Three-dimensional nanoscale composition mapping of semiconductor nanowires.
ABSTRACT We demonstrate the three-dimensional composition mapping of a semiconductor nanowire with single-atom sensitivity and subnanometer spatial resolution using atom probe tomography. A new class of atom probe, the local electrode atom probe (LEAP) microscope, was used to map the position of single Au atoms in an InAs nanowire and to image the interface between a Au catalyst and InAs in three dimensions with 0.3-nm resolution. These results establish atom probe tomography as a uniquely powerful tool for analyzing the chemical composition of semiconductor nanostructures.
Composition Mapping of Semiconductor
Daniel E. Perea,†Jonathan E. Allen,†Steven J. May,†Bruce W. Wessels,†,‡,§
David N. Seidman,†,§and Lincoln J. Lauhon*,†,§
Department of Materials Science and Engineering, Department of Electrical
Engineering and Computer Science, and Materials Research Center,
Northwestern UniVersity, EVanston, Illinois 60208
Received August 12, 2005; Revised Manuscript Received September 19, 2005
We demonstrate the three-dimensional composition mapping of a semiconductor nanowire with single-atom sensitivity and subnanometer
spatial resolution using atom probe tomography. A new class of atom probe, the local electrode atom probe (LEAP) microscope, was used
to map the position of single Au atoms in an InAs nanowire and to image the interface between a Au catalyst and InAs in three dimensions
with 0.3-nm resolution. These results establish atom probe tomography as a uniquely powerful tool for analyzing the chemical composition
of semiconductor nanostructures.
Semiconducting nanowires of controlled composition and
doping1-4show great promise as multifunctional components
in a number of emerging device technologies.5-7The
continued advancement of these nanometer-scale devices will
depend critically on knowledge of their atomic-scale struc-
ture8because compositional fluctuations as small as a single
dopant atom can affect device performance. It is therefore
highly desirable to determine the composition of individual
nanowires with the utmost precision. The spatial resolution
of secondary ion mass spectroscopy (SIMS) has been pushed
below 100 nm,9but the nanowire length-scales of interest
are much smaller. Transmission electron microscopy (TEM)
is capable of imaging single dopant atoms under specific
conditions10but TEM cannot yet be considered a general
tool for the volumetric mapping of low-concentration ele-
ments in nanostructures. The important challenge of doping
atoms into the “bulk” of nanowires and nanocrystals while
avoiding surface segregation further emphasizes the need for
three-dimensional composition characterization in these
Here we demonstrate the three-dimensional composition
mapping of a semiconductor nanowire with single-atom
sensitivity and subnanometer spatial resolution using atom-
probe tomography.11A new class of atom probe, the local
electrode atom probe (LEAP) microscope,12,13was used to
map the position of single Au atoms in an InAs nanowire
and to image the interface between a Au catalyst and InAs
nanowire in three dimensions with 0.3-nm resolution. These
results establish atom probe tomography as a uniquely
powerful tool for analyzing the chemical composition of
Functional one-dimensional semiconductor nanostructures
have been synthesized by a number of methods.14In the
present study, electron beam lithography followed by metal
evaporation and lift-off was used to pattern an array of Au
catalyst disks on a GaAs(111)B wafer. The Au seeds
catalyzed the growth of InAs nanowires in a quartz metal-
organic vapor-phase epitaxy (MOVPE) reactor with tri-
methylindium and arsine as reactant chemical precursors and
hydrogen as the carrier gas under conditions described
previously.15In this manner, arrays of epitaxial vertically
oriented InAs nanowires 100-140 μm long and spaced 500
μm apart were generated, which facilitated analysis with the
LEAP microscope (Figure 1a).
The capability of mapping nanowire composition at the
atomic scale originates from the sequential field evaporation
of individual atoms from a nanowire tip (Figure 1a inset,
Figure 1b). When a positive voltage is applied to a nanowire
through the growth substrate, a very large electric field
develops at the nanowire tip because of the high local radius
of curvature. This local electric field reduces the potential
energy barrier that bonds an atom to the surface, resulting
in field evaporation of a positive ion (Figure 1c). In the LEAP
microscope, subnanosecond voltage pulses are applied at 200
kHz to a conical electrode positioned above the nanowire,
* Corresponding author. E-mail: email@example.com.
†Department of Materials Science and Engineering.
‡Department of Electrical Engineering and Computer Science.
§Materials Research Center.
