Kinetic control of ferroelectricity in ultrathin epitaxial Barium Titanate capacitors

TL;DR

This work demonstrates that tuning the kinetic energy of PLD plume via laser spot size can dramatically improve ultrathin BaTiO$_3$ capacitors, producing high-quality SRO/BTO/SRO/GSO heterostructures with significantly reduced leakage. The optimized 20 nm BTO devices exhibit ultralow switching voltages (below $0.3$ V), long retention times (> $10^4$ s), and extraordinary endurance (> $10^{11}$ cycles), enabled by improved bulk and interfacial quality and a broadened growth window. Structural and electrical characterizations (RHEED, XRD, PUND, C-V, and TEM) reveal that non-equilibrium growth kinetics can mitigate extrinsic loss mechanisms in ultrathin ferroelectrics. These findings establish a scalable pathway to practical, low-voltage ferroelectric switches for logic and memory applications and underscore the role of plume dynamics in non-equilibrium oxide deposition.

Abstract

Ferroelectricity is characterized by the presence of spontaneous and switchable macroscopic polarization. Scaling limits of ferroelectricity have been of both fundamental and technological importance, but the probes of ferroelectricity have often been indirect due to confounding factors such as leakage in the direct electrical measurements. Recent interest in low-voltage switching electronic devices squarely puts the focus on ultrathin limits of ferroelectricity in an electronic device form, specifically on the robustness of ferroelectric characteristics such as retention and endurance for practical applications. Here, we illustrate how manipulating the kinetic energy of the plasma plume during pulsed laser deposition can yield ultrathin ferroelectric capacitor heterostructures with high bulk and interface quality, significantly low leakage currents and a broad "growth window". These heterostructures venture into previously unexplored aspects of ferroelectric properties, showcasing ultralow switching voltages ($<$0.3 V), long retention times ($>$10$^{4}$s), and high endurance ($>$10$^{11}$cycles) in 20 nm films of the prototypical perovskite ferroelectric, BaTiO$_{3}$. Our work demonstrates that materials engineering can push the envelope of performance for ferroelectric materials and devices at the ultrathin limit and opens a direct, reliable and scalable pathway to practical applications of ferroelectrics in ultralow voltage switches for logic and memory technologies.

Figures (23)

• Figure 1: The effect of (a) a diffuse laser spot with a large area and (e) a focused laser spot with a smaller area on the plume dynamics and the kinetic energy of the species provided that all other growth parameters, including laser fluence, remain the same. The bombardment effects on the film can be greater with the larger spot, as illustrated in (a). (b)-(d) shows the HAADF-STEM image of a 20 nm SrRuO$_3$/20 nm BaTiO$_3$/20 nm SrRuO$_3$ heterostructure on GdScO$_3$ substrate where in the spotsize and the plume dynamics were akin to the one shown in (a). Utilizing a low angle annular dark field (LAADF) with the collection angle from 33 to 199 mrad, the defects were examined throughout the 20 nm SrRuO$_3$ /20 nm BaTiO$_3$/20 nm SrRuO$_3$ heterostructure. (b) Cross-sectional view on large area of the heterostructure reveals multiple defective regions. Notably in (c), the bottom SrRuO$_3$/GdScO$_3$ features a thin layer of distorted atomic columns, depicted in the white box. (d) High-resolution image of the BaTiO$_3$/SrRuO$_3$ interface from the labeled region in (b), indicated by the white box. Furthermore, the BaTiO$_3$ layer has defective atomic columns. (f)-(h) shows the HAADF-STEM image of a 20 nm SrRuO$_3$ /20 nm BaTiO$_3$/20 nm SrRuO$_3$ heterostructure on GdScO$_3$ substrate where in the spotsize and the plume dynamics were akin to the one shown in (e). (f) Cross-sectional view of a large area shows a sharp interface on both sides of the BaTiO$_3$/SrRuO$_3$ layers. No obvious crystallographic defects were found over a 10 um field of view. (g) High resolution image of the BaTiO$_3$/SrRuO$_3$ interface from labeled region in (f), white box. (h) Intensity profile of the HAADF image from yellow line in (g) showing the atomically sharp interface.
• Figure 2: (a) Large leakage currents in 20 nm BTO films. (b) and (c) show P-V loops for ferroelectric 20 nm BTO films. (d) Phase space of fluence and thickness that renders ferroelectric films. (e) and (f)-(g) show P-V loops for ferroelectric 100 nm and 60 nm BTO films respectively. All BTO layers were grown at 0.11 mbar oxygen partial pressure.
• Figure 3: (a) Phase space of SRO and BTO spot sizes rendering ferroelectricity in 20 nm SRO/20 nm BTO/20 nm SRO/GSO heterostructures. All BTO films were grown at a laser fluence of 2.13 J/cm$^2$ and a pressure of 0.11 mbar. (b) c axis, (c) a axis lattice parameters for the 20 nm BTO films grown at different spotsizes.
• Figure 4: (a)Retention characteristics in 20nm BTO film measured till $10^4$ seconds. The applied waveform is shown in the inset of the figure. The pulse width t$_p$ and the wait time between two positive pulses t$_w$ was 100$\mu$s. The Delay between the first and second pulse is increased after each measurement and is represented as t$_{Dn}$ for the n$^{th}$ measurement. The pulse Amplitude was 0.5 V. (b) Endurance characteristics in the same 20 nm BTO film. A continuous square pulse of amplitude 0.7V and pulse width 100ns is applied.
• Figure S1: (a) GdScO$_{3}$ (110) substrate before growth (b) RHEED pattern of SrRuO$_{3}$ (SRO) bottom electrode (c) RHEED pattern of BaTiO$_{3}$ (BTO) (d) RHEED pattern of SRO top electrode at the end of growth (e) RHEED oscillation during BTO growth.