characterisation of ge10se80te10 chalcogenide...

CHARACTERISATION OF Ge10Se80Te10

CHALCOGENIDE GLASS

SUBMITTED BY

DIVYA NARAYANAN

Master of Technology

INTERNATIONAL SCHOOL OF PHOTONICS,COCHIN UNIVERSITY OF

SCIENCE AND TECHNOLOGY

WORK CARRIED OUT AT

INTERNATIONAL SCHOOL OF PHOTONICS

COCHIN UNIVERSITY OF SCIENCE AND TECHNOLOGY

KOCHI –682 022

December 2014

COCHIN UNIVERSITY OF SCIENCE AND TECHNOLOGY

INTERNATIONAL SCHOOL OF PHOTONICS

KOCHI –682 022

CERTIFICATE

This is to certify that the Project report entitled “CHARACTERISATION OF

Ge10Se80Te10 CHALCOGENIDE GLASS” submitted by DIVYA NARAYANAN

is a bonafide record of the project done under my supervision.

Dr.Sheenu Thomas

Supervising guide

Associate Professor

International School of Photonics,CUSAT

ACKNOWLEDGEMENT

Firstly I thank Lord almighty for guiding me in every step on my way to the completion

of this project.

This project would not have been successfully materialized, had it not been for the

several people who have helped me. I am extremely indebted to Ms Ajina.C for her valuable

support throughout the work.

I am extremely grateful to Dr.M.KAILASNATH (Director ,Intetnational School of

Photonics,CUSAT) for his valuable support and guidance.

I also thank Dr.Sheenu Thomas, my project guide for her valuable guidance in choosing

the topic, progress of my work and preparation of project report.

I sincerely thank all my friends and classmates who in one way or the other have helped

me in this work.

I truly admire my family for their constant encouragement and enduring support which was

inevitable for the success of all my ventures.

Divya Narayanan

ABSTRACT

Here the chalcogenide glass Ge10Se80Te10 was synthesized. Thin film , nanocolloid and filim

from nanocolloid were prepared.. The nanocolloid was prepared by mixing the powdered sample

in solutions like butylamine, diethyl amine, ethanol amine and ethylyne diamine The film from

nanocolloid was prepared by spin coating. The properties were evaluated for the bulk , thin film

nanocolloid,spin coated film of Ge10Se80Te10. The studies include absorption, transmission,

nonlinear, photoinduced etc. An application of the sample was also evaluated ie Temperature

sensor.

Table of Contents

1) INTRODUCTION................................................................................................................. 1

1.1 Chalcogens ....................................................................................... 1

1.2 Chalcogenide Glass .......................................................................... 2

1.3 Recent Trends inChalcogenide Glass.............................................. 4

1.4 Properties of Chalcogenide Glass ..................................................... 8

1.4.1 Structural Property .......................................................................................... 8

1.4.2 Electrical Properties & Electronic Band Structutre .................................... 10

1.4.3 Thermal Properties ......................................................................................... 12

1.4.4 Optical Properties ........................................................................................... 13

1.5 Applications .................................................................................... 18

2) MATERIALS & METHODS ............................................................................................. 20

2.1 Preparation Techniques ................................................................. 20

2.2 GeSeTe ... ........................................................................................ 20

2.3 Experimental Tools& Techniques used for ChG ......................... 20

2.3.1 Structural Characterisation ........................................................................... 20

2.3.2 Optical Characterisation ................................................................................ 23

2.3.3 Photodarkening Experiment .......................................................................... 27

3) BULK GLASS ..................................................................................................................... 29

3.1 Preparation…… ............................................................................. 29

3.2 Studies on bulk sample .................................................................. 30

3.2.1 XRD ................................................................................................................ 30

3.2.2 Absorption ....................................................................................................... 31

3.3 Conclusion ................................................................................................... 33

4) THIN FILM ........................................................................................................................ 34

4.1 Preparation…… ............................................................................. 34

4.2 Studies on thin film ........................................................................ 36

4.2.1 EDAX ............................................................................................................... 36

4.2.2 Absorption ....................................................................................................... 37

4.2.3 Transmission ................................................................................................... 39

4.2.4 Photodarkening Experiment ..................................................................... 40

4.2.5 Nonlinear studies ......................................................................................... 50

4.3 Conclusion………………………………………………………54

5)NANOCOLLOID…………………………………………………………………………..55

5.1 Experiment on A………………………………………………………………55

5.1.1 Absorption…………………………………………………………………………..56

5.1.2 Nonlinear studies………………………………………………………………….57

5.2 Experiment on B………………………………………………………………58

5.2.1 Absorption…………………………………………………………………………..58

5.2.2 Nonlinear studies……………………………………………………………………59

5.3 Experiment on Cand D ……………………………………………………….55

5.4 Experiment on E………………………………………………………………60

5.4.1 Absorption…………………………………………………………………………..60

5.4.2 Nonlinear studies…………………………………………………………………..61

5.5 Experiment on F………………………………………………………………64

5.5.1 Absorption…………………………………………………………………………..64

5.6 Experiment on G………………………………………………………………65

5.6.1 Absorption…………………………………………………………………………..65

5.6.2 Nonlinear studies…………………………………………………………………..66

5.7 Conclusion…………………………………………………………………………….69

6 ) SPIN COATING………………………………………………………………………...70

6.1 Experiment on E………………………………………………………………71

6.1.1 Absorption…………………………………………………………………………..71

6.1.3 Transmission………………………………………………………………………...74


6.2 Experiment on F………………………………………………………………75

6.2.1 Absorption…………………………………………………………………………..75

6.2.3 Transmission………………………………………………………………………...78


6.3 Experiment on G………………………………………………………………81

6.3.1 Absorption…………………………………………………………………………..81

6.3.3 Transmission………………………………………………………………………...84


6.4 Conclusion……………………………………………………………………………..87

7) Temperature sensor using GeSeTe glass……………………………………………88

7.1 Absorption after 0 degree celsius……………………………………………………..88







8) CONCLUSION .................................................................................................................. 98

9) REFERENCES ................................................................................................................. 100

CHARACTERISATION OF Ge Se Te CHALCOGENIDE GLASS

International School of Photonics Page 1

INTRODUCTION

1.1 CHALCOGENS

The chalcogens are the chemical elements in group 16 of the periodic table. This group is

also known as oxygen family. The members of this group show increasing metal character as

the atomic number increases.

It consists of the elements oxygen (O), sulfur (S), selenium (Se), tellurium (Te), and

the radioactive element polonium (Po). The synthetic element livermorium (Lv) is predicted

to be a chalcogen as well. Often, oxygen is treated separately from the other chalcogens,

sometimes even excluded from the scope of the term "chalcogen" altogether, due to its very

different chemical behavior from sulfur, selenium, tellurium, and polonium. The word

"chalcogen" is derived from a combination of the Greek word khalkόs (χαλκός) principally

meaning copper (the term was also used for bronze/brass, any metal in the poetic

sense, ore or coin), and the Latinised Greek word genēs, meaning born or produced.

Sulfur has been known since antiquity, and oxygen was recognized as an element in the 18th

century. Selenium, tellurium and polonium were discovered in the 19th century, and

livermorium in 2000. All of the chalcogens have six valence electrons, leaving them two

electrons short of a full outer shell. Their most common oxidation states are −2, +2, +4, and

+6. They have relatively low atomic radii, especially the lighter ones.

Lighter chalcogens are typically nontoxic in their elemental form, and are often critical to

life, while the heavier chalcogens are typicallytoxic. All of the chalcogens have some role in

biological functions, either as a nutrient or a toxin. The lighter chalcogens, such as oxygen

and sulfur, are rarely toxic and usually helpful in their pure form. Selenium is an important

nutrient but is also commonly toxic. Tellurium often has unpleasant effects (although some

organisms can use it), and polonium is always extremely harmful, both in its chemical

toxicity and its radioactivity.

Sulfur has more than 20 allotropes, oxygen has nine, selenium has at least five, polonium has

two, and only one crystal structure of tellurium has so far been discovered. There are

http://en.wikipedia.org/wiki/Chemical_element

http://en.wikipedia.org/wiki/Group_(periodic_table)

http://en.wikipedia.org/wiki/Periodic_table

http://en.wikipedia.org/wiki/Oxygen

http://en.wikipedia.org/wiki/Sulfur

http://en.wikipedia.org/wiki/Selenium

http://en.wikipedia.org/wiki/Tellurium

http://en.wikipedia.org/wiki/Radioactive_decay

http://en.wikipedia.org/wiki/Polonium

http://en.wikipedia.org/wiki/Synthetic_element

http://en.wikipedia.org/wiki/Livermorium

http://en.wikipedia.org/wiki/Copper

http://en.wikipedia.org/wiki/Bronze

http://en.wikipedia.org/wiki/Brass

http://en.wikipedia.org/wiki/Ore

http://en.wikipedia.org/wiki/Coin

http://en.wikipedia.org/wiki/Valence_electron

http://en.wikipedia.org/wiki/Oxidation_state

http://en.wikipedia.org/wiki/Atomic_radius

http://en.wikipedia.org/wiki/Toxicity

http://en.wikipedia.org/wiki/Toxicity



numerous organic chalcogen compounds. Not counting oxygen, organic sulfur compounds

are generally the most common, followed by organic selenium compounds and organic

tellurium compounds. This trend also occurs with chalcogen pnictides and compounds

containing chalcogens and carbon group elements.

Oxygen is generally extracted from air and sulfur is extracted from oil and natural gas.

Selenium and tellurium are produced as byproducts of copper refining. Polonium and

livermorium are most available in particle accelerators. The primary use of elemental oxygen

is in steelmaking. Sulfur is mostly converted into sulfuric acid, which is heavily used in the

chemical industry. Selenium's most common application is glassmaking. Tellurium

compounds are mostly used in optical disks, electronic devices, and solar cells. Some of

polonium's applications are due to its radioactivity.

FIGURE 1.1- PERIODIC TABLE

1.2 CHALCOGENIDE GLASSES

Chalcogenide glasses (ChG) belong to an important class of amorphous solids which contain

at least one chalcogen element(sulphur, selenium and tellurium) as a major constituent.An

amorphous solid is defined as any solid that shows a short range order molecular structure or

medium range order but does not show any long range order. Glass, an amorphous solid is

defined in American Society for Testing and Materials as ‘an inorganic product of fusion

which has been cooled to a rigid condition without crystallization. Glass formation is possible

in a system of any composition provided that it contains sufficient ‘network

modifier’.Network modifiers produce three dimensional random network of strong bonds in a

system. Glass is an isotropic material, where as crystalline materials are generally

anisotropic. Like all glasses ChG‘s exhibit a glass transition temperature, a fact which

http://en.wikipedia.org/wiki/Pnictogen

http://en.wikipedia.org/wiki/Carbon_group

http://en.wikipedia.org/wiki/Sulfuric_acid



becomes very important for the processing of bulk glasses into thin film and fiber form as

required for most applications.Chalcogenide glasses have certain unique properties that make

them of interest compared to other materials for optoelectronic applications. Their good

infrared transparency (as shown in Figure1.2), high refractive index, photosensitivity,

amenability to doping and alloying, low phonon energy and compatibility with low

temperature processing make them smart materials for optical integration. Moreover,due to

their amorphous nature, chalcogenide glasses do not need to be grown on single crystalline

substrates and can potentially be incorporated within the interconnect levels of a CMOS chip.

This backend compatibility minimizes the need for dedicated processing facilities,which

reduces fabrication costs and leverages the existing and welldeveloped semiconductor

technologies. Many of the unique properties of ChG are a direct result of low phonon energy

associated with this material resulting from the heaviness of the chalcogen nuclei.ChG with

heavier nuclei and weaker bonding have lower vibrational frequency and thus lower phonon

energies.

