脈衝雷射沉積高導電性之碲化鎵/碲週期排列奈米複合結構於熱電轉換之應用

Abstract

碲化鎵(Ga2Te3)是具有閃鋅礦結構之III-VI族半導體材料，為了維持閃鋅礦(Zinc blende)結構中陰陽離子之劑量比，故在碲化鎵晶粒中存在大量週期性排列之二維空孔平面(2-dimensional vacancy planes)，其可有效散射聲子，大幅降低材料之熱傳導係數(κ)。此外，從歷年來僅有之少數熱電研究中發現，碲化鎵奈米結構塊材之常溫席貝克(Seebeck)係數(~800 μVK-1)遠高於廣為研究之碲化鉍材料(~200 μVK-1)，由此吾人推估其為極具發展潛力之熱電材料系統。然而，從目前文獻可知，碲化鎵各式材料之導電率由於空孔平面大量存在而始終偏低，導致其熱電優值(Thermoelectric figure of merit, ZT)仍遠低於可應用之基本要求(ZT~1)。 奈米化工程被視為是可以有效提升ZT值的方法之ㄧ，這是由於材料奈米化後，新生成之表面或介面能大幅降低熱傳導係數，同時理論已証明，奈米尺寸引發之量子侷限效應(Quantum confinement)將有助於Seebeck係數之提升。除上述奈米工程概念之外，在本研究中，更進一步導入奈米複合結構之全新概念，藉由導電性第二相的析出，對奈米結構之介面進行修飾，進而提高導電率，甚至，改變第二相之介面析出含量，將有機會控制奈米複合結構之Seebeck係數，從而達到提升ZT值之目標。 本研究利用脈衝雷射沉積技術，於常溫下將碲化鎵奈米粒子(Nanoparticles)薄膜沉積於1.5×1.5 cm2之SiO2/Si絕緣基板上，再透過後續退火處理，可以在碲化鎵奈米粒子薄膜之介面析出第二相(碲)，形成週期性排列之碲化鎵/碲奈米複合結構。藉由退火溫度及時間之精準控制，在主相碲化鎵之奈米區塊間隔中，可獲得點對點單晶橋樑(200 oC)、完整填充(225 oC)及部分填充(250 oC)單晶河流等第二相網絡狀形貌，這些導電性奈米結構析出物可作為導電載子之新通道，使得遷移率大幅提升，結果發現與碲化鎵單晶材料(~28 cm2V-1s-1)相當。另外，載子濃度(~3×1017 cm-3)亦較碲化鎵單晶材料高出約5個數量級，因此常溫導電率最高可提升至177.7 Sm-1，從而使得功率因子達19.6 μWm-1 K-2，約是2008年所發表最佳功率因子（碲化鎵奈米結構塊材）之87倍。

Gallium telluride (Ga2Te3), one of the typical A2IIIB3VI semiconductors, has a crystal structure of zinc blende (ZnS). To keep the stoichiometry of the Ga3+ and Te2- in the zinc blende structure, a large number of periodically assembled two-dimensional vacancy planes spontaneously form, which could greatly scatter the phonons and thus effectively decrease the thermal conductivity (κ). In addition, the Seebeck coefficient of Ga2Te3 found from the very few thermoelectric data available can reach a very high level of ~800 μVK-1, which is four times higher than that of the well-studied bismuth telluride (Bi2Te3, ~200 μVK-1). As a conclusion, it is reasonable to believe that Ga2Te3 should be a very promising thermoelectric material system not only for the fundamental researches but also for the practical applications for the next generation. However, according to the relevant studies, the electrical conductivity of all kinds of Ga2Te3 structures is rather low probably due to the great presence of the 2D vacancy planes, which also leads extremely low values of thermoelectric figure of merit (ZT). Nanostructuring is recognized as one of the most effective strategies for improving the ZT values since not only could the largely created surfaces or interfaces inhibit the thermal conductivity but the Seebeck coefficient could be improved through the induced quantum confinement. In this work, in addition to the above mentioned concepts, we introduce a completely new concept, that is, the nanocomposites for fundamentally resolving the problem of interface resistance. By precipitating the conductive second phase, each separated nano-domains of the main phase can be well linked by point-to-point or face-to-face approaches. By changing the amount of precipitations, we could further control the Seebeck coefficient of the nanocomposites to achieve the goal of ZT enhancements. In this work, by using the pulsed laser deposition (PLD) technique, the Ga2Te3 nanoparticles were deposited on the insulated SiO2/Si substrates (1.5×1.5 cm2). Through the post annealing, the Te second phase could precipitate between the Ga2Te3 nano-domains and thus formed unusual periodically-aligned Ga2Te3/Te nanocomposites. By precisely controlling the annealing temperature and time, the morphology of the Te second phase varies from the point-to-point single-crystal bridge (200 oC) to fully (225 oC) or partially (250 oC) filled river-like nanostructures for providing brand-new pathways for carriers. As a result, the carrier mobility is greatly improved and is comparable to the value found from the single crystalline Ga2Te3 (~28 cm2V-1s-1). Interestingly, the carrier concentration (~3×1017 cm-3) also dramatically increases up to 5 orders compared to that of the single crystalline Ga2Te3. The present Ga2Te3/Te nanocomposites show an extremely high electrical conductivity of 177.7 Sm-1 at room temperature. The obtained power factor of 19.6 μWm-1K-2 is about 87 times higher than that of the best value been reported up to date.