**1. Introduction**

The interest in the surface structures with their special properties has increased considerably due to extensive applications in micro- and optoelectronics. It is known that the properties of films of submicron size can be different from those of structures having macroscopic dimensions. The parameters that change the properties of films, are the thickness, number of layers, uniformity of the films, the size of clusters and nanocrystals. The presence of small particles and nano-sized elements leads to changes in material properties such as electrical conductivity, refractive index, band gap, magnetic properties, strength, and others (Suzdalev, 2006; Kobayashi, 2005).

One of the most promising materials in this regard is tin dioxide. Such advantages as high transparency in a visible range of wavelengths and high conductivity make SnO2 very suitable as transparent conductive electrodes in such devices as solar sells, flat panel displays, etc (Rembeza et al., 2001; Jarzebski et al., 1976; Das and Banerjee, 1987; Song, 1999). Wide-gap semiconductor SnOx films exhibit quantum confinement effect with decreasing of crystallite sizes, i.e. the band gap becomes size dependent and is increased from 3.6 to 4.2 eV.

Semiconductor gas sensors on the base of nanoscale SnO2 films are manufactured (Evdokimov et al., 1983; Buturlin et al., 1983b; Watson et al., 1993). The possibility of tin dioxide layers to change their electrical conductivity upon adsorption of gases due to the reactions of reduction and oxidation, is used (Bakin et al., 1997; Srivastava, R. et al., 1998a; Jiang et.al., 2002; Vigleb, 1989). An increase in adsorption possibilities of SnO2 films during the transition from single-crystal to nanocrystalline system (Srivastava, R. et al., 1998b) is one of the main directions of work to improve the sensitivity and reduce of response time (Bakin et al., 1997; Ramamoorthy et al., 2003; Karapatnitski et al., 2000; Xu et al., 1991; McDonagh et al., 2002).

As is known, tin dioxide (SnО2) is a crystal of white color, the density is 7.0096 g/cm3, melting point is about 2000°C (Knuniants, 1964). The SnO2 films have predominantly amorphous or polycrystalline structure with a tetragonal lattice of rutile with parameters a = b = 0.4737 nm, c = 0.3185 nm, with two tin atoms and four oxygen atoms in the unit cell (Dibbern et al., 1986; Shanthi et al., 1981; Weigtens et al., 1991). Depending on the method of

Influence of Crystallization on the Properties of SnO2 Thin Films 221

from 4 nm with increasing annealing temperature to 13 nm at 400ºC and 27 nm at 900ºC. The dependences of the electrical resistance of the films in air (Ra) and a mixture of hydrogen-air (Rg) at 300ºС on the crystallite sizes were presented. The values of Ra and Rg increase with decreasing D for D ≤ 6 nm, as well as increase with increasing D for D ≥ 10 nm. The value of D at the boundary between two regions is equal to 2L and the optimum value for L is obtained by 3 nm. The gas sensitivity of film was determined as Ra/Rg. For the both H2 and СО the sensitivity increases if the value D ≤ 2L (~6 нм). It follows, that the gas-sensitive properties of the tin dioxide films are determined by their nanostructure, formed during crystallization process. This makes it extremely important to study the processes of crystallization in tin dioxide films, depending on the conditions of their preparation and

Good quality SnO2 films have been grown by several techniques (Buturlin et al., 1983a). As is known, the method of preparation of SnOx films significantly influences on their characteristics (Bakin et al., 1997; Srivastava R. et al, 1998; Jiang et al., 2002; Srivastava R. et al., 1998b; Minami et al, 1989). Not all techniques of deposition are used for production of functional metal oxide films (Buturlin et al., 1983b; Jarzebski, 1982; Kholʹkin and Patrusheva, 2006). The physical methods of obtaining films of metal oxides include the method of thermal evaporation of metal in vacuum with deposition on the dielectric substrate and oxidation in air (Buturlin et al., 1983b) at 720–770К. The particles of deposited material one can obtain also by evaporation in a gas. In an atmosphere of reactive gas material is heated to evaporation, then the atoms of substance reacts with residual gas atoms, resulting in formation of ultrafine particles of size in range 1–100 nm. The resulting film has an extremely developed surface and requires no further heat treatment (Buturlin et al., 1983a). During thermal sputtering of an oxide target (Das and Banerjee, 1987; Feng et al., 1979) an annealing is not required, too. The method is based on the sputtering of target made of pressed powder of desired oxide (Jarzebski, 1982) or in plasma at high frequency in a threeelectrode circuit, or by an ion beam. The spraying process is carried out in an plasma of argon of high purity with the addition of oxygen. Das and Banerjee (1987) show the relation between a structure and other physical properties of the films. For the deposited by electronbeam evaporation SnOx films a change of *x* value by varying the substrate temperature TS was revealed. The films deposited at TS = 150°C were amorphous, but after annealing in air (TA = 550°C, 2h) the presence of SnO2 by X-ray diffraction was revealed. Films deposited at TS = 225°C were also amorphous, but after annealing under identical conditions consisted of a mixture of phases SnO and β-SnO. These amorphous films are characterized by high electrical resistivity (1010 ohm·cm at room temperature), which sharply decreased after annealing in air (550°C, 2 h) to the values ρ = 9,5×10-2 ohm·cm at room temperature, seemingly, due to the formation of 0.015 eV donor level (TS = 150°C) and 0.26 eV (TS = 250°C) below the bottom of the conduction band of tin oxide. For TS = 150°C the band gap Eg = 3.46 eV corresponds to SnO2 and decreases with increasing TS up to 225°C due to the presence of different oxide phases, and increases again at TS>225°C. Annealing in air (550°C, 2h) leads to deterioration of the transmission T in the wavelength range of 400−850 nm for the films deposited at TS = 150°C and to the increase of transmission for films deposited at TS ≥ 250°C. At TA = 650°C the full transformation into the SnO2 phase with the dominant lines (101) and (200) is completed. The increase of the substrate temperature (TS = 350°C) causes a decrease in the resistance of the film, indicating that activation occurred because of

processing.

donors at a depth of 0.26 eV.

film synthesis, the band gap varies in the range 3.35–4.3 eV and the refractive index – in the range 1.8–2.0 (Martin et al., 1986; Melsheimer et al., 1986; Nagamoto Takao et al., 1990).

