Download IEEE J. Quantum Electron. 33, 970 (1997)

970
IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 33, NO. 6, JUNE 1997
Kinetic Model for Degradation
of Light-Emitting Diodes
Shun-Lien Chuang, Fellow, IEEE, Akira Ishibashi, Member, IEEE, Satoru Kijima,
Norikazu Nakayama, Masakazu Ukita, and Satoshi Taniguchi
Abstract— We present a kinetic model for the optical output degradation of light-emitting diodes based on the carrierrecombination enhanced defect motion. Our model leads to analytical solutions and universal curves for the optical output
power and the defect density as a function of the normalized
aging time with the initial quantum efficiency as the determining
parameter. The theoretical results explain very well the time
dependence of the II–VI light-emitting diodes under constant
current aging condition. The faster aging rate with increasing bias
current or temperature is also investigated both experimentally
and theoretically, resulting in a very good agreement. Our model
provides a quantitative description of the light-emitting diode aging characteristics for compound semiconductors in the presence
of electron-hole recombination-enhanced defect generation.
Index Terms— Light-emitting diodes, quantum-well devices,
semiconductor lasers.
I. INTRODUCTION
W
IDE-BANDGAP compound semiconductors using
II–VI and III–V materials for blue-green laser
diodes (LD’s) and light-emitting diodes (LED’s) have been
subjected to intensive research in recent years. With the
successful demonstrations of blue–green LD’s using ZnSebased materials [1]–[3] and GaN-based materials [4], [5],
researchers have made significant progress toward the ultimate
goal of making long-lifetime blue-green LD’s operating in CW
mode at room temperature. Blue and green LD’s will be useful
for the next generation of high-density optical disks because
of their shorter wavelength compared to that for the LD’s
in the current systems. A red–green–blue (RGB) full-color
display of all-solid-state light sources will be available in a
highly compact manner and for mass production mode. For
an overview on II–VI LD’s, see [6]–[9].
CW operation of II–VI blue-green LD’s at room temperature
for more than 101 h has been reported [10]. A value for the
dark-spot density of less than 3 10 cm is used to explain
the high performance of these LD’s. This is significant since
for one dark spot in an area of a typical LD stripe of 10
m
600 m, the density is 1.7
10 cm . Reduction
of defect density has been the primary effort in achieving
a long lifetime of II–VI light-emitting devices. It has been
recognized that pre-existing defects and their influence on
Manuscript received July 22, 1996; revised January 21, 1997.
S.-L. Chuang is with the Department of Electrical and Computer Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801 USA.
A. Ishibashi, S. Kijima, N Nakayama, M. Ukita, and S. Taniguchi are with
Sony Corporation Research Center, Hodogaya-ku, Yokohama 240, Japan.
Publisher Item Identifier S 0018-9197(97)03800-1.
the II–VI material degradation in light-emitting devices play
a significant role. Yet very little is understood about the
physical mechanisms of the degradation in II–VI materials.
On the other hand, in the early days of III–V laser-diode
and LED research, the dislocations, dark-line and dark-spot
defects were reported to play a significant role in determining
the rapid device degradation [11]. Similar studies on the
microstructure changes, such as stacking faults and dark-line
and dark-spot defects in the presence of optical or electrical
injection in II–VI light-emitting materials, have also been
reported [12]–[16]. It has also been recognized that for GaAs
or wider bandgap materials, point defects are the limiting
factor determining the gradual degradation of the LD’s [11],
[17]–[21]. Recombination-enhanced defect motion [22]–[25],
such as dislocation climb and dislocation glide causing rapid
degradation as well as structral changes due to creation and
diffusion of point defects causing gradual degradation, have
been investigated. The role of dislocation and point defect
interaction with recombination enhancement has been pointed
out in [26] and [27]. When an electron is captured by a
defect level with a subsequent capture of a hole, multiphonon
emissions occur [28]–[30]. This will result in strong vibration
of the defect atoms and reduce the energy barrier for the defect
motion such as migration, creation, or clustering. Usually, an
activation energy is measured and the possibility of different
activation energies at different temperature operation ranges
has been discussed [24], [31]. For an aging LED under a
constant driving current, the decay behavior of output light
intensity as a function of time is believed to be an exponential
function [11], [31]. There are very few studies which directly
compare the experimental data of an aging LED or LD with
theoretical models.
Recently, we proposed a transient recombination-enhanced
defect generation model [32], [33]. Our model shows that the
time dependence can be nonexponential in character, and its
dependence, which decays at
long-time behavior has a
a much slower rate than an exponential function. This
dependence is also related to the growth rate of the defect
dependence. We also
density, which should behave as a
found that universal curves can be plotted for the normalized
optical output power as a function of the normalized time
parameter with the initial radiative quantum efficiency as the
only input parameter. Our theoretical results are compared with
experimental data measured using five LED’s with two different strained quantum-well structures. This comparison shows
that the experimental data are well explained by the electron-
0018–9197/97$10.00  1997 IEEE
CHUANG et al.: KINETIC MODEL FOR DEGRADATION OF LIGHT-EMITTING DIODES
hole recombination-enhanced defect motion. For convenience
in the previous experiment, these are short-lived LED’s with
a large number of pre-existing defects, but it does not mean
that the model is only applicable to short-lived LED’s. In fact,
our model successfully explains aging characteristics of LED’s
with a lifetime of about 1000 h, as shown below.
In this paper, we present a detailed description of our
kinetic model, including the analytical solution as well as
asymptotic solutions for both short-time and long-time aging
characteristics. New experimental data on the optical output
aging of II–VI LED’s as a function of the bias current under
constant current aging condition with an aging time close to
1000 h and the temperature dependence are presented. We
show that our universal curve derived from our kinetic model
can be used to explain our experimental data very well.
