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   TABLE OF CONTENTS

Introduction:

a) AlGaAs.

b) GaAs.

Occurance

a) Aluminium.

b) Gallium.

c) Arsenide.

Properties:

a) Chemical Properties.

b) Electrical Properties.

c) Mechanical Properties.

d) Thermal Properties.

e) Optical Properties.

f) Laser Properties.

g) Structural Properties.

Uses:

a) Leds.

b) Optical Fibers.

c) Laser pointers.

d) Bragg mirrors.

e) Solar Cell.

f) PIN Diodes.

g) Micro-Machining.

   Aluminium gallium arsenide

Aluminium gallium arsenide (also aluminum gallium arsenide) (AlxGa1-xAs) is a semiconductor material which has almost the same crystal lattice that of   GaAs, However the bandgap of AlGaAs is larger. The x in the formula above is a number between 0 and 1 . This corresponds to some arbitrary alloy between GaAs and AlAs.

We take the general formula to be  AlGaAs,rather than any particular ratio.The alloy system GaAs/AlGaAs is potentially very important for many high-speed electronics and optoelectronic devices, because the lattice parameter difference between GaAs and AlGaAs is very small, which promises an insignificant concentration of unwanted interface states. Due to this prominent feature, a number of interesting properties and phenomena, such as high-mobility, low-dimensional carrier gases, resonant tunnelling and fractional quantum Hall effect, have been  found in the GaAs/A1GaAs heterostructure system. New devices, such as modulation-doped FETs, heterojunction bipolar transistors, resonant tunnelling transistors, quantum-well lasers, and other photonic and quantum-effect devices, have also been developed recently using this material system.

 

Gallium arsenide

Gallium arsenide is a chemical compound of the elements gallium and arsenic. It is a compound from groups  III and V with direct bandgap .it is a semiconductor with a zinc blende crystal structure.

IUPAC ID:  Gallium arsenide

Formula:   GaAs

Density:   5.32 g/cm''

Melting point : 2,260''F (1,238''C)

Molar mass:  144.645 g/mol

OCCURRENCE  OF ALUMINIUM:

Aluminium  is  very  reactive element that's why it doesn't  occur  in  free  state. It is  the third  most  abundant  element  in  the  universe  after oxygen and silicon. It  is approximately present  ( 7.5%  to  8.3% )  on the earth crust.  It also occurs in sea water  but  its concentration is  very  low  about  ( 0.01 mg ).

In 1827 AD , F Wholer , heated the Anhydrous  Aluminium Cholride , AlCl3 , with Potassium metal and obtain a grey metallic powder and that was  aluminium.  Commercially it is formed by the process of electrolysis  of fused mixture of Bauxite, Al203.2H2O,  and Cryolite , AlF3.3HF

Aluminuim  is  metallic  element  and  it  belongs  to  Group III-B  of  the  periodic table.

Atomic  number  :  13 Atomic  mass   :  26.97 Melting  point :  660 degC  Boiling  point   :  2467 degC  Density :  2.7

OCCURRENCE OF GALLIUM (Ga) :

A  Russian chemist Dmitri Mendeleev , predicted the existence and properties of gallium before its discovery.He said that an element 'number 31' did exist and according to his prediction this element would be similar to aluminum (above  31) and indium (below 31). So he named the hypothetical element  EKA  ALUMINIUM.

Later on Gallium was discovered  by French chemist  Paul  E Lecoq de Boisbaudran through an optical  instrument  used  for  spectrographic  analysis  in  1875. He was s doing work on spectra of chemical elements. Spectra are the lines that produced by heating a chemical element. Each chemical element has its own spectra and we can determine the chemical  from its spectrum lines. Paul knew about the prediction of Medeleev and at that time a chemical element was missing between the aluminum and indium then he started to learn more about spectrum.He start working to find the gallium element and he thought that element number 31 would present in zinc ore because atomic number of zinc is 30.Finally Pual extracted gallium from zinc ore from the Pyrenees and obtain a small amount of gallium which is  ('0.65 grams ') from 430 kg of zinc ore

Gallium is also an plentiful element on earth .Its presence is estimated about 5 parts per million .Mostly it is found in zinc and aluminium ores. The largest producers are australlia,Russia,France and Germany.