FIGURE 1.2-Optical transmission of three families of chalcogenide glass

compared to silica and fluride glass

The historical development of ChG as optical materials in infrared systems began with the

rediscovery of arsenic trisulfide glass.Development of the glass as a practical optical material

was continued by W. A. Fraser and J. Jerger in 1953 at Servo Corporation. Jerger, Billian,

and Sherwood extended their investigation of arsenic glasses containing selenium and

tellurium,and later adding germanium as a third constituent. Research in this fascinating area



of chalcogenides glasses gained its momentum in with the discovery that ChG’s behave like

intrinsic semiconductors by Kolomiets. Later on Eaton, Ovshinsky and Pearson observed

their semiconductor properties and switching phenomenon. The discovery of switching and

memory effects in ChG was a turning point which attracted many researches to the world of

amorphous chalcogenide semiconductors.

The chalcogenide glasses share two common properties that have a profound impact on

their interaction with light; their electronic structure and their phonon vibrations.

Electronically, the chalcogenide glasses behave as semiconductors. They have a bandgap, and

consequently they are transparent to a certain range of wavelengths of light. The disorder of

the network creates localized electronic states that extend into the forbidden bandgap. These

states have a significant effect on the electrical and optical properties of the chalcogenide

glasses. The transparent window extends far into the infrared because the infrared side of the

transparent region is determined by the phonon energy of the material. The presence of large,

heavy atoms shifts the phonons to lower energy and therefore longer wavelengths. The low

phonon energy makes the chalcogenide glasses attractive as infrared optical materials.

1.3 RECENT TRENDS IN CHALCOGENIDE GLASSES

Device applications using ChG usually require the glass to be processed into either fiber or

thin film form. Conventionally,chalcogenide films are deposited using physical vapour

deposition (PVD) techniques such as thermal evaporation, pulsed laser deposition or

sputtering from the bulk melt quenched . These methods suffer from several shortcomings

which intermittently confine their use. They are in general limited to largely two dimensional

surfaces and require high-vacuum processing and sometimes difficult target preparation in

the case of laser deposition and sputtering. Another impediment, particularly for thermal

evaporation, is the observation that the resulting film often has a different composition

(stoichiometry) from that of the parent glass target, or is inhomogeneous in thickness, owing

to differential volatility in multi-component materials. A common solution to this problem is

solution-based coating methods. Solution-based coating methods offer a prospective pathway

to overcome these limitations by controlling the chemical composition of the solution phase

and hence the chemistry of the film with high accuracy. Solution casting method offers

higher production rate, simpler processing and opportunity to incorporate other materials like

metals, semiconductors, nano particles, polymers etc.



1.3.1 Chalcogenide glass as nanomaterial

Nano colloids of chalcogenide glass have gained a lot of interest in the research field

recently. Understanding the chemical stability of the glasses, and finding the suitable solvent

for making solutions is very important in nano colloid preparations. Laser ablation method

was employed for the preparation of aqueous As2S3 colloidal solution by R A Ganeev et al.

Their investigation of nonlinear-optical parameters on the prepared nano colloids showed the

non linear refractive index to be -7.5 *10 -18 m 2W -1 and nonlinear absorption coefficient to

be 1 cmGW1.Eventhough the samples showed good optical non linearity, the ageing occurs

in the solution due to spontaneous nanoparticles clusterization. So in order to avoid this

ageing, stabilizer have to be added.Another method for the preparation of chalcogenide nano

colloid was by reorganisation of dissolution of As2S3 in liquid ammonia by Berzelius and

Bineau. Then onwards a lot of structural and optical studies were carried out on chalcogenide

nano colloid prepared by dissolution in different solvents like ethylenediamine,

nbutylamine,n-propylamine, diethylamine, triethylamine and other organic solvents. Even

though attempts to understand the mechanism of dissolution of chalcogenides in amines and

the existence of chalcogenide clusters with dimensions of several nanometres were reported

earlier, factors such as the use of suitable solvent for bulk glass dissolution solubility and

solution viscosity have only been studied recently by Song et al. G.C. Chern and I.Lauks had

made important contribution in the area of nano colloid chalcogenide glass like As2S2,

As2Te3, As2Te3 and Ge-Se. The proposed dissolution product of As2S3 and Ge23Sb7S70 in

amine solvent is as given in Figure 1.3.

FIGURE 1.3-Structure of dissolution products

Thin films prepared from nano colloid Ge 23S 7S 70 ChG are reported to be of promising

optical properties. Shanshan Song et al. have shown that photo-responsive nano colloid ChG



can be used for tuning quantum cascade (QC) lasers. The photosensitivity of the cladded

chalcogenide layer is utilized for tuning of over 30 nm, by deep-etched distributed Bragg

gratings in cladding layer. ChG based QC lasers offers high power and room temperature

operation making them a promising choice for trace-gas detection in the mid-infrared where

the spectroscopic fingerprints of majority atmospheric trace gas are found. Recently in 2010

Chalcogenide opal and inverse opal photonic crystals were successfully fabricated from nano

colloid As 30S 70 chalcogenide glass by Tomas Kohoutek et al. The fabricated photonic

structures from nano colloid As 30S70 and silica as shown in Figure 1.4, are proposed for

designing novel flexible colloidal crystal laser devices, photonic waveguides and chemical

sensors.

FIGURE 1.4 –Photonic Structures

1.3.2 Chalcogenide/Polymer Composite Filim

Presently, great interest has been devoted to the fabrication of new materials suitable for

photonics applications. Among these, the amorphous chalcogenide structures are of

considerable interest due to their effectiveness in nonlinear optical characteristics. The

Quantum wave stacks (QWS) devices based on high refractive index chalcogenides and low

refractive index polymers seem to be promising for applications challenging favourable

performance ratio. There exist certain challenges associated with the use of chalcogenide

glasses in integrated optics like toxicity, durability, large coefficient of thermal expansion

(CTE) etc. This can be overcome by making composite films using chalcogenide glass and



polymer. As it was shown by many authors it is reasonable to combine the properties of

these two groups of materials by getting nano-composites from polymers and ChG for

optimization of the sensitive parameters, simplification of the technology of fabrication,

improving the stability of the registration media, solving problems related to ecological

outputs etc. Tomas Kohoutek recently fabricated Ge-Se/Polystyren (PS) dielectric and

Au/Ge-Se/PS /dielectric reflectors from amorphous chalcogenide and polymer films with the

optical reflectivity (R) higher than R > 99 % near λ ~1550 nm using low-temperature and

inexpensive deposition techniques. Thin polymer films anchored to ChG are widely used for

modulation of the surface properties and for fabrication of versatile adaptive surfaces capable

of responding to changes in the environment. Diffractive structures by holographic and e-

beam recording technologies were recently reported by

Andriesh et al using rare-earth-doped chalcogenide glasses and polymer nanocomposite.

1.3.3 Chalcogenide glass fiber

Optical fibers need low-phonon-energy materials which exhibit excellent resistance to

moisture corrosion and good glassforming ability. The only vitreous materials that

accomplish these requirements are glasses based on sulphur, selenium, or

tellurium.Chalcogenides fibers and ChG fibers doped with rare earths have been studied for

active applications in the near- and mid-IR. The low phonon energy of chalcogenide glasses

activates many mid-IR transitions for rare-earth ions that are usually absent in materials with

higher phonon energies. Arsenic trisulfide glasses suffer from poor rare-earth solubility and

shows signs of crystallization coinciding with the temperature for fiber-drawing. Ge–As–Ga–

Sb–S glass doped with neodymium chalcogenide fibers exhibited an optical amplification at

1.083 μm with a maximum internal gain of 6.8dB achieved for a pump power of 180 mW.

ChG fibers find application in all optical switches and fiber lasers. Holey fibers based on Ga-

La-S glasses chalcogenide glasses are recently demonstrated. In these structures, the holes

generate a low effective index in the cladding and permit light guiding in the solid core by

internal reflection at the core-clad interface. Review on chalcogenide holey fibers by J.Trolès

et al. says holey fibers possess the potential3for new applications in the fields of high

nonlinear optics and largemode-area propagation together with single-mode operation at all

wavelengths.



1.4 Properties of chalcogenide glasses

1.4.1 Structural property

The atomic structure and related properties in chalcogenide glasses depend upon preparation

methods and history after preparation. This prehistory dependence is common in all

nonequilibrium glass systems. Various experimental techniques like Xray,neutron, electron

diffractions, the anomalous x-ray scattering, the molecular vibrational (IR and Raman)

spectra, the Electron Spin Resonance (ESR), the Nuclear Magnetic Resonance (NMR) and

the Extended X-ray Absorption Fine Structure (EXAFS) are used to probe the microscopic

structures of ChG.Amorphous chalcogenide materials are reported to have a disordered

structure even though the disorder in the structure of an amorphous material is not complete

on the atomic scale. ChG lacks a long range periodic ordering of the constituent atoms.

Chemical ordering has a significant effect on the control of the atomic correlation in these

glassy solids. This is particularly important if one approaches from the nonstoichiometric to

the stoichiometric compositions. The chemical bonding between atoms, which result in the

short-range order, is responsible for most of the properties of amorphous materials. The

semiconducting property of chalcogenide glasses is, however, a direct consequence of the

covalent bonding existing in these materials. In chalcogenide glasses the covalent bonded

atoms are arranged in an open network with order extending up to the third or fourth nearest

neighbours. So chalcogenide glasses are also referred to as network glasses. Various

structural models5 have been developed for amorphous materials depending upon their

chemical nature. The Continuous Random Network (CRN) or Zachariasen model was

developed for directional bonding, present in covalent solids. Another model Random Close

Packing (RCP) or Bernal model was developed for non-directional bonding present in

metallic solids. The Random Coil (RC) [Flory model] deals with one dimensional bonding

present in polymers.The CRN model suggested for amorphous semiconductors is based on

certain features of these materials like directional nature of bondings, well-defined SRO (the

bond length and bond angle) and the topological constraints. The main drawback of the CRN

model is that it assumes that all constituent atoms satisfy their valence requirements (8-N),

where N is the valency. Hence, the structural defects such as dangling bonds, voids etc. are

not taken into account.The Random Close Packed (RCP) model is applicable for non-



directional bonding present in metallic solids providing a satisfactory model for the structure

of amorphous metals. Like the CRN model and RCP model, Random Coil model, is

essentially a homogeneous single-phase model. This model is widely applied to one-

dimensional bonding present in polymers.To understand the chemical ordering and structural

model consider the simple case of binary alloy Ax B100-x. The atoms A and B,say belongs to

the 'a' and 'b' groups of the periodic table, respectively.There are two models to count the

fraction of A-A, A-B and B-B bonds by assuming that all atoms satisfy the (8-N) rule. These

are the Random Covalent Network (RCN) model and the Chemically Ordered Network

(CON) model. The RCN considers these bond distributions as purely statistical and neglects

the relative bond energies i.e. in random. The fraction of these bonds depends upon their

coordination numbers (8-a), (8-b) and the concentration x.Therefore it gives A-A, A-B and B-

B bonds at all the concentrations except at x = 0 and 1. Where as the CON model considers

thepreferential ordering i.e. the heteropolar A-B bonds are more preferred entities. Thus, this

model predicts a chemically ordered phase at critical composition Xc = Ya/(Ya + Yb) (the

coordination of the A and B atoms are Ya = 8 - a and Yb = 8 - b respectively). The structural

controversy is less for selenium.

1.4.2 Electrical properties and electronic band structure

Structural defects play an important role in electrical properties than the role of impurities in

ChG. The band of the states existing near the centre of the gap arise from specific defect

characteristics of the material like dangling bonds, interstitals etc.Thus the band structure of

the ChG specifically defines its property.Density of states (DOS) diagram is used to explain

or predict the properties of a material in the band theory. It denotes the number of electron

states per unit energy per electron a material will have at an energy level and it is used

successfully to describe many of the characteristic found in a crystalline solids. Band theory

for amorphous materials was first explained by Mott by extending the band theory of

crystalline materials. Mott suggested that the spatial fluctuations in the potential caused by

the configurational disorder in amorphous materials could lead to the formation of localized

states based on Anderson's localization principle. The diagram of the DOS for crystalline and

the modification of it for amorphous materials by Mott is given in figure.



