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High-Performance Quantum Cascade Lasers Grown by Metal-Organic

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High-Performance Quantum Cascade Lasers Grown by Metal-Organic 3534 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 26, NO. 21, NOVEMBER 1, 2008 High-Performance Quantum Cascade Lasers Grown by Metal-Organic Vapor Phase Epitaxy and Their Applications to Trace Gas Sensing Mariano Troccoli, Member, IEEE, Member, OSA, Laurent Diehl, David P. Bour, Fellow, IEEE, Scott W. Corzine, Nanfang Yu, Christine Y. Wang, Mikhail A. Belkin, Gloria Höfler, Rafal Lewicki, Gerard Wysocki, Frank K. Tittel, Fellow, IEEE, and Federico Capasso, Fellow, IEEE, Fellow, OSA (Invited Paper) Abstract—We present an overview of our results on the design, I. INTRODUCTION material growth, device characterization, and spectroscopic appli- cations of MOVPE-grown quantum cascade lasers (QCLs). These devices are capable of room-temperature (RT) continuous-wave UANTUM CASCADE (QC) lasers have experienced a operation and high power emission. The first section focuses on Q constant development since their invention at Bell Labs growth of laser material, device fabrication, and quantum design in 1994 [1] and represent so far the most interesting new The second section discusses RT pulsed operation, in particular source for optical systems operating in the mid-infrared (mid- the doping dependence of laser performance and broadband IR), i.e. –25 , and in the THz region of the spectrum emission. Near-field measurements performed on the devices’ facets correlating lateral modes to device size are also discussed. ( –180 ) [2], [3]. Innovation in device engineering Section III deals with continuous-wave high-temperature oper- of QC lasers has been supported all along by an equally inten- ation from lasers with different active region designs, including sive material development effort. The operating principle of QC their spectral characteristics and in the emergence of coherent lasers has in fact been demonstrated in different material sys- phenomena at high power levels. Section IV analyses the devices tems [4]–[6] and with various growth techniques [7]–[9], overall thermal dissipation capabilities, while in Section V we report reliability data. The final section focuses on spectroscopic ap- proving QC lasers to be a robust technology platform for the plications and tunability. Optofluidic narrow ridge lasers and mid-infrared. their application to chemical sensing are reported along with The QC laser material structure poses several challenges to recent data on a broadband on chip spectrometer consisting of the growth process: the total thickness can easily reach ten mi- individually addressable distributed feedback QCLs. Various crons, the number of interfaces is typically in the hundreds, the spectroscopic techniques and in particular quartz-enhanced photoacoustic absorption spectroscopy and its use in gas sensing smallest layer thickness can be as little as few monolayers, and systems are discussed. Finally, optofluidic narrow ridge lasers and the laser performance is sensitive to background doping and their applications to fluid sensing are presented. interface roughness. Having a consistently reproducible high- quality source of material for QC lasers is therefore one of the Index Terms—Infrared spectroscopy, semiconductor device fab- key elements to its manufacturability. rication, semiconductor lasers. The main characteristic that sets QC lasers apart from Manuscript received October 11, 2007; revised February 22, 2008. Current common diode lasers is that they are unipolar devices, i.e. they version published January 28, 2009. This work was supported by Agilent Tech- employ only one type of carrier/doping. QCL operation relies nologies, by the Air Force Office of Scientific Research (AFOSR MURI on Plas- monics), by the DARPA Optofluidics Center under Grant HR0011-04-1-0032, on transitions between electronic states created by quantum by the U.S. Army Research Laboratory and the U.S. Army Research Office confinement within the conduction band of the laser materials under Grant W911NF-04-1-0253, and by the Center for Nanoscale Systems (intra-band), as opposed to the inter-band transitions taking (CNS) at Harvard University. Harvard-CNS is a member of the National Nan- otechnology Infrastructure Network (NNIN). place in p-n junction diode lasers. Band structure engineering M. Troccoli was with Harvard University, Cambridge, MA 02138 USA. He is in QC devices allows significant control over important laser now with Argos Tech LLC, Santa Clara, CA 95051 USA (e-mail: mtroccoli@ar- characteristics such as its wavelength, threshold, optical power, gostechinc.com). L. Diehl, N. Yu, C. Y. Wang, M. A. Belkin, and F. Capasso are with the and even its high-temperature performance. School of Engineering and Applied Sciences, Harvard