298 IEEE JOURNAL ON SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 8, NO. 2, MARCH/APRIL 2002

AlGaN–GaN UV Light-Emitting Diodes Grown onSiC by Metal–Organic Chemical Vapor Deposition

Ting Gang Zhu, Jonathan C. Denyszyn, Uttiya Chowdhury, Michael M. Wong, Member, andRussell D. Dupuis, Fellow, IEEE

Abstract—In this paper, we report the study of the electricaland optical characteristics of AlGaN–GaN quantum-well (QW)ultraviolet light-emitting diodes grown on SiC by metal–organicchemical vapor deposition. These devices exhibit room-tempera-ture electroluminescence emission peaked at = 363 nm with anarrow linewidth of � = 9 nm under high-current-density dcinjection. We have also applied a Mg-doped AlGaN–GaN super-lattice structure as a p-cladding layer and vertical-geometry holeconduction improvement has been verified. A comparative studyof the performance of light-emitting devices with single-QW andmultiple-QW structures indicates that the single-QW structure ispreferred.

Index Terms—GaN, AlGaN, UV LED, metal–organic chemicalvapor deposition, superlattice, quantum-well structure.

I. INTRODUCTION

RECENTLY, wide bandgap nitride semiconductor ma-terials have attracted attention for ultraviolet (UV)

optoelectronic devices [1]–[3]. A compact, high-efficiencyUV light source could be used in chemical identification,medical processes, lighting and displays, and optical datastorage. While the InGaN-based III-nitride materials are welldeveloped for emission in the visible range [4], the widerbandgap AlGaN-based nitride materials, candidates for UVlight-emitting diodes (LEDs) and injection lasers, are less de-veloped. As the quality of III-nitride semiconductor materialshas continued to improve, there have been some preliminaryreports on nitride-based UV light emitters [1]–[3]. However,the development of efficient AlGaN-based UV light emittersis fraught with many difficulties. One of these is the relativelylow hole concentrations reported for p-type AlGaN layers, andthus, high device series resistance and limited current injectionthrough p-AlGaN films. One approach to reduce the resistanceof the p-type and n-type wide-bandgap cladding layers is theuse of doped AlGaN–GaN superlattices [5]. We have exploreda Mg-doped AlGaN–GaN superlattice structure to improve thevertical hole conduction in the p-type cladding layer of a UV

Manuscript received December 26, 2001; revised February 5, 2002. Thiswork was supported in part by the Defense Advanced Research ProjectsAgency/ (DARPA/MTO) under Grant N66001-01-1-8934 (J. C. Carrano), andGrant N00014-99-1-0231 (E. Martinez), by the National Science Foundation(NSF) under Grant BES-0 086 736 and Grant DMR-9 312 947 (L. Hess), andby the Office of Naval Reseach (ONR) under Grant N00014-99-1-0304 (Y. S.Park). R. D. Dupuis acknowledges the support of the Judson S. SwearingenRegents Chair in Engineering.

T. G. Zhu, J. C. Denyszyn, U. Chowdhury, M. M. Wong, and R. D. Dupuis arewith the Microelectronics Research Center, The University of Texas at Austin,Austin TX 78758 USA.

Publisher Item Identifier S 1077-260X(02)03761-9.

light-emitting device. There are many reports describing theimproved electrical conductivity and transport properties ofp-AlGaN–GaN superlattices (SL) [6], [7], but most of these arelimited to a study of the lateral conduction in these structures.In this paper, we compare the vertical conduction of p-typeMg-doped superlattices and conventional p-type AlGaN:Mglayers and confirm the improvement in the vertical conductionin the superlattice structures. By applying this p-type superlat-tice approach to a light-emitting QW heterostructure, devicescan be obtained having relatively good current–voltage (– )characteristics with differential resistances of 20–30. Whilethis value is high relative to that achieved for InGaN LEDs, itis state-of-the-art for LEDs emitting at UV wavelengths. Theroom-temperature electroluminescence (EL) spectra exhibiteda sharp emission peak around the GaN bandedge of363nm, and a narrow emission linewidth of9 nm.

