7] M.-Y. Ryu, C.Q. Chen, E. Kuokstis, J.W. Yang, G. Simin, M. Asif Khan, Appl. Phys. Lett. 80 (2002) 3730. [8] D. Xu, Y. Wang, H. Yang, L. Zheng, J. Li, L. Duan, R. Wu, Sci. China (a) 42 (1999) 517. [9] H. Hirayama, A. Kinoshita, A. Hirata, Y. Aoyagi, Phys. Stat. Sol. (a) 188 (2001) 83. [10] Y. Chen, T. Takeuchi, H. Amano, I. Akasaki, N. Yamada, Y. Kaneko, S.Y. Wang, Appl. Phys. Lett. 72 (1998) 710. [11] Ig-Hyeon Kim, Hyeong-Soo Park, Yong-Jo Park, Taeil Kim, Appl. Phys. Lett. 73 (1998) 1634. [12] K. Watanabe, J.R. Yang, S.Y. Huang, K. Inoke, J.T. Hsu, R.C. Tu, T. Yamazaki, N. Nakanishi, M. Shiojiri, Appl. Phys. Lett. 82 (2003) 718

Amabile, D.

Artus, L.

Cusco, R.

Hernandez, S.

Martin, R.W.

Nogales, E.

O'Donnell, K.P.

Pastor, D.

Wang, K.

Watson, I.M.

