Quantum Cascade Laser based on a Waveguide Design with Ge Layer

(Master Diploma work at the Mesoscopic Physics group, University of Neuchatel, Switzerland)

Optical confinement in typical edge-emitting semiconductor lasers is provided by a dielectric waveguide[1]. This is also true when considering the mid-infrared quantum cascade lasers[2]. Quantum cascade lasers have been demonstrated successfully using InP[2], GaAs[3] and InAs[4] based materials. Progress has also been achieved in the direction of the achievement of a Si-based QCL[5]. However, to the exception of InP-based devices, in all the other cases the fabrication of the waveguide is a challenging task because of lattice mismatch or poor electrical and thermal conductivity of the cladding materials. In addition, the fact that the QC lasers operate in the mid-infrared require the growth of prohibitively thick layers.

Other waveguide approaches have already been explored, most notabely the use of interface plasmons formed at the metal-semiconductor interface[6]. These waveguides tend however to perform best in the Terahertz region of the spectrum[7]. In this work, we propose a waveguide structure that has a potential to operate efficiently in the mid-infrared and that combines high confinement factor and low waveguide losses.

Shown in Fig. 2 is the refractive index and mode profile of such a waveguide structure applied to the InP-based material system. The optical mode is confined in the active region due to the presence of a high refractive index Ge layer evaporated onto the top surface of the laser active region. A TM mode is propagating in the structure, in agreement to the polarization-selection rule for the intersubband transitions. The boundary conditions induced by the TM polarization are also responsible for the depression of the optical field in the high refractive index Ge layer and therefore to the high overlap factor of the optical mode with the active region. Mode overlap and optical waveguide losses are shown as a function of the Ge layer thickness in Fig. 2b). For the optimum thickness value of 750nm and the operation wavelength of 9um, the overlap factor of 74% and a computed waveguide loss of only 8.8 cm^-1 is predicted for a 25 period active region (assuming no loss in the bulk Ge). As a comparison, a conventional InP waveguide exhibits 64% overlap factor for a computed waveguide loss of 10cm^-1 . Note however that this value of the overlap factor is achieved for 35 period of the active region.

In the present study two lasers are investigated. Both employ the same gain medium, based on a two-phonon resonance active region in the InP material system operating at a wavelength of 9um [10], inserted in two different waveguides. The first one has a conventional waveguide structure that uses two InP claddings, the top one grown by MOCVD and the lower one formed by the InP substrate, that confine the optical mode in the active region. [11]. The second device uses the waveguide design shown in Fig. 1. The Molecular Beam Epitaxy starts with a 200nm thick InGaAs buffer layer, followed by 25 periods of the active region. In contrast to previous approaches that used a lateral injection[12], the layer responsible for the current injection is not located at the minimum of the optical field, and therefore should display the best possible compromise between optical losses and electrical conductivity. To this end, the active region is followed by a two-dimensional electron gas formed by a 30nm undoped InGaAs, followed by a 5nm AlInAs spacer layer and a doped (Si: 2x10¹⁸cm-3) 4nm thick AlInAs layer. A 20nm AlInAs and a 10nm InGaAs undoped layers complete the growth. Fig.2a shows the effect of the thickness of the Ge layer on the mode confinement in the waveguide [8]. In the absence of the Ge overlayer, the mode is mainly confined in the substrate and the overlap factor is only 30%. The optical confinement has a maximum for the Ge thickness of 750nm and decreases in the case of thicker Ge layers as the mode confines more in the Ge layer. Computations of threshold current density for both waveguide designs were done using the formula in [15]. With the use of parameters given in [16] we calculated Jth=1.4 kA/cm² and Jth=1.75 kA/cm² for the lasers with the conventional and the studied designs, respectively.

As shown in Fig. 1, the Ge layer is laterally patterned into a narrow, 8um wide ridge that also achieves a tight lateral confinement of the optical mode. According to the simulation results using the BeamProp/Fullwave software package displayed in Fig 1b, the optical mode extends laterally only over a characteristic length of approximatly 2um. As a result, no excess loss is expected from the interaction between the guided mode and the lateral contacts. Our simulations showed that ridges as narrow as 5um could be realized while keeping a large mode overlap factor.

The first experiments were performed using a simplified process in which the devices were processed into micro-cylinder cavities[17]. The process started by a relatively deep wet chemical etching of the active region and into the InP substrate, to a depth of 7-8um using an isotropic HBr etch solution. This process step formed cylindrical mesa of 160um diameter with nice vertical sidewalls. As a second step, a Ge/Au/Ag/Au top contact was deposited over a 100um diameter centered in the middle of the mesa and alloyed at 355 ⁰C . The process was terminated by an E-gun evaporation of a 750nm thick Ge layer over the mesa, where a hole was left to enable wire bonding. Apart from its simplicity, this processing had the advantage that the whispering gallery modes exhibit almost no outcoupling loss, and that the lateral patterning of the Ge is automatically achieved by the smooth sidewalls of the etched mesas. As shown in Fig 3, cylinders manufactured of the InP-cladded samples did exhibit the low thresholds expected from these epilayers. For the samples processed using the Ge overlayer, laser operation was achieved only at low temperatures and with a much larger threshold current density (4.3kA/cm2) close to the onset of the negative differential conductivity. For this reason, these devices could not operate at higher temperatures than about 100K.

