by measuring product yields vs laser repetition rate from 1-250 Hz, which varies the adsorption time between laser pulses. No dependence was observed. Sample irradiation was perfor
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