NANO
LETTERS
Hybrid Devices from Single Wall Carbon
Nanotubes Epitaxially Grown into a
Semiconductor Heterostructure
2004
Vol. 4, No. 2
349-352
Ane Jensen,*,† Jonas Rahlf Hauptmann, Jesper Nygård, Janusz Sadowski,‡ and
Poul Erik Lindelof
Nano-Science Center, Niels Bohr Institute, UniVersity of Copenhagen,
UniVersitetsparken 5, DK-2100 Copenhagen, Denmark
Received November 10, 2003
ABSTRACT
To take advantage of nanoscale molecular electronic components in semiconductor technology, there will be a desire to integrate new elements
such as one-dimensional (1D) carbon nanotubes in conventional 2D or 3D semiconductor systems. We report on hybrid devices based on
single wall carbon nanotubes encapsulated in epitaxially grown semiconductor heterostructures of GaAs/AlAs and (Ga,Mn)As below and
above the carbon nanotube. In our devices the semiconductor serves as leads to the nanotubes, forming the first reported electronic hybrid
nanotube-semiconductor devices.
We have successfully fabricated electronic devices from
single wall carbon nanotubes (SWNTs) encapsulated in a
semiconductor heterostructure, grown by molecular beam
epitaxy (MBE). SWNTs are tubular molecules of carbon with
unique electron transport properties. These tubes may be
either metallic or semiconducting, determined by their chiral
vector, which circumferences the tube. The electron transport
in the tubes is one-dimensional (1D) and ballistic over a µm
length scale.1 They have the capability of extraordinarily high
current densities, up to about 109 A/cm2,2 even at room
temperature. Given these unique electronic properties, many
possible applications have been proposed for devices consisting of carbon nanotubes.3,4 A promising device is the tube
field-effect transistor (FET), reported in 1998 from semiconducting SWNTs.5,6 High quality semiconductor material,
grown by MBE, already enables semiconductor materials to
be designed into molecular layer-by-layer nanostructures, to
create 2D systems or band gap engineered devices.7 Epitaxially grown semiconductor materials are the foundation
of advanced electronic and photonic devices, and thereby
are of tremendous importance to the electronics and optoelectronics industry.8 The ability to merge conventional
semiconductor techniques with nanoscale molecular electronics is likely to be essential for future integrated nanoelectronics. We find that epitaxial growth can indeed be
compatible with carbon nanotube devices. By incorporating
tubes in semiconductor materials, nanosize circuits with many
* Corresponding author. E-mail: ane@fys.ku.dk
† Also at Department of Physics, DTU, Denmark.
‡ Also at MAX-lab, Lund University, Sweden.
10.1021/nl0350027 CCC: $27.50
Published on Web 01/10/2004
© 2004 American Chemical Society
potential applications may be attained. For instance, onedimensional tubes could be utilized as interconnects in
conventional semiconductor devices, and tube FETs may be
integrated in epitaxially grown semiconductor circuits.
Carbon nanotubes have been interesting for basic research
in 1D electron systems. For example Coulomb blockade (CB)
has been detected at low temperatures,1 in addition measurements indicate the formation of a Luttinger liquid (LL) state
in the 1D tubes.9-11 Since, 2D electron systems in GaAs
based heterostructures have been important for fundamental
mesoscopic physics so far,12 our work could possibly lead
to new combinations of 1D SWNTs and 2D semiconductor
systems that would be exciting for future basic investigations.
