Effect of Ge concentration on the temperature dependence of photoluminescence from Ge-doped ZnO

The temperature dependence of photoluminescence (PL) has been studied for Ge-doped ZnO films grown on Si wafers by RF-magnetron sputtering and subsequently treated by rapid thermal annealing. A new PL line (named the G line) seen at 3.324 eV for Ge doping and its phonon replicas are most clearly observed at T < 200 K. At T ≥ 200 K, the G line is not seen and the free-exciton (FX) line and its phonon replicas are dominant. As the Ge concentration (n_Ge) is increased from 0.25 to o.75 mol%, the activation energy E_a in the Arrhenius plots of the G line increases gradually from 45.4 to 82.6 meV, but above n_Ge = 0.75 mol%, it increases sharply to 130.0 meV, which is thought to result from an increase in the Ge suboxide states with increasing n_Ge, as confirmed by the analysis of the near-edge X-ray absorption fine structure (NEXAFS). We suggest that the new NEXAFS line found at n_Ge = 1 mol% originates from the O 2p state hybridized with the Ge 3d state.