Epitaxial Ferroelectric Films Research Articles

Distinguishing the Rhombohedral Phase from Orthorhombic Phases in Epitaxial Doped HfO2 Ferroelectric Films.

Epitaxial strain plays an important role in the stabilization of ferroelectricity in doped hafnia thin films, which are emerging candidates for Si-compatible nanoscale devices. Here, we report on epitaxial ferroelectric thin films of doped HfO2 deposited on La0.7Sr0.3MnO3-buffered SrTiO3 substrates, La0.7Sr0.3MnO3 SrTiO3-buffered Si (100) wafers, and trigonal Al2O3 substrates. The investigated films appear to consist of four domains in a rhombohedral phase for films deposited on La0.7Sr0.3MnO3-buffered SrTiO3 substrates and two domains for those deposited on sapphire. These findings are supported by extensive transmission electron microscopy characterization of the investigated films. The doped hafnia films show ferroelectric behavior with a remanent polarization up to 25 μC/cm2 and they do not require wake-up cycling to reach the polarization, unlike the reported polycrystalline orthorhombic ferroelectric hafnia films.

Influence of oxygen pressure on the ferroelectricity of pulsed laser deposition fabricated epitaxial Y-doped HfO2

Ferroelectric properties of hafnium-based thin films have gained significant interest, yet the fundamental mechanisms responsible for the emergence of the ferroelectric phase continue to be inadequately investigated. In contrast with polycrystalline films fabricated by atomic layer deposition or sputter methods, which possess uncertainty in polarization orientation, epitaxial ferroelectric HfO2-based materials are less investigated, especially for factors such as electric field and oxygen vacancy, which are proposed and examined for their potential impacts on phase stability. In this study, Y-doped hafnium oxide (HYO) ferroelectric epitaxial films were fabricated using pulsed laser deposition, with variations in oxygen pressure during the deposition process. Structural and electrical analyses of HYO epitaxial ferroelectric films prepared under differing oxygen pressures revealed a correlation between the ferroelectric properties of the films and the oxygen content. An optimal selection of oxygen pressure was found to be conducive to the formation of HYO epitaxial ferroelectric films, presenting a promising avenue for future ferroelectric memory applications.

Strain-driven high energy storage in BaTiO3-based lead-free epitaxial thin films

In this work, the epitaxial 0.85BaTiO3-0.15Bi(Mg1/2Ti1/2)O3 (BT-BMT) films with large compressive strain were fabricated on SrTiO3 (001). The expansion of the unit cell volume and out-of-plane lattice parameter and the large built-in electric field (Ebi) in BT-BMT films indicate the existence of defect dipoles. It was found that the polarization and the breakdown strength can be optimized by the strain and the defects, respectively. Ultimately, a desirable energy density of 90.3 J/cm3 with efficiency of 62.3% was achieved. It suggests that strain can serve as a practical means to modulate the energy storage performance of ferroelectric epitaxial film capacitors.

Buffer electrode layers tuned electrical properties, fatigue behavior and phase transition of KNN-based lead-free ferroelectric films

Herein, the impact of conductive buffer electrode layers (p- or n-type) on the electrical properties and fatigue behavior of KNN-based ferroelectric films are investigated.

Tunable crystalline structure and electrical properties of (Pb,Sr)TiO3 films grown by liquid phase epitaxy

Ferroelectric epitaxial films of Pb1−xSrx TiO3 (PST) with nominal compositions of x = 0, 0.33, 0.39, and 0.43, supported by (100) SrTiO3 (STO) substrates, have been successfully grown by liquid phase epitaxy (LPE).

Patterning of large area nanoscale domains in as-grown epitaxial ferroelectric PbTiO3 films

Effective tuning of nanoscale domain structures provides fundamental basis for controlling and engineering of various functionalities in ferroelectric materials. In this work, we demonstrate the precise patterning of nanoscopic domain structures in as-grown epitaxial PbTiO3 (PTO) films by merely introducing an ultrathin pre-patterned doping layer (e.g., Fe-doped PTO). The doping layer can effectively reverse the interfacial built-in bias, consequent to a reversed initial polarization reorientation in the as-grown film, which makes it possible to transfer the nano-patterns in the doping layer into the domain structure of ferroelectric films. For instance, we have successfully fabricated large area ordered array of nanoscale cylindrical domains (downward polarization) embedded in the matrix domain with opposite polarization (upward polarization) in PTO film. These nanoscale cylinder domains also allow deterministic and reversible erasure and creation induced by biased tip scanning. The results provide an effective pathway for on-demand patterning of large area nanoscale domains in the as-grown films, which may find applications in a wide range of nanoelectronic devices.

