Investigation of the Effect of the Short-Term Exposure of Oxygen and Hydrogen Plasma on the Composition and Structure of Thin Tin Dioxide Films

Modern technologies cannot function without the production of thin films of tin dioxide, which are most widely used mainly in three areas: as transparent electrodes, catalysts, and solid-state sensors of various gases. Their use as transparent electrodes is related to the high transmittance of tin dioxide layers in the optical range, as well as with their low electrical resistivity. The effect of short-term exposure to plasma on the composition and structure of thin films of tin dioxide obtained from a solution of pentahydrate tin tetrachloride in 97% ethanol with different concentrations of tin ions is considered. The linear character of the dependence of the thickness of the tin dioxide SnO2 films on the concentration of the solution and the number of layers applied is revealed. A decrease in the electrical resistance of the films is found with an increase in the concentration of the initial solution and an increase in the number of layers. It is shown that processing SnO2 films of hydrogen plasma makes it possible to reduce their electrical resistance without decreasing transparency. The oxygen plasma treatment reduces the transparency of SnO2 films, and the resistance of the films increases with an increase in the duration of such treatment.


INTRODUCTION
Modern technologies cannot function without the production of thin films of tin dioxide, which are most widely used mainly in three areas: as transparent electrodes, catalysts, and solid-state sensors of various gases [1,2]. Their use as transparent electrodes is related to the high transmittance of tin dioxide layers in the optical range and their low electrical resistivity.
Among the various methods used to improve the functional properties of metal oxide layers, plasma treatment is of particular interest. Analysis of the changes observed in the optical parameters and structural characteristics of tin dioxide after plasma treatment makes it possible to better understand the dynamics of the changes in the physical and structural properties of thin tin dioxide films.
As is known, the method of obtaining tin dioxide films substantially affects practically all of their characteristics [3][4][5]. The distinctive characteristics of films obtained using various methods are the initial composition of the synthesized film and its stoichiometry, which have a strong effect on the properties of the film both after deposition and after processing.
The aim of this paper is to study the effect of shortterm exposure to plasma on the composition, structure, and properties of thin films of tin dioxide, obtained from a solution of pentahydrate tin tetrachloride (SnCl 4 ⋅5H 2 O) in 97% ethanol with different concentrations of tin ions. EXPERIMENTAL A series of solutions of pentahydrate tin tetrachloride in 97% ethanol with different concentrations of tin ions were prepared. The solutions were applied to substrates (glass slides measuring 76 × 26 × 1 mm 3 ) by the modified dipping method. The film was applied in several stages to one side of the substrate. After deposition, the samples were dried in air for at least 30 min, then annealed in a muffle furnace at 400°С for 15 min.
The obtained samples of SnO 2 films were briefly exposed to oxygen and hydrogen plasma. The film samples, which were then exposed to oxygen plasma, were prepared from solutions with tin ion concentrations of 0.08, 0.11, and 0.14 mol/L. Oxygen plasma treatment was carried out in a quartz tube. Oxygen was obtained by the pyrolytic decomposition of potassium permanganate. Plasma treatment was carried out at a pressure of 6.5 Pa and a power of ~20 W. The oscillation frequency generated by the generator was 27.12 ± 0.6% MHz. The temperature of the samples during processing did not exceed 100°C. The processing times were 30 and 60 s. In order to avoid a decrease in the transmittance of the films upon treatment with oxygen plasma, the transmission spectra of thin SnO 2 films were monitored after each processing step.
Film samples, which were then exposed to hydrogen plasma, were prepared from solutions with tin ion concentrations of 0.12, 0.16, and 0.2 mol/L. Hydrogen plasma treatment was also carried out in a quartz tube. Hydrogen was obtained by the electrolysis of water. To remove water vapor, hydrogen was passed through a heated platinum filter. Hydrogen plasma treatment was carried out at a pressure of 6.5 Pa. The plasma's power was ~20 W and the frequency of oscillations created by the generator was 27.12 MHz ± 0.6%. The temperature of the samples during processing did not exceed 100°C. The processing times were 3, 6, and 9 minutes. After processing thin films of SnO 2 , the spectrophotometric analysis of the samples was carried out with hydrogen plasma.
