Paramagnetic defects of silicon nanowires

Article abstract of DOI:10.1063/1.1775288

Conventional electron spin resonance spectroscopy was used to analyze the paramagnetic defects in and on silicon nanowires (SiNW) obtained by oxide-assisted growth. Raw materials were dissolved in dichloromethane by ultrasonification and spread on a quartz glass substrate, followed by evaporation of the solvent to obtain thin layers of SiNWs. The spectra were collected over the range of 8000-400 cm^-1. The optical absorption spectra of SiNW were found to be comparable to those of microcrystalline silicon and showed a similar decrease of defect density upon H termination.

Full text of DOI:10.1063/1.1775288

Paramagnetic defects of silicon nanowires
A. Baumer, M. Stutzmann, M. S. Brandt, F. C.K. Au, and S. T. Lee
Citation: Applied Physics Letters 85, 943 (2004); doi: 10.1063/1.1775288
View online: http://dx.doi.org/10.1063/1.1775288
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APPLIED PHYSICS LETTERS
VOLUME 85, NUMBER 6
9 AUGUST 2004
Paramagnetic defects of silicon nanowires
A. Baumer,a) M. Stutzmann, and M. S. Brandt
Walter Schottky Institut, Technische Universität München, Am Coulombwall 3, 85748 Garching, Germany
F. C.K. Au and S. T. Lee
Center of Super-Diamond and Advanced Films and Department of Physics and Materials Science,
City University of Hong Kong, Hong Kong SAR, China
(Received 20 April 2004; accepted 3 June 2004)
The paramagnetic defects in and on Si nanowires ͑SiNWs͒ obtained by oxide-assisted growth were
studied by conventional electron spin resonance spectroscopy. For the as-grown nanowires, three
different defects were found: Dangling bonds or Pb-centers with g=2.0065, located at the interface
of the crystalline core to the surrounding oxide, E -centers with g=2.0005 and EX-centers with
Ј
g=2.00252, located in the oxide. For the EX-centers, the characteristic hyperfine lines separated
by 16.4 G were detected. The as-grown SiNWs showed a spin density of about 1018 cm−3.
H termination of the nanowires via hydrofluoric acid decreases the spin density drastically to
3ϫ1016 cm−3. The optical absorption spectra of SiNWs determined by photothermal deflection
spectroscopy are comparable to those of microcrystalline silicon and show a similar decrease of
defect density upon H termination. © 2004 American Institute of Physics.
[DOI: 10.1063/1.1775288]
Semiconductor nanowires have received considerable
scientific interest during the last few years, encouraged by
synthetic techniques and their potential use in applications
ranging from integrated circuit interconnects to functional
electronic and optical devices1,2 as well as chemical and bio-
logical sensors.3,4 Specifically, Si nanowires ͑SiNWs͒ attract
a great deal of attention as they might allow easy integration
of nanowire devices with standard microelectronics. Various
fabrication methods for SiNWs have been reported, such as
synthesis by vapor–liquid–solid growth5,6 or oxide-assisted
growth.7,8 With the objective of using SiNWs for nanoscale
devices, their electronic and optical properties are of particu-
lar interest. While electronic transport through SiNWs has
been studied, e.g., by current–voltage measurements,9 noth-
ing is known about electronically active defects in SiNWs
which can have a very significant influence on the electronic
transport properties leading to compensation or recombina-
tion. In this letter, we present an investigation of the defects
in SiNWs with the help of electron spin resonance ͑ESR͒ and
photothermal deflection spectroscopy ͑PDS͒ measurements.
In as-grown SiNWs, three different types of defects are iden-
tified at the Si/SiO2 interface as well as in the SiO2 shell. We
further show that these defects can be removed quantitatively
via H termination.10,11
SiNWs used in this study were prepared by oxide-
assisted growth7,8 on quartz substrates at 900°C using ther-
mal vaporization of SiO and are nominally undoped. As de-
scribed in Refs. 11 and 12, the as-grown SiNWs have a
crystalline core measuring 10–15 nm in diameter, which is
covered with a 3–5 nm thick oxide layer. For the H termi-
nation, the as-grown oxide was removed by immersing the
SiNWs in an aqueous solution of 5%HF for 5 min. Subse-
quently, the HF was removed by repeated dipping into H2O.
