Molecular-beam epitaxy of Inx Ga1-x N alloy on GaN(0001) is investigated by scanning tunneling microscopy. The Stranski-Krastanov mode of growth of the alloy is followed, where the newly nucleated three-dimensional islands are initially coherent to the underlying GaN and the wetting layer, but then become dislocated when grown bigger than about 20 nm in the lateral dimension. Two types of islands show different shapes, where the coherent ones are cone shaped and the dislocated ones are pillar like, having flat-tops. Within a certain range of material coverage, the surface contains both coherent and dislocated islands, showing an overall bimodal island-size distribution. The continued deposition on such surfaces leads to the pronounced growth of dislocated islands, whereas the sizes of the coherent islands change very little. © 2005 The American Physical Society.published_or_final_versio

Cao, YG

Liu, Y

Tong, SY

Wu, HS

Xie, MH

Physical Review B

HKU Scholars Hub

Title Coherent and dislocated three-dimensional islands of Inx Ga1-xN self-assembled on GaN(0001) during molecular-beam epitaxyAuthor(s) Liu, Y; Cao, YG; Wu, HS; Xie, MH; Tong, SYCitation Physical Review B - Condensed Matter And Materials Physics,2005, v. 71 n. 15Issued Date 2005URL http://hdl.handle.net/10722/43469Rights Creative Commons: Attribution 3.0 Hong Kong LicenseCoherent and dislocated three-dimensional islands of InxGa1−xN self-assembled on GaN(0001)during molecular-beam epitaxyY. Liu, Y. G. Cao, H. S. Wu, and M. H. Xie*Physics Department and the HKU-CAS Joint Laboratory on New Materials, The University of Hong Kong, Pokfulam Road,Hong Kong, ChinaS. Y. TongDepartment of Applied Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong KongsReceived 24 August 2004; revised manuscript received 1 November 2004; published 26 April 2005dMolecular-beam epitaxy of InxGa1−xN alloy on GaNs0001d is investigated by scanning tunneling micros-copy. The Stranski-Krastanov mode of growth of the alloy is followed, where the newly nucleated three-dimensional islands are initially coherent to the underlying GaN and the wetting layer, but then becomedislocated when grown bigger than about 20 nm in the lateral dimension. Two types of islands show differentshapes, where the coherent ones are cone shaped and the dislocated ones are pillar like, having flat-tops. Withina certain range of material coverage, the surface contains both coherent and dislocated islands, showing anoverall bimodal island-size distribution. The continued deposition on such surfaces leads to the pronouncedgrowth of dislocated islands, whereas the sizes of the coherent islands change very little.DOI: 10.1103/PhysRevB.71.153406 PACS numberssd: 68.55.2a, 61.14.Hg, 81.15.HiThe III-V nitrides continue to attract intensive interest dueto their promises in optoelectronic applications.1 By adjust-ing the indium sInd content x in InxGa1−xN alloys, the energyband gap of the material can be tuned from 0.7 eV to 3.4 eV,so the band-edge emission covers the whole visiblespectrum.1,2 However, due to a large lattice mismatch se.g.,,10% between InN and GaNd, the epitaxial growth ofInGaN on GaN has been difficult. The lattice mismatch strainleads to the Stranski-Krastanov sSKd growth mode, wherethree-dimensional s3Dd islands nucleate following an initialtwo-dimensional s2Dd wetting-layer formation. Dependingon the amount of strain, the SK islands can be coherent to thelattice of the substrate, or they may be dislocated.3,4 Thedriving force for the spontaneous formation of the 3D islandsis to accommodate or relieve the strain in the system. Thecoherent islands of semiconductors have drawn special atten-tion, as they may act as the “quantum dots” in modern opto-electronic and/or microelectronic devices.5 In contrast toother semiconductor systems, the heteroepitaxial growthmode and island characteristics of nitrides remain poorlycharacterized so far.In this paper, we follow the SK growth of the InGaN alloyon GaNs0001d during molecular-beam epitaxy sMBEd. Thenewly nucleated islands are shown to