奈米結構之製作與多閘極奈米結構在交流偏壓下之量子抽運效應

Abstract

摘要 隨著磊晶技術的進步與次微米製程的發展，使我們可以成功的製作出具介觀性質的元件。也就是說在此系統中，電子的平均自由路徑以及相位同調長度可以超越元件的尺寸大小。在此論文中，我們研究了在量子點接觸中的量子傳輸特性，以及在量子窄通道上加上一時間調變的位能，觀察其特殊的量子抽運效應(quantum pumping effect)。 在論文的第一部份，我們在低溫下對Hall bar結構以及量子窄通道元件做電性量測，並分析其量測結果。這些元件都是建構在利用分子束磊晶(MBE)所成長的二維電子氣體之上。我們利用在Hall bar結構上量測得到的結果( 、 )，推算出在二維電子系統中的電子密度(electron density)與遷移率(mobility)。我們也利用在閘極上加負偏壓的方式，定義出量子窄通道，並做電性量測，得到電導量子化的結果。 在論文的第二部份中，我們探討了在量子窄通道上加一可隨時間調變的位能的量子傳輸特性。利用數值計算的方法，我們精確的描述出電子在窄通道內的行為。當電子的化學能(chemical potential)為窄通道內的共振態時，將會在傳輸上發生特殊的溪谷結構(valley structure)我們將不需要外加偏壓於系統上，只需藉著週期變化的位能(pumping potential)影響，即會產生因量子抽運效應而造成的電流。最後，位能(pumping potential)的頻率及強度對於傳輸的影響也將被探討。

Abstract With the tremendous improvements in the qualities of semiconductor materials and submicron fabrication technology, it has become possible to fabricate mesoscopic devices: devices in which its dimension is exceeded by the phase coherent length and even, in the ballistic regime, by the elastic mean free path. In this dissertation, we fabricate a few quantum point contacts and measure their quantized conductance characteristics. We also explore theoretically the quantum pumping effect in a narrow constriction due to a time modulated potential. In the first part of this thesis, we present the results of our low temperature transport measurement on a Hall bar structure, and on a quantum narrow channel induced by split-gates. These devices are made out of a molecular beam epitaxy (MBE) grown two-dimensional electron gas (2DEG) in GaAs/AlGaAs heterostructures. Important physical parameters such as election density and mobility are determined from the magnetoresistance and Hall resistance measurements in Hall bar structures. The conductance in the narrow channel is also measured by similar techniques. In the second part of this work, we study the transmission of electrons through a narrow channel acted upon by a time modulated potential. This pumping effect is nonadiabatic. We have calculated nonperturbatively the effect of a spatiotempral pumping potential on a narrow constriction. Valley structures are found in the transmission whenever the chemical potential aligns with the resonant states formed in the pumping region of the channel. The pumped current is driven by a cyclic change in the pumping potential and not by an externally imposed source-drain bias. Dependencies of pumping on the frequency and amplitude of the pumping potential are also investigated.