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GaInP quantum well lasers grown by gas-source molecular beam epitaxy
Journal of Crystal Growth 227–228 (2001) 541–544
Strain-compensated GaInNAs/GaAsP/GaAs/GaInP quantum well lasers grown by gas-source molecular beam epitaxy Wei Li*, Jani Turpeinen, Petri Melanen, Pekka Savolainen, Petteri Uusimaa, Markus Pessa Optoelectronics Research Center, Tampere University of Technology, P.O. Box 692, FIN-33101 Tampere, Finland
Abstract We report the growth of strain-compensated GaInNAs/GaAsP quantum well structures and lasers by gassource molecular beam epitaxy with a RF-coupled plasma source for nitrogen activation. The influence of growth temperature and rapid thermal annealing (RTA) on the optical properties of GaInNAs/GaAsP quantum well structures were studied. RTA was found to significantly increase the photoluminescence from the quantum wells and reduce the threshold current density of the lasers, mainly due to a removal of N induced non-radiative centers from GaInNAs wells. # 2001 Elsevier Science B.V. All rights reserved. PACS: 81.15.Hi; 81.05.Zx; 78.66.Fd; 78.55.Cr Keywords: A3. Molecular beam epitaxy; B2. Semiconducting III–V materials; B3. Laser diodes
1. Introduction Semiconductor lasers emitting at 1.3 mm are important for high-speed optical data links and optical networks [1]. GaInNAs has been recently proposed as a novel material for near-infrared lasers on GaAs [2]. When combining with GaAs barriers, GaInNAs alloy allows an increased conduction band offset, which should greatly improve high temperature laser performance compared with conventional GaInAsP/InP material system. To date, GaInNAs semiconductor alloy is being intensively studied for both its fundamental properties [3,4] and its potential for *Corresponding author. Tel.: +358-3-365-2452; fax: +3583-365-3400. E-mail address: [email protected].fi (W. Li).
1.3 mm laser applications [2,5]. It has been reported that the incorporation of nitrogen in GaInAs can reduce the band-gap energy and allows emission wavelengths as long as 1.3 mm to be reached [5]. However, the optical material quality deteriorates significantly with increasing N mole fractions [6], resulting in a much higher threshold current density of GaInNAs/GaAs lasers compared with that of GaInAs/GaAs lasers. In order to improve the performance of 1.3 mm GaInNAs/GaAs quantum well (QW) lasers, the nitrogen composition of GaInNAs well should be reduced, although, this leads to an increased strain in the quantum wells. By introducing a strain compensated barrier to this system it is possible to grow highly strained GaInNAs wells free of misfit dislocations. In this paper, we report on the growth of highly strained GaInNAs layers with strain-compensating GaAsP
0022-0248/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 0 7 6 3 - 1
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W. Li et al. / Journal of Crystal Growth 227–228 (2001) 541–544
barriers by gas-source molecular beam epitaxy. Especially, effects of rapid thermal annealing (RTA) on the optical properties of GaInNAs/ GaAsP quantum well structures and lasing properties of the laser diodes were investigated.
2. Experimental procedures The GaInNAs/GaAsP QW and laser structures were grown by gas-source molecular beam epitaxy on (0 0 1) GaAs substrates. Elemental Ga and In were used to produce the group-III beam fluxes, while As2 and P2 were produced from cracked AsH3 and PH3. A radio frequency-coupled nitrogen plasma source was used to generate reactive nitrogen species. N and In mole fractions were estimated from previous calibrations in which GaIn(N)As/GaAs superlattice structures were grown and measured by X-ray diffraction. Room temperature photoluminescence (PL) measurements were carried out using the 514.5 nm line of an Ar+ laser as the excitation source. A PbS detector was used to detect the signal at the exit of a 50 cm monochromator associated with a standard lock-in technique. After growth, ex situ RTA was applied by using halogen lamps and flowing N2 gas ambient.
Fig. 1. Room temperature PL from 7 nm Ga0.6In0.4(N)As/ GaAsP SQW grown at different temperatures.
