The efficiency of spin-to-charge conversion is a key parameter in determining the performance of emerging spintronic devices. In the topological surface state of a topological insulator (TI), the spin-momentum locking effect offers a great possibility for efficient spin-to-charge conversion. Here, we report the relation between the Fermi level position E F and spin-to-charge conversion efficiency in heterostructure Ni80Fe20 (Py)/(Bi1–x Sb x )2Te3. The band structure of (Bi1–x Sb x )2Te3 films becomes tailored by tuning the ratio of bismuth to antimony so that the position of the Fermi level E F varies from the top side of the valence band to the bottom side of the conduction band through the in-gap surface Dirac cone. The result is consistent with the electronic behavior of the majority carriers varying from n-type to p-type. In spin-pumping measurements, we observed that the inverse Edelstein effect length (λIEE) with a tuned E F near the Dirac point is significantly enhanced, indicating that the spin-charge conversion is determined mainly by the topological surface state. These results demonstrate that fine-tuning of E F in a TI-based heterostructure is critical to maximizing the efficiency of spin-to-charge conversion using a spin-momentum locking mechanism.

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The spin-to-charge conversion in Permalloy (Py)/Cu/Bi2Se3 is tunable by changing the Cu layer thickness. The conversion rate was studied using the spin pumping technique. The inverse Edelstein effect (IEE) length λIEE is found to increase up to ~2.7 nm when a 7 nm Cu layer is introduced. Interestingly, the maximized λIEE is obtained when the effective spin-mixing conductance (and thus Js) is decreased due to Cu insertion. The monotonic increase in λIEE with decreasing Js suggests that the IEE relaxation time (τ) is enhanced due to the additional tunnelling barrier (Cu layer) that limits the interfacial transmission rate. The results demonstrate the importance of interface engineering in the magnetic heterostructure of Py/topological insulators (TIs), the key factor in optimizing spin-to-charge conversion efficiency.

(Bi1−xSbx)2Se3 thin films were prepared by molecular beam epitaxy (MBE). The existence of strong and robust topological surface states was demonstrated in the (Bi1−xSbx)2Se3 ternary system by angle-resolved photoemission spectroscopy (ARPES). The sheet carrier density n2D was found to be decreased by 75% by doping Sb into Bi2Se3, compared with that in the case of undoped Bi2Se3. The enhancement of the surface state transport due to Sb doping was also revealed via the high-field Hall effect and weak antilocalization measurement. Our results reveal the potential of this system for the study of tunable topological-insulator based device physics.

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