APPLIED PHYSICS LETTERS
VOLUME 73, NUMBER 24
14 DECEMBER 1998
GaAs/Alx Ga12 x As quantum cascade lasers Carlo Sirtori,a) Peter Kruck, Stefano Barbieri, Philippe Collot, and Julien Nagle Thomson–CSF, Laboratoire Central de Recherches, 91404 Orsay, France
Mattias Beck and Je´roˆme Faist Institute of Physics, University of Neuchaˆtel, 2000 Neuchaˆtel, Switzerland
Ursula Oesterle Ecole Polytechnique Fe´de´rale de Lausanne, Lausanne, Switzerland
~Received 4 September 1998; accepted for publication 14 October 1998! A unipolar injection quantum cascade ~QC! laser grown in an AlGaAs/GaAs material system by molecular beam epitaxy, is reported. The active material is a 30 period sequence of injectors/active regions made from Al0.33Ga0.67As/GaAs-coupled quantum wells. For this device a special waveguide design, which complies with a GaAs heavily doped substrate and very short Al0.90Ga0.10As cladding layers, has been optimized. At a heat-sink temperature of 77 K, the laser emission wavelength is 9.4 mm with peak optical power exceeding 70 mW and the threshold current density is 7.3 kA/cm2. The maximum operating temperature is 140 K. This work experimentally demonstrates the general validity of QC laser principles by showing laser action in a heterostructure material different from the one used until now. © 1998 American Institute of Physics. @S0003-6951~98!01150-4#
The intersubband lifetime in GaAs single quantum wells is >0.8 ps for T,80 K and for transitions energies ranging between 120 and 130 meV.8 To increase the excited-state lifetime of the laser transition, the active region of the present laser is designed following an ‘‘anticrosseddiagonal’’ scheme.9 The conduction-band profile and the relevant wave functions of an ‘‘injector/active region/injector’’ are represented in Fig. 1. In this configuration the wave function corresponding to the n53 state is delocalized across the
Since their birth, almost five years ago, quantum cascade ~QC! lasers1 have shown tremendous performance improvements and technological progress. At present they are the only semiconductor lasers operating at and above room temperature in the 5–12 mm wavelength range,2 with peak output power exceeding 100 mW at 300 K.3 Using QC structures, distributed-feedback lasers4 and microcavity disk lasers5 were also demonstrated. All these results, however, were accomplished using the same semiconductor system: Al0.48In0.52As/Ga0.47In0.53As/InP. Indeed, to the best of our knowledge no unipolar injection QC lasers have been reported in any other material system.6 This is somewhat in contradiction with the principle of QC lasers, which are based on intersubband transitions and, therefore, should be essentially independent of the specific semiconductor system used. In this letter we report a QC laser in GaAs/Alx Ga12x As proving that these laser fundamentals are truly not bound to a particular material system. Moreover, the use of ~Al!GaAsbased heterostructures, which are the most widespread and developed among compound semiconductors, confers an additional technological value to this device. Although the overall principles accounting for population inversion are basically the same as in QC lasers based on Al0.48In0.52As/Ga0.47In0.53As/InP, the use of GaAs/ Alx Ga12x As imposes significant modifications to the active region design, the waveguide, and the fabrication of QC lasers. The lasers are grown by molecular beam epitaxy on a heavily doped GaAs substrate (n si52 – 431018 cm23). For the active region/injector, Alx Ga12x As with x50.33 is used as the barrier material @Fig. 1~a!#. With this Al concentration, the conduction-band discontinuity at the G point is DE c >295 meV and the AlGaAs is a direct gap semiconductor.7
FIG. 1. Schematic conduction-band diagram of a portion of the laser heterostructure at threshold bias. The thicknesses of the layers are indicated in nm, the underlined numbers denote the four layers which are n-type doped ~sheet density 3.931011 cm22 per period!. The wavy arrow indicates the transition 3→2 responsible for the laser action. The solid curves represent the moduli squared of the relevant wave functions. The calculated energylevel differences are E 325134 meV and E 21538 meV. In the calculation, nonparabolicities were taken into account following Ref. 13. The dashdotted line in the injector represents the effective conduction-band edges of the digitally graded region. Note the reduced spatial overlap between the n53 and n52 wave functions, a typical feature of a laser based on a diagonal transition.