Atomic weight 69.723

State: solid

Melting 29.76 oC, 302.91 K

Boiling point : 2200 oC, 2473 K

Electrons: 31

Proton: 31

Density 20oC: 5.907 g/cm3

OCCURANCE OF ARSENIDE (As):

Arsenic does not  occur as a pure element . It is present in the compound form  and these compounds are occur from the mining and purification of silver metal. Mostly common ores are Arsenopyrite (FeAsS), Orpiment (As2S3), and Realgar (As4S4). It  is  present  5 parts  per million in  the  earth. China, Chile, Mexico, Namiba  and Philippines are the world largest producer countries while United state does not produce it. Natural source of arsenic is volcanic actions .

Actually arsenic was discovered by the alchemist named Albert . He heated the orpiment (As2S3 ) with soap and obtain a pure arsenic . arsenic is recover from their ore by common method firstly the ore is heated in air to chemically convert arsenic  sulfide to arsenic oxide , than arsenic oxide is heated with pure carbon. Carbon react with oxygen in arsenic oxide and produces pure arsenic.

Arsenic occurs in the form of 3 allotropes:

Grey (mostly common) , yellow and black

Symbols   : As Atomic number   : 33 Atomic mass   :74.921 amu   Melting point  :817.0 C   Boiling points  :613.0 C Classification   :Metalloid   Density  :5.72 g/cm^3

    PROPERTIES OF AlGaAs!

Chemical Properties:

Chemical formula AlGaAs

Periodic Group III-V

Band Gap 1.42v ' 2.16v

Band Gap Type Direct (when x<0.4)

Crystal Structure Zinc Blende

Symmetry Group TD2-F43m

Lattice Constant 5.6533+0.0078x ''

Electrical properties:

Intrinsic Carrier Concentration

x=0.1   2.1'105 cm-3

x=0.3   2.1'103 cm-3

x=0.5   2.5'102 cm-3

x=0.8   4.3'101 cm-3

Electron Mobility 02 V-1 s-1

0.452 V-1 s-1

Hole Mobility 370-970x+740x2 cm2 V-1 s-1

Electron Diffusion Coefficient 02/s

0.452/s

Hole Diffusion Coefficient 9.2-24x+18.5x2 cm2/s

Electrical Resistivity X=02/s

0.452/s

Mechanical Properties:

Melting Point 1414 ''C

Density 2.329 g cm-3

Young's Modulus 130-188 GPa

Shear Modulus 51-80 GPa

Bulk Modulus (7.55+0.26x)'1011 dyn cm-2

Specific Heat (@ 298 K) 0.33+0.12x J g-1''C-3

Thermal Properties:

Thermal Conductivity 0.55-2.12x+2.48x2 W cm-1 ''C-1

Thermal Diffusivity 0.31-1.23x+1.462 cm2s-1

Thermal Expansion Coefficient 2.6x10-6 ''C-1

Optical Properties:

Infrared Refractive Index (@ 300 K) n=3.3-0.53x+0.09x2

follows Kramers'Kronig relations

Laser Properties:

Laser type Solid

Pump source Electrical Discharge

Operating wavelength 0.63-0.9''m

Structural Properties of AlGaAs

The lattice characteristics of AlGaAs alloys determine on AlGaAs growing  a crystal layer on

 mineral of another mineral in such a way that its crystalline orientation is grown on GaAs.On cooling on room temperature ,an AlGaAs layer is distorted tetragonally.For measuring the values of stress-free the lattice characteristics must be considered elasticity aeolotropic.Using a linear cypher scheme we can calculate the lattice parameters of AlGaAs in stress free state.We can calculate linear equation of AlGaAs alloy by the least squares method.

