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Introduction

Chapter - IIntroduction

1.1 Semiconductors and Metal oxidesThe last 60 years or so have been shaped by the silicon age. The expression information

age is a misnomer really, as it would not have materialized without silicon and other

semiconducting materials. The first transistor device, or crystal triode, was invented in

the Bell labs in 1947. Unbeknown to many, it was built using Germanium, a

semiconductor.

The semiconductor industry is still a main driver of today’s innovations. A life

without semiconductors is unimaginable today: no information highway, no cell

phones, no personal computers and no satellites, to name a few. Besides silicon, which

has been the ultimate material for being a semiconductor for the last 60 years, scientists

have turned their efforts towards other materials to fulfill the needs of various

applications. It is a little known fact that the semiconducting properties of zinc oxide

(ZnO) were already discovered in the 1920’s. Based on this fact, they had put effort to

study the semiconducting properties of various metal oxides.

Comparing with the simple semiconductors, the electrical properties of oxides

show certain characteristic features. One is the metal – insulator transition, in which at a

certain temperature or pressure, an insulator turns into metal. The word transition is

usually used even when this change occurs at a certain composition. This phenomenon

has attracted much attention and was the most popular research theme before high-

temperature superconductivity exploded onto the scene. The mechanism of the

transition is not simple and the phenomenon itself seems to become more and more

complex as research progresses.

Another characteristic feature of oxide conductivity is a temperature dependance

of resistivity that is stronger than T4. This is mostly due to scattering by optical phonons

but for certain transition element oxides, such as cuprates, electron-electron scattering is

not negligible. In transition metal oxides, the conduction bands are mostly formed with

oxygen 2p and metal d-orbitals. The s-electrons enter into deeper bonding orbitals. The

oxygen orbitals are usually located lower in energy, which leads to the observed

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ionicity. At the same time the directionality of the p and d – orbitals and the strong

coulomb interaction in the d – orbital cloud manifest more directly in the transport

phenomena, whereas in the simple metals, s – orbitals are the main constituent of the

conduction band. Apart from these, many features are there when oxides are considered

than the elemental semiconductors. They are,

➢ The ionicity is large and even infrared absorption is observed

➢ Distribution of conduction electron of the oxides occupy only part of the

material and sometimes the average density has no meaning

➢ Carrier density covers wide range and in some cases it may be much smaller

than the metals

➢ In oxides the main impurity is not other elements but an excess or insufficiency

in the constituent element itself which occurs as a deviation from stoichiometric

composition

There are many good conductors among oxides. Phenomenologically they are

classified into the following groups [1].

➢ Metallic conductors in which a deviation from the stoichiometric composition is

essential to have metallic conductivity. For example SnO2, In2O3, CdIn2O4,

Zn2SnO4, Cd2SnO4, etc.

➢ Normal metallic conductors which show metallic conductivity in their

stoichiometric compositions. Examples for these types are ReO3, TiO, SrCrO3,

RuO2, OsO2, MoO2, WO2, etc.

1.2 Multi-component Metal oxide semiconductorsMulti-component metal (multication) oxide semiconductors are of interest for many

device application and fundamental research. They have high luminous transmittance,

wide bandgap, good electrical conductivity, excellent substrate adherence, hardness and

chemical inertness. Therefore they are frequently used in variety of opto-electronic

devices. To have favorable characteristics of a wide-gap semiconductors, two necessary

conditions are to be satisfied. The first is to restore high mobility of electrons in the

conduction band. Most of the electrical insulators are truly insulators because of their

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low mobility in addition to wide band gap. However, in the case of oxides the

conductivity can be drastically enhanced by doping. The second requirements relates to

the possibility of doping. In the case of n-type oxides, electron carriers are in principle

introduced by substitution of Mi+ ions with M(i+1)+ ions [2, 3] and of O2- oxygen vacancy

or interstitial positively chargeable atoms [4]. However, there is no established science

that enables one to predict which technique is applicable to a specified material,

therefore the prediction are based only on the condition - high mobility.

For having high mobility in n-type wide gap semiconductor, one of the cation

must belongs to p-block with ns0 electronic configuration [eg, Zn2+, Hg2+, Ga3+, In3+,

Ti3+, Ge4+, Sn4+, Pb4+, Sb5+, Bi5+ and Te6+]. In this case, bottom edge of the conduction

band is composed mainly of ns atomic orbital of the M-cation. In addition to this, a

linear chain of edge-sharing octahedral (rutile chain) is preferred in which the p block

cations occupy a central position. This is the case of the oxides having high mobility.

There is no intervening oxygen between the two neighboring M cation in the rutile

chain. Consequently, the M-M distance becomes short compared with that in the M-O-

M linear structure frequently seen in a vertex sharing structure. Direct overlap between

ns atomic orbital of the neighboring M cations is possible for the heavy p-block cations.

This feature as well as covalency in M-O bonds gives rise to large dispersion of the

conduction band, which is appropriate for high mobility of the electron carrier.

Normally in multi-cation oxides, mobility decreases with increasing the second

cation proportion. The lowering of mobility has been explained in terms of the

scattering of carrier electrons by the substituted ions in the lattice [5]. Indium oxide

possesses a single type of cation site in the lattice and that the drop in the mobility by

the doping may be avoidable or reducible, if in the crystalline lattice, the doping site

and conduction path of carrier electrons are separated spatially by selecting the crystal

structure having two different cation sites.

1.2.1 Importance of the Multi-component metal oxide semiconductors

1.2.1.1 Bandgap tailoring due to multi-cationBand gap tailoring refers to the ability to tune the band structure of a semiconductor

which is useful for tailoring the performance of optoelectronic devices. Band gap

tailoring of semiconductors can be typically achieved by:

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➢ alloying two or more semiconductors

➢ use of heterostructures to cause quantum confinement or formation of

superlattices

For example, the bandgap of ZnMgO can be tuned from 3.3 eV to 7.8 eV by

tuning the Mg content from 0 – 100% [6].

1.2.1.2 Self doping of cations:In particular cases, the second cation itself replaces the site of the first cation. For

example, the inverse spinel exists with the general formula of AB2O4, where the

tetrahedral voids are occupied by B atoms and the octahedral voids are occupied

randomly by an equal number of A and B atoms. This causes the tuning of carrier

density.

