Structural and magnetic properties of annealed ZnO–Co digital alloys

Abstract

Diluted magnetic semiconductor (DMS) materials, which can utilize both the spin and charge properties of carriers, have attracted attention because the combination of two degrees of freedom holds promise for potential applications in spintronic devices [1, 2]. Among the various DMS materials, Ga1−x Mnx As and In1−x Mnx As DMS thin films grown on GaAs substrates have been the most extensively studied [3–6]. However, until now, the highest ferromagnetic transition temperature (Tc) obtained from III–V Ga1−x Mnx As DMS thin films has been 110 K [7]; thus, various studies on increasing the Tc of DMS materials have driven considerable efforts with the aim of realizing spintronic devices operating at higher temperature. Among several candidate DMS materials with a high Tc, (Zn1−x Cox )O DMSs are of current interest because they are theoretically expected to have high Tc values, as a consequence of the large energy gap and effective mass of (Zn1−x Cox )O [8, 9]. Since there are inherent problems due to possible existence of excess Co and CoO, the direct formation of (Zn1−x Cox )O films is very difficult. Thus, an alternative growth method of the (Zn1−x Cox )O films necessary for improving the crystallinity of films, and the improvement of the crystallinity of the films is very important in enhancing the value of Tc. Even though few works concerning the formation and the magnetic properties of the (Zn1−x Cox )O films on sapphire substrates were performed [10, 11], studies on the formation and the magnetic properties of the (Zn1−x Cox )O films on Si substrates have not been performed yet. Since Si substrates with large areas and good qualities are relatively cheap and extensively available in comparison with sapphires, Si technologies offer the potential applications for fabricating spintronic devices. This letter reports the effects of thermal annealing on the structural and optical properties of ZnO–Co digital alloys grown on p-Si (100) substrates by using the radio-frequency magnetron sputtering method. X-ray diffraction (XRD) measurement were performed to investigate the crystallization of the as-grown and the annealed ZnO–Co digital alloys, and superconducting quantum interference device (SQUID)