The Influence of Nitrogen Doping on the Chemical and Local Bonding Environment of Amorphous and Crystalline Ge 2 Sb 2 Te
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The Influence of Nitrogen Doping on the Chemical and Local Bonding Environment of Amorphous and Crystalline Ge2Sb2Te5 J. S. Washington1, E. Joseph2,1, M. A. Paesler1, G. Lucovsky1, J. L. Jordan-Sweet2, S. Raoux3,2, C. F. Chen2, A. Pyzyna2, R. K. Dasaka2, A. Schrott2, C. Lam2, B. Ravel4 and J. Woicik4 1
Physics Department, North Carolina State University, Campus Box 8202, Raleigh, NC 27695 IBM/Macronix PCRAM Joint Project, T. J. Watson Research Center, Yorktown Heights, NY 10598 3 IBM Almaden Research Center, San Jose, California 95120 4 National Institute of Standards and Technology, Gaithersburg, MD 2
ABSTRACT Recent interest in phase change materials (PCMs) for non-volatile memory applications has been fueled by the promise of scalability beyond the limit of conventional DRAM and NAND flash memory [1]. However, for such solid state device applications, Ge2Sb2Te5 (GST), GeSb, and other chalcogenide PCMs require doping. Doping favorably modifies crystallization speed, crystallization temperature, and thermal stability but the chemical role of the dopant is not yet fully understood. In this work, X-ray Absorption Fine Spectroscopy (XAFS) is used to examine the chemical and structural role of nitrogen doping (N-) in as-deposited and crystalline GST thin films. The study focuses on the chemical and local bonding environment around each of the elements in the sample, in pre and post-anneal states, and at various doping concentrations. We conclude that the nitrogen dopant forms stable Ge-N bonds as deposited, which is distinct from GST bonds, and remain at the grain boundary of the crystallites such that the annealed film is comprised of crystallites with a dopant rich grain boundary. INTRODUCTION As interest in non-volatile memory cells based on phase change materials has increased, research has turned to the optimization of device performance through the control of material properties. Doping allows for the tuning of the properties of Ge2Sb2Te5 (GST), GeSb, and other currently widely accepted PCMs. Nitrogen doping in these materials improves thermal stability [2] and affects crystallite size and transition temperature [3]. Furthermore, finer control of crystallization temperature, Tx, through doping leads to better management of set and reset currents. Tx must be sufficiently above device operation temperature for thermal stability but well below the melting point [4]. The present study employs XAFS to clarify the structural role of nitrogen in N-GST in amorphous and annealed thin films. EXPERIMENT One micron thick films of nitrogen doped GST were deposited at room temperature on bare Si wafers and/or glass substrates via RF sputtering from a Ge2Sb2Te5 compound target in a mixture of argon and nitrogen feedgas chemistries. For crystallization, samples were annealed in a helium ambient using a tube furnace; nitrogen containing samples (2.5, 5.0, and 6.1 atomic percent (at %)) were annealed for 30 minutes at 300 ºC while the undoped (0.0 at %) sample was
annealed at 200 ºC for 10 minutes. XAFS measuremen
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