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CHAPTER 2

RESEARCH BACKGROUND WITH LITERATURE

SURVEY

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CHAPTER – 2

RESEARCH BACKGROUND WITH LITERATURE SURVEY

2.1 QUANTUM CONFINEMENT – A PREVIEW

Quantum size effects in semiconductors occur when the size of the

particle is small in comparison with Bohr excitonic radius which is the natural

length scale of the electron – hole pair. This effect is a direct consequence of the

confined electron and hole motions in three dimensional spaces. Quantum

confinement effect is characterized by electronic transitions which have been

shifted to higher energies upon decrease of the size of the particle. Qualitatively this

confinement effect is similar to the problem of particle in a box (Beck et al. 1992;

Wang.Y. et al. 1987; Alfassi et al. 1982; Goldstein et al. 1992). This leads to

discrete energy levels depending on the size of the structure as it is known from the

simple potential well treated in introductory quantum mechanics. Following this

line, artificial structures with properties different from those of the corresponding

bulk materials can be prepared. Control over dimensions as well as composition of

structures make it possible to tailor the properties of materials to specific

applications. Both semiconductor and metal nanostructures have been investigated

over the years by various research groups (Basu.P.K. et al. 1997; Harrison 2005).

The applications of semiconductor electronic and optoelectronic components based

on structures with quantum size effects have been on the market for several years.

The present research will therefore treat the quantum size effect with semiconductor

materials like CdSe as a base material.

.

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2.2 TYPES OF CONFINEMENTS

Semiconductor nanostructures signify a class of materials in which

quantum confinement effects are investigated in greater detail. They are also

referred to as ‘semiconductor nanocrystals’. The nanostructures treated here can be

divided into the following classes:

2.2.1 Two-dimensional object (or) Quantum Well

Thin films with thickness of the order of a few nanometers are usually

deposited on a bulk material. Their properties may be dominated by surface and

interface effects or they may reflect the confinement of electrons in the direction

perpendicular to the film (a quantum well). In the two dimensions as shown in

Figure 2.1, parallel to the film, the electrons behave like in a bulk material ie.,

electrons can easily move in two directions. The quantum well notation implies that

the electrons feel a potential well as they are trapped in the film.

Figure 2.1 Quantum well structures in two dimensional films, plates and networks where electron is quantized in one direction (nz) and freely moving in 2 dimensions (kx and ky).

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2.2.2 One-dimensional object (or) Quantum Wire

In the case of a one –dimensional system as shown in Figure 2.2, the

electrons are free to move only in one direction. Cylinder-like objects like wires and

tubes with diameters on the nanoscale and lengths typically in the micrometer

range. Confinement effects for electrons may appear in the transverse direction

while electrons are free to move in one dimension (along the structure).

2.2.3 Zero- dimensional object (or) Quantum dot

The electrons are confined to a point in this system as shown in Figure

2.3 wherein the electrons are not free to move at all. Usual names used to represent

zero-dimensional objects are quantum dots, nanoparticles, clusters, colloids,

nanocrystals, and fullerenes. They are composed of several tens to a few thousand

atoms. Here, the movement of electrons is restricted in all three directions.

Figure 2.3 Quantum dot structures in zero dimensional spheres and clusters where electrons confined in all three directions (ny,ny,nz).

Figure 2.2 Quantum wire structures in one dimensional nanofibres, wires and rods where electron is confined in two directions (ny,nz) and freely move in one dimension (kx).

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2.3 BASIC THEORY OF QUANTUM CONFINEMENT

Nanomaterials are closer in size to single atoms and molecules than to

bulk materials. To explain their behaviour, it is necessary to use quantum

mechanics. Quantum mechanics is a scientific model that was developed for

describing the motion and energy of atoms and electrons. In a macroscopic

semiconductor crystal, the energy levels form bands. The valence band is filled and

the conduction band is completely empty at 0 K. The bands are separated with a

specific energy gap, Eg. When an electron gets excited due to thermal excitations,

en electron-hole pair is created. The electron in the conduction band and the hole in

the valence band can be bound when they approach each other at a finite distance.

This bound pair is called an ‘exciton’, which is delocalized through out the crystal.

The Bohr radius of the exciton can be given as,

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where, � is the dielectric constant of the material, � and �� are the effective

masses of electron and hole respectively and � is the elementary charge. Quantum

size effects are manifested when the size of the nanocrystal is comparable to the

exciton radius, a. This is 56 Å for CdSe semiconductor (Adair et al. 1998). The de

Broglie wavelength of the materials is in the range of nanometers and strong

confinement effects are manifested only when the particle dimension approaches

this value. The electronic structure of materials is strongly related to the nature of

the material. In this section, basic theory behind the electronic structure of bulk

materials as well as in confined structures is given in detail.

