Effect of Adding Selected Impurities the Optical and Electrical Properties of Silicon

Mohamed Osman Ahmed Ibrahim

Postgraduate Diploma in Physics-University of Gezira (2014)

B.Sc. in Education physics &mathematics, University of Gezira (2012)

A Dissertation

Submitted to the University of Gezira in Partial Fulfillment of the

Requirements for the Award of the Degree of Master of Science

in

Physics Sciences

Department of Electronics Engineering

Faculty of Engineering and Technology

May, 2018

i

Effect of Adding Selected Impurities the Optical and Electrical Properties of Silicon

Mohamed Osman Ahmed Ibrahim

Supervision Committee:

Name Position Signature

Dr. Hasaballrasoul Gesmallh Ismail Main Supervisor …………….

Prof. Mubarak Dirar Abdullah Yagoup Co. Supervisor ………….....

April /2018

ii

Effect of Adding Selected Impurities the Optical and Electrical Properties of Silicon

Mohamed Osman Ahmed Ibrahim

Examination Committee:

Name Position Signature

Dr. Hasaballrasoul Gesmallh Ismail Chairperson ………......

Dr. Nooreldain Edriss Fadol Adam External Examiner …………...

Dr. Murtada Mohammed Abdelwahab Internal Examiner …………...

Date of Examination: 28/April, 2018.

iii

Holly Quran

قَاَل تَ َعاََل: م آ آ م آ ﴿يرفَ آ َنُ َ ني َ َ ُ َ ُ َ ني َ آ ٍ آ أُ تُ َ َ ع ل َ َدَرَجات َنُ مباتَعَملُ َن َخبريٌ ﴾

سورةالمجادلة ﴿11﴾

iv

Dedication To those who taught me how to catch the pen a

speak to my mother

Who taught me contestation

My father

My brothers

To my teachers my friends

My colleagues in the study

To Family of department of physics

&mathematics, Faculty of Education –El

Hasaheisa, University of Gezira to all those who

are the source of their nation`s progress.

v

Acknowledgements

Thanks to the University of Gezira, college of

Engineering and technology, especially the electronics department that we have and to Dr.

Hasaballrasoul Gesmallh Ismail Professor Mubarak

Dirar Abdullah "University of Sudan", Dr.

Mohammed Saeed Dawalbait and Dr. Murtada

Abdul Wahab Mohammed "University of Gezira. Dr.

Abdalsaki University of Elnileen.

vi

Effect of Adding Selected Impurities the Optical and Electrical Properties of Silicon Mohamed Osman Ahmed Ibrahim M.Sc. in Physics

Abstract

The Impurities are used to improve optical and electrical properties for semiconductor, in this piece of study impurities are used to improve the optical and electrical properties of a silicon semiconductor. The aim of this study effect of adding impurities to improve the optical and electrical properties of silicon. For this reason three different samples are prepared from silicon doped with different types of impurities having the different concentration. One sample is doped with iodine pointedly (1.03%), another one is doped with Coumarin 500 pointedly (1.05%) and the third one is doped with Rohdamin B pointedly (1.07%). Spectrometer is used to measure absorption, absorption coefficient, extintion coefficient, and energy gap each of the above three mentioned samples. The results showed that for absorption the maximum is absorbed in the case of iodine sample and the minimum is in the case of the Coumarin 500 while the absorption of the Rohdamin B is found to lie in between that of these two samples. For the absorption coefficient the results showed that the maximum is in the case of the Rohdamin B sample and the minimum is for the Coumarin 500 sample while the coefficient of absorption for the iodine sample is found to lie in between that of these two samples. As for the excitation coefficient it is found that the maximum occurred in the case of the Rohdamin B sample and the minimum is in the case of the Coumarin 500 sample while that for the iodine sample is found to lie in between that of the above two cases. Finally the results of measurements of the energy gap are found to be 2.11 ev for the Coumarin 500 sample, 2.08 ev for the iodine sample, 2.03 ev for the Rohdamin B sample. The outcome of the results found in this piece of study indicate that the energy gap and the absorption increase of the three doped samples. However, the absorption coefficient and excitation coefficient are found to depend on the type of the impurity used. Here they are found to increase in the case of the iodine and Rohdamin B impurities and to decrease in the case of Coumarin 500. This study recommends to use same types of impurities added to anther semiconductor like germanium having the same concentration used have so as to see whether the results obtained here constitute a general rule or they just a special singular case.

vii

أثر تغيير إضافة الشوائب في تحسين الصفات الضوئية والكهربية للسليكون

محمد عثمان أحمد إبراهيم

ماجستير العلوم في الفيزياء

ملخص الدراسة

الشووووووها ا لدووووووو ال ووووووو ة علوووووووى تحسوووووووات الضوووووو ال وووووووه اة والكدرب اوووووووة ألشوووووووو ال ال ه ووووووو . فووووووا جوووووواا السووووووائ ووووووت ال ا ووووووة الشووووووها ا تسووووووكو لكحسووووووات الضوووووو ال ووووووه اة والكدرباووووووة للسوووووولاكه شووووووو ال ه ووووو . والدووووو ا وووووت ال ا وووووة ا وووووة تووووور صفووووو فة الشوووووها ا فوووووا تحسوووووات الضووووو ال وووووه اة والكدرباوووووة للسووووولاكه . لدووووواا السووووووا توووووا تح وووووار تووووو ن عاوووووو وكل وووووة وووووت السووووولاكه وشوووووهب بووووو ها وكل ووووووة ووووووت الشووووووها ا بكرا اووووووا وكل ووووووة. ال اوووووووة األولووووووى شووووووهب ب وضوووووور الاووووووه بكر اووووووا 1.03%(، وال اوووووووة الش اووووووة شووووووهب ب وضوووووور الكووووووه ر ت 500بكر اووووووا %1.05(، وال اوووووووة الش لشووووووة شووووووهب ب وضوووووور الووووووووووورو ا ات بكر اوووووووووووا %1.07(. وجدووووووووووو إ اووووووووووو ا وووووووووووه ص بوووووووووووا 2000 سوووووووووووكو ل اووووووووووو اإل كضووووووو ، ووووووو اإل كضووووووو ، ووووووو الكوووووووهجات، وفسوووووووهة ال نوووووووة لل اوووووووو ال وووووووا ه ة عووووووو ال. الوكووووووو وفوووووووح علوووووووى ص كضووووووو ووووووو لل اووووووووة ال شوووووووهبة ب وضووووووورالاه و نووووووو نا وووووووو لل اووووووووة ال شووووووهبة ب وضوووووور الكووووووه ر ت 500، باو وووووو اإل كضوووووو ل اوووووووة الوووووورو ا ات وجوووووو فووووووا ال وووووو بووووووات ال اوكووووووات عوووووو ال. ل وووووو اإل كضوووووو الوكوووووو وفووووووح علووووووى نا ووووووة فووووووا لووووووة ال اوووووووة ال شووووووهبو ب وضووووور الووووورو ا ات و نووووو نا وووووة فوووووا لوووووة ال اووووووة ال شوووووهبو ب وضووووور الكوووووه ر ت 500 باو ووووو لل اووووووة ال شوووووهبو ب وضووووور الاوووووه وجووووو بوووووات ال اوكوووووات عووووو ال. ب لوسووووووة ل ووووو الكوووووهجات وجووووو علوووووا نا وووووة لوووووو فوووووا ال اووووووة ال شوووووهبة ب وضووووور الووووورو ا ات و نووووو نا وووووة لل اووووووة ال شوووووهبة ب وضووووور الكوووووه ر ت 500 باو ووووو لل اووووووة ال شوووووهبو ب وضووووور الاوووووه وجووووو فوووووا ال ووووو بوووووات ال اوكوووووات عووووو ال. اووووورا كووووو نا ووووو فسووووووووووهة ال نووووووووووة وووووووووو 2.11 صلككوووووووووورو فهلوووووووووو لل اوووووووووووة ال شووووووووووهبو ب وضوووووووووور الكووووووووووه ر ت 500، 2.08صلككوووووووورو فهلوووووووو لل اوووووووووة ال شووووووووهبو ب وضوووووووور الاووووووووه و2.03 صلككوووووووورو فهلوووووووو لل اوووووووووة ال شووووووووهبو ل وضووووور الووووورو ا ات . ورجووووو جووووواال السا اوووووو وووووت ال ا وووووو وجووووو دووووو تشوووووار صلوووووى فسوووووهة ال نوووووو واإل كضووووووو ا ووووووو ا فوووووووا لوووووووة الش توووووووة عاوووووووو ال شوووووووهبة، باو ووووووو ووووووو اإل كضووووووو و ووووووو الكووووووهجات وجوووووو د وووووو ك وووووو ا علووووووا هعاووووووة الشووووووها ا ال سووووووكو و اوووووو د وووووو ا وووووو ا فووووووا لووووووة الكشووووووه ا بوووووو لاه والوووووورو ا ات وت وووووو فووووووا لووووووة الكووووووه ر ت 500. جوووووواال ال ا ووووووة ته ووووووا ب ووووووكو ا وووووو الشوووووها ا وصفووووو فكد لشووووووو ه ووووو ووووور لسر ووووو اه بوووووو و الكرا اوووووا ل رفوووووة جووووو جووووواال الوكووووو ال ة ت كور ن ع ة ع و لة ة ب لسلاكه .

