Transition from mobility-activated small polaron to carrier density-activated conduction of sol-gel-derived highly-oriented CuAlO2 thin film and enhanced thermoelectric properties

CuAlO2 is a technologically important material having diverse applications, including superior thermoelectric properties. Its unique crystallographic structure manifests an anisotropic environment for the charge carriers and phonons, which is considered to be the reason for the enhancement of the thermopower. To exploit this novel property, a controlled sol-gel deposition technique is adopted to realize highly c-axis-oriented growth of CuAlO2 thin film on Si and glass substrates. Thermoelectric measurements are performed in such a way that the carriers are confined along the a-b plane of the nanocrystal, which is parallel to the substrate. This allows a two-dimensional confinement of the charge carriers, leading to enhanced thermoelectric properties. Additionally, the temperature-dependent electrical characterizations depict two different charge-transport regimes with a cross-over at 360 K. The low-temperature region corresponds to the mobility-activated small-polaron conduction and the high-temperature region belongs to the semiconductor-type carrier-density-activated conduction. Calculation of polaron activation energy from low-temperature regime indicates considerable influence of band carriers (hole) on the polaronic levels, due to which the above-mentioned transition is manifested. Calculations of activation and Fermi energy from high-temperature regime reveal a deep acceptor level and shallow Fermi level, which is typical of a non-degenerate semiconductor with acceptors not fully ionized at room temperature.