National Institute Of Materials Physics - Romania

Multifunctional Materials and Structures Laboratory

Group of Complex Heterostructures and Perovskite Oxides

Main research topics:

1) ferroelectric materials and related structures for electronic, optoelectronic and sensing applications (including non-volatile memories, UV and IR detectors, piezoelectric devices);

2) materials and structures for photovoltaic conversion and light/particle detection (including perovskite solar cells and Si particle detectors);

3) bio-compatible materials and other materials with potential applications in medicine or life sciences.

Main scientific achievements (015-2019).

  • Development of novel ferroelectric multilayers for non-volatile memory applications. The multi-layers are formed by alternating ferroelectric and insulating films so that the structure starts and ends with a ferroelectric film. It was found that such structures presents: multiple polarization states, therefore can be used to dramatically increase the memory density from only 2 states in classic ferroelectric memories to 2n in multilayers, with n being the number of ferroelectric layers in the structure; the memory state can be read non-destructively by measuring the capacitance, resulting 2n-1 independent capacitance states; logic memories can be performed with the multi-layer structure, and the final result can be stored in the same cell, concluding the new structure acts as a memcapacitor (see figure 5). The results were published in Nanoscale and Physical Review Applied (NANOSCALE 9(48), 19271-19278 (2017); PHYSICAL REVIEW APPLIED 12, 024053  (2019); PHYSICAL REVIEW APPLIED 8, 034035 (2017))
Fig. 5 Tri-layer ferroelectric-insulator-ferroelectric structure acting as memcapacitor (PHYSICAL REVIEW APPLIED 12, 024053  (2019)).
  • Research on Perovskite Solar Cells (PSC) – Small and large area devices with PCE of 15.4% and 7%, respectively, stable over 3 years time (10% loss in PCE in the first month) have been developed, all the fabrication steps being performed in normal open laboratory conditions. It has been demonstrated that the electro-migration of iodine through the solar cell and subsequent degradation of the PSC performance, is triggered by the presence of metallic contacts with significantly different workfunction compared to front electrode and/or high chemical reactivity with the compounds of the layers in proximity (J. PHYS. CHEM. LETTERS   7, 5168-5175, 2016). Theoretical models for layers / interfaces have been developed as well as a thorough description of the dynamic electrical behaviour within this type of solar cells. This way a unified description of different hysteresis types (normal, inverted, mixed) have been obtained and a set of guidelines was proposed for a proper characterization of the PCE and of the hysteresis phenomena (J. PHYS. CHEM. C 121, 11207-11214, 2017; SOLAR ENERGY MATERIALS AND SOLAR CELLS   159, 197-203, 2017;  SOLAR ENERGY  173,  976-983, 2018;  J. PHYS. CHEM. C 7, 5267-5274, 2019). In addition, specific printing technique and equipment for depositing large area of mesoporous TiO2 layers in a PSC have been developed (A00364/2018 & A00195/2017 patents; ENERGY TECHNOL. 8, UNSP 1900922, 2020).
Fig. 6 Evolution of power conversion efficiency for the solar cells produced in the group; schematic structure of the solar cell; the reticulated TiO2 layer.
  • Novel results were obtained regarding the intrinsic properties of epitaxial PZT thin films. Self-doping was evidenced as primarily compensation mechanism in very thin epitaxial films. The lack of structural defects in such films leads to very low values of the background static dielectric constant, of about 20-25. Novel theoretical models were developed to calculate the polarization value and to assess the polarization stability in very thin ferroelectric films. Results were published in Scientific Reports and New Journal of Physics (SCIENTIFIC REPORTS 9, 14698 (2019); SCIENTIFIC REPORTS 5, 14974 (2015); NEW JOURNAL OF PHYSICS 21, 113005 (2019)).
  • The development of new materials as electrodes for solid oxide fuel cells (SOFCs). Due to their advantages, such as high theoretical efficiency (above 80%), low costs, flexibility regarding the fuel and good stability in time, with good performance in intermediate temperature (600–800 °C) SOFCs are receiving much attention compared to other fuel cells. By tuning the chemical composition and designing the synthesis methods, the catalytic materials investigated in our studies exhibit high resistance to carbon deposition and sulfur tolerance and therefore a high stability over long operating period due to their higher reducibility degree and surface oxygen vacancies. Our findings open a new perspective on the assessment of efficient electrodes for SOFCs. The results were published in Catalysis Science & Technology and Applied Catalysis B: Environmental (CATALYSIS SCIENCE & TECHNOLOGY, 8, 1333–1348 (2018); CATALYSIS SCIENCE & TECHNOLOGY, 9, 2351–2366 (2019); APPLIED CATALYSIS B: ENVIRONMENTAL, 241, 393-406 (2019)).
  • Delineation of structural and compositional architectures of biofunctional silica-based glass implant-type coatings (see figure 6). The implant coatings consist of silica-based bioactive glasses with designed degrees of network connectivity which can facilitate the release of therapeutic ions (i.e., osteogenic: Ca, Sr; antioxidant:  Sr; and/or antimicrobial: Zn) at certain rates into the biological environment. Magnetron sputtering was selected as deposition method of choice, due to its ability to be scaled up to industrial level. It was found that by the reduction or elimination of alkalis (and their partial or complete substitution with CaO, MgO, Sr and/or ZnO) from the glass formulation, one can tailor the coefficient of thermal expansion to match more closely that of titanium and its medical grade alloys. This is consequenting into improved mechanical properties of the silica-based coatings deposited onto both flat or real titanium implants (the latter were assessed by “cold-implantation”). Overall, the silica-based glass sputtered bioglass coatings elicited remarkable in vitro biomineralization capacity (under the correct homeostatic conditions) and cytocompatibility in fibroblast, endothelial and stem cell cultures, opening the path towards their future in vivo testing in animal model. The results were published in: J MECH BEHAV BIOMED MATER, 51 (2015) 313–327; ACS APPL MATER INTERFACES, 8 (2016) 4357–4367; INT J NANOMED, 12 (2017) 683–707; MATERIALS, 11 (2018) 2530; CERAM INT 45 (2019) 4368–4380.