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Table of Contents

NO Subject page 1 Halley Quran iv 2 Dedication v 3 Acknowledgment vi 6 English Abstract vii 7 Arabic Abstract viii 8 Table of Contents ix 9 List of Figures xi 10 List of Abbreviations xii Chapter One Introduction 1.1 Overview 1 1.2 A study Problem 2 1.3 Aims of research 3 1.4 Methodology 3 1.5 Lay out of these 3 Chapter Two Literature Review 2.1 Introduction 4 2.2 Semiconductor 4 2.3 Semiconductor types 4 2.4 Semiconductor devices 6 2.5 Energy bands and electrical conduction 10 2.6 Conductivity 21 2.7 Impurities 23 2.8 Doping of Semiconductor 25 2.9 Silicon 26 2.10 Absorption 28

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2.11 Absorption Coefficients 28 2.12 Determination Of Band Gaps 29 2.13 Literature Review 29 Chapter Three Material and Method 3.1 Material 32 3.1.1 Iodine 32 3.1.2 Coumarine500 32 3.1.3 Rohdamin B 32 3.1.4 Spin Coating 32 3.1.5 USB2000 33 3.2 Methods 33 Chapter Four Results and Discussion 4.1 Results 35 4.2 Discussion 37 Chapter Five Conclusion and Recommendations 5.1 Conclusion 39 5.2 Recommendation 39 5.3 Reference 40

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List of Figures

No Figures Page

2.1 Fermi levels to metal, semimetal, semiconductor and insulator 10

2.2 Two-dimensional representation of the silicon lattice 18

2.3 (a) Donor and (b) acceptor levels in extrinsic semiconductors 19

2.4 Schematic representation of the number of electrons per cubic 20

centimeter in the conduction band versus temperature for an

extrinsic semiconductor with low doping

2.5 Conductivity of two extrinsic semiconductor 21

3.1 USB 2000 spectrometer 33

4.1 relationship between absorbance and wavelength 35

4.2 the relation between the Absorption coefficient and 35

wavelengths

4.3 the relation between the Excitation coefficient and wavelengths 36

4.4 relationship between ( (αhf)² ev. m⁻ ¹ and (hf) ev) 36

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List of Abbreviations

Abbreviation Full name

ICs Integrated circuits.

LED Light Emitting Diode

USB Universal Serial Bus

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Chapter One

Introduction

1.1 Overview

A semiconductor material has an electrical conductivity value falling between that of a conductor, such as copper, and an insulator, such as glass. Their resistance decreases as their temperature increases, which is behavior opposite to that of a metal. Their conducting properties may be altered in useful ways by the deliberate, controlled introduction of impurities ("doping") into the crystal structure, which lowers its resistance but also permits the creation of semiconductor junction between differently- doped regions of the extrinsic semiconductor crystal The behavior of charge carriers which include electrons, ions and electron holes at these junctions is the basis of diodes, transistors and all modern electronics ,Semiconductor devices can display a range of useful properties such as passing current more easily in one direction than the other, showing variable resistance, and sensitivity to light or heat. Because the electrical properties of a semiconductor material can be modified by doping, or by the application of electrical fields or light, devices made from semiconductors can be used for amplification, switching, and energy conversion, the modern understanding of the properties of a semiconductor relies on quantum physics to explain the movement of charge carriers in a crystal lattice. Because the electrical properties of a semiconductor material can be modified by doping, or by the application of electrical fields or light, devices made from semiconductors can be used for amplification, switching, and energy conversion, the modern understanding of the properties of a semiconductor relies on quantum physics to explain the movement of charge carriers in a crystal lattice. (Chan, 1994) Doping greatly increases the number of charge carriers within the crystal. When a doped semiconductor contains mostly free holes it is called "p-type", and when it contains mostly free electrons it is known as "n-type". The semiconductor materials used in electronic devices are doped under precise conditions to control the concentration and

xiii regions of p- and n-type dopants. A single semiconductor crystal can have many p- and n-type regions; the p–n junctions between these regions are responsible for the useful electronic behavior. In general, dopants that produce the desired controlled changes are classified as either electron acceptors or donors. Semiconductors doped with donor impurities are called n-type, while those doped with acceptor impurities are known as p- type. The n and p type designations indicate which charge carrier acts as the material's majority carrier. The opposite carrier is called the minority carrier, which exists due to thermal excitation at a much lower concentration compared to the majority carrier For example, the pure semiconductor silicon has four valence electrons which bond each silicon atom to its neighbors (Richard, 1963). The conductivity of semiconductors may easily be modified by introducing impurities into their crystal lattice. The process of adding controlled impurities to a semiconductor is known as doping. The amount of impurity, or dopant, added to an intrinsic (pure) semiconductor varies its level of conductivity. Doped semiconductors are referred to as extrinsic. By adding impurity to the pure semiconductors, the electrical conductivity may be varied by factors of thousands or millions. The materials chosen as suitable dopants depend on the atomic properties of both the dopant and the material to be doped. Absorption coefficient the absorption coefficient determines how far into a material light of a particular wavelength can penetrate before it is absorbed. In a material with a low absorption coefficient, light is only poorly absorbed, and if the material is thin enough, it will appear transparent to that wavelength. The absorption coefficient depends on the material and also on the wavelength of light which is being absorbed. In physics and electrical engineering the reflection coefficient is a parameter that describes how much of an electromagnetic wave is reflected by an impedance discontinuity in the transmission medium. It is equal to the ratio of the amplitude of the reflected wave to the incident wave, with each expressed as phases .The reflection coefficient is closely related to the transmission coefficient. The reflectance of a system is also sometimes called a reflection coefficient (Kittle, 1995).

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1.2 A study Problem

The effect of impurities in improving optical and electrical properties. Determine the physical parameters that affect the optical and physical parameters. Need knowing empirical relation that.

1.3 Research objective

-This objective of this research is determination of the effect of impurities on the reflection and absorption coefficient. -Beside studying the electrical conductivity of impurities.

1.4 Methodology

-Prepare some semiconductor samples having common substrate and changing the concentrations of impurities. -Display absorption and reflection spectrum by using USB 2000 spectrometer. -Relative absorption and reflection to impurities to concentration.

1.5 Thesis layout

The Research Include Five Chapters , Chapter One Is The Introduction , Chapter Two Is The Article Back Ground And Previous Studies , Chapter Three Is The Methodology , Chapter Four Is The Result And Discussion , Chapter Five Is For Conclusion And Recommendation

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Chapter Two

Literature Review

2.1 Introduction

Impurities play an important role in the nucleation of other phase transitions specially semiconductor. This chapter is devoted to do this.

2.2 Semiconductor

A semiconductor is a material that has certain unique properties in the way it reacts to electrical current. It is a material that has much lower resistance to the flow of electrical current in one direction than in another. The electrical conductivity of a semiconductor is between that of a good conductor (like copper) and that of an insulator (like rubber). Hence, the name semi-conductor. A semiconductor is also a material whose electrical conductivity can be altered (called doping) through variations in temperature, applied fields, or adding impurities. While a semiconductor is not an invention and no one invented the semiconductor, there are many inventions which are semiconductor devices. The discovery of semiconductor materials allowed for tremendous and important advancements in the field of electronics. We needed semiconductors for the miniaturization of computers and computer parts. We needed semiconductors for the manufacturing of electronic parts like diodes, transistors, and many photovoltaic cells (Shockley and William, 1950).

2.3 Semiconductor types:-

Semiconductor materials include the elements silicon and germanium, and the compounds gallium arsenide, lead sulfide, or indium phosphide. There are many other semiconductors, even certain plastics can be made semiconducting, allowing for plastic light-emitting diodes (LEDs) which are flexible, and can be molded to any desired shape. A semiconductor is a substance, usually a solid chemical element or compound that can conduct electricity under some conditions but not others, making it a good

xvi medium for the control of electrical current. Its conductance varies depending on the current or voltage applied to a control electrode, or on the intensity of irradiation by infrared (IR), visible light, ultraviolet (UV), or X rays .Elemental semiconductors include antimony, arsenic, boron, carbon, germanium, selenium, silicon, sulfur, and tellurium. Silicon is the best-known of these, forming the basis of most integrated circuits (ICs). Common semiconductor compounds include gallium arsenide, indium antimonite, and the oxides of most metals. Of these, gallium arsenide (Ga As) is widely used in low-noise, high-gain, weak-signal amplifying devices .A semiconductor device can perform the function of a vacuum tube having hundreds of times its volume. A single integrated circuit (IC), such as a microprocessor chip, can do the work of a set of vacuum tubes that would fill a large building and require its own electric generating plant See also atom, transistor, bipolar transistor, and field-effect transistor. Materials that contain both the properties of a conductor and an electrical insulator are known as semiconductors. Although the concept had been discussed before, the first documented observation of a semiconductor was made in 1833 when British physicist Michael Faraday noted the electrical resistance of Silver sulfide declines with falling temperatures. After the initial discovery of semiconductor material, the next big breakthrough came with the realization that these materials could be used in rectification, or the conversion of alternating current (AC) in to direct current (DC). In 1874, British physicist Arthur Schuster observed rectification with a circuit made of copper wires. Schuster noted that rectification only occurred in his circuit after it had been left to sit for a while. Then, after Schuster cleaned off the end of the wires, the rectification effect was gone. The reason for this was when Schuster cleaned off his wires; he was effectively removing copper oxide, a semiconductor. (Al-Azzawi, 2007)

The property of rectification makes semiconductors essential to the creation of a diode. Permitting an electric current to pass in just one direction, diodes are the basis for radio and TV tuners and LEDs (light-emitting diodes), among other things. In addition to being useful at the metal-semiconductor junction, physicists found semiconductor materials to be useful at junction between a semiconductor and an electrolyte, where it has a photovoltaic effect in 1876, William Grylls Adams and Richard Evans Day discovered that lighting up a junction between selenium and platinum could change the

xvii direction of a flowing electric current. This discovery led to the creation of the first solar cell, by Charles Fritts in 1883. The efficiency of this cell was less than 1 percent.

