International Conference and Summer School on Silicide Green Technology(ICSS-SILICIDE 2014) , Jul. 21, 2014
Tokyo University of Science, Katsushika Campus

Light Emission from b-FeSi2
Yoshihito Maeda
Department of Computer Science and Technology
Kyushu Institute of Technology (Kyutech)
Advanced Science Research Center, Japan Atomic Energy Agency

1International Conference and Summer School on Silicide Green Technology(ICSS-SILICIDE 2014) , Jul. 21, 2014
Tokyo University of Science, Katsushika Campus

Acknowledgements

This lecture is based on many research results on light emission form IBS
b-FeSi2 since 1995. YM would like to acknowledge both to dear graduates ofOsaka Prefecture University (1995-2003), Kyoto University (2003-2012) andKyushu Institute of Technology, Kyutech (2012-present) and to students ofKyutech for their serious efforts to sample preparations and opticalmeasurements. And also he would like to give many thanks to Professor Y.
Terai of Kagoshima University for continuous and fruitful discussion andsuggestion on understanding of PL data. Finally, he expresses his appreciation
to organizing committee of ICSS Silicides 2014 for invitation to thisinternational summer school.

2International Conference and Summer School on Silicide Green Technology(ICSS-SILICIDE 2014) , Jul. 21, 2014
Tokyo University of Science, Katsushika Campus

Outline of my lecture
1. Introduction and Research background
2. Light emission from b-FeSi2 crystals
and its fundamental scheme
3. Light emission from b-FeSi2/Si nano-composite phase
4. Some examples of enhancement of PL from b-FeSi2/Si
5. Future study required moreover3ICSS-SILICIDE 2014 , Jul. 21, 2014
Tokyo University of Science Katsushika Campus

1
Introduction andResearch background
First, introduction relating to general scheme of semiconducting
Silicides and technology toward application to optoelectronics.

4Variety of Semiconducting Silicides
toward device applications
Wavelength(mm)
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
b-FeSi2/Si, Ru2Si3

GaP(2.27)

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

Light receiver materials
b-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
b-FeSi2/Si waveguide
Photovoltaic materials
b-FeSi2/Si,
BaSi2(Schottky
junction)2

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

1.0

b-FeSi2 2unit
単位胞
b-FeSi
cell

0
0

0.5

Applications(realized)

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

2.5

3.0

Thermoelectric materials
Mg2Si,
b-FeSi2(~600K)

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

5

Silicon-based approaches for photonics
Full Contribution of Active Silicides
Advanced New
silicide technology
Si/b-FeSi2/Si
stacking structure
SiO2/b-FeSi2/Si
PhC waveguide
SiO2/Fe3Si/Si PhC
MO modulator

b-FeSi2 PhC
defect amplifier?
b-FeSi2/Si
p-n junction
Ch. Buchal et al.: MRS Proc. 486 (1988) 3-19.

6

Iron-Silicon binary compounds
Reconsidering their functional properties.
Ferromagnetics
Spin-polarized
electrodes

Electron
Correlated matter

Semiconductor
1.5mm lightemitter

Narrow-gap

Fe(SixGe1-x)

-FeSiFull Heusler
alloysFe2(MnSi)
Fe2(MnSiGe)
Fe2(CoSi)

Kondo insulator-ferromagnetics
Fe2Si5bFeSi2Fe3Si

FeSi2

Schottkycontact with b

Buffer-layer
for epitaxy with Si
7

Family of iron silicides
-FeSi2(metallic)

Tc=~950oC

Tc=~600oC

b-FeSi2(semiconductor)

Fe3Si (D03: Ferromagnetics)

-FeSi2 (metallic)

FeSi(B20 strong electron
correlation matter)8

What do we achieve?
Si Optoelectronic Integrated Circuit (OEIC)
with Silicides photonics
01

LSI-chips

1

1
1

1
0

0

Optically logical operation for XOR

Electricalconnection

1

b-FeSi2/Si
PhotodiodeH

SiO2/Fe3Si/Si
MO-Modulator
ElectroMagneto

p+Si/b-FeSi2/n-Si
stacking LED

Photonic connection
SiO2/Si waveguideSiO2 or Si/b-FeSi2/Si
Photonic crystal circuits withlogic operations

9

Development of b-FeSi2 LEDs and related LEDs
Univ. of Surrey

Univ. of Tsukuba

8/40

RT-LED

Wavelength [mm]

1.7

1.6

1.5

RT
20A/cmFI

77K ELEL Intensity [arb.units]

MBE

1.4

1.3

2

1.2

300K

2

18A/cm2
16A/cm2
14A/cm

LED structure
Problem:
Improvement of injection efficiency

0.7

Dots -> multi-layers

Low limit of an InGaAs PMT

0.8

0.9

1.0

1.1

Photon Energy [eV]

After T. Suemasu et al. JJAP 39 (2000) L1013.

D. Leong et al. :NATURE 387 (1997) 686

Dislocation engineered LED (DELED)HAMAMATSU PHOTONICS

300K
b
Si
10
M. Lourenco et al.: Thin Solid Films 461 (2004) 219.

Fabrication of IBSD-b-FeSi2 photonic patterns on Si
b-FeSi2 PhC waveguides on Si**

Optical advantage
high index contrast for a Si matrix

Refractive index n

6.0

5.0

4.0

b-FeSi2 0.8eVhigh contrast materials for a SiO2 matrixnGe 0.66eV
Si 1.12eV

3.0

2.0
1.0

GaAs 1.42eVtransparent region
SiO2
1.0eV
photon energy (eV)A highindexsputtering
contrast:
difference in the index n>2.0*IBSD:
Ion-beam
deposition**NLD-RIE using a conventional SF6 gas

Y. Maeda: Appl. Surf. Sci., 254 (2008) 6242.

11Perspective of Active Silicides
towards Realization of Silicon Photonics and Spintronics
Active Silicides
SemiconductingbFeSi2
BaSi2
Mg2Si

B20-type
Electron correlation
matters

Ferromagnetic
Heusler alloys
Fe3-xTXSi
Co3-xT’xSi(T=Fe, Mn, Co, Ni)

