Capabilites and Reliability of LEDs and Laser Diodes

Transcription

1 Capabilies and Reliabiliy f LEDs and Laser Dides Melanie O Technlgy Validain Assurance Grup Swales Aerspace Cmpnen Technlgies and Radiain Effecs Branch NASA Gddard Space Fligh Cener melanie@gsfcnasagv updaed 9/3/97 In general, high emperaure esing is used deermine LED and laser dide lifeimes, even hugh laser dide failure mechanisms are mre sensiive increases in curren densiy As a measured parameer f degradain, he curren densiy is f grea significance when searching fr failure mdes in a laser dide Raising he curren densiy hwever, is n really indicaive f lifeime since i is mre likely a siuain be avided han ne ha simulaes nrmal lifeime degradain The reliabiliy f semicnducr surces is very dependen n he degradain mdes This repr inends summarize sme f he degradain mdes and capabiliies f ypical LEDs and laser dides currenly used in many cmmunicain and sensing sysems LED s are ypically used in mulimde ransmissin sysems where daa raes n larger han 50 Mb/s are required They have larger specral widhs and can add he prblem f dispersin in cmmunicains sysems Laser dides are used in sysems ha require cheren and fen single mde ligh such as high daa rae cmmunicains and sensing applicains In cmparisn laser dides, LED s can generally be driven harder, are less expensive, have lwer pwer, have larger emiing regins, and lnger lifeimes Lasers, unlike LED s will n perae belw a hreshld curren Meaning, nly when he hreshld curren is reached will he dide cmmense lasing (funcining) As menined previusly, LEDs and laser dides are emperaure sensiive when cnsidering verall lifeime, fr example, peraing a laser dide a 10 C higher han raed will half he life f he dide Als a laser usually will sp funcining a 100 C The degradain mdes ha resul in failures r gradual degradain f hese devices can be mdelled using Arrhenius relainships where each degradain mde carries a specific acivain energy Fr example in reliabiliy ess in which lifeime is based n emperaure aging he relainship is life = A e Ea/kT

2 Capabiliies: Table 1: Cmparisn f Typical Parameers f Ineres fr LEDs and Laser Dides Aribue/ Parameer LEDs Laser Dides Radiaive Recmbinain Spnaneus Emissin Simulaed Emissin Paricle Phase Incheren Cheren Plarizain sae Randmly plarized linearly plarized Direcin Randm linear 665 nm GaAsP GaAlAs nm Ga 1-x Al x As Ga 1-x Al x As 1300, 1550 nm InGaAsP InGaAsP Specral Widh λ 145 λ 2 kt λ in µm, kt in ev, k = Blsman s cnsan, T = juncin emp Specral Widh, GaAlAs 10s f nm < 15 nm Specral Widh, InGaAsP surface emiing, 100 nm 1 10 nm edge emiing, nm Significan Parameers BW vs Pwer BW increases a he expense f pwer Threshld curren, Index guided: ma Gain guided: ma Reliabiliy lifeimes hurs 10 5 hurs Temperaure Effecs increases wavelengh by 6 nm/ C wavelengh varies by 25 nm/ C hreshld curren rises by 5mA/ C Rise Time ns < 1 10 ns Oupu Pwer (highpwer) µw mw Mdulain 3 Mhz Mhz > 350 Mhz In general he bandwidh-rise ime relainship is calculaed as BW = 35 / rise ime x is beween 0 and 1 in Ga 1-x Al x As Table 1 summarizes available infrmain n a wide range f LEDs and laser dides Specialy devices are n included in his summary and he parameers specified are highly generalized This daa is included as a reference when cnsidering which device, due aribue r parameer, is ms useful fr he applicain Degradain Mdes The main degradain mdes are: dislcains ha affec he inner regin, meal diffusin and ally reacin ha affec he elecrde, slder insabiliy (reacin and migrain) ha affec he bnding pars, separain f meals in he hea sink bnd, and defecs in buried heersrucure devices These mdes are enhanced by curren during ambien emperaure perains Face damage due xidain is enhanced by ligh r misure and is paricular laser dides Degradain f he Inner Regin: Pin Defecs lead Dark Line Defecs