Under normal conditions in air, at the surface of tin dioxide, a layer of the adsorbed oxygen molecules is formed. Oxygen molecules carry out the electron trapping from the surface layer (Kaur et al., 2007). In the simpler schematization, in clean air the conductivity of SnO2 is low because the conduction electrons are bound to surface oxygen, whereas in the presence of a reducing gas, electrons are no longer bound to the surface states and the conductivity increases. The transfer of electrons from SnO2 to oxygen leads to the creation near the surface of a crystal a space charge layer (depletion layer), the concentration of electrons in which is less than in bulk. The net charge at the surface generates an electric field, which causes a bending of the energy bands in the SnO2. A negative surface charge bends the bands upward, i.e. pushes the Fermi level into the band gap of the SnO2, effectively reducing the charge carrier concentration and resulting in an electron depletion zone. In other words, this is an electron trapping process at the surface, which leads to an increase in the electrical resistance (or decrease in conductivity) of the surface layer. When a reducing gas, e.g. CO (or Н2, СО, Н2S, NH3, etc.), reacts with the adsorbed oxygen to form CO2, the concentration of oxygen in layer is decreased. The electrons released in this reaction are injected into the conduction band of the SnO2, which results in a decrease in the electrical resistance (or increase in conductivity) of materials of n-type of conductivity (Kaur et al., 2007; Bakin et al., 1997; Andryeeva et al., 1993). According to barrier model, reaction of adsorbed oxygen species with reducing gases decreases the potential barrier heights, resulting in a huge decrease in resistance.

Reduced tin dioxide is characterized by a deficiency of oxygen – SnO2-δ, where 10-5 <δ <10-3 deviation from stoichiometry (Rumyantseva et al., 2003; Kaur et al., 2007). Vacancies of ionized oxygen are the main intrinsic defects and define the electrical properties of the material – n-type of conductivity and free carrier concentration. Energy levels of oxygen vacancies VО+ and VО2+ lie at a depth of 30−40 and 140−150 meV below the conduction band edge, respectively (Ryabtsev et al., 2008). The concentration of oxygen vacancies can be reduced by annealing of the material in an oxygen atmosphere.

In the transition from single crystal to a polycrystalline system, each crystal grain is considered as a closed volume, near the surface of which is located a depletion layer. Reducing of the charge carrier concentration at the grain boundaries leads to a formation of intergranular energy barriers, the magnitudes of which determines the electrical conductivity of polycrystalline material as a whole. The greatest influence of the surface state on the electrical properties of the material occurs when the condition: *l*/2 ≤ *L*, where *l* is the size of the crystallites; *L* − length of the depletion layer, which for different oxides varies

$$\text{from 3 to 10 nm: } L = L\_D \sqrt{\frac{eV\_s^2}{kT}} = \sqrt{\frac{\text{e}kT}{2\pi e^2 N\_d}} \cdot \sqrt{\frac{eV\_s^2}{kT}} = \sqrt{\frac{\text{e}V\_s^2}{2\pi e N\_d}}, \text{ where } \text{e is the dielectric constant.}$$

constant, VS – surface/Schottky potential, Nd is the concentration of the donor impurity, LD is the Debye length, k is the Boltzmann constant, T is the temperature. Wide range of sizes of the SnO2 crystallites (from several to several hundred nanometers) affects the mobility of charge carriers, which may vary from a few to 100 cm3/V·s. For example, Xu Chaonan et al. (1991) investigated the dependence of gas sensitivity of film on the diameter D of SnO2 crystallites (in the range 5–32 nm) and the Debye length LD. For undoped SnO2 D increased

film synthesis, the band gap varies in the range 3.35–4.3 eV and the refractive index – in the range 1.8–2.0 (Martin et al., 1986; Melsheimer et al., 1986; Nagamoto Takao et al., 1990).

Under normal conditions in air, at the surface of tin dioxide, a layer of the adsorbed oxygen molecules is formed. Oxygen molecules carry out the electron trapping from the surface layer (Kaur et al., 2007). In the simpler schematization, in clean air the conductivity of SnO2 is low because the conduction electrons are bound to surface oxygen, whereas in the presence of a reducing gas, electrons are no longer bound to the surface states and the conductivity increases. The transfer of electrons from SnO2 to oxygen leads to the creation near the surface of a crystal a space charge layer (depletion layer), the concentration of electrons in which is less than in bulk. The net charge at the surface generates an electric field, which causes a bending of the energy bands in the SnO2. A negative surface charge bends the bands upward, i.e. pushes the Fermi level into the band gap of the SnO2, effectively reducing the charge carrier concentration and resulting in an electron depletion zone. In other words, this is an electron trapping process at the surface, which leads to an increase in the electrical resistance (or decrease in conductivity) of the surface layer. When a reducing gas, e.g. CO (or Н2, СО, Н2S, NH3, etc.), reacts with the adsorbed oxygen to form CO2, the concentration of oxygen in layer is decreased. The electrons released in this reaction are injected into the conduction band of the SnO2, which results in a decrease in the electrical resistance (or increase in conductivity) of materials of n-type of conductivity (Kaur et al., 2007; Bakin et al., 1997; Andryeeva et al., 1993). According to barrier model, reaction of adsorbed oxygen species with reducing gases decreases the potential barrier heights,

