Novel Materials And Their Switching Mechanisms Of Resistive Random Access Memory

Resistive switching random access memory (RRAM) is one of the promisingcandidates for the next generation nonvolatile memory application, due to its merits ofhigh-speed, high-density and nonvolatility. Exploring appropriate storage materials fordifferent kinds of RRAM, demonstrating the definite mechanism for resistiveswitching and developing the high performance RRAM devices are the hot topics bothin the scientific community and semiconductor market. In this dissertation, AlN, Ta2O5and ZnO thin film were deposited by magnetron sputtering and two kinds of RRAMdevices (anion-migration type and cation-migration type) were fabricated with thesematerials. The influences of storage medium thickness, device size, top electrodematerials, current compliance and ambient temperature on the memory performancewere systematically studied. By combining the microstructure and chemical valencestate analysis of the storage medium and the interfaces with the resistive switchingcharacteristics, this dissertation tries to demonstrate the microscopic mechanisms indifferent types of resistive switching systems and clarify the influence of processingparameters on the resistive switching characteristics, for the sake of developing highperformance RRAM devices for next generation nonvolatile memory application.The results show that the AlN film is one of the excellent storage medium forRRAM. Memory characteristics including steep resistive switching, large memorywindow of103and long retention time of>106s were confirmed in the (Ag, Cu)/AlN/Pt structure. The temperature dependent resistance on low resistance state andhigh resistance state indicates that the resistive switching effect is mediated by theformation and rupture of Ag/Cu conducting filament in AlN film by means of redoxreaction. Unipolar RRAM device with a structure of Pt/AlN:Cu/Pt was fabricated usingCu-embedded AlN film in which Cu impurity is dispersed into the AlN matrix. TheAlN based unipolar memory device shows high-speed response to the electric stimuli,which can be programmed and erased under100-ns pulses in unipolar operation mode.Moreover, AlN-based complementary resistive switching memory was obtained byanti-serially connecting two bipolar memory elements. Thus, we have demonstratedthe AlN-based bipolar, unipolar and complementary resistive memory, whichintroduces the nitride as a kind of outstanding materials to the field of RRAMapplications. In the Ta2O5-based bipolar resistive switching memory, the migration of oxygenions/vacancies plays an important role to the resistive switching. By comparing thechemical valence state of Ta between high and low resistance states at W/Ta2O5andTa2O5/Pt interfaces by X-ray photoelectron spectroscopy, the switching mechanism inW/Ta2O5/Pt memory device is confirmed to be dominated by the oxygen ions driftunder electric field in the Ta2O5film. The interaction of various metals with oxygenatoms at the electrode/Ta2O5interface under electric stress and the effect of topelectrode on the characteristic variations of Ta2O5-based memory device wereinvestigated. It is indicated that the metals (Al, Ti and W) whose chemical activity isclose to Ta are the optimal top electrodes for Ta2O5-based memory device whichexhibit the minimum variations of threshold voltages and resistance. In addition,quantized conductance was observed in the Ta2O5-based memory after electrodeoptimization, demonstrating that conductance quantization is an intrinsic feature of thenanoscale filaments in RRAM devices regardless of the type of the filaments.For Al/ZnO/Si memory device, bipolar resistive switching behaviors withself-rectifying effects in the low resistance state were observed. It is proposed that theresistive switching originates from the formation and dissolution of the AlOxbarrierlayer which are induced by the migration of oxygen ions under electric field. TheAl/ZnO/Si device show memory window of103at0.5V and rectifying ratio of102at±0.5V in low resistance state.