The impacts of surface conditions on the vapor-liquid-solid growth of germanium nanowires on Si (100) substrate

Article abstract of DOI:10.1063/1.2968201

The impacts of surface conditions on the growth of Ge nanowires on a Si (100) substrate are discussed in detail. On Si O2 -terminated Si substrates, high-density Ge nanowires can be easily grown. However, on H-terminated Si substrates, growing Ge nanowires is more complex. The silicon migration and the formation of a native Si O2 overlayer on a catalyst surface retard the growth of Ge nanowires. After removing this overlayer in the HF solution, high-density and well-ordered Ge nanowires are grown. Ge nanowires cross vertically and form two sets of parallel nanowires. It is found that nanowires grew along 〈110〉 directions.

Full text of DOI:10.1063/1.2968201

The impacts of surface conditions on the vapor-liquid-solid growth of germanium
nanowires on Si (100) substrate
C. B. Li, K. Usami, T. Muraki, H. Mizuta, and S. Odal
Citation: Applied Physics Letters 93, 041917 (2008); doi: 10.1063/1.2968201
View online: http://dx.doi.org/10.1063/1.2968201
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/93/4?ver=pdfcov
Published by the AIP Publishing
Articles you may be interested in
The growth and radial analysis of Si/Ge core-shell nanowires
Appl. Phys. Lett. 97, 251912 (2010); 10.1063/1.3531631
Patterned epitaxial vapor-liquid-solid growth of silicon nanowires on Si(111) using silane
J. Appl. Phys. 103, 024304 (2008); 10.1063/1.2832760
Vapor-liquid-solid growth of silicon nanowires by chemical vapor deposition on implanted templates
J. Appl. Phys. 100, 084323 (2006); 10.1063/1.2357342
Vapor-liquid-solid growth of germanium nanostructures on silicon
J. Appl. Phys. 96, 7556 (2004); 10.1063/1.1815051
Nanopillar growth mode by vapor-liquid-solid epitaxy
Appl. Phys. Lett. 84, 5302 (2004); 10.1063/1.1766076
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
128.59.226.54 On: Wed, 10 Dec 2014 09:18:28

