This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Journal of Applied Physics 109, 083104 (2011) and may be found at https://doi.org/10.1063/1.3574406.The parameters for reducing the threshold current density of InAs/InGaAsP/InP quantum-dot (QD) lasers suitable for high temperature operation are studied. The structures were grown using metalorganic vapor phase epitaxy. Increasing the number of QD layers leads to a substantial improvement of the optical confinement and a markedly reduced threshold per dot layer in broad area devices. A reduction of the spacer thickness between the QD layers was not found to significantly affect device characteristics. Depending upon the device length, an optimum number of QD layers was deduced. Based upon optimized QD stacks, buried-heterostructure lasers with a medium device length emitting at 1.5 μm were fabricated. Laterally single-mode devices show promising low threshold currents near 10 mA and good thermal stability with a characteristic temperature of 65 K up to 90 °C.DFG, 43659573, SFB 787: Halbleiter - Nanophotonik: Materialien, Modelle, Bauelement

Bimberg, Dieter

Franke, D.

Künzel, Harald

Moehrle, M.

Pohl, Udo W.

Sigmund, A.

Journal of Applied Physics

DepositOnce

J. Appl. Phys. 109, 083104 (2011); https://doi.org/10.1063/1.3574406 109, 083104© 2011 American Institute of Physics.Improved threshold of buriedheterostructure InAs/GaInAsP quantum dotlasersCite as: J. Appl. Phys. 109, 083104 (2011); https://doi.org/10.1063/1.3574406Submitted: 25 January 2011 • Accepted: 11 March 2011 • Published Online: 18 April 2011D. Franke, M. Moehrle, A. Sigmund, et al.ARTICLES YOU MAY BE INTERESTED INTemperature-stable operation of a quantum dot semiconductor disk laserApplied Physics Letters 93, 051104 (2008); https://doi.org/10.1063/1.2968137Fast gain and phase recovery of semiconductor optical amplifiers based on submonolayerquantum dotsApplied Physics Letters 107, 201102 (2015); https://doi.org/10.1063/1.4935792Formation of InAs/InGaAsP quantum-dashes on InP(001)Applied Physics Letters 95, 203105 (2009); https://doi.org/10.1063/1.3265733Improved threshold of buried heterostructure InAs/GaInAsP quantum dotlasersD. Franke,1 M. Moehrle,1 A. Sigmund,1 H. Kuenzel,1,a) U. W. Pohl,2 and D. Bimberg21Fraunhofer Institute for Telecommunications, Heinrich-Hertz-Institut, Einsteinufer 37, 10587 Berlin,Germany2Institut für Festkörperphysik, Technische Universität Berlin, Hardenbergstr. 36, 10623 Berlin, Germany(Received 25 January 2011; accepted 11 March 2011; published online 18 April 2011)The parameters for reducing the threshold current density of InAs/InGaAsP/InP quantum-dot (QD)lasers suitable for high temperature operation are studied. The structures were grown usingmetalorganic vapor phase epitaxy. Increasing the number of QD layers leads to a substantialimprovement of the optical confinement and a markedly reduced threshold per dot layer in broadarea devices. A reduction of the spacer thickness between the QD layers was not found tosignificantly affect device characteristics. Depending upon the device length, an optimum numberof QD layers was deduced. Based upon optimized QD stacks, buried-heterostructure lasers with amedium device length emitting at 1.5 lm were fabricated. Laterally single-mode devices showpromising low threshold currents near 10 mA and good thermal stability with a characteristictemperature of 65 K up to 90 C. VC 2011 American Institute of Physics. [doi:10.1063/1.3574406]I. INTRODUCTIONQuantum dot (QD) based gain media are advantageousfor semiconductor lasers due to the modified electronic den-sity of states which strongly differs from that of higherdimensional systems.1 QD lasers based on GaAs demon-strated improved characteristics compared to their quantumwell (QW) counterparts, such as reduced threshold currentdensity and improved temperature stability.2,3 Devices basedon InP using InAs QDs embedded in an InGaAsP matrix arepresently