1.3 Amorphous oxide semiconductorsAmorphous oxide semiconductors (AOS), have captured manufacturers interest as both

a replacement for hydrogenated amorphous silicon (a-Si:H) in flat screen displays and

for high-volume reel-to-reel processing applications involving flexible substrates. AOSs

consist of the combinations of several optically-transparent transition and post-

transition metal oxides with (n - 1)d10 ns0 (where n ≥ 4) cation electronic configurations.

These oxides derive their conduction bands from the cation's ns orbital, which is

spherically-symmetric with a large orbital radius. High electron mobilities with low

sensitivity to crystalline disorder are found in these AOS materials as a result of the

significant overlap between these large-radius ns orbitals. The most common AOS

materials are synthesized from Ga2O3, In2O3, SnO2, and ZnO into multicomponent

oxides such as indium gallium oxide (IGO), indium gallium zinc oxide (IGZO), zinc

indium oxide (ZIO), and zinc tin oxide (ZTO). Searching of new materials of AOS are

still under research. Compared to binary oxides, the multicomponent metal oxides

largely remain amorphous, and thus atomically smooth, over a wider range of

processing conditions and allow for more control over electrical properties via changes

in film stoichiometry.

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1.4 Importance of thin filmsThe positions of the surface ions of thin films may or may not be the same as those

defined by simple extension of the bulk structure, depending on whether the free

surface reconstructs or not. The driving force for reconstruction is to lower the surface

Gibbs energy per unit surface area to attain a thermodynamically more stable system.

However, metastable surface structure can exist if the energy barrier for reconstruction

is too high. Thus thin film plays a main role in the materials science. A great advantage

with a thin film device is the small amount of material used and the compact volume of

the device. Thin film science and technology play a crucial role in microelectronics,

communications, optoelectronics, integrated optics, photovoltaic devices and

waveguides.

There are three steps in formation of a deposit:

a. Synthesis or creation of the depositing species

b. Transport from source to substrate

c. Deposition onto the substrate and film growth

These steps can be completely separated from each other or be superimposed on

each other depending upon the process under consideration. The important point to note

is that if, in a given process, these steps can be individually varied and controlled, there

is much greater flexibility for such a process as compared to one where they are not

separately variable. This is analogous to the degrees of freedom in Gibbs phase rule.

1.5 Physical dimension of thin filmsThe physical dimension of thickness was used to make the distinction between thick

films and thin films. Unfortunately, the critical thickness value depends on the

application and discipline. In recent years, a Confucian solution has been advanced. It

states that if a coating is used for surface properties such as electron emission, catalytic

activity, it is a thin film; whereas, if it is used for bulk properties, corrosion resistance,

etc., it is a thick film. Thus, the same coating material of identical thickness can be a

thin film or a thick film depending upon the usage [7]. This represents a reasonable way

out of the semantic problem.

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1.6 Transparent Conducting Oxides (TCOs)

1.6.1 Background of TCOThe technological uses of wide bandgap conducting oxides have come to the forefront

in the past decade because of an increasing need for photonic devices, high power, high

frequency electronic devices and also transparent conducting devices because of their

breakthroughs in growth techniques for these materials. While many research efforts

have focused on doped ZnO, In2O3:Sn and doped SnO2 or which have some defects in

their atomic sites. The materials which have the properties of the above said oxides

make up a small class of specialized materials called Transparent Conducting Oxides

(TCOs) and the search is not only limited to common TCO materials and mixtures

thereof [8]. Bӓdeker [9] prepared the first transparent conducting oxide in 1907 by

thermal oxidation of glow discharge sputtered Cd films. Since then, TCOs have

received a great attention because of their various technological applications. The most

widely used TCO in optoelectronic devices is ITO. Other TCOs are also available and

find use in specialized applications where ease of deposition, cost, or IR reflectivity is

favored over optimum optical transmission and minimum sheet resistance. Recent work

has begun to explore new binary-oxide combinations and even to move into phase

fields of ternaries. Cd2SnO4, Zn2SnO4, MgIn2O4, CdSb2O6:Y, ZnSnO3, GaInO3, GdInOx,

Zn2In2O5, and In4Sn3O12 are just a few of the ternary n-type materials under

investigation [101112-131415]. The efficiency and performance of these devices depend on the

electrical and optical properties of the TCO material used in their construction.

1.6.2 Properties of TCOAs mentioned above, transparent conducting oxides are materials possesses high optical

transparency and high electrical conductivity. The transparency of the TCOs is derived

from a large band gap (Eg > 3 eV), which prevents absorption of visible wavelengths,

and a lack of d-d transitions in the metal cations which could act as color centers. The

d-d transitions cannot occur if the d orbitals of the metal cation are full, and therefore

many TCOs incorporate this type of cations [16]. This yields a transmittance in the

visible often greater than 85% (T>85%). They are basically doped semiconductors

categorized most often by their carrier type (n-type or p-type) or by the mechanism of

charge transport (free carrier or bound). Each classification of carrier type or

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conduction mechanism exhibits complementary electrical and optical properties.

Enhancing the properties of TCOs, specifically the conductivity, can be done

either by increasing the charge carrier concentration (doping levels) or improving the

mobility of those carriers [17], since conductivity is given as σ = μeN ,where μ is the

mobility of the carriers in a TCO , e is electronic charge and N is the carrier

concentration. Increasing the carrier concentration can be achieved by heavily doping

the TCO materials. For example, in ZnO, Zinc atom contributes two electrons to the

formation of the bond with oxygen. When ZnO is aluminum doped, aluminum replaces

some of the Zinc. But when oxidized aluminum provides three electrons, one more than

Zinc, this electron would disrupt the electrostatic distribution of the lattice; this electron

is not free to appreciably move about the lattice. It is unstable and can be easily

elevated to the conduction band with the external application of energy. In the case of,

the most commonly used TCOs SnO2 and In2O3, the materials are doped with Fluorine

and Tin respectively. Fluorine doping gives superior performance compared with

metallic dopants in TCOs. A theoretical understanding of this advantage of fluorine can

be obtained by considering that the conduction band of oxide semiconductors is derived

mainly from metal orbitals. If a metal dopant is used, it is electrically active when it

substitutes for the primary metal (such as zinc or tin). The conduction band thus

receives a strong perturbation from each metal dopant, the scattering of conduction

electrons is enhanced, and the mobility and conductivity are decreased. In contrast,

when fluorine substitutes for oxygen, the electronic perturbation are largely confined to

the filled valence band and the scattering of conduction electrons is minimized [18].

