20

CHAPTER – 2

SOL-GEL PREPARATION AND CHARACTERIZATION TECHNIQUES

2.1 Deposition techniques

The growth technique played a significant role in controlling the properties of ZnO

films, because the same material deposited by two different techniques, usually had different

physical properties. This is due to the fact that the electrical and optical properties of the

films strongly depended on the structure, morphology and nature of impurities present.

Moreover the films grown using any particular technique might have different properties due

to the variation of the deposition parameters and hence the properties can be tailored by

controlling the deposition parameters. Therefore, it is important and necessary to make a

detailed investigation on the different techniques used for the deposition of ZnO films. A lot

of methods have been extensively used for oriented ZnO films synthesis, including MBE,

pulsed laser deposition, sol-gel method, metal-organic chemical vapour deposition, sputtering

[37], cathodic magnetron sputtering, reactive electron beam evaporation [97–101], spray

pyrolysis [102-105] and electrodeposition [106, 107]. However, sol–gel processes are

particularly adapted to produce ZnO colloids [108–119] and films [120–126] due to the

following advantages such as,

Low cost.

Simple deposition equipment.

Easy fabrication of large-area films.

The idea behind sol-gel synthesis is to “dissolve” the compound in a liquid

and bring it back as a solid in a controlled manner.

Multi component compounds may be prepared with a controlled stoichiometry

by mixing sols of different compounds.

21

The sol-gel method prevents the problems encountered with co-precipitation

where inhomogeneity occurs during a gelation reaction.

Enables mixing at an atomic level.

Results in small particles, which can be easily sintered.

The sol–gel process, also called soft chemistry (‘chimie douce’), allows elaborating a solid

material from a solution by using a sol or gel as an intermediate step (Scheme 1) and at much

lower temperatures are possible by traditional methods of preparation. It enables the powder

less processing of glasses, ceramics and thin films or fibers directly from solution. The

synthesis of solid materials via ‘chimie douce’ often involves wet chemistry reactions while

sol–gel chemistry is based on the transformation of molecular precursors into an oxide

network by hydrolysis and condensation reactions [127, 128]. Scheme 1 shows the main

steps of preparation of thin films and powder by the sol-gel process.

The film preparation contains three parts: (i) preparation of the precursor solution,

(ii) deposition of the prepared sol on the substrate by the chosen technique and (iii) heat

treatment of the xerogel film. The xerogel is the dried gel at ambient pressure (dried gel in a

supercritical condition is called an aerogel). ZnO thin film synthesis involves several

parameters:

Nature of the precursor and its concentration,

Type of solvent and the acidity of the medium,

The type of additive species and their concentrations,

Aging time of the early mixture,

The method of coating of substrates and its speed,

Nature of the substrate,

Pre and post-heat treatment of the materials.

22

Scheme 1 Overview showing two synthesis examples by the sol–gel method; (a) films from a

colloidal sol; (b) powder from a colloidal sol transformed into a gel.

A survey of the literature shows that all these parameters play a key role in the evolution of

the texture in zinc oxide films.

2.2 Sol–gel method for zinc oxide synthesis

In the sol–gel process, a molecular precursor in a homogeneous solution undergoes a

succession of transformations: (a) hydrolysis of the molecular precursor; (b) polymerization

23

via successive bimolecular additions of ions, forming oxo-, hydroxyl or aquabridges; (c)

condensation by dehydration; (d) nucleation and (e) growth [129, 130]. Depending on the

nature of the molecular precursors, two sol–gel routes are currently used: metal alkoxides in

organic solvents or metal salts in aqueous solutions. The main methods of ZnO film

elaboration, as reported in the literature, involve several steps and are in fact intermediate

between the sol–gel methods since they use metal salts in alcoholic solutions. Indeed, ZnO

films are obtained starting from inorganic salts such as nitrates, chlorides, perchlorates or

organic salts like acetates and acetylacetonates, dissolved in alcoholic media. It is believed

that in such media the process involves two steps. The first one consists of in situ formation

of alkoxide or alkoxy-complexes. In the second step, these complexes undergo

transformation through hydrolysis and polymerization leading to the oxide formation.

2.2.1 Precursors

Several zinc precursors have been used in the preparation of ZnO. They are nitrate,

chloride, perchlorate, acetylacetonate and alkoxides such as ethoxide and propoxide, but the

most often used one is the acetate dihydrate. Metal alkoxides, although offer several chemical

advantages are not suitable because they are very sensitive to moisture, highly reactive and

expensive. Because of their low cost, facility of easy handling and commercial availability,

metal salts are interesting as precursors which could be more appropriate for large-scale

applications. Since metal salts include inorganic and organic ones, comparisons can be made

between them as reported by some authors [131]. Inorganic salts like nitrates are often used,

as precursors for sol–gel ZnO-based materials where the main drawback is related to the

inclusion of anionic species or their removal from the final product. Using zinc acetate as a

precursor, the acetate groups, as contaminants of the gel, decompose under annealing

producing combustion volatile by-products.

24

Bahnemann et al [110] synthesized transparent colloidal suspensions of zinc oxide in

water, 2-propanol and acetonitrile using different zinc salts. They reported that the anion, in

zinc salt is critical for the preparation of transparent and stable ZnO colloids. Also they

mentioned that the use of zinc perchlorate instead of zinc acetate yields a turbid suspension;

i.e coagulation of the particles takes place, the acetate act as a stabilizer of the colloidal sol.

The experiments with ZnCl2 or Zn(NO3)2 reveal a faster coagulation than in the case of

Zn(ClO4)2 following the initial formation of a clear colloidal suspension. Matijevic [108]

reported that the preparation of “monodispersed” sols of metal (hydrous) oxides from metal

salts is very sensitive to such factors as salt concentration, nature of the anion, pH and

temperature. He demonstrated that entirely different products result when the anions are

changed in the studied systems.

2.2.2 Solvents

The solvent must have a relatively high dielectric constant in order to dissolve the

inorganic salts [132, 133]. Most alcohols are dipolar, amphiprotic solvents with a dielectric

constant that depends on the chain length. Alcohols with low carbon number, up to 4 are the

most used solvents: methanol, ethanol, 1-propanol, 2-propanol, 1-butanol and

2-methoxyethanol. In addition, a few works use ethylene glycol (HOCH2CH2OH) as a

solvent that has a dielectric constant of 40.61 (at 25◦C) and a boiling point of 197.4◦C [134].

Among all the monoalcohols, the most used ones are ethanol and 2-propanol. It should be

noted that 2-methoxyethanol, despite having good physical properties, is toxic to

reproduction, because it is labelled with Risk Phrase R60 (category 2: ‘May impair fertility’)

by the International Programme on Chemical Safety, among others. Hosono et al. [133],

made a comparative study of chemical reactions from zinc acetate dihydrate to ZnO using

different types of alcoholic solvents, i.e. methanol, ethanol, and 2-methoxyethanol.

