Magnetotransport in epitaxial films of the degenerate semiconductor Zn1-xCoxO

Magnetotransport measurements are performed over a broad range of temperature (T) and magnetic field (H) on highly degenerate n-type Zn1-xCoxO (0 less than or equal to x less than or equal to 0.30) epitaxial films. The cobalt-free samples are characterized by a metallic resistivity p(T) down to 2 K, a negative and predominantly isotropic magnetoresistance (MR) and optical transmission above 85% in the visible range of the electromagnetic spectrum. X-ray diffraction measurements show that while for x less than or equal to 0.20, all cobalt atoms occupy the tetrahedral sites of the wurtzite structure of ZnO, a phase separation into CoO is seen for x > 0.2. In the solution phase, we do not observe any signatures of a spontaneous ordering of the cobalt spins despite a large concentration of mobile electrons (> 10(20) cm(-3)). The absence of anomalous Hall resistance is consistent with this observation. The carrier concentration (n) over the entire range of x remains above the Mott limit for the insulator-to-metal transition in a doped semiconductor. However, while the Co-free samples are metallic (T > 2 K), we see a resistivity (rho) minimum followed by ln T divergence of rho(T) at low temperatures with increasing x. The magnetoresistance of these samples is negative and predominantly isotropic. Moreover, the MR tends to follow a log H behaviour at high fields. These observations, including the Kondo-like minimum in the resistivity, suggest s-d exchange dominated transport in these dilute magnetic semiconductors.