Trivial band topology of ultra-thin rhombohedral Sb2Se3 grown on Bi2Se3

Thin films of rhombohedral Sb2Se3 with thicknesses from 1 to 5 quintuple layers (QL) were grown on Bi2Se3/Si(1 1 1) substrate. The electronic band structure of the grown films and the Sb2Se3/Bi2Se3 interface were studied using angle-resolved photoemission spectroscopy. It was found that while Sb2Se3 has an electronic band structure generally similar to that of Bi2Se3, there is no fingerprints of band inversion in it. Instead, the one-QL-thick Sb2Se3 films show direct band gap of about 80 meV. With growing film thickness, the Fermi level of the Sb2Se3 films gradually shifts by 200 meV for 5 QL-thick film revealing the band bending of the Sb2Se3/Bi2Se3 hetero-junction.


Introduction
Nowadays, tetradymite family plays crucial role in a explo sively growing field of topological states of matter. In par ticular, Bi 2 Se 3 , Bi 2 Te 3 and Sb 2 Te 3 have the same rhombohedral crystalline arrangement and similar inverted band structure with a descent gap of 0.2-0.3 eV [1]. As a result, their surfaces or interfaces with trivial insulators host conducting electronic states with helical spin texture. These unique topological sur face states (TSS) define topological insulators (TI) as a per spective playground for observation of a variety spinphysics phenomena [2][3][4].
The above mentioned materials have a layered structure (figure 1(a)) where each socalled quintuple layer (QL) con sists of alternating XAXAX atomic planes, where X is Se or Te and A-Bi or Sb. Within the layer, the atoms are cova lently bonded, while layers are weakly bonded with each other via van der Waals (vdW) forces. Such structure allows precise control of the film thickness in the epitaxy process that can be used to tune the topological properties [5][6][7]. Moreover, using vdW epitaxy approach [8,9], one can combine these topo logical insulators with other materials, that opens a way for observation such striking phenomena as quantum anomalous Hall effect [2] and Majorana or Weyl quasiparticles [10,11].
Among the four members of the chalcogenide family with rhombohedral crystalline structure [1] only the Sb 2 Se 3 stays unexplored. Bulk Sb 2 Se 3 arranges in the orthorhombic phase with trivial insulator behavior and band gap of ∼1.1 eV [12,13]. However, ab initio calculations shows that rhom bohedral structure of Sb 2 Se 3 is kinetically stable [14]. In the experiments with the (Bi (1−x) Sb x ) 2 Se 3 thin films (15-25 QL), rhombohedral phase was found to be stable for x of up to 0.5 on Al 2 O 3 substrate [15] and up to 0.7 on Bi 2 Se 3 substrate [16]. Whether the rhombohedral Sb 2 Se 3 phase has a band inversion or not appeared to depend on the calculation procedure [1,14,17,18]. Furthermore, for the Sb 2 Se 3 /Bi 2 Se 3 heterostruc ture, calculations predict penetration of the Bi 2 Se 3 topological interface states through the slab of the trivial Sb 2 Se 3 [18]. Motivated by these findings and existed discrepancy in the calculation results, we performed an experimental study of the crystalline and electronic band structures of the Sb 2 Se 3 ultra thin films (1-5 QL) grown on the thick Bi 2 Se 3 buffer layer.
Thin films of rhombohedral Sb 2 Se 3 with thicknesses from 1 to 5 quintuple layers (QL) were grown on Bi 2 Se 3 /Si(1 1 1) substrate. The electronic band structure of the grown films and the Sb 2 Se 3 /Bi 2 Se 3 interface were studied using angleresolved photoemission spectroscopy. It was found that while Sb 2 Se 3 has an electronic band structure generally similar to that of Bi 2 Se 3 , there is no fingerprints of band inversion in it. Instead, the oneQLthick Sb 2 Se 3 films show direct band gap of about 80 meV. With growing film thickness, the Fermi level of the Sb 2 Se 3 films gradually shifts by 200 meV for 5 QLthick film revealing the band bending of the Sb 2 Se 3 /Bi 2 Se 3 heterojunction.

