Influence of W-doping on the Phase Transition in Vanadium Dioxide Thin Films

R.ALIEV, V. KLIMOV, E. SHADRIN

Ioffe Physico-Technical Institute of RussianAcademy of Sciences,

Politekhnicheskaya26, St.Petersburg 194021

RUSSIA

A. ILINSKI, F. SILVA-ANDRADE

Sciences Institute,

Autonomous University of Puebla,

Privada 17 Norte 3417, Puebla, 72050

MEXICO

Abstract: -Thin films of V1-x Wx O2 (0 < x < 0,017) with a different x but identical degree of oxygen stoichiometry are synthesized and investigated. On the base of model supposing, that the polycrystalline film is constructed from grains, which are distributed both on the sizes, and the W-contents, the transformation of a hysteresis loops of film’s conductivity, caused by an increase of W-concentration is explained.

Key-Words: - Semiconductor-metal phase transition, Thermal Hysteresis, Vanadium Dioxide

1 Introduction

Vanadium dioxide (VO2) undergoes the metal - semiconductor phase transition (MSPT) at Тс=670С [1]. Above Тс this substance has a tetragonal symmetry of a crystal lattice and metal conductivity. Below Tс the symmetry of a lattice changes to monoclinic, and vanadium dioxide gets semiconductor properties (with band gap ~0,7 eV). MSPT is accompanied by an appreciable change of conductivity and optical properties VO2. These changes are the base an action principle of a lot of devices using VO2 thin films (thermorelay, bolometer, optical limiters etc.) [1,2].

The improvementof a design of the mentioned devices requires decreasing of temperature of MSPT, which results in decrease of a power threshold of their operation. This threshold is determined, except for the transition latent heat (~1000 cal/mol), difference between room temperature and MSPT-temperature. In addition to this, the decreasing of Tc up to room temperature improve conditions of thermostabilization of devices that use VO2-films as optical memory elements or optical limiters. The decreasing of MSPT-temperature in vanadium dioxide is carried out by introduction of impurities, which valency does not coincide with valency of ions of the basic lattice [1]. In this paper the tungsten as such impurity is chosen.

In VO2 crystal lattice at replacement of V4+ ion to W6+ ion for maintenance of an electroneutrality in the crystal lattice arise two V3+ ions. It means that the doping results in formation of valencies donor defects [3]. Influence of these defects on properties of a material results in increase of conductivity of a semiconductor phase and in reduction of MSPT temperature. According to [1] the degree of reduction of Tc linearly depends on concentration of arisen ions V3+. The factor of proportionality of this dependence is equal 12.1o for each atomic percent [1].

2 Results and discussion

V1-xWxO2 films withthickness of 90050 Å have been synthesized by a method of simultaneous laser evaporation of metallic V and W targets in a oxygen stream of low pressure. At film’s synthesis the oxygen pressure in the synthesizing device was ~ 40 mTorr, and the temperature of Si (100) single crystal substrates was near 500о C.

Earlier we used the laser evaporation method for synthesis on rutile and sapphire substrates of not doped epitaxial films ofvanadium dioxide[4, 5]. It is necessary to note, that the laser evaporation method allows to synthesizeVO2 thin filmswith the set of an impurity contents by insertin the working chamber of the synthesizing device of an additional target from doping element. After preliminary calibration experiments the concentration W in VO2 filmcan be established by fixation of a ratio of evaporation times of targets from tungsten and vanadium. The last conclusion is fair only in the case of exact reproduction of synthesis conditions of filmswith various contents of W (substrate temperature, oxygen pressure, energy of laser pulses, etc.)

Fixing of a constancy of synthesis conditions is important for creation of a stoichiometry constancy of samples with different contents of W. The point is that the reduction of the oxygen contents can result in a case of dissociation ofneutral oxygen vacancies to formation of V3+ions and thus, to reduction of MSPT temperature.

Fig. 1 Change of the form of a thermal hysteresis loop of the conductivity of V1-хWхO2 films at increase of a degree of W-doping.

A: x=0, B: x=0,006, C: x=0,008, D: x=0,017.

On Fig. 1 are shown hysteresis loops of the conductivity of V1-xWxO2 films, that are registered with the help of an alternating current at frequency of 200 Hz – 200 kHz. Obviously a reduction of Tc with increasing of x is observed. Dependence of Tc from x with good accuracy toexpressionТс = -24.2o x is represented. This result corresponds to conclusions of [1]. It is the additional evidence that the stoichiometry of films with the different contents of tungsten is identical. It is visible the change of the form of a hysteresis loop and its narrowing at increase of x.

