室溫下於金屬鍍膜之氮化鎵奈米共振腔之雷射特性

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國立交通大學

光電工程研究所

碩士論文

室溫下於金屬鍍膜之氮化鎵奈米共振腔

之雷射特性

Lasing characteristics of metal-coated GaN nanocavity at room

temperature

研究生：王祐國

指導教授：郭浩中教授

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室溫下於金屬鍍膜之氮化鎵奈米共振腔之雷射特性

Lasing characteristics of metal-coated GaN nanocavity at room temperature

研究生：王祐國 Student：Yow-Gwo Wang

指導教授：郭浩中教授 Advisor：Prof. Hao-Chung Kuo

國立交通大學

光電工程研究所

碩士論文

A Thesis

Submitted to Institue of Electro-Optical Engineering

College of Electrical and Computer Engineering

National Chiao Tung University

in Partial Fulfillment of the Requirements

for the Degree of Master

in

Electro-Optical Engineering

June 2011

Hsinchu, Taiwan, Republic of China

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室溫下於金屬鍍膜之氮化鎵奈米共振腔之雷射特性

研究生：王祐國指導教授: 郭浩中教授國立交通大學光電工程研究所碩士班

摘要

本論文中，我們展現了在室溫下從金屬鍍膜之氮化鎵奈米共振腔觀察到雷射訊號的可能性。首先我們利用電子束微影在未參雜的氮化鎵薄膜上定義出條狀的結構，並在此結構上方覆蓋上一層介電質材料和鋁金屬。由微光致螢光量測系統所得的實驗結果，我們觀察到了單一模態的雷射訊號，其波長約略三百七十奈米，並由實驗結果反推出其品質因數大約為 150。除此之外，藉由模擬的方式來分析此實驗結果，我們也證明了鍍在奈米結構上的鋁金屬大大提升了此結構對於光場的局限能力。我們相信此雷射模態是由波導的模態和表面電漿的模態所混合而成的。第二個部分，我們驗證了室溫下在金屬鍍膜之環狀奈米結構中雷射的可能性。我們利用迴音壁模態和表面電漿模態做一結合，達到在奈米結構中觀察到雷射訊號的目標。最小的環形共振腔其直徑為三微米，環的寬度約為310 奈米。和前一章之條狀結構之元件特性做比較，我們可以得到更高的品質因子及更低的閥值能量密度，且環形的寬度更可以達到次波長的領域，並且大幅度的縮小整個元件大小。可見環形結構一如預期可利用回音壁模態來大幅提升奈米級半導體雷射的元件表現。

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Lasing characteristics of meta-coated GaN nanocavity at room temperature

Student : Yow-Gwo Wang Advisor: Prof. Hao-Chung Kuo

Institute of Electro-Optical Engineering National Chiao Tung University

Abstract

In this thesis, we demonstrated metal-coated GaN nanocavity laser operable at room temperature by optical pumping.

We first utilized fabrication process to define the nanostripe pattern on undoped GaN layer grown on a sapphire substrate, with a thin dielectric layer and aluminum layer coated on it. From micro-photoluminescence measurement result, we observed a single mode lasing at room temperature with lasing wavelength around 370nm and the quality factor was about 150. Moreover, by simulation analysis, we proved the aluminum layer coated on the nanostripe strongly enhance the optical confinement of this nanocavity. We believed that this lasing mode was a combination of waveguide mode and surface plasmon mode.

Second, we demonstrated lasing in metal-coated GaN nanoring at room temperature. We utilize whispering-gallery-mode with surface plasmon mode to achieve lasing in a nanoring cavity. The diameter of the smallest ring is 3μm, and the width of the nanoring is about 310nm. Compare with the device performance of metal-coated GaN nanostripe, we got a higher quality factor and lower threshold power density. The width of the nanoring could shrink to subwavelength scale and the size of the device can be reduced significantly. This result proved our assumption that whispering-gallery mode would improve the performance of nanolaser.

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誌謝

兩年的碩士班生涯一下子就走到終點了，雖然兩年不是段很短的時間，但我依然記得剛進來時跟學長們一起學做實驗討論問題的那些時光。首先，我要感謝中研院應科中心的施閔雄老師，沒有老師的指導就不會有這本碩士論文和這些研究成果。老師的耐心指導以及所提供的意見，讓我在碩士班這段期間學到很多做研究和分析實驗的方法。這些無價之寶相信會在我未來研究之路上依舊受用無窮的。另外，我還要謝郭浩中老師的所給予的幫忙，沒有郭老師的推薦，我是沒有機會前往莊順連老師的實驗室學習的。此外，謝謝老師提供了很好的研究環境，還有平時對我的幫助與鼓勵，使我在碩士班兩年的路上能夠專注於研究上。感謝在我初進實驗室，帶著我學習的清華學長以及Joseph 學長，在你們的指導下我熟習到理論知識與製程技術，紮根了我對GaN-based LED 研究的基礎，讓我可以可以學習到更多更廣的知識。再來要感謝的是振昌學長的照顧，除了一起在無塵室寫 E-beam 和做 micro-PL 的量測。學長也不厭其煩的教導我分析實驗結果和撰寫文章的技巧。另外，我還要感謝中研院應科中心的研究助理旻彥，在旻彥跟振昌學長的幫忙以及合作下，我才能有這些研究成果和發表，並且有機會可以順利完成我的碩士班學位。此外，我要感謝李博、鏡學、閔安、智凱學長對於我在報告時提供不少想法與建議，令我收穫良多。感謝板弟、信助、Gibu、小昕、阿祥、阿菲、翁翁、哭哭、David 和小邱等學長姐，無私的和我分享實驗上的經驗與並給予我鼓勵與叮嚀，使我能有所成長。感謝羿蓁、大寶、Jolin、肉圓、祐慶、小杜、家齊、阿 po、瑋婷、Just、SGG、 KAKA、冠霖、峰瑜等碩二的同學們，因為有你們使我的碩班生活增添不少快樂回憶，我會永遠記得一起做實驗討論，在無塵室打拼的時光，一起分享歡笑與宣洩壓力，很開心可以認識到可愛的大家。此外，還要感謝碩一的學弟妹們，有你們的幫忙讓我們的實驗順利許多，祝福你們明年也能順利。此外，我要感謝幫忙維護機台的各位學長跟徐先生，有你們細心照料實驗設備，讓我能順利完成實驗，做出元件。

