Investigation of Phase Transition in Stacked Ge-Chalcogenide/SnTe Phase-change Memory Films
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Investigation of Phase Transition in Stacked Ge-Chalcogenide/SnTe Phase-change Memory Films Feiming Bai1, Surendra Gupta2, Archana Devasia1, Santosh Kurinec1, Morgan Davis3, and Kris A Campbell3 1 Department of Microelectronic Engineering, Rochester Institute of Technology, Rochester, NY, 14623 2 Department of Mechanical Engineering, Rochester Institute of Technology, Rochester, NY, 14623 3 Department of Electrical and Computer Engineering, Boise State University, Boise, ID, 83725
ABSTRACT Phase transitions in stacked GeTe/SnTe and Ge2Se3/SnTe thin layers for potential phasechange memory applications have been investigated by X-ray diffraction using a twodimensional area detector system. The as-deposited underlying GeTe or Ge2Se3 layer is amorphous, whereas the top SnTe layer is crystalline. In the GeTe/SnTe stack, the crystallization of GeTe phase occurs near 170oC, and upon further heating, the GeTe phase disappears, followed by the formation of rocksalt-structured GexSn1-xTe solid solution. In the Ge2Se3/SnTe stack, the phase transition starts with the separation of a SnSe phase due to the migration of Sn ions into the Ge2Se3 layer. SnSe is believed to facilitate the crystallization of Ge2Se3-SnTe solid solution at ~360oC, which is much lower than the crystallization temperature of Ge2Se3, therefore consuming less power during the phase transition.
I. INTRODUCTION Phase-change random access memory, also called phase-change memory (PCM), is a promising non-volatile electronic memory technology due to its low-voltage operation, high cycling endurance, scaling potential, and excellent radiation resistance.[1,2] Ideal materials for PCM include those that exhibit a large, reversible change in resistance between an amorphous and a crystalline phase. An example phase-change material is Ge2Sb2Te5 (GST) which has been widely studied, and commercially developed, for use in optically induced phase-change memory, and exhibits a fast crystallization speed (2%) observed in Fig.1. Therefore, composition of SnTe layer must be changing during annealing above 200oC. To investigate the composition of 600 SnTe during annealing, a specimen was measured stress 500 heated at the rate of 30°C/min under thermal stress 400 flowing N2, and a 3 minute diffraction 300 frame was captured at 30°C intervals. Fig. 200 3(a) shows the level view of SnTe (200) 100 peak as a function of temperature. A broad 0 d200 transition can be observed, initiating from ~170 oC until 400 oC. The d200 versus -100 temperature is plotted in Fig.3(b) to better -200 show the details of phase transitions. Phase 0 100 200 300 400 500 o transitions can be described by two stages: Annealing temperature ( C) (i) the first stage occurs from 170 oC to 210 o C, accompanyin g a gradual decrease in Fig.2. The dependence of residual stress in d200; and (ii) the second stage occurs from GeTe/SnTe stacks on annealing temperatures. 210 oC to 400 oC, where a first sharp decrease then increase in d200 is observed. According to the results of after-annealed sample in
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