Jan. 10, 2008
Winter School on New Functionalities in Glass

Vacuum-ultraviolet transparency of silica glass and its relation to processes
involving mobile interstitial species
Tokyo Metropolitan University
Koichi Kajihara

Overview
1. Introduction
2. Structure and optical properties of defects• Strained Si-O-Si bonds• Network modifiers (≡SiX)• Interstitial hydrogen molecules (H2 )3. Improvement of UV-VUV transparency of silica glasses(a) Effects of structural disorder (strained Si-O-Si bonds) on VUV transparency(b) Removal of strained Si-O-Si bonds by doping with network modifiers(c) Role of mobile interstitial H2 molecules
4. Silica glasses for UV-VUV spectral region• Silica glasses for excimer laser photolithography• Deep-UV optical fibers
5. Interstitial oxygen in silica glass

1. Introduction
Why silica glass?
• One of the simplest light metal amorphous oxides• Large-size crystalline polymorph (α-quartz) is available• Good mechanical properties and chemical stability• High purity products are commercially available• Various practical applications– Optical components– Gate dielectric films– Catalysts and catalyst supports

1. Introduction
Silica glass (amorphous SiO2 ) – A promising UV optical material
1. Largest bandgap among glasses commercially available (absorption edge ∼8eV)
Mercury UV lamp

2. Good shape workability
3. Good physical and chemical properties
Transparency region of various optical materials
400

300

3

4

Hg i line XeCl
VIS

UV

250
5

Wavelength (nm)
200

170

Photon energy (eV)
6
7

KrF

ArF

deep−UV (DUV)

150
8

140
9

F2
vacuum−UV (VUV)O2 Schumann−Runge band

CaF2
Al 2O3α −quartz (c−SiO 2)
SiO 2 glass
Pyrex glass

Glass plate

Luminescent glass

1. Introduction

Characteristic types of silica glasses [after Brückner(1998)]

Fused silica . . . Prepared from natural quartz
Good thermal stability; for crucibles and reactor chambers.
• Type I

Electric melting in crucibles. Contain metallic impurities(e.g. Al, Na), low (<5ppm) OH concentration.

• Type II

Crucible-free H2 -O2 flame fusion. Concentrations ofmetallic impurities are lower than Type I. Medium(∼100ppm) OH concentration.

From product catalog,
Covalent Materials Co.

1. Introduction

Characteristic types of silica glasses [after Brückner(1998)]

Synthetic silica . . . Prepared by vapor-phase decomposition of silane compounds
High purity, various doping techniques; for optical components• Type III

Directly deposited by H2 -O2 hydrolysis.
High (∼1,000ppm) OH concentration.

• Type IIIa,b Prepared by “soot”-remelting.
Suitable for dehydration and doping.
• Type IV

Prepared by O2 -Ar plasma CVD method.
Nealy OH-free but contains O2
molecules.

There are various types of silica glasses!

1. Introduction

Effect of point defects (color centers)• Different types of silica glasses
different optical properties . . . different concentrations of point defects
Control of point defects is important!
Mizuguchi et al., Opt. Lett. 24, 863(1999), Hosono et al., APL 74, 2755(1999)

Absorption spectra
8.5

8Induced absorption spectra
7.5

7

6.5

800 600
3

1-1

Transmittance

0.8
0.6

Dry
Wet
F-doped

0.4
0.2

300

200

(157nm)Dry
Wet
F-dopedreflection loss
∆absorption coefficient (cm )(7.9eV)

400

2

1
x10

0

(157nm)
0
140

150

160
170
180
Wavelength (nm)

190

200

2

3

4
5
6
Photon energy (eV)78

1. Introduction

Defect concentration

Optical properties of silica glass is often influenced by trace amounts of defects!
log[Conc.(cm−3 )]22

Lattice atom (O: 4.4×1022 cm−3 )21

Solubility limit of fluorine (SiF) (several wt%)

20

SiOH in “wet” silica glass (∼1000wtppm、∼1020 cm−3 )Detection limit by X-ray fluorescence spectroscopy

