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ESD sensitivity of AlGaAs and InGaAsP based Fabry–Perot laser diodes
Microelectronics Reliability 50 (2010) 1563–1567
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
ESD sensitivity of AlGaAs and InGaAsP based Fabry–Perot laser diodes H.C. Neitzert * Department of Electronics (DIIIE), Salerno University, Via Ponte Don Melillo 1, 84084 Fisciano (SA), Italy
a r t i c l e
i n f o
Article history: Received 2 July 2010 Accepted 16 July 2010 Available online 8 August 2010
a b s t r a c t The sensitivity to electrostatic discharges of Fabry–Perot laser diodes with InGaAsP as active layer material has been tested and compared to Fabry–Perot lasers based on AlGaAs as active layer material. In the case of the forward-bias ESD pulses we observed a substantially lower degradation threshold voltage for the AlGaAs type lasers as compared to the InGaAsP type lasers. A detailed analysis of the optical and electrical parameters before and after ESD test with particular emphasis on the characteristic temperature and optical emission spectra changes has been done. Effective suppression of the optical emission on a ns-time scale due to device heating during the forward-bias ESD pulses has been evidenced by monitoring the light emission during ESD pulses. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction InGaAsP based laser diodes emitting around 1300 nm and 1550 nm are the standard devices for long- and medium-haul fiber-optical links. Electrostatic discharge (ESD) related degradation is one of the main obstacles regarding the reliability of these devices. There is a wide spread in the ESD damage threshold values reported in literature for 1300 nm lasers, ranging from very low values between 0.5 and 1.6 kV [1,2] up to more than 12 kV [3]. Often a lower damage threshold value is found during reverse-bias ESD as compared to forward-bias ESD tests [3,4]. This is easily explained by the fact, that the current distribution under reverse-bias breakdown conditions is strongly inhomogeneous, leading to localized high current densities. On the other hand do we have to consider under forward-bias conditions potentially an additional failure mechanism, namely the facet degradation due to excessive optical power, leading to the well-known catastrophical optical damage (COD). Low-power AlGaAs based laser diodes are low-cost devices, that play, regarding fiber-optical data links, an important role in short-haul communication systems. Comparing both material systems, it can be stated, that AlGaAs based lasers have a better temperature behaviour, for example regarding the increase of the threshold current and the decrease of the differential quantum efficiency with increasing temperature. Higher characteristic temperature (T0) values have been reported for AlGaAs based devices as compared to InGaAsP based lasers [5]. A positive consequence regarding the device reliability of the lower T0 values and of the stronger decrease of the differential quantum efficiency with
* Tel.: +39 089 964304; fax: +39 089 964218. E-mail address: [email protected] 0026-2714/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2010.07.094
increasing temperature of InGaAsP based lasers is the suppression of excessive optical powers during forward-bias high voltage pulses. This suppression has been clearly shown by monitoring the optical power transients during positive high voltage ESD pulses [3,6]. It should also be noted that an increasing catastrophical optical damage level has been found for InP/InGaAsP based lasers, as compared to GaAs/AlGaAs based devices due to the higher probability of dark line defect migration in the latter case [7]. 2. Experimental The electrostatic discharge tests have been performed using a human body model (HBM) type commercial ESD tester. The discharge current has been sensed by an inductive current probe and in the case of the optical emitter characterization under forward bias stress also the resulting optical emission transients during the ESD tests have been measured using a fast amplified photodiode. In some experiments both optical and electrical transients have been recorded during each forward-bias ESD. ESD pulses with successively increasing pulse amplitudes have been applied. In general for each pulse amplitude we exposed the device under test to one ESD pulse. The pulse amplitude step width has been varied depending on the degradation threshold pulse amplitude to be expected. For all devices the dark current–voltage (I–V) characteristics has been measured in discrete intervals using a source measurement unit. The optical power vs. bias current (P–I) characteristics has been also measured in discrete intervals. In order to identify the start of the degradation of the devices, after each pulse the current at a given reverse bias voltage and additionally the emitted optical power at a given bias current have been monitored. As a failure criterion we used either the increase of the reverse bias dark current in one region of the I–V characteristics over
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In the case of the Fabry–Perot (FP) InGaAsP/InP laser diodes, commercial 1300 nm MiniDIL housed devices with a maximum bitrate of 622MBIT/s with incorporated InGaAs monitor diodes have been chosen. The devices have been designed for extended temperature operation and a characteristic temperature (T0) of 81 K has been measured. Six of theses lasers have been tested under forward-bias conditions and six devices under reverse bias conditions up to a maximum pulse amplitude of 17 kV. As a comparison commercial 780 nm FP-lasers with selfaligned-MBE (SAM) structure with AlGaAs active layer and with silicon monitor diodes have been chosen.
