SnO₂ thin films have a wide bandgap and are used as transistor devices for sensor elements, solar cells, and photoconductors based on the excellent electrical and optical properties of SnO₂ [1–6]. There are various methods of growing SnO₂ thin films, including chemical vapor deposition [7, 8], sputtering [9, 10], and the sol-gel method [11,12]. Sputtering methods have the merit of being able to produce a large amount of thin films easily. The electrical and chemical properties of thin films vary depending on the deposition conditions [13]. Even if the deposition conditions are the same, the shape of the thin film depends on the stoichiometry and morphology [14]. The deposition temperature of the thin film, amount of oxygen introduced, type of substrate, vacuum state in which the thin film is formed, intensity of the supplied power, and deposition time all affect thin film formation. Among these factors, the initial growth conditions of the thin film have a significant influence on the formation of the thin film [15–17]. In this study, the initial growth of the thin film is changed by varying the deposition time of the thin film as 5, 20, and 30 min after formation of the SiO₂ layer by treating the Si substrate with oxygen for 30 min.

Si substrate has been washed initially for 20 s in acetone solution followed by pure water and exposed in spilled nitrogen gas in the vacuum state. Flowing oxygen (purity: 99.99%) was introduced into a chamber for 30 min using a radio-frequency sputterer to form a SiO₂ thin film. The SnO₂ thin film was formed by introducing oxygen into the chamber while simultaneously forming a plasma in which the Sn⁺ ions were dropped onto the Si substrate from the Sn target while maintaining the conditions under which the SiO₂ layer was formed. The SnO₂ thin film was fabricated by maintaining the same conditions used for forming the SiO₂ layer and generating plasma in the chamber, causing the Sn⁺ ions to fall onto the SiO₂ thin film. The amount of oxygen entering the chamber was maintained constant at a rate of 10 sccm, and the deposition time of the thin film was adjusted to 5, 20, and 30 min, respectively, for growth of the SnO₂ thin film. The experimental conditions are listed in Table 1.

Table 1 Conditions for formation of SnO₂/SiO₂ thin films.

Sample	Power [W]	Pre-oxygen flow rate [sccm]	Deposition Temp.[℃]	Deposition Time[min.]	Pre-oxygen cond.[ min.]	Flow rate Ar:O2[sccm]
S1	200	10	500	5	30	10:10
S2	200	10	500	20	30	10:10
S3	200	10	500	30	30	10:10

The structure of the SnO₂/SiO₂ films was investigated by X-ray diffraction (Rigaku, DMAX 2000, Japan). The X-rays were generated from the Cu-Kα line with a wavelength of 1.5405 Å, over an angular range of 10 -− 60°. The X-ray diffraction pattern obtained from the measurement was compared with the interval and peak intensity of the crystal plane given in the JCPDS card (00-005-0467), and the crystal planes of the measured diffraction pattern were analyzed. The chemical compositions of the SnO₂/SiO₂ film samples were calculated by converting the peak intensity and area of the characteristic X-rays emitted from each element constituting the sample using energy dispersive X-ray spectroscopy (EDS; Horiba, 7593-H, UK). The electrical and chemical properties of the SnO₂ thin films were measured using a potentiostat (EG&G). The interface between the thin film and the substrate was evaluated by transmission electron microscopy (TEM; Technai, F20, Phillips, The Netherlands) based on the electron density penetrating through the layer using an electron beam emitted at 200 kV.

Figure 1 shows the X-ray diffraction data for S1, S2, and S3 (see Table 1). In the case of S1, the XRD pattern of the SnO₂/SiO₂ thin film exhibits broad peaks of amorphous species because the intensities of the major peaks of the (110), (101), and (200) crystal planes were very weak. When the deposition time is short, the initial time for satisfying the stoichiometry by chemical bonding of the Sn⁺ ion and the O− ion during growth of the thin film is insufficient. As the deposition time increases, the intensity of the main peaks of the (110), (101), and (211) planes of the thin film increase and the sample becomes more crystalline. The initial growth condition is an important parameter in the growth of the thin film, and changes in the growth of the thin film will change the particle shape, particle size, and electrical characteristics of the thin film.

Figure 1. X-ray diffraction patterns of the SnO₂/SiO₂ thin films.

Figure 2 shows the EDS spectra of the SnO₂ thin films S1, S2, and S3. The elements constituting the sample were quantitatively analyzed by converting the area of the characteristic EDS peaks of Si, Sn, and O in the profiles of the SnO₂/SiO₂ thin films. Table 2 shows the deposition time increased, the weight ratio of Sn:O changed from 54:42 to 56:40 to 65:32, indicating that the ratio of Sn increased as the deposition time of the thin film increased. As the deposition time increased during the growth of the tin oxide thin films, oxygen initially formed on and adhered to the silicon substrate; however, with the increasing deposition time, the Sn ions deposited to the substrate even more, causing an apparent increase in the Sn ion ratio.

Table 2 Atomic element percentages of SnO₂/SiO₂ thin films.

Element	S1		S2		S3
	Wt%	At%	Wt%	At%	Wt%	At%
O K	42.11	82.07	40.49	81.57	32.62	76.85
Si K	03.21	03.57	02.59	02.97	01.71	02.30
Sn L	54.68	14.36	56.92	15.46	65.66	20.85

Figure 2. EDS spectra of the surface of the SnO₂/SiO₂ thin films.

Figure 3 shows the Nyquist plot based on impedance spectroscopy. As the deposition time of the thin films increased, the Nyquist plot of the thin films tended to rise uniformly in a semicircle. The capacitance of the thin films changed to 6.59 µF, 2 nF, and 5 nF. The electrical capacity of the thin film is determined by the surface area of the thin film, electric resistance, potential difference, and defects in the thin film. As the deposition time increased, the thickness of the thin film increased, whereas the electrical capacity of the thin film did not increase in proportion to the thickness of the thin film. As the deposition time increased, the resistance of the thin film increased constantly from 37.5 to 172.5 to 297.4 kΩ. As the deposition time increased, the thickness of the thin film increased and the number of defects in the thin film decreased. Therefore, the electrical resistance of the thin film also increased.

Figure 3. (Color online) Nyquist plot of the SiO₂/SnO₂ thin films. The x and y axes correspond to the real and imaginary parts of the impedance, respectively.

Figure 4 shows a TEM photograph of a section of the SiO₂/SnO₂ thin film. The SiO₂ layer on the Si substrate was 316.82 nm thick and the SnO₂ layer was 48.81 nm thick. During deposition of the thin film, oxygen was introduced onto the Si substrate for 30 min to form the SiO₂ layer, and the SnO₂ thin film was then deposited on the Si substrate through the transmission electron microscope. The electrical properties of the thin film may be defective at the surface of the thin film or at the interface between the substrate and the thin film. The oxygen defects will affect the electrical properties of the SnO₂ thin films.

Figure 4. (Color online) TEM image of the surface of the SiO₂/SnO₂ thin films.

SnO₂/SiO₂ thin films are affected by the deposition time because the intensity of the growth surface increases constantly as the deposition time increases for the (110), (101), and (211) planes. The electrochemical properties of SnO₂/SiO₂ thin films also affect the impedance of the thin films based on the deposition time, where the resistance increases as the deposition time increases. TEM analysis showed that a SiO₂ layer was formed between the substrate and the SnO₂ thin film, affecting the electrical properties of the thin film.

This study was supported by research fund from Chosun University, 2021.