Đây là một bài báo khoa học về dây nano silic trong lĩnh vực nghiên cứu công nghệ nano dành cho những người nghiên cứu sâu về vật lý và khoa học vật liệu.Tài liệu có thể dùng tham khảo cho sinh viên các nghành vật lý và công nghệ có đam mê về khoa học
Trang 1Growth kinetics of silicon nanowires by platinum assisted vapour–liquid–solid mechanism
H Jeong, T.E Park, H.K Seong, M Kim, U Kim, H.J Choi*
Department of Materials Science and Engineering, Yonsei University, Seoul 120-749, Republic of Korea
a r t i c l e i n f o
Article history:
Received 9 April 2008
In final form 5 November 2008
Available online 12 November 2008
a b s t r a c t
The growth kinetics of Si nanowires produced by a vapour–liquid–solid (VLS) mechanism in conjunction with Pt and Au catalysts, respectively, was investigated and compared Pt was employed as a VLS catalyst for single-crystal Si nanowires in a SiCl4-based chemical vapour deposition process The growth rates were higher with Pt than with Au under all processing conditions The activation energy was measured
as 80 and 130 kJ/mol with the Pt and Au catalysts, respectively The present results suggest that the rate-determining step is the incorporation of Si atoms in the lattice at the liquid/solid interfaces and, further-more, the metal catalysts affect this step, resulting in different activation energy
Ó 2008 Elsevier B.V All rights reserved
1 Introduction
Silicon (Si) nanowires have novel properties as well as
comple-mentary metal oxide semiconductor (CMOS) compatibility As
such, they are among the most promising materials in terms of
serving as building blocks for the next generation of nano devices
Among the many available methods to grow Si nanowires,
chem-ical vapour deposition (CVD) processes via a vapour–liquid–solid
(VLS) mechanism have been widely used since Wagner and Ellis
contrived this process in 1964[1]for Si whiskers The bulk of
re-search in this area has focused on the use of an Au catalyst to
grow Si nanowires However, Au acts as a deep-level impurity
in the Si band gap [2], thereby preventing direct integration of
Si nanowires in the CMOS process In this regard, Pt is a good
can-didate catalyst for the growth of Si nanowires via the VLS
mech-anism It is known that Pt does not yield an undesirable electronic
effect in Si[3], and thus will not degrade the performance of Si
nanowire devices even if it diffuses into the nanowires Some
studies have reported on the growth of Si nanowires using Pt
with a precursor of SiH4 or SiCl4 [4–6] However, because the
VLS mechanism was not utilized and/or single-crystalline
nano-wires were not employed in these studies, the processes are not
ideal for device application or for understanding the growth
kinetics, which is essential to integrating nanowires into the
CMOS process In this study, we have investigated the growth
kinetics of Si nanowires fabricated with Pt via the VLS
mecha-nism, and compared the results with those for nanowires
fabri-cated with Au
2 Experimental Silicon nanowires were fabricated by the conventional CVD pro-cess using silicon tetrachloride (SiCl4) as a precursor Using an E-beam evaporator, Si (1 1 1) wafers were coated with Au and Pt as catalysts, respectively, to a thickness of 5 nm Portions of the cata-lyst-coated substrates were then placed in a quartz tube inside a CVD furnace As the furnace temperature was increased, flow of di-luted H2(100 standard cubic centimeter per minute, sccm) and Ar (100 sccm) gas was initiated SiCl4 was carried into the reactor quartz tube by H2 that had been passed through a bubbler that maintained SiCl4in a liquid state at 0 °C To investigate the growth kinetics, processing time (1, 5, 10, and 20 min), temperature (900–
1100 °C), and SiCl4/H2concentration (5, 10, 15, and 20 sccm) were manipulated
3 Results and discussions Both Pt and Au successfully catalyzed the growth of Si nano-wires Fig 1 shows scanning electron microscopy (SEM) images and transmission electron microscopy (TEM) observations The SEM images shown inFig 1a and b reveal Si nanowires with diam-eters of 100 nm and lengths of severallm The Si nanowires in Fig 1a and b were grown under identical conditions with the exception of the metal catalyst that was used The figures show that the nanowires grew vertically on the substrate in both cases
It is well known that the epitaxial relationship between substrate and nanowires can be attributed to the vertical growth of nano-wires [7,8]and thus, similar to Au, Pt yields epitaxial interfaces between the nanowires and Si substrate The SEM image also shows that the Si nanowires grown using Pt was far longer than those grown using Au
0009-2614/$ - see front matter Ó 2008 Elsevier B.V All rights reserved.
