Influence of defects on structural and electrical properties of VO ₂ thin films

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-VO ₂, 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 ΔEpi-VO₂. The larger tensile strain or thicker strained layer leads to lower carrier density and higher E_a, while the constant strain produces saturated ΔE_a.