Development of nanosphere lithography and its applications 1

v 2.3 Applications of colloidal crystal as templates 2.3.1 Introduction 2.3.2 Ordered spherical nanocavities 2.3.3 Creating nanostructures with multi-features in one step 2.3.4 Shape en

DEVELOPMENT OF NANOSPHERE LITHOGRAPHY AND ITS

APPLICATIONS

WANG BENZHONG (B.Sc, M Eng., Jinlin Univ)

A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY

DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING

NATIONAL UNIVERSITY OF SINGAPORE

2014

I would like to thank Dr Soh Chew Beng, Dr Zang Keran, Mr Rayson Tan Jen Ngee, Dr Dong Jianrong, and Mr Eng Cher Sing, for their helpful discussions and assistance in MOCVD growth

I would like to thank Dr Teng Jinghua, Dr Han Mingyong, Lau Jun Yong, Teo Siew Lang, Yong Anna Marie, Chew Ah Bian, Dr Liu Hong, Dr Liu Yanjun Dr Ke Lin, and Ang Soo Seng, for their assistance in fabrication processes and materials characterizations They are my colleagues in the Institute of Materials Research and Engineering

I would like to thank Mr Tan Beng Hwee and Ms Musni bte Hussain for their assistance in the use of facilities in the Centre for Optoelectronics, Department of Electrical and Computer Engineering, NUS

Special thanks go to Gao Hongwei for the help in many aspects

1.2 Review of nanofabrication technologies

1.2.1 Lithography with Photons

1.2.2 Lithography with Particles

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2.3 Applications of colloidal crystal as templates

2.3.1 Introduction 2.3.2 Ordered spherical nanocavities 2.3.3 Creating nanostructures with multi-features in one step 2.3.4 Shape engineering of nanostructures

2.3.4.1 Shape control through multi-cycle etching (MCE) 2.3.4.2 Shape control through 3D mask (3DM)

2.3.4.3 Shape control through dry etching mechanism 2.4 Summary

Reference Chapter 3 Nanosphere lithography applied to nano-

growth of III-V compounds

3.1 Introduction 3.2 Basics of MOCVD 3.3 GaN film grown on an array of Si (111) nanopillars

3.3.1 Introduction 3.3.2 Experiments 3.3.3 Results and discussion

3.4 Dual-sized (In)GaAs/GaAs nanobars grown by one step MOCVD

3.4.1 Introduction 3.4.2 Experiments 3.4.3 Results and discussion

3.4 Summary Reference Chapter 4 Nanosphere lithography applied to

formation of metal nanostructures 4.1 Introduction

4.2 Fabrication and characterization of metal

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nanostructures

4.2.1 Quasi-ordered 2D Au nanostructures with holes

4.2.1.1 Introduction 4.2.1.2 Experiments

4.2.1.3 Results and discussion

4.2.2 3D Au nanostructures formed by a 2D array of

nanospheres

4.2.2.1 Introduction 4.2.2.2 Experiments 4.2.2.3 Results and discussion

4.2.3 Ag nanoparticle superlattices formed by template

guided annealing

4.2.3.1 Introduction 4.2.3.2 Experiments 4.2.3.3 Results and discussion 4.3 Summary

Reference

Chapter 5 Nanosphere lithography applied to

LEDs for light extraction 5.1 Introduction

5.2 Effects of ordered surface nanostructures on LEDs

5.2.1 GaAs based red LEDs

5.2.1.1 Experiments 5.2.1.2 Results and discussion

5.2.2 GaN based blue LEDs

5.2.2.1 Experiments

5.2.2.2 Results and discussion

5.3 Effects of ordered surface Au nanostructures on

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Abstract

Nanosphere lithography (NSL) has been recognized as an inexpensive, high throughput and flexible technology to fabricate nanostructures used in many fields However, the weaknesses of this technology limit its widespread applications For example, a single layer of nanospheres is difficult to obtain

in a large area due to the nature of self-assembly; only limited shapes and arrangements of nanostructures are obtained due to the nature of spherical particles and its 2D hexagonal arrangement Here, I provide my solutions to overcome these weaknesses and widen its applications into the nano-growth of semiconductor, nano-formation of metals and in light extraction of light emitting diodes

