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GaAsN quantum dots grown by solid source molecular beam epitaxy
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Journal of Crystal Growth 271 (2004) 8–12 www.elsevier.com/locate/jcrysgro
Growth temperature dependence and study of multilayer self-assembled GaInNAs/GaAsN quantum dots grown by solid source molecular beam epitaxy K.C. Yew, S.F. Yoon, Z.Z. Sun School of Electrical and Electronic Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore Received 27 May 2004; accepted 15 July 2004 Available online 26 August 2004
Abstract Self-assembled GaInNAs quantum dots (QD) with inserted GaAsN strain-reducing layer were grown on GaAs (0 0 1) substrate by solid source molecular beam epitaxy. The highest room temperature photoluminescence (PL) intensity at wavelength of 1.57 mm was obtained from the sample grown at 500 1C, which has the most uniform QD size observed from atomic force microscopy measurement. A number of samples with different spacer thickness and stacked QD layers were grown to investigate the effect of growth condition for GaIn0.5N0.01As QDs. Multilayer structures with 5 QD stacks and spacer layer thickness of 2 nm were found to show the optimum result in terms of PL intensity. r 2004 Elsevier B.V. All rights reserved. PACS: 81.05.Ea; 81.15.Np; 81.40.–z Keywords: A3. GaInNAs/GaAs; A3. Self-assembled quantum dots; A3. Solid source molecular beam epitaxy
1. Introduction Long-wavelength lasers emitting at 1.3 and 1.55 mm are important active components in advanced optical fiber communication systems. Extension of the emission wavelength on GaAsbased materials and devices is an important Corresponding author. Tel.: +6567905428; fax: +6567933318.
E-mail addresses: [email protected] (K.C. Yew), [email protected] (S.F. Yoon).
priority, driven primarily by the lower GaAs substrate cost compared to that of InP. There are several options available for materials grown on GaAs that have the potential for longwavelength emission, such as GaInNAs/GaAs quantum wells (QWs), GaAsSb/GaAs QWs, and, most recently, GaInNAs/GaAsN quantum dots (QDs) [1–3]. Recent reports have demonstrated laser emission at 1.3 mm [4,5] based on GaInNAs. However, emission at longer wavelength towards 1.55 mm still remains a challenge due to the
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.07.030
ARTICLE IN PRESS K.C. Yew et al. / Journal of Crystal Growth 271 (2004) 8–12
fundamental limit of critical thickness in pseudomorphic growth. The usage of GaInNAs QDs to extend the emission wavelength beyond 1.3 mm is therefore being proposed. Although photoluminescence (PL) exceeding 1.3 mm has been reported [1,2], there have been few studies on surface morphology and growth characteristic of solid source molecular beam epitaxy-grown (SSMBE) GaInNAs QDs of emission wavelength approaching 1.55 mm. This paper reports an investigation on the growth characteristic of GaInNAs QDs of different spacer thickness (2, 4 and 6), and samples with 3-, 5-, and 7-stacked QD layers. A series of experiments to vary the growth temperature from 440 to 540 1C was carried out to investigate the effects of growth temperature. Atomic force microscopy (AFM) images indicate the best QD uniformity could be obtained around 480–5001C. Furthermore, increase in growth temperature red shifts the emission wavelength. Beyond 520 1C, the PL spectrum degrades and the wavelength exhibits a blue shift. The results provide an insight into the optimum growth temperature for GaIn0.5N0.01As QDs, and show that (room temperature) PL emission at 1.55 mm can be achieved through systematic growth optimization.
2. Experimental procedure The self-assembled GaInNAs QD samples were grown on (0 0 1) GaAs substrate in an SSMBE system equipped with a radio frequency (RF) plasma-assisted nitrogen source. Our previous work [2,3] has shown that GaIn0.5N0.01As QDs grown by the above method has reasonably high PL intensity and possibility of 1.5 mm emission. Therefore, to be consistent with previously established experimental conditions, the N composition was fixed at 1%, In composition at 50% and thickness at 5 ML for all QD samples in this work. The sample structure (Fig. 1(a)) comprises a 100 nm-thick GaAs buffer layer grown at 580 1C. This is followed by a GaInNAs QD layer grown at different temperature to obtain the optimized growth temperature range for the above GaInNAs QD composition.