Semiconductors are also crucial to the development of transistors. The first semiconductor transistor was developed by America physicists John Bardeen and Walter Brattain in 1947. The physicists used the semiconductor germanium with two tightly- spaced gold contacts placed against it with a spring. The chunk of germanium had an exterior layer with an overabundance of electrons, and when an electric signal passed in through the gold foil, it inserted points without electrons, making a thin layer with an electron deficiency. A semiconductor material has an electrical conductivity value falling between that of a conductor – such as copper, gold etc. – and an insulator, such as glass. Their resistance decreases as their temperature increases, which is behavior opposite to that of a metal. Their conducting properties may be altered in useful ways by the deliberate, controlled introduction of impurities ("doping") into the crystal structure. Where two differently-doped regions exist in the same crystal, a semiconductor junction is created. The behavior of charge carriers which include electrons, ions and electron holes at these junctions is the basis of diodes, transistors and all modern electronic (Feynman and Richard ,1963).

2.4 Semiconductor devices:

Can display a range of useful properties such as passing current more easily in one direction than the other, showing variable resistance, and sensitivity to light or heat. Because the electrical properties of a semiconductor material can be modified by doping, or by the application of electrical fields or light. The modern understanding of the properties of a semiconductor relies on quantum physics to explain the movement of charge carriers in a crystal lattice. Doping greatly increases the number of charge carriers within the crystal. When a doped semiconductor contains mostly free holes it is called "p-type", and when it contains mostly free electrons it is known as "n-type". The semiconductor materials used in electronic devices are doped under precise conditions to control the concentration and regions of p- and n-type dopants. A single semiconductor crystal can have many p- and n-type regions; the p–n junctions between these regions

xviii are responsible for the useful electronic behavior .Although some pure elements and many compounds display semiconductor properties, silicon, germanium, and compounds of gallium are the most widely used in electronic devices. Elements near the so-called "metalloid staircase", where the metalloids are located on the periodic table, are usually used as semiconductors. Some of the properties of semiconductor materials were observed throughout the mid19th and first decades of the 20th century. The first practical application of semiconductors in electronics was the 1904 development of the cat's-whisker detector, a primitive semiconductor diode widely used in early radio receivers. Developments in quantum physics in turn allowed the development of the transistor in 1947 and the integrated circuit in 1958. Variable conductivity (Peter, 2010).Semiconductors in their natural state are poor conductors because a current requires the flow of electrons, and semiconductors have their valence bands filled, preventing the entry flow of new electrons. There are several developed techniques that allow semiconducting materials to behave like conducting materials, such as doping or gating. These modifications have two outcomes: n-type and p-type. These refer to the excess or shortage of electrons, respectively. An unbalanced number of electrons would cause a current to flow through the material. Heterojunctions occur when two differently doped semiconducting materials are joined together. For example, a configuration could consist of p-doped and n-doped germanium. This results in an exchange of electrons and holes between the differently doped semiconducting materials. The n-doped germanium would have an excess of electrons, and the p-doped germanium would have an excess of holes. The transfer occurs until equilibrium is reached by a process called recombination, which causes the migrating electrons from the n-type to come in contact with the migrating holes from the p-type. A product of this process is charged ions, which result in an electric field. Excited electrons a difference in electric potential on a semiconducting material would cause it to leave thermal equilibrium and create a non- equilibrium situation. This introduces electrons and holes to the system, which interact via a process called am bipolar diffusion. Whenever thermal equilibrium is disturbed in a semiconducting material, the number of holes and electrons changes. Such disruptions can occur as a result of a temperature difference or photons, which can enter the system and create electrons and holes. The process that creates and annihilates electrons and

xix holes are called generation and recombination. Light emission in certain semiconductors, excited electrons can relax by emitting light instead of producing heat. These semiconductors are used in the construction of light-emitting diodes and fluorescent quantum dots. Thermal energy conversion semiconductors have large thermoelectric power factors making them useful in thermoelectric generators, as well as high thermoelectric figures of merit making them useful in thermoelectric coolers (Allen,1960).

A large number of elements and compounds have semiconducting properties, including: Certain pure elements are found in Group 14 of the periodic table; the most commercially important of these elements are silicon and germanium. Silicon and germanium are used here effectively because they have 4 valence electrons in their outermost shell which gives them the ability to gain or lose electrons equally at the same time. Binary compounds, particularly between elements in Groups 13 and 15, such as gallium arsenide, Groups 12 and 16, groups 14 and 16, and between different group 14 elements, e.g. silicon carbide. Certain ternary compounds, oxides and alloys. Organic semiconductors, made of organic compounds. Must common semiconducting materials are crystalline solids, but amorphous and liquid semiconductors are also known. These include hydrogenated amorphous silicon and mixtures of arsenic, selenium and tellurium in a variety of proportions. These compounds share with better known semiconductors the properties of intermediate conductivity and a rapid variation of conductivity with temperature, as well as occasional negative resistance. Such disordered materials lack the rigid crystalline structure of conventional semiconductors such as silicon. They are generally used in thin film structures, which do not require material of higher electronic quality, being relatively insensitive to impurities and radiation damage. Almost all of today's electronic technology involves the use of semiconductors, with the most important aspect being the integrated circuit (IC), which are found in laptops, scanners, cell-phones, etc. Semiconductors for ICs are mass-produced. To create an ideal semiconducting material, chemical purity is paramount. Any small imperfection can have a drastic effect on how the semiconducting material behaves due to the scale at which the materials are used. A high degree of crystalline perfection is also required, since faults in crystal structure (such as dislocations, twins, and stacking faults) interfere

xx with the semiconducting properties of the material. Crystalline faults are a major cause of defective semiconductor devices. The larger the crystal, the more difficult it is to achieve the necessary perfection. Current mass production processes use crystal ingots between 100 and 300 mm (3.9 and 11.8 in) in diameter which are grown as cylinders and sliced into wafers (Yacobi, 2003). There is a combination of processes that is used to prepare semiconducting materials for ICs. One process is called thermal oxidation, which forms silicon dioxide on the surface of the silicon. This is used as a gate insulator and field oxide. Other processes are called photo masks and photolithography. This process is what creates the patterns on the circuitry in the integrated circuit. Ultraviolet light is used along with a photoresist layer to create a chemical change that generates the patterns for the circuit. Etching is the next process that is required. The part of the silicon that was not covered by the photoresist layer from the previous step can now be etched. The main process typically used today is called plasma etching. Plasma etching usually involves an etch gas pumped in a low- pressure chamber to create plasma. A common etch gas is chlorofluorocarbon, or more commonly known Freon. A high radio-frequency voltage between the cathode and anode is what creates the plasma in the chamber. The silicon wafer is located on the cathode, which causes it to be hit by the positively charged ions that are released from the plasma. The end result is silicon that is etched anisotropic ally. The last process is called diffusion. This is the process that gives the semiconducting material its desired semiconducting properties. It is also known as doping. The process introduces an impure atom to the system, which creates the p-n junction. In order to get the impure atoms embedded in the silicon wafer, the wafer is first put in a 1,100 degree Celsius chamber. The atoms are injected in and eventually diffuse with the silicon. After the process is completed and the silicon has reached room temperature, the doping process is done and the semiconducting material is ready to be used in an integrated circuit (Mott, 1969).

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2.5 Energy Bands and Electrical Conduction for Semiconductor

Fig (2.1)Fermi level to metal, semimetal, semiconductor and insulator: Spiral modified from (Allen, 1960) Filling of the electronic states in various types of materials at equilibrium. Here, height is energy while width is the density of available states for a certain energy in the material listed. The shade follows the Fermi–Dirac distribution (black = all states filled, white = no state filled). In metals and semimetals the Fermi level EF lies inside at least one band. In insulators and semiconductors the Fermi level is inside a band gap; however, in semiconductors the bands are near enough to the Fermi level to be thermally populated with electrons or holes. Semiconductors are defined by their unique electric conductive behavior, somewhere between that of a conductor and an insulator. (Sze, 2002) The differences between these materials can be understood in terms of the quantum states for electrons, each of which may contain zero or one electron (by the Pauli Exclusion Principle). These states are associated with the electronic band structure of the material. Electrical conductivity arises due to the presence of electrons in states that are delocalized (extending through the material), however in order to transport electrons a state must be partially filled, containing an electron only part of the time. If the state is always occupied with an electron, then it is inert, blocking the passage of other electrons via that state. High conductivity in a material comes from it having many partially filled states and much state delocalization. Metals are good electrical conductors and have