Experimental details
In the present study, MBE growth of selenides was con ducted in the ultrahigh vacuum (UHV) chamber with a base pressure less than 5.0 × 10 −10 Torr, equipped with reflec tionhighenergy electron diffraction (RHEED) facility. Atomicallyclean Si(1 1 1)7 × 7 surface was prepared in situ by flashing the Si(1 1 1) samples to 1280 °C after they were first outgassed at 600 °C for 6 h. Details of the Bi 2 Se 3 / Si(1 1 1) sample prep aration could be found elsewhere [19]. Bismuth, antimony and selenium were deposited from the Knudsen cells heated to 470, 380 and 195 °C, respectively. Thus, the flux ratio of Bi (Sb) to Se was about 1:10. Growth rate of Sb 2 Se 3 was about 0.5 QL min −1 . Substrate temper ature during Sb 2 Se 3 growth was about 170 °C. The prepared samples were transferred, using evacuated transfer unit, into the UHV Omicron MULTIPROBE system, equipped with scanning tunneling microscopy (STM), xray photoemission spectr oscopy (XPS) and angleresolved photoemission spectr oscopy (ARPES) facilities. Spectroscopy measurements were conducted using VG Scienta R3000 electron analyzer, Mg xray source (hν = 1253.7 eV) and highflux He discharge lamp (hν = 21.2 eV).

Results and discussion
Figure 1(b) shows RHEED patterns from the initial Bi 2 Se 3 substrate and Sb 2 Se 3 films of 2 QL and 5 QL thickness grown atop of it. It turns out that film preserves the crystal structure of the substrate for the thicknesses of up to 5 QL. There is a slight increase of reciprocal lattice constant of about 2.5%. Bearing in mind that lattice constant of thick Bi 2 Se 3 film on Si(1 1 1) a Bi2Se3 = 4.147 Å [20] one can obtain a Sb2Se3 = 4.048 ± 0.003 Å . For the 5 QLthick Sb 2 Se 3 film, one can notice faint addi tional spots marked by blue arrows. We attribute them to the islands of the orthorhombic phase that should be more stable for the thick films. Thus, rhombohedral Sb 2 Se 3 is believed to persist in a relatively narrow region of the film thicknesses.
In the STM images (figures 1(c) and (d)), Sb 2 Se 3 films exhibit morphology similar to that of the Bi 2 Se 3 substrate with screw dislocations and steps of ≈1 nm height. One can also notice weakly periodic Moiré pattern that does not show longrange order. Moiré periodicity can be roughly estimate from the set of the STM profiles (figure 1(e)) which gives A= 19 ± 2 nm. Similar value can be obtained from the fastFouriertransform (FFT) profile (figure 1(f)) where Lorentzian fit gives A= 19.2 ± 0.8 nm.
Due to a low growth temperature of the Sb 2 Se 3 films, we expect that diffusion of Bi atoms into the film is negligible. Observed Moiré pattern in the STM images also suggests that lattice constant changes stepwise after formation of the first Sb 2 Se 3 layer and interface is sharp. This conclusion is sup ported by the results of the XPS measurements conducted for the Sb 2 Se 3 films of various thicknesses (figure 2). Bismuth signal was found to decrease exponentially with the thick ness and already vanishes for the 5 QLthick film. Extracted inelastic mean free pass λ = 1.22 ± 0.04 nm is in a agreement with universal curve value (1.54 nm) [21] for corresponded electron energy (E k = 810 eV).
We will now discuss overall band structures and then take a close look on a Γ point. Figure 3 shows the ARPES data recorded along the main directions of surface Brillouin zone for the Bi 2 Se 3 substrate and the Sb 2 Se 3 films (1 and 5 QL). One can notice that film and the substrate have similar band structures. According to the calculations [1], in the crystals of Bi 2 Se 3 family the highest valence band is mainly occu pied by chalcogen p z orbitals and the lowest conduction band is mainly occupied by Bi (Sb) p z orbitals. Both calculated [1,14,18,23] and experimental spectra for Sb 2 Se 3 show valence band maximum (VBM) in the Γ point. In the ΓM direction the experimental dispersion of the highest valence band shows additional maximum that is ∼200 meV lower then VBM and minimum in M point that is ∼600 meV lower then VBM. Calculations [1,14,18,23] show similar disper sion along ΓM with slightly closer position in energy of the additional maximum to VBM. In the ΓK direction the exper imental dispersion of the highest valence band shows addi tional maximum that is ∼500 meV lower then VBM and then stays relatively flat until it reach the K point and show maximum in the middle of KM that is ∼500 meV lower then VBM. This particular behavior well coincide with one obtained in calculations by Liu et al [23] and contradicts with calculations by Cao et al [14] that show maximum in K point.