In Section II, we present our theoretical model including
analytical and numerical results. We also compare our model
with other models for defect generation. In Section III, we
discuss our experiment and present our experimental data. Our
theoretical results are then compared with the experimental
data in Section IV. The extraction of the intrinsic radiative
quantum efficiency from the optical power versus current data
is also shown. We then discuss the significance and impact
of our findings in Section V and present our conclusions in
Section VI.
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Fig. 1. Energy-band diagram showing the capture of an electron with a
subsequent capture of a hole at a defect site that causes strong defect vibrations
and results in defect generation.
with experiments. Our kinetic model is consistent with the
REDM picture, and, when coupled with the carrier-continuity
equation, provides a direct measure of the optical output
degradation and correlates the optical power to the defect
generation in semonconductors.
B. Carrier-Continuity Equation
The rate equation for the carrier density in the quantumwell active region is governed by the continuity equation
[34]
II. THEORETICAL MODEL
(3)
A. Kinetic Model for Defect Generation in LED’s
The defect generation rate for the defect density
assumed to be of the form
is
(1)
depends on the physical processes
where the coefficient
of defect generation. Here we consider a defect-carrier interaction process such that
where the first term
is the current injection rate, the
second term
accounts for the spontaneous emission
rate, and the last term
accounts for the nonradiative carrier captures at the defects. Here is the current
density, is the unit charge, is the thickness of the active
region, and
and
are the nonradiative and radiative
recombination coefficients, respectively. The above equation
also assumes explicitly that nonradiative recombination occurs
at the defects, i.e.,
instead of
is used.
(2)
C. Analytical Solutions
where we have used the fact that the electron and hole
concentrations are equal
in the undoped active region
and assume that defect annealing effects can be ignored since
the injected carrier density is much larger than its thermal
equilibrium value.
Fig. 1 shows the physics of the electron-hole recombinationenhanced defect generation mechanism. The band diagram
shows that an electron in the conduction band is captured at
a defect level with a subsequent capture of a hole (or a hole
capture first followed by an electron capture), resulting in the
release of an energy of the bandgap. This energy goes into
multiple phonon emissions, which cause a violent vibration
at the defect site, resulting in the lowering of the energy
barrier for defect motion such as diffusion or generation of
new defects. Previous work on recombination-enhanced defect
motion (REDM) focused on the temperature dependence of
the defect annealing and the extraction of the activation
energy. There has been little work on the kinetic model for
defect generation in light-emitting devices and comparison
We define the normalized defect density as
(4)
the normalized carrier density as
(5)
and the normalized time as
(6)
We also use the initial radiative quantum efficiency before
aging starts:
(7)
We can write (1) and (3) as
(8)
(9)
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IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 33, NO. 6, JUNE 1997
Since the radiative and nonradiative recombination lifetimes
are of the nanosecond scale (
10 s ), while the defect
generation rate is of the order of seconds or longer (
1 s ) depending on the aging processes and temperature, the
ratio in the prefactor,
, is of the order 10 . Therefore, we can ignore the time derivative of the carrier density in
(9), which is the same as the quasi-statics approximation [35].
Physically, the carrier density reaches its equilibrium value
very quickly compared with the device degradation rate caused
by defect growth and we set
in (3) and solve for
as a function of
(or vice versa), which is a slowly
varying function. For example, we obtain
Our model also leads to the asymptotic behavior for the
defect density,
, as
. Therefore, the defect density behaves like
, which coincides with the result concluded from the
logarithmic plot of the degradation time as a function of the
pre-existing defect density in II–VI LD’s and LED’s [8].
D. Theoretical Results Using the Analytical Solutions
An analytical expression for the normalized half-lifetime,
, can be found by setting
, and we obtain
(16)
(10)
is proportional to the
The normalized output power
spontaneous emission rate normalized by its initial value;
therefore,
The aging rate depends on the prefactor
is related to the activation energy
the constant
temperature
by
, and
and
(17)
(11)
The second-order case corresponds to a degradation process
activated by the recombination of an electron and a hole at a
defect site. We obtain from (8), (10), and (11)
where
lifetime,
is Boltzmann’s constant. Therefore, the half, is given by
(12)
(18)
which can be integrated analytically by moving all the terms
on the right-hand side to the denominator of the left-hand side
and moving the
term to the right-hand side, given the initial
condition
. We find the most general solution for the
normalized optical power degradation of an LED
Our definition of the activation energy in the defect generation coefficient
is consistent with the previous work
in the aging studies of semiconductor devices, and it leads
naturally to the conventional approach of measuring the device
half-lifetime as a function of the temperature. The activation
energy is then extracted from the semilogarithmic plot of the
device half-lifetime as a function of the inverse temperature.
Physically the activation energy represents an effective energy
barrier for defect reactions such as creation and migration. If
we assume Boltzmann statistics for simplicity, we can write
the defect reaction coefficient as
(13)
Numerical results for the normalized optical output power
as a function of the normalized aging time can be calculated
from the above analytical expression by calculating as a
for
, and then plotting
as a
function of
function of . The normalized defect density is then obtained
from the normalized optical power using
(14)
as
, i.e.,
From (13), we find that
as goes to infinity, which is a simple algebraic decay
instead of an exponential dependence as commonly believed
in the aging studies of LD’s and LED’s. The above theoretical
results also indicate that a slope approaching
is expected
from a
versus
plot as increases. Furthermore, a
versus
plot should differ from the
versus
plot by a constant shift in the horizontal coordinate
without change in the shape of the curve since
(15)
Therefore, universal curves can be plotted with the initial
radiative quantum efficiency immediately before the aging
test as the only determining parameter.
