1

CHAPTER 1

INTRODUCTION

This chapter explains about the background information of nanotechnology,

definition of nanowires, fabrication of NWs, problem statement, objectives, scopes,

and the significance of the study that had been conducted.

1.1 General Information

Nanotechnology generates a lot of attention these days and therefore it

expectation is not only in the academic community but also among investors, the

governments and industries. It is predicted that nanotechnology would be able to

provide better lasting, cleaner, safer and smarter products for homes,

communications, medicine, transportation, and agriculture. This atomic scale

structure has produced novel materials and devices with great potential applications

in a wide number of fields as stated by Serrano E. et al. (2009).

2

Actually nanotechnology is a field which concentrated on nanometer scale of

natural and artificial structures. It is stated that the range of nano scale is in the range

of one nanometer (10−9 m) to a few hundreds of nanometer. 1 nm is approximately

the distance from one end to the other of a line of five neighbouring atoms in an

ordinary solid (Bruus H., 2004).

NW, which is one of the basic products, plays a key role in this area. NW is a

two-dimensional nano scale structure that has future application in integrated

electronics, and optoelectronic devices, such as light emitting diodes (LEDs), solar

cells and sensors. There are two approaches to produce quantum wire structures,

which is either top-down or bottom-up. Top-down method involves lithography,

metallization, and etching. Bottom-up methods begin with arrangement of atoms or

molecules to build up nanostructures and in some cases through smart use of self-

organization.

As time pass by, a lot of advancement has been made in instrumentation for

observation and manipulation of individual atoms and molecules such as atomic

force microscope (AFM), field emission-scanning electron microscope (FE-SEM)

and transmission electron microscope (TEM).

1.1.1 Definition of a Nanowire

Samuelson L. (2003) had reported that, a NW can be defined as an extremely

thin wire with a diameter in the order of a few nanometers (nm) to ~100nm. A free

standing NW is also called nano whisker. Earlier, NWs have a promising future as

one-dimensional (1D) building blocks for a wide range of applications such as in

electronics and photonics devices including photo detectors, chemical and gas

sensors, waveguides, LEDs, micro cavity lasers, solar cells and non linear optical

converters also in life-sciences as described by Yan R. et al. (2009). NWs have

many interesting properties that are not seen in bulk or 3-D materials because

3

electrons in NWs are quantum confined laterally and thus occupy discrete energy

levels.

There are three types of nanostructures. They are NWs, quantum dots (QDs)

and quantum wells (QW). A quantum dot is a semiconductor whose excitations are

confined in all three spatial dimensions. As a result, they have properties that are

between those of bulk semiconductors and those of discrete molecules, while

quantum well is a potential well that confines particles, which were originally free to

move in three dimensions (3D), to two dimensions (2D), forcing them to occupy a

planar region as described by Kang J.H. et al. (2009).

In Science and Technology Dictionary, Quantum Wire (QWR) is defined as a

strip of conducting material about 10 nanometers or less in width and thickness that

displays quantum-mechanical effects and universal conductance fluctuations. A

quantum wire can also be described as an electrically conducting wire, in which

quantum effects affects transport properties by limiting the motion of electrons or

holes in two spatial directions or in two-dimensional (2D) but at the same time still

allowing free propagation in the third-dimensions (3D).

1.2 Statements of Problem

Different methods have been used to synthesize group III-V materials

nanostructures. That includes metal-organic chemical vapor deposition (MOCVD).

NWs have been grown with the aid of catalyst before and the kinetics and

mechanism of III-V NWs growth using Au catalyst has also been investigated and is

relatively well understood. Gold (Au) colloid usage as the catalyst is one of the vital

aspects in NWs growth. The main function of these gold colloids is as a controller on

where the wires growth is significant.

4

Many research before had been focusing on the effect of the growth time,

growth temperature, and also the pressure variation on the growth of NWs. But not

much of them are really discussing in depth the tapering factor and phenomenon due

to the source mole fraction that occurring in the InGaAs NWs and the angle of grown

InGaAs NWs on GaAs (100) substrate. The source mole fraction and the substrate

orientation were observed to be an important factor for controlling the morphologies

of InGaAs structures.

The aim of this work is to investigate the tapering phenomenon due to Indium

mole fraction in InGaAs NWs using liquid Au droplets and to calculate the angle of

grown InGaAs NWs on GaAs (100) substrate. FE-SEM reveals that the shapes of the

nanostructures can be precisely controlled in the MOCVD process to synthesize

wires.

