用掃描穿隧顯微鏡測量硒化鉛量子點陣列之電子傳輸行為

Abstract

硒化鉛為一具有窄能隙0.28 V的半導體，是個適合在長波長及紅外光波段之光電元件應用上的材料。近年來，除了單一量子點的研究外，更發現到硒化鉛所組成的量子點陣列有著場效電晶體跟記憶等特性，因此對於量子點陣列的研究成為近年來熱門的課題。

本實驗是利用低溫超高真空掃描穿隧電子顯微鏡去量測在金平面上，硒化鉛量子點陣列的集體電子傳輸行為。硒化鉛顆粒原本溶在甲苯溶液中，直徑約為15奈米，我們藉由控制溫度，將溶液滴在金平面上，可以成長出奈米顆粒陣列，並可控制陣列大小，並利用掃描穿隧顯微鏡，測量奈米顆粒陣列之電流-電壓曲線，結果可用雙穿隧接面模型與MW模型分析。如量子點陣列可視為一島嶼，並用雙穿隧接面模型擬合，我們取得陣列大小與兩穿隧接面之等效電阻與電容的關係，因此可推測量子點之間有強電性耦合效應。如回到量子點陣列模型，利用MW模型分析，我們可估計臨界電壓及電流通道數與陣列大小的關係，並觀察到電子在量子點陣列的集體傳輸行為。由MW模型分析結果，又可得知量子點間有強電容耦合效應，此結果與雙穿隧接面模型分析結果一致。

The PbSe semiconductor having a narrow band gap of 0.28 eV is suitable for applications in optoelectronic device, especially in long wave length and infrared waveband. Recently, PbSe quantum-dot arrays attract much attention due to a demonstration to build field-effect transistors and memory effect devices.

In this study, we try to investigate electron transport in PbSe QD arrays by using a low-temperature scanning tunneling microscope in a ultra-high vacuum. The PbSe QDs dispersing in toluene were deposited on a gold surface. Two-dimensional islands of QD arrays with various sizes formed on the substrate via a self-assembly process. I-V curves on QD arrays were obtained through scanning tunneling spectroscopy measurements. We adopted double-tunneling-junction model and MW model to analyze our data.

Taking the QD array as an isolated metal island, we can obtain resistances and capacitances according to the double-tunneling-junction model. The estimated resistances and capacitances show a dependence on the size of the QD array. On the other hand, the MW model was used to analyze I-V curves in voltage range above a threshold voltage. We obtained the size dependence of threshold voltage (Vth) and the dimensional parameter ζ. We believe that the result come from a strong capacitive coupling between QDs. We have explored collective electron transport in the PbSe QD arrays as a function of the array sizes.