Magnetotransport in epitaxial films of the degenerate semiconductor Zn_1-xCo_xO

Abstract

Magnetotransport measurements are performed over a broad range of temperature (T) and magnetic field (H) on highly degenerate n-type Zn_1-xCo_xO (0 ≤ x ≤0.30) epitaxial films. The cobalt-free samples are characterized by a metallic resistivity ρ(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 ≤ 0.20 , all cobalt atoms occupy the tetrahedral sites of the wurtzite structure of ZnO, a phase separation into CoO is seen for xgt;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 (gt;10²⁰ 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 (Tgt;2 k), we see a resistivity (ρ) minimum followed by lnT divergence of ρ(T) at low temperatures with increasing x. The magnetoresistance of these samples is negative and predominantly isotropic. Moreover, the MR tends to follow a logH 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.