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The range of VCSEL wearout reliability acceleration
behavior and its effects on applications
James Guenter, Luke Graham, Bobby Hawkins, Robert
Hawthorne, Ralph Johnson, Gary Landry, Jim Tatum
Finisar, 600 Millennium Drive, Allen, TX 75044
Proc. SPIE 863917, 2013
The range of VCSEL wearout reliability acceleration behavior and its effects on applications
James Guenter∗, Luke Graham, Bobby Hawkins, Robert Hawthorne, Ralph Johnson, Gary Landry, Jim Tatum, Finisar, 600 Millennium Drive, Allen, TX 75044
ABSTRACT
For nearly twenty years most models of VCSEL wearout reliability have incorporated Arrhenius activation energy near 0.7 eV, usually with a modest current exponent in addition. As VCSEL production extends into more wavelength, power, and speed regimes new active regions, mirror designs, and growth conditions have become necessary. Even at more traditional VCSEL 850-nm wavelengths instances of very different reliability acceleration factors have arisen. In some cases these have profound effects on the expected reliability under normal use conditions, resulting in wearout lifetimes that can vary more than an order of magnitude. These differences enable the extension of VCSELs in communications applications to even greater speeds with reliability equal to or even greater than the previous lower-speed devices. This paper discusses some of the new applications, different wearout behaviors, and their implications in real-life operation. The effect of different acceleration behaviors on reliability testing is also addressed. Keywords: VCSEL, vertical cavity surface emitting laser, reliability, InGaAs
1. INTRODUCTION The reliability goal for a mature optoelectronic product is to have wearout irrelevant during the planned operating life, a goal difficult to achieve in an environment where the operating requirements double every few years. In this paper we describe reliability of a device designed for 14 Gbps operation, but there are implications for devices of the next generations as well.
It is a common complaint that certain knowledge about reliability is only available for parts that have actually failed. For all parts not yet failed one must rely on models and statistics, both of which may take many years and thousands—or even millions—of parts to generate for new technologies. This leads to a preferred conservatism: when an equation has been found to apply, we are reluctant to move to a different technology where its applicability is unknown, or to make modifications to model parameters for similar devices even in the face of substantial new data. Sometimes, however, there is no choice, because new operating requirements require new devices.
The acceleration factor of VCSEL failure rate at an operating condition relative to some reference condition is almost invariably described by the modified Arrhenius equation, = exp 1 − 1
where I and T are the current and the absolute temperature of the active region, k is Boltzmann’s constant, and Ea is an activation energy. The exponent N can take on any value. The combination of the power law with parameter N and the exponential term with parameter Ea allows modeling of many different possible behaviors. (It should be noted that if there are multiple failure mechanisms, each can be described by the equation above, each with their own values of Ea and N. If that is the case the net reliability is based on the superposition of all the mechanisms. Even in the case of multiple mechanisms in practice it is usually found that one mechanism will dominate the reliability behavior within a range of current and temperature. The fact that another mechanism could be dominant within a different range provides much of the motivation for this paper.)
The earliest 850-nm VCSEL reliability tests found Ea near 1 eV, but they ignored the current except to the extent that it affected junction temperature, effectively assuming N=0.1,2 With substantially more data and more sophisticated analysis, most VCSEL manufacturers subsequently found that for 850-nm AlGaAs VCSELs Ea near 0.7 eV and N ≈ 2 more accurately described the reliability behavior over a wide range of conditions,3,4,5,6 though at least some manufacturers find that Ea=0.7 eV, N=0 better describes their devices.7 The fact that at multiple manufacturers ∗ jim.guenter@finisar.com
activation energy near 0.7 eV has been found to describe AlGaAs 850-nm VCSEL reliability for both MOCVD and MBE epitaxial growth techniques, for both proton implant and oxide aperture fabrication techniques, and over a wide variety of current aperture sizes leads to high confidence in the predictive power of reliability models for VCSELs with GaAs quantum wells surrounded by AlGaAs barriers. This is all the more remarkable since the trajectories of degradation for different manufacturers sometimes differ, suggesting that the degradation mechanisms are not precisely identical.8 But what if the active regions change and the quantum wells are no longer in the AlGaAs system?
1.1 InGaAs quantum wells in VCSELs
VCSELs dominate optical data communications, with many times more fielded devices than any other technology. Most of the fielded devices have had GaAs quantum wells and AlGaAs barriers. Increasing speeds have necessitated increasing current densities, and those speeds combined with the push for increased packing density have increased the required operating temperature.