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最後要感謝我的好朋友們以及我的女朋友Tina，感謝你們的支持，使我可以堅持下去，即使是面對低潮和困難的打擊。最後要感謝我的父母跟我的家人，你們在背後支持我、鼓勵我，並提供這麼好的環境讓我可以專心於課業上，使我能順利的取得碩士學位，謝謝你們！

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Content

摘要...i Abstract...ii 誌謝...iii Content...v List of Tables...vii List of Figures...viii Chapter 1 Introduction 1.1 History of Semiconductor Laser...1

1.2GaN-based Optoelectronic Devices...2

1.3 Surface Plasmon Effect...3

1.4 Metal-coated Nano Devices...5

1.5 Motivation...6

Reference...12

Chapter 2 Experimental Instruments and Methods 2.1 Electron Beam Lithography and Scanning Electron Microscope...15

2.2 Dry Etching Process and E-gun Evaporation...16

2.3 Micro-Photoluminescence System...18

Chapter 3 Lasing in Metal-coated GaN Nanostripe at Room Temperature 3.1 Epitaxial Characteristics and Fabrication Process of Metal-coated GaN Nanostripe...23

3.2 Lasing Characteristics of Metal-coated GaN Nanostripe...27

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3.4 Summary...31

Reference...41

Chapter 4 Lasing in Metal-coated GaN Nanoring at Room Temperature 4.1 Epitaxial Characteristics and Fabrication Process of Metal-coated GaN Nanoring...42

4.2 Lasing Characteristics of Metal-coated GaN Nanoring...45

4.3 Results and Discussion...47

4.4 Summary...49 Reference...59 Chapter 5 Conclusion 5.1 Conclusion...60 5.2 Future Work...61 Appendix 1. Thermal Issue...63

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List of Tables

Table 1.4 Recent Research Results on Metal-coated Nanocaviy...11

Table 4.2 Lasing Characteristics of Different Metal-coated GaN Nanoring...58

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List of Figures

Figure 1.1 Dr. Robert N. Hall with Semiconductor laser...8

Figure 1.2 Basic structure of GaN-based LDs and LEDs...8

Figure 1.3.1 Schematic representation of SPPs and its electric field distribution at the interface...9

Figure 1.3.2 Electric Field distribution of dielectric waveguide and plasmonic waveguide...9

Figure 1.3.3 Applications of surface plasmon effect: (a) Biosensor. (b) Bioimaging. (c) Lithography (d)Nanolaser...10 Figure 2.1.1 Schematic Diagram of E-beam Lithography System...18

Figure 2.1.2 JSM-6500 E-beam Lithography System...18

Figure 2.1.3 JSM-7000F SEM System...19 Figure 2.2.1 ICP-RIE System (Oxford Plasmalab System100)...19

Figure 2.2.2 ICP-RIE System (SAMCO RIE-101PH)...20

Figure 2.2.3 E-gun Evaporation System (ULVAC EBX-8C)...20

Figure 2.3.1 Schematic Diagram of Micro-Photoluminescence System...21

Fig. 3.1.1 Process Flow Chart for Metal-coated GaN nanostripe...33

Fig. 3.1.2 SEM Image of Metal-coated GaN Nanostripe (a) Top View of the GaN Nanostripe before Deposition of Shielding Layers. (b) Angle View of One side of the GaN Nanostripe before Deposition of Shielding Layer...34

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Fig. 3.1.3 Angle View of the GaN Nanostripe after the Deposition of SiO2 and Aluminum

Layers...35

Fig. 3.1.4 Schematic Diagram of Metal-coated GaN Nanostripe...35

Fig. 3.2.1 PL Spectrum of Undoped GaN Layer with and without Metal and Dielectric Shielding Layers...36

Fig. 3.2.2 PL Spectrum of Metal-coated GaN Nanostripe Above (Red) and Below (Black) Threshold...36

Fig. 3.2.3 Light-in and Light-out Curve and the Linewidth of Lasing Peak...37

Fig.3.3.1 (a) The Ez Mode Profile of the Nanostripe without Shielding Layers. (b) The Ez Mode Profile of the Nanostripe with Shielding Layers...38

