**2. Experimental**

A solution to obtain the nanostructured films by sol-gel technique (the method of spreading) was prepared from crystalline hydrate of tin tetrachloride (SnCl4 . 5H2O) by dissolution in 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

Influence of Crystallization on the Properties of SnO2 Thin Films 227

2

3

imperfect structure.

0

20

40

60

Т %

concentration in the colloidal solution.

**SnO2(110)**

**SnO2(101)**

**0.1**

100°С; c) annealing at 200°С.

**10 15 20 25 30** θ**, градус**

**0.2**

**0.3**

**0.4**

**I, относит. единицы**

**I, arb. units** 

**0.5**

**0.6**

80

1

100

and remains practically unchanged after annealing for 1 hour at 100°С (Fig. 2b). Separation of the broad band into two SnO2 lines, indicating the formation of crystallites, visually observed only after annealing at 200°С (Fig. 2c). Crystallite size in the planes (110) and (101) of SnO2 is ~ 1.5 nm, i.e. the crystallites are small and have an

> 0 500 1000 1500 2000 2500 λ, nm

Fig. 1. The transmission spectra of tin oxide film synthesized by sol-gel technique: 1 − glass substrate, 2 − SnO2 film after annealing at 100°C, 3 − SnO2 film after annealing at 400°C.

b. A similar pattern after the deposition and annealing at 100°С is observed for films of SnO2, produced at lower tin concentrations of 0.41 mol/L (~ 0.08 ml aliquot) and 0.30 mol/L (~ 0.11 ml aliquot). After annealing at 200°С the differences are appeared, which manifest themselves in increasing intensities of SnO2 lines with decreasing of tin

**SnO2(110)**

a) b) c)

**SnO2(101)**

**10 15 20 25 30** θ**, градус**

Fig. 2. X-ray diffraction patterns and intensity curves for the SnOx films on glass substrates obtained by the sol-gel technique (0.04 ml solution with tin concentration of 0.83 mol/L, d = 320 nm) after: a) deposition and drying at a temperature of 80°С, and b) annealing at

θ**, degree** θ**, degree** θ**, degree** 

**10 15 20 25 30** θ**, градус**

**SnO2(110)**

**SnO2(101)**

micropipettes were deposited on the cleaned surface of the microscope glass slides. It was assumed that the number of tin atoms in the films will be identical (~ 3.25 × 10-5 mole), and the film thickness is ~ 350 nm. After the deposition, the films were dried for 1 hour at 80ºC. Then the samples were annealed at 100, 200 or 400ºC for 1 hour. The thickness of the films, estimated from the change in mass of the sample, was 360±40 nm.

The SnO2 films with thickness of 300 nm were also fabricated by method of centrifugation. A solution was obtained by dissolving of anhydrous SnCl4 in 97% ethanol. The solution was deposited onto a glass substrate located on a table of centrifuge rotor, the rotation speed of which was ~ 3800 rpm. Centrifugation time was 3–5 s. Substrate with a layer was dried using an infrared radiation at 80°C for 3−5 minutes, and then in a muffle furnace at 400°C for 15 minutes. After cooling, the cycle repeats. Number of deposited SnO2 layers ranged from 12 to 15.

The SnO2 films with thickness of 300 nm were also deposited on cleaned microscopy glass slides by magnetron sputtering. The magnetron sputtering mode parameters were: cathode voltage Uc = 470 V, the ion beam current Iion = 35 mА, the argon-oxygen mixture pressure inside the chamber ~ 1−2.7 Pa, the oxygen concentration in the Ar-O2 mixture ~10%, deposition rate of films ~ 0.05 nm/c, the temperature of the substrate ~ 200°С.

The SnO2 film's structure was investigated by X-ray diffraction using a narrow collimated (0.05×1.5 mm2) monochromatic (CuKα) X-ray beam directed at an angle of 5º to the sample surface. The average crystallite size was estimated from the width of X-rays lines by Jones method. The surface of the layers was analyzed by Atomic force microscopy (JSPM 5200, Jeol, Japan) using AFM AC technique. The optical transmittance spectra of SnOx films were measured in the wavelength range from 190 to 1100 nm by means of the SF-256 UVI and from 1100 to 2500 nm by means of the SF-256 NIR spectrophotometers (LOMO, Russia). Electrical resistance of the films was measured by a four probe technique at room temperature. For the measurements of electrical characteristics and parameters of gas sensitivity of the thin films in a wide temperature range was used a specialized experimental setup.

The glow discharge hydrogen plasma was generated at a pressure of 6.5 Pa with a capacitive coupled radio frequency (r.f.) power (27.12 MHz) of about 12.5 W. The temperature of processing did not exceed 100°С. The processing time was 5 min.