19

H2 in H2 -loaded silica, chlorine (SiCl) in dry silica

18

SiOH in silica glass for KrF and ArF photolithography (10-100wtppm)
Metallic impurities (e.g. Al) in fused silica

17-16

Detection limit by IR and Raman spectroscopy (bulk glasses)

17-15

Common radiation-induced defects

15-14

Detection limit by PL and EPR spectroscopy (bulk glasses)
SiOH in optical telecom fibers

13

Problematic defect concentration for DUV optical fibers

1. Introduction

Transparency region of silica glass• Excellent transparency from infrared to vacuum-ultraviolet• “Blue shift” of the main research field
Wavelength (nm)
400 300
200

2000 800

150

IR

4

VIS

UV

Deep−UV

Optical telecom

KrF

Vacuum−UVArF F22

10
8

Defects

6

Weak abs. tail0α-Quartz

Radiation-induced defects
OH impurity-2

4

Defects
OH impurity-4

Rayleigh scattering
Defects

0-6
Mutiphonon edge

Urbach edge

-8
0

2

2

4
6
Photon energy (eV)

8

-2
10

log[Attenuation (dB km-1)]-1

log[Absorption coefficient (cm )]6

After Griscom,
J.Ceram.Soc.Jpn,
99,923(1991)

2. Structure and optical properties of defects
Ideal structure. . . Corner-shared SiO4 tetrahedra, built only from Si-O bonds• Chemical defects . . . Local nonstoichiometry(vacancy, interstitial, dangling bonds, impurity atoms)• Physical defects. . . Topological disorder (strained Si-O-Si bonds)
Normal bond

Oxygen deficiency−related defects
O

O
O

O
Si

O

O

Si

O

Si

O
O

O

E’ center

O
O

O

Si

O

Si

O

O

Si−Si bondDivalent Si

O

O

O

Strained Si−O−Si bond

O

O

Si
O
O

O
O

Si
O
O

Si

Si
F

O

O

SiOH group

SiF group

O

O

O

Si

Si

O
O
O

O
O
Molecular
oxygenO

Peroxy linkage (POL)

O
Ozone

H
O
AtomicoxygenOO

Non−bridging oxygen Peroxy radical (POR)
hole center (NBOHC)OHInterstital chemical species

O

O

O

O

Oxygen excess−related defects

O

O
Si

O

O

O

O

Physical disorder

Si

Si
O

O

Dopant−related structures

H

MolecularhydrogenH
Atomichydrogen2. Structure and optical properties of defects

Optical absorption bands

Improvement of transparency and radiation hardness . . . Control of point defects
1

150

8

F2

7
ArF

200

5

3

4
9

6

8 13

12
7
8

KrF
Nd:YAG4ω

250

5

XeCl
XeF
Nd:YAG3ω

300

4

11

400

3

9 11

Nd:YAG2ω

500

Nd:YAG

2
1000

1

9

14

12 10

12

8
13
13

6

2

VUV absorption edge
1. Fluoride groupSiF
2. Hydride group
SiH
3. Chloride group
SiCl
4. Oxygen vacancy [SiODC(I)]
Si−Si
5. Hydroxyl group
SiOH
6. Peroxy linkage [POL]SiOOSi
7. E’ center
Si
8. Peroxy radical [POR]
SiOO
9. Divalent Si / Oxygen divacancy[SiODC(II)]O−Si−O
10. Ozone molecule
O3
11. Chlorine moleculeCl2
12. Non−bridging oxygen
SiOhole center [NBOHC]
13. Oxygen molecule
O2
14. Self−trapped hole [STH]
Si−O−Si
2
Peak absorption cross section(cm )<10−19 10−19−10−17 >10−17
Verified0

Tentative

After Skuja et al.,Proc.SPIE
4347,155(2001)

3a. Strained Si-O-Si bonds

A comparison among SiO2 polymorphs
Silica glass (disordered SiO4 units)

α-quartz (ordered SiO4 units)• Larger bandgap than silica glass• F2 laser irradiation does not form persistent defects
Materials

Band gap

Bandgap excitation causes

Amorphous silicon∼1.7eV

Staebler-Wronski effectChalcogenide glasses∼2eV

Photo darkening

Silica glass

∼9eV

?

