Wen-Yuan Chang

Single-ZnO-Nanowire Memory

Single-ZnO-nanowire (NW) memory based on resistive switching is demonstrated for the first time. The NW memory is stable, rewritable, and nonvolatile with on/off ratio up to 7.7 × 105. The O vacancies at the surfaces of ZnO NWs and around the interface of Ti/ZnO NWs observed using X-ray phototelectron spectroscopy, transmission electron microscopy (TEM), selected-area electron diffraction, and high-resolution TEM might play a role in the resistive switching behavior. The endurance of resistive switching can be enhanced by further increasing the sweeping voltage. This paper brings an exciting possibility of building next-generation memory devices based on NWs.

Fully Transparent Resistive Memory Employing Graphene Electrodes for Eliminating Undesired Surface Effects

A ZnO-based transparent resistance random access memory (TRRAM) employs atomic layered graphene exhibiting not only excellent transparency (less than 2% absorptance by graphene) but also reversible resistive switching characteristics. The statistical analysis including cycle-to-cycle and cell-to-cell tests for almost 100 cells shows that graphene plays a significant role to suppress the surface effect, giving rise to the notable increase in the switching yield and the insensitivity to the environmental atmosphere. The resistance variation of high-resistance state of ZnO is greatly suppressed by covering graphene as well. The device reliability investigation, such as the endurance more than 102 cycles and the retention time longer than 104 s, reveals the robust passivation of graphene for TRRAM applications. The obtained insights show guidelines not only for TRRAM device design and optimization against the undesired switching parameter variations but also for developing practically useful applications of graphene.

Eliminating surface effects via employing nitrogen doping to significantly improve the stability and reliability of ZnO resistive memory

Metal oxides suffering from oxygen molecule chemisorption display environment-dependent metastability, leading to unstable resistive memory characteristics and performance degradation. To obtain ambient-independent characteristics, we introduced nitrogen into ZnO resistive memory devices, compensating for the native defects and suppressing oxygen chemisorption, giving rise to a significant improvement in switching behavior without undesired surface effects. Moreover, by thermal activation of the nitrogen doping via annealing, an increased yield ratio from 50% to 82%, a reduced current compliance from 15 mA to 5 mA, and more stable cycling endurance are obtained. Our findings give physical insight into designing resistive memory devices.

Effect of ultraviolet illumination on metal oxide resistive memory

We investigate the photoelectrical and resistive switching properties of Pt/ZnO/Pt capacitor operated in unipolar mode under ultraviolet (UV) illumination. The oxygen photodesorption under UV illumination explains the photoconduction observed in initial and high resistance states. Meanwhile, oxygen readsorption at surface-related defects justifies the different photoresponses dynamics in both states. Finally, UV illumination significantly reduces the variations of resistance in high resistance state, set voltage and reset voltage by 58%, 33%, and 25%, respectively, stabilizing Pt/ZnO/Pt capacitor. Our findings in improved switching uniformity via UV light give physical insight into designing resistive memory devices.