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Influence of driving conditions on the stability of ZnSe-based cw-laser diodes
Journal of Crystal Growth 214/215 (2000) 1040}1044
In#uence of driving conditions on the stability of ZnSe-based cw-laser diodes M. Klude*, M. Fehrer, V. Gro{mann, D. Hommel Institut fu( r Festko( rperphysik, Universita( t Bremen, Kufsteiner Str. NW1, D-28359 Bremen, Germany
Abstract ZnSe-based laser diodes grown by molecular beam epitaxy are tested under various current driving conditions. We found that the driving conditions do not in#uence the degradation process itself. Lifetime measurements show no dependence on the pulse width of the applied current, whereas the duty cycle has a strong in#uence on the lifetime. This indicates that heat is one of the driving forces of the degradation process. It is found that stable high-power pulsed operation can be maintained over a long time when the delay between the current pulses is long enough. In pulsed mode light pulses with 128 mW and in continous wave (cw)-mode an output power of more than 37 mW have been obtained. 2000 Elsevier Science B.V. All rights reserved. PACS: 42.55.Px; 42.60.Lh; 42.60.P Keywords: Laser diodes; Degradation; Pulsed operation; cw-Operation
1. Introduction Laser diodes operating in the green spectral region are of great interest for a variety of applications. So far only ZnSe-based devices have shown electrically pumped stimulated emission in this region. However, their limited lifetime in cw-mode hampers commercialization. One of the key issues on the way towards longer lifetimes is the understanding of the degradation process. There have been a number of studies using di!erent approaches to determine the nature of this process, such as transmission electron microscopy [1,2], electroluminescence topography [3,4] and photo* Corresponding author. Tel.: #49-421-218-2856; fax: #49421-218-4581. E-mail address: [email protected] (M. Klude).
luminescence spectroscopy [5,6]. In this paper we want to investigate the degradation process from a more device related point of view.
2. Experimental setup The tested laser structures have been grown in a twin-chamber molecular beam epitaxy system (EPI 930/95) on GaAs(0 0 1) substrates. A GaAs bu!er layer has been deposited prior to the growth of the II}VI heterostructure which has a conventional double heterostructure, seperate con"nement design, using MgZnSSe cladding layers, ZnSSe waveguide layers and a 3 nm ZnCdSSe quantum well. For the p-contact a ZnTe/ZnSe multi-quantum well structure was used. All layers were grown pseudomorphically. The emission
0022-0248/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 0 ) 0 0 2 7 0 - 0
M. Klude et al. / Journal of Crystal Growth 214/215 (2000) 1040}1044
wavelength of the tested structures is between 505 and 525 nm. Details of the structures and the growth can be found in Refs. [4,7]. After growth, 10 lm wide stripes were formed on top of the structure, using Al O as insulator and Pd/Au as metal contact. The devices were then cleaved into bars of roughly 1 mm length and mounted epi-side up onto a copper heat sink using silver paste. No facet coating or further special processing technology was applied. The devices were operated using a pulsed current source. For lifetime measurements the operating current was adjusted to maintain a constant output power which was measured using an optical multimeter with a silicon power head. During these tests the laser bar was placed directly in front of the power head so that all light coming from one facet of the tested laser stripe was collected. The same setup was also used for measuring the light output characteristics. All output powers reported in this paper are measured values. They have not been rescaled to cw-values unless otherwise stated and are taken with respect to one facet. To determine the defect density, etch pit density (EPD) measurements have been performed. For these tests the p-side contact layers were removed using a K Cr O solution. The thus exposed p MgZnSSe cladding layer was then etched for 20 s in 603C hot HCl. Pictures from this etched surface were taken using a Normaski interference microscope.
