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UETTAXILA
OptoelectronicDevicesMakingVisible
BY
MUHAMMADRASHID
14MSENC15
Presentedto:
Dr.YaseerDurrani
UETTaxila
17TH
December,2014
[Thephysical interactionsbetweenphotonsandsemiconductordeviceshavetobeexploitedto
designandoptimizeavarietyofinformationprocessingdevices.]
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Thispageisintentionallyleftblank
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To my parents, who showed me the
path of intellectual pursuits
To my wife for her continuing
guidance and support along the way
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TableofContents
1 WHATISOPTOELECTRONICS?.......................................................................................6
2 PHOTODIODES[1]...........................................................................................................6
2.1 SolarCells................................................................................................................7
2.1.1 OperatingPrinciple[12]
....................................................................................7
2.1.2 Applications[2]...............................................................................................10
2.1.3 Materials........................................................................................................10
2.1.4 NewResearch................................................................................................10
2.2 PhotoDetectors[1]................................................................................................11
2.2.1 Applications...................................................................................................13
2.2.2 NewResearch................................................................................................13
3 LIGHTEMITTINGDIODES.............................................................................................14
3.1 Applications..........................................................................................................15
3.1.1 Indicatorsandsigns.......................................................................................15
3.1.2 Lighting..........................................................................................................16
3.2 NewResearches....................................................................................................17
3.2.1 WhitelightLEDs[2].........................................................................................17
3.2.2 Organiclightemittingdiodes(OLEDs)[2]......................................................18
3.2.3 OptimizationofLightExtractionEfficiencyofIIINitrideLEDsWithSelf
AssembledColloidalBasedMicrolenses[8].................................................................18
3.2.4 LightExtractionEfficiencyEnhancementofIIINitrideLightEmittingDiodes
byUsing2DClosePackedTiO2MicrosphereArrays[9]..............................................20
3.3 FiberOpticCommunications................................................................................20
3.3.1 Applications...................................................................................................22
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3.3.2 Sensors..........................................................................................................24
3.3.3 Powertransmission.......................................................................................24
4 LASERS.........................................................................................................................24
4.1 Specialpropertiesoflasers...................................................................................24
4.2 Applications..........................................................................................................25
4.3 Laserphysics.........................................................................................................25
4.3.1 Stimulatedemission[2]..................................................................................25
4.3.2 Gainmedium.................................................................................................27
4.3.3 Populationinversion......................................................................................28
4.4 Someconditions...................................................................................................28
5 SEMICONDUCTORLASERS...........................................................................................28
5.1 PopulationInversion.............................................................................................28
5.2 TheBasicSemiconductorLaser............................................................................29
5.3 MaterialsforSemiconductorLasers.....................................................................29
5.4 LatestResearches.................................................................................................30
5.4.1 ENERGYEFFICIENTANDENERGYPROPORTIONALOPTICALINTERCONNECTSFORMULTICOREPROCESSORS:DRIVINGTHENEEDFORONCHIPSOURCES
[7].......30
6 REFERENCES................................................................................................................32
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OPTOELECTRONICDEVICES
1 WHATISOPTOELECTRONICS?
Optoelectronicsisthestudyandapplicationofelectronicdevicesthatinteractwithlight,
alsoreferredaselectroopticsorOptronics.
Figure1:FieldofOptoelectronics
The emission and absorption of light by semiconductors gives rise to useful
optoelectronic devices.Carrier generation and recombination are processes by which
mobilechargecarriers(electronsandelectronholes)arecreatedandeliminated.Carrier
generation and recombination processes are fundamental to the operation of manyoptoelectronicsemiconductordevices,suchasphotodiodes,LEDsandlaserdiodes.
2 PHOTODIODES[1]
Aphotodiode isasemiconductordevicethatconverts light intocurrent.Thecurrent is
generatedwhenphotonsareabsorbed inthephotodiode.Asmallamountofcurrent is
alsoproducedwhenno light ispresent.Photodiodesmaycontainopticalfilters,builtin
lenses, and may have large or small surface areas. The common, traditional solar cell
usedtogenerateelectricsolarpowerisalargeareaphotodiode.
The device is operated either in 3rd or 4
th quadrant. Photo detectors are used in 3
rd
quadrantwhilesolarcell isoperated in4thquadrant (power isdelivered to loadby the
junction)Figure2below
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Figure2
Figure:Operationofdeviceindifferentquadrants
2.1 SolarCells
Asolarcell,orphotovoltaiccell, isanelectricaldevicethatconvertstheenergyof light
directly into electricity by the photovoltaic effect. It is a form of photoelectric cell,
defined as a device whose electrical characteristics, such as current, voltage, or
resistance,varywhenexposedtolight.Solarcellsarethebuildingblocksofphotovoltaic
modules,otherwiseknownassolarpanels.
2.1.1 OperatingPrinciple[12]
Photovoltaiceffect:Theappearanceofaforwardvoltageacrossanilluminatedjunctionisknownasthephotovoltaiceffect.Figure3showsapnjunctionunderthermalequilibrium
conditions.
Figure3
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ptypeSi ntypeSi
Figure:pnjunctionatthermalequillibrium
2.1.1.1 Photogenerationofchargecarriers
Whenaphotonhitsapieceofsilicon,oneofthreethingscanhappen:
1. Thephotoncanpassstraight through thesilicon this (generally)happens for
lowerenergyphotons.
2.
Thephotoncanreflectoffthesurface.
3.
Thephotoncanbeabsorbedbythesiliconifthephotonenergyishigherthanthe
silicon band gap value. This generates an electronhole pair and sometimes heat
dependingonthebandstructureasshowninfigure4.
(a) (b)
Figure4:whenphotonsstrikethepnjunction
Whenaphotonisabsorbed,itsenergyisgiventoanelectroninthecrystallattice.Usuallythis electron is in the valence band and is tightly bound in covalent bonds with
neighboringatoms,andthereforeunabletomovefar.Theenergygiventotheelectron
by the photon "excites" it into the conduction band where it is free to move around
withinthesemiconductor.
