臺灣博碩士論文加值系統

在本論文中，我們研究了數種半導體奈米材料之光電性質，包括了InAs/GaAs量子點，InAs/GaAs[0.7]Sb[0.3]量子點，ZnO奈米線及Si/Si[0.5]Ge[0.5]多重量子井。藉由光激螢光光譜 (PL)，電致螢光光譜 (EL)，拉曼光譜 (Raman)，掃描式電子顯微鏡 (SEM)等實驗來研究其物理性質。這些研究對於此類材料的應用有十分大的幫助。本文中共包含四個部分：
1. InAs/GaAs量子點超晶格之線性特徵應用於光電元件
藉由表面光電壓光譜及光激螢光光譜，我們可以證明InAs/GaAs量子點超晶格之線性特徵是由上下相鄰的量子點之電子耦合效應所造成的。藉由電子耦合的效應，表面光電壓光譜之強度可以被增強100倍以上。我們同時發現了樣品側面的光激螢光具有偏振性，並且可以藉由改變GaAs的厚度來調整電子耦合效應的強度進而改變偏振的程度。此外，具有電子耦合的量子點樣品相較於不具有電子耦合的量子點樣品，其電致螢光光譜有較高的強度及較窄的半高寬。
2. 第二型InAs/GaAs[0.7]Sb[0.3]量子點之特殊光學性質在光激螢光光譜
我們藉由光激螢光光譜來研究第二型InAs/GaAs[0.7]Sb[0.3]量子點之光學性質。我們發現隨著激發光強度的上升樣品之光激螢光能量也隨之藍移。這個明顯的藍移是由於在第二型能帶結構中，空間中分離的電子和電洞對所造成的能帶彎曲影響所致。我們也同時發現樣品之光激螢光具有很強的偏振性，約有24%。這個偏振性是由於在InAs/GaAs[0.7]Sb[0.3]的異質接面上不對稱的化學鍵結所造成的。
3. ZnO奈米線之光致應力效應
我們發現ZnO奈米線具有之光致應力效應會造成許多有趣的現象。首先，隨著激發光強度的增加，我們發現A1(LO)聲子頻率的紅移及光激螢光能量的藍移。此外，變溫度的光激螢光光譜在高激發及低激發強度下有相當程度的不同。這些現象都可以用光致應力效應或者被光激的載子所屏避的內建電場來解釋，因為ZnO是一種壓電材料，隨著內建電場的改變，內建的應力也會跟著改變。我們研究的對ZnO奈米線有更深入的瞭解，相信對未來光電元件的應用有十分的幫助。
4. 藉由奈米牆的結構來增強SiGe/Si多重量子井之電致螢光強度
我們藉由奈米牆的結構來增強SiGe/Si多重量子井之電致螢光強度。奈米牆則是由ECR電漿蝕刻而成，經由實驗我們發現，ECR電漿蝕刻並沒有嚴重的破壞樣品的晶體結構。相較於一般的多重量子井，在相同的工作電流下(5.5×106 A/m2)，具有奈米牆結構之多重量子井其電致螢光強度可以被增強50%。此外，奈米牆之結構也同時表現出應力鬆弛的現象，使樣品具有較好的光學性質。我們的研究提供了一個可能的方法，來製造高功率的發光二極體。

In this thesis, we have reported the optoelectronic properties of the semiconductor nanostructures. In part I, the optical anisotropy in type-I and type-II semiconductor nanostructures have been well investigated through the photoluminescence (PL) and scanning electron microscopy (SEM) measurements. In part II, we have investigated the photoelastic effect and the strain relaxation in semiconductor nanostructures through the electroluminescence (EL), PL, and Raman scattering measurements, which will shift the frequencies of the phonon modes and the band-edge transition energies. The studied samples in this thesis including InAs/GaAs quantum dots (QDs) superlattices, InAs/GaAs0.7Sb0.3 QDs, ZnO nanowires, and Si/Si0.5Ge0.5 multiple quantum wells (MQWs).

Part I. Optical Anisotropy in Type-I and Type-II QDs

1. Wire-like characteristics in stacked InAs/GaAs quantum dots superlattices for optoelectronic devices

The wire-like characteristics of stacked InAs/GaAs QDs superlattices induced by vertically electronic coupling effect were demonstrated by surface photovoltaic and PL measurements. It was found that the surface photovoltaic signal can be enhanced by up to more than one hundred times due to the wire-like behavior along the growth direction. We also found that the emission from the cleaved edge surface is strongly anisotropic, which suggests a possibility to fine tune the polarization by changing the spacer thickness. Additionally, the EL of stacked QDs near 1.3 μm based on the wire-like characteristics has a much better performance than that of uncoupled QDs.

2. Unusual optical properties of type-II InAs/GaAs[0.7]Sb[0.3] quantum dots revealed by photoluminescence

The optical properties of type-II InAs/GaAs[0.7]Sb[0.3] QDs were investigated by PL. It is found that the peak position of PL spectra exhibits a significant blueshift under a moderate excitation level. The observed blueshift can be well explained by the band-bending effect due to the spatially separated photoexcited carriers in a type-II band alignment. We also found that the PL spectra exhibit a strong in-plane polarization with a polarization degree up to 24 %. The observed optical anisotropy is attributed to the inherent property of the orientation of chemical bonds at InAs/GaAs[0.7]Sb[0.3] heterointerfaces.