less well developed. The best performance reportedto date in terms of threshold current, internal efficiency, andcharacteristic temperature was achieved with structuresgrown by molecular beam epitaxy (MBE).4,5 InAs QDsgrown by MBE usually feature a lateral elongation alignedalong [011], referred to as quantum dashes.6,7 In contrast,dots grown by metalorganic vapor phase epitaxy (MOVPE)were found to be in-plane symmetric.8–10 Such dots form ina wide range of MOVPE growth conditions. Dashes likethose found in MBE are only formed by applying unusualgrowth parameters in terms of growth temperature and/or in-dium flux.11 Recently, high quality and thermally stableInAs/InGaAsP/InP QD structures emitting at 1.55 lm weregrown by MOVPE.12 These thermally stable QDs are prom-ising for fabricating QD lasers with characteristics compara-ble to their MBE-grown counterparts.13A single QD layer generally has a lower amount ofactive medium as compared to a single QW layer, leading tolower gain saturation levels and output power for QD laserscompared to QW devices with the same number of activelayers. The fraction of emitting QD material in the wave-guide, i.e., the optical confinement, can be enhanced byincreasing the number of QD layers and by reducing thethickness of the InGaAsP matrix spacer-layers. In this contri-bution, we present the effect of these parameters on the per-formance of QD broad-area lasers and demonstrate animproved status achieved in corresponding buried hetero-structure (BH) devices.II. CONDITIONS FOR QD GROWTHQD samples were grown by low pressure MOVPE usingconventional sources. The formation of QD structuresrequires flat surfaces. We achieved two-dimensional low-temperature growth (Tg 500 C) of GaInAsP layers with abandgap equivalent to 1.15–1.20 lm emission wavelength(Q1.15–1.20). These quaternary layers were lattice-matchedto InP and were employed as waveguide and matrix materi-als in our laser structures. InAs QD formation in the range470–510 C leads to dot densities between 5 1010 cm2and 1.7 1010 cm2, respectively (not shown here). In thisstudy we applied InAs deposition at 500 C, yielding2.5 1010 cm2 QDs per layer. The structures contained upto 13 stacked QD layers with typically 30 nm thick lattice-matched Q(1.15–1.20) spacer layers. Stacks incorporatingmore than 13 QD layers yielded inferior material quality.The surface morphology degraded due to strain relief by theformation of dislocation networks.III. EXPERIMENTAL RESULTSThe optical quality of structures containing 3 to 13 QDlayers was found to be identical as assessed from high excita-tion density 300 K photoluminescence (PL) measurements.Only a structure with a single QD layer showed a sub-linearincrease for excitation densities above 1 kW cm2 indicatingsome saturation.12 For all structures with three or more QDlayers a linear intensity increase was observed with increas-ing excitation density within the measured range up to6 kW cm2. The majority of the excited carriers recombinevia quantum dot states as concluded from PL data. Only aa)Author to whom correspondence should be addressed. Electronic mail:harald.kuenzel@hhi.fraunhofer.de.0021-8979/2011/109(8)/083104/4/$30.00 VC 2011 American Institute of Physics109, 083104-1JOURNAL OF APPLIED PHYSICS 109, 083104 (2011)small fraction of the carriers recombines in the quaternarymatrix material as indicated by the weak PL feature at 1150nm in Fig. 1. No additional broadening of the PL is found asthe number of QD layers is increased, indicating good inter-layer uniformity. The large full width at half maximum ofthe QD emission of 110 meV originates from intralayer QDsize fluctuations. Fluctuations of the QD height are the mainreason for this, since the height is markedly smaller than thelateral dimensions (not shown here) and thus essentiallydetermines the quantization energy.A. Fabrication of broad area QD lasersTo study the effect of the number of QD layers on theoptical