Imaging ambipolar two-dimensional carriers induced by the spontaneous electric polarization of a polar semiconductor BiTeI

TL;DR

This study demonstrates that spontaneous electric polarization in the polar semiconductor BiTeI induces ambipolar 2D carriers with spin splitting at the surface, enabling the formation of lateral p-n junctions without complex fabrication.

Contribution

The paper provides experimental evidence that spontaneous electric polarization in BiTeI induces ambipolar 2D carriers and lateral p-n junctions at atomically-sharp interfaces, a novel mechanism for 2D carrier control.

Findings

Ambipolar 2D carriers are induced by spontaneous electric polarization.

Surface electron densities are independent of bulk densities.

Lateral p-n junctions form along domain boundaries.

Abstract

Two-dimensional (2D) mobile carriers are a wellspring of quantum phenomena. Among various 2D-carrier systems, such as field effect transistors and heterostructures, polar materials hold a unique potential; the spontaneous electric polarization in the bulk could generate positive and negative 2D carriers at the surface. Although several experiments have shown ambipolar carriers at the surface of a polar semiconductor BiTeI, their origin is yet to be specified. Here we provide compelling experimental evidences that the ambipolar 2D carriers at the surface of BiTeI are induced by the spontaneous electric polarization. By imaging electron standing waves with spectroscopic imaging scanning tunneling microscopy, we find that positive or negative carriers with Rashba-type spin splitting emerge at the surface correspondingly to the polar directions in the bulk. The electron densities at the surface are constant independently of those in the bulk, corroborating that the 2D carriers are induced by the spontaneous electric polarization. We also successfully image that lateral $p$-$n$ junctions are formed along the boundaries of submicron-scale domains with opposite polar directions. Our study presents a novel means to endow non-volatile, spin-polarized, and ambipolar 2D carriers as well as, without elaborate fabrication, lateral $p$-$n$ junctions of those carriers at atomically-sharp interfaces.