29

CHAPTER 2

EXPERIMENTAL TECHNIQUES USED FOR THIN FILMS

SYNTHESIS AND CHARACTERIZATIONS

2.1 INTRODUCTION

In this chapter different aspects of the experimental techniques employed in the present

work are described in brief. At the outset an introduction to the basic aspects of the thin films

growth processes and deposition technique is provided. Afterwards, the characterization

techniques like X-ray diffraction (XRD), scanning and transmission electron microscopy

(SEM/TEM), atomic force microscopy (A.F.M.), and magnetic and magnetotransport

measurements techniques are discussed.

2.1.1 BASIC ASPECTS OF THIN FILMS

A thin film can be defined as a quasi two-dimensional material created by condensing,

atomic/molecular/ionic species of matter [142-146]. The fabrication of thin films on a single

crystal substrate is done by the deposition of individual atoms. On the other hand thick films

can be defined in a different way, as a low-dimensional material created by thinning a three-

dimensional material or assembling large clusters/aggregates/grains of atomic/molecular/ionic

species. For making different devices like, electronic devices, instrument hard coatings,

optical coatings, decorative parts etc. thin films have been widely used for more than a half

century. Although the thin film technology is a well-established materials technology, the

demand of twenty first century, for the development of new materials such as nanostructured

materials and/or a man made superlattices; it is still evolving on a daily basis. Thin film

technology is both an old and a current key material technology. Thin film materials and

deposition processes have been reviewed in several publications and books [142-146].

There are several techniques available for thin films deposition on a single crystal

substrate like thermal evaporation, chemical decomposition, and the evaporation of source

materials by the irradiation of energetic species or photons. In general the growth process of

thin films exhibits the following features:

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30

1. Thin films of all materials created by any deposition technique starts with a random

nucleation process followed by nucleation and growth stages.

2. Nucleation and growth stages are dependent upon various deposition conditions, such

as growth temperature, growth rate, the chemistry of the material and the substrate and

their structure.

3. The nucleation stage can be modified significantly by external agencies, such as

electron or ion bombardment.

4. Film microstructure, associated defect structure, and film stress depend on the

deposition conditions at the nucleation stage.

5. The crystal phase and the orientation of the films are governed by the deposition

conditions as well as by the structure of the substrate.

Film composition, crystal phase and orientation, film thickness, and microstructure, are

the basic properties of film, and can be controlled by the deposition conditions. Some unique

features like quantum size effects, impact of strain, consequence multilayer aspects that cause

variety of proximity effects are observed in thin films and cannot be realized in bulk materials

Thin films have been extensively studied in relation to their applications for making

electronic devices in the latter part of fifties. Wiemer proposed thin film transistors (TFTs)

composed of cadmium sulfide (CdS) semiconducting films in the early sixties and at the end

of sixties the bulk Si-MOS (metal-oxide semiconductor) devices were successfully developed

[147]. In seventies, different kinds of novel thin-film devices were proposed, including thin-

film surface acoustic wave (SAW) devices [148], integrated thin-film bulk acoustic wave

(BAW) devices [149], and thin-film integrated optics [150] and several other wide variety of

thin-film devices were developed. Variety of multi-layered materials including giant

magnetoresistance (GMR) materials have been developed by using sputtering technology,

with its precise, controlled deposition. Prof. K. L. Chopra, one of the pioneers of the thin film

deposition technology once commented that, "The thin film was in past considered as the fifth

state of matter next to plasma, since the reliable materials properties could not be obtained

and thin films were considered to be different from bulk materials", at present the thin films

are considered as the first state of matter [151].

2.1.2 THIN FILM GROWTH PROCESS

Three major steps that constitute a typical thin-film deposition process are, (i) production of

the appropriate atomic, molecular, or ionic species, (ii) transport of these species to the

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31

substrate through a medium, and (iii) condensation on the substrate, either directly or via a

chemical and/or electrochemical reaction, to form a solid deposit. Based on the various

experimental and theoretical studies a general picture of the step-by-step growth process is as

follows:

The unit species, on impacting the substrate, lose their velocity component normal to

the substrate (provided the incident energy is not too high) and are physically adsorbed

on the substrate surface.

The adsorbed species are not in thermal equilibrium with the substrate initially and

move over the substrate surface. In this process they interact among themselves,

forming bigger clusters.

The clusters or the nuclei, as they are called, are thermodynamically unstable and may

tend to desorb in time, depending on the deposition parameters. If the deposition

parameters are such that a cluster collides with other adsorbed species before getting

desorbed, it starts growing in size. After reaching a certain critical size, the cluster

becomes thermodynamically stable and the nucleation barrier is said to have been

overcome. This step involving the formation of stable, chemisorbed, critical-sized

nuclei is called the nucleation stage.

The critical nuclei grow in number as well as in size until a saturation nucleation

density is reached. The nucleation density and the average nucleus size depend on a

number of parameters such as the energy of the impinging species, the rate of

impingement, the activation energies of adsorption, desorption, thermal diffusion, and

the temperature, topography, and chemical nature of the substrate. A nucleus can grow

both parallel to the substrate by surface diffusion of the adsorbed species, and

perpendicular to it by direct impingement of the incident species. In general, however,

the rate of lateral growth at this stage is much higher than the perpendicular growth.

The grown nuclei are called islands.

The next stage in the process of film formation is the coalescence stage, in which the

small islands start coalescing with each other in an attempt to reduce the substrate

surface area. This tendency to form bigger islands is termed agglomeration and is

enhanced by increasing the surface mobility of the adsorbed species, for example, by

increasing the substrate temperature. In some cases, formation of new nuclei may

occur on areas freshly exposed as a consequence of coalescence.

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32

Larger islands grow together, leaving channels and holes of uncovered substrate. The

structure of the films at this stage changes from discontinuous island type to porous

network type. Filling of the channels and holes results in the formation of a completely

continuous film.

Thus statistical process of nucleation, surface-diffusion controlled growth of the three-

dimensional nuclei, and formation of a network structure and its subsequent filling to give a

continuous film, these processes constitute the growth process. Growth stages and the initial

nucleation, depends on the thermodynamic parameters of the deposit and the substrate

surface, can be categorized as (a) island type, called Volmer-Weber (VW) type, (b) layer type,

called Frank-Van der Merwe (FV) type, and (c) mixed type, called Stranski-Krastanov (SK)

type. This is shown in Figure 2.1.

Figure 2.1: Three modes of thin film growth processes (taken from [142]).

Island type is the most common growth process, available in almost all practical cases.

Except under special conditions, the crystallographic orientations and the topographical

details of different islands are randomly distributed, so that when they touch each other during

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33

growth, grain boundaries and various point and line defects are incorporated into the film due

to mismatch of geometrical configurations and crystallographic orientations. If the grains are

randomly oriented, the films show a ring-type diffraction pattern and are said to be

polycrystalline. Even if the orientation of different islands is the same throughout, as obtained

under special deposition conditions, on suitable single crystal substrates, a single-crystal film

is not obtained. Instead, the film consists of single crystal grains oriented parallel to each

other and connected by low-angle grain boundaries. These films show diffraction patterns

similar to those of single crystals and are called epitaxial single-crystal films.

2.1.3 THIN FILM GROWTH MODES

Thin film growth modes in materials as reported in literature [142-146] can be

characterize in three modes Volmer–Weber (VW) or island growth, Frank–Van der Merwe

(FV) or layer-by-layer growth, and Stranski–Krastanov (SK) or mixed type growth. These

growth mechanisms are shown in Figure 2.1 and described below one by one.

Volmer–Weber or island growth: Shown in Figure 2.1 (a) occurs when the smallest stable

clusters nucleate on the substrate and grow into three-dimensional island features [152]. One

simplistic explanation for this growth behavior is that the atoms or molecules being deposited

are more strongly bonded to each other than to the substrate material. This is often the case

when the film and substrate are dissimilar materials. There are a few example of such

behavior in the growth of oxide films on oxide substrates, but this growth mode is typically

observed when metal and semiconductor (i.e., Group IV, III–V, etc.) films are grown on oxide

substrates.

Frank–Van der Merwe or layer-by-layer growth: The opposite characteristics of Volmer–

Weber or island growth, however, are displayed in Frank–Van der Merwe or layer-by-layer

growth (Figure 2.1 (b)) which occurs when the extension of the smallest nucleus occurs in

two dimensions resulting in the formation of planar sheets [153]. In layer-by-layer growth the

depositing atoms or molecules are more strongly bonded to the substrate than each other and

each layer is progressively less strongly bonded than the previous layer. This effect extends

continuously until the bulk bonding strength is reach. A typical example of this is the epitaxial

growth of semiconductors and oxide materials. The field of oxide thin film growth has

developed around the ability to control materials through this and other similar growth modes.

