Ultrafast-laser Modification of Thermoelectric Sb 2 Te 3 Thin Films
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Ultrafast-laser Modification of Thermoelectric Sb2Te3 Thin Films Yuwei Li1,2, Vladimir A. Stoica1,2, Lynn Endicott1, Guoyu Wang1,2, Huarui Sun2,3, Kevin P. Pipe2,3, Ctirad Uher1,2,4, and Roy Clarke1,2,4 1 Department of Physics, University of Michigan, Ann Arbor, MI 48109 2 Center for Solar and Thermal Energy Conversion, University of Michigan, Ann Arbor, MI 48109 3 Department of Mechanical Engineering, University of Michigan, Ann Arbor, MI 48109 4 Applied Physics Program, University of Michigan, Ann Arbor, MI 48109 ABSTRACT We have modified Sb2Te3 thin film thermoelectric materials by scanning a femtosecond laser across the film surface to create track-like nanostructures. These nanotracks have widths of 50-80 nm and a periodicity of ~ 130 nm. We show that the nanotrack morphology is highly dependent on laser fluence and scan speed. Moreover, we performed transient thermoreflectance measurements on a laser-irradiated film and found a thermal conductivity reduction of 4.5% in the nanostructured regions compared to that of the unmodified regions. These results suggest the potential use of femtosecond pulsed lasers to create nanostructured thermoelectric materials with improved performance.
INTRODUCTION Antimony Telluride (Sb2Te3) has emerged as a very important technological material, with applications in thermoelectrics[1], topological insulators[2], and phase change memory[3]. In the context of thermal energy conversion, it has been predicted that low dimensional structures such as nanowires and quantum dots can further increase the thermoelectric figure of merit compared to the bulk form[4,5]. A type of nanostructure, produced with ultrafast lasers and having dimensions much smaller than the laser wavelength, has recently attracted considerable interest[6-8]. The majority of these laser-induced nanostructures were studied in semiconductors having a bandgap that is larger than the laser photon energy. In this work we demonstrate that surface nanostructures can also be formed in narrow-gap materials like Sb2Te3 which is absorbing in the visible and near-infrared spectral bands[9]. We show the dependence of the surface morphology of these nanostructures on various experimental conditions, including laser fluence and scan speed. In addition, we performed transient thermoreflectance measurements and compared the thermal conductivities in the laser-irradiated nanostructured regions and the non-irradiated smooth regions, observing a significant reduction. This is one route to improved energy conversion performance.
EXPERIMENT The Sb2Te3 thin films were grown by molecular beam epitaxy (MBE) on Al2O3 (sapphire) substrates. We used a fiber laser with a wavelength of 1580 nm, a pulse duration of 100 femtoseconds and a repetition rate of 100 MHz. The fluence of the laser can be varied by using a half waveplate followed by a polarizer. The linearly polarized laser beam was focused by a parabolic mirror at normal incidence onto the Sb2Te3 film. The film was mounted on an XYZ translation stage and moved transversely, enabling
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