Applied Surface Science 255 (2008) 676–678
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Proton radiation effects in quantum dot lasers Shun-ichi Gonda a,*, Hiroyuki Tsutsumi a, Ryoya Ishigami b, Kyo Kume b, Yoshifumi Ito b, Mitsuru Ishida c, Yasuhiko Arakawa c a
Department of Space Communication Engineering, Fukui University of Technology, 3-6-1 Gakuen, Fukui 910-8505, Japan The Wakasawan Energy Research Center, 64-52-1 Nagatani, Tsuruga, Fukui 914-0135, Japan c Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan b
A R T I C L E I N F O
A B S T R A C T
Proton beams with energies of 10 and 200 MeV were irradiated onto InAs quantum dot lasers with a wavelength of 1.3 mm. The increase in threshold current by proton irradiation was small compared with those of the previously reported other quantum dot lasers with larger active region and 1.3-mm InGaAsP quantum well lasers. These results were discussed by taking account of non-ionizing energy loss and effective volume of active region. ß 2008 Elsevier B.V. All rights reserved.
Available online 4 July 2008 PACS: 42.55.Px 61.80Jh 78.55.Cr 78.67.Hc Keywords: Quantum dot Semiconductor laser InAs Proton radiation Radiation hardness
1. Introduction In the use of semiconductor lasers in radiation environments such as space, the investigation of radiation hardness of semiconductor lasers is needed. As for radiation hardness of quantum dots (QDs) and QD lasers, there are several papers. For example, Piva et al. reported the characteristics of InAs/GaAs QD lasers and quantum well (QW) lasers irradiated with P+ ions of energy of 8.56 MeV . Ribbat et al. irradiated proton beams of 2.4 MeV onto InGaAs/GaAs QD lasers with a wavelength of 1.16 mm and QW lasers and found that the change of threshold current by irradiation in QD lasers is smaller . After these works, QD lasers have been made in great progress and QD lasers with temperature-insensitive characteristics and eye-opening under 10 Gb/s modulation are realized using p-doped QD. However, no work has been reported on the radiation hardness of high-performance QD lasers. In this paper, proton radiation effects in a similar type of highperformance InAs/GaAs QD (undope) lasers with a wavelength of
* Corresponding author. Tel.: +81 776 29 2562; fax: +81 776 29 7891. E-mail address: [email protected]
(S.-i. Gonda). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.07.037
1.3 mm were investigated and the results were compared with those of the previously reported other QD lasers and InGaAsP QW lasers with a wavelength of 1.3 mm. 2. Experimental The QD lasers used in this experiment are of active regions with ten stacks of self-organized undope InAs/GaAs quantum dots. The clad layers are p-AlGaAs and n-AlGaAs. The stripe structure is of ridge type and the mesa widths are 1–4 mm (narrow stripe). The front facet is cleavage surface and the back facet is coated with high reﬂectance materials (93%). The cavity length is 0.5 mm (short cavity). The oscillation wavelength is about 1.28 mm. The radiative transition is made via the ground levels in quantum dots. Proton beams with energies of 10 and 200 MeV were irradiated onto the light emitting surface (front facet) of the laser using the multi-purpose accelerator of the Wakasawan Energy Research Center. Energy of 10 MeV is selected because this energy is often used and suitable for comparison with other data obtained in various works. 10-MeV proton irradiation was done in vacuum for an appropriate time and the measurements were made in air after taking out the samples from the vacuum
S.-i. Gonda et al. / Applied Surface Science 255 (2008) 676–678
Fig. 1. Current vs. voltage and current vs. light-output characteristics for QD lasers before and after proton irradiation.