2.1.1.2 Chargecarrierseparation
Therearetwomainmodesforchargecarrierseparationinasolarcell:
1.
driftofcarriers,drivenbyanelectricfieldestablishedacrossthedevice2. diffusionofcarriersduetotheirrandomthermalmotion,untiltheyarecaptured
bytheelectricalfieldsexistingattheedgesoftheactiveregion
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Figure5
Figure:Movementofchargecarriers
2.1.1.3 Connectiontoanexternalload
Metalsemiconductorcontactsaremadetoboththentypeandptypesidesofthesolar
cell,andtheelectrodesconnectedtoanexternalload.Electronsthatarecreatedonthe
ntype side,orhavebeen "collected"by thejunctionand sweptonto the ntype side,
may travelthrough thewire,power the load,andcontinue through thewireuntil they
reach the ptype semiconductormetal contact as shown in figure 6 below. Here, they
recombinewithaholethatwaseithercreatedasanelectronholepairontheptypeside
ofthesolarcell,oraholethatwassweptacrossthejunctionfromthentypesideafter
beingcreatedthere.
Figure6
Figure:currentpassingthroughaload
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2.1.2
Applications[2]
Solarphotovoltaicenergyconversionisusedtodayforbothspaceandterrestrialenergy
generation. The success of solar cells in space applications is well known (e.g.,
communications satellites, manned and unmanned space exploration). On earth, solar
cells have a myriad of applications varying from supplementing the grid to powering
emergencycallboxes.However,theneedformuchmoreextensiveuseofsolarcells in
terrestrialapplications isbecoming clearerwith thegrowingunderstandingof the true
costoffossilfuelsandwiththewidespreaddemandforrenewableandenvironmentally
acceptableterrestrialenergyresources.
Itisimportantthattheseriesresistanceofthedevicebeverysmallsothatpowerisnot
losttoheatduetoohmiclossesinthedeviceitself.Aseriesresistanceofonlyafewohms
can seriously reduce the output power of a solar cell. Since the area is large, the
resistanceoftheptypebodyofthedevicecanbemadesmall.However,contactstothe
thinnregionrequirespecialdesign.Ifthisregion iscontactedattheedge,currentmustflow along the thin n region to the contact, resulting in a large series resistance. To
preventthiseffect,thecontactcanbedistributedoverthensurfacebyprovidingsmall
contact fingers. These narrow contacts serve to reduce the series resistance without
interferingappreciablywiththeincominglight.
2.1.3 Materials
Monocrystalline silicon, Polycrystalline silicon, Ribbon silicon, Monolikemulti silicon
(MLM), Cadmium telluride, Copper indium gallium selenide, Silicon thin film, Gallium
arsenidethinfilmareusedforsolarcellmanufacturingaccordingtotheirpropertiesand
application.
2.1.4 NewResearch
2.1.4.1DesignofPhotovoltaicSolarCellModelforStandaloneRenewable
System[11]
ThisresearchpaperpresentsasimulationmodelofsolarcellsinMatlab.Theinfluenceof
temperatureandsolar irradiationonphotovoltaicmodule is investigatesandcompared
toIVcharacteristicsofPVcell.
As the resources of fossil fuels as coal, oil and gas are getting minimized, renewable
energy is very much important to focus on. Inpast PVenergy was used only in space
applicationsandincalculators.Itslimitedusewasduetoitssmallerefficiency.Research
in last decades has increased the efficiency of solar cells. A new concept is thin film
technology. In this technology thePV solarcell ismadebyoneormore thin layerson
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substrate. Multiple layers allow higher usage of spectrum. The highest recorded
efficiencyis44%.
Nowadayssolarpower isusedastheenergysource inpowersystemsused forbattery
chargers and in street lighting. The author has discussed different formulas for its
computationsandhasdemonstrateddifferentequivalentcircuitsforaPVsolarcell.The
PVModuleoutputvoltageisincreasedbyincreasethenumberofthecellsconnectionsin
seriesandoutputcurrentisincreasedbyincreasethenumberofthecellsconnectionsin
parallel.
The authors have shared simulated IV characteristics of PV module under different
conditions and concluded that the main disadvantage is the dependence on weather
conditions.Thesolar irradiation influencewasbiggerthantemperatureespeciallywhen
wefocusonthedecreaseofthesolar irradiation.The impactontheoutputpowerwas
muchheavier.
2.2
PhotoDetectors[1]
WhenaphotodiodeisusedinthirdquadrantofIVgraph,thecurrentisproportionalto
theillumination.Thisisuseful inmeasurementoflightintensityorconversionofoptical
pulsestoelectricalsignals.
Indetection applications two parameters are important response time and detectable
bandwidth. Photodiodes are similar to regular semiconductor diodes except that they
maybeeitherexposed (todetectvacuumUVorXrays)orpackagedwithawindowor
optical fiber connection to allow light to reach the sensitive part of the device. Many
diodesdesignedforusespecificallyasaphotodiodeuseaPINjunctionratherthanapn
junction, to increase the speed of response. A photodiode is designed to operate in
reversebiasasshowninfigure7.
IfwidthofdepletionregionWiswide,mostoftheincidentphotonswillbeabsorbedin
the depletion region, leading to a high sensitivity. Also, a wide W results in a small
junction, thereby reducing the RC time constant of the detector circuit. On the other
hand,Wmustnotbesowidethatthetimerequiredfordriftofphotogeneratedcarriers
outofthedepletionregionisexcessive,leadingtolowbandwidth.
Oneconvenientmethodofcontrollingthewidthofthedepletionregionistobuildapin
photodetector. The"i"regionneednotbetrulyintrinsic,aslongastheresistivityishigh.
