Identification of degradation mechanisms in high-power laser barsusing by-emitter degradation studies

Stephen Bull Æ Jens W. Tomm Æ Eric C. Larkins

Received: 30 September 2007 / Accepted: 9 January 2008 / Published online: 25 January 2008

� Springer Science+Business Media, LLC 2008

Abstract An investigation into the degradation of an

808 nm AlGaAs/GaAs compressively strained high-power

laser bar is performed using a range of experimental

techniques that focus on understanding the performance of

the individual emitters that make up the laser bar—a

so-called by-emitter analysis. We find that thermally

induced current runaway, beginning in the edge emitters, is

an important factor in the degradation of this bar and its

eventual failure. Facet temperature measurements reveal

that temperature distributions where the emitters are hotter

at the bar edges as compared to those in the bar centre can

occur in aged laser bars. This is in contrast to the facet

temperature distributions observed in new devices and also

in measurements of the bulk temperature.

1 Introduction

The state-of-the-art for reliable high-power laser diodes has

advanced greatly over the last few years. Today, individual

diodes and laser bars have been demonstrated with

output powers of tens of watts and hundreds of watts,

respectively [1–4]. Devices are also available covering a

large wavelength range, thereby increasing their potential

applications. Typical applications for these high-power

devices include the pumping of solid-state lasers, materials

processing (soldering, cutting and welding of both plastics

and metals) and medicine [5]. For applications that require

the highest output powers, laser bars consisting of a linear

array of laser emitters are used. However, as the demand

grows for more reliable and higher power laser bars, the

study of degradation processes in such devices has become

more important.

A high-power laser bar, as depicted in Fig. 1, can be

more properly considered as an array of individual diodes

connected in parallel, but sharing the same electrical con-

nections, substrate and heatsink. Consequently, the

electrical, optical and thermal interactions between emit-

ters are very complex and need to be considered when

investigating both the operation and degradation mecha-

nisms of laser bars.

Laser bars are known to degrade faster than single

emitters. The lifetimes of single emitter devices can exceed

100,000 h, whilst high-power laser bars have lifetimes of

up to 10,000 h. The reasons for increased emitter degra-

dation rates in laser bars are not well understood, but seem

to be related to a combination of increased and inhomo-

geneous packaging-induced stress and current competition

between emitters. Indeed, one can think of the laser bar as

a system, the stability (i.e. degradation rate) of which is

influenced by effects such as strain and current competi-

tion. In a broad sense, the current competition between

emitters in a laser bar can be compared to the constant

power vs. constant current degradation of a single emitter,

where device lifetimes are generally shorter for emitters

that are operated in a constant power mode.

While there are many reports on reliability studies of

single emitters, there are fewer reports on the same issues

in high-power laser bars. Furthermore, when conventional

S. Bull (&) � E. C. Larkins

School of Electrical and Electronic Engineering, University

of Nottingham, University Park, Nottingham NG7 2RD, UK

e-mail: steve.bull@nottingham.ac.uk

J. W. Tomm

Max-Born-Institut fur Nichtlineare Optik und

Kurzzeitspektroskopie, Max-Born-Straße 2A,

Berlin 12489, Germany

123

J Mater Sci: Mater Electron (2008) 19:S145–S149

DOI 10.1007/s10854-008-9577-5

aging experiments, as used for the lifetime testing of single

emitters, are extended to the case of laser bars, such

experiments only monitor changes in the bar as a whole.

This means that little is known about the operating con-

ditions and degradation of individual emitters. While a

conventional analysis allows the main degradation mode(s)

to be identified (i.e. gradual, rapid, catastrophic), it cannot

identify the exact cause of the degradation or its location

within the bar [6]. The degradation or failure of one or

more emitters can have a severe and rapid impact on the

performance and reliability of a laser bar [7].

In previous work, we have developed a by-emitter

analysis technique, which combines several experimental

methods to examine degradation processes at both the bar

and emitter level, making it easier to identify bar degra-

dation mechanisms [8–11]. The benefits of this

methodology are evident from our earlier studies. In some

bars, a packaging-induced strain threshold was found to

exist, above which the rate of degradation during operation

increased [8, 9]. In another study, a relationship was found

between the level of packaging-induced strain and defect

density. This allowed a packaging technology that was

optimised for both a minimum level of strain and a mini-

mum defect density to be identified [10]. In this work, we

again use the by-emitter methodology to study an 808 nm

AlGaAs/GaAs high-power laser bar. The results of this

analysis suggested that thermally induced current runaway

is an important factor in the bar degradation and eventual

device failure [11]. In order to confirm this suggestion,

temperature measurements on a series of devices are also

performed and the results of these further investigations

indeed support the results of the by-emitter analysis.