4. Results and discussion 4.1. Degradation of InGaAsP/InP based devices
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wavelength (nm) Fig. 2. I–V (upper figure) and P–I characteristics (center figure) and optical emission spectra (lower figure) of an InGaAsP laser diode measured before and after application of a forward-bias ESD pulse of 15,500 V amplitude.
Besides the electro-optic characterization after each ESD pulse, we registrated, as mentioned before, also the optical emission transients during the pulses. In Fig. 3 typical transients, as recorded during the laser ESD stress test, characterized in Figs. 1 and 2, are shown. For low pulse amplitude values, here represented by the +500 V pulse, the optical transient amplitude is increasing linearly with increasing electrical pulse amplitude and the decay is approxi-
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In the case of the 1300 nm laser diodes we found a very asymmetric behaviour, regarding the ESD damage threshold voltages for forward and reverse bias stressing. In the case of the forward biased ESD stressed laser diodes, there was no degradation found below 13 kV and some devices survived even the maximum ESD voltage of 17 kV. In Fig. 1 the values of the reverse bias current at an applied voltage of 1 V and the emitted optical power monitoring for a laser current of 30 mA are shown as function of the ESD pulse amplitude for a laser diode, that failed at 15,500 V. Besides small variations of the emitted optical power for ESD-amplitudes below 7000 V, no degradation has been found up to an amplitude value of 15,500 V, where a sudden optical power decrease of more than three orders of magnitude and a reverse bias current increase of again more than three orders of magnitude is observed. The electrical and optical characteristics of this laser diode, before and after ESD-induced degradation are shown in Fig. 2. The current–voltage (I–V) characteristics change indicates no modifications at high forward bias and the development of a shunt resistance parallel to the laser diode. The optical power vs. current (P–I) characteristics, displayed on a logarithmic scale, shows that after the 15,500 V pulse application, no laser threshold (before degradation at 20 mA) is observed any more. The optical spectra before and after the positive ESD stress confirm, that after degradation stimulated emission is absent. The peak of the optical emission decreased strongly from 1302 nm to a value of about 1270 nm.
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time (ns) Fig. 3. Optical emission transients, measured during application of positive bias ESD pulses with four different pulse amplitudes (as indicated in the figure) to an InGaAsP laser diode, emitting at 1300 nm.
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mately following the exponentially decaying ESD pulse current. For higher ESD pulse amplitudes, the optical pulse amplitudes saturate. As an example in Fig. 4 we see that the 10,000 V optical pulse has only a 3 higher amplitude than the 500 V pulse. Additionally it is decaying much faster due to strong device heating within few nanoseconds. This efficient light suppression may effectively save the laser from catastrophical optical damage (COD) during ESD pulses. A second optical emission peak is observed, starting about 350 ns after the start of the 10,000 V ESD pulse. It is due to the – in the meantime – partially cooled device, that can emit light again. Compare similar results obtained on other InP based laser diodes in [3,6]. The subsequent decrease of the second optical emission peak is following the ESD pulse current. This is shown more clearly in Fig. 4, where for the case of the 10,000 V ESD pulse, electrical and optical pulses are plotted together. The electrical signal amplitude of about 7A is consistent with the HBM tester equivalent circuit, consisting of a 100pF capacitor that is discharged during the pulse through the series connection of the DUT (assumed for high voltages as a short circuit) and a 1.5 kX resistor. The calculated time constant of the exponential decay is also consistent with the measured current pulse. Increasing the pulse amplitude further, we see in the case of the next optical pulse (15,000 V trace in Fig. 3), that the initial signal amplitude is not increasing anymore, the first pulse is decaying even faster within 20 ns and the second optical peak is now absent. This means, that the cooling time down to reasonable temperatures is now longer than 600 ns, a time, where the ESD pulse current is almost completely vanished. Finally, the last pulse, with an amplitude of 15,500 V, results in a very small optical transient signal, due to device damaging, as evidenced for this ESD pulse amplitude value in Figs. 1 and 2. As already shown for the case of the monitoring of ESD-testing of VCSELs [8] and of LEDs [9], the optical pulse monitoring during forward-bias ESD tests of laser diodes gives a clear indication of device failure without the need of more time consuming other electro-optical characterizations after every ESD pulse. Under reverse-bias ESD stress, typical damage thresholds between 2400 V and 2600 V have been found for all six investigated InGaAs/InP lasers. In Fig. 5 we see, that also under reverse bias conditions, electrical and optical degradation start together. In the presented case, no degradation is visible for ESD pulse amplitudes as high as 2400 V. After a 2500 V pulse the optical power at 30 mA decreases for more than two orders of magnitude and after application of a pulse with 2600 V amplitude the optical power decreases to a five orders of magnitude lower value than before ESD-testing. The P–I characteristics (see Fig. 6b) shows, that the device after the 2500 V pulse is still lasing, but that the lasing threshold current at room temperature doubled from 20 mA to 40 mA. The optical emission spectra in Fig. 6c shows a strong blue
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ESD-Amplitude (V ) Fig. 5. Monitoring of the reverse bias current at 1 V and of the optical emitted power at 30 mA, measured after every step of the positive bias ESD pulses with increasing amplitudes of an InGaAsP laser diode.
