臺灣博碩士論文加值系統

半導體雷射的微小化是未來發展高速、寬頻、低功耗光運算器與光通訊系統的關鍵。尤其將半導體雷射的三維(3D)尺寸縮小至次波長等級，以與積體電路中的電晶體尺寸相匹配，更是當今光電科技的研究焦點。然而傳統半導體雷射受限於光學繞射極限，要能達到雷射所需的回饋機制，必須有至少光波長長度的光共振腔，相較於現今只有幾十個奈米的電晶體尺寸，有著一段不小的差距。在 2003 年，Bergman 和 Stockman 提出了 Spaser (Surface Plasmon Amplification by Stimulated Emission of Radiation，受激輻射引致表面電漿子放大)的理論[1]，提倡以表面電漿子 (Surface Plasmon)達成雷射所需的回饋機制來克服此困難。表面電漿子是透過激發貴金屬材料表面產生之自由電子密度集體振盪波，它可以在貴金屬與介電質界面的次波長範圍內有效地將耦合埸壓縮到奈米尺度。
本論文利用此概念開發一種可突破繞射極限的新型奈米半導體雷射: 以電漿子共振腔取代傳統的光學共振腔，成功的將雷射的元件體積縮小到遠小於光波波長的奈米尺度。此種電漿子奈米雷射與電晶體同為「金屬－氧化物－半導體」結構，未來將很適合用於奈米光子學之實際應用
。本論文中，我們利用原子層平坦的磊晶銀膜作為低損耗的電漿子傳遞平台，搭配5 nm的低折射率介電材料以及分子束磊晶技術成長的高光增益係數之氮化銦鎵/氮化鎵 (InGaN/GaN) 核殼 ( Core－Shell ) 結構奈米柱為雷射必須的增益介質; 由於所用材料皆具有極高品質的材料特性，因而可以成功實現最小的半導體雷射。這種新型雷射結構所需的回饋機制:由於銀的表面電漿子共振頻率在可見光波段，當氮化銦鎵受激發出可見光時，兩者間會形成光子與電漿子的耦合態，以構成雷射必需的回饋機制，並將電磁波能量有效地侷限在中間等效折射率較低的介電層內。不同於過去文獻之多晶結構的銀膜(嚴重的電漿子散射及損耗)必需由高功率脈衝雷射激發的奈米雷射
，此研究首次揭露以極高光增益係數 (10,000 cm-1) 的全彩發光氮化物半導體增益介質搭配上低能量損耗的電漿子共振腔，實現了可以連續波 (Continuous Wave) 的方式運作且具有極低的閥值並可於液態氮環境下工作，尺寸可比擬電晶體的半導體奈米雷射。此外，雙光子相干實驗也首次證實了此類電漿子雷射的雷射光具有時間同調性。模擬結果並指出，金屬歐姆損耗26% 的能量，32%能量輻射入矽基板，14% 的能量以遠場雷射光形式釋放，28% 能量於銀膜表面以近場(near-field)的同調表面電漿子波存在。其中我們直接觀察的量值只有遠場雷射光。結合時間同調性證據可間接推衍得此元件同時存在近場的奈米受激輻射表面電漿子放大源 (nanospaser)。特別的是，本論文亦實驗證實了氮化物半導體本身優越材料特性，此材料在光電元件方面具有相當高的應用潛力: (i) 電性:氮化鎵(GaN)已證實可成功製作p型及n型摻雜,並具有良好的二極體電性。(ii)光性: 氮化銦鎵(InGaN)合金是一個發光波長可連續調控的寬能隙半導體,其發光波長可涵蓋全可見光波段。結合此光電特性，我們可利用奈米柱結構實現真正的多彩發光次波長光源。其遠程的發展目標是將此雷射運用於光運算系統以取代目前電晶體為主的電運算系統，理論預測其運算速度可提升1000倍[2]。此外，透過將光子元件尺寸縮小到與電晶體相仿，可促成在單一矽晶片上整合電漿子及奈米電子元件的發展平台拓展其實用的價值。結合氮化物半導體的優越光電特性以及搭配上低能量損耗的電漿子共振腔，研發電激發光的奈米雷射來取代現有的光激發奈米雷射為當前的主流課題，可為未來發展積體整合型高速、寬頻、低功耗光世代科技有所助益。另外，在生物醫學應用上也可發展超高解析生物影像。在半導體雷射問世後的五十年後，本研究成果寫下一個全球最小的半導體雷射的紀錄，並證實雷射微型化將不再受制於共振腔的光學繞射極限。

Size mismatches between electronics and photonics have been a huge barrier to realize on-chip optical communications and computing systems. The minimum size of conventional semiconductor lasers utilizing dielectric cavity resonators is governed by the optical diffraction limit (λ/2n)3. The recent surge of research interest in nanoplasmonics has been largely due to its capability to break the diffraction limit. In this dissertation, we present a record smallest semiconductor nanolaser based on surface plasmon amplification by stimulated emission of radiation (spaser). In plasmonic cavities, the coupling of photons and plasmons (a hybrid system) on noble metal surfaces at optical frequencies constitutes the necessary spaser feedback mechanism. However, direct observation of spaser has not been conducted. Only the far-field radiation (lasing) of spaser (so-called plasmonic nanolasers) was observed, and the plasmonic nanolaser emissions were first spectrally and temporally resolved in this study.
