IEEE JOURNAL ON SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 7, NO. 2, MARCH/APRIL 2001
231
Sensitivity of Proton Implanted VCSELs to
Electrostatic Discharge Pulses
Heinz-Christoph Neitzert, Agnese Piccirillo, and Barbara Gobbi
Abstract—The sensitivity of vertical cavity surface-emitting
lasers to electrostatic discharge (ESD) pulses has been investigated
under human body model test conditions. Very similar degradation behavior has been found for vertical-cavity surface-emitting
lasers (VCSELs) from two different manufacturers, both with
proton-implantation for lateral current confinement. For all
investigated devices we observed during forward bias stress that
the optical degradation precedes the electrical degradation and
the forward bias damage threshold pulse amplitudes were only
slightly higher than the reverse bias values. At the initial stage
of the VCSEL degradation, damage of the upper p-DBR mirror
region has been observed without modification of the active layer.
During the ESD tests we monitored the electrical and the optical
parameters of the VCSELs and measured during forward bias
stress additionally the optical emission transients. The optical
transients during ESD pulsing enable a fast evaluation of the
damage threshold and give also an indication of the time scale of
the junction heating during ESD pulses.
Index Terms—Electrostatic discharge sensitivity, laser reliability, optical emission, proton implantation, thermal resistance,
vertical-cavity surface-emitting lasers.
I. INTRODUCTION
T
HE POSSIBLE applications of vertical cavity surface-emitting lasers (VCSELs) range from smart pixels
[1] and parallel optical links for computers [2] to low-cost
optical access network transceivers [2]. Small signal frequency
responses of more than 10 GHz have been reported as well
for proton-implanted [3] for oxide-confined VCSELs [4] and
more than 10 Gb/s data transmission over 500-m graded-index
multimode optical fiber with a bit-error rate better than 10
has been achieved using vertical cavity surface-emitting lasers
[4]. Reliability studies of proton-implanted VCSELs report a
lifetime of more than 5 million hours at 40 C [5]. Another important aspect of device reliability, however, is the sensitivity to
electrostatic discharge pulses. Most results regarding the electrostatic discharge (ESD) sensitivity of optoelectronic emitters
can be found on 1.30 and 1.55 m Fabry–Pérot (FP) and distributed-feedback (DFB) edge-emitting lasers [6]–[9].The ESD
damage threshold amplitudes during reverse bias ESD stress of
1.30- m lasers were generally around 2000 V [6]–[8], whereas
the data for the forward bias damage threshold range from 600
to 17 000 V [6]–[8]. It has been found that edge-emitting lasers
were more sensitive to reverse bias pulses compared to forward
Manuscript received October 16, 2000; revised March 2, 2001.
H.-C. Neitzert is with the Università di Salerno, Dipartimento di Elettronica,
I-84084 Fisciano (SA), Italy.
A. Piccirillo and B. Gobbi are with the Centro Studi e Laboratori Telecomunicazioni (CSELT), I-10148 Torino, Italy.
Publisher Item Identifier S 1077-260X(01)08010-8.
bias pulses [8], [10]. It is, however, much easier to protect
the lasers against reverse bias ESD stress, and therefore, it is
particularly important to determine the forward bias damage
threshold amplitudes [6]. While degradation under reverse bias
pulses is mainly due to excessive device heating, additionally
problems due to high optical power densities during the ESD
pulse should be considered under forward bias stress conditions
[6], [11].
Few data have been published so far on the ESD sensitivity of
VCSELs and no detailed study of the kinetics of ESD-induced
VCSEL degradation has been reported. Particular problems of
VCSELs with respect to their ESD robustness are due to the poor
heat dissipation as compared to edge-emitting lasers, leading to
severe heating problems [12]. It has been shown that thermal
lensing causes considerable differences between pulsed and CW
operation [13]. This makes the calculation of the expected temperature rise during the short ESD pulses rather difficult.
Threshold damage values around 800 V under human body
model (HBM) conditions have been measured for proton implanted GaAs–GaAlAs based VCSELs [5] comparable with the
values found for edge emitting GaAs–GaAlAs-based compact
disc (CD) lasers. In this paper, we present results of ESD step
stress tests under forward and reverse bias condition on two
types of commercially available vertical cavity surface-emitting
lasers from different manufacturers. We present in particular
the optical and electrical property data of the laser monitoring
during and after ESD step stress tests, followed by a discussion
of the possible degradation mechanisms.
II. MEASUREMENT SETUP AND TEST PROCEDURE
The electrostatic discharge tests have been performed using
an IMCS System 700 ESD tester, generating pulses conforming
to the HBM [14]. The discharge current has been sensed by an
inductive current probe and in the case of the forward bias ESD
stress the resulting optical emission transients during the tests
have been measured using a fast photodiode (NEW FOCUS,
Model 1611). Both optical and electrical transients have been
recorded during each pulse using a digitizing oscilloscope. In
Fig. 1 electrical and optical transients measured during forward
bias ESD testing of a VCSEL from manufacturer A using a pulse
amplitude of 100 V are shown. The pulse shape of the current
transient confirms that our tester conforms to the HBM specifications. For this low value of the pulse amplitude substantial
heating of the VCSEL can be excluded and the optical signal
shape follows strictly the exponential decay of the current pulse
transient.
During the step stress test, ESD pulses with successively increasing pulse amplitudes have been applied, with a pulse am-
1077–260X/01$10.00 © 2001 IEEE
Authorized licensed use limited to: ACADEMIA SINICA COMPUTING CENTRE. Downloaded on February 28,2010 at 08:37:20 EST from IEEE Xplore. Restrictions apply.
