Ferroelectric YMnO3 Research Articles

Origin of the Critical Thickness in Improper Ferroelectric Thin Films

Improper ferroelectrics are expected to be more robust than conventional ferroelectrics against depolarizing field effects and to exhibit a much-desired absence of critical thickness. Recent studies, however, revealed the loss of ferroelectric response in epitaxial improper ferroelectric thin films. Here, we investigate improper ferroelectric hexagonal YMnO3 thin films and find that the polarization suppression, and hence functionality, in the thinner films is due to oxygen off-stoichiometry. We demonstrate that oxygen vacancies form on the film surfaces to provide the necessary charge to screen the large internal electric field resulting from the positively charged YMnO3 surface layers. Additionally, we show that by modifying the oxygen concentration of the films, the phase transition temperatures can be substantially tuned. We anticipate that our findings are also valid for other ferroelectric oxide films and emphasize the importance of controlling the oxygen content and cation oxidation states in ferroelectrics for their successful integration in nanoscale applications.

Charged domain boundaries stabilized by translational symmetry breaking in the hybrid improper ferroelectric Ca3\u2013xSrxTi2O7

Charged domain walls and boundaries in ferroelectric materials display distinct phenomena, such as an increased conductivity due to the accumulation of bound charges. Here, we report the electron microscopy observations of atomic-scale arrangements at charged domain boundaries in the hybrid improper ferroelectric Ca2.46Sr0.54Ti2O7. Like in the prototype improper ferroelectric YMnO3, we find that charged domain boundaries in Ca2.46Sr0.54Ti2O7 correspond to out-of-phase boundaries, which separate adjacent domains with a fractional translational shift of the unit cell. In addition, our results show that strontium ions are located at charged domain boundaries. The out-of-phase boundary structure may decrease the polarization charge at the boundary because of the ferrielectric nature of Ca2.46Sr0.54Ti2O7, thereby promoting the stabilization of the charged state. By combining atomic-resolution imaging and density-functional theory calculations, this study proposes an unexplored stabilization mechanism of charged domain boundaries and structural defects accompanying out-of-phase translational shifts.

Prospects for application of ferroelectric manganites with controlled vortex density

There is an urgent need for multifunctional materials that can reduce the energy demands of microelectronic devices. In ferroelectric manganites (RMnO3), R = Tm, Lu, Er, Ho, Y, Yb, the spontaneous formation of one-dimensional (1D) closed and open loop vortices has been observed when the ferroelectric manganite is cooled over its ferroelectric ordering temperature [Li et al., Phys. Chem. Chem. Phys. 22, 14415–14432 (2020)], namely, 621 K (TmMnO3), 730 K (LuMnO3), 833 K (ErMnO3), 875 K (HoMnO3), 914 K (YMnO3), and 1350 K (YbMnO3). The applicability of ferroelectric YMnO3 thin films as an electroforming-free, unipolar memristor for artificial intelligence [Rayapati et al., Nanotechnology 31, 31LT01 (2020); J. Appl. Phys. 126, 074102 (2019); 124, 144102 (2018)] as the light-emitting material for double-sided electroluminescent devices [Schmidt et al., German patent pending DE102018117210.0 (17 July 2018)] and as a p-type conducting material with a large Seebeck coefficient for thermoelectrics has been demonstrated. It is expected that the vortices in ferroelectric manganites are highly conducting at room temperature. In this report, envisioned disruptive innovations based on ferroelectric manganite thin films with a top electrode and a bottom electrode where the vortex density can be reconfigured by an electric field are presented and discussed, namely, electroluminescence illumination, ultrasensitive thermoelectrics, and artificial intelligence and cryptography. Finally, a short outlook to potential applications of manganites whose vortex density is controlled by temperature gradients, electric field ramps, and light pulses in the area of low loss transformers, single photon detectors, and 5G components is given.

Tailoring the band gap of ferroelectric YMnO3 through tuning the Os doping level

Perovskite-oxide materials have grabbed enormous attention from various research groups all over the world due to their large application areas. The band-gap engineering of those materials are important for optoelectronic researches especially for ferroelectric (FE) solar cells that have unique features such as having higher open circuit voltages than the band gap and their spontaneous polarization which leads to photovoltaic effect. Nevertheless, the most of the perovskite FE materials have wide band gaps that hamper the absorption of large solar spectrum. In the present study, it has been demonstrated the band gap of YMnO3 (YMO), which is one of the mostly studied FE materials, can be tuned via doping osmium (Os) into manganese (Mn) site. The band gap of YMO, 2.10 eV successfully is lowered to 1.61 eV. Polycrystalline YMnO3 and YMn1−xOsxO3 (YMOO) (x = 0.01, 0.05, 0.10) thin films were synthesized on indium tin oxide (ITO) substrates at 500 °C by magnetron sputtering method. Their structural, chemical and optical band-gap properties were studied and the results showed the Os doped YMO compounds could be a potential candidate for future ferroelectric solar cell studies.