Therefore, the growth of InGaAs will be conducted in MOCVD system to

establish the mechanism that will help out wire growth and to optimize each growth

conditions. It is being proposed that the crystal morphology is determined by the

relative rates of InGaAs deposition via the vapor-liquid-solid (VLS).

1.3 Objectives

The objectives of this research are as follows:

i. To grow the InGaAs NWs on GaAs (111)B substrate with different Indium

mole fraction.

i. To determine the tapering factor in InGaAs NWs due to different Indium

mole fraction grown on GaAs (111)B substrate.

ii. To grow InGaAs NWs on GaAs (100) substrate.

iii. To determine the growth direction of InGaAs NWs with respect to GaAs

(111)B and (100) substrate.

5

1.4 Scope of study

In line with the objectives, the experiments will be done under the following scopes;

i. Growing of the InGaAs NWs on GaAs (111) and GaAs (100) substrates using

the MOCVD technique.

ii. Observation on the distribution, size of the wires, tapering factor and

uniformity of the InGaAs NWs using FE-SEM.

1.5 Significant of Study

This study may help other researchers to find out the tapering phenomenon in

InGaAs NWs using liquid gold (Au) droplets and the growth direction of InGaAs

NWs with respect to its substrates grown by MOCVD technique. This study will help

to determine the tapering factor in NWs thus lead to the development of

nanotechnology in the near future especially in semiconductor fields.

6

CHAPTER II

LITERATURE REVIEW

This chapter explains the theories and the early information related to the

study which helps in the pursuing of this project. The information includes NWs,

vapor-liquid-solid (VLS) mechanism, gold (Au) colloid as catalyst, MOCVD system,

tapering phenomenon in NWs and the relationship between the angle of NWs and

substrate orientation, also the analysis and characterization of NWs.

2.1 Nanowires (NWs)

Semiconductor NWs represent a unique system for exploring phenomena at

the nano scale and are also expected to play a critical role in future. It is predicted to

have lower threshold currents, lower temperature sensitivity, and narrower spectral

line widths rather than quantum well devices. The properties make NWs an

extremely attractive in future applications involving large scale integration of laser

diodes, sensors and other photonic and electronic devices.

7

As defined earlier, NW is defined as a strip of conducting material about 10

nanometers (nm) or less in width and thickness that displays quantum-mechanical

effects. Works by Nemanja Isailovic (2004), have shown that the basic difference

between quantum wire and the classical wire is that quantum information cannot be

copied instead, it must be transported which means destroying the information at the

source and recreating it at the destination. Inside quantum wire there are electrons

confined to a narrow one-dimensional (1D) channel with motion perpendicular to the

channel quantum mechanically frozen out.

As time past by, modern semiconductor technologies could be applied to

fabricate the wires. There are several types of NWs which were metallic, semi-

conducting, insulating and molecular NWs. Metal NWs are made from platinum (Pt),

gold (Au), or nickel (Ni). Whereas semi-conducting NWs are made by the

composition of phosphate (InP), arsenide (GaAs), silicon (Si), or nitride (GaN).

Insulating NWs are made up from silicon dioxide (SiO2) or titanium dioxide (TiO),

while molecular NWs involves process of repeating organic or inorganic molecular

units in particular order. As for this project we used InGaAs and GaAs NWs, it can

be stated that, semi-conducting NWs were used for the project.

Many systematic studies have been published on the growth of NWs using

different methods of growth. As mentioned before, the fabrication methods are

generally classified as top-down and bottom-up approaches. The later approach

refers to the arrangement of smaller components into more complex assemblies,

which include chemical vapor deposition (CVD) and also molecular beam epitaxy

(MBE). There are a number of forms of CVD used. Each process differs in the means

by which chemical reactions are initiated and the process conditions. For this project,

MOCVD is used rather then MBE due to its versatility to the sources also because of

its flux uniformity is better than MBE.

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2.2 Vapor-Liquid-Solid (VLS) Mechanism

The vapor-liquid-solid method (VLS) is a mechanism for the growth of one-

dimensional (1D) structures. It can be stated as one of the most popular method

commonly used to fabricate semiconductor NWs. This mechanism was introduced by

Wagner & Ellis in 1964. According to this method, a small nanometer size Au

particle on the semiconductor surface forms a eutectic liquid alloy with the host

material at growth temperature. Based on Wagner R. S. and Ellis W. C. (1964) the

growth of a crystal through direct adsorption of a gas phase onto a solid surface is

generally very slow. Thus, VLS mechanism improved this by introducing a catalytic

liquid alloy phase which can rapidly adsorb a vapor to supersaturation level, and

from which crystal growth can subsequently occur from nucleated seeds at the

liquid-solid interface.