Itcanbegrownepitaxiallyonthentypesubstrate,andthepregioncanbeobtainedbyimplantation. When this device is reverse biased, the applied voltage appears almost
entirelyacrosstheiregion.Ifthecarrier lifetimewithinthe iregion is longcompared
withthedrifttime,mostofthephotogeneratedcarrierswillbecollectedbythenandp
regions.
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If lowlevel optical signals are to be detected, it is often desirable to operate the
photodiode in the avalanche region of its characteristic. In this mode each photo
generated carrier results in a significant change in the current because of avalanche
multiplication, leading togainandexternalquantumefficienciesofgreater than100%.
Avalanchephotodiodes(APDs)areusefulasdetectorsinfiberopticsystems.
Figure7
Figure:PINphotodetector
IfhvislessthanEg,thephotonswillnotbeabsorbed;ontheotherhand,ifthephotons
aremuchmoreenergeticthanEg,theywillbeabsorbedverynearthesurface,wherethe
recombination rate ishigh. Therefore, it isnecessary to choose a photodiodematerial
withabandgapcorrespondingtoaparticularregionofthespectrum.
MaterialWavelength range
(nm)
Silicon(Si) 1901100
Germanium(Ge) 4001700
Indiumgalliumarsenide(InGaAs) 8002600
Leadsulfide(PbS)
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2.2.1
Applications
Photodetectors may be used to generate an output which is dependent upon the
illumination (analog; formeasurementetc),or to change the state of circuitry (digital;
eitherforcontrolandswitching,ordigitalsignalprocessing).
Photodiodes are used in consumer electronics devices such as compact disc players,
smokedetectors,and thereceivers for infrared remotecontroldevicesused tocontrol
equipmentfromtelevisionstoairconditioners.Formanyapplicationseitherphotodiodes
or photoconductors may be used. Either type of photosensor may be used for light
measurement,asincameralightmeters,ortorespondtolightlevels,asinswitchingon
streetlightingafterdark.
Photosensorsofalltypesmaybeusedtorespondtoincidentlight,ortoasourceoflight
whichispartofthesamecircuitorsystem.Aphotodiodeisoftencombinedintoasingle
componentwithanemitteroflight,usuallyalightemittingdiode(LED),eithertodetect
the presence of a mechanical obstruction to the beam (slotted optical switch), or to
coupletwodigitaloranalogcircuitswhilemaintainingextremelyhighelectricalisolation
betweenthem,oftenforsafety(optocoupler).
Photodiodesareoftenused foraccuratemeasurementof light intensity inscienceand
industry.Theygenerallyhaveamorelinearresponsethanphotoconductors.
PINdiodesaremuchfasterandmoresensitivethanpnjunctiondiodes,andhenceare
oftenusedforopticalcommunicationsandinlightingregulation.
Pnphotodiodesarenotusedtomeasureextremelylowlightintensities.Instead,ifhigh
sensitivity is needed, avalanche photodiodes, intensified chargecoupled devices or
photomultiplier tubesareused forapplicationssuchasastronomy,spectroscopy,night
visionequipmentandlaserrangefinding.
2.2.2 NewResearch
2.2.2.1HighPowerSiliconGermaniumPhotodiodesforMicrowave
PhotonicApplications[10]
This research paper introduces the concept of photo detectors developed on SiGeplatformformicrowaveapplications.Thesedetectorsaredesiredtogivehighdatarate
and wide bandwidth along with better quantum efficiency. The majority of work on
powermicrowavephotodiodes ison InPplatform.Thermaleffectsposeabigchallenge
for achieving high current operation. In surface illuminated InGaAsInP devices, the
thermal conductivity of the InP substrate is 0.68 W/cm K. thermal conductivity for
waveguidephotodiodesonInPplatformis0.05W/cmK.whichis30timeslessthanthat
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ofsilicon.Heatflowoutof it isverymuchrestrictedwhichresults inthermal failure.Si
hasthermalconductivityof1.5W/cmKisovertwotimesofInP(0.68W/cmK).
The author has shown the device structure, DC characteristics as well as thermal and
microwave measurements. A 7.4 um x 500 um device was able to dissipate 1W of
electricalpower.Thermalsimulations indicateheattrapped intheabsorbercontributes
to device failure. Since Ge has nearly 12x higher thermal conductivity than an InGaAsabsorber,wepredictandexperimentallyverifythethermalperformanceoftheseSiGe
waveguidedevicestobesignificantlybetterthancomparableInPdevices.
3 LIGHTEMITTINGDIODES
TheLEDconsistsofachipofsemiconductingmaterialdopedwithimpuritiestocreatea
pnjunction.Asinotherdiodes,currentflowseasilyfromthepside,oranode,tothen
side,orcathode,butnotinthereversedirection.Chargecarrierselectronsandholes
flowintothejunctionfromelectrodeswithdifferentvoltages.Whenanelectronmeetsa
hole, it falls into a lowerenergy level and releases energy in the form of a photonas
showninthefigure8.[6]
Figure8
Figure:LightEmittingDiodebanddiagram
Thewavelengthofthelightemitted,andthusitscolor,dependsonthebandgapenergy
ofthematerialsformingthepnjunction.
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LED development began with infrared and red devices made with gallium arsenide.
Advances in materials science have enabled making devices with evershorter
wavelengths,emittinglightinavarietyofcolors.
LEDsareusuallybuiltonanntypesubstrate,withanelectrodeattachedtotheptype
layerdepositedonitssurface.Ptypesubstrates,whilelesscommon,occuraswell.
Recently,OnOctober7,2014,theNobelPrizeinPhysicswasawardedtoIsamuAkasaki,
Hiroshi Amano and Shuji Nakamura for "the invention of efficient blue lightemitting
diodeswhichhasenabledbrightandenergysavingwhitelightsources"or,lessformally,
LEDlamps.[2]
3.1 Applications
3.1.1
Indicatorsandsigns
Thelowenergyconsumption,lowmaintenanceandsmallsizeofLEDshasledtousesas
statusindicatorsanddisplaysonavarietyofequipmentandinstallations.LargeareaLED
displays are used as stadium displays and as dynamic decorative displays. Thin,
lightweightmessagedisplaysareusedatairportsandrailwaystations,andasdestination
displaysfortrains,buses,trams,andferries.
