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Phase-change materials, such as Ge2Sb2Te5 (GST), have recently drawn much attention as promising materials for nano-scale non-volatile memories (PRAM, Phase-change Random Access Memory). The atomic structure of GST changes by annealing at 180 °C from amorphous to crystalline (fcc with 20% vacancy) phase. However, chemical states related to the structural phase-changes are not yet fully understood, because the research of GST has been mainly focused on the device applications. Also, since native oxide forms on the GST surface when the surface exposes to air, it has been difficult to measure the original chemical states of GST by VUV techniques. In this research, we present a Ne+ ion sputtering method with which the native oxide of amorphous GST is removed without breaking stoichiometry. We in-situ annealed the amorphous GST in UHV, preventing incorporation of impurities (oxygen or carbon), in order to phase-change to crystalline GST with the phase of face-centered cubic (fcc). Chemical states of the Te 4d, Sb 4d, and Ge 3d core-levels of amorphous and crystalline GST were measured by high-resolution x-ray photoelectron spectroscopy (HRXPS) with the beam energy of 250 eV at the 8A1(U7) Spectro-microscopy beamline at Pohang Light Source (PLS). Chemical state of the Te 4d didn¡¯t change between the phase-changes. However, the peak width and position of the Sb 4d and Ge 3d core-levels changed. We attribute the change of chemical states to the structural phase-change from amorphous to crystalline. |