1.1 Introduction
Low-dimensional materials, such as nanowires, have attracted
considerable attention over the last decade due to their potential
for forming structures with tailored composition, crystallinity,
and morphology and hence with tunable electronic, optical,
magnetic, mechanical, and other properties. The most widely used
approach to grow nanowires involves deposition from vapor phase
in the presence of a liquid catalyst and is commonly known as
vapor–liquid–solid (VLS) process.
1
proposed in the early 1960s for the Au-mediated growth of Si
whiskers.
2–5
In this process, the metal alloys (e.g., Au–Si eutectic)
with the depositing material and forms a liquid alloy which promotes
material incorporation at the liquid–solid interface and results in the
Chapter 1
In situ Observations of Vapor–Liquid–Solid
Growth of Silicon Nanowires
Silicon and Silicide Nanowires: Applications, Fabrication, and Properties
Edited by Yu Huang and King-Ning Tu
Copyright © 2014 Pan Stanford Publishing Pte. Ltd.
ISBN 978-981-4303-46-0 (Hardcover), 978-981-4303-47-7 (eBook)
www.panstanford.com
S. Kodambaka
Department of Materials Science and Engineering, University of California,
Los Angeles, CA 90095, USA
2
In situ Observations of Vapor–Liquid–Solid Growth of Silicon Nanowires
formation of one-dimensional (1-D) pillars or “wires.” Variants of
VLS method include the so-called vapor–solid–solid (VSS) process,
where solid instead of liquid catalyst promotes 1-D growth;
6
self-
catalyzed process, where one of the components of the growing
material acts as a catalyst;
7
and oxide- and template-assisted
processes, where oxygen and patterns aid wire formation.
8
Over the
past 50+ years, these methods have been used to grow whiskers and
nanowires of a wide variety of elemental metals, semiconductors
as well as compound arsenides, borides, carbides, nitrides, oxides,
phosphides, selenides, sulphides, and tellurides.
9
An exhaustive list
of all the materials grown, methods employed, catalysts and growth
parameters used can be found in Ref. 10.
Recently, nanowires of silicon have generated a lot of interest for
applications in Li ion batteries and in thermoelectric devices.
11 –14
Given the growing demand for sustainable energy, large-scale, low-
advantage in the fabrication of such devices because they can form
single-crystalline, defect-free structures with atomically abrupt
interfaces and surfaces at desired locations even on amorphous or
lattice mismatched substrates.
15
To-date, Si nanowires have been
grown via VLS using Au,
1
Ag,
1
Ga,
16
Au–Ga,
17
and In,
18
and via VSS
using Ti,
19
Al,
20
Ni,
21
Cu,
22
and Pd.
23
Over the last decade, remarkable
progress has been made in the areas of nanowire synthesis, device
fabrication, and characterization. Readers interested in the growth
by Schmidt and co-workers.
24
Recent attempts to obtain desired
nanowire heterostructures have yielded limited success and
have proven to be a challenging task.
25–27
Improvements in the
performance of nanowire-based devices and the development of
large-scale fabrication procedures, however, require a fundamental
understanding of and precise control over morphological,
compositional, and structural evolution of nanowires.
Pioneering studies by Bootsma and Gassen
28
and by Givargizov
29–31
provided important insights into the VLS growth process. The
classic analysis of Givargizov
29
predicted that narrower wires
should grow more slowly than wider wires, that is, the growth rate
dL/dt increases with wire diameter d, due to the Gibbs–Thomson
32
with a critical diameter below which growth cannot occur.
The opposite behavior, that is, dL/dt decreasing with d, is expected
3
33
For the growth of Si nanowires, some experiments
29
agree with
Givargizov’s prediction while others do not.
34–37
In most of these
reports, ex situ post-growth measurements of nanowire lengths
were used to determine the growth rates. While post-growth
characterization is essential for the determination of chemical
composition, interface structure, and wire morphologies, inference
In situ
In situ experiments are ideally suited to follow the kinetics of
nucleation and growth of individual nanowires and to quantitatively
determine the underlying mass transport mechanisms and reaction
pathways. The advent of sophisticated in situ characterization
techniques has enabled monitoring of dynamic materials phenomena
such as nucleation and growth. A variety of methods, such as
scanning and transmission electron microscopies [scanning electron
microscopy (SEM) and transmission electron microscopy (TEM)],
to follow the nucleation and growth of nanowires.
