不同載子密度下串聯量子尖端接觸的電性傳輸

Abstract

本論文是研究低溫時在不同載子密度下串聯量子尖端接觸的電性傳輸。我們利用微影製程技術在高遷移率的GaAs/AlGaAs異質結構上製作多對串聯型式的金屬閘極，並在串聯傳輸通道上方隔著絕緣層製作控制電極，藉此控制下層傳輸通道內的載子密度並觀察其電性傳輸。 在數據分析上，我們利用Beenakker的理論模型分析串聯通道下的電性傳輸情形，直接穿透率係數Td可以描繪電子從某通道穿越至另一通道的彈道式傳輸比例多寡，並藉此分析電性傳輸的特性。實驗結果顯示Td隨著控制閘極電壓下降而下降，載子傳輸在高載子密度區展現部分的絕熱傳輸，但在低載子密度區卻呈現完全的歐姆傳輸，我們把它歸因於低載子密度下平均自由徑與同調長度的減少。

We have studied the ballistic electron transport of double quantum point contacts(QPCs) in series with different carrier densities at low temperatures. Our sample is fabricated by the lithographic technology on a high mobility GaAs/AlGaAs heterostructure. In this device, two pairs of metal gate are placed longitudinally and sequentially with an edge-to-edge distance of 600nm. Isolating from an insulating layer, a top gate is also fabricated on the top of the quantum wires to modify the carrier density in the quantum wires and the two dimensional electron gas as well. In our analysis, we used Beenakker model to understand the electric transport behaviors. The electric transport is characterized by the direct transmission coefficient Td which represents the portions of electrons traveling ballistically from one quantum wire to the other. Our results show that the parameter Td decreases with decreasing carrier density. The transport is partially adiabatic in high electron densities and transits to completely ohmic regimes in low densities. Because of the correlation between the coherence length and transmission coefficient, we attribute the result to the reduction of the mean free path and the coherence of electron.