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November 2004
Beta Iron Disilicide (-FeSi2)
As an Environmentally Friendly Semiconductor
for Space Use
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
1Yunosuke MAKITA, 1Zhengxin LIU, 1Teruhisa OOTSUKA, 1Naotaka OTOGAWA, 1Masato OSAMURA, 1Yasuhiro FUKUZAWA, 2Ryo KURODA,
1Yasuhiko NAKAYAMA, 2Yasuyuki HOSHINO, 3Hisao TANOUE
Abundance of chemical elements in the earth’s crust
-FeSi2
Log C (C=mole/ton Seawater)
Log
C (
C=
mol
e/to
n A
nim
al L
iver
Tis
sue)
Contents of chemical elements in animal liver tissue and seawater
Comparison of -FeSi2 with Si and GaAs as a photovoltaic semiconductor for space use
-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 (m) 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
Mobility (cm2/Vs)
(at 300K)
Electron 17,000 at 77K(1016cm-3)
1350 5-8,000
Hole 3,800 at 77K(1016cm-3)
480 300
-FeSi
Fe5Si3 Fe2Si
Fe-Si compounds
Fe3Si
Semiconductor-FeSi2
Properties and possible applications of -FeSi2
Light emitting diode (LED)Photosensor for quartz fiber communication (1.5m)
・ Direct band gap: Eg = 0.85 eV
・ Optical absorption coefficient: > 105 cm-1
・ Thermoelectric power >10-4 K-1
・ No-toxicity and abundance of the constituent chemical elements (Fe, Si)
Thin film solar cellThermoelectric generator
Environmentally friendly semiconductor
Metallic -Fe2Si5
Bright future of -FeSi2
a
b c
Si(100)
Si(001)
5.43Å
-FeSi2(100)
b
c
2.0%
1.4%
Si(111)
7.68
Å
a
b (or c)1.4%(2.0) 5.3%
-FeSi2(101)/(110)
Si(111)
Crystal structure of -FeSi2 Possible epitaxial growth on Si
Crystal structure: orthorhombic (Cmca)a=9.86Å, b=7.79Å, c=7.83Å
Fe
Si
Electron empty space
Electronic density distribution map of -FeSi2 measured by 4-axis X-ray diffractometer and calculated by MEM
Owing to a large volume of electron empty space , -FeSi2 has high resistance against the exposition of cosmic rays and radiation.
40 50 60 70 80 90 100
1700T
emp
erat
ure
(o C
)
Metallic -Fe2Si5
Semiconductor -FeSi2
++εε+Si+Si
αα++SiSiαα++εε
εε
12121212ooCC
982982ooCC
14141414ooCC
12071207ooCC
937937ooCC
14101410ooCC1500
1300
1100
900
700
500
Fe Si/Fe Ratio (%) Si
Fe-Si phase diagram
Possibility of transforming semiconductor -FeSi2 into metallic -Fe2Si5 by laser heating
Metallic -Fe2Si5 can be used as a deposition- and step-free electrode for -FeSi2 devices.
2. High optical absorption coefficient
(>1105cm-1).
1. A large volume of electron empty
space.
3. Semiconductor -FeSi2 to metallic
-Fe2Si5 phase transformation by
laser heating.
4. Growth on stainless steel substrate.
Advantages of -FeSi2 as a photovoltaic semiconductor for space use
Small electronic density cross-sectional area, High resistance against the exposure of cosmic rays and radiation.
Thin film solar cell (thinner than 1 m), Elevation of payload.
Use of metallic -Fe2Si5 as a deposition- and
step-free electrode, Improvement of mechanical strength, High reliability at elevated temperatures,Elevation of payload.High resistance against cosmic rays and radiation, Elimination of thick Si substrates,Elevation of payload.
Pole figure of (202)/(220) peakPole figure of (202)/(220) peak
(010)/(001)
(101)/(110)
Si(110)
Si(111)
-FeSi2(110) or (101)//Si(111)
Epitaxial relationshipEpitaxial relationship
XRD spectrumXRD spectrum
20 30 40 50 60 70
-F
eSi 2(2
20)/
(202
)
-F
eSi 2(4
40)/
(404
)
Si(2
22)
Si(1
11)
Inte
nsit
y (a
. u.)
