Tin dioxide ultra-thin films become more mobile

Materials World magazine
3 Jun 2020

Researchers claim to have achieved the highest mobility in thin films of tin dioxide ever reported.

An ultra-thin film of tin dioxide with very high mobility is the focus of research at the University of Tokyo, Japan. The increased mobility is said to make the material highly conductive and transparent, allowing visible and near-infrared light to pass through. The μH value of mobility was recorded at 130 cm²V−1s−1 at room temperature – thin films of tin dioxide usually show a range of μH under 100 cm²V−1s−1. 

According to the team, transparency and conductivity do not usually coexist within a material. Conducting materials tend to be opaque, and transparent materials such as glass or plastic are insulating. They point to potential applications of their discovery in solar cells, enhanced touchscreen displays with increased accuracy and responsiveness, and more efficient LED lights.
 Shiochiro Nakao, Researcher from the Department of Chemistry at the University, notes that tin dioxide is a desirable material, found in many industrial applications such as gas sensors and transparent electrodes for its high mobility and durability in harsh environments. ‘Mobility is a parameter that is specific to a material – every material has its own upper limit of mobility,’ says Nakao. 

‘Electrical properties of semiconductors are expressed by mobility and carrier density. Because conductivity is proportional to mobility, higher is better.’ 

Materials in the form of thin films are often unable to exhibit expected mobility, the team has adopted a method using a highly focused laser to evaporate pellets of pure tin dioxide to manipulate the material. ‘Such a process allows us to explore different growth conditions as well as how to incorporate additional substances,’ says Nakao. ‘This means we can endow tin dioxide semiconductors with high mobility and useful functionality.’ 

The team made the thin films of tin dioxide systematically and achieved record values in (001)-oriented films. This was tested by measuring the electrical properties of tin dioxide using four-point resistivity and hall effect measurements. 

The team wants to extend the technique to a scalable and industrial deposition method – sputter-deposition. ‘In this study, we used expensive substrates – TiO₂ (001) single crystals. We must make films on inexpensive substrates such as glass or plastic,’ says Nakao. ‘We have already successfully fabricated (110)- and (100)-oriented films on glass substrates. Thus, we are trying to make them (001) as the next step.’