Includes bibliographical references.We report the high yield fabrication and reproducible performance of selectively oxidized vertical-cavity surface emitting lasers. We show that linear oxidation rates of AlGaAs without an induction period allows reproducible fabrication of buried oxide current apertures within monolithic distributed Bragg reflectors. The oxide layers do not induce obvious crystalline defects, and continuous wave operation in excess of 650 h has been obtained. The high yield fabrication enables relatively high laser performance over a wide wavelength span. We observe submilliamp threshold currents over a wavelength range of up to 75 nm, and power conversion efficiencies at 1 mW output power of greater than 20% over a 50-nm wavelength range.The work at Sandia National Laboratories was supported in part by the United States DOE under contract No. DE-AC04-94AL85000

Choquette, Kent D.

Figiel, J. J.

Geib, Kent M.

Hull, Robert

Lear, Kevin L.

Schneider, Richard P.

English

We report the high yield fabrication and reproducible performance of selectively oxidized vertical-cavity surface emitting lasers. We show that linear oxidation rates of AlGaAs without an induction period allows reproducible fabrication of buried oxide current apertures within monolithic distributed Bragg reflectors. The oxide layers do not induce obvious crystalline defects, and continuous wave operation in excess of 650 h has been obtained. The high yield fabrication enables relatively high laser performance over a wide wavelength span. We observe submilliamp threshold currents over a wavelength range of up to 75 nm, and power conversion efficiencies at 1 mW output power of greater than 20% over a 50-nm wavelength range.The work at Sandia National Laboratories was supported in part by the United States DOE under contract No. DE-AC04-94AL85000

Hull, Robert, author

Figiel, J. J., author

Geib, Kent M., author

Schneider, Richard P., author

Lear, Kevin L., author

Choquette, Kent D., author

IEEE, publisher

Mountain Scholar (Digital Collections of Colorado and Wyoming)

IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 7, NO. II, NOVEMBER 1995Fabrication and Performance of SelectivelyOxidized Vertical-Cavity LasersKent D. Choquette, K. L. Lear, R. P. Schneider, Jr., K. M. Geib, 1. 1. Figiel, and Robert Hull1237Abstract- We report the high yield fabrication and repro-ducible performance of selectively oxidized vertical-cavity surfaceemitting lasers. We show that linear oxidation rates of A1GaAswithout an induction period allows reproducible fabrication ofburied oxide current apertures within monolithic distributedBragg reflectors. The oxide layers do not induce obvious crys-talline defects, and continuous wave operation in excess of 650 hhas been obtained. The high yield fabrication enables relativelyhigh laser performance over a wide wavelength span. We observesubmilliamp threshold currents over a wavelength range of up to75 om, and power conversion efficiencies at 1 mW output powerof greater than 20% over a 50-nm wavelength range.WET OXIDATION of AIGaAs [1], [2] has been suc-cessfully employed in the fabrication of edge emittinglasers, [3] and has recently been applied to various vertical-cavity surface emitting laser (VCSEL) diode structures [4]-[9].For example, VCSEL's incorporating GaAs-Al-oxide layersfor high-index contrast distributed Bragg reflector (DBR)mirrors have been reported [4]. Partial oxidation of AlAs layersin the DBR mirror stack has been pursued to form a high-index cladding around the laser cavity [5]. Finally, using oxidelayers to form current apertures under hybrid dielectric DBR's[6] or monolithic semiconductor DBR's [7] has also beendemonstrated. The latter oxide-confined lasers have exhibitedthe highest power conversion efficiency (>50%), [8] lowestthreshold current density (90 Ncm2 per quantum well), [9]and lowest threshold voltage (50 mV above photon energy)[7] reported to date for VCSEL's. In this letter we reporton the high-yield fabrication and reproducible performanceof monolithic selectively oxidized VCSEL's. As opposed torecord operation, we show that samples of large arrays ofthese lasers also exhibit relatively high performance levelsover a wide lasing wavelength span that may be attractivefor emerging VCSEL applications.Fig. 1 is a schematic of our top-emitting VCSEL, whichemploys selective oxidation to produce buried oxide currentapertures [7]. This oxide-confined VCSEL has several advan-tages. First, in this monolithic structure we fully exploit lowresistance p-type DBR designs (such as parabolic heterointer-Manuscript received June 23, 1995; revised July 24, 1995. The work atSandia National Laboratories was supported in part by the United States DOEunder contract No. DE-AC04-94AL85000.K. D. Choquette, K. L. Lear, R. P. Schneider, Jr., K. M. Geib, and J. J. Figie1are with the Photonics Research Department, Sandia National Laboratories,Albuquerque, NM 87185 USA.R. Hull is with the Department of Materials Science, University of VIrginia,Charlottesville, VA 22903 USA.IEEE Log Number 9414875.Oxidized CurrentLayers AperturesFig. 1. Sketch of an oxide-confined VCSEL showing oxidized layers andthe resulting current apertures on each side of the optical cavity.face grading [10] and C-doping [11D in utilizing the entire topmirror to conduct current into the active region. Thus, currentcrowding effects of thin intracavity contact layers [6] or ionimplantation damage in the top DBR are both avoided. Thecurrent apertures immediately surrounding the optical cavityalso eliminate sidewall nonradiative recombination present inetched air-post VCSEL's [12], and minimize lateral currentspreading to outside of the laser cavity. Finally, the smallerrefractive index of the Al-oxide layer [13] also inducesindex-guided optical confinement [14], [15], but in a planarVCSEL configuration amenable to efficient current flow andheat extraction. Comparing oxide-confined VCSEL's to ionimplantation-defined VCSEL's, the enhanced electrical andoptical confinement of the former produces reduced thresholdcurrent/voltage and the concomitant parasitic ohmic heating[9]. In the following, we describe the properties of 980 nmVCSEL's, although similar 850 and 