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Mansoura University Faculty of Science Physics Department Research Blue Light Emitting Diode Prepared by Mohamed Hasanin Farg Premaster degree

Blue light Emitting Diode

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Page 1: Blue light Emitting Diode

Mansoura University

Faculty of Science

Physics Department

Research

Blue Light Emitting Diode

Prepared by

Mohamed Hasanin Farg

Premaster degree

Page 2: Blue light Emitting Diode

Introduction

Energy, when we hear that word must offer thanks and respect to their

importance, It has also become a part of modern life and one cannot

think of a world without it. The most important types of energy are

electricity. Electricity has many uses in our day to day life. It is used for

lighting rooms, Modern means of transportation and communication,

battery cars are quick means of travel, radio, television and cinema.

For that important, we must be preserved, in this research will talk about

the best inventions that has earned a well-deserved Nobel Prize for

Physics, Because it helped keep the energy used in lighting by ten times

less than last use . It is Blue Light Emitting Diode.

Difference between materials

We all know the important roles of electrons in our life , but I will talk in

this research about special role of electrons in solid state material , not all

electrons but some special of them that can do that role

Talk about electrons of the most outer shell in atoms

These electrons have the stick of magic that can change any material from

type to another, how????

Solid materials divided into three types in ability of conduct electricity to

(conductors, insulator and semiconductor)

Let’s discuss about the Fermi level first. Fermi level is the energy level

taken up by an electron at the temperature range of zero Kelvin. So, at the

temperature of zero Kelvin the energy levels which are lower than i.e.

below the Fermi level are filled completely by electrons. Where below

Fermi level is valence band and above it conduction band.

Materials to be able conduct electricity, when it has some electrons in

conduction band.

In conductor (metal)

When there is in outer shell from (1 to 3) electrons material is conductors,

binding energy that bonded these electrons by nucleus is small and

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electrons can be free by small energy in solids material (small or there is

no energy is energy gab between valence band and conduction band) and

do the role of electrical conduction. But in the insulator materials have (5

to 8) so there is high binding energy between these electrons and nucleus

and can not to be free (high energy gab between valence and conduction

band ) and can not conduct electricity ,but in semiconductors have 4

electrons that inter between conductors and insulator in ability of electric

conduction.

Types of Semiconductors

Semiconductors are mainly classified into two types: Intrinsic and

Extrinsic.

Intrinsic Semiconductor

An intrinsic semiconductor material is chemically very pure and

possesses poor conductivity. It has equal numbers of negative carriers

(electrons) and positive carriers (holes). A silicon crystal is different from

an insulator because at any temperature above absolute zero temperature,

there is a finite probability that an electron in the lattice will be knocked

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loose from its position, leaving behind an electron deficiency called a

"hole".

If a voltage is applied, then both the electron and the hole can contribute to a

small current flow.

The conductivity of a semiconductor can be modeled in terms of the band

theory of solids. The band model of a semiconductor suggests that at ordinary

temperatures there is a finite possibility that electrons can reach the

conduction band and contribute to electrical conduction.

The term intrinsic here distinguishes between the properties of pure

"intrinsic" silicon and the dramatically different properties of doped n-type or

p-type semiconductors.

Extrinsic Semiconductor

Where as an extrinsic semiconductor is an improved intrinsic

semiconductor with a small amount of impurities added by a process,

known as doping, which alters the electrical properties of the

semiconductor and improves its conductivity. Introducing impurities into

the semiconductor materials (doping process) can control their

conductivity.

Doping process produces two groups of semiconductors: the negative

charge conductor (n-type) and the positive charge conductor (p-type).

Semiconductors are available as either elements or compounds. Silicon

and Germanium are the most common elemental semiconductors.

Compound Semiconductors include InSb, InAs, GaP, GaSb, GaAs, SiC,

GaN. Si and Ge both have a crystalline structure called the diamond

lattice. That is, each atom has its four nearest neighbors at the corners of a

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regular tetrahedron with the atom itself being at the center. In addition to

the pure element semiconductors, many alloys and compounds are

semiconductors. The advantage of compound semiconductor is that they

provide the device engineer with a wide range of energy gaps and

mobilities, so that materials are available with properties that meet

specific requirements. Some of these semiconductors are therefore called

wide band gap semiconductors.

The Doping of Semiconductors

The addition of a small percentage of foreign atoms in the regular crystal

lattice of silicon or germanium produces dramatic changes in their

electrical properties, producing n-type and p-type semiconductors.

Pentathlon impurities

(5 valence electrons) produce n-type semiconductors

by contributing extra electrons.

Trivalent impurities

(3 valence electrons) produce p-type semiconductors

by producing a "hole" or electron deficiency

N-Type Semiconductor

The addition of Pentathlon (5) impurities such as antimony, arsenic or

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phosphorous contributes free electrons, greatly increasing the

conductivity of the intrinsic semiconductor. Phosphorous may be added

by diffusion of phosphine gas (PH3).

