Effects of Substrate Temperature on Properties of Sb-doped SnO₂ Thin Film

1. INTRODUCTION

Transparent conductive oxide (TCO) films having exceptional optoelectrical properties, such as high transparency and electrical conductivity, are widely applied in devices such as flat panel displays, solar cells, sensors, and optoelectronic devices. [1–6] Indium tin oxide (ITO) is one of the most widely used materials for TCO films owing to its high transparency in the visible wavelength range, and low electrical resistivity. However, indium (In) is a relatively rare material, and ITO is brittle. [1]

Another promising candidate for a TCO material is tin oxide (SnO₂) because of its wide bandgap and good chemical and mechanical stability. [1] SnO₂ is known to be a wide bandgap (~3.6 eV) n-type semiconductor material with high transmittance in the visible wavelength range (380–780 nm), low transmittance in the near-infrared wavelength range (780–2500 nm), and good adhesion to glass. [3, 6–8] To meet the practical requirement of TCO films, transition metal ions can be doped into SnO₂ films to modify the bandgap of SnO₂. [3, 7] Among various dopants, In, F, and Sb are attractive because they improve electrical conductivity. As In is expensive, films doped with the relatively cheaper elements like F and Sb are of interest, and they show excellent chemical and thermal stability. [3, 6, 7] Door and window glasses with these films can save building energy owing to the above properties. [6] A blue-colored coating can be produced by doping Sb into SnO₂ films. [3]

Sb-doped SnO₂ (ATO) can be prepared by various deposition methods, including sputtering [9-11], spray pyrolysis [12], and sol-gel [7, 13]. Among the deposition techniques, the sputtering method has been widely used for fabricating TCO thin films because of its advantages of high growth rate, low cost, easy control, high growth efficiency for a good quality film, and suitability for large area deposition. [14] In addition, the deposition of very thin ATO films with under 100 nm thickness is important from the viewpoint of microelectronic application. Accordingly, studies on this topic will be of benefit in helping the design and fabrication of miniaturized devices.

In this work, ATO thin films were prepared by an RF (radio frequency) magnetron sputtering system. The effect of substrate temperature on the properties of ATO films was investigated.

2. EXPERIMENTAL

ATO films were prepared on a SiO₂/glass substrates by an RF-magnetron sputtering system. A ceramic target of 95% SnO₂ and 5% Sb₂O₃ with a diameter of 3 inches was used. After establishing a vacuum chamber of 1.5 × 10^-6 Torr in the RF magnetron sputtering system, Ar gas was introduced at a flow rate of 20 sccm. The working power, sputtering time, and target-to-substrate distance (T-S distance) were 30 W, 10 min, and 100 mm, respectively. The substrate temperature was varied from 200 to 500 °C. The sputtering conditions for ATO film deposition are summarized in Table 1.

Table 1.

Sputtering conditions for ATO film deposition.

The crystallinity of the ATO films was determined with an X-ray diffractometer (XRD, Rigaku, D/Max-IC), and the film thickness was measured using an alpha-step (α-step, KLA, Tencor). The surface morphology of the deposited films was observed by the scanning electron microscope (SEM, Hitachi S-4200). Transmittance spectra were produced using an ultraviolet-visible spectrometer (Shimadzu, UV-1601PC). The electrical characteristics of the ATO films, such as sheet resistance and carrier concentration, were measured by Hall effect measurement (Ecopia, HMS-3000).

3. RESULTS AND DISCUSSION

3.1 Structural properties

Fig. 1 shows XRD patterns of ATO films deposited on SiO₂/glass substrates at different substrate temperatures. Three diffraction peaks at approximately 26.6°, 33.8°, and 51.7° are observed for the deposited ATO films. These peaks correspond to the (110), (101), and (211) planes, respectively, and they indicate the cassiterite tetragonal structure for SnO₂ (according to JCPDS card no 41-1445). [12] These peaks do not change significantly with the substrate temperature in the range of 200–500 °C; thus, crystallinity is not affected by temperature.

Fig. 1.

XRD patterns of ATO films deposited on SiO2/glass substrates at different substrate temperatures.

Fig. 2 shows the surface morphology of the ATO films. It can be observed that the grain size decreases, and the grain boundaries become less distinct as the substrate temperature increases from 200 °C to 400 °C. At 500 °C, there are a few large particles resulting from grain aggregation. This may be caused by an increase in the surface energy of the grains at this high substrate temperature. [6]

Fig. 2.

SEM images of ATO films deposited on glass SiO2 substrates at different substrate temperatures.