Figure9
Figure:Redandgreentrafficsignals
Onecolor light iswellsuited for traffic lightsandsignals,exitsigns,emergencyvehicle
lighting,ships'navigation lightsor lanterns (chromacityand luminancestandardsbeing
set under the Convention on the International Regulations for Preventing Collisions at
Sea 1972, Annex I and the CIE) and LEDbased Christmas lights. In cold climates, LED
traffic lights may remain snow covered. Red or yellow LEDs are used in indicator and
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alphanumeric displays in environments where night vision must be retained: aircraft
cockpits,submarineandshipbridges,astronomyobservatories,andinthefield,e.g.night
timeanimalwatchingandmilitaryfielduse.
Figure10
Figure:LEDsusedintaillightsofanAudicar
Because of their long life, fast switching times, and their ability to be seen in broad
daylightduetotheirhighoutputandfocus,LEDshavebeenusedinbrakelightsforcars'
highmountedbrakelights,trucks,andbuses,andinturnsignalsforsometime,butmany
vehiclesnowuseLEDsfortheirrearlightclusters.WhiteLEDheadlampsarestartingtobe
used.UsingLEDshasstylingadvantagesbecauseLEDscanformmuchthinnerlightsthan
incandescentlampswithparabolicreflectors.
3.1.2
Lighting
WiththedevelopmentofhighefficiencyandhighpowerLEDs,ithasbecomepossibletouseLEDsinlightingandillumination.Replacementlightbulbshavebeenmade,aswellas
dedicatedfixturesandLEDlamps.
LEDsareusedasstreet lightsand inotherarchitectural lightingwherecolorchanging is
used.Themechanicalrobustnessandlonglifetimeisusedinautomotivelightingoncars,
motorcycles,andbicyclelights.
LEDstreetlightsareemployedonpolesandinparkinggarages.LEDsareusedinaviation
lighting. Airbus has used LED lighting in their Airbus A320 Enhanced since 2007, and
Boeing plans its use in the 787. LEDs are also being used now in airport and heliportlighting. LED airport fixtures currently include mediumintensity runway lights, runway
centerline lights, taxiway centerline and edge lights, guidance signs, and obstruction
lighting.
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LEDsarealsosuitableforbacklightingforLCDtelevisionsandlightweightlaptopdisplays
andlightsourceforDLPprojectors(LEDTV).RGBLEDsraisethecolorgamutbyasmuch
as 45%. Screens for TV and computer displays can be made thinner using LEDs for
backlighting.
LEDsaresmall,durableandneedlittlepower,sotheyareusedinhandhelddevicessuchasflashlights.LEDstrobe lightsorcameraflashesoperateatasafe,lowvoltage,instead
of the250+voltscommonly found inxenon flashlampbased lighting.This isespecially
useful in camerason mobile phones, where space isat apremium andbulky voltage
raisingcircuitryisundesirable.
LEDsareusedfor infrared illumination innightvisionuses includingsecuritycameras.A
ringofLEDsaroundavideocamera,aimedforwardintoareflectivebackground.
LEDsarenowusedcommonlyinallmarketareasfromcommercialtohomeuse:standard
lighting, AV, stage, theatrical, architectural, and public installations, and whereverartificiallightisused.
3.2 NewResearches
3.2.1
WhitelightLEDs[2]
Asweknowthatwhite lightconsistsofmanycolors (minimumthree)andLEDgivesus
lightofaspecificwavelengththenhowwhiteLEDproduceswhitelight
Therearetwoprimarywaysofproducingwhitelightemittingdiodes(WLEDs),LEDsthat
generatehighintensitywhitelight.OneistouseindividualLEDsthatemitthreeprimarycolors
red,green,andblueandthenmixallthecolorstoformwhitelight.Theotheris
touseaphosphormaterial to convertmonochromatic light fromablue orUV LED to
broadspectrumwhitelight,muchinthesamewayafluorescentlightbulbworks.
TherearethreemainmethodsofmixingcolorstoproducewhitelightfromanLED:
blue LED + green LED + red LED (color mixing; can be used as backlighting for
displays)
nearUVorUV LED+RGBphosphor (an LEDproducing lightwithawavelength
shorterthanblue'sisusedtoexciteanRGBphosphor) blueLED+yellowphosphor (twocomplementarycolorscombine to formwhite
light;moreefficientthanfirsttwomethodsandmorecommonlyused)
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3.2.2
Organiclightemittingdiodes(OLEDs)[2]
Inanorganiclightemittingdiode(OLED),theelectroluminescentmaterialcomprisingthe
emissive layerof thediode isanorganiccompound.Theorganicmaterial iselectrically
conductiveduetothedelocalizationofelectrons,andthematerialthereforefunctionsas
an organic semiconductor. The organic materials can be small organic molecules in acrystallinephase,orpolymers.
The potential advantages of OLEDs include thin, lowcost displays with a low driving
voltage,wideviewingangleasshown in figure11,andhigh contrastandcolorgamut.
PolymerLEDshavetheaddedbenefitofprintableandflexibledisplays.OLEDshavebeen
used tomakevisualdisplays forportableelectronicdevicessuchascellphones,digital
cameras,andMP3playerswhilepossiblefutureusesincludelightingandtelevisions.
