從奈米到微米結構設計以達成氮化銦鎵/氮化鎵元件之高效能光擷取/光萃取

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國立臺灣大學電機資訊學院光電工程學研究所碩士論文

Graduate Institute of Photonics and Optoelectronics College of Electrical Engineering and Computer Science

National Taiwan University Master Thesis

從奈米到微米結構設計以達成氮化銦鎵/氮化鎵元件之高效能光擷取/光萃取

Efficient Light Harvesting/Extraction Schemes Employing Structure Designs from Microscale to

Nanoscale for InGaN/GaN Devices

研究生: 何政翰 Cheng-Han Ho

指導教授：何志浩博士 Advisor:Jr-Hau He, Ph.D.

中華民國 101 年 6 月

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致謝

在研究所短短兩年時間，能完成這篇碩士論文及其餘的研究成果，我要特別

感謝我的指導教授何志浩老師、賴昆佑老師及林冠中學長，也要感謝實驗室的每

一位成員對我的幫助與給我的建議，這段時間能有你們的陪伴真的很好，不論是

課業、研究實驗及日常生活上，大家彼此互助，有困處也不吝於給予幫助，使我

們實驗室像個溫暖的家，謝謝大家。另外，也要感謝所有的口試委員，能特地撥

空前來給予我指導，使本篇論文內容更加地完整。

回想剛進研究所的前半年，實驗上遭遇了種種的不順利及不愉快，多虧了實

驗室政營學長、伯康學長的建議及鼓勵，才使我的實驗漸漸步上軌道，一步步的

從做實驗、模擬到寫出論文，也感謝老師平時在研究之外，教導我們做人處事應

有的態度及觀念，對我受益良多，也確實讓我學到了課本以外學不到的東西，我

想老師、學長教給我的想法觀念，應該會一輩子在我的生活、工作裡時時提醒、

督促著我，讓我不斷進步、成長。研究所的這兩年，我覺得是我求學生涯裡過得

最有價值的兩年，最後，感謝並祝福所有曾給予我幫助的老師、同學們。

政翰謹誌於台大

2012 年 6 月

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摘要

在本篇論文中，我們將先討論氮化鎵系的太陽能電池，接著為氮化鎵系的發光二極體，最後是我們的總結。

首先，在氮化銦鎵系的多重量子井太陽能電池上，利用自組裝的銀奈米小球當作蝕刻遮罩，去做反應式離子蝕刻，製做出二氧化矽奈米柱陣列。由於光捕捉效應及折射率的匹配(在空氣及元件間)，使此二氧化矽奈米柱陣列可有效地降低元件的表面反射率(從 330 至 570 奈米波段)。電池在模擬太陽光源(air mass 1.5G) 的照射下，其短路電流明顯提升，而轉換效率可增加 21 %。模擬軟體的分析也進一步証明此表面結構能改善電池的光伏特性。

第二，將太陽能電池的 p 型氮化鎵層製做成微米鐘的結構，也可以顯著的提升其轉化效率達 102 %之多。此微米鐘結構能降低元件表面的反射率，增加電池的光吸收能力，並提升短路電流及填充因子。此經由磊晶直接成長出微米鐘的方法，可有效的改善元件的光伏特性。

第三，二氧化矽奈米柱陣列/p 型氮化鎵微米鐘的分層結構被應用在氮化銦鎵的多層量子井太陽能電池上，以當作光擷取層。同樣以自組裝的銀球當作蝕刻遮罩來做反應式離子蝕刻，來將二氧化矽奈米柱陣列製作於 p 型氮化鎵微米鐘之上。由於此粗糙結構的光捕捉效應以及奈米柱具匹配的折射率，使得介面的菲涅耳反射(Fresnel reflection)能被更有效地降低。具此分層結構的電池表現出優異的光伏特性，能提升短路電流及填充因子，進而使轉換效率增加 1.47 倍。此外，元件光吸收能力的增加與以有限差分時域法 (finite-difference time-domain, FDTD)分析的結果相吻合。

最後，我們將此分層結構應用在 LED 上，發現能增加 LED 的出光強度。與表面未經粗化的 LED 相比，在 20mA 注入電流下，微米鐘 LED 出光強度增強 16.7 %，

而奈米柱/微米鐘 LED 則增強了 36.8 %之多。此結果歸因於粗化結構能使出射光

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散射並提供一等效折射率，來降低元件的內部全反射，進而提高光萃取率。此 LED 出光強度的增加也同樣可由有限差分時域法來分析得到。

關鍵字: 太陽能電池，氮化銦鎵/氮化鎵，反應式離子蝕刻，奈米柱，抗反射，

光擷取，微米鐘，發光二極體，內/外部量子效率，光萃取效率。

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Abstract

In this thesis, we will firstly focus on InGaN/GaN solar cells, and secondly we move to GaN/InGaN light emitting diodes. The final is our conclusion.

First, SiO₂ nanorod arrays (NRAs) are fabricated on InGaN-based multiple quantum well (MQW) solar cells using self-assembled Ag nanoparticles as the etching mask and subsequent reactive ion etching. The SiO₂NRAs effectively suppress the undesired surface reflections over the wavelengths from 330 to 570 nm, which is attributed to the light-trapping effect and the improved mismatch of refractive index at the air/MQW device interface. Under the air mass 1.5 global illumination, the conversion efficiency of the solar cell is enhanced by ~21 % largely due to increased short-circuit current from 0.71 to 0.76 mA/cm². The enhanced device performances by the optical absorption improvement are supported by the simulation analysis as well.

Second, InGaN-based multiple quantum well (MQW) solar cells (SCs) employing the p-GaN microdome were demonstrated to significantly boost the conversion efficiency by 102 %. The improvements in short-circuit current density (Jsc, from 0.43 to 0.54 mA/cm²) and fill factor (from 44 % to 72 %) using the p-GaN microdome are attributed to enhanced light absorption due to surface reflection suppression. The concept of microdome directly grown during SC epitaxial growth preserving mechanical robustness and wafer-scale uniformity proves a promising way in promoting the photovoltaic performances of SCs without any additional process.

Third, the hierarchical structure of SiO₂nanorod arrays/p-GaN microdomes was applied as a light harvesting scheme on InGaN-based multiple quantum well solar

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cells. Using self-assembled Ag nanoparticles as the etching mask and subsequent reactive ion etching, SiO2NRAs were fabricated upon the p-GaN microdomes. Due to the light trapping effect of the roughness and the improved match of refractive index by SiO2 nanorod arrays, the undesired Fresnel reflections are effectively suppressed.

