以脈衝雷射所沉積碲化錫薄膜之熱電特性

Abstract

碲化錫（SnTe）為無鉛狹窄能隙IV-VI 族半導體材料。此材料對於環境極為友善，且為熟知鉛硫族化合物的高潛能替代材料，一樣可應用於600 – 900 K的熱電（TE）發電機中。本研究中，改變基板溫度（Ts）從25至400 oC 與220 mTorr的氬氣壓力下，利用脈衝雷射沉積（PLD）SnTe 薄膜在SiO2/Si (100)基板上。進一步系統性地分析薄膜結構、成分和形貌對於室溫下TE特性的影響。形貌在不同的Ts下展現出奈米三角形(25 – 120 oC)、奈米柱(200 oC)、奈米葉片(250 oC)、奈米雕塑品(300 oC)、奈米方塊(350 oC)以及緊湊多晶形(400 oC)。Sn成分比例略微提高時，能有效地抑制Sn空缺的生成進而獲得較低的電洞濃度Np ~ 1020 cm-3。Seebeck係數與理論預測相比有顯著地提高，可被歸因於(1)較低的費米能階讓SnTe的兩價帶(L+Σ)同時貢獻，使得狀態密度大幅的提升(2)獨特的奈米結構。此外，電洞遷移率(µ)單方向地隨著Ts從25 oC (0.37 cm2/Vs) (對應奈米三角形)至400 oC (345.2 cm2/Vs) (對應緊湊多晶)增加。µ隨著Ts的增加而增加，歸因於當Ts增加使得Np（缺陷）減小、晶粒尺寸略微增大也大幅改善晶體結晶度。Ts在250 oC下，奈米葉片薄膜擁有最好的Seebeck係數(41.1 µV/K)和最佳的TE 功率因子(3.53 µWcm-1K-2)，其中σ = 2089 Scm-1。這些奈米結構像是奈米葉片因大量的聲子在晶界產生散射而有利於降低晶格熱傳導係數，其結果展現了奈米結構的SnTe薄膜有希望的提高TE優質係數(ZT)與其應用。

Tin telluride (SnTe), a lead-free narrow band gap IV-VI semiconductor, is an environment friendly and a potential substitution material of the well-known lead chalcogenides for thermoeletric (TE) power generator in middle range temperatures (600 – 900 K). In this study, nanostructured p-type SnTe thin films were grown on SiO2/Si (100) substrates using pulsed laser deposition (PLD) at substrate temperatures (Ts) from 25 to 400 oC under an argon gas pressure of 220 mTorr. Effects of the films structure, composition, and morphology on the room-temperature TE properties were systematically studied. The morphologies exhibited nanotriangulars at Ts, 25 – 120 oC, nanocolumns at 200 oC, nanoleaves at 250 oC, nanosculptures at 300 oC, nanotetris at 350 oC, and compact-polycrystalline at 400 oC. A composition of slightly Sn-rich suppressed Sn vacancies, resulting in a low hole concentration of Np ~ 1020 cm-3. The Seebeck coefficient was considerably enhanced as compared with the theoretical prediction, due to (1) the enhancement of density of states as lowering the Fermi level to have simultaneously contribution of the well-known two-valence (L+Σ) bands of SnTe, and (2) the distinctive nanostructures. Moreover, the hole mobility (µ) monotonically increased from 0.37 cm2/Vs (for nanotriangular) to 345.2 cm2/Vs (for compact-polycrystalline) with increasing Ts from 25 to 400 oC. The increasing µ with Ts can be attributed to the decreasing Np (defects), the slightly increasing grain size, and the improved crystallinity with increasing Ts. At Ts of 250oC, the nanoleaves film possessed the best Seebeck coefficient (41.1 µV/K) in conjunction with the best TE power factor (PF), 3.53 µWcm-1K-2, where σ = 2089 Scm-1. The nanostructures such as nanoleaves are beneficial in reducing the lattice thermal conductivity due to the extensive phonon scattering at grain boundaries. As a result, nanostructured SnTe thin films have been demonstrated their promises for enhancing the figure of merit (ZT) and TE applications.