- Email: [email protected]
InP 1.3 μm microcavity LEDs with high quantum efficiency
Journal of Crystal Growth 221 (2000) 674}678
InP 1.3 lm microcavity LEDs with high quantum e$ciency B. Depreter*, I. Moerman, R. Baets, P. Van Daele, P. Demeester Department of Information Technology, University of Gent } IMEC, Sint-Pietersnieuwstraat 41, 9000 Gent, Belgium
Abstract We report on the growth and the fabrication of InP-based microcavity light-emitting diodes (MCLEDs) operating at 1300 nm wavelength. Thus far, the MCLED concept was mainly used in the GaAs material system. Due to the reduced refractive index contrast available in the InP material system, a transfer to longer wavelengths is certainly not straightforward. The transition from the ternary A1GaAs to the quaternary InGaAsP system poses an additional growth-technical challenge. First, results of our 1300 nm MCLEDs indicate excellent output power and high "bercoupling e$ciency. 2000 Elsevier Science B.V. All rights reserved. PACS: 81.15.K; 85.60.J; 85.30.V; 78.66.F Keywords: MOVPE; InP; LED; Microcavity
1. Introduction The microcavity concept has proven to be very successful in improving several characteristics of light-emitting diodes [1]. Especially in the GaAs material system, huge increases in the overall quantum e$ciency were demonstrated [2]. It was also shown that the e$ciency of coupling light into a "ber could be improved, reducing or even eliminating the need for extra optics [3]. Essentially, a microcavity LED consists of three building blocks: the active region, the cavity, and the mirrors. The interaction between these components will in#uence the "nal properties of the device. It becomes clear that a simultaneous
* Corresponding author. Tel.: #32-9-2643316; fax: #32-9264-3593. E-mail address: [email protected] (B. Depreter).
control over di!erent growth parameters becomes a necessity. The active region emits at a certain wavelength, which must be tuned very accurately to the cavity length in order to produce the desired microcavity e!ects [4]. This poses a restriction on thickness uniformity for the cavity, and shows a need for composition and thickness control of the quantum wells. The DBR mirror must re#ect the emitted light while not absorbing any of it. Re#ection at the desired wavelength requires, again, control over thickness variations, while absorption can be avoided by control over the composition. When attempting to transfer the microcavity approach of improving LEDs from the (Al)GaAs to the In(Ga)(As)P material system, already some fundamental di$culties arise. In view of the constraints on growth mentioned above, it is clear that fabricating a MCLED in a quaternary material
0022-0248/00/$ - 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 7 9 8 - 3
B. Depreter et al. / Journal of Crystal Growth 221 (2000) 674}678
system is more strenuous than in a ternary material system. Furthermore, the reduced refractive index contrast available in InP poses an additional hurdle when creating a good DBR. This problem is well known for long-wavelength VCSELs, although for a di!erent reason. VCSELs need highly re#ective mirrors, and thus a very high number of mirror pairs in their DBRs. Since MCLEDs are spontaneous emission devices, there is no need for such high number of mirror pairs; it is even necessary that the out-coupling mirror has a relatively low re#ectivity. The problem with the low refractive index contrast is that it enlarges the penetration depth into the DBR. This increases the cavity length, and m , the number of modes supported by the cavity. The maximum extraction e$ciency of a microcavity LED is inversely proportional to m . We have previously demonstrated the "rst 1.55 lm microcavity LED [5]. The available refractive index contrast is reduced even further when moving to 1.3 lm. Devices fabricated by other groups have not been able to achieve high quantum e$ciency [6,7]. In this paper, we demonstrate the successful fabrication of InP-based microcavity light-emitting diodes, obtaining high total external quantum e$ciency, and high coupling e$ciency into multimode glass "ber. A possible use for such LEDs could be found in LAN applications.
2. Experimental procedure
675
ration runs were performed before the actual device run. This also allowed us to characterize both composition and thickness uniformity over a single wafer in our reactor. In a "rst test run, the uniformity of the quantum wells to be used for the active region of the MCLEDs was investigated. A mapping of the photoluminescence spectrum over an entire 2 in wafer is presented in Fig. 1. We obtained a standard deviation of 6.1 nm over the entire wafer and 2.9 nm if an edge of 2 mm is dropped. The DBR calibration run consisted of 10 periods of InGa As P alternated with InP. The photoluminescence mapping of the DBR is shown in Fig. 2. Here we have a standard deviation of
Fig. 1. Photoluminescence mapping of quantum well.