xxii many partially filled states with energies near their Fermi level. Insulators, by contrast, have few partially filled states, their Fermi levels sit within band gaps with few energy states to occupy. Importantly, an insulator can be made to conduct by increasing its temperature: heating provides energy to promote some electrons across the band gap, inducing partially filled states in both the band of states beneath the band gap (valence band) and the band of states above the band gap (conduction band). An (intrinsic) semiconductor has a band gap that is smaller than that of an insulator and at room temperature significant numbers of electrons can be excited to cross the band gap. A pure semiconductor, however, is not very useful, as it is neither a very good insulator nor a very good conductor. However, one important feature of semiconductors (and some insulators, known as semi-insulators) is that their conductivity can be increased and controlled by doping with impurities and gating with electric fields. Doping and gating move either the conduction or valence band much closer to the Fermi level, and greatly increase the number of partially filled states some wider-band gap semiconductor materials are sometimes referred to as semi-insulators. When undoped, these have electrical conductivity nearer to that of electrical insulators, however they can be doped (making them as useful as semiconductors). Semi-insulators find niche applications in micro-electronics, such as substrates for HEMT. An example of a common semi- insulator is gallium arsenide. Some materials, such as titanium dioxide, can even be used as insulating materials for some applications, while being treated as wide-gap semiconductors for other applications. (Kittle, 1995) Temperature dependence of extrinsic semiconductors, on the other hand is totally different. For example, an n-type conductor exhibits three regions in the temperature vs. carrier concentration curve. Extrinsic Semiconductors in the low temperature region known as Freeze-out region, the charge carriers cannot be excited from the donor level to conduction band due to insufficient thermal energy. In the intermediate temperature range (150 – 450 K) almost all the donor atoms are ionized and electron concentration is approximately equal to donor content. This region is known as extrinsic region. In the high temperature region sufficient thermal energy is available for electrons to get excited from the valence to the conduction band and hence it behaves like an intrinsic semiconductor. The partial filling of the states at the bottom of the conduction band can

xxiii be understood as adding electrons to that band. The electrons do not stay indefinitely (due to the natural thermal recombination) but they can move around for some time. The actual concentration of electrons is typically very dilute, and so (unlike in metals) it is possible to think of the electrons in the conduction band of a semiconductor as a sort of classical ideal gas, where the electrons fly around freely without being subject to the Pauli exclusion principle. (Feynman and Richard, 1963) In most semiconductors the conduction bands have a parabolic dispersion relation, and so these electrons respond to forces (electric field, magnetic field, etc.) much like they would in a vacuum, though with a different effective mass. Because the electrons behave like an ideal gas, one may also think about conduction in very simplistic terms such as the Drude model, and introduce concepts such as electron mobility For partial filling at the top of the valence band, it is helpful to introduce the concept of an electron hole. Although the electrons in the valence band are always moving around, a completely full valence band is inert, not conducting any current. If an electron is taken out of the valence band, then the trajectory that the electron would normally have taken is now missing its charge. For the purposes of electric current, this combination of the full valence band, minus the electron, can be converted into a picture of a completely empty band containing a positively charged particle that moves in the same way as the electron. Combined with the negative effective mass of the electrons at the top of the valence band, we arrive at a picture of a positively charged particle that responds to electric and magnetic fields just as a normal positively charged particle would do in vacuum, again with some positive effective mass. This particle is called a hole, and the collection of holes in the valence band can again be understood in simple classical terms (as with the electrons in the conduction band. Carrier generation and recombination. When ionizing radiation strikes a semiconductor, it may excite an electron out of its energy level and consequently leave a hole. (Kittle, 2005) This process is known as electron–hole pair generation. Electron-hole pairs are constantly generated from thermal energy as well, in the absence of any external energy source. Electron-hole pairs are also apt to recombine. Conservation of energy demands that these recombination events, in which an electron loses an amount of energy larger than the band gap, be accompanied by the emission of thermal energy (in the form of

xxiv phonons) or radiation (in the form of photons). In some states, the generation and recombination of electron–hole pairs are in equipoise. The number of electron-hole pairs in the steady state at a given temperature is determined by quantum statistical mechanics. The precise quantum mechanical mechanisms of generation and recombination are governed by conservation of energy and conservation of momentum. As the probability that electrons and holes meet together is proportional to the product of their numbers, the product is in steady state nearly constant at a given temperature, providing that there is no significant electric field (which might "flush" carriers of both types, or move them from neighbor regions containing more of them to meet together) or externally driven pair generation. The product is a function of the temperature, as the probability of getting enough thermal energy to produce a pair increases with temperature, being approximately exp (−EG/kT), where k is Boltzmann's constant, T is absolute temperature and EG is band gap. The probability of meeting is increased by carrier traps—impurities or dislocations which can trap an electron or hole and hold it until a pair is completed. Such carrier traps are sometimes purposely added to reduce the time needed to reach the steady state (Allen, 1960). The history of the understanding of semiconductors begins with experiments on the electrical properties of materials. The properties of negative temperature coefficient of resistance, rectification, and light-sensitivity were observed starting in the early 19th century. Thomas Johann See beck was the first to notice an effect due to semiconductors, in 1821. In 1833, Michael Faraday reported that the resistance of specimens of silver sulfide decreases when they are heated. This is contrary to the behavior of metallic substances such as copper. In 1839, Alexandre Edmond Becquerel reported observation of a voltage between a solid and a liquid electrolyte when struck by light, the photovoltaic effect. In 1873 Willoughby Smith observed that selenium resistors exhibit decreasing resistance when light falls on them. In 1874 Karl Ferdinand Braun observed conduction and rectification in metallic sulfides, although this effect had been discovered much earlier by Peter Munck of Rosenschold writing for the Anna Len der Physic und Chime in 1835, and Arthur Schuster found that a copper oxide layer on wires has rectification properties that ceases when the wires are cleaned. William Grylls Adams and Richard Evans Day observed the photovoltaic effect in selenium in 1876. A

xxv unified explanation of these phenomena required a theory of solid-state physics which developed greatly in the first half of the 20th Century. In 1878 Edwin Herbert Hall demonstrated the deflection of flowing charge carriers by an applied magnetic field, the Hall Effect. (Al-Azzawi, 2007). The discovery of the electron by J.J. Thomson in 1897 prompted theories of electron- based conduction in solids. Karl Baedeker, by observing a Hall Effect with the reverse sign to that in metals, theorized that copper iodide had positive charge carriers. Johan Koenigsberger classified solid materials as metals, insulators and "variable conductors" in 1914 although his student Josef Weiss already introduced the term Halbleiter (semiconductor in modern meaning) in PhD thesis in 1910. Felix Bloch published a theory of the movement of electrons through atomic lattices in 1928. In 1930, B. Godden stated that conductivity in semiconductors was due to minor concentrations of impurities. By 1931, the band theory of conduction had been established by Alan Harries Wilson and the concept of band gaps had been developed. Walter H. Schottky and Neville Francis Mott developed models of the potential barrier and of the characteristics of a metal-semiconductor junction. By 1938, Boris Davydov had developed a theory of the copper-oxide rectifier, identifying the effect of the p–n junction and the importance of minority carriers and surface states. Agreement between theoretical predictions (based on developing quantum mechanics) and experimental results was sometimes poor. This was later explained by John Bardeen as due to the extreme "structure sensitive" behavior of semiconductors, whose properties change dramatically based on tiny amounts of impurities. Commercially pure materials of the 1920s containing varying proportions of trace contaminants produced differing experimental results. This spurred the development of improved material refining techniques, culminating in modern semiconductor refineries producing materials with parts-per-trillion purity. (Morris, 1990) Devices using semiconductors were at first constructed based on empirical knowledge, before semiconductor theory provided a guide to construction of more capable and reliable devices. Alexander Graham Bell used the light-sensitive property of selenium to transmit sound over a beam of light in 1880. A working solar cell, of low efficiency, was constructed by Charles Frits in 1883 using a metal plate coated with selenium and a thin

xxvi layer of gold; the device became commercially useful in photographic light meters in the 1930s. Point-contact microwave detector rectifiers made of lead sulfide were used by Jag a dish Chandra Bose in 1904; the cat's-whisker detector using natural galena or other materials became a common device in the development of radio. However, it was somewhat unpredictable in operation and required manual adjustment for best performance. In 1906 H.J. Round observed light emission when electric current passed through silicon carbide crystals, the principle behind the light-emitting diode. Oleg Losev observed similar light emission in 1922 but at the time the effect had no practical use. Power rectifiers, using copper oxide and selenium, were developed in the 1920s and became commercially important as an alternative to vacuum tube rectifiers. These devices were used for detecting ships and aircraft, for infrared rangefinders, and for voice communication systems. The point-contact crystal detector became vital for microwave radio systems, since available vacuum tube devices could not serve as detectors above about 4000 MHz; advanced radar systems relied on the fast response of crystal detectors. Considerable research and development of silicon materials occurred during the war to develop detectors of consistent quality. Detector and power rectifiers could not amplify a signal. Many efforts were made to develop a solid-state amplifier and were successful in developing a device called the point contact transistor which could amplify 20db or more. In 1922 Oleg Losev developed two-terminal, negative resistance amplifiers for radio, and he perished in the Siege of Leningrad after successful completion. In 1926 Julius Edgar Lilienfeld patented a device resembling a modern field-effect transistor, but it was not practical. R. Hirsch and R. W. Pohl in 1938 demonstrated a solid-state amplifier using a structure resembling the control grid of a vacuum tube; although the device displayed power gain, it had a cut-off frequency of one cycle per second, too low for any practical applications, but an effective application of the available theory. At Bell Labs, William Shockley and A. Holden started investigating solid-state amplifiers in 1938. The first p–n junction in silicon was observed by Russell Ohl about 1941, when a specimen was found to be light-sensitive, with a sharp boundary between p-type impurity at one end and n-type at the other. (Kasap, 2006)