3. Results and discussion When measuring the lifetimes of the devices, it is found that the principle course of degradation is the same under all driving conditions, including cw-operation. This is shown in Fig. 1 where the operation current versus time at constant output power is shown. Here, the time axis has been normalized to the maximum lifetime of each device to visualize the universality of the curves. It has to be noted that not only the driving conditions di!er in pulse width as well as in duty cycle and output power level, but also that di!erent laser structures are shown. All curves show a drop in operating current at the begining of the measurement. This
1041
Fig. 1. Lifetime measurements under di!erent driving conditions of di!erent laser structures. The time axis has been normalized to the maximum operating time. The inset shows a measurement of another structure in cw-mode.
might be due to defect annealing which reduces the number of non-radiative recombination centers in the active region [8,9]. During most of the time the operating current raises rather slowly, it is just in the last 10% of the total lifetime that the current increases drastically. The inset of Fig. 1 shows the degradation of a device in cw-mode at an output power of 2 mW. The lifetime of this structure is 4 min and the form of the curve does not di!er signi"cantly from the ones in pulsed mode. This indicates that the underlying degradation mechanism is the same in all cases and that the driving conditions do not in#uence the degradation process itself. We now study the in#uence of heat on the degradation process. It is well known that the device temperature greatly in#uences the performance of the laser structure, for example the threshold current density [10]. Not only can the external heating change the device temperature, but also heat is generated during current injection in the device itself due to the series resistance of the semiconductor material and potential barriers at the layer interfaces. It is known that in ZnSe-based laser diodes at a poorly performing p-contact extensive
1042
M. Klude et al. / Journal of Crystal Growth 214/215 (2000) 1040}1044
Fig. 3. Dependence of the lifetime on the duty cycle. The pulse width is 1 ls and the output power level is 1.5 mW. The tested stripes come from the same bar of the same structure. The lines are guide to the eye.
Fig. 2. Etch pit density measurement of a laser structure. The sample has been etched for 20 s in hot HCl. Clearly a cluster of etch pits can be seen. The sample area is roughly 0.01 cm. The resulting etch pit density is around 1;10 cm\.
heat can be generated and thus lead to thermal index guiding along the current injection path, effectively reducing the threshold current density of the device [11]. On the other hand, the in#uence of heat on the lifetime and therefore the degradation process is not so obvious. Using epi-side up-mounting and pulsed mode operation it is possible to investigate this topic. It was found that the lifetime of the devices does not depend signi"cantly on the pulse width of the applied current within the same laser bar. Therefore extensive heat generation can be excluded. However, comparing di!erent bars, the lifetimes di!er noticeably under the same driving conditions,
ranging from roughly 10 min for one bar to 200 min for another bar (at 10% duty cycle, 1 ls pulse width and 1.5 mW output power). An explanation of these di!erences was found by EPD measurements. Whereas most of the tested sample piece (area roughly 1 cm) had an EPD of 1;10 cm\, clusters of etch pits up to 1; 10 cm\ were also observed. One of these clusters is shown in Fig. 2. The visible area is roughly 0.01 cm. Furthermore we observed that professional handling of the devices is necessary to obtain optimal and reproducible results. Additional investigations of this special problem are underway. The dependence of the lifetime on the duty cycle of the applied current can be found in Fig. 3. For these measurements a pulse width of 1 ls and a constant output power of 1.5 mW was chosen. All tested stripes were taken from the same laser bar. When changing the duty cycle from 10 to 20% a drastic reduction of the lifetime from 200 to 20 min was observed. A further increase of the duty cycle up to 70% does only moderately reduce the lifetime. In cw-mode the lifetime of this particular laser structure was roughly 2 min. Obviously, the lifetime of the device does not scale linearly with the duty cycle where one would expect e.g. a lifetime of
M. Klude et al. / Journal of Crystal Growth 214/215 (2000) 1040}1044
1043
was 0.128 mW, corresponding to a pulse power of 128 mW. The inset of Fig. 4 shows the ¸/Icharacteristic for the same structure in cw-mode. Here a maximum output power of 37.5 mW is reached. These powers are su$ciently high for a number of technical, environmental and medical applications.