FeSi-FeGe
FeSi-CoSi

Epitaxial Growth Technology
Si

Photonics
Si-Photonic
LED
PD
Ph-V Th-V

Ge

Magneto-photonics Spintronics
Photonic Crystal
Waveguides

SiGe

Electron correlation

Spin Polarized
electron injection

Interlayer magnetic
coupling devices

Magneto-optical Kerr modulator

12

Available epitaxial structure for Application of junctions
towards Si-photonics or Si-spintronics(Lattice mismatch <5%)
Film

Si

Substrate

Semiconductors

Ferromagnetics
Metal

-1.42
-3.8

0.09

B20FeSi-0.04-0,29*

DO3Fe3Si

-3.9

-3.5
-2.8

0.56

-4.2
-4.0

-FeSi2

FeSi

1.5
-4.0

Si
b-FeSi2

b-FeSi2Fe3Si

-FeSi2

4.06

-0.56

3.3
2.5

-3.4
3.9

-4.4
0.64
*FeSi(111)//b-FeSi2(100)13A variety of synthesis of b-FeSi2 and properties
Growth/Epitaxial

Carrier
controlHall Mobility(cm2/Vs)

Light
emission

Photo
voltaic

IBS

Epi/SiOR/Sin(Co),
p(Mn,Al)300 - 450

PL(<120K)

Heteron-Si

RDE

Epi/Sin,p550

PL(RT)
EL(RT)----------

MBE

Epi/Sin,p2~500

EL(RT)-----------

SPE

Epi/Sin~120----------------------MOCVD

Epi/Si
/YSZ

n(P),
p(B)

~450

PL(10K)

Hetero

IBSD

OR/Si

n,p

<100

PL(10K)----------

Sputter

OR/Si

n,p

0.5~300

EL(RT)

homo

PLD

OR/Si

n

------

PL(10K)-----------

VE1

OR/Si

n

100~500---------------------

Bulk2

needlen,p0.1-10----------------------

Bulk3

facet

n,p

~50

PL(10K)-----------

Method

OR: highly oriented growth, YSZ: Yttria stabilized Zirconia
1: Vacuum evaporation, 2: Chemical vapor transport, 3:Metal solution growth14

Ion Beam Synthesis (IBS) : b-FeSi2(A) High Dose Ion implantation
Mass Spectral Analyzer
50100keV

56Fe+

High purity synthesis

Impurity ions

Doping control
Fe

Si(001)(111)x x x x x x x x x

Implanted damage layer(B) Rapid Thermal Annealing >700oC
High power
IR lamp

Damage(SF, DL)
recovery

Silicidation
Fe+2Si=FeSi2
Si

Fe:
100 keV 1017 cm-2
Mn, Co:100 keV 1016 cm-2
Al:
50 keV 1016 cm-2

15

Structure and function controls of b-FeSi2
by IBS conditions
Enhancement of
Photoluminescence

Nanocrystals
embedded in Si

16

Luminescence spectra corresponding tomorphology of IBS b-FeSi2
Intrinsic A-band
56+
Doses of Fe ions
C A B
1.3x10 cm
5x10 cm
-2

a

16

-2

100K

b

b

Si
5x1015 cm-2cb微細粒

5x1014 cm-2d

PL Intensity (arb. unit)

17

x20

(a)

D
(b)

(c)

Si

(d)

0.70

Si

0.75

0.80

0.85

0.90

0.95

Photon Energy (eV)17

Y. Maeda et al. :Thin Solid Films 461 (2004) 160.

Electronic band structure of b-FeSi2
m*h=0.85 m0, m*e=0.80m0 at G

Indirect gap
c (//L)a (//S)b(//)

b-FeSi2

0.63 eV
0.64 eV

0.68 eVy

L

S

D. B. Migas and Leo Miglio: Phys. Rev. B62 (2000) 11063.

BZ (Rhombic base-centered)18

Optical band-edge and intrinsic PL at 1.54mm
Schematic figures

Direct band-gap

Indirect band-gap

10K0.140eV

Indirect abs.
Exind

Exdir
Egd
0.803eV

0.943eV

Luminescence is not related tothe direct band-gap and excitons.

0.803eV
Luminescence may come froman exciton process at the indirect
band-gap via phonon contribution.
19

ICSS-SILICIDE 2014 , Jul. 21, 2014
Tokyo University of Science Katsushika Campus

2
Light emission from b-FeSi2 crystals
and its fundamental scheme

20

First observation of PLs from b-FeSi2 films deposited on
Si substrates
free exciton related
emission

TG=900oC

TG=800oC

phonon replica
emission

TG=850oCNote
The luminescence at 0.84eV assigned
as emission from a free or bound exciton
was not observed in other samples.

C. A. Dimitriadis, H. H. Werner, S. Logothetidis and M. Stutzmann, J. Weber and R. Nesper:
J. Appl. Phys., 68 (1991), 1726- 1734.

21

Dislocation related PL spectra on Si(111)

V. V. Kvederet al., PRB, 51, 10520 (1995)
R. Sauer, J. Weber, and J. StolzAppl. Phys. A, 36, 1 (1985)22

1.55mm-Light emission from various samples of b-FeSi2
b-FeSi2
nanometer
sized
precipitates
formed by
Ion BeamSynthesis
(IBS)

50

PL excitedby a tungsten
lamp

~5meV

K. Akiyama:
Doctoral thesis
Tokyo Tech. 2003

( b-FeSi2/Si)
multilayer
formed by
MBE

10K

40

30

50meV
20

T. Sumemasu et al.
Univ. of Tsukuba(private communication)

10

K. Homewood et al.:Thin Solid Films
381 (2001) 188.

0
0.7

0.75

0.8

0.85

0.9

0.95

1

1.05

1.1

Photon Energy (eV)10K

0.802eV

0.800eV
10K

PL Intensity (a.u.)

Epitaxiallygrown
b-FeSi2 filmon Si(111)by MOCVD

0.802eV

PL Intensity (a.u.)

10K

30meV

SEM image ofdroplets~84meVb-FeSi2
droplets
formed byexcimerlaser
ablation
A. Narazaki et al.
AIST

0.7

0.8

0.9

Photon Energy (eV)23
A. Narazaki et al.: Appl. Phys. Lett. 82 (2003) 3078.

Defect-related Photoluminescences
Very confusing situation near 1.5 mm of wavelength in IBS samples

140

8000

10 K

Si (100keV, 5e15, 800deg. 8h)
Al (50keV, 5e15, 800deg. 4h)
Mn (100keV, 5e15, 800deg. 2h))O (100keV, 5e15, 800deg. 2h))Fe (b-FeSi2)
0.805eV200
X 35

x 40

160

6000

120

D1

100
80
60

120

PL intensity

PL Intensity (mV)160

PL spectra from impurity doped Si(100) sub.

