Finite-size effect on the first-order metal-insulator transition inVO2films grown by metal-organic chemical-vapor deposition

We studied the finite-size effect on the first-order metal-insulator phase transition and the accompanying tetragonal-to-monoclinic structural transition of ${\mathrm{VO}}_2$ films. The ${\mathrm{VO}}_2$ films were epitaxially grown by a metal-organic-chemical-vapor-deposition technique on the (101) growth plane of a 125-Å-thick ${\mathrm{TiO}}_2$ buffer layer which was also epitaxially predeposited on polished sapphire (112¯0) substrates. The thickness of the ${\mathrm{VO}}_2$ films in this study ranges from 60 to 310 Å. We find that ${\mathrm{VO}}_2$ films grow isomorphically on the ${\mathrm{TiO}}_2$ buffer layer resulting in a high degree of epitaxial ${\mathrm{VO}}_2$ films. We determined structural correlation lengths of the ${\mathrm{VO}}_2$ films parallel and normal to the growth plane from the x-ray-diffraction widths of ${\mathrm{VO}}_2$ reflections at room temperature. The structural order parameter associated with the monoclinic distortion and the change in resistivity associated with the metal-insulator phase transition were simultaneously measured using x-ray-diffraction and resistivity measurements. It was found that the transition temperature, width of the transition, and the estimated electronic gap are dependent on the structural correlation length normal to the growth plane. These dependences are discussed in terms of finite-size and substrate effects on the first-order phase transition.