APPLIED PHYSICS LETTERS 90, 012105 共2007兲
Electrical properties of nominally undoped silicon nanowires grown
by molecular-beam epitaxy
Jan Bauer,a兲 Frank Fleischer, Otwin Breitenstein, Luise Schubert, Peter Werner,
Ulrich Gösele, and Margit Zacharias
Max Planck Institute of Microstructure Physics, Weinberg 2, Halle D-06120, Germany
共Received 9 August 2006; accepted 12 October 2006; published online 3 January 2007兲
Single undoped Si nanowires were electrically characterized. The nanowires were grown by
molecular-beam epitaxy on n+ silicon substrates and were contacted by platinum/iridium tips. I-V
curves were measured and electron beam induced current investigations were performed on single
nanowires. It was found that the nanowires have an apparent resistivity of 0.85 ⍀ cm, which is
much smaller than expected for undoped Si nanowires. The conductance is explained by hopping
conductivity at the Si– SiO2 interface of the nanowire surface. © 2007 American Institute of
Physics. 关DOI: 10.1063/1.2428402兴
In contrast to the lithography and etching techniques
used in the “top down” approach of microelectronics, the
“bottom up” approach is focused on the direct growth of
nanostructures from the very beginning. Such a controlled
growth of one-dimensional structures such as nanowires
共NWs兲, based on conventional semiconductors, has been under discussion for future semiconductor devices. Many examples for electrical applications of NWs have been
shown1,2 and concepts for NW transistors have been
discussed,3–6 but mostly these applications are based on
semiconductor NWs, which were separated from their substrates on which they were grown. It was also shown that
epitaxial semiconductor NWs can be grown on different substrates and can be arranged in nearly any pattern.7,8 The electrical transport properties of epitaxial NWs, which are still
connected to their substrates, are not very well known so far
but are critical for designing NWs for vertically aligned
semiconductor devices. For that reason it is necessary to understand the electronic properties of these NWs.
We present results of the electrical characterization and
of first electron beam induced current 共EBIC兲 measurements
of undoped Si NWs. A method was developed to contact
NWs without the use of electron beam lithography. The electrical properties of single as-grown Si NWs were measured
by probing them in the scanning electron microscope 共SEM兲,
using a nanomanipulator, which was equipped with a metal
tip. The EBIC method was used for investigating the interface between the Si NWs and the Si substrate, which influences the overall electrical properties. Using the obtained
experimental data, a model will be presented based on the
Shockley two-diode model. It will turn out that the resistivity
of the NWs cannot be described in the framework of a model
assuming only volume conductivity. It is argued that hopping
conduction via interface states between the silicon and native
oxide provides another conduction path along the NWs.
Details of the growth process of the Si NWs characterized in this work were described previously.9 Briefly, highly
n-doped 共As, 2 ⫻ 1019 cm−3兲 5 in. Si 共111兲 wafers were used
as substrates for the growth. The molecular-beam epitaxy
共MBE兲 growth process starts by a thermal annealing at
850 ° C to desorb the native oxide layer. Afterwards, a thin
gold layer with a nominal thickness of 2 nm was deposited
in situ by electron beam evaporation at a substrate temperature of 525 ° C to produce gold nanocluster on the silicon
surface. During the growth of the wires a very pure Si source
共␳ ⬇ 5000 ⍀ cm兲 was used. The resulting Si NWs are standing perpendicular to the substrate surface 共Fig. 1兲. Transmission electron microscopy 共TEM兲 experiments confirmed a
growth direction of 具111典 for the Si NWs. Note that there is
an additional growth of a Si layer on the substrate.9 The
electrical characterization of these Si NWs was carried out
using a high-resolution SEM JEOL JSM 6400, equipped with
a nanomanipulator 共Kleindiek兲. The manipulator is equipped
with a platinum/iridium tip, which is used for connecting and
manipulating the NWs. The tips were electrochemically
etched from 250 ␮m thick wires 共Pt/ Ir 80:20, GoodFellow兲
using an etching procedure as described in Ref. 10. A picoamperemeter with an internal voltage source 共Keithley 6487,
艋2 fA noise兲 was connected to the Pt/ Ir tip for the I-V measurements, which were performed under computer control.
