Single-electron effects in side-gated point contacts fabricated in low-temperature deposited nanocrystalline silicon films

Article abstract of DOI:10.1063/1.1350428

Single-electron effects have been observed up to 60 K in a side-gated point contact device fabricated in nanocrystalline silicon films. The films were phosphorus-doped and deposited at 30°C by plasma enhanced chemical vapor deposition. Using transmission electron microscopy and Raman spectroscopy, the grain size, crystalline volume fraction, and grain boundary thickness are determined. The single-electron effects are associated with islands formed by crystalline silicon grains ～4 nm in size, isolated by amorphous silicon regions ～0.5 nm in thickness. The structural characteristics of the nc-Si film are correlated to the electrical behavior. The electrical transport mechanism at high temperatures is attributed to percolation conduction across a distribution of tunnel barriers with a maximum height of 40 meV.

Full text of DOI:10.1063/1.1350428

Single-electron effects in side-gated point contacts fabricated in low-temperature
deposited nanocrystalline silicon films
Y. T. Tan, T. Kamiya, Z. A. K. Durrani, and H. Ahmed
Citation: Applied Physics Letters 78, 1083 (2001); doi: 10.1063/1.1350428
View online: http://dx.doi.org/10.1063/1.1350428
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/78/8?ver=pdfcov
Published by the AIP Publishing
Articles you may be interested in
Evaluation of microstructures and carrier transport behaviors during the transition process from amorphous to
nanocrystalline silicon thin films
J. Appl. Phys. 105, 054901 (2009); 10.1063/1.3087500
Low temperature deposition of nanocrystalline silicon carbide films by plasma enhanced chemical vapor
deposition and their structural and optical characterization
J. Appl. Phys. 94, 5252 (2003); 10.1063/1.1609631
Large-grain polycrystalline silicon films with low intragranular defect density by low-temperature solid-phase
crystallization without underlying oxide
J. Appl. Phys. 91, 2910 (2002); 10.1063/1.1448395
Characteristics of chemical vapor deposition diamond films for x-ray mask substrates
J. Vac. Sci. Technol. B 17, 3144 (1999); 10.1116/1.590969
Hydrogen elimination and phase transitions in pulsed-gas plasma deposition of amorphous and microcrystalline
silicon
J. Appl. Phys. 81, 2847 (1997); 10.1063/1.364309
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
140.254.87.149 On: Sat, 20 Dec 2014 03:23:37

APPLIED PHYSICS LETTERS
VOLUME 78, NUMBER 8
19 FEBRUARY 2001
Single-electron effects in side-gated point contacts fabricated
in low-temperature deposited nanocrystalline silicon films
Y. T. Tan, T. Kamiya,a) Z. A. K. Durrani, and H. Ahmedb)
Microelectronics Research Center, University of Cambridge, Cavendish Laboratory, Madingley Road,
Cambridge CB3 0HE, United Kingdom
͑
Received 2 October 2000; accepted for publication 21 December 2000͒
Single-electron effects have been observed up to 60 K in a side-gated point contact device fabricated
in nanocrystalline silicon films. The films were phosphorus-doped and deposited at 300 °C by
plasma enhanced chemical vapor deposition. Using transmission electron microscopy and Raman
spectroscopy, the grain size, crystalline volume fraction, and grain boundary thickness are
determined. The single-electron effects are associated with islands formed by crystalline silicon
grains ϳ4 nm in size, isolated by amorphous silicon regions ϳ0.5 nm in thickness. The structural
characteristics of the nc-Si film are correlated to the electrical behavior. The electrical transport
mechanism at high temperatures is attributed to percolation conduction across a distribution of
tunnel barriers with a maximum height of 40 meV. © 2001 American Institute of Physics.
