Structural Optical and Thermal Studies of Rare Earth Sm Doped Ge2Sb2Te5 Phase Change Material

Abstract

The advancements in memory devices for achieving efficient computing performance is one of the recent areas in research and development. Phase change (PC) memories are among the leading next generation memory technology. The basic principle behind the working of PC memories is based upon the repeated reversible phase transformation of chalcogenide PC materials which are induced thermally. The unique features of Ge2Sb2Te5 (GST) such as higher crystallization temperature (~160⁰C), faster switching speed (~60ns) and better contrast in the amorphous and crystalline phases for electrical resistivity and optical reflectivity makes it a novel material for such purpose. The GST exhibits the amorphous, rocksalt cubic (fcc) and hexagonal crystal structure at different temperatures. The addition of impurity element to GST is an effective method for the improvement of switching performance by inducing the change in chemical bonding nature and local structural network which alters the optical, electrical and thermal characteristics. In the current work the outcome of Sm doping on the local structure, optical behavior, crystallization kinetics and electrical properties has been studied. The bulk alloys of (Ge2Sb2Te5)100-xSmx PC material have been prepared by means of the melt-quenching process. The thin film deposition for Sm added GST samples have been carried out by employing thermal evaporation. The crystal structure of powder material of (Ge2Sb2Te5)100-xSmx bulk alloys has been studied by employing the X-ray diffraction (XRD) analysis and rietveld refinement by Fullprof software for phase quantification thereafter. The local structures of these samples have been probed employing the Raman spectroscopy analysis using 785 nm laser excitation and the FTIR measurements in 30 cm-1 to 300 cm-1 range. The XRD analysis of (Ge2Sb2Te5)100-xSmx thin films show amorphous nature. The elemental analysis of the thin films has been done using the FESEM mapping and EDX investigations. The chemical bonding nature and compositional analysis of these films have been studied using XPS measurements employing Al K𝛼 X-ray source. UV-Vis-NIR spectroscopy has been utilized for investigation of optical behavior of (Ge2Sb2Te5)100-xSmx thin films. Non-isothermal crystallization kinetics of (Ge2Sb2Te5)100-xSmx thin films has been studied using the differential scanning calorimetry (DSC) at 10°C/min from 30-700⁰C in Ar environment. The current voltage characteristics of (Ge2Sb2Te5)100-xSmx thin films with temperature have been probed in coplanar geometry. The (Ge2Sb2Te5)100-xSmx thin films annealed in vacuum at 50°C, 160°C and 250°C have been investigated for structural analysis using XRD. The physical parameters viz. mean coordination number (Z), bond bending and bond stretching constraints, density, cohesive energy, energies of conduction as well as valence band edge and the glass transition temperature for (Ge2Sb2Te5)100-xSmx samples have been computed for calculating the glass forming ability and rigidity of structural networks. The degree of cross-linking with increase in Sm concentration has been theoretically examined. The structural modification efficiency for amorphous (Ge2Sb2Te5)100-xSmx system has been theoretically estimated. The stability of threshold switching voltage has been analyzed theoretically through the evaluation of metallicity and lone pair electrons which causes the bond switching and creation of valence alternation pairs and leads to the change in threshold switching voltage. The crystalline nature of (Ge2Sb2Te5)100-xSmx alloys formed after melt quenching has been analyzed from the XRD analysis. The rietveld refinement of the XRD data of these samples has been carried out for the phase quantification and lattice parameter evaluation which reveals the presence of mixed amorphous, fcc and hexagonal phases. The Raman analysis of the samples shows change in local structure of Sb on incorporation of Sm. The signature of Sm—Te, Ge—Ge, Sb—Sb, Ge—Te, and Sb—Te bond vibrations has been observed through the FTIR investigation. The XPS measurements of (Ge2Sb2Te5)100-xSmx thin films indicates creation of Sm—Ge, Sm—Sb and Sm—Te bonds. These also Sm occurrence together in oxidation states +2 as well as +3. The elemental fraction of constituent elements of (Ge2Sb2Te5)100-xSmx thin films has been observed to be in close agreement with the stoichiometric composition. The uniformity of as-deposited thin films and homogeneous elemental distribution along the thin film’s surface has been observed using the FESEM elemental mapping and imaging measurements.