Download - Inorganic/Organic Hybrid Nanocomposite and its Device Applications

Inorganic/Organic Hybrid Nanocomposite and its Device Applications

S.K. Tripathi

Department of Physics, Panjab University, Chandigarh-160 014, India

[email protected]

Keywords: Chemical Synthesis, Metal-Semiconductor Contact, Nanocomposite, Polymer, Semiconductor

Abstract. II-VI semiconductors are promising nanomaterials for applications as window layers in

low-cost and high-efficiency thin film solar cells. These nanoparticles are considered to be the

model systems for investigating the unique optical and electronic properties of quantum-confined

semiconductors. The electrical and optical properties of polymers are improved by doping with

semiconductor materials and metal ions. In particular, nanoparticle-doped polymers are considered

to be a new class of organic materials due to their considerable modification of physical properties.

In this paper, I review the present status of these types of Inorganic/Organic hybrid nanocomposite

materials. CdSe nanorods dispersed in polyvinyl alcohol (PVA) matrix have been prepared by

chemical routes. Different characterization techniques like structural, optical and electrical have

been used to characterize these nanocomposites. The devices like Schottky diodes and MOS

structures have been fabricated and the results have been discussed in this review. The results have

been compared with the reported literature by other groups also.

Table of Contents

1. Introduction

2. What is a Polymer Nanocomposite?

2.1. Types of Polymer Nanocomposites

2.2. Properties of Polymer Nanocomposite

2.2.1. Physical properties

2.2.2. Mechanical properties

2.2.3. Electrical properties

2.2.4. Crystallinity and barrier properties

2.2.5. Dielectric and magnetic properties

2.2.6. Optical properties

3. Polymer Surface at the Nanoscale

4. Factors that affect Polymer Nanocomposites Structure

4.1. Synthesis Method

5. An Over View of Nanorods

6. Characterization of Nanostructures

7. II-VI Semiconductor Nanocomposites

7.1. CdSe/PVA Nanocomposite

7.2. Preparation of II-VI Semiconductor Nanocomposites

8. Device Fabrication and Characterization

8.1. Metal-Semiconductor Contact

8.2. Metal-Oxide-Semiconductor Interfaces

9. II-VI Semiconductor Devices

10. Summary

References

Solid State Phenomena Vol. 201 (2013) pp 65-101Online available since 2013/May/14 at www.scientific.net© (2013) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/SSP.201.65

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1. Introduction

Nanoparticles are thought to hold some keys for solving many present and future technological

demands. However, direct applications of these nanoparticles are limited due to their size and

stability. They aggregate easily because of their high surface energy, and are quickly oxidized as

well. To overcome the aggregation and stability problems, these nanoparticles are incorporated in a

dielectric matrix thereby forming a nanocomposite. Polymers, when used as a dielectric matrix

result in a wide range of useful characteristics in the nanocomposites.

Composites are used when a combination of properties is required that cannot be found in a single

material. Particularly interesting are combinations of organic polymers and inorganic materials as

the properties of the pure components are very distinct. In general, organic polymers are flexible,

tough, and are easy to process, but they can also be relatively easily damaged, either chemically or

mechanically. In contrast, inorganic materials are typically much harder, have better barrier

properties, and have a good chemical stability, but are also brittle and are difficult to process.

Organic/inorganic composites may yield a combination of these properties, resulting in a hard,

chemically stable and durable material that is still easy to process.

The properties of a composite are not simply the average properties of its components but it

involves their volume fraction, size, shape and the distribution. In a composite one component may

be enclosed by another component that forms a continuous phase, but it is also possible that the

components form continuous phases resulting in interpenetrating networks. The interactions

between the different components may induce changes in the chemical or physical structure of the

components, especially in the first few nanometers from the interface [1-3]. The interfacial area

increases with decreasing domain size in the composite. For nanocomposites, with domain sizes of

about 10 nm, 1 cm3 of composite may contain several hundred square meters of interface. The

addition of a third component that concentrates at the interface and alters the interactions can have

strong effects on the composite properties [2-6]. The third component may be a surfactant that

assembles at the interface by physical adsorption, or it may be a reactive species that is grafted on

the surface of the filler or it may even react with both phases forming a chemical bond between the

two phases [5]. This modification of the interface is often used to improve the mechanical

properties of composites.

2. What is a Polymer Nanocomposite?

As the name indicates “Polymer Nanocomposite” means composed of polymer + nanocomposite.

A composite material is a combined material created from two or more components, selected filler

or reinforcing agent and a compatible matrix, binder (i.e. resin) in order to obtain specific

characteristics or properties that were not there before. The matrix is the continuous phase, and the

reinforcement constitutes the dispersed phase. It is the behavior and properties of the interface that

generally control the properties of the composite. [7]

The load acting on the matrix has to be transferred to the reinforcement via the interface. Thus,

reinforcements must be strongly bonded to the matrix, if their high strength and stiffness are to be

imparted to the composite. The fracture behavior is also dependent on the strength of the interface.

A weak interface results in low stiffness and strength, but high resistance to fracture, whereas a

strong interface produces high stiffness and strength but often a low resistance to fracture, i.e.,

brittle behavior. The exact role of interface may differ with the type of reinforcement. The interface

can be viewed as a planar region of only a few atoms in thickness across in which there is a change

in properties from those of the matrix to those of the reinforcement. Thus, the interface is usually a

discontinuity in chemical nature, crystal and molecular structure, mechanical and other properties.

66 Functional Nanomaterials and their Applications

Nanocomposites are materials that are created by introducing nanoparticulates (often referred to as

filler) into a macroscopic sample material (often referred to as the matrix). This is part of the

growing field of nanotechnology. After adding nanoparticulates to the matrix material, the resulting

nanocomposite may exhibit drastically enhanced properties. For example, adding carbon nanotubes

tends to drastically add to the electrical and thermal conductivity. Other kinds of nanoparticulates

may result in enhanced optical properties, dielectric properties or mechanical properties such as

stiffness and strength. In general, the nanosubstance is dispersed into the matrix during processing.

Fig. 1 Comparison between microcomposites and nanocomposites

In materials research, the development of polymer nanocomposites is rapidly emerging as a

multidisciplinary research activity whose results could broaden the applications of polymers to the

great benefit of many different industries. Nanocomposites are a distinct form of composite

materials, which involve embedding nano or molecular domain sized particles into organic polymer,

metal or ceramic matrix materials. In all cases, it is perceived that the intimate inclusion of these

nanoparticles in these matrices can completely change the properties of these materials. The

nanoparticles can serve as matrix reinforcement as well as change the electrical behavior of these

base materials. The reason for this is that with such small inclusions, a large amount of interfacial

phase material is now included in the bulk of these nanocomposites. A complete transformation of

the material’s chemical, mechanical and morphological domain structure can be achieved by the

proper addition of nanoparticles.

Polymer Nanocomposites (PNC) [8] are polymers (thermoplastics, thermosets or elastomers) that

have been reinforced with small quantities (less than 5% by weight) of nano-sized particles having

high aspect ratios (L/h > 300). PNCs represent a radical alternative to conventional filled polymers

or polymer blends [9-10]. In contrast to conventional composites, the reinforcement and PNCs are

of the order of microns and few nanometers respectively. The transition from micro- to nano-

particles leads to change in its physical as well as chemical properties as shown in figure 1. Two of

the major factors in this case are the increase in the ratio of the surface area to volume, and the size

of the particle, also it increases the strength, and heat resistance etc. and many factors do change for

the mixture. As PNC is a polymer or copolymer having dispersed in it nanoparticles, these may be

of different shape (e.g., platelets, fibers, spheroids), but at least one dimension must be in the range

of 1 to 50 nm.

Solid State Phenomena Vol. 201 67

For the fabrication of polymer nanocomposites, there are three main material constituents in any

composite:

The matrix,

The reinforcement (fiber), and

The interfacial region.

Fig. 2 Different methods of preparation of polymer nanocomposites

There are many methods to modify polymer/clay surface properties for specific applications, e.g.

preparing nanocomposites through polymerization, preparing nanocomposites using melt blending

technologies, preparing nanocomposites from polymer blends, developing nanocomposite

characterization techniques: rheology, microstructure, thermal, thermodynamic and mechanical

analyses (in the short- and long- term) , forming of nanocomposites (injection moulding, blow

moulding, film blowing, blowing, foam extrusion, etc). The different methods of preparation of

polymer nanocomposites are shown in figure 2.

