1. Chapter 8
InGaN Laser Diode Degradation
Piotr Perlin and Łucja Marona
Abstract We discuss the current knowledge of degradation processes in InGaN
laser diodes. It is quite surprising that after quite a few years of intensive studies,
there is still no clear picture of physical mechanisms lying behind these aging
processes. First of all, in contrast to GaAs counterparts, the nitride laser degradation
seems to be independent from extended defects movement and multiplication, is
uniform across the device surface, and is rather unrelated with optical phenomena.
Involvement of point defects in this process is very tempting but not yet
sufficiently experimentally supported hypothesis. Bipolar GaN devices will very
likely teach us new things about the physics and technology of wideband gap
devices – a truly new thing in the optoelectronics.
8.1 Introduction
Since the first appearance of nitride optoelectronic devices, the problem of their
reliability and understanding the mechanism of degradation processes turned out to
be crucial. The early studies that focused on light-emitting diodes (LEDs) revealed
that the degradation of these devices is dominated by the packaging issues (epoxy
darkening and carbonization) [1, 2]. Packaging improvement enabled manufacturing
blue-green LEDs operating at current densities close to 20 A/cm2
and of the lifetime of
50–100 kh, the value satisfying for the most of the manufacturers.
P. Perlin (*)
Institute of High Pressure Physics, Semiconductors Laboratory, Polish Academy of Sciences,
Sokołowska 29/37, 01-142 Warsaw, Poland
TopGaN Ltd., Warsaw, Poland
e-mail: piotr@unipress.waw.pl
Ł. Marona
Institute of High Pressure Physics, Semiconductors Laboratory, Polish Academy of Sciences,
Sokołowska 29/37, 01-142 Warsaw, Poland
O. Ueda and S.J. Pearton (eds.), Materials and Reliability Handbook for Semiconductor
Optical and Electron Devices, DOI 10.1007/978-1-4614-4337-7_8,
# Springer Science+Business Media New York 2013
247

2. In contrast to LEDs, nitride laser diodes (LDs) reliability issues were found to be
much more complex. The early laser diodes were grown on sapphire substrates
which are not lattice matched to GaN. Therefore, these devices were characterized
by a very large density of defects of the order of 109
to 1010
dislocation per square
centimeter. It was not too surprising that the lifetime of the first InGaN laser diodes
was very short. For example, the first CW operated laser demonstrated by
Nakamura lived only 1 s [3]. An obvious approach was to increase the lifetime of
laser diodes by reducing the dislocation density. Epitaxial lateral overgrowth
(ELOG) technology allowed to reduce the dislocation density approximately by
two orders of magnitude [4]. In parallel, availability of freestanding GaN substrates
(dislocation densities <104
cm2
) was gradually improving. Using of both, ELOG
technique and GaN substrates, allows to increase lifetime of nitride laser diodes to
desirable 10,000 h [4]. Though the described progress in the longevity of nitride
laser diodes was impressive, the mass production of these devices started with a
significant delay (6–7 years), which may mean that the production yield and
reliability problems were not completely solved at that time. At present, though a
limited number of companies apparently possess a long-living nitride laser diodes
know-how, our understanding of the degradation processes in this important mate-
rial system is still very limited.
8.2 Structure of InGaN Laser Diode
Nitride-based laser diode structures are grown by two epitaxial method: mostly
by metal-organic vapor phase epitaxy (MOVPE) [4] but also by molecular
beam epitaxy (MBE) [5] on bulk GaN substrates. There are only few GaN substrates
suppliers, and they use different fabrication technologies. Sumitomo Electrics uses
“DEEP” method [6], Ammono Ltd. uses ammonothermal growth method [7],
TopGaN Ltd. is specializing in high-pressure gallium nitride growth [8], and
Furukawa, Hitachi Cables, Mitsubishi, Lumilog, and Kyma use various
modifications of hydride vapor phase epitaxy (HVPE) to manufacture GaN
substrates. Gallium nitride substrates have many quality disadvantages like crystal
bowing, dislocations, and too small available sizes. Substrate problems influence
epitaxial growth often leading to cracking, defects propagation, and poor uniformity
of growth. Ability to grow good quality nitride layers is a crucial issue for obtaining
satisfying parameters of the device.
The conventional epistructure of InGaN laser diode is a separate confinement
heterostructure (SCH) consisting of 1–3 InGaN quantum wells [4]; see Fig. 8.1.
Usually on top of the active layers, a p-type AlGaN layer, acting as a electron
blocker, is placed. In minority cases, EBL can be moved up and put between the
upper waveguiding layer and the upper cladding layer. Active region is covered by
waveguides, usually formed by GaN layers of the thickness of approximately
100 nm. Waveguiding layers may be Si- or Mg-doped, but they may be also left
248 P. Perlin and Ł. Marona

