GaN (0001) films grown by molecular beam epitaxy (MBE) were studied using scanning tunneling microscopy (STM). 'Ghost' islands were observed on surfaces grown under excess Ga conditions. These ghost islands were associated to a metastable, intermediate nucleation state of the surface.published_or_final_versio

Cheung, SH

Seutter, SM

Tong, SY

Xie, MH

Zheng, LX

Physical Review Letters

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Title Observation of 'ghost' islands and surfactant effect of surfacegallium atoms during GaN growth by molecular beam epitaxyAuthor(s) Zheng, LX; Xie, MH; Seutter, SM; Cheung, SH; Tong, SYCitation Physical Review Letters, 2000, v. 85 n. 11, p. 2352-2355Issued Date 2000URL http://hdl.handle.net/10722/42455Rights Creative Commons: Attribution 3.0 Hong Kong LicenseVOLUME 85, NUMBER 11 P H Y S I C A L R E V I E W L E T T E R S 11 SEPTEMBER 2000Observation of “Ghost” Islands and Surfactant Effect of Surface Gallium Atomsduring GaN Growth by Molecular Beam EpitaxyL. X. Zheng,1,2 M. H. Xie,1 S. M. Seutter,1 S. H. Cheung,1 and S. Y. Tong11Department of Physics, University of Hong Kong, Pokfulam Road, Hong Kong2State Key Laboratory on Integrated Optoelectronics, Institute of Semiconductors, Chinese Academy of Sciences, Beijing, China(Received 26 April 2000)We observe “ghost” islands formed on terraces during homoepitaxial nucleation of GaN. We attributethe ghost islands to intermediate nucleation states, which can be driven into “normal” islands by scanningtunneling microscopy. The formation of ghost islands is related to excess Ga atoms on the surface.The excess Ga also affect island number density: by increasing Ga coverage, the island density firstdecreases, reaching a minimum at about 1 monolayer (ML) Ga and then increases rapidly for coveragesabove 1 ML. This nonmonotonic behavior points to a surfactant effect of the Ga atoms.PACS numbers: 68.55.–a, 61.16.Ch, 68.35.Bs, 81.10.–hThe growth of GaN films has attracted much attentionbecause of applications that this material has in fabricatingintense blue/green light emitting diodes, lasers, and hightemperature/power electronic devices [1–3]. However, un-til recently, little is known of the microscopic details ofthe growth process. Recent experiments have shown thatfor the Ga-terminated face, i.e., GaN (0001), the “1 3 1”surface is terminated by up to 2 monolayers (ML) of Gaadatoms beyond the last bilayer. The top-layer excess Gaatoms are thought to exist in a completely disordered, fluidphase [4]. It has also been established that film growth un-der excess-Ga conditions generally results in better qualityfilms with a smoother surface morphology. Thus, it seemsthat the excess Ga atoms on the surface of a film play animportant, if not fully understood, role in affecting the mi-croscopic processes governing growth.The purpose of this Letter is to report a novel observa-tion of an intermediate nucleation state occurring at thesurface of the film. Using scanning tunneling microscopy(STM) to study GaN (0001) films grown by molecularbeam epitaxy (MBE), we observe “ghost” islands onsurfaces grown under excess Ga conditions. The ghostislands can be irreversibly driven into normal islands bysimply continuous imaging or altering scanning voltage ofthe STM (at no time does the tip touch the sample). Thus,we associate the ghost islands to a metastable, interme-diate nucleation state of the surface. We further establishthat the ghost islands appear only when there are excessGa atoms on the surface. An increase in N depositionchanges the ghost islands into normal islands. Details ofthe experimental conditions and these observations aregiven below.A second set of observations is that the island numberdensity during growth depends sensitively on the amountof excess Ga atoms on the surface. The island density firstdecreases with increasing excess Ga coverage, up to 1 ML.This indicates that in the less than 1 ML regime, excessGa atoms enhance diffusion of the arriving N atoms. How-2352 0031-90070085(11)2352(4)$15.00ever, at excess Ga coverages greater than 1 ML, the islanddensity rapidly increases. This result suggests a surfactant-mediated growth process. Surfactant-mediated growth isa recently discovered phenomenon where the growthmode can be changed from three-dimensional (3D) is-landing to 2D layer-by-layer growth [5,6]. It is achievedby depositing on a substrate 1 ML or more of a thirdmaterial, different from the substrate and the epilayer.During growth, this third material or “surfactant layer”will decrease the surface energy and thus float on topthrough an exchange action between atoms of the surfac-tant layer and the deposit. The presence of a surfactantlayer on the surface can change the growth kineticsfrom the traditional diffusion-limited process [7,8] to anexchange reaction-limited process [9–11]. Consequently,it