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Growth of gallium nitride nanowires by lowpressure chemical vapor deposition (LPCVD)

Wang, Jianbo

2012

Wang, J. (2012). Growth of gallium nitride nanowires by low pressure chemical vapordeposition (LPCVD). Master’s thesis, Nanyang Technological University, Singapore.

https://hdl.handle.net/10356/54694

https://doi.org/10.32657/10356/54694

Downloaded on 26 Jan 2022 05:21:09 SGT

1

TABLE OF CONTENTS

CHAPTER 1: INTRODUCTION ............................................................................... 6

1.1 Background .......................................................................................................... 6

1.2 Motivations and Objectives ................................................................................... 7

1.3 Organization ......................................................................................................... 8

CHAPTER 2: LITERATURE REVIEW ..................................................................... 9

2.1 Properties of Gallium Nitride and its Nanowires ................................................... 9

2.2 Synthesis of Nanowires ....................................................................................... 13

2.2.1 Catalyst and Catalyst-Free Synthesis of Nanowires ...................................... 13

2.2.2 Different Growth Techniques for the Synthesis of GaN Nanowires .............. 16

CHAPTER 3: LOW PRESSURE CHEMICAL VAPOR DEPOSITION (LPCVD)

SYSTEM SETUP AND EXPERIMENTS ................................................................ 27

3.1 Low Pressure Chemical Vapor Deposition (LPCVD) System and Experiment .... 27

3.2 Design of Experiments ........................................................................................ 29

CHAPTER 4: RESULTS AND DISCUSSION ......................................................... 31

4.1 Influence of Catalysts on GaN Nanowire Formation ........................................... 31

4.2 Influence of Temperature on GaN Nanowire Formation ...................................... 34

4.3 Influence of Pressure on GaN Nanowire Growth ................................................. 38

CHAPTER 5: CONCLUSION AND FUTURE WORK ............................................ 43

5.1 Conclusion .......................................................................................................... 43

5.2 Future Work ....................................................................................................... 45

2

REFERENCES ......................................................................................................... 46

3

LIST OF FIGURES

Figure 2.1 Wurtzite structure of Gallium Nitride [9]. ..................................................... 10

Figure 2.2 Illustration of the growth of a silicon nanowire based on VLS mechanism [15].

.............................................................................................................................. 14

Figure 2.3 An APCVD system for the synthesis of GaN nanowirs showing the gas flow

and the location of the substrates with respect to the gallium source [5]. ................ 17

Figure 2.4 Schematic of a MOCVD reactor for the growth of III-V materials [17] ......... 21

Figure 2.5 Schematic of a MBE Chamber for the growth of compound semiconductor

materials. The chamber is cooling by liquid N2 during growth. .............................. 23

Figure 2.6 SEM images of GaN nanowires grown by MBE. (a) Bunches of GaN

Nanowires grown without catalyst (b) Arrays of GaN nanowires grown through a

patterned SiNx mask [21]. ...................................................................................... 25

Figure 3.1 (a) Configuration of the LPCVD system used in this study. The system

equipped with a three-zone heater and a mechanical pumping system with pressure

controller. (b). A close look to the Gallium source and Si samples on the quartz boat.

.............................................................................................................................. 28

Figure 4.1 GaN nanostructures formed on Si (100) substrates grown at 800 oC (a) 3 nm

Au as the catalyst, (b) 3 nm Ni as the catalyst. ....................................................... 33

Figure 4.2 SEM images of GaN nanowires on (100) Si grown at different temperatures. 3

nm Ni was used as the metal catalyst. .................................................................... 35

Figure4.3 Effect of growth temperature on average length of GaN nanowires. ............... 37

Figure4.4 Effect of growth temperature on GaN nanowires density. .............................. 37

Figure4.5 Effect of growth temperature on average diameter of the GaN nanowires grown.

.............................................................................................................................. 38

Figure 4.6 SEM images for the GaN nanowire grown on Si (100) with 3 nm Ni catalyst

under 250 mTorr and 600 mTorr. ........................................................................... 39

Figure 4.7 X-ray diffraction scan of GaN nanowire sample grown on Si (100) with 3 nm

Ni catalyst at 800 oC. ............................................................................................. 40

4

Figure 4.8 Room temperature PL spectrum of GaN NWs grown on Silicon (100)

substrate at 800 oC using Ni as the catalyst. ........................................................... 41

5

LIST OF TABLES

Table 3.1 The experimental conditions for the growth of GaN nanostructures. .............. 28

6

CHAPTER 1: INTRODUCTION

1.1 Background

In the semiconductor industry, Gallium Nitride has been an interesting wide band gap

(3.4eV) semiconductor with excellent thermal stability among III-V nitride

semiconductors due to its large bandgap, large dielectric breakdown field, superior

electron transport properties, and good thermal conductivity, which make it ideal in

short-wavelength optoelectronic and high power/high temperature electronic devices

applications [1, 2]. In recent years, In addition to the research towards GaN thin film or

GaN bulk materials, there have been significant attentions in the growth and

characterization of GaN nanostructures for their perspective applications. Nanowires in

particular have shown promising properties and are envisioned as building blocks for

nano-electronic and photonic devices. Photonic and electronic devices based on GaN-

based single nanowire have already been demonstrated opening the path to the

realization of functional devices based on one-dimensional (1-D) GaN and related

structures. There has been demonstrated by several research groups that GaN nanowires

have its potential applications towards single nanowire light emitting diode (LED) [3],

Nanowire field effect Transistors [4] and GaN nanowires Hydrogen Sensors [5], etc.

