Transport characteristics of thermally degraded ZnO films doped with Al

• Effect of annealing on the properties of AZO films were examined.
• Air-annealed AZO film showed thermally activated mobility behavior.
• Thermally degraded Hall mobility could be recovered by subsequent vacuum-annealing.
• Barrier height (Eb) and Fermi energy (EF) were calculated.
• The influence of GBS on electron transport was analyzed by the ratios Eb/EF.

The thermal degradation characteristics of Al doped ZnO (AZO) films, which were prepared by radio frequency magnetron sputtering at room temperature and subsequently annealed in air, were examined by making comparisons with those of the as-deposited and the vacuum-annealed AZO films. AZO films annealed in air at temperature higher than 300 °C exhibited severely degraded conductivity due to the decrease of carrier concentration and Hall mobility, which could be interpreted as grain boundary scattering from the thermally activated mobility behavior. Experimental observations, that vacuum-annealing of the as-deposited AZO film led to substantial reduction in carrier concentration and that subsequent vacuum-annealing of the air-annealed AZO film could not restore the carrier concentration unlike the restoration of Hall mobility up to 80%, led to a perception that substantial amount of free carriers in the as-deposited AZO film were generated from oxygen vacancies. It was proposed that ratios of the barrier height to the Fermi level, which were determined by taking into account the non-parabolic nature of the conduction band, could be used as a measure for the influence of grain boundary scattering on the overall electron transport mechanism in AZO films.