Chemical potential gradient induced formation of Kirkendall voids at the epitaxial TiN/MgO interface.

We report the observation of Kirkendall voids at the epitaxial titanium nitride (TiN)/magnesium oxide(MgO)(001) interface. While epitaxial growth of TiN on MgO has been known for years, many reports show a perfectly sharp epitaxial interface. Because TiN is a prototypical diffusion barrier material, observing the consequence of rapid diffusion at a TiN interface is interesting. Structural characterization of the interface using X-ray diffraction and electron microscopy confirms the diffuse nature of the interface. Rectangular voids that form at the TiN/MgO(001) interface and extend into both TiN and MgO result from a large chemical potential gradient at the interface, which contributes a strong chemical driving force for diffusion. The spatial localization of the observed voids is limited to within ∼10 nm from the interface, consistent with a chemical potential gradient driving force. A composition gradient on the nanometer scale is also observed. Observation of Kirkendall voids at this nitride/oxide interface suggests possibilities for engineering oxygen and nitrogen vacancies at thin film interfaces.