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

## TL;DR

Researchers optimized bismuth telluride thin films for thermoelectric performance by adjusting deposition and annealing temperatures, achieving a top power factor and revealing topological properties.

## Contribution

Systematic optimization of PLD-grown Bi2Te3 thin films via substrate and annealing temperature control, achieving a 19% improvement in thermoelectric power factor.

## Key findings

- Near-stoichiometric Bi2Te3 films deposited at 250–300°C showed reduced carrier concentration and enhanced mobility.
- Post-annealing at 250°C improved the power factor to 23.8 µW·cm−1·K−2, a 19% increase over as-deposited films.
- Low-temperature transport measurements revealed two-dimensional weak antilocalization behavior, indicating topological surface states.

## Abstract

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 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 (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.

This work systematically optimizes PLD-grown Bi2Te3 thin films via substrate and annealing temperature control, achieving a PF of 23.8 µW cm−1K−2, enhanced mobility, and uncovering thermoelectric–topological coupling via magnetotransport and ultrafast spectroscopy.