Such capabilities have ushered in an era of unprecedented control of oxide materials down to

the single (or even half-) unit cell level.

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34

Stranski–Krastanov mode: This is the final growth mechanism shown in Figure 2.1 (c)

which is a combination of the layer-by-layer and island growth [154]. In this growth mode,

after forming one or more monolayers in a layer-by-layer fashion, continued layer-by-layer

growth becomes energetically unfavorable and islands begin to form. This sort of growth is

fairly common and has been observed in a number of metal-metal and metal–semiconductor

systems. These different growth modes can be described in more detail with simple

thermodynamic models for the nucleation and growth of film materials.

In addition to these three well-known classical epitaxial growth modes mentioned

above, there are four distinct growth modes: columnar growth, step flow mode, step bunching,

and screw-island growth (as shown in Figure 2.2) [155].

Figure 2.2: Schematic cross sections of substrate-film, in three successive growth stages, of the four extended

growth modes for epitaxial thin film (taken from [155]): (a) Columnar-Growth, (b) Step-Flow, (c) Step-

Bunching and (d) Screw-Island modes.

Columnar growth mode: Starting with the Volmer-Weber (VW) growth mode, the column

of individual island coagulates to form a continuous film. Coalescence of growth islands and

columnar growth cause high density of defects (grain boundaries, dislocations, voids,

antiphase boundaries, etc.)

(a)

(b)

(c)

(d)

Step-Flow

Step-Bunching

Screw-Island

Columnar

Dislocation lines and

Anti-phase boundaries

(d)

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35

Step flow growth mode: The Step flow mode is clearly distinct from layer-by layer growth in

FV mode. Unidirectional step flow is induced by substrate misorientation (off cut angle). This

trick is often used to avoid island formation, their coalescence and following columnar growth

in epitaxy from the vapor phase.

Step bunching growth mode: Step bunching is observed when a high density of steps moves

with large step velocities over the growth surface. By fluctuations, higher steps catch up with

lower steps and then move together as double; triple and so on or in general as macro steps

that can exceed thickness of thousands of monosteps. The microsteps cause different

incorporation rates of impurities and dopants due to locally varying growth rate.

Screw island growth mode: Coalescence of larger number of initial growth islands may lead

to screw dislocations due to the layer structure resulting in spiral-island growth mode. This

has been observed in the high temperature superconductor thin films. The FV growth mode

arises because the atoms of the deposit material are more strongly attracted to the substrate

than they are to themselves. In the opposite case, where the deposit atoms are more strongly

bound to each other than they are to the substrate, the island, or VW mode results. An

intermediate case, the layer-plus-island, or SK growth mode is much more common.

In almost all practical cases, the growth takes place by island formation. After a

continuous film is formed, anisotropic growth takes place normal to the substrate in the form

of cylindrical columns. The initial nucleation density determines the lateral grain size, or

crystallite size. However, if recrystallization takes place during the coalescence stage, the

lateral grain size is larger than the average separation of the initial nuclei, and the average

number of grains per unit area of the film is less than the initial nucleation density. The grain

size normal to the substrate is equal to the film thickness. For thicker films, re-nucleation

takes place at the surface of previously grown grains, and each vertical column grows multi-

granularly with possible deviations from normal growth.

2.2 DIFFERENT THIN FILM DEPOSITION METHODS

Based on the nature of deposition process the methods employed for thin oxide film

deposition can be divided into two group i.e. physical and chemical methods.

All possible deposition processes are shown in Figure 2.3, the physical deposition

processes include vacuum evaporation [156], laser ablation [157], molecular beam epitaxy

(MBE) [158], and sputtering [159]. The chemical deposition processes comprise gas phase

deposition methods and solution techniques. The gas phase methods are chemical vapour

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36

deposition (CVD) [160, 161] and atomic layer epitaxy (ALE) [162], while spray pyrolysis

[163], sol-gel [164], spin- [165] and dip-coating [166] methods employ precursor solutions.

Figure 2.3: Different physical and chemical thin film deposition processes.

Vapor deposition technique describes any process in which a solid immersed in a

vapor becomes larger in mass due to transference of material from the vapor onto the solid

surface. The deposition is normally carried out in a vacuum chamber to enable control of the

vapor composition. If the vapor is created by physical means without a chemical reaction, the

process is classified as physical vapor deposition (PVD), if the material deposited is the

product of a chemical reaction; the process is classified as CVD. Many variations of these

basic vapor deposition methods have been developed in efforts to balance advantages and

disadvantages of various strategies based on the requirements of film purity, structural

quality, the rate of growth, temperature constraints and other factors.

PVD is a technique whereby physical processes, such as evaporation, sublimation or

ionic impingement on a target, facilitate the transfer of atoms from a solid or molten source

onto a substrate. Evaporation and sputtering are the two most widely used PVD methods for

depositing films. This section described briefly the typical physical and chemical vapor

deposition processes which are commonly used to grow epitaxial and polycrystalline thin

films of transition metal oxides. DC magnetron sputtering and spray pyrolysis techniques are

PHYSICAL DEPOSITION PROCESSES

THERMAL PROCESS SPUTTERING

THIN FILM DEPOSITION

VACUUM EVAPORATION

LASER ABLATION

MBE

CHEMICAL DEPOSITION PROCESSES

SOLUTION GAS PHASE

CVD

ALE DIP COATING

SOL-GEL

SPIN COATING

SPRAY PYROLYSIS DC/RF SPUTTERING DC/RF MAGNETRON SPUTTERING

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37

described here in more detail, because these techniques have been used to deposit the single

crystalline epitaxial and polycrystalline thin films in the present thesis work.

2.2.1 PHYSICAL VAPOUR DEPOSITION

Vacuum Evaporation: The thermal evaporation process comprises evaporating

source materials in a vacuum chamber below 10-6

torr and condensing the evaporated particles

on a substrate [156]. In this process, thermal energy is supplied to a source from which atoms

are evaporated for deposition onto a substrate. Heating of the source material can be

accomplished by any of several methods. The simplest is resistance heating of a wire or stripe

of refractory metal to which the material to be evaporated is attached. Larger volumes of

source material can be heated in crucibles of refractory metals, oxides or carbon by resistance

heating, high frequency induction heating, or electron beam evaporation. The evaporated

atoms travel through reduced background pressure in the evaporation chamber and condense

on the growth surface. The deposition rate or flux is a function of the travel distance from the

source to the substrate, the angle of impingement onto the substrate surface, the substrate

temperature TS, and the base pressure.

Pulsed-Laser Ablation-Based Techniques: Pulsed laser deposition (PLD) is an improved

thermal process used for the deposition of alloys and/or compounds with a controlled

chemical composition. In laser deposition, a high-power pulsed laser (1 J/shot) is irradiated

onto the target of source materials through a quartz window. In general, laser sources of PLD

for manganite thin films, are KrF (248 nm) [167], ArF (193 nm) [168] and Nd-YAG (266 or

355 nm) [169]. A quartz lens is used to increase the energy density of the laser power on the

target source. Atoms that are ablated or evaporated from the surface are collected on nearby

substrate surfaces to form thin films [170]. The target material is locally heated to the melting

point, melted, and vaporized in a vacuum. The laser pulse may also provide photoemitted

electrons from the target to make a plasma plume and the evaporation mechanism may be

complex since the process includes the thermal process and the plasma process. By

optimizing various parameters such as ablation energy, base vacuum level, background

oxygen pressure, distance between target and substrate and the temperature of substrates, one

can have desired deposition rate and structural quality. Advantage of PLD technique is direct

monitoring of cell-by-cell growth by reflective high-energy electron diffraction (RHEED)

pattern [171]. Major drawbacks of this excellent technique are the limited area of uniform

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38

deposition and particle/particulate ejection from the target [172]. To avoid the deposition of

the microsized ejections, the substrates are settled at an off-axis position.

Molecular Beam Epitaxy (MBE): MBE is an example of an evaporative method. This

growth technique can provide film materials of extraordinarily good quality which are ideal

for research purposes. However, the rate of growth is very low compared to other methods,

which makes it of limited use for production of devices. In MBE, the deposition of a thin film

can be accurately controlled at the atomic level in an ultra-high vacuum (10−10

torr). A

substrate wafer is placed in the ultra-high vacuum chamber. It is sputtered briefly with a low

energy ion beam to remove surface contamination. This step is followed by a high

temperature anneal to relax any damage done to the growth surface during preparation. The

substrate is then cooled to the growth temperature, typically between 400 and 700 C, and

growth commences by directing atomic beams of the film material, as well as a beam of

dopant material if necessary, toward the growth surface of the substrate. The beams are

emitted from crucibles of the growth materials which have been heated to temperatures well

above the substrate temperature to induce evaporation and condensation. The film

composition can be properly selected by accurate control of atomic ratio of each metallic

electron beam sources. O'Donnell et al. have used MBE to prepare manganite thin films and

investigated anisotropic magnetotransport [173].