All layers were grown by means of metal-organic vapor-phase epitaxy in a 3;2 in Thomas Swan vertical reactor. The precursors are TMI, TMG, TMA, pure PH and pure AsH . H S was used as n-dopant, DEZ for p-doping. The substrate is a 2 in 11 0 02 exact oriented n-type wafer supplied by Japan Energy. All device layers are grown at a pressure of 76 Torr and at a temperature of 6303C. Prior to the growth, the wafer is baked out at 6303C under a PH overpressure. 3. Growth calibration In view of the sensitivity of the microcavity effects to di!erent growth parameters, several calib-
Fig. 2. Photoluminescence mapping of high index DBR layer.
676
B. Depreter et al. / Journal of Crystal Growth 221 (2000) 674}678
Fig. 4. 1300 nm MCLED-structure.
Fig. 3. Re#ection spectrum of DBR calibration structure.
4.4 nm over the entire 2 in wafer and 2.8 nm if an edge of 2 mm is dropped. We also acquired information about the thickness uniformity from this sample. We measured the re#ectivity spectrum of the DBR on di!erent positions on the wafer. This spectrum was compared to a simulation using 101 nm InP and 95.5 nm InGaAsP. The results are shown in Fig. 3. By comparing the relative peak positions we obtain the thickness variation over the wafer. The lateral thickness variation was limited to $0.5%, the radial thickness variation was $2% ($1% when discarding 2 mm of the edges). Since our processing facilities do not allow us to process an entire 2 in wafer, no detailed experimental study of the in#uence of the uniformity on the LED characteristics was performed. However, measurements on devices from a processed quarter wafer indicated a maximum variation in output power of only 5%.
The bottom DBR consists of 5.5 periods of alternating 101 nm InP and 95 nm InGa As P. The composition of the quaternary layer is limited by the fact that this outcoupling DBR mirror must remain transparent for the light emitted. One quarter of the wafer was processed as follows. On top of the devices 200 nm of Au was deposited to serve both as the highly re#ective top mirror and as the p-type contact. Contact diameters of 25, 40, 70, 115, 215, 520, 1030 and 2030 lm are realized. Mesas are de"ned by a shallow self-aligned wet etch, using the metal mirrors as mask pattern. On the bottom side AuGeNi n-contact stripes were deposited. After measuring the processed devices, it became clear that the cavity length was slightly short. We then performed a regrowth on another quarter from the original run, depositing an additional 5 nm of p-doped InGaAsP on top of the contact layer. This was done to tune the cavity length to the emission wavelength of the quantum well in view of optimizing for high "ber coupling e$ciency. This quarter wafer was subsequently processed in an identical fashion as the "rst quarter. In addition, the substrate was thinned and polished.
4. Device fabrication 5. Output power InP MCLEDs with the structure as presented in Fig. 4 have been fabricated. The active region emitting at 1315 nm consisted of 3 compressively strained InGa As P quantum wells (4.5 nm) embedded within 10 nm InGa As P barriers.
The e!ect of the regrowth on the output power of the devices is shown in Fig. 5 for 2 mm diameter devices. We can note an increase in power of 20%. This indicates the importance of tight growth control.
B. Depreter et al. / Journal of Crystal Growth 221 (2000) 674}678
677
Fig. 5. Output power and quantum e$ciency of a 2 mm device before (dashed line) and after regrowth (full line).
From the batch with the tuned cavity, we investigated the output power of several devices with di!erent diameters. The results are shown in Fig. 6(a). It is immediately apparent that the larger devices show much higher output powers. When we plot the e$ciency of the devices with respect to the current density (see Fig. 6(b)), it becomes apparent that the di!erences in output power are only generated by a di!erence in current density. We can therefore conclude that no photon recycling is taking place in our devices. The highest output power obtained was 2.6 mW at a drive current of 50 mA, the highest total external quantum e$ciency 6.2%, both for a 2 mm device.
Fig. 6. (a) Output power vs. drive current for di!erent device sizes. (b) E$ciency vs. current density for di!erent device sizes.
6. Fiber coupling Coupling a large portion of the generated light into a multimode "ber, without using any optics, could prove to be very interesting for low-cost "ber-based applications. We therefore investigated the coupling e$ciency of our MCLEDs into 62.5 lm core multimode glass "ber. We obtained a maximum coupling e$ciency of 9.5% for the 40 lm diameter devices, as shown in Fig. 7. Although the 70 lm device showed a lower coupling e$ciency of only 5%, the maximum power
Fig. 7. Fiber coupling e$ciency of 9.5% for a 40 lm MCLED. £ designates the power in "ber, and the full line the power in air.
