Download Carbon nanotube quantum dots on hexagonal boron nitride

Carbon nanotube quantum dots on hexagonal boron nitride
Carbon nanotube quantum dots on hexagonal boron nitride
A. Baumgartner,1 G. Abulizi,1 T. Taniguchi,2 and C. Schönenberger1
1
Institute of Physics, University of Basel, Klingelbergstrasse 82, 4056 Basel, Switzerland
A. Baumgartner 1, G. Abulizi 1, T. Taniguchi 2, and C. Schönenberger 1
⇤
2
Advanced Materials Laboratory, National Institute for Material Science, 1-1 Namiki, Tsukuba, 305-0044, Japan
(Dated: April 6, 2014)
1
Institute of Physics, University of Basel, Klingelbergstrasse 82, 4056 Basel, Switzerland
Advanced Materials Laboratory, National Institute for Material Science, 1-1 Namiki, Tsukuba,
305-0044, Japan
[email protected]
We report
the fabrication details and low-temperature characteristics of the first carbon nanotube
2
(CNT) quantum dots on flakes of hexagonal boron nitride (hBN) substrate. We demonstrate that
CNTs can be grown on hBN by standard chemical vapor deposition and that standard scanning
electron microscopy imaging and lithography can be employed to fabricate nanoelectronic structures
when using optimized parameters. Our findings pave the way to more complex devices on hBN,
with more predictable and reproducible characteristics and electronic stability.
PACS numbers: 73.23.-b
Carbon nanotubes (CNTs) are a versatile fundamental building block for classical small-scale
Carbon nanotubes
(CNTs) are aand
versatile
fundamenricate hBN-graphene
multi-layer
one
electronics
quantum
electronics
- in theory.
In structures
practice,(REF?),
the ideal
properties of CNTs are
tal building block for classical small scale electronics and
dimensional contacts14 and combinations with other layusually
masked
by Inelectrical
fluctuations induced by the substrate. For graphene, a new
ered materials.
quantum electronics
applications
- in theory.
practice, potential
the ideal properties of CNTs are usually masked by elecThe nextnanostructures,
step, however, namely
the usethe
of hBN
approach
has
recently
lead
to
'clean'
namely
useasof thin layers of hexagonal
trical potential fluctuations induced by the substrate. An
substrate for independent, more complex nanostructures
boron
nitride
(hBN)
[1].
contrast to the standard Si/SiO2
intuitive solution to
avoid the
noise induced
by theused
sub- aslikeinsulating
quantum dotssubstrates
(QDs), has not
beenIn
demonstrated
strate is to suspend the active device volume above the
so far. Specifically, no experiments on CNTs grown on
substrates,
hBN
exhibits
significantly
less
charge
traps
and
leads
to larger charge puddles in
substrate, which has lead fundamental experiments, both
hBN substrates have been reported. The main reason
1,2,5,6
3
on suspended CNTs
and on[2],
suspended
graphene
whichgraphene
results. in for
a reduction
noisetoinlocate
transport experiments.
this probablyofis the
thatelectronic
CNTs are difficult
on hBN, because optical microscopy lacks the required
Suspended devices, however, su↵er from limitations in
resolution, the contrast in scanning electron microscopy
the scalability, geometry and in the choice of the conThe next step, however, namely
the use
hBN
as substrate
for independent, more complex
(SEM) images
can beof
poor
(see below)
and AFM imaging
tact and gate materials. For example, it is difficult to
isdots
rather(QDs),
demandinghas
because
largedemonstrated
lateral and verfind a superconductor
or a ferromagnet like
suitable
for the
nanostructures
quantum
notof the
been
so far. Specifically, no
tical scales involved, while retaining the nanometer resCVD growth of CNTs at temperatures around 1000 C.
experiments on CNTs grownolution
on tohBN
been
reported. Here, we report the
image substrates
the CNTs. Herehave
we report
the fabricaStamping techniques5? are more versatile, but strongly
tion details andcharacteristics
low-temperature characteristics
of CNT
depend on the interface
characteristics
of the contacts.
In
fabrication
details
and low-temperature
of the first
CNT QDs on flakes of hBN
quantum dots (QDs) on top of hBN flakes. We demoncontrast, devices fabricated on a substrate o↵er a much
substrate.
