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p s scurrent topics in solid state physics

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caphys. stat. sol. (c) 5, No. 9, 2935–2937 (2008) / DOI 10.1002/pssc.200779283

Process-free estimation of threshold current density of InAs quantum dot laser

N. Kumagai*, 1, 2, K. Watanabe1, 2, M. Ishida1, 2, Y. Nakata1, 2, N. Hatori1, 2, H. Sudo4, T. Yamamoto4, M. Sugawara4, 5, and Y. Arakawa1, 2, 3

1 Institute for Nano Quantum Information Electronics (INQIE), The University of Tokyo, 4-6-1 Komaba, Meguro-ku,

Tokyo 153-8505, Japan 2 Institute of Industrial Science (IIS), The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan 3 Research Center for Advanced Science and Technology (RCAST), The University of Tokyo, 4-6-1 Komaba, Meguro-ku,

Tokyo 153-8505, Japan 4 Fujitsu Laboratories Ltd., 10-1 Morinosato-Wakamiya, Atsugi 243-0197, Japan 5 QD Laser Inc., 5F Sanso-Kudan Bld., 1-14-17 Kudan-Kita, Chiyoda-ku, Tokyo 102-0073, Japan

Received 9 October 2007, revised 7 March 2008, accepted 10 March 2008

Published online 13 June 2008

PACS 42.55.Px, 78.55.Cr, 78.67.Hc, 81.05.Eq, 81.07.Ta, 81.15.Hi

* Corresponding author: e-mail nkumagai@iis.u-tokyo.ac.jp, Phone: +81-3-5452-6098 (57711), Fax: +81-35452-6247

© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction Self-assembled quantum dot (QD) by Stranski-Krastanow growth mode has been attractive for not only physical interests in zero dimensional system but also applications to optoelectronics devices such as a laser and an optical amplifier [1]. Especially, InAs QDs laser on GaAs based material system is attractive for one of next-generation sources for 1.3 µm optical communication band. To realize high performance InAs QD laser, high density and high uniformity of QDs are required. As known well, optimizations for growth condition of InAs QDs are car-ried out by photoluminescence (PL) and atomic force scanning microscopy characterization [2, 3]. Besides, op-timizations for stacking of QD layer are required in order to increase in modal gain [3-6]. However, laser characteristics like threshold current density (Jth) or gain were able to be hardly related with PL characterizations in our knowledge. Therefore it seems that non-destructive characterization method for as-grown wa-fer of InAs QD laser structure is significant to estimate

quality of wafers before process to fabricate laser diode and immediate feedback on optimizing to growth condi-tions. In this study we have performed PL measurements for wafers of 1.3 µm InAs QD laser by high power excitation and related to threshold current density of InAs QDs laser. Additionally, we discuss to relate enhancement of modal gain by increase of stacked QD layer with PL peak inten-sity. 2 Experimental All wafers of InAs QD laser struc-ture with 5 and 10 stacked QD layers were grown on (100) n-GaAs substrate by molecular beam epitaxy. In an active region, thickness of GaAs interlayer between each InAs QDs layers with InGaAs strain reduced layer is 40 nm. QD density per a layer has been fixed at 3x1010 or 6x1010 cm-2. The active region is sandwiched between n- and p-type AlGaAs cladding layers. Various InAs QD laser wafers were grown by changing growth parameters. Growth tem-

We have associated threshold current density of 1.3 µm InAs

quantum dot (QD) laser diodes with the photoluminescence

(PL) peak intensity of as-grown wafers. These InAs QD laser

wafers with stacking structure of 5 and 10 QD layers were

grown by molecular beam epitaxy. PL measurements have

carried out by high power excitation at room temperature.

The PL peak intensity from ground state of InAs QDs has

been inversely proportional to the threshold current density.

Additionally, enhancement of PL peak intensity by increase

in stacked QD layers has been corresponded to increase in

maximum modal gain of QD laser. These results indicate that

PL characterization by high power excitation is useful to es-

timate quality of as-grown wafers without fabrication of QD

laser diode.

2936 N. Kumagai et al.: Threshold current density of InAs quantum dot laser

© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-c.com

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perature of GaAs interlayer and p-AlGaAs cladding layer were varied from 500 to 560 oC and from 530 to 580 oC, respectively. And nominal InAs coverage per a layer was 2.6 or 2.7ML. These growth parameters were linked to create a compound condition. Wafers have the same laser diode structure. The detail of optimizing of growth condi-tions will be reported elsewhere [7]. Threshold current density and modal gain have been evaluated using broad area type laser diode with the cavity length of 500 µm and its stripe width of 80 µm. The both facet are treated by high reflection (HR) coating with Rf/ Re = 81%/ 93%. PL measurements have performed at room temperature by pulsed Titanium -sapphire laser at 800 nm with repeti-tion frequency of 80 MHz in order to increase in peak power of excitation. High power excitation is desirable to close to the lasing situation. Since AlGaAs layer is almost transparent for the wavelength, most of carriers are excited from GaAs matrix in active region.

Figure 1 (a) Typical evolution of PL spectra by high power exci-

tation. (b) I-V and I-L characteristics of one of InAs QD lasers.