For the EBIC measurements, a variable-gain low-noise current amplifier 共Femto DLPCA-S兲 was connected to the Pt/ Ir
tip. The tip contacts the Au dot of the NW and the back
contact was realized by the Si n+ substrate using silver paste.
The electron beam of the SEM was blanked during the I-V
measurements to avoid any influence of the electron beam on
the results.
a兲
FIG. 1. 共Color online兲 SEM image of the Si NWs and the Pt/ Ir tip. The dark
hemispheres on the top of the wires are gold droplets.
Author to whom correspondence should be addressed; electronic mail:
jbauer@mpi-halle.de
0003-6951/2007/90共1兲/012105/3/$23.00
90, 012105-1
© 2007 American Institute of Physics
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012105-2
Appl. Phys. Lett. 90, 012105 共2007兲
Bauer et al.
FIG. 2. I-V curve of an individual freestanding Si NW. The characteristics
correspond to a diode behavior. The inset shows the setup for the electrical
measurements. The Si n+ substrate 共1兲 is used for the back side contact. The
Si NW 共2兲 is covered by a gold droplet 共3兲 which is contacted by a Pt/ Ir tip.
For demonstration, Fig. 1 shows the Pt/ Ir tip as it approaches one selected NW. In Fig. 2, a typical I-V characteristic is shown for such a Si NW having a diameter of 70 nm
and a length of 500 nm. The measured NW shows a diodelike behavior. To determine the electronic structure of the
gold-dot–Si-NW–Si-substrate system, an EBIC measurement
was performed on a contacted Si NW. As Fig. 3共b兲 shows,
the EBIC signal appears only at the interface between the Si
NW and the Si substrate. If our NW would be n type, we
would expect an EBIC signal, which occurs mainly at the
NW–Au-dot interface. In contrast, if the NW would be p
type, we expected an EBIC signal of opposite sign also there,
and in addition an EBIC signal at the substrate-NW interface
related to the expected pn junction. However, only an EBIC
signal at the interface to the substrate was observed. The
only possible explanation is that our NW is intrinsic; hence
the Fermi level is lying close to midgap. Then, as Fig. 3共c兲
shows, there is an electric field at the Si-substrate–NW interface, but only a negligible field at the NW–Au-dot interface.
It can be assumed that at this interface the Fermi level is
pinned also close to midgap. For the growth of the wires a
very pure Si source was used. Any residual doping of the
NW will be compensated by interface states of the outer
native oxide of the wire and by gold atoms, which are embedded in the NW volume. If there is an interface state density of about 1 ⫻ 1012 cm−2 at the native oxide interface of
the NW, which is typical for a native oxide, a possible residual doping of ⬇5 ⫻ 1017 cm−3 can be compensated. Note
that such interface states always consist of deep donor and
acceptor states; hence they may trap and thereby compensate
free carriers. For describing this interface it is not sufficient
to consider only fixed charges.11 Moreover, gold generates
deep acceptors and deep donors in Si, which also may compensate an amount of free charge carriers comparable to their
concentration. Therefore it can be expected that all residual
free carriers are compensated in our NWs. Hence, the carrier
density of intrinsic silicon material of n ⬇ p ⬇ 1010 cm−3 at
room temperature will be assumed to exist in our Si NWs.
We consider a two-diode model 共double exponential IV characteristics兲 for describing the Si NWs. In Fig. 2 it is
obvious that the exponential behavior of the I-V characteristics turns into a linear one at higher positive voltages. From
the linear part of the I-V characteristics 共from 0.4 to 0.55 V兲,
the series resistance was estimated to be Rs = 1.1 M⍀. Referred to the dimensions of the NW and assuming that the
current flows homogeneously through the wire, this corresponds to a resistivity of 0.85 ⍀ cm. Regarding the series
resistance Rs, the total forward current INW is given by
冉
INW = I01 exp
e共U − INWRs兲
−1
n01kT
冉
+ I02 exp
冊
冊
e共U − INWRs兲
−1 ,
n02kT
共1兲
where e is the electronic charge, U is the applied voltage, Rs
is the series resistance of the NW, n01 is the ideality factor of
the diffusion current and n02 that of the recombination current, k is Boltzmann’s constant, T is the absolute temperature
共T = 300 K兲, and I01 is the saturation current of the diffusion
current and I02 is that of the recombination current, respectively. We imply that the diffusion current has an ideality
factor n01 = 1. I01 is hardly visible in the I-V characteristic
because of the series resistance. We have used a value of
I01 = 1.25⫻ 10−13 A, which is expected for a hole concentration of 1010 cm−3 and a diffusion length of about the diameter of the NW. I02 is dependent on the interface state density
of the n+ − i interface. According to the elementary diode
theory n02 should be 2. However, in the presence of a high
density of states, which is provided by the growth interface
and by the native oxide layer at the Si– SiO2 interface, also
larger ideality factors are frequently measured.12 To find I02
and n02 we fitted Eq. 共1兲 to the measured data. The best fit
occurs for I02 = 3.5⫻ 10−10 A, n02 = 4.7, and Rs = 1.1 M⍀. As
can be seen in Fig. 4, the model agrees quite well with the
observed I-V curve.
