Microstructural Investigations of Epitaxial GaN Films Grown on MgF2 using Transmission Electron Microscopy
by Tobias Wolfgang Günter Niemeyer
Date of Examination:2023-09-07
Date of issue:2023-10-27
Advisor:Prof. Dr. Michael Seibt
Referee:Prof. Dr. Michael Seibt
Referee:Prof. Dr. Hans Christian Hofsäss
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EnglishFor the development of novel solar-blind UV multichannel plate detectors planned for applications in astronomy, it would be advantageous to grow GaN acting as the photocathode directly on a typical window material like MgF$_2$ providing a high transmission down to the deep UV range. For this study, GaN films have been grown with plasma-assisted molecular beam epitaxy on $(100)$ MgF$_2$ (by K. Meyer and D. Schaadt at TU Clausthal) at growth temperatures of 525 °C and 650 °C in order to investigate the microstructure and the chemical composition of the GaN films with scanning transmission electron microscopy. As a first step for transmission electron microscopy experiments, a good lamella preparation is crucial for reliable results. Besides the standard preparation with the focused ion beam in cross-section geometry, two approaches for plan-view preparation by using the benefits of the focused ion beam are demonstrated in this work. With these methods, site-specific and high-quality lamella preparation can be performed in each desired geometry. To analyze the microstructure, nanobeam electron diffraction is used, for which a diffraction pattern at each scan position in real space is collected. With this data, an orientation mapping with a resolution on the nanometer scale is performed. As during the acquisition of the nanobeam electron diffraction data, several thousands of diffraction patterns are recorded an automated analysis is needed which is performed by virtual dark field mapping using a plug-in in Gatan DigitalMicrograph written by T. Meyer (PhD thesis 2020) and by automated crystal orientation mapping using the python library py4DSTEM. The chemical analysis, performed by energy dispersive X-ray spectrometry, shows in-diffusion of Mg and F into the first 20-30 nm of the GaN film grown at 650 °C while no significant in-diffusion is observed in the GaN film grown at 525 °C. For the general structure of the film, a higher surface roughness and hole formation in the MgF$_2$ at the interface of GaN and MgF$_2$ is observed at the sample with the higher growth temperature. The microstructural analysis reveals the growth of five different orientations of cubic (zincblende) GaN with growth directions along $\langle110\rangle$, $\langle111\rangle$ and $\langle115\rangle$ and no significant amount of hexagonal (wurtzite) GaN. A detailed analysis of the five c-GaN orientations shows that the orientations are related as (higher-order) twins. In combination with experiments in cross-section and plan-view geometry, there is evidence that the twinning occurs only on two out of in general four possible glide planes. This observation is supported by a calculation of the force on partial dislocations caused by the anisotropic misfit strain, assuming the initial growth to be either along $\langle110\rangle$ or $\langle111\rangle$. For the initial growth, there is an indication that the growth along $\langle110\rangle$ might be preferred but at this point further investigations are necessary. The sample grown at 525 °C shows a much lower twin density and has a better film quality regarding the surface roughness and the interface to the MgF$_2$. This makes the lower growth temperature more suitable for the application of GaN films as photocathode in a UV multichannel plate detector with a MgF$_2$ window.
Keywords: FIB preparation; Transmission electron microscopy; Gallium nitride; Nanobeam electron diffraction; Twinning