9
GaInNAs QDs layer
100nm GaAs buffer layer (a) GaInNAs QDs
GaAsN layer
(b)
100nm GaAs buffer layer
Fig. 1. (a) Schematic diagram of sample structure for AFM measurements, (b) schematic diagram of multilayer QD sample.
Further samples prepared for process optimization comprise a 100-nm-thick GaAs buffer layer, followed by a GaAsN strain-reducing layer (SRL) grown at 500 1C. Next, a 5-ML-thick GaInNAs QD layer was grown at 500 1C, followed by a 5nm-thick GaAsN spacer layer grown at the same temperature. Another layer of GaInNAs QDs was grown at 500 1C, before the growth was terminated by a 5-nm-thick layer of GaAsN. This growth sequence is repeated to achieve the desired number of stacked QD layers. Fig. 1(b) shows the schematic diagram of the sample structure. The growth rate of the GaInNAs QD layer was 0.5 ML/s. Ex -situ surface morphology inspection was carried out using atomic force microscopy (AFM). The PL spectrum was measured using the 514.5 nm Ar+ laser line and detected using a liquid-N2-cooled Ge detector in conjunction with a standard lock-in technique.
3. Results and discussion The growth temperature was varied from 440 to 540 1C to investigate its effect on surface morphology. Fig. 2 shows a series of AFM images of 5ML-thick GaInNAs QD samples grown at 440–5401C. The QD layer appears to show the best uniformity at 480 and 500 1C, (Fig. 2(c) and Fig. 2(d)). Beyond 520 1C, the QD uniformity
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K.C. Yew et al. / Journal of Crystal Growth 271 (2004) 8–12
Fig. 2. AFM images of 5-ML-thick GaInNAs QD samples grown at: (a) 440 1C, (b) 460 1C, (c) 480 1C, (d) 500 1C, (e) 520 1C and (f) 540 1C.
appears to degrade. Furthermore, the lateral dot size increases following increase in the growth temperature, from 15 nm at 440 1C to 28 nm at 500 1C. As seen in Fig. 2(e), the dots started to coalesce beyond 520 1C, and at 540 1C, humps and valleys began to appear on the surface, further
deteriorating the surface morphology. The increase in surface migration length of the adatoms is believed to have contributed to the change in dot size, as the chances of collision of the adatoms to form larger islands increase following increase in temperature.
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Fig. 3. Plot of room temperature PL peak wavelength and peak intensity of samples grown at 440–540 1C.
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PL measurements were taken to examine the optical property of the QD samples. Fig. 3 shows the RT PL peak wavelength and peak intensity change in samples grown at 440–540 1C. The PL peak wavelength shifts from 1260 nm in the sample grown at 440 1C to 1570 nm in the sample grown at 500 1C. The shift to longer wavelength could be attributed to an increase in the QD size following an increase in growth temperature. The decrease in quantum confinement energy following increase in dot size results in PL shift to longer wavelength. Further increase in the growth temperature towards 520 1C causes reduction in the peak wavelength, due probably to increasing onset of indium desorption, which is commonly observed above 500 1C. It is noted that the variation in PL intensity of the samples follows the trend of the growth temperature. The increase of the QDs density from 440 to 500 1C as observed in AFM images of Fig. 2 could be the reason for the increase in intensity. Beyond 500 1C, coalescence of the QDs will lead to decrease in the QDs density, and hence decrease in PL intensity as observed in Fig. 3. Under the optimized growth temperature condition of 500 1C as deduced from the previous experiment, a study was carried out to examine the effects of multiple stacked QD layers and GaAsN spacer layer thickness. Fig. 4(a) plots the PL intensity as function of number of stacked QD layers (3-, 5- and 7-stacked layers) for three
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K.C. Yew et al. / Journal of Crystal Growth 271 (2004) 8–12
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Fig. 4. (a) Plot of 5 K PL-integrated intensity as function of number of stacked QD layers in samples of different spacer thickness. (b) Plot of 5 K PL peak wavelengths as function of number of stacked QD layers in samples of different spacer thickness (c) Plot of FWHM as function of number of stacked QD layers in samples of different spacer thickness.
