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Title  Light emitting diodes based on ZnO nanowires 

Author(s)  Chan, Hiu Laam (陳曉嵐) 

Citation Chan, H. L. (2011). Light emitting diodes based on ZnO nanowires  (Outstanding Academic Papers by Students (OAPS)). Retrieved from City University of Hong Kong, CityU Institutional Repository. 

Issue Date  2011 

URL  http://hdl.handle.net/2031/6445 

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i

CITY UNIVERSITY OF HONG KONG

DEPARTMENT OF

PHYSICS AND MATERIALS SCIENCE

BACHELOR OF ENGINEERING (HONS) IN MATERIALS ENGINEERING

2010– 2011

DISSERTATION

Light emitting diodes based on ZnO nanowires

by

CHAN Hiu Laam

March 2011

ii

Light emitting diodes based on ZnO nanowires

by

CHAN Hiu Laam

Submitted in partial fulfillment of the

Requirements for the degree of

BACHELOR OF ENGINEERING (HONS)

IN

MATERIALS ENGINEERING

from

City University of Hong Kong

March 2011

Project Supervisor: Prof. Igor Bello

i

Acknowledgement

I would like to express my deep gratitude to my supervisor, Prof. Igor Bello, and my

assessor, Dr. Xu Z K for their encouragement, expertise and valuable guidance over

the period of the project.

I am also grateful to Dr Oleksandr Kutsay and Dr Shrawan Kumar Jha for their

generous support and assistance throughout the project.

ii

Table of Contents

Page

Acknowledgement i

Table of Contents ii

List of Figures iv

List of Tables vi

Abstract vii

1 Introduction and Objectives 1

2 Literature Review 2

2.1 Light Emitting Diodes 2

2.1.1 Basic Physics of Light Emitting Diodes 2

2.1.2 Application of Light Emitting Diodes 3

2.2 Zinc Oxide Nanowires 3

2.2.1 Basic Properties of Zinc Oxide 4

2.2.2 Growth of Vertically Aligned Zinc Oxide Nanowires 5

2.2.3 Characterization of Zinc Oxide Nanowires 6

2.3 Zinc Oxide Nanowires Based Heterojunction Light Emitting

Diodes

9

2.3.1 Characterization 9

2.3.2 Waveguiding Behavior of Nanowires Based Unit 10

3 Experimental Methods 11

3.1 Hydrothermal Growth of Vertical n-Zinc Oxide Nanowire Arrays

on Diverse Substrates

12

3.1.1 Cleaning Substrates 12

iii

3.1.2 Zinc Oxide Seed Layer Patterning on the Substrates 12

3.1.3 Growth of n-Zinc Oxide Nanowires 13

3.2 Characterization of n-Zinc Oxide Nanowires 13

3.2.1 Scanning Electron Microscopy 13

3.2.2 X-ray Diffraction Analysis 13

3.2.3 Raman Scattering 14

3.3 Assembly of n-ZnO Nanowires/ p-GaN Thin Film Based Light

Emitting Diodes

14

3.4 Characterization of Assembled Light Emitting Diodes 15

3.4.1 Four Point Hall Measurement of p-GaN Substrates 15

3.4.2 Current-voltage Characterization 16

3.4.3 Electroluminescence 16

4 Results and Discussions 17

4.1 Characteristics of Hydrothermal Grown n-Zinc Oxide Nanowires 17

4.1.1 Morphology of n-Zinc Oxide Nanowires 17

4.1.2 Structure and Crystallinity of n-Zinc Oxide Nanowires 20

4.1.3 Raman Spectrum of n-Zinc Oxide Nanowires 20

4.2 Characteristics of Assembled n-ZnO Nanowires/p-GaN Thin Film

Based LEDs

21

4.2.1 Crystallographic Characteristics of n-Zinc Oxide

nanowires grown on p-GaN substrates

21

4.2.2 Four Point Hall Measurement of Gallium Nitride

Substrates

22

4.2.3 Current-voltage Characteristics of Assembled Light

Emitting Diodes

24

4.2.4 Electroluminescence of Assembled Light Emitting Diodes 28

5 Summary and Conclusion 32

6 Reference 34

iv

List of Figures

Page

Figure 2-1 Schematic representation of a p-n junction in a LED under

forward bias [2].

2

Figure 2-2 The wurtzite structure of a ZnO crystal [10]. 3

Figure 2-3 (a) Top view and (b) side view SEM images of pristine ZnO

nanowires grown on a silicon surface by a hydrothermal

method [7].

6

Figure 2-4 (a) TEM micrograph of a bunch of pristine ZnO nanowires

abstracted from the nanowire array. The inset shows the [100]

SAD pattern of a single nanowire [17] and (b) XRD patterns

of ZnO nanowires and acetate-derived ZnO seeds [18].

7

Figure 2-5 Photoluminescence spectrum of pristine ZnO nanowires

grown over 1.5 hours on a silicon substrate by a hydrothermal

method [17].

8

Figure 2-6 A schematic diagram of a reported ZnO/GaN heterojunction

LED [5].

9

Figure 3-1 A flow chart showing the experimental methods in this study. 11

Figure 3-2 A schematic diagram of the assembled n-ZnO/p-GaN

heterojunction LED.

15

Figure 3-3 Configuration of electric contacts used in four-point Hall

measurements.

16

Figure 4-1 SEM micrographs of n-ZnO nanowires grown on Si substrates

(a) Top view showing the distribution of n-ZnO nanowires

over a large substrate area, (b) top view of n-ZnO nanowires

without alignment, (c) a sample with well-aligned n-ZnO

nanowires, (d) cross-sectional image of well-aligned n-ZnO

nanowires and (e) schematic diagram of (0001) facet of a

n-ZnO nanowire showing its hexagonal shape and size

parameter, diameter, d

17

Figure 4-2 Arbitrary squares were drawn on the SEM micrographs of (a)

sample N1 and (b) sample N2 to determine the density of

n-ZnO nanowires.

18

v

Figure 4-3 XRD θ-2θ scan result of the n-ZnO nanowires grown on Si

substrates.

20

Figure 4-4 A typical Raman spectrum of the n-ZnO nanowires grown on

Si substrates.

21

Figure 4-5 Comparison of XRD θ-2θ scan result of the ZnO nanowires

grown on Si and p-GaN substrates.

22

Figure 4-6 I-V characteristics of Ni/Au electrical contacts on the GaN

substrates (a) before rapid thermal annealing and (b) after

rapid thermal annealing.

