auger study of indium over high index si(112)

8/10/2019 Auger Study of Indium Over High Index Si(112)

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Kinetically Controlled Growth & Effect Of

Temperature on Indium Superstructures

Grown on High Index Si(112) Surface

M.Tech Dissertation

BY

VIDUR RAJ

AMITY INSTITUTE OF NANOTECHNOLOGY

AMITY UNIVERSITYSETOR 125, NOIDA-201301, UP (INDIA)

JUNE, 2014


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DECLARATION

I, Vidur Raj , student of B.Tech. + M.Tech. (Dual Degree), Enrolment no.

A1223309001, hereby declare that the project titled Kinetically Controlled

Growth & Effect Of Temperature on Indium(In) Superstructures Grown on

High Index Si(112) Surface. which is submitted by me to Amity Institute of

Nanotechnology, Amity University Uttar Pradesh, Noida-201301, in partial

fulfilment of requirement for the award of the degree of B.Tech. + M.Tech. (Dual

Degree) in Nanotechnology, has not been previously formed the basis for the award

of any degree, diploma or other similar title or recognition.

The Author attests that permission has been obtained for the use of any copy

righted material appearing in the Project report other than brief excerpts requiring

only proper acknowledgement in scholarly writing and all such use is

acknowledged.

Vidur Raj

A1223309001

\


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CERTIFICATE- II

On the basis of declaration submitted by Vidur Raj, student of B.Tech. +

M.Tech. (Dual Degree) (Nanotechnology), Enrolment no. A1223309001, I hereby

certify that the project titled Kinetically Controlled Growth & Effect Of

Temperature on Indium(In) Superstructures Grown on High Index Si(112)

Surface. which is submitted to Amity Institute of Nanotechnology, Amity

University Uttar Pradesh, Noida, in partial fulfilment of the requirement for the award

of the degree of B.Tech. + M.Tech. (Dual Degree) in Nanotechnology, is an

original contribution with existing knowledge and faithful record of work carried out

by him under my guidance and supervision.

To the best of my knowledge this work has not been submitted in part or full

for any Degree or Diploma to this University or elsewhere.

Dr. Monika Joshi

(Faculty Guide)

Asst. Professor

AINT, Amity University, Noida


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ACKNOWLEDGMENTS

I would like to express my gratitude to all those who gave me the possibility to

complete Major Project at NPL, New Delhi, India. I very sincerely thank Dr. Govind,Senior Scientist at NPL and my mentor during my M.Tech thesis whose regular

guidance and caring helped me to achieve this honourable thesis. I also thank Amit

Chauhan, PhD scholar at NPL who helped me during my thesis and made sure that I

dont do any mistake. It my duty to thank all my lab members including Lalit

Goswami, Saket Vihari, Shibin Krishna, Monu Mishra, and Neha Aggarwal.

I am thankful to my parents, my big brother and my sister who regularly motivated

me opt a research career.

I would also like to thank Dr. Monika Joshi (Inernal Guide) for her regular motivation

and caring. Nevertheless, I pay gratitude to honourable Director, Amity institute of

Nanotechnology, Amity University, UP who helped me a lot during my thesis.


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ABSTRACT

In current work I have studied the kinetics of adsorption and desorption of Indium

grown at different temperature on high index Si (112) surface. We found that In

superstructures grown at room temperature at a flux rate of around 0.11ML/min

follows frank-van der Merve growth mode. In grown at 2000 C shows similar

behavior to that of room temperature growth but a significant amount of decrement in

flux rate was observed which was attributed to weakening of In-In bond over Si(112)

at high temperature. RTD performed at both RT and 2000C shows temperature

induced rearrangement of In atoms over Si(112) leading to change in layer to layer

growth mode to Stranski-Krastanov growth mode. High temperature study at 4500C

shows, after deposition of 0.75 ML no more deposition was possible, also In followed

Volmer-Weber growth mode. The RTD study also demonstrates the effect of

temperature on growth kinetics as well as on the multilayer/monolayer desorption

pathway. The calculated bilayer as well as monolayer desorption energy was found to

be different for RT and HT. For room temperature monolayer and bilayer desorption

energy was found to be 2.5 eV and 1.52 eV respectively. While for HT the monolayer

and bilayer desorption energy was found to be 1.6 eV and 1.3 eV respectively.


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TABLE OF CONTENT

1. Introduction 10 -12

2. Literature Review

2.1 Metal Semiconductor Surfaces 12 - 16

2.1.1 Introduction

2.1.2 Basic M-S junction Physics

2.2 Adsorption and Desorption 16 - 20

2.2.1 Adsorption Kineticsa) Coverage Dependence

b) Temperature Dependence

2.2.2 Thermal Desorption

a) Desorption Kinetics

b) Temperature Programmed

Desorption

3. Experimental Techniques

3.1 Ultra High Vacuum 21 - 33

3.1.1 Introduction

3.1.2 Pumps Used

a) Rotary Pump

b) Turbo Pump

c) Ion Pump3.1.3 Gauges

a) Single Gauge

b) Ion Gauge

3.1.4 Baking and Degassing

3.2Auger Electron Spectroscopy 34- 40

3.2.1 Introduction

3.2.2 Basic Principle


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3.2.3 Growth Modes

3.3 Low Energy Electron Diffraction 41-45


3.4.2 Experimental Set-Up3.4 4- Axis Precision Manipulator 46

4. Results and Discussion

4.1 Experimental Details 47

4.2 Room Temperature adsorption and desorption 48- 50

4.3 HT - 200 adsorption and desorption 50 -51

4.4 HT450 adsorption and desorption 51 -53

5. Conclusion 54

6. References 54- 57


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CHAPTER 1

INTRODUCTION

In contemporary electronics presence of metal-semiconductor junctions in every

semiconductor devices has made them very important topic of research in field of

solid state physics. In some of the cases these MS junction are used make contact

whereas in other cases MS junction itself perform vital electronic function. Due to MS

junction an electrostatic barrier i.e. Schottky Barrier arises in the semiconductor

which produces rectifying behavior.[1] this rectifying behavior makes MS junction

crucial to operation in many electronic devices. [2] Some of the examples of circuit

elements that include MS junctions are metalsemiconductor field-effect transistors

(MESFETs), , high electron- mobility transistors (HEMTs), Schottky diodes, and

heterojunction bipolar transistors (HBTs) varactor diodes. MS junction is most

important solid-state component in microwave integrated circuits. This shows the

importance of MS junction in the world of semiconductor electronics. [3]