APPLIED PHYSICS LETTERS 93, 041917 ͑2008͒
The impacts of surface conditions on the vapor-liquid-solid growth
of germanium nanowires on Si „100… substrate
1
,4,a͒
1,4
1
2,3,4
1,4
C. B. Li,
K. Usami, T. Muraki, H. Mizuta,
and S. Oda
1
Quantum Nanoelectronics Research Center, Tokyo Institute of Technology, Tokyo 152-8552, Japan
School of Electronics and Computer Science, The University of Southampton, Highfield, Southampton,
2
Hampshire SO17 1BJ, United Kingdom
3
Department of Physical Electronics, Tokyo Institute of Technology, Tokyo 152-8552, Japan
4
Solution-Oriented Research for Science and Technology, Japan Science and Technology (SORST-JST) ,
Japan
͑
Received 27 June 2008; accepted 15 July 2008; published online 1 August 2008͒
The impacts of surface conditions on the growth of Ge nanowires on a Si ͑100͒ substrate are
discussed in detail. On SiO -terminated Si substrates, high-density Ge nanowires can be easily
2
grown. However, on H-terminated Si substrates, growing Ge nanowires is more complex. The
silicon migration and the formation of a native SiO overlayer on a catalyst surface retard the growth
2
of Ge nanowires. After removing this overlayer in the HF solution, high-density and well-ordered
Ge nanowires are grown. Ge nanowires cross vertically and form two sets of parallel nanowires. It
is found that nanowires grew along ͗110͘ directions. © 2008 American Institute of Physics.
͓
DOI: 10.1063/1.2968201͔
One-dimensional semiconductor nanostructures have at-
tracted much attention because of their potential applications
in the design of nanoelectronic, photonic, and sensing
islands with a wetting layer, as shown in Figs. 1͑c͒ and 1͑d͒.
All Ge nanowires were grown at 300 °C for 20 min by a
low-pressure chemical vapor deposition method with 10%
GeH4 precursors ͑in an atmosphere of hydrogen͒ in a total
pressure of 5 Torr. On a SiO2-terminated Si substrate, with-
out any pretreatment to the catalysts, high-density Ge nano-
wires with diameters of 5–20 nm were grown on Au cata-
lysts with thicknesses of 1 nm ͑Fig. 2͒ and 0.1 nm. Both
high-resolution transmission electron microscopy and x-ray
diffraction results reveal high-quality single-crystalline Ge
nanowires with a cubic diamond structure as we discussed
1
devices. Due to their high mobility of electrons and holes,
Ge nanowires show their potential application in high-speed
2
–4
field-effect transistors. Moreover, Ge nanowires are poten-
tially useful for high-speed quantum computing because
of long decoherence time due to the predominance of
5
,6
spin-zero nuclei and the advantage of a large excitonic
Bohr radius in Ge ͑24.3 nm͒, allowing for quantum confine-
7
,8
ment to be observed in relatively large structures and at
high temperatures. In order to realize these applications, con-
trollable and high-quality nanowire growth is important.
Much attention has been focused on the growth of Ge
1
4
before. It was found that the thickness of the SiO layer,
2
either a few nanometers of native layer or a thicker thermal
oxidized SiO , has no influence on the growth of Ge nano-
2
9
–12
wires.
nanowires.
However, only few papers discuss the impacts
However, very few Ge nanowires were grown on the
H-terminated Si substrate under the same growth conditions
of surface conditions on the vapor-liquid-solid synthesis of
Ge nanowires. In this letter, we will discuss them in detail.
Au catalyst layers with thicknesses of 0.1 and 1 nm were
evaporated by electron beam evaporation at room tempera-
͑
Fig. 3͒. Even after high temperature preannealing ͑650 °C
in vacuumed condition͒ to dewet the Au wetting layer, it had
no great influence on the growth of Ge nanowires.
ture on two kinds of substrates: SiO -terminated ͑a few na-
2
When observing the topography of Au catalysts evapo-
rated on the H-terminated substrate by a scanning electron
microscope ͑SEM͒, we noticed that the contrast between the
nometers of native SiO layer or 170 nm thermal oxidized
2
SiO ͒ and H-terminated ͑the wafers were dipped in 1.5% HF
2
solution for 1 min and were immediately loaded in the cham-
ber for electron beam evaporation͒ silicon ͑100͒ substrates.
In order to satisfy the rules of minimum surface energy, the
evaporated Au shows different topographies at different sub-
strate surface conditions, as shown in Figs. 1͑a͒–1͑d͒. On the
(a)
(b)
(d)
SiO -terminated substrate, because the condensing Au ada-
(c)
2
toms are more strongly bound to each other than to the
1
3
substrate, these atoms encounter other atoms, nucleate, and
agglomerate to form stable islands with diameters of
(e)
2
–10 nm ͓Figs. 1͑a͒ and 1͑b͔͒. The thicker Au layer will
induce the formation of bigger Au dots. On the other hand,
on the H-terminated Si substrate, the Au catalyst prefers to
deposit in the Stranski–Krastanov mode to form Au small
FIG. 1. SEM images of Au catalysts evaporated on SiO2-terminated silicon
substrates with a thickness of ͑a͒ 0.1 nm and ͑b͒ 1 nm. Au catalysts evapo-
rated on H-terminated silicon substrates with a thickness of ͑c͒ 0.1 nm and
͑d͒ 1 nm. ͑e͒ SEM image of 1-nm-thick Au evaporated on H-terminated
silicon after HF treatment for 2 min. The scale bars in the figures are 30 nm.
a͒Tel.: ϩ81-͑0͒3-5734-2542. FAX: ϩ81-͑0͒3-5734-2542. Electronic mail:
cbli@neo.pe.titech.ac.jp.
0
003-6951/2008/93͑4͒/041917/3/$23.00
93, 041917-1
28.59.226.54 On: Wed, 10 Dec 2014 09:18:28
© 2008 American Institute of Physics
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
1

