Tuning Carrier Type and Density in Highly Conductive and Infrared-Transparent (Bi1-xSbx)2Te3 films

Infrared transparent conductors have long been sought due to their broad optoelectronic applications in the infrared wavelength range. However, the search for ideal materials has been limited by the inherent trade-off between electrical conductance and optical transmittance. Band engineering offers an effective approach to modulate carrier type and density, enabling concurrent tuning of both conductance and transmittance. In this work, we present a band engineering strategy that enables effective tuning of both infrared transmittance and electrical conductance in topological insulator (Bi1-xSbx)2Te3, bridging the gap and paving the way for applying topological insulators to infrared photoelectric devices. More importantly, with the combination of high carrier mobility and a large optical dielectric constant as suggested by previous report, Sb2Te3 achieves a high electrical conductance (1000 S/cm) and outstanding infrared transmittance (92.3%) in the wavelength range of 813 um, demonstrating strong potential as an infrared transparent conductor. Our findings reveal that concurrent enhancement of both carrier mobility and optical dielectric constant is key to overcoming the conductance-transmittance trade-off. This work provides valuable insight for the exploration of high-performance infrared transparent conducting materials.

💡 Research Summary

This paper addresses the long‑standing challenge of simultaneously achieving high electrical conductivity and high infrared (IR) transmittance in a single material, a trade‑off that limits the performance of conventional transparent conductors such as indium tin oxide (ITO) in the far‑IR region. The authors propose a band‑engineering strategy based on the ternary topological‑insulator alloy (Bi₁₋ₓSbₓ)₂Te₃, exploiting the distinct defect chemistry of the binary end members Bi₂Te₃ (n‑type, dominated by Te vacancies) and Sb₂Te₃ (p‑type, dominated by Sb‑Te antisite defects). By varying the Sb composition x from 0 to 1, they are able to continuously tune the carrier type, density, and mobility while preserving the layered quintuple‑layer crystal structure.

The films (≈20 nm thick) were grown by atomic‑layer‑by‑layer molecular‑beam epitaxy (MBE) on double‑sided polished BaF₂ (111) substrates. A two‑step growth (seed layer at 150 °C, main growth at 240 °C) and a high Te‑to‑(Bi,Sb) flux ratio (1:10) were employed to suppress Te vacancies. Structural quality was confirmed by high‑resolution X‑ray diffraction (XRD) and X‑ray photoelectron spectroscopy (XPS); the appearance of the (009) reflection for x ≥ 0.45 indicated successful Sb substitution. Atomic‑force microscopy showed atomically flat terraces, confirming epitaxial growth.

Electrical transport was measured using a Physical Property Measurement System (PPMS) in a van‑der‑Pauw configuration. Temperature‑dependent resistivity revealed metallic behavior for the pure binaries, with Sb₂Te₃ reaching ~1000 S cm⁻¹ at room temperature and >2000 S cm⁻¹ at 2 K. As Sb content increased, the conductivity first decreased due to compensation between n‑type Bi₂Te₃ and p‑type Sb₂Te₃, reaching a minimum near x ≈ 0.62 (the charge‑neutral point, CNP). Beyond this composition, conductivity rose again as p‑type carriers dominate. Hall measurements confirmed a clear n‑to‑p transition, with carrier densities as low as 2 × 10¹⁸ cm⁻³ (2 K) and 5 × 10¹⁸ cm⁻³ (300 K). Mobility was notably high for Sb₂Te₃, reaching 624 cm² V⁻¹ s⁻¹ at 2 K and 389 cm² V⁻¹ s⁻¹ at 300 K, surpassing previously reported IR‑transparent conductors (e.g., Bi₂Se₃‑based alloys). The high mobility implies a long carrier relaxation time (τ), which is a key factor in reducing free‑carrier absorption in the IR.

Angle‑resolved photoemission spectroscopy (ARPES) on a (Bi₀.₃₈Sb₀.₆₂)₂Te₃ film grown on TiN confirmed the presence of a V‑shaped Dirac surface state with the Dirac point only 0.09 eV below the Fermi level, indicating proximity to the CNP and supporting the transport data.

Fourier‑transform infrared spectroscopy (FTIR) was performed on all compositions, with the BaF₂ substrate contribution subtracted using a calibrated equation. In the far‑IR window (8–13 µm), Sb₂Te₃ exhibited an average transmittance of 92.3 %, exceeding that of the other alloy compositions despite not having the lowest carrier density. The authors attribute this superior transmittance to Sb₂Te₃’s larger high‑frequency dielectric constant (ε∞), as previously reported, which reduces the plasma‑edge absorption independent of carrier concentration. MnTe films showed even higher transmittance (≈97 %) but suffered from very poor conductivity, while Bi₄Te₃ displayed the opposite behavior.

To quantitatively compare performance, the transparent‑conductor figure of merit (FOM = –1/(R□·ln T)) was calculated for the 8–13 µm range. Sb₂Te₃ achieved a FOM of 2.35 × 10⁻², an order of magnitude higher than Bi₂Se₃‑based alloys (≈5 × 10⁻³) and other chalcogenide systems. Bi₂Te₃ ranked second, while the composition near the CNP (x ≈ 0.62) yielded the lowest FOM due to a disproportionate drop in conductivity relative to the modest gain in transmittance.

The study concludes that the simultaneous enhancement of carrier mobility (long τ) and optical dielectric constant ε∞ is essential to break the conventional conductance‑transmittance trade‑off in IR transparent conductors. Sb₂Te₃, with its high mobility and large ε∞, emerges as a promising candidate, delivering both ~1000 S cm⁻¹ conductivity and >90 % IR transmittance. The authors also propose MnSb₂Te₄—a natural superlattice combining the high‑transmittance MnTe and high‑conductivity Sb₂Te₃—as a future direction for achieving even better performance. This work provides a clear design roadmap for next‑generation infrared transparent conductors based on topological‑insulator materials.