Processing of ridge-cavities differs from the one of micro-disk cavities in the variety of process steps. The first step of the process is 2-line metal deposition by electron beam. The alloy of Ge/Au/Ag/Au is annealed at 355⁰C so that Ge diffuses into the semiconductor breaking the Schottky barrier. After chemical etching of the ridge mesas (28;38;48um wide, 1.75um deep) Si₃N₄ insulation layer is deposited by plasma-enhanced chemical vapour deposition (PECVD) and reactive-ion (RI) - etching is applied to open the layer for the electrical contacts. Having deposited TiAu top-electrode the substrate is thinned mechanically down to 120-150um and Ge/Au/Ag/Au back-electrode is deposited followed by alloying in the same way as for the 2-line metal contact. As the last step of the processing the Ge polycrystalline layer is deposited by the E-beam evaporation. The Ge stripe is 8um wide and 750nm thick. Schematic representation of a processed device is shown in Fig.1a. As the last step the samples are cleaved into separate cavities (1-3mm long), In-soldered to Cu-bars and Au-bonded to contact pads.

As expected for a device with a reduced number of period, the turn-on voltage (about 6V) is proportionally lower than the one observed in the InP-cladded devices. The measured differential series resistance (= 2.1 Ohms) is only 0.5 Ohms larger than the one measured in the InP-cladded sample, demonstrating the efficiency of the two-dimentional electron gas for the lateral transport. The L-I curves, however, show only spontaneous emission that saturates when the device reaches the negative differential resistance region.

The ridge laser devices suffer from additional optical losses that completely prevent the laser action. Additional experiments performed on microcylinder resonators have shown that an Ar+ cleaning prior to the Ge evaporation is able to reduce the threshold current. However, the fabricated devices show aging behavior with a significant threshold current increase over periods of few weeks of storage in ambiant air. Our interpretation is that a significant loss mechanism is the formation of an interfacial oxide at the Ge/InGaAs interface. This oxide is expected to have a low refractive index. As the mode is TM polarized, continuity of the displacement electric field requires that the electric field in the oxide layer will be enhanced by the ratio of the dielectric constant in the oxide and in the Ge. Therefore, even a very thin layer of Oxide may yield very large optical losses. This loss mechanism could be circumvented by a proper surface preparation prior to the Ge deposition, followed by a passivation of the device using Si3N4.

References

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[7] R. Ko¨hler, A. Tredicucci, F. Beltram, H. Beere, G. Davies, E. Linfield, D. Ritchie, R. C. Iotti, and F. Rossi, Terahertz semiconductor-heterostructure laser, Nature, London, 417, 156, 2002.

[8] 1D simulations are realised solving Maxwell equations by transfer matrix algorithm.

[9] 2D simulations are done by D.Gerber using “Full-wave” software.

[10] The layer sequence of one period, in nanometers, starting with the injection barrier, is as follows: 4.0/1.9/0.7/5.8/0.9/5.7/0.9/5.0/2.2/3.4/1.4/3.3/1.3/3.2/1.5/3.1/1.9/3.0/2.3/2.9/2.5/2.9, where InAlAs barrier layers are in bold, InGaAs well layers are in roman, and n-doped layers (2x10¹⁷ cm^-3) are underlined.

[11] Mattias Beck, Daniel Hofstetter, Thierry Aellen, Jérôme Faist, Ursula Oesterle, Marc Ilegems, Emilio Gini, Hans Melchior, Continuous-wave operation of a mid-infrared semiconductor laser at room-temperature, Science, Vol.295, 2002.

[12] D. Hofstetter, J. Faist, M. Beck, A. Müller, U. Oesterle " Demonstration of high performance 10.16um quantum cascade distributed feedback lasers fabricated without eptaxial re-growth ", Appl. Phys. Lett. 75, 665, 1999.

[13] J. Faist, Daniel Hofstetter, Mattias Beck, Thierry Aellen, Michel Rochat, and Stéphane Blaser, "Bound-to-continuum and two-phonon resonance quantum cascade lasers for high duty cycle, high temperature operation", IEEE J. Quantum Electron., Vol.38, 2002.

[14] Mattias Beck, “Growth and characterization of strained InAlAs/InGaAs heterostructures for high-frequency applications”, PhD thesis EPFL, 1996.

[15] J_th=[e₀n/(4pq)]×[L_p/(N_pG_p)]×[l(2g₃₂)/z²₃₂]×[(a_m+a_w)/(t₃-t₂t₃/t₃₂)], J. Faist in the book of H.C. Liu, F. Capasso, Intersubband Transitions in Quantum Wells: Physics and Device Applications II, Academic Press, 2000.

[16] Parameters used for computations of J_th at 80K for studied InP-based design (for conventional design, in brackets ), period length Lp=60nm, number of periods Np=25 (35), overlap with one period Gp=0.024 (0.0183), broadening of the transition 2g32=24.8meV (22meV), losses am=2cm^-1 and aw=8.8cm^-1 (10cm^-1), dipol matrix element z32=3nm, intersubband lifetimes t2=0.5psec, t3=0.52psec, t32=1.45psec, effective index n=3.41.

[17] S. Anders, W. Schrenk, E. Gornik, and G. Strasser, Room-temperature operation of electrically pumped quantum-cascade microcylinder lasers, Appl. Phys. Lett. 80, 4094, 2002.