Recent development in the field of nanotechnology and its application in material

sciences has further accelerated the research in field of M-S Junctions. Metal

superstructures/nanostructures on semiconductor surfaces exhibit interesting low-

dimensional phenomena such as two-dimensional (2D) gas formation, 1D charge

density wave phase transition and 1D bandgap engineering (Shibin MRX). New

potential application of nanoscale metal-semiconductor junctions such as enhaced

photocatalytic effect, enhanced solar cell efficiency etc. has already been applied in

modern electronics. In some cases the unique electronic and physicochemical

properties of MS interfaces affected by modification of the interfacial geometry,

which the metal adsorbate induces on the semiconducting substrate within the regime

of the first monolayer[4]

Using modern semiconductor fabrication processes, the metalsemiconductor junction

is very easy to create. [3]Metalsemiconductor junctions represent the essential and

basic building blocks of Silicon based devices. Although In/Si is a prototypical MS

interface but still the physics of this interface is not well understood in terms ofadsorption and desorption kinetics followed by them. Formation of self-assembled 1D


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and 2D nanostructures of metal adsorbates on high index Si surfaces has attracted

attention of nanotechnology and surface science researchers to investigate into

adsorption and desorption kinetics of metal adatoms on Si surfaces. Previous research

works suggest that among the metal adsorbates, indium adsorption on the Si surface

gives a number of coverage- and temperature-dependent structural phases. Unlike

other metals such as Au[5], Ti [6], In was found to be unreactive toward the Si surface

on adsorption. However, the In-Si bonding is quite strong and probably of covalent

type since it forms various surface reconstructions.[4] Zheng Gai et. el. studied atomic

structure of Si(112)/In surface but they didnt discussed about adsorption and

desorption kinetics of In over Si (112) which is of great importance. Here in this paper

we have tried give a detailed description of adsorption and desorption kinetics of in

over Si at various temperature ranging from room temperature to 600 0C.

In present work Si(112) was chosen among other high index surfaces because

previous studies suggest that Si(113) and Si(112) are relatively more stable than most

of other high index silicon surfaces. In addition, the Si(112) and Si(113) surfaces are

likely candidates for the growth of self-assembled wires on the nanometer scale. [7].

Already there are a lot research on study of adsorption and desorption kinetics of In

on low index and high index surfaces but till now none of the paper has been reported

on In deposition of In on Si(112).

The (112) plane corresponds to a vicinal Si (111) surface tilted toward the [-1-12]

direction by 19.5. The unit cell of the ideal bulk-terminated Si(112)11 surface is

3.84 wide in the [-110] direction and 9.41 high in the [-1-11] direction. The unit

cell consists of a (111)-oriented terrace with a width of 8.87 , and a step with a

height of 3.14 . The scanning tunneling microscopy STM studies reported that the

clean Si(112) surface reconstructs into quasiperiodic, nanometer-scale facets

snanofacetsd.16 The reconstructed Si(112) surface is composed of unit-cell-width

reconstructed (111) planes, 60100--width (337) facets, and a unit-cell width

horizontal (112) plane with a 9.4 width. The area of the (337) facet is wider than

the area of the (111) face. In addition, the unit cell is 15.7 (5a=53.14 ) wide and

1.11 high.


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AES has long been incorporated for surface based studies. In our current work AES

was utilized to find the various adsorption and desorption based study of In on

Si(112). The Auger intensity ration of In/Si was used to investigate various

temperature based layering and clustering effect of In on Si. The adsorption kinetics

was studied at room temperature, at 2000C and 4500C based on initial investigation of

desorption kinetics of layer grown at room temperature. At room temperature

desorption study suggested temperature based transition from layer into 2-D/3-D

island formation and vice-versa. At 2000C, desorption kinetics suggested that

monolayer formed at 2000C was more stable than the room temperature growth. It has

also been found that the layer into island transition happened twice during desorption.

As suggested by the initial desorption study of room temperature growth, we unable

to deposit more than 0.75 ML of In on Si at 4500C. This complete thesis is organized

as follows: in chapter 2 we have discussed about the literature review related to

current work and in chapter 3 a short description of experimental techniques used in

during the thesis has been given Chapter 4 deals with the experimental background

related to measurements made during experiments and the results and discussion. In

results and discussion section we have discussed adsorption and desorption kinetics at

different temperature in different sections.


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CHAPTER 2

LITERATURE REVIEW

2.1 Metal Semiconductor Interface

2.1.1 Introduction

The earliest solid-state device reported consisted of a wire tip pressed into a lead-

sulfide crystal. This simple MS junction was the first solid-state device and came to

known as a whisker contact rectifier. A few examples of circuit elements that include

metalsemiconductor junctions are Schottky diodes, varactor diodes, metal

semiconductor field-effect transistors (MESFETs), highelectron- mobility transistors

(HEMTs), and heterojunction bipolar transistors (HBTs).

Using modern semiconductor fabrication processes, the metalsemiconductor junction

is very easy to create. Besides its ease of fabrication, the junction is very versatile.

Just by varying the type of semiconductor or metal doping level, the junction can be

made into a non-rectifying or rectifying junction. Rectifying junctions preferentially

permit current to flow in one direction versus the other. For example, electrons may

flow easier from the metal into the semiconductor than the opposite. Therefore, a

rectifying junction acts as a gate keeper to stop current from flowing in the reverse

direction. The rectifying junction is commonly called a Schottky contact or a Schottky

barrier junction. The no rectifying junction or ohmic contact permits current to flow

across the junction in both directions with very low resistance. Metalsemiconductor

junctions represent the essential and basic building blocks of Si-based devices.

Therefore, it is essential to get an understanding of the MS junction structure and

operation.


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MS Junction Physics

When a metal and a semiconductor with different work functions are brought into

contact at thermal equilibrium, their Fermi levels are forced to align. The energy banddiagrams for the metal to p-type semiconductor interface are depicted in Figure 2.1.

Figure 2.1Energy band diagram of metal and semiconductor (a) separate fromeach other and (b) in intimate contact.

If the semiconductor Fermi level is greater than the metal Fermi level then when the

metal and semiconductor are put in intimate contact, electrons will diffuse from the

semiconductor to the metal. As electrons are depleted from the semiconductor, a net

positive charge is created at the junction of the semiconductor. This positive chargewill exert a force on the electrons that opposes the diffusion current. Equilibrium is

established when these two forces are equal. Figure 2.1 shows the contact in

equilibrium. Notice that the semiconductor energy bands bend in response to the

forces just described. It is within this region, called the depletion region, that all of the

junctions electrical properties are established. The amount of band bending is called

the built-in potential, Vbi .


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For an electron to cross from the semiconductor to the metal, it must overcome Vbi,

whereas an electron moving from the metal to the semiconductor must overcome the

barrier potential, fb. To a first approximation, the barrier height is independent of the

semiconductor properties, whereas Vbi is dependent on the doping level. If an

external potential is applied across the junction, the added electric field will disturb

the equilibrium conditions.

