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This document is downloaded from DR‑NTU (https://dr.ntu.edu.sg)Nanyang Technological University, Singapore.
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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