II. EXPERIMENTAL

The Al Ga N–GaN LED structures of this work weregrown by low-pressure metal–organic chemical vapor de-position (MOCVD) in an EMCORE TurboDisc D125 UTMhigh-speed rotating-disk reactor on 2-in diameter n-type6H-SiC substrates. While SiC is expected to absorb the lightgenerated in the active region, this substrate has a higher thermalconductivity than sapphire and is expected to offer advantagesfor high-power LED operation. Furthermore, as determinedby X-ray diffraction and TEM, III-N epitaxial films grown onSiC substrates have a higher quality than those grown directlyon sapphire. The epitaxial films were grown in a hydrogenambient using adduct-purified trimethylgallium (TMGa),trimethylaluminum (TMAl), and ammonia (NH). The AlGaNepitaxial layers were grown at pressures50 torr; the 1- mGaN:Si template layer and 15-nm GaN:Mg p-type contactlayer were grown at 200 torr. The n-type and p-type dopantsemployed were silane (SiH) and bis(cyclopentadienyl)magne-sium (Cp Mg), respectively. A two-temperature growth processwas employed with a high-temperature AlN buffer layer grownat 1 080 C and the high-temperature AlGaN device layersgrown at 1 060 C. After the AlN buffer layer was grown onthe (0001) Si surface of the SiC substrate, a 1-m-thick GaN:Sitemplate layer was grown, followed by the growth of the n-sideof the device structure with a multiple-layer cladding region:100 nm of Al Ga N:Si, 20 nm of Al Ga N:Si, then50 nm of Al Ga N:Si. The active region consisted of a20 nm of a Al Ga N:Si waveguide layer, and a 10-nmGaN:Si QW for the single QW (SQW) structure; or three 10-nm

1077-260X/02$17.00 © 2002 IEEE

ZHU et al.: AlGaN–GaN UV LEDs GROWN ON SiC BY METALORGANIC CHEMICAL VAPOR DEPOSITION 299

Fig. 1. Schematic diagram of the circular-geometry UV LED structures grownon SiC substrates. The SQW active region consists of a 10 nm GaN QW andAl Ga N barriers. The MQW active region includes three 10-nm GaNQWs with 5 nm Al Ga N barriers.

GaN:Si quantum wells (QWs) with 5-nm Al Ga N:Sibarriers, for the multiple-QW (MQW) active region structure.The 50-nm Al Ga N:ud and 20-nm Al Ga N:udcladding layers of the p-side of the structure were left undopedto help prevent Mg diffusion into the active region of thedevice. The five-pair [Al Ga N:ud–Al Ga N:ud]SL that followed the cladding layers was also left un-doped. The p-type cladding layer consisted of a twenty-pair[Al Ga N:Mg–Al Ga N:Mg] SL. Finally, a15-nm-thick Mg-doped p-GaN layer was grown to serve asthe p-contact cap layer. The epitaxial layer structure for the UVlight emitters of this study is shown schematically in Fig. 1.

Samples were processed with conventional photolithographyto define a circular device geometry with mesas of various diam-eters. The mesa diode structures were formed using reactive-ionetching (RIE) with BCl as etching reagents. The semitrans-parent Ohmic contact to the top p-GaN layer was made by con-ventional liftoff of e-beam-evaporated Pd (90) and a low-tem-perature anneal. The electrode contact on this current spreadinglayer was formed by e-beam evaporated Pd–Au (200/1000

). The Ohmic metallization to the bottom n-type GaN:Si con-tact layer was Ti–Al–Pt–Au (200 /700 /200 /700 ) thatwas annealed in Nat 850 C for 30 s.

