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    題名: 半導體量子點奈米功能元件之製作與物理特性研究(I);Semiconductor Quantum Dot Functional Devices---Physics and Fabrication(I)
    作者: 李佩雯;徐子民;張亞中;綦振瀛;郭明庭
    貢獻者: 奈米科技研究中心
    關鍵詞: Single-electron;single photon;semiconductor quantum dot;物理類
    日期: 2008-07-01
    上傳時間: 2010-12-28 15:19:18 (UTC+8)
    出版者: 行政院國家科學委員會
    摘要: 本計畫的目標是研究實作高品質的半導體量子點奈米功能元件如:單電子電晶體電荷檢測器與單光子源放射器所需的尖端製作技術與其相關物理特性,以建立單電子電晶體電荷檢測器與單光子源放射器的學理基礎與實作驗證平台。近年來在奈米量子點結構中所觀測到的量子力學現象已廣泛地引起基礎物理與應用電子產業的矚目。在目前相關量子點元件的材料與結構研究發展中雖然有多種嘗試,基於製程可調變性與未來電路積體化的考量,其中以半導體材料的基礎與應用研究最為廣泛。半導體的奈米量子點因為具有分立的能態密度,使得電荷量子訊息可以達到潛在應用的要求。因此美英德日的量子資訊團隊紛紛將奈米量子點的應用視為重點課題。特別是,當積體電路製造技術日益進步且元件尺寸持續縮小以改善切換速度與消耗功率進而加強電路的積集密度與功能性之際,單電子元件更是倍受矚目。單電子元件的運作在庫倫阻斷效應的作用下僅有單一或極少數個電子得以自源極通過奈米量子點到達汲極,因此不僅可精確控制導通電子數目更可藉此讀取量子點內的電荷訊息。對於高積體化之記憶體發展而言,單電子電晶體已被列入30 nm 技術以下之主要記憶單元,而對量子電腦技術的發展而言,單電子電晶體則被公認是最有能力來讀取量子訊息的最小元件。展望未來,單電子元件極可能如同二十世紀的半導體電晶體對於超大型積體電路的發展一樣成為下一世紀電子元件的主角。量子點單光子光源發展於2000 年(P. Michler et. al., Science),為了增強發光效率,量子點光源必須放置在共振腔內,目前已經研發成功的單光子共振腔共有三種:微型圓盤(micro-disk)、微型柱體(micro-rod)及光子晶體奈米共振腔(photonic crystal nanocavity)。其中光子晶體共振腔的體積較小,光源容易與腔內電場產生作用而改變發光特性(即Purcell 效應),是單光子光源研發的重要方向。但單量子點在共振腔內的定位及輻射波長和共振腔波長的匹配是目前國際上眾人追求之目標。本團隊先後接受國科會奈米國家型計劃的補助製作『單電子電晶體』與『單光子光源』及研究其相關物理特性。在單電子電晶體方面,我們成功地研發出「選擇性氧化矽鍺」平面」」以形成鍺奈米量子點(約3~10 nm)的方法。利用此方法所形成的鍺量子點之大小取決於矽鍺材料在熱氧化過程中鍺原子的釋放與聚集,因此可突破目前一般製作量子點所需的微影與蝕刻技術瓶頸。同時,這種製程方法完全相容於目前的 CMOS 製程技術,因此可以順利地將奈米量子點元件與目前的超大型積體電路技術互相橋接與整合。最重要的是,我們成功地製作出可在高溫下操作的鍺單電子/電洞電晶體,不僅在室溫下清楚地展現庫倫震盪,而且得以觀察探討電子在鍺奈米量子點內的傳輸特性與微觀的電子能帶結構等物理現象。經由實驗估算單一電子在鍺量子點內的填充能量(addition energy)約為125 meV。除了首要之務製作奈米尺度的量子點外,如何得以精準地放置單一顆的奈米量子點於源/汲極電極之間、有效地縮減穿隧障層的厚度及製作低接觸阻抗且可對準於奈米量子點的奈米尺度電極,更是決定單電子電晶體實作成功與否的關鍵製程技術。此外,單電子/電洞元件的結構設計之優化與製程整合亦是亟待研究開發的重點。這不僅需要創新的製程技術與觀念更需要深刻認識與瞭解其中的量子理論,這也正是可以發揮學術創意及深入研究箇中之物理意義與製程工藝技術的空間所在。我們期待經由製作高品質的鍺量子點單電子/電洞電晶體,進而提供一極佳的測試平台,得以觀測微觀的量子傳輸行為、電子/電洞在奈米量子點內的穩態與暫態時變響應,並探討單電子/電洞元件的雜訊機制與影響。而在單光子光源方面,我們先後開發了低密度量子點成長、單光子光譜量測及光子晶體製作技術並成功地將量子點放置在光子晶體共振腔內,製作出一高效率且高純度之光激發單光子源。在此基礎上,我們將進一步考量實際應用之需求,研究發展電激發單光子源並延伸至具雷射功能之奈米光源的探究。主要研究重點分述如下: (1)實作高品質的量子點單電子電晶體以探討流經量子點基態及激發態之穿隧電流。製作單電子電晶體的首要之務除了製作奈米尺度的量子點外,如何得以精準地放置單一顆的奈米量子點於源/汲極電極之間、有效地縮減並控偫穿隧障層的厚度及製作低接觸阻抗的奈米歐姆電極,亦是決定單電子電晶體實作成功與否的關鍵製程技術。我們擬發展可自我對準於量子點的閘極/源極/汲極等尖端製作技術以有效地降低寄生電容效應及提升訊號/雜訊比。近來由於單電子電晶體及單光子放射器的研究突破與提昇該新穎元件朝向高溫操作的需要,使得對流經量子點激發態之穿隧電流行為的瞭解變得相形重要。特別是如何藉由流經量子點基態及激發態之穿隧電流/電壓特性曲線,來決定基態及第一激發態的能階分離與電子彼此間的庫倫斥力的大小。我們將利用多能階之Anderson 模型與非平衡態格林函數法,來理論建構電流電壓特性曲線,並與實驗的穿隧電流比對來決定出量子點的電子結構。 (2)探討流經量子點之光電流與光子放射譜線。在適當的光激發下,量子點內部可能會激發出電子/電洞對的激發態進而改變了量子點內的電子能階結構。因此我們擬調變光激發的條件來探討穿隧電流特性曲線的變化,進而探討各種激發態的束縛能。此外,若量子點中發生電子與電洞共同穿隧的現象,經由電子與電洞的結合,應該會產生精細的光子放射譜線。因此我們將透過實驗檢測精細的光子放射譜線來確認電子與電洞共同穿隧的物理現象存在與否。 (3)探討流經量子點基態之暫態時變電流。當單電子電晶體應用於電荷檢測器時,我們該如何從單電子電晶體之電流來判別是何種電荷信號被偵測到?假若單電子電晶體的量子點附近存有一些與時間相依的被檢測電荷源(例如:被檢測的電荷形式是從電子轉為電洞或是從一個電子增加為兩個電子等情況),由於在量子點內的電子會透過庫倫交互作用與被檢測的電荷耦合在一起,當電子從電極進入該量子點基態時,所對應的穿隧電流將會如何被影響呢?我們擬利用時間相依的Anderson 模型理論論證,並與以脈衝電荷注入方式所實驗量測到的穿隧電流交互比對來釐清此問題。由此,我們或可延伸單電子電晶體來檢測運動中的分子之穿隧電流行為。如果分子在空間的位子是固定的,那麼此分子的角色和單電子電晶體的量子點並無不同。因此若能檢測運動中分子的穿隧電流如何隨著時間而變化,如DNA 分子在奈米圓柱空洞中流動時所造成的穿隧電流改變,將可應用於DNA 基因檢測。我們擬在源極與汲極之間製作一奈米圓柱,利用縱向電極(延著奈米圓柱方向)將溶於氯化鉀溶液裡的DNA 導入奈米空洞內。當電子從源極穿隧到汲極時,帶負電的DNA 將會影響其穿隧電流。因此我們可藉由如此的元件佈局來判讀DNA 四種不同氮基的排序。我們擬分別以理論與實驗先探討氯化鉀離子體對源極穿隧電流(暗電流)的影響,再接續著理論討論DNA 氮基的電子結構。最後預測短單股DNA 被源極檢測出的DNA 排序。 (4)為實現光子晶體單光子功能性光源,我們將設計新穎之光子晶體共振腔結構,利用其品質因子(Q factor)及模態體積(mode volume)之組合來提高量子點發光效率。以分子束磊晶成長之砷化銦鎵量子點將置於一p-i-n 二極體結構中,並結合電子束微影及選擇性蝕刻技術以製作電激元件。為符合現有光纖通訊系統,我們將利用最近開發的含銻異質結構發展1.3 微米波長之單光子源。此外,在適當的品質因子及激發之下,我們預期亦可將光子晶體量子點結構製成一高速量子點雷射。這些研究涵蓋物理科學及工程應用技術之探討,將在奈米光電領域開拓新的一頁。 