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Growth of GaN Nanocolumns and Their Growth of GaN Nanocolumns and Their Coalescence Overgrowth Using Metalorganic Coalescence Overgrowth Using Metalorganic

Chemical vapor Deposition and the Chemical vapor Deposition and the Characterization StudyCharacterization Study

以有機金屬氣相沉積法從事氮化鎵奈米柱生長和接合再生長以及其特性研究

研究生：唐宗毅 (Tsung-Yi Tang)指導教授：楊志忠博士 (Dr. C. C. Yang)

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Special Recognition

XRD: Wen-Yu Shiao TEM: Yung-Sheng Chen

33

Acknowledgements

Advisor: C. C. Yang Nanoimprint lithography: Epistar

Corporation MBE: Dr. Kent Averett Raman measurement: Hsu-Cheng Hsu CL measurement: Wei-Chao Chen PECVD: Cheng-Hung Lin and Kun-Ching

Shen

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Outline

IntroductionIntroduction Limiting Factors for the Nitride-based LED Limiting Factors for the Nitride-based LED

DevelopmentDevelopment Methods of Reducing Threading Dislocation DensityMethods of Reducing Threading Dislocation Density Motivations of the ResearchMotivations of the Research Overview of Nitride Nanocolumn (NC) or Nanowire Overview of Nitride Nanocolumn (NC) or Nanowire

GrowthGrowth Part 1:Part 1: MOCVD Overgrowth on MBE-grown GaN NCsMOCVD Overgrowth on MBE-grown GaN NCs Part 2:Part 2: MOCVD Overgrowth on MOCVD-grown GaN NCsMOCVD Overgrowth on MOCVD-grown GaN NCs Conclusions Conclusions

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Because of the 36 % lattice mismatch between GaN and sapphire Because of the 36 % lattice mismatch between GaN and sapphire substrate, the density of threading dislocation in GaN is too high (10substrate, the density of threading dislocation in GaN is too high (1099 – – 10101010 cm cm-2-2))

Because of the 11 % lattice mismatch between GaN and InN, high-Because of the 11 % lattice mismatch between GaN and InN, high-indium incorporation is difficult and the internal quantum efficiency is indium incorporation is difficult and the internal quantum efficiency is low when the indium content is high (green-red range).low when the indium content is high (green-red range).

Threading dislocations

Limiting Factors for the Nitride-based LED Development

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Methods of Reducing Threading Dislocation DensityEpitaxial Lateral Overgrowth

APL 71, 2639 (1997)

Facet-controlled ELOGJCG 221, 316 (2000)

PendeoepitaxyAPL 75, 196 (1999)

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Methods of Reducing Methods of Reducing Threading Dislocation DensityThreading Dislocation Density

Insertion of inter-mediate layer Patterned sapphire

Insertion of LT AlN and SiNJAP 99, 123518 (2006)

Multiple insertions of SiNJAP 101, 093502 (2007)

Cantilever epitaxyAPL 77, 3233 (2000)

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Motivations of the ResearchReduction of residual strain and threading Reduction of residual strain and threading

dislocation densitydislocation density

Journal of Crystal Growth 287, 500 (2006)

Dislocation-free NCs

Nano letters 6, 1808 (2006)

Strain-free NCs

Jpn. J. Appl. Phys. Vol. 40 (2001)

99

Motivations of the Research

Mechanisms for enhancing LED efficiency using nitride NCs

1. Lower dislocation density or higher crystal quality

2. Lateral strain relaxation for increasing indium content

3. Scattering for enhancing light extraction

4. High-quality GaN template with coalescence overgrowth

Substrate

Overgrown thin film

Threading dislocation Coalescence overgrowth of NC

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Overview of Nitride Nanocolumn or Nanowire Growth with MBE

• Self-organized growth

--- vertical to the template and high column density; but random distribution and non-uniform size distribution --- E. Calleja et al., Mater. Sci. Eng. B 82, 2 (2001). K. Kusakabe et al., Jpn. J. Appl. Phys. 40, L192 (2001). L. W. Tu et al., Appl. Phys. Lett. 82, 1601 (2003). J. E. Van Nostrand et al., J. Cryst. Growth 287, 500 (2006). R. Calarco et al., Nano Lett. 7, 2248 (2007).