shift of the optical emission, with the peak wavelength shifted for about 10 nm after the 2500 V pulse. After the 2600 V pulse, again the lasing is completely suppressed. The resulting broad emission spectrum is shown in Fig. 6d. Interestingly, now two distinct peaks, one at 1305 nm in coincidence with the maximum of the laser emission after the 2500 V peak and one at a much lower wavelength of about 1270 nm, are observed. This indicates a non homogeneous degradation of the active layer. Regarding the changes in the electrical characteristics with increasing ESD pulse amplitude (see Fig. 6a) we observe again the gradual formation of a shunt resistance and basically no modification of the I–V characteristics in the high forward bias current regime. It should also be noted, that after forward bias degradation, the T0 values decreased from typical values of 80 K before degradation down to values around 60 K after degradation. This may be an indication, that part of the strong decrease of the optical emission efficiency is due to an increase of non-radiative recombination or due to partial loss of the charge-carrier confinement within the active layers. As an intermediate conclusion, we observe during both, forward and reverse bias degradation, contemporarily a worsening of the optical and the electrical characteristics. Only that this modifications occur at much lower ESD pulse amplitudes in the case of the reverse bias testing as compared to the forward bias testing. In literature the defect formation at the periphery of the active area during the reverse-bias ESD-induced degradation of GaInAsP/InP laser diodes has been reported and by comparison with simulation results it has been stated, that the local concentration of the electric field in this region is the main reason for the degradation [10]. Also a recent study on InP based buried-heterostructure DFB lasers found a clear asymmetry in the forward/reverse-bias ESD thresholds, with reverse ESD-amplitude degradation thresholds between 2.4 kV and 4 kV and no degradation at all under forward-bias ESD stress up to 5 kV [11]. The authors did not test their lasers at still higher ESD pulse amplitudes. As an alternative active layer material for 1300 nm lasers, AlGaInAs/InP has been proposed for applications, were good temperature stability is an important issue [12]. With lasers, based on this material system, T0 values higher than 90 K have been achieved [12] and numerical simulations indicate that values up to 109 K are feasible [13]. Forward-bias ESD tests on this type of lasers, however, indicate very low damage threshold levels between 500 V and 1500 V [14]. A detailed failure analysis showed, that the degradation started in the region of the highest photon density and is hence optically initiated [15]. It should be noted, that the authors choose a purely electrical criterion for damage threshold determination [14]. In this way, however, optical degradation, that preceeds the electrical degradation is not detected. An earlier optical degradation than electrical degradation has been for example reported during ESD tests on VCSELs [16].
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wavelength (nm) Fig. 6. (a) I–V, (b) P–I characteristics and (c and d) optical emission spectra of an InGaAsP laser diode, measured before and after application of successive reversebias ESD pulses with increasing pulse amplitude (as indicated).
4.2. Degradation of AlGaAs/GaAs based devices A complete different picture emerges from the ESD-induced degradation of the AlGaAs lasers, emitting around 780 nm. Here we find a lower forward bias damage threshold with values between 900 V and 1500 V as compared to the reverse-bias value between 3500 V and 4000 V. In Fig. 7 we see for example the slight shift of the threshold currents and a slight decrease of the differential quantum efficiency, but surprisingly no decrease of the characteristic temperature (124 K before degradation and 122 K after degradation, as shown in Fig. 8).
An analysis of the changes of the emission spectra (Fig. 9) revealed no clear 1red-or blue-shift after degradation, but an pronounced increase of the wavelength dependence on ambient temperature instead. The influence of the reverse-bias ESD stress on the laser current – optical power characteristics for the laser diode with AlGaAs active layer is shown in Fig. 10. Instead of the laser power, we plotted, as in Fig. 7, the monitor diode photocurrent (Im). Similar as in the case of the forward-bias ESD stress, we observe a slight increase of the lasing threshold current and a decrease of the differential quantum efficiency, but now the damage threshold is found, as mentioned before, at a higher ESD pulse amplitude as compared to the case of the forward-bias ESD stress.
