Fabrication and optical properties of a self-organized ferromagnetic Ni/Si-nanocomposite

Article abstract of DOI:10.1016/j.jmmm.2007.02.025

A mesoscopic composite consisting of porous silicon (PS) and two kinds of Ni precipitations is investigated by FTIR-spectroscopy and SQUID-magnetometry, respectively. The electrochemically produced PS/Ni-composite system consists of wires and non-spherical particles with a diameter around 60 nm and a one-dimensional extension of a few tens of micrometers, which imposes a significant out of plane magnetic anisotropy. Transmission in the mid-infrared is found near 10% for both, bare PS and the Ni-filled system. The transmission spectra of unprocessed Si and PS are similar, apart from the Si-H_x vibration modes due to the PS template. In the presence of Ni a broad absorbance structure at wavenumbers of 3500 cm^-1 is found, caused by NiO_x or Schottky barrier absorption across the Si/Ni-interface between Ni nanoparticles/wires and the silicon skeleton.

Full text of DOI:10.1016/j.jmmm.2007.02.025

ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 316 (2007) 114–117
www.elsevier.com/locate/jmmm
Fabrication and optical properties of a self-organized
ferromagnetic Ni/Si-nanocomposite
Ã
K. Rumpfa, , P. Granitzera, P. Poltb, A. Reichmannb, H. Krenna
¨
aInstitute of Physics, Karl Franzens University Graz, Universita¨tsplatz 5, 8010 Graz, Austria
bInstitute for Electron Microscopy, University of Technology Graz, Steyrergasse 17, 8010 Graz, Austria
Available online 27 February 2007
Abstract
A mesoscopic composite consisting of porous silicon (PS) and two kinds of Ni precipitations is investigated by FTIR-spectroscopy and
SQUID-magnetometry, respectively. The electrochemically produced PS/Ni-composite system consists of wires and non-spherical
particles with a diameter around 60 nm and a one-dimensional extension of a few tens of micrometers, which imposes a significant out of
plane magnetic anisotropy. Transmission in the mid-infrared is found near 10% for both, bare PS and the Ni-filled system. The
transmission spectra of unprocessed Si and PS are similar, apart from the Si-Hx vibration modes due to the PS template. In the presence
of Ni a broad absorbance structure at wavenumbers of 3500 cmꢀ1 is found, caused by NiOx or Schottky barrier absorption across the Si/
Ni-interface between Ni nanoparticles/wires and the silicon skeleton.
r 2007 Elsevier B.V. All rights reserved.
PACS: 78.55.Mb; 79.60.Jv
Keywords: Porous silicon; Optical properties; Magnetic nanocomposite; Metal deposition; FTIR-spectroscopy
1. Introduction
the sample-surface. Electrochemical filling of these chan-
nels leads to a PS/Ni-nanocomposite system with an
interesting magnetic behavior with potential for magnetic
high-field sensor applications [6]. Furthermore, this system
is a promising candidate for spin injection investiga-
tions due to the long spin-lifetime in silicon. Optical
properties are investigated using an FTIR-spectrometer. PS
can be considered as a two-phase material, and thus the
dielectric functions can be determined by effective medium
theory [7].
Silicon is one of the most popular basic materials in
today’s integrated electronic devices, but because of its
indirect band gap not straightly usable for optical
applications. In the last decade microporous silicon,
exhibiting luminescence in the visible spectral range, has
been intensively investigated [1]. But also PS in the
macroporous range is of great interest for waveguide
applications and photonic crystals [2]. Composite systems
consisting of embedded magnetic nanowires are also of
interest for optical/magneto-optical applications [3]. Many
nanocomposite systems are based on a porous aluminum
oxide template [4] or a polycarbonate matrix [5]. These
materials are not immediately integrable in electronic
devices; thus, we use silicon as template-material. This
matrix is fabricated electrochemically during an anodiza-
tion process in hydrofluoric acid leading to a porous
structure with highly oriented channels perpendicular to
2. Preparation and characterization of the ferromagnetic
PS/Ni-composite
N-type Si wafer with a doping density of 1018 cmꢀ3 is
used as a substrate. The achieved porous structure is
composed of channels with a diameter of about 60 nm and
remaining Si-slabs in the same lateral range (see Fig. 1).
The thickness of the porous layer ranges up to 40 mm.
During a cathodic process, this PS-matrix is filled with a
ferromagnetic metal (e.g. Ni) [8], during a pulsed current
deposition in a NiCl2-solution 185 g/l NiCl2, 45 g/l H3BO3)
Ã
Corresponding author. Tel.: +43 316 380 5191; fax: +43 316 380 9816.
E-mail address: klemens.rumpf@uni-graz.at (K. Rumpf).
0304-8853/$ - see front matter r 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2007.02.025

