Photoluminescence and electrical properties of epitaxial Al-doped ZnO transparent conducting thin films

Epitaxial ZnO and Al-doped ZnO (AZO) thin films were grown on (0001)-sapphire substrates using pulsed laser deposition. The photoluminescence spectrum of the highly conductive (1.3×10 ³ S cm ^-1), as-grown AZO shows a poor near band edge (NBE) emission (3.30 eV) and no deep level emission at room temperature. In addition, the peak (3.386 eV) for the free excitons of AZO showed thermal quenching behavior with two activation energies (38.2 and 10.0 meV). The poor NBE emission is attributed to the nonradiative recombination center created by Al doping. Highly conductive (6.0×10 ² S cm ^-1) and intense NBE emitting AZO films could be achieved by the reduction of the nonradiative recombination centers through hydrogen annealing.