Magnetron Sputtering: The basic concept of sputtering is ejection of surface atoms from the

target surface by momentum transfer by bombarding ions. The main advantage of sputtering

is that it is a non-thermal physical process. One of advantage of sputtering is to be utilized for

etching process as well as deposition depending on the ion energy.

Figure 2.4: The schematic diagram of DC and RF sputtering system (taken from [142]).

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39

Several variants of the sputtering process such as (i) DC, (ii) RF, (iii) DC/RF

magnetron and (iv) reactive are used for thin films fabrication. The schematic diagram of DC

and RF sputtering is shown in Figure 2.4. However, that there are important variants within

each category (e.g., DC bias) and even hybrids between categories (e.g., reactive RF). RF and

DC magnetron sputtering techniques are the popular methods for growing high quality

manganite thin film [96].

Sputtering with transverse magnetic field has several advantages compared to other

sputtering depositions such as low heating of substrate and low radiation damage. Therefore,

magnetron sputtering techniques are suitable for temperature sensitive or surface sensitive

material deposition. The detail of this deposition technique will be discussed in Section 2.3,

because some of the thin films which investigated in this Ph.D thesis have been prepared by

DC magnetron sputtering technique.

2.2.2 CHEMICAL VAPOUR DEPOSITION

Chemical vapor deposition (CVD) is the process of chemically reacting volatile

compound of a material to be deposited, with other gases, to produce a nonvolatile solid that

deposits atomistically on a suitably placed substrate. It has emerged one of the powerful

techniques of thin film growth. Among the reasons for the growing adoption of CVD methods

is the ability to produce a large variety of films and coatings of metals, semiconductors, and

compounds in a crystalline or vitreous form, possessing high purity and desirable properties.

Furthermore, the capability of controllably creating films of widely varying stoichiometry

makes CVD unique among deposition techniques. Other advantages include relatively low

cost of the equipment and operating expenses, suitability for both batch and semicontinuous

operation, and compatibility with other processing steps. Hence, many variants of CVD

processing have been researched and developed in recent years, including low-pressure

(LPCVD), plasma-enhanced (PECVD), metal-organic (MOCVD) and laser-enhanced

(LECVD) chemical vapor deposition. Hybrid processes combining features of both physical

and chemical vapor deposition have also emerged. MOCVD has presently assumed

considerable importance in the deposition of epitaxial compound semiconductor films.

However, the main obstacle of MOCVD for high temperature superconductor (HTSC) and

rare earth manganite oxides is the lack of thermally stable precursors. MOCVD technique has

been utilized to grow different compositions of thin CMR manganite films [174].

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Metal-Organic Chemical Vapor Deposition: Metal-organic chemical vapor deposition

(MOCVD) is of great importance for large scale production of oxide thin films [175]. It is

routinely used in the electronics industry, has excellent film uniformity over large areas, is

capable of conformal coating of arbitrary geometries, can be done at relatively high partial

pressures of oxygen, has easy and reproducible control of film stoichiometry, has relatively

high deposition rates, and allows for multilayer growth, superlattices, and graded

compositions [176,177]. MOCVD works on the principle that one can create a complex

organic molecule decorated with the material desired for thin film growth. By passing an inert

gas through a bubbler of a liquid precursor, these molecules are transported to the reaction

chamber and passed over a substrate at high temperature. The heat helps to break the

molecules and deposits the desired material on the surface. One of the biggest challenges for

MOCVD growth of oxide materials is identification of the appropriate metal-organic

precursors. Precursors for materials with high atomic number typically have limited vapor

pressure at room temperature and thus it is essential to heat the bubblers and all the lines in

the system to avoid clogging. This requires careful attention so as to avoid hot spots where

premature deposition might occur as well as cool spots where condensation of the precursor

can occur. In the end, very high quality thin films of oxide materials can be created using this

technique. Again, for a more thorough review of the MOCVD process, precursors, and

specific details in reference to ferroelectric materials please see Ref. [177].

Solution-Based Thin Film Deposition Techniques: There are a variety of solution-based

approaches for the creation of complex oxide materials including sol–gel, chelate, and

metaloorganic decomposition (for some good reviews, see Refs. [178,179]). Very briefly,

solution deposition usually involves four steps:

(1) Synthesis of the precursor solution,

(2) Deposition by spin-casting or dip-coating,

(3) Low-temperature heat treatment for drying and/or pyrolosis of organics, and formation of

amorphous films (typically 300–400°C),

(4) High temperature heat treatment for densification and crystallization (anywhere from 600

to 1100 ºC).

Such processes are highly scalable, cheap, and very quick. Great strides have been made in

utilizing such techniques to make high quality and highly oriented films for devices.

Low-Temperature Aqueous Solution Depositions: In stark contrast to the previously

reported growth techniques, there is a set of aqueous solution-based deposition techniques that

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41

enable the creation of films at lower temperatures (25–100°C). Processes such as chemical

bath deposition (CBD), successive ion layer adsorption and reaction (SILAR), liquid phase

deposition (LPD), electroless deposition (ED), as well as more modern variants such as

photochemical deposition (PCD), deposition assisted by applied fields, ferrite plating, liquid

flow deposition, and more can be used to create films of oxide materials at low temperatures.

More detail given in Ref. [180].

2.3 SPUTTERING PROCESS FOR THIN FILM DEPOSITION

In this section the details of sputtering phenomenon such as, the sputtering yield and

its dependence on various factors is provided. Then the types of sputtering mechanisms have

been discussed, and finally describing the complete sputtering system used for the preparation

of manganite thin film in present work.

2.3.1 SPUTTERING PHENOMENON

When a solid surface is bombarded with energetic particles such as accelerated

ions, the surface atoms of the solid are scattered backward due to collisions between the

surface atoms and the energetic particles, as shown in Figure 2.5. This phenomenon is called

back-sputtering, or simply sputtering. When a thin foil is bombarded with energetic particles,

some of the scattered atoms transmit through the foil. The phenomenon is called transmission

sputtering.

Figure 2.5: The physical sputtering processes showing the collision process of incident ions and the target atoms

particles (taken from [142]).

The word “spluttering” is synonymous with “sputtering.” Cathode sputtering, cathode

disintegration, and impact evaporation are also used in the same sense. The details of

sputtering deposition technique for thin film of various materials are given in the books by

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42

Wasa, Kitabatake & Adachi and McClanahan & Laegreid [142,181]. Among the sputtering

techniques, the simplest one is the DC sputtering. The DC sputtering system is composed of a

pair of planar electrodes. One of the electrodes is a cold cathode and the other is the anode.

The front surface of the cathode is covered with target material to be deposited. The substrates

are placed on the anode. The sputtering chamber is filled with sputtering gas, typically argon

gas at 5 Pa (4×10-2

torr). The glow discharge is maintained under the application of DC

voltage between the electrodes. The Ar+ ions generated in the glow discharge are accelerated

towards the cathode and hits the target surface and results in the sputtering of the target.

Before reaching the target surface the Ar+ is converted to neutral Ar atom by capturing the

stray electron on the surface of the target or the surrounding. The sputtered atoms from the

target then fall on the heated substrate resulting in the deposition of the thin films. Sputtered

atoms are generally composed of neutral single atoms of the target material when the target is

sputtered by bombardment with ions having a few hundred electron volts. These sputtered

atoms are partially ionized (i.e., a few 1-5 % of the sputtered atoms are ionized) in the

discharge region.

The removal of the target atoms by sputtering is initiated by the first collision between

incident ions and target surface atoms, followed by the second and third collisions between

the target surface atoms. The displacement of target surface atoms will eventually be more

isotropic due to successive collisions, and atoms may finally escape from the surface.

Depending on the incident ion energy which hit the target, different models has been proposed

for the collision mechanisms in sputtering. The first is the elastic-collision theory in which the

maximum possible energy is transferred in the first collision only. The second theory is called

linear cascade collision theory started in the 1960s by Sigmund, Thomson, and Wehner (did

the seminal works) on this theory [182-185]. Sigmund assumed that sputtering of the target by

energetic ions or recoil atoms results from cascades of atomic collisions [184]. The sputtering

yield is calculated under the assumption of random slowing in an infinite medium. The

theoretical formula was compared with the experimental results given by Rosenberg and

Wehner [182]. Sigmund’s cascade collision theory is the most acceptable for understanding

sputtering phenomena in the keV range of the incident energy. Another model is probably due

to the evaporation at local patches of the target surface heated by the subsiding cascade

similar to thermal evaporation. This process is considered as a nonlinear cascade collision

and/or thermal sputtering, in contrast to the linear cascade collision model.

