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

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ONCLUSIONliability of 850conditions we

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even as testingvides a basis foppears wearout

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well devices are

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e only differenanisms coexistid always accoulures at lowerby default assition is “How c

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NS 0-nm data com

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28 Gbps, re

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t study have los-well devices ntum wells. W

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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).