Semiconductor QDs are extremely interesting nanostructures that can improve performance of a variety of existing devices and even offer unique functionalities. In particular, semiconductor QDs have been subjected to energetic researches on single electron transistors (SETs) and single photon (SP) functional light sources since they can provide a fully tunable two-state system where a single electron or exciton could be controlled. A SET is an ultimate scheme of electronic device in the purpose of controlling current with one electron precision. It employs the so called Coulomb blockade effect exhibiting by the interaction between electrons within a nano-QD, whereby any further tunneling of charges is suppressed during the process of a tunneling charge through a QD. Motivation to study a SE device is strong in light of its new operating principle and precise charge sensitivity for single charge/photon detection and future quantum information application. While the interests in SP functional light sources arise from their potential application in quantum cryptography for an ultimately secure communication. In particular, electrically driven SP source and self-tuned single QD light source have been proposed from the viewpoint of practical application. The first QD SP light source generated through laser excitation was demonstrated by P. Michler et al. in 2000. Based on the so-called Purcell effect, QDs embedded in optical microcavities offer the ability to create new, efficient optical sources of specified wavelength once the emission wavelength is coupled to the cavity resonant mode, which is the key technology for the development of recent quantum light sources, such as SP source and low-threshold lasers. Several cavities such as micro-disk, micro-rods and photonic crystal nanocavity have been employed to improve the photon emissions. In particular, photonic crystal nanocavity has been viewed as the most efficient cavity to make QD light source due to the smallest cavity mode volume. However, the effective solutions to positioning a single QD in a cavity as well as coupling the QD emission wavelength to the cavity resonant mode remain to be explored. We have been granted by 「National research program for nanoscience and technology NSC」 to explore the feasibility of 「high-temperature single electron transistors」 and 「single photon nanoemitters」, respectively. In the previous phase on SETs, we have developed a simple and CMOS-compatible method for forming Ge QDs embedded in a SiO2 matrix using selective oxidation of SiGe/Si-on-insulator (SGOI) plane by Ge atoms segregation and agglomeration. Instead of being determined by the resolutions of e-beam lithography and etching, the properties of Ge QDs such as diameter, shape, spatial density, and crystallinity could be well controlled by modulating conditions of thermal oxidation, SGOI layer structure, and Ge content in Si1-xGex. Tiny Ge QDs (dot size/spatial density = 3~10 nm/0.1~2.8×1012 cm-2) could be formed by oxidizing a Si0.95Ge0.05/Si-on-insulator 「plane」, which is well suitable for SETs application. Thereby, we have experimentally demonstrated Ge-QD SETs with a single electron addition energy of 125 meV for room temperature operation. To our best knowledge, very few demonstrations of room temperature Si-based SET have been reported so far (only Tokyo Univ., NTT, and Princeton Univ.), and we have demonstrated the first room-temperature Ge-QD SET. For practical application, in addition to a nano-scale QD, a successful SET requires precisely