APPLIED PHYSICS LETTERS
VOLUME 80, NUMBER 15
15 APRIL 2002
Controlling doping and carrier injection in carbon nanotube transistors
V. Derycke, R. Martel,a) J. Appenzeller, and Ph. Avouris
IBM Research Division, T. J. Watson Research Center, Yorktown Heights, New York 10598
共Received 7 December 2001; accepted for publication 15 February 2002兲
Carbon nanotube field-effect transistors 共CNTFETs兲 fabricated out of as-grown nanotubes are
unipolar p-type devices. Two methods for their conversion from p- to n-type devices are presented.
The first method involves conventional doping with an electron donor, while the second consists of
annealing the contacts in vacuum to remove adsorbed oxygen. A comparison of these methods
shows fundamental differences in the mechanism of the transformation. The key finding is that the
main effect of oxygen adsorption is not to dope the bulk of the tube, but to modify the barriers at
the metal–semiconductor contacts. The oxygen concentration and the level of doping of the
nanotube are therefore complementary in controlling the CNTFET characteristics. Finally, a method
of controlling individually the contact barriers by local heating is demonstrated. © 2002 American
Institute of Physics. 关DOI: 10.1063/1.1467702兴
Carbon nanotubes 共CNTs兲 are attractive to both fundamental science and technology.1 They are quasi onedimensional 共1D兲 conductors with unique electrical properties such as reduced carrier scattering rates2 and high current
carrying capabilities.3 CNTs are also promising materials for
building electronic devices, in particular field effect transistors 共FETs兲.4,5 Single-wall carbon nanotube FETs 共CNTFETs兲 built from as-grown tubes are found to be unipolar
p-type, i.e., no electron current flows even at large positive
gate biases. This behavior suggests the presence of a
Schottky barrier at the metal–nanotube contact. In air, the
barrier for electrons is high because of the pinning of the
Fermi level (E F) close to the valence band maximum at the
nanotube–metal interface.6
The capability to produce n-type transistors is important
technologically, as it allows the fabrication of CNT-based
complementary logic devices and circuits.7 Experiments
have shown that p- to n-type conversion of the CNTFETs
can be made either by doping the surface of the tube using
alkali metals,7,8 or by simply annealing the device in
vacuum7 or in an inert gas.6 Here, we compare the characteristics of CNTFETs produced by both methods. Although
the resulting devices have rather similar electrical behaviors,
the mechanisms involved are quite different. The annealing
in vacuum removes the adsorbed oxygen and results in the
direct modification of the Schottky barrier height at the contacts. In contrast, doping changes the barrier thickness and
introduces significant shifts of the threshold voltage of the
device. These results are consistent with a simple model in
which the main role of oxygen is not to dope the CNT but to
modify directly the line-up of the CNT bands at the metal–
nanotube junction.
In all of our experiments we used single-wall carbon
nanotubes 共SWCNTs兲 with a diameter of 1.4 nm and a band
gap of ⬃600 mV.9 These tubes were dispersed on gold electrodes prepatterned on an oxidized 共120 nm兲 silicon wafer.
Figure 1 shows the evolution of the transfer characteristics 共I D vs V GS curves兲 at V DS⫽0.5 V for incremental exposures to pure oxygen 关Fig. 1共a兲兴, or potassium 关Fig. 1共b兲兴. In
a兲
Electronic mail: martelr@us.ibm.com
Fig. 1共a兲, the initial p-type device is first converted into a
unipolar n-type transistor by annealing at 600 K in vacuum
( P⬍10⫺6 Torr, open circles兲 for several minutes. Oxygen is
then introduced in the system. At each oxygen exposure step,
the current at positive gate bias decreases while the current at
negative gate bias increases. For intermediate oxygen doses,
the device is ambipolar; i.e., it conducts either electrons or
FIG. 1. 共a兲 Effect of oxygen on an n-FET produced by thermal annealing
共open circles兲. The O2 exposures are: 2 min. at P⫽10⫺4 Torr 共black triangles兲, P⫽5⫻10⫺4 Torr 共open squares兲, P⫽5⫻10⫺3 Torr 共gray diamonds兲, P⫽10⫺1 Torr 共open triangles兲 and exposure to the ambient 共black
circles兲. 共b兲 Effect of potassium doping on a CNTFET. The FET is initially
p-type 共curves 1–7兲. After seven doping cycles, no more current can be
detected 共open circles 8, 9兲. At higher doping levels, the device becomes
n-type 共curves 10–12兲.
