High Performance 1.45 μm InAs Quantum Dot Lasers on GaAs

The conventional light source for long-haul optical communications has been InGaAsP/InP double heterostructure or multi-quantum well lasers, but these devices characteristically have high Ith, small To (40 50 K), small T1, and large values of chirp (> 2 A) and a-factor (2 5). Two different material systems are being investigated for long wavelength lasers, both based on GaAs substrates. In the first, the active region consists of GaInNAsSb quantum wells and the lasers (2 1.45 1.6 ptm) are usually characterized by high Jth (> 1000 A/cm2).' The modulation characteristics are hitherto unknown. The other alternative is the use of In(Ga)As/GaAs quantum dots (QDs) as the gain material. While extraordinary performance has been reported for 1.3 ptm QD lasers (low Ith, To =o, a e 0, chirp < 1 A, f3dB = 12 GHz),2' the development of 1.55 ptm QD lasers, wherein metamorphic QD heterosructures have to be used due to the large strain, has not been so optimistic. The devices reported (2 1.45 ptm) have Jth> 800 A/cm2, poor luminescence of the QDs with linewidth > 70 meV and no data is available on the dynamic characteristics.4 By detailed investigation of the growth kinetics of the metamorphic heterostructures, we have realized InAs QDs on GaAs that are comparable in PL intensity and linewidth to state-of-the-art 1.1 and 1.3 ptm InAs QDs. 1.45 ptm lasers made with these heterostructures exhibit, for the first time, ultra low Jth (70 A/cm2), To oo,f3dB= 5 GHz, chirp < 0.3 A, a 1.0, and present a practical alternative to the InGaAsP/InP technology. The Ino.15Gao.85As/Ino. 5Al0.35Gao.5oAs separate confinement heterostructure lasers with InAs QD active region, as illustrated in Fig. 1(a), were grown on (001) GaAs substrates by MBE. The active region consists of four or eight QD layers, which are either undoped or modulation doped p-type using Be (20 holes per dot). A 0.6 ptm Ino 15Gao 85As buffer layer was grown at a relatively low temperature (390 °C), which can accommodate most of the misfit dislocations. Multiple steps of thermal cycle anneal (700 °C) were then utilized to further reduce defect densities and suppress their propagation into the active region. A thin (15 A) AlAs layer was first grown as a protective layer to avoid any potential indium desorption during the anneal. The surface of the laser sample grown under optimized conditions is free of any micro-structural roughness or stacking faults. Each InAs QD layer consists of 2.9 ML InAs, capped by an additional 50 A In0.33Ga0.67As layer. To smooth the growth front and avoid phase separation, thin (20 A) GaAs layers were grown before and after each InAs QD layer. After the growth of each QD layer, an in situ anneal at 600 °C is performed, which can reduce any surface undulations, and therefore allow the growth of multiple layers of defect-free QDs. The room temperature PL spectra of InAs QDs grown at various temperatures are shown in Fig. l(b). With optimum growth conditions, the QDs exhibit intense PL emission with narrow linewidth (30 meV), which is comparable to the state-of-the-art 1.3 ptm pseudomorphic InAs QDs.2 Uncapped InAs QDs were characterized by atomic force microscopy, as shown in Fig. l(c). The dots have an average base width of 45 nm and height of 14 nm. The dot surface density is 2.6 1010 cmI 2 Light-current (L-I) measurements were performed under pulsed mode operation (1% duty cycle) at various temperatures. A threshold current of 70 A/cm2 is measured from a 1000X80 ptm2 device with 9500 and 500O high reflectivity coating on both facets, as shown in Fig. 2(a). The laser output is peaked at 1.45 ptm. From temperaturedependent L-I measurements, we derived a To of 556 K in the temperature of 263 305 K for the p-doped laser. The effect of p-doping level on To is being investigated. The variation of threshold current and output slope efficiency with temperature for the p-doped laser are shown in Fig. 2(b). The very large To is primarily due to the increase in Auger recombination in QDs upon p-doping and its decrease with the increase of temperature, which offsets the increasing trends of other recombination currents. From small signal modulation bandwidth measurements, we measure a maximum 3-dB bandwidth of 5 GHz, as shown in Fig. 3(a). The differential gain is 5X 10-15 cm2 . The chirp and u-factor, as shown in Fig. 3(b) and (c), are 0.2 A and 1.0, respectively, at X 2 1.45 pim. The structural properties of the laser structures are currently being characterized by transmission electron microscopy. We will also investigate 1.5 ptm metamorphic QD lasers that incorporate the scheme of tunnel injection to achieve large modulation bandwidth and near-zero u-factor. These results, together with the reliability of the lasers, will also be presented. The work is being supported by the Army Research Office and DARPA. 'Z. C. Niu, S. Y. Zhang, H. Q. Ni, D. H. Wu, H. Zhao, H. L. Peng, Y. Q. Xu, S. Y. Li, et al., Appl. Phys. Lett., 87, 231121 (2005). 2Z. Mi, P. Bhattacharya, and S. Fathpour, Appl. Phys. Lett., 86, 153109 (2005). 3S. M. Kim, Y. Wang, M. Keever, and J. S. Harris, IEEE Photon. Tech. Lett., 16, 377 (2004). 4N. N. Ledentsov, A. R. Kovsh, A. E. Zhukov, N. A. Maleev, S. S. Mikhrin, A. P. Vasil'ev, E. S. Semenova, et. al., Electron. Lett., 39, 1126 (2003).