Structure And Properties Of La-doped HfO₂ Epitaxial Films

With the development of microelectronics technology,the degree of integration of integrated circuits(IC)is further increased,and the characteristic size of transistors as their basic units is gradually reduced,resulting in a series of problems such as leakage current are brought.As a high-k material with good Si-based process compatibility,HfO₂has been widely used as a gate dielectric layer in IC below 45nm.Recently,the ferroelectricity of HfO₂-based material has been found.As a new ferroelectric material,it is not only compatible with the Si-based process,but also has robust ferroelectricity below 10nm thickness.These characteristics make HfO₂-based ferroelectrics to become one of the most potential candidate materials for the new generation of ferroelectric memory,and set off an upsurge of research on HfO₂-based ferroelectrics.Considering the unclear inducement of ferroelectric formation of HfO₂-based ferroelectrics,this paper studies the structure and properties of La-doped HfO₂epitaxial films.At the same time,analyzing the effects of defects,epitaxial stress,and ionic doping on the ferroelectric regulation of HfO₂films.The following results are obtained:(1)La:HfO₂thin films epitaxial growth:La:HfO₂(HLO)thin films were successfully epitaxially grown on SrTiO₃substrate with La_0.7Sr_0.3MnO₃-buffer layer via PLD method.The effects of growth temperature and film thickness on their structure and ferroelectricity were systematically studied.By comparing the ferroelectricity of HLO films grown at different temperatures,the optimum epitaxial growth temperature is 730?.It shows that higher or lower growth temperature will affect the crystalline quality of HLO film and bring non-ferroelectric phases(such as amorphous phase),which will reduce the proportion of ferroelectric phase and weaken the ferroelectricity.The effect of thickness on the crystal structure and ferroelectricity of HLO films shows that ferroelectricity deteriorate with the increase of thickness.Due to the strain relaxation,the surface of HLO film away from the substrate will exhibit poor ferroelectricity.Meanwhile,owing to the part away from the substrate can shied part of the electric field applied to HLO film,the weakening of the ferroelectricity of HLO film.(2)Phase evolution mechanism of HfO₂film caused by La doping concentration change:La-doped HfO₂films with different concentrations were successfully prepared via the PLD method.The effect of doping concentration on their phase evolution was systematically studied.It shows that when the doping concentration is 0-5mol%,the films exhibit ferroelectricity,and the optimal doping concentration is 3mol%.Meanwhile,the phase evolves from the monoclinic to the orthorhombic and subsequently to the cubic phase with increasing La concentration.Oxygen vacancies and chemical pressure play an important role in the process of phase evolution.The existence of oxygen vacancies and chemical pressure reduce the formation energy of orthorhombic/cubic phase and promote the transformation from monoclinic phase to orthorhombic/cubic phase.Besides,the ferroelectricity of pure HfO₂ film shows that the epitaxial stress can change its structure from monoclinic phase to orthorhombic phase.Therefore,it is considered that the phase evolution mechanism of La:HfO₂ thin film is caused by the combined action of epitaxial strain,oxygen vacancy,and chemical pressure.(3)Study on the structure and properties of 3mol%La-doped HfO₂film:Further study on the epitaxial mechanism of HLO films shows that the HLO layer is easier to grow on La(Sr)O terminated surface of the LSMO layer,which is due to the MnO₂^-0.7layer is substituted by the HLO layer on the MnO₂-terminated surface of LSMO.The results also show that oxygen vacancy is very important to the variable temperature ferroelectric and fatigue properties of HLO films.Additionally,the leakage current mechanism of Au/Cr/HLO/LSMO capacitor is the Schottky emission mechanism regardless of whether the top or bottom electrode is connected to the positive bias voltage.