We have fabricated integrated devices of SWNTs and
MBE grown heterostructures on III-V GaAs substrates. To
contact the nanotubes we use (Ga,Mn)As, which is a
ferromagnetic semiconductor and has potential in the rapidly
growing field of spintronics.13-15 We explore the new aspects
provided by these structures, e.g., spin transport phenomena16
and magnetically contacted LL.17-19 These results will be
published elsewhere.20
The tube-semiconductor devices were fabricated as schematically presented in Figure 1. First, the substrate was
prepared by MBE, Figure 1a. The base was a heavily n-doped
GaAs layer, serving as a back-gate. On top of this, a 100
period superlattice, which has proven to be an efficient
insulating barrier,21 of 2 nm GaAs plus 2 nm AlAs ended
by 20 nm GaAs was added. Finally, the wafer was capped
by a layer of amorphous As, which is crucial for the later
successful epitaxial overgrowth. Laser ablated SWNTs from
Figure 2. AFM images of tube-semiconductor devices. (a) A
device with two (Ga,Mn)As electrodes (SC-NT-SC type). (b) A
device with one metal (Cr/Au) and one (Ga,Mn)As electrodes (MNT-SC). (c) A schematic image of the SWNT encapsulated in
the MBE grown semiconductor crystal.
Figure 1. Fabrication of tube-semiconductor devices. (a) The MBE
grown substrate of n-doped GaAs with the superlattice barrier and
amorphous As. (b) Nanotubes are dispersed on the As surface. (c)
The substrate with tubes is reloaded in the MBE chamber. Here
the amorphous As is evaporated, leaving the nanotubes on the clean
GaAs crystal surface. (d) The tubes are overgrown by (Ga,Mn)As.
(e) A trench is etched in the (Ga,Mn)As, leaving a nanotube
connecting two islands of (Ga,Mn)As. (f) In a different device, a
metal (Cr/Au) lead is evaporated to contact one end of a tube.
Rice University were deposited on the surface of the
amorphous As, by applying a suspension of the tubes in
dichloroethane while spinning the substrate, Figure 1b. After
tube deposition, the sample was loaded into the MBE
chamber dedicated to (Ga,Mn)As growth. To overgrow the
tubes, the amorphous As was removed by desorption at a
temperature of about 400 °C and the GaAs surface was kept
in As flux at 500-600 °C for several minutes. This process
leaves the nanotubes on the clean and molecular smooth
GaAs crystal surface. Subsequently, the sample was overgrown epitaxially by the ferromagnetic semiconductor
Ga0.95Mn0.05As at 250 °C. The epitaxial growth of singly
crystalline, smooth (Ga,Mn)As layer was confirmed by twodimensional reflection-high energy electron diffraction
(RHEED) patterns. Details of the MBE growth were reported
elsewhere.22 (Ga,Mn)As films of thicknesses from 20 to 50
nm have been prepared. The (Ga,Mn)As was capped by 3
nm GaAs to prevent oxidation. To optimize the magnetic
properties of the semiconductor, annealing of the
(Ga,Mn)As film was performed, by keeping the sample in
the MBE system at a temperature 20 °C higher than the
350
growth temperature for 0.5 to 1.5 h after the growth. The
result is SWNTs encapsulated in the semiconductor, beneath
the epitaxial (Ga,Mn)As film, see Figure 1d.
Lithographic techniques were used to fabricate devices
from the grown structures. First, UV-lithography and wet
etching (using H3PO4/H2O2/H2O (1:1:38)) was employed to
create mesas of (Ga,Mn)As with the encapsulated tubes. By
use of electron-beam lithography, approximately 1 µm wide
and 5-10 µm long trenches were etched in the (Ga,Mn)As,
with the wet etchant, leaving nanotubes as connectors
between separated islands of (Ga,Mn)As, Figure 1e. The
trenches were etched about 20 nm deeper than the
(Ga,Mn)As film, to prevent electrical short cut of the tube
devices by the semiconductor base. Another type of device
has been made. Here, metallic leads (5 nm Cr and 20 nm
Au) defined by e-beam lithography and lift-off technique
were applied next to an edge of the (Ga,Mn)As mesa. In
this manner reference devices consisting of tubes contacted
by one metal lead and one semiconducting lead were
fabricated, Figure 1f.
