Electrical Detection of the Helical Spin Texture in a p-type Topological Insulator Sb2Te3

The surface states of 3D topological insulators (TIs) exhibit a helical spin texture with spin locked at right angles with momentum. The chirality of this spin texture is expected to invert crossing the Dirac point, a property that has been experimentally observed by optical probes. Here, we directly determine the chirality below the Dirac point by electrically detecting spin-momentum locking in surface states of a p-type TI, Sb2Te3. A current flowing in the Sb2Te3 surface states generates a net spin polarization due to spin-momentum locking, which is electrically detected as a voltage on an Fe/Al2O3 tunnel barrier detector. Measurements of this voltage as a function of current direction and detector magnetization indicate that hole spin-momentum locking follows the right-hand rule, opposite that of electron, providing direct confirmation that the chirality is indeed inverted below Dirac point. The spin signal is linear with current, and exhibits a temperature dependence consistent with the semiconducting nature of the TI film and freeze-out of bulk conduction below 100 K. Our results demonstrate that the chirality of the helical spin texture of TI surface states can be determined electrically, an enabling step in the electrical manipulation of spins in next generation TI-based quantum devices.

Scientific RepoRts | 6:29533 | DOI: 10.1038/srep29533 detect bias current generated spin polarization. From measurements of the carrier type and sign of the spin voltage, we deduce a counter-clockwise chirality with carrier spin-momentum locking following a right-hand rule, confirming the chirality inversion of the helical spin texture of TI surface state above and below the Dirac point. The direct electrical access to the helical spin texture in the TI surface states is an enabling step in the electrical manipulation of spins in topological devices for spintronics and quantum computation applications.

Results
MBE growth and characterization of Sb 2 Te 3 . Sb 2 Te 3 thin films 30 nm thick are grown by molecular beam epitaxy (MBE) on epitaxial graphene/SiC(0001) substrates at 275-325 °C (see Methods). A conductive nitrogen doped n-type 4H-SiC substrate (0.05 Ohms-cm) is used to facilitate the in situ scanning tunneling microscopy/spectroscopy (STM/STS) monitoring of the surface morphology and electronic structure to ensure optimal layer-by-layer spiral growth (Fig. 2a) 27 . The n-type substrate does not contribute to transport through the p-type TI film due to the depletion region at the p-n junction interface, as discussed in the Supplementary Information. Tunneling spectrum shown in Fig. 2b taken on the as-grown Sb 2 Te 3 surface in situ at 77 K exhibits a minimum conductance at + 80 meV above the Fermi level, indicating the Dirac point (E D ) (identified by an arrow in the inset). This indicates p-type carriers in the TI surface states, with an estimated hole concentration of ~4.7 × 10 11 /cm 2 . The p-type conductivity is as also confirmed by ex situ Hall measurements, as shown in the Supplementary Information. This value is slightly enhanced due to the shift of the Dirac point to accommodate charges induced by the electric field between the STM tip and sample, as observed in earlier STS studies of Sb 2 Te 3 28 . In addition, quantum well (QW) states (marked by arrows in Fig. 2b) are also observed, consistent with earlier STS 28,29 and ARPES studies 30 .
Temperature dependent resistivity measurements show over two orders of magnitude increase in resistivity with decreasing temperature (Fig. 2c), indicating semiconducting behavior, i.e., the Fermi level lies within the bandgap. A weakly temperature dependent plateau is seen below 100 K, characteristic of metallic behavior, and attributed to freeze-out of bulk carriers and conduction only in metallic states such as the TI surface states. These findings are consistent with earlier work on MBE grown Sb 2 Te 3 films 28-30 , where p-type conductivity is typically found and attributed to excess Sb leading to Sb Te acceptor-like antisites with an activation energy of ~7 meV 31 , where the Fermi level lies in the gap and intersecting only the surface Dirac cone 30 . This is in contrast to bulk Sb 2 Te 3 single crystals synthesized by melting mixtures of Sb and Te, where much larger p-type doping is typically found with the Fermi level lying within the bulk valence band 32 .