As formulated by Wagner & Ellis (1964), the VLS mechanism is typically

described in three stages. The stages include the preparation of a liquid alloy droplet,

usually (Au) upon the substrate from which a wire is to be grown. Next is an

introduction of a vapor (sources) to the substance which is then adsorbs on to the

liquid surface, and thus diffuses into the droplet. Finally is the formation of

supersaturation and nucleation at the liquid or solid interface leading to axial crystal

growth to produce NWs. A schematic of the elementary processes that occur during

the deposition is presented in Figure 2.1.

Recently, the VLS growth technique has been extended to nanometer-sized

Au particles on a variety of semiconductor materials including III-V compound.

With a reduction in size, unique electrical, mechanical, chemical and optical

properties would result in large surface area and quantum confinement effects.

9

Figure 2.1: Schematic diagram of the VLS growth mechanisms for Self-Catalyzed InP NWs in MOCVD (after Robyn Woo L. et al. (2009).

2.3 Catalyst

There are several requirements for catalyst particles. As wrote by Wagner R.

S. et al. (1975), catalyst particle must form a liquid solution with the crystalline

material to be grown at the NWs growth temperature. The solid solubility of the

catalyzing agent needs to be low in the solid and liquid phases of the substrate

material as well. The equilibrium vapor pressure of the catalyst over the liquid alloy

also have to be small, so that the droplet does not vaporize, shrink in volume, and

decrease the radius of the growing wire until, ultimately, growth is terminated.

Above and beyond, the catalyst must be inert (non-reacting) to the reaction products

during CVD NWs growth.

Works by Yicheng L. and Jian Z. (2004) have shown that during VLS growth

process, the rate of wire growth is dependent on its diameter. The larger the wire’s

diameters, the faster the NWs grow axially. This is due to the fact that the

supersaturation of the catalyst is the main driving force for NWs growth and

decreases with decreasing wire diameter.

10

Usually, for the growth of NWs, gold (Au) colloid is used as catalyst. Spirkosa D. et

al. (2009) in their report have stated that Au is used because it works well with wide

range of semiconductor materials and this surely helps in better performance of

devices. It is in line with what Zhang X. et al. (2006) had described that the

concentration of gold (Au) colloid particles certainly gives significant consequence

to the height of NWs.

2.4 Metal-Organic Chemical Vapor Deposition (MOCVD)

Chemical vapor deposition (CVD) is a chemical process and can be classed

as a system where chemical precursors react either in gas phase or on the substrate to

produce a high-purity, high performance solid materials or film which has a

composition different from any of the precursors. It is a technique to produce II-VI

and III-V semiconductor structures.

In recent years, a number of different CVDs are widely used. These processes

differ in the means by which chemical reaction are initiated and process conditions.

In the semiconductor industry two major techniques are well established. There are

MOCVD and MBE. The difference of these two epitaxy techniques is that MOCVD

are associated with the epitaxy of compound semiconductors using gas sources like

arsenides (AsH3), phosphides (PH3), and nitrides (NH3) and MBE are associated with

high purity solid source materials such as arsenic, gallium, indium or aluminium.

The MOCVD is a process that utilizes the chemical reaction between the

Metal-Organics (MO) and or between the MOs and hydrides. MOCVD is

distinguished by the chemical nature of the precursor gases. As the name implies,

metal organic compounds like trimethlygallium (TMGa), trimethlyindium (TMIn),

and trimethylaluminium (TMAl) are employed. They react with group V hydrides.

11

As what Ramdani J. and Vaccari G. wrote in Semiconductor Manufacturing

Handbook (2004), for III-V compound semiconductor epitaxy, an additional

parameter which is the V/III ratio simply the mole fraction ratio of group V elements

over group III elements must be taken into account. The V/III ratio needs to be high

enough to compensate for the low decomposition rates of hydrides and to drive

carbon and oxygen from group V lattice sites. For example GaAs grown at a typical

temperature of 700°C, a V/III of about 100 is sufficient while for GaN using

ammonia for group V, a V/III of about 1000 at 1050°C is needed for high-quality

material.

It is desired that the chemical reaction does not occur at room temperature but

occurs only at the elevated temperatures. So the reactant is supplied to the substrates

that are heated to the desired temperature. Then when the reactants reach on the

surface of the substrate, they will undergo thermal decomposition and react on it to

form the film. The metal organic (MO) sources are delivered to the reactor chamber

by the carrier gas (high purity hydrogen) that passes through the MO bubblers. The

hydrides are usually gaseous and directly delivered from the cylinder. When different

kinds of compounds are desired to be grown on the same substrate, the injections of

reactants are switched by the injection manifold.

