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 (I_Zn) or hole levels with zinc vacancies (V_Zn) is necessary to induce ferromagnetism, whereas the formation of electron levels with oxygen vacancies (V_O) 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 (V_O, I_Zn), an optimization of defect-related ferromagnetism in ZnO:Co is not possible at the moment. PACS 78.30.Fs; 91.60.Ed; 91.60.Mk