(19)
and
are the band-edge density parameters [34],
where
and
are the conduction and valence band edges,
and
the quasi-Fermi levels (both measured from the valence
band edge) for the electrons and holes, respectively, and
is the bandgap energy. From the argument
in the exponent,
, we can see clearly that
the effective barrier height is reduced because of the electron
and hole injections. Thus, the electron and hole recombination
enhances the defect reaction rate by the
factor. We have
assumed that the carrier population is large enough such that
only defect generation is dominant and ignored the defect
annealing, which should be proportional to
, where
and
are the thermal equilibrium values of the electrons
and holes, respectively, in the absence of carrier injection.
In Fig. 2, we plot (a) the normalized optical output power
and (b) the normalized defect density versus the normalized
aging time on a linear scale for
0.2, 0.4, 0.6, and 0.8.
CHUANG et al.: KINETIC MODEL FOR DEGRADATION OF LIGHT-EMITTING DIODES
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(a)
(a)
(b)
(b)
Fig. 2. Theoretical results for (a) normalized optical output power P and
(b) normalized defect density Nd (t)=Nd (0) are plotted as a function of the
normalized time in linear scales for initial radiative quantum efficiency =
0.2, 0.4, 0.6, and 0.8.
Fig. 3. Logarithmic plots for (a) log(P ) and (b) log[Nd (t)=Nd (0)] versus
log( ) for the same theoretical curves in Fig. 2. Note that the optical power
approaches 1= with a slope of 1 and the defect density approaches 1=2
Since is related to time by a scaling factor depending on
the degradation coefficient
, the horizontal axis can
be stretched or compressed uniformly by a constant factor to
directly compare the theoretical curves with the experimental
data. This procedure will determine the scaling factor
.
On the other hand, the
versus
plot in Fig. 3(a)
and the
versus the
plot in Fig. 3(b)
will have the same shape for each with a possible constant
horizontal shift determined by
. The preservation
of the shape for each is very useful to explain experimental
data since our model shows that the shape is invariant once
the initial is determined, while the aging rate appearing in
the
factor depends on the material parameters,
injection condition, and the temperature. As a matter of fact,
all of our data show the general shapes in the log–log plot of
the normalized optical output power versus the aging time.
an exponential function as
0
with a slope of 1/2 in the log–log scales.
:
(20)
and the normalized defect density is obtained from (14).
Equation (20) agrees with the previous observation that the
III–V LED output power decays exponetially as a function
of aging time under a constant current aging condition. Our
model shows that this exponential dependence is only valid
for the inital period and the decay rate depends on
.
2) Long-Time Behavior: Since the optical output intensity
approaches zero as time goes to infinity, while the
defect density grows to infinity, the continuity (3) can be
simply written as
(21)
E. Approximate Solutions for Asymptotic
Short- and Long-Time Behaviors
1) Short-Time Behavior: Using the exact expression (13),
we find that the normalized optical output power behaves as
or
(22)
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IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 33, NO. 6, JUNE 1997
(a)
(b)
(c)
(d)
Fig. 4. Comparison of the asymptotic results for short-time [long-dashed lines, (20) and (14)] and long-time [dotted lines, (24) and (23)] behaviors with the
exact results for (a) normalized optical output power, P , and (b) normalized defect density, x(t) = Nd (t)=Nd (0). The above results are calculated assuming
that the quantum efficiency of the LED before aging is 0.5. (c) and (d) are the same as (a) and (b), except that = 0.8 .
which can then be substituted into the defect-generation model
(1) and we obtain the defect density as a function of time
(23)
is only valid for a very short time near
0. The long-time
behavior for the defect density [dotted lines in Fig. 4(b) and
(d)] seems to follow the exact solution (solid curves) with a
small amount of error.
F. Comparison with Other Models
and the normalized optical output power is
(24)
The above results also lead to the time dependencies that
and the power
the defect density approaches
approaches
as time goes to infinity. This result was
unreported in previous studies of II–VI and III–V LED aging.
In Fig. 4(a)–(d), we compare the above simplified asymptotic behaviors with those using the exact expressions for the
optical power decay and the growth of the normalized defect
density. For an inital quantum efficiency of
0.5 in Fig. 4(a)
and (b), we find that the exponential dependence (long-dashed
curves) agrees very well with the exact results (solid curves)
for the optical power starting from
0 to the half-lifetime.
However, for
0.8 in Fig. 4(c), the exponential dependence
1) Case A: First-Order Process: We
assume
in the defect generation model, i.e.,
that
(25)
This first-order case corresponds to a process in which the
degradation proceeds with the presence of either an electron
or a hole at a pre-existing defect site. The first-order defect
dependence in the optical
generation model leads to a
output power, which does not agree so far with experimental
observation.
2) Case B: Silicon Weak-Bond-Breaking Model [35], [36]:
This defect generation model corresponds to a process in
which the degradation proceeds due to the breaking of weak
silicon bonds caused by electron-hole recombination, yet the
CHUANG et al.: KINETIC MODEL FOR DEGRADATION OF LIGHT-EMITTING DIODES
975
Fig. 5. Typical device structure grown by MBE for the II–VI LED’s under
study.
generation rate depends only on the
defect density does not appear explicitly:
(a)
product and the
(26)
This case has been investigated in silicon photoconductors
[35], [36] and silicon solar cells [37]. It was found that
optical illumination creates electron-hole pairs, and electronhole recombination-enhanced defect or weak silicon-silicon
bond breaking plays an important role in the degradation of
silicon materials. The degradation was measured from the
decrease in the photoconductivity as a function of optical
illumination time (or aging time). This model, coupled with
the carrier continuity equation, leads to a time dependence of
for the defect density as time goes to infinity. If we apply
this model to the study of an aging LED, we obtain a time
dependence of
for the optical output power as time goes
to infinity, which has not been observed, to the best of our
knowledge.