1 For interpretation of color in Fig. 9, the reader is referred to the web version of this article.
normalized optical power (dB)
H.C. Neitzert / Microelectronics Reliability 50 (2010) 1563–1567
The corresponding forward bias current–voltage characteristics, measured before degradation and after application of the 3700 V ESD pulse (see Fig. 11), does not evidence any electrical degradation. It should be mentioned, that in this case also the reverse-bias breakdown voltage did no change at all after ESD-induced degradation. A sharp, avalanche type breakdown has been observed at 10.30 V.
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wavelength (nm) Fig. 9. Normalized optical emission spectra, measured at three different temperatures before and after degradation of an AlGaAs laser diode during forward-bias ESD tests after degradation at +1500 V.
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ESD degradation tests under Human Body model conditions of Fabry–Perot laser diodes with different active layer material revealed that in AlGaAs lasers lower ESD damage thresholds under forward bias have been observed in contrast to InGaAsP based lasers. In the latter case a very good ESD stability under forward bias has been measured and assigned to a self-protection mechanism due to the lower characteristic temperature values. Monitoring the optical emitted pulses during forward-bias ESD step-stress tests is a good measure for the evaluation of the device heating during ESD pulses and enables a faster determination of the device failure threshold amplitudes than classical electro-optic laser characterization after each ESD pulse application.
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References
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[1] DeChiaro LF, Unger BA. Degradation in InGaAsP semiconductor lasers resulting from human body model ESD. J Electrostat 1993;29:227–50. [2] Wallon J, Terol G, Bauduin B, Devoldère P. Sensitivity to electrostatic discharges of ‘‘low-cost” 1.3 lm laser diodes: a comparative study. Mater Sci Eng B 1994;28:314–8. [3] Twu Y, Cheng LS, Chu SNG, Nash FR, Wang KW, Prayanthal P. Semiconductor laser damage due to human-body-model electrostatic discharge. J Appl Phys 1993;74:1510–20. [4] Ichikawa H, Kumagai A, Hamada K, Yamaguchi A, Nakabayashi T. Analysis on reverse electrostatic discharge induced degradation of GaInAsP/InP laser diodes. In: Proceedings of the 2005 IPRM Conf, Glasgow; 2005. p. 629–32. [5] Fukuda M. Reliability and degradation of III–V optical devices. Boston: Artech House; 1991. [6] Vanzi M, Giansante M, Zazzetti L, Magistrali F, Sala D, Salmini G, Fantini F. Interpretation of failure analysis results on ESD-damaged InP laser diodes. In: Proceedings of ISTFA’91. Los Angeles; 1991. p. 305–14. [7] Waters RG. Diode laser degradation mechanisms: a review. Prog Quant Electron 1990;15:153–74. [8] Neitzert HC, Gobbi B, Piccirillo A. Sensitivity of proton implanted VCSELs to electrostatic discharge pulses. IEEE J Select Top Quant Electron 2001;7:231–41. [9] Neitzert HC, Picirillo A. Sensitivity of multimode bidirectional optoelectronic modules to electrostatic discharges. Microelectron Reliab 1999;39:1863–71. [10] Ichikawa H, Kumaga A, Hamada K, Yamaguchi A, Nakabayashi T. Analysis of reverse-biased electrostatic-discharge-induced degradation of GaInAsP/InPburied heterostructure laser diode. Jpn J Appl Phys 2009;48:022201. [11] Huang J-S, Olson T, Isip E. Human-body-model electrostatic-discharge and electrical-overstress studies of buried-heterostructure lasers. J Appl Phys 2009;106:083101. [12] Savonlainen P, Toivonen M, Orsila S, Saarinen M, Melanen P, Vilokkinen V, et al. AlGaInAs/InP strained-layer quantum well lasers at 1.3 m grown by solid source molecular beam epitaxy. J Electron Mater 1999;28:980–5. [13] Hsieh S-W, Kuo Y-K. A numerical study of characteristic temperature of shortcavity 1.3-lm AlGaInAs/InP MQW lasers. Appl Phys A 2006;82:287–92. [14] Ichikawa H, Matsukawa S, Hamada K, Ikoma N, Nakabayashi T. Electrostaticdischarge-induced degradation of 1.3 lm AlGaInAs/InP buried heterostructure laser diodes. J Appl Phys 2009;106:083101. [15] Ichikawa H, Matsukawa S, Hamada K, Yamaguchi A, Nakabayashi T. Failure analysis of InP-based edge-emitting buried heterostructure laser diodes degraded by forward-biased electrostatic discharge tests. Jpn J Appl Phys 2009;49:052102. [16] Neitzert HC. ESD-induced degradation of vertical-cavity-surface-emitting lasers. Electron Lett 2000;36:1620–1.
