Asian School-Conference on Physics and Technology
of Nanostructured Materials
Vladivostok, Russia, 21 – 28 August, 2011

Article number

Iron Silicides for Optoelectronics;
A New Technology Using “Elements of Hope”
Yoshihito Maeda*
Department of Energy Science and Technology, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan*e-mail: maeda@energy.kyoto-u.ac.jp
Abstract. In the 21st century, we are facing global resource depletion in industry and environment problems. In this lecture,
we would discuss problems of sustainability in optoelectronics and report results obtained in basic and application researchesof iron silicides as a new technology based on elements of hope that may be one of solutions for problems of scarcity andenvironment hidden in the present optoelectronics.

origin of orthorhombic -FeSi2 from -FeSi2 with a cubic(fluorite) lattice and a metastable metallic phase can beunderstood by a significant lattice deformation due to a
Jahn-Teller effect between the d like electron bands andthe lattice system. The equilibrium phase above 900oC is-Fe2Si5 (that exists as -FeSi2) with a tetragonal andmetallic phase. The -FeSi2 forms by a rapid  to  phasetransition above 850oC, however, the formation of -FeSi2
by the reverse transition requires very long hours. The
monosilicide -FeSi with a B20 structure (distorted NaCl
structure) is a semiconductor with a narrow-gap of ~50
meV [8]. The origin of its semiconducting nature has been
explained by characteristics of a Kondo insulator due tostrong correlation between d-electrons. The narrow
bandgap corresponds to the Kondo temperature TK=600 K.
The quasi-binary system FeSi-FeGe: Fe (SixGe1-x) shows ametal-insulator transition at x=0.25at%.
A Heusler alloy of Fe2(FeSi)=Fe3Si has a DO3 typeordered ferromagnetic phase. The L21 type ordered alloys
Fe2MnSi with B site sensitivity and Fe2CoSi with a half
metallicity (completely spin polarized state at the Fermi
level) can be formed by using lattice site preference
characteristics of Mn and Co atoms in the Fe3Si lattice.
These silicides written above may be applied to
optoelectronics, photonics or spintronics. Moreover, there
are great advantages that these silicides can be synthesized
directly on a silicon substrate by a silicide reaction whereiron atoms are supplied onto or into Si wafers.

1. Sustainable materials
Present electronics and optoelectronics have been still
supported by rare resources and toxic compound
semiconductors such as III-V (GaAs, InGaAs etc.) or IIVI (CdTe etc.) group compound semiconductors. We are
surrounded with such electronic or optoelectronic devices
in our daily life. Most of semiconductors employed inthose devices give huge ecological impact to environment.
We have to reconsider these problems that have been
hidden inside glamorous looks of optoelectronics andcannot expect its further progress without technologicalresolution of them. Since 1997, my colleague and myself
have stated necessity of new semiconductor technology
and named it “Kankyo-Semiconductor” (ecologicallyfriendly semiconductors, in English) [1]. The word
“Kankyo” in Japanese means environment. Researchersand scientists mainly in Japan have investigated Kankyosemiconductors that consist of most of abundant resources
existing on the earth crust and being less toxic elements
such as Silicon, Iron, Aluminum, Calcium, Magnesium,
Oxygen etc. in order to establish sustainable
optoelectronics. These elements are called “Elements ofHope” as respected concept [2]. Electronics based on
sophisticated silicon technology can be regarded as themost sustainable one, therefore, future optoelectronics
also adapt to the Si-based electronics, that is Si photonics
or Si-optoelectronic integrate circuits (Si-OEICs)[3-6].
The most essential condition of new semiconductors
required for Si photonics is compatibility of production
processes with Si LSI processes. Moreover, the new
semiconductor should be deposited or synthesized directlyon Si or related materials, more preferably can epitaxiallygrow on Si substrates. From these conditions, ironsilicides with semiconducting or ferromagnetic properties
may be candidates for Si optoelectronics and photonics,
furthermore for spintronics as introduced them in thefollowing sections.
2. Binary system of Iron-Silicon
An iron-silicon binary system shown in Fig. 1
reminds ourselves to grub up some silicides valuable forapplications. There are three disilicides: - and FeSi2 with different structures and properties [7]. In a Si
rich region, -FeSi2 is a semiconductor with the bandgapof ~0.87eV @300K. This optical property of -FeSi2 may
be applicable for some telecom devices such as lightemitters and photodetectors near 1.5m-wavelengths. The

Fig.1. Iron-silicon binary system and their silicides withsome functional properties for electronic, optoelectronics,
photonics or spintronics.

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3.