Whether a given metal-semiconductor junction is an ohmic contact, or Schottky

barrier, depends only on the Schottky barrier height, B, of the junction. For a

sufficiently large Schottky barrier height, where B is significantly higher than the

thermal energy kT, the semiconductor is depleted near the metal and behaves as a

Schottky barrier. For lower Schottky barrier heights, the semiconductor is not

depleted and instead forms anohmic contact to the metal.

The Schottky barrier height is defined differently for n-type and p-type

semiconductors (being measured from the conduction band edge and valence band

edge, respectively). The alignment of the semiconductor's bands near the junction is

typically independent of the semiconductor's doping level, so the n-type and p-type

Schottky barrier heights are ideally related to each other by:

where Egis the semiconductor'sband gap.

In practice, the Schottky barrier height is not precisely constant across the interface,

and varies over the interfacial surface.
http://en.wikipedia.org/wiki/Depletion_regionhttp://en.wikipedia.org/wiki/Schottky_barrierhttp://en.wikipedia.org/wiki/Ohmic_contacthttp://en.wikipedia.org/wiki/Band_gaphttp://en.wikipedia.org/wiki/Band_gaphttp://en.wikipedia.org/wiki/Ohmic_contacthttp://en.wikipedia.org/wiki/Schottky_barrierhttp://en.wikipedia.org/wiki/Depletion_region


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2.2 Adsorption and Desorption

Adsorption is a physical or chemical phenomenon where the attractive energy

between adsorbate and adsorbent becomes more pronounced then the thermal

disordering effect. In other words Adsorption is the adhesion of molecules, atoms,

ions to a solid or liquid or gases surface. Based on the attractive forces acting between

the adsorbate and adsorbent, adsorption can broadly be classified in two categories,

physisorption and chemisorption. In physisorption the forces that act between

adsorbate and adsorbent is van-del-walls forces whereas in case of chemisorption the

forces acting between adsorbate and adsorbent is overlap of molecular orbitals of

adsorbent and adsorbate. Chemisorption is an active process where a barrier is to be

overcome in order to form chemisorptive bond.

When a uniform solid surface is exposed to a gaseous adsorbate the according to

kinetic theory the rate I with which the gaseous particles impinges a solid surface is

given by the following equation

I= P/sqrt(2pi*m*KB*T)

In the above equation P is the partial pressure of the gas imping the solid surface, m is

the molecular mass of the molecule and T is the temperature at which the substrate iskept. But in real experiment all the molecules in gaseous form striking the surface

doesnt get adsorbed to the surface, the ratio of the adsorption rate to the impingement

rate is given by the sticking coefficient or sticking probability s. Expression for s is

given as

s = f() exp ( -Eact/KBT)

In the above expression is the the condensation coefficient and is responsible for theorientation effect (steric factor and the energy accommodation of the adsorbed

molecules. f() is the coverage dependent function which gives the information or

probability of finding the adsorbed sites. Temperature dependent Boltzmann term is

related to the energetics of activated adsorption.


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2.2.1 Adsorption Kinetics

a) Coverage Dependence:

Langmuir adsorption model:

In Langmuir adsorption model following assumptions are made:

Adsorption is limited to monolayer coverage.

All adsorption sites are equivalent

At one adsorption site only one molecule can reside.

Non Dissociative Langmuir Adsorption: For non-dissociative adsorption the,

impinging molecules have access to readily available free sites and f() is simply

f() = 1

and the Langmuir adsorption kinetics is as following

d/dt = s0I(1-)

Here s0 is the sticking probability at zero coverage.

Dissociative Langmuir Adsorption:This kind of adsorption is valid for diatomic

molecule where the impinging molecule dissociates into two atoms and then these aretrapped in the adsorption sites, and f() is given as

f() = (1- )2

If the dissociative products are mobile

f() = (Z/(Z))*( 1- )2

Simple Langmuir kinetics for an adsorbate that dissociates into n number of species

the Langmuir kinetics can be given by

f() = (1- )n

where n gives the order of kinetics.


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b) Temperature Dependence:

Sticking coefficient is very closely related to the energetics of the adsorption. Lets

consider a molecule that is trapped in the precursor state and can either desorb back togas phase or can get adsorbed to at the chemisorption state. The rate of the desorption

and adsorption from the precursor state can be expressed as follows:

Kd= pvd exp (- d/kBT)

and

Ka= pvd exp (- a/kBT)

Where vd and va are rate constants and p is the coverage inthe precursor state.

Consequently, the initial sticking coefficient can be written as

s0= ka/(ka+kd)

i.e. if d < a s0 increases as the substrate temperature increases and if d> a s0

decreases as the substrate temperature increases. The value of (a- d) can be extracted

from the slope of the experimental Arrhenius plot of (1/s01) versus 1/T.


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2.2.2 Thermal Desorption

a) Desorption Kinetics

The process in which the adsorbates are supplied with enough energy to escape from

the adsorption well and thus leaves the surface under the influence of thermal

vibrations is known as thermal desorption. In kinetic approach, desorption is

described in terms of desorption rate, rdes, which is the number of particles desorbing

from the unit surface area per unit time. In more general form the desorption rate can

be written as

rdes= *f*() exp (-Edes/ kBT)

where f*( ) describes the coverage dependence and * is the desorption coefficient

standing for steric and mobility factors.

Polanyi-Wigner Equation: In this equation it is assumed that all adsorbate atoms or

molecules occupy the identical sites and interact with each other, the deposition rate is

expressed by

Rdes= -d/dt = kn0

n

exp(-Edes/kBT)here Edes is the energy of desorption, n is the order of desorption kinetics and kn is the

desorption rate constant.

Kinetic Order: The kinetic order of desorption is given by the value of the exponent n

in above equation.

Zero Order Kinetics: The desorption rate is not coverage dependent i.e.

is constant for a fixed temperature. It takes place in desorption ofhomogenous multilayer film.

First Order Kinetics: The desorption rate is proportional to . It

corresponds to the simplest case when single atoms desorb directly and

independently from their sites.

Second Order Kinetics: The desorption rate is proportional to 2.


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b) Temperature-Programmed Desorption

It is used for the determination of the thermodynamics and kinetics of desorption

process at an elevated temperature. In this process the sample is heated at different

temperature and the atoms or molecules leaving the surface are measured using aspectroscopic technique. In our case the spectroscopic technique used was AES.

When the complete desorption process is done in a UHV chamber in controlled

environment then this technique is also known as thermal desorption spectroscopy.

In ours case In deposited Si (112) sample was mounted on a high precision 4-axis

manipulator and the sample was heated at a given temperature for 1 min. Heating was

done in resistive mode. Every time after heating the sample was left for cooling for

almost 10 minutes and once the sample is sufficiently cool Auger spectroscopy was

performed. To understand the desorption kinetics a graph was plotted with In/Si (112)

Auger intensity and temperature. This whole experiment of temperature based

desorption was performed in a controlled UHV environment.


