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«By JOSEPH A. CIANFRONE A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR ...»

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FUNCTIONAL COMPLEX OXIDE THIN FILMS AND RELATED SUPERLATTICES

GROWN VIA PULSED LASER DEPOSITION

By

JOSEPH A. CIANFRONE

A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL

OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT

OF THE REQUIREMENTS FOR THE DEGREE OF

DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA

© 2011 Joseph A. Cianfrone To the pursuit of knowledge “I believe in evidence. I believe in observation, measurement, and reasoning, confirmed by independent observers. I'll believe anything, no matter how wild and ridiculous, if there is evidence for it. The wilder and more ridiculous something is, however, the firmer and more solid the evidence will have to be.” – Isaac Asimov “I don't know anything, but I do know that everything is interesting if you go into it deeply enough.” –Richard Feynman

ACKNOWLEDGMENTS

I could not have achieved this accomplishment without the help, guidance and love of those around me. It is their continual support that I would like to acknowledge.

I would like to thank my parents, Adele and Joe, who raised me to have an inquisitive and stubborn disposition of character. I thank my brother, John, and my sister, Beth, for their support and for continually achieving remarkable accomplishments driving me further to try, in vain, to keep pace. I thank my friends, especially Galileo Sarasqueta, Edward McKenna, and Fernando Lugo, for their continual support and presence in my life helping keep me sane during the doctoral process.

I thank my advisor, Dr. David Norton, without whom I would not have the direction or understanding of how to conduct scientific research. I would like to thank my committee members, Dr. Stephen Pearton, Dr. Brent Gila, Dr. Valentin Craciun, and Dr. Art Hebard, who have helped guide me through the doctoral research.

I would like to thank my colleagues in the Materials Science department for their teaching, help and support: Dr. Seemant Rawal, D. Li-Chia Tien, Dr. Jean Marie Erie, Dr. Li-Leu Cherng, Dr. Mat Ivill, Dr. Patrick Sadik, Dr. Fernando Lugo, Ryan Pate, Kyeongwon Kim, Seonhoo Kim. I would like to acknowledge Blake Darby for significant help with high resolution transmission electron microscope imaging. I would also like to acknowledge Dr. Ritesh Das, Siddartha Ghosh and Patrick Mickel in the Department of Physics for their collaboration and discussion on several research topics.

I would also like to acknowledge funding from the Army Research Office, Department of Energy, and the National Science Foundation. I would like to thank Paula Mathis, Doris Harlow, Jennifer Horton, and Martha McDonald for their support as well.

–  –  –

ACKNOWLEDGMENTS

LIST OF TABLES

LIST OF FIGURES

Abstract

CHAPTER

1 INTRODUCTION

2 BACKGROUND AND MOTIVATIONS

Complex Oxide Structures

Magnetic Materials

Magnetic Measurements

New Concepts for Magnetic Technologies

Polarizable Materials

Ferroelectricity

Thin Film Effects

Ferroelectric and Dielectric Measurements

Multiferroics

Superlattices and Interfaces

Literature Review

ZnCo2O4

K(Ta,Nb)O3

BaFeO3

Figures

3 EXPERIMENTAL DESIGN

Thin Film Synthesis

Pulsed Laser Deposition

Substrate Preparation

System Parameters

Target Creation

Structural and Chemical Characterization

X-Ray Diffraction

Reflection High Energy Electron Spectroscopy

X-Ray Photoelectron Spectroscopy (XPS)

Scanning Probe Microscopy (SPM)

Electronic Characterization

Hall Effect

Capacitance Measurements

Optical Characterization

Magnetic Characterization

4 STRUCTURAL, OPTICAL AND ELECTRONIC PROPERTIES OF ZnCoO4 THIN FILMS

Experimental Methods

Results and Discussion

Structural and Chemical Properties

Transport and Optical Properties

Magnetic Properties

Conclusions

Figures

5 REFLECTION HIGH ENERGY ELECTRON DIFFRACTION STUDY OF K(Ta,Nb)O3 THIN FILMS

Experimental Procedures

Discussion of Results

Conclusions

Figures

6 ELECTRONIC PROPERTIES OF K(Ta,Nb)O3-SrTiO3 SUPERLATTICES............. 91

Experimental Procedures and Results

Conclusions

Figures

7 MULTIFERROIC PROPERTIES of BaFeO3-K(Ta,Nb)O3 Superlattices................ 104

Experimental Procedure

Results and Discussion

Figures

8 CONCLUSIONS

APPENDIX: EXTRACTED CAPACITANCE AND MATLAB DIELECTRIC

CONSTANT MODEL

LIST OF REFERENCES

BIOGRAPHICAL SKETCH

–  –  –

2-1 (100) and (111) planes of Spinel crystal structure in Normal orientation............ 39 2-2 Several unit cells of the perovskite crystal structure exhibited by K(Ta,Nb)O3... 39 2-3 Magnetic hysteresis loop indicative of ferromagnetism





2-4 Polarization mechanisms and their changes with frequency

2-5 Debye curves showing the relationship between frequency and the dielectric constant

2-6 Ferroelectric displacement in K(Ta,Nb)O3 perovskite ferroelectrics

3-1 A typical PLD setup w/RHEED.

3-2 Unit cell step surface measured by AFM measurement of SrTiO3 substrate after etching in buffered HF and annealing in oxygen at 900°C

3-3 Recipes for PLD target fabrication

3-4 RHEED geometry.

3-5 XRD geometry

3-6 Monitored specular spot intensity as a function of surface coverage.................. 58 3-7 Thin film growth modes

3-8 Photolithography recipe for patterning and deposition of interdigitated capacitors

4-1 X-ray diffraction results for ZnCo2O4 film grown on sapphire, MgO, and MgAl2O4.

4-2 X-ray diffraction results for ZnCo2O4 films grown on (0001) sapphire in 100 mTorr oxygen at various temperatures.

4-3 X-ray diffraction results for ZnCo2O4 films grown on (0001) sapphire at 400ºC in various oxygen pressures.

X-ray diffraction Ω-rocking curve about the ZnCo2O4 (333) for a film grown at 4-4 400ºC in 50 mTorr oxygen.

4-5 X-ray diffraction phi-scan of a ZnCo2O4 film grown on c-plane sapphire at 400ºC in 300 mTorr.

4-6 Atomic force microscopy image of ZnCo2O4 film grown at 400ºC in 150 mTorr Oxygen

4-7 X-Ray Photoelectron Spectroscopy results for a ZnCo2O4 film grown at 400ºC in 150 mTorr oxygen, showing the Co 2p3/2 and 2p1/2 peaks

4-8 Energy Dispersive Spectroscopy results of ZnCo2O4 films growth at 400ºC and various oxygen pressures

4-9 Characterization of semiconducting properties of ZnCo2O4 films grown at 400ºC showing (1) carrier concentration, (2) resistivity, and (3) mobility as a function of oxygen growth pressure.

4-10 Plot of resistivity as a function of temperature, showing exponential T -1/4 behavior indicative of variable range hopping

4-11 Optical transmittance vs. photon energy for ZnCo2O4 films grown at 400 ºC in 50 and 150 mTorr Oxygen.

4-12 Magnetization versus applied applied magnetic field for ZnCo2O4 films grown at 400ºC and various oxygen partial pressure.

X-ray diffraction results for K(Ta,Nb)O3 films deposited on (ℓ00) oriented 5-1 MgO, LaAlO3, KTaO3, and SrTiO3 at 750°C in 10 mTorr O2.

X-ray diffraction Ω-rocking curve for K(Ta,Nb)O3 film grown at 750°C, 10 5-2 mTorr O2 on (100) SrTiO3.

X-ray diffraction θ-scan for K(Ta,Nb)O3 film grown at 750°C, 10 mTorr O2 on 5-3 (100) SrTiO3.

5-4 RHEED patterns obtained during a growth of K(Ta,Nb)O3 on (100) SrTiO3 at 750°C in 10 mTorr oxygen (1) before growth, (2) during early growth, and (3) indicating 3D island growth

5-5 RHEED specular intensity as a function of time during growth of KTN on (100) SrTiO3 at 770°C in 10 mTorr oxygen at laser energy density of 1.7 J/cm2

5-6 RHEED specular intensity as a function of time during growth of KTN on (100) SrTiO3 at 730°C in 10 mTorr oxygen at laser energy density of 1.5 J/cm2

5-7 RHEED specular spot intensity as a function of time showing ablation on the K(Ta,Nb)O3/KNO3 target, ablating on one side of the target at a time onto (100) SrTiO3 at 750°C in 10 mTorr oxygen..

5-8 RHEED specular spot intensity as a function of time during for a single pulse with full relaxation on each side of the K(Ta,Nb)O3/KNO3 target onto (100) SrTiO3 at 750°C in 10 mTorr oxygen

5-9 Auger Electron Spectroscopy results after KTN/KNO3 deposition on SrTiO3 substrates at 750°C, 10 mTorr O2

5-10 AFM scan for a K(Ta,Nb)O3 film grown on (100) SrTiO3 at 750°C in 10 mTorr oxygen.

5-11 AFM scan for an ultra-thin K(Ta,Nb)O3 film grown on (100) SrTiO3 at T=770°C, PO2=50 mTorr (Laser energy=150 mJ/pulse, 1 Hz).

5-12 Capacitance measurements for a 150 nm KTa0.6Nb0.4O3 film grown on SrTiO3 at 750°C in 10 mTorr oxygen.

XRD θ-2θ scan of a 12x12 KTN-STO superlattice grown at 750°C in 10 6-1 mTorr of oxygen on a (100) SrTiO3 substrate.

6-2 Reactive component of the complex impedance measured for superlattice and thin film samples as a function of frequency at 25°C. Note: IDC geometry for STO and KTN thin films are different with larger finger length, L... 98 6-3 Dispersive component of the complex impedance measured for superlattice and thin film samples as a function of frequency at 25°C.

6-4 Measured capacitance values from KTN-STO superlattices of varied periodicity and individual films measured at 25°C.

6-5 Measured dielectric loss tangent from KTN-STO superlattices of varied periodicity and individual films measured at 25°C.

6-6 Real component of the extracted dielectric constant as a function of frequency from KTN-STO superlattices of varied periodicity and individual films measured at 25°C.

6-7 Imaginary component of the dielectric constant as a function of frequency from KTN-STO superlattices of varied periodicity and individual films measured at 25°C

6-8 Extracted capacitance as a function of temperature from a 24x24 KTN-STO superlattice measured at 1 MHz

6-9 Measured loss tangent as a function of temperature for a 24x24 KTN-STO superlattice measured at 1 MHz.

6-10 Electronic properties of a 6x6 KTN-STO superlattice showing: (1) resistivity, (2) mobility, and (3) carrier concentration, measured as a function of temperature.

6-11 HR-TEM images of 6x6 KTN-STO superlattice grown at 750°C in 10 mTorr of oxygen on a (100) SrTiO3 substrate.

7-1 X-ray diffraction scan of a 24x24 BaFeO3-KTa0.47Nb0.53O3 superlattice deposited on a SrTiO3 substrate, comparing as-grown and post-anneal samples.

7-2 Magnetic hysteresis loops of BaFeO3-KTa0.47Nb0.53O3 superlattices and BaFeO3 thin films deposited on SrTiO3 substrates ex situ annealed in flowing oxygen at 900°C for 3 hours.

7-3 Magnetic properties of BFO-KTN superlattices and thin films, measured at 300 K, compared as a function of superlattice periodicity.

7-4 Field cooled and zero field cooled magnetization measured as a function of temperature and Curie temperature estimation for a 6x6 BaFeO3KTa0.47Nb0.53O3 superlattice deposited on a SrTiO3 substrate and ex situ annealed in flowing oxygen at 900°C for 3 hours.

7-5 X-ray photoelectron spectroscopy data for 6x6 BaFeO3-KTa0.47Nb0.53O3 superlattice deposited on a SrTiO3 substrate and annealed in flowing oxygen at 900°C for 3 hours. Fe 2p3/2 peak locations from [178].

7-6 Superlattice layering observed via transmission electron microscopy in a 24x24 BaFeO3-KTa0.47Nb0.53O3 superlattice deposited on a SrTiO3 substrate as grown (1) and annealed (2) in flowing oxygen at 900°C for 3 hours............ 113 7-7 Atomic ordering and superlattice layering observed via transmission electron microscopy of a 24x24 BaFeO3-KTa0.47Nb0.53O3 superlattice deposited on a SrTiO3 substrate after annealing in flowing oxygen at 900°C for 3 hours......... 113 7-8 Energy dispersive spectroscopy measured via scanning transmission electron microscopy for a 24x24 BaFeO3-KTa0.47Nb0.53O3 superlattice deposited on a SrTiO3 substrate annealed in flowing oxygen at 900°C for 3 hours.

7-9 Nanoscale precipitates exhibiting (1) crystalline and (2) amorphous regions observable via transmission electron microscopy in a 24x24 BaFeO3KTa0.47Nb0.53O3 superlattice deposited on a SrTiO3 substrate annealed in flowing oxygen at 900°C for 3 hours.

A-1 Final equation solving for film dielectric constant as a function of known values

A-2 Matlab Code for Excel read/write of data

A-3 Matlab Code for dielectric constant estimation from measured capacitance.... 121 A-4 Top and side views of IDC geometry.

–  –  –

Chair: David Norton Major: Materials Science and Engineering Oxide electronics offer a wide array of interesting properties, including ferromagnetism, ferroelectricity and superconductivity. This dissertation investigates the transport, magnetic, and structural properties of several perovskite and spinel material systems, with particular interest in their manipulation at the nanoscale through the growth of functional heterostructures. Spinel phase ZnCo2O4 is investigated as a ferromagnetic semiconductor whose carrier type changes with oxygen concentration.

The perovskite system of K(Ta,Nb)O3 is investigated for its interesting properties as a solid solution which exhibits a ferroelectric transition dependent on composition. Its thin film epitaxial growth modes are investigated via reflection high energy electron diffraction, with particular attention paid to the volatile potassium ion. Superlattices of K(Ta,Nb)O3 and SrTiO3 are also investigated for the possibility of dielectric enhancement and conductive interfaces. The multiferroic properties of BaFeO3K(Ta,Nb)O3 superlattices are investigated, with particular attention paid to the role of magnetoelectric coupling and strain as stabilizing mechanisms.

–  –  –



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