650 nm [16] oxide-confined lasers have also been characterized.The two-inch diameter VCSEL wafers are grown by met-alorganic vapor phase epitaxy on a rotating susceptor, and havethree InGaAs quantum wells in the active region. The mono-lithic DBR mirrors consist of GaAs-Alo.92Gao.osAs layerswith parabolic heterointerface grading. The low index layersadjacent to the optical cavity are adjusted to Alo.9sGao.o2Asto increase their oxidation rates relative to the other DBRlayers [7]. Fabrication of oxide-confined VCSEL's begins withthe lift-off deposition of a top Ti-Pt-Au contact. A nitridemask is then deposited and patterned to encapsulate the metalcontact and form a mesa etch mask. Reactive ion etchingwith BCI3-Ch is used to define the mesas and expose thesidewalls for oxidation. The samples are oxidized at 450°Cunder flowing nitrogen, which is bubbled through water at1041-1135/95$04.00 © 1995 IEEE1238 IEEE PHOTONICS TECHNOLOGY LEITERS. VOL. 7. NO. II, NOVEMBER 199584 nm thick AIo.98Gao.ozAs5040S~..t: 30'60c:~c:200';:l'""0';<0 10100 200OxidationTime (min)300-- 200nmFig. 3. Cross-section transmission electron micrograph (g = (311) brightfield image) of a selectively oxidized VCSEL showing the oxide layer abovethe quantum-well (QW) active region. The dark (light) layers in the DBRmirror are GaAs (AlGaAs). The terminus of the buried oxide layer, whichforms the perimeter of the current aperture, is at the vertical arrow.Fig. 2. Lateral extent of oxidation at 400, 425, and 450°C for a 84-nm-thickAlo.98Gao.ozAs layer contained within a distributed Bragg reflector, showinglinear oxidation rates without induction periods.85 DC. After oxidation the nitride mask is removed and abackside Ge-Au-Ni-Au contact is deposited and annealed. Inthis work we fabricate repeating rows of square mesas thatvary in size from 48-90 JLm on a side. Oxidation conditionsare selected to give an oxide penetration length of 24 JLmto "pinch off" the smallest mesa, thus producing arrays ofVeSEL's with current apertures of 2 x 2 JLm and larger. Foreach wafer examined, multiple samples are processed undersimilar oxidation conditions, where each sample extends fromthe wafer center to the edge.To develop reproducible oxidation processes, test sampleshave been oxidized using different Al compositions, furnacetemperatures and oxidation times. The extent of the lateraloxidation of a buried 84-nm-thick AlO.98Gao.02As layer withina DBR mirror is plotted in Fig. 2. Fig. 2 shows that the lateraloxidation has a temperature dependent linear oxidation ratewithout an induction time preceding the onset of oxidation.Hence, reproducible fabrication of oxide apertures can beachieved on similar samples from a given wafer. However,the oxidation rate of AlxGal-xAs is very sensitive to Alcomposition, where x varying from 1-0.82 changes the relativeoxidation rate by more than two orders of magnitude [7].Thus, the scatter of the oxidation rates in Fig. 2 may bepartially related to small composition variations between thetest samples. For example, near the edge of a wafer theoxidation rate is consistently found to monotonically decrease,presumably reflecting a slight increase of the Ga contentarising from a growth edge effect. In addition, nominallyidentical wafers may have oxidation rates (at the center) thatdiffer by ~1O%. Therefore, stringent compositional controlmay be necessary for wafer scale manufacture of uniformlysized oxide apertures.A cross-section transmission electron micrograph of a se-lectively oxidized VeSEL is shown in Fig. 3. The roundedterminus of the oxide layer in the top mirror is denoted atthe vertical arrow in Fig. 3; the unoxidized region beyondthis point corresponds to the interior of the current aperturewhich defines the laser cavity. In Fig. 3, and all other samplesthat have been examined by transmission electron microscopy,no dislocations or other crystalline defects are apparent alongthe oxide/semiconductor interface or near the oxidation ter-minus. Hence, the formation of the oxide layer does notinduce obvious defects which would lead to degraded VeSELoperation or lifetime. This is consistent with our observationof approximately 