Coulomb blockade behavior in an indium nitride nanowire with disordered surface states

Article abstract of DOI:10.1063/1.3216071

We present electron transport phenomena in a single electron transistor based on an individual indium nitride nanowire. Meticulous Coulomb oscillations are observed at low temperatures. While the device shows single period Coulomb oscillation at high temp

Full text of DOI:10.1063/1.3216071

Coulomb blockade behavior in an indium nitride nanowire with disordered surface
states
K. Aravind, Y. W. Su, I. L. Ho, C. S. Wu, K. S. Chang-Liao, W. F. Su, K. H. Chen, L. C. Chen, and C. D. Chen
Citation: Applied Physics Letters 95, 092110 (2009); doi: 10.1063/1.3216071
View online: http://dx.doi.org/10.1063/1.3216071
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APPLIED PHYSICS LETTERS 95, 092110 ͑2009͒
Coulomb blockade behavior in an indium nitride nanowire with disordered
surface states
K. Aravind,1,2 Y. W. Su,2,3 I. L. Ho,2 C. S. Wu,4 K. S. Chang-Liao,1 W. F. Su,3
K. H. Chen,5 L. C. Chen,6 and C. D. Chen2,7,a͒
1Department of Engineering and System Science, National Tsing Hua University, Hsinchu 300, Taiwan
2Institute of Physics, Academia Sinica, Nankang, 115 Taipei, Taiwan
3Department of Materials Science and Engineering, National Taiwan University, Taipei 106, Taiwan
4Department of Physics, National Chang-Hua University of Education, ChangHua 500, Taiwan
5Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 106, Taiwan
6Center for Condensed Matter Sciences, National Taiwan University, Taipei 106, Taiwan
7Department of Physics, National Cheng Kung University, Tainan 701, Taiwan
͑Received 22 June 2009; accepted 28 July 2009; published online 4 September 2009͒
We present electron transport phenomena in a single electron transistor based on an individual
indium nitride nanowire. Meticulous Coulomb oscillations are observed at low temperatures. While
the device shows single period Coulomb oscillation at high temperatures or at high bias voltages,
additional satellite peaks along with the main Coulomb peak appear at low temperatures and low
bias voltages. The quasiperiodic structure is attributed to the mixing of dissimilar Coulomb
oscillations arising from two serially coupled islands embedded inadvertently in the surface metallic
states of the nanowire. The proposed model is numerically simulated with good agreement with the
experimental data. © 2009 American Institute of Physics. ͓DOI: 10.1063/1.3216071͔
Semiconductor nanowires are promising candidates to
serve as building blocks for next generation electronics with
SET were made of Ni ͑50 nm͒ covered with Au ͑50 nm͒
protection layer. The top-left inset of Fig. 1 shows an
FESEM image of the measured device. The wire diameter is
about 55 nm and the separation between the source and drain
electrodes is approximately 80 nm. The low temperature
measurements were performed with a symmetrical biasing
scheme in a He3/He4 dilution refrigerator. The current volt-
age I-Vb characteristics of the SET device measured at tem-
perature of 300, 4.2, and 0.38 K are marked in green, red and
blue respectively ͓Fig. 1͔. The zero bias resistance at room
temperature is about 70 k⍀ and increases to 1.46 M⍀ at
0.38 K, marking the onset of Coulomb charging effects and
the corresponding nonlinear current-voltage ͑I-Vb͒ character-
istics. The offset voltage of the I-Vb characteristics at low
temperatures ͑not shown here͒ correspond to a charging en-
ergy of about 13 K ͑or, e2/2C⌺=1.12 meV, C⌺ is the total
capacitance seen by the nanowire island͒, which accounts for
the clear observation of I-Vg Coulomb oscillations at 4.2 K.