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CHAPTER 3

EXPERIMENTAL TECHNIQUES

3.1 Ultra High Vacuum

3.1.1 Introduction

Level of vacuum in any given chamber can roughly be classified as:

Rough Vacuum 1- 10-3Torr

Medium Vacuum -10-4-10-6Torr

High Vacuum 10-9 Torr

UHV 10-9- 10-12Torr

Using residual gas analyzer we can find the various gaseous constituent present at a

given vacuum. Below given table shows in rough vacuum there is high possibility to

find water vapor (75%-85%). At a level 10-6 major gaseous content is H2O and CO.

Once a pressure of 10-6 is achieved the UHV chamber is baked at around 100-130 0C

using heating tape. At high pressure major content that can be seen in RGA are H2O,

N2, CO, H2,and O2. In UHV condition above 10-11 only H2peak leads.

Table 1 Gas composition at different pressure range

Each type of vacuum discussed above shows certain characteristics and behavior

which are discussed below:


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Rough Vacuum - There are many pumps by which rough vacuum can be

achieved but in ours case rough vacuum has been achieved using rotary pump

(discussed below). Roughing leads to removal of original atmosphere from the

chamber. A rough vacuum is characterized by viscous flow (MFP/ Diameter

of pipe less than 0.01) of gases and the composition of N2 :O2 remains the

same as atmosphere i.e. 80:20.

High Vacuum Like rough vacuum for high vacuum generation also many

pumps can be used but in ours case we used turbo pump for high vacuum

generation. As high as 10-8 can be achieved by turbo. Once 10-8 level of

pressure is achieved ion pump can be switched on. In high vacuum condition

gases originates from wall and surfaces. The gases move at molecular flow

(MFP/ Diameter of pipe >1) such that MFP > Chamber dimensions. Pressure

and pump down time is determined by Surface area, material type and pump

speed. Comparison remains constant through high vacuum (80% H20 and

20% N2, CO, H2, CO2)

Ultra High Vacuum - Ion pump is able to achieve vacuum as high as 10-11

range. Gases in UHV condition originates from walls and surfaces. Here also

gases are at molecular flow and the primary source of gas is hydrogen.

Other than above mentioned parameter i.e. pressure, there are many other parameters

on basis of which vacuum can be classified. Below given table represents some of the

parameters and their dependence on label of vacuum inside the chamber.

Table 2 Different vacuum range divided on basis of physical characteristics


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Figure- Table showing basic properties on basis of which vacuum label inside any

chamber can be described and identified.

3.1.2 Pumps Used

a) Rotary Pump

As shown in the figure a rotary vane pump consists of a housing to protect inner parts,

vanes which move radially under spring force, a rotor installed eccentrically and an

inlet and outlet. The outlet valve is oil-sealed. The inlet valve which is always openduring the operation of rotary pump acts as a vacuum safety valve. The working

chamber is located inside the housing. Working chamber is divided into two parts by

vanes and rotor. Gas flows into the enlarging suction chamber as the rotor turns, until

the suction chamber is sealed off by the second vane. The enclosed gas is compressed

until the outlet valve opens against atmospheric pressure. In the case of gas ballast

operation, a hole to the outside is opened, which empties into the sealed suction

chamber on the front side.

Figure 3.1 Working of a Oil sealed Rotory vane Pump


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Operating fluid, oil

Pump oil, which is also called as operating fluid, has multiple tasks to perform in a

rotary vane pump. It lubricates all moving parts, fills both the harmful space under the

outlet valve as well as the narrow gap between inlet and outlet. It compresses the gap

between the vanes and the working chamber and additionally ensures an optimal

temperature balance through heat transfer.

Multi-stage pumps

Rotary vane vacuum pumps are built in single- and two-stage versions. Two-stage

pumps achieve lower ultimate pressures than single-stage pumps. Moreover, the

effects of the gas ballast on the ultimate pressure are lower, as the ballast gas is only

admitted in the second stage.

Vacuum safety valve

Depending upon the type of pump in question, rotary vane vacuum pumps can be

equipped with a vacuum safety valve. The vacuum safety valve disconnects the pump

from the vacuum recipient in the event of intentional or unintentional standstill, and

uses the displaced gas to vent the pumping system in order to prevent oil from rising

into the recipient. After switching on the pump, it opens after a delay once the

pressure in the pump has reached the approximate pressure in the recipient.


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b) Turbomolecular Pumps

Here in this section I will describe about the working of turbomolecular pumps. Turbo

molecular pumps consists of a rotor-stator combination.aligned at an specific angle

Figure 3.2: Shematic diagram showing rotor-stator working combination

to impart a continuous directional impulse. On collision with the rotor surface the gas

molecules get adhered to the surface and leave the blade after sometime. Blade speed

adds to the thermal speed of the gaseous molecules. So make sure that the speed

component imparted by the blade is not lost due to unwanted collision with the

neighboring molecules the molecular flow must prevail in the pump i.e. the mean free

path must be greater than the blade spacing. This is why a turbo molecular pump is

switched on only when roughing is completed and the system is ready for molecular

flow.

Once the turbo is switched on it will remain pumping till a pressure of around 10 -7

Torr to 10-8Torr is achieved. After this point no more lowering down of pressure is

possible. This situation arises mainly because of three effects:

a) At lower pressure the desorption of materials from bearing and seals becomes very

high and thus the overall pressure of the system increases and further reduction is not

possible


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b) There is significant increase in leaks at lower pressure due an increased difference

between the internal and external pressure. .

c) The turbopump reaches its maximum compression ratio, which is defined to be the

ratio of the outlet pressure to the inlet pressure.

In our UHV system the leak is decreased to a large extent by using the copper gaskets.

Also leak detection technique was incorporated to avoid any unwanted leakage. To

avoid any desorption of gases from UHV chamber surfaces after achieving a pressure

of around 10-7 Torr the system was kept on baking at 1000C1200C.


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c) Ion Pump

As the name suggest ion pumps are based on the ionization of molecules followed by

sorption by Ti plates. The basic principle is transfer of momentum from accelerated

electrons to the atoms which leads to expulsion of electrons from the gaseous

molecules present inside the system. This ionization leads to an overall positive

charge on the atom and positively charged atoms are thus attracted toward the Ti

plates which are maintained at negative potential.

Figure 3.3: Working of an Ion Pump

An Ion pump consists of following:

a) Pump Envalop

b) Powerful Permanent Magnets

c) Titanium Cathodes

d) Anode Cell Array

In order to increase the probability of collision between atoms and electrons, electrons

must be accelerated in a helical path with the help of high magnetic field. These


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electrons accelerating in a helical path have enough energy to cause ionization of

gaseous molecules. There are array of cells inside the ion pumps which are

maintained at high anode potential which repels the positively charged gaseous atoms

and ensure maximum sorption at titanium plates which are maintained at a negative

potential.

