Influences of Cr Content on the Phase Transformation Properties and Stress Change in V–Cr–O Thin-Film Libraries

VO2-based thin-film libraries with a continuous composition spread of Cr were obtained by reactive cosputtering. Gradual changes in the crystalline structures of VO2 were observed in the thin-film libraries at room temperature as the M1 phase exists for Cr < 1.2 at. %, the M2 phase for Cr > 4.2 at. %, and the T phase in between. Although X-ray diffraction indicates that only VO2 phases exist in the library, X-ray photoelectron spectroscopy reveals an increased V5+/V4+ ratio with increasing Cr content along the V–Cr–O library. A V–Cr–O phase diagram was assessed based on the results of temperature-dependent X-ray diffraction of the libraries. Microstructures of the V–Cr–O libraries were studied by scanning electron microscopy and atomic force microscopy. High-throughput temperature-dependent electrical resistance [R(T)] and stress [σ(T)] measurements were performed on the V–Cr–O libraries to systematically study the influence of Cr on the transformation properties. The transformation temperature Tc was increased by 4.9 K/at. % in the composition range 2.8 at. % < Cr < 7.3 at. % and by 1.2 K/at. % for Cr > 7.3 at. %. The resistance change across the phase transformation was decreased from 3 to 1 order of magnitude with Cr content increasing from 1.1 at. % up to 12.6 at. %, and the R(T) curves became less abrupt. The addition of Cr increased the stress change across the phase transformation up to 1.3 GPa for a Cr content of 3.3 at. %. However, for increased Cr contents from 3.3 to 9 at. %, the stress change decreased to 380 MPa. This could be because of the increased fraction of an O-rich VOx phase in the films and a changed crystallographic orientation for Cr-rich V–Cr–O.