The processes parameters include the temperature, pressure, and the amount

of reactants supplied to the substrate. Since it is desired to grow films with uniform

thickness and composition on a large area of substrate, the reactants are fed into the

growth chamber through the shower head.

The MOCVD reactor is usually equipped with the components listed below in order

to fulfill the function above:

i. Growth chamber with a shower head, a heater, a susceptor on which the

substrate is sit.

ii. Vacuum system that control the pressure of the growth chamber.

iii. Injection manifold that switches the reactants into the growth chamber.

iv. MO bubblers in the temperature bath.

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v. Hydrides in the cylinders.

vi. Gas distribution system equipped with Mass Flow Controllers (MFCs),

Electronic Pressure Controllers (EPCs), valves, and SUS tubing.

vii. Exhaust lines through which the by-product of the chemical reaction passes

out.

The gas manifold is made of semiconductor-grade stainless steel tubing with

pneumatic valves, mass flow controllers, and pressure regulators and transducers.

The growth chamber is made of either quartz or stainless steel and consists of a

susceptor and elements of heating (resistive, induction heating, and lamp heating)

and finally the exhaust system. Figure 2.2 (a) and (b) below shows the pressure

controller and toxic gas detector used at Ibnu Sina Institute MOCVD system.

Figure 2.2 (a): Pressure controller

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Figure 2.2 (b): Toxic Gas detector

Nowadays, two reactor designs are available which are the vertical reactors in

which the gas flow is perpendicular to the substrate surface and horizontal reactors in

which the gas flow is parallel to the substrate. In addition, for batch or multiwafer

processes, two types of susceptor designs exist which are the barrel type and the

pancake type. These reactors are suitable for small diameter substrates (less than 200

mm) standard complementary metal oxide semiconductors (CMOS) and for discrete

device applications where the requirements in thickness and resistivity uniformity are

not tight. Advanced CMOS epitaxy systems for 200 and 300 mm diameter wafers are

of the horizontal type equipped with cassette load locks, a transfer chamber, and a

cool-down chamber. Figure 2.3 and 2.4 below show the MOCVD system and its

schematic diagram located at Ibnu Sina Institute, Universiti Teknologi Malaysia used

for the fabrication of III-V compound semiconductor.

14

Figure 2.3: MOCVD system at Ibnu Sina Institute for Fundamental Science Studies

15

Figure 2.4: Schematic Diagram of MOCVD Gas lines

16

2.5 Tapering in the Growth of NWs

Figure 2.5 below had been reported by Cho H. S. et al. (2010). In this study,

Ge nanocones were grown by an Au-catalyzed process in a chemical vapor

deposition (CVD) chamber using germane (GeH4) as the source gas with temperature

variation. The corresponding variation of taper in the segments was observed. When

the growth was conducted at an initial temperature of 350oC then increased to 400oC,

faceted pillars consisting of a base segment of largely uniform diameter and a conical

tapered tip were formed. When the growth was conducted at 395oC and 375oC, then

at 350oC, the NWs featured a base with a wider taper and tips with a smaller taper.

Figure 2.5: SEM images and respective schematic diagrams of Ge NWs grown at:

(a) 380oC, (b) 350oC, then increases to 400oC, and (c) 395, 375, then 350oC (After

Cho H. S. et al., 2010).

17

Thus, from all of the images, Cho H. S. et al. (2010) in their report had concluded

that, the tapering phenomenon occur depending on the growth temperature.

Catalyzed unidirectional growth at lower temperatures approaching the Au–Ge

eutectic point, and increasingly isotropic and facet-bounded epitaxial grow that

higher temperatures up to 400oC.

The effect of temperature during the growth is not only seen through the

morphology of the quantum wires produced, but it could also influence the sources

ratios (net carrier concentration). Kasemset D. et al. (1984) in their report had

reported that the growth temperature does affect the carrier concentration for

different V/III sources ratios. In general, the net carrier concentration decreases at

higher growth temperature. This could be due to the differences in the relative

incorporation of donors and acceptors. At higher growth temperatures, the

concentrations of donors and acceptors move closer together resulting in lower net

carrier concentration.

In other research conducted by Plante M.C. et al. (2007), the Au-assisted

growth of GaAs NWs by gas source molecular beam epitaxy has been investigated.

This paper generally investigates about the tapering, sidewall faceting and crystal

structure of GaAs NWs. Figure 2.6 shows 45° titled view of GaAs NWs grown on

GaAs (111) B substrate with growth time variations.

