Directed Self-Assembly of Ge Nanostructures on ... - ResearchGate

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Directed Self-Assembly of Ge Nanostructures on ... - ResearchGate

ong>Directedong> ong>Selfong>-ong>Assemblyong> ong>ofong> ong>Geong>ong>Nanostructuresong> on Very High Index,Highly Anisotropic Si(hkl) SurfacesNANOLETTERS2005Vol. 5, No. 2369-372Kenji Ohmori,* Y. L. Foo, † Sukwon Hong, ‡ J. G. Wen, J. E. Greene, and I. PetrovFrederick-Seitz Materials Research Laboratory and Department ong>ofong> Materials Science,UniVersity ong>ofong> Illinois, 104 South Goodwin AVenue, Urbana, Illinois 61801Received October 8, 2004; Revised Manuscript Received December 16, 2004ABSTRACTFamilies ong>ofong> very high-index planes, such as those which bifurcate spontaneously to form a hill-and-valley structure composed ong>ofong> opposingfacets, provide natural templates for the directed growth ong>ofong> position-controlled self-organized nanostructures with shapes determined by thefacet width ratio R. For example, deposition ong>ofong> a few ML ong>ofong> ong>Geong> on Si(173 100 373), corresponding to R(113/517) ) 1.7, results in a field ong>ofong>40-nm-wide ong>Geong> nanowires along [72 187 84] with a uniform period ong>ofong> 60 nm.The controlled fabrication ong>ofong> nanostructures and surfacenanopatterns is a crucial step underlying all fields ong>ofong>nanotechnology. For example, nanoscale architectures on Sisurfaces are presently ong>ofong> intense interest due to theircompatibility with conventional microelectronic device processing,and hence, their potential for future generations ong>ofong>novel electronic devices. Current strategies to achieveuniform nanostructural arrays include both top-down andbottom-up approaches. In the former case, nanostructurefabrication is generally accomplished through conventionaloptical lithography techniques in which feature sizes arelimited by the radiation wavelength, 1 while bottom-upmethodologies have been utilized primarily in self-assemblytechniques such as film growth in the strain-induced Stranski-Krastanov(SK) mode. A major bottleneck in bothtactics, however, is the lack ong>ofong> control in one or more ong>ofong> thefollowing: size, shape, position, and/or areal density distributions.ong>Geong> growth on Si(001) has been studied extensively as amodel SK system due to a combination ong>ofong> the relatively largelattice constant mismatch, 4.2%, as well as compatibility ong>ofong>ong>Geong> with Si device processing. A variety ong>ofong> ong>Geong> nanostructuresincluding pyramids (huts), domes, and elongated islands areobtained as a function ong>ofong> temperature, growth rate, and filmthickness. 2-4 Some progress in controlling the density, size,and spatial uniformity ong>ofong> these structures has been achievedusing stacked multilayer growth, 5 ion implantation, 6 and* Corresponding author. E-mail: ohmori@mrl.uiuc.edu.† Present address: Institute ong>ofong> Materials Research and Engineering(IMRE), 3 Research Link, S117602, Singapore.‡ Present address: School ong>ofong> Materials Science and Engineering, SeoulNational University, San 56-1, Shilim-dong, Kwanak-ku, Seoul 151-744,Korea.lithographically-assisted deposition. 7 Since the SK growthmode is driven by strain and Si is elastically asymmetric,the shape ong>ofong> deposited ong>Geong> nanostructures also dependsstrongly on substrate orientation. 8 Until now, the primarysubstrate orientations used have been (001), (113), (110),and (111) due to their stable surface reconstructions. 9We have chosen to take a different route and havedeveloped a novel and completely general method for rapidlyevaluating all possible substrate orientations, within a givencrystal structure, for self-organized nanostructure growth. Weuse computer-controlled focused ion beam (FIB) etching tong>ofong>abricate concave circular cones and square pyramids withvarying depths. In the case ong>ofong> Si(001) substrates, bysystematically changing both the tilt angle θ ong>ofong> the sidewallswith respect to the (001) plane as well as the in-planeazimuthal direction φ with respect to the [1h10] direction,we show that very-high-index Si(hkl) planes provide naturaltemplates for the directed growth ong>ofong> uniformly spaced selforganizednanostructures with shapes ranging from isotropicdomes to triangles to teardrops to one-dimensional wiresunder the same deposition conditions. Based upon initialexperimental results, we use a surface structural model forthe formation ong>ofong> families ong>ofong> nanoscale grooves in order topredict precise θ and φ values for the substrate growth planesrequired for shape-, size-, and position-controlled selfassemblyong>ofong> as-deposited nanostructures. For example,regular fields ong>ofong> 40-nm-wide, several-µm-long ong>Geong> nanowireswith a highly uniform period ong>ofong> 60 nm are grown onSi(173 100 373), corresponding to θ ) 28.2° with φ ) 75°with respect to the initial Si(001) substrate surface, alongthe [72 187 84] direction. The Si(173 100 373) surface has10.1021/nl048340w CCC: $30.25Published on Web 01/12/2005© 2005 American Chemical Society


a hill-and-valley structure which consists ong>ofong> alternating (113)and (517) facets with a (113)/(517) width ratio R ) 1.7.The Si(001) substrates used in these experiments have amiscut angle ong>ofong> 0.3° at 2.2° from the [100] direction. Thesamples are first oxidized at 1000 °C to form a 200-nmthickSiO 2 cap layer in order to avoid excess deformationand penetration ong>ofong> Ga + ions, typically 30 keV and 3 nA,during FIB sample preparation. After processing, the oxidelayer is removed and a protective oxide layer formed usinga UV/ozone process. 10 The samples are then introduced intoa multichamber gas-source molecular beam epitaxy system(details in refs 11 and 12), where they are degassed at650 °C and then flashed at 1100 °C to remove the oxide.Si buffer layers, 50-nm-thick, are grown at 800 °C fromSi 2 H 6 , after which 7 ML ong>ofong> ong>Geong> is deposited at 600 °C fromong>Geong> 2 H 6 (see ref 12 for deposition procedures). Surface morphologiesare examined by tapping-mode atomic forcemicroscopy (AFM), while ong>Geong>/Si interfaces are analyzed usinghigh-resolution cross-sectional transmission electron microscopy(XTEM) and scanning TEM (STEM).Figure 1a is an AFM image ong>ofong> one quadrant ong>ofong> a concavetruncated circular-cone structure with a depth ong>ofong> 2.3 µm,fabricated in Si(001), on which 7 ML ong>ofong> ong>Geong> has beendeposited. The image, 9.0 × 9.0 µm 2 , is high-pass filteredto highlight fine structures on the inclined surface. The imagescan direction, for this and all AFM images shown here, isalong [1h10] projected onto the (001) plane. Outside the conestructure (i.e., on the Si(001) surface), the ong>Geong> islands aredome shaped with a density ong>ofong> 30.9 µm -2 , as expected fromprevious results. 13 Note that the islands decorate the circularconerim where local deviations from (001) provide surfacesteps that are favorable sites for island nucleation. 