Revealing surface-state transport in ultrathin topological crystalline insulator SnTe films

Ke Zou,Udo D Schwarz,Sohrab Ismail-Beigi,Eric I Altman,Frederick J Walker,Subhasish Mandal,M D Morales-Acosta,Stephen D Albright,Omur E Dagdeviren,Georg H Simon,Charles H Ahn,Chao Zhou

doi:10.1063/1.5096279

Abstract

SnTe is a topological crystalline insulator that exhibits crystal symmetry protected topological surface states (SS), which are useful for the development of novel devices, such as low-dissipation transistors. However, major obstacles remain to probe the SS and realize the application of SnTe and other topological insulators. Due to unintentional doping by Sn vacancies, bulk conduction may overwhelm the transport through SS in SnTe. Synthesis of SnTe films thin enough to suppress bulk conduction has proven difficult due to the formation of discontinuous domain structures. By introducing a novel deposition method that builds upon molecular beam epitaxy, we achieve ultrathin continuous films of single-orientation SnTe (001) on SrTiO3 (STO) (001) substrates. We separate the carrier concentrations in the bulk and in the SS and discover that conduction through the SS dominates (a majority of hole carriers occupy the SS) in films thinner than 40 unit cells, with a large temperature independent hole density of SS nS = 5 × 1014 cm−2. Unlike the depletion of SS carriers observed at the vacuum/SnTe interface that inhibits topological behavior, we show that SS carriers are buried and protected from depletion at the SnTe/STO interface, which is enabled by the relatively large bandgap of STO and its favorable band alignment with SnTe. This work provides an important pathway for probing and realizing SS transport in SnTe and other TIs even when bulk conduction coexists.

Highlights

We initially grow SnTe films using a conventional molecular beam epitaxy (MBE) approach by thermally evaporating SnTe source material onto atomically flat single-crystal (001) SrTiO3 (STO)
SnTe is a topological crystalline insulator that exhibits crystal symmetry protected topological surface states (SS), which are useful for the development of novel devices, such as low-dissipation transistors
We develop a novel method that builds upon molecular beam epitaxy (MBE) in order to grow ultrathin continuous SnTe films on SrTiO3 with a single orientation, where SS host a majority of carriers and the conduction through SS dominates

Summary

Introduction

We initially grow SnTe films using a conventional MBE approach by thermally evaporating SnTe source material onto atomically flat single-crystal (001) SrTiO3 (STO). The Hall measurements of SnTe films are linear up to B = 9 T, indicating that the differences in mobility for the three channels are small enough to assume that the measured 2D carrier density is the sum of the contributing channels, n2D(t) = nBt + ns,SnTe/vacuum + ns,SnTe/STO.

Results

Conclusion