Zinc ion and neutral emission from single crystal zinc oxide during 193-nm excimer laser exposure

Mass resolved time-of-flight measurements on neutral zinc atoms and zinc ions show energetic ions and neutrals during 193-nm irradiation of single crystals of semiconducting zinc oxide. Typical Zn+ kinetic energies are 3-5 eV. At fluences (energy per unit area per pulse) below 200 mJ/cm2, the ion intensities (per laser pulse) decrease monotonically to low values with laser pulse number. The depletion kinetics change from exponential to second order near 50 mJ/cm2. We attribute this change to the annihilation of defects yielding Zn+ emission when Zn+ or other surface defects become mobile. At fluences between 200 and 300 mJ/cm2, Zn+ emission becomes more sustained due to defects created by the laser. In this same fluence range, we observe the onset of detectable neutral atomic zinc emission. These neutral atoms display Maxwell-Boltzmann kinetic energy distributions w th effective surface temperatures that approach 5000 K as the fluence is raised to 350 mJ/cm2. These high surface temperatures are remarkable given the low etch rates observed at these fluences, suggesting that heated layer is extremely thin. We propose emission mechanisms and experiments to resolve outstanding questions.