Session B6: Focus Session: Proper and Improper Ferroelectrics

Ferroelectric Transition in Compressively Strained Epitaxial SrTiO$_{3}$

Strontium titanate (SrTiO$_{3})$ is a transition metal oxide semiconductor that crystallizes in the cubic perovskite crystal structure and exhibits incipient ferroelectricity. The dielectric constant of bulk unstrained SrTiO$_{3}$ crystals saturates at temperatures below 4K while above $\sim$ 50K, its dielectric constant decreases following the Curie-Weiss law of ferroelectricity [Muller et al., Phys. Rev. B 19, 3593 (1979)]. Based on the Landau-Ginzburg-Devonshire theory of ferroelectrics, it is theoretically predicted that under biaxial compressive or tensile strain, SrTiO$_{3}$ thin films should become ferroelectric [Pertsev et al., Phys. Rev. B 61, R825 (2000)]. Heteroepitaxial growth on lattice-mismatched substrates was used earlier to demonstrate near room temperature in-plane ferroelectricity in tensile strained SrTiO$_{3}$ thin films [Haeni et al., Nature 430, 758 (2004)]. In this work, we have epitaxially grown compressively strained SrTiO$_{3}$ thin films on (001) LSAT substrates, fabricated Pt/SrTiO$_{3}$ Schottky diodes, and performed temperature-dependent capacitance-voltage (CV) measurements of these diodes. As predicted by the theory, the out-of plane dielectric constant of SrTiO$_{3}$ extracted from these CV measurements shows a divergence, implying a ferroelectric transition in compressively strained SrTiO$_{3}$.   

Structural Stability of Nano-scale SrTiO$_{3}$ Under Pressure

The bulk phase of SrTiO$_{3}$ (STO) is paraelectric and exhibits a structural phase transition near $\sim$ 6 GPa under hydrostatic pressure. It has recently been found that nano-scale STO is polar under ambient conditions. We have conducted pressure dependent structural measurements on monodispersed nano-scale samples with 10 nm and 83 nm particle size. The structures of both samples were explored and the results are compared with the reported studies of bulk STO. This work is supported by DOE Grant DE-FG02-07ER46402.   

Hybrid Improper Ferroelectricity

Utilizing trilinear coupling of two types of octahedron rotations, hybrid improper ferroelectricity has been theoretically predicted in double layered compounds such as (Ca,Sr,Ba)$_3$(Mn,Ti,Sn,Zr,Ge)$_2$O$_7$. On the other hand, there exists little theoretical prediction on practical properties of the potentially-ferroelectric compounds such as ferroelectric transition temperature, switchability of polarization, and chemical phase stability. We have attempted to fabricate single crystals of some of these compounds, and examined the physical properties of the crystals with the aim of discovering new bulk ferroelectrics with switchable polarization at room temperature. We will report the results of our comprehensive experimental investigation.   

Experimental Demonstration of Hybrid Improper Ferroelectric in the Layered Ruddlesden-Popper Compounds

Geometric ferroelectrics are called as improper ferroelectrics where geometric structural constraints, rather than typical cation-anion paring, induce proper ferroelectric polarization. Hybrid improper ferroelectricity, one kind of geometric ferroelectricity, results from the combination of two or more of non-ferroelectric structural order parameters. In recent, hybrid improper ferroelectricity has been theoretically predicted in ordered perovskites and the Ruddlesden-Popper compounds. However, the ferroelectricity of these compounds has never been experimentally confirmed and even their polar nature has been under debate. In this talk, we report our experimental results of exploring switchable electric polarization and domain structures in the single crystals of the $n$ = 2 layered Ruddlesden-Popper compounds.\\[4pt] In collaboration with Xuan Luo, Laboratory for Pohang Emergent Materials, Postech; Fei-Ting Huang, Department of Physics \& Astronomy, Rutgers University; Yazhong Wang, Department of Physics \& Astronomy, Rutgers University; and Sang-Wook Cheong, Department of Physics \& Astronomy, Rutgers University.   

Single crystal ternary oxide ferroelectric integration with Silicon

Integrating single crystal, ternary oxide ferroelectric thin film with Silicon or other arbitrary substrates has been a holy grail for the researchers since the inception of microelectronics industry. The key motivation is that adding ferroelectric materials to existing electronic devices could bring into new functionality, physics and performance improvement such as non-volatility of information, negative capacitance effect and lowering sub-threshold swing of field effect transistor (FET) below 60 mV/decade in FET [Salahuddin, S, Datta, S. Nano Lett. 8, 405(2008)]. However, fabrication of single crystal ferroelectric thin film demands stringent conditions such as lattice matched single crystal substrate and high processing temperature which are incompatible with Silicon. Here we report on successful integration of PbZr$_{\mathrm{0.2}}$Ti$_{\mathrm{0.8}}$O$_{\mathrm{3}}$ in single crystal form with by using a layer transfer method. The lattice structure, surface morphology, piezoelectric coefficient d33, dielectric constant, ferroelectric domain switching and spontaneous and remnant polarization of the transferred PZT are as good as these characteristics of the best PZT films grown by pulsed laser deposition on lattice matched oxide substrates. We also demonstrate Si based, FE gate controlled FET devices.   

