Control Of Phase Transition And Dielectric Properties Of HfO₂-Based Nanofilms

In recent years,as one of the most representative high-dielectric constant materials,HfO2 based thin films are used in various electronic devices.Research shows that the dielectric properties of HfO2 based films are mainly dependent on phase structure.In order to control the electric properties of films,the phase stabilization mechanism of HfO2 should be investigated.At present,the research of ascertaining the doping content for the phase transition is mainly relying on experiments,which will lengthen product development cycle time and increase cost.Furthermore,typical preparation methods for HfO2 based films are vacuum deposition.The vacuum methods have some drawbacks,such as high fabrication cost and technical difficulty for large-scale production.The use of solution processes for the fabrication of the HfO2 based thin films should be investigated.According to these ideas,a series of designed HfO2 based thin films with various compositions,thickness and nanolaminate structures were grown using a solution processing method with aqueous precursors.The structure and surface morphology,along with the accompanying dielectric performance and band structure,were systematically investigated.The phase transition mechanism of the HfO2 based thin films was discussed in depth.Main conclusions are listed as follows:(1)TiN thin films with high densities,low surface roughness and resistivity were prepared by direct current reactive magnetron sputtering on Si substrates.The thickness of the films could be controlled from a few to hundreds of nanometers.The influence of thickness on microstructure and properties was investigated in detail.Results indicated that with increasing thickness,surface roughness at first decreased,then increased.The resistivity at first decreased,then increased with increasing thickness.The lowest resistivity was 5.4×10-7?·m.Considering the excellent film properties,these films would be suitable when they were used as electrode materials for electrical measurement of the HfO2 based thin flims.(2)Y doped HfO2 based films were prepared by chemical solution deposition with aqueous solution precursors.The films showed high densities,low surface roughness,high dielectric constant,low leakage current densities and high breakdown fields.The effect of Y content and thickness on the phase structure of HfO2 based thin films was studied by XRD measurements.Results indicated that the increase of Y content and decrease of film thickness helped to stabilize cubic HfO2.We introduced surface energy to correct the classical thermodynamic conditions of phase transition,and established a thickness-composition phase diagram of Y doped HfO2 based thin films at room temperature.Theoretical results agreed well with the experimental results obtained not only in our work,but also in previous work reported by Muller et al.and Niinisto et al,confirming that this theory was correct.We also investigated the valence band offset,energy gap and conduction band offset between Y doped HfO2 thin films and Si substrates by XPS measurements.The valence band offset,energy gap and conduction band offset changed linearly with Y content.(3)Hf1-xZrxO2 films were prepared by chemical solution deposition with aqueous solution precursors.The films showed high densities,low surface roughness,high dielectric constant,low leakage current densities and high breakdown fields.The effect of Zr content and thickness on the phase structure of Hf1-xZrO2 films was studied by XRD measurements.Results indicated that the increase of Zr content and decrease of film thickness helped to stabilize tetragonal phase.We introduced surface energy to correct the classical thermodynamic conditions of phase transition,and established a thickness-composition phase diagram of Hf1-xZrxO2 films at room temperature.Theoretical results agreed well with the experimental results obtained in our work,confirming that this theory was correct.We also investigated the valence band offset,energy gap and conduction band offset between Hf1-xZrxO2 thin films and Si substrates by XPS measurements.The valence band offset was almost constant,but energy gap and conduction band offset decreased linearly with increasing Zr content.(4)The 16 nm thick undoped t/c-HfO2 film was prepared through layer by layer crystallization process.Further increasing film thickness,the monoclinic phase occured.With incorporation of 1 or 4 mol%Y into these films,a single phase of t/c-structure was obtained.The films showed high densities,low surface roughness,high dielectric constant,low leakage current densities and high breakdown fields.(5)A series of HfO2/ZrO2/Si and ZrO2/HfO2/Si bilayer films were prepared by chemical solution deposition with aqueous solution precursors.The films showed high densities,low surface roughness,high dielectric constant,low leakage current densities and high breakdown fields.Results indicated that the underlying or capping ZrO2 layers helped to stabilize the t/c-HfO2 films.Based on the collaborative control of epitaxial-like growth and mechanical stress,the 50 nm thick undoped t/c-ZrO2-HfO2 nanolaminate film was prepared by chemical solution deposition with aqueous solution precursors.The nanolaminate film showed high density,low surface roughness,high dielectric constant,low leakage current density and high breakdown field.