Figure11
Figure:wideviewangleof170o
3.2.3 OptimizationofLightExtractionEfficiencyofIIINitrideLEDsWith
SelfAssembledColloidalBasedMicrolenses[8]
ThisresearchpublicationfocusesonmethodoflightextractionfromIIINitrideLEDusing
SiO2 /polystyrene (PS) microlens arrays. Most of the light generated by the activeregionsoftheLEDsistrappedinthehigherrefractiveindexsemiconductormaterial.The
largerefractiveindexdifferenceofGaN(n=2.5)andair(n=1)attheinterfaceresultsin
totalinternalreflectionthatleadstolowlightextractionefficiency.
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Figure12
Theescapecone inGaNmaterial isonly23.5 withaphotonescapeprobabilityofonly
4%asshowninfigure12.
Theauthorsdemonstratedanovelapproachtosignificantlyenhancethelightextraction
efficiency of IIInitride LEDs by fabricating SiO2/polystyrene (PS) microlens arrays as
showninfigure13
Figure13
This publication also shares the experimental results as well as deposition process of
thesemicrolenses.TheauthorsconcludefromtheirexperimentsthattheuseofSiO2/PS
microlensarraysledtoimprovementoflightextractionefficiencyintherangeof1.8up
to 2.7 times, depending on the thickness of the PS layer. The increase in the light
extractionefficiencyforwavelengthsof420525nmdependsstronglyonthediameterof
SiO2microspheres(dSiO2)andthicknessofPSlayer(hPS).
TheSiO2/PS microlensarrayswere also depositedon InGaN LEDsemitting in480 nm
spectralrange.Theuseof1.0mdiameterSiO2/0.5mPSmicrolensarraysontheLEDs
ledtoa2.49timesimprovementinthelightextractionefficiencyincomparisonwiththat
ofplanarLEDs.
Theauthorsaysthatthey foundthattheuseofSiO2/PSmicrolensarraysontopof III
nitride LED device leads to a low cost and practical approach to increase its light
extractionefficiency.
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3.2.4 LightExtractionEfficiencyEnhancementofIIINitrideLight
EmittingDiodesbyUsing2DClosePackedTiO2Microsphere
Arrays[9]
This research publication shows TiO2 microspheres arrays used for light extraction
efficiencyenhancement.TheauthorshaveexperimentedonLEDswithandwithoutTiO2
microspheres arrays. The results show that this process increases the light extractionefficiencyupto1.9times.Thisprocessislowcostandhighlyefficientaswellasitcanbe
doneonlargescale.
Furtherenhancementofextractionefficiency isexpectedwithmicrolensstructuresby
introducingplanarmaterials (i.e.,polystyrene)which results in semiburiedTiO2micro
lensarrays,which isexpectedtoresult inhigher lightcoupling fromGaNLEDs intothe
TiO2/PSmicrolensarraysincomparisontotheSiO2/PS microlensarrays.
3.3 FiberOpticCommunications
Fiberoptic communication isa methodof transmitting information fromone place to
another by sending pulses of light through an optical fiber. Optical fiber is pipe or a
waveguidefortransmissionoflight.Thelightformsanelectromagneticcarrierwavethat
is modulated to carry information. As LEDs can cycle on and off millions of times per
second,veryhighdatabandwidthcanbeachieved.
The process of communicating using fiberoptics involves the following basic steps:
Creatingtheopticalsignalinvolvingtheuseofatransmitter,relayingthesignalalongthe
fiber, ensuring that the signal does not become too distorted or weak, receiving the
opticalsignal,andconvertingitintoanelectricalsignal.
Figure14
Figure:fiberopticsystem
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Optical fiberstypically includeatransparentcoresurroundedbyatransparentcladding
material with a lower index of refraction. Light is kept in the core by total internal
reflection.Total internalreflection isaphenomenonthathappenswhenapropagating
wavestrikesamediumboundaryatanangle largerthanaparticularcriticalanglewith
respecttothenormaltothesurface.Iftherefractiveindexislowerontheothersideof
theboundaryandthe incidentangle isgreaterthanthecriticalangle,thewavecannot
passthroughandisentirelyreflected.Forexamplelightrayfromwaterpassingintoairat
certainangleisreflectedbackintothewater.[2][4]
Figure15
Figure:Totalinternalreflection
Figure16
Figure:Thestructureofatypicalsinglemodefiber.
1.Core:8mdiameter
2.Cladding:125mdia.3.Buffer:250mdia.
4.Jacket:400mdia.
Core has high refractive index than that of cladding. Difference in refractive indices
causes total internal reflection to takeplace.Wecanobservedifferentkindsofoptical
fiber in the figure. Single mode fiber has much thin core and used for single mode
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transmissionandhasvery lessattenuation.Step index fiberormultimode fiber isused
for multimode transmission. It has larger core so that multiple light signals can be
entered within the limitsof its allowed aperture. As different light rays have different
pathlengths,lightatoutputisreceivedwithdifferentdelays.Oneofthetypesisgraded
indexfiber.Itsrefractiveindexgraduallydecreasesoutwardswhichcausesthelightrays
to smoothly bend back towards the centre of the fiber. This curved path reduces the
multipathdispersions.Choiceoflightwavelengthisverynecessaryaseverywavelength
will not be very efficient for transmissions. Proper wavelength is calculated for best
performanceandlessattenuation.
Figure17
Figure:differenttypesofopticalfibers
3.3.1 Applications
3.3.1.1Communication
Opticalfibercanbeusedasamediumfortelecommunicationandcomputernetworking
becauseitisflexibleandcanbebundledascables.Itisespeciallyadvantageousforlong
distance communications, because light propagates through the fiber with little
attenuationcomparedtoelectricalcables.Thisallowslongdistancestobespannedwith
fewrepeaters.
Theperchannel light signalspropagating in the fiberhavebeenmodulatedat ratesas
highas111gigabitspersecond (Gbit/s),although10or40Gbit/s istypical indeployed
systems.
In June 2013, researchers demonstrated transmission of 400 Gbit/s over a
singlechannelusing4modeorbitalangularmomentummultiplexing.