The tube-semiconductor devices were imaged by atomic
force microscopy (AFM), as presented in Figure 2. The
surface roughness of the GaAs compound semiconductor
structures was around 0.5-1 nm. The tubes were visible in
the trenches between the leads, Figure 2a. The height of the
tubes above the bottom of the trench was typically a few
nm. From this we conclude that the tubes bend in order to
reach the bottom of the trenches. In Figure 2a, where the
tube is covered at both ends by the epitaxially grown
(Ga,Mn)As, no structure of the underlying tube is visible
on the surface of the contacts. This indicates that the
nanotube has been successfully incorporated in the epitaxially
Nano Lett., Vol. 4, No. 2, 2004
Figure 4. (a) Conductance G vs the gate voltage Vg at 4 K for the
SC-NT-SC device. (b) G vs Vg for M-NT-SC device at 300
mK. (c) Gray scale plot of the differential conductance as a function
of bias voltage V and Vg (high dI/dV ) dark). Solid lines indicate
a Coulomb blockade diamond structure.
Figure 3. (a) The resistivity of the 40 nm (Ga,Mn)As film versus
temperature. (b) A typical room temperature I-V curve of the tube
device SC-NT-SC (solid line) and a trench in the (Ga,Mn)As
without a tube SC-SC (dashed line). (c) The conductance versus
temperature for the two devices: SC-NT-SC (solid line) and
M-NT-SC (dashed line). (d) The temperature dependence for
SC-NT-SC on a log-log scale. The line is a power law fit. (e)
T ) 10 K, the differential conductance for SC-NT-SC as a
function of the applied bias voltage on a log-log scale, together
with a power-law fit (line).
grown material. Figure 2c presents schematically how the
SWNT appears in the single-crystal heterostructure. Contrary,
in Figure 2b one tube end is covered by a polycrystalline
metal electrode of the same thickness as the (Ga,Mn)As layer,
and made by thermal evaporation. Here the surface bears an
imprint of the tube beneath it. Similar images of a nanotube
under metal leads are often reported in the literature.23
The transport properties of bulk (Ga,Mn)As films were
investigated. The resistivity as a function of the temperature
for a 40 nm thick film is presented in Figure 3a. The
resistivity of the film shows a local maximum, which is
typical for such films.14 The maximum in the resistivity, here
70 K, provides an estimate of the Curie temperature of the
magnetic semiconductor.
The transport measurements of the tube-semiconductor
devices were two-terminal dc measurements, as is shown
schematically in Figure 1e. The bias voltage V is applied to
the source, and the current I is measured from drain to
ground. On the third terminal, the gate, a voltage Vg is
applied. We have focused on measurements from two
devices: (i) A SWNT contacted by two semiconducting leads
of (Ga,Mn)As, denoted by SC-NT-SC and (ii) A SWNT
with a metal (Cr/Au) source contact and (Ga,Mn)As drain,
denoted M-NT-SC. These devices are chosen to represent
results from a collection of 20 measured devices.
The I-V characteristics of the SC-NT-SC device
displayed in Figure 3b (solid) is nearly linear at room
temperature with a conductance G ≈ 0.2e2/h In the same
plot is shown the I-V measured on another device with no
tube between the two contacts (dashed curve). Here the
current is negligible (below 1 nA with 0.5 V bias) compared
to device SC-NT-SC, proving that leak currents through
the substrate can be neglected and that the measured transport
Nano Lett., Vol. 4, No. 2, 2004
in device SC-NT-SC indeed occurs through the contacted
nanotube. The present tube devices displayed no gate voltage
dependence of G at room temperature (not shown), i.e., these
incorporated nanotubes are metallic.1,24
The temperature dependence of G is shown in Figure 3c
for the two devices SC-NT-SC and M-NT-SC. A
decrease in conductance with decreasing temperature is found
in all our investigated devices. The behavior of SC-NTSC is consistent with a power law, G ∝ TR with R ≈ 0.6, as
presented in Figure 3d. We find similar power law-like
behavior in the majority of our devices, with the scaling
coefficients 0.5 e R e 1. The nonlinear I-V characteristics
for SC-NT-SC, as shown in Figure 3e, can be fitted to a
similar power law of dI/dV ∝ VR with R ≈ 0.7. Power lawlike characteristics have previously been found in nanotube
devices.11,24,25 Here it has been attributed to the suppression
of tunneling into an LL in the tubes due to strong e-e
interactions in the 1D electron system.9,10 The power R is
determined by the strength of the e-e interactions and the
screening provided by the environment. Although the
environment of our nanotubes embedded in a GaAs crystal
(r ≈ 12) is different from typical devices prepared on SiO2
substrates (r ≈ 4) we find surprisingly that the scaling
coefficients are comparable to such devices, see, e.g.,
ref 11.
