Post-metal annealing temperature-dependent forming-free resistive switching memory characteristics, Fowler-Nordheim (F-N) tunneling at low resistance state, and after reset using a new W/WO3/WOx/W structure have been investigated for the first time. Transmission electron microscope image shows a polycrystalline WO3/WOx layer in a device with a size of 150 × 150 nm2. The composition of WO3/WOx is confirmed by X-ray photo-electron spectroscopy. Non-linear bipolar resistive switching characteristics have been simulated using space-charge limited current (SCLC) conduction at low voltage, F-N tunneling at higher voltage regions, and hopping conduction during reset, which is well fitted with experimental current-voltage characteristics. The barrier height at the WOx/W interface for the devices annealed at 500 °C is lower than those of the as-deposited and annealed at 400 °C (0.63 vs. 1.03 eV). An oxygen-vacant conducting filament with a diameter of ~34 nm is formed/ruptured into the WO3/WOx bilayer owing to oxygen ion migration under external bias as well as barrier height changes for high-resistance to low-resistance states. In addition, the switching mechanism including the easy method has been explored through the current-voltage simulation. The devices annealed at 500 °C have a lower operation voltage, lower barrier height, and higher non-linearity factor, which are beneficial for selector-less crossbar memory arrays.

Negative voltage modulated multi-level resistive switching with quantum conductance during staircase-type RESET and its transport characteristics in Cr/BaTiOx/TiN structure have been investigated for the first time. The as-deposited amorphous BaTiOx film has been confirmed by high-resolution transmission electron microscopy. X-ray photo-electron spectroscopy shows different oxidation states of Ba in the switching material, which is responsible for tunable more than 10 resistance states by varying negative stop voltage owing to slow decay value of RESET slope (217.39 mV/decade). Quantum conductance phenomenon has been observed in staircase RESET cycle of the memory devices. By inspecting the oxidation states of Ba+ and Ba2+ through measuring H2O2 with a low concentration of 1 nM in electrolyte/BaTiOx/SiO2/p-Si structure, the switching mechanism of each HRS level as well as the multi-level phenomenon has been explained by gradual dissolution of oxygen vacancy filament. Along with negative stop voltage modulated multi-level, current compliance dependent multi-level has also been demonstrated and resistance ratio up to 2000 has been achieved even for a thin (<5 nm) switching material. By considering oxidation-reduction of the conducting filaments, the current-voltage switching curve has been simulated as well. Hence, multi-level resistive switching of Cr/BaTiOx/TiN structure implies the promising applications in high dense, multistate non-volatile memories in near future.

We report a switching model that directly explains the change in activation energy (E AC ) at different RESET stop voltages (V stop ) in HfO 2 -based resistive random access memory devices. The dependence of oxygen vacancy-driven conductive filaments (V o 2+ ) density (n D ) on V stop was validated by a kinetic Monte Carlo (kMC) simulation and hopping conduction mechanism. A wide operating range of temperatures from −40 to 175 °C is achieved with stable endurance of 100 ns short pulses and high retention of more than 10 years at 125 °C. Distinct exponentially increased multilevel high-resistance states are observed at increasing V stop and is attributed to the increase in E AC with V stop . The increase in E AC due to the increase in V stop and depletion of n D during RESET was explained using our proposed switching model. A kMC simulation further emphasizes this relation due to the depletion of V o 2+ during RESET, which was supported by the increase in trap-to-trap distance in the hopping conduction analysis.

The resistive switching characteristics of a scalable IrO/AlO/W cross-point structure and its mechanism for pH/HO sensing along with glucose detection have been investigated for the first time. Porous IrO and Ir/Ir oxidation states are observed via high-resolution transmission electron microscope, field-emission scanning electron spectroscopy, and X-ray photo-electron spectroscopy. The 20 nm-thick IrO devices in sidewall contact show consecutive long dc cycles at a low current compliance (CC) of 10 μA, multi-level operation with CC varying from 10 μA to 100 μA, and long program/erase endurance of >10 cycles with 100 ns pulse width. IrO with a thickness of 2 nm in the IrO/AlO/SiO/p-Si structure has shown super-Nernstian pH sensitivity of 115 mV per pH, and detection of HO over the range of 1-100 nM is also achieved owing to the porous and reduction-oxidation (redox) characteristics of the IrO membrane, whereas a pure AlO/SiO membrane does not show HO sensing. A simulation based on Schottky, hopping, and Fowler-Nordheim tunneling conduction, and a redox reaction, is proposed. The experimental I-V curve matches very well with simulation. The resistive switching mechanism is owing to O ion migration, and the redox reaction of Ir/Ir at the IrO/AlO interface through HO sensing as well as Schottky barrier height modulation is responsible. Glucose at a low concentration of 10 pM is detected using a completely new process in the IrO/AlO/W cross-point structure. Therefore, this cross-point memory shows a method for low cost, scalable, memory with low current, multi-level operation, which will be useful for future highly dense three-dimensional (3D) memory and as a bio-sensor for the future diagnosis of human diseases.

Resistive switching characteristics and urea sensing have been investigated by using annealed GdO x film in IrO x /GdO x /W cross-point memory for the first time. The annealed GdO x film shows larger polycrystalline grains as compared to as-deposited films, which is observed by high-resolution transmission electron microscope (HRTEM) and X-ray diffraction patterns (XRD). Surface roughness of the GdO x films on W nano-dome is observed by atomic force microscope (AFM). The annealed IrO x /GdO x /W cross-point memory shows resistance ratio of 1000× times higher, multi-level operation with varying current compliance (CC) from 10-300 μA, good non-linearity factor of 8.3, good dc switching cycles of > 1000 at CC of 10 μA, long read endurance of >10 9 cycles with pulse width of 1 μs at higher read voltage of −0.5 V, and high speed operation of 100 ns. Repeatable resistive switching characteristics at low CC of 10 μA and mechanism are due to the electric field enhancement on the W nano-dome simulated by MATLAB, which controls the O 2− ions migration through polycrystalline GdO x grain boundary as well as Schottky barrier height modulation (0.59 vs. 0.39 eV). In addition, the annealed GdO x membrane in electrolyte-insulator-semiconductor (EIS) structure shows higher pH sensitivity than the as-deposited film (53.2 vs. 45.1 mV/pH) and lower drift (1.8 vs. 2.6 mV/hr) as well as lower detection of pH change (0.034). Detection of pH and urea sensing from 6 to 24 mg/dl have been measured by using cross-point memory, and the sensing mechanism is also discussed, which will be very useful for real healthcare unit in near future.