Free-standing wafers (50 mm diameter) of GaN were grown by halide vapor phase epitaxy on lattice-matched gamma-LiAlO2. We report a transmission electron microscopy study of defects and defect densities in these wafers. The growth direction is [10 (1) over bar0]. Stacking faults in the basal plane are seen when viewing the specimen in the [1 (2) over bar 10] direction with an average spacing of less than 100 nm. Convergent beam electron diffraction measurements show no switch in the polarity and thus the faults are proposed to be ABABACAC changes in the stacking. Threading dislocations are found to have a correlated arrangement with a density of 3x10(8) cm(-2) when viewing the [1 (2) over bar 10] direction and widely varying (depending upon location) when viewing in the [0001] direction. These dislocations act as  seeds  for postgrowth surface features that directly exhibit the correlated nature of these threading dislocations

Chai, B. H.

Chou, M. M. C.

Hill, D. W.

Maruska, H. P.

Simmons, J. A.

Vanfleet, R. R.

English

University of Central Florida (UCF): STARS (Showcase of Text, Archives, Research & Scholarship)

University of Central Florida STARS Faculty Bibliography 2000s Faculty Bibliography 1-1-2003 Defects in m-face GaN films grown by halide vapor phase epitaxy on LiAlO2 R. R. Vanfleet University of Central Florida J. A. Simmons University of Central Florida H. P. Maruska D. W. Hill M. M. C. Chou See next page for additional authors Find similar works at: https://stars.library.ucf.edu/facultybib2000 University of Central Florida Libraries http://library.ucf.edu This Article is brought to you for free and open access by the Faculty Bibliography at STARS. It has been accepted for inclusion in Faculty Bibliography 2000s by an authorized administrator of STARS. For more information, please contact STARS@ucf.edu. Recommended Citation Vanfleet, R. R.; Simmons, J. A.; Maruska, H. P.; Hill, D. W.; Chou, M. M. C.; and Chai, B. H., "Defects in m-face GaN films grown by halide vapor phase epitaxy on LiAlO2" (2003). Faculty Bibliography 2000s. 4089. https://stars.library.ucf.edu/facultybib2000/4089 Authors R. R. Vanfleet, J. A. Simmons, H. P. Maruska, D. W. Hill, M. M. C. Chou, and B. H. Chai This article is available at STARS: https://stars.library.ucf.edu/facultybib2000/4089 Appl. Phys. Lett. 83, 1139 (2003); https://doi.org/10.1063/1.1599962 83, 1139© 2003 American Institute of Physics.Defects in m-face GaN films grown by halidevapor phase epitaxy on Cite as: Appl. Phys. Lett. 83, 1139 (2003); https://doi.org/10.1063/1.1599962Submitted: 05 March 2003 . Accepted: 06 June 2003 . Published Online: 05 August 2003R. R. Vanfleet, J. A. Simmons, H. P. Maruska, D. W. Hill, M. M. C. Chou, and B. H. ChaiARTICLES YOU MAY BE INTERESTED INStructural characterization of nonpolar  a-plane GaN thin films grown on  r-planesapphireApplied Physics Letters 81, 469 (2002); https://doi.org/10.1063/1.1493220Microstructural evolution of a-plane GaN grown on a-plane SiC by metalorganic chemicalvapor depositionApplied Physics Letters 84, 1281 (2004); https://doi.org/10.1063/1.1650545Growth of a-plane InN on r-plane sapphire with a GaN buffer by molecular-beam epitaxyApplied Physics Letters 83, 1136 (2003); https://doi.org/10.1063/1.1599634Defects in m-face GaN films grown by halide vapor phase epitaxyon LiAlO2R. R. Vanfleeta) and J. A. SimmonsAdvanced Materials Processing and Analysis Center (AMPAC) and the Department of Physics,University of Central Florida, Orlando, Florida 32816H. P. Maruska, D. W. Hill, M. M. C. Chou, and B. H. ChaiCrystal Photonics Inc., 5525 Benchmark Lane, Sanford, Florida 32773~Received 5 March 2003; accepted 6 June 2003!Free-standing wafers ~50 mm diameter! of GaN were grown by halide vapor phase epitaxy onlattice-matched g-LiAlO2 . We report a transmission electron