APPLIED PHYSICS LETTERS 95, 053105 共2009兲

Crucial role of doping dynamics on transport properties of Sb-doped
SnO2 nanowires
Annop Klamchuen,1,2 Takeshi Yanagida,1,3,a兲 Kazuki Nagashima,1 Shu Seki,3,4Keisuke Oka,1 Masateru Taniguchi,1,3 and Tomoji Kawai1,a兲
1Institute of Scientific and Industrial Research, Osaka University, 8–1 Mihogaoka, Ibaraki,
Osaka 567-0047, Japan
2NANOTEC, NSTDA, 111 Thailand Science Park, Paholyothin Rd. Klong 1, Klong Luang,Pathumthani 12120, Thailand
3PRESTO, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
4
Division of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka,Suita, Osaka 565-0871, Japan

共Received 10 June 2009; accepted 2 July 2009; published online 4 August 2009兲
Impurity doping on semiconductor nanowires grown by vapor-liquid-solid 共VLS兲 mechanism
remains an important challenge. Here we demonstrate the importance of doping dynamics to control
the transport properties of Sb-doped SnO2 nanowires. Sb doping decreased the resistivity of SnO2nanowires down to 10−3 ⍀ cm range, while there was the lower bound of resistivity even increasing
further the dopant concentration from supplied source. We found that the doping limitation is related
to the re-evaporation events of dopant through vapor-solid growth process rather than VLS process.
Thus understanding the dopant incorporation dynamics is essential to control the transport
properties of SnO2 nanowires by impurity doping. © 2009 American Institute of Physics.
关DOI: 10.1063/1.3186080兴

a兲Authors to whom correspondence should be addressed. Electronic addresses: yanagi32@sanken.osaka-u.ac.jp and kawai@sanken.osaka-u.ac.jp.

0003-6951/2009/95共5兲/053105/3/$25.00

vestigate the transport properties of Sb-doped SnO2 nanowires grown by VLS and the dopant incorporation mechanisms. Our results highlight the crucial role of dopant
incorporation dynamics on the transport properties ofSb-doped SnO2 nanowires.
SnO2 nanowires were grown on Al2O3 共110兲 single crystal substrate by Au catalyst-assisted pulsed laser deposition
technique 共ArF excimer laser, ␭ = 193 nm兲.7 The backgroundpressure of the chamber was 10−5 Pa. Sn and Sb2O3 mixedpowders 共Koujundo Chemical兲 were used as the target withvarying the Sb dopant concentration. The Sb dopant concentration 共at. %兲 is hereafter defined as the atomic ratio of Sb toSn. Oxygen and argon mixed gas was introduced into thechamber with controlling the ambient total pressure 10 Pa
with the flux ratio of oxygen and argon 共1:1000兲. Prior to thelaser ablation, the Au-coated Al2O3 substrate was preheated
at 750 ° C for 20 min. The typical growth temperature was(a)

(b)
[101]

0.26 nm

Al2O3(110)

Al2O3(220)(d)
Intensity (a.u.)(c)
SnO2(101)

SnO2(202)
20304050 60 70
2θ (degree)

Cu K

Sn L

Intensity (a.u.)

500nm

Intensity (a.u.)

Semiconductor nanowires grown via the vapor-liquidsolid 共VLS兲 mechanism are attracting much attention due tonot only the nanoscale scientific interests but also the potential device applications.1 Impurity doping is an important
process to control the transport properties of semiconductors.
Feasibility of the impurity doping for VLS grown nanowires
has been intensively investigated due to the complex natureof dopant incorporation dynamics.2 The essential difference
between VLS grown nanowires and thin films on the impurity doping is the presence of a metal catalyst, which is aliquid state during VLS, and the mass transport events
through liquid-solid, vapor-liquid, and vapor-solid 共VS兲 interfaces. Thus the complex dynamics of the impurity dopingon VLS growth has held back the arbitral controllability ofthe electric properties. Although the dopant incorporation
processes on growths of nanowires comprised of single element including Si and Ge are complex enough,2–4 the doping
process for nanowires comprised of multielements would be
more complex. Although metal oxides exhibit many interesting physical properties,5 current knowledge as to the impurity doping on VLS grown oxide nanowires is still scarce andfar from the comprehensive understanding. For example,
impurity-doped SnO2 nanowires by VLS have been investigated due to the transparent conductivity of SnO2−␦.6 The
wide variation of the measured conductivity was found;6however, the mechanism of such wide variation has not been
identified due to the lack of understanding of the dopant
incorporation process. In addition, the uniform dopant distribution within nanowires has been assumed in previous
investigations.6 The lack of understanding and the variationof transport properties would be detrimental for the reliable
device applications. These backgrounds motivated us to in-

