Origins of low resistivity and Ge donor level in Ge ion-implanted ZnO bulk single crystals

The energy level of Ge in Ge-ion implanted ZnO single crystals is studied by Hall-effect and photoluminescence (PL) methods. The variations in resistivity from ∼103 Ωcm for un-implanted samples to ∼10−2 Ωcm for as-implanted ones are observed. The resistivity is further decreased to ∼10−3 Ωcm by annealing. The origins of the low resistivity are attributed to both the zinc interstitial (Zni) related defects and the electrical activated Ge donor. An activation energy of Ge donors estimated from the temperature dependence of carrier concentration is 102 meV. In PL studies, the new peak at 372 nm (3.33 eV) related to the Ge donor is observed in 1000 °C annealed samples.