Low-threshold field emission from cesiated silicon nanowires

Article abstract of DOI:10.1063/1.2136217

Field-emission studies on Si nanowires (Si NWs) grown by the vapor-liquid-solid (VLS) technique are presented. The field-emission properties of the Si NWs were characterized in ultrahigh vacuum following several postgrowth processes such as catalyst etching, in situ annealing, and cesiation. The average threshold field of cesiated Si NWs was found to be ～7.76±0.55 Vμm and showed a significant improvement over that of as-grown NWs (average threshold field ～11.58 Vμm). The superior field-emission characteristics are attributed to the combination of cesiation and quality of the NWs' surface grown via hydrogen reduction of silicon tetrachloride.

Full text of DOI:10.1063/1.2136217

Low-threshold field emission from cesiated silicon nanowires
N. N. Kulkarni, J. Bae, C.-K. Shih, S. K. Stanley, S. S. Coffee, and J. G. Ekerdt
Citation: Applied Physics Letters 87, 213115 (2005); doi: 10.1063/1.2136217
View online: http://dx.doi.org/10.1063/1.2136217
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/87/21?ver=pdfcov
Published by the AIP Publishing
Articles you may be interested in
Integrated silicon nanowire diodes and the effects of gold doping from the growth catalyst
J. Appl. Phys. 102, 054310 (2007); 10.1063/1.2778290
Er-doped silicon nanowires with 1.54 μ m light-emitting and enhanced electrical and field emission properties
Appl. Phys. Lett. 91, 093133 (2007); 10.1063/1.2777181
Field emission from randomly oriented ZnO nanowires
J. Vac. Sci. Technol. B 25, 1249 (2007); 10.1116/1.2752517
Field emission of silicon nanowires grown on carbon cloth
Appl. Phys. Lett. 90, 033112 (2007); 10.1063/1.2428543
Synthesis of single crystalline silicon nanowires and investigation of their electron field emission
J. Vac. Sci. Technol. B 24, 20 (2006); 10.1116/1.2137338
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:
142.244.5.161 On: Tue, 02 Dec 2014 03:55:04

