Phosphorus doping and hydrogen passivation of donors and defects in silicon nanowires synthesized by laser ablation

Article abstract of DOI:10.1063/1.2721377

Phosphorus (P) doping was performed during the synthesis of silicon nanowires (SiNWs) by laser ablation. At least three types of signals were observed by electron spin resonance (ESR) at 4.2 K. Phosphorus doping into substitutional sites of crystalline Si in SiNWs was demonstrated by the detection of an ESR signal with a g value of 1.998, which corresponds to conduction electrons in crystalline Si, and by an energy-dispersive x-ray spectroscopy spectrum of the P Kα line. The ESR results also revealed the presence of defects. These defects were partially passivated by hydrogen and oxygen atoms.

Full text of DOI:10.1063/1.2721377

Phosphorus doping and hydrogen passivation of donors and defects in silicon
nanowires synthesized by laser ablation
N. Fukata, J. Chen, T. Sekiguchi, S. Matsushita, T. Oshima, N. Uchida, K. Murakami, T. Tsurui, and S. Ito
Citation: Applied Physics Letters 90, 153117 (2007); doi: 10.1063/1.2721377
View online: http://dx.doi.org/10.1063/1.2721377
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APPLIED PHYSICS LETTERS 90, 153117 ͑2007͒
Phosphorus doping and hydrogen passivation of donors and defects
in silicon nanowires synthesized by laser ablation
N. Fukata,a͒,b͒ J. Chen, and T. Sekiguchi
Advanced Electronic Materials Center, National Institute for Materials Science, 1-1 Namiki,
Tsukuba 305-0044, Japan
S. Matsushita, T. Oshima, N. Uchida, and K. Murakamib͒
Institute of Applied Physics, University of Tsukuba, 1-1-1 Tennodai, Tsukuba 305-8573, Japan
T. Tsurui and S. Ito
Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
͑Received 14 January 2007; accepted 9 March 2007; published online 12 April 2007͒
Phosphorus ͑P͒ doping was performed during the synthesis of silicon nanowires ͑SiNWs͒ by laser
ablation. At least three types of signals were observed by electron spin resonance ͑ESR͒ at 4.2 K.
Phosphorus doping into substitutional sites of crystalline Si in SiNWs was demonstrated by the
detection of an ESR signal with a g value of 1.998, which corresponds to conduction electrons in
crystalline Si, and by an energy-dispersive x-ray spectroscopy spectrum of the P K␣ line. The ESR
results also revealed the presence of defects. These defects were partially passivated by hydrogen
and oxygen atoms. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2721377͔
The doping and activation of carriers in silicon nano-
wires ͑SiNWs͒ is a key step in realizing nanoscale silicon
devices using SiNWs. The vapor-liquid-solid mechanism has
been widely used for growing SiNWs, and carrier doping has
been performed during growth. The doping effect has been
investigated by electrical transport measurements.1–7 To
evaluate the electrical properties of SiNWs, Si/SiO2 inter-
face defects such as Pb centers and other impurities need to
be investigated, since compensation by interface defects and
contamination by metal catalysts need to be considered. On
the other hand, it is possible to distinguish information on
boron as a dopant from other types by optical measurements.
We recently reported a result for boron ͑B͒ doping in SiNWs,
based on spectroscopic results, such as the observation of the
local vibrational mode of B and the Fano effect due to heavy
B doping in SiNWs by Raman scattering measurements.8 In
the case of phosphorus ͑P͒ doping, the mass of P atoms and
that of Si atoms are very similar, making it difficult to detect
the local vibrational mode of P atoms in SiNWs by Raman
scattering measurements. Electron spin resonance ͑ESR͒ is a
sensitive method for investigating P donors or conduction
electrons in Si.9,10 It is also sensitive to defects with dangling
bonds. This method makes it possible to obtain information
on both P donors and defects in SiNWs.
The passivation of P by hydrogen ͑H͒ has been exten-
sively studied for P-doped bulk Si.11–15 When H atoms are
introduced into P-doped Si, the H atoms locate at the anti-
bonding site of the nearest Si neighbor to a P atom, forming
a P–Si–H passivation center.11–15 This effect can be used to
control the active carrier concentration in SiNWs. Indeed, we
have proved the H passivation of B in SiNWs in our previous
paper.8 Defects with dangling bonds can be also passivated
by hydrogen. Interfacial defects between the surface oxide
and Si core of SiNWs inevitably exist and affect the electri-
cal properties of SiNWs. The control of interfacial defects by
powerful techniques such as H passivation is also an impor-
tant subject for the realization of nanoscale silicon devices
using SiNWs.
In this study, P doping was performed during the forma-
tion of SiNWs by laser ablation of a Si target with P and
nickel ͑Ni͒ atoms. The former acts as a dopant impurity and
the latter as a catalyst. The states of P atoms in SiNWs were
investigated by ESR at low temperatures and energy-
dispersive x-ray spectroscopy ͑EDX͒ measurements. These
results showed that P atoms were doped in the substitutional
sites in crystalline Si core of SiNWs and were activated in
the sites. We also investigated the passivation effect of P
donors and defects in P-doped SiNWs by H and oxygen
treatments.
