Unraveling the optical bandgap and local structural change during phase transition in In3SbTe2 material through UV–Vis–NIR and XPS studies

Abstract

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 (Eg) and structural disorder (B12) in In3SbTe2 (IST) phase change material during the transition from amorphous to crystalline phases employing UV–Vis–NIR spectroscopy. Eg 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 (B12) 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 (EU) 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.