Comparable microcavities with 3/2 (~240 nm) active regions containing distributed (In,Ga)N quantum wells, grown on GaN substrates and bounded by two dielectric mirrors, have been fabricated by two different routes: one using laser lift-off to process structures grown on GaN-on-sapphire templates and the second using freestanding GaN substrates, which are initially processed by mechanical thinning. Both exploit the properties of an Al0.83In0.17N layer, lattice matched to the GaN substrate and spacer layers. In both cases cavity quality factors >400 are demonstrated by measurements of the cavity-filtered room-temperature excitonic emission near 410 nm

Bejtka, K.

Dawson, M.D.

Edwards, P.R.

Gu, E.

Kang, X.N.

Martin, R.W.

Rizzi, F.

Semond, F.

Watson, I.M.

Zhang, G.Y.

English

F. Rizzi

P. R. Edwards

K. Bejtka

F. Semond

X. N. Kang

G. Y. Zhang

E. Gu

M. D. Dawson

I. M. Watson

R. W. Martin

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(In,Ga)N∕GaN microcavities with double dielectric mirrors fabricated by selective removal of an (Al,In)N sacrificial layer

University of Strathclyde Institutional Repository

Strathprints Institutional RepositoryRizzi, F. and Edwards, P.R. and Bejtka, K. and Semond, F. and Kang, X.N. and Zhang, G.Y. andGu, E. and Dawson, M.D. and Watson, I.M. and Martin, R.W. (2007) (In,Ga)N/GaN microcavitieswith double dielectric mirrors fabricated by selective removal of an (Al,In)N sacrificial layer. AppliedPhysics Letters, 90 (11). 111112-1-111112-3. ISSN 0003-6951Strathprints is designed to allow users to access the research output of the University of Strathclyde.Copyright c© and Moral Rights for the papers on this site are retained by the individual authorsand/or other copyright owners. You may not engage in further distribution of the material for anyprofitmaking activities or any commercial gain. You may freely distribute both the url (http://strathprints.strath.ac.uk/) and the content of this paper for research or study, educational, ornot-for-profit purposes without prior permission or charge.Any correspondence concerning this service should be sent to Strathprints administrator:mailto:strathprints@strath.ac.ukhttp://strathprints.strath.ac.uk/„In,Ga…N/GaN microcavities with double dielectric mirrors fabricatedby selective removal of an „Al,In…N sacrificial layerF. RizziaInstitute of Photonics, SUPA, University of Strathclyde, 106 Rottenrow, Glasgow G4 0NW, United KingdomP. R. Edwards and K. BejtkabDepartment of Physics, SUPA, University of Strathclyde, 107 Rottenrow, Glasgow G4 0NG,United KingdomF. SemondCRHEA-CNRS, Rue Bernard Gregory, Parc Sophia Antipolis, 06560 Valbonne, FranceX. N. Kang and G. Y. ZhangResearch Center for Wide-Gap Semiconductors, State Key Laboratory for Mesoscopic Physics,Peking University, Beijing 100871, People’s Republic of ChinaE. Gu, M. D. Dawson, and I. M. WatsonInstitute of Photonics, SUPA, University of Strathclyde, 106 Rottenrow, Glasgow G4 0NW, United KingdomR. W. MartincDepartment of Physics, SUPA, University of Strathclyde, 107 Rottenrow, Glasgow G4 0NG,United KingdomReceived 29 November 2006; accepted 5 February 2007; published online 15 March 2007Comparable microcavities with 3 /2 240 nm active regions containing distributed In,GaNquantum wells, grown on GaN substrates and bounded by two dielectric mirrors, have beenfabricated by two different routes: one using laser lift-off to process structures grown onGaN-on-sapphire templates and the second using freestanding GaN substrates, which are initiallyprocessed by mechanical thinning. Both exploit the properties of an Al0.83In0.17N layer, latticematched to the GaN substrate and spacer layers. In both cases cavity quality factors 400 aredemonstrated by measurements of the cavity-filtered room-temperature excitonic emission near410 nm. © 2007 American Institute of Physics. DOI: 10.1063/1.2712786Applications of III-nitride-based semiconductors con-tinue to expand and diversify. Commercialized devices in-clude UV blue-green light-emitting