The authors report on the achievement of optically pumped III-V nitride blue microdisk lasers operating at room temperature. Controlled wet chemical etching of an AlInN interlayer lattice matched to GaN allows forming inverted cone pedestals. Whispering gallery modes are observed in the photoluminescence spectra of InGaN∕GaN quantum wells embedded in the GaN microdisks. Typical quality factors of several thousands are found (Q>4000). Laser action at ∼420nm is achieved under pulsed excitation at room temperature for a peak power density of 400kW/cm2. The lasing emission linewidth is down to 0.033nm

BUHLMANN, H-J

BUTTE, R'

CARLIN, J-F

CASTIGLIA, A

FELTIN, E

GRANDJEAN, N

MOSCA, M

SIMEONOV, D

ZHU, T

English

Archivio istituzionale della ricerca - Università di Palermo

Reuse of AIP Publishing content is subject to the terms at: <a href="https://publishing.aip.org/authors/rights-and-permissions">https://publishing.aip.org/authors/rights-and-permissions</a>. Downloaded to: 147.163.19.185 on 14 November 2018, At: 06:25Blue lasing at room temperature in high quality factor  microdisks withInGaN quantum wellsD. Simeonov, E. Feltin, H.-J. Bühlmann, T. Zhu, A. Castiglia, M. Mosca, J.-F. Carlin, R. Butté, and N. GrandjeanCitation: Appl. Phys. Lett. 90, 061106 (2007); doi: 10.1063/1.2460234View online: https://doi.org/10.1063/1.2460234View Table of Contents: http://aip.scitation.org/toc/apl/90/6Published by the American Institute of PhysicsArticles you may be interested in III-Nitride-on-silicon microdisk lasers from the blue to the deep ultra-violetApplied Physics Letters 109, 231101 (2016); 10.1063/1.4971357Low threshold, room-temperature microdisk lasers in the blue spectral rangeApplied Physics Letters 103, 021112 (2013); 10.1063/1.4813471Lasing in GaN microdisks pivoted on SiApplied Physics Letters 89, 211101 (2006); 10.1063/1.2392673Stimulated emission and lasing in whispering-gallery modes of GaN microdisk cavitiesApplied Physics Letters 75, 166 (1999); 10.1063/1.124307High quality nitride based microdisks obtained via selective wet etching of AlInN sacrificial layersApplied Physics Letters 92, 171102 (2008); 10.1063/1.2917452Optically pumped whispering-gallery mode lasing from 2-µm GaN micro-disks pivoted on SiApplied Physics Letters 104, 221106 (2014); 10.1063/1.4881183Blue lasing at room temperature in high quality factor GaN/AlInNmicrodisks with InGaN quantum wellsD. Simeonov,a E. Feltin, H.-J. Bühlmann, T. Zhu, A. Castiglia,b M. Mosca,b J.-F. Carlin,R. Butté, and N. GrandjeanÉcole Polytechnique Fédérale de Lausanne (EPFL), Institute of Quantum Electronics and Photonics,CH-1015 Lausanne, SwitzerlandReceived 27 November 2006; accepted 8 January 2007; published online 6 February 2007The authors report on the achievement of optically pumped III-V nitride blue microdisk lasersoperating at room temperature. Controlled wet chemical etching of an AlInN interlayer latticematched to GaN allows forming inverted cone pedestals. Whispering gallery modes are observed inthe photoluminescence spectra of InGaN/GaN quantum wells embedded in the GaN microdisks.Typical quality factors of several thousands are found Q4000. Laser action at 420 nm isachieved under pulsed excitation at room temperature for a peak power density of 400 kW/cm2. Thelasing emission linewidth is down to 0.033 nm. © 2007 American Institute of Physics.DOI: 10.1063/1.2460234High quality factor Q= / three-dimensional semi-conductor microcavities MCs and microdisks disks at-tract much attention because of their potential use for fabri-cating low-threshold lasers.1,2 They are equally interestingfor studying cavity quantum electrodynamics CQED phe-nomena in the solid state such as strong light-mattercoupling3,4 or the Purcell effect.5,6 Furthermore