We
demonstrate
that
CNTs
can
be
grown
on
hBN
by
strate that for a range of hBN thicknesses and SEM set- standard chemical vapor
larger variety of design options and possible materials,
tings rapid feed-back
large scale SEM
imagingimaging
of
but the stability and
quality of the
electronic
structure scanning
deposition
and
that standard
electronandmicroscopy
(SEM)
and lithography can
CNTs on hBN is possible, also shedding light on the
is usually compromised. Standard cleaning techniques
be
employed
to
fabricate
nanoelectronic
structures
when
using
optimized
parameters. Our
imaging contrast for CNTs on dielectrics. Based on these
cannot be deployed because they also remove the carbon
results,
we
fabricate
CNT
QDs
on
hBN,
the
most
fundastructures4 and thefindings
thermal coupling
to
the
substrate
is
pave the way to more complex devices on hBN, with more predictable and reproducible
mental non-trivial electronic nanostructure, and report
too large for in-situ current annealing.
characteristics
and electronic stability.
For graphene, a new approach has recently lead to
’clean’ nanostructures, namely the use of thin layers
(flakes) of hexagonal boron nitride (hBN) used as insu(a)
metal
hBN
lating substrates. This approach allows the implemencontacts
Pd
tation of substrate supported graphene in high-mobility
CNT
CNT (QD)
hBN
transistors7 and the observation of fundamental physiPd
SiO
SiO
cal phenomena like the formation of a superlattice band
++
Si p
9
10
structure or the cloning of Dirac Fermions . Using
hBN as a substrate has also enabled the fabrication of
(b) optical
(c) SEM
(d) AFM
highly efficient monolayer WSe2 light emitting diodes11 .
In contrast to the standard Si/SiO2 substrates, hBN exhibits significantly less charge traps and leads to larger
charge puddles in graphene8 , which results in a reduction
2 m
4 m
4 m
of the electronic noise in transport experiments, e.g. in
graphene quantum dots12 and double layer graphene13 .
FIG. 1: (a) Schematic of the quantum dot structure on a hBN
hBN can be cleaved (mechanically exfoliated) with sim(a) Schematic
quantum
a hBN
flake.
The inset
shows a device with Pd contacts. (b)
flake. dot
The structure
inset showson
a device
with
Pd contacts.
(b) optical
ple methods7 with Figure
resulting1:thicknesses
down of
to the
single
(c)
SEM
and
(d)
AFM
image
of
a
CNT
(⇠
8
nm
radius)
on a(~ 28 nm thickness) on SiO2.
Optical,
(c)
SEM,
and
(d)
AFM
image
of
a
CNT
(~
8
nm
radius)
on
a
hBN
flake
atomic layers. To date, only few graphene-based devices
hBN flake (⇠ 28 nm thickness) on a SiO2 substrate.
have been demonstrated, but big e↵orts are made to fab2
2
40
20
height (nm)
60
0
The structure of our devices is shown schematically in figure 1a. Figure 1b shows the optical
microscopy image of a ~ 28 nm thick hBN flake after CNT growth. The contrast and color allow
for an initial screening for suitable flakes on a marker field before the device fabrication [3]. The
CNTs are not visible using an optical microscope and can only be found by SEM or atomic force
microscopy (AFM) imaging, as shown for the same hBN flake in figure 1c and 1d, respectively.
Figure 2 shows a series of SEM images of a ~ 1 nm radius CNT on a hBN flake with several
steps in the thickness for different SEM acceleration voltages Uacc. The apparent CNT diameter
is considerably larger on hBN than on SiO2 and wider for lower voltages. The CNT contrast is
1
G (G0)
VSD (mV)
a recipe optimized to obtain clean CNTs and reliable
number of secondary electrons in the
contacts4 and thermal evaporation of Pd to fabricate
gies lower than in the primary beam,
electrical contacts for two-terminal devices on CNTs
han the material’s energy gap. These
on top of hBN flakes. The contacts are separated by
ns can leave the substrate through the
⇠ 300 nm so that we expect the formation of quantum
orbed in the material. The total charge
dots (QDs) at cryogenic temperatures. The conductance
nds on the balance between the number
of a device at 4.2 K is plotted in Fig. 4 as a function of
ry and back-scattered secondary elec(a) (dark) around U (b) ≈ 1.5 kV, then (c)
bestthe
andprimary
positive
for the
voltages
and
turns negative
acc
thelowest
backgate
voltage V
BG and the source-drain bias VSD .
cceleration voltages,
elecpositive
again
Uacc >The
2 kV.
Weexhibits
find anclear
optical
Uacc for
imaging
both, the hBN flakes and the
device
Coulomb
blockade
diamonds
trate deep into the
substrate
andfor
more
a series
of resonances
due to
excited states.
The images of CNTs on
CNTs,
between
0.4 kVand
and
1.1 kV.
