Aman Baunthiyal
University of Bremen, Germany
Growth and electrical characterization of RF-sputtered Ga₂O₃ on Ru(0001) for non-volatile memory technology
Resistive switching (RS) in metal oxides has emerged as a promising phenomenon for achieving nextgeneration non-volatile memory devices, owing to their low power consumption, high scalability, and fast switching times. Gallium oxide (Ga₂O₃), extensively explored for power electronics applications due to its ultra-wide bandgap, high breakdown field, and tunable electrical conductivity through oxygen stoichiometry, is also increasingly studied for RS-based memory devices. While highly crystalline Ga₂O₃ is desirable for high-power applications, defect-rich polycrystalline films offer significant advantages for RS functionalities by facilitating the formation and modulation of conductive pathways. Although comprehensive efforts have been dedicated to growing Ga₂O₃ on insulating substrates like AlN and Al₂O₃, the growth of crystalline Ga₂O₃ on metal substrates remains largely uninvestigated.
In this study, Ru(0001)/α-Al₂O₃(0001) templates served as substrates, selected for their significant lattice mismatch with the preferred (-201) orientation of β-Ga₂O₃, thereby enabling a systematic investigation of mismatch-driven polycrystallinity and its impact on RS behavior. The influence of Ru surface morphology, Ga₂O₃ deposition temperature (ranging from room temperature to 600 °C), and post-deposition annealing (up to 900 °C) on the structural and microstructural evolution of Ga₂O₃ films was examined. Comprehensive characterization using Raman spectroscopy, AFM, XRD, LEED, and STEM was carried out. Raman spectroscopy confirmed the formation of β-Ga₂O₃ above 200°C and revealed an improvement in short-range order with increasing deposition temperature, while XRD analysis indicated the emergence of broad (-402) peaks starting at 200°C, with no notable enhancement in crystallinity at higher temperatures. After annealing, films deposited at room temperature on rougher Ru templates transitioned from an amorphous structure to nanocrystalline grains of approximately 10 nm along the [-201] direction. In contrast, films deposited at 600°C showed no further structural improvement upon annealing, as indicated by XRD. Additionally, STEM displayed randomly oriented crystallites of approximately 40 nm, suggesting their pre-existing nature. These post-annealed samples revealed the growth of predominantly polycrystalline β-Ga₂O₃ with some γ-phase Ga₂O₃. Initial nucleation studies confirmed that Ru step edges act as preferential sites for Ga₂O₃ island formation, accompanied by a (3×3) surface reconstruction, indicative of strained epitaxial β-Ga₂O₃ growth. Electrical characterization of Al/Ga₂O₃/Ru devices revealed robust bipolar resistive switching with ON/OFF ratios exceeding 10³ and stable operation over 70 switching cycles. The RS is attributed to the formation and dissolution of oxygen vacancy-mediated conductive filaments, governed by deposition parameters and film thickness.