Fig. 3 shows the variation in ATO film thickness as a function of substrate temperature. The film thicknesses were 76.7, 71.3, 68.4, and 57.0 nm at substrate temperatures of 200 °C, 300 °C, 400 °C, and 500 °C, respectively. Thus, it can be concluded that the thickness of the deposited film decreases with increasing substrate temperature.

Fig. 3.

Thickness of ATO films deposited on SiO2/glass substrates at different substrate temperatures.

3.2 Optical properties

Optical transmittance spectra of the ATO films in the range of 220–800 nm are shown in Fig. 4. In the wavelength range between 380 and 780 nm, the average transmittance of the ATO films deposited at the substrate temperature of 200 °C, 300 °C, 400 °C, and 500 °C is 76.9%, 77.9%, 77.5%, and 74.5%, respectively. For the films deposited at 300, and 400 °C, the transmittance is higher than the other two samples, and in particular, the transmittance in the blue region is higher than that in the other wavelength regions. Thus, these two films have an intense blue color, which has been frequently reported in literatures. [3, 15, 16]

Fig. 4.

Optical transmittance spectra of ATO films deposited on SiO2/glass substrates at different substrate temperatures.

3.3 Electrical properties

The effect of the substrate temperature on the electrical properties of the ATO films is illustrated in Figs. 5 and 6. Fig. 5 shows that as the substrate temperature increases from 200 to 400 °C, the sheet resistance and resistivity decrease significantly, while from 400 to 500 °C, the increase of both properties is quite small. In other words, the ATO film deposited with substrate temperature at 400 °C has the best conductivity. The sheet resistance and resistivity at this temperature were 8.43 × 10² Ω/sq and 0.3991 × 10^-2 Ω·cm, respectively, which are lower than those of ATO films deposited by metal organic decomposition. [1]

Fig. 5.

Sheet resistance and resistivity of ATO films deposited on SiO2/glass substrates at different substrate temperatures.

Fig. 6.

Electron concentration and mobility of ATO films deposited on SiO2/glass substrates at different substrate temperatures.

For the application of the ATO films as transparent electrodes, the films must have low electrical resistance and high optical transparency. A useful tool for evaluating the performance of transparent electrodes is a calculation of the figure of merit (FOM). FOM is defined by

• FOM = T¹⁰/Rs,

where T is the average optical transmittance and Rs is the sheet resistance of the ATO films. [17] FOM value of the ATO films deposited at the substrate temperature of 200 °C, 300 °C, 400 °C, and 500 °C was 3.04×10⁻⁵ Ω⁻¹, 8.17×10⁻⁵ Ω⁻¹, 9.37×10⁻⁵ Ω⁻¹, and 6.01×10⁻⁵ Ω⁻¹, respectively. The highest FOM was at 400 °C, showing the optimal substrate temperature of the ATO films.

Fig. 6 shows the electron concentration and mobility of the ATO films deposited at different substrate temperatures between 200 °C and 500 °C. Since Sb has five valence electrons and is substituted for Sn, which has four valence electrons, the majority charge carrier is excess electrons, resulting in an n-type semiconductor. As the substrate temperature increases from 200 °C to 400 °C, the carrier concentration and mobility increase. This is likely because a more uniform surface reduces lattice scattering, which can improve mobility. At 500 °C however, as shown in Fig. 2(d), the aggregation of grains increases the level of defects in the deposited ATO film, resulting in a decrease in the mobility of the charge carrier. Thus, the optimized substrate temperature is 400 °C, and at this temperature, the charge carrier concentration and mobility were 2.36 × 10²¹ cm^-3 and 6.627 × 10^-2 cm²V^-1s^-1, respectively.

4. CONCLUSIONS

ATO thin films were prepared on a SiO₂/glass substrate by an RF magnetron sputtering system. The effects of substrate temperature on the structural, optical, and electrical characteristics were investigated. The substrate temperature during deposition did not affect the basic crystal structure of the films, but the grain boundaries became less distinct as the substrate temperature increased to 400 °C. In addition, the thickness of the deposited ATO film decreased with increasing substrate temperature. The optical transmittance of the ATO films deposited at different substrate temperatures was over 70%. The lowest sheet resistance and resistivity were 8.43 × 10² Ω/sq, 0.3991 × 10^-2 Ω·cm, respectively, and the highest carrier concentration and mobility were found to be 2.36 × 10²¹ cm^-3 and 6.627 × 10^-2 cm²V^-1s^-1, respectively, at a substrate temperature of 400 °C. Considering the electrical and optical properties of the ATO films, the optimized substrate temperature is 400 °C.

Acknowledgments

This work was supported by research grants from Daegu Catholic University in 2022.

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