APPLIED PHYSICS LETTERS 94, 073109 共2009兲

Self-organization of In nanostructures on Si surfaces
Maojie Xu, Arifumi Okada, Shoji Yoshida, and Hidemi Shigekawaa兲
Institute of Applied Physics, CREST-JST, University of Tsukuba, Tsukuba 305-8573, Japan

共Received 30 December 2008; accepted 29 January 2009; published online 18 February 2009兲
Methods of forming various In nanostructures on Si surfaces are demonstrated. Using a high-index
Si共311兲 surface, isolated nanotriangles and wires were grown by optimizing the deposition rate andsubstrate temperature. In contrast, nanodots were formed by the deposition of In on aSi共111兲 – In-冑31⫻ 冑31 surface at room temperature 共RT兲 deposition. On a Si共111兲 – In-4 ⫻ 1 / 冑31⫻ 冑31 coexisting surface, nanowires were selectively grown in the Si共111兲 – In 4 ⫻ 1 area by RTdeposition through the nucleation promoted by the boundary barrier produced by the surrounding冑31⫻ 冑31 area. Details were studied using scanning tunneling microscopy. © 2009 American
Institute of Physics. 关DOI: 10.1063/1.3085960兴
Nanometer-sized low-dimensional structures, such asnanowires and nanodots, have been attracting growing interest because they provide typical systems for exploring interesting physical phenomena, such as quantized conductance,
charge density wave, metal-insulator transition 共MIT兲, andquantum confinement in reduced dimensions. Technologically, they also have great potential for application, forexample, in optoelectronic and nanoelectronic devices.
Among various methods employed to fabricate nanoscale
structures, self-organization or self-assembly processes onmaterial surfaces are used to create large-scale arrays ofnanostructures. For example, strain in lattice-misfit heteroepitaxial growth gives rise to the self-organization of ordered arrays of semiconductor quantum wires and dots withuniform size and shape.1–3 Reconstructions, dislocations,steps, and facets also induce selective growth on a substratesurface.4–6
Indium 共In兲 is an attractive material widely used in semiconductor technologies as well as in fundamental studies.
The growth of 1.0 ML of In on a Si共111兲 surface, for example, forms a self-assembled In nanowire array, In-共4 ⫻ 1兲.
It is a typical quasi-one-dimensional 共quasi-1D兲 metal system that induces MIT.7 A recent study has revealed that theMIT of this system can be well controlled by optical doping,8making this system a more applicable nanoscale building
block that can controllably transport electrical charges. Owing to its anisotropic structure, the In-共4 ⫻ 1兲 surface has
been used as a template for the growth of nanostructures.
Self-organized Pb islands of uniform width and elongatedshape were grown on the surface by taking advantage ofsubstrate-mediated anisotropic strain relaxation.9 Adsorbed
Co atoms showed a self-alignment on the surface byquasi-1D electron-gas-mediated interactions.10 The epitaxial
Ag film on In-共4 ⫻ 1兲 surface formed a well-ordered stripe
superstructure.11
However, only a uniform wire structure has been reported, and the method, for example, of forming isolated In
nanowires or In nanodots on a Si surface has not been realized despite the interest of their applications. For more effective applications of the various attractive characteristics ofIn/Si system, development of the methods that enable thefabrication of In nanostructures in more applicable forms isa兲

URL: http://dora.ims.tsukuba.ac.jp.

