The stress release of morphological change on thermochromic properties of nitrogen-incorporated VO2 thin films

Abstract

VO2 thin films are fabricated by the reactive high power impulse magnetron sputtering technique. Their thermochromic properties are found to be greatly affected by the addition of nitrogen during the deposition process. These include an effect of localized surface plasmon resonance due to isolated island structures. Furthermore, low transition temperatures below 45 °C are observed due to oxygen-deficient conditions. Also, the transition temperature decreases with an increase in the thickness of the TiO2 buffer layer. The reduction in transition temperature could be as low as 39 °C with a solar modulating ability of 4% at a thicker buffer of 300 nm under a high gas ratio of 21.7% for nitrogen. The crystalline phase is identified by x-ray diffraction, showing that the intensity of monoclinic crystallites at a diffraction angle of 2θ = 27.8° for (011) phase decreases with an increase in the amount of nitrogen, whereas a relaxing shift is detected near the diffraction angle of 2θ = 37.0° for (2¯11) phase. Similar behavior is seen in the peak shift of the (004) phase for TiO2. The d-spacing of the crystallization phase with island structure is identified by high-resolution transmission electron microscopy. The evolution of stress release, which is strongly dependent on surface morphology, is consistent with x-ray pole figure representation. The visible transmittance and solar modulation ability as functions of the amount of nitrogen and the buffer thickness of TiO2 are discussed.