Section 2 gives brief details of the laser devices investi-

gated in this work. More details are then given in Sect. 3 on

the experimental techniques with which these devices have

been studied. Both the results of the conventional aging tests

and the by-emitter investigations are presented and discussed

in Sect. 4. Conclusions are presented in Sect. 5.

2 Device details

The principal high-power laser bar studied in this work was

an 808 nm AlGaAs/GaAs device grown by metal-organic

vapour phase epitaxy. The total bar width was 1 cm (a

so-called ‘cm bar’) and comprised of a linear array of

25 emitters, each 200 lm wide, 900 lm long and separated

by 200 lm isolation regions. The bar was designed for

30–40 W continuous-wave operation and was mounted

p-side down onto an actively cooled Cu heatsink using In

solder. This device was aged under constant current con-

ditions for 600 h at 60 A, before being measured using the

various by-emitter techniques described in Sect. 3. Fol-

lowing the measurements, electro-optical characterisation

of the bar confirmed that the by-emitter measurements did

not result in any measurable change in the bar’s perfor-

mance. A further 500 h of aging was then performed at a

higher current of 75 A. Several other devices of this type,

taken from the same batch, were also used for temperature

measurements and were measured either as new devices or

after *2,500 h of aging at a current of 60 A.

3 Experimental methods

Many experimental techniques exist for the characterisation

laser emitters and laser bars. For a by-emitter analysis study

one must select non-destructive techniques (i.e. they must

not cause any change in the degradation rate of the device),

which can be applied to individual emitters within a laser

bar. In previous studies, the techniques used have included

micro-photoluminescence spectroscopy, photocurrent

spectroscopy, laser beam induced current, photolumines-

cence microscopy and electroluminescence microscopy

[8–11].

In this work, photocurrent spectroscopy was used to

determine the effective bandgap of the quantum well of

each emitter. A photocurrent spectrum was measured at the

centre of each emitter in the laser bar. From the measured

spectra, the first derivative was calculated allowing the

spectral position of the lowest quantum-confined transition

to be found [12, 13].

Near-field images were measured for each emitter as a

function of bias current both below and above the bar

threshold. By integrating the image intensities, Pi vs. IBAR

curves of individual emitters were plotted, where Pi is the

power of an individual emitter, but IBAR is the current of

the total bar. From such data, an apparent threshold current

and apparent efficiency for each emitter can be determined.

As in previous work [8, 9], we refer to these as ‘‘apparent’’

quantities since they are not determined using individual

emitter currents (as these cannot be measured in a parallel-

connected array).

Fig. 1 Schematic representation of a high-power laser bar consisting

of a linear array of emitters connected in parallel and sharing the same

physical substrate, contacts and heatsink. This representation depicts

only 5 emitters—the laser bars examined in this work consisted of 25

emitters

S146 J Mater Sci: Mater Electron (2008) 19:S145–S149

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The emission spectra of the individual emitters were

also measured as a function of bias current and the shift of

the spectra with increasing current examined. In this study,

we focus on the sub-threshold wavelength shift, which is

primarily a measure of the quasi-Fermi level separation

(i.e. band filling of the quantum well). Above threshold,

wavelength shifts are often used as a measure of the

junction temperature [14].

Finally, Raman spectroscopy measurements were used

to determine facet temperature profiles of several laser bars

and a thermal camera was used to capture typical bulk

thermal profiles during laser operation [15–17].

4 Results and discussion

Figure 2 shows the results of the conventional constant

current aging test described in Sect. 2, which provides

information only on the bar as a single entity. As can be

seen, there was a gradual power loss, amounting to a 1.4%

decrease, during the first 600 h. At the start of the second

aging period, following an increase in the bar current from

60 to 75 A, catastrophic degradation occurred with a power

loss in excess of 70%. The inset in Fig. 2 shows that the

degradation began *5 h into this aging test. Several sud-

den drops are then observed in the output power, each of

which represents the failure or one or more emitters.

Figure 3 shows the by-emitter measurement results from

the device investigated. These are plotted as a function of

emitter number in order to see variations in the emitter

performances at different positions within the bar.