(a) (b)

FIGURE 1.5:-(a)-DOS of crystalline semiconductor (b)DOS models proposed by MOTT

for amorphous semiconductor

These localized states do not occupy all the energy states in the band but it form a tail above

and below the band. An electron in a localized state will not diffuse at zero temperature to

other allowed states with corresponding potential fluctuations. Several models have been

proposed for the band structure of amorphous semiconductors using the concept of localised

states in band tails. The first diagrammatic representation of the band structure of amorphous

semiconductor was given by Cohen and is referred to as the Cohen-Fritzsche-Ovshinsky

(CFO) model. This model suggests that the tail states extend across the gap in a structure-less

distribution. The bandpicture of CFO model is shown in Figure 1.6. The relevant features of

this model are 1) continuous tailing of the localized states, which is so high that they overlap

in the middle of the gap. Thus some of the normally filled valence band tails would be at a

higher energy than then normally empty conduction tails. Obviously, a redistribution of

charge carriers must take place.Thus electrons transfer takes place from the high lying

valence band tails to the low-lying conduction band tails. Hence, there is a deep electron trap

above and below the E F which is shown in Figure 1.6 a. This model predicts the existence of

the average mobility gap between the valence and conduction bands. There is a finite density

of these states g (E F) at EF. If g(E F)> g (Ec), it would make materials to be metallic

otherwise they would remain as semiconductors. The existence of high density of g (E F)

turns materials into undopable ones.The band picture of model proposed by Marshall and

Owen is given in Figure 1.6.b. The main difference between the CFO and the David Mott

(DM) model is the origin of deep traps in the middle of the gap.There is also another model

proposed by Emin called Small polaron model. Emin suggested that the extra free carriers in



some amorphous semiconductor may enter as a self trapped (small polaron) state as a result

of the surrounding atomic lattice. All of the chalcogenide glasses appear to share a common

electronic band structure. The chalcogen atoms all have six valence electrons in an

s2p4configuration. The two half-filled p shells participate in the formation of covalent bonds,

so the chalcogen atoms are normally two fold coordinated. The valence band of chalcogenide

glasses consists of states from the p bonding (σ) and loan pair ( LP) orbitals.The LP electrons

have higher energy than the bonding electrons, so the full LP band forms the top of the

valence band. The conduction band is formed by the antibonding (σ*) band. The LP band

falls between the σ and σ* bands, so the bandgap is about half of the bonding-antibonding

splitting energy. Because the electrical properties are determined by the LP band, these

chalcogenide glasses are called lone-pair semiconductors.

FIGURE 1.6: (a)-DOS Models by CFO (b)-DOS model by Marshell and Owen

1.4.3 Thermal properties

Glass transition, crystallisation and melting temperatures,along with the coefficient of

thermal expansion, thermal diffusivity etc are the thermal properties associated with

chalcogenide glass. The glass transition temperature of ChG is related to the

magnitude of cohesive forces within the network and these forces must be overcomed

to allow atom movement. Thermal conductivity is critical to many electronic devices.

Thermal conductivity, of a material results from transport of energy via electrons or

via lattice vibrations(phonons). The total thermal conductivity is the sum of both.

Thermal conductivity is related to phonon mean free path. Phonon mean free path in

ChG is considerably shorter and correspondingly thermal conductivity is less.

Thermal diffusivity (TD) has a major role in switching exhibited by a chalcogenide



glass. TD decides the rate at which heat can be dissipated away from the conducting

channel. It has been recently pointed out that there is a strong correlation between the

thermal diffusivity and the switching behaviour of chalcogenide glasses. ChG with

low thermal diffusivity are likely to exhibit memory behaviour and those with higher

values of thermal diffusivity may show threshold-type switching. Consequently, the

measurement of thermal diffusivity of switching glasses is important for identifying

suitable materials for phase change memory applications. Like optical absorption

coefficient, thermal diffusivity is unique for each material and is often ideal over

conductivity measurements due to its insensitivity to radiative heat losses as the latter

involves heat fluxes that are difficult to control.

1.4.4 Optical properties

Chalcogenide glasses are promising candidates for photonic applications due to their

attractive optical properties, such as high refractive index, high photosensitivity and large

optical nonlinearity.The investigation of the optical properties of chalcogenide glasses is of

considerable interest and affords critical information about the electronic band structure,

optical transitions and relaxation mechanisms. The optical and electrical properties of

chalcogenide glasses are generally much less sensitive to non stoichiometry and the presence

of impurities is less sensitive than crystalline semiconductors.

1.4.4.1Absorption spectroscopy

The typical absorption spectra of chalcogenide glass is shown in Figure 1.9. In amorphous

semiconductors, the optical absorption edge spectra generally contain three distinct

region2,15: A)High absorption region (α=104 cm-1) ,which involves the optical transition

between valence band and conduction band and determines the optical bandgap, B) Spectral

region with α=10 2-10 4 cm is called Urbach’s exponential tail (In this region most of the

optical transitions take place between localized tail states and extended band states) andC)

The region α ≥ 10 2 cm -1 involves low energy absorption and originates from defects and

impurities.



FIGURE 1.8-ABSORPTION CURVE

1.4.4.3 Photoinduced properties

The photoinduced phenomena exhibited in ChG can be grouped into three categories: the

photon mode, in which the photoelectronic excitation directly induces atomic structural

changes;the photothermal mode, in which photoelectronic excitation induces some structural

changes with the aid of thermal activation; and the heat mode, in which the temperature rise

induced by optical absorption is essential. Interestingly, these three kinds of phenomena are

likely to appear in sulfides, selenides, and tellurides,respectively. The photon effects are of

particular interest from the viewpoint of fundamental science and modern applications.One of

the interesting properties of the chalcogenide materials is their sensitivity to the action of

light and other electromagnetic radiations.Therefore, many effects discovered in

chalcogenide disordered materials are3based on the action of light. Some of the important

photoinduced effects are photo-darkening, photo-bleaching, photo-plastic effect, photo-

induced fluidity, photo-induced ductility, optomechanical effect, polarization dependent

photoplastic, light-stimulated interdiffusion effect, photoexpansion,photocontraction,

athermal photo-induced transformation effect,photo-induced amorphisation effect, laser-

induced suppression of photocrystallization, photoinduced softening and hardening effect,

photoamplified oxidation effect, photo-dissolution, photo-doping effects,

photopolymerization effect, photo-anisotropy effect, photo-induced dichroism,photoinduced



scattering of light, photo-elastic birefringence effect,switching(Ovshinsky) effect,

photoluminescence etc. Many types of photosensitive processes observed in the chalcogenide

glasses are accompanied by changes in the optical constants, i.e., changes in the electronic

band gap, refractive index and optical absorption coefficient. These light-induced changes are

favoured in chalcogenide glasses due to their structural flexibility (low coordination of

chalcogens) and also due to their high-lying lone-pair p states in their valence bands.

Annealing chalcogenide glasses can affect the photoinduced changes, in particular

irreversible effects occur in as-deposited films, while reversible effects occur in well-

annealed films as well as bulk glasses. Changes in local atomic structure are observed on

illumination with light having photon energy near the optical band gap of the chalcogenide.

(a) PHOTODARKENING

In chalcogenide glasses the photodarkening (PD) process refers to a shift of the optical

absorption edge to lower energies upon the application of light whose energy is near that of

the band gap. All chalcogenide glasses appear to exhibit the PD process to varying degrees.

The role of specific defects in the photodarkening process has yet to be established because a

microscopic description of this effect does not exist yet.

(b) PHOTOLUMNISENCE

The defect states in the band-gap of ChG are expected to play an important role in the

occurrence of most of the photo-induced phenomena, since the defects are considered to alter

their charge conditions or their mutual interactions by trapping photo-excited carriers. To

investigate these gap-states, photoluminescence (PL) measurement is an effective tool since

their spectra provide detailed information on the relaxation process of photo-excited

electron–hole pairs.



1.4.4.4 OPTICAL NONLINEARITY

Different techniques, such as two-photon absorption spectroscopy, degenerate four wave

mixing (DFWM), Z-scan, thirdharmonic generation (THG) and optical Kerr shutter (OKS)

have been used to measure the non-linear refractive index as well as the nonlinear absorption

coefficient of chalcogenide materials. Spectral dependence of different nonlinear parameters

like two-photon absorption (β) and intensity-dependent refractive index (n2) on linear

absorption and refractive index for an ideal amorphous semiconductor is shown in Figure 1.9

refractive index n 0, two-photon absorption β, and intensity-dependent refractive index n2 in

an ideal amorphous semiconductor with energy gap Eg. J.S. Sanghera et al. have compared

the nonlinear optical properties of these chalcogenide glasses in both bulk and fiber form.The

first investigations of the non-linear absorption of nanosecond laser pulses with hν <Eg in

ChG were reported by Lisitsa et al.. The dynamics of such induced absorption with

subpicosecond and picosecond time resolution have been investigated by Fork, Shank

etal.and by Ackley, Tauc et al . These authors showed that as a result of strong excitation of

ChG with excitation energy less than bandgap,an additional induced absorption appears,

which exhibits maximum amplitude during the excitation pulse and relaxes with several time

constants. This kind of photo-induced absorption (when hν is far from the absorption edge

Eg) appears only at strong laser excitation of ChG. The mechanisms of two-photon (or two-

step) absorption and of the carrier localization and redistribution on states in the gap were

proposed to explain the photo-induced absorption in ChG. In telecommunications based

applications, chalcogenide glasses stand out because they exhibit third-order optical

nonlinearities (Kerr,Raman and Brillouin) between two to three orders of magnitude greater

than silica.Microscopically, the nonlinear terms arise through several mechanisms such as

electronic, atomic (including molecular motions), electrostatic, and thermal processes.

Among these, the electronic process can provide the fastest response with nano second and

lower time scales, which will be needed for optical information technologies. The nonlinear

absorptions usually exhibited in amorphous semiconductors are shown in Figure 1.10.

Nonlinear absorption refers to the change in transmittance of a material as a function of

intensity or fluence.



FIGURE 1.9-SPECTRAL DEPENDENCE

(a) (b) (c) (d)

FIGURE 1.10-(a)-One photon absorption (b) Two photo absorption (c) Two step

absorption via midgap state (c)Raman scattering process in a semiconductor

Many reports indicate that a two-photon absorption process is responsible for the optical non-

linearities observed in chalcogenide glasses. Studying on the spectral dependence of

absorption using a tunable source, showed that two-photon and two-step absorption occurs in

As2S3, and the two-photon absorption spectrum appeared to vary exponentially with energy.

This exponential form implies that the two photon process is resonantly enhanced by the gap

states which cause the weak absorption tail found in this glass.



1.5 APPLICATIONS

Much more must be learned about the structure-property relationships in chalcogenide

glasses before we know the true potential for these glasses. Chalcogenide glasses have been

studied as promising integrated optics materials since early 1970. The IR transparency of

chalcogenide glasses (ChGs) leads to a wide variety5 of optical applications as shown in

Figure 1.11. Chalcogenide glasses have the potential to be the basis for future optical

computers, much as silicon is the basis for today’s microprocessors and computer memories.

Any optical microcircuit will require passive devices such as waveguides and gratings to

control the flow of optical information between the active elements. These elements can be

fabricated in a chalcogenide glass by several methods including photodarkening and

photodoping. Grating can be recorded on chalcogenide bulk and thin films. The

photosensitive response of the chalcogenide glasses can be used to produce high-resolution

images and photolithographic resists16. High-speed optical switching has been demonstrated

with chalcogenide glasses. Demultiplexing signals of 50 Gbit/s was achieved and the system

has the potential to exceed 100 Gbit/s operations. Infrared fibers based on ChG are of great

technological importance for communication, imaging, remote sensing and laser power

delivery. ChG fibers find application in the various fields due to their high

bandgap,longwavelength multiphonon edge and low optical attenuation. They are also

chemically stable in air and can be drawn into long core-clad fibers.They also have the

potential to permit new applications that are unachievable with current infrared materials.