* Corresponding author Fax: +82 2 365 5882.
E-mail address: hjc@yonsei.ac.kr (H.J Choi).
Contents lists available atScienceDirect Chemical Physics Letters
j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / c p l e t t
Trang 2The TEM images and selected area electron diffraction (SAED)
patterns inFig 1c and d clearly show that the Si nanowires are
sin-gle crystalline, without any structural defects Alloy globules of
Pt–Si or Au–Si were formed at the tips of the nanowires An energy
dispersive spectroscopy (EDS) analysis across the catalyst-nanowire interface at the tip of an individual catalyst-nanowire clearly shows that Pt has operated as a VLS catalyst, like the Au catalyst (Fig 1e and f) Note that a Si-rich globule was formed when using
Fig 1 Synthesis of Si nanowires using Au and Pt as catalysts (a) Vertically-aligned Si nanowires using Au (b) Vertically-aligned Si nanowires using Pt Si nanowires using Pt are longer than those using Au (c) HRTEM image of Si nanowire using Au (d) HRTEM image of Si nanowire using Pt Both were single crystal (e) EDS line mapping of Si nanowires using Au catalyst (f) EDS line mapping of Si nanowires using Pt catalyst (g) HRTEM image of interfaces between the globule and nanowire grown with Pt Arrows indicates the platinum silicides showing different lattice fringe to Si (h) HRTEM image of interfaces between the globule and nanowire grown with Au.
Trang 3the Pt catalyst, while an Au-rich globule was formed when using
the Au catalyst One reason for this compositional difference may
be the differential solubility of Si in Pt and Au Specifically, the
sol-ubility of Si in Au is 18.6 at%[9], whereas in Pt it is as high as 67 at%
[10]at their eutectic points Therefore, it is possible that the Pt
cat-alyst under a liquid state has a Si-rich composition The
composi-tional differences may also be attributable to the formation of
platinum silicide upon cooling We characterized the catalyst
glob-ule/nanowires interfaces by HRTEM and observed platinum silicide
layers showing different lattice fringes from Si in the nanowires
grown with Pt (Fig 1g) However, no such layers were observed
at the interfaces with Au (Fig 1h)
holding time for the Pt and Au catalysts, respectively, at 1000 °C
with a SiCl4flow rate of 20 sccm The growth rate of the Si
nano-wires was constant with time in both cases This indicates that Pt
maintains a catalytic role throughout the growth of the Si
nano-wires, as does Au, without loss of components by chemical reaction
or evaporation Thus, it has been demonstrated that Pt is a stable
catalyst for the VLS mechanism The growth rate of Si nanowires
with Au was 5.20lm/min while Pt showed a 2.28 times faster
growth rate (11.86lm/min), with a 20 sccm SiCl4flow rate in both
cases
rate of SiCl4at 1000 °C The holding time was fixed at 20 min The
figure shows that the growth rate with Pt is higher than that with
Au under all flow rates of SiCl4 It also shows that the growth rate
decreased with the SiCl4flow rate and was nearly saturated at a
flow rate of 20 sccm The VLS process consists of four main steps:
(1) mass-transport in the gas phase; (2) chemical reaction on the
vapour–liquid interface; (3) diffusion in the liquid phase; and (4)
incorporation of Si atoms in a crystal lattice[11–14] Identification
of the rate-determining step among these steps is important for
understanding the kinetics However, this is very complicated,
since three phases, two interfaces (that is, vapour–liquid and
liquid–solid interfaces), and chemical reactions are involved[11]
Nevertheless, it may be possible to draw some insights based on
previous and current studies
Among those steps, step (3) can be excluded, since Si atoms
dif-fuse in liquid metals very quickly [12] and thick nanowires or
whiskers do not grow more slowly than thin nanowires while
the shape of the liquid droplet is maintained to be almost
hemi-spherical, and thus holds longer diffusion length [11] Step (1)
can also be excluded since the diffusion coefficient in the gaseous
phase usually follows a power law D = D0(T/T0)n(P/P0), n = 1.5–2 [11,12], and thus the growth rate should follow the power law However, this is not the case in the present study as well as in pre-vious studies[11–14]
The primary evidence for regarding step (2) as the rate-limiting step is that the growth rate is proportional to the partial pressure
of reactant gas However, this does not fully support the argument since the growth process consists of two activated steps in series [11] The dependence of the growth rate on the reactant vapour concentration is not in itself evidence that any of them is the rate-determining step Rather, it simply reflects the dependence
of the growth rate on supersaturation Furthermore, as shown in
20 sccm; i.e., in the present growth conditions, the growth rate is not proportional to the partial pressure of reactant gas Therefore, the rate-determining step would be step (4), incorporation of Si atoms in a crystal lattice Accordingly, Pt would affect this step, resulting in a different growth rate
We determined the activation energy for the growth of the Si nanowires, as shown in Fig 4, by plotting the growth rate with
0
50
100
150
200
Time (min)
Fig 2 Length of Si nanowires using Au and Pt versus time at holding temperature
0 50 100 150
SiCl4/H2 (sccm)
Fig 3 Growth rate versus SiCl 4 flow rate at 1000 °C.
