Doping efficiency and electron transport in Al-doped ZnO films grown by atomic layer deposition

Abstract

Transparent conducting Al-doped ZnO films were grown by atomic layer deposition (ALD). Al-doping was introduced by inserting 1 Al2O3 cycle per 28 ZnO cycles. The x-ray photoelectron spectroscopy showed that the density of the Al donors is 2×1021–3×1021 cm−3, while the Hall-effect measurements showed a ten times lower electron density. This low doping efficiency is a well-known inherent problem of the ALD method, and we wanted to explain its origin. We have found that the electron density is reduced by electron traps at the grain surface; however, the effect was too weak to explain the low doping efficiency. Therefore, the mechanism of the Al2O3 doping was analyzed. We have proposed that each Al2O3 molecule ideally provides two single-electron Al donors accompanied by one Zn vacancy, which acts as a two-electron acceptor. This would cause a perfect compensation; however, the compensation is in reality not perfect, which results in weakly efficient doping. Calculations also showed that each Zn vacancy creates a bound pair with an Al donor. To verify our doping model experimentally, it was inserted into the metallic transport theory and compared with the electron transport measurements. A good agreement was found for a broad range of experimental conditions. In the regime of weak localization, the conductivity showed the temperature dependence σ(T)=a+bT3/4, which is a signature of weak localization and electron–electron scattering in a 3D dirty metal.