The ChG find application in different fields as shown in Figure 1.11.



FIGURE 1.11-APPLICATION OF CHALCOGENIDE GLASSES



2.MATERIALS AND METHODS

2.1 PREPARATION TECHNIQUES

The methods used for preparation of bulk glass,thin filim and nanocolloid is discussed in

detail in preceeding chapters.

2.2 GeSeTe

The Ge-Se-Te system forms a stable glass. The properties of this glass shows that it is

similar to many other chalcogenide glasses with respect to infrared transmission and

semiconducting electrical behavior. The composition GeSeTe does not lie close to other well

investigated regions of glass-forming composition, and so a more thorough study of GeSeTe

glass was made. Here Ge10Se80Te10 composition is studied.

2.3 EXPERIMENTAL TOOLS AND TECHNIQUES USED FOR

CHARACTERISATION OF ChG GLASSES

In this section the structural, thermal and optical characterization and the tools used for

characterizing chalcogenide based materials are included.

2.3.1 STRUCTURAL CHARACTERISATION

The structural characterization of the investigated samples of chalcogenide glass was done

using X -Ray diffraction technique,Scanning electron microscopy.

(a) X-Ray DIFFRACTION( XRD)

Macroscopically, the distinction between crystalline solids and non crystalline solids can be

made just by observation. The crystals have definite shapes which reflect the regular atomic

arrangement in the atomic scales for example the cubic faces of common salt and glasses on

the other hand have curved surfaces. Microscopically, the distinction can be made using X-

ray diffraction (XRD). It is a rapid analytical technique primarily used for phase



identification of a crystalline material and can provide information on unit cell dimensions.

The analyzed material is finely ground, homogenized,and average bulk composition is

determined. Hence it is also called the powder diffraction method. Max von Laue, in 1912,

discovered that crystalline substances act as three-dimensional diffraction gratings for X-ray

wavelengths similar to the spacing of planes in a crystal lattice. X-ray diffraction is now a

common technique for the study of crystal structures and atomic spacing. X-ray diffraction is

based on constructive interference of monochromatic X-rays and a crystalline sample. The

interaction of the incident rays with the sample produces constructive interference (and a

diffracted ray) when conditions satisfy Bragg's Law (nλ=2d sin θ). This law relates the

wavelength of electromagnetic radiation to the diffraction angle and the lattice spacing in a

crystalline sample. These diffracted X-rays are then detected, processed and counted. By

scanning the sample through a range of 2θ angles, all possible diffraction directions of the

lattice should be attained due to the random orientation of the powdered material. Conversion

of the diffraction peaks to d-spacings allows identification of the mineral because each

mineral has a set of unique d-spacings. If a material does not show this diffraction peaks it

proves that the material is not a crystal and must be non crystalline.We have used the Bruker

AXS D8 Advance diffractometer whose source of X rays is Cu, Wavelength 1.5406 A°.

(b) Scanning Electron Microscopy(SEM)

Surface imaging of the chalcogenide nano clusters in the spin coated thin films are studied

using Scanning electron microscope(JEOL Model JSM - 6390LV) equipped with EDS (JEOL

Model JED –2300) for the qualitative elemental analysis.



2.3.2 OPTICAL CHARACTERISATION

(a) Absorption,reflection,transmission measurement

JASCO V-570 UV/VIS/NIR Spectrophotometer was used for the absorption, transmission

and reflectance measurements of the samples. The spectrometer consist of Optical system33:

single monochromatic, UV/ VIS region 1200 lines/ mm plane grating, NIR region: 300 lines/

nm plane grating, Czenry –Turner mount double beam type Resolution: 0.1 nm (UV/ VIS

region) 0.5 nm (NIR region).Wavelength range: 200 nm to 2200 nm. The beam from the light

source is converged and enters the monochromator. It is dispersed by the grating in the

monochromator and the light passes out through the exit slit. This light is split into two light

paths by a sector mirror,one incident on the sample to be measured and the other on the

reference sample such as solvent. The light that has passed through the sample or reference

sample is incident on the photomultiplier tube and PbS photoconductive cell which are the

detectors. In the reflectance measurement the set up has to be changed.The Model SLM-468

single reflection attachment is designed to measure the relative reflectance of sample using

the forward reflected light from the aluminum-deposited plane mirror as reference. It permits

the measurement of the reflectance of metal deposited film,metal plating etc. The wavelength

range is 220 nm to 2200 nm with a beam port diameter of 7 mm and angle of incidence

approximately 5º.

(b) Optical absorption spectroscopy of amorphous semiconductors

Absorption spectroscopy of the materials investigated in this thesis was studied using Jasco

V570 spectrophotometer. The typical absorption spectrum of chalcogenide glass is shown in

Figure 2.5. In amorphous semiconductors, the optical absorption edge spectra generally

contain three distinct region:

(A) High absorption region (α=104 cm-1), which involves the optical transition between

valence band and conduction band6which determines the optical bandgap. The

absorption coefficient in this region is given by

α hυ=B(hυ-Eg)p ( 2.1)



where Eg is the optical bandgap and B is a constant related to band tailing parameter. In the

above equation, p=1/2 for a direct allowed transition=3/2 for a direct forbidden transition,

p=2 for an indirect allowed transition and p=3 for an indirect forbidden transition.

FIGURE 2.1-ABSORPTION CURVE OF CHALCOGENIDE GLASSES

(B) Spectral region with α=102-104 cm-1 is called Urbach’s exponential tail region in

which absorption depends exponentially on photon energy 36and is given by

α hυ= αo exp(hυ/Ee) ( 2.2)

where αo is a constant and Ee is interpreted as band tailing width of localized states, which

generally represents the degree of disorder in amorphous semiconductors. In this region most

of the optical transitions take place between localized tail states and extended band states.

(C) The region with α ≥ 102 cm-1 involves low energy absorption and originates from defects

and impurities.



(a)Analysis of Transmission Spectra of thin films using Swanepoels method

Basically, the amount of light that gets transmitted through a thin film material depends on

the amount of reflection and absorption that takes place along the light path. The transmission

spectrum which depends on the material will have two distinctive features, it will either have

interference fringes or it will not. The schematic representation of behaviour of light passing

through the material is shown in Figure 2.2.

FIGURE 2.2-Schematic diagram of light passing through thin film

The optical constants can be measured by examining the transmission through a thin film

deposited on a transparent glass or other (e.g. sapphire) substrate. Figure 2.3 shows a

spectrum taken from a thin film on glass substrate. Swanepoel has critically reviewed how a

single transmission spectrum as shown in figure can be used to extract the optical constants

of a thin film.



FIGURE 2.3- Transmission spectra showing interference fringes

The refractive index of the thin film with uniform thickness can be calculated from the two

envelopes, TM (λ) and Tm(λ), by considering the extremes of the interference fringes.

Maxima T M= Ax/(B-Cx+Dx 2) (2.3)

Minima T m= Ax/(B+Cx+Dx 2) (2.4)

Subtracting the reciprocal of above first equation 2.3 from second equation 2.4 yields an

expression that is independent of the absorbance, x

(1/T m)-(1/T M)=2C/A (2.5)

Where A =16n2s (2.6)

C=2(n2-1)(n2-s) (2.7)

Rearranging for n



In this equation, s is the refractive index of glass substrate and its values are obtained from

transmission spectra of substrate Ts,using the relation.

In the region of weak and medium absorption, where α≠0,transmittance decreases mainly due

to the effect of absorption coefficient, α and Eq.(2.9) modifies to

where TM and Tm are the transmission maximum and corresponding minimum at a certain

wavelength .

If n1 and n2 are refractive indices of two adjacent maxima or two adjacent minima at

wavelengths λ1 and λ2, respectively, then the thickness d1 of the film is given by



2.3.3 PHOTODARKENING EXPERIMENT

Photoinduced darkening experiments were done on the thin films using the experimental set

up as shown in Figure 2.4. Photoinduced studies were carried out using above band gap and

near band gap laser sources. We have used 30mW DPSS(532nm) to study the

photosensitivity of the unannealed films. The laser power was made stable during exposure to

avoid significant uncertainty in the total supplied energy. The laser beam was expanded using

a plano-concave lens and collimated with a second plano-convex lens. The absorbance

spectra of the film at normal incident condition in the spectral range 200–2200 nm were

recorded by a double beam UV–VIS–NIR spectrophotometer (Jasco V570) after and before

exposure. All the measurements were executed at room temperature and the samples were

kept in the dark between experiments.

FIGURE 2.4-Photodarkening experiment

2.3.4 Z-SCAN for analyzing nonlinear properties of the sample

Z-scan technique introduced by Sheik Bahae is a single beam method for measuring the sign

and magnitude of nonlinear refractive index that has a sensitivity compared to interferometric

methods. It provides direct measurement of nonlinear absorption coefficient.Previous

measurements of nonlinear refraction have used a variety of techniques including nonlinear

interferometry, degenerate four wave mixing, nearly degenerate three wave mixing, ellipse

rotation and beam distortion measurements. The first three methods namely nonlinear

interferometry and wave mixing are potentially sensitive techniques, but all require complex

experimental apparatus. The propagation of laser beam inside such a material and the ensuing

self refraction can be studied using the z-scan technique. Thus it enables one to determine the



third order nonlinear properties of solids, ordinary liquids, and liquid crystals. The

experimental set up for single beam z-scan technique is given in Figure 2.5. In the ordinary

single beam configuration, the transmittance of the sample is measured, as the sample is

moved along the direction of the focussed guassian beam. A laser beam propagating through

a nonlinear medium will experience both amplitude and phase variation. If transmitted light is

measured through an aperture placed in the far field with respect to focal region, the

technique is called closed aperture z-scan. In this case, the transmitted light is sensitive to

both nonlinear absorption and nonlinear refraction. In a closed aperture z-scan experiment,

phase distortion suffered by the beam while propagating through the nonlinear medium is

converted into corresponding amplitude variations. On the other hand, if transmitted light is

measured without an aperture, the mode of measurement is referred to as open aperture z-

scan. In this case, the output is sensitive only to nonlinear absorption. Closed and open

aperture z-scan graphs are always normalized to linear transmittancei.e., transmittance at

large values of |z|.Closed and open aperture z-scan methods yield the real part and imaginary

part of nonlinear susceptibility χ(3) respectively. Usually closed aperture z-scan data is

divided by open aperture data to cancel the effect of nonlinear absorption contained in the

closed aperture measurements. The new graph, called divided z-scan,contains information on

nonlinear refraction alone.

FIG 2.5-ZCAN TECHNIQUE

An important requirement in the z-scan measurement is that,it is assumed that the sample

thickness is much less than Rayleigh’s range z0 (diffraction length of the beam [z0=k ω0/2,

where k is the wave vector and ω0 is the beam waist radius. The beam waist radius ω0 is

given by ω0=fλ/D, where f is the focal length of the lens used, λ is the wavelength of the



source and D is the beam radius at the lens. This is essential to ensure that the beam profile

does not vary appreciably inside the sample because z-scan technique is highly sensitive to

the profile of the beam and also to the thickness of the sample. Any deviation from gaussian

profile of the beam and also from thin sample approximation will give rise to erroneous

results.

2.3.4.1 Open aperture ZScan

Non linear absorption of a sample is manifested in the open aperture z- Scan measurement. If

the sample is having nonlinear absorption such as two photon absorption (TPA), it is

manifested in the measurement as a transmission minimum at the focal point.Otherwise if the

sample is a saturable absorber, the transmission increases with increase in incident intensity

and results in transmission maximum at the focal region. In the case of an open aperture z-

scan, the aperture as shown in Figure 2.5 is absent. In the absence of an aperture the

transmitted light measured by the detector is sensitive only to intensity variations.