be cone-shaped andcoherent to the underlying GaN. As they grow bigger, theislands become dislocated. Over a range of material cover-age, both coherent and dislocated islands exist on the samesurface, showing an overall bimodal size distribution. Insuch situations, continued deposition results in the pro-nounced growth of the dislocated islands, whereas the small,coherent islands change little in size.The experiments were conducted in a multichamber ultra-high vacuum sUHVd system, where the MBE reactor andsome surface analysis tools were connected via vacuum in-terlocks. In the MBE chamber, the effusion cells for galliumsGad and In were installed together with a radio-frequencysrfd plasma unit for nitrogen sNd. Before depositing theInGaN alloy, a thick sø1 mmd GaN layer was grown on6H-SiCs0001d at about 600 °C using an excess Ga flux,6which then acted as the pseudosubstrate for subsequent In-GaN deposition at a lower temperature of 390 °C. Thesample heating was achieved by flowing a direct currentthrough the long side of the rectangular sample piece, andthe temperature was measured by a focus infrared pyrometer.During the alloy deposition, the flux ratio between Ga and Nwas 0.3, while that between In and N was 0.9. The growthrate of the alloy was ,0.03 bilayers per second sBLs/sd ac-cording to the reflection high-energy electron-diffractionsRHEEDd intensity oscillation measurements. The In compo-sition x of the alloy was about 70% according to the mea-surements of the strain and the estimation from the sourcefluxes.7,8 After depositing a nominal thickness of the film, thegrowth was stopped and the sample quenched by switchingoff the heating current. Then the surfaces were examined byscanning tunneling microscopy sSTMd in an adjacent UHVchamber. A constant-current mode of STM was conducted atroom temperature under a sample bias of −2.0 V and a tun-neling current of 0.1 nA.Figure 1 shows a set of STM images depicting the evolu-tion of surfaces as the deposition of InGaN proceeds. At thenominal coverage of 2.1 bilayers sBLsd, no 3D islands areobserved fFig. 1sadg. Rather, the surface contains triangularlyshaped 2D islands that are 1 BL high, suggesting the growthto be 2D nucleation at this stage. At the coverage of 2.5 BLsfFig. 1sbdg, the surface starts to show minute 3D islands,signifying the 2D to 3D transition and thus the SK growthmode. Upon depositing 3.3 BLs fFig. 1scdg, the surfaceshows some large 3D islands in addition to the small ones.The densities of both large and small islands increases as thedeposition continues fFig. 1sdd for 7 BLsg until it reaches theaggregation regime, where the existing islands coarsen, andthe overall density of the islands remains unchanged.PHYSICAL REVIEW B 71, 153406 s2005d1098-0121/2005/71s15d/153406s4d/$23.00 ©2005 The American Physical Society153406-1The large and small islands identified above show dis-tinctly different shapes, as illustrated by the line profiles inFig. 2. The small islands are cone shaped with circular bases.On the other hand, the large islands are pillarlike, having flattops. The bases of the large islands are predominantly hex-agonal. A line-profile analysis of many of the islands doesnot suggest faceting for the cone-shaped islands. The data forthe island aspect ratio fsee Fig. 3sbd belowg are very scat-tered, indicating the kinetic nature of the island shape selec-tion. The pillar-shaped islands, on the other hand, arebounded by s0001d planes at the top. For the sidewalls, theSTM measurements at different scan lengths and speeds in-dicate that they are likely vertical h112¯0j and h101¯0j planes,though the convoluted STM line profiles all show nonverti-cal edges. We further measured the island heights H andlateral sizes L, and the results are summarized in Figs. 3sadand 3sbd for the height and aspect ratio H /L, respectively.From Fig. 3sad, one observes that for the cone-shaped is-lands, the height increases rapidly with increasing lateralsize, whereas for the pillar-shaped islands, the heights varyslowly with the lateral dimension. From