25

Zinc acetate dihydrate (ZAD) was more soluble in methanol than in ethanol or

2-methoxyethanol according to dielectric constants of these alcohols. The reflux time

necessary for the formation of ZnO increases with the solutions in the order, MeOH (12 h)

EtOH (48 h) < 2-ME (72 h). Besides, the same authors demonstrated that the XRD analysis

of particles, obtained from the three alcoholic solutions of ZAD revealed, after refluxing, the

formation of intermediate product as Zn5(OH)8(Ac)2·2H2O called layered hydroxide zinc

acetate (LHZA). This complex (LHZA) was also observed by Meulenkamp [117] using

ethanol as a solvent and by Fujihara et al. [136] using methanol. On the other hand,

Tokumoto et al. [138] have shown by EXAFS and UV spectroscopy that zinc acetate

dissolved in ethanol forms zinc oxy-acetate, Zn4O(Ac)6. Whatever be their nature, those

complexes undergo hydrolysis and inorganic polymerization leading to the formation of sols

consisting of zinc oxide nanoparticles. On the other hand, Hosono et al. [133] also showed

the washing role and the effect of the reflux time on the crystallinity and the size of particles.

It was found that LHZA was removed from the mixture particles, obtained with different

reflux times (from 8 to 48 h) in methanolic solution, by washing with MeOH. Both the XRD

peak intensities and the crystallite size are determined by using the Scherrer’s equation. They

increase monotonically with increasing reflux time, indicating the growth of the ZnO

particles where the crystallinity improves with time.

2.2.3 Additives

Additives are chemical species which present at least one functional group that

enables these species to play several roles. They act as basic or acid and/or chelating agents.

Alkali metal hydroxides, carboxylic acids, alkanolamines, alkylamines, acetylacetone and

polyalcohols are used for this purpose. They may facilitate the zinc salt dissolution in some

alcoholic media. For example, ZAD has a limited solubility in alcohols like ethanol and

2-propanol in the absence of other agents or heating.

26

The agents like (mono to tri) ethanolamines or lactic acid help in complete dissolution

and formation of a stable sol [135]. Furthermore, the additives are believed to play the role of

chelating and stabilizing ligands, which avoid the rapid precipitation of zinc hydroxide and

allow stable dispersions or solutions to be formed. The amino groups and/or the hydroxyl

groups of alkanolamines are known to coordinate the metal atoms of alkoxides, improving

the solubility and stability against hydrolysis of the alkoxides. This is also the case with zinc

acetate and the addition of alkanolamine provides a clear solution. There are two possible

ways for monoethanolamine (MEA) a bidentate ligand to coordinate the zinc atoms; one is to

act as a chelating ligand and the other is to bridge two zinc atoms [136]. Among inorganic

bases, lithium or sodium hydroxide is often used to form stable dispersions of colloids.

Altogether, solvents and additives are likely to react with zinc cations to give zinc complexes

that appear to be the precursor for zinc oxide. Several works have been devoted to the

structural characterization of such complexes. It has been proposed in previous works [137,

138], a mechanism for the reaction with the main species constituting the precursor solutions

(ZAD, ethanol and MEA) where additives such as MEA are employed. In this medium,

forced hydrolysis ([H2O]/[Zn2+] = 2) and condensation of the Zn2+ cation are relatively slow

due to the low quantities of water (resulting from zinc salt). MEA acts as a complexing agent,

also retarding Zn2+ condensation; however, the presence of this amine also increases the pH,

which should promote the formation of ZnO. The acetate group plays a very relevant role by

complexing Zn2+ in competition with the MEA. The complex chemical relationships of the

main species are indicated in Scheme 2. In fact, three nucleophilic species (MEA, OH− and

CH3COO−) compete for the Zn2+ Lewis acid center: attack of an HO− group leads to the

formation of small zinc-oxo-acetate oligomers, which are expected to be formed in the initial

stage, from gradual forced hydrolysis of Zn-MEA or Zn-OCOCH3 soluble complexes during

aging.

27

The progressive condensation of the hydrolysed moieties gives rise to colloids or

precipitates. Stable acetate-capped colloidal nanometrics to sub-micronic particles are likely

to be formed in dilute solutions.

Scheme 2 Sketch of the chemical equilibria is taking place in the initial solutions. Hydrolysis and condensation

are spurred by temperature. Soluble or colloidal condensed moieties can result, which can be deposited as film

precursors.

2.3 Particle formation

Several works have been keen to the study of the parameters that affect the particle

size of zinc oxide. In addition, other works have been devoted to the study of the influence of

the precursor, additive concentrations, the solution aging conditions (time and temperature)

on the colloids stability and the zinc oxide particles formation (morphology and size). Before

this discussion, we recall the possible growth mechanisms proposed by some authors.

2.3.1 Growth mechanisms

Spanhel and Anderson [113] have reported that there are two possible ways of

describing the growth of ZnO crystals: Ostwald ripening and aggregation. As soon as the

smallest stable molecular clusters are formed, they rapidly combine to give the next most

stable aggregates. The primary aggregates would further rapidly combine to give the next

28

most stable secondary aggregates and so on. The authors observed that the primary clusters

were stable aggregates and they could be a result of rapid aggregation rather than that of

Ostwald growth. Hence they concluded that the Ostwald mechanism should be considered as

the only one possible approach to the formation of bulk materials. Meulenkamp [117] also

suggested that the particle growth in colloidal systems could be viewed from two standpoints.

The first one corresponds to Ostwald ripening: large particles grow at the expense of smaller

particles, which have a higher solubility according to the Ostwald–Freundlich equation. This

relation is derived from thermodynamic equilibrium considerations and does not, therefore,

provide information on the growth rate. The second standpoint describes growth by the

addition of reactive precursors available in solution to already existing particles. The rate of

particle growth is governed by the concentration of precursors or dissolved species and their

reactivity, which depends on the number of particle surface atoms and the solution

composition. Tokumoto et al. [138] have reported that the formation of ZnO colloidal

particles in an alcoholic solvent consists of two stages. During the early stage of phase

transformation, small oligomers are continuously formed. At advanced stages, the

aggregation of the oligomers leads to wurtzite structure of the primary colloidal particles. The

primary particles then aggregate and form a third family of the secondary colloidal particles.

The growth of the colloidal particles is a stepped and discontinuous process indicating that

the predominant mechanism of aggregation is a heterogeneous coagulation. This mechanism

of formation and growth leads to a hierarchical structure.

2.3.2 Nucleation and growth – particle size

Spanhel [113] has reported that at least four different “primary particles or clusters”

serving as initiators of the ZnO colloidal growth from ZAD as precursor, could be identified

till date. The nature of these “primary particles or clusters” strongly depends on the synthesis

conditions chosen: initial salt concentration, the temperature, time of the thermal treatment,

29

the nature of alcohol solvent as well as the overall humidity, storage and analysis conditions.