University, Cambridge, Band structure engineering makes it possible to tailor the MA 02138 USA (e-mail: [email protected]). laser design so that its optical emission characteristics can be tar- D. P. Bour was with Agilent Labs, Palo Alto, CA 94304 USA. He is now with Photodigm, Inc., Richardson, TX 75081 USA. geted to specific purposes. QC lasers design innovations so far S. W. Corzine was with Agilent Labs, Palo Alto, CA 94304 USA. He is now have included: short-wavelength [5], [10] or long-wavelength with Infinera HQ, Sunnyvale, CA 94089 USA. [11] emission, high power output [12], nonlinear optical gener- G. Höfler was with Agilent Labs, Palo Alto, CA 94304 USA. She is now with Argos Tech LLC, Santa Clara, CA 95051 USA. ation [13]–[15] vertical emission [16], and Terahertz generation R. Lewicki, G. Wysocki and F. K. Tittel are with the Electrical and Computer [17], among others. Engineering Department, Rice University, Houston, TX 77005 USA. The resulting complexity of the active region structure has Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. been in most cases a drawback for the optimization of the Digital Object Identifier 10.1109/JLT.2008.925056 laser thermal performances. A typical QC laser has a 1- to 0733-8724/$25.00 © 2008 IEEE Authorized licensed use limited to: Mariano Troccoli. Downloaded on February 11, 2009 at 14:51 from IEEE Xplore. Restrictions apply. TROCCOLI et al.: HIGH-PERFORMANCE QCLS GROWN BY METAL-ORGANIC VAPOR PHASE EPITAXY 3535 Fig. 1. Compact tunable source can enable high-sensitivity portable photoacoustic sensors for gas analysis in the molecular fingerprint region (the mid-IR region of the electromagnetic spectrum). Several important compounds have their signature absorption resonances in this region. 2- -thick active region including hundreds of interfaces, tion of present applications is also concerned with high-power whose typical thermal conductivity is therefore low (about 10 laser operation over long distances (IR countermeasures, free- K/W) [18]. In addition, the active region transport is controlled space optics, remote sensing). In all these fields, QCLs can have by intersubband transitions with intrinsically short lifetimes an impact because of their portability, wavelength agility, ro- (sub-picosecond) and typically requires a bias of several volts bustness, and power. at threshold for laser action. This results in high threshold current densities (1 or more) and a large amount of II. MATERIAL GROWTH AND CHARACTERIZATION heat to be dissipated ( 5–10 W) in a device with low thermal For growth of high-performance QCL heterostructures, sev- conductivity, thus pushing the QC active region temperature eral requirements must be fulfilled. Foremost among these is several tens of degrees (30–80 K) above the heat sink one. the ability to form atomically abrupt interfaces between layers As a consequence, while room-temperature (RT) pulsed of nanometer or even sub-nm thickness. Molecular beam epi- operation was achieved soon after the first laser demonstration, taxy (MBE) is exemplary in this regard, however a properly de- continuous-wave (CW) RT operation has been elusive for quite signed MOCVD reactor can approach the interface abruptness a while. Room temperature CW operation was first demon- associated with MBE. Low background doping, in concert with strated in a laser grown by MBE [19] but more recently we were controlled intentional-doping profiles, are also critical for QC able to demonstrate CW RT operation in an MOVPE-grown lasers in order to minimize the gain spectrum width and to re- QC laser, both for lattice matched and for strained material duce thresholds. This requirement may represent a limitation systems [20]–[24]. This is a key step towards the industrial for AlGaAs-based QCLs by MOCVD, since carbon contami- implementation and production of QC technology. nation is always present in MOCVD-grown AlGaAs. However, CW RT operation of QC lasers with high reliability and low carbon contamination is generally not present in the AlInAs and powerconsumptionwillenablecompactandversatileopticalsen- GaInAs alloys grown by MOCVD. Therefore these InP-based sors(seeFig.1foraschematicillustrationofopticalgasanalysis). alloys are a natural choice for MOCVD-grown QCLs. In addi- In the case of the more mature p-n diode lasers , com- tion, MOCVD may even offer some advantages over MBE for pactness and reliability have been the key to high-volume low- QCL growth. cost applications of optical technology in data storage, imaging, Among the anticipated benefits of MOCVD are stability of and telecommunications. Analogously,the emergence of volume growth rate and composition over very long runs, the ease of applicationsforQCL-basedsystemswilldependontheefficiency achieving low oxygen contamination in aluminum-containing and reliability of the optical
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