Although Ion pump can be used any pressure ranging from 10-3to 10-11but it is

beneficial to use an Ion pump only when a pressure of around 10-6 is already achieved.

This is suggested to maintain long life efficiency of the pump, because at lower

pressure there will be very high sorption at Ti plates and high sorption will lead to

rapid saturation of Ti which is the most basic requirement of an Ion Pump.


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d) Titanium Sublimation Pump

In a sublimation pump the getter material is evaporated by applying high current and

is deposited on a cold inner wall of vacuum chamber as getter film. These getter filmsact as a sorption material as they form stable solid compound with gas molecules

which have an immeasurably low pressure. A regular evaporation is required to

maintain the reactivity of getter film toward the gaseous molecules present in the

UHV chamber. Mostly titanium is used as the getter material so we call it a Titanium

Sublimation pump. The titanium is evaporated from a wire made of a special alloy of

a high titanium content which is heated by an electric current. As reported the

optimum sorption capacity (about one nitrogen atom for each evaporated titaniumatom) can scarcely be obtained in practice, titanium sublimation pumps have an

extraordinarily high pumping speed for active gases. [FUNDAMENTALSOF

VACUUMTECHNOLOGY ]. In most of the cases sublimation pumps act as an

auxiliary pump to the sputter-ion pump and turbomolecular pump so their installation

is often indispensable.


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3.1.3 Baking and Degassing

As mentioned above in section 3.1.2, pumping down speed of UHV chamber is

limited to a large extent due to desorption of gases especially water vapor molecules

from chamber surfaces. In order to minimize this desorption it is essential that remove

these adsorbed gases from the surface of UHV systems. Baking is done by heating the

UHV chamber at a temperature of around 100-1300 C. During the baking whole

chamber is wrapped with aluminum foil along with the heating element in order to

reduce any loss of heat during baking. Also special care is taken while baking the 4-

axis manipulator to avoid melting of bellow contacts which are simply welded.

Figure 3.4: shows the pressure profile of degassing by baking.

As shown in the graph due to rapid desorption of adsorbed gaseous molecules thepressure increases rapidly as soon as the UHV chamber gets heated. A decrement is

observed in the pressure because the desorbed gaseous molecules are pumped out of

the chamber as soon as they desorb from the surface. The dotted line plot in the above

shown graph shows the pressure profile of system without baking. That means after

baking a better vacuum can be achieved in comparison to without baked system.

Degassing or Outgassing is also done by supplying current to the instrumental parts

available inside the UHV chamber. Degassing of each and every instruments part is


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required because the gas that desorbs during baking gets adsorbed to relatively cold

surfaces i.e. instrumental part. In order to remove these adsorbed gas molecules the

current is increased slowly with the help of voltage source. In ours case mainly CMA,

Electron Gun, RGA, Source, Sample, Ion Pump, Ion Gauges and RGA were degassed.

It is mandatory to mention that during degassing current should be supplied very

slowly to the instruments also special care should be taken that the pressure doesnt

rise above the prescribed limit. Also in some of the cases sudden rise to high current

can lead to breaking of filaments on which the current is applied.


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3.1.4 Gauges

A) penning gauge:

This gauge also called cold cathode ionization gauge. This gauge is based on

the glow discharge, which occurs in the gas at low pressure in the presence of a

magnetic field. The geometry of the gauge is quite simple. It consists of two cathode

plates parallel to each other with a ring shaped anode in a space between them. A

direct current voltage of 2000 volts is supplied between cathodes and anode and a

magnetic field approx. 400 gauss is normal applied to the cathode surface. Electrons

that originated in one or other cathodes do not go directly to the anode because the

magnetic field gives a spiral path to the electrons.

Figure 3.5:Penning gauge.

This increase the path length of the electron due to which no. of collisions increases as

a result the ionization probability also increases even at lower pressure. The positive

ions attracted by the cathodes. The total discharge current that is the sum of the

positive ion current to the cathodes and the electron current from the cathode is used

to measure the pressure. These gauges are used from the upper limit of the Pirani

gauge and measure thepressure up to 10-7 torr.


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b) Ion Gauges

Ion gauges are used to measure pressure below 10-6Torr range. Based on the heating

of cathode filament Ion gauges are of two types, cold cathode gauges and hot cathode

gauges.

Principles of Operation

The ion gauge consists of three distinct parts; the filament, the grid, and the collector.

The working of Ion gauge is very much similar to the working of Ion pumps. In Ion

gauges filment produces electrons by thermionic emission and these electrons are

attracted by the grids maintained at a positive potential. These electrons circulatearound the grid passing through the fine structure many times until eventually they

collide with the grid. During the circulation electrons collides with the gaseous

molecules present in the grid. Collision between electron and gaseous molecules leads

to ionization of molecules. The collector inside the grid is negatively charged and

attracts these positively charged ions. The more will be the number of gaseous

molecules present in the system the more will be ionization and thus more ions will be

collected at collector. The number of ions collected by the collector is directly

proportional to the number of molecules inside the vacuum system. Thus by

measuring the collected ion current we can get direct reading of the pressure.

The above is a simplification of what happens. The design of the gauge head affects

how efficiently electrons are produced, how long they survive, and how likely they

are to collide with a molecule. These factors combine together to result in the gauge

sensitivity. As a general rule, the higher is the sensitivity, the more efficient is the

operation of the gauge.


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3.2 Auger Electron Spectroscopy

3.2.1 Introduction

On interaction of electron beam with sample a lot of eletrons are emitted in different

form from the sample depending on the energy of electron beam interacting with the

sample. Electrons emitted from the sample carry valuable information about the

structural, physical and electronic characteristics of sample. Auger electron

spectroscopy is a non distructive surrface characterization technique that gives

information of few atomic layers of the sample i.e. about 0.4-5 nm of depth of the

sample. Except hydrogen and helium all elements can be detected with high precision

using AES with a detactibility limit of 0.1 to 1 atomic percent.

Figure 3.6 Schematic representation of various phenomenons arising due to

Interaction of electron-beam with the substrate


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AES high sensitivity for chemical analysis in the 0.4-5 nm region near the surface, a

rapid data acquisition speed, its ability to detect all elements above helium, and its

capability of high-spatial resolution has made AES an inevitable tool for surface

analysis in the field of solid state physics, the metallurgy, advanced materials,

electronics, semiconductor and micro engineering segments.[9]

With an emergence of nanotechnology and ultra-thin film era the importance of AES

for surface analysis has further increased. In the biomaterials field also AES plays

great role in surface characterization. It cannot be ignored that although bulk

properties dictate the mechanical properties of biomaterials, tissue biomaterials

interactions are a surface phenomenon and are governed by surface properties. AES

has so far been successful in surface and interface characterization of biomaterial in

nanometer regime.

