低維度多組閘極侷域之電子系統的電性傳輸

Abstract

本論文探討由分離閘極技術侷域之低維度電子系統的電性傳輸，利用微影技術在具有高遷移率的GaAs/AlxGa1-xAs異質接面結構之二維電子氣樣品上製作出多對串聯型式的金屬閘極以及量子空腔，利用外加負偏壓於金屬閘極對二維電子氣產生位障形成低維度電子系統，在低溫的環境中來研究類一維窄通道(串聯)和類零維量子空腔的電性傳輸。 在類一維窄通道系統中，我們製作出四種不同通道長度的量子線，並排組合出三種不同的串聯窄通道形式，(I)通道長度近似零和1μm的量子線，間距D=1μm, (II) 通道長度0.25μm和 2μm的量子線，間距D=1μm，以及(III) 通道長度0.25μm和2μm的量子線間距D=1.7μm，電子在此串聯形式窄通道間的傳輸可以用直接穿透率係數Td來描述，Td值越接近1代表電子有較高比例是由彈道式傳輸通過兩窄通道；由實驗分析的結果，間距越遠的串聯窄通道會因為有限同調長度的關係而有較小的Td值，另外Td也會被窄通道產生的位障形式所影響，在通道開口附近具有緩慢增寬位障形式的窄通道具有將電子流對準下一個窄通道的效應，具有這種對準效應的窄通道可以增加電子直接穿透下一個窄通道的機率，具有較高的Td值。 我們製作出兩種面積的電子空腔，分別為0.48和0.12μm2，在電導值與源極,汲極電位分析中發現有震盪產生，震盪發生的電壓條件是空腔與環境有較大的耦合強度所以震盪形式並不屬於庫倫阻絕式震盪；在外加垂值磁場的縱向磁電阻量測中發現兩種不同的量子干涉效應，在高磁場部分磁電阻隨磁場變化出現週期性震盪的磁電阻，經過分析發現與Aharonov-Bohm效應與磁通量週期變化的干涉效應相吻合，在低磁場的部分則是有負磁阻效應產生，這是由弱侷域效應所造成，並且在零磁場負磁阻波峰形式隨閘極電壓的變化，發現空腔形式在越負的閘極電壓電壓之下會由chaotic空腔變成regular空腔。

We have investigated the electrical properties of low dimensional electron systems defined by the split-gate technique. Our samples are QWs of different channel lengths placed longitudinally and sequentially with an edge-to edge distance D, and cavities of different area enclosed formed in a high mobility GaAs/AlxGa1-xAs heterostructure by negatively biasing split gates. In the work of serially connected QWs, three configurations are employed for QWs in series: (I) a quasi-zero and a 1μm QWs with D=1μm, (II) a 0.25μm and a 2μm QWs with D=1μm, and (III) a 0.25μm and a 2μm QWs with D=1.7μm. The transport of serially connected QWs is characterized by the direct transmission coefficient Td which represents the portions of electrons traveling ballistically from one quantum wire to the other. By a combination of measurements on the individual QWs and on the composite structure, Td can be determined. The transmission through a pair of QWs depends on details of confinement potentials and the 2DEG region between the two QWs. That Td is smaller for larger D by comparing configurations (II) and (III) can be understood under the consideration of the finite coherence length. For a fixed D, Td of configuration (I) is smaller than that of configuration (II). The phenomenon is attributed to the collimation effect of injected electrons from one QW to the other by channel length. In a fixed configuration while both QWs are confined at different subbands, a series of Td is also analyzed and made comparison with theory taken account of collimation effect. By negatively biasing a pair of metal gates, electrons can be confined in a small region forming a cavity.Two cavities with topological area of 0.48 and 0.12μm2,respectively, are studied. Oscillations of conductance are observed in conductance-gate voltage characteristics and source-drain bias spectroscopy. These oscillations are not the well-known “Coulomb blockade oscillations”, since both cavities are strongly coupled to their reservoirs in the observation range. In the presence of perpendicular magnetic field, two quantum interference effects take place in longitudinal resistance measurement. In high magnetic fields, oscillations of resistance occurs with the periodicity in magnetic flux in consistence with interference condition of Aharonov-Bohm effect. In low magnetic fields, a negative magnetoresistance is present and is ascribed to Weak localization effect. By varying gate voltages, the line shape of the low field MR curve would transits from Lorentzian to linear. We suggest that the transition may be due to the shape change of cavity from chaotic to regular with further decreasing negative gate bias.