Cells with the hierarchical surfaces exhibit excellent photovoltaic performances including enhanced short-circuit current densities and fill factor, and the measured conversion efficiency is enhanced by 1.47-fold. The improved light absorption in device is consistent with the finite-difference time-domain analysis.

Finally, we report the enhanced light extraction efficiency of the hierarchical structure, SiO2 nanorods/p-GaN microdomes, fabricating on InGaN/GaN LEDs.

Compared with conventional flat LEDs, the light output intensity of bare microdome LED presents an improvement of 16.7 % at 20 mA, yet it boosts to 36.8 % for SiO2

NRA/p-GaN microdome LED. The results are attributed to the scattering effect and the effective refraction indexes of the textured structures that reduce the total internal reflection, contributing to the most light extraction. The enhanced optical performances are supported by the improved light output power calculated by finite-difference time-domain analysis.

Keywords: Solar cell (SC), InGaN/GaN, Reactive ion etching (RIE), Nanorod, Antireflection, Light harvesting, Microdome, Light-emitting diode (LED), Internal/External quantum efficiency (IQE/EQE), Light extraction efficiency.

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List of Contents

口試委員會審定書...I 致謝...II 摘要...III Abstract...V List of Contents...VII List of Figures and Tables...IX Chapter 1 Introduction

Introduction ...1

Chapter 2 Experimental setup Experimental setup……….3

Chapter 3 An efficient light-harvesting scheme using SiO2nanorods for InGaN MQW solar cells 3-1 Introduction ...4

3-2 Experiment...6

3-3 Results and Discussion ...8

3-4 Summary...15

3-5 References…...16

Chapter 4 Microdome InGaN-Based Multiple Quantum Well Solar Cells 4-1 Introduction ...19

4-2 Experiment...21

4-4 Summary ...30

4-5 References...31

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Chapter 5 Efficient light harvesting scheme for InGaN-based quantum well solar cells employing the hierarchical structure: SiO₂ nanorods/p-GaN microdomes

5-1 Introduction ...34

5-2 Experiment...37

5-4 Summary ...46

5-5 References...47

Chapter 6 Light emission enhancement of GaN-based light -emitting diodes via the hierarchical structure: SiO₂nanorods/p-GaN microdomes 6-1 Introduction ...49

6-2 Experiment...52

6-4 Summary ...60

6-5 References...61

Chapter 7 Conclusion Conclusion...64

Cheng-Han Ho curriculum vitae………,…….………66

Publication list………..….……….67

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List of Figures and Tables

Chapter 3

Figure 3.1 Schematic of fabrication procedures for the antireflective nanostructures on InGaN MQW solar cells.

Figure 3.2 (a) Top-view and (b) 45-degree tilted-view SEM images of the SiO₂ NRAs.

Figure 3.3 Specular reflectance measured on the MQW solar cells with bare and SiO₂ NRA surfaces.

Figure 3.4 Time-averaged, normalized TE electric field distribution, |Ey|, simulated by FDTD analysis for the MQW solar cells with (a) bare and (b) SiO2

NRA surfaces with a 380-nm incident light. (c) Normalized optical power, integrated over the MQW region, as a function of times for two kinds of solar cells.

Figure 3.5 (a) EQEs and (b) J–V characteristics measured on the MQW solar cells with bare and SiO2NRA surfaces.

Table 3.1 Device characteristics of the MQW solar cells with bare and NRAs surfaces.

Chapter 4

Figure 4.1 45 degree-tilted SEM image of the MQW SCs with p-GaN microdomes.

The inset shows the cross-sectional SEM image.

Figure 4.2 Specular reflection measured on the MQW SCs with flat and microdome-like p-GaN surfaces.

Figure 4.3 Time-averaged and normalized TE electric field distribution simulated by FDTD analysis with two surface structures: (a) flat and (b) p-GaN microdomes. (c) Normalized optical power, integrated over the MQW region, as a function of times for the two kinds of SCs at 400 nm wavelength.

Figure 4.4 J–V characteristics measured on the MQW SCs with two kinds of surface structures.

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Figure 4.5 (a) EQE curves and (b) IQE curves for the SCs with two kinds of surface structures.

Figure 4.6 The absorption spectra of InGaN MQW SCs with and without microdome structures.

Table 4.1 PV characteristics of InGaN MQW SCs with two kinds of surface structures.

Chapter 5

Figure 5.1 Schematic of fabrication procedures for the SiO₂ nanorod/p-GaN microdome structure on InGaN/GaN SCs.

Figure 5.2 (a) and the inset are cross-sectional and 45 degree SEM images of p-GaN microdome device, respectively. (b) and the inset are cross-sectional and 45 degree SEM images of SiO2 NRA/p-GaN microdome device, respectively.

Figure 5.3 Specular reflection measured on the MQW SCs with planar, p-GaN microdome and SiO2NRA/p-GaN microdome surfaces.

Figure 5.4 Time-averaged and normalized TE electric field distribution, |Ey| , simulated by FDTD analysis with different surface structures, (a) Planar, (b) p-GaN microdomes, (c) SiO2NRAs/p-GaN microdomes with a 400-nm incident light. (d) Normalized optical power, integrated over the MQW region, as a function of times for the three SCs.

Figure 5.5 (a) J–V characteristics and (b) EQE curves measured on the MQW SCs with three surface conditions.

Table 5.1 Device characteristics of InGaN MQW SCs with three surface structures.

Chapter 6

Figure 6.1 (a) and the inset are 45-degree tilted and cross-sectional surface images of p-GaN microdome LED respectively. (b) and the inset are 45-degree tilted and cross-sectional surface images of SiO2NRA/p-GaN microdome LED respectively.

Figure 6.2 Light-output intensity versus injection currents (L－I curves) of flat, p-GaN microdome, and SiO2NRA/p-GaN microdome LEDs. The insets are the corresponding I－V characteristics and EL spectra at the 20 mA

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driving current, respectively.

Figure 6.2 Light-output intensity versus injection currents (L－I curves) of flat, p-GaN microdome, and SiO2NRA/p-GaN microdome LEDs. The insets are the corresponding I－V characteristics.

Figure 6.3 Time-averaged and normalized TE electric field distribution, |Ey|, simulated by FDTD analysis with different surface conditions at 460 nm wavelength, (a) Flat, (b) p-GaN microdome, (c) SiO2 NRA/p-GaN microdome, (d) Normalized optical power, integrated over the framed regions, as a function of times for the three LEDs.

