IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 11, NO. 1, JANUARY 1999 9 Strain-Compensated 1.3- m AlGaInAs Quantum-Well Lasers with Multiquantum Barriers at the Cladding Layers Jen-Wei Pan, Ming-Hong Chen, Jen-Inn Chyi, Senior Member, IEEE, and Tien-Tsorng Shih Abstract—Strain-compensated 1.3- m AlGaInAs graded-index separate confinement heterostructure (GRINSCH) lasers with multiquantum barrier (MQB) at both the n- and p-cladding layers are comprehensively studied and compared with the con- ventional GRINSCH lasers. It is found that the lasers with MQB’s exhibit lower threshold current, higher maximum output power and better temperature characteristics because of the enhanced barrier height for carrier leakage. The characteristic temperature is improved as much as 10 K and the vertical far-field angle is also reduced from 38 to 32 as compared to the conventional counterpart. Index Terms— Multiple-quantum barrier, semiconductor la- sers, strain-compensation. F OR CONVENTIONAL long-wavelength lasers, more than 50% of the lasing mode is propagating in the doped cladding layer, where the optical loss due to free-carrier absorption is significant. The internal loss can be reduced using broadened waveguide structures and/or low doping concentration in the cladding layer. For instance, the internal loss of long-wavelength GaInAsP–InP multiquantum-well (MQW) lasers can be reduced to as low as 1.3 cm , because the optical confinement factor in the doped cladding layer of the laser is decreased to less than 5% [1]. However, a broadened waveguide may cause adverse effects as well. It has been shown that the carrier transport time across the guiding layer into the quantum well varies with the square of the width of the guiding layer [2]. The increased carrier transport time will in turn increase the population of carriers in the guiding layers and facilitate the carrier recombination [3], [4]. Consequently, the temperature sensitivity of the slope efficiency is deteriorated at high temperature. The modulation bandwidth will also be degraded for the broadened waveguide lasers since the carrier transit time is increased [2], [4]. Although higher slope efficiency, lower internal loss and lower threshold current can be achieved using low doping concentration in the cladding layer, electron leakage current over the low doped cladding layer has been found to become Manuscript received July 16, 1998; revised September 25, 1998. This work was supported by the National Science Council of R.O.C. under Contract NSC87-2215-E008-012 and by the Chunghwa Telecom Co., Ltd. under Contract TL-86-5106. J.-W. Pan, M.-H. Chen, and J.-I. Chyi are with the Department of Electrical Engineering, National Central University, Chung-Li, Taiwan 320, R.O.C. T.-T. Shih is with the Telecommunication Labs, Chunghwa Telecom Company, Ltd., Yang-Mei, Taiwan 326, R.O.C. Publisher Item Identifier S 1041-1135(99)00374-2. Fig. 1. Schematic conduction band diagram of the GRINSCH laser with MQB’s. significant with temperature, and renders threshold current and slope efficiency highly temperature-sensitive [5]. A multiquantum barrier (MQB) structure is, therefore, proposed to reduce carrier leakage so as to alleviate this problem [4], [6]. Moreover, the resultant longer escape time for the lasers with MQB is believed to be beneficial for high-speed modulation [4]. In this work, we carry out a comprehensive study on phosphorus-free 1.3- m AlGaInAs linearly graded- index separate confinement heterostructure (GRINSCH) MQW lasers with MQB’s at both the n- and p-cladding layers. Since introducing MQB’s at the cladding layers may lead to a minor modification on the effective refractive index of the waveguide, the resultant far-field angle is also measured and compared with the conventional GRINSCH laser. The lasers studied in this work were grown on S-doped (100)-oriented InP substrates by solid source molecular beam epitaxy (SSMBE). As shown in Fig. 1. the active region of these lasers consists of six 6-nm compressively strained Al Ga In As quantum wells and five 10- nm tensile-strained Al Ga In As barrier layers, and is sandwiched by two undoped 0.15- m-thick Al Ga In As ( ) guiding layers. In order to reduce the internal loss, the doping concentrations of the n- and p-cladding layers adjacent to the guiding layer are chosen to be 5 and 2 10 cm , respectively. For the GRINSCH laser with MQB’s, MQB’s are 54 nm away from the outer edge of the guiding layer. The structure of the two-stack Al In As–Al Ga In As MQB is , where the first and second terms in parenthesis are the widths of the barrier and well in the unit of monolayer, respectively. The devices fabricated are 1041–1135/99$10.00  1999 IEEE