In Chapter 1, the background context of photonic and plasmonic nanolaser research is provided. The theoretical background of spaser is also briefly described. In Chapter 2, the related experimental techniques are presented to explore the lasing features of nanolasers. In Chapters 3 and 4, the non-polar nitride semiconductor (InGaN) nanorods with highly crystalline quality is discussed, grown using plasma-assisted molecular beam epitaxy (PAMBE), served as efficient gain media for realizing plasmonic nanolasers. Chapter 4 presents a plasmonic nanolaser consisting of an epitaxial growth silver film coupled with a single epitaxial growth InGaN/GaN nanorod. Nevertheless, overcoming the losses inherent to metals remains a fundamental challenge. The epitaxial approach reported in this study provides a scalable platform for low-loss, active nanoplasmonics. Therefore, 3D diffraction-unlimited nanolasers were realized and operated in continuous-wave conditions, above liquid nitrogen temperature, and in a nearly 100% polarized lasing mode. According to theoretical studies, a large proportion of energy emissions are transformed into in-plane directional and coherent surface plasmon (as a spaser). In Chapter 5, we introduce the single-mode, all-color plasmonic nanolasers. By optimizing the metal-oxide-semiconductor (MOS) structure design, nanolasers exhibiting an ultralow threshold can be fabricated. The temporal coherence signature denotes the “thresholdless” blue plasmonic nanolaser. In Chapter 6, the electrical properties of axial p–n junctions in GaN nanorods are described; the results indicate that nitride semiconductors are a prerequisite for electrically driven photonic devices in the future. Chapter 7 presents a single InGaN nano light emitting diode (LED) that comprises a single InGaN nanodisk embedded in a GaN p-n nanorod. This nano-LED can be used as a subwavelength light source that possesses spatial, spectral, and polarization controlling capabilities, which are crucial for extending optical imaging and lithography beyond the diffraction limit. In this study, the growth of high-quality nitride semiconductor nanorods by using PAMBE offered several advantages for broadband-tunable light emission in the full visible spectrum and amphoteric doping (for both n- and p-type GaN) for solid state lighting. Therefore, electrically driven nitride semiconductor-based plasmonic nanolasers are expected to be employed in multi-functional on-chip optoelectronic devices in the near future.