3a. Strained Si-O-Si bonds

Physical disorder in silica glass

Short-range physical disorder. . . Distribution in Si-O-Si angle
c.f. α-quartz. . . No distribution in Si-O-Si and O-Si-O angles, Si-O length
Si-O-Si angle

O-Si-O angle

Si-O length

150

500

300

(b)

(a)

50

Number of bonds

100(Si-O)3(c)

α-quartz

300

200

0
100

120
140
Si-O-Si angle (deg)

160

200

100

100

(Si-O)4080α-quartz

400
Number of sites

Number of sites

α-quartz

180

80

100

120
140
O-Si-O angle (deg)

160

180

0
0.14

0.15

0.16
Si-O length (nm)

0.17

0.18

*Calculated from a periodic silica structure reported in Mukhopadhyay et al., PRB70,195203 (2004)

3a. Strained Si-O-Si bonds
Typical strained Si-O-Si bonds. . . 3- and 4-membered rings

3−membered ring

4−membered ringD2 band (606cm−1)

D1 band (495cm−1)

Galeener, JNCS49,53(1982)• Do not exist in α-quartz• The concentration depends on thermal
annealing (fictive) temperature

O

O

Si
O

O

Si

Si

Si

O

O

Si

Si

O

Si

Hosono et al., PRL87, 175501(2001)
0.8

Energy (eV)-1

Absorption coefficient (cm )Modified based on discussion
by Galeener in JNCS 49,53(1982)

0.6(Si-O)2
0.4(Si-O)30.2

α-quartz(Si-O)4

144°
0
110

120

130
140
150
160
Si-O-Si bond angle (deg)

170

180

10

5

Normalized intensity

15
Newton and Gibbs,
Phys. Chem. Minerals, 6,221(1980)

1

ω1

f

900°C

D2
0

Tf =1400°C
1200°C
1100°C
900°C

D1 T =1400°C

ω3∆I (x2)
400

600

800
-1

Raman shift (cm )

0
7.8

7.9
8
Photon energy (eV)8.1

3a. Strained Si-O-Si bonds

Defect formation• <10mJ cm−2 . . . One-photon processeshν(7.9eV)≡Si-O-Si≡−→≡Si• (E’ center) +

KrF

ArF

5.0eV

6.4eV

F2

7.9eV

• O-Si≡(NBOHC)• >10mJ cm−2 . . . Two-photon processes (Yield. . . F2  KrF, ArF)
Strained Si-O-Si bonds . . . Real intermediate states for defectformation via two-step absorption processes
Hosono et al., PRL87,175501(2001)

1
1

0.1
0.1
1
10
Power (mJ cm-2)

Tf =1400°C

1
Tf =900°C

0.1

Peak power (MW cm-2)
0.2
0.5
1
2

NBOHC, Tf =1400°C
900°C
E’ center, Tf =1400°C
900°C
1
1
0.7

-3

1

0.05
18

5.8eV (E’ center)
4.8eV (NBOHC)

5.8eV
4.8eV

TSA

Kajihara et al., APL81,3164(2002)

log[Defect concentration (cm )]2∆α (cm-1)-1∆Absorption coefficient (cm )

10

TPA

17

16

5

1.5
1.3

0.5

2.0
2.1

0.8

15

2

E’ center,
Tf =900°C
ArF

1

1.0
14

KrF

0
0

2

4

6

Number of pulses (x106)

8

1

2

5

10

20

50

Pulse energy (mJ cm-2pulse-1)100

3a. Strained Si-O-Si bonds

Chemical annealing

Elimination of strained Si-O-Si bonds• Low temperature heating (“physical” annealing) . . . time consuming• Breaking up glass network by network modifiers (SiF, SiCl, SiOH, SiH)(“chemical” annealing). . . structural relaxation by lowered viscosity
Hosono and Ikuta, NIMB166, 691(2000)
15

high Tf
α-quartz low Tf∆α (cm-1)