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
ESD sensitivity of AlGaAs and InGaAsP based Fabry–Perot laser diodes H.C. Neitzert * Department of Electronics (DIIIE), Salerno University, Via Ponte Don Melillo 1, 84084 Fisciano (SA), Italy
a r t i c l e
i n f o
Article history: Received 2 July 2010 Accepted 16 July 2010 Available online 8 August 2010
a b s t r a c t The sensitivity to electrostatic discharges of Fabry–Perot laser diodes with InGaAsP as active layer material has been tested and compared to Fabry–Perot lasers based on AlGaAs as active layer material. In the case of the forward-bias ESD pulses we observed a substantially lower degradation threshold voltage for the AlGaAs type lasers as compared to the InGaAsP type lasers. A detailed analysis of the optical and electrical parameters before and after ESD test with particular emphasis on the characteristic temperature and optical emission spectra changes has been done. Effective suppression of the optical emission on a ns-time scale due to device heating during the forward-bias ESD pulses has been evidenced by monitoring the light emission during ESD pulses. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction InGaAsP based laser diodes emitting around 1300 nm and 1550 nm are the standard devices for long- and medium-haul fiber-optical links. Electrostatic discharge (ESD) related degradation is one of the main obstacles regarding the reliability of these devices. There is a wide spread in the ESD damage threshold values reported in literature for 1300 nm lasers, ranging from very low values between 0.5 and 1.6 kV [1,2] up to more than 12 kV [3]. Often a lower damage threshold value is found during reverse-bias ESD as compared to forward-bias ESD tests [3,4]. This is easily explained by the fact, that the current distribution under reverse-bias breakdown conditions is strongly inhomogeneous, leading to localized high current densities. On the other hand do we have to consider under forward-bias conditions potentially an additional failure mechanism, namely the facet degradation due to excessive optical power, leading to the well-known catastrophical optical damage (COD). Low-power AlGaAs based laser diodes are low-cost devices, that play, regarding fiber-optical data links, an important role in short-haul communication systems. Comparing both material systems, it can be stated, that AlGaAs based lasers have a better temperature behaviour, for example regarding the increase of the threshold current and the decrease of the differential quantum efficiency with increasing temperature. Higher characteristic temperature (T0) values have been reported for AlGaAs based devices as compared to InGaAsP based lasers [5]. A positive consequence regarding the device reliability of the lower T0 values and of the stronger decrease of the differential quantum efficiency with
* Tel.: +39 089 964304; fax: +39 089 964218. E-mail address: [email protected] 0026-2714/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2010.07.094
increasing temperature of InGaAsP based lasers is the suppression of excessive optical powers during forward-bias high voltage pulses. This suppression has been clearly shown by monitoring the optical power transients during positive high voltage ESD pulses [3,6]. It should also be noted that an increasing catastrophical optical damage level has been found for InP/InGaAsP based lasers, as compared to GaAs/AlGaAs based devices due to the higher probability of dark line defect migration in the latter case [7]. 2. Experimental The electrostatic discharge tests have been performed using a human body model (HBM) type commercial ESD tester. The discharge current has been sensed by an inductive current probe and in the case of the optical emitter characterization under forward bias stress also the resulting optical emission transients during the ESD tests have been measured using a fast amplified photodiode. In some experiments both optical and electrical transients have been recorded during each forward-bias ESD. ESD pulses with successively increasing pulse amplitudes have been applied. In general for each pulse amplitude we exposed the device under test to one ESD pulse. The pulse amplitude step width has been varied depending on the degradation threshold pulse amplitude to be expected. For all devices the dark current–voltage (I–V) characteristics has been measured in discrete intervals using a source measurement unit. The optical power vs. bias current (P–I) characteristics has been also measured in discrete intervals. In order to identify the start of the degradation of the devices, after each pulse the current at a given reverse bias voltage and additionally the emitted optical power at a given bias current have been monitored. As a failure criterion we used either the increase of the reverse bias dark current in one region of the I–V characteristics over
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H.C. Neitzert / Microelectronics Reliability 50 (2010) 1563–1567
In the case of the Fabry–Perot (FP) InGaAsP/InP laser diodes, commercial 1300 nm MiniDIL housed devices with a maximum bitrate of 622MBIT/s with incorporated InGaAs monitor diodes have been chosen. The devices have been designed for extended temperature operation and a characteristic temperature (T0) of 81 K has been measured. Six of theses lasers have been tested under forward-bias conditions and six devices under reverse bias conditions up to a maximum pulse amplitude of 17 kV. As a comparison commercial 780 nm FP-lasers with selfaligned-MBE (SAM) structure with AlGaAs active layer and with silicon monitor diodes have been chosen.
