Bubble-Free SrTiO3 Crystals 30 mm In Diameter By Flame-Fusion Growth Method
Different thin film applications require the availability of high-quality single crystal substrates. A popular oxide for such purposes is SrTiO3 (STO). STO is also studied as an active material for device fabrication taking advantage of new ideas, namely, that the oxygen-deficient regions will act as metallic conductors for nano-inclusions in an insulating matrix. Applications where STO is expected to make an impact are microelectronics, power, and energy, and the list is open for further developments and concepts.
STO has a high melting temperature (2350 K) and, owing to this, fabrication of single crystals is not trivial. The flame-fusion (Verneuil) method is the only approach employed industrially to grow STO single crystals. The other methods are not suitable for commercial production due to small growth rates, sizes, or low yield. In the flame-fusion method, mixtures of powders of Sr and Ti oxides added with SrCO3 and are melted in a flame produced by burning a mixture of oxygen and hydrogen. The melt is solidified on a seed of an STO single crystal.
Difficulties are related to evaporation of Sr, control of thermal gradients, and dynamic changes of the growth parameters during processing. The last feature is due to the influence of the growing crystal on the furnace environment. In addition, the process control is also limited because some of the key growth parameters are more or less dependent on each other. Under presented circumstances and for larger crystals, growth stability changes, and during one production shift, one may easily end up with micron size or larger bubbles in the crystal. Especially critical in this regard is the end of the growth.
The larger the crystal is, the higher the probability is of obtaining crystals with unwanted defects. Another problem is that after growth, larger crystals are increasingly difficult to be uniformly oxygenated. Considering these addressed problems, it is easy to understand that, usually, the diameter of the STO crystals on the market does not exceed 20 mm.
In the work performed by researchers from Furuuchi Chemical Co, Japan, Kanazawa Institute of Technology, Japan, National Institute for Materials Science, Japan, INOE 2000, Romania, and National Institute of Materials Physics, Romania, bubble-free single crystals of STO with a diameter of 30 mm were grown under industrial conditions and within one production shift (~5h of growth). The composition of the source powder mixture, in particular, the amount of SrCO3 additive, and other growth parameters were optimized. After oxygenation, single crystals were characterized from the structural, microstructural, optical and THz spectroscopy viewpoints.
These crystals show similar properties with commercial ones of smaller sizes. This enables their use as substrates for thin film growth. It also suggests the possibility of growing single crystals of larger sizes and for commercialization of large area substrates.