The structure, metal-insulator transition (MIT), and related Terahertz (THz) transmission characteristics of VO₂ thin films obtained by sputtering deposition on c-, r-, and m-plane sapphire substrates were investigated by different techniques. On c-sapphire, monoclinic VO₂ films were characterized to be epitaxial films with triple domain structure caused by β-angle mismatch. Monoclinic VO₂ β angle of 122.2° and the two angles of V⁴⁺–V⁴⁺ chain deviating from the a_m axis of 4.4° and 4.3° are determined. On r-sapphire, tetragonal VO₂ was determined to be epitaxially deposited with VO₂ (011)_T perpendicular to the growth direction, while the structural phase transformation into lower symmetric monoclinic phase results in $\left(\overline{2}11\right)$ and (200) orientations forming a twinned structure. VO₂ on m-sapphire has several growth orientations, related with the uneven substrate surface and possible inter-diffusion between film and substrate. Measurements of the electrical properties show that the sample on r-sapphire has MIT property superior to the other two samples, with a resistivity change as large as 9 × 10⁴ times and a transition window as narrow as 3.9 K, and it has the highest resistivity with the lowest free carrier density in the insulating phase. THz transmission measurements on VO₂ films grown on r-plane sapphire substrates revealed intensity modulation depth as large as 98% over a broadband THz region, suggesting that VO₂ films are ideal material candidates for THz modulation applications.

We present the structural and electrical properties of (011) preferred polycrystalline (Poly) and multidomain (020) epitaxial (Epi) VO₂ thin films grown at different temperature (T_s) and on different substrates with variable defects. These defects cause variation in strain, metal-insulator transition (MIT) temperature (T_MIT), activation energy (ΔE_a), and charge carrier type in insulating phase. Both the Poly- and Epi-VO₂ behave n-type conductivity when grown at relative low T_S. As T_S increases, defects related acceptor density increases to alter conductivity from n- to p-type in the Poly-VO₂, while in the Epi-VO₂ donor density increases to maintain n-type conductivity. Moreover, the strain along monoclinic a_m axis dramatically reverses from tensile to compressive in both the Poly- (848 K < T_S < 873 K) and Epi-VO₂ (873 K < T_S < 898 K), and eventually approaches to a constant in the Poly-VO₂ (T_S ≥ 898 K) in particular. T_MIT decreases with increasing the carrier density independent of the conductive type in the lightly doped Poly- and Epi-VO₂; however, this trend is reversed in heavily doped n-type Epi-VO₂ with a higher T_MIT due to the formation of large quantity of small polarons related with V⁴⁺-V²⁺ pair. ΔE_a is associated with the carrier density and thus the strain or strained interfacial layer thickness in the Poly- or Epi-VO₂. The larger tensile strain or thicker strained layer leads to lower carrier density and higher ΔE_a, while the constant strain produces saturated ΔE_a.

We report the epitaxial relationship of ${\text{VO}}_2$ thin-films on $c$ -plane sapphire and their terahertz transmission modulation with temperature. The films exhibit a triple-domain structure caused by the lattice mismatch between monoclinic ${\text{VO}}_2$ and sapphire hexagon. The epitaxial relationship is determined to be ${\text{VO}}_2\left[010\right]\parallel {\text{Al}}_2{\text{O}}_3\left[0001\right]$ and ${\text{VO}}_2\left(\overline{2}02\right)\parallel {\text{Al}}_2{\text{O}}_3\left\{11\overline{2}0\right\}$ , with the in-plane lattice mismatch of 2.66% (tensile) along $\left[\overline{2}02\right]$ and the out-of-plane lattice mismatch of −2.19% (compressive). Terahertz measurements revealed that ${\text{VO}}_2$ films have over fourfold change in transmission during the metal-insulator transition, indicating a strong potential for terahertz wave switching and modulation applications.