Structural and physical properties of Co films DC-bias-plasma-sputter-deposited on MgO(001)

Co films 90 nm thick are deposited on MgO(001) substrates at a temperature of 250°C by DC plasma sputtering at 2.5 kV in pure Ar gas. A DC bias voltage Vs between 0 and −140 V is applied to the substrate during deposition. The structural and physical properties of the film are studied as a function of Vs by the use of reflection high-energy electron diffraction (RHEED), cross-sectional transmission electron microscopy (XTEM), X-ray photon spectroscopy (XPS) and by measuring the temperature coefficient of electrical resistance (TCR) from 50 to 150°C and the saturation magnetization 4πMf at room temperature. As ∥Vs∥ increases, the structure of the Co film which is always fcc transforms from a polycrystalline state to a textured state such as Co(001) ⊥MgO(001) and Co[010]⊥MgO[010] with larger grains at Vs = −50 V. The impurity which consists mainly of O and C decreases with increasing ∥Vs∥ from 0 V but reaches a slight maximum at −Vs = 110 V. The XTED lines of CoO are observed strongly at Vs = 0 V and weakly at Vs = − 110 V. Consistent with the above mentioned behavior of the structural properties, 4πMf as well as TCR (>0) take smaller values at Vs = 0 and −110 V. In conclusion, the properties of the Co film are most improved at −Vs = 140 V.