Figure 6.4 Radiation patterns of the three LEDs under a 20 mA injected current.

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Chapter 1 Introduction

Recently, since much attention has been paid to the issue of energy shortage,

III-V based optoelectronic devices, such as solar cells (SCs) and light-emitting diodes

(LEDs), gradually become the promising elements with the remarkable potential of

energy reserve. Extensive researches have been devoted to the development of

InGaN-based SCs and LEDs. The light-harvesting and light-extracting layers are

crucial for eliminating optical loss for optical elements, and have many practical

applications to SCs and LEDs.

In this thesis, we firstly use an efficient method to have the SiO2 nanorods to

harvest much light. The approach by using process of self-assembled Ag

nanopraticles and RIE for the SiO2 nanorod fabrication is a useful and low cost

technique. To further enhance the solar-energy harvesting of SCs, approaches to

fabricate antireflection coatings or subwavelength structures with the additional

photo-lithography and wet/dry etching processes have been devoted. However, these

approaches are relatively complex and may further damage the devices. In this regard,

we secondly performed self-assembled p-GaN microdome structure which is directly

grown during the epitaxial growth of SCs. This simple and direct fabrication

preserves the nanostructured device with mechanical robustness, wafer-scale

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uniformity and stability. The microdomed structure improves the device performances

by reducing the internal piezoelectric ﬁeld and increasing the light absorption by light

trapping effect. Integrating microdomes with SiO2 nanorods is supposed to bring

about the most energy-harvesting ability for both SC and LED devices, which will be

demonstrated in the following contents. This would in turn lead to nitride-based SCs

and LEDs with high efficiency. We believe that this approach is of great interest for

researchers working in solar cell devices and III-Nitride optoelectronics device

technologies.

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Chapter 2 Experimental setup

The SEM images were recorded by a JEOL JSM-6500 field emission scanning

electron microscope (SEM). The specular reflection spectra were obtained using a

standard UV-visible spectrometer (JASCO ARN-733) at the incident angle of 5^o. An

external quantum efficiency (EQE) measurement was carried out under the

monochromatic illumination by a halogen lamp coupled to a monochromator. The

current density-voltage (J-V) curves of solar cells were measured with a Keithley

4200 source meter under the illumination of air mass 1.5 global solar simulator (100

mW/cm²). The electrical characteristics (I－V curves) of LEDs were measured with a

Keithley 2400 source meter. The electroluminescence (EL) spectrum was measured

using an Ocean Optics USB2000 spectrometer.

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Chapter 3 An efficient light-harvesting scheme using SiO

₂

nanorods for InGaN MQW solar cells

§ 3-1 Introduction

InGaN-based solar cells have recently drawn much research attention due to

many desired photovoltaic (PV) characteristics, including the potential to realize

nearly full absorption of solar spectrum, high absorption coefficients, and high

mobilities.^[1-3] In addition, InGaN alloys have superior radiation resistance, thermal

stability, and chemical tolerance.^[3] In consideration of the structure design for active

regions, multiple quantum wells (MQWs) consisting of the InGaN layers thinner than

the critical thickness on GaN not only prevent the formation of undesired crystal

defects, but also provide an additional control of light absorption through the

quantized energy levels.^[4]

A number of methods have been developed to improve the performances of

InGaN-based MQW solar cells.^[5-8]For example, Ag nanoparticles have been utilized

to induce plasmonic resonance on the device surface to enhance light absorption in

the InGaN active region.^[7] Kuwahara et al. reported that the PV performances of

InGaN solar cells with supperlattice active layers can be enhanced by optimizing the

barrier thickness.^[8]

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Recently, it is reported that subwavelength nanostructures exhibit superior

antireflective (AR) performances to their quarter-wavelength thin-film

counterparts.^[9,10] It can be clarified by the light trapping effect at short wavelength

regions and the effective medium effect at long wavelength regions ^[11,12], where the

nanostructures can be regarded as an effective medium with the effective refractive

index (neff) increasing gradually from air to device surfaces. As the incident light is

reflected at different depths in nanostructured layers, the suppression of the reflection

over a wide range of wavelengths can occur through destructive interferences among

the reflected waves. Periodic subwavelength nanostructures have been fabricated with

various techniques, such as the nanosphere lithography^[13,14]and the anodic aluminum

oxide templates.^[15,16] For the practical applications, the interests in the AR

nanostructures have been extended to disordered nanostructures using a simple yet

scalable method with low cost.[6,9,17,18]

In this study, we fabricated AR SiO2 nanorod arrays (NRAs) utilizing

self-assembled Ag nanoparticles as a mask and reactive ion etching (RIE) techniques

with photoresist-free and wafer-scale-uniformity capabilities. SiO2 NRAs effectively

reduce surface reflections over a wide wavelength range and thus InGaN MQW solar

cells employing the SiO2 NRAs generate additional photocurrents, corresponding to

the conversion efficiency enhancement of 21 % due to increased short-circuit current

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(Jsc) from 0.71 to 0.76 mA/cm². Simulation results based on finite-difference

time-domain (FDTD) analysis also indicate that the improved device performance is

due to the enhanced optical absorption in the MQW layers upon the application of

SiO2 NRAs. The proposed concept in this study is applicable to other optoelectronic

devices.

§ 3-2 Experiment

The MQW solar cells were grown by metal-organic chemical vapor deposition

on c-plane sapphire substrates. The layer structures consist of nine periods of

intentionally undoped In0.3Ga0.7N (3 nm)/GaN (17 nm) MQWs, sandwiched by a

2.5-μm n-type and a 0.2-μm p-type GaN layer. In content in the MQWs determined by

x-ray diffraction is around 30 %. Following the growth, transparent Ohmic contacts to

p-GaN were formed with indium tin oxide (ITO) deposited by the electron beam

evaporation. The 1×1 mm²diode mesas were then defined by chlorine-based plasma

etching. The contact scheme consists of fingered Ti/Al/Ni/Au metal grids deposited

on the ITO and the n-GaN.

