Electron transport characteristics of silicon nanowires by metal

Electron transport characteristics of silicon nanowires by metal-assisted chemical
etching
Yangyang Qi, Zhen Wang, Mingliang Zhang, Xiaodong Wang, An Ji, and Fuhua Yang
Citation: AIP Advances 4, 031307 (2014); doi: 10.1063/1.4866578
View online: http://dx.doi.org/10.1063/1.4866578
View Table of Contents: http://scitation.aip.org/content/aip/journal/adva/4/3?ver=pdfcov
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AIP ADVANCES 4, 031307 (2014)
Electron transport characteristics of silicon nanowires
by metal-assisted chemical etching
Yangyang Qi, Zhen Wang, Mingliang Zhang, Xiaodong Wang,a An Ji,
and Fuhua Yang
Engineering Research Center for Semiconductor Integrated Technology,
Institute of Semiconductors, Chinese Academy of Sciences, Beijing, 100083, China
(Received 24 November 2013; accepted 2 January 2014; published online 18 February 2014)
The electron transport characteristics of silicon nanowires (SiNWs) fabricated by
metal-assisted chemical etching with different doping concentrations were studied.
By increasing the doping concentration of the starting Si wafer, the resulting SiNWs
were prone to have a rough surface, which had important effects on the contact
and the electron transport. A metal-semiconductor-metal model and a thermionic
field emission theory were used to analyse the current-voltage (I-V) characteristics.
Asymmetric, rectifying and symmetric I-V curves were obtained. The diversity of
the I-V curves originated from the different barrier heights at the two sides of the
SiNWs. For heavily doped SiNWs, the critical voltage was one order of magnitude
larger than that of the lightly doped, and the resistance obtained by differentiating the
I-V curves at large bias was also higher. These were attributed to the lower electron
tunnelling possibility and higher contact barrier, due to the rough surface and the
C 2014 Author(s). All
reduced doping concentration during the etching process. article content, except where otherwise noted, is licensed under a Creative Commons
Attribution 3.0 Unported License. [http://dx.doi.org/10.1063/1.4866578]
I. INTRODUCTION
Silicon nanowires (SiNWs) have attracted much attention due to their potential application in
the field of nanoelectronics,1 thermoelectrics2 and photovoltaics.3 However, with the reduction in
the diameter of the SiNWs, it is difficult to obtain ideal ohmic contact between the SiNWs and the
metal electrode, which has limited the application of SiNWs. Several factors, such as the contact
metal, the fabrication method of the test structure and the process should be considered in the ohmic
contact formation. Currently, Ti,4–7 Ni,8 Al7, 9 and Au7, 10 are widely used as the test electrodes,
because they have work functions that permit alignment of the Fermi levels. In the main, there are
two kinds of strategy to fabricate the test structure. In the first, the SiNWs are directly patterned
on the silicon-on-insulator substrate, and then the electrical contact is defined at the ends of the
SiNWs.2, 6, 10 In the second, the SiNWs are first fabricated by vapour-liquid-solid growth4, 5, 7, 8 or
metal-assisted chemical etching (MACE) methods,11 and then the individual SiNW is transferred to
an insulating substrate with the electrodes ready or not. For these strategies, special facilities, such
as e-beam lithography and focused ion beam, should be used.12 Surface treatment processes, such as
dipping in diluted HF acid,5 or thermal annealing treatments,5–7 are also performed to achieve ideal
ohmic contact. It has also been found that, with p-type and heavily doped SiNWs, ohmic contact
is easy to achieve.9 Therefore, it is necessary to understand the electron transport behaviours of
SiNWs, which are affected by the dimension, doping concentration and material characteristics.13
MACE is a convenient method of fabricating large scale SiNW arrays.14, 15 By increasing the
doping concentration of the starting Si wafer, the resulting SiNWs evolve from smooth surface
to rough surface.16 To date, the electron transport characteristics of SiNWs fabricated by MACE
a Corresponding author: xdwang@semi.ac.cn
2158-3226/2014/4(3)/031307/6
4, 031307-1
C Author(s) 2014
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AIP Advances 4, 031307 (2014)
FIG. 1. (a) A test structure of an individual SiNW synthesized from a light-doped Si wafer (resistivity 3–7 · cm). The
length of the nanowire is about 2 μm and the diameter is about 160 nm. (b) Energy band diagram of the MSM structure,
where φ B1 and φ B2 denote the Schottky barrier heights at the two contacts, V1 , VNW and V2 are the voltage loaded on the
barrier φ B1 , SiNW and barrier φ B2 , respectively.