Fig. 6: (a,b) SEM and (c–e) TEM images of (a) arrays and (b–e) individual BG submicrometer cones grown by radio-frequency magnetron sputtering at a working pressure of 0.4 Pa argon onto Ti substrates. Inset (e): Typical SAED pattern of a BG cone (ACS APPL MATER INTERFACES, 8 (2016) 4357–4367).
  • . It was reported for the first time the successful integration of aluminum nitride (AlN) as gate dielectric and In:Ga:Zn:O (IGZO) as channel semiconductor in transparent thin film transistors (TTFT). The original idea of using AlN films as a viable TTFT gate dielectric solution, derives from its low cost, abundant source materials, low temperature synthesis (~50 °C), good thermal conductivity, coefficient of thermal expansion matching IGZO, and excellent stability at high temperatures. The architecture used for the analyzed transistors is the staggered bottom-gate, the devices patterning being performed using photolithography and lift-off processes. In order to have good electrical performance, the IGZO channel length and width were fixed to 20 µm and 10 µm, respectively. The devices showed an Ion/Ioff of 2.5×106, a subthreshold slope of 0.44 V/dec, low leakage currents of ~10-13A and saturation voltages lower than 5 V (for VGS up to 5 V). The AlN/IGZO interface was investigated through capacitance/conductance-voltage (C/G-V) measurements. The results were published in Applied Surface Science 379 (2016) 270; WOS:000376819300036.
  • Mechanical, Corrosion and Biological Properties of Room-Temperature Sputtered Aluminum Nitride Films with Dissimilar Nanostructure. It was evidenced the potential use of AlN in biological sensors. This inter-disciplinary study brought new insights about AlN cytotoxicity and its corrosion rate in biomimetic medium. The corrosion assays, relevant in view of developing in vivo sustainable devices, were performed in both saline solution and DMEM-FBS biomimetic medium. The results emphasized the superior performance of the well-crystallized and highly c-axis textured AlN films. A corrosion rate of ~0.13 µm/year was deduced for such AlN structures. The in vitro tests in fibroblast cell (Hs27) cultures endorsed the cytocompatibility of AlN thin films, regardless of their structural quality, allowing good adherence, healthy morphology and great proliferation of the cells (with respect to both bare Si substrate and polycarbonate biological control). The cellular mortality index, determined by quantifying the activity of the LDH enzyme, is comparable to control, having values below 2% from the total cell number. The results were published in Nanomaterials 7 (2017) 394; WOS:000416783800049.
  • Tunable microwave capacitors based on ferroelectric thin films. Ba2/3Sr1/3TiO3 (BST) and 0.92(Bi0.5Na0.5)TiO3-0.08BaTiO3 (BNT–BT0.08) ferroelectric thin films were integrated in planar inter-digitated capacitors (IDC) and in out of-plane metal-insulator-metal (MIM) devices and their specific properties (dielectric tunability and losses) were investigated in the whole 100 MHz–15 GHz frequency domain. Low tunability under high bias voltage was obtained for the IDCs components based on BNT-NT which make them incompatible with autonomous devices integrating WiFi applications. Instead, BNT-BT-based MIM capacitors shows 50% of tunability for DC-bias voltages lower than 20 V and microwave applications could foresee the integration of BNT-BT films. High-performance devices based on complex oxides require epitaxial growth (or at least highly textured) oxide thin films, since the film’s properties strongly depend on their crystallographic orientation. In this regard, heteroepitaxial BST/Ir/MgO structures were grown by magnetron sputtering. Iridium thin films with (001), (111) and (110) crystallographic orientations grown on single-crystal substrates, i.e. MgO (001), (111) and (110), respectively were successfully achieved despite the fact that the growth of thin films with fcc-type structure generally is governed by the minimization of the surface energy, and thus the (111) crystalline direction is often preferred. The high-frequency properties of heteroepitaxial BST(001)/Ir(001)/MgO(001) were investigated and exhibit a very high capacitance tuning abilities of 82% with dielectric losses of several percents at 2.45 GHz under applied voltages as low as 10 V. By combining high tunability with low resistive losses under low applied voltages, these devices are opening promising avenues for their integration in high-performance tunable devices in the microwave domain and particularly at 2.45 GHz, corresponding to the widely used ISM (industrial, scientific and medical) frequency band. The results were published in JOURNAL OF MATERIALS SCIENCE 51 (2016) 8711, JOURNAL OF APPLIED PHYSICS 120, (2016) 184101, JOURNAL OF APPLIED PHYSICS 119, (2016) 144103, IEEE Microwave and Wireless Components Letters 26 (2016) 504, JOURNAL OF MATERIALS SCIENCE 55 (2020), PROC. ROMANIAN ACAD. SERIES A 20 (2019), APPLIED SURFACE SCIENCE 506 (2020).
  • Epitaxial heterostructures: manufacturing, non-destructive characterization and mangneto-electrical properties