Optical second-harmonic characterization of ferroelectricity in double perovskites Ca$_{\mathrm{2-x}}$Mn$_{\mathrm{x}}$Ti$_{2}$O$_{6}$

Perovskite-type ferroelectric oxides such as BaTiO$_{3}$ are used widely as actuators and memory storage devices. Recently ferroelectricity was demonstrated in the double perovskite CaMnTi$_{2}$O$_{6}$, which represents a fundamental new class of ferroelectrics in which dipoles from Mn$^{2+}$ at the A-site and Ti$^{4+}$ at the B site are cooperatively coupled [1]. However, synthesis of CaMnTi$_{2}$O$_{6}$ from CaTiO$_{3}$-MnTiO$_{3}$ required pressure as high as 7GPa. We are developing spark plasma sintering (SPS) methods to synthesize Ca$_{\mathrm{2-x}}$Mn$_{\mathrm{x}}$Ti$_{2}$O$_{6}$ at pressures as low as 50 MPa, and using Second Harmonic Generation (SHG) microscopy to characterize the strength of ferroelectricity. Preliminary SHG results show that ferroelectric CaMnTi$_{2}$O$_{6}$ can be synthesized at low pressure with stronger ferroelectricity achieved with higher x and synthesis pressure. We will present comparative SHG results for SPS-synthesized and high-pressure-synthesized CaMnTi$_{2}$O$_{6}$ and relate them to the underlying origins of ferroelectricity.\\[4pt] [1] A. Aimi \textit{et al}., Chem. Mat. 26, 2601 (2014).   

Local Structure Study of the Diffuse Phase Transitions in 0.75Pb(Mg$_{1/3}$,Nb$_{2/3})$O$_{3}$-0.25PbTiO$_{3}$

Relaxor ferroelectrics have been of scientific interest and importance due to their fascinating properties such as a giant piezoelectric response, high permittivity over a broad temperature range, and unique dielectric response with frequency dispersion with the diffuse phase transitions. The experimental results undoubtedly explain the transitions with the widely accepted model of polar nanoregions (PNRs), appear at $T_{b}$, in a non-polar matrix. Local structure distortions using diffuse scattering (DS) technique had been reported already. Intensities of DS raise up below the intermediate temperature $T*$, a few hundreds degrees below $T_{b}$, and can be seen even at low temperature where system undergoes the frozen phase. Formations of local dipoles which give rise to the DS intensities are still unclear. We study DS by performing molecular dynamics simulations with the first-principle-based potential for 0.75Pb(Mg$_{1/3}$,Nb$_{2/3})$O$_{3}$-0.25PbTiO$_{3}$ without invoking the PNRs. Our results show that DS patterns form the experimentally reported shapes and integrated DS intensities as a function of temperature reveal a similar trend to the experimental results. Our results indicate that the local structure correlations in lead-based relaxors can arise from local random fields without PNRs. Instead, we find that the DS patterns are due to formations of slush-local-dipole multidomains..   

Relative Stability of FE and AFE States in (Na$_{0,5}$Bi$_{0,5})$TiO$_{3}$-based Solid Solutions

Changes of the relative stability of antiferroelectric (AFE) and ferroelectric (FE) phases in the [(Na$_{0.5}$Bi$_{0.5})_{0.80}$Ba$_{0.20}$](Ti$_{\mathrm{1-y}}$B$_{\mathrm{y}})$O$_{3}$ system of solid solutions with the $B$-site ion substitutions have been studied. Ions of zirconium and tin along some ions complexes such as (InNb), (FeNb) and several others were used for substitutions. The increase in the substituent ion content leads to nearly linear variation of the crystal cell size along with changes of the relative stability of the AFE and FE phases according to the tolerance factor variation. Substituent ions with ionic radii larger than the ionic radius of original ion evoke a decrease of the FE-AFE phase transition temperature. The substituent ions with smaller ionic radii have the opposite effect. Our results demonstrate that the size of the substituent ion causes a predominant influence on the relative stability of the FE and AFE states in (Na$_{0.5}$Bi$_{0.5})$TiO$_{3}$-based solid solutions. Our studies also indicate the way to raise the FE-AFE phase transition temperature.   