have been rarely investigated. In this paper, the current-voltage (I-V) characteristics of SiNWs are
investigated. The SiNWs were synthesized by MACE using an Si wafer with two kinds of resistivity,
that is, 3–7 · cm and <0.0035 · cm. Asymmetric, rectifying and symmetric I-V curves were
obtained. A metal-semiconductor-metal (MSM) model,17, 18 including two Schottky barriers at the
interface of the metal and the SiNWs and a resistance between these two Schottky barriers, was
used to analyse the I-V characteristics, which were determined by the reverse-biased Schottky
barrier. The electron transport behaviour at this reverse-biased Schottky barrier was explained by
the thermionic field emission (TFE) method, which added the effect of tunnelling current compared
with the conventional thermionic emission theory.19
II. EXPERIMENTAL
Two kinds of Si (100) wafers, the lightly doped with resistivity 3–7 · cm and the heavily doped
with resistivity <0.0035 · cm, were used to fabricate SiNWs by Ag assisted chemical etching.11
The lightly doped SiNWs were produced in a mixture of 8.5 M HF and 0.6 M H2 O2 for two hours.
Considering that the heavily doped Si wafer was more sensitive to the etching condition, the etching
condition was changed to be 8 M HF and 0.4 M H2 O2 for 15 min.20 Then these SiNWs were released
from the substrate by sonication in ethanol. Several drops of SiNW suspension were placed on a
200 nm SiO2 coated Si sample and dried with N2 flow. Measurement electrodes were formed at
the two ends of the individual SiNWs by electron beam lithography, electron beam evaporation of
200 nm Al and the lift-off process. All samples were loaded on a Lakeshore CR6-4K probe stage
and I-V curves were obtained using an Agilent B1500A semiconductor analyser.
III. RESULTS AND DISCUSSION
The I-V measurements were taken on an individual SiNW between two electrodes. The SiNW
has a diameter of 160 nm and length of 2 μm. Figure 1(a) shows a scanning electron microscopy
(SEM) image of an I-V test structure for lightly doped SiNWs. It can be seen that this is an MSM
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AIP Advances 4, 031307 (2014)
FIG. 2. Typical I-V curves of lightly doped SiNW (a) asymmetric, and heavily doped SiNW (b) rectifying, (c) symmetric.
FIG. 3. SEM images of the surface morphology of SiNWs. Lightly doped SiNW has a smooth surface (a), while heavily
doped SiNW has a rough surface (b).
structure, which can be modelled by an SiNW sandwiched with two Schottky barriers back to
back.18, 21, 22 The corresponding energy band diagram is shown in Figure 1(b), where φ B1 and φ B2
denote the Schottky barrier heights at the two contacts, and V1 , VNW and V2 are the voltages loaded
on the barrier φ B1 , SiNW and barrier φ B2 , respectively. When a positive voltage V is applied to the
right metal-semiconductor contact, the barrier φ B2 is forward-biased while φ B1 is reverse-biased.
The resistance of the reverse-biased barrier is much larger than that of the forward-biased, so most
voltage is loaded onto barrier φ B1 . The reverse-biased barrier φ B1 determines the I-V characteristics.