samples with GaAsN spacer thickness of 2-, 4and 6- nm, respectively. It is noted that following increase in the number of stacked QD layers, the integrated PL intensity increases accordingly, and degrades beyond 5 stacked layers. This trend is true for samples with 2- and 4-nm-thick GaAsN spacer layer but not so for samples with 6-nmthick spacer layer. It is possible that for samples with thin spacer layers (2- and 4-nm-thick GaAsN spacer), coupling exists between the different dot layers to form a columnar structure of QDs. This has the effect of mitigating the non-uniformity effects, since each columnar structure behaves as a single QD column. This helps to increase the PL intensity [6]. Further increase in the number of stacked QD layers (beyond 5-stacked QD layers) increases the total strain around the stacked columnar dots. This could lead to dislocation formation and hence adversely affect the PL intensity. The PL peak wavelength variation as function of number of stacked QD layers is shown in Fig. 4(b) for samples with three different GaAsN
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spacer thickness. Samples with 2-nm-thick spacer layer exhibit blue shift in the wavelength following an increase in the number of stacked QD layers. On the contrary, samples with 4-nm-thick spacer layer exhibit red shift in wavelength. The difference in the observed wavelength shift phenomena in the 2- and 4-nm-thick GaAsN spacer layer samples could be due to two factors. Firstly, there exists the coupling effect (or columnar effect) of the QDs. As the number of stacked QD layer increases, the columnar QD height increases and this could cause a red shift in the wavelength. Secondly, there exists the ‘‘cap layer’’ effect as reported by Fariba et al. [7,8]. Under such an effect, when a cap layer covers the QDs, the dot size will be forced to reduce due to the mass of the cap layer, resulting in a blue shift of the PL wavelength. Furthermore, it was reported that the rate of wavelength decrease is inversely proportional to the layer thickness. Hence, in samples with a 2-nm-thick GaAsN spacer layer, it is possible that this effect could be more dominant in influencing the PL wavelength shift as compared to the columnar effect. On the other hand, for samples with a 4-nm-thick GaAsN spacer layer, the columnar effect appeared to be more dominant over the ‘‘cap layer’’ effect, resulting in the wavelength red shift as observed. Fig. 4(c) shows the full-width at half-maximum (FWHM) as a function of the number of stacked QD layers. The FWHM of the 2-nm-thick spacer samples are observed to reduce from 190 to 120 meV following an increase in the number of stacked QD layers from 3 to 7. This suggests that columnar QDs could reduce the height nonuniformity as the number of stacked layers increases. However, samples with 4-nm-thick spacer showed a reverse trend, where the FWHM increases following increase in the number of stacked QD layers. This could possibly be due to
an increase in the non-coupled dots as the thickness increases. The mixture of columnar QDs and non-coupled dots gives rise to the increase in FWHM.
4. Conclusion In summary, this paper reports a set of experiments to optimize the growth of selfassembled GaInNAs QDs using the RF N plasma-assisted SSMBE system. AFM imaging shows a growth temperature window of 480–500 1C, where uniformly sized QDs of lateral dimension from 25 to 28 nm are achieved. PL spectrum of samples grown at different temperatures (Ts) shows the best optical characteristic in terms of wavelength and intensity at Ts=500 1C. Further experiments using different numbers of stacked QD layers and different spacer thickness in samples grown at Ts=500 1C show the best result in the sample with 5-stacked QD layers and spacer thickness of 2 nm. References [1] T. Hakkarainen, J. Toivonen, M. Sopanen, H. Lipsanen, J. Crystal Growth 248 (2003) 339. [2] K.C. Yew, S.F. Yoon, Z.Z. Sun, S.Z. Wang, J. Crystal Growth 247 (2003) 279. [3] K.C. Yew, S.F. Yoon, Z.Z. Sun, J. Vac. Sci. Technol. B 21 (2003) 2428. [4] M. Kondow, K. Uomi, A. Niwa, T. Kitatani, S. Watahiki, Y. Yazawa, Jpn. J. Appl. Phys. 35 (Part 1) (1996) 1273. [5] K. Nakahara, M. Kondow, T. Kitatani, M. Larson, K. Uomi, IEEE Photonics Technol. Lett. 10 (1998) 487. [6] M. Sugawa, Self-Assemble InGaAs/GaAs Quantum Dots. Vol. 60, Academic Press, New York, (Chapter 2). [7] F. Ferdos, S. Wang, Y.Q. Wei, A. Larsson, M. Sadeghi, Q.X. Zhao, Appl. Phys. Lett. 81 (2002) 1195. [8] H. Saito, K. Nishi, S. Suguo, Appl. Phys. Lett. 73 (1998) 2742.