Impact of dislocation density on the ferroelectric properties of ScAlN grown by molecular beam epitaxy

We report on the effect of dislocation density on the ferroelectric properties of single-crystalline ScAlN thin films grown by molecular beam epitaxy. Wurtzite phase and atomically smooth ScAlN films have been grown on bulk GaN, GaN on sapphire, and GaN on Si substrates with dislocation densities ranging from ∼107 to 1010 cm−2. Despite the significant difference in dislocation density, ferroelectricity is observed in all three samples. The presence of high densities of dislocations, however, results in enhanced asymmetric P–E loops and overestimated remnant polarization values. Further measurements show that the leakage current and breakdown strength can be improved with decreasing dislocation density. Detailed studies suggest that trapping/detrapping assisted transport is the main leakage mechanism in epitaxial ferroelectric ScAlN films. This work sheds light on the essential material quality considerations for tuning the ferroelectric property of ScAlN toward integration with mainstream semiconductor platforms, e.g., Si, and paves the way for next-generation electronics, optoelectronics, and piezoelectronics.

A Robust Memristor Based on Epitaxial Vertically Aligned Nanostructured BaTiO3 -CeO2 Films on Silicon.

With the exploration of ferroelectric materials, researchers have a strong desire to explore the next generation of non-volatile ferroelectric memory with silicon-based epitaxy, high-density storage, and algebraic operations. Herein, a silicon-based memristor with an epitaxial vertically aligned nanostructures BaTiO3 -CeO2 film based on La0.67 Sr0.33 MnO3 /SrTiO3 /Si substrate is reported. The ferroelectric polarization reversal is optimized through the continuous exploring of growth temperature, and the epitaxial structure is obtained, thus it improves the resistance characteristic, the multi-value storage function of five states is achieved, and the robust endurance characteristic can reach 109 cycles. In the synapse plasticity modulated by pulse voltage process, the function of the spiking-time-dependent plasticity and paired-pulse facilitation is simulated successfully. More importantly, the algebraic operations of addition, subtraction, multiplication, and division are realized by using fast speed pulse of the width ≈50ns. Subsequently, a convolutional neural network is constructed for identifying the CIFAR-10 dataset, to simulate the performance of the device; the online and offline learning recognition rate reach 90.03% and 92.55%, respectively. Overall, this study paves the way for memristors with silicon-based epitaxial ferroelectric films to realize multi-value storage, algebraic operations, and neural computing chip applications.

Electrical property and phase transition analysis of KNN-based lead-free ferroelectric films

The electrical properties, phase construction, thermal stability and phase transition behavior are investigated in 0.95(K0.49Na0.49Li0.02)(Ta0.2Nb0.8)O3–0.05BaZrO3 with 2 wt% MnO2 (KNNLT-BZM) lead-free epitaxial ferroelectric film on LaNiO3 (LNO)-coated SrTiO3 (STO) (001) substrate. The x-ray diffraction results show that a mixed orthorhombic (O) and tetragonal (T) phase is obtained in KNNLT-BZM film. The MnO2 doping can effectively suppress its leakage current, which greatly improves the electrical performance featured by a lower leakage value of 8 × 10–11 A cm−2, a twice remnant polarization of 44.7 μC cm−2, and frequency-dependent ferroelectricity between 50 Hz and 10 kHz. Strikingly, the KNNLT-BZM film maintained ferroelectric nature up to 200 °C, and exhibit a phase transition from O + T mixed phase to T phase (T Mix-T) at 300 °C with a high Curie temperature above 440 °C. These results suggest great application potentials of KNN-based films in lead-free micro-electronic devices.

Giant tunneling electroresistance in epitaxial ferroelectric ultrathin films directly integrated on Si

Retaining information with the least loss of energy is of interest for ubiquitous mobile devices. Various forms of nonvolatile memory have been pursued based on the resistance change in the memory architecture. The nonvolatility of spontaneous polarization in a ferroelectric material also has been considered a promising ingredient for information storage applications, including traditional nonvolatile memory applications and emerging neuromorphic synaptic devices. Here, we demonstrate a colossal resistance change in the ferroelectric tunnel junction with an On/Off ratio of 106 in a ferroelectric HfO2 thin film integrated directly on a silicon wafer, which is inevitable for practical application. To achieve this large On/Off ratio, we integrated the epitaxial fluorite-structure HfO2 thin film on the silicon substrate. The polarization direction in the metal-ferroelectric-semiconductor junction altered the depletion width, leaving behind the change in the tunnel barrier. Industry-relevant HfO2 with high CMOS compatibility could lead to fast adoption of a ferroelectric tunnel barrier with newly observed ferroelectricity in fluorite-structured HfO2 films.

