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Thin Solid Films 469–4

Self-forming silicide/SiGe-based tube structure on Si(001) substrates

H.C. Chen, K.F. Liao, S.W. Lee, L.J. Chen*

Department of Materials Science and Engineering, National Tsing Hua University, 101 Sec 2 Kuang Fu Rd, Hsinchu 300, ROC Taiwan

Available online 28 September 2004

Abstract

Silicide/SiGe-based tube structures have been fabricated onto silicon by precise transformation from two-dimensional structures to three-

dimensional objects. By using the strain in a pair of lattice-mismatched epitaxy layers, a method was developed to create the tube structure by

their release from a substrate. The tube structures combining semiconductor (SiGe) and metallic silicide (NiSi2) may find applications in

advanced devices.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Self-forming; SiGe; Silicide; Tube

1. Introduction

Recently, a new approach for the fabrication of semi-

conductor structures with a precise control in all three

dimensions was introduced. This technique was generalized

to a technology, in which 3-D objects of various shapes,

sizes, and material combinations could be rolled and folded

on a substrate surface, consequently creating a technology

of released layers on the micro- and nanometer scale [1–4].

Different strained material systems, such as Si/SiGe,

InGaAs/GaAs have been used to fabricate a rich variety

of differently shaped micro- and nano-objects [1–8].

Furthermore, nanotubes with hybrid structures integrating

semiconductors, insulators, and metals have been reported

[6].

NiSi2 is of a cubic CaF2 structure with lattice constant

a=0.5406 nm and the lattice mismatch with strained

diamond cubic Si0.7Ge0.3 layer is as small as 0.4%,

epitaxial NiSi2 can be grown readily on Si0.7Ge0.3 layer.

In this paper, we put forward a new structure combining

semiconductor (SiGe) and metallic silicide (NiSi2) into

single nano-tubes by self-forming of the strained Si1�xGex/

NiSi2 bilayers. The use of strained Si1�xGex/NiSi2 hetero-

0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.tsf.2004.06.167

* Corresponding author. Tel.: +886 3 5731166; fax: +886 3 5718328.

E-mail address: ljChen@mx.nthu.edu.tw (L.J. Chen).

structures takes advantage of not only NiSi2 layers with

metallic conductivity and cubic crystal lattice to obtain

tubes with ultra-thin wall and good conducting properties,

but also being compatible with the Si/SiGe-based integra-

tion technology.

2. Experimental procedures

A multiplayer structure grown by ultra-high vacuum

chemical vapor deposition (UHV/CVD) at 600 8C forms the

basis for all samples. The layer stack is grown on a Si(001)

substrate and consists of a 100-nm-thick Si buffer layer and

a bilayer of Si (8 nm)/SiGe (12 nm) layers. The lower SiGe

layer incorporates 30% Ge and the upper layer contains only

Si. Both undoped and heavily p-type-doped Si layers have

been used. The growth of the bilayer is pseudomorphic.

The strained Si/Si1�xGex wafers were cleaned by a

standard RCA process and then dipped in a dilute HF

solution before loading into the deposition chamber. Ni thin

films (2.5 nm thick) were evaporated with an electron beam

evaporation system on the strained Si/Si0.7Ge0.3 substrates at

room temperature in a vacuum of about 5�10�7 Torr. The

samples were then annealed at 400–600 8C for 2 min by rapid

thermal annealing (RTA) in N2 ambient. The Si buffer layer

can be selectively removed by dissolving it in a dilute

tetramethylammonium hydroxide [(CH3)4NOH] solution. A

70 (2004) 483–486

H.C. Chen et al. / Thin Solid Films 469–470 (2004) 483–486484

JEOL-6500F field-emission scanning electron microscope

(FESEM) was used to examine the surface morphology. A

JEOL-2010 transmission electron microscope (TEM) oper-

ating at 200 kV was utilized for the examination of the

structure quality and phase identification. For high-resolu-

tion TEM (HRTEM) observation, a JEOL 4000EX TEM

operating at 400 kV with a point-to-point resolution of 0.18

nm was used.

Fig. 2. Top-view SEM image of NiSi2/Si0.7Ge0.3 tubes along different sites

on a Si buffer layer, (a) cylindrical-shaped tube from the sample edge and

(b) rugged tube from the trench.

3. Results and discussion

The tube structures were fabricated onto the substrate

surfaces by a precise transformation from two-dimensional

to three-dimensional objects. Fig. 1(a) shows a cross-

section TEM image of the initial layer structure. The layer

stack contains a NiSi2 (8 nm)/Si0.7Ge0.3 (12 nm) structure.

Fig. 1(b) shows that NiSi2 grows epitaxially on the

Si0.7Ge0.3 layer. The growth of the NiSi2 layer is induced

by rapid thermal annealing in N2 ambient at 600 8C. Fromthe examination of the XTEM image, no defects such as

dislocations were observed to introduce into the Si0.7Ge0.3

Fig. 1. (a) Cross-sectional TEM image of the strained NiSi2/Si0.7Ge0.3bilayer on a Si buffer layer, (b) high-resolution XTEM image of the

interface between NiSi2/Si0.7Ge0.3 bilayer.

layer. This indicates that the Si0.7Ge0.3 layer is still in a

pseudomorphic state after the growth of the NiSi2 layer. On

the other hand, some parts of the interface between

epitaxial NiSi2 and Si0.7Ge0.3 layer are seen to be faceted

along {111} planes shown in Fig. 1(a). The facetted

structure for epitaxial NiSi2 and Si has been reported

previously [9,10].

