INSTITUTE OF PHYSICS PUBLISHING SEMICONDUCTOR SCIENCE AND TECHNOLOGY Semicond. Sci. Technol. 19 (2004) 615–625 PII: S0268-1242(04)70311-9 Design optimization of InGaAsP–InGaAlAs 1.55 μm strain-compensated MQW lasers for direct modulation applications M Nadeem Akram, Christofer Silfvenius, Olle Kjebon and Richard Schatz Department of Microelectronics and Information Technology, Royal Institute of Technology, ELECTRUM 229, SE-164 40, Stockholm, Sweden Received 10 October 2003 Published 19 February 2004 Online at stacks.iop.org/SST/19/615 (DOI: 10.1088/0268-1242/19/5/010) Abstract In this paper, a simulation study of InGaAsP(well)/InGaAlAs(barrier) 1.55 µm strain-compensated multi-quantum well (MQW) lasers is presented. Due to a large conduction band discontinuity in this material system, a higher material gain and differential gain can be obtained from such a quantum well (QW) as compared to a traditional InGaAsP/InGaAsP quantum well. The deeper electron well should also improve elevated temperature operating characteristics and reduce the electron spillover from QWs. For MQWs, a uniform vertical distribution of holes is achieved due to a reduced effective hole confinement energy by optimizing the bandgap and the strain in the barriers. A large number of quantum wells can be uniformly pumped, reducing the carrier density in each individual well. A uniform and low carrier density in all the wells help reduce the total Auger recombination current. High p-doping in the active region is shown to enhance the carrier and gain non-uniformity in the MQWs. A simulated high modulation bandwidth has been demonstrated, promising directly modulated lasers as a low-cost source for short to medium distance (1–10 km) high speed optical links. 1. Introduction Semiconductor lasers utilizing strained quantum wells (QWs) are important components for modern day optical fibre telecommunication systems. Directly modulated lasers operating around 1300 nm or 1550 nm can be used as a low-cost source for digital communication at high bit-rate. The device modulation bandwidth has reached 30 GHz [1, 2] at 1550 nm and devices have recently been tested for 40 Gbit s −1 transmission over 1 km fibre distance [3]. These devices are also useful for analogue communication due to their low distortion characteristics at microwave frequencies [4]. Traditionally, the InGaAsP(well)/ InGaAsP(barrier)/InP(substrate) material system has been the mainstay for 1300 and 1550 nm lasers. However, the large valence band discontinuity (E v ≈ 60%E g ) [5] in this material system increases the effective hole quantum well confinement energy and limits the vertical hole transport through the multi-quantum wells (MQW). This can result in a hole pile-up on the p-side of the active region. The increased hole population at the p-side of the MQW also increases the electron population on the p-side due to Coulomb attraction and higher mobility of electrons. This largely affects the total material gain and carrier density-dependent losses. For a given total carrier density n in the MQW, the sum of the material gain from all wells is less in the non-uniform case than that if the carrier density is uniform in all the wells. This is due to a sublinear relationship of the material gain g with carrier density n (g ∝ ln(n)). In a similar fashion, the carrier losses increase superlinearly with carrier density (Auger recombination ∝ n 3 and spontaneous recombination ∝ n 2 ) and hence the total carrier losses are enhanced in the non-uniform case. The wells on the n-side may have low 0268-1242/04/050615+11$30.00 © 2004 IOP Publishing Ltd Printed in the UK 615