English

Name not available

Structural and optical properties of MOCVD AllnN epilayers

University of Strathclyde Institutional Repository

Strathprints Institutional RepositoryHernandez, S. and Wang, K. and Amabile, D. and Nogales, E. and Cusco, R. and Pastor, D. andArtus, L. and Martin, R.W. and O’Donnell, K.P. and Watson, I.M. (2005) Structural and opticalproperties of MOCVD AllnN epilayers. MRS Online Proceedings Library, 388-393 (6). pp. 388-393. ISSN 0272-9172Strathprints 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/Journal of Crystal Growth 260 (2004) 388–393Structural and optical properties of quaternary AlInGaNepilayers grown by MOCVD with various TMGa ﬂowsJ.P. Liua,*, B.S. Zhanga, M. Wua, D.B. Lib, J.C. Zhanga, R.Q. Jina,J.J. Zhua, J. Chena, J.F. Wanga, Y.T. Wanga, H. YangaaState Key Laboratory on Integrated Optoelectronics, Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912,Beijing 100083, ChinabKey Laboratory of Semiconductor Materials, Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912,Beijing 100083, ChinaReceived 29 July 2003; accepted 5 September 2003Communicated by M. SchieberAbstractAlInGaN quaternary epilayers have been grown with various TMGa ﬂows by metalorganic chemical vapordeposition to investigate the inﬂuence of growth rate on the structural and optical properties. Triple-axis X-raydiffraction measurements show AlInGaN epilayers have good crystalline quality. Photoluminescence (PL)measurements show that the emission intensity of AlInGaN epilayers is twenty times stronger than that of AlGaNepilayer with comparable Al content. V-shaped pits are observed at the surface of AlInGaN epilayers by atomic forcemicroscopy (AFM) and transmission electron microscopy (TEM). High growth rate leads to increased density and sizeof V-shaped pits, but crystalline quality is not degraded.r 2003 Elsevier B.V. All rights reserved.PACS: 68.65.+g; 78.55.Cr; 81.15.GhKeywords: A1. Triple-axis X-ray diffraction; A1. Atomic force microscopy; A3. Metalorganic chemical vapor deposition; B1.AlInGaN Quaternary alloys1. IntroductionThe group III-nitride wide-band-gap semicon-ductors have been recognized as leading materialsfor many optoelectronic devices, such as blueultraviolet (UV) light-emitting diodes (LEDs),laser diodes (LDs), and high-temperature/high-power electronic devices. Nearly all the high-efﬁciency blue ultraviolet LEDs and LDs useInGaN quantum wells (QWs) with InGaN or GaNbarrier layers. However, AlInGaN quaternaryalloy has attracted much research interest becauseuse of this quaternary material should allowalmost independent control of the lattice mismatchand band offset in AlInGaN-based heterostruc-tures [1]. Enhanced luminescence in InGaN multilequantum wells with quaternary AlInGaN barrierswas demonstrated, in which emission mechanismARTICLE IN PRESS*Corresponding author. Tel.:+8610-82304586 ; fax: +8610-82305033.E-mail address: jpliu@red.semi.ac.cn (J.P. Liu).0022-0248/$ - see front matter r 2003 Elsevier B.V. All rights reserved.doi:10.1016/j.jcrysgro.2003.09.007was considered to change from localized statebased to that of a conventional quantum wellstructure [2–4].High-power UV LEDs are needed for efﬁcientpumping of phosphors for solid-state white-lightdevices. UV LEDs with AlGaN QWs as activelayer have very low efﬁciency. However, byintroducing In into AlGaN, a similar effect tothat obtained in InGaN QWs is expected forquaternary AlInGaN. In fact, enhanced lumines-cence in quaternary AlInGaN with In-segregationeffect have been reported by several authors [5–7].Although UV LD and LED with AlInGaN-based quantum wells structures have already beendemonstrated, fewer reports are available onAlInGaN material properties and optical charac-teristics. Especially, few authors mentioned thedetailed parameters about the crystalline quality ofquaternary AlInGaN, such as FWHM of X-rayrocking curve or FWHM of photoluminescence(PL) spectrum. In this paper, AlInGaN quaternaryalloys were grown with different TMGa ﬂows.Triple-axis X-ray diffraction (XRD) and PL wereused to study the material and optical properties ofAlInGaN quaternary epilayers. The surface mor-phology of AlInGaN quaternary alloy was ob-served using atomic force microscopy (AFM).Cross-section transmission electron microscopy(TEM) was used to study the pits formation. Theeffect of growth rate on the structural and opticalproperties of AlInGaN quaternary alloy wasdiscussed.2. Experimental procedureThe quaternary AlInGaN epilayers were grownby low pressure metalorganic chemical vapordeposition (MOCVD). A 3 mm GaN templatelayer was ﬁrst deposited on the sapphire