Lattice Parameters:

The relative parameter differences in stress free states computed have been used for computing stress free lattice parameters of AlGaAsalloys.By using least sqaure method the relationship of lattice parameters of AlGaAS is computed as

  Y=565.330+0.809a pm

Standard deviation of  23 fm.

The consolidation of phosphorous in small amount in AlGaAs growing layers cause improvement performances  in devices such as laser diodes and solar cells.

  Relative Parameter Differences

  X  in AlGaAs Ax-Ao/Ao (10^-4)  Ax   (pm)

  0.05 0.9975 565.381

  0.31 4.625 565.586

  0.12 1.84 565.429

    0.54 8.47 565.804

    1.00 14.8 566.162

    0.68 10.27 565.906

    0.79 11.14 565.955

Molecular Densities:

The AlGaAs alloys form crystals with zinc blende structure and in unit cell it has four molecules.The molecular density Dm is determined by the following equation

Dm=4/(ax)^3

Crystal Densities:

The distinct experimental values on crystal densities of AlGaAs have been found through following equation

  Z=4Mx/Na(Ax)^3

Where Mx is a molecular weight of AlGaAs and is given as

Mx=144.6446-42.7415x g

Na is avogadro's number having value 6.022*10^23 per mole.

Long Range Ordering InAlGaAs alloys

It crystallise in  ZnS structures which consists of  two face centered cubic sublattice replace from each other by one quarter.Onesublattice is fill by Al ,one by Ga and one by As.The arrangement of Al and Ga atoms in their sublattice are always considerd to becompletely random.

Ordered Structure:

The order in AlGaAs is of Cu-Au type with the priority Al and Ga placed as a monolayer superlatticeas shown in figure.

The presence of ordering was determine by the appearance of superstructure reflections in an electron diffraction pattern.The reflection from ZnS  have a monolayer super lattice.A thin structure is observed of AlGaAs  layers growing over a wide range of compositions.The order of Cu-Au in AlGaAs has a symmetry having a tetragonal unit cell containing 2 cells in each aliminnium,gallium and arsenide.The higher order determine in AlGaAs  layers is less than 0.3.

Lattice Dislocations:

We will studied the line defects and planar defects that are in AlGaAs heterostructures.It is expertful that we consider the core structure of dislocations in ZnS structure.Perfect lattice dislocations have vectors representation.Dislocations which are present on a fine plane are  split up ,so their stacking fault energy is low and stacking fault is inherent.

To maintain tetrahedral co-ordination stacking fault itself occur between closely spaced atom layers it is shown in figure.

In ZnSstructure we have to differeniate between dislocations having half  plane of the crystal at the top and half of  the crystal at the bottom in such a way we have different core structures.

   USES OF AlGaAs!!

1. In high-efficiency Led:

There are two different structures used in leds.namely,

a. Quantum Well structure : in this type of structure, the barriers are made of AlGaAs while the well or deep region in between is made up of GaAs.We make use of size quantization to increase the amount of emission energy. The drawback is that we need very thin layer of GaAs.in case of MQW(Multiple Quantum Well),when are used for vertical transport, the carrier distribution becomes non-uniform.unless,the GaAs layer is very thin.Thus , second structure is more preferred.

b. Double Heterostructure: in this type of structure, the barriers and the wells are both made up of AlGaAa. The well in this case is not so deep.Most efficient of these are called DH-TS (Double Heterostructure Transparent Substrate) devices. They are developed on temporary GaAS layer and they consist of thick AlGaAs lower layer Al mole fraction (x) less than 60%, an AlGaAs  active layer(x equal to 35%) and a thick AlGaAs upper layer,also with Al mole fraction (x) less than 60%. Thickness of such thick layers is usually around 125 micro meters(um).In case of devices which emit in IR (Infra -Red) region, the Al mole fractions can be lowered.

