Toward Thick Piezoelectric HfO₂‐Based Films

Abstract

For HfO2‐based films, which are mainly prepared by atomic layer deposition, the desired ferroelectric (FE) properties typically vanish while extending the thickness beyond a limit of about 50 nm. Herein, the successful fabrication of a 1 μm‐thick piezoelectric La:HfO2 film is demonstrated using chemical solution deposition, paving the way toward sensor and actuator applications. After identifying the optimal La content, the film thickness is increased from 45 nm to 1 μm. Polarization and strain measurements evidence the persistence of the FE properties at all thicknesses and even a slight improvement due to a better orientation of the polar axis at higher thicknesses. Scanning electron microscopy and X‐ray reflectivity reveal a fine‐grained microstructure and a density of only 80% of the theoretical value, which seems to be a common issue of solution‐deposited HfO2‐based films, based on the performed literature survey. Using tilt‐angle‐dependent X‐ray diffraction scans, homogeneous nucleation is found to be the likely root cause of the observed microstructure. It is suggested that this microstructure issue is key for the optimization of cycling stability of solution‐based films to exploit the full potential of the HfO2 for cost‐efficient, thick piezoelectric films.

Multi‐Step Chemical Solution Deposition‐Annealing Process Toward Wake‐Up Free Ferroelectricity in Y:HfO2 Films

“…The obtained pattern for 7 × 4N is shown in Figure a along with reference patterns of tetragonal, cubic, monoclinic, and orthorhombic phases of HfO 2 . [ 7,9,10,22 ] The pattern matches well with the orthorhombic phase pattern without any secondary pattern similar to previous reports on doped‐HfO 2 . [ 22–24 ] The possibility of the formation of tetragonal and cubic phases was excluded using the High‐resolution transmission electron microscopy (HRTEM) technique.…”

“…The curves exhibit typical ferroelectric characteristics with remnant polarization around 17 µC cm −2 and maximum polarization of 28 µC cm −2 at an applied voltage of 22 V. It should be noted that these P‐V loops were measured on a pristine capacitor without any initial field cycling for the typical wake‐up treatment. Earlier reports demonstrated proper ferroelectric hysteresis after wake‐up treatment; [ 3–5,7,10,13,14,28 ] however, we achieved similar hysteresis with comparable remnant polarization without any wake‐up process. The dielectric constant and lost tangent (Figure 3b) values are comparable to those obtained in the previous reports.…”

“…Using the metal‐organic decomposition (MOD) route, Y‐doped HfO 2 thin film samples are prepared through multi‐step deposition and annealing processes. Compared with recent references, [ 7,8,18–21 ] the process is systematically improvised to achieve wake‐up‐free ferroelectricity and defect‐free microstructure. We have labeled our samples as D × AT, where “D” corresponds to the number of deposited layers, “A” represents the number of multi‐step annealing carried out between the depositions, and T is the type of annealing (N = normal furnace annealing / R = RTA).…”

“…reported the maximized polarization of La‐doped HfO 2 prepared by chemical solution deposition (CSD) after 10 4 cycles of wake‐up treatment. [ 7 ] In Y‐doped HfO 2 prepared by CSD, the requirement was 10 3 wake‐up cycles. [ 8,9 ] On the other hand, Mart et al.…”

Electric‐Field‐Induced Ferroelectricity in 5%Y‐doped Hf0.5Zr0.5O2: Transformation from the Paraelectric Tetragonal Phase to the Ferroelectric Orthorhombic Phase

“…Ferroelectricity in fluorite oxide-based materials, such as HfO 2 , has renewed the applications of ferroelectric memories due to these materials' high compatibility with complementary metal-oxide-semiconductor (CMOS) technology. [1] The recent development of fluorite-structure ferroelectric materials has given rise to applications, such as neuromorphic devices, [2,3] negative capacitance logic devices, [4,5] piezoelectric, [6,7] and pyroelectric sensors. [8,9] The ferroelectricity of fluorite materials is attributed to their metastable orthorhombic structure.…”