4. Conclusion
Fig. 4. Light output measurement in pulsed mode. The inset shows a measurement of the same structure in cw-mode.
20 min for a duty cycle of 10% for this structure. This shows that heat is an important driving force of the degradation process. If the delay between the current pulses is long enough, so that the heat generated in the current path can be completely removed from the active region, the degradation is slowed down. An increasing duty cycle shortens this delay and thus heat starts to accumulate in the device, which leads to faster degradation. Another important point has to be noticed in Fig. 3. For all measurements the output power was kept constant at a measured level of 1.5 mW. Therefore, the light power in each pulse was considerably higher, e.g. 15 mW at a duty cycle of 10% but only 3 mW at a duty cycle of 50%. Still the lifetime is higher for lower duty cycles. This implies that the devices are capable of delivering stable high-power light pulses over a long time if the delay between the pulses is long enough. The maximum achievable output power is shown in Fig. 4, where the light output power versus driving current (¸/I-characteristics) of one sample in pulsed mode is depicted. Here the driving current was pulsed with a duty cycle of 0.1% and a pulse width of 1 ls and increased up to the breakdown of the device. The maximum measured output power
In this paper we investigated the in#uence of the current driving conditions on the degradation of ZnSe-based laser diodes. In pulsed operation it was found that the generation of heat is one of the driving forces of the degradation process. If the delay between the current pulses is long enough, operation with stable high-power light pulses can be maintained over a long time. We therefore conclude that industry standard back-end technology should provide a drastic increase in lifetime compared to the naturally limited university approach used in this paper.
Acknowledgements The authors thank S. Hesselmann for device preparation. This work was supported by the Deutsche Forschungsgemeinschaft in the project DFG HO 1388/11-1.
References [1] G.C. Hua, N. Otuska, D.C. Grillo, Y. Fan, J. Han, M.D. Ringle, R.L. Gunshor, M. Hovinen, A.V. Nurmikko, Appl. Phys. Lett. 65 (1994) 1331. [2] S. Tomiya, E. Morita, M. Ukita, H. Okuyama, K. Nakano, A. Ishibashi, Appl. Phys. Lett. 66 (1995) 1208. [3] S. Guha, J.M. DePuydt, M.A. Haase, J. Qiu, H. Cheng, Appl. Phys. Lett. 63 (1993) 3107. [4] H. Wenisch, M. Behringer, M. Fehrer, M. Klude, A. Isemann, K. Ohkawa, D. Hommel, Jpn. J. Appl. Phys. 38 (1999) 2590. [5] M. Behringer, K. Ohkawa, V. Gro{mann, H. Heinke, K. Leonardi, M. Fehrer, D. Hommel, M. Kuttler, M. Strassburg, D. Bimberg, J. Crystal Growth 184/185 (1998) 580.
1044
M. Klude et al. / Journal of Crystal Growth 214/215 (2000) 1040}1044
[6] D. Albert, J. NuK rnberger, V. Hock, M. Ehinger, W. Faschinger, G. Landwehr, Appl. Phys. Lett. 74 (1999) 1957. [7] M. Behringer, H. Wenisch, M. Fehrer, V. Gro{mann, A. Isemann, M. Klude, H. Heinke, K. Ohkawa, D. Hommel, FestkoK rperprobleme/Adv. Solid State Phys. 38 (1999) 47. [8] Y. Ichimura, K. Kishino, M. Kuramoto, M. Satake, A. Yoshida, J. Electron. Mater. 24 (1995) 171.
[9] M. Momose, A. Taike, M. Kawata, J. Gotoh, S. Nakatsuka, Appl. Phys. Lett. 69 (1996) 3572. [10] J.M. Gaines, R.R. Drenten, K.W. Haberern, T. Marshall, P. Mensz, J. Petruzzello, Appl. Phys. Lett. 62 (1993) 2462. [11] A. Isemann, M. Behringer, H. Wenisch, M. Fehrer, K. Ohkawa, D. Hommel, Acta Phys. Pol. A 94 (1998) 355.