embeddingquantum dots QDs in such structures opens the way to therealization of single photon emitters.7 In this respect, III-Vnitride semiconductors are particularly attractive for observ-ing CQED phenomena at room temperature RT due to theirlarge oscillator strength and exciton binding energies.8Recent progress in the fabrication of planar nitride-basedMCs has led to the successful demonstration of high Q2800 MCs,9 as well as laser emission under opticalpumping in vertical cavity surface emitting laser structures.10The interest on semiconductor disks resides in their high Qwhispering gallery modes WGMs and their small modalvolume.2 Furthermore, while the fabrication of high qualitynitride distributed Bragg reflectors and MCs usually requiresstrain engineering solutions,11 disks remain rather simplephotonic structures in comparison. In addition, the latter pos-sess the same advantages of compactness and ability to beintegrated in two-dimensional arrays. However, the fabrica-tion of disks has proven to be challenging in III-V nitridesmainly due to the lack of efficient underetching techniques.Indeed a convenient wet etching process suitable for the for-mation of thin disks d /2neff optically isolated from anysurrounding higher index materials, as it is the case, e.g., forthe III-V arsenide system,6 does not exist for standard AlGaNand InGaN alloys. Furthermore at short wavelengths, theemitted light is more sensitive to processing defects leadingto scattering losses.WGMs in nitrides were first observed in circularmesas.12 The mode structure was studied but neither lasingaction nor high Q factors were demonstrated. Recently thefabrication of nitride disks using photoelectrochemicalband gap selective etching was successfully achieved leadingto disks optically isolated from the underneath template.13Low temperature T=10 K lasing in these structures wasreported under pulsed optical pumping with a peak powerdensity threshold of 27 MW/cm2.In this letter we report the fabrication of disk struc-tures, containing InGaN quantum wells QWs, in the lattice-matched GaN/AlInN system. Typical WGM Q factors in thespontaneous emission regime of about 4000 are found. Thesedisks exhibit a RT lasing in the blue wavelength range witha threshold at a peak power density of about 400 kW/cm2.The growth was carried out by metal organic vaporphase epitaxy MOVPE on 2 in. c-plane sapphire substrates.After deposition of a standard GaN template, a 200 nm thickGaN:Si doped layer is grown. It is followed by a 400 nmthick nonintentionally doped nid GaN layer to reduce thecurrent flowing out of the mesas during the oxidation pro-cess. A 400 nm thick Si doped AlInN layer xIn18% nearly lattice matched to GaN is then deposited for subse-quent lateral oxidation. The cavity and the active region areformed by a 100 nm thick GaN layer with three In0.15Ga0.85NQWs about 3 nm thick separated by 12 nm thick GaNbarriers.After the MOVPE growth step, 200 nm thick circularSiO2 masks are formed and 550 nm high circular mesas3–10 m in diameter are etched by Cl2 /Ar reactive ionetching to give access to the GaN:nid layer and the AlInNlayer sidewall. Then the bottom part of the AlInN layer islaterally etched using high current densities during a lateraloxidation process similar to that described in Ref. 14. Thisprocedure gives access to the N polarity face of the AlInNlayer. The mesas are then wet etched in a HCl:H2O=1:1solution. Here etching occurs both laterally and in the verti-cal direction through the previously revealed N polarity face.This process seems to be isotropic as no specific crystallo-graphic planes appear Fig. 1. After the etching step, theremaining protective SiO2 layer is dissolved in a BHFsolution.A typical disk structure is presented