Properties of Transparent Conductive Oxides Deposited by Magnetron Sputtering

Transparent conductive oxides (TCOs) are attracting an increasing attention as materials for transparent electrodes in thin film solar cells. The characteristic features of TCOs are low formation energy of defects, which enables n-type doping, and low probability of compensating defects formation. These materials are mainly based on In2O3, SnO2 and ZnO, wide band gap (>3 eV) semiconductors, which may be easily converted by doping to degenerate semiconductors with electrical resistivity as low as ~10^-4 Ohm cm and high transmittance (>80%) in the visible spectral range. In addition to these properties, photovoltaic applications require TCO materials with increased near-IR transmittance, adjustable work function and defined surface morphology, which improves light trapping in PV absorber layer (e.g. a-Si:H-based cells). The need to decrease TCO production costs has stimulated research activity on reactive pulsed magnetron sputtering (RPMS) as a versatile deposition method.
In order to explore potential advantages of RPMS, the relationship between the deposition parameters and structure, phase composition, and physical properties of TCOs should be established. Understanding the mechanisms of donor impurity incorporation and its electrical activation is of special importance. For this purpose, indium oxide (IO), ITO, ZnO, and AZO films (with Al concentrations 0.7-8.7 at. %) were grown by RPMS with a precise control of the oxygen partial pressure at substrate temperatures ranging from ~40 °C to 580 °C. The magnetron plasma parameters were determined by a Langmuir probe. The resulting films were characterized by spectroscopic ellipsometry, Hall effect measurements, X-ray diffraction (XRD) and, in case of ZnO and AZO films, by X-ray absorption near edge spectroscopy (XANES). The Sn concentration in ITO was determined by Auger analysis, while the Al concentration in ZnO matrix was estimated by elastic recoil detection analysis.
The comparison of the real-time behavior of the IO and ITO film structure and electrical properties during annealing provides a direct evidence of Sn donor activation (with an estimated efficiency of 40%) in ITO due to amorphous-to-crystalline transition. The ITO film crystallinity always improves with increasing substrate temperature or during isothermal annealing, with the electrical resistivity decreasing. In contrast, the electrical resistivity of ZnO:Al films shows a clear minimum at a certain substrate temperature (250-400 °C), which depends on Zn/O2 flux ratio and correlates with a maximum in crystallinity (grain size). In this case, the highest mobility value of 46 cm2 V-1 s-1 is comparable to the best values achieved in AZO films grown by less cost-efficient techniques. This value is achieved at the free electron density of 6x10²⁰ cm-3 which corresponds to maximum ~30% electrical activation of Al impurity. At higher temperatures, the AZO electrical properties and crystalline quality deteriorate abruptly. This is likely due to formation of a new metastable phase, which is identified by XANES as homologous (ZnO)3(Al2O3). Its formation is triggered by an increase of the Al/Zn ratio in the film, and leads to electrical deactivation of the Al impurity in AZO. In order to improve electrical activation of the donor impurity and to extend the functionality of TCOs, the formation of epitaxial AZO and TiO2:Nb films by RPMS is proposed.