Electron Self-trapping in Ge 2 Se 3 and its Role in Ag and Sn Incorporation

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Electron Self-trapping in Ge2 Se3 and its Role in Ag and Sn Incorporation Arthur H. Edwards1, Kristy A. Campell2, and Andrew C. Pineda3 1 Space Vehicles Directorate, AFRL, Bldg. 914, 3550 Aberdeen Ave. SE, Kirtland AFB, NM 87117-5776, U.S.A. 2 Department of Electrical and Computer Engineering, Boise State University, Boise, ID 83725 U. S. A. 3 Department of Electrical and Computer Engineering, University of New Mexico, Albuquerque, NM 87131-0001 U. S. A ABSTRACT We present a set of density functional theory (DFT) calculations on the electronic structure of Ag and Sn in Ge2 Se3 in a periodic model. We show that electron self-trapping is a persistent feature in the presence of many defects. Ag and Sn autoionize upon entering Ge2 Se3 becoming Ag+ and Sn2+ , respectively, and the freed electrons self trap at the lowest energy site. Both Ag and Sn can substitute for Ge, and we present formation energies as a function of Fermi level that show that Sn can substantially alter the incorporation of Ag into the Ge2Se3 network. INTRODUCTION Since the seminal article by Chua [1], memristor technology has been exploited using a variety of materials systems and physical principles. Borghetti et al. have developed metal oxide-based memristors, wherein positively charged oxygen vacancies migrate under the influence of an applied electric field [2]. Another class of devices exploits electroplating within solid electrolytes. In these devices, positively charged metal ions traverse a thin insulating or semiconducting film to a bottom electrode. Rather than forming a uniform conductive layer, conductive dendrites grow toward a top electrode until complete pathways are formed, lowering the device resistance by orders of magnitude. This dendritic growth can be at least partially reversed, so that the device resistance can be reliably varied over orders of magnitude. Recently, Campbell et al. have developed a novel device shown schematically in Figure 1 [3]. Note that the silver is separated

Figure 1: Schematic diagram of layers for device of Campbell et al. Gray (outer) is tungsten. In present case, M-Se (blue) is SnSe. from the active Ge2Se3 by a layer of tin selenide. Silver is introduced into the Ge2 Se3 solely through the influence of an electric field impressed by the top and bottom electrodes. Devasia et al. have shown that during heating, tin can enter Ge2Se3 from a SnTe layer [4], and Campbell has

inferred from a variety of measurements that tin enters Ge2Se3 from the SnSe layer under positive bias. So, to understand the physics of the device in Figure 1, we need to understand the physics of both silver and tin in Ge2Se3 , including the interaction between the two. To this end, we report a set of electronic structure calculations, based on density functional theory (DFT), on a crystalline model of Ge2Se3 with one or more Ag and/or Sn atoms. We incorporate recent results showing that electrons self-trap in this model system [5]. This is appropriate because in other GexSe(100-x) compounds, negatively charged defects have been inv