Fig. 3sbd, one notesthat for the pillar-shaped islands, the aspect ratio decreasescontinuously with increasing lateral size, which is not obvi-ous for the cone-shaped islands. All of these differences, webelieve, reflect the very different characters of the two is-lands.Previous studies on the characteristics of the SK islandsshowed that the island aspect ratio increases continuouslywith size for coherent and unfacetted islands.9,10 For facettedislands, which is generally the case for crystalline semicon-ductors, the shape transition may take place at some criticalsize, above which island shapes with larger aspect ratios be-come favorable.4,11–13 The latter was verified experimentallyfor Ge/Si and InAs/GaAs systems.14,15 Therefore, the de-creasing trend of the aspect ratio seen in Fig. 3sbd for thepillar-shaped InGaN islands is inconsistent with these previ-ous investigations. However, in a recent study of the het-eroepitaxial growth of binary InN on GaNs0001d, a similardecreasing trend of island aspect ratio was reported.16,17 Inthat case, the measurements of strain by in situ RHEED in-dicated a partial relaxation of the misfit strain by defects inthe islands.18 Taking this factor into account, a decreasingisland aspect ratio with size is less surprising.16 Indeed forsuch islands, the strain-relieving defects diminish the drivingpotential for the atoms’ up-diffusion, and so the increase inthe island height becomes gradually arrested. Based on thisanalysis, we infer that the cone-shaped InGaN islands, whichare nucleated initially following the completion of the wet-ting layer, are coherent SK islands, whereas the large pillar-shaped islands are dislocated upon growing above a criticalsize s,20 nm in the lateral dimensiond.While the dynamic and kinetic details of the island shapechange from cones to pillarlike are unclear, the stagnation ofthe island height growth for the large dislocated islands isevident in Fig. 3sad. At this stage, continued growth in thelateral dimension naturally creates a plateau at the island top.A strong anisotropy in the growth rate sets in between thevertical and lateral directions. By measuring the island di-mension following the deposition of different coverages, thedegree of growth-rate anisotropy is estimated. The resultsshow that the lateral growth is about 5 times faster than thegrowth in the vertical direction.Lastly, Figs. 4sad and 4sbd plot the size distributions of theislands following 10 and 13 BL materials deposition, respec-tively. For both coverages, the two types of islands coexistFIG. 1. STM images of surfaces following InxGa1−xN depositionon GaNs0001d for the nominal thicknesses of sad 2.1 BLs, sbd 2.5BLs, scd 3.3 BLs, and sdd 7.0 BLs. Image sizes: 2003200 nm2 forsad–scd and 5003500 nm2 for image sdd.FIG. 2. sad A prospective STMimage showing 3D InGaN islandsformed on GaNs0001d. sbd and scdare line profiles depicting theshape of small and large islands,respectively.BRIEF REPORTS PHYSICAL REVIEW B 71, 153406 s2005d153406-2on surfaces. From these distribution curves, one firstly reaf-firms the vastly different sizes corresponding to the two is-land shapes. The overall size distributions are bimodal. Simi-lar bimodal distribution curves have been recorded in otherheteroepitaxial systems as well,15,19,20 which were associatedwith the shape change of islands.20,21 In our case, since thecone-shaped islands are coherent while the pillarlike islandsare defected, the two islands show different growth rates.According to Refs. 22 and 23, if coherent and dislocatedislands coexisted on the surface, the latter would grow fasterthan the former. This is understood by the fact that thegrowth of coherent islands leads to an increase of the strainenergy; whereas for dislocated islands, since the lattice mis-match strain has been mostly relieved by defects, the growthof such islands will clearly be energetically preferred.24 Theresult of Fig. 4, where the increase in material coverage hasled to the significant growth of the pillar-shaped islands butlittle change for the