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2.3.1 Electron states in bulk semiconductor crystals

To discuss the origin of energy bands in bulk semiconductors a single

electron in a crystal is considered. The time-independent Schrodinger equation for a

free electron can be written as,

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Here, �� is the mass of the free electron, $ is the kinetic energy and

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%(& � %&%)&�. The general solution of the above equation is given by,

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where, r is the position vector � ! "! #� and k is the wave vector � 1 234 � ! ��is the

lattice constant. The factor ,- is due to the normalization condition, which requires

that the particle be present in the sample volume V. The linear momentum of the

electrons is 5 �6 and the kinetic energy is given by,

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The density of energy states is just,

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In bulk material of large size, the electronic structure is not restricted by the

dimension of the material. The wavelength of electrons is much smaller than the

typical length of the material. Therefore, the density of states with respect to energy

given in equation (2.4) is smoothly varying in bulk materials as depicted in Figure

2.4. It shows that there are no discrete energy levels in either valence or conduction

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bands. As particle size decreases from bulk to nanomaterial, the valence band splits

up in to discrete energy levels as shown in Figure 2.4 and 2.5 (Chestnoy et al.

1986).

Figure 2.4 Density of states versus energy graph showing smooth variation in bulk material

Figure 2.5 Splitting of energy levels in quantum dots due to the quantum confinement effect, semiconductor band gap increases with decrease in size of the nanocrystal.

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2.3.2 Electron states in confined structures

Some of the properties which change drastically as a function of particle

size are the optical properties, including both absorption and emission of light.

Nanoparticles have discrete orbitals. The energy of the first level will be shifted

from the position of the bulk value by $ � �&;<=4& where ‘a’ is the diameter of the

particle. The energy gap increases with decrease in ‘a’. As a consequence of this,

the CdSe nanocrystals emit light anywhere from 4500 to 6500 Å, so that any color

from blue to red is achievable, depending on the size of the particle, if the size of

the particle is lesser than the Bohr exciton diameter, quantum confinement effect is

detectable.

Based on the degree of freedom on electron’s motion, materials can be categorized

as quantum wells, quantum wires and quantum dots. The graphs between energy

and density of states for two, one and zero dimensional confinement in objects are

given in Figure 2.6 (Pradeep 2007). The graph shows discontinuities in confined

Figure 2.6 Quantization of electronic density of states as a result of variation in the dimensionality of materials

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systems. This will lead to steps in two dimensional confinement, singularities in one

dimensional confinement and discrete lines in zero dimensional confinement.

Electron band structure diagrams can serve as an efficient and informative tool to

understand the behaviour of charge carriers in both macroscopic and microscopic

materials. To discuss the effect of quantum confinement on the electronic structure

of materials, the behaviour of electron in the confined direction of a quantum well

can be considered. The possible energy states in the confined directions will depend

on the boundary conditions which the structure imposes on the electron wave

function. If the electron is assumed to move in a constant potential within the

structure and is subject to an infinite potential barrier at each side, the wave function

is then zero everywhere on the barrier as shown in Figure 2.7.

Figure 2.7 Schematic diagram of particle in a box (or) potential well problem

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Hence,

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Now the motion in x and y directions is a free one and hence the total kinetic energy

can be written as,

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In quantum wires, electrons are confined to two directions (y and z) and free to

move along x direction. Therefore, the total energy of electron becomes,

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In quantum dot, confinement is in all the directions (x, y and z), hence it becomes,

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As depicted in Figure 2.6, the density of states as a function of energies given in

equations (2.7), (2.8) and (2.9) gives additive staircase structure in quantum wells,

additive staircase decayed structure in quantum wires and independent discrete line

structure in quantum dots (Pradeep 2007).

2.4 ROLE OF MULTILAYER IN CARRIER CONFINEMENT

The exclusive properties of CdSe semiconductor were discussed in the

first chapter. Though CdSe semiconductor provides all the possibilities to analyze

quantum confinement effect, it is not easy to produce stable CdSe nanocrystallites

using thermal evaporation technique. As it is very rare that the nanostructures

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appear as free standing structures, one should create artificial constraints over

charge carriers to experience the quantum confinement phenomenon. Hence,

methodology is very important rather than method and material for carrying a

research successfully. Based on the literature survey, it is found that the alternate

stacking of two different materials in a multilayer form creates limitations to the

movement of charge carriers. Multilayer structure of the combination of a

semiconductor and an insulating material is considered as an efficient system to

study the confinement effects. Mainly, multilayer thin films are focused for

confinement effects, high and fast optical non linearity, formation of nanoclusters

and quantum dots (Nesheva et al. 2002). Moreover, preparation of multilayer thin

films showed that the layers deposited in a step-by-step manner were smoother than

those made in one step (Zhou-yao et al. 1987). The stress produced on the surface of

layers can be easily increased in multilayers. According to these suggestions, thin

layers of CdSe material is deposited with its own compound (Se) ie., super lattice

structure, with heterostructure materials (ZnSe and CdTe) and with insulating

material (SiOx) in the present research work.