Figure 2 shows three typical I-V curves, which are asymmetric, rectifying or symmetric. The
diversity of the I-V curves originates from the difference in barrier heights φ B1 and φ B2 . Only when
barrier heights φ B1 and φ B2 are equal, a symmetric I-V curve can be obtained. It should be noted that
even though all experimental conditions were the same, it was difficult to produce the same barrier
heights at the two interfaces due to the different contact surfaces.
Comparing the I-V curves of the heavily doped SiNWs (Fig. 2(b) and 2(c)) with the lightly
doped (Fig. 2(a)), it can be seen that the critical voltage of the heavily doped SiNWs, at which
the current began to increase, is one order of magnitude larger than that of the lightly doped. The
critical voltage for the heavily doped SiNWs (Fig. 2(b) and 2(c)) is about 20 V, while for the lightly
doped (Fig. 2(a)) it is only 2 V. This indicates that a higher Schottky barrier exists at the interface
of the heavily doped SiNW and the metal. This high Schottky barrier may be attributed to the rough
surface of the heavily doped SiNW.23 As shown in Figure 3, compared with the lightly doped SiNW
(Fig. 3(a)), the heavily doped (Fig. 3(b)) has a rough surface. Large surface roughness increases the
surface-to-volume ratio, which results in a thick oxide layer. In general, the natural oxide layer on the
surface of Si is only several nanometres and can be ignored in the measurement process. However,
for the heavily doped SiNW, the thick oxide layer cannot be ignored. There is less possibility that
electrons pass the barrier.
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AIP Advances 4, 031307 (2014)
TABLE I. Parameters of samples a-c corresponding to Fig. 2.
RNW a [M]
Restimated b [M]
q/kT-1/E0 [V−1 ]
E0 [mV]
Nd [cm−3 ]
a The
b The
Sample a
Sample b
Sample c
7.18 × 103
5
0.621
26.3
2.7 × 1018
5.747 × 104
1 × 10−2
0.228
26.0
9.6 × 1017
6.443 × 104
1 × 10−2
0.206
26.0
9.6 × 1017
RNW include the intrinsic resistance of the SiNWs and the contact resistances.
Restimated are obtained based on the resistivity of the bulk Si, the length and the diameter of the SiNWs.
TFE theory has been used to analyse the MSM structure.18 According to the influence of the
contact resistance and intrinsic resistance of SiNW on the I-V characteristics, the I-V curves can be
divided into two stages. In the first stage, the applied voltage is small, and most voltage is loaded
on the reverse-biased Schottky barrier. Until the voltage V is large enough, tunnelling at the reversebiased barrier occurs. In this case, the I-V characteristic is contact-limited. With further increasing
the voltage, the I-V curve enters the second stage. The effect of the intrinsic resistance of the SiNW
becomes significant. Most voltage is loaded on the SiNW, and the I-V curve is bulk-limited and
nearly linear. The resistance of the SiNW can be obtained by directly differentiating the I-V curve at
high voltage.
In the first stage, the current at the reverse-biased voltage V, is given by18
1
1
q
q
= Isr exp V
,
−
−
I = S × JT F E (V, φ B ) = S × Jsr (V, φ B ) exp V
kT
E0
kT
E0
(1)
where S is the cross-sectional area of SiNW and Jsr is the current density. Jsr is a slowing varying
function of voltage and is written as
⎤ 12
⎡
1
A∗ T (πq E 00 ) 2
φ
φB
B
⎦ .
Jsr =
(2)
× ⎣q(V − ζ ) +
exp −
2 q E 00
k
q E0
cosh
kT
E0 is defined by
q E 00
E 0 = E 00 coth
KT
with
E 00
≡
2
Nd
m ∗ εs
12
,
,
(3)
(4)
where Nd , m∗ and εs are doping concentration, effective mass of electron and relative permittivity
of SiNWs, respectively. To simplify the analysis, Equation (1) can be reduced to
q
1
.
(5)
ln I = ln Isr + V
−
kT
E0
Based on Equation (5), the ln I-V curve is a straight line and has a slope of q/kT-1/E0 .
