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