4. Results and discussion 4.1. Degradation of InGaAsP/InP based devices
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wavelength (nm) Fig. 2. I–V (upper figure) and P–I characteristics (center figure) and optical emission spectra (lower figure) of an InGaAsP laser diode measured before and after application of a forward-bias ESD pulse of 15,500 V amplitude.
Besides the electro-optic characterization after each ESD pulse, we registrated, as mentioned before, also the optical emission transients during the pulses. In Fig. 3 typical transients, as recorded during the laser ESD stress test, characterized in Figs. 1 and 2, are shown. For low pulse amplitude values, here represented by the +500 V pulse, the optical transient amplitude is increasing linearly with increasing electrical pulse amplitude and the decay is approxi-
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ESD-Amplitude (V) Fig. 1. Monitoring of the reverse bias current at 1 V and of the optical emitted power at 30 mA, measured after every step of the positive bias ESD pulses with increasing amplitudes of an InGaAsP laser diode.
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In the case of the 1300 nm laser diodes we found a very asymmetric behaviour, regarding the ESD damage threshold voltages for forward and reverse bias stressing. In the case of the forward biased ESD stressed laser diodes, there was no degradation found below 13 kV and some devices survived even the maximum ESD voltage of 17 kV. In Fig. 1 the values of the reverse bias current at an applied voltage of 1 V and the emitted optical power monitoring for a laser current of 30 mA are shown as function of the ESD pulse amplitude for a laser diode, that failed at 15,500 V. Besides small variations of the emitted optical power for ESD-amplitudes below 7000 V, no degradation has been found up to an amplitude value of 15,500 V, where a sudden optical power decrease of more than three orders of magnitude and a reverse bias current increase of again more than three orders of magnitude is observed. The electrical and optical characteristics of this laser diode, before and after ESD-induced degradation are shown in Fig. 2. The current–voltage (I–V) characteristics change indicates no modifications at high forward bias and the development of a shunt resistance parallel to the laser diode. The optical power vs. current (P–I) characteristics, displayed on a logarithmic scale, shows that after the 15,500 V pulse application, no laser threshold (before degradation at 20 mA) is observed any more. The optical spectra before and after the positive ESD stress confirm, that after degradation stimulated emission is absent. The peak of the optical emission decreased strongly from 1302 nm to a value of about 1270 nm.
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time (ns) Fig. 3. Optical emission transients, measured during application of positive bias ESD pulses with four different pulse amplitudes (as indicated in the figure) to an InGaAsP laser diode, emitting at 1300 nm.
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mately following the exponentially decaying ESD pulse current. For higher ESD pulse amplitudes, the optical pulse amplitudes saturate. As an example in Fig. 4 we see that the 10,000 V optical pulse has only a 3 higher amplitude than the 500 V pulse. Additionally it is decaying much faster due to strong device heating within few nanoseconds. This efficient light suppression may effectively save the laser from catastrophical optical damage (COD) during ESD pulses. A second optical emission peak is observed, starting about 350 ns after the start of the 10,000 V ESD pulse. It is due to the – in the meantime – partially cooled device, that can emit light again. Compare similar results obtained on other InP based laser diodes in [3,6]. The subsequent decrease of the second optical emission peak is following the ESD pulse current. This is shown more clearly in Fig. 4, where for the case of the 10,000 V ESD pulse, electrical and optical pulses are plotted together. The electrical signal amplitude of about 7A is consistent with the HBM tester equivalent circuit, consisting of a 100pF capacitor that is discharged during the pulse through the series connection of the DUT (assumed for high voltages as a short circuit) and a 1.5 kX resistor. The calculated time constant of the exponential decay is also consistent with the measured current pulse. Increasing the pulse amplitude further, we see in the case of the next optical pulse (15,000 V trace in Fig. 3), that the initial signal amplitude is not increasing anymore, the first pulse is decaying even faster within 20 ns and the second optical peak is now absent. This means, that the cooling time down to reasonable temperatures is now longer than 600 ns, a time, where the ESD pulse current is almost completely vanished. Finally, the last pulse, with an amplitude of 15,500 V, results in a very small optical transient signal, due to device damaging, as evidenced for this ESD pulse amplitude value in Figs. 1 and 2. As already shown for the case of the monitoring of ESD-testing of VCSELs [8] and of LEDs [9], the optical pulse monitoring during forward-bias ESD tests of laser diodes gives a clear indication of device failure without the need of more time consuming other electro-optical characterizations after every ESD pulse. Under reverse-bias ESD stress, typical damage thresholds between 2400 V and 2600 V have been found for all six investigated InGaAs/InP lasers. In Fig. 5 we see, that also under reverse bias conditions, electrical and optical degradation start together. In the presented case, no degradation is visible for ESD pulse amplitudes as high as 2400 V. After a 2500 V pulse the optical power at 30 mA decreases for more than two orders of magnitude and after application of a pulse with 2600 V amplitude the optical power decreases to a five orders of magnitude lower value than before ESD-testing. The P–I characteristics (see Fig. 6b) shows, that the device after the 2500 V pulse is still lasing, but that the lasing threshold current at room temperature doubled from 20 mA to 40 mA. The optical emission spectra in Fig. 6c shows a strong blue
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ESD-Amplitude (V ) Fig. 5. Monitoring of the reverse bias current at 1 V and of the optical emitted power at 30 mA, measured after every step of the positive bias ESD pulses with increasing amplitudes of an InGaAsP laser diode.