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Published on Web 10/18/2005
© 2006 American Chemical Society
leading to sequential field evaporation of single ions at the
pulsing frequency (Figure 1d).16The mass-to-charge state
ratio, m/n, of an evaporated ion is determined by measuring
the time delay between the local electrode voltage pulse and
the signal generated when the ion strikes a two-dimensional
position-sensitive detector;17the calibrated time-of-flight
between the nanowire tip and the detector is proportional to
the square root of m/n. The initial position of an atom on
the nanowire tip is determined by reconstructing the flight
path between the detector and the tip. In this manner, a three-
dimensional reconstruction of the nanowire composition may
be generated atom-by-atom with subnanometer resolution.11,18
Figure 2 presents a three-dimensional reconstruction of
an InAs nanowire generated from a data set of 1.3 × 106
atoms evaporated sequentially in the LEAP microscope.19
Single atoms are represented by dots, and the dot color
indicates its chemical identity as determined by the time-
of-flight measurement. The side view of Figure 2a shows
that analytical volumes comparable to an entire nanowire-
based device, such as the channel of a nanowire transistor,20
are well within the capabilities of this technique. When
looking along the nanowire axis (Figure 2b), hexagonal facets
are clearly observed, and comparison with a scanning
electron microscopy image (Figure 2c) verifies that we have
reconstructed the nanowire cross-section accurately.21More
significantly, a magnified view perpendicular to the nanowire
axis reveals distinct planes of atoms extending across the
nanowire (Figure 2d). These planes lie perpendicular to the
wurtzite  growth direction15at a spacing of 0.35 nm,
as determined by transmission electron microscopy (TEM),
and the observation of atomic planes in the LEAP recon-
struction demonstrate the achievement of subnanometer
It is important to note that while Figure 2 presents useful
two-dimensional projections, the three-dimensional position
and chemical identity of every atom is retained in the original
data set, which is not generally the case for projections
generated in transmission electron microscopy (TEM) mea-
surements. To emphasize this point, Figure 2e presents an
enlarged section of the nanowire of Figure 2a using spheres
to represent each individual Au atom found within the
analyzed volume. From the figure, it is obvious that dopant
concentrations and fluctuations on any length scale and along
any axis can be extracted by averaging over a chosen volume.
The implications of our observation of catalyst atoms within
the nanowire are not discussed here, but we can immediately
draw two important conclusions regarding the potential for
LEAP tomographic analysis of nanostructures. First, one can
now determine whether the catalyst used in nanowire growth
schemes becomes incorporated in the bulk of the nanowire
for nanowires of arbitrary composition. This question is of
critical importance to the performance of nanowire devices
because metal atoms such as gold can strongly influence
electronic properties. Second, and more generally, LEAP
microscopy can be used to reveal the concentration and
distribution of dopants in a range of chemically synthesized
nanostructures,14and may therefore play a critical role in
addressing the major challenge of dopant incorporation and
segregation in nanomaterials.
Another important capability of the LEAP microscope is
the imaging of interfaces within a nanostructure because
functional heterointerfaces form the basis of most semicon-
ductor devices.22In particular, one would like to be able to
analyze the compositional abruptness of junctions within
nanowires without averaging over the nanowire diameter.
Analysis of a reconstructed nanowire section including a Au
catalyst/InAs nanowire heterophase interface is displayed in
Figure 3. As expected, the gold catalyst lies atop the
nanowire during and after growth. The four “slices” of Figure
3a, two on either side of the interface, demonstrate that
Figure 1. Scheme for three-dimensional nanowire composition mapping. (a) InAs nanowires grown epitaxially on a GaAs(111) substrate
using gold catalyst arrays. The nanowire in the foreground was analyzed by LEAP microscopy; the background nanowires, indicated by the
dashed white lines, form a fiducial cross used to facilitate specimen alignment (scale bar 20 μm). The inset shows the foreground nanowire
tip (scale bar 50 nm). (b) A large positive voltage creates an electric field at the nanowire tip sufficient to induce field evaporation of atoms.
Enhanced field evaporation at surface asperities leads to the development of an approximately hemispherical tip shape. (c) Potential energy
diagram of a surface atom with no applied voltage (light green curve) and at high voltage (dark green). (d) Schematic of LEAP microscope.
