Formation of reliable electrical and thermal contacts between graphene and metal electrodes by laser annealing S.A. Moshkalev a,⇑ , V.A. Ermakov a , A.R. Vaz a , A.V. Alaferdov a,b , R. Savu a , J.V. Silveira a,c , A.G. Souza Filho c a Center for Semiconductor Components, University of Campinas – UNICAMP, C.P. 6061, Rua João Pandia Calógeras, 90, 13083-870 Campinas, SP, Brazil b Lobachevsky State University of Nizhni Novgorod, Gagarine Av. 32/3, Nizhni Novgorod 603950, Russia c Physics Department, Federal University of Ceará, P.O. Box 6030, 60455-900 Fortaleza, CE, Brazil article info Article history: Received 28 October 2013 Received in revised form 6 March 2014 Accepted 22 March 2014 Available online 1 April 2014 Keywords: Graphene Multi-wall carbon nanotube Laser annealing Raman spectroscopy Contact resistivity abstract A new approach for electrical and thermal improvement of contacts between carbon nanostructures (multi-wall carbon nanotubes – MWCNTs and multi-layer graphene – MLG) and metal electrodes by localized laser heating is presented. The nanostructures were deposited over electrodes using the dielec- trophoresis (DEP) technique. A focused laser beam was used for direct heating the samples in ambient atmosphere. The Raman spectroscopy was used to determine the process temperature by observations of the graphitic G-line downshift. In the laser annealing experiments, the G-line position was found first to downshift linearly with laser power indicating gradual heating of the sample proportional to the absorbed power. However, with increasing power the shift was found to saturate at levels that depend on the metal and the contact area. This saturation was attributed to gradual increase of the contact area and improvement of the thermal contacts between the nanostructures and metal electrode that can occur during sample heating. The maximum sample temperature in the beginning of the annealing process was always higher for MLG samples, due to smaller area of contact established between rigid multi-layer graphene and initially rough metal surface. The final result is the increased heat losses to the electrodes and, subsequently, the reduction of the samples temperature. The main advantage of this method, when compared with traditional and rapid thermal annealing, is that the thermal treatment is localized in a small pre-determined region, allowing individually controlled annealing process. Ó 2014 Elsevier B.V. All rights reserved. 1. Introduction One of fundamental issues in nanofabrication is formation of reliable, low-resistance and stable contacts between 1D or 2D nanostructured materials like carbon nanotubes or graphene and metal electrodes [1–4]. For constantly decreasing sizes of contact area in novel nanodevices, the reduction of electrical and thermal contact resistivities becomes increasingly important. However, the studies of these properties are challenging and currently very scarce. A frequently used approach to connect nanotubes or graph- ene sheets to patterned metal electrodes is based on a controlled deposition from solutions by a.c. dielectrophoresis (DEP) process [5–7]. However, poor electrical and thermal contacts are usually formed between the DEP-deposited structures and metallic films [7–9]. Thus, further processing is required in order to improve the performance of the nanostructured-based devices. This can be done by means of conventional (global) thermal treatment or localized annealing. High-quality thermal and electrical contacts between graphene and metals usually are obtained using conventional high-tempera- ture (875–1175 K) [7,10] annealing in vacuum. However, high- temperature processing of samples may be not compatible with other fabrication steps. Furthermore, as shown in our previous study [11], this can induce strain in multi-layer graphene due to formation of tight mechanical contacts between graphene and me- tal electrodes during annealing (for metals that form carbides) fol- lowed by the electrodes shrinkage, and thus stretching the graphene flake, that eventually can damage the structure. There- fore, it is desirable to perform annealing by heating locally the graphene flake and the contact areas without substantial heating of the electrode bodies. Localized annealing of carbon nanostructures, in turn, can be done using heating by focused laser beam, resulting eventually in http://dx.doi.org/10.1016/j.mee.2014.03.028 0167-9317/Ó 2014 Elsevier B.V. All rights reserved. ⇑ Corresponding author. Address: Centro de Componentes Semicondutores (CCS) – Universidade de Campinas (UNICAMP), Rua João Pandia Calógeras, 90, C.P. 6061, 13083-870, Campinas, SP, Brazil. Tel.: +55 (19) 3521 7282; fax: +55 (19) 3521 5226. E-mail address: stanisla@ccs.unicamp.br (S.A. Moshkalev). Microelectronic Engineering 121 (2014) 55–58 Contents lists available at ScienceDirect Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee