Temperature-Dependent Electrical Characterization of Multiferroic BFO Thin Films Danielle Hitchen, Sid Ghosh, K. Hassan, K. Banerjee, J. Huang Electrical and Computer Engineering Rutgers University

Outline  Motivation  Multiferroics  Hysteresis: The Enabling Property  Ferroelectricity  Bismuth Ferrite: Material of Choice  Procedures  Challenges  Data Results  Conclusion  Acknowledgements

Motivation Storage limitations on existing memory devices as well as the desire for faster write/erase capability on non-volatile memory devices has increased the demand for better materials that accord with standard integrated circuit requirements. [3] [1][2] [1] [2] [3] Zambrano, Raffaele. “Applications and issues for ferroelectric NVMs.” Materials Science in Semiconductor Processing 5 (2003)

Multiferroic Materials  Discovered less than a century ago, ferroics relate to the ancient study of magnetism  Ferroic materials can be: Ferroelectric Ferromagnetic Ferroelastic  Multiferroics exhibit two or more of these properties simultaneously

Hysteresis: The Enabling Property Hysteresis: the ‘memory’ a material retains of a previously applied energy field [4] [4]

Ferroelectricity Ferroelectric materials possess a spontaneous, stable polarization that switches hysteretically in an applied electric field. [5] [5]

Ferroelectricity  Polarization characteristics change when subjected to varying Pressure Temperature Applied Voltage  These unique properties make the material useful for many different applications

Bismuth Ferrite: Our Material of Choice  BFO is multiferroic at room temperature– a rarity among multiferroics  Has strong ferroelectric, but weak ferromagnetic properties  Crystalline structure, as well as polarization, alters in varying temperature  We hope to see how well BFO functions as a capacitor  Goal: document the changes in polarization that occur as the temperature changes Fractal ferroelectric domains in thin films of multiferroic BiFeO3. [6] [6]

Procedure The probe (left) controls temperature and pressure in the chamber housing the sample. Leakage current is plotted in the semiconductor precision analyzer (above).

Challenges  The samples were not uniformly dielectric; finding good contacts was difficult  Careful probing was necessary due to the properties of the material  Equipment broke down several times  Redeposition of the contacts appeared to influence the functionality of the devices

Data: Varied Dielectric Behavior Data is from ten contacts on a single sample taken at room temperature.

Data: Polarization Device became more resistive as temperature increased; this is evidenced by the shape of the curve. Ideal Hysteresis [4]

Data: Remanent Polarization Polarization shows an increasing trend at higher temperatures; this is not what is expected, and may relate to the increasing current leakage.

Data: Current Leakage At increasing temperatures, our device leaks more current, as expected. The curved data points are representative of a dielectric; a linear slope would be a purely resistive device.

Data: Remanent Current Leakage As temperatures increase, we see an increasingly leaky device. (All data was taken At -1.5V.) Current leakage is high at high temperatures (20nA/cm 2 vs. 2.0 E 5 nA/cm 2 ).

Conclusion  Dielectric behavior did not characterize the behavior of this material  There was non-uniformity in the samples that DID exhibit capacitive polarization  The contact deposition process may have influenced functionality  Dielectric behavior degraded at higher temperatures, as expected

Acknowledgements I would like to thank the National Science Foundation and the US Department of Defense for funding my research (EEC-NSF Grant # and CMMI-NSF Grant # ), as well as the University of Illinois at Chicago for hosting my undergraduate research program. I would also like to express my thanks to the directors of my program, Professors Christos Takoudis and Greg Jursich, as well as to Professor Siddhartha Ghosh who advised me in my research. Finally, thank you Koushik Banerjee, Jun Huang, Khaled Hassan and Hsu Bo for informing my research, assisting with the equipment, and providing me with necessary literature.