Reduced temperature sensitivity of lasing wavelength in near-1.3 lm InAs=GaAs quantum-dot laser with stepped composition strain-reducing layer H.Y. Liu, T.J. Badcock, C.Y. Jin, E. Nabavi, K.M. Groom, M. Hopkinson and D.J. Mowbray An InGaAs strain-reducing capping layer with a stepped composition is shown to significantly reduce the temperature sensitivity of the lasing wavelength in a 1.3 mm InAs=GaAs quantum-dot laser. With this technique, the sensitivity is reduced from 0.48 nm=K for a laser with standard capping layer to 0.11 nm=K for the new design over the temperature range 20–130 C.
Introduction: Recently, high-performance 1.3 mm quantum dot (QD) lasers have been successfully demonstrated exhibiting a very low threshold current density ( Jth) [1–3], high power output [4], a temperature-insensitive Jth [5, 6], and a high modulation speed [7]. However, the temperature sensitivity of the lasing wavelength is as high as 0.5 nm=K, primarily reflecting the temperature-induced change of the bandgap of the InAs QDs [7, 8]. This temperature sensitivity is a potential drawback for fibre optics datacom applications near 1.3 mm, for which a stable lasing wavelength is essential. Recently, the concept of the tilted cavity laser has been proposed to achieve wavelength-stable operation of near-1.3 mm InAs=GaAs QD lasers [9]. Applying this technique, a very low temperature coefficient of 0.165 nm=K has been demonstrated for operation between 200 and 70 C. An alternative technique involves the modification of the strain distribution around InAs dots by overgrowing with an InGaAs strain-reducing layer (SRL). This has been shown to suppress the temperature sensitivity of the wavelength of the QD photoluminescence (PL) when the In composition in the SRL is above 25% [10]. However, such high In compositions dramatically reduce the roomtemperature PL efficiency [11]. In addition, the effect of a high In content InGaAs SRL on the temperature sensitivity of the lasing wavelength is still unclear. In this Letter, we show that the temperature sensitivity of the lasing wavelength for a near-1.3 mm QD laser can be significantly reduced by overgrowing the InAs QDs with an InGaAs stepped composition strainreducing layer (SCSRL). A temperature sensitivity of the lasing wavelength of 0.11 nm=K between 20 and 130 C is achieved for a five-layer device with room-temperature lasing at 1256 nm. This temperature sensitivity is much lower than the values of 0.5 nm=K typically observed for 1.3 mm InAs=GaAs QD lasers.
sequence was separated by a 50 nm GaAs spacer layer of which the initial 15 nm was grown at 510 C, following which the temperature was increased to 580 C for the remaining 35 nm. This growth technique is referred to as the high growth temperature spacer layer (HGTSL), and acts to reduce surface roughness, which in turn suppresses the formation of dislocated dots in the second and subsequent dot layers [12]. The active region was grown at the centre of an undoped 150 nm GaAs=AlGaAs waveguide, with n-type lower and p-type upper 1.5 mm Al0.4Ga0.6As cladding layers. A 300 nm p GaAs contact layer completed the growth.