041917-2
Li et al.
Appl. Phys. Lett. 93, 041917 ͑2008͒
[
-110]
5
00 nm
~45o [110]
~
45o
1µm
(
a)
1
µm
[
011]
[
-101]
[001]
[0-11]
FIG. 2. Top-view SEM image of high-density Ge nanowires grown on
-nm-thick Au catalysts evaporated on SiO -terminated silicon substrates.
[101]
110]
1
2
The inset shows their cross-section image.
[
[
-110]
[010]
(
b)
Au metal and the Si substrate at the edge area is smaller and
the brightness of the Au metal in the SEM image is weaker
compared with that evaporated on the SiO2 substrate. It
seems that Au catalysts are covered by some insulator layers.
So maybe the formation of a very thin silicon oxide over-
layer due to the gold catalyzed migration of silicon through
[
0-10]
FIG. 4. ͑Color online͒ ͑a͒ Top-view SEM image of high-density and well-
ordered Ge nanowires grown on 1-nm-thick Au catalysts evaporated on
H-terminated silicon ͑100͒ after being dipped into the HF solution to remove
the SiO2 overlayer. The bottom edge is the ͓110͔ cleavage direction. The
inset shows its side-view SEM image, and the scale bar is 500 nm. ͑b͒
Three-dimensional schematic of the Ge nanowire growth direction on a Si
͑100͒ wafer. Four dashed arrows directed to ͓101͔, ͓011͔, ͓−101͔, and
1
5–18
the gold film
retards the growth of Ge nanowires.
In order to verify this hypothesis, Au catalysts evapo-
rated on H-terminated substrates were dipped into a 1.5% HF
solution for 2 min and then were immediately loaded into a
growth chamber to grow Ge nanowires. On 1-nm-thick Au
catalyst substrates, high-density and well-ordered Ge nano-
wires were grown, as shown in Fig. 4͑a͒. Obviously, it has a
great improvement for Ge nanowire growth by removing this
͓
0−11͔ are the Ge nanowire growth directions.
͓
111͔, were found for Ge nanowires grown on silicon
2,3,5,10–12
substrates.
Based on this fact and our experiment
results, we believe that Ge nanowires are grown orderly
along ͓101͔, ͓011͔, ͓−101͔, and ͓0−11͔ directions as the four
dashed arrows shown in Fig. 4͑b͒, in which these four
growth directions are crossed vertically and have a 45° de-
gree with ͓110͔ and ͓−110͔ cleavage directions. It agrees
well with the experiment results as shown in Fig. 4͑a͒. The
two dashed-dotted arrows in Fig. 4͑b͒ are ͓110͔ and ͓−110͔
cleavage directions for ͑100͒ wafer as labeled in Fig. 4͑a͒.
During the growth of Ge nanowires on Au catalysts
evaporated on the H-terminated Si substrate after HF treat-
overlayer, and it confirms that the SiO overlayer formed on
2
the catalyst surface prevents the growth of Ge nanowires.
Figure 1͑e͒ shows the SEM image of 1-nm-thick Au cata-
lysts after HF treatment. Clear Au dots can be found after
removing the SiO overlayer. In the case of 0.1-nm-thick Au
2
catalysts, even after HF treatment, no nanowire was grown.
We did not find any Au dots after HF treatment in the SEM
image since they are still in the wetting layer stage. So there
are no nucleation centers for Ge atoms to grow in the axial
direction. The GeH will get more chances to be decomposed
ment, GeH will decompose in the AuSi eutectic alloy lo-
4
4
and deposited to form a film since the AuSi eutectic alloy
exists on the whole surface.
cated at the interface between the single-crystalline Si sur-
face and Au layer, and then the supersaturation Ge atom will
separate out to grow in the axial direction. This ordered
structure at the interface can induce the Ge nanowires to
grow along only ͗110͘ directions. This capability to control
the nanowire growth direction would be very important in
engineering the transport characteristics for electrons and
holes separately to get the best switching performance of a
Compared with the random-directed Ge nanowires
grown on the SiO -terminated substrate, as shown in Fig. 2,
2
Ge nanowires grown on the H-terminated Si ͑100͒ substrate
show a more ordered structure. Almost all Ge nanowires
cross vertically and form two sets of parallel nanowires. It
should be pointed out that each set of Ge nanowires has
around 45° angle with a ͓110͔ cleavage direction as labeled
in Fig. 4͑a͒. Until now, only two growth directions, ͓110͔ and
19
complementary logic circuit configuration. Also it is quite
attractive for future large-scale nanowire integration.
In the case of Au catalysts evaporated on the
SiO -terminated substrate, maybe the strong bond between
2
Si and O atom retards the migration of silicon to the surface
and the formation of the SiO overlayer. Therefore Ge nano-
2
wires can be grown without HF treatment.
3
00 nm
In conclusion, high-density Ge nanowires can be easily
grown on SiO -terminated Si ͑100͒ substrates. However, the
2
SiO overlayer formed on the surface of Au catalysts evapo-
2
rated on the H-terminated Si substrate prevents the growth of
3
µm
Ge nanowires. After removing this SiO overlayer by HF
2
solution, high-density and well-ordered Ge nanowires can be
obtained. Moreover, nanowires can be grown orderly along
͓101͔, ͓011͔, ͓−101͔, and ͓0−11͔ directions.
FIG. 3. Top-view SEM image of Ge nanowires grown on H-terminated
silicon substrates. Inset shows the enlarged image of one wire, and it exhib-
its the same diameter as that grown on SiO -terminated substrates.
This article is copyrighted as indicated in the ar2ticle. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
128.59.226.54 On: Wed, 10 Dec 2014 09:18:28

