Activities are groupd in 6 workpackages (WP):

WP1. Theory and simulations. CO and P2 will collaborate for theoretical modelling of dopants in ferroelectrics.

WP2. Target preparation. CO and P3 will collaborate to realize the undoped and doped targets.

WP3. Growth of the epitaxial ferroelectric films. CO will grow the epitaxial ferroelectric layers by PLD.

WP4. Structural and chemical characterization of the epitaxial films. CO and P1 will collaborate for structural and chemical characterization using XRD, TEM, XPS techniques.

WP5. Electrical characterization of the epitaxial films and ferrotronic devices. CO will investigate the physical properties of epitaxial layers by complex electrical measurements and will characterize the ferrotronic devices for memory and photovoltaic applications.

WP6. Management, dissemination and patenting. Reporting, publishing and patenting activities.

Relation between WPs.

During the project implementation interesting results were obtained regarding the presence of negative capacitance effect in ferroelectric capacitors of PZT type.  This effect can help to reduce the power consumption in field effect transistors, including for non-volatile memories. The topic is intensively studied, with publications in prestigious journals, such as Nature. The Project Director is co-author to the article published in Nature 565, 464 (2019).s41586-018-0854-z

Considering the importance of the subject, research was initiated to investigate the presence of the negative capacitance effect in the samples grown in the frame of the project. The obtained results were published in Physical Review Applied.

The Project Director was the main organizer of the 6th edition of the International Workshop of Materials Physics (IWMP). Below are the first announcement and the program of IMP.

First Announcement

14-16 of September 2021

The National Institute of Materials Physics (NIMP) announces the organization of the 6^th edition of the International Workshop of Materials Physics (IWMP). The topic for 2021 edition is dedicated to ferroelectric and multiferroic materials, with special emphasis on thin films, multilayers, super-lattices and nano-objects. Aspects related to modeling, fabrication, characterization and potential applications will be presented and discussed.

Similar to the first five editions, the 6^th edition of IWMP is organized on invitation only. The aim is to attract well known researchers in the field, the final purpose being to establish new collaborations concretized in common publications, projects and exchange of personnel.

Young researchers willing to present their latest results on topics related to the main topic of the workshop are invited to submit a 2 page abstract (A4, Times New Roman 11, single spacing, 2 cm margins, including figures and references) to the organizers (pintilie@infim.ro). The best abstracts will be selected for oral presentations during the workshop.

The workshop will take place at NIMP premises located in Magurele, Romania.

Workshop Program

14 of September

9:00 – 9:10 Opening

9:10-9:50: Liliana Mitoseriu: Scale-dependent properties in BaTiO₃ ceramics with structural instability

9:50-10:30: Antonio Casares (on-line): Multi-Modal-Correlative Microscopy (sponsor presentation, Zeiss)

10:30-11:10: Maryline Guilloux-Viry (on-line): Niobates Ferroelectric Thin Films: i/ Growth and Characterization of Perovskite and TTB Phases in the K-Na-Nb-O System; ii/ Potential of Application in High Frequency Miniature Tunable Devices

11:10 – 11:30 coffee break

11:30-12:10: Michael Springborg: Shapes (of) Matter

12:10-12:50: Ibrahim Burc Misirlioglu (on-line): Interface limited stability of ferroelectricity in thin film heterostructures: Electrostatic interactions, elasticity effects and phase coexistence

12:50-13:30: Jorge Iniguez: Optimizing steady-state negative capacitance

13:30 – 14:30 lunch

14:30-15:10: Josep Fontcuberta: Electron transport and plasmonic response of metallic oxides

15:10-15:50: Valerie Demange: Oxide nanosheets as seed layers for epitaxial growth of complex oxides

15:50-16:30: Brian Rodriguez: AFM tip-induced strain effects in BiFeO₃ films: from structural phase changes to polarization switching and nanofabrication

16:30 – 16:45 coffee break

16:45-17:25: Marty Gregg (on-line): Conducting Ferroelectric Domain Walls: Fundamentals of Transport and Device Opportunities

17:25-18:05: Alexei Gruverman (on-line): Unconventional Dynamics of Domain Walls in Uniaxial Ferroelectric Lead

Germanate

19:15 dinner

15 of September

9:00-9:40: Marin Alexe: Incommensurate spin crystal phases in ferromagnetic and ferroelectrics

9:40-10:20: Brahim Dkhil: Playing with domains in ferroelectric polymers and hidden states in relaxors for neuromorphic computing

10:20-11:00: Gustau Catalán: Switching dynamics and “giant” electrocooling effect of antiferroelectric PbZrO3.

