Ab Initio Calculations of Crystalline and Amorphous In 2 Se 3 Compounds for Chalcogenide Phase Change Memory
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Ab Initio Calculations of Crystalline and Amorphous In2Se3 Compounds for Chalcogenide Phase Change Memory Renyu Chen and Scott T. Dunham Department of Electrical Engineering, University of Washington, Seattle, WA 98195-2500, U.S.A. ABSTRACT Ab initio calculations of various configurations of In2Se3 compounds are used to gain insight into the transition from crystalline to amorphous phase. The structures considered are based on wurzite structures with 1/3 of indium sites vacant as observed experimentally. From extensive calculations for possible vacancy configurations in In2Se3 compounds, predictions based on the local coordination of In/Se atoms are made for the energetically favorable vacancy ordering structures. Results indicate that in the most stable In vacancy configurations, Se atoms have coordination of either 2 or 3 (In atoms have coordination of 4). Other coordinations lead to significantly higher formation energies. Results from analyzing the total energy and electronic structure of a range of off-stoichiometry, including vacancies, interstitials and anti-site, configurations, suggest that the energetically most favorable way to form In-rich material is via incorporation of Se vacancies, while Se occupying a vacant site is the most favorable for formation of Se-rich phase. Based on these calculations, predictions are made on how stoichiometry deviations impact structural evolution during phase change. INTRODUCTION Phase-change memory (PCM) is one of the most promising non-volatile memories for future electronic devices. Unlike the most commonly used non-volatile memory, flash memory, which works by modulating electric charge within the gate of a MOS transistor, PCM uses the reversible phase change behavior of chalcogenides. Compared to its traditional counterpart, PCM has several advantages such as faster switching speed, slower rate of degradation, and better scalability. However several issues related to PCM have to be addressed, such as its high programming current density, integration difficulties, and long-term resistance and threshold voltage shift [1]. Therefore, extensive investigations have been carried out to overcome these challenges and improve device performance [2]. The basic switching mechanism of PCM involves transition from crystalline to amorphous state under a high voltage and short duration pulse, and from amorphous to crystalline state under a low voltage and long duration pulse. During the former process, the chalcogenide glass is heated above its melting temperature and then quenched; while during the latter process, it is slowly heated below the melting temperature to allow crystal growth. The resistivity of the amorphous state is orders of magnitude higher than that of crystalline state, facilitating data storage through a resistance change in different phases of the material. The currently most widely used phase-change material is Ge2Sb2Te5 (GST). However, the In based chalcogenides, such as In2Se3, have several advantages over GST. These include a wider
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