Abstracts
Contents…………………………………………………………………………………………… I
List of Figures……………………………………………………………………………………..IV
Chapter 1 Introduction………………………………………………………………………….. .1
1.1 Background of Semiconductor Lasers……………………………………………………...1
1.2 Motivation for the Development of Plasmonic Nanolasers………………………………...2
1.3 Theoretical Background of Plasmonic Nanolasers and Spasers…………………………....4
1.3.1 Basic Theory of Surface Plasmon Polaritons…………………………………………4
1.3.2 Surface Plasmon Amplification by Stimulated Emission of Radiation……………….6
1.4 Plasmonic versus Photonic Cavities…………………………………………………………9
1.5 Requirements of Low-Loss Plasmonic Materials and High-Gain Media…………………..16
Chapter 2 Experimental Methods………………………………………………………………...21
2.1 Sample Preparation………………………………………………………………………….21
2.1.1 III-Nitride Nanorods Growth by PAMBE…………………………………………….21
2.1.2 Epitaxial Silver Film Growth by MBE………………………………………………..23
2.1.3 Atomic Layer Deposition (ALD)……………………………………………………...23
2.2 Optical Measurements………………………………………………………………………25
2.2.1 Temperature Dependent Micro-Photoluminescence (μ-PL) Measurement….………..25
2.2.2 Time-resolved μ-PL Measurement…………………………………………………....26
2.2.3 Second Order Photon Correlation Function Measurement…………………………....28
2.3 Electrical Measurements…………………………………………………………………..... 32
2.3.1 Device Fabrication…………………………………………………………………….32
2.3.2 μ-Electroluminescence (EL) Measurement…………………………………………....35
Chapter 3 Characteristics of InGaN/GaN Nanorods…………………………………………….36
3.1 Introduction to the Properties of InxGa1-xN Alloys…………………………………………. 36
3.2 Optical Properties of Non-Polar Nitride Heterostructures………………………………….. 38
3.2.1 Absence of Polarization Effects………………………………………………………. 38
3.2.2 Large Optical Gain Coefficient……………………………………………………….. 40
3.3 Optical Properties of Non-Polar InxGa1-xN/GaN Nanorods Growth by PAMBE…………... 41
3.3.1 InGaN/GaN Core-Shell Nanorods……………………………………………………. 41
3.3.2 Single InGaN Nanodisk Embedded in GaN p-n Nanorods…………………………… 48
3.4 Possible Device Applications of InGaN Alloy Semiconductors…………………..………... 49
Chapter 4 Single InGaN Nanorod Plasmonic Nanolaser………………………………………...51
4.1 Motivation…………………………………………………………………………………... 52
4.2 Device Structure……………………………………………………………………………..52
4.2.1 Epitaxial Silver Film…………………………………………………………………...54
4.2.2 Gain Medium…………………………………………………………………………..55
4.3 Experimental Signatures of Lasing…………………………………………………………. 59
4.4 Emission Polarization Properties of the Plamonic Nanolaser……………………………… 65
4.5 Discussion..……………………………………………………………………………......…69
4.5.1 Comparison between Epitaxial Ag Films and Thermal Ag Films………………..……69
4.5.2 Laser versus Spaser………………………………………………………………….... 72
Chapter 5 InxGa1-xN Nanorod Based All-color Plasmonic Nanolasers……………………...…. 75
5.1 Motivation for the All-Color Nanolasers …………………………………………………....76
5.2 InxGa1–xN@ GaN Nanorods as Efficient Gain Media across the Visible Spectrum……….. .77
5.3 All-Color InGaN@GaN Nanorod Plasmonic Nanolasers…………………………………... 80
5.4 Evidence for Thresholdless Blue Plasmonic Nanolaser……………………………………. .85
5.4.1 Temperature-Dependent Lasing Threshold………………………………………….....85
5.4.2 Lasing/Spasing Mechanism………………………………………………………….....88
5.4.3 Temporal Coherence…………………………………………………………………...93
Chapter 6 Dynamic Visualization of Axial p–n Junctions in Single Gallium Nitride Nanorod
under Electrical Bias…………………………………………………………………. ..96
6.1 Introduction to One Dimensional Semiconductor Axial p–n Junctions……………………...96
6.2 Structural Characterization of GaN nanorods with Axial p–n Junctions………………….. ..98
6.3 Secondary Electron Imaging Technique for Electrostatic Potential Mapping of
Semiconductor Nanostructures…………………………………………………………….101
6.4 in-situ Electrical Measurements of Single GaN Nanorod p–n Junction Device in FESEM..104
6.5 Visualization of Electrostatic Potential Variation across the in-situ Biased p–n Junctions..106
6.6 Discussion of the Depletion Widths and the Carrier Concentrations………………………108
Chapter 7 Single InGaN Nano-LED as Full-Color Subwavelength Light Sources………...… 111
7.1 Motivation…………………………………………………………………………………. 111
7.2 Optoelectronic Characteristics of InGaN Nano-LEDs…………………………………...…113
7.2.1 Sample Structure……………………………………………………………………113
7.2.2 Electrical Properties of Single InGaN/GaN Nanodisk-in-Nanorod LED…………..114
7.2.3 Single InGaN/GaN Nanodisk-in-Nanorod LED as Full-Color Nano-Emittors…….116
7.2.4 Additional Optical Polarization Properties of the Nano-LEDs……………………..117
7.3 Single-Nanorod LED as an Exposure Light Source for Subwavelength Photolithography…………………………………………………………………………..120
7.3.1 Nanolithographic Process……………………………………………………………..120
7.3.2 Using Single-Nanorod LED as a Local Exposure Light Source……………………...122
Chapter 8 Conclusions and Future Perspective .………………………………………………..126
References…………………………………………………………………………………………….i

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