Resistive switching in HfO2-based metal-insulator-metal structures for non-volatile memory

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Non-volatile memory (NVM) is of high interest for several applications within Si-based CMOS technology such as high-value and mobile computers, consumer electronics, sensors and medical health care. Among existing NVM technologies resistive random access memory (RRAM) where the resistance of a metal-insulator-metal (MIM) structure is switched by applying a voltage, is a promising candidate. From the large variety of switchable materials we have chosen HfO2-based devices since this material is already integrated into CMOS technology. One important issue is the controlled induction of conducting filaments by applying a forming voltage. Within the current project we have shown that for strongly oxygen deficient HfO2-x almost electroforming free devices can be realized. Conducting defect states in the vicinity of the Fermi edge induced by these oxygen vacancies reduce the forming voltage considerably. The applicants have newly developed an in operando measurement technique by which microstructural and chemical changes as well as the influence of impurities (such as carbon) during switching can be investigated by hard X-ray photoelectron spectroscopy (HAXPES) at the modern synchrotron source PETRA III. Within this follow-up project we want to understand the influence of isovalent and aliovalent substitutional elements as Zr4+ and La3+/Y3+. First experiments indicate that trivalent doping also reduces the forming voltage, and that more stable devices with higher on/off ratios can be obtained. By investigating the device area dependence of the forming voltage and by using in operando micro(nano)beam X-ray absorption finestructure (µ-XAS), we intend to achieve a clear understanding of the conducting filaments by characterizing the complete MIM-cell. Using HAXPES and µ-XAS the fatigue mechanisms of substituted devices will be studied in operando, e.g. in order to stabilize pulse-induced switching by material scientific approaches. Supported by an external collaboration in the field of materials modeling we want to understand the defect physics and chemistry of switchable (Hf,X)O2-x (X = Zr, La, Y, Nb) and the involved interfaces to the electrode materials and correlate them with synthesis parameters and switching behavior. In summary, our proposal is expected to make a fundamental contribution to the materials science of an optimized device structure for HfO2-based RRAM and to the understanding of its fatigue behavior.