Journal of Crystal Growth 271 (2004) 8–12 www.elsevier.com/locate/jcrysgro
Growth temperature dependence and study of multilayer self-assembled GaInNAs/GaAsN quantum dots grown by solid source molecular beam epitaxy K.C. Yew, S.F. Yoon, Z.Z. Sun School of Electrical and Electronic Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore Received 27 May 2004; accepted 15 July 2004 Available online 26 August 2004
Abstract Self-assembled GaInNAs quantum dots (QD) with inserted GaAsN strain-reducing layer were grown on GaAs (0 0 1) substrate by solid source molecular beam epitaxy. The highest room temperature photoluminescence (PL) intensity at wavelength of 1.57 mm was obtained from the sample grown at 500 1C, which has the most uniform QD size observed from atomic force microscopy measurement. A number of samples with different spacer thickness and stacked QD layers were grown to investigate the effect of growth condition for GaIn0.5N0.01As QDs. Multilayer structures with 5 QD stacks and spacer layer thickness of 2 nm were found to show the optimum result in terms of PL intensity. r 2004 Elsevier B.V. All rights reserved. PACS: 81.05.Ea; 81.15.Np; 81.40.–z Keywords: A3. GaInNAs/GaAs; A3. Self-assembled quantum dots; A3. Solid source molecular beam epitaxy
1. Introduction Long-wavelength lasers emitting at 1.3 and 1.55 mm are important active components in advanced optical fiber communication systems. Extension of the emission wavelength on GaAsbased materials and devices is an important Corresponding author. Tel.: +6567905428; fax: +6567933318.
E-mail addresses: [email protected] (K.C. Yew), [email protected] (S.F. Yoon).
priority, driven primarily by the lower GaAs substrate cost compared to that of InP. There are several options available for materials grown on GaAs that have the potential for longwavelength emission, such as GaInNAs/GaAs quantum wells (QWs), GaAsSb/GaAs QWs, and, most recently, GaInNAs/GaAsN quantum dots (QDs) [1–3]. Recent reports have demonstrated laser emission at 1.3 mm [4,5] based on GaInNAs. However, emission at longer wavelength towards 1.55 mm still remains a challenge due to the
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.07.030
ARTICLE IN PRESS K.C. Yew et al. / Journal of Crystal Growth 271 (2004) 8–12
fundamental limit of critical thickness in pseudomorphic growth. The usage of GaInNAs QDs to extend the emission wavelength beyond 1.3 mm is therefore being proposed. Although photoluminescence (PL) exceeding 1.3 mm has been reported [1,2], there have been few studies on surface morphology and growth characteristic of solid source molecular beam epitaxy-grown (SSMBE) GaInNAs QDs of emission wavelength approaching 1.55 mm. This paper reports an investigation on the growth characteristic of GaInNAs QDs of different spacer thickness (2, 4 and 6), and samples with 3-, 5-, and 7-stacked QD layers. A series of experiments to vary the growth temperature from 440 to 540 1C was carried out to investigate the effects of growth temperature. Atomic force microscopy (AFM) images indicate the best QD uniformity could be obtained around 480–5001C. Furthermore, increase in growth temperature red shifts the emission wavelength. Beyond 520 1C, the PL spectrum degrades and the wavelength exhibits a blue shift. The results provide an insight into the optimum growth temperature for GaIn0.5N0.01As QDs, and show that (room temperature) PL emission at 1.55 mm can be achieved through systematic growth optimization.
2. Experimental procedure The self-assembled GaInNAs QD samples were grown on (0 0 1) GaAs substrate in an SSMBE system equipped with a radio frequency (RF) plasma-assisted nitrogen source. Our previous work [2,3] has shown that GaIn0.5N0.01As QDs grown by the above method has reasonably high PL intensity and possibility of 1.5 mm emission. Therefore, to be consistent with previously established experimental conditions, the N composition was fixed at 1%, In composition at 50% and thickness at 5 ML for all QD samples in this work. The sample structure (Fig. 1(a)) comprises a 100 nm-thick GaAs buffer layer grown at 580 1C. This is followed by a GaInNAs QD layer grown at different temperature to obtain the optimized growth temperature range for the above GaInNAs QD composition.