24

Figure 4-7 Room temperature I-V behaviors of the assembled n-ZnO

nanowires/p-GaN thin film heterojunction LEDs based on (a)

substrate S1 and (b) substrate S2.

25

Figure 4-8 Schematic energy level diagrams of the p-n junction formed

by n-ZnO and p-GaN at (a) reverse bias, (b) zero bias and (c)

forward bias.

26

Figure 4-9 Blue emission of assembled n-ZnO/p-GaN heterojunction

LED based on substrate S1 at a forward bias of 10 V as

captured with a digital camera. (a) Test chip image and (b)

higher magnification image of the blue emission from the

LED.

28

Figure 4-10 EL spectra of the assembled n-ZnO/p-GaN heterojunction

LEDs collected at normal incidence, based on different GaN

substrates, (a) substrate S1 and (b) substrate S2 under various

forward bias voltages.

39

vi

List of Tables

Page

Table 4-1 Densities of n-ZnO nanowires of samples N1 and N2 19

Table 4-2 Room temperature Four-point Hall Measurements of p-GaN

substrate S1 at four different magnetic fields

23

Table 4-3 Room temperature Four-point Hall Measurements of p-GaN

substrate S2 at four different magnetic fields

23

vii

Abstract

The fabrication and characterization of light emitting diodes (LEDs) based on n-ZnO

nanowires/p-GaN thin film heterojunction were studied. Closely packed and vertically

aligned n-ZnO nanowire array was first grown on Si substrates by a hydrothermal

method and then optimized on the p-GaN substrates. The average diameter and length

of the n-ZnO nanowires were around 95 nm and 520 nm, respectively. The perfect

wurtzite structure of n-ZnO nanowires was verified by the dominant E2 high mode

(439 cm-1

) in the Raman spectrum of the n-ZnO nanowires. The sharp (002)

diffraction peak with a small full width at half maximum (FWHM) of 0.3° in X-ray

Diffraction pattern shows the good crystallinity and [001] preferred growing direction

of the n-ZnO nanowires.

LEDs based on n-ZnO nanowires/p-GaN thin film heterojunctions were successfully

fabricated by a hydrothermal method. Arrays of n-ZnO nanowires were grown on two

commercially available p-GaN substrates (S1 and S2). According to the results of

Four-point Hall Measurement, the carrier density of substrate S1 (2.00 × 1017

cm-3

) is

double of that of S2. Substrate S2 on the other hand has higher carrier mobility (176

cm2/Vs) than S1 (33 cm

2/Vs). Both assembled LEDs exhibit diode-like rectifying

behavior in current-voltage (I-V) measurements. Blue emission (440 nm) is

dominated in the assembled LED based on substrate S1, while weak UV (370 nm)

and green (550 nm) emissions are also detected. The blue emission is visible even at a

forward bias as small as 7 V. Interference fringes found in the Electroluminescence

emission of the assembled LED based on substrate S2 indicate that those well-faceted

n-ZnO nanowires act as waveguiding cavity in the photon emission.

1

1 Introduction and Objectives

Light emitting diode (LED) is a semiconductor device and its performance greatly

depends on the properties of semiconductor material used to fabricate the p-n junction.

Zinc Oxide (ZnO) is an important semiconductor with inherent n-type conductivity,

intrinsic wide and direct band gap, 3.37eV, and high exciton binding energy of 60meV

[11]. Due to the desired electronic properties and simple growth method of n-ZnO

nanowires, the design and fabrication of arrays of light emitting devices based on

n-ZnO nanowires can be achieved in a practical and cost effective approach. Because

of the immature p-doping process of ZnO, ZnO based homojunction LEDs is not

desired and fabrication of n-ZnO based heterojunction LEDs is a good alternative.

Among various p-type semiconductors, gallium nitride (GaN) exhibits similar

physical properties as ZnO, such as the same wurtzite structure and similar band gap

energy and lattice constant. Due to the good compatibility of ZnO and GaN, n-ZnO

nanowires/p-GaN thin film heterojunction LED is proposed to fabricate in the

dissertation.

The objectives of the dissertation are

i) To study hydrothermal growth of n-ZnO nanowire array

ii) To characterize the hydrothermal grown n-ZnO nanowires

iii) To assemble n-ZnO nanowires/p-GaN thin film heterojunction LED

iv) To investigate the effect of p-type GaN thin film on the characteristics of the

assembled LEDs

2

2 Literature Review

2.1 Light Emitting Diodes

Light emitting diodes (LEDs) are a type of diodes that are specially designed for light

emission purpose. LEDs are fabricated from semiconductor materials in which a p-n

junction is formed. The p-n junction formed by joining a p-type semiconductor to an

n-type semiconductor is responsible for the light emission of LEDs.

2.1.1 Basic Physics of Light Emitting Diodes

LEDs convert electrical energy into optical energy during their operation. When a

light emitting diode is under forward bias conditions, electrons are injected into the

p-type semiconductor and holes are injected into the n-type semiconductor. The

recombination of these minority carriers with the majority carriers at the p-n junction

leads to the release of photons with energy corresponding to the band gap energy [1].

The wavelength of the emitted light is determined by the band gap energy of

semiconductor materials. The physics governing light emission of LEDs is known as

electroluminescence (EL) of semiconductor materials. A schematic representation of a

radiative recombination at a p-n junction in a forward biased LED is shown in Figure

2-1.

Figure 2-1 Schematic representation

of a p-n junction in a LED under

forward bias [2].

3

2.1.2 Application of Light Emitting Diodes

LEDs are ideal light sources owing to their high brightness, low power consumption

and long durability compared to conventional incandescent bulbs or fluorescent lamps

[3]. The advantages of LEDs over the conventional light sources contribute to the

diverse applications of LEDs. Some examples of LEDs applications are home lighting,

navigation lighting, automotive lighting, traffic signals, electronic displays and also

indicator lights for electronic equipment.

2.2 Zinc Oxide Nanowires

Zinc oxide (ZnO) nanostructures have attracted considerable interest as an important

II-VI semiconductor with extensive application potentials. The variety of its practical

applications includes acting as ideal building blocks for light-emitting diode [4, 5],

piezoelectric transducers [6], dye sensitized solar cells [7], and chemical and gas

sensors [8]. Among the nanostructures of ZnO, one-dimensional nanostructures such

as nanowires, nanotubes and nanoribbons have been extensively studied for

electronics and optoelectronics applications owing to the efficient carriers transfer at

nanosized junctions [9].