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FIG. 2. ~a! Mode intensity profile of the waveguide in the direction perpendicular to the layers. The numbers on top of the figure are their thicknesses in mm. Material composition and the doping concentration in cm23 are also indicated. ~b! Refractive index profile in the direction perpendicular to the layers.
two wells on the left side of the active region. The spatial overlap between the wave functions (n53 and n52) of the laser transition is tailored so that it decreases the electron– optical-phonon scattering matrix elements while preventing a strong reduction of its dipole matrix element z 32 ~1.6 nm!. The corresponding laser transition energy is E 32 5134 meV. The calculated lifetime, due to electron–opticalphonon interaction, is for the n53 state t 3 51.5 ps, a factor 2 longer than a QC structure at the same wavelength based on a vertical transition. Our calculations show also that the population inversion condition t 322 t 2 .0 is well satisfied, here, t 3252.4 ps is the relaxation time from the n53 state into the n52 state and t 2 50.3 ps is the lifetime of the n 52 state. Contrary to InP, GaAs substrates have a higher refractive index than the active region and, therefore, cannot be used as a lower cladding layer for the waveguide. In our case, the growth of the laser structures is then further thickened by the presence of the lower cladding. To reduce the total thickness and hinder the appearance of leaky modes, the waveguide is designed with a ‘‘double plasmon-enhanced confinement’’ ~Fig. 2!. This structure can be described as a dielectric waveguide with short Al0.90Ga0.10As cladding layers, therefore, leaky, sandwiched between two GaAs n 11 regions which complete the optical confinement. As one can see from Fig. 2, these regions are the heavily doped substrate (n si5331018 cm21 ) and the GaAs (n si5931018 cm21 ! layer below the top contact. Due to the plasma resonance, the real part of the refractive index of the heavily doped regions strongly decreases @Fig. 2~b!#. The resulting enhanced index contrast dumps the leaky tail of the fundamental mode into the substrate side and suppresses its coupling with the high-
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loss surface-plasmon mode on the other. This effect has to be traded against increased optical losses due to free-carrier absorption. Our calculations show that there are no significant changes in the losses of our waveguide if compared with a ‘‘plasmon-enhanced waveguide,’’ 10 commonly used for quantum cascade laser. The cladding layers are made of AlGaAs with 90% Al concentration. At a 9.4 mm wavelength the refractive index of this material is n52.87, that of GaAs is n53.23, and the average refractive index of the active region is n53.21. The index profile of the waveguide is reported in the lower part of Fig. 2. The high Al-content alloys tend to exhibit poor conductivity due to low electronic mobility and incomplete ionization of the donors ~;65 meV binding energy!.7 To minimize series resistances, much care is taken in the waveguide design to reduce the total thickness of the Al0.90Ga0.10As layer. This is obtained by adding thick lowdoped GaAs layers around the active region in order to increase the average refractive index of the core and push the claddings on the tails of the mode. The overlap factor between the mode and the core ~active region1low-dopedGaAs layers! in our waveguide is ;93%, hence, only 7% is penetrating in the cladding regions. Moreover, the GaAs layers show good conductivity with much lower doping concentration than the AlGaAs ones. This allows us to decrease the total amount of free electrons in the structure, thus reducing the optical losses. Lasers have been processed into ridge waveguides of different widths ~20, 24, and 28 mm at their