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Electric and thermoelectric response of hBN/WSe2 bsed transistors

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Transition metal dichalcogenides represent an emerging platform for new nanoscale energy conversion solutions [1-3]. The thermoelectric performance of devices based on two-dimensional materials depends not only on the electrical and thermal properties of the materials used but also on the engineering of the devices.
In real devices, hybridization effects at the semiconductor/metal interface strongly affect the local band structure with important consequences on charge injection and thermoelectric response.
We study the thermoelectric response of 2D materials such as transition metal dichalcogenides and graphene by optimizing the nature of the used metal contacts and by decoupling the 2D material from the substrate via a thin layer of hexagonal boron nitride (hBN), 2D dielectric.
We have recently studied the role of different metal contacts (Ag, Pd, Co, Ti) on the electric and thermoelectric properties of tungsten sidelenide (WSe2) based transistors with a low number of layers (4L, 6L) deposited on hBN.
We reveal a Seebeck response which depends on the metallic contact used with high values ​​of the Seebeck coefficient (S), up to 180 μV/K, and power factors (PF= S2σ) of the order of 2.4 μW/cm K2 (Co), in agreement with the state of the art [3-4].
The metallic electrodes for which a low interface hybridization is theoretically expected (Ag) have the lowest electrical conductivity and the highest Seebeck coefficient. On the contrary, for strong interface hybridization (Pd, Co, Ti), the electrical conductivity increases and slightly reduced S values ​​are measured [5].
Our work reveals the importance of metal contact engineering to optimize the thermoelectric performance of transistors based on transition metal dichalcogenides.

Fig.1 : Optical image of a WSe2-based field effect transistor for charge transport and thermoelectric measurements. Seebeck coefficient and electrical conductivity at low bias as a function of gate voltage in the WSe2-based transistor with (from left to right) Ag, Pd, Co and Ti contacts. Seebeck coefficient (top row) and conductivity low-bias voltage (bottom row) as a function of gate voltage in the WSe2-based transistor with, from left to right, the Ag, Pd, Co, and Ti contacts.

[1] D. Wickramaratne et al., J. Appl. Phys. 118, 075101 (2015)
[2] M. Buscema, et al. Nano Lett. 13, 358 (2013)
[3] M. Yoshida, et al. Nano Lett. 16, 2061 (2016)
[4] S. Timpa, M. Rahimi, J. Rastikian, S. Suffit, F. Mallet, P. Lafarge, C. Barraud, and M. L. Della Rocca, J. Appl. Phys. 130, 185102 (2021)
[5] Y. Wang ey al., Nanoscale 8, 1179 (2016).