FERROELECTRICITY IN MAGNESIUM SUBSTITUTED ZINC OXIDE THIN FILMS
Open Access
- Author:
- Ferri, Kevin
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- February 19, 2021
- Committee Members:
- Jon-Paul Maria, Dissertation Advisor/Co-Advisor
Jon-Paul Maria, Committee Chair/Co-Chair
Susan E Trolier-Mckinstry, Committee Member
Clive A Randall, Committee Member
Brian Foley, Outside Member
John C Mauro, Program Head/Chair - Keywords:
- ZnO
Ferroelectriccs
MgO
ZnMgO
wakeup
room temperature
sputtering - Abstract:
- Since its discovery, ZnO has been utilized for numerous applications from medicines and food additives, to UV absorbents, coatings, and pigments. Perhaps the most ubiquitous usage for ZnO however, has been in the electronics industry; historically it has been used in varistors for surge protection, but it also has more modern day applications such as solar cells, light emitting diodes, ultraviolet lasers, and gas sensors. Given that the electrical properties of ZnO can be tuned from insulating to conducting by modifying oxygen vacancy concentrations and cation substitutional species, the non-linear properties of ZnO based materials and devices has garnered interest throughout the early 20th and 21st centuries. As modern technology pushes the limits of electronic components and demands for devices with increased functionality skyrocket, research into low-cost materials systems with tunable properties has subsequently increased. In particular, this dissertation assess modern demands for non-linear functional properties in ZnO-based materials, specifically within the context of ferroelectric thin film systems. Modern demands from the microelectronics community has led to renewed interest regarding the integration between ferroelectric capacitors and mainstream semiconductors within the same device structure. To do so would allow for new device architectures which could result in enhanced functionalities for modern electronics. In order to achieve this, new ferroelectric materials must be developed which can be integrated in conditions conforming to modern microelectronics processing; for example the Si-CMOS process which requires strict temperature and pO2 regulation. Historically, ferroelectric materials such as lead ziconate-titanate (PZT), strontium bismuth tantalate (SBT), and bismuth titanate (BiT) have been used most commonly for integrated circuitry. For modern applications, these materials possess several disadvantages including low paraelectric transition temperatures and minimal compatibility with common integrated manufacturing processes. Following work by Kiel University in 2019 on ferroelectricity in Al(1-x)Sc(x)N, it is important to assess whether such an effect is unique to III-V nitrides, or whether perturbations along composition driven phase boundaries is a general approach for finding new ferroelectric materials; a theory this dissertation refers to as "ferroelectrics everywhere". This dissertation provides support for this new theory by demonstrating ferroelectric switching behavior in the Zn(1-x)Mg(x)O system for the first time. Thin film ZnO substituted with MgO in concentrations approaching phase separation show hysteretic switching with remanent polarization values in excess of 100 uC/cm2 and coercive field values ranging from 1.7 - 3 MV/cm. In order to achieve robust switching behavior, Zn(1-x)Mg(x)O capacitors must undergo a "wakeup" process in which stable remanent polarization values are achieved only after a capacitor has been cycled under a constant field for 100 cycles. In addition to ferroelectric properties, structural trends reveal that with increasing Mg content the wurtzite c-lattice parameter decreases while the a-lattice parameter increases, though the c/a -ratio largely remains constant. Optical measurements via UV-Vis spectroscopy demonstrate that bandgap values increase with Mg additions, pushing to a value of 4.1 eV at x = 0.34. Dielectric trends show relative permittivity values approaching 20 with 34% Mg, and a steady decrease in loss tangent values pushing below 0.01 with increasing Mg content. Surface morphology trends show the formation of abnormally oriented grains for Mg concentrations exceeding 29% though these can be eliminated by increasing the partial pressure of oxygen during deposition. Room temperature experiments revealed that robust ferroelectric switching behavior was possible for capacitors fully fabricated at ~ 26C. Future studies are proposed with the goal of increasing film resistance at lower thickness values through a refinement of the deposition process or utilization of alternative substrates. Lastly, several experiments are proposed with the goal of better understanding the switching mechanisms within the new wurtzite class of ferroelectric materials.