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 (<i>T</i> _<i>S</i> ) ranging from 25°C to 350°C under 220 mTorr He. Film morphology evolved from nanoparticles to layered hexagonal platelets with increasing <i>T</i> _<i>S</i> , accompanied by a shift in preferred orientation from (015) to highly (00 l)-oriented textures. Composition varied from Te-rich at low <i>T</i> _<i>S</i> 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 the TE performance, Bi₂Te₃ films grown at 200, 250, and 300°C were in-situ annealed in helium gas at 220 mTorr for 60 min at annealing temperatures (<i>T</i> _<i>A</i> ) of 200, 250, 300, and 350°C. Simultaneous tuning of <i>T</i> _<i>S</i> and <i>T</i> _<i>A</i> 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.