SUPERVISOR’S TESTIMONIAL on the master's thesis Shapenkov Sevastyan Vladimirovich " Core structure of dislocations and recombination properties of gallium nitride” Shapenkov Sevastyan Vladimirovich entered the magistracy at the Department of Solid State Electronics after graduating from the Geological Faculty of St. Petersburg State University in the Department of Crystallography. During two years of the master’s course study he successfully completed the missing basic knowledge in general physics and acquired extensive knowledge of the special disciplines of the master's program. In addition, he underwent a one-month internship in Germany at the University of Goettingen, where he obtained the skills in transmission electron microscopy (TEM) studies.  As a research topic, he proposed using the TEM technique to obtain data on the core structure of freshly introduced screw dislocations in gallium nitride, which, as was recently discovered in our laboratory, are sources of the intrinsic luminescence band. Shapenkov S.V. successfully coped with the task, despite the technical complexity of its implementation. He developed a new, very time-consuming method of preparing foils for TEM, and applied nanoindentation techniques to introduce dislocations. His research allowed the first time to establish a direct correlation between the luminescent properties and the structure of dislocation nuclei in gallium nitride.  His findings were included in a scientific article published in the high-ranking journal Journal of Applied Physics, which he co-authored, as well as in the abstracts of papers at two international scientific conferences.  I think that Sevastyan Vladimirovich Shapenkov deserves the assignment of a master's degree in physics, and his thesis is worthy of excellent evaluation in terms of its content and design.  1.06.2018							prof. O.F. Vyvenko