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43

2.3.2 SPUTTERING YIELD

The sputter yield, S, which is the removal rate of surface atoms due to ion

bombardment, is defined as the mean number of atoms removed from the surface of a solid

per incident ion and is given by,

S = (atoms removed) / (Incident Ions) (2.1)

The sputter yield is influenced by several factors as explained below:

(a) Energy of incident particles

Figure 2.6 shows a typical variation of the sputtering yield with incident ion energy. In a

low energy region, threshold energy exists for sputtering which is typically 20 ~ 30 eV for

most of metallic targets. Above threshold value sputtering yield 'S' follows direct dependence

to the √E up to the ion energy of E ~ 100 eV. In the mid-range between 100 eV and few keV

ion energy the 'S' follows direct dependence on the ion energy E [142].

Figure 2.6: Variations of sputter yield with incident ion energy, (taken from page 73 of Ref [142]).

The sputter yield shows maximum value in a high-energy region (typically of the range of

keV). In this energy region, the incident ions collide with the surface atoms of the target, and

the number of displaced atoms due to the collision will be proportional to the incident energy.

At higher ion energies of 10 to 100 k eV, the incident ions travel beneath the surface and the

sputter yields are not governed by surface scattering, but by the scattering inside the target.

Above 10 k eV, the sputter yields will decrease due to energy dissipation of the incident ions

deep in the target.

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44

(b) Target materials

Sputter yields 'S' varies periodically with the element’s atomic number of the target atom.

Comparing various materials, the yields increase consistently as the electronic d shells of

target material are filled, as Gold (Au), Silver (Ag) and Copper (Cu) having the highest

yields. Conversely, elements with the most open electronic structure result in the least

sputtering yield. The sputtering yields are proportional to the energy transfer factor, and are a

function of the hardness of the atoms since hard collisions increase the yield. Periodicity is

also observed in sputtering thresholds [169,170].

(c) Incident angles of particles

Sputter yields vary with the angle of incident ions. Metals such as Gold (Au), Silver (Ag),

Copper (Cu) and Platinum (Pt), which have high sputtering yields, show very slight “angle

effects.” Iron (Fe), Tantalum (Ta), and Molybdenum (Mo), that have low sputtering yields

and show very pronounced angle effects. The yield increases with the incident angle and

shows a maximum at angles between 60° and 80°, while it decreases rapidly for larger angles.

The influence of the angle is also governed by the surface structure of the target [185].

(d) Crystal structure of the target surface

It is well known that the sputtering yield and angular distribution of the sputtered

particles are affected by the crystal structure of the target surface. The angular distribution

may be either under cosine law or over cosine law when the target is polycrystalline. Non-

uniform angular distribution is often observed from the single-crystal target. Wehner studied,

in detail, the nonuniform angular distribution of single crystals and found that different

patterns appeared in the deposited film [183]. He suggests that, near the threshold, the

sputtered atoms are ejected in the direction of close-packed atoms. For instance, in fcc Ag, the

close-packed direction corresponds to ⟨110⟩ and the sputtering yield is also maximum along

this direction.

Thus sputter yield, S, can be measured by the following (i) weight loss of target, (ii)

decrease of target thickness, (iii) collection of the sputtered materials and (iv) detection of

sputtered particles in flight.

2.3.3 REACTIVE SPUTTERING

When a reactive gas species such as oxygen or nitrogen is introduced into the

chamber, thin films of compounds (i.e., oxides or nitrides) are deposited by the sputtering of

the appropriate metal targets. This technique is known as reactive sputtering and may be used

CHAPTER 2

45

in either the DC or RF mode. Reactive sputtering is used in practice for the high-rate

deposition of insulating metal oxide films. However, DC sputtering from a metal target is

unstable in a standard large-scale DC sputtering operation due to the deposition of an

insulating layer on the cathode leading to the buildup of a charge. To reduce the charge-up

phenomenon, medium-frequency sputtering with twin targets is used in practice. A medium-

frequency power source is connected to the twin targets. The targets act as anode and cathode

alternatively at the frequency of several tens of kilohertz, and deposition can be carried out

with no appreciable decrease in the deposition rate.

2.3.4 TYPICAL MAGNETRON SPUTTERING SYSTEM

A typical sputtering system is shown in the Figure 2.7 which consists of the entire

component required to grow a single crystalline epitaxial thin films under reactive sputtering

condition. The figure also shows the sputtering system which has been used to prepare the

thin films of manganites in the present work. The components are marked with numbers and

will be explained below with their respective functions.

1. DC Diode: Two inch diameter copper plate acting as cold cathode attached to one foot

long metallic rod with embedded water inlet and outlet. One end of the rod is fitted

with input DC/RF power supply attachment. The front surface of copper plate is used

to place the target and the reverse side is attached to water cooled jacket. The

permanent magnets are tight fitted within the copper plate itself in the configuration.

2. A dark space shield, known as a ground shield, surrounds the target with small

separation so that only target material is sputtered. The spacing between the target and

the ground shield must be less than the thickness of the dark space, λ0/p (where λ0 is

mean free path of ions and p is pressure of the discharge gas). If we design the

maximum operating pressure to be 20 Pa (0.15 torr), taking λ0 = 0.05 cm (Ar), and p =

0.15 torr, λ0/p becomes 3 mm. The spacing between the target and the ground shield is

kept at 2 mm in present case of thin film deposition DC magnetron sputtering.

3. The DC power supply can deliver output power from 1W to 2.5 kW and provided with

safety interlock system to protect from sudden voltage fluctuation.

4. Cylindrical heater is of 2 inch diameter and 2 cm height metallic body that

encapsulates the heating element (isolating the element from the gases mixture to

prevent oxidation). The maximum temperature is 1000°C and it can be operated

continuously even at 800°C for six to eight hours.

CHAPTER 2

46

Figure 2.7: A typical Magnetron Sputtering system one which is used in the present investigation.

5. Rotary pump providing the initial rough vacuum up to ~10-3

mbar pressure to the

vacuum chamber (7) and the pressure is read out using Pirani gauge. It also acts as

backing pump for the silicone oil diffusion pump (6). It is also used for maintain

dynamic condition at constant sputtering pressure.

6. Diffusion pump provides the high vacuum to the chamber up to ~ 10-6

mbar and the

pressure is read out using Penning gauge attached to the chamber. Such high vacuum

condition is required for the removal of atmospheric gases so that the chamber is clean

for the growth of high quality films.

7. Vacuum glass chamber which can stand up to ~ 10-7

mbar pressure.

8. Gas cylinder provides the gases mixture required to sustain the sputtering during the

thin film deposition.

9. Fine Gas regulator is used to break the vacuum by sputtering gas into the chamber.

10. Temperature sensor which is attached to the heater whose output is digitally read out

by this Temperature Indicator.

CHAPTER 2

47

11. Pressure Indicator attached directly to the chamber, which digitally read out the

pressures of the chamber during sputtering.

2.4 DC MAGNETRON SPUTTERING TECHNIQUE

Prior to the film deposition processes by magnetron sputtering one needs a bulk form

of thin film material generally called “target”. Target is compactly-pressed sintered disk (as

per the shape of the cathode) of metal, alloy or compound oxides etc. of same chemical

stoichiometric as required for the thin film. Sputtering is an inefficient process, and most of

the power input to the system appears ultimately as target heating; thus these targets are

usually mounted on a water-cooled backing plate which also acts as cathode. The target is

fixed on the backing plate by mounting clips or other mechanical support. Solid state reaction

route for targets preparation has been used in the present investigations for the deposition of

Nd0.51Sr0.49MnO3 thin film, results of this work is reported in Chapter 3 and 4 of this thesis.