Auger electron spectroscopy (AES) utilizes a focused beam of electrons directed on

the sample surface with sufficient energy to cause the necessary core level ionization.

The ability to focus such a beam of electrons into an extremely small spot (


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When an electron beam strikes at surface of any sample, it causes the ionization of

atoms present in the sample and the core level electrons are ejected out. This vacancyof core level can be filled by radiative energy loss in form of X-ray or a non-radiative

Auger process. In Auger process the final sate is left with two vacancies. The sum of

the Auger yield and the fluorescence yield is unity, since an excited ion must relax by

either Auger electron emission or x-ray emission. A schematic diagram for both of

these processes is shown in figure.Auger electron mission is the more probable decay

mechanism for low energy transitions, i.e., for low atomic number elements with an

initial vacancy in the K shell and for all elements with initial vacancies in the L or Mshells. By choosing an appropriate Auger transition, all elements (except H and He)

can be detected with high sensitivity.

Figure 3.7 (a) Schematic diagram of radiative(X-ray) and Non-radiative (Auger)

process of relaxation of core level electrons.[AES- LPD lab Services] (b)Fluorescence and Auger electron yields as a function of atomic number for K

shell vacancies. CosterKronig (i.e. intra-shell) transitions are ignored in this

analysis. [ Auger Electron Spectroscopy Wikipedia]

The incident electrons entering a solid are scattered both elastically and inelastically.

At the primary beam energy a sharp peak is observed, caused by electrons that have

been elastically scattered back out of the specimen. For a crystalline specimen, these

electrons carry the crystal structure information, which is exploited in techniques such


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as low-energy electron diffraction and reflection high-energy electron diffraction. At

slightly lower energies there are smaller peaks due to electrons that have undergone

characteristic energy losses. The information contained in this region is exploited in

the technique of low-energy electron loss spectroscopy. At the other end of the

spectrum (that is, on the low-energy side of the spectrum) there is a large peak

corresponding to the secondary electrons. AES usually performed using electron

source not x-rays (experimentally simpler and cheaper) [Auger Electron

Spectroscopy by A. R. Chourasia and D. R. Chopra]

Figure 3.8 Basic steps in Auger electron creation: (1) Creation of core hole (2)

Creation of Auger electron by relaxation

The energy distribution of emitted electrons, N(E), plotted against kinetic energy, E,

constitutes the fundamental AES measurement because the Auger peaks are of

relatively low intensity, and for historical reasons, it is common to differentiate this

N(E) spectrum and display (dN(E)/dE) vs. E. In Auger electron spectroscopy,

elemental identification is determined by the energy positions of the Auger peaks. Thekinetic energy of an Auger electron is equal to the energy difference of the singly

ionized initial state and the doubly ionized final state.

For an arbitrary ABC transition of an atom of atomic number z, the measured Auger

electron energy, referenced to the Fermi level, is given by:

EABC(z) = EA (z)EB(z)E*C(z)s

E* is the binding energy of a level in the presence of a core hole and is greater thanthe binding energy of the same level in a neutral atom. Each element has a unique set


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of Auger peaks. The kinetic energies of the most useful Auger peaks are typically

between 40 eV and 2500 ev. Variations in chemistry may change binding energies,

relaxation energies, and Auger transition probabilities.

In Auger electron spectroscopy, quantification of the observed elements is determined

from the relative intensities of the Auger peaks. The measured intensity of an arbitrary

Auger peak is a complicated function of a large number of sample and instrumental

factors.[APPLIED SPECTROSCOPY REVIEWS, 34(3), 139158 (1999)]These include:

the number of atoms of that element per unit volume,

the primary electron current,

the Auger transition probability for that element,

the ionization cross section of that element by incident electrons,

the ionization cross section of that element by scattered electrons,

the mean free path of the emitted Auger electron,

the angle between the collected Auger electron and the surface normal,

the analyzer acceptance solid angle,

the analyzer transmission function,

the electron detector efficiency, and the surface roughness


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3.2.3 Growth Modes

Condensation of a substance from the vapor phase on a surface can result in various

structures of the deposits, ranging from two and three-dimensional clusters to acompletely closed layer. The system can be characterized by the specific surface free

energies and interface energy

Figure 3.9 Schematic representation of different growth modes.

The growth process of a deposited material can be subdivided into three modes

i) The Frank-van der Merwe mode, characterized by a two-dimensional (2-D)

growth. The substance grows on a surface by forming consecutive closed layers

ii) The Stranski-Krastanov mode characterized by the formation of one or more

monolayers (ML) followed by island growth.

ii) The Volmer-Weber (VW) growth with formation of three-dimensional structures.


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Figure 3.10 Interpretation of Various growth modes using Auger Electron

Spectroscopy

As shown in the above figure the growth modes during thin film can closely be

monitered by monitoring the Auger intensity of the adsorbate and the adsorbent. As

shown in the figure to have frank vander merve growth mode the intensity of

adsorbate should decrease whereas the intensity of adsorbent should increase, 1 ML is

completed where a change in slope is observed. In case of stranki-kastranov growth

mode the Auger intensity of adsorbent increases linearly but changes slope very

readily but increase in intensity is slow in comparison to Frank vander merve growth

mode, also change in intensity adsorbate is not very rapid. In case of volmer-weber

growth mode the change in intensity of adsorbate and adsorbent is very slow in

comparison to other two growth modes.

The surface may not be completely covered until a large deposition has been made.

Layer plus island type of growth (b) is observed in some systems when the lattice

parameter of the overlayer is slightly larger than the substrate. When several layers

grow on each other at some point the strain inside the deposited layer may become too

large to allow the growth of a homogenous layer. Small islands can emerge as a result

of the interplay between strain and surface tension.


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3.3 Low Energy Electron Diffraction

3.3.1 Introduction

A diffraction method of surface characterization utilizes electrons or X-ray photons as

the incident beam. Pioneering experiment done by Davisson and Germer confirmed

the wave behavior of particle i.e. electrons, which further became the basis of electron

diffraction experiments. The wave associated with an electron is described by de

Broglies relation:

= h/P

where is the wavelength associated with the particles, h is the plancks constant and

P is the momentum associated with the particle. Equally spaced arrays of crystal

atoms act as a grating for the wave associated with electrons and thus create a

diffraction pattern. These diffraction patterns are analyzed to get information about

atomic arrangements in a lattice. None of the experimental technique except LEED

can give the information at a level of single atomic layer thickness, this special feature

of LEED makes LEED a very important diffraction based technique for surface

characterization.[ LEED by LESTER H. Germer]

Surface structural information is gained by analysis of particles or waves scattered

elastically by the crystal. The spatial distribution of diffracted beams tells us about the

crystal lattice and surface symmetry. The intensity of diffracted beams gives the

information about the atomic arrangement inside a unit cell. The diffraction pattern is

scaled version of reciprocal lattice and can directly be related to reciprocal lattice by

below give relation i.e.