The contrast
mechanism
for SEM
ns are scattered back
through
the surdashed
lines
in
the
figure
trace
the
edges
of
the
Coulomb
to a positive charging
of
the
substrate
insulating substrates can be understood qualitatively in a simple picture [4]: the primary beam
blockade diamonds and suggest a four-fold symmetry as
. At higher voltages,
the freea electrons
generates
large number
of secondary electrons in the dielectric at energies lower than in the
found in clean CNT quantum dots due to the spin and
into the substrate and the secondary
primary
beam,
but
still
larger
than the material’s
energy
gap.Uacc = 0.3energy
valley degeneracies
in CNTs.
The charging
and
KV
Uacc = 0.5 KV
Uacc = 0.9 KV
1µm
1µm
1µ
e layers with reduced probability, which
the energy of the first excited state are ⇠ 7 meV and
o a negative charging of the structure
(d)
(e)
(a)
(b)
(c)
⇠ 2 meV, respectively,
and the lever arm is 0.1, comlectrons. Only when the penetration
parable to bare SiO2 dielectrics, in accordance with the
e oxide layer thickness, the secondary
Schematic of contrast mecha
similar dielectric constants.
absorbed by the conducting backgate
rge of the structure can again become
We observe up to 6 excited states and negative di↵erential conductance between excited states resonances, as
indicated in Fig. 4 by arrows (...). The excited states
ation voltages the electrons interact
1µm
1µm
= 1.5the
KV
Uacc = 20 KV
Uacc = 0.3
KV
Uaccenergies
= 0.5 KV
= 0.9 all).
KV
1µm
are 1µm
..., ... Uacc
(list
We 1µm
note Uacc
that
ground
CNTs and the image
contrast
stems
state
transitions
are
weaker
in
some
Coulomb
blockade
hanges in the electron
density near the (e) images of CNTs (~ 1 nm radius) on a hBN flake (first step ~ 35 nm thickness, second
(d) Figure 2: Seriers of SEM
can be seen as charge reservoirs
(or~ 80 nm thickness) on a SiO substrate obtainedFIG.
2: (a)Schematic
a CNT quantum dot fabricated
second step
for different
accelerationofvoltages.
2
ot connected to an electrical contact)
Schematic of contrast mechanism hBN flake as substrate. (b) optical, (c) SEM and (d) A
−3
rons to the substrate if it is positively
image of
x 10CNTs on a hBN flake on a SiO2 surface.
4
pts electrons if the substrate is negaFor positive substrates electrons move
10
3
c and this additional electron density
1µm
= 1.5 KV
tering rates and theUaccgeneration
of1µm
sec- Uacc = 20 KV
2
5
leading to a positive contrast in the
1
Ts. This explains FIG.
qualitatively
the two of a CNT quantum dot fabricated on a
2: (a)Schematic
0
he CNT contrast hBN
in SEM
images
and
flake as substrate. (b) optical, (c) SEM and (d) AFM
0
is almost identical
on the
two rather
image
of CNTs
on a hBN flake on a SiO2 surface.
−5
es.
−1
the contrast depends less on the di−2
−10
of the substrate (✏hBN ⇡ 4 (depends
tation) and ✏SiO2 ⇡ 4), but rather on
−3
ility to supply electrons, i.e. on its to−15.1
−15
−14.9
−14.8
−14.7
VBG (V)
which scales with the length of the tube
the logarithm of its diameter. This is
h our findings of about the
contrast
beFigure
3: Coulomb
blockade and excited states resonances on a CNT QD fabricated on top a hBN flake.
FIG. 4: Coulomb blockade and excited states resonances on
bstrates. The apparent width depends
QD fabricated on top an hBN flake (temperature T =
s electron di↵usion length
for
the
quasi
The conductance of a4.2
aCNT
device
(figure 1a) at 4.2 K is plotted in figure 3 as a function of the
K).
ove the energy gap,
and on voltage
the elec-V
back-gate
and the source-drain bias V . The device exhibits clear Coulomb
BG
SD
blockade diamonds and a series of resonances due to excited states. The dashed lines in the
figure trace the edges of the Coulomb blockade diamonds and suggest a four-fold symmetry as
found in clean CNT QDs due to the spin and valley degeneracies in CNTs. The charging energy
and the energy of the first excited state are ~ 7 meV and ~ 2 meV, respectively, and the lever arm
(a)
(b)
(c)
is 0.1, comparable to bare SiO2 dielectrics, in accordance with the similar dielectric constants.
Height (nm)
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Uacc = 0.6 KV
2µm
Uacc = 0.6 KV
2µm
2µm
0
FIG. 3: (a)Schematic of a CNT quantum dot fabricated on a
2