多閘極奈米結構製作與其量子抽運傳輸

Abstract

在本論文中，我們研究了在GaAs/AlGaAs異質結構中利用分離閘極定義一維窄通道的量子傳輸性質，除了在單純窄通道之元件觀察到了量子化電導(quantized conductance)的現象，也在窄通道上製作指狀閘極，藉以外加一位能對窄通道造成額外的靜電侷限，並觀察此造成電子傳輸的影響。

在樣品製作方面，我們成功地利用微影製程在GaAs/AlGaAs異質結構上定義出一些不同幾何結構的雙層次(bi-layer)金屬閘極，利用電子阻劑PMMA作一電絕緣層隔離分離閘極層與指狀閘極層，使不同層次的金屬閘極能獨立控制。外加負偏壓於金屬閘極來驅趕(deplete)底層的二維電子氣體，並藉由控制不同的金屬閘極，可以使得二維電子氣形成一窄通道(narrow channel)或量子點(quantum dot)，進而觀察電子傳輸的性質。

在製程過程，我們發現淺式蝕刻(shallow etching)可避免樣品與閘極間漏電流(leakage current)發生，另外，以遠紅外光在低溫下照射樣品，可提升載子濃度與遷移率。這兩個關鍵技術是量測到低雜訊且高重複性數據的重要因素。

我們先量測了二維電子氣體的磁電阻Rxx與霍爾電阻RH，以決定其遷移率μ及電子密度ns，並估算電子平均自由路徑le。利用分離閘極外加負偏壓，造成其有效電位障對二維電子氣形成窄通道結構，並觀察到似一維的量子化電導現象。此外，又在分離閘極上層的指狀閘極外加負偏壓，造成額外進出窄通道的位障，我們藉由調變此指狀閘極的負偏壓，觀察窄通道與外界耦合的強弱所導致的電子傳輸特性變化。

In this thesis, we have studied the quantum transport properties of one dimensional channels defined by split-gates in GaAs/AlxGa1-xAs heterostructures. In addition to the observation of the usual quantized conductance plateaus in the clean narrow channels, other interesting transport properties have been investigated with the additional electrostatic confinements via overlaying finger-gates in the channel.

In the aspect of nano-pattern fabrication, we have successfully produced different geometric bi-layer metal gates on GaAs/AlGaAs heterostructures by electron beam lithography. A layer of PMMA between split-gate and finger-gate layers was made to be an insulator, so that all gates can be independently controlled. When a negative voltage is applied to split-gate, the underlying two dimensional electron gas is electrostatically squeezed into a one dimensional channel. Finger-gates on the top layer can be used to deplete electrons within the channel. Moreover, they can introduce QPCs in both ends of the channel to act as the entrance and exit barrier to a dot.

Through the processing, we also learned that shallow etching can prevent the electric leakage between mesa and gates. In order to increase the carrier density and mobility of the samples, they were first illuminated with the far infrared light at low temperature upon the measurement.

Magnetoresistance Rxx and Hall resistance RH have been measured to determine the carrier density and mobility of the two dimensional electron gas, and estimate the mean free path of electrons. The conductance measurements as a function of split-gate voltage demonstrate conductance plateaus at multiples of consistent with theoretical prediction for 1D channel. Applying a negative bias to one finger-gate atop the channel causes extra channel width confinement and dominate mainly the transport when its defined width is less than the original channel width. We also present the conductance measurements for a dot configurations with different coupling strengths between dot and its environment by controlling both finger-gates that affect the entrance and exit barriers.