Different methods for the synthesis of GaN nanowires have been reported including

Metal Organic Chemical Vapor Deposition (MOCVD) [6] , Molecular beam epitaxy

7

(MBE) [7] as well as catalytic hydride vapor phase epitaxial [8], etc. Among all of the

growth techniques, the use of a CVD process to synthesize GaN nanowires has gained

much interest over the years as this technique offers lesser cost and greater simplicity

than other processes like Molecular Beam Epitaxy or MOCVD [8]. CVD synthesis

benefits from a fast growth rate compared to MOCVD and MBE processes that makes

possible to achieve nanowires of several tens of micrometer in a short growth time (e.g.,

half an hour). However a high temperature is often required to yield the desired 1-D

nanostructures; temperatures of CVD processes were reported typically in the range

from 850°C to 1100°C. . So far, the most of growth of GaN nanowires using a

conventional CVD system has been reported at high pressures (from 400 Pa to

atmospheric pressure) focusing mainly on the effects of temperature and catalysts on the

morphology of the nanowires. The information regarding the synthesis of GaN

nanowires by CVD in low pressure conditions is limited.

1.2 Motivations and Objectives

The motivation of my Master of Engineering project is to perform a systemic study on

the synthesis of GaN nanowires using a Low Pressure Chemical Vapor Deposition

(LPCVD) system. The aim of this study was to optimize different growth parameters in

order to obtain nanowires with a high aspect ratio. Successful synthesis of nanowires

will enable to investigate their electrical properties and used them for future device

fabrication.

8

Through the project, the following objectives are to be achieved:

(1) Studies on the impact of catalysts and substrate orientation on the formation of GaN

nanowires for LPCVD;

(2) Optimization of the growth conditions such as growth temperature and pressure for

high density and high aspect ratio;

(3) Characterization of the GaN nanowires grown by LPCVD using scanning electron

microscope (SEM), photoluminescence (PL), X-ray diffraction (XRD), etc.

1.3 Organization

The report has been organized into the following chapters: Chapter 1 briefly introduces

the motivation and objectives of this thesis; Chapter 2 provides a detailed literature

review of the current research effort on the growth of Gallium Nitride nanowires and

their material properties; In Chapter 3, the experimental details on the growth of GaN

nanowires are presented. The experimental results and discussion are summarized in

Chapter 4. Finally, Chapter 5 concludes and summarizes the contributions of this thesis

and makes corresponding suggestions to the future work towards the topic.

9

CHAPTER 2: LITERATURE REVIEW

Gallium nitride is one of the semiconductor compounds in the III-V family. Since 1990s,

it has become the focus of intensive research because of its attractive properties. This

binary material presents strong potentialities for high power/high temperature electronic

devices. Moreover, optoelectronic devices based on GaN that emit and detect in the bleu-

ultraviolet part of the spectrum are of great interest for many applications. In this chapter,

a literature review on GaN and its nanostructures with an emphasis on its electrical and

optical properties will be presented.

2.1 Properties of Gallium Nitride and its Nanowires

Gallium nitride naturally crystallizes in the wurtzite form. This structure consists of two

embedded hexagonal close packed (hcp) lattices, one for gallium atoms and the other for

nitrogen elements. The two hcp lattices are offset along the c-axis by 3/8 of the cell

height. Figure 1 shows the wurtzite structure of GaN. Alternate layers of Ga and N atoms

are stacked according to the ABABA scheme in the [0001] direction. GaN can also be

found in the zincblende form if grown epitaxially on a cubic substrate. The zincblende

structure is the cubic analog of the wurtzite structure. It is formed by joining two face-

centered cubic lattices with a shift of ¼ along the longest diagonal of the cube.

10

Figure 2.1 Wurtzite structure of Gallium Nitride [9].

Two polarities can be found in the Wurtzite structure: a metal polarity if the gallium-

nitrogen bond is directed towards the surface or a nitride-polarity in the reverse case. This

absence of centrosymmetry and highly ionic bonds are responsible for the piezoelectricity

of GaN which also exhibits a spontaneous polarization. This polarization field has been

proved beneficial for transistors based on GaN-heterostructures as it significantly

improves the current density. However polarization effects are not desirable for

optoelectronic devices in which they provoke a red-shift in the emission wavelength as

well as a decrease of the recombination efficiency.

11

GaN has interesting optical properties based on its direct and wide bandgap of 3.44eV at

room temperature [10]. With a direct bandgap, GaN is a natural candidate for light

emission and detection (unlike Silicon which cannot be used for optical applications

because of its indirect bandgap). Besides, GaN belongs to the family of wide bandgap

semiconductors similar to AlN (6.2 eV) and InN (0.7 eV). Its large bandgap places it as a

material of choice for optoelectronic devices that operate in the blue and UV part of the

spectrum which were for a long time inaccessible.

Furthermore, GaN is well-known for its superior electrical properties and resistance to

temperature and strain. GaN has above average mechanical and thermal stability.