Relative fraction

Tf =1400°C
900°C
10
F-doped
5
0
7.8

Si
O

Lowering Tf
or F-doping(157nm)

O

Si
O

7.9
8
8.1
Photon energy (eV)

Si

7.9eV
photon

O
Si

Si

Si

(SiO)3,D2(SiO)4,D1

planarpuckered planar

O

O

Si
O

120

140

Si-O-Si bond angle (deg)

160

O

Si

F

Si

F−doping
O
Si

Si
F

100

O

O

Si

3b. Network modifiers

Types and the VUV absorption bands
Hydrogen−related species

Hydrogen−related species

O
O

O

Si
O

O

H

O

O

Si
H

O

SiOH groupOO

SiH group

O

Si
F

O

SiF group&7.4eVSi

SiH

Not known (& Eg )SiF

Not known (& Eg )

SiCl

&7eV

Cl

Awazu et al. JAP69,1849(1991)

-18
X 2O
SiOX

-20

7
7.5
8
Photon energy (eV)Awazu et al.(1994)
Morimoto et al.(1999)

SiOX (x5)0Sintered in He
Treated in CCl4, Sintered in He
Treated in CCl4, Sintered in Cl2/He=5/10
3/10
1/10
Awazu et al.,

-1

X2O

X=H
D

log[Absorption coefficient (cm )]

X=H
D

6.5

5

SiOH

1

-16
log[σ (cm2)]

2

cm )

-18

Absorption cross section, σ (1010OSiCl group

Kajihara et al. PRB72,214112(2005)
15

Absorption band

0.5

JAP69,1849(1991)
0

-0.5

-1
6.5

7

7.5
Photon energy (eV)

8

6.5

7

7.5
Photon energy (eV)88.5

3b. Network modifiers

Absorption edge (nm)

10
8
154
6
4
156
2
0

E’ center concentration (1016cm-3)

152• Increase in SiF concentration– Improve VUV transparency– Decrease defect concentration• Most effective at <1 % SiF doping(Effects do not proportionally with
SiF concentration)
Structural relaxation by SiF doping

158
0

1

2

3

4

5

6

7

8

Hosono and Ikuta, NIMB166, 691(2000)

Si-F concentration (mol%)VUV OA

Photolysis

Cost

Applications

No

No

High

Excimer laser lithography, DUV fiber

SiOH (Wet)&7.4eV

SiO• + H0

SiCl (Dry)&7.7eVSi• + Cl0Med

IR optical telecom

No?

Si• + H0 ?

–

–

SiF

SiH

(F-doped)

Low-Med. UV-DUV laser optics

3c. Interstitial H2 molecules
Silica glass • Low density as compared with crystalline SiO2 , Al2 O3 . . . large free volume• Easy diffusion and reaction of small chemical species
e.g. Doremus, “Diffusion of reactive molecules in solids and melts”, Wiley(2002)• Neutral interstitial species• Hydrogen-related. . . H0 、H2• Oxygen-related . . . O0 、O2
Density (g cm−3 )Silica glass

2.21

Tridymite

2.33

Cristobalite

2.33

α-quartz

2.65

Soda-lime silicate

2.47

Alumina (Al2 O3 )3.97

3c. Interstitial H2 molecules
H2 in silica glass. . . fast diffusion (He > H2 > Ne  Ar, H2 O), high reactivity• Hydrogen corrosion in telecom fibers (≡Si-O-Si≡ + H2 → ≡SiOH + ≡SiH)• Sensitization of photoencoding of Bragg gratings• Termination of dangling bonds (R• + H2 → RH + H0 )• Improvement of KrF and ArF laser hardness
Mochizuki et al,
H2-loaded
IEEE J.Quantum.Electron. H desorbed at 80°C
2
QE-20,694(1984)
220°C

Canning et al., Opt.Lett. 20,2189(1995)
1

15

Transmittance

Optical loss (dB)

20

10

5

0.8
0.6
0.4
0.2

1

1.1

1.2
1.3
1.4
Wavelength (µm)

