IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 17, NO. 6, JUNE 2005 1145 High-Performance Laser Diodes With Emission Wavelengths Above 1100 nm and Very Small Vertical Divergence of the Far Field F. Bugge, H. Wenzel, B. Sumpf, G. Erbert, Member, IEEE, and M. Weyers Abstract—The effect of variations in the vertical structure on the performance of AlGaAs–GaAs laser diodes with an InGaAs quantum well (QW) emitting around 1120 nm was investigated. With very thick waveguide layers, more than 95% of the output power is enclosed in an angle smaller than 35 . This allows the use of fast axis collimators with a small numerical aperture. Broad area laser diodes with 100- m stripe width, an optimized doping profile, and a double QW emit more than 12 W and show reliable opera- tion at 5 W. Index Terms—Epitaxial growth, laser reliability, quantum well (QW) lasers, semiconductor lasers. I. INTRODUCTION D UE TO their high output power potential, GaAs-based laser diodes in the wavelength range at and beyond 1100 nm are interesting as pump sources for Raman amplifiers in telecommunication systems, for pumping up-conversion fiber lasers or direct material processing without transfer of optical power to fiber or solid state lasers [1], [2]. For several applications, it is necessary to adjust the emis- sion wavelength exactly to obtain high efficiencies of the whole system; for other applications, additionally, a high output power or a low vertical beam divergence is required. Such a low ver- tical beam divergence allows the use of fast axis collimators with a numerical aperture of only 0.5 and, thus, relatively large adjustment tolerances. For high optical output power structures with a high differential efficiency and a large equivalent ver- tical spot size ( is the quantum-well (QW) thickness, the confinement factor) are necessary [3]. Laser diodes emitting beyond 1100 nm need a high indium content in the InGaAs QW. Such highly strained layers are grown favorably at low temperatures to avoid strain relaxation or three-dimensional growth. Contrary, Al containing layers are grown typically at higher temperatures for better layer properties, especially a low oxygen contamination [4]. On the other hand, at these long wavelengths, GaAs already forms sufficient barriers for the carriers so that GaAs can be used as waveguide layer (WL) around the QW. The lower affinity of GaAs to oxygen and point defect incorporation allows the use of lower growth temperatures. An additional advantage of GaAs compared to AlGaAs is its higher thermal and electrical Manuscript received November 29, 2004; revised February 8, 2005. The authors are with the Ferdinand-Braun-Institut für Höchstfrequen- ztechnik, Berlin D-12489, Germany (e-mail: bugge@fbh-berlin.de). Digital Object Identifier 10.1109/LPT.2005.846927 conductivity. GaAs WLs will reduce resistive heating and facilitate the removal of heat from the active region. Therefore, such structures have the potential for a very high optical output power [5]. This letter deals with the effect of different vertical layer de- signs on the performance of laser diodes with emission wave- lengths between 1120 and 1150 nm. Based on these investiga- tions, high output powers of more than 10 W are demonstrated. II. GROWTH AND FABRICATION Growth by metal–organic vapor phase epitaxy was carried out in an Aixtron 200/4 reactor on exactly oriented (001) GaAs substrates. Precursors were pure arsine, phosphine, and the trimethyl compounds of gallium (TMGa), indium (TMIn), and aluminum (TMAl). For p-type doping dimethyl zinc or carbon and for n-type doping disilane diluted in hydrogen were used. The laser structure consists of a single or double InGaAs QW embedded in thick GaAs WLs and Al Ga As cladding layers (CLs). On top of the p-CL is a highly p-doped GaAs contact layer. The structure is designed for a small vertical far field by using very thick WLs. The target specification is to include more than 95% of the output power in an angle smaller than 35 . Higher order vertical modes are suppressed by loss discrimination due to radiation into the substrate. The thickness of the n-CL is optimized in order to achieve low losses only for the fundamental mode. The broad GaAs WLs are undoped in the inner part and lowly doped cm toward the CLs. This results in reduced free carrier absorption and low internal optical losses . Therefore, the use of very long cavity lengths is possible, which again improves the heat removal. In all cases, the InGaAs QWs were grown at 530 C, while the (Al)GaAs WLs and CLs were grown at temperatures be- tween 570 C and 770 C. To adjust the necessary growth tem- perature, the growth was interrupted between the spacer layers surrounding the QW and the WLs. The growth process is de- scribed more in detail in [6]. Although the QW thickness of 6 nm at an In content of 35% in the QW is near the critical thickness of about 8 nm for the single QW (SQW) and exceeds the critical thickness for the double QW (DQW), cathodoluminescence images show no hints for the formation of defects. Also high resolution X-ray investigations show no indications for strain relaxation in the QWs. The structures were processed into broad-area laser diodes with 60-, 100-, and 200- m stripe width and cavity lengths from 1 to 4 mm. The transparency current density and other 1041-1135/$20.00 © 2005 IEEE