Restoring the Properties of Transparent Al-Doped ZnO Thin Film Electrodes Exposed to Ambient Air

The properties of Al-doped ZnO (AZO) films are known to degrade with exposure to humidity. Different AZO films deposited using reactive direct current magnetron sputtering (DCMS) and high power impulse magnetron sputtering (HiPIMS) have been aged in ambient laboratory conditions and annealed at temperatures between 160 and 180 degrees C in a N-2 atmosphere. Their electrical and optical properties, which have been investigated both ex situ and in situ during the annealing, are improved. The results of the in situ measurements are interpreted in terms of a diffusion process, where hydroxyl groups are decomposed and water is diffusing out of the films. As hydroxyl groups are known to act as a trap for charge carriers in ZnO, their removal from the film can explain the improvement of the electrical properties by the annealing.