placing a single QD between S/D electrodes with very thin tunneling barriers (~10 nm) as well as low contact-resistance nanoelectrodes self-aligned to the single QD. However, it has been a major task in any practical nanofabrication scheme as making a well controlled tunnel contact to the self-assembled QDs, even though advanced electron-beam lithography and etching processes have been adopted. In this project, we will develop the cutting-edge fabrication technologies to realize high-performance SETs for next generation integrated circuit. This device technology will then be extended to demonstrate the features of a SET as a charge or photon detector. The fundamental steady-state and ac-driven carrier quantum transports through zero dimensional systems will also be investigated. While on the subject of 「single photon nanoemitters」, we had accomplished (1) low density InGaAs QDs growth, (2) single QDs detection, (3) single photon anti-bunching detection, and (4) photonic crystal nanocavity fabrication. Thereby a laser-excited SP source has been successfully demonstrated with high efficiency and high purity. From the viewpoint of practical application, we will further proceed to develop the extraordinary fabrication technologies for realizing electrically driven SP light sources, which will be further extended to the exploration of self-tuned single QD functional light source. The research topics are planned as follows: (1) High-performance self-aligned Ge-QD SETs: As aforementioned, in addition to a nano-scale QD, the precise control on the tunnel junctions/tunneling rates (Γin, Γout) and the minimization of gate-source or gate-drain coupling parasitic capacitance is more imperative to ensure a SET with a high signal-to-noise ratio and a less background charge fluctuation. However, this has been rarely discussed in the past due to the difficulty on a small QD nanocontacted to electrodes. The solution we propose to surmount this difficulty is control of selective oxidation of Si1-xGex/Si-on-insulator (SGOI) 「nanowires」. The oxidation of a SGOI nanowire will proceed from not only top surface but also sidewalls concurrently, so that segregated Ge atoms would be squeezed to the core of the nanowire once it is fully oxidized. Meanwhile, the so-formed Ge QDs in the core of an oxidized nanowire would also be self-aligned to the nearby unoxidized pads, which could be fabricated as source/drain electrodes. We will systematically investigate the effects of layer structure, geometric pattern, and oxidation conditions on the control of QD number, size, and position within the oxidized nanowire. Besides, we will also proceed to device structure design and IC-oriented fabrication process development for a SET with a gate nano-electrode (L ~ 10 nm) self-aligned to a single QD based on the FinFET technology. (2) Steady-State and ac-driven quantum transports: The steady-state and ac-driven electrical characteristics of Ge-QD SETs/SHTs at various temperatures are solid data to be studied. Owing to the interplay between carrier Coulomb interactions and energy levels, the electronic structure such as charging energies and energy level separations of a QD could not be directly determined from experimental measurement, in particular, in the high applied bias regime or at room temperature, although these physical parameters are crucial in the optimization of a SET. Consequently, a theoretical model to derive a tunneling current formula, which can be applied to a single QD with arbitrary