Application of a composite plasmonic substrate for the suppression of anelectromagnetic mode leakage in InGaN laser diodesPiotr Perlin, Katarzyna Holc, Marcin Sarzyński, Wolfgang Scheibenzuber, Łucja Marona, Robert Czernecki,Mike Leszczyński, Michał Bockowski, Izabella Grzegory, Sylwester Porowski, Grzegorz Cywiński, Piotr Firek,Jan Szmidt, Ulrich Schwarz, and Tadek Suski Citation: Applied Physics Letters 95, 261108 (2009); doi: 10.1063/1.3280055 View online: http://dx.doi.org/10.1063/1.3280055 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/95/26?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Indium-tin-oxide clad blue and true green semipolar InGaN/GaN laser diodes Appl. Phys. Lett. 103, 081103 (2013); 10.1063/1.4819171 Direct generation of 20 W peak power picosecond optical pulses from an external-cavity mode-locked GaInNlaser diode incorporating a flared waveguide Appl. Phys. Lett. 99, 111105 (2011); 10.1063/1.3640499 Low-threshold-current-density AlGaN-cladding-free m -plane InGaN/GaN laser diodes Appl. Phys. Lett. 96, 231113 (2010); 10.1063/1.3443719 Mode dynamics of high power ( InAl ) GaN based laser diodes grown on bulk GaN substrate J. Appl. Phys. 101, 083109 (2007); 10.1063/1.2718881 Microsecond time scale lateral-mode dynamics in a narrow stripe InGaN laser Appl. Phys. Lett. 84, 2473 (2004); 10.1063/1.1691497

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Application of a composite plasmonic substrate for the suppressionof an electromagnetic mode leakage in InGaN laser diodes

Piotr Perlin,1,2,a� Katarzyna Holc,1,2 Marcin Sarzyński,1 Wolfgang Scheibenzuber,3

Łucja Marona,1 Robert Czernecki,1,2 Mike Leszczyński,1,2 Michał Bockowski,1,2

Izabella Grzegory,1,2 Sylwester Porowski,1 Grzegorz Cywiński,1 Piotr Firek,4 Jan Szmidt,4

Ulrich Schwarz,3 and Tadek Suski11Institute of High Pressure Physics, Unipress, Sokolowska 29/37, 01-142 Warsaw, Poland2TopGaN, Ltd., 01-142 Warsaw, Poland3Fraunhofer IAF, Tullastr. 72, 79104 Freiburg, Germany4Institute of Microelectronics and Optoelectronics, Warsaw University of Technology, 00-661 Warsaw,Poland

�Received 15 September 2009; accepted 7 December 2009; published online 30 December 2009�

We demonstrate an InGaN laser diode, in which the waveguiding quality of the device is improvedby the introduction of highly doped �plasmonic� layer constituting an upper part of the GaNsubstrate. Thanks to this, we were able to suppress the electromagnetic mode leakage into thesubstrate without generating additional strain in the structure, in contrast to the typical designrelying on thick AlGaN claddings. The plasmonic substrate is built as a stack of gallium nitridelayers of various electron concentrations deposited by a combination of hydride epitaxy andhigh-pressure solution method. The mentioned improvements led to the reduction of the thresholdcurrent density of our devices down to 2 kA /cm2 and to the optimization of the near and far fieldpattern. © 2009 American Institute of Physics. �doi:10.1063/1.3280055�

Strong confinement of the optical mode in the edge emit-ting semiconductor laser diodes �LDs� is crucial for con-structing devices characterized by low threshold current andgood beam characteristics. The transversal confinement ofthe mode is always achieved by the refractive indexengineering.1 In the traditional GaAs/AlaAs system, one hasalmost full freedom of choice of layers content and thick-ness, owing to the good lattice match existing between GaAsand AlAs. In case of InGaN–GaN–AlGaN system, widerband gap AlGaN alloy forms cladding layers providing opti-cal confinement. However, as a result of significantly smallerlattice constants of this compound, flexibility in design ofLDs cladding is severely limited due to the presence of ten-sile strain in AlGaN layers. Excessive buildup of this strainleads to numerous problems, such as macroscopic epistruc-ture bowing, cracking, and/or creation of misfit dislocations.2