PL(Si:TO)

180

Si
80

4000

O
Mn

40

Al

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

Photon Energy (eV)very weak luminescence fromthese impurity implanted Si(100)substrates.

Note The intensity of the intrinsic photoluminescence from b-FeSi2 ismuch larger than that of defect-related emissions.
Y. Maeda: Appl. Surf. Sci., 254 (2008) 6242.

24

1.2

PL spectra corresponding to morphology
Surface precipitates

40

30

10K
18K
30K
50K
60K
70K
80K
90K
100K
110K
120K
130K
140K
150K
160K

b/Si(100)
800℃/16H/VAC

Broad
Band
emission

20

A
200

PL Intensity (mV)

PL Intensity (mV)

50

Nanocrystals in Si

150

100

B

50

C

10K
20K
30K
50K
60K
70K
77K
90K
100K
110K
120K
130K
140K
150K
160K
170K

10

0
0.70

0.75

0.80

0.85

Photon Energy (eV)

0.90

0
0.70

0.75

0.80

0.85

Photon Energy (eV)

0.90

25

Inhomogeneous spectra of combination of
PL bands; A, B, C, and D
The peak separation was not unique.

apparent peak energy
800°
C/16H

PL Intensity (arb. unit)

PL Intensity (arb. unit)b/Si(100)

800°
C/16H

b/Si(111)

A
B

A

B
C

C

D
0.70

0.75

0.80

0.85

0.90

Photon Energy (eV)

0.95

0.70

0.75

0.80

0.85

0.90

0.95

Photon Energy (eV)Note The apparent spectrum is a usually inhomogeneous shape by changing
26
of combination of A, B, C and D emission bands.

Inhomogeneous spectrum due to IBS conditions
９４Ｋ

100keV
35

120

D

5x1014

30

100

140

5x1015

A

120

25

A

1016

100
80

B

20

80

60
15

60

C

B

40

10

40

A
5

0
12000

13000

14000

15000

16000

D

20

C
17000

18000

0
12000

13000

Wavelength (A)

14000

20

15000

16000

17000

18000

A
120

A

1017

B

20

100

C

14000

15000

25

60

50

13000

16000

17000

18000

17000

18000

Wavelength (A)140

5x1016012000

Wavelength (A)

80

70

C

B

A
100-80-30keV
1017

C

15

80

40
60

10

30
40
20

5

B

10

0
12000

20

13000

14000

15000

16000

Wavelength (A)

17000

18000

0
12000

13000

14000

15000

16000

Wavelength (A)

17000

18000

0
12000

13000

14000

15000

16000

Wavelength(A)27b-FeSi2 NCs/Si nano-composite phase
HRSEM image

b-FeSi2 NC

Size distribution of NCs
35

30

Exciton size predicted

Freaquency (%)

25

20

15

10

5

0
4

6

9

11

13 15

17 19 21

26 28 30 32 34 39 41 43

Diameter d (nm)Note The average size of nanocrystals (NCs) is 11 nm, in which the exciton
can be confined inside them. This situation brings high efficiency of radiative
recombination of exciton. We will discuss effect of the exciton confinement onradiative efficiency later.
Y. Maeda et al.: physica status solidi (c) 9 (2012) 1888.

28

Intrinsic luminescence from b-FeSi2 nanocrystals(NCs)
IBS b-FeSi
precipitates
b-FeSi2 NC/Si
nano-composite
phase
2
4000
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)Note The nano-composite phase consisting of high density of b-FeSi2 NCs embedded in the
Si matrix indicated the intrinsic PL . Their sizes were close to the Winner-Mott type exciton
29
size (~16 nm=2xaB) in b-FeSi2.

Empirical Varshni’s law for interband radiative
recombination(eV)
PL peak
Peakenergy
Energy (eV)

0.810

Ep(T) = Ep (0)  T /(b+T)(Varshni’s law)
2

A

Pure A-band
A+B

0.805
0.800

Ep(0) (eV) (x10-4)

0.795

IP

0.803

2.8

300

SP

0.8074

6.4

300

Si (ref.)

1.17

4.73

636

0.790
0.785
0.780

A+B mixed
bands

0.775
0.770
0

b

IBS β－FeSi2
20

40

60

80 100 120 140 160 180

Temperature (K)Temperature (K)Yang, et.al(absorption)

0.9046

5.6

600

Martinelli

0.80524.2

600

A: Intrinsic emission(PL)
A+B:Intrinsic & defect emissions

Y. Maeda et al. Thin Solid Films 461 (2004) 160.

Y. Maeda et al.: Thin Solid Films 461 (2004) 160.

30

Excitation power dependence of PL peak energy

Energy (eV)
Peak
(eV)
energyPL peak

0.807
0.806 (b) Intrinsic & defectrelated emissions
0.805

very small
scale

Burstein-Moss shift

6 meV

0.804
0.803
0.802
0.801
0.01

(a) Intrinsic emission fromb-FeSi2 (A-band)

0.1

1

10

Excitation
Powerdensity(W/cm(W/cm)
2)
Excitation
power
Note We can understand the difference between the intrinsic (A) and extrinsic or defectrelated band (B) luminescence by investigating the excitation power dependence. 31

Unique PL spectrum from b-FeSi2 bulk crystals
20 K

Crystal plateof b-FeSi2
77 K

H. Udono et al.: Thin Solid Films 461 (2004) 182.

H. Tatsuoka et al.:Thin Solid Films 515 (2007) 8268.

Unique PL spectra from the bulk crystal have been observed.
In this case, the PL cannot be affected by defects in Si substrates.
32
However, the present intensity is much smaller than those from thin film samples.
The reason for weak PLs may come from unavoidable and unintentional impurities.

ICSS-SILICIDE 2014 , Jul. 21, 2014
Tokyo University of Science Katsushika Campus

3
Photoluminescence properties of b-FeSi2/Si
nano-composite phases
In this section, we discuss light emission due to exciton
recombination related to material purity by using a simple
model. The material purity required for sufficient efficiencyof light emission will be roughly obtained.
33

Nano-composite phases
b-FeSi2 nanocrystals embedded in a Si matrix
b-FeSi2 nanocrystals

Si

The average size of the nanocrystal = ~10 nm
which depends on growth conditions.