energy levels, will be developed in this project. Once this current formula is constructed, the experimental tunneling current spectra could be analyzed through a comparison with theoretical current characteristics. From the viewpoints of scientific insight and practical application, the ac-driven quantum transport has recently received a lot of attention since it will provide real-time carrier dynamics in response to external fields or waves. The study of quantum transport under light irradiation is a natural extension into the realm of quantum optical effects in mesoscopic transport through a QD. An additional time-dependent electric field in general will give insights into quantum dynamics of electrons, and a large number of interesting adiabatic phenomena such as charge pumping and operations relevant for quantum information processing would be observed. The time-dependent field is not from the beginning treated as a perturbation, but is rather considered as inherent of the system itself. By this, we will deal with conditions of a non-equilibrium system under which the quantity of interest, e.g. a tunnel current or the screening of a static potential, has to be determined. (3) Single-Charge Detectors: The final milestone we plan to achieve is to realize single-charge or single-photon detectors for sensing the dynamic response of coupled QDs (CQDs). By advancing the above techniques for self-aligned SETs, we will integrate CQDs and a SET, which would be fabricated in close proximity to the CQDs. The number of injected electrons into the CQDs would be controlled by side-electrodes; and the electrical control over wavefunction overlap and the exchange coupling between the individual QD can be manipulated by CQDs gate bias, which could be precisely monitored by the nearby SET with its current-voltage (I-V) curves shift accordingly due to the potential change in QD. Therefore, the corresponding quantum states/charges could be precisely measured with the I-V shifts in SET. (3) Single-photon functional light sources: To realize photonic-crystal single-photon functional light sources, we will design novel photonic crystal nanocavity structure with a high Q factor and a small mode volume to enhance the QD emission rate. An electrical-driven SP functional light source will be developed by growing InGaAs QDs in a p-i-n light-emitting diode structure, in which a submicron hole will be defined on the p-type contact layer using e-beam lithography to confine current injection. We will also develop 1.3 μm single photon light source for prevailing optical-fiber communication applications by taking advantage of our recent success on the Sb-based heterostructure growth. Besides, we intend to explore the feasibility of a high speed self-tuned QD laser using a few QDs embedded in photonic crystal nanocavity structure with suitable Q factor. The implication of this technology will lead to the development of high efficient nano-scale light sources. There are scientific and technologic significances involved in this proposed project. The research of single charge/photon detectors and SP function light sources are closely related to quantum information, which is a frontier research field. To implement SE devices and SP functional light sources demands smart device design, precise lithography resolution, and process integration, which will become the key technologies for the industry of nano-optoelectronics. 研究期間:9608 ~ 9707
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