2.5 EXPERIMENTAL OBSERVATION OF CARRIER CONFINEMENT

When the size of the semiconductor nanocrystal becomes small, the

electronic structure of the crystal is governed by the laws of quantum physics. Very

small group II-VI semiconductor nanoparticles, in the order of 2 - 10 nm, exhibit

significantly different optical and electronic properties from their bulk counterparts.

The characterization of size dependent optical properties of group II-VI

semiconductor particles provide a lot of qualitative and quantitative information

about them – size, quantum yield, monodispersity, shape and presence of surface

defects. A combination of information from both the UV-visible absorption and

fluorescence, complete the analysis of the optical properties.

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Quantum confinement effect of a material is easily observable by

analyzing its physical and optical properties using appropriate analytical

instruments such as HRSEM, TEM, UV-Vis spectrophotometer, Photofluorimeter

and so on. Invention of electron imaging techniques upgrades the nanotechnology to

the higher level. Though XRD and SEM methods reveal the morphological

properties of the multilayer systems with some approximations, the phenomena like

spin-orbit coupling, splitting of valence bands and energy level shifting could not be

characterized by these techniques. Therefore, confinement effects are mostly

discussed with optical absorption and emission data by researchers. Just imagine a

small dot is not enough to say that it is confined. Optical data alone allows insight

into confinement structure of carriers. The following two analyses are very useful to

explain quantum confinement effect

• Optical Absorption

• Photoluminescence Spectroscopy

2.5.1 Optical Absorption

Optical Absorption is a technique that allows one to directly probe the band

gap. Typically, absorption edge is related with transition that takes place between

highest valence band and lowest conduction band. The absorption edges get shifted

to lower wavelength side when particle size decreases. As a result of spin-orbit

interaction, valance band of a semiconductor splits into an energetically higher and

lower components which will be seen in absorption spectra. The band gap edge of a

material should be blue shifted if the material is confined in nanoscale.

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Norris et al. (1996) present the optical absorption of CdSe nanocrystals as a

function of crystallite sizes which is shown in Figure 2.8. As the dot decreases in

size there is a systematic shift of the band gap edge towards shorter wavelengths.

2.5.2 Photoluminescence spectroscopy

Photoluminescence spectroscopy, in general, refers to a characterization

technique that measures the emission of radiation by a material that has been

excited. Fluorescence spectroscopy is one type of emission spectroscopy which

records the intensity of light radiated from the material as a function of wavelength.

It is a nondestructive characterization technique. After an electron is excited from

the ground state, it needs to relax back to the ground state. This relaxation or loss of

energy to return to the ground state, can be achieved by a combination of non-

radiative decay (loss of energy through heat) and radiative decay (loss of energy

through light). This is because loss of energy through vibrational modes across the

band gap can result in breaking the bonds of the crystal. This phenomenon is shown

in Figure 2.9.

Figure 2.8 Fluorescence image of CdSe nanocrystals as a function of size (Left). Absorbance spectrum as a function of size (Right)

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Figure 2.10 Photoluminescence spectra of CdSe particles showing blue shift upon particle size decrease

Figure 2.9 Emission of luminescence photon for group II-VI semiconductor quantum dot�

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A combination of absorbance and emission spectra is shown in Figure 2.10 for four

different sized particles emitting green, yellow, orange, and red fluorescence.

Different sized CdSe particles have different colored fluorescence spectra. The

FWHM of the ML samples is a noteworthy entity. FWHM of the spectra becomes

broad when size of the particle decreases (Kammerer et al. 2001; Kako et al. 2002).

The main reason of this phenomenon is the size fluctuation of the quantum dots

(Mao et al. 2005). Very narrow absorption should allow for production of lasers. At

present QD lasers outperform other solid state lasers at low temperatures (below

room temperature).

2.6 CONFINEMENT EFFECTS IN CdSe BASED ML STRUCTURES

Artificial constraints imposed on charge carriers lead to confinement.

Reducing the size of the particles lesser than the Bohr exciton diameter of the

corresponding material creates restriction on their motion. The multilayers of CdSe

semiconductor coated with various structures such as Se, ZnSe, CdTe, SiOx were

prepared and analysed structurally and optically using analytical instruments. At

this point, the appropriate research background has been presented with useful

literatures and the new ideas incorporated in each combinations of CdSe material.

