臺灣博碩士論文加值系統

本論文的主題乃以磷化銦為基板，使用N型調變摻雜砷化鋁鎵銦應變平衡多重量子井，設計雷射/半導體光放大器之結構，且利用分子束磊晶成長技術製備材料，進而完成元件之製作。此一量子井之組成，乃由一晶格匹配之砷化鎵銦為其核心，填入一壓縮應變之砷化鎵銦在其核心之兩旁，配合一伸張應變之砷化鋁鎵銦做為與位障之間的緩衝隔離區。兩種具有相似結構，但設計在不同之基本發光波長(1.55微米與1.48微米)的晶片，將分別地被準備用來研究: (1)順偏壓時，光之放大特性。(2)逆偏壓時，電致吸收的特性。在實驗上，以固態源分子束磊晶系統成長高品質之砷化鋁鎵銦的技術已經被建立，且利用雙晶X光繞射儀、穿透式電子顯微鏡、電致螢光與光致螢光光譜量測技術，所成長之塊材與多重量子井晶片的特性也更進一步被檢測確定。
針對波長為1.55微米的晶片，使用一新發展之多次濕蝕刻技術完成脊狀波導雷射的製備，包含有Fabry-Perot (FP)與tilted-end-facet (TEF)兩種形式。電致螢光光譜顯示，當注入電流密度大於20 (A/cm^2)，此量子井之第一電子能階至第二電洞能階的光增益(e1-hh2，波長為1460奈米)大於第一電子能階至第一電洞能階的躍遷(e1-hh1，波長為1550奈米)。FP-laser操作在其臨限電流時，雷射出的波長為1514奈米。隨著注入電流的增加，將依序觀察到兩個額外的雷射發光波長(1528奈米與1545奈米)。然而，在TEF-laser僅發現唯一雷射發光波長在1511奈米。我們也進一步的發現，這些TE極化的雷射發光波長與光電流光譜中發現之δ-like吸收峰之位置呈現一致。此雷射行為與量子線或量子點的光能量躍遷極為相似，極可能工作在激發態。
為了研究調變摻雜半導體光放大器之電致吸收調變的行為，波長為1.48微米的晶片，將被用來量測其在電場下之穿透係數變化量，進而求得其微分吸收頻譜。比較六片不同的樣品，其中，在一結合電洞阻擋位障、具有N型調變摻雜且載子薄片密度為3.5 × 10^11 (cm^-2 per QW)之量子井結構，發現此一結構工作在反向偏壓下，具有最大的揪譜參數。因此，此結構將提供一絕佳的平台讓我們有機會去實現，那些需要較大的折射率變化量，但希望有較小的吸收係數變化量之電致折射元件。
此外，在波導元件的設計上，利用一漸變寬度波導的概念，針對耦和係數為0.15與0.28的兩元件，我們已經成功地使得傳統上固定寬度之多模干涉器之長度縮短超過32%。進一步延伸此一設計概念，我們展示了一個利用串接兩段長度較短之多模干涉器，形成具有新的能量分配比(7%, 64%, 80% 與93%) 之2x2波導耦合器。最後，在兩段長度較短之多模干涉器之中，插入一對不等寬的波導作為相位調整之用，我們更加實現了一任意分光比之波導耦合器。這些耦合器具有簡單的幾何形狀與低損耗的特性，進而在爾後的光積體晶粒之設計上，提供了一新穎元件可供選擇。