Hence,phase variations of the beam are not taken into consideration

2.3.4.2 Closed aperture ZScan

The basis of closed aperture z-scan is the self refraction and self phase modulation effects.

The technique relies on the transmittance measurement of a nonlinear medium through a

finite aperture in the far field as a function of the sample position z with respect to the focal

plane using a single gaussian beam in a tight focus geometry.Consider, for instance, a

material with a negative nonlinear refraction and thickness smaller than the diffraction length

of the focused beam being positioned at various points along the z-axis. This assumption

implies that the sample acts as a thin lens of variable focal length due to the change in

refractive index at each position ( n = n0 + n2 I ).



3.BULK GLASS

3.1 PREPARATION

Bulk glasses were prepared using melt quenching method. Melt quenching technique was the

only method used for the preparation of bulk glasses before the development of chemical

vapour deposition and sol gel technique. One of the important features of the melt quenching

technique is the high flexibility of geometry and composition and the advantage of obtaining

materials of large size in comparison to other methods. The doping or codoping of active ions

or transition metals are quiet easy using this method. This method can be used for the

preparation of silicate, borate,phosphate, oxide or non oxide systems. One of the main

disadvantages of this method is the lack of ultra high purity as compared to other chemical

methods. In order to avoid contamination, the crucibles made of noble metals can be used.

Melt quenching method applied for chalcogenide glass preparation is as follows. This method

is based on the fusion of raw materials in to a viscous solid, followed by forming in to a

shape and quenching to a glass. The electronic grade (5N purity) constituent elements are

weighed in proportion to their atomic weight percentages. The raw materials used in the

present study are Ge, Se,Te. For each composition approximately around 4gm of material is

transferred to clean quartz ampoules of 8 mm diameter and 8cm length. The ampoule is then

evacuated at a pressure of 10-3 m bar for half an hour and then flame sealed at this vacuum

using oxygen -indane flame torch. Precleaning and evacuating helps to avoid the presence of

impurities. The ampoule is then placed in a rocking and rotating furnace as shown in Figure

3.1.



FIGURE 3.1-Rocking and rotating furnace

The furnace can attain a maximum temperature of 1100ºC. The temperature controller in this

unit is C961 Blind Temperature Controller with single thermocouple or RTD input and one

relay output with user specific control action and relay logic. Before keeping the ampoule, the

furnace can be programmed to the desired temperature. The samples presented in this thesis

are prepared at a temperature of 1050ºC. In order to homogenize the melt continuous rotation

and rocking in an interval of 1hour is given. The melt is then rapidly quenched to ice cold

water. The samples are then taken out from the sealed ampoules by dipping it in Hydrofluoric

acid (HF) solution. HF solution etches the quartz ampoule leaving the bulk glass.

3.2 STUDIES ON BULK Ge 10Se 80Te 10

3.2.2 XRD

XRD is explained in the previous section.



FIGURE 3.1-XRD of Ge10Se80Te10

The absence of sharp peaks prove that the prepared sample is amorphous in nature.

3.2.2 Absorption

Absorption spectroscopy is studied using Jasco V570 spectrophotometer.The specifications

of which are given in previous chapter.The absorbance corresponding to wavelength from

200-2200nm is taken.

Ge Se Te SAIF COCHIN BRUKER D8 Cochin Uni

Operations: Smooth 0.150 | Background 0.214,1.000 | Import

File: SAIFXR140821A-01(GeSeTe).raw - Step: 0.020 ° - Step time: 65.6 s - WL1: 1.5406 - kA2 Ratio: 0.5 - Generator kV: 40 kV - Generator mA: 35 mA - Type: 2Th/Th locked

Lin

(C

ounts

)

0

1000

2000

3000

4000

5000

6000

7000

8000

2-Theta - Scale

3 10 20 30 40 50 60 70 80



200 400 600 800 1000 1200

-2

-1

0

1

2

3

4A

bso

rba

nce

Wavelength(nm)

B

FIGURE 3.2 –ABSORPTION SPECTRUM OF BULK Ge 10 Se 80Te 10

This graph can be modified to get bandgap of the glass. The energy hv vs ( αhv)1/2 graph is

extrapolated to X-axis and point is noted from which the bandgap of the bulk material is

calculated.



1 2 3

1.5

2.0

2.5

3.0

3.5

4.0(

ah

v)^

1/2

( cm

-1e

V)

Energy(eV)

BANDGAP GRAPH

FIGURE 3.3-Bandgap graph

The bandgap is found to be 1.22 eV by this method.

3.3 Conclusion

1) X-ray diffraction studies conducted on the bulk sample show no prominent peaks

which reveals the amorphous nature of the samples .

2) Optical bandgap determined by UV-Vis-NIR spectroscopy is found to be 1.22eV



4.THIN FILM

4.1 PREPARATION

Thin film preparation can be based on physical deposition or chemical deposition.

Depositions that happen because of a chemical reaction are Chemical Vapour Deposition

(CVD), Electrodeposition, Epitaxy, Thermal oxidation etc and depositions that happen

because of a physical reaction are Physical Vapour Deposition, Evaporation, Sputtering and

Casting. The vapour deposition methods such as thermal evaporation, sputtering and

chemical vapour deposition methods can yield amorphous thin films deposited on a substrate.

Here thermal evaporation technique is used.

4.1.1 Thermal Vapourisation

It is perhaps the simplest vapour deposition technique which involves resistive or electron

beam heating in vacuum of a reservoir containing the material to be evaporated. The melt so

produced then evaporates and the vapour is condensed on to a substrate, forming a thin film.

In chalcogenide glass the deposition of the film at the oblique incidence may result to

structural inhomogeneity which may lead to formation of columnar growth morphology for

the films.The making of an amorphous chalcogenide thin film by thermal evaporation in

vacuum coating unit is done in the following way. The unit used for coating is India High

Vacuum pumps (12A4-D). Firstly, bulk sample is weighed and 0.39g loaded in tungsten boat

in the system as shown in Figure 4.1. After this the bell jar is closed and the system pumped

down to around 2*10-5 torr through a diffusion pump. At this level of air pressure, the entire

environment inside the deposition chamber is free of impurities and the sample is ready for

deposition.The chamber is evacuated by INDVAC Diffpak pump Model 114D abd backed by

250 liters per minute, doublestage, direct driven,Rotary vacuum pump, ModelIVP.



FIGURE 4.1-BELL JAR IN THERMAL EVAPORATION UNIT

With the shutter in the closed position, the temperature of the substrate is set to the desired

level heated till the sample loaded starts to evaporate. The heating element in the system is

conal sheated nichrome having a power rating of 500 watts, 120 V to 140V. Once the

evaporation rate is stabilized and the substrate reaches its desired temperature, the vapour is

allowed to come into contact with the substrate. The rate of evaporation is maintained to be

10A0/s. The evaporation rate as well as the film thickness can be controlled using a quartz

crystal in Digital thickness Monitor Model-CTM-200 attached to the bell jar. When the

desired thickness is reached, theshutter is closed. The amorphous film is maintained at the

substrate temperature until the boat and the chamber is cooled down to a level suitable for the

film to be removed from the system.

FIGURE 4.2-Ge10Se80Te10



4.2 STUDIES ON Ge 10Se 80Te 10

4.2.1 EDAX

To confirm the composition of the elements in the sample EDAX spectra have been taken.

Table 4.1 compares the atomic percentage of components obtained from EDAX analysis with

respect to the nominal composition. Figure 4.3 shows EDAX spectra of Ge10Se80Te10 thin

film.

Element Mass% Atom%

Ge 3.81 4.36

Se 81.98 86.37

Te 14.21 9.27

Table 4.1-EDAX of the thin film



The EDAX image is

FIGURE 4.3-EDAX of thin film

4.2.2 ABSORPTION SPECTRUM

Absorption spectroscopy is studied using Jasco V570 spectrophotometer. The specifications

of which are given in previous chapter. The absorbance corresponding to wavelength from

200-2200nm is taken.



0 500 1000 1500 2000 2500

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Ab

so

rba

nce

(a.u

)

Wavelength(nm)

Absorbance spectrum

FIGURE 4.4-ABSORBANCE SPECTRUM OF THIN FILM Ge 10 Se 80Te10

This graph can be modified to get bandgap of the glass. The wavelength is known so we plot

the energy hv in xaxis and ( αhv)^1/2 in yaxis. The graph is extrapolated to xaxis and point is

noted,this is the bandgap of the bulk material.

1 2 3

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

( a

hv)^

1/2

(cm

-1e

V)

Energy(eV)

FIGURE 4.5- ( αhv)^1/2 Vs energy graph

The bandgap is obtained from the graph is 1.478 eV.



4.2.3 TRANSMISSION SPECTRUM

The transmission spectrum of the sample fabricated obtained from a spectrometer is as shown

in Figure 4.4. The plot shows fringes due to interference at various wavelengths. A

continuously oscillating maxima and minima at different wavelengths confirm the optical

homogeneity of deposited thin films. Optical transmission is very complex functions and

strongly depend on the absorption coefficient. Various optical parameters are calculated for

the prepared thin film as given in Chapter 2 using a straight forward method proposed by

Swanepoel, which is based on the use of extrema’s of the interference fringes of the

transmittance spectrum.

0 500 1000 1500 2000 2500

0

20

40

60

80

100

Tra

nsm

itta

nce

(a.u

)

Wavelength(nm)

Figure 4.6-Transmission spectra of Ge 10Se 80Te10

Using Swapnoel’s method the refractive index is found at wavelength 917.02nm is 2.84 and

that at 1045.32nm is 2.75. These values are obtained by using eqns(2.3 to 2.11). Now by

using eqn 2.12 we get the thickness of film to be 3.55µm. The variation of refractive index

(n) with wavelength for the thin film is shown in Figure 4.7. The decrease in refractive index

with wavelength shows the normal dispersion behavior of the material.



WAVELENGTH(nm) REFRACTIVE INDEX

917.02 2.84

1045.32 2.75

1237.50 2.68

1529.10 2.66

2013.44 2.42

800 1000 1200 1400 1600 1800 2000 2200

2.4

2.5

2.6

2.7

2.8

2.9

refr

active

in

de

x

wavelength (nm)

refractive index

FIGURE 4.7-Variation of refractive index with wavelength

4.2.4 Photoinduced darkening

Chalcogenide glasses when exposed to above- and near bandgap light, absorption coefficient

over a broad range of frequencies increases. The amount of increase depends on the

wavelength of the inducing light, the duration of exposure and the intensity of the light. The

photodarkening process involves a shift of the optical absorption edge to lower energy and an

increase in the band tail absorption. The absorption change is permanent and can only be

removed by annealing the glass at a temperature near its glass transition temperature. Because

the optical changes can be removed this is known as reversible photodarkening. Reversible



photodarkening has been observed in bulk glasses and well as in thin films. The

photodarkening can be induced with above-bandgap or below-bandgap light, so long as the

light has sufficient energy to excite electrons from the LP band. The index of refraction of the

glass also changes with photodarkening. The associated index change may prove useful for

the fabrication of optical structures in bulk glasses and in thin films.

EXPERIMENTAL SETUP:-

The photodarkening experiment was arranged as shown in Figure.4.6. Diode pumped solid

state laser (DPSS) of 532nm is used to study the photosensitivity of the films. The laser

power was made stable during exposure to avoid significant uncertainty in the total supplied

energy. The laser beam was expanded using a plano-concave lens and collimated with a

second plano-convex lens. The transmittance and absorption spectra of films at normal

incident condition in the spectral range 200–2200 nm were recorded by a double beam UV-

VIS-NIR spectrophotometer (Jasco V 570).

FIGURE 4.8-SETUP FOR PHOTODARKENING EXPERIMENT

There will not be any change to the thickness of the film. In order to study the kinetics

involved in the photoinduced process, time dependence of bandgap of the sample on laser

irradiation was made. The sample were irradiated at different time intervals and the

absorption and transmission spectra were recorded using double beam UV-VIS-NIR

spectrophotometer (Jasco V 570) at each interval.



