Abstract
In this thesis, we focus on HfO2-based resistive random access memory (RRAM) researches in improvement and application. All the RRAM devices using HfO2 as a main material are prepared by the atomic-layer-deposition (ALD) technique with remote-plasma system. The transition metal oxide, HfO2, is already widely used in semiconductor industries because of its superior physical properties, such as large permittivity, subsequent band gap, and excellent thermal stability. In addition to its use as high-k/metal gate stacks, HfO2-based RRAM has attracted significant attention for its potential in next-generation nonvolatile memory. HfO2-based RRAM devices are formed by an electric-field induced conductive filaments formation/rupture process, and possess superior bipolar resistive switching for future RRAM applications. However, the indefinite resistive switching mechanism and unstable resistive switching behaviors prevent RRAM from being put into practice. The localized filamentary conducting paths in the thin films are diverse in each switching, leading to the nonuniform distributions of switching voltages and resistance states, which result in irresolvable errors in the RRAM operations. Thus, how to effectively improve the stability of switching behavior is an essential issue for practical application of the RRAM. In Chapter 4 and Chapter 5, we use methods of doping and inserting metal layer to control the filaments formation and to stable the resistive switching behaviors of HfO2 RRAM devices. By the ALD doping of Al:HfO2 films, Hf-Al-O bonding formed in the Al:HfO2 films decreases the formation energy of oxygen vacancies and forms controllable conducting filaments along Al atoms. Inserting a Hf metal layer not only improves resistive switching characteristics but also makes polarity operation reversed. The interface HfOx generated in the Pt/Hf/HfO2/TiN devices makes redox fixed near the interface and the conduction mechanism switch to SCLC mechanism from Poole-Frenkel emission, leading to a polarity reversion. Chapter 6 proposes an oxide selector (Pt/NiO/HfO2/TiN) which was verified to exhibit similar, stable threshold switching characteristics by applying mutual bias. The mutual bias could balance the internal defects, which are produced by the interaction between conductive NiO and HfO2 thin films, to present such a similar threshold switching behavior. This volatile oxide selector with a similar threshold switching property is compatible with bipolar RRAM crossbar array applications.