Reducing the Impact of Bulk Doping on Transport Properties of Bi‐Based 3D Topological Insulators

Abstract

The observation of helical surface states in Bi-based three-dimentional topological insulators has been a challenge since their theoretical prediction. The main issue raises when the Fermi level shifts deep into the bulk conduction band due to the unintentional doping. This results in a metallic conduction of the bulk which dominates the transport measurements and hinders the probing of the surface states in these experiments. In this study, we investigate various strategies to reduce the residual doping in Bi-based topological insulators. Flakes of Bi2Se3 and Bi1.5Sb0.5Te1.7Se1.3 are grown by physical vapor deposition and their structural and electronic properties are compared to mechanically exfoliated flakes. Using Raman spectroscopy, we explore the role of the substrate in this process and present the optimal conditions for the fabrication of high quality crystals. Despite of this improvement, we show that the vapor phase deposited flakes still suffer from structural disorder which leads to the residual n-type doping of the bulk. Using magneto-measurements we show that exfoliated flakes have better electrical properties and are thus more promising for the probing of surface states. arXiv:2001.04368v2 [cond-mat.mes-hall]

Effect of Sn Doping on Surface States of Bi2Se3 Thin Films

“…In a post-CMOS paradigm, extending the fast computational gains envisioned by Moore’s law for high-performance computing requires novel devices utilizing alternate state variables. , Spintronic devices, such as various MRAM designs and spinFETs, utilize the spin degree of freedom, which can allow for lower power dissipation, higher device density, and nonvolatility unlike conventional Si and CMOS-based electronics. − Such devices require efficient spin-to-charge conversion, long spin-diffusion lengths, and controllable spin relaxation. , Control of the spin relaxation can be achieved through spin–orbit coupling (SOC), although it is detrimental to long spin diffusion lengths as it introduces additional scattering. , Topological insulators (TIs) can be used to circumvent this roadblock via topologically protected surface states. , TIs are characterized by a linear dispersion in their surface bands, referred to as a Dirac cone, at the center of which the conductance and valence bands meet at a single “Dirac” point where the conductivity is expected to be minimized. − The surface states associated with these bands are accompanied by spin-momentum locking and are thus protected from backscattering by the band topology. Finally, TIs have significant SOC which should enable control of the spin relaxation.…”

“…Finally, TIs have significant SOC which should enable control of the spin relaxation. Unfortunately, TIs are often intrinsically doped such that the non-topological bulk conduction overwhelms that of the surface, resulting in metallic behavior. , …”

“…Unfortunately, TIs are often intrinsically doped such that the non-topological bulk conduction overwhelms that of the surface, resulting in metallic behavior. 15,16 Among the TIs, Bi 2 Se 3 is widely studied due to the relative ease of thin-film growth and the simplicity of the band structure, with one Dirac cone at the gamma point and a large band gap (∼0.3 eV). 13 However, because of Se vacancies, samples are heavily n-doped leading to a significant contribution from the bulk.…”

Profiling novel high-conductivity 2D semiconductors

“…Three of these 2D materials are very well-known and studied (P 4 [13][14][15][16] , WSe 2 13 , InSe 44 ). The Sb 2 X 3 and Bi 2 X 3 (X = Se, Te, S) compounds are better known for the topological properties of their 3D parents 45,46 . Monolayers have been recently studied in the context of phonon-limited transport 17,47,48 , but only within the approximate Tagaki formalism 18 .…”

Non-linear hybrid surface-defect states in defective Bi$_2$Se$_3$

“…Selenium vacancies (VSe) are known to be a common defect in Bi 2 Se 3 , and are assumed to be responsible for the n-type doping of the as-grown crystals. 10,11 Due to high volatility of selenium, 12 formation of additional vacancies on the surface of cleaved Bi 2 Se 3 slabs is expected. This is possibly responsible for the observed aging of the slabs, as they show changes in electronic properties with time.…”