This will, however, degrade the transparency due to increased free carrier absorption

and lower the carrier mobility due to an increase in ionized impurity scattering and

neutral impurity scattering.

Additionally, introduction of point defects or oxygen vacancies in the lattice of

the oxides can increase the conductivity of a TCO. In an oxide, oxygen vacancies

produce donor sites. This is done by the reduction of the cation to which the oxygen

atoms were bound, thus creating a dangling bond - an electron pair. These electrons,

upon receipt of a specific amount of energy (the activation energy for the defect) can

enter the conduction band. TCOs can have oxygen vacancies introduced in two ways.

The first is by synthesizing the material with a deficiency of O2 as one of the reactants

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or having a reducing ambient while synthesizing. Alternatively, the compound can be

subjected to a reducing post synthetic ambient, such as H2 often at elevated

temperatures [19].

Increasing the mobility of charge carriers in TCOs will allow the conductivity to

increase without compromising the transparency, thereby enhancing the overall

performance of the TCO material. In order to obtain films with high conductivity, high

carrier concentration and mobility should be simultaneously realized. The electrical

properties of the oxides depend critically upon the oxidation state of the metal

component, stoichiometry of the oxide and also on the nature and quantity of impurities

incorporated in the films, either intentionally or unintentionally.

Seeing the electrical and optical properties of TCOs with the band structures of the

materials [20], A good performing n-type TCO must simultaneously satisfy two

requirements: (i) large bandgap as well as large energy separation between the

conduction band minimum (CBM) and the second conduction band (SCB), for

transparency; (ii) a low CBM with respect to the vacuum level, for high dopability, as

well as a small effective mass, for good conductivity. As a consequence of the second

condition, a low valance band maximum (VBM), which is a common characteristic of

oxides, is also required to meet the first condition. Taking theses criteria into account,

the recent growing demand has led to an extensive search for new TCOs with high

performance and low cost materials with higher transparency and conductivity [21, 22].

Among many binary and ternary oxides, Cd2SnO4 and Zn2SnO4 have emerged as

promising TCO's. Cd2SnO4 have a high carrier mobility [23] compared to most

conventional TCO's. Zn2SnO4 has better optical transparency than Cd2SnO4, although its

electrical conductivity is lower [23] than that of Cd2SnO4. Hence, the Cadmium

Stannate (Cd2SnO4) and also the Zinc Stannate (Zn2SnO4) have been chosen for the

current research.

1.6.3 Applications of TCOTransparent conducting oxides have largest technological applications such as solar

cells, optoelectronics, low-emissivity windows, gas sensors, wear resistant applications,

flat panel displays, touch screens, IR reflectors, solar cells and optical sensor and thin

film resistors [24]. A multitude of applications exist in which transparent conductors are

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involved in active or passive roles. Some important applications of transparent

conductors are listed below.

1.6.3.1 Solar CellsWidespread utilization of solar energy required the production of durable, low-cost and

optically efficient solar selective coatings. Transparent conducting coatings are

preferred due to their application in a wide variety of fields of interest in solar energy

conversion. During the past few years, heterojunction solar cells consisting of wide

band gap semiconductor mated to a much narrower band gap semiconductor have

gained considerable prominence. The performance of these devices is strongly

controlled by the presence of a thin interfacial insulation layer. Heterojunctions with

this interfacial layer constitute SIS solar cells.

1.6.3.2 Transparent electrode for heterojunctionThin films of In2O3 (IO) and In2O3:Sn (ITO) have been extensively used to provide a

transparent conducting base for back wall heterojunction solar cells yielding

efficiencies of about 6-8%.

1.6.3.3 Anti reflection coatings for solar cellsSince SnO2(TO) layers reduces the surface recombination velocity, the short circuit

current increases in a solar cells. Also due to their refractive indices in the range 1.8-

2.0, this makes them useful as anti-reflection coatings on silicon solar cells.

1.6.3.4 Heat mirror

Transparent conducting oxide films with plasma edge at about 1.5 µm are well suited

for keeping heat radiation confined in a closed space.

1.6.3.5 Reflector-absorber tandem for photothermal conversionA coating of a transparent conductor on any semiconductor absorber surface will yield a

selective absorber tandem. These materials can also be used as anti-reflection coatings

in photothermal conversions.

1.6.3.6 Opto-electronic devicesConducting oxides have been extensively used as transparent electrodes in various

display devices, which includes electrochromic displays, light emitting diodes and

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liquid crystal displays. Also TCO films have been used as transparent gates on charge

injection devices (CIDs) and charge-coupled devices (CCDs).

1.6.3.7 Thin film resistorsTCO materials exhibit low temperature co-efficient of resistivity (TCR) and high

degree of stability. As a consequence of these attractive properties, highly stable TO

resistors are being manufactured commercially.

1.6.3.8 Gas sensorsOxides thin films have shown promising results in the detection of gases such as CO,

CO2, H2, H2S, alcohols and hydrocarbons. Usually a change in electrical conductivity

occurs when a change in the ambient occurs and this property forms the basis of gas

sensing. The change in conducting can be either due to direct transfer of electrons from

the absorbed gas to the oxide semiconductor or due to a reaction of the absorbed gas

with previously chemisorbed surface oxygen.

1.6.3.9 Wear resistance applicationsIn order to reduce damaging of glass containers during manufacturing or filling, it is

desirable to apply a low friction wear resistant coating to the glass surface.

They are applied as thin films using various deposition techniques such as spray

pyrolysis, sputter deposition, chemical vapor deposition, molecular beam epitaxy, and

laser ablation [2526-272829]. For optoelectronic applications, the transparent conductor must

be carefully processed to maximize optical transmissivity in the visible regime, while

achieving minimum electrical resistivity. Optimization of these properties will depend

on the application, but in general, achieving the required performance in the as-

deposited condition requires careful process control [19].

1.7 Properties of Cd2SnO4: ReviewDicadmium Stannate (Cd2SnO4 or CTO) despite having somewhat lower carrier

concentration than the conventional TCO's such as ITO and SnO2, is actually a better

conductor due to its higher carrier mobility [23, 30]. This results in overall improved

performance. Cd2SnO4 is also a more robust material and can survive in process

environments where SnO2-based materials have difficulties. It is, in fact, more readily

etchable and generally smoother than SnO2 films. Cd2SnO4 is a TCO with exceptional

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electrical and optical properties. This particularly illustrates the complex nature of the

electro-optical properties in these materials and their potential for improvement.