232
IEEE JOURNAL ON SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 7, NO. 2, MARCH/APRIL 2001
(a)
Fig. 1. Electrical current (filled diamonds) and optical emission (open circles)
transients measured for a VCSEL from manufacturer A during the application
of an ESD pulse with an amplitude of 100 V.
+
plitude difference of 50 V between successive steps. For each
pulse amplitude we exposed the lasers to one ESD pulse. The
dark current–voltage (I–V) and the optical power–current (P–I)
characteristics have been measured in discrete intervals and the
optical emission spectra of the lasers have been measured before and after device degradation.
III. CHARACTERIZED DEVICES AND VCSEL PROPERTIES
BEFORE ESD TESTING
The devices investigated in this study are commercially available VCSELs from two different manufacturers. Both types of
VCSELs are GaAlAs–GaAs-based devices with lateral confinement by proton implantation and 840-nm multimode emission.
More details about the device structure from manufacturer A
can be found in [5]. The type A VCSELs were mounted in a
TO case and the optical power measurements have been calibrated by using an optical power measurement system with an
integrating sphere. For this type of VCSEL we measured over a
wide range between 10 C and 70 C an almost temperature-independent quantum efficiency, threshold current values between
4 and 5 mA, and a temperature coefficient of the peak wavelength of 0.060 nm/ C. Above threshold the peak wavelength
increases linearly with increasing laser current with a coefficient
of 0.092 nm/mA.
The investigated VCSELs from manufacturer B are used as
emitters in a receptacle module for bidirectional full duplex
data transmission together with InGaAs pin-photodiodes. The
optical power—reported here—is, therefore, the optical power
coupled to a multimode silica fiber with a 50– m core diameter.
Between 0 C and 70 C for this laser type monotonically increasing threshold currents between 2.8 and 4.4 mA have been
measured, and the fiber coupled laser output power at a given
current of 20 mA decreases monotonically with increasing temperature from a value of 540 W at 0 C to a value of 340 W
at 70 C.
(b)
Fig. 2. Optical power at given bias current (filled circles) and reverse bias
current at a fixed applied bias voltage (open circles) during: (a) forward bias
ESD step stress and (b) reverse bias ESD step stress testing of VCSELs from
manufacturer A.
IV. ESD TEST RESULTS
A. Type A VCSEL Degradation
In order to determine the minimum ESD pulse amplitude
that causes degradation of the VCSEL performances, after each
pulse the current at a given reverse bias voltage and the emitted
optical power at a given bias current have been monitored. The
bias parameters have been chosen in order to stay well below
the reverse bias breakdown voltage of typically 9 V and well
above the typical laser threshold currents before degradation of
4 mA. Typical monitoring results of this two entities during positive and negative ESD stress tests of lasers from manufacturer A
are displayed in Fig. 2. We observe that below a given threshold
value, in the displayed case at 1200 V, during ESD stress
test with pulses in reverse bias direction [Fig. 2(b)], the optical
power and the reverse bias current are constant. Exceeding the
damage threshold value, the optical emitted power starts to decrease and the reverse bias current increases abruptly for more
than one order of magnitude. With further increasing negative
Authorized licensed use limited to: ACADEMIA SINICA COMPUTING CENTRE. Downloaded on February 28,2010 at 08:37:20 EST from IEEE Xplore. Restrictions apply.
NEITZERT et al.: SENSITIVITY OF PROTON IMPLANTED VCSELs
233
(a)
(a)
(b)
(b)
Fig. 3. (a) Optical P–I and (b) I–V characteristics before and at different steps
during forward bias ESD step stress test of a VCSEL from manufacturer A.
Fig. 4. (a) Optical P–I and (b) I–V characteristics before and at different steps
during reverse bias ESD step stress test of a VCSEL from manufacturer A.
pulse amplitudes the optical power drops monotonically to very
low values and the reverse bias current is still slightly increasing.
During forward bias ESD stress tests [see Fig. 2(a)] a different
picture has been found. In this case, instead of a single ESD
pulse amplitude degradation threshold value, we observe different thresholds at which optical and electrical properties start
to deteriorate. While the optical power starts to decrease already
after application of the 1900-V pulse, the reverse bias current
remains stable up to 2150 V and increases sharply after applying the 2200-V pulse.
In Fig. 3(a) the development of the P–I characteristics during
the degradation of the type A VCSEL under positive pulse ESD
stress is shown. The degradation starts initially with a slight
increase of the threshold current without decrease of the slope
efficiency—seen here after the 2150-V pulse application.
After application of the 2200-V pulse, the threshold current
increased by 70% and the slope efficiency decreased roughly
by a factor 2 as compared to the unstressed sample. The
development of the I–V characteristics of the type A VCSEL
during forward bias ESD stress is shown in Fig. 3(b). Before
degradation, we observe already a relatively small reverse bias
breakdown voltage of about 9 V. The reverse bias current
below a reverse bias voltage of 8 V and the forward current
below a forward bias of 1 V are dominated by a parallel
leakage current path and stay below 10 nA. Fitting the slope of
the exponential part of the forward bias I–V characteristics—that
holds for six orders of magnitude of the forward current—we
determine a diode ideality factor of 1.36. Up to an ESD pulse
amplitude of 2150 V we see no change at all, both in the forward and in the reverse bias I–V characteristics of this VCSEL.