KK0 = Ghklwhere K is scattered wave vector, K0 is incidence wave vector and Ghkl is reciprocal

lattice vector.

Reasons that make LEED perfect for surface analysis:

Energy of electrons used for LEED lies in a range of 30-200 eV. On

calculation the wavelength of the beam is found to be in a range of 1-2 . For


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diffraction to occur the wavelength of the wave should be in order of or less

than the interatomic distance.

Most elastic collision occur in very top layer of the sample as mean free path

of the electron is very short and lies in range of few atomic layers.

When diffraction occurs from a 2-D surface the periodicity of crystal lacks in

perpendicular direction to the surface so the relation between wave vector and crystal

lattice changes as follows.

K|| - K0|| = Ghk

Also the law of conservation of momentum applies only to the components thats

parallel to the surface. Ewald sphere is constructed to represent the diffraction by

LEED.


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3.3.2 LEED Experimental Set-Up

The figure shown below is the schematic representation of actual LEED experimental

setup. Basically any LEED instrument consists of three main components

An electron gun which produces a collimated beam of low energy electrons of

range 30200 eV.

A sample holder to hold the sample under investigation.

A hemispherical fluorescent screen which is available with a set of four grids.

This screen is used to observe the diffraction patterns available from the

elastically scattered electrons. Sample is placed at the center of curvature of

grids and the screen.

The electro gun unit consists of a cathode filament with a Wehnelt cylinder followed

by an electrostatic lens. The cathode remains at negative potential while the sample,

the grid and the last aperture of the lens remains at the earth potential. Thus the

electrons emitted by the cathode are accelerated to energy of eV within the gun and

then propagate and scatter from the sample in the field free space. The additional

grids i.e. second and third grids are used to reject the idealistically scattered electron

so as to minimize the background and make the diffraction spots brighter. The

negative potential applied to second and third grids are almost equal to that of cathode

but the magnitude is slightly lower than the magnitude of potential applied to cathode.

If a voltage (-V) is applied to cathode then -( V V) is applied to suppressors. The

greater will be V, the brighter will be LEED pattern but the background intensity

will increase. So the grid voltages are adjusted to get maximum spot-to-background

intensity. The fourth grid is maintained at ground potential and is used to screen other

grids from field of the fluorescent screen which is maintained at high potential ofabout 5 kV. Thus the elastically scattered electron which slows down due to retarding

potential applied at grids, and is reaccelerated at a high energy to the screen to cause

the bright fluorescence at the screen.


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Figure 3.11: LEED Experimental- Setup.

There are two kinds of LEED system from the viewpoint of LEED pattern

observation:

Normal View Arrangement: In this the LEED pattern is viewed past the

sample through the grids and the viewport is placed in of the back side of the

sample. The size of sample holder should be reasonably small.

Reverse View Arrangement: LEED pattern is viewed through a viewport that

is placed phosphorescent screen. ELctron gun size is to to be miniaturized but

no limitation in shape and size of the sample holder.

Ertl/Kppers fig. 9.7, p.210


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The wavelength of electron can be decreased by increasing the energy of electron and

consequently the edwald sphere radius can be increased and more spots can be seen

on the screen.


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3.4 4-Axis precision manipulator

A UHV Manipulator (VG Scienta, UK) is a combination of the translator, sample

holder and rotary drive modules, connected to the vacuum chamber and used for

sample manipulation in UHV environment. This allows an object, which is inside avacuum chamber and under vacuum to be mechanically positioned. It may provide

rotary motion, linear motion, or a combination of both. The manipulator or sample

holder may include features which allow additional control and testing of a sample,

such as the ability to apply heat, cooling, voltage, or a magnetic field.

Figure 3.12: UHV manipulator

The sample (substrate) of 10 mm width and 20 mm length can hold by sample

holder. The XY slide has range from 0 to 25 mm, for each, with screw gauge of 0.005

mm least count. While Z motion starts from 0 to 150 mm. The rotary drive has motion

form 0 to 360. There is also a power feedthrough at 16 CF port, for current supply to

the sample, with maximum of 10 ampere current. Figure 6.3 shows the image of an

UHV manipulator, used by us.


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CHAPTER 4

RESULTS AND DISSCUSION

4.1 Experimental Details:

Complete experiment was performed in a UHV system with a base pressure of around

110-10Torr and equipped with cylindrical mirror analyzer for AES analysis. Si (112)

sample was mounted on a high precision 4-axis manipulator and an in-situ cleaning

process was adopted to get an atomically cleaned surface. For In-situ cleaning the

sample was heated at 6000C for 4 hours afterward flashed at 11500C and then the

sample was brought to room temperature very slowly. During the complete cleaning

procedure temperature was monitored using pyrometer. After cleaning, the impurity

level on Si surface was found to be below the detection limit of AES and thus we

obtained an atomically cleaned Si(112) surface. For In deposition a homemade

tantalum evaporator assembly was used. The flux rate during deposition was

maintained at constant rate by keeping the current constant during whole experiment.

Sample was heated in resistive mode by applying a current using high voltage source

and simultaneous monitoring of temperature by pyrometer and thermocouple (WRe

(5%25%)).To carry out the residual thermal desorption process the sample was

heated at a given temperature for 1 min and left for cooling for 10 min. Once the

sample was cool enough, AES was performed for the sample and In/Si ratio was

obtained for any further adsorption and desorption analysis.

4.2 Results and Discussion

4.2.1 Room temperature adsorption and desorption

Figure 4.1 gives the In/Si(112) Auger intensity plot versus time. Adsorption profile as

shown by the figure 4.1 shows change in the slope at 0.2 In/Si intensity and confirms1ML completion. Change in slope was confirmed by plotting the sum of the square of

errors (SSQ) [36] in the least-square fits of a set of two straight lines near the change

in slope, whose minima identify the inflexion points that suggest the completion of

one monolayer (ML). Time taken to complete one monolayer adsorption was found to

be around 8.0 min which gives a calibration of the In flux rate as 0.125 ML min. Our

experiment is in accordance to previously reported results which suggest one


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Figure 4.1: Auger Uptake curve for In growth on Si (112) at RT

Figure 4.2: Desorption Profile of In growth on Si (112) at RT

Si (112)

Si (112)

Si (112)

Si (112)

Si (112)

Si (112)

Si (112)

Si (112)Si (112)


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monolayer deposition of In on Si at In/Si intensity ration of 0.2. On further deposition

In/Si Auger intensity ration was found to increase linearly until a change in slope was

observed at 16 min. Change in slope confirmed 2ndmonolayer deposition of In on Si.