9
GaInNAs QDs layer
100nm GaAs buffer layer (a) GaInNAs QDs
GaAsN layer
(b)
100nm GaAs buffer layer
Fig. 1. (a) Schematic diagram of sample structure for AFM measurements, (b) schematic diagram of multilayer QD sample.
Further samples prepared for process optimization comprise a 100-nm-thick GaAs buffer layer, followed by a GaAsN strain-reducing layer (SRL) grown at 500 1C. Next, a 5-ML-thick GaInNAs QD layer was grown at 500 1C, followed by a 5nm-thick GaAsN spacer layer grown at the same temperature. Another layer of GaInNAs QDs was grown at 500 1C, before the growth was terminated by a 5-nm-thick layer of GaAsN. This growth sequence is repeated to achieve the desired number of stacked QD layers. Fig. 1(b) shows the schematic diagram of the sample structure. The growth rate of the GaInNAs QD layer was 0.5 ML/s. Ex -situ surface morphology inspection was carried out using atomic force microscopy (AFM). The PL spectrum was measured using the 514.5 nm Ar+ laser line and detected using a liquid-N2-cooled Ge detector in conjunction with a standard lock-in technique.
3. Results and discussion The growth temperature was varied from 440 to 540 1C to investigate its effect on surface morphology. Fig. 2 shows a series of AFM images of 5ML-thick GaInNAs QD samples grown at 440–5401C. The QD layer appears to show the best uniformity at 480 and 500 1C, (Fig. 2(c) and Fig. 2(d)). Beyond 520 1C, the QD uniformity
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Fig. 2. AFM images of 5-ML-thick GaInNAs QD samples grown at: (a) 440 1C, (b) 460 1C, (c) 480 1C, (d) 500 1C, (e) 520 1C and (f) 540 1C.
appears to degrade. Furthermore, the lateral dot size increases following increase in the growth temperature, from 15 nm at 440 1C to 28 nm at 500 1C. As seen in Fig. 2(e), the dots started to coalesce beyond 520 1C, and at 540 1C, humps and valleys began to appear on the surface, further
deteriorating the surface morphology. The increase in surface migration length of the adatoms is believed to have contributed to the change in dot size, as the chances of collision of the adatoms to form larger islands increase following increase in temperature.
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Fig. 3. Plot of room temperature PL peak wavelength and peak intensity of samples grown at 440–540 1C.
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PL measurements were taken to examine the optical property of the QD samples. Fig. 3 shows the RT PL peak wavelength and peak intensity change in samples grown at 440–540 1C. The PL peak wavelength shifts from 1260 nm in the sample grown at 440 1C to 1570 nm in the sample grown at 500 1C. The shift to longer wavelength could be attributed to an increase in the QD size following an increase in growth temperature. The decrease in quantum confinement energy following increase in dot size results in PL shift to longer wavelength. Further increase in the growth temperature towards 520 1C causes reduction in the peak wavelength, due probably to increasing onset of indium desorption, which is commonly observed above 500 1C. It is noted that the variation in PL intensity of the samples follows the trend of the growth temperature. The increase of the QDs density from 440 to 500 1C as observed in AFM images of Fig. 2 could be the reason for the increase in intensity. Beyond 500 1C, coalescence of the QDs will lead to decrease in the QDs density, and hence decrease in PL intensity as observed in Fig. 3. Under the optimized growth temperature condition of 500 1C as deduced from the previous experiment, a study was carried out to examine the effects of multiple stacked QD layers and GaAsN spacer layer thickness. Fig. 4(a) plots the PL intensity as function of number of stacked QD layers (3-, 5- and 7-stacked layers) for three
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K.C. Yew et al. / Journal of Crystal Growth 271 (2004) 8–12
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Fig. 4. (a) Plot of 5 K PL-integrated intensity as function of number of stacked QD layers in samples of different spacer thickness. (b) Plot of 5 K PL peak wavelengths as function of number of stacked QD layers in samples of different spacer thickness (c) Plot of FWHM as function of number of stacked QD layers in samples of different spacer thickness.
