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Carbon nanotube-based light emitting FETs could yield cheap organic lasers

Technology News |
By Julien Happich

Publishing their results in Nature Materials under the paper title “Electrical pumping and tuning of exciton-polaritons in carbon nanotube microcavities”, the researchers disclose a seemingly simple to manufacture device with a strong potential for laser applications.

The authors explain that while organic semiconductors are actively researched to design exciton-polariton lasing devices, low charge carrier mobilities have been hindering the applicability of such devices. Here they used single-walled carbon nanotubes (SWCNTs) embedded in a polymer matrix as a planar semiconducting material between source and drain electrodes, topping it up with a gate doubling up as a top mirror to form a light-emitting field-effect transistor (LEFET). The carbon nanotubes are readily processed from solution and boast exceptionally high electron and hole mobilities.

Schematic geometry of a bottom-contact/top-gate
LEFET (top stack). By extending the structure with a
bottom mirror an optical microcavity is formed
between the top gate and the bottom mirror.

They built the whole device within an optical cavity formed perpendicular to the direction of charge transport, by the top gate mirror and a semitransparent bottom mirror electrically isolated by a layer of aluminium oxide, on a glass substrate. Through multiple experiments, they were able to tune the narrow-band polariton electroluminescence (EL) from 1,060 nm up to 1,530, by simply adjusting the cavity through different spacer thicknesses. 


Operated at room temperature and under ambient conditions, the LEFETs exhibited ambipolar charge transport with high on/off current ratios of up to 106 at low drain bias (Vd = −0.5 V), reports the paper. And the high electron and hole mobilities of 4.2 and 3.5 cm2 V−1 s−1 , respectively allowed the device to operate continuously at high current densities in excess of 10kA cm−2.

Interestingly, in the ambipolar regime of the LEFETs (Vd = −8 V), electrons and holes are injected into the channel simultaneously and meet within a narrow zone and the position of the emission zone (about 1µm wide) within the channel can be adjusted by the applied gate voltage Vg.

Transfer characteristics of the cavity-embedded
transistor for low and high source–drain bias (Vd).

The researchers estimate the polariton density in cavity-embedded LEFETs with SWCNTs to be around 3.6×1011 cm−3 and ground state occupancy at 0.004, which they suggest makes polariton lasing devices within realistic reach, pending further cavity and device fabrication optimization.

“We demonstrate thermalization of SWCNT polaritons, exciton-polariton pumping rates ∼104 times higher than in current organic polariton devices, direct control over the coupling strength (Rabi splitting) via the applied gate voltage, and a tenfold enhancement of polaritonic over excitonic emission”, the paper reads.

The paper concludes that cavity-embedded LEFETs based on SWCNTs are a promising material/device combination that could ultimately lead to electrically pumped, carbon-based polariton lasers operating at room temperature.


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