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In-situ Variable Temperature Structural Inspection and Phase Transition Mechanism of Ge Family Materials

Author: LiMing
Tutor: ZuoYiMing
Course: Materials Processing Engineering
Keywords: Germanium XRD in situ variable temperature structure detection Rietveldstructure refinement first-principle
Type: Master's thesis
Year: 2013
Downloads: 1
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Since chalcogenide phase change materials were reported for semiconductor memory,researches on phase change materials have accelerated the phase-change memorytechnology and rapid development in1980s. The key of developing phase change memorytechnology is to research better performance of phase change material.Among themGermanium compounds are very important item of phase change materials.This work focuses on crystal structure change with in situ variable temperature XRDdetection technology for Germanium phase change materials. That is to say there will bedifferent crystal structures at different temperatures. Meanwhile Rietveld whole spectrumfitting method was used to refine crystal structure,and plane wave pseudopotential methodwas combined with under the framework of density functional theory to research crystalstructure and phase change process of germanium family phase change materials atdifferent temperatures.Highscore is a kind of analysis software for X-ray diffraction which leads aconvenience way to understand crystal structure, full spectrum analysis of XRD data andRietveld structure refinement can be applied to research crystal structures of sample atdifferent temperatures.Through the results of the intensive experimental data, theconclusion could been obtained: as temperature rising, GeTe structure occurred a changefrom amorphous into hexagonal structure at300℃, with an expanded volume; anotherchange from hexagonal structure into face-centered cubic structure occurred at400℃with shrunk volume, the changes of a and b directions were not so obviously than the cdirection. Ge1Bi2Te4structure changed at135℃from amorphous into hexagonalstructure. As temperature rising, its Full Width Half Maximum(FWHM) narrowedgradually, its crystallinity became better, its cell volume became smaller, but temperaturebecame more higher, its cell volume changed less. When the temperature rose to160℃,peaks of Ge2Sb2Te5crystal began to appear, its unit cell volume first became smaller for the substrate hold, as the temperature increased, then became larger,at higher temperature,its cell volume changed less. The analysis showed that the doping can reduce the phasetransition temperature point of GeTe phase change materials. Using first-principle, theelectron density distribution、energy band structure and density of states of GeTe werecalculated,, its band gap was0.264eV at300℃,up to0.372eV at400℃, whentemperature return to room temperature, its band gap was0.266eV, band gap changed withtemperature, indicating that temperature affected the conductive properties of phase changematerials.

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