量子點系統製作與其電性傳輸

Abstract

隨著磊晶技術與次微米元件製程技術的進步，我們成功地在GaAs/AlGaAs異質結構上製作不同幾何形狀的量子尖端(quantum point contacts，QPCs)閘極結構與量子點(quantum dots,，QDs)閘極結構，並得以研究電子在似一維(quasi-1D)窄通道與零維量子點中的傳輸特性。 在樣品製作過程中，我們利用淺式蝕刻(shallow etching)的技術，成功地降低樣品與閘極之間的漏電流(leakage current)。在電子束微影製程上，我們分別微調每支閘極的曝光劑量(expose dosage)，克服了電子近接效應(proximity effect)的問題，進而製作出我們的量子點結構。 我們在量子尖端結構(dgap ~ 0.5□m)上，清楚地觀察到一維窄通道傳輸的量子化電導平台N(2e2/h)。並進一步研究兩個QPC串聯的電子傳輸現象，我們發現在兩QPC通道寬度為大於一個電子傳輸態下(N≧1)，量子化平台依然會出現，兩量子電阻耦合形式與古典上的電阻串聯相加相似。不過當其中一個QPC通道寬度小於一個電子傳輸態(N<1)，量子化平台即消失，無法以古典電阻串聯的形式解釋。 我們量子點的形成是在二維電子氣介面上的五支閘極加負偏壓，使閘極下的電子排開，形成一個封閉區域，得以觀察量子點的傳輸特性。我們發現當量子點通道與外界接近隔離時，會有一些電導峰值產生。造成這些電導峰值出現，通常和量子點內電荷效應的影響以及電子利用穿隧(tunneling)的方式進出量子點兩邊的電位能障有關。

With the great progress of semiconductor epitaxy technology and sub-micron device fabrication technology, we have successfully made the quantum point contacts (QPCs) and quantum dots (QDs) structures with different geometries on GaAs/AlGaAs heterostructures. This allows us to study the transmission nature of electron transport in quasi-one dimensional narrow channel and zero-dimensional quantum dot. In the process of the micro-pattern fabrication, we utilized technology of shallow etching process effectively, that succeeded in reducing the leaked electric currents between mesa and gates. In the E-beam lithography process, the electron expose dosage on each gate has been finely adjusted independently to overcome the problem of electron proximity effect and hence, make out the quantum dot structure. We have clearly observed the quantized conductance plateaus G = N(2e2/h) in our QPC (dgap ~ 0.5µm) in consistence with 1D ballistic transport. Moreover, the measurement of two QPCs in series has been performed. The quantum resistors seems add classically and conductance plateaus are present when both QPCs with at least one subband in the constriction (N≧1). However, it does not work when one QPC is almost pinched off (N<1). In the study of the electron transport in the quantum dot that was formed by applying negative bias on five metal gates atop 2DEG. We found that there are additional conductance peaks when the dot is nearly isolated. The peaks are usually attributed to the charging effect of the dot and tunneling nature through both entrance and exit barriers of the dot.