The Finite Size Effect on The Metal-Insulator Transition of VO 2 Films Grown by MOCVD
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THE FINITE SIZE EFFECT ON THE METAL-INSULATOR TRANSITION OF V0 FILMS GROWN BY MOCVD HYUNG KOOK KIM, R.P. CHIARELLO, HOYDOO YOU, M.H.L. CHANG, ZHANG AND D. J. LAM Argonne National Laboratory, Materials Science Division Argonne, Illinois 60439-4837
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T.J.
ABSTRACT We studied the finite size effect on the metal-insulator phase transition and the accompanying tetragonal to monoclinic structural phase transition of V0 2 films grown by MOCVD. X-ray diffraction measurements and electrical conductivity measurements were done as a function of temperature for V0 2 films with out-of plane particle size ranging from 60-310A. Each V02 film was grown on a thin TiO2 buffer layer, which in turn was grown by MOCVD on a polished sapphire (1120) substrate. The transition was found to be first order. As the out-of plane particle size becomes larger, the transition temperature shifts and the transition width narrows. For the 60A film the transition was observed at -610 C with 0 a transition width of -10 C, while for the 310A film the transition temperature was -590 C and the transition width -2 0 C. We also observed thermal hysteresis for each film, which became smaller with increasing particle size. INTRODUCTION The metal-insulator phase transition (MIPT) is a subject of intense theoretical(l] and experimental[2-6] interest. In spite of the similarities in the structural and electronic properties of metal oxide systems the nature of the MIPT can be first order in some systems such as V0 2 [1,2] or continuous in others such as NbO2 (3]. MIPT for bulk system occurs at -67"C and is accompanied by a slight structural distortion from a tetragonal rutile structure in the high temperature metallic phase to a monoclinic structure in the low temperature insulating phase[2,4]. In this paper we present the result of simultaneous x-ray diffraction and electrical conductivity measurements of MIPT for V0 2 films grown by MOCVD as a function of the out-of plane particle size (d(A)) of the film. We observed thermal hysteresis for each film which is consistent with the first order nature of the transition. Also, as the out-of plane particle size increases, the transition temperature shifts and the transition width becomes narrower. We attempt to compare this result with the finite size effect on the first-order transition[7]. EXPERIMENTAL The detailed description of our deposition system and sample preparation procedure was presented elsewhere[5,8,9].
Mat. Res. Soc. Symp. Proc. Vol. 237. 01992 Materials Research Society
418
The V02 films were grown on a uniformly thin (-125A) TiO2 buffer layer which in turn was grown using MOCVD on a polished sapphire (1120) substrate. By growing the V0 2 at 500"C on TiO2 we could obtain good quality epitaxial films of monoclinic structure with in-plane particle size of -1000A at room temperature[5,8,9]. The V0 2 growth plane was (200) which is, in effect, the tetragonal rutile (101) plane. The (313) diffraction peak was used to determine the particle size of V0 2 films and to study the structural transition. I
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