A special thin film structure has been developed at NIMP with a maximum magnetoresistance effect (MRE) at room temperature, which is one of the key operating requirements for many applications. The La.67Ba.33Ti.02Mn.98O3 (LBTMO) single epilayer obtained by pulsed laser deposition onto (001) SrTiO3 single crystal substrates exhibits the highest MRE, ΔR/R(H)≈150% or ΔR/R(0)≈60% under 5 T, at 300 K, a temperature near to the corresponding Curie temperature (TC). The titanium doping as well as the induced stress due to lattice mismatch between the thin film and the substrate contribute to a decrease in TC as compared to the pristine LBMO compound and therefore to the decrease of the temperature where the highest MRE can be recorded. The magnetization easy axis lies in the film plane along the [100] direction of the (001) substrate. The magnetic entropy change (Delta S-M) associated with the second-order magnetic phase transition can be determined via magnetization measurements in the temperature range between 210 and 350 K under different magnetic fields. The relative cooling power (RCP) of this film is about 220 J/kg, around half of the one of expensive bulk Gd (410 J/kg ) for a 50 kOe field change, thus making the LBTMO ferromagnetic thin films a promising candidate for micro/nanomagnetic refrigeration around room temperature. The results were published in Appl. Phys. Lett. 111, 182409 (2017); and Dalton Transactions 45, 15034;