Electron beam‐induced dynamic evolution of vortex domains and domain walls in single crystalline YMnO3

AbstractThe ability to reversibly switch ferroelectric polarization directions with small driving force is of interest both for fundamental researches and for novel applications. Here, the dynamic evolution of vortex domains in hexagonal ferroelectric YMnO3 is systematically demonstrated. Driven by propagations of charged domain walls (CDWs), vortex domains can be reversibly switched by merely utilizing electron beam irradiation. Dislocations pining at domain walls impede their motions, contributing to the irreversible evolution. Moreover, none of six ferroelectric domains in a vortex can vanish during such evolution due to the topological protection. Our results suggest that even with small external stimuli, CDWs in improper ferroelectrics can be considerably active and altered, providing a perspective on potential applications in ferroelectric storage.

Spontaneous formation of circular and vortex ferroelectric domain structure in hexagonal YMnO3 and YMn0.9Fe0.1O3 prepared by low temperature solution synthesis

We report an experimental study of the domain structure of ferroelectric YMnO3 and YMn0.9Fe0.1O3 using polycrystalline samples prepared by direct hydrothermal crystallisation at 240 °C, well below their structural phase transition temperatures. Powder X-ray diffraction shows the expected P63cm space group for both samples with an increase in a and a small decrease in c with Fe incorporation, consistent with an adjustment of MnO5 tilting, while XANES spectra at the Mn and Fe K edges show the oxidation state of both metals are maintained at +3 in the doped sample. High resolution TEM shows that curved stripe, annular and vortex domains can all be observed in the YMnO3 crystals, proving that the structural phase transition is not the only driving force for the occurrence of the annular and vortex domains. Furthermore, the absence of the annular and vortex domains in YMn0.9Fe0.1O3 indicates that the tilting state of MnO5 bipyramids plays an important role in the domain formation. Atomic resolution STEM images confirm that the ferroelectric domain walls correspond to structural antiphase boundaries similar to the crystals made via high temperature solid-state reactions.

Investigation of magnetic, dielectric and electrical properties of Ce-substituted YMn0.8Fe0.2O3 multiferroic ceramics

Y1−xCexMn0.8Fe0.2O3 (x = 0, 0.05 and 0.1) solid solution ceramics were prepared and the magnetic, dielectric and electrical properties were investigated. The structures of the present ceramics were maintained in hexagonal P63cm space group as the ferroelectric YMnO3. The magnetic structures of the solid solution ceramics were not uniform, and all the compositions were predominantly antiferromagnetic with weak ferromagnetic properties at low temperature. The antiferromagnetic transition temperatures of the present ceramics were decreased upon Ce substitution, originating in the weakened antiferromagnetic exchange interactions of Mn3+ ions. Varied valence distributions of Fe and Mn ions led to the effective paramagnetic moment decreasing with Ce introduction. Compared with YMn0.8Fe0.2O3, the dielectric loss of Y0.95Ce0.05Mn0.8Fe0.2O3 ceramics was reduced and the resistivity was enhanced, which should be related to the decreased Mn2+ content in the mixed structure of Mn ions. Whereas, the presence of Mn3+–Mn4+ ions pair led to the increased conductivity and dielectric loss in the Y0.9Ce0.1Mn0.8Fe0.2O3 ceramics.

Electrostatic topology of ferroelectric domains in YMnO3

Trimerization-polarization domains in ferroelectric hexagonal YMnO3 were resolved in all three spatial dimensions by piezoresponse force microscopy. Their topology is dominated by electrostatic effects with a range of 100 unit cells and reflects the unusual electrostatic origin of the spontaneous polarization. The response of the domains to locally applied electric fields explains difficulties in transferring YMnO3 into a single-domain state. Our results demonstrate that the wealth of nondisplacive mechanisms driving ferroelectricity that emerged from the research on multiferroics are a rich source of alternative types of domains and domain-switching phenomena.

Y–O hybridization in the ferroelectric transition of YMnO3

The electron density distribution of paraelectric and ferroelectric YMnO3 are investigated by analyzing high temperature synchrotron radiation powder diffraction data using the maximum entropy method (MEM) and MEM-based pattern fitting combined with the Rietveld method. The results show that chemical bonding by orbital hybridization is established between the Y and in-plane O ions along the polar c-axis below the ferroelectric transition temperature. We suggest that the hybridization observed in the ferroelectric phase is the driving force for ferroelectricity in YMnO3.