98% yield of all oxide-confined VeSEL'stested to date. Moreover, the first lifetime experiment from arandomly chosen VeSEL has shown unchanged characteristicsduring >650 hours of ew operation at room temperature(7 rnA constant current producing 1.1 mW output light) ona probe station.In Fig. 4, we plot the threshold current, I t h , of VeSEL'sfrom two wafers grown more than a month apart with thesame epitaxial structure, which utilizes relatively high reflec-tivity top mirrors. The data points in Fig. 4 correspond tolasers that are spatially separated by 1 mm, showing thatboth I t h and threshold wavelength vary radially, that latterdue to thickness nonuniformity of the VeSEL wafers. TheI t h wavelength variation arises from the spectral mismatchbetween the cavity resonance and the laser gain. The mini-mum Ith occurs at ~970 nm, corresponding to the optimaloverlap of cavity resonance/peak gain. Notice Ith around thiswavelength are relatively insensitive to the lasing wavelength.Specifically over the wavelength span of 945-980 nm, theaverage threshold current is 292JLA ± 6.5% for 5 X 5-JLmVeSEL's in wafer XF1l4, and is 360 JLA ± 5.3% for 7 x7-JLm VeSEL's in wafer XF225. For comparison, implantedVeSEL's (15 JLm diameter) fabricated from wafer XF1l4have an average threshold current of 4.4 rnA ± 10% overthe same wavelength span of 945-980 nm. All of the oxide-confined VeSEL's in Fig. 4 have threshold currents less than1 rnA, corresponding to an operating wavelength span of up to75 nm. The maximum output power for the VeSEL's in Fig. 4vary from 0.5-1.5 mW, in spite of the relatively high outputcoupler reflectivity. Fig. 4 illustrates that submilliamp Ith canbe reproducibly obtained from oxide-confined VeSEL's.Fig. 5 depicts the power conversion (wall plug) efficiencyobtained at I-mW output power from VeSEL's fabricatedfrom two other wafers. The lasers in Fig. 5 have reducedoutput coupler reflectivity where wafer XF130e has a singlecurrent aperture in the p-type DBR, and wafer XF130B hasdouble apertures, as depicted in Fig. 1. For these wafers thehighest efficiency again occurs at ~ 970 nm. The highestefficiency measured was 30%, where the maximum typicallyCHOQUEITE et al.: FABRICATION AND PERFORMANCE OF OXIDIZED VERTICAL-CAVITY LASERS 123930,--------------------,1000,..------------------,REFERENCES[1] W. T. Tsang, "Self-terminating thermal oxidation of AlAs epilayersgrown on GaAs by molecular beam epitaxy," Appl. Phys. Lett., vol. 33,pp. 426-429, 1978.[2] J. M. Dallesasse, N. Holonyak, Jr., A. R. Sugg, T. A. Richard, andN. EI-Zein, "Hydrolyzation oxidation of AlxGal_xAs-AlAs-GaAsquantum well heterostructures and superlattices," Appl. Phys. Lett.,vol. 57, pp. 2844-2846, 1990.[3] 1. M. Dallesasse and N. Holonyak, Jr., "Native-oxide stripe-geometryAlxGal-xAs-GaAs quantum well heterostructure lasers," Appl. Phys.Lett., vol. 58, pp. 394-396, 1991.[4] M. H. MacDougal, P. D. Dapkus, V. Pudikov, H. Zhao, and G. M. Yang,"Ultralow threshold current vertical-cavity surface emitting lasers withAIAs-{lxide-GaAs distributed Bragg reflectors," IEEE Photon. Technol.Lett., vol. 7, pp. 229-231, 1995.[5] Y. Hayashi, T. Mukaihara, N. Hatori, N. Ohnoki, A. Matsutani,F. Koyama, and K. Iga, "Record low threshold index-guidedInGaAs/AIGaAs vertical-cavity surface emitting laser with a nativeoxide confinement structure," Electron. Lett., vol. 31, pp. 560-561,1995.[6] D. L. Huffaker, D. G. Deppe, K. Kumar, and T. J. Rogers, "Native-oxidedefined ring contact for low threshold vertical-cavity lasers," Appl. Phys.Lett., vol. 65, pp. 97-99, 1994.[7] K. D. Choquette, R. P. Schneider, Jr., K. L. Lear, and K. M. Geib,"Low threshold voltage vertical-cavity lasers fabricated by selectiveoxidation," Electron. Lett., vol. 30, pp. 2043-2044, 1994.[8] K. L. Lear, K. D. Choquette, R. P. Schneider, Jr., S. P. Kilcoyne,and K. M. Geib, "Selectively oxidized vertical-cavity surface emittinglasers with 50% power conversion efficiency," Electron. Lett., vol. 31,pp. 208-209, 1995.[9] K. D. Choquette, K. L. Lear, R. P. Schneider, Jr., and K. M. Geib,"Cavity characteristics of selectively oxidized vertical-cavity lasers,"Appl. Phys. Lett., vol. 66, pp. 3413-3415, 1995.[10] R. P. Schneider, Jr., J. A. Lott, M. Hagerott Crawford, andK. D. Choquette, "Epitaxial design and performance of AIGainP red(650--690 nm) VCSEL's," Inter. J. High Speed Electron. and Syst.,vol. 5, pp. 625--666, 1994.[11] K. L. Lear, R. P. Schneider, Jr., K. D. Choquette, S. P. Kilcoyne,J. J. Figiel, and 1. C. Zolper, "Vertical-cavity surface emitting lasers with21% efficiency by metalorganic vapor phase epitaxy," IEEE Photon.Technol. Lett., vol. 6, pp. 1053-1055, 1994.[12] K. D. Choquette, G. Hasnain, Y. H. Wang, J. D. Wynn, R. S. Freund,A. Y. Cho, and R. E. Leibenguth, "GaAs vertical-cavity surface emittinglasers fabricated by reactive ion etching," IEEE Photon. Technol. Lett.,vol. 3, pp. 859-862, 1991.[13] F. A. Kish, S. J. Caracci, N. Holonyak, Jr., J. M. Dallesasse, K. C. Hsieh,M. J. Ries, S. C. Smith, and R. D. Burnham, "Planar native-oxide index-guided AlxGal-xAs quantum well heterostructure lasers," Appl. Phys.Lett., vol. 59, pp. 1755-1757, 1991.[14] D. L. Huffaker, J. Shin, and D. G. Deppe, "Lasing characteristics of lowthreshold microcavity lasers using half-wave spacer layers and lateralindex confinement," Appl. Phys. Lett., vol. 66, pp. 1723-1725, 1995.[15] K. L. Lear, K. D. Choquette, R. P. Schneider, Jr., and S. P. Kilcoyne,"Modal analysis of a small surface emitting laser with a selectivelyoxidized waveguide," Appl. Phys. Lett., vol. 66, pp. 2616-2618, 1995.[16] K. D. Choquette, R. P. Schneider, Jr., M. H. Crawford, K. M. Geib,and J. J. Figiel, "Continuous wave operation of 640--660 nm selectivelyoxidized AIGainP vertical-cavity lasers," Electron. Lett., vol. 31, pp.1145-1146, 1995.ACKNOWLEDGMENTThe authors would like to thank 1. Walker of FEI Europe,Inc. for assistance with focused ion beam preparation of theTEM samples.confined VeSEL's, appropriate for a variety of wavelengths,should benefit emerging applications and markets consideredfor VeSEL's.••rI'III•[J[J[J•[J.•o•• 5x511m XF130B[J 6x611m XF130C••••• 5x5l1m XF114[J 7x711m XF225[J[J[J •[J[J[J[J[J •••• ••• ••••••••[J•[J [J [J940 960 980 1000Wavelength (nm) Wafer Center940 960 980 1000Wavelength (nm) Wafer Center[J•• [J[J[J[Jp •200+----r----,.......,----~----,...-1920Wafer Edge1O+--~-~----r--~~_r_~~-_1920Wafer EdgeFig. 5. Wall-plug efficiency at l-mW output power at room temperature foroxide-confined VCSEL's. The lasers in wafer XF130B have a single 5 x5-pm sized current aperture, while VCSEL's in wafer XF130C have two 6 x6-pm-sized current apertures surrounding the optical cavity.occured between 1-3 mW output power. As shown in Fig. 5 atI-mW output power, greater than 20% efficiency is obtainedover more than a 50-nm wavelength range for wafer XFI30B,and similar results are found from wafer XF130e (note thecenter resonance wavelength ofthis wafer is shorter). Over thiswavelength range I t h varies from 0.8-1.4 rnA for veSEL'sfrom both wafers. Since 1 mW output is appropriate for manyVeSEL applications, achieving high efficiency at this poweris desirable. Maintaining wavelength insensitive I t h and wallplug efficiency will impart higher yield to specification forveSEL arrays in the presence of wafer nonuniforrnity.In conclusion, monolithic oxide-confined VeSEL's not onlydemonstrate benchmark performance records, they also exhibitreproducible and wavelength insensitive performance levelswhich are attractive for potential applications. We have devel-oped a reproducible and high yield selective oxidation processfor fabrication of robust oxide-confined VeSEL's. Moreover,selectively oxidized VeSEL's have been fabricated at emis-sion wavelengths of 980, 850, and 650 nm, which indicates theuniversality of this VeSEL structure. High-performance oxide-Fig. 4. Threshold current versus threshold wavelength at room temperaturefor oxide-confined VCSEL's from two wafers. The lasers in wafer XF114(XF225) have 5 x 5-pm (7 x 7-pm) sized current apertures.

Fabrication and performance of selectively oxidized vertical-cavity lasers

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Fabrication and performance of selectively oxidized vertical-cavity lasers

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