1.5

1.6

33wt% P-doped fiber,
after exposure to ArF laser
H2-free, 30s
150s
H2-loaded

0
1520

1521

1522
Wavelength (nm)1523

3c. Interstitial H2 molecules

In-situ study of diffusion and reactions

F2 -laser-irradiated “wet” silica glass
F2 laser (7.9eV)≡SiO-H −→ ≡SiO• + H0 (quantum yield ∼0.1–0.2 )Nd:YAG 4HG (4.7eV)≡SiO• −→ ≡SiO• (1.9eV PL)• Concentration of radiation-induced NBOHC(≡SiO• ) . . . insensitive to H2 loading• NBOHC does not accumulate in H2 -loaded glass

H2-free18-3H2-im (3x10 cm )Intensity (a.u.)

Relative NBOHC concentration (arb. units)

Kajihara et al., APL79,1575(2001); NIMB33,323(2004); PRB74,094202(2006)
H2-free
H2-im

18

H2 -free≡SiOH H2 -loaded→−≡SiOH ⇐

H2 band-1

4135cm-3(H2:1x10 cm )

4000
4100
4200
-1
Raman shift (cm )F2 -excited
NBOHC 1.9eV PL≡SiO• + H0
≡SiO• + H0 , H2
YAG 4 ω -excited
NBOHC 1.9eV PL

measurement~20 µ s03060Time (s)

90

120

150

F2 pulse
YAG 4 ω pulse

time

t IF
tD

t IY

t IY

3c. Interstitial H2 molecules

Various effects of interstitial H2

1. Termination of dangling bonds[≡Si• (5.8eV), ≡SiO• (4.8eV, 6.8eV)]
2. Acceleration of oxygen vacancy formation [≡Si-Si≡ (7.6eV)]. . . Photoreduction (≡Si-O*-Si≡ + H2 → ≡Si-Si≡ + H2 O)
3. Crack formation . . . Stress corrosion (≡Si-O-Si≡ + H2 O → 2≡SiOH)
H2 conc. should be strictly optimized

Ikuta et al., APL80,3916(2002); Appl.Opt.43,2332(2004)

Si

Photo−reduction of Si−O−Si bond
H

+H2

O
Si

Si

O
Si

H

0.1
(157nm)

-1

-1

O
Si H

Si

∆Absorption coefficient (cm )

O
Si

H

+H2

(b) F2 , OH-free, H2-free
H2-im∆Absorption coefficient (cm )0.15

Termination of dangling bonds

0.05
1

0
(a) ArF, OH-doped, H2-free
H2-im

0.5

(193nm)

0

Si
3

4

5
6
Photon energy (eV)78

4. Silica glasses for UV-VUV spectral region
Type

Defect species

Wet

SiOH

Dry

SiCl, Si-Si

F-dopedSiF

F2 laser optics

Conventional applications

7.9eV Transparency

UV optics

Poor(OA by SiOH)

IR telecom. fibers

Poor(OA by Si-Si)

X- and γ-resistant fibers

Good

Fluorine-doped silica . . . Suitable for photomask substrates in F2 laser photolithography
Hosono et al. APL74,2755(1999), Mizuguchi et al. JVSTB17,3280(1999)
8.5

8

7.5

7

6.5

1
(7.9eV)

reflection loss

Transmittance

0.8
0.6

Dry
Wet
F-doped

0.4
0.2(157nm)
0
140

150

160
170
180
Wavelength (nm)

190

200

4. Silica glasses for UV-VUV spectral region

DUV optical fibers

Conventional fibers (Ge-doped core and pure-silica cladding)• Not transparent for UV light• High viscosity – drawing-induced defects• High radiation sensitivity
1. F-doped core and cladding⇒
2. Defect annihilation by H2 impregnation
Oto et al, IEEE Photo. Technol. Lett. 13, 978(2001); J. Non-Cryst. Solids 349,133(2004)
1

7

6

Photon energy (eV)
5

4

1

0.2

0.8

3
2
1
0

As drawn, irradiatedbefore irradiation,
H2-loaded, irradiated

0.6
0.4

E’ center
0.2

500
1000
1500
Wavelength (nm)H2-loaded
As drawn(193nm)1
Norm. T

0.4

ArF laser(193nm)4
Transmittance

0.6

Optical loss (dB m-1)

Transmittance (m-1)

0.8H2-loaded
As drawn
0
0

0

5
10
Pulses (x104)

0
180

200

220
240
260
Wavelength (nm)

280

300

180

200

220
240
260
Wavelength (nm)

280

300

320

4. Silica glasses for UV-VUV spectral region

Processing of fiber ends

• End sharpening by chemical etching in hydrofluoric acid. . . Possible application to scanning nearfield optical microscopy (SNOM)

10 µ m

−→HF etchingCore:
SiF 200ppm
Clad:
SiF 10,000ppm∆ n = 0.7%

5. Interstitial oxygen in silica glass••• Oxygen-deficiency related defects. . . Si-Si, ≡Si , –Si–, . . .
– Main color centers in DUV fibers•• Oxygen-excess related defects. . . ≡SiOO• , O2 , Si-O-O-Si, . . .
– May be used to oxidize oxygen-deficiency related color centers– Chemical and optical properties remain largely unclear
1

Transmittance

0.8
E’ center

0.6

0.4

Si-Si
Fiber
Preform

0.2

Oto et al. JNCS349,133(2004)
0
160

180
200
Wavelength (nm)220

5. Interstitial oxygen in silica glass
Interstitial O2 . . . The most common form of excess oxygen in silica glass• Nassau and Shiever (1975) Preparation of low-OH a-SiO2 by plasma-CVD method• Heitmann et al.(1983) Sharp loss bands of unknown origin in telecom fibers by PCVD
• Carvalho et al.(1985) Identification of interstitial O2 by Raman spectroscopy• Awazu et al.(1990) Observation of VUV absorption band of interstitial O2-1

Attenuation (dB km )20

Heitmann et al.,
Eectron.Lett. 19,616(1983)

O2 b-X

O2 a-X
15

10

5

0
600

800

1000
1200
Wavelength (nm)

1400

1600

5. Interstitial oxygen in silica glass

Detection by photoluminescence• Shikama et al.(1994) Discovery of 1270nm PL band in optical fiber in an nuclear reactor• Skuja et al.(1996) PL detection of interstitial O2 via 1064nm excitation• Skuja et al.(1998) PL detection of interstitial O2 via 765nm excitation
Sensitive, selective, and non-destructive detection of interstitial O2 in a-SiO2

Optical radiation intensity (dB m)-60
Shikama et al. NIMB91,342(1994)
Cherenkov radiation

O2 energy level1 +b Σg 0-70765nm(Ti:Al2O3)
2νOH

-80

1
1
a ∆g 01272nm-90

400

1064nm(Nd:YAG)

600

800

1000 1200 1400
Wavelength (nm)

1600

1800

3 _
X Σg 0O2 2p π∗5. Interstitial oxygen in silica glass

Thermal diffusion• O2 PL measurements of silica glasses thermally annealed in air. . . Solubility and diffusion coefficient of interstitial O2 in silica glasscw Ti:Al 2O3 laser16

cw Nd:YVO 4 laser
SHG

0.6

900°C, 120h
840h

PL ampl. (a.u.)

765nm

-3

532nm

O2 concentration (10 cm )Kajihara et al. J.Ceram.Soc.Jpn.112,559(2004); JAP98,013529(2005)

IR spectrometer(FT−Raman detector)
1272nm0.4-17860cmO2 PL(λex=765nm)

8500
8000
7500
Wavenumber (cm-1)
0.2

Sample

0
-0.2

-0.1

0

0.1

0.2

Position (cm)Oxidant in the thermal oxidation of silicon isinterstitial O2

Oxygen transport throughthermally−grown SiO2 film
growth front
Si substrate

5. Interstitial oxygen in silica glass

Concentration calibration• Thermal desorption spectroscopy
8.3×1016 molecules∼

22 % decrease of PL intensity

O2 concentration∼

2.7×1016 cm−3 ∆APL peak /ARaman@1200cm−1

1000

(a)

