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Ionic gate spectroscopy of 2D semiconductors

Nature Reviews Physics volume 3, pages 508–519 (2021)Cite this article

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Abstract

Reliable and precise measurements of the relative energy of band edges in 2D semiconductors are needed to determine band gaps and band offsets, as well as to establish the band diagram of devices and heterostructures. However, commonly employed techniques such as optical studies and scanning tunnelling microscopy need to be accompanied by modelling for quantitative results. Over the last decade, ionic gate spectroscopy has emerged as a technique that can quantitatively determine the relative alignment of band edges of 2D semiconductors directly from transport measurements. The technique relies on the extremely large geometrical capacitance of ionic gated devices that, under suitable conditions, enables a change in gate voltage to be directly related to a shift in chemical potential. Here, we present an overview of ionic gate spectroscopy and illustrate its relevance with applications to different 2D semiconductors and their heterostructures.

Key points

  • The capacitance of ionic gates is two to three orders of magnitude larger than that of gates commonly used in most of the experiments in the field of 2D materials.

  • Owing to the large geometrical capacitance of ionic gates, a change in gate voltage gives rise to an equal change in chemical potential within the band gap of a semiconductor.

  • Ionic gate spectroscopy allows the semiconducting band gap to be determined in a simple and reliable manner, directly from the transfer characteristics of a transistor, without the need for data modelling.

  • Ionic gate spectroscopy straightforwardly enables the precise determination of the relative band offsets between different semiconductors, a task commonly difficult to achieve.

  • Ionic gate spectroscopy is a relatively young technique that is ideally suited to probe and characterize 2D semiconducting materials and their heterostructures.

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Fig. 1: Evolution of transistor characteristics upon increasing gate capacitance.
Fig. 2: Determination of the band gap.
Fig. 3: Precision and reliability of ionic gate spectroscopy.
Fig. 4: Gap values for common 2D semiconductors.
Fig. 5: Band alignment in different semiconductors.

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Acknowledgements

We acknowledge Alexandre Ferreira for his continuous and precious technical support over the years. We acknowledge the scientific contributions of several former and current group members who have been actively involved in the development of the technique: Daniele Braga, Sanghyun Jo, Marc Philippi, Aditya Reddy and Haijing Zhang. Financial support from the Swiss National Science Foundation and from the European Union Graphene Flagship Project is also gratefully acknowledged.

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Authors and Affiliations

  1. Department of Quantum Matter Physics, University of Geneva, Geneva, Switzerland

    Ignacio Gutiérrez-Lezama, Nicolas Ubrig, Evgeniy Ponomarev & Alberto F. Morpurgo

  2. Department of Applied Physics, University of Geneva, Geneva, Switzerland

    Ignacio Gutiérrez-Lezama, Nicolas Ubrig, Evgeniy Ponomarev & Alberto F. Morpurgo

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  1. Ignacio Gutiérrez-Lezama
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Gutiérrez-Lezama, I., Ubrig, N., Ponomarev, E. et al. Ionic gate spectroscopy of 2D semiconductors. Nat Rev Phys 3, 508–519 (2021). https://doi.org/10.1038/s42254-021-00317-2

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