In#uence of driving conditions on the stability of ZnSe-based cw-laser diodes M. Klude*, M. Fehrer, V. Gro{mann, D. Hommel Institut fu( r Festko( rperphysik, Universita( t Bremen, Kufsteiner Str. NW1, D-28359 Bremen, Germany
Abstract ZnSe-based laser diodes grown by molecular beam epitaxy are tested under various current driving conditions. We found that the driving conditions do not in#uence the degradation process itself. Lifetime measurements show no dependence on the pulse width of the applied current, whereas the duty cycle has a strong in#uence on the lifetime. This indicates that heat is one of the driving forces of the degradation process. It is found that stable high-power pulsed operation can be maintained over a long time when the delay between the current pulses is long enough. In pulsed mode light pulses with 128 mW and in continous wave (cw)-mode an output power of more than 37 mW have been obtained. 2000 Elsevier Science B.V. All rights reserved. PACS: 42.55.Px; 42.60.Lh; 42.60.P Keywords: Laser diodes; Degradation; Pulsed operation; cw-Operation
1. Introduction Laser diodes operating in the green spectral region are of great interest for a variety of applications. So far only ZnSe-based devices have shown electrically pumped stimulated emission in this region. However, their limited lifetime in cw-mode hampers commercialization. One of the key issues on the way towards longer lifetimes is the understanding of the degradation process. There have been a number of studies using di!erent approaches to determine the nature of this process, such as transmission electron microscopy [1,2], electroluminescence topography [3,4] and photo* Corresponding author. Tel.: #49-421-218-2856; fax: #49421-218-4581. E-mail address: [email protected] (M. Klude).
luminescence spectroscopy [5,6]. In this paper we want to investigate the degradation process from a more device related point of view.
2. Experimental setup The tested laser structures have been grown in a twin-chamber molecular beam epitaxy system (EPI 930/95) on GaAs(0 0 1) substrates. A GaAs bu!er layer has been deposited prior to the growth of the II}VI heterostructure which has a conventional double heterostructure, seperate con"nement design, using MgZnSSe cladding layers, ZnSSe waveguide layers and a 3 nm ZnCdSSe quantum well. For the p-contact a ZnTe/ZnSe multi-quantum well structure was used. All layers were grown pseudomorphically. The emission
0022-0248/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 0 ) 0 0 2 7 0 - 0
M. Klude et al. / Journal of Crystal Growth 214/215 (2000) 1040}1044
wavelength of the tested structures is between 505 and 525 nm. Details of the structures and the growth can be found in Refs. [4,7]. After growth, 10 lm wide stripes were formed on top of the structure, using Al O as insulator and Pd/Au as metal contact. The devices were then cleaved into bars of roughly 1 mm length and mounted epi-side up onto a copper heat sink using silver paste. No facet coating or further special processing technology was applied. The devices were operated using a pulsed current source. For lifetime measurements the operating current was adjusted to maintain a constant output power which was measured using an optical multimeter with a silicon power head. During these tests the laser bar was placed directly in front of the power head so that all light coming from one facet of the tested laser stripe was collected. The same setup was also used for measuring the light output characteristics. All output powers reported in this paper are measured values. They have not been rescaled to cw-values unless otherwise stated and are taken with respect to one facet. To determine the defect density, etch pit density (EPD) measurements have been performed. For these tests the p-side contact layers were removed using a K Cr O solution. The thus exposed p MgZnSSe cladding layer was then etched for 20 s in 603C hot HCl. Pictures from this etched surface were taken using a Normaski interference microscope.