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A slice cut from the specimen at the p–n boundary developed a voltage when exposed to light. According to G. Busch the term “semiconducting” was used for the first time by Alessandro Volta in 1782. The first documented observation of a semiconductor effect is that of Michael Faraday (1833), who noticed that the resistance of silver sulfide decreased with temperature, which was different than the dependence observed in metals. An extensive quantitative analysis of the temperature dependence of the electrical conductivity of Ag2S and Cu 2S was published in 1851 by Johann Hittorf. For some years to come the history of semiconductors focused around two important properties, i.e., rectification of metal-semiconductor junction and sensitivity of semiconductors to light. In 1874 Karl Ferdinand Braun observed conduction and rectification in metal sulfides probed with a metal point. Although Braun’s discovery was not immediately appreciated, later it played a significant role in the development of the radio and detection of microwave radiation in WWII radar systems (in 1909 Braun shared a Nobel Prize in physics with Marconi). In 1874 rectification was observed by Arthur Schuster in a circuit made of copper wires bound by screws. Schuster noticed that the effect appeared only after the circuit was not used for some time. As soon as he cleaned the ends of the wires (that is removed copper oxide), the rectification was gone. In this way he discovered copper oxide as a new semiconductor. In 1929 Walter Schottky experimentally confirmed the presence of a barrier in a metal-semiconductor junction. Photoconductivity and Photovoltaic In 1839 Alexander Edmund Becquerel (the father of a great scientist Henri Becquerel) discovered the photovoltaic effect at a junction between a semiconductor and an electrolyte. The photoconductivity in solids was discovered by Willoughby Smith in 1873 during his work on submarine cable testing that required reliable resistors with high resistance. Smith experimented with selenium resistors and observed that light caused a dramatic decrease of their resistance. (Kittle, 2005) Adams and Day were the first to discover the photovoltaic effect in a solid material (1876). They noticed that the presence of light could change the direction of the current flowing through the selenium connected to a battery. The first working solar cell was constructed by Charles Frits in 1883. In 1878 Edwin Herbert Hall discovered that charge carriers in solids are deflected in magnetic field (Hall effect). This phenomenon was later

xxviii used to study the properties of semiconductors. Shortly after the discovery of the electron by J. J. Thomson several scientists proposed theories of electron-based conduction in metals. The theory of Eduard Riecke (1899) is particularly interesting, because he assumed the presence of both negative and positive charge carriers with different concentrations and mobilities. Around 1908 Karl Baedeker observed the dependence of the conductivity of copper iodide on the stoichiometry (iodine content). He also measured the Hall effect in this material, which indicated carriers with positive charge. In 1914 Johan Koenigsberger divided solid-state materials into three groups with respected to their conductivity: metal Insulators and “variable conductors”. In 1928 Ferdinand Bloch developed the theory of electrons in lattices. In 1930 Bernhard Godden reported that the observed properties of semiconductors were due exclusively to the presence of impurities and that chemically pure semiconductor did not exist. In 1930 Rudolf Peierls presented the concept of forbidden gaps that was applied to realistic solids by Brillion the same year. Also in 1930 Kronig and Penney developed a simple, analytical model of periodic potential. In 1931 Alan Wilson developed the band theory of solids based on the idea of empty and filled energy bands. Wilson also confirmed that the conductivity of semiconductors was due to impurities. In the same year Heisenberg developed the concept of hole (which was implicit in the works of Rudolf Peierls). In 1938 Walter Schottky and Neville F. Mott (Nobel Prize in 1977) independently developed models of the potential barrier and current flow through a metal- semiconductor junction. A year later Schottky improved his model including the presence of space charge. In 1938 Boris Davydov presented a theory of a copper-oxide rectifier including the presence of a p-n junction in the oxide, excess carriers and recombination. Semiconductors only a very small number of electrons (about 109 electrons per cubic centimeter) contribute to the conduction of the electric current. In most semiconductor devices, a considerably higher number of charge carriers are, however, present. They are introduced by doping, i.e., by adding small amounts of impurities to the semiconductor material. In most cases, elements of group III or V of the periodic table are used as dopants (Morris, 1990). They replace some regular lattice atoms in a substitutional manner. Let us start our discussion by considering the case where a small amount of phosphorus (e.g., 0.0001%)

xxix is added to silicon. Phosphorus has five valence electrons, i.e., one valence electron more than silicon. Four of these valence electrons form regular electron-pair bonds with their neighboring silicon atoms (Fig. 2.2). The fifth electron, however, is only loosely bound to silicon, i.e., the binding energy is about 0.045 eV.

Figure 2.2 Two-dimensional representation of the silicon lattice: Spiral modified from (Morris, 1990).

An impurity atom of group V of the periodic table (P) is shown to replace a silicon atom. The charge cloud around the phosphorus atom stems from the extra phosphorus electron. Each electron pair between two silicon atoms constitutes a covalent bond (electron sharing). The two electrons of such a pair are indistinguishable, but must have opposite spin to satisfy the Pauli principle.

At slightly elevated temperatures this extra electron becomes disassociated from its atom and drifts through the crystal as a conduction electron when a voltage is applied to the crystal. Extra electrons of this type are called "donor electrons." They populate the conduction band of a semiconductor, thus providing a contribution to the conduction process. It has to be noted that at sufficiently high temperatures, in addition to these donor electrons, some electrons from the valence band are also excited into the conduction band in an intrinsic manner. The conduction band contains, therefore,

xxx electrons from two sources, the amount of which depends on the device temperature. Since the conduction mechanism in semiconductors with donor impurities (P, As, Sb) is predominated by negative charge carriers (electrons) these materials are called n-type semiconductors. The electrons are the majority carriers. A similar consideration may be done with impurities from the third group of the Periodic Chart (B, Al, Ga, In). They possess one electron less than silicon and, therefore, introduce a positive charge cloud into the crystal around the impurity atom. The conduction mechanism in these semiconductors with acceptor impurities is predominated by positive carriers (holes) which are introduced into the valence band. They are therefore called p-type semiconductors. Band Structure the band structure of impurity or extrinsic semiconductors is essentially the same as for intrinsic semiconductors. It is desirable, however, to represent in some way the presence of the impurity atoms by impurity states.

Figure2.3 (a) Donor and (b) acceptor levels in extrinsic semiconductors Spiral modified from (Kittle, 2005)

It is common to introduce into the forbidden band so-called donor or acceptor levels (Fig. 2.3). The distance between the donor level and the conduction band represents the

xxxi energy that is needed to transfer the extra electrons into the conduction band. (The same is true for the acceptor level and valence band.) It has to be emphasized, however, that the introduction of these impurity levels does not mean that mobile electrons or holes are found in the forbidden band of, say, silicon. The impurity states are only used as a convenient means to remind the reader of the presence of extra electrons or holes in the crystal. Temperature Dependence of the Number of Carriers At 0 K the excess electrons of the donor impurities remain in close proximity to the impurity atom and do not contribute to the electric conduction. We express this fact by stating that all donor levels are filled. With increasing temperature, the donor electrons overcome the small potential barrier (Fig.2.3 (a)) and are excited into the conduction band. Thus, the donor levels are increasingly emptied and the number of negative charge carriers in the conduction band increases exponentially. Once all electrons have been excited from the donor levels into the conduction band, any further temperature increase does not create additional electrons and the Ne versus T curve levels off (Fig. 2.3). As mentioned before, at still higher temperatures intrinsic effects create additional electrons which, depending on the amount of doping, can outnumber the electrons supplied by the impurity atoms. Similarly, the acceptor levels do not contain any electrons at 0 K. At increasing temperatures, electrons are excited from the valence band into the acceptor levels, leaving behind positive charge carriers. Once all acceptor levels are filled, the number of holes in the valence band is not increased further until intrinsic effects set in. (Kittle, 2005)

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Figure2.4 Schematic representation of the number of electrons per cubic centimeter in the conduction band versus temperature for an extrinsic semiconductor with low doping. Spiral modified from (Kittle, 2005)

2.6 Conductivity

Figure2.4 shows the temperature dependence of the conductivity. We notice that the magnitude of the conductivity, as well as the temperature dependence of s, is different for various doping levels. For low doping rates and low temperatures, for example, the conductivity decreases with increasing temperature (Fig.2.4 (b)). This is similar to the case of metals, where the lattice vibrations present an obstacle to the drifting electrons (or, expressed differently, where the mobility of the carriers is decreased by incoherent scattering of the electrons). However, at room temperature intrinsic effects set in, which increase the number of carriers and therefore enhance the conductivity. As a consequence, two competing effects determine the conductivity above room temperature: an increase of s due to an increase in the number of electrons, and a decrease of s due to a decrease in mobility. (It should be mentioned that the mobility of electrons or holes also decreases slightly when impurity atoms are added to a semiconductor). (Kasap, 2006)

Figure 2.5 Conductivity of two extrinsic semiconductors, (a) relatively high doping and (b) low doping. N d = number of donor atoms per cubic centimeter. Spiral modified from (Kasap, 2006)

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For high doping levels, the temperature dependence of s is less pronounced due to the already higher number of carriers (Fig. 2.4 (a)).

