The Local Structure and Kinetics of Ge-Sb-Te Phase Change Materials for use in Solid State Applications

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Title: The Local Structure and Kinetics of Ge-Sb-Te Phase Change Materials for use in Solid State Applications
Author: Washington, Joseph St. Paul
Advisors: Michael A. Paesler, Committee Chair
Nicholas C.M. Fuller, Committee Member
Eric A. Joseph, Committee Member
John E. Rowe, Committee Member
David E. Aspnes, Committee Member
Gerald Lucovsky, Committee Co-Chair
Abstract: Recent interest in phase change materials (PCMs) for non-volatile memory applications has been fueled by the promise of scalability beyond the limit of conventional DRAM and NAND flash memory. Typical PCMs such as Ge2Sb2Te5 (GST) require significant nitrogen doping to shift their crystallization temperature (Tx) sufficiently above standard CMOS device operation ranges (~ 80ºC ) but also well below the melting point for thermal stability. Reactive ion etching (RIE) in an Ar/Cl2/CHF3 plasma chemistry is another crucial step en-route to fabricating energy efficient, high density, nano-scaled PCM memory devices, yet it can lead to unfavorable, irreversible modification of the GST material. Chalcogens such as Te in GST can easily diffuse and interact unfavorably with the adjacent materials in the device structure thus negatively impacting the lifetime of a PCM device cell. In light of these implications for the final solid state device, it is necessary to understand and articulate the nature and structural implications of doping/alloying GST, the local structural changes that occur post etch processing, and the nature of the Ge-Sb binary and the Ge-Sb-Te ternary alloys. Fourier Transform Infrared Spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), X-ray absorption fine spectroscopy (XAFS), and in-situ, time resolved, X-ray diffraction (XRD) is used to understand the local structure of nitrogen in GST, and results point to the formation of preferential Ge-N bonding in a chemically ordered germanium nitride local bonding environment in as-deposited and annealed films. XAFS of various GeSb and Ge-Sb-Te glasses in the binary and ternary systems, in conjunction with time resolved XRD, show that Te in thin films of GeSb with gradually increasing Te atomic concentration prevents phase segregation, promotes stability, and induces nucleation. A multi-edge refinement of as-deposited thin films of Ge2Sb2Tex (x=4, 5, 6, 7), shows that Ge-Sb bonds are present in Ge2Sb2Te4 and Ge2Sb2Te5 from EXAFS fits, and these Ge-Sb bonds can also be isolated in the Ge near edge spectra in light of inelastic losses, i.e. shake-up / shake-off effects. The chemical and structural effects of RIE on the crystallization of N-GST is examined via XPS, XAFS, time resolved laser reflectivity and XRD. Time resolved laser reflectivity and XRD show that exposure to various etch and ash chemistries significantly reduces the crystallization speed, while the transition temperature from the rocksalt to the hexagonal phase is increased. XPS and XAFS attribute this to the selective removal and oxidization of N, Ge, Sb, and Te, thus altering the local bonding environment to the detriment of device performance.
Date: 2010-04-12
Degree: PhD
Discipline: Physics

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