APPLIED PHYSICS LETTERS 87, 213115 ͑2005͒
Low-threshold field emission from cesiated silicon nanowires
N. N. Kulkarni
Department of Materials Science & Engineering, University of Texas at Austin, Austin, Texas 78712
J. Bae
Department of Physics, University of Texas at Austin, Austin, Texas 78712
a͒
C.-K. Shih
Department of Physics, and Texas Materials Institute and Center for Nanotechnology and Molecular
Science, University of Texas at Austin, Austin, Texas 78712
S. K. Stanley, S. S. Coffee, and J. G. Ekerdt
Department of Chemical Engineering, University of Texas at Austin, Austin, Texas 78712
͑
Received 3 June 2005; accepted 27 September 2005; published online 17 November 2005͒
Field-emission studies on Si nanowires ͑Si NWs͒ grown by the vapor-liquid-solid ͑VLS͒ technique
are presented. The field-emission properties of the Si NWs were characterized in ultrahigh vacuum
following several postgrowth processes such as catalyst etching, in situ annealing, and cesiation.
The average threshold field of cesiated Si NWs was found to be ϳ7.76±0.55 V/␮m and showed a
significant improvement over that of as-grown NWs ͑average threshold field ϳ11.58 V/␮m͒. The
superior field-emission characteristics are attributed to the combination of cesiation and quality of
the NWs’ surface grown via hydrogen reduction of silicon tetrachloride. © 2005 American Institute
of Physics. ͓DOI: 10.1063/1.2136217͔
One-dimensional ͑1D͒ nanostructures ͑nanotubes and
nanowires͒ have gained attention over the past few years for
their potential use as electron field emitters in displays, x-ray
sources, sensors, and other vacuum microelectronic
Field-emission measurement was carried out on as-
grown samples and subsequent measurements were per-
formed at the end of each of the following postgrowth pro-
cessing steps: ͑a͒ catalyst removal ͑Au-selective etching͒, ͑b͒
in situ annealing, and ͑c͒ in situ cesiation. The I–V charac-
teristics ͑truncated to current value of 30 ␮A͒ of the Si NWs
for different postgrowth processing steps are shown as fol-
lows: as-grown case ͓Fig. 2͑a͔͒, Au-selective etching + in
situ annealed case ͓Fig. 2͑b͔͒, and finally, Au-selective etch-
ing + in situ annealed + in situ cesiated case ͓Fig. 2͑c͔͒. The
I-V curves in all cases were obtained after repeated forward
and reverse sweeps until subsequent sweeps were almost
identical. The threshold field is defined as the electric field
1
–4
devices.
have been characterized for their field-emission
Nanostructures of different materials systems
5
–11
properties.
Silicon has been the backbone of the micro-
electronics industry for decades and it would be desirable to
have silicon field emitters to facilitate integration onto sili-
con substrates along with the driving circuitry.
We report the electron field-emission characteristics of
silicon nanowires ͑Si NWs͒ grown by the vapor-liquid-solid
1
2
͑
VLS͒ technique and the reduction of threshold field via
2
required for an emission current density of 10 mA/cm , cor-
postgrowth processing steps such as in situ annealing and
in situ cesiation. The NWs were grown on n -Si ͗111͘ sub-
+
responding to the saturation current density of a phosphor in
displays. The emission area in our case was calculated ac-
strates by atmospheric pressure chemical vapor deposition
͑
APCVD͒ of SiCl +H ͑mole ratio SiCl :H =0.02͒ in the
4 2 4 2
temperature range of 850–950 °C. A thin Au film ͑5–20 nm͒
was used as the catalyst for VLS growth. The average NW
diameter was ϳ100 nm and the average areal density was
7
−2
3
ϫ10 cm . Figure 1 shows a scanning electron micro-
scope ͑SEM͒ image of a Si NW sample used in this study.
The field-emission measurement was carried out in a home-
built ultrahigh-vacuum ͑UHV͒ system using a spherical Au
ball ͑500 ␮m diam͒ as the anode. The sample holder was
equipped with a heater for in situ annealing and the chamber
contained a cesium ͑Cs͒ evaporation source to perform®
in situ cesiation. The data were acquired using LABVIEW
and a homebuilt current amplifier was used to measure the
field-emission current. A recessed window allowed for
measuring the interelectrode separation with an accuracy
of ϳ5 ␮m using a long working distance objective lens
͑
Mitutoyo 10ϫ͒. All field-emission measurements were per-
−
9
formed in vacuum with a base pressure of Ͻ5ϫ10 Torr.
FIG. 1. SEM image of Si NWs grown by APCVD of SiCl +H ͑mole ratio
4
2
SiCl :H =0.02͒ at 850 °C using a 20-nm Au film as a catalyst. The average
4
2
a͒Electronic mail: shih@physics.utexas.edu
NW diameter ϳ100 nm.
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:
003-6951/2005/87͑21͒/213115/3/$22.50 87, 213115-1 © 2005 American Institute of Physics
42.244.5.161 On: Tue, 02 Dec 2014 03:55:04
0
1

