Growth, characterization, and properties of bulk SnO₂ single crystals

Abstract

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.

High-mobility BaSnO3 grown by oxide molecular beam epitaxy

“…There are only a few studies in the oxide MBE literature using evaporation of the metal oxide from an effusion cell. [19][20][21][22][23] Vapor pressure data indicate that SnO 2 sublimes in the form of suboxides (SnO x ) with vapor pressures sufficient for reasonable flux rates at practically feasible effusion cell temperatures, i.e., ∼10 −5 Torr at 1300 • C. 24 Figure 1(b) shows RHEED during BaSnO 3 growth on (001)SrTiO 3 at a substrate temperature of 800 • C, using the SnO 2 source. The RHEED pattern is streaky from the start of the growth and remains so throughout, without any significant decrease in the intensity.…”

Perovskite Solar Cells Based on Nanocrystalline SnO2 Material with Extremely Small Particle Sizes

“…The electron mobility of bulk SnO 2 is up to 200 cm 2 V À1 s À1 . [5] This value decreases to ,1.0 cm 2 V À1 s À1 for porous nanomaterial, [6] nevertheless, is two orders of magnitude higher than that of TiO 2 . Hossain et al have reported a very high photocurrent density that was achieved with quantum dots-sensitised solar cells using SnO 2 as electron-transport material.…”

Growth Measures to Achieve Bulk Single Crystals of Transparent Semiconducting and Conducting Oxides

“…Another growth from the vapor phase includes the PVT [65][66][67][121][122][123], which involves the decomposition of SnO 2 into SnO and oxygen at high temperatures (vaporization zone) and reoxidation of SnO to SnO 2 at lower temperatures (growth zone). Marley et al [65,121] used an alumina ceramic muffle tube furnace with a vaporization zone held at 1650 C and a longer zone with a temperature gradient of 20 K/in, where crystal growth occurred on a removable mullite tube.…”

“…Galazka et al [67] applied the PVT method with an iridium crucible containing the SnO 2 starting material, which was covered by a ring-shaped seed holder acting as a nest for a circular sapphire or rutile crystal seed (or substrate). The crucible was placed inside thermal insulation and was heated up inductively.…”

“…[67]. In the case of this class of electronic materials, the main properties include, among other ones, electrical properties FIGURE 6.14 Relationship between free electron concentration and growth conditions: substrate temperature and p(SnO)/p(O 2 ) partial pressure.…”