[14p-501-7~10] 10.4 Semiconductor spintronics, superconductor, multiferroics

[14p-501-10] Work function of bulk-insulating topological insulator Bi_2-xSb_xTe_3-ySe_y

〇Daichi Takane¹, Seigo Souma^2,3, Takafumi Sato^1,3, Takashi Takahashi^1,2,3, Kouji Segawa⁴, Yoichi Ando⁵ (1.Dept. Physics, Tohoku Univ., 2.WPI-AIMR, Tohoku Univ., 3.CSRN Tohoku Univ., 4.Kyoto Sangyo Univ., 5.Univ. of Cologne)

Keywords:Topological insulator, Work function

Topological insulator (TI) realizes a novel quantum state of matter characterized by the metallic Dirac-cone surface state with a helical spin texture which traverses the bulk band gap. Exotic topological phenomena of TIs as well as their device applications largely rely on the bulk-insulating nature and the dominant Dirac transport at the surface. However, it is known that the bulk-insulating nature is hard to be achieved in most of known TIs mainly due to defects in the crystals. Tetradymite solid-solution system Bi_2−xSb_xTe_3−ySe_y (BSTS) provides an excellent platform for studying genuine TIs, because the bulk crystal is highly insulating at certain compositions. The discovery of the BSTS system has stimulated the investigations of electronic structure with various spectroscopies.After establishing the basic electronic structure of BSTS, the next important step is to utilize the novel Dirac-carrier properties such as the spin-momentum locking in actual spintronic devices. An important consequence of the spin-momentum locking is the spin polarization induced by charge current and vice versa.While such studies have opened a pathway toward application of TIs in spintronics, the performance of devices so far achieved is not as high as one would expect from theory. This is largely related to the TI-metal contact necessary for operating the electrical circuits and devices. It is thus essential to well characterize the TI-metal interface to integrate bulk insulating TIs into efficient devices. In this regard, it is of great importance to know the work function of bulk insulating TIs, in particular, BSTS which is actually used in devices, since the band bending caused by a difference in the work functions between a TI and a metal largely influences the interface property and thereby plays a crucial role for device operation. However, the work function of TIs has been reported only for limited prototypical TIs.In this presentation, we report our measurements of the work function of BSTS at various bulk- insulating compositions, determined precisely by high-resolution photoemission spectroscopy. We observed that the work function of BSTS is 4.95 - 5.20 eV for x = 0.0 - 1.0. We also determined the work function of other TIs such as Pb- and Tl-based solid solutions, and compared them with those of BSTS.