Reduced tin dioxide is characterized by a deficiency of oxygen – SnO2-δ, where 10-5 <δ <10-3 deviation from stoichiometry (Rumyantseva et al., 2003; Kaur et al., 2007). Vacancies of ionized oxygen are the main intrinsic defects and define the electrical properties of the material – n-type of conductivity and free carrier concentration. Energy levels of oxygen vacancies VО+ and VО2+ lie at a depth of 30−40 and 140−150 meV below the conduction band edge, respectively (Ryabtsev et al., 2008). The concentration of oxygen vacancies can be

In the transition from single crystal to a polycrystalline system, each crystal grain is considered as a closed volume, near the surface of which is located a depletion layer. Reducing of the charge carrier concentration at the grain boundaries leads to a formation of intergranular energy barriers, the magnitudes of which determines the electrical conductivity of polycrystalline material as a whole. The greatest influence of the surface state on the electrical properties of the material occurs when the condition: *l*/2 ≤ *L*, where *l* is the size of the crystallites; *L* − length of the depletion layer, which for different oxides varies

> 2 2 2 <sup>2</sup> 2 2 *<sup>s</sup> s s <sup>D</sup>*

*kT e N kT eN*

constant, VS – surface/Schottky potential, Nd is the concentration of the donor impurity, LD is the Debye length, k is the Boltzmann constant, T is the temperature. Wide range of sizes of the SnO2 crystallites (from several to several hundred nanometers) affects the mobility of charge carriers, which may vary from a few to 100 cm3/V·s. For example, Xu Chaonan et al. (1991) investigated the dependence of gas sensitivity of film on the diameter D of SnO2 crystallites (in the range 5–32 nm) and the Debye length LD. For undoped SnO2 D increased

*eV kT eV V L L*

<sup>ε</sup> <sup>ε</sup> = = ⋅= <sup>π</sup> <sup>π</sup>

*d d*

, where ε is the dielectric

resulting in a huge decrease in resistance.

from 3 to 10 nm:

reduced by annealing of the material in an oxygen atmosphere.

from 4 nm with increasing annealing temperature to 13 nm at 400ºC and 27 nm at 900ºC. The dependences of the electrical resistance of the films in air (Ra) and a mixture of hydrogen-air (Rg) at 300ºС on the crystallite sizes were presented. The values of Ra and Rg increase with decreasing D for D ≤ 6 nm, as well as increase with increasing D for D ≥ 10 nm. The value of D at the boundary between two regions is equal to 2L and the optimum value for L is obtained by 3 nm. The gas sensitivity of film was determined as Ra/Rg. For the both H2 and СО the sensitivity increases if the value D ≤ 2L (~6 нм). It follows, that the gas-sensitive properties of the tin dioxide films are determined by their nanostructure, formed during crystallization process. This makes it extremely important to study the processes of crystallization in tin dioxide films, depending on the conditions of their preparation and processing.

Good quality SnO2 films have been grown by several techniques (Buturlin et al., 1983a). As is known, the method of preparation of SnOx films significantly influences on their characteristics (Bakin et al., 1997; Srivastava R. et al, 1998; Jiang et al., 2002; Srivastava R. et al., 1998b; Minami et al, 1989). Not all techniques of deposition are used for production of functional metal oxide films (Buturlin et al., 1983b; Jarzebski, 1982; Kholʹkin and Patrusheva, 2006). The physical methods of obtaining films of metal oxides include the method of thermal evaporation of metal in vacuum with deposition on the dielectric substrate and oxidation in air (Buturlin et al., 1983b) at 720–770К. The particles of deposited material one can obtain also by evaporation in a gas. In an atmosphere of reactive gas material is heated to evaporation, then the atoms of substance reacts with residual gas atoms, resulting in formation of ultrafine particles of size in range 1–100 nm. The resulting film has an extremely developed surface and requires no further heat treatment (Buturlin et al., 1983a). During thermal sputtering of an oxide target (Das and Banerjee, 1987; Feng et al., 1979) an annealing is not required, too. The method is based on the sputtering of target made of pressed powder of desired oxide (Jarzebski, 1982) or in plasma at high frequency in a threeelectrode circuit, or by an ion beam. The spraying process is carried out in an plasma of argon of high purity with the addition of oxygen. Das and Banerjee (1987) show the relation between a structure and other physical properties of the films. For the deposited by electronbeam evaporation SnOx films a change of *x* value by varying the substrate temperature TS was revealed. The films deposited at TS = 150°C were amorphous, but after annealing in air (TA = 550°C, 2h) the presence of SnO2 by X-ray diffraction was revealed. Films deposited at TS = 225°C were also amorphous, but after annealing under identical conditions consisted of a mixture of phases SnO and β-SnO. These amorphous films are characterized by high electrical resistivity (1010 ohm·cm at room temperature), which sharply decreased after annealing in air (550°C, 2 h) to the values ρ = 9,5×10-2 ohm·cm at room temperature, seemingly, due to the formation of 0.015 eV donor level (TS = 150°C) and 0.26 eV (TS = 250°C) below the bottom of the conduction band of tin oxide. For TS = 150°C the band gap Eg = 3.46 eV corresponds to SnO2 and decreases with increasing TS up to 225°C due to the presence of different oxide phases, and increases again at TS>225°C. Annealing in air (550°C, 2h) leads to deterioration of the transmission T in the wavelength range of 400−850 nm for the films deposited at TS = 150°C and to the increase of transmission for films deposited at TS ≥ 250°C. At TA = 650°C the full transformation into the SnO2 phase with the dominant lines (101) and (200) is completed. The increase of the substrate temperature (TS = 350°C) causes a decrease in the resistance of the film, indicating that activation occurred because of donors at a depth of 0.26 eV.

