A simple turn gets the most from rutile tin dioxide

Polycrystalline tin dioxide (SnO₂) has been used for years in applications such as gas sensors, heat-reflecting coatings on windows and transparent contacts on displays and solar cells. In recent years, many in the research community have also been exploring single-crystal forms of SnO₂ and similar oxides for potential uses in next-generation electronic devices. The high band gap of SnO₂ allows those devices to operate at greater temperatures or voltages than other semiconductor materials.

Now researchers have found a simple way to get the most from single crystal SnO₂ by taking advantage of one of its natural properties. Rutile tin dioxide, the most common stable structure of SnO₂, crystalizes into a form whose unit cell is naturally anisotropic. Seeking to uncover how that crystal anisotropy maps to its electrical properties, the research group tweaked an experimentally simple approach called “van der Pauw measurements,” a method for determining average conductivity of a material.

Instead of the average conductivity, they determined the conductivity along the two inequivalent crystal axes by measuring a single-crystalline, square-shaped wafer of SnO₂. The wafer’s edges were aligned according to these crystal axes in combination with a calibration derived from finite element method calculations. Reporting in the journal Applied Physics Letters, authors showed that the material’s conductivity was anisotropic and that the direction of highest conductivity was the direction of the lowest effective electron mass — 1.26 times higher conductivity than in the perpendicular direction.

This suggests that devices using SnO₂ components could realize performance gains simply by aligning the material in the optimal direction. The work, part of the growth and fundamentals of oxides for electronic applications collaboration (GraFOx), also demonstrates how van der Pauw measurements can be exploited to determine anisotropic transport properties.

Source: “The inherent transport anisotropy of rutile tin dioxide (SnO₂) determined by van der Pauw measurements and its consequences for applications,” by Oliver Bierwagen and Zbigniew Galazka, Applied Physics Letters (2018). The article can be accessed at https://doi.org/10.1063/1.5018983 .