Overview fabrication and working of LASER diode.

2. OVERVIEWOVERVIEW
►Laser diodeLaser diode
 diodediode
 laser actionlaser action
►Laser diode fabrication.Laser diode fabrication.
 An overviewAn overview
 Process in detailProcess in detail

4. LASERLASER
►Light amplification by stimulated emission ofLight amplification by stimulated emission of
radiation, or laser in short, is a device thatradiation, or laser in short, is a device that
creates and amplifies electromagnetic radiationcreates and amplifies electromagnetic radiation
of specific frequency through process ofof specific frequency through process of
stimulated emission. In laser, all the light raysstimulated emission. In laser, all the light rays
have the same wavelength and they are coherenthave the same wavelength and they are coherent

5. BASIC IDEABASIC IDEA
►Basic principle of laser were first given byBasic principle of laser were first given by
American scientist Charles hard TownesAmerican scientist Charles hard Townes
However, TH Miamian of the Hughes ResearchHowever, TH Miamian of the Hughes Research
Laboratory, California, was the first scientistLaboratory, California, was the first scientist
who Experimentally demonstrated laser bywho Experimentally demonstrated laser by
flashing light through a ruby crystal, in 1960.flashing light through a ruby crystal, in 1960.

6. WORKINGWORKING

7. Difference between LED ANDDifference between LED AND
LASERLASER
►Laser threshold current is higer then LEDLaser threshold current is higer then LED
threshold.threshold.
►Optical feedback is used in laser diodes thisOptical feedback is used in laser diodes this
result in two important properties of laser.result in two important properties of laser.
 Laser beam is highly directional.Laser beam is highly directional.
 Laser beam is coherentLaser beam is coherent

8. FABRICATION OVERVIEWFABRICATION OVERVIEW

9. EPITAXIAL GROWTHEPITAXIAL GROWTH
► GaAs is used as the substrateGaAs is used as the substrate
material because of lattice match tomaterial because of lattice match to
AlGaAs and InGaAsP, the directAlGaAs and InGaAsP, the direct
bandgap materials from which thebandgap materials from which the
epitaxial structure is engineered. Inepitaxial structure is engineered. In
this fabrication process, a Si-dopedthis fabrication process, a Si-doped
GaAs substrate is used for n-sideGaAs substrate is used for n-side
contact. The choice of n-typecontact. The choice of n-type
substrate is due to it having highersubstrate is due to it having higher
mobility than p-type, whichmobility than p-type, which
provides lower resistivity in theprovides lower resistivity in the
substrate (which is the thickest partsubstrate (which is the thickest part
of the structure).of the structure).

10. EPITAXIAL GROWTHEPITAXIAL GROWTH
► Epitaxy is grown byEpitaxy is grown by
using a technique namelyusing a technique namely
MOCV.MOCV.

11. PLG for mesaPLG for mesa
► The mesa structure is neededThe mesa structure is needed
to confine the mode in theto confine the mode in the
lateral direction and helplateral direction and help
prevent excessive currentprevent excessive current
spreading. After growth ofspreading. After growth of
the epitaxial layers, athe epitaxial layers, a
photolithography process isphotolithography process is
performed, which produces aperformed, which produces a
pattern on the surface of thepattern on the surface of the
wafer using light sensitivewafer using light sensitive
photoresist material andphotoresist material and
controlled exposure to light.controlled exposure to light.

12. PLGPLG

13. DIELECTRIC DIPOSITIONDIELECTRIC DIPOSITION
► An oxide layer is used toAn oxide layer is used to
insulate the metal from the p-insulate the metal from the p-
side of the device in all areaside of the device in all area
except where current is to beexcept where current is to be
injected. The oxide layer isinjected. The oxide layer is
deposited by an E-beamdeposited by an E-beam
evaporation or PECVD afterevaporation or PECVD after
a cleaning process isa cleaning process is
performed. Deposition is aperformed. Deposition is a
process that places the filmprocess that places the film
layer on the waferlayer on the wafer

14. PECVDPECVD
►PECVD reffers to Plasma Enhanced ChemicalPECVD reffers to Plasma Enhanced Chemical
Vapour Dipostion.Vapour Dipostion.
►The films are either thermally grown throughThe films are either thermally grown through
oxidation of silicon or deposited by thermal oroxidation of silicon or deposited by thermal or
plasma-enhanced chemical vaporplasma-enhanced chemical vapor
deposition(PECVD).deposition(PECVD).

