Two shallow donors related to Zn interstitial in S-ion implanted ZnO epitaxial film

Two shallow donors are observed in sulfur-ion implanted ZnO epitaxial films on sapphire substrates. The resistivity varies from ~103 Ω cm for un-implanted samples to 1.7×10−2 Ω cm for as-implanted ones. This low resistivity is attributed to zinc interstitial (Zni), corresponding to the activation energy (26 meV) estimated from the temperature dependence of electron concentration. The presence of Zni is evaluated from ion channeling by Rutherford backscattering spectroscopy. The disordered concentration is ~40% of Zn atoms at a maximum point (300 nm) from the surface in as-implanted samples and recovers to ~80% after 1000 °C annealing. In 1000 °C annealed samples, a shallower donor level of 12 meV is observed, suggesting the presence of complex defects consisting of remaining Zni and sulfur atoms replaced to the oxygen atom.