Device fabrication and measurement geometry. Ferromagnet/oxide tunnel contacts, Fe/Al 2 O 3 , are utilized as spin sensitive probes to detect the bias current-generated spin polarization in the Sb 2 Te 3 surface states. Such contacts have enabled electrical detection of spin current and accumulation in both semiconductors and metals 33 and recently spin-momentum locking in topological insulators such as Bi 2 Se 3 15,16,[18][19][20] . MBE grown Sb 2 Te 3 films were processed into the device structures illustrated in Fig. 3a,b for transport measurements, consisting of two Au/Ti current leads on opposite ends of the Sb 2 Te 3 mesa, with several pairs of ferromagnetic (red) detector and corresponding non-magnetic Au/Ti (yellow) reference contacts in between. The unpolarized current flowing between the two outer contacts produces a spontaneous spin polarization in the Sb 2 Te 3 surface states throughout the channel due to spin-momentum locking. The projection of this spin onto the magnetization of the ferromagnetic detector contact is recorded as a voltage with a high-impedance voltmeter (> 1 Giga-ohm). Note that no current flows through the detector contact. An in-plane magnetic field is applied to switch the magnetization of the detector contact, so that the spins generated by carriers populating the Sb 2 Te 3 surface states are either parallel or antiparallel to the magnetization, which changes the magnitude of detector   voltage. Here we define the positive current to be holes flowing from left to right along the + x axis, and the positive magnetic field to be pointing in the + y direction.
Electrical detection of spin-momentum locking. Transport measurements were carried out in a closed cycle cryostat equipped with an electromagnet (10-300 K, ± 1000 Oe). Figure 3c,d show the detector voltage as a function of the applied in-plane magnetic field when a bias current flows between the two outer Ti/Au contacts. A simple linear background was subtracted 15 , and the data centered on the vertical axis. For a constant + 400 uA current (Fig. 3c), a constant low voltage is observed when the detector magnetization is saturated in the + y direction (> 70 Oe positive magnetic field), and a constant high voltage is observed with the detector magnetization in the − y direction (negative field). From our earlier work on the electrical detection of spin-momentum locking in Bi 2 Se 3 using the same detector contacts 15 , when the detector magnetization is parallel to the TI spin, a low voltage is detected. (A complete description of the measurement and model is contained in ref. 34.). Given the same spin detecting contacts (Fe/Al 2 O 3 ), which are only sensitive to the orientation of the spin regardless of the source, this indicates that in Fig. 3c the low voltage observed correspond to a spin orientation in the + y direction, locked to a hole momentum in the + x direction. This counter-clockwise helicity deduced here below the Dirac point in a p-type TI is indeed opposite to that above the Dirac point as seen in a n-type TI, where the + x moving electron induces a spin locked in the − y direction. This is further confirmed when the detector magnetization is saturated in the − y direction (negative field), where a high voltage is observed, indicating an antiparallel alignment between the detector magnetization and spin orientation, i.e., a spin orientation in the + y direction.
Another hallmark of the current generated spin in the TI surface states due to spin-momentum locking is that the spin orientation can be changed by reversing the current direction, as shown in Fig. 3d for − 400 uA current, i.e., holes flowing in the − x direction. Following the discussion above, this should induce a spin direction in − y. A parallel alignment between this TI surface spin and that of detector magnetization (< − 70 Oe, negative magnetic field) again results in a low voltage, while the antiparallel alignment (> + 70 Oe, positive field) yields a high voltage. The resulting curve is a step-like hysteric field dependence of the FM detector voltage that flips around the zero voltage axis relative to that in Fig. 3c. The voltage measured with a non-magnetic detector exhibits no such step-like behavior. These results clearly show that the current generated spin orientation in the Sb 2 Te 3 surface states is locked in a counterclockwise helicity to the hole momentum, demonstrating that chirality in spin-momentum locking is indeed inverted below the Dirac point. produced when the detector magnetization reverses (the spin of the Sb 2 Te 3 surface states is parallel or antiparallel to the detector magnetization) depends linearly upon the applied bias current that flows in the Sb 2 Te 3 film, as shown in Fig. 4. ΔV is determined from the data as illustrated in the inset. This behavior is consistent with a spin polarization generated by a bias current, and also consistent with earlier theoretical work 35 as discussed in more detail below.
