單分子電晶體內部之力學震盪對電子傳輸的影響

Abstract

由於微製成技術的進步,分子電子學在這幾年成為熱門的議題。當電子穿越低維尺度元件,如一維的奈米碳管(carbon nanotubes),零維的半導體量子點(semiconductor quantum dot)或分子聚合物(single molecules based on C60), 電流隨bias的增長呈現非線性關係, 傳統的歐姆定律不再適用。這時候,我們必須從能階量化角度研究這個問題。不同於傳統的半導體量子點(GaAs/AlGaAs)單純考慮能階量化，以分子當SET元件必須考慮van der Walls力造成的分子震盪, 該力可簡諧位勢用近，所以我們額外獲得聲子自由度。因為是SET元件，分子裡的電子與金屬導線裡的電洞形成靜電力，該力造成位勢的偏移。最後，聲子-聲子之間的hoping行為改變電子的傳輸性質。一般，聲子輔助傳輸分佈範圍很廣，即使在bias-transport區域(Coulomb Blockade)外也可以測到. 這些通道的數量可經由溫度, bias-voltage, EPI強度來調節。在這份論文裡,我們分析了單分子電晶體裡的電流與雜訊受熱與bias的影響。在這個研究,我們使用非平衡態Green function的方法求得標準的EPI 電流公式。經由電流的計算, 我們瞭解系統的電導如何受聲子的影響. 除此之外,我們也建立廣義的EPI noise公式, 其中包涵對導線裡thermal fluctuation與系統裡shot noise的描述,也包涵了各級(e.g., sequential tunneling與co-tunneling) tunneling的行為。更重要的,這個方法預測的結果與實驗很接近。

In recent years, molecular electronics has become a very popular topic of great importance due to the advance in micro-manufacture technology. The conventional Ohm’s law is no long applicable when electrons pass through low-dimensional devices such as 1D carbon nanotubes, 0D semiconductor quantum dot, or single molecules based on C60, where the current increases non-linearly with the increment of bias. To investigate this phenomenon, we consider the quantization of energy level. Different from the conventional semiconductor quantum dot (GaAs/AlGaAs), the molecule vibration caused by the van der Waals forces needs to be taken into account when using molecules as SET devices, for example, the degree of freedom of phonon. The hoping behavior among phonons changes the transport properties of electrons because the static electric forces between molecules and the wire lead to the shift of potential energy. In general, phonons have a vast range of assistance in transport, even outside the bias-transport region. The numbers of these channels can be controlled through temperature, bias-voltage and the EPI. In this dissertation, we analyze the influences of heat and bias on the current and the noise in SETs. We study how the conductance in a system is affected by phonons using the standard EPI derived from non-equilibrium Green function. Additionally, we also establish a generalized formula for EPI noise, which concludes the description about the thermal fluctuation in the wire and the shot noise in the system as well as various tunneling behavior. These theoretical predications are very close to the experimental results.