Meulenkamp [117] has prepared ZnO nanoparticles by addition of LiOH to an ethanolic zinc

acetate solution. He showed that control of the particle size was improved by recognizing the

influence of temperature, water and reaction products during the aging of ZnO sols. Water

and acetate induced considerably accelerated particle growth. Hu et al. [119] have

synthesized ZnO nanoparticles by precipitation from zinc acetate in a series of n-alkanols

from ethanol to 1-hexanol as a function of temperature. The kinetics of nucleation and growth

are expected to be strongly dependent on the properties of the solvent. For the shorter chain

length alcohols, ethanol and 1-propanol, nucleation and growth are retarded compared to

longer chain length alcohols from 1- butanol to 1-hexanol, where nucleation and growth are

fast. The particle size increases with increasing temperature for all solvents and increases also

with alkanol chain length. In fact, during the growth of colloidal ZnO nanoparticles, the

alcohols not only provide the medium for the reactions, but also act as ligands to help control

the morphology and particle size of ZnO.

2.3.3 Morphology

Chittoffrati and Matijevic [112] have clearly showed that the precursor concentration

and the aging conditions as well as the nature and the concentration of the additive have an

important effect on the morphology of zinc oxide particles. Zinc oxide dispersions were

obtained by mixing at room temperature zinc nitrate or sulphate with different bases NaOH or

KOH, NH4OH, triethanolamine or ethylenediamine and cationic or anionic surfactants with

varied concentrations. It is followed by aging for different periods of time at constant

elevated temperatures. The authors have demonstrated that the precipitates formed by aging

zinc nitrate solutions at elevated temperatures, always consisted of zincite in ZnO, but the

shape and the size of the particles were strongly dependent on the concentration and nature of

the reactants. As a rule, particles uniform in shape and in some cases in size was obtained

30

only under a certain range of concentrations of the reacting compounds. For example, for the

system Zn(NO3)2–NH4OH turbidity appeared at room temperature, just after mixing, it with

[Zn(NO3)2] ≥ 3.2 × 10−4 mol L−1 and [NH4OH] ≥ 1.0 × 10−3 mol L−1. At much lower

concentrations of zinc nitrate and ammonia, no precipitation could be detected on mixing the

solution at room temperature. On the other hand by heating the diluted solution to 90◦C,

reasonably uniform ellipsoidal particles were obtained in small amounts. The particle shape

depends also on the concentrations and aging time since intertwined particles were obtained

with aging a solution of [Zn(NO3)2] = 5.0 × 10−3 mol L−1 and [NH4OH] = 1.9 × 10−2 mol L−1

at 90◦C for 3 h, when ellipsoidal particles appeared with [Zn(NO3)2] = 1.0 × 10−4 mol L−1 and

[NH4OH] = 3.2 × 10−4 mol L−1 aged at 90◦C for 1 h. The particle shapes were changed by

Zn(NO3)2 in different solvents. For example, intertwined particles for Zn(NO3)2–

ethylenediamine, compact spheroidal or ellipsoidal particles for Zn(NO3)2–triethanolamine

and rod-like particles for Zn(NO3)2–KOH. Substituting ZnSO4 for Zn(NO3)2 yielded

polydispersed platelets in the case of Zn(NO3)2–triethanolamine system. The concentration

domain yielding uniform dispersions depend on the nature of the base employed. Indeed it

appeared that the stronger the Zn base complex, the lower the concentrations required to

generate well defined particles.

2.3.4 Colloids stability and thermal behaviour

The procedure for ZnO colloids synthesis, first developed by Spanhel and Anderson

[113], consists of two major steps: (1) preparing the precursor by reacting zinc acetate with

ethanol and (2) hydrolyzing the precursor to form the colloids by using lithium hydroxide.

Sakohara et al. [115], using this method, studied the concentration effect of zinc acetate

(ZnAc) and lithium hydroxide on the colloids stability as well as their behaviours during the

thermal analysis. The stability of ZnO colloids was observed to depend on the concentration

ratio, ZnAc/LiOH. The prepared colloids with a high ratio (e.g. ZnAc/LiOH = 0.1/0.025 M)

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were quite stable and maintained transparency for more than a month, while a low ratio (e.g.

ZnAc/LiOH = 0.025 M/0.025 M) leads to a precipitate within 2 days. For thermal analysis of

ZnO particles, a weight loss was observed from about 200◦C with high ratios (e.g.

ZnAc/LiOH = 0.1 M/0.0125 M) but only from 300◦C for low ratios (e.g., ZnAc/LiOH =

0.0125 M/0.025 M). The acetate groups act so to stabilize colloids and this result was also

found by Bahnemann et al. [110] and Yang et al. [142]. Sun et al. [134] studied purification

and stabilization of colloidal ZnO nanoparticles synthesized from ZAD and potassium

hydroxide in methanol. They compared the stability of the purified ZnO nanoparticles

redispersed in methanol and in methanol/hexane solutions at two different volume ratios (7/1

and 3/1). The redispersed purified ZnO nanoparticles were unstable in methanol alone

because they become aggregated within several minutes at room temperature. In the case of

redispersion of ZnO nanoparticles in the mixture of methanol/hexane solutions, ZnO

nanoparticles would begin to agglomerate at room temperature after a couple of hours and 1

day for the mixtures with a volume ratio 7/1 and 3/1, respectively. So, hexane can stabilize

the purified ZnO nanoparticles in methanol and it has also demonstrated that the higher

concentration of hexane in the solution is more stable. The same authors have reported that a

possible reason why hexane can stabilize the purified ZnO nanoparticles in methanol could be

that a small amount of organic non-solvent, i.e. hexane, dissolved in methanol helps to reduce

the polarity of the solvent around the acetate-containing ZnO nanoparticle surfaces, thus

isolating the ZnO nanoparticles from each other and preventing them from aggregating.

2.4 Formation of oriented film

In general, films are prepared by dip or spin coating of substrate from solutions or sols

as freshly prepared or aged at 60◦C or around room temperature. The heat treatment of the

deposited films is carried out in two steps. In the first step, a pre-heat treatment (40–500◦C) is

applied for a short time for solvent evaporation and organic compounds removal [139–142].

32

In the second, a post-heat treatment is employed in order to obtain a well-crystallized films

and the final decomposition of organic by-products varying from 250 to 900◦C, according to

the substrate nature among others. The following examples of ZnO thin film elaboration are

given for illustration [143]. In these works, two approaches were explored to produce dense

films or colloid-based films. In the direct dense film formation ZAD was dissolved in a

2-methoxyethanol-MEA solution. The ratio ([MEA]/[Zn(II)]) was fixed at 1 or 2 and the final

concentration of ZAD was fixed at 0.75 mol L−1. The resultant mixture was allowed to stand

at 60◦C for 120 min under stirring leading to a clear and colourless solution, which remained

stable for several days. Freshly prepared solution was then deposited on silica glass by spin

coating at 3000 rpm (30 s). These films were pre-heated at 300◦C for 10 min after each

coating and this procedure was repeated up to six times. The films were post-heated at 550◦C

(1–2 h) in order to obtain crystallized ZnO in wurtzite structure. For the colloid-based films,

0.2 mol L−1 solution of Zn(II) was prepared by refluxing ZAD for 3 h at 80◦C in dry ethanol.