substratewith a 25 nm low temperature GaN buffer layer,followed by the deposition of a 250 nm AlInGaNepilayers. The growth temperature and pressurefor the underlying GaN epilayer were 1040C and76Torr, respectively. Ammonia (NH3), trimethy-laluminum (TMAl), trimethylgallium (TMGa) andtrimethylindium (TMIn) were used as precursors.H2 was used as carrier gas. The molar ﬂows ofTMGa during AlInGaN growth were 3, 6, 10,13 mmol/min for the R1, R2, R3 and R4 samples,respectively. The ratios of TMAl to TMGa andTMIn to TMGa were held constant. Growth rateof sample R1, R2, R3 and R4 is 2.9, 5.4, 8 and11.8 nm/min, respectively, obtained from the insitu laser reﬂection spectrum.Triple axis X-ray diffraction was carried out ona Rigaku SLX-1AL to obtain the composition andto evaluate crystalline quality of AlInGaN epi-layers. The 325 nm line of an He–Cd laser was usedas an exciting source for the photoluminescence(PL) measurements. A tapping mode AFM(Digital Instruments Nanoscope II) was used toobtain the AFM images. The observed area was3 3 mm2. Cross-section TEM was used to studythe pits formation.3. Results and discussionLattice constant can be obtained by triple-axis2y=o scan, and the FWHM of triple-axis o scanreﬂects a degree of crystalline quality [8]. Fig. 1shows (0 0 0 2) triple-axis 2y=o X-ray diffractionpatterns for the four AlInGaN samples withdifferent growth rates. The peaks at 34.56Coriginate from the underlying GaN layers and allAlInGaN peaks locate at the left side of the GaNpeaks. This indicates the AlInGaN epilayersundergo the compressive strain, which, however,ARTICLE IN PRESSFig. 1. Triple-axis (0 0 0 2) 2y=o XRD patterns of AlInGaNepilayers grown with various TMGa ﬂows.J.P. Liu et al. / Journal of Crystal Growth 260 (2004) 388–393 389is very small. The largest strain experienced by theR4 sample is 3.6 103. The other samples havealmost equal strain, suggesting the same c-axislattice constant. Fig. 2 shows triple-axis X-ray oscan of the AlInGaN peak in sample R1 with afull-width at half maximum (FWHM) of 266arcsec, indicating good crystalline quality. TheFWHM of triple-axis X-ray o scan of the under-lying GaN layer is 280 arcsec. To our knowledge,this narrow FWHM value is the best resultobtained from quaternary AlInGaN epilayer upto now. o scans of other samples have almost thesame FWHM, which indicates that crystallinequality is not degraded with increasing growthrate.Fig. 3 shows room temperature PL spectra forthese AlInGaN epilayers. The Al and In contentsof sample R1, R2 and R3 are estimated to be 0.075and 0.04, respectively. Al contents are obtainedbased on the Rutherford backscattering spectro-metry (RBS) measurement for the previouslygrown samples with the same TMAl, TMGa, andammonia ﬂows. In contents are obtained based onthe measurement of the lattice constant. Inaddition In contents are conﬁrmed by roomtemperature PL peak energy. It is assumed thatthe aluminum and gallium incorporation rate issame at different growth rates. The In content ofsample R4 lightly increases to 0.05. This resultsuggests the In incorporation is not enhanced withthe increase of TMGa ﬂow until TMGa ﬂowreaches 13 mmol/min which is the TMGa ﬂow forsample R4. This trend is different from theobservation of InGaN growth. This may be dueto the effect of Al incorporation because the In–Al–N bond is stronger than In–Ga–N bond.Hirayama et al. [9] also observed much more Inwas incorporated into AlInGaN epilayers com-pared with InGaN because of the effect of Alincorporation. All samples show strong band edgeemission peaks as we can see in Fig. 3. Theemission intensity is twenty times stronger thanthat of AlGaN epilayer with comparable Alcontent as shown in Fig. 3. The enhanced emissionis due to the In-segregation effect as reported byother authors [5–7]. The peak position for sampleR1 is at 3.39 eV and the narrow FWHM of46.5meV conﬁrm good crystalline quality. TheFWHM of sample R4 increases to 99.4meV andthe integral emission intensity is twice more thanthat of sample R1 because of larger compositionalﬂuctuations.Fig. 4 shows AFM images of these AlInGaNepilayers. The surface morphologies of thesesamples show atomic steps besides hexagonal pits.The pit has a hexahedron cone morphology asshown in a typical SEM image in Fig. 5. Thesurface region outside V-shaped pits is verysmooth, indicating step-ﬂow growth. The densityand the size of V-shaped pits in these samples aresummarized in Table 1. The density and the sizeincrease with increasing growth rate. Fig. 6 is aARTICLE IN PRESSFig. 