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Forshortdistanceapplication,suchasanetworkinanofficebuilding,fiberopticcabling
cansavespace incableducts.This isbecausea single fibercancarrymuchmoredata
thanelectricalcablessuchasstandardcategory5Ethernetcabling,whichtypicallyruns
at100Mbit/sor1Gbit/sspeeds.Fiberisalsoimmunetoelectricalinterference;thereis
nocrosstalkbetweensignalsindifferentcables,andnopickupofenvironmentalnoise.
Nonarmoredfibercablesdonotconductelectricity,whichmakesfiberagoodsolution
forprotectingcommunicationsequipment inhighvoltageenvironments,suchaspower
generationfacilities,ormetalcommunicationstructuresproneto lightningstrikes.They
canalsobeusedinenvironmentswhereexplosivefumesarepresent,withoutdangerof
ignition.Wiretapping(inthiscase,fibertapping) ismoredifficultcomparedtoelectrical
connections,andthereareconcentricdualcorefibersthataresaidtobetapproof.
3.3.1.1.1Advantagesovercopperwiring
Theadvantagesofopticalfibercommunicationwithrespecttocopperwiresystemsare:
Broadbandwidth
Asingleopticalfibercancarry3,000,000fullduplexvoicecallsor90,000TVchannels.
Immunitytoelectromagneticinterference
Lighttransmissionthroughopticalfibersisunaffectedbyotherelectromagneticradiation
nearby.Theopticalfiberiselectricallynonconductive,soitdoesnotactasanantennato
pickupelectromagneticsignals.Informationtraveling insidetheopticalfiber is immune
to electromagnetic interference, even electromagnetic pulses generated by nuclear
devices.
Lowattenuationlossoverlongdistances
Attenuationlosscanbeaslowas0.2dB/kminopticalfibercables,allowingtransmission
overlongdistanceswithouttheneedforrepeaters.
Electricalinsulator
Optical fibers do not conduct electricity, preventing problems with ground loops and
conduction of lightning. Optical fibers can be strung on poles alongside high voltage
powercables.
Materialcostandtheftprevention
Conventionalcablesystemsuselargeamountsofcopper.Insomeplaces,thiscopperisa
targetfortheftduetoitsvalueonthescrapmarket.
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3.3.2
Sensors
Opticalfiberscanbeusedassensorstomeasurestrain,temperature,pressureandother
quantitiesbymodifyingafibersothatthepropertytomeasuremodulatestheintensity,
phase,polarization,wavelength,ortransittimeoflightinthefiber.Sensorsthatvarythe
intensityoflightarethesimplest,sinceonlyasimplesourceanddetectorarerequired.Aparticularlyusefulfeatureofsuchfiberopticsensorsisthattheycan,ifrequired,provide
distributed sensing over distances of up to one meter. In contrast, highly localized
measurementscanbeprovidedbyintegratingminiaturizedsensingelementswiththetip
of the fiber. These can be implemented by various micro and nanofabrication
technologies, such that they do not exceed the microscopic boundary of the fiber tip,
allowingsuchapplicationsasinsertionintobloodvesselsviahypodermicneedle.
Extrinsicsensorscanbeused in thesameway tomeasure the internal temperatureof
electrical transformers, where the extreme electromagnetic fields present make other
measurement techniques impossible. Extrinsic sensors measure vibration, rotation,
displacement, velocity, acceleration, torque, and twisting. A solid state version of the
gyroscope,usingtheinterferenceoflight,hasbeendeveloped.Thefiberopticgyroscope
(FOG)hasnomovingparts,andexploitstheSagnaceffecttodetectmechanicalrotation.
3.3.3 Powertransmission
Opticalfibercanbeusedtotransmitpowerusingaphotovoltaiccelltoconvertthelight
into electricity.
While this method of power transmission is not as efficient as
conventionalones, it isespeciallyuseful insituationswhere it isdesirablenottohavea
metallic conductor as in the case of use near MRI machines, which produce strong
magnetic fields.Otherexamplesare forpoweringelectronics inhighpoweredantenna
elementsandmeasurementdevicesusedinhighvoltagetransmissionequipment.
4 LASERS
Alaserisadevicethatemitslightthroughaprocessofopticalamplificationbasedonthe
stimulated emission of electromagnetic radiation. The term "laser" originated as an
acronymfor"lightamplificationbystimulatedemissionofradiation".
4.1
Specialproperties
of
lasers
Coherent light: Coherence can be defined as an ideal property of waves that enables
stationary(i.e.temporallyandspatiallyconstant)interference.Alaserdiffersfromother
sourcesof lightbecause itemits lightcoherently.Spatialcoherenceallowsa lasertobe
focused toa tight spot,enablingapplications like laser cuttingand lithography.Spatial
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coherence also allows a laser beam to stay narrow over long distances (collimation),
enablingapplicationssuchaslaserpointers.
Monochromatic: As obvious from the word having single color. Lasers are
monochromaticsinceeachphotonhaveanenergyofpreciselyhv=E2 E1
4.2
Applications
Someofitsapplicationsare:
Medicine: Bloodless surgery, laser healing, surgical treatment, kidney stone
treatment,eyetreatment,dentistry
Industry: Cutting, welding, material heat treatment, marking parts, noncontact
measurementofparts
Military: Marking targets, guiding munitions, missile defence, electrooptical
countermeasures(EOCM),alternativetoradar,blindingtroops.
Law enforcement: used for latent fingerprint detection in the forensic
identificationfield
Research: Spectroscopy, laser ablation, laser annealing, laser scattering, laser
interferometry,LIDAR,lasercapturemicrodissection,fluorescencemicroscopy
Productdevelopment/commercial: laserprinters,opticaldiscs (e.g.CDsand the
like),barcodescanners,thermometers,laserpointers,holograms.
Laserlightingdisplays:Laserlightshows
Cosmeticskintreatments:acnetreatment,celluliteandstriaereduction,andhair
removal.