diodes, edge-emittingblue laser diodes, and transistors.1 Future devices will in-clude vertical cavity surface emitting lasers and related struc-tures. In particular, the high exciton binding energies andoscillator strengths for bulk GaN and III-nitride heterostruc-tures make microcavity MC structures very promising forroom-temperature operation of devices such as polariton la-sers or optical amplifiers.2 GaN-based MCs have been fabri-cated using a variety of processing routes. Recently, goodprogress has been made on fully epitaxial MCs, in whichboth distributed Bragg reflectors DBRs comprise III-nitridemultilayers.3,4 However, growth of such MCs requireslengthy and demanding growth processes to combat strainand cracking issues, and the resulting III-nitride DBRs stillhave relatively narrow high-reflectivity stop bands. In othercases, a so-called hybrid approach was used, combining oneepitaxial semiconductor mirror with a second metallic or di-electric mirror.5,6 The so-called fully hybrid approach ex-ploits the well-established properties of high-reflectivity di-electric DBRs for both upper and lower mirrors, and offersthe potential for higher finesse MCs.7–10 Fabricating a MC ofthis type requires accessing the underside of the active regionby removal of the substrate and buffer layers. Laser lift-offLLO from a sapphire substrate,7,8 plasma etching of a SiCsubstrate,9 and wet chemical etching of a silicon substrate10have all been employed to achieve this. After substrate re-moval, further thinning of buffer layers is usually required toaccurately define the desired cavity length, and it is essentialto have a reliable method to achieve this objective.In this letter, we describe the properties of two types offully hybrid III-nitride MCs fabricated using a sacrificialAl0.83In0.17N layer to perform key roles in the growth andfabrication sequence. Both processing routes utilize recentlyavailable substrates with low threading dislocation densitiesand consequent benefits for the structural quality of the MCactive region. One approach uses 1010 mm2 freestandingGaN FS-GaN substrates, and here postgrowth processingbegins with mechanical grinding of the substrate back side.Larger FS-GaN substrates have since become more readilyavailable and are expected to give better results due to re-duced edge effects. The other approach employs 2 inch GaN-on-sapphire templates, using LLO to remove the substratefrom 66 mm2 squares in a single rapid step. In both cases,an Al0.83In0.17N layer is included in the epitaxial structureimmediately below an In,GaN/GaN MC active region.This ultimately acts as a sacrificial layer in a selective wetetching step,11 but also allows endpoint detection duringplasma etch steps which precede this. The sacrificialAl0.83In0.17N is lattice matched to GaN,3,12 and the high qual-ity of GaN grown above such a layer has recently been con-firmed by spectroscopic and structural characterization.13The refractive index contrast between Al0.83In0.17N and GaNis also valuable in facilitating in situ reflectometry duringepitaxy on FS-GaN.14,15aAlso at: Department of Physics, University of Strathclyde, Glasgow, UK.bAlso at: Institute of Photonics, University of Strathclyde, Glasgow, UK.cElectronic mail: r.w.martin@strath.ac.ukAPPLIED PHYSICS LETTERS 90, 111112 20070003-6951/2007/9011/111112/3/$23.00 © 2007 American Institute of Physics90, 111112-1Downloaded 19 Mar 2007 to 130.159.248.222. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jspGrowth of the nitride heterostructures used an Aixtron200 series metal-organic vapor phase epitaxy MOVPE re-actor, fitted with a multiwavelength, normal-incidence reflec-tometer. FS-GaN and ultralow defect ULD GaN-on-sapphire templates were supplied by Lumilog Vallauris,France. Both had 0001 orientation with a Ga-polar topsurface. The FS-GaN substrates were approximately 300 min thickness, while the ULD-GaN templates consisted of8–10 m layers of GaN grown by MOVPE, using propri-etary dislocation filtering techniques. More details on growthconditions and the substrates are reported elsewhere.13–15Growth of each MC structure began with a GaN buffer layer,followed by the Al0.83In0.17N sacrificial layer with a nominalthickness of 70 nm, chosen to give a readily interpretable insitu reflectance waveform at a monitoring wavelength of625 nm.15 The MC active region was grown directly abovethe Al0.83In0.17N layer and contained two groups of three2.5 nm thick In0.08Ga0.92N quantum wells QWs, posi-tioned symmetrically within the MC at electric field antinodepositions, as illustrated in Fig. 1. The optical thickness of3 /2 corresponds to a total physical thickness of 240 nm.Both DBRs comprised ten repeat periods of SiO2/ZrO2quarter-wavelength layers deposited by electron beam evapo-ration, with an optimum reflectivity of 99.7% and a centerwavelength at 410 nm, closely matched to the measured QWpeak emission wavelength. The first of the DBRs was depos-ited on top of the active MC layers following the epitaxialgrowth. The second was deposited on the lower MC surfacefollowing the processing steps described below.After deposition of the top mirror, the top surface of thisDBR was bonded to a sapphire carrier using UV-curable op-tical cement. As a result N-face GaN 0001¯ faces away fromthe carrier and will be subsequently processed. The processflow now followed one of two routes, depending on whichsubstrate type had been used in the growth. A summary isprovided here, but more details on several steps, particularlyoptimization of plasma etching for N-polar GaN, is reportedelsewhere.16 For MCs on FS-GaN, mechanical grinding wasemployed as a fast method to reduce the substrate thicknessto a few tens of microns. A chemomechanical polishing stepwas used to recover a smooth surface after grinding. For theMCs on ULD-GaN templates, the first step involved LLOusing a KrF excimer laser to remove the epitaxial layers fromthe sapphire substrate. Next, in both variants of the processflow, the remaining few tens of microns of GaN above theAl0.83In0.17N were removed, stopping within the ternarylayer. This was achieved by plasma etching employing areflectivity-based endpoint detection technique.17 Havingstopped the plasma etch just after penetration into the sacri-ficial layer, the remaining Al0.83In0.17N was removed usingselective wet etching an aqueous solution of a chelatingamine MC.11,17 Ideally this process will stop cleanly at theepitaxial interface between Al0.83In0.17N and GaN, removingany requirement for the plasma etching steps themselves toleave an optically smooth surface. Thus the scale of surfaceroughness must only be limited to values below theAl0.83In0.17N thickness. Finally, a second dielectric DBR, ofsimilar specification to the first, was deposited on the N-faceGaN surface of the active region.The optical properties of the MCs were analyzed usingphotoluminescence PL and cathodoluminescence CLspectra. CL spectra were collected in the surface-normal di-rection from “half-cavities,” prior to deposition of the secondDBR, using an electron probe microanalyzer and a defocusedelectron beam 10 m diameter.18 The optical propertiesof the completed MCs were assessed by PL excited witheither a 325 nm HeCd or a 244 nm frequency-doubled Ar+laser. The laser was focused by eye using only a singlet lens,giving a spot size in the region of 50 m. Both PL excitationand collection were performed through the N-face mirror,with a laser incidence angle of 30°–40° from the normal forthe He-Cd laser and almost normal for the doubled Ar+ laser.The emitted light was collected in the surface-normal direc-tion in all cases, apart from the angle-resolved measurementsmentioned below.Figures 2 and 3 show typical luminescence spectra, eachFIG. 