microscopy study of defects anddefect densities in these wafers. The growth direction is @101I 0# . Stacking faults in the basal planeare seen when viewing the specimen in the @12I 10# direction with an average spacing of less than100 nm. Convergent beam electron diffraction measurements show no switch in the polarity andthus the faults are proposed to be ABABACAC changes in the stacking. Threading dislocations arefound to have a correlated arrangement with a density of 33108 cm22 when viewing the @12I 10#direction and widely varying ~depending upon location! when viewing in the @0001# direction. Thesedislocations act as ‘‘seeds’’ for postgrowth surface features that directly exhibit the correlated natureof these threading dislocations. © 2003 American Institute of Physics. @DOI: 10.1063/1.1599962#Nitride-based semiconductor products have burst ontothe scene in the last several years, and have now become amajor player in the optoelectronics market. Revenues fromthe use of blue and green light-emitting diodes ~LEDs! in thehome and commercial lighting business are predicted toreach almost three billion dollars by 2009. Although the bluelaser diode ~LD! optical storage market is presently stillsmall, it is predicted to reach over the two billion dollar levelby 2009. However, continued market growth is in somedoubt, because further increases in unit brightness levels forLEDs, which would allow penetration into the illuminationindustry, may not be possible because the quality of presentnitride materials is quite poor. It is difficult to coax evenmarginal performance levels and operating lifetimes out ofthe blue LDs, since their structures are filled with deleteriousdislocations. Therefore, we have undertaken an effort to pre-pare low defect density gallium nitride substrates suitable forthe homoepitaxial growth of near defect-free device material.We report here our progress in understanding the structuraldefects which are found in our wafers.We have used Czochralski grown1 g-LiAlO2 as a start-ing substrate for GaN growth. g-LiAlO2 has an unusualcrystal structure consisting of tetrahedra which share edgesas well as vertices.2 Although the unit cell has square sym-metry when looking down the c axis, the a – c ~100! planehas the same atomic arrangement as the prismatic face(101I 0) plane of the wurtzite structure. In fact, the c param-eter of LiAlO2 , 6.268 Å, is close to 2 times ah56.378 Å forGaN, while the a parameter of LiAlO2 , 5.168 Å, is basicallya perfect match to ch55.165 Å for GaN. In addition, theGaN hexagonal cell will be oriented sideways, with the polarch axis in the plane of growth, which will have profoundeffects on polarization issues ~Fig. 1!. In 1998, Ke et al.,deposited GaN films3 featuring the (101I 0) orientation4 bymetalorganic vapor phase epitaxy ~MOVPE! on polishedg-LiAlO2 . We note that conventional growth of GaN on~0001! sapphire substrates gives material with a permanentelectric dipole along the c axis, leading to the quantum con-fined Stark effect in GaN quantum well structures.5 This in-ternal electric field leads to spatially indirect optical transi-tions for UV-emitting LEDs, creating much dimmer devices.It was recently demonstrated that growth of GaN on ~100!LiAlO2 gives films with the (101I 0) orientation and, hencebrighter emission levels.6,7 Recent work using (11I 02) sap-phire has renewed interest in placing the c, axis in the planeof GaN which has the (112I 0) orientation.8In recent years, a number of researchers have pursuedhalide vapor phase epitaxy ~HVPE! as a quasibulk techniquefor the growth of thick ~.20 mm!, large-area GaNsubstrates.9–11 Growth rates in the HVPE machines werevery fast, ranging as high as 200 mm/h ~3–4 mm/min!.12 Thethick layer growth is facilitated by the near-equilibriumnature of the process, which can be exploited to