Fig. 3.3.2 Schematic Diagram of the Nanostripe Model in Effective Index Method...39

Fig. 3.3.3 Effective Structure of Nanostripe under TE Mode...39

Fig. 3.3.4 TE Mode Band Diagram of Nanostripe...40

Fig. 4.1.1 Process Flow Chart for Metal-coated GaN Nanostripe...51

Fig. 4.1.2 SEM Image of different size of GaN Nanoring before shielding layer deposition (a) 7μm in diameter. (b) 5μm in diameter. (c) 3μm in diameter...52

Fig.4.1.4Schematic Diagram of Metal-coated GaN Nanostripe...53

Fig. 4.2.1 Lasing Characteristics of Metal-coated GaN Nanoring with 7μm in diameter (a)

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Lasing Peak...54

Spectrum Above (Red) and Below (Black) Threshold. (b)L-L Curve and the Linewidth of the

Lasing Peak...55

Spectrum Above (Red) and Below (Black) Threshold. (b)L-L Curve and the Linewidth of the

Lasing Peak...56

Fig. A.1 The Lasing Peak Wavelength of GaN Nanoring Laser with (Red) or without (Black)

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Chapter 1 Introduction

1.1 History of Semiconductor Laser

Robert N. Hall, a researcher worked for General Electric, invented first semiconductor

laser in 1962. Since then, the development of semiconductor laser had a huge progress thanks

to scientists and engineers around the globe. At 1962, Nick Holonyak Jr. demonstrated first

visible wavelength semiconductor laser at red wavelength region. Other groups in the world

also contributed to the invention of semiconductor laser in different ways.

Since invention of semiconductor laser, it promises a huge potential for its application

in the very near future. It generally becomes one of the most important types of laser in the

world. It was first used for optical communication in fiber. Then with the utilization of

different materials to fabricate the Laser Diodes (LDs), now we could see semiconductor

lasers in our daily life like optical storage, or used in a micro projector for display, even for

military purposes. Its low cost and mass-production ability make it suitable for marketing than

other types of laser.

The first laser diode was made by gallium arsenide (GaAs), which emits light from

optical communication to red wavelength region. Other types of material including gallium

nitride (GaN) also adopted to make a laser diode with a lasing wavelength from visible to

shorter wavelength region. Nowadays common lasing wavelength for laser diode is ranging

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1.2 GaN-based Optoelectronic Devices

Gallium nitride (GaN) is a compound semiconductor with direct bandgap, which is a

crucial characteristic for radiative recombination. It is a wide band gap (3.4 eV) material

which makes it a suitable candidate for the application of optoelectronic devices, including

GaN-based Light Emitting Diodes (LEDs) and LDs operating in blue or ultra-violet (UV)

wavelength regions and other devices.

GaN-based LEDs fill the void on the primary color, and make white light LEDs possible

with the combination of phosphor or LED with different colors. Thanks to intensive

development in recent years [1-4], the manipulation of GaN material system has been mature

enough to create high-brightness GaN LEDs in blue wavelength. This progress makes LEDs

possible for the applications in display or solid-state lighting, and might one day replace the

light bulb which used by human beings for many years.

At the same time, GaN-based semiconductor lasers’ lasing wavelength in blue or UV

wavelength region has also developed by scientist. Since the demonstration by Nakamura et

al. [5] the first GaN-based laser lasing operated under room temperature continuous-wave

(CW) condition at 1996, many applications of GaN-based have been achieved in recent years.

Its application like blue-ray disks or as a light source in a micro-projector already exists in our

daily life.Fig. 1.2 shows typical structures of GaN-based LDs and LEDs.

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developed for many years [6-9]. At 2008, T. C. Lu et al. developed GaN-based VCSELS

operated under continuous wave condition at room temperature [7]. It has some advantages

like single-longitudinal-mode operation, symmetric and low divergence angle, and two

dimensional array capabilities makes VCSELs very competitive compared with edge emitting

laser. These merits make GaN-based VCSELs have many applications in our daily life.

1.3 Surface Plasmon Effect

Surface plasmon polaritons (SPPS) are light waves trapped on the surface because of

electron gas couple with photons. It is a kind of electromagnetic wave propagating on the

interface between two materials which change sign on their real part of dielectric constant.

For example: a metal-dielectric interface. The SPPs existed at the interface between dielectric

materials and metal could attribute to resonant coupling of photons from the polarized light

with the oscillation of metal free electron. Fig. 1.3.1 shows the schematic representation of an

electron density wave propagating along a metal-dielectric interface and the electric field

distribution around the interface.

SPPs could concentrate and channel light under subwavelength scale, which has a huge

potential on exploring science phenomenon under such small scale, and this might one day

help scientists and engineers to make photonic integrated circuit with a smaller size than we

have now. This kind of structure also solves the problem that dielectric waveguide would

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optical field for these two waveguide structures. The other advantage is surface plasmon

effect concentrates light in this way leads to an electric field enhancement that can be used to

manipulate light-matter interactions and boost non-linear phenomena. Therefore, lots of

applications have been developed for many years and exist in our daily life [10-13]. For

example, bioimage , biosensor , advanced lithography technique, and nanolaser.

It is the most difficult task for researchers to observe the behavior of a bio molecule.