MEA and ethanol were subsequently added to adjust the final concentration (0.05 mol L−1 of

Zn2+) and the solutions were then aged at different temperatures (25–100◦C, in closed vessels)

during variable periods (10–240 h). Depending on the [Zn2+] and the ratio ([MEA]/[Zn2+]),

transparent solutions or colored colloidal dispersions (bluish to yellow) are obtained. The

solution aged at 60◦C for 72 h leads to a translucent yellow colored sol (colloidal dispersion).

The precursor sol was then filtered (Millipore cellulose membranes, 0.45 m) and

deposited on glass substrates by spin-coating 3000 rpm for 30 s. These films were dried at

100◦C for 5 min between successive spin-coating steps. Subsequently, they were pre-treated

at 135◦C for 36–72 h and further post-treated at 450–550◦C for 2 h.

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2.4.1 Role of the heat treatments: pre-heat treatment and post-heat treatment

The heat treatment appears to be one of the most important factors governing the film

orientation. As discussed above, the pre-heat treatment appears to be a crucial step. Indeed, it

governs the orientation of the crystallites during solvent evaporation and removal of the

organic compounds. The temperature of this treatment varies from case to case, but a clear

rule can be deduced from a proper choice. It should be higher than the boiling point of the

solvent, the additives and near the crystallization temperature of ZnO. A temperature around

300◦C appears to be the most appropriate [144, 145] in the case of 2-methoxyethanol and

MEA as solvent and additive, respectively, to produce (002) oriented films. This temperature

has also been observed for the system consisting of methanol and lactic acid. On the contrary,

a pre-heat temperature of 100◦C for the films obtained from 2-propanol and DEA was

sufficient for observing full (002) orientation. A similar low pre-heat temperature (80◦C) has

also been used for the chemical systems consisting of methanol as solvent without using any

additives [146]. Beside the pre-heat and post-heat temperature should be carefully chosen.

The preferential orientation, for instance (002), increases with increasing temperature. For, a

great number of systems, a temperature range of 500–600◦C appears to be the most

appropriate one. However, it should be noted that there exists for each system an upper limit

in temperature above which the loss of orientation is observed. The pre-heat temperature

seems to be the most important factor for obtaining ZnO films with preferred orientation

[147], affecting the solvent vaporization, zinc acetate decomposition and zinc oxide crystal

growth. Removal of the solvent and organic substances produced by acetate decomposition

prior to crystallization may be one of the key factors that provide oriented crystal growth

[148, 149].

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2.5 Spin Coating Method

Spin coating is currently the predominant technique employed to produce uniform

thin films of photosensitive organic materials with thickness of the order of micrometers and

nanometers. The pioneering analysis of spin coating was performed more than fifty years ago

by Emisle et al [151] who considered the spreading of a thin axisymmetric film of Newtonian

fluid on a planner substrate rotating with constant angular velocity. In many cases the coating

material is polymeric and is applied in the form of a solution from which the solvent

evaporates. Spin coating was first studied for coating of paint and pitch [150]. This process

has been widely used in the manufacture of integrated circuits [151], optical mirrors, color

television screens and magnetic disk for data storage [152]. Centrifugal force drives the liquid

radially outward. The viscous force and surface tension causes a thin residual film to be

retained on the flat substrate. The thin nature of the film depends on the combination of

outward fluid flow and evaporation [153]. The spin coating can be broken down into several

key stages: fluid dispense, spin-up, stable fluid out flow and finally evaporation dominated

drying [154]. Flow controlled and evaporation controlled are the two stages that have the

most impact on final coating thickness. Clearly it’s describe the two processes that must be

occurring simultaneously throughout all times (viscous flow and evaporation). However, at

an engineering level the viscous flow effect dominates early on, while the evaporation

processes dominate later. Fluid flow on a flat spinning substrate is one of the most important

physical processes involved in spin coating [155]. Several processing parameters involved in

the spinning process are: dispense volume, final spin speed (w), final film thickness, solution

viscosity, solution concentration (c), spin time, etc. Using different parameters film thickness

and radius of spreading are calculated [156]. The film forming process is primarily driven by

two independent parameters viscosity and spin speed. The range of the film thickness easily

achieved by spin coating is 1–200 µm [157, 158].

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2.5.1 Modelling of spin coating

Even though spin coating has been universally used in the semiconductor industry

long before, hardly any theoretical study has been initiated. An accurate theory of spin

coating would permit better design and control of the process in its various applications.

Decades of industrial production have shown that the depositions of thin photoresists films,

which constitute the main utilization of the spin coating process are quasi-perfectly

controlled. However, any progress in developing mathematical models should be probably

more helpful in the case of heat resistant dielectric insulating films (polyamides or other

materials). In contrast to the photoresists which are often low molecular weight materials

dissolved in low boiling solvents at low concentration, polyimide precursors are high

molecular weight polymers. These materials are supplied in organic solvents with boiling

temperatures close to 200°C per concentration levels (10–20%), where the non-Newtonian

behaviors cannot be neglected. This implies that spin analytical models will be based on

sophisticated equation taking in consideration a number of physical parameters.

2.5.2 Key stages of spin coating

The physics of spin coating can be effectively modeled by dividing the whole process

into four stages is sketched in Fig.2.1. They are deposition, spin-up, spin-off and evaporation

of solvents. The first three are commonly sequential, but spin-off and evaporation usually

overlap.

36

Fig.2.1 Key stages of spin coating process.

37

Deposition

During this stage, solution is allowed to fall on rotating substrates from microsyrings

and the substrate is accelerated to the desired speed. Spreading of the solution takes place due

to centrifugal force and height is reduced to critical height. This is the stage of delivering an

excess of the liquid to be coated to the surface of the substrate a portion of which is

immediately covered or “wetted”. In this stage liquid can be deposited in different ways (a)

as a heavy rain that inundates the entire disk, (b) as a bolus at the centre or around the

perimeter of hole if the centre has been cut out, the liquid then spreading over the rest of the

disk, (c) as a continuous stream at the centre or all the way around the inner perimeter, the

liquid then flowing outward over the entire disk, (d) as a continuous stream from an elevated

delivery port that moves racially over the disk. No matter what way is used the amount of

liquid deposited through excessive is limited and this stage ends when delivery ceases.

Spin-up

The second stage is when the substrate is accelerated up to its final, desired, rotation

speed. This stage is usually characterized by aggressive fluid expulsion from the wafer

surface by the rotational motion. Because of the initial depth of fluid on the wafer surface,

spiral vortices may briefly be present during this stage; these would form as a result of the

twisting motion caused by the inertia that the top of the fluid layer exerts while the wafer

below rotates faster and faster. Eventually, the fluid is thin enough to be completely

co-rotating with the wafer and any evidence of fluid thickness differences is gone.