ZnO crystal has a wurtzite structure with a hexagonal unit cell as shown in Figure 2-2.

ZnO nanowires are easy to synthesize because of the enhanced facial growth along

the c axis direction of the wurtzite crystal structure [7].

Figure 2-2 The wurtzite structure of a

ZnO crystal [10].

4

2.2.1 Basic Properties of Zinc Oxide

ZnO is an important candidate for electronics and optoelectronics applications

because of its direct and wide band gap (3.37 eV) feature and high exciton binding

energy of 60 meV [11]. Owing to the low efficiency of LEDs covering green to violet

light available in the market, intense development of those LEDs are still carrying on.

ZnO is an appealing material for the development of high efficiency UV or blue LEDs

because of its direct band gap feature and appropriate band gap energy. Furthermore,

the high exciton binding energy of ZnO enables significant excitonic emission even at

room temperature.

Change in ZnO resistivity on exposure to various gases and chemical species makes

ZnO attractive for sensing applications [8]. The surface to volume ratio of a sensing

unit can greatly increase when nanostructures are employed as the building blocks.

ZnO also exhibits piezoelectric property originating from its non-centrosymmetric

structure. Dimension changes were resulted from the application of voltage.

Piezoelectric characterization of an individual ZnO nanobelt has been carried out [6].

The results of the study show that the ZnO nanobelt has much higher piezoelectric

coefficient when compared to bulk ZnO, supporting the piezoelectric applications of

ZnO.

Pristine ZnO nanostructures commonly possess n-type conductivity because of

contamination of the growth environment and structural defects formed in the ZnO

crystal lattice [11]. Defects like Zn interstitials, oxygen vacancies or hydrogen

contribute to the intrinsic n-type doping of ZnO. Group III elements such as Al, Ga,

5

and In are typically used as dopants to increase the n-type conductivity of ZnO. ZnO

can easily be n-type doped, but p-type doping of ZnO nanostructures is difficult to

achieve due to compensation of dopants by low-energy native defect, low solubility of

the dopants and deep impurity level [12].

2.2.2 Growth of Vertically Aligned Zinc Oxide Nanowires

Synthesis of high quality and single crystalline ZnO nanowires can be achieved by

metal-organic chemical vapor deposition (MOCVD) [13, 14], vapor–liquid–solid

(VLS) mechanism [15], pulsed laser deposition (PLD) [16] and hydrothermal

synthesis [4, 5, 7, 17]. Among these methods, hydrothermal growth of ZnO nanowires

is suggested owing to the low growth temperature (~90 °C), its simplicity, low cost

and capability for mass production. Those ZnO nanowires synthesized by MOCVD,

VLS mechanism and PLD require elevated growth temperatures of 450-950 °C.

Sample uniformity, substrate selection and low production yield also limit the use of

those processes.

Hydrothermal growth of ZnO nanowires involves heterogeneous nucleation and

subsequent crystal growth of ZnO nanocrystal seeds deposited on a substrate. The

ZnO nanocrystal seeds were prepared by first coating the substrate with zinc nitrate

solution in ethanol and then heating the substrate at 350 °C for 20 minutes. The

thermal decomposition of the zinc nitrate film led to the formation of a layer of

textured ZnO seeds with their (0001) planes parallel to the substrate surface [7, 18].

The coating and annealing procedure is required to repeat twice or more to ensure

uniform coverage of ZnO seeds on the substrate. The subsequent crystal growth of

ZnO seeds into vertical nanowire array was carried out by immersing the prepared

6

substrate into an aqueous solution of zinc nitrate hydrate, hexamethylene tetramine

(HMTA) and polyethylenimine at 90 °C for 2.5 hours [4] or at 95 °C for 2 hours [5].

The hydrolysis and condensation reactions of zinc nitrate in water with the addition of

HMTA at temperature >80 °C result in ZnO nanowire growth. Introduction of

polyethylenimine inhibits the radial growth but promotes the axial growth of ZnO

nanowires. The concentration of zinc nitrate and HMTA should be less than 0.1 M

and with the pH ranged from 5 to 8 [7]. This synthesis using a seeded substrate

produced wires with diameters down to 200 nm. The orientation and alignment of the

nanowires can be significantly improved by using the acetate-derived ZnO seeds

rather than ZnO nanoparticles [18].

2.2.3 Characterization of Zinc Oxide Nanowires

In the literature, various characterization techniques like scanning electron

microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD)

and photoluminescence (PL) were used to characterize the ZnO nanowires after the

growing process.

Figure 2-3 (a) Top view and (b) side view SEM images of pristine ZnO nanowires

grown on a silicon surface by a hydrothermal method [7].

(a)

(b)

7

SEM was employed to study the morphology of the nanowire array [7, 19]. Top and

side view SEM images of the pristine ZnO nanowires can be obtained as shown in

Figure 2-3 (a). The packing density, length, diameter and aspect ratio of the nanowires

can be determined from the micrographs. The hexagonal cross sections shown in

Figure 3a are evident to the wurtzite hexagonal crystal structure of ZnO nanowires.

The side view image in Figure 2-3 (b) shows absence of the intermediate ZnO thin

film between the substrate and the array.

TEM analyses provide better understanding on the nanowire structure and

morphology [7, 17]. Both high resolution TEM (HRTEM) image and diffraction

pattern of a single pristine nanowire give information on the crystallography of ZnO

nanowire. The TEM image of a brunch of ZnO nanowires in Figure 2-4 (a) shows a

typical morphology. The discrete diffraction spots depict in the inset of a selected area

diffraction (SAD) pattern in Figure 2-4 (a) indicate the single crystallinity and [0001]

growth direction of those ZnO nanowires.

Figure 2-4 (a) TEM micrograph of a bunch of pristine ZnO nanowires abstracted

from the nanowire array. The inset shows the [100] SAD pattern of a single nanowire

[17] and (b) XRD patterns of ZnO nanowires and acetate-derived ZnO seeds [18].

90nm

(a) (b)

8

XRD was performed for the study of crystallography and growth direction of ZnO

nanowires [18]. In Figure 2-4 (b), a XRD pattern of wurtzite ZnO nanowires with a

high intensity (0002) peak is shown. The intense (0002) diffraction peak demonstrates

the orientation of the nanowires along their c-axes.