The development of PNCs, as with any multicomponent material, must simultaneously balance four

interdependent areas:

Constituent selection,

Cost-effective processing,

Fabrication, and

Performance

Fig. 3(a) Intercalated structure Fig. 3(b) Exfoliated structure


2.1 Types of Polymer Nanocomposites

Polymer/layered nanocomposites in general, can be classified [8] into three different types, namely:

(i) Intercalated nanocomposites,

(ii) Flocculated nanocomposites,

(iii) Exfoliated nanocomposites

In the first case polymer chains are inserted into layered structures such as clays, which occur in a

crystallographically regular fashion, with a few nanometers repeat distance, irrespective of the ratio

of polymer to layered structure as shown in figure 3(a).

In the second case, flocculation of intercalated and stacked layers to some extent takes place due to

the hydroxylated edge–edge interactions of the clay layers.

Finally, separation of the individual layers in the polymer matrix occurs in the third type by average

distances that depend only on the loading of layered material such as clay.

In this new family of composite materials, high storage modulus, increased tensile and flexural

properties, heat distortion temperature, decrease in gas permeability, and unique properties such as

self extinguishing behavior and tunable biodegradability are observed, compared to matrix material

or conventional micro and macro-composite materials. The exfoliation of layered minerals and

hence the preparation of a homogeneous nanocomposite material is seriously hindered by the fact

that sheet-like materials display a strong tendency to agglomerate due to their big contact surfaces

as shown in figure 3(b).

2.2 Properties of Polymer Nanocomposite

There are two main challenges to develop polymer nanocomposite materials after the desired

nanoparticle has been selected for polymer of interest. First, the choice of nanoparticle requires an

interfacial interaction and/or compatibility with the polymer matrix and secondly, the proper

processing technique should be selected to synthesize uniformly disperse and distributed

nanoparticle within the polymer matrix. In most cases, the polymer nanocomposites exhibit

multifunctional properties. Some of them are given below:

2.2.1 Physical properties. The physical properties of nanocomposites depend greatly on the

chemistry of polymer matrices, nature of nanofillers, and the way in which they are prepared. The

uniform dispersion of nanofillers in the polymer matrices is a general pre-requisite for achieving

desired mechanical and physical characteristics.

2.2.2 Mechanical properties. The incorporation of nanoceramics such as layered silicate clays,

calcium carbonate or silica nanoparticles arranged on the nanometer scale with a high aspect ratio

and/or an extremely large surface area into polymers improves their mechanical performances

significantly.

2.2.3 Electrical properties. Electronic properties can be tailored to be suitable for device

applications such as in photovoltaic applications by varying the particle size. Potential applications

of nanocomposites as functional materials include organic field emitting displays, photovoltaic

cells, highly sensitive strain sensors, and electromagnetic-wave interference materials. Small

changes of the configuration of the composite concerning the metal fraction as well as the size and

shape of the nanoparticles can lead to dramatic changes in the electrical properties. By minimizing

interface effects and through variation of the material composition, properties such as mechanical

strength, electrical conductivity, and coefficient of thermal expansion can be controlled.


2.2.4 Crystallinity and barrier properties. Polymer nanocomposites have shown potential to

improve barrier properties. The primary mechanism of improvement has been attributed to the

development of a tortuous path. However, the degree of improved barrier properties has been varied

among different polymers.

2.2.5 Dielectric and magnetic properties. Nanocomposites are media made of nanoparticles

embedded in a matrix. Depending on the nanoparticle types and shapes, nanocomposites can have

different properties. At low frequencies, dielectric and magnetic properties can be enhanced by

using elongated nanoparticles. At high frequencies [11], various photonic, plasmonic and

polaritonic effects can occur in nanocomposites, in particular in those with periodic arrangement of

nanoparticles. The negative refraction can be achieved in nanocomposites with simultaneously

negative dielectric function and magnetic permeability. For future magnetic applications, significant

advantages are expected on the basis of independent control of the size (which determines the

quantum effect) and interparticle distance (which determines particle-to-particle interactions) of

metal nanoparticles. In addition, the macroscopic morphology of nanocomposites (e.g., films and

powders with various particle shapes) may be designed for practical applications.

2.2.6 Optical properties. Plasmon resonance allows for the enhancement and manipulation of local

electromagnetic fields at nanoparticle surface due to surface effects and as a result of these singular

optical features, noble metal nanoparticles stimulate great interest for implementation in photonics,

biotechnology and space applications [12, 13]. Owing to interesting optical properties of

nanocomposites which are different from the ones of individual metals, these materials have drawn

much attention. Combining metal and polymer together enhances the optical properties of

nanometals and also alter the mechanical behavior of the polymer. The former is of prime

importance due to its possible application in polarizers, light stable colour filters, solar cells and

optical sensor. If the properties of polymer can also be tuned in correct direction with enhanced

optical properties of metals, these materials could be best candidates for space application.

In general, the size dependent structural, optical, thermal and electronic properties can be tailored to

be suitable for device applications by varying the particle size. Small changes of the configuration

of the composite concerning the metal fraction as well as the size and shape of the nanoparticles can

lead to dramatic changes in the electrical and optical properties [14, 15]. However, as attractive

such nanocomposites may be, the process of blending or dispersing nanoparticles in a polymer

matrix has proven to be problematic.

3. Polymer Surface at the Nanoscale

A few trends for the behavior of polymer matrix nanocomposites has been observed based on the

nature of the polymer matrix, particularly crystalline or amorphous nature of the polymer, and the

interaction between the filler and matrix.

The crystallinity of crystalline and semi-crystalline polymers is not affected very much by the

addition of nanoparticles. There may be some changes in particular nanocomposite systems, but

overall no major differences in crystallinity of nanocomposites versus neat polymers have been

observed in any of the systems examined.

The glass transition temperature (Tg) is influenced by the addition of particles [16]. When there

is good filler–particle interaction, Tg tends to increase with a decrease in the size of particles for

amorphous polymers. For crystalline polymers, Tg decreases with an increase in particle

concentration. For an amorphous system with poor filler–polymer interfacial interaction, Tg

decreased overall.


Interaction between matrix and filler may play an important role in the effects of the

nanoparticles on composite properties. For composites with good interaction between filler and

matrix, the yield stress tends to increase with increasing volume fraction and decreasing particle

size, similarly to the increase in modulus under same conditions. The pattern changes when

there is poor interaction between the matrix and particles.

The degree of crystallinity is not significantly affected by the presence of particles, however, Tg

is very dependent upon this factor. There was another mechanism which was causing these

same properties to increase. The increase occurred when there was a strong interaction between

the polymer and filler.

The small interparticle distance in nanocomposites was used as another parameter to explain the

changes in the elastic modulus and strength of these materials when compared with the

composites with micron-sized particles. The same parameter also plays a role in Tg changes

observed in nanocomposites versus composites with micron-sized reinforcement.

When there is little or no interfacial interaction between the filler and matrix and the inter-particle

distance is small enough, the polymer between two particles acts as a thin film. For a thin film, Tg

decreases as film thickness decreases. As the filler concentration increases, the inter-particle

distance and the resulting thickness of the film, decrease.

4. Factors Affecting Polymer Nanocomposite Structure

Synthesis method (melt compounding, solvent blending, in-situ polymerization, emulsion

polymerization etc).

Type of nanoparticles and their surface treatments

Polymer matrix (crystallinity, molecular weight, polymer chemistry, blocks…)

Nanocomposite morphology: Control of location and orientation of NP.

4.1 Synthesis Method

There are two approaches to the synthesis of nanomaterial and fabrication of nanostructures: top-

down and bottom-up. A bottom-up approach refers to the build-up of a material from bottom: atom-

by-atom, molecule-by-molecule, or cluster-by-cluster. In polymer science, polymers are synthesized

by connecting individual monomers together. Bottom-up approach has a better chance to obtain

nanostructures with less defects, more homogeneous chemical composition, and better short and

long range ordering.

A top-down approach is essentially breaking down a system to gain insight into its compositional

sub-systems. In a top-down approach an overview of the system is first formulated, specifying but

not detailing any first-level subsystems. The biggest problem with top-down approach is the

imperfection of surface structure. Top-down approach most likely introduces internal stress, in

addition to surface defects and contaminations. In the synthesis of polymer nanocomposites bottom-

up approach is used. Nanocomposites are generally synthesized by adding nanoparticulates into

polymer. The physical mixture of a polymer and nanoparticle may not form a nanocomposite; in

this case a separation into discrete phases takes place. The poor physical interaction between the

organic and the inorganic components leads to poor mechanical and thermal properties. In contrast,

strong interactions between the polymer and nanoparticle lead to the organic and inorganic phases

being dispersed at the nanometer level. As a result, nanocomposites exhibit unique higher properties

than conventional composites.