Because of its wide bandgap, GaN becomes intrinsic at a much higher temperature than

Si or GaAs. This allows the devices made from GaN to be operated in high temperature

environments. GaN benefits also from a high thermal conductivity (compared to Gallium

Arsenide) that helps heat dissipation. Another feature that shows the strength of GaN

material is its high breakdown field, estimated to be superior to 4 MV/cm which is

equivalent from ten to sixteen times the breakdown voltage of GaAs and Si respectively.

Also, GaN possesses remarkable electronic properties. It has high electron mobility and

saturated drift velocity (3×107 cm/s, which is three times superior to that of silicon).

Because of its outstanding transport properties, GaN is widely used for high frequency

and high power devices such as High Electron Mobility Transistors (HEMT) based on

AlGaN/GaN heterostructures.

12

Evolution of the technology towards nanoscale devices has triggered a growing interest

for nanostructures such as nanowires, nanotubes, nanorods, nanobelts, etc. These

nanostructures can serve as important building blocks for miniaturized optical and

electrical devices with improved performance.

The nanostructure refers to a system in which at least one dimension is smaller than 100

nm. By reducing a bulk material in two dimensions, one-dimensional structures such as

nanowires can be obtained. Because of their low dimensionality, nanowires could behave

differently from their bulk counterparts. Interesting optical, electrical and mechanical

properties are expected. In 1-D structures, the density of defects could be reduced

compared to the bulk material. As a result, their mechanical and electrical properties are

improved. For example, high carrier mobility has been reported for nanowires due to a

defect-free structure: electron mobility in GaN nanowires was reported to reach 150 to

650 cm2/V.s against 100 to 300 cm

2/V.s in thin films of GaN for the same carrier

concentration [11]. Semiconductor nanowires are also promising for lasing applications

because of the strong confinement of holes, electrons and photons in their narrow

cylindrical structure. Nanowires can act as both waveguide and optical cavities to

produce coherent UV light due to the wide bandgap of the material. UV-lasing has been

reported in GaN nanowires. Furthermore, the high surface-to-volume ratio make

nanowires well suited for sensing applications. The high sensitivity of 1-D nanostructures

has been exploited for hydrogen detection by Pd and GaN nanowires.

13

2.2 Synthesis of Nanowires

The first attempts for the synthesis of nitride-based nanowires involved the use of

templates such as alumina membranes [12] or carbon nanotubes (CNTs). In 1997, Han et

al. reported the successful growth of GaN and SiN3 nanorods via the use of CNTs [13]

[14]. Later, the synthesis of GaN nanowires was extended towards catalyst-assisted

methods that present some significant advantages over the nanotube confined reaction,

such template-free, lower growth temperature, etc. The catalyst-assisted methods make

use of metal nanoclusters that acts as nucleation centers for the growth of the nanowires.

Each particle gives rise to a nanowire according to the Vapor-Liquid-Solid mechanism

which will be explained in details in this chapter. Moreover, the synthesis of GaN

nanowires was also proved possible by using catalyst-free methods. The mechanism

under the growth of 1-D structures and the major growth techniques for the growth of

GaN Nanowires will be discussed in the following parts.

2.2.1 Catalyst and Catalyst-Free Synthesis of Nanowires

The ideas to use a catalyst particle to grow one dimensional structure is not recent but go

back to 1964 when Wagner and Ellis published the first paper on what is known today as

the Vapor-Liquid- Solid (VLS) mechanism [15]. Their experiment dealt with the growth

of silicon whiskers on a Si (111) wafer and set the basis for the understanding of

nanowire growth via a catalytic procedure. The synthesis of the single crystal nanowires

14

based on VLS mechanism is shown in Figure 2.2. The first stage is the nucleation phase

which corresponds to the formation and activation of the catalyst. The role of the catalyst

particles is to decrease the energy barrier that exists for incorporating the growth material.

During this phase, the radius of the catalyst particle increases with time as growth

material is added to it until saturation is reached. Then, the steady-state phase follows

during which the nanowire length increases as more material reaches the growth interface.

In the ideal case, the incorporation of new material occurs only at the catalyst location

and not on the nanowire sidewalls resulting in the growth of a nanowire with a constant

radius. Finally, the nanowire growth ends because of a change in growth conditions:

decrease of temperature, suppression of gases flux or entire consumption of the catalyst.

The nanowire diameter decreases until growth definitively stops.

Figure 2.2 Illustration of the growth of a silicon nanowire based on VLS mechanism [15].

The choice of the catalyst is of prime importance for the success of the nanowire growth.

The catalyst influences the nucleation at the initial stage. Moreover the diameter of the

final wire is directly related to the diameter of the catalyst. The key to synthesize

15

nanowires with a diameter of a few tens of nanometers is to control the catalyst size.

Concerning the growth of GaN nanowires, efficient catalysts reported are mostly

transition metals such as Au and Ni. Iron, cobalt, indium chloride (InCl3) and nickel

nitrate (Ni(NO3)2) have also been cited as efficient catalysts. The catalyst layer can be

deposited directly on the substrate via sputtering, electron-beam or thermal evaporation.

At high temperature, this catalyst layer will break up and generate small particles all over

the substrate.