Influence of Crystallization on the Properties of SnO2 Thin Films 223

dependence of the concentration of free electrons on the partial pressure of oxygen for the film deposited by rf magnetron sputtering of target, prepared from powder of SnO2 doped with CuO, followed by a two-hour annealing in a flowing oxygen at a temperature of 700°C. Polycrystalline SnO2 films with an average grain size of 100 nm and a thickness of 1 micron were monophasic and had a texture in the (110). In the range of partial pressure of oxygen in the chamber from 0.013 to 1.3 Pa a power dependence of the conductivity of the films on the pressure with an exponent of 0.6–0.8 is observed. Rumyantseva et al. (2001) showed that the effect of porosity on gas sensitivity of the film is more significant than the influence of the partial pressure of oxygen. Saturation by vacancies and the formation of pores in the gassensitive layers was carried out by doping the initial loading of tin by volatile impurities iodine and tellurium. The porosity of the material with a grain size comparable with the wavelength of the radiation manifests itself in the form of reducing the magnitude of the

Chemical methods of preparation of SnO2 films can be divided as obtained from the gas phase and from solution. Hydrolysis of chemical compounds in the gas phase occurs at relatively low temperatures. The chlorides and other compounds easily hydrolyzed in the vapor phase and are deposited as thin films of hydroxides (Suikovskaya, 1971). Obtaining of the corresponding oxides is carried out by annealing or by deposition on the heated substrate. Depending on the temperature of the substrate, the amorphous or polycrystalline films can be produced (Melsheimer and Teshe, 1986). In chemical vapor deposition (Kern and Ban, 1978), the laminar flow of an inert carrier gas completely covers the substrate in the reaction chamber, forming over it motionless boundary layer. The reagents are fed as a vapor-gas mixture, the deposition rate of which depends on the speed of their diffusion through the boundary layer. Growth of the SnO2 films (Kim and Chun, 1986) can occur in several modes. The optimal regime is the nucleation on the substrate surface and their growth by surface diffusion of the incoming material. This leads to the texturing of the film

To obtain homogeneous films of SnO2 from solutions one must take into account the properties of precursors and solvents. As a precursor the tin chlorides are often used. Suitable solvents include ethanol and acetone containing a small amount of water (Suikovskaya, 1971). Aerosol methods of producing of tin oxide films are based on the decomposition at high temperature (720−970 K) of chloride or organometallic tin compounds on the surface of the substrate. At spraying under the influence of the inert gas, the solution was transferred to the substrate surface in the form of small droplets. The deposition rate of films is 100 nm/min. Dimitrov et.al. (1999) by the hydrothermal method (tsint = 130−250°С, τsint = 2−6 h) synthesized nanocrystalline samples of SnO2. Crystallite size of SnO2, synthesized by hydrothermal treatment of amorphous gel of tin acid is 4−5 nm and

An actively developing chemical method of producing of SnO2 thin films is a sol-gel method. It is based on transfer of a substance in the sol and gel states. The method allows obtaining the multilayer films with a thickness of one layer of 2-3 nm, and it does not require a vacuum and complex installations. Deposition of the film can occur on a cold substrate. The main methods of film deposition are dip-coating, centrifugation (spin-onfilms) and spray pyrolysis (Kim and Chun, 1986; Aranowich et al.,1979; Sanz Maudes and

refractive index, averaged over the volume.

due to the formation of oriented nuclei on the surface.

is virtually independent of temperature and duration of treatment.

In the method of ion-stimulated deposition (Song, 1999) flow of neutral Sn atoms was obtained in a high vacuum using partially ionizing radiation source, the flow of oxygen ions with energies in the range 0-1000 eV was obtained using ion gun with hollow cathode. The properties of SnO2 films on substrates of glass, amorphous SiO2/Si and Si (100) were studied. In films deposited on glass, with increasing of the energy of the ion flux both the surface grain size up to 7−10 nm and roughness are increased. The increase of energy of the oxygen ions led to the increase of crystal perfectness, stoichiometry, porosity of the films and their transmission in the shortwave region. All tin oxide films on the glass after deposition had very low optical absorption in the visible wavelength region and high absorption in the shortwave region.

Method for production of SnO2 films by cathode sputtering of tin provides the creation of plasma in an inert gas (argon) at a pressure in the vacuum chamber from 50 to 10-2 Pa. A target is fixed to the cathode , substrate − to the anode. When the diode sputtering is used, between the anode and cathode DC voltage is applied, the growth rate of SnO2 films is small (~ 1.7 nm/min) and only conductive targets can be used. In the high-frequency sputtering, between the cathode and the anode an alternating RF voltage is applied. As a result, highspeed deposition of the layer up to 800 nm/min is provided, depending on the voltage and the partial pressure of O2 and Ar. One can use a non-conductive target. Conductivity of SnO2 films varies in the range 10-9−102 ohm-1·cm-1.