unprecedented operation speeds and packing densities.1,2
A
single electron transistor ͑SET͒ geometry consists of an in-
dividual nanowire island and involves the quantization of
electron charge as well as energy. The device thus provides a
versatile platform to probe the physics in the quasione di-
mension and also paves way for next generation electronics
and optoelectronics converging to quantum limit. Indium ni-
tride ͑InN͒ has attracted recent attention because of its re-
vised direct band gap of 0.7–0.8 eV in the visible range3 and,
to this end, we study the single electron transport through an
individual single crystal wurtzite InN nanowire in the SET
configuration. The measured devices show a clear crossover
from high voltage, high temperature single period Coulomb
oscillation to low bias, low temperature multiple period os-
cillations. In this paper we disclose this unexplored phenom-
enon in disordered nanowires and associate it with the coex-
istence of multiple Coulomb islands in the granular surface
states of nanowire.
Single crystalline wurtzite nanowires used in this device
were grown by guided stream thermal chemical vapor depo-
sition with trimethyl indium as indium source, ammonia as
nitrogen source, and gold as catalyst.4 The nanowires were
first dispersed in isopropanol solution. A few droplets of this
solution were then placed on Si substrates with micron-sized
Pt/Ti measurement pads prefabricated on 300-nm-thick SiO2
oxide surface. Subsequently, nanoscaled electrodes were fab-
ricated by standard electron beam lithographic technology,
which served as interconnectors between nanowires and
Pt/Ti pads. Electron beam exposure and identification of the
positions of the dispersed nanowires for e-beam writing pur-
pose were performed using a field emission scanning elec-
tron microscope ͑FESEM͒. The nanometer electrodes of the
FIG. 1. ͑Color online͒ I-Vb characteristics at T=0.38 K ͑blue dotted line͒,
4.2 K ͑red dashed line͒, and 300 K ͑green dot-dashed line͒ showing unam-
biguous Coulomb blockade effect at low temperature. Top left inset is the
scanning electron micrograph of the measured device. The bottom right
inset is the schematic employed in modeling the measured device.
a͒
Author to whom correspondence should be addressed. Electronic mail:
chiidong@phys.sinica.edu.tw.
0003-6951/2009/95͑9͒/092110/3/$25.00
95, 092110-1
© 2009 American Institute of Physics
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092110-2
Aravind et al.
Appl. Phys. Lett. 95, 092110 ͑2009͒
FIG. 2. ͑Color online͒ Coulomb oscillations at T=0.8 K, 0.44 K, and 0.25
K clearly showing satellite peaks in the vicinity of major peak at lower
temperatures. All the curves are taken at Vb =1 mV. The vertical red-dotted
lines are guide to the eye, representing the major peak position at several
temperatures.
FIG. 3. ͑Color online͒ Coulomb oscillations at Vb =3 mV, 2 mV, 1.3 mV,
and 0.2 mV showing clear onset of satellite peaks in the vicinity of major
peak at lower bias voltages. All the curves are taken at T=0.35 K. The
vertical red-dotted lines are guide to the eye, denoting the major peak posi-
tion at different bias voltages.
Figure 2 shows such oscillations at a constant bias volt-
age Vb of 1 mV at three different temperatures. Unlike the
orthodox single island Coulomb blockade model which pre-
dicts a sharp and prominent single period oscillation corre-
sponding to e/Cg at low temperatures,5 we observe addi-
tional peaks along with the main peak as the temperature was
lowered. The newly added ͑short͒ periods can be attributed
to the additional ͑larger͒ Coulomb islands with smaller
charging energy, forming inadvertently in the vicinity of the
initial smaller island. The schematic of the model is pre-
sented in the bottom inset of Fig. 1. There are two Coulomb
islands, denoted as Il and Is, participating in the electron
transport which are weakly coupled to the source and drain
electrodes via left and right tunnel junctions, respectively,
and the two islands are capacitively coupled to the gate elec-
trode via two dissimilar capacitors Cgl and Cgs. The interac-
tion between the two islands is tuned by a tunnel junction
with a capacitance Cc and a resistance Rc. The applied gate
voltage alters the chemical potential of the individual islands
and this corresponds to a periodic oscillation in gate voltage
with a period of e/Cg1 and e/Cg2 for the large and small
islands, respectively. Now, the thermal energy kBT ͑kB is the
Boltzman constant͒ is the average translational kinetic en-
ergy experienced by the electrons which can be viewed as a
finite broadening or background energy providing excitation
to the electron transport. The higher thermal energy smears
the Coulomb charging levels contributed by large dot ͑Il͒ and
the single period corresponding to Is alone survives. The pe-
riodicities in gate voltage determine as to which island plays
a role in the single electron transport in the form of Coulomb
oscillations. For instance, at higher temperatures, only the
smaller island ͑Is͒ with larger charging energy would be ap-
preciable and at lower temperatures the additional bigger is-
land ͑Il͒ with a smaller charging energy emerges.