  • Preparation of various ceramic compunds and their characterization.
    1. Ferroelectric PZT-BT ceramic compounds. Ceramic compounds of ferroelectric lead zirconate titanate (PZT) and ferroelectric barium titanate (BT) were prepared from nanopowders corresponding to the formula (1-x)PZT.xBT with x = 0; 0.1; 0.2 … 0.9; 1. The piezoelectric properties show unusual behavior discussed in terms of different grain size and composition. A remarkably high enhancement of the relative dielectric constant, and maxima of, piezoelectric charge constant d33 and the piezoelectric voltage constant g33, is observed for x = 0.4 while the values of the coupling coefficient kp are still high, similar to those of pure PZT.
    2. Lead free ferroelectric ceramics with increased piezoelectric response for applications based on piezo/piro-effects. Barium titanate (BT) is a perovskite ferroelectric, well known among lead free materials. After doping with 1 and 2 mol% Zr, orthorhombic-tetragonal phase transition was shift near room temperature, resulting an increasing number of polarization orientations in the vicinity of this transition, with enhanced piezoelectric and ferroelectric response.
    3. PZT ceramics for transducers applications: effects of vanadium doping on sintering conditions and functional properties of Nb-Li co-doped PZT ceramics. The effect of 1 and 2 mol % V addition on sintering conditions, structural and functional properties of Nb-Li co-doped PZT ceramics, was systematically studied. The reported results show that the increasing amount of vanadium, as a substitute for niobium, decreases the optimal sintering temperature with 100-150 degrees C, but slightly reduces the piezoelectric, ferroelectric, and electromechanical properties of these ceramics. The presence of vanadium increases the lattice tetragonality and grain size, promoting a harder piezoelectric behaviour, mostly in the absence of niobium. EPR spectra exhibit well-defined hyperfine splitting characteristics for V4+ state, dispersed in the lattice as VO2+ ion.
    4. Synthesis and characterization of double-perovskite A2TM2O6 compounds TM being a transition metal. Pr2CoMnO6 and Ho2CoMnO6 were synthesized as bulk ceramic by conventional ceramic method from oxide precursors. They were characterized by measurements of DC magnetization (at 1.8 K and 14 T field), specific heat, electrical resistivity and polarization. The results indicated a ferromagnetic order of both compounds at low temperatures and a complex interplay between ferro-and antiferromagnetisme.
    5. Nanostructured Titanium Doped Iron Oxide Photoelectrodes for Water Splitting. The effect of Ti doping on the structure, electrical and photoelectrochemical properties alpha-Fe2O3 were investigated. Optimum results were obtained for samples doped with 5 at. % titanium and sintered at 1200 degrees C. Photocurrents as high as 8.4 mA/cm(2), for illumination from a 300 W xenon lamp, were recorded for such samples. The results were published in: JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS,20, pp.558-565 (2018); CERAMICS INTERNATIONAL,43, pp.4919-4925 (2017); JOURNAL OF ALLOYS AND COMPOUNDS,685, pp.159-166 (2016)

Main infrastructure:

