Structure, performance regulation and typical device applications of HfO₂-based ferroelectric films

The rapid developments of big data, the internet of things, and artificial intelligence have put forward more and more requirements for memory chips, logic chips and other electronic components. This study introduces the ferroelectric origin of HfO₂-based ferroelectric film and explains how element doping, defects, stresses, surfaces and interfaces, regulate and enhance the ferroelectric polarization of the film. It is widely accepted that the ferroelectricity of HfO₂-based ferroelectric film originates from the metastable tetragonal phase. The ferroelectricity of the HfO₂-based film can be enhanced by doping some elements such as Zr, Si, Al, Gd, La, and Ta, thereby affecting the crystal structure symmetry. The introduction of an appropriate number of oxygen vacancy defects can reduce the potential barrier of phase transition between the tetragonal phase and the monoclinic phase, making the monoclinic phase easy to transition to tetragonal ferroelectric phase. The stability of the ferroelectric phase can be improved by some methods, including forming the stress between the substrate and electrode, reducing the film thickness, constructing a nanolayered structure, and reducing the annealing temperature. Compared with perovskite oxide ferroelectric thin films, HfO₂-based films have the advantages of good complementary-metal-oxide-semiconductor compatibility and strong ferroelectricity at nanometer thickness, so they are expected to be used in ferroelectric memory. The HfO₂-based 1T1C memory has the advantages of fast reading and writing speed, more than reading and writing 10¹² times, and high storage density, and it is the fast reading and writing speed that the only commercial ferroelectric memory possesses at present. The 1T ferroelectric field effect transistor memory has the advantages of non-destructive reading and high storage density. Theoretically, these memories can achieve the same storage density as flash memory, more than reading 10¹⁰ times, the fast reading/writing speed, low operating voltage, and low power consumption, simultaneously. Besides, ferroelectric negative capacitance transistor can obtain a subthreshold swing lower than 60 mV/dec, which greatly reduces the power consumption of integrated circuits and provides an excellent solution for further reducing the size of transistors. Ferroelectric tunnel junction has the advantages of small size and easy integration since the tunneling current can be largely adjusted through ferroelectric polarization switching. In addition, the HfO₂-based field effect transistors can be used to simulate biological synapses for applications in neural morphology calculations. Moreover, the HfO₂-based films also have broad application prospects in antiferroelectric energy storage, capacitor dielectric energy storage, memristor, piezoelectric, and pyroelectric devices, etc. Finally, the current challenges and future opportunities of the HfO₂-based thin films and devices are analyzed.