0003-6951/2009/94共7兲/073109/3/$25.00

required. In this letter, we present the methods we found forforming various In nanostructures on Si surfaces.
Experiments were carried out in an ultrahigh vacuum
system with a base pressure of 1.5⫻ 10−8 Pa. Clean Si共111兲
and Si共311兲 surfaces were prepared as substrates by standard
procedures. A heat-resistant evaporator was used to deposit
In atoms at a flux of 0.3–6.0 ML/min. Scanning tunnelingmicroscopy 共STM兲 images were taken at room temperature
共RT兲.
First, we show the methods of forming isolated nanotriangles and wires. After preparing a clean Si共311兲 surface
with a 3 ⫻ 2 structure,12,13 the deposition of more than 1 MLof In on the surface at ⬃300 ° C formed a homogeneous
Si共311兲 – In-3 ⫻ 2 structure, as shown in Figs. 1共a兲 and 1共b兲.
On the other hand, for a substrate temperature lower than
50 ° C, a nanoscale structure was formed on the surface, butjust small clusters 关Figs. 1共c兲 and 1共d兲兴. Since they do notfollow the underlying symmetry, they are considered asamorphous. In contrast, triangular islands were grown on thesurface for the substrate temperature at 200– 250 ° C 关Figs.
1共e兲 and 1共f兲兴, whose atomic structure is similar to that of theSi共311兲 – In-3 ⫻ 2 surface. The distribution of the size of thetriangular island measured from the STM image is shown in
the inset in Fig. 1共e兲. Furthermore, when the temperature waskept at 50– 150 ° C, a single nanowire was formed along theupper edge of the triangular island in the 关1̄10兴 direction关Figs. 1共g兲 and 1共h兲兴. The edge of the triangular structure
here is considered to function as a barrier that traps In atoms,
promoting the formation of In nanowires.14,15 To grow Innanowires, a high In deposition rate 共⬃2.0 ML/ min兲 wasneeded with the optimization of the substrate temperature. As
the inset in Fig. 1共g兲 shows, the distribution is narrower thanthat in Fig. 1共e兲.
Second, we represent the method of forming In nanodots
on a Si surface. With the deposition of 0.6 ML of In atoms
on a clean Si共111兲-7 ⫻ 7 surface at 300 ° C, a uniform
Si共111兲 – In-冑31⫻ 冑31 structure is formed,16 which we usedas the substrate. When In atoms were deposited with thesubstrate temperature being kept at RT, In nanodots weregrown on the surface, as shown in Fig. 2. Figures 2共a兲–2共c兲
show the STM images obtained for the nanodots grown by
depositions of about 0.1, 0.2, and 0.3 ML of In, respectively.
From the diameter distributions of the nanodots shown in theinsets, the size of the nanodots increases with deposition ra-94, 073109-1© 2009 American Institute of PhysicsDownloaded 18 Feb 2009 to 130.158.131.51. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp

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Appl. Phys. Lett. 94, 073109 共2009兲

FIG. 2. STM images of In nanodots grown at RT with In coverages of 共a兲
0.1 ML 共Vs = −1.0 V, It = 0.2 nA兲, 共b兲 0.2 ML, 共Vs = −2.0 V , It = 0.5 nA兲,and 共c兲 0.3 ML 共Vs = −1.0 V , It = 0.2 nA兲. The diameter 共d兲 distribution ofIn dots is shown in the inset, the average value of which is shown in 共d兲 asa function of coverage.

FIG. 1. STM images obtained for the large and small areas of 共a兲 and 共b兲
Si共311兲 – In-3 ⫻ 2 共substrate temperature Ts = ⬃ 300 ° C, 1–15 ML: large islands are also formed at high coverage, Vs = −2.0 V, It = 0.2 nA兲, 关共c兲 and共d兲兴 small In clusters 共Ts = ⬃ 50 ° C, 1 ML, Vs = 2.0 V, It = 0.2 nA兲, 关共e兲 and共f兲兴 triangular structures 共Ts = 200– 250 ° C, 1 ML, Vs = −2.0 V, It = 0.2 nA兲,and 关共g兲 and 共h兲兴 triangular structures with nanowires 共Ts = 50– 150 ° C,
1 ML, Vs = −2.0 V, It = 0.2 nA兲. The insets in 共e兲 and 共g兲 show the distributions of the triangular size 共length of side without wire兲 and the wire length
reduced by the averaged lengths of L0 共16 and 8.1 nm兲, respectively.

