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 E-mail address: pintilie@infim.ro Timisoara, 25-28 Aprilie 2016 1
Outlines Introduction Interfaces in ferroelectrics Self-doping in epitaxial ferroelectrics Polar heterostructures Light and polar materials Next steps Timisoara, 25-28 Aprilie 2016 2
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, 25-28 Aprilie 2016 3
The Nobel Prize in Physics 2014 - 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, 25-28 Aprilie 2016 4
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, 25-28 Aprilie 2016 5
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, 25-28 Aprilie 2016 6
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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, 25-28 Aprilie 2016 8
Polarization (C/cm 2 ) Current (ma) Polarization (C/cm 2 ) Current (A) Polarization (C/cm 2 ) Current (ma) Polarization (C/cm 2 ) Current (A) 100 80 60 40 20 0-20 -40-60 -80-100 100 80 60 40 20 0-20 SRO Pt Cu Au -6-4 -2 0 2 4 6 SRO Pt Cu Au Voltage (V) a) 300 K 0,8 0,6 0,4 0,2 0,0-0,2-0,4-0,6-0,8-40 -0,2-60 c) -80 300 K -100-0,4-6 -4-2 0 2 4 6 Voltage (V) 0,4 0,2 0,0 100 80 60 40 20 0-20 -40-60 -80-100 50 40 30 20 10 0-10 -20-30 -40-50 SRO Cu -8-6 -4-2 0 2 4 6 8 SRO Voltage (V) b) 150 K -6-4 -2 0 2 4 6 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, 25-28 Aprilie 2016 9 80 60 40 20 0-20 -40-60 -80 80 60 40 20 0-20 -40-60 -80
C F R c C S R F Schottky contacts Timisoara, 25-28 Aprilie 2016 10
Potential barrier not correlated with work function Large density of free carriers Timisoara, 25-28 Aprilie 2016 11
Dielectric constant 160 140 120 PRB 75, 104103 2007 100 Polarization term 0.7-0.9 ev P=1 C/m 2 ε op =6 ε st =60 or 40 80 60 40 20 0 200 400 Thickness (nm) Potential barrier around 1 ev XPS results 1.4-1.7 ev the potential barriers at electrode interfaces ACS Appl. Mater. Interfaces 6, 2929 2939 (2014) J. Phys. D: Appl. Phys. 44, 255301 (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, 25-28 Aprilie 2016 12
S factor S S 0 ed 0 1 1 0.1 1 2 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, 25-28 Aprilie 2016 13
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 0.2-0.5 ev for both PZT and BTO Timisoara, 25-28 Aprilie 2016 14
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, 122915 (2007) Timisoara, 25-28 Aprilie 2016 15
Interfacial polarization Timisoara, 25-28 Aprilie 2016 16
Self-doping in epitaxial ferroelectrics AFM/PFM studies revealing that upward polarization is dominant in epitaxial PZT films Timisoara, 25-28 Aprilie 2016 17
Polarization (C/cm 2 ) 98 97 96 95 94 0 50 100 150 200 250 300 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, 14974 (2015) Timisoara, 25-28 Aprilie 2016 18
There is a variation of stoichiometry with thickness, suggesting different densities of cation and oxygen vacancies as the thickness increases Timisoara, 25-28 Aprilie 2016 19
Timisoara, 25-28 Aprilie 2016 20 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
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, 104103 (2012); Appl. Phys. Lett. 96, 012903 (2010) Timisoara, 25-28 Aprilie 2016 21
Artificial multiferroic structures-combination of ferroelectric and ferrimagnetic materials. Timisoara, 25-28 Aprilie 2016 22
Light and polar materials Light photovoltaic pyroelectric Intrinsic internal electric field to separate the charge carriers J. Mater. Chem. A, 2014, 2, 6027 6041 6029 i p Ap( dt / dt) p dps / dt A current is generated when there is a temperature variation of the sample Timisoara, 25-28 Aprilie 2016 23
Photovoltaic Short-circuit photocurrent dependes on the electrode materials J. Appl. Phys. 110, 044105 (2011) Timisoara, 25-28 Aprilie 2016 24
Necache et al., Nature Photonics DOI: 10.1038/NPHOTON.2014.255 Efficiency 8.1 % by band-gap engineering in a multilayer! Timisoara, 25-28 Aprilie 2016 25
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, 25-28 Aprilie 2016 26
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 10 5 10 3 10 5 STO SRO 10 4 PZT 10 3 10 2 10 1 10 0-50 0 50 100 150 200 250 300 350 Phi ( 0 ) PZT 10 1 18 20 22 24 26 2 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, 25-28 Aprilie 2016 27
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, 232902 (2013) Phys. Rev. Lett. 109, 257602 (2012). p~19x10-4 C/m 2 K Timisoara, 25-28 Aprilie 2016 28
Optical amplification of the pyroelectric signal through the photogeneration of free carriers by the continuous UV illumination Timisoara, 25-28 Aprilie 2016 29
Pyroelectric signal (V) 100 AlN 10 ZnO 10 100 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, 25-28 Aprilie 2016 30
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, 25-28 Aprilie 2016 31
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, 25-28 Aprilie 2016 32
Funding Funding Program/Project: IFA-CEA INTERFACING OXIDES (IFOX) Project 8 SEE/2014 (EEA-JRP-RO-NO-2013-1) Complex Idea Project Effect of interfaces on charge transport in ferroic/multiferroic heterostructures Project NOPYDET Timisoara, 25-28 Aprilie 2016 33
THANK YOU FOR YOUR ATTENTION! Timisoara, 25-28 Aprilie 2016 34