Laser ablation of wide band-gap materials: The role of defects in single crystal MgO

Abstract

Laser ablation has important applications in materials analysis, surface modification, and thin film deposition. We have been examining the details of processes that lead to the emission of particles (atomic/molecular ground state neutrals, excited neutrals, ±ions, electrons) when wide band gap materials are irradiated with pulsed UV laser light. In model wide bandgap materials such as single crystal alkali halides and MgO (nominally transparent), exposure to repeated pulses of 248 nm excimer laser radiation of a few J/cm2 results in substantial interaction including extensive biaxial deformation and cleavage. Significant surface heating also occurs, consistent with strong free‐carrier/laser interactions. 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. We report here our observations of damage induced by single pulses of 248 nm excimer laser radiation on cleaved MgO surfaces. Cleaved surfaces which yield significant densities of excited neutrals also show highly localized regions of laser damage, producing μm‐scale holes and/or regions of superficial melting. We present evidence that these features are due to localized heating along individual dislocation cores. These structures are often associated with highly deformed structures, such as undercut regions along cleavage steps. Luminescence images confirm the pressure of high densities of deformation‐induced defects along cleavage steps. These localized damage processes are precursors of the more generalized response accompanying extensive laser‐induced deformation during exposure to multiple pulses.