Significant speed increase can be achieved without modifying the quantum well materials. The threshold current can be reduced and operating current density increased by reducing aperture size; the photon lifetime can be tailored by adjusting mirror reflectance profiles; capacitance can be reduced by adding dielectric layers within the structure or by decreasing the radius of conducting material outside the current aperture; and high-temperature performance can be improved by adjusting composition and doping of nearby layers to minimize carrier losses. At some point, however, these and similar modifications reach a limit and further improvements in modulation speed or operating temperature range require new quantum well gain materials. GaAs-well devices have improved to meet the changing demands, but at some speed above 14 Gbps it appears that achievable performance with those wells will be insufficient. Even at 14 Gbps an inherently faster well would lead to better margin or enable operation at higher temperatures. Compressively-strained indium-containing quantum wells are one path to such higher speed.
VCSELs have been so reliable that it has often been the case that wearout lifetime to some commercially relevant failure rate, say, 0.1% failures, was many times longer than the time for a similar rate from random failures due to rare manufacturing defects or to damaging events. As current densities and temperatures rise the wearout failure rate increases at a rate much greater than the random rate increase, however. At a high enough stress (the combination of temperature and current) the wearout rate is completely dominant, which is what enables accelerated testing in the first place. If wearout rates cannot be improved eventually the faster and higher packing density systems will be less reliable than their slower predecessors. The addition of indium to the quantum wells improves their speed, but its effect on wearout reliability in 850-nm AlGaAs-based VCSELs has not been extensively studied.
To be sure, the addition of indium in many types of devices has been a source of reliability improvement, and it has often been suggested that addition of indium to 850-nm VCSELs would also improve reliability.9 These expectations are primarily based on one failure mode, climb dislocation growth, which has been a substantial life limiter for lasers and LEDs of many types, and which has certainly been observed in some VCSELs. Compressively strained indium-containing quantum wells in long wavelength edge-emitting lasers appear to be almost completely immune to dislocations of this kind.9
However, 850-nm VCSELs differ from these long wavelength devices in a number of ways, each of which is generally associated with reduced reliability. The photon energy and the operating voltage are higher, resulting in higher energies available for defect creation and propagation; the operating current densities are generally higher, almost always over 10 kA/cm2; and the quantum wells are generally thinner, leading to higher volumetric current density as well. In addition, at 850 nm the quantum well barriers generally must contain aluminum and devices containing adjacent Al-containing and In-containing layers have been problematic.10,11,12 Published reliability studies of 850-nm VCSELs fabricated with InGaAs wells and AlGaAs barriers have been scant, low-stress, and of short duration.13 Excellent reliability has been demonstrated for InGaAs-well VCSELs at longer wavelengths, but at those wavelengths the barriers usually do not contain much aluminum.14
At Finisar we had additional reasons to be skeptical about the potential reliability improvement the addition of indium might provide. We had tested indium-containing wells in various VCSEL designs over more than a decade. We saw the performance differences we were looking for, but no improvements in reliability. In fact, until recently the reliability of our InGaAs test devices was often worse than for similar designs with GaAs wells. One important reason is that dislocations had never been the source of wearout failure in Finisar VCSELs, so immunity to climb dislocations would not affect our wearout results at all. The other reason is even more important. Since it was likely that indium would eventually be necessary for speed, in the last few years we embarked on an extensive experimental campaign to
determine whGaAs well pgrowth rates,variability inat an extremenot particularthe best resul
Figucond
The very shonominal use require a muc
1.2 Evolutio
Until substanmodels will lgroups will hof degradatioearlier in testcan only be trajectory is l
Figufailuposs
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tress groups wimodel already es occur later tction. If the ne
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g InGaAs reliaratures, precurion of the wellsfigure represencombination others since the
us range.
VCSELs with difer three orders of
times at the vany years of opextreme than th
hose with lowee Ea and N.) Ey to model geno failure, allowectory is similaed to parts alreoups must be te
the midpoint ofit is pessimistic
CSELs.8
entually failureer stress groupdicts, or downwmodel that stil
ability up to thrsor types, ratis themselves. W
nts the statisticsof the variables
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very high streperation, to be he nominal use
est stress, the pEarly in testing neration. Depewing more comar to (a) of Figueady very nearested to actual
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2.1 Wearou
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TablsubsstudNote
2.2 Test res
A large numbincluded in Tadditional waAlso not in consistent res
The preliminfailures werecurrent and tacceleration increase as thµm VCSELsvalues have nmodel appea
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le 1. Burn-in cosequent validatiody parts, after foue from typical LI
sults
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nary results ofe verified by eltemperature wexponent N > he model evolvs16 and both Snot previously rs quite robust
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e is not just du
ution, the new parent bimodali
UCTION InGest description
tion of a GaAsfor the VCSELd.15 Aiming fregion using I
bles described nd put into extoriginal emitteditions and adence. The grou
ps had accumuland some pa
as possible (bection of a prelim
onditions and quon groups. Sampurteen months aI every 30°C fro
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Sale and Honebeen reported , as demonstra
using the modality.