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Figure 2.6: 45°tilted view of GaAs NWs grown on a GaAs (111) B substrate for (a)

3 min (b)10 min and(c) 30 min at a temperature of 525 ºC and V/III ratio of 1.5. The

insets are TEM images illustrating the typical morphology of the NWs from each

specimen (After Plante M.C. et al., 2007).

It is important to note that the NWs diameter is essentially constant up to a

certain height from the NWs base, and than reduces rapidly near the tip thus

proposed that the radial growth occurs at some expense of axial growth via layer-by-

layer and step-flow growth modes on the NWs sidewalls.

2.6 Substrate Orientation

Mostly NWs growth is carried out on (111) substrates that commonly results

orthogonal oriented NWs to the substrate surface (Rosnita Muhammad et al., 2009).

In addition, NWs with cubic crystal structure are often epitaxially grown on (111)

substrates to achieve vertically aligned, out-of-plane NWs growth along the surface

normal. Vertically oriented NWs on (100) substrates is also preferred to grow. But

unfortunately, NWs growth on (100) substrates were typically yields tilted growing

along the equivalent (111) crystallographic directions (Fortuna S. A. et al., 2008).

19

Based on Wagner and Ellis (1964), NWs generally grow in the crystal direction that

minimizes the total free energy which, in most cases, is dominated by the surface

free energy of the interface between the semiconductor and the metal catalyst.

Additionally, in the origin of thermodynamics, a system or any material was in it

most stable state only when it is in a state with the lowest energy. This were

supported by research conducted by (Ghosh S. C.et al., 2009) where the NWs growth

on polar III–V substrates such as GaAs (100) results in NWs grown along the (111)B

directions where As-faced planes are preferable for nucleation.

2.7 Characterization of NWs

The information on nanostructures has open up ideas for possibility of

measuring the parameters associated with relevant theories and mechanism. Selected

techniques have to be used to characterize the NWs since the surface morphology of

the NWs cannot be seen by unaided human eyes.

For the characterization of NWs, SEM is used to characterize the surface

morphology of the grown NWs. Interaction of electrons with atoms that make up the

sample producing signals that contain information about the sample’s surface

topography, composition and other properties.

The SEM has many advantages over traditional microscopes. Some of its

main advantages are that, the SEM has a large depth of field, which allows more of a

specimen to be in focus at one time. The SEM also has much higher resolution, so

closely spaced specimens can be magnified at much higher levels. Figure 2.7 shows

an image of NWs that being characterized using SEM.

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Figure 2.7: Image of InP NWs (After Woo R. L. et al., 2009).

Additionally, the NWs samples also can be viewed with the aid of FE-SEM

(Rosnita Muhammad et al., 2008). A field-emission cathode in the electron gun of a

scanning electron microscope provides narrower probing beams at low as well as

high electron energy, resulting in both improved spatial resolution and minimized

sample charging and damage. FE-SEM produces clearer, less electrostatically

distorted images with spatial resolution down to 1 1/2 nm, which is 3 to 6 times

better than conventional SEM. High quality, low voltage images are obtained with

negligible electrical charging of samples. Figure 2.8 shows the image of NWs that

being characterized using FE-SEM.

Figure 2.8: Image of GaAs NWs (After Rosnita Muhammad et al., 2008)

21

CHAPTER III

METHODOLOGY

This chapter discusses the research method that was used while conducting

the study. It includes the sample preparation of GaAs (111) and (100) substrates until

the analysis and characterization of InGaAs (InGaAs) quantum wires using FE-SEM.

The experiments were done in RF heated reactor cell under low pressure

MOCVD condition (100 mbar). Trimethylgallium (TMGa), Trimethylindium

(TMIn), Arsine (AsH3) and GaAs (111) and (100) substrates were used in this

experiment.

3.1 Sample Preparation

The experiments started with semi insulating GaAs (111) and (100) wafer

immersed in 0.1% poly-L-lysine (PLL) solution for 3 minutes. After washing with

22

de-ionized (DI) water and subsequently blow dried with N2, the wafer was cut into

desired sizes depending on application. The 30 nm diameter gold colloids were

dispersed onto the wafer surface by using micro littre pipette and immediately

washed after 20 seconds, followed by drying with N2. The PLL layer on the surface is

positively charged and attracts the negatively charged of the gold colloids.

The growth recipes were first keyed-in into the computer. It consisted of all

information needed for the experiment including the pressure (Torr), temperature

(oC), time and also the sources ratio for the experiment to the controller computer

that was connected and controlling the MOCVD machine.