Control of the epitaxial orientation and reduction of the interface leakage current in YMnO3/GaN heterostructures

Epitaxial ferroelectric YMnO3 (YMO) thin films with high crystalline quality were fabricated on (0 0 0 1) GaN substrates by pulsed laser deposition followed by rapid thermal annealing. X-ray diffraction and transmission electron microscopy measurements reveal that YMO layers are of (0 0 0 1) orientation with an in-plane epitaxial relation (0 0 0 1)YMO‖(0 0 0 1)GaN, . The orientation of the lattice of YMO was found to be controlled by the annealing conditions. Y2O3 nanocrystallites observed near the interface were possible seeds to stabilize the orientation of the epitaxial YMO thin films. The appropriate annealing treatment could improve the crystalline quality and reduce the interface leakage current.

Temperature- and field-dependent leakage current of epitaxial YMnO3/GaN heterostructure

Epitaxial ferroelectric YMnO3 (YMO) thin films were fabricated on (0001) GaN substrates by pulsed laser deposition followed by rapid thermal annealing. The temperature and field dependence of the leakage current of YMO/GaN interface was studied in a temperature range from 150 to 300 K and for an applied voltage up to 10 V. In a low temperature region from 180 to 220 K, the YMO/GaN interface acted as a Schottky barrier with a height of 0.27 eV for a field below 1.4 MV/cm, while the leakage mechanism was governed by the Fowler–Nordheim tunneling for a field above 1.4 MV/cm. Moreover, a space-charge-limited-current behavior was observed in a high field for a temperature above 270 K, while an Ohmic behavior was observed in a low field. In comparison, the dominant leakage mechanism of In/YMO interface was an Ohmic behavior in the whole measured voltage and temperature ranges.

Electric field effects on magnetotransport properties of multiferroic Py/YMnO3/Pt heterostructures

We report on the exchange bias between antiferromagnetic and ferroelectric hexagonal YMnO3 epitaxial thin films sandwiched between a metallic electrode (Pt) and a soft ferromagnetic layer (Py). Anisotropic magnetoresistance measurements are performed to monitor the presence of an exchange bias field. When the heterostructure is biased by an electric field, the exchange bias field is suppressed. We discuss the dependence of the observed effect on the amplitude and polarity of the electric field. Particular attention is devoted to the role of current leakage across the ferroelectric layer.

Electric-Field Control of Exchange Bias in Multiferroic Epitaxial Heterostructures

The magnetic exchange between epitaxial thin films of the multiferroic (antiferromagnetic and ferroelectric) hexagonal YMnO3 oxide and a soft ferromagnetic (FM) layer is used to couple the magnetic response of the FM layer to the magnetic state of the antiferromagnetic one. We will show that biasing the ferroelectric YMnO3 layer by an electric field allows control of the magnetic exchange bias and subsequently the magnetotransport properties of the FM layer. This finding may contribute to paving the way towards a new generation of electric-field controlled spintronic devices.

Influence of Schottky and Poole–Frenkel emission on the retention property of YMnO3-based metal/ferroelectric/insulator/semiconductor capacitors

The relationship between the memory retention properties and the leakage current density of Pt/YMnO3/Y2O3/Si capacitors was discussed. The leakage current of the ferroelectric YMnO3 layer was varied by temperature and by annealing the capacitors. It was found that the retention time became shorter upon increasing the leakage current density at the retention voltage. The retention time was prolonged from about 103 to up to 104 s when the leakage current density was reduced to 2×10−9 A/cm2 by annealing under N2 ambient. The analysis of the leakage current revealed that the retention degradation mechanism was related to the Schottky emission. From the temperature dependence of the leakage current and the pseudo isothermal capacitance transient spectrum, it was found that applied voltage with an unnecessarily long time to polarize the ferroelectric layer generated Frenkel defects in the ferroelectric layer, and that the amount of the defects greatly affected the memory retention time. These results suggest that to improve the memory retention properties, reducing the defect density of the ferroelectric layer is important, as well as lowering the Schottky current.

Electrical properties of ferroelectric YMnO3 films deposited on n-type Si(111) substrates

YMnO3 thin films were grown on an n-type Si substrate by nebulized spray pyrolysis in the metal–ferroelectric–semiconductor (MFS) configuration. The capacitance–voltage characteristics of the film in the MFS structure exhibit hysteretic behaviour consistent with the polarization charge switching direction, with the memory window decreasing with increase in temperature. The density of the interface states decreases with increasing annealing temperature. Mapping of the silicon energy band gap with the interface states has been carried out. The leakage current, measured in the accumulation region, is lower in well-crystallized thin films and obeys a space-charge limited conduction mechanism. The calculated activation energy from the dc leakage current characteristics of the Arrhenius plot reveals that the activation energy corresponds to oxygen vacancy motion.