III. EXPERIMENT
Most of the experiments were conducted at room temperature under constant current aging of LED’s using CdZnSe
quantum-well structures. We have investigated many LED’s
with different structures and lifetimes ranging from on the
order of hours to 1000 h, all showing the power dependence
as a function of aging time following the general shapes
in Fig. 3(a), predicted by our theory. Previously, we have
reported results using five LED’s made from two samples
with the lifetime of a few hours. A sample consists of three
40-Å Cd Zn Se quantum wells with 90-Å ZnS Se
barriers, and another sample consists of five 40-Å Cd Zn Se
barriers.
quantum wells with 75-Å ZnS Se
In this paper, we show new experimental results on blue
LED’s with a room-temperature lifetime of the order of 1000
h and investigate their aging behavior as a function of driving
current and temperature. Most of these devices are candelaclass blue LED’s (wavelength around 486 nm) producing 1.5
mW at a direct current of 20 mA with an external quantum
efficiency as high as 2.9% at room temperature. Typical LED
devices are shown in the schematic diagram in Fig. 5. These
LED’s were grown on Si-doped n-type GaAs substrates using
a molecular-beam epitaxy (MBE) system equipped with two
(b)
Fig. 6. Experimental data showing the optical output power normalized to
its initial value at t
0 of II–VI LED’s using Sample A under a constant
current aging condition in (a) linear scales and (b) log-log scales at nine
1004
driving currents. The current injection area of the LED is about 6.7
cm2 . Therefore, 10 mA corresponds to a current density of 15 A/cm2 .
=
2
growth chambers for III–V and II–VI compounds. The details
on these LED structures are reported in [39]. Sample A has
Se
an active region consisting of three undoped Cd Zn
0.198) quantum wells and two undoped ZnSSe barriers,
(
surrounded by doped ZnSSe guiding layers ( 100 nm) and
doped ZnMgSSe cladding layers of about 1 m. The epitaxial
wafer was processed into surface-emitting LED’s. The current
was injected through a light-extracting window about 6.7
10
cm covered by a thin layer of Au. The chip was
encapsulated in a clear epoxy dome, 3 mm in diameter and 5
mm in height, to enhance the light-extracting efficiency. The
absolute light output power was measured using a LD tester
equipped with a silicon p-i-n photodiode with an area of 1
cm . The emitted light was collected by an integrating cone
surrounding the device.
In Fig. 6(a), we show the experimental data for the optical
output aging characteristics of nine LED’s fabricated from
the same wafer, Sample A, under a constant-current aging
10, 15, 20, 25, 30, 35,
condition with a bias currents
45, 50, and 70 mA. Using the current injection area of 6.7
10 cm , we obtain the current density of 15 A/cm at 10
mA. The output powers decay with aging time. To look for
the asymptotic behavior, we plot the data on Fig. 6(b) on a
log-log scale. It can be seen clearly that the trend of the slope
approaches 1 as the aging time becomes long and the power
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IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 33, NO. 6, JUNE 1997
Fig. 7. Experimental data (symbols) for the integrated optical output power
P versus current (L–I curves) for two LED’s using Sample A before aging
started. The solid curves for the power are the fits using (27). The extracted
radiative quantum efficiencies using (28) for both L–I curves as a function
of the operation current are also plotted.
decays to less than 20% of its initial value. More experimental
data will be discussed in the next section. It is also noted
that the optical output power at 10 mA increases first for a
short time and then decreases. The initial rise is similar to the
previous findings in III–V compound semiconductor lasers for
which a reduction of the threshold current was observed [21],
which is due to the annealing of the defects in the presence
of carrier injection. Note the slowly decaying behavior at long
aging times. Since our model is based on the defect generation
and the decrease of the optical power, we ignore the annealing
behavior, which only lasts a very short time at a small current
operation.
IV. COMPARISON BETWEEN THEORY AND EXPERIMENT
Since our theoretical model requires the initial intrinsic
quantum efficiency as the input parameter, we measured the
light output versus the injection current using LED’s fabricated
from the same Sample A. The – curves for two LED’s are
20 mA,
shown in Fig. 7, for illustration. For example, at
the output optical power is about 1.5 mW, which corresponds
to an external quantum efficiency of 2.9% at a wavelength
around 486 nm.
Using the experimental – curve and the continuity (3)
with
, we can determine the initial radiative
quantum efficiency
as a function of
current before aging starts. First, we solve the carrier density
as a function of
, where is the area of current
injection. Since the output power is proportional to
, we
have
(27)
where
is a constant proportional to the optical coupling
factor, and
. The intrinsic quantum
efficiency is then given by
(28)
Fig. 8. Comparison between the theoretical results (solid curves) and the
experimental data (symbols) from LED’s using Sample A for five constant
15, 25, 35, 45, and 70 mA in Fig. 6. At 70 mA, the current
currents: I
density is about 105 A/cm2 .
=
Since the intrinsic quantum efficiency only depends on
and , it can be calculated once the constant parameter
is
obtained from fitting the experimental – data using (27).