1

6
4
2
0
0
0
0
0

600
m/e=
32
400

44
28

200

18
2
0

100

200
Time (min)

300

0
400

(b)

7500

Before (B)After (A)(A) - (B), x2

800

Normalized intensity

8

Absolute wavenumber (cm-1)
9000
8500
8000

Temperature (°C)

Desorption rate (x1014-1molecules min )Kajihara et al. JNCS, in press

0.8
0.6
0.4

x5

0.2
0
500

1000
Raman shift (cm-1)

1500

2000

5. Interstitial oxygen in silica glass

VUV OA cross section• Simultaneous measurement of VUV absorption and O2 concentration changes
1. Red-shift of VUV absorption edge
Weak attractive interaction between⇒
O2 and a-SiO2 framework
2. Increase in absorption intensity
Kajihara et al. JAP98,013527(2005)-16
log[Absorption cross section (cm )]20

10

Normalized intensity

1O2 band(1539cm-1)

0
1000
Raman shift (cm-1)(b)2O2-loaded
O2-desorbed-1

Absorption coefficient (cm )

30

2000

This study
Gaseous O2

-17

-18

-19

-20

0
6.5

7

7.5
Photon energy (eV)

8

6.5

7

7.5
8
Photon energy (eV)

8.59

5. Interstitial oxygen in silica glass

Reactions (1)• Reaction of a-SiO2 with H2 . . . Cracking of Si-O bond
≡Si-O-Si≡ + H2 −→ ≡SiOH + HSi≡
• Shelby(1980)

SiOH creation with little accompanying SiH formation in O2 -rich a-SiO2

1. 1/2O2 + H2

Two-step reactions

−→

H2 O

2. ≡Si-O-Si≡ + H2 O−→

Shelby, JAP51,2589(1980)

Suprasil W
800°C, 5.8atm H2

1.5

1

0.5
SiH
0
4000

9000

Shelby, JAP51,2589(1980)
Normalized emission intensity

SiOH

Kajihara, JAP98,043515(2005)-1

Absorption coefficient (cm )

2

≡SiOH

Absolute wavenumber (cm-1)
8500
8000

7500

Before (O2-loaded)
After H2 loading

1
0.8

-1

O2 PL(7860cm )

0.6
0.4
0.2
0

3500

3000

2500
-1

Wavenumber (cm )

500

1000
1500
-1
Raman shift (cm )2000

5. Interstitial oxygen in silica glass

Reactions (2)• Reaction with Si-Si bonds ≡Si-Si≡ + 1/2O2−→• Reaction with E’ center≡Si• + O2• Reaction with SiCl

1/2O2 + 2≡SiCl−→• Reaction with H0

O2 + H 0

HO2 •Pfeffer (1998)

14
Degassed
13

300

1.5
Pristine (A)O2-loaded (B)-11

Gaseous Cl2 (2x1018cm-3)[Gibson and Bayliss (1933)]

0.5

E’ center
POR
400

0.5

Fit: Position: 3.80eV
FWHM: 0.70eV-1
Peak ∆α: 0.48cm(B)-(A)

0
500
600
700
Temperature (K)

800

900

∆Absorption coefficient, ∆α (cm )-1

Absorption coefficient, α (cm )-3log[Spins per sample (cm )]O2-loaded (500atm)

12≡Si-O-Si≡ + Cl2

Kajihara, JAP98,043515(2005)
Pfeffer, in "The Physics and Technology ofAmorphous SiO2" (1988)

15≡SiOO•−→−→≡Si-O-Si≡2

3

4Photon energy (eV)56

5. Interstitial oxygen in silica glass
Configuration. . . Peroxy linkage form

Interstitial oxygen atoms• Anion part of the Frenkel paire.g. Hamann, PRL81,3447(1998)
Szymanski et al. PRB63,224207(2001)