Giant conventional and rotating magnetocaloric effects in TbScO3 single crystal

Magnetocaloric effect (MCE) shows great potential in refrigeration field and especially fundamental scientific research with cooling requirement. In this work, we studied the anisotropic magnetic and magnetocaloric properties of TbScO3 single crystal, a popular substrate used for depositing epitaxial ferroelectric films. Our results indicate a strong antiferromagnetic ordering below TN ~ 3.1 K in orthorhombic TbScO3 single crystal. Large magnetocrystalline anisotropy exists between the magnetic easy ab plane and the hard c axis, leading to a giant anisotropic MCE. In field changes of 20 and 50 kOe along the a axis, large magnetic entropy changes reach 15.25 and 23.71 J/kg K at 3 K and 7 K, respectively, while the magnetic entropy changes along the c axis are negligible. As a result, large rotating magnetic entropy changes of 15.27 and 23.63 J/kg K are obtained by rotating TbScO3 single crystal with constant fields of 20 and 50 kOe lying in the ac plane. The large anisotropic MCE of TbScO3 single crystal indicates this material as a promise refrigerant candidate for rotating magnetic refrigeration technique near the helium temperature.

Influence of growth oxygen pressure on the electrical properties and phase transformation of the epitaxial (K,Na)NbO3-based lead-free ferroelectric films

The influence of the growth oxygen pressure (GPO2) on the performance of the epitaxial 0.95(K0.49Na0.49Li0.02)(Ta0.2Nb0.8)O3–0.05CaZrO3 with 2 wt. % MnO2 addition (KNNLT-CZM) lead-free ferroelectric films grown on the La0.07Ba0.93SnO3-coated SrTiO3 (001) substrates is investigated. The x-ray diffraction results show that the tetragonality of the KNNLT-CZM films is dependent on GPO2, which varies from 0.999 at 15 Pa to 1.006 at 35 Pa. Since the polarization direction with applied electrical field of the (010)-oriented KNN-based film is along [110]/[011], the relationship between the ferroelectricity and GPO2 is well explained from the perspective of the tetragonality change. The leakage current density of the KNNLT-CZM films is suppressed and the dielectric constant is enhanced from 427 to 1538 at 1 kHz with increasing the GPO2. Moreover, the orthorhombic to tetragonal phase transition temperature (TO-T) of the KNNLT-CZM films grown at 15 Pa is ∼180 °C, which is much lower than ∼210 °C of those grown at 25/35 Pa. GPO2 is proven to be an important factor that regulating the ferroelectricity and TO-T of the epitaxial KNNLT-CZM films.

Emerging Fluorite‐ and Wurtzite‐Type Ferroelectrics: From (Hf,Zr)O2 to AlN and Related Materials

Emerging Fluorite‐ and Wurtzite‐Type Ferroelectrics: From (Hf,Zr)O₂ to AlN and Related Materials

Reduced coercive field in epitaxial thin film of ferroelectric wurtzite Al0.7Sc0.3N

Epitaxial ferroelectric wurtzite films exhibiting clear polarization-electric field hysteresis behavior are presented. The coercive field of this epitaxial Al0.7Sc0.3N film on the W/c-sapphire substrate is 0.4 ± 0.3 MV cm−1 (8%) smaller than that of a conventional fiber textured film on a Pt/TiOx/SiO2/Si substrate, attributed to the 0.01 ± 0.007 Å smaller c-axis lattice parameter in the epitaxial film. The strain and decrease in the coercive field most likely originate from epitaxial strain rather than the mismatch in the thermal coefficient of expansion. These results provide insight for further coercive field reduction of wurtzite ferroelectrics using epitaxial mismatch strain.

Ultrafast photoinduced strain in super-tetragonal PbTiO3 ferroelectric films

The ultrafast photoinduced strain (UPS) resulting from the coupling of piezoelectric and photovoltaic effects in ferroelectric has been focused in the last decade, endowing them with extensive applications including ultrafast optical memories, sensors and actuators with strain engineering. The mechanism of screening of the depolarization field by photoinduced carriers is generally accepted for UPS in ferroelectrics, while the thermal component of the strain is usually diluted as the offset and has not been systematically confronted, leading to unnecessary confusion. Herein, both the positive and negative thermal expansion effects in composite ferroelectric epitaxial films are investigated by use of high-repetition-rate ultrafast X-ray diffraction, along with the piezoelectric and photovoltaic effects. The coupling of the positive/negative thermal effects and the piezoelectric/photovoltaic effects in ultrafast strain is evidenced and can be regulated. The opposite lattice responses due to different thermal effects of the samples with different axial ratios are observed. The maximum UPS is up to 0.24%, comparable to that of conventional ferroelectric. The interaction between the thermal and ferroelectric effects in the induced strain could promote the diversified applications with the coupling of light, heat and electricity.

