Our new work on SrHfO3, jointly led by recent Auburn Ph.D. Patrick Gemperline and Arashdeep Thind from Robert Klie’s group at UIC, is out today in ACS Applied Electronic Materials. Patrick employed a Hf metal-organic precursor to grow SrHfO3 epitaxial films via hybrid MBE for the first time on GdScO3 and TbScO3, inducing large compressive strains. Arashdeep did some very impressive 4D STEM analysis to look at octahedral tilt patterns on the films and found that they change from the bulk Pnma space group to I4/mcm. We backed this up with DFT analysis by Boris Kiefer, Hf L-edge X-ray absorption by George Sterbinsky, and second harmonic generation measurements by Chunli Tang and Wencan Jin. It turned into a very large undertaking but a really fun project to bring to fruition. Everyone deserves a big round of applause!
What is particularly interesting about the results is that SrHfO3 has long been considered as a possible ferroelectric material if it could be grown with significant compressive strain (>3%). That would be a big breakthrough for gate dielectrics in transistors, given how much interest HfO2 has generated for the reported ferroelectric phase. Our films on the scandate substrates have the largest reported strain in the literature, because past efforts on SrTiO3 were almost fully relaxed. These results show that there is not likely to be any way to grow homogeneous SrHfO3 films that become ferroelectric. There may still be other ways to achieve the goal of ferroelectric hafnates, but we are pretty confident that epitaxial strain is not enough by itself.
Thanks to the National Science Foundation for support of our research on film synthesis through DMR-2045993, computational resources, and the MRI program (DMR-2018794) for the XRD instrument we used. Thanks also to the U.S. Department of Energy (DOE) for the synchrotron work, second harmonic generation (DE-SC0023478) , and electron microscopy (DE-SC0025396)!