FFT spectra of I-Vg curves taken at two bias voltages, Vb
=1.3 mV and 3 mV, and the two amplitude peaks in
each spectrum correspond to the two dominant periods
of 1.81 and 0.95 V. Note that the FFT amplitude ratio,
A1.81 V/A0.95 V, increases with increasing bias voltage. This
is further illustrated in the main panel, which shows the nor-
malized oscillation amplitude of these two peaks as a func-
tion of bias voltages with a clear bias voltage dependence of
A1.81 V and A0.95 V and the oscillation amplitude of smaller
period ͑A0.95 V͒ increases with decreasing bias voltage. This
confirms our observation of fine structures emerging on both
FIG. 4. ͑Color online͒ ͑a͒ FFT analysis of the measured I-Vg Coulomb
oscillations at several bias voltages. The inset is the FFT spectra at Vb =3
and 1.3 mv which can be used to identify the two dominant periods pre-
sented in the oscillations. The first peak corresponds to a period of 1.81 V in
Vg and the second peak represents 0.95 V in Vg. The main panel is the
normalized oscillation amplitude extracted as a function of bias voltage. The
black solid circles indicate the amplitude of 1.81 V period oscillation while
the red solid circles denote the amplitude of 0.95 V period oscillation. Pan-
els ͑b͒ and ͑c͒ are simulated I-Vg Coulomb oscillations. ͑b͒ shows I-Vg at
Vb =0.15 mV at four temperatures: 0.5 K, 1 K, 1.5 K, and 2 K; ͑c͒ shows
I-Vg at T=1 K at Vb =1.2 mV and 0.1 mV. We can clearly see the fine
structures on both sides of the original peak at low bias voltages and at low
temperatures.
The measured I-Vg oscillations of the device at fixed T
=0.35 K and at various bias voltages are presented in Fig. 3.
The single period oscillation at high bias voltage evolves into
multiple periodic oscillations with additional short periodici-
ties as we lower the bias voltage. A close inspection reveals
that satellite peaks gradually emerge on the both sides of the
main peaks and the most interesting observation is that peri-
odicity of the original peak still prevails. This is clearly dem-
onstrated by the fast Fourier transform ͑FFT͒ of the mea-
sured data shown in Fig. 4͑a͒. The inset of Fig. 4͑a͒ shows
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092110-3
Aravind et al.
Appl. Phys. Lett. 95, 092110 ͑2009͒
sides of the original peak at low bias voltages in Fig. 2. A
similar analysis is performed ͑not shown͒ as a function of
temperature at a fixed low bias voltage and the result clearly
indicates the onset of satellite peaks on both sides of the
main peak at lower temperatures. The two prominent periods
in the Coulomb oscillations inspire us to model the device
using two coupled dissimilar Coulomb islands, and we nu-
merically simulate this intuitive model using the standard
Master equation approach in the sequential tunneling limit5
and find qualitative agreement with the experimental results.
The device parameters chosen for the simulation are C1
=100 aF, R1=26 k⍀, C2=130 aF, R2=26 k⍀, Cc=4 aF,
and Rc=50 k⍀. The gate capacitances are Cg1=230.3 aF
and 77.7 aF for the big and the small island, respectively.