Catalyst-free growth has also been carried out successfully. For the synthesis of

nanowires without the mediation of a catalyst two main schemes are currently discussed:

the Vapor-Liquid-Solid and the Vapor-Solid mechanisms (VS). The VLS route is in fact

a self-catalytic process that substitutes Ga particles to the usual metal catalyst. This way,

no foreign substance is needed to activate the growth and the risks of quality degradation

due to catalyst incorporation are eliminated. In the experiment conducted by Stach et al. a

thin GaN layer was decomposed at high temperature (1050°C) in vacuum to provide

liquid Ga droplets as well as vapor containing Ga and N atoms [2]. Adsorption of these

gaseous elements led to the super-saturation of the liquid droplets followed by

precipitation of GaN material and subsequent growth of nanowires. Other reports have

explained of the nanowire growth on the VS mechanism for which no liquid phase is

required to mediate the growth. For example, nanowires have been obtained by thermal

evaporation of GaN powders or by reaction of metallic gallium with ammonia at high

temperature [16].

16

2.2.2 Different Growth Techniques for the Synthesis of GaN Nanowires

Under the framework of Vapor-Liquid-Solid growth, different growth techniques have

been investigated for the synthesis of GaN-based nanowires. Chemical Vapor Disposition

(CVD), Metal Organic Chemical Vapor Disposition (MOCVD) and Molecular Beam

Epitaxy (MBE) are the three major growth techniques for GaN Nanowire growth in the

recent years. A brief summary on the main features and advantages of the three

techniques for GaN nanowire formation is given below.

CHEMICAL VAPOR DEPOSITION (CVD)

Chemical Vapor Deposition (CVD) is a technique widely used in the semiconductor

industry for thin film deposition. A CVD system is composed of a furnace (oriented

vertically or horizontally) in which the desired gaseous species are injected to react and

produce the film constituents. CVD is a general term which encompasses several

techniques from the simplest one that operates at atmospheric pressure (APCVD) or

LPCVD (Low Pressure Chemical Vapor Deposition) to more sophisticated systems such

as PECVD (Plasma Enhanced Chemical Vapor Deposition), MOCVD (Metal Organic

Chemical Vapor Deposition). But here, we only refer the CVD to those simple systems

such as APCVD or LPCVD.

17

The use of a CVD process to synthesize GaN nanowires has gained increased interest

over the years as this technique offers lower cost and greater simplicity than other

processes like MBE or MOCVD. The precursors for the growth of GaN nanowires are

usually ammonia gas (to provide Nitrogen) and gallium under metallic or powder form.

During the growth, the substrates are placed downstream to the ammonia flow and in

close proximity to the Gallium source (between 3 mm to 3 cm). A typical APCVD

system for the growth GaN nanowires is shown in Figure 2.3 [5]. The growth of

nanowires in such a system is highly dependent on the distance between the source and

the substrates, the temperature, the gases ratio and the pressure within the furnace tube.

CVD synthesis benefits from a fast growth rate compared to MOCVD and MBE

processes that makes possible to achieve nanowires of several tens of micrometer in half

an hour. However a high temperature is often required to yield the desired 1-D

nanostructures; temperatures of CVD processes were reported typically in the range from

850°C to 1100°C.

Figure 2.3 An APCVD system for the synthesis of GaN nanowirs showing the gas flow

and the location of the substrates with respect to the gallium source [5].

18

Compared to APCVD, operating the conventional CVD processes at lower pressures,

which is Low Pressure Chemical Vapor Deposition (LPCVD) offers great advantages.

The typical pressure range in such a system is 0.25 to 2.0 torr (30-270 Pa). LPCVD

deposition is widely used in the semiconductor industry for the deposition of high quality

thin films with good uniformity and step coverage. The typical pressure range in such a

system is 0.25 to 2.0 torr (30-270 Pa). LPCVD deposition is widely used in the

semiconductor industry for the deposition of high quality thin films with good uniformity

and step coverage.

Two main regimes are considered for the CVD process depending on the temperature: (1)

The mass transfer regime or diffusion limited regime: The deposition rate is dominated

by the transport of the reacting gases. In order to have a uniform deposition, the fluxes of

gases have to be the same at every location of the chamber; (2) The surface reaction

limited regime: The deposition rate is limited only by the reactions at the substrate

surface. This regime is reached at lower deposition temperatures.

19

For a vapor phase deposition the deposition rate is given by:

(2.1)

where ks is the surface reaction coefficient, hG is the mass transfer coefficient, CT is the

total concentration of gas species, N is the number of atoms incorporated per unit volume

in the film or film density, and Y Mole fraction of the incorporated species in the gas

phase.

The mass transfer coefficient hG is inversely proportional to the total pressure. It is more

difficult for the gas species to go through a “crowded” atmosphere chamber as the

number of collisions is higher. By lowering the pressure, the increased (higher) diffusion

of the gas species is expected and the surface reaction becomes predominant over the

mass transfer during the growth. For instance, by lowering the total pressure from 1 atm

to 1 Torr, the mass transfer coefficient is increased by 100 times. A hG>>ks could be

expected at low pressure. Therefore, equation (2.1) can be simplified as:

, (2.2).

20

which suggests that the growth will be controlled by the surface reaction. CVD in surface

reaction regime is preferred for the deposition of high quality films because it promotes

surface reactions and reduces the probability of gas phase reactions. The gas phase

reaction generally results in poor adhesion and a higher defect density.

