Charge Transport in Nanoglasses of Phase-Change Memory
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Charge Transport in Nanoglasses of Phase-Change Memory M. Simon1, M. Nardone1, S. A. Kostylev 1, I. V. Karpov2, V. G. Karpov1 1 Department of Physics and Astronomy, University of Toledo, Toledo, OH 43606, U.S.A. 2 Intel Corporation, SC9-09, 3601 Juliette Ln., Santa Clara, CA 95054-1579, U.S.A. ABSTRACT We discuss possible mechanisms for Poole-Frenkel type of non-ohmic conduction in chalcogenide glasses in the range of room temperatures. Overall, we list 8 such mechanisms, only one of which (Schottky emission) can be ruled out as inconsistent with the observations. Seven others can give more or less satisfactory fits of the observed non-linear IV curves. Our analysis calls upon indicative facts that would enable one to discriminate between the various alternative models. INTRODUCTION The operation of phase change memory (PCM) depends on charge transport in its constituent inclusions of chalcogenide glasses. It has long been established that low field room temperature conduction in bulk chalcogenide glasses is dominated by band transport.[1] The commonly observed nonlinear current-voltage (IV) characteristics (above ~ 103 - 104 V/cm) are often referred to as the Poole-Frenkel (PF) effect after the classical work [2,3] suggesting their plausible interpretation. An experimental signature of PF conduction is a region of linearity in the plot of ln(I / I0) vs. either √V or V where I0 is the pre-exponential factor. The underlying mechanism is commonly related to field induced increase in free carrier concentration [4-7]. In spite of the general agreement about the observed PF type of non-ohmicity and the fact that I0 ~ exp(Ea / kT) where Ea is approximately half of the mobility gap, particular features and especially their interpretations vary dramatically between researchers. Furthermore, some data [8,9] point at two different domains in ln I vs. V exhibiting different proportionality coefficients and temperature dependencies. Here, we present a survey of mechanisms of dc conduction in chalcogenide glasses including thin films down to nanoscale region. CONDUCTION MECHANISMS Poole-Frenkel effect . The originally suggested physics of the PF effect is the decrease in the ionization energy of a single coulombic potential well in the direction of an applied field or a pair of coulombic centers. The corresponding barrier change ∆ increases the center ionization rate, proportional to which is the free carrier concentration and the activation electric current I / I0 ~ exp (∆ / kT). For the case of two centers separated by distance 2a in the electric field of strength F, the electron energy along the axis is given by
q2 q2 − − Fqx , (1) ε (a − x) ε (a + x) where q is the electron charge and ε is the dielectric permittivity. The barrier maximum is found from dU/dx = 0. The original PF result is ∆=√(4q3F/ε), valid for fields F> q/4εa2 ~ 104 V/cm for U ( x) = −
the typically assumed center concentration of ~ 1018 cm-3. In the opposite limiting case of 'weak' fields, F Ft ~ 105 V/cm. Schottky emission. The Schottky eff
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