Demonstration of high-performance 10.16 μm quantum cascade distributed feedbacklasers fabricated without epitaxial regrowthDaniel Hofstetter, Jérôme Faist, Mattias Beck, Antoine Müller, and Ursula Oesterle Citation: Applied Physics Letters 75, 665 (1999); doi: 10.1063/1.124475 View online: http://dx.doi.org/10.1063/1.124475 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/75/5?ver=pdfcov Published by the AIP Publishing Articles you may be interested in High-performance uncooled distributed-feedback quantum cascade laser without lateral regrowth Appl. Phys. Lett. 100, 112105 (2012); 10.1063/1.3693425 High-temperature operation of distributed feedback quantum-cascade lasers at 5.3 μm Appl. Phys. Lett. 78, 396 (2001); 10.1063/1.1340865 High-frequency properties of 1.55 μm laterally complex coupled distributed feedback lasers fabricated byfocused-ion-beam lithography Appl. Phys. Lett. 77, 325 (2000); 10.1063/1.126965 Surface-emitting 10.1 μm quantum-cascade distributed feedback lasers Appl. Phys. Lett. 75, 3769 (1999); 10.1063/1.125450 1.55 μm single mode lasers with complex coupled distributed feedback gratings fabricated by focused ion beamimplantation Appl. Phys. Lett. 75, 1491 (1999); 10.1063/1.124732

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Demonstration of high-performance 10.16 mm quantum cascade distributedfeedback lasers fabricated without epitaxial regrowth

Daniel Hofstetter,a) Jerome Faist, Mattias Beck, and Antoine MullerUniversity of Neuchaˆtel, Institute of Physics, 1 Rue A.-L. Breguet, Neuchaˆtel, CH 2000, Switzerland

Ursula OesterleSwiss Federal Institute of Technology Lausanne, Institute of Micro- and Optoelectronics,Physics Department, PHB Ecublens, CH 1015 Lausanne, Switzerland

~Received 29 March 1999; accepted for publication 7 June 1999!

We present measurement results on high-power low threshold quantum cascade distributedfeedback lasers emitting infrared radiation at 10.16mm. A lateral current injection scheme allowedthe use of a strongly coupled surface grating without metal coverage and epitaxial regrowth.Although this design resulted in a simplified processing, the fabrication of high performance deviceswas demonstrated. The laser emitted 230 mW of pulsed power at 85 K, and 80 mW at roomtemperature. Threshold current densities of 1.85 kA/cm2 at 85 K and 5.4 kA/cm2 at roomtemperature were observed. Since the spectrum showed single mode behavior for all temperaturesand power levels of the operating range, this device will be ideal for optical sensor applications.© 1999 American Institute of Physics.@S0003-6951~99!01931-2#

The development of high-performance midinfrared lightsources has experienced tremendous progress during the lastfive years. Pacemakers of this progress were the appearanceand the subsequent improvements of the quantum cascade~QC! laser.1–3 For several potential applications, especiallyin the area of optical sensors for atmospheric trace gases, it isadvantageous to operate with single mode, single frequencylasers. For this purpose, distributed feedback~DFB! QC la-sers have been extensively investigated and characterized.4–6

Although DFB lasers have obvious performance benefits,there are some severe fabrication drawbacks. One of them isthe requirement of epitaxial regrowth, which makes fabrica-tion rather complicated and prolonged. This is not only dueto the regrowth process itself, but also because of the factthat the material can be tested only after grating fabricationand overgrowth.

A simple method to avoid epitaxial regrowth in DFBlasers consists of fabricating the grating directly on top of thewaveguide.4 However, in order to prevent the top contactmetal from introducing a large waveguide loss~which is es-pecially true for QC lasers operating in TM mode!, one hasto either decrease the grating coupling coefficient,4 choose ametal with small refractive and absorption indices,7 or com-pletely avoid the metal on top of the waveguide.8 The lastpossibility, which comes close to what we will describe inthis article, was published under the name ‘‘surface skim-ming’’ laser. Such devices consist basically of a waveguidewith a semiconductor lower cladding layer and air acting astop cladding. The heavilyn-doped InGaAs cap layer, whichserves as host layer for the grating, is highly conducting toallow lateral current injection and distribution throughout thedevice. The most important consequences of such a designare obviously that there is a large refractive index step be-tween semiconductor and air, and that there are low calcu-

lated losses of 12 cm21. This results in both a high couplingcoefficient of the grating and a relatively high net gain of thelaser; thus it potentially allows the fabrication of short de-vices with a low threshold current.

Growth of this material was based on molecular beamepitaxy ~MBE! of lattice matched InGaAs/InAlAs layerson top of an n-doped InP ~Si, 231017cm23! substrate.The growth process started with the lower waveguidelayers ~InGaAs, Si, 131017cm23, total thickness 1.3mm!,proceeded with an active region~thickness 1.75mm!and was finished by a thicker set of upper waveguidelayers ~thickness 2.1 mm! and a 0.5-mm-thick highlyn-doped cap layer on top. This cap layer was also thehost layer for the grating, as mentioned in theintroduction. The active region, which thus formed thecentral part of the waveguide, consisted of 35 superlatticeperiods; those were alternatingn-doped funnel injector re-gions and undoped triple quantum well active regions. Thelaser transition in the latter was diagonal, similar as de-scribed in Ref. 9. The layer sequence of the structure, innanometers, starting from the injection barrier, is as follows:3.9/1.0/3.8/1.2/3.7/1.5/3.9/1.7/4.0/4.2/3.1/0.9/6.4/1.0/6.0/2.8 nm. In0.52Al0.48As layers are in bold, In0.53Ga0.47Aslayers are in roman, andn-doped layers ~Si, 2.531017cm23) are underlined. A more detailed description ofthe layer thicknesses and compositions, as well as a sche-matic drawing of the band gap of one superlattice periodincluding the relevant electronic transitions, will be pub-lished in a companion letter about wavelength tunable Fabry-Perot lasers fabricated from material using the same activeregion.10

The fabrication of these DFB lasers was based on holo-graphically defining a grating with 1.59mm period (neff

53.21), and wet chemical etching of the grating in aHBr:H2O2:H2O solution to a depth of 0.4mm. We used a488 nm Ar-ion laser and a 90° corner reflector mounted on arotational stage for the grating exposure. Standard processinga!Electronic mail: daniel.hofstetter@iph.unine.ch

APPLIED PHYSICS LETTERS VOLUME 75, NUMBER 5 2 AUGUST 1999

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techniques were then used to define ridge waveguides with awidth of 35–55mm ~etch depth 4.5mm! and a length of1–1.5 mm.3 300 nm of ZnSe served as an electrical passiva-tion layer and Ti/Au~10/400 nm! was used as top contactmetal. Thinning, back contacting~Ge/Au/Ag/Au, 12/27/50/100 nm!, and cleaving completed the processing. As shownby the schematic cross-section in Fig. 1~a! and the scanningelectron microscopy picture in Fig. 1~b!, the contact metalcovered only the edges~about 5mm on each side! of theridge to prevent large absorption losses in the waveguide, butstill allow lateral current injection. The devices, whose facetswere left uncoated, were mounted ridge side up on copperheatsinks and operated at different temperatures between 85and 300 K. The samples were then placed into a temperaturecontrolled N2 flow cryostat. The light of the QC DFB laserwas collected byf /0.8 optics and fed into a high resolutionFourier transform spectrometer~Nicolet type Magna-IR860!, where we detected it using a liquid nitrogen-cooledHgCdTe detector. For the measurement of light–current(L – I ) curves, we measured the intensity with a calibrated5003500mm2 room temperature HgCdTe detector. TypicalL – I and current–voltage (I –V) curves of a 45-mm-wideand 1.2-mm-long device are shown in Fig. 2. The currentpulses were 100 ns long, and a pulse repetition frequency of5 kHz was used for all temperatures. At low temperatures,we observed a threshold current of 1 A and a maximumoutput power of 230 mW for 9.7 V bias voltage. The slopeefficiency at this temperature was 220 mW/A and a threshold

current density of 1.85 kA/cm2 was determined. At roomtemperature, we still obtained 80 mW optical output powerwith a slope efficiency of 80 mW/A; however, the thresholdcurrent increased to 2.9~threshold current density of 5.4kA/cm2!, and an operating voltage of 12.5 V was seen. Fromthe increase in threshold current, we were able to derive acharacteristic temperatureT0 of 204 K.

In Fig. 3, we present three luminescence spectra mea-sured at 85, 105, and 150 K. In all of them, the spontaneousemission peaks around 980 cm21, and there occur regularFabry–Pe´rot modes with a spacing of 1.7 cm21 ~cavitylength: 850mm!. The Bragg reflector’s stop band with awidth of 2.5 cm21 is clearly visible at 995.7~85 K!, 994.6~105 K!, and 992.3 cm21 ~150 K!. From the stop bandwidth,we determined the coupling coefficient of the grating to bek5Dlpneff /l

2524 cm21; this number agrees well with avalue obtained from an estimation based on the effectiverefractive index difference ofDn51.831022 between areaswith and without the grating layer (k5pDneff/2l528 cm21). A relatively small free carrier absorption loss of12 cm21 was calculated for this device, whereas a laser uti-lizing our standard waveguide design with a 2.2-mm-thickInAlAs/InGaAs upper cladding layer and a metal-covered

FIG. 1. ~a! Schematic cross section through the laser waveguide showingthe exact position of the grating with respect to the active layer and themetal top contact.~b! Scanning electron microscopy picture of a QC DFBlaser. Please note that there is no metal in the central top part of the ridgewaveguide.

FIG. 2. L – I and I –V curves of a 45-mm-wide and 1.2-mm-long DFB QClaser measured at different temperatures. The inset shows a plot of thethreshold current vs device temperature.

FIG. 3. Spontaneous emission spectra of a 45-mm-wide and 850-mm-longDFB QC laser at 85, 105, and 150 K.

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grating would suffer from a waveguide loss of 30 cm21. Inaddition, the refractive index contrast would be reduced byalmost two orders of magnitude, namely to a value ofDn52.331024. Since the partial removal of the contact layerleads to a slight gain variation, a small amount of loss cou-pling might also be present in this device.

Figure 4 shows the lasing spectra at temperatures be-tween 85 and 300 K. We observed single mode operation forall temperatures and, in particular, at maximum power foreach individual temperature. We determined the linewidth tobe on the order of 0.3 cm21, which corresponds to the reso-lution limit of our experimental setup. The emission wave-length at 85 K was 996 cm21; at room temperature, it de-creased to 982 cm21. As already mentioned above, theluminescence peak was found in the vicinity of 975 cm21 forall temperatures. The temperature tuning coefficient of thelasing peak was constant over the entire temperature range,and its magnitude was 1/l3Dl/DT56.531025 K21

(Dn/DT520.063 cm21/K). These numbers are consistentwith what has been reported in the literature.5

By utilizing a second processing run with a changedgrating period of 1.63mm ~instead of 1.59mm!, we wereable to perform some measurements regarding the influenceof a detuning between Bragg peak and gain peak. The firstseries of DFB lasers with a detuning of about 5–10 cm21 atroom temperature (L51.59mm) showed high performanceat all temperatures. The second series with a detuning ofabout 30 cm21 (L51.63mm) did not lase at room tempera-ture, and at low temperatures, the output power did not ex-

ceed 60 mW. Since we determined a full width at half maxi-mum ~FWHM! of the gain peak ofDn565 cm21, it is clearthat the laser performance will degrade rapidly with increas-ing detuning between Bragg reflection maximum and gainpeak.

In conclusion, we have shown device results for a DFBQC laser operating at 10.16mm. This DFB laser functionswithout upper cladding layer; the grating is therefore directlyexposed to air. Current injection is accomplished laterallythrough the grating layer; this design avoids large waveguidelosses due to metal absorption. Although the fabrication ofthis device is straightforward, without epitaxial regrowth, weachieved a very good performance. At room temperature, thelaser emitted 80 mW optical power at single mode operation.Pulsed threshold current densities of 5.4 and 1.85 kA/cm22,and slope efficiencies of 220 and 80 mW/A at 85 and 300 K,respectively, were obtained. This device will have importantapplications in optical sensing of ammonia and other atmo-spheric trace gases.

We would like to thank Thierry Aellen, Ste´phane Blaser,and Michel Rochat for their advice during processing, elec-trical, and spectral measurements on these samples, Hans-Peter Herzig, Philippe Nussbaum, and Rene´ Dandliker fromthe Institute for Microtechnology at the University of Neu-chatel for their support in grating fabrication, and the SwissNational Science foundation and the Science Foundation ofthe European community for their financial support~projectUNISEL, BRITE/EURAM, No. CT97-0557!.

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FIG. 4. Lasing spectra of a 45-mm-wide and 1.2-mm-long DFB QC laser atdifferent temperatures between 85 and 300 K. All spectra were measuredwith the maximum possible output power at each individual temperature.

667Appl. Phys. Lett., Vol. 75, No. 5, 2 August 1999 Hofstetter et al.

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