At low temperatures clear oscillations appeared in G vs
Vg as shown in Figure 4a (SC-NT-SC at 4 K) and Figure
4b (M-NT-SC at 0.3 K). These are signs of CB,26,27
showing that a quantum dot is formed in the nanotube
segment confined by the source and drain contacts.1,28,29 The
CB behavior furthermore indicates that tunnel barriers are
formed between the semiconductor as well as the metal
contacts and the nanotube.
In Figure 4c a plot of the differential conductance against
V and Vg is shown for device M-NT-SC. Dashed lines
indicate a CB diamond pattern from which we can deduce
the electrostatic charging energy U ≈ 1.5 meV. The
asymmetric diamond structure is common in this type of
devices. The left sloping lines represent transition where the
highest unoccupied level of the nanotube is aligned with the
source, i.e., the metal contact. These diamond edges have
the highest conductance (darkest) and the smallest absolute
351
slope, indicating that the metal source contact has the most
efficient capacitative coupling to the tube.26,27
We find that the CB peaks are spaced by around ∆V GaAs
g
) 10-20 mV in our devices with a 400 nm GaAs/AlAs
dielectric between tube and gate. In previous nanotube
devices on Si substrates with SiO2 gate oxide barriers of
2
similar thickness, the typical peak spacing is ∆V SiO
) 50g
100 mV.23,24,29 For any quantum dot we have ∆Vg ≈ e/Cg,
where Cg is the gate capacitance which we expect to scale
with the dielectric constant, Cg ∝ r. The smaller peak
spacings in our devices reflect the stronger gate coupling
due to the about three times higher dielectric constant for
the GaAs/AlAs superlattice compared to the dielectric
constant for SiO2.
In all previously reported nanotube transport experiments
the tubes were contacted by evaporated polycrystalline metal
electrodes. We have successfully incorporated single-wall
carbon nanotubes in semiconductor heterostructures by
epitaxial overgrowth. We find that the semiconductor material can provide electrical contacts to the nanotubes. In our
devices we observe CB at low temperature as well as
indications of LL behavior, confirming that the electron
transport is dominated by the properties of the embedded
nanotubes. Further work is needed in order to explore routes
for improving the nanotube-semiconductor contacts. We
have utilized a ferromagnetic semiconductor for these
experiments. In a subsequent publication we will describe
in detail the effect of the spin valve properties of the devices.
In this context we note that new aspects of Luttinger liquids
in nanotubes may be accessed at temperatures below Tc
where our contacts are magnetic.17-19
Generally, the novel fabrication technique combining MBE
and nanotube devices may enable new nanoscale electronic
and photonic devices, based on hybrids of epitaxially grown
semiconductor structures and nanotubes. Future work may
investigate the compatibility of nanotube devices and other
material systems and growth methods such as Si MOVPE,
which is important for industrial semiconductor manufacturing.
Acknowledgment. We thank Brian Skov Sørensen, Søren
Erfurt Andresen, and Jørn Bindslev Hansen for fruitful
discussions, and Kasper Grove-Rasmussen for assisting with
the AFM. The MBE growth was carried out in the MBE
352
systems located at MAX-lab, Lund University and III-V
nanolab, operated jointly by the NBIfAPG, University of
Copenhagen, and Research Center COM, the Technical
University of Denmark. This work is supported by NEDO
spintronics and the Danish Research Councils (STVF and
SNF).