Fig 4 Construction of log-scaled growth rate and reciprocals of growth
Trang 4Temper-temperature, assuming that the growth rate and temperature are
satisfied by the Arrhenius equation We determined the activation
energy with a SiCl4flow rate of 20 sccm Under these conditions,
the growth rate is saturated and thus supersaturation of the
cata-lyst is nearly maximized Therefore, step (2) can be excluded as the
rate-determining step.Fig 4shows that the growth rate does not
follow the power law of D = D0(T/T0)n(P/P0) for gas diffusion
coeffi-cient[11,12]but strongly depends on temperature This supports
the previous argument that step (1) is not the rate-determining
step FromFig 4, the calculated activation energy values were 80
and 130 kJ/mol for the Pt and Au catalysts, respectively These
val-ues are much higher than the activation energy for the diffusion of
Si atoms throughout most liquid metals ranges, i.e., between 10.5
and 36 kJ/mol, as well as the activation energy for Si diffusion in
Au–Si liquid (about 25 kJ/mol) and that in Pt–Si liquid (54 kJ/
rate-determining step Since steps (1)–(3) can be excluded again
as described above, it is concluded that step (4), incorporation of
Si atoms in a crystal lattice at the liquid/solid interfaces, is the
rate-limiting step under the present conditions, for both the Pt
and Au catalyst cases
The lower activation energy with Pt implies that step (4) is
dependent on the type of catalyst This could be explained by
the effect of the catalyst on the barrier energy to the nucleation
of a new step at the growth front of the nanowires (i.e., the
li-quid/solid interfaces where the incorporation of the Si atoms in a
crystal lattice occurs) The (1 1 1) plane of Si is a smooth surface
where the formation of a new lattice plane is difficult[17] and,
thus the incorporation of Si atoms on the plane is rather difficult
Under these conditions, the incorporation rate may critically
de-pend on the nucleation of new steps and the barrier to this
nucle-ation may depend on the metal catalyst It is noted that the
activation energy for the deposition of Si through a continuous
Pt–Si liquid film is lower than that through Au–Si films[18] This
is coincident with our observations and supports the argument
that Pt enhances the rate of incorporation of Si from the liquid
phase to the lattice
The present results suggest that Pt is a good candidate for the
VLS growth of Si nanowires However, the growth temperature of
the present study is sufficiently high to induce substantial dopant
diffusion and thus too high for application to CMOS processing
Nevertheless, the results illustrate some fundamental aspects of
growth kinetics of Si nanowire with Pt The development of a
CMOS compatible Si nanowire growth process using Pt as a VLS catalyst is potentially a fruitful direction for future research
4 Summary
Si nanowires were successfully synthesized using Au and Pt via the VLS mechanism The growth rate of Si nanowires using Pt was 2.3 times faster than that of Si nanowires using Au The rate-deter-mining step of the overall VLS mechanism under the present growth conditions was the incorporation of Si atoms in a crystal lattice at liquid/solid interfaces The difference in the activation en-ergy between two catalysts (i.e., 130 kJ/mol with the Au catalyst and 80 kJ/mol with the Pt catalyst) may be attributable to the effect
of the metal catalyst on the rate-determining step
Acknowledgments This work was supported by the Program of the National Re-search Laboratory (Grant R0A-2007-000-20075-0) of the Korean Ministry of Science and Technology (MOST), and the Korean Re-search Foundation (MOEHRD, KRF-2005-042-D00203)
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