Finally, the SiNWs were dried in a stream of N2. To verify
the H termination, Fourier-transform infrared ͑FTIR͒ trans-
mission spectra were acquired at room temperature using a
Bruker IFS 113V FTIR system. The spectra were collected
over the range of 8000–400 cm−1 by averaging 100 scans at
a resolution of 1 cm−1. To this end, SiNWs were dispersed
in dried spectroscopic grade potassium bromide (KBr,
Sigma Aldrich). After correction for the KBr signal, the in-
frared absorbance was calculated. For the ESR measure-
ments, approximatley 1 mg of SiNWs was filled into a small
Teflon tube which was then sealed with Teflon tape and put
inside standard ESR quartz glass sample holder. ESR mea-
surements were performed at room temperature in a conven-
tional continuous-wave X-band ESR spectrometer (Bruker
ESP-300 with a TE102 cavity) operating at 9.27 GHz. A
magnetic field modulation amplitude of 0.2 G and a micro-
wave power of 0.3 mW were used. Calibration of the
magnetic field was done at 11 K using Si doped with
P ͑g=1.998 69͒. The optical absorption of SiNWs was deter-
mined by PDS measurements.13 In this technique, the energy
of the light absorbed by the thin film is dissipated into a
surrounding deflection medium ͑C6F14͒, resulting in a tem-
perature gradient. This gradient in turn gives rise to a gradi-
ent in the refractive index of the medium, which is detected
via the deflection of a laser beam propagating parallel to the
film surface. To obtain thin layers of SiNWs, raw materials
were dissolved in dichloromethane by ultrasonification and
spread on a quartz glass substrate, followed by evaporation
of the solvent.
Before turning to the discussion of the defects in the
SiNWs, we briefly summarize the effect of HF treatment on
the SiNWs as observed by FTIR spectroscopy. In Fig. 1, the
infrared absorption spectra of the as-grown and H-terminated
SiNWs are shown, which are in agreement with the previous
report.10 For the as-grown SiNWs, a broad peak around
1100 cm−1 is detected, which is caused by asymmetric
Si–O–Si stretching vibrations14,15 originating from the na-
tive oxide. At 800 cm−1, the corresponding Si–O–Si bend-
ing modes are found. After HF etching, both peaks are re-
duced by a factor of 30, indicating that almost all oxide has
been removed. While for the as-grown samples, no signal
a)Electronic mail: baumer@wsi.tum.de
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0003-6951/2004/85(6)/943/3/$20.00 943 © 2004 American Institute of Physics
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944
Appl. Phys. Lett., Vol. 85, No. 6, 9 August 2004
Baumer et al.
cancies is found, with a typcial ⌬BPP=0.8 G.20 The most
prominent feature is the line at g=2.002 52±0.000 10 with a
linewidth of ⌬BPP=0.54±0.05 G. This ESR line was re-
ported by Stesmans21 for thermal SiO2 grown at typically
800°C on ͑111͒ Si. At low temperatures, Stesmans21 ob-
served a narrow symmetric line with ⌬BPP=1.0±0.05 G and
a g value of 2.002 46±0.000 03, together with a symmetric
hyperfine doublet with a total splitting ahf=16.1±0.1 G and
a width of each hyperfine satellite of ⌬BPP,hfϷ1.2,G. For
the intensity ratio R of the spin density in the hyperfine lines
to the spin density in the central signal, which is a key quan-
tity for identification of the defect, Stesmans21 determined a
value of 0.142±0.022. Further studies revealed that this so-
called EX-center consists of a hole delocalized over three or
four oxygen dangling bonds at a Si vacancy.22 In our experi-
ments, the EX resonance and the hyperfine splitting can be
observed even at room temperature. We find ahf
=16.4±0.1 G for the separation of the two hyperfine lines
and a linewidth ⌬BPP,hf=1.0 G. These hyperfine lines are
shown in Fig. 2, enlarged by a factor of 20 after subtraction
of the background signal. For the intensity ratio R, we obtain
a value of 0.14±0.02. These results are in good agreement
with the values reported by Stesmans,21 allowing us the as-
signment of the resonance in the as-grown SiNWs to the
EX-center.
The EX-center has also been observed in other Si-based
nanostructures. In porous silicon oxidized at 800°C for a
short period of time,23 a g-factor of 2.002 45±0.000 10 with
a linewidth ⌬BPP=0.7 G for the central resonance as well as
two hyperfine lines separated by 16.5±0.3 G with a line-
width of ⌬BPP,hfϷ1.5 G comprising about 10% –12% of the
total integrated intensity have been found. Dohi et al.24 ob-
served two EX-centers in ultrafine silicon particles at g
=2.0024 and at g=2.0025. From the saturation behavior of
the two lines, they identified the second as the EX-center
reported by Stesmans, appearing after a heat treatment of the
Si particles at 900−1000°C. The other line they attributed to
a defect observed by Kusumoto et al.25 in Si powders
crushed in air. Also taking into account the growth tempera-
ture of the SiNWs studied here, the EX-center seems to be
characteristic for SiO2 grown at comparatively low tempera-
tures of 750−1000°C.26
FIG. 1. FTIR absorption spectra of as-grown and H-terminated SiNWs ob-
tained by oxide-assisted growth. (a) shows predominantly oxide-related and
(b) hydride-related vibrations. The oxide covering the as-grown SiNWs is
removed by HF etching. At the same time, H termination is generated.
due to H can be detected, the HF-treated SiNWs show a
broad Si–H stretching mode absorption feature around
2100 cm−1. Three main absorption lines can be resolved, ac-
companied by several smaller peaks and shoulders. Based on
the comparison to the well-known FTIR modes of H on crys-
talline silicon surfaces,16 Sun et al.10 have assigned the dif-
ferent vibration modes observed to the Si3Si–H stretching
vibration ͑2085 cm−1͒, to a dimer structure formed by the
coupling of two adjacent Si–H units via a Si–Si bond
͑2099 cm−1͒, and to trihydride ͑−SiH3͒ vibrations
͑2136 cm−1͒. The complexity of the spectra suggests that
facets with different crystallographic orientations exist on the
H-terminated SiNWs, as well as boundaries between the fac-
ets showing specific Si–Hx configurations. At 630 cm−1, the
corresponding Si–H bending modes are observed.
