(A) transmission electron microscopy study on the crystallization of $Ge_2Sb_2Te_5$ thin films deposited by sputtering method

Abstract

A non-volatile memory device using $Ge_2Sb_2Te_5$ thin films are one of the most promising candidates for high-speed mass storage in computer systems. Especially, in the viewpoint of the phase-change random access memory, the research on $Ge_2Sb_2Te_5$, phase-change materials, is very important. Up to date, microscopic analyses for identifying the electrical characteristics of $Ge_2Sb_2Te_5$ have been mainly carried out. Even though there were many researches on the microstructure of $Ge_2Sb_2Te_5$, a high-resolution transmission electron microscopy study on an atomic level was little investigated. In other words, the fundamental studies of the crystal structure and atomic arrangement of the $Ge_2Sb_2Te_5$ on an atomic level leave much to be desired. Therefore, atomic arrangement and phase transition mechanism of the $Ge_2Sb_2Te_5$ should clearly be understood in order to overcome the technical barrier of non-volatile memory device. In this thesis, a transmission electron microscopy study was performed to investigate the fundamental theory on the atomic arrangement and phase transition mechanism. The $Ge_2Sb_2Te_5$ thin films deposited by a sputtering method on a %SiO_2/Si$ substrate were annealed and subjected to transmission electron microscopy in order to investigate the crystallization and growth of the metastable $Ge_2Sb_2Te_5$. The metastable $Ge_2Sb_2Te_5$was initially crystallized with 10-nm-sized grains and its sheet resistance was still as high as in the amorphous state. However, resistivity value was abruptly decreased at the grain growth stage after the crystallization. On the basis of the results of the microstructural and electrical characteristics from the annealing time and temperature variation, it can be stated that the electrical properties of the $Ge_2Sb_2Te_5$ film are directly related to the grain growth and thus, the grain size. The crystal structure and atomic arrangement of the metastable $Ge_2Sb_2Te_5$ were studied by a high-resol...