The electrical resistance of the films was determined by a two-probe method by ten measurements in different parts of the samples. Student's coefficient for ten measurements with a reliability of 0.95 is 2.262. The error was calculated by the formula where is the absolute error of the measurements; is Student's coefficient; is the value of the ith measurement; is the arithmetic mean; and n is the number of dimensions.
The transmission spectra of the films were measured on two-beam spectrophotometers SF-256 UVI (wavelength 190-1100 nm) and SF-256 BIK (wavelength 1000-2500 nm). The surface structure of the films was studied using an MPE-11 optical microscope. A television camera for a VEC-535 microscope was used to output the data to a personal computer.
The SnO 2 films were also investigated by X-ray diffractometry immediately after deposition and after treatment in plasma for different times. Figure 1 shows the transmission spectra of SnO 2 films obtained at various concentrations of the filmforming solution, after application and after treatment in oxygen plasma for 30 and 60 s.

RESULTS AND DISCUSSION
From Fig. 1 we can see that the effect of oxygen plasma on the transparency of SnO 2 films depends on the concentration of the tin ions in the film-forming system. After treatment with oxygen plasma of the films obtained from the SnCl 4 /EtOH film-forming system with the tin ion concentration of 0.08 mol/L,  T, % the transparency increased by 3-5% in the visible region of the spectrum (see Fig. 1a). Treatment for 30 s in oxygen plasma of films obtained from a filmforming system with a tin ion concentration of 0.11 mol/L led to an increase in the transparency in the visible region of the spectrum by 1-3%. An increase in the processing time to 60 s caused a decrease in transparency in the visible region of the spectrum and an increase in the long-wavelength part of the spectrum. The transparency of samples obtained from the SnCl 4 /EtOH film-forming system with a tin ion concentration of 0.14 mol/L decreased with an increase in the duration of treatment [6][7][8]. Table 1 shows the average values of the electrical resistance of the films under study. As can be seen from Table 1, the electrical resistance of the films, regardless of the concentration of the tin ions in the film-forming solution, decreases with an increase in the duration of treatment with oxygen plasma. Treatment with oxygen plasma should lead to an increase in the electrical resistance of the films due to the filling of oxygen vacancies. However, in practice, the opposite picture is observed. The mechanism of this phenomenon requires further study [9][10][11][12]. Figure 2 shows SEM images of the surface of the SnO 2 films obtained from the SnCl 4 /EtOH film-forming system with a tin ion concentration of 0.08 mol/L, after treatment in oxygen plasma. In Fig. 2, a section with clear outlines is observed, along the contour of which cracks are visible.
On the surface of the film obtained from the SnCl 4 /EtOH film-forming system with a tin ion concentration of 0.11 mol/L, blisters (bubbles) were formed. The presence of solvent vapors and the reaction products at the film-substrate interface leads to the formation of bubbles in the still gel-like film [13][14][15][16]. During annealing, the gel-like film transforms into ceramics, and the resulting blisters solidify. Figure 3 shows the surface of the SnO 2 (0.14 mol/L) films before and after treatment with oxygen plasma. From Fig. 3, it can be seen that after treatment with oxygen plasma, surface areas appeared from which, under the influence of the plasma, the upper layer split off. A cellular structure was found under the top layer (see Fig. 3b).
The X-ray diffraction analysis showed a decrease in the intensity of reflections upon treatment for 60 s in oxygen plasma for all the studied samples. From Fig. 4 it can be seen that, with an increase in the duration of the treatment with oxygen plasma, the intensity of reflections from all the observed planes decreases. After treatment in oxygen plasma for 60 s, the reflection from the plane with the Miller indices (211) could not be identified. Plasma exposure led to the destruction of crystallites in the film [17,18].      [6,19,20].
The transmission spectra were recorded after applying SnO 2 films and their treatment with hydrogen plasma (Fig. 7). As seen from Fig. 7, on the transmission spectra of films obtained from solutions with tin ion concentrations of 0.12, 0.16, and 0.2 mol/L (Figs. 7a-7c, curve 2), there are weakly pronounced interference peaks. The transparency of the samples in the visible region of the spectrum was 85-90%. The processing of SnO 2 thin film by hydrogen plasma for 3 min (Figs. 7a, 7c, curve 3) leads to a decrease in transparency by 0.5-2% for samples obtained from solutions with tin ion concentrations of 0.12 and 0.2 mol/L. The transparency of the sample obtained from a solution with a tin ion concentration of 0.16 mol/L decreased by 3% in the visible region of the spectrum. The further increase in the duration of exposure to hydrogen plasma to 6 and 9 minutes leads to a decrease in the transparency of the films by 0.5-1.0% (Figs. 7a-7c, curves 4, 5). In the long-wavelength region of the spectrum, the transparency of films obtained from a solution with a tin ion concentration of 0.2 mol/L varies within the permissible measurement accuracy.