Ultimately, the wafer reaches its desired speed and the fluid is thin enough that the viscous

shear drag exactly balances the rotational accelerations

Stable fluid out flow

The third stage is when the substrate is spinning at a constant rate and fluid viscous

forces dominate fluid thinning behaviour. This stage is characterized by gradual fluid

38

thinning. Fluid thinning is generally quite uniform, though with solutions containing volatile

solvents; it is often possible to see interference colors “spinning off” and doing so

progressively more slowly as the coating thickness is reduced. Edge effects are often seen

because the fluid flows uniformly outward, but must form droplets at the edge to be flung off.

Thus, depending on the surface tension, viscosity, rotation rate, etc., there may be a small

bead of coating thickness difference around the rim of the final wafer. Mathematical

treatments of the flow behavior show that if the liquid exhibits Newtonian viscosity (linear)

and if the fluid thickness is initially uniform across the wafer (albeit rather thick), then the

fluid thickness profile at any following time will also be uniform.

Evaporation

When spin-off stage ends the film drying stage begins. During this stage centrifugal

out flow stops and further shrinkage is due to solvent loss. This results in the formation of

thin film on the substrate. The fourth stage is when the substrate is spinning at a constant rate

and solvent evaporation dominates the coating thinning behaviour. In this, the rate of

evaporation depends on two factors (a) the difference in partial pressure (actually chemical

potential) of each solvent species between the free surface of the liquid layer and the bulk of

the gas flowing nearby. (b) during the evaporation stage the suspended or dissolved solids

may grow so concentrated at the liquid surface as to form a high viscosity, low diffusivity

layer or solid skin.

2.6 Characterization tools

The optimisation of the preparation conditions is the main task in order to get

device quality films. This has to be carried out on the basis of detailed structural,

compositional, morphological, optical and electrical properties of the films obtained at

different growth conditions. In the following sections the techniques used for the film

characterizations are discussed briefly.

39

2.6.1 Thin film thickness

Thickness plays an important role in the film properties unlike a bulk material.

Reproducible properties are achieved only when the film thickness and the deposition

parameters are kept constant. Film thickness may be measured either by in-situ monitoring

of the rate of deposition or after the film deposition. The thicknesses of the thin films

prepared for the work presented in this thesis were measured either by optical interference

method or by a stylus profiler.

i) Optical interference method

Film thickness can be measured accurately from interference fringes using multiple

beam interferometry. This technique was first used by Weiner [161] and later was modified

by Tolansky [162]. Two reflecting surfaces are brought in close proximity such that a small

wedge with a small air gap in between them is formed. If a monochromatic light falls at

normal incidence on it, interference of light due to the multiple reflected beams results in a

series of fringes (Fizeau). The distance between the fringes depends on the air gap as well

as the wavelength of the monochromatic light. The film whose thickness is to be measured

is deposited on a flat surface so as to leave a sharp edge between the film and the uncoated

region of the substrate. Over this film a highly reflecting coating of aluminium forms a sharp

step on the film edge. Another optically flat glass slide known as the reference plate with a

partially transparent aluminium film coated over it is then placed over the specimen with the

metal coated surfaces in contact with each other so as to leave a small air gap at the step. A

monochromatic parallel beam of light (λ) is made to fa l l on the two plate’s assembly

and the reflected light is then observed through a microscope. A set of sharp fringes

perpendicular to the step with equal displacements will be observed.

40

The thickness (t) can be determined using the relation,

𝑡 = 𝑏𝜆

2𝑎 (1)

where ‘b’ is the displacement of the fringes at the step and ‘a’ is the distance between

consecutive fringes. The sharpness of the fringes depends on the reflectivity of the metal

coating, the spread of the incident beam, air gap etc.

ii) Stylus Profiler

The stylus profiler records measurements electromechanically by moving the sample

beneath a diamond tipped stylus (Fig. 2.2).

Fig. 2.2 Schematic diagram illustrating the determination of thin film

thickness with stylus profiler

The high precision stage moves the sample according to a user defined scan length,

speed and stylus force. The stylus is mechanically coupled to the core of a linear variable

differential transformer (LVDT). At this stage the stylus moves over the sample surface.

Surface variations cause the stylus to be translated vertically. Electrical signals corresponding

to the stylus movement are produced as the core position of the LVDT changes. The LVDT

measures an AC reference signal proportional to the position change, which in turn is

conditioned and converted to a digital format through a high precision, integrating, analog-to-

digital converter [159]. The film whose thickness has to be measured is deposited with a

41

region masked this creates a step on the sample surface the thickness of the sample can

be measured accurately by measuring the vertical motion of the stylus over the step.

2.6.2 X-ray diffraction studies

Electrical and optical properties of the thin films grown are influenced by the

crystallographic nature of the films. X-ray diffraction (XRD) studies are carried out to study

the crystallograoghic properties of the thin films prepared. A given substance always

produces a characteristic X-ray diffraction pattern irrespective of the fact that the substance

is present in the pure state or as one constituent of a mixture of substances. This fact is the

basis for the diffraction method of chemical analysis. The particular advantage of X-ray

diffraction analysis is that it discloses the presence of a substance which is not in terms of its

constituent chemical elements. Diffraction analysis is useful whenever it is necessary to

know the state of chemical combination of the elements involved or the particular phase in

which they are present. Compared with ordinary chemical analysis the diffraction method

has the advantage that it is much faster, requires only very small sample and is non-

destructive [160]. The basic law involved in the diffraction method of structural analysis is

the Bragg’s law. When monochromatic X-rays impinge upon the atoms in a crystal

lattice, each atom acts as a source of scattering. The crystal lattice acts as series of

parallel reflecting planes. The intensity of the reflected beam at certain angles will be

maximum when the path difference between two reflected waves from two different planes

is an integral multiple of ‘λ’. This condition is called Bragg’s law and is given by the

relation,

2𝑑𝑆𝑖𝑛𝜃 = 𝑛𝜆 (2)

where ‘n’ is the order of diffraction, ‘λ’ is the wavelength of the X-rays, ‘d’ is the spacing

between consecutive parallel planes and ‘θ’ is the glancing angle (or the complement of

42

the angle of incidence) [161]. X-ray diffraction studies gives a whole range of information

about the crystal structure, orientation, average crystalline size and stress in the films.

Experimentally obtained diffraction patterns of the sample are compared with the

standard powder diffraction files published by the International Centre for Diffraction Data

(ICDD). The average grain size of the film can be calculated using the Scherrer’s formula

[162],

𝑑 = 0.9𝜆

𝛽𝐶𝑜𝑠𝜃 (3)

where, ‘λ’ is the wavelength of the X-ray and ‘β’ is the full width at half maximum

intensity in radians. The lattice parameter values for different crystallographic systems can

be calculated from the following equations using the (hkl) parameters and the interplanar

spacing (d).