4.3 Laserphysics
4.3.1
Stimulatedemission[2]
Theenergyofanelectronorbitinganatomicnucleusislargerfororbitsfurtherfromthe
nucleus of an atom. However, quantum mechanical effects force electrons to take on
discrete positions in orbitals. Thus, electrons are found in specific energy levels of an
atom,twoofwhichareshownbelowinfigure18:
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Figure18
Figure:Stimulatedemission
When an electron absorbs energy either from light or heat, it receives that incident
quantumofenergy.But transitionsareonlyallowed inbetweendiscreteenergy levels
suchasthetwoshownabove.
Whenanelectronisexcitedfromalowertoahigherenergylevel,itwillnotstaythatway
forever.Anelectron inanexcitedstatemaydecaytoa lowerenergystatewhich isnot
occupied.Whensuchanelectrondecayswithoutexternalinfluence,emittingaphotonis
called"spontaneousemission".
The stimulus isprovidedby thepresenceofphotonsof theproperwavelength. Letus
visualizeanelectroninstateE2waitingtodropspontaneouslytoE1withtheemissionof
aphotonofenergyhv12=E2E1.Nowweassumethatthiselectronintheupperstateis
immersed inan intensefieldofphotons,eachhavingenergyhv=E2E1,and inphase
with the other photons. The electron is induced to drop in energy from E2 to E1,
contributingaphotonwhosewaveisinphasewiththeradiationfield.Atransitionfrom
thehighertoa lowerenergystate,however,producesanadditionalphoton;this isthe
processofstimulatedemission.
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Figure19
Figure:Componentsofatypicallaser:
1.Gainmedium
2.Laserpumpingenergy
3.Highreflector
4.Outputcoupler
5.Laserbeam
4.3.2 Gainmedium
Thegainmediumisexcitedbyanexternalsourceofenergyintoanexcitedstate.Inmost
lasersthismediumconsistsofpopulationofatomswhichhavebeenexcited intosuchastatebymeansofanoutsidelightsource,oranelectricalfieldwhichsuppliesenergyfor
atomstoabsorbandbetransformedintotheirexcitedstates.
Thegainmediumofalaserisnormallyamaterialofcontrolledpurity,size,concentration,
and shape,which amplifies thebeam by theprocessof stimulatedemissiondescribed
above.Thismaterialcanbeofanystate:gas, liquid,solid,orplasma.Thegainmedium
absorbs pump energy, which raises some electrons into higherenergy ("excited")
quantumstates.Particlescaninteractwithlightbyeitherabsorbingoremittingphotons.
Emissioncanbespontaneousorstimulated. Inthe lattercase,thephoton isemitted in
thesamedirectionasthe lightthat ispassingby.Whenthenumberofparticles inone
excited state exceeds the number of particles in some lowerenergy state, population
inversion is achieved and the amount of stimulated emission due to light that passes
through is largerthantheamountofabsorption.Hence,the light isamplified.By itself,
this makes an optical amplifier. When an optical amplifier is placed inside a resonant
opticalcavity,oneobtainsalaseroscillator.
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4.3.3
Populationinversion
Apopulation inversionoccurswhenasystem (suchasagroupofatomsormolecules)
existsinastatewithmoremembersinanexcitedstatethaninlowerenergystates.The
productionofapopulation inversion isanecessary step in theworkingsofa standard
laser.
4.4 Someconditions
1.
Overall lightgain (amplification) shouldbegreater than1. Thegain inphotons
per pass between the end plates must be larger than the transmission at the ends,
scatteringfromimpurities,absorption,andotherlosses.
2.
Reflectorandoutputcouplershouldbeexactlyparalleltoeachother
3.
Distancebetweenreflectorandoutputcouplershouldbeintegralmultipleofhalf
ofthewavelengthtohavecoherenceasmentionedinfigure20.
Figure20
Figure:resonantmodes
5 SEMICONDUCTORLASERS
Semiconductor lasers or laser diodes play an important part in our everyday lives by
providingcheapandcompactsize lasers.Theyconsistofcomplexmultilayerstructures
requiringnanometerscaleaccuracyandanelaboratedesign.
5.1 PopulationInversion
Whenpnjunctiondiodeisforwardbiased,thentherewillbeinjectionofelectronsinto
theconductionbandalongnsideandproductionofmoreholesinvalencebandalongp
sideofthejunction.Ifthereishighnumberofelectronsinconductionbandcomparable
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tovalenceband, thenpopulation inversion isachieved.This isachieved throughheavy
doping.
5.2
TheBasicSemiconductorLaser
Tobuildabasicsemiconductorlaseritisrequiredtohaveheavilydopedpandnregions.
Thefrontandbacksidesshouldbecompletelyflatandparalleltoeachother.ThefirstpnjunctionlaserswerebuiltinGaAs(infrared)andGaAsP(visible)materials.
In choosing a material forjunction laser fabrication, it is necessary that electronhole
recombination occur directly, rather than through trapping processes such as are
dominantinSiorGe.Galliumarsenideisanexampleofsucha"direct"semiconductor.
Figure21
Figure:simplesemiconductorlaser
5.3
MaterialsforSemiconductorLasers
In principle, a diode laser can be produced from any directbandgap semiconductor.
However,efficient,electrically injectedlasersrequirepreciselydopedlayeredstructures
ofvaryingalloys that areall latticematched tooneanotherand toa substrate.These
requirements place some limitations on the available materials. Commercial
semiconductorlasersareallIIIVcompoundsalloysofGroupIIIandGroupVelements
intheperiodictable.Therearetwomajorcommercialfamiliesofsemiconductorlasers
those grown on GaAs substrates and those grown on InP substrates.