METAL ORGANIC CHEMICAL VAPOR DEPOSITION (MOCVD)

Metal Organic Chemical Vapor Deposition (MOCVD) is one of the most popular

methods in the industry for the epitaxy of III-V semiconductor materials [17]. Currently,

MOCVD can produce GaN heterostructures of very high quality. It naturally attracts the

researchers to use this technique for the synthesis of GaN nanowires.

A MOCVD reactor consists of three main parts: the deposition chamber, the gas

distribution system and the exhaust. The substrate is placed inside the reaction chamber

on a susceptor usually made of graphite as shown in Figure 2.4. The precursors of the

film are organometallics compounds: molecules with carbon-metal bonds. For the case of

GaN thin film/nanowires, the trimethylgallium molecule (TMGa: Ga(CH3)3) is used as

the gallium source and Nitrogen is provided by ammonia. A carrier gas (H2 or N2) is

passed through a bubbler which contains the organometallic source in liquid phase. The

carrier gas collects some organic molecules and transports them to the reactor. In the

deposition chamber, the molecules decompose on the heated substrate depositing atoms

21

of the desired element at the surface. The by-products of the reaction desorb form the

surface and are reinjected in the chamber atmosphere. MOCVD growth of thin films is

usually performed between 400°C and 700°C at a pressure of 100 to 700 Torr. The most

important parameters of an MOCVD process are the temperature and the III-V ratio. A

high temperature can increase the desorption of the atoms at the surface and hinder the

film growth whereas a low temperature reduces the surface mobility of the atoms and is

also responsible of the incorporation of impurities in the layer. Both catalyst-based and

catalyst-free growth of GaN nanowires using MOCVD were reported in [18] and [19].

GaN nanowires grown by MOCVD usually exhibit triangular or hexagonal cross-sections

with well-defined facets. The use of different catalysts such as Ni, Au and Fe for GaN

nanowires were also investigated.

Figure 2.4 Schematic of a MOCVD reactor for the growth of III-V materials [17]

22

MOLECULAR BEAM EPITAXY (MBE)

Molecular beam epitaxy (MBE) is a film growth technique that produces high quality

thin films by successive depositions of atomic layers. The films are crystalline and their

orientation is determined by the substrate, hence the name epitaxy. Unlike CVD, a MBE

process operates under ultra high vacuum conditions (lower than 10-10

torr) that

guarantees the purity of the layers and makes possible to realize abrupt interfaces.

Therefore, the deposition rate is very low: 0.5 to 1 µm/h against a few µm/h for CVD. An

MBE system as shown in Figure 2.5 features a main chamber where the film growth

takes place [20]. The film precursors are placed in source ovens (also called effusion cells)

where they are evaporated by resistive heating to produce beams of atoms directed on the

heated substrate. Mechanical shutters positioned in front of the source ovens are used to

regulate or stop the beam fluxes. MBE enables to grow a crystalline layer of elemental

and compound semiconductor materials such as Si, SiGe, and III-V and II-V

semiconductors. In particular, MBE has found applications in the fabrication of

heterostructures for high performance electronic and photonic devices. Although a MBE

system presents a high degree of complexity which results in high equipment cost, it

offers some advantages such as in situ characterization of the film during the growth.

Examples of analysis tools that can be included in the MBE system are electron

diffraction and mass spectroscopy, etc.

23

Figure 2.5 Schematic of a MBE Chamber for the growth of compound semiconductor

materials. The chamber is cooling by liquid N2 during growth.

The mechanism for the MBE growth of GaN nanowires is believed to be either based on

a self-catalytic reaction where molten Ga droplets serves as catalyst seeds or on the

difference in the sticking coefficients of Ga that favors a vertical growth over a lateral

enlargement. The last explanation is likely more probable as no report ever mentioned the

presence of a catalyst particle at the tip of the nanowires grown by MBE.

24

In early days, the MBE growth of GaN nanowires required the initial deposition of a

buffer layer (AlN or GaN layer) to prevent the formation of amorphous layers and ensure

that the nanowires grew vertically. As the technique developed, it was possible to

synthesized nanowires directly on silicon or sapphire substrates. MBE-grown nanowires

are generally synthesized in a lower temperature zone than those grown by MOCVD

because higher growth temperature would increase the amount of Ga desorption. Typical

temperatures reported range between 750°C and 850°C. The precursors’ fluxes are

extremely low allowing a precise control over the growth process. N elements are

provided by nitrogen dissociated using RF plasma while Ga elements come from a

molten source of Gallium. MBE-grown nanowires although synthesized in different

systems and under various conditions share consistent features: nanowires grow normal

to the substrate along the c-axis; they exhibit hexagonal cross sections and faceted

sidewalls as shown in Figure 2.6(a). Nanowires grow densely in bunches which leads

sometimes to the coalescence of very close nanowires. This is one of the main issues with

MBE synthesis. Synthesis through a mask could help in defining the emplacement of the

nanowires. In a recent report Bertness et al. used a SiNx patterned substrate to realize

arrays of GaN nanowires as illustrated in Figure2.6 (b) [21].

25

Figure 2.6 SEM images of GaN nanowires grown by MBE. (a) Bunches of GaN

Nanowires grown without catalyst (b) Arrays of GaN nanowires grown through a

patterned SiNx mask [21].