Part II. Strain Effect in Semiconductor Nanostructures

3. Photoelastic effect in ZnO nanorods

A novel phenomenon called photoelastic effect has been observed in ZnO nanorods, which causes several intriguing behaviors. With increasing excitation power, it is found that the A1(LO) phonon shows a redshift in frequency, and a blueshift of PL peak energy has also been observed. In addition, the temperature dependent PL spectra behave quite differently under high and low excitation power. All our results can be interpreted well in terms of the photoelastic effect, in which the built-in surface electric field is screened by photoexcited electrons and holes. Through the conversed piezoelectric effect, the internal strain is therefore changed. Our results open a new opportunity to manipulate the physical properties of ZnO nanorods, which should be very useful in the application of optoelectric devices.

4. Electroluminescence enhancement of SiGe/Si multiple quantum wells through nanowall structures

The enhancement of light extraction from Si[0.5]Ge[0.5]/Si MQWs with nanowall structures fabricated by electron cyclotron resonance (ECR) plasma etching is presented. It is shown that the ECR plasma treatment does not damage crystalline quality. At a driving current of 5.5×106 A/m2, the light output intensity of the MQWs with nanowall structures shows an enhancement of about 50% compared with that of the original MQWs. In addition to the enhanced light extraction, the improved optoelectronic properties are also attributed to the strain relaxation in nanowall structures. Our result shown here offers a promising potential for creating high power light-emitted-diodes.

Chapter 1 Introduction 1
1.1 Nanotechnology………….......................................................................................1
1.2 Novel properties of coupling in quantum dots ........................................................3
1.3 Unusual properties of quantum dots with type-II band alignment ..........................3
1.4 Photoelastic effect in nanostructure ........................................................................4
1.5 Enhancement of light extraction from semiconductor nanostructures ....................5
1.6 Overview of the thesis …………………………………………………………….6

Chapter 2 Experimental Details and Theoretical Background 11
2.1 Photoluminescence (PL) …………………………………………….....………..11
2.1.1 Introduction …………………………………………….....…………..…...11
2.1.2 Brief Description of Photoluminescence ………….…………….....….…...12
2.1.3 Conventional Photoluminescence Setup ...………...………...………….....15
2.2 Raman scattering ..........…………………………...………………………..........16
2.2.1 Introduction …………………………………………….....…………...…..16
2.2.2 Brief Description of Raman Scattering ………………........................……16
2.2.3 Conventional Raman scattering Setup ………………………………….....18
2.3 Scanning Electron Microscopy (SEM) ……………………………......................19
2.3.1 Introduction ................................……………………………......................19
2.3.2 Brief Description of SEM ……………....................….......................…….20
2.3.3 Conventional SEM Setup …………………………….................................22
2.4 Optical anisotropy of semiconductor heterostructures ..........................................23
2.4.1 Optical anisotropy of the type-I semiconductors .........................................23
2.4.2 Optical anisotropy of the type-II semiconductors ........................................24
2.5 The Strain effect in semiconductor heterostructures..............................................25
2.4.1 Characteristics of the Strain effect in Raman scattering…......................….25
2.4.2 Characteristics of the Strain effect in PL transition energy ………..............26

Chapter 3 Sample preparation 41
3.1. InAs/GaAs quantum dots superlattices …...……………….......................……..41
3.2. InAs/GaAs0.7Sb0.3 quantum dots ……………………..........……………..……..42
3.3. ZnO nanorods …………………………………………………………….……..42
3.4. Si0.5Ge0.5/Si multiple quantum wells …………………………………..………..43


Part I. Optical Anisotropy in Type-I and Type-II QDs

Chapter 4 Wire-like characteristics in stacked InAs/GaAs quantum
________ _dots superlattices for optoelectronic devices 45
4.1 Introduction …………………………………………………………...…………45
4.2 Experiments ……………………………………………….……………………..46
4.3 Results and Discussion ……………………………………………….………….46
4.4 Summary ……………………………………………………………...…………51

Chapter 5 Unusual optical properties of type-II InAs/GaAs0.7Sb0.3
________ quantum dots revealed by photoluminescence 61
5.1 Introduction ……………………………………………………………………...61
5.2 Experiments ……………………………………………….……………………..62
5.3 Results and Discussion …………………………….…………………………….62
5.4 Summary ………………………………………………………………………...67

Part II. Strain Effect in Semiconductor Nanostructures

Chapter 6 Photoelastic effect in ZnO nanorods 80
6.1 Introduction …………………………………………………………………...…80
6.2 Experiments ………………………………………………………………….…..81
6.3 Results and Discussion ……………………………………………….………….82
6.4 Summary …………………………………………………………………...……86

Chapter 7 Electroluminescence enhancement of SiGe/Si multiple
_____ quantum wells through nanowall structures 95
7.1 Introduction …………………………………………………………………...…95
7.2 Experiments ……………………………………………………………….……..96
7.3 Results and Discussion …………………………………………….…………….98
7.4 Summary …………………………………………………………………...…..102

Chapter 8 Conclusion 111
8.1 Optical Anisotropy in Type-I and Type-II QDs ……..........……......………..….111
8.2 Strain Effect in Semiconductor Nanostructures ..................................................112

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