cone-shaped islands, is consistent withthis expectation.To summarize, in the SK growth of InxGa1−xN onGaNs0001d, the initial 3D islands are coherent, having aconical unfacetted shape. When they have grown to above acritical size sø20 nm in lateral dimensiond, defects are intro-duced in the islands. The shape of the islands also changes toa pillarlike shape, developing flat tops. At this stage, a stronganisotropy in the growth rate in the lateral versus the verticaldirections is noted, which gives rise to a decreased aspectratio with an increasing island size. When both coherent anddislocated islands are present on the same surface, furtherdeposition leads to the rapid growth of the dislocated islands,while the coherent islands remain little changed in size.We would like to acknowledge technical support from W.K. Ho. The project is financially supported by grants fromthe Research Grants Council of the Hong Kong Special Ad-ministrative Region, China sProject Nos. HKU7118/02P,7035/03Pd. M.H.X. also acknowledges support from the Na-tional High-Tech Research and Development s863d Programof China under Grant No. 2003AA311060.*Electronic address: mhxie@hkusua.hku.hk1 S. Strite and H. Morkoc, J. Vac. Sci. Technol. B 10, 1237 s1992d.2 A. G. Bhuiyan, A. Hashimoto, and A. Yamamoto, J. Appl. Phys.94, 2779 s2003d.3 J. W. Matthews, D. C. Jackson, and A. Chambers, Thin SolidFilms 26, 129 s1975d.4 C. Ratsch and A. Zangwill, Surf. Sci. 293, 123 s1993d.5 D. Bimberg, M. Grundmann, and N. N. Ledentsov, Quantum DotHeterostructures sJohn Wiley & Sons, Chichester, UK, 1999d.6 S. M. Sean, M. H. Xie, W. K. Zhu, L. X. Zheng, H. S. Wu, and S.FIG. 3. sad Island height H and sbd aspect ratio H /L, plotted asa function of the lateral dimension L.FIG. 4. Histograms showing the distribution of sizes for thecone-shaped and pillarlike islands following nominal sad 10-BL andsbd 13-BL material deposition. The curves represent the Gaussianfits to the distributions.BRIEF REPORTS PHYSICAL REVIEW B 71, 153406 s2005d153406-3Y. Tong, Surf. Sci. 445, L71 s2000d.7 Y. Liu, Y. G. Cao, H. S. Wu, M. H. Xie, and S. Y. Tong, J. Appl.Phys. 97, 023502 s2005d.8 T. Böttcher, S. S. Einfeldt, V. Kirchner, S. Figge, H. Heinke, D.Hommel, H. Selke, and P. L. Ryder, Appl. Phys. Lett. 73, 3232s1998d.9 H. T. Johnson and L. B. Freund, J. Appl. Phys. 81, 6081 s1997d.10 B. J. Spencer and J. Tersoff, Phys. Rev. Lett. 79, 4858 s1997d.11 L. G. Wang, P. Kratzer, N. Moll, and M. Scheffler, Phys. Rev. B62, 1897 s2000d.12 I. Daruka, J. Tersoff, and A.-L. Barabási, Phys. Rev. Lett. 82,2753 s1999d.13 J. Tersoff and R. M. Tromp, Phys. Rev. Lett. 70, 2782 s1993d.14 H. Saito, K. Nishi, and S. Sugou, Appl. Phys. Lett. 74, 1224s1999d.15 G. Medeiros-Ribeiro, A. M. Bratkovski, T. I. Kamins, D. A. A.Ohlberg, and R. S. Williams, Science 279, 353 s1998d.16 Y. G. Cao, M. H. Xie, Y. Liu, Y. F. Ng, H. S. Wu, and S. Y. Tong,Appl. Phys. Lett. 83, 5157 s2003d.17 Y. G. Cao, M. H. Xie, Y. Liu, S. H. Xu, Y. F. Ng, H. S. Wu, andS. Y. Tong, Phys. Rev. B 68, 161304sRd s2003d.18 Y. F. Ng, Y. G. Cao, M. H. Xie, X. L. Wang, and S. Y. Tong,Appl. Phys. Lett. 81, 3960 s2002d.19 S. Anders, C. S. Kim, B. Klein, M. W. Keller, R. P. Mirin, and A.G. Norman, Phys. Rev. B 66, 125309 s2002d.20 F. M. Ross, J. Tersoff, and R. M. Tromp, Phys. Rev. Lett. 80, 984s1998d.21 R. E. Rudd, G. A. D. Briggs, A. P. Sutton, G. Medeiros-Ribeiro,and R. S. Williams, Phys. Rev. Lett. 90, 146101 s2003d.22 M. Krishnamurthy, J. S. Drucker, and J. A. Venables, J. Appl.Phys. 69, 6461 s1991d.23 J. Drucker, Phys. Rev. B 48, 18 203 s1993d.24 S. C. Jain, J. R. Willis, and R. Bullough, Adv. Phys. 39, 127s1990d.BRIEF REPORTS PHYSICAL REVIEW B 71, 153406 s2005d153406-4

Coherent and dislocated three-dimensional islands of Inx Ga1-x N self-assembled on GaN(0001) during molecular-beam epitaxy

http://hub.hku.hk/bitstream/10722/43469/1/97951.pdf

Coherent and dislocated three-dimensional islands of Inx Ga1-x N self-assembled on GaN(0001) during molecular-beam epitaxy

Abstract

Similar works

Full text

Available Versions

HKU Scholars Hub