Mechanisms of Excimer Laser Ablation of Wide Band-Gap Materials: The Role of Defects in Single Crystal MgO
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MECHANISMS OF EXCIMER LASER ABLATION OF WIDE BAND-GAP MATERIALS: THE ROLE OF DEFECTS IN SINGLE CRYSTAL MgO J. T. DICKINSON, L. C. JENSEN, R. L. WEBB, M. L. DAWES, AND S. C. LANGFORD Department of Physics, Washington State University, Pullman, WA 99164-2814 ABSTRACT Laser ablation has important applications in surface modification, materials analysis, and thin film deposition. We have been examining the details of processes that lead to the emission and formation of particles (atomic/molecular ground state neutrals, excited neutrals, tions, electrons) when wide band gap materials are irradiated with pulsed UV laser light. Etching and deposition of wide bandgap materials is of particular interest due to their excellent insulating and optical properties. Our studies bear directly on achieving control of emission intensities and particle characteristics for use in film deposition and materials analysis. In model wide bandgap materials such as single crystal alkali halides and MgO (nominally transparent materials), exposure to repeated pulses of 248 nm excimer laser radiation of a few J/cm 2 results in substantial interaction including extensive biaxial deformation and cleavage. Significant surface heating also occurs, consistent with strong free-carrier/laser interactions. We present strong evidence that achieving intense emission of atomic, molecular, and ionic particles actually depends on point defect production by laser-induced deformation and fracture. Defect production via dislocation motion yields orders of magnitude increases in laser vaporization of these wide bandgap materials, including cluster ion formation. The dependence of the lasermaterial interaction on dislocation density and mobility, as well as point defect density, suggests several novel strategies for the enhancing the ablative response or preventing laser damage. I. INTRODUCTION The desorption and ablation of material from nominally transparent, wide bandgap materials by UV excimer radiation can be greatly enhanced by the presence of any lattice defects which allow single-photon excitations to the conduction band. 1 ,2 In the previously unexposed solid, absorbing defects may be produced during material processing (e.g., crystal growth, impurities) and sample preparation (e.g., polishing, 3 ,4 electron irradiation 5 ). The relatively high energy of UV radiation can promote single photon absorption from defects which are much less important at visible wavelengths. While polishing increases the laser damage resistance of many materials at visible wavelengths (due to the removal of surface scratches etc.), at UV wavelengths mechanically produced defects can actually make polished surfaces more susceptible to laser damage. In MgO (Eg = 7.9 eV) and NaCl (E = 8.6 eV bandgap), these defects can strongly enhance desorption and ablation by 248 nm (5 e-V photon energy) UV laser light. 3- 6 Cleaved MgO surfaces show considerably less sensitivity to single pulse laser irradiation at 248 nm radiation than polished or abraded surfaces. 3 We attribute these diff
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