After the growth , the temporary GaAs layer is removedby processes of polishing and selective wet chemical etching.

Earlier, the growth method preferred was AlGaAs DH-TS leds was liquid phase epitaxy(LPE)in this method, the growth is performed at a high growth rate and high quality AlGaAs layers which have high Al content.LPE can be scaled up for high volume production.

2.In optical fiber communications:

A fiber optic cable comprises of a bundle of glass threads. Each of these cables is capable of sending messages modulated onto light waves. It has  many advantages over older ways of metal communications lines.Fiber optics cables have a much  greater bandwidth than metal cables.

The GaAs/AlGaAs double hetrostructure system is used for fabricating lasers for region of shorter wavelength. The bandgap in such a material can be tailored to span the entire 0.8 to 0.9 um wavelength band by changing the composition of AlGa. There is a little lattice mismatch (approximately 0.17%)

between the AlGaAs layer and the gas substrate , which makes it more internally efficient.  In the region of longer wavelength (1.1 to 1.6 um) number of alloys from groups 3 and 4 have been used which must also be compatible with GaAs ,InP and GaSB substrates. Ternary alloys such as GaAs(1-x)Sb(x) and In(x)Ga(1-x)AS fall in this category and thus are grown on GaAs.

Ternary alloys may allow bandgap tailoring , but their lattice perimeters are fixed however quaternary alloys allow both bandgap tailoring and control of lattice perimeters. They offer a range of lattice perimeters for each bandgap. So they are considered more useful, in case of longer wavelength region.

Such alloys based on nitride which have the same properties of GaAS , only nitride reduces the bandgap sufficiently ,are used in optical devices such as optical switches , wavelength routers, detectors , polarization effects in hetrostructures,  modulation efficiency and modulation induced polarization effects.

3.In Laser Pointers:

Lasers from 780 to 830 nm (nano meter) are made using  AlGaAsdevices . Those at 940 nm are made from InGaAs. 780 nm AlGaAs laser is used by  CD (Compact disc ) players for reading the structure on CD surface. They occupy a storage space of 650 megabytes on CD ROM .whereas, DVD players and DVD ROMs use 640 nm AlGaInP laser. This gives higher resolution than CD players since it uses shorter wavelength light.

Higher power red lasers are used in laser levelers and laser markers. They are also used in plastic optical fiber (POF) communications, as discussed earlier. This type of fiber can only be used for short and medium range optical fiber networks.They can be employedinhomes,offices and also in digital communication between electronic devices such as CD-player and other audio-visual systems.

Low power lasers are used for laser pointers. Laser pointer is also a large application for AlGaInP laser diodes.

4. In Bragg Mirrors:

Due to its refractive index mentioned in table of optical properties below,it allows the construction of Bragg mirrors used in VCSELs and RCLEDs.

The mirror consists of a total of 81 layers (thickness, 6.83 ''m) of alternating quarter-wave GaAs (high index, 3.480) and Al0.92Ga0.08As (low index, 2.977) grown on a (100)-oriented GaAs substrate via MBE. The base of the mirror incorporates a 270-nm-thick Al0.92Ga0.08As etch stop layer for protection during the substrate removal process.

following figure shows a Cross-sectional schematic of the crystalline multilayer. Inset: the zincblende unit cell (space group  )..

Figure b shows  Scanning electron micrograph of a cleaved facet of the epitaxial structure.It demonstrating smooth and abrupt interfaces. The etch stop can be seen as the dark band above the legend.

Figure c shows Fitted reflectance spectrum (red points, spectrophotometer measurements; blue line, transmission matrix theory) of the AlGaAs multilayer after transfer to a glass carrier. An excellent fit is achieved, with the absolute reflectance value limited by the wavelength resolution (1 nm) of the instrument.