Only after the 2200-V pulse, the ideality factor—as determined
in the forward bias characteristics—increases to a value of
2.50, and the reverse bias dark current increases between 2
and 9 V. The breakdown voltage, however, remains constant.
During the development of the P–I characteristics of the same
VCSEL type during negative bias ESD [see Fig. 4(a)] we observe no change at all up to pulse amplitudes of 1200 V, followed by a small threshold current increase of about 1 mA
and successively a dramatic decrease of the slope efficiency at
1300 V. The development of the I–V characteristics during negative bias ESD of this VCSEL type [see Fig. 4(b)] shows no
change up to 1200 V. Increasing the ESD pulse further, as
in the forward bias stress case, first an increase of the ideality
factor and of the reverse bias current between 3 and 9 V
Authorized licensed use limited to: ACADEMIA SINICA COMPUTING CENTRE. Downloaded on February 28,2010 at 08:37:20 EST from IEEE Xplore. Restrictions apply.
234
IEEE JOURNAL ON SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 7, NO. 2, MARCH/APRIL 2001
Fig. 5. Laser threshold current as a function of temperature measured for a
VCSEL from manufacturer A before and after reverse bias ESD step stress.
Fig. 7. Positions of the maxima of the optical emission spectra as a function
of the bias current of a VCSEL from manufacturer A, taken before and after
reverse bias ESD step stress.
Fig. 6. Optical emission spectra at a bias current of 11 mA for a VCSEL from
manufacturer A, taken before and after reverse bias ESD step stress.
after ESD-induced degradation. In order to take into account
the influence of the bias current on the spectral behavior of the
VCSELs, we compared the spectra before and after degradation
over a wide range of bias currents. In Fig. 7, the position of
all observed emission peaks as a function of the bias current is
shown. The peaks have been labeled in order to indicate also
their relative amplitudes. In this scheme, we see four rows,
parallel to each other and spaced apart at constant current by a
wavelength difference of about 0.1 nm. The lines indicate the
shift of single emission peaks as a function of the bias current.
Again we find a coefficient of 0.092 nm/mA, similar to the
value reported before. These results suggest that the slight
changes in the peak positions and the number of emission peaks
after degradation can be explained essentially by a change of
the charge carrier injection efficiency into the active layer and
that the active layer itself seems to be unmodified. The typical
images of the VCSELs from manufacturer A before degradation [see Fig. 8(a)] show a homogeneous surface morphology,
while another VCSEL of the same family after forward bias
ESD-induced degradation shows clearly surface modifications,
as revealed by the inhomogeneous color distribution at the
laser surface [see Fig. 8(b)]. This means that the high optical
power emitted during ESD testing at elevated pulse amplitude
values is sufficient to modify either the upper p-DBR mirror
region or the surface passivation layer. The emission window,
as observed in Fig. 8 as the innermost circular feature, has a
diameter of approximately 12 m.In Fig. 9, the development of
the optical power at a given laser current and of the current at a
fixed reverse bias voltage during typical positive and negative
ESD step stress tests of the VCSELs from manufacturer B are
shown.
is observed. Moreover, we see two new features: a monotonically decreasing reverse bias breakdown voltage for increasing
ESD pulse amplitudes above damage threshold and, after the
1300-V pulse, a further increase of the leakage current. The
latter becomes visible in an symmetric current increase for reverse bias and low forward bias voltages.
For this device we measured the temperature dependence of
the threshold current up to 70 C before degradation and after
the 1250-V ESD pulse (Fig. 5). The degradation results in a
parallel shift of this characteristic to higher threshold current
values without changes of the position of the threshold current
minimum at 35 C. The position of the minimum enables us
to obtain—before degradation—threshold currents below 5 mA
over the full operating temperature range, specified by the manufacturer to be 0 C–70 C.
The laser emission spectrum can give valuable information
regarding eventual modifications of the active layer of the
VCSEL. In Fig. 6, the emission spectrum of the type A VCSEL
before ESD and after a 1300-V pulse application is shown for
the same laser current of 11 mA. Instead of the multimode spectrum before degradation, we observed a single-mode spectrum
B. Type B VCSEL Degradation
The development of the constant bias optical power during
forward bias step stress [see Fig. 10(a)] shows only slight power
variations up to a first threshold pulse amplitude of 1500 V,
followed by a gradual monotonic decrease of the output power
to about 80% of its original value up to pulse amplitudes of
Authorized licensed use limited to: ACADEMIA SINICA COMPUTING CENTRE. Downloaded on February 28,2010 at 08:37:20 EST from IEEE Xplore. Restrictions apply.
NEITZERT et al.: SENSITIVITY OF PROTON IMPLANTED VCSELs
235
(a)
(a)
(b)
Fig. 9. Optical power at given bias current (filled circles) and reverse bias
current at a fixed applied bias voltage (open circles) during: (a) forward bias
ESD step stress and (b) reverse bias ESD step stress testing of VCSELs from
manufacturer B.
(b)
Fig. 8. Image of VCSELs from manufacturer B before and after ESD-induced
degradation during forward bias ESD step stress test.
2100 V, and finally a slightly faster decreasing output power,
reaching 50% of the original value after the 2350-V pulse application. On the other hand, in the development of the reverse
bias current we do not observe any change at the first threshold,
a minor decrease at the second threshold (after the 2200-V
pulse), and a small increase at 2350 V.
