Doping and interface effects on the ferroelectric properties of epitaxial HfO2-based thin films

Abstract

Ferroelectric HfO2-based thin films have aroused great interest in the research field of memory devices, because it is a complementary metal oxide semiconductor compatible material with excellent scalability. However, it is needed to further clarify the nature of ferroelectricity and enhance the ferroelectric properties. Epitaxial thin films, with more controlled microstructure than polycrystalline films, can help to this objective. This thesis is focused on epitaxial HfO2-based films, investigating new deposition conditions, the use of different substrates and doping and their impact on basic functional properties. Switching and fatigue mechanisms are also investigated. In order to tailor the epitaxial growth of HfO2-based thin films, first, we grown Hf0.5Zr0.5O2 (HZO) films under lower oxidizing condition with inert Ar gas to decrease the pulsed laser deposition (PLD) plasma energy when reducing the oxygen pressure in the deposition process. Optimized mixing Ar and O2 atmosphere allows increase of polarization around 50% respect films grown by conventional pulsed laser deposition. Second, we have investigated the effect of seed layers. Ultrathin HZO seed layer was grown on the bottom electrode to obtain crystallization of HZO film at lower deposition temperature. Third, we have found that orthorhombic phase can also be epitaxially stabilized on LSMO/STO(110) and LSMO/STO(111), having the films different in-plane epitaxial relationship than films on LSMO/STO(001) Doping effect of Zr and La on the ferroelectricity of HfO2 epitaxial films has been systematically studied. First, Hf1-xZrxO2 films in all the explored compositions are ferroelectric without wake-up effects. Endurance is better in HZO film than that of HfO2 and ZrO2 films. Second, we investigated the influence of La content, film thickness, and substrate induced epitaxial stress on the ferroelectricity of La:HfO2 films. The optimized La content is 2-5 at%. 2 at% La:HfO2 films with thickness less than 7 nm show a high Pr of about 30 μC/cm2, slight wake-up, an endurance of at least 1010 cycles and a retention of more than 10 years. 2 at% La:HfO2 films show the highest Pr for films grown on scandate substrates when compared with other perovskite substrates. Third, the Zr and La co-doping on HfO2-based films is explored. In 1% La doped HZO films, it is found that, as for other compositions, there is a reduction of polarization with thickness increase in the explored range; however, for the studied composition the reduction is less than for films without La doping. Endurance in epitaxial La-doped HZO films is more than 1010 cycles, and it is accompanied by excellent retention of more than 10 years at same voltage. This proves that there is no endurance-retention dilemma in La-doped HZO films as stated in previous literature. Furthermore, phases and polarization of the HfO2-ZrO2-La2O3 ternary system has been mapped observing that indeed 1%La doped HZO show the best performance Impact of crystal phases on ferroelectric properties, such as endurance and switching dynamics, is also investigated. Epitaxial HZO films almost free of parasitic monoclinic phase suffer severe fatigue. In contrast, fatigue is mitigated in films with greater amount of paraelectric phase. On the other hand, switching analysis reveals that pure orthorhombic phase films follow the Kolmogorov-Avrami-Ishibashi switching model. Instead, the mixed orthorhombic/monoclinic phase films show nucleation limited switching. Films having larger fraction of non-ferroelectric phase with concomitant larger number of incoherent grain boundaries show 2 orders of magnitude faster switching time. Both studies allow to conclude that the films showing the largest amount of ferroelectric orthorhombic phase do not show the best functional properties.