Fig. 3.1 describes the fabrication procedure for SiO2NRAs. First, the SiO2/Ag

layers with the thickness of 300/15 nm were deposited on the ITO layer of

InGaN/GaN MQW solar cells by the electron beam evaporation [Fig. 3.1(a)]. For

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forming Ag nanoparticles, a thermal annealing process was carried out in the furnace

at 270 ℃ for 2 min [Fig. 3.1(b)]. Using these Ag nanoparticles as etching masks, the

SiO2layer was patterned by the RIE process with CHF3gas (30 SCCM) and rf power

of 90 W for 9 min [Fig. 3.1(c)]. After removing the remaining Ag nanoparticles by

HNO3, the SiO2NRAs were obtained, as shown in Fig. 3.1(d). It should be mentioned

that we had controlled Ag thickness from 5 nm to 30 nm, annealing temperatures from

250 ℃ to 350 ℃, and annealing times from 1 min to 30 min, which contribute to the

different geometries of self-assembled Ag nanoparticles. Changing etching times from

5 min to 10 min of RIE increased nanorod lengths. After experimental parameter

optimization, the best photovoltaic conversion efficiency of our solar cells devices can

be obtained using 15-nm-thick Ag films combining an annealing process at 270 ℃ for

2 min and subsequent RIE process for 9 min.

Figure 3.1 Schematic of fabrication procedures for the antireflective nanostructures forming Ag nanoparticles, a thermal annealing process was carried out in the furnace

Figure 3.1 Schematic of fabrication procedures for the antireflective nanostructures

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on InGaN MQW solar cells.

§ 3-3 Results and Discussion

Fig. 3.2(a)-(b) respectively reveal the top-view and 45-degree cross-sectional

images of the fabricated MQW device with SiO2 NRA surfaces. With Ag

nanoparticles as etching masks, the underlying SiO2 layer was selectively etched

using a CHF3 RIE. Because the metal Ag was slowly eroded away during the RIE,

slightly tapered NRAs can be created on the MQW solar cells through prolonged

etching. Using the CHF3 gas for RIE maintains a smoth surface finish. The average

lengths of the NRAs are around 230 nm, and the diameters are in the range of 50-100

nm, which are controlled by annealing times and temperatures for Ag thin films.

According to Fig. 3.2(a), the area density of the NRAs, defined as the number of

nanorods per unit area, is approximately 1.5×10¹⁰ cm^-2. The coverage of NRAs is

characterized by the fill factor, which is defined by the area ratio of NRAs to the

entire substrate surfaces using the top-view SEM images of Fig. 3.2(a). The fill factor

of the NRA surface is 0.68. The determined fill factor will be used later for

calculating effective refractive index (neff). It is worth mentioning that the dependence

of AR properties on geometric features for nanorods has been widely studied.^[9,19,20]In

particular, it is found that increasing the diameter and the length of the naorods leads

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to reduced surface reflectance and improved omnidirectionality. However, it is also

found that the over-lengthened nanorods may lead to the degraded Jsc despite their

lowest reflection, which is attributed to the excessive absorption of the lengthened

nanorods.^[6] Accordingly, an appropriate length of nanorods is needed for maximizing

the Jscenhancement. The robustness of the SiO2nanorods can be confirmed during the

fabrication process. For the removal of Ag nanoparticles after etching, the SiO2

nanorods were immersed in 70 % HNO₃solvent at 40 ℃－50 ℃ for a few minutes.

Fig. 3.2 shows that the geometries of the overall NRAs are still well preserved after

HNO3 treatment. These results indicate that the rod-like nanostructure can be

fabricated with controllable manner and good yield for achieving low-cost solar

devices.

Figure 3.2 (a) Top-view and (b) 45-degree tilted-view SEM images of the SiO2

NRAs.

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Fig. 3.3 is the specular reflection spectra with the wavelengths ranging from 330

to 570 nm. The surface reflectance of the SiO2NRA surface is much lower than that

on the bare surface for the entire studied wavelengths. The significantly low

reflectance at 330-450 nm is particularly important to the efficiency enhancement of

the MQW solar cell, which will be shown in the EQE measurements later. The

suppressed reflection by NRA structures is attributed to several effects. As the

incident wavelength is much higher than the geometric size of NRAs at the long

wavelength region, the reduced reflectance can be explained by the effective medium

theory. Due to the subwavelength dimensions and the refractive index (around 1.56 at

400 nm) of SiO2 [21]

, the SiO2 NRAs behave like an effective medium with the neff

between the refractive indices of air (~1) and ITO (~2.3) layer. neff of the NRAs can

be estimated by the equation^[22]:

n_eff={fn_SiO^q

2+ [1- fn_air^q ]}^1/q (1)

where q is 2/3,

SiO2

n and n_air are respectively the refractive indices of SiO₂ and air,

and f is the fill factor. The calculated neffis 1.37 with f = 0.68. Such low neff is rarely

found in natural materials, but is of great importance to achieve broadband AR at the

air/device interface. For the short incident wavelength region, the gaps between NRAs

lead to the light trapping effect. As the light impinges on the nanostructrued surface, it

diffracts to several beams with different diffraction angles, and then re-bounces

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between NRAs until the light propagates into MQW regions at a high angle from the

normal, which increases the opportunity of optical absorption by the MQWs due to

the increase of light propagation paths in MQW regions. This speculation will be

further demonstrated by PV measurements and simulations later. We note that the

subwavelength nanostructures result in the suppressed reflectance not only over a

wide range of wavelengths, but also a wide range of incident angles, which can be

clarified by the effective medium theory and the light trapping effect.^[23-25]

Figure 3.3 Specular reflectance measured on the MQW solar cells with bare and SiO2

NRA surfaces.

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In order to reveal the light propagation across the interfaces, the distributions of

electromagnetic fields within the device structures were simulated by FDTD analysis.

We modeled the devices with bare and SiO2NRA surfaces. Fig. 3.4(a)-(b) visualize

the time-averaged TE-polarized electric field intensity distributions, |Ey|, for the

MQW solar cells with two surface conditions at 380 nm as considering n and k of all

materials.^[26] All of the calculated values are normalized to the ones of the excitation

source. It can be seen that the field intensities inside the InGaN MQW region are

enhanced with SiO2NRAs. In the inset of Fig. 3.4(b), where the region of SiO2NRAs

is enlarged, one can see strong field intensity between nanorods, indicating that the

nanorods behave as effective scattering centers. The strong scattering within the

NRAs therefore prevents the incident waves bouncing back to the air and prolongs the

optical path, giving rise to the light-trapping effect. Fig. 3.4(c) shows the normalized

optical power, integrated over the MQW region, as a function of times for bare and

NRA surfaces. The steady-state power values for the cells with bare and NRA

surfaces are 0.744 and 0.824, respectively. The results indicate that the NRAs increase

the number of the photons reaching the MQW region, which benefits the conversion

efficiency of the MQW solar cell.