O2−

M2+
O2−

M2+

O
O

Si
O

O

Si

M2+

M2+

O2−
M2+

O2−

O
O

O2−

O2−

M2+

M2+

O2−

O2−

M2+

O2−

O2−

M2+

O
M2+

O

• Low-temperature oxidant of silicon
1. Radiolytic decomposition of Si-O-Si bondshν≡Si-O-Si≡ −→ ≡Si-Si≡+O0 (or 1/2O2 )2. VUV photolysis of interstitial O2
O2

hν

−→

2O0

3. UV photolysis of peroxy radical≡SiOO•

hν−→≡SiO• + O0

log[Parabolic rate constant, B (Å2 min−1)]e.g. Ishikawa et al. JJAP31,1148(1992)

Formation

Temperature (° C)
600
500
400

800

300

3
Ishikawa et al., JJAP31,1148(1992)

Dry O2 + UVDry O2

2.5

2

1.5
0.8

1

1.2
1.4
1000/[Tempeature (K)]

1.6

1.8

5. Interstitial oxygen in silica glass

Optical absorption and diffusivity
hν

−→
←−

O2

2O0

Heat

• Optical absorption. . . Use O0 -rich sample prepared by F2 laser irradiation• Diffusivity . . . Probe O2 generated by recombination of O0
Skuja et al. NIMB191,127(2002)

Skuja et al. PCG43C,145(2002)
20

3
10

7.1eV
2
4.73eV

5

1
0

0
3

4

5
6
7
Photon energy (eV)

8

-3

4

17

15

Concentration of O2 (10 cm )4

5
Before irradiation

pristine (A)F2-laser irradiated (B)17-3Destroyed 6.1x10 cm O2 (C)
B-A+C

Absorption coefficient (cm-1)-1∆Absorption coefficient (cm )5

F2-laser irradiation
at 77K

3

2

Experiment
FittedFit parameters:
D=D0exp(-∆E/kT)
D0 = 1x10-3cm2s-1

1∆E = 1.1eV
0
0

200

400
600
800
Annealing temperature (K)

1000

1200

5. Interstitial oxygen in silica glass

Conversion of dangling bondshν(∼5eV)
•

≡SiOO

−→
←−
Heat

Kajihara et al. PRL92,015504(2004)

Skuja et al. Glass.Sci.Technol. 75,24(2002)

3

2
Before F2 laser
irradiation

1-1

Before
After

-1

4

Absorption coefficient (cm )E’ center
NBOHC
POR
KrF 10Hz, 1min
O2

16

-3

0.4

Skuja et al.,Glass Sci. Technol
75,24(2002)

0.3

0.1

0.2
0

0
0

30

60

90

Cumulative anneal time (min)120

3∆Absorption coefficient (cm )5Concentration (10 cm )≡SiO• + O0

4

5Photon energy (eV)6

5. Interstitial oxygen in silica glass

Absorption cross section “map”

E’ center
NBOHC
Si-Si
SiOH
O2
H2 O
Si-O-O-Si

2

log[Absorption cross section (cm )]

-15

-16

-17

-18

-19

-20
3

4

5
6
Photon energy (eV)78Summary

Opticalisotropy

Wide−gap
α −quartz

Workability

Process
engineering

Fundamental
research

Structuralmodification

Raw material
Production method
Fiber drawing

Optical spectroscopy
EPR
Simulation

Network topology
Stoichiometry
Doping(H,F,P,RE, ...)

Deep−UV
opticsPhotomasks
Hard pellicles
Lenses

Opticalfibers

DUV fibers
Bragg grating devices
Fiber lasers

Acknowledgment
This work has been made in collaboration with
• Professor Hideo Hosono(Japan Science and Technology Agency, Tokyo Institute of Technology)• Professor Masahiro Hirano(Japan Science and Technology Agency, Tokyo Institute of Technology)• Dr. Linards Skuja (University of Latvia)• Dr. Yoshiaki Ikuta (Asahi Glass Company Co. Ltd.)• Dr. Masanori Oto (Showa Device Technology Co. Ltd.)