26

2.3 Characterizations of GaN nanowires

The properties of GaN nanowires grown by different research groups were characterized

by using different characterization tools such as scanning electron microscope (SEM), X-

ray diffraction (XRD) and Photoluminescence (PL), etc. [8, 22-24]. Scanning electron

microscope (SEM) has been utilized mostly for the inspection of the morphology and

structural parameters of the nanowires in terms of the length and diameter, etc. The X-ray

diffraction (XRD) has also been used for characterization of the crystal structures of the

GaN nanowires. In addition, the other materials properties such as the bandgap and

defect energy levels in the GaN nanowires were also evaluated by using

photoluminescence [25]. These characterization techniques will also be used to

investigate the properties of the GaN nanowires grown by this work.

27

CHAPTER 3: LOW PRESSURE CHEMICAL VAPOR DEPOSITION

(LPCVD) SYSTEM SETUP AND EXPERIMENTS

In this chapter, the LPCVD system setup used in this study will be introduced. Detailed

information on the design of experiment to investigate the impact of growth conditions

such as growth temperature, gallium source to sample distance, and pressure on the

formation of GaN nanowires will be summarized.

3.1 Low Pressure Chemical Vapor Deposition (LPCVD) System and

Experiment

In this work, the synthesis of GaN nanowires was performed on silicon substrates (Si

(100) and Si (111)) in a LPCVD system. Figure 3.1(a) illustrates the system

configuration. The Gallium and Si samples on the quartz boat before they are loaded into

the quartz tube are shown in Figure 3.1(b). The system is equipped with a three-zone

heater to ensure good temperature uniformity. The chamber pressure is monitored by a

pressure sensor. During the growth, the Gallium source was loaded into a crucible in the

quartz tube. The substrates were placed vertically in a quartz boat located downstream of

the Gallium source. Six to Seven samples were examined for the each rounds of

experiments, and totally six rounds of experiments were conducted in this study.

28

(a)

(b)

Figure3.1 (a) Configuration of the LPCVD system used in this study. The system

equipped with a three-zone heater and a mechanical pumping system with pressure

controller. (b). A close look to the Gallium source and Si samples on the quartz boat.

29

After being pumped to a low pressure of 10 mtorr, the furnace was ramped up to the

desired temperature (between 700°C and 850°C) in approximately 30 to 40 minutes.

Then ammonia and hydrogen gases were flown in the tube furnace at a rate of 100 sccm

and 150 sccm, respectively. The pressure inside the chamber was regulated by a pumping

system: pressures ranging from 250 mTorr to 800 mTorr could be maintained in the

deposition chamber. For the current study, the growth time was fixed at 30 minutes for

different growth conditions. After growth, the furnace was cooled down to room

temperature before unloading the samples.

The morphology of the as grown samples was inspected by using a Scanning Electron

Microscope (SEM). Characteristic features of the nanowires such as length, diameter and

density were measured based on the SEM image.

3.2 Design of Experiments

In order to gain a thorough understanding of the impact of growth condition such

temperature, pressure, substrate orientation on the formation of GaN nanowire and

determine the optimal growth window, the experiment conditions were carefully

designed to cover a reasonable range in terms of temperature, pressure, etc. Table 3.1

summarizes the experimental conditions which we have performed during this work.

30

Metal Catalyst

Gas flow

(NH3/H2)

Pressure

(mTorr)

Temperature

(°C)

Gold

(3 or 10 nm)

Nickel

(3 or 10 nm)

100/150

250

400

500

600

700

750

800

850

Table 3.1 The experimental conditions for the growth of GaN nanostructures.

After the growth, the samples were inspected by a scanning electron microscopy (SEM)

to evaluate the morphology of the GaN nanostructures including the information on the

density, length and diameter of the nanowires. The crystal structure of the GaN nanowire

was also investigated by X-ray diffraction (XRD). Preliminary investigations of the

optical properties using PL and Raman Spectroscopy were also carried out on some of the

samples. The detailed analysis and discussion on the experimental results will be

presented in Chapter 4.

31

CHAPTER 4: RESULTS AND DISCUSSION

In this chapter, experimental results on the synthesis of GaN nanowire using our LPCVD

system will be summarized and discussed in terms of the effect of metal catalyst and their

thickness, growth temperature, pressure on the nanowire morphology. The optical

properties from some of the samples measured by XRD, PL amd Raman spectroscopy

will also be presented and discussed.

4.1 Influence of Catalysts on GaN Nanowire Formation

The growth of GaN nanowires was experimented on both Si (100) and (111) substrates

coated with two different catalysts: Au and Ni using electron beam evaporation. Both

catalysts were deposited with two different thickness 3 nm and 10 nm. The nanowires

were grown at a temperature of 800°C and a pressure of 250 mTorr. The density of

nanowires was very low for both Au and Ni samples with 10 nm thickness. However,

significant difference was observed between the Au and Ni samples with 3 nm thickness.

