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1251-H06-02

Stress Limited Scaling in Ge2 Sb2 Te5 Robert E. Simpson1 , Milos Krbal1 , Paul J. Fons1&2 , Alexander V. Kolobov1&2 , Tomoya Uruga2 , Hajime Tanida2 and Junji Tominaga1 1 Nanodevice Innovation Research Centre, Advanced Industrial Science and Technology 1-1-1 Higashi, Tsukuba, Japan 2 SPring-8, JASRI, Mikazuki Hyogo 679-5198, Japan ABSTRACT The influence of stress on the phase change behaviour of Ge2 Sb2 Te5 encapsulated by ZnSSiO2 and TiN is investigated using temperature dependent Extended X-ray Asbsorption Fines Structure and Ellipsometry to determine the crystallisation temperature. The encapsulation material surrounding the Ge2 Sb2 Te5 has an increasingly dominant effect on the material’s ability to change phase and can cause a profound increase in its crystallization temperature. We have experimentally shown that the increased crystallization temperature originates from compressive stress exerted from the encapsulation material. By minimizing the stress we have maintained the bulk crystallization temperature in Ge2 Sb2 Te5 films just 2 nm thick. INTRODUCTION Phase Change RAM (PCRAM) is a memory technology which, unlike the current silicon based technologies, does not suffer from problems associated with the storage of charge. Data is stored in the form of structural differences in a thin film of the material. Ge2 Sb2 Te5 is the leading candidate material for such technology[1] and changes in its rate of crystallization have been observed for films thinner than 30 nm however the true limit to which Ge2 Sb2 Te5 can be scaled yet still retain the ability to crystallise needs to be proven. Scaling Ge2 Sb2 Te5 to smaller volumes has the added virtue that the switching power, accomplished by Joule heating, linearly improves with reducing cell size [2]; clearly this is beneficial with respect to the recent increase in portable devices which require large solid state memories. Ge2 Sb2 Te5 can exist in two crystalline phases, the metastable cubic phase, and the equilibrium hexagonal phase; in addition it can also exist in an amorphous phase. The cubic and hexagonal phases are formed by increasing the temperature of the as-deposited amorphous material to approximately 150 ◦ C and 300 ◦ C respectively. Substantial optical and electrical differences manifest as a result of the atomic scale structural differences between the amorphous and cubic crystalline phases; generally, the crystalline phase exhibits a higher refractive index, optical absorption and electrical conductivity in comparison to the amorphous phase. Clearly, changing phase between the amorphous and the metastable cubic crystalline state causes a large optical and electrical contrast which can be utilized for data storage applications[3]. Raoux et al. studied in situ x-ray diffraction from ultra-thin films of Ge2 Sb2 Te5 , GeSb and SbTe as a function of temperature[4, 5]. It was found that for Ge2 Sb2 Te5 films capped

with Al2 O3 , the crystallization temperature sharply increased with decreasing film thickness. But since long range order is ne