The available interference peaks were used to calculate the film thickness, extinction coefficient, refractive index of the film, and the absorption coefficient. The calculation results are presented in Table 2.
As can be seen from the data in Table 2, after treatment with hydrogen plasma for 3 min, the film thickness increased, probably due to a decrease in density. There was an increase in the absorption coefficient. As noted earlier [10][11][12], hydrogen, being a reducing agent and being in a chemically active (ionized) state, reduces some of the SnO 2 molecules to SnO: The resulting tin oxide (SnO) has a black-blue or brownish-black color. The formation of SnO molecules leads to a decrease in transparency (see Fig. 8).
The reduction of tin dioxide to metallic tin is unlikely due to the absence of a decrease in the transmittance in the long-wavelength region of the spectrum. Based on theoretical considerations, treatment with oxygen plasma leads to the filling of oxygen vacancies, which causes an increase in the surface resistance of tin dioxide films. The resistance of films without treatment depends on the concentration of the film-forming solution. Films obtained from solutions with a lower concentration have a smaller thickness, which leads to an increase in the resistance of the sample. Treatment with hydrogen plasma for 3 min led to a decrease in the resistance of the samples, approximately by a factor of 1.5, due to the increase in oxygen vacancies under the influence of hydrogen plasma. Treatment in hydrogen plasma for 6 min led to a further decrease in resistance. For example, the resistance of films obtained from a solution with a tin ion concentration of 0.2 mol/L decreased by a factor of 1.2. There is a further increase in oxygen vacancies under the action of plasma; however, an amorphous and crystalline SnO phase is formed. Tin oxide SnO is a semiconductor of the p-or n-type of conductivity, depending on the production conditions, but with greater resistance than that of SnO 2 . Treatment for 9 min in hydrogen plasma caused an increase in resis-tance, probably due to the destruction of the SnO 2 crystallites. Figure 8 shows the change in the transparency of the films depending on the duration of exposure to hydrogen plasma at a wavelength of 550 nm.
As seen from Fig. 8, curve 1, the transparency of the film obtained from a solution with a tin ion concentration of 0.12 mol/L changes from 90.1 to 87.4%. The transparency of the remaining samples depends more strongly on the duration of exposure to hydrogen plasma [21][22][23]. The transparency of the sample obtained from a solution with a tin ion concentration of 0.2 mol/L changes from 88.0 to 84.2%. The hydrogen plasma had the greatest effect on the film obtained from a solution with a tin ion concentration of 0.16 mol/L. Its transparency (at λ = 550 nm) varies from 89.5 to 81.3%. The amount of black SnO inclusions formed on the surface of the samples (see Fig. 9) affects the transparency of the films. The largest number of such inclusions was observed on the surface of samples obtained from a solution with a tin ion concentration of 0.16 mol/L: the transparency of this sample decreased by 8.2%. The smallest number of such inclusions was observed on the surface of the samples obtained from a solution with a tin ion concentration of 0.12 mol/L: the transparency decreased by 2.7%. Figure 9 shows an X-ray diffraction pattern of a thin SnO 2 film obtained from a film-forming solution with a concentration of 0.36 mol/L.   Figure 9 shows that the obtained spectrum has a high noise level. A halo is observed with the maximum at 20°-25°, corresponding to the amorphous component. The peaks from the investigated thin film are only slightly indistinguishable. To isolate the signal, a method was used to increase the level of the signal-tonoise ratio when using the principle of interference attenuation, which consists in synchronizing the timefrequency elements of the input signal and then summing all the frequency-time elements of the input signal formed on the transmitting side, for which the frequency-time parameters of the implementation of the noise received with a signal in the same band must satisfy the independence conditions for random variables [14].