tio, but their density remains almost constant, as shown inFig. 2. Remarkably, the size distribution of the nanodots becomes narrow with In deposition 关Fig. 2共d兲兴, which is attributed to the kinetically self-limiting effect and/or thermodynamic effect of strain-induced energy minimization.1 This is
the demonstration of the formation of well-shaped regularnanodots with the diameter of a few nanometers for an In/Sisurface.17
Third, a selective formation of nanowires on aSi共111兲 – In-4 ⫻ 1structureisdemonstrated.
TheSi共111兲 – In-4 ⫻ 1 surface has been extensively studied to reveal its atomic structure and electronic properties.7,18–21
However, the wire array of the Si共111兲 – In-4 ⫻ 1 surface isuniform; despite the interest in the formation of threedimensional In nanowires and the theoretical studies of thestructure, it is still a challenging issue to experimentally
grow In wire arrays on a homogeneous In-4 ⫻ 1 surface because of the fast diffusion of In adatoms on the surface evenat RT.
Here, we realized the formation of In nanowires on theIn-4 ⫻ 1 surface with the combination of the RT growth anduse of an In/ Si共111兲-4 ⫻ 1 / 冑31⫻ 冑31 coexisting phase as
the substrate. Figure 3共a兲 shows an STM image of the template surface grown by the deposition of ⬃0.7 ML of In ona Si共111兲-共7 ⫻ 7兲 surface at ⬃400 ° C. As shown in Fig. 3共a兲,the surface consists of two phases with the 4 ⫻ 1 and 冑31⫻ 冑31 structures. The In coverage of the 冑31⫻ 冑31 phase isabout 0.6 ML,16 while that of the 4 ⫻ 1 phase is 1 ML.14
Therefore, the density and size of the 4 ⫻ 1 and 冑31⫻ 冑31
phase areas can be controlled by adjusting the substrate temperature and In coverage from 0.6 to 1.0 ML. After the structure with the two phases at a certain ratio was formed, thesample was cooled to RT, and In atoms were deposited.
Figure 3共b兲 shows an STM image obtained after thedeposition of ⬃0.2 ML of In on the template surface shownin Fig. 1共a兲. As was observed in Fig. 2, In nanodots areformed on the 冑31⫻ 冑31 phase that surrounds the 4 ⫻ 1phase area. However, In nanowires are selectively formed in
the In-共4 ⫻ 1兲 area. Since no wires but large islands wereformed on a homogeneous 4 ⫻ 1 surface without the 冑31⫻ 冑31 phase at RT, the boundary barrier may play an important role in the nucleation and growth of the In wires on aSi共111兲 – In-4 ⫻ 1 / 冑31⫻ 冑31 surface. Similar boundary effect may work in part in the case shown in Figs. 1共g兲 and1共h兲. To clarify the detailed mechanism, further study isnecessary.
Figures 3共c兲–3共f兲 show the STM images of the nanowire
arrays obtained for three different In coverages and theircross sections. The wire array of 0.1 ML In coverage showsa wide spacing. With increasing In coverage to 0.2 ML, thespace becomes uniform with an eight times periodicity in theDownloaded 18 Feb 2009 to 130.158.131.51. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp

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method, the sizes and the ratio of the areas with nanodots
and nanowires can be controlled. The interaction may produce new properties, such as a change in criticaltemperature.23
In summary, methods of forming various In nanostructures on Si surfaces are demonstrated. A variety of nanoscaleelements will enable further development in designing andcontrolling nanoscale electronic and optical devices. The
findings are expected to make the system more applicable tofuture semiconductor technologies as well as to fundamental
research studies. Studies on the detailed atomic structuresand electronic properties of the nanostructures are inprogress.
We thank Ms. Rie Yamashita in our group at Universityof Tsukuba for her help in preparing this paper.
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12FIG. 3. STM images of 共a兲 In/Si共111兲 surface with coexisting 4 ⫻ 1 and冑31⫻ 冑31 phases 共0.7 ML, Vs = −1.4 V, It = 0.2 nA兲. 共b兲 In nanowires andnanodots formed after the growth of 0.2 ML of In at RT on the templatesurface of 共a兲 共Vs = −0.6 V , It = 1.0 nA兲. STM images of nanowire
arrays formed on the 4 ⫻ 1 phase with In coverages of 共c兲 0.1 ML共Vs = −1.0 V , It = 0.5 nA兲, 共d兲 0.2 ML 共Vs = −1.5 V , It = 0.3 nA兲, and共e兲 0.4 ML 共Vs = −1.0 V , It = 0.2 nA兲. The white arrows in 共c兲 indicate theIn adatoms adsorbed on top of substrate In chains and the black arrow in 共e兲
indicates the wire orientation. 共f兲 Cross sections of the wire arrays in 共c兲–共e兲.

关112̄兴 direction. Then the wire array exhibits a four times
periodicity for the growth of 0.4 ML of In. These observations suggest the existence of an interaction between nanowires. This surface consists of wires formed by two zigzag In
chains separated by a Si chain on the almost undistorted Sisubstrate.19 It is theoretically predicted that In adatoms preferto adsorb on the sites between two neighboring In wires,
which yields a positional shift in the In atoms in the chains.22
As indicated by white arrows in Fig. 3共c兲, the In atoms areadsorbed on the ridge of the substrate wires. The induced
lattice deformation results in a local strain and may cause theobserved interaction between the nanowires. With thisDownloaded 18 Feb 2009 to 130.158.131.51. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp