Photoluminescence studies on MBE grown Co-doped ZnO thin films fabricated through ion implantation and swift heavy ion irradiation

The temperature dependant photoluminescence of the Co-doped ZnO thin films, prepared by ion implantation on the MBE grown ZnO thin films followed by swift heavy ion irradiation, were investigated. The phenomenon of negative thermal quenching (NTQ), where the photoluminescence (PL) intensity increases with temperature, in contrast to the usual behavior of decrease in intensity with temperature, has been observed. The I3 peak and the peaks (a, b, c, d, and e), corresponding to t2g and eg levels of the crystal field split Co d orbitals exhibit the NTQ behavior. The NTQ temperature range 35–45 K observed in un-doped ZnO shifts towards lower temperature with the Co doping. The increased number of dopant related and/or the vibrational/rotational resonance states with lower activation energies, from which the thermal excitation of the electrons takes place to the initial state of the PL transition, are responsible for the NTQ behavior.