Microstructure and Electrical Property Correlations in Ga:ZnO Transparent Conducting Thin Films

Abstract

ABSTRACTWe report the correlations between processing, microstructure and electrical properties of Ga doped ZnO films. Films with varying grain size were grown on amorphous glass 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 build up 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 2000C and 1 mtorr of oxygen partial pressure, we attained a minimum resistivity value of 1.8 × 10-4Ω-cm. As a comparison, the epitaxial films on sapphire substrates showed a resistivity of 1.4 × 10-4 Ω-cm, deposited at 4000C and pressure of 2.4 × 10-2 torr. Role of grain boundaries and defects in controlling the carrier generation and transport is considered in detail and possible mechanisms limiting the electrical conductivity in films with different microstructures are identified.