In this work, we have reported the design, MBE-growth and fabrication of strain-balanced n-type modulation-doped (MD) InGaAlAs/InGaAs multiple quantum wells laser/SOAs on InP. The quantum well contains a lattice-matched InGaAs core, a compressive-strained InGaAs padding, and a tensile-strained InGaAlAs spacer. Two kinds of samples having similar structure but different fundamental transition wavelength of 1.55 μm and 1.48 μm are separately prepared for investigating their characteristics in optical amplification under forward bias and electro-absorption under reversed bias. Also, the technique of growing high-quality InGaAlAs with solid-source molecular beam epitaxy has been established and the resulting InGaAlAs bulk and QWs samples are extensively characterized by double-crystal X-ray diffraction, transmission electron microscopy, electroluminescence, and photoluminescence measurements.
For λ = 1.55 μm samples, ridge-waveguide lasers of Fabry-Perot (FP) type and tilted-end-facet (TEF) type were fabricated by a new developed multi-step wet-etching process. When injection current density > 20A/cm^2, electroluminescence spectra show higher optical gain for the quantum well e1-hh2 transition at λ = 1460 nm than the e1-hh1 transition at λ = 1550 nm. The FP laser shows a lasing peak of λ = 1514 nm at threshold. Additional lasing wavelength at λ =1528 nm and 1545 nm were observed sequentially as the injection current increased. However, for the TEF laser, only the emission at λ = 1511 nm was observed. These TE-polarized lasing wavelengths are consistent with the δ-like absorption peaks in photocurrent spectra. The lasing performance is possible attributed to optical transitions within quantum dots/wires which are formed by the strain-field profile and alloy segregation/migration.
For λ = 1.48 μm samples, the differential absorption spectroscopy, which measures the change of transmission (ΔT/T) in the presence of electric field, is used to study the electro-absorption modulation behavior of MD-SOA’s. A sample with n-type modulation-doping amounting to a sheet density of 3.5 × 10^11 cm^-2 per QW and combining with a hole-stopping barrier represents the largest chirp parameter (Δn/Δk) under reversed bias, which offers an excellent platform to realize electro-refractive devices with larger refractive index changes (Δn) but lower differential absorption (Δα) near λ = 1.55 μm, which is also our interested region of operation.
In addition, we have succeeded in reducing the length of conventional constant-width multimode interference (MMI) coupler of K = 0.15 and 0.28 more than 32% by a novel stepped-width design concept. By extending the stepped-with idea, we show that it is possible to obtain 2x2 waveguide couplers with new power splitting ratios of 7%, 64%, 80% and 93% for cross coupling by cascading two short MMI sections. We further realize freely chosen power splitting ratio by interconnecting a pair of unequal-width waveguides as the phase-tuning section into the middle of two short MMI sections. These compact and low loss MMI-based devices use only rectangular geometry without any bent, curved, and tapered waveguides. They offer valuable new possibilities for designing waveguide-based photonic integrated circuits.

Contents

Chapter 1. Introduction 1
1.1 Benefits of InGaAlAs, strain QWs, and n-type modulation doping 3
1.1.1 Advantage of InGaAlAs 3
1.1.2 Compressively Strained QWs 3
1.1.3 N-type modulation doping 5
1.2 Outline of the thesis 7
Reference 10

Chapter 2. Design of N-type Modulation-Doped InGaAlAs/InP Strained-Balanced MQWs Laser/SOA’s 12
2.1 Important issues for TE-polarized laser/SOA’s QW 12
2.2 Laser/SOA’s QW design for active devices 15
2.3 p-i-n laser/SOA’s structure 19
Reference 26

Chapter 3. MBE Growth of InGa(Al)As Materials and Laser/SOA’s Structures 27
3.1 Calibration of cell flux and growth rate 28
3.1.1 RHEED intensity oscillations 29
3.1.2 Flux measurements 31
3.2 Experiment: MBE-growth of p-i-n laser/SOA’s structures 42
Reference 46

Chapter 4. InGaAlAs/InP Strain-Balanced MQWs Laser/SOA’s 48
4.1 Mesa diode 49
4.2 Ridge waveguide fabrication 49
4.2.1 Multi-step wet-etching process 49
4.2.2 Double-layer photoresist method 51
4.3 Devices: experimental results and discussions .....54
Reference 63

Chapter 5. Electro-Absorption Characteristics in N-type Modulation-Doped Strain-balanced MQWs 64
5.1 Six blue-shifted samples (λ = 1.48 μm) 66
5.2 Photocurrent and electroluminescence (EL) spectra 68
5.3 Transmission and Absorption 69
5.3.1 Kramers-Kronig Transform (KKT) 69
5.3.2 Experimental results and discussions 69
Reference 85

Chapter 6. Compact Multimode Interference Couplers with Arbitrary Power Splitting Ratio 86
6.1 Simulation methods 94
6.1.1 Multi-Mode Interference, MMI 94
6.1.2 An approximate 2-D Waveguide for representing a real 3-D waveguide. 94
6.1.3 Approximations in Guided-Mode Propagation Analysis (MPA). 95
6.2 Conventional 2 x 2 MMIs (K ≥ 0.5) 99
6.2.1 Transfer functions of MMI’s 101
6.3 Cascaded 2 x 2 MMI couplers 105
6.3.1 Transfer Matrix of Cascaded 2 x 2 MMI
Couplers 105
6.3.2 Possible K-values of Cascading MMI’s 106
6.3.3 Wavelength sensitivity of MMI’s 109
6.3.4 Applications 110
6.4 Design of 2 x 2 Couplers with Arbitrary Power Splitting Ratio 117
6.5 Transfer functions of half MMI-D 123
Reference 126

Chapter 7. Summary 129

APPENDICES
A. Band Lineup and Effective Mass of Strained Layers 133
B. Achievable Epitaxial Materials 150
C. Riber Compact 21T MBE System 155
D. MBE Growth Process and Conditions 160
E. Ring-Resonator Loop-Mirror Laser 167

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