(a) ABSORPTION SPECTRUM

The sample were irradiated for 5 minutes,10 minutes,20 minutes,30 minutes and 60

minutes. The absorption at each interval was taken using spectrometer mentioned

above.

0 500 1000 1500 2000 2500

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Ab

so

rba

nce

(a.u

)

Wavelength(nm)

Absorbance spectrum

Figure 4.9-Absorption spectrum without illumination



0 500 1000 1500 2000 2500

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Ab

so

rba

nce

(a.u

)

Wavelength(nm)

Figure 4.10-Absorbance spectrum at 5minute illumination

With the help of absorption spectrum

0 500 1000 1500 2000 2500

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Ab

so

rba

nce

(cm

-1)

wavelength(nm)

Figure 4.11-Absorption spectrum after 10min illumination



0 500 1000 1500 2000 2500

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Ab

so

rba

nce

(a.u

)

Wavelength(nm)


0 500 1000 1500 2000 2500

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Ab

so

rba

nce

(a.u

)

Wavelength(nm)

B




0 500 1000 1500 2000 2500

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5A

bso

rba

nce

(a.u

)

Wavelength(nm)

FIGURE 4.14-Absorption spectrum after 60min illumination

From the absorption spectrum the hv Vs ( αhv)1/2 graph is plotted corresponding to each

interval.They are given below.

1 2 3

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

( a

hv)^

1/2

(cm

-1e

V)

Energy(eV)

FIGURE 4.15- hv Vs ( αhv)1/2 without illumination

The bandgap found by this method is 1.478 eV.



1 2 3

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

( a

hv)^

1/2

(cm

-1e

V)1

/2

hv(eV)

FIGURE 4.16- hv Vs ( αhv)1/2 with 5 minutes illumination

The bandgap obtained is 1.472 eV.

1 2 3

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

( a

hv)^

1/2

(cm

-1e

V)1

/2

Energy(eV)





1 2 3

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

( a

hv)^

1/2

(cm

-1e

V)

Energy(eV)


The bandgap obtained is 1.466eV.

1 2 3

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

( a

hv)^

1/2

(cm

-1e

V)

Energy(eV)





1 2 3

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0 (

ah

v)^

1/2

(c

m-1

eV

)1/2

Energy(eV)

FIGURE4.20- hv Vs ( αhv)1/2 with 60 minutes illumination


Although darkenening in films has attracted attention for its possible applications in optical

memory elements, the mechanism causing the change in optical gap remains unclear. One of

the possible mechanism can be due to the dissociation of the bonds or bonding rearrangement

leading to large absorption of light ie, illumination above the band gap alters the bonstatistics

resulting in the randomness of the bond distribution and the creation of defect states which

leads to an increase in disorder. As discussed earlier the bandgap decreases with increase in

exposure.The above values are tabulated in the Table 4.2 given below:

EXPOSURE TIME BANDGAP(eV)

0 MINUTES 1.478

5 MINUTES 1.472

10 MINUTES 1.472

20 MINUTES 1.466

30MINUTES 1.466

60 MINUTES 1.461

Table 4.2-Bandgap variation with exposure time



Figure 4.22 shows the plot of Bandgap variation with exposure time

0 10 20 30 40 50 60

1.460

1.465

1.470

1.475

1.480

Ba

nd

ga

p (

eV

)

Time (minutes)

Figure 4.22-Bandgap variation with exposure time

The photodarkening effects induced by Diode pumped solid state laser (DPSS) of was very

small hence the photo induced experiment was repeated with He-Ne laser of 635nm

wavelength . The results are tabulated below.

EXPOSURE TIME BANDGAP

0 minute 1.48

5 minutes 1.47

10 minutes 1.41

20 minutes 1.29

30 minutes 1.24

45 minutes 1.21

Table 4.3-Bandgap variation with exposure time



0 10 20 30 40 50

1.20

1.25

1.30

1.35

1.40

1.45

1.50

BA

ND

GA

P(e

V)

EXPOSURE TIME(MINUTES)

Figure 4.23-Variation of bandgap with exposure time

Studies show that photoinduced effects on thin film is higher for He-Ne laser than Diode

Pumped Solid State Laser. The difference in the magnitude of photodarkening can be due to

the difference in effective penetration depth.

4.2.5 Nonlinear studies

Z-Scan is used for studying the nonlinear properties. Z-scan technique introduced by Sheik

Bahae is a single beam method for measuring the sign and magnitude of nonlinear refractive

index that has a sensitivity compared to interferometric methods. It provides direct

measurement of nonlinear absorption coefficient. Previous measurements of nonlinear

refraction have used a variety of techniques including nonlinear interferometry, degenerate

four wave mixing, nearly degenerate three wave mixing, ellipse rotation and beam distortion

measurements. The first three methods namely nonlinear interferometry and wave mixing are

potentially sensitive techniques, but all require complex experimental apparatus. The

propagation of laser beam inside such a material and the ensuing self refraction can be

studied using the z-scan technique. Thus it enables one to determine the third order nonlinear

properties of solids, ordinary liquids, and liquid crystals. The experimental set up for single

beam z-scan technique is given in Figure 2.5. In the ordinary single beam configuration, the

transmittance of the sample is measured, as the sample is moved along the direction of the

focussed guassian beam. A laser beam propagating through a nonlinear medium will

experience both amplitude and phase variation. If transmitted light is measured through an

aperture placed in the far field with respect to focal region, the technique is called closed

aperture z-scan. In this case, the transmitted light is sensitive to both nonlinear absorption and



nonlinear refraction. In a closed aperture zscan experiment, phase distortion suffered by the

beam while propagating through the nonlinear medium is converted into corresponding

amplitude variations. On the other hand, if transmitted light is measured without an aperture,

the mode of measurement is referred to as open aperture z-scan. In this case, the output is

sensitive only to nonlinear absorption. Closed and open aperture z-scan graphs are always

normalized to linear transmittance i.e., transmittance at large values of |z|.Closed and open

aperture z-scan methods yield the real part and imaginary part of nonlinear susceptibility χ(3)

respectively.Usually closed aperture z-scan data is divided by open aperture data to cancel the

effect of nonlinear absorption contained in the closed aperture measurements. The new graph,

called divided z-scan,contains information on nonlinear refraction alone.

Figure 4.25-Z –Scan setup

An important requirement in the z-scan measurement is that,it is assumed that the sample

thickness is much less than Rayleigh’s range z0 (diffraction length of the beam [z0=k ω0/2,

where k is the wave vector and ω0 is the beam waist radius. The beam waist radius ω0 is

given by ω0=fλ/D, where f is the focal length of the lens used, λ is the wavelength of the

source and D is the beam radius at the lens. This is essential to ensure that the beam profile

does not vary appreciably inside the sample because z-scan technique is highly sensitive to

the profile of the beam and also to the thickness of the sample. Any deviation from gaussian

profile of the beam and also from thin sample approximation will give rise to erroneous

results.

The nonlinear studies are done at 120 µJand 77 µJ.Both open and closed Z-Scan are done.



Figure 4.26-Open aperture Z-Scan of thin film at 120 µJ

Figure 4.27-Open aperture Z-Scan of thin film at 77µJ

The curve shows that the sample exihibits saturable absorption. When the absorption cross-

section from excited state is smaller than that from the ground state, the transmission of the

system will be increased when the system is pumped with high intensity laser beam. This

process is called saturable absorption. if the sample is a saturable absorber, transmission

increases with increase in incident intensity and results in a transmission maximum at the

focal region.The nonlinear absorption coefficient and imaginary part of nonlinear

susceptibility χ(3) can be found out by following equations.

L eff=1-exp(-(αl))/α (4.1)

-6 -5 -4 -3 -2 -1 0 1 2 30

50

100

150

200

250

z(cm)

Norm

alis

ed T

ransm

itta

nce(a

.u)

-6 -5 -4 -3 -2 -1 0 1 2 30

50

100

150

200

250

300

z(cm)

Norm

alis

ed T

ransm

itta

nce(a

.u)



β=q/I0Leff (4.2)

α=absorption at 532 nm

l=cuvette thickness

β=nonlinear absorption coefficient

q=First value obtaines from matlab program

I0=E0/Power Width*Area (4.3)

Power width(z)=7 ns

Area=π*ω02 (4.4)

ω0= fλ/D, (4.5)

f=focal length=20cm

D=radius of aperture=6mm

Im(χ(3)=n02C2β/240π2 ω (4.6)

From the calculations above the nonlinear absorption coefficient and imaginary parts of

susceptibility are tabulated below.

E0(µJ) 77 120

β(m/W)(10-10) -4.4814 -2.699

Im(χ(3) (e.s.u)(10-11) -3.4277 -2.06437

The nonlinear absorption coefficient and imaginary part of the susceptibility decrease with

the energy. The measured value of β for the samples decreases with increasing input intensity

due to the removal of an appreciable fraction of photocarriers from the ground state.



4.3 Conclusion

1) EDAX spectra show that the thin film does not have the same composition as that of bulk

sample.

2) The bandgap calculated for the thin film was 1.474 eV which is different from bulk

sample.

3) Studies show that a low power radiation is sufficient enough to provide a shift in the

absorption edge. This mechanism in Ge10Se80Te10 glasses can be used to realize continuous

wave laser written waveguide and for fabricating photosensitive optical components for

various applications. The photo induced effect was more prominent for He-Ne laser of 635

nm wavelength.

4) Nonlinear absorption coefficient and imaginary part of susceptibility was calculated and

nonlinear studies show that these samples exihibit saturable absorption. The third order

susceptibility, Im(χ(3) is of interest because of its importance in applications such as

nonlinear propagation in fibers, fast optical switching, self-focusing, damage in optical

materials and optical limiting in semiconductors. Saturable absorbers are useful in laser

cavities. The key parameters for a saturable absorber are its wavelength range (where it

absorbs), its dynamic response (how fast it recovers), and its saturation intensity and fluence

(at what intensity or pulse energy it saturates). They are commonly used for passive Q-

switching. However, saturable absorbers are also useful for purposes of nonlinear filtering

outside laser resonators, e.g. for cleaning up pulse shapes, and in optical signal processing.

https://en.wikipedia.org/wiki/Optical_cavity

https://en.wikipedia.org/wiki/Optical_cavity

https://en.wikipedia.org/wiki/Wavelength

https://en.wikipedia.org/wiki/Q-switching

https://en.wikipedia.org/wiki/Q-switching

http://www.rp-photonics.com/laser_resonators.html



5. NANOCOLLOID

The finely grounded powder is mixed in different solutions and all the solutions are kept for

many months. Some got partially dissolved and some did not show any sign of

dissolution.The prepared solutions are as follows.

A-0.0275g of powdered sample is mixed in 50ml of butyl amine

B-0.0275g of powdered sample is mixed in 50ml of diethyl amine

C-0.051g of powdered sample is mixed in 50ml of ethanol amine

D-0.0272g of powdered sample is mixed in 30ml of ethanol amine

E-0.014g of powdered sample is mixed in 30 ml of ethanol amine

F-0.0272g of powdered sample in 30ml of ethanol amine

G-0.025g of powdered sample is mixed in 30ml of ethylyne diamine

For every sample absorption and nonlinear effects are studied

5.1 Experiments on A

A is kept nearly for 3 months and it showed colour change.Considering this as a sign of

dissolution the studies are carried out.



FIGURE 5.1-Powdered sample in butyl amine

5.1.1 Absorption

0 500 1000 1500 2000 2500

-3

-2

-1

0

1

2

3

4

5

6

Ab

so

rba

nce

(a.u

)

Wavelength(nm)

B

Figure 5.2-Absorbance of sample in butylamine



0 1 2 3 4 5 6 7

0

1

2

3

4

5(

ah

v)^

1/2

(c

m-1

eV

)^1

/2

Energy(eV)

Figure 5.3-- hv Vs ( αhv)^1/2 graph of A

The bandgap of prepared solution is found to be 2.21eV.