One of the most promising physical methods of production of the oxide films is the reactive magnetron sputtering of a metal target (Geoffroy et al., 1991; Kisin et al., 1999; Lewin et al., 1986; Semancik and Cavicchi , 1991). As a result of the deposition process, a mixture of two phases - SnO and SnO2 - is formed. By changing the parameters of deposition (substrate temperature, chamber pressure, the rate of film growth) both amorphous and nanocrystalline films can be prepared. For example, Semancik S. and Cavicchi R.E. (1991) by this method have deposited the SnO2 films on substrates of sapphire and TiO2. It is shown that the use of substrates with perfect structure and an increase in their temperature up to 500°C leads to the epitaxial growth of the film and the formation of a single crystal structure. Gas sensitivity of the films was close to sensors based on bulk single-crystal SnO2. Using a VUP-4 installation, Rembeza et al. (2001) have deposited polycrystalline SnO2 films with thickness of 1–5 microns on glass substrate by magnetron sputtering of tin target doped by antimony (3% vol.) in a gas mixture of Ar (25%) and O2 (75%). In the films has been revealed only a well-crystallized tetragonal phase of SnO2 with average grain size 11–19 nm after annealing. After annealing of the films (600°C, 4h) the concentration of electrons decreases with the increase of temperature up to 400°C in the range ~ 2×1018 – 3×1017 cm-3, and the mobility of free charge carriers increases from 70 to 150 cm2/(V·s) at 130ºC and further up to 400ºC does not change. Stjerna and Granqvist (1990) studied the optical and electrical properties of SnOx films, obtained by magnetron sputtering of tin in a mixture of argon and oxygen. The optical properties show a transparency window in the wavelength range of ~ 0.4–2 microns. It is noted that the behaviors of the curves of transmission and reflection strongly depend on the ratio of values of partial pressure of O2 and Ar during the growth of the films, and is particularly strong changed near ratio of ~ 4%. Near this point (from 4.1 to 4.2% vol. O2) a sharp minimum of the surface resistance is observed. If the power discharge is increased from 10 to 60 watts, the position of minimum is shifted toward higher concentrations of oxygen – up to 11.6–11.8%. Kissin et al. (2000) gave a description of

In the method of ion-stimulated deposition (Song, 1999) flow of neutral Sn atoms was obtained in a high vacuum using partially ionizing radiation source, the flow of oxygen ions with energies in the range 0-1000 eV was obtained using ion gun with hollow cathode. The properties of SnO2 films on substrates of glass, amorphous SiO2/Si and Si (100) were studied. In films deposited on glass, with increasing of the energy of the ion flux both the surface grain size up to 7−10 nm and roughness are increased. The increase of energy of the oxygen ions led to the increase of crystal perfectness, stoichiometry, porosity of the films and their transmission in the shortwave region. All tin oxide films on the glass after deposition had very low optical absorption in the visible wavelength region and high

Method for production of SnO2 films by cathode sputtering of tin provides the creation of plasma in an inert gas (argon) at a pressure in the vacuum chamber from 50 to 10-2 Pa. A target is fixed to the cathode , substrate − to the anode. When the diode sputtering is used, between the anode and cathode DC voltage is applied, the growth rate of SnO2 films is small (~ 1.7 nm/min) and only conductive targets can be used. In the high-frequency sputtering, between the cathode and the anode an alternating RF voltage is applied. As a result, highspeed deposition of the layer up to 800 nm/min is provided, depending on the voltage and the partial pressure of O2 and Ar. One can use a non-conductive target. Conductivity of

One of the most promising physical methods of production of the oxide films is the reactive magnetron sputtering of a metal target (Geoffroy et al., 1991; Kisin et al., 1999; Lewin et al., 1986; Semancik and Cavicchi , 1991). As a result of the deposition process, a mixture of two phases - SnO and SnO2 - is formed. By changing the parameters of deposition (substrate temperature, chamber pressure, the rate of film growth) both amorphous and nanocrystalline films can be prepared. For example, Semancik S. and Cavicchi R.E. (1991) by this method have deposited the SnO2 films on substrates of sapphire and TiO2. It is shown that the use of substrates with perfect structure and an increase in their temperature up to 500°C leads to the epitaxial growth of the film and the formation of a single crystal structure. Gas sensitivity of the films was close to sensors based on bulk single-crystal SnO2. Using a VUP-4 installation, Rembeza et al. (2001) have deposited polycrystalline SnO2 films with thickness of 1–5 microns on glass substrate by magnetron sputtering of tin target doped by antimony (3% vol.) in a gas mixture of Ar (25%) and O2 (75%). In the films has been revealed only a well-crystallized tetragonal phase of SnO2 with average grain size 11–19 nm after annealing. After annealing of the films (600°C, 4h) the concentration of electrons decreases with the increase of temperature up to 400°C in the range ~ 2×1018 – 3×1017 cm-3, and the mobility of free charge carriers increases from 70 to 150 cm2/(V·s) at 130ºC and further up to 400ºC does not change. Stjerna and Granqvist (1990) studied the optical and electrical properties of SnOx films, obtained by magnetron sputtering of tin in a mixture of argon and oxygen. The optical properties show a transparency window in the wavelength range of ~ 0.4–2 microns. It is noted that the behaviors of the curves of transmission and reflection strongly depend on the ratio of values of partial pressure of O2 and Ar during the growth of the films, and is particularly strong changed near ratio of ~ 4%. Near this point (from 4.1 to 4.2% vol. O2) a sharp minimum of the surface resistance is observed. If the power discharge is increased from 10 to 60 watts, the position of minimum is shifted toward higher concentrations of oxygen – up to 11.6–11.8%. Kissin et al. (2000) gave a description of

absorption in the shortwave region.

