A Single-atom-thick semiconductor sandwich is a significant step toward ultra-low-energy electronics

Left: Electron-hole pairs in atomically-thin WS2 on a substrate where dielectric disorder is similar size to excitons. Right: Hybridisation of excitons and photons leads to formation of polaritons in an all-dielectric high-Q optical microcavity, reducing effect of dielectric disorder. Credit: FLEET
Left: Electron-hole pairs in atomically-thin WS2 on a substrate where dielectric disorder is similar size to excitons. Right: Hybridisation of excitons and photons leads to formation of polaritons in an all-dielectric high-Q optical microcavity, reducing effect of dielectric disorder. Credit: FLEET

A new sandwich-style fabrication process placing a semiconductor only one atom thin between two mirrors has allowed Australian researchers to make a significant step towards ultra-low energy electronics based on the light-matter hybrid particles exciton-polaritons. The study, led by the Australian National University, demonstrated robust, dissipationless propagation of an exciton mixed with light bouncing between the high-quality mirrors. Conventional electronics rely on flowing electrons, or 'holes' (a hole in the absence of an electron, ie a positively charged quasiparticle).

However, a major field of future electronics focuses instead on the use of excitons (an electron bound to a hole) because, in principle, they could flow in a semiconductor without losing energy by forming a collective superfluid state. And excitons in the novel, actively studied atomically thin semiconductors are stable at room temperature. Atomically thin semiconductors are thus a promising class of materials for low-energy applications such as novel transistors and sensors. However, precisely because they are so thin, their properties, including the flow of excitons, are strongly affected by disorder or imperfections, which can be introduced during fabrication.

The ANU-led FLEET team with colleagues at Swinburne University and FLEET Partner institution Wroclaw University has coupled the excitons in an atomically thin material to light to demonstrate for the first time their long-range propagation without any dissipation of energy, at room temperature. When an exciton (matter) binds 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 allows this to happen.

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

A high-quality optical microcavity that ensures the longevity of the light (photonic) component of exciton-polaritons is the key to these observations. The study found that exciton-polaritons can be made remarkably stable if the microcavity is constructed in a particular way, avoiding damage to the fragile semiconductor sandwiched between the mirrors during fabrication.

Lead and corresponding author Matthias Wurdack said, the choice of the atomically thin material in which the excitons travel is far less important.

Matthias said, we found that construction of that microcavity was the key, And while we used tungsten sulfide (WS2) in this particular experiment, we believe any other atomically thin TMDC material would also work.

(Transition metal dichalcogenides are excellent hosts for excitons, hosting excitons that are stable at room temperature and interact strongly with light). The team built the microcavity by stacking all its components one by one. First, a bottom mirror of the microcavity is fabricated, then a semiconductor layer is placed onto it, and then the microcavity is completed by placing another mirror on top. Critically, the team did not deposit the upper mirror structure directly onto the notoriously fragile atomically thin semiconductor, which is easily damaged during any material deposition process.

Matthias said, instead, we fabricate the entire top structure separately, and then place it on top of the semiconductor mechanically, like making a sandwich. Thus we avoid any damage to the atomically thin semiconductor and preserve the properties of its excitons.

Importantly, the researchers optimized this sandwiching method to make the cavity very short, which maximized the exciton-photon interaction.

Matthias said, we also benefitted from a bit of serendipity. An accident of fabrication that ended up being key to our success!

The serendipitous accident came in the form of an air gap between the two mirrors, making them not strictly parallel. This wedge in the microcavity creates a voltage/potential slope for the exciton-polaritons, with the particles moving either up or down the incline. The researchers discovered that a proportion of exciton-polaritons travel with conservation of total (potential and kinetic) energy, both up and down the incline. Traveling down the slope, they convert their potential energy into an equal amount of kinetic energy, and vice versa.

That perfect conservation of total energy means no energy is being lost in heat (due to friction), which signals 'ballistic' or dissipationless transport for polaritons. Even though the polaritons in this study do not form a superfluid, the absence of dissipation is achieved because all scattering processes that lead to energy loss are suppressed.

Group leader Prof Elena Ostrovskaya (ANU) said, this demonstration, for the first time, of ballistic transport of room-temperature polaritons in atomically-thin TMDCs, is a significant step towards future, ultra-low energy exciton-based electronics.

Apart from creating the potential slope, that same fabrication accident created a potential well for exciton-polaritons. This enabled the researchers to catch and accumulate the traveling exciton-polaritons in the well an essential first step for trapping and guiding them on a microchip.

Furthermore, the researchers confirmed that exciton-polaritons can propagate in the atomically thin semiconductor for tens of micrometers (easily far enough for functional electronics), without scattering on material defects. This is in contrast to excitons in these materials, the travel length of which is dramatically reduced by these defects. Moreover, the exciton-polaritons were able to preserve their intrinsic coherence (correlation between signals at different points in space and time), which bodes well for their potential as information carriers.

Matthias Wurdack said, this long-range, coherent transport was achieved at room temperature, which is important for the development of practical applications of atomically thin semiconductors.

If future excitonic devices are to be a viable, low-energy alternative to conventional electronic devices, they must be able to operate at room temperature, without the need for energy-intensive cooling.

Matthias said, in fact, counterintuitively, our calculations show that the propagation length is getting longer at higher temperatures, which is important for technological applications.

Motional narrowing, ballistic transport, and trapping of room-temperature exciton-polaritons in an atomically thin semiconductor were published in Nature Communications in September 2021.

Journal Information: M. Wurdack et al, Motional narrowing, ballistic transport, and trapping of room-temperature exciton polaritons in an atomically-thin semiconductor, Nature Communications (2021). DOI: 10.1038/s41467-021-25656-7

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