Phase transformation properties and stress changes in V_1-xM_x(W, Cr)O₂ thin-film libraries

Xiao Wang, Ruhr-Universität Bochum, Bochum, Germany
Detlef Rogalla, Ruhr-Universität Bochum, Bochum, Germany
Alfred Ludwig, Ruhr-Universität Bochum, Bochum, Germany

VO₂ film undergoes a semiconductor-to-metal transformation at a critical temperature Tc, which is accompanied by crystallographic structure change from the monoclinic phase (M1) to rutile phase (R). Due to the strain change of up to 1% along the rutile axis and high Young’s modulus of about 140 GPa, high stress change in the thin film-substrate combination is associated with the phase transformation, which make the VO₂ promising in application of micro/nano-actuators. In present study, V_1-xW_xO₂ and V_1-xCr_xO₂ thin film libraries were fabricated by reactive co-sputtering, and high-throughput characterizations of their compositions, crystalline structures, transformation properties and stress changes were performed. In the V_1-xW_xO₂ library, the evolution of crystalline structure from M1 phase to R phase was observed at room temperature as the W amount increases from x=0.007 to 0.044. Stress changes across the phase transformation decrease from ~700 MPa to ~250. In V_1-xCr_xO₂ libraries, the crystalline structure evolves from M1 to T then to M2 phase as Cr amount increases from x= 0.008 to 0.128. The stress changes increase from ~900 MPa to ~1250 MPa with addition of Cr. In addition, the effect of microstructure of the thin films on the stress change was studied. Tc decreases with W in V_1-xW_xO₂, while it increases with Cr amount in V_1-xCr_xO₂. A phase diagram of V_1-xM_x (W, Cr)O₂ was assessed in dependence of temperature and concentration of W and Cr.

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