SnO2 films varies in the range 10-9−102 ohm-1·cm-1.

dependence of the concentration of free electrons on the partial pressure of oxygen for the film deposited by rf magnetron sputtering of target, prepared from powder of SnO2 doped with CuO, followed by a two-hour annealing in a flowing oxygen at a temperature of 700°C. Polycrystalline SnO2 films with an average grain size of 100 nm and a thickness of 1 micron were monophasic and had a texture in the (110). In the range of partial pressure of oxygen in the chamber from 0.013 to 1.3 Pa a power dependence of the conductivity of the films on the pressure with an exponent of 0.6–0.8 is observed. Rumyantseva et al. (2001) showed that the effect of porosity on gas sensitivity of the film is more significant than the influence of the partial pressure of oxygen. Saturation by vacancies and the formation of pores in the gassensitive layers was carried out by doping the initial loading of tin by volatile impurities iodine and tellurium. The porosity of the material with a grain size comparable with the wavelength of the radiation manifests itself in the form of reducing the magnitude of the refractive index, averaged over the volume.

Chemical methods of preparation of SnO2 films can be divided as obtained from the gas phase and from solution. Hydrolysis of chemical compounds in the gas phase occurs at relatively low temperatures. The chlorides and other compounds easily hydrolyzed in the vapor phase and are deposited as thin films of hydroxides (Suikovskaya, 1971). Obtaining of the corresponding oxides is carried out by annealing or by deposition on the heated substrate. Depending on the temperature of the substrate, the amorphous or polycrystalline films can be produced (Melsheimer and Teshe, 1986). In chemical vapor deposition (Kern and Ban, 1978), the laminar flow of an inert carrier gas completely covers the substrate in the reaction chamber, forming over it motionless boundary layer. The reagents are fed as a vapor-gas mixture, the deposition rate of which depends on the speed of their diffusion through the boundary layer. Growth of the SnO2 films (Kim and Chun, 1986) can occur in several modes. The optimal regime is the nucleation on the substrate surface and their growth by surface diffusion of the incoming material. This leads to the texturing of the film due to the formation of oriented nuclei on the surface.

To obtain homogeneous films of SnO2 from solutions one must take into account the properties of precursors and solvents. As a precursor the tin chlorides are often used. Suitable solvents include ethanol and acetone containing a small amount of water (Suikovskaya, 1971). Aerosol methods of producing of tin oxide films are based on the decomposition at high temperature (720−970 K) of chloride or organometallic tin compounds on the surface of the substrate. At spraying under the influence of the inert gas, the solution was transferred to the substrate surface in the form of small droplets. The deposition rate of films is 100 nm/min. Dimitrov et.al. (1999) by the hydrothermal method (tsint = 130−250°С, τsint = 2−6 h) synthesized nanocrystalline samples of SnO2. Crystallite size of SnO2, synthesized by hydrothermal treatment of amorphous gel of tin acid is 4−5 nm and is virtually independent of temperature and duration of treatment.

An actively developing chemical method of producing of SnO2 thin films is a sol-gel method. It is based on transfer of a substance in the sol and gel states. The method allows obtaining the multilayer films with a thickness of one layer of 2-3 nm, and it does not require a vacuum and complex installations. Deposition of the film can occur on a cold substrate. The main methods of film deposition are dip-coating, centrifugation (spin-onfilms) and spray pyrolysis (Kim and Chun, 1986; Aranowich et al.,1979; Sanz Maudes and

Influence of Crystallization on the Properties of SnO2 Thin Films 225

One of the ways to improve the selectivity of sensors based on SnO2 and increase the contribution of molecules of this type in the gas phase to the total electrical signal is the introduction of alloying elements into highly dispersed oxide matrix (Rumyantseva et al., 2003; Fantini and Torriani, 1986; Okunara et al., 1983 ; Bestaev et al., 1998). Dopants are usually divided into two groups: catalytic (Pt, Pb, Ru, Rh) and electroactive (In, Sb, Cu, Ni, Mn). Of considerable interest is also the effect of processing by plasma on the properties of films. Minami et al. (1989) placed the transparent conductive films of tin oxide (TCO) in a quartz tube filled with hydrogen gas. Films at high temperature treatment are completely painted in dark gray. Transmittance of ITO indium doped and undoped ТО films decreased rapidly during annealing above 300ºC for 30 minutes. Decrease the transparency and color of the films are attributed to the formation of the oxygen-depleted surface level due to chemical reduction of these films by hydrogen. The upper surface of the films passed into the metallic state, and then indium and tin were thermally evaporated at a higher temperature. Hydrogen glow discharge plasma was obtained at a pressure of 400 Pa in the installation with a capacitive circuit (frequency of 13.56 MHz or 2.45 GHz) and power of 300 watts. Transmittance and film thickness are greatly reduced when exposed by hydrogen plasma at 250ºC. ECR hydrogen plasma was obtained in the volumetric resonator cavity at a pressure of 6.5×10-2 Pa, microwave power (2.45 GHz) 300 watts when a magnetic field was 8.75×10-2 T. Indium containing (ITO) film and undoped (TO) SnO2 were stained even when

An analysis of published data shows that in papers a great attention is paid to influence of the structure of the synthesized tin dioxide films on their sensitivity to different gases and transparency in a wide range of wavelengths. Usage of different methods of preparation, as well as modification of the films by different types of treatments leads to a change of phase composition and microstructure of the films causing a change in their optical, electrical and gas-sensitive properties. It is of considerable interest to study the effect of the concentration component, nanoclusters, the composition of films and their heat or plasma treatment on the crystallization processes and clustering, the size of the nanocrystals and, consequently, the

