應用於熱電之立方摻銻碲化鍺化合物之第一原理研究

First-Principles Studies of Cubic Sb-Doped GeTe Compounds for Thermoelectric Applications

張光遠(Benjamin K Chang)

指導教授 ： 周美吟

國立臺灣大學/理學院/物理學研究所/碩士(2017年)

https://doi.org/10.6342/NTU201701378

摘要

鍺銻碲化合物(Ge-Sb-Te compound) 因其相變的特質， 最早被應用於非揮發性記憶體。近年來， 該類化合物應用於熱電(thermoelectric) 方面的可能性亦被探討。經實驗上證實，經由適當的製程方式，該化合物的熱電優值(zT) 可在約攝氏300 度的操作環境下增加至2.5 以上，表現極佳。
本研究旨在使用第一原理計算，以立方碲化鍺(cubic GeTe) 晶體為基底，探討立方銻碲鍺化合物的晶體結構、電子結構、傳輸性質，以及銻在該化合物中扮演的角色。我們首先說明了立方碲化鍺極易產生鍺空缺(Ge vacancies) 和銻取代缺陷(Sb substitutions) 以形成銻碲鍺化合物。接著，我們發現銻碲鍺化合物能在存在大量缺陷之情況下維持晶體結構之穩定性。而後，我們發現該系統的能帶結構在有大量缺陷的情況下，仍可與完美的立方碲化鍺晶體之能帶結構相去不遠。這些證據說明，缺陷在鍺銻碲化合物中扮演調整費米能級的功能。也因為如此，我們提出剛性能帶模型，利用完美的立方碲化鍺晶體之能帶結構進行波茲曼方程式的計算，以估計鍺銻碲化合物的傳輸性質。最後，在與實驗結果比對後，我們推測：對於鍺銻碲薄膜，其生長的基板可能是決定其熱電性質的一大重要因素。
銻碲鍺化合物是極有潛力的熱電材料，本研究為未來該化合物方面的研究奠定了基礎。

關鍵字

熱電 ； 能量學 ； 能帶展開 ； 剛性能帶模型 ； 波茲曼傳輸理論 ； 第一原理

並列摘要

Ge-Sb-Te (GST) compounds have been known for their application to non-volatile memories due to their good phase change property. Recently, their applicability to thermoelectric usage has also been discussed. It has been shown that by proper preparation procedures, their thermoelectric figure of merit (zT) can be boosted over 2.5 near 300◦C.
In this study, we adopt first-principles calculations and use cubic-phase GeTe as an example to investigate the crystal structure, electronic structure, transport properties, and the role of Sb in cubic GST. First, we show that a considerable amount of Ge vacancies and Sb substitutions are both easily introduced into cubic GeTe to form cubic GST. Second, we find that the crystal structure of cubic GST can sustain a large amount of defects. Third, we show that the band structure of cubic GST remains similar to that of cubic GeTe in the presence of many defects. These findings indicate that the role of the defects in GST is largely tuning the Fermi level. Thus, we adopt a rigid band model and use the cubic GeTe band structure to estimate the transport properties of cubic GST. Finally, by directly comparing our calculational results to experiment results, we conclude that the substrate plays a substantial role in determining the transport properties of GST thin films.
GST is a very promising type of thermoelectric materials. We believe that this study provides a guideline for the future development on GST.

並列關鍵字

thermoelectric ； energetics ； band unfolding ； rigid band model ； Boltzmann transport theory ； first-principles

參考文獻

[1] H. Alam and S. Ramakrishna. A review on the enhancement of figure of merit from bulk to nano-thermoelectric materials. Nano Energy, 2(2):190 – 212, 2013.

[3] P. Bauer Pereira, I. Sergueev, S. Gorsse, J. Dadda, E. Müller, and R. P. Hermann. Lattice dynamics and structure of GeTe, SnTe and PbTe. physica status solidi (b), 250(7):1300–1307, 2013.

[4] L. E. Bell. Cooling, Heating, Generating Power, and Recovering Waste Heat with Thermoelectric Systems. Science, 321(5895):1457–1461, 2008.

[5] L. Bellaiche, S.-H. Wei, and A. Zunger. Composition dependence of interband transition intensities in GaPN, GaAsN, and GaPAs alloys. Phys. Rev. B, 56:10233–10240, Oct 1997.

[6] P. E. Blöchl. Projector augmented-wave method. Phys. Rev. B, 50:17953–17979, Dec 1994.