Fig. 2. Schematic representation of the strain-compensated InGaNAs/GaAsP QW structures.
3. Results and discussions Ga0.6In0.4NAs layer was grown under As-stable conditions. It was found that the N content was only determined by the N flux. The experiments on growing GaInNAs QW structures show that growth temperature is one of the most important factors affecting the QW quality. Samples grown around 4708C exhibited the best PL characteristics with high intensity and narrow peak width as shown in Fig. 1. The lower PL intensities from the samples grown at lower temperatures were mainly due to the reduced migration length of cations resulting in many defects into the material. At higher temperatures, the highly strained layer could relax through formation of three-dimensional islands and/or dislocations which can act as non-radiative centers.
In comparison, two QW structures were grown: one having tensile-strained GaAsP barriers and the other with GaAs barriers at the same condition. It was found that the luminescence intensity from strain-compensated structure was slightly higher than that with GaAs barrier, and effects of strain compensation on PL efficiency were more remarkable for structures having two or more wells. In the following paragraphs, we only discuss the results of GaInNAs QW structures with tensile-strained GaAsP barriers. The schematic structure of strain-compensated GaInNAs/GaAsP QW structures is shown in Fig. 2. It consists of a compressively strained GaInNAs single QW sandwiched between two identical GaAsP under tensile strain. Fig. 3 shows room temperature PL of such structures. All
W. Li et al. / Journal of Crystal Growth 227–228 (2001) 541–544
543
Fig. 4. Room temperature PL peak intensity from GaInNAs/ GaAsP SQW structures annealed at different RTA temperatures for 10 s. Fig. 3. Room temperature PL from 7 nm Ga0.6In0.4(N)As/ GaAsP SQW with different N composition.
samples were annealed at 7508C for 10 s. A strong emission band (the PL full width at half maximum (FWHM)=19.85 meV) was observed at 1.15 mm, indicating that this structure is of good structural quality. The PL peak intensity remains nearly the same and the linewidth increases slightly when only a small amount of nitrogen is incorporated. With higher nitrogen concentration the peak intensity decreases significantly, and the peak broadens. The PL intensity of as-grown GaInNAs/GaAsP QWs is quite weak, mainly due to N related defects and damage induced by N ions in the GaInNAs layer. In addition, due to the reduced migration length of the cations at lower temperature many point defects incorporate into the GaInNAs material. In order to improve the crystalline quality we have studied the PL features of the samples annealed at different temperatures for various time intervals. For both GaInAs/GaAsP and GaInNAs/GaAsP structures the PL intensity were improved significantly. Fig. 4 shows effects of RTA on the optical properties of a GaInNAs/ GaAsP samples. The PL intensity increases with annealing temperature to a maximum at 7508C for 10 s (FWHM=23.5 meV), and then decreases. With increasing RTA temperature, the PL peaks blue shift and broaden for all the QWs, mainly due to interdiffusion of In and N. P diffusion may also be one cause.
The evolution of PL features of a sample as a function of RTA time was investigated at 7508C. The PL intensity increases in the initial stage of RTA and reaches its maximum in 10 s, indicating that the defects are quickly removed from the QW. As annealing continues the intensity decreases, probably due to the reduced quantum efficiency of the QWs with graded interfaces [7]. The FWHM and peak wavelength also change rapidly in the initial stage of RTA. This could be mainly caused by the interdiffusion of N and In atoms. At the beginning of RTA for a defect-rich GaInNAs layer, the diffusion of N and In atoms can be very high. After the defects are removed by annealing, it decreases gradually. Effects of RTA on the lasing properties of Aluminum-free GaInNAs/GaInP SQW laser diodes were also studied. The active region of the laser consists of a single 6-nm-thick GaInNAs quantum well with two 100-nm-thick GaAs space layers sandwiched between Ga0.49In0.51P cladding layers. After growth, uncoated broad Fabry-Perot cavity lasers having a cavity length of 600 mm were fabricated by a standard method. The lasers, soldered on submounts, were tested under a pulsed mode operation at temperature of 208C. Fig. 5 shows the threshold current density (Jth) and emission wavelength of lasers as a function of RTA temperature. Jth decreases significantly as RTA temperature increases, while the lasing wavelength slightly blue shifts.