Fig. 4 Synthesis approaches for ex-situ and in-situ methods

(i) Ex-situ methods for synthesizing metal/polymer nanocomposites. The nanoclusters prepared

as described above are dispersed in solvents like water, toluene and dichloromethane. Hence a

homogenous mixture of the nanoclusters with the polymer solution can be easily obtained. The

steps followed in the synthesis of nanocomposites by the ex-situ method are given in figure 4. The

resulting nanocomposites can remain either as dispersion, film, powder, or flakes.

(ii) In-situ methods for synthesizing metal/polymer nanocomposites. The in-situ method is

newer and involves the synthesis of nanoclusters in the presence of a polymer or polymerization in

the presence of nanoclusters (with the monomers being the capping agent). In the in-situ methods

the polymer that serves as the dielectric matrix also acts as the capping agent, unlike the ex-situ

methods that use separate capping agents like thiols and amines. The steps followed in the synthesis

of nanocomposites by the in-situ method are given in figure 4. The advantage of the in-situ method

is the absence of surfactants and the lack of intermediate purification steps, thereby resulting in a

higher nanoclusters yield. However, this results in the poor size distribution of the nanoclusters in

the nanocomposites. Various in-situ methods have been reported for the preparation of

metal/polymer nanocomposites.

The uniformity of nanoparticles is found to be very good in the prepared nanocomposites. It was

also noted that the nanocomposites formed have good strength. These methods enable the final

product with the following characteristics:

Nanosized particle

Narrow particle size distribution

High surface area

Homogenous

Pure, and

Improved properties.


5. An Over View of Nanorods

The particles with small size in the range from a few to several tens of nanometers are called quasi

zero-dimensional microscopic system, quantum dots, quantized particles, etc. One dimensional

nanostructure [17] has been called by a variety of names including: whiskers, fibers, nanowires and

nanorods. Nanorods are considered to be shorter than fibers and nanowires. Nanorods may be

synthesized from metals, polymers or semiconducting materials. Standard aspect ratio (length

divided by width) is 3-5. Nanorods are produced by direct chemical synthesis. A combination of

ligands acts as shape control agents and bond to different facets of the nanorod with different

strengths. This allows different faces of nanorod to grow at different rates, producing an elongated

object.

Nanorods have wide application in display technology. By changing the orientation of nanorods

with respect to an applied electric field, the reflectivity of nanorods can be altered, resulting in

superior displays. Nanorods based flexible, thin film computers can revolutionize the retail industry,

enabling customers to check out easily without the hassles of having to pay cash. These are used in

various biomedical applications. Nanorods are nowadays particularly used in electronics devices.

Nanoscale electronic devices that include components other than silicon offer attractive alternatives

to traditional devices made using photolithographic methods and by other methods. Various wires,

with dimensions less than one micron have been fabricated or grown and some of them have been

demonstrated to function as active electronic devices and have been used as components of

electronic devices. For example, devices have been made which include Si and related nanowires

and nanorods and other semiconducting nanowires or nanorods including those made from Group

III-V or Group II-VI compounds. Electronic devices that utilize carbon nanotubes have been

demonstrated as possible replacements for fabricated devices. Diode, transistor and logic element

operation has been demonstrated.

6. Characterization of Nanostructures

Characterization of nanostructures is necessary to establish understanding and control of

nanostructures synthesis and applications [15]. It therefore leads to various microscopies that will

play a central role in characterization and measurements of nanostructed materials and

nanostructures. Common techniques are given below:

i. Electron microscopy (TEM,SEM)

ii. Atomic force microscopy (AFM)

iii. Dynamic light scattering (DTS)

iv. X-Ray photoelectron spectroscopy (XPS)

v. Powder X-ray infrared spectroscopy (XRD)

vi. Fourier transform infrared spectroscopy (FTIR)

vii. UV spectroscopy.

viii. Nanoparticle tracking analysis (NTA) allows direct tracking of the Brownian motion and

this method therefore allows the sizing of individual nanoparticles in solution.

ix. Cone calorimeter (CC), and

x. Mass loss calorimeter (MLS) is used for detection of polymer nanocomposites.

7. II-VI Semiconductor Nanocomposites

Semiconducting polymer nanocomposites are materials in which nanoscopic inorganic particles,

typically 10-100 Å in at least one dimension, are dispersed in an organic polymer matrix in order to

dramatically improve the performance of the polymers. In the broadest sense, nanocomposites can

also include porous media, colloids, gels and polymers, because in these materials the particles or


structures are in nano scale. There are many factors that affect the polymer nanocomposite

properties: Synthesis methods such as melt compounding, solvent blending, in-situ polymerization,

and emulsion polymerization, polymer nanocomposite morphology, types of nanoparticles and their

surface treatments, Polymer matrix such as crystallinity, molecular weight, polymer chemistry, and

whether thermoplastic or thermosetting.

The synthesis of semiconductor nanocomposites adopts two main approaches: in-situ synthesis and

ex-situ technique. In situ technique, the matrix material and metal ions are mixed in solution and

then exposed to the counterion (S2-

, Se2-

) in the form of gas or as ions dissolved in solution [18].

The composite can be cast as a film before or after exposure to the counterion. An alternative in situ

polymerization method is also developed; a well-defined nano-sized semiconductor is first prepared

using the monomer as a capping agent, then the nanoparticles undergo the homopolymerization or

copolymerization process with other monomers to get polymer nanocomposites. In ex-situ method,

semiconductor clusters are first prepared by using capping agents as the stabilizers, and then the

nanoparticles are dissolved in a solvent along with a soluble polymer. This mixed solution can be

cast to produce a polymer film doped with the semiconductor cluster. This simple approach

provides some new examples of interesting photoconductive or photovoltaic nanocomposites. For

instance, CdS, CuS and ZnSe in various polymer matrixes have been reported by ex-situ technique

[19]. Such polymers include polyester with a thiol end group, starburst dendrimers and amino-

derivatized polysaccharides.

There are different types of commercially available nanoparticles that can be incorporated into the

polymer matrix to form polymer nanocomposites. Depending on the application, the researcher

must determine the type of nanoparticle needed to provide the desired effect. Based on the matrix

materials, the nanocomposites can be classified as ceramic-matrix nanocomposites, metal-matrix

nanocomposites and polymer-matrix nanocomposites etc. In all of three types, inorganic/polymer

nanocomposites is of great interest for technologically useful applications as well as for

fundamental studies of nanocrystal-matrix interactions. Polymers offer opportunities for flexible,

lightweight, and mechanically stable nanocrystal nanocomposites. Semiconductor nanocrystal-

polymer composites combine the advantages of both components and have been realized as LED

covers or in solar concentrators [20], optical bar coding [21], photocatalyst [22] and photovoltaics

[23]. The main challenge in the preparation of nanocrystal-polymer composites is preventing a

macroscopic phase separation and the aggregation of nanocrystals in the hybrid material which

would lead to film inhomogeneities and fluorescent quenching effects limiting the respective optical

device performance.

Colvin et al. [24] reported on the first hybrid nanocrystal-polymer light emitting diodes (LEDs) in

1994. A thin layer of CdSe nanocrystals was deposited on a conductive support, and combined with

a 100 nm thick soluble poly(p-phenylenevinylene) PPV derivative layer. Since then, a lot of

progress has been achieved for optimizing all parameters of nanocrystal-polymer hybrid LEDs. The

introduction of CdSe@CdS core-shell NCs made a significant improvement for the nanocrystals-

polymer hybrid LED [24]. The efficiency was increased twenty times by increasing the efficiency

of the radiative recombination and device internal quantum efficiency, while the lifetime was

increased as well by a factor of hundred. Other potential application for nanocrystal-polymer based

LEDs are offering large area lighting systems and backlighting for flat panel displays. Such

applications require LEDs emitting multi-colour light or white light. Recently, Wood et al. [25]

fabricated a full colour AC-driven display based on inject-printed nanocrystals/polymer composites.

