Near order structure of transparent conducting oxides: X-ray absorption study of Al-doped ZnO and ZnMgO in low doping regime

ZnO belongs to the class of wide band gap semiconductors (Eg>3eV), with Eg=3.37eV, and exhibits very interesting optical and electrical properties. Used as a transparent conductive oxide (TCO) electrode in optoelectronic devices [1], ZnO has been developed as a low costs material, an alternative to commonly used and indium tin oxide (ITO). Due to the required transparency, at least in the solar spectral range and a low resistivity, ZnO has to be doped degenerately. While doping with Al leads to the required high carrier concentration [1], alloying with Mg increases the band gap up to 4.5 eV [2]. However, the increase of the conductivity by the increase of the carrier concentration (i.e. reaching the level of N=10^21cm^-3) can lead to a significant increase of the absorption of the light. Therefore, many researches focus on the increase of the electron mobility μ, in accordance with formula ρ=(eNμ)^-1.
The electron mobility is determined by the typical scattering processes in semiconductors, among other the extrinsic scattering on dopants and defects in the film. Since Al doped ZnO and ZnMgO layers, used for transparent electrodes are polycrystalline films, this process can be connected directly with dopants but also with local structure defects as Zn or/and O vacancies, stacking faults and grain boundaries are. These together with the small crystallographic domain size, in the range of 100nm, preferred growing orientation in c axis (perpendicular to the substrate), together with the Al and Mg dopants ionic radius smaller than for Zn [3] (respective crystal radii in tetrahedral coordination are: Al3+ 0.53Å, Mg2+ 0.71Å and Zn2+ 0.74Å), can cause the decrease of the local ordering of the samples.
In this paper we present XAS measurements on the Al doped ZnO and ZnMgO. The XANES spectra were fitted with the simulation program FEFF9 [4]. First results on the Al K edge, see Figure 1, show that the Al substitutes preferably the Zn lattice site in the material. For both dopants, Al and Mg, the spectra show a similar behaviour. The only clearly visible difference, the decrease of the intensity of the first peak for Al and Mg doped ZnO, in comparison to Al doped ZnO, can be attributed to the higher doping level (a similar effect is observed for double Al substitution of ZnO). The comparison of the experimental data with the simulation shows that the measured samples (3 at. % Al doped ZnO and 3 at. % Al doped ZnMgO (6 at.% Mg)) are in the low doping regime, where only single ion doping can be considered. The effect of the expansion of the c axis and the compression of a axis for the growth on glass substrate is also observed.

Acknowledgments: This work was supported by the German Ministry for Economy (AiF Köln).

Reference
[1] K. Ellmer, A. Klein, B. Rechs (Eds.), "Transparent Conductive Zinc Oxide: Basics and Applications in Thin Film Solar Cells", Springer, Berlin (2008)
[2] S. Choopun, R. D. Vispute, W. Yang, R. P. Sharma, T. Venkatesan, H. Shen, “Realization of band gap above 5.0 eV in metastable cubic-phase MgxZn1-xO alloy films” Appl. Phys. Lett. 80 (2002) 1529
[3] R.D. Shannon, “Revised Effective Ionic Radii and Systematic Studies of Interatomic Distances in Halides and Chalcogenides”, Acta Cryst. A32 (1976) 751
[4] J.J. Rehr, J.J. Kas, M.P. Prange, A.P. Sorini, Y. Takimoto, F. Vila, “Ab initio theory and calculations of X-ray spectra”, Comptes Rendus Physique 10 (2009) 548-559