FIG. 3. SEM image of a contacted Si NW 共a兲 and the corresponding EBIC
image 共b兲. The dotted lines frame the Pt/ Ir tip and the nanowire. An EBIC
signal occurs mainly at the NW-substrate interface and hardly at the golddot–Si-NW interface. In 共c兲 the band structure is shown for the metal–SiNW–Si-substrate system, which is indicated by the EBIC measurement. CB
and VB are the conduction and valence bands, and EF is the Fermi level.
Between n+ substrate and intrinsic Si NW a band bending occurs. In 共d兲 a
high-resolution TEM image of the edge of a NW is shown; the dark contrast
at the interface between the Si NW and the native SiO2 demonstrates the
presence of Au clusters.
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012105-3
Appl. Phys. Lett. 90, 012105 共2007兲
Bauer et al.
FIG. 4. Semilogarithmical diagram of the I-V measurement from Fig. 2:
measured data 共circles兲, two-diode model without resistance correction 共dotted line兲, and two-diode model with resistance correction with Rs
= 1.1 M⍀ from Eq. 共1兲 共solid line兲.
However, the series resistance of 1.1 M⍀ does not fit
well with our assumed intrinsic NW material. A resistivity of
0.85 ⍀ cm of the NW would correspond to a doping concentration of ⬇1016 cm−3. As discussed above, this is in contradiction to the EBIC results and also cannot be expected from
our undoped NW material. Therefore, we propose that the
unexpectedly high electrical conduction of the NWs is a
property of their outer surface. TEM investigation has revealed the existence of nanometer-sized Au clusters at the
Si-NW– SiO2 interface 关Fig. 3共d兲兴. These clusters were
partly overgrown by the native oxide. Such Au clusters are
typical for an oxygen-free UHV environment.13,14 For
chemical-vapor deposition growth of Si NWs Au nanoclusters on the surface of NWs have not been observed and reported so far. Hence a high density of surface or interface
states can be expected to exist at our MBE NW surface. We
propose that these interface states are responsible for the
measured conductivity, rather than the conductivity in the
NW volume. Such “hopping conductivity” has been described theoretically15 and has been proposed to be responsible also for leakage currents in solar cells.12
In summary, single, nominally undoped MBE-grown Si
NWs were electrically characterized as grown. The I-V char-
acteristics of single Si NWs showed a diodelike behavior,
with a well pronounced asymmetry. The series resistance of
the wire was calculated to be Rs = 1.1 M⍀. An EBIC signal
occurred only at the interface between the Si NW and the Si
substrate, which is evidence for the intrinsic behavior of the
Si NW. The adopted two-diode model, corrected by Rs, describes the I-V curve quite well, but the high conductance of
the NW is not compatible with our EBIC results, suggesting
intrinsic behavior. We propose that the high conductance is
due to hopping conductivity across surface and interface
states at the surface of the NWs. This phenomenon shows
that nominally undoped Si NWs with a native oxide layer
may be conductive, but the properties of the conduction
come essentially from the SiO2 layer and not from the Si
NW itself. This has to be considered for Si NWs grown
under UHV conditions and their application in devices.
The authors thank W. Erfurth and K.-U. Aßmann for the
technical support of the SEM and the nanomanipulator system. This work was supported by the DFG SPP 1165 共Za
191/17-1兲.
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