Microstructural analysis of GaN films grown on (1 0 0) MgF$_2$ substrate by 4D nanobeam diffraction and energy-dispersive X-ray spectrometry

TL;DR

This study investigates the microstructure of GaN films grown on MgF2 substrates using advanced electron microscopy and X-ray diffraction, revealing grain orientations, twin formations, and impurity diffusion effects at different growth temperatures.

Contribution

It provides detailed microstructural characterization of GaN films on MgF2, highlighting the effects of growth temperature on grain orientation, twin density, and impurity diffusion, which was not previously comprehensively analyzed.

Findings

Predominant cubic GaN orientations with no preferred nucleation.

Higher growth temperature increases twin density and surface roughness.

Mg and F in-diffusion causes cavity formation at the interface.

Abstract

The use of highly efficient and solarblind GaN photocathodes as part of multichannel plate UV detectors for applications in astronomy would strongly benefit from the direct growth of GaN on typical window materials with high transmission down to the deep UV range. GaN growth on MgF$_2$ substrates by plasma-assisted molecular beam epitaxy has recently been demonstrated. Here, we report an extensive scanning transmission electron microscopy study of the thin film microstructure for growth at 525 $^{\circ}$C and 650 $^{\circ}$C on (100) MgF$_2$. These results are systematically supported by X-ray diffraction reciprocal space maps. For both growth temperatures predominant cubic (111), (115) and (110) GaN is found with no preferred nucleation on the substrate and typical grain sizes of 100-200 nm. All observed orientations can be understood as the result of first and second order twins on different $\{111\}$ planes related to the underlying substrate. The higher growth temperature shows a strongly increased twin density along with a higher surface roughness. Furthermore, grains with cubic (110) GaN growth show a reduced density and a reduced size of about 20 nm. Furthermore, in-diffusion of Mg and F into the GaN is observed, which is accompanied by the formation of cavities in the MgF$_2$ directly at the interface.