Intermediates in Ammonothermal GaN Crystal Growth under Ammonoacidic Conditions



Invited for the cover of this issue is the group of Rainer Niewa from the Institute of Inorganic Chemistry of the University of Stuttgart. The cover image illustrates several gallium-containing species as possible intermediates of GaN crystal growth in supercritical ammonia under ammonoacidic conditions as well as crystals of solid compounds recovered from ammonia.

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What prompted you to investigate this topic?

I gained my first experience in ammonothermal synthesis during my studies at University of Dortmund in the group of Prof. Herbert Jacobs, an early pioneer in this field. This technique was largely abandoned for a number of years, but colleagues working on crystal growth and chemical engineering recently convinced me of its technological importance for the development of superior nitride-based semiconductor materials. In nearly three years, a cluster of six groups with expertise in chemical engineering, development of high-pressure apparatus, in situ investigation, crystal growth, and solid-state chemistry was formed, and funding from the German Science Foundation within the research unit FOR 1600 was obtained. Investigation of the chemical processes underlying this technique is for me at the same time a journey in time to my own scientific past and a new challenge beyond the purely synthetic use.

Is this research topic mainly fundamental or rather applied?

Although our experimental approach has the point of view of basic science and is mostly curiosity-driven, it is expected to lay the foundations for applied research and development. The information gained should enable the modeling of the GaN crystal growth process and the optimization of the whole procedure with respect to temperature fields and gradients, process pressure, and control of fluid flow on the engineering side, as well as choice of nutrients, mineralizers, and their concentrations on the chemical side. This would lead to the production of large native substrates for homoepitaxial growth of materials for optoelectronic devices at reasonable costs. We should be able to prepare more binary and possibly multinary nitride-based semiconductor materials in the future.

IIn one word, how would you describe your research?

As I learned many new facets of the field through our cooperation and as I believe that we produce important information for other disciplines, I would like to call it “insightful”.

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