Temperature and thickness dependent dielectric functions of MoTe2 thin films investigated by spectroscopic ellipsometry

Understanding the physical origin of temperature and thickness dependent optical properties of 2D MoTe2 is essential and vital for their applications in nanoelectronics and optoelectronics. Here, we investigate the dielectric functions of 5-25 nm MoTe2 thin films over temperatures from 100 K to 450 K and energy range of 0.75-5.91 eV by spectroscopic ellipsometry. Six critical points (CPs) A-F are observed in the dielectric functions, and their center energies and strengths exhibit novel thickness dependencies due to thickness scaling effects. Optical transition positions of CPs A-D are pointed out in bandstructure and partial density of states (PDOS). Besides, by tuning temperature from 100 K to 450 K, the amplitude of CPs decreases, the broadening of CPs increases, and center energies of CPs shows a Bose-Einstein attenuation trend. The difference Delta E between center energies of CPs A and B indicates the spin-orbit splitting value in MoTe2, which is found to remain around 320 meV and is independent to temperature and thickness. Results demonstrate that the strengths of electron-phonon interactions in CPs A and B show "V" evolution trend with thickness increasing, which could be assigned to the alternating effects of thickness scaling effects and increase of surface roughness.