0003-6951/2002/80(15)/2773/3/$19.00
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© 2002 American Institute of Physics
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Appl. Phys. Lett., Vol. 80, No. 15, 15 April 2002
FIG. 2. Schematic energy band diagram in the region of the metal-nanotube
contact at V DS⫽0. 共a兲 Bands of the starting p-type device in air, 共b兲 the
device after annealing in vacuum, 共c兲 a nanotube FET that is not out-gassed
but is n-doped by potassium 关curves 8 and 9 of Fig. 1共b兲兴, and 共d兲 the same
as in 共c兲 but at a higher doping level. W denotes the barrier thickness for
electron injection. The examples in 共a兲 and 共b兲 are for an intrinsic nanotube.
However, a low dopant level is expected to present a similar behavior.
holes depending on the gate bias. At higher oxygen doses,
the device is converted back into the original unipolar p-type
device. This conversion from n-type to p-type with the intermediate ambipolar stages is fully reversible. The above
results clearly show that the ambient gas is responsible for
the observed p-type behavior in air.
Now we concentrate on the p- to n-type conversion introduced by direct doping of the CNTFET with an electron
donor. In Fig. 1共b兲, we show the evolution of the I D⫺V GS
characteristics of a p-CNTFET upon doping with potassium.
Twelve different curves are shown, corresponding to increasing doping levels. At low doping, the effect of potassium is
to shift the curves toward more negative gate biases 共curves
labeled 1–7兲. This behavior is typical for doping of a
semiconductor.10 At intermediate doping 共open circles labeled 8 and 9兲, no current can be detected within the full
range of gate biases. Electron conduction 共V GSⰇ0兲 becomes
observable only for the highest doping levels 共curves 10 to
12兲. Further doping results in a shift of the curves to the left.
Most importantly, no ambipolar behavior is seen in the range
of gate biases used 共⫺15–15 V兲: the flat I – V curves extend
over this full voltage range. This behavior is reproducible
and is different from that produced by annealing 关Fig. 1共a兲兴.
The behavior of the devices clearly indicates that the
mechanisms for converting from p- to n-type are fundamentally different depending on the method of conversion. A
simple phenomenological model 共Fig. 2兲 can describe the
impact of barrier height and width at the metal–nanotube
junctions as a function of the dopant level and the concentration of adsorbed oxygen. Initially, the nanotube is essen-
Derycke et al.
tially undoped and the p-type character of the device in air is
due to the presence of a Schottky barrier that prevents electrons from being injected into the device. From this initial
state, the band diagram evolves in two different ways as
shown in Figs. 2共a兲 and 2共b兲 for the removal of oxygen, and
as in Figs. 2共c兲 and 2共d兲 for n-doping with potassium.
The effect of oxygen on the electrical properties of a
nanotube device has previously been observed.11 In particular, an increase in the resistivity of nanotubes has been reported when they are brought from air to vacuum. In former
studies, the effect associated with the adsorption of oxygen
was assumed to involve doping.11–14 Doping implies a significant charge transfer from the nanotube to the adsorbed
oxygen molecules. However, doping by oxygen cannot account for the experimental observations in Fig. 1共a兲 for several reasons. 共i兲 It does not explain the absence of electron
conduction for large positive gate biases when the device is
in air. Indeed, doping can, to a large extent, be compensated
for by the gate field. The application of a sufficiently high
positive gate bias should allow electron conduction, unless
the hole doping level is extremely high. However, high doping levels are easy to detect since they lead to a significant
current at zero gate bias, in contradiction to the measurements. 共ii兲 Assuming that the CNTs are p-doped by oxygen,
its removal should therefore give intrinsic CNTs, not the observed n-type. Thus, the conversion of the CNFETs to make
unipolar n-type devices is unaccounted for since it would
require doping as well. 共iii兲 A change in the doping level
should give rise to a shift in the threshold voltage of the
transistor 关see Fig. 1共b兲兴. However, the gradual conversion in
Fig. 1共a兲 from n- to p-type does not show any significant
change in the threshold voltage.
Charge transfer from the nanotube to the adsorbed gas
can be present. It would lead to a shift of the Fermi level in
the bulk of the tube. However, the interaction of O2 with a
SWCNT is weak,15 and the charge transfer does not account
for our experimental findings. The direct comparison with
potassium doping in Fig. 1 shows striking differences. The
main effect of out-gassing the device is rather consistent with
a change in the barrier height at the contact. It implies that
the pinning condition at the metal–CNT interface depends
on the concentration of oxygen. The position of E F at this
interface can change gradually within the band gap as a function of the concentration of oxygen. This effect changes the
line-up of the bands at the interface but does not affect the
bulk of the CNT. When E F at the junction is close to the
center of the band gap, the barrier allows efficient tunneling,
and ambipolar transport is observed.6 With gold contacts in
air, only holes can be injected in the device 关Fig. 2共a兲兴, while
after annealing, only electrons can be injected because the
barrier is now too high for hole injection 关Fig. 2共b兲兴.