P-doped SiNWs were synthesized at 1200 °C in flowing
argon ͑Ar͒ gas at 50 SCCM ͑SCCM denoter cubic centime-
ter per minute at STP͒ by catalytic laser ablation of a Si
target with Ni2P compounds, namely, Si100−x ͑Ni2P͒ ͑x
x
=0.5, 5, and 10͒ targets. For comparison, a Si99Ni1 target was
used for the synthesis of undoped SiNWs. The growth is
conducted by the so-called vapor-liquid-solid mechanism.16
A frequency-doubled Nd doped yttrium aluminum garnet la-
ser ͑532 nm, 7 ns pulse width, 10 Hz, 150 mJ/pulse͒ was
used to ablate the targets. SiNWs were directly deposited on
a Si substrate, then transferred to a quartz capsule for ESR
measurements or a molybdenum ͑Mo͒ grid for electron mi-
croscopy with EDX measurements. ESR measurements were
carried out at 4.2 K using an X-band ESR spectrometer with
a magnetic field modulation of 100 kHz to investigate the
state of the P donor in SiNWs. Scanning transmission elec-
tron microscopy ͑STEM͒ ͑Hitachi: S-5500, 30 kV; JEOL:
JEM3000F, 300 kV͒ and transmission electron microscopy
͑TEM͒ ͑JEOL: JEM4000EX, 400 kV͒ were used to observe
SiNWs and to investigate the details of their structures. EDX
measurements were also performed in the TEM to confirm P
doping in SiNWs. To investigate the passivation of defects in
SiNWs, some specimens were thermally oxidized at 900 °C
for 30 min in O2 gas at a pressure of 200 torr after the syn-
thesis of SiNWs. Hydrogen atom treatments ͑HATs͒ using
remote downstream high flux H atoms were also performed
at 120 and 400 °C for 30 min to investigate the passivation
a͒
Electronic mail: fukata.naoki@nims.go.jp
Also at Special Research Project on Nanoscience, University of Tsukuba,
b͒
1-1-1 Tennodai, Tsukuba 305-8573, Japan.
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0003-6951/2007/90͑15͒/153117/3/$23.00 90, 153117-1 © 2007 American Institute of Physics
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Fukata et al.
Appl. Phys. Lett. 90, 153117 ͑2007͒
FIG. 2. ͑Color online͒ EDX spectra observed for SiNWs synthesized using
a Si90͑Ni2P͒10 target. The STEM image in the inset shows the analysis point
in a P-doped SiNW.
values of 1.998, 2.002, and 2.005, while no component with
a g value of 1.998 was observed for undoped SiNWs. The g
value of 1.998 corresponds to that of conduction electrons in
Si, indicating that the ESR signal can be attributed to the
conduction electrons in the Si core of the SiNWs, and that P
atoms were clearly doped in substitutional sites of the crys-
talline Si core during laser ablation. Figure 3͑b͒ shows the
dependence of the peak-to-peak width of the ESR signal of
the conduction electrons on the Ni2P content in the target.
The ESR signal width of conduction electrons depends sig-
nificantly on the P concentration, i.e., it increases with in-
creasing Ni2P content in the target. A similar dependence
was observed in heavily P-doped bulk Si. It can be explained
by an increase in scattering among conduction electrons and
with the P atoms, resulting in a decrease in the spin-lattice
relaxation time T1, leading to a broadening of the signal
width with increasing P concentration.9,10 Assuming that the
data in P-doped SiNWs correspond to those in bulk Si, the
concentration of P in SiNWs is estimated to probably reach
about 1020 cm−3.
FIG. 1. Representative ͑a͒ STEM and ͑b͒ a high-resolution TEM images of
the SiNWs synthesized using a Si99.5͑Ni2P͒ target.
0.5
of both defects and P dopants. In this method, the specimens
are placed in a quartz tube at a distance of 15 cm from the
plasma source and hydrogenated only by H atoms. Thus, in
this remote method, no damage is caused by H plasma. The
details of the laser ablation and hydrogenation procedures
are reported in Refs. 17 and 18 and in Ref. 15, respectively.
A large number of SiNWs were synthesized for ESR
measurements. The STEM image of typical SiNWs synthe-
The other ESR signals at 2.002 and 2.005 are related to
certain defects. Similar signals were observed in SiNWs.19,20
These results suggest that the former is due to defects in the
surface oxide of SiNWs called EX centers,19,20 and the latter
is probably attributable to interfacial defects between the sur-
sized using a Si99.5͑Ni2P͒ target is shown in Fig. 1͑a͒. The
0.5
high-resolution TEM image is shown in Fig. 1͑b͒. The Si
lattice fringes are seen inside the SiNW, showing that the
SiNW is sheathed in an amorphous SiOx ͑xഛ2͒ layer and the
core is crystalline Si. The average Si core diameter of SiNWs
synthesized using a Si99.5͑Ni2P͒ target was about 5–7 nm.