3 The main causes f degradain in he inner regin is direcinally dependen n he crysal srucure as well as dependen n he maerial used fr he surce In paricular dislcains alng he 100 direcin grw as a resul f inersiial am r vacancy pin defecs AlGaAs/GaAs shw a much higher rae f dislcain grwh han surces fabricaed in InGaAs(P)/InP In general, he lnger he wavelengh respnse f he maerial, he less sensiive i is his pin defec Pin defecs can als lead a slw degradain r a rapid degradain when he defec leads a plane defec in he crysal srucure Imprving crysal grwh echniques is he nly way f making hem less likely Oher Types f Dark Line Defecs Dislcains alng he 110 crysal axis will grw and frm as a resul f mechanical bnd sresses The resul f bh ypes f dislcains are Dark Line Defecs (DLD) and induce rapid degradain f he device Anher degradain in InGaAs(P)/InP surces is precipiain f hs ams ha resul in an elevain in pulse hreshld curren (driving curren required fr lasing) The higher his curren is driven he faser degradain f her mechanisms will ccur as a resul as well as dark line defecs In lking fr hese ypes f degradain mechanisms i is mre revealing mnir he hreshld curren as ppsed he upu pwer The hreshld curren is mre sensiive defecs han he upu pwer As he curren is driven saurain, nise will develp in he laser signal Surface Degradain Face Damage Frm Oxidain in Laser Dides Oxidain f a surce face can lead slw degradain Surces ha cnain higher cncenrains f aluminum end inhibi he xide grwh Alumium paricles are acive and inhibi diffusin he face by decreasing he juncin emperaure AlGaAs/GaAs surces will develp xide hickness prprinal heir upu pwer levels when peraing a lw pwer and will grw hickness prprinal he square f he upu pwer levels when peraing a higher pwer levels Caasrphic Opical Damage (COD) COD ccurs as a resul f face meling due curren cncenrain and pical absrpin Opical absrpin ha encurages nnradiaive recmbinain resuls in heaing and meling a he face The hea generaed will als cause he bandgap shrink and herefre as a resul he curren cncenrain increases creaing mre hea and he cycle cninues The AlGaAs/GaAs surces are much mre sensiive his ype f damage han he InGaAs(P)/InP surces Where he firs is cnsidered unsable agains xidain and has high raes f face xidain, he secnd has a much lwer rae f xidain wih respec ime and upu pwer The same is rue fr COD as he firs will experience his a levels less han 1 MW/cm 2 he secnd will experience n unil pwer densiies f ens f MW/cm 2 have been reached One sluin his is a nn-absrbing mirrr srucure r NAM The Japanese are wrking n fabricaing his ype f echnlgy bu i is difficul manufacure a presen

4 Ally Elecrdes In surces (and phdeecrs) wih ally elecrdes, here develps degradain as a resul f he meal diffusing in wards he inner regin One example f an ally ype elecrde is AuZnNi During perain he meal will diffuse creaing spikes alng wih he direcin f curren flw The resul is dark sp defecs in he inner regin f he semicnducr The Schky-ype elecrdes such as Ti/P/Au d n seem cause he same degradain The meal frms an iner inerface wih beween he elecrde and he semicnducr surface Bnding Pars Sf-slders can reduce mechanical sresses n he bnding surface bu end add early degradain f he device In, Sn, and Sn-rich Au-Sn are amng he ype f sf (lw meling pin) slders ha are aribuable slder insabiliies like whisker grwh, hermal faigue, vid frmain a he bnding par, and diffusin similar wha ccurs wih he ally elecrdes when in cnac wih he semicnducr surface The insabiliies direcly lead sudden premaure failures The higher meling pin slders, r hard slders which include such maerials as Au-rich AuSn eliminae many f he insabiliies ha plague devices ha have prblems wih sf slders Buried Heersrucure In Buried Heersrucure dides (BH), he cnfigurain and index f refracin changes nearby he acive regin f he laser dide creaes a waveguide fr ligh emerging frm he ineracins This ype f laser is cnsidered an index-guided laser The n-ype InGaAsP acive emiing regin is surrunded n bh sides by he p-ype InP The degradain mde in hese lasers is assciaed wih a breakdwn r degradain f he acive regin due a decrease in injeced carriers The degradain f he BH inerface is cnsidered a wear u failure and is n a sudden ype failure Abve is a summary f he ms generally cmmn characerisics and degradain mdes f laser dides and LEDs Face degradain is specific laser dides Sme degradain mdes can be eliminaed hrugh redesign f he semicnducr srucures r hrugh packaging echniques Fr mre infrmain please review he references a he end f his paper Reliabiliy Equains: There are several mehds f exraplaing surce lifeime including mehds f calculaing lifeime given pwer upu peraing emperaure, device drive currens, and decrease in upu pwer Belw are several f he expraplain equains fr predicing surce lifeime Oupu Pwer: The lifeime f a laser dide r LED can be apprximaed by he fllwing relainship Given an iniial pwer upu f he device P and he expnenial lifeime τ, he pwer upu ver ime, can be exraplaed