shift of the optical emission, with the peak wavelength shifted for about 10 nm after the 2500 V pulse. After the 2600 V pulse, again the lasing is completely suppressed. The resulting broad emission spectrum is shown in Fig. 6d. Interestingly, now two distinct peaks, one at 1305 nm in coincidence with the maximum of the laser emission after the 2500 V peak and one at a much lower wavelength of about 1270 nm, are observed. This indicates a non homogeneous degradation of the active layer. Regarding the changes in the electrical characteristics with increasing ESD pulse amplitude (see Fig. 6a) we observe again the gradual formation of a shunt resistance and basically no modification of the I–V characteristics in the high forward bias current regime. It should also be noted, that after forward bias degradation, the T0 values decreased from typical values of 80 K before degradation down to values around 60 K after degradation. This may be an indication, that part of the strong decrease of the optical emission efficiency is due to an increase of non-radiative recombination or due to partial loss of the charge-carrier confinement within the active layers. As an intermediate conclusion, we observe during both, forward and reverse bias degradation, contemporarily a worsening of the optical and the electrical characteristics. Only that this modifications occur at much lower ESD pulse amplitudes in the case of the reverse bias testing as compared to the forward bias testing. In literature the defect formation at the periphery of the active area during the reverse-bias ESD-induced degradation of GaInAsP/InP laser diodes has been reported and by comparison with simulation results it has been stated, that the local concentration of the electric field in this region is the main reason for the degradation [10]. Also a recent study on InP based buried-heterostructure DFB lasers found a clear asymmetry in the forward/reverse-bias ESD thresholds, with reverse ESD-amplitude degradation thresholds between 2.4 kV and 4 kV and no degradation at all under forward-bias ESD stress up to 5 kV [11]. The authors did not test their lasers at still higher ESD pulse amplitudes. As an alternative active layer material for 1300 nm lasers, AlGaInAs/InP has been proposed for applications, were good temperature stability is an important issue [12]. With lasers, based on this material system, T0 values higher than 90 K have been achieved [12] and numerical simulations indicate that values up to 109 K are feasible [13]. Forward-bias ESD tests on this type of lasers, however, indicate very low damage threshold levels between 500 V and 1500 V [14]. A detailed failure analysis showed, that the degradation started in the region of the highest photon density and is hence optically initiated [15]. It should be noted, that the authors choose a purely electrical criterion for damage threshold determination [14]. In this way, however, optical degradation, that preceeds the electrical degradation is not detected. An earlier optical degradation than electrical degradation has been for example reported during ESD tests on VCSELs [16].
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Fig. 7. Monitor current–laser current characteristics, measured at three different temperatures before (full lines) and after (broken lines) degradation of an AlGaAs laser diode during forward-bias ESD tests up to 1500 V.
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wavelength (nm) Fig. 6. (a) I–V, (b) P–I characteristics and (c and d) optical emission spectra of an InGaAsP laser diode, measured before and after application of successive reversebias ESD pulses with increasing pulse amplitude (as indicated).
4.2. Degradation of AlGaAs/GaAs based devices A complete different picture emerges from the ESD-induced degradation of the AlGaAs lasers, emitting around 780 nm. Here we find a lower forward bias damage threshold with values between 900 V and 1500 V as compared to the reverse-bias value between 3500 V and 4000 V. In Fig. 7 we see for example the slight shift of the threshold currents and a slight decrease of the differential quantum efficiency, but surprisingly no decrease of the characteristic temperature (124 K before degradation and 122 K after degradation, as shown in Fig. 8).
An analysis of the changes of the emission spectra (Fig. 9) revealed no clear 1red-or blue-shift after degradation, but an pronounced increase of the wavelength dependence on ambient temperature instead. The influence of the reverse-bias ESD stress on the laser current – optical power characteristics for the laser diode with AlGaAs active layer is shown in Fig. 10. Instead of the laser power, we plotted, as in Fig. 7, the monitor diode photocurrent (Im). Similar as in the case of the forward-bias ESD stress, we observe a slight increase of the lasing threshold current and a decrease of the differential quantum efficiency, but now the damage threshold is found, as mentioned before, at a higher ESD pulse amplitude as compared to the case of the forward-bias ESD stress.
