A. G. Cullis

A. Nylandsted Larsen

D. E. Jesson

D. J. Eaglesham

F. Jensen

F. Jonsdottir

H. Sunamura

J. Lundsgaard Hansen

J. Tersoff

J. W. P. Hsu

J. Wulff Petersen

K. Häusler

K. M. Chen

L. J. Gray

M. A. Lutz

M. Albrecht

Q. Xie

S. Christiansen

S. P. Baker

S. Yu. Shiryaev

Y.-W. Mo

New capabilities of misfit dislocations for spatial manipulation of islands in Si1-xGex/Si heteroepitaxial systems have been elucidated. Formation of highly ordered Ge-island patterns on substrates prestructured by slip bands of misfit dislocations is revealed. The major sources leading to the ordering are identified to be dislocation strain fields at the surface and modifications of the nearsurface-layer morphology induced by dislocation slip

Shiryaev, S.Y.

Jensen, Flemming

Hansen, J. Lundsgaard

Petersen, Jon Wulff

Larsen, Arne Nylandsted

Online Research Database In Technology

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Lundsgaard; Petersen, Jon Wulff; Larsen, ArneNylandstedPublished in:Physical Review LettersLink to article, DOI:10.1103/PhysRevLett.78.503Publication date:1997Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Shiryaev, S. Y., Jensen, F., Hansen, J. L., Petersen, J. W., & Larsen, A. N. (1997). Nanoscale Structuring byMisfit Dislocations in Si1-xGex/Si Epitaxial Systems. Physical Review Letters, 78(3), 503-506. DOI:10.1103/PhysRevLett.78.503VOLUME 78, NUMBER 3 P HY S I CA L REV I EW LE T T ER S 20 JANUARY 1997Nanoscale Structuring by Misfit Dislocations in Si12xGexySi Epitaxial SystemsS. Yu. Shiryaev,1 F. Jensen,2 J. Lundsgaard Hansen,1 J. Wulff Petersen,2 and A. Nylandsted Larsen11Institute of Physics and Astronomy, University of Aarhus, DK-8000 Aarhus C, Denmark2Mikroelektronik Centret, Building 345 East, The Technical University of Denmark, DK-2800 Lyngby, Denmark(Received 22 March 1996)New capabilities of misfit dislocations for spatial manipulation of islands in Si12xGexySiheteroepitaxial systems have been elucidated. Formation of highly ordered Ge-island patterns onsubstrates prestructured by slip bands of misfit dislocations is revealed. The major sources leadingto the ordering are identified to be dislocation strain fields at the surface and modifications of the near-surface-layer morphology induced by dislocation slip. [S0031-9007(96)01997-7]PACS numbers: 68.55.–a, 61.16.Bg, 61.16.Ch, 61.72.LkThe interplay between surface morphology and misfitdislocations in heteroepitaxial semiconductor systemshas recently been a subject of numerous studies [1]. Itmanifests itself in a diverse range of phenomena, suchas formation of gradual surface undulations (cross-hatchpatterns) arising from the surface mass transport drivenby the strain fields of the misfit dislocations [2–4],elastic [5] and plastic [6] displacements of the surface bymisfit dislocations, and dislocation nucleation at surface-ripple structures in continuous heteroepitaxial layers[7–10]. Alignment of the surface-ripple domains alongdislocation lines was observed in Si12xGexySi hetero-epitaxial systems and attributed to ripple-dislocation in-teractions mediated by strain [7,10]. An ordering of InPislands on a strained InGaP buffer was found in Ref. [11]and explained by a preferential island nucleation drivenby the misfit-dislocation strain. In this Letter we reporton a new phenomenon—formation of highly ordered pat-terns of isolated Ge islands on SiySi12xGexySi substratesprestructured by the introduction of misfit dislocations.An outstanding feature of these patterns is that theyare almost entirely composed of well-separated rows ofsharply aligned Ge islands and mimic the underlyingdislocation-slip-band morphology.The basic idea of our experiments is illustrated inFig. 