The inset is lasing spectra at room temperature.

3 Results and discussion Figure 1(a) shows typical evolution of power dependence of PL spectra for as-grown InAs QD laser wafer at room temperature. PL peak from 2nd excited state has clearly observed as excitation power increase. And the PL peak intensity from ground state has commenced to saturate at 40 mW excitation. Therefore we associated PL intensity of these wafers by the highest exci-tation power of 50 mW with their threshold current densi-

ties. Figure 1(b) shows the typical I-V and I-L characteris-tics. The lasing spectrum under CW operation at 1.34 µm at room temperature is shown in inset. Figure 2 shows that PL peak intensity of various as-grown wafers of InAs QD laser as a function of 1/Jth. Jth has been measured under pulsed operation at room tem-perature. All plotted data in Fig. 2 have been obtained from wafers with 5 stacked QD layers. The PL peak intensity has been inversely proportional to Jth. The solid line is fit-ted by the least square method. This result indicates that emission efficiency of InAs QDs in active region is di-rectly reflected to PL peak intensity by high power excita-tion. Shimizu et al mentioned that Jth is inversely propor-tional to spontaneous emission efficiency at threshold in approximated equation [8]. Our results show experimen-tally that PL peak intensity is proportional to 1/Jth. About the scattering of plots for samples with medium PL peak intensity in Fig. 2, the influence of uniformity of QDs has been considered. However, it is hard to connect the size distribution to full width at half maximum (FWHM) of PL spectra by high power excitation. Size uni-formity of QDs should be characterized by FWHM with low power excitation at low temperature. In spite of same PL peak intensity, low uniformity of QDs layer may lead to increase in Jth.

Figure 2 PL peak intensity of as-grown wafers of InAs QD laser

at ground state as a function of 1/Jth. The solid line is the fitted re-

sult by the least-squares method.

Figure 3 shows that power dependences of PL intensity of 2 groups (group A & group B) of InAs QD laser wafers at room temperature. PL measurements have performed in the same condition to that in Fig. 1(a). Each group consists of 5- and 10-stacked QD layers wafers. QD density of group A and B were 3x1010cm-2 and 6x1010 cm–2, respec-tively. As excitation power increase, stacking effect from 5 to 10 layers has been clearer. For the group A, the maxi-mum modal gain has increased up to 31 cm–1 from 16 cm–1 as numbers of stacked QD layer increase from 5 to 10 lay-ers. PL peak intensity also has become almost twice due to increase in number of QDs in an active region [9]. On the other hand, group B hasn’t showed two times PL peak intensity despite of increase in stacked QD layers.

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phys. stat. sol. (c) 5, No. 9 (2008) 2937

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Contributed

Article

Although the maximum modal gain of sample B (5 stacked QD layers) has achieved 28 cm-1, that of sample B’ has been 38 cm–1. The PL enhancement is less than twice (about 1.4 times). At least, this result suggests that amount of contributing QDs in the stacking structure has not be-come twice and/ or huge increase in non radiative centres like defects. The reason for inadequate stacking effect may be defects due to stacking that generated during the stack-ing growth from 5 to 10 layers [4, 5]. This result indicates that further optimization of stacking condition for the case of high QD density of 6x1010 cm–2. Therefore it seems that PL characterization by high power excitation is also useful to evaluate stacking effect of QD layers. Although the maximum modal gain of sample B’ (10 stacked QD layers) is 38 cm–1 due to the increase in QD density [9], the threshold current density has become 3.6 times of sample A’ and the PL peak intensity has been lower than that of sample B as shown in Fig. 3.

Figure 3 Power dependence of PL intensity of group A and B at

room temperature. Group A is a pair of 5 and 10 stacked QD laser

wafer with QD density of 3.0x1010cm–2. Group B is a pair of 5

and 10 stacked QD laser wafer with QD density of 6.0x1010 cm–2.

4 Conclusion We have grown various wafers of InAs QDs laser structure and carried out PL measurement for them by high power excitation. And their laser characteris-tics like Jth and modal gain have evaluated by broad type laser diodes. The PL intensity of as-grown wafers has been associated with threshold current density. The intensity has been inversely proportional to Jth. For the more detailed characterization, size uniformity of QDs should be also taken in association with Jth. Additionally we have investi-gated stacking effect of QD layers in active regions by re-lating between enhancement of PL intensity and increase in modal gain. Our results suggest that PL characterization by high power excitation is useful to evaluate Jth without process for fabrication of laser diodes. By characterization of as-grown wafer, efficient development and feedback to opti-mization for growth condition will be expected.

Acknowledgements This work is support from the Special

Coordination Funds for Promoting Science and Technology,

MEXT of Japan, and OITDA trusted by NEDO as Photonic net-

work technology Project. The authors thank Dr. Nomura for his

cooperation in optical measurements.

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relayed Materials), paper FrB1-3 (2007).

0 10 20 30 40 50 60 70

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PL

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Excitation power [mW]

A ( x5, 3.0x1010

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A' (x10, 3.0x1010

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B (x5, 6.0x1010

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B' (x10, 6.0x1010

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~x1.5