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W. Li et al. / Journal of Crystal Growth 227–228 (2001) 541–544
effect on the optical quality of the GaInNAs QW structures and performance of laser diodes processed from these materials. The optimal annealing temperature was found to be around 7508C.
References
Fig. 5. Threshold current density and lasing wavelength for GaInNAs/GaInP SQW broad area lasers diodes processed at different RTA temperatures.
4. Conclusions Strain-compensated GaInNAs/GaAsP quantum well structures and lasers were grown by gassource molecular beam epitaxy using a RF-plasma nitrogen radical beam source. Our results show that rapid thermal annealing has a significant
[1] F. Koyama, D. Schlenker, T. Miyamoto, Z. Chen, A. Matsutani, T. Sakaguchi, K. Iga, Electron. Lett. 35 (1999) 1079. [2] M. Kondow, K. Uomi, A. Niwa, T. Kitatani, S. Watahiki, Y. Yazawa, Jpn. J. Appl. Phys. Lett. 73 (1998) 3703. [3] W. Shan, W. Walukiewicz, J.W. Ager III, E.E. Haller, J.F. Geisz, D.J. Friedman, J.M. Olson, S.R. Kurtz, Phys. Rev. Lett. 82 (1999) 1221. [4] P. Perlin, S.G. Subramanya, D.E. Mars, J. Kruger, N. Shapiro, H. Siegle, E.R. Weber, Appl. Phys. Lett. 73 (1998) 3703. [5] C. Elmers, F. Hohnsdorf, J. Koch, C. Agert, S. Leu, D. Karaiskaj, M. Hofmann, Appl. Phys. Lett. 74 (1999) 2271. [6] H.P. Xin, C.W. Tu, Appl. Phys. Lett. 72 (1998) 2442. [7] G. Zhang, J. Na¨ppi, A. Ovtchinnikov, H. Asonen, M. Pessa, J. Appl. Phys. 72 (1992) 3788.
Strain-compensated GaInNAs/GaAsP/GaAs/GaInP quantum well lasers grown by gas-source molecular beam epitaxy Wei Li*, Jani Turpeinen, Petri Melanen, Pekka Savolainen, Petteri Uusimaa, Markus Pessa Optoelectronics Research Center, Tampere University of Technology, P.O. Box 692, FIN-33101 Tampere, Finland
Abstract We report the growth of strain-compensated GaInNAs/GaAsP quantum well structures and lasers by gassource molecular beam epitaxy with a RF-coupled plasma source for nitrogen activation. The influence of growth temperature and rapid thermal annealing (RTA) on the optical properties of GaInNAs/GaAsP quantum well structures were studied. RTA was found to significantly increase the photoluminescence from the quantum wells and reduce the threshold current density of the lasers, mainly due to a removal of N induced non-radiative centers from GaInNAs wells. # 2001 Elsevier Science B.V. All rights reserved. PACS: 81.15.Hi; 81.05.Zx; 78.66.Fd; 78.55.Cr Keywords: A3. Molecular beam epitaxy; B2. Semiconducting III–V materials; B3. Laser diodes
1. Introduction Semiconductor lasers emitting at 1.3 mm are important for high-speed optical data links and optical networks [1]. GaInNAs has been recently proposed as a novel material for near-infrared lasers on GaAs [2]. When combining with GaAs barriers, GaInNAs alloy allows an increased conduction band offset, which should greatly improve high temperature laser performance compared with conventional GaInAsP/InP material system. To date, GaInNAs semiconductor alloy is being intensively studied for both its fundamental properties [3,4] and its potential for *Corresponding author. Tel.: +358-3-365-2452; fax: +3583-365-3400. E-mail address: [email protected].fi (W. Li).