͓
DOI: 10.1063/1.1350428͔
Nanocrystalline silicon (nc-Si) is a chemically-tailored
material where crystalline silicon grains Ͻ10 nm in size are
embedded in an oxide or amorphous silicon matrix. These
nanoscale grains can form isolated charging islands which
show single-electron effects, raising the possibility of room
temperature single-electron devices fabricated in silicon.
Several large-area nc-Si devices have been demonstrated,
such as the nanodot memory structure of Tiwari and
frequency ͑VHF͒ PECVD from a SiF :H :SiH gas mixture.
4
2
4
The films were deposited on a 150 nm thick silicon oxide
layer thermally grown on top of n-type crystalline silicon.
The flow rates of the SiF , H , and SiH were 30, 40 and
4
2
4
0.25 sccm. The films were doped in situ with PH ͑1% di-
3
luted with H ) where the concentration of PH in SiH was
2
3
4
2%. The VHF frequency was 100 MHz, the VHF power was
40 W and the reactor pressure was 200 mTorr. The carrier
concentration and electron mobility, measured at room tem-
1
2
co-workers and the tunneling diode of He and co-workers.
An atomic force microscope has been used to characterize
2
0
3
perature by Hall measurements, were 3ϫ10 cm and 1.8
2
individual nc-Si grains embedded in SiO by Fukuda and
cm /Vs, respectively.
2
3
4
co-workers and Otobe and co-workers. Resonant tunneling
and single-electron effects have been observed at room tem-
perature in these systems.
Our silicon point contact structures were defined using
electron-beam lithography in polymethyl methacrylate resist
and reactive-ion etching in a mixture of SiCl4 and CF4 gases
͑20 sccm each͒ at 13.56 MHz, 300 W, and 20 mTorr. The
point contact has a width of 20 nm. Ohmic contacts were
formed to the structure by first wet etching the surface oxide
on the nc-Si film in SILOX ͑Laporte proprietary chemical͒
and then depositing aluminum contact pads. Figure 1 shows
Nc-Si can be prepared using a low-temperature plasma-
enhanced chemical vapor deposition ͑PECVD͒5 process.
Typically, a plasma of SiH or SiF at temperatures р300 °C
4
4
is used. The nc-Si can be deposited on glass or silicon-on-
insulator substrates for faster device operation and better
electrical isolation. The low process temperature reduces fab-
rication costs and makes large-area fabrication easier. This
raises the possibility of device applications such as quantum
devices fabricated on flexible plastic sheets.
In this letter, we report the observation of single-electron
effects up to a temperature of 60 K in an in-plane, side-gated,
point contact device fabricated in phosphorous-doped nc-Si.
Our nc-Si film is deposited by PECVD at 300 °C. We struc-
turally characterize the nc-Si film and correlate this to the
electrical characteristics. We attribute electron transport in
our device to a thermally-assisted single-electron tunneling
process in the low-temperature range and percolation con-
duction through a distribution of barriers heights in the high-
temperature range.
We prepared 30 nm thick nc-Si films using very high
a͒Also at: Materials and Structure Laboratory, Tokyo Institute of Technol-
ogy, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan.
Electronic mail: ha10@phy.cam.ac.uk
FIG. 1. Scanning electron micrograph of the side-gated point contact device.
The inset shows an enlarged view of the source–drain point contact.
b͒
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
003-6951/2001/78(8)/1083/3/$18.00 1083 © 2001 American Institute of Physics
40.254.87.149 On: Sat, 20 Dec 2014 03:23:37
0
1

1084
Appl. Phys. Lett., Vol. 78, No. 8, 19 February 2001
Tan et al.
FIG. 3. ͑a͒ Ids–Vds characteristics at 8 K. Vgs is varied from Ϫ2 to 2 V in
steps of 50 mV. Each I–V curve is displaced by 20 pA for clarity. ͑b͒
Ids–Vgs characteristic at 4.2 K. Vds is varied from Ϫ44 to 44 mV in steps of
4
mV.