Semiconductor colloidal nanocrystals integrated in solvent based polymers have a potential to

compete with other technologies such as OLEDs and full-colour quantum dot displays [26]. The

lifetime of hybrid organic- nanocrystal based LED devices is still limited to some extent by the

instability of the metal contacts and degradation of organic components under high current


operation conditions. In order to avoid such limitations, some non-conductive polymers are

combined with semiconductor NCs forming a photoluminescent conversion layer for commercially

available, e.g., blue LEDs [27]. The first down-conversion LED was achieved by coating a

transparent CdSe@ZnS core-shell polylaurylmethacrylate (PLMA) hybrid composite on the surface

of a GaN light-emitting diode. Saturated-colour light with different wavelengths has been generated

by tuning the size of nanocrystals.

7.1 CdSe/PVA Nanocomposite

CdSe is a typical direct band [Eg = 1.75 eV] semiconductor, which has been a model material in the

studies of quantum confinement effects because of its large exciton Bohr radius (54 Å) [28]. In thin

films form, CdSe exhibits very interesting properties, such as direct transition, proper band gap

width (the band gap energy of CdSe is in the visible spectrum range, which simplifies the optical

detection), short penetration length of light, high absorption coefficient, n-type of conductivity, etc.

These properties make CdSe particularly suitable for various technical applications in solar cells

[29], gamma-ray detectors [30], TFTs [31], PEC cells [32], etc. Other interesting applications of

CdSe include gas sensors [33,34], acousto-optic devices [35] etc. CdSe NCs feature attractive

optical properties. By appropriate choice of CdSe NC size, the absorption edge can be made to fall

anywhere in the visible region. This is directly visible from the clear changes in colour of CdSe

NCs, synthesized in our lab, with different sizes as shown in figure 5. Many workers have studied

the growth conditions of thin films for use in device fabrication.

Fig. 5 Variation of colour of CdSe with size

Kalandaragh et al. [36] have reported the monodispersed and highly water dispersed CdSe spherical

nanocrystals prepared by a simple ultrasound-assisted technique. They reported the control of

crystal shape through controlling different concentrations of PVA as a stabilization agent and also

prevent coagulation of particles. Badr et al [37] have studied the CdSe nanocrystallites of different

sizes which were prepared in PVA photopolymer films at different ratios of Cd:Se. The

photoluminescence spectrum of CdSe varies from 562 to 575 nm with variation of cadmium to

selenium ratio from 16:1 to 1:1. Bozanic et al [38] have developed the synthetic procedure for the

encapsulation of cadmium selenide (CdSe) nanoparticles in a sago starch matrix. Khanna et al [39]

have studied the light emitting CdSe quantum dots in commercial polymethylmethacrylate showing

excellent optical properties. Mansur et al [40] have developed a relatively simple colloidal route

using a single-step method to produce CdSe nanoparticles using acid-functionalized poly(vinyl

alcohol) (PVA–COOH) polymer as capping ligands that offers a window of opportunities to explore

these novel nanohybrid materials. CdSe nanoparticles formation at the interaction between CdCl2


and Na2SeSO3 in aqueous solutions of sodium polyphosphate and gelatin has been reported by

Raevskaya et al [41]. They have used the Flash photolysis technique to show the rate-limiting step

of the photoreaction.

Ramrakhiani et al [42] has studied the photo- and electro-luminescence of cadmium selenide

nanocrystals and nanocomposites. Photoluminescence (PL) spectra show single peak between 345

and 400 nm at 300 nm excitation wavelength. In case of nanocomposites, the emission spectra show

two peaks at 525 and 575–585 nm at 475 nm excitation wavelength. Electroluminescence (EL)

study of CdSe nanopowder and CdSe/PVA nanocomposite show that the emission starts at a

threshold voltage and then increases with voltage. Seoudi et al [43] have prepared CdSe and

polyvinyl alcohol (PVA)-capped CdSe nanoparticle using a one-step solution growth technique.

Singh et al [44] have synthesized CdSe nanoparticles in aqueous solution containing equimolar

ammoniated CdSO4 and Na2SeSO3 as the starting materials without any capping agents, using

gamma and electron beam irradiation under a reducing condition. These bare CdSe nanoparticles

exhibit room temperature ferromagnetic (RTFM) behavior. A facile aqueous route to synthesize

CdSe nanoparticles at room temperature has been developed by Yang et al [45]. The nearly

monodisperse CdSe nanoparticles have been prepared by 10 min of reaction between CdCl2 and

Na2SeSO3 in the presence of thioglycerol and poly(vinylpyrrolidone) (PVP) as capping agent.

Considering all these aspects, the author have decided to study polymer/inorganic nanocomposites

synthesized by in-situ technique and the surface of the particles is covered by organic polymer

(Polyvinyl Alcohol) attached to the surface. Polyvinyl alcohol (PVA) was chosen as the polymer

matrix for its aqueous solubility. The high viscosity of the polymer solution would be helpful in

controlling the growth of selenide nanocrystals. Furthermore, from the application point of view,

the polymer matrix would protect the selenide particles against photooxidation.

7.2 Preparation of II-VI Semiconductor Nanocomposites

Many organic polymeric species have been successfully incorporated within inorganic networks by

different synthetic approaches. The chemical bond between inorganic and organic phases can be

introduced mainly by three approaches: (1) functionalize organic polymeric species with silane,

silanol, or other functional groups that can undergo hydrolysis and condensation with metal

alkoxides; (2) utilize already existing functional groups within the organic polymeric species; (3)

use alkoxysilanes (R’Si(OR)3) as the sole or one of the precursors of the sol-gel process with R’

being a second-stage polymerizable organic group often carried out by either a photochemical or

thermal curing following the sol-gel reaction. Table 1 lists many polymers that have been

incorporated into an inorganic network via this method.

In the synthesis of II-VI semiconductor nanocomposites, group II sources can be metal alkyls, metal

oxide or organic salts. The group VI sources are organophosphine chalcogenides (R3PE) or

bistrimethylsilylchalcogenides TMS2E where (E = S, Se, and Te). CdO, CdCO3, Cd(Ac)2, Me2Cd,

etc., can be used for Cd sources. By tuning the reaction parameters, different shapes and sizes of

CdSe, CdS, CdTe, ZnSe has been prepared in different polymer matrix. The author has prepared

CdSe nanorods dispersed in PVA matrix by in-situ technique.

Selenide anion For synthesis of CdSe, sodium selenosulphate has been used as a common precursor for

selenium ion source. Selenide anion can also be obtained using different precursors like

selenourea, dimethylselenourea, etc. But we have used sodium selenosulphate (Na2SeSO3)

because it is more stable, simpler to prepare and cheaper. It can be prepared by adding elemental

selenium in an aqueous hot solution of sodium sulphite. This mixture is magnetically stirred for

several hours at 80oC, kept overnight and then filtered the excess of selenium. The prepared


solution will slowly deposit selenium as a black precipitate. The solution should be freshly

prepared prior to film deposition process due to the reason that by using freshly prepared

Na2SeSO3 solution, the reaction will proceed much faster than if an aged solution is used.

Na2SeSO3 can only be used in alkaline solutions (with pH greater than 7, since at lower pH

values selenosulphate immediately decomposes to red selenium).

Preparation of the cation source (cadmium ion or zinc ion) For cationic source (cadmium ion or zinc ion), cadmium acetate (zinc acetate) has been used as

cadmium ion (zinc ion) source.

Capping Agent PVA is used as capping agent for formations of CdSe, ZnSe and CdS nanocrystals. PVA is

chosen as the polymer matrix for its aqueous solubility. The high viscosity of the polymer

solution is helpful in controlling the growth of nanocrystals. Furthermore, from the application

point of view, the polymer matrix would protect the particles against photo oxidation.

Sodium selenosulphate (Na2SeSO3) solution (0.50M) is prepared by adding 1.0M of sodium

sulphite in 50 ml of distilled water, by adding 0.05 mol of selenium powder. The solution has been

stirred for 7 hours at 70oC. The solution is kept overnight. Upon filtration, sodium selenosulfate

solution is sealed and stored in the dark at 60oC to prevent decomposition. PVA solution is prepared

by adding 6.0 gram of PVA to 100 ml deionized water and stirring at 60oC until a viscous

transparent solution is obtained. 0.1 M of cadmium acetate source has been dissolved in 20 ml of

deionized water to obtain metal salt solution. Ammonia or sodium hydroxide solution (2.0 M) is

used to turn metal ions into complex ions and to reduce the free metal ion concentration. In a 50 ml

flask, 20 ml PVA solution is placed and 16.0 ml cadmium salt solution (0.10 M) is added with

constant stirring. Ammonia solution is then slowly added dropwise until a clear solution is obtained.