5. In Solar Cells:

The AlGaAs/GaAs heterostructure solar cells are developed using a technique called molecular beam epitaxy (MBE), which provides ultra-thin molecular layers in the design. The typical efficiency of the solar cells fabricated and their characteristics are '' = 17%, Voc = 0.73 V, Isc = 33 mA/cm2 , and F.F. = 0.7. Spectral response of the solar cells gives a broad spectrum ranging from  wavelengths 500 to 900 nm, corresponding to the window effect of the AlGaAs and the band edge of the GaAs materials.

The AlGaAs/GaAs heterostructure is a suitable structure for several optoelectronic devices as GaAs is a direct band gap material . Furthermore, realization of the wide-gap window effect is very important for solar cell applications. It permits us to broaden the spectral region considerably and control it precisely.

It is well known that in photovoltaic applications, AlGaAs/GaAs heterostructure solar cells yield high efficiency, which results from a wide gap optical window of AlGaAs and a high absorbing effect of GaAs due to its direct band gap property at 1.4 eV . Such solar cells can be used in high intensity, high temperature and high radiation conditions; and the solar cells produced in this manner are usually used in satellite technology . Because of its large absorption coefficient, the active thickness of this III-V compound semiconductor in the solar cell structure must be carefully designed for optimal energy output from solar radiation. Ultra-thin layers of the structure in nanometers will lead to a better device performance and also material saving, and this is feasible by employing the MBE technique.

6. In PIN Diodes:

The main use of a GaAs/AlGaAs heterojunction is to form a PIN diode, with reduced RF resistance (RS) and no change in junction capacitance (CT).It has been analyzed and employed in the development of several different PIN diode switches of various circuit topologies. Series designs demonstrate improved insertion loss, shunt designs show improved isolation, and series-shunt designs have improvements in both parameters. These switches demonstrate superior broadband performance, with low insertion loss and high isolation from 50MHz to almost 80GHz, and series-shunt switches exhibit 50% increased input power capability over equivalent homojunction GaAs PIN diode switches.

7. In Machining:

GaAs/AlGaAs micromachining technique  is compatible with laser diode fabrication process.AlGaAs structural layers and GaAs sacrificial layers are prepared by method called metal organic vapor phase epitaxy. Reactive dry etching with chlorine is used to fabricate high-aspect structures. Peroxide/ammonium hydroxide solution is used for selective etching of the sacrificial layer.Precise undeformed microstructures are obtained because the epitaxial layer has low stress.Good compatibility with the LD process makes it possible to integrate microcantilever beams with LD's without degradation of LD characteristics. Microcantilever beams of AlGaAs are characterized by directly measuring stiffness and natural frequencies. A fracture test is also performed on the AlGaAs microcantilever beams. The average fracture stress of AlGaAs is found to be 1 GPa at 1% strain, which shows that the material is strong enough to support the micrometer scale structures.The main advantage of GaAs/AlGaAs micromachining is that heterostructures can be grown epitaxially with abrupt interfaces of high lateral smoothness.A thin AlGaAs layer of high Al concentration  is used as etch stop layer.

References:

Books:

1.Light-Emitting Diodes (First Edition, 2003)

By E. Fred Schubert

2. Optical Fiber Communications: Principles and Practice

By Senior John M

3.Laser Fundamentals

By William T. Silfvas

4.Textbook of inorganic (2003)

By Arora Amit.

5.Properties of Aluminium Gallium Arsenide

By sadao Adachi

Links:

http://www.azom.com/article.aspx?ArticleID=8365

http://www.chemistryexplained.com

http://ieeexplore.ieee.org/xpl/login.jsp

http://amsacta.unibo.it/1801/1/GAAS_94_016.pdf

https://en.m.wikipedia.org/wiki/Gallium

http://periodictable.com/Properties/A/UniverseAbundance.v.log.html

http://www.chemistryexplained.com/elements/A-C/Aluminum.html

https://en.m.wikipedia.org/wiki/Aluminium

http://www.mindat.org/min-2901.html

https://en.wikipedia.org/wiki/Arsenic

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