confinement we processed broad-area lasers usingstructures with 3, 5, 7, and 13 QD layers grown at 500 C.The 50 nm Q(1.15) undoped waveguide layers with 2 1017cm3 n-doped (Si) and p-doped (Zn) InP cladding layers,and a 200 nm InGaAs:Zn contact layer doped to a level of1 1019 cm3 were used. The temperature for growth belowthe active layers was 640 C, while it was lowered to 620 Cfor layers deposited after the QD stacks to reduce potentialintermixing. The growth of the whole structure was com-pleted by an in situ Zn indiffusion to increase the p-dopinglevel to 8  1019 cm3 near the surface.Broad-area lasers 50 lm in width, along with seven QDlayers were processed, mounted, and operated in continuouswave (cw) mode. From the length dependence a thresholdper dot layer of 100 A cm2 for an infinite length at roomtemperature was deduced. Furthermore, a characteristic tem-perature T0  65 K, an absorption coefficient a¼ 5 cm1,and an internal quantum efficiency g ¼ 0.65 were achieved.B. Influence of design parametersThreshold data of broad-area devices with 50 lm ridgewidth and a varied number of QD layers were recorded on achip in pulsed mode for simplicity. The results are summar-ized in Fig. 2. The data depict the threshold per dot layer ver-sus the inverse cavity length for devices with differentnumbers of QD layers. A marked influence of the number ofQD layers on the threshold current density per dot layer isobvious. From the exponential fits, a decrease of the thresh-old per dot layer at infinite length L from 210 to 90 A cm2is found as the number of dot layers is increased from threeto seven. The slope of the ln(J) versus 1/L dependence allowsus to determine the optimum resonator length.14 These val-ues, marked by arrows in Fig. 2, decrease from 640 lm forthree QD layers to 370 lm for seven QD layers. By plottingthe threshold current density at the optimum resonator lengthversus the inverse number of QD layers a transparency cur-rent density of 77 A cm2 and a gain constant of 9 A cm1were deduced. Based upon these results, a further improve-ment of the optimum length can mainly be achieved by fur-ther increasing the number of QD layers incorporated in theactive core of the laser structure. Devices with up to 13 QDlayers still show some minor improvement of the thresholdper dot layer at infinite length to 70 A cm2. Additionally,the slope of the relation ln(J) versus 1/L shows some minorimprovement.The decrease of threshold per dot layer at infinite lengthis accompanied by a linear increase of the absorption coeffi-cient with the number of dot layers. The values increasedfrom 5.7 (five QD layers) to 11.5 cm1 (13 QD layers). Bycomparing these results with the data of a reference structurecontaining eight QWs, given by the gray line in Fig. 2, weconclude: i) the threshold per dot layer at infinite length ofthese devices is virtually identical with comparable QWstructures, however ii) the threshold for short cavity QDlasers is still inferior due to the smaller slope of the ln(J) ver-sus 1/L dependence as is also found for quantum dash lasersgrown by MBE.4 For 200 lm long devices the difference inFIG. 1. Room-temperature photoluminescence spectra of five-fold (black)and eleven-fold (gray) stacked InAs/InGaAsP QD layers nonresonantlyexcited with 500 W cm2.FIG. 2. Device-length dependence of the threshold current density per dotlayer of broad-area QD lasers (width ¼ 50 lm) emitting at 1.55 lm. Thedevices contain various numbers of QD layers and thicknesses of spacersbetween the QD layers as indicated in the figure. Arrows mark optimum cav-ity lengths and the gray line refers to a similar structure with eight quantumwells.083104-2 Franke et al. J. Appl. Phys. 109, 083104 (2011)threshold amounts to a factor of 3 as deduced from Fig. 2 for1/L¼ 50 cm1. To attain similar values as those of the QWstructures an increase of the density of the QD layers mightbe a viable way. Alternatively, an increase of the dot densityper QD layer may also lead to quantum-dash formation forMOVPE growth due to the required change in depositionconditions.15We kept the growth parameters constant and fabricateda device with seven QD layers, where the spacer thicknesswas varied between 40 and 10 nm. The data included inFig. 