1. A heteroepitaxial system is grown consisting ofa compositionally graded Si12xGex layer on a (001) Sisubstrate and—on top of that—a uniform Si12xGex layercomprising a quantum well (QW) used as a marker layer,and finally a silicon cap layer. The structure is metastableand has been shown to relax during postgrowth annealingthrough a self-organized process leading to the formationof very narrow, shear bands of misfit dislocations in thegraded layer [12]. This has dramatic consequences forthe initial structure, such as the following: (i) formation ofshear steps both on the surface and at the heterointerfacesin the bulk of the epitaxial system and (ii) appearance ofa surface, elastic-strain pattern associated with the highlynonuniform distribution of the misfit dislocations in thegraded layer. After relaxation, Ge is deposited on thesurface to study the effect of the surface morphology andsurface strain on the growth of three-dimensional (3D) Geislands on Si.The samples were grown at 530 –C by molecular-beamepitaxy (MBE) and included graded layers identical tothose used in our previous work [13]. The top uniformSi0.8Ge0.2 layer included a 5 nm thick, Si0.6Ge0.4 QW and a10 nm thick Si cap. The relaxation of the mismatch strainin the samples was achieved directly in the MBE chamberby flashing the substrate temperature up to 800 –C for 15 s.After the flash the temperature was reduced to 740 –C and aGe film with a nominal thickness of 1.2 nm was depositedon top of the relaxed structure, at a deposition rate of0.02 nmys. For comparison, the same amount of Ge wasalso deposited on the surface of a virgin Si substrate and onthe surface of the above heterostructure without a Si cap.The atomic force microscopy (AFM) images displayedin Fig. 2 illustrate the results of the Ge deposition onthe surface of a virgin (001)Si wafer (a) and of prestruc-tured Si12xGexySi (b) and Si-capySi12xGexySi substrates(c). Well-defined Ge islands are resolved in all the im-ages indicating 3D growth in all the cases. The islandsare randomly distributed on the surface of the Si substrate[Fig. 2(a)] in agreement with previous observations of Geislanding on (001) Si (see, for example, Ref. [14]). Incontrast to this structure, ordered Ge-island patterns areFIG. 1. Schematic diagram illustrating the structuring ofSiySi12xGexySi substrate with dislocations in a compositionallygraded layer. The dashed lines illustrate the regions of dislo-cation slip. C and QW denote a Si cap and a quantum well,respectively.0031-9007y97y78(3)y503(4)$10.00 © 1997 The American Physical Society 503VOLUME 78, NUMBER 3 P HY S I CA L REV I EW LE T T ER S 20 JANUARY 1997FIG. 2. AFM image of a 7.7 3 7.7 mm2 surface area show-ing the result of deposition of a 1.2 nm thick Ge film on the sur-face of a bare s001d Si substrate (a) and patterned Si12xGexySi(b) and SiySi12xGexySi (c) systems. The height scale (blackto white) is (a) 41 nm, (b) 53 nm band, and (c) 63 nm, respec-tively. A height profile (d) through an island in (c) along theline P-Q is shown.found on the surfaces modified by the misfit dislocations[Figs. 2(b) and 2(c)]. The islands formed on the surfaceof bare Si0.8Ge0.2 [Fig. 2(b)] are distributed dominantly inrelatively wide straps oriented along k110l surface direc-tions. Although a weak geometrical correlation can beseen on this surface (a part of the islands accumulatesaround the transition regions between elevated and de-pressed areas of the surface), the islands do not preferen-tially populate the depressions or elevations.A spectacular effect is observed on the surface of theheterostructure with the Si cap. The island pattern foundin this case [Fig. 2(c)] mimics the slip morphology of thesurface and almost all (about 90%) Ge islands are alignedin sharp rows parallel to the k110l surface directions.Height profiles through the islands [see Fig. 2(d)] revealthat the islands are located at the edges of the shear steps.Furthermore, in case (a) the fraction of the surface areacovered with the islands (18%) is much higher than that incase (c) (8%). Note also that the island rows in case (c)are separated by large rectangular areas completely freeof islands. These observations suggest that Ge from theseareas redistributes to the islands in the rows [15] leavingthe Ge-film thickness in these areas in the range of thewetting-layer thickness (ø3 ML [16]).The spatial relationship between the Ge islands andthe misfit dislocations in the sample shown in Fig. 2(c)becomes apparent in cross-sectional TEM images (Fig. 3).The Ge islands are now seen to be localized in thevicinity of the slip regions on the surface (the so calledFIG. 