34Band-gap formation in FeSi2
Note Deformation of g-FeSi2 lattice by Jahn-Teller effect⇒Band-gap formation between d bands：Semiconducting b-FeSi2
This situation also means presence of large interaction between electrons
and lattice in FeSi2.
d-band of Fe EF
: Lattice distortion

Metallic
-FeSi2
a=b=c=5.43A
Si a=5.4306A

Semiconducting
b-FeSi2The -phase easily makes
a coherent interface with Si.
This condition can be applied
to improvement of interface
quality between b and Si.
L. Miglio et al.: PRB 52 (1995) 1448.

35

Radiative recombination of
b-FeSi2 crystals
CBM
CBM atat L
L
Fe(I,II)p
Fe(I,II) p 22
dd 83
83
ss 11
Si(I,II)
Si(I,II) pp 99
dd 33
ss 22

Indirect excitonE
me*=1.0

Ex(1)=3 meV

Eg indphoton emissionh

0.803 eVphonon emission
Eph=31 meV0
VBM
VBM atat Y
YFe(I,II)
Fe(I,II) pp 28
28
dd 11
11
ss 22
Si(I,II)
Si(I,II) pp 29
29
dd 23
23
ss 77

exciton state

Egind=EPL(0.803eV)+Ex(1)(3meV)+Eph=0.834eV
m*h=0.25

Y

Absorption: Egind=Egx+Eex=0.834eV

Lk

Note In b-FeSi2 crystal, we can understand the radiative recombination of
exciton due to interband transition with LO phonon emission from the minimum
point on the L point on the conduction band to the Y point of the valence band.
36
Y. Maeda, Appl. Surf. Sci., 254 (2008) 6242.

Radiative and Non-radiative
recombination processes

37

Thermal resolution of excitons as a
non-radiative recombination
Non-radiative recombination centers (NRC)
Deep centers due to transition metal impurities
Atomic vacancy, defects etc.

NRC(e-h)ex

XaB*
Wannier-Mott exciton(e-) in CB + (h+) in VB+ heat

NT: Density of NRC<n>: Number of NRC in the volume of exciton
a*B: Effective Bohr radius in a semiconductor4 * 3n ( aB ) NT ,3

In case of <n>=1, we determine the critical radius (a*B)c by

4 * 3 4
*1
(aB ) c NT , (aB ) c  
NT 
3
 3


1/ 3
38

Calculation of effective Bohr radius in b-FeSi2

 (0)  29.9 ~ 32.5 : static dielectric constantm  0.2m0 : reduced mass of electron-hole pairaB  0.529   (0)*m0~ 8 nm

3 times larger than that in Sim
The emission efficiency η is proportional to the density of exciton
remaining <nex>after thermal non-radiative recombination

 ~ nex 

* 3
B c
*3

(a )
aB

  nT 

Note In higher purity or less defective crystals, (a*B)c is larger,
so that efficiency η becomes larger.
39

Possible conditions of crystal purity for excitonradiative recombination
30

100(aB*)c (nm)2520<nex>≦015<nex>≧1

10

5(%) 2 (at%)
of b-FeSi
Purity Purity

Non-radiative
recombination(NRC)

Exciton radiative
recombination (RR)

0
16
10

17

18

10

10

-3

N (cm )
T
Density of NRC traps

19

10

RR

5Ns

99.999<nex>≧1
99.998<nex>≦099.997NRC
99.99699.995
5

6

7

8

9

10

(aB*)c (nm)Note The exciton radius in b-FeSi2 with a large static dielectric
constant is so large that for exciton radiative recombination the purityof crystal should be higher than that of usual semiconductors withsmall static dielectric constants.
40

20

Radiative Efficiency and crystal purity

Radiative efficiency  ~ nex 

* 3
B c
*3

(a )
aB

  nT 

a*B =8nm
10Actual purity of
IBS b-FeSi2
crystals＜nT＞＝1
1
99.9990

99.9992

99.999499.9996

Crystal Purity (at%)

99.999841

PL spectrum and its temperature dependence
200keV- b-FeSi
Fe 1e172/ nanocrystals
Si(100) 800°C/2h/800°C/4h/Vac
in Si
56

+

p

PL Intensity (V)

0.8

0.6

0.4

0.2

0
12000

EPL(T)LD(641nm)
P=20.00mW
8K1
10K
30K
45K
50K
70K
90K
100K
110K
120K
130K

13000

Ａ

Note
The B and C bands
emissions should be
removed before
Analysis of the spectrum.

IPL(T)

G(T)
C
B
14000

15000

16000

Wavelength (Å)

17000

18000

42

Emission efficiency dominatedby thermal quenching and crystal quality
Time dependent density for excitons
Generation rate of
excitons
Life time of radiative recombination

Life time of non-radiative recombination (NR)

Activation energy into NR process

In a stationary state
Emission efficiency η(T)43

Internal quantum efficiency for light emission(Fundamental formula)
tr: life time for a radiative
recombination
tnr: life time for the non-radiativei-th process that is activatedthermally
Note Decrease of radiative efficiency
due to thermal activation to non-radiative
processes

44

Analysis of radiative efficiency
as a function of temperature1 (T ) 
1  C1 exp(  E1 / k BT )  C2 exp(  E2 / k BT ) 
E1  ET

ET: trap depth in energy

tr:life time of electrons or holes
NT: density of trap for a non-radiative process
: Cross section of the trap
m: effective mass of electron or hole
Thermal velocity of electrons or holes
1/ 2

 3k BT 
C1  t r N T  * 
 m 

 (T ) 1
3k BT 
1  t r N T  *  exp(  ET / k BT )  C2 exp(  E2 / k BT ) 


m




E2  Eex for free exciton
E2  Eex  Eb

1/ 2

for bound exciton
45

Actual case of analysis
10

1/ 2

 3k BT 
C1*  t r NT   * 
 m 

4

I

p

A

b-FeSi2 NCs in Si

10

10

3

C1*
ET (meV)138.19
43.13

C2

5.29

E2 (meV)
Imax

7.43
16624

2

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

-1

1/T (K )46

Some parameters required in
calculations(1) Life time of radiative recombination
tr=100ns
T<100K
Y. Terai et al.
JSAP Annual
meeting
40K(2)Trap for minority carriers

Effective mass （Γ）me*=1.0m0

20K

1
1 E D / k BTtr eBn Bn0

Cross section σ＝5.2×10-20 m2 in p-type b-FeSi2
Lefki et al.: J. Appl. Phys. 69(1)(1991)47352.