We will now consider the electron band structure in the vicinity of Γ point. As mentioned in the introduction, depending on the calculation procedure, numerical studies show topological trivial [1] and nontrivial [14] behavior for the rhombohedral Sb 2 Se 3 as well as transition among these phases under the pressure [22,23]. Figure 4 presents detailed spectroscopy data in the Γ point of Sb 2 Se 3 /Bi 2 Se 3 system as a function of film thickness (measurements were done in the Γ 1 point instead of the Γ 0 point due to better resolution and positioning). As was shown by Zhang et al [1] strong spinorbit interaction leads to inversion of orbital character near the Γ point for Bi 2 Se 3 , Bi 2 Te 3 and Sb 2 Te 3 and corresponded topological insulator behavior with gapless states on a sur face. Indeed, on a spectrum of Bi 2 Se 3 substrate one can see the conduction band touching the Fermi level and the Dirac cone of the topological surface states ( figure 4(a)) with the position of the Dirac point at ∼240 meV. On energy distribu tion curve (EDC) in figure 4(c) the Dirac point appears as the spectral intensity maximum. On the the spectrum for the 1 QLthick Sb 2 Se 3 film ( figure 4(b)), one can see conduction and valence bands as two cones touching each other. Their apexes are separated by the tiny gap of about 79 ± 15 meV as one can see on a EDC curve fit in figure 4(d) where instead of single Dirac point maximum double peak feature appears. Similar gap size was predicted for bulk Sb 2 Se 3 by Zhang et al [1] and Liu et al [23].
For this Sb 2 Se 3 /Bi 2 Se 3 system the total thickness is 25 QL. In their comprehensive numerical study of the Bi 2 Se 3 family heterostructures Aramberri and Muñoz [17] showed that in case of the topological insulator/topological insulator hetero structure (in their example, Sb 2 Te 3 /Bi 2 Te 3 ) even if one of the topological insulators is below penetration depth of the TSS, there is no gap in the spectrum, as well as no interface states. Thus, topological insulator/topological insulator heterostruc ture acts as a whole in terms of coupling of bottom and top TSS. Similar picture was found in the experimental study of Sb 2 Te 3 /Bi 2 Te 3 heterostructure [24]. In the current case of 1 QLthick Sb 2 Se 3 film on thick Bi 2 Se 3 , the observed gap sup ports trivial insulator behavior of the Sb 2 Se 3 .
With growing film thickness (figures 4(e) and (f)), the Sb 2 Se 3 bands continuously shift to lower binding energy by 200 meV for the 5 QLthick film with respect to the 1 QLthick film. Thus, the Fermi level lies in the conduction band (electronlike dispersion) at lower thicknesses and in the valence band (holelike dispersion) at higher thicknesses with position inside the gap for 3 QLthick film. While elec tron doping of Bi 2 Se 3 usually is explained by Se vacancies, slight hole doping of the Sb 2 Se 3 can be explained by forma tion of Sb vacancies in analogy with the Sb 2 Te 3 case [25]. This leads to pn junction formation and corresponded band alignment observed in the ARPES measurements. A similar trend was observed for (Bi 1−x Sb x ) 2 Se 3 /Bi 2 Se 3 heterostruc tures [16].

Conclusions
The goal of this study was to clarify experimentally the elec tronic and topological properties of the rhombohedral Sb 2 Se 3 and Sb 2 Se 3 /Bi 2 Se 3 interface. While we did succeed in growing the rhombohedral Sb 2 Se 3 phase on Bi 2 Se 3 substrate, we did not find any fingerprints of its nontrivial topology. Instead, 1 QLthick Sb 2 Se 3 film grown on 24 QLthick Bi 2 Se 3 substrate shows trivial gap of about 80 meV, which is unlikely for topo logical insulator/topological insulator heterostructure [16,17]. Observation of the gap also contradicts the predicted penetra tion of the Bi 2 Se 3 topological interface states through the slab of the trivial Sb 2 Se 3 [18]. In calculations [17], the Sb 2 Se 3 topo logicalinsulator phase was found to realize in a narrow region of lattice parameters. Thus, a principal possibility to realize Sb 2 Se 3 topologicalinsulator phase still remains choosing an appropriate substrate, provided that orthorhombic phase issue would be satisfied. The present experimental results on the electronic band structure of the rhombohedral Sb 2 Se 3 phase could help for future studies of its family.