Revealing Oxygen Donor Level in Tungsten Oxide Films for Neuromorphic Applications with Parallel Dipole-Line Hall System

Electrochemical random-access memory (ECRAM) devices stand as key candidates for realizing analog cross-point array-based AI computation accelerators thanks to its excellent programmability driven by ion movement, high stability, low cycle-to-cycle and device-to-device variation<i>(1)</i>. However, there have been limited efforts to investigate fundamental physical parameters of the key channel material, WO_3-x, that control ECRAM switching characteristics. Tungsten oxide serves as a channel layer since its conductivity can be modulated depending on the ion concentration, rendering it a fitting choice to realize the analog switching<i>(2)</i>. In this work, we fabricate ECRAM devices in a multi-terminal Hall-bar structure and conduct Parallel Dipole Line (PDL) Hall measurements to investigate the essential electrical properties of tungsten oxide films, including resistivity, mobility, carrier density and activation energy. The AC Magnetic PDL Hall system, based on strong magnetic field generation and lock-in detection, offers high sensitivity, allowing for sensing even weak Hall signals from low mobility and high resistivity samples<i>(3)</i>. Furthermore, we measure the mobility and carrier density of tungsten oxide as a function of temperature using variable temperature PDL Hall measurements. The observed Hall mobility in WO_3-x films reaches to 4.66 cm²/Vs at room temperature. At lower temperatures, a slight decrease in Hall mobility is observed due to impurity scattering from ionized centers. We extract the activation energy of the oxygen donor level in tungsten oxide thin films using Arrhenius plots. Our findings show the experimental access of the key variables that change during switching in ECRAM, which is not only crucial for enhancing ECRAM's performance but also essential for providing vital insights into neuromorphic applications.<br/><br/>1. P. Chen, F. Liu, P. Lin, P. Li, Y. Xiao, B. Zhang, G. Pan, Open-loop analog programmable electrochemical memory array. <i>Nature Communications</i> <b>14</b>, (2023).<br/>2. K. Miyake, H. Kaneko, Y. Teramoto, Electrical and optical properties of reactively sputtered tungsten oxide films. <i>Journal of Applied Physics</i> <b>53</b>, 1511-1515 (1982).<br/>3. O. Gunawan, S. R. Pae, D. M. Bishop, Y. Virgus, J. H. Noh, N. J. Jeon, Y. S. Lee, X. Shao, T. Todorov, D. B. Mitzi, B. Shin, Carrier-resolved photo-Hall effect. <i>Nature</i> <b>575</b>, 151-155 (2019).