in the scanningelectron microscope images in Fig. 1. The sidewall is smoothand inclined at 45°. In such disk structures, part of theemitted light is waveguided in the GaN layer. While the topaElectronic mail: dobri.simeonov@epfl.chbAlso at DIEET, Università degli Studi di Palermo, Palermo, Italy.APPLIED PHYSICS LETTERS 90, 061106 20070003-6951/2007/906/061106/3/$23.00 © 2007 American Institute of Physics90, 061106-1confinement is only ensured by air, the bottom one is ensuredby air at the edge of the disk and AlInN refractive indexn2.28 at =420–440 nm15 in the central part. As a con-sequence, the effective refractive index neff in the GaN wave-guide varies between 2.10 and 2.28 when going from theedge to the central part of the disk. The guided light will belaterally confined by total internal reflections and thus willform a disk cavity.The modal structure of the disks was investigated byphotoluminescence PL using the 244 nm line of a cw Ar+laser frequency doubling unit. Laser action was achieved us-ing a pulsed 266 nm neodymium-doped yttrium aluminumgarnet laser with an average output power of 2 mW, a rep-etition rate of 8 kHz, and a pulse length of 500 ps. Measure-ments were performed in backscattering geometry using anUV microscope objective 80 with a numerical aperture of0.55. Samples were fixed normally to the optical axis. Whilerecording spectra the laser is intentionally defocused in orderto excite a whole disks 4 m in diameter. The spectraare recorded with a spectrometer offering a maximum spec-tral resolution of 0.019 nm. All measurements were carriedout at RT.The 300 K PL spectrum of InGaN/GaN QWs beforeprocessing, measured under cw excitation, is presented inFig. 2a. The broad PL linewidth 20 nm is a usual fea-ture of InGaN QWs. No degradation of their optical proper-ties due to the growth on AlInN instead of a GaN buffer isobserved. Note that the multiple peak line shape originatesfrom the interference occurring in the GaN template. Forcomparison, a PL spectrum of a single disk is presented inFig. 2b. It is worth emphasizing that such a broad lumines-cence allows probing simultaneously several optical modes,as it is the case with InAs QD ensemble see below.16 Theexcitation power density of the cw laser was kept intention-ally low, namely, 200 W/cm2. Multiple modes are ob-served but narrow ones can be found only in the low energypart of the spectra. The modal picture suggests that theycorrespond to both TE and TM WGMs.12 The existence ofhigher order radial modes is also possible. However, theirexact assignment requires complex simulations and will bethe subject of future work. The narrowest linewidths are ofthe order of 0.07 nm but they are likely reduced by the shortrange interference due to internal reflections occurring in thesapphire substrate. After fitting the envelope of these oscil-lations we obtain typical linewidths of 0.1 nm correspond-ing to Q factors of 4000. These relatively high Q factorsare promising for both CQED studies and the achievement oflow-threshold nitride-based lasers.It is worth discussing the origin of the parameters limit-ing the Q factor. The latter can be decomposed as17Q−1 = Qrad−1 + Qabs−1 + Qscat−1 . 