2.4.1 TARGET PREPARATIONS BY SOLID STATE REACTION METHOD

There are several methods for preparing target like solid state reaction (SSR), sol-gel

method etc. We have used solid state reaction method for the preparation of Nd0.51Sr0.49MnO3

and Nd0.50Sr0.50MnO3 targets. Solid state reaction method is most widely used method for

synthesizing the polycrystalline solids (powders). This is the direct reaction, in the solid state,

of a mixture of solids as starting materials. Solids do not usually react together at room

temperature over normal time scale so it is necessary to heat them at much higher temperature

for long time duration for reaction to occur at an appreciable rate. There are two factors,

namely thermodynamic and kinetic, which are important in solid state reaction, the former

determines the possibilities of any chemical reaction to occur by the free energy

considerations which are involved while the later determines the rate at which the reaction

occurs [186, 187]. The atoms diffuse through the material to form a stable compound of

minimum free energy. Different compounds or phases might have the lowest free energy at

various temperatures or pressures or the composition of the gas atmosphere might affect the

reaction. In order to prepare a single-phase sample, the conditions during any reaction are

very important. During synthesis, the parameters such as temperature, pressure, gas flow and

time for the reaction are needed to be varied according to the phase requirements in the

samples.

All the bulk polycrystalline manganite samples studied during present work were

synthesized using SSR method as per the steps described below:

CHAPTER 2

48

1. All starting materials were high purity (99.9%) powders of oxides, nitrides, viz., Nd2O3,

Sr(NO3)2 and MnO2. They were preheated for appropriate time and temperature. After

preheating powders were weighed for desired composition using high precision electronic

weighing machine.

2. In the solid state reaction, for the reaction to take place homogeneously, it is very important

to mix and grind the powders thoroughly for long duration to obtain homogeneous

distribution of components (starting materials) in required proportions of the desired

stoichiometric compound.

3. After mixing of stoichiometric amounts of all powder materials, proper grinding using

pestle-mortar is very important. Thorough grinding decreases the particle size of mixed

powders. This is necessary for obtaining close contact among the atoms so the right

material is formed.

4. This powdered mixture was then heated (calcined) at 800ºC for 24 hours, in air for the first

time. During the first calcination, CO2 is liberated from the mixture.

5. After the first heating, obtained powder was ground thoroughly for three to four hours. To

maintain uniform particle size.

6. The powders then sintered at 900oC for 24 hours with intermittent grindings to obtain

single phase samples.

7. After this grinding the powder was pressed in the form of pellet (diameter, Φ = 2 inch) with

a thickness of 3 mm. For various analytical studies a second pellet of dimension 12 x 5 x 2

cubic mm, was also made and then was palletized at 4-5 T pressure using hydraulic press.

8. Both target and second pellet was finally sintered at 1300oC for 24 hours to obtain the

desired structural phase.

The solid state reaction method has proved to be the most suitable for synthesizing

reproducible samples of CMR manganites. Pictures of target before deposition and after

deposition shown below (Figure 2.8).

Figure 2.8: Pictures of Nd0.51Sr0.49MnO3 target which is used in the present investigation. (a) Target before film

deposition, and (b) after film deposition.

(a

)

(b)

CHAPTER 2

49

2.4.2 THIN FILM PREPARATION PROCEDURE

a) The target is placed on the cathode and mounted properly to the chamber. The

substrates are cleaned and placed on the top surface of the heater.

b) The chamber is evacuated using the rotary pump for half an hour. In the mean time the

diffusion pump is switched on so that it reaches to its optimum condition to carried out

the further evacuation of the chamber. These processes roughly take more than two

hours to attain pressure ~10-6

torr.

c) During the evacuation process, independently the power of the heater was increased

slowly so that its temperature reaches to desired value (~750°C - 800°C).

d) The chamber is flushed twice during the evacuation with gas mixture (Argon ~ 80% +

Oxygen ~ 20%) which is used during the sputtering.

e) Once completing these procedures the heater temperature is fixed to the desired value

(~ 800°C) and the gas mixture is introduced in the chamber up 200 mTorr and the DC

power to the cathode is switched on to maintain the self-sustain plasma (~ 210 Volts

DC, 0.15 A).

f) After the plasma formation the sputtering of target is triggered and the shutter is

removed from the substrate. The layer of film gets deposited on the plasma exposed

surface of the substrate. Once the desired thickness of the film is achieved the plasma

is switched off.

g) The film is then kept for an hour on the same temperature (deposition temperature

~ 800°C) and 1000 mTorr. Then followed by slow cooling of films to room

temperature in more than three hours.

h) The as prepared film has oxygen deficiency and to overcome this, films were

oxygenated for more than 6 hours at 800°C.

i) Some of the films were deposited on half of the substrate to get a step-film

configuration for thickness measurement by using Stylus profilometer.

The above mentioned parameters in step (e) and other condition in above steps are optimized

for certain growth rate for thin film. In present investigations the average growth rate is found

to be ~5 nm/minute. The set of films with 30 nm and 100 nm thicknesses, prepared using

target Nd0.51Sr0.49MnO3 on substrates LaAlO3 (LAO) and 100 nm thickness on Yttria-

stabilized ZrO2 (YSZ) single crystal substrates. Films of thicknesses 30 and 100 nm on LAO

will hereafter be referred as L30 and L100 respectively. The films on YSZ with thickness 100

nm will be inferred as Z100.

CHAPTER 2

50

2.5 SPRAY PYROLYSIS PROCESS FOR THIN FILM DEPOSITION

A wide variety of thin films has been deposited by applying the spray pyrolysis

technique. Various devices such as solar cells, sensors, and solid oxide fuel cells have been

prepared by using these films. Preparation conditions are mainly responsible for different

properties for such deposited thin films. Most critical parameter which influences the films

roughness, cracking, crystallinity, etc. is the substrate surface temperature. Atomization of the

precursor solution, aerosol transport, and decomposition of the precursor are the processes

mainly involved in spray pyrolysis technique.

For preparing dense and porous oxide films, ceramic coatings, and powders, spray

pyrolysis is the most suitable processing technique. Spray pyrolysis represents a very simple

and relatively cost-effective method, especially regarding equipment cost. In the glass

industry [188] and in solar cell production to deposit electrically conducting electrodes [189]

spray pyrolysis has been used for several decades. Typical spray pyrolysis equipment consists

of an atomizer, precursor solution, substrate heater, and temperature controller. The following

atomizers are usually used in spray pyrolysis technique: air blast (liquid is exposed to a stream

of air) [190], ultrasonic (ultrasonic frequencies produce the short wavelengths necessary for

fine atomization) [191] and electrostatic (liquid is exposed to a high electric field) [192].

In literature there are various reviews concerning spray pyrolysis techniques have been

published. Mooney and Radding have reviewed the spray pyrolysis method, properties of the

deposited films in relation to the conditions, specific films (particularly CdS), and device

application [193]. Tomar and Garcia have discussed the preparation and the properties of

sprayed films as well as their application in solar cells, anti-reflection coatings and gas

sensors [194]. Albin and Risbud presented a review of the equipment, processing parameters

and optoelectronic materials deposited by spray pyrolysis technique [195]. Pamplin has

published a review of spraying solar cell materials as well as a bibliography of references on

the spray pyrolysis technique [196]. Recently thin metal oxide and chalcogenide films

deposited by spray pyrolysis and different atomization techniques were reviewed by Patil

[197]. Bohac and Gauckler have discussed the mechanism of chemical spray deposition and

presented some examples of sprayed YSZ films [198].

Doped manganite thin film deposition using spray pyrolysis will be discussed in

Chapter 5 and 6 of this thesis. The study of magnetism and magnetotransport of these thin

films will be described. A schematic of spray pyrolysis equipment shown in Figure 2.9.

CHAPTER 2

51

Figure 2.9: Schematic of spray pyrolysis equipment.

Spray pyrolysis is a versatile and effective technique to deposit metal oxide films. It is an

attractive method to prepare a wide variety of powders and thin film materials for various

industrial applications. Metal oxide, chalcogenide and even metal films have been deposited

using this technique. Spray pyrolysis opens up the possibility to control the film morphology.

The quality and properties of the films depend largely on the process parameters. The most

important parameter is the substrate surface temperature. The higher the substrate

temperature, the rougher and more porous are the films. If the temperatures are too low the

films are cracked. In between, dense smooth films can be obtained. The deposition

temperature also influences the crystallinity, texture and other physical properties of the

deposited films. The precursor solution is the other important spray parameter which affects

the morphology and the properties of the deposited films. In addition, the film morphology

and properties can be drastically changed by using various additives in the precursor solution.

It is often suggested that a modified CVD process occurs in film formation close to the

surface of the substrate. However many observations contradict the involvement of a model

with a CVD character. Further efforts are necessary to clarify the model for film deposition in

more detail.

2.5.1 MAIN STEPS FOR FILM DEPOSITION BY SPRAY PYROLYSIS

As reported in literatures, there are too many processes that occur either sequentially

or simultaneously during film formation by spray pyrolysis. These include precursor solution

atomization, droplet transport and evaporation, spreading on the substrate, drying and

decomposition of the precursor salt. Understanding these processes will help to improve film

quality. Thin film deposition using spray pyrolysis can be divided into three main steps:

atomization of the precursor solution, transportation of the resultant aerosol and

decomposition of the precursor on the substrate.

