Influence of defects on structural and electrical properties of VO2 thin films

Abstract

We present the structural and electrical properties of (011) preferred polycrystalline (Poly) and multidomain (020) epitaxial (Epi) VO2 thin films grown at different temperature (Ts) and on different substrates with variable defects. These defects cause variation in strain, metal-insulator transition (MIT) temperature (TMIT), activation energy (ΔEa), and charge carrier type in insulating phase. Both the Poly- and Epi-VO2 behave n-type conductivity when grown at relative low TS. As TS 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 am axis dramatically reverses from tensile to compressive in both the Poly- (848 K < TS < 873 K) and Epi-VO2 (873 K < TS < 898 K), and eventually approaches to a constant in the Poly-VO2 (TS ≥ 898 K) in particular. TMIT 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 TMIT due to the formation of large quantity of small polarons related with V4+-V2+ pair. ΔEa 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 ΔEa, while the constant strain produces saturated ΔEa.