generatematerial with somewhat lower defect densities than othermethods.a!Electronic mail: vanfleet@physics.ucf.edu FIG. 1. Atomic arrangement at the interface between LiAlO2 and GaN.APPLIED PHYSICS LETTERS VOLUME 83, NUMBER 6 11 AUGUST 200311390003-6951/2003/83(6)/1139/3/$20.00 © 2003 American Institute of PhysicsFollowing established practices, Crystal Photonics hasdesigned and constructed a HVPE reactor for the rapidgrowth of GaN films. The prepared LiAlO2 wafer is mountedon a SiC-coated graphite chuck. The three furnace zones aretypically set at 875–875–850 °C. Nitrogen is set to flowthrough all gas lines, except for the GaCl generation tube, inwhich hydrogen flows. The chuck is spun at 60 rpm, theammonia flowrate is 1 slm, and the HCl flowrate is 20 sccm.A run usually proceeds for 6 h. The growth rate is about 50mm/h. The GaN wafers are at least 300 mm thick. The result-ing wafer has the GaN c axis in the plane of the wafer andparallel to the a axis of the LiAlO2 as expected. The @101I 0#GaN direction is normal to the wafer. Many analytical pro-cedures have been undertaken in order to understand thecharacteristics of these wafers. Results based on a transmis-sion electron microscopy ~TEM! study will be presentedherein.TEM specimens were prepared using the focused ionbeam ~FIB! ex situ liftout method.13 In this method, the FIBgallium ion beam is used to form and cut free a thin sliverfrom a specific site of the specimen. This sliver is approxi-mately 20 mm long by 5 mm deep by ;500 nm thick. Amicromanipulator is then used to remove the specimen andplace it on a carbon film for TEM observation. The Ga ionbeam creates a Ga implanted amorphized layer on the sur-face along with redeposited material that at times obscuresareas of the final specimen. The abundance of these redepos-ited features depends upon the details of the FIB processing.Thus, with care and a little bit of luck, good specimens arerapidly produced from specific sites on the wafer.FIB liftout specimens from the top surface of (101I 0)GaN can of course easily give any direction perpendicular tothis @101I 0# normal. Two convenient choices are @0001# and@12I 10# which are looking into the c axis and looking per-pendicular to the c-axis ~into the a-axis!.TEM results have shown significant variability depend-ing upon location on the wafer, first or last grown surface,and wafer to wafer. However, several features have beenwidely seen and appear to be consistent across specimensand a range of processing conditions. Threading dislocationsare seen that are qualitatively similar to those commonlyseen with other samples grown on sapphire. Differing sig-nificantly from what is seen on more traditional substrates,we see a significant number of planar defects that we identifyas stacking faults. Such stacking faults have also recentlybeen identified for (112I 0) oriented GaN films grown on(11I 02) sapphire substrates.14These planar defects are only seen in the @12I 10# speci-mens. When viewed directly on the @12I 10# axis the defectsare not visible, tilting about the @101I 0# growth direction toput the basal planes at a small incline to the viewing direc-tion makes them visible. Figures 2~a! and 2~b! shows anexample of these faults. Figure 2~b! shows a region wherethe thickness of the specimen is increasing from right to left.The single fringe that not only splits into several fringes butincreases in apparent width as the thickness is increased istypical of the contrast expected from an inclined planar de-fect in this approximately two-beam bright field image g5101I 0. These defects are absolutely parallel to each otherand run perpendicular to the c direction. The average spacingbetween these defects is ;100 nm. They are only seen whenthe c axis is tilted relative to the viewing direction and arenever seen when the specimen is