Negative Capacitance transients in a ferroelectric capacitor

The Boltzmann distribution of electrons poses a fundamental barrier to lowering energy dissipation in conventional electronics, often termed as Boltzmann Tyranny [1,2]. Negative capacitance in ferroelectric materials, which stems from the stored energy of phase transition, could provide a solution, but a direct measurement of negative capacitance has so far been elusive. Here we demonstrate the negative differential capacitance in a thin, single crystalline ferroelectric film, by constructing a simple R-C network and monitoring the voltage dynamics across the ferroelectric capacitor6. When a voltage pulse is applied, the voltage across the ferroelectric capacitor is found to be decreasing with time--in exactly the opposite direction to which voltage for a regular capacitor should change. The results are analyzed on the basis of the Landau-Khalatnikov equation, which shows that as the ferroelectric polarization switches its direction, it passes through the unstable negative capacitance region resulting in the characteristic ``negative capacitance transients.'' Analysis of this ``inductance''-like behavior from a capacitor allows us to calculate the value of the negative capacitance directly and presents an unprecedented insight into the intrinsic energy profile of the ferroelectric material.\\[4pt] [1] Salahuddin et al.~Nano Lett.~8, 40 (2008).\\[0pt] [2] Zhirnov et al. ~Nature Nanotechnology~3, 77 (2008).\\[0pt] [3] Khan et al. Nature Materials (in press).   

Double hysteresis in BaTiO$_{3}$/PbZr$_{0.2}$Ti$_{0.8}$O$_{3}$ ferroelectric bilayer thin film

We observed two hysteresis loops in BaTiO$_{3}$/PbZr$_{0.2}$Ti$_{0.8}$O$_{3}$ (BTO/PZT) bilayer thin film. The first loop with polarization of 27 $\mu$ C/cm$^{2}$ was measured in the applied voltage of $\pm$ 20 V. The second hysteresis loop with polarization of 76 $\mu$ C/cm$^{2}$ was measured in the applied voltage of $\pm$ 55 V. Both hysteresis loops showed characteristic shape with concave region followed by saturation region in the broad range of applied voltage frequencies providing strong evidence for ferroelectric origin of both loops. We performed computational analysis of BTO/PZT bilayer based on Landau-Ginzburg-Devonshire model including contributions of electronic band structure. We found an increased concentration of free charge carriers at the interface between BTO and PZT which provides compensation for the bound charge due to polarization mismatch of the layers. Moreover, as the free charge effectively screens polarization in one layer from another, polarization switching of individual layers can be possible. This leads to the conclusion that two hysteresis loops can be a result of polarization switching of the individual layers.   

Ferroelectric Field Effect in Ultrathin Epitaxial Sm$_{0.5}$Nd$_{0.5}$NiO$_{3}$ Films

We report the study of ferroelectric field effect modulation of the metal-insulator transition in ultrathin Sm$_{0.5}$Nd$_{0.5}$NiO$_{3}$ (SNNO) films. We have fabricated high quality epitaxial SNNO thin films and Pb(Zr,Ti)O$_{3}$ (PZT)/SNNO heterostructures on (001) LaAlO$_{3}$ substrates using off-axis RF magnetron sputtering. X-ray diffraction and atomic force microscopy studies reveal (001) oriented films with highly crystallinity and surface roughness of 3-4 {\AA}. Thin SNNO films (4-6 nm) typically have the transition temperature $T_{MI}$ around 230 K, showing thermally activated transport below $T_{MI}$ followed by 3D variable range hoping at low temperature. Hall effect measurements reveal p-type conduction with $\sim$ 4 holes/uc in the metallic phase. Working with films one to two unit cells thicker than the electrical dead layer thickness ($\sim$ 4 nm), we have demonstrated nonvolatile, reversible ferroelectric field effect modulation of $T_{MI}$ in SNNO by up to 10 K. The maximum resistance ratio $R_{high}$/$R_{low}$ is 1.7 at 140 K, which is in the thermally activated regime. In the metallic phase, the carrier density has been modulated by 1x10$^{15}$ cm$^{-2}$, corresponding to the polarization field of PZT of 80 $\mu $C/cm$^{2}$.   

Electrical Properties of Tetragonal-PZT Thin Film Capacitors from 5 K to 300 K.

Rapid assessment of ferroelectric device characteristics is critical to improving ferroelectric materials processing as well as developing accurate ferroelectric device models. Here, we demonstrate automated electrical testing of thin PZT and Nb-doped PZT thin film devices at temperatures ranging from 300 K down to 5 K in a cryogenic probing environment. In this configuration, temperature-dependent dielectric constant, remanent polarization, coercive voltage, switching speed, and leakage are measured in a single pass on a single sample. From these measurements, it appears that tetragonal PZT does not have a phase boundary from room temperature down to 5 K. Retention tests conducted on several capacitors while transitioning from room temperature to 200 K, 100 K, and 6.5 K showed no loss of remanent polarization indicating 20/80 PZT and its niobium-doped cousins remain fully functional as memory devices down to 5 K.