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Figure 3.4 Time-averaged, normalized TE electric field distribution, |Ey|, simulated by FDTD analysis for the MQW solar cells with (a) bare and (b) SiO2NRA surfaces with a 380-nm incident light. (c) Normalized optical power, integrated over the MQW region, as a function of times for two kinds of solar cells.

Fig. 3.5(a) presents the spectra of the EQE for the MQW solar cells with two

kinds of surface conditions, showing the influence of the light-harvesting SiO2NRAs

on PV performances. The EQE for MQW solar cells with SiO2NRAs is improved at

370-440 nm, which agrees with the reduced reflectance seen in Fig. 3.3. Fig. 3.5(b)

shows the J-V curves of the two kinds of solar cells. The PV data summarized from Figure 3.4 Time-averaged, normalized TE electric field distribution, |Ey|, simulated by FDTD analysis for the MQW solar cells with (a) bare and (b) SiO2NRA surfaces with a 380-nm incident light. (c) Normalized optical power, integrated over the MQW region, as a function of times for two kinds of solar cells.

shows the J-V curves of the two kinds of solar cells. The PV data summarized from

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the J-V curve is listed in Table 3.1. The agreement between the results in Fig. 3.3, Fig.

3.4, and Fig. 3.5 indicates that the light-harvesting SiO2 NRAs increase the optical

transmission and light propagation paths through the device surface, and hence

enhance the light absorption in the MQW region, giving rise to the additional

photocurrent of the MQW devices with NRA surfaces. The enhanced Jschence boosts

the conversion efficiency from 0.37 % to 0.45 %, which is an efficiency improvement

of ∼21 %.

Figure 3.5 (a) EQEs and (b) J–V characteristics measured on the MQW solar cells with bare and SiO₂NRA surfaces.

(a) (b)

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Table 3.1 Device characteristics of the MQW solar cells with bare and NRAs surfaces.

Note that FF stands for the fill factor of solar cells, which is defined as the ratio of the actual maximum obtainable power, to the product of J_sc and open circuit voltage (Voc).

AR Layers J_sc(mA/cm²) V_oc(V) FF (%) η (%)

Bare 0.71 1.95 27.28 0.37

SiO₂NARs 0.76 1.93 31.04 0.45

§ 3-4 Summary

Using self-assembled Ag nanoparticles as an etching mask and RIE method, we

successfully fabricated low-cost light-trapping SiO2 NRAs to improve the optical

absorption of InGaN MQW solar cells. The light-trapping SiO2NRA layer increases

the EQE of the solar cell mostly at 370-440 nm, corresponding to the improvement of

conversion efficiency by up to ~21 %. The superior AR performance of the SiO2

NRAs is attributed to the subwavelength dimensions and the nanorod-structured

geometry, effectively suppressing the surface reflection at the wavelengths from

330–570 nm via the light trapping effect and the graded neff, which have been

demonstrated by PV measurements and simulations. Presented concepts and

manufacturing techniques for light-harvesting nanostructures would be a viable way

to boost the efficiency for a variety of PV devices.

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§ 3-5 References

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[2] K. Y. Lai, G. J. Lin, Y.-L. Lai, J. H. He, Origin of hot carriers in InGaN-based quantum-well solar cells, IEEE Electron Dev. Lett. 32 (2011) 179–181.

[3] J. Wu, W. Walukiewicz, K. M. Yu, W. Shan, J. W. Ager, E. E. Haller, H. Lu, W.

J. Schaff, W. K. Metzger, S. Kurtz, Superior radiation resistance of In_1−xGa_xN alloys: full-solar-spectrum photovoltaic material system, J. Appl. Phys. 94, (2003) 6477–6482.

[4] K. W. J. Barnham, G. Duggan, A new approach to high-efficiency multi-band-gap solar cells, J. Appl. Phys. 67 (1990) 3490–3493.

[5] E. Matioli, C. Neufeld, M. Iza, S. C. Cruz, A. A. Al-Heji, X. Chen, R. M.

Farrell, S. Keller, S. DenBaars, U. Mishra, S. Nakamura, J. Speck, C. Weisbuch, High internal and external quantum efficiency InGaN/GaN solar cells, Appl.

Phys. Lett. 98 (2011) 021102.

[6] G. J. Lin, K. Y. Lai, C. A. Lin, Y.-L. Lai, J. H. He, Efficiency enhancement of InGaN-based multiple quantum well solar cells employing antireflective ZnO nanorod arrays, IEEE Electron Dev. Lett. 32 (2011) 1104–1106.

[7] I. M. Pryce, D. D. Koleske, A. J. Fischer, H. A. Atwater, Plasmonic nanoparticle enhanced photocurrent in GaN/InGaN/GaN quantum well solar cells, Appl. Phys. Lett. 96 (2010) 153501.

[8] Y. Kuwahara, T. Fujii, T. Sugiyama, D. Iida, Y. Isobe, Y. Fujiyama, Y. Morita, M. Iwaya, T. Takeuchi, S. Kamiyama, I. Akasaki, H. Amano, GaInN-based solar cells using strained-layer GaInN/GaInN superlattice active layer on a

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freestanding GaN substrate, Appl. Phys. Express 4 (2011) 021001.

[9] Y. C. Chao, C. Y. Chen, C. A. Lin, Y. A. Dai, J. H. He, Antireﬂection effect of ZnO nanorod arrays, J. Mater. Chem. 20 (2010) 8134–8138.

[10] K. Y. Lai, Y. R. Lin, H. P. Wang, J. H. He, Synthesis of anti-reﬂective and hydrophobic Si nanorod arrays by colloidal lithography and reactive ion etching, Cryst. Eng. Comm. 13 (2011) 1014–1017.

[11] P. Beckman, A. Spizzichno, The scattering of electromagnetic waves from rough surfaces, Pergamon, Oxford, 1963.

[12] P. B. Clapham, M. C. Hutley, Reduction of lens reflexion by the ‘moth eye’

principle, Nature 244 (1973) 281–282.

[13] L. Li, T. Y. Zhai, H. B. Zeng, X. S. Fang, Y. Bando, D. Golberg, Polystyrene sphere-assisted one-dimensional nanostructure arrays: synthesis and applications, J. Mater. Chem. 21 (2011) 40–56.

