Influence of the Ozone Dose Time during Atomic Layer Deposition on the Ferroelectric and Pyroelectric Properties of 45 nm-Thick ZrO₂ Films

Abstract

Over a decade ago, ferroelectricity was discovered in doped HfO 2 thin films. The HfO 2 -based thin films have attracted much attention due to their remarkable scalability and CMOS compatibility. Other than the HfO 2 -based thin films, the undoped ZrO 2 thin films are understudied despite their commonly reported antiferroelectric behavior. However, being of the same fluorite structure as HfO 2 -based thin films, the undoped ZrO 2 also displayed considerable ferroelectricity as demonstrated in recent studies. In this work, 45 nm-thick polycrystalline undoped ZrO 2 films are synthesized using atomic layer deposition with different ozone dose times. The ZrO 2 films are crystallized after atomic layer deposition at 350 °C without anneals. In general, the longer ozone dose time causes a lower in-plane tensile stress and oxygen vacancy content, which help facilitate an irreversible non-polar tetragonal to polar orthorhombic phase transition with electric-field cycling. However, the lower in-plane tensile stress and oxygen vacancy content also stabilize the monoclinic phase so that a long ozone dose time (>17.5 s) reduces the ferroelectric behavior. After wake-up cycles, the ZrO 2 thin film with an ozone dose time of 17.5 s exhibits a remanent polarization of 6 μC•cm −2 and a pyroelectric coefficient of −35 μC•K −1 •m −2 . Moreover, the wake-up behavior is consistent between the ferroelectric and pyroelectric response. As essential factors in optimizing the growth of fluorite-structure thin films for ferroelectric applications, the in-plane tensile stress and oxygen vacancy content significantly influence the ferroelectric and pyroelectric properties. Additionally, the low thermal budget for processing ferroelectric ZrO 2 thin films is valuable for semiconductor back-end-of-line processes.

Strain as a Global Factor in Stabilizing the Ferroelectric Properties of ZrO2

“…As recently discussed, the relationship between the strain/stress and ferroelectric properties in fluorite-structured thin films is an important issue. [27][28][29][30]47] Therefore, the biaxial strain and grain size, which plays an essential role in the crystallite surface energy, are characterized and discussed in this section. [15,48] The grain size is measured on the top surface of the ALD ZrO 2 films and in cross-sections for the CSD ZrO 2 layers.…”

“…The relationship between ferroelectric behavior and strain/stress has recently received increasing attention. [27][28][29][30][31] The strain and stress can be influenced by lattice mismatch, [32] coefficient of thermal expansion (CTE) difference between the film and substrate, [33,34] grain size and surface energy, [15,35] defect content, [36,37] and densification during crystallization. [1,35] Density functional theory (DFT) suggests that large in-plane biaxial compressive stress may stabilize the polar phase and t-phase over the m-phase thermodynamically.…”

Pure ZrO2 Ferroelectric Thin Film for Nonvolatile Memory and Neural Network Computing

“…The ferroelectricity of the ZrO 2 thin film was experimentally revealed in a Pt/ZrO 2 (6.1–19.6 nm)/Pt structure . Further investigations revealed that the ferroelectricity of ZrO 2 films can be influenced by many factors, such as doping, − film thickness, , epitaxial strain, interfacial layers, electrode material, and oxygen source dose time . It was found that the macroscopic ferroelectric responses can be ascribed to the formation of the orthorhombic or rhombohedral (r) phase in the ZrO 2 films. ,− …”

Defect Engineering of Hafnia-Based Ferroelectric Materials for High-Endurance Memory Applications

“…These defects can affect the leakage current and breakdown of HfZrO x during operation. Previously, it was shown that the formation of the non-ferroelectric monoclinic phase could be suppressed and the growth of the ferroelectric orthorhombic phase could be promoted by optimizing the ozone exposure time. − Also, due to different phase compositions, the ferroelectric characteristics such as remnant polarization and coercive electric field could be affected by the ozone exposure time. , However, most of the previous studies analyzed the effect of ozone exposure time on the defect and electrical properties of thin HfZrO x films with a thickness of <10 nm. − The impact of the ozone exposure time may be different in thicker HfZrO x films due to different in-plane stress induced in the films . Also, as thicker HfZrO x films are beneficial for memory applications due to their large memory window, the effect of ozone exposure on the defect and electrical properties of HfZrO x should be further analyzed.…”

“… 19 − 22 The impact of the ozone exposure time may be different in thicker HfZrO x films due to different in-plane stress induced in the films. 23 Also, as thicker HfZrO x films are beneficial for memory applications due to their large memory window, the effect of ozone exposure on the defect and electrical properties of HfZrO x should be further analyzed.…”