[Paper Review] Work function of bulk-insulating topological insulator Bi2-xSbxTe3-ySey

Daichi Takane, S. Souma+4 more|Kölner Universitäts PublikationsServer (Universität zu Köln)|Jun 25, 2016

Topological Materials and Phenomena3 citations

TL;DR

This study measures the work function of bulk-insulating bismuth antimony telluride selenide (Bi2-xSbxTe3-ySe y) topological insulators using high-resolution photoemission spectroscopy, revealing a systematic increase from 4.95 eV to 5.20 eV as antimony content (x) rises from 0.0 to 1.0. The work function shift correlates directly with the chemical potential shift in the Dirac cone, confirming electron affinity pinning and providing essential interface design guidelines for TI-based spintronic devices.

ABSTRACT

Recent discovery of bulk insulating topological insulator (TI) Bi2-xSbxTe3-ySey paved a pathway toward practical device application of TIs. For realizing TI-based devices, it is necessary to contact TIs with a metal. Since the band-bending at the interface dominates the character of devices, knowledge of TIs' work function is of essential importance. We have determined the compositional dependence of work function in Bi2-xSbxTe3-ySey by high-resolution photoemission spectroscopy. The obtained work-function values (4.95-5.20 eV) show a systematic variation with the composition, well tracking the energy shift of the surface chemical potential seen by angle-resolved photoemission spectroscopy. The present result serves as a useful guide for developing TI-based electronic devices.

Motivation & Objective

• To determine the compositional dependence of the work function in bulk-insulating topological insulators of the Bi2-xSbxTe3-ySe y system.
• To establish a quantitative link between the work function and the chemical potential shift in the Dirac cone surface state.
• To provide essential interface parameters for designing efficient topological insulator-based spintronic devices.
• To compare the work functions of various topological insulators and common contact metals for device integration.
• To validate the intrinsic nature of the work function measurement through bias voltage independence in photoemission spectra.

Proposed method

• High-resolution photoemission spectroscopy (PES) with a He Iα source (hν = 21.218 eV) and a toroidal grating monochromator at Tohoku University.
• In-situ cleaving of single-crystal Bi2-xSbxTe3-ySe y samples to achieve atomically clean, mirror-like surfaces.
• Application of variable bias voltage to the sample to measure the kinetic energy cut-off (E_cut) and Fermi edge (E_0) in PES spectra.
• Work function (Φ) calculated using the formula: Φ = hν - E_0 + E_cut, with energy resolution set to 20 meV.
• Systematic measurement across three compositions: x = 0.0, 0.25, and 1.0, corresponding to different Sb-doped Bi2Te3-ySey phases.
• Comparison of work function values with those of prototypical TIs (Bi2Te3, Bi2Se3) and common contact metals (Cu, Ag, Au, Fe, CoFeB, permalloy).

Experimental results

Research questions

• RQ1 How does the work function of bulk-insulating Bi2-xSbxTe3-ySe y vary with antimony doping (x)?
• RQ2 Is the measured work function intrinsic and independent of applied bias voltage?
• RQ3 How does the work function correlate with the chemical potential shift observed in angle-resolved photoemission spectroscopy?
• RQ4 What is the relative work function of Bi2-xSbxTe3-ySe y compared to other topological insulators and common contact metals?
• RQ5 What implications does the work function difference have for band bending and contact engineering in topological insulator devices?

Key findings

• The work function of Bi2-xSbxTe3-ySe y increases systematically from 4.95 eV at x = 0.0 to 5.20 eV at x = 1.0.
• The measured work function is independent of applied bias voltage, confirming its intrinsic nature.
• The 0.25 eV increase in work function closely matches the 0.3 eV downward shift in the chemical potential of the Dirac cone, indicating consistent electron affinity pinning.
• The work function of Bi2-xSbxTe3-ySe y (4.95–5.20 eV) is lower than that of Bi2Te3 (5.25 eV) and Bi2Se3 (5.60 eV), but higher than PBST (4.80 eV) and TBS (4.70 eV).
• Copper (4.65 eV), silver (4.26 eV), and titanium (4.33 eV) have lower work functions than Bi2-xSbxTe3-ySe y, suggesting favorable Ohmic contact potential.
• Ferromagnetic metals like Fe (4.5 eV), CoFeB (4.8 eV), and permalloy (4.83 eV) exhibit work functions lower than Bi2-xSbxTe3-ySe y, implying significant band bending (0.15–0.7 eV) at their interfaces.