Ab Initio Study of the Sub-threshold Electron Transport Properties of Ultra-scaled Amorphous Phase Change Material Germa
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Ab Initio Study of the Sub-threshold Electron Transport Properties of Ultra-scaled Amorphous Phase Change Material Germanium Telluride Jie Liu1, Xu Xu1, and M. P. Anantram1 1 Department of Electrical Engineering, University of Washington, Seattle, WA 98195, USA ABSTRACT The sub-threshold electron transport properties of amorphous (a-) germanium telluride (GeTe) phase change material (PCM) ultra-thin films are investigated by using ab initio molecular dynamics, density function theory, and Green’s function simulations. The simulation results reproduce the trends in measured electron transport properties, e.g. current-voltage curve, intra-bandgap donor-like and acceptor-like defect states, and p-type conductivity. The underlying physical mechanism of electron transport in ultra-scaled a-PCM is unraveled. We find that, though the current-voltage curve of the ultra-scaled a-PCM resembles that of the bulk a-PCM, their physical origins are different. Unlike the electron transport in bulk a-PCM, which is governed by the Poole-Frenkel effect, the electron transport in ultra-scaled a-PCM is largely dominated by tunneling transport via intra-bandgap donor-like and acceptor-like defect states. INTRODUCTION Phase change memory is one of the most promising candidates to offer next-generation ultra-dense nonvolatile data storage solutions. Driven by the promising application scenario, phase change memory technology has attracted intensive research efforts, in order to understand the device physics and to optimize the design [1-13]. It is well known that the amorphous chalcogenide phase change materials (PCM) have a lot of interesting and practically useful electron transport properties, e.g. the sub-threshold linear (exponential) current-voltage curve when the bias is low (large), the threshold switching, and the current-voltage curve snap-back. Interestingly, the stat-of-art PCM scaling research has found that these peculiar electron transport properties exist not only in the bulk PCM (tens of nm or larger) measurements, but also in the ultra-scaled PCM nanostructure (sub-10 nm) measurements [1]. However, though the electron transport of the bulk PCM has been well studied [2-3], the understanding of the electron transport of ultra-scaled PCM nanostructures is still missing. It is important to study the electron transport properties of the ultra-scaled PCM nanostructures, which are crucial to enable ultradense PCM device technologies. In this study [7,9], we investigate the sub-threshold electron transport properties of the prototypical PCM GeTe ultrathin film in the amorphous phase, by using the ab initio molecular dynamics (AIMD), density functional theory (DFT), and non-equilibrium Green’s function (NEGF) simulations. THEORY To investigate the sub-threshold electron transport properties of ultra-scaled amorphous GeTe, purely ab initio simulations are applied [6, 7, 9]. Firstly, the melt-quench AIMD
simulations are applied to generate the amorphous GeTe model. Then, the DFT-based conjugate gradient relaxation simulations are
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