References
(1) Carbon Nanotubes. Synthesis, Structure, Properties, and Applications.
In Topics in Applied Physics; Dresselhaus, M. S., Dresselhaus, G.,
Avouris, Ph., Eds.; Springer-Verlag: Berlin, 2001; Vol. 80.
(2) Yao, Z.; Kane, C. L.; Dekker, C. Phys. ReV. Lett. 2000, 84, 2941.
(3) McEuen, P. L.; Fuhrer, M.; Park, H. IEEE Trans. Nanotech. 2002,
1, 78.
(4) Baughman, R. H.; Zakhidov, A. A.; de Heer, W. A. Science 2002,
297, 787.
(5) Martel, R.; Schmidt, T.; Shea, H. R.; Hertel, T.; Avouris, Ph. Appl.
Phys. Lett. 1998, 73, 2447.
(6) Tans, S.; Verschueren, A. R. M.; Dekker, C. Nature 1998, 393, 49.
(7) Weisbuch, C.; Vinter, B. Quantum Semiconductor Structures,
Fundamentals and Applications; Academic Press Inc.: San Diego,
1991.
(8) Davies, J. The Physics of Low-dimensional Semiconductors: An
Introduction, Cambridge University Press: Cambridge, 1998.
(9) Kane, C. L.; Balents, L.; Fischer, M. P. A. Phys. ReV. Lett. 1997,
79, 5086.
(10) Egger, R.; Gogolin, A. O. Phys. ReV. Lett. 1997, 79, 5087.
(11) Bockrath, M. et al. Nature 1999, 397, 598.
(12) Stormer, H. L. et al. Phys. ReV. Lett. 1983, 50, 1953.
(13) Wolf, S. A. et al. Science 2001, 294, 1488.
(14) Ohno, H. Science 1998, 281, 951.
(15) Ohno, H. J. Magn. Magn. Mater. 1999, 200, 110.
(16) Tsukagoshi, K.; Alphenaar, B. W. Nature 1999, 401, 572.
(17) Balents, L.; Egger, R. Phys. ReV. Lett. 2000, 85, 3464.
(18) Si, Q. Phys. ReV. Lett. 1998, 81, 3191.
(19) Mehrez, H.; Taylor, J.; Guo, H.; Wang, J.; Roland, C. Phys. ReV.
Lett. 2000, 84, 2682.
(20) Jensen, A. et al., to be published.
(21) Kawaharazuka, A.; Saku, T.; Hirayama, Y.; Horikoshi, Y. J. Appl.
Phys. 2000, 87, 952.
(22) Sadowski, J. et al. J. Vac. Sci. Technol. B 2000, 18, 1697.
(23) Cobden, D. H.; Nygård, J. Phys. ReV. Lett. 2002, 89, 046803.
(24) Nygård, J.; Cobden, D. H.; Bockrath, M.; McEuen, P. L.; Lindelof,
P. E. Appl. Phys. A 1999, 69, 297.
(25) Yao, Z.; Postma, H. W. C.; Balents, L.; Dekker, C. Nature 1999,
402, 273.
(26) Kouwenhoven, L. P.; Schön, G. Mesoscopic Electron Transport,
Sohn, L. L., Kouwenhoven, L. P., Schön, G., Eds.; Kluwer:
Dordrecht, 1997; pp 105-214.
(27) Van Houten, H.; Beenakker, C. W. J.; Staring, A. A. M. Single
Charge Tunneling; Grabert, H., Devoret, M. H., Eds.; Plenum
Press: New York, 1992; pp 167-216.
(28) Tans, S. J. et al. Nature 1997, 386, 474.
(29) Bockrath, M. et al. Science 1997, 275, 1922.
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Nano Lett., Vol. 4, No. 2, 2004