2 show virtually no difference in performance within thescatter of the data. This indicates that a more dense stackingof QD layers does not lead to a significant improvement ofthreshold. Further reduction towards a 10 nm spacer did notdegrade laser performance and gives practically identicallaser characteristics as with 20 or 30 nm spacers. In contrast,an increase in QD-layer separation to 40 nm leads to anincrease of the threshold per dot layer at infinite length from90 to 135 A cm2 (data not included in Fig. 2).For the practical design of lasers with improved injec-tion current, as is necessary for stable high temperature oper-ation, the overall threshold current is of concern. Accordingto Ref. 14 (Eq. 17), the overall threshold in conventionalmultiple quantum well lasers is mainly dominated by its de-pendence upon the number of quantum wells. This depend-ence comprises both; a factor being linearly dependent and afactor decreasing exponentially with the number of quantumwells. As a result, a minimum overall threshold in depend-ence of the number of quantum wells can be deduced for afixed laser length. We have applied the same evaluation pro-cedure to our QD lasers. The corresponding results aredepicted in Fig. 3 for three different laser lengths. Besidesthe expected appearance of a minimum threshold, it canclearly be seen in Fig. 3 that due to the additional depend-ence on laser length, the number of QD layers suitable for aminimum threshold increases for finite lengths. For lasers ofinfinite length (extrapolated) the optimum number of QDlayers is five. This value increases to six layers for very longlasers (L¼ 1000 lm) and to seven layers for devices with amedium length (L¼ 400 lm).C. Buried heterostructure QD lasersTo demonstrate the achieved capability of InAs/InGaAsP QD structures grown using MOVPE, we fabricated1.4 lm wide laterally single mode BH lasers. We appliedgrowth conditions also used for the broad-area devices andincorporated seven QD layers as an optimum for mediumlength devices. The current flow to the 1.4 lm wide laserridge was restricted by p/n-blocking. A scanning electronmicrograph of the cleavage plane showing the 1.4 lm wideridge with the active QD region and the p/n blocking layersis given in Fig. 4(a). The emission of these devices was blue-shifted by typically 50–100 nm with respect to that of theunprocessed structure. The shift originates from the regrowthof the blocking and contact layers and may be balanced instructures thermally optimized with respect to the regrowthprocess. The effect may also be suppressed by a decrease ofthe regrowth temperature and a slightly higher growth tem-perature of the QD layers to obtain a higher thermal stabilityof the QDs.13 BH-type QD laser devices were prepared bycleaving the structures to lengths between 300 and 400 lm.Pulsed light output versus current LI-characteristics of theseuncoated devices given in Fig. 4(b) showed typical thresholdcurrents between 10 and 20 mA at room temperature. Devi-ces were operated up to 90 C with the threshold increasingonly up to 30 mA in agreement with a characteristic temper-ature, T0, of 65 K similar to broad-area devices. An outputpower per facet of 10 mW at 200 mA drive current of theunmounted lasers was achieved. Gain spectra were deducedfrom cw emission spectra recorded close to threshold asdepicted in Fig. 5(a). By analyzing the longitudinal spectralmode behavior using the Hakki–Paoli method, a 80 nm widegain spectrum peaking at 1490 nm was extracted for a 330lm long device driven at 10 mA, cf. Fig. 5(b). A peak netgain of 30 cm1 corresponding to a value per dot layer closeto 5 cm1 was determined. This compares with a net gain atFIG. 