This thesis addresses two aspects of work viz forming the nanostructures which could also be used as template and applying them to enhance the intensity and tailor the luminescence spectrum of semiconductors In forming the nanostructures, the main considerations are:

i) Position control of the nanosphere arrays A simple and efficient

method, combining photolithography and the self-assembly characteristics, has been devised to control an entire area filled with either single or double layer on an area as large as 400 µm2 to match the standard size of a LED In addition, the nanosphere arrays can be formed at specified areas of the substrates by modifying the surface properties

ii) Shape engineering of the nanostructures created through the

nanospheres A multi-step-etch technique has been invented to control

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the vertical profiles of the created nanostructures A 3D SiO2 network formed through a bilayer of polymer nanospheres acting as a mask is demonstrated to fabricate various 2D and 3D surface nanostructures Three applications of the nanostructures created through NSL are described They are: i) Fabrication of an ordered array of InGaAs/GaAs nanobars using the SiO2 template created by the one-step-NSL technique They are grown on the selected regions of a GaAs substrate by MOCVD Ordered arrays of InGaAs/GaAs nanobars with two-sized features are obtained

by a one-step MOCVD growth In addition, GaN films have been successfully grown on a nanopillar array created by NSL on a Si (111) substrate Strong enhancement (7 times) of PL intensity has been observed from the GaN film I have also demonstrated that surface energies play a main role in the initial growth stages on top of the nanopillars ii) Formation of 3D Au nanostructures on the template created through the multi-cycle-etching technique A honeycomb of holes in SiO2 template created through NSL and combined with thermal annealing, enables Ag nanoparticles to be formed on a

Si substrate Surface energy and the boundary formed by the SiO2 template play a main role in forming the well arranged Ag nanoparticles as deduced from temperature annealing studies These metal nanostructures show unique surface plasmon properties iii) Enhancement of light extraction in LEDs through NSL An increase in the output power by 2.4 times and 1.9 times is obtained for the red and blue LEDs, respectively Strong enhancement of output power is also observed for the red LEDs with a thin Au honeycomb nanostructure created through NSL

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List of Tables

Chapter 2

Table 2.1 Etching duration for PS spheres vs opening sizes of the

nanostructures created onto the SiO2 film

Table 2.2 Etching conditions for the wafers of wafer A, B and C

Chapter 3

Table 3.1 Growth parameters for the wafer of the Si nanopillar and flat Si

substrate, which was started from AlN growth

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List of Figures

Chapter 1

Fig 1.1 Quantum effects of matter

Fig 1.2 Basic outline of optical lithography processes The diagram shows

the optical radiation entering the system, which is then filtered by the

chromium mask The image is then projected on to the resist, and any

non-exposed material is removed during developing

Fig 1.3 Basic electron optical column in which the beam is formed The

image is formed on the resist, and the deflectors control the position of the

beam on the resist

Fig.1.4 (a) Schematic of the originally proposed NIL process (b) Scanning

electron microscopy (SEM) image of a fabricated mold with a 10 nm

diameter array (c) SEM image of hole arrays imprinted in poly(methyl

methacrylate) by using such a mold [34]

Fig 1.5 (a) side and (b) top-view of self-assembly of nanospheres

Fig 1.6 Nanosphere lithography used to create various nanostructures

Fig 1.7 Schematic illustration (a) and representative AFM image (b) of SL

PPA The AFM image was captured from a SL PPA fabricated with D = 542

nm nanospheres and d m = 48 nm thermally evaporated Ag metal after

removing the nanospheres; Schematic illustration (c) and representative AFM

image (d) of DL PPA The AFM image was captured from a SL PPA

fabricated with D = 400 nm nanospheres and d m = 30 nm thermally

evaporated Ag metal after removing the nanospheres [39] (e) and (f) show

the definition of the parameters of D, a and d ip for single and double layer

arrangement, respectively

Fig 1.8 Schematic illustration (a) and representative AFM image (b) of

nanoring and SL PPA fabrication The AFM image was captured from a

sample fabricated with D = 979 nm nanospheres and d m = 50 nm e-beam

deposited Ni metal after removing the nanospheres [39]

Fig 1.9 Schematic of the angle resolved deposition process (a) Samples

viewed at 0° (a), 30°, (b) and 45°, (c), respectively [46]

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Fig 2.2 (a) Photograph and (b) SEM images of the monolayered PS spheres

of 2 μm diameter self-assembled on a Si substrate (c) A SEM image of the

monolayered PS spheres of 400 nm diameter self-assembled on a Si substrate

Fig 2.3 (a) A cross-section viewed SEM image showing the bilayer arranged

PS spheres with 400 nm diameter (b) A top viewed microscopy image

showing the distribution of the bilayer array (green color) and the single layer

(purple color) formed by the 400 nm spheres

Fig 2.4 Microscopic images (up panel) of different layered nanospheres

assembled on glass substrates (as indicated in bottom panel)