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Coalescence overgrowth Coalescence overgrowth

InGaN/GaN LEDsInGaN/GaN LEDs

Jpn. J. Appl. Phys., Part 2 Jpn. J. Appl. Phys., Part 2 4040, L192 (2001), L192 (2001)J. Vac. Sci. Technol. BJ. Vac. Sci. Technol. B 25, 964 (2007) 25, 964 (2007)

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Patterned growth with focused-ion-beam or electron-beam lithography --- K. Kishino et al., J. Cryst. Growth 311, 2063 (2009). --- S. Ishizawa et al., applied physics express 1, 015006 (2008)

Selective-Area Growth of GaN nanocolumns on Si(111) substrates using nitrided Al nanopatterns by rf-plasma-assisted molecular-beam epitaxy

Ti-mask selective-area growth (SAG) by rf-plasma-assisted molecular beam epitaxy demonstrating extremely uniform GaN nanocolumn arrays

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Overview of Nitride Nanocolumn or Overview of Nitride Nanocolumn or Nanowire Growth with HVPENanowire Growth with HVPE

Self-organized growth of GaN nanorods Appl. Phys. Lett. 81, 2193 (2002)

Self-organized growth of InGaN nanorods Phys. Stat. Sol. (b) 241, 2802 (2004)

Blue emission from InGaN/GaN QW nanorod arrays

Appl. Phys. Lett. 87, 093115 (2005)

Fabrication of free-standing GaN Phys. Stat. Sol. (c) 4, 2268 (2007)

InGaN

GaN

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Overview of Nitride Nanocolumn Overview of Nitride Nanocolumn or Nanowire Growth with VLS or Nanowire Growth with VLS

methodmethod--- Most of them have random orientations and not vertical to the template---Impurity incorporation into NC or nanowire due to the use of a catalyst may degrade device performance.

phys. stat. sol. (b) 241, 2775 (2004)

(111) MgO

Nature materials 3, 524 (2004)

J. Am Chem. Soc.123, 2793 (2001)

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Overview of Nitride Nanocolumn Overview of Nitride Nanocolumn or Nanowire Growth with MOCVDor Nanowire Growth with MOCVD

• Top-down method

--- Electron-beam lithography has been used. --- The dry etching procedure normally generates defect states on the column surfaces.

• Patterned growth

Appl. Phys. Lett. 89, 233115 (2006). (interferometry lithography) J. Appl. Phys.100, 054306 (2006). (AAO lithography)

Nanotechnology 17, 1454 (2006)

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Overview of Nitride Nanocolumn Overview of Nitride Nanocolumn or Nanowire Growth with MOCVDor Nanowire Growth with MOCVD

Pattern growth- pulsed growth modePattern growth- pulsed growth mode Nano Lett. Nano Lett. 66, 1808 (2006), 1808 (2006)

Our considerationOur consideration High quality (surface defect High quality (surface defect

state, impurity incorporation)state, impurity incorporation) High densityHigh density Regular arrangementRegular arrangement

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OutlineOutline

IntroductionIntroduction Part 1:Part 1: MOCVD Overgrowth on MBE-grown GaN NCsMOCVD Overgrowth on MBE-grown GaN NCs

Sample Structure and Growth ConditionsSample Structure and Growth Conditions Characterization: PL, CL, AFM, SEM, XRD, and TEMCharacterization: PL, CL, AFM, SEM, XRD, and TEM SummarySummary

Part 2:Part 2: MOCVD Overgrowth on MOCVD-grown GaN NCsMOCVD Overgrowth on MOCVD-grown GaN NCs Conclusions Conclusions

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MBE-grown GaN Nanocolumns MBE-grown GaN Nanocolumns TemplateTemplate

Si (111)

NCs (810OC)

AlN (710OC)

Column diameter: 100nmColumn density: 109 /cm2

Grown by Dr. Kent Averett

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Pressure: 200 torrTMG flow rate: 17 mol/minNH3 flow rate: 1000 sccmV/III ratio: 2600Growth rate: 0.44 nm/secTemperature: 800 OC, 900 OC, 1000 OCThickness: 700 nm, 2.5 m

Si

Overgrown GaN

NCs

AlN

Growth Parameters of Growth Parameters of MOCVD-Overgrown GaNMOCVD-Overgrown GaN

2020

SEM and CL ImagesSEM and CL Images

The overgrown layer shows relatively stronger emission when compared with that from the NCs.