In our model, the adsorption of oxygen at the contacts
affects the local bending of the nanotube bands through
charge-transfer and the dipole fields generated by the adsorbates. Analogous changes in field-emission barriers induced
by adsorbates are well known.16
The case of doping with potassium is different. The
depletion width, W, of a nanotube device at low doping levels is expected to depend exponentially on the inverse of the
doping fraction.17 At large doping fractions, W is in the na-
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Derycke et al.
Appl. Phys. Lett., Vol. 80, No. 15, 15 April 2002
FIG. 3. 共a兲 I D(V GS) curves of a p-type FET in air for V DS⫽0.5 V and
V DS⫽⫺0.5 V. 共b兲 Same in vacuum after a heating current has been passed
through one of the contacts. 共c兲 Same after a second local heating step. 共d兲
Schematic of the contact design.
nometer regime and it becomes possible for the electrons to
tunnel through this barrier. This trend is totally consistent
with the results at Fig. 1共b兲. The finding in Fig. 1共b兲 of flat
I–V characteristics at intermediate doping levels clearly
shows that doping involves changes in the width of the
depletion region near the contacts. When the doping level of
the nanotube is sufficiently high, a high negative gate bias is
required to remove the high concentration of charges 共electrons兲 on the tube. Therefore, no hole conduction can take
place 共up to at least V GS⫽⫺15 V兲. Furthermore, because of
the pinning of the Fermi level at the valence band maximum,
the resulting barrier is too high and wide to allow electron
injection 关Fig. 2共c兲兴. Thus, there is no current at any gate
bias. At even higher doping levels, the barrier becomes increasingly thinner because of the increased screening. In this
regime, the device shows n-type characteristics as seen in the
curves 10–12 of Fig. 1共b兲, as is illustrated in Fig. 2共d兲.
If a highly potassium-doped CNTFET is exposed to oxygen, the curves revert to the initial p-type character following the same path without an ambipolar intermediate stage
共not shown兲. The oxygen oxidizes the adsorbed potassium
atoms and therefore reduces the charge transfer from the potassium to the tube. This simple experiment again shows
clearly that the effect of oxygen on an n-doped or an outgassed nanotube is different, revealing the difference in the
mechanisms involved.
Given that the Schottky barriers at the contacts can determine the device characteristics, gaining control of these
barriers is very important. Next, we describe an experimental
approach that allows a local control of the barrier height.
Resistive contacts are used in place of the usual gold electrodes. These gold electrodes are connected at both ends and
thinned down in the vicinity of the nanotube. With this design, it is possible to anneal separately each contact 共by passing a current兲 instead of annealing the complete device. Figure 3 presents this new electrode design as well as typical
results. A CNTFET is first probed 关Fig. 3共a兲兴 in air for two
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opposite drain–source biases 共⫾0.5 V兲. The two curves correspond to hole conduction under forward and reverse bias
conditions. Initially, the curves are symmetric. That is, the
absolute value of the ratio between the current at V GS⫽⫾15
V is close to one 共R⫽0.94兲. The device is then placed in
vacuum and a dc current is passed through one of the two
contacts 关Fig. 3共d兲兴. Note that, after this treatment, 共i兲 both
currents are smaller than in air, and 共ii兲 the curves are no
longer symmetric, with one having almost twice as much
current as the other 共R⫽1.8兲. A second step corresponding to
a higher current through the same contact is displayed on
Fig. 3共c兲. The ratio between the two curves is now close to
five.
The global decrease of the current indicates that the
whole device is partially out-gassed by the heating of one
contact. That implies some thermal conduction through the
tube and thus a partial desorption of oxygen from the other
contact. The drastic asymmetry in the I – Vs results from this
nonuniform annealing, i.e., the directly annealed contact is
much more affected by the heating. These results strongly
support the importance of the contact barriers in determining
the characteristics of the nanotube FETs. The behavior of the
FETs can be fully understood in terms of the action of oxygen on the Schottky barriers without any assumptions concerning doping. Moreover, the new technique gives us a way
to individually adjust these Schottky barriers.
We thank B. Ek for expert technical assistance and J.
Bucchignano for help with e-beam lithography.
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