14 Figure1b is a line prong>ofong>ile along the azimuthal direction φ ) 0 (i.e.,along [1h10]) ong>ofong> the cone in Figure 1a showing both the(negative) feature height h below the rim and the tilt angleθ as a function ong>ofong> position. The spikes in the θ vs positioncurve are due to ong>Geong> islands.The sample in Figure 1 contains a sidewall regionapproximately 2.5 µm wide with a constant tilt angle θ ong>ofong>28.2(1.5°. A wide variety ong>ofong> island shapes and densitiesare formed on this conically curved surface as a function ong>ofong>azimuthal position φ within the etched crater. For example,a magnified view ong>ofong> the region highlighted by the whitesquare in Figure 1a is presented in 1c, where we broadlyclassify the ong>Geong> nanostructures as (1) domes which areelongated along φ ) n90° with n ) 0, 1, 2, 3 (see upperleft, Figure 1c), (2) curved nanowires extending approximately2 µm long, located near φ = (n90(15)° with n ) 0,1, 2, 3 (see middle region: 69° e φ e 83°), and (3) arcshapedfacetted regions terminated with arrowheads locatednear φ ) (n45(10)° with n ) 1, 3, 5, 7 (lower right). Anextensive library ong>ofong> ong>Geong> nanostructure shapes, distributions,and densities has been obtained by systematically varyingthe dimensions ong>ofong> FIB-produced conical structures etchedin the Si(001) substrate (i.e., by varying the orientations ong>ofong>the exposed Si planes).Here we focus on isolating a high-order Si plane whichallows the self-organized growth ong>ofong> straight ong>Geong> nanowiresFigure 1. Examples ong>ofong> ong>Geong> nanostructures formed on the sidewallsong>ofong> a FIB-fabricated truncated conical structure in Si(001). (a) AFMimage, 9.0 × 9.0 µm 2 , from one quadrant ong>ofong> the conical structure,with a depth ong>ofong> 2.3 µm, on which 7 ML ong>ofong> ong>Geong> was deposited. (b)Height h and tilt angle θ as a function ong>ofong> position along φ ) 0([1h10]) from the center ong>ofong> the conical structure toward the rim. (c)Higher resolution AFM image, 3.0 × 3.0 µm 2 , ong>ofong> the region in (a)outlined by the white square (45 < φ < 90°).with a regular pitch. Based upon analyses ong>ofong> the results inFigure 1c showing curved nanowires, we fabricate a truncatedinverse square pyramid structure in Si having sidewalls withθ ) 28.2° and φ p ) (n90-15)° (n ) 0, 1, 2, 3), where wedefine the azimuthal direction φ p to identify the exposed tiltedplane orientations. The base ong>ofong> the pyramidal pit is 15 µmon a side. This results in four =3 × 5 µm 2 trapezoid-shapedSi{173 100 373} planes, which after flashing and Si buffergrowth form a regular corrugated surface morphology asrevealed by AFM (not shown). Figure 2a is an AFM imageong>ofong> one ong>ofong> the sidewalls (φ p ) 75°), corresponding to370 Nano Lett., Vol. 5, No. 2, 2005


Figure 2. ong>Selfong>-organized ong>Geong> nanowires formed on Si(173 100 373),corresponding to φ p ) 75° with θ ) 28.2°. (a) AFM image witha scan size ong>ofong> 2.0 × 2.0 µm 2 . (b) Height h as a function ong>ofong> positionalong the white line in (a). (c) Z-contrast STEM image ong>ofong> ong>Geong>nanowires shown in (a). (d) High-resolution [121h] zone-axis XTEMimage.(173 100 373), after depositing 7 ML ong>ofong> ong>Geong>. Straight coherentong>Geong> nanowires are formed with lengths extending to severalµm, limited only by the size ong>ofong> the FIB-fabricated structure.The ong>Geong> nanowires are aligned along a direction φ d ) 114°(i.e., along [72 187 84]) with respect to [1h10]. Figure 2b, anAFM prong>ofong>ile along the white line in Figure 2a, shows thatthe nanowires are 40 nm wide with a period ong>ofong> 60 nm. Thesmall bumps between two neighboring nanowires, indicatedby the arrow in Figure 2b, suggest that ong>Geong> nanowires, asdemonstrated below, grow along the template grooves onthe Si(173 100 373) plane.Figure 2c is a Z-contrast high-angle annular dark-fieldSTEM image ong>ofong> the ong>Geong> nanowires shown in Figure 2a. Themicrograph was obtained along the [121h] zone-axis, whichis 5.3° from the nanowire direction [72 187 84]. The imagecontrast is due