Light Emission from -FeSi2
Yoshihito Maeda1,2
1

Department of Computer Science and Electronics, Kyushu Institute of Technology, Iizuka, Fukuoka 820-8502, Japan
2
Advanced Science Research Center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1110, Japan
Phone: +81-948-29-7653 E-mail: maeda@cse.kyutech. ac.jp

ABSTACT
Orthorhombic -FeSi2 is one of a few semiconductors which can be applied to IR-light emitters at opticaltelecom at 1.55-1.3 m of wavelength. Actual situation of its light emission is not close to realization of practicaluses in optoelectronics, however, efficiency of rad-FeSi2 iative recombination has been uplifted by controlling
crystal growth and by fabricating nanometer-sized structures (nanostructures).
My lecture in ICSS-Silicide 2014 is based on our group’s researches on IR-light emission from a hetero-system of -FeSi2 and Si and aims at giving the whole phenomena relating to light emission from -FeSi2 inorder to draw a true picture for understanding it as much as possible. We emphasize important facts that IR radiative processes are strongly dependent upon micro-structures of crystalline units for radiative recombination. In
this meaning, nano-composite phases consisting of -FeSi2 nanocrystals are appropriate to light emitters.
This lecture contains the following topics.
Contents of lectures (tentative)
1. Research Background
1.1 Green technology toward sustainable optical telecom technology
1.1.1 What are problems in optical telecom technology? (see Figure 1)
1.1.2 Why do we need -FeSi2?
1.1.2 What is light emission from -FeSi2
1.1.3 How do we do the crystal growth?
2. Light emission observed in some crystal phases
2.1 PL behaviors dependent upon growth or synthesis methods
2.1.1 High purity synthesis of crystals by ion-beam technology
2.1.2 Film deposition methods
2.1.3 Bulk crystals
2.2 Understanding of emission bands observed in PL spectra
2.2.1 Intrinsic A band emission (see Figure 2)
2.2.2 Impurity-related C band emission (see Figure 2)
2.2.3 Defects-related bands emission (see Figure 3)
3. Photoluminescence properties of nano-composite phases
3.1 -FeSi2/Si nano-composite phases
3.1.1 Microstructure observations
3.1.2 Band structures
3.2 A band emission
3.2.1 Temperature and pumping dependences
3.2.2 Time resolved measurements
3.2.3 Photo-carrier injection PL measurements (see Figures 5-8)
3.3 Impurity related emission
3.2.1 Temperature and pumping dependences
3.2.2 Related impurity levels
4. Enhancement of PL
4.1 Impurity doping
4.1.1 Al doping
4.1.2 Cu doping (see Figures 4-8)

4.2 -> phase transition
4.3 Oxidation (see Figure 9)
5. Future study required moreover
5.1 Further enhancement of radiative recombination rate
5.2 Thermal quenching (damping) problem (see Figure 9)
6. Conclusions
Some key figures and their explanations
1. Research Background

Variety of Semiconducting Silicides
adapted to photonic device applications
Wavelength(m)
波 長/μ m5.0 3.0 2.0 1.5 1.2 1.0

0.8

0.7 0.6

0.5

R

光通信
Telecom.
applications
III-V
Si(1.12)IV族
IV group

Mismatch on Si (%)
Lattice
格子不整合率（％）

Ge(0.66)

FeSi(0.06)
3.0

G

0.4
B

Visiblelight
可視光

GaAs(1.42)

Ir3Si5(1.2)

Light emitter materials
-FeSi2/Si, Ru2Si3

GaP(2.27)

OsSi2(1.8) Os2Si3(2.3)
Ca2Si(1.9)

Light receiver materials
-FeSi2/Si, FeSi/Si,
Ca2Si/Si

WSi2(0.07)
OsSi(0.34)
MnSi2-x (0.7)
Mg2Si (0.78)
MoSi2(0.07)
Ru2Si3(0.8) BaSi (1.3)

2.0

Photonic crystal materials
 -FeSi2/Si waveguide
Photovoltaic materials
 -FeSi2/Si,
 BaSi2(Schottky j
unction)2

ReSi1.75(0.12)
-FeSi(0.78)
-FeSi22(0.78)
CrSi2(0.35)

1.0

-FeSi2 2unit
単位胞
-FeSi
cell

0
0

0.5

Applications(realized)

1.0
1.5
2.0
バンドギャップ
EgBand-gap Energy /eV
(eV)

2.5

3.0

Thermoelectric materials
Mg2Si（LT),-FeSi2(~600K)