leads to very different growth behaviors, one of which isan increase in the island number density. However, unlikethe conventional surfactant-mediated growth, the newobservation here is that the surfactant is itself a constituentcomponent of the GaN film. This novel process has theadvantage of improving a film’s quality during growth butavoiding undesirable incorporation of foreign (surfactant)atoms in the film.The experiments are conducted in a multifunctional ul-trahigh vacuum system containing, among other intercon-nected chambers, an MBE reactor and an STM system.The MBE chamber consisted of an effusive cell for Gasource and a radiofrequency plasma generator (Oxford Ap-plied Research, CARS-25) for N. Prior to GaN depositionat low temperatures, a thick GaN buffer layer is first grownon a SiC(0001) substrate at 650 ±C under the excess-Gacondition. The buffer layer grown under such conditionshas atomically flat surfaces with relatively large terraces.The film has the Ga polarity as judged by surface recon-struction observations [12,13]. The buffer layer growthis interrupted by closing the source shutters, and the sub-strate temperature is lowered to 400 500 ±C, conditionswhich promote 2D island nucleation [12] due to reduced© 2000 The American Physical SocietyVOLUME 85, NUMBER 11 P H Y S I C A L R E V I E W L E T T E R S 11 SEPTEMBER 2000diffusion length of adatoms on the surface. GaN growthis reinitiated at the low temperatures for a specified periodbefore the sample is thermally quenched and transferred tothe STM chamber for analysis. Room temperature STMexperiments are carried out at a tunneling current of 0.1 nAand a sample bias of 23.0 V, unless stated otherwise.Figure 1(a) shows a typical STM image of the sur-face following growth under excess Ga conditions andat low temperatures. It is seen that triangularly shapedislands populate the terraces of the buffer layer. Thefaster growth speeds of double-bonded bilayer edges ver-sus single-bonded bilayer edges result in the triangularlyshaped islands, as explained in an earlier paper [14]. Mea-surements of height differences by STM confirm that theterraces have single bilayer height differences and the tri-angular islands have single bilayer heights on the terracesthemselves. What is surprising in Fig. 1(a) is the pres-ence of ghost islands having low STM contrasts, espe-cially in the interior of the islands. The ghost islands havethe triangular shape, with the inside appearing “hollow”and surrounded by bright frames. Moreover, the ghostFIG. 1. STM images of GaN surface (a) immediately afterthe growth; (b) after reversing the gap voltage to 12.5 V; and(c) after restoring the original scanning condition as used in (a).The image sizes are 135 nm 3 75 nm for all three images.islands can be converted into normal, bilayer-height is-lands following continuous STM imaging with or with-out altering the imaging conditions. An example showingthe converted islands by simply changing the sample-tipvoltage is presented in Fig. 1(b). There is a clear one-to-one correspondence between the ghost and “normal”islands. The converted normal islands have the same is-land height (1 bilayer) and the distinct triangular shape. Itis further found that island conversion is irreversible; i.e.,once a ghost island has been converted to a normal island,it will remain normal and cannot be changed back to aghost through STM imaging. An STM image of the samesurface obtained immediately after Fig. 1(b) but using thesame imaging voltage as in Fig. 1(a) is shown in Fig. 1(c).In Figs. 1(b) and 1(c), some ghost islands have not yetbeen converted. However, with additional STM scanning,all ghost islands will be eventually converted into normalislands.Since the normal and ghost islands are both present andrandomly scattered in an STM image, we cannot attributethe ghosts to an STM artifact, although the scanning caninduce a conversion as demonstrated in Figs. 1(a)–1(c).Further experiments show that lowering the growthtemperature and increasing Ga coverage will increase thepopulation of ghost islands. Another interesting findingis that when growth is under the stoichiometric or excessN condition, only normal islands are observed by STM.Thus, we believe that the ghost islands are real surfacefeatures related to the presence of excess Ga atoms onthe surface. Because of the one-to-one correspondencebetween ghost and normal islands and the irreversibleconversion demonstrated above, we suggest that the ghostislands probably correspond to an intermediate nucleationstate involving Ga atoms. This intermediate nucleationstate is converted to a normal state by STM scanning. Wenow report experimental observations which suggest thatthe ghost islands play an important role in the growthkinetics of GaN films.To show this, we have conducted a series of experimentsin which Ga and N are