Microstructure and electrical property correlations in Ga:ZnO transparent conducting thin films

In this paper, we report correlations between processing, microstructure, and electrical properties of Ga doped ZnO films. Films with different grain sizes were grown on amorphous glass substrate by changing the substrate and pulsed laser deposition variables. The results corresponding to these films were compared with those from epitaxial single crystal films grown on (0001) sapphire. Microstructural characteristics were analyzed in detail by using x-ray diffraction and transmission electron microscopy. Electrical properties were evaluated by resistivity measurements in the temperature range of 15–300K and Hall measurements at room temperature. It was observed that the grain boundaries and orientation of grains (texture characteristics) affected the carrier concentration and the mobility considerably in nanocrystalline films deposited on glass substrates. This effect is envisaged to occur as a result of trapping of electrons and buildup of a potential barrier across the grain boundaries. However, the resistivity in nanocrystalline films could be decreased significantly by carefully controlling the deposition conditions. For a film deposited on glass at 200°C and 1mtorr of oxygen partial pressure, we attained a minimum resistivity value of 1.8×10−4Ωcm. The epitaxial films on sapphire substrates showed a resistivity of 1.4×10−4Ωcm, deposited at 400°C and pressure of 2.4×10−2torr. The role of grain boundaries and defects in controlling the carrier generation and transport has been discussed in detail and the possible mechanisms limiting the electrical conductivity in films with different microstructures have also been identified.