Calculation results of NT
b-FeSi2 NCs
Anneal conditions

NT(cm-3)
calculated

Purity（at％）
consent value

200keV 1e17 Fe
800℃/2ｈ NCs

9.08×1018

99.9657

200keV 1e17 Fe
800℃/6ｈ NCs

3.90×1018

99.9852

3.90×1018

99.9852

2.75×1018

99.9896

4.67×1019

99.8241

3.98×1019

99.8500

200keV 1e17 Fe
DTT：500℃/4ｈ＋800℃/6ｈ
NCs
200keV 1e17 Fe
DTT：500℃/8ｈ＋800℃/6ｈ
NCs
100keV 5e16 Fe
800℃4ｈSurface precipitates
IBS b-FeSi2**Katsumata et al.：J. Appl. Phys.
80 (1996) 1784.

48

How do we need purity of crystals
toward RT emission?
1

Consent purity
NT=Cimpurity

Quantum Efficiency

NT=1e19
1e18
1e17
1e16

m*

99.99996%

0.1

99.9996%0.01

99.9962%
99.962%
0.001
0

50

100

150

200

250

Temperature (K)

300

350

m0

(m2)

5.2x1020

ET (meV)

50.0

C2

10

E2 (meV)10

Note
At T>50K, a strong
Dependence ofthe purity on a quantum
efficiency of emission
is expected.
49

Emission efficiency vs purity of crystals
10K
0

10

Quantum Efficiency

50K

10K
30K

1

77K

-1

10

100K

100K

0.1

200K
10-2

200K
0.01

300K

10-3

10-4
99.98

300K

99.985

99.99

Purity (at%)

99.9950.001
100 99.9990

5Ns

99.9992

99.999499.9996

99.9998

100.0000

Purity (at%)

Note The purity is more than 5 nines 3, η≧1％ at RT can be expected.
50

Analysis of temperature dependence ofemission peak energy
We can get information of phonons relating to emission processes.
T 2
1. Varshni’s empirical law
EPL T   EPL 0 T b
α：electron-lattice interaction constant（eV/K), β：Debye temperature in K（approx.)

2. Multiple phonon coupling modelm j jj 1exp( j / T )  1

EPL T   EPL 0  

αj : coupling constant of lattice with the i-th phonon，kBj : average energy of the j-th phonon

3. Einstein model (Harmonic oscillator model)

 

 

EPL T   EPL 0  S  E ph coth E ph / 2k BT  1

S： Huang-Rhys factor，＜Eph＞: average energy of related phononsS   / 2kB   / 1.72  10451

Physical meaning of Haung-Rhys S factor
The value of S corresponds to the average number of emitted phonons
which should be required by the lattice relaxation from the point a at （q=0,
E=E0+） to the point b at （q=q0, E=E0）ab

Δ：Lattice relaxation energy
Ω：average energy of phonon
Non-radiative
process

Excitation

Ｓ：smaller⇒Δ：smaller
The smaller  means
Radiative processelectrons more weakly
bounded at localized centers.

Adiabatic potential curves of a ground state
and an excited state.
D. R. Vij ed.: Luminescence of Solids, (Plenum, 1998, New York) p.108.
52

Haung-Rhys factor andSpectrum shapenS S 
Poisson distribution I 0,n  e 
 n! , n  0, 1, 2, Peak position n=S-1/2

53
D. R. Vij ed.: Luminescence of Solids, (Plenum, 1998, New York) p.111.

Spectrum shape (1)In the case of small Ｓ＝0.84， ＜Eph＞＝47meV
Shallow DAP luminescence

1LO
2LO
3LO

Phonon side band

LO: Longitudinal optical phonon

Zero-phonon emission
54Spectrum shape (2)In the case of large S=~3, <Eph>=26 meV
Mirror relationshipAbsorption

Emission

55

Analysis by multi-phonon model2 j jj 1exp( j / T )  1

EPL T   EPL 0  

β-FeSi2nanocrystals

0.81

EPL(T) / eV

0.8

0.79

Zerophononemission

0.78

Phononside band
emission witha one phononemission process

0.77

0.76

T>100K
α１
Ｓ１

1.76x10-4 (eV/K)
1.02<Eph>1=kB1

27.7 (meV)

T<~50K
α2
Ｓ2

4.72x10-5 (eV/K)
0.275<Eph>2=kB2

4.27 (meV)

LO phonon：ELO=3１meV

0.75
0

50

100

150

200

Temperature / K

250

300

56

β-FeSi2: S factor reported
SamplesS<Eph>meV

Ref.

Ref.(2)(a) Optical absorption measurements
Single crystal

2.2 (id)
2.4 (d)

2.7
2.9

(1)

IBS
sample

2.15

34.5

(2)

Polycrystal
films

6.22

71.0

(3)
Ref.(3)(b) PL measurements
IBS
nanocrystals

0.97~
1.02

4~30

Un
published

IBS films or
precipitates

1.2~8

10~20

Un
published(1) H. Udono et al.: Thin Solid Films 461 (2004) 182.
(2) Z. Yang et al.: J Appl. Phys. 78 (1995)1945.
(3) C. Giannini et al.: Phys. Rev. B 45 (1992) 8822.

Crystal Si: S=1.49

57

A Debye-Waller factor
Note Computing the Debye-Waller factor, we can know anintensity ratio of the zero-phonon emission to the phononside band emissions. This ratio determines the spectrum
shape.

Planck function for phonon statistics

58
Graduate School of Energy Science, Kyoto University

Debye-Waller factor R0 vs S factor
1

1

100K
Debye-Waller factor

Debye-Waller factor

10K
0.8

0.6

0.4

0.8

0.6

<Eph>(meV)30, 40

0.4

10 20

0.2

0.2

5
0

0
0

0.5

1

1.5

S

2

2.5

0

0.5

1

1.5

2

2.5

S

Note As the S factor is larger, the R0 factor (contribution of the zerophonon emission) is smaller. As the average energy of phonon relating to
optical transitions is smaller, the R0 factor is smaller at the same S factor.
It suggests that the R0 factor can be regarded as an evaluation parameter of59
thermal quenching of the emission intensity.