3. undoped. Whole structure is sandwiched by n- and p-doped AlGaN claddings to
avoid optical mode leakage.
8.3 Packaging of InGaN Laser Diodes
Nitride laser diodes are typically mounted into 5.6 mm TO cans (TO18 package)
(Fig. 8.2) or quite rarely into 9 mm cans. In most cases, the laser diodes are
assembled in a p-up configuration. Depending on the manufacturer, laser chips
are mounted on the top of a highly thermally conducting submount (heat spreader)
or directly on the stem of the package. Very high thermal conductivity of the GaN
allows for heat-spreader elimination.
The important aspect of nitride laser diodes is hermeticity of the package. Since
the high photon energy of the light emitted from a laser enhances the chemical
reactions on the mirrors, it is important to keep moisture and hydrocarbons out of
the operating devices. This issue will be discussed in more detail in the next
chapters.
8.4 Symptoms of the Degradation
Degradation of the semiconductor laser diodes can be divided into three groups
depending on the device region where a damage occurs as shown in the Table 8.1
below.
In the next sections, problems with facet, active layer, and contact degradation
will be discussed in more details.
Fig. 8.1 Scheme of nitride laser diode epitaxial structure
8 InGaN Laser Diode Degradation 249

4. 8.4.1 Facet Degradation
Quite frequently, the first symptom of laser diodes degradation occurs at their
facets. Mirror degradation is a phenomenon well known in case of arsenide devices
[9]. We can roughly divide it into two classes: first one is a catastrophic optical
mirror damage (COMD), a process which manifests through the sudden increase of
the mirror’s temperature leading to melting of the facets. Increase of the tempera-
ture is caused by an optical absorption in the near-facet area of a device and a
positive feedback loop between the increase of device temperature and the increase
of the optical absorption. The threshold of COMD was determined to be around
40–57 MW/cm2
[10, 11] for InGaN laser diodes. These values are an order of
magnitude higher than for GaAs counterparts, thus showing the potential of nitride
lasers for high optical power emission. However, a mechanism of COMD is far
from being well understood in the nitride system.
The second effect related with mirror degradation is the formation of carbon
deposits on the surface of the output mirror. It was discovered that under the operation
in the oxygen-free atmosphere, a complicated process of photo-assisted hydrocarbons
decomposition occurs leading to the formation of carbon deposit and fast laser diode
Fig. 8.2 Packaged nitride laser diode: (a) nitride laser diode installed on top of diamond heat
spreader in TO 18 package, (b) laser chip soldered directly to the stem of the package (A courtesy
of TopGaN Ltd.)
Table 8.1 Degradation mechanisms in nitride laser diodes
Region of a device Phenomena Factor
Laser facet Catastrophic mirror damage (COD),
oxidation, different chemical reactions
Humidity, light, atmospheres,
surface states, temperature
Active layer Nonradiative recombination centers Current flow, temperature
Contacts Metal diffusion Current flow, temperature
250 P. Perlin and Ł. Marona

5. degradation. Similar processes were observed in GaAs-based lasers and were called
package-induced failure (PIF). This term was introduced by Julia Sharps to name the
phenomenon of very fast degradation of 980 nm, high power laser diode operated in
hermetic packages under neutral gas atmosphere [12]. The fingerprint of this degra-
dation mode was the formation of carbon deposit on the mirrors of the laser during its
operation and high, positive, susceptibility to the presence of oxygen in the laser
package [13]. It was shown that the reaction is related to the hydrocarbon photoin-
duced decomposition and the source of the hydrocarbons may be the package
atmosphere or the package material [13]. Quite the same observations have been
made recently by groups studying the facet degradation of nitride laser diode.
Schoedl et al. [14] found that operating laser diode in dry nitrogen atmosphere
leads to the formation of a thick deposit. This deposit can be altered or even remove
by adding oxygen to the atmosphere. The authors also claim that the presence of a
water vapor speeds up the mirror oxidation. Observation of the carbon deposit
formation was also reported by Kim et al. [15]. They limited this process by an
additional cleaning and careful sealing of the package. Our experiments performed
on laser diodes in argon or nitrogen atmospheres with or without an addition of
oxygen show the following:
1. The fastest degradation occurs in dry nitrogen atmosphere.
2. The addition of the oxygen slows down the degradation but does not eliminate it
completely until the proper cleaning (oxygen plasma asher and UV ozone
cleaning) of the structure is performed.
3. The facet degradation involves the formation of carbon deposit not only in the
region of the laser stripe but also around the chip at the level of p-n junction.
4. The progress of degradation is irregular; the oscillation of the threshold current
may occur, related to the deposit growth and delamination.
5. The source of hydrocarbons is most likely the electroplated gold on the laser
chips but also very likely gold layers on the commercial package material.
The carbon deposit formed on the facet of our laser diode is presented in Fig. 8.3.
Judging from our experience, the carbon deposit formation may limit the lifetime of
devices from few tens up to hundreds of hours. The chemistry of this process is still
not well understood, and the resemblance to classical 980 nm PIF mechanism has to
be investigated.
8.4.2 Active Layer Degradation
Elimination of facet degradation reveals much more fundamental process occurring
in the whole volume of the active area of a nitride laser diode. As a first step, let us
analyze symptoms observed during the aging of a device. Degradation in InGaN-
based laser diodes occurs the most frequently through the increase of the threshold
current like it is shown in Fig. 8.4 [16, 17]. What can be easily seen is that the strong
increase of the threshold current is not accompanied by the proportional change in
the slope efficiency.
8 InGaN Laser Diode Degradation 251