The solid curves for the light output power are our fits and the
open circles and the solid triangles are the experimental data.
The good agreement shows that the simple rate equation and
the – relation work very well. Using the above procedure
for both – curves in Fig. 7, we obtain the radiative quantum
efficiency plotted on the same figure. Both – curves give
the same curve as expected since they were made from the
same sample A.
Once the intrinsic quantum efficiency is determined from
the experimental – curves for the LED’s, we plot the –
curves using (13) with the scaling coefficient as the only
adjustable parameter. The theoretical results are shown as solid
curves in Fig. 8 and compared with the experimental data
(symbols) for
15, 25, 35, 45, and 70 mA. The general
agreement is very good. Note that the long-time decaying
behavior of each LED shows a slope of 1 indicating a
dependence as expected from our theoretical model.
We also fabricated LD’s with a stripe width of 10 m and
a cavity length of 600 m using another sample (Sample B).
These LD’s have a lifetime of a few hours and a threshold of
about 34 mA. We biased these devices at a constant current
mA (corresponding to a current density of 500 A/cm )
and kept them in a constant temperature chamber at
20 ,
40 , 60 , and 80 C, respectively. The temperature-dependent
aging behavior is shown in Fig. 9. Again, the theoretical results
(solid curves) agree with the experimental data (symbols) very
well. The data also show a long-time behavior of a slope of
1 on the
–
plot. The half-lifetimes at 40 –80
also give an estimate of the activation energy of
0.8 eV
on a plot of the half-lifetime versus the inverse temperature.
Since our LED’s measured for Fig. 6 at different bias
currents had no heat sink, we found that the heating effects
also accelerate the aging process with increasing current. In
Fig. 10, we plot the half-lifetime of the LED’s using Sample
A as a function of the bias current. The open circles are the
CHUANG et al.: KINETIC MODEL FOR DEGRADATION OF LIGHT-EMITTING DIODES
Fig. 9. Optical output power degradation of four II–VI LD’s using Sample
B operating in an LED mode at four temperatures under a constant current
30 mA, which is below threshold ( 34 mA). The area
aging condition, I
600 m. Therefore, 30 mA corresponds to a current
of the LD is 10 m
density of 500 A/cm2 .
=
=
2
Fig. 10. Half-lifetime of the LED’s using Sample A is plotted as a function
of operation current. The open circles are the experimental data obtained from
Fig. 6 and the solid curve is our theoretical result taking into account thermal
heating.
experimental data and the solid curve is our theory, taking into
account the thermal heating in the junction temperature:
(29)
where the junction temperature is increased by the thermal
effects
(30)
is the ambient temperature. We use an activation energy
0.8 eV,
5.5 V and
150 K/W to fit the
temperature-dependent half-lifetime. The thermal resistance is
comparable with previously reported values ranging from 37
to 98 K/W using six samples mounted at a 170-K heatsink
temperature in [38], noting that our devices made using Sample
A were measured at room temperature without a heatsink.
and
V. DISCUSSION
Our model indicates that the defect density grows sublinearly with a power of
indicating defect diffusion enhanced
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by carrier recombination processes. This time dependence
leads to a
dependence in the asymptotic behavior of the
optical output power of an LED under the constant current
aging condition. The commonly believed exponential decay
behavior is only valid for an initial period of time, which
depends on the initial radiative quantum efficiency. Our model
indicates that the nonradiative recombination of an electron
and a hole at a defect site drives the degradation. The carrier
recombination near the defects releases an energy of the order
of the bandgap via a multiphonon emission process. This
bandgap energy reduces the barrier for defect diffusion or
generation and increases the probability of defect reactions
[22], [28], [30]. Many of our devices were prepared from
samples that contain no dark-line defects (DLD’s) or darkspot defects (DSD’s) and a uniform degradation was observed.
These devices have a LD configuration, e.g., 10- m stripe
width and 600–1000- m cavity length. For a DSD density
of 3000–10 000 cm , no DSD’s or DLD’s are supposed
to be in the stripe. For various samples with or without
pre-existing DSD’s or DLD’s, the statistical variation of the
lifetime is affected by the normalization constant,
, on
the horizontal time scale in the universal curves. The initial
carrier density and the activation energy in the coefficient
may be affected by the densities of DSD’s and DLD’s. For a
uniform degradation, the point defects are the recombination
centers, which cause the LED degradation. Although various
point defects such as Se vacancies and Zn interstitials and the
formation of complexes have been investigated theoretically
[40]–[43] for the compensation of p-type doping in ZnSe,
very little work has been reported on point defects in ternary
compounds such as CdZnSe materials. Microscopically, the
REDR mechanism occurs because the electronic energy released upon carrier captures is transferred nonradiatively to the
vibrational freedom of the defect system and then redistributed
in statistical fluctuation to a specific vibrational mode and
causes defect motions such as creation or migration. The
reaction of defects in the active CdZnSe quantum wells is
further complicated by the presence of compressive strain.
The presence of strain could increase the defect reaction rate
activated by the nonradiative carrier–recombination processes.
The LED’s of this paper are of two types. One is surfaceemitting LED’s, which have on top a transparent Au thin film,
through which photons come out. This transparent window has
fringe through which no current is injected. The cleavage is
done in the middle of the fringe. Thus, this type of LED has
no mesa, but cleaved facet at the fringe. Since current does not
flow at the fringe, we may ignore the DLD’s from the cleaved
facet, except for the case of REDR due to carriers generated by
the photons that are recycled at the facet. The second type has
an LD configuration as discussed above, which has no mesa
either but cleaved facets at both ends, for which REDR at
periphery (facet) may be important, but we do not necessarily
see DLD’s at the facets.