Turning to electronic defects, Fig. 2 shows the ESR
spectra of the as-grown SiNWs and the H-terminated SiNWs
as a function of the external magnetic field. The as-grown
samples show rather complicated specta caused by three dif-
ferent defects. The broad ESR feature at gϷ2.0065 with a
peak-to-peak linewidth of ⌬BPPϷ9 G is attributed to Si dan-
gling bonds or Pb-centers located at the interface of the crys-
talline core and the surrounding oxide.17–19 The varying
chemical and structural surrounding of the Si dangling bonds
as well as their random orientation leads to an inhomoge-
neous broadening of the resonance. At g=2.0005, the weak
In the H-terminated SiNWs, the intensities of all ESR
features decrease drastically. The disappearance of the E -
Ј
and EX-centers is obviously caused by the removal of the
oxide by the HF treatment. The reduction of the broad reso-
nance caused by dangling bonds or Pb-centers results from
the termination of the Si dangling bonds at the surface of the
SiNWs with H atoms. The number of paramagnetic defects
was calculated from the ESR signals by double integration of
the experimental curve and comparison to a known reference
standard. For the as-grown SiNWs, an overall defect density
of 6.5ϫ1014 mg−1 is obtained. About 95% of the spin den-
sity is due to the dangling bonds or Pb-defects at the Si/SiO2
interface, while 5% is caused by the oxide defects. Assuming
the density of the SiNWs to be similar to the density ␳
=2.33 g cm−3 of bulk crystalline Si, this corresponds to a
defect density of 1.5ϫ1018 cm−3. After HF treatment, the
defect density is reduced by a factor of almost 50 to 3.3
ϫ1016 cm−3. The nearly complete removal of the spin den-
sity for the resonance at gϷ2.0065 also argues against dan-
gling bonds inside the crystalline core of the SiNWs or re-
resonance of E -centers in the oxide caused by oxygen va-
Ј
FIG. 2. ESR spectra of undoped as-grown and H-terminated SiNWs. In the
as-grown nanowires, P -centers at gϷ2.0065, E -centers at g=2.0005 and
Ј
b
EX-centers at g=2.002 52 are found. The hyperfine lines of the EX-center
are plotted enlarged by a factor of 20. After HF treatment, all ESR lines
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nearly vanish.
gions of amorphous Si as the origin of the signal in the
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Appl. Phys. Lett., Vol. 85, No. 6, 9 August 2004
Baumer et al.
945
line core to the surrounding oxide, E -centers and
Ј
EX-centers located in the oxide. For the EX-centers, the two
characteristic hyperfine lines were detected. A total spin den-
sity of 1.5ϫ1018 cm−3 was found for the as-grown SiNWs.
After HF etching, the spin density was reduced by a factor of
50. PDS measurements of the as-grown and H-terminated
SiNWs showed that the optical absorption properties of the
nanowires above photon energies of 1.5 eV are essentially
identical to those of microcrystalline Si. In addition, the PDS
experiments confirmed the decrease in defect density after
HF etching.
The work done in Germany was supported by the
Deutsche Forschungsgemeinschaft through SFB 563, while
that in China was supported by the Research Grants Council
of Hong Kong SAR (CAV project City U 3/01C) and by a
grant from the Chinese Academy of Sciences.
FIG. 3. Optical absorption coefficient of as-grown and H-terminated SiNWs
determined by PDS plotted as a function of the incident photon energy. As a
reference, the absorption spectrum of microcrystalline Si (Ref. 27) is
included.
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densely packed thin films impedes the determination of the
absorption via classical transmission measurements. Rather,
experimental techniques such as photothermal deflection are
needed to directly detect the absorption of the samples. Fig-
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absorption, correcting for different layer thicknesses. For en-
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and H-terminated SiNWs agree within the experimental er-
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which is due to direct optical transitions in crystalline Si. For
energies above E1, the absorption coefficient of the SiNWs
seems to decrease again. However, this is an experimental
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To summarize, we have shown H termination of SiNWs
by FTIR measurements after the removal of the as-grown
oxide layer by HF etching. ESR measurements on the
as-grown SiNWs revealed several paramagnetic defects:
Dangling bonds or Pb-centers at the interface of the crystal-
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