Optimization of incident electron energy for internal-detector electron holography with Monte Carlo simulation

Internal-detector electron holography is one of the atomic resolution holography methods to analyze local structures around specific elements by reconstructing 3D atomic images, and it can measure the holograms with table-top electron microscopes. Since the internal-detector electron holography uses electron beams as holographic waves, understanding of behaviors of the electron beams in solids is important to optimize the performance of the internaldetector electron holography. Here, we defined an evaluation function for the hologram measurements, f(E), which is obtained from characteristic X-ray intensity and the holographic amplitude with Monte Carlo simulation. Using this formula, the best electron energy for the Ti-K hologram from SrTiO3 bulk was estimated to be 0.5 keV above the Ti-K X-ray ionization energy. Next, the best energies of the electron beam for Pt thin film was calculated with varying its thickness since the quality of holographic data are degraded by the characteristic X-rays from the deep region of the sample due to breaking of the coherence of the incident electron beams. The energy profile of f(E) for the thin film is quite different from that for bulk. The estimated energies were as wide as 5.0-10.0 keV above the Pt-M X-ray ionization energy. Moreover, the best thickness of thin film for hologram measurement was obtained to be about 10 nm, which is close to the inelastic mean free path of the electron in the sample.