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November 2004
Beta Iron Disilicide (b-FeSi2)
As an Environmentally Friendly Semiconductor
for Space Use
1Yunosuke
MAKITA, 1Zhengxin LIU, 1Teruhisa OOTSUKA, 1Naotaka OTOGAWA,
1Masato OSAMURA, 1Yasuhiro FUKUZAWA, 2Ryo KURODA, 1Yasuhiko
NAKAYAMA, 2Yasuyuki HOSHINO, 3Hisao TANOUE
1.Kankyo Semiconductors Co., Ltd.
2.Nippon Institute of Technology
3.National Institute of Advanced Industrial Science and Technology
NI-AIST Central-2, Umezono 1-1-1, Tsukuba, Ibaraki, 305-8568 Japan
Abundance of chemical elements in the earth’s crust
b-FeSi2
Log C (C=mole/ton Animal Liver Tissue)
Contents of chemical elements in
animal liver tissue and seawater
Log C (C=mole/ton Seawater)
Comparison of b-FeSi2 with Si and GaAs as a
photovoltaic semiconductor for space use
b-FeSi2
c-Si
GaAs
Band-gap (eV)
0.85
1.11
1.43
Optical absorption coefficient (cm-1)
>105
103-104
104
Theoretical Conversion Efficiency (%)
16-23
24
25
Thickness required for solar cell (mm)
1
300
10
Specific gravity (g/cm3)
4.93
2.33
5.33
Payload (relative value)
1
1/150
1/5
Resistance against the exposition of
cosmic rays & radiation
High
Low
Low
Thermal stability (oC)
937
1000
500
Electron
17,000 at 77K
1350
5-8,000
Hole
3,800 at 77K
480
300
Mobility (cm2/Vs)
(at 300K)
(1016cm-3)
(1016cm-3)
Properties and possible applications of b-FeSi2
Fe5Si3
Fe3Si
Fe2Si
Fe-Si compounds
Metallic a-Fe2Si5
e-FeSi
Semiconductor
b-FeSi2
・Direct band gap: Eg = 0.85 eV
Light emitting diode (LED)
Photosensor for quartz fiber communication (1.5mm)
・Optical absorption coefficient: > 105 cm-1
Thin film solar cell
・Thermoelectric power >10-4 K-1
Thermoelectric generator
・ No-toxicity and abundance of the constituent chemical elements (Fe, Si)
Environmentally friendly semiconductor
Bright future of b-FeSi2
Crystal structure of b-FeSi2
Possible epitaxial growth on Si
b-FeSi2(100)
Si(100)
c
b
a
5.43Å
Si(001)
b (or c)
Si(111)
a
Si(111)
b
c
Crystal structure: orthorhombic (Cmca)
a=9.86Å, b=7.79Å, c=7.83Å
b-FeSi2(101)/(110)
Electronic density distribution map of b-FeSi2 measured by
4-axis X-ray diffractometer and calculated by MEM
Si
Fe
Electron empty space
Owing to a large volume of electron empty space , b-FeSi2 has high
resistance against the exposition of cosmic rays and radiation.
Fe-Si phase diagram
1700
Temperature (oC)
1500
Metallic
a-Fe2Si5
1410oC
1414oC
1207oC
1212oC
1300
α+ε
1100
α+Si
982oC
900
937oC
ε
b+Si
b+ε
700
Semiconductor
b-FeSi2
500
40
Fe
50
60
Possibility of transforming
semiconductor b-FeSi2 into
metallic a-Fe2Si5 by laser
heating
70
80
Si/Fe Ratio (%)
90
100
Si
Metallic a-Fe2Si5 can be
used as a depositionand step-free electrode
for b-FeSi2 devices.
Advantages of b-FeSi2 as a photovoltaic
semiconductor for space use
1. A large volume of electron empty
space.
Small electronic density cross-sectional area,
High resistance against the exposure of
cosmic rays and radiation.
2. High optical absorption coefficient
(>1105cm-1).
Thin film solar cell (thinner than 1 mm),
Elevation of payload.
3. Semiconductor b-FeSi2 to metallic
a-Fe2Si5 phase transformation by
laser heating.
Use of metallic a-Fe2Si5 as a deposition- and
step-free electrode,
Improvement of mechanical strength,
High reliability at elevated temperatures,
Elevation of payload.
4. Growth on stainless steel substrate.
High resistance against cosmic rays and
radiation, Elimination of thick Si substrates,
Elevation of payload.
XRD measurements for b-FeSi2 films grown on Si(111) substrates
Pole figure of (202)/(220) peak
b-FeSi2(440)/(404)
Si(222)
b-FeSi2(220)/(202)
Intensity (a. u.)
Si(111)
XRD spectrum
20
30
50
40
2/ (degree)
60
Epitaxial relationship
b (101)/(110)
70
b (010)/(001)
Si(111)
b-FeSi2(110) or (101)//Si(111)
Si(110)
TEM images of b-FeSi2 films grown on Si(111) substrates
Cross-sectional TEM image and
diffraction patterns
High resolution TEM image
Si(111)
b(200)
b(220)
-Si(111)
b(020)
Si(002)
b -FeSi
b-FeSi
2 2
Si
Si
Si(111)
b(110)/(101)
0.94nm
Interface
Impurity doping technologies for b-FeSi2 bulks and thin films
IVa
Va
VIa
VIIa
VIII
VIII
VIII
Ib
IIb
IIIb
IVb
Vb
p-type
B
C
N
n-type
Al
Si
P
Ti
V
Cr
Mn
Fe
Co
Ni
Cu
Zn
Ga
Ge
As
Zr
Nb Mo
Tc
Ru
Rh
Pd
Ag
Cd
In
Sn
Sb
Hf
Ta
Re
Os
Ir
Pt
Au
Hg
Tl
Pb
Bi
W
Dopants used in thermoelectric devices
・ Substitution at two Fe sites
・ Doping efficiency is very low: ~ several atm %
・ Formation of undesired silicides: MnSi1.7, CoSi2,
CrSi2, NiSi2 etc.
