本論文探究鍺/矽與砷化銦/砷化鎵量子點之微結構與物理性質之研究。吾人利用原子力顯微鏡(AFM)與高解析穿透式電子顯微鏡(HRTEM)探討鍺量子點之微結構及成長機制，並利用導電式原子力顯微鏡(conductive-AFM)進行單顆鍺量子點之電性研究。另一方面探討砷化銦/砷化鎵多重結構在後成長階段，其量子點尺寸、形貌及成分的變化對於光學性質上的影響。
本論文依研究主題可區分為四大部分。第一部份，吾人以超高真空離子濺鍍法成長鍺量子點，利用AFM及TEM來探討量子點的成長演化，可知其形貌的演化不同於一般常見的金字塔至蒙古包的轉變。新型態量子點由{103}及{105}面構成一低高寬比的形貌，而{103}與{105}面的比例決定了量子點的高寬比。在500C下成長的量子點，量子點的形貌由低高寬比的小蒙古包轉變為高高寬比的大蒙古包。由TEM及micro-Raman可量測到鍺量子點具有高的殘留應變。此外藉由實驗結果與理論的比較，新型態量子點的形成與較大的特徵長度(characteristic length, L)及較長的表面波長(surface wavelength, λ)有關。
第二部份，吾人則以conductive-AFM來進行單一顆鍺量子點的電性研究。首先利用不同的成長條件，可成功的得到契合性(coherent)及非契合性(incoherent)量子點。接著利用conductive-AFM可量測到單一顆量子點的電流-電壓(I-V)曲線，在契合性量子點上得到一I-V曲線，其低壓下呈現線性而高壓下呈現非線性的關係。在非契合性量子點上則得到一階梯狀的I-V曲線，推論應為電子經由穿遂通過量子點量化的能階所造成。
第三部分，吾人利用HRTEM及螢光光譜分析儀(PL)來研究砷化銦量子點之微結構及光學性質。30 nm AlAs嵌入層被用來改變砷化銦量子點的形貌及尺寸，因而形成較大尺寸與高寬比(0.4)之量子點。由HREM影像可觀察到量子點經覆蓋後顯示出平台金字塔狀(truncated pyramid)之形貌，此意味著量子點與覆蓋層發生In/Ga原子間的相互混合。變溫PL顯示在低溫下由band-tail transition主導發光，而高溫下由ground state transition主導發光。InAs量子點會受GaAs/InAs/GaAs基材影響，其PL的強度明顯的被增強，而量子點之活化能則相對降低。此外，在文中亦利用TEM及PL來探討In/Ga原子相互混合的行為。
最後一部份，吾人研究砷化銦/砷化鎵多重結構中表面(surface)及內層(buried)量子點形貌的改變與成分之間的關係。利用能量過濾穿透式電子顯微鏡(energy filter TEM, EFTEM)中三視窗法(three-window)與影像光譜(spectrum imaging)，可得到各個元素的分佈圖。由銦的元素分佈圖可知，內層的量子點在後成長過程中形貌由透鏡狀(lens)轉變為平台金字塔狀(truncated pyramid)，推論與量子點內部的成分改變有關。此外，吾人發現在不對稱的PL光譜中，主要由純砷化銦量子點的量子效應及銦/鎵相互擴散所造成。

This dissertation explores the microstructure and physical properties of Ge/Si and InAs/GaAs quantum dots. We have investigated the microstructures and growth mechanism of the self-assembled Ge quantum dots by high resolution transmission electron microscopy (HRTEM) and atomic force microscopy (AFM). The electrical properties of the samples were investigated by conductive-AFM. In addition, we have investigated the chemical composition and optical properties of InAs/GaAs quantum dots.
The main focus of this dissertation can be divided into four parts. First, we report on the formation of self-assembled Ge coherent islands on silicon (001) substrates using higher ion energy with ultra-high vacuum ion beam sputter deposition. At 500C, the shape of the islands is transformed from a small dome of low aspect ratio, to a large dome of high aspect ratio. The growth evolution appears to differ from conventional pyramid-to-dome transition. Lower-aspect-ratio islands surrounded by {310} and {510} facets are found without trench formation and alloying. The largest shallow dome islands can only relax about 4% of the lattice mismatch from transmission electron microscopy strain measurements. By comparing these results with theoretical calculations, the shallow domes correspond to a larger characteristic length, and the mechanisms involved are subsequently discussed
Then, Electrical properties of self-assembled quantum dots have been the subject of intensive research due to quantum confinement. Here we report on the fabrication of Ge self-assembled quantum dots and the electrical properties of individual quantum dots. Ge quantum dots were deposited onto silicon (100) substrates by ultra-high-vacuum ion beam sputtering. AFM results show that the Ge island shape is semi-spherical with a round base. Dark-field TEM images show that samples with incoherent or coherent islands can be produced under different ion energies. Subsequently the current-voltage (I-V) characteristics from individual islands were directly measured with conductive-AFM at room temperature. Whereas the I-V curves from coherent islands exhibit linear behavior at low bias and non-linear behavior at large bias, the staircase structures are clearly observed in the I-V curves from incoherent islands which are attributed to electron tunneling through the quantized energy levels of a single Ge quantum dot.
In the third part, we have investigated the microstructure and the optical properties of InAs/GaAs quantum dots (QDs) by using TEM and PL measurements. A 30nm thick AlAs insertion layer was employed to change the shape and size of the InAs QDs. The InAs QDs composed of an AlAs layer exhibit a larger dot size of 20 nm and a higher aspect ratio of 0.4. The temperature-dependent PL spectra showed that the main emission originated from the band-tail state transition at low temperature and the ground state transition at high temperature. The PL intensity of the InAs dots was significantly enhanced by the GaAs/AlAs/GaAs capped layers, and the thermal activation energy of the InAs dots was decreased. Furthermore, the In-Ga intermixing behavior for InAs QDs is also discussed based on the TEM and PL measurements.
In the last, we have investigated the shape and composition profiles of buried and surface InAs/GaAs Stranski-Krastanow quantum dots (QDs) by the spectrum-imaging (SI) method with energy-filtered transmission electron microscopy (EFTEM). Indium maps from EFTEM SI reveal lens and truncated pyramid shapes for the surface and buried QDs, with an increase in composition variations for the buried QDs. Photoluminescence measurements reveal an emission at 1.075 eV, associated with confined states in the buried QDs, along with a high energy shoulder, associated with band-tail states due to In-Ga intermixing in the vicinity of the buried QDs.

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