1. Schematic representation of the band gap variation and calculatedelectric field distribution in the In,GaN/GaN MC active region.FIG. 2. Color online RT-PL spectra from the MC grown on FS-GaN,plotted as continuous lines. Spectra a and b correspond to different areas.A RT-CL spectrum from the back side of the active region before depositionof the second DBR is plotted as a dashed line.FIG. 3. Color online PL and CL spectra from the MC grown on an ULD-GaN template. PL were acquired with a a doubled Ar+ laser at 70 K andb a HeCd laser at RT from different areas. The dashed line shows theRT-CL spectrum from the back side of the active region before deposition ofthe second DBR.111112-2 Rizzi et al. Appl. Phys. Lett. 90, 111112 2007Downloaded 19 Mar 2007 to 130.159.248.222. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jspcomparing CL from a half-cavity structure with PL emissionfrom completed MCs. Figure 2 shows results from a MCgrown on FS-GaN, while Fig. 3 shows results from a struc-ture grown on ULD-GaN. The room-temperature RT CLspectra show the expected excitonic luminescence peak fromthe QWs, with maxima at wavelengths of 417 and 409 nm,respectively, for the two samples. The full widths at halfmaximum FWHMs are 111 and 95 meV. These reflect theconventional narrowing of the QW luminescence as the peakwavelength decreases. Single QWs from our reactor, withpeak emission wavelengths at 390 nm, show low-temperature FWHM down to 45 meV. After completion ofthe MCs by deposition of the second mirror, the lumines-cence is strongly filtered by the cavity mode, and consider-ably sharper peaks were observed. Figure 2 shows twoRT-PL spectra from the completed MC, representative ofmeasurements across several mm2 of the sample. One peak,labeled a, has a FWHM of 1.0 nm 7 meV and is centeredon a wavelength of 415 nm. This corresponds to a cavityquality factor Q= / of 415. A different region of thesample gives a shifted peak centered at 424 nm, labeled b,with a slightly broader FWHM 1.9 nm. Characterization ofthe LLO-processed sample included low-temperature PLmeasurements at 70 K, using 244 nm excitation. The result-ing spectra showed a sharper cavity mode peak, centered ona wavelength of 405 nm, and superimposed on an excitonicpeak very similar to that observed in the CL from the half-cavity Fig. 3a. The FWHM of 0.7 nm 5 meV gives acavity Q factor of 580. Figure 3b shows a typical RT-PLspectrum obtained with HeCd excitation, giving a broadercavity-filtered PL peak at slightly longer wavelength. Angle-resolved PL measurements, in which the detection wasangled at up to 32°, show a conventional dispersion of thiscavity mode. In this case the excitonic mode is too broad andat too low an energy to see polariton coupling effects.5,6,9The measured Q factors lie within the range of the best re-ported values 400–750 for similar MCs with double dielec-tric Bragg mirrors7–9 but here the use of GaN substrates withlow threading dislocation densities gives the added benefit ofimproved epitaxial quality.As will be apparent from the examples discussed, mea-surements at different points on the two MCs produced sig-nificantly different PL spectra. Transfer matrix simulationsindicate that fluctuations in the active region thicknesses of12 nm for the sample grown on FS-GaN and 5 nm forthe sample grown on ULD-GaN are sufficient to account forthe shift of the cavity mode. Such fluctuations are likely toresult from issues in control of the final etch-based fabrica-tion steps at their current stage of development. Figure 4shows a cross-sectional scanning electron microscope imageof the MC fabricated on FS-GaN, taken following the opticalmeasurements. Undulations on a scale consistent with theapparent MC thickness variations from simulations are evi-dent in the upper DBR, reflecting surface roughness resultingfrom the processing. Optimization of the etching of theAlInN sacrificial layer is under way to reduce this.In summary, we have demonstrated two processingroutes for epitaxial GaN-based MCs with double dielectricBragg mirrors. An Al0.83In0.17N sacrificial layer, latticematched to GaN, is used to facilitate controlled removal ofmaterial from under the active region. Postgrowth processingbegan with mechanical grinding for structures grown onfreestanding GaN or laser