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K. Rumpf et al. / Journal of Magnetism and Magnetic Materials 316 (2007) 114–117
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Fig. 3. EDX spectrum showing the Ni–L line at ꢂ0.9 keV.
Fig. 1. SEM-micrograph of a 35 mm thick porous silicon layer consisting
of oriented pores with a diameter of about 65 nm on top of an n+-type Si
wafer. Insets: top view of the surface of the sample and cross-section of the
pores (upper panel).
Fig. 4. Hystereses loops of a Ni-loaded porous silicon template for both
magnetization directions perpendicular and parallel to the surface as well
as for bulk Ni.
Fig. 2. Pore-diameter distribution from the top view of a typical porous
silicon sample within an area of 3 mm ꢁ 3 mm. The diameter scatters mainly
between 55 and 65 nm.
presence of strong anisotropy of the deposited metallic Ni
within the pores. The hysteresis differs from the one of thin
metal layers [9] as well as from bulk Ni.
or a Watts solution (45 g/l NiCl2, 300 g/l NiSO4, 45 g/l
H3BO3) as electrolyte.
The difference is noticed between an external magnetic
field applied perpendicular (H ) and parallel (H ) to the
?
jj
The pore-diameter and the pitch can be varied in a small
range by adjusting the current density. In an individual
sample the size distribution of the pores is quite homo-
geneous, as shown in Fig. 2.
The investigation of Ni-loaded porous silicon samples by
EDX-spectrometry demonstrates the presence of Ni within
the porous layer over the entire pore-length. A typical
EDX-spectrum gained at an interior position of the cleaved
edge of a sample is shown in Fig. 3. The presence of Ni is
clearly seen by the Ni–L line at about 0.9 keV. Analogous
spectra are observed along the total pore-length from the
surface down to the pore tips [8].
surface as shown in Fig. 4. Depending on the Ni-loading
coercivity ranges between 600 and 200 Oe for an applied
magnetic field H . If a magnetic field H is applied the
coercivity decreases to values between 150 and 90 Oe. With
respect to bulk Ni the coercivity is about 30 times larger
(HC, bulk ¼ 20 Oe). The squareness depends also on the
orientation of the external field and lies between 70% and
20%, which clearly shows one-dimensional anisotropy for
the former one.
?
jj
3. Optical investigations of the PS/Ni-system
Magnetization measurements of the Ni/PS-samples show
clearly a ferromagnetic behavior and demonstrate the
The optical characterization of the samples has been
carried out by a Bruker Equinox 55 FTIR-spectrometer

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K. Rumpf et al. / Journal of Magnetism and Magnetic Materials 316 (2007) 114–117
116
Fig. 6. Transmission of a Ni-filled PS sample. Additional absorption
peaks due to O–Si–H modes are identified as well as a broad absorption at
wavenumbers of 3500 cmꢀ1 due to Schottky barrier absorption across Ni/
Si interfaces.
Fig. 5. FTIR-spectra of a bare Si wafer and porous silicon. The identified
Si–H vibration modes are shown in detail in the inset.
and the properties of PS are discussed in the mid infrared
range. First, the transmittance of a bare n-type silicon
wafer has been recorded (Fig. 5). It shows free carrier
absorption at wavenumbers below 1000 cmꢀ1 due to the
high doping density. The porous silicon sample has a main
peak around 2100 cmꢀ1, which corresponds to the Si–H
vibration modes (Fig. 5). In the inset of Fig. 5, this peak
splits into three ones. The first at wavenumbers of
2087 cmꢀ1 is due to a Si2–Si–H2 stretching mode, the
second peak at 2115 cmꢀ1 corresponds to a Si3–Si–H
stretching mode and the third one at 2138 cmꢀ1 is identified
as a Si2–Si–H2 stretching mode [7].
This absorption can either be due to some NiOx-species
[14] or Schottky barrier absorption across the interface
between the Ni and the silicon skeleton. The barrier
energies of Ni-Si interfaces are between 0.66 and 0.75 eV,
which means 5300–6050 cmꢀ1 [15]. These values reported
for planar interfaces exceed the measured ones but never-
theless, if considering a possible barrier height reduction
due to a quantum confinement effect of the porous silicon
matrix these values from the literature are not too far from
the observed ones.
4. Conclusion
Considering the overall transmission of the PS sample,
interference fringes are visible due to multiple reflections
within the porous layer. The condition to obtain such
undulations is the optical flatness of the porous layer with
sharp termination against bulk Si, which can be seen in Fig.
1. Also, the surface of the sample has to be of sufficient
smoothness for observing interferences in the mid-IR
range. This can be achieved by special drying techniques
(supercritical drying or replacement of water by a liquid
with lower surface tension) to avoid cracks [10].
The porosity of the produced PS samples can be
estimated to be typically 40%. The refractive index
associated to this porosity is 2.7 (at 0.49 eV) [11]. Bulk Si
offers at 0.49 eV a refractive index of 3.44 [12]. Examining
the observed undulations of the considered sample, a
period with a wavenumber of 50 cmꢀ1 is found. The
undulation period leads to a thickness of 37 mm of the
porous silicon layer, which is in good accordance with the
value obtained from SEM-micrograph.
A strongly anisotropic PS/Ni-nanocomposite is electro-
chemically produced and its soft ferromagnetic behavior is
demonstrated by SQUID-measurements. The magnetic
anisotropy is caused by differently shaped incorporated
metallic structures. The verification of Ni within the porous
layer is also carried out by EDXS. The PS-templates as well
as the PS/Ni-composites show transmission in the mid
infrared. IR-spectra of unloaded and Ni-loaded samples
are significantly different due to the interfaces between
different materials (Si/Ni) along the pores, and due to the
quantum confinement.
Porous silicon, a constituent of silicon, air and of silicon
oxide acts as effective medium for probe wavelengths much
larger than the considered structures. Furthermore, the Ni-
loaded PS-matrix represents a ferromagnetic effective
medium.
Considering a Ni-filled PS-sample there are additional
peaks at wavenumbers of 2200 and 2250 cmꢀ1, which
correspond to O–Si–H modes [13] and can be seen in the
transmission spectrum in Fig. 6. Furthermore, a broad
absorption peak at wavenumbers of 3500 cmꢀ1 is found.
Acknowledgments
We thank the Austrian FWF-fund for his grant under
project P18593 and acknowledge the support of the
Austrian MNA-nanonetwok.

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117
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