Substrate induced composition change during Ge₂Sb₂Te₅ atomic layer deposition and study of initial reactions on SiO₂ surface

Ge₂Sb₂Te₅ (GST) is a well-known phase change material used in nonvolatile memory devices, photonics, and nonvolatile displays. This study investigates the impact of precursor sequence during atomic layer deposition (ALD) of GST from GeCl₂.C₄H₈O₂, SbCl₃, and Te[(CH₃)₃Si]₂ on Si/SiO₂ substrates, focusing on growth per cycle, morphology, and composition along with initial precursor reactions on the substrate. We found that while thick layers approach stoichiometric Ge₂Sb₂Te₅, a Ge-rich interfacial layer is initially formed, regardless of the binary ALD sequence used to start the deposition (GeTe vs Sb₂Te₃). Starting the ALD process with the GeTe-Sb₂Te₃ sequence results in a higher Ge content near the GST/SiO₂ interface compared to the Sb₂Te₃-GeTe sequence. To understand these phenomena, we examine the initial precursor reactions on the SiO₂ substrate by total reflection x-ray fluorescence spectrometry. The analysis reveals that SbCl₃ exhibits lower reactivity with the SiO₂ substrate than GeCl₂.C₄H₈O₂, and not all Ge adspecies react with the Te[(CH₃)₃Si]₂ precursors. Additionally, the impact of the initial SiO₂ surface on deposition extends over several ALD cycles, as the SiO₂ surface only gradually gets covered by GST due to island growth. These processes contribute to the formation of a Ge-rich GST layer at the interface, irrespective of the precursor pulse sequence. These insights from the study may contribute to optimizing the initial deposition stages of GST and other ternary ALD processes to enable better composition control and enhanced device performance.