6. Very slow evolution of slope efficiency of the degraded devices seems to be
quite reproducible feature of nitride LDs [16, 17]. This behavior agrees with the
simple model of the increase of the nonradiative recombination within the active
layer [16]. According to this model, constant slope efficiency implies constant
Fig. 8.3 Carbon deposit protruding from the facet of an InGaN laser diode (Courtesy of TopGaN
Ltd.)
0 40 80 120 160 200 240
0.0
2.5
5.0
outputpower(mW)
current (mA)
t=0 h t=600 h t=900 h
Fig. 8.4 The evolution of light-current curves with the degradation time
252 P. Perlin and Ł. Marona

7. injection efficiency and no increase of optical losses (in nitride devices cavity losses
are dominated by the optical absorption on magnesium states). The slope efficiency
(SE) can be expressed by:
SE ¼ j
am
ai þ am
; (8.1)
where j is injection efficiency, am mirror losses, and ai internal losses.
The threshold current was reported [16–18] to increase with the square root of
time like it is shown in Fig. 8.5.
The square root dependence was quite frequently associated with diffusion
according to the Einstein-Smoluchowski formula:
Dx ¼
ffiffiffiffiffi
Dt
p
; (8.2)
where Dx is change of the stoichiometry along one direction, D is diffusion
coefficient, and t is time. However, the question arises: what actually diffuses?
Though the interdiffusion of magnesium was a tempting candidate for the degrada-
tion mechanism, there is only one very indirect evidence provided by secondary ion
mass spectroscopy (SIMS)-smeared magnesium profile measured at large area
light-emitting diode [19]. The lack of SIMS profiles measured at the laser diodes
tested under standard conditions is related to the difficulty in performing
200 400
1.0
1.1
1.2
1.3
square root - fit
experiment
Relativechangeoftheoperatingcurrent
Aging time (hours)
Fig. 8.5 The increase of operation current as a function of time in comparison with a square root
function
8 InGaN Laser Diode Degradation 253

8. SIMS measurements over very tiny area of the laser stripe. However, we recently
managed to accurately measure SIMS profiles of many dopants (Mg, H, Si) [20]
over a stressed area of a laser diode (Fig. 8.6).
In contrast to previously reported data, our new results show no magnesium nor
hydrogen profile changes with the degradation time. Thus, we can safely say that
these two elements are most likely not involved directly in the relevant diffusion
process. This of course does not exclude local rearrangements of atoms around
defects centers, complexes, etc. Additionally, there is still open question concerning
possibility of native defects diffusion.
Though the square root dependence of degradation dominates in some cases,
also linear behavior of the current versus time is observed like the one shown in
Fig. 8.7. At the present moment, it is not clear why in some cases linear degradation
mode dominates and if it means that the mechanism of the degradation is different.
Degradation of laser diodes is often characterized by a value of so-called
activation energy (EA). This value can be estimated by using Arrhenius formula:
D ¼ C exp
EA
kT
; (8.3)
where D is degradation rate, C is a constant, EA activation energy, T temperature,
and k Boltzmann constant. Degradation rate can be defined in various ways, as, for
example, change of the threshold current in the unit of time or a number of
accumulated failures as a function of time. Arrhenius equation shows that degrada-
tion is thermally activated process. The values of the activation energy for nitrides
are reported to be between 0.3 [11, 16] and 0.5 eV [21].
Finally, the additional argument that the degradation is related to the active area
of the device (volume) and is nor related with complicated laser diode topology was
provided by Meneghini et al. [17]. They showed that if instead of laser diode you
fabricate a completely planar light-emitting diode (LED) of the identical
epistructure to the laser, then it degrades in the identical manner scaled only by
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
1016
1017
1018
1019
1020
initial
after aging
concentration(cm−3
)
Distance from the surface (µm)
MgA
Fig. 8.6 Magnesium SIMS
profiles measured over the
aged area and virgin area
of the heavily stressed laser
diode (10,000 h of operation)
254 P. Perlin and Ł. Marona

