Date of Graduation

5-2018

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Microelectronics-Photonics (PhD)

Degree Level

Graduate

Department

Graduate School

Advisor

Gregory Salamo

Committee Member

Laurent Bellaiche

Second Committee Member

Hugh Churchill

Third Committee Member

Morgan Ware

Fourth Committee Member

Rick Wise

Keywords

Ferroelectrics, Molecular Beam Epitaxy, Piezoforce Microscopy, X-ray Photoelectron Spectroscopy

Abstract

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.

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.

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.

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