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    題名: 低維度系統之熱電特性;Thermoelectric properties of low-dimensional structures
    作者: 曾彥鈞;Tseng,Yen-chun
    貢獻者: 電機工程學系
    關鍵詞: 熱電;奈米結構;量子點;量子井;低維度結構;熱整流;應力;熱電優值;thermoelectric;nanostructure;quantum dot;quantum well;low-dimensional structures;heat rectification;strain;figure of merit
    日期: 2014-03-31
    上傳時間: 2014-06-19 14:08:06 (UTC+8)
    出版者: 國立中央大學
    摘要: 本論文探討低維度系統於線性與非線性響應區間之傳輸及熱電特性。此低維度系統包括單層P型量子井結構和耦合量子點的結構。線性響應區間, P型量子井結構的熱電特性研究,討論平坦價電能帶結構(來自於混合價電帶與應力效應) 將如何影響電導率、席貝克係數、電子熱導率和熱電優值。聲子熱導的部分,我們則是採用二維聲子輻射傳遞的方程式加以計算。我們發現量子井內的最大熱電優值可利用調變合適的應力大小而將有所提升。熱電優值的提升可歸因於: 席貝克係數的平方項受應力介入提升的量強於電導率的下降量。另外,我們亦探討了量子點陣列在庫倫阻斷區間的熱電特性系統,並使用了延展的哈伯模型和安德森模型來描述量子點陣列接面之電子總能。凱帝旭格林函數則是被用以計算電導、席貝克係數和電子熱導。除此之外,我們求解一維聲子輻射傳遞來計算奈米線的聲子熱導。於庫倫阻斷範疇內,聲子熱導是遠大於電子熱導。因此,我們可以分別地藉由提升電導和降低聲子熱導以得到最佳化熱電優值。藉此,我們於砷化銦鎵/砷化鎵的一維量子點陣列接面系統中,論證室溫下的熱電優值可以接近於1.5。不過,當維度從一維量子點陣列嵌入奈米線開始過渡至二維量子點陣列嵌入量子井時,最佳的熱電優值將有顯著的下降。其原因來自於聲子熱導將隨著量子點陣列系統從一維過渡至二維而大幅提升。
    接下來,我們理論地研究串接耦合量子點連接金屬電極之非線性傳輸特性。在包利自旋阻斷的條件下,串接耦合量子點的電流整流和負微分電導之行為,主要來自於受偏壓方向不同而有所不同的機率權重和共振能階的失去而成。除此之外,我們亦觀察到齊曼效應的自旋偏振方向相依的電流整流現象。其中,最大的自旋偏振相依電流發生在順向偏壓的情況之下。這與前述電流整流的現象有所不同。最後,我們論證了串接耦合量子點系統,電子具有方向相依的熱流行為(熱整流)。此熱整流行為源於非對稱性量子點能階分佈。我們估算奈米線的聲子熱流,討論串接耦合量子點系統如何降低聲子熱流以期觀測到電子的熱整流行為。; The thermoelectric properties of low-dimensional systems including a single p-type quantum well (QW) and a serially coupled quantum dot chain (CQDC) are investigated in the linear response regime. In a p-type QW, we focus on the influences of flat valence subbands arising from the valence-band mixing and strain on the thermoelectric properties, such as electrical conductivity (, Seebeck coefficient (S), electrical thermal conductivity and figure of merit (ZT). The phonon thermal conductivity is calculated by the equation of phonon radiative transfer (EPRT) method. In this work, we find the maximum ZT of QWs can be enhanced under strain. Such an enhancement of ZT results from the fact that the enhancement of S2 is stronger than the reduction of .
    The thermoelectric properties of CQDC connected to electrodes are illustrated by using a combination of the extended Hurbbard model and Anderson model. The electrical conductance, Seebeck coefficient and electron thermal conductance are calculated by the Keldysh Green’s function technique. In addition, we calculate the phonon thermal conductivity of 1D-nanowire using the EPRT method. In the Coulomb blockade regime, phonon thermal conductance is much larger than electron thermal conductance. Therefore, the optimal ZT can be obtained by increasing the electrical conductance and decreasing phonon thermal conductance, simultaneously. We find that the optimal ZT value can reach 1.5 at room temperature in an InGaAs/GaAs 1D-CQDC junction system. The optimal ZT values are significantly suppressed in the 2D-CQDCs. It is mainly attributed to a large enhancement of phonon thermal conductivity. Finally we theoretically study the transport and thermoelectric properties of serially coupled QDs (SCQDs) in the nonlinear response regime. We observe the spin-polarization current rectification under the Zeeman effect. We also demonstrate that the electron heat current of SCQDs exhibits a heat rectification behavior in the low temperature regime.
    顯示於類別:[電機工程研究所] 博碩士論文

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