The dark regions are always located around the boundaries of the domains. In other words, the optical property near the center of a domain is much better than that near its boundary.

Growth temperature: 1000OC, thickness: 2.5m

2121

Comparison between the Overgrown Sample and a GaN Thin Film (PL

measurement)

a high-quality GaN thin film Substrate: sapphireFWHM of the peak of (0002) XRD curve: 190 arcsecFWHM of the peak of (10-12) XRD curve: 296 arcsecThickness of GaN layer: 2-3m

The comparison shows that the overgrown sample has better optical quality than the GaN thin film.

Wavelength (nm)


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AFM and PL MeasurementsAFM and PL Measurements

20nm

-20nm

0

0 2 4 6m

An AFM image of 7 m x 7 m in dimensions demonstrating part of a hexagon. The difference in height between the maximum and the minimum, indicated with the two marks, is about 14nm, and the surface roughness is about 5.7nm.


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Two-beam X-ray DiffractionTwo-beam X-ray Diffraction (Conventional Measurement)

MOCVD overgrowth samples:A: 800 oC – 700 nm thick (1274 arcsec)B: 900 oC – 700 nm (1435)C1: 1000 oC – 700 nm (2653)C2: 1000 oC -- > 2.5 m (6245)

Comparison samples:GaN1: good GaN film – 2 m (201)GaN2: poor GaN film – 2 m (1012)

XRD results: courtesy of Wen-Yu Shiao

(0002) plane

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1m

1m

Sample A

Sample B

Sample C1

Sample C2

Cross-section SEM ImagesCross-section SEM Images

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Three-beam Depth-dependent X-ray Three-beam Depth-dependent X-ray Diffraction ResultsDiffraction Results

Three-beam X-ray diffraction geometry

Depth-dependent X-ray diffraction results

c-axis

C1

C2

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SummarySummary

Higher growth temperature leads to better crystal quality. For the growth temperature of 1000OC, the overgrown GaN layer even shows better optical properties than conventional GaN grown on sapphire.

The overgrown GaN layer shows stronger CL emission than nano-columns, which may be attributed to the relative high growth temperature in MOCVD growth process.

Hexagonal structures are observed on the surface. It is believed that the surface morphology can be improved by using regular and uniform NCs.

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OutlineOutline

IntroductionIntroduction Part 1:Part 1: MOCVD Overgrowth on MBE-grown GaN NCsMOCVD Overgrowth on MBE-grown GaN NCs Part 2:Part 2: MOCVD Overgrowth on MOCVD-grown GaN NCsMOCVD Overgrowth on MOCVD-grown GaN NCs

Overgrown Undoped GaNOvergrown Undoped GaN Overgrown QWs and LEDOvergrown QWs and LED SummarySummary

Conclusions Conclusions

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MOCVD Patterned Growth of GaN MOCVD Patterned Growth of GaN NanocolumnsNanocolumns

Sapphire

2m undoped GaN

80nm SiO2

Holes fabricated with nano-imprint lithography (courtesy of Epistar)

Sapphire

2m undoped GaN

time

NH3

TMGa

Flow rate

GaN NCs maintain their geometry after they emerge from the growth mask if the growth conditions are changed into a pulsed MOCVD growth mode before the NCs emerge from the growth mask.

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Growth Conditions NC growth

Continuous growth (5sec) Temperature: 1050OC Pressure: 100 torr V/III ratio: 1100

Pulsed growth mode Temperature: 1050OC Pressure: 100 torr Duration and flow rate of TMG: 20 sec and 12.5

mol/min Duration and flow rate of ammonia: 30 sec and 500

sccm Growth rate: 2 m/ hr

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Growth Conditions Coalescence overgrowth

Temperature: 1050OC Pressure: 200 torr V/III ratio: 3900 Growth rate: 1.3 m/ hr Thickness: 2 m

Four patterns are fabricated by nanoimprint lithography. The hole diameters of the four patterns are 250, 300, 450, and 600 nm, corresponding to center-to-center spacing sizes of 500, 600, 900, and 1200 nm, respectively.