chamber. Proton ﬂuence was calculated from the current ﬂowed into the sample and its neighborhood. This process was repeated several times. 200-MeV proton irradiation was conducted in air at room temperature. Proton ﬂuence was estimated from the current of the ionization chamber placed on the rear side of the sample. 3. Results Current vs. voltage and current vs. light-output characteristics are shown in Fig. 1. In the curves designated by 200 MeV, the uppermost curve is that of non-irradiated sample. The rest of curves are those of the sample irradiated with 200 MeV proton. The proton ﬂuences are in the downward direction 0.9, 2.7, 4.6, 7.1 and 9.8 1013 proton/cm2. The curve designated by 10 MeV is due to the sample, which is different from 200-MeV irradiated sample, irradiated with 5 1013 proton/ cm2. The current–voltage characteristics and the threshold current before irradiation of the 10-MeV sample is almost the same as that of 200-MeV sample, but the light output is a little bit different. So, a small adjustment was made in plotting it to the ﬁgure. After irradiation the current increases a little bit at each voltage. In the current vs. light-output curve, the threshold current Ith increased with increasing proton ﬂuence F. The slope efﬁciency decreases after irradiation, but the magnitude is not so large.
Fig. 2. Proton ﬂuence dependence of normalized threshold current for QD lasers irradiated with 10 and 200-MeV proton.
Fig. 3. Proton energy dependence of NIEL and damage factor KI. Lines are NIEL of GaAs (broken line), InP (lower line)  and InGaAs, which are nearly the same on the logarithmic scale. Square points are KIs of our QD lasers, where the KI of 10 MeV is normalized to the NIEL of 10 MeV. Circled rhombic point is KI of QD lasers by Ribbat et al. . Rhombic point is the extrapolated value at 10 MeV taking account of proton energy dependence of NIEL. Triangular points are KIs of InGaAsP lasers.
As shown in Fig. 2, the normalized threshold current Ith/Ith0 (Ith0 is Ith of pre-irradiated laser) is proportional to F and this relation can be written as Ith ¼ 1 þ K I F; Ith0 where KI is called as threshold current damage factor . The change of the threshold current of the laser irradiated with 10-MeV proton is larger than that of laser irradiated with 200-MeV proton. From Fig. 2, KI (10 MeV) is estimated to be 2.4 1014 cm2/proton and KI (200 MeV) is 0.16 1014 cm2/proton. 4. Discussion By proton irradiation to semiconductors, displacements of constituent atoms take place and defects are formed. These defects (displacement damage) form defect levels in the forbidden band of semiconductors. The levels work as nonradiative recombination centers. This is the main origin of the change of laser characteristics. In discussing proton energy dependence of characteristics change, the concept of NIEL (non-ionizing energy loss) is useful. Fig. 3 shows proton energy dependence of NIEL of GaAs, InP  and InGaAs. The threshold current damage factor KIs of QD lasers are plotted as squares. Here, The KI of 10 MeV is normalized to the NIEL of 10 MeV. In InGaAsP quantum well lasers with a wavelength of 1.3 mm, KI (10 MeV) is 5.6 1014 and KI (200 MeV) is 1.1 1014 cm2/ proton . These values are plotted as triangular points in Fig. 3. These lasers have the nearly same volume of active region as QD lasers in this work. The energy required to displace an atom is called the displacement energy Ed. Ed is nearly proportional to (lattice constant)1/3 of the materials. Hence, The formation of defects is rather easier in InAs than in InGaAsP. In spite of it, The KIs of QD lasers are smaller than those of InGaAsP lasers. Ribbat et al. reported the results of QD lasers irradiated with 2.4-MeV proton . From their ﬁgure KI is estimated, in the same manner as before, to be 30 1014 cm2/proton. This value is plotted as a circled rhombic point in Fig. 3. To compare this with
S.-i. Gonda et al. / Applied Surface Science 255 (2008) 676–678
our results, the extrapolated value at 10 MeV is estimated using NIEL and plotted as a rhombic point. This is large compared with our results. This may be due to the larger volume of active region. The stripe width is 200 mm (broad) and cavity length is 1 mm. The formation of defects is considered to be uniform in the crystal. If the same light output is obtained in the smaller volume, the number of defects related with light emission is small and the effects to the laser characteristics are considered to be small. 5. Summary The change of threshold current by proton irradiation in highperformance QD lasers was small compared with other 1.2–1.3mm lasers. This is because the laser oscillation occurs effectively in very small volume of quantum dot active region.
Acknowledgement Thanks are due to M. Yamamoto of Fujitsu Laboratory Ltd., for the work of InGaAsP QW lasers.
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