A coincidence between the inset of the optical power decrease and the inset of the reverse bias current increase at a given
voltage is also found for the type B VCSELs only during reverse bias ESD stress [see Fig. 9(b)]. In the displayed case the
degradation starts at 800-V pulse amplitude and the optical
power decreases to 50% of its original value at 1500 V. Again
the damage threshold amplitude is lower compared to the pulse
amplitude needed to degrade the lasers during forward bias ESD
step stress. The reverse bias current at 10 V increases from
values lower than 1 nA before degradation, to about 4 nA directly above damage threshold, and to 100 nA after 1000-V
ESD pulse application, while remaining nearly constant for further increasing pulse amplitudes.
As seen in Fig. 10(a), in the case of this type of VCSEL the
degradation during positive ESD step stress is initially only visible in the P–I characteristics for ESD pulse amplitudes between
1550 and 1750 V. For laser currents above 8 mA the slope efficiency starts to decrease slightly. For higher pulse amplitudes
we see the start of a successively increasing threshold current, a
further decreasing slope efficiency with increasing pulse amplitude, and finally a highly nonlinear P–I characteristic after the
2350-V pulse. The I–V characteristic before degradation [full
line in Fig. 10(b)] shows a sharp inset of avalanche breakdown
at a reverse bias voltage of 15 V, low reverse bias currents
( 1 nA) below breakdown, and an exponential forward current
regime holding over more than seven orders of magnitude of
forward bias current. Before degradation, an ideality factor of
1.84 is found. Up to ESD pulse amplitudes of 2000 V, well
Authorized licensed use limited to: ACADEMIA SINICA COMPUTING CENTRE. Downloaded on February 28,2010 at 08:37:20 EST from IEEE Xplore. Restrictions apply.
236
IEEE JOURNAL ON SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 7, NO. 2, MARCH/APRIL 2001
(a)
(b)
Fig. 10. (a) Optical P–I and (b) I–V characteristics before and at different steps
during forward bias ESD step stress test of a VCSEL from manufacturer B.
above the inset of optical degradation at 1550 V, no modifications of the I–V characteristics could be detected, except of small
current variations for reverse bias voltages below 4 V. These
modifications below 4-V bias voltage are due to charging effects, which can be deduced by the fact that the minimum current is slightly displaced from 0 V and additionally from the fact
that a hysteresis in the I–V characteristic has been observed also
in this region [not shown in Fig. 10(b)]. After the application
of the 2200-V pulse we do not yet see any modifications in
the forward bias regime, but we observe a small increase of the
reverse bias current directly before the inset of avalanche multiplication. This increase is probably due to a lowering of the
inset of electron tunneling directly from the valence band into
the conduction band. After the 2350-V pulse this inset of tunneling already starts at about 8 V, and also a modification of
the forward bias characteristics can be observed, resulting in an
increase of the ideality factor to a value of 2.32. Such an increase
is indicative of an enhancement of nonradiative recombination
processes. No change of the avalanche breakdown voltage is observed. In Fig. 11 the optical emission spectra for a laser current
of 15 mA before and after ESD-induced degradation is shown.
Fig. 11. Room-temperature optical emission spectra of a VCSEL from
manufacturer B, measured at a bias current of 15 mA before and after forward
bias ESD stress test.
The laser emission exhibits a wide multimode spectrum with a
peak wavelength of about 838 nm; relevant changes after the
2200-V ESD pulse application cannot be observed.
The degradation of the type B VCSELs during reverse bias
ESD step stress test results initially in a decrease of the slope
efficiency [see Fig. 12(a) after the 900-V ESD pulse] and successively in an increase of the laser threshold current value [e.g.,
after the 1500-V pulse shown in Fig. 12(a)]. As for the type
A VCSEL, and also for the type B devices after reverse bias
ESD-induced degradation, we observe in the I–V characteristics [see Fig. 12(b)] the inset of avalanche multiplication at progressively lower reverse bias voltages. Furthermore, we see the
formation of a parallel leakage current path leading to a substantial increase of the current in the whole reverse bias voltage
regime as well as for small forward bias voltages [ 0.6 V, as
shown in the inset of Fig. 12(b)]. As can be seen in Fig. 13,
where the monitoring results of the optical power during eight
different ESD step stress tests of type B VCSELs are shown, the
damage threshold pulse amplitude value for degradation is very
reproducible. The inset of degradation is always found around
1200 V during positive pulse stress (five samples) and around
800 V during negative pulse ESD stress (three samples). This
confirms that the obtained damage threshold values are representative for this type of device, despite the limited number of
investigated samples in this study. The same has been found for
the values obtained during step stress of the type A VCSELs.
C. Optical Emission During Positive Pulse ESD Stress
As mentioned before, during positive ESD pulse application
we measured the electrical current transients during pulsing in
addition to the optical emission transients. In Fig. 14(a), the development of this transient for different amplitude values, all
below degradation threshold, is shown for the case of the type
A VCSELs. It can be observed that at 100 V, the resulting optical transient is monotonically decaying, basically following
the exponentially decaying pulse current (see also Fig. 1). For
the transients with higher pulse amplitudes we observe a first
peak within 10 ns after the ESD pulse start, followed by a second
Authorized licensed use limited to: ACADEMIA SINICA COMPUTING CENTRE. Downloaded on February 28,2010 at 08:37:20 EST from IEEE Xplore. Restrictions apply.
NEITZERT et al.: SENSITIVITY OF PROTON IMPLANTED VCSELs
237
(a)
(a)
(b)
(b)
Fig. 12. (a) Optical P–I and (b) I–V characteristics before and at different steps
during reverse bias ESD step stress test of a VCSEL from manufacturer B.