1 For interpretation of color in Fig. 9, the reader is referred to the web version of this article.
normalized optical power (dB)
H.C. Neitzert / Microelectronics Reliability 50 (2010) 1563–1567
The corresponding forward bias current–voltage characteristics, measured before degradation and after application of the 3700 V ESD pulse (see Fig. 11), does not evidence any electrical degradation. It should be mentioned, that in this case also the reverse-bias breakdown voltage did no change at all after ESD-induced degradation. A sharp, avalanche type breakdown has been observed at 10.30 V.
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ESD degradation tests under Human Body model conditions of Fabry–Perot laser diodes with different active layer material revealed that in AlGaAs lasers lower ESD damage thresholds under forward bias have been observed in contrast to InGaAsP based lasers. In the latter case a very good ESD stability under forward bias has been measured and assigned to a self-protection mechanism due to the lower characteristic temperature values. Monitoring the optical emitted pulses during forward-bias ESD step-stress tests is a good measure for the evaluation of the device heating during ESD pulses and enables a faster determination of the device failure threshold amplitudes than classical electro-optic laser characterization after each ESD pulse application.
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[1] DeChiaro LF, Unger BA. Degradation in InGaAsP semiconductor lasers resulting from human body model ESD. J Electrostat 1993;29:227–50. [2] Wallon J, Terol G, Bauduin B, Devoldère P. Sensitivity to electrostatic discharges of ‘‘low-cost” 1.3 lm laser diodes: a comparative study. Mater Sci Eng B 1994;28:314–8. [3] Twu Y, Cheng LS, Chu SNG, Nash FR, Wang KW, Prayanthal P. Semiconductor laser damage due to human-body-model electrostatic discharge. J Appl Phys 1993;74:1510–20. [4] Ichikawa H, Kumagai A, Hamada K, Yamaguchi A, Nakabayashi T. Analysis on reverse electrostatic discharge induced degradation of GaInAsP/InP laser diodes. In: Proceedings of the 2005 IPRM Conf, Glasgow; 2005. p. 629–32. [5] Fukuda M. Reliability and degradation of III–V optical devices. Boston: Artech House; 1991. [6] Vanzi M, Giansante M, Zazzetti L, Magistrali F, Sala D, Salmini G, Fantini F. Interpretation of failure analysis results on ESD-damaged InP laser diodes. In: Proceedings of ISTFA’91. Los Angeles; 1991. p. 305–14. [7] Waters RG. Diode laser degradation mechanisms: a review. Prog Quant Electron 1990;15:153–74. [8] Neitzert HC, Gobbi B, Piccirillo A. Sensitivity of proton implanted VCSELs to electrostatic discharge pulses. IEEE J Select Top Quant Electron 2001;7:231–41. [9] Neitzert HC, Picirillo A. Sensitivity of multimode bidirectional optoelectronic modules to electrostatic discharges. Microelectron Reliab 1999;39:1863–71. [10] Ichikawa H, Kumaga A, Hamada K, Yamaguchi A, Nakabayashi T. Analysis of reverse-biased electrostatic-discharge-induced degradation of GaInAsP/InPburied heterostructure laser diode. Jpn J Appl Phys 2009;48:022201. [11] Huang J-S, Olson T, Isip E. Human-body-model electrostatic-discharge and electrical-overstress studies of buried-heterostructure lasers. J Appl Phys 2009;106:083101. [12] Savonlainen P, Toivonen M, Orsila S, Saarinen M, Melanen P, Vilokkinen V, et al. AlGaInAs/InP strained-layer quantum well lasers at 1.3 m grown by solid source molecular beam epitaxy. J Electron Mater 1999;28:980–5. [13] Hsieh S-W, Kuo Y-K. A numerical study of characteristic temperature of shortcavity 1.3-lm AlGaInAs/InP MQW lasers. Appl Phys A 2006;82:287–92. [14] Ichikawa H, Matsukawa S, Hamada K, Ikoma N, Nakabayashi T. Electrostaticdischarge-induced degradation of 1.3 lm AlGaInAs/InP buried heterostructure laser diodes. J Appl Phys 2009;106:083101. [15] Ichikawa H, Matsukawa S, Hamada K, Yamaguchi A, Nakabayashi T. Failure analysis of InP-based edge-emitting buried heterostructure laser diodes degraded by forward-biased electrostatic discharge tests. Jpn J Appl Phys 2009;49:052102. [16] Neitzert HC. ESD-induced degradation of vertical-cavity-surface-emitting lasers. Electron Lett 2000;36:1620–1.