SEM revealed the presence of GaN nanostructures with a rather low aspect ratio as

shown in Figure 4.1(a) for the sample using Au as the catalyst. The Si substrate is

uniformly covered with hillocks. It can be seen that elongated features emerged from the

hillocks. However the width of their base was quite large (100-200 nm) compared to that

of nanowires. The mechanism for the growth of these sharper structures seemed to be a

VS process rather than a VLS process as the thinner features grew at the corners of edges

of larger structures. Nevertheless, the density of the sharpest structures is rather low

32

compared to the density obtained with nickel catalyst shown in Figure 4.1 (b). Scratched

areas on the wafers were almost free of nanowires which indicated that the presence of

the catalyst is crucial to promote the growth of the nanowires. Absence of growth with

Gold catalyst has been reported previously by Duan et al. and was explained by the poor

solubility of Nitrogen in Gold [26]. Contrary to the samples with gold catalyst, the

samples coated with 3 nm nickel showed a high density of nanowires. As seen in

Figure4.1 (b), the nanowires had a superior aspect ratio: average diameter of the

nanowires was around 120 nm while their length reached several micrometers. Similar

results were also observed on the other samples grown at different temperatures. In

addition, no obvious difference in terms of nanowire morphology between the Si (100)

and (111) substrates was observed. In our case, it seems that the influence of substrate

orientation on nanowire formation is negligible. Based on the results obtained, in the later

part of the experiments, we will focus on the samples using nickel as the metal catalyst.

33

(a)

(b)

Figure 4.1 GaN nanostructures formed on Si (100) substrates grown at 800 oC (a) 3 nm

Au as the catalyst, (b) 3 nm Ni as the catalyst.

34

4.2 Influence of Temperature on GaN Nanowire Formation

The effect of growth temperature on the formation of nanowires was studied on the Si

(100) substrates. The following experiments were performed by using nickel as the

catalyst as successful growth of nanowires was obtained with this metal. Temperature is a

very important parameter to tune in nanowire growth as changes of a few tens of degrees

have been reported to produce nanowires with very different morphologies. In the

literatures, for the growth of GaN nanowires using the conventional atmosphere pressure

CVD systems, a large temperature range has been reported. The optimal temperature to

form GaN nanowire was reported in the range of 850 oC to 1000

oC. In our case, four

temperatures were tested: 700°C, 750°C, 800°C, and 850°C. SEM images (Figure 3.3)

below show typical nanowire morphology obtained for each temperature. Si (100)

substrates with 3 nm Ni as the catalyst were used. The growth time was fixed at 30

minutes.

35

(a) T = 850

oC (b) T = 800

oC

(c) T = 750

oC (d) T = 700

oC

Figure 4.2 SEM images of GaN nanowires on (100) Si grown at different temperatures. 3

nm Ni was used as the metal catalyst.

It can be seen that the morphology of the nanowires could be largely affected by the

growth temperature. For the sample grown at the temperature of 850°C, most of the Si

surface is covered with 3-D nano- or submicron- structures exhibiting triangular faceting

as shown in Figure4.2 (a). A few nanowires emerged laterally from the already grown

nano- and submicron-structures were occasionally observed. The nanowire structures

36

started to form when the growth temperature was lowered down to 800 oC. At T=800

oC,

although some nanowires grew straight with a smooth surface most of them exhibited a

rough surface. The nanowire body could be seen as a succession of cones stacked along

the main axis. The “stacked-cone” GaN nanowires was reported in the literature [27] [28].

Further reduction of the growth temperature results in a much smooth nanowire surface

with a drastic decrease in the nanowire diameter at T=700°C. The SEM observation tends

to suggest that, for the Si substrate using Ni as the catalyst, reduction of growth

temperature could result in the transition of nanowire growth from 3-D to 1-D. The effect

of growth temperature on the formation of GaN nanowires are quantitively summarized

in Figure 4.3 to Fig 4.5. The calculation of the average size of the nanowires were based

on the SEM inspections in a 5 µm x 5 µm area. A high temperature will give a high

growth rate (~4.5 µm/hr at 800 oC) with large diameter. To achieve a small nanowire

diameter, a lower temperature is needed. An average diameter of 17 nm was obtained at

the growth temperature of 700 oC. If the growth of high density GaN nanowires is the

goal, a growth temperature around 750 oC is required under the given experimental

conditions.

37

650 700 750 800 8500.0

0.5

1.0

1.5

2.0

2.5

3.0

Avera

ge L

ength

(µm

)

Growth Temperature, T (oC)

Si (100)

Figure 4.3 Effect of growth temperature on average length of GaN nanowires.

700 720 740 760 780 8000

2

4

6

8

10

Den

sity (

µm

-2)

Growth Temperature T (oC)

Si (100)

Figure 4.4 Effect of growth temperature on GaN nanowires density.

38

650 700 750 800 8500

50

100

150

Eve

rag

e D

iam

ete

r (n

m)

Growth Temperature, T (oC)

Si (100)

Figure 4.5 Effect of growth temperature on average diameter of the GaN nanowires

grown.

4.3 Influence of Pressure on GaN Nanowire Growth

For the growth of GaN nanowires using the LPCVD system, it is interesting to see how

the synthesis of GaN nanowire is affected by the pressure. Figure 4.6 shows the SEM

images for the GaN nanowire grown under 250 mTorr (a) and 600 mTorr (b) . It is

obvious that growth of nanowire at high pressure results in “stacked-cone” like nanowires.

39

(a)

(b)

Figure 4.6 SEM images for the GaN nanowire grown on Si (100) with 3 nm Ni catalyst of

250 mTorr (a) and 600 mTorr (b).

