脈衝雷射沉積之硒化鉍與碲化鉍熱電薄膜: 結構，組成和形貌對熱電性質之影響

Abstract

Bismuth selenide (Bi2Se3) and bismuth telluride (Bi2Te3) are well-known compounds for thermoelectric (TE) cooling and generation applications near room-temperature. The performance of TE materials is quantified by a dimensionless figure of merit, ZT = α2σT/κ, in which α, σ, κ, and T are the Seebeck coefficient, the electrical conductivity, the thermal conductivity, and absolute temperature, respectively. Currently, enhancing the TE power factor (PF = α2σ) of Bi2Se3 and Bi2Te3 thin-films remains a challenge due to the coupling amongst TE material properties and the difficulty of growing stoichiometric films under elevated substrate temperatures (Ts), at which is beneficial for enhancing the σ. In this thesis study, n-type TE Bi2Se3 and Bi2Te3 thin films were grown on SiO2/Si substrates using pulsed laser deposition (PLD). The effects of the structure, composition, and morphology on the TE properties of Bi2Se3 and Bi2Te3 thin films were investigated by controlling background ambient pressures (P) and Ts in PLD depositions. We found that the deposition in relatively high P (≥ 40 Pa) could obtain stoichiometric films at extended Ts up to 300 °C for Bi2Se3 and 340 °C for Bi2Te3, which can reduce the carrier concentration (n) and significantly enhance the Seebeck coefficient (α), following the α~n-2/3 relation approximately. Furthermore, at high Ts- growths, the obtained structures of highly (00l)-oriented – layered of large crystallites led to the substantial increase in the carrier mobility µ and thus improve the σ (= nµe). For example, the stoichiometric Bi2Se3 films grown at grown at 300 °C and 40 Pa with highly (00l) oriented and layered-hexagonal platelets possessed the highest PF of 5.54 µWcm-1K-2, where ׀α׀ = 75.8 µV/K and σ = 963.8 S/cm. Similarly, the stoichiometric Bi2Te3 films grown at Ts = 220–340 °C and PAr = 80 Pa with highly (00l)-oriented and layered structures showed the best properties, with a carrier mobility µ of 83.9 – 122.3 cm2/Vs, an ׀α׀ of 172.8 – 189.7 µV/K, and a remarkably high PF of 18.2 – 24.3 µWcm-1K-2. In contrast, the Te-rich films deposited at Ts ≤ 120 °C with (015)-preferred orientations and columnar–small grain structures or the Te-deficient film deposited at 380 °C with Bi4Te5 polyhedron structure possessed poor properties, with µ < 10.0 cm2/Vs, ׀α׀ < 54 µV/K, and PFs ≤ 0.44 µWcm-1K-2. This study provides a comprehensive understanding the interrelationships between PLD processing conditions, microstructures, and TE properties of Bi2Te3-based thin films, promising for further improving the TE performance of materials and applications. In brief, the morphology of highly (00l) oriented–layered large crystallite structures and the stoichiometry predominantly contribute to the substantial enhancement of µ and ׀α׀, respectively, resulting in remarkable enhancement in PF.