3.2 Substrate Loading

Before the chamber was opened and the substrate was loaded into the

chamber, the pressure in the growth chamber should always be set to the atmospheric

pressure. It was opened by lowering the SUS plate at the bottom of the SUS growth

chamber using the up-down system operated by N2 gas. It was important to flush the

growth chamber first when it were opened. Since N2 is an inert gas, N2 purge was the

best for this propose. It was also possible to used H2 gas rather than N2 gas but, it

should be made sure that the H2 gas was properly ventilated.

Next, the substrate was loaded onto the graphite susceptor by using tweezers.

Afterwards, the growth chamber were closed by raising the lower SUS plate by using

the up-down system and the carrier gas was started to flow into the chamber.

Usually, H2 gas was used as the carrier gas and it was supposed that the carrier gas

flows through all the lines that will be used to grow.

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After the chamber was closed, the growth chamber was evacuated. The

carrier gas flow was started and the pressure was set to the desired value. This

process was done by opening the valves in between the growth chamber and the

vacuum pump. The vacuum pump was supposed to be run all the times unless in

maintenance process or in an emergency case. The pressure in the growth chamber

was automatically controlled by the pressure control system that were composed of

the Baratron (Absolute Pressure Transducer), Pressure Controller, and the butterfly

valve.

After the substrate was loaded, the growth chamber was then evacuated to the

desired pressure. Then the valve in between the injection manifold and the growth

chamber was opened in order to flow H2 gas into the growth chamber. Next, wait

until the pressure settles down to the desired value.

3.3 MOCVD Growth Process

The heater of MOCVD was turned on and the temperature was raised to the

pre-bake or generally known as soft bake temperature. The temperature was

automatically controlled by the Temperature Controller that could be manually or

computer set. Before it was heated up to the actual process temperature, the

temperature was usually set around 250 to 300oC. The rationale of this temperature is

to prepare the susceptor and the growth chamber in the MOCVD. Pre-bake or soft

bake temperature usually holds for about 10 minutes.

To provide overpressure, the flow of hydrides was started. If the substrate

contained the atomic element that easily evaporates, then reactants were needed to be

supplied to compensate its evaporation. As in this project, InGaAs NW was the

24

desired outcome, therefore AsH3 or As reactant was supplied to provide

overpressure. The overpressure is then kept until the growth run were finished.

The substrate was then annealed or heated in-situ at 600oC for 10 minutes under

AsH3 ambient to desorb surface contaminants. This was also called as the hard-

baking process. At the same time, Au particles were alloyed with GaAs, primarily by

an up-take of Ga particle in the Au droplet. It was stabilized for 10 minutes after the

process.

The carrier gas was then started to flow through the MO bubbler. The

stabilization period was needed for the MOs in the bubbler to be saturated by the

carrier gas, usually about 10 minutes. With the overpressure, the temperature was

raised for the process. The reactants which were trimethylgallium (TMGa) and

trimethylindium (TMIn) were then flowed into the process by switching the injection

manifold valves to the run position from the vent position that was done through the

run program prepared on the computer.

3.4 Sample Unloading

Before the growth chamber was opened, the carrier gas was switched from H2

to N2. In order to open the growth chamber to unload the grown sample, it was

required to follow the same procedure as the sample loading process. Subsequently,

when the susceptor was already cooled down to room temperature, the growth

chamber was then ready to open to remove the sample out from the growth chamber.

When the sample was taken out, the growth chamber was evacuated to

maintain it in vacuum condition. The air in the growth chamber could oxidize the

25

reactants in the growth chamber wall. Later then, the growth chamber is filled back

with H2 or N2 gas.

3.5 Sample Analysis and Characterization Technique

In this project, the samples were analyzed by using Field-Emission Scanning

Electron Microscope (FE-SEM) as it has much higher resolution, therefore closely

spaced specimens can be magnified at much higher level. Besides, it has an ability to

produce strikingly clear images. In order to analyze the sample, it should be prepared

first by coating the sample with Auto Fine Coater (JOEL JFC 1600). The purpose of

Auto Fine Coater coating was to hold the grown nanowires in place by increasing the

conductivity between the nanowires and the surface of the substrate. After the

coating process was finished, the sample was then attached to the FE-SEM specimen

holder to be observed.

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CHAPTER IV

RESULTS AND DISCUSSION

This chapter showed the results of the experiment that had been conducted.

The results were in the form of the FE-SEM images of the NWs which showed the

distribution and area density (number of NWs per unit area) of NWs in each sample.

From the FE-SEM images the length and tapering factor (diameter) of NWs were

determined including the direction or orientation of the NWs.

4.1 FE-SEM Images of InGaAs NWs

The images of InGaAs NWs grown using gold (Au) droplets as a function of

Indium mole fraction on GaAs (111) substrate as characterized by Field-Emission

Scanning Electron Microscope (FE-SEM) were shown in Figure 4.1 below.

