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IEEE PHOTONICS TECHNOLOGY LETTERS VOL 18 NO 12 JUNE 15 2006 1347 Room Temperature Continuous Wave Quantum Cascade Lasers Grown by MOCVD Without Lateral Regrowth Zhijun Liu Daniel Wasserman Scott S Howard Anthony J Hoffman Claire F Gmachl Senior Member IEEE Xiaojun Wang Tawee Tanbun Ek Liwei Cheng and Fow Sen Choa Senior Member IEEE Abstract We report on room temperature continuous wave CW operation of 8 2 m quantum cascade lasers grown by metal organic chemical vapor deposition without lateral regrowth The lasers have been processed as double channel ridge waveguides with thick electroplated gold CW output power of 5 3 mW is measured at 300 K with a threshold current density of 2 63 kA cm2 The measured gain at room temperature is close to the theoretical design which enables the lasers to overcome the relatively high waveguide loss Index Terms Continuous wave CW lasers midinfrared quantum cascade QC lasers semiconductor lasers I INTRODUCTION Q UANTUM cascade QC lasers are promising and influential midinfrared light sources with potential for applications as varied as chemical sensing wireless communication and counter measures Since their first demonstration in 1994 1 constant and significant performance improvements have been made for QC lasers through improved laser design material growth and packaging Up to now room temperature continuous wave CW operation an important milestone for compact noncryogenic laser sources has been demonstrated for QC lasers grown by solid source molecular beam epitaxy MBE or gas source MBE at wavelengths of 9 1 and 4 6 m 2 7 Metal organic chemical vapor deposition MOCVD has recently attracted research interest because it is a technology preferred by industry and is promising for the commercialization of QC lasers 8 MOCVD has been reported as a high performance QC laser growth technique first with low threshold pulsed operation 9 and very recently room temperature CW operation of an MOCVD grown 7 2 m QC laser and an MOCVD grown 5 1 m strained QC laser using a buried heterostructure design 10 11 In this letter we report on an MOCVD grown m without buried room temperature CW QC laser at Fig 1 a Portion of the conduction band diagram and the moduli squared of the relevant wave functions of a 8 2 m QC laser with a four quantum well active region based on a two phonon resonance An electric field of 51 kV cm is applied The arrow indicates the laser transition b Intensity profile of the fundamental mode layer structure and profile of the real part of the refractive index of the dielectric waveguide used heterostructure The laser is processed as a double channel ridge waveguide with thick electroplated gold on top allowing the omission of the more complex lateral InP regrowth step II LASER DESIGN AND FABRICATION Manuscript received February 22 2006 revised April 7 2006 This work was supported in part by the Defense Advanced Research Projects Agency DARPA L PAS Z Liu D Wasserman S S Howard A J Hoffman and C F Gmachl are with the Department of Electrical Engineering and the Princeton Institute for the Science and Technology of Materials Princeton University Princeton NJ 08544 USA e mail cgmachl princeton edu X Wang and T Tanbun Ek are with AdTech Optics City of Industry CA 91748 USA L Cheng and F S Choa are with the Department of Computer Science and Electrical Engineering University of Maryland Baltimore MD 21250 USA Digital Object Identifier 10 1109 LPT 2006 877006 The laser active region is based on a two phonon resonance design The layer sequence in ngstr ms of one period of active region and injector is 44 18 9 57 11 54 12 45 25 34 14 33 13 32 15 31 19 29 23 27 25 27 where In Al As barrier layers are in bold In Ga As well layers are in roman and the n doped cm layers are underlined The electron energy band diagram is shown in Fig 1 a The energy of the laser transition between levels 4 and 3 is designed as 154 meV 1041 1135 20 00 2006 IEEE 1348 IEEE PHOTONICS TECHNOLOGY LETTERS VOL 18 NO 12 JUNE 15 2006 and levels 1 2 and 3 are each separated by about one optical phonon energy The relatively large energy separation between level 3 and the ground state of the next down stream injector 147 meV is intended to suppress the thermal back filling effect The lifetime of the upper laser level is designed as ps and that of lower laser level is ps The is 1 8 nm dipole matrix element Thirty five periods are used as the active core and sandcm wiched between two 0 5 m thick n doped In Ga As layers The upper cladding layers consists cm InP followed by a of 2 m thick n doped 1 cm InP cap layer The 1 m thick n doped calculated intensity profile of the fundamental mode is shown is calculated as 6 6 cm in Fig 1 b The waveguide loss and the confinement factor is 0 67 The double channel ridge waveguide lasers were fabricated by conventional wet chemical etching A 0 3 m thick Si N layer was deposited for side wall insulation After evaporation m thick of Ti Au 30 300 nm for the top contact a gold layer was electroplated around the laser ridge for efficient heat transfer To facilitate cleaving small gaps were left in the thick electroplated gold layer After the wafer was thinned to 150 m the back Ge Au 15 300 nm contact was evaporated The laser was mounted epilayer up to a copper submount with In solder and wire bonded Finally the back facet was high reflection HR coated with SiO Ti Au SiO 400 15 100 100 nm III LASER CHARACTERIZATION For testing the lasers were loaded onto a cryostat with the temperature measured on the cryostat cold finger next to the laser mount Fig 2 a shows the CW light current curves of an HR coated 8 m wide 3 5 mm long QC laser at different heat sink temperatures The voltage current curve at 300 K is also given A CW optical output power of 5 3 mW is obtained with a threshold current of 0 73 A corresponding to kA cm at 300 K The typical emission spectrum at 300 K is given by the inset of Fig 2 a The lasing wavelength is 8 2 m Fig 2 b shows the threshold current density of the laser as a function of the heat sink temperature for both pulsed and CW operation For pulsed operation the threshold current density increased from 1 21 kA cm at 240 K to 2 26 kA cm at 380 K The CW threshold current density increased from 1 56 kA cm at 240 K to 3 2 kA cm at 315 K The solid lines are the result of exponential fits is 217 K and The extracted characteristic temperature 107 K for pulsed operation and CW operation respectively By comparing the two curves in Fig 2 b the


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