The ever-increasing demands for high-performance computing and data storage have made the development of high-density, high-speed, and highly reliable memory technologies a critical challenge in contemporary industrial progresses. The innovative research and development of memory materials have become particularly crucial. HfO2-based thin films, as a key dielectric material, have not only been successfully applied in high-dielectric capacitors for conventional dynamic random-access memory (DRAM), but the discovery of their ferroelectric properties has also provided an ideal material choice for ferroelectric capacitors in emerging ferroelectric random-access memory (FeRAM). Particularly, they can be used to construct new electronic devices of ferroelectric field effect transistor (FeFET, utilizing a ferroelectric thin film as the gate dielectric) and ferroelectric tunnel junction (FTJ, using an ultrathin ferroelectric film as the tunneling barrier) for memristors as artificial synapses for neuromorphic computing.
This review systematically elucidates the fundamental phase structures of HfO2 materials in both bulk and thin-film forms, with a focused analysis on dielectric and ferroelectric performance manipulation strategies for HfO2-based thin films. In terms of dielectric characteristics, we highlight effective methods for achieving high-dielectric-constant (high-κ) morphotropic phase boundary (MPB) structures through phase regulation, along with an in-depth exploration of technical approaches to effectively reduce leakage current density. Regarding ferroelectric properties, this review summarizes optimization strategies for enhancing ferroelectric polarization, improving endurance characteristics, and reducing coercive field. Finally, we provide a systematic overview of the specific applications of HfO2-based dielectric and ferroelectric thin films in relevant information devices, including DRAM, FeRAM, FeFET and FTJ.
HfO2-based materials have been successfully commercialized applications for DRAM capacitors and field-effect transistor gate dielectrics due to their excellent CMOS compatibility, superior thermochemical stability, wide bandgap, and high dielectric constant. The recent discovery of their ferroelectric properties has further expanded the application prospects of HfO2-based thin films in information storage technologies.
1) Dielectric properties: HfO2-based thin films with MPB structures located at the phase boundary between orthorhombic (o) and tetragonal (t) phases, exhibit high dielectric constants under relatively low electric fields. Precise control of phase composition is crucial for realizing MPB structures, with approaches including: superlattice design, elemental doping, oxygen vacancy and carbon defect engineering, annealing process optimization, grain size control, and electrode material selection, and so on.
2) Ferroelectric properties: HfO2-based thin films demonstrate good ferroelectricity especially at a thickness below 10 nm. However, relatively poor endurance and high coercive field remain major bottlenecks for practical applications. Researchers have developed various improvement strategies. Introducing oxide interlayer, optimizing domain switching ratio, elemental doping, and adopting oxide electrode can significantly enhance endurance; While superlattice design, elemental doping, and new ferroelectric phase engineering can effectively reduce coercive fields.
3) Devices: HfO2-based high-κ dielectrics have been utilized in commercial DRAM capacitors. However, with continuously shrinking feature size, further increasing dielectric constant while maintaining low leakage current require deeper investigation. Successful 3D trench deposition of HfO2-based ferroelectric thin films has enabled high-density integration of FeRAMs with advantages including large polarization, fast switching, and good endurance. Novel device architectures like FeFETs and FTJs demonstrate great potential in neuromorphic computing through nonvolatile multi-state manipulation. However, challenges remain in FeFET retention characteristics and FTJ endurance. Future efforts should focus on improving thin-film quality and scaling up from single-cell to array-level applications, thereby fully realizing the potential of HfO2-based FeFETs and FTJs.
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