The optical properties of the ZnO nanowire array have been characterized by PL

spectroscopy [17, 20]. PL characterization was performed by using a 325nm HeCd

laser as the excitation source and a fiberoptic spectrometer for the collection of the

photonic emission. Various visible emissions such as violet, blue, green, yellow, and

orange-red can be observed from ZnO nanowires. Violet and blue emissions result

from the interband transitions of the ZnO material while green, yellow and orange

emissions are attributed to the defect emissions [17]. The PL spectrum collected from

pristine ZnO nanowires grown on a silicon substrate reveals two peaks as seen in

Figure 2-5. The first peak at around 380nm corresponding to violet emission

originates in interband transitions of the ZnO material. The second peak at around

600nm corresponds to defect emission and gives orange emission [5, 17].

Figure 2-5 Photoluminescence spectrum of pristine ZnO nanowires grown over 1.5

hours on a silicon substrate by a hydrothermal method [17].

9

2.3 Zinc Oxide Nanowires Based Heterojunction Light Emitting Diodes

As p-type doping of ZnO is not well-developed, fabrication of ZnO based

homojunction LED is not suggested. Alternatively, ZnO based heterojunction LEDs

using different well-established p-type materials like Si [17], GaN [14] and AlGaN

[21] are under extensive studies to develop reliable LEDs. n-ZnO nanowires/p-type

GaN thin film heterojunction is ideal to fabricate LEDs with UV or blue emission

spectra because of the epitaxial growth of aligned n-ZnO nanowires from p-type GaN

thin film. ZnO and GaN have the same wurtzite structure and similar band gap energy

and lattice constant. One of the proposed LED architectures based on n-ZnO

nanowires and p-type GaN thin film is shown in Figure 2-6.

Figure 2-6 A schematic diagram of a reported

ZnO/GaN heterojunction LED [5].

2.3.1 Characterization

Current-voltage (I –V) behavior and electroluminescence are commonly used for

characterization of fabricated LEDs. LEDs with different architectures show different

I - V characteristics and emission spectra. Good rectifying property was observed in

some fabricated LEDs [5], while other showed almost symmetric I - V characteristics

[4]. Several abnormal light-emitting behaviours of ZnO nanowire based

heterojunctions were found. Light emission of ZnO based LEDs was not only

observed under forward bias [5], but also under both forward and reverse bias [4] or

only under reverse bias [22]. The emission spectra for forward bias were not the same

Ti/Au

Ni/Au

Sapphire

p-GaN

ZnO PMMA

10

as those for reverse bias. In addition, UV, violet, blue and yellow emissions have been

reported in electroluminescence of ZnO based heterojunctions [4, 5].

2.3.2 Waveguiding Behavior of Nanowires Based Unit

Vertical nanowire array provided possible waveguiding in the light extraction process

of nanowire-based photonic devices [5, 15]. The vertical nanowire array serves as

waveguiding cavities. Light emitted from one end of the nanowire array is guided by

the cavities to travel to the opposite end of the array. Improvement in the light

extraction of LEDs can be achieved. Waveguiding behavior of the nanowires is

greatly affected by the quality of the nanowires [15]. Any defects found in the

nanowires will act as scattering centers of the guided light, resulting in lower

extraction efficiency

11

3 Experimental Methods

A flow chart showing the experimental methods of fabrication and characterization of

n-ZnO nanowires/p-GaN thin film heterojunction based LEDs is shown below.

Figure 3-1 A flow chart showing the experimental methods in this study.

Cleaning Si substrates

Hydrothermal growth of

ZnO nanowires

SEM XRD

Vertical array of

high crystalline

ZnO nanowires?

No

ZnO seed layer patterning

on the substrates

Raman scattering

Four-point Hall

measurement of p-GaN

substrates

Electron beam evaporation

of Ni/Au contact on p-GaN

substrates

Rapid thermal annealing

Ohmic

Ni/Au contact?

Yes

No

Cleaning p-GaN substrates

Hydrothermal growth of

ZnO nanowires

ZnO seed layer patterning

on the substrates

Formation of Ag contact

on ZnO nanowires

I-V characterization

EL

I-V characterization of the

LEDs

Yes

12

3.1 Hydrothermal Growth of Vertical n-Zinc Oxide Nanowire Arrays on Diverse

Substrates

Well-aligned n-ZnO nanowire arrays were synthesized by a hydrothermal method

because of the low growth temperature, the low fabrication cost and the production of

arrays with high density of nanowires. Synthesis of vertical n-ZnO nanowires was

investigated on Silicon (Si) substrate and then optimized on two commercially

available p-GaN substrates, namely S1 and S2. The p-GaN substrates are composed of

MOCVD grown p-type doped GaN layer on top of undoped GaN layer over c-plane

sapphire. The p-type GaN film thickness of the S1 and S2 substrates are 2 µm and 3

µm, respectively.

3.1.1 Cleaning Substrates

Substrates of 1 cm × 1 cm size were cut from Si and p-GaN wafers and used to grow

vertical n-ZnO nanowires. Cleaning of the substrates was carried out before the

growth process. The substrates were cleaned with acetone in an ultrasonic bath for 1

minute to wash away the grease, followed by washing with ethanol for 1 minute to

remove the acetone. The substrates were then rinsed with deionized water and dried in

nitrogen flow.

3.1.2 Zinc Oxide Seed Layer Patterning on the Substrates

Forming layer of ZnO nanocrystal seeds on the substrate was achieved by wetting the

substrate with a droplet of 0.005 M zinc acetate dehydrate in ethanol. The substrate

was dried in nitrogen flow after each wetting. The dropping and drying process was

repeated three- to five-times to ensure uniform coating of the solution. The seeded

substrate was annealed at 350 °C for 20 minutes to ensure seed adhesion to the

substrate surface and to allow seed orientation to well defined crystallographic

13

direction.

3.1.3 Growth of n-Zinc Oxide nanowires

Hydrothermal growth of n-ZnO nanowires was carried out by suspending the

substrate face-down in an aqueous solution of 0.025 M zinc nitrate (Aldrich, 99.999%)

and 0.025 M hexamethylenetetramine (Aldrich, 99%) at 90 °C for 3 hours. After the

growth process, the substrates were removed from the solution, cleaned with

deionized water in an ultrasonic bath for 5 minutes and subsequently dried in air.

3.2 Characterization of n-Zinc Oxide Nanowires

Morphology, structure, crystallinity and optical properties of the n-ZnO nanowires

were studied using various characterization techniques.