Gallium arsenide (a compound of group III and group V elements of the Periodic Table) is of great technical interest, partially because of its large band gap, 12 which essentially prevents intrinsic contributions in impurity semiconductors even at elevated temperatures, partially because of its larger electron mobility, 12 which aids in high- speed applications, and particularly because of its optical properties, which result from the fact that GA As is a "direct-band gap" material. The large electron mobility in GA As is caused by a small value for the electron effective mass, which in turn results from a comparatively large convex upward curvature of the conduction electron band near r. The electrons which have been excited into the conduction band (mostly from donor levels) most likely populate this high curvature region near r. The atomic bonding in III- V and II-VI semiconductors resembles that of the group IV elements (covalent) with the additional feature that the bonding is partially ionic because of the different valences of the participating elements. The ionization energies12 of donor and acceptor impurities in GA As are as a rule one order of magnitude smaller than in germanium or silicon, which ensures complete ionization even at relatively low temperatures. (Busch, 1989)

The crystal structure of GA As is similar to that of silicon. The gallium atoms substitute for the corner and face atoms, whereas arsenic takes the places of the four interior sites (zinc-blende structure). The high expectations that have been set for GA As as the semiconductor material of the future have not yet materialized to date. It is true that GA As devices are two and a half times faster than silicon-based devices, and that the "noise" and the vulnerability to cosmic radiation is considerably reduced in GA As because of its larger band gap. On the other hand, it’s ten-time higher price and it’s much greater weight (d Si = 2.3 g/cm3 compared to d Ga As = 5.3 g/cm3) are serious obstacles to broad computer-chip usage or for solar panels. Thus, Ga As is predominantly utilized for special applications, such as high-frequency devices (e.g., 10 GHz), certain military projects, or satellite preamplifiers. One of the few places, however, where Ga As seems to be, so far, without serious competition is in optoelectronics (though even this domain appears to be challenged according to the most

xxxiv recent research results). We will learn in Part III that only direct band-gap materials such as Ga As are useful for lasers and light-emitting diodes (LED). Indirect-band gap materials, such as silicon, possess instead the property that part of the energy of an excited electron is removed by lattice vibrations (phonons). Thus, this energy is not available for light emission. Ga As is, of course, not the only compound semiconductor material which has been heavily researched or is being used. Indeed, most compounds consisting of elements of groups III and V of the periodic table are of some interest. Among them are Ga P, Ga N, In P, In As, In Sb, and Al Sb, to mention a few.12 But also, group II-VI compounds, such as Zn O, Zn S, ZnSe, CdS, CdTe, or HgS are considered for applications. These compounds have in common that the combination of the individual elements possesses an average of four valence electrons per atom because they are located at equal distances from either side of the fourth column. Another class of compound semiconductors is the group IV-VI materials, 12 which include Pbs. PbSe, and PbTe. Finally, ternary alloys, such as AlxGa1—xAs, or quaternary alloys, such as AlxGa1—xAsySb1—y, are used. Most of the compounds and alloys are utilized in optoelectronic devices, e.g., GaAs1—xPx for LEDs, which emit light in the visible spectrum (see Part III). AlxGa1—xAs is also used in modulation-doped field-effect transistors (Kittle , 1995)

2.7 Impurities

Impurities play an important role in the nucleation of other phase transitions. For example, the presence of foreign elements may have important effects on the mechanical and magnetic properties of metal alloys. Iron atoms in copper cause the renowned Kondo effect where the conduction electron spins form a magnetic bound state with the impurity atom. Magnetic impurities in superconductors can serve as generation sites for vortex defects. Point defects can nucleate reversed domains in ferromagnetism and dramatically affect their coercivity. In general impurities are able to serve as initiation points for phase transitions because the energetic cost of creating a finite-size domain of a new phase is lower at a point defect. In order for the nucleus of a new phase to be stable, it must reach a critical size. This threshold size is often lower at an impurity site. Diffusion and Ion implantation. N & P Dopants determine the resistivity of material.

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Very low levels for change 1 cm3 Silicon has 5.5x1022 atoms. Significant resistivity changes at even 1010 dopant atoms/cc. Typical doping begins at 1013 atoms/cc NA or ND. Note N lower resistivity than p: due to higher carrier mobility. Near linear relationship below 0.2 ohm-cm (~1016 cm-3). Above that high doping effects. At 1019 get significant degeneracy effects. There quantum effects become important. Typical Si wafer substrate is about 1-10 ohm-cm or 1015-1016 cm-3. Diffusion and Dopant Location. Doping is adding impurities to Silicon. Thermal diffusion process easiest. Directly implanting (injecting) more expensive. Dopant Atoms Substitution AL – replaces Si: Called activated dopants – i.e. n and p carriers created. Interstitial dopant: pushes out Si. True Interstitial dopant atoms: not activated – no carriers. Diffusion under Concentration Gradient. Dopant moves from heavy concentration area to lower concentration area. Reason: simple statistics of motion: More dopant in heavy area. Hence more heading in lower dopant direction. Higher the temperature the faster dopants move. Hence for doping done in a furnace. (Mitchell, 2004)

Silicones are truly jacks of all trades, capable of standing up to the toughest of requirements. Modern life would be inconceivable without silicones. And, thanks to the freedom for designing the silicone molecule, there are countless future applications still to come. Impurities are chemical substances inside a confined amount of liquid, gas, or solid, which differ from the chemical composition of the material or compound. Impurities are either naturally occurring or added during synthesis of a chemical or commercial product. During production, impurities may be purposely, accidentally, inevitably, or incidentally added into the substance. The levels of impurities in a material are generally defined in relative terms. Standards have been established by various organizations that attempt to define the permitted levels of various impurities in a manufactured product. Strictly speaking, then a material's level of purity can only be stated as being more or less pure than some other material. Impurities can be destructive when they obstruct the working nature of the material. Examples include ash and debris in metals and leaf pieces in blank white papers. The removal of impurities is usually done chemically. For example, in the iron, calcium carbonate is added to the blast furnace to remove silicon dioxide manufacturing from the iron ore. Zone refining is an economically important method for the purification of semiconductors. However, some

xxxvi kinds of impurities can be removed by physical means. A mixture of water and salt can be separated by distillation, with water as the distillate and salt as the solid residue. (Peter. Yu, 2010)

Impurities are usually physically removed from liquids and gases. Removal of sand particles from metal ore is one example with solids. No matter what method is used, it is usually impossible to separate an impurity completely from a material. The reason that it is impossible to remove impurities completely is of thermodynamic nature and is predicted by the second law of thermodynamics. Removing impurities completely means reducing the entropy of the system to zero. This would require an infinite amount of work and energy as predicted by the second law of thermodynamics. What technicians can do is to increase the purity of a material to as near 100% as possible or economically feasible. When an impure liquid is cooled to its melting point the liquid, undergoing a phase transition, crystallizes around the impurities and becomes a crystalline solid. If there are no impurities then the liquid is said to be pure and can be supercoiled below its melting point without becoming a solid. This occurs because the liquid has nothing to condense around so the solid cannot form a natural crystalline solid. The solid is eventually formed when dynamic arrest or glass transition occurs, but it forms into an amorphous – a glass, instead, as there is no long-range order in the structure (Shockley and William ,1950).

2.8 Doping of Semiconductor

Doping this involves substituting Si by neighboring elements that contribute excess electrons. For example, small amounts of P or As can substitute Si. Since P/As have 5 valence electrons, they behave like Si plus an extra electron. This extra electron contributes to electrical conductivity, and with a sufficiently large number of such dopant atoms, the material can displays metallic conductivity. With smaller amounts, one has extrinsic n-type semiconducting. Rather than n and p being equal, the n electrons from the donor usually totally outweigh the intrinsic n and p type carriers so that. The donor levels created by substituting Si by P or As lie just below the bottom of the conduction band. Thermal energy is usually sufficient to promote the donor electrons

xxxvii into the conduction band. Doping this involves substituting Si by neighboring atom that has one less electron than Si, for example, by B or Al. The substituent atom then creates a “hole” around it that can hop from one site to another. The hopping of a hole in one direction corresponds to the hopping of an electron in the opposite direction. Once again, the dominant conduction process is because of the dopant. (Mitchell, 2004)

The plot above shows typical variation of the logarithm of the carrier concentration with inverse temperature. At high temperatures (small 1/T) the data follows usual activated behavior of an intrinsic semiconductor. At lower temperatures (larger 1/T) extrinsic behavior dominates. Semiconductor devices the p−n junction is formed when the two different sides of semiconductor are doped, respectively with holes (for example, Al for Si) and electrons (for example, P for Si). One of the properties of the p−n junction is that it rectifies — it allows an electric current to pass only in one direction. (Łukasiak and Jakubowski, 2010).