After the pH value is adjusted to 10, 1 ml of selenosulfate solution is introduced in order to achieve

the required Cd:Se::16:1 ratio. The mixture is stirred for 3h at room temperature to obtain a solution

Table 1 Organic Polymers Used in the Preparation of Organic/Inorganic Hybrid Materials [46]

Organic Polymers Phase Connection Reference

Poly(dimethylsiloxane) (PDMS) Chemical bond [47]

Poly(methyl methacrylate) (PMMA) Chemical bond/no chemical bond [48,49]

Polystyrenes Chemical bond [50]

Polyoxazolines (POZO) Chemical bond/no chemical bond [51,52]

Polyimides Chemical bond/no chemical bond [53,54]

Polyamide No chemical bond [55]

Poly(ether ketone) (PEK) Chemical bond [56]

Poly(ethylene oxide) Chemical bond [57]

Poly(butadiene) No chemical bond [58]

Epoxy Chemical bond [59]

Polycarbonate No chemical bond [60]

Poly(vinyl alcohol) No chemical bond [61]

Poly(methyloxazoline) Chemical bond [62]

Poly(ethyloxazoline) Chemical bond [63]

Poly(vinyl acetate) No chemical bond [60]

Poly(acrylic acid) No chemical bond [64]

Poly(ethyleneimine) Chemical bond [65]


Poly(2-vinylpyridine) No chemical bond [66]

Poly(p-phenylenevinylene) No chemical bond [67]

Poly(N-vinylpyrrolidone) No chemical bond [62]

Poly(ε-caprolactam) No chemical bond [68]

Polyurethane No chemical bond [68]

Poly(N,N-dimethylacrylamide) No chemical bond [60]

Cellulosics No chemical bond [69]

Poly(silicic acid esters) No chemical bond [70]

Polyacrylics No chemical bond [71]

Poly(arylene etherphosphine oxide) Chemical bond [72]

Poly(oxypropylene) Chemical bond [73]

Poly(arylene ether sulfone) (PSF) Chemical bond [74]

Cellulose acetate Chemical bond [75]

Figure 6 shows TEM micrograph of CdSe nanorods dispersed in PVA matrix. TEM image shows

three arm nanorods or tripods structure. The lengths of the arms are not very uniform, while their

widths have a relatively narrow size distribution (about 4-5 nm). There is no clamping of PVA and

CdSe. In literature, there are reports regarding the synthesis of CdSe nanorods prepared by

Chemical method (CBD). Xiao-Dong Ma et al [77] have synthesized PVA-capped CdSe nanorods

at room temperature and the diameter of the particles was confined within 8 nm. Yang et al [78]

have reported the CdSe nanorods in PVA matrix at high temperature (160-180oC). The diameter of

these CdSe nanorods is 15-100 nm and length 10-30 μm. Seoudi et al [43] have reported PVA

capped CdSe nanorods with diameter 25nm. In this study, we have obtained PVA:n-CdSe nanorods

that having smaller dimensions (i.e. average diameter ~1.9-5.1 nm).

Fig. 6 TEM image of CdSe nanorods [76]

The fundamental absorption, which corresponds to the transition from valence band to conduction

band, can be used to determine the band gap of the material. The relation between α and the

incident photon energy (hν) can be written as [79]


(1)

where A is a constant, Eg is the optical band gap of the material and the exponent n depends on the

type of transition. The n may have values 1/2, 2, 3/2 and 3 corresponding to the allowed direct,

allowed indirect, forbidden direct and forbidden indirect transitions, respectively. Figure 7 inset

shows the plot of (αhν)2 vs. hν for CdSe nanorods. The value of Eg is calculated by extrapolating the

straight line portion of (αhν)2 vs. hν graph to hν axis taking n = 0.5. The value of optical band gap

for CdSe is 2.69 eV [76]. The wavelength corresponding to absorption edge is 443 nm calculated

from simple relation, . The observed values of Eg are higher than the value of bulk

optical band gap of CdSe nanorods due to quantum confinement. The factor which has a great

effect on the size of CdSe nanorods is the PVA matrix effect. This blue shift could be attributed to

the size reduction effect of PVA matrix to CdSe nanorods.

350 400 450 500 550 600 650 700 750

0.20

0.25

0.30

0.35

0.40

0.45

1.8 2.1 2.4 2.7 3.0 3.3 3.60

2000

4000

6000

8000

10000

12000

14000

(h (

eV

/m)

2

heV)

Ab

so

rban

ce

Wavelength (nm)

Fig. 7 UV/Vis spectra of CdSe nanorods[76]

Figure 8 shows the PL spectra of CdSe nanorods excited at excitation wavelength 400 nm. The

graph clearly shows the photoemission peaks at about 540 nm [76]. This emission peak is blue

shifted as compared to bulk CdSe (709 nm). This feature indicated the quantum-confined effect of

the PVA:n-CdSe nanorods. The band gap of PVA:n-CdSe nanorods obtained from PL peak is 2.29

eV. The full width at half maximum (FWHM) of the band edge is maintained at 29.18 nm for full

excitation wavelength scan. There are two types of photo-emission; an excitonic and a trapped

emission. The trapped emission is broad and it may be due to the formation of deep or shallow

traps. An excitonic emission is normally sharp [80-81]. In our case, the observed peak at 540 nm is

an excitonic emission peak with 30 meV binding energy. The energy of the light emitted is not

equal to the bandgap.

480 500 520 540 560 580 600

0.0

0.2

0.4

0.6

0.8

1.0

PL

In

ten

sit

y (

a.u

.)

Wavelength (nm) Fig. 8 PL spectra of CdSe nanorods [76]


It is less by an amount equal to the exciton energy (Ex). The value of Ex depends on the material and

it is given by the following relation:

hν = Eg - Ex (2)

where hν is emission energy calculated from PL spectra and Eg is band gap of material and Ex is

excitonic energy. The band gap measured experimentally from PL spectra and calculated

theoretically from Brus relation is 2.29 eV and 2.26 eV, respectively. The value of Ex is 30 meV

from equation (12). Shabaev et al. [82] and Chen et al. [83] have also reported that the optical

properties of CdSe nanorods are controlled by excitons with binding energy. This excitonic

emission peak is Stokes shifted with respect to the absorption edge wavelength. The appearance of

such peak is also reported by Ramrakhiani et al. [42] for chemically synthesized CdSe doped in

PVA.

8. Device Fabrication and Characterization

Nanoparticles (NPs) with polymer fillers relate to the strong current interest in device applications.

These polymer nanocomposites have long-term stability and reprocesses ability. Metal-

Semiconductor/Metal-Organic-Insulator-Semiconductor structures or organic/inorganic

semiconductor structures [84-87] are of great importance for the fabrication of microelectronic

devices due to their unique electrical, optical and magnetic properties. The recent researches have

been carried out for applying semiconducting organic materials to electronic devices such as

organic light emitting diodes, Schottky diodes based on organic materials, organic solar cells and

organic field effect transistors [88-91]. It is well known that the interfacial properties of

metal/semiconductor (MS) contacts have a dominant influence on the device performance,

reliability and stability. There is always native thin insulating layer of oxide on the surface of the

semiconductor in most practical MS contacts. This layer converts the MS structure into a

metal/oxide/semiconductor device [92-93]. However, organic thin film or other interfacial layer

between metal and inorganic semiconductor can be constructed by many methods. This film

modifies some electrical parameters of the devices [94]. The performance and reliability of metal–

oxide–semiconductor (MOS) Schottky diodes especially depend on the formation of an oxide layer,

active metal/semiconductor interface, the interface states distribution at the semiconductor, oxide

interface, series resistance and inhomogeneous barrier heights [95-97].

Fig. 9 Energy band diagram of (a) n-type semiconductor and metal separated from each other, (b) at

thermal equilibrium.


8.1 Metal-Semiconductor Contact

A rectifying metal-semiconductor contact was known as a Schottky Barrier after W. Schottky who

realized the potential barrier at the interface of metal semiconductor contact. A Metal-

Semiconductor contact is formed when a metal and a semiconductor are brought into intimate

contact with each other. Depending on the work-functions of the metal and the semiconductor, the

contact may be either Schottky (rectifying) or Ohmic (non-rectifying). According to Schottky and

Mott model, the difference in the work functions of metal and semiconductor causes the barrier

[98]. Suppose that the metal and semiconductor are both electrically neutral and isolated from each

other. The energy-diagram in figure 9(a) is for an n-type semiconductor whose work function (Φs)

is less than that of the metal (Φm). If the metal and semiconductor are electrically connected by a

wire, electrons will pass from the semiconductor into the metal and the two Fermi levels are forced

to align.

