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

在熱電冷卻及室溫發電應用方面、硒化鉍（Bi2Se3）與碲化鉍（Bi2Te3）是廣為人知的熱電材料。熱電材料的性能取決於ZT = α2σT/κ值，其中 α、σ、κ和T分別為Seebeck係數、電導率、熱導率和絕對溫度。目前，以高沉積溫度，有利於增高σ值，但所形成薄膜之組成為非化學計量型。因此如何提昇Bi2Se3和Bi2Te3薄膜之熱電功率因子（PF = α2σ）仍然有其困難及挑戰性。 本論文研究中，以脈衝雷射 (PLD) 沉積 n型TE Bi2Se3 與Bi2Te3 熱電薄膜在SiO2/Si基板上。進而探討壓力（P）與沉積溫度對於Bi2Se3及Bi2Te3熱電薄膜的結構、組成、與形貌,及其熱電性質之影響。吾人發現，在較高沉積壓力下（≥ 40 Pa），Bi2Se3沉積基板溫度達300℃，而Bi2Te3基板溫度達340℃時，可製備出具化學計量之熱電薄膜。此不僅降低載流子濃度（n），而且依著α ~ n-2/3之關係，顯著提高Seebeck係數（α）。此外，在較高的基板溫度下沈積，可得到具高度（00l）晶向的大晶粒，層狀結構，此促使載流子遷移率 (μ) 大幅增加，進而提高電導率 (σ = nμe）。例如，在300℃及40 Pa下沈積製備之Bi2Se3薄膜，其結構是具有高程度（00l）方向的層狀、六方晶片，此薄膜展現最高的PF 值，5.54 μWcm-1K-2 ，其中 |α| = 75.8 μV/ K、σ = 963.8 S/cm 。 同樣地，在220 – 340°C基板溫度和80 Pa氬氣壓力下，可製備出具化學計量之Bi2Te3熱電薄膜，其具有高程度之（00l）方向的層狀結構，並展現最佳熱電性能，其中載流子遷移率μ = 83.9 – 122.3 cm2/Vs、|α|=172.8 – 189.7 µV/K、以及非常高的PF值，24.3 μWcm-1K-2。反之，在基板溫度(Ts) ≤ 120℃下 成長的Bi2Te3薄膜，含較多的Te元素，並具有（015）優選方向之小晶粒、柱狀結構或者在380℃所沉積製備之薄膜，含有Te-空缺，另呈現Bi4Te5多面體結構，導致較差的熱電特性。其PFs 值， ≤ 0.44 μWcm-1K-2，此中μ < 10.0 cm2/Vs、|α| < 54 μV/ K。 本研究全面性探討PLD製程參數、對 Bi2Se3 與Bi2Te3熱電薄膜的微觀結構、組成和形貌對熱電性質之影響， 及其相互關係，進而改善熱電材料的性能和應用。簡而言之，具高度（00l）晶向的大晶粒、層狀結構和化學計量之組成乃為影響μ及|α|之主要因素，進而顯著提高PF值。

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.