Memory Characteristics Of γ-Fe₂O₃Nano-particle Doped HfO_x Films

Rapidly developed modern information technologies require new generation of non-volatile memory devices with much higher charge storage density, smaller device size and larger reading/writing speed. Since1960s, resistive switching random access memory devises, based on resistive switching characteristics of oxide film with metal-insulator-metal structure, have been highlighted as a candidate due to their large reading/writing speed, low power consumption, simple structure, and high integrated compatibility etc. Recently, it was found that both HfOx films and γ-Fe2O3nano-particle films exhibit resistive switching characteristics, however, the ratio of high and low resistance for the former and the durability of memory properties for the latter need to be further improved.In this thesis work,100nm HfOx/γ-Fe2O3nano-particles/50nm HfOx sandwich structures have been fabricated on Pt-covered p-Si (100) substrates by radio frequency magnetron sputtering combined with spin-coating methods. Scanning electron microscopy, x-ray diffraction and x-ray photoelectron spectra investigations reveal that the oxygen-deficient HfOx film layers exhibit a monoclinic polycrystalline structure, and the self-organized γ-Fe2O3nano-particles have an average diameter of34.3nm. Current-voltage (I-V) curves reveal that:after doped with γ-Fe2O3nano-particles, HfOx films exhibit significant bipolar resistive switching characteristics:the ratio of high and low resistance at the reading voltage of-0.8V is about18.7. This on/off ratio can be well maintained after100switching cycles, suggesting a good durability of memory characteristics. The logarithmic plots and power-law fittings for current-voltage curves demonstrate that a trap-controlled space-charge limited current conduction process is responsible for the current transport in the high resistance state, while an ohmic conduction process is dominated in the low resistance state. The drift of oxygen ions at the interface between Ag top electrodes and the HfOx buffer layer results in the formation and annihilation of a conductive filament, which is responsible for the resistive switching. Capacitance-voltage (C-V) characteristics illustrate that γ-Fe2O3nano-particle doped HfOx films exhibit a clockwise C-V hysteresis loop. The width of memory window is related to the electrode area and gate voltage. When scanning the gate voltage back and forth from4to-4volt, the sample with the electrode diameter of0.1mm obtains a memory window of0.39V, corresponding to the charge storage density of1.18X1010/cm2. Magnetic moment measurements indicate that:after doped nano-particles, diamagnetic HfOx films become ferromagnetic film with anisotropic magnetic moment, but no ferromagnetic hysteresis loop is observed.Due to their excellent resistive switching properties, significant charge storage characteristics and notable nano-magnetic properties of γ-Fe2O3nano-particle doped HfOx films should have potential application in multifunctional magnetic/electronic memory devices.