1.7.1 Crystal structureThe binary oxide CTO have the general chemical formula AB2O4 and usually exist in

either the orthorhombic structure or the cubic spinel structure. Many experimental and

theoretical studies have been carried out to understand the structural, electrical and

optical properties of these binary oxides [22, 23, 31]. Generally, the CTO is more stable

in the spinel structure [31, 32]. But, CTO has been observed in two crystallographic

forms which have orthorhombic and cubic symmetry, respectively [33]. Relatively

orthorhombic Cd2SnO4 can be easily prepared by heating the mixtures of CdO and SnO2

powders at some elevated temperatures [34]. The crystal structure of the orthorhombic

Cd2SnO4 is shown in Fig. 1.1.

In the orthorhombic structure with space group Pbam, the Sn atom is in a

slightly distorted octahedral (2a) site with six (4+2) Sn-O bonds, whereas the Cd atom

is in a lower symmetry (4h) site and can form either four, six, or eight Cd-O bonds. For

the orthorhombic structure, three external and six internal structural parameters

determine the atomic positions [20]. Even though the bulk CTO normally crystallizes in

the orthorhombic crystal structure, the CTO obtained from sputtered thin films and

from a high temperature, high pressure procedure has now been shown, on the basis of

X-ray diffraction investigation, to crystallize in the cubic system [35]. The crystal

structure of the cubic spinel structure is shown in Fig. 1.2, where the A atom is Sn and

B atom is Cd.

In the normal spinel structure with space group Fd 3 m , one-eighth of the

tetrahedral voids in a face-center-cubic (fcc) close-packed oxygen sublattice are

occupied by A atoms and one-half of the octahedral voids are occupied by B atoms.

There exists also an inverse spinel structure, where the tetrahedral voids are occupied

by B atoms and the octahedral voids are occupied randomly by an equal number of A

and B atoms [31].

The normal spinel structure is determined by two parameters: the lattice

constant a and the anion displacement u. The bond length between A atom at the center

of a AO4 tetrahedron and its four nearest-neighbor oxygen atoms is given by [31]

Rtetra=3u−0.25a ...........................................................................................(1.1)

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whereas the bond length between the B atoms at the corner of the octahedron and their

six nearest oxygen atom is

Rocta=u−0.625 22 u−0.3752 a ..................................................................(1.2)

For the Inverse spinel, which has the same lattice parameters as normal spinel,

the effective u parameter is obtained using the averaged tetrahedral and octahedral bond

lengths and the formula given above.

1.7.2 Band structureA clear understanding of the band structure is of critical importance to explain the

electrical properties of Cd2SnO4. In the determination of the band structure of a

semiconductor, both experimental method and theoretical modeling is followed by

Segev et al. [20]. The band structure calculations are performed using the local density

of approximation (LDA) [36, 37], with the Caperley-Alder exchange correlation

potential [36], as parametrized by Perdew and Zunger [37]. The calculated results of

Segev et al. [20] was in good agreement with the all-electron full-potential linearized

augmented plane-wave (FLAPW) calculations, as well as with the available

experimental data [22, 31, 38, 39].

Figure 1.3(a) shows the band structure of Cd2SnO4 along the L-Γ-X line for the

normal and inverse spinel structures and along the X-Γ-Z line for the orthorhombic

structure. Figure 1.3(b) shows the atom- and angular-momentum resolved local density

of states (LDOS) of Cd2SnO4 in the normal spinel structure. Only the dominent

components of the LDOS are shown. The inverse and orthorhombic structures show

very similar features at the vicinity of the band gap edges and are not shown here.

At the Γ point, the valence band maximum (VBM) state has the Γ12v

representation and consists mostly of O p and cation d states of the octahedral site, with

some d character from the cation at the tetrahedral site. The Conduction band minimum

(CBM) has the Γ1c representation with predominantly O s and cation s states of the

tetrahedral site, as well as some s character from the cation at the octahedral site. A

minority of O p and Cd d characters can also be observed for this state. The second

conduction band (SCB) has the Γ2'c representation and consists of O s and p, of cation s

states of the tetrahedral site only, as well as of some p state from the cation at the

octahedral site. With this analysis, it can be explained that the change in the band gap

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and the splitting between the first two conduction bands, E12 = ESCB – ECBM, as a function

of the crystal structure.

The atomic size of Cd is much larger than that of the Sn. When Cd moves to the

tetrahedral site in the Cd2SnO4 inverse spinel structure, the tetrahedral bond length

increases, whereas the octahedral bond length decreases, as reflected by the different u

parameters observed for the normal and inverse spinel structures. Consequently, this

lowers the energy of the CBM state, which is an antibonding state centered mostly on

the tetrahedral site. Thus the strain induced deformation effect partially cancels the

chemical effect in the Cd2SnO4 and the volume of the Cd2SnO4 is small which explains

the smaller band gap variation in Cd2SnO4. But, the energy of the SCB increases more

than that of the CBM state because the former contains only Cd 4s orbital but no Sn 5s

orbital. This explains why the E12 is larger in the inverse spinel than in the normal spinel

structure.

It is evident from the figure that the orthorhombic Cn2SnO4 has an even larger

band gap than the inverse spinel structure. However the E12 energy separation is smaller

than that in the inverse spinel structure. Due to the smaller volume of the orthorhombic

structure compared to that of the spinel structure, the CBM in the orthorhombic

structure has a higher energy [31]. Furthermore, the p-d coupling is reduced at the

VBM, which is due to the lower symmetry of the orthorhombic structure. This explains

the larger band gap of the orthorhombic structure compared to the spinel structure.

Moreover, in contrast to the spinel structure, the SCB state has a mixed Cd 4s and Sn 5s

character in the orthorhombic structure. Therefore the energy separation E12 in the

orthorhombic structure is in between that of the normal and the inverse spinel structure.

1.7.3 Properties and applications of Cd2SnO4 filmsEven though the bulk Cd2SnO4 normally crystallizes in the orthorhombic crystal

structure, the Cd2SnO4 films obtained from sputtering and from a high temperature,

high pressure procedure has now been shown to crystallize in the cubic system [35].

Such phenomenon makes the material a unique compound and not a mere mixture of

CdO and SnO2. Cd2SnO4 being a defect semiconductor attracts many coworkers [30,

40] because of their excellent optical and electrical properties not the structural

properties of the films, since the as prepared films are mostly amorphous in nature or

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amorphous-like [41] state in nature. The amorphous films are crystallized after the post

heat treatment of the films, temperature ranging from room temperature to 700oC [23,

42-4344] in the Argon atmosphere or CdS atmosphere.

