Fabrication of Orientation-Controlled Fluorine-Doped SnO2 Film Grown By Atmospheric Spray Pyrolysis
Authors: Yoshino Kenji , Yoshiyama Koichi , Higashi Tomohiro 
Published: December 2024

Wide bandgap oxide semiconductors are attracting attention for applications such as optical devices, liquid crystal displays, and solar cells. In particular, Sn-doped In2O3 (ITO) is the most widely used transparent conductive oxide (TCO) material. Recently, due to the problem of decreasing indium resources, ZnO has been investigated as an alternative TCO material. Similarly, F-doped SnO2 (FTO) has been investigated in gas sensors, photovoltaic solar energy conversion devices, and electrochromic devices. FTO is an n-type semiconductor with a tetragonal structure similar to the rutile structure and a wide energy gap. Although various dopants are possible, films doped with fluorine exhibit high transparency and good conductivity. Many techniques have been used to prepare SnO2 thin films, including radiofrequency magnetron sputtering, chemical vapor deposition, metal-organic chemical vapor deposition, sol-gel methods, and spray pyrolysis. Spray pyrolysis is attractive for thin film growth because plasma damage to the substrate is avoided, high vacuum is not required, and equipment costs are low. However, there are not many papers on the FTO grown by spray pyrolysis.
In our previous work [1], FTO films were successfully grown on glass substrates by the spray pyrolysis at 500 °C. The (200) orientation became strong with increasing fluorine concentration. In addition, F-doping caused the resistivity to decrease and the carrier concentration to increase. The lowest resistivity, 1.4 × 10-3 Ω cm, was obtained at a fluorine concentration of 4 mol%. Its carrier concentration and mobility were 6.2 × 1020 cm-3 and 7 cm2 V-1 s-1, respectively. Furthermore, the average optical transmittance in the visible region was nearly constant (at more than 80%) with increasing fluorine concentration.
In this study, FTO films with various thicknesses (70 - 1550 nm) were successfully grown on glass substrates by spray pyrolysis method below 500 °C under atmospheric atmosphere. All the peak positions of XRD spectrum of the synthesized FTO film were in good agreement with the peak positions of ICDD. Furthermore, no fluorine related signals were observed in the spectra. The three-dimensional growth was associated with an increase in both grain size and film thickness and a change in orientation from (110) to (101) and (200).
The preferred orientation of FTO changed at a film thickness of approximately 600 nm. It was considered to be influenced by the critical film thickness. Additionally, as the film thickness increased, the transmittance in the infrared region decreased. This was due to the absorption of free carriers at the electron plasma frequency within the film. The resistivity showed a tendency to decrease as the film thickness increased.
On the other hand, the carrier concentration remained constant, but the mobility increased with increasing film thickness. This meant that the grain boundary scattering due to increasing grain size was dominant. Carrier concentration, surface structure, and film thickness were important factors in solar cells. In this study, the IR absorption increased despite the carrier concentration being constant. This result implied that at a constant high carrier concentration, the absorption was enhanced by increasing the film thickness.
[1] M. Oshima and K. Yoshino: J. Electron. Mater. 39 (2010) 819.