678
B. Depreter et al. / Journal of Crystal Growth 221 (2000) 674}678
launched into "ber was higher: 17 lW at 60 mA. This is due to the lower current density in the larger devices leading to a better saturation behavior.
ported by the European Contract ESPRIT 24,997 SMILED.
References 7. Conclusions We have demonstrated the fabrication of high e$ciency InP-based MCLEDs. The largest devices exhibit a total external quantum e$ciency of over 6% at low drive currents and a maximum output power of 2.6 mW at 50 mA. Smaller devices show excellent "ber coupling e$ciency into multimode "ber, as high as 9.5% for a 40 lm diameter MCLED.
Acknowledgements We greatly appreciate the work of Steven Verstuyft for the device processing. This work is sup-
[1] E.F. Schubert, Y.-H. Wang, A.Y. Cho, L.-W. Tu, G.J. Zydzik, Appl. Phys. Lett. 60 (1992) 921. [2] H. De Neve, J. Blondelle, P. Van Daele, P. Demeester, R. Baets, G. Borghs, Proceedings of the Conference SPIE Photonics West, San Jose, CA, 1997, p. 74. [3] R. Bockstaele, C. Sys, J. Blondelle, B. Dhoedt, I. Moerman, P. Van Daele, P. Demeester, R. Baets, Phot. Tech. Lett. 11 (1999) 158. [4] H. Benisty, H. De Neve, C. Weisbuch, IEEE J. Quantum Electron. 34 (1998) 1612. [5] B. Depreter, J. Blondelle, I. Moerman, R. Baets, P. Van Daele, P. Demeester, Proceedings of the 10th Annual Meeting of LEOS, San Francisco, CA, 1997, p. 241. [6] N.E.J. Hunt, E.F. Schubert, R.A. Logan, G.J. Zydzik, Appl. Phys. Lett. 61 (1992) 336. [7] S. Orsila, J. KoK ngaK s, M. Toivonen, P. Savolainen, M. Jalonen, M. Pessa, 10th International Conference on Molecular Beam Epitaxy, Cannes, France, 1998.
InP 1.3 lm microcavity LEDs with high quantum e$ciency B. Depreter*, I. Moerman, R. Baets, P. Van Daele, P. Demeester Department of Information Technology, University of Gent } IMEC, Sint-Pietersnieuwstraat 41, 9000 Gent, Belgium
Abstract We report on the growth and the fabrication of InP-based microcavity light-emitting diodes (MCLEDs) operating at 1300 nm wavelength. Thus far, the MCLED concept was mainly used in the GaAs material system. Due to the reduced refractive index contrast available in the InP material system, a transfer to longer wavelengths is certainly not straightforward. The transition from the ternary A1GaAs to the quaternary InGaAsP system poses an additional growth-technical challenge. First, results of our 1300 nm MCLEDs indicate excellent output power and high "bercoupling e$ciency. 2000 Elsevier Science B.V. All rights reserved. PACS: 81.15.K; 85.60.J; 85.30.V; 78.66.F Keywords: MOVPE; InP; LED; Microcavity
1. Introduction The microcavity concept has proven to be very successful in improving several characteristics of light-emitting diodes [1]. Especially in the GaAs material system, huge increases in the overall quantum e$ciency were demonstrated [2]. It was also shown that the e$ciency of coupling light into a "ber could be improved, reducing or even eliminating the need for extra optics [3]. Essentially, a microcavity LED consists of three building blocks: the active region, the cavity, and the mirrors. The interaction between these components will in#uence the "nal properties of the device. It becomes clear that a simultaneous
* Corresponding author. Tel.: #32-9-2643316; fax: #32-9264-3593. E-mail address: [email protected] (B. Depreter).