Cd2SnO4 is now the subject of numerous investigations as alternative material to

Indium Tin Oxide (ITO) for photovoltaic device structures [45]. Cd2SnO4 films have

been used in the CdTe/CdS solar cells because of their outstanding optical and electrical

properties which yields highly efficient solar cells [46, 47]. Cd2SnO4 films are n-type

defect semicondutor material exhibiting promising properties such as low metal-like

electrical resistivity (10-4 Ωcm) [42] and high transmission (> 90%) [46] in the visible

range of the light spectrum and high reflectivity in the IR range. Another importance of

the Cd2SnO4 films is the carrier mobility of the films even at a higher carrier

concentration. Electron mobility as high as 65 cm2 V-1s-1 at a carrier concentration of

2X1020 cm-3 have been achieved [46]. Possible applications of Cd2SnO4 films as a

perspective electrode material for various electrochemical applications, including bio

catalytic redox transformations were recently suggested [48-4950].

1.7.4 Some literature on Cd2SnO4 filmsThe properties of cadmium stannate films were first reported by Nozik [30], who

prepared amorphous films by RF sputtering and reported mobilities as high as

100 cm2 V-1 s-1 at a carrier concentration of 5×1018 cm-3. Nozik attributed this unusually

high mobility to a low carrier effective mass (m* = 0.04 me). Haacke [51, 52]

investigated the effect of deposition and annealing parameters on the polycrystalline

Cd2SnO4 films prepared by RF sputtering. Miyata et al. [41, 53, 54] studied the effect of

thickness and the number of charge carriers on the electrical conductivity of the

Cd2SnO4 films. They reported the resistivity of the CTO films as 4×10-4 Ωcm in the film

thickness range of 2500-15000 Å, and the average transmission was 90% over the

visible region. The lowest band gap of the Cd2SnO4 was determined by Koffyberg et al.

[55]. They have determined the band gap of the Cd2SnO4 by the photoelectrolysis of

water with the Cd2SnO4 films as the anode and a platinum as the cathode arranged in an

electrochemical cell. They have reported the minimum band gap of Cd2SnO4 as 2.12 eV

which is attributed to the indirect transition and the band gap of interband transition as

2.79 eV. Howson et al. [56] calculated the carrier effective mass of Cd2SnO4 films by

applying the thin film interference theory and sheet resistance measurements. They

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have compared the results of Cd2SnO4 with the results of ITO, and believed that the

Cd2SnO4 films are best over ITO. Carl. M. Lampert [57] studied the properties of the

films to use Cd2SnO4 films as heat mirror coatings for energy conserving windows.

Armando Ortiz [58] studied the surface morphology of the Cd2SnO4 and showed that

the films are more uniform in their topography. Stapinski [59, 60] suggest the existence

of oxygen vacancies, interstitial cadmium ions and neutral CdO as the defects

influencing electrical properties of the film. Shetty et al. [61] observed firstly the

Burstein effect in the Cd2SnO4 films which are heavily doped. Apart from these specific

results, many coworkers [46, 62, 63] just studied only the electrical and optical

properties of Cd2SnO4. Even though several literatures dealing with the electrical and

optical properties, there is no any reports on the theoretical phenomena behind the

electrical and optical properties of the Cd2SnO4 films upto the year of 1999. After 1999,

Many research coworkers [23] from national renewable laboratory, Golden, Colorodo

and Sun Power corporation, California, started to study the physical phenomena behind

the electrical and optical properties of the Cd2SnO4 films. Mason et al. in 2002 [64, 65]

studied the defect chemistry and the physical properties of the Cd2SnO4 films. Shang et

al. [66] reported the defect formation energy in the Cd2SnO4 films. Mamazza et al. in

2005 [42] studied the effect of Cd Sn ratio on the conductivity of the Cd2SnO4 films.

Apart from the optoelectronic properties, Dou et al. [67] studied the surface

composition analysis of the bulk Cd2SnO4 with XPS, UPS and EELS studies. Possible

applications of Cd2SnO4 films as a perspective electrode material for various

electrochemical applications, including bio catalytic redox transformations were

recently suggested [48-4950] after the electrochemical studies of Cd2SnO4 films by

Valincius et al. [48] on the Cd2SnO4 films.

1.8 Properties of Zn2SnO4: ReviewDizinc Stannate (Zn2SnO4 or ZTO) is superior to CTO in optical properties but has poor

electrical conductivity and mobility due to its structure [68] comparing to Cd2SnO4 and

does not suffer from the toxicity problem associated with Cd in Cd2SnO4 [20].

1.8.1 Crystal structurePolycrystalline ZTO thin films form in the 'inverse' spinel crystal structure. This

structure is viewed as a combination of the rock salt and zinc-blend structure. The

16

crystal structure of the inverse spinel cubic Zn2SnO4 is given in Fig. 1.2. The inverse

spinel exists where the tetrahedral voids are occupied by Zn atoms and the octahedral

voids are occupied randomly by an equal number of Zn and Sn atoms [31]. However,

the spinel lattice is locally distorted enough to form two distinct octahedrally

coordinated Sn and Zn sites. The disorder in the Sn and Zn sites in the lattice

significantly limits the mobility of the carriers, possibly by disrupting the edge-sharing

nature [69] of the octahedrally coordinated cations. For this inverse structure, the cation

inversion energy ∆E is slightly negative, which is a strong indicative of stable structure

in the inverse spinel structure. Inter mediate phases with the formula (A1-xBx)[AxB2-x]O4

also exists. Here the cations in the square bracket occupy the octahedral sites and

cations in the parenthesis occupy the tetrahedral sites. The cation inversion parameter

‘x’ ranges from one for a normal spinel to one for an inverse spinel.

Inverse spinel structure in Zn2SnO4 is a favorable crystal structure for TCOs,

composed of closely packed oxygen ions and rutile chains which are connected by

cations in the tetrahedral sites and which run through the lattice. The rutile chains are

considered as conduction paths for electrons as unoccupied orbital of cations

significantly overlap in the chains because of short cation-cation separation distances

due to the edge sharing of cation octahedra.

1.8.2 Band structureThe band structure and the LDOS of the normal spinel Zn2SnO4 are as shown in Fig.

1.4. The variation of the band gap and the changes in E12 are similar to those in the

Cd2SnO4 and can be explained in an analogous way to that described in section 1.7.2

for Cd2SnO4.

