Growth and electrical characterization of RF Sputtered Ga2O3 on Ru(0001) for non-volatile memory technology

The performance of modern computing is fundamentally limited by bottlenecks within its memory hierarchy, such as the von Neumann bottleneck between CPU and main memory and the memory bottleneck between fast volatile memory and slower non-volatile storage. To overcome these limitations, resistive random access memory (ReRAM) has emerged as a promising candidate among various emerging non-volatile memories, targeting both bottlenecks, due to its simple metal–insulator–metal structure, fast switching, high scalability, low-power operation, and CMOS compatibility. It is based on the resistive switching (RS) phenomena, where the device’s resistance is electrically switched between high and low resistance states.

Within this field, Ga2O3 has shown promise as an active layer in ReRAM devices, owing to its wide bandgap and defect-sensitive conductivity. However, to date, the vast majority of research has focused on Ga2O3 growth on wide-bandgap insulating substrates such as Al2O3, which restricts device architecture to lateral configuration only. In contrast, the growth of crystalline Ga2O3 on metallic substrates remains largely unexplored. The present thesis addresses this gap by investigating the growth of Ga2O3 films on Ru(0001)/Al2O3(0001) templates prepared by radio frequency magnetron sputtering.

First, the structural evolution of the Ga2O3 films was systematically studied as a function of deposition temperature and the surface morphology of the underlying Ru film. A key finding is the strong dependence of the film’s crystallinity and phase on these parameters. Films deposited at RT on rough Ru films exhibit an amorphous nature that crystallizes upon annealing into nanocrystalline β-Ga2O3 with some residual of the γ-phase. In contrast, films deposited at higher temperature show pre-crystallization of Ga2O3 in which annealing primarily drives the γ → β phase transformation. Films grown on smoother Ru templates, however, show negligible changes upon annealing, highlighting the crucial role of surface defects in the crystallization process. These findings collectively indicate a two-step crystallization pathway along with the structural change, from amorphous to the metastable γ and then to the stable β phase. Furthermore, initial nucleation studies further reveal that Ga2O3 islands preferentially form at Ru step edges, resulting in a newly found strained (3 × 3) surface reconstruction on (2 × 2) Ru (0001).

Building on these insights, Al/Ga2O3/Ru devices were fabricated using the rougher Ru templates. The optimized devices exhibit excellent performance metrics (ON/OFF > 104, retention > 104 s). The RS behavior of these devices is governed by the oxygen vacancies-based filamentary switching. However, device reliability was found to depend strongly on the thickness of the Ga2O3 layer. Devices with thinner films suffered from premature failure due to filament overgrowth, which was successfully addressed by increasing the Ga2O3 film thickness, resulting in stable device operation.

Overall, this thesis bridges the material science behind the growth of Ga2O3 on Ru(0001) with its applications in ReRAM technology, establishing a framework for engineering reliable Ga2O3-based RS devices.