1.3 mm laser applications [2,5]. It has been reported that the incorporation of nitrogen in GaInAs can reduce the band-gap energy and allows emission wavelengths as long as 1.3 mm to be reached [5]. However, the optical material quality deteriorates significantly with increasing N mole fractions [6], resulting in a much higher threshold current density of GaInNAs/GaAs lasers compared with that of GaInAs/GaAs lasers. In order to improve the performance of 1.3 mm GaInNAs/GaAs quantum well (QW) lasers, the nitrogen composition of GaInNAs well should be reduced, although, this leads to an increased strain in the quantum wells. By introducing a strain compensated barrier to this system it is possible to grow highly strained GaInNAs wells free of misfit dislocations. In this paper, we report on the growth of highly strained GaInNAs layers with strain-compensating GaAsP
0022-0248/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 0 7 6 3 - 1
542
W. Li et al. / Journal of Crystal Growth 227–228 (2001) 541–544
barriers by gas-source molecular beam epitaxy. Especially, effects of rapid thermal annealing (RTA) on the optical properties of GaInNAs/ GaAsP quantum well structures and lasing properties of the laser diodes were investigated.
2. Experimental procedures The GaInNAs/GaAsP QW and laser structures were grown by gas-source molecular beam epitaxy on (0 0 1) GaAs substrates. Elemental Ga and In were used to produce the group-III beam fluxes, while As2 and P2 were produced from cracked AsH3 and PH3. A radio frequency-coupled nitrogen plasma source was used to generate reactive nitrogen species. N and In mole fractions were estimated from previous calibrations in which GaIn(N)As/GaAs superlattice structures were grown and measured by X-ray diffraction. Room temperature photoluminescence (PL) measurements were carried out using the 514.5 nm line of an Ar+ laser as the excitation source. A PbS detector was used to detect the signal at the exit of a 50 cm monochromator associated with a standard lock-in technique. After growth, ex situ RTA was applied by using halogen lamps and flowing N2 gas ambient.
Fig. 1. Room temperature PL from 7 nm Ga0.6In0.4(N)As/ GaAsP SQW grown at different temperatures.
Fig. 2. Schematic representation of the strain-compensated InGaNAs/GaAsP QW structures.
3. Results and discussions Ga0.6In0.4NAs layer was grown under As-stable conditions. It was found that the N content was only determined by the N flux. The experiments on growing GaInNAs QW structures show that growth temperature is one of the most important factors affecting the QW quality. Samples grown around 4708C exhibited the best PL characteristics with high intensity and narrow peak width as shown in Fig. 1. The lower PL intensities from the samples grown at lower temperatures were mainly due to the reduced migration length of cations resulting in many defects into the material. At higher temperatures, the highly strained layer could relax through formation of three-dimensional islands and/or dislocations which can act as non-radiative centers.
In comparison, two QW structures were grown: one having tensile-strained GaAsP barriers and the other with GaAs barriers at the same condition. It was found that the luminescence intensity from strain-compensated structure was slightly higher than that with GaAs barrier, and effects of strain compensation on PL efficiency were more remarkable for structures having two or more wells. In the following paragraphs, we only discuss the results of GaInNAs QW structures with tensile-strained GaAsP barriers. The schematic structure of strain-compensated GaInNAs/GaAsP QW structures is shown in Fig. 2. It consists of a compressively strained GaInNAs single QW sandwiched between two identical GaAsP under tensile strain. Fig. 3 shows room temperature PL of such structures. All
W. Li et al. / Journal of Crystal Growth 227–228 (2001) 541–544
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Fig. 4. Room temperature PL peak intensity from GaInNAs/ GaAsP SQW structures annealed at different RTA temperatures for 10 s. Fig. 3. Room temperature PL from 7 nm Ga0.6In0.4(N)As/ GaAsP SQW with different N composition.
