Electrical and optical spectroscopy on ZnO : Co thin films

Magnetic oxide semiconductors, for example the highly transparent and intrinsically n-type conducting zinc oxide doped with the 3d transition metal Co (ZnO:Co), are promising for the emerging field of spintronics [1]. We investigated n-conducting ZnO:Co thin films with a Co content of nominal 0.02, 0.20, or 2.00 at.%. The substitution of Co cations in the tetrahedral sites of wurtzite ZnO with Zn was confirmed at low temperature by the 1.877 eV photoluminescence between crystal field split d-levels of Co2+ (d7) ions. Based on theoretical studies, it is predicted that the formation of electron levels with zinc interstitials (IZn) or hole levels with zinc vacancies (VZn) is necessary to induce ferromagnetism, whereas the formation of electron levels with oxygen vacancies (VO) is detrimental for ferromagnetism in ZnO:Co [2]. Cobalt generates a hole level in ZnO [3]. We investigated the generation of electron levels in n-conducting ZnO:Co in dependence on the Co content by means of deep level transient spectroscopy (DLTS). However, because of the ambiguous categorization of deep defects in n-conducting ZnO (VO, IZn), an optimization of defect-related ferromagnetism in ZnO:Co is not possible at the moment.