Optoelectronic Devices Re-edited_p

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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

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