[14] X. D. Wang, E. Graugnard, J. S. King, Z. L. Wang, Large-scale fabrication of ordered nanobowl arrays, Nano Lett. 4 (2004) 2223–2226.

[15] Z. Fan, J. C. Ho, Self-assembly of one-dimensional nanomaterials for cost-effective photovoltaics, Int. J. Nanoparticles 4 (2011) 164–183.

[16] Z. Fan, R. Kapadia, P. W. Leu, X. Zhang, Y.-L. Chueh, K. Takei, K. Yu, A.

Jamshidi, A. A. Rathore, D. J. Ruebusch, M. Wu, A. Javey, Ordered arrays of dual-diameter nanopillars for maximized optical absorption, Nano Lett. 10 (2010) 3823–3827.

[17] Y. J. Lee, D. S. Ruby, D. W. Peters, B. B. McKenzie, J. W. P. Hsu, ZnO nanostructures as efﬁcient antireﬂection layers in solar cells, Nano Lett. 8 (2008) 1501–1505.

[18] H. C. Chang, K. Y. Lai, Y. A. Dai, H. H. Wang, C. A. Lin and J. H. He,

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Nanowire arrays with controlled structure profiles for maximizing optical collection efficiency, Energy Environ. Sci. 4 (2011) 2863–2869.

[19] Y. R. Lin, H. P. Wang, C. A. Lin and J. H. He, "Surface proﬁle-controlled close-packed Si nanorod arrays for self-cleaning antireﬂection coatings," J.

Appl. Phys. 106 (2009) 114310.

[20] Y. A. Dai, H. J. Chang, K. Y. Lai, C. A. Lin, R. J. Chung, G. R. Lin and J. H. He,

"Subwavelength Si nanowire arrays for self-cleaning antireflection coatings," J.

Mater. Chem. 20 (2010) 10924–10930.

[21] E. D. Palik, Handbook of Optical Constants of Solids, Academic Press.

[22] D. G. Stavenga, S. Foletti, G. Palasantzas, K. Arikawa, Light on the moth-eye corneal nipple array of butterflies, Proc. R. Soc. B 273 (2006) 661–667.

[23] J. Q. Xi, M. F. Schubert, J. K. Kim, E. F. Schubert, M. Chen, S. Y. Lin, W. Liu, J. A. Smart, Optical thin-ﬁlm materials with low refractive index for broadband elimination of Fresnel reﬂection, Nat. Photonics 1 (2007) 176–179.

[24] J. Q. Xi, J. K. Kim, E. F. Schubert, Silica nanorod-array films with very low refractive indices, Nano Lett. 5 (2005) 1385–1387.

[25] S. L. Diedenhofen, G. Vecchi, R. E. Algra, A. Hartsuiker, O. L. Muskens, G.

Immink, E. P. A. M. Bakkers, W. L. Vos, J. G. Rivas, Broad-band and omnidirectional antireﬂection coatings based on semiconductor nanorods, Adv.

Mater. 21 (2009) 973–978.

[26] S. Laux, N. Kaiser, A. Zoller, R. Gotzelmann, H. Lauth, H. Bernitzki, Room-temperature deposition of indium tin oxide thin films with plasma ion-assisted evaporation, Thin Solid Films 335 (1998) 1–5.

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Chapter 4 Microdome InGaN-Based Multiple Quantum Well Solar Cells

§ 4-1 Introduction

To date, InGaN-based MQW SCs have drawn much research attention due to

their favorable photovoltaic (PV) characteristics, including direct and tunable

bandgaps covering nearly the entire solar spectrum, high absorption coefficient, high

mobility, superior radiation resistance, and additional control of the light absorption

through the quantized energy levels.^1,2 Despite these promising characteristics, the

conversion efficiencies of InGaN-based MQW SCs are still unsatisfactory. To

improve the PV performances of InGaN SCs, much research has been reported.^3-7 In

order to mitigate the abrupt change in refractive indices from GaN (or ITO) to air,

which blocks a great portion of photons propagating through the air-GaN (or air-ITO)

interface,⁵a variety of roughened structures have been employed on the top of InGaN

MQW SCs.^3,6,7 Periodic subwavelength structures can be fabricated with various

techniques, such as the anodic aluminum oxide templates⁸ and the nanosphere

lithography.^9,10 For the practical applications, the interests in the subwavelength

structures have been extended to disordered structures.^3,11,12However, the processing

complexity and the mechanical robustness of the additional roughened structures on

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the top of SCs might be a limit to PV applications.

Among various roughened structures, the microdomes (or lens-like

microstructures) have been used for increasing the light escape cone in InGaN

light-emitting diodes (LEDs),^13-15resulting in improved light extraction efficiency and

engineered far-field.¹⁵ Recently, the use of colloidal microlens arrays had also been

implemented for increasing the light extraction in organic light-emitting diodes,¹⁶ as

well as improving the light collection in SCs.¹⁷ Additionally, it is found that

roughened p-GaN structures in InGaN LEDs can effectively reduce reflectance loss in

GaN,¹⁸ and provides an effective way to make p-metal contact deposition without

using a filling process with the insulating materials in the air gaps caused by

nanostructing/microstructing processes at the device surface, which is required for the

axial p-n nanorod devices to form electrical connection.¹⁹ Moreover, the roughened

p-GaN also leads to a reduction of piezoelectric ﬁeld and improved crystal qualities

due to the relaxation of in-plane strain.^20,21Roughening p-GaN is expected to benefit

the PV performances of InGaN MQW SCs since numerical studies have demonstrated

the detrimental effects of strain-induced piezoelectric polarization, forcing the

photocurrent to the opposite direction and thus resulting in low Jsc and open-circuit

voltage (Voc).²² Thus, the pursuit of microdome/lens-like structure via roughening

p-GaN is of great importance for increasing the critical angle and collection efficiency

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in InGaN SCs.

In this study, we realize the p-GaN microdome surfaces with the attempt to boost

the conversion efficiencies (η) of InGaN MQW SCs. Compared with flat p-GaN, the

p-GaN microdomes not only generate additional photocurrents (from 0.43 to 0.54

mA/cm²) by suppressing surface reflections considerably but also exhibit an improved

fill factor (from 44 % to 72 %), indicating the strain relaxation and the piezoelectric

ﬁeld reduction. Accordingly, the p-GaN microdome leads to a 102 % enhancement of

η. The optical enhancement is confirmed using the simulation based on

finite-difference time-domain (FDTD) analysis, reflection measurements and external

quantum efficiency (EQE) measurements. The internal quantum efficiency (IQE)

measurements indicate the possible improvement in photocarrier separation/collection

due to the strain relaxation. The concept of the microdome directly grown during SC

growth preserving mechanical robustness and wafer-scale uniformity without any

additional process represents a viable, promising path toward high-efficiency SCs.

§ 4-2 Experiment

The multiple quantum well (MQW) solar cells (SCs) were grown by

metal–organic chemical vapor deposition on c-plane sapphire substrates. The layer

structures consist of fifteen periods of intentionally undoped In Ga N (2.4

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nm)/GaN (14 nm) MQWs, sandwiched by a 2.5-μm n-type (Si-doped) and a 0.2-μm

p-type (Mg-doped) GaN layer. The free carrier concentrations for n-type and p-type

GaN are around 2×10¹⁸ cm^-3 and 5×10¹⁷ cm^-3, respectively. Ammonia (NH3),

trimethylgallium (TMGa) for n-GaN and p-GaN, triethylgallium for MQWs, and

trimethylindium were used as the precursors. Surface morphologies of p-GaN were

controlled by TMG flows and substrate temperatures during the p-GaN growth. In

device fabrication, ITO was deposited by electron beam evaporation on p-GaN to

form transparent ohmic contacts. The 1×1 mm² mesas were then defined by

chlorine-based plasma etching. The contacting scheme consists of fingered

Ti/Al/Ni/Au metal grids with the thicknesses of 20/400/20/2000 nm deposited on the

ITO and the n-GaN.

§ 4-3 Results and Discussion

Surface morphologies of p-GaN were controlled by TMGa flows and substrate

temperatures during the p-GaN growth. For flat p-GaN, TMGa flow rate was 40-50

μmol/min and substrate temperature was in the range of 950-1100 °C. To fabricate the

micro-roughened p-GaN, TMGa flow rate was increased to more than 55 μmol/min

and substrate temperature was decreased to lower than 920 °C. Fig. 4.1 shows

scanning electron microscopy (SEM) images of the MQW SCs with p-GaN

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microdome surfaces. The p-GaN microdomes are 530 250 nm in height and the base

of microdomes are 600370 nm in diameter. Fig. 4.2 is the specular reflection

spectra obtained on flat and microdome surfaces at the wavelengths ranging from 340

to 600 nm. One can see that the MQW SC with p-GaN microdome surfaces exhibits a

low reflectance for all studied wavelengths, demonstrating that surface reflectance can

be effectively suppressed by the p-GaN microdomes.

Figure 4.2 Specular reflection measured on the MQW SCs with flat and microdome surfaces. The p-GaN microdomes are 530 250 nm in height and the base

of microdomes are 600 370 nm in diameter. Fig. 4.2 is the specular reflection

Figure 4.2 Specular reflection measured on the MQW SCs with flat and microdome surfaces. The p-GaN microdomes are 530 250 nm in height and the base

of microdomes are 600 370 nm in diameter. Fig. 4.2 is the specular reflection

Figure 4.2 Specular reflection measured on the MQW SCs with flat and

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For the incident wavelengths comparable or shorter than the geometric sizes of the

microdomes, the low reflectance can be caused by the light trapping effect due to

severe light scattering. As light impinges on the structured surface, it is diffracted to

several beams with different diffraction angles. The diffracted beams re-bounces

between the p-GaN microdomes, which prevents the light from escaping back to air,

and thus increases the opportunity of optical absorption by the underneath material. It

should be mentioned that the light trapping effect results in the suppressed reflectance

over not only a wide range of wavelengths but also a wide range of incident angles.

23,24

In order to reveal the light propagation across the surface structures, the

distributions of electromagnetic fields within the device structures were simulated by

FDTD analysis. We modeled two kinds of device surfaces: flat p-GaN and p-GaN

microdomes. Fig. 4.3 visualizes the time-averaged TE-polarized electric field

intensity distributions for the MQW SCs with two surface conditions at the incident

wavelength of 400 nm. All of the calculated values in Fig. 4.3 are normalized to those

of the excitation source. It can be seen that the light propagating in the MQW region

for the SC with p-GaN microdomes is strongly and widely scattered, as compared

with the case of flat surface. One can also notice that strong fields are confined within

the p-GaN microdomes. The normalized optical power integrated over the MQW

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region as a function of times for the SCs with p-GaN flat and microdome surfaces is

shown in Fig. 4.3(c). The steady-state power values for the two devices are 0.65 and

0.75, respectively. These results indicate that the roughened p-GaN surface not only

helps light propagating across the interfaces but also widens the field distribution

within the device by increasing the light scattering on the surface. Strong light

scattering by the microdome surface is desired for improving the efficiencies of SCs

due to the increase in optical paths, which benefits the light absorption in MQW

regions.

Figure 4.3 Time-averaged and normalized TE electric field distribution simulated by FDTD analysis with two surface structures: (a) flat and (b) p-GaN region as a function of times for the SCs with p-GaN flat and microdome surfaces is

regions.

Figure 4.3 Time-averaged and normalized TE electric field distribution simulated by FDTD analysis with two surface structures: (a) flat and (b) p-GaN region as a function of times for the SCs with p-GaN flat and microdome surfaces is

regions.

Figure 4.3 Time-averaged and normalized TE electric field distribution simulated by FDTD analysis with two surface structures: (a) flat and (b) p-GaN

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microdomes. (c) Normalized optical power, integrated over the MQW region, as a function of times for the two kinds of SCs at 400 nm wavelength.

Fig. 4.4 shows the measured current density–voltage (J–V) curves of the SCs with

two kinds of surfaces. PV characteristics obtained by the J–V curves are listed in the

inset table of Fig. 4.4. One can clearly see that the microdome surfaces lead to

enhanced Jsc, which confirms that the microstructured surface enhances light

absorption. It is also found that the fill factors are enhanced for the SCs with

microdome surfaces suspectedly due to the strain relaxation and the piezoelectric ﬁeld

reduction caused by the roughened structure.^18,20,21 Accordingly, for the SCs with

p-GaN microdomes, η is increased from 0.43 % (with flat surface) to 0.87 %, which is

an improvement of 102 %. The results prove that the microdomes are effective in

boosting the PV performances of the MQW SCs. Note that previous simulation works

focusing on wire SCs reported that increasing the wire length could lead to a constant

decrease in Voc in spite of increases in η.^25,26 Consequently, a slight decrease in Voc is

also observed in the MQW SCs with microdome surfaces. We note that the

optimization considerations must be made in future with respect to the geometric

parameters of microdome structures due to the inverse behavior of Jscand Voc.^25,26

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Figure 4.4 J–V characteristics measured on the MQW SCs with two kinds of surface structures. The inset table shows PV characteristics of InGaN MQW SCs with two kinds of surface structures. FF is the fill factor of SCs, which is defined as the ratio of the actual maximum obtainable power, to the product of J_scand V_oc.