2.9 Silicon

We encounter silicones every day, though we hardly every notice them. Under the hood, silicone rubber protects the car electronics against moisture and dirt; in car lacquers silicone additives provide gloss; in washing machines, silicone antifoam agents prevent the detergent from foaming over; in shampoo they give hair its sheen; they provide woolen garments with a typical soft hand, and, as silicone resin emulsion paints, they give masonry water repellency, while allowing water vapor and carbon dioxide to diffuse out of its interior. Silicones also perform superbly in medical applications, where high resistance or a state-of-the-art product is required: as a highly pure material for medical tubes, plasters or orthopedic products, as a reliable sealant and insulating material in electrical equipment or insulators. Pyrogenic silica is also used as a thickening additive in adhesives for the rotor blades of wind generators (Morris, 1990)

This outstanding versatility is the result of silicone chemistry: silicones are modern synthetic products based on a raw material, quartz sand, which is available in practically

xxxviii unlimited quantities. Their versatile performance is due to the chemical structure and the many different ways it can be modified. As a result, silicones can be provided with tailor-made properties that are fascinating and offer continually new possibilities. On the following pages, let us guide you through the world of silicones. Discover the unique chemical and physical properties, and gain insights into the versatile applications they open up. In nature, silicon occurs exclusively in oxidized form, as the compounds silicon dioxide and silicates. Silicon is the second commonest element in the Earth’s solid crust, accounting for 25.8 percent by weight, and the most important component of inorganic materials. Since silicon is very rarely found as an element in nature, it was not isolated until relatively recently. On the other hand, siliceous construction and engineering materials, such as sand, clay and ceramics, have been available since time immemorial. The Term “Silicone” The term “silicone” was coined by F. S. Kipping (1863-1949), and refers to the formal analogy between these silicon compounds and the equivalent oxygen compounds of carbon (polysilicoketones). However, the Si-O-Si group is better described by the term “siloxane.” Strictly speaking, therefore, all silicones should correctly be termed “polysiloxanes.” Nowadays, the term silicone is principally used in conjunction with the technical applications of polysiloxanes. (Sconza and Torzo, 1989)

The Silicone Structure Silicones, known to chemists as polydiorganosiloxanes, have a structure that resembles quartz modified with organic groups. They consist of an “inorganic” backbone built up of alternating silicon and oxygen atoms. The other two bonds of the silicon atoms are occupied with organic groups (preferably methyl’s), which are responsible for silicones’ semi organic nature. The conductivity of silicon is increased by adding a small amount of pentavalent (antimony, phosphorus, or arsenic) or trivalent (boron, gallium, indium) atoms (~ part in 108). This process is known as doping and resulting semiconductors is known as doped or extrinsic semiconductor (Morris, 1990).

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2.10 Absorption

The intensity of the net absorbed radiation is dependent on the character of the medium as well as the path length within. The intensity of transmitted or non-absorbed radiation continuously decreases with distance x that the light traverses:

−βx IT = Ioe (2.1)

Where is the intensity of the non-reflected incident radiation and ß the absorption Coefficient (in mm-1), is characteristic of the particular material; furthermore, varies with wavelength of the incident radiation. The distance parameter x is measured from the incident surface into the material. Materials that have large values are considered highly absorptive (Bogatin and Eric, 2004).

2.11 Absorption Coefficients

Much of the information about the properties of materials is obtained when they interact with electromagnetic radiation. When a beam of light (photons) is incident on a material, the intensity is expressed by the Lambert-Beer-Bouguer law:

I = Io exp(−αd) (2.2) If this condition for absorption is met, it appears that the optical intensity of the light wave, (2.6.I), is exponentially reduced while traveling through the film. If the power that is coupled into the film is denoted by I0, gives the transmitted intensity that leaves the film of thickness d. (α) Is called “absorption coefficient”. From (2.2) it follows that 1 I α = − Lin( ) (2.3) d Io It is clear that α must be a strong function of the energy hν of the photons. For hν < Eg (direct), no electron hole pairs can be created, the material is transparent and α is small. For hν ≥ Eg (direct), absorption should be strong. All mechanisms other than the fundamental absorption may add complications (e.g. "sub band gap absorption" through exactions), but usually are not very pronounced. (Allen, 1960) Optical transmission measurements were carried out to determine the film thickness, the wavelength dependence of the refractive index and optical absorption coefficient. The

xl transparent substrate has a thickness several orders of magnitude larger than (d) and has index of refraction (n) and absorption coefficient (α = 0). The index of refraction for air is taken to be n0 =1. In the transparent region (α =0) the transmission is determined by n and s through multiple reflections. In the region of weak absorption α is small and the transmission begins to decrease. In the medium absorption region α is large and the transmission decreases mainly due to the effect of α. In the region of strong absorption the transmission decreases drastically due almost exclusively to the influence of α. If the thickness d is uniform, interference effects give rise to the spectrum (Allen, 1960) 2.12 Determination of Band Gaps The fundamental absorption is related to band-to-band or to exaction transition. The transitions are classified into several types, according to the band structure of a material. The relation between absorption coefficient and optic band gap for direct transition (k=0) is given by: opt √αhν = B(hν − Eg ) (2.4) And for indirect transition (k ≠ 0) the relation becomes

( ℏω−E ) 2 α(hω) ∝ gap (2.5) ℏω

From the αhν versus hν one obtains Eg and B parameters. B is also a useful diagnostic of the material since it is inversely proportional to the extent of the tail state (Δ E) at conduction and valance band edges (Sconza and Torzo, 1989)

2.13 Literature Review

*Band Structure and Electrical Conductivity in Semiconductors. This work done by Amrozia Shaheen. Calculate the energy band gap in the intrinsic region and the temperature dependence of the majority carrier mobility in the extrinsic region. Apparatus. The experiment involves the following major components. Cryostat, Sample cell containing the sample and wound with heater wire, Temperature controller, Thermocouple, Voltmeter with high input resistance, Constant current source, Power supply for the constant current source, Solid state relay (SSR) and Data acquisition system (DAQ). For high temperature measurements, the semiconductor sample is slowly

xli heated inside the sample cell wound with Ni chrome heater wire (Ni chrome 37). It is essentially a copper pipe (20 mm diameter with one end closed, for low temperature. (Shaheen, etal, 2010).

**Doping effects in amorphous oxides, This paper reviews three pronounced doping effects on optical and electrical properties of amorphous oxides; i.e., (i) F-doping of silica glass to improve the vacuum-ultraviolet optical transmission and radiation toughness, (ii) co doping effects on solubility enhancement of rare earth ions in silica glass melt, and (iii) electron-carrier generation in transparent amorphous oxide semiconductors. It is emphasized that effectiveness of electron doping is determined by the magnitude of electron affinity and stabilization energy of a dopant. Importance of the local structure formed around a dopant ion and the location of conduction band minimum measured from the vacuum level is addressed to understand the doping effects in amorphous oxides. Successful examples of doping of amorphous oxides were described along with the respective mechanisms. Amorphous materials are commonly characterized by lack of long-range structural order, and there is a wide range of structural randomness strongly depending on material systems and fabrication conditions. Some types of amorphous materials have local structures similar to those in the corresponding crystalline phases. Substitutional doping would work to some extent for such a material system like amorphous silicon, 92) but off-stoichiometry/ interstitial doping is much more effective in general for carrier generation. In amorphous oxides, electron doping is possible like in the crystalline oxides when the stabilization energy of dopant is large, the CB minimum is located far from the vacuum level, and cation-anion stoichiometry is maintained. A cation which forms a stronger bond with constituent oxygen works as an effective dopant. Interstitial H ion is a typical example as demonstrated in TAOS thin films by ion implantation89), 96), 110) and hydrogen annealing.98), 100) Here, we should consider what the essential bottleneck of the doping ability is. If it is determined by an average structure like chemical composition, a further improvement would be impossible. In silica glass, the minor strained Si•O bonds responsible for the tail state control the optical transmission in the vacuum- UV Region. The concentration of such a strained bond is as low as ³1% estimated from the Raman spectral data.111), this explains well why a small amount of fluorine improves the

xlii vacuum-UV transmittance. The same argument is valid for the co doping effects of P and Al to enhance the solubility of RE ion in silica glass melt. Finally, we would like to mention about an example in which electron doping would alter the bulk structure of amorphous material. 12CaO·7Al2O3 (C12A7) is a typical transparent insulator, but all the O2-, which accommodate in the crystallographic cages as the counter anion to compensate the positive charge of the cage framework structure, may be exchanged by electrons through certain physical/chemical reduction treatments, and the resulting material C12A7:e¹ exhibits metallic conduction (conductivity at RT is 1,500Scm¹1).112) When this C12A7:e¹ is melted at 1800K under a very low oxygen pressure and quenched, we may have C12A7:e¹ glass in which almost the same concentration (³1021cm¹3) of electrons occupy the interstitial positions. (Funabiki, etal, 2012).

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Chapter Three

Materials and Methods

3.1 Materials

3.1.1 Iodine

Iodine is a chemical element with symbol I and atomic number 53. The heaviest of the stable halogens, it exists as a lustrous, purple-black metallic solid at standard conditions that sublimes readily to form a violet gas. The elemental form was discovered by the French chemist Bernard Courteous in 1811.

3.1.2 Coumarin500

Is used in certain perfumes and fabric conditioners. Coumarin is been used as an aroma enhancer in pipe tobaccos and certain alcoholic drinks, although in general it is band as aflavorant food additive, due to concerns regarding its hepatotoxicity in animal models. Is found naturally in many plants.

3.1.3 Rohdamin B

Is a chemical compound and a dye. It often used as a tracer dye within water to determine the rate and direction of flow and transport. Rhoda mine dyes fluoresce and can thus be detected easily and inexpensively with instruments called fluorometers. Rhoda mine dyes are used extensively in biotechnology application such as fluorescence microscopy, flow cytometer, fluorescence correlation spectroscopy and ELISA.

3.1.4 Spin Coating

Spin coating has been used for several decades for the application of thin films. A typical process involves depositing a small puddle of a fluid resin onto the center of a substrate a then spinning the substrate at high speed (typically around 3000 rpm).

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Centripetal acceleration will cause the resin to spread to, and eventually off, the edge of the substrate leaving a thin film of resin on the surface. Final film thickness and other properties will depend on the nature of the resin (viscosity, drying rate, percent solids, surface tension, etc.) and the parameters chosen for the spin process. Factors such as final rotational speed, acceleration, and fume exhaust contribute to how the properties of coated films are defined.