An individual nanowire is positioned beneath the local electrode by using two cameras to locate the fiducial cross, shown in a, and position
the sample stage accordingly. Negative voltage pulses on the local electrode produce field evaporation of atoms from the nanowire tip. The
positively charged ions travel through an orifice in the local electrode and toward the detector, where the position and time-of-flight is
Nano Lett., Vol. 6, No. 2, 2006
tomographic analyses can reveal both radial and axial
composition variations within a nanowire. Although the
catalyst-nanowire interface appears qualitatively abrupt in
Figure 3b, the interface width is more readily ascertained
by plotting a one-dimensional composition profile derived
from a cylindrical section perpendicular to the interface
(Figure 3c); the interface between the catalyst (Au0.9In0.1)
and nanowire (In0.5As0.5) is extremely abrupt with a width
of less than 0.5 nm. Because nanowire growth occurs at this
interface, its structure may influence the widths of intrawire
semiconductor heterojunctions formed by similar nanowire
growth processes, particularly the metal-catalyzed vapor-
liquid-solid growth process.23
Nanowire heterointerfaces have also been analyzed in
some detail by TEM,1,3,24,25but the one-dimensional com-
position profiles determined by energy-dispersive X-ray
spectroscopy or electron energy loss spectroscopy may show
artificially large interface widths because the signal is
averaged over the entire volume through which the electron
beam passes. This represents a serious limitation when
Figure 2. Three-dimensional reconstruction of an InAs nanowire. (a) Side view (perpendicular to growth axis) of a 25 × 25 × 300 nm3
reconstruction of nanowire. In, As, and Au atoms are rendered as green, purple, and yellow dots, respectively. Only 5% of the atoms are
shown to provide a sense of depth. (b) A 21 × 21 nm2end-on view of nanowire reconstruction showing hexagonal faceting. (c) Scanning
electron micrograph of an InAs nanowire showing hexagonal cross section (1.7 μm2). (d) Magnified side view of the nanowire showing
(0001) atomic planes. The dimensions are 23 × 14 nm2. The slight curvature of the planes is an artifact; the software used for the reconstruction
assumes a hemispherical end-shape for the evaporating nanowire. (e) A 27 × 27 × 29 nm3reconstruction of the nanowire with Au atoms
enlarged and 2% of In and As atoms shown for clarity; the growth axis runs left to right. The 18 Au atoms within the volume correspond
to a concentration of 100 atomic parts per million.
Nano Lett., Vol. 6, No. 2, 2006 183
analyzing nanostructures of varying radial composition, such
as core-shell nanowires.2Furthermore, TEM elemental
analysis has a sensitivity of approximately 1% for general
chemical identification. In contrast, the LEAP microscope
provides single-atom sensitivity that could be used to image
small concentrations of dopant atoms on either side of an
intrawire p-n junction.1Cross-sectional scanning tunneling
microscopy (XSTM) is also a powerful tool for analyzing
interfaces in nanowires with atomic resolution,26but we do
not believe that the XSTM will be as generally useful as
atom-probe tomography because it cannot provide general
chemical identification and is restricted to two dimensions.
Historically, atom-probe tomography has been applied more
extensively to metals27than semiconductors,28but atom-probe
analysis of semiconductors and semiconductor devices is
likely to increase greatly because advances in instrumentation
now enable the analysis of sample volumes comparable to
the active region of semiconductor devices.
The present demonstration of three-dimensional composi-
tion mapping of a semiconductor nanowire with subnanom-
eter resolution establishes that LEAP microscopy can play
an important role in the development of semiconductor
nanostructure device technology by providing critical insight
into the connection between synthesis schemes and nano-
scale composition. LEAP microscopy has the potential to
be applied to other semiconductor nanostructures, including
zero dimensional nanocrystals, by depositing samples on
arrays of posts elevated from the substrate.28Furthermore,
considering the concurrent development of nanoscale prop-
erty measurements via scanned probes,29LEAP microscopy
promises to advance materials science by extending our
understanding of structure-property relationships to the
Acknowledgment. The assistance of Dr. Chantal K.
Sudbrack and Dr. Dieter Isheim in initiating the atom
probe measurements is gratefully acknowledged. Helpful
discussions with Imago Scientific Instruments are also
acknowledged. This work was supported by the Office of
Naval Research (L.J.L., N00014-05-1-0566), Northwestern
University, and the National Science Foundation through
the CAREER program (L.J.L., DMR-0449933), MRSEC
seed funding (L.J.L. and B.W.W.), and the Spin Elec-
tronics program (B.W.W., ECS-02242100). Atom-probe
measurements were performed at the Northwestern Center
for Atom Probe Tomography (NUCAPT), and the LEAP
microscope was purchased with funding from the NSF-MRI
(DMR-0420532) and ONR-DURIP (N00014-0400798) pro-
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