Improving electrical conductivity and its thermal stability of Al-doped ZnO polycrystalline films using ultrathin Al film as a passivation layer

Aluminum-doped ZnO (AZO) has been considered as a prospective material for application as transparent electrodes in solar cells. In this application, an improvement of the Hall mobility is desired to achieve the high conductivity, because an increase in the carrier concentration results in a decrease in transmission at near-infrared wavelengths. However, the achievement of high Hall mobility is still a challenge. Another challenge associated with AZO is the low thermal stability of the electrical properties, which limits the application of AZO. This limitation originates from Zn desorption and migration of oxygen from the environment into the AZO film. It has been shown that using an ultrathin Al capping layer on an AZO film can greatly improve its thermal stability. An improvement in the Hall mobility was obtained for an Al capped AZO film after annealing in N-2 gas at 400 degrees C. The Hall mobility reached 49.1 cm(2)/V, which is close to the theoretical limit of the mobility for an AZO polycrystalline film, with a moderate carrier concentration of 2.8 x 10(20) cm(-3), ensuring high transparency in the visible and near-infrared ranges. The obtained results are due to the passivation effect of the AlOx film formed by natural oxidation of Al film. Zn desorption was suppressed and the migration of O from the environment was reduced, leading to an improvement in the thermal stability. Once Zn desorption is prevented, improvement of the crystalline perfection due to thermal annealing can contribute to an improvement in the Hall mobility.