Sn K Sn KSb K

2426
2830
Energy (keV)

Cu K
8090

010

Sn K
2030

Energy (keV)

FIG. 1. 共a兲 FESEM image of Sb 共5 at. %兲-doped SnO2 nanowires. 共b兲 HRTEM image and SAED pattern. 共c兲 X-ray diffraction 共XRD兲 data. 共d兲 EDS
data. The inset shows the data around Sn K and Sb K shell peaks.

95, 053105-1© 2009 American Institute of Physics

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Appl. Phys. Lett. 95, 053105 共2009兲

Klamchuen et al.

-30.001

10Sb 7 atm%
doping-40.0001

10TABLE I. Comparison between MCM and FPCM on the resistivity data ofnondope SnO2 nanowires and Sb 共7 at. %兲-doped SnO2 nanowires.

0.3
0.2
0.1

4-probe2-probeResistivity at RT 共⍀ cm兲

0-0.1

1μμm-0.2-50.00001
8980000000 9000000000 9020000000 9040000000 9060000000

109.009.02

9.04

Frequency (GHz)

9.06

-0.3
-0.05

-0.02500.0250.05

NondopeSb: 7 at % in target

MCM

FPCM

6.67⫻ 10−1
4.35⫻ 10−3

3.86⫻ 10−1
7.12⫻ 10−3

Voltage (V)FIG. 2. 共Color online兲 共a兲 Resonance curves of MCM. Data of nondoped
system and Sb 共7 at. %兲-doped SnO2 nanowires were shown. 共b兲 I-V curves
measured by FPCM for Sb 共7 at. %兲-doped SnO2 nanowires. Two-probe andfour-probe data were shown. The inset shows FESEM image of FPCMdevice structure.

