Taimur Ahmed Chalmers University of Technology, Sweden

Background Ferromagnetics Ferroelectrics Multiferroics BiFeO3 (BFO)

Design & modeling of dielectric response Device fabrication Characterization & Analysis Conclusion Future outlook

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Ferromagnetic materials Materials retain their magnetic polarization Curie-Weiss law (susceptibility is inverse dependent

on Curie temperature)

T>Tc: Ferromagnetics changes to paramagnetics T=Tc: Material is in phase transition

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( )CTTC−

=χC = Curie constant of material T = Absolute temperature Tc= Curie temperature

Ferroelectric materials Materials retain their electric (dipole) polarization Curie temperature defines Polar (ferroelectric) and non-polar (paraelectric) phase

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Tc

T

Polar phase Non-polar phase

T < Tc T > Tc

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P E

P E

Perovskite - ABO3

A B O

Non-polar phase Polar phase

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Non-polar phase Polar phase

S. Gevorgian. Springer, 2009.

( )EPE∂∂

=0

1ε

ε

Hans Schmid (1990): “A material that combines two (or more) of the

primary ferroic orders in one phase”

In practice often:

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Multiferroic = Magnetoelectric

ME = Ferromagnetic +

Ferroelectric

L. W. Martin et al. Mater. Sci. Eng., R, 68(4-6):89-133, May 2010.

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Tunable resonator Tunable filter Tunable phase shifter/delay line

Adapted from: Ce-Wen Nan et al., J. Appl. Phys., 103, 03101 (2008)

Magnetoelectric memories Magneto-mechanical actuators Tunable microwave devices Sensors

Bismuth ferrite – BiFeO3 (BFO) High Curie temperature, TC (850⁰C)

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8 9 17 24 34 72

120

254 274

345

161

Published articles

Data collected from: www.webofknowledge.com June 2010.

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Fe 2.022

2.022

2.022

1.998

1.998

1.998

Distorted Rhombohedral Perovskite Oxygen octahedron

Adapted from: C. Michel et al. Solid State Commun., 7(9), 1969.

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

Weak magnetoelectric coupling

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Optimization of growth temperature of BiFeO3 (BFO) thin films To have stoichiometric thin films To have low leakage current

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Background Design & modeling

Design configurations Grain structure of BiFeO3 films Parallel plate configuration Co-planar configuration Co-planar configuration with buried ID electrode

Permittivity models Device fabrication Characterization & Analysis Conclusion

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R. Zheng et al. J. Am. Ceram. Soc. 91 (2008) 463. M.S. Kartavtseva et al. Surface and Coatings

Technology 201 (2007) 9149.

MOCVD RF sputtering

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

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Bottom electrode

Substrate

Top electrode

Ci1

Ci2

Cg1 Cb1 Cbi Cgi CgN CbN

( )∑ +∑ −−+=−

gibeq CCCC 11 1

Multiferroic film Grain boundary E-Field

Cb = Capacitance of grain boundaries

Cg = Capacitance of grains

Ci = Capacitance of interface layers

Interfacial layer

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∑∑∑−−−−

++= CCCC gibeq1111

Substrate

Electrode Electrode

Ci1

Cg1

Cb1

Cgi

Cbi

CgN

Ci2

Multiferroic film Grain boundary E-Field

Cb = Capacitance of grain boundaries

Cg = Capacitance of grains

Ci = Capacitance of interface layers

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Substrate

Electrode Electrode

Adhesion layer

Ci1

Cg

Ci2

Cb

( )∑∑ ++=−−−

CCC bgieq111

∑−−

≈ CC geq11

Cb = Capacitance of grain boundaries

Cg = Capacitance of grains

Ci = Capacitance of interface layers

Grain boundary E-Field

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

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Ion Beam Milling 45⁰ tilt

Pulsed Laser Deposition (PLD)

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Parameters Comments Laser source KrF

Laser wavelength 248 nm

Energy 1.5 mJ/cm2

Target Bi1.1FeO3

Oxygen pressure 1 x 10-2 mbar

Repetition rate 10 Hz

Substrate Au/Ti/SiO2

Substrate temperature 500⁰C -750⁰C

Substrate-target distance 6cm

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Substrate

Pads Pads

E-beam evaporator Ti: 50 nm Au: 500 nm

ID pads ID finger

ID pads BFO film

Background Design & modeling of dielectric response Device fabrication Characterization & Analysis

Microstructure of deposited thin films Dielectric response Magnetoelectric response

Conclusion

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600⁰C

700⁰C 750⁰C

550⁰C

6μm

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XRD Spectra

As – deposited BFO thin films

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Better stoichiometry

Better microstructure

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EcC Epitaxial BiFeO3 thin films Tg = 600⁰C

80kV/cm

H. W. Jang et al. Phys. Rev. Lett. 01

(2008) 107602-1

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As deposited Tg = 500⁰C Tg =500⁰C & Annealed at 700⁰C

6μm

Ex – situ annealed (700⁰C) BFO thin film

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εBFO=26 Bulk BiFeO3 ceramic

Yu.E. Roginskaya, et al. Sov.

Phys. JETP, 23 (1966) 47

Dielectric constant (Farnell’s model)

31 ME tunability = ∆ε/ε =0.21%

Tg=600⁰C

B ⊥ E

F. B. A. Ahad, J. Appl. Phys. 105, 07D912 (2009)

BiFeO3/quartz substrate

B=0.2T

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

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Effect of Tg 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), Ec (80kV/cm) and ME tunability of 0.21% by 0.2 T @ Tg=600⁰C are close to bulk BFO

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

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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. 4470-4474 (2012).

A. Vorobiev, Taimur Ahmed, S. Gevorgian, “Microwave

response of BiFeO3 films in parallel-plate capacitors” Integrated Ferroelectrics, 134 pp. 111-117 (2010).

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FG FW λ

Farnell’s model

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−+

−

=

−

129.0

5.1

Lh

K

sKCBFO

εε

Parameters Dimensions

(μm)

FL Finger length 700

FW Finger width 3

FG Finger gap 6

h Finger/electrode

thickness 0.55

P Number of finger

pairs 25

37.208.15.6

2

++=

LFW

LFW

K

C = Measured capacitance εs= Dielectric constant of substrate (SiO2) L = (FW + FG)