Preparation And Properties Of La:HfO₂ Thin Films

As people’s demand for data storage increases,more and more storage devices emerge at the historic moment.Because of its special hysteresis loop,ferroelectric materials are widely used in memory devices such as ferroelectric tunnel junction and memristor.Perovskite-type ferroelectric materials are the most common ferroelectric materials.However,the conventional ferroelectric perovskite thin films,which were applied to Si-based ferroelectric memories,have been certified to have a lot of difficulties,such as ferroelectric size effect,small band gap,small coercive field,and interface mismatch with silicon,etc.These problems greatly limit the application of ferroelectric memory devices.The binary oxide HfO₂,Zr O₂ are always widely used in the field effect transistor due to its high dielectric constant,small band gap as well as good compatibility with CMOS process.The discovery of ferroelectricity in doped hafnium oxide has attracted extensive attention.Doped HfO₂ thin films have attracted considerable attention due to promising applications in CMOS-compatible ferroelectric memories.However,important physical mechanisms such as the source of ferroelectric hafnium oxide and fatigue mechanism have not been recognized.In this thesis,the main work and conclusions are as follows:1.The preparation process of La:HfO₂ thin films was explored,and the causes of ferroelectricity generation were analyzed in combination with structure characterizations and performance measurements.Finally,La:HfO₂ thin films with smooth surface were prepared with the maximum switchable polarization of 32.3μC/cm².2.By varying epitaxial strain and film thickness,the effect of stress on La:HfO₂ thin films was investigated.The formation of metastable o-phase may be ascribed to the epitaxial strains and thus improve the ferroelectric properties of La:HfO₂ films.3.The conduction mechanism of La:HfO₂ thin films with thickness was investigated.The leakage current characteristics of La:HfO₂ films of different thicknesses were measurements in the temperature range of 30 K-350 K and fitted with different models,and the conduction mechanism of the 4 nm-thick La:HfO₂ thin film was finally fitted by direct tunneling,and Poole–Frenkel emission became the dominant conduction mechanism with the gradual increase of thickness.4.The storage characteristics and fatigue mechanism of hafnium oxide were studied.The capacitor exhibits an excellent retention extrapolated to 10 years.Fatigue measurements are carried out using designed pulse sequences,in which the voltage,pulse width,and interval time are changed to observe the evolution of switchable polarization with increasing cycles.Further analysis of the fatigue behaviors with the nucleation-limited-switching model shows that the meaning time and activation field for polarization switching are increased in fatigued La:HfO₂ capacitors.These results support the domain wall pinning as a dominant effect accounting for the fatigue in the epitaxial La:HfO₂ thin films.