0.5
EDX analyses were performed for individual P-doped
SiNWs. A typical result is shown in Fig. 2. The P K␣ peak
was observed at 2.01 keV. The peak at 1.74 keV is due to the
Si K␣ line, and the peak at 2.29 keV is due to the Mo K␣
line caused by the Mo grid. This result indicates the presence
of P atoms in the SiNW, including the surface oxide layer.
The content of P in SiNWs is roughly estimated to be in the
order of 1–7 at. % from the peak intensity; this is in agree-
ment with the values deduced from the ESR measurements.
ESR measurements were performed to further investi-
gate the P-doping sites in SiNWs. Figure 3͑a͒ shows the
typical results of ESR measurements for undoped and
FIG. 3. ͑Color online͒ ͑a͒ ESR spectra observed for undoped and P-doped
SiNWs synthesized using Si99Ni1 and Si95͑Ni2P͒ targets, respectively. The
5
P-doped SiNWs synthesized using Si99Ni1 and Si95͑Ni2P͒
5
ESR signal for P-doped SiNWs was deconvoluted into three components.
͑b͒ Dependence of the peak-to-peak width of the ESR signal of the conduc-
tion electrons on the Ni P content in the target.
targets, respectively. The ESR signal observed for P-doped
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SiNWs is deconvoluted at least to three components with g
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Fukata et al.
Appl. Phys. Lett. 90, 153117 ͑2007͒
heavily P-doped specimens,10 i.e., the reduction of active
carrier concentration by HAT cannot be estimated from the
change in intensity. Instead of using intensity, the passivation
efficiency can be estimated from the ESR linewidth, which
closely depends on P concentration.9,10 If the ESR linewidth
decreased for heavily P-doped Si, it would indicate the pas-
sivation of P by H. Figure 4͑b͒ clearly shows this, giving
evidence for the passivation of P by H. If the data reported
for bulk Si are assumed to correspond to those for SiNWs, it
can be estimated that 30% of active P dopants were passi-
vated by HAT at 120 °C.9,10
In conclusion, P doping in SiNWs during laser ablation
was confirmed by the ESR signal of conduction electrons
and the EDX peak of the P K␣ line. The concentration of P
donors was controlled by the content of P in the Si target.
The donor concentration of electrically active donors was
also controlled by hydrogen passivation. In addition to the
ESR signal of conduction electrons, the ESR results showed
the presence of defects in the surface oxide layer and at the
interface between the surface oxide and crystalline Si core.
The defects were also passivated by hydrogen and oxygen
atoms.
This work was partly supported by a Grant-in-Aid for
Scientific Research ͑2005͒ on Young Scientists ͑B͒
͑17760003͒ and the 21st Century COE ͑Center of Excel-
lence͒ Program “Promotion of Creative Interdisciplinary Ma-
terials Science for Novel Functions” under the Ministry of
Education, Culture, Sports, Science and Technology ͑MEXT͒
of Japan. TEM observations were also partly supported by
MEXT’s “Nanotechnology Support Project.” This work was
also performed under the interuniversity cooperative Re-
search Program of the Institute for Materials Research, To-
hoku University.
FIG. 4. ͑Color online͒ ͑a͒ Dependence of ESR signals on oxidation and
HAT temperature, and ͑b͒ change in the ESR signal of conduction electrons
by oxidation and HAT. The SiNWs were synthesized using a Si95͑Ni2P͒
5
target.
face oxide and Si core of SiNWs which are so-called Pb
centers. We also observed a similar signal for nanocrystalline
Si embedded in SiO2.21 Wang reported that amorphous Si
states exist around the interface between the surface oxide
and Si core of SiNWs, and defects in such amorphous Si
states can be the origin of the signal at 2.005.20 It is, how-
ever, not clear at the present time.
Next, the passivation of defects and P donors in SiNWs
was investigated, as shown in Fig. 4. Annealing in O2 gas at
900 °C reduced the ESR signal intensity of interfacial de-
fects ͑g=2.005͒, indicating that oxidation is effective against
their passivation. On the other hand, the ESR signal at 2.002
increases rather than decreases. For further passivation of the
interfacial defects and EX centers, H atoms were introduced
into P-doped SiNWs by HAT at 400 °C. The interfacial de-
fects were almost completely passivated by hydrogenation.
The intensity of the EX centers decreased by more than two-
thirds, showing that hydrogenation is effective against the
passivation of the EX centers.
The ESR signal of conduction electrons showed no
change after oxidation at 900 °C and subsequent HAT at
400 °C. In general, H passivation of P donors, i.e., the de-
crease in the concentration of conduction electrons, is
achieved by hydrogenation at less than 200 °C. The most
appropriate temperature is around 120 °C, which is due to
the thermal stability of the P–Si–H passivation center.12,15 To
investigate the H passivation of P donors, HAT was per-
formed at 120 °C after HAT at 400 °C. The results are also
shown in Fig. 4͑a͒. The interfacial defects and EX centers
were further passivated by HAT at 120 °C. The change in
the ESR signal of conduction electrons is shown in Fig. 4͑b͒.
It has been reported that the ESR signal intensity of conduc-
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