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| 題名: | 自聚性砷化銦鎵量子點之光電特性;Electronic and Optical Properties of InxGa1-xAsSelf-Assembled Quantum Dots |
| 作者: | 張文豪;Chang,Wen-Hao |
| 貢獻者: | 物理研究所 |
| 日期: | 2001-07-05 |
| 上傳時間: | 2014-05-08 15:59:51 (UTC+8) |
| 出版者: | 國立中央大學 |
| 摘要: | 本論文旨於探討自聚性砷化銦鎵量子點的光電特性，其主要研究內容涵蓋兩
個主題。首先，在論文的第一部份，我們以不同的光譜技術研究在砷化銦鎵量子
點中侷限能階的物理特徵。我們使用了三種不同的光譜技術，分別是光激發螢光
光譜、光電流光譜及電調制反射光譜。這些光譜技術均可有效地檢測量子點能帶
間的光學躍遷，但卻各自具有不同的物理特徵。就量子點的螢光光譜而言，可研
究量子點的零維度特徵、填態效應、載子動態行為及溫度效應對載子分布的影響。
而螢光光譜亦被利用來研究經熱處理後的量子點侷限能階。由光譜特性可推斷，
即使經過嚴重的相互擴散後，量子點依然保有其零維度的物理特性。光電流光譜
則是被利用來檢測溫度及電場效應對量子點能階的影響。利用測量光電流的溫度
變化，可探究載子因熱效應而逃脫出量子點的過程。而低溫的光電流量測，則可
研究載子藉由電場的幫助而穿遂到量子點外的現象。同時，此外加於量子點的電
場，不僅會造成所謂的史塔克效應，也會造成不同量子點大小的選擇性穿遂。針
對量子點的電場效應，我們亦利用電場調制反射光譜進行更深入的探討。我們觀
察到一種不對稱的量子侷限史塔克效應，此意味在量子點中的電子電洞對具有內
建的電偶極矩。
在建立量子點侷限能階的概念後，在論文的第二部份，我們將介紹如何操控
電子於這些量子點的能階中。在這類的研究中，量子點是被設計於空間電荷的結
構中，因此可以藉由外加適當的電壓來對量子點充電或放電。我們發展出一種光
譜技術，稱為電子填充調制反射光譜，用以研究砷化銦鎵量子點的充放電行為。
這種電子填充反射譜基本上是一種新形式的電調製反射譜，但其光譜特徵卻更類
似傳統用於半導體的空間電荷技術，如電容電壓量測及導納量測。首先，我們結
合電容電壓量測及電子填充反射譜來研究量子點的電子分布及能階佔有率。利用
電容電壓的特徵曲線，我們建構出量子點內的能帶結構，並以此推算出對量子點
充電時所需要的庫倫充電能量大小。電子的能階佔有率則是利用電子填充反射譜
的強度來估算。我們發現在費米能階附近的電子分布相當不均勻，而此現象則可
歸因於不同量子點間的電子耦合分布。同時，溫度效應對電子能階佔有率的影響
也在這個研究中一併的探討。另一方面，結合電子填充反射譜及導納量測，亦可
用來研究砷化銦量子點的充放電行為。藉由導納量測，我們可以研究量子電充電
的動態行為。我們解析出對於量子點不同能階的充電行為，也因此得探討電子自
不同能階逃脫的物理機制。量子點經過充電後的光學躍遷可以由電子填充反射譜
來觀察。我們清楚的觀察到隨著電子填入量子點，光學躍遷因鮑利阻斷而造成強
度減弱的現象。同時，因量子點帶電而產生的帶電激子，及其所造成的能量修正
也一併被觀察到。最後我們將這些量測到的帶電激子能量與理論推算的庫倫作用
力比較，並提出合理的模型解釋
;This dissertation is devoted to the electronic and optical properties of InxGa1-xAs
self-assembled quantum dots. The main focus of this dissertation can be divided into
two parts. First, we present optical investigations with regard to the physical features of
the confined states in InxGa1-xAs quantum dots. Three optical spectroscopes have been
employed: photoluminescence, photocurrent and electroreflectance. These
spectroscopes in principle can all be utilized to probe the interband transitions in the
InxGa1-xAs quantum dots, but possess characteristic features specific to the different
physical mechanisms involved in each. The general features of the quantum-dot
photoluminescence, including the state-filling effect and its interplay with carrier
dynamics, and the temperature effects on carrier distributions, are comprehensively
discussed. The photoluminescence spectroscopy was further utilized to study the tuning
of confined energy levels in InAs self-assembled dots via rapid thermal annealing.
Intense and sharp interband transitions were observed, which demonstrates
unambiguously that the investigated quantum dots retained their optical quality and
zero-dimensional properties even after the strongest condition of interdiffusion.
Photocurrent spectroscopy was used to investigate both temperature and electric-field
effects on the InAs dots. The path for thermal escapes of photogenerated electron-hole
pair from the dot states is clarified. Low-temperature photocurrent also revealed a clear
feature of field-induced escapes via direct tunneling out of the quantum dots. The
applied electric field not only leads to an energy shift due to quantum-confined Stark
effects, but also causes a size selective tunneling. A more detailed study of electric-field
ii
effects on the quantum dot interband transitions was presented by electroreflectance
spectroscopy. Asymmetric Stark shifts in transitions energies were observed, implying
that the optically excited electron-hole pairs exhibit built-in dipole moments in the
quantum dots.
After having the idea of confined states in the InxGa1-xAs self-assembled dots, in
the second part of this dissertation, we present how to manipulate and corral electrons in
these confined states. The quantum dots were incorporated into a space-charge structure,
so that the charging of quantum dots can be achieved by suitably applied bias voltage,
forming charged quantum dots. We developed a novel spectroscopic technique, called
electron-filling modulation reflectance (EFR), to study the charging of InxGa1-xAs
self-assembled dots. The EFR technique is essentially a new kind of electroreflectance,
but possessing characteristic features that are more similar to the conventional
space-charge techniques, such as capacitance-voltage and admittance spectroscopes.
Electron distribution and level occupation in quantum dot ensemble were investigated
by combining the EFR with the capacitance-voltage spectroscopy. We used the
capacitance-voltage characteristics to construct the electronic structures of the
investigated In0.5Ga0.5As quantum dots. The Coulomb-charging energy required for
adding electrons into the dots were also deduced from the capacitance-voltage
characteristics. The electron level occupations were investigated by monitoring the
measured EFR intensity. We found that the electron distribution in the dot ensemble was
inhomogeneous near the Fermi level, which was attributed to the correlated charge
transfer among different dots. The temperature effects on electron thermal population in
the dots are demonstrated. We also present a combination of EFR with admittance
spectroscopy to study the charging of InAs quantum dots. Charging dynamics of the
InAs dots were characterized by the admittance spectroscopy. Clear features for
different electronic shells of the InAs dots were resolved, enabling a separate
investigation of the electron escape behaviors in different dot shells. The interband
transitions of charged quantum dots were obtained from EFR measurements. We
demonstrate clear Pauli blocking of the transition strength caused by the electrons being
charged into the quantum dots. Remarkable energy modification due to the formation of
negatively charged exciton was observed. The experimental determined energy shifts
were finally compared with the theoretical calculation of Coulomb interactions in a
quantum dot with a parabolic confining potential. |
| 顯示於類別: | [物理研究所] 博碩士論文
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