3.2.1 Scanning Electron Microscopy

A Philips FEG SEM XL30 scanning electron microscope was used to study the

morphology and size of the hydrothermal grown n-ZnO nanowires on Si substrates.

Prepared samples were mounted on the sample stage of the SEM. The analysis

chamber was pumped down to pressure on the order of 10-4

Pa before testing. SEM

images of the n-ZnO nanowires were obtained with 15 kV electron acceleration

voltages. The packing density and size of the nanowires were determined from the

SEM micrographs.

3.2.2 X-ray Diffraction Analysis

The crystallography and crystallinity of the n-ZnO nanowires grown on the Si and

p-GaN substrates were studied by X-ray Diffraction (XRD) using a Philips X’Pert

14

X-ray differactometer. The incident X-ray has a wavelength of 1.54 Å . The diffraction

angle 2θ was set to scan from 25° to 45°. The step width was 0.02° while the dwell

time was 1 second. Diffracted X-rays were collected and analyzed by the software

“XRD software data collection” to produce the XRD diffraction pattern of the n-ZnO

nanowires grown on the Si substrates.

3.2.4 Raman Scattering

The Raman scattering spectra were induced by a He–Ne laser operating with a

wavelength of 632.8 nm and collected at room temperature using a Renishaw inVia

Raman Microspectroscope.

3.3 Assembly of n-ZnO Nanowires/p-GaN Thin Film Based Light Emitting

Diodes

The GaN substrates, S1 and S2, were used to fabricate LEDs based on n-ZnO

nanowires. First, Ni/Au electrical contacts were evaporated at two ends of the GaN

substrate using electron beam evaporation (Edward E-beam system) by covering the

central part of the substrate with aluminum foil. The Ni/Au electrical contacts

deposited on the p-GaN thin film were then thermally treated, using a rapid thermal

annealing, to achieve Ohmic behaviour. The annealing was carried out in a dynamic

nitrogen atmosphere at a flow rate of 500 sccm and 550 °C for a minute in two

subsequent cycles. Ohmic contact formation was confirmed by current-voltage (I-V)

measurements using a Keithley 4200 Semiconductor Parameter Analyzer. Then,

n-ZnO nanowires were grown on the central part of the substrate by a hydrothermal

method as described in section 3.1. Finally, silver conductive paint was applied onto

the exposed n-ZnO nanowire tips to form an ohmic contact. The assembled LED

structure is shown in Figure 3-2.

15

3.4 Characterization of Assembled Light Emitting Diodes

The electrical and optical properties of the assembled LEDs based on the GaN

substrates S1 and S2 were studied and compared. The effect of the p-type GaN thin

film on the performance of the assembled LEDs was investigated.

3.4.1 Four-point Hall Measurement of p-GaN Substrates

A four-point Hall method was used to determine the type, concentration and mobility

of charge carriers in the p-GaN thin film. Samples of 5 mm × 5 mm size were cut

from the two p-GaN wafers and Ni/Au electrical contacts were evaporated in the four

corners by Edward E-beam system in the Van der Pauw geometry as shown in Figure

3-3. Rapid thermal annealing was performed immediately after metal deposition to

achieve Ohmic electrical contacts. Four-point Hall measurements of the two

substrates were performed using a Lake Shore Hall Measurement System at room

temperature. In the four-point Hall measurements, a current I13 was applied to flow

from electrode 1 to electrode 3 and a positive magnetic field is acted along the

positive z direction. The corresponding voltage V24P across electrode 2 and 4 was

measured. Then, the direction of the current was reversed, current I31 was applied and

Figure 3-2 A schematic diagram of the

assembled n-ZnO/p-GaN heterojunction LED.

Ni/Au

Ag

Ni/Au

p-GaN (0002)

GaN (0002)

Al2O3 (0002)

ZnO

16

voltage V42P was measured. V13P and V31P were measured with I42 and I24 by repeating

the steps. The polarity of the magnetic field was reversed. V24N, V42N, V13N, and V31N

were measured respectively with I13, I31, I42, and I24. The Hall voltage of the sample

was determined from the measured voltage. The corresponding concentration and

mobility of the charge carriers were calculated by the measuring system. Four sets of

four-point Hall measurements were done on each GaN substrate at room temperature.

The applied magnetic field was 0.1, 0.4, 0.7 and 1.0 tesla, respectively.

Figure 3-3 Configuration of electric

contacts used in four-point Hall

measurements.

3.4.2 Current-voltage Characterization

The current-voltage (I-V) characteristics of the Ni/Au metal contacts on the p-GaN

thin film, before and after, rapid thermal annealing, as well as the p-n junction of the

assembled LEDs were measured using a Keithley 4200 Semiconductor Parameter

Analyzer at room temperature.

3.4.3 Electroluminescence

Room temperature electroluminescence (EL) at different voltages was performed on

the assembled LEDs. In the EL measurement, the LED was biased at different

voltages using a Keithley 220 current source and emission was detected using an

ACTON SpectraPro 500i monochromator coupled with an Ocean Optics CCD matrix

spectrometer.

A 1

2 3

4

17

4 Results and Discussions

4.1 Characteristics of Hydrothermal Grown n-Zinc Oxide Nanowires

4.1.1 Morphology of n-Zinc Oxide Nanowires

SEM images of the n-ZnO nanowires grown on Si substrates are shown in Figure 4-1.

18

Figure 4-1 SEM micrographs of n-ZnO nanowires grown on Si substrates (a) Top

view showing the distribution of n-ZnO nanowires over a large substrate area, (b) top

view of n-ZnO nanowires without alignment, (c) a sample with well-aligned n-ZnO

nanowires, (d) cross-sectional image of well-aligned n-ZnO nanowires and (e)

schematic diagram of (0001) facet of a n-ZnO nanowire showing its hexagonal shape

and size parameter, diameter, d

As shown in Figure 4-1 (a), a dense array of n-ZnO nanowires over a large substrate

area was achieved by a hydrothermal growing method. Samples with different

nanowire alignment and density were observed as shown in Figure 4-1 (b) and (c).

Three squares, namely P1, P2 and P3, were randomly drawn on the SEM images of

the samples, as shown in Figure 4-2, to determine the density of the n-ZnO nanowires

grown on Si substrates. Number of n-ZnO nanowires per unit centimeter square,

which implies the density of the n-ZnO nanowires, was determined by counting the

number of n-ZnO nanowires in each square. The average density of n-ZnO nanowires

of two chosen samples (N1 and N2) were measured and shown in Table 4-1.