2.6.1 Cadmium Selenide with Selenium

Selenium which is a constituent element of CdSe semiconductor, is

constructed of random chains, in such a way that all atoms are two-fold coordinated

in chains with a constant dihedral angle, but this angle is changing its sign randomly

(Benkhedir et al. 2006). This makes selenium a mixture of chain and ring fragments

that allows both electrons and holes to attain measurable drift mobilities. This

property of selenium will increase strain and constraints to the adjacent layers in the

multilayer systems. Moreover, the flexibility of selenium strongly reduces the

requirements for matching the lattice constants of the constituent materials in the

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fabrication of CdSe/Se multilayers as shown in Figure 2.11. When CdSe material is

coated over Se in a stacked multilayer film, the strain and constraints to the adjacent

CdSe layers will increase in the system. This will create artificial constraints to the

particle motion, so that possibility for carrier confinement is more. Again, selenium

is a flexible material that can adjust itself according to lattice constants of CdSe

material in the preparation of CdSe/Se multilayers.

2.6.2 Cadmium Selenide with Zinc Selenide

CdSe nanoparticles are apprehended when it is coated with lattice

matched heterostructure such as ZnSe, CdTe in multilayer structure. When two

Figure 2.11 Wurtzite crystal structure of cadmium selenide and rings and chains structure of selenium

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heterostructures with matched lattice parameters as shown in Figure 2.12 are coated

over one another, new lattice boundaries are created and therefore particle motion is

restricted. In this system, the band gap of each material plays an important role

(Toropov et al. 1999). The smaller band gap material usually undergoes

confinement effect. Moreover, this system gives possibility to analyse type I and

type II band structure alignments.

Much attention has been paid to the properties of the CdSe/ZnSe

systems, which have been considered as an attractive system for green-blue opto-

electronic applications (Pejova 2008; Maehashi et al. 2001; Shubina et al. 1998;

Kurtz et al. 2002; Kapitonov et al. 2005; Ohishi et al. 2000). Mostly molecular

beam epitaxy (MBE) (Cardona, 1963; Baldereschi et al. 1971) and chemical route

(Yu.P.Y. 1999; Kurtz et al. 1999) have been used to prepare CdSe/ZnSe systems by

various research groups. A series of profound works by other researchers

Figure 2.12 Wurtzite crystal structure of cadmium selenide and cubic structure of zinc selenide

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(Chinyama et al. 1999; Reiss et al. 2003) have been devoted to correct assignments

of higher-energy transitions detected in the photoluminescence and optical spectra

of CdSe quantum dots (QDs) in various size regimes. In the present work,

alternative CdSe and ZnSe thin film layers are stacked over glass substrates using

physical vapor deposition method. Moreover the changes that take place in

structural and optical properties of CdSe/ZnSe multilayer (ML) thin films have been

thoroughly studied by varying the sublayer thicknesses as well as the number of

sublayers.

2.6.3 Cadmium Selenide with Cadmium Telluride

Recent investigations of type II CdTe/CdSe heterostructure

nanocrystallites are ideal materials for their long range photo induced charge

separation and could be applied in photovoltaic devices (Lee et al. 2009; Sandeep et

al. 2007; Peng et al. 2005). CdSe, CdTe and CdTe/CdSe tetropod nanocrystals

perform well in nanocrystal-polymer hybrid solar cells (Huynh et al. 2002; Sun et

al. 2003; Gur et al. 2006). Few researchers have prepared type II CdTe/CdSe

tetropod nanocrystals in chemical synthesis route (Jing.W. et al. 2010; Saad et al.

2011; Kim et al. 2003). In the present work, the formation of type II nanocrystals in

CdTe/CdSe multilayer thin films prepared by physical vapour deposition method

has been reported. Alternate coating of CdTe and CdSe heterostructure

semiconductors under high vacuum condition provides uniform sequential

arrangement of layers in the order of few nanometers so that trapping of electron-

hole pair is made possible in simple steps.

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2.6.4 Cadmium Selenide with Silicon Monoxide

II-VI semiconductor materials such as CdSe, ZnSe, CdS in an insulator

matrix like SiOx or in a strained system are mainly focused for confinement effects,

high and fast optical non linearity, formation of nanoclusters and quantum

dots(Rabe et al. 1997; Kurtz et al. 1999; Zhang et al. 1997).

Silicon monoxide is an insulator which has the structure of valleys on its surface as

shown in Figure 2.13. These valleys can hold the charge carriers and easily

contribute much to confinement effect. The depth of these valleys can be increased

when thickness of silicon monoxide layer increases (Nesheva et al. 2000). As the

valleys and chains present in SiOx matrix structure with a length up to several

hundreds of nanometers, nanoclusters of semiconductor materials such as CdSe,

ZnSe might be formed on its surface (Nesheva et al. 2000). This combination of II-

VI semiconductor and insulator drastically alter the band structure alignment and

become very interesting system to analyse.

Figure 2.13 Wurtzite crystal structure of cadmium selenide and Silicon monoxide with surface valleys