Exploring substrate and Structure-Induced ultrafast phonon dynamics in Bi2Te3 thin films

Abstract

The investigation of phonon and charge carrier dynamics in Bi₂Te₃ overlayers requires consideration of key factors such as substrate orientation, annealing-induced modifications, and lattice mismatch, emphasizing the need for employing a range of substrates. In this study, Bi₂Te₃ thin films were grown on Silicon (Si) and Silicon nitride (SiN) substrates under room temperature and annealed conditions. Despite identical growth conditions, significant differences in film morphology were observed due to substrate orientation and lattice mismatch. Morphology due to annealing processes affected the vibration modes of Bi₂Te₃ and coherent acoustic phonon oscillations in the NIR range. Annealing altered phonon frequencies, leading to oscillation disappearance, attributed to breaking quintuple layers and reforming crystallographic planes, reducing crystallite size. For the as-grown samples, the crystallite size is 19.8 nm for Si and 38.9 nm for SiN, with a respective phonon frequency of 30.8 GHz and 31.3 GHz. After annealing, the morphology changed, reducing the crystallite size to 2.34 nm for Si and 0.54 nm for SiN, respectively. The reduction of crystallite size highly influenced the phonon frequency of Bi₂Te₃, which has implications for various optoelectronics applications, including tunable frequency generators and GHz-range radar applications.