Intrinsic- and extrinsic-defect formation in silica glasses by radiation

Intrinsic- and extrinsic-defect formation in silica glasses by radiation

IOURNAL ELSEVIER OF Journal of Non-Crystalline Solids 179 (1994) 202-213 Intrinsic- and extrinsic-defect formation in silica glasses by radiation ...

1MB Sizes 7 Downloads 18 Views

IOURNAL

ELSEVIER

OF

Journal of Non-Crystalline Solids 179 (1994) 202-213

Intrinsic- and extrinsic-defect formation in silica glasses by radiation H i r o a k i I m a i *, H i r o s h i H i r a s h i m a Department of Applied Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku, Yokohama 223, Japan

Abstract

The dose dependence of the defect concentration produced by radiation was studied for many tTnes of silica glass in order to discuss the contribution of 'extrinsic' and 'intrinsic' processes to the paramagnetic defect formation. A linearly increasing concentration of paramagnetic defects with dose accompanied by a saturating tendency is observed for the 'extrinsic' defect formation due to transformation of pre-existing precursors. The concentration of E' centers substantially equals that of non-bridging oxygen hole centers, and both are approximately proportional to the square-root of the accumulated dose for the 'intrinsic' defect formation involving cleavage of the Si-O network. Since the dose dependence of the defects is independent of the incident photon energy, electron-hole pairs having the band gap energy of silica are implied to have an essential role for either 'extrinsic' or 'intrinsic' defect formation.

1. Introduction

Paramagnetic defects are formed in silica glass exposed to energetic photons and particles. The typical paramagnetic defects in silica glass subjected to ~/- and X-irradiation and ultraviolet (UV) laser light are E' centers [1], consisting of an unpaired electron on a silicon bonded to three oxygens ( - S i . ) , and non-bridging oxygen hole centers (NBOHCs) [2], which is a hole trapped on a non-bridging oxygen ( - S i - O • ). Investigation of the defect formation processes is important for many current applications of silica glass and for understanding the fundamental interaction of amorphous materials with radiation.

The formation of paramagnetic defects in silica glass has been studied from two points of view: transformation of pre-existing diamagnetic point defects and the cleavage of intrinsic S i - O bonds. Trapping of a hole at oxygen-deficient centers (ODCs), such as an Si-Si bond or neutral oxygen vacancy, was proposed to be a formation mechanism of E' centers [3]: - S i - S i - + h +--> -Si" + + Si-.

A radiochemical reaction of pre-existing defects consisting of impurities bonded to silicon was suggested as a mechanism that forms paramagnetic defect centers, e.g. [4-9] -Si-H ~-Si-

*Corres~nding author. Tel: +81-45 563 1141. Telefax: + 81-45 563 0446.

+ H °,

(2)

--Si-CI ~ - S i . + CI °,

(3)

- S i - O H ~ - S i - O • + H°.

(4)

0022-3093/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0022-3093(94)00258-0

(1)

H. lmaL H. Hirashima / Journal of Non-Crystalline Solids 179 (1994) 202-213

X rays

UV lasers

"~~Cleavage intrinsic

~re-existing defects - Si-H'---

Knock-on

pairs

Cleavage of Si • +H °

oxygen hole centers (OHCs) increase non-linearly with the X-ray dose, and they ascribed this behavior to the coexistence of two processes: the 'activation' of pre-existing defects and the 'creation' of new defects. Imai et al. [17] reported on the dose dependence of E' centers and NBOHCs for silicas irradiated by ~/-irradiation. They suggested that ~/-irradiation fundamentally creates defect pairs of an E' center and an NBOHC from intrinsic S i - O bonds; this is in addition to a large number of E' centers induced from precursors. In this paper, we discuss the published data for the dose dependence of E' centers and NBOHCs in many types of silica glass irradiated with ~- and X-rays and UV lasers. From the discussion of the data for various materials, general models are expected to be found for defect formation in silica glasses by irradiation with energetic photons.

~,rays

Electron-hole

Photolysis

!f bonds

- S i - O - S i - - ~ = Si • + • O - S i -

- S i - C I - , - S i • +C1 °

- Si-O-Si-- *-

-= S i - O H -, -= 5 i - 0 • + H °

S i - S i -= + 0

[Intrinsic]

Trapping at pre-existing defects = S i - S i =- +h ° - * - - - Si • + +Si =

[Extrinsic] F i g . 1. T h e

schematic

processes

of defect

formation

by ener-

getic photons.

2. Dose dependence of E' centers and NBOHCs

Since the preparation history of silica glass influences the concentration of pre-existing diamagnetic defects and impurities, E' formation from precursors therefore depends on the 'types' of silica glass. Thus, defect formation due to the transformation of precursors is seen as 'extrinsic'. On the other hand, an E' center and an NBOHC could be created according to the reaction [10,11] = S i - O - S i - --* =Si-O • + . Si-.

203

(5)

The cleavage was suggested to be induced at a strained S i - O bond [10,11] and through decay of self-trapped excitons [12-14]. Since this mechanism of defect formation should be independent of the contents of pre-existing defects and impurities, it is therefore an 'intrinsic' process. Fig. 1 shows the schematic formation processes of paramagnetic defects by irradiation with energetic photons. The dose dependence of defects produced by ionizing radiation reflects the formation processes because it is influenced by the concentration of pre-existing defects. Galeener et al. [15,16] reported that the numbers of E' centers and

2.1. Formation of E' centers by y-irradiation in 'dry' [18] silicas containing E' precursors All the spin concentrations were determined by electron spin resonance (ESR). Fig. 2 shows the dependence of the E' concentration on the 6°Co ~/dose at room temperature for oxygen-de-

10 +a

!

E

o

"~ 1017 r.I

• a

!

S2H" HR5 HR3

|

|

[]

O

Q.