Therefore, in order to observe this tiny molecule with the size in just a few nanometers,

researchers utilize surface plasmon effect to improve the extraction of light. With this new

technique, researchers could observe their samples and distinguish its details even in

subwavelength region. For biosensor, the utilization of surface plasmon effect makes the test

sample become label free and can be detected directly in real time. This has been applied to

drug screening, kinase analysis and research on antibody development[10]. To further lower

the cost of production for a single chip, a better lithography is needed to put more patterns on

a single wafer, the e-beam lithography and other techniques are still struggling on their

throughput, which is too low to become commercialize. A set of plasmonic mirrors takes the

advantages of surface plasmon effect and demonstrate lithography technique with a linewidth

only 80nm recently [11]. This new method has showed a promising way to the development

of nanolithography. Moreover, taking the advantages of energy confinement and field

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scale even with the lossy metal coated on it [13]. Many research teams have reported their

result using different metals and cavity. This will discuss thoroughly in the following section.

Fig. 1.3.3 shows some examples of the application of surface plasmon effect.

1.4 Metal-coated Nano Devices

Metal-coated cavity has been intensely researched by scientist around the world. It could

reduce the size of a semiconductor laser to nano scale, even in subwavelength scale. This result

breaks the diffraction limit which constrains the size of a laser that cannot be smaller than its

nature wavelength. The lasing characteristics also be studied by researchers trying to explain

the physical meaning behind the experimental result.

At 2007, Martin T. Hill et al. demonstrated lasing in metal-coated nanocavity [13]. They

coated silver and dielectric layer on the nanorod and observe lasing signal at 77 K. After that,

different designs of metal-coated nanocavity have reported experimentally or theoretically.

Metal-coated waveguide structure had also been demonstrated by the same research team and

bow-tie nanostructure had been proposed by S. W. Chang et al. [14]. The combination of

surface plasmon effect and bow-tie structure shows a promising way theoretically in forming a

semiconductor laser. Moreover, different kinds of metal have been use to form metal-coated

nanocavity. From recent research results [13-17], people use silver, gold and aluminum toform

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with aluminum layer and SiO2 as the dielectric layer. They also proposed a theoretical analysis

to show that optimization of the dielectric layer could have a higher chance to get a better

quality factor to achieve lasing action. K. Y. Yu et al. at 2010 demonstrated a nano-patch laser

with metal coated above and below the gain medium, and analyze the lasing characteristics of

the device [16]. In 2011, M. W. Kim et al. demonstrated lasing in a metal-clad microring [17].

In conclusion, metal-coated nanocavity has been demonstrated experimentally in different

structures includes nanorod, waveguide, and ring. In sum, recent research results mainly focus

on InGaAsP material system, which has a lasing wavelength from red to Infar-red region. Table

1.4 shows some important research results did by researchers around the world in recent years.

1.5 Motivation

Metal-coated nanocavity shows huge potential to reduce the size of semiconductor laser

into subwavelength scale. However, their research results mainly focus on InGaAsP material

system and optical communication to infra-red wavelength region. Shorter lasing wavelength

and other material systems have not yet been reported by any groups in the world.

In this thesis, we utilize GaN as the gain medium for metal-coated nanocavity, combining

with aluminum and SiO2 layer. We use E-beam lithography to define two different types of

nano structure, and then we use dry etching processes and other fabrication process to form the

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devices which is optical pumped under room temperature pulsed condition, then we use finite

element method and effective index method to further simulate our structure and discuss the

physics behind the experiment result.

In chapter 2, we briefly introduce the instruments which are used to fabricate the device

and measure its characteristics. Then in chapter 3, we present experiment and simulation result

of lasing in a metal-coated GaN nanostripe at room temperature. The waveguide structure is

only 500nm in its width. In chapter 4, we use E-beam lithography to define a nanoring pattern

on the undoped GaN layer. We also perform measurement on different sizes of nanoring laser

range from 7μm to 3μm, coated with the aluminum layer. By the combination of nanoring

structure and the advantages of metal-coated nanocavity, we could shrink the size of our

nanostructure into smaller scale and better device performance. Finally in chapter 5, we will

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Fig. 1.1 Dr. Robert N. Hall with Semiconductor Laser.

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Fig. 1.3.1 Schematic representation of SPPs and its electric field distribution at the interface.

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Fig. 1.3.3 Applications of surface plasmon effect: (a) Biosensor. (b) Bioimaging. (c) Lithography (d) nanolaser.

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References

[1]S. Nakamura, M. Senoh, N. Iwasa, and S. Nagahama, “High brightness InGaN blue, green,

and yellow light-emitting diodes with quantum structures,” Jpn. J. Appl. Phys., vol. 34, pp.

L797, 1995.

[2]M. H. Kim, M. F. Schubert, Q. Dai, J. K. Kim, and E. F. Schubert, J. Piprek, and Y. Park,

“Origin of efficiency droop in GaN-based light-emitting diodes,” Appl. Phys. Lett., vol. 91,

pp. 183507, 2007.

[3]C. H. Wang, S. P. Chang, W. T. Chang, J. C. Li, Y. S. Lu, Z. Y. Li, H. C. Yang, H. C. Kuo, T.

C. Lu, and S. C. Wang, “Efficiency droop alleviation in InGaN/GaN light-emitting diodes by

graded-thickness multiple quantum wells,” Appl. Phys. Lett., vol. 97, pp. 181101, 2010.