to atomic mass; thus the bright regionscorrespond to ong>Geong>. The interface forms a corrugated hill-andvalleystructure composed ong>ofong> two different facet planes. Theong>Geong> nanowires, embedded in a thin wetting layer, have aparallelogram-shaped cross-section and reside in valleysformed by the opposing Si facets. The substrate surfacestructure shown here is analogous to that predicted theoreticallyby Shchukin 15 for a GaAs(311)A surface consisting ong>ofong>a hill-and-valley structure composed ong>ofong> equivalent (31h3) and(331h) facets. We determined the facet orientations at the ong>Geong>/Si interface based upon high-resolution bright-field XTEMimages (see, for example, Figure 2d) to be (113) and (517).Note that (517) is 2.4° ong>ofong>f (15 3 23), one ong>ofong> seven majorfacets observed on well-annealed ong>Geong> and Si surfaces. 16 Thecorresponding facets on the ong>Geong> nanowire surface are also(113) and (517). No dislocations were observed in ong>Geong>nanowires formed on the Si hill-and-valley template surface.Next, we consider the general conditions required to obtaina (113)/(517) hill-and-valley template structure at tilt angleθ HV as a function ong>ofong> φ p , i.e., θ HV (φ p ), and use the results tong>ofong>orm other types ong>ofong> nanostructures with controlled shapes.The ridgeline ong>ofong> the hill-and-valley structure, which is formedby the intersection ong>ofong> (113) and (517) facets, corresponds tothe [121h] direction. From geometric considerations, any planeobtained by rotating (113) toward (517) around the pivotaxis [121h], which can be expressed as (k 100 200+k) with100 < k < 500, will form a (113)/(517) hill-and-valleystructure with varying ratios R ong>ofong> the widths ong>ofong> (113) to (517)facets. This rotation is equivalent to changing θ HV from 36.1°with φ p ) 56.3° (the (517) plane) to θ HV ) 25.2° with φ p )90.0° (the (113) plane) with the ridgeline direction ong>ofong> thehill-and-valley structure constant at φ d ) 108.4° (i.e., [121h]).In Figure 1, for example, we observe nanowires within theinterval 69° e φ e 83° for which we calculate θ HV valuesranging from 30.1 to 26.3°, which includes the actual tiltangle θ ) 28.2° ong>ofong> the conical structure used in thisexperiment, and R values from 0.85 to 5.09. This implies atunable periodicity in hill-and-valley structures composed ong>ofong>(113) and (517) facets, simply by varying θ and φ.These initial results indicate that directed self-assemblyong>ofong> nanostructures with virtually any shape can be achievedby suitable choice ong>ofong> high-index template surfaces. Note thatφ p is different for (113) and (517) facets and that neitherare equal to φ p for (173 100 373), which is equivalent toφ p ) 75.0° with θ ) 28.2°. This allows us, through thechoice ong>ofong> θ and φ p during sample preparation by FIB, tochange the direction ong>ofong> the intersection lines between the(113)/(517) facets and the exposed growth plane. As a finalillustration ong>ofong> this geometric representation for surfacenanogroove transformations, we demonstrate results fortransforming ong>Geong> nanostructures from nanowires to “teardrops”.Figures 3a and 3b are AFM images ong>ofong> ong>Geong> nanostructuresgrown on planes with tilt angles θ ong>ofong> 24.7 and 31.9°,respectively, and an azimuthal tilt direction φ p ong>ofong> 75°. Thiscorresponds to -3.5 and +3.7° ong>ofong>f the (173 100 373) planetoward [173 100 107]. By decreasing (increasing) θ slightlyfrom θ ) 28.2° (which results in long straight ong>Geong> nanowires),the shape ong>ofong> as-deposited ong>Geong> nanostructures is transformedNano Lett., Vol. 5, No. 2, 2005 371

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