Mechanisms of electrical transition using a conformal MoO_x cap on Mo-doped VO₂ thermochromics

Mo-doped VO₂ thin-films are deposited on the soda-lime glass by a reactive high-power impulse magnetron co-sputtering technique (R-HiPIMS). Rapid thermal annealing at 500 °C for 3 min is performed to achieve the fabrication under low thermal budget. To prevent VO₂ from further oxidation, a conformal and semi-conducting MoO_x film is applied as the capping layer by the plasma-enhanced atomic layer deposition (PE-ALD). After MoO_x film is deposited, an electrical transition in the resistance ratio of 6–10 orders of magnitude has been found, especially for MoO_x thicknesses less than 20 nm. A secondary phase of V₂O₃ plays an important role in abrupt change of current transportation. Also, the hysteresis of optical transition in transmittance at a wavelength of 2500 nm shows that both the width and transition temperature (T_C) decrease with increasing the thickness of the MoO_x cap. Moreover, the effect of localized surface plasmonic resonance (LSPR) due to the Karst-like structure has been detected by the absorption spectrum. The variations in LSPR peak, such as intensity, blue-shifting, and red-shifting, originated from either excess carrier or structure change are discussed. The high aspect ratio of surface morphology is further examined by atomic force microscopy. In addition, the coverage of MoO_x on Mo-doped VO₂ concerning the friction behavior is evaluated by lateral force microscopy (LFM), which is helpful in clarifying the responsible mechanisms during T_C transition.