Europium Doping Impact on the Properties of MBE Grown Bi₂Te₃ Thin Film

The impact of europium doping on the electronic and structural properties of the topological insulator Bi₂Te₃ is studied in this paper. The crystallographic structure studied by electron diffraction and transmission microscopy confirms that grown by Molecular Beam Epitaxy (MBE) system film with the Eu content of about 3% has a trigonal structure with relatively large monocrystalline grains. The X-ray photoemission spectroscopy indicates that europium in Bi₂Te₃ matrix remains divalent and substitutes bismuth in a Bi₂Te₃ matrix. An exceptional ratio of the photoemission 4d multiplet components in Eu doped film was observed. However, some spatial inhomogeneity at the nanometer scale is revealed. Firstly, local conductivity measurements indicate that the surface conductivity is inhomogeneous and is correlated with a topographic image revealing possible coexistence of conducting surface states with insulating regions. Secondly, Time of Flight Secondary Ion Mass Spectrometry (TOF-SIMS) depth-profiling also shows partial chemical segregation. Such in-depth inhomogeneity has an impact on the lattice dynamics (phonon lifetime) evaluated by femtosecond spectroscopy. This unprecedented set of experimental investigations provides important insights for optimizing the process of growth of high-quality Eu-doped thin films of a Bi₂Te₃ topological insulator. Understanding such complex behaviors at the nanoscale level is a necessary step before considering topological insulator thin films as a component of innovative devices.