3. Results and discussion When measuring the lifetimes of the devices, it is found that the principle course of degradation is the same under all driving conditions, including cw-operation. This is shown in Fig. 1 where the operation current versus time at constant output power is shown. Here, the time axis has been normalized to the maximum lifetime of each device to visualize the universality of the curves. It has to be noted that not only the driving conditions di!er in pulse width as well as in duty cycle and output power level, but also that di!erent laser structures are shown. All curves show a drop in operating current at the begining of the measurement. This
1041
Fig. 1. Lifetime measurements under di!erent driving conditions of di!erent laser structures. The time axis has been normalized to the maximum operating time. The inset shows a measurement of another structure in cw-mode.
might be due to defect annealing which reduces the number of non-radiative recombination centers in the active region [8,9]. During most of the time the operating current raises rather slowly, it is just in the last 10% of the total lifetime that the current increases drastically. The inset of Fig. 1 shows the degradation of a device in cw-mode at an output power of 2 mW. The lifetime of this structure is 4 min and the form of the curve does not di!er signi"cantly from the ones in pulsed mode. This indicates that the underlying degradation mechanism is the same in all cases and that the driving conditions do not in#uence the degradation process itself. We now study the in#uence of heat on the degradation process. It is well known that the device temperature greatly in#uences the performance of the laser structure, for example the threshold current density [10]. Not only can the external heating change the device temperature, but also heat is generated during current injection in the device itself due to the series resistance of the semiconductor material and potential barriers at the layer interfaces. It is known that in ZnSe-based laser diodes at a poorly performing p-contact extensive
1042
M. Klude et al. / Journal of Crystal Growth 214/215 (2000) 1040}1044
Fig. 3. Dependence of the lifetime on the duty cycle. The pulse width is 1 ls and the output power level is 1.5 mW. The tested stripes come from the same bar of the same structure. The lines are guide to the eye.
Fig. 2. Etch pit density measurement of a laser structure. The sample has been etched for 20 s in hot HCl. Clearly a cluster of etch pits can be seen. The sample area is roughly 0.01 cm. The resulting etch pit density is around 1;10 cm\.
heat can be generated and thus lead to thermal index guiding along the current injection path, effectively reducing the threshold current density of the device [11]. On the other hand, the in#uence of heat on the lifetime and therefore the degradation process is not so obvious. Using epi-side up-mounting and pulsed mode operation it is possible to investigate this topic. It was found that the lifetime of the devices does not depend signi"cantly on the pulse width of the applied current within the same laser bar. Therefore extensive heat generation can be excluded. However, comparing di!erent bars, the lifetimes di!er noticeably under the same driving conditions,
ranging from roughly 10 min for one bar to 200 min for another bar (at 10% duty cycle, 1 ls pulse width and 1.5 mW output power). An explanation of these di!erences was found by EPD measurements. Whereas most of the tested sample piece (area roughly 1 cm) had an EPD of 1;10 cm\, clusters of etch pits up to 1; 10 cm\ were also observed. One of these clusters is shown in Fig. 2. The visible area is roughly 0.01 cm. Furthermore we observed that professional handling of the devices is necessary to obtain optimal and reproducible results. Additional investigations of this special problem are underway. The dependence of the lifetime on the duty cycle of the applied current can be found in Fig. 3. For these measurements a pulse width of 1 ls and a constant output power of 1.5 mW was chosen. All tested stripes were taken from the same laser bar. When changing the duty cycle from 10 to 20% a drastic reduction of the lifetime from 200 to 20 min was observed. A further increase of the duty cycle up to 70% does only moderately reduce the lifetime. In cw-mode the lifetime of this particular laser structure was roughly 2 min. Obviously, the lifetime of the device does not scale linearly with the duty cycle where one would expect e.g. a lifetime of
M. Klude et al. / Journal of Crystal Growth 214/215 (2000) 1040}1044
1043
was 0.128 mW, corresponding to a pulse power of 128 mW. The inset of Fig. 4 shows the ¸/Icharacteristic for the same structure in cw-mode. Here a maximum output power of 37.5 mW is reached. These powers are su$ciently high for a number of technical, environmental and medical applications.
4. Conclusion
Fig. 4. Light output measurement in pulsed mode. The inset shows a measurement of the same structure in cw-mode.