CHAPTER 2

52

2.5.1.1 ATOMIZATION OF PRECURSOR SOLUTION

Normally used atomizers in spray pyrolysis techniques are air blast, ultrasonic and

electrostatic. Various reports were published on the mechanism of liquid atomization.

Rizkalla and Lefebvre examined the influence of liquid properties on air blast atomizer spray

characteristics [199]. Lampkin presented results concerning the application of the air blast

atomizer in a spray pyrolysis set-up [200]. Recently a theory of ultrasonic atomization was

published [201]. Ganan-Calvo et al. have studied the electrostatic atomization of liquids and

derived scaling laws for droplet size from a theoretical model of charge transport [202].

Compared with other spray techniques, the nebulized spray (air blast) deposition

technique has been only recently used [203, 204] for deposition of manganite and

multiferroics thin films.

2.5.1.2 AEROSOL TRANSPORT

The droplet of solution is transported and eventually evaporates in an aerosol. For

making dense thin films, it is important that during transportation as many droplets as possible

fly to the substrate without forming particles before reaching the surface. Sears et al.

investigated the mechanism of SnO2 film growth [205]. The influence of forces which

determine both the trajectory of the droplets and evaporation were examined and a film

growth model was proposed. Four types of forces i.e. gravitational, electric, thermophoretic

and Stokes forces were taken into account. The thermophoretic force pushes the droplets

away from a hot surface, because the gas molecules from the hotter side of the droplet

rebound with higher kinetic energy than those from the cooler side.

Figure 2.10: Schematic of aerosol transport by Sears et al. [205].

CHAPTER 2

53

Thermophoretic forces keep most droplets away from the surface in non-electrostatic

spray process. It was concluded that the film grows from the vapour of droplets passing very

close to the hot substrate in a manner of chemical vapour deposition (Figure 2.10).

Droplets that strike the substrate form a powdery deposit. The authors suggest that

forcing droplets closer to the substrate while avoiding actual contact would improve the

efficiency of film growth. Siefert described the transport processes in corona spray pyrolysis.

Here the droplets enter a corona discharge and are transported in an electric field to the

substrate [206]. Lenggoro et al. investigated powder production by spray pyrolysis using a

temperature-graded laminar flow aerosol reactor [207]. Oh and Kim have studied the

behaviour of an evaporating droplet in a non-isothermal field [208].

2.5.1.3 DECOMPOSITION OF PRECURSOR

Many processes occur simultaneously when a droplet hits the surface of the substrate:

evaporation of residual solvent, spreading of the droplet, and salt decomposition. Many

models exist for the decomposition of a precursor. Most of the authors suggest that only a

kind of CVD process gives high quality films by spray pyrolysis. Viguie and Spitz proposed

the following processes that occur with increasing substrate temperature [209]. In the lowest

temperature regime (process A in Figure 2.11) the droplet splashes onto the substrate and

decomposes.

Figure 2.11: Description of the deposition processes initiated with increasing substrate

temperature by Viguie and Spitz (taken from [209]).

CHAPTER 2

54

At higher temperatures (process B) the solvent evaporates completely during the flight

of the droplet and dry precipitate hits the substrate, where decomposition occurs. At even

higher temperatures (process C) the solvent also evaporates before the droplet reaches the

substrate. Then the solid precipitate melts and vaporizes without decomposition and the

vapour diffuses to the substrate to undergo a CVD process. At the highest temperatures

(process D) the precursor vaporizes before it reaches the substrate, and consequently the solid

particles are formed after the chemical reaction in the vapour phase. It was speculated that the

processes A and D lead to rough or non adherent films. Adherent films were obtained by

CVD at low temperatures (process C). Choy proposed a deposition model for the so called

electrostatic spray-assisted vapour deposition process [210]. This technique is also known as

electrostatic spray deposition. The precursor solution is atomized using an electric field. Chen

et al. investigated the correlations between film morphologies and deposition parameters

[192]. The films were deposited using the so-called cone-jet mode. The substrate temperature

was indicated as the most important parameter. The concentration of the precursor solution

had a minor influence on the film morphology.

2.6 SPRAY PYROLYSIS TECHNIQUE

Prior to the film deposition processes by spray pyrolysis one needs a solution of

required material. The molarity (M) of solution is being optimized by making solution of

different molarity followed by several characterizations.

Figure 2.12: A typical spray pyrolysis system which is used in the present investigation.

CHAPTER 2

55

Here for thin film preparation we have used nebulizer for spray of solution and hot

plate for required substrate temperature. A typical spray pyrolysis system shown in Figure

2.12. This thin film preparation technique has been used in the present investigations for the

deposition of Nd1-xSrxMnO3 (0.50 x 0.62) and La1-xCaxMnO3 (0.45 x 0.60) thin films,

results of this work is presented in Chapter 5 and 6 of this thesis.

2.6.1 SOLUTION PREPARATION

Aqueous solutions of a well-homogenized material of required optimized molarity (0.2

M) have been prepared by dissolving high-purity of required metal nitrates in an appropriate

cationic ratio.

2.6.2 THIN FILM PREPARATION PROCEDURE

a) Solutions of required molarity (0.2 M) have been prepared by using required metal

nitrates.

b) Prior to deposition, the substrates were cleaned by using acetone and placed on the top

surface of the heater maintained at temperature of ~250 ± 2ºC.

c) Then the solution was sprayed with the help of nebulizer on different substrates.

d) After deposition, the films were slowly cooled down to room temperature and then

annealed at ~1000ºC for 12h in air by using furnace. For nanostuctured films we have

annealed at ~920ºC for 2h in air.

e) Some of the films were deposited on half of the substrate to get a step-film

configuration for thickness measurement.

The above mentioned parameters in step (a,b,d) are optimized for certain growth rate for

thin film. In present investigations the average growth rate is found to be ~150 nm/minute.

2.7 CHARACTERIZATIONS TECHNIQUES

2.7.1 STRUCTURAL CHARACTERIZATION

For structural characterization we have employed different experimental techniques

briefly explained in following sections.

2.7.1.1 CHEMICAL CHARACTERIZATION OF THIN FILM

The elemental and chemical nature of the films has been analyzed by Scanning

Electron Microscopy (Model: LEO 0440 SEM equipped with ISIS 300 Oxford Microanalysis

CHAPTER 2

56

system) having the attached detector for Energy Dispersive X-ray spectroscopy (EDS or

EDX). EDS is one of the variants of X-ray fluorescence spectroscopy which relies on the

investigation of a sample through interactions between electromagnetic radiation and matter,

analyzing X-rays emitted by the matter in response to being hit with charged particles. Its

characterization capabilities are due in large part to the fundamental principle that each

element has a unique atomic structure allowing X-rays that are characteristic of an element's

atomic structure to be identified uniquely from one another. To stimulate the emission of

characteristic X-rays from a specimen, a high-energy beam of charged particles such as

electrons or protons or a beam of X-rays, is focused into the sample being studied (see Figure

2.13).

Figure 2.13: The schematic diagram of a scanning electron microscope (SEM) equipped with EDS.

At rest, an atom within the sample contains ground state (or unexcited) electrons in

discrete energy levels or electron shells bound to the nucleus. The incident beam may excite

an electron in an inner shell, ejecting it from the shell while creating a hole where the electron

was. An electron from an outer, higher-energy shell then fills the hole, and the difference in

energy between the higher-energy shell and the lower energy shell may be released in the

form of an X-ray. The number of photon and energy of the X-rays emitted from a specimen

can be measured by an energy dispersive spectrometer. As the energy of the X-rays is

characteristic of the difference in energy between the two shells, and of the atomic structure

EDS analysis

part

CHAPTER 2

57

of the element from which they were emitted, this allows the respective ratio of elemental

composition of the specimen to be measured. EDS systems are most commonly found on

scanning electron microscopes (SEM-EDS) and electron microprobes.

Accuracy of EDS spectrum can be affected by many factors. Windows in front of the

detector can absorb low-energy X-rays (i.e. EDS detectors cannot detect elements with atomic

number less than 4, that is H, He, and Li). Over-voltage settings in EDS alter the peak size-

raising and shift the spectrum to the larger energies, making higher-energy peaks larger and

lower-energy peaks smaller. The accuracy of the spectrum can also be affected by the nature

of the sample. X-rays can be generated by any atom in the sample that is sufficiently excited

by the incoming beam. These X-rays are emitted in any direction, and so they may not all

escape the sample. The likelihood of an X-ray escaping the specimen, and thus being

available to detect and measure depends on the energy of the X-ray and the amount and

density of material it has to pass through. This can result in reduced accuracy in

inhomogeneous and rough samples.