Morphology of NWs looks very different even grown at narrow range of Indium

mole fractions. At 0.41 the InGaAs NWs grown were up straight with relatively

cylindrical shape. While at 0.47, it grown with bigger size at the bottom leaded to

27

tapered wire. At 0.65, InGaAs NWs grown with low height and highly tapered form

whereby the top of the NWs was highly small compared to the based of the NWs.

Figure 4.2 shows the image of InGaAs NWs grown using gold (Au) on GaAs (100)

substrate. The NWs growth on (100) substrates were shown to yields tilted growing

along the equivalent crystallographic directions.

Figure 4.1: FE-SEM images of InGaAs NWs grown with different value of Indium

mole fraction: (a) 0.41, (b) 0.47, and (c) 0.65. Images were viewed at an angle of 50o

from the surface normal.

ba

c

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Figure 4.2: Images of InGaAs NWs grown on GaAs (100) substrate

From these images, there are several parameters that can be discussed as in

section below. Those parameters are the area density, growth direction, length and

tapering factor of the grown NWs.

4.2 Effect of Indium Mole Fraction Value towards Area Density of InGaAs

NWs

The morphology of the InGaAs NWs varies depending on the process

condition. From Figure 4.1 the area density initially increases from 2.24 x 1012 m-2 at

indium mole fraction 0.41 to 2.53 x 1012 m-2 at indium mole fraction 0.47. However,

when the indium mole fraction is increased to 0.65 the area density decreased to 1.19

x 1012 m-2. The change in the trend of area density was due to the change in the

growth of the NWs. Besides, it can be observed that InxGa1-x As NWs grown were

different in diameter, surface density and length.

29

Figure 4.3: The change of area density with Indium mole fraction.

4.3 Effect of Indium Mole Fraction Value to the Length and Tapering of

InGaAs NWs

The NWs’ length and diameter can be simply controlled by adjusting the

growth parameter. In this study, the NWs were varied by the change in Indium mole

fraction. To study the gradation of NWs tapering, the tapering factor (TF) is used

(Wibowo Edy, 2011). The smaller the tapering factor value, the lower the NWs

tapering. The poorer value that TF can take place is zero (0.00). Meaning that, the

diameters along the grown NWs become more uniform at much lower value of TF.

Therefore, the tapering factor TF can be calculated using equation 4.1 (c) below

(Wibowo Edy, 2011). The average length, average diameter, and tapering factor of

the NWs grown with different Indium mole fraction (x) are shown in Table 4.1.

30

= 4.1 (a)

TFi= 4.1 (b)

= 4.1 (c)

: Tapering factor ( 0.00)

TFi : Tapering factor for individual NW

n : The number of NW sample

: Average diameter of individual NW

dTi, dMi, dBi : Diameter of tip, middle and bottom of individual NW, respectively

(After Wibowo Edy, 2011)

Table 4.1: The length, diameter, and tapering factor of the InGaAs NWs at different

Indium mole fraction (x)

Indium Mole

Fraction (x)

Average Length

(nm)

Average

Diameter (nm)

Tapering Factor,

(TF)

0.41 1000 1070 11.00

0.47 1180 966 26.59

0.65 455 1140 30.78

31

Figure 4.4 shows that, the dependence of length and tapering of the NWs towards the

Indium mole fraction value. Based on the figure, the length of InGaAs NWs were

increased when the Indium mole fraction increasing from 0.41 up to 0.47, while

started to decreased when the Indium mole fraction were further increased from 0.47

to 0.65. At the same time the tapering of the NWs increases continuously from 0.41

up to 0.65.

Figure 4.4: Plot of InGaAs NWs length versus Indium mole fraction

The change in the trend of the length and tapering was due to the change in

the growth mechanism from direct impinging to combination of direct impinging and

sideway diffusion of source atoms. At lower Indium mole fraction most atoms were

favorable for the direct impinging mechanism (Figure 4.5(a)). As the Indium mole

32

fraction increased to 0.65 the mechanism has changed to a combination of direct

impinging and sideways diffusion (Figure 4.5(b)).

Figure 4.5: Different growth mechanism models for sources atom incorporated into

the growth of InxGa1-xAs NWs. The metal droplet (seed particle) is either in solid or

partially molten state, (a) direct impinging mechanism, (b) combination of direct

impinging and diffusion of source atoms from substrate surface (c) domination of

diffusion of source atoms mechanism from substrate surface.