In this paper we consider the influence of composition and structure on the optical and electric characteristics of SnOx thin films deposited on glass substrates by sol-gel technique. For comparison, the data for films prepared by magnetron sputtering, are presented. Using various techniques the effect of both thermal and plasma treatments on the structure and

A solution to obtain the nanostructured films by sol-gel technique (the method of spreading)

pure ethanol. SnO2 sol of desired concentration was obtained under strong stirring to obtain a colorless and transparent solution. Freshly prepared solution has a neutral pH value of ~ 7. After maturation of the solution, which lasted more than six hours, pH was equal about 0.88, indicating the release of HCl during the dissolution of SnCl4. To study the effect of concentration of components in the solution on the film properties were obtained the solutions with a concentration of tin atoms: 0.82, 0.41, 0.30 and 0.14 mol/L. Aliquots of these solutions with volumes of 0.04 ml, 0.08 ml, 0.11 ml, 0.23 ml, respectively, using

5H2O) by dissolution in

was prepared from crystalline hydrate of tin tetrachloride (SnCl4.

treated for 2 and 5 minutes.

physical properties of the films.

**2. Experimental** 

properties of the SnOx films were studied.

Rodriguez, 1980; Fantini and Torriani, 1986; Jitianu et al., 2003; Chatelon et al., 1997). It is one of the simplest and economic among known techniques. The advantages of the sol-gel process are the possibility of doping and its potential to fabricate films of large area substrates. The flexibility of the technological cycle management provides opportunities to obtain the necessary surface morphology and sizes of structural formations. One can obtain the films with a high degree of homogeneity and controlled stoichiometry by sol-gel method at a sufficiently low temperature of synthesis . Often (Suykovskaya NV, 1971; Zang and Liu, 1999) salt volatile acids, such as chlorides, as film-forming substances are used. As a solvent mainly ethanol is used. Chlorine can be included in the formed SnO2 film or by replacing the oxygen, or by interstitial mechanism (Bosnell and Waghorne, 1973; Aboaf et al., 1973). The admixture of chlorine causes an increase in carrier concentration, leads to the stabilization of the parameters of the oxide layer, to a low density of surface states, and to high values of the lifetime of the major carriers. Growth of SnO2 crystallite size after annealing is noted by many authors (Asakuma et al., 2003; Brito et al., 1997). However, the introduction of additional components in the composition of the film-forming mixture (Torkhov et al., 2003) leads to a decrease in the rate of growth of crystallites of the both SnO2 and substance injected with increasing temperature. Torkhov et al. (2003) synthesized the nanocrystalline SnO2 and WO3 and nanocomposite Sn:W = 1:9, 1:1, 9:1 by α-SnO2·n·H2O and WO3·H2O (series Х) gel deposition and krizol-method (series K). For nanocomposites of Sn (X, K) and Sn9W1 (X, K) after annealing at Т ≥ 150°C a phase of tin dioxide SnO2 (cassiterite) was detected, the degree of crystallinity of which increases with increasing temperature. An increase in the crystallite size of SnO2 (2−15 nm) and WO3 (10−50 nm) is also observed. For SnO2 films the change of grain size in range from 10 to 6 nm is accompanied by increase of resistance from 2×104 up to 6×104 ohms. Anishchik et al. (1995) annealed the films obtained by centrifugation from an aqueous solution at temperatures of 400 or 500ºC for 30 minutes with further rapid cooling in air ("hardening of soft"). In SnO2 films, subjected to cooling from 400ºC, the SnO phase also found. Rapid cooling from 500ºC leads to an appearance of Sn3O4 phase, while at slow cooling this phase is not detected. Rapid cooling of the samples from temperatures of 400 and 500ºC leads to "freezing" on the surface of films of elemental tin and its oxides, as they do not have time to oxidize up to SnO2.

The films to ensure maximum sensitivity are heated during operation of sensors. The temperature effect causes an increase in the crystallite sizes of SnO2. On the one hand, there is a reduction of the effective activation energy of the interaction of SnO2 with oxygen, and, on the other hand, increasing the concentration of electrons with sufficient energy to overcome the barrier created by the negatively charged surface (Rumyantseva et al., 2008). The degree of surface coverage by chemisorbed oxygen О- (ads), О2-(ads) is increased. The films with a fixed size of the crystallites SnO2 are required to ensure a long-term operation of the sensors. Kukuev and Popov (1989) used a SnO2 film in the manufacture of microelectronics devices in installations for heating of ultrapure deionized water, as well as in commercially available installations for infrared heat treatment of photo resist as heating elements. The most suitable as heaters are the films with grain size ~ 0.2-0.3 μm, in which such processes as crystallization, condensation and oxidation do not occur. Adamyan et.al. (2006) obtained high-quality films using sodium stannate as a precursor. 1 M of H3PO4 was added into Na2SnO3 solution with constant stirring to neutralize the solution (pH = 7). The transparent, stable sol, indicating the small size of its particles and the stability of their surface, was obtained.