11:00 – 11:20 coffee break

11:20-12:00: Vincent Garcia: Controlling antiferromagnetic textures in BiFeO₃ multiferroic thin films

12:00-12:40: Nathalie Jedrecy (on-line): Electro- and magneto- resistance in perovskite-based multiferroic junctions

12:40-13:20: Pavlo Zubko (on-line): Domains and lattice curvature in ferroelectric superlattices and supercrystals

13:20 – 14:30 lunch

14:30-15:10 Torsten Granzow: Anomalous photovoltaic effect in low-leakage solution-deposited BiFeO₃ films: Influence of doping and substrate stress

15:10-15:50: Igor Stolichnov: Switching in HfO₂-based ferroelectrics: an insight from nanoscopic analysis

15:50-16:30: Marie-Helen Chambrier:  Ferroelectric state in a α-Ln2WO6 polymorphes stabilized in thin film

16:30 – 16:45 coffee break

16:45-17:25: Gregory S. Rohrer (on-line): High Throughput Studies of Metal Oxide Water Splitting Catalysts for the Development of Structure-Property Relations

17:25: 18:05: Eric L. Altman (on-line): Coupling Elastic, Electrostatic and Magnetic Responses in Transition Metal Silicate Monolayers

19:15 dinner

16 of September

9:00-9:40: Nick Barrett: Interface chemistry, oxygen vacancies, charge injection and polarization stability in ferroelectric hafnia-based films for non-volatile memories

9:40-10:20: Uwe Schroeder: Stabilization of the Ferroelectric Phase in Doped Hafnium Oxide Films: Influence of Dopants and Oxygen Vacancies

10:20-11:00: Athanasios Dimoulas (on-line): Scaling of HZO ferroelectric in Ge MFS for low power FTJ

11:00 – 11:20 coffee break

11:20-12:00: Bertrand Vilquin: Nanostructuration effect on the wake-up effect of Hf_0.5Zr_0.5O₂ capacitor

12:00-12:20: Cristian Mihail Teodorescu: Charge accumulation, conduction band filling and ferroicity

12:20-12:40: Georgia Boni: Dynamic dielectric characterization of ferroelectric capacitors- Evidencing negative capacitance

12:40-13:00: Leontin Padurariu: Dynamic Finite Element Method for describing complex dielectric properties in ferroelectric-based composites

13:00-13:20: Marius Husanu: Ferroelectricity and rhombohedral distortion in the electronic band structure of strained PbZrTiO₃

13:20 – 14:30 lunch

Group photo with the participants to 6th edition of IWMP.

Final Report 2022

Scientific Report Phase 2, 2019

Report Phase 3, 2020

Report Phase 4, 2021

Important results 

Theory and modelling-effect of dopants on PTO energetic structure 

Figure 1: Comparison of total DOS between the non doped PTO structure (a) and three cases of unsuccessful p-type dopant: b) Na substitution of Pb atom; c) Pb vacancy and d) Na substitution of Ti atom.

Figure 2: Total DOS comparison between the non doped PTO (top plot) and the spin polarized total DOS for the Fe substitution of Ti atom case (bottom plot). The red and blue lines represent the contribution to the DOS of the Fe atom (spin-up in red and spin-down in blue).

PZT thin films-properties 

It was evidenced that the background static dielectric constant in PZT is of low value and depends on the amount of structural defects and on the magnitude of the applied electric field. 

Fig. 3a) The thickness dependence of the dielectric constant evaluated from C-V measurements performed at 100 kHz. Evaluation was performed in three cases: at 0V “static”; at 0V “dynamic”; at maximum applied voltage “dynamic”; b) TEM images for 20 nm and 150 nm thick samples (inside each image the notations are a-low magnification image cross-section; b-SAED image; c-low magnification HR-TEM image; d-high magnification HR-TEM image of PZT/SRO interface and SRO/STO interfaces; these images demonstrate the high quality of the epitaxial growth). 

Memcapacitance and memcomputing was demonstrated in layered ferroelectric structures 

FIG. 4. Logic operation using an F-I -F capacitor. The representation of the polarization states in an F-I -F structure during different stages (initialization and computation) of a logic operation for the OR/NOR case (a) and for the AND/NAND case (b), together with the corresponding simulations of the logic operations obtained by changing the capacitance state (HCS or LCS) of the system using different combinations of pulses. The HCS and LCS states can have 0 or 1 values associated for logic operations and the results are memorized on the computation cell and can be accessed at any time. 