4
to 44 mV in 4 mV steps. A dominant oscillation with
smaller superimposed peaks is seen in the characteristics. A
Fast Fourier Transform of the characteristics indicates a pe-
FIG. 2. TEM planar image of nc-Si. The arrows indicate examples of
twinning in the grain structure.
riod ⌬V of 500 mV for the dominant oscillation and periods
g
of 100 and 60 mV for the superimposed minor oscillations.
This behavior can be attributed to single-electron conduc-
9
a scanning electron micrograph of the structure.
tance oscillations in a multiple tunnel junction ͑MTJs͒. Each
We have characterized the structure of the nc-Si films
using transmission electron microscopy ͑TEM͒. We used a
Philips CM 300 FEG with an accelerating voltage of 300 kV.
Figure 2 shows a TEM image of uniformly distributed nc-Si
grains embedded in an amorphous silicon matrix. The grain
sizes range from 4–8 nm. The diffraction rings shown in the
inset demonstrate that the film contains crystalline silicon
and not entirely amorphous. Additional high-resolution TEM
analysis using Fresnel contrast indicated that the grain
boundaries were decorated with fine layers of amorphous
silicon less than 1 nm thick.
oscillation is associated with a charging island in the MTJ
and a corresponding island—gate capacitance Cgϭe/⌬Vg
͑Ref. 10͒ can be extracted. The island—gate capacitances
corresponding to the three oscillation periods are 0.3 aF, 1.6
aF, and 2.7 aF, respectively. There exist switching effects at
V ϳ200–400 mV which may be associated with defect
gs
states near the charging islands. We have observed the con-
ductance oscillations at temperatures up to 60 K. The minor
oscillations disappear at lower temperatures then the domi-
nant oscillation, which persists up to 60 K.
We can estimate the minimum size of the charging is-
lands from the oscillations periods. Assuming equal tunnel-
ing capacitances C ϭC ϭC along the conducting path
We have also characterized the structure of the nc-Si
films with Raman spectroscopy, using an argon–ion beam
with an excitation wavelength of 514.5 nm. The film struc-
ture was analyzed6 by fitting the Raman spectra with a
combination of the correlation length model for the crystal-
1
2
across each island,10 the total capacitance of the island C⌺
–8
ϭ2CϩC . The temperature at which an oscillation disap-
g
pears may be used to estimate the total charging energy of
Ϫ1
2
line silicon peak at ϳ521 cm and a Gaussian function for
each associated charging island using E ϭe /(2C )
C
⌺
Ϫ1
the amorphous silicon peak at ϳ480 cm . The results dem-
Ϸ3k T ͑Ref. 11͒ and larger oscillation periods imply a
B
onstrated that the average grain size in the nc-Si film is 4 nm
and the crystalline volume fraction is 70%. Assuming spheri-
cal nc-Si grains with a radius of 2 nm, uniformly encapsu-
lated by an amorphous silicon layer of thickness D, we esti-
mate Dϭ0.25 nm from the crystalline volume fraction of
smaller island–gate capacitance. The dominant oscillation of
500 mV disappears at 60 K and can be associated with an
island with E ϳ15 meV, C ϳ5.3 aF, and Cϭ(C ϪC )/2
C
⌺
⌺
g
ϭ2.5 aF. Assuming that C is a simple parallel plate capacitor
where the plate separation is equal to the amorphous silicon
tunnel barrier width dϳ0.5 nm ͑estimated earlier from the
volume fraction͒ and ignoring any depletion regions on ei-
ther side of the amorphous silicon region, the island size is
ϳ4 nm. This is in good agreement with the grain size ob-
tained from TEM analysis and Raman spectroscopy. Simi-
larly, we can obtain island sizes for the minor oscillations
periods. The 100 mV oscillations disappear at ϳ20 K and the
60 mV oscillations disappear at ϳ6 K. For these oscillations,
70%. Therefore, the average thickness of the grain boundary
region is dϭ2Dϭ0.5 nm.