A schematic view of layer by layer deposition has been presented along with the

adsorption profile. After the growth of two monolayers a linear increase in the

intensity curve of In/Si was reported which shows the layer to layer adsorption or

Frank-van der Merwe growth mode. Previously reported work on Si(113) suggested

that In deposition on Si follows Frank-van der Merwe growth mode which is similar

to what we report here in this report.

Residual thermal desorption method was used to understand the thermal stability and

desorption kinetics of room temperature layer by layer deposited In on Si(112). The

system was subjected to 1 min annealing at different temperature until complete

desorption i.e. In/Si Auger intensity touched ~0. Figure 4.2 shows the Graph plotted

with Auger In/Si intensity and annealing temperature gives an idea of In desorption

from Si(112) surface. A very slow desorption rate of In from Si(112) was observed till

annealing temperature reached from RT to 3000C. As the temperature is increased

after 3000C a significant decrease in the In/Si intensity ratio was observed. At around

4500

C it looked like almost 1 monolayer has desorbed from the Si(112). But furtherincrease in temperature shown increase in In/Si peak ratio i.e. an increase in the In

over Si(112). This bizarre phenomenon observed at high temperature can be

understood in terms of temperature induced rearrangement of In atoms on Si. A

figurative description of above phenomenon has been presented along with the graph

shown in fig 4.2. In the temperature range of 450 to 5000C a transition between layer

by layer growth mode to 2D/3D island formation can be seen. Between 5000C to

600

o

C a significant amount of In desorbed from the surface and such kind sudden risein In desorption from Si(112) surface can be attributed to high thermal energy which

leads to easy breaking of In-In bond from the surface of Si(112). A complete

desorption was observed at around 7000C.

Based on the desorption behavior obtained for room temperature grown In on Si(112)

we further investigated adsorption and desorption behavior at higher temperature. For

higher temperature studies 2000C, and 4500C were found suitable based on the

desorption curve (figure 4.2). 2000C was selected because desorption started at this


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temperature. 4500C shown significant rearrangement during desorption so we were

keen to study adsorption and desorption behavior at these temperatures.

4.2.2 Adsorption and desorption at 2000C

Figure 4.3: Auger Uptake curve for In growth on Si (112) at HT -2000C

This shows completion of 1 monolayer of In on Si(112) at 2000 C. But further

deposition of In shown a change in slope after 23 minutes where In/Si intensity ratio

touched a value of 0.4. This suggests completion of second monolayer at 2000C takes

more time compared to RT growth. The longer time taken in second monolayer

deposition can be attributed to weakening of In-In bond due to higher substrate

temperature. On further deposition a linear increase in In/Si Auger intensity with time

can be observed, which suggest layer by layer deposition or Frank-van der Merwe

growth mode was followed at 2000C substrate temperature which similar to room

temperature grown In/Si(112) system.


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Figure 4.4: Desorption curve of In grown on Si (112) at HT-2000C

The desorption curve for In grown at 200

0

C on Si (112) is shown in figure 4.4. In atemperature range of 2000C to 3500C no significant desorption was observed.

Although small rise and fall in Auger intensity with temperature was observed but this

rise and fall can be seen as small scale rearrangement of In atoms over Si (112). After

3500C a significant downfall in Auger intensity from 0.5 to 0.25 was observed which

can either be due mass scale desorption of In from Si(112) surface or rearrangement

of In over Si(112) surface. But on further rise in temperature In/Si intensity

unexpectedly increased to almost 0.5 at around 4700 C. This unusual rise in the

intensity ratio with temperature suggests that no desorption happened in a temperature

range of 200-4400C. Instead of desorption it was temperature induced rearrangement

because of which the down fall was observed. As it is clear from the desorption plot

that these rearrangements (cluster to layer transition) happen above the monolayer

coverage. Increasing temperature beyond 4700C continuous decrement in the In/Si

Auger intensity can be observed which suggest In desorption from Si(112) surface

was started and the complete desorption was observed at 7000C.


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The multilayer (bilayer) and monolayer desorption energy of In/Si(113) is calculated

from the Arrhenius equation which is given by

ln I(T) ED/kT

where I(T) is the change in the density of adatoms(coverage), ED is the desorption

energy, k is the Boltzmann constant and T is the temperature. By plotting ln I(T)

versus 1/T, the resultant slope (ED/kT) determines the desorption energy at a

particular temperature T. The multilayer (InIn) desorption energy for RT-grown

In/Si(112) is calculated to be 1.52 eV, while the monolayer desorption energy is

calculated to be 2.50 eV. For HT In grown on Si (112) the multilayer and monolayer

desorption energy was found to be 1.30 eV and 1.60 eV respectively.

4.2.3 Adsorption and desorption at 4500C

figure 4.5 shows the Auger uptake curve for In growth at 4500C. As shown in figure

Auger intensity ratio increases linearly and gets saturated at value 0.15 which

corresponds to 0.75 monolayer of In coverage. This adsorption profile suggests that at

4500C substrate temperature even single monolayer is no formed. Instead of that In

2D/3D islands were formed on Si(112) surface, i.e. In follows Volmer-Weber growth

mode. This kind of adsorption behavior at higher substrate temperature may be due to

higher desorption which is dominant at higher temperature in comparison to the

adsorption.

Figure 4.6 shows the desorption profile of HT-4500 C grown In/Si(112) system.

Between 4700C to 5700C stability in In/Si Auger Intensity ratio was observed which

shows the stability of 2D/3D islands in this particular temperature range. Other than

this no anomalous behavior was observed during desorption of In from Si (112)

surface.


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0 2 4 6 8 10 12 14

0.00

0.05

0.10

0.15

0.20

0.25

AESIntensityRatio(InIn/Si)

Deposition Time(min)

Figure 4.5: Auger Uptake for In growth on Si (112) at HT- 4500C

440 460 480 500 520 540 560 580 600 620 640

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

AESIntensityRatio(InIn/Si)

Desorption Temperature(td)(

0C)

Figure 4.6 Desorption Curve for In growth on Si (112) at HT-4500C


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Chapter 5

Conclusion

In conclusion, the controlled growth kinetics of Indium metal atoms on high index Si

(112) surface has been carried out. This study investigates the kinetically controlledRT and HT adsorption/desorption of In on Si (112) surface under sub-monolayer

regime using Auger electron spectroscopy. The results show the formation of In/Si

interface, where In adsorption at RT follows Frank van-der Merve (layer-by-layer)

growth mode. At HT 200 In adsorption follows Frank van-der Merve (layer-by-layer)

growth mode whereas at 450oC adsorption curve shows the SK growth mode.

Desorption studies revealed the monolayer and bilayer desorption energy changes for

In grown on Si(112) at RT and HT. The K completely desorbed from Si surface attemperature ~700oC. The study contribute to the fundamental understanding of

kinetics of Indium (group iii) metal adsorption/desorption on high index surfaces and

metal/semiconductor interfaces mechanism on Si (112).


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