Comprehensive Development and Analysis of Sputter-Grown Superlattice Films

Abstract

Phase change materials (PCMs) hold great promise for next-generation neuromorphic hardware. Although standard Ge-Sb-Te (GST) alloys face challenges with high reset currents and resistance drift, limiting scalability and precision, chalcogenide superlattices (CSLs) offer a compelling solution. Particularly GST/Sb2Te3 combinations offer a solution by reducing reset currents through enhanced control over van der Waals (vdW) gaps and interfaces [1]. Unlike GeTe/Sb2Te3 CSLs, which suffer from intermixing, GST/Sb2Te3 maintains better interface control [2], essential for reset current reduction. Our research provides a systematic study of the optimal sputter growth conditions for highly oriented GST/Sb2Te3 CSLs on SiO2 and carbon substrates, offering an overview of the deposition window exploring various growth parameters. Substrates were pre-cleaned with acetone and isopropanol, followed by Ar+ inverse sputtering etching. The deposition involved a two-step process [3]: room temperature deposition of a Sb2Te3 seed layer, followed by annealing and high T deposition of the rest of the film. X-ray diffraction (XRD) and spectroscopic ellipsometry were used to analyze structural properties and energy bandgaps, respectively. High-temperature deposition of Sb2Te3 can cause Te desorption, impacting CSL quality. By fine-tuning deposition parameters, we mitigated these effects on Sb2Te3 and also grew ordered cubic GST films with different stacking. We developed a diagram of optimal growth conditions for both materials, facilitating the fabrication of CSLs with various periodicities. XRD confirmed satellite peaks, indicative of periodicity, consistent across different substrates. Resistivity measurements revealed strong anisotropy, with significant in-plane/cross-plane differences. The impact of the Sb2Te3 seed layer was explored, with high-temperature deposited seed introduced for stress mitigation at the crystallization interface. Ellipsometry showed that the energy bandgap remained consistent across different periodicities. Finally, TEM analysis provided insights into the local structure of low periodicity superlattices. REFERENCES 1. A. I. Khan, et al, Nano Lett. 2022, 22, 6285−6291 2. S. Cecchi, et al. APL Mater. 5, 026107 (2017) 3. Y. Saito, et al. AIP advances 6, 045220 (2016)