3. Cross-sectional TEM images of the sample shown inFig. 2(c). (a) Rows (a, b, and c) of Ge islands are aligned alongthe f110g surface direction. The sample is tilted off the f110gzone axis to visualize the (001) surface thereby displaying therows of Ge islands. (b) Enlarged TEM image of one island (Fand G mark the steps produced by a shear band on, respectively,the Si0.6Ge0.4 marker, and the interface between the Si capand Si0.8Ge0.2). The dashed lines indicate h111j slip planescorresponding to particular bands in the graded layer.band-yield regions). In Fig. 3(a) island rows a, b, and cpropagate along the f110g-surface directions perpendicularto the cross-sectional plane and are positioned at the yieldregions of, respectively, bands A, B, and C composed ofdislocations gliding on the (111) (A) and s111d (B and C)planes perpendicular to the cross-sectional plane.Figure 3(b) is a magnified image of one island and theunderlying structure. The yield region of the slip bandcorresponding to this island is defined in Fig. 3(b) bythe (111) plane intersecting slip step F on the Si0.6Ge0.4marker and slip step G at the interface between the Si capand the underlying Si0.8Ge0.2 layer. It can be recognizedthat the position of the Ge island is displaced from theband-yield region to the elevated part of the step structure.Detailed examination of cross-sectional images showsthat this spatial relationship is characteristic of islandsaligned along the band-yield regions. This observation isconsistent with the AFM images where such a relationshipis found for a number of Ge rows [marked by arrows inFig. 2(c)]. On the other hand, inspection of the cross-sectional images of the noncapped samples revealed nosuch spatial correlation between the islands and slip-bandregions; the islands in the straps were found at both sidesof these regions. In addition, a heavy strain contrast wasfound in both the islands and the substrate just below theislands for all the samples shown in Fig. 2. Hence, weconclude that they belong to the category of islands whichare partially relaxed through elastic displacements in boththe island and the substrate [14].504VOLUME 78, NUMBER 3 P HY S I CA L REV I EW LE T T ER S 20 JANUARY 1997A characteristic feature of the capped samples is that theSi-cap thickness is locally decreased at the elevated sidesof the bulk slip steps [G in Fig. 3(b)] and locally increasedat the opposite sides. In the areas remote from the band-yield regions this thickness reaches its nominal value of10 nm. We found that this modification of the Si-caplayer occurs during annealing in the MBE chamber beforethe Ge deposition, and attribute this effect to a surfaceredistribution of Si which results in a smoothing of thesharp slip-step bunches highly energetically unfavorableon the free surface.In general, there are several sources which may provideenergetically favorable sites for island location on the sur-face. They include surface steps [17,18], local deviationsof the surface from planar orientation [19], and the nonuni-form strain at the surface which provides the sites with areduced mismatch between Ge and Si [10,20,21]. The is-land positions found in our case do not coincide with theslip steps on the surface either for noncapped or cappedsamples. Therefore, we conclude that the island orderingobserved in the present work is not mediated by the slipsteps. Furthermore, the islands in the noncapped samplesdo not preferentially populate the elevations or depressionson the surface. Hence, the only source which can