低維度系統中的新奇電導現象及聲電流效應

Novel Conductance Phenomenon and Acoustoelectric Effect in Low-Dimensional System

溫崇賢(Chung-Hsien Wen)

指導教授 ： 陳正中

國立清華大學/理學院/物理學系/博士(2013年)

摘要

在此論文中我所探討的研究主要分成兩個部分,分別是在準一維度系統下的 電性傳輸特性及低維度系統下的聲電傳遞行為。首先以電子束微影製程製作出 量子點接觸元件於砷化鎵及鋁砷化鎵異質結構中的高電荷遷移率二微電子氣系 統,藉此來探討量子點接觸平台上的電導振盪現象。量子點接觸通道中的位勢 可分別藉由加偏壓於分離電極(split-gate)及跨越電極(cross-gate)來作局部調整。 分離電極和跨越電極彼此之間以PMMA做電性上的絕緣。在跨越電極上施予正 偏壓時,電導的量化平台上展現了震盪的特徵,相反的施予負偏壓時振盪現象 會消失,同時平台寬度也會縮減。我們的探討認為電導平台上的振盪現象是來 自於縱向電子於通道中共振所造成。主導此現象的主要因子為通道的寬長比及 入射電子的費米波長。

接著我們探討的是存在於量子線(準一維系統通道)中的雜質對零偏壓異常現 象(zero-bias anomaly)的影響。線性電導呈現一突出特徵,此突出特徵會隨著雜 質在通道中的位置有所演變,而雜質在通道中的相對位置可藉由分離電極及跨 越電極來連續調控。零偏壓異常現象無論在有無雜質的存在下都可被觀察到。 近籐模型已不再適合描述電導及零偏壓異常的行為。儘管存在著雜質散射,在 乾淨傳輸通道中的零偏壓異常行為如同於其他研究著發表的結果。我們的研究 結果推論出零偏壓異常現象是量子線裡面的本質效應,其隨著溫度及磁場的行 為不適合以量子點中的近籐模型來描述。

論文最後一部分所探討的是利用表面聲波來探測由單層石墨所構成的二維載 子與聲子之間的交互作用,以及雙層二維傳輸系統之間的壓電耦合效應。我們整合了指叉狀能量轉換器,化學氣相沈積的單層石墨以及砷化鎵異質結構成為 一混合元件。此元件的優勢是,表面聲波除了可以在砷化鎵的表面傳遞之外, 砷化鎵異質結構中的二維電子氣可當做單層石墨稀背部電極用來調控其載子濃 度。我們於實驗上藉由直接量測單層石墨稀理由表面聲波所感應出的聲電流, 來得到隨著不同載子濃度的表面聲波強度衰減量。然而實驗結果與理論預測呈 現相反行為。儘管如此,我們首開先例的探討了表面聲波與單層石墨稀中可調 變載子濃度的交互作用。除此之外,聲電流在石墨稀及二維電子氣的傳輸在透 過壓電效應會使其之間有偶和效應。

關鍵字

量子點接觸 ； 零偏壓異常 ； 近藤模型 ； 表面聲波 ； 石墨稀

並列摘要

In this thesis, we present research concerning both electrical transport in a quasi-one-dimensional system and acoustic transport in a two-dimensional system.

We investigated the resonance conductance of a quantum point contact (QPC) defined in a two-dimensional electron gas of a high-mobility GaAs/AlGaAs het- erojunction. The potential profile of the QPC channel can be locally tuned by separately biasing the split gate and a cross gate, which is electrically isolated on the top of the QPC. The conductance, which evolves with the cross-gate voltage, exhibits an oscillatory feature superimposed on the quantized plateau for positive bias voltages and a suppression of the plateau for negative bias voltages. Our investigation suggests that the oscillations on the conductance result from the longitudinal resonance through the channel. The governing parameters of the res- onance are the aspect ratio of the channel and the Fermi wavelength of the incident electrons.

Next, we investigated the behavior of the zero-bias anomaly (ZBA) in quan- tum wires embedded with impurities. The linear conductance G can exhibit cusp features that evolve with the positions of the impurities, and these features can be continuously tuned using a combination of split-gate and top-gate voltages. The ZBA is observed regardless of the presence of impurities. The Kondo model is in- adequate to describe the behaviors of both G and the ZBA. Despite the presenceof impurity scattering, various ZBA behaviors resembling those reported in clean quantum wires can be observed. Our results suggest that ZBA is an intrinsic phe- nomenon in a quantum wire, and its temperature and magnetic-field dependence do not pertain to Kondo correlations in the quantum dot system.

The last portion of this thesis reports a study of the carrier density dependence of the acoustoelectric effect in graphene and the coupling effect between double two-dimensional electronic systems. We integrated interdigital transducers, chem- ical vapor deposition (CVD) graphene and a GaAs/AlGaAs heterostructure into a hybrid device. Our device has two advantages: the SAW can propagate on the GaAs cap layer, and the 2DEG can operate as a back gate for tuning the Fermi energy in the graphene. We obtained the density dependence of the SAW atten- uation by directly measuring the induced acoustoelectric current in the graphene and obtained a result that differs from the theoretical prediction. Despite this dis- agreement, we believe that the interaction between the SAW and the adjustable carrier density in the graphene is significant. In addition, some data reveal that the acoustoelectric transport property in each 2DES has some correlation with the hybrid system.

並列關鍵字

Quantum point contact ； Zero bias anomaly ； Kondo model ； Surface acoustic wave ； graphene

參考文獻

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