・ High residual carrier concentrations: ~ 1020 cm-3
Low mobilities: < 10 cm2/Vs
Possible dopants on Si site
・ Substitution at Si sites.
・ Established doping technologies for Si
device manufacturing can be used.
・ Expecting high doping efficiencies with
low carrier concentrations and high Hall
mobilities
Boron-doping for p-type b-FeSi2 films
100
-3
Net Hole Concentration(cm )
120
19
2
Hall Mobility (cm /Vs)
10
10
18
80
60
40
20
10
0
17
0.0
0.5
1.0
1.5 2.0 2.5 3.0
B/Si Area Ratio (%)
3.5
4.0
0.0
0.5
1.0
1.5 2.0 2.5 3.0
B/Si Area Ratio (%)
3.5
4.0
120
80
2
Hall Mobility (cm /Vs)
100
Effective doping of boron
atoms for p-type b-FeSi2 films
60
40
20
0
17
10
18
19
10
10
-3
Net Hole Concentration (cm )
8x10
17
6x10
17
4x10
17
2x10
17
300
Non-doped
2
Hall Mobility (cm /Vs)
-3
Net Electron Concentration(cm )
Arsenic-doping for n-type b-FeSi2 films
Non-doped
1x10
250
200
17
150
0.0
2.0
4.0
6.0
8.0
Area Ratio of As-doped Si Chips (%)
0.0
2.0
4.0
6.0
8.0
Area Ratio of As-doped Si Chips (%)
300
2
Hall Mobility (cm /Vs)
Non-doped
250
Effective doping of arsenic atoms
for n-type b-FeSi2 films
200
150
1x10
17
2x10
17
4x10
17
Net Electron Concentration (cm )
-3
6x10
17
Phase-transformation from b-FeSi2 to a-Fe2Si5 by laser heating
Process image
(110) (111)
Laser light
--(515)
Metallic a-Fe2Si5
(130)
(001)
b-FeSi2
a-Fe2Si5
- ‐
(425)
b-FeSi2
n-Si
Surface image
a-Fe2Si5
Metal a-Fe2Si5 electrode
b-FeSi2
200nm
100nm
Si
b-FeSi2
Locally phase-transformed a-Fe2Si5 can be used as a delineated metal contact
Phase-transformed a-Fe2Si5 used as an electrode for
b-FeSi2 devices
Ohmic contact between
b-FeSi2 & a‐Fe2Si5
I-V measurement for a p-b-FeSi2/n-Si
heterojunction device
a-Fe2Si5
0.015
0.14
0.01
1mm
1mm
p-b-FeSi2
5mm
0.005
p-b-FeSi2
0.1
A
A
5mm
0.08
n-Si
I (mA)
I (mA)
a-Fe2Si5
0.12
0
n-Si
0.06
0.04
-0.005
0.02
-0.01
0
-0.015
-0.02
-1
-0.5
0
V (V)
0.5
1
-2
-1.5
-1
-0.5
0
0.5
1
1.5
V (V)
Metallic a-Fe2Si5 can be used as an electrode for b-FeSi2 devices
2
n-b-FeSi2/p-Si heterojunction solar cell
Cell structure
I-V Curve under sun light
16
2
Current Density (mA/cm )
AM1.5, 100 mW/cm
Light
2
Electrode
n-b-FeSi2 (0.3mm)
Ａ
12
p-Si
=3.7%
Back electrode
8
Area: 4x4 mm2
Voc=0.45 V
4
0
0.0
Jsc=14.84 mA/cm
2
FF=0.551
=3.7%
0.1
0.2
0.3
Voltage (V)
0.4
0.5
20
XRD spectrum
30
40
70
Fe
Fe3Si
Fe
Fe3Si
b(422)
50
60
2 (degree)
80
90
Raman spectrum
Intensity (arb. u.)
b-FeSi2
Fe
b(312)
Fe3Si
b(220)/(202)
SEM surface image
Fe3Si
Intensity (arb. u.)
Fe3Si
Formation of b-FeSi2 films on stainless steel substrates
Semiconductor b-FeSi2 thin films
grow on stainless steel substrates
100
200
300
400
-1
Raman Shift (cm )
500
b-FeSi2 solar cells under development
a-Fe2Si5 electrode
Wide-gap semiconductors
(e. x., ZnO, CuAlO2, etc.)
Metal electrode
n-b-FeSi2
p-b-FeSi2
Stainless steel
Stainless steel
p-b-FeSi2
Buffer layer
Structure 1
Structure 2
Summary
b-FeSi2 as a semiconductor for space-use solar cell
1. A large volume of electron empty
space.
Small electronic density cross-sectional area,
High resistance against the exposure of
cosmic rays and radiation.
2. High optical absorption coefficient
(>1105cm-1).
Thin film solar cell (thinner than 1 mm),
Elevation of payload.
3. Semiconductor b-FeSi2 to metallic
a-Fe2Si5 phase transformation by
laser heating.
Use of metallic a-Fe2Si5 as a deposition- and
step-free electrode,
Improvement of mechanical strength,
High reliability at elevated temperatures,
Elevation of payload.
4. Growth on stainless steel substrate.
High resistance against cosmic rays and
radiation, Elimination of thick Si substrates,
Elevation of payload.
Bright future of b-FeSi2