inducethe highly nonuniform island distribution in this case is thenonuniform elastic strain from the dislocation bands. Thisconclusion is supported by the results of recent experimen-tal and theoretical studies of the vertical self-alignment ofislands in multilayered heteroepitaxial systems [20,21] andalignment of InP islands on strained InGaP buffers alongdislocation bunches [11]. Although these studies focus onthe different aspects of the island formation process (eitherisland-nucleation events [20] or surface diffusion leadingto a nonuniform thickness of the wetting layer [21]), theyimply a common driving force for the island alignmentwhich is nonuniform strain at the surface.The above results show that the presence of the Sicap has a dramatic influence on the island pattern whichmanifests itself in the extremely sharp island alignmentalong the band-yield regions. There is also a significantdifference in the island-size distributions for the cappedand noncapped samples [see Figs. 2(b) and 2(c)]. In thiswork we focus on the most striking effect which is thesharp alignment. To explain this effect we shall considerthe near-surface layer structure which includes the Si capand the interface between the cap and the Si0.8Ge0.2 layer.There are two features of this structure that may contributeto such an alignment. The first feature is the sharp stepsat the interface between the tensile-strained Si cap andthe relaxed Si0.8Ge0.2 layer [G in Fig. 3(b)]. Becausethese slip steps perturb the biaxial strain between theSi cap and Si0.8Ge0.2, and lie very close to the surface,they induce local surface-strain variations and, thus, mayprovide low-energy sites for the islands. The secondfeature is the local variation in the Si cap thickness aroundthe band-yield regions. It appears that the positions of theislands correspond to those sides of the regions where thethickness of the Si cap film is smaller [Fig. 3(b)]. Thiscorrelation suggests that the substrate sites with a thinnerSi cap may be energetically more favorable for the islandsthan the sites with a thicker Si cap. A reason for thismay be that the substrate in these regions is “softer,” i.e.,it allows larger elastic deflections of the GeySi interfacefrom the planar orientation providing a higher degree ofthe elastic relaxation in the islands. This suggestion isbased on recent calculations which show that the elasticdisplacement fields originating from islands penetrateto substrate depths larger than the island heights [22].Therefore, since the Si cap thickness in our case is smallerthan the island heights, both the Si cap and underlyingSi0.8Ge0.2 alloy volumes are involved in the relaxation.Note that the substrate regions under the islands wherethe cap is thinner include smaller Si volumes per unitsurface area. Hence, these regions are softer because Sihas larger elastic constants than Si0.8Ge0.2 [23]. Finally,the present results do not allow for a decoupling of theeffects of the near-surface steps and the Si-cap thickness.Thus we cannot make any clear conclusions concerningtheir relative role in the sharp island confinement. Aseparation of these effects would, however, be of greatinterest not only from a scientific but also a technologicalpoint of view since it may provide new routes for thealignment of nanodot structures.In summary, new capabilities of misfit dislocations forspatial structuring of heteroepitaxial semiconductor sys-tems at the nanometer length scale have been elucidated.Formation of ordered Ge-island patterns on Si12xGexySiand Si-capySi12xGexySi substrates prestructured by dislo-cation bands is revealed. The most spectacular orderingwas observed on surfaces of samples with Si caps wherethe islands appear to be confined to one-dimensional rowsaligned along slip regions on the surface. It was demon-strated that the ordering of the islands is not mediated bythe slip-step bunches on the surface. The results suggestthat the ordering on the noncapped