To gain insight into the correlation between Jsc enhancement, optical absorption,

and carrier separation/collection efficiency in the active layer, EQE and IQE spectra

were investigated. Fig. 4.5(a) presents the EQE spectra for the two kinds of SCs,

showing the influence of p-GaN microdomes on PV performances. EQEs of the SCs

with microdomes are mostly improved in the region of 360–450 nm, which reveals

the enhancement in light collection efficiency and agrees with the suppressed

reflection on the microdome surface shown in Fig. 4.2. The results also echo with

those revealed by FDTD analysis in Fig. 4.3. Since E. Matioli et al. had found the Figure 4.4 J–V characteristics measured on the MQW SCs with two kinds of surface structures. The inset table shows PV characteristics of InGaN MQW SCs with two kinds of surface structures. FF is the fill factor of SCs, which is defined as the ratio of the actual maximum obtainable power, to the product of J_scand V_oc.

those revealed by FDTD analysis in Fig. 4.3. Since E. Matioli et al. had found the

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possible IQE change in roughened SCs and Wierer et al. had demonstrated that SC

performances including IQE can be further improved by optimizing the barrier

thickness of QWs,^4,27 IQE measurements are then taken to evaluate the carrier

separation/collection occurring in the active region. EQE is converted into IQE

through

IQE = EQE/abs(λ) (1)

where abs(λ) the absorption spectra of the SCs, which is shown in Fig. 4.6. For

absorption spectrum measurements, by measuring reflection and transmission in the

spectral range from 360 to 540 nm, the absorption spectra can be obtained by

subtracting reflection and transmission from unity at every wavelength. Accordingly,

the obtained IQE in Fig. 4.5(b) may point out the possibility of IQE changes due to

p-GaN roughening. The variation in IQE after introducing p-GaN microdomes

indicates the possible change in carrier separation/collection efficiency resulted from

relaxation of in-plane strain and consequent reduction of piezoelectric ﬁelds during

p-GaN microdome growth.²² One should note that the possible inaccuracy in

determining the absorption spectra due to the difference in the surfaces for both SCs

might not enable one to provide accurate IQE comparison data for both devices. For

correctly determining the IQE for both devices, more detailed experiments in

providing conclusive comparison are required in future. In short, the p-GaN

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microdome results in the increases in EQE, which are attributed to the suppressed

interface reflection and possibly efficient separation/collection of photocarriers.

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§ 4-4 Summary

In conclusion, InGaN MQW SCs with microdome surfaces were fabricated. In

comparison with the flat surface, the microdome structures show improved fill factor

and Jsc, leading to the η enhancement by up to 102 %. The p-GaN microdomes exhibit

the enhanced optical absorption due to light trapping effects and possible

improvement of photocarrier separation/collection for the MQW SCs due to the strain

relaxation, resulting in enhanced EQE and IQE. With the advantages of the simple

process and the mechanical robustness, the microdomes grown during SC epitaxial

growth offer a viable way to boost solar efficiency of a variety of SCs.

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§ 4-5 References

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Chapter 5 Efficient light harvesting scheme for InGaN-based quantum well solar cells employing the

hierarchical structure: SiO

₂

nanorods/p-GaN microdomes

§ 5-1 Introduction

InGaN-based multiple quantum well (MQW) solar cells (SCs) have a direct and

tunable bandgap, which covers nearly the entire solar spectrum. In addition, they also

possess advantages as high absorption coefficients, high mobilities, and superior

radiation resistance, which allow the operation under harsh environments.^[1-3]

Employing MQW structures can provide the independent design between the

short-circuit current (Jsc) and open-circuit voltage (Voc) ^[4], and also avoid the

undesired trade-off between solar response and crystalline quality ^[4-6]. However, the

conversion efficiencies (η) of InGaN-based SCs are still limited. Methods for either

internal quantum efficiency (IQE) or external quantum efficiency (EQE)

improvements of SCs have been devoted to boost their photovoltaic (PV)

performances.^[7-9]

Recently, a variety of subwavelength structures (SWSs) have been demonstrated

to effectively suppress the undesired Fresnel reﬂections. These roughened structures

從奈米到微米結構設計以達成氮化銦鎵/氮化鎵元件之高效能光擷取/光萃取

國立臺灣大學電機資訊學院光電工程學研究所 碩士論文

National Taiwan University Master Thesis

從奈米到微米結構設計以達成氮化銦鎵/氮化鎵元件 之高效能光擷取/光萃取

Efficient Light Harvesting/Extraction Schemes Employing Structure Designs from Microscale to

Nanoscale for InGaN/GaN Devices

研究生: 何政翰 Cheng-Han Ho

指導教授：何志浩 博士 Advisor:Jr-Hau He, Ph.D.

中華民國 101 年 6 月

致謝

摘要

Abstract

List of Contents

List of Figures and Tables

Chapter 3

Chapter 4

Chapter 5

Chapter 6

Chapter 1 Introduction

Chapter 2 Experimental setup

Chapter 3 An efficient light-harvesting scheme using SiO

nanorods for InGaN MQW solar cells

§ 3-1 Introduction

§ 3-2 Experiment

§ 3-3 Results and Discussion

(a) (b)

§ 3-4 Summary

§ 3-5 References

Chapter 4 Microdome InGaN-Based Multiple Quantum Well Solar Cells

§ 4-1 Introduction

§ 4-2 Experiment

§ 4-3 Results and Discussion

§ 4-4 Summary

§ 4-5 References

Chapter 5 Efficient light harvesting scheme for InGaN-based quantum well solar cells employing the

hierarchical structure: SiO

nanorods/p-GaN microdomes

§ 5-1 Introduction

國立臺灣大學電機資訊學院光電工程學研究所碩士論文

從奈米到微米結構設計以達成氮化銦鎵/氮化鎵元件之高效能光擷取/光萃取

指導教授：何志浩博士 Advisor:Jr-Hau He, Ph.D.