Seebeck Coefficient of ${\mathrm{Au}}_{x}{\mathrm{Ge}}_{1\ensuremath{-}x}$ Thin Films Close to the Metal-Insulator Transition for Molecular Junctions

Seebeck Coefficient of ${Au}_{x} {Ge}_{1 - x}$ Thin Films Close to the Metal-Insulator Transition for Molecular Junctions

C. Salhani, J. Rastikian, C. Barraud, P. Lafarge, and M. L. Della Rocca

Phys. Rev. Applied 11, 014050 – Published 25 January 2019

Abstract

We experimentally study the thermoelectric properties of an ${Au}_{x} {Ge}_{1 - x}$ thin-film alloy close to the metal-insulator transition ( $x = 19.5 %$ ). The thermoelectric characterization of the thin-film alloy shows a Seebeck coefficient comparable to that of $Au$ thin film while preserving good thermal sensor properties, revealing the potential interest in its use in nanoscale thermoelectric systems. In particular, we demonstrate the ability to directly evaporate an ${Au}_{x} {Ge}_{1 - x}$ thin film on different nanometric-thick molecular layers. A device engineering is proposed paving the way for investigation of the thermoelectricity of large-area nanometric-thick molecular layers, where the alloy is integrated as a contact electrode, fulfilling the double role of an in situ local heater and a high-resolution thermometer.

Received 24 June 2018
Revised 6 November 2018

DOI:https://doi.org/10.1103/PhysRevApplied.11.014050

Physics Subject Headings (PhySH)

Electrical conductivity Energy sources Seebeck effect Thermoelectric effects

Alloys Molecular junctions Ultrathin films

Transport techniques

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

C. Salhani, J. Rastikian, C. Barraud, P. Lafarge, and M. L. Della Rocca^*

Laboratoire MPQ, Université Paris Diderot, Sorbonne Paris Cité, UMR 7162, CNRS, 10 rue Alice Domon et Léonie Duquet, 75205 Paris Cedex 13, France

^*maria-luisa.della-rocca@univ-paris-diderot.fr

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Vol. 11, Iss. 1 — January 2019

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Figure 1
(a) A schematic representation of the device. The four $Ni$ /Au contact probes, labeled $a$ , $b$ , $c$ , and $d$ , are spaced at $d_{a b} = 10 μ m$ , $d_{b c} = 10 μ m$ , and $d_{c d} = 70 μ m$ . The numbers from 1 to 6 define zones of the device with different temperatures. (b) A SEM image of a ${Au}_{x} {Ge}_{1 - x}$ thin film with an area of $50 \times 150 μ m^{2}$ and a thickness of $100 nm$ .
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Figure 2
(a) EDX analysis of the ${Au}_{19.5} {Ge}_{80.5}$ alloy. Characteristic $K$ , $L$ , and $M$ lines of $Au$ , $Ge$ , and $Si$ are visible. The alloy composition of ${Au}_{80.5} {Ge}_{19.5}$ is determined by averaging on 30 different zones. Inset: the temperature dependence of the resistivity in the range $4 K \leq T \leq 250 K$ for three samples fabricated as described in the main text but at different moments. The atomic composition in each case is indicated following the same color code as the measured curve. (b) A SEM image of the alloy surface. (c) An AFM image of the alloy surface.
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Figure 3
(a) A three-dimensional color plot of the temperature in the ${Au}_{x} {Ge}_{1 - x}$ thin-film alloy for an applied voltage $V_{heat}$ of $2 V$ between contacts $a$ and $b$ at a temperature $T_{env} = 150 K$ of the surrounding environment. (b) Temperature profiles in the longitudinal direction of the device [red dotted line in (a)] calculated by varying the $k_{Au Ge}$ value from 1 to 150 W/m K. A temperature gradient of the order of 20 K develops between contacts $c$ and $d$ . The positions of the contact probes are indicated by the vertical dark yellow bars. Inset: an enlargement of the calculated temperature profiles between contacts $c$ and $d$ .
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Figure 4
(a) The temperature dependence of the resistance of the $c d$ segment, $R_{c d}$ , of the ${Au}_{19.5} {Ge}_{80.5}$ thin-film alloy in the range 150–185 K. Inset: the resistance of the $a b$ segment, $R_{a b}$ , measured in the temperature range 3.7–260 K. (b) The thermoelectric voltage measured on the $c d$ segment of the ${Au}_{19.5} {Ge}_{80.5}$ thin-film alloy heated following a nonlocal scheme at a temperature of the environment of $T_{env} = 150 K$ . Inset: the temperature increase $Δ T_{c d}$ between contacts $c$ and $d$ when applying a dc voltage $V_{heat}$ between contacts $a$ and $b$ . (c) The temperature dependence of the resistance of the $c d$ segment, $R_{c d}$ , of the ${Au}_{19.5} {Ge}_{80.5}$ thin-film alloy in the range 200–260 K. (d) The thermoelectric voltage measured on the $c d$ segment of the ${Au}_{19.5} {Ge}_{80.5}$ thin-film alloy heated following a nonlocal scheme at a temperature of the environment of $T_{env} = 200 K$ . Inset: the temperature increase $Δ T_{c d}$ between contacts $c$ and $d$ when applying a dc voltage $V_{heat}$ between contacts $a$ and $b$ . In all of the figures, the black dots are the experimental data and the solid red lines are fitting curves.
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Figure 5
The current-voltage ( $I$ - $V$ ) characteristics at $T = 2$ K of junctions including a molecular layer, namely BTB (a), $NB$ (b), and AQ (c), and having an ${Au}_{x} {Ge}_{1 - x}$ electrode. All $I$ - $V$ curves show the typical nonlinear behavior of molecular junctions. Insets: the temperature dependence of the resistance of the top electrodes realized with an ${Au}_{x} {Ge}_{1 - x}$ thin-film alloy of each large-area molecular junction.
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Physical Review Applied

Seebeck Coefficient of AuxGe1−x Thin Films Close to the Metal-Insulator Transition for Molecular Junctions

C. Salhani, J. Rastikian, C. Barraud, P. Lafarge, and M. L. Della Rocca

Phys. Rev. Applied 11, 014050 – Published 25 January 2019

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Seebeck Coefficient of ${Au}_{x} {Ge}_{1 - x}$ Thin Films Close to the Metal-Insulator Transition for Molecular Junctions