Figure 4.6: Plot of InGaAs NWs tapering factor versus Indium mole fraction

Substrate

(b)

1

3

(a)

2

1

2

3

(c)

33

Based on the above results, the tapering of the NWs at high value of indium mole

fraction is due to the effect of radial growth. A significant amount of reaction species

arrived at the NWs sidewall or GaAs (111) substrate and diffused towards the gold

nanoparticle. During the migration process, the incorporation of the reaction species

into the GaAs (111) surface and NWs sidewall takes place. This incorporation was

known as the 2-D growth, results in the tapering of NWs (Tan H. H. et al. 2006).

Besides, the decrease of apparent NWs length from 0.47 to 0.65 is due to the increase

of 2-D growth at GaAs (111) substrate.

4.4 Effect of Different Substrate Orientation to the Growth Direction of

InGaAs NWs

The substrate often chose such that the NWs growth direction were normal to

the growth plane. From this experiment, the growth direction for InGaAs NWs

grown on GaAs (111) were in (111) direction which is perpendicular (90°) to the

substrate, while for InGaAs NWs grown on GaAs (100) they grew at 52° to the

substrate surface.

The above result were obtained by the condition that NWs generally grow in

the crystal direction that minimizes the total free energy which in most cases, it were

dominated by the surface free energy of the interface between the semiconductor and

the metal catalyst. For zinc-blende crystals like GaAs, it has been widely observed

that the semiconductor catalyst interface often forms a single surface at the lowest-

34

energy (111) plane and thus NWs tend to grow in the (111) direction for most growth

conditions (Wagner and Ellis, 1964).

For compound zinc-blende semiconductors and their alloys the (111)B is the

lowest energy plane (Braun W et al., 2001) and therefore NWs have been generally

observed to grow in the (1 11)B direction (Hiruma K et al., 1995). For the case of

InGaAs NWs grown on GaAs (100) substrate, the growth direction were in (111)

direction but with an angle of 38° to the normal or simply 52° from the substrate

surface. This was obtained by the condition that GaAs (100) substrate has much

higher surface energy compared to that of GaAs (111) substrate.

35

CHAPTER V

CONCLUSION AND SUGGESTION

This chapter presented particular conclusions that had been obtained from all

the results of the conducted experiment. Several suggestions were also presented in

the purpose for future research to improve the previous conducted research.

5.1 Conclusion

From the results and discussions that had been presented, there are several

conclusions that can be attained:

36

i) The growth of InGaAs using vapor-liquid-solid (VLS) technique by MOCVD

can be done by setting the growth temperature at 400°C and the growth time

at 30 minutes using GaAs (111)B substrate.

ii) For the increasing of Indium mole fractions value from 0.41 to 0.47, both

length and the tapering (diameter) of NWs were increased while further

increased of Indium mole fractions from 0.47 to 0.65 leaded to the decreased

in NWs length but, on the contrary the tapering were continuously increased.

iii) The growth of InGaAs using vapor-liquid-solid (VLS) mechanism by

MOCVD can be done by setting the growth temperature at 400°C and the

growth time at 30 minutes using GaAs (100) substrate.

iv) The InGaAs NWs were grown at an angle of 38° to the normal or simply 52°

from the GaAs (100) substrate surface.

5.2 Suggestions

Based on the experiment, there are some suggestions for improving the research for

future purposes:

i) Further investigation on the effect of Indium mole fraction towards NWs with

more samples and wider range of Indium mole fraction to other material

besides InGaAs NWs.

ii) Investigate more on the characterization of NWs, such as EDAX or other

characterization techniques rather than FE-SEM only for advance

understanding of NWs composition.

iii) Investigate the relationship of InGaAs NWs growth direction towards various

substrate orientations besides GaAs (100) and GaAs (111) substrate.

37

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41

APPENDIX A

MOCVD Software System

Auto Fine Coater (JEOL JFC 1600)

42

Coating Process

43

APPENDIX B

InGaAsNWs Images by FE-SEM

44

Temperature : 400°C

Time : 30 minutes

Type :InGaAs NWs

Substrate :GaAs {111}

Indium mole fraction : 0.41

Temperature : 400°C

Time : 30 minutes

Type :InGaAs NWs

Substrate :GaAs {111}

Indium mole fraction : 0.47

45

Temperature : 400°C

Time : 30 minutes

Type :InGaAs NWs

Substrate :GaAs {111}

Indium mole fraction : 0.65

46

Temperature : 400°C

Time : 30 minutes

Type :InGaAs NWs

Substrate = GaAs (100)

Indium mole fraction = 0.41

APPENDIX C

47

Graphs of Area Density, Length and Tapering Factor of InGaAs NWs versus Indium

Mole Fraction

48