Semiconducting -FeSi2

E(eV)

The primitive cell of -FeSi2 involves 24 atoms (8formula units) in the base-centered orthorhombic latticewith D12h18 symmetry. The lattice constants are a=0.98792,
b=0.77991, c=0.78388 nm. Both 8 iron and 16 silicon
atoms are grouped into two equivalent sites (Fe(I), Fe(II),Si(I), Si(II)) with slightly different distances. The crystal
looks a complicated structure with stacked and wavy
structures of iron and silicon atoms in the lattice. There
are no Fe-Fe and Si-Si bonds. The lattice spacing along<110> of Si is so close to the b- or c-axes of -FeSi2 that
two typical epitaxial growth of -FeSi2(100) planes on
Si(001) or -FeSi2(110),(101) on Si(111) can be realized.
Highly oriented or epitaxial growth of -FeSi2 on Si has
been realized by ion-beam synthesis (IBS), reactive
deposition epitaxy (RDE), solid phase epitaxy (SPE),
sputtering deposition, metal organic chemical vapor
deposition (MOCVD), and molecular beam epitaxy(MBE).
4.

Indirect exciton
me*=1.0
Ex(1)=3meV

CBM
CBM atat 
Fe-d
Fe-d 83%
83%
VBM
VBM atat Y
Y
Fe-p
Fe-p 28%28%Si-p
Si-p 29%
29%

h
Egind

EPL=0.805eV
phononEph=31meV0
m*h=0.25

Y

Egind=EPL+Ex(1)+Eph=0.805+0.003+0.031=0.839eVkFig.2. Radiative transition at the indirect bandgap of FeSi2 between the Y point on the valence band (including
p orbital component of Fe 28% and Si 29%) and the point on the conduction band (d orbital component of Fe73%). This band conditions indicate that the electric
dipole interaction partially allows this optical transition.
When considering the photoluminescence peak at
Ep=0.805 eV, the edge absorption due to excitons with thebiding energy Ex=3 meV and phonon emission with
Eph=31 meV estimated from Raman measurements, theenergy conservation among them gives the indirect
bandgap energy Egind=0.839 eV at 10 K [10].

Physics of light emission

Optical absorption () spectra of bulk -FeSi2
crystals at a low  range were measured by Udono et al.
[9] and presence of clear exciton absorption at the indirectabsorption edge coupled with phonons with the energy of
0.031eV (250 cm-1) was confirmed. Some theoretical
calculations of electronic band structure have reported theindirect band gap between valence band maxima (VBM)
at Y-points and conduction band minima (CBM) on lines [10,11]. Photoreflectance measurements of epitaxial
films on Si(100) revealed that the direct gap energy was
0.934eV and that some exciton effects at the direct bandgap exist [12]. The observed energy (0.805eV) of intrinsic
photoluminescence is smaller than the direct gap energy(0.934eV), therefore, the intrinsic luminescence should be
related to the transition at the indirect gap and exciton
effects.
Figure 2 shows schematic explanation of a radiative
process due to dissociation of exciton at the indirect-gap
between the Y and  points. At low temperature, theexciton dissociation requires emission of one phonon with
Eph=31 meV (LO=250 cm-1) which corresponds to total
symmetric (Ag mode) vibration of the orthorhombic lattice[10]. The oscillator strength of electron transition between
the valence band maxima (VBM) with a p-component
from Fe and the Si conduction band minima (CBM) with ad-component from Fe is allowed by electric dipole
approximation. The 1st exciton binding energy in the bulk
crystal can be calculated to be Ex1st=~3 meV (Ex1st/kB=35
K) which indicates low thermal stability because of boththe large dielectric constant and reduced mass of -FeSi2.
Under such a condition, we can expect enhancementof luminescence efficiency due to exciton confinement in
-FeSi2 nanocrystals. The indirect band-gap energy can becalculated by
Egind  EPL  Eex1st (r )  E ph ,The IBS of semiconducting -FeSi2 using possible
conditions can present variety of morphologies from thenanocrystal to the polycrystalline layer [11]. Each crystal
structures may be employed for appropriate applications.
Our long-standing research on light emission and its
enhancement teaches us that the -FeSi2 nanocrystals
embedded in a Si crystal matrix are best for a light emitter
device. It is because the nanocrystal can makecrystallographic coherent growth on Si(111) planes andless defective interfaces with Si(111) planes. This
coherent interface may reduce both a nonradiative
recombination centers and a dislocation related radiative
centers. When photoluminescence (PL) is excited byphotons with a higher energy than the band-gap energy,
electrons and holes generated in the Si matrix can beinjected effectively through such a coherent interface.
Moreover, in such small sized crystals diffusion of Si
atoms from the Si matrix to the inside of -FeSi2 lattice
can be easy so that Si vacancies in the lattice, which arenonradiative centers or play as accepters, can beeliminated by the diffusion. The enhancement of lightemission due to the Si diffusion has been confirmed insamples with different morphologies and fabricated bydifferent methods. As described above, we have foundappropriate situations for enhancement of light emission
in the -FeSi2 nanocrystal embedded in the Si matrix. The
nanocrystal size can be controlled by thermal annealing
temperature and time. We have discussed radiative
recombination of exciton confirmed in the nanocrystal andemission enhancement and thermal stability coming fromquantum confirmation [10]. And it was observed that theEindg=0.839 eV may be consistent to the indirect gap
energy of 0.83eV reported by Giannini et al. [13].