Y. Maeda and Y. Terai：Bull. Inst. of Metals Japan, MATERIA 2005 Jun.

Figure 1
Semiconducting silicides that are available to applications optoelectronics, photonics, photovoltaics andthermoelectronics on Si platforms. Their band-gaps (or band-gap wavelengths) and lattice mismatch ratios on the
Si substrate are crucial parameters for applications to telecom technology. Orthorhombic FeSi 2 (-FeSi2), cubic
Mg2Si, MnSi2-x, and Ru2Si3 have their band-gap wavelengths near the telecom wavelength. Radiative processes ofIR light have been confirmed in -FeSi2, and reported in Ru2Si3. BaSi2 with the band-gap of 1.3 eV as been studied in order to apply it high efficient solar cells.
(Cited from Y. Maeda and Y. Terai, Article published in Bulletin of Japan Inst. Metals, Jun vol. 2005)

2. Light emission observed in some crystal phases

intrinsic PL and the microstructure
-FeSi2 nanocrystals
embedded in Si
IBS -FeSi precipitates
24000
0.805eV3500

very strong
and narrow
band emission

Exciton size 16nm

PL Intensity

3000

2500

2000
FWHM=18.9meV

1500

R=14nm
1000

500

R=7nm

100nm

0
0.7

0.75

0.8

0.85

0.9

0.95

Photon Energy (eV)Figure 2
A typical photoluminescence (PL) spectrum with an intrinsic emission due to the radiative indirect transitionin -FeSi2 (A band emission) and the impurity (or defect)-related emission (C band emission). This PL spectrum
mainly consists of the A band emission with a Lorentz function shape and the C band emission weakly appears at
its shoulder. This observation tells us the important fact that the correct or pure observation of the radiative process in -FeSi2 can be only in the nano-composite phase as shown in the left figure). The nanocrystals size issame to or less than the effective Bohr size of 16 nm of Mott-Wannier type excitons in -FeSi2. This structuralevidence teaches us that the migration of the M-W type excitons in the crystal is limited (exciton confinement).
This confinement contributes enhancement of a radiative recombination rate of electrons-holes. This better electronic situation due to exciton confinement realized in the nano-composite phase leads high efficient radiative
recombination at the A band emission. The C band emission at ~0.77 eV at 8 K is originated from impurity-band transitions. In the case of non-doped samples the origin of impurity level (band) is silicon atomic vacancy(VSi) in the -lattice, in the case of impurity doped samples acceptor or donor levels are formed. It has been observed that the C band intensity is dependence upon an impurity doping dose. In Cu-doping case, enhancement of
the C band emission depending on the Cu doping dose was clearly observed. (see Fig4)

Comparison between -FeSi2 and defect-related PLs
We designedly made defects in Si substrates by implantation of some ionsother than an iron ion.
PL spectrafrom intentionally
Si substrates
PL spectrafrom impurity madedopeddefectSi(100) in
sub.
180

Si

Fe (-FeSi2)
0.805eV200

intrinsic PL
X 35

x 40

PL(Si:TO)140

Si (100keV, 5e15, 800deg. 8h)
Al (50keV, 5e15, 800deg. 4h)
Mn (100keV, 5e15, 800deg. 2h))O (100keV, 5e15, 800deg. 2h))

160

6000

120
120

D1

100

PL intensity

PL Intensity (mV)

160

8000

10 K

80
60

Si
80

4000

O
Mn

40

Al

D2

40

0

2000

0.7

0.8

0.9

1

1.1

1.2

Photon Energy (eV)20
010000 11000 12000 13000 14000 15000 16000 17000

Wavelength (A)

0
0.7

0.8

0.9

1

1.1

1.2

Photon Energy (eV)Figure 3
Some PL spectra observed from Si substrates implanted by some ions with high energies and high doses. In
these ion implantation gives damages into implantation depth of Si substrates. In the damaged region, atomic vacancy, interstitial atoms and higher ordered crystal defects such as dislocations (DL), looped DLs. It has been reported that one of these defects, especially lopped DLs, can emit IR light, unfortunately, the emission energy of
the D1 line (the strongest DL-related emission line) at ~1550 nm (~0.81-0.82eV) is very close to that of intrinsic
A band emission. However, this situation about IR light emission may be more complicated than that estimatedby us. The left spectra show this complicated situation well, and very broad emission bands but their peak energies and intensities are different each other. These similarity between such a defect-related PL (DRPL) and the
intrinsic A band PL have made chaos for understanding nature of light emission from -FeSi2 itself. Compering
the defect-related PL spectrum and the intrinsic A band emission as shown in the right figure, we found that the
DRPL intensity was much smaller than the A band one as shown in an inset of the right figure. This means thatthe DRPL intensity may be proportional to the implanted dose under the competition with nonradiative processes
brought by the implantation at the same time. In general, the density of LDs and their density of state (DOS) as aradiative unit may be much smaller than a joint density of state (JDOS) at interband transition. These big differences in the JDOS and DOS between them are reason of observed difference in the PL intensities. (cited from Y.
Maeda, Appl. Surf. Sci., 254 (2008) 6242. )3. Photoluminescence properties of nano-composite phases
4. Enhancement of PL