There is an electric field in the gap and there is a negative charge on the surface of the metal, which

is balanced by a positive charge in the semiconductor. If the metal and the semiconductor approach

each other, the potential difference between the surfaces of the metal and the semiconductor tends

to zero, since the electric field is finite. When they finally touch, the barrier due to the gap vanishes

altogether and we get an ideal metal-semiconductor contact.

To describe this model, consider the energy band diagrams of n-type semiconductor and metal

isolated from each other. When an intimate contact is made between metal and semiconductor, the

electrons in the conduction band of the semiconductor move into the metal till the Fermi level of

two sides are coincident. This creates a depletion region at the semiconductor interface. Decrease in

the electron concentration of semiconductor boundary region cause bending up at the conduction

band boundary. At thermal equilibrium; the quantity of electrons transferring in both ways are the

same; and so, there is no net current flow as shown in figure 9 (b). Since there are a few mobile

carriers in the depletion region, its resistance is very high in comparison to the metal and neutral

semiconductor. Thus, applied voltage appears at this region. Applied forward voltage reduces the

depletion region width, W, and voltage across this region from Vi to Vi-VF. Therefore, electrons on

the semiconductor sides come across a lower barrier. However, on the metal sides barrier doesn't

change. As a result, the flow from semiconductor to metal increases but the one from metal to

semiconductor doesn't change. Consequently, there is a net current flow from metal to

semiconductor and these current increases by increasing VF. Applied reverse voltage increases the

width of the depletion region and voltage across this region increases from Vi to Vi + VR. Thus the

electrons on the semiconductor side meet with an increased barrier. However, the barrier on the

metal side is again the same. Therefore, a net current flow from metal to semiconductor occurs and

it increases by increasing VR. When compared to forward current this is a smaller current.

Current Transport Mechanisms in Schottky Barriers

The current flow in Schottky barriers is mainly due to the majority carriers. There are four different

mechanisms by which carrier transport can occur:

a) Thermionic emission-diffusion over the barrier

b) Tunnelling through the barrier

c) Carrier recombination (or generation) in the depletion region

d) Carrier recombination in the neutral region

(a) Thermionic emission-diffusion over the barrier. For the electrons to move from the

conduction band of the semiconductor into the metal they have to first be transported through the

space charge region and then emitted over the barrier. There have been two theories proposed for

this phenomenon. One of them is the diffusion theory and the other is the theory of thermionic


emission [99]. According to diffusion theory, the current is limited by diffusion and drift in the

depletion region. The electrons in the conduction band of the semiconductor are in equilibrium with

the electrons in the metal near the interface. The applied voltage has no effect on the concentration

of electrons at the interface. Hence, the quasi Fermi level in the semiconductor coincides with the

Fermi level in the metal at the junction as shown in Figure 10(b). Since the gradient of the quasi

Fermi level is the driving force for the electrons to move from the semiconductor to the metal, the

transportation of electrons in the space charge region is the reason for the current flow [100-105].

Thermionic emission theory suggests that the current is limited by emission of electrons over the

barrier, similar to the thermionic emission of electrons from a metal into vacuum. The transported

electrons are not in thermal equilibrium with the electrons in the metal. They lose energy as they

move into the metal and the quasi Fermi level approaches the Fermi level in the metal. Hence, the

electrons are not in thermal equilibrium at the interface and the quasi Fermi level does not coincide

with the Fermi level of the metal at the boundary, but remains constant throughout the barrier

region.

Fig. 10 (a) Current Transport Mechanisms in Schottky Barriers, (b) Electron quasi Fermi level in a

forward-biased Schottky barrier.

The condition for thermionic emission theory to be applicable is that the electron mean free path be

greater than the distance d, in which the barrier falls by kT from its maximum value. Experimental

data has shown that the thermionic emission theory is a better approximation than the diffusion

theory [104]. The thermionic emission and diffusion theories were combined to give the thermionic

emission diffusion theory. This theory suggests that the total current in the Schottky diode is due to

diffusion and thermionic emission and the equation for the current density is given by the Richard

Dushman equation

(3)

where V, n, q, IRS, and T are the applied bias voltage, the ideality factor, the electronic charge, the

voltage drop across series resistance, and the temperature in Kelvin, respectively. Is is the reverse

saturation current. Equation (3) makes it easy to understand the effect of the dominant carrier

transport mechanism on the J-V characteristics.

(b) Tunneling through the barrier. Quantum mechanical tunneling of carriers though the barrier

is another important current conduction mechanism, which has a significant effect at low

temperatures and high doping concentrations. In the case of heavily doped semiconductors, the

depletion region and the barrier width are narrow, allowing carriers to readily tunnel through the


barrier. Tunneling of hot carriers near the top edge of the barrier is called thermionic field emission,

while emission of electrons throughout the entire barrier is called field emission [106]. The

tunneling current density is given by

(4)

where (5)

(6)

where, Js is the saturation current density, T is the temperature, E00 is the diffusion potential of a

Schottky barrier, η =h/2π where h is the Planck’s constant, m* is the effective mass of an electron,

ND is the donor doping concentration, and εs is the permittivity of the semiconductor. Tunneling is

the main current transport mechanism in Ohmic contacts. The devices we fabricated had ohmic

contacts with very low contact resistance; therefore, we can conclude that tunneling is the dominant

mechanism of current conduction in our ohmic contacts.

(c) Recombination in the depletion region. Recombination of electrons and holes in the depletion

region may play an important role in the case of metal semiconductor contacts at low temperatures

and at low bias voltages [102]. The recombination in the depletion region normally takes place due

to the localized states in the semiconductor. The localized states are often referred to as “traps”

since they tend to capture minority carriers. The localized states are formed due to a number of

reasons such as defects, surface states, dangling bonds, and impurities. These traps have an energy

level associated with them, which is usually located in the forbidden energy gap. The most effective

trap centres are those with energies lying near the center of the forbidden gap. The theory of current

due to such recombination centers is similar to that for p-n junctions, and is predicted by the S-H-R

(Shockley, Hall and Read) model. Recombination current is a common cause for non-ideal behavior

in Schottky diodes. Such departures from ideal behavior are more pronounced at low voltage and

low temperature conditions. The current density due to recombination is given by:

(7)

where (8)

where, ni is the intrinsic electron concentration which is proportional to exp (- Eg/2kT), d is the

thickness of the depletion region, A its area, and τ′ is the lifetime within the depletion region. In

cases where the recombination current is significant, the temperature variation of the forward

current shows two activation energies. Above room temperature, the activation energy tends

towards the barrier height Φb, characteristic of thermionic emission, while below room temperature

it approaches Eg/2, characteristic of the recombination current [106]. The effect of recombination

causes a deviation from the ideal Schottky behavior, either by a deviation called the ideality factor,

n, from unity, or deviation from the exponential behavior of current as predicted by thermionic

emission.

(d) Hole Injection. Bardeen, Brattain (1949) [107] and Banbury (1953) [108] suggested the theory

of hole injection from the metal. This theory states that when the barrier height exceeds one half the

band gap, the region in the semiconductor near the interface becomes inverted. The hole density in

this region exceeds the electron density and hence it becomes p-type. Thus, holes are injected from

the metal near the interface into the bulk of the semiconductor on the application of a forward bias,

which recombine with electrons in the neutral region of the semiconductor.


8.2 Metal-Oxide-Semiconductor Interfaces

MOS diode consists of a semiconductor substrate with a thin insulating layer and a top metal

contact. A second metal layer forms an ohmic contact to the back of the semiconductor.

The band diagram for metal, oxide and semiconductor when they are in contact and under zero bias

is shown in Figure 11. As can be seen at zero applied voltage there is no energy difference between

the metal work function Φm and the semiconductor work function Φs. In other words the band is flat

(flat-band condition) when the applied voltage is zero. If Φms is the work function difference then

(9)

where is semiconductor electron affinity, Ec the band gap and the potential difference

between the Fermi level and intrinsic Fermi level.

Fig. 11 Band Diagram for Ideal MOS Diode

When an ideal MOS structure is biased with positive or negative voltages, basically three cases may

exist at the semiconductor surface. Consider p-type semiconductor. Now when a negative voltage V

is applied to the metal plate, the top of the valence band bends upward and is closer to the Fermi

level. Since the carrier density depends on the energy difference (EF - EV) and Fermi level remains

constant because of no current flow in an ideal MOS, this band bending causes accumulation of

majority carriers (holes for p-type and electrons for n-type) near the semiconductor surface. This

phenomenon is termed as accumulation process.

