# In-plane Topological p-n Junction in the Three-Dimensional Topological Insulator Bi$_{2-x}$Sb$_x$Te$_{3-y}$Se$_y$

Research paper by Ngoc Han Tu, Yoichi Tanabe, Yosuke Satake, Khuong Kim Huynh, Katsumi Tanigaki

Indexed on: 24 Mar '16Published on: 24 Mar '16Published in: Physics - Materials Science

#### Abstract

A topological p-n junction (TPNJ) is an important concept to control spin and charge transport on a surface of three dimensional topological insulators (3D-TIs). Topological surface states with up and down helical spin locking provide unique rectification in the flow of current, such as Klein tunneling and Veselago lensing. Although theoretical predictions have been reported on the p-n junction of 3D-TIs, intrinsic experimental observations on the electric transport arising from such intriguing p-n junction composed of spin helical Dirac fermions have not yet been made. Here we report successful fabrication of such TPNJ on a surface of 3D-TI Bi$_{2-x}$Sb$_x$Te$_{3-y}$Se$_y$ (BSTS) thin films and experimental observation of the electrical transport. Tuning the chemical potential of n-type topological Dirac surface of BSTS in its top half by employing tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) as an acceptor molecule, the half surface can be converted to p-type with leaving the other half side as the opposite n-type, and consequently a TPNJ can be created. By sweeping the back-gate voltage ($V_{\rm G}$) in the field effect transistor structure fabricated on a SiO$_2$/Si substrate, the topological p-n junction was controlled both on the bottom and the top surfaces. A sharp and clearly discontinuous jump in resistivity is seen in a four probe measurement at the p-n junction by sweeping $V_{\rm G}$. The robust Shubnikov-de Haas (SdH) oscillations associated with the $\pi$ Berry phase on the topological surface gives an unambiguous proof that the observed resistivity jump at the p-n junction is indeed ascribed to the spin-locked Dirac cone surface states. The dramatic change in electrical transport observed at the TPNJ on 3D-TI thin films promises novel spin and charge transport of 3D-TIs for future spintronics.