Fig. 2 Temperature variation of lasing spectrum over range 20–130 C Cavity dimensions are 3 mm 15 mm Injection level is 1.1 threshold at each temperature
Fig. 3 Comparison of temperature dependence of lasing wavelength for QD laser devices with stepped composition InGaAs strain-reducing layer and with normal strain-reducing layer
Fig. 1 Schematic diagram of laser structure containing InGaAs stepped composition strain-reducing layer
Experiment: A schematic diagram of the five-layer InAs=GaAs QD laser structure incorporating an InGaAs SCSRL is shown in Fig. 1. The structure was grown in a solid-source VG Semicon V90H Molecular Beam Epitaxy system on a three-inch Si-doped GaAs (100) substrate. The dots were formed from three monolayers of InAs deposited on 2 nm of In0.15Ga0.85As. Above the QDs, the growth sequence of the InGaAs SCSRL consisted of 2 nm In0.22Ga0.78As, 2 nm In0.15Ga0.85As and 2 nm In0.08Ga0.92As. This sequence gives a high In composition immediately next to the QDs but minimises the total amount of strained material. Each InGaAs=InAs=InGaAs
Results: Standard ridge waveguide lasers with 15 mm width and 3 mm length were fabricated. No facet coatings were applied. Laser characteristics were measured for pulsed current injection (5 ms, 10 kHz). Room temperature (30 C) lasing occurs via the QD ground state at a wavelength at 1256 nm. The threshold current density is 104 A=cm2. Ground-state lasing is achieved up to 130 C where the emission wavelength is 1269 nm. Fig. 2 shows lasing spectra recorded at 1.1 threshold over the temperature range 20– 130 C. The temperature dependence of the lasing wavelength is shown in Fig. 3. For comparison, data for a standard 1.3 mm InAs=InGaAs dot-in-a-well(DWELL) laser previously reported in [12], where the QDs are capped with a 6 nm In0.15Ga0.85As SRL, is also shown. The temperature coefficient of the lasing wavelength for the device with the SCSRL is only 0.11 nm=K, significantly smaller than the value of 0.48 nm=K for the standard DWELL device, which is typical of other reports for 1.3 mm DWELL devices and atomic-layer-epitaxy QD lasers. The reason for this decreased temperature sensitivity is unclear, but may result from different thermal expansion coefficients between the QDs and surrounding material. In principle this could result in a temperature dependent strain in the QDs, which acts to partly cancel the direct thermally mediated bandgap variation [10]. Fig. 4 shows the temperature
ELECTRONICS LETTERS 7th June 2007 Vol. 43 No. 12
dependence of Jth for the SCSRL containing device. The characteristic temperature, T0, is 45 K over the temperature range 20–130 C.
Fig. 4 Temperature dependence of threshold current density for QD laser with InGaAs SCSRL
Conclusions: A significant reduction of the temperature sensitivity of the lasing wavelength has been demonstrated in a near-1.3 mm InAs=GaAs QD laser with an InGaAs stepped composition strain-educing layer. The temperature coefficient is reduced to 0.11 nm=K over the temperature range 20–130 C. This technique could find application in lasers designed for optical fibre systems. Acknowledgment: This work is supported by the UK Engineering and Physical Sciences Research Council (EPSRC). # The Institution of Engineering and Technology 2007 12 March 2007 Electronics Letters online no: 20070716 doi: 10.1049/el:20070716 H.Y. Liu, C.Y. Jin, K.M. Groom and M. Hopkinson (Department of Electronic & Electrical Engineering, EPSRC National Centre for III-V Technologies, University of Sheffield, Sheffield S1 3JD, United Kingdom) E-mail:
[email protected] T.J. Badcock, E. Nabavi and D.J. Mowbray (Department of Physics & Astronomy, University of Sheffield, Sheffield S3 7RH, United Kingdom)
2 Huang, X., Stintz, A., Hains, C.P., Liu, G.T., Cheng, J., and Malloy, K.J.: ‘Very low threshold current density room temperature continuous-wave lasing from a single-layer InAs quantum-dot laser’, IEEE Photonics Technol. Lett., 2000, 12, pp. 227–229 3 Park, G., Shchekin, O.B., Hufferaker, D.L., and Deppe, D.G.: ‘Lowthreshold oxide-confined 1.3-mm quantum-dot laser’, IEEE Photonics Technol. Lett., 2000, 12, pp. 230–232 4 Zhukov, A.E., Kovsh, A.R., Ustinov, V.M., Shernyakov, Yu.M., Mikhrin, S.S., Maleev, N.A., Kondrat’eva, E.Yu., Livshits, D.A., Maximov, M.V., Volovik, B.V., Bedarev, D.A., Musikhin, Yu.G., Ledentsov, N.N., Kop’ev, P.S., Alferov, Zh.I., and Bimberg, D.: ‘Continuous-wave operation of long-wavelength quantum-dot diode laser on a GaAs substrate’, IEEE Photonics Technol. Lett., 1999, 11, pp. 1345–1347 5 Fathpour, S., Mi, Z., Bhattacharya, P., Kovsh, A.R., Mikhrin, S.S., Krestnikov, I.L., Kozhukhov, A.V., and Ledentsov, N.N.: ‘The role of Auger recombination in the temperature-dependent output characteristics (T0 ¼ 1) of p-doped 1.3 mm quantum dot lasers’, Appl. Phys. Lett., 2004, 85, pp. 5164–5166 6 Liu, H.Y., Liew, S.L., Badcock, T., Mowbray, D.J., Skolnick, M.S., Ray, T.L., Choi, T.L., Groom, K.M., Stevens, B., Hasbullah, F., Jin, C.Y., Hopkinson, M., and Hogg, R.A.: ‘P-doped 1.3-mm InAs=GaAs quantumdot laser with a low threshold current density and high differential efficiency’, Appl. Phys. Lett., 2006, 89, p. 073113 7 Kim, S.M., Wang, Y., Keever, M., and Harris, J.S.: ‘High-Frequency Modulation Characteristics of 1.3 mm InGaAs Quantum Dot Lasers’, IEEE Photonics. Technol. Lett., 2004, 16, pp. 377–379 8 Kim, K.W., Chao, N.K., Pyu, S.P., Song, J.D., Choi, W.J., Lee, J.I., and ParK, J.H.: ‘InAs=GaAs quantum-dot laser diode lasing at 1.3 mm with triplestacked-layer dot-in-a-well structure grown by atomic layer epitaxy’, Jpn. J. Appl. Phys., 2006, 45, pp. 8010–8013 9 Ledentsov, N.N., Shchukin, V.A., Mikhrin, S.S., Krestnikov, I.L., Kozhukhov, A.V., Kovsh, A.R., Karachinsky, L.Ya, Maximov, M.V., Novikov, I.I., and Shernyakov, Yu.M.: ‘Wavelength-stabilized tilted cavity quantum dot laser’, Semicond. Sci. Technol., 2004, 19, pp. 1183– 1188 10 Mukaia, K. and Sugawara, M.: ‘Suppression of temperature sensitivity of interband emission energy in 1.3 mm-region by an InGaAs overgrowth on self-assembled InGaAs=GaAs quantum dots’, Appl. Phys. Lett., 1999, 74, pp. 3963–3965 11 Liu, H.Y., Hopkinson, M., Harrison, C.N., Steer, M.J., Frith, R., Sellers, I.R., Mowbray, D.J., and Skolnick, M.S.: ‘Optimizing the growth of 1.3 mm InAs=InGaAs dots-in-a-well structure’, J. Appl. Phys., 2003, 93, pp. 2931–2936 12 Liu, H.Y., Sellers, I.R., Badcock, T.J., Mowbray, D.J., Skolnick, M.S., Groom, K.M., Gutierrez, M., Hopkinson, M., Ng, J.S., David, J.P.R., and Beanland, R.: ‘Improved performance of 1.3 mm multilayer InAs=GaAs quantum-dot lasers using a high-growth-temperature GaAs spacer layer’, Appl. Phys. Lett., 2004, 85, pp. 704–706
References 1 Liu, H.Y., Childs, D.T., Badcock, T.J., Groom, K.M., Sellers, I.R., Hopkinson, M., Hogg, R.A., Robbins, D.J., Mowbray, D.J., and Skolnick, M.S.: ‘High-performance 3-layer 1.3 mm InAs=GaAs quantum-dot lasers with very low continuous-wave room-temperature threshold currents’, IEEE Photonics Technol. Lett., 2005, 17, pp. 1139– 1341
ELECTRONICS LETTERS 7th June 2007 Vol. 43 No. 12