3. Dependence of the threshold current density on the number of QDlayers for broad-area lasers of different lengths.FIG. 4. BH-type laser containing seven InAs QD layers and a Q(1.15) ma-trix. (a) Scanning electron micrograph of the cleavage plane of the deviceshowing the active QD region and p/n-blocking layers. (b) Static LI-charac-teristics for different operation temperatures of a 330 lm long and 1.4 lmwide device in pulsed mode.083104-3 Franke et al. J. Appl. Phys. 109, 083104 (2011)threshold of 19 cm1 in a MBE grown buried ridge stripe de-vice with six QD layers and 40 nm spacers5 demonstratingthe competitiveness of our MOVPE approach.IV. CONCLUSIONSTo conclude, the main parameters for reducing thethreshold of MOVPE-grown InP-based QD lasers have beenpresented. Only the number of QD layers was found to mark-edly affect the optical confinement and thus the laser thresh-old. Optimum performance was obtained for devicescomprising seven QD layers. First structures for BH-type de-vice fabrication operating laterally single mode near 1.5 lmshowed a promising low threshold near 10 mA without facetcoating, a high net gain of 30 cm1, and a good thermal sta-bility with T0¼ 65 K up to 90 C.ACKNOWLEDGMENTSPart of the work was funded by the German ResearchFoundation DFG within the SFB787 project A3. The authorsthank W.-D. Molzow for expert help in laser processing andcharacterization.1M. Asada, Y. Miyamoto, and Y. Suematsu, IEEE J. Quantum Electron.QE-22, 1915 (1986).2D. Bimberg, J. Phys. D: Appl. Phys. 38, 2055 (2005).3O. B. Shchekin, J. Ahn, and D. G. Deppe, Electron. Lett. 38, 712 (2002).4J. P. Reithmaier, A. Somers, S. Deubert, R. Schwertberger, W. Kaiser, A.Forchel, M. Calligaro, P. Resneau, O. Parillaud, S. Bansropun, M. Kra-kowski, R. Alizon, D. Hadass, A. Bilenca, H. Dery, V. Mikhelashvili, G.Eisenstein, M. Gioannini, I. Montrosset, T. W. Berg, M. van der Poel, J.Mork, and B. Tromborg, J. Phys. D: Appl. Phys. 38, 2088 (2005).5F. Lelarge, B. Rousseau, B. Dagens, F. Poingt, F. Pommerau, and A.Accard, IEEE Photon. Technol. Lett. 17, 1369 (2005).6O. Bierwagen and W. T. Masselink, Appl. Phys. Lett. 86, 113110 (2005).7G. Saint-Girons, A. Michon, I. Sagnes, G. Beaudoin, and G. Patriarche,Phys. Rev. B 74, 245305 (2006).8J. W. Jang, S. H. Pyun, S. H. Lee, I. C. Lee, W. G. Jeong, R. Stevenson,P. D. Dapkus, N. J. Kim, M. S. Hwang, and D. Lee, Appl. Phys. Lett. 85,3675 (2004).9K. Kawaguchi, M. Ekawa, A. Kuramata, T. Akiyama, H. Ebe, M. Suga-wara, and Y. Arakawa, Appl. Phys. Lett. 85, 4331 (2004).10S. Anantathanasarn, R. Nötzel, P. J. van Veldhoven, T. J. Eijkemans, andJ. H. Wolter, J. Appl. Phys. 98, 013503 (2005).11A. Lenz, F. Genz, H. Eisele, L. Ivanova, R. Timm, D. Franke, H. Künzel,U. W. Pohl, and M. Dähne, Appl. Phys. Lett. 95, 203105 (2009).12D. Franke, P. Harde, J. Boettcher, M. Moehrle, A. Sigmund, and H. Kuen-zel, Proc. 19th International Conference on InP and Related Materials,Matsue, 2007, IEEE Catalog # 07CH37859C, ISBN # 1-4244-0874-1,p. 559-562, 2007.13D. Franke, M. Moehrle, J. Boettcher, P. Harde, A. Sigmund, and H. Kuen-zel, Appl. Phys. Lett. 91, 081117 (2007).14M. Rosenzweig, M. Moehrle, H. Dueser, and H. Venghaus, IEEE J. Quan-tum Electron. 27, 1804 (1991).15D. Franke, J. Kreissl, W. Rehbein, H. Künzel, U. W. Pohl, and D. Bim-berg, Appl. Phys. Express 4, 014101 (2011).FIG. 5. (a) Mode spectrum of the BH QD laser shown in Fig. 4 takenslightly below threshold at a 10 mA injection currrent in cw-mode. (b)Wavelength dependence of the net gain as deduced from the data of (a)using the Hakki–Paoli method.083104-4 Franke et al. J. Appl. Phys. 109, 083104 (2011)

Improved threshold of buried heterostructure InAs/GaInAsP quantum dot lasers

https://depositonce.tu-berlin.de/bitstream/11303/16161/1/franke_etal_2011.pdf

Improved threshold of buried heterostructure InAs/GaInAsP quantum dot lasers

Abstract

Similar works

Full text

Available Versions

DepositOnce