1The value of Qrad is directly linked to the ratio of thedisk diameter to the mode wavelength. For disks with adiameter of 4 m at 440 nm, this value exceeds 1025 andconsequently does not limit the performance of oursystem.2,17 The value of Qabs is intrinsic to the grown mate-rials and is defined as Qabs−1 = /2neff, where  is the meanabsorption coefficient in the cavity. Absorption usually oc-curs both in the guiding layer and in the QWs. If QW ab-sorption was the dominant factor, we should observe a nar-rowing of the optical modes when increasing the pumpingpower until QWs reach the transparency regime.16 Since wedo not observe such a behavior for over four decades ofexcitation power change we can conclude that the impact ofQW absorption on  is marginal for these narrow modes.This is in agreement with the large Stokes shift usually re-ported for InGaN QWs arising mainly from the quantumconfined Stark effect.18 Furthermore, it reminds the weakmodal absorption observed in microdisks using QDs as anactive region,16 a picture consistent with the QD-like behav-ior seen in InGaN/GaN QWs.19 Consequently in this casethe average absorption coefficient in the cavity does matchclosely that of GaN. For the studied wavelengths, the typicalvalue for the GaN sub-band-gap absorption ranges between20 and 40 cm−1,20 leading to values for Qabs comprising be-tween 16 000 and 8000, respectively. However, a strongerabsorption is expected in the QWs at higher energy due tothe reduced Stokes shift between their absorption and emis-sion. This could explain why high quality factors are onlyFIG. 1. Scanning electron micrograph of a 4 m diameter disk: a perspec-tive view, b edge close-up view, and c top view.FIG. 2. a RT photoluminescence spectrum of bare InGaN/GaN QWs. bMultimode structure of a single 4 m disk. The typical mode linewidth is0.1 nm which corresponds to a quality factor 4000.061106-2 Simeonov et al. Appl. Phys. Lett. 90, 061106 2007measured on the low energy part of the PL spectraFig. 2b. For the low energy modes we can estimate Qscat,which represents the combined influence of surface scatter-ing, external coupling template and light extraction. UsingEq. 1, the obtained value is in the range of 4000-8000. Thismeans that the influence of absorption and that of leakymodes are comparable and cannot be easily discriminated.The values found for Qscat can, however, be tentatively as-cribed to an increased light extraction in the vertical direc-tion due to the inclined sidewall. Indeed, under homoge-neous laser excitation an increased extraction of theluminescence near the disk edge is observed in optical mi-croscope images Fig. 3a. The width of this bright ring,about half a micron, corresponds to that of the in plane pro-jection of the sidewall Fig. 1b.Lasing action was investigated under pulsed excitation.The power dependence of the integrated intensity of the las-ing mode as well as its line shape are presented in Figs. 4aand 4b, respectively. It should be noted that laser action hasbeen only achieved for modes in the high energy part of theQW PL spectra which is consistent with the larger absorptionin this spectral region, i.e., a larger oscillator strength. Asuperlinear power dependence, corresponding to single modelasing operation at 422.5 nm, is observed for an averagepower density Pav1.5 W/cm2 at RT. The threshold isfound at a peak power density of 400 kW/cm2. For densitiessuperior to 2 MW/cm2, a degradation and even a completecollapse of the lasing mode are observed. Such behavior isattributed to overheating of the disk samples causing theirmechanical failure. However a stable lasing action at roomtemperature is observed for a range of powers up to four tofive times the threshold power which is a significant im-provement over what has been previously reported for nitridedisks.13 This improvement is assigned to increased heatsink capabilities of the present disks due to their large ped-estal. The measured linewidth of the laser mode under strongexcitation 1.8 MW/cm2 peak power density in the stablelasing regime goes down to 0.033 nm, i.e.,  /12 800Fig. 4b. Finally the light emission pattern above lasingthreshold in the normal direction is presented in Fig. 3b.The structured pattern demonstrates well coherent lightemission.In conclusion, we have demonstrated the