0.8

PL Intensity (V)

0.7

8K
Pex=10mW

A band(a)Cu doped -NCs
after 5.5h annealing

0.6(b)non-doped -NCs
0.5

after 8h annealing

0.4
0.3

(c)non-doped -NCs
after 2h annealing

C band

0.2
0.1
0
0.7

0.75

0.8

0.85

0.9

Photon Energy (eV)Fig.1. PL spectra of non-doped and Cu-doped
-FeSi2 nanocrystals (NCs) samples.

Figure 4
PL spectra of Cu-doped -FeSi2/Si nano-compositedphase thermally treated for different time. The thermal
treatment time corresponds to doping dose of Cu into thenano-composite phase. We found that Cu causes pronounced enhancement of both the A band and the C band
emissions. The C band emission enhancement can be understood by increase of the DOS at the acceptor level due to
Cu+ activated in the -lattice. However, the enhancement of
the A band emission at 0.803 eV at 8 K cannot be understood by the same reason. One of the possible reasons is
improvement of heterointerfaces between -FeSi2 and Si by
Cu-doping. TEM observations revealed that semi-coherent
interfaces can be realized in Cu-doped -FeSi2/Si heterointerfaces. However, in the -FeSi2 nanocrystals (-NCs)

such a coherent interface with Si can be realized because the -NC size is very small (~10 nm in size) and totallattice mismatch is small (less than the critical length for generation of screw dislocations). The Mott-Wannier(M-W) type exciton generated in -NCs has a large effective Bohr radius (aB=~8 nm) because of the large static
dielectric constant (0) of -FeSi2. In the -NCs of less than ~10 nm in size, the M-W excitons can be confined
well if the band-offsets at the conduction and valence bands between -FeSi2 and Si. Photocurrent measurements
have made it evident that the band-off energy of holes (EV in Fg.5) at the -FeSi2/Si hetero-interface is too small
to realize confinement of holes in the -NCs. Therefore, a part of holes can go out from the -NCs to the Si matrix by thermal activation. Radiative recombination rate at steady state can be controlled by a rate of holes goingout from -FeSi2 as shown in Fig. 5. Even if the confinement of excitons can be realized, the excitons are dissociated by thermal activation and the some holes as a part of them goes out before radiative recombination atthe-NCs.

-FeSi2SiEcelectrons

Radiative
Recombinationof electrons and holes

Ev

Ec

Non-radiative
Recombination

Ev
holes going out
from -FeSi2 bythermal activation

Figure 5Band diagram in -FeSi2/Si heterointerfaces. Ec and Ev are band offset energies at the conduction and valence bands, respectively. Radiative recombination processes (RRP) between electrons at the conduction bandand holes at the valence band and non-radiative processes due to escape of holes from -FeSi2 to Si. The RRP
rate may be controlled by NRRP due to escape of holes.
Therefore, the RRP efficiency is controlled by the escape process of holes. Photocarrier-injection PL(PCI-PL) measurements is powerful in order to know dynamic processes (migration, recombination, repeated trap
processes during migration) of minority carriers. The arrangement of PCI-Pl measurements is shown in Fig. 6.
The big difference between PL and its measurements is the way of their excitation light (usually lasers with suitable power and wavelength). In the PCI-PL measurements, the Si substrate is irradiated by the laser and pairs ofelectrons and holes are optically generated near the back-side surface. The minority carrier can diffuse into thereverse side (toward -FeSi2) frequently is trapped at the trap centers due to doped impurities.
In cases of Cu-doping where we observed pronounced enhancement of the A band emission in Fig. 4, we
think that such a dynamic migration process of minority carriers (holes) are related to the PL enhancement. Under
this assumption, we investigated the PCI-PLs in Fig. 6, dependences of excitation frequency on their intensityand phase-shift in Fig.7 (response delay due to migration rate, or trap frequency).