The only noticeable distinctions between Zn2SnO4 and Cd2SnO4 are that the

band gap of Zn2SnO4 is larger than the band gap of Zn2SnO4, due mostly to the smaller

volume of Cd2SnO4 than Zn2SnO4. Since Zn and Sn have similar atomic sizes, there is

no any change in bond lengths as seen that for Cd2SnO4. Further, when Sn is at the

octahedral site, the VBM energy decreases, which is due to the lower Sn 4d orbital

energy compared to that of the Zn 3d orbital and hence the reduced p-d repulsion at the

octahedral site. Moreover, because Zn has a much higher 4s orbital energy than the Sn

5s orbital, both of the first two conduction bands energies increase. However the energy

17

of the SCB increases more than that of the CBM state because the former contains only

Zn 4s orbital but no Sn 5s orbital. This explains why the fundamental bandgap Eg as

well as the E12 of Zn2SnO4 is much larger in the inverse spinel structure than in the

normal spinel structure.

1.8.3 Properties and applications of Zn2SnO4 filmsFrom the crystal structure, it is cleat that the Inverse spinel structure is the most stable

form of Zn2SnO4 and the Band structure reveals that the inverse spinel has the large

band gap. This makes the Zn2SnO4 as the promising material for the TCO applications

even though its conductivity is slightly smaller than the Cd2SnO4. Most often the

literature [20, 31] deals both the Zn2SnO4 and Cd2SnO4, simultaneously, since the

Cd2SnO4 is well known for its high conductivity due to its high carrier mobility and the

Zn2SnO4 for its high optical transparency, because of its large energy separation

between the CBM and the SCB which causes the transparency. Sometimes the Zn2SnO4

is considered as a non-toxic alternative of Cd2SnO4, despite concerns about the toxicity

of cadmium [46].

The Zn2SnO4 is also crystallizes in the amorphous state in the as-prepared

conditions. Many coworkers [70-7172] analyzed its properties in the as-prepared

condition, because amorphous Zn2SnO4 (a-ZTO) is considered to be a promising

material for the channel layer of transparent thin film transistors (TFTs) [73-7475] which

is currently using the Indium Gallium oxide (IGO) or Indium Gallium Zinc oxide

(IGZO). ZTO shows equivalent TFT performance to In or Ga based AOS, which

employs inexpensive atomic constituents and is able to be deposited by sputtering itself.

The as deposited films are amorphous but subsequent annealing at 600ºC give

polycrystalline uniaxially oriented films with resistivity of 10-2-10-3 Ωcm [72, 76, 77],

mobilities of 16–26 cm2/Vs [78], and n-type carrier concentrations in the low 1019 cm3

range [79]. Zn2SnO4 films are being used as the buffer layer for the CdTe/CdS with the

combination of Cd2SnO4 recently [80].

1.8.4 Some literature on Zn2SnO4 filmsCoutts et al. (2000) [23] and Wu et al. [46, 81], while searching for the new transparent

conducting oxide and the remedy for the toxicity of cadmium in Cd2SnO4, they started

to develop Zn2SnO4, in the belief that the spinel compounds should have to behave in a

18

similar manner, but they found this is clearly not the case. They believed that the grain

boundary scattering which is due to the poor crystallinity of the films plays a major role

in the conductivity and the very high effective mass (0.1me) and the short relaxation

time of 8X10-6 s lead to the reduced carrier mobility. The results of Young et al. [69,

79] also confirms the electrical property results of Coutts and Wu et al., even though the

optical properties of Zn2SnO4 is superior to Cd2SnO4. Many researchers tried to achieve

better conductivity in Zn2SnO4 by varying the film processing parameters such as

applying the post heat treatment [70, 72] or intentionally doping [47] and varying the

films Sn/Zn concentration [82, 83]. Segev et al. (2005) theoretically described the band

structures of the Zn2SnO4 and Cd2SnO4. The results made clear the era of investigation

on Zn2SnO4. The all the researchers came to know the basic phenomenon behind the

transparency and the conductivity of Zn2SnO4 films. This lead, the analysis of Zn2SnO4

as TCO geared back in literature.

Wu et al. [45] in (2001) suggests the Zn2SnO4 as the buffer layer for the CdTe

solar cells and [47, 80] analyzed it as a buffer layer for CdTe solar cells. But being a

amorphous semiconductor which has a smooth surface like Cd2SnO4 made the Zn2SnO4

as a promising material for the channel layer of AOS based transparent thin film

transistors (TFTs) [73-7475] which is currently using the Indium Gallium oxide (IGO) or

Indium Gallium Zinc oxide (IGZO) ZTO shows equivalent TFT performance to In or

Ga based AOS, which employs inexpensive atomic constituents and is able to be

deposited by sputtering itself.

In 2005, Chiang et al. [73] reported the first TFT with a ZTO channel layer.

ZTO is sputtered onto the substrate, which is held at ~175 oC, in a 90/10 Ar/O2

atmosphere, and then subsequently annealed for one hour at temperatures up to 600 oC,

where the channel material remains amorphous with target stoichiometries with

ZnO:SnO2 ratios of 2:1 and 1:1 (Zn2SnO4 and ZnSnO3, respectively). According to this

study, there is little to no performance difference between the targets. Since then, many

coworkers [74, 75, 84, 85] studied the device structures like TFTs, Field effect

transistors (FET) and Organic LEDs which uses Zn2SnO4 as its constituents.

Zn2SnO4 has another importance role in the field of gas sensors which is used to

sense the ethanol (C2H5OH) [86], Carbon Monoxide (CO) [87, 88], Nitrogen dioxide

19

(NO2) [89], isobutane (i-C4H10) [90] and humidity [91] after Zhang et al. [92] who

observed that Zn2SnO4 has gas-sensitivities to reductive gases.

1.9 Scope of the present workThe principal aim of this thesis is to prepare the transparent and conducting Cd2SnO4

and Zn2SnO4 films by RF Magnetron sputtering technique by varying the deposition

parameters, since the goal of the work is to prepare the optimized films in the as-

prepared conditions without any post deposition treatments.

The objective of the study are:

➢ Prepare the Cd2SnO4 and Zn2SnO4 thin films by RF Magnetron sputtering

technique by varying the deposition parameters such as the substrate

temperature and the RF power.