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PL intensity increased and the evident shift of peak energy
was not caused with decreasing the average size of FeSi2 nanocrystals. Further studies on exciton effects withheavy effective masses due to d-like conduction and p-dhybridized valence bands in -FeSi2 and their contribution
to the radiative exciton recombination should be interestin physics and be needed in order to realize high efficient
light emission at room temperature.

can contribute to increase in an electromotive force (emf)near 0.36V of open circuit voltage Voc, however, thelonger annealing gives saturation of the emf [17]. The
doping of Aluminum atoms into -FeSi2 was very
effective to enhancement both of photovoltaic property
and PL [18].
The Al-doping can bring us the same effect as thelong hour annealing. The Al atom has an atomic size close
to Si size and prefers occupation on Si sites or Si vacancy
sites in the -FeSi2 lattice. The doping can contribute toelimination of Si vacancy, so that the recombination
process can be inhibited well and the photovoltaic
response can be enhanced. From above experimental
results, the p-n junction between the Al-doped -FeSi2
with a p-type electrical property and n-type Si with asuitably designed density of electron may be promising
for the photodetector directly fabricated on a Si OEIC
platform [19-21].

Silicon-based approaches for photonics
Full Contribution of Active Silicides
Si-OEIC assisted bysilicide technology
Si/-FeSi2/Si
barrier structure
SiO2/-FeSi2/Si
PhC waveguide
SiO2/Fe3Si/Si PhC
MO modulator

5.2 Photonic crystal on Si
Semiconducting -FeSi2 has a higher refractive index
at an infrared region than Ge, GaAs and Si. This opticalproperty is valuable for fabrication of waveguides andphotonic crystals on Si platform. Photonic crystals (PhCs)
for telecom applications have been developed by using
mainly InGaAsP/InP, InGaAs/GaAs or Si on insulators.
These materials have a high refractive index and canrealize high index-contrast PhCs just only on SiO2. For
telecom-applications we need high performance PhCs inwhich light propagation can be controlled in wavelengths
1.3-1.55 m. -FeSi2 has a high refractive index of over
5.3 in the transparent region. We have applied -FeSi2 tohigh index-contrast PhCs with wide photonic band-gaps.
For submicron etching processes of the -FeSi2 film on Si
substrates, we have developed a reactive ion etching (RIE)
process consistent in the conventional Si processing andwe succeeded in fabrication of -FeSi2 photonic crystals,
waveguides etc. [10, 22] Moreover, a new photonic
structure, that is named a conjugated photonic crystal [23],was fabricated in order to reduce a Fresnel reflection lossof light propagating at the interface with a high contrast in
the refractive index. We have confirmed that improvementof incident efficiency at the inter face of inverted photonic
lattices of high refractive silicide semiconductors can berealized by using these unique incidence structures [23].

-FeSi2 PhC
defect amplifier?
-FeSi2/Si junction

Fig.3. Silicon-based approaches for Si-photonics (oroptoelectronics) using hybrid technology of some different
materials, which may teach us their difficulties, while
approaches using components of silicides may be more
possible to be realized. (this figure was reproduced byusing a table reported by Ch. Buchal [14].
5. Towards optoelectronics on Si
For realization of Si optoelectronics and photonics,
monolithic fabrication of some devices on Si platform
should be crucial. Figure 3 shows both approaches basedon hybrid technology of some materials for realization of
Si-optoelectronics proposed by Ch. Buchal at the MRS
Fall meeting in 1988 [14] and our approaches based on
silicide technology. In this section we would like tointroduce some researches towards realization ofnecessary devices (except for the light emitter) using ironsilicides on Si.
5.1 IR photodetector on Si
Photodetectors with photosensitivity in a 1.5mwavelength region have been fabricated by using a p type-FeSi2 layer/n-type Si heterojunction [15]. The photodetector showed both low responses of photovoltaic andphotoconduction near the band-gap. Thermal annealing
effects on enhancement of the photoresponse were
examined [16]. After many efforts, it was found that thethermal annealing in a vacuum at 800-850 oC for longhours was very effective to enhancement of photovoltaic
response near the band-gap. Key issues for high sensitive
photoresponse are how we can eliminate both interfacial
defects at the p-n junction and Si vacancies in the -FeSi2
layer. In the case of the annealing below 700 oC, weobserved formation of a metastable -FeSi2 phase with ametallic property, which resulted significant degradationof photoresponse. The long hour annealing (~360 min)

