FINITE-SIZE EFFECT ON THE 1ST-ORDER METAL-INSULATOR-TRANSITION IN VO2FILMS GROWN BY METAL-ORGANIC CHEMICAL-VAPOR-DEPOSITION

We studied the finite-size effect on the first-order metal-insulator p hase transition and the accompanying tetragonal-to-monoclinic structur al transition of VO2 films. The VO2 films were epitaxially grown by a metal-organic chemical-vapor-deposition technique on the (101) growth plane of a 125-angstrom-thick TiO2 buffer layer which was also epitaxi ally predeposited on polished sapphire (1120BAR) substrates. The thick ness of the VO2 films in this study ranges from 60 to 310 angstrom. We find that VO2 films grow isomorphically on the TiO2 buffer layer resu lting in a high degree of epitaxial VO2 films. We determined structura l correlation lengths of the VO2 films parallel and normal to the grow th plane from the x-ray-diffraction widths of VO2 reflections at room temperature. The structural order parameter associated with the monocl inic distortion and the change in resistivity associated with the meta l-insulator phase transition were simultaneously measured using x-ray- diffraction and resistivity measurements. It was found that the transi tion temperature, width of the transition, and the estimated electroni c 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.