2.7.1.2 THICKNESS MEASUREMENT

Thickness measurement was carried out on Veeco make Dektek stylus profilometer.

This profilometer is used to determine the thickness of specially fabricated step-thin film

(when the film is deposited only on some part of the substrate leaving other part naked). The

stylus profilometer uses a diamond tipped stylus to scan across the sample surface and

measures the surface topography of thin and thick films (see Figure 2.14 for basic design).

Figure 2.14: Left (a) show typical stylus profile measurement idea and right (b) typical thickness measurement

curve for the 100 nm step thin film of Nd0.51Sr0.49MnO3 on LAO(001).

(a)

(b)

)))

)

))

CHAPTER 2

58

A diamond stylus is moved vertically in contact with a sample and then moved

laterally across the sample for a specified distance and specified contact force. A profilometer

can measure small surface variations in vertical stylus displacement as a function of position.

A typical profilometer can measure small vertical features ranging in height from 10 nm to

1 mm. The height position of the diamond stylus generates an analog signal which is

converted into a digital signal stored, analyzed and displayed. The radius of diamond stylus

ranges from 20 nm to 25 μm, and the horizontal resolution is controlled by the scan speed and

data signal sampling rate. The stylus tracking force can range from less than 1 to 50 mg.

Figure 2.14 also show a typical thickness measurement curve for the 100 nm (film deposit

time ~15 minutes) step thin film of Nd0.51Sr0.49MnO3 on grown on LaAlO3 (LAO).

Thickness of films prepared by using spray pyrolysis, was measured by using atomic

forced microscopy (A.F.M.) and explained in next sub-section.

2.7.1.3 ATOMIC FORCED MICROSCOPY

Surface analysis of thin films was carried out by Atomic force microscopy (Model-

Nanoscope Vicco III). Atomic force microscopy (A.F.M.) is a very high-resolution type of

scanning probe microscopy, with demonstrated resolution on the order of fractions of a

nanometer, more than 1000 times better than the optical diffraction limit (see Figure 2.15 for

block diagram).

Figure 2.15: Block diagram of atomic force microscope (A.F.M.).

CHAPTER 2

59

The A.F.M. is one of the foremost tools for imaging, measuring, and manipulating matter at

the nanoscale. The information is gathered by "feeling" the surface with a mechanical probe.

Piezoelectric elements that facilitate tiny but accurate and precise movements on (electronic)

command enable the very precise scanning. In some variations, electric potentials can also be

scanned using conducting cantilevers. The A.F.M. consists of a cantilever with a sharp tip

(probe) at its end that is used to scan the specimen surface. The cantilever is typically silicon

or silicon nitride with a tip radius of curvature of the order of nanometers. When the tip is

brought into proximity of a sample surface, forces between the tip and the sample lead to a

deflection of the cantilever according to Hooke's law. Depending on the situation, forces that

are measured in A.F.M. include mechanical contact force, van der Waals forces, capillary

forces, chemical bonding, electrostatic forces, magnetic forces (known as magnetic force

microscope, MFM), etc. Typically, the deflection is measured using a laser spot reflected

from the top surface of the cantilever into an array of photodiodes. If the tip was scanned at a

constant height, a risk would exist that the tip collides with the surface, causing damage.

Hence, in most cases a feedback mechanism is employed to adjust the tip-to-sample distance

to maintain a constant force between the tip and the sample. Traditionally, the sample is

mounted on a piezoelectric tube that can move the sample in the 'z' direction for maintaining a

constant force, and the x and y directions for scanning the sample. The resulting map of the

area s = f(x,y) represents the topography of the sample. The height resolution of an atomic

force microscope on a flat surface is down to less than an atomic layer (in many cases < 1

˚A), whereas the lateral resolution is much lower (under good conditions down to 1 nm) and

depends on the size and shape of the tip. The A.F.M. can be operated in a number of modes,

depending on the application. In general, possible imaging modes are divided into static (also

called contact) modes and a variety of dynamic (or non-contact and tapping mode) modes

where the cantilever is vibrated.

Contact Mode (DC Mode): In the static mode operation, the static tip deflection is used as a

feedback signals because the measurement of a static signal is prone to noise and drift, low

stiffness cantilevers are used to boost the deflection signal. However, close to the surface of

the sample, attractive forces can be quite strong, causing the tip to 'snap-in' to the surface.

Thus static mode A.F.M. is almost always done in contact where the overall force is repulsive.

In contact mode, the force between the tip and the surface is kept constant during scanning by

maintaining a constant deflection. This mode has also been used for thickness measurement of

thin film with step.

CHAPTER 2

60

Non-Contact Mode (AC Mode): In this mode, the tip of the cantilever does not contact the

sample surface. The cantilever is instead oscillated at a frequency slightly above its resonance

frequency where the amplitude of oscillation is typically a few nanometers (< 10 nm). The

Van der Waals forces, which are strongest from 1 nm to 10 nm above the surface, or any other

long range force which extends above the surface acts to decrease the resonance frequency of

the cantilever. This decrease in resonance frequency combined with the feedback loop system

maintains a constant oscillation amplitude (tapping mode) or frequency (non-contact mode)

by adjusting the average tip-to-sample distance. Measuring the tip-to-sample distance at each

(x,y) data point allows the scanning software to construct a topographic image of the sample

surface.

Thickness measurement by using A.F.M.: Thickness measurement of specially fabricated

step-thin films of given material by using spray pyrolysis technique can also be done by using

A.F.M. The A.F.M. scan across the step film onto given substrate and measures step height of

the thin film and thick films, typical graphs of thickness measurements at two different places

for a given film is shown in Figure 2.16.

Thickness Measurement Using AFM

Figure 2.16: Typical thickness measurement graph for step thin films (La0.50Ca0.50MnO3) deposited using

nebulized spray pyrolysis measured using A.F.M. at two different places.

0 1 2 3 4 5 6 7 8-200

-150

-100

-50

0

50

100

150

Z (

Heig

ht)

(n

m)

X (m)

290 nm

0 2 4 6 8-200

-150

-100

-50

0

50

100

150

200

Z (

Heig

ht)

(n

m)

X (m)

310 nm

CHAPTER 2

61

2.7.1.4 X-RAY DIFFRACTION

X-ray diffraction is one of the basic tools employed for the determination of structural

phase and order of crystallinity of any single crystal, bulk solid matter and thin films. There

are certain modification to diffractometer used for bulk is needed, to get the detail information

of structural phase of thin films [211,212]. The difference between the diffraction peak profile

of a bulk and thin film is directly dependent on the availability of the diffracting volume. The

relative intensity of characteristic peak does not show any variation with one found in the

JCPDS (Joint Committee on Powder Diffraction Standards) file for the bulk sample. But for a

film with specific thickness, or say in the range of penetration depth of the X-rays, relative

intensity of characteristic peak does show a variation with incident angle ‘θ’ due to the

asymmetric irradiating volume distribution in the thin film. If we consider a film with a

specific thickness, the relative diffracted intensity measured for each characteristic peak is

equal to the one found in the JCPDS file, multiplied by the ratio between the actual diffracting

volume and the volume that would have been irradiated had the sample been infinite.

Generally speaking, diffractometers designed for the study of epitaxial thin films need to

present two essential features. First, the incident beam on the sample and the beams detected

after diffraction by the sample have to be as parallel and as monochromatic as possible.

Second, it has to be possible to orient the sample very precisely with respect to the incident

beam, meaning that the diffractometer has to be equipped with a multi-axis sample holder

able to move with an angular precision in the range of a thousandth of a degree.