samples were annealed at 7508C for 10 s. A strong emission band (the PL full width at half maximum (FWHM)=19.85 meV) was observed at 1.15 mm, indicating that this structure is of good structural quality. The PL peak intensity remains nearly the same and the linewidth increases slightly when only a small amount of nitrogen is incorporated. With higher nitrogen concentration the peak intensity decreases significantly, and the peak broadens. The PL intensity of as-grown GaInNAs/GaAsP QWs is quite weak, mainly due to N related defects and damage induced by N ions in the GaInNAs layer. In addition, due to the reduced migration length of the cations at lower temperature many point defects incorporate into the GaInNAs material. In order to improve the crystalline quality we have studied the PL features of the samples annealed at different temperatures for various time intervals. For both GaInAs/GaAsP and GaInNAs/GaAsP structures the PL intensity were improved significantly. Fig. 4 shows effects of RTA on the optical properties of a GaInNAs/ GaAsP samples. The PL intensity increases with annealing temperature to a maximum at 7508C for 10 s (FWHM=23.5 meV), and then decreases. With increasing RTA temperature, the PL peaks blue shift and broaden for all the QWs, mainly due to interdiffusion of In and N. P diffusion may also be one cause.
The evolution of PL features of a sample as a function of RTA time was investigated at 7508C. The PL intensity increases in the initial stage of RTA and reaches its maximum in 10 s, indicating that the defects are quickly removed from the QW. As annealing continues the intensity decreases, probably due to the reduced quantum efficiency of the QWs with graded interfaces [7]. The FWHM and peak wavelength also change rapidly in the initial stage of RTA. This could be mainly caused by the interdiffusion of N and In atoms. At the beginning of RTA for a defect-rich GaInNAs layer, the diffusion of N and In atoms can be very high. After the defects are removed by annealing, it decreases gradually. Effects of RTA on the lasing properties of Aluminum-free GaInNAs/GaInP SQW laser diodes were also studied. The active region of the laser consists of a single 6-nm-thick GaInNAs quantum well with two 100-nm-thick GaAs space layers sandwiched between Ga0.49In0.51P cladding layers. After growth, uncoated broad Fabry-Perot cavity lasers having a cavity length of 600 mm were fabricated by a standard method. The lasers, soldered on submounts, were tested under a pulsed mode operation at temperature of 208C. Fig. 5 shows the threshold current density (Jth) and emission wavelength of lasers as a function of RTA temperature. Jth decreases significantly as RTA temperature increases, while the lasing wavelength slightly blue shifts.
544
W. Li et al. / Journal of Crystal Growth 227–228 (2001) 541–544
effect on the optical quality of the GaInNAs QW structures and performance of laser diodes processed from these materials. The optimal annealing temperature was found to be around 7508C.
References
Fig. 5. Threshold current density and lasing wavelength for GaInNAs/GaInP SQW broad area lasers diodes processed at different RTA temperatures.
4. Conclusions Strain-compensated GaInNAs/GaAsP quantum well structures and lasers were grown by gassource molecular beam epitaxy using a RF-plasma nitrogen radical beam source. Our results show that rapid thermal annealing has a significant
[1] F. Koyama, D. Schlenker, T. Miyamoto, Z. Chen, A. Matsutani, T. Sakaguchi, K. Iga, Electron. Lett. 35 (1999) 1079. [2] M. Kondow, K. Uomi, A. Niwa, T. Kitatani, S. Watahiki, Y. Yazawa, Jpn. J. Appl. Phys. Lett. 73 (1998) 3703. [3] W. Shan, W. Walukiewicz, J.W. Ager III, E.E. Haller, J.F. Geisz, D.J. Friedman, J.M. Olson, S.R. Kurtz, Phys. Rev. Lett. 82 (1999) 1221. [4] P. Perlin, S.G. Subramanya, D.E. Mars, J. Kruger, N. Shapiro, H. Siegle, E.R. Weber, Appl. Phys. Lett. 73 (1998) 3703. [5] C. Elmers, F. Hohnsdorf, J. Koch, C. Agert, S. Leu, D. Karaiskaj, M. Hofmann, Appl. Phys. Lett. 74 (1999) 2271. [6] H.P. Xin, C.W. Tu, Appl. Phys. Lett. 72 (1998) 2442. [7] G. Zhang, J. Na¨ppi, A. Ovtchinnikov, H. Asonen, M. Pessa, J. Appl. Phys. 72 (1992) 3788.