Fig. 13. Optical power at a given bias current, normalized to the power value
before ESD testing, measured during ESD forward bias (five devices) and
reverse bias (three devices) step stress tests of VCSELs from manufacturer B.
peak at about 15 ns after the start of the ESD pulse. The optical emission is completely suppressed during the 500-V pulse
after 200 ns, during the 1000-V pulse after about 120 ns, and
during the 1500-V pulse after 40 ns. In order to correct for the
Fig. 14. (a) Optical emission transients measured for different pulse amplitude
values during a forward bias ESD step stress test of a VCSEL from manufacturer
A and (b) ratio between the optical emitted power and the laser current during
ESD transients for different pulse amplitudes, obtained during a forward bias
ESD step stress test of a VCSEL from manufacturer A.
different electrical current values during the different pulses, we
plotted the ratio between the optical power and the electrical current as a function of the time for the above shown optical pulses
[see Fig. 14(b)]. This value is proportional to the quantum efficiency and should therefore, monotonically decrease with increasing junction temperature above 40 C. However, there is
not a simple linear relation between temperature and quantum
efficiency and, as mentioned before, there are strong differences
between the pulsed and CW optical power–laser current characteristics. Nevertheless, the development of the quantum efficiency during pulsing is related to the dynamics of the heating
of the VCSEL during exposure to the ESD pulses. At 100 V,
the quantum efficiency remains constant for the whole impulse
duration, indicating that no substantial heating of the junction
takes place. At 500 V, the quantum efficiency drops initially
very fast to 30% of its original value and decreases after the
above-mentioned second peak at 15 ns monotonically, but after
220 ns it is only reduced to 20% of its original value. With further increasing the pulse amplitude, a faster and faster decrease
of the quantum efficiency during the first 10 ns of the ESD pulse
is observed. The quantum efficiency approaches zero after 100
Authorized licensed use limited to: ACADEMIA SINICA COMPUTING CENTRE. Downloaded on February 28,2010 at 08:37:20 EST from IEEE Xplore. Restrictions apply.
238
IEEE JOURNAL ON SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 7, NO. 2, MARCH/APRIL 2001
Fig. 15. Comparison of the optical emission transients during the application
of 1000-V ESD pulses for a VCSEL from manufacturer A and an InGaAsP
MQW FP laser with an emission wavelength around 1300 nm.
+
ns for the 1000-V pulse and after 50 ns for the 1500-V pulse.
Other than for edge emitting lasers, for VCSELs are at minimum in the threshold current versus temperature characteristics in coincidence with a maximum in the quantum efficiency
versus temperature characteristics in order to obtain small variations of the optical power with varying ambient temperatures.
VCSELs are projected to have the minimum threshold current
at temperatures in the middle of the intended operating temperature range. It should be expected that for laser temperatures far
above this optimal temperature the quantum efficiency is monotonically decreasing with increasing laser temperature. A local
maximum in the quantum efficiency versus temperature relation
at relatively high temperatures—exceeding the maximum temperature induced during the 100-V ESD pulse—could explain
the second peak in the transients, but we have no explanation for
the physical origin of such a local maximum so far.
How do the optical transients of proton implanted VCSELs
compare with the optical transients during positive bias ESD
testing of edge-emitting lasers? In Fig. 15, we see two optical
transients, both taken during 1000-V ESD pulse application,
respectively, for a GaAs–GaAlAs VCSEL and for an MQW
InGaAsP FP laser. It can be noted that the suppression of the
optical emission during pulsing is much faster in the case
of the VCSELs, despite the fact that GaAs–GaAlAs-based
VCSELs have in general a better high temperature behavior
than InGaAsP-based edge-emitting lasers. This means that the
heating of the VCSEL junction during ESD pulse application
takes place on a very short time scale as compared to the edge
emitting FP lasers.
In Fig. 16, optical transients measured during a forward bias
ESD test of a type B VCSEL are shown. As in the case of the
type A VCSEL (see Fig. 14), the optical pulse amplitude is
monotonically increasing with increasing electrical pulse amplitude up to 1500 V, while the optical pulse duration is decreasing
due to the above-mentioned fast suppression of the optical emission by heating of the laser diode junction. For this type of
VCSEL, however, we do not observe the double peak as for the
type A VCSEL. Further increasing the ESD pulse amplitude, we
observe that the optical pulse duration does not change anymore,
Fig. 16. Optical emission transients measured for different electrical pulse
amplitude values during a forward bias ESD step stress test of a VCSEL from
manufacturer B.
Fig. 17. Optical power at 10-mA laser current (open circles) and pulse
amplitude of the optical emission transients (filled circles) as a function of the
ESD pulse amplitude, during a forward bias ESD step stress test of a VCSEL
from manufacturer B.
but the pulse amplitude starts to saturate at about 1700 V and
decreases after subsequent application of the 2250- and 2300-V
ESD pulses. In Fig. 17, the amplitude of the optical transients
measured during ESD step stress has been compared to the continuous current optical emission measured after the respective
pulse for a bias current of 10 mA. The small decrease of the optical emission after the 1500-V ESD pulse coincides with the
beginning of the saturation of the optical pulse amplitude. At
2000 V both the optical pulse amplitude and the continuous
emission at 10-mA laser current decrease. Later on they saturate and at 2250 V, both entities decrease sharply, indicating the
final degradation of the laser. It should be mentioned that also in
this case the beginning of the optical degradation preceded the
electrical degradation that started after the 2200-V ESD pulse
Authorized licensed use limited to: ACADEMIA SINICA COMPUTING CENTRE. Downloaded on February 28,2010 at 08:37:20 EST from IEEE Xplore. Restrictions apply.
