Abstract
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•The dielectric function of freestanding GaN nanomembranes is studied.•The dielectric function is described with the help of monochromated valence EELS.•Optical characteristics n, k, α and R are numerically deduced.
In the present study, the dielectric function of freestanding GaN nanomembranes was described using valence electron energy loss spectroscopy coupled to a monochromated electron probe in a Sub-Electron-volt Sub-Angstrom transmission electron Microscope. Such a technique has proven to be very efficient in handling nanometer-scale freestanding nanomembranes and provided local and direct measurements of optical characteristics. Indeed, core-state along with outer-shell electrons transitions were first measured in the configuration where the GaN c→-axis was normal to the transferred momentum q→. With the help of Kramers-Kroning analyses and density functional theory, critical points energies were extracted and assigned. Finally, optical data such as the real refractive index n, extinction coefficient k, absorption coefficient α and reflectivity R were numerically deduced from the real and imaginary parts of the dielectric function.