FIG. 5. Multiple stable states with continuous capacitive values. (a) A continuous spectrum of capacitance values, with stable intermediate states measured for the STO interlayer case at 1 kHz frequency with 0.5-V ac signal; insets show schematic illustrations of polarization configurations associated with distinct capacitive states. (b) An example of a voltage sequence combining pulses with different amplitudes and polarities used to access different capacitive states. (c) The piezoresponse phase signal obtained using the poling map: the upper PZT layer present totally reverses polarization toward the surface for negative applied bias (bright central rectangular zone) while for positive bias the polarization remains partially reversed, forming with 180◦ domain structure. (d) The piezoresponse phase signal obtained by applying the poling map with a voltage gradient on the totally reversed polarization area from (c): the switching of polarization takes place gradually with increasing the amplitude of voltage; different degrees of partial switching of polarization are obtained in the 8–37 V range. 

It was shown, by XPS analysis, that the compensation mechanism changes from intrinsic (with carriers generated in ferroelectric by self-doping) to extrinsic (with carriers from metal electrodes, as the electrode thickness increases.  

Variation of the atomic O/Ti ratio indicating the source of oxygen vacancy (a) for the system ferroelectric/metal, with different electrode metals and polarization orientations (b). (c) the charge profile associated to oxygen vacancy density for different thicknesses of the metal layer. (d) the band bending at the interface.  

Important results 2020

The effect of negative capacitance (NC) was evidenced in PZT epitaxial capacitors (Phys. Rev. Applied 14, 014080 (2020)). NC occurs during polarization switching, being accompanied by a large increase of the current flowing through the capacitor.

Fig. 2 a) the opening of the polarization hysteresis loop as the amplitude of the triangular voltage pulse is gradually increased (the red arrow mark the increase); b) the C-V characteristics derived from the hysteresis loops presented in fig. 2a. The arrows mark the increase and decrease of negative capacitance observed for voltages corresponding to the domains marked in fig. 2a).

Fig. 3 a) current hysteresis recorded during the hysteresis measurements (associated to hysteresis loops presented in fig. 2a); b) the voltage dependence of the maximum value of negative capacitance in the positive voltage range (see also the arrows in fig. 2a), together with the voltage dependence of the current recorded at maximum applied voltage during the hysteresis measurement.

It was proposed that, in high quality epitaxial PZT films, the polarizations switching may have place without formation of domains with opposite orientation of polarization, as suggested by the Figure below.

Fig. 4 a) the poling map; b) the voltage variation while the PFM tip scanned the line in Figure 4 a)-it starts with +5.33 V, then after about 0.1 µm drops to -5.33 V and starts to slowly increase to +5.33 V while the tip is moving on the surface of the sample, after which drop again to -5.33 V for about 0.1 µm and suddenly change to +5.33 V for the final 0.1 µm; c) the phase contrast after applying the poling map from Figure 4 a); the phase variation while the tip scans the line in Figure 4 c).

Pure target was prepared (99.99 % purity) and films were deposited both from this target and from a commercial target with 99.9 % purity. The structural and physical characterization revealed important differences: c-lattice constant is slightly larger for the film deposited from commercial target; polarization is slightly lower for the film deposited from pure target; leakage current is lower in the film deposited from pure target and the potential barrier at electrode interfaces are higher. All these differences are attributed to anion impurities that are present in the commercial target and that an affect the electronic properties but also the strain in the lattice.

TEM and SAED images of the films deposited from commercial target (upper line) and pure target (lower line)

Measured leakage current in films deposited from commercial target (line) and pure target (markers).

First attempts were made to deposit doped films. Co-deposition method was used meaning successive deposition of layers from the pure PZT target and from Nb oxide or Fe oxide targets. TEM studies revealed that co-deposition is perturbing the epitaxial growth. However, significant differences were obtained in the electrical properties.

TEM images for the Nb doped PZT by co-deposition.

An interesting results was obtained when graphene was deposited on ferroelectric PZT. The graph below shows the conductance hysteresis in graphene, produced by polarization switching (published in RSC Adv., 2020, 10, 1522).

Left-the resistance hysteresis in graphene deposited on PZT; right-schematic showing how polarization switching in PZT can produce the hysteresis in the resistance of graphene sheet.

Some important results were obtained in 2021.

First, it was shown that there is s direct link between structural quality, polarization reversal (homogeneous versus non-homogeneous) and the amplitude of the dynamic/transitory, differential negative capacitance effect. The experimental results from electric and PFM investigations point out that the switching in high quality epitaxial films is, most probably, homogeneous, with a large amplitude of the dynamic/transitory NC effect associated to polarization switching, while in polycrystalline films the switching is non-homogeneous, with weak NC effect.

TEM images of PZT films deposited by PLD on SRO/STO substrates (epitaxial quality) or by sol-gel on Pt/Si substrates (polycrystalline).

Electric measurements on PZT epitaxial (left) and polycrystalline films (right)

PFM images (topography, amplitude and phase) for epitaxial PZT (left) and polycrystalline (right) films.

A last hour result is the change of polarization orientation by changing the doping of the epitaxial PZT from n-type (Nb doping) with polarization upward to p-type (Fe doping) with polarization downward. The evidence is in the PFM phase images below.

