Study on the internal structure of ferroelectric Hf1-xZrxO2 thin film systems

Abstract

Ferroelectric (FE) property of HfO2 thin films was first reported in 2011 NaMLab in Dresden, Germany, which had a fluorite structure, doped with a few amounts of Si. It was a very intriguing issue on the FE community because the fluorite-type film has only ~ 10 nm thickness, whereas the conventional perovskite type ferroelectrics have > 100 nm thickness. It has merit for fabrication of 3-dimensional structure due to its small thickness. Also, the band gap of the HfO2 thin film is 5.5 eV which is high enough to prevent leakage currents flowing through devices. Having titanium nitride as a metal electrode, combined with an excellent compatibility with Si, HfO2 as a thin film could be the representative industrial-friendly materials for the adoption of memory production technology. It has been widely accepted that the emergence of unexpected ferroelectricity in HfO2 thin films is due to the formation of non-centrosymmetric orthorhombic Pca21 phase. However, it still lacks researches on the emergence of ferroelectricity in this material systems. Therefore, this dissertation aims to resolve the ambiguity of the origin of the emergence of ferroelectricity in thin films through researches on the internal structure of the FE HfO2 thin films. For its robust ferroelectricity, many dopants were induced. (Si, Zr, Y, Al, Gd, Sr, La, etc.) Among these dopants, Zr doped HfO2 has its wide composition range for emerging various electrical characteristics and lower processing temperature for crystallization of films. Therefore, Hf1-xZrxO2 thin films are up-and-coming FE materials for analyzing the mechanism of emerging ferroelectricity. As the first step, the degradation of the FE properties of atomic layer deposited Hf0.5Zr0.5O2 films with increasing thickness was examined. When the thickness of the film increases over 20 nm, the FE properties of the films start to degrade whereas the 10 nm-thick film shows robust FE properties in previous reports. The origin of the degradation was elucidated by phase transition of non-FE monoclinic phase. According to general thin film growth theory, meanwhile, the grain size of the film increases with increasing film thickness. The grain size is the critical factor to get FE properties of Hf0.5Zr0.5O2 films because the surface energy and volumetric energy are affected by the grain size. Therefore, control of the grain size of films is key point to interrupt degradation of FE properties despite of increasing film thickness. In this dissertation, the grain size is successfully controlled by inserting 1nm-thick Al2O3 interlayer at the middle position of the thickness of the FE film. The Al2O3 interlayer could hinder the continual growth of Hf0.5Zr0.5O2 films, and the resulting decrease of grain size prevented the formation of the non-FE monoclinic phase. The Al2O3 interlayer also principally decreased the leakage current of the Hf0.5Zr0.5O2 films. As the next step, a wake-up effect on the FE Hf0.5Zr0.5O2 films was examined which refers to the increase in remanent polarization with increasing electric field cycling number before the occurrence of fatigue effect. In this work, the wakeup effect from the Hf0.5Zr0.5O2 was carefully examined by the pulse-switching experiment. At the pristine state, the Hf0.5Zr0.5O2 film mostly showed the FE-like behavior with a small contribution of antiferroelectric(AFE)-like distortion, which could be ascribed to the involvement of AFE phase. The field cycling of only 100 cycles almost wholly transformed the AFE phase into FE phase by depinning the pinned domains. The influence of field cycling on the interfacial layer was also examined through the pulse-switching experiments. In addition to that, the broken FE hysteresis loops achieved from a Hf0.4Zr0.6O2 film was interpreted based on the first order phase transition theory. The two-step polarization switching, which was expected from the theory, could be observed by dynamic pulse switching measurement. The variations in the interfacial capacitance values along with switching time and number of switching cycles could also be estimated from the pulse switching test. Being different from the one-step polarization switching in other FE films, two-step polarization switching produced two slanted plateau regions where the estimated interfacial capacitance values were different from each other. This could be understood based on the quantitative model of the two-step polarization switching with the involvement of an intermediate nonpolar phase. The Hf0.4Zr0.6O2 film changed from AFE-like to FE-like with increasing number of electric field cycling, which could be induced by the field driven phase change. Finally, this thesis presents a new strategy for extending conventional scaling trend in dynamic random access memory (DRAM) by utilizing newly found morphotropic phase boundary (MPB) of solid solution of the HfO2-ZrO2 system. For the purpose, the schematic phase diagram of HZO films with various thickness and Hf:Zr ratio was presented based on the previous works, and the MPB of tetragonal and orthorhombic phase for an abnormal increase in dielectric constant (r) was found. From the C-V characterizations, the extraordinary r values could be observed in the MPB, and it could be confirmed that the composition of films changes with changing film thickness. The Zr contents for MPB decreased with decreasing film thickness owing to the relative decrease of the free energy of o-phase compared to that of t-phase. The minimum tox of 0.59nm could be achieved for 8.1 nm-thick Hf0.5Zr0.5O2 films.