本研究利用有機金屬化學氣相沉積法，以S-K模式成長自聚性硒化鋅量子點於砷化鎵基板上，以原子力顯微鏡來探測表面的量子點與光激光譜（PL）來研究量子點之光學特性。改變自聚性硒化鋅量子點的成長溫度與硒化氫的流量，探討自聚性硒化鋅量子點之密度、高度與直徑等表面形貌。然後，在自聚性量子點上覆蓋不同厚度之披覆層，以光激光譜儀探討自聚性量子點的光學特性。
首先實驗結果顯示，硒化氫的流量會明顯影響自聚性硒化鋅量子點密度與成長溫度之關係。在硒化氫的流量為25 sccm時，當成長溫度由140℃增至380℃時，量子點密度反而增加至3.96×108 cm-2。當硒化氫的流量為30 sccm時，量子點成長溫度增加時，量子點密度並無明顯改變。在硒化氫的流量為40 sccm時，量子點密度隨著成長溫度增加而呈現下降之趨勢。
本研究在自聚性硒化鋅量子點上覆蓋一層硫化鋅。實驗結果指出第一，隨著硫化鋅披覆層厚度的增加，使自聚性量子點遭受到更多來自披覆層的壓力而造成，其光激光譜有明顯藍移的趨勢。第二，由於硫化鋅披覆層厚度的增加，硫化鋅披覆層提供了一些額外的載子給自聚性硒化鋅量子點，使得其光激光譜明顯的增強。第三，自聚性硒化鋅量子點的密度隨著硫化鋅披覆層厚度的增加而減少。本論文已成功在砷化鎵基板上成長高密度之自聚性硒化鋅量子點，並在其上覆蓋硫化鋅披覆層。在此基礎上，未來將可發展出多層量子點結構。

In this thesis, ZnSe self-assembled quantum dots (SAQDs) was grown on GaAs substrate by organic-metal vapor phase epitaxy (OMVPE) with Stranski-Krastanow (S-K) growth mode. The contact-mode atomic force microscopy (AFM) and photoluminescence (PL) are used to measure the surface morphology and optical properties of ZnSe SAQDs, respectively.
Experimental data show that the flow rate of H2Se have a significant influence on the relation between the density and the growth temperature of ZnSe SAQDs. At the H2Se flow rate of 25 sccm, the density of ZnSe SAQDs increases up to 3.96×108 cm-2 as the growth temperature increase from 140℃ to 380℃. However, the growth temperature has a negligible effect of the density of ZnSe SAQDs at the flow rate of 30 sccm. At the H2Se flow rate of 40 sccm, the density of ZnSe SAQDs decreases as the growth temperature increases due to the coalescences of SAQDs.
Furthermore, a cap layer of ZnS was deposited on ZnSe SAQDs. Experimental data indicate that the increase of the thickness of ZnS cap layer results in blue-shifted emission due to the ZnSe SAQDs experience more biaxial strain. Besides, the ZnS cap layer provides an additional source of carriers, which thermalize to the ZnSe SAQDs before recombination, resulting in a significantly stronger photoluminescence signal. In AFM images, the density of ZnSe SAQDs decreases as the increase of the thickness of ZnS cap layer. In conclusion, we have successfully grown the high density of ZnSe SAQDs on the GaAs substrate and deposit the ZnS cap layer on it. Based on the technique, the multi-quantum dots will be developed in the future.

CONTENTS

CONTENTS I
LIST OF FIGURES Ⅳ
ABSTRACT Ⅷ

Chapter 1 Introduction

1.1 A basic concept of nanotechnology…………………………………..1
1.2 Characterization of quantum dots…………………………………….3
1.2.1 The density of states for quantum dots……………..……….….4
1.3 Formation of quantum dots…………………………………………..4
1.4 Quantum dot structures by self-assembling…………………………..8
1.5 Effect of dot size…………………………………………………….10
1.6 The potential device application of self-assembled
quantum dots……………………………………………………….11