Fig 2.5 (a) Schematic of the self-assembly within the wells (b) Microscopic

image of the patterned Si substrate

Fig 2.6 Microscopic images of 300 nm diameter nanospheres self-assembled

in device-sized wells (a) Monolayer formation at the conditions of

concentration, ~7 wt%, spin speed, 1900 rpm, (b) Monolayer formation at the

conditions of concentration, ~7 wt%, spin speed, 1800 rpm, (c) Bilayer

formation at the conditions of concentration, ~15 wt%, spin speed, 900 rpm

Fig 2.7 (a) Schematic illustration of the patterns created on a SiO2 surface by

photolithography from top (up) and side views (bottom) (b) A SEM image of

the monolayer arrays of 300 nm PS spheres formed at a SiO2 film with

micro-wells

Fig 2.8 (a) A SEM image of the 300 nm PS spheres selectively formed inside

a circular well created on a GaAs substrate (b) A microscopic image of the

arrangement of the wells created by photolithography

Fig 2.9 (a) A microscopic images of the 400 nm spheres selectively formed

inside micro-wells created on hydrophobic polymer substrate by imprinting

(b) The front form of the colloidal solution indicating the selection of the

self-assembly inside the micro-wells

Fig 2.10 A schematic diagram of the procedure to make the ordered array of

nanocavities

Fig 2.11 A cross-section view of a SEM image of the silica nanocavities

Fig 2.12 Top-view SEM images of the periodic ordered nanocavities To

form the nanocavities the top of the silica film was etched down (a) 150 nm

and (b) 170 nm to expose the PS spheres

Fig 2.13 (a) Perspective view of the sample, where the silica was etched

below the horizontal diameter plane of the sphere at step g shown in Fig 10

(b) and (c) show line scan taken along the directions of mm’ and nn’ as

illustrated in (d) (d) Top view of AFM image for the same sample (e)

Schematic illustrations of the evolution [cross-section views along mm’ and

nn’ direction, respectively, as shown in (d)] of the hexagonal close-packed

silica nanostructure arrays at different stages of etching

Fig 2.14 Schematic illustration of the principle to make nanoholes with

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Fig 2.15 The procedure for fabricating ordered SiO2 nanostructures via dry

etching using two layers of PS nanospheres of ~300 nm in diameter (a) A

side-view SEM image of the bilayered nanospheres on substrate coated with

SiO2 of ~100 nm in thickness Schematic top-views of the bilayered

nanospheres (b) before and (c) after O2 RIE etching (d) A top-view SEM

image and (e) A schematic side-view of the as-formed SiO2 nanoholes after

further O2/CHF3 RIE etching of the bilayered nanospheres and the SiO2 films

underneath

Fig 2.16 (a) SEM image of the ordered arrays of nanoholes with two-size

features (before removing the PS spheres) (b) A tilted-view (~100) of SEM

image of the nanoholes created on the thicker SiO2 region (after removing the

PS spheres)

Fig 2.17 SEM images of the nanoholes formed by a bilayer of PS spheres

with 600 nm diameter Before etching the SiO2 film and PS spheres, the PS

spheres were etched by O2 RIE for (a) 75, (b) 115 and (c) 155s

Fig 2.18 Schematic illustration of the principle to make surface

nanostructures with different cross-section shapes

Fig 2.19 SEM images showing the nanostructures of (a) pillars, (b)

candle-like, and (c) and (d) bell-candle-like, respectively, formed by the multi-cycle etching

technique

Fig 2.20 SEM images of the lens-like microstructures formed by the

multi-cycle etching technique The number of repeat multi-cycles was: (a) 6 for sample A,

(b) 9 for sample B, and (c) 12 for sample C

Fig 2.21 (a) Schematic illustration of the formation of 3D silica

nanostructures (b) A simulation result of the 3D network formed by

bilayered PS spheres (c) A SEM image of the 3D silica network formed by

bilayered PS spheres of 300 nm diameter obtained from experiments

Fig 2.22 Simulation results of the 3D networks formed by (a) two and (b)

three layers of PS spheres The silica network is colored in red, green and

white, the black color represents the watched substrate SEM images of the

3D silica networks formed by (c) two and (d) three layer of PS spheres with

300 nm diameter (e) A top-view of SEM image showing the 2D nanoholes

created by the mask shown in (c) (f) A tilted-view of SEM image showing

the 2D nanoholes created by the mask shown in (d)

Fig 2.23 Schematic illustration showing the formation of 3D silica

nanostructures at different dry etching stages (a)-(d) SEM images showing

the relative 3D nanostructures (e)-(h)

Fig 2.24 SEM images showing the effects of O2 RIE for bilayered PS

spheres on the template The bright spots show the top layer of PS spheres

The etching duration is (a) 90s and (d) 140s (b) and (e) show the templates

formed from (a) and (d) (c) and (f) show the surface structures created

through (b) and (e), respectively

Fig 2.25 A cross-section (a) and top (b) views of schematic illustration of the

template formed by bilayered nanospheres (c) A top-view of schematic