Challenges in melt growth of high melting point oxide single crystalsWednesday (26.09.2018) 11:15 - 11:45 S1/03 - 123 Part of:
In the past decades Czochralski pulling has become the technique of choice to synthesize single crystals of many types of materials including metals, semiconductors and isolators. This broad spread is due to the unique combination of available crystals sizes and growth rates and the extraordinary structural and chemical perfection of the obtained crystals.
Many oxide materials of scientific or technological impact have melting point temperatures near or beyond 2000°C. For one thing, this usually entails and extended application temperature range of single crystals, then again it also brings great challenges to the fabrication process.
Due to the entropic contribution to Gibbs energy, thermodynamic stability of metal oxides generally decreases with increasing temperature and a temperature-dependent oxygen minimum activity in the ambient is required to avoid decomposition. In case of transition metals and some rare-earth metals having several stable oxidation states, additional constraints may result from the necessity to avoid unwanted oxidation. Moreover, a typical setup for melt growth contains many different materials at significantly different temperatures, for example crucible and heaters, thermal insulation and constructive parts, and selection of an inert atmosphere, i.e. an atmosphere that does not change oxidation state of any of the involved materials, becomes an ambitious task that can be accomplished efficiently by thermodynamic modelling. In this paper, examples of successful application of this modelling e.g. to control the effective dopant distribution during growth will be presented.
A growth instability often occurring in Czochralski pulling of oxides of very high melting point temperatures known as spiraling or footing seriously degrades quality and reduces yield of the growing crystal. The origin of this type of instability is impeded heat transport through the crystal due to infrared absorption. A technique to avoid or delay the onset will be discussed for the case of rare-earth scandium oxides, REScO3, a family of crystals with perovskite structure widely used as substrates for ferroelectric films.