According to the TFE theory, the resistances of SiNWs can be obtained by differentiating the I-V
curves at high voltage. In fact, the calculated values of the resistances include the contact resistance.
The oxide layer on the surface of the SiNWs reduces the electron transmission probability, which
presents as contact resistance. The contact resistance can affect the I-V characteristics of the SiNWs.
The heavily doped SiNWs have a thicker oxide layer due to the rough surface, as mentioned above,
and thus their contact resistances are greater. Therefore, the influence of the oxide layer is greater
for the heavily doped SiNWs. The calculated values of the resistances are listed in Table I. As a
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AIP Advances 4, 031307 (2014)
FIG. 4. Logarithmic forms of I-V curves corresponding to three typical cases, asymmetric (a), rectifying (b) and symmetric
(c). The dots and the solid line are the experiment and fitted results, respectively.
comparison, the estimated values of the resistances, which are obtained based on the resistivity of
bulk Si and the dimension of the SiNWs, are also presented. The resistance of the lightly doped
SiNW is three orders of magnitude larger than the estimated value, which may be attributed to the
contact between the metal and the SiNW. The resistances of the heavily doped SiNWs are six orders
of magnitude larger than the estimated value. Such high resistances are due to not only the contact
resistance but also three other factors: (1) the electron transport channel is actually very small due
to the existence of a porous shell on the surface of the heavily doped SiNW; (2) the porous structure
may produce an electron trap or increase the electron scattering in the transport process, which
decreases electron mobility; and (3) because the etching preferentially removes the dopants,23 the
doping concentration of the SiNW is lower than that of the bulk Si, which results in the higher
resistivity of the SiNW.
The I-V curves in Figure 2 are re-plotted in logarithmic scale at intermediate bias. As shown
in Figure 4, the lnI-V curves are almost linear. Their slopes are listed in Table I. E0 can be easily
obtained based on the slopes of the lnI-V curves and Equation (5). The E0 of the lightly doped SiNW
is 26.3 mV, slightly larger than the 26.0 mV of the heavily doped SiNWs. The small E0 of the heavily
doped SiNW further confirms that the possibility of tunnelling in the heavily doped SiNW is low.
According to Equations (3) and (4), the doping concentrations of the SiNWs can be obtained, and
are also listed in Table I. It is noted that the SiNWs fabricated from the heavily doped Si wafer have
a lower doping concentration, which indirectly proves that the etching can remove the dopants. The
low doping concentration is one of the reasons for the large contact and intrinsic resistance of the
SiNWs.
IV. CONCLUSIONS
In conclusion, the electron transport characteristics of SiNWs with two kinds of doping concentration were investigated. The SiNWs were fabricated by the MACE method. With the increase
in doping concentration of the starting Si wafer, the surfaces of the resulting SiNWs evolved from
smooth to rough, which had important effects on the contact and electron transport. An MSM model
was used to analyse the I-V characteristics of the SiNWs. Asymmetric, rectifying and symmetric
I-V curves were obtained due to the different barrier heights at the two sides of the SiNWs. The
reverse-biased barrier determined the electron transport characteristics. When the voltage was high
enough, tunnelling at the reverse-biased barrier occurred. The possibility of tunnelling in the heavily
doped SiNWs was lower than in the lightly doped because of the rough surface, which resulted in a
thick oxide layer. The resistances of the SiNWs, which were about 104 M and much larger than
the estimated values, were obtained by differentiating the I-V curves at high voltage. The doping
concentrations of the heavily doped SiNWs, calculated based on the TFE theory, were lower than
that of the starting Si wafer. Therefore, the large resistances of heavily doped SiNWs were attributed
to the rough surface and the low doping concentration.
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ACKNOWLEDGMENTS
The authors gratefully acknowledge the support from the National Natural Science Foundation
of China under the Grants 61076077, 61372059 and 61274066.
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