Electrical and optical properties of epitaxial and polycrystalline undoped and Al-doped ZnO thin films grown by pulsed laser deposition

Undoped and Al-doped (1.6%) ZnO films were prepared on (0001) sapphire and fused silica substrates using a pulsed laser deposition technique. The ZnO films on sapphire substrates were epitaxially grown, while the ZnO films on fused silica substrates were texturally grown. The films on sapphire substrates were ordered along the in-plane direction and had grains in which the c-axis was well aligned normal to the substrate. However, the films on fused silica were randomly oriented along the in-plane direction and had poor c-axial aligned grains. The structure analyses showed that the epitaxial ZnO films had low-angle grain boundaries, while the textured polycrystalline ZnO films had high-angle tilt and twist grain boundaries. The nature of the grain boundaries influenced the electrical and optical properties of the undoped and Al-doped ZnO films. Resistivity, Hall mobility, carrier concentration, and near band edge emission of the films were measured at room temperature and discussed in connection with the nature of grain boundaries.