The nonlinear studies of A is carried out.Z Scan is carried out . Z-Scan is explained in the

previous chapter in detail.The studies are carried out 43-47 µJ and 104 µJ.The curve didn’t fit

for both open and closed Z-Scan. The sample did not mix properly in butyl amine and hence

the coefficients could not be found out



5.2 Experiments on B

The sample was kept for 8 months and it showed only partial dissolution.

5.2.1 Absorption

0 500 1000 1500 2000 2500

-2

-1

0

1

2

3

4

Ab

so

rba

nce

(a.u

)

Wavelength(nm)

Figure 5.4-Absorbance of sample in diethylamine

To find the bandgap the another graph is plotted.



2 3 4 5 6 7

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0(

ah

v)^

1/2

(c

m-1

eV

)^1

/2

Energy(eV)

Figure 5.5-- hv Vs ( αhv)^1/2 graph of B

The bandgap is determined to be 2.289 eV.


The nonlinear studies of B is carried out.Z Scan is carried out . ZScan is explained in the

previous chapter in detail. The studies are carried out 43-47 uJ and 104 uJ.The curve didn’t fit

for both open and closed ZScan.

The sample didn’t mix properly in butyl amine.So the coefficients could not be found out

5.3 Experiments on C and D

The samples did not mix in the solutions.



5.4 Experiments on E

5.4.1 Absorption

0 500 1000 1500 2000 2500

-2

-1

0

1

2

3

Ab

so

rba

nce

(a.u

)

Wavelength(nm)

Figure 5.6-Absorbance of sample in E

0 1 2 3 4 5 6 7

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

( a

hv)^

1/2

(c

m-1

eV

)^1

/2

Energy(eV)

Figure 5.7-- hv Vs ( αhv)^1/2 graph of E

The bandgap was found to be 2.97eV.



5.4.2 Nonlinear Studies

The experiments are carried out at 70 and 100 µJ. Both open aperture and closed aperture Z-

Scans were undertaken.

Figure 5.8-Open aperture Z-Scan at 70µJ


The sample exihibits reverse saturable absorption. When the absorption cross-section from

excited state is larger than that from the ground state, the transmission of the system will be

less under intense laser fields. This process is called reverse saturable absorption. The reverse

saturable absorption, which is generally associated with a large cross section of absorption

from excited levels, brings about optical limiting effects in colloidal solutions. In

semiconductor materials the optical limiting is governed by two photon absorption as

observed in the present studies. An important parameter in the optical limiting phenomena is

-4 -3 -2 -1 0 1 2 3 4 50.4

0.5

0.6

0.7

0.8

0.9

1

1.1

z(cm)

Norm

alis

ed T

ransm

itta

nce(a

.u)

-4 -3 -2 -1 0 1 2 3 4 50.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

z(cm)

Norm

alis

ed t

ransm

itta

nce(a

.u)



the limiting threshold. It is obvious that lower the optical limiting threshold, better the

efficiency of optical limiting material.

The β and Im(χ(3) decreases with increase in E0. The experimental data shows best fit for two

photon absorption confirming that TPA may be the basic mechanism involved in thenonlinear

absorption process. Possibility of higher order nonlinear processes such as free carrier

absorption contributing to induced absorption cannot be ruled out. The measured value of β

for the samples decreases with increasing input intensity due to the removal of an appreciable

fraction of photocarriers from the ground state. Thus when the incident intensity exceeds the

saturation intensity, the nonlinear absorption coefficient of the medium decreases.

The closed aperture Z Scan was also carried out to get real part of susceptibility.

The nonlinear refractive index n2 is given by the relation

n2=C n0λΔϕ0/80π2 I0Leff (5.1)

Re(χ(3)= n0 n2/3π (5.2)

E0(µJ) 70 100

β(m/W)(10-10) 1.488 1.178

Im(χ(3) (e.s.u)(10-12) 4.1898 3.75



Figure 5.10-Closed aperture Z-Scan at 100µJ

The curve didnot fit at 70µJ.The calculations were done for 100 µJ.

n2=-7.6046*10-11

Re(χ(3)=-1.307*10-11

Here the real part is larger than imaginary part which shows that nonlinear refraction is more

than absorption.

-4 -3 -2 -1 0 1 2 3 40

0.5

1

1.5

2

2.5

3

3.5

z(cm)

No

rma

lise

d T

ransm

itta

nce

(a.u

)



5.5 Experiments on F

5.5.1 Absorption

0 500 1000 1500 2000 2500

-2

-1

0

1

2

3

4

Ab

so

rba

nce

(a.u

)

Wavelength(nm)

Figure 5.11-Absorbance of sample in F

0 1 2 3 4 5 6 7

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

( a

hv)^

1/2

(c

m-1

eV

)^1

/2

Energy(eV)

Figure 5.12-- hv Vs ( αhv)1/2 graph of F

The bandgap is 1.786 eV.



5.6 Experiments on G

5.6.1 Absorption

0 500 1000 1500 2000 2500

-3

-2

-1

0

1

2

3

4

5

6

Ab

so

rba

nce

Wavelength(nm)

Figure 5.13-Absorbance of sample G

The bandgap can be calculated from below graph

.



0 1 2 3 4 5 6 7

0

1

2

3

4

5(

ah

v)^

1/2

(c

m-1

eV

)^1

/2

Energy(eV)

Figure 5.14-- hv Vs ( αhv)^1/2 graph of G

The bandgap is 1.91348 eV



-4 -3 -2 -1 0 1 2 3 4 50.4

0.5

0.6

0.7

0.8

0.9

1

1.1

z(cm)

No

rma

lise

d T

ransm

itta

nce

(a.u

)



Figure 5.16-Open aperture Z-Scan at 100µj

The sample exihibits reverse saturable absorption. When the absorption cross-section from

excited state is larger than that from the ground state, the transmission of the system will be

less under intense laser fields. This process is called reverse saturable absorption.

The β and Im(χ(3) decreases with increase in E0. The experimental data shows best fit for two

photon absorption confirming that TPA may be the basic mechanism involved in thenonlinear

absorption process. Possibility of higher order nonlinear processes such as free carrier

absorption contributing to induced absorption cannot be ruled out. The measured value of β

for the samples decreases with increasing input intensity due to the removal of an appreciable

fraction of photocarriers from the ground state. Thus when the incident intensity exceeds the

saturation intensity, the nonlinear absorption coefficient of the medium decreases.

The closed aperture results are

-4 -3 -2 -1 0 1 2 3 4 50.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

z(cm)

No

rma

lise

d T

ransm

itta

nce

(a.u

)

E0(µJ) 70 100

β(m/W)(10-10) 1.462 1.107

Im(χ(3) (e.s.u)(10-12) 4.219 3.194





-5 -4 -3 -2 -1 0 1 2 3 40

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

z/z0

Norm

alis

ed T

ransm

itta

nce

-4 -3 -2 -1 0 1 2 3 4-1

-0.5

0

0.5

1

1.5

2

2.5

3

3.5

4

z/z0

Norm

alis

ed T

ransm

itta

nce(a

.u)

E0(µJ) 70 100

n2 (10-10) -1.15587 -1.53

Re(χ(3) (e.s.u)(10-11) -2.0113 -2.663



5.7 Conclusion

Investigations on the nanocolloids prepared show that it takes a long time for dissolution

and some solutions were highly unstable. Sample A,B shows large bandgap compared to

bulk samples. Samples C and D were not completely dissolved. On exposure to light it was

seen that the sample F and solvent got separated and hence nonlinear properties could not be

evaluated..F has the bandgap somewhat near to bulk.The dissolution was complete for F and

G only.Rest all other showed partial dissolution.The sample E and F exihibits reverse

saturable absorption and the nonlinear absorption coefficient and imaginary part of

susceptibility increases when the energy is increased.For sample E nonlinear refraction is

more than absorption. From closed aperture z-scan nonlinear refractive index can be found

out.Nonlinear index of refraction is the change in refractive index or the spatial distribution

of the refractive index of a medium due to the presence of optical waves and has generated

significant and technological interest. It has been utilized for a variety of applications such as

nonlinear spectroscopy, correcting optical distortions, optical switching, optical logic gates,

optical data processing, optical communications, optical limiting, passive laser mode-locking,

wave guide switches and modulators. The nonlinear studies show that materials are

promising candidates for light-emitting devices, optoelectronic devices and optical limiters

for the development of nonlinear optical devices with a relatively small limiting threshold.



6. Spin coating

SPIN 150-v3 was used for spin coating the films. Spin Coating involves the acceleration of a

liquid puddle on a rotating substrate .In this technique the coating material is deposited in the

center of the substrate either manually or by a robotic arm. The physics behind spin coating

involve a balance between centrifugal forces controlled by spin speed and viscous forces

which are determined by solvent viscosity. Some variable process parameters involved in

spin coating are Solution viscosity, Solid content, Angular speed and Spin Time.

Figure 6.1-Spin coating

Here the nanocolloids explained in the previous chapter are coated into films. The

nanocolloids D,E and F are coated into films.

Composite film preparation

2 grams of polyvinyl alcohol is mixed in 18ml of hotwater. It is continuously stirred till the

sample mixed properly.Polyvinyl alcohol and sample are mixed. Then this mixture is coated

using spin coating technique. Then the properties of the coated filims are studied.



6.1 Experiment on E

1ml polyvinyl alcohol and 1ml E are mixed. Two films film 1 and film 2 are prepared from

the mixture.

Absorption,transmission and nonlinear studies are conducted for each film.

.

6.1.1 Absorption

0 500 1000 1500 2000 2500

-0.4

-0.3

-0.2

-0.1

0.0

0.1

Ab

so

rba

nce

(a.u

)

Wavelength(nm)

Figure 6.2-Absorption spectrum of spin coated filim from E (film1)



1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

0.0

0.1

0.2

0.3

0.4

0.5

( a

hv)^

1/2

(c

m-1

eV

)^1

/2

Energy(eV)

Figure 6.3- hv Vs ( αhv)1/2 graph of film1

The bandgap is found to be 1.335eV.



0 500 1000 1500 2000 2500

-0.02

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

Ab

so

rba

nce

(a.u

)

Wavelength(nm)

Figure 6.4-Absorption of film2

2 3 4 5 6 7

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

( a

hv)^

1/2

(c

m-1

eV

)^1

/2

Energy(eV)


The bandgap is found to be 2.0874 eV



6.1.2 Transmission spectrum

0 500 1000 1500 2000 2500

80

100

120

140

160

180

200

220

240

Tra

nsm

itta

nce

(a.u

)

Wavelength(nm)

Figure 6.6-Transmission spectrum of film1

The maxima and minimas are not obtained.So the thickness cannot be calculated.

0 500 1000 1500 2000 2500

70

75

80

85

90

95

100

105

Tra

nsm

itta

nce

(a.u

)

Wavelength(nm)

Figure 6.7- Transmission spectrum of film2





The studies are carried out for film1 and film2 but the curves didn’t fit.The studies are carried

out 100uJ.

6.2 Experiment on F

1ml of pva and 1 ml of F taken and mixed. Three films film 1 ,film 2,film 3 are prepared at

600rpm,1000rpm and 1500rpm.

