Scientists Visualize Electronic Structure in Microelectronic Devices

For the first time, researchers have successfully observed the electronic structure in a microelectronic device that could pave the way for finely-tuned, high-performance electronic devices.

At the University of Washington and the University of Warwick, physicists have devised a novel method to determine electrons’ momentum and energy in operating microelectronic devices composed of the so-called atomically thin, two-dimensional (2D) materials.

With this data, the physicists can generate visual representations of the materials’ optical and electrical characteristics to guide engineers in increasing their potential in electronic components.

Reported in Nature on July 17^th, 2019, the experimentally-led research can also open up opportunities for the 2D semiconductors that could play a major role in state-of-the-art electronics, in applications like quantum computers, mobile devices, and photovoltaics.

A material’s electronic structure describes the way electrons behave inside that material, and thus the nature of the current passing through it. That behavior can differ based on the voltage—that is, the amount of “pressure” on its electrons—used on the material, and hence variations to the electronic structure with voltage establish the microelectronic circuits’ efficiency.

It is these variations in the electronic structure of operating devices that underpin all the latest electronics. However, until now, no direct method was available to observe these changes to help understand how they influence the electrons’ behavior.

Applying this method will allow researchers to have the data they require to fabricate “fine-tuned” electronic components that operate at high performance without consuming excess power and also work more efficiently.

The technique will also help in developing 2D semiconductors that are perceived as promising components for sophisticated electronics, with applications in spintronics, photovoltaics, and flexible electronics. 2D semiconductors are different from today’s 3D semiconductors and contain only a few layers of atoms.

How the electronic structure changes with voltage is what determines how a transistor in your computer or television works. 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.

Dr Neil Wilson, Department of Physics¸ University of Warwick

Dr Wilson continued, “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 behaviour, from what wavelength of light they emit, to how they switch current in a transistor.”

Angle-resolved photoemission spectroscopy, or ARPES, is used by the technique to “excite” electrons in the selected material. When a beam of X-ray or ultra-violet light is focused on atoms in a localized area, the excited electrons are displaced from their atoms. The electrons’ energy and their direction of travel can be subsequently measured and from this, scientists can work out the momentum and energy they had within the material (applying the laws of the conservation of energy and momentum).

That establishes the material’s electronic structure, which can be subsequently compared against hypothetical predictions. These predictions were based on advanced electronic structure calculations conducted in this case by the research team of co-author Dr Nicholas Hine.

To test the technique, the researchers used graphene and then applied it to 2D transition metal dichalcogenide (TMD) semiconductors. The measurements were done at the Spectromicroscopy beamline at the ELETTRA synchrotron in Italy, in association with Dr Alexei Barinov and his research team there.

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. 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.

Dr David Cobden, Professor, Department of Physics, University of Washington

Dr Xiaodong Xu, from the Department of Physics and the Department of Materials Science & Engineering at the University of Washington, stated, “this powerful spectroscopy technique will open new opportunities to study fundamental phenomena, such as visualisation of electrically tunable topological phase transition and doping effects on correlated electronic phases, which are otherwise challenging.”

The Engineering and Physical Sciences Research Council, part of UK Research and Innovation, and the U.S. Department of Energy and the National Science Foundation supported the study.