Figure 4͑b͒ is the simulated I-Vg Coulomb oscillation sub-
jected to a constant bias Vb=0.15 mV at several tempera-
tures and Fig. 4͑c͒ is the I-Vg oscillation at a fixed tempera-
ture T=1 K under two different bias conditions. We can
clearly see that the fine structures on either side of the main
peak emerge at low bias and low temperatures. Higher bias
voltages open a window which surmounts the charging level
spacing corresponding to the e/Cgl of the big dot; the small
dot ͑Is͒ alone prevails leading to a single period oscillation
corresponding to e/Cgs. At low enough voltages, both the
dots become appreciable and the Coulomb oscillation has
complicated gate dependence depending on the ratio of Cgl
and Cgs. The newly added short periods are thus attributed to
additional Coulomb islands forming inadvertently in the vi-
cinity of the initial island. This crossover from periodic os-
cillation at high temperatures to aperiodic oscillations at
lower temperatures is not a measurement artifact or noise
and has survived three complete thermal cycles ͑three differ-
ent runs͒ from He3/He4 fridge to room temperature. If the
aperiodic oscillation at low temperatures were to come from
noise effects, it would not be reproducible in repeated mea-
surements and the FFT of the measured signal would be a
random conglomeration of oscillation periods. It is also
straight forward to extend our model to multiple islands
which become important at much lower temperatures. We
have measured the I-Vg oscillations at 75 mK ͑not shown͒
and the interesting cross over from single period high bias
oscillation to multiple period low bias traces is still retained.
The physics of InN is very unique compared to other
III-V materials and has attracted a lot of recent attention. Ab
initio calculations of band structures predict that the branch
point energy of InN lies within the conduction band at the
⌫-point and is thus responsible for the surface states.6 The
high-resolution electron-energy-loss spectroscopy measure-
ments on clean InN surfaces provides an unambiguous proof
of metallic or donor type surface accumulation states up to a
thickness of about 5 nm.7 The origin of surface states is
twofold. First, it is an accepted inherent electronic band
structure property of clean InN nanowires and second, the
ambient oxygen forms native indium oxide, which is be-
lieved to cause large surface accumulation layer.8 These two
phenomena together can account for the robust surface accu-
mulation states and it is thus justified to consider that in our
device the electrons hop along the nanowire surface states.
Moreover, these states by nature fall in disordered metallic
regime. Similar crossover from periodic to quasi periodic
I-Vg oscillations in disordered systems has already been re-
ported and is well understood in GaAs wires.9 Further,
charge transport at very low temperatures in disordered sys-
tem was also modeled as arising from mixed Coulomb oscil-
lations of two dissimilar metal dots unintentionally formed in
the disordered potential10 and the crossover from periodic to
quasiperiodic oscillations was attributed to incommensurate
energy scales required to add charges to the two dots.
Further, the I-Vg Coulomb oscillations are taken under
external magnetic fields up to 5 T. The striking observation is
that satellite peak structure along with the main peak sur-
vives even in high magnetic fields. There is neither shift in
the peak position nor observable change in the peak height. It
is widely believed that electron transport through Coulomb
charging system with continuous density of states is insensi-
tive to the magnetic field, and our model of weakly coupled
Coulomb islands are thus justified.
In summary we present a single electron transistor based
on an individual indium nitride nanowire. The interesting
crossover from high voltage, high temperature single period
Coulomb oscillation to low bias, low temperature multiple
period oscillations implies the interesting interplay of Cou-
lomb charging effects from more than one Coulomb island.
The tunnel barriers between the islands arise inadvert-
ently due to large fluctuations in the underlying disordered
potential. We propose and simulate an intuitive model ex-
plaining the observed phenomena. The present work sheds
light on the unexplored single electron charging behavior and
transport of InN nanowires and is an encouraging develop-
ment toward the InN based electronics and optoelectronics
devices.
This research was funded by the National Science Coun-
cil of Taiwan under Grant No. NSC 95-2112-M-001-062-
MY3. Technical support from NanoCore, the Core Facilities
for Nanoscience and Nanotechnology at Academia Sinica, is
acknowledged. The first two authors, K.A and Y.W.S., con-
tributed equally to this work.
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