0

A

0.6

PL

PL

PCI-PL
(X10)

PCIPL

-10

Phase  (degree)

PL, PCI-PL Intenisty (V)0.7

Si Laser
0.5
0.4

T

-NC/Si

C

0.3
0.2
0.1

-20

88Hz
f =88Hz

-30

-40

-50

200Hz
310Hz

(f)

500Hz
-60

1000Hz
0
0.7

0.75

0.8

0.85

0.9

0.95

Photon Energy (eV)

1

-70
0.7

0.75

0.8

0.85

0.9

0.95

1

Photon Energy (eV)Figure 6 (left) PL and PCI-PL spectra for Cu-doped -NC/Si nano-composite phases on substrates (see the inset). The both spectra were measured at 8 K. This sample was annealed for 5.5 h. The PCI-PL spectrum includes
information of holes migration with a repeated trap process and radiative recombination of electrons and holes atthe -NCs.
Figure 7 (right) Phase spectra of PCI-PL measured at each excitation frequency (chapping frequency of excitation laser). (f) is defined as a difference in phases at ~0.8 and ~0.9eV.
Figure 7 shows the phase spectra of PCI-PL signals measured at each frequency f. We observed clear
phase-changes in the energy region where the A and C emissions in PL and PCI-PL spectra were observed. This
result indicates that these phase-changes are attributed to migration of minority carriers (holes in this study) fromback-side of n-Si substrates to the -NCs. We also observed a pronounced frequency dependence of the phase
spectra as the frequency increased. Phase shift () is defined as a difference in phases at emission bands at ~0.8
eV and the non-emission region at ~0.9 eV and can be regarded as a delay response due to the migration of holes
with a repeated trap process brought by Cu-doping into n-type Si substrates.
Therefore, we can deduce a time constant () of such a delay response from frequency dependence of the 
by using the following equation; ( f )  360 (s)  f ( Hz )   (0) (in degrees),

Phase Shift  (degree)where  (0) is an off-set of phase shift coming from some mechanical reasons.
25

Figure 8
Phase shift () defined in Fig. 7 as a function of
frequency. Two slopes were observed and mean two delay
responses with different time constants 1=87.8 s and2=21.8 s for migration of holes. The longer time constant obtained by this analysis corresponds to migration
process with repeated trap process of holes. This trap
comes from Cu-doping. However, the measurements at 50
K the time constant was close to that in usual Si substrates.
This fact suggests that the energy depth of trap may be
4~5 meV.

8K
20

15

10

5
0

200

400

600

800 1000 1200 1400

Frequency (Hz)The was computed from the PCI-PL phase spectrum at 8 K as a function of the frequency f. We found
two slopes with different time constants 1 and 2 in Fig.8 and obtained two time constants 1=87.8 s and2=21.8 s. This 2=21.8 s is the same to = 22 s obtained in the non-doped -NCs. These results are very important and indicate that doping of Cu atoms into n-Si substrates introduces a hole migration process with larger
1 which may be attributed to an repeated trap process of holes.
PCI-PL measurements reveal that enhancement of the A band emission is attributed to controlled migrationof holes with the repeated trap process due to Cu-doping into the Si phase. However, this trap center may form ashallow energy well of 4~5 meV so that the migration control of holes above 50 K is not effective to enhancement of PL. For light emission possible at room temperature, we need formation of trap centers with energy wellof more than 26 meV.

PL Intensity

250 K

Pex=40 mW

260 K
270 K

0.7

0.75

0.8

0.85

0.9

Figure 9
PL spectra from the oxidized non-composite phases nearroom temperature. The oxidation of -NCs/Si nano-composite
phase was carried out at 900oC for 6 hours. The FTIR absorption
measurements revealed that the Si phase was mainly oxidized.
This means that a -NCs/SiO2 nano-composite phase forms. SiO2
has a wide band-gap and makes large offset energies for bothconduction and valence bands with -FeSi2. So we can expect sufficient confinement of electron-hole pairs at the -NCs in the oxidized nano-composite phase. This situation may contribute to observable PL spectra near room temperature.

Photon Energy (eV)

5. Future study required moreover
Toward realization of room temperature light emission from -FeSi2 we need sufficient confinement ofelectrons and holes (or M-W excitons) in the -NCs at room temperature. So we cannot expect it to -NCs/Si
hetero-structures because of the too small valence band offset as described above and shown in Fig. 5, and weneed to change other hetero-structures fabricated in SiO2 or SiC which can realize sufficient confinement of electrons and holes in the -NCs and may give the maximum efficiency of radiative recombination except for
non-radiative recombination processes due to the inherent interface problems. We have already observed enhancement of PL from -NCs/SiO2 hetero-structures which is fabricated by oxidation of -NCs/Si hetero-structures and evident PL maintained near room temperature as shown in Fig. 9.
Acknowledgements
This lecture is based on many research results on light emission form IBS -FeSi2 since 1995. YM would
like to acknowledge both to dear graduates of Osaka Prefecture University (1995-2003), Kyoto University(2003-2012) and Kyushu Institute of Technology, Kyutech (2012-present) and to students of Kyutech for theirserious efforts to sample preparations and optical measurements. And also he would like to give many thanks toProfessor Y. Terai of Kagoshima University for continuous and fruitful discussion and suggestion on understanding of PL data. Finally, he expresses his appreciation to organizing committee of ICSS Silicides 2014 forinvitation to this international summer seminar.