Temperature dependence of the conductivity in large-grained boron-doped ZnO films

The temperature dependence of the conductivity is investigated as a function of boron doping in large-grained, degenerate polycrystalline ZnO films prepared by low-pressure chemical vapor deposition. Carrier transport in undoped and lightly doped films is mainly controlled by the grain boundary; field emission through grain boundaries limits the conductivity below 90 K, while thermally activated thermoionic-field emission leads to an increase in the conductivity with the temperature near room temperature. In contrast, carrier transport in highly doped films is mainly governed by intra-grain scattering, which does not depend on the temperature for degenerate electron gases, limits the mobility below 120 K, whereas a metallic behavior (decrease in conductivity with increasing temperature) is observed at room temperature, which is linked to the ionized impurity scattering. The transition between the “semiconductor”-like and metallic-like behavior at room temperature takes place for a film with carrier concentration between 6×1019 and 9×1019 cm−3.