prepared to look down thec axis. This data leads us to identify these as planar defectsconfined to the basal planes.The question of stacking fault ~with no polarity inver-sion! versus polar inversion domains is of practical impor-tance. Polar inversion domains have been shown to havedifferent intensities in the g50002 two-beam imaging con-ditions due to the underlying nonsymmetry in the crystalstructure.15 We saw no evidence for this contrast mechanismin our specimens. This image contrast is, however, a weakerfeature that is usually only suggestive, with more conclusiveevidence coming from convergent beam electron diffraction~CBED! methods. CBED is sensitive to the polarity of GaNwhere an asymmetry in the intensity distribution of the~0002! and (0002I ) diffraction disks is both predicted andseen.16 However, in the symmetrical orientation of the ~0002!peaks required, these defects are not visible. Thus, we tiltedthe specimen to identify a region with these defects, tiltedback to the on-axis condition and performed CBED linescans across the identified region. The asymmetry of the~0002! type peaks was evident, but no evidence for flippingof that asymmetry was seen.In the c-axis direction hexagonal GaN has an ABABstacking where each A ~or B! layer is composed of both Gaand N sublayers with a polarity built in. There is, however, aC site that is equally acceptable and thus an ACAC or BCBCstacking is also possible. There is physically no differencebetween these variants ~shifting the coordinate system willmap one into the other! and thus no reason to discuss themunless they are found in the same specimen as we are sug-gesting here. We propose to identify our observed planardefects as stacking faults between these variants with anABABACAC stacking. They are thus atomically sharp pla-nar defects that would be invisible if seen edge on. There isno change in the polarity and no extended strain field, con-sistent with our observations.GaN when viewed down the c axis has a hexagonal sym-metry with each of the hexagonal faces being ^101I 0& direc-tions. Thus, each new nucleation site on LiAlO2 can beginwith any one of these six faces as the growth direction aswell as in principle either orientation of the c axis. UniformFIG. 2. ~a! Stacking faults. ~b! Viewed closer in a wedge shaped region.1140 Appl. Phys. Lett., Vol. 83, No. 6, 11 August 2003 Vanfleet et al.alignment of ‘‘arrowhead’’ features on the final wafersurface17 and the lack of observed inversion domains implyLiAlO2 is able to fix the c-axis direction and suppress inver-sion domain formation. The six ^101I 0& directions are not allidentical if seen in the same crystal. Three of those faceswould give ABAB stacking and the other three ACAC stack-ing. This suggests random nucleation of the various ^101I 0&directions as a mechanism for the proposed stacking faultformation.Scanning electron microscopy ~SEM! studies of the finalgrowth surface of these wafers show regions with ‘‘plowedfield’’ features as shown in Fig. 3. These surface featuresconsistently match with crystal orientation with the stripesrunning parallel to the ~0002! planes. Our TEM studies haveshown these surface features to be polycrystalline GaN thathas ‘‘seeded’’ or decorated subsurface features. Figure 4shows a TEM image of a @12I 10# specimen ~looking alongthe rows! illustrating one of these surface features associatedwith a threading dislocation. This implies that the threadingdislocations are correlated and predominantly confined tolines that run perpendicular to the c axis. The initial nucle-ation of these correlated dislocations has not been seen and isnot understood. However, once formed they would be con-fined to move only perpendicular to the c axis by the ob-served stacking faults. A further manuscript to discuss thedislocations and their exact nature