Interpretation of the results is based on model [6,7] according to which the hysteresis loop of a film is the sum of elementary loops corresponding to each grain of a film. It is supposed, that elementary loops have the rectangular form, and their width and position on a temperature scale are distributed in the intervals determined by conditions of synthesis of a film and from substrate material. For each grain the width of an elementary loop is determined by a grain size and value of a superficial tension of a grain, and the phase equilibrium temperature Tc is determined by concentration of W and a degree of nonstoichiometry ofvanadium dioxide in each grain. In this model the increasing of temperature extention is connected with expansion of grains distribution against phases balance temperatures. The expansion of grains distribution by increasing ofxis explained by increasing of interval in which temperatures of phases equilibrium are distributed. The narrowing of a main hysteresis loop by increasing of W-contents is explained by reduction of width of elementary loops (by increasing x).

As was told above, width of an elementary loop is determined by both the size of a grain and the size of a superficial tension of a grain. But it is known [8], that the size of grains is determined by the temperature of substrate and pressure of oxygen during the process of synthesis of a film. As the conditions of film's synthesis with the different W-contents we supported constant, it is reasonable to consider, that both average size of grains, and the distribution of number of grains versus the sizes in films synthesized by us are not depending on the W-contents.

It is demonstrated by results AFM - investigation (Fig. 2 and Fig. 3).

Fig.2 AFM-image and hystogram of distribution of number of grains versus its sizes for a non-doping vanadium dioxide film.

At the same time, the introduction of an impurity can change elastic constants of VO2 and the value of the superficial tension. If the ion of an impurity have a radius which does not coincide with radius of an ion V4+, this ion is favorable to be going near to defects for reduction of elastic energy of a lattice. Such defects are dislocations and surfaces of division of differing environments. The W-ions concentrate on surfaces of grains and lowered the energy of a superficial tension of these surfaces. It results in narrowing of elementary loops of each grain. Due to the such narrowing the total hysteresis loop a

Fig.3 AFM-image and hystogram of distribution of number of grains versus its sizes for W–doped vanadium dioxide film with Х = 0,017.

sample with growth of film with concentration of W is narrowed too (Fig. 1). Similar results are described in the literature for V1-x NbxO2 films [9].In work [10] the strong influence of value of a superficial tension of "substrate-film" boundary on width of the main hysteresis loop was established: than less this value, than less the width of the main loop. It is reasonable to assume, that the reduction of value of superficial tension of "substrate-film" boundary is equivalent to reduction of coupling between substrate and film. For the benefit of reduction of this coupling at increase of concentration W speaks such fact, established by us: a film with high concentration of an W-impurity (х0,008) are separated from a substrate at their storage in a damp atmosphere. A film with small concentration of an W impurities demonstrate stability to be influenced by the moisture.

Fig.4. Frequency dependence of conductivity vanadium dioxide films: Continuous line - conductivity at frequency of 200 kHz. Dotted line- conductivity at frequency of 200 Hz.

The received results did not depend on frequency of an alternating current, which varied in an interval from 200 Hz up to 200 kHz. On all frequencies we see the narrowing of a hysteresis loops and increasing of their temperature extent. However, for each concentration of W with increasing of frequency the shift of the main hysteresis loop to low temperatures (Fig. 4) took place. Qualitatively this fact can possible to explain to that infinite cluster, due to which begins a percolation [11], on higher frequencies is formed at lower temperatures. Namely, in contrast to a constant current, on an alternating current the percolation through intercontact capacity begins at those smaller temperatures, than thefrequency of current fluctuations becomes higher.

3 Conclusions

Thus, in this work is shown, that the W-doping of VO2 thin films during their synthesis by a laser ablation method displaces a temperature of phases equilibrium to the low temperatures range on 30-40 оС (almost up to room temperature). It is achieved by that each grain of a polycrystalline film synthesized by a laser ablation method contain defects, which presence narrows an elementary hysteresis loop of each grain and moves, as a consequence, to the range of low temperatures the total hysteresis loop of a polycrystalline film. Let's notice, that the AFM measurements show, that the morphological quality of VO2-films is not worsened at W-doping (size of grains and distribution of their number on the sizes are practically identical with W and without it), but the W-doping allows to receive new and useful properties of films.

Acknowledgments

We thank to SEP/SESIC/DGES grant No. 2003-21-001-023 for supporting this study.

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