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1 m

(a) (b)

100 nm

SiO2SiO2250 nm (hole)

300 nm (column)

Regularly Arranged GaN Nanocolumns - 1Regularly Arranged GaN Nanocolumns - 1

Template hole diameter: 250 nm

Hexagonal column size: ~300 nm

Center-to-center spacing: 500 nm

Clean NC bottom

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Hole diameter: 450 nm Column diameter: 500nm Separation: 900nm

Hole diameter: 600 nm Column diameter: 800nm Separation: 1200nm

Regularly Arranged GaN Nanocolumns - 2Regularly Arranged GaN Nanocolumns - 2

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(a)

1 m

(b)

1 m

I

II

III

(c)

SiO2 SiO2SiO2

Coalescence Overgrowth ResultsCoalescence Overgrowth ResultsHole diameter for column growth: 250 nm

The overgrown surface is quite smooth.The overgrown layer (I), the NC layer (II), and the GaN template layer (III) can be identified.The original column walls are depicted by the vertical dashed lines.

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CL and AFM Results

Overgrown layer

Template

Nanocolumns

1 m1 m

Overgrown layer

Nanocolumns

Template

Pit density: 2X107cm-2

Roughness: 0.411nm

AFM image

5 m*5m

Control sample

Pit density: 3X108cm-2

Roughness: 0.843nm

Hole diameter: 250 nm

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PL Measurements

354 357 360 3630.0

0.2

0.4

0.6

0.8

1.0

Nor

mal

ized

inte

nsity

Wavelength (nm)

Overgrowth NCs Control

10 K RT

For control sample, spectra at 10 K include one major peak around 355.5 nm due to donor-bound exciton (DBE) recombination and two minor peaks at 354.8 and 356 nm, corresponding to free exciton A (FE(A)) and acceptor-bound exciton (ABE), respectively. By using the proportionality factor of K = 21.2 meV/GPa, we find that at room temperature a stress of 0.66 GPa is built during the overgrowth process.


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Raman and XRD Measurements

NC is almost strain-free. Based on the stress-shift coefficient of 4.2 cm-1 GPa-1, one can estimate the stress built in the overgrown layer to give 0.67 GPa.The FWHMs of the control, NCs, and overgrowth samples are 220, 303, and 256 arcsec, respectively. The rocking curve of the NCs sample can be decomposed into two components, including the broader one from the NCs and the narrower one from the template beneath. different columns still have significant variations in crystal orientation even though they stem from the same GaN template.


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Comparisons of Internal Quantum Comparisons of Internal Quantum efficiencyefficiency

Sample E: GaN templateSamples A, B, C, D: NCs with hole sizes at 250, 300, 450, and 600 nmSamples AO, BO, CO,DO: Overgrowth samples with hole sizes at 250, 300, 450, and 600 nm

NCs Overgrown layers

10X 7X

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Comparisons of Dislocation DensityComparisons of Dislocation Density Based on a depth-dependent X-ray diffraction measurement technique

Edge and screw dislocation densities at the level of 107 cm-2 are achieved.

The lateral domain size has been significantly increased.

>10X

>3X

>3X

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Comparison of Dislocation Density, Internal Quantum

Efficiency, and Surface Roughness

Overgrowth sample

Surface roughness

(nm)

IQE(%)

Screw dislocation density (cm-2)

Edge dislocation density (cm-2)

Lateral domain size (m)

AO/A(250 nm)

0.411 6.7/9.9 3.09 x 107 5.04 x 107 2.24

BO/B(300 nm)

0.425 4.1/7.1 5.09 x 107 6.21 x 107 2.01

CO/C(450 nm)

0.473 3.1/4.2 8.11 x 107 9.24 x 107 1.73

DO/D(600 nm)

0.665 1.6/3.9 9.81 x 107 1.32 x 108 1.71

E (GaN template)

0.834 1.1 1.09 x 108 6.63 x 108 0.81

4040

c-axis

SiO2

NC

100 nm

templateSiO2

NCc-axis

200 nm

template

Cross-sectional TEM Images of NanocolumnsCross-sectional TEM Images of Nanocolumns

Threading dislocation is terminated at the bottom of a

hole when the hole size is small.