Emission spectrum（Case of S near 1.0）
β-FeSi2 nanocrystals
1

Ratios of Zero phonon bands

100K
0.8

10K

S=0.275

X11

0.6

C

0.4

S=1.021
0.2

0
0

50

100

150

200

Temperature (K)

250

300

0.7

0.75

0.8

0.85

Photon Energy (eV)0.9Note Above 100 K, the electron-lattice interaction becomes larger
and the S factor also larger, so that contribution of phonon side bands
60
is larger to the radiative process.

Emission spectrum（Case of Large S）
PL spectrum from β-FeSi2 precipitates
100

Ratios of Zero phonon bands

Ratios of Zero phonon bands

1

10-1

S=0.912

10-6

0.8

0.6

0.4

100K

S=0.912

10K

10-2

10-3

X20

10-4

S=8.74
10-5

0

50

100

150

200

250

300

Temperature (K)0.2

S=8.74
0
0

50

100

150

200

Temperature (K)

250

300

0.7

0.75

0.8

0.85

Photon Energy (eV) 61

0.9

Emission spectra
corresponding to S factors
Note : Zero-phonon emission line＋Phonon side bands
n


S
S
I 0,n  e  , n! 

1n  0, 1, 2,
S=0.275
S=1
S=2
S=4
S=8

<Eph>=27.7meV

Efficiency
Emission efficiency

Radiative processes
with a large Huang-Rhys
factor S indicate

0.8

Zero-phonon emission
line

0.6

0.4

0.2

0
500

400

300

200

100

0

Energy(eV) E=E0-n<Eph>(1) Decease of zero-phononemission intensity(2) Red shift ofapparent emission peak(3) Decrease of
total emission intensity(4) Increase ofemission band widths
62

Analysis of spectrum shape1

S=1.1<Eph>=10meV

n=1

PL Intensity and I0n

0.8

0.6

n=2

The A band emission

0.4=Zero-phonon line
0.2

n=3
n=4

+phonon side bands
Zero-phonon line

0

0.76

0.78

0.8

0.82

at n=1,2,3,4…….

0.84

Photon Energy (eV)63Normalized PL Intensity (PL/mW)

Emission efficiency and S factor
103

Zero-phonon line (n=0)
102

I∝e-S

101

IBS b-FeSi2
nanocrystals

100

IBS precipitates
or polycrystalline films

10-1

Single crystals
10-2

10-3
0.1

at 10Kat 10K1

Huang-Rhys S factor

1064

ICSS-SILICIDE 2014 , Jul. 21, 2014
Tokyo University of Science Katsushika Campus

4Some example of enhancement of PL fromb-FeSi2/Si
In this section, we report some cases about
how enhancement of the PL intensity from b-FeSi2 crystals
has been realized.
Keywords to understand are
Improvement of defective heterointerface,
Si vacancy in the lattice,and Band-diagram at the heterointerface.
65PL enhancement of b-FeSi2 by long hours ofannealing Reduction of interfacial defects and PL enhancement
1.54mm (A-band)
24h

94 K

b-FeSi2
Si fault layer I

Si fault layer

PL Intensity (arb. units)

Dislocation loops

b-FeSi2

A

94K 2/Si(111)b-FeSiSi fault layer

16h

24 h

8h

b-FeSi2B

Si fault layer II

4h

C

D

Si fault layer

2ｈ

10nm

Annealing time: 2 hAnneal temperature=800oC

1200

1300

1400

1500

1600

Wavelength (nm)

1700

1800

16 h

Y. Maeda et al., Thin Solid Films 461 (2004) 160.

Note In many cases, we need post-annealing at high temperature after deposition in order toobtain high quality crystals. Long hours of annealing is effective to reduce interfacial defects
with nonradiative recombination centers and the efficiency of intrinsic PL can be improved.

66

Photoluminescence spectra affected by interfacialconditions
Films,
Staking fault(SF)Structures

Surface islands,
Dislocation +SFs

Nanocrystals withcoherent interface

PL Intensity

XTEM
images

x 1/6

0.7

0.8

0.9

0.7

0.8

0.9

0.7

0.8

0.9

Photon energy (eV)NoteA-band: intrinsic emission from b-FeSi2
B, C-bands: defect-related emission
D-band: Carbon-related emission at the surface.

67

Nanocrystals
Importance of coherent
Interfaces for luminescence Nano-composite phase
Coherent interface realized atb-FeSi2(220),(202)//Si(111)800°C/16H

Si(100), 200keV
Si(100), 100keV
Si(111), 100keV
Sputter b-FeSi2

200

PL intensity
PL Intensity (mV)(mV)

β-FeSi2b(220)

Si(111)150

Large size precipitates100

Si(111)b(220)

50

00.70

Lattice strains
stacking fault
dislocations
0.75

0.80

0.85

0.90

0.95

Photon Energy (eV)Photonenergy (eV)

SF

10nm
68

Orthorhombic b-FeSi2: Unit Cellc

Wavy structureFeaLayer structure

Si

b

c,b

a=9.8792A, b=7.7991A, c=7.8388A
Si a=5.4306A √2xa=7.6789A
Note The lattice structure shows different view on each plain. A formation energy of Si vacancy
is smaller than that of Fe vacancy, therefore, a large amount of Si vacancy forms in the lattice.

69

Al doped

Al doped

non doped
non doped

Al doping is very effective toenhance the PL intensity ofdefective samples.
Al atoms can diffuse inside of b-FeSi2 andoccupy Si vacancies, so that the vacancy
sites which are nonradiative recombination
centers can be reduced much effectively.
After thermal annealing
Al dopedb-FeSi2
nanocrystals
cases
Si vacancy
Nonradiative center

Al atom occupying thevacancy

Y. Terai and Y. Maeda, Appl. Phys. Lett. 84 (2004) 903.

Al or B doping is not effective for PL enhancement
because of less defective structures of nanocrystals.
Y. Terai and Y. Maeda, Optical Materials 27 (2005) 925.

70

TEM observation of IBS Al-doped b-FeSi2/Si photodiodes
Al doping into IBS b-FeSi2 can enhance epitaxial growth on Si
SF//<111>
004
002

000

400

222
440044220

Epitaxial relationships:

b-FeSi2(Al)b-FeSi2 (101), (110) // Si(111),
b-FeSi2 [010], [001] // Si[110].