6.2.1 Absorption

0 500 1000 1500 2000 2500

0.55

0.60

0.65

0.70

0.75

0.80

Ab

so

rba

nce

(a.u

)

Wavelength(nm)




0 1 2 3 4 5 6 7

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2(

ah

v)^

1/2

(c

m-1

eV

)^1

/2

Energy(eV)



0 500 1000 1500 2000 2500

-0.02

0.00

0.02

0.04

0.06

0.08

0.10

0.12

Ab

so

rba

nce

(a.u

)

Wavelength(nm)

Figure 6.10-Absorption of film 2



1 2 3 4 5 6 7

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

( a

hv)^

1/2

(c

m-1

eV

)^1

/2

Energy(eV)

Figure 6.11- hv Vs ( αhv)1/2 graph of film 2


0 500 1000 1500 2000 2500

0.92

0.94

0.96

0.98

1.00

1.02

1.04

1.06

1.08

1.10

Ab

so

rba

nce

(a.u

)

Wavelength(nm)




0 1 2 3 4 5 6 7

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6(

ah

v)^

1/2

(c

m-1

eV

)^1

/2

Energy(eV)



6.2.2 Transmission

0 500 1000 1500 2000 2500

16

18

20

22

24

26

28

Tra

nsm

itta

nce

(a.u

)

Wavength(nm)





0 500 1000 1500 2000 2500

80

85

90

95

100

105

Tra

nsm

itta

nce

(a.u

)

Wavength(nm)

Figure 6.15-Transmission spectrum of film 2


0 500 1000 1500 2000 2500

8.0

8.5

9.0

9.5

10.0

10.5

11.0

11.5

Tra

nsm

itta

nce

(a.u

)

Wavelength(nm)






Experiments are conducted for film 1.

Figure 6.17-Open aperture ZScan at 44µJ


-5 -4 -3 -2 -1 0 1 2 3 40.8

0.85

0.9

0.95

1

1.05

1.1

1.15

z(cm)

No

rma

lise

d T

ransm

itta

nce

(a.u

)

-4 -3 -2 -1 0 1 2 3 4 5

0.7

0.8

0.9

1

1.1

1.2

1.3

z(cm)

Norm

alis

ed T

ransm

itta

nce(a

.u)

E0(µJ) 44 110

β(m/W)(10-11) 9.52 6.6762

Im(χ(3) (e.s.u)(10-12) 2.29816 1.611664



The sample exihibits reverse saturable absorption. The β and Im(χ(3) decreases with increase

in E0.

6.3 Experiment on G

1ml of pva and 2 ml of G taken and mixed.

6.3.1 Absorption

0 500 1000 1500 2000 2500

0.85

0.90

0.95

1.00

1.05

1.10

1.15

1.20

1.25

Ab

so

rba

nce

(a.u

)

Wavelength(nm)




0 1 2 3 4 5 6 7

0.5

1.0

1.5

2.0

2.5

( a

hv)^

1/2

(c

m-1

eV

)^1

/2

Energy(eV)



0 500 1000 1500 2000 2500

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

Ab

so

rba

nce

(a.u

)

Wavelength(nm)




0 1 2 3 4 5 6 7

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5(

ah

v)^

1/2

(c

m-1

eV

)^1

/2

Energy(eV)


The bandgap is 0.5486eV

0 500 1000 1500 2000 2500

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Ab

so

rba

nce

(a.u

)

Wavelength(nm)




0 1 2 3 4 5 6 7

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

( a

hv)^

1/2

(c

m-1

eV

)^1

/2

Energy(eV)



6.3.2 Transmission

0 500 1000 1500 2000 2500

6

7

8

9

10

11

12

13

14

Tra

nsm

itta

nce

(a.u

)

Wavelength(nm)




0 500 1000 1500 2000 2500

0

10

20

30

40

50

60

70

80

Tra

nsm

itta

nce

(a.u

)

Wavelength(nm)



0 500 1000 1500 2000 2500

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Tra

nsm

itta

nce

(a.u

)

Wavelength(nm)






Experiments are performed for film 3.



At 44µJ saturable absorption is obtained ,as the energy is increased reverse saturable

absorption is obtained.

-4 -3 -2 -1 0 1 2 3 4 50.98

1

1.02

1.04

1.06

1.08

1.1

1.12

1.14

z(cm)

Norm

alis

ed T

ransm

itta

nce

-3 -2 -1 0 1 2 3 4 5 6-0.2

0

0.2

0.4

0.6

0.8

1

1.2

z(cm)

Norm

alis

ed T

ransm

itta

nce



6.4 Conclusion

The transmission spectrum of films did not have maxima and minima so the thickness of the

filim cannot be obtained. One film coated from E showed bandgap near the bulk sample ie

1.335 eV. The nonlinear studies are done but the coefficients could not be evaluated as the

graph did not fit . The films coated from G shows large bandgap variation from the bulk.One

filim coated from F exihibits bandgap 1.391eV ie near bulk bandgap. All other shows large

variation. Nonlinear effects are studied for film 1 coated from F and film 3 coated from G. G

shows a transition from saturable absorption reverse saturable absorption when the energy is

increased. F exihibits reverse saturable absorption. The nonlinear absorption coefficient and

imaginary part of susceptibility increases when the energy is increased. Nonlinear optical

characterisation of the films studied by the z-scan technique shows reverses saturable

absorption(except filim from G at 100µJ )which makes it useful for optical limiting

applications. Thus depending the nano colloid solution used for the fabrication of nano

composite films, varying nonlinear response can be obtained,enabling a pathway to new

materials for optoelectronic devices.

E0(µJ) 44 110

β(m/W) -3.4636*10-11 1.11*10-10

Im(χ(3) (e.s.u) -8.3698*10-13 2.298*10-10



7. Temperature sensor using GeSeTe glass

There are numerous applications for chalcogenide glass as discussed in chapter 1. One of its

application is discussed here. The glass sample can be used as an active medium for a

temperature sensor. The investigations are done in prepared sample glass.Here thin film of

the sample is used. The bulk sample is coated as thin film by a vacuum coating unit as

discussed earlier. The thin film is heated at different temperature using a heater and the

temperature is noted by a thermometer. The absorption at each temperature is taken by

JASCO-V spectrometer from which bandgap is found out. The glass transition temperature

of this sample is 150 degree Celsius. The experiment is conducted over a range of 0-100

degree Celsius temperature.

7.1 Absorption at 0 degree celsius

0 500 1000 1500 2000 2500

0

1

2

3

4

5

Ab

so

rba

nce

(a.u

)

Wavelength(nm)

Figure 7.1-Absorption at degree Celsius

For finding the band gap ,graph is plotted as shown below.



0 1 2 3 4 5 6 7

0

1

2

3

4

5

6(

ah

v)^

1/2

(c

m-1

eV

)^1

/2

Energy(eV)

FIGURE7.2- hv Vs ( αhv)1/2 without heating

The bandgap is found to be 1.445 eV.

7.2 Absorption after 5 degrees heating

The thin filim is heated and temperature is noted by a thermometer.The heating was stopped

when the sample attained 5 degree Celsius.Then absorption was taken using JASCO-V

spectrometer from which the bandgap was calculated using hv Vs ( αhv)1/2 graph.The

procedure is same for all the temperatures.



0 500 1000 1500 2000 2500

0

1

2

3

4

5A

bso

rba

nce

(a.u

)

Wavelength(nm)

Figure 7.3-Absorption at 5 degree

0 1 2 3 4 5 6 7

0

1

2

3

4

5

6

( a

hv)^

1/2

(c

m-1

eV

)^1

/2

Energy(eV)

Figure 7.4- hv Vs ( αhv)1/2 after 5 degree heating




7.3 Absorption after 15 degree heating

0 500 1000 1500 2000 2500

0

1

2

3

4

5

Ab

so

rba

nce

(a.u

)

Wavelength(nm)

Figure 7.5-Absorption after 15 degree heating

0 1 2 3 4 5 6 7

0

1

2

3

4

5

6

( a

hv)^

1/2

(c

m-1

eV

)^1

/2

Energy(eV)


The bandgap is found to be 1.336 eV




0 500 1000 1500 2000 2500

0

1

2

3

4

5

Ab

so

rba

nce

(a.u

)

Wavelength(nm)


0 1 2 3 4 5 6 7

0

1

2

3

4

5

6

( a

hv)^

1/2

(c

m-1

eV

)^1

/2

Energy(eV)


The bangap is found to be 1.245 eV




0 500 1000 1500 2000 2500

0

1

2

3

4

5

Ab

so

rba

nce

(a.u

)

Wavelength(nm)


0 1 2 3 4 5 6 7

0

1

2

3

4

5

6

( a

hv)^

1/2

(c

m-1

eV

)^1

/2

Energy(eV)

Figure 7.10- hv Vs ( αhv)^1/2 after 55 degree heating

The bandgap is found to be 1.118eV.




0 500 1000 1500 2000 2500

0

1

2

3

4

5

Ab

so

rba

nce

(a.u

)

Wavelength(nm)


0 1 2 3 4 5 6 7

0

1

2

3

4

5

6

( a

hv)^

1/2

(c

m-1

eV

)^1

/2

Energy(eV)





Temperature(degree Celsius) Bandgap(eV)

0 1.445

5 1.389

15 1.336

35 1.245

55 1.118

75 0.862

100 0.537

Table 7.1-Bandgap variation with temperature

0 20 40 60 80 100

0.6

0.8

1.0

1.2

1.4

1.6

Ba

nd

ga

p(e

V)

Temperature(degree Celsius)

Figure 7.15-Bandgap variation with temperature



The glass transition temperature of this sample is 150 degree Celsius. A quasi linear

behaviour below transition temperature is exhibited for these glasses which is similar to the

studies already reported “ On the Application of Chalcogenide Glasses in Temperature

Sensors”(M. Shpotyuk, D. Chalyy, O. Shpotyuk, M. Iovu, A. Andriesh4, M. Vakiv and S.

Ubizskii).



8. CONCLUSION

Ge10Se80Te10 sample was prepared by melt quench technique. The sample was charecterised

in 4 different forms ie bulk glass, thin film, nanocolloid and spin coated nanocolloid film. X-

ray diffraction studies conducted on the bulk sample show no prominent peaks which

reveals the amorphous nature of the samples . Optical bandgap determined by UV-Vis-NIR

spectroscopy is found to be 1.22eV. The bulk glass is not used as such for application

purpose. Device applications using ChG usually require the glass to be processed into either

fiber or thin film form. Thin film was coated using India High Vacuum pumps (12A4-D).

EDAX spectra show that the thin film does not have the same composition as that of bulk

sample. The bandgap calculated for the thin film was 1.474 eV which is different from bulk

sample. The photo induced effects was prominent for He-Ne laser of 635 nm than diode

pumped solid state laser of 532 nm. Nonlinear absorption coefficient and imaginary part of

susceptibility was calculated and nonlinear studies show that these samples exihibit

saturable absorption. Nano colloids of chalcogenide glass have gained a lot of interest in the

research field recently. Understanding the chemical stability of the glasses, and finding the

suitable solvent for making solutions is very important in nano colloid preparations. 7

different samples are prepared by using solvents like butylamine,diethylamine,ethanol amine

and ethylyne diamine. They were named A, B, C. D, E, F and G. The absorption and

transmission taken for all except C and D. The nonlinear optical properties are evaluated for

E and G. E and G showed reverse saturable absorption so it can be used for optical limiters.

Due to the difficulty in dissolution the concentration dependence could not be evaluated. The

samples E, F, G were mixed with polyvinyl alcohol and coated to film using spin coating.

The transmission spectrum of films did not have maxima and minima so the thickness of the

film cannot be obtained. The absorption spectrum of all films taken and the bandgap is found

out. Nonlinear effects are studied for film 1 coated from F and film 3 coated from G. G shows

a transition from saturable absorption to reverse saturable absorption when the energy is

increased. F exihibits reverse saturable absorption. So this can be used as optical limiters.

Among thin film, nanocolloid and spin coated film, thin film has bandgap near to the bulk.

The thin film can be used as saturable absorber and nanocolloid and film coated from F can

be used as reverse saturable absorber. Nonlinear absorption coefficient is maximum for the

thin film compared to others. The spin coated film is cheaper compared to thin film. One of



the application of sample was evaluated. The glass sample can be used as an active medium

for a temperature sensor. A quasi linear behaviour below transition temperature is exhibited

for these glasses which is similar to the studies already reported.



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