Growth, characterization, and properties of bulk SnO2 single crystals.

SnO₂ is a semiconductor with a wide optical bandgap (3.5 eV), which makes it an attractive transparent semiconducting oxide (TSO) for electronic and opto-electronic applications. At elevated temperatures it is, however, much more unstable than other TSOs (such as ZnO, Ga₂O₃, or In₂O₃). This leads to a rapid decomposition even under very high oxygen pressures. Our experiments showed that stoichiometric SnO₂ does not melt up to 2100 °C, in contradiction to earlier published data. Bulk SnO₂ single crystals, that could provide substrates for epitaxial growth, have not been reported so far. Hereby we report on truly bulk SnO₂ single crystals of 1 inch diameter grown by physical vapor transport (PVT). The most volatile species during SnO₂ decomposition is, in addition to oxygen, SnO, which is stable in the gas phase at high temperature and reacts again with oxygen at lower temperatures to form SnO₂. We identified a relatively narrow temperature window, temperature gradients and a ratio of SnO/O₂ for providing the best conditions for SnO₂ single crystal growth. X-ray powder diffraction (XRD) proved the single SnO₂ phase. Moreover, by selecting a suitable SnO/O₂ ratio it was possible to obtain either n-type conductivity with electron concentrations up to 2 × 10¹⁸ cm⁻³ and electron mobilities up to 200 cm² V⁻¹ s⁻¹, or insulating behavior. The crystals exhibited an optical absorption edge located at 330-355 nm, depending on the crystal orientation, and a good transparency over visible and near infrared (NIR) spectra. [ABSTRACT FROM AUTHOR]