Since II–VI materials have been considered to be fragile
because the bonding energy is typically smaller than the
bandgap energy, researchers in the II–VI community have
shown great concern that the II–VI wide-gap light emitters
would die catastrophically before the REDR process comes
978
IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 33, NO. 6, JUNE 1997
into playing a role. Our finding that the degradation of the
II–VI light-emitting materials is governed by the co-presence
of an electron and a hole at the defect site as in the III–V
materials is significant since it indicates that II–VI compounds,
when well prepared, have high potential as light-emitting
materials.
We have applied our model to the LED’s with and without
dark-line or dark-spot defects with a lifetime varying from
hours to 1000 h. Our theoretical model successfully explains
the time dependence of the LED aging from the early exponential decay to the
behavior. The theory seems to be
applicable to both rapid and gradual degradation of LED’s,
as long as the defect reactions are caused by the carrierrecombination-enhanced defect motion. Our finding is consistent with the previous work on the studies of recombinationenhanced defect reaction causing rapid degradation involving
dark-line defects as well as gradual degradation involving
point defects [26], [30].
VI. CONCLUSION
We have presented a kinetic model for defect generation
and applied it successfully to the study of II–VI LED’s to
explain their power decay as a function of aging time at various
bias currents and temperatures. The physics is based on the
carrier-recombination-enhanced defect motion and our kinetic
model leads to a universal analytical solution for the optical
output power as a function of the normalized aging time. Our
model should also be applicable to III–V LED’s, especially
wide bandgap materials, for which the carrier–recombination
enhanced defect motion is believed to play a significant role at
room temperature because the energy released is proportional
to the bandgap energy.
So far, the studies of degradation physics have been usually
based on empirical parameters such as the activation energy.
Our model successfully shows, for the first time to the best of
our knowledge, that one can obtain universal curves for the
time dependence of the optical output power of LED’s, for a
given initial radiative quantum efficiency determined from the
– curves immediately before the aging test starts. Increases
in current, temperature, and thermal heating accelerate the
aging rate of the LED’s. Our transient recombination-enhanced
defect generation model also provides a more physically
intuitive explanation and quantitative description than the
previous qualitative descriptions of the degradation behavior.
ACKNOWLEDGMENT
A major part of this work was done while S. L. Chuang
was visiting SONY Research Center. The authors thank K.
Nakano, H. Okuyama, H. Yoshida, J. Seto, and T. Yamada for
their comments.
REFERENCES
[1] M. A. Haase, J. Qiu, J. M. DePuydt, and H. Cheng, “Blue-green laser
diodes,” Appl. Phys. Lett., vol. 59, pp. 1272–1274, 1991.
[2] H. Jeon, J. Ding, W. Patterson, A. V. Nurmikko, X. Xie, D. C. Grillo,
M. Kobayashi, and R. L. Gunshor, “Blue-green injection laser diodes
in (Zn,Cd)Se/ZnSe quantum wells,” Appl. Phys. Lett., vol. 59, pp.
3619–3621, 1991.
[3] H. Okuyama, T. Miyajima, Y. Morinaga, F. Hiei, M. Ozawa, and K.
Akimoto, “ZnSe/ZnMgSSe blue laser diode,” Electron. Lett., vol. 28,
pp. 1798–1799, 1992.
[4] S. Nakamura, M. M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, T.
Matsushita, H. Kiyoku, and Y. Sugimoto, “InGaN-based multi-quantumwell-structure laser diodes,” Jpn. J. Appl. Phys., vol. 35, 1996.
[5] I. Akasaki, S. Sota, H. Sakai, T. Tanaka, M. Koike, and H. Amano,
“Shortest wavelength semiconductor laser diode,” Electron. Lett., vol.
32, pp. 1105–1106, 1996.
[6] A. V. Nurmikko and R. L. Gunshor, “Semiconductor lasers with widegap II–VI materials,” in Semiconductor Lasers: Past, Present, and
Future. Woodbury, NY: Amer. Inst. Phys., 1995, ch. 7, pp. 208–242.
[7] R. L. Gunshor and A. V. Nurmikko, “The first compact blue/green
diode lasers-wide-bandgap II–VI semiconductors,” Proc. IEEE, vol. 82,
pp. 1503–1513, 1994.
[8] A. Ishibashi, “II–VI blue-green laser diodes,” IEEE J. Select. Topics
Quantum Electron., vol. 1, pp. 741–748, 1995.
, “II–VI blue-green light emitters,” J. Cryst. Growth, vol. 159,
[9]
pp. 555–565, 1996.
[10] S. Taniguchi, T. Hino, S. Itoh, K. Nakano, N. Nakayama, A. Ishibashi,
and M. Ikeda, “100h II–VI blue-green laser diode,” Electron. Lett., vol.
32, pp. 552–553, 1996.
[11] M. Fukuda, Reliability and Degradation of Semiconductor Lasers and
LEDs. Boston: Artech, 1991.
[12] S. Guha, J. M. DePuydt, M. A. Hasse, J. Qiu, and H. Cheng, “Degradation of II–VI based blue-green light emitters,” Appl. Phys. Lett., vol.
63, pp. 3107–3109, 1994.
[13] S. Guha, H. Cheng, M. A. Hasse, J. M. DePuydt, J. Qiu, B. J. Wu, and
G. E. Hofler, “h100i dark line defect in II–VI blue-green light emitters,”
Appl. Phys. Lett., vol. 65, pp. 801–803, 1994.
