Electrons ejected by a beam of light focused on a two-dimensional semiconductor device are collected and analyzed to determine how the electronic structure in the material changes as a voltage is applied between the electrodes.Nelson Yeung/Nick Hine/Paul Nguyen/David Cobden
Scientists have visualized the electronic structure in a microelectronic device for the first time, opening up opportunities for finely tuned, high-performance electronic devices.
Physicists from the University of Washington and the University of Warwick developed a technique to measure the energy and momentum of electrons in operating microelectronic devices made of atomically thin – so-called 2D – materials.
Using this information, the researchers created visual representations of the electrical and optical properties of the materials to guide engineers in maximizing 2D materials’ potential in electronic components.
The study, published July 17 in the journal Nature, could also pave the way for the types of 2D semiconductors that are likely to play a role in the next generation of electronics, in applications such as photovoltaics, mobile devices and quantum computers.
The electronic structure of a material describes how electrons behave within that material, and therefore the nature of the current flowing through it. That behavior varies depending upon the voltage – the amount of “pressure” on its electrons – applied to the material, and so changes to the electronic structure with voltage determine the efficiency of microelectronic circuits.
These changes in electronic structure in operating devices are what underpin all of modern electronics. But until now there has been no way to directly see these changes to help us understand how they affect the behavior of electrons.
Their technique used angle-resolved photoemission spectroscopy, or ARPES, to “excite” electrons in the chosen material.
“It used to be that the only way to learn about what the electrons are doing in an operating semiconductor device was to compare its current-voltage characteristics with complicated models,” said co-author David Cobden, a University of Washington professor of physics. “Now, thanks to recent advances, which allow the ARPES technique to be applied to tiny spots, combined with the advent of two-dimensional materials where the electronic action can be right on the very surface, we can directly measure the electronic spectrum in detail and see how it changes in real time.”
By applying this technique, scientists will have the information they need to develop “fine-tuned” electronic components that work more efficiently and operate at high performance with lower power consumption. It will also help in the development of 2D semiconductors that are seen as potential components for the next generation of electronics, with applications in flexible electronics, photovoltaics and spintronics. Unlike today’s 3D semiconductors, 2D semiconductors consist of just a few layers of atoms.
“How the electronic structure changes with voltage is what determines how a transistor in your computer or television works,” said co-author Neil Wilson, an associate professor of physics at the University of Warwick. “For the first time we are directly visualising those changes. Not being able to see how [structure] changes with voltages was a big missing link. This work is at the fundamental level and is a big step in understanding materials and the science behind them.”
By focusing a beam of ultraviolet or x-ray light on atoms in a localized area, the excited electrons are knocked out of their atoms. Scientists can then measure the energy and direction of travel of the electrons, which – thanks to laws for the conservation of energy and momentum – allows them to work out the energy and momentum they had within the material. That determines the electronic structure of the material, which can then be compared against theoretical predictions based on state-of-the-art electronic structure calculations performed by co-author Nicholas Hine‘s group at the University of Warwick.
“This powerful spectroscopy technique will open new opportunities to study fundamental phenomena, such as visualization of electrically tunable topological phase transition and doping effects on correlated electronic phases, which are otherwise challenging,” said co-author Xaiodong Xu, a University of Washington professor of both physics and materials science and engineering.
The team first tested the technique using graphene before applying it to 2D transition metal dichalcogenide, or TMD, semiconductors.
“The new insight into the materials has helped us to understand the band gaps of these semiconductors, which is the most important parameter that affects their behavior, from what wavelength of light they emit, to how they switch current in a transistor,” said Wilson.
“This changes the game,” said Cobden.
Co-authors are Paul Nguyen, Nathan Wilson and Joshua Kahn, all University of Washington doctoral students in the Department of Physics; Natalie Teutsch and Xue Xia with the University of Warwick Department of Physics; Viktor Kandyba with the Elettra – Sincrotrone Trieste; and Gabriel Constantinescu with the University of Cambridge. The research was funded by the U.S. Department of Energy, the U.K. Engineering and Physical Sciences Research Council, the National Science Foundation, the University of Warwick, the Winton Programme for the Physics of Sustainability, the Cambridge Trust European Scholarship and the University of Cambridge.