250-nm hole size 450-nm hole size

Two threading dislocations merge into one.

Courtesy of Yung-Sheng Chen

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16

5

43

2

7

500 nm

c-axis

SiO2

NC layer

template

overgrowth

c-axis

SiO2

1 m

NC layer

overgrowth

template

2

3

5

9

67

81

4

10

Cross-sectional TEM Images of Overgrowth Cross-sectional TEM Images of Overgrowth SamplesSamples

New dislocations are formed on the masks when they are narrow. Such dislocations may disappear along overgrowth.

250-nm hole size

600-nm hole size

Similar to ELOG

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Edge effect due to different Edge effect due to different Thermal Expansion Thermal Expansion

CoefficientCoefficient The termination or bending of a

TD around the hole region is caused by the strain, compressive or tensile, induced in GaN by the SiO2 mask due to their different thermal expansion coefficients.

TDs can easily penetrate into the overgrown layer through the large windows, leading to the poor-quality window regions and high-quality mask regions in the lateral dimension.

mask

Thin solid films 514, 344 (2006)

4343

OutlineOutline

IntroductionIntroduction Part 1:Part 1: MOCVD Overgrowth on MBE-grown GaN NCsMOCVD Overgrowth on MBE-grown GaN NCs Part 2:Part 2: MOCVD Overgrowth on MOCVD-grown GaN NCsMOCVD Overgrowth on MOCVD-grown GaN NCs

Overgrown Undoped GaNOvergrown Undoped GaN Overgrown QWs and LEDOvergrown QWs and LED SummarySummary

Conclusions Conclusions

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Sample StructuresSample Structures

Sapphire

Overgrown thin film

5 QWs

1 m

4m nGaN/1m uGaN

Well:3nmBarrier: 15nm

Undoped GaN

Sapphire

Overgrown thin film

5 QWs

120nm pGaN

Undoped GaN

Quantum well (QW) structure LED structure

Conventional GaN template

2 m

5 m uGaN

80 nm SiO2 mask

Growth temperatures for blue and green emission are 715 OC and 675 OC, respectively. The growth temperature of barrier is 850 OC.

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Blue LED Structure on Coalescence Blue LED Structure on Coalescence Overgrown GaN TemplateOvergrown GaN Template

IQE

A quick test shows ~80 % increase in output intensity.

Scattering of the residual NC pattern may also help in enhancing light extraction.

Wavelength: 460 nm

49.2%

20.1%

~80 %

L-I curves

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Green QW and LED on Coalescence Green QW and LED on Coalescence Overgrown GaN TemplateOvergrown GaN Template

9.1 % increase

IQE of green QW structure

14.1 %

10.4 %

IQE of the green LED

The reduction of dislocation density does not seem to significantly help in enhancing the efficiency of a green LED. The low miscibility between GaN and InN is the major cause for the low efficiency of a green LED.

Wavelength: 520 nm

21.2 %

12.4 %

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SummarySummary

The coalescence overgrowth has been successfully implemented on the NCs grown by patterned growth and pulsed growth mode. The measurement results of depth-dependent XRD, PL, and AFM show the superior properties of the overgrown thin film than those of a standard GaN thin film directly grown on sapphire substrate. Smaller NC and spacing size lead to higher overgrowth quality, including a lower TD density and a larger lateral domain size. We presented the emission enhancement results of the blue and green-emitting InGaN/GaN QW and LED structures based on NC growth and coalescence overgrowth. Significant enhancements (up to 80 % output intensity increase in the blue LED) were observed. For LED application, the TD density reduction in an overgrown GaN template could more effectively enhance the emission efficiency of a blue LED, when compared with a green LED.

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ConclusionsConclusions The coalescence overgrowth of MBE-grown

NC is demonstrated. The characterization results show superior properties of overgrowth sample compared to the control sample although hexagonal structures can be observed on the surface.

The coalescence overgrowth of MOCVD-grown NC is demonstrated. The overgrowth process improve the properties of GaN crystal. Smaller NC and spacing size lead to higher overgrowth quality.

Significant enhancement of output intensity of the overgrown blue LED is observed.