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GaAs QUANTn
s quantum welL structure.15for even betterInGaAs quantu
above. Qualiftended stress teed power at 6 dditional samplups in the wealated over 10,0arts in the lowehavior like (a)minary reliabil
uantities of unitsples are from muall but 116 in lowom -10 to +170°C
produced test reeptance tests ase those tests haburn-in was c00 additional t
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ated by several
del to accelerat
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one that will mthat more than
TUM WELL
l device for thReliability test
r performance um wells was fication epitaxests at multiplemA and 25°C.les at some ex
arout test are su000 operating hwest stress gr) in Figure 2 ality model.
s, including 128ultiple epitaxial rw stress groups C that most devi
esults entirely at 150°C and ave fixed test d
conducted in wtest devices at 1
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nprecedented inN ≈ 5 in 670-n
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L DESIGN R
he 14 Gbps Fibting of that Gamargin and toplaced in this
xial runs and e conditions. F. As the first fxisting test conummarized in
hours. At least roups had sufand at least -0
1 parts in main runs and multiplhave actual or c
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case in previoucation-free, butl shows activat failures, bothn VCSELs: Grnm VCSELs,17
50 nm.* While
o a common st
N=2 in their red
ly predict actuechanism is pre
RELIABILIT
bre Channel maaAs well devicoward future p
mature designfabrication lot
For purposes offailures occurrenditions were Table 1. As osome failures
fficient degrad0.5 dB degrada
wearout study le fabrication lotconfident extrapnder all burn-in
h those of the more than 6700r than continuother than on a
mA.
us Finisar VCt the acceleratiation energy Eah of these valueraham found E7,18 but to our the acceleratio
tress condition
d-emitting VC
ual operating cesent.
TY TEST
arket for some ce found the traroducts at even using the prots were produf the study, faied in the higheadded to estab
of the fourth quhad occurred i
dation that reaation at last te
and another 180ts. Of the 1281 m
polated failure ticonditions.
main study, bu0 units from doous operation tossembled com
CSEL designs, on of degradata >1.3 eV andes will almost c
Ea=1.2 and N=4knowledge su
on factors are h
shows a reaso
CSELs.19
condition
time, so aditional
en higher oduction
uced and ilure was est stress blish the uarter of in all but asonable
est time),
04 in main imes.
ut are not ozens of o failure.
mponents,
wearout tion with d current certainly 4 in 1.3-uch high high, the
onable fit
Figufromfrom
Second, Figuthat it is simdistributions our case theaccelerated f
FigutimeOnean efor i
ure 3. Reliabilitym model fit line m model failure t
ure 4 shows themilar to the σ=
have a locatione distribution ifailure times of
ure 4. (a) Cumule distributions co group at 125°Crror in applied s
individually relia
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time. See main te
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lative failure disompared to the s
C, 12 mA differsstress, but even table statistics.)
GaAs-well VCSEer parts (from twext for additiona
ailure distributoverall lognor
hat varies with twith location ken separately
stributions of easolid line overall more than 2×; athis group is less
EL at worst casewo different streal information.
tions of each strmal cumulativtest condition aparameter µ fall close to th
ach group of Tabl median model as it is intermeds than 3×. (All g
e system operatiess groups), each
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he overall mod
ble 1, showing cprediction at no
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ing conditions. h less than a fac
n taken separatetribution modearameter that isrameter σ.) In
del median.
consistent σ. (b)ominal use condirent and temperaexcept the two w
Maximum deviactor of two diffe
ely have similael. (Cumulativs presumed conn addition, the
) Accelerated faition of 55°C, 6 ature it may indwith too few fail
ation ferent
ar σ, and e failure nstant. In e median
ailure
mA. dicate lures
Third, we hamany months
As with Finisis shown in F
Figu
The high—aimplications.higher than tthe InGaAs wpredicted lifeextreme condifferences ctypes. The dInGaAs winn
ave seen consis apart.
sar GaAs-well Figure 5.
ure 5. Degradatio
and likely to . The most impthat of previouwell devices c
fe at worst casditions require
can be quite lardotted line sepaning in the low
istent results fr
parts, degrada
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go higher aportant one is tus designs. In aan provide thee operation th
ed for short-terge, as shown arates the regi
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3. Das the reliabilthat it appears addition to the e required perfhat is substantirm reliability in Figure 6, w
ime where theion containing
own to the sam
ul, with gradual
he 85°C, 12 mA g
DISCUSSIONity test continthat wearout rrapid reductio
formance at a lially longer fotests the InGa
which plots the InGaAs modethe worst case
me design but
l reduction in p
group.