750 ° C. The nanowire morphology was characterized byfield emission scanning electron microscopy 共FESEM兲 共S4300兲 at an accelerating voltage of 30 kV. High-resolution
transmission electron microscopy 共HRTEM兲 共JEM-3000F兲
coupled with energy dispersive spectroscopy 共EDS兲 at anaccelerating voltage of 300 kV was used to evaluate the diameter, the crystallinity, and the composition of the fabricated nanowires. We employed two different methods tomeasure the transport properties of nanowires, including a
microwave conductivity measurement 共MCM兲 and a fourprobe conductivity measurement 共FPCM兲. Detail apparatusof MCM can be seen elsewhere.7,8 All the above experiments
were carried out at room temperature in Ar or SF6 atmosphere. For FPCM, we utilized devices composed of individual nanowires, which were fabricated by combining photolithography and electron-beam 共e-beam兲 lithography
processes. Then the metal deposition of Pt/Au 20/100 nm
were carried out. The transport properties were measured byusing semiconductor analyzer 共Keithley 4200兲.
Figure 1共a兲 shows the typical FESEM image of Sbdoped SnO2 nanowires grown on Al2O3 共110兲 substrate. The
Sb composition in the target was 5 at. %. It can be seen thatthe fabricated SnO2 nanowires grow almost perpendicular tothe substrate. The presence of nanowire growth was consistently confirmed for the range of Sb-concentration 共1–7
at. %兲 in the target. Figure 1共b兲 shows the HRTEM image offabricated SnO2 nanowire and the selected area electron diffraction 共SAED兲 pattern. The single crystalline nature and共101兲 oriented growth of fabricated SnO2 nanowires can beseen. X-ray diffraction 共XRD兲 data also supports the 共101兲
oriented growth, as shown in Fig. 1共c兲. Figure 1共d兲 shows theEDS data in HRTEM, which indicates the presence of Sb
dopant within nanowires. Note that it is crucial to identify
the presence and the concentration of Sb dopant within nanowires via Sb K shell peaks rather than Sb L shell peaks dueto the significant overlapping of Sn and Sb peaks. The estimated Sb concentration was almost 2 at. %, which is smaller
than the nominal value 5 at. %. Figure 2共a兲 shows the absorption curves of MCM for nondope SnO2 nanowires andSb 共7 at. %兲-doped SnO2 nanowires. The attenuation factor
Q around the resonance for Sb-doped nanowires was foundto be much larger than that of nondope nanowires, indicatingthat the conductivity of Sb-doped nanowires is higher. Figure
2共b兲 shows the typical data of current 共I兲 versus voltage 共V兲
of FPCM for Sb 共7 at. %兲-doped SnO2 nanowire. The inset ofFig. 2共b兲 shows the FESEM image of fabricated device
structure. The linear relationship of I-V data was seen in both
two-probe and four-probe configurations, indicating theOhmic contact. In addition, the I-V data obtained from thetwo-probe method is almost identical to that obtained fromthe four-probe method. Table I shows the comparison between MCM and FPCM in the resistivity data for both nondope and Sb 共7 at. %兲-doped SnO2 nanowires. The reasonable agreement between MCM and FPCM data can be seen.
The agreement between ac and dc measurements is theoretically understandable8 when considering the mobility range ofimpurity-doped SnO2, which is typically ranged from20– 50 cm2 V−1 s−1.9 Note that the MCM data is an averaged value of 1011 nanowires. Thus the reliable transport
data of MCM affords us to focus our studies on the MCM
data in the following discussions.
Figure 3 shows the dependence of supplied dopant concentration on the resistivity. Interestingly there was the lower
bound of resistivity even increasing further the dopant concentration from supplied source above 3 at. %. The inset
shows the Sb concentration data within nanowires, demonstrating that the incorporated dopant concentration approached the limited value around 2 at. % above 3 at. % ofsupplied dopant concentration. Clearly there is a reasonable
correspondence between the resistivity and the incorporated
dopant concentration. As such the lower bound of resistivity
is closely related to the dopant incorporation dynamics. The
dopant incorporation dynamics is related to three major processes, including 共1兲 the supplied process of dopant from thesource, 共2兲 the re-evaporation process of supplied dopantfrom the surface, and 共3兲 the dopant incorporation process
into nanowires. We examined the effects of each process inthe following discussions. First in 共1兲, the supplied process,the dopant concentration supplied onto the surface was investigated by measuring the dopant concentration of thinfilms formed under RT, where the re-evaporation process isnegligible. The electron probe microanalysis on the Sb 共20
at. %兲-doped SnO2 films demonstrated that the dopant concentration within the film was 20.07 at. %, which is almost
identical to the nominal value. Thus there is no significant
difference between the dopant concentration supplied onto
the surface and the nominal dopant concentration in thepresent experiments. Second, the re-evaporation process ofdopant on the surface was studied by measuring the dopant
810(a)

Resistivity at RT (Ω cm)(b)

Non-dope

Sb % in nanowire-2
0.01
10Current (μA)(a)

Reflected power (W)053105-2

64
2002

46

Sb % in target

8(b)

110-1
10-2
10-3
02468Sb atm % in target

FIG. 3. 共a兲 The Sb concentration data within nanowires by EDS whenvarying the supplied dopant concentration. 共b兲 Dependence of supplied dopant concentration on the resistivity measured by MCM.