P-Type Semiconductor

The addition of trivalent impurities such as boron, aluminum or

gallium to an intrinsic semiconductor creates deficiencies of

diborane 6H2valence electrons,called "holes". It is typical to use B

gas to diffuse boron into the silicon material.

is a boundary or interface between two types of n junction–p A

inside a single crystal type-n and type-p semiconductor material,

n junctions are –, pdoping . It is created bysemiconductor of

ductor electronic semicon elementary "building blocks" of most

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. LEDs such devices

are able electrons is applied to the leads, voltage When a suitable

within the device, releasing electron holes to recombine with

. This effect is photons energy in the form of

, and the color of the light is electroluminescence called

of the semiconductor. band gap determined by the energy

Idea of creating Light emitting diode

optical ) is anEL( Electroluminescence

in which a material emits electrical phenomenon and phenomenon

or to a electric current in response to the passage of an light

. electric field strong

Electroluminescence is the result of radiative

recombination of electrons and holes in a material, usually

a semiconductor. The excited electrons release their energy

as photons - light. Prior to recombination, electrons and holes may

be separated either by doping the material to form a p-n

junction or through excitation by impact of high-energy electrons

accelerated by a strong electric field.

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Using Gallium nitride (GaN), which is the material used to create blue

LEDs .GaN material has a direct band gap, i.e. the optical transitions

across the band gap are “allowed” and therefore much stronger than in the

case of indirect band gaps. GaN is hard to grow.

In order to make an LED you need to make a P-N junction, meaning a

layer of p-type material (positively doped) on top of n-type material

(negatively-doped). "Doping" a material is the process of changing the

carrier concentration (number of electrons or holes) and this can be done

by adding other elements with a different charge state, such as silicon

(adds an electron) or magnesium (takes an electron). Unfortunately,

adding these "dopants" introduces defects into the host material. Defects

are bad and GaN is very sensitive to certain types of defect.

Method of growth crystal of LED

Epitaxy: Deposition and growth of monocrystalline structures/layers.

Epitaxial growth results in monocrystalline layers differing from

deposition which gives rise to polycrystalline and bulk structures.

Epitaxy types:

Homoepitaxy: Substrate & material are of same kind.

(Si-Si)

Heteroepitaxy: Substrate & material are of different kinds. (Ga-

As)

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Epitaxy Techniques

Vapor-Phase Epitaxy (VPE)

Modified method of chemical vapor deposition (CVD).

Undesired polycrystalline layers

Growth rate: ~2 µm/min.

Liquid-Phase Epitaxy (LPE)

Crystal layers are from the melt existent on the substrate.

Hard to make thin films

Growth rate: 0.1-1 µm/min.

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Molecular Beam Epitaxy (MBE)

Relies on the sublimation of ultrapure elements, then

condensation of them on wafer

In a vacuum chamber (pressure: ~10-11 Torr).

“Beam”: molecules do not collide to either chamber walls or

existent gas atoms.

Growth rate: 1µm/hr.

Start to made GaN crystals by HVPE

At the end of the 1960s, GaN crystals were more efficiently produced by

growing GaN on a substrate using the HVPE technique (Hydride Vapour

Phase Epitaxy). There are some problems in this method. The surface

roughness was not controlled, the HVPE-grown material was

contaminated with transition metal impurities and p-doping was

passivated due to the presence of hydrogen, forming complexes with

acceptor dopants. The role of hydrogen was not understood at that time.

The major goals in the technology of GaN should be: (1) the synthesis of

strain free Single crystals, (2) the incorporation of a shallow acceptor in

high concentrations" (to provide effective p-doping).

New growth techniques

In the 1970s, new crystal growth techniques, MBE (Molecular Beam

Epitaxy) and MOVPE (Metalorganic Vapour Phase Epitaxy) were

developed. Efforts were made to adapt these techniques for growing GaN.

MBE: Working Principle

Epitaxial growth: Due to the interaction of molecular or atomic

beams on a surface of a heated crystalline substrate.

The solid source materials sublimate

They provide an angular distribution of atoms or molecules

in a beam.

The substrate is heated to the necessary temperature.

The gaseous elements then condense on the wafer where

they may react with each other.

Atoms on a clean surface are free to move until finding correct

position in the crystal lattice to bond.

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Growth occurs at the step edges formed: More binding forces at an

edge.

Isamu Akasaki began studying GaN as

a) Growth of GaN on sapphire using an AlN layer.

b) Resistivity of Mg doped GaN as a function of annealing temperature.

The breakthrough was the result of a long series of experiments and

observations. A thin layer (30 nm) of polycrystalline AlN was first

nucleated on a substrate of sapphire at low temperature (500 °C) and then

heated up to the growth temperature of GaN (1000 °C) . During the

heating process, the layer develops a texture of small crystallites with a

preferred orientation on which GaN can be grown. The density of

dislocations of the growing GaN crystal is first high

, but decreases rapidly after a few nm growths. A high quality surface

could be obtained, which was very important to grow thin multilayer

structures in the following steps of the LED development. In this way,

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high quality device-grade GaN was obtained for the first time. GaN could

also be produced with significantly lower background n-doping.

Main applications of blue GaN LEDs

Traffic signals

Automotive head lamps

Backlighting for liquid crystal displays (LCD)

Mobile phones

Laptop computers

Desktop computers

TVs

General lighting

Industrial

Street lighting

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References

.Article of Nobel Prize 2014 in physics

. Shuji Nakamura, Gerhard Fasol, Stephen J Pearton “The Blue Laser

Diode: The Complete Story” ISBN 3540665056, Springer-Verlag,

(Second Edition, September 2000)

.http://iramis.cea.fr/en/Phocea/Vie_des_labos/Ast/ast_sstechnique.php?id

_ast=494