NEITZERT et al.: SENSITIVITY OF PROTON IMPLANTED VCSELs
239
TABLE I
COMPARISON OF THE DAMAGE THRESHOLD PULSE AMPLITUDE VALUES AND THE CHANGE OF SOME IMPORTANT ELECTRICAL PARAMETERS DURING FORWARD
BIAS AND REVERSE BIAS ESD STEP STRESS FOR REPRESENTATIVE SAMPLES OF VCSELS FROM TWO DIFFERENT MANUFACTURERS
and that the VCSEL after the 2300-V pulse did not lase anymore. These results demonstrate that in situ measurements of
the optical transients during forward bias ESD step stress tests
enable a fast evaluation of the damage threshold value without
the necessity to measure the continuous current emitted power.
This may facilitate and accelerate the ESD testing of lasers, important for the evaluation of their robustness against high optical
power transients, that may also be present during normal operation due to short electrical overstress pulses.
V. DISCUSSION OF THE ESD TEST RESULTS
In Table I, an overview is given regarding the electrical pulse
amplitude values, corresponding to the beginning of the degradation of the different VCSELs and to the change of some key
parameters of the I–V characteristics during this degradation. In
particular the values of the diode ideality factor and the series resistance are reported, as obtained by fitting respectively the exponential region—for intermediate forward bias voltages—and
the linear region—at high forward currents—of the VCSEL’s
I–V characteristics. Also the values of the reverse bias breakdown
voltage before and after ESD-induced degradation are shown. It
is worth noting that the discussion will be focused on the qualitative development of these entities and their kinetics rather than
on the absolute values, because the degree of these changes in
some cases depend strongly on the instant when the ESD stress
test has been terminated. In all reported cases, we continued
the test until the emitted optical power at a given forward bias
has dropped to 50% of its original value and defined the corresponding value of the ESD pulse amplitude as the damage
threshold for the ESD-induced degradation of the device. In
Table I, however, we reported the pulse amplitude values corresponding to the beginning of the VCSEL degradation, respectively, for modifications of the optical and electrical laser properties. As already mentioned, during a positive bias ESD stress
test, the inset of the optical degradation starts earlier than the
inset of electrical degradation. Other common features for the
degradation of both VCSEL types during positive bias ESD include the fact that, in contrast to the negative bias degradation,
the reverse bias breakdown voltage does not change at all and
that we do not observe the formation of an additional leakage
current path parallel to the diode junction, even for heavily damaged VCSELs that did not lase any more after ESD stress [see
Figs. 3(b) and 10(b)]. However, we do observe an increased reverse bias current below the inset of avalanche breakdown that
is most probably due to an increase of the tunneling current.
In the development of the I–V characteristics during the negative bias ESD step stress [see Figs. 4(b) and 12(b)] we also observe initially the above-mentioned inset of tunneling but subsequently—after the 1300-V ESD pulse for the type A VCSEL
in Fig. 4(b) and after the 900-V pulse for the type B VCSEL in
Fig. 12(b)—a parallel shunt resistance is formed, as indicated by
the increase of the laser current in the whole reverse bias range
and for small forward bias voltages. Also, for both laser types
the reverse bias breakdown voltage is substantially lower after
the negative bias ESD stress compared to the breakdown voltage
of the unstressed lasers. Common to all VCSEL step stress tests
is the increase of the ideality factor and of the series resistance.
A possible explanation of the kinetics of the forward bias-induced degradation is the following: The high optical power
emitted during forward bias ESD transients leads to local
heating in the surface mirror region. This induces first an optical degradation, leading to enhanced absorption in this surface
region as evidenced for example by the initial decrease of the
quantum efficiency of the VCSELs. After further increasing
the forward bias stress amplitudes, the heating of the surface
regions induces finally the propagation of lateral surface near
defects, originating very probably from the proton implantation, toward the VCSEL center region. In a second step then
these defects change the electrical characteristics of the device,
as evidenced by the increase of both the diode ideality factor
and the series resistance. The high VCSEL series resistance is
mainly due to the tradeoff between optical and electrical optimization of the top p-DBR mirror region. A similar degradation
scheme has also been reported in literature during accelerated
lifetime testing of proton implanted VCSELs [15]–[17]. It
should be noted that, in this study, for reverse bias ESD stress
observed, decrease of the reverse bias breakdown voltage
during degradation has already been reported on FP GaAlAs
lasers [11] and may be due to inhomogeneous breakdown after
degradation resulting in locally enhanced electric fields.
In Table II, the typical VCSEL damage threshold values, as
presented in this study, are compared to the values of other
types of optoelectronic emitters that are used for local area networks and telecom fiberoptic access networks. The reported
values correspond to the ESD pulse amplitudes, where the op-
Authorized licensed use limited to: ACADEMIA SINICA COMPUTING CENTRE. Downloaded on February 28,2010 at 08:37:20 EST from IEEE Xplore. Restrictions apply.
