Electrical transport and Al doping efficiency in nanoscale ZnO films prepared by atomic layer deposition

In this work, the structural, electrical, and optical properties as well as chemical bonding state of Al-doped ZnO films deposited by atomic layer deposition have been investigated to obtain insight into the doping and electrical transport mechanisms in the films. The range in doping levels from 0% to 16.4% Al was accomplished by tuning the ratio of ZnO and Al2O3 ALD cycles. With X-ray photoelectron spectroscopy depth profiling and transmission electron microscopy, we could distinguish the individual ZnO and AlOx layers in the films. For films with a thickness of 40 nm, the resistivity improved from 9.8 m Omega cm for intrinsic ZnO to an optimum of 2.4 m Omega cm at 6.9 at. % Al. The binding energy of Zn 2p(3/2) increased by 0.44 eV from the intrinsic ZnO to the highest Al-doped ZnO. This shift can be ascribed to an increase of the Fermi level. Ex-situ spectroscopic ellipsometry and Fourier transform infrared spectroscopy were used to measure the optical properties from which the carrier concentration and intra-grain mobility were extracted. The results showed that with increasing Al content, the grain boundary mobility increased at first due to an increased Fermi level, and then decreased mainly due to the scattering at AlOx/ZnO interfaces. For the same reasons, the doping efficiency of Al for highly Al-doped ZnO dropped monotonically with increasing Al. Furthermore, a blue shift of the optical band-gap Delta E-g up to 0.48 eV was observed, consistent with the shifts of the Fermi level and the binding energy of the Zn 2p(3/2) state. (C) 2013 AIP Publishing LLC.