deposited alternately rather than atthe same time. The same procedure, known as “atomiclayer epitaxy,” has been used to study the growth of II-VIcompounds [15]. Figure 2 shows STM images of surfacesfollowing different N exposures but having the same ex-cess Ga coverage. The starting surfaces are prepared bygrowing thick GaN buffer films under the identical excessGa condition and for the same time interval. It is interest-ing to observe that following a 0.14 ML deposition of N,the surface shows sizable islands; some are normal whilemost others are mixtures of normal islands formed inside“ghostlike” triangles [Fig. 2(a)]. By increasing the N ex-posure to 0.3 ML, almost all the islands are normal, asshown in Fig. 2(b). Further increase of the N exposureproduces little change in the characteristics of the islandscompared to that of Fig. 2(b), including the island arealdensity. Figures 2(c) and 2(d) are close-up STM images2353VOLUME 85, NUMBER 11 P H Y S I C A L R E V I E W L E T T E R S 11 SEPTEMBER 2000FIG. 2. STM images after exposing the Ga-terminated GaNsurfaces to N for (a) 0.14 ML and (b) 0.30 ML at 450 ±C. Theimage sizes are 200 nm 3 200 nm. (c),(d) Close-up images40 nm 3 40 nm of the surface similar to (a) showing mor-phological features of partially converted islands.of a surface similar to Fig. 2(a), which show detailed mor-phological features of the partially converted islands. InFig. 2(c), converted normal islands are seen residing ina large triangular area partially bordered by voids. Theislands appear fragmented. If the normal islands in the tri-angular areas are formed by converting a triangular ghostseen in Fig. 1(a), then Fig. 2(c) suggests a conversion pro-cess which occurs in parts rather than as a whole for theghost island. These observations indicate possibly differ-ent mechanisms between the conversion of ghost to normalislands by STM scanning versus N deposition. Figure 2(d)shows another morphological feature of the N-depositionconverted islands. The fractal-like normal island becomesdominant when the rate of deposition is lowered, in agree-ment with recent kinetic Monte Carlo (KMC) simulationswhen an exchange process between surfactant and depositis considered [11].Figure 3 shows a series of STM images of surfaces fol-lowing various Ga depositions but at the same N expo-sure. To achieve this, we first prepare GaN buffer films (at650 ±C) under the stoichiometric condition [12], followedby deposition of different amounts of Ga from the Knud-sen cell at low sample temperatures 400 500 ±C. Thesurfaces are then exposed to a fixed amount (0.25 ML)of N before thermal quenching for STM analysis. Fig-ures 3(a)–3(d) show the resulting surfaces for Ga cov-erages of 0.4, 1.0, 1.5, and 3 ML, respectively. Severaldistinct features are revealed: (1) For a Ga coverage of lessthan 1 ML [Fig. 3(a)], the majority of islands appear nor-mal, which contrasts the situation where the Ga coverage is2354FIG. 3. STM images of GaN surfaces after depositing(a) 0.4 ML, (b) 1 ML, (c) 1.5 ML, and (d) 3 ML Ga andfollowed by 0.25 ML deposition of N at 450 ±C. The imagesizes are 200 nm 3 200 nm for all images.above 1 ML, with most islands being either ghosts or par-tially converted normal-ghost mixtures [Figs. 3(b)–3(d)].(2) As the Ga coverage increases, the island density firstdecreases, reaching a minimum at about the 1 ML cover-age. After that, the island density increases at a rapid rateand saturates when the coverage is about 2 ML. The islanddensity versus excess Ga coverage is plotted in Fig. 4. Fea-ture (1) is consistent with the observation made in Figs. 1and 2; i.e., ghost and normal-ghost mixed islands formonly where there is excess Ga on the surface. Feature (2)shows that the excess Ga atoms behave as a surfactant thathas a profound effect on nucleation.FIG. 4. Island density dependence on excess Ga coverage. Theisland densities (filled circles) are calculated according to 1l2,where l is the island-island distance away from the region nearthe surface steps, therefore, excluding the effect of steps onnucleation. The error bars represent standard deviation of themean, while the solid line is drawn to guide the eye.VOLUME 85, NUMBER 11 P H Y S I C A L R E V I E W L E T T E R S 11 SEPTEMBER 2000Recent theoretical and KMC simulation studies [9,11] ofsurfactant-mediated growth have indicated modificationsto classical nucleation theory [7,8]. The surfactant atomscan block surface reaction sites; therefore, nucleationinvokes site exchange action between the deposited andsurfactant atoms. The exchange action requires overcom-ing an energy barrier leading to grossly different islanddensity distribution, as well as novel island shape transi-tion from fractal to compact [10,11]. Our observations oftriangularly shaped ghost islands and fractal-like normalor mixed islands are thus consistent with such