213115-2
Kulkarni et al.
Appl. Phys. Lett. 87, 213115 ͑2005͒
FIG. 3. J-E characteristics of the Si NWs for different electrode separations
for ͑a͒ Au-selective etched + in situ annealed and ͑b͒ Au-selective etched
+
in situ annealed + in situ cesiated cases.
in Fig. 3͑a͒, from which we deduce a threshold field of
ϳ9.9±0.3 V/␮m, indicating that the threshold field de-
creased upon in situ annealing. From the simplified Fowler–
4
2
2
3/2
Nordheim ͑F–N͒ equation, J=(A␤ E /␾)exp(−B␾ /␤E),
FIG. 2. I-V characteristics for different electrode separations ͑truncated to
current value of 30 ␮A͒ of Si NWs for ͑a͒ as grown, ͑b͒ Au-selective etched
−
10
−2
3
where A=1.56ϫ10 AV eV and B=6.83ϫ10 V
−
3/2
−1
+
in situ annealed, and ͑c͒ Au-selective etched + in situ annealed + in situ
eV
␮m ,␤ was estimated to be ϳ500 using the work
cesiated cases.
function of intrinsic Si ͑␾=4.5 eV͒ ͑Ref. 15͒.
In situ cesium evaporation was then carried out in UHV
1
6
using a Cs source from SAES Getters. The cesium was
evaporated for 5 min corresponding to a nominal coverage of
ML. As can be seen from the J-E characteristics of the
cesiated Si NWs ͓Fig. 3͑b͔͒, the average threshold field re-
duced to ϳ7.76±0.55 V/␮m, which is lower compared to
prior reports on silicon NWs ͑Refs. 17 and 18͒ and Si
microstructures. The superior field-emission characteristics
can be explained based on the fact that the Si NWs were
grown by H reduction of SiCl , in which the by-product
HCl is simultaneously etching the Si surface as the growth
proceeds, rendering NWs with a superior surface quality.
Figure 4 shows the F–N plot for different postgrowth
processing conditions at an electrode separation of 250 ␮m.
From the F–N equation, the ratio of slopes in the F–N plot of
the cesiated sample to that of the as-grown sample is equal to
cording to a modification of the formula in Ref. 13, and is
given by A=2␲(R+Z/2)Z͑2͑1/n͒−1͒, and is valid for Z
5
Ͻ2R, where R=radius of anode, Z=interelectrode spacing,
and n=(V/I)dI/dV. The quantity dI/dV in our case was ob-
tained by differentiating the smoothed I-V curves obtained
from experiment ͑shown in Fig. 2͒.
Field-emission measurement on the as-grown sample
was performed initially and the average threshold field was
found to be 11.58 V/␮m. Catalyst removal was performed
1
9
2
4
by etching the sample in aqua regia ͑HNO :HCl=1:3͒ for
3
20
10 min to selectively etch away Au and this was confirmed
by energy dispersive x-ray spectroscopy ͑EDS͒ in a SEM
͑
LEO 1530͒. After the Au-selective etch, the sample was
reloaded into the vacuum chamber for field-emission mea-
surements. All subsequent postgrowth processes were per-
formed in situ. The field-emission measurement on Au-
selective etch alone yielded similar I-V characteristics as the
as-grown case ͑not shown͒ and the same threshold field, in-
dicating that the Au-Si Schottky barrier did not play a deter-
ministic role in the field-emission characteristics of the Si
NWs. In situ annealing was carried out by passing a dc cur-
3
/2
͑
␾cesiated/␾as-grown
͒
.
From Fig.
4
we deduce that
2
rent of 1.2 A ͑equivalent to a current density of 16 A/cm ͒
for 4 s. Based on the color of the sample ͑red hot͒ we esti-
mated the temperature was raised to 700 °C. The sample was
then rapidly cooled down. The field-emission measurement
was carried out after a reasonable time ͑Ͼ30 min͒ had
elapsed from the point it was heated so as to avoid any ther-
mionic contribution to the emission current. This procedure
1
4
removed the surface adsorbates as evidenced from the
smooth I-V characteristics ͓shown in Fig. 2͑b͔͒. The plot of
emission current density, J, versus electric-field strength, E,
FIG. 4. F–N plot for different postgrowth processing conditions with re-
spective linear fits. The decrease in slope in the Au-selective etched + in situ
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:
for the Au-selective etched + in situ annealed case is shown
annealed + in situ cesiated case indicates the lowering of the work function.
142.244.5.161 On: Tue, 02 Dec 2014 03:55:04