23,38–49
Of all these techniques, in situ
method that was used over 40 years ago to observe the growth of
nanowires. (Readers are advised to check the references within the
book chapter by Givargizov.
10
) In situ microscopy permits direct
determination of nanowire growth, helps minimize the uncertainties
in growth-rate measurements, and, since the structures are observed
under growth conditions, often reveals surprising and previously
unknown aspects of the growth process as we show in the following
sections. Moreover, the ability to directly observe the changes as a
not only saves enormous amount of time required otherwise but
Hence, in situ characterization has the potential to promote rapid
progress in the development of methods for the fabrication of complex
nanowire architectures. Here, we review the recent in situ ultra-high
vacuum transmission electron microscopy (UHV TEM) observations
of the nucleation and growth of Si nanowires via the VLS process at
in situ TEM studies of nanowire
23,47
In the
following sections, we will present experimental results focused on
Introduction
4
In situ Observations of Vapor–Liquid–Solid Growth of Silicon Nanowires
the kinetics of nucleation and growth of Si nanowires using Au as the
catalyst.
1.2 Experimental
All the experimental results presented in this chapter are obtained
using a custom-designed Hitachi H-9000 UHV (base pressure
2 × 10
–10
Torr) TEM with a LaB
6
The microscope is part of a multi-chamber system, located at the
IBM T. J. Watson Research Center, and is equipped with a thermal
evaporator and gas-dosing facilities for physical and chemical vapor
deposition (PVD and CVD), respectively (see Fig. 1.1).
50
Figure 1.1 (Left) Schematic of the custom-designed sample column, part
of the IBM UHV TEM. (Right) The top and bottom images show
used for nanowire growth and nucleation experiments,
respectively.
For CVD, precursor gases, such as disilane (Si
2
H
6
) for Si deposition,
are introduced into the sample region through UHV leak valves which
facilitate precise control of the total pressure in the system. In this
particular UHV-TEM, the maximum attainable pressures are limited
to ~10
–4
Torr during the operation of the TEM. (In other systems,
however, it is possible to vary the pressure over wider range, e.g.,
5
Experimental
high-pressure cell.
51,52
) Since the permissible pressures are lower
than that typically used for nanowire growth in a CVD reactor, the
nanowire nucleation and growth rates are also lower, which is ideal
for real-time monitoring of the growth phenomena. Moreover, low
doses of oxygen or water vapor, the common contaminants in a CVD
reactor, can be controllably introduced via the leak valves and their
.
In order to carry out the in situ experiments, a specially designed
the samples. For the nanowire nucleation and growth experiments,
view samples work best for observation of nanowire nucleation and
suited for the observation of the growth of nanowires.
53
Chemically-
thinned polished Si(111) wafers with ~80 nm–thick amorphous
silicon nitride (a-SiN
x
a-SiN
x
membranes are thermally stable, chemically inert, relatively
easy to clean, and hence are ideal for the nucleation experiments.
These samples are cleaned in the UHV preparation chamber,
attached to the microscope by slow heating to ~600 °C, and held
until the base pressure is below 5 × 10
–10
Torr. For the nanowire
growth experiments, rectangular pieces (4 mm × 1.5 mm × 0.5 mm)
of Si(111) are cut from a Si(111) wafer, cleaned chemically by dipping
in HF solution to remove the native oxide, and mounted such that one
of the polished surfaces is parallel to the electron beam as shown in
Fig. 1.1. The samples are cleaned in the preparation chamber by a
and held for a few hours until the base pressure in the system is below
5 × 10
–10
Torr. Then, the samples are heated rapidly to ~1250 °C
and held for 30–60 s after which they are cooled to ~600 °C. This
procedure yields oxide-free Si surface, desirable for accurate
description of the growth phenomena. For both the nucleation and
Au and Al was prepared by sequential evaporation with an atomic
ratio of 2:1.) The samples are then transferred without breaking
vacuum to the microscope and heated to the desired temperature
(450 ~ 650 °C) in presence of Si
2
H
6
gas at pressures between
1 × 10
–8
and 1 × 10
–5
Torr. TEM images are acquired at video rate (30
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