Title: Structural disorder of natural <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>Bi</mml:mi><mml:mi>m</mml:mi></mml:msub><mml:msub><mml:mi>Se</mml:mi><mml:mi>n</mml:mi></mml:msub></mml:mrow></mml:math> superlattices grown by molecular beam epitaxy
Abstract: The structure and morphology of ${\mathrm{Bi}}_{m}{\mathrm{Se}}_{n}$ epitaxial layers with compositions ranging from ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ to the ${\mathrm{Bi}}_{1}{\mathrm{Se}}_{1}$ grown by molecular beam epitaxy with different flux compositions are investigated by transmission electron microscopy, high-resolution x-ray diffraction, and atomic force microscopy. It is shown that the lattice structure changes significantly as a function of the beam flux composition, i.e., Se/BiSe flux ratio that determines the stoichiometry of the layers. A perfect ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ phase is formed only with a sufficiently high additional Se flux, whereas ${\mathrm{Bi}}_{1}{\mathrm{Se}}_{1}$ is obtained when only a BiSe compound source without additional Se is used. For intermediate values of the excess Se flux during growth, ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3\ensuremath{-}\ensuremath{\delta}}$ layers are obtained with the Se deficit $\ensuremath{\delta}$ varying between 0 and 1. This Se deficit is accommodated by incorporation of additional Bi-Bi double layers into the ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ structure that otherwise exclusively consists of Se-Bi-Se-Bi-Se quintuple layers. While a periodic insertion of such Bi double layers would result in the formation of natural ${\mathrm{Bi}}_{m}{\mathrm{Se}}_{n}$ superlattices, we find that this Bi double-layer insertion is rather stochastic with a high degree of disorder depending on the film composition. Therefore, the structure of such epilayers is better described by a one-dimensional paracrystal model, consisting of disordered sequences of quintuple and double layers rather than by strictly periodic natural superlattices. From detailed analysis of the x-ray diffraction data, we determine the dependence of the lattice parameters $a$ and $c$ and distances of the individual (0001) planes ${d}_{j}$ as a function of composition, evidencing that only the in-plane lattice parameter $a$ shows a linear dependence on composition. The simulation of the diffraction curves with the random stacking paracrystal model yields an excellent agreement with the experimental data and it brings quantitative information on the randomness of the stacking sequence, which is compared to growth modeling using Monte Carlo simulations. The analysis of transmission electron microscopy data furthermore confirms that the Bi-Bi bilayers contain a large amount of vacancies of up to 25%. Conductivity and Hall data confirm that ${\mathrm{Bi}}_{m}{\mathrm{Se}}_{n}$ phases containing Bi-Bi double layers exhibit a rather semimetallic behavior.
Publication Year: 2018
Publication Date: 2018-05-07
Language: en
Type: article
Indexed In: ['crossref']
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Cited By Count: 17
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