Figure 4-2 Arbitrary squares were drawn on the SEM micrographs of (a) sample N1

and (b) sample N2 to determine the density of n-ZnO nanowires.

19

Table 4-1 Densities of n-ZnO nanowires of samples N1 and N2

Samples Density of

nanowires in

P1 (108 cm

-2)

Density of

nanowires in

P2 (108 cm

-2)

Density of

nanowires in

P3 (108 cm

-2)

Average density of

nanowires (108 cm

-2)

N1 63.28 55.69 54.42 57.80

N2 93.40 75.31 90.47 86.39

Sample N1 with a less dense n-ZnO nanowire array showed poor vertical alignment.

A high density of nanowires of around 86 × 108 nanowires per cm

-2 was achieved in

sample N2. Owing to the crowding effect in high density nanowire array, well-aligned

array could be formed. The preparation of the ZnO nanocrystal seeding layer in the

hydrothermal growth plays an important role in forming a closely packed vertical

nanowire array. Since each nanowire is grown from a deposited nanocrystal, a dense

distribution of deposited nanocrystals and elimination of contamination on the

substrate surface lead to the formation of vertically-aligned dense nanowire array. The

magnified image, as shown in Figure 4-1 (c), shows the (0001) facet ends of n-ZnO

nanowires. The hexagonal shape characterizes the wurtzite crystal structure of ZnO.

Both regular and irregular hexagonal cross-sectional areas of the n-ZnO nanowires

were observed. Different size nanowires over the substrate surface were attributed to

the original size of deposited nanocrystals. The cross-sectional SEM image of the

dense nanowire array confirms the good vertical alignment of n-ZnO nanowires on

the substrate, as shown in Figure 4-1 (d). The average length of n-ZnO nanowires is

520 nm. The typical diameter of n-ZnO nanowires in a dense array is around 95 nm,

which is measured based on Figure 4-1 (e).

20

4.1.2 Structure and Crystallinity of n-Zinc Oxide Nanowires

The result of the XRD θ-2θ scan of the n-ZnO nanowires grown on Si substrates

is presented in Figure 4-3.

Figure 4-3 XRD θ-2θ

scan result of the

n-ZnO nanowires

grown on Si

substrates.

Only the (002) diffraction peak centered at 34° is shown in the XRD pattern. The

result suggests that the n-ZnO nanowires have the growth direction of [001]. A small

full width at half maximum (FWHM) value of 0.3° was found, showing the good

crystallinity of the n-ZnO nanowires grown by the hydrothermal method.

4.1.3 Raman Spectrum of n-Zinc Oxide Nanowires

A Raman spectrum of the n-ZnO nanowires grown on Si substrates is shown in Figure

4-4. Raman spectra were collected by using a 632.8 nm He–Ne laser as the excitation

source. ZnO with wurtzite structure belongs to the space group. Only

the optical phonons at Γ point of the Brillouin zone are participated in Raman

scattering of the single-crystalline ZnO. The optic modes predicted by group theory

are as follows: Γopt = A1 + 2B1 + E1 + 2E2. A1, E1 and E2 modes are Raman active,

while B1 mode is Raman silent. Both A1 and E1 modes are split into transverse optical

(TO) and longitudinal optical (LO) components. E2 mode also consists of a low

(002)

21

frequency mode (E2 low) and a high frequency mode (E2 high) [23].

Figure 4-4 A typical Raman spectrum of the n-ZnO nanowires grown on Si

substrates.

The intense peak at 439 cm-1

in the Raman spectrum, corresponding to the E2 high

mode of ZnO, verifies the perfect wurtzite structure of n-ZnO nanowires. This mode

is associated with the vibration of oxygen atoms. The obvious peak at 389 cm-1

is

attributed to A1 (TO) mode. A weak peak at 339 cm-1

was observed, which is the

optical phonon overtone with A1 symmetry [23].

4.2 Characteristics of Assembled n-ZnO Nanowires/p-GaN Thin Film Based

LEDs

4.2.1 Crystallographic Characteristics of n-Zinc Oxide nanowires grown on

p-GaN substrates

Both crystallinity and growth direction of n-ZnO nanowires grown on Si and p-GaN

substrates were studied and compared. Two typical XRD θ-2θ scan results of the

n-ZnO nanowires grown on Si and p-GaN substrates are shown in Figure 4-5.

ZnO E2 high

22

ZnO/p-GaN: 34.71°

ZnO/Si: 34.83°

Intense (002) diffraction peak of n-ZnO nanowires grown on Si substrates is centered

at 34.71°, while that of n-ZnO nanowires grown on p-GaN substrates is centered at

34.83°. The n-ZnO nanowires grown on Si and p-GaN substrates show similar XRD

patterns, those patterns denoting the good crystallinity of n-ZnO and [100] growth

direction. Therefore, it is assumed that n-ZnO nanowires grown on p-GaN substrates

have similar properties as those grown on Si substrates.

4.2.2 Four-point Hall Measurement of Gallium Nitride Substrates

Two commercially available p-GaN substrates, namely S1 and S2, were used to

fabricate n-ZnO nanowires/p-GaN thin film heterojunction LEDs. The charge carrier

type, density and mobility of substrates S1 and S2 measured with a four-point Hall

method at room temperature and different magnetic fields are listed in Table 4-2 and

Table 4-3, respectively.

Figure 4-5 Comparison of XRD

θ-2θ scan result of the ZnO

nanowires grown on Si and p-GaN

substrates.

23

Table 4-2 Room temperature Four-point Hall Measurements of p-GaN substrate S1 at

four different magnetic fields

Magnetic Field

[tesla]

Carrier Type Carrier Density

[cm-3

]

Hall Mobility

[cm2/Vs]

0.1 p 1.45×1016

113

0.4 p 3.04×1016

5.39

0.7 p 2.94×1017

5.57

1.0 p 1.88×1017

8.70

Average: 2.00×1017

33.11

Table 4-3 Room temperature Four-point Hall Measurements of p-GaN substrate S2 at

four different magnetic fields

Magnetic Field

[tesla]

Carrier Type Carrier Density

[cm-3

]

Hall Mobility

[cm2/Vs]

0.1 p 9.97×1016

176

0.4 p 9.97×1016

176

0.7 p 9.97×1016

176

1.0 p 9.97×1016

176

Average: 9.97×1016

176

The positive values of the measured hall coefficients confirm the p-type doping of

GaN films in both substrates. Holes are the major carriers in both the substrates.