The temperature dependence of the spin signal [V(M) − V(−M)] measured at ± 1 mA is shown in Fig. 5a. It initially exhibits a plateau as the temperature decreases below 100 K, where the transport measurements shown in Fig. 2c indicate the freeze out of some of the bulk carriers, and metallic states such as the TI surface states dominate transport. At higher temperatures, a significant decrease in magnitude is observed, and the signal disappears at this bias current above 175 K. The data taken at 150 K are shown in Fig. 5b,c, where a clear step-like hysteric behavior is seen. This temperature dependence is consistent with the semiconducting nature of the TI film, mirroring the temperature dependence of the resistivity (green curve in Fig. 5a). Since the conduction of the metallic states is expected to be weakly temperature dependent 36 , the significant decrease in resistance above 100 K is attributed to activation of bulk carriers such as Sb Te antisites in the semiconducting TI bulk, which shunts increasing fractions of the current, and dilutes the portion that flows through the surface states. By increasing the total bias current from ± 1 mA to ± 3.7 mA at 175 K, a clear hysteretic step-like behavior is again seen (Fig. 5d,e).

Discussion
The linear behavior of the spin signal is consistent with the theoretical work by Hong et al. on current-induced spin polarization in a TI in both diffusive and ballistic regimes 35  where I b is the (hole) bias current in the + x direction, R B is the ballistic resistance of the channel, and P FM is the transport spin polarization of the FM detector metal. M u is a unit vector along the detector magnetization M, and p is the degree of spin polarization induced per unit current by both spin-momentum locking in TI Dirac surface states and Rashba spin-orbit coupling in the two-dimensional electron or hole gas that may form on the surface due to band bending.
Though it has been shown that the surface of Sb 2 Te 3 is much more stable against adsorbate-induced band bending and shifting of the Fermi level 30 , and exhibits less aging or photo-induced doping typical of Bi-based materials such as Bi 2 Se 3 32,37 , the formation of a two dimensional hole (or electron) gas on the surface post processing cannot be ruled out. Due to the breaking of inversion symmetry at the Sb 2 Te 3 surface, these states can exhibit Rashba-type spin-orbit splitting with spins in-plane and also at approximately right angles to the carrier momentum, and therefore may also contribute to the spin signal measured. However, these states exist as spin-split pairs with opposite spin orientation at each momentum, therefore the resulting current-induced spin densities tend to cancel. Hence the net spin polarization should be dominated by that from the TI surface state, as predicted by model calculations 35 . Furthermore, the TI surface state and Rashba contributions can also be distinguished since they are expected to exhibit opposite sign. Following Eqn. 1 and models presented in ref. 34, the sign of the spin voltage signal [V(M) − V(−M)] that we measure (Fig. 3c,d) is consistent with contribution from the TI surface states. Finally, the sign of the spin signal/polarization measured here for a p-type TI is the same as we've shown previously for the n-type Bi 2 Se 3 15 . This is also consistent with the prediction by Hong et al. (c.f. Fig. 3c of ref. 35), where the polarization of the TI surface states is shown to be constant with energy, and work of ref. 19 where the chemical potential of (Bi,Sb) 2 Te 3 was tuned by a back gate 19 .
While a quantitative experimental determination of the TI spin polarization p can be obtained from the equation above, it is limited by the precise determination of the fraction of the current that flows through the TI surface states (which generates the spin polarization due to spin-momentum locking). Even though temperature dependent resistivity measurements suggest freeze out of bulk carriers at lower temperatures (< 100 K, Fig. 2c), there may still be other conducting channels that contribute to transport and shunt the bias current. This is evident from the large disparity in carrier concentration measured in tunneling spectroscopy measurements carried out in ultrahigh vacuum that are most sensitive to surface (states) and Hall measurements which integrate over a range of energies. The Dirac point of + 80 meV in the STS spectrum (Fig. 2b, taken at 77 K) indicates an Sb 2 Te 3 surface state carrier density of 4.7 × 10 11 /cm 2 in the as-grown sample, while Hall measurements at the same temperature yield a sheet density of 8.0 × 10 13 /cm 2 in the processed sample corresponding to the data of Fig. 2c. Although adsorbates are known to introduce additional doping into the TI system on the order of ~10 12 /cm 2 in the case of Bi 2 Se 3 , Sb 2 Te 3 has been shown to be much less susceptible to such adsorbates induced effects 30 , and therefore is likely not the culprit for such drastic difference in sheet density.
