On the T2 deep level in zinc oxide thin films

For the majority of deep levels studied in n - type conducting ZnO by means of capacitance spectroscopy only the activation energy and the high temperature limit of the electron capture cross - section are known since these quantities can be evaluated easily from the temperature dependence of the trap’s thermal electron emission rate. We focused on the T2 level occuring in pulsed laser deposition grown ZnO thin films. In order to tune the T2 concentration in the samples, we employed different growth and annealing conditions as well as the implantation of oxygen and zinc ions, respectively. The physical properties of T2 were studied by sophisticated deep level transient spectroscopy and photo - capacitance experiments. These experiments revealed a strong dependence of the thermal activation energy, 185meV < Ea < 280meV, on the concentration of T2 in the sample as well as on the electric field (Poole - Frenkel effect). T2 was found to be preferentially generated under zinc rich conditions as both, the implantation of zinc ions and thermal annealing at low oxygen partial pressures increase its concentration. From photo - capacitance transients the photo - ionisation cross - section spectrum was calculated.