240
IEEE JOURNAL ON SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 7, NO. 2, MARCH/APRIL 2001
TABLE II
COMPARISON OF THE DAMAGE THRESHOLD PULSE AMPLITUDE FOR DIFFERENT
TYPES OF OPTOELECTRONIC EMITTERS FOR ACCESS NETWORK APPLICATIONS
1900 V. The beginning of the optical degradation during
forward bias ESD can be attributed to a modification of the
upper p-DBR mirror region due to the excessive optical power
during the pulses, confirmed also by an inhomogeneous surface
morphology as visible after degradation.
From a comparison with ESD test results performed on various other optoelectronic emitters that are frequently used in
optical access network and local area network (LAN) applications, it can be concluded that negative and positive bias stress
ESD damage threshold values of the proton implanted VCSELs
are substantially lower than the values reported for 820- and
1300-nm LEDs and state-of-the-art 1300-nm FP laser diodes.
REFERENCES
tical emission decreased to 50% of its initial value. In particular we compared the VCSELs to light-emitting diodes based on
the GaAs–GaAlAs and InP–InGaAsP material systems [18], to
GaAs–GaAlAs-based FP lasers emitting at 780 nm [19], mainly
used for compact disc (CD) players, and to InP–InGaAsP-based
FP laser diodes emitting at 1300 nm. In the latter case we reported the values for commercial state-of-the-art lasers [19] that
had much improved results regarding forward bias ESD sensitivity compared to earlier reported values on other lasers of
this type [6]–[8]. It can be seen that the VCSELs under negative bias ESD stress are more sensitive to degradation then all
other emitters and under forward bias stress only the CD lasers
had still lower degradation threshold values. The low values
of the forward bias damage thresholds of both the CD lasers
and the VCSELs may also be due to the fact that in general
GaAs–GaAlAs-based devices are more sensitive to catastrophical optical damage (COD) than InP–InGaAsP-based devices
due to higher probability of dark line defect migration [20]. It
can therefore, be expected that the development of long-wavelength VCSELs, emitting at 1.30 and 1.55 m may also give
better performance regarding the ESD sensitivity. Due to the
lower optical power density compared to lasers, the above-mentioned problem is not so severe for light emitting diodes and
hence the 820-nm LEDs have a rather high damage threshold
under positive bias ESD stress.
VI. CONCLUSION
Monitoring of the ESD-induced modifications of the electrooptical properties of proton implanted GaAs–GaAlAs
VCSELs from two different manufacturers shows very similar
degradation behavior. In all cases, we found that the laser
diodes degraded at slightly lower values of the ESD pulse
amplitudes during reverse bias ESD stress as compared to
forward bias stress. Negative bias ESD stress resulted always
in a sudden increase of the reverse bias dark current and in the
same instant the power of the emitted light started to decrease.
The degradation process damage started at pulse amplitudes
between 800 and 1250 V. During forward bias ESD step
stress, we found that a degradation of the optical emission
properties preceded the electrical degradation and the inset of
degradation has been determined to be between 1500 and
[1] F. R. Beyette, K. M. Geib, C. M. St. Clair, S. A. Feld, and C. W. Wilmsen,
“Optoelectronic exclusive-OR using hybrid integration of phototransistors and vertical cacity surface emitting lasers,” IEEE Photon. Technol.
Lett., vol. 5, pp. 1322–1324, 1993.
[2] R. A. Morgan, J. A. Lehman, M. K. Hibbs-Brenner, Y. Liu, and J. P.
Bristow, “Vertical-cavity surface-emitting lasers: The applications,”
SPIE Proc., vol. 3004, pp. 91–103, 1997.
[3] A. V. Krishnamoorthy, L. M. F. Chirovsky, W. S. Hobson, J. Lopata, J.
Shah, R. Rozier, J. E. Cunningham, and L. A. D’Asaro, “Small-signal
characteristics of bottom-emitting intracavity contacted VCSELs,”
IEEE Photon. Technol. Lett., vol. 12, pp. 609–611, 2000.
[4] U. Fiedler, G. Reiner, P. Schnitzer, and K. J. Ebeling, “Top surface-emitting vertical-cavity laser diodes for 10-Gb/s data transmission,” IEEE
Photon. Technol. Lett., vol. 8, pp. 746–748, 1996.
[5] R. A. Hawthorne, III, J. K. Guenter, D. N. Granville, M. K. HibbsBrenner, and R. A. Morgan, “Reliability study of 850 nm VCSEL’s
for data communications,” in Proc. 34th IEEE Rel. Symp., 1996, pp.
203–210.
[6] L. F. DeChiaro and B. A. Unger, “Degradation in InGaAsP semiconductor lasers resulting from human body model ESD,” J. Electrostat.,
vol. 29, pp. 227–250, 1993.
[7] J. Wallon, G. Terol, B. Bauduin, and P. Devoldére, “Sensitivity to electrostatic discharges of ‘low-cost’ 1.3 m laser diodes: A comparative
study,” Mat. Sci. Eng., vol. 28, pp. 314–318, 1994.
[8] Y. Twu, L. S. Cheng, S. N. G. Chu, F. R. Nash, K. W. Wang, and P.
Parayanthal, “Semiconductor laser damage due to human-body-model
electrostatic discharge,” J. Appl. Phys., vol. 74, pp. 1510–1519, 1993.
[9] J. Jeong, K. H. Park, and H. M. Park, “Wavelength shifts of 1.5 m
DFB lasers due to human-body-model electrostatic discharge followed
by accelerated aging experiments,” J. Lightwave Technol., vol. 13, pp.
186–190, 1995.
[10] H. C. Neitzert and A. Piccirillo, “Comparison of the electrostatic discharge sensitivity of 1300 nm lasers and light emitting diodes,” in Proc.