Although the four-point Hall measurements were conducted at different magnetic

fields, the carrier density and mobility should be more or less the same in the sample.

The fluctuation in the carrier density and Hall mobility of substrate S1 is probably due

to not good enough metal contacts. Average carrier density and mobility are therefore

calculated for comparison. The average carrier density of p-GaN film on substrate S1

is 2.00×1017

cm-3

, which is double that on substrate S2. P-GaN film on substrate S2 on

the other hand has higher average carrier mobility than substrate S1. Under the

influence of electric field, holes of substrate S2 are 5-times more mobile than those of

substrate S1. This difference is closely related to the charge carrier concentration; the

higher concentration the lower mobility of charge carries.

24

4.2.3 Current-voltage Characteristics of Assembled Light Emitting Diodes

The I-V characteristics of the Ni/Au electrical contacts evaporated on p-GaN films by

electron beam evaporation before and after rapid thermal annealing are shown in

Figure 4-6 (a) and (b) respectively.

Figure 4-6 I-V characteristics of Ni/Au electrical contacts on the GaN substrates (a)

before rapid thermal annealing and (b) after rapid thermal annealing.

Before rapid thermal annealing, Ni/Au metal contacts formed on p-type GaN films

exhibit non-linear I-V behavior as shown in Figure 4-6 (a). The characteristic with a

rectifying-like feature is symmetric. The linear I-V curve in Figure 4-6 (b) indicates

(a)

(b)

25

that good Ni/Au Ohmic contacts are formed on the p-GaN film after rapid thermal

annealing. The formation of Ohmic contacts could be attributed to the interfacial

reactions between Ni, Au and GaN [24]. Rapid thermal annealing of Ni/Au metal

contacts right after electron beam evaporation is required to obtain low resistance

Ohmic contacts for effective transfer of electrons and holes. Silver contacts to n-ZnO

nanowires also show linear I-V characteristics.

The I-V curves of the n-ZnO nanowires/p-GaN thin film heterojunction LEDs grown

on substrates S1 and S2 are shown in Figure 4-7 (a) and (b) respectively.

(a)

(b)

26

Figure 4-7 Room temperature I-V behaviors of the assembled n-ZnO

nanowires/p-GaN thin film heterojunction LEDs based on (a) substrate S1 and (b)

substrate S2.

Both the assembled LEDs show nonlinear and rectifying diode-like I-V curves, which

are typical electrical characteristics of LED devices. Since the Ni/Au and Ag metal

contacts on p-GaN and n-ZnO show Ohmic behavior, the rectifying characteristics

originate in the nanosized heterojunctions formed by growing n-ZnO nanowire arrays

on p-GaN films. Owing to the small lattice mismatch between ZnO and GaN,

epitaxial growth of the n-ZnO nanowires on p-GaN film was achieved and nanosized

rectifying p-n junctions were formed in both the assembled LEDs. Energy level

diagrams of the p-n junction at zero, forward and reverse biases as seen in Figure 4-8,

are studied to illustrate the rectifying characteristics of the assembled LEDs.

Figure 4-8 Schematic energy

level diagrams of the p-n junction

formed by n-ZnO and p-GaN at

(a) reverse bias, (b) zero bias and

(c) forward bias.

+

+

-

-

27

When p-GaN is coupled with n-ZnO, electrons of n-ZnO flow to p-GaN and holes

vice versa until the Fermi energies of both materials reach equilibrium. Interdiffusion

of free electrons and holes takes place at the p-n junction, leading to recombination of

carriers. Carrier at the contact surface of the p-GaN and n-ZnO has lower

concentration than the interior of the solid. A depletion layer is formed at the p-n

junction where the carrier concentration is depressed. A potential barrier is

consequently developed to hinder the movement of either electrons from n-ZnO to

p-GaN or holes from p-GaN to n-ZnO. Schematic energy level diagram of

n-ZnO/p-GaN heterojunction with no applied voltage is shown in Figure 4-8 (b).

Under forward bias, n-ZnO is connected to the negative terminal of a power supply.

The potential barrier and the width of the depletion layer decreases, which are shown

in the schematic energy level diagram in Figure 4-8 (c). The electron flow from

n-ZnO to p-GaN contributes to the forward current. As the bias voltage increases, the

potential barrier, which hinders the electron flow from n-ZnO to p-GaN, decreases.

Therefore, a nonlinear increase of forward current is observed under forward bias. As

shown in Figure 4-8 (a), the potential barrier increases and the depletion layer

becomes wider under reverse bias. The suppressed electron flow from n-ZnO to

p-GaN under reverse bias leads to the low reverse current.

The assembled LED based on substrate S2 has a turn-on voltage of ~2.5 V. Under the

forward bias, nonlinear increase in current was observed in both LEDs. The current

increased abruptly when the applied voltage beyond the turn-on voltage. The turn-on

voltage of the assembled LED based on substrate S1 is ~1.0 V, while the assembled

LEDs based on substrate S2 has a turn-on voltage of ~2.5 V. Among the two

fabricated heterojunction LEDs, the assembled LED based on substrate S2 exhibits

28

high degree of rectification at low voltage. The forward current of the assembled LED

based on substrate S2 is at least an order of magnitude larger than the reverse current

at a bias voltage larger than 5 V. However, the same degree of rectification was

observed in the assembled LED based on substrate S1 at applied voltage beyond 30 V.

Noisy I-V characteristic of LED made by substrate S1 is probably caused by not good

enough metal contacts as making a reliable contact to nanowire is difficult.

Differences in the I-V behavior could be due to the differences in carrier density and

mobility of p-GaN film on substrates and also the quality of nanosized ZnO/GaN

material interface in the heterojunction. The assembled LEDs based on substrate S2,

which reveals noticeable rectifying property, is expected to have better quality of

electrical interface at the p-n junction.

4.2.4 Electroluminescence of Assembled Light Emitting Diodes

Both the assembled n-ZnO/p-GaN heterojunction LEDs showed blue emission under

visual observation. Emission of the fabricated LEDs based on substrate S1, at a

forward bias of 10 V, captured with a digital camera is shown in Figure 4-9. Although

the picture was taken in a dark condition, the emission of both the assembled LEDs

was strong enough to be seen with bare eyes at normal lighting conditions. Emission

of the assembled LEDs based on substrates S1 and S2 is visible at a forward bias as

small as 7 V and 10 V, respectively. The low driving voltage required to induce visible

emission makes both the assembled LEDs suitable for practical application.

