POLAR MATERIALS: FROM PHYSICAL PHENOMENA TO APPLICATIONS

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1 Conferinţa Diaspora în Cercetarea Ştiinţifică şi Invăţământul Superior din România Eveniment aflat sub Înaltul Patronaj al Preşedintelui României. POLAR MATERIALS: FROM PHYSICAL PHENOMENA TO APPLICATIONS Lucian Pintilie National Institute of Materials Physics, Atomistilor 405A, Magurele, Romania address: Timisoara, Aprilie

2 Outlines Introduction Interfaces in ferroelectrics Self-doping in epitaxial ferroelectrics Polar heterostructures Light and polar materials Next steps Timisoara, Aprilie

3 Introduction Polar materials (inorganic) Lack of center of symmetry-polar axis-polarizationpyroelectricity, ferroelectricity, piezoelectricity, non-linear optical effects Polar semiconductors (e.g. GaN, AlN, ZnO) Ferroelectrics, especially those with perovskite structure (e.g. BaTiO 3, Pb(Zr,Ti)O 3, or multiferroics (BiFeO 3 ) NTT Technical Review 8 (2010) Timisoara, Aprilie

4 The Nobel Prize in Physics awarded jointly to Isamu Akasaki, Hiroshi Amano and Shuji Nakamura "for the invention of efficient blue lightemitting diodes which has enabled bright and energy-saving white light sources. AlN for high voltage, high power electronic devices Timisoara, Aprilie

5 Applications of ferroelectrics Non-volatile memories (exploiting the reversible polarization); ultrasound generation and reception, micro-motors, actuators, MEMS (exploiting the piezoelectriv effect); thermal imaging (exploiting the pyroelectric effect); optical switches; energy harvesting (solar reactor, photovoltaic). Timisoara, Aprilie

6 Interfaces in ferroelectrics In many cases the sample is a capacitor-electrode interfaces How interact polarization with the interface? Multi-layers and super-lattices-internal interfaces How do the interfaces affect the macroscopic properties? Samples prepared by pulsed laser deposition-epitaxy (minimizing extrinsic effects) PZT, BaTiO3, BiFeO3; single crystal substrate-srtio3; bottom SrRuO3 electrode Timisoara, Aprilie

7 Timisoara, Aprilie

8 The electrode-ferroelectric interface Polarization controlled properties of electrode interface C-V characteristics obtained for different metals as top electrodes on: a) PZT grown on STO (001); b) BTO grown on STO (001); c) PZT grown on STO (111); d) BTO grown on STO (111). Timisoara, Aprilie

9 Polarization (C/cm 2 ) Current (ma) Polarization (C/cm 2 ) Current (A) Polarization (C/cm 2 ) Current (ma) Polarization (C/cm 2 ) Current (A) SRO Pt Cu Au SRO Pt Cu Au Voltage (V) a) 300 K 0,8 0,6 0,4 0,2 0,0-0,2-0,4-0,6-0, ,2-60 c) K , Voltage (V) 0,4 0,2 0, SRO Cu SRO Voltage (V) b) 150 K Voltage (V) Hysteresis loops recorded for the PZT layer deposited on (001)STO (a), for the BTO layer deposited on (001)STO (b), for PZT film deposited on (111)STO (c), and for BTO film deposited on (111)STO. d) 150 K Timisoara, Aprilie

10 C F R c C S R F Schottky contacts Timisoara, Aprilie

11 Potential barrier not correlated with work function Large density of free carriers Timisoara, Aprilie

12 Dielectric constant PRB 75, Polarization term ev P=1 C/m 2 ε op =6 ε st =60 or Thickness (nm) Potential barrier around 1 ev XPS results ev the potential barriers at electrode interfaces ACS Appl. Mater. Interfaces 6, (2014) J. Phys. D: Appl. Phys. 44, (2011) The barrier is not the simple difference between metal work function Φ m and the electron affinity of the ferroelectric material χ f interface states? Timisoara, Aprilie

13 S factor S S 0 ed JAP 36, 3212 (1965) APL 74, 1168 (1999) PRL 58, 1260 (1986) S~0.3 for PZT Dδ~1.6x10 14 cm -2 ev -1 (ε=40) P~90 μc/cm 2 (E g ~3.5 ev)! It can be that polarization acts as interface states, pinning the Ferrmi level and controlling the height of the potential barriers Timisoara, Aprilie

14 XPS investigation Metal deposition-2 monolayers; 4 monolayers 10 nm XPS spectra recorded after each deposition The shift of the binding energy was monitored as function of metal thickness Band bending occurs at the interface ev for both PZT and BTO Timisoara, Aprilie

15 Tranzitie de faza indusa de grosime in superretele PZT-PZO (feroelectric-antiferoelectric). Cu scaderea grosimii straturilor componente faza antiferoelectrica devine feroelectrica. Applied Physics Letters 91, (2007) Timisoara, Aprilie

16 Interfacial polarization Timisoara, Aprilie

17 Self-doping in epitaxial ferroelectrics AFM/PFM studies revealing that upward polarization is dominant in epitaxial PZT films Timisoara, Aprilie

18 Polarization (C/cm 2 ) Thickness (nm) The density of the free carriers increases with decreasing the thickness of the PZT films The deduced empirical law is: n( m 3 ) 27 2x10 d( nm) Polarization induced selfdoping Scientific Reports 5, (2015) Timisoara, Aprilie