III. RESULTS AND DISCUSSIONS

To investigate the vertical hole conduction improvementby employing the p-type AlGaN–GaN superlattice struc-ture, we have compared the resistance of a p-type Mg-dopedAlGaN–GaN SL with that of a conventional uniform alloy com-position p-AlGaN:Mg layer. The SL structure consists of an AlNbuffer layer, a 360-nm-thick Mg-doped ( cm )GaN layer, a 75-pair Mg-doped Al Ga N–Al Ga NSL with a period of 3 nm (a total thickness of 225 nm), and 25nm of Mg-doped ( cm ) GaN as a cap layer. Thecomparison p-AlGaN layer structure consists of the same struc-ture except that the AlGa N-AlyGa N Mg-doped SL is

replaced by a 225 nm thick of Mg-doped Al Ga N layer.The inset of Fig. 2 shows a schematic diagram of the structure.The – characteristics of both of the structures are illustratedin Fig. 2. The mesa height is 275 nm, and the distance betweenthe bottom contact to the mesa edge is 5m. The – dataindicates a lower- resistance is obtained for the SL structure.The measured resistance of this structure is contributed by thetop contact resistance, the top 25-nm GaN layer resistance,the resistances due to the GaN-AlGaN heterointerfaces, theSL resistance (or the p-Al Ga N layer resistance), thebottom 25-nm GaN layer resistance, and the bottom contactresistance, plus the contact sheet resistance. From the measuredsheet resistance and the mesa geometry, the series resistancecontribution of the region between the mesa edge and the metalcontact is estimated to be100 , which is much smaller thanthe actual mesa resistance extracted from Fig. 2, i.e.,25k . Transmission-line method (TLM) measurements in theneighboring areas on the wafers yielded a specific contactresistance in the range of cm , confirmingthat the contribution from the contact resistance and the sheetresistance is small compared to the measured resistance, andcan be ignored. Furthermore, the mesas for the two wafershave the same lateral resistance contributed by the identicallower p-type GaN:Mg layers. Thus, the resistance differencesindicated by the – plots are mainly derived from the verticalconduction difference of these two structures. Furthermore,these two test structures underwent identical processing steps,therefore, the measured resistance difference should be dueto the fundamental design differences of the structures. Weconclude from Fig. 2 that the SL structure effectively increasesthe p-type vertical conduction.

Fig. 3 shows the – and – characteristics of anAlGaN–GaN SQW LED grown on a SiC substrate using thisp-type AlGaN SL cladding layer (see Fig. 1). The “on-wafer”light output measurements for circular-mesa devices having anarea of cm are carried out using a probe station and acalibrated Si p-i-n photodiode (area cm ), placed about2 cm above the device. No special light extraction measures areemployed, e.g., antireflection coatings or molded epoxy domes.While it is difficult to estimate the actual light output fromthese devices using this detector geometry, we have measuredthat the light output power is more than 12W (the signal fromthe uncorrected photodetector response) under a dc injectioncurrent of 240 mA. The output power did not saturate beyonda dc current of 240 mA ( 240 A/cm ). We believe that thisis due to the SiC substrate providing effective heat extraction.The – characteristics show a forward voltage of 4.2 V at aninjection current of 20 mA. This forward voltage is relativelylow for structures having thick p-type and n-type Al Ga Nlayers for optical and carrier confinement. The SL structure(Mg-doped Al Ga N–Al Ga N) employed here toimprove the p-type vertical conduction has a 15% difference inthe Al molar fraction, which was intended to further increasethe hole concentration [8]. Analysis of the– data indicatesthat the devices exhibit differential resistances of20–30 .

The room-temperature electroluminescence (EL) spectra ofa UV SQW LED operated at various dc forward bias currentsare shown in Fig. 4. The peak wavelength and the full-width

300 IEEE JOURNAL ON SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 8, NO. 2, MARCH/APRIL 2002

Fig. 2. I–V characteristics of the test structure consisting of a Mg-doped AlGa N–Al Ga N superlattice layer and a conventionalAl Ga N:Mg p-layer of the same total thickness. The inset shows a schematic diagram of the test structure.