The basic principle of determining the crystal structure using X-ray is based on the

diffraction phenomenon of electromagnetic wave. So when matter is irradiated with a beam of

X-ray photons, the interaction of photon with the bounded electron mostly result in a coherent

scattering of these photons which can be detected using electromagnetic photon detectors. The

scattered or emitted X-ray beams are of equal wavelength or very close to that of the incident

beam. In the case where scattering occurs without a modification of the wavelength (coherent

scattering) and when the scattering centers are located at non-random distances (~ integral

multiple of wavelength of X-ray) from one another, then the scattered waves interfere to give

rise to diffracted waves with higher intensities. The analysis of the diffraction figure, that is,

the analysis of the distribution in space of the diffracted intensity, makes it possible to

characterize the structure of the material being studied. Basic formulation for the

determination of crystal structure using the angular distribution of diffraction peaks was

established by Bragg's in 1913 known as Bragg’s law

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(2.2)

'n' denotes the order of diffraction, 'λ' is the wavelength of X-ray used, 'dhkl' is distance

between the lattice plane and 'θ' is angle at which diffraction peak is detected. Pictorial view

of the diffraction phenomenon is shown in Figure 2.17. All the atoms in a lattice can be

assumed to be as a part of set of planes characterized by three integers (h, k, l), referred to as

the Miller indices. Within each set of defined planes the lattice points (atoms) are equally

spaced throughout the plane. Equation (2.2) shows how the interplaner distances in a given

crystal can be calculated from the measurements of the diffraction angles 'θ' and the known

value of 'λ'. After calculating the value of dhkl using equation (2.3) and applying certain rules

(depending on the crystal structure) for determining the Miller indices h, k and l we can

accurately find the lattice parameter of a crystal being studied [18]. For example the relation

between the lattice parameters (a, b and c), Miller indices of (h, k and l) and the interplaner

distance dhkl is for a simple cubic system is given as,

(2.3)

These methods of calculation are bit cumbersome for certain type of crystal system

such as rhombohedral, triclinic etc. To avoid such calculation several softwares such as

Fullprof Rietveld Refinement, Poudrix, Index etc. are available for the direct refinement of

experimental data to the theoretically calculated data.

Figure 2.17: The pictorial view of X-ray Diffraction by crystals.

Different modes of scan can be employed on the sample to find the various structural

parameters, 'θ-2θ' scan in which the angle between X-ray and sample plane is varied up to 'θ'

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and simultaneously the detector is moved up to '2θ'. By employing this scan one can find the

phase purity, crystal structure and type of symmetry does sample have. The '-2θ' scan of a

polycrystalline sample results in numbers of possible diffracted peak at certain angles where

the Bragg’s law is satisfied. The '-2θ' scan of single crystalline sample also result in numbers

of peak but of higher intensity as compared to the polycrystalline sample. In epitaxial or

oriented thin films, 'θ-2θ' scan results in only few diffraction peaks depending on the plane

orientation of the substrate. The high intensity peak in epitaxial thin films results from the

substrate and along with it low intensity peak correspond to the thin films of same orientation.

Using the Bragg's law (equation 2.2) it is possible to calculate the length of the out-of-plane

lattice parameter for the different layers.

2.7.2 MAGNETIC CHARACTERIZATION OF THIN FILMS

Magnetic characterization of thin films is more difficult as compare to the bulk

magnetic material because of the small volume fraction available for the analysis and also

receiving simultaneous contribution from the substrate ions. Generally the Bulk magnetic

material can easily be characterized on the low resolution Vibrating Sample Magnetometer

(VSM), Susceptometers/Magnetometers, Fluxmeters, etc. Typical the resolution of VSM is of

the order of 10-4

~ 10-6

emu/gm. But these instruments are not suitable if the order of magnetic

field ~ 10-7

emu/gm, which is the typical value what one gets in low dimensional system such

as thin films. To have a precise magnetic character of thin films one has to employ

superconducting quantum interference device (SQUID) magnetometer. Magnetic

characterization of all the films in this thesis has been characterized by SQUID

magnetometer. Some simple aspects of SQUID based magnetometers are discussed below.

A superconducting quantum interference device (SQUID) is the most sensitive

magnetic field/flux sensor and is capable of measuring extremely weak signals (~few 10-15

T),

such as subtle changes in the human body's electromagnetic energy field. Based on Josephson

junctions, a SQUID can detect a change of energy as much as 100 billion times weaker than

the electromagnetic energy that moves a compass needle. SQUIDs have been used for a

variety of measurements that demand extreme sensitivity and have been used in engineering,

medical, and geological equipment. Because they measure changes in a magnetic field with

ultra-high sensitivity, they do not have to come in contact with a system that they are testing.

A Josephson junction is made up of two superconductors, separated by an insulating layer it is

so thin that electrons can tunnel through it. SQUIDs are usually made of either a lead alloy

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(with 10% gold or indium) and/or niobium, often consisting of the tunnel barrier sandwiched

between a base electrode of niobium and the top electrode of lead alloy. A direct current (DC)

SQUID, which is much more sensitive, consists of two Josephson junctions employed in

parallel so that electrons tunneling through the junctions demonstrate quantum interference,

dependent upon the strength of the magnetic field within a loop. DC SQUIDs demonstrate

resistance in response to even tiny variations in a magnetic field, which is the capacity that

enables detection of such minute changes. The configuration of the DC SQUID is

schematically shown in Figure 2.18.

Figure 2.18: Configuration of the DC SQUID (a) Schematic drawing showing the super conducting loop with

two Josephson junction and B denotes the magnetic flux enclosed within the loop area; (b) Equivalent circuit

where ‘R’ is resistance and ‘C’ is capacitance is in parallel with Josephson junction; (c) voltage vs. flux Φa/Φ0

for constant bias current (I).

It consists of a superconducting ring biased with a current I. An external magnetic

field H = B/µ0 is applied to the loop. A Josephson junction is incorporated into each of the

two arms of the DC SQUID.

The Josephson junctions limit the maximum supercurrent Ic that can flow across the

ring to a maximum value given by the sum of the critical currents of the two junctions. The

magnetic flux enclosed inside the SQUID ring modulates Ic periodically, with a period of one

flux quantum (Φ0 = h/2e) [213]. This modulation, caused by an interference of the

superconducting wave functions in the two SQUID arms, forms the basis of the working

principle of the DC SQUID. The direct way to detect this modulation is to read out Ic directly,

e.g., by increasing the bias current at fixed magnetic field until a nonzero DC voltage

develops across the junctions.

A much simpler method, however, can be used at least for DC SQUIDs with over-

damped junctions having a non-hysteretic current-voltage characteristic. Here, the SQUID is

(b

) (

c

)

(a)

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biased with a current slightly above the maximum value of Ic and the DC voltage V across the

junctions is read out directly as a function of external magnetic field or applied magnetic flux

Φa. When Ic is maximum, V is minimum and vice versa. Reading out in this way, the DC

SQUID thus directly acts as a flux-to-voltage transducer. To measure small changes δΦa in

applied flux (flux originating from the sample to be investigated) one generally chooses the

bias current to maximize the amplitude of the voltage modulation, and the applied flux to be

(2n+1) Φ0/4 (n = 0, 1, 2,...) so that the flux-to-voltage transfer coefficient, │(dV/dΦa)│I, is a

maximum, which we denote as VΦ. Thus, the SQUID produces a maximum output voltage

signal dV = VΦδΦa in response to a small applied flux signal δΦa. The SQUID is operated in

the flux locked loop mode (FLL) to achieve linearity in the output voltage.

2.7.3 MAGNETOTRANSPORT CHARACTERIZATION

Temperature dependence of resistivity (ρ) and magnetoresistance (MR) of all the films

has been measured using home made four-probe resistivity measurement setup equipped with

electromagnet capable of producing magnetic field up to 4 kOe. A schematic diagram of four-

probe resistivity measurement shown in Figure 2.19. In this technique, four contacts are made

on the sample to measure the electrical resistivity [214,215].

Figure 2.19: A schematic diagram of resistivity measurement setup using a four-point collinear probe on

film.

Figure 2.19, shows a schematic diagram of four-point collinear probe resistivity

measurement involves bringing four equally spaced probes in contact with the material of

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unknown resistance (such as conductive film). The probe array is placed in the center of the

material. A known current is passed through the two outside probes and the voltage is sensed

at the two inside probes. The resistivity of the films are calculated by using following

formula:

(2.4)

where = resistivity in ohm-cm, V = voltage in volts, I = current in ampere, w = width of the

film in cm, t = thickness of the film in cm, and l = distance between inner two probes in cm.

Our home made setup consist of doubled walled dewar placed between the poles of

electromagnet. The dewar is filled with liquid nitrogen and the sample holder, having four

contact point for resistivity and a calibrated temperature sensor (Pt-100), is suspended with

help of long non-magnetic steel hollow rod. The temperature can be varied manner between

liquid nitrogen temperature (77 K) and 400 K during the measurements. To measure the ‘ρ’ at

certain temperature, Keithley 220/224-constant current source, 181-nanovoltmeter and 195-A

digital multi-meter as temperature indicator were employed. A separated temperature

controlled heating arrangement was used to heat the sample. For contact conventional silver

paste and copper wire contact method of making the four contacts to the thin film instead of

pressure contact (generally used for bulk semiconductor). High magnetic field up to 70 kOe,

characterization of ‘ρ’ and ‘MR’ are carried out on physical property measurement system

(Quantum design, PPMS). PPMS was equipped with superconducting magnets and

temperature variation from 4.2 to 400 K.