SF
Si(111)
10nm

Si(001)SF

56Fe+ ion

Implantation region
001

DL

110
XTEM image (FEI Tecnai F20)

Si (001) sub.

100 nm

SF: Stacking Fault, DL: Dislocation Loop
after Y. Maeda et al.: Jpn. J. Appl. Phys. 44 (2005) 2502.

71

Improvement of surface morphology with manypin holes by doping Al

Epitaxial
interface

Growth of large grains

Solid phase epitaxy (SPE)also is enhanced by Al

Growth of fine grains withsmall pinholes

72

Photoluminescence from CVD b-FeSi2 films
highly oriented on Si substrates
β -FeSi2（100)//Si(100)CA

β -FeSi2（101)//Si(111)C

B
Annealingtime
アニール時間A
B
アニール時間DDafter K. Akiyama, PhD. Thesis (Tokyo Tech.)73

b-FeSi2 films grown on the Cu mediated surface can emit 1.54mm lightwithout any annealing at high temperature. This is the first PL observationfrom the as-deposited films. This shows importance of interfacial quality forluminescent processes.

intrinsic

K. Akiyama, Appl. Phys. Lett. 91 (2007) 71903.

Coherent interfaces between
b-FeSi2 grains and Si can be grown.
74PL enhancement by isolating b-FeSi2 layers
from implantation damage layers

too thinNonradiative recombination at
the interface can be eliminated.

Surface nonradiative
recombination affects
a PL efficiency.

The pre-annealing at 600oC is very effective to cause surface segregation of Fe
atoms implanted deeply into Si substrates and b-FeSi2 precipitates at the surface
where is isolated from implantation damage layers. The best case of preannealing for PL enhancement was 4 h.
Y. Ando et al., Thin Solid Films 515 (2007) 75
8133.

Radiative and nonradiative recombinations in IBS samples

b-FeSi2
Si defective layer

The usual IBS samples with incoherent interfaces and dislocation loops in initial damage
layers show inhomogeneous PL spectra, where the A band PL has some defect-related
bands around it. The A band PL can be increased by reducing nonradiative processes atthe surface and the incoherent interface. This situation has confused correct understanding 76
of nature of the intrinsic luminescence.

Dislocation free PL spectrum in IBS
samples( Y. Gao et al.： Appl. Phys. Lett. 83 (2003) 42.)
PL spectra obtained from IBS samples with nodislocation loops which result in D-line emissions.
d
c
a

c
a

b

d

Implantation temp. (a)350 oC ⇒ (d)RTProf. Wang’s group found the IBS process where dislocation loops (DL) did not form in Si
substrates. They observed the intrinsic PL (red line) from b-FeSi2 precipitates under muchless influence of DLs. It is easy to see a complicated situation where both PLs overlap each77
other at the same energy region.

Introduction(100)

Semiconducting b-FeSi2
IR light emission at the telecom
wavelength, 1.55 mm
Large Optical absorption >105 cm-1
Epitaxial growth on Si substrates(010)

Optimal for
applications to
infrared
optoelectronics
and photonics

b-FeSi2 nanocrystals (NCs) for IR light emitting*

IBS b-FeSi precipitates
24000
0.805eV

3500

3000

PL Intensity

Exciton size

intrinsic
emission(A band)

Strong intrinsic light emission
observed from the exciton sizedb-NCs embedded in Si.

R=14nm

2500

R=7nm

100nm

2000
FWHM=18.9meV

1500

Si(111)

1000

500

3 nm
0
0.7

0.75

0.8

0.85

0.9

Photon Energy (eV)b-FeSi2(220)The heterointerface was partiallycoherent and included some
defective structures because oflattice mismatch with Si. These
defects may control the efficiencyof light emission.

0.95*Y. Maeda: Appl. Surf. Sci. 254 (2008) 6242．

PL enhancement observed in Cu-doped b-NCs/Si
nano-composite phase
0.8

PL Intensity (V)

0.7

8K
Pex=10mW

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

0.6(b)non-doped b-NCs
0.5
0.4(c)non-doped b-NCs

0.3

after 2h annealing

X

C band

0.2

X

0.1
0
0.7

slow

after 8h annealing

0.75

0.8

0.85

fast

0.9

Photon Energy (eV)Fig.1. PL spectra of non-doped and Cu-doped
Note Dynamics
of positive holes going out from b-NCs before radiative
b-FeSi nanocrystals (NCs) samples.
recombination dominates light emission efficiency. Cu in Si forms shallow
traps of holes, so that transport rate of holes should be controlled by therepeated trap process. This dynamics may contribute to the PL enhancement.
2

Y. Maeda et al., Phys. Stat, Sol. (c) (2014) (in press). (E-MRS Proc. 2014 Spring)79Note The -b phase transition can be applied
to enhancement of PL intensity.

80Why we focus on the -FeSi2?

b-FeSi2 nanocrystal (NC) formation by direct silicidation
ion implanted Fe + Si substrate->Silicidation Fe+2Si=FeSi2->growth of b-NC
Light emission efficiency is controlled by interface defects
Crystallographic relation between -FeSi2
and b-FeSi2
-b phase transition
metastable
CaF2 structure
cF12
a=0.5431nm~Si

above 600oC

Cmca
a=0.9865nm
b=0.7791nm
c=0.7833nm*G. Shao, This Solid Films 519 (2011) 8446.

After the transition, the following plane relationship can be maintained.
b(202),(220)//(111)//Si(111)
Our idea, Toward enhancement of the light emission efficiency,
we employ the phase transition to improve the interface between b-NC and Si
In this study, we have investigated photoluminescence properties of theb-NCs which are formed by controlling the phase transition

Fe channel RBS spectrum

Experiments

1600

b-NCs preparations
Backscattering Yield

High purity crystal growth by
Ion beam synthesis (IBS)

Ion implantation
Energy: 200keV, Dose : 1017ions/cm-2
Substrate: CZ-Si(100)Post anneal
Double annealing process by RTA(Rapid Thermal Annealing) in a vacuum.