𝐶𝑢𝑏𝑖𝑐 𝑠𝑦𝑠𝑡𝑒𝑚, 1

𝑑2=

ℎ2 + 𝑘2 + 𝑙2

𝑎2

𝑇𝑒𝑡𝑟𝑎𝑔𝑜𝑛𝑎𝑙 𝑠𝑦𝑠𝑡𝑒𝑚, 1

𝑑2=

ℎ2 + 𝑘2

𝑎2+

𝑙2

𝑐2

𝐻𝑒𝑥𝑎𝑔𝑜𝑎𝑛𝑎𝑙 𝑠𝑦𝑠𝑡𝑒𝑚, 1

𝑑2=

4

3 (

ℎ2 + ℎ𝑘 + 𝑘2

𝑎2) +

𝑙2

𝑐2

X-ray diffraction measurements of the films in the present studies were done by copper Kα

(λ=1.5406 A ) radiation was used for recording the diffraction pattern.

2.6.3 Scanning electron microscope (SEM)

The scanning electron microscope is a microscope that uses electrons rather than

light to form an image. There are many advantages in using SEM instead of an optical

microscope [163]. The SEM has a large depth of field, which allows a large amount of the

sample to be in focus at one time. The SEM also produces images of high resolution, which

means that closely spaced features can be examined at a high magnification. Preparation of

43

the samples is relatively easy since most SEMs only require that the sample should be

conductive. The combination of higher magnification, larger depth of focus, greater

resolution and ease of sample observation make the SEM as one of the appropriate

instruments used in current research and development. The electron beam comes from a

filament, made of various types of materials. The most common is the tungsten hairpin gun.

This filament is a loop of tungsten that functions as the cathode. A voltage is applied to the

loop, causing it to heat up.

The anode, which is positive with respect to the filament, forms powerful attractive

forces for electrons. This causes electrons to accelerate toward the anode. The anode is

arranged, as an orifice through which electrons would pass down to the column where the

sample is held. Other examples of filaments are lanthanum hexaboride filaments and field

emission guns. The streams of electrons that are attracted through the anode are made to

pass through a condenser lens and are focused to very fine point on the sample by the

objective lens (Fig. 2.3). The electron beam hits the sample, producing secondary electrons

from the sample. These electrons are collected by a secondary detector or a backscatter

detector, converted to a voltage and amplified. The amplified voltage is applied to the grid

of the cathode ray tube (CRT) that causes the intensity of the spot of light to change. The

image consists of thousands of spots of varying intensity on the face of a CRT that

correspond to the topography of the sample.

Fig. 2.4 shows the interaction of electrons with matter in order to make sure that the

information comes from the sample surface only, when a SEM is used, the column must

always be at a vacuum. Otherwise there are chances for contamination of the sample and

also the electrons beam would induce ionisation in any background gas that would affect

the measurement being made on the sample.

44

Fig. 2.3 The focusing of electrons in SEM

Fig. 2.4 In SEM set up, interaction of electron with the sample produces both

photons and electrons.

2.6.4 Energy dispersive X-ray (EDX) spectra

EDX analysis stands for energy dispersive X-ray analysis. It is sometimes referred to

as EDAX analysis. It is a technique used for identifying the elemental composition of the

specimen, or an area of interest thereof. The EDX analysis system works as an integrated

45

feature of a scanning electron microscope (SEM) and cannot operate on its own without the

latter [164, 165]. During EDX analysis, the specimen is bombarded with an electron beam

inside the scanning electron microscope. The bombarding electrons collide with the

specimen atom’s own electrons, knocking some of them off in the process. A higher-energy

electron from an outer shell eventually occupies a position vacated by an ejected inner shell

electron. To be able to do so, however, the outer electron must give up some of its energy

by emitting X-ray. The amount of energy released by the transferring electron depends on

which shell it is transferring from, as well as which shell it is transferring to. Furthermore,

the atom of every element releases X-rays with unique amounts of energy during the

transferring process. Thus, by measuring the energy of the X-rays emitted by a specimen

during electron beam bombardment, the identity of the atom from which the X-ray was

emitted can be established. The EDX spectrum is a plot of intensity of X-ray Vs energy of

the emitted X-ray. An EDX spectrum normally displays peaks corresponding to the energy

levels for which the most X-rays have been received. Each of these peaks is unique to an

atom, and therefore corresponds to a single element. The higher intensity of a peak in the

spectrum is more concentrated element in the specimen. An EDX spectrum plot not only

identifies the element corresponding to each of its peaks, but the type of X-ray to which it

corresponds as well. For example, a peak corresponding to the amount of energy possessed

by X-rays emitted by an electron in the L-shell going down to the K-shell is identified

as Kα peak. The peak corresponding to X-rays emitted by M-shell electrons going to the

K-shell is identified as a Kβ peak as shown in Fig. 2.5.

46

Fig. 2.5. Electronic transitions are showing the emission of X- rays.

2.6.5 Atomic force microscopy (AFM)

Following the invention of the scanning tunnelling microscope (STM), a number of

new scanning probe microscopes (SPM) has been developed that use the key

components of the STM. One of the most important SPM is the atomic force microscope

(AFM) [166]. In atomic force microscopy a tip is integrated to the end of a spring

cantilever, is brought within the interatomic separations of a surface, such that the atoms of

the tip and the surface are influenced by interatomic potentials. As the tip is restored across

the surface, it bounces up and down with the contours of the surface. By measuring the

displacement of the tip (i.e. the deflection of the cantilever), one can theoretically map out

the surface topography with atomic resolution. The first generated AFM's measured

cantilever deflections using a piggy-backed STM. Later instruments are used for more

practical optical techniques. The AFM is essentially identical in concept to the scanning

profilometer, except that the deflection-sensitivity and resolution are improved by several orders

of magnitude. There are a large number of applications for the AFM, including biological

systems, polymers and a host of insulator and semiconductor materials.

47

Fig. 2.6 The essential elements of an AFM

An AFM images a surface in a manner analogous to the gramophone stylus sensing

the groves of gramophone disk. The essential elements of an AFM are shown in Fig.2.6.

The tip is attached to a cantilever type spring as shown in the Fig.2.7.

Fig. 2.7 Attachment of the tip to the cantilever in an AFM.

48

As the tip and sample interact, forces act on the tip and cause the cantilever (spring)

to deflect. The cantilever position is monitored by a position detector. The output of the

detector is connected to a feedback controller that regulates the force between the sample

and the tip by moving the sample up or down. The sample is moved by a PZT (lead

zirconate titanate) scanning actuator. The cantilever must be soft enough to deflect a

measurable amount without damaging the surface features of the sample. The amount of

deflection is proportional to the force acting on the tip.

𝐹𝑠𝑝𝑟𝑖𝑛𝑔 = −𝑘 . ∆𝑍 (4)

where ‘F’ is the force on the sample, ‘k’ is the spring constant of the cantilever and ‘∆Z’ is the

deflection of the cantilever. Various modes of AFM measurements are contact mode, dynamic

force mode and phase mode. Friction force microscope, magnetic force microscope, surface

potential microscope, etc., are all scanning probes microscopes with slight variation in the working

principles from that described above.

2.6.6 Raman spectroscopy

Raman spectroscopy is a commonly used vibrational, optical and mostly non-

invasive research method, for example used in chemical analysis and in solid state physics.

