ZnO-based TCOs

Zinc oxide is an II-VI semiconductor with a hexagonal wurtzite crystal structure and wide direct band gap (EG=3.37 eV). This is a low-cost material, which, dependently on composition and structure, shows piezoelectric effect, optically pumped lasing (due to exciton binding energy as high as 60 meV), and can be degenerately n-type doped to form TCO.

ZnO-based TCOs attract a special attention as potential ITO-replacing materials. Addressing the following problems is crucial for wider applications of ZnO-based TCOs in thin film photovoltaics, solid-state lighting and liquid crystal display technologies:

- establishing the physical limits for the free electron mobility and increase of existing mobility values in ZnO
- understanding of the impurity incorporation and electrical activation mechanisms
- control of interfaces between ZnO-based films and typical semiconductor materials
- establishing relationship between the magnetron plasma parameters and the film properties, structure/morphology, phase composition.

Epitaxial films of ZnO-based materials may be used as a model system for these studies. The differences between two- and single-domain undoped ZnO epitaxial films, grown by reactive pulsed magnetron sputtering (RPMS), were studied [1]. RPMS in combination with an oxygen RF plasma pretreatment of the Al2O3(0001) single crystal substrate enables growth of single-domain ZnO epitaxial films (see Figure for TEM cross-section) at a growth rate as high as 1.2 nm/s [1].

 

Epi ZnO XTEM

Recent research is devoted to investigations of Al-doped ZnO (AZO) thin films grown by RPMS. A fine tuning of the oxygen partial pressure using gettering of O2 by sputtered metal atoms, along with substrate temperature variation, yielded AZO films with the highest mobility values so far reported for the present deposition technique [2].

It is also shown that deterioration of the film electrical properties above an optimum deposition temperature is due to formation of metastable secondary phase [3]. The latter also leads to a substantial decrease of the film crystallinity.

Projects

SAB/SMWA Project 11815/1854 SOLARMETALL, 2006-2007
AiF Project “Grenzflächenoptimierung zwischen transparenten Zinkoxidelektroden und photovoltaisch aktiven Schichten”, 2010-2012
BMBF-TUBITAK „RainbowEnergy“ Project, 2010-2013

Collaboration

  • Institut Solare Brennstoffe und Energiespeichermaterialien, Helmholtz Zentrum Berlin
    Instituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Cientificas, Madrid, Spain
    Middle East Technical University, Ankara, Turkey
    Bilkent University, Ankara, Turkey
    Roth & Rau AG
    VON ARDENNE Anlagentechnik GmbH
    Dresden Thin Film Technology GmbH i.G.
    Solayer GmbH

  • Vaciontec GmbH
    CreaPhys GmbH
    Heliatek GmbH
    GfE Fremat GmbH
    INTERPANE Glas Industrie AG
    Signet-Solar GmbH
    Robert Bosch GmbH
    Euroglas GmbH Silverstar & Solar

Publications

1. M. Vinnichenko, N. Shevchenko, A. Rogozin, R. Grötschel, A. Mücklich, A. Kolitsch, and W. Möller: Structure and dielectric function of two- and single-domain ZnO epitaxial films. J. Appl. Phys. 102, 113505 (2007).
2. S. Cornelius, M. Vinnichenko, N. Shevchenko, A. Rogozin, A. Kolitsch, and W. Möller: Achieving high free electron mobility in ZnO:Al thin films grown by reactive pulsed magnetron sputtering. Appl. Phys. Lett. 94, 042103 (2009).
3. M. Vinnichenko, R. Gago, S. Cornelius, N. Shevchenko, A. Rogozin, A. Kolitsch, F. Munnik, and W. Möller: Establishing the mechanism of thermally induced degradation of ZnO:Al electrical properties using synchrotron radiation. Appl. Phys. Lett. 96, 141907 (2010).

 

Contact: Dr. Vinnichenko, Mykola