  • PLD (pulsed lase deposition) work station with (fig. 1): 2 deposition chambers, each with 4 target carousel, sample heater up to 1000 K, fluence control, vacuum system and control of working gases pressure; one chamber has high pressure RHEED; a KrF excimer laser with 248 nm wavelength, 10 Hz repetition rate and maximum 700 mJ energy. One chamber is used to deposit ferroelectric layers from materials with perovskite structure and other simple metal oxides (ZnO, HfO2), and the other chamber is used to deposit superconductor materials.
  • Matrix assisted pulsed laser deposition (MAPLE): 1 deposition chamber with 2 frozen and 2 solid targets; sample holder heated up to 800 K; a KrF excimer laser with 248 nm wavelength, 10 Hz repetition rate and maximum 700 mJ energy. This machine is used to deposit for example nanoparticles from frozen suspensions in a dielectric matrix.
  • RF sputtering equipment (with 4 confocal magnetron sputtering for 2 inch targets, and 1 central magnetron sputtering with 3-inch target, see figure 2).
  • Chemistry laboratory, with various spin-coaters, annealing furnaces, glove boxes and other laboratory equipment for preparation of nanopowders and thin films.
  • X-ray diffraction equipment for thin films (XRD) from Rigaku (figure 3), and other two older machines from Bruker, one for powders and one for thin films. This are used for structural characterization, allowing identification of crystalline phases, crystalline strain, quality of epitaxy, etc.
Fig. 1 PLD work station-the chamber for deposition of ferroelectric thin films.
Fig. 2 RF magnetron sputtering equipment.
Fig. 3 XRD equipment for thin film characterization, produced by Rigaku.
  • Laboratory for electrical measurements (see figure 4), including: 2 Lake Shore cryo-probers, one with vertical magnetic field up to 2.5 T, and one with horizontal magnetic field up to 1.5 T, each has at least 3 micro-manipulated arms with contact needles allowing electrical measurements from liquid helium to 425 K with various electric fields and illumination conditions; several close cycled cryostats for measurements between 10 K and 400 K; DLTS system for trap investigation; set-up for pyroelectric measurements; ferritester from AiXACCT; various instruments to measure low currents, high resistance; RLC bridges. This lab is used to perform complex investigation of the electrical properties (hysteresis loops, C-V and I-V characteristics, impedance spectroscopy; thermally stimulated currents, etc.).
  • Laboratory for testing solar cells, with 1 SUN solar simulator and other accessories.
  • Laboratory for characterization of materials for microwave devices, as well as laboratory models of microwave devices, including: network analyzers working on various frequency domains up to 500 GHz; THz spectrometer.
  • Laboratory for optical spectroscopies including a spectroscopic ellipsometer and FTIR.

The group has access to other infrastructures located at NIMP, through collaborative research activities, such as: TEM and SEM equipment; XPS characterization (including at Ellettra Synchrotron Trieste); magnetic measurements (SQUID, PPMS); other optical spectroscopies (Raman, UV-Vis-NIR, luminescence); clean room facility; small bio-laboratory for testing bio-materials.

Fig. 4 Laboratory for electrical characterization of dielectric, ferroelectric and semiconductor materials and related structures.

Projects (2015-2020)