Etching mechanism of YMnO3 thin films in Cl2/Ar gas chemistries

Ferroelectric YMnO3 thin films are excellent dielectric materials for high integrated ferroelectric random access memory with a metal–ferroelectric–silicon field effect transistor structure. In this study, YMnO3 thin films were etched with Cl2/Ar gas chemistries in inductively coupled plasma. The maximum etch rate of YMnO3 thin films is 285 Å/min under Cl2/(Cl2+Ar) of 1.0, 600 W/−200 V and 15 mTorr. The selectivities of YMnO3 over CeO2 and Y2O3 are 2.85 and 1.72, respectively. The results of x-ray photoelectron spectroscopy (XPS) reflect that Y is removed dominantly by chemical reaction between Y and Cl, while Mn is removed more effectively by Ar ion bombardment than chemical reaction. The results of the secondary ion mass spectrometer were equal to these of XPS. The etch profile of the etched YMnO3 film is approximately 65° and free of residues at the sidewall.

Characteristics of ferroelectric YMnO3 thin films for MFISFET by MOCVD

The ferroelectric YMnO3 and Y2O3 thin films for the Metal/Ferroelectric/insulator/Semiconductor field effect transistors (MFISFET) structure were deposited in-situ using metalorganic chemical vapor deposition (MOCVD). The C-V characteristics had a hysteresis curve with a clockwise rotation, which indicates ferroelectric polarization switching behavior. When the gate voltage sweeps from +5 to −5 V, the memory window of the Pt/YMnO3/Y2O3/Si gate capacitor annealed at 850°C in 75 mTorr(oxygen pressure) was about 1.4V. The typical leakage current density of the films annealed at 850°C in 75 mTorr(oxygen pressure) was about 10−7A/cm2 at applied voltage of 5 V.

Low-temperature synthesis in vacuum of c-axis oriented ferroelectric YMnO3 thin films using alkoxy-derived precursors

Hexagonal YMnO3 thin films were prepared on Pt(111)/TiOx/SiO2/Si and Pt(111)/Al2O3(0001) substrates using alkoxy-derived precursor solutions. The films were prepared by spin coating the YMnO3 precursor solutions and then, the films were calcined and crystallized via rapid thermal annealing in oxygen or vacuum ambient. The annealing conditions and substrates were critical for crystallization of ferroelectric YMnO3 films. When annealed in vacuum, the films both on Pt(111)/TiOx/SiO2/Si and Pt(111)/Al2O3(0001) substrates crystallized to hexagonal YMnO3 and the orientation depended on the substrates. The film on Pt(111)/Al2O3(0001) had c-axis orientation and the film on Pt(111)/TiOx/SiO2/Si had no preferred orientation. In addition, it was found that crystallization behavior, orientation and morphology of YMnO3 films on Pt(111)/TiOx/SiO2/Si substrates depended on the annealing condition. The heat-treatment in vacuum at initial stage for crystallization affected the restraint of perovskite phase and formation of hexagonal phase. The following heat-treatment in oxygen promoted the c-axis orientation and grain growth. The optimum annealing procedure for crystallization of the c-axis oriented YMnO3 films on Pt(111)/TiOx/SiO2/Si was addressed.

Aging effect on the ferroelectric property of YMnO3 thin film

Preferential [0001] oriented ferroelectric YMnO3 thin films were prepared on Si substrates using inorganic precursors by sol-gel method. X-ray diffraction (XRD) measurement shows that the peak shifts were reduced through aging. Wider C-V window was obtained on the aged sample at room temperature. This is attributed to that the film stress relieves through aging and so the ferroelectricity can be fully displayed.

Characteristics of Pt/YMnO[sub 3]/Y[sub 2]O[sub 3]/Si structure using a Y[sub 2]O[sub 3] buffer layer grown by pulsed laser deposition

The Pt/YMnO3/Y2O3/Si structure for a metal/ferroelectric/insulator/semiconductor (MFIS) field effect transistors was fabricated and the effect of a Y2O3 layer on the properties of MFIS structure was investigated. The Y2O3 thin films on p-type Si(111) substrate growth by pulsed laser deposition were crystallized along (111) orientation irrespective of the deposition temperatures. Ferroelectric YMnO3 thin films deposited directly on p-type Si (111) by metalorganic chemical vapor deposition resulted in a Mn deficient layer between Si and YMnO3. However, YMnO3 thin films having good quality and stoichiometric composition can be obtained by adopting a Y2O3 buffer layer. The memory window of the YMnO3 thin films with a Y2O3 film is greater than that of the YMnO3 thin films without a Y2O3 film after the annealing at 850 °C in vacuum ambient (100 mTorr). The memory window is 1.3 V at the applied voltage of 5 V.