➢ Study the structural, surface morphological and the chemical composition and

the energy states of the films by X-Ray diffraction, High resolution transmission

electron microscopy, Scanning electron microscopy, atomic force microscopy

and X-ray photoelectron spectroscopy

➢ Study the optical properties of the films by optical absorption spectroscopy,

photoluminescence spectroscopy, Raman spectroscopy, Photoacoustic

spectroscopy

➢ Study the thermal properties by photoacoustic spectroscopy and the electrical

properties by conductivity measurements and the Hall measurements

➢ Fabricate Cd2SnO4 and Zn2SnO4 based devices and study its performance.

References

[1] N. Tsuda, K. Nasu, A. Fujimori, K. Siratori, Electronic conduction in oxides, Springer – Verlag, Berlin Heidelberg (2000).

[2] H.J.J. Van Boort, R. Groth, Phil. Tech. Rev, 29 (1968) 17.[3] H. Knako, K. Miyake, J. Appl. Phys., 53 (1982) 3629.[4] V.E. Henrich, P.A. Cox, Inorganic chemistry, Cambridge University Press,

Cambridge (1994).[5] J.L. Bosnell, R. Wahorn, Thin Solid Films, 15 (1973) 141.[6] I. Takeuchi, W. Yang, K.S. Chang, M. A. Aronova, T. Venkatesan, R. D. Vispute,

and L. A. Bendersky, J. Appl Phys, 94 (2003) 7336.

20

[7] Rointan F. Bunshah, Handbook of deposition technologies for films and coatings, Noyes Publications, New Jersey, USA (1994).

[8] T. Minami. T. Kakumu, K. Shimokawa, S. Takata, Thin Solid Films, 317 (1998) 318.

[9] K. Bӓdeker, Ann. Phys. (Leipzig), 22 (1907) 749.[10] X. Wu, P. Sheldon, Y. Mahathongdy, R. Ribelin, A. Mason, H.R. Moutinho, and

T.J. Coutts, Proc. AIP Conf., CPV Photovoltaics Program Review, (1999).[11] R.J. Cava, J.M. Phillips, J. Kwo, G.A. Thomas, R.B. van Dover, S.A. Carter, J.J.

Krajewski, W.F. Peck, Jr., J.H. Marshall, and D.H. Rapkine, Appl. Phys. Lett, 64 (1994) 2071.

[12] D.D. Edwards, T.O. Mason, F. Goutenoire, and K.R. Poeppelmeier, Appl. Phys. Lett., 70 (1997) 1706.

[13] T. Minami, S. Takata, T. Kakumu, and H. Sonohara, Thin Solid Films, 270 (1995) 22.

[14] T. Minami, T. Kakumu, K. Shimokawa, and S. Takata, Thin Solid Films, 317 (1998) 1.

[15] T. Omata, N. Ueda, K. Ueda, and H. Kawazoe, Appl. Phys. Lett, 64 (1994) 1077.[16] H. Kawazoe, H. Yanagi, K. Ueda, and H. Hosono, MRS Bulletin, 25 (2000) 28.[17] H. Kawazoe, H. Yanagi, K. Ueda and H. Hosono, MRS Bulletin, 25 (2000) 28.[18] Roy G. Gordon, MRS Bulletin, 25 (2000) 52.[19] B.G. Lewis and D.C. Paine, MRS Bulletin, 25 (2000) 22.[20] David Segev and Su-Huai Wei, Phys. Rev. B, 71 (2005) 125129.[21] Y. Dou and G. Egdell, Phys. Rev. B, 53 (1996) 405.[22] B. Li, L. Zeng and F. Zhang, Phys. Status. Solidi A, 201 (2004) 960.[23] T.J. Coutts, D. L. Young, X. Li, W. P. Mulligan and X. Wu, J. Vac. Sci. Technol.

A, 18 (2000) 2646.[24] E.J.J. Martin, M. Yan, M. Lane, J. Ireland, C.R. Kannewurf and R.P.H. Chang,

Thin Solid Films, 461 (2004) 309.[25] K. Ellmer, J. Phy. D: Appl. Phys., 33 (2000) R17.[26] S. E. Demian, J. Mat. Sci.: Mat Elec, 5 (1994) 360.[27] J. S. Kim, H. A. Marzouk, P. J. Reucroft, and C. E. Hamrin Jr, Thin Solid Films,

217 (1992) 133.[28] V. Srikant, V. Sergo, and D. Clarke, J. Am. Cer. Soc., 78 (1995) 1931.[29] V. Craciun, J. Elders, J. G. E. Gardeniers, J. Geretovsky, and I. W. Boyd, Thin

Solid Films, 259 (1995) 1931.[30] A.J. Nozik, Phys. Rev. B, 6 (1972) 453.[31] Su-Huai Wei and S. B. Shang, Phys. Rev. B, 63 (2001) 045112.[32] X. Z. Wu, R. Ribelin, R. G. Dhere, D. S. Albin, T. A. Gessert, S, Asher, D. H.

levi, A. Mason, H. R. Moutinho and P. Sheldon, Proceedings of 28th IEEE PVSC, 2000.

[33] M. E. Bowden, J. mat. Sci. lett., 9 (1990) 735.[34] A.J. Smith, Acta Cryst., 13 (1960) 749.[35] L. A. Siegel, J. Appl. Cryst., 11 (1978) 284.[36] D.M. Ceperley and B.J. Alder, Phys. Rev. Lett., 45 (1980) 566.[37] J.P. Perdew and Z. Zunger, Phys. Rev. B, 23 (1981) 5048.[38] R.D. Shannon, J.L. Gillson, R.J. Bouchard, J. Phys. Chem. Solids, 38 (1977) 877.[39] M.E. Bowden and C.M. Cardile, Powder Diffr., 5 (1990) 36.[40] Armando Ortiz R, J. Vac. Sci. Technol., 20 (1982) 7.

21

[41] N. Miyata, K. Miyake and S. Nao, Thin Solid Films, 58 (1979) 385.[42] Robert Mamazza Jr., Don L. Morel and Christos S. Ferekides, Thin Solid Films,

484 (2005) 26.[43] W.L. Wang, K.J. Liao, C.Z. Cai, G.B. Liu and Y. Ma, Surface and Coatings

Tech., 167 (2003) 284.[44] Walter Wohlmuth and Ilesanmi Adesida, Thin Solid Films, 479 (2005) 223.[45] X.Wu, S. Asher, D.H. Levi, D.E. King, Y. Yan, T.A. Gessert and P. Sheldon, J.

