Research On Electric Properties Of Hf_xZr_（1-x）O₂ Ferroelectric Thin Films

Ferroelectric thin films have been widely used in information technology fields. 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, the degradation caused by heat treatment during the crystallization, small band gap, and mismatch with Si interface. The binary oxide HfO2, ZrO2 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. Experimental results demonstrate that under certain conditions, HfO2 film mixed with Zr exhibits ferroelectricity. Compared with the conventional ferroelectric thin film, the HZO which has polycrystalline structure reduces the requirements to substrate. Besides, the mature technology combined with silicon guarantee the reliability and stability of the device. Meanwhile, smaller thickness restriction makes the miniaturized ferroelectric devices possible.In this thesis, the main work and conclusions are as follows:1. The HZO thin films were prepared by pulsed laser deposition(PLD) method, the reason of the ferroelectric nature of the deposited samples was confirmed. Combined with structural characterization and electric performance tests, it was showed that HZO ferroelectric thin film had a polycrystalline structure. A structure investigation revealed the orthorhombic phase to be of space group Pbc21, whose noncentrosymmetric nature was deem responsible for the spontaneous polarization.2. By changing the ZrO2 content, adjusting the ratio of hafnium and zirconium, we had explored the effect of the process parameters(oxygen partial pressure and growth temperature) on the structure and properties of HZO films. On the basis of the optimized HZO film growth process, the impact of Zr mixing content on HZO film was studied. Test results showed a tetragonal to orthorhombic to monoclinic phase displacive transformation in HfO2-ZrO2 film with reducing of Zr content. The remanent polarization presented a trend of first increase and then decrease. It was found that the structural changes correspond to the properties changes. The highest remanent polarization is actually achieved for samples with 60 mol% ZrO2 content(Hf0.4Zr0.6O2).3. Many efforts had been devoted to studying the electrical properties and microstructure of the thin films with different electrode materials. On the basis of the best ratio of hafnium zirconium composition, we selected these four electrode structures: Au/Ni/HZO/Pt, Au/Ni/HZO/TiN, TiN/HZO/TiN, TiN/HZO/Pt. Experimental results showed that in TiN / HZO / TiN structure, the film had better ferroelectric properties. When electric field strength was 1000kV/cm, the remanent polarization(2Pr) was about 50.6 μC/cm2, which is nearly fourfold than that of the Au/Ni/HZO/Pt electrode structures. Confinement of the thin films by the TiN electrodes assisted in the suppression of the volume expansion of tetragonal to monoclinic transition during the crystallization process, resulting in an facilitation of the tetragonal to orthorhombic phase transition in Hf O2, a lower probability of orthorhombic to monoclinic phase transition will take place, and the ferroelectric properties of the film were improved.