[4]S. D. Lester, F. A. Ponce, M. G. Craford, and D. A. Steigerwald, “High dislocation

densities in high efficiency GaN-based light-emitting diodes,” Appl. Phys. Lett., vol. 66, pp.

1249, 1995.

[5]S. Nakamura, M. Senoh, S. I. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, Y. Sugimoto,

and H. Kiyoku, “Room-temperature continuous-wave operation of InGaN

multi-quantum-well structure laser diodes,” Appl. Phys. Lett., vol. 69, pp. 4056, 1996.

[6]J. T. Chu, T. C. Lu, H. H. Yao, C. C. Kao, W. D. Liang, J. Y. Tsai, H. C. Kuo, and S. C.

Wang, “Room-temperature operation of optically pumped blue-violet GaN-based

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pp. 2556, 2006.

[7]T. C. Lu, C. C. Kao, H. C. Kuo, G. S. Huang, and S. C. Wang, “ CW lasing of current

injection blue GaN-based vertical cavity surface emitting laser,” Appl. Phys. Lett., vol. 92,

pp. 141102, 2008.

[8]C. C. Kao, T. C. Lu, H. W. Huang, J. T. Chu, Y. C. Peng, H. H. Yao, J. Y. Tsai, T. T. Kao, H.

C. Kuo, S. C. Wang, and C. F. Lin, “The lasing characteristics of GaN-based vertical-cavity

surface-emitting laser with AlN/GaN and Ta2O5/SiO2 distributed bragg reflectors,” IEEE

Photonic Tech. Lett., vol.18, pp. 877, 2006.

[9]T. C. Lu, S. W. Chen, T. T. Wu, P. M. Tu, C. K. Chen, C. H. Chen, Z. Y. Li, H. C. Kuo, and

S. C. Wang, “Continuous wave operation of current injection GaN vertical-cavity surface

emitting laser at room temperature,” Appl. Phys. Lett., vol. 97, pp. 071114, 2010.

[10]X. D. Hoa, A. G. Kirk, M. Tabrizian, “Towards integrated and sensitive surface plasmon

resonance biosensors: A review of recent progress,” Biosensors and Bioelectronics, vol. 23,

pp. 151, 2007.

[11]W. Srituravanich, L. Pan, Y. Wang, C. Sun, D. B. Bogy, and X. Zhang, “Flying plasmonic

lens in the near field for high-speed nanolithography,” Nat. Nanotechnology vol. 3, pp. 733,

2008.

[12] M. F. Garcia-Parajo, “Optical antennas focus in on biology,” Nat. Photonics, vol. 2, pp.

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[13]M. T. Hill, Y. S. Oei, B. Smallbrugge, Y. Zhu, T. de Vries, P. J. van Veldhoven, F. W. M.

van Otten, T. J. Eljkemans, J. P. Turkiewicz, H. de Waardt, E. J. Geluk, S. H. Kwon, Y. H.

Lee, R. Notzel, and M. K. Smit, “Lasing in metallic-coated nanocavities,” Nat. Photonics,

vol. 1, pp. 589, 2007.

[14]S. W. Chang, C. Y. A. Ni, S. L. Chuang, “Theory for bowtie plasmonic nanolasers,”

Opt. Express, vol. 16, pp. 10580, 2008.

[15]M. P. Nezhad, A. Simic, O. Bondarenko, B. Slutsky, A. Mizrahi, L. Feng, V. Lomakin,

and Y. Fainman, “Room-temperature subwavelength metallo-dielectric lasers,” Nat.

Photonics, vol. 4, pp. 395, 2010.

[16]K. Yu, A. Lakhani, and M.C. Wu, “Subwavelength metal-optic semiconductor nanopatch

lasers,” Opt. Express, vol. 18, pp. 8790, 2010.

[17]M. W. Kim, and P. C. Ku, “Lasing in metal-clad microring resonator,” Appl. Phys. Lett.,

vol. 98, pp. 131107, 2011.

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Chapter 2 Experimental Instruments and Methods

2.1 Electron Beam Lithography and Scanning Electron Microscope (SEM) Electron Beam Lithography System

Electron beam lithography, usually call as E-beam lithography, is a technique to utilize a

beam of electrons to exposing the photo resist on the surface of a material, then selectively

removing the photo resist on the film to transfer the pattern we want onto the film. This could

break the diffraction limit which constrains conventional photo lithography technique, and

shows a promising chance to fabricate device in nano-scale.

The E-beam lithography system usually consists of an electron gun as source of electron,

lenses for focusing, stage for moving the sample precisely under the electron beam, a beam

blanker to control the exposure time of electron beam and a computer to control the whole

system and the pattern. Fig. 2.1.1 shows the schematic diagram.

For most of E-beam lithography system used for commercial applications are very

expensive; therefore, for academic purpose, people usually convert an electron microscope

into an E-beam lithography system with a relatively low cost. For thesis, we use an E-beam

lithography system converted from a scanning electron microscope using JSM-6500 made by

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Scanning Electron Microscope

Scanning electron microscope (SEM) is one of the most important equipment for people

to observe objects in nano-scale. The electrons interact with atoms that make up the sample

producing signals that contain information’s about itself. Moreover, preparation of the

samples for SEM is relatively easy due to the fact that SEM only require the sample to be

conductivity. The combination of higher magnification, larger depth of focus, greater

resolution, and ease of sample observation makes the SEM becomes one of the most widely

used equipment used for commercial and research purposes. Fig. 2.1.3 shows the JSM-7000F

made by JEOL.