Amongst others, it has been successfully applied to investigate semiconductor systems, e.g.

in heterostructures and at interfaces. Raman spectroscopy is based upon the inelastic

scattering of monochromatic light within the studied sample, accompanied by the

generation or annihilation of elementary excitations. In this thesis mostly the elementary

excitations of vibrations and the particularly lattice vibrations (phonons) are dealt. Raman

spectroscopy then provides access to the lattice dynamics of a sample and thereby delivers

information on structural properties like chemical composition, orientation or crystalline

quality. In addition, electronic and magnetic properties can be addressed, e.g. by Raman

49

resonance effects and Raman scattering from magnons, respectively. All this information is

gathered by the analysis of the Raman signals with regard to their frequency position,

frequency width, recorded intensity and line shape in the Raman spectra. In a typical

spectrum, the intensity is plotted with the so-called Raman shift. The former is proportional

to the number of photons of a certain frequency reaching the detector, whereas the Raman

shift is given by the frequency difference between the scattered light and the

monochromatic excitation source. This shift corresponds to the energy of the generated or

annihilated elementary excitation. It is usually given in wavenumber 𝜈 = ν/c, where [𝜈] =

cm−1 (1 cm−1∼= 300 GHz ∼= 0.124 meV). Conventionally, if the inelastic light scattering

generates an elementary excitation and the scattered light therefore exhibits a lower

frequency, the Raman shift is denoted positive (stokes Raman scattering). In the case of an

annihilated elementary excitation, the wavenumber value becomes negative (anti-stokes

Raman scattering). In experiments with T ≤ room temperature, usually only a few phonons

are thermally excited and so the stokes process is dominant. The inelastic Raman scattering

processes (Stokes and anti-Stokes) and the elastic Rayleigh scattering are illustrated in an

energy diagram using the concept of virtual electronic states as shown in Fig.2.8.

Fig.2.8 Energy diagram for inelastic (Raman) and elastic (Rayleigh) scattering.

50

While in the case of resonant Raman scattering an actual electronic transition is involved,

the other Raman processes are described by the introduction of virtual electronic states. The

frequency difference between the scattered light and the monochromatic excitation source

is conventionally called Raman shift. Therefore, a Raman shift of zero corresponds to

elastic Rayleigh scattering.

General setup of Raman experiments

The general setup for Raman scattering experiments is shown in Fig.2.9 and it

consists of (i) a monochromatic light source for excitation, usually a laser, (ii) optical

equipment to bring the laser beam on the sample and collect the scattered light, (iii) a

spectrometer to analyse the scattered light and (iv) a detector to collect the signal.

Fig.2.9 Schematic experimental setup for Raman scattering experiments.

Complementary information on the sample may be derived if the exciting and

analysed light is manipulated with optical filters, polarizers, etc. Today, light sources in

Raman scattering setups are usually realized by laser systems. Gas lasers, solid-state lasers,

dye lasers and other laser devices provide a quasi-continuous variety of wavelengths from

the IR to the UV as well as continuous wave (cw) power out-puts from µW to several W.

51

Monochromatic laser light is focused on a sample; the scattered light is collected and

analysed by a spectrometer and a detector, for example a CCD. For magneto-Raman and low

temperature Raman experiments, samples are placed in a sample chamber within a liquid

helium bath or a liquid helium continuous flow cryostat. The choice of the spectrometer

setup can result in a trade-off between sensitivity and resolution. For the detection, multi-

channel detectors are essential and CCD arrays were used.

2.6.7 Optical characterisation

i) Determination of band gap energy

Intrinsic optical absorption of a single photon across the band gap is the dominant

optical absorption process in a semiconductor. When the energy of the incident photon (hν)

is larger than the band gap energy the excitation of electrons from the valence band to the

empty states of the conduction band occurs. The light passing through the material is then

absorbed and the number of electron hole pairs generated depends on the number of incident

photons S0(ν) (per unit area, unit time and unit energy). The frequency ν is related to the

wavelength λ by the relation, λ= c/ν, where c is the velocity of light. The photon flux S(x,

ν) decreases exponentially inside the crystal according to the relation,

𝑆(𝑥, 𝜈) = 𝑆0(𝜈)𝑒−𝛼𝑥 (5)

where, ‘α’ is the absorption coefficient, (α(ν) = 4πkυ/c) is determined by the absorption

process in semiconductors and ‘k’ is the extinction coefficient. For the parabolic band

structure, the relation between the absorption coefficient (α) and the band gap of the material

is given by [167],

𝛼 = 𝐴

ℎ𝜈 (ℎ𝜈 − 𝐸𝑔)𝑟 (6)

where, r = 1/2 for allowed direct transitions, r = 2 for allowed indirect transitions, r = 3 for

forbidden indirect transitions and r = 3/2 for forbidden direct transitions. ‘A’ is the

52

parameter which depends on the transition probability. The absorption coefficient can be

deduced from the absorption or transmission spectra using the relation,

𝐼 = 𝐼0𝑒−𝛼𝑡 (7)

where, ‘I’ is the transmitted intensity, ‘I0’ is the incident intensity of the light and ‘ t’ is the

thickness of the film. In the case of direct transition, (αhν)2 will show a linear

dependence on the photon energy (hν). A plot of (αhν)2 against ‘hν’ will be a straight line

and the intercept on energy axis at (αhν)2 equal to zero will give the band gap energy.

ii) Photoluminescence

Luminescence in solids is the phenomenon in which electronic states of solids

are excited by some energy from an external source and the excited states release energy as

light. When energy comes from short-wavelength light, usually UV, the phenomenon is

called photoluminescence (PL) [168]. PL is divided into two major types: Intrinsic and

extrinsic depending on the nature of electronic transition producing it.

a) Intrinsic luminescence

There are three kinds of intrinsic:

1) Band-to-band luminescence

2) Exciton luminescence

3) Cross luminescence.

Band–to–band luminescence:

Luminescence owing to the band-to-band transition occur due to the recombination of

an electron in the conduction band with a hole in the valance band, can be seen in pure

crystal at relatively high temperature. This has been observed in Si, Ge and III(B)-V(B)

compounds such as GaAs.

53

Exciton luminescence:

An exciton is a composite particle where an excited electron and a hole interact with

one another. It moves in a crystal conveying (creates) energy and produces luminescence

owing to the recombination of the electron and the hole. There are two kinds of excitons:

Wannier exciton and Frenkel exciton. The Wannier exciton model express an exciton

composed of an electron in the conduction band and a hole in the valence band bound

together by coulomb interaction. The expanse of the wave function of the electron and hole

in Wannier exciton is much larger than the lattice constant. The excitons in III(B)-V(B) and

II(B)-VI(B) compounds are examples of Wannier exciton. The Frenkel exciton model is used

in cases where expanse of electron and hole wave function is smaller than lattice constant.

The excitons in organic molecular crystals are examples of Frenkel exciton.