bases! by optical contact lithography and deep wet chemical etching in a HCl:H2O25200:10 solution. After thinning the substrate down to ;100 mm, a AuGe/Ni/Au alloyed contact is evaporated on the device backside. A 300-nm-thick Si3N4 insulation is deposited by sputtering, and windows are opened by reactive ion etching on top of the ridges. Nonalloyed Ti/Au Ohmic contacts are evaporated on the top surface. The lasers are cleaved in bars 1–3 mm long and the facets left uncoated. They are then soldered epilayer up on copper holders ~Au coated!, wire bonded, and mounted on a temperature controlled cold head. Figure 3 shows the voltage @Fig. 3~a!# and the peak optical power from a single facet @Fig. 3~b!# versus the injection current for a device 2.5 mm long and 24 mm wide. The output light is collected using an f /0.8 optics and a calibrated fast room-temperature HgCdTe detector. The collection efficiency of the apparatus is estimated to be 50%. Various evidences of laser action are presented in Fig. 3: the abrupt change ~by orders of magnitudes! of the optical power above the threshold current density, reaching ;70 mW @Fig. 3~b!#, the line narrowing of the emission spectra shown in the insets of Fig. 3~a!, and, although less obvious, the change of slope at threshold in the voltage current characteristic.9 This last effect, associated with the pinning of the gain above the threshold, is a feature typical of all semiconductor lasers. The laser emission peaks at 131.6 meV (l59.4 m m) are in very good agreement with the calculated laser transition energy (E 235134 meV). 11
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ment is caused by the intervalley scattering of electrons which, from the high-energy tail of the quasidistribution in the n53 state, transfer into the L valleys of GaAs and/or X valleys of AlGaAs and from there eventually return into G in other subbands than the excited one. This effect reduces population inversion, thus decreasing the gain. Systematic studies as a function of the temperature and the cavity length should provide important insights to the comprehension of this effect and are underway. Preliminary results are shown in the inset of Fig. 3~b!. Lasers at longer wavelength, where the upper state of the laser transition has deeper confinement, can become of great interest in this material system particularly in view of the reduced waveguide losses compared to AlInAs/GaInAs. In summary, we have reported a QC laser fabricated in the AlGaAs/GaAs material system. This device operates up to 140 K in the technologically important ~8–12 mm! atmospheric window. Efforts aiming at increasing the optical output power and the operating temperature, as well as achieving cw operation are in progress.
FIG. 3. ~a! Voltage vs current characteristic measured at 77 K. In the insets, three emission spectra at various drive currents: 0.8 A, 2.8 A below threshold, and 5.1 A above it. In the spectra below threshold, the high-energy peak corresponds to the 3→1 transition. The measured separation between the two peaks is 35 meV. Note the strong line narrowing signature of laser action. The spontaneous emission and the laser radiation are polarized normal to the layer ~TM polarization!. ~b! Collected peak optical power from a single facet vs drive current. The device is 24 mm wide and 2.5 mm long. The inset shows the threshold current density in pulsed operation as a function of the heat-sink temperature.
The authors acknowledge the help of J. Croizier and M. Stellmacher for sample growth, A. Peugnet for sample processing, and V. Berger for useful discussions. This work has been partially funded by the European Community under the Brite/Euram ‘‘UNISEL’’ research project ~Contract No. CT97-0557!. P.K. is grateful for the financial support from EC grant under Contract No. ERB FMB ICT 972589.
