Bismuth telluride (Bi2Te3) has been recently established as an archetype for the three-dimensional topological insulator with a single Dirac cone on the surface, as determined experimentally from angle-resolved photoemission spectroscopy. Topological insulators are insulating in the bulk and exhibit gapless metallic surface states with linear energy-momentum dispersion shaped like a Dirac cone. Due to the strong spin-orbit coupling, these conducting surface states have electron momentum locked to the spin orientation and are protected from scattering mechanisms by time reversal symmetry. Consequently, high-mobility spin polarized surface currents can be produced without external magnetic fields, offering possibilities to new applications in spintronics or quantum computing .
Conductivity measurement of the metallic surface states in Bi2Te3 is hindered by the bulk conductivity due to intrinsic defects, like vacancies and anti-sites. Counter doping (Ca, Sn or Pb) is a way to control the Fermi level and suppress the bulk contribution . Another way is to grow films of high structural quality. Intrinsic conduction through topological surface states has been obtained in very thin insulating Bi2Te3 films grown by molecular beam epitaxy (MBE) [3,4].
Structural defects determine the n/p type and density of electrical carriers in the bulk. In MBE growth, the most probable structural defect to be formed are vacancies. Bismuth vacancies act like an acceptor (holes), while tellurium vacancies result in free electrons in the bulk. Depending on the growth kinetics, determined by the experimental parameters (substrate temperature, extra Te offer and deposition rate), different electrical behavior can be obtained. Theoretical modeling based solely on vacancy formation as a function of substrate temperature predicts gradual transitions from p-type (holes) to n-type (electrons) of charge carrier. However, experimental data have show different behaviour, implying in more complex structure of defects in epitaxial films than can be seen by either local probes or X-ray diffraction along the growth direction [5,6].
Here a multi-axis single crystal diffractometer is used to access diffraction vectors with different in-plane and out-plane components, revealing another dimension in terms of crystalline perfection to be taken into account when synthesizing epitaxic films of Bi2Te3. A more sensitive probe of latteral coherence in the films—based on second-order X-ray dynamical diffraction—is also used to shown that there is a very narrow gap in terms of growth parameters where in-plane long-range order in the films can be achieved.
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