Thickness-dependent Crystallization Behavior of Phase Change Materials
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1072-G05-02
Thickness-dependent Crystallization Behavior of Phase Change Materials Simone Raoux1, Jean L. Jordan-Sweet2, and Andrew J. Kellock3 1 IBM/Qimonda/Macronix PCRAM Joint Project, IBM Almaden Research Center, 650 Harry Road, San Jose, CA, 95120 2 IBM T. J. Watson Research Center, P. O. Box 218, Yorktown Heights, NY, 10598 3 IBM Almaden Research Center, 650 Harry Road, San Jose, CA, 95120 ABSTRACT We have investigated the crystallization behavior of phase change materials as a function of their thickness. Thin films of variable thickness between 1 and 50nm of the phase change materials Ge2Sb2Te5 (GST), N-doped GST (N-GST), Ge15Sb85 (GeSb), Sb2Te, and Ag and In doped Sb2Te (AIST) were deposited by magnetron sputtering, and capped in situ by a 10nm thick Al2O3 film to prevent oxidation. The crystallization behavior of the films was studied using time-resolved X-ray diffraction. For each material we observed a constant crystallization temperature Tx that was comparable to bulk values for films thicker than 10 nm, and an increased Tx when the film thickness was reduced below 10 nm. The thinnest films that showed XRD peaks were 2 nm for GST and N-GST, 1.5 nm for Sb2Te and AgIn-Sb2Te, and 1.3 nm for GeSb. The observed increase in the phase transition temperature with reduced film thickness and the fact that very thin films still show clear phase change properties are indications that Phase Change Random Access Memory technology can be scaled down to several future technology nodes.
INTRODUCTION The scaling of Phase Change Random Access Memory (PCRAM) devices to smaller and smaller dimensions calls for the investigation of the scaling properties of the phase change materials themselves. Many material properties change when the material is present in a thin film form compared to bulk properties, e. g. electrical resistivity or thermal conductivity of thin films can differ substantially from those of their bulk counterparts. For phase change materials used in optical storage it was found that the film thickness can have a substantial effect on the crystallization speed [1, 2]. Complete erasure times CET (defined as the minimum duration of the erasure pulse for complete crystallization of a written amorphous mark) were measured on optical disks consisting of phase change materials sandwiched between ZnS-SiO2 [1, 2] or carbide and nitride interfacial layers [1]. The CETs which are a measure for the crystallization speed and data rate for optical disks showed a substantial and complex dependency on the phase change material and interfaces. Zhou [1] found an increase in CET for the nucleation dominated phase change material Ge2Sb2Te5 (GST), a decrease for the growth-dominated
Ag-In-Sb-Te and a reduction in CET by introducing crystallization-enhancing carbide or nitride layers. Martens et al. [2] observed a minimum in CET at a thickness of 9 nm for a doped SbTe material. For PCRAM applications, the crystallization time is also one of the most important parameters determining the data rate, and it is likely that it will
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