control over di!erent growth parameters becomes a necessity. The active region emits at a certain wavelength, which must be tuned very accurately to the cavity length in order to produce the desired microcavity e!ects [4]. This poses a restriction on thickness uniformity for the cavity, and shows a need for composition and thickness control of the quantum wells. The DBR mirror must re#ect the emitted light while not absorbing any of it. Re#ection at the desired wavelength requires, again, control over thickness variations, while absorption can be avoided by control over the composition. When attempting to transfer the microcavity approach of improving LEDs from the (Al)GaAs to the In(Ga)(As)P material system, already some fundamental di$culties arise. In view of the constraints on growth mentioned above, it is clear that fabricating a MCLED in a quaternary material
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 7 9 8 - 3
B. Depreter et al. / Journal of Crystal Growth 221 (2000) 674}678
system is more strenuous than in a ternary material system. Furthermore, the reduced refractive index contrast available in InP poses an additional hurdle when creating a good DBR. This problem is well known for long-wavelength VCSELs, although for a di!erent reason. VCSELs need highly re#ective mirrors, and thus a very high number of mirror pairs in their DBRs. Since MCLEDs are spontaneous emission devices, there is no need for such high number of mirror pairs; it is even necessary that the out-coupling mirror has a relatively low re#ectivity. The problem with the low refractive index contrast is that it enlarges the penetration depth into the DBR. This increases the cavity length, and m , the number of modes supported by the cavity. The maximum extraction e$ciency of a microcavity LED is inversely proportional to m . We have previously demonstrated the "rst 1.55 lm microcavity LED [5]. The available refractive index contrast is reduced even further when moving to 1.3 lm. Devices fabricated by other groups have not been able to achieve high quantum e$ciency [6,7]. In this paper, we demonstrate the successful fabrication of InP-based microcavity light-emitting diodes, obtaining high total external quantum e$ciency, and high coupling e$ciency into multimode glass "ber. A possible use for such LEDs could be found in LAN applications.
2. Experimental procedure
675
ration runs were performed before the actual device run. This also allowed us to characterize both composition and thickness uniformity over a single wafer in our reactor. In a "rst test run, the uniformity of the quantum wells to be used for the active region of the MCLEDs was investigated. A mapping of the photoluminescence spectrum over an entire 2 in wafer is presented in Fig. 1. We obtained a standard deviation of 6.1 nm over the entire wafer and 2.9 nm if an edge of 2 mm is dropped. The DBR calibration run consisted of 10 periods of InGa As P alternated with InP. The photoluminescence mapping of the DBR is shown in Fig. 2. Here we have a standard deviation of
Fig. 1. Photoluminescence mapping of quantum well.
All layers were grown by means of metal-organic vapor-phase epitaxy in a 3;2 in Thomas Swan vertical reactor. The precursors are TMI, TMG, TMA, pure PH and pure AsH . H S was used as n-dopant, DEZ for p-doping. The substrate is a 2 in 11 0 02 exact oriented n-type wafer supplied by Japan Energy. All device layers are grown at a pressure of 76 Torr and at a temperature of 6303C. Prior to the growth, the wafer is baked out at 6303C under a PH overpressure. 3. Growth calibration In view of the sensitivity of the microcavity effects to di!erent growth parameters, several calib-
Fig. 2. Photoluminescence mapping of high index DBR layer.
676
B. Depreter et al. / Journal of Crystal Growth 221 (2000) 674}678
Fig. 4. 1300 nm MCLED-structure.
Fig. 3. Re#ection spectrum of DBR calibration structure.
4.4 nm over the entire 2 in wafer and 2.8 nm if an edge of 2 mm is dropped. We also acquired information about the thickness uniformity from this sample. We measured the re#ectivity spectrum of the DBR on di!erent positions on the wafer. This spectrum was compared to a simulation using 101 nm InP and 95.5 nm InGaAsP. The results are shown in Fig. 3. By comparing the relative peak positions we obtain the thickness variation over the wafer. The lateral thickness variation was limited to $0.5%, the radial thickness variation was $2% ($1% when discarding 2 mm of the edges). Since our processing facilities do not allow us to process an entire 2 in wafer, no detailed experimental study of the in#uence of the uniformity on the LED characteristics was performed. However, measurements on devices from a processed quarter wafer indicated a maximum variation in output power of only 5%.
The bottom DBR consists of 5.5 periods of alternating 101 nm InP and 95 nm InGa As P. The composition of the quaternary layer is limited by the fact that this outcoupling DBR mirror must remain transparent for the light emitted. One quarter of the wafer was processed as follows. On top of the devices 200 nm of Au was deposited to serve both as the highly re#ective top mirror and as the p-type contact. Contact diameters of 25, 40, 70, 115, 215, 520, 1030 and 2030 lm are realized. Mesas are de"ned by a shallow self-aligned wet etch, using the metal mirrors as mask pattern. On the bottom side AuGeNi n-contact stripes were deposited. After measuring the processed devices, it became clear that the cavity length was slightly short. We then performed a regrowth on another quarter from the original run, depositing an additional 5 nm of p-doped InGaAsP on top of the contact layer. This was done to tune the cavity length to the emission wavelength of the quantum well in view of optimizing for high "ber coupling e$ciency. This quarter wafer was subsequently processed in an identical fashion as the "rst quarter. In addition, the substrate was thinned and polished.