Fig. 3. TheI–V and current-light output plot of the UV LED SQW devicegrown on a SiC substrate.

at half-maximum (FWHM) were nm andnm, respectively. The peak wavelength corresponds well tothat of the bandedge emission of the GaN in the QW activeregion. Another peak at nm in the EL spectra is alsoobserved. It results from emission from the p-type SL region(Al Ga N–Al Ga N). This result clearly impliesthat shorter wavelengths are obtainable from properly designedAlGaN-based QW structures. The brighter nmemission peak in the EL spectra indicates that the dominant ra-diative recombination occurs in the active region. At increasingcurrents, the emission peak remains at a constant wavelengthof 363 nm. A red shift is not observed, again confirming thatthe SiC substrate helps to remove the heat from the activeregion. Also, a blue-shift of the recombination radiation due to

Fig. 4. Room-temperature dc electroluminescence spectra of the UV LEDSQW at various injection currents. The output powers are only relative estimatesof a fraction of the total light output.

the piezoelectric field is not observed [9], perhaps because theinjected carriers screen this charge.

The performance of UV LED devices fabricated from SQWand MQW structures are summarized in Table I. From thesedata, it is clear that both structures exhibit similar device char-acteristics. However, we observe that the output power from theSQW LEDs is higher at all current densities than the power fromthe MQW LEDs by comparing them under the same measure-ment conditions. While precise total UV power measurementshave not been made, these diodes operate to250 A/cmand exhibit linear - curves up to this current density. Morestudies of the effect of the QW and cladding layer design on theAlGaN-based UV LED device performance is underway.

ZHU et al.: AlGaN–GaN UV LEDs GROWN ON SiC BY METALORGANIC CHEMICAL VAPOR DEPOSITION 301

TABLE ICOMPARISON OF THE CHARACTERISTICS OF

THE SQWAND THE MQW UV LEDS.

IV. SUMMARY AND CONCLUSION

In summary, we have grown, fabricated and characterizedAlGaN–GaN UV SQW- and MQW-structure LEDs grown byMOCVD on 6H-SiC substrates. The EL exhibits a peak emis-sion wavelength of nm with a linewidth of nm.Our study shows that p-type AlGaN–GaN:Mg superlattices caneffectively improve the vertical conduction in p-type structures,which indicates that such SLs are useful in UV LED applica-tions. Furthermore, we believe that the number of QWs in theactive region has an effect on the device emission characteristicsand more study of this is underway.

REFERENCES

[1] M. M. Wong, J. C. Denyszyn, C. J. Collins, U. Chowdhury, T. G. Zhu,K. S. Kim, and R. D. Dupuis, “AlGaN/AlGaN double-heterojunctionultraviolet light-emitting diodes grown by metal organic chemical vapordeposition,”Electron. Lett., vol. 37, pp. 1188–1190, 2001.

[2] A. Kinoshita, H. Hirayama, M. Ainoya, Y. Aoyagi, andA. Hirata, “Room-temperature operation at 333 nm ofAl Ga N/Al Ga N quantum-well light-emitting diodeswith Mg-doped superlattice layers,”Appl. Phys. Lett., vol. 77, pp.175–177, 2000.

[3] T. Nishida, H. Saito, and N. Kobayashi, “Efficient and high-powerAlGaN-based ultraviolet light-emitting diode grown on bulk GaN,”Appl. Phys. Lett., vol. 79, pp. 711–712, 2001.

[4] S. Nakamura and G. Fasol,The Blue Laser Diode, Berlin: Springer-Verlag, 1997.

[5] E. F. Schubert, W. Grieshaber, and I. D. Gopfert, “Enhancement of deepacceptor activation in semiconductors by superlattice doping,”Appl.Phys. Lett., vol. 69, pp. 3737–3739, 1996.

[6] I. D. Goepfert, E. F. Schubert, A. Osinsky, P. E. Norris, and N. N. Faleev,“Experimental and theoretical study of acceptor activation and transportproperties inp-type Al Ga N/GaN superlattices,”J. Appl. Phys., vol.88, pp. 2030–2038, 2000.