Appl. Phys., 89 (2001) 4564.[46] X. Wu, W.P. Mulligan and T.J. Coutts, Thin Solid Films, 286 (1996) 274.[47] N. Romeo, A. Bosio, V. Canevari, M. Terheggen and L. Vaillant Roca, Thin Solid

Films, 431-432 (2003) 364.[48] G. Valincius, V. Reipa, V. Vilker, J.T. Woodward and M. Vaudin, J.

Electrochem. Soc., 148 (2001) E341.[49] G. Valincius, V. Reipa, V. Vilker and M. Mayhew, Electrochem. Commun., 4

(2002) 314.[50] T. Hashemi, J. Illingsworth and F. Golestani-Fard, J. Am. Ceram. Soc, 74 (1991)

662.[51] G. Haacke, Appl. Phys. lett, 28 (1972) 622.[52] G. Haacke, W.E. Mealmaker and L.A. Seigel, Thin Solid Films, 55 (1978) 67.[53] N. Miyata, K. Miyake T. Fukushima and K. Koga, Appl. Phys. Lett., 35 (1979)

542.[54] N. Miyata, K. Miyake and Y. Yamaguchi, Appl. Phys. Lett., 37 (1980) 180.[55] F.P. Koffyberg and F.A. Benko, Appl. Phys. Lett., 37 (1980) 320.[56] R.P. Howson, M.I. Ridge and C.A. Bishop, Thin Solid Films, 80 (1981) 137.[57] Carl. M. Lampert, Solar Energy Materials, 6 (1981) 1.[58] Armando Ortiz R., J. Vac. Sci. Technol., 20 (1982) 7.[59] T. Stapinski, Solar & Wind Technol., 3 (1986) 195.[60] T Stapinski, E Leja and T Pisarkiewicz, J. Phys. D: Appl. Phys., 17 (1984) 407.[61] M.S. Shetty and A.P.B. Sinha, Thin Solid Films, 144 (1986) 7.[62] K. Pisarkiewicz, K. Zakrzewska and E. Leja, Thin Solid Films, 153 (1987) 479.[63] J.M. Blackmore, Thin Solid Films, 185 (1990) 97.[64] T.O. Mason, G.B. Gonzalez, D.R. Kammler, N. Mansourian-Hadavia and B.J.

Ingram, Thin Solid Films, 411 (2002) 106.[65] T.O. Mason, D.R. Kammler, B.J. Ingram, G.B. Gonzalez, D.L. Young and T.J.

Coutts, Thin Solid Films, 445 (2003) 186.[66] S.B. Zhang and Su-Huai Wei, Appl. Phys. Lett., 80 (2002) 1376.[67] Y. Dou and R.G. Egdell, Surface Science, 377 (1997) 181.[68] X. Wu, P. Sheldon, T.J. Coutts, D.H. Rose, and H. Moutinho, 26th PVSC,

Anaheim, California (1997).[69] D. L. Young, D. L. Williamson and T. J. Coutts, J. Appl. Cryst., 91 (2002) 1464.[70] E. Cetinorgu, S. Goldsmith and R.L. Boxman, Thin Solid Films, 515 (2006) 880.[71] Kazuo Satoh, Yoshiharu Kakehi, Akio Okamoto, Shuichi Murakami, Kousuke

Moriwaki and Tsutom Yotsuya, Thin Solid Films, 518 (2007) 213.[72] J.D. Perkins, J.A. del Cueto, J.L. Alleman, C. Warmsingh, B.M. Keyes, L.M.

Gedvilas, P.A. Parilla, B. To, D.W. Readey and D.S. Ginley, Thin Solid Films, 411 (2002) 152.

[73] H. Q. Chiang, J. F. Wager, R. L. Hoffman, J. Jeong and D. A. Keszler, Appl. Phys. Lett., 86 (2005) 013503.

22

[74] D. Hong and J. Wager, J. Vac. Sci. & Technol. B, 23 (2005) L25.[75] W. B. Jackson, R. L. Hoffman, and G. S. Herman, Appl. Phys. Lett., 87 (2005)

193503.[76] T. Minami, MRS Bull., 25 (2000) 38.[77] D.L. Young, D.L. Williamson and T.J. Coutts, J. Appl. Phys., 91 (2002) 1464.[78] J.H. Ko, I.H. Kim, D. Kim, K.S. Lee, T.S. Lee, B. Cheong and W.M. Kim, Appl.

Surf. Sci., 253 (2007) 7398.[79] David L. Young, Helio Moutinho, Yanfa Yan, and Timothy J. Coutts, J. Appl.

Phys., 92 (2002) 310.[80] S. Gayam, S. Bapanapalli, H. Zhao, L. Nemani, D.L. Morel and C.S. Ferekides,

Thin Solid Films, 515 (2007) 6060.[81] X.Wu, T.J. Coutts and W.P. Mulligan, J. Vac. Sci. Technol. A, 15 (1997) 1057.[82] E. Holmelund, J. Schou, S. Tougaard and N.B. larsen, Appl. Surf. Sci., 197-198

(2002) 467.[83] A. Kurz, K. Brakecha, J. Puetz and M.A. Aegerter, Thin Solid Films, 502 (2006)

212.[84] R. Hoffman, Solid state Elect., 50 (2006) 784.[85] P. GÄorrn, M. Sander, J. Meyer, M. KrÄoger, E. Becker, H.H. Johannes, W.

Kowalsky, and T. Riedl, Adv. Mater., 18 (2006) 738.[86] Zhouguang Lu and Yougen Tang, Mater. Chem. Phys., 92 (2005) 5.[87] Ji Haeng Yu and Gyeong Man Choi, J. Electroceram., 8 (2002) 249.[88] J.H. Yu and G.M. Choi, Sensors and Act. B, 72 (2001) 141.[89] Yasushi Yamada, Yoshiki Seno, Yumi Masuoko and Katsuji Yamashita, Sensors

and Act. B, 49 (1998) 248.[90] N. Hiratsuka, A. Hosoi, H. Kobayashi and K. Kakizaki, J. Ceram, Soc. Jpn., 104

(1996) 1173.[91] I. Stambolova, K. Konstantinov, D. Kovacheva, P. Peshev and T. Donchev, J.

Solid State Chem., 128 (1997) 305.[92] Z.T. Zhang and Z.G. Zhou, Proc. of Int. Sci. Symp. on Sensors, Harbin, Republic

of China, (1986) 95.