Subproject 6



Technological, thermodynamic and kinetic aspects of the ammonothermal crystal growth of nitrides as semiconductor and NL optic materials


One goal of this project is the determination of existing contamination sources in the ammonothermal synthesis and their potential for contaminating the growing crystals. Moreover, we have to figure out how these impurities affect the growth process and the electronic properties of the crystal. This leads to a continuous improvement of the process purity, whereat in the first phase of the project the focus is on the purity of the low pressure gas supply and the raw materials used. At the end of the first part of the project (project years 1-3), an autoclave for ammonothermal growth of GaN should be available, which would ensure the production of GaN for demonstration. For the growth of larger crystals, the control of the actual temperature fields with high accuracy is extremely important. Therefore, this project focuses on the realization of the corresponding heater geometry, which allows the control and adjustment of the temperature fields with accuracy better than 1 K.

Based on the theoretical models of Chen, as already suggested in literature, the temperature fields will be simulated in the selected setup. In order to verify the model and the correctness of the calculated temperature distribution, it is essential to compare the simulation with the experimental data. This requires a temperature measurement in the inner autoclave.

The design of the low-pressure gas supply will take place in close cooperation with sub-project 3 and will be implemented in the local laboratory plant in order to optimize the supply of several autoclaves with high-purity gases. A suitable autoclave for the crystal growth is divided into three temperature zones. In addition, the in situ measurement of temperature in the reaction chamber requires thermocouples in central positions. Only the direct measurement inside the autoclave delivers a reliable validation of the simulation data and assures that effects of additional substances compared to pure supercritical ammonia, are detectable. At the end of the first phase, there should be an experimental setup, which allows the growth of nitride crystals on GaN as an example. Therefore we intend to develop both an ammonobasic and an ammonoacidic reference process.

The necessary precursors and the essential chemical knowledge regarding the intermediates will be developed in close cooperation with sub-projects 1 and 2. If possible, the results of thermodynamic investigations of sub-project 5 could be used to improve the thermal process control.

This sub-project is characterized by an intensive interaction with the other sub-projects and the combination of their results. The aim is to develop an overall picture of the processes inside the reactors and transfer the knowledge to establish a usable growth process of nitride crystals. In conclusion, the overall aim is the ammonothermal synthesis for the manufacture of modern key materials for the electronics, optics and energy technology as well as to provide new materials not only as powder but also as large single crystals in Germany. Because of the strong interdisciplinary features of the subject this aim can only be realized as a team.