On the other hand, the lack of the good transversal modeconfinement transforms the transparent gallium nitride sub-strate into a parasite waveguide for the LD emission, takingaway significant fraction of the radiation traveling throughthe cavity. The importance of this effect was pointed out bySmolyakov et al.3 who gave the nickname “a ghost mode” tothe leaking mode. This effect is also referred to as “substratemodes.”4 The strict applicability of either of these terms de-pends on the thickness and absorption coefficients of a GaNsubstrate. In either case, the field in the substrate increasesthe internal losses and decreases the confinement factor forthe waveguide mode. In order to suppress the mode leakage,the leading technological groups use as thick AlGaN clad-ding as it is feasible taking into account the strain accumu-lation. For instance, the group from Nichia Chemical re-ported using very thick, 5 �m, Al0.05Ga0.095N:Si lowercladding layer.5 Osram OS achieved a suppression of sub-

strate modes by increasing the thickness of the lower clad-ding layer to 2 �m.4

In the present paper, we propose an alternative methodof the mode leakage suppression. On the top of bulk GaNsubstrate, an additional layer of GaN highly doped withO-donors was deposited. This enabled us to suppress themode leakage completely, without increasing the thickness ofthe AlGaN cladding layer. This choice was inspired by wellknown fact in semiconductor physics; strong dependence ofthe dielectric function of semiconductors on free carrier con-centration in the vicinity of the plasma frequency. In thisregion, neglecting the electron damping term, the dielectricfunction can be expressed as

���� = �0�1 −�p

2

�2� , �1�

where �0 is the optical dielectric constant and �p is theplasma frequency: �p=ne2 /m� �n is the carrier concentrationand e is an electron charge�. The plasma frequency in theplasmonic material is in the range of 2500–2800 cm−1.More information on plasma frequency in highly doped GaNcan be found in Ref. 6.

For the � values reasonably close to the plasma fre-quency, a substantial decrease of the dielectric constant andconsequently of the refractive index occurs. Materials withhigh enough electron concentration can thus serve as a clad-ding layer �we refer to them as plasmonic claddings in thistext�. The additional advantage of a plasmonic material isthat the lattice mismatch between low and high electron con-centration material is rather low in contrast to high strainintroduced by increasing Al content in AlGaN alloys used ascladding layers.7

The InGaN LDs, with plasmonic claddings werefabricated as follows �see Fig. 1�. First, the freestandingbulk GaN crystal obtained via hydride vapor phasea�Electronic mail: piotr@unipress.waw.pl.

APPLIED PHYSICS LETTERS 95, 261108 �2009�

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epitaxy �HVPE� �Ref. 8� with electron concentration of1�1018 cm−3 was mechanically polished in such a way thatits surfaces �Ga-polar and N-polar� were misoriented by 0.5°with respect to the crystallographic c-plane of the crystal.9

Subsequently, such wafer was mechanochemically polishedboth sides and placed into a high-pressure reactor where itwas overgrown with about 10 �m of high electron densityGaN layers on each Ga and N-polar faces. Free electronconcentration in these layers was not smaller than 5�1019 cm−3, and this high doping level is attributed to thehigh content of oxygen donors. The merit of this compositestructure �used in the next step as LD substrates� is the pos-sibility of combining the advantage of large lateral sizes andlow defect density of HVPE crystals with ultrahigh dopinglevel available in the high pressure method. The laser struc-ture deposited subsequently on the top of the gallium side ofthe crystal �by metalorganic vapor phase epitaxy methoddescribed elsewhere10� consisted of 600 nm Si-dopedAl0.08Ga0.92N bottom cladding, 50 nm Si-doped lower GaNwaveguide, 50 nm In0.02Ga0.98N injection layer, three pairsof In0.1Ga0.9N / In0.02Ga0.98N:Si �3.5/8 nm� quantum wells,20 nm Al0.2Ga0.8N-p+ electron blocking layer, 80 nm GaNwaveguide layer, 330 nm Al0.08Ga0.92N:Mg upper claddinglayer and 30 nm GaN:Mg p+-subcontact layer. It is importantto note that, in order to avoid the formation of the parasiticwaveguide, we did not grow the low carrier density galliumnitride layer typically inserted between the substrate andAl0.08Ga0.92N bottom cladding layer. As a reference device,we used identical epistructure but deposited on the top ofstandard HVPE gallium nitride substrate.

The lasers were fabricated as a ridge-waveguide, oxide-isolated devices. The mesa structures, with typical height of300 nm, were fabricated using reactive ion etching. Thestripe width for various devices varied from 3 to 20 �m.The mirrors were coated with quarter-wavelength layers ofSiO2 and TiO2 providing 95% and 20% reflectivity for theback and front mirrors, respectively.

The comparison between the current-optical power �IP�curves of these two laser structures is shown in Fig. 2. Strik-ing reduction in the threshold current density is clearly vis-

ible. It decreases from 5.2 kA /cm2 for the reference struc-ture to 2.3 kA /cm2 for the structure equipped with theadditional plasmonic cladding. Note that the slope efficiencyfor both structures remains similar, close to 0.5 W/A. Thisreduced value of the efficiency is caused by the pronouncedlight absorption in the p-side layers of the structure and canbe improved by introducing asymmetric claddings.11 Inter-estingly enough, the device equipped with the plasmonicwaveguide does not show any kink in its IP curve, which isattributed to the improvement in the vertical waveguidingproperties of the modified structure. The decrease of thethreshold current density is accompanied by the pronouncedchanges in the near field pattern of the laser beam, as shownin Fig. 3. The near field images were taken using a setupknown as Gaussian telescope with an aspheric lens of highnumerical aperture. A detailed description of the setup can befound in Ref. 12. It consists of two lenses aligned at a dis-tance equal to the sum of their focal lengths fi and providesa magnified image of the LD’s near field with magnificationM=f1 / f2. It can be seen that in the transversal direction themode extends deep into the substrate �few microns deep� forthe conventional structure, while in the case of the plasmonicsubstrate the mode is well confided around the active area.