9. the value of the operating current density (much lower for LEDs). The authors
establish also a linear relation between the aging rate and the current density for all
tested devices.
8.4.2.1 Optical Gain in Stressed Laser Diodes
Optical gain is a decisive parameter which defines the laser threshold. Optical gain
is a function of the carrier density in the active area of the laser diode and thus
depends on the recombination kinetics. One of the first studies of the optical gain in
the stressed laser diode was performed by Goddard [22] from Xerox Palo Alto
Group. They demonstrate that the laser diode degradation (an increase of the
threshold current) is accompanied by a gradual decrease of the optical gain. We
performed systematic study of the gain spectra taken during laser diode aging. On
Fig. 8.8, we can see the results of our study. The gain spectra were recorded using a
Hakki-Paoli method.
We see that after degradation a magnitude of the gain peak is decreased when
measured under constant current conditions. However, if current is increased again,
we can see that gain curve returns to its original strength and form. This result can
be interpreted in such a way that under same operating current, due to an increase
nonradiative recombination or carrier escape from the quantum wells, the number
0 50 100 150 200 250 300
60
70
80
90
100
ILD[uA]
t[hours]
LD5800 d41
Fig. 8.7 An example of a close to linear degradation behavior of nitride laser diode. Vertical axis
shows a device operating current measured at constant optical power of 10 mW
8 InGaN Laser Diode Degradation 255

10. of carriers in the InGaN quantum wells is lower which lowers the gain value
correspondingly. The stability of the gain spectral shape confirms the material
stability excluding, for example, the quantum well interdiffusion.
8.4.2.2 Nonradiative Lifetime Measurements
As we mentioned before, an expected reason for laser diode degradation is genera-
tion of nonradiative recombination centers within the active layer of the stressed
device. This is why it is so important to determine the nonradiative recombination
time (or rate) during the aging process. Direct measurements of the recombination
rates on completely processed and packaged devices are hard to perform. There-
fore, an important approach to determine these parameters is to use an analysis of
subthreshold light-current characteristics of laser diodes. The starting point for such
an analysis is always the so-called A-B-C rate equation [23]:
jI
qV
¼ AN þ BN2
þ CN3
; (8.4)
where j is an injection efficiency, I is a current, q is a charge on electron and V is an
active area volume, A is nonradiative recombination rate coefficient, B is
400 410 420 430 440 450 460 470
−50
−40
−30
−20
−10
0
10
gaincm−1
Wavelenght (nm)
before aging
after aging, 50 mA
after aging, 80mA
Fig. 8.8 Gain spectra in the aged device. Black curve shows an initial gain peak, a red curve
presents the spectrum measured at the same current, and a green curve shows the gain for the
device with increased current to compensate the gain loss
256 P. Perlin and Ł. Marona

11. bimolecular radiative rate coefficient, and C is typically related with Auger recom-
bination or similar mechanism.
Optical power is related with the recombination coefficient through the follow-
ing equation:
P ¼ kBN2
: (8.5)
Using the above two formulas, relation between optical power and current can be
derived:
I
ffiffiffi
P
p ¼ a þ b
ffiffiffi
P
p
; (8.6)
where parameters a and b are defined by:
a ¼
qV
j
A
ffiffiffiffiffiffi
kB
p ; (8.7)
b ¼
qV
j
1
k
: (8.8)
The application of the Eq. 8.6 for the determination of the nonradiative lifetime
in nitrides laser diode was demonstrated by Ryu et al. [24].
On Fig. 8.9, we show the measured a parameter which is basically proportional
to nonradiative recombination rate A (we assume here that bimolecular radiative
recombination B is constant). As it is visible, a parameter steadily grows suggesting
a steady increase of nonradiative recombination rate.
The increase of the nonradiative recombination in the active layer of the
device can be intuitively associated with the increasing density of deep levels
0 20 40 60 80 100
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
parametera(arb.units)
Time (h)
Fig. 8.9 Depending of
nonradiative recombination
on the aging time of the
device
8 InGaN Laser Diode Degradation 257