2.2 Dry Etching Process and E-gun Evaporation Dry Etching Process

Dry etching process is critical for scientists to fabricate the device according to their

plan, and there are two types of etching processes: wet etching and dry etching. Dry etching

process uses plasma to etch the semiconductor material and it is a kind of anisotropic etching

process. The linewidth of dry etching process is smaller than wet etching, therefore, dry

etching process gradually replace wet etching process after 1980.

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diffuse to chamber under ultra-low pressure. Second, when the pressure is stable, plasma is

produced by RF frequency. Third, the radicals produced by bombardment of high speed

electron would diffuse to the wafer and attach to its surface. Fourth, with the help of ion

bombardment, these radicals would react with the atoms on the surface and form by-product

as gas. At the end, these volatile by-products would then leave the surface of wafer and

discharge from chamber. Fig. 2.2.1 shows the inductively coupled plasma and reactive ion

etching (ICP-RIE) system used to etch Si3N4 layer to transfer the patter from PMMA layer,

and Fig. 2.2.2 shows the ICP-RIE equipment used to etch GaN layer.

E-gun Evaporation

E-gun evaporation, also called as E-beam evaporation, is one kind of physical vapor

deposition (PVD). The difference of E-gun evaporation is that it uses electron beam to heat

up the source material. The advantage of E-gun evaporation is that it could heat only one

small part of the surface of the source material. This will reduce the energy consumption

used by PVD. Therefore, it is a common way to use E-gun evaporation to deposit metal on to

the device.

For an E-gun evaporation system, it consists of an electron beam evaporation gun, a

system controller, power supply, evaporation material, its crucibles, and sample to be coated.

All the processes are conducted under very low pressure, in a vacuum chamber. Fig. 2.2.3

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2.3 Micro-Photoluminescence (μ-PL) System

Micro-Photoluminescence is an advanced Photoluminescence system to measure

sample in micrometer even nanometer scale. The spot size of the light source for μ-PL

system has shrunk to micrometer scale to observe the optical properties of sample. In our

μ-PL system, we use Nd:YVO4 laser with lasing wavelength 355nm as a pumping source,

and the spot size is about 50 μm, frequency of the laser is 1 kHz, and the pulse width is

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Fig. 2.1.1 Schematic Diagram of E-beam Lithography System.

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Fig. 2.1.3 JSM-7000F SEM System.

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Fig. 2.2.2 ICP-RIE System (SAMCO RIE-101PH).

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Chapter 3 Lasing in Metal-coated GaN Nanostripe at Room Temperature

3.1 Epitaxial Characteristics and Fabrication Process of Metal-coated GaN Nanostripe

We use undoped GaN layer grown on C-plane (0001) sapphire substrate as gain

medium. The thickness of the undoped GaN layer is about 2μm. Metal-organic chemical

vapor deposition (MOCVD) system (EMCORE D-75) is used to grow undoped GaN layer

on the polished optical-grade C-plane (0001), 2 inches sapphire substrate. Trimethygallium

(TMGa) and Ammonia (NH3) were used as the Ga and N sources respectively.

First, a thermal cleaning process was carried out at 1080℃ for 10 minutes in a stream

of hydrogen ambient before the growth of epitaxial layers to clean the sample surface.

Second, the 30nm thick GaN nucleation layer was first grown on the sapphire substrate at

530℃, and at the end the 2μm thick undoped GaN layer was grown on it at 1040℃.

After we finish the preparation of sample, we adopt some fabrication processes to

complete our device. The complete process flow chart for our device is shown in Fig. 3.1.1.

First, we deposit 300nm thick Si3N4 layer on the undoped GaN layer as an etching mask for

the following E-beam lithography process. Second, for E-beam lithography process, we use

spin coater to put a thin polymethylmethacrylate (PMMA) on the sample, then the nanostripe

pattern was defined using E-beam lithography system. After this, we use ICP-RIE dry

etching system to transfer the pattern on the PMMA layer onto the Si3N4. And then transfer

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3.1.2 shows the SEM image of nanostripe from top view and angled view after these

processes.

Third, before we start to do the next step, we use wet etching methods to clean our

sample, washing away the particles created in the previous dry etching processes to promote

the performance of our device. After this, we use plasma enhanced chemical vapor

deposition (PECVD) to deposit a thin SiO2 layer on the GaN nanostripe for just 20nm. At

the end, we use E-gun evaporation to deposit a 60nm thick aluminum layer to complete our

device. The SEM image of our device after the deposition of dielectric layer and metal is

shown in Fig. 3.1.3 in angled view. The detail recipe of each process is shown in the

following paragraphs, and the schematic diagram of metal-coated GaN nanostripe is shown

in Fig. 3.1.4. PECVD (SAMCO PD220) z Si3N4 film deposition: SiH4/Ar: 20sccm NH3:10sccm N2:490sccm Temperature: 300℃ RF Power: 35W Pressure: 100Pa

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Time: 31min. for 300nm thick Si3N4

z SiO2 film deposition:

