Abstract
Enrichment of GeSbTe alloys with germanium has been proposed as a valid approach to increase the crystallization temperature and therefore to address high-temperature applications of non-volatile phase change memories, such as embedded or automotive applications. However, the tendency of Ge-rich GeSbTe alloys to decompose with the segregation of pure Ge still calls for investigations on the basic mechanisms leading to element diffusion and compositional variations. With the purpose of identifying some possible routes to limit the Ge segregation, in this study, we investigate Ge-rich Sb2Te3 and Ge-rich Ge2Sb2Te5 with low (<40 at %) or high (>40 at %) amounts of Ge. The formation of the crystalline phases has been followed as a function of annealing temperature by X-ray diffraction. The temperature dependence of electrical properties has been evaluated by in situ resistance measurements upon annealing up to 300 °C. The segregation and decomposition processes have been studied by scanning transmission electron microscopy (STEM) and discussed on the basis of density functional theory calculations. Among the studied compositions, Ge-rich Ge2Sb2Te5 is found to be less prone to decompose with Ge segregation.
Highlights
Materials and MethodsAmorphous chalcogenide Ge-rich GST films were deposited either on Si(111)-( 3 × 3). R30◦ -Sb passivated surfaces or on SiO2 substrates
Paul-Drude-Institut für Festkörperelektronik, Leibniz-Institut im Forschungsverbund Berlin e.V., Department of Materials Science, University of Milano-Bicocca, via R
A GIF Quantum ER system (Gatan AMETEK, Pleasanton, CA, USA) was used for energy loss spectroscopy (EELS) measurements. Both lowand core-loss EELS spectra were acquired with the Dual EELS tool through Gatan DigitalMicrograph software Version 3.4 (Gatan AMETEK, Pleasanton, CA, USA), in spectrum imaging (SI) mode
Summary
Amorphous chalcogenide Ge-rich GST films were deposited either on Si(111)-( 3 × 3). R30◦ -Sb passivated surfaces or on SiO2 substrates. For Ge-rich GST225 samples, the temperature of the Ge effusion cell was correspondingly set to achieve the two compositions, while all the other growth parameters were unchanged. The electrical properties have been investigated by in situ sheet resistance measurements of the samples grown on SiO2 with a four-point probe, during annealing in air using a Temptronic ThermoChuck system (inTEST Thermal Solutions GmbH, Müllrose, Germany). The temperature dependence of the resistance has been evaluated in the range from 30 to 300 ◦ C by employing a Agilent HP 4156B parameter analyser (Agilent Technologies, Inc., Santa Clara, CA, USA). A GIF Quantum ER system (Gatan AMETEK, Pleasanton, CA, USA) was used for EELS measurements Both lowand core-loss EELS spectra were acquired with the Dual EELS tool through Gatan DigitalMicrograph software Version 3.4 (Gatan AMETEK, Pleasanton, CA, USA), in spectrum imaging (SI) mode. H-Ge-ST and H-Ge-GST alloys, by using the DFT data on the formation free energy of cubic alloys in the central part of the ternary Ge-Sb-Te phase diagram reported in our previous work, [15] which we refer to for all details
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