For V > 0 (positive voltage across the metal plate), metal Fermi level is lowered by qV relative to

its equilibrium position. As a result, the oxide conduction band is again tilted (moving the metal

oxide down relative to the semiconductor side). The positive voltage deposits positive charge on the

metal and calls for a corresponding net negative charge at the surface of the semiconductor. Such a

negative charge in the p- type material arises from the depletion of holes from the near surface,

leaving behind uncompensated ionized acceptors. This is the depletion case as shown in Figure

12(a). If the voltage is further increased in the positive direction, holes are repelled to a large extent.

The bands bend even more downward such that the intrinsic level Ei at the surface crosses over the


Fig. 12 (a) Band Diagram for Ideal MOS Diode under biasing, (b) Inversion region of MOS diode

Fermi level as seen in Figure 12(b). At this point electrons become the majority carriers. The

surface is thus “inverted” and this process is termed as inversion. In the case of an n-type

semiconductor, with the polarity of voltages reversed, the same sequence of events takes place.

With a positive gate voltage the majority carriers (electrons) accumulate at the surface making the

semiconductor more n-type. With a negative gate voltage, a depletion layer is formed as electrons

are pushed back from the surface. At inversion, the depletion layer reaches a maximum width. Any

further increase in gate bias is balanced by the inversion layer charge, which consist of mobile

holes.

9. II-VI Semiconductor Devices

Most researchers have done work on II-VI or III-V semiconductors devices. Compared to these

materials, II-VI semiconductors such as CdSe has wide band gap (Eg ~ 1.74eV). CdSe exhibits

strong-confinement effect, which cannot be accessed in other materials. CdSe is a very attractive

semiconductor material due to its wide variety of applications in photovoltaic absorbers, IR

detectors, photographic and memory-switching devices [109-111], thermoelectric cooling materials,

laser materials, optical sensors, supersonic materials, optical recording materials, and solar cells

[112-115]. PVA is chosen as the polymer matrix for its aqueous solubility. The high viscosity of the

polymer solution would be helpful in controlling the growth of selenide nanocrystals. Furthermore,

from the application point of view, the polymer matrix would protect the selenide particles against

photo oxidation. Normally PVA is a poor electrical conductor, and the conductivity of the polymer

is of major importance in constructing a Schottky barrier. Its electrical conductivity depends on the

thermally generated carriers and the addition of suitable dopant materials. When a polymer is doped

with semiconductor, especially II-VI semiconductor such as PbSe, ZnSe and CdSe in various

quantities and forms, their incorporation within a polymeric system may be expected to improve the

conductivity [116-117]. In other words, diffusion of the dopant material into polymer matrix plays

an important role in the conduction process. The doping process affects the chemical structure,

crystallinity, and electrical conductivity of polymers [118]. A low doping of the polymer associated

with a low number of charge carriers can lead to an extended depletion layer in the structure. On the

other hand, high-charge carrier densities in the polymer may give rise to a thin barrier with a high

probability of tunneling through the barrier.

However, the formation of the Schottky barrier at the metal–semiconductor (MS) interface has been

a subject of extensive investigations for several years and the fundamental mechanisms that

determine the barrier height are still not fully understood [119-122]. The interface properties of the

MS contacts have a dominant influence on the device performance, reliability and stability.


Schottky barrier inhomogeneity at MS interfaces has been considered as an important factor in

explaining the non-ideal behaviour of the Schottky diodes. The Schottky contact’s quality with a

sufficient height barrier (Φb) and low leakage current are critical factors for the realization of

Schottky diode. In fact, the barrier height (Φb) is a key parameter of the junction, controlling both

the width of the depletion region in the semiconductor and the electron current across the interface

[123-124]. The analysis of the current voltage (I–V) characteristics of the SBDs at room

temperature does not only give detailed information about their conduction process or the nature of

barrier formation at the MS interface but, the temperature dependence of the I–V characteristics

allows us to understand different aspects of conduction mechanisms [125-128]. From I–V

characteristics, parameters like the ideality factor (n), the effective barrier height (Φb) has been

calculated. C-V measurements, in the reverse bias, of Al/n-PbSe-PVA have been performed. The

values of barrier height (ΦC-V), the built-in–voltage (Vbi) and carrier concentration (ND) and

depletion layer width (W) have been calculated.

The samples of Schottky diode consist of at least two layers to form a p-n junction. A Schottky

diode is a p-n junction which measures I-V characteristic and allows current conduction in the

forward direction and blocks the current in the other (reverse) direction. The Metal-Semiconductor

(MS) diode always possesses a thin interfacial native oxide layer between the metal and

semiconductor. The existence of such an insulating layer can turn MS diode into Metal-Oxide-

Semiconductor (MOS) diode. There are several possible reasons of errors that cause deviation of

the ideal behavior of Schottky diodes with and without an oxide layer. The effect of oxide layer in

between semiconductor and metal has been studied. The Alumina (Al2O3) is used as oxide layer and

prepared by electrolytic anodization. The film prepared by this method has advantages: (1) easy

preparation with self-organized by anodizing an aluminum substrate; (2) high uniformity and

controllability of nano-parameters of samples by anodizing conditions; (3) a variety of depositable

materials into the nonporous as catalyst for preparation of nanoscale structures.

Deposition of Oxide layer Anodization is divided into two steps which were similar to that reported by Masuda and Satooh et

al [129-130]. In this work, aluminum anodization was conducted under a constant cell potential 12

V in 0.3 mol/L oxalic acid as electrolyte. Plumbum sheet has been chosen as cathode in the

experiment as shown in figure 13(a). The temperature of the electrolyte is maintained constant at 20 oC during the anodization. After the specimen has been first anodized for 2 hrs, the anodic oxide

layer was then eroded in a mixture of phosphoric acid (6 wt. %) and chromic acid (1.8 wt. %) at 65 oC for 10~20 min. Thus, numerous flat pores have been obtained in the surface of the Al substrate,

which will be propitious to nucleate for the second step anodization. The specimen got after erosion

is anodized in 0.3 mol/L oxalic acid for another 3 hours. After the second step anodization, the

specimen is dipped into the 5 wt% phosphoric acid at 50oC for broadening the pores. The ideal

porous structure of anodic alumina is shown in figure 13 (b).

The as prepared diode is characterized by current-voltage (I-V) and Capacitance-voltage (C-V)

measurement. The analysis of the I-V and C-V characteristics of the both MS and MOS diode at

room temperature does not give detailed information about their conduction process or the nature of

barrier formation at the metal/semiconductor interface. The study of temperature dependence of I-V


Fig. 13 (a) Electrolytic deposition of anodic alumina, (b) Ideal porous structure of anodic alumina

and C-V characteristics is necessary to understand conduction mechanism. The temperature

dependence I–V measurements are taken with Keithley’s electrometer 6517 A and C-V

measurements are taken with Hioki 3532-50 LCR Hi-Tester, at different frequencies and

temperature range.

Fig. 14 SEM image of Alumina deposited by electrolytic anodization on Al substrate

The SEM image of porous Alumina deposited by electrolytic anodization on Al substrate is shown

in figure 14. The alumina particles film is in homogenous and aggregated phase. There are no

cracks or pinholes seen in the film. The oxide layer thickness is of order of 20 Å.

Forward Current-Voltage Characteristics A schottky barrier refers to a metal-semiconductor contact having a large barrier height and low

doping concentration that is less than the density of the states in conduction band or valence band.

The current transport in Schottky diode is mainly due to majority carriers, in contrast to p-n

junction. For the Schottky diodes operated at moderate temperature (e.g. 300K), the dominate

transport mechanism is thermionic emission of majority carriers from the semiconductor over the

potential barrier into the metal. At the semiconductor surface, carriers may recombine with the

recombination centers due to dangling bonds of the surface region. In addition, if the carriers have

the sufficient energy, they may be “thermionically” emitted into the vacuum. At the thermal

equilibrium, the current is balanced by two equal and opposite flows of carriers, thus there is zero

net currents. Electrons in the semiconductor flows into the metal, there is opposing balanced flow of

electrons from metal to semiconductor. At the semiconductor surface an electron can be

thermionically emitted into the metal if its energy is above the barrier height, then


(10)

At thermal equilibrium,

(11)

where, is the current from metal to the semiconductor is the current from the

semiconductor to the metal, and C1 is the proportionality constant. When a forward bias is applied

to the contact, the electrostatic potential difference across the barrier is reduced, the electron density

at the surface increases to

(12)

Then, the net current under forward bias is then

(13)

Using same argument for the reverse- bias condition, the expression for the net current is identical

to above equation except VF is replaced by -VR. The coefficient C1NC is found to be equal to A*T

2,

where A* is called the effective Richardson constant and T is the absolute temperature. Its value

depends on the effective mass.

