半導體量子點陣列中電性傳輸與電子結構對尺寸之相關性

Abstract

近年來科學家藉由化學溶膠法可製作出尺寸為十幾到幾奈米的單顆半導體量子點並且自組裝成量子點陣列。此量子點及其陣列可用來研究量子侷限效應、庫侖阻滯與類似原子的電子能態等相關研究。硒化鉛的激子波耳半徑相較於其他二六族或三五族材料來得大，使得硒化鉛製成量子點時其量子侷限效應更顯著。此外，硒化鉛具有較大的介電常數，因此電荷屏蔽效果明顯並且電容耦合效應較強。從過去的研究可知量子點陣列中的電性是複雜且無法使用簡單理論解釋的，而且對於量子點陣列中電子結構對尺寸的相關性仍缺乏有系統地研究。 在本實驗中，我們將硒化鉛量子點在Au(111)平面上自組裝成大小不同的二維陣列，接著分別在室溫、液態氮與液態氦的溫度下使用掃描穿隧顯微鏡來量測陣列的掃描穿隧能譜，得到不同大小陣列的I-V曲線。首先我們使用雙穿隧接面模型來分析室溫與液氮溫度下陣列的電性傳輸行為，發現樣品與基板間的等效電容無法隨著陣列顆粒數增加而等倍數增加。此結果顯示陣列不能被視為單一導電島嶼，電子在量子點陣列中並非像在導體中一樣自由運動；因此我們進一步使用集體傳輸理論來解釋量子點之間電容耦合與量子點陣列尺寸的關係。我們發現電子從針尖傳輸到量子點之後，有可能先傳輸到相鄰的量子點再傳輸到基板上，因此電子的導通路徑隨著陣列的尺寸變大而增加。最後我們將液氦溫度量測的I-V曲線經微分處理得到電導能譜，使用多峰高斯擬合得到電子與電洞的能階；經由統計發現隨著陣列顆粒數增加，能階的半高寬會變寬且能階間距會變小，顯示陣列中量子點的耦合效應使電子與電洞去侷域化，能態由分裂逐漸趨向於連續。

Various diameters (5-20 nm) of PbSe semiconductor quantum dots were synthesized through a colloidal synthesis technique and used for the investigation of quantum confinement, Coulomb blockade, and atomic-like electronic states. In this work, the size dependence of charge transport and electronic structure of PbSe QD arrays were studied. Various sizes of QD self-assembled, two-dimensional islands (clusters or arrays) were deposited on flat gold surface for the scanning tunneling spectroscopy measurement at room temperature, liquid-nitrogen, or liquid-helium temperatures. For data taken at room and liquid-nitrogen temperatures, the double tunneling junction model was used to analyze the current-voltage (I-V) curves of the QD arrays to derive the tip-to-array and the array-to-substrate capacitances. It is found that the array-to-substrate capacitances are not linearly proportional to the size of the QD arrays, so the array cannot be taken as a simple island. We therefore used another analysis method, applying the collective Coulomb blockade theory to exploration of these I-V curves. For data taken at 5 K (liquid-helium temperature), the differential conductance (dI/dV versus V) spectra of the QD arrays reveal atomic-like electronic states. The data were fitted by using the Gaussian function to find multiple quantized electron and hole states. It is observed that the quantized states become broadened and energy gap narrowed down as the number of QD is increased. This result implies that there must be interdot, electrical coupling between the PbSe QDs, resulting in delocalization of quantized electrons and holes in the QD arrays.