2/ (degree)
XRD measurements for -FeSi2 films grown on Si(111) substrates
-FeSi2
Si
Cross-sectional TEM image and diffraction patterns
Cross-sectional TEM image and diffraction patterns
(220)
(020)
(200)
Si(002)-
Si(111)- -
Si(111)-
Si
-FeSi2
TEM images of -FeSi2 films grown on Si(111) substrates
Si(111)(110)/(101)
0.94nm
High resolution TEM imageHigh resolution TEM image
Interface
IVa Va VIa VIIa VIII VIII VIII Ib IIb IIIb IVb Vb
B C N
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 W Re Os Ir Pt Au Hg Tl Pb Bi
p-type
n-type
・ 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
Dopants used in thermoelectric devices 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
Impurity doping technologies for -FeSi2 bulks and thin films
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.01017
1018
1019
Net
Hol
e C
once
ntr
atio
n (
cm-3
)
B/Si Area Ratio (%)0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
0
20
40
60
80
100
120
B/Si Area Ratio (%)
Hal
l Mob
ilit
y (c
m2 /V
s)
Boron-doping for p-type -FeSi2 films
1017 1018 10190
20
40
60
80
100
120
Hal
l Mob
ilit
y (c
m2 /V
s)
Net Hole Concentration (cm-3)
Effective doping of boron atoms for p-type -FeSi2 films
1x1017 2x1017 4x1017 6x1017
150
200
250
300
Non-doped
Hal
l Mob
ilit
y (c
m2 /V
s)
Net Electron Concentration (cm-3)
Arsenic-doping for n-type -FeSi2 films
Effective doping of arsenic atoms for n-type -FeSi2 films
0.0 2.0 4.0 6.0 8.0
1x1017
2x1017
4x1017
6x1017
8x1017N
et E
lect
ron
Con
cen
trat
ion
(cm
-3)
Area Ratio of As-doped Si Chips (%)
Non-doped
0.0 2.0 4.0 6.0 8.0150
200
250
300
Hal
l Mob
ilit
y (c
m2 /V
s)
Area Ratio of As-doped Si Chips (%)
Non-doped
Si
200nm100nm
n-Si
FeSi2
Laser light
(110)
(001)
(111)
-Fe2Si5
-Fe2Si5
-FeSi2
(130)
(515) - - - (425) - ‐
-FeSi2
Metallic -Fe2Si5
Process imageProcess imageProcess imageProcess image
-FeSi2
Metal -Fe2Si5 electrode
Surface imageSurface imageSurface imageSurface image
Phase-transformation from -FeSi2 to -Fe2Si5 by laser heating
Locally phase-transformed -Fe2Si5 can be used as a delineated metal contact
-0.02
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
-2 -1.5 -1 -0.5 0 0.5 1 1.5 2
I (m
A)
V (V)
-0.015
-0.01
-0.005
0
0.005
0.01
0.015
-1 -0.5 0 0.5 1
I (m
A)
V (V)
Ohmic contact between -FeSi2 & ‐Fe2Si5
Ohmic contact between -FeSi2 & ‐Fe2Si5
Phase-transformed -Fe2Si5 used as an electrode for -FeSi2 devices
Metallic-Fe2Si5 can be used as an electrode for -FeSi2 devices
1mm
5mm
n-Si
A
-Fe2Si5
p--FeSi2
I-V measurement for a p--FeSi2/n-Si heterojunction device
I-V measurement for a p--FeSi2/n-Si heterojunction device
A
p--FeSi2
-Fe2Si5
1mm
5mm
n-Si
0.0 0.1 0.2 0.3 0.4 0.50
4
8
12
16AM1.5, 100 mW/cm2
=3.7%
Voc
=0.45 V
Jsc=14.84 mA/cm2
FF=0.551=3.7%C
urr
ent
Den
sity
(m
A/c
m2 )
Voltage (V)
I-V Curve under sun lightI-V Curve under sun light
p-Si
n--FeSi2 (0.3m)Electrode
Light
A
Area: 4x4 mm2
Back electrode
Cell structureCell structure
n--FeSi2/p-Si heterojunction solar cell
Semiconductor -FeSi2 thin films grow on stainless steel substrates
20 30 40 50 60 70 80 90
Fe
Fe F
e
Fe 3S
i
Fe 3S
i
Fe 3S
i
Fe 3S
i
Fe 3S
i
(4
22
)
(3
12
)(2
20
)/(2
02
)
Inte
nsi
ty (
arb
. u
.)
2 (degree)
XRD spectrumXRD spectrum
100 200 300 400 500
-FeSi2
Inte
nsi
ty (
arb
. u
.)
Raman Shift (cm-1)
Raman spectrumRaman spectrum
SEM surface imageSEM surface image
Formation of -FeSi2 films on stainless steel substrates
Stainless steel
Buffer layer
n--FeSi2
-Fe2Si5 electrode
p--FeSi2
Structure 1
Wide-gap semiconductors (e. x., ZnO, CuAlO2, etc.)
Stainless steel p--FeSi2
Metal electrode
Structure 2
-FeSi2 solar cells under development
2. High optical absorption coefficient
(>1105cm-1).
1. A large volume of electron empty
space.
3. Semiconductor -FeSi2 to metallic
-Fe2Si5 phase transformation by
laser heating.
4. Growth on stainless steel substrate.
Summary-FeSi2 as a semiconductor for space-use solar cell
Small electronic density cross-sectional area, High resistance against the exposure of cosmic rays and radiation.
Thin film solar cell (thinner than 1 m), Elevation of payload.
Use of metallic -Fe2Si5 as a deposition- and
step-free electrode, Improvement of mechanical strength, High reliability at elevated temperatures,Elevation of payload.High resistance against cosmic rays and radiation, Elimination of thick Si substrates,Elevation of payload.
Bright future of -FeSi2