Effects of N-Doping on Magnetic Properties of ZnCoO Diluted Magnetic Semiconductor Thin Films

Radio-frequency magnetron sputtering technique was used in manufacturing thin films of Co doped and (Co, N) co-doped ZnO (Co:ZnO and Co:Zn(N, O)). The sputtering target was a commercial Zn-Co-O compound with 5% Co in weight. The sputtering gas of pure argon was used for Co:ZnO films; and two other different gases, a mixture of Ar and N₂ (gas flow-rate ratio 15 : 5 seem) and a pure N₂, was used for (Co, N):ZnO films. Without N-doping, the films show n-type electrical conduction. Upon N-doping, films become poorly conductive and even exhibit p-type conduction for pure N₂ sputtering-gas. Paramagnetism is observed in the film with no N-doping (Co:ZnO); but superparamagnetism and then a mixture of ferromagnetism plus paramagnetism were observed with increasing N-doping. Based on the magnetic phase diagram proposed by Coey, we explain in detail the distinct magnetic properties which are closely related with the microstructure and electrical properties of each sample.