In order to further elucidate this strong improvement, weperformed calculations of the transversal mode distribution.Needed for this refractive index of the high-electron concen-tration GaN plasmonic cladding was measured experimen-

FIG. 1. The schematic representation of two structures studied in the presentpaper.

FIG. 2. IP curves measure for the laser grown on the standard HVPEsubstrate and on the plasmonic substrate. The stripe dimensions are20�700 �m2, measured cw, at room temperature.

FIG. 3. �Color online� The comparison of the near field distributions for twostructures: upper one grown on plasmonic substrate while the lower one onstandard HVPE substrate. The near field images are arbitrarily shifted in thevertical direction for the better comparison. The stripe width is 3 �m for theplasmonic structure and 7 �m for the structure on HVPE.

261108-2 Perlin et al. Appl. Phys. Lett. 95, 261108 �2009�

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tally using spectrally resolved elipsometry. As can be seen inthe inset of Fig. 4, which shows the ratio between low carrierand high carrier density materials, at 400 nm the refractiveindex of the high electron concentration gallium nitride crys-tal is 2% lower than that of the low electron concentrationmaterial.

To calculate the transversal mode in our laser structurewe used a solver based on two-dimensional plane-wave ex-pansion method, which includes optical gain and losses inthe structure.13 The refractive indices of AlGaN and InGaNwere taken from Ref. 14.

The results are shown in Fig. 4. The calculations areperformed for identical structures with the only exceptions oflower refractive index of the plasmonic part of the substrate.In the calculations, we neglected the presence of the lowcarrier density material existing below the plasmonic layer,but large thickness of the latter ��10 �m� prevents themode from entering the middle part of this composite sub-strate. Figure 4�a� confirms the existence of the oscillatingmode in the substrate, for the structure with Al0.08Ga0.92Nlayer serving as a lower cladding. Indeed, as we mentionedearlier in this paper, many groups developing LDs wereforced to apply much thicker cladding in order to stop themode leakage.4,5 In Fig. 4�b�, we see the calculations per-formed for LDs with the plasmonic structure. The refractiveindex of the plasmonic GaN is almost identical to that ofA0.08Ga0.92N. This low refractive index value is sufficient toprevent the mode from leaking into the substrate, as shownin Fig. 4�b�. Additional advantage is clear from the measure-ment of the lattice parameter a for A0.08Ga0.92N and

plasmonic GaN. The x-ray diffraction measurements yieldsthe mismatch between the a—lattice parameters inGaN–GaNplasmonic system of roughly 0.02%. This is one or-der of magnitude less then the lattice mismatch of 0.2% forA0.08Ga0.92N–GaN system, meaning that the plasmonic clad-ding practically does not contribute to the increase of strainin the structure. Looking at the similarities of Al0.08Ga0.92Nand plasmonic-GaN refractive indices, the question ariseswhether it is possible to eliminate completely the lower Al-GaN cladding layer. This step would be beneficial in terms ofstructure simplicity and further reduction of strain. However,it could lead to increased free carrier losses in the waveguidedue to the presence of the electron plasma. The right com-promise between strain reduction and low waveguide losshas to be found in order to design a fully optimized device.

In summary, we demonstrate an optical-mode leakage-free nitride laser with a plasmonic substrate. Owing to theuse of highly n-doped GaN we were able to reduce the re-fractive index of the substrate by 2% at 400 nm, which issufficient to confine the mode in the original waveguide ofthe device without the use of thick AlGaN cladding. Thismodification of the laser design led to greatly improved de-vice parameters with jthr=2.3 kA /cm2 for the device with20 �m wide stripe.

The research was partially supported by the EuropeanUnion within European Regional Development Fund,through grant Innovative Economy �Grant No.POIG.01.01.02-00-008/08� and by Polish Ministry of Sci-ence and State Committee for Scientific Research, ProjectNo. R00-O0025/3.

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FIG. 4. �Color online� Plane-wave expansion method calculation of thetransversal mode distribution �a� for standard HVPE based structure, �b� forthe plasmonic substrate. The inset shows the ratio of refractive indices mea-sured for plasmonic GaN �n=5�1019 cm−3� and standard HVPE material�n=1�1018 cm−3�.

261108-3 Perlin et al. Appl. Phys. Lett. 95, 261108 �2009�

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