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4(a)

10-4

10-3

3210-2

110-1

0500

550 600 650 700 750o
Growth Temperature ( C)800

Resistivity at RT (Ω cm)

Sb atm% in nanowires

5Appl. Phys. Lett. 95, 053105 共2009兲

Klamchuen et al.
Resistivity at RT (Ω cm)

053105-3

10-1(b)

10-2

10-3

10-4
01

234

Sb atm% in nanowires

5FIG. 4. 共Color online兲 共a兲 Dopant concentration data within nanowires andresistivity data when varying the growth temperature from 550 to 750 ° C.
共b兲 Relationship between the incorporated dopant concentration and the resistivity of nanowires for various conditions.

concentration of thin films grown at 400 ° C. The remaining
dopant concentration within the film was 1.73 at. %, which ismuch lower than the nominal value 20 at. %. Although above
experiments were performed in the absence of Au catalysts,
the re-evaporation process must be substantial when performing a nanowire growth above 400 ° C. Based on above
experimental implications, an enhancement of the dopant incorporation might be achieved by decreasing the growthtemperature, i.e., the reduction of re-evaporation events. Figure 4共a兲 shows the dopant concentration data within nanowires when varying the growth temperature from 550 to750 ° C. Note that there was no SnO2 nanowire growth below 550 ° C. The nominal dopant concentration in supplied
source was 5 at. %. The significant enhancement of dopant
incorporation was observed when reducing the growth temperature from 750 to 650 ° C. The dopant concentration
within nanowires grown under 650 ° C was nearly 4%,
which is in fact twice when compared with the value for
750 ° C. As such the reduction of re-evaporation process canbe achieved by controlling the nanowire growth temperature.
Further decreasing the growth temperature from 650 to
550 ° C resulted in the decrease of the dopant concentration
within nanowires. This is presumably due to 共i兲 the suppression of diffusion length of dopant adatoms on the surface and共ii兲 the presence of an activation energy barrier for incorporating dopant adatoms into nanowires. Although Sb reevaporation from the SnO2 nanowire surface must occur, ourresults indicate the dominance of Sb re-evaporation events
before the dopant incorporation process rather than the reevaporation from the solid surface. Figure 4共a兲 also shows
the resistivity data of nanowires when varying the growthtemperature. Clearly there is a remarkable correspondence
between the incorporated dopant concentration and the conductivity of nanowires when varying the growth temperature.
Figure 4共b兲 summarizes the relationship between the incorporated dopant concentration and the resistivity of nanowires
for various conditions. The reasonable correspondence canbe seen. Thus it is very important to control the dopant incorporation process rather than varying the supplied dopantconcentration. Further remained issue is the dopant incorporation pathway. There are two distinct incorporation
pathways of dopant adatoms, including a VLS process via
metal catalysts and a VS process through the nanowire surface. The VS mechanism assumes the inhomogeneous
dopant distribution within nanowires,4 whereas the VLS

mechanism assumes the homogeneous dopant distribution.
Assuming the homogeneous dopant distribution within nanowires, the experimental data of resistivity should agreewith theoretical value estimated by the following formula:␳ = 1 / e␮n, where e is an elementary electric charge, ␮ is amobility, and n is a carrier density. Adapting the data of the␮ 共40– 50 cm2 V−1 s−1兲 and n 关共3.0– 8.0兲 ⫻ 1020 cm−3兴 forour experimental configurations,6,9 the theoretical value
is ␳theory : 共1.7– 5.2兲 ⫻ 10−4 ⍀ cm, which is an order ofmagnitude lower than our experimental value ␳exp : 4.3⫻ 10−3 ⍀ cm. The discrepancy cannot be solely explained interms of the variation of ␮ and n data for the estimation.
Thus this indicates the inhomogeneous dopant distribution
within nanowires, in other words, the dominance of VS incorporation mechanism rather than the VLS. Recently Lauhon and co-workers4 reported such inhomogeneous dopant
distribution of P-doped Ge nanowires with the relation to thedifferences in precursor decomposition rates, and the similar
mechanisms might govern our experimental system. These
results highlight that understanding the dopant incorporation
dynamics is rather essential to control appropriately thetransport properties of SnO2 nanowires by impurity doping.
This work is partially funded by SCOPE.
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