15. PECVD SYSTEMPECVD SYSTEM

16. PECVD SYSTEMPECVD SYSTEM

17. ► PECVD uses electrical energy to generate a glow dischargePECVD uses electrical energy to generate a glow discharge
(plasma) in which the energy is transferred into a gas mixture.(plasma) in which the energy is transferred into a gas mixture.
This transforms the gas mixture into reactive radicals, ions,This transforms the gas mixture into reactive radicals, ions,
neutral atoms and molecules, and other highly excited species.neutral atoms and molecules, and other highly excited species.
These atomic and molecular fragments interact with a substrateThese atomic and molecular fragments interact with a substrate
and, depending on the nature of these interactions, either etchingand, depending on the nature of these interactions, either etching
or deposition processes occur at the substrate. Since theor deposition processes occur at the substrate. Since the
formation of the reactive and energetic species in the gas phaseformation of the reactive and energetic species in the gas phase
occurs by collision in the gas phase, the substrate can beoccurs by collision in the gas phase, the substrate can be
maintained at a low temperature. Hence, film formation canmaintained at a low temperature. Hence, film formation can
occur on substrates at a lower temperature than is possible in theoccur on substrates at a lower temperature than is possible in the
conventional CVD process, which is a major advantage ofconventional CVD process, which is a major advantage of
PECVD.PECVD.

18. ADVANTAGESADVANTAGES
► High purity films can be achieved;High purity films can be achieved;
► Great variety of compositions can be deposited;Great variety of compositions can be deposited;
► Some films cannot be deposited with adequate filmSome films cannot be deposited with adequate film
properties by any other methodproperties by any other method
► Good economy and process control are possible for manyGood economy and process control are possible for many
films.films.
► Good step coverage.Good step coverage.
► That is why PECVD is nowadays one of the key sectors ofThat is why PECVD is nowadays one of the key sectors of
the industrial production of silicon-based films, notably,the industrial production of silicon-based films, notably,
CMOS, thin film transistors, display technology and solarCMOS, thin film transistors, display technology and solar
cells.cells.

19. DISADVANTAGESDISADVANTAGES
► Toxic byproductToxic byproduct
► High cost of equipmentHigh cost of equipment

20. PECVD PRODUCTPECVD PRODUCT
► After PECVD, PLG is done to obtain a window.After PECVD, PLG is done to obtain a window.
Final productFinal product

21. METALIZATIONMETALIZATION
► For P-side ohmic contact, chromiumFor P-side ohmic contact, chromium
(Cr) and Gold (Au) are deposited by(Cr) and Gold (Au) are deposited by
E-beam evaporator these metals areE-beam evaporator these metals are
used because GaAs and Au contactsused because GaAs and Au contacts
have different work function whichhave different work function which
makes Schottky contact or Ohmicmakes Schottky contact or Ohmic
contact. Ohmic contact is ancontact. Ohmic contact is an
electrical junction between metalelectrical junction between metal
and semiconductor material that hasand semiconductor material that has
a linear current-voltage curve.a linear current-voltage curve.
► ForFor N-side ohmic contacts areN-side ohmic contacts are
deposited with AuGe, Ni and Au ondeposited with AuGe, Ni and Au on
the backside, which is N-type GaAsthe backside, which is N-type GaAs
substrate. The electrical propertiessubstrate. The electrical properties
of alloyed AuGe offers relativelyof alloyed AuGe offers relatively
lower contact resistance to N-typelower contact resistance to N-type
GaAs.GaAs.

22. METALIZATIONMETALIZATION

23. CLEAVING AND FACETCLEAVING AND FACET
COATINGCOATING
► In this step, each laser isIn this step, each laser is
cleaved by hand. Aftercleaved by hand. After
that, the facets of lasers arethat, the facets of lasers are
coated with asymmetriccoated with asymmetric
reflectivity. This is calledreflectivity. This is called
Partial-reflecting (PR)Partial-reflecting (PR)
which the laser comes out;which the laser comes out;
the other side is calledthe other side is called
High-reflecting (HR). TheHigh-reflecting (HR). The
facet direction isfacet direction is
considered a crystalconsidered a crystal
orientation of the GaAsorientation of the GaAs
wafer to get perpendicular.wafer to get perpendicular.

24. LASER BARLASER BAR

25. COATINGCOATING

26. E-BEAM EVAPORATORE-BEAM EVAPORATOR

27. CLEAVING AND FACETCLEAVING AND FACET
COATINGCOATING

28. SCRIBING AND BONDINGSCRIBING AND BONDING
► The last step is the bonding processThe last step is the bonding process
between chip and heat-sink withbetween chip and heat-sink with
indium solder.indium solder.
► Indium solder is widely used toIndium solder is widely used to
bond semiconductor lasers due tobond semiconductor lasers due to
its simplicity and it can bondits simplicity and it can bond
directly to copper. AuSn is anotherdirectly to copper. AuSn is another
option for higher reliability, as itoption for higher reliability, as it
requires expansion to be matchedrequires expansion to be matched
heat sinks.heat sinks.
► Scribing is done to isolateScribing is done to isolate
individual diodes.individual diodes.

29. CHARACTERIZATIONCHARACTERIZATION
►After fabrication of diode its resones is checked :After fabrication of diode its resones is checked :
 DirectionalityDirectionality
 MonochromacityMonochromacity
 CoherenceCoherence

30. NEERAJ MISHRA
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