  • Ideas Project 72/2011 “Comprehensive studies regarding the irradiation induced damage in controlled unpurified Si-from point defects to clusters (2011-2016)
  • Young Team project TE/11/2013 “Metal-ferroelectric interfaces: from theoretical simulation to experimental optimization (2013-2015)
  • Young Team Project TE/96/2015 “Field effect transistors based on new transparent heterostructures deposited at low temperature” (2015-2017)
  • Young Team Project TE/73/2015 “Augmentation of bio-integration for dental implants by coatings with bioglasses with osteoinductive and anti-microbial properties” (2015-2017)
  • IFA-CEA Project C04/2016 “Pyroelectricity in PZT thin films and structures” (2014-2016)
  • Partnership project 238/2014 “Pyroelectric materials optimized by the concept of polarization gradient and experimental pyroelectric detector with potential of applications for monitoring high power/high energy lasers” (2014-2016)
  • FP7 project IFOX 127EU/2011 “Interfacing oxides” (2011-2015)
  • M-ERA NET Project 10/2015 “New generation of pyroelectric detectors based on polar semiconductors” (2015-2016)
  • SEE Project (Norwegian funds) 8/2014 “Perovskites for photovoltaic efficient conversion technology” (2014-2017)
  • Complex Ideas Project 3/202 “Effect of interfaces on the charge transport in ferroelectric/multiferroic structures” (2012-2016)
  • IFA-CEA Project 03/2016 “Optimization of pyroelectric elements on Si substrate for sensor and energy harvesting applications” (2016-2019)
  • ANR Project 21 RO-FR/2013 “Agile compact integrated antennas with tunable ferroelectric materials” (2013-2016)
  • Project POC type G 54/2016 “Intelligent multifunctional materials for high technology applications” (2016-2021)
  • MERA NET Project 61/2016 “Optimized materials for integration in intelligent sensors for millimeter waves” (2016-2019)
  • PCCDI Project 0062/2017 “New diagnosis and treatment methodologies: present challenges and technological solutions based on nanomaterials and biomaterials” (2017-2020)
  • Complex Ideas Project 16/2018 “Control of electronic properties in heterostructures based on ferroelectric perovskites: from theories to applications” (2018-2022)
  • Complex Ideas Project 18/2018 “Multilayer particulate nanostructures with high dielectric constant for applications in energy storage and nanoelectronic devices” (2018-2022; partner)
  • H2020 Project GA 7803302/2018 “Energy Efficient Embedded Non-volatile Memory & Logic based on Ferroelectric Hf(Zr)O2” (2018/2021)
  • Young Team Project 4/2018 “Sinergy of antimicrobial agents incorporated in bio-glass resistant coatings for osteo-implants” (2018-2020)
  • Young Team Project 134/2018 “Intrinsic properties of dielectric materials for microwaves investigated with THz spectroscopy in the temporal domain” (2018-2020)
  • Ideas Project 177/2017 “New approaches for the synthesis of hybrid perovskite type organic-inorganic materials with possible ferroelectric properties for photovoltaic applications” (2017-2019)
  • Post doc grant 75/2018 “Studies to enhance the efficiency and stability of planar perovskite solar cells” (2018-2020)
  • Post doc grant 16/2018 “Development of photo-transistors based on halogen lead free perovskites for a new generation of OLET displays” (2018-2020)
  • Post doc grant 133/2020 “Size-driven phenomena as origin for novel traits of advanced ferroelectric nanostructured (Ba,Sr)TiO3 ceramics” (2020-2022)
  • Post doc grant 128/2020 “Control of ferroelectric negative capacitance in multilayer systems for low power electronics” (2020-2022)
  • PED project 455/2020 “Mini-module with perovskite solar cells” (2020-2022)
  • PED project 487/2020 “Multifunctional dielectric materials produced by spark plasma sintering for passive microwave devices” (2020-2022)
  • PED project 433/2020 “PZT based piezo elements for rocket propelled grenade ordnance” (2020-2022)
  • PED project 509/2020 “Optimizing perovskite photoactive materials through automatic learning” (2020-2022; partner)
  • PED project 472/2020 “Development doped compounds from vanadium oxide/graphene for ultra-performant batteries and super-capacitors through physical vapor deposition for sustainable and ecological applications for energy storage” (2020-2022)
  • Young Team Project “Optimization of multiple polarization states in ferroelectric heterostructures” (2020-2022; not yet contracted)
  • Young Team Project “Robocasting manufacturing of porous bioceramic implants: towards a new generation of synthetic bones substituents” (2020-2022; will start in 2021)
  • Young Team Project 102/2020 “High quality HZO and AlN thin films deposited by industry scalable techniques for a new generation of sensors and electronic devices” (2020-2022)

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