Growth of Delafossite Substrate Crystals

• Applicant: Professor Dr. Dietmar Siche
• Subject Area: Synthesis and Properties of Functional Materials ; Thermodynamics and Kinetics as well as Properties of Phases and Microstructure of Materials
• Term: from 2015 to 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 267463722

Only in recent years Delafossites came again to the focus of research. They are known as transparent, p-type ABO2 oxide semiconductors. So far their popular properties such as p-type conduction for oxide electronics and high power yield in thermoelectrics, could not be adequately used, due to insufficiently developed growth methods. The crystalline perfection of thin, heteroepitaxial, preferentially by Pulsed Laser Deposition grown layers is low. Therefore, the potential of the material does not come to bear. The literature research on this proposal and the preliminary work on thermodynamics and crystal growth support the assumption, that CuAlO2 is a very promising Delafossite. Therefore, a novel growth technology will be developed for CuAlO2 substrate crystals for homoepitaxy. It is based on a refined experimental investigation of the phase diagram and on using a reactive atmosphere in the Top Seeded Solution Growth from the Cu2O native flux. Thus, there is a real chance to overcome the reasons for the slow progress of the last three decades regarding crystal size (currently 5 x 5 x 0.3 mm3 without seed) and phase purity (Cu+ - Cu2+ equilibrium is sensitive to oxygen and temperature). The aim of this project is to study the suitability of the concept of reactive atmospheres for the growth process and to understand this process. On this basis hitherto existing barriers could be overcome, which limited crystal size and perfection and thus the crystals appropriateness as epitaxial substrate could be proved. This goal will be achieved by growth experiments in a Czochralski system to be modified, where already melting behavior studies of copper oxide was studied in different crucibles. Here are the essential steps aptitude tests of various crucible materials, the pressing and sintering of powder mixtures, the seed preparation, growth experiments to optimize growth rates and crystal perfection and the material characterization promptly for growth, to use the results for choice of parameters in subsequent experiments. At last, after study of the influence of crystal perfection on the conductivity (free crystallites and bulk crystals compared to layers on foreign substrates), doping with alternative acceptors will be done to improve the electrical properties without essential limitation of crystals optical transparency.

• DFG Programme: Research Grants
• Co-Investigators: Privatdozent Dr. Detlef Klimm; Dr. Detlev Schulz