Investigation of Mg 2 (Si,Sn) thin films for integrated thermoelectric devices Codrin Prahoveanu a, b, c , Ana Lacoste a, b, * , St  ephane B  echu a, b ,C  edric de Vaulx d , Kamel Azzouz d , Laetitia Laversenne b, c, e, ** a LPSC, Universit e Grenoble-Alpes, CNRS/IN2P3, 53 rue des Martyrs, 38026 Grenoble, France b Univ. Grenoble Alpes, F-38000 Grenoble, France c CNRS, Inst NEEL, F-38000 Grenoble, France d ValeoThermal Systems, 8 rue Louis Lormand, BP 517 La Verri ere, 78321 Le Mesnil Saint Denis Cedex, France e School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA article info Article history: Received 9 October 2014 Received in revised form 12 June 2015 Accepted 4 July 2015 Available online 7 July 2015 Keywords: Thermoelectric materials Mg 2 (Si,Sn) Thin films Microwave plasma co-sputtering Thermal stability Film-substrate reactivity abstract The ongoing miniaturization of thermoelectric (TE) modules requires scaling down to thin films of TE materials with high efficiency. Moreover, thin film-based integrated devices contain interfaces (between TE materials and dielectric or conductive components) that must show chemical and mechanical stability in the whole range of the operating temperature. In search for such materials, thin films of Mg 2 (Si,Sn) solid solutions have been deposited by microwave plasma-assisted co-sputtering method with a fine control over their composition. Three types of substrates were chosen (SiO 2 /Si, glass and Ni substrates) to examine their potential use as insulators and electrodes in a miniaturized thermoelectric module based on Mg 2 (Si,Sn). The electrical conductivity and thermo-mechanical properties, as well as the thermal stability, of the thin films have been investigated in the intermediate range of temperature (300e700 K). It is shown that the deposition process, as well as the substrates on which the films are grown, determine the subsequent adherence of the films. Also, the metastability of the Mg 2 Si 0.4 Sn 0.6 solid solution for small variations in composition (possibly bordering the edge of the miscibility gap in the phase diagram) has been observed, which can lead to a separation into 2 phases during the first annealing treatment at intermediate temperatures. © 2015 Published by Elsevier B.V. 1. Introduction Within the continuous development of different technologies dealing with power generation, a significant emphasis can be found in the field of thermoelectricity [1e5]. Many thermoelectric (TE) materials have been studied, using the figure of merit ZT as crite- rion for establishing their efficiency. Among these, the Mg 2 (Si,Sn) solid solutions stand out as promising TE materials not only due to their ZT which has been reported to have surpassed unity after doping [6e11] with values comparable to that of state-of-the-art materials [12,13], but also on account of the abundance of the constituent elements and their environmentally friendly feature. Also, by appropriately doping these solid solutions it can result to both n-type and p-type TE materials that can subsequently be implemented in TE modules [14]. The potential of these solid solutions arises from the relatively low thermal conductivity due to the enhanced point defect phonon scattering and strain fluctuations stemmed from the great differ- ence in atomic mass between Si and Sn [15]. The power factor can be improved as well on account of the increase of the Seebeck coefficient determined by the degeneracy of the conduction band minima characteristic to the Mg 2 (Si,Sn) solid solutions and the possibility to fine-tune it by controlling the Sn content [8]. After a number of studies reported on these solid solutions with different stoichiometries, it was established that the best TE properties correspond to the materials of composition Mg 2 Si x Sn 1x with x between 0.35 and 0.6 in the temperature range of 500e850 K [8,16,17]. These solid solutions were doped with Sb, known to improve the TE properties of the ternary materials by increasing the * Corresponding author. LPSC, Universit e Grenoble-Alpes, CNRS/IN2P3, 53 rue des Martyrs, 38026 Grenoble, France. ** Corresponding author. CNRS, Inst NEEL, F-38000 Grenoble, France. E-mail addresses: ana.lacoste@ujf-grenoble.fr (A. Lacoste), laetitia.laversenne@ neel.cnrs.fr (L. Laversenne). Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom http://dx.doi.org/10.1016/j.jallcom.2015.07.043 0925-8388/© 2015 Published by Elsevier B.V. Journal of Alloys and Compounds 649 (2015) 573e578