lift-off in the case of those grownon GaN-on-sapphire templates. Cavity-filtered excitonic PLpeaks were observed in both cases, giving Q factors in therange of 400–600. These approaches open the way to im-proved structural quality of the active region, resulting fromthe use of substrates with low threading dislocation densities.The authors thank S. Christopolous and J. J. BaumbergSouthampton University, UK for providing the angle-resolved PL data and acknowledge funding from the EUprojects CLERMONT2 MRTN-CT-2003-503577 andSTIMSCAT STREP Contract No. 517769.1S. Nakamura, G. Fasol, and S. J. Pearton, The Blue Laser Diode: TheComplete Story Springer, New York, 2000.2G. Malpuech, A. Di Carlo, A. V. Kavokin, J. J. Baumberg, A. Zamfirescu,and P. Lugli, Appl. Phys. Lett. 81, 412 2002.3J.-F. Carlin, J. Dorsaz, E. Feltin, R. Butté, N. Grandjean, M. Ilegems, andM. Läugt, Appl. Phys. Lett. 86, 031107 2005.4X. H. Zhang, S. J. Chua, W. Liu, L. S. Wang, A. M. Yong, and S. Y. Chow,Appl. Phys. Lett. 88, 191111 2006.5F. Semond, I. R. Sellers, F. Natali, D. Byrne, M. Leroux, J. Massies, N.Ollier, J. Leymarie, P. Disseix, and A. Vasson, Appl. Phys. Lett. 87,021102 2005.6E. Feltin, G. Christmann, R. Butté, J.-F. Carlin, M. Mosca, and N.Grandjean, Appl. Phys. Lett. 89, 071107 2006.7Y.-K. Song, H. Zhou, M. Diagne, I. Ozden, A. Vertikov, A. V. Nurmikko,C. Carter-Coman, R. S. Kern, F. A. Kish, and M. R. Krames, Appl. Phys.Lett. 74, 3441 1999.8R. W. Martin, P. R. Edwards, H.-S. Kim, K.-S. Kim, T. Kim, I. M. Watson,M. D. Dawson, Y. Cho, T. Sands, and N. W. Cheung, Appl. Phys. Lett. 79,3029 2001.9T. Tawara, H. Gotoh, T. Akasaka, N. Kobayashi, and T. Saitoh, Phys. Rev.Lett. 92, 256402 2004.10T. K. Kim, S. S. Yang, J. K. Hong, and G. M. Yang, Appl. Phys. Lett. 89,041129 2006.11F. Rizzi, K. Bejtka, P. R. Edwards, R. W. Martin, and I. M. Watson,J. Cryst. Growth 300, 254 2007.12K. Lorenz, N. Franco, E. Alves, I. M. Watson, R. W. Martin, and K. P.O’Donnell, Phys. Rev. Lett. 97, 085501 2006.13K. Bejtka, R. W. Martin, I. M. Watson, S. Ndiaye, and M. Leroux,Appl. Phys. Lett. 89, 191912 2006.14I. M. Watson, C. Liu, E. Gu, M. D. Dawson, P. R. Edwards, and R. W.Martin, Appl. Phys. Lett. 87, 151901 2005.15K. Bejtka, F. Rizzi, P. R. Edwards, R. W. Martin, E. Gu, M. D. Dawson,I. M. Watson, I. R. Sellers, and F. Semond, Phys. Status Solidi A 202,2648 2005.16F. Rizzi, E. Gu, M. D. Dawson, I. M. Watson, R. W. Martin, X. N. Kang,and G. Y. Zhang, J. Vac. Sci. Technol. A 25, 252 2007.17F. Rizzi, P. R. Edwards, I. M. Watson, and R. W. Martin, SuperlatticesMicrostruct. 40, 369 2006.18R. W. Martin, P. R. Edwards, K. P. O’Donnell, M. D. Dawson, C.-W. Jeon,C. Liu, G. R. Rice, and I. M. Watson, Phys. Status Solidi A 201, 6652004.FIG. 4. Cross-sectional scanning electron microscopy image of the micro-cavity on FS-GaN.111112-3 Rizzi et al. Appl. Phys. Lett. 90, 111112 2007Downloaded 19 Mar 2007 to 130.159.248.222. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

(In,Ga)N/GaN microcavities with double dielectric mirrors fabricated by selective removal of an (Al,In)N sacrificial layer

Comparable microcavities with 3??2 (??240 nm) active regions containing distributed (In,Ga)N quantum wells, grown on GaN substrates and bounded by two dielectric mirrors, have been fabricated by two different routes: one using laser lift-off to process structures grown on GaN-on-sapphire templates and the second using freestanding GaN substrates, which are initially processed by mechanical thinning. Both exploit the properties of an Al0.83 In0.17 N layer, lattice matched to the GaN substrate and spacer layers. In both cases cavity quality factors &gt;400 are demonstrated by measurements of the cavity-filtered room-temperature excitonic emission near 410 nm. ? 2007 American Institute of Physics.EI0119

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(In,Ga)N/GaN microcavities with double dielectric mirrors fabricated by selective removal of an (Al,In)N sacrificial layer

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