Highly oriented nanocrystalline bismuth telluride thin films obtained by radio-frequency magnetron sputtering with a magnetic field applied to the substrate via an affixed permanent magnet

We deposited nanostructured bismuth telluride (Bi2Te3) thin films on glass substrates with NdFeB permanent magnets of varying magnetic flux densities attached to the backside, and then thermally annealed them in an electric furnace. Surface scanning electron microscopy (SEM) revealed that the thin films deposited without the additional magnetic field and in the presence of an additional magnetic field with a magnetic flux density of 189 mT were both composed of nano-sized crystal grains with flat surfaces. XRD patterns indicated that the thin film deposited at 189 mT exhibited very high crystal orientation along the c-axis. As a result, this thin film exhibited high mobility and a power factor of 14.1 μW/(cm·K2), which was 65% higher than that of the thin film deposited in the absence of an additional magnetic field. When the magnetic flux density was increased to 378 mT or more, the thin film surface was covered with irregularly shaped powder depositions, resulting in a decrease in the crystal orientation, and by extension, the desirable thermoelectric properties. Therefore, we conclude that application of an additional magnetic field with a moderate magnetic flux density can improve the structural and thermoelectric properties of Bi2Te3 thin films deposited by RF magnetron sputtering.