2. Triple-axis X-ray o scan of the AlInGaN peak insample R1.Fig. 3. Room temperature PL spectra for Al0.1Ga0.9N andAlInGaN epilayers grown with various TMGa ﬂows.J.P. Liu et al. / Journal of Crystal Growth 260 (2004) 388–393390typical [10–12] cross-sectional dark-ﬁeld TEMimage of AlInGaN epilayer and underlying GaNtemplate layer. It can be seen that each pit isassociated with a threading dislocation from theunderlying GaN template layer at its vertex. Pitswith similar morphology have been reportedpreviously in InGaN/GaN multiple quantum wellsand bulk InGaN ﬁlm [10–12]. The strain energyinduced by the lattice mismatch between InGaNand GaN and the reduction of surface mobilitydue to the low growth temperature of InGaN aremajor attributors in the pit formation. We suggestthat V-shaped pits in our AlInGaN epilayers havethe same formation mechanism due to the similargrowth conditions. In atoms tend to congregate inthe dislocation core areas due to the low growthtemperature. Thus local In segregation may slowdown or inhibit the crystal growth in this area,which initiates the pit formation. It has also beenproposed that pits start at thread dislocations orIn-rich dots in InGaN/GaN multiple quantumwells and bulk InGaN ﬁlm [10–12]. Surfaceadatoms have longer time to migrate and thus toﬁnd energetically favorable sites before beingincorporated into alloy thereby enhancing two-dimensional growth at lower growth rate, whichmay suppresses the In segregation and also theARTICLE IN PRESSFig. 4. AFM images of the surface morphologies of AlInGaN epilayers, The scan area is 3 3 mm2.Fig. 5. A typical SEM image of V-shaped pit in AlInGaNepilayer.J.P. Liu et al. / Journal of Crystal Growth 260 (2004) 388–393 391pits formation. On the other hand, pits aremore easy and thus more early to form duringAlInGaN growth at higher growth rate. That maybe the reason why the density and the size of V-shaped pits increase with increasing growth rate.Further investigation about V-shaped pits isunderway.4. ConclusionAlInGaN quaternary epilayers have beengrown with different TMGa ﬂows by MOCVD.Triple-axis X-ray diffraction measurementsshow AlInGaN epilayers have good crystallinequality, which doesn’t degrade with increasinggrowth rate. Intense UV emission at wavelengthsin the range of 360–370 nm has been obtained.The In incorporation doesn’t increase with in-creasing growth rate until TMGa ﬂow reaches13 mmol/min. The density and the size of V-shapedpits are found to increase with increasing growthrate.AcknowledgementsThe authors gratefully acknowledge Dr. Y.L.Wang for his help in AFM measurement.References[1] M. Asif Khan, J.W. Yang, G. Simin, R. Gaska, M.S. Shur,H.-C. zur Loye, G. Tamulaitis, A. Zukauskas, Appl. Phys.Lett. 76 (2000) 1161.[2] J.P. Zhang, J. Yang, G. Simin, M. Shatalov, M. AsifKhan, M.S. Shur, R. Gaska, Appl. Phys. Lett. 77 (2000)2668.[3] A. Chitnis, A. Kumar, M. Shatalov, V. Adivarahan,A. Lunev, J.W. Yang, G. Simin, M. Asif Khan, R. Gaska,M. Shur, Appl. Phys. Lett. 77 (2000) 3800.[4] M. Shatalov, A. Chitnis, V. Adivarahan, A. Lunev,J. Zhang, J.W. Yang, Q. Fareed, G. Simin, A. Zakheim,M. Asif Khan, R. Gaska, M. Shur, Appl. Phys. Lett. 78(2000) 817.[5] H. Hirayama, A. Kinoshita, T. Yamabi, Y. Enomoto,A. Hirata, T. Araki, Y. Nanishi, Y. Aoyagi, Appl. Phys.Lett. 80 (2002) 207.[6] C.H. Chen, L.Y. Huang, Y.F. Chen, H.X. Jiang, J.Y. Lin,Appl. Phys. Lett. 80 (2002) 1397.ARTICLE IN PRESSTable 1Summary of AFM results of V-shaped pits in AlInGaN epilayers grown with various TMGa ﬂowsSample no. Growth rate (nm/min) Pit density (cm2) Pit width (nm) Pit depth (nm) RMS (nm) (for 3 3mm2)R1 2.9 2.3 108 234 33 4.4R2 5.4 2.4 108 234 45 5.7R3 8 3.1 108 280 54 8.7R4 11.8 4.0 108 300 115 15.3Fig. 6. A typical (10–12) cross-section TEM image of AlInGaN epilayer.J.P. Liu et al. / Journal of Crystal Growth 260 (2004) 388–393392[7] M.-Y. Ryu, C.Q. Chen, E. Kuokstis, J.W. Yang,G. Simin, M. Asif Khan, Appl. Phys. Lett. 80 (2002)3730.[8] D. Xu, Y. Wang, H. Yang, L. Zheng, J. Li, L. Duan,R. Wu, Sci. China (a) 42 (1999) 517.[9] H. Hirayama, A. Kinoshita, A. Hirata, Y. Aoyagi, Phys.Stat. Sol. (a) 188 (2001) 83.[10] Y. Chen, T. Takeuchi, H. Amano, I. Akasaki, N. Yamada,Y. Kaneko, S.Y. Wang, Appl. Phys. Lett. 72 (1998) 710.[11] Ig-Hyeon Kim, Hyeong-Soo Park, Yong-Jo Park, TaeilKim, Appl. Phys. Lett. 73 (1998) 1634.[12] K. Watanabe, J.R. Yang, S.Y. Huang, K. Inoke, J.T. Hsu,R.C. Tu, T. Yamazaki, N. Nakanishi, M. Shiojiri, Appl.Phys. Lett. 82 (2003) 718.ARTICLE IN PRESSJ.P. Liu et al. / Journal of Crystal Growth 260 (2004) 388–393 393

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Structural and optical properties of MOCVD AllnN epilayers

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