12. (within Schottky-Read model). One of the few methods suitable for determining the
concentration of these nonradiative recombination centers is deep level transient
spectroscopy (DLTS). In this method, we measure the junction-related capacitance
transient in relatively broad temperature range. Meneghini et al. [25] performed
DLTS measurements during the degradation of InGaN laser diodes. They detected a
new signal of the amplitude increasing with the degradation time. This signal can be
attributed to the electron trap situated 0.35–0.45 eV below the conduction band
minimum. Meneghini et al. pointed out that such a level was previously attributed
to VGa-(ON)3 complex [26]; however, this important problem needs further studies.
8.4.2.3 Role of the Extended Defects
From the very beginning of nitride laser diode fabrication, the dislocations were
considered as a key factor influencing the lifetime of these devices. As we men-
tioned before, the first CW operated violet laser of Nakamura et al. lased only for
one second [3] and contained most likely around 109
dislocations per centimeter
squared. Systematic study performed by Takeya et al. [18] demonstrated very steep
dependence of the laser diodes’ lifetime on the number of threading dislocations
existing in the laser diode structure. The lifetime of the device varies from hundreds
to ten thousand hours when defect density decreases from 107
down to 106
cmÀ2
.
Though the strong dependence of the laser diode lifetime on the defect density is a
fact, the physical interpretation of this relation is far from being clear. In the
classical III-V systems, the dislocation glide and multiply, leading to the formation
of so-called dark lines [9] is a main cause of degradation. Movement and multipli-
cation lead to the effective increase of the number of nonradiative recombination
centers. In the nitride laser diodes, the dislocation does not multiply. Gallium
nitride, with its large cohesion energy and strong, short interatomic bonds, should
not be too much susceptible to these phenomena. Indeed, most of the existing
papers claim that the dislocations in the stressed and unstressed regions of the aged
laser diodes look the same, like it is demonstrated by Tomiya et al. [27] by using
electron transmission microscopy (TEM) characterization. It means that no new
dislocations are created, though the authors of that paper reports on the observed
recombination-enhanced dislocation glide (REDG) mechanism. The dislocation
activity was observed in situ in TEM microscope by irradiating the laser structures
with electrons. These dislocations were, most likely, the dislocation loops lying in
(0001) basal plane. However, the authors of the previously discussed paper claim
that this mechanism cannot lead to multiplication of threading dislocations being
extended along the c axis of the crystal.
The only reports on the formation of the localized dislocation network come
from UK Sharp group [28, 29]. This group fabricated their devices by using a gas
source MBE method. After a few-hour-long operation, the laser diodes were
deprocessed and examined by TEM. The electron microscopy revealed the network
of dislocations confined in the active region of the device. The authors claim that
the mechanism of dislocation network formation may be similar to REDG
258 P. Perlin and Ł. Marona

13. mentioned before [27]. However, considering the uniqueness of this observation,
the discussion about the true importance of this mechanism for nitride laser diode
degradation has to be postponed until more facts are gathered.
Another piece of the puzzle which does not fit too well is that the laser diode
containing 106
dislocations in square centimeter has only around 10 dislocations
within its stripe. If the diffusion is fast along the dislocation, the nonradiative
recombination centers should be concentrated close to the dislocations forming
dark areas. This effect is up to our knowledge never observed.
8.4.3 Contact Degradation: Operation Voltage
and the Series Resistance
Another typical feature of the degradation is an increase of the diode operation
voltage under the prolonged operation. Figure 8.10 shows the I-V curves measured
for the virgin laser diode and after 900 h of operation at 5 mW optical output power.
The increase of 0.5 V of the operation voltage at the current of 150 mA was
observed. Gradual degradation of the device series resistance and the operating
voltage is most likely related to the deterioration of the metal contact to the p-type
layer. There is however a lack of broader experimental base to evaluate this
phenomenon. Data coming from the transistor reliability studies show no metal
diffusion to semiconductor from the hardly stresses metal contacts.
0 50 100 150 200 250
0
1
2
3
4
5
6
7
voltage(V)
current (mA)
t=900 h
t=0 h
Fig. 8.10 Comparison between current–voltage characteristics of virgin and stressed laser diode
8 InGaN Laser Diode Degradation 259

14. 8.5 Summary
The degradation of nitride laser diodes seems to be dominated by the increase of the
nonradiative recombination in InGaN quantum wells. The increase of nonradiative
recombination leads to the increase of threshold current, while the differential
efficiency of the device remains roughly constant. This increase can be associated
with the generation of point defects in the active layer. The generation of the defects
is proportional to the current density and also proportional to the square root of the
degradation time. Though diffusion is still considered as a plausible mechanism
responsible for degradation, however, it has not been attributed to any dopant or
impurity. The role of dislocations though apparently important is totally unclear.
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