(a)

(b)

29

Figure 4-9 Blue emission of assembled n-ZnO/p-GaN heterojunction LED based on

substrate S1 at a forward bias of 10 V as captured with a digital camera. (a) Test chip

image and (b) higher magnification image of the blue emission from the LED.

Room temperature electroluminescence (EL) spectra were collected from both the

assembled LEDs to provide a clear description of their emission under forward bias.

Comparison of the EL spectra of the assembled n-ZnO/p-GaN heterojunction LEDs

collected at normal incidence, based on different GaN substrates, substrate S1 and

substrate S2 under various forward bias voltages are illustrated in Figure 4-10 (a) and

(b) respectively.

400 450 500 550 600 650

100

200

300

400

500

600

700

800

900

1000

1100

1200

EL

inte

nsity (

arb

. u

nits)

5 V

7 V

10 V

15 V

20 V

25 V

p-GaN/ZnO nanorods LED

Wavelength nm)

350 400 450 500 550

0

100000

200000

300000

400000

500000

600000

EL

inte

nsity (

arb

. u

nits)

p-GaN/ZnO nanorods LED

5 V

10 V

15 V

20 V

25 V

32 V

Wavelength nm)

(a)

(b)

30

Figure 4-10 EL spectra of the assembled n-ZnO/p-GaN heterojunction LEDs

collected at normal incidence, based on different GaN substrates, (a) substrate S1 and

(b) substrate S2 under various forward bias voltages.

EL spectrum of the assembled n-ZnO/p-GaN heterojunction LEDs based on substrate

S1 was detectable even at a bias as small as 5V, as shown in Figure 4-10 (a). The

spectrum indicates a strong peak centered at 440 nm, corresponding to the violet-blue

emission originated from the conduction band to deep acceptor-level transitions in the

p-GaN film [25]. With increasing bias voltage, a broad emission centered at 550 nm is

observed as well as a weak ultraviolet (UV) emission around 370 nm. The broad

green (550nm) emission is likely attributed to recombination of electrons in the

conduction band with holes trapped in oxygen vacancy in n-ZnO [12]. The low

intensity of the defect peak relative to the main peak suggests saturation of deep level

traps, allowing interband transitions to dominate. The weak UV emission is possible

due to the band-edge emission of ZnO [14]. The increase in EL intensity with the bias

voltage is in agreement with the band bending model of a p-n junction as shown in

Figure 4-8. The reduction in band bending between p-GaN and n-ZnO with increasing

bias voltage drives the holes in the valence band of p-GaN towards n-ZnO and vice

versa the electrons in the conduction band of n-ZnO to p-GaN.

Broad emission extending from UV to blue is observed from the assembled

n-ZnO/p-GaN heterojunction LEDs based on substrate S2, as illustrated in Figure

4-10 (b). Band-edge emission of ZnO (370nm) is dominated in the spectrum. The

difference in the EL spectrum of the assembled LEDs is due to the difference in

carrier mobility of the two p-GaN substrates. Since holes in substrate S2 have a higher

mobility, they have a higher tendency to be driven across the interface without

31

recombination. These holes lead to the band-edge emission of ZnO (370nm). Small

superimposed peaks with a periodic spacing are revealed in the spectrum, indicating

the presence of interference phenomena in photon emission. The single-crystalline

and well-faceted n-ZnO nanowires are considered to act as natural resonance cavity,

within which the light is repeatedly reflected by the (0001) facets. The spectrum of

this device is expected to be a ordinary emission with wave-guiding effect. This

suggests that the assembled n-ZnO/p-GaN heterojunction LEDs based on substrate S2

has the potential to develop into violet-blue nanolaser.

32

5 Summary and Conclusion

Hydrothermal growth of well-aligned n-ZnO nanowires assembled into arrays was

conducted in the study. The preparation of ZnO nanoparticle layer in the growing

process is a critical parameter determining the density and alignment of n-ZnO

nanowires. Dense distribution of deposited ZnO nanoparticles favors the formation of

closely packed and vertically aligned n-ZnO nanowire array. The average diameter

and length of n-ZnO nanowires grown on Si substrate were around 95 nm and 520 nm,

respectively. Hexagonal facet ends of the n-ZnO nanowires as shown in top view

SEM images characterize the wurtzite crystal structure of ZnO. Their good

crystallinity is further confirmed by the sharp (002) diffraction peak in the presented

XRD patterns and the intense E2 high mode at 439 cm-1

of n-ZnO in the Raman

spectra.

Dense and vertically aligned n-ZnO nanowire arrays were grown, by hydrothermal

process, on two commercially available p-GaN substrates, namely S1 and S2, to

fabricate n-ZnO nanowires/p-GaN thin film heterojunction LEDs. The results of

four-point Hall measurement showed that both the substrates are p-type conductivity

with charge carrier concentrations on the order of 1017

. The average carrier density of

substrate S1 is double of that of S2, which affect the Hall mobility. As a result the

substrate S2 has higher carrier mobility since it has the lower density of charge

carriers. Ohmic metal contacts were formed on the p-GaN substrates and the tips of

n-ZnO nanowires for effective transfer of electrons and holes. Nonlinear and

rectifying diode-like I-V characteristics were observed from both assembled LEDs,

which are attributed to the nanosized p-n junctions formed by growing n-ZnO

nanowire arrays on p-GaN film. The assembled LED based on substrate S2 shows

33

better rectifying property than the other assembled LED. The assembled LED based

on substrate S1 mainly emits violet-blue light (440 nm), accompanying with weak UV

(370 nm) and green (550 nm) emissions. The strong violet-blue emission is due to the

conduction band to deep acceptor-level transitions in the p-GaN. This suggests that

the light emitted from the LED made of S1 is dominantly associated with the p-GaN.

The blue emission is visible at a forward bias as small as 7 V. The low supply voltage

required for blue emission made both assembled LEDs suitable for practical

application. Furthermore, broad emission extending from UV to blue is observed from

the assembled LED based on substrate S2. The light emitted from the LED made of

S2 is dominantly from band-edge emission of ZnO. Interference fringes found in the

emission indicate wave-guided effect provided by the nanowires. Single-crystalline

and well-faceted n-ZnO nanowires act as natural resonance cavity. The assembled

n-ZnO/p-GaN heterojunction LEDs based on substrate S2 show a high potential to

develop into violet-blue nanolasers.

34

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