23rd Workshop Compound Semiconductors Devices and Integrated Circuits (WOCSDICE), Chantilly, France, May 26–28, 1999, pp. 43–44.
[11] S. P. Sim and M. J. Robertson, “Catastrophic and latent damage in
GaAlAs lasers caused by electrical transients,” J. Appl. Phys., vol. 55,
no. 11, pp. 3950–3955, 1984.
[12] T. E. Sale, J. S. Roberts, J. P. R. David, R. Grey, J. Woodhead, and P. N.
Robson, “Temperature effects in VCSELs,” SPIE Proc., vol. 3003, pp.
100–110, 1997.
[13] G. Hasnain, K. Tai, L. Yang, Y. H. Wang, R. J. Fischer, J. D. Wynn, B.
Weir, N. K. Dutta, and A. Y. Cho, “Performance of gain-guided surface
emitting lasers with semiconductor distributed Bragg reflectors,” IEEE
J. Quantum Electron., vol. 27, pp. 1377–1385, 1991.
[14] “Sensitivity testing, human body model, component level,” ESD Association, Rome, ESD STM5.1, 1998.
[15] R. W. Herrick, Y. M. Cheng, J. M. Beck, P. M. Petroff, J. W. Scott, M.
G. Peters, G. D. Robinson, L. A. Coldren, R. A. Morgan, and M. K.
Hibbs-Brenner, “Analysis of VCSEL degradation modes,” SPIE Proc.,
vol. 2683, pp. 123–132, 1996.
[16] W. Jiang, C. Gaw, P. Kiely, B. Lawrence, M. Lebby, and P. R. Claisse,
“Effect of proton implantation on the degradation of GaAs/GaAlAs vertical cavity surface emitting lasers,” Electron. Lett., vol. 33, pp. 137–139,
1997.
[17] Y. M. Cheng, R. Herrick, P. M. Petroff, M. K. Hibbs-Brenner, and R. A.
Morgan, “Degradation mechanisms of vertical cavity surface emitting
lasers,” in Proc. 34th IEEE Rel. Symp., 1996, pp. 211–213.
Authorized licensed use limited to: ACADEMIA SINICA COMPUTING CENTRE. Downloaded on February 28,2010 at 08:37:20 EST from IEEE Xplore. Restrictions apply.
NEITZERT et al.: SENSITIVITY OF PROTON IMPLANTED VCSELs
[18] H. C. Neitzert and A. Piccirillo, “Sensitivity of multimode bidirectional
optoelectronic modules to electrostatic discharges,” Microelectron. Rel.,
vol. 39, pp. 1863–1871, 1999.
[19] H. C. Neitzert, , unpublished results.
[20] R. G. Waters, “Diode laser degradation mechanisms: A review,” Prog.
Quantum Electron., vol. 15, pp. 153–174, 1991.
Heinz-Christoph Neitzert was born in Mayen,
Germany, in 1957. He received the M.Sc. degree
in electrical engineering from the RWTH Aachen,
Germany, in 1985, and the Ph.D. degree from the
TU Berlin, Germany, in 1991. His Ph.D. work
focused on the in situ characterization of the thin
film semiconductor growth by microwave reflection
measurements.
After holding post-doctoral positions 1992 at the
Ecole Polytechnique, Palaiseau, France and from
1993 and 1994 at the Centro Studi e Laboratori
Telecommunicazioni (CSELT), Torino, Italy, he joined CSELT in Torino,
Italy, in 1994, as a Senior Researcher and worked there until 1998 in the
Fiberoptic Device Reliability Group. In 1998, he was appointed Associate
Professor in the Electrical Engineering Department of Salerno University in
Fisciano, Italy, where he is currently teaching electronic devices and circuit
theory. His research interests include the development of new techniques
for semiconductor material and device characterization, photovoltaics, and
nonlinear optics. He holds a patent and has co-authored more than 50 papers.
Dr. Neitzert is a member of the German Physical Society (DPG), the IEEE
Lasers and Electro-Optics Society, and is currently secretary of the Central and
South Italy chapter of the IEEE Electron Device Society.
241
Agnese Piccirillo received the degree in physics from
the University of Turin, Italy.
She joined CSELT, Centro Studi e Laboratori
Telecommunicazioni in 1969. Initially, she worked
in the Mathematic and Traffic Division for the
definition of mathematical models for the telephone
network, then she moved to the Reliability Department of the Technological Division, where she
was involved in silicon and optoelectronic devices
reliability physics. From 1986 until 1992, she was
in charge of the Microelectronics Technology Group
and from 1992 until 1996 the Optical Receiver and Amplifier Device Group.
Since 1997, she has been involved with component and system reliability
studies. She has authored papers in both silicon and optoelectronic devices
reliability and technology studies.
Mrs. Piccirillo is a member of the IEC SC86B/WG5 and IEC SC86C/WG4
for passive and active optical semiconductor components.
Barbara Gobbi received the degree in electronical
engineering from the Politecnico di Torino, Torino,
Italy, in 1998.
She worked until 1999 at CSELT, Centro Studi e
Laboratori Telecomunicazioni on the reliability of
VCSELs. Since 1999, she has been with the ARTIS
Company, Torino, working in the field of computer
simulation of optical networks.
Authorized licensed use limited to: ACADEMIA SINICA COMPUTING CENTRE. Downloaded on February 28,2010 at 08:37:20 EST from IEEE Xplore. Restrictions apply.