1200

1000

Rp=162nm

200keV56Fe+
1017ions/cm2

Surface

800

600

Rp+Rp=209nm

400

200

0
380

390

400

410

420

430

440

450

Channel Number

Profile of double annealing

Temperature

Measurements
Structural observations
SEM, RBS
Optical measurements
Infrared photoluminescence (PL)detected by LN2 cooled Ge PIN PDs
Infrared reflection measurements

Si(100)1400

-phaseformation
preannealing

postannealing
at 800oC

b-NCs
formation

at 400-500oC
Coolingin a vacuum
Rapid heating
RTtime (h)460

Toward enhancement of PL
by controlling growth of b-FeSi2 on Si(111)Thin films or
Precipitatesb
Si(100) substrateb(100)//Si(001)d=1.5~2.1%

b-NCsbSi(111)First step
Precipitation of -phasewith epitaxy on Si(111) below 500oC.

Deep implantation(E~200keV)

Si(111)

Second step
->b phase transition withepitaxy at 800oC
b(220),(202)//Si(111)

Thermal anneal
Temp.<600-800oC

Temp.>750oCb

PL properties
Weak & broad peaks
with defect related
emissions

(111)//Si(111)b(101),(110)//Si(111)d=1.45~2.0%

IBS conditions
Shallow implantation(E<100keV)

New steps process

Si(111)

Strong & sharp peak(Intrinsic A band)

Third step
very coherent
and least
defective
interfaces*Theoretically, less defective interfaces were realized onb(202),(220)//Si(111).

Structures , Sizes, Phonon properties of b-NCs in Si
Single annealing at 800oC

Phonon abs. of b-NC

30
25

13 nm

(a)

20
15
10
5

Absorbance (arb. units)

Freaquency (%)

35

0
4 6 9 11 13 15 17 19 21 26 28 30 32 34 39 41 43

Double anneling at 400oC and 800oC

500

450

400

350

300

250

300

250

-1

Diameter d (nm)

Wavenumber (cm )

30

11 nm

25
20
15
10
5
0

4 6 9 11 13 15 17 19 21 26 28 30 32 34 39 41 43

Diameter d (nm)Absorbance (arb. units)

Freaquency (%)

35

500

450

400

350

-1

Wavenumber (cm )We confirmed that morphology of b-NCs formed in Si and their phonon properties
showed no large differences in both annealing cases, except for size decrease.

Photoluminescence spectra of b-NCs embedded in Si
4.0

10000

0.803eV b-NCs by
double annealing

PL Intensity

6000

3.5

8h
0.5h
4h

8000

Intensity
enhancedby 360%4000

b-NCs bysingle annealing
at 800oC

2000

Optical Gains

8K

3.0

Increase of
postannealing timefrom 2h to 6h

2.5
2.0

Singleannealing

1.5
1.0

0
0.70

0.75

0.80

0.85

0.90

0.95

Photon Energy (eV)Comparison of PL intensities between b-NCs
formed by single annealing and by double
annealing. We observed 360% enhancement of
PL peak intensity in the maximum.

0.5

0

2

4

6

8

10

Preannealing time (h)Enhancement of PL intensity (optical gain)
depended on the time of postannealing at
800oC, where the phase transition to bNCs was enhanced.

We confirmed that bNCs formed by the phase transition have higher efficiencyof light emission than b-NCs directly formed by silicidation.

Discussion
Possible reasons of the PL enhancement,(1) Change of phonon states adapting a radiative indirect transition?
Answer NO!
because of no change in phonon states.
(2) Realization of coherent interface between the b-NCs and Si?
Answer. Possible! because of epitaxy among b-, -phases andSi can be maintained even after the ->b phase transition.

Coherent interface

Partial coherent interface
Si(111)

b-NC

b-NC
3 nm

Si

Si

b-NC
b-NC

b-FeSi2(220)Si(111)electrons

R: Radiative
recombination

EcEX
NR: Nonradiative
recombination

R’

R

EX

NR

prevented
R’

R

NR’<<NREv
holes

PL controlled by NR process

PL enhancement

Carbon doping increases a binding energy of excitons,
so that the A band PL an be enhanced.
87

ICSS-SILICIDE 2014 , Jul. 21, 2014
Tokyo University of Science Katsushika Campus

5
Future study: enhancement of PL
In the final section, we report trial study of PL enhancement
in order to reduce a thermal quenching effect which has
been observed in b/Si heterostuructures.
This pronounced thermal quenching comes from situation ofband offsets at the heterojunction.

88

b-FeSi2Si

Ecelectrons

Radiative
Recombinationof electrons and holes

Ev

Ec

Non-radiative
Recombination

Ev
holes going out
from b-FeSi2 bythermal activation

Band diagram in b-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 band and holes at the valence band and non-radiative processes due to escape of holes fromb-FeSi2 to Si. The RRP rate may be controlled by NRRP due to escape of holes.
89Band offset problem at b-FeSi2/Si heterojunction

Thermally activatednear RT

Note The valence band offset Ev at b-FeSi2/Si heterojunction is small (~20meV),so that holes in b-FeSi2 can be thermally activated into Si sides before radiative
recombination near room temperature. Toward RT light emission, we need aheterojunction with large band offests like a b/SiO2 heterojunction.

90

PL properties of b/SiO2 heterostructures

PL Intensity

250 K

Pex=40 mW

260 K
270 K

0.7

0.75

0.8

0.85

0.9

Photon Energy (eV)PL spectra from the oxidized non-composite phases near room temperature. The oxidation of bNCs/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 b-NCs/SiO2 nano-composite phaseforms. SiO2 has a wide band-gap and makes large offset energies for both conduction and valence bands
with b-FeSi2. So we can expect sufficient confinement of electron-hole pairs at the b-NCs in theoxidized nano-composite phase. This situation may contribute to observable PL spectra near room
temperature.
91

Analysis of reflection interferences
50
45
40

phonon abs

-FeSi2formation
IR

35
30

+Si
(n=7.2)25

Volume fractions of each phase can beobtained from the change of refractive index.

c-Si

1.2
20

7000

6000

5000

4000

3000

2000

1000

After phase transition
40 to b-NCs
45

phonon abs

IR

35
30

b+Si
(n=5.8)25
20(=~25%)

1

50

Volume fraction

Reflectance (%)

Changes of refractive index due to the ->bphase transition can be calculated byanalyzing the reflection interferences.

0.6

6000

5000

4000

Wavenumber (cm-1)

2000

-FeSi2

0.2
0

c-Si
3000

->b phase transition

0.4

-0.2
7000

b-NC

0.8

1000

0

2

4

6

8

10

12

Annealing time (h) at 800oC

After postannealing at 800oC, in the analysis errors, whole of the phase formed by preannealing have transformed into b-NCs.