40

4.4 Characterization of GaN Nanowires by XRD, PL and Raman

Spectroscopy

X-ray diffraction (XRD) was used to investigate the structural properties of the GaN

nanowire sample. Figure 4.7 shows a XRD curve measured from a sample grown at 800

oC. The primary planes observed in the x-ray diffraction scan for GaN were (100), (002),

and (101) which is in consistent with the previously reported XRD results for the

hexagonal wurtzite structure of GaN. This indicates the GaN nanowires on Si substrate

could be predominantly with the single crystal hexagonal wurtzite structures.

30 40 50 60 700

10

20

30

40

50

60

70

(101)

(002)

Co

un

ts

2 theta (degree)

(100)

Figure 4.7 X-ray diffraction scan of GaN nanowire sample grown on Si (100) with 3 nm

Ni catalyst at 800 oC.

41

Figure 4.8 Room temperature PL spectrum of GaN NWs grown on Silicon (100)

substrate at 800 oC using Ni as the catalyst.

Figure 4.8 shows the room-temperature photoluminescence (PL) emission spectrum of

GaN nanowire on Si (100) grown at 800 oC. The excitation ultraviolet light was obtained

from a white color lamp and its excitation wavelength was set at 320 nm. A strong

emission peak at 359 nm (3.4536 eV) which could be attributed to the band-edge

emission of GaN was obtained. The emissions at 364 nm could be due to defect related

emission. The full-width at half maximum (FWHM) of PL spectrum is around 101 meV,

suggesting a reasonably good crystal quality of the GaN nanowires.

42

The Raman spectrum provides further structural information about the GaN material.

Figure 4.9 shows a room temperature Raman spectrum measured from a GaN nanowire

sample grown at 800 oC. The first-order phonons of E1(TO), E2

H and A1(LO),[29 –31]

which peak at 556, 568, and 724 cm−1

respectively are illustrated in the Figure. The peak

at 525 cm−1

is the emission from the Si substrate. The three features clearly show that the

hexagonal phase of GaN appears to dominate the nanowires grown under this condition.

400 500 600 700 8000

3000

6000

9000

12000

Si

A1(LO)

E2H

Inte

nsity (

a.u

.)

Wave number (cm-1

)

Si (100)

E1(TO)

Figure 4.9 Room temperature Raman spectrums measured from a GaN nanowire sample

grown at 800 oC. The first-order phonons of E1(TO), E2

H and A1(LO) for hexagonal

phase of GaN are at the wave number of 556, 568, and 724 cm−1

, respectively.

43

CHAPTER 5: CONCLUSION AND FUTURE WORK

5.1 Conclusion

In this thesis, the synthesis of GaN nanowire using LPCVD has been investigated. The

effect of metal catalysts and their thickness, growth temperature, pressure on the

formation of GaN nanowires was explored. The influence of catalysts and growth

conditions on the morphology of the GaN nanowires was studied and analyzed. In

addition, the characterizations of GaN nanowires using XRD, PL and Raman

Spectroscopy were also performed to understand the structural and optical properties of

GaN nanowires.

Through the project, GaN nanowires have been successfully grown on both Si (100) and

(111) substrates by low pressure chemical vapor deposition (LPCVD) using the reaction

of gallium with the combination of ammonia and hydrogen. The use of Au and Ni as the

catalyst were studied. It was found that, compared with Au, the use of Ni results in a

much high yield for the synthesis of GaN nanowire. A thinner Ni (3 nm) tends to give

better results as compared to the thicker one (10 nm). The impact of substrate orientation

on the formation of GaN nanowires is very small. SEM characterization showed that

highest aspect ratio and highest density were obtained at 750 oC. Lowering down the

grown temperature to 700 oC may result in a much smaller diameter. An average

diameter in the range of 17 to 26 nm was realized. In general, we found that the optimal

temperature for the synthesis of GaN nanowire using LPCVD in the range of 700 to 800

44

oC, which is lower than that reported in the literatures using conventional APCVD

systems.

The grown GaN nanowires were characterized by using XRD, PL and Raman

spectroscopy. The XRD and Raman measurements suggest that grown GaN nanowires

are predominantly by hexagonal GaN phase with the wurtzite structure.

Photoluminescence spectra at room temperature revealed a sharp emission peak at 359

nm. A full width at half maximum (FWHM) of 101 meV was obtained, which is smaller

than the most of reported values for GaN nanowires in the literatures. The material could

be potentially used for electronic and photonic devices.

45

5.2 Future Work

In this work the influence of catalyst, temperature and pressure on the growth of GaN

nanowires has been studied. Further experiments can be performed in order to explore the

impact of other growth parameters. For example, it would be interesting to adjust the

thickness of catalyst layer to understand how it influences on the diameter of the

nanowires. Reducing the catalyst layer thickness is expected to decrease the size of the

catalyst droplets and lead to the growth of the nanowires with smaller size. The flow rate

of the gaseous species (ammonia and hydrogen) is another important parameter to

investigate. It has indeed been reported that the density and morphology of GaN

nanowires could be affected by gas ratio and flow rates.

So far, a valid recipe has been established to obtain a high density of nanowires, the

second step will be to characterize the electrical properties of the nanowires.

Determination of electron mobility and electrical conductivity will give further insight

into their electronic behavior. Growth of the GaN nanowire on patterned Si and

fabrication of nanowire based devices for different applications such as sensors and

transistors are also of great interests, which could be investigated in the future.

46

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