Bali, Katalin

Hevesi, Imre

Michailovits, Lehel

English

Bali Katalin

Michailovits Lehel

Hevesi Imre

University of Szeged

PREPARATION OF AMORPHOUS V2Os THIN FILMS BY CHEMICAL VAPOR DEPOSITION METHOD By K. BALI, L. MICHAILOVITS and I. HEVESI Institute of Experimental Physics, Attila József University, Szeged (Received January 30,1979) Amorphous vanadium pentoxide thin films have been prepared by chemical vapor decom-position of VOCI3 with HaO in inert atmosphere at room temperature. The films crystallize at aro-und 240°C irreversibly. The d. c. electrical resistivity in the amorphous state is approximately 1-2 orders of magnitude lower than in the polycrystalline state. Vanadium pentoxide, V2Os, is known to contain both V4 + and V5 + ions and the electronic conduction is due to the hopping motion of the unpaired 3d electron from V4 + to V5 + ions (small polaron hopping). Due to the difficulty of preparing pure vitreous V205 most of the previous studies on its semiconducting properties were carried out on glasses containing vanadium pentoxide as a component. Until now vacuum deposition [1-3] and splat cooling [4-6] have been succesfully applied to prepare amorphous V205 . Chemical vapor deposition (CVD) proved to be a convenient method for growing crystalline vanadium oxides [7]. Relatively high reaction temperatures (above 600° C) are needed to grow single crystals of good quality. Since the decomposition of VOCI3 vapor with HaO vapor is a strongly exothermic process, the reaction already takes place at room temperature according to the equation: 2 VOCI3 + 3 H 2 0 - V2Os + 6 HC1. Based on this reaction the CVD method can be applied to obtain amorphous vana-dium pentoxide. In this paper we report the successful preparation of amorphous V205 "thin films by chemical vapor decomposition of VOCl3 with H 2 0 in inert atmosphere. The schematic diagram of the apparatus used is shown in Fig. 1. The vapor of VOCI3 (FLUKA; Switzerland) and H 2 0 thermostated at 22° C was led to the reac-tion chamber by high purity N2 gas flow. To avoid the chemical reaction of water vapor traces in the VOCl3 containing ampulla, the carrier gas was led through an LN2 baffle. As substrate material quartz plates (HERASIL I; Germany) were used. The colour of the freshly prepared films varied from yellow to orange depending on the reaction time typically 10 through 20 minutes. The thickness of the films determined by optical method was of some thousand A. 44 Ä. SÜLI, L. MICHAILOV1TS and I. HEVESI (Corning, England) T 1 =T 2 =T 3 =22° C, V ^ l lit/hours, V2=0.7 lit/hours LIVAGE et al. pointed out that the solubility of the two phases is extremely dif-ferent. Crystalline V 2 0 5 can not be dissolved in water while the amorphous oxide readily dissolves [5, 6]. Our films prepared by the CVD method easily dissolved in water, yielding strongly acidic solution with pH ~ 2, in accordance with the results of LIVAGE and COLLONGUES. The results of microscopic examination in polarized light and electron diffraction pictures unambigously verified that these films were amorphous. The crystallization temperature of the amorphous oxide obtained by splat cooling is between 180 and 200° C [5, 6]. The crystallization is an irreversible, strongly exothermic process. Because the crystallization can be followed by electrical measure-ments, we studied the temperature dependence of the d.c. electrical resistivity of the amorphous V 2 0 5 thin films from room temperature up to 300° C in dry 0 2 atmosphere. The platinium electrode were deposited on the substrates by vacuum evaporation before the preparation of the films. Applying a constant heating rate of 0.5° C/min, a decrease in the conductivity could be observed at around 240° C in each case. We found a ratio of 10—102 between the conductivity of the two phases. The material of the films heated above 240° C was insoluble in water. The PREPARATION OF AMORPHOUS V 20 5 THIN FILMS 4 5 microscopic re-examination in polarized light demonstrated, that the films had been crystallized (Fig. 2). To determine more precisely the va-lue of the crystallization temperature the influence of the annealing was studied on initially amorphous films. The samples were held for 3 hours at different tempe-ratures: (a) 180° C, (b) 200° C, (c) 220° C, and (d) 240° C. The resistivity of the samples was steadily measured during this annealing process. Samples (a), (b), and (c) exhibited no changes in the resistivity, while the resistivity of sample (d) increased by approximately two orders of magnitude. The microscopic examination showed that the film when annealed at 240° C crystal-lized completely. Summing up we conclude that the chemical vapor decomposition of VOCl3 with HaO in inert atmosphere at room temperature proved to be a reliable method for preparing amorphous vanadium pentoxide thin films. The films crystallize at around 240° C irreversibly. The d.c. electrical conductivity in the amorphous state is approximately two orders of magnitude higher than in the polycrystalline state. References [1] Kennedy, T. N.. R. Hakim and J. D. Mackenzie: Mat. Res. Bull. 2, 193 (1967). [2] Rjanjel, E. F., V. I. Gaman and V. M. Kaligina: Izv. Vuz. Fiz. no. 2, 102 (1976). [3] Allersma, T., R. Hakim, T. N. Kennedy and J. D. Mackenzie: J. Chem. Phys. 46, 154 (1967). [4] Kahn, A., J. Livage and R. Collongues: phys. stat. sol. (a) 26, 175 (1974). [5] Rivolen, L., A. Revcolevschi, J. Livage and R. Collongues: J. Non-Cryst. Solids 21. .171 (1976) [6] Livage, J. and R. Collongues: Mat. Sci. Eng. 23, 297 (1976). [7] Takei, H.: Jap. J. Appl. Phys. 7(8), 827 (1968). ПРИГОТОВЛЕНИЕ АМОРФНЫХ ТОНКИХ СЛОЕВ У205 МЕТОДОМ РАЗЛОЖЕНИЯ ГАЗОВОЙ ФАЗЫ К. Бали, Л. Михайлович и И. Хевеиш Описано приготовление тонких слоев У205 путем химического разложения газовой фазы УОС13 с Н 2 0 в инертном газе при комнатной температуре. Пленки кристаллизируются около 240 °С по необратимому пути. Электрическое сопротивление аморфного состояния по посто-янному току ниже на 1—2 порядка по сравнению сопротивлением поликристаллического состояния. Fie. 2 

Preparation of amorphous V2O5 thin films by chemical vapour deposition method

http://acta.bibl.u-szeged.hu/24115/1/phys_chem_025_fasc_001_002_043-045.pdf

Preparation of amorphous V2O5 thin films by chemical vapour deposition method

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