Alternatively, earlier work has shown robust and well-defined QW states due to confinement in MBE grown Sb 2 Te 3 thin films 29,30 , which is also clearly seen in our tunneling spectroscopy data (Fig. 2b) with a peak-to-peak separation of ~80 meV. These states are two-dimensional in nature and can exhibit a weak temperature dependence. However, these states can contribute to the sheet density measured by Hall measurements, and certainly contribute to transport and provide a parallel conduction path. As a first order approximation, we assume equal conduction through the bulk and the surface states, and that only the current flowing through the top surface contributes to the spin polarization arising from the Dirac states. With this assumption, and taking P FM (Fe) ~ 0.4, and k F ~ 0.07 Å −1 38 we estimate |p| ~ 0.15 from the data in Fig. 4.
In summary, we demonstrate that the chirality of the helical spin texture of a topological insulator can be determined using transport measurement, by electrically detecting the spin-momentum locking of the TI surface states. In the p-type TI, Sb 2 Te 3 , where the carrier type is confirmed by Hall measurement, using Fe/Al 2 O 3 tunnel barrier contacts to detect bias current generated spin polarization, our results indicate a counter-clockwise chirality with carrier spin-momentum locking following a right-hand rule, confirming that the chirality is indeed inverted below the Dirac point. The spin signal arising from spin-momentum locking of the surface states is linear with current, and exhibits a temperature dependence consistent with the semiconducting nature of the TI film. With the efficient spin-momentum driven spin transfer torque 21 and magnetization switching 22 already demonstrated in TIs such as Bi 2 Se 3 , the direct electrical access to the spin texture of the helical TI surface states is an enabling step towards realizing next generation TI-based spintronics devices.

Methods
The growth of Sb 2 Te 3 films was carried out on epitaxial graphene/SiC(0001) substrates in an ultrahigh vacuum (UHV) system (base pressure ~1 × 10 −10 Torr) that integrates two MBE chambers and a low temperature (5-300 K) scanning tunneling microscope (STM). As-received nitrogen-doped 4H-SiC(0001) substrates were first etched in H 2 /Ar atmosphere in a separate vacuum chamber at ~1500 °C to remove polishing damage. They were then transferred to the MBE system and annealed in UHV in a Si flux (~0.1 ML/min) at 950 °C to produce a (3 × 3) reconstructed surface, and further annealed at temperatures 1000-1300 °C without Si flux to grow epitaxial graphene 39 . For the growth of Sb 2 Te 3 , the substrate was held at 275-325 °C, and Sb and Te were supplied via separate Knudsen cells at 460 and 250 °C, respectively. In situ STM imaging is used to monitor surface morphology and electronic structure and ensure optimal layer-by-layer spiral growth, as shown in Fig. 2a. The as-grown film exhibits a Dirac point 80 meV above the Fermi level, as shown in the tunneling spectra in Fig. 2b, indicating p-type conductivity with an estimated carrier concentration of ~4.7 × 10 11 /cm 2 . These values represent an upper bound due to the upward shift of the Dirac point accommodating charges induced by the electric field between the STM tip and sample 28 .
The Fe/Al 2 O 3 contacts were formed on the air-exposed surface of a 20 nm thick Sb 2 Te 3 film in a separate MBE system as follows. A 0.7 nm layer of polycrystalline Al was first deposited by MBE, and then oxidized in 200 Torr O 2 for 20 min in the presence of UV light in the load-lock chamber. This step was then repeated for a total Al 2 O 3 thickness of 2 nm. The sample was then transferred under UHV to an interconnected metals MBE chamber, where 20 nm of polycrystalline Fe was deposited at room temperature from a Knudsen cell.
The samples were processed into the device structures illustrated in Fig. 3a,b to enable transport measurements. Standard photolithography and chemical etching methods were used to define the Fe contacts, which ranged in size from 20 × 20 μ m 2 to 80 × 80 μ m 2 , with adjacent contact separation ranging from 45 to 200 μ m. Ion milling was used to pattern the Sb 2 Te 3 mesa. Large Ti/Au contacts were deposited by lift-off in an electron beam evaporator as non-magnetic reference contacts and bias current leads. The Fe contacts were capped with 10 nm Ti/100 nm Au, and bond pads for wire bonded electrical connections are electrically isolated from the SiC using 100 nm of Si 3 N 4 .
Transport measurements were performed in a closed cycle cryostat equipped with an electromagnet (4-300 K, ± 1000 Oe). An unpolarized bias current was applied through the outer Ti/Au contacts on the opposite ends of the Sb 2 Te 3 mesa, and the voltage on the detector contact was recorded as a function of the in-plane magnetic field applied orthogonal to the electron bias current direction in the TI.