Rodriguez, 1980; Fantini and Torriani, 1986; Jitianu et al., 2003; Chatelon et al., 1997). It is one of the simplest and economic among known techniques. The advantages of the sol-gel process are the possibility of doping and its potential to fabricate films of large area substrates. The flexibility of the technological cycle management provides opportunities to obtain the necessary surface morphology and sizes of structural formations. One can obtain the films with a high degree of homogeneity and controlled stoichiometry by sol-gel method at a sufficiently low temperature of synthesis . Often (Suykovskaya NV, 1971; Zang and Liu, 1999) salt volatile acids, such as chlorides, as film-forming substances are used. As a solvent mainly ethanol is used. Chlorine can be included in the formed SnO2 film or by replacing the oxygen, or by interstitial mechanism (Bosnell and Waghorne, 1973; Aboaf et al., 1973). The admixture of chlorine causes an increase in carrier concentration, leads to the stabilization of the parameters of the oxide layer, to a low density of surface states, and to high values of the lifetime of the major carriers. Growth of SnO2 crystallite size after annealing is noted by many authors (Asakuma et al., 2003; Brito et al., 1997). However, the introduction of additional components in the composition of the film-forming mixture (Torkhov et al., 2003) leads to a decrease in the rate of growth of crystallites of the both SnO2 and substance injected with increasing temperature. Torkhov et al. (2003) synthesized the nanocrystalline SnO2 and WO3 and nanocomposite Sn:W = 1:9, 1:1, 9:1 by α-SnO2·n·H2O and WO3·H2O (series Х) gel deposition and krizol-method (series K). For nanocomposites of Sn (X, K) and Sn9W1 (X, K) after annealing at Т ≥ 150°C a phase of tin dioxide SnO2 (cassiterite) was detected, the degree of crystallinity of which increases with increasing temperature. An increase in the crystallite size of SnO2 (2−15 nm) and WO3 (10−50 nm) is also observed. For SnO2 films the change of grain size in range from 10 to 6 nm is accompanied by increase of resistance from 2×104 up to 6×104 ohms. Anishchik et al. (1995) annealed the films obtained by centrifugation from an aqueous solution at temperatures of 400 or 500ºC for 30 minutes with further rapid cooling in air ("hardening of soft"). In SnO2 films, subjected to cooling from 400ºC, the SnO phase also found. Rapid cooling from 500ºC leads to an appearance of Sn3O4 phase, while at slow cooling this phase is not detected. Rapid cooling of the samples from temperatures of 400 and 500ºC leads to "freezing" on the surface of films of elemental

tin and its oxides, as they do not have time to oxidize up to SnO2.

The degree of surface coverage by chemisorbed oxygen О-

obtained.

The films to ensure maximum sensitivity are heated during operation of sensors. The temperature effect causes an increase in the crystallite sizes of SnO2. On the one hand, there is a reduction of the effective activation energy of the interaction of SnO2 with oxygen, and, on the other hand, increasing the concentration of electrons with sufficient energy to overcome the barrier created by the negatively charged surface (Rumyantseva et al., 2008).

with a fixed size of the crystallites SnO2 are required to ensure a long-term operation of the sensors. Kukuev and Popov (1989) used a SnO2 film in the manufacture of microelectronics devices in installations for heating of ultrapure deionized water, as well as in commercially available installations for infrared heat treatment of photo resist as heating elements. The most suitable as heaters are the films with grain size ~ 0.2-0.3 μm, in which such processes as crystallization, condensation and oxidation do not occur. Adamyan et.al. (2006) obtained high-quality films using sodium stannate as a precursor. 1 M of H3PO4 was added into Na2SnO3 solution with constant stirring to neutralize the solution (pH = 7). The transparent, stable sol, indicating the small size of its particles and the stability of their surface, was

(ads), О2-(ads) is increased. The films

One of the ways to improve the selectivity of sensors based on SnO2 and increase the contribution of molecules of this type in the gas phase to the total electrical signal is the introduction of alloying elements into highly dispersed oxide matrix (Rumyantseva et al., 2003; Fantini and Torriani, 1986; Okunara et al., 1983 ; Bestaev et al., 1998). Dopants are usually divided into two groups: catalytic (Pt, Pb, Ru, Rh) and electroactive (In, Sb, Cu, Ni, Mn). Of considerable interest is also the effect of processing by plasma on the properties of films. Minami et al. (1989) placed the transparent conductive films of tin oxide (TCO) in a quartz tube filled with hydrogen gas. Films at high temperature treatment are completely painted in dark gray. Transmittance of ITO indium doped and undoped ТО films decreased rapidly during annealing above 300ºC for 30 minutes. Decrease the transparency and color of the films are attributed to the formation of the oxygen-depleted surface level due to chemical reduction of these films by hydrogen. The upper surface of the films passed into the metallic state, and then indium and tin were thermally evaporated at a higher temperature. Hydrogen glow discharge plasma was obtained at a pressure of 400 Pa in the installation with a capacitive circuit (frequency of 13.56 MHz or 2.45 GHz) and power of 300 watts. Transmittance and film thickness are greatly reduced when exposed by hydrogen plasma at 250ºC. ECR hydrogen plasma was obtained in the volumetric resonator cavity at a pressure of 6.5×10-2 Pa, microwave power (2.45 GHz) 300 watts when a magnetic field was 8.75×10-2 T. Indium containing (ITO) film and undoped (TO) SnO2 were stained even when treated for 2 and 5 minutes.

An analysis of published data shows that in papers a great attention is paid to influence of the structure of the synthesized tin dioxide films on their sensitivity to different gases and transparency in a wide range of wavelengths. Usage of different methods of preparation, as well as modification of the films by different types of treatments leads to a change of phase composition and microstructure of the films causing a change in their optical, electrical and gas-sensitive properties. It is of considerable interest to study the effect of the concentration component, nanoclusters, the composition of films and their heat or plasma treatment on the crystallization processes and clustering, the size of the nanocrystals and, consequently, the physical properties of the films.

In this paper we consider the influence of composition and structure on the optical and electric characteristics of SnOx thin films deposited on glass substrates by sol-gel technique. For comparison, the data for films prepared by magnetron sputtering, are presented. Using various techniques the effect of both thermal and plasma treatments on the structure and properties of the SnOx films were studied.