However, the separation of the reflected X-ray signal from crystals smaller than 100 nm did not occur. It is possible to separate the signal of the reflected X-rays from crystals of about 5 nm on an amorphous substrate using the photographic method of recording the signal, a small tilt angle (5°) of X-ray radiation, and exposure of the samples for 35 h [15]. This requires specific X-ray equipment and long-term exposure. The method [16] based on the fact that the useful signal is separated from the noise by a combination of two actions-accumulation of deviations from the average spectrum value along the spectrum (horizontal accumulation) and spectrum averaging over time (vertical accumulation)-reduces the exposure time. The method makes it possible to achieve the same signalto-noise ratio as the standard method of time averaging (vertical accumulation of the spectrum) used in magnetic resonance spectroscopy, by about two orders of magnitude shorter [24]. However, significant distortion of the signal shape can occur when studying nanoobjects on amorphous or polycrystalline substrates due to the low signal-to-noise ratio and the presence of a distorting background signal. In addition, the initial signal as a result of horizontal accumu-lation is obtained in integral form, i.e., in the form of an absorption signal, which in some cases presents significant inconveniences for analysis [6,25]. In this study, we took the spectrum of an empty substrate and accumulated a background signal along the spectrum in the interval limited by the order parameter. Next, the accumulated signal of the substrate was subtracted from the accumulated signal of the substrate with a thin film of SnO 2 . Figure 10 shows the original signal and the accumulated spectrum from the glass substrate.
From Fig. 10a, it can be seen that the observed wide peak with an apex at 20°-25°, corresponding to the amorphous component, belongs to the glass substrate. Figure 10b shows that the white noise level is significantly reduced. Accumulation along the spectrum takes place according to the rule Si = .
In this case, the white noise is reduced by a factor of . The order parameter, in this case, was n = 50. Figure 11 shows the accumulated signal from a thin SnO film on a glass substrate and the spectrum, after subtracting the accumulated signal of the substrate from the accumulated signal of the thin film substrate. Figure 11b shows that the white noise level is significantly reduced. However, the analysis of the crystallographic planes of the sample under study is difficult. As can be seen from Fig. 11b, the signal from a thin film of SnO 2 is sufficiently isolated and can be analyzed. Figure 11b shows four peaks at angles 2θ = 26.6°, 33.9°, 37.8°, and 51.7°, corresponding to reflections from the crystallographic planes of SnO 2 (110), SnO 2 (101), SnO 2 (200), and SnO 2 (211), respectively. This shows the possibility of increasing the signal-tonoise ratio for spectra from thin (nanometer thickness) ( ) S i SnO 2 films on amorphous or polycrystalline substrates, using the techniques described in [5,6,26,27].

CONCLUSIONS
Oxygen and hydrogen plasmas have a greater effect on SnO 2 films obtained from a pentahydrate tin tetrachloride solution in 97% ethanol with different concentrations of tin ions. Perhaps this is due to the higher porosity of the sample and, consequently, to the increase in the contact surface of ionized gases with the film material. After treatment with the oxygen plasma of the films obtained from a film-forming system with a tin ion concentration of 0.08 mol/L, the transparency increased by 3-5% in the visible region of the spectrum. Treatment for 30 s in the oxygen plasma of films obtained from a film-forming system with a tin ion concentration of 0.11 mol/L led to an increase in the transparency in the visible region of the spectrum by 1-3%. An increase in the processing time to 60 s caused a decrease in transparency in the visible region of the spectrum and an increase in the longwavelength part of the spectrum. The transparency of the samples obtained from a film-forming system with a tin ion concentration of 0.14 mol/L decreased with an increase in the duration of the treatment.
It is assumed that the formation of tin oxide occurs under the influence of the reducing properties of hydrogen plasma. The reduction of tin dioxide to metallic tin is unlikely due to the absence of a decrease in the transmittance in the long-wavelength region of the spectrum. Treatment with hydrogen plasma for 3 min led to a decrease in the resistance of the samples by a factor of about 1.5 due to the increase in oxygen vacancies under the influence of hydrogen plasma. Treatment in hydrogen plasma for 6 min leads to a further decrease in resistance due to the formation of the amorphous and crystalline phases of tin oxide SnO 2 , which has a higher resistance. Treatment for 9 min in hydrogen plasma leads to an increase in resistance, probably due to the destruction of SnO 2 crystallites. Reducing the resistance of thin SnO 2 films after treatment in hydrogen plasma for 3 min, without a significant decrease in transparency, promotes the use of layers of tin dioxide as transparent electrodes.