Electrical gating visualised in microelectronic devices

Physicists from the University of Warwick and the University of Washington have developed a new technique, described in the journal Nature, to measure the energy and momentum of electrons in operating microelectronic devices made of atomically thin, two-dimensional materials. Using this information, the scientists can create visual representations of the electrical and optical properties of the materials to guide engineers in maximising their 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 behaviour can vary depending on 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. Until now, however, there has been no way to directly see these changes to help us understand how they affect the behaviour of electrons.

The new technique will give scientists 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 two-dimensional semiconductors that are seen as potential components for the next generation of electronics, with applications in flexible electronics, photovoltaics and spintronics. Unlike today’s three-dimensional semiconductors, two-dimensional 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 Dr Neil Wilson from the University of Warwick. “For the first time we are directly visualising those changes. Not being able to see how that 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.

“The new insight into the materials has helped us to understand the bandgaps of these semiconductors, which is the most important parameter that affects their behaviour — from what wavelength of light they emit to how they switch current in a transistor.”

The technique uses angle-resolved photoemission spectroscopy (ARPES) to ‘excite’ electrons in the chosen material. By focusing a beam of ultraviolet or X-ray light on atoms in a localised area, the excited electrons are knocked out of their atoms. Scientists can then measure the energy and direction of travel of the electrons, from which they can work out the energy and momentum they had within the material (using the laws of the conservation of energy and momentum). 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 the researchers.

The team first tested the technique using graphene before applying it to two-dimensional transition metal dichalcogenide (TMD) semiconductors. The measurements were taken at the Spectromicroscopy beamline at the Elettra synchrotron in Italy.

“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 Dr David Cobden from the University of Washington. “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. This changes the game.”

Image caption: Electrons ejected by a beam of light, focused on a two-dimensional semiconductor device, are collected and analysed to determine how the electronic structure in the material changes as a voltage is applied between the electrodes. Image credit: Nelson Yeung/Nick Hine/Paul Nguyen/David Cobden.

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