Investigation of dissolution- and crystrallization equilibria as a basis for the ammonothermal crystalgrowth of nitrides

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The knowledge on the solubility of GaN in ammonothermal synthesis is a crucial prerequisite to understand and control the process as well as to optimize the operating conditions. In order to generate the solubility data and to obtain the optimal conditions for the ammonothermal process four different systems available in the second funding period: (i) In the first phase, an experimental set-up was built with six autoclaves having a possibility of autoclave filling station and three heating ovens with three zones for different synthesis functions. With this experimental set-up, solubility measurements under uniform heating conditions and the crystallization experiments with a temperature gradient along the length of autoclave are being performed in collaboration with TP 3, 6 and Z. (ii) A prototype double reactor consists of two autoclaves was designed to understand the sampling technique and to improve the optimization of the process. In the second period of the project, com-prehensive solubility measurements will be performed by collecting the samples from the double reactor and analyzing through ex situ characterization methods. (iii) In the second period of the project, the above-mentioned double reactor will be upgraded to multiple reactors with the integration of more sampling reactors to understand the crystallization kinetics. Mass balance of the ammonothermal process and the characterization of the products will be performed through this multiple reactor sampling and ex situ characterization methods, respectively. (iv) To the best of our knowledge, for the first time we will determine the solubility of GaN and the density of the supercritical ammonia using in situ measurement of ultrasonic velocity. In addition, knowledge on the crystallization kinetics will be obtained through this in situ ultrasonic method.