We have electrically characterized the point contact de-
vices at temperatures ranging from 4.2 to 300 K. Figure 3͑a͒
shows the drain–source current–voltage (I –V ) character-
ds
ds
istics of the device at 8 K as the gate voltage Vgs is varied
from Ϫ2 to 2 V in 50 mV steps. A Coulomb gap VC ,
strongly modulated by V , is observed at low V . The
gs
ds
maximum width of V is ϳ40 mV. Periodic peaks separated
C Ϸ15 aF and 52 aF and the island size is ϳ6.4 nm and 12
⌺
C
by ϳ20 mV are observed in the differential conductance
characteristic ͑not shown͒. A nonlinearity in the I –V
characteristics corresponding to the Coulomb gap is ob-
served up to a temperature 70 K.
nm, respectively. The TEM analysis indicated some twinning
of grains and this is likely to form larger coupled islands.
Figure 4 shows an Arrhenius plot of the conductance of
the device as a function of inverse temperature. The device
ds
ds
Figure 3͑b͒ shows oscillations in I as V is swept from
conductivity ␴ is measured at a bias of V ϭ50 mV ͑circles͒
ds
gs
ds
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
Ϫ1 to 1 V and V is varied from Ϫ44 to Ϫ4 mV and from
and at zero bias ͑triangles͒. For the measurement at V
ds
ds
1
40.254.87.149 On: Sat, 20 Dec 2014 03:23:37

Appl. Phys. Lett., Vol. 78, No. 8, 19 February 2001
Tan et al.
1085
The variation in the amorphous silicon tunnel barrier
height may be explained by considering the dopant distribu-
tion in the nc-Si film. A nc-Si grain of 4 nm diameter con-
tains on average thirty phosphorous atoms while a 0.5 nm
thick amorphous silicon grain boundary contains on average
only six phosphorous atoms. This implies that the grain
boundaries would be strongly influenced by any fluctuation
in the doping concentration, leading to a variation in the
energy difference between the conduction band edge and the
Fermi level in the amorphous silicon, and a variation in the
depletion width at the crystalline silicon/amorphous silicon
interface. We note that if the tunnel barrier on average is 40
meV high and 0.5 nm wide, the tunnel resistance is ϳ10 k⍀,
which is less than the quantum resistance R ϭ25.8 k⍀ and
K
1
1
Coulomb blockade effects are not possible. However, in
our measurements there must be a wider than average grain
boundary because of statistical fluctuations in the grain
boundary width together with additional depletion regions on
either side. This would create a wider tunnel barrier with a
higher tunnel resistance, for example if the barrier is 40 meV
high and 2 nm wide, the tunnel resistance is ϳ10RK and
Coulomb blockade effects would occur.
We have also fabricated up to 60 nm wide devices which
exhibit a maximum tunnel barrier height from 16–22 meV.
In these devices a larger number of percolation paths may
exist for electron transport and a lower tunnel barrier height
is more likely observed. Even in the point contact device of
Fig. 4, the maximum barrier height of 40 meV is relatively
low for electronic confinement at higher temperatures closer
FIG. 4. ͑a͒ Arrhenius plot of Vds biased at 50 mV ͑circles͒ and at zero
biased ͑triangles͒. T1 indicates the transition temperature, ϳ60 K, and EAl is
the maximum gradient obtained from the region for 60 KϽtemperature
Ͻ300 K, as indicated by the first solid line. The second and third solid line
to room temperature. At temperatures ϾT , single-electron
1
Ϫ1
effects disappear and there is a transition to percolation con-
duction.
shows the T dependence. ͑b͒ For 60 KϽtemperatureϽ300 K, the results
Ϫ1/4
follow a T
dependence as shown by the solid lines.
In conclusion, we have observed single-electron effects
in nc-Si point contacts up to a temperature of 60 K. We have
shown that the charging islands are nc-Si grains as small as
ϳ4 nm, isolated by amorphous silicon regions ϳ0.5 nm
thick. Electron transport is attributed to a thermally assisted
single-electron tunneling process at low temperature and per-
colation conduction at high temperature. Our work is funded
by the Japan Science and Technology Agency CREST pro-
gram.
