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. (Image courtesy of NelsonYeung/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 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 applied to the material, and so changes to the electronic structure with voltage determine the efficiency of microelectronic circuits. But until now there has been no way to directly see these changes to help 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-corresponding 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 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 the 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.

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 co-corresponding author Neil Wilson, an associate professor of physics at the University of Warwick.