20 min for a duty cycle of 10% for this structure. This shows that heat is an important driving force of the degradation process. If the delay between the current pulses is long enough, so that the heat generated in the current path can be completely removed from the active region, the degradation is slowed down. An increasing duty cycle shortens this delay and thus heat starts to accumulate in the device, which leads to faster degradation. Another important point has to be noticed in Fig. 3. For all measurements the output power was kept constant at a measured level of 1.5 mW. Therefore, the light power in each pulse was considerably higher, e.g. 15 mW at a duty cycle of 10% but only 3 mW at a duty cycle of 50%. Still the lifetime is higher for lower duty cycles. This implies that the devices are capable of delivering stable high-power light pulses over a long time if the delay between the pulses is long enough. The maximum achievable output power is shown in Fig. 4, where the light output power versus driving current (¸/I-characteristics) of one sample in pulsed mode is depicted. Here the driving current was pulsed with a duty cycle of 0.1% and a pulse width of 1 ls and increased up to the breakdown of the device. The maximum measured output power
In this paper we investigated the in#uence of the current driving conditions on the degradation of ZnSe-based laser diodes. In pulsed operation it was found that the generation of heat is one of the driving forces of the degradation process. If the delay between the current pulses is long enough, operation with stable high-power light pulses can be maintained over a long time. We therefore conclude that industry standard back-end technology should provide a drastic increase in lifetime compared to the naturally limited university approach used in this paper.
Acknowledgements The authors thank S. Hesselmann for device preparation. This work was supported by the Deutsche Forschungsgemeinschaft in the project DFG HO 1388/11-1.
References [1] G.C. Hua, N. Otuska, D.C. Grillo, Y. Fan, J. Han, M.D. Ringle, R.L. Gunshor, M. Hovinen, A.V. Nurmikko, Appl. Phys. Lett. 65 (1994) 1331. [2] S. Tomiya, E. Morita, M. Ukita, H. Okuyama, K. Nakano, A. Ishibashi, Appl. Phys. Lett. 66 (1995) 1208. [3] S. Guha, J.M. DePuydt, M.A. Haase, J. Qiu, H. Cheng, Appl. Phys. Lett. 63 (1993) 3107. [4] H. Wenisch, M. Behringer, M. Fehrer, M. Klude, A. Isemann, K. Ohkawa, D. Hommel, Jpn. J. Appl. Phys. 38 (1999) 2590. [5] M. Behringer, K. Ohkawa, V. Gro{mann, H. Heinke, K. Leonardi, M. Fehrer, D. Hommel, M. Kuttler, M. Strassburg, D. Bimberg, J. Crystal Growth 184/185 (1998) 580.
1044
M. Klude et al. / Journal of Crystal Growth 214/215 (2000) 1040}1044
[6] D. Albert, J. NuK rnberger, V. Hock, M. Ehinger, W. Faschinger, G. Landwehr, Appl. Phys. Lett. 74 (1999) 1957. [7] M. Behringer, H. Wenisch, M. Fehrer, V. Gro{mann, A. Isemann, M. Klude, H. Heinke, K. Ohkawa, D. Hommel, FestkoK rperprobleme/Adv. Solid State Phys. 38 (1999) 47. [8] Y. Ichimura, K. Kishino, M. Kuramoto, M. Satake, A. Yoshida, J. Electron. Mater. 24 (1995) 171.
[9] M. Momose, A. Taike, M. Kawata, J. Gotoh, S. Nakatsuka, Appl. Phys. Lett. 69 (1996) 3572. [10] J.M. Gaines, R.R. Drenten, K.W. Haberern, T. Marshall, P. Mensz, J. Petruzzello, Appl. Phys. Lett. 62 (1993) 2462. [11] A. Isemann, M. Behringer, H. Wenisch, M. Fehrer, K. Ohkawa, D. Hommel, Acta Phys. Pol. A 94 (1998) 355.