Box-in-box poling map (a) of PZT-Nb (b), PZT-Fe (c) and undoped PZT (d). One can observe that the surface outside the boxes has the same color as the inside box for PZT-Nb and undoped PZT, meaning polarization upward in n-type PZT (undoped PZT is n-type due to oxygen vacancies). For the PZT-Fe the surface outside the boxes has the same color as the outer box, meaning polarization downward in p-type PZT (results published in Scientific Reports 12, 755 (2022)).

The electric and electronic properties of PZT-Nb and PZT-Fe are summarized in the next table.

Interesting results were further obtained in 2022, in collaboration with project partners but also in collaboration with external partners.

For example, in collaboration with researchers from Demokritos research center from Greece, we have demonstrated the presence of a metastable ferroelectric phase in very thin HZO layers (results published in COMMUNICATIONS PHYSICS 5, 178 (2022))

Structural investigation of the very thin HZO layer, demonstrating a good crystallization and the presence of the orthorhombic (O) ferroelectric phase.

Another interesting result is that ferroelectricity modulates the polaronic coupling at multiferroic interfaces. The study was performed on PZT and BTO thin films grown on LSMO substrates, and the results were recently published in COMMUNICATIONS PHYSICS 5, 209 (2022).

a High resolution high-angle annular dark-field scanning transmission electron microscopy image at the
PbZr0.2Ti0.8O3-La0.7Sr0.3MnO3 (PZT|LSMO) interface, and Ti L, Mn L and La M maps at atomic level. The black arrow indicates the first ferroelectric
unit cell (b) Theoretical Hubbard U - corrected density functional theory band structure for SrTiO3-strained pseudocubic LSMO and (insets) local
density of states integrated ± 0.2 eV around the Г and A bands bottom. (c) Calculated Fermi surface (FS) for strained LSMO. d, e In-plane FS maps for
respectively bare LSMO and BaTiO3 (BTO) interface, LSMO|BTO in the ГMX planes recorded with hv = 643 eV. f, g In-plane FS maps for respectively
bare LSMO and BaTiO3 (BTO) interface, BTO|LSMO in the XZAR.plane recorded with hv = 708 eV. h Experimental out-of-plane FS maps for the BTO|
LSMO heterostructure in the XYMR and ΓXxXz planes. Colored frames enclosing the out-of-plane FSs encode the probed planes with the corresponding
colors from (c).

Table showing modulation of carrier concentration due to ferroelectricity.

There are also other interesting results which were not published yet. For example, it was found that the I-V characteristic of a ferroelectric p-n homojunction based on tetragonal PZT is quasi-linear, suggesting a resistive like behavior (see the graph below for a p-n homojunction with equal thicknesses of the component layers, of 100 nm). n-type conduction was obtained by doping with Nb, while p-type conduction was obtained by doping with Fe.

This behavior was explained assuming that the total current flowing through the ferroelectric p-n homojunctions is formed by summing the electron and hole currents injected at the electrode interfaces:

These results were presented for the first time at the international conference E-MRS Fall Meeting, held in Warsaw.

At present the PI is working on a new model for polarization switching in displacive ferroelectrics, based on the hypothesis that the switching is completely homogeneous, flip-flop type, and is triggered by the charge injection at electrode interfaces (invited presentations at E-MRS Spring Meeting and AMSE 2022, both presentations were on-line).

The hypothesis is base on the observation that the increasing part of the current peak recorded during polarization switching has a quasi-linear dependence on the applied voltage, therefore a conductance/resistance can be estimated from the slope of the I-V representation. This has voltage, frequency and temperature dependence, as shown in the above graphs.

The equation for the current density in the case of a Schottky injection influenced by the bulk mobility of the injected carriers suggests that the exponent can be negligible for high polarization values, low values of the dielectric constant, and large values of the applied voltage. In this case, when the exponential term is negligible (close to unity), the current density has an ohmic like form, as observed experimentally.

A sketch showing how the injected chares can trigger the polarization switching, just because they destabilize the previous charges involved in the compensation of the depolarization field.

The Project Director is Professor (associated) to the Doctoral School of Physics of the University of Bucharest.

Currently he has 4 PhD students, 2 of them being in the team of the project.

The Responsible of P1 is also Professor (associated) to the Doctoral School of Physics of the University of Bucharest.

He has 2 PhD students in the team.

The Responsible of P3 is Professor at University Politechnica Bucharest and has 2 PhD students in the team.

Updated list of publications

Deposition of epitaxial ferroelectric films by PLD.

Complex electrical, photovoltaic and pyroelectric characterization of ferroelectric based structures.

Development of applications, such as memory devices, field effect transistors or pyroelectric detectors.