Fermi Level Manipulation through Native Doping in the Topological Insulator Bi₂Se₃

Abstract

The topologically protected surface states of three-dimensional (3D) topological insulators have the potential to be transformative for high-performance logic and memory devices by exploiting their specific properties such as spin-polarized current transport and defect tolerance due to suppressed backscattering. However, topological insulator based devices have been underwhelming to date primarily due to the presence of parasitic issues. An important example is the challenge of suppressing bulk conduction in BiSe and achieving Fermi levels ( E) that reside in between the bulk valence and conduction bands so that the topologically protected surface states dominate the transport. The overwhelming majority of the BiSe studies in the literature report strongly n-type materials with E in the bulk conduction band due to the presence of a high concentration of selenium vacancies. In contrast, here we report the growth of near-intrinsic BiSe with a minimal Se vacancy concentration providing a Fermi level near midgap with no extrinsic counter-doping required. We also demonstrate the crucial ability to tune E from below midgap into the upper half of the gap near the conduction band edge by controlling the Se vacancy concentration using post-growth anneals. Additionally, we demonstrate the ability to maintain this Fermi level control following the careful, low-temperature removal of a protective Se cap, which allows samples to be transported in air for device fabrication. Thus, we provide detailed guidance for E control that will finally enable researchers to fabricate high-performance devices that take advantage of transport through the topologically protected surface states of BiSe.

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

“…We will henceforth refer to vacancies in the surface (bulk) QL with a subscript 's'('b'). An Se vacancy in a 6×6×6 QLs slab corresponds to a defect concentration of ≈ 10 19 cm −3 (in-plane concentration = 2.6 × 10 13 cm −2 ), comparable to the values reported in experiments, 15,22 which range from 10 11 to 10 20 cm −3 . Additional details of charge-states of the VSe defects and their stability under different conditions are given in the Supplement (S4).…”

Device Applications of Synthetic Topological Insulator Nanostructures

“…However, sometimes the position of the Fermi level cannot be properly tuned only by using electrical gating to the same or the adjacent regime to the intrinsic Fermi level, because it might fall into the valence band under negative voltages. Elemental doping has been confirmed to be an effective approach to assist the manipulation of Fermi level in topological insulator nanostructures [42][43][44]. In 2012, Wang et al reported the Na elemental doping in Bi 2 Te 3 nanoplates synthesized by solvothermal method [45].…”

Influence of Doping on the Topological Surface States of Crystalline Bi2Se3 Topological Insulators

“…Therefore, high quality p-type crystals are critically important for basic as well as applied research on Bi 2 Se 3 , and a great deal of research work has been devoted to manipulating the Fermi level by searching for suitable p-type dopants. These efforts include volumetric doping with elements, such as Mn [ 10 ], Sn [ 4 ], Fe [ 11 , 12 ], Ca [ 13 , 14 ], Mg [ 15 ], Cr [ 16 , 17 ], Pb [ 18 ], surface adsorption with Fe [ 19 , 20 ], Co [ 21 ], and controlling a number of native anti-site defects [ 22 , 23 , 24 ]. Doping with magnetic materials is particularly important due to the fact that the magnetic order leads to the breaking of the time reversal symmetry, and thus, can destroy the topological order.…”