Crystallization Characteristics of Ge-Sb Phase Change Materials
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1160-H14-07
Crystallization Characteristics of Ge-Sb Phase Change Materials Simone Raoux1, Cyril Cabral2, Jr., Lia Krusin-Elbaum2, Jean L. Jordan-Sweet2, Martin Salinga3, Anita Madan4 and Teresa L. Pinto4 1
IBM/Macronix PCRAM Joint Project, Almaden Research Center, San Jose, California
95120, USA 2
IBM T. J. Watson Research Center, Yorktown Heights, New York 10598, USA
3
1. Physikalisches Institut (1A), RWTH University of Technology, 52056 Aachen,
Germany 4
IBM Hudson Valley Research Park, Hopewell Junction, New York 12533, USA
ABSTRACT The crystallization behavior of Ge-Sb phase change materials with variable Ge:Sb ratio X between 0.079 and 4.3 was studied using time-resolved x-ray diffraction, differential scanning calorimetry, x-ray reflectivity, optical reflectivity and resistivity vs. temperature measurements. It was found that the crystallization temperature increases with Ge content from about 130 ºC for X = 0.079 to about 450 ºC for X = 4.3. For low X, Sb x-ray diffraction peaks occurred during a heating ramp at lower temperature than Ge diffraction peaks. For X = 1.44 and higher, Sb and Ge peaks occurred at the same temperature. Mass density change upon crystallization and optical and electrical contrast between the phases show a maximum for the eutectic alloy with X = 0.17. The large change in materials properties with composition allows tailoring of the crystallization properties for specific application requirements. INTRODUCTION Re-writable optical data storage based on phase change materials is a mature technology and much effort was spent in the design and optimization of the materials currently in use [1]. In the early stages of Phase Change Random Access Memory (PCRAM) technology the same materials that are used in optical storage (most prominently Ge2Sb2Te5) were applied. However, PCRAM materials require a different set of materials properties compared to optical storage. In particular, the thermal stability and data retention are problematic for Ge2Sb2Te5-based PCRAM devices. Failure times of PCRAM cells of 10 years at 85 ºC have been reported [2], but these times become much shorter at higher temperature and are just a few days at 150 ºC which is the operation temperature for automotive applications, and even just a few seconds at temperatures above 200 ºC. It has been reported recently that PCRAM devices with Ge-Sb phase change materials (with X = 0.17, the eutectic alloy) show very promising switching behavior [3]. The re-crystallization temperature of RESET devices was measured as a function of RESET conditions (the RESET operation is the melt-quench operation to switch the
device to high resistance). It was observed that the crystallization temperature can be as high as 220 ºC (depending on the RESET conditions) which is very promising in terms of data retention. In addition, very fast SET operations with pulses as short as 4 ns were reported (the SET operation is the re-crystallization to switch the device to low resistance). Here we have studied the crystallization properties of
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