Mobile and immobile boundaries in ferroelectric films

The intrinsic mobile interfaces in ferroelectrics—the domain walls can drive and enhance diverse ferroelectric properties, essential for modern applications. Control over the motion of domain walls is of high practical importance. Here we analyse theoretically and show experimentally epitaxial ferroelectric films, where mobile domain walls coexist and interact with immobile growth-induced interfaces—columnar boundaries. Whereas these boundaries do not disturb the long-range crystal order, they affect the behaviour of domain walls in a peculiar selective manner. The columnar boundaries substantially modify the behaviour of non-ferroelastic domains walls, but have negligible impact on the ferroelastic ones. The results suggest that introduction of immobile boundaries into ferroelectric films is a viable method to modify domain structures and dynamic responses at nano-scale that may serve to functionalization of a broader range of ferroelectric films where columnar boundaries naturally appear as a result of the 3D growth.

Misfit strain-temperature phase diagram of multi-domain structures in (111)-oriented ferroelectric PbTiO3 films

High-index-oriented epitaxial ferroelectric films have recently attracted great attention due to their emergent functional properties related to the multi-domain structures. Here, the equilibrium phases and domain structures in (111)-oriented ferroelectric PbTiO3 films were investigated via phase-field simulations and the misfit strain-temperature equilibrium phase diagram was constructed. It was found that the epitaxial strain can effectively control the orientations of polarization vectors, which uniquely determine the domain variant patterns, domain wall orientations and densities. In particular, two types of tetragonal-like domain patterns can form under different strains with their distinct structural characteristics, ferroelectric properties and thermal conductivities. These results provide guidance for interpreting and understanding experimental data as well as designing of (111)-oriented ferroelectric films with specified domain structures and properties.

Domain-Matching Epitaxy of Ferroelectric Hf0.5Zr0.5O2(111) on La2/3Sr1/3MnO3(001)

Epitaxial ferroelectric HfO2 films are the most suitable to investigate intrinsic properties of the material and for prototyping emerging devices. Ferroelectric Hf0.5Zr0.5O2(111) films have been epitaxially stabilized on La2/3Sr1/3MnO3(001) electrodes. This epitaxy, considering the symmetry dissimilarity and the huge lattice mismatch, is not compatible with conventional mechanisms of epitaxy. To gain insight into the epitaxy mechanism, scanning transmission electron microscopy characterization of the interface was performed, revealing arrays of dislocations with short periodicities. These observed periodicities agree with the expected for domain matching epitaxy, indicating that this unconventional mechanism could be the prevailing factor in the stabilization of ferroelectric Hf0.5Zr0.5O2 with (111) orientation in the epitaxial Hf0.5Zr0.5O2(111)/La2/3Sr1/3MnO3(001) heterostructure.

Ultrafast Photoinduced Strain in Super-Tetragonal PbTiO3 Ferroelectric Films

The ultrafast photoinduced strain in ferroelectric coupling of piezoelectric and photovoltaic effects have been focused in the last decade, making them extend numerous applications including ultrafast optical memories, sensors and actuators with strain engineering. The mechanism of screening of the depolarization field by photoinduced carriers is generally accepted for ultrafast photoinduced strain in ferroelectric, while the thermal component of the strain is usually diluted as the offset and have not been systematically confronted, leading to unnecessary confusion. Herein, both the positive and negative thermal expansion effect in composite ferroelectric epitaxial films have been investigated by use of high repetition rate ultrafast X-ray diffraction, along with the piezoelectric and photovoltaic effects. The coupling of the positive/negative thermal effects and the piezoelectric/photovoltaic effects in ultrafast strain have been evidenced and regulated. The opposite lattice response due to different thermal effect of the different axial ratio samples have been observed. The maximum ultrafast photoinduced strain is up to 0.24%, comparable to that of conventional ferroelectric. The interaction of thermal and ferroelectric effect in the induced strain could promote the diversified application with the coupling of light, heat and electricity.

Electric-Field-Driven Nanosecond Ferroelastic-Domain Switching Dynamics in Epitaxial Pb(Zr,Ti)O_{3} Film.

Epitaxial oxide ferroelectric films exhibit emerging phenomena arising from complex domain configurations even at pseudoequilibrium, including the creation of domain states unfavored in nature and abrupt piezoelectric coefficients around morphotropic phase boundaries. The nanometer-sized domain configurations and their domain switching dynamics under external stimuli are directly linked to the ultrafast manipulation of ferroelectric thin films; however, complex domain switching dynamics under homogeneous electric fields has not been fully explored, especially at the nanosecond timescale. This Letter reports the nanosecond dynamics of ferroelastic-domain switching from the 90° to 180° direction using time-resolved x-ray microdiffraction under homogeneous electric fields onto an epitaxial Pb(Zr_{0.35},Ti_{0.65})O_{3} film capacitor. It is found that the application of electric fields induces spatially heterogeneous domain switching processes via intermediate domain structures with rotated polarization vectors. In addition, the domain switching time is shown to be inversely proportional to the magnitude of the applied electric field, and electric fields higher than 480 kV/cm are found to complete the ferroelastic switching within nanoseconds.