Ferroelectricity of wurtzite Al_1−xHf_xN heterovalent alloys

Thin films of aluminum hafnium nitride (Al1−xHfxN) were synthesized via reactive magnetron sputtering for Hf contents up to x = 0.13. X-ray diffraction showed a single c-axis oriented wurtzite phase for all films. Hard x-ray photoelectron spectroscopy demonstrated homogeneous Al:Hf distribution through the thin films and confirmed their insulating character. A collection of complementary tests showed unambiguous polarization inversion, and thus ferroelectricity in multiple samples. Current density vs electric field hysteresis measurements showed distinct ferroelectric switching current peaks, the piezoelectric coefficient d_33,f,meas measured using a double beam laser interferometer (DBLI) showed a reversal in sign with similar magnitude, and anisotropic wet etching confirmed field-induced polarization inversion. This demonstrates the possibility of using tetravalent–and not just trivalent–alloying elements to enable ferroelectricity in AlN-based thin films, highlighting the compositional flexibility of ferroelectricity in wurtzites and greatly expanding the chemistries that can be considered for future devices.. Authors: Bernstein, Nate S. P. [Colorado School of Mines, Golden, CO (United States)] (ORCID:0009000914041157); Drury, Daniel [Army Research Lab., Adelphi, MD (United States)] (ORCID:0000000284135927); Lee, Cheng-Wei [Colorado School of Mines, Golden, CO (United States)] (ORCID:000000028317588X); Shimada, Tatau [TAIYO YUDEN Co., Ltd., Tokyo (Japan)] (ORCID:0009000904995600); Sakai, Yuki [TAIYO YUDEN Co., Ltd., Tokyo (Japan)] (ORCID:000900018426610X). DOE Contract: AC36-08GO28308. Subjects: 36 MATERIALS SCIENCE; crystal structure; crystallographic defects; ferroelectric; heterovalent alloy; magnetron sputtering; nitride; nitrides; piezoelectricity; sputtering; thin films; wurtzite; x-ray photoelectron spectroscopy