GaAsbasedlasersareformedfromalloysofGa,Al,InandAs,P,grown incompositions
latticematchedtoGaAs.Theycanemitatanywavelengthfromabout630toabout1100
nm,themostcommoncommercialonesbeing635,650,680and780nm,whichareused
inopticalstorageanddisplays;785,808,830,920and940nm,whichareusedforvarious
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pumping and printing applications; and 980 nm, which is used for pumping fiber
amplifiersintelecommunications.[3]
InPbasedlasersareformedfromalloysofthesameconstituents,Ga,Al,InandAs,P,but
incompositionsthatarelatticematchedtoInP.Theyrangefromabout1100to2000nm,
butbyfarthemostcommonareemittersat1300,1480and1550nm,whichareusedinfiberopticcommunications.
Longer wavelengths out to about 10 m are possible by incorporating other
materials (e.g., Sb) into the alloy and/or by using different substrates. Shorter
wavelengths, down to about 400 nm, have been demonstrated by growing GaN and
relatedalloys,andcommerciallasersinthenearUVareunderdevelopment.
5.4
LatestResearches
5.4.1 ENERGYEFFICIENTANDENERGYPROPORTIONALOPTICALINTERCONNECTSFOR
MULTICORE
PROCESSORS:
DRIVING
THE
NEED
FOR
ON
CHIP
SOURCES
[7]
Thispublication focusesontheenergyefficiencyofoptical interconnects formulticore
processorsandthenumberofcoresare increasingtoovercometheheatingdissipation
and data synchronization. It allows more efficient parallel processing. Quadcore
processors are common in market and processors having 80 cores are high end. It is
expectedthatnumberofcoreswillincreaseinfuture.Theglobalonchipcommunication
betweenprocessorcoresconsumesalargeportionofthetotalpowerbudget.
Opticallinkshaveallbutreplacedelectricallinksfortelecommunicationsapplicationsand
are replacing thedatacommunications interconnect links. It isexpected that toenable
high speed data communications within the chips, optics will penetrate into these
modules. This is only possible when optical interconnects will outperform electricalinterconnectsintermsofbandwidthdensity,energyefficiencyandlatency.
Studiesbasedonthecurrentstateofthetechnologyconcludethatopticalinterconnects
arenotafeasibleoptionforonchipcommunications,dueto lackoflatencyandenergy
consumption improvements, or not even offer enough bandwidth for offchip
communications,when memorybandwidth is included. Looking forward,however, the
22nm technology node seems promising for optical interconnects. In this node the
circuittransistorcapacitancesaresmallenoughtobedrivendirectlybyaphotodetector,
thereby eliminating power hungry transimpedance amplifiers and hence greatly
reducingthepowerconsumptionofthelink.
Thispublicationcomparesanoffchipcomb laser, fibercoupledtothechipwithanon
chipsinglewavelengthdistributedfeedback(DFB)laserarray.Byuseofsomegraphsand
simulationsitconcludes:
Coupling lossesofoffchipcomb lasersaregenerallyunderestimated, improving
thecaseforonchiplasers;
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Multipleonchiplasersallowforbetterenergyproportionality;
Makingtheonchiplaserspartofthearchitecturedesignequationandpartofthe
loaddistributiondesign,additionalpowersavingscanbeachieved;
TheflexibilityofplacingonchipsourcesatanydesiredpositionintheNoClayout
leadstoimprovedenergyefficiency.
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6 REFERENCES
1.
SOLID STATE ELECTRONIC DEVICES by Ben G. Streetman and Sanjay Kumar
BanerjeetheUniversityofTexasatAustin
2.
wikipedia.org
3. OPTOELECTRONIC SEMICONDUCTOR DEVICES Principals and Characteristics by
IrinaStateikina
4. howstuffworks.com
5. INTRODUCTION TO OPTOELECTRONIC DEVICES (Lecture notes) by Dr. Jing Bai
UniversityofMinnesotaDuluth
6. ecee.colorado.edu
7.
MartijnJ.R.Heck,Member,IEEE,andJohnE.Bowers,Fellow,IEEE.Energy
EfficientandEnergyProportionalOpticalInterconnectsforMultiCoreProcessors:
DrivingtheNeedforOnChipSourcesIEEEJournalofSelectedTopicsInQuantum
Electronics,Vol.20,no.4,July/August20148201012
8.
YikKhoonEe,StudentMember,IEEE,PisistKumnorkaew,RonaldA.Arif,Hua
Tong,HongpingZhao,JamesF.Gilchrist,andNelsonTansu,Member,IEEE
OptimizationofLightExtractionEfficiencyofIIINitrideLEDsWithSelfAssembled
ColloidalBasedMicrolensesIEEEJournalofSelectedTopicsInQuantum
Electronics,Vol.15,No.4,July/August2009
9.
XiaoHangLi,PeifenZhu,GuangyuLiu,JingZhang,RenboSong,YikKhoonEe,PisistKumnorkaew,JamesF.Gilchrist,andNelsonTansuLightExtraction
EfficiencyEnhancementofIIINitrideLightEmittingDiodesbyUsing2DClose
PackedTiO2MicrosphereArraysJournalofDisplayTechnology,Vol.9,NO.5,
May2013
10.
AnandRamaswamy,StudentMember,IEEE,MollyPiels,StudentMember,IEEE,
NobuhiroNunoya,Member,IEEE,TaoYin,Member,IEEE,andJohnE.Bowers,
Fellow,IEEEHighPowerSiliconGermaniumPhotodiodesforMicrowavePhotonic
ApplicationsIEEETransactionsonMicrowaveTheoryAndTechniques,Vol.58,
NO.11,November2010
11.
DesignofPhotovoltaicSolarCellModelforStandaloneRenewableSystem,
publishedinELEKTRO,2014,DateofConference:1920May2014.Link:
http://ieeexplore.ieee.org/xpl/articleDetails.jsp?tp=&arnumber=6848903
12.http://ocw.tudelft.nl/fileadmin/ocw/courses/SolarCells/res00018/!3620504e206a
756e6374696f6e.pdf