0
41917-3
Li et al.
Appl. Phys. Lett. 93, 041917 ͑2008͒
Appl. Phys. Lett. 59, 3168 ͑1991͒.
This work is partially supported by SORST-JST ͑Japan
9
L. J. Lauhon, M. S. Gudiksen, D. Wang, and C. M. Lieber, Nature
Science and Technology͒ and Japan Society for the Promo-
tion of Science ͑JSPS͒.
10͑
London͒ 420, 57 ͑2002͒.
D. Wang and H. Dai, Angew. Chem., Int. Ed. 41, 4783 ͑2002͒.
A. B. Greytak, L. J. Lauhon, M. S. Gudiksen, and C. M. Lieber, Appl.
Phys. Lett. 84, 4176 ͑2004͒.
11
1
H. J. Fan, P. Werner, and M. Zacharias, Small 2, 700 ͑2006͒.
J. Xiang, W. Lu, Y. J. Hu, Y. Wu, H. Yan, and C. M. Lieber, Nature
London͒ 441, 489 ͑2006͒.
G. C. Liang, J. Xiang, N. Kharche, G. Klimeck, M. Charles, C. M. Lieber,
and M. Lundstrom, Nano Lett. 7, 642 ͑2007͒.
S. M. Sze, Physics of Semiconductor Devices ͑Wiley, New York, 1981͒.
Y. J. Hu, H. O. H. Churchill, D. J. Reilly, J. Xiang, C. M. Lieber, and C.
M. Marcus, Nat. Nanotechnol. 2, 622 ͑2007͒.
A. M. Tyryshkin, S. A. Lyon, T. Schenkel, J. Bokor, J. Chu, W. Jantsch, F.
Schaffler, J. L. Truitt, S. N. Coppersmith, and M. A. Eriksson, Physica E
2
12
13
S. Kodambaka, J. Tersoff, M. C. Reuter, and F. M. Ross, Science 316, 730
͑2007͒.
J. A. Venables, J. Vac. Sci. Technol. B 4, 870 ͑1986͒.
C. B. Li, K. Usami, H. Mizuta, and S. Oda, IEEE SNW 2008, Honolulu.
A. Hiraki, M. A. Nicolet, and W. J. Mayer Appl. Phys. Lett. 18, 178
3͑
1
1
4
5
4
5
6͑
1971͒.
1
1
1
6
G. Le Lay, Surf. Sci. 132, 169 ͑1983͒.
7
8
A. Hiraki, Surf. Sci. Rep. 3, 357 ͑1984͒.
H. Jagannathana, Y. Nishi, M. Reuter, M. Copel, E. Tutuc, S. Guhab, and
R. P. Pezzia, Appl. Phys. Lett. 88, 103113 ͑2006͒.
B. Yu, X. H. Sun, G. A. Calebotta, G. R. Dholakia, and M. Meyyappan, J.
Cluster Sci. 17, 579 ͑2006͒.
7͑
Amsterdam͒ 35, 257 ͑2006͒.
X. H. Sun, G. Calebotta, B. Yu, G. Selvaduray, and M. Meyyappan, J. Vac.
Sci. Technol. B 25, 415 ͑2007͒.
1
9
8
Y. Maeda, N. Tsukamoto, Y. Yazawa, Y. Kanemitsu, and Y. Masumoto,
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
28.59.226.54 On: Wed, 10 Dec 2014 09:18:28
1