[7] P. Kozodoy, Y. P. Smorchkova, M. Hansen, H. Xing, S. P. DenBaars, U.K. Mishra, A. W. Saxler, R. Perrin, and W. C. Mitchel, “Polarization-enhanced Mg doping of AlGaN/GaN superlattices,”Appl. Phys. Lett.,vol. 75, pp. 2444–2446, 1999.

[8] P. Kozodoy, M. Hansen, S. P. DenBaars, and U. K. Mishra, “EnhancedMg doping efficiency in Al Ga N/GaN superlattices,”Appl. Phys.Lett., vol. 74, pp. 3681–3683, 1999.

[9] H. S. Kim, J. Y. Lin, H. X. Jiang, W. W. Chow, A. Botchkarev,and H. Morkoç, “Piezoelectric effects on the optical properties ofGaN/Al Ga N multiple quantum wells,”Appl. Phys. Lett., vol. 73,pp. 3426–3428, 1998.

Ting Gang Zhu received the B.E. degree in applied physics from ShanghaiUniversity and the M.S. degree in electrical and computer engineering fromThe University of Texas at Austin, Austin, TX, in 1991 and 2000, respectivley.He is working toward the Ph.D. degree in electrical engineering studying wide-bandgap device, processing and characterization.

Soon after his graduation from Shanghai University, he immigrated to Singa-pore and worked in a semiconductor memory manufacturer, in a joint venturebetween Texas Instruments, EDB of Singapore, Hewlett-Packard, and Canon,as a process and equipment engineer. In 1998, he matriculated in the College ofEngineering at The University of Texas at Austin to pursue his graduate studies.

Jonathan C. Denyszynreceived the B.S. degree in electrical and computer en-gineering from The University of Texas at Austin, Austin, TX, in 1999. He isworking toward the M.S. degree at The University of Texas at Austin.

After graduation, he participated in intern training at Motorola and Intel.

Uttiya Chowdhury received the B.Sc. Eng. degree in electrical and electronicengineering at Bangladesh University of Engineering and Technology, in 1993.He is working toward the Ph.D. degree on compound semiconductor growth atThe University of Texas, Austin, TX.

After graduation, he joined Spectrum Engineering Consortium, a softwarehouse in Bangladesh as a junior programmer. He then received the M.S. degreein electrical and computer engineering from The University of Texas at Austin,in 1999.

Michael M. Wong (S’87–M’91) received the B.S. degree in electrical engi-neering from Arizona State University, and the M.S. degree in electrical en-gineering from The University of Texas at Austin, in 1991 and 1999. respec-tively. He is working towards the Ph.D. degree in electrical engineering in widebandgap nitride material growth and characterization.

After graduating, he was employed as a Product Engineer by Texas Instru-ments, Incorporated, responsible for application specific integrated circuits(ASICs) and digital signal processors (DSPs). In 1997, he matriculated at TheUniversity of Texas at Austin for graduate studies.

Russell D. Dupuis(S’68–M’84–F’87) received the Ph.D. degree in electricalengineering from the University of Illinois in 1973.

From 1973 to 1975, he worked at Texas Instruments, on the growth of GQPlight-emitting diodes. In 1975, he joined Rockwell International, where hewas the first to demonstrate that metal–organic chemical vapor deposition(MOCVD) could be used for the growth of high-quality semiconductor thinfilms and devices. In 1979, he joined AT&T Bell Laboratories where heextended his work to the growth of InP–InGaAsP by MOCVD. In 1989, hejoined The University of Texas at Austin as a Chaired Professor. His technicalspecialties include semiconductor materials and devices, epitaxial growth byMOCVD, and heterojunction structures in compound semiconductors. He iscurrently studying the growth of III–V compound semiconductor devices byMOCVD, including materials in the InAlGaN–GaN, InAlGaAsP–GaAs, andInAlGaAsP–InP systems.

He is a member of the NAE and a Fellow of the OSA.