Chapter 2 Experiments

2.1 The procedure of epitaxial growth…………………………….……18
2.2 Organic-metal vapor-phase epitaxy (OMVPE)……………….…….19
2.2.1 Cold wall system and single hot zone………………………...20
2.2.2 Simplicity, flexibility and versatility………..…………..…….21
2.2.3 Halid-free……………………………………….……..……...22
2.2.4 Stoichiometry easily controlled……….………..…….……….22
2.2.5 Low temperature and low pressure growth…………………...23
2.2.6 Capability of double heterostructure………………..…..…….24
2.3 Components of OMVPE………………………………...…....……..24
2.3.1 Design of growth reactor……………………………..….…....24
2.3.2 Reactant gases handling system………………………..…..…26
2.3.3 Heating system………………………………………..…..…..27
2.3.4 Exhaust disposal system………………………………..…..…27
2.3.5 Safety considerations…………………………………..….…..28
2.4 GaAs substrate preparation………………………………….…...….29
2.5 Characterization techniques……………………………….….….….30
2.5.1 Atomic Force Microscopy (AFM)………………….……......30
2.5.2 Photoluminescence (PL)……………………………….….....31

Chapter 3 Results and Discussion

3.1 Growth of ZnSe SAQDs on GaAs by LP-OMVPE…………………34
3.1.1 ZnSe SAQDs……………….………………………………..34
3.1.2 ZnSe SAQDs growth procedure……………………………..35
3.2 Study of ZnSe SAQDs by modulating the growth time of ZnS
buffer layer……………………………………………......….……37
3.3 Study of ZnSe SAQDs by modulating the growth time of
ZnSe SAQDs …………...………………………..……………......38
3.4 Study of ZnSe SAQDs under the influence of the growth
temperature………………………………………………………...41
3.4.1 Growth of ZnSe SAQDs with 25 sccm hydrogen selenium
flow rate………………………………...…………………….42
3.4.2 Growth of ZnSe SAQDs with 30 sccm hydrogen selenium
flow rate………………………………………………………43
3.4.3 Growth of ZnSe SAQDs with 40 sccm hydrogen selenium
flow rate………………....……………………………………43
3.5 Ostwald ripening effect of ZnSe SAQDs …………………….…….45
3.6 Characteristics of ZnSe SAQDs under the influence of cap layer thickness………………………………………………………...…46