ϭ50 mV, the device operates outside the Coulomb gap.
From 6 K to a transition temperature T ϳ60 K, log(␴) fol-
1
Ϫ1
lows a T
T
dependence and above T , log(␴) follows a
1
Ϫ1/4
Ϫ1/4
dependence. In the region of the T
dependence, the
conduction mechanism is likely to be dominated by percola-
tion conduction through a distribution of potential barrier
1
2
heights with various activation energies. The maximum
Ϫ1/4
gradient obtained from the region of T
dependence cor-
responds to an activation energy EA1ϳ40 meV, which can
1
S. Tiwari, F. Rana, H. Hanafi, A. Hartstein, E. F. Crabbe, and K. Chan,
Appl. Phys. Lett. 68, 1377 ͑1996͒.
Y. L. He, G. Y. Hu, M. B. Yu, M. Liu, J. L. Wang, and G. Y. Xu, Phys.
be associated with the maximum height of the amorphous
Ϫ1
silicon tunnel barrier in the nc-Si. The region of T depen-
2
dence corresponds to a smaller activation energy EA2
ϳ3 meV. This value is too low for electronic confinement
and we speculate that it corresponds to the difference be-
tween the Fermi level and conduction band edge in the nc-Si
Rev. B 59, 15352 ͑1999͒.
M. Fukuda, K. Nakagawa, S. Miyazaki, and M. Hirose, Appl. Phys. Lett.
3
70, 2291 ͑1997͒.
4
5
M. Otobe, H. Yajima, and S. Oda, Appl. Phys. Lett. 72, 1089 ͑1998͒.
K. Nakahata, K. Ro, A. Suemasu, T. Kamiya, C. M. Fortmann, and I.
Shimizu, Jpn. J. Appl. Phys., Part 1 39, 3294 ͑2000͒.
grains. Electron transport in this temperature range is con-
6
7
8
trolled by a thermally-assisted tunneling process.2
P. M. Fauchet and I. Campbell, Crit. Rev. Solid State Mater. Sci. 14, S79
͑
1988͒.
For ␴ measured at zero V , the device operates within
ds
Y. He, C. Yin, G. Cheng, L. Wang, and X. Liu, J. Appl. Phys. 75, 797
͑1994͒.
the Coulomb gap. At temperatures above T , the behavior is
1
B. Garrido, A. Perez-Rodriguez, J. R. Morante, A. Achiq, F. Gourbilleau,
R. Madelon, and R. Rizk, J. Vac. Sci. Technol. B 16, 1851 ͑1998͒.
K. Nakazato, R. J. Blaikie, and H. Ahmed, J. Appl. Phys. 75, 5123 ͑1994͒.
D. K. Ferry, Transport in Nanostructures ͑Cambridge University Press,
New York, 1996͒.
H. Grabert and M. H. Devoret, Single Charge Tunneling, Coulomb Block-
ade Phenomena in Nanostructures, NATO ASI Series B, Vol. 294 ͑Ple-
num, New York, 1992͒.
similar to that measured at V ϭ50 mV. This is because the
ds
Coulomb gap in the device disappears at ϳT and the zero
1
9
bias conductivity is similar to the conductivity at V
10
ds
Ϫ1
ϭ50 mV. Below T , there again exists a T
dependence.
1
11
However, the activation energy EA3ϳ10 meV is larger. This
energy is close to the Coulomb charging energy ϳ15 meV
obtained for the dominant island from the gate oscillations of
12
D. Adler, L. P. Flora, and S. D. Senturia, Solid State Commun. 12, 9
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
Fig. 3͑b͒.
͑1973͒.
1
40.254.87.149 On: Sat, 20 Dec 2014 03:23:37