The effect of oxygen annealing on ZnO:Cu and ZnO:(Cu,Al) diluted magnetic semiconductors

Here we report a systematic study of magnetic, electronic and microstructural properties of Cu-doped and Cu and Al codoped ZnO thin films, grown epitaxially on a (0001) sapphire substrate by the pulsed laser deposition technique. The films were annealed in oxygen at high temperature (600 degrees C) and their properties were compared with the as deposited films in order to study the role of defects like oxygen vacancies on the ferromagnetic properties. The doping of ZnO : Cu specimens with Al increased the carrier concentration by three orders of magnitude (from 10(17) to 10(20) cm(-3)) without altering the ferromagnetic ordering. On the other hand, a reduction in oxygen vacancies concentration brought about by high temperature annealing in oxygen had a large detrimental effect on ferromagnetism. These results tend to eliminate a free carrier mediated mechanism and point towards a defect mediated mechanism, such as a bound-magnetic-polaron mediated exchange, as being responsible for stabilizing long-range ferromagnetic ordering in these systems.