Observation of Surface Metallic States in Ultrathin Bi2Se3 Topological Insulator Epitaxially Grown on AlN
Topological insulators (TI), Bi2Se3 in particular, exhibit robust 2D surface states in which spin is strongly coupled to the orbital motion creating the prospect for the realization of electronic devices with novel switching mechanism and novel functionalities. An important requirement is that Bi2Se3 can be grown epitaxially in the form of very thin films, so that the surface properties dominate over the unwanted bulk conduction. Unfortunately in most cases, in order to observe the surface states, the films must be at least 6 quintuple (QL) layer thick (~6 nm) [1, 2], which limits their applicability in functional devices. Here in this paper we show by ARPES that well defined 2D surface metallic states are formed in ultrathin (only about 3 QL thick) Bi2Se3 grown epitaxially on AlN(0001)/Si(111) substrates by Molecular Beam Epitaxy (MBE). The use of AlN, not reported before, opens the possibility of employing insulating substrates first to improve electrical conduction measurements, second to integrate Bi2Se3 with gate dielectrics in order to exploit the quantum capacitance of TIs. The Bi2Se3/AlN will be compared with our control Bi2Se3/Si (111) samples more typically obtained in the literature [3, 4] in order to assess the advantages of growth on AlN. Employing RHEED, XRD/XRR and HRTEM we demonstrate a high epitaxial quality of Bi2Se3 on AlN. Using HRTEM, we identify interfacial misfit dislocations (MDs) that accommodate the high lattice mismatch between Bi2Se3 and AlN as well as rotation and lamellar twins. Strain mapping is performed to reveal the distribution of the strain field of the MDs at the atomic scale. Using in-situ XPS we show that, Bi2Se3 does not react with AlN allowing the growth of high quality ultrathin films at an optimum temperature of 300 oC without the need to grow low temperature buffer layers, as in the case of Si substrates. HRTEM observations reveal a crystalline, albeit distorted, interface between the two materials. Using ARPES we perform a systematic investigation of the dependence of the 2D surface states as a function of the Bi2Se3 thickness. On both substrates we observe a shift of the Dirac point closer to the Fermi level as thickness increases, but remarkably, in the case of Bi2Se3/AlN, the shift is much larger, bringing the Dirac point at about 0.15 eV below EF for a 12 QL sample, which is highly desirable for several envisaged applications. Finally, we will show that Bi2Se3 on AlN may be suitable substrate for the growth of a few layer MoSe2 dichalcogenide, yielding good quality epitaxial growth with well-defined valence bandstructure as evidenced from ARPES.
A. Dimoulas acknowledges an ERC Advanced Grant through project SMARTGATE-291260
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