3.1.5 USB 2000

Spectrometer is a versatile, general-purpose UV-Vis-NIR Spectrometer for absorption, transmission, reflectance, emission, color other applications. This popular spectrometer can be custom-configured for maximum flexibility. Choose from a wide range of optical bench options and sampling accessories to build your system. USB2000 spectrometers have a small footprint and take the measurement to the sample. As spectrometer have become smaller, faster and more powerful, applications once considered impractical outside of the laboratory are now feasible. USB spectrometers connect via free-space optics or via fiber to light sources and sampling accessories to measure liquids, solids and other samples.

Figure (3.1) USB2000 spectrometer: Spiral modified from (Chan, 1994)

3.2 Methods

Three sample of silicon were prepared by using cutter and liquid materials Iodine, Rohdamin B and Coumarin 500 were prepared in laboratory after that three sample of silicon were weight by the sensitive devise before the doped.

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Spin coating were uses to made doped in the three sample, the first sample added iodine ((2.5 x 2.15). cm²) was taken and was put in spin coating to doped pointedly (1.03%) and let it to drying, then the second sample added coumarin500 (( 2.5 x 2.8) cm²) was taken in spin coating to doped pointedly (1.05%) and let to drying too and the third sample added Rohdamin B (( 2.5 x 2.75.) cm²) was taken and was but in spin coating to doped pointedly (1.07%) and let to drying after that the sample was weight by sensitive devise too.

At last the three sample were taken in the USB 2000 and calculate the Absorption, and Absorption coefficient and Excitation coefficient and Energy Gap the measurement sprouted at four diagrams, the first diagram symbolizes relationship between absorbance and wavelength and the second diagram symbolizes relationship between the absorption coefficient (α)and wavelength and the third diagram symbolizes relationship between extinction coefficient (K) and wavelength and the last diagram symbolizes relationship between Band Gab and wavelength.

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Chapter Four

Results and Discussion

4.1 Results

2.464 Control Iodeen 2.420 Coumarin 500 Rohadmin B 2.376

2.332

2.288

2.244

2.200

Absorption ( a.u ) a.u ( Absorption 2.156

2.112

396 429 462 495 528 561 594 627 660 Wavelngth ( nm )

Fig (4.1) relationship between absorbance and wavelength

3.96x107 Control 3.52x107 Iodeen

) Coumarin 500

-1 3.08x107 Rohadmin B

( m

 2.64x107

2.20x107

1.76x107

1.32x107

8.80x106

Absorption Coefficient Coefficient Absorption 4.40x106

0.00 396 429 462 495 528 561 594 627 660 Wavelength ( nm )

Fig (4.2) relationship between The absorption coefficient (α)and wavelength

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1.28

1.12 Control 0.96 Iodeen Coumarin 500 0.80 Rohadmin B 0.64

0.48

0.32

Exctintion Coefficient ( k ) ( Coefficient Exctintion 0.16

0.00 396 429 462 495 528 561 594 627 660 Wavelength ( nm )

Fig (4.3) relationship between Extinction coefficient (K) and wavelength

13 5.70x10 Control Eg = 1.58 eV

Iodeen Eg = 2.08 eV 5.13x1013 Coumarin 500 Eg = 2.11 eV

2 13 Rohadmin B E = 2.03 eV ) 4.56x10 g

-1

3.99x1013

( eV.m ( 13

2 3.42x10

)



h 2.85x1013



( 2.28x1013

1.71x1013

1.14x1013

2.016 2.058 2.100 2.142 2.184 2.226 h ( eV )

Fig (4.4) relationship between Band Gab and wavelength

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4.2 Discussion

In figure (4.1) was reported the relation between absorbance and wavelengths , found the behavior of curves is the maximum absorption observed at wavelength 561 nm for iodine sample and the minimum absorption observed at wavelength 415.8 nm for undoped sample. The effects of concetration of samples , absorbance value increased with iodine and rohdamin B increase, absorbance value decreased with coumarine 500 increase.

Show as figure (4.2) the absorption coefficient (α) of the prepared samples also of the concetration were found from the following relation α = 2.303 A/t . where (A) is the absorbance and (t) is the optical length in the samples. This figure show that the maximum value of the absorption coefficient was rohdamin B sample and the minimal was coumarin 500 sample. The effects of concetration of samples , absorption coefficient value increased with rohdamin B increase, absorption coefficient value decreased with coumarine 500 increase.

Extinction coefficient (K) was calculated using the related K = λ α /4 π . The variation at the (K) values as a function of (λ) are shown in fig. (4.3) to for all samples Figure (4.3) was showing the relation between the excitation coefficient and wavelength. In this figure show the maximum value of the excitation coefficient for the rohdamin B sample and the minimal for coumarin 500 sample, this means that he at increased rohdamin B the value of excitation coefficient increase, the value of excitation coefficient decreased at the coumarin 500 increase.

The optical energy gap (Eg) has been calculated by using the relation (αhυ)2 = C(hυ – Eg) where (C) is constant. By plotting (αhυ)2 vs photon energy (hυ) as shown in fig.(4.4 ) to f for all sample, and by extrapolating the straight thin porting of the curve to intercept the energy axis. The value of the energy band gab has been calculated. The value of (Eg) obtained was 1.58 ev for undoped sample and obtained value 2.11 ev in for coumarin500 sample, that is means the energy gap increased as the roumarin 500 increase. Band structure and Electrical conductivity in Semiconductors. This work done by Amrozia Shaheen. Calculate the energy gap in the intrinsic region and the

xlix temperature dependence of majority carrier mobility in the extrinsic region. Apparatus the experiment involves the following major components. Cryostat, sample cell containing the sample and wound with heater wire Temperature, Voltmeter with high input resistance, Constant current source, Power supply for the constant current source, Solid state relay (SSR) and Data acquisition system (DAQ). For high temperature measurements, the semiconductor sample slowly heated inside the sample cell wound with Ni chrome heater wire (Ni chrome 37). It is essentially a copper pipe (20 mm diameter with one end closed, for low temperature.

The two studies improved the electrical properties of semiconductors by means of high temperature and the addition of credit. here, the optical characteristics were improved rather than electrical properties.

l

Chapter Five

Conclusion and Recommendations

5.1 Conclusion

 Absorbency Company the amount of energy received by the compound. The disappearance is the rate of decrease energy inside the compound in order to decode electrons link within the valence package in order to more to contain pay package. The inter in rate of energy needed to transport electrons parity package to plus a power people that can more and increase the interim thing positive for applications of photoelectric. Rise and fall in the absorption and decay related to the length corresponding more length.  These indicate to Extinction coefficient, absorption coefficient, band gap and absorption increase to impurities increase. The extinction coefficient and absorption coefficient increase to add some impurities such as Rohdamin B and Iodine and decrease to add some impurities like a Coumarine500. 5.2 Recommendations

 A study were added another impurity to know the effect and more measurements particularly the structural and electrical by XRD dives and ESM or TSM. 5.3 References

Al-Azzawi, Abdul. (2007) "Light and Optics: Principles and Practices, Circuited Beirut for Print able and Publication, Beirut, pp.53-67.

Allen, J. W, (1960). "Gallium Arsenide as a semi-insulator". Nature. 187 (4735): 403– 405. Bibcode:1960Natur.187..403A. Doi:10.1038/187403b0.

Busch, G, (1989). "Early history of the physics and chemistry of semiconductors-from doubts to fact in a hundred years". European Journal of Physics. 10 (4): 254–264.

Chan, J. (1994), “Four-Point Probe Manual”, EECS 143 Micro fabrication Technology.

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Kasap, S.O, (2006) “Principles of Electronic Materials and Devices”, Boston, McGraw- Hill, pp. 378-405, 114-122.

Kittle, C, (2005) “Introduction to Solid State Physics”, John Wiley and Sons, pp. 216- 226.

Kittle, C. (1995) Introduction to Solid State Physics, 7th ed. Wiley, ISBN 0-471-11181- 3.

Łukasiak, Lidia and Jakubowski, Andrzej, (2010). "History of Semiconductors" (PDF). Journal of Telecommunication and Information Technology, pp135-155.

Mitchell, B.S, (2004) “An Introduction to Materials Engineering and Science”, New Jersey, John Wiley and Sons, Hoboken, pp. 550-557.

Morris, (1990), A History of the World Semiconductor Industry, IET, ISBN 0-86341- 227-0, pp10-15,44-62.

Mott, N, (1969). As in the Mott formula for conductivity, "Observation of Anderson Localization in an Electron Gas". Physical Review. 181 (3): 1336. Bibcode:1969PhRv..181.1336C. Doi:10.1103/PhysRev.181.1336.

Peter, Yu, (2010). Fundamentals of Semiconductors. Berlin: Springer-Verlag. ISBN 978-3-642-00709-5.

Sconza, A and Torzo. G, (1989) “An undergraduate laboratory experiment for measuring the energy gap in semiconductors”.

Shaheen, Amozia and Zia, Wasif and Anwar, Mohammed, (2010). Band Structure and Electrical Conductivity in Semiconductors

Shockley, William. (1950), Electrons and holes in semiconductors: with applications to transistor electronics. R. E. Krieger Pub. Co. ISBN 0-88275-382-7.

Sze, S. M, (2002) “Semiconductor Devices”, John Wiley and Sons, pp33-37.

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Yacobi, B.G, (2003), Semiconductor Materials: An Introduction to Basic Principles, Springer 2003 ISBN 0-306-47361-5, pp. 1–3

Funabiki, Fuji and Kamiya Toshio, and Hosono, Hideo (2012) (Frontier Research Center, Tokyo Institute of Technology Yokohama 226-8503.

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