Invited Lecture of IRTG 2495: Defects in Nitrides, Fundamentals of Texturing, and Optical Sintering

Prof. Geoff Brennecka
Colorado School of Mines, USA

Defects in Nitrides, Fundamentals of Texturing, and Optical Sintering

This presentation will provide a brief overview of the research in Prof. Geoff Brennecka's group at Colorado School of Mines, including work on textured ceramics, optical sintering, and sputtered nitride ferroelectrics.
Templated grain growth (TGG) is a clever method for imparting crystallographic texture in polycrystalline ceramics, but simultaneous densification and grain growth in inherently heterogeneous (and often bimodal) microstructures make reliably achieving full densification difficult. As part of an effort to effectively simulate this TGG process across multiple length scales, aerosol deposition (AD) is used to create dense, fine-grain matrices into which single-crystal templates can be ripened. Grain growth studies on these and complementary sample sets via traditional compaction and tape-cast TGG with seed platelets provide key input parameters for simulations as well as parallel model validation. A low cost optical furnace for rapid densification has also been developed; using simple broadband blackbody radiation, BaTiO3, PMN-PT can be sintered, and many other samples in seconds.
Prof. Geoff Brennecka's group also works extensively on sputtered AlN-based thin films, which have dominated the market of piezoelectric microelectromechanical system (MEMS) resonators for many years and have recently attracted increased interest for their ferroelectric response. Their work focuses on the roles of structurally-disruptive isovalent (e.g., Sc, B, La, Gd for Al) substitutions and on nominally donor (e.g., O for N; Si, Hf, Zr for Al) defects in AlN. The large bandgap of AlN combined with non-equilibrium sputter deposition provides a great deal of flexibility for both iso- and hetero-valent alloy / defect engineering of AlN-based films for ferroelectric and piezoelectric applications. In fact, films with upwards of 10%donor substitution can not only remain sufficiently insulating for piezoelectric and/or ferroelectric applications but may also exhibit reduced coercive field and/or increased piezoelectric response over baseline AlN. Work from the group and others is revealing effects of such defects and non-stoichiometry on polarity inversion, both static during growth and dynamic during ferroelectric switching, and associated impacts on domain walls and inversion domain boundaries.