N nues—accelerreliability at noon in failure ralower operatin
or the InGaAs aAs well partse log of the ratel predicts lone system operat
t in different r
power over tim
ation factors ormal use cond
ate with the lowng current. Thi
design but at s will show shtio of the lifetinger life than tating conditions
eactors, and p
me. One examp
have some pditions will actwer stress at ops combination the same tim
horter lifetimeimes of the twthe GaAs mods.
rocessed
ple group
profound tually be peration, leads to
me, at the s. These
wo device del, with
Figutypicwors
As noted prewhile the exarecombinatioassume that fapplied stresIf that is the lower acceleassumed in TeV, guarantewearout mec
In a sense thstress for thestress conditibound on wtraditional Eawe actually oacceleration
While there InGaAs quandevices can bahead to theperformance
In reliability we have learmodel for a n
ure 6. Relative rcal burn-in condst-case condition
eviously, GaAsact physics of fon-enhanced crfor both designs, it is at least case, the mech
eration mechanTelcordia GR-eing their domhanisms?”
e question is ue full product lions in our stu
what lower-accea=0.7, N=2 meobserve. Our cmodel above a
were reasons ntum wells, witbe even more e next speed that the additi
testing there irned from the new generation
reliability of Gaditions, GaAs wns of 95°C and 8
s-well devices failure remain reation or mulns the specific dpossible that whanism with thnism might ac468 testing, w
minance at low
unanswerable blifetime, typicady to that predeleration mechechanism wouconclusion is tand that we can
to question thth proper desigreliable than thnode, where on of indium p
is no substitutedevices descri
n of Finisar VC
aAs and optimizwell devices deg8.5 mA the final
otherwise the elusive, at Finltiplication of defects are idewe have two dihe higher accectually accountwhere “random”
stress.) So the
because certainally a decade odicted by each hanism could
uld already predthat Finisar InGn draw inferenc
4. COhe wearout relgn and growth heir GaAs-welVCSELs oper
provides.
e for time, so eibed here prov
CSELs and it ap
zed InGaAs quagrade more slow
design InGaAs w
same as thosenisar both GaA
point defects ntical, with theifferent mechaleration wouldt for most fail” failures are be obvious quest
nty would onlyor two. But wemodel separatebe present. Wdict a greater nGaAs VCSELces about lifetim
ONCLUSIONliability of 850conditions we
ll predecessorsrate at 25 or
even as testingvides a basis foppears wearout
antum wells in wly, but at all ac
well devices are
e of the presents- and InGaAsnear the quan
e only differenanisms coexistid always accoulures at lowerby default assition is “How c
y be available ae can compareely. Based on
With fourteen mnumber of fail
L wearout reliames at lower s
NS 0-nm data com
e have demonsts. This is an es
28 Gbps, re
g proceeds on tor optimism. Tt reliability onc
the same VCSctual operating ce more reliable.
t study have los-well devices ntum wells. W
nce being the raing in the indiuunt for failuresr stresses. (Thiigned activatiocan we know th
after operation e the actual faithat comparisomonths of conlures in the lowability is well-dtress condition
mmunication Vtrated that undspecially impoquiring the in
the next generaThe data compce again meets
EL structure. Aconditions inclu
ower acceleratiappear to degr
While it is temate of acceleratum containing s at high stressis situation is on energy of ohat we do not h
with zero acceilure rate at thon we can at lentinuous operawest stress grodescribed by t
ns based on tha
VCSELs incorder use conditioortant result as nherently bette
ation of VCSEpels a new acces its goal.
At all uding
ion. And rade with
mpting to tion with devices.
s, but the actually
only 0.35 have two
eleration he lowest east set a ation the ups than
the high-at model.
rporating ons these we look
er speed
ELs what eleration
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[14] Takaki, K., et al., “1060nm VCSEL for inter-chip optical interconnection,” Proc. SPIE 795204, (2011). [15] Graham, L., Chen, H., Gazula, D., Gray, T., Guenter, J., Hawkins, B., Johnson, R., Kocot, C., MacInnes, A.,
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[19] Duggan, G., Barrow, D., Calvert, T., Maute, M., Hung, V., McGarvey, B., Lambkin, J., Wipiejewski, T., “Red vertical cavity surface emitting lasers (VCSELs) for consumer applications,” Proc. SPIE 69080G, (2008).
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