5.3 Light modulator on Si
Magnetooptical Kerr effects (MOKE) taking place on
ferromagnetic materials are available for control ofpolarized states of lights propagating on the Si-OEIC. We
have found that Fe3Si based Heusler alloy films, Fe4Si,
Fe3Si, Fe2CoSi films, cause a larger Kerr rotation than Fe
films [24]. These Heusler alloy films have the following
advantages, they can be grown epitaxially on Si(111) orGe(111)[25, 26], and have smaller coercive fields than theamorphous Fe film. However, sub-micron processing ofthese iron rich alloy films such as ferromagnetic Fe3Si wasdifficult only by using a conventional RIE process. We
have developed a self rift-off process in which the Heusler
alloy film is deposited into Si pits with photonic crystal
patterns and is employed as a mask film during RIE, then3

Article number
two types of Heusler alloy photonic crystal patterns arefabricated as shown in Fig.4 left and right.
It was found that the MOKE of this structure named
magneto-photonic crystals (MPhCs) was very sensitive toin-plane magnetic fields. We tried to apply the Fe3Si/Si
MPhCs to optical modulators of light propagating on the
waveguide fabricated in the SiO2/-FeSi2/Si photonic
crystal.

6.

Conclusions

We have introduced basic research of physics onlight emission from -FeSi2 and its enhancement andresearches of iron silicides towards optoelectronics which
includes photovoltaic detectors, photonic crystals, andlight modulators using magneto-photonic crystals. The
further valuable properties of iron silicides areinvestigating now, and will be found more and applied tophotonic devices compensating to Si-optoelectronics.
Acknowledgements
The Author (Y.M) would like to acknowledge senior
members of the professional group on Semiconducting
silicides and related materials, Japan Society of Applied
Physics (JSAP) for valuable discussion on researches ofsilicides and Dr. K. Akiyama of Kanagawa Industrial
Technology Research Center for deposition of Fe3Si films
on Si. A part of this study was supported by a Grant-inAid for Scientific Research (B) (Grant No.20360009)
from the Ministry of Education, Culture, Sports, Science,
and Technology in Japan.

Fig. 4. SEM image of magnetic photonic crystal (MPhC)
patterns of Fe3Si/Si. The line structure at the center canwork for Evanescent coupling with the waveguide inphotonic crystals. The left picture is air columns in Fe3Si
and the right one is Fe3Si columns.

References
Alternate magnetic field B
SiO2 coupling

Fe3Si MPhCs

P

Longitudinal MO Kerr effects
cause a polarization changeof lights at the junction.

InGaAsLaser Diode-FeSi2/Si PhCs
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Thin Solid Films 381 (2001) 171-310.
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InGaAsfemto-wattdetectors

Analyzer
pin hole

B= 100 mT

Signal intensity

x2.5

The amplitude
was small but
reproducible.

Cross Nicole level

Turn of magnetic field

Turn of magnetic field

Fig.5. Demonstration of modulation of propagating lights
in PhCs by longitudinal magnetooptical Kerr effects. The
polarization state of propagating light was effected by
Evanescent coupling with Fe3Si magneto-photonic
crystals (MPhCs) controlled by an applied magnetic field.

Figure 5 shows an experimental set-up and a structureof the MOKE modulator. The TE mode light from an
InGaAs laser diode was input into the SiO2 waveguide in
the SiO2/-FeSi2/Si PhC and modulated by Fe3Si MPhCs
shown in Fig. 4 through Evanescent coupling when the inplane magnetic field of 10-100 mT was applied. The
modulation behavior of the light was sensed at a crossNicolet configuration with an analyzer and an InGaAs
femto-watt detector. The figures in the down of Fig. 5
show signals corresponding to turn of applied magnetic
field. We succeeded in clear modulation of the polarized
light propagating on the waveguide in the PhCs. The lightintensity was modulated by changing the magnetic field
applied to MPhCs.

4