Taimur Ahmed. Chalmers University of Technology, Sweden

Transcription

1 Taimur Ahmed Chalmers University of Technology, Sweden

2 Background Ferromagnetics Ferroelectrics Multiferroics BiFeO 3 (BFO) Design & modeling of dielectric response Device fabrication Characterization & Analysis Conclusion Future outlook 2

3 Ferromagnetic materials Materials retain their magnetic polarization Curie-Weiss law (susceptibility is inverse dependent on Curie temperature) χ = C ( T ) T C C = Curie constant of material T = Absolute temperature T c = Curie temperature T>T c : Ferromagnetics changes to paramagnetics T=T c : Material is in phase transition 3

4 Ferroelectric materials Materials retain their electric (dipole) polarization Curie temperature defines Polar (ferroelectric) and non-polar (paraelectric) phase Polar phase T c Non-polar phase T < T c T > T c T 4

5 Non-polar phase Perovskite - ABO 3 Polar phase A B O E P E P 5

6 Polar phase Non-polar phase ε ( E) = 1 ε 0 P E S. Gevorgian. Springer,

7 Hans Schmid (1990): A material that combines two (or more) of the primary ferroic orders in one phase ME = Ferromagnetic + Ferroelectric In practice often: Multiferroic = Magnetoelectric L. W. Martin et al. Mater. Sci. Eng., R, 68(4-6):89-133, May

8 Tunable resonator Tunable filter Tunable phase shifter/delay line Magnetoelectric memories Magneto-mechanical actuators Tunable microwave devices Sensors Adapted from: Ce-Wen Nan et al., J. Appl. Phys., 103, (2008) 8

9 Bismuth ferrite BiFeO 3 (BFO) High Curie temperature, T C (850⁰C) 345 Published articles Data collected from: June

10 Distorted Rhombohedral Perovskite Oxygen octahedron Fe Adapted from: C. Michel et al. Solid State Commun., 7(9),

11 Leakage current Non-stoichiometric thin films (Bi depletion) Microstructural defects (grain boundaries etc.) Weak magnetoelectric coupling 11

12 Optimization of growth temperature of BiFeO 3 (BFO) thin films To have stoichiometric thin films To have low leakage current 12

13 Background Design & modeling Design configurations Grain structure of BiFeO 3 films Parallel plate configuration Co-planar configuration Co-planar configuration with buried ID electrode Permittivity models Device fabrication Characterization & Analysis Conclusion 13

14 RF sputtering MOCVD R. Zheng et al. J. Am. Ceram. Soc. 91 (2008) 463. M.S. Kartavtseva et al. Surface and Coatings Technology 201 (2007)

15 Design configurations Parallel plate configuration Co-planar inter digital (ID) configuration 15

16 E-Field Multiferroic film Interfacial layer Top electrode Bottom electrode Substrate Grain boundary C i1 C b = Capacitance of grain boundaries C g = Capacitance of grains C i = Capacitance of interface layers C g1 C b1 C gi C bi C gn C bn C eq = C b + ( ) 1 1 C C + i g 1 C i2 16

17 Multiferroic film Electrode E-Field Substrate Grain boundary Electrode C b = Capacitance of grain boundaries C g = Capacitance of grains C i = Capacitance of interface layers C i1 C b1 C bi C i2 C eq = C + C + C b i C g1 g 1 C gi C gn 17

18 Adhesion layer Electrode E-Field Substrate Grain boundary Electrode C b = Capacitance of grain boundaries C g = Capacitance of grains C i = Capacitance of interface layers C b C i1 C i2 C eq 1 = 1 ( ) C g b Ci C g C 1 eq C g 1 18

19 Objective Background Design & modeling of dielectric response Device micro fabrication Patterning of IDC electrodes BFO thin film growth (PLD) Contact pads deposition Characterization & Analysis Conclusion Future outlook 19

20 Ion Beam Milling 45⁰ tilt 20

21 Pulsed Laser Deposition (PLD) Laser source Parameters Laser wavelength Comments KrF 248 nm Energy 1.5 mj/cm 2 Target Bi 1.1 FeO 3 Oxygen pressure Repetition rate 1 x 10-2 mbar 10 Hz Substrate Au/Ti/SiO 2 Substrate temperature Substrate-target distance 500⁰C -750⁰C 6cm 21

22 E-beam evaporator Ti: 50 nm Au: 500 nm Pads ID finger ID pads Pads Substrate ID pads BFO film 22

23 Background Design & modeling of dielectric response Device fabrication Characterization & Analysis Microstructure of deposited thin films Dielectric response Magnetoelectric response Conclusion 23

24 550⁰C 600⁰C 6μm 700⁰C 750⁰C 24

25 XRD Spectra 25

26 As deposited BFO thin films Better stoichiometry Better microstructure 26

27 T g = 600⁰C E c C Epitaxial BiFeO 3 thin films H. W. Jang et al. Phys. Rev. Lett kV/cm (2008)

28 As deposited T g = 500⁰C T g =500⁰C & Annealed at 700⁰C 6μm 28

29 Ex situ annealed (700⁰C) BFO thin film 29

30 Dielectric constant (Farnell s model) ε BFO =26 Bulk BiFeO 3 ceramic Yu.E. Roginskaya, et al. Sov. Phys. JETP, 23 (1966) 47 30

31 T g =600⁰C BiFeO 3 /quartz substrate B E B=0.2T F. B. A. Ahad, J. Appl. Phys. 105, 07D912 (2009) ME tunability = ε/ε =0.21% 31

32 Background Objective Design & modeling of dielectric response Device fabrication Characterization & Analysis Conclusion 32

33 Effect of T g on BFO films growth Strong dependence on film s microstructure and stoichiometry BFO films grown over Au IDC electrodes Improved microstructure/large in-plane grains Reduced parasitics/less grain boundaries Permittivity (26), E c (80kV/cm) and ME tunability of 0.21% by 0.2 T g =600⁰C are close to bulk BFO 33

34 Post deposition ex-situ annealing results higher permittivity than as-deposited films Improved microstructure/increased grain size Simultaneously preserving stoichiometry 34

35 Taimur Ahmed, A. Vorobiev, S. Gevorgian, Growth temperature dependent dielectric properties of BiFeO3 thin films deposited on silica glass substrates Thin Solid Films, 520 (13) pp (2012). A. Vorobiev, Taimur Ahmed, S. Gevorgian, Microwave response of BiFeO3 films in parallel-plate capacitors Integrated Ferroelectrics, 134 pp (2010). 35

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38 FW FG λ 38

39 Farnell s model ε K BFO = K + C 0.29 K ε h L s 1.5 = FW L 2 FW L C = Measured capacitance ε s = Dielectric constant of substrate (SiO 2 ) L = (FW + FG) Parameters Dimensions (μm) FL Finger length 700 FW Finger width 3 FG Finger gap 6 h P Finger/electrode thickness Number of finger pairs