Palladium Germanides for Mid- and Long-Wave Infrared Plasmonics
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Palladium Germanides for Mid- and Long-Wave Infrared Plasmonics Evan M. Smith1,2*, William H. Streyer3, Nima Nader2,4, Shivashankar Vangala2,5, Richard Soref6, Daniel Wasserman7, and Justin W. Cleary2 1 KBRwyle, Beavercreek, OH, 45431, USA 2 Air Force Research Laboratory, Sensors Directorate, Wright-Patterson Air Force Base, Ohio, 45433, USA 3 Department of Electrical and Computer Engineering, University of Illinois, Urbana-Champaign, IL, 61801, USA 4 Solid State Scientific Corporation, Nashua, NH 03060, USA 5 Azimuth Corporation, Dayton, OH 45431, USA 6 The Engineering Program, University of Massachusetts at Boston, MA 02125, USA 7 Department of Electrical and Computer Engineering, University of Texas Austin, Austin, TX 78758, USA ABSTRACT Palladium germanide thin films were investigated for infrared plasmonic applications. Palladium thin films were deposited onto amorphous germanium thin films and subsequently annealed at a range of temperatures. X-ray diffraction was used to identify stoichiometry, and Scanning Electron Micrographs, along with Energy Dispersive Spectroscopy (EDS) was used to characterize composition and film quality. Resistivity was also measured for analysis. Complex permittivity spectra were measured from 0.3 to 15 µm using IR ellipsometry. From this, surface plasmon polariton (SPP) characteristics such as propagation length and mode confinement were calculated and used to determine appropriate spectral windows for plasmonic applications with respect to film characteristics. Films were evaluated for use with on-chip plasmonic components. INTRODUCTION Plasmonic devices utilize sub-wavelength structures to confine and guide electromagnetic waves along the interface between dielectric and conductive media. Plasmonic waveguides enable the propagation of surface plasmon polaritons (SPP) based upon the structure geometry and the complex optical permittivity of the conductor. Applications include chemical and biological sensing, information processing, and integrated on-chip optoelectronic circuits [1-3]. Early research in plasmonics focused on noble metals due to their high conductivity and low loss. However, these materials are not useful in the infrared spectrum as their plasma frequency is typically in the UV or visible range [3, 4]. For mid-wave infrared (MWIR, 3-5µm) and longwave infrared (LWIR, 8-12µm) applications, materials such as doped silicon and germanium [5, 6], transparent conducting oxides [7], metal silicides [5, 8] and metal germanides [9] have been investigated. A comprehensive review of these materials can be found in [2]. Metal silicides and metal germanides have been investigated extensively for their use as contact materials in transistor designs, and have been shown to have similar properties [10, 11]. This work seeks to expand upon the metal germanide system for plasmonic applications by studying palladium germanide films in a similar fashion to the Pt-Ge work presented in [9]. PdGe has been found to be stable over a wide temperature range and to have a low resistivity
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