Topological surface states in ultrathin Bi1−xSbx layers

Abstract

Topological insulator spin-polarized surface states are attractive for spintronic applications, in particular for spin-charge current interconversion, where extremely high conversion efficiencies are predicted. However, the contribution of topologically trivial bulk states is often disregarded although it may play a crucial role in the experimental results and extracted conversion efficiencies. The presence of bulk states at the Fermi level can be avoided by increasing the gap using the confinement effect appearing as the film thickness is reduced. We address this topic by growing Bi 1−x Sb x thin films (2.5-15 nm) by molecular beam epitaxy on InSb, BaF 2 , and Si substrates. The surface electronic band structure is studied by angle-resolved photoemission spectroscopy. Two Bi 1−x Sb x surface states are observed in the gap for several Sb concentrations and thicknesses, across the topological insulator phase, scanning x between 7% and 30%. Tight-binding calculations of the surface states are in good agreement with the experiments, revealing their polarized nature. Surface states are still present at the point for the thinnest films (2.5 nm), suggesting highly confined polarized states at the surface.

Growth of BiSb on GaAs (001) and (111)A surfaces: A joint experimental and theoretical study

“…This suggests a non uniform growth mode where the deposited atoms do not form early a connected layer, but agglomerate into isolated islands before coalescence, as confirmed in our previous work. 15 We note that topological surface states of BiSb are known to appear above 2.5 nm, 36 but the resistivity of the wetting layer in Fig. 3 decreases only above 5 nm due to the roughness of the sample surface that prevents the BiSb surface conduction channels for centimeter large devices.…”

Spin‐Momentum Locking and Ultrafast Spin‐Charge Conversion in Ultrathin Epitaxial Bi1 − xSbx Topological Insulator

“…BiSb films with thicknesses from 2.5 to 15 nm and with various compositions (0.03 < x < 0.3) were grown using this method and their crystallographic quality was demonstrated by RHEED, X-ray-diffraction, or STEM. [19] All the samples were grown in the MBE chamber of the CASSIOPEE beamline installed on the SOLEIL synchrotron. After their elaboration, they can be transferred in UHV to an ARPES or a SARPES experiments where their electronic structure can be characterized.…”

“…Once the Green function of the multilayer system is defined as The modeling of IREE were performed by TB method. The contributions were summed from the different Fermi surface pockets within the 2D-BZ after i) having introduced the Rashba potentials at the BiSb surfaces, required to match both the TSS electronic dispersion and SML measured in (S)ARPES experiments; [19] The typical value of 𝜏 s = 10 fs corresponded to an energy broadening Γ = ℏ/(2𝜏 s ) ≃ 50 meV) and a Fermi velocity of ≈5 × 10 5 m.s −1 like extracted from ARPES data. The trace on 𝜅 xy was performed by summing the BL contributions from the top surface down to the middle of the BL, typically matching the typical finite TSS evanescence or extension length.…”

“…Figure 1b displays the energy dispersion along the $$\overline{{\Gamma}}\bar{\text{M}}$$ direction. The signature of two surface S 1 and S 2 states [ 18,19 ] are visible in the bandgap. The valence band state energy dispersion is also visible, from a series of confined states with energy splitting increasing with reducing thickness.…”

“…In this respect, Bi 1 − x Sb x alloys, although displaying clear topological surface states, [ 16–18 ] have been mostly neglected for spintronic applications as a result of their modest bulk bandgap (about 40 meV for x = 0.07) and relatively complex band structure. However, quantization effects in ultrathin films have shown to lead to much larger gap [ 19,20 ] while retaining their band inversion near the $$\bar{\text{M}}$$ point in the x = 0.07–0.3 composition range, [ 21 ] unlike pure Bi. [ 22–24 ] BiSb therefore has considerable potential as candidate for spintronics applications as well as for recently engineered efficient spintronic THz emitters.…”