SiH4/Ar: 25sccm N2O:500sccm N2:250sccm Temperature: 250℃ RF Power: 35W Pressure: 120Pa

Time: 1min. for 20nm thick SiO2

ICP-RIE (Oxford Plasmalab System 100)

z Si3N4 film etching: Ar/O2: 5sccm CHF3: 50sccm RF Power: 150W Pressure: 7.5x10-9Torr Temperature: 20℃

Time: 3min. 40sec. to etch 300nm thick Si3N4 film

ICP-RIE (SAMCO RIE-101PH)

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Cl2: 25sccm

Ar: 10sccm

ICP Power: 200W

Bias Power: 200W

Pressure: 0.33Pa

Time: 55sec. to etch 500nm thick GaN film

E-beam Lithography System (JEOL JSM-6500)

z Spin coating use PMMA (A5)

First step: 1000 rpm for 10sec.

Second step: 3500 rpm for 25sec.

z Hard bake: hot plate 180℃, 90sec.

z Exposure:

Beam voltage: 25KeV

Dosage: 1.4~1.7 (point does)

z Development: MIBK: IPA (1:3) 70sec.

z Fixing: IPA 40sec.

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z Source: Aluminum

z Pressure: 3x10-6Torr

z Current: 170mA for first 5nm

200mA for the rest 55nm

3.2 Lasing Characteristics of Metal-coated GaN Nanostripe at Room Temperature

To explore experimentally about our device, we use the micro-Photoluminescence

system mentioned in the previous chapter to measure its characteristics. We pump our device

directly from the metal side to avoid the absorption of undoped GaN layer beneath our

device, if we choose to pump the device from the back of the wafer.

First, to ensure that the lasing action is truly originated from our nano structure, we use

a He-Cd 325nm continuous-wave laser to pump the flat region of undoped GaN layer, before

and after the deposition of dielectric layer and metal to see the Photoluminescence (PL)

spectrum of them. As shown in Fig. 3.2.1, the peak wavelength of undoped GaN layer is

about 362nm. However, the spectrum after the deposition of SiO2 and aluminum layer is

totally different, the peak around 362nm no longer exists because the energy from 350nm to

380nm had been totally absorbed by shielding layer. Therefore, we could use Fig. 3.2.1 to

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PL spectrum of the metal-coated nanostripe below (black) and above (red) the threshold

power density has been presented. This device has a single lasing mode at 370nm and the

difference between two spectrums further ensure that we observe a lasing action in our

structure. From the light-in light-out curves of this mode shows in Fig. 3.2.3 and the

linewidth of the lasing mode, we could observe linear behavior after soft turn-on which

indicates the lasing action, and also the narrowing linewidth shows that the transition from

spontaneous emission to stimulated emission of the optical mode. These evidences prove the

lasing action in our metal-coated GaN nanostripe. The threshold pump power density is

about 0.055kW/cm2 (55mJ/cm2) and the quality factor estimated by the wavelength to

linewidth around the transparency is 150. Compared to the sample without metal or SiO2

shielding layer, the high thermal conductivity and high reflectivity of aluminum make

measurement easier and increase the possibility of lasing at room temperature. Moreover, the

SiO2 layer would passive the surface roughness created by the dry etching processes and

therefore reduce the nonradiative center on the surface of the nanostripe, and it reduce

energy loss if the metal layer is directly coated on to the gain medium, this would reduce the

absorption and lower the threshold gain of this device, making lasing action possible [1].

The nanostripe without shielding layer exhibits high optical loss and lower Q value. It was

for these reasons that we can’t observe lasing action in uncoated GaN nanostripe. The

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metal-coated device is that aluminum has a higher reflectivity which could show a better

optical confinement at UV wavelength region compared with silver [2], even aluminum is

also quite lossy at this wavelength region. Also, the adhesive ability to SiO2 is better for

aluminum than gold.

3.3 Simulation Results and Discussion

To get a better understanding of our experiment results, we use finite-element-method

(FEM) to simulate the optical mode in our metal-coated nanocavity. The simulation model

consists of a sapphire layer, an undoped GaN layer, a thin SiO2 layer, and an aluminum

shielding layer. The refractive indexes of aluminum and undoped GaN layers established by

Peng and Piprek[3], and Rakic et al. [4], with the refractive index of the SiO2 are 1.46. This

model also includes a perfectly matched layer surrounds the nanocavity to absorb redundant

signal which might reflect back to the metal-coated nanocavity, thereby influencing the

simulation result. Fig.3.3.1 shows the simulated optical mode profiles for a GaN nanostripe

cavity with or without the shielding layers. In Fig.3.3.1 (a), the model incorporating metal

and SiO2 layers had an optical mode well confined within the nanostripe, demonstrating a

clear standing wave pattern with 3.5 nodes. However, for the nanostripe without shielding

layers in Fig. 3.3.1 (b), the optical mode leaks into the region of air with a standing wave

pattern lacked uniformity, compares with the metal-coated nanostripe. The wavelength of the

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ten times larger than the case in Fig. 3.3.1 (b). This illustrates the difficulty of achieving

GaN nanostripe lasing without dielectric and metal layer coated on it. The small differences

between the experimental and simulation results might comes from the imprecise fabrication

processes and the imprecision of material indices used in the simulation model. From the

discussion above, we confirmed that the metal shielding layer in this structure plays an

important role in lasing at room temperature. And also, from the thermal conductivity of

aluminum (237W/m-k) and air (0.025W/m-k), it indicates that under room temperature

pulsed condition, metal-coated nanostripe has a better opportunity to observe lasing action

before it breaks down than the nanostripe without shielding layers.