Figure 3a shows that the bandgap, as determined by pho-

tocurrent spectroscopy, is smaller for emitters at the bar

edges compared to those in the centre. This may be caused

by lower levels of strain at the edges of the bar. The dif-

ferent bandgaps will clearly cause a small variation in the

emitter turn-on voltages and consequently more significant

variation in their operating currents. Figure 3b shows the

individual emitter sub-threshold wavelength shifts as

determined from the bias-dependent near-field spectra of

each emitter. The larger negative wavelength shift with

increasing bias in the edge emitters suggests that the cur-

rent in these edge emitters is increasing faster than that of

the emitters in the centre and furthermore that the edge

emitters are hotter than those in the centre. Figure 3c

shows the relative powers of the emitters for a total bar

current of 30 A (*1.5 times the bar threshold current). A

clear variation is observed across the bar, with emitters

near the edges emitting up to 60% less power than those in

the centre. This is consistent with the higher apparent

threshold currents (up to 20% higher) and lower apparent

Fig. 2 Aging data (power vs. time) measured during the constant

current aging tests. During the first 600 h the current was 60 A

(corresponding to an initial facet load of 10 mW/lm), after which it

was increased to 75 A (corresponding to an initial facet load of

12 mW/lm). The dashed lines represent the initial power levels at

each of the aging currents. The inset figure highlights the rapid

degradation that results from sudden emitter failures between 600 and

650 h

Fig. 3 Emitter performance characteristics measured after 600 h of

aging and plotted as a function of emitter number: (a) quantum well

bandgap energy Eg; (b) sub-threshold injection-induced shift in the

emission wavelength Dk/DI; (c) relative emitter power at 30 A; (d)

apparent threshold current Ith_app; (e) apparent external differential

efficiency gext_app

J Mater Sci: Mater Electron (2008) 19:S145–S149 S147

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slope efficiencies (up to 40% lower) observed for the edge

emitters, as shown in Fig. 3d, e, respectively. As threshold

current increases and slope efficiency decreases with

temperature, these results further support the idea that the

edge emitters are hotter.

The conclusion of the by-emitter analysis, that the

temperature distribution has a minimum at the centre of the

bar appears to contradict previously observed results for

both actively and passively cooled bars. To investigate this

further, facet temperature measurements were made on the

emitters of other devices from the same batch using Raman

spectroscopy. Bulk temperatures were also investigated

using a thermal camera. Both new devices and devices that

had been aged for *2,500 h were investigated. Figure 4a

shows a typical bulk thermal profile. Small temperature

variations (DT * 1 �C) are observed between the centre of

each emitter and the centres of the isolation regions that

separate the emitters. Furthermore, as expected, the highest

temperatures are found for the emitters at the centre of the

bar. However, the facet temperature measurements shown

in Fig. 4b, c for new and aged devices, respectively, reveal

that a temperature distribution can occur in aged devices,

whereby the facet temperature is higher at the bar edges.

These facet temperature increases may be due to an

increased number defects in the edge emitters, which

may have been introduced during packaging or may be

caused by larger emitter currents or even by higher

surface currents at the bar edges. Whatever the cause,

any increased non-radiative recombination will lead to

increases in emitter current and temperature, thereby

forming a positive feedback mechanism to enhance

defect generation and propagation. This ultimately leads

to the thermal runaway of the emitter current and the

onset of more rapid and catastrophic degradation (i.e. the

positive feedback causes the system to become unsta-

ble)—as was indeed observed during the further aging of

this bar (see Fig. 2).

5 Conclusion

Using the by-emitter analysis technique, we have consid-

ered the degradation scenario of a particular high-power

laser bar. The results obtained in this by-emitter study

suggested, against common belief, that thermally induced

current runaway of the edge, rather than the more central,

emitters was an important factor in the degradation of this

bar and its eventual failure. Facet and bulk temperature

measurements of new and aged laser bars further revealed

that the thermal runaway process occurred at the facet, not

in the bulk, thereby confirming the correctness of the both

the by-emitter conclusions and common understanding of

bulk temperature distributions in laser bars. This work,

together with the previously cited examples, clearly dem-

onstrate that the by-emitter degradation analysis technique

provides a much more detailed understanding of individual

Fig. 4 Temperature measurements: (a) a typical bulk thermal profile

of a laser bar at 30 A and Raman facet temperature measurements

made on (b) fresh devices and (c) devices aged at 60 A for *2,500 h.

The lines are plotted as a guide to the eye and represent the general

trend of the temperature variations along the bar in the two cases

S148 J Mater Sci: Mater Electron (2008) 19:S145–S149

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emitter operation within a laser bar and that it is useful for

determining bar degradation mechanisms.

Acknowledgments The authors gratefully acknowledge support

from the European Commission through the IST projects POWER-

PACK (IST-2000-29447), WWW.BRIGHT.EU (IST-511722) and

WWW.BRIGHTER.EU (IST-035266). We also wish to thank the

project partners at the Fraunhofer ILT for performing the aging tests.

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