Chapter 4 Conclusions………………………....…………………….50

REFERENCES.…………………………………………………………53
FIGURES……..………………………………………………………..58











LIST OF FIGURES

Figure 1.2-1 Variations in active regions in semiconductor lasers and
the density of states for bulk, quantum well, quantum wire,
and quantum dots……………………………………..…………58
Figure 1.3-1 (a) Frank-van der Merwe growth, (b) Volmer-Weber, and (c) Stranski-Krastanow growth……………………………………..59
Figure 1.4-1 Schematics of self-assembling in the Stranski-Krastanow
Growth mode……………………….………………………...…60
Figure 1.5-1 Effect of dot size on the excited states of electrons
and holes……………….………………………………………..61
Figure 2.1-1 The various processes during an epitaxial growth……...…62
Figure 2.3-1 Illustration of the OMVPE system………………………..62
Figure 2.3-2 Cross-section of the rectangle quartz tube………………...63
Figure 2.3-3 Vapor pressure of organometallic precursors…………...…63
Figure 2.5-1 Plot of atomic force……………………………………..…64
Figure 2.5-2 Possible PL emission……………………………………...65
Figure 3.1-1 The relations of band-gap and lattice constants, between
the barrier of SAQDs and substrate materials………...…….…..66
Figure 3.2-1 Growth parameters of ZnSe SAQDs at different ZnS
buffer layer growth time………………...….…….……….…….67
Figure 3.2-2 AFM images of ZnSe SAQDs at different ZnS buffer
layer growth time…………………………………….……….....68
Figure 3.2-3 The density of ZnSe SAQDs as a function of the ZnS
buffer layer growth time…………………………….…….….…69
Figure 3.3-1 Growth parameters of ZnSe SAQDs at different ZnSe
SAQDs growth time……………………………………….……70
Figure 3.3-2AFM images of ZnSe SAQDs at different ZnSe SAQDs
growth time……………………………………………….…….71
Figure 3.3-3 (a) Density, (b) diameter, and (c) height of ZnSe SAQDs
as a function of the ZnSe SAQDs growth time.……….………..72
Figure 3.3-4 77 K PL spectrum of 60 sec ZnSe SAQDs growth with
5 min ZnS buffer layer………………………………………….73
Figure 3.3-5 77 K PL spectra wavelength and intensity as a function
of ZnSe SAQDs growth time with 5 min ZnS buffer layer…….74
Figure 3.4-1 Growth parameters of ZnSe SAQDs at different ZnSe SAQDs growth temperatures. The flow rate of hydrogen
selenium is fixed at 25 sccm……………………………..….…..75
Figure 3.4-2 AFM images of ZnSe SAQDs as a function of the ZnSe
SAQDs growth temperature. The flow rate of hydrogen
selenium is fixed at 25 sccm………………………………..…...76
Figure 3.4-3 (a) Density, (b) diameter, and (c) height of ZnSe SAQDs
for various growth temperatures. The flow rate of hydrogen selenium is fixed at 25 sccm…………….………...…..….…..77
Figure 3.4-4 Growth parameters of ZnSe SAQDs at different ZnSe SAQDs growth temperatures. The flow rate of hydrogen
selenium is fixed at 30 sccm……………………………….……79
Figure 3.4-5 AFM images of ZnSe SAQDs as a function of the ZnSe
SAQDs growth temperature. The flow rate of hydrogen
selenium is fixed at 30 sccm……………………………………80

Figure 3.4-6 (a) Density, (b) diameter, and (c) height of ZnSe SAQDs
for various growth temperatures. The flow rate of hydrogen
selenium is fixed at 30 sccm……………………………………81
Figure 3.4-7 Growth parameters of ZnSe SAQDs at different ZnSe SAQDs growth temperatures. The flow rate of hydrogen
selenium is fixed at 40 sccm…………………………………….83
Figure 3.4-8 AFM images of ZnSe SAQDs as a function of the ZnSe
SAQDs growth temperature. The flow rate of hydrogen selenium is fixed at 40 sccm……………………………………...……….84
Figure 3.4-9 (a) Density, (b) diameter, and (c) height of ZnSe SAQDs
for various growth temperatures. The flow rate of hydrogen selenium is fixed at 40 sccm……………………………………85
Figure 3.4-10 Density of ZnSe SAQDs for various flow rates of
hydrogen selenium. The ZnSe SAQDs growth temperatures
are 140℃, 300℃, and 380℃……………………………………87
Figure 3.5-1 Growth parameters of uncapped ZnSe SAQDs to observe ripening effect…………………………….……….…………….83
Figure 3.5-2 AFM images of ZnSe SAQDs as a function of the ripening days……………………………………………….……………..89
Figure 3.5-3 (a) Density, (b) diameter, and (c) height of ZnSe SAQDs
for various ripening days………..………………………………90
Figure 3.5-4 SEM morphologies of uncapped ZnSe SAQDs…………..92
Figure 3.6-1 Growth parameters of ZnSe SAQDs capped with ZnS...…93
Figure 3.6-2 Evolution of the surface morphology of ZnSe SAQDs capped with ZnS………………………………………………...94
Figure 3.6-3 (a) Density, (b) diameter, and (c) height of ZnSe SAQDs for various cap layer growth time……………………………..……95
Figure 3.6-4 PL spectra of ZnSe SAQDs for different cap layer growth time. The ZnS cap layer growth time is (a) 50 sec, (b) 70 sec, (c) 110 sec, and (d) 130sec…………………………………....….…97
Figure 3.6-5 (a)The peak position and (b) PL intensity of emission as
a function of ZnS cap layer growth time………………...……...98

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