Cross luminescence

Cross luminescence is produced by the recombination of an electron in the valance

band with a hole created in the outer most core band. This is observed in number of alkali

and alkaline-earth halides and double halides. This takes place only when the energy

difference between the top of valance band and that of conduction band is smaller than the

band gap energy. This type of luminescence was first observed in BaF2.

b) Extrinsic luminescence

Luminescence caused by intentionally incorporated impurities, mostly metallic

impurities or defects is known as extrinsic luminescence. Most of the observed type of

luminescence of practical application belongs to this category. Intentionally incorporated

impurities are activators and materials made luminescent in this way are called phosphors.

Extrinsic luminescence in ionic crystals and semiconductors is classified into two types:

unlocalized and localized

54

Unlocalized type:

. The electrons and holes of the host lattice participate in the luminescence process of

unlocalized type. In a semiconductor, donors and acceptors act as luminescence activators.

There are two types of luminescence transitions i.e. the transition of a free carrier to a bound

carrier and the transition of a bound electron at a donor to a bound hole at an acceptor.

These kinds of luminescence lines and bands are usually observed in compound

semiconductors such as III(B)-V(B) and II(B)-VI(B) compounds.

Localised type:

In localized type the luminescence excitation and emission process are confined in a

localized luminescence centre. Various kinds of metallic impurities intentionally

incorporated in ionic crystals and semiconductors create efficient localized luminescence

centres. Localized type centres are classified into: (a) allowed transition type (b) forbidden

transition type in terms of electric dipole transitions. The electric dipole transition can take

place between energy levels with different parities. The selection rule for this transition in

atoms is ∆l= ±1. When atoms and ions are incorporated in crystals, the forbidden character

of the dipole transition is altered by the perturbation of the crystal electric field, so that the

forbidden transition becomes allowed to some degree. Many phosphors that are important

from a practical point o f v i e w are synthesized by incorporating the following activators.

a) Allowed transition type:

(i) s↔p transition–F centre↔( an electron trapped at an anion vacancy)

(ii) s2↔sp transition Tl+, Sn2+, Pb2+, Sb3+, Bi3+

(iii) f↔d transition Eu2+, Ce3+

55

b) Forbidden transition type:

(i) d↔d transition Ti3+, Cr3+, Cr2+, Mn4+, Mn2+, Fe3+, Fe2+

(ii) f↔f transition Pr3+, Nd3+, Sm3+, Eu3+, Tb3+, Tm3+ ( and other

trivalent rare earth ions)

Iron group ions show luminescence owing to the 3dn ↔ 3dn (n = 2 – 8) transition

from visible to infrared region. Among them, Cr3+ and Mn4+ ions with the 3d3

configurations and the Mn2+ ion with the 3d5 configurations are important for phosphor

application. The luminescence spectra of Mn2+ are always bands, which vary over a wide

visible range from blue to red [169]. Two types of luminescence spectra can be

distinguished: excitation and emission. In the case of an excitation spectrum the wavelength

of the exciting light is varied and the intensity of the emitted light at a fixed emission

wavelength is measured as a function of the excitation wavelength. The excitation spectrum

gives information on the position of excited states just as the absorption spectrum does,

except that the former reveals only the absorption bands that result in the emission of light.

The observed differences between the absorption and excitation spectra can yield useful

information.

An emission spectrum provides information on the spectral distribution of the light

emitted by a sample. The time resolved PL measurements are a powerful tool for the

determination of the radiative efficiency. The radiative efficiency specifies the fraction of

excited states, which de-excite by emitting photons. The emission and excitation spectra for

the powder and thin film samples are recorded using Fluorolog–3 spectroflurometer

consisting of 450W Xenon arc monochromatic lam and a CCD detector.

56

2.6.8 Electrical characterisation

i) Resistivity by two probe method

The resistivity of the films is determined by the two-probe method where the

electrodes are in planar geometry. Evaporated indium layers or high conducting silver paste

was used as the electrodes. The resistivity (ρ) of the films is calculated applying ohm’s law,

using the relation ρ = RA/L. Here ‘ R’ is the resistance given by the slope of the

current – voltage characteristic curves, ‘ A’ is the area of the film in t h e planar

geometry which is given by the product of the film thickness and the width of the film, ‘L’

is the spacing between the electrodes.

ii) Hall measurement

The electrical resistivity of a semiconductor thin film can be written using Ohm’s

law,

𝜌 = 1

𝑒𝑛𝜇 (8)

where ‘ρ’ is the film resistivity, ‘e’ is the electronic charge, ‘n’ is the number of carriers

corresponding to the carrier concentration and ‘µ’ is the carrier mobility. According to

Ohm’s law, the carrier mobility affects resistivity. Low resistivity can be achieved by

increasing the carrier concentration, mobility or both. Increasing carrier concentration is self-

limiting because at some point the increased number of free carriers decreases the mobility

of the film due to carrier scattering. Hence there is a trade- off between the carrier

density and carrier mobility for achieving low resistivity. In the case of zinc oxide films, the

resistivity (ρ), carrier concentration (n), carrier mobility (µ) and sheet resistance were

measured using four point probe in the Van der Pauw configuration [170]. Samples used

were 1cm x 1cm in size. The ohmic contacts were made using silver paste. The silver paste

was applied at the corners of the sample symmetrically as shown in Fig.2.10.

57

C D

A . . B

. .

Fig.2.10 Symmetric connections for taking the Hall measurements

Let ‘R1’ be the potential difference between ‘A’ and ‘B’ per unit current through ‘C’ and

‘D’ or vice versa. Similarly ‘R2’ the potential difference between ‘B’ and ‘C’ per unit current

through ‘D’ and ‘A’. Then sheet resistance ‘Rs’ can be calculated using the relation,

𝑅𝑠 = (𝜋

𝑙𝑛2) (

𝑅1+𝑅2

2) 𝑓 (

𝑅1

𝑅2) (9)

Where 𝑓(𝑅1 𝑅2⁄ ) is the Van der Pauw function and is given by the relation,

𝑓 (𝑅1

𝑅2) = 1 − 0.3466 (

𝑅1−𝑅2

𝑅1+𝑅2)

2 (10)

The Hall signal was measured between two ends while passing the current through the

other two ends. Hall mobility is,

𝜇 = ∆𝑅 × 108

𝐵𝑅𝑠 (11)

where ‘∆R’ is the change in resistance due to magnetic field (B), which was applied to

measure Hall voltage. Hall coefficient RH is given by,

𝑅𝐻 = 𝜇𝜌 (12)

The type of carriers can be understood from the sign of the RH. The negative values

of RH correspond to the electrons (n-type) and positive values to holes (p-type) which are

responsible for conduction.

58

iii) Nature of conductivity – Carrier type

Thermo power measurement is a useful technique for the evaluation of the type of

carriers responsible for the conduction. Thermo power measurements were carried out using

an automated set up. The set-up is similar to the one designed by Young et al [171]. The

slope of the thermo emf versus ∆T curve gives the Seebeck coefficient. A positive Seebeck

coefficient is obtained when the majority carriers are holes and a negative Seebeck

coefficient is obtained when the majority carriers are electrons. This technique is especially

useful for characterisation of type of carriers in semiconductor materials with low

mobility for charge carriers.