6nm (Bi Sb ) Te Films Demonstrate Hybridization Gap, Enabling Observation of Ultrathin Topological Insulator Electrical Transport

The behaviour of electrons in ultrathin materials holds promise for next-generation electronics, and recent research focuses on the topological insulator (Bi Sb)Te as a potential building block. Feike van Veen, Sofie Kölling, and Stijn R. de Wit, all from the MESA+ Institute for Nanotechnology at the University of Twente, alongside colleagues including Daniel Rosenbach and Chuan Li, investigate how reducing the thickness of this material impacts its electrical properties. They demonstrate the emergence of an insulating phase in 6-nanometre-thick films, a result that confirms the opening of a hybridization gap, a key prediction for achieving a spin Hall state. This achievement represents an important step towards creating novel electronic devices that exploit the unique quantum properties of these ultrathin films, and highlights the complex interplay between material thickness and electron behaviour.

Topological insulators conduct electricity along their surfaces while behaving as insulators internally, a characteristic stemming from their unique electronic structure. Reducing the material’s thickness significantly alters its surface states, leading to hybridization and potentially opening a gap in the electronic spectrum, profoundly impacting conductivity and overall electronic properties. The research systematically varied film thickness and measured the resulting changes in electrical conductivity.

Films ranging from several to tens of nanometres in thickness were created with careful control of the deposition process. Electrical transport measurements, including four-point probe resistance and Hall effect measurements, determined carrier density, mobility, and conductivity as a function of film thickness at low temperatures. The results demonstrate a clear evolution of electrical transport properties with decreasing film thickness, showing a significant suppression of conductivity below a critical thickness attributed to the opening of the surface hybridization gap. Data reveals a shift in carrier density and a reduction in carrier mobility, consistent with gap formation, providing direct evidence for the surface hybridization gap in ultrathin (Bi1−xSbx)Te3 films and contributing to a deeper understanding of two-dimensional topological insulators.

Three-dimensional topological insulator (Bi1−xSbx)Te3 exhibits unique properties when reduced to its ultrathin limit, specifically when the thickness approximates the surface state penetration depth. Theoretical predictions suggest that this reduction induces a hybridization gap at the Dirac point, potentially leading to a quantum spin Hall phase or an insulating state, dependent on the material’s thickness. Hall bar devices fabricated from (Bi1−xSbx)Te3 with thicknesses of 6 and 9nm underwent investigation, revealing an insulating phase around the Dirac point only in the 6nm films at low bias and sub-Kelvin temperatures, confirming the presence of a hybridization gap.

Hybridization Gap and Transport in BST Films

This supplementary information document details the experimental setup, data analysis, and interpretation of results concerning the electrical properties of ultrathin bismuth selenide telluride (BST) films, a topological insulator. It provides a deeper understanding of the data and reasoning behind the conclusions presented in the main publication, demonstrating a hybridization gap, a disordered potential landscape with fluctuating charge puddles, and a geometry dependence where longer devices require a lower voltage for conduction. The study shows that device geometry influences the observed threshold voltage for conduction, as longer devices require a lower voltage due to the increased number of insulating regions. The temperature dependence of the gap is found to be independent of device geometry. Researchers demonstrated that the observed insulating behaviour is reproducible across multiple devices fabricated from different BST films, suggesting a consistent underlying mechanism. A helpful analogy was drawn between the disordered potential landscape and a series of resistive and capacitive circuit elements, providing a conceptual framework for understanding the transport behaviour.

Ultrathin Film Insulating Phase Tuned by Voltage

Researchers experimentally observed an insulating phase in ultrathin films of bismuth selenide telluride, specifically those measuring six nanometres in thickness. This insulating behaviour, consistently measured across multiple devices fabricated from the same film, can be tuned by applying a gate voltage or a sufficiently large bias. The largest gap size occurs when the chemical potential is close to its intrinsic position, controlled by adjusting the ratio of bismuth to antimony within the material. Crucially, this insulating phase vanishes in thicker films of nine nanometres, leading researchers to attribute the effect to a hybridization phenomenon occurring within the ultrathin material.

These findings represent an important step towards understanding and potentially realizing novel electronic states in topological insulators. Researchers also investigated the influence of a magnetic field on the observed gap, revealing variations in its response depending on the device and contact material. Some devices exhibited a gap closing at low magnetic fields potentially due to superconductivity, while others showed an increase in gap size with increasing field. Researchers acknowledge that variations in gap size between samples may be linked to the presence of charge puddles or a non-uniform distribution of the applied bias across the sample, suggesting that device geometry can influence the measured characteristics. Further research will focus on clarifying these effects and fully understanding the interplay between hybridization, magnetic fields, and device geometry in these ultrathin films.