Notesale: Turn your study into money

Document Preview

Extracts from the notes are below, to see the PDF you'll receive please use the links above

---

Mansoura University
Faculty of Science
Physics Department

Research
Blue Light Emitting Diode

Prepared by

Mohamed Hasanin Farg

Premaster degree

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
...
Electricity has many uses in our day to day life
...

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
...


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
...
So, at the
temperature of zero Kelvin the energy levels which are lower than i
...

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

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


Intrinsic Semiconductor
An intrinsic semiconductor material is chemically very pure and
possesses poor conductivity
...
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

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

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


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
...
Introducing impurities into
the semiconductor materials (doping process) can control their
conductivity
...

Semiconductors are available as either elements or compounds
...

Compound Semiconductors include InSb, InAs, GaP, GaSb, GaAs, SiC,
GaN
...
That is, each atom has its four nearest neighbors at the corners of a

regular tetrahedron with the atom itself being at the center
...
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
...


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
...

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

phosphorous contributes free electrons, greatly increasing the
conductivity of the intrinsic semiconductor
...


P-Type Semiconductor
The addition of trivalent impurities such as boron, aluminum or
gallium to an intrinsic semiconductor creates deficiencies of
valence electrons,called "holes"
...


A p–n junction is a boundary or interface between two types of
semiconductor material, p-type and n-type inside a single crystal
of semiconductor
...

When a suitable voltage is applied to the leads, electrons are able
to recombine with electron holes within the device, releasing
energy in the form of photons
...


Idea of creating Light emitting diode
Electroluminescence (EL) is an optical
phenomenon and electrical phenomenon in which a material emits
light in response to the passage of an electric current or to a
strong electric field
...
The excited electrons release their energy
as photons - light
...


Using Gallium nitride (GaN), which is the material used to create blue
LEDs
...
e
...
GaN is hard to grow
...
"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)
...
Defects
are bad and GaN is very sensitive to certain types of defect
...

Epitaxial growth results in monocrystalline layers differing from
deposition which gives rise to polycrystalline and bulk structures
...

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

 Undesired polycrystalline layers
 Growth rate: ~2 µm/min
...

 Hard to make thin films
 Growth rate: 0
...


 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)
...

 Growth rate: 1µm/hr
...
There are some problems in this method
...
The role of hydrogen was not understood at that time
...


New growth techniques
In the 1970s, new crystal growth techniques, MBE (Molecular Beam
Epitaxy) and MOVPE (Metalorganic Vapour Phase Epitaxy) were
developed
...

MBE: Working Principle
 Epitaxial growth: Due to the interaction of molecular or atomic
beams on a surface of a heated crystalline substrate
...

 The substrate is heated to the necessary temperature
...

 Atoms on a clean surface are free to move until finding correct
position in the crystal lattice to bond
...


Isamu Akasaki began studying GaN as
a) Growth of GaN on sapphire using an AlN layer
...

The breakthrough was the result of a long series of experiments and
observations
...
During the
heating process, the layer develops a texture of small crystallites with a
preferred orientation on which GaN can be grown
...
A high quality surface
could be obtained, which was very important to grow thin multilayer
structures in the following steps of the LED development
...
GaN could
also be produced with significantly lower background n-doping
...
Article of Nobel Prize 2014 in physics

...
http://iramis
...
fr/en/Phocea/Vie_des_labos/Ast/ast_sstechnique

---