Bulk Crystal Growth
Stability Fields of the Sn-О System
To grow bulk Sn02 single crystals, it is important to conclude how Sn02 is thermally stable. The stability fields of the Sn-0 system are shown in Fig. 6.3. The calculations show no liquid phase of Sn02(l), which leads to the conclusion that growing Sn02 crystals from the melt is technically difficult, if possible at all. Although there are reports indicating melting point of 1999°C [104], this value may not relate to stoichiometric or near stoichiometric Sn02 but rather to Sn+Sn02, where metallic Sn acts as a self-flux for Sn02 and lowers its melting point. Indeed, more comprehensive experiments by Galazka et al. [90], performed at high temperatures (>2000°C) and high oxygen partial pressure, p(02) (>100 atm.), did not show any melting signs of Sn02. Sn02 seems to be one of the most unstable binary compounds at high temperatures among other TSOs.
High thermal instability of Sn02 promotes, therefore, the growth from the gas phase, as it can be easily converted into gas phase. Indeed, the stability diagram clearly shows that the gas phase (ideal gas field) can be easily reached just above 1500°C at low p(02) =
10'5 bar. The gas phase field can be reached from both sides, when using Sn02 or metallic Sn (e.g., as solid Sn(s), which is heated up to liquid Sn(l)). At very low p(02) < 10~5 bar, metallic Sn is formed at temperatures even below 1550°C. The higher p(02) used for growing Sn02 crystals, the higher the operating temperature utilized.
These considerations are valid for the physical vapor transport (PVT) method, but not necessary for the chemical vapor transport (CVT) method, where Sn is transported via a chemically active transport agent at lower temperatures.
Figure 6.3 Stability diagram of Sn02 versus temperature. The field "ideal gas" enables the growth of Sn02 crystals from the gas phase.
Historical Events in Bulk SnО2 Crystal Growth
The first report on growing bulk Sn02 single crystals from the gas phase by the PVT method was published in 1961 by Marley and MacAvoy [105]. In 1965, the first Sn02 single crystals were grown by the flux and hydrothermal methods by Kunkle et al. [106] and Harvill and Roy [107], respectively. In that year the first Sn02 crystals obtained by the CVT method were reported by Nagasawa et al. [108] as well. In 2014, large, 1-inch diameter Sn02 crystals obtained by the PVT method were reported by Galazka et al. [90]. The dominating growth methods for Sn02 crystals are those involving gas phase, mainly PVT. Main historical events in growing bulk Sn02 single crystals are collected in Fig. 6.4.
Figure 6.4 Main events in obtaining bulk Sn02 single crystals.
Vapor Phase
The growth of Sn02 single crystals from the vapor (gas) phase can be divided into two main groups:
(i) Physical vapor transport (PVT) or sublimation technique, where gaseous SnO(g) is created at high temperatures in an evaporation zone and transported to a cooler growth zone, where it is oxidized to solid Sn02(s) in the form of single crystals. Here, we can distinguish two subgroups, where the starting (source) material is:
a. Sn02(s), which thermally decomposes and creates SnO(g), or
b. metallic Sn(s), which is oxidized at high temperatures to SnO(g);
(ii) Chemical vapor transport (CVT), where Sn is transported via a chemically active transport agent, such as Cl or I.
