AVS 59th Annual International Symposium and Exhibition
    Thin Film Tuesday Sessions
       Session TF-TuM

Paper TF-TuM5
Optimization of Properties of Al-doped ZnO Films Deposited by Atomic Layer Deposition

Tuesday, October 30, 2012, 9:20 am, Room 11

Session: ALD Reactions and Film Properties
Presenter: Y. Wu, Eindhoven University of Technology, the Netherlands
Authors: Y. Wu, Eindhoven University of Technology, the Netherlands
P.M. Hermkens, Eindhoven University of Technology, the Netherlands
B.W.H. van de Loo, Eindhoven University of Technology, the Netherlands
H.C.M. Knoops, Eindhoven University of Technology, the Netherlands
F. Roozeboom, Eindhoven University of Technology, the Netherlands
W.M.M. Kessels, Eindhoven University of Technology, the Netherlands
Correspondent: Click to Email
ZnO is widely used in solar cell windows, and as active layers in gas sensors. Often Al-doping is applied to decrease its resistivity. However, the chemical environment and electrical properties of Al-doped ZnO are not fully understood and the doping efficiency of Al is not optimized yet. In this work, 40 nm Al-doped ZnO layers were deposited on 450 nm SiO2/Si-substrate at 250 °C by ALD using ZnEt2, AlMe3 and water vapor as precursors. The Zn-O/Al-O cycle ratios were varied corresponding to an Al-content ranging from 0 at.% to 17.4 at.%. The resistivity improved from 8.2 mΩ·cm for intrinsic ZnO to an optimum of 2.2 mΩ·cm at 6.8 at.% Al-content. The stoichiometry, distribution and chemical environment of Zn, Al and O elements were studied by angular-resolving and depth-profiling X-ray photoelectron spectroscopy (XPS). With XPS sputter depth profiling we could distinguish the individual ZnO and Al-O lamellae in the films grown with high cycle ratios, whereas films grown with low cycle ratios showed a more homogeneous composition. The binding energies of Al 2p3 and Zn 2p3 increase by 0.23eV and 0.44eV for intrinsic ZnO to highest doped AZO, respectively. This shift is ascribed to an increase of the Fermi level, and secondly, to the delocalization of bonded electrons from ZnZn0 to AlZn+.

Ex-situ SE and Fourier transform infrared spectroscopy were applied to measure the optical properties, from which the carrier concentration and intra-grain mobility were extracted by modeling. The relative permittivities ԑ1 and ԑ2 were obtained from the modeling as well and the optical band gap was determined by Tauc-plot fitting. The optical band gap increases from 3.29 eV for intrinsic ZnO to 3.77eV for the highest doped AZO (17.4 at.% Al), corresponding to the Burstein-Moss effect and an increase of the Fermi level. Meanwhile, the total mobility was determined by Hall measurement. Combined with the intra-grain mobility, the mobility at grain boundaries (GB) can be calculated. The result shows that with increasing Al%, the barrier at GB decreases at first due to an increased Fermi level and increases next due to alumina clustering at the GB. The Al-doping efficiency, as calculated from the carrier concentration, shows that the doping of Al in ZnO phase is saturated at 6.8 at.% Al. Above this value, the Al incorporated mainly forms alumina at GB, which decreases the mobility while hardly leading to higher carrier concentrations.

In summary, the chemical and electrical properties of Al-doped ZnO were measured and explained properly, and the doping efficiency was optimized at 6.8 at.% Al, which is useful for further study and applications.