Electron microscopy and spectroscopy demonstrate nanoscale features of barium titanate thin films

Ferroelectric thin films and capacitors have garnered widespread use for applications such as non-volatile memory and tunable microwave circuits. New designs require much thinner ferroelectric layers, so understanding their detailed atomic structure, especially ferroelectric-conductor interfaces, becomes increasingly important. One paper looks to explore such properties on the nanoscale.

Wu et al. have characterized the structural features of dielectric barium titanate (BTO) after breakdown. After using molecular beam epitaxy or atomic-layer deposition to grow thin BTO films on niobium-doped strontium titanate substrates, the group electrically stressed the material and explored the mechanisms of its degradation using advanced electron microscopy techniques and electron energy-loss spectroscopy.

The 20- to 60-nanometer thick BTO-based dielectrics failed at electric fields of 1.5 to 3.0 megavolts per centimeter. Electron microscopy revealed that the crystalline films suffered widespread structural damage, developing substantial polycrystallinity and becoming amorphous in some regions.

Electron energy-loss analysis indicated loss of stoichiometry that the group attributed to the accumulation of oxygen vacancies. Stressed heterostructures that had been patterned by electron lithography likewise showed degraded crystallinity as well as oxygen loss.

“The perovskites are relatively easy to grow. They can tolerate being off stoichiometry and seem fine in X-ray diffraction analysis,” said author John Ekerdt. “One might think one has grown a good crystal. However, these electrical studies required very tight tolerances on stoichiometry between barium and titanium, and very low concentrations of oxygen vacancies were essential.”

Ekerdt said the group looks to continue studying the role of defects and how they affect dielectric and nonlinear properties for this class of complex oxides.

Source: “Dielectric breakdown in epitaxial BaTiO₃ thin films,” by HsinWei Wu, Patrick Ponath, Edward L. Lin, Robert M. Wallace, Chadwin Young, John G. Ekerdt, Alexander A. Demkov, Martha R. McCartney, and David J. Smith, Journal of Vacuum Science & Technology B (2020). The article can be accessed at https://doi.org/10.1116/6.0000237 .