Towards a More Accurate Determination of Thermoelectric Properties of Bi₂Se₃ Epifilms by Suspension via Micromachining Techniques

We report on the measurement of thermoelectric properties of Bi₂Se₃ epifilms. MBE-grown Bi₂Se₃ epifilms on SI GaAs (111)A are micromachined with integrated Pt elements serving as local joule heaters and integrated thermometer, and voltage probes. Seebeck coefficients (<i>S</i>), as well as electrical conductivities (<i>σ</i>), are measured on a patterned 2 µm x 30 µm Bi₂Se₃ from 50 K to 300 K. To minimize substrate effects in determining the thermal conductivity (<i>k</i>) of Bi₂Se₃ by self-heating 3-w method, we suspend a 2 µm x 300 µm Bi₂Se₃ by micromachining techniques, namely selectively etching GaAs buffer/substrate layers by citric acid solution followed by critical point drying method. From temperature dependent electrical conductivity measurements, we find little difference between unsuspended and suspended devices. From suspended devices, we find the absolute value as well as temperature dependence to be similar to nanoribbon samples reported elsewhere. The measured thermoelectric properties of 200 nm thick Bi₂Se₃at room temperature are <i>k</i> = 1.37 W / m K, <i>S</i> = -160.4 µV / K, <i>σ </i>=94687 S / m, and the figure of merit, <i>ZT</i> = 0.53. We show the thickness dependence of figure of merit from 100nm up to 150 nm and 200 nm thick Bi₂Se₃thin films, and discuss the optimized thickness of Bi₂Se₃. The study not only introduces a method to measure the thermal conductivity accurately by suspending thin films, but also provides the understanding of the thickness effects on thermoelectric properties.