is in process.Because the positions of these dislocations are corre-lated, the exact location and direction in which theyare viewed is important. When viewing along the rows(@12I 10# direction! we expect to see dislocations that matchthe linear density of the rows. TEM estimates give 1.33104 dislocations/cm in reasonable agreement with SEM re-sults of 1.63104 rows/cm. The average spacing betweenrows is ;0.6 mm. From those TEM images and the approxi-mate thickness, the dislocation density per area is estimatedto be 33108 cm22. TEM specimens are 0.5 mm or less inthickness. When looking into the row or in the c direction,we expect to get significant variation depending upon wherethe TEM specimen was extracted. If the specimen is from arow, then we would expect to see a higher dislocation densitythan if the specimen is between the rows. When looking inthe c direction, with specimens from the plowed field re-gions, we have seen dislocation counts as high as 109 cm22to lower than 107 cm22.We have observed by TEM GaN grown on LiAlO2 withthe c axis in the plane of the specimen and the (101I 0)growth normal. Basal plane stacking faults are seen through-out with approximate spacing of 100 nm. Threading disloca-tions are seen that are correlated in their positioning, occur-ring in rows that are spaced ;0.6 mm apart.Note added in proof. Recently, Sun et al. have discussedstacking faults in M-plane GaN films grown by MBE onLiAIO2.18This work was partially supported by DARPA MTO un-der the SUVOS program with Joseph Lorenzo of Air ForceResearch lab as technical monitor.1 B. Cockayne and B. Lent, J. Cryst. Growth 54, 546 ~1981!.2 M. Marezio, Acta Crystallogr. 19, 396 ~1965!.3 X. Ke, X. Jun, D. Peizhen, Z. Yongzong, Z. Guoqing, Q. Rongsheng, andF. Zujie, J. Cryst. Growth 193, 127 ~1998!.4 K. Xu, J. Xu, P. Deng, R. Qiu, and Z. Fang, Phys. Status Solidi A 176, 589~1999!.5 P. Lefebvre, B. Gil, J. Allegre, H. Mathieu, N. Grandjean, M. Leroux, J.Massies, and P. Bigenwald, MRS Internet J. Nitride Semicond. Res. 4S1,G3 ~1999!.6 P. Waltereit, O. Brandt, M. Ramsteiner, A. Trampert, H. T. Grahn, J. Men-niger, M. Reiche, R. Uecker, P. Reiche, and K. H. Ploog, Phys. StatusSolidi A 180, 133 ~2000!.7 E. Kuokstis, C. Q. Chen, M. E. Gaevski, W. H. Sun, J. W. Simin, M. AsifKhan, H. P. Maruska, D. W. Hill, M. M. C. Chou, J. J. Gallagher, and B.Chai, Appl. Phys. Lett. 81, 4130 ~2002!.8 M. D. Craven, S. H. Lim, F. Wu, J. S. Speck, and S. P. DenBaars, Phys.Status Solidi A 194, 541 ~2002!.9 T. Detchprohm, K. Hiramatsu, H. Amano, and I. Akasaki, Appl. Phys.Lett. 61, 2688 ~1992!.10 R. J. Molnar, K. B. Nichols, P. Maki, E. R. Brown, and I. Melngailis,Mater. Res. Soc. Symp. Proc. 378, 479 ~1995!.11 N. R. Perkins, M. N. Horton, and T. F. Kuech, Mater. Res. Soc. Symp.Proc. 395, 243 ~1996!.12 M. Ilegems, J. Cryst. Growth 13Õ14, 360 ~1972!.13 L. A. Giannuzzi and F. A. Stevie, Micron 30, 197 ~1999!.14 M. D. Craven, S. H. Lim, J. S. Speck, J. Appl. Phys. ~to be published!.15 J. L. Rouviere, M. Arlery, B. Daudin, G. Feuillet, and O. Briot, Mater. Sci.Eng., B 50, 61 ~1997!.16 B. Daudin, J. L. Rouviere, and M. Arlery, Appl. Phys. Lett. 69, 2480~1996!.17 H. P. Maruska, D. W. Hill, M. M. C. Chou, J. J. Gallagher, and B. Chai,Opto-Electron. Rev. 11, 7 ~2003!.18 Sun, O. Brandt, U. Jahn, T. Y. Liu, A. Trampert, S. Cronenberg, S. Dhar,K. H. Ploog, J. Appl. Phys. 92, 5714 ~2002!.FIG. 3. SEM image of surface features.FIG. 4. Cross section showing surface features and their correlation withthreading dislocations.1141Appl. Phys. Lett., Vol. 83, No. 6, 11 August 2003 Vanfleet et al.

Defects in m-face GaN films grown by halide vapor phase epitaxy on LiAlO2

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Defects in m-face GaN films grown by halide vapor phase epitaxy on LiAlO2

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