4. Device fabrication 5. Output power InP MCLEDs with the structure as presented in Fig. 4 have been fabricated. The active region emitting at 1315 nm consisted of 3 compressively strained InGa As P quantum wells (4.5 nm) embedded within 10 nm InGa As P barriers.
The e!ect of the regrowth on the output power of the devices is shown in Fig. 5 for 2 mm diameter devices. We can note an increase in power of 20%. This indicates the importance of tight growth control.
B. Depreter et al. / Journal of Crystal Growth 221 (2000) 674}678
677
Fig. 5. Output power and quantum e$ciency of a 2 mm device before (dashed line) and after regrowth (full line).
From the batch with the tuned cavity, we investigated the output power of several devices with di!erent diameters. The results are shown in Fig. 6(a). It is immediately apparent that the larger devices show much higher output powers. When we plot the e$ciency of the devices with respect to the current density (see Fig. 6(b)), it becomes apparent that the di!erences in output power are only generated by a di!erence in current density. We can therefore conclude that no photon recycling is taking place in our devices. The highest output power obtained was 2.6 mW at a drive current of 50 mA, the highest total external quantum e$ciency 6.2%, both for a 2 mm device.
Fig. 6. (a) Output power vs. drive current for di!erent device sizes. (b) E$ciency vs. current density for di!erent device sizes.
6. Fiber coupling Coupling a large portion of the generated light into a multimode "ber, without using any optics, could prove to be very interesting for low-cost "ber-based applications. We therefore investigated the coupling e$ciency of our MCLEDs into 62.5 lm core multimode glass "ber. We obtained a maximum coupling e$ciency of 9.5% for the 40 lm diameter devices, as shown in Fig. 7. Although the 70 lm device showed a lower coupling e$ciency of only 5%, the maximum power
Fig. 7. Fiber coupling e$ciency of 9.5% for a 40 lm MCLED. £ designates the power in "ber, and the full line the power in air.
678
B. Depreter et al. / Journal of Crystal Growth 221 (2000) 674}678
launched into "ber was higher: 17 lW at 60 mA. This is due to the lower current density in the larger devices leading to a better saturation behavior.
ported by the European Contract ESPRIT 24,997 SMILED.
References 7. Conclusions We have demonstrated the fabrication of high e$ciency InP-based MCLEDs. The largest devices exhibit a total external quantum e$ciency of over 6% at low drive currents and a maximum output power of 2.6 mW at 50 mA. Smaller devices show excellent "ber coupling e$ciency into multimode "ber, as high as 9.5% for a 40 lm diameter MCLED.
Acknowledgements We greatly appreciate the work of Steven Verstuyft for the device processing. This work is sup-
[1] E.F. Schubert, Y.-H. Wang, A.Y. Cho, L.-W. Tu, G.J. Zydzik, Appl. Phys. Lett. 60 (1992) 921. [2] H. De Neve, J. Blondelle, P. Van Daele, P. Demeester, R. Baets, G. Borghs, Proceedings of the Conference SPIE Photonics West, San Jose, CA, 1997, p. 74. [3] R. Bockstaele, C. Sys, J. Blondelle, B. Dhoedt, I. Moerman, P. Van Daele, P. Demeester, R. Baets, Phot. Tech. Lett. 11 (1999) 158. [4] H. Benisty, H. De Neve, C. Weisbuch, IEEE J. Quantum Electron. 34 (1998) 1612. [5] B. Depreter, J. Blondelle, I. Moerman, R. Baets, P. Van Daele, P. Demeester, Proceedings of the 10th Annual Meeting of LEOS, San Francisco, CA, 1997, p. 241. [6] N.E.J. Hunt, E.F. Schubert, R.A. Logan, G.J. Zydzik, Appl. Phys. Lett. 61 (1992) 336. [7] S. Orsila, J. KoK ngaK s, M. Toivonen, P. Savolainen, M. Jalonen, M. Pessa, 10th International Conference on Molecular Beam Epitaxy, Cannes, France, 1998.