Direct nanoscopic observation of plasma waves in the channel of a graphene feld-effect transistor


Direct nanoscopic observation of plasma waves in the channel of a graphene feld-effect transistor

Soltani, A.; Kuschewski, F.; Bonmann, M.; Generalov, A.; Vorobiev, A.; Ludwig, F.; Wiecha, M. M.; Cibaraite, D.; Walla, F.; Winnerl, S.; Kehr, S. C.; Eng, L. M.; Stake, J.; Roskos, H. G.

Plasma waves play an important role in many solid-state phenomena and devices. They emerge as signifcant also in electronic device structures as the operation frequencies of these devices increase. A prominent example are feld-effect transistors which are increasingly being used as rectifying detectors and mixers of electromagnetic waves at gigahertz and terahertz frequencies, where they exhibit very good sensitivity even high above the cut-off frequencies which limit their application in amplifers and switches. Transport theory predicts that coupling of radiation at THz frequencies into the channel of a feld-effect transistor leads to the development of a gated plasma wave collectively involving the charge carriers of both the two-dimensional electron gas and of the gate electrode. Because of the small spatial separation of the channel from the gate, such a wave propagates with a speed much lower than the vacuum speed of light. In this paper, we present the first direct visualization of such waves. Employing graphene FETs with a buried gate electrode, we utilize near-field THz nanoscopy at room temperature to probe the electric field amplitude of the propagating wave directly on the exposed graphene sheet. Mapping of the feld distribution allows us to determine the decay length and the gate-voltage-dependent propagation speed of the plasma waves which is found to lie in the range of 3.5-7 x 10^6 m/s, in good agreement with theory.

Keywords: graphene field-effect transistor; plasma waves; near-field microscopy

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Publ.-Id: 29461