Evolution of atomic structure and electronic transport properties in n-type Bi2Te3 films via Bi2 planar defects

Abstract

Bi2Te3 films always exhibit n-type transport characteristics even under the Bi-rich condition, which, however, was not clarified clearly. Herein, by virtue of advanced techniques such as scanning tunneling microscopy, angle-resolved photoelectron spectroscopy, scanning transmission electron microscopy, and x-ray photoelectron spectroscopy, we are able to identify the structural evolution on the atomic scale for Bi-rich Bi2Te3 films. The excess of Bi content will lead to the formation of p-type BiTe antisite defects; however, there is a doping limit for the excess of Bi to form BiTe antisites. Beyond this limit, the excess of Bi will form the n-type Bi2 planar defects in the van der Waals gap, the excellent electron donors, which can enhance the electron density by over one order of magnitude and up to the 1021 cm−3 range for Bi-rich Bi2Te3 films. Benefiting from the remarkable increase in the electron density and the suppression of carrier intrinsic excitations, Bi2Te3 films with Bi2 planar defects possess a much improved thermoelectric power factor, with a maximum value of 1.4 mW m−1 K−2 at 450 K, showing about 130% enhancement compared to that of the film without Bi2 intercalations. The discovery opens a new avenue to improve the thermoelectric properties of Bi2Te3 films utilizing the Bi2 planar defects.

A comprehensive review on Bi2Te3‐based thin films: Thermoelectrics and beyond

“…[ 135 ] To suppress the formation of V Te and Bi Te , one needs to significantly increase the nominal Te content and promote the formation of n‐type antisites Te Bi during the deposition of Bi 2 Te 3 films. Zhang et al [ 101,105,131 ] and other researchers [ 96,136 ] have shown that a low T sub (e.g., ∼250°C) and a high Te/Bi flux ratio (e.g., Te/Bi ≈ 18/1) are effective in promoting the formation of Te Bi and improving the n e . The formation of Te Bi in Bi 2 Te 3 films by increasing the nominal Te content can be expressed by the following formula, …”

“…The effective approach to verify the above theoretical speculation is to synthesize single-crystalline Bi 2 Te 3 films with a controlled amount of V Te , Bi Te , and Te Bi point defects, and accurately characterize their electronic band structure. The challenging task was carried out by Zhang et al [101,105,131] who achieved the precise control and direct characterization of the point defects in single-crystalline Bi 52 m e , that is, markedly smaller than in the two previous cases where V Te and Te Bi defects dominated. The in-plane band effective mass m* of Bi 2 Te 3 films was then calculated, [141] and showed m* of 2.46 m e , 2.13 m e, and 1.50 m e for films containing the dominating defects of V Te , Te Bi , and Bi Te , respectively.…”

“…Moreover, the advanced tools of scanning tunneling microscopy and angle-resolved photoemission spectroscopy (ARPES) have recently been added as powerful tools to study directly the influence of atomic defects on the electronic transport properties of n-type Bi 2 Te 3 films. [101,105,131] The comprehensive analysis has unambiguously revealed that the carrier density n e , the carrier mobility μ, the effective mass m*, and PF of n-type Bi 2 Te 3 films could be conveniently tailored through the manipulation of atomic defects.…”

“…The effective approach to verify the above theoretical speculation is to synthesize single‐crystalline Bi 2 Te 3 films with a controlled amount of V Te , Bi Te , and Te Bi point defects, and accurately characterize their electronic band structure. The challenging task was carried out by Zhang et al [ 101,105,131 ] who achieved the precise control and direct characterization of the point defects in single‐crystalline Bi 2 Te 3 films by making use of the combined in situ techniques of MBE, STM, and ARPES. Their results are depicted in Figure 5A in which the white dashed lines represent the momentum–energy relation at the conduction band edge.…”

Quantum Transport and Potential of Topological States for Thermoelectricity in Bi$_2$Te$_3$ Thin Films

“…Depending on the growth temperature of Bi 2 Te 3 films, the orientation of the triangle mounds varied. intercalations [113].…”