Fig. 2 shows top-view FESEM images of self-forming

NiSi2/Si0.7Ge0.3 nanotubes detached from the substrate

surface. A NiSi2/Si0.7Ge0.3 based nanotube, which has

formed along the edge of the sample, is shown in Fig.

2(a). The tube is of straight cylindrical shape, 200 nm in

diameter and 10 Am in length, with a high aspect ratio of

about 50. The tube appears homogeneous in size and

geometry. On the other hand, for scratches introduced

into the sample surface with a cutting knife, a tube

structure was formed with under-etching at the edge as

shown in Fig. 2(b). The tube with 250 nm in diameter is

somewhat rugged compared to that formed at the sample

edge.

Despite the formation of rugged tubes, cylindrical-shape

tubes with a low aspect ratio were also found near the

trench. Fig. 3(a) shows an end of such a tube. The tube is 2

Am long and has a diameter of 200 nm as shown in Fig.

3(b). It is thought that the facets present between the

bilayer, seen in Fig. 1(a), might act as a bsourceQ for thenucleation of the defects during the self-forming process.

During rolling, the internal mechanical stress at the stress

concentration point may exceed the strength of the bilayer,

leading to the tearing off the tube from the substrate. As a

result, cylindrical-shape tubes with a low aspect ratio were

formed. It indicated that the rugged shape of the trench and

the crystal quality of the bilayer might lead to the

irregularity in tube geometry.

Solid-state tubes with a wide range of properties are

appropriate for many applications since semiconductor epi-

layers possess remarkable control over material composition,

doping concentration, and layer thicknesses. In order to

improve the uniformity of the interface between NiSi2 and

Si0.7Ge0.3 to form nanotubes with high aspect ratio, the

method of growing epitaxial NiSi2 on strained Si0.7Ge0.3layer at low temperatures has been employed. We replace the

intrinsic Si layer with heavily doped boron Si layer to form

Fig. 5. Top-view SEM image of the cylindrical-shaped tube with a high

aspect ratio along the edge of the sample.

Fig. 3. Top-view SEM image of the cylindrical-shaped tube with a low

aspect ratio near the trench.

H.C. Chen et al. / Thin Solid Films 469–470 (2004) 483–486 485

the initial layer structure. The concentration of the boron

incorporated into the p+-Si layer amounted to 1020 cm�3. For

the growth of epitaxial NiSi2 layer, samples were annealed in

Fig. 4. (a) Cross-sectional TEM image of the strained NiSi2/Si0.7Ge0.3bilayer on a Si buffer layer by employing p+-Si/Si0.7Ge0.3 bilayer structure,

(b) top-view SEM image of rugged tubes along edge of the trench.

N2 ambient by rapid thermal furnace at 400 8C. Fig. 4(a)shows the NiSi2 (8 nm)/Si0.7Ge0.3 (12 nm) layered structure

after the growth of the NiSi2 layer. The interface between the

bilayer is rather smooth without facetted planes, as seen in

Fig. 1(a). An epitaxial NiSi2 layer with smooth interface was

grown at 400 8C by employing strained p+-Si/Si0.7Ge0.3substrates. Since the size of the boron atom is smaller than

that of the Si atom, stress was induced in the heavily boron

doped Si layer. The stress present at the heavily boron doped

Si layer apparently promotes the growth of epitaxial NiSi2 by

lowering the activation energy [11,12].

Fig. 4(b) shows an irregular tube with 250 nm in

diameter and 10 Am in length at the edge of the trench.

The sections of the tube have higher aspect ratios than

that tube shown in Fig. 2(b). It is apparent with the

improvement in the flatness of the interface of the

bilayer that the self-forming process was less disrupted

by local stress concentration. A straight cylindrical shape

nanotube with 200 nm in diameter has been formed

along the edge of the sample as shown in Fig. 5. The

tube appears to be rather regular along the entire length.

Consequently, the improvement in the quality of the

epitaxial NiSi2/SiGe interface leads to the fabrication of

high aspect ratio self-forming NiSi2/Si0.7Ge0.3 nanotubes

on the Si(001) surface.

The present work represents a successful attempt to

form coaxial NiSi2/SiGe tube structures on SiGe. For

applications in nanotechnology, efforts are needed to

scale down the tube diameter to less than 100 nm. This

in principle can be achieved by growing thinner epitaxial

SiGe film and is currently being pursued. It is worth-

while to mention that micrometer-sized tubes with an

aspect-ratio as high as 1943 have been reported [8].

4. Summary and conclusions

Silicide/SiGe-based tube structures were fabricated

onto substrate surfaces by precise transformations of

H.C. Chen et al. / Thin Solid Films 469–470 (2004) 483–486486

two-dimensional into three-dimensional objects. By using

the strain in a pair of lattice-mismatched epitaxy layers, a

method was developed to create tube structures released

from a substrate. Solid-state tubes with a wide range of

properties are appropriate for many applications since

semiconductor epi-layers possess remarkable control over

material composition, doping concentration, and layer

thicknesses. A new structure combining semiconductor

(SiGe) and metallic silicide (NiSi2) into a single tube

structure was achieved by employing strained p+-Si/

Si0.7Ge0.3 substrates.

Acknowledgments

The research is supported by the Republic of China

National Science Council grant No. NSC 91-2215-E-007-

015 and Ministry of Education grant No. 91-E-FA04-1-4 as

well as ERSO, ITRI.

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