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The threshold current density is 7.2 kA/cm2 at 77 K and the corresponding bias voltage V th511.4 V. The maximum operating temperature is 140 K ~inset, Fig. 3!. The potential drop per period required to line up the ground state of the injector with the upper state of the laser transition ~the condition shown in Fig. 1! is 0.23 V. This value times the number of periods ~30 in our laser! gives, in a first approximation, the expected potential drop at threshold, V th56.9 V, in loose agreement with the measured value. This discrepancy can be explained by a residual series resistance of the order of 1 V which likely originates from the Al0.90Ga0.10As cladding layers. Preliminary results on devices with different lengths and direct waveguide absorption measurements12 indicate that waveguide losses a W are in the range of 20 cm21. Assuming a linear dependence of the material gain G M on the injection current, the threshold current density can be estimated from the expression: G M 5gJ th5( a M 1 a W )/G , where a M are the mirror losses, J th is the current density, g is the gain coefficient, and G is the overlap factor between the mode and the active region. Substituting the calculated G50.42, the value of a M 55.3 cm21, the expected a W and the measured J th , we derive g58.7 cm21/~kA/cm2!. This value is lower than the one theoretically predicted using, e.g., Eq. ~3! of Ref. 13, which gives g530 cm21/~kA/cm22!. We believe that this disagree-
J. Faist, F. Capasso, D. L. Sivco, C. Sirtori, A. L. Hutchinson, and A. Y. Cho, Science 264, 553 ~1994!. 2 C. Sirtori, J. Faist, F. Capasso, D. L. Sivco, A. L. Hutchinson, and A. Y. Cho, IEEE Photonics Technol. Lett. 9, 294 ~1997!; J. Faist, C. Sirtori, F. Capasso, D. L. Sivco, J. N. Baillargeon, A. L. Hutchinson, and A. Y. Cho, ibid. 10, 1100 ~1998!. 3 J. Faist, F. Capasso, C. Sirtori, D. L. Sivco, J. N. Baillargeon, A. L. Hutchinson, and A. Y. Cho, Appl. Phys. Lett. 68, 3680 ~1996!. 4 C. Gmachl, J. Faist, J. N. Baillargeon, F. Capasso, C. Sirtori, D. L. Sivco, S. N. G. Chu, and A. Y. Cho, Appl. Phys. Lett. 70, 2670 ~1997!. 5 J. Faist, C. Gmachl, M. Striccoli, C. Sirtori, F. Capasso, D. L. Sivco, and A. Y. Cho, Appl. Phys. Lett. 69, 2456 ~1996!. 6 Recently, optically pumped intersubband lasers and electroluminescent devices in GaAs/AlGaAs have been demonstrated: O. Gauthier-Lafaye, P. Boucaud, F. H. Julien, S. Sauvage, S. Cabaret, J.-M. Lourtioz, V. ThierryMieg, and R. Planel, Appl. Phys. Lett. 71, 3619 ~1997!; G. Strasser, P. Kruck, M. Helm, J. N. Heyman, L. Hvozdara, and E. Gornik, ibid. 71, 2892 ~1997!. 7 N. Chand, T. Henderson, J. Klem, W. T. Messelink, R. Fisher, Y.-C. Chang, and H. Morkoc¸, Phys. Rev. B 30, 4481 ~1984!. 8 A. Bonvalet, J. Nagle, V. Berger, A. Migus, J.-L. Martin, and M. Joffre, Phys. Rev. Lett. 76, 4392 ~1996!; K. L. Vodopyanov, V. Chazapis, and C. C. Phillips, Appl. Phys. Lett. 69, 3405 ~1996!; J. Faist, F. Capasso, C. Sirtori, D. L. Sivco, A. L. Hutchinson, S. N. G. Chu, and A. Y. Cho, ibid. 63, 1354 ~1993!. 9 C. Sirtori, F. Capasso, J. Faist, A. L. Hutchinson, D. L. Sivco, and A. Y. Cho, IEEE J. Quantum Electron. 34, 1722 ~1998!. 10 C. Sirtori, J. Faist, F. Capasso, D. L. Sivco, A. L. Hutchinson, and A. Y. Cho, Appl. Phys. Lett. 66, 3242 ~1996!. 11 Calculations are made taking into account nonparabolicity following C. Sirtori, F. Capasso, J. Faist, and S. Scandolo, Phys. Rev. B 50, 8663 ~1994!; the nonparabolicity coefficient is g 54.9310219 m2. 12 O. Gauthier-Lafaye and F. H. Julien ~unpublished data!. 13 C. Sirtori, J. Faist, F. Capasso, D. L. Sivco, A. L. Hutchinson, S. N. G. Chu, and A. Y. Cho, Appl. Phys. Lett. 68, 3242 ~1996!.