5 P ( ) = P e u τ Assume ha fr a given ime, he pwer upu f he device has drpped a percenage frm he iniial pwer level such ha Pwer rai, P R = P u /P and slve fr τ such ha, τ = / ln( P R ) Nw wih τ knwn, as well as he iniial pwer upu P, he pwer upu P u () can be exraplaed ver ime Drive Curren: Anher way f predicing surce lifeime is by exraplain f he curren densiy Elevaed currens can bring u many f he degradain mechanisms assciaed wih pin defecs in devices such as AlGaAs If J is defined as he curren densiy, he lifeime f he device is defined as, and he empirical value parameer is defined as n, hen here exiss a relainship such ha J n Therefre if he lifeime f he device, is knwn fr a given peraing curren, I hen a relainship beween drive curren and device lifeime can be deduced frm J J I = n = J = J I Slving fr 2 such ha a relainship exiss where lifeime can be prediced as a resul f elevaed r decreased peraing drive curren, I 2 I2 = I 2 0 The values f n range frm 15 20, wih he larger n indicaing mre f a reducin in perainal lifeime r greaer sensiiviy f he device increased currens Temperaure: Fr deermining he relainship beween emperaure f he device predic lifeime an Arrhenius relainship can be expressed as, ce 0 Ea/ kt where E a is he acivain energy fr he device in unis f ev, k is Blzman s cnsan = 138 * Jules/Kelvin, T is abslue emperaure, ( C ) in unis f Kelvin, c is he device cnsan in unis f ime, and e is elecrn charge = 16 * jules/ev =

6 The lifeime in his relainship is defined as unexcepable drive currens fr lasers where he drive currens elevae imes he raed drive curren and fr LEDS can be upu pwer lss belw ha f he raed value due pin defecs Fr life ime versus emperaure calculains he fllwing E a, acivain energies can be used: AlGaAs/GaAs lasers ~ 7 ev AlGaAs LEDs ~5 ev InGaAsP/InP (lnger wavelengh) ~ 16 ev InGaAsP/InP Buried Heersrucure ~ 9 ev GaAlAs Duble Heersrucure LED ~ 56 ev Given a knwn acivain energy E a, peraing emperaure T and lifeime f he device, he cnsan can be calculaed by c = e Ea/ kt Or as a rai, 2 can be slved fr in erms f T 2 given T and such ha 2 = e e Ea/ kt Ea/ kt 2 Simplifying slve fr 2 as a funcin f he emperaure fr acceleraed life esing, 2 = e Ea 1 1 k T T2 Ne ha fr phdeecrs he degradain mechanisms are differen bu he same Arrhenius relainship can be used deermine lifeime f he device given differen peraing emperaures The same relainship hlds wih he acivain energy being ~ 7 ev fr infrared deecrs Als i is impran ne ha he crieria fr deecr lifeime degradain is based n receiving an unaccepable signal nise rai upu as a resul f he acceleraed emperaure life es References 1 Misu Fukuda, Semicnducr Laser and LED Reliabiliy, OFC Turial 1996, San Jse, Califrnia 2 Misu Fukuda, Laser and LED Reliabiliy Updae, Jurnal f Lighwave Technlgy, vl6, pp , Misu Fukuda, Degradain Mdes f Semicnducr Lasers used in Opical Fiber Transmissin Sysems, SPIE vl 1634, pp , 1992

7 4 Jhn P Pwers, An Inrducin Fiber Opic Sysems, Aksen Assciaes Inc,1993