The current-voltage characteristics for Schottky diode [131] are as follows:

(14)

(15)

where, I is current flowing, Is the saturation current density and n is the ideality factor, A is diode

area, A* is the Richardson constant, T is the absolute temperature, is the Schottky barrier height

and k is the Boltzmann constant (8.62 ×10−5

eV / k). This method fails when the influence of the

series resistance is already significant at medium or small forward voltages or even at reverse bias.

Another conduction mechanism such as generation–recombination current or leakage current is also

taken into account to explain nonideality of the curves. When a metal–semiconductor (MS)

structure with the Rs is considered, according to the TE theory, it is assumed that the relation

between the applied forward bias voltage V, and the current I is expressed as

(16)

where I0 is the reverse saturation current derived from the straight line intercept of the current zero

bias, and is given by

(17)


From equation (16) and (17), the barrier height ( and ideality factor (n) of Schottky diode is

calculated from following relation

(18)

(19)

The semi logarithmic forward current-voltage characteristics for Schottky diode are plotted. By

extrapolating the values of current density to zero voltage axes give the saturation current. The

barrier height and ideality factor is obtained from the slope and intercept of this graph.

Reverse current–voltage characteristics

Reverse leakage current in a semiconductor device is the current from that semiconductor device

when the device is reverse-biased. When a semiconductor device is reverse biased it should not

conduct any current at all, even though, as a temperature effect, it will form electron-hole

generation in the depletion layer and therefore produced a very small current, which is named

reverse leakage current. The reverse leakage current (IR or Ileakage) is given by

(20)

According to equation (20), the reverse leakage current is exponentially dependent on the operating

temperature.

Figure 15 show the forward and reverse Current-Voltage Characteristics of Al/PVA:n-CdSe,

Schottky and MOS diode at room temperature. The inset of figure shows the structure of PVA:n-

CdSe, based Schottky diode. In this geometry of PVA:n-CdSe films are grown by using casting

method at 300 K using polyvinyl alcohol as capping agent. As a positive bias is applied to the

metal, the Fermi energy of the metal is lowered with respect to the Fermi energy in the

semiconductor. This results in a smaller potential drop across the semiconductor. The balance

between diffusion and drift is disturbed and more electrons will diffuse towards the metal than the

number drifting into the semiconductor.

This leads to a positive current through the junction at a voltage comparable to the built-in potential.

As the negative voltage is applied, the Fermi energy of the metal is raised with respect to the Fermi

energy in the semiconductor. The potential across the semiconductor now increases, yielding a

larger depletion region and a larger electric field at the interface. The barrier, which restricts the

electrons to the metal, is unchanged so that barrier, independent of the applied voltage, limits the

flow of electrons.

The metal-semiconductor junction with positive barrier height has therefore a pronounced rectifying

behavior. A large current exists under forward bias, while small current exists under reverse bias.

This shows good diode characteristics. The semi logarithmic forward current-voltage characteristics

show an appreciable increase in the forward bias conditions. By extrapolating the values of current

density to zero voltage axis give the saturation current. The barrier height and ideality factor is

obtained from the slope and intercept of this graph the linear region of the forward-bias ln(I) versus

V plot.


-3 -2 -1 0 1 2 3-1.5x10

6

-1.0x106

-5.0x105

0.0

5.0x105

1.0x106

1.5x106

2.0x106

2.5x106

Cu

rre

nt

(pA

)

Voltage(V)

-4 -3 -2 -1 0 1 2 3-1.0x10

5

-5.0x104

0.0

5.0x104

1.0x105

1.5x105

2.0x105

Cu

rren

t (p

A)

Voltage (V)

Fig. 15 Forward and Reverse Current-Voltage Characteristics of Al/n-PVA:n-CdSe Schottky diode

and Al/Al2O3/n-PVA:n-CdSe MOS diode at room temperature

The Schottky barrier height Φb and ideality factor is found to be 0.74 eV and 2.3. The MOS barrier

height Φb and ideality factor is found to be 0.78 eV and 1.9. The diode shows a large ideality factor.

There may be a portion of the device nonideality attributable to generation recombination currents

due to deep levels in CdSe. However, tunnelling mechanisms probably provide a greater

contribution to both the high device currents and device nonideality. Another possibility for the

high leakage current is due to oxide layer on the CdSe, even though the removal of oxide layer

during Schottky diode fabrication has been conducted, the exposure from the atmosphere was

unavoided resulting in a native oxide on CdSe surface. The oxide layer degraded the rectifying

properties of metal contact deposited on the surface. The greater value of ideality factor is caused

by the interfacial layer and surface states.

C-V Measurements

In order to study nature of depletion region in the Schottky barrier, C-V characteristics are studied

[132]. The mode of measurement depends on the capacitance, when the element is reverse-biased.

Under the reverse-bias conditions, there is a region of uncovered charge on the either side of the

junction that together make up the depletion region and define the depletion width W. As the

reverse-bias potential increases, the width of the depletion region increases, which in turn reduces

the capacitance [133]. The depletion- layer capacitance C can be given by the equation

(21)

and (22)

The depletion-layer width W is obtained from the equation (21)

(23)


From the intercept and slope of the graph, the built-in–voltage and carrier concentration is

calculated. The carrier concentration is obtained by the following equation

(24)

-5 -4 -3 -2 -1 07.20E+019

7.25E+019

7.30E+019

7.35E+019

7.40E+019

7.45E+019

1/C

2 (

F-2)

V(V)

Fig. 16 Reverse bias Capacitance-Voltage Characteristics of Al/n-PVA:n-CdSe Schottky diode of

Al/Al2O3/n-PVA:n-CdSe at room temperature

As the barrier height is the sum of built-in-voltage Vbi and Vn. The Vn can be calculated from the

equation

(25)

where is acceptor concentration and ND is the donor concentration. Figure 16

shows reverse C-V characteristics of Al/PVA:n-CdSe Schottky diode at room temperature. The

values of built-in-voltage, barrier height, carrier concentration and depletion layer width can be

obtained from equations (21) to (25), respectively, as shown in Table 2.

Table 2 Diode Parameters calculated from C-V measurement of CdSe Schottky diode

10. Summary

The author has given an overview about the inorganic/organic hybrid materials, their synthesis and

properties. The detailed descriptions of the II-IV semiconductor nanoparticles have been given. The

synthesis method and the factors affecting their properties have been discussed in detail. The

structure, conduction mechanism, diode-theory, formation of the diode and diode characteristics of

metal-semiconductor and metal-oxide-semiconductor contact have been discussed in detail.

Temperature (T) 273K

Built-in-Voltage (Vbi) 1.00 V

Fermi Energy (EF) 5.17×10-2

eV

Donor Concentration (ND) 3.37×1018

cm-3

Depletion layer width (W) 1.75×10-6

cm


CdSe/PVA nanocomposites have been synthesized by in-situ technique. The structural and optical

properties of nanocrystalline semiconductor CdSe/PVA nanorods have been described in detail. The

results obtained from the present work are in good agreement with some of those reported in the

literature. The fabrication and characterization of Schottky and MOS diode of CdSe/PVA

nanocomposites have been discussed. The various parameters, the ideality factor (n) and the

effective barrier heights (Фb), have been calculated from forward current-voltage characteristics.

The values of barrier height (ФC-V), the built-in–voltage (Vbi) and carrier concentration (ND) and

depletion layer width (W) have been calculated in reverse bias capacitance-voltage characteristics.

In general, the experimental values of n are greater than one indicating that the thermionic emission

is not the dominant conduction mechanism, but it is rather a recombination-tunneling mechanism

assisted by traps at the metal/semiconductor interface in all Schottky diodes. The barrier

inhomogeneities also play an important role in explaining the transport properties of metal-

semiconductor/ metal-oxide-semiconductor contact of these diodes.

Acknowledgment

Author is thankful to Ms Mamta Sharma and Ms Ramneek Kaur for the help during the preparation

of the manuscript.

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