fabrication ofIII-V nitride disk resonators in the blue wavelength range.Narrow WGMs with a typical quality factor of 4000 areobserved. RT lasing action was achieved under pulsed opti-cal excitation for a peak power density at threshold as low as400 kW/cm2. In the stable lasing regime, the linewidth isless than 0.033 nm. Such system seems to be promising withregard to its light extraction and thermal properties.The authors are grateful to M. Ilegems for his continuoussupport. This work was carried out in the framework of theNCCR Quantum Photonics program and the Swiss NationalScience Foundation. Two of the authors N.G. and R.B. areindebted to the Sandoz Family Foundation for its support.1J. L. Jewell, A. Scherer, S. L. McCall, Y. H. Lee, S. Walker, J. P. Harbison,and L. T. Florez, Electron. Lett. 25, 1123 1989.2S. L. McCall, A. F. J. Levi, R. E. Slusher, S. J. Pearton, and R. A. Logan,Appl. Phys. Lett. 60, 289 1992.3J. P. Reithmaier, G. Sek, A. Löffler, C. Hofmann, S. Kuhn, S. Reitzenstein,L. V. Keldysh, V. D. Kulakovskii, T. L. Reinecke, and A. Forchel,Nature London 432, 197 2004.4E. Peter, P. Senellart, D. Martrou, A. Lemaître, J. Hours, J.-M. Gérard, andJ. Bloch, Phys. Rev. Lett. 95, 067401 2005.5J.-M. Gérard, B. Sermage, B. Gayral, B. Legrand, E. Costard, and V.Thierry-Mieg, Phys. Rev. Lett. 81, 1110 1998.6B. Gayral, J.-M. Gérard, B. Sermage, A. Lemaître, and C. Dupuis,Appl. Phys. Lett. 78, 2828 2001.7M. Pelton, C. Santori, J. Vučković, B. Zhang, G. S. Solomon, J. Plant, andY. Yamamoto, Phys. Rev. Lett. 89, 233602 2002.8A. Kavokin and B. Gil, Appl. Phys. Lett. 72, 2880 1998.9G. Christmann, D. Simeonov, R. Butté, E. Feltin, J.-F. Carlin, and N.Grandjean, Appl. Phys. Lett. 89, 261101 2006.10T. Someya, R. Werner, A. Forchel, M. Catalano, R. Cingolani, and Y.Arakawa, Science 285, 1905 1999.11R. Butté, E. Feltin, J. Dorsaz, G. Christmann, J.-F. Carlin, N. Grandjean,and M. Ilegems, Jpn. J. Appl. Phys., Part 1 44, 7207 2005.12R. A. Mair, K. C. Zeng, J. Y. Lin, H. X. Jiang, B. Zhang, L. Dai, A.Botchkarev, W. Kim, H. Morkoç, and M. A. Khan, Appl. Phys. Lett. 72,1530 1998.13E. D. Haberer, R. Sharma, C. Meier, A. R. Stonas, S. Nakamura, S. P.DenBaars, and E. L. Hu, Appl. Phys. Lett. 85, 5179 2004.14J. Dorsaz, H.-J. Bühlmann, J.-F. Carlin, N. Grandjean, and M. Ilegems,Appl. Phys. Lett. 87, 072102 2005.15J.-F. Carlin and M. Ilegems, Appl. Phys. Lett. 83, 668 2003.16B. Gayral, J.-M. Gérard, A. Lemaître, C. Dupuis, L. Manin, and J. L.Pelouard, Appl. Phys. Lett. 75, 1908 1999.17R. E. Slusher, A. F. J. Levi, U. Mohideen, S. L. McCall, S. J. Pearton, andR. A. Logan, Appl. Phys. Lett. 63, 1310 1993.18E. Berkowicz, D. Gershoni, G. Bahir, E. Lakin, D. Shilo, E. Zolotoyabko,A. C. Abare, S. P. Denbaars, and L. A. Coldren, Phys. Rev. B 61, 10 9942000.19H. Schömig, S. Halm, A. Forchel, G. Bacher, J. Off, and F. Scholz,Phys. Rev. Lett. 92, 106802 2004.20F. Omnès, N. Marenco, B. Beaumont, Ph. de Mierry, E. Monroy, F. Calle,and E. Muñoz, J. Appl. Phys. 86, 5286 1999.FIG. 3. Color online Optical microscope images of a 4 m disk underhomogeneous Gaussian beam excitation: a below and b above lasingthreshold.FIG. 4. Color online a Mode intensity plotted as a function of the peakpower density demonstrating a laser threshold at 400 kW/cm2. b diskemission line above threshold.061106-3 Simeonov et al. Appl. Phys. Lett. 90, 061106 2007

Blue lasing at room temperature in high quality factor GaN/AlInN microdisks with InGaN quantum wells

Crossref

https://iris.unipa.it/bitstream/10447/19209/1/Blue%20lasing.pdf

Blue lasing at room temperature in high quality factor GaN/AlInN microdisks with InGaN quantum wells

Abstract

Similar works

Full text

Available Versions

Archivio istituzionale della ricerca - Università di Palermo

Crossref