Thermoelectric, magnetotransport, and ultrafast dynamics of bismuth telluride thin films grown using pulsed laser deposition: effects of substrate temperature and post-annealing

n-Type Bi2Te3 and Bi4Te5 thin films were grown on SiO2/Si substrates via pulsed laser deposition (PLD) at substrate temperatures (TS) ranging from 25°C to 350°C under 220 mTorr He. Film morphology evolved from nanoparticles to layered hexagonal platelets with increasing TS, accompanied by a shift in preferred orientation from (015) to highly (00 l)-oriented textures. Composition varied from Te-rich at low TS to Te-deficient at 350°C. Near-stoichiometric and (00 l)-textured Bi2Te3 thin films deposited at 250–300°C exhibited reduced carrier concentration (~9.5 × 1019 cm−3), significantly enhanced mobility (up to 81.2 cm2/V·s), and a maximum thermoelectric (TE) power factor (PF) of 20.0 µW·cm−1·K−2. To further enhance TE performance, Bi2Te3 films grown at 200, 250, and 300°C were in-situ annealed in helium gas at 220 mTorr for 60 minutes at annealing temperatures (TA) of 200, 250, 300, and 350°C. Simultaneous tuning of TS and TA revealed a processing window for optimized PFs, achieving a peak value of 23.8 µW·cm−1·K−2 for the film grown at 250°C and annealed at 250°C– a 19% improvement over the as-deposited counterpart. Additionally, low-temperature transport measurements exhibited two-dimensional weak antilocalization behavior in the optimized TE Bi2Te3 thin film, suggesting the presence of topological surface states. Ultrafast spectroscopy further revealed coherent optical and acoustic phonon modes at 1.87 THz and 37.3 GHz, respectively.