predictions.It can be inferred that excess Ga atoms on a GaN surfaceact as surfactants for epitaxial growth. Indeed, it hasbeen observed that up to 2 ML of Ga exist and wet thesurface of SiC substrates [16] and GaN epifilms [4]. Inorder for GaN epitaxy to proceed, the incoming N atomsmust exchange sites with the surfactant Ga atoms. Suchan exchange can be hindered at low temperatures by anexchange barrier. According to this scenario, when thesurfactant Ga layer is less than a certain coverage, thedeposited N atoms can easily “see” the GaN surface andgrowth proceeds without invoking site-exchange action.Classical nucleation theory applies and the presence ofexcess surface Ga atoms enhances surface diffusion, lead-ing to fewer islands being nucleated in this regime. Thisexplains the initial decrease of island density versus Gacoverage up to this critical value. When the Ga coverageis above the critical value, the GaN surface becomes fully“hidden” and the nucleation is limited by a “surfactant-deposit” exchange rate. This manifests in an increase inthe island density, as shown in Fig. 4. The increase isexplained by the fact that already-nucleated islands maynot effectively trap the surface diffusing adatoms, becauseof the exchange barrier. Adatoms then have more chancesto form new islands near preexisting islands. As suggestedby Kandel [9], adatoms do not see the step edges of anexisting island for incorporation; therefore, they nucleatenew islands on the terraces, resulting in more islands.From Fig. 4, the critical coverage is about 1 ML, althoughthe exact value is not accurately determined due to thelack of data points between 0.5–1 ML.To summarize, nucleation of GaN during MBE growthshows a novel behavior under the excess Ga growth con-dition. STM images show ghost islands and these are at-tributed to an intermediate nucleation state. The overallisland density is found to depend critically on the surfaceGa coverage. The results point to a surfactant effect ofthe excess Ga atoms. Depending on the coverage, the ki-netics are different: at less than 1 ML coverage, classicalnucleation theory applies; however, above 1 ML, exchangeand incorporation rates of adatoms limit growth. The tworegimes result in a nonmonotonic behavior of the islanddensity versus Ga coverage. From a practical viewpoint,the surfactant effect of Ga atoms may be utilized to bettercontrol growth, thereby improving the films’ quality. How-ever, unlike traditional surfactant-mediated growth, wherea third material is added to the growing surface, Ga is aconstituent component of GaN, thus having the added ad-vantage of avoiding contamination by surfactant atoms infilms. Finally, we point out that at high temperatures ofMBE, the growth mode becomes step-flow and the sur-factant effect observed at low temperatures becomes lessevident.The authors thank Ho Win kin for technical support,S. J. Xu, H. S. Wu, and X. Q. Dai for fruitful discussions.This study is supported in part by HK RGC GrantsNo. HKU7118/98P, No. 7142/99P, and No. 260/95P.[1] S. Strite and H. Morkoc, J. Vac. Sci. Technol. B 10, 1237(1992).[2] S. Nakamura, T. Mukai, and M. Senish, Appl. Phys. Lett.64, 1687 (1994).[3] I. Akasaki, H. Amono, S. Sota, H. Sakai, T. Tanaka, andM. Koike, Jpn. J. Appl. Phys. 34, L1517 (1995).[4] A. R. Smith, R. M. Feenstra, D. W. Greve, M. S. Shin,M. Skowronski, J. Neugebauer, and J. E. Northrup, J. Vac.Sci. Technol. B 16, 2242 (1998).[5] M. Copel, M. C. Reuter, E. Kaxiras, and R. M. Tromp,Phys. Rev. Lett. 63, 632 (1989).[6] M. H. von Hoegen, F. K. Legoues, M. Copel, M. C. Reuter,and R. M. Tromp, Phys. Rev. Lett. 67, 1130 (1991).[7] J. A. Venables, G. D. T. Spiller, and M. Hanbücken, Rep.Prog. Phys. 47, 399 (1984).[8] M. C. Bartelt and J. W. Evans, Phys. Rev. B 46, 12 675(1992).[9] D. Kandel, Phys. Rev. Lett. 78, 499 (1997).[10] T.-C. Chang, I.-S. Hwang, and T. T. Tsong, Phys. Rev. Lett.83, 1191 (1999).[11] B.-G. Liu, J. Wu, E. G. Wang, and Z. Zhang, Phys. Rev.Lett. 83, 1195 (1999).[12] S. M. Seutter, M. H. Xie, W. K. Zhu, L. X. Zheng,Huasheng Wu, and S. Y. Tong, Surf. Sci. 445, L71 (2000).[13] A. R. Smith, R. M. Feenstra, D. W. Greve, M.-S. Shin,M. Skowronski, J. Neugebauer, and J. E. Northrup, Appl.Phys. Lett. 72, 2114 (1998).[14] M. H. Xie, S. M. Seutter, W. K. Zhu, L. X. Zheng,Huasheng Wu, and S. Y. Tong, Phys. Rev. Lett. 82, 2749(1999).[15] M. Ahonen, M. Pessa, and T. Suntola, Thin Solid Films65, 301 (1980).[16] L. X. Zheng, M. H. Xie, and S. Y. Tong, Phys. Rev. B 61,4890 (2000).2355

Observation of 'ghost' islands and surfactant effect of surface gallium atoms during GaN growth by molecular beam epitaxy

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Observation of 'ghost' islands and surfactant effect of surface gallium atoms during GaN growth by molecular beam epitaxy

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