To further improve the performance of this device, vertical confinement would be the

key issue. Distributed Bragg Reflector (DBR) would significantly reduce the energy loss

from the bottom of the nanostripe as shown in Fig. 3.3.1 (a). For example, C. Y. Lu et al.

demonstrated this idea by an n-type DBR with metal coated on the microrod [5]. The device

operated under room temperature continuous-wave condition, and showed a high thermal

resistance. This would form a three dimensional cavity which would definitely improve the

performance of the device and even operate under electrically pumped condition. Also,

heterostructure with precise calculation to confine the optical field in the active region would

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Moreover, we adopt effective index method to calculate the band diagram of this

structure. The effective index method is an efficient method to analysis rectangular

waveguide structure, which is really common structure for semiconductor laser. The ridge

waveguide structure has been split into different part first, and calculates the effective

indexes of each part, then combined these parts together again and calculate the true

effective index of this structure. A representation schematic diagram is shown in Fig. 3.3.2.

This way put the complex two dimensional calculations into one dimensional, and still keeps

a high accuracy with the experimental result. In our case, we split our nanostripe structure

into three parts as shown in Fig. 3.3.3 and then calculate the band diagram. The TE mode

band diagram is shown in Fig. 3.3.4, as you could see, there is only one waveguide band in

this structure. Moreover, around 370nm there is waveguide mode quite fit to our

experimental result. We believe that the single mode lasing we observe in the experiment is

a combination of TE waveguide mode with surface plasmon mode from the Al/SiO2/GaN

interfaces. The calculation helps us to clarify the observation result and further prove that

there would be only one optical mode with a clear standing wave pattern confine in the

nanostripe, which makes lasing action possible.

3.4 Summary

In sum, we demonstrated lasing in metal-coated GaN nanostripe at room temperature

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was about 370nm and the threshold power density is about 0.055kW/cm2. The quality factor

was about 150 estimated from the experimental result. FEM simulation result showed the

importance of aluminum layer coated on the nanostripe in the lasing action, and from

effective index method, the single mode lasing was confirmed by the band diagram of the

nanostripe structure, which had only one waveguide band around 300nm to 400nm

wavelength region. From experimental and simulation results, the GaN nanostripe without

metal and SiO2 shielding layers was really hard to achieve lasing at room temperature with

the width of the nanostripe only 500nm due to its poor optical confinement and worse

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Fig. 3.1.2 SEM Image of Metal-coated GaN Nanostripe (a) Top View of the GaN

Nanostripe before Deposition of Shielding Layers. (b) Angle View of One side of the GaN

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Fig. 3.1.3 Angle View of the GaN Nanostripe after the Deposition of SiO2 and

Aluminum Layers.

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Fig. 3.2.1 PL Spectrum of Undoped GaN Layer with and without Metal and Dielectric Shielding Layers.

360

370

380

390

400

410

420

0.0

0.2

0.4

0.6

0.8

1.0 Intensity (a.u.)

Wavelength (nm)

0.015kW/cm

2 0.065kW/cm

2

Fig. 3.2.2 PL Spectrum of Metal-coated GaN Nanostripe Above (Red) and Below (Black)

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0.00

0.02

0.04

0.06

0.08

0.0

0.2

0.4

0.6

0.8

1.0 Pump Power Density (kW/cm

2

)

Intensity (a.u.)

0

2

4

6

8

10 Linewidth (nm)

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(a)

(b)

Fig.3.3.1 (a) The Ez Mode Profile of the Nanostripe without Shielding Layers. (b) The

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y

z

Fig. 3.3.2 Schematic Diagram of the Nanostripe Model in Effective Index Method.

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References:

[1] A. Mizrahi, V. Lomakin, B. A. Slutsky, M. P. Nezhad, L. Feng, and Y. Fainman, “Low

threshold gain metal coated laser nanoresonators,” Opt. Lett., vol. 33, pp. 1261, 2008.

[2] W. M. Rohsenow and H. Choi, “Heat Mass and Momentum Transfer,” Prentice Hall,

New York, (1961).

[3]T. Peng, and J. Piprek, “Refractive index of AlGaInN alloys,” Electron. Lett., vol. 32, pp.

2285, 1996.

[4]A. D. Rakic, A. B. Djurisic, J. M. Elazer, and M. L. Majewski, “Optical properties of

metallic films for vertical-cavity optoelectronic devices,” Appl. Opt., vol. 37, pp. 5271,

1998.

[5]C. Y. Lu, S. W. Chang, S. L. Chuang, T. D. Germann, and D. Bimberg, “Metal-cavity

surface-emitting microlaser at room temperature,” Appl. Phys. Lett., vol. 96, pp. 251101,

2010.

[6]M. T. Hill, “Status and prospects for metallic and plasmonic nanolasers,” J. Opt. Soc. Am.

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Chapter 4 Lasing in Metal-coated GaN Nanoring at Room Temperature

4.1 Epitaxial Characteristics and Fabrication Process of Metal-coated GaN Nanoring