samples with GaAsN spacer thickness of 2-, 4and 6- nm, respectively. It is noted that following increase in the number of stacked QD layers, the integrated PL intensity increases accordingly, and degrades beyond 5 stacked layers. This trend is true for samples with 2- and 4-nm-thick GaAsN spacer layer but not so for samples with 6-nmthick spacer layer. It is possible that for samples with thin spacer layers (2- and 4-nm-thick GaAsN spacer), coupling exists between the different dot layers to form a columnar structure of QDs. This has the effect of mitigating the non-uniformity effects, since each columnar structure behaves as a single QD column. This helps to increase the PL intensity [6]. Further increase in the number of stacked QD layers (beyond 5-stacked QD layers) increases the total strain around the stacked columnar dots. This could lead to dislocation formation and hence adversely affect the PL intensity. The PL peak wavelength variation as function of number of stacked QD layers is shown in Fig. 4(b) for samples with three different GaAsN
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spacer thickness. Samples with 2-nm-thick spacer layer exhibit blue shift in the wavelength following an increase in the number of stacked QD layers. On the contrary, samples with 4-nm-thick spacer layer exhibit red shift in wavelength. The difference in the observed wavelength shift phenomena in the 2- and 4-nm-thick GaAsN spacer layer samples could be due to two factors. Firstly, there exists the coupling effect (or columnar effect) of the QDs. As the number of stacked QD layer increases, the columnar QD height increases and this could cause a red shift in the wavelength. Secondly, there exists the ‘‘cap layer’’ effect as reported by Fariba et al. [7,8]. Under such an effect, when a cap layer covers the QDs, the dot size will be forced to reduce due to the mass of the cap layer, resulting in a blue shift of the PL wavelength. Furthermore, it was reported that the rate of wavelength decrease is inversely proportional to the layer thickness. Hence, in samples with a 2-nm-thick GaAsN spacer layer, it is possible that this effect could be more dominant in influencing the PL wavelength shift as compared to the columnar effect. On the other hand, for samples with a 4-nm-thick GaAsN spacer layer, the columnar effect appeared to be more dominant over the ‘‘cap layer’’ effect, resulting in the wavelength red shift as observed. Fig. 4(c) shows the full-width at half-maximum (FWHM) as a function of the number of stacked QD layers. The FWHM of the 2-nm-thick spacer samples are observed to reduce from 190 to 120 meV following an increase in the number of stacked QD layers from 3 to 7. This suggests that columnar QDs could reduce the height nonuniformity as the number of stacked layers increases. However, samples with 4-nm-thick spacer showed a reverse trend, where the FWHM increases following increase in the number of stacked QD layers. This could possibly be due to
an increase in the non-coupled dots as the thickness increases. The mixture of columnar QDs and non-coupled dots gives rise to the increase in FWHM.
4. Conclusion In summary, this paper reports a set of experiments to optimize the growth of selfassembled GaInNAs QDs using the RF N plasma-assisted SSMBE system. AFM imaging shows a growth temperature window of 480–500 1C, where uniformly sized QDs of lateral dimension from 25 to 28 nm are achieved. PL spectrum of samples grown at different temperatures (Ts) shows the best optical characteristic in terms of wavelength and intensity at Ts=500 1C. Further experiments using different numbers of stacked QD layers and different spacer thickness in samples grown at Ts=500 1C show the best result in the sample with 5-stacked QD layers and spacer thickness of 2 nm. References [1] T. Hakkarainen, J. Toivonen, M. Sopanen, H. Lipsanen, J. Crystal Growth 248 (2003) 339. [2] K.C. Yew, S.F. Yoon, Z.Z. Sun, S.Z. Wang, J. Crystal Growth 247 (2003) 279. [3] K.C. Yew, S.F. Yoon, Z.Z. Sun, J. Vac. Sci. Technol. B 21 (2003) 2428. [4] M. Kondow, K. Uomi, A. Niwa, T. Kitatani, S. Watahiki, Y. Yazawa, Jpn. J. Appl. Phys. 35 (Part 1) (1996) 1273. [5] K. Nakahara, M. Kondow, T. Kitatani, M. Larson, K. Uomi, IEEE Photonics Technol. Lett. 10 (1998) 487. [6] M. Sugawa, Self-Assemble InGaAs/GaAs Quantum Dots. Vol. 60, Academic Press, New York, (Chapter 2). [7] F. Ferdos, S. Wang, Y.Q. Wei, A. Larsson, M. Sadeghi, Q.X. Zhao, Appl. Phys. Lett. 81 (2002) 1195. [8] H. Saito, K. Nishi, S. Suguo, Appl. Phys. Lett. 73 (1998) 2742.