[14] G. C. Hua, N. Otsuka, D. C. Grillo, Y. Fan, J. Han, M. D. Ringle,
R. L. Gunshor, M. Hovinen, and A. V. Nurmikko, “Microstructure
study of a degraded pseudomorphic separate confinement heterostructure
blue-green laser diode,” Appl. Phys. Lett., vol. 65, pp. 1331–1333, 1994.
[15] L. H. Kuo, L. Salamanca-Riba, B. J. Wu, and J. M. DePuydt, “Misfit strain induced tweed-twin transformation on composition modulation Zn10x Mgx Sy Se10y layers and the quality control of the ZnSe
buffer/GaAs interface,“ J. Electron Mater., vol. 24, pp. 155–161, 1995.
[16] S. Tomiya, E. Morita, M. Ukita, H. Okuyama, S. Itoh, K. Nakano, and A.
Ishibashi, “Structural study of defects induced during current injection
to II–VI blue light emitter,” Appl. Phys. Lett., vol. 66, pp. 1208–1210,
1995.
[17] Y. L. Khait, J. Salzman, and R. Baserman, “Kinetic model for gradual
degradation in semiconductor lasers and light-emitting diodes,” Appl.
Phys. Lett., vol. 53, pp. 2135–2137, 1988.
[18] J. Salzman, Y. L. Khait, and R. Baserman, “Material evolution and
gradual degradation in semiconductor lasers and light emitting diodes,”
Electron. Lett., vol. 25, pp. 244–246, 1989.
[19] K. Kondo, O. Ueda, S. Isozumi, S. Yamakoshi, K. Akita, and T. Kotani,
“Positive feedback model of defect formation in gradually degraded
GaAlAs light emitting devices,” IEEE Trans. Electron. Devices, vol.
ED-30, pp. 321–326, 1983.
[20] H. Imai, K. Isozumi, and M. Takusagawa, “Deep level assciated with
slow degradation of GaAlAs DH laser diodes diodes,” Appl. Phys. Lett.,
vol. 33, pp. 330–332, 1978.
[21] T. Kobayashi and Y. Furukawa, “Recombination enhanced annealing
effect in AlGaAs/GaAs remote junction heterostructure lasers,” IEEE J.
Quantum Electron., vol. QE-15, pp. 674–684, 1979.
[22] D. V. Lang and L. C. Kimerling, “Observation of recombinationenhanced defect reactions in semiconductors,” Phys. Rev. Lett., vol. 33,
pp. 489–492, 1974.
[23] J. D. Weeks, J. C. Tully, and L. C. Kimerling, “Theory of recombinationenhanced defect reactions in semiconductors,” Phys. Rev. B, vol. 12, pp.
3286–3292, 1975.
[24] L. C. Kimerling, “Recombination-enhanced defect reactions,” Solid
State Electron., vol. 21, pp. 1391–1401, 1978.
[25] A. Sibille, “Electronically stimulated deep-center reactions in electronirradiated InP: Comparison between experiment and recombinationenhanced theories,” Phys. Rev. B, vol. 35, pp. 3929–3936, 1987.
[26] P. W. Hutchinson, P. S. Dobson, B. Wakefield, and S. O’Hara, “The
generation of point defects in GaAs by electron-hole recombination at
dislocations,” Solid State Electron., vol. 21, pp. 1413–1417, 1978.
[27] M. M. Sobolev, A. V. Gittsovich, M. I. Papentsev, I. V. Kochnev, and
B. S. Yavich, “Degradation mechanism of GaAs/AlGaAs quantum well
laser,” Sov. Phys. Semicond., vol. 26, pp. 985–989, 1992.
[28] C. H. Henry and D. V. Lang, “Nonradiative capture and recombination
by multiphonon emission in GaAs and GaP,” Phys. Rev. B, vol. 15, pp.
989–1015, 1977.
CHUANG et al.: KINETIC MODEL FOR DEGRADATION OF LIGHT-EMITTING DIODES
[29] Y. Toyozawa, “Multiphonon recombination processes,” Solid State Electron., vol. 21, pp. 1313–1318, 1978.
[30] I. N. Yassievich, “Recombination-induced defect heating and related
phenomena,” Semicond. Sci. Technol., vol. 9, pp. 1433–1453, 1994.
[31] B. Mroziewicz, M. Bugajski, and W. Nakwaski, Physics of Semiconductor Lasers. Amsterdam, The Netherlands: Elsevier, 1994.
[32] S. L. Chuang, M. Ukita, S. Kijima, S. Taniguchi, and A. Ishibashi,
“Optical output degradation of II–VI blue-green light-emitting diodes,”
in Conf. Lasers Electro-Optics, Tech. Dig. Ser., 1996, vol. 9, p. 455.
[33]
, “Universal curves for optical power degradation of II–VI lightemitting diodes,” Appl. Phys. Lett., vol. 69, pp. 1588–1590, 1996.
[34] S. L. Chuang, Physics of Optoelectronic Devices. New York: Wiley,
1995.
[35] M. Stutzmann, W. B. Jackson, and C. C. Tsai, “Light-induced metastable
defects in hydrogenated amorphous silicon: A systematic study,” Phys.
Rev. B, vol. 32, pp. 23–47, 1985.
[36] S. Vignoli, R. Meaudre, and M. Meaudre, “Equilibrium temperature
in intrinsic hydrogenated amorphous silicon under illumination,” Phys.
Rev. B, vol. 50, pp. 7378–7383, 1994.
[37] R. A. Street, “Current-induced defect creation and recovery in hydrogenated amorphous silicon,” Appl. Phys. Lett., vol. 59, pp. 1084–1086,
1991.