19 There is a variation of stoichiometry with thickness, suggesting different densities of cation and oxygen vacancies as the thickness increases Timisoara, Aprilie

20 Timisoara, Aprilie Suggested model-oxygen vacancies p m v O Free charge from oxygen vacancies Free charge from the bottom SRO electrode Bound polarization charges 0 F B O F 0 F O ) ( 2 2exp 1 ) ( 2 1 d e E g n e P d f T k E E d e E g n e P d n m m v v d P B A n V c v O EC O ) ( 3 ) ( 10 2 ) ( 27 3 nm d m n x e P d e E g n V f P B A m EC 2 ) ( ) ( 3 0 F T k E E f v B O F 2exp 1 A= 3.09 ± 0.05 f~10 10 % of oxygen vacancies are ionized T k E E n n v v v B O F O O 2exp 1

21 Polar heterostructures policristalin epitaxial columnar 450K 350K Pt-PZT-ZnO (MFS) structures 150K Orientation of the C-V hysteresis dependent on structural quality and temperature J. Appl. Phys. 112, (2012); Appl. Phys. Lett. 96, (2010) Timisoara, Aprilie

22 Artificial multiferroic structures-combination of ferroelectric and ferrimagnetic materials. Timisoara, Aprilie

23 Light and polar materials Light photovoltaic pyroelectric Intrinsic internal electric field to separate the charge carriers J. Mater. Chem. A, 2014, 2, i p Ap( dt / dt) p dps / dt A current is generated when there is a temperature variation of the sample Timisoara, Aprilie

24 Photovoltaic Short-circuit photocurrent dependes on the electrode materials J. Appl. Phys. 110, (2011) Timisoara, Aprilie

25 Necache et al., Nature Photonics DOI: /NPHOTON Efficiency 8.1 % by band-gap engineering in a multilayer! Timisoara, Aprilie

26 Solar cells with (MA)PbI 3 perovskit TiO 2 HTM-spiro-OMeTAD FTO Standard structure 12 %-best performance ever obtained in Romania for any type of solar cells studied up to now-possibility to improve by optimizing the process and component materials. Au electrodes, maximum area of 5x5 mm 2 SEM image of the stack (MA)PbI 3 molecules (blue-yellow) have electric dipole moments- tend to be correlated resulting in a net polarization of the cell Timisoara, Aprilie

27 Pyroelectric Epitaxial PZT films grown by PLD on SRO/STO substrates and with top SRO contacts Polycrystalline films grown by spin coating on Pt/Si substrates and with top Pt contacts Intensity (cps) Intensity (arb. unit.) PZT SRO STO STO SRO 10 4 PZT Phi ( 0 ) PZT Theta (degrees) Structural characterization of the PZT films: XRD (left); PFM (middle); TEM (right) AlN and ZnO thin films grown by RF-sputtering on Si substrates and with top Cu electrodes ZnO AlN Timisoara, Aprilie

28 a) the frequency dependence of the pyroelectric signal obtained from the Pt/PZT/SRO/STO active element); b) the dependence of the pyroelectric signal on 1/ω, evidencing the frequency ω 0 where the heating conditions for the sample are changing from relatively uniform to non-uniform; c) the PFM image of the free surface of the PZT layer, evidencing the presence of the 90 domains. Applied Physics Letters 103, (2013) Phys. Rev. Lett. 109, (2012). p~19x10-4 C/m 2 K Timisoara, Aprilie

29 Optical amplification of the pyroelectric signal through the photogeneration of free carriers by the continuous UV illumination Timisoara, Aprilie

30 Pyroelectric signal (V) 100 AlN 10 ZnO Frequency (Hz) p=12.4 μcm -2 K -1 for AlN p~3 μcm -2 K -1 for ZnO Inferior to ferroelectrics but may work at elevated temperatures, where ferroelectrics are no longer pyroelectrically active Timisoara, Aprilie

31 Next steps strain engineering-induction of ferroelectric phase in nominally non-ferroelectric materials rectifying and ohmic contacts on ferroelectricferrotronics? doping-manipulating the concentration of free carriers in close relation with polarization band-gap tuning-adjusting band-gap for optimal light absorption in ferroelectric solar cells pyro-photo combination-novel concepts for light sensors (optical amplification and filtering) Timisoara, Aprilie

32 Samples Collaborators Structure Dr. Chirila Cristina Dr. Luminita Hrib Dr. Stancu Viorica Dr. George Stan Dr. Pasuk Iuliana Drd. Raluca Negrea Dr. Corneliu Ghica XPS, PFM Dr. Cristian Mihail Teodorescu Dr. Apostol Nicoleta Drd. Laura Abramiuc Dr. Trupina Lucian Photovoltaic Dr. Cristina Besleaga Electric, magnetic charac. Drd. Andra Georgia Boni Drd.Mihaela Botea Dr. Iuga Alin Drd. Simona Greculeasa Dr. Pintilie Ioana Dr. Kuncser Victor Timisoara, Aprilie

33 Funding Funding Program/Project: IFA-CEA INTERFACING OXIDES (IFOX) Project 8 SEE/2014 (EEA-JRP-RO-NO ) Complex Idea Project Effect of interfaces on charge transport in ferroic/multiferroic heterostructures Project NOPYDET Timisoara, Aprilie

34 THANK YOU FOR YOUR ATTENTION! Timisoara, Aprilie