CM – A single atom thick semiconductor sandwich is a significant step towards ultra-low-energy electronics


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October 5, 2021


A new sandwich-style manufacturing process, in which a semiconductor is placed just an atom thin between two mirrors, has enabled Australian researchers to take a significant step towards ultra-low-energy electronics based on light-matter. Hybrid particles to make exciton polaritons.

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The study, led by the Australian National University, showed robust, lossless propagation of an exciton mixed with light that is reflected between the high-quality mirrors.

Conventional electronics rely on flowing electrons or « holes » (a hole is the absence of an electron, i.e. a positively charged quasiparticle).

However, an important field of future electronics will instead focus on the use of excitons (an electron bound to a hole), as they could in principle flow in a semiconductor, without losing energy by forming a collective superfluid state. And excitons in new, actively investigated, atomically thin semiconductors are stable at room temperature.

Atomically thin semiconductors are therefore a promising class of materials for low-energy applications such as new types of transistors and sensors. But precisely because they are so thin, their properties, including exciton flow, are greatly affected by clutter or imperfections that may be introduced during manufacture.

The ANU-led FLEET team – with colleagues from Swinburne University and the FLEET partner facility of the University of Wroclaw – has coupled the excitons in an atomically thin material with light in order to demonstrate for the first time their long-distance propagation in a room without any energy dissipation.

When an exciton (matter) interacts with a photon ( Light), it forms a new hybrid particle – an exciton polariton. Trapping light between two parallel high quality mirrors in an optical microcavity makes this possible.

In the new study, a new sandwich-style manufacturing process for the optical microcavity allowed researchers to minimize damage to the atomically thin semiconductor and the interaction between to maximize the excitons and the photons. The exciton polaritons formed in this structure were able to spread over dozens of micrometers without loss of energy, the typical scale of an electronic microchip.

A high-quality optical microcavity that ensures the longevity of the light component (photonic) of exciton polaritons is the Key to these observations.

The study found that exciton polaritons can be made remarkably stable if the microcavity is designed in a specific way to avoid damaging the fragile semiconductor that sits between the mirrors during manufacture. to avoid.

« The choice of the atomically thin material in which the excitons move is far less important, » says first author and corresponding author Matthias Wurdack.

« We found that the construction of this microcavity of the The key was, « says Matthias, » and although we used tungsten sulfide (WS2) in this particular experiment, glau Let’s practice that any other atomically thin TMDC material would work too. « 

(Transition metal dichalcogenides are excellent hosts for excitons because they host excitons that are stable at room temperature and interact strongly with light).

That Team built the microcavity by stacking all of the components individually. First a lower mirror of the microcavity is made, then a semiconductor layer is placed on top, and then the microcavity is completed by placing another mirror on top. It is crucial that the team did not apply the top mirror structure directly to the notoriously fragile, atomically thin semiconductor, which can be easily damaged in any material deposition process.

« Instead, we manufacture the entire top structure separately and then lay it mechanically like a sandwich on the semiconductor, « says Matthias.

 » In this way we avoid any damage to the atomically thin semiconductor and preserve the properties of its excitons. « 

It is important that the researchers have optimized this sandwiching method, to make the cavity very short, which maximizes the exciton-photon interaction.

« We were also a bit lucky, » says Matthias. « A manufacturing accident that was ultimately the key to our success! »

The random « accident » occurred in the form of an air gap between the two mirrors, which meant that they were not exactly parallel.

This wedge in the microcavity creates a voltage / potential « slope » for the exciton polaritons, with the particles moving either up or down kinetic) energy both uphill and downhill. As they go down the slope, they convert their potential energy into the same amount of kinetic energy and vice versa.

This perfect conservation of total energy means that no energy is lost in heat (due to « friction »), which means  » ballistic “or lossless transport for polaritons signals. Although the polaritons do not form a superfluid in this study, freedom from dissipation is achieved because all scattering processes that lead to energy losses are suppressed.

“This first demonstration of the ballistic transport of room temperature polaritons in atomically thin TMDCs is an important step towards future, ultra-low-energy exciton-based electronics ”, says group leader Prof. Elena Ostrovskaya (ANU).

Apart from the generation of the potential » slope « , the same manufacturing accident generated a potential source of exciton polaritons. This enabled the researchers to capture and accumulate the migrating exciton polaritons in the borehole – an essential first step in capturing and directing them on a microchip. « 

In addition, the researchers confirmed that exciton polaritons are located in the atomically thin semiconductor can propagate over several dozen micrometers (for functional electronics) without scattering at material defects. This is in contrast to excitons in these materials, the path length of which is dramatically reduced by these defects.

In addition, the exciton polaritons could be intrinsic Maintain coherence (correlation between signals at different points in space and time), which promises their potential as an information carrier.

« This far-reaching, coherent transport was achieved at room temperature, which is important for the development of practical applications of atomically thin semiconductors » says Matthias Wurdack.

If future excitonic devices are a prak tic, energy-saving alternative to conventional electronic devices, they must be able to be operated at room temperature without energy-intensive cooling. said Matthias.

« Motional Narrowing, Ballistic Transport, and Trapping of Room-Temperature Exciton Polaritons in an Atomically-thin Semiconductor » was published in Nature Communications in September 2021.

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Similar title :
Semiconductor sandwiches with a single atom thickness are an important step towards ultra-low energy electronics
Semiconductor sandwiches with a single atom thickness are an important step towards ultra-low-energy electronics


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