substrates originatesfrom the nonuniform dislocation strain fields and that thesharp lateral confinement of the islands on the cappedsubstrates is mediated by the near-surface layer structuremodified by the dislocation slip. Although the analysisdone in this work is qualitative and omits aspects of is-land nucleation and growth [8,18,24], it captures, in ouropinion, the major sources which may lead to the sharplateral island confinement, and thus provides a frameworkfor further, detailed studies.This work was supported by the Danish Minister ofIndustry and Research in the framework of CNAST.Special thanks to Pia Bomholt for the help in thepreparation of the cross-sectional TEM specimens.[1] For a review, see articles in Mater. Res. Soc. Bull. 21,No. 4 (1996).[2] J.W. P. Hsu, E. A. Fitzgerald, Y.-H. Xie, P. J. Silverman,and M. J. Cardillo, Appl. Phys. Lett. 61, 1293 (1993).505VOLUME 78, NUMBER 3 P HY S I CA L REV I EW LE T T ER S 20 JANUARY 1997[3] M. Albrecht, S. Christiansen, J. Michler, W. Dorsch, H. P.Strunk, P.O. Hansson, and Bauser, Appl. Phys. Lett. 67,1232 (1995).[4] F. Jonsdottir and L. B. Freund, Mater. Res. Soc. Symp.Proc. 317, 309 (1994).[5] M.A. Lutz, R.M. Feenstra, F.K. LeGoues, P.M. Mooney,and J. O. Chu, Appl. Phys. Lett. 66, 724 (1995).[6] S.Yu. Shiryaev, F. Jensen, and J. Wulff Petersen, Appl.Phys. Lett. 64, 3305 (1994).[7] D. E. Jesson, K.M. Chen, S. J. Pennycook, T. Thundat,and R. J. Warmack, Science 268, 1161 (1995).[8] D. E. Jesson, K.M. Chen, and S. J. Pennycook, Mater. Res.Soc. Bull. 21, 31 (1996).[9] A.G. Cullis, A. J. Pidduk, and M. T. Emeny, Phys. Rev.Lett. 75, 2368 (1995).[10] A.G. Cullis, D. J. Robbins, S. J. Barnett, and A. J. Pidduk,J. Vac. Sci. Technol. A 12, 1924 (1994).[11] K. Häusler, F. Noll, and K. Eberl, Solid State Electron.40, 803 (1996).[12] S.Yu. Shiryaev, J. Lundsgaard Hansen, A. NylandstedLarsen, F. Jensen, and J. Wulff Petersen, Phys. Rev. B52, 15 881 (1995).[13] S.Yu. Shiryaev, F. Jensen, J. Wulff Petersen, J. Lunds-gaard Hansen, and A. Nylandsted Larsen, J. CrystalGrowth 157, 132 (1995).[14] D. J. Eaglesham and M. Cerullo, Phys. Rev. Lett. 64, 1943(1990).[15] To estimate the total amount of Ge atoms in the islands inthe sample shown in Fig. 3(c), we used the average islanddimensions (diameter equal to 100 nm and height equalto 21 nm) determined from plan-view and cross-sectionalTEM images. The fraction of the surface covered withislands was determined from large-scan-area AFM images.We found that the Ge content of the islands amountsto ø2 ML which means that roughly 50% of the totalamount of the deposited Ge sø4 MLd is redistributed tothe islands.[16] H. Sunamura, S. Fukatsu, N. Usami, and Y. Shiraki,J. Crystal Growth 157, 265 (1995).[17] K.M. Chen, D. E. Jesson, S. J. Pennycook, M. Mostoller,T. Kaplan, T. Thundat, and R. J. Warmack, Phys. Rev.Lett. 75, 1582 (1995).[18] Y.-W. Mo, D. E. Savage, B. S. Swartzentruber, and M.G.Lagally, Phys. Rev. Lett. 65, 1020 (1990).[19] Recent calculations by L. J. Gray, M. F. Chisholm, andT. Kaplan [Appl. Phys. Lett. 66, 1924 (1995)] show thatstrain energies in the GeySi system are very sensitive torelatively small deviations of the interface from the planarorientation.[20] J. Tersoff, C. Teichert, and M.G. Lagally, Phys. Rev. Lett.76, 1675 (1996).[21] Q. Xie, A. Madhukar, P. Chen, and N. P. Kobayashi, Phys.Rev. Lett. 75, 2542 (1995).[22] S. Christiansen, M. Albrecht, H. P. Strunk, and H. J.Maier, Appl. Phys. Lett. 64, 3617 (1994).[23] S. P. Baker and E. Arzt, in Properties of Strained andRelaxed Silicon Germanium, edited by E. Kasper, EMISData Reviews Series No. 12 (INSPEC, IEE, London,1995), p. 67.[24] J. Tersoff and F. K. LeGoues, Phys. Rev. Lett. 72, 3570(1994).506

Nanoscale Structuring by Misfit Dislocations in Si1-xGex/Si Epitaxial Systems

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Nanoscale Structuring by Misfit Dislocations in Si1-xGex/Si Epitaxial Systems

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