[38] R. R. Drenten, K. W. Haberern, and J. M. Gaines, “Thermal characteristics of blue-green II–VI semiconductor lasers,” J. Appl. Phys., vol. 76,
pp. 3988–3993, 1994.
[39] N. Nakayama, S. Itoh, A. Ishibashi, and Y. Mori, “High-efficiency
ZnCdSe/ZnSSe/ZnMgSSe green and blue light-emitting diodes,” in
SPIE Meet., San Jose, CA, 1996.
[40] R. W. Jansen and O. F. Sankey, “Theory of relative native- and impuritydefect abundances in compound semiconductors and the factors that
influence them,” Phys. Rev. B, vol. 39, pp. 3192–3206, 1989.
[41] D. B. Laks, C. G. Van de Walle, G. F. Newmark, P. E. Blochl, and S. T.
Pantelides, “Native defects and self-compensation in ZnSe,” Phys. Rev.
B, vol. 45, pp. 10965–10978, 1992.
[42] C. G. Van de Walle and D. B. Laks, “Theory of defects, impurities, and
doping in ZnSe,” J. Lumin., vol. 52, pp. 1–8, 1992.
[43] A. Garcia and J. E. Northrup, “Compensation of p-type doping in ZnSe:
The role of impurity-native defect complexes,” Phys. Rev. Lett., vol. 74,
pp. 1131–11134, 1995.
Shun-Lien Chuang (S’78–M’82–SM’88–F’97) received the B.S. degree in electrical engineering from
National Taiwan University in 1976, and the M.S.,
E.E., and Ph.D. degrees in electrical engineering
from the Massachusetts Institute of Technology,
Cambridge, in 1980, 1981, and 1983, respectively.
While in graduate school, he held research and
teaching assistantships and also served as a Recitation Instructor. In 1983, he joined the Department
of Electrical and Computer Engineering at the University of Illinois at Urbana-Champaign, where he
is currently a Professor. He was a Resident Visitor at AT&T Bell Laboratories,
Holmdel, NJ, in 1989. He was a Consultant at Bellcore and Polaroid in 1991,
and a Senior Visting Professor at the Sony Corporation Research Center,
Yokohama, Japan, in 1995. He was also an Associate at the Center for
Advanced Study at the University of Illinois in 1995. He is conducting
research on strained quantum-well semiconductor lasers and femtosecond
nonlinear optics. He developed a graduate course on integrated optics and
optoelectronics. He is the author of Physics of Optoelectronic Devices (New
York: Wiley, 1995). He is an Associate Editor for the IEEE JOURNAL OF
QUANTUM ELECTRONICS and was a Feature Editor for a special issue of the
Journal of the Optical Society of America B on Terahertz Generation, Physics
and Applications, in 1994. He has published more than 150 conference
and journal papers and was invited to give talks at conferences including
the American Physical Society March Meeting, Optical Society of America
Annual Meeting, and Integrated Photonics Research.
Dr. Chuang is a fellow of the Optical Society of America and a member
of the American Physical Society. He has been cited several times for
Excellence in Teaching. He received the Andersen Consulting Award from
the University of Illinois for excellence in advising in 1994. He was also
awarded a Fellowship from the Japan Society for the Promotion of Science
to visit the University of Tokyo in 1996.
979
Akira Ishibashi (M’91) was born in Saga, Japan, in
1958. He received the Ph.D. degree from the University of Tokyo, Tokyo, Japan, in 1990, studying
GaAs JFET, electrons and phonons in AlAs/GaAs
superlattices.
He joined the Sony Corporation Research Center,
Yokohama, Japan, in 1983 after studying elementary particle physics at the Department of Physics,
University of Tokyo, KEK, and Lawrence Berkeley
National Laboratory. As a Visiting Faculty Member
at Loomis Laboratory, University of Illinois, he
extended the study of GaAs-based nanostructures in 1990–1991. Returning
to Sony, finishing the development of low-noise AlGaInP red-emitting laser
diodes, he has been engaged in the physics of ZnMgSSe-based blue–green
laser diodes.
Satoru Kijima received the B.Sc. and M.Sc. degrees from the Division of Electronic and Information Engineering, Faculty of Technology, Tokyo
University of Agriculture and Techonology, Tokyo,
Japan, in 1991 and 1993, respectively.
In 1993, he joined the Sony Corporation Research
Center, Yokohama, Japan, where he has been engaged in the growth and the characterization of
II–VI semiconductor laser diodes.
Norikazu Nakayama received the B.S. and M.S.
degrees in electronics from Toyama University,
Japan, in 1986 and 1988, respectively.
He joined the Sony Corporation, Atsugi, Japan,
in 1988 and worked as a test engineer for bipolar
IC’s. Since 1992, he has been with the Sony
Corporation Research Center, Yokohama, Japan,
where he is involved in process development and
device characterization of II–VI semiconductor
lasers.
Masakazu Ukita was born in Tokyo, Japan, in
1965. He received the M.Sc. and D.Sc. degrees in
physics from Keio University, Yokohama, Japan, in
1989 and 1992, respectively.
In 1992, he joined the Sony Corporation Research
Center, Yokohama, Japan, and since then he has
been working in the field of semiconductor lasers.
Satoshi Taniguchi received the B.Sc. and M.Sc.
degrees in electrical engineering from Kyoto University, Japan, in 1990 and 1992, respectively.
In 1992, he joined the BLD project team at
the Sony Corporation Research Center, Yokohama,
Japan, where he has been engaged in studying
growth by molecular beam epitaxy and the characterization of II–VI semiconductor laser diodes and
related materials.