Correlating the structure and the defects of the solution-processed WOx films with RRAM devices' performance for PUF applications

Rapid advancements in information technology mandate the need for unpredictable yet reliable security encryption tools. Hardware-based solutions, such as physically unclonable functions (PUFs), have been identified as viable candidates for exploiting inherent device-level abnormalities. Resistive random-access memory (RRAM) devices exhibit high entropy and can be ideal for PUF implementation. Beyond intrinsic variability, the process conditions and material structure have been recognized as critical determinants of RRAM performance and suitability for PUFs, yet these factors have never been systematically assessed. Therefore, here we report a solution-processed WOx that has been annealed at 300 °C, 350 °C, 400 °C, and 450 °C, resulting in dissimilar structures (crystallinity, morphology, surface roughness) and defect densities, leading to different device performance metrics, including HRS/LRS ratio, endurance, and retention. All samples, except the 450 °C-annealed one, show good endurance (103 cycles) and retention (103 seconds). In terms of variability, amorphous samples have shown the least temporal and spatial variation compared to crystalline samples. However, the crystalline films, particularly those annealed at 400 °C, have shown a higher HRS/LRS ratio, because larger grains provide a stable path for filament formation; moreover, these devices also show excellent retention stability when extrapolated for 10 years. Furthermore, analysis of PUF metrics for these 400 °C annealed samples also reveal superior values compared to others, in terms of uniformity (50%), bit-error rate (0%), diffuseness (47.62%), and uniqueness (48%), all close to ideal. While the inferior performance of 450 °C annealed samples can be attributed to pervasive porosity, which has promoted leakage, unstable filaments, and accelerated drift, thereby degrading endurance and retention, the absence or scarcity of grain boundaries in amorphous and semicrystalline films has reduced defect-assisted pathways, lower variability and high HRS/LRS contrast. It is believed that the optimized RRAM devices fabricated for PUF applications in this study are highly suitable for cost-effective hardware security.