The unique property of fast and reversible switching between SET (crystalline, highly conductive) and RESET (amorphous, highly resistive) phases of phase change materials has led to its usage in non-volatile memory applications. The quest for new phase change materials with enhanced properties is of utmost importance for developing memory devices that meet the current demand for high speed, better data retention, and multi-bit storage capabilities. We report the systematic changes occurring in the optical bandgap ([Formula: see text]) and structural disorder ([Formula: see text]) in In3SbTe2 (IST) phase change material during the transition from amorphous to crystalline phases employing UV–Vis–NIR spectroscopy. [Formula: see text] in IST ranges from 0.998 (amorphous) to 0.449 eV (crystalline), revealing higher bandgap values compared to widely used Ge2Sb2Te5. An increment of 22.7% in the Tauc parameter ([Formula: see text]) slope, which governs the structural disorder, is also observed during the cubic transition in IST, revealing a more ordered nature of IST in the crystalline phase. Moreover, a rise in Urbach energy ([Formula: see text]) from 33.4 (amorphous) to 150.2 meV (crystalline) exhibits an increase in disorder at elevated temperatures owing to film defects. These findings are supported by the change in the atomic bonding upon crystallization, which is studied using X-ray Photoelectron Spectroscopy (XPS). Our XPS findings demonstrate that the amorphous phase of IST is composed of In2Te3, InSb, and InTe species with a peak area of ∼52.97%, ∼51.26%, and ∼39.83%, respectively. XPS spectra of annealed samples reveal the phases separation of IST alloy into crystalline InSb (∼60.89%) and InTe (∼64.69%) around 300 °C and then the formation of stable cubic In3SbTe2 (∼47.54%) at 400 °C. These experimental findings of the optical properties with structural changes would help distinguish the IST from the conventional phase change materials.

Temperature-dependent local structural details of In3SbTe2 (IST) and Ge2Sb2Te5 (GST) phase change materials are explored with the aid of Raman scattering and X-ray Photoelectron Spectroscopy (XPS) techniques. Significant temperature-induced changes occur in the local structure of the phase change material, facilitating the amorphous to crystalline phase transformation. These two phases exhibit a large resistivity contrast, which is utilized to store data in the Phase Change Memory (PCM) application. The Raman spectra recorded for IST material suggest that the as-deposited sample (amorphous phase) first crystallized around 300 ℃ annealing temperature with two binary phases InTe and InSb formations. InTe and InSb phases were obtained at ~85.5 cm-1 and ~180 cm-1 Raman shift, respectively, having vibration modes associated with B1g symmetry and TO phonons with Г15 symmetry. Further annealing at a higher temperature of 400℃, a ternary IST phase is obtained at ~163 cm-1 Raman shift indicating the transformation to a fully crystalline state. On the other hand, the amorphous GST material forms a metastable face-centered cubic (FCC) phase and stable hexagonal (HEX) phase upon crystallization. The Raman findings demonstrate the changes in vibration modes of Ge-Te and Sb-Te bonds during phase switching without any phase separation. Crystalline GST (HEX phase) comprises rising peaks of GeTe4 and GeTe4-nGen (n=1, 2) corner-sharing tetrahedra with A1 mode at ~104.5 cm-1 and ~137 cm-1 Raman shift, respectively, and a declining Sb2Te3 peak at ~175 cm-1, having A1g (2) vibration mode. Furthermore, the XPS analysis displays the changes in the bonding mechanism of the elements present in the phase change material during the amorphous to crystalline phase transition, firmly supporting the Raman observations.

The amorphous to crystalline phase transition in Ge2Sb2Te5 (GST) phase change material is investigated using XRD and the systematic variation in the optical band-gap (Eg ) and structural disorder (B 1/2) employing UV-Vis-NIR spectroscopy. The amorphous phase is observed to have Eg value of 0.708 eV while crystalline phase (200 °C) shows 0.442 eV. Variation in B 1/2 slope of 13.4 % is noted around the crystallization temperature (150 °C), depicting structural disorder reduction and hence structural ordering in the material forming the cubic phase. The change in the optical band-gap and local structural disorder upon crystallization occurs due to alternation in the atomic bonding configurations, which is explored using XPS technique. The findings reveal Ge-Te (~1218.35 eV binding energy) and Sb-Te (~528.8 eV) bonds in the amorphous phases. However, their bond lengths increase (hence binding energy reduces) as the annealing temperature rises, demonstrating phase switching occurs upon reaching the crystallization temperature. Insight into the optical band-gap, local structural disorder, and atomic arrangement governs many vital features of phase change material, such as fast crystallization speed, better thermal stability, high endurance, and large resistance contrast, which provide the path for non-volatile memory and nanophotonic applications.