P 73 ISSRNS 2012: Abstracts / Synchrotron Radiation in Natural Science Vol. 11, No 1 – 2 (2012)
NEAR-ORDER STRUCTURE OF TRANSPARENT CONDUCTING OXIDES: X-RAY ABSORPTION STUDY OF Al-DOPED ZnO
AND ZnMgO FILMS
D.A. Zajac1∗, A. Bikowski1, M. Vinnichenko2, and K. Ellmer1
1Institut of Solar Fuels and Energy Storage Materials, Helmholtz-Zentrum Berlin f¨ur Materialien und Energie GmbH, Hahn-Meitner-Platz 1, 14109 Berlin, Germany
2Institut f¨ur Ionenstrahlphysik und Materialforschung, Helmholtz Zentrum Dresden-Rossendorf, Bautzner Landstraβe 400, 01328 Dresden, Germany
Keywords: X-ray absorption spectroscopy, transparent conductive oxides, solar cells, ZnO, ZnMgO, zinc oxide
∗e-mail : dariusz.zajac@helmholtz-berlin.de
ZnO belongs to the class of wide band gap semi- conductors with Eg = 3.37 eV and exhibits very interesting optical and electrical properties. ZnO is also under development as a cost-efficient alterna- tive to commonly used indium tin oxide (ITO) for applications in optoelectronic devices as a transpar- ent conductive oxide (TCO) electrode [1]. Such ap- plications require simultaneously high transparency in the solar spectral range and a low electrical re- sistivity, therefore ZnO has to be doped degener- ately. While doping with Al leads to the required high carrier concentration [1], alloying with Mg in- creases 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 = 1021 cm−3) can lead to a significant increase of light absorption in the near-IR spectral range.
Therefore, many researches focus on the increase of the electron mobility µ at fixed N , in accordance with formula ρ = (eN µ)−1.
The electron mobility is determined by the typ- ical scattering processes in semiconductors, among other the extrinsic scattering on dopants and de- fects in the film. Since Al doped ZnO and ZnMgO layers, used for transparent electrodes are polycrys- talline films, this process can be connected directly not only with dopant but also with local structure defects as Zn or/and O vacancies, stacking faults and grain boundaries. Small crystallographic do- main size (< 100 nm), misorientation of domains with respect to the surface normal and incorpora- tion the Al and Mg ions having ionic radius smaller than that of Zn3+ (respective crystal radii in tetra- hedral coordination are: Al3+ 0.53 ˚A, Mg2+ 0.71 ˚A and Zn2+ 0.74 ˚A [3]), 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 films. The XANES spectra were simulated with the program FEFF9 [4].
First results on the Al K edge (Figure 1), show that the Al substitutes preferably the Zn lattice site in the material. For both host materials (ZnO and Zn- MgO), Al K edge spectra show a similar behaviour.
The only clearly visible difference, the decrease of the intensity of the first peak for Al in ZnMgO com- pared to Al in ZnO, can be attributed to the higher substitution level (a similar effect is observed for higher Al doping 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 Al doping ion 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.
Figure 1 : Comparison of Al:K edge XANES simulations for Al doped ZnO and ZnMgO and the experimental data for Al 3 at.% doping of ZnO and ZnMgO (Mg 6 at.%). For the simulation a 0.6 eV broadening has been added according to the energy resolution of the experi- ment.
Acknowledgments: The authors acknowledge finan- cial support by the IGF Project No. 16595 BG (BMWi/AiF, Germany).
References
[1] K. Ellmer, A. Klein, B. Rechs, Transparent Conduc- tive 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. A 32 (1976) 751.
[4] J.J. Rehr, J.J. Kas, M.P. Prange, A.P. Sorini, Y. Takimoto, F. Vila, “Ab initio theory and calcu- lations of X-ray spectra,” Comptes Rendus Physique 10 (2009) 548 – 559.
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