Characteristics of cobalt-doped zinc oxide thin films prepared by pulsed laser deposition

We have characterized Zn_1–xCo_xO thin films grown on sapphire (0001) substrates by pulsed laser deposition. We have found for x = 0.25 that inhomogeneous Zn_1–xCo_xO films with wurtzite ZnO phase mixed with rock-salt CoO and hexagonal Co phases are formed when the growth temperature (T_g) is relatively high (⩾600 °C) and the O pressure (Po₂) is fairly low (⩾10^–5 torr). The presence of the Co clusters leads to room temperature ferromagnetism. Homogeneous Zn_1–xCo_xO alloy films with wurtzite structure are predominantly paramagnetic. High-quality epitaxial Zn_1–xCo_xO alloy films are formed at a growth condition of T_g= 600 °C and Po₂ = 10^{–5 torr. Under this optimized growth condition, Co can be dissolved into ZnO up to around 40%. Photoluminescence spectra and electrical properties of Zn_1–xCo_xO strongly depend on the Co content.}