Self-Assembled Barium Titanate Nanoscale Films by Molecular Beam Epitaxy

<p>One challenge of investigating ferroelectrics at the nanoscale has been controlling the stoichiometry during growth. Historically, the growth of barium titanate (BaTiO3) by molecular beam epitaxy has relied on a growth technique called shuttered RHEED. Shuttered RHEED controls the stoichiometry of barium titanate through the precise deposition of alternating layers of BaO and TiO2. While this approach has achieved 1% control of stoichiometry, finding self-limiting mechanisms to lock-in stoichiometry has been the focus of the growth community. The Goldschmidt tolerance factor predicts an unstable perovskite when barium sits in the titanium lattice site. The BaO-TiO2 phase diagram predicts a low-solubility (<100 ppm) of excess barium oxide at molecular beam epitaxy (MBE) growth temperatures of 600-800 °C. We show that excess barium provided during MBE growth is a self-limiting mechanism to grow stoichiometric barium titanate thin films. </p> <p>Features in RHEED oscillations were identified for both shuttered RHEED and co-deposition that confirm barium rich growth condition. Barium-rich growth condition was confirmed to lead to bulk BTO values for out-of-plane lattice constant, Ti/Ba ratio, and piezoelectric coefficient for 40 nm thick BTO thin films. Angle-resolved x-ray photoelectron spectroscopy studies show that excess barium accumulates at the surface in the form of a barium-rich surface layer referred to here as BaO. For titanium-rich growth condition, the layer assumed stoichiometric bulk BTO values. The excess barium accumulated at the surface was removed with methanol sonication. </p> <p>Barium titanate thin films were shown to self-assemble when excess barium was provided during co-deposition. A systematic comparison of 5 nm thick BTO films grown comparing the shuttered RHEED and co-deposition growth approaches was performed to prove that excess barium doesn’t incorporate into the film but only as BaO at the surface. Both growth approaches produce identical out-of-plane lattice parameter, Ti/Ba ratio, and piezoelectric coefficients. An enhancement in the d33 for the 5 nm thin films compared to the 40 nm thin films was also observed. The compressive strain on 5 nm thin films enhanced the polarization over fully relaxed 40 nm thin films. </p>

Name thesis:degree_name

Doctor of Philosophy in Microelectronics-Photonics (PhD)

Level thesis:degree_level

Dissertation

Year dc:date.available

2018

Author dc:creator

• Morgan, Timothy Allen

Advisor dc:contributor.advisor

• Salamo, Gregory J.

Contributors dc:contributor

• Bellaiche, Laurent
• Churchill, Hugh O.H.

• Ferroelectrics
• Molecular Beam Epitaxy
• Piezoforce Microscopy
• X-ray Photoelectron Spectroscopy
• Metallurgy
• Nanoscience and Nanotechnology
• Polymer and Organic Materials

Repository record dc:identifier

https://scholarworks.uark.edu/etd/2817

OAI identifier oai:identifier

oai:scholarworks.uark.edu:etd-4366