2
13115-3
Kulkarni et al.
Appl. Phys. Lett. 87, 213115 ͑2005͒
Vacuum Microelectronics ͑Wiley, New York, 2001͒.
F. G. Tarntair, C. Y. Wen, L. C. Chen, J.-J. Wu, K. H. Chen, P. F. Kuo, S.
W. Chang, Y. F. Chen, W. K. Hong, and H. C. Cheng, Appl. Phys. Lett.
4
5
͑
␾cesiated/␾as-grown͒=0.4147, indicating that there is a clear
reduction in the work function of the Si NWs’ surface after
cesiation.
In conclusion, we have reported field-emission studies
on VLS-grown Si NWs and studied the effects of various
postgrowth processing steps on their field-emission charac-
teristics. The cesiated Si NWs exhibit a low-threshold field
2
1
7
6, 2630 ͑2000͒.
6
7
8
9
0
Y. B. Li, Y. Bando, D. Golberg, and K. Kurashima, Appl. Phys. Lett. 81,
5048 ͑2002͒.
Y. H. Lee, C. H. Choi, Y. T. Jang, E. K. Kim, B. K. Ju, N. K. Min, and J.
H. Ahn, Appl. Phys. Lett. 81, 745 ͑2002͒.
C. J. Lee, T. J. Lee, S. C. Lyu, Y. Zhang, H. Ruh, and H. J. Lee, Appl.
Phys. Lett. 81, 3648 ͑2002͒.
H. L. Lai, N. B. Wong, X. T. Zhou, H. Y. Peng, F. C. K. Au, N. Wang, C.
S. Lee, S. T. Lee, and X. F. Duan, Appl. Phys. Lett. 76, 294 ͑1999͒.
C. T. Hsieh, J. M. Chen, H. H. Lin, and H. C. Shih, Appl. Phys. Lett. 83,
3383 ͑2003͒.
͑
ϳ7.76±0.55 V/␮m͒
compared
to
most
silicon
1
7–19
nanostructures,
with the exception of carbon-coated Si
2
2
cones. This low-threshold field emission from cesiated Si
NWs is comparable to that reported from various 1D
materials,
added advantage of being potentially integrated with micro-
electronic driving circuitry, making vacuum microelectronic
devices, including displays based on Si NWs, viable in the
future. Further improvement in the field-emission character-
istics can be obtained by incorporating the Si NWs into a
triode structure, as compared to the diode device reported in
this study.
1
5
–11
13
including carbon nanotubes, and Si has the
11
Y. B. Li, Y. Bando, and D. Golberg, Appl. Phys. Lett. 82, 1962 ͑2003͒.
R. S. Wagner, in Whisker Technology, edited by A. P. Levitt ͑Wiley, New
York, 1970͒, pp. 47–199.
1
1
1
1
2
3
4
5
W. Zhu, C. Bower, O. Zhou, G. Kochanski, and S. Jin, Appl. Phys. Lett.
7
5, 873 ͑1999͒.
M. K. Sanganeria, M. C. Ozturk, G. Harris, K. E. Violette, I. Ban, C.
Archie Lee, and D. M. Maher, J. Electrochem. Soc. 142, 3961 ͑1995͒.
N. W. Ashcroft and N. D. Mermin, Solid State Physics ͑Saunders College,
Philadelphia, 1981͒.
1
1
6
7
SAES Getters S. p. A., Milano, Italy.
The authors gratefully acknowledge support of this work
by the Texas Advanced Technology and Research Programs,
NSF Grants No. DMR-0210383, DMR-0306239, and DMR-
F. C. K. Au, K. W. Wong, Y. H. Tang, Y. F. Zhang, I. Bello, and S. T. Lee,
Appl. Phys. Lett. 75, 1700 ͑1999͒.
1
8
Y. L. Chueh, L. J. Chou, S. L. Cheng, J. H. He, W. W. Wu, and L. J. Chen,
Appl. Phys. Lett. 86, 133112 ͑2005͒.
0
312491, and the Advanced Materials Research Center.
19
W. K. Wong, F. Y. Meng, Q. Li, F. C. K. Au, I. Bello, and S. T. Lee, Appl.
Phys. Lett. 80, 877 ͑2002͒.
1
20
W. B. Choi, Y. W. Jin, H. Y. Kim, S. J. Lee, M. J. Yun, J. H. Kang, Y. S.
Choi, N. S. Park, N. S. Lee, and J. M. Kim, Appl. Phys. Lett. 78, 1547
2001͒.
G. Z. Yue, Q. Qiu, B. Gao, Y. Cheng, J. Zhang, H. Shimoda, S. Chang, J.
P. Lu, and O. Zhou, Appl. Phys. Lett. 81, 355 ͑2002͒.
A. Modi, N. Koratkar, E. Lass, B. Q. Wei, and P. M. Ajayan, Nature
S. K. Ghandhi, VLSI Fabrication Principles, 2nd ed. ͑Wiley, New York,
1994͒.
1
2
2͑
K. Wandelt, in Physics and Chemistry of Alkali Metal Adsorption, edited
by H. P. Bonzel, A. M. Bradshaw, and G. Ertl ͑Elsevier, New York, 1989͒,
pp. 25–44.
22
3
X. D. Bai, C. Y. Zhi, S. Liu, E. G. Wang, and Z. L. Wang, Solid State
Commun. 125, 185 ͑2003͒.
͑London͒ 424, 171 ͑2003͒.
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:
42.244.5.161 On: Tue, 02 Dec 2014 03:55:04
1