" 1016 o L_

o tj c o

1015

O 1014 102

I

10 3

I

104

I

I

10 s 10 s 10 z Dose (Gy) Fig. 2. The concentrations of E' centers generated at room temperature in 'dry' silicascontainingSi-Si or Si-H bonds, as a function of the accumulated ",/ dose [17,19]. The lines indicate growth calculated using Eq. (9).

H. Imai, H. Hirashima /Journal of Non-Crystalline Solids 179 (1994) 202-213

204

Table 1 The preparation m e t h o d s and d o m i n a n t defects in samples [8,17] Sample

H1 H2 P2 $2 S2H $3 SF1 SF3 SF4 SF5

Preparation method

flame hydrolysis flame hydrolysis plasma oxidation soot remelting H 2 t r e a t m e n t of $2 soot remelting soot remeiting soot remelting soot remelting soot remelting

C o n c e n t r a t i o n (cm - 3) OH

Si-Si a

OEC b

Si-H

CI c

4 × 10 t9 6 × 10t9 ND ND ND l × 1019 f ND ND ND ND

ND ~ ND ND 1 X 10 TM ND ND ND ND ND ND

ND ND 3 × 10 ts ND ND ND ND ND ND ND

ND ND ND ND ,-, 10 TM ND ND ND ND ND

6× ND 1X 3x 3× 3× 1x 1× 1× 1×

F d l0 ts 1019 l0 ts l0 TM l0 TM 1019 1018 10 ts 1018

ND ND ND ND ND ND ND 5 × 10 20 6 × 102o 7 × 10 2°

" T h e concentration of Si-Si bonds was e s t i m a t e d from the 7.6 e V absorption band [8]. b The concentration of oxygen-excess c e n t e r s ( O E C ) was e s t i m a t e d f r o m the increase in the O H absorption b a n d by the reaction with H 2 [8]. Dissolved oxygen molecules and peroxy linkages are p r o p o s e d for candidates o f O E C . c D e t e r m i n e d by chemical analysis. a Estimated from the refractive index. e ND: not d e t e c t e d . f This value is relatively small c o m p a r e d with ' w e t ' silicas.

ficient 'dry' silicas containing Si-Si ($2) or Si-H bonds (S2H, HR1, HR3 and HR5) [17,19]. The preparation methods and the concentrations of pre-existing defects and impurities for the samples are listed in Tables 1 and 2. $2 and S2H were confirmed to contain Si-Si and Si-H bonds, respectively [8]. HR1, HR3 and HR5, which were prepared in a reducing condition, were suggested to contain silicon lone-pair centers (two-coordi-

nated silicons) and S i - H bonds [19]. Both Si-Si and Si-H bonds were proposed to be precursors of E' centers. The concentration of E' centers in these silicas increases linearly in the low-dose region and shows a tendency to saturate at doses above 10 4 Gy. Fig. 3 shows the "y-dose dependence of the E' concentration at room temperature in 'dry' silicas containing a lot of chlorine (SF1, CL1, CL3 and CL5) [17,19]. These silicas

Table 2 The preparation m e t h o d s and d o m i n a n t defects in samples [19,20] Sample

Preparation m e t h o d

Suprasil l Suprasil W1 VADWET VAD1 VAD7 VADI0 HR 1 ttR3 HR5 CL1 CL3 CL5

flame hydrolysis plasma oxidation soot remelting soot remelting in soot remelting in soot remelting in soot remelting in soot remelting in soot remelting in soot remelting in soot remelting in soot remelting in

a' Not r e p o r t e d

0 2 0 2 O2 H2 H2 H2 0 2 CI 2 CI 2

C o n c e n t r a t i o n ( c m - 3) OH

oxygen

,~ 1020

_ a

_

" 1017 7.8 × l0 ts '-" l0 TM " 18 ts ~ l0 TM ND ND ND

excess excess excess excess deficient deficient deficiu~,t -

ND 6.7 × 1019 1 1 × 10 I'~ 10 × l0 w

-

-

chlorine

H. lmai, H. Hirashima / Journal of Non-Crystalline Sofids 179 (1994) 202-213

A

10 le

10 le

¢o 0

E 0

t/) t'¢1 v

tO

(5

SF1

1017

CL3 *

E l 01 r O

CL1

/f

10 is

1015 (1) ¢.) ¢::

U) cO. ¢/) v

= 10 le

.o

i 0

P2

[]

S3

A

SF3

[]

SFS

205

i

i

SF4



Suprasll

X

VADWET

[]

VAD1



VAD7



VAD10

Wl

C

0

® 1015

s

1014

10 2

I

I

I

I

10 a

10 4

105

10 s

¢-

10 7

O

Dose (Gy) Fig. 3. The concentrations of E' centers generated at room temperature in 'dry' silicas containing a lot of chlorine, as a function of the accumulated -y dose [17,19]. The lines indicate growth calculated using Eq. (9).

are presumed to contain Si-CI bonds which are proposed to be a precursor of E' centers. (Most of the chlorine atoms in silica glasses are incorporated as Si-Cl bonds.) For these silicas, the concentration of E' centers also increases linearly in the low-dose region and shows a tendency to saturate. On the other hand, the concentrations of NBOHCs in $2, S2H and SF1 containing Si-Si, Si-H and Si-CI bonds, respectively, (see Fig. 4)

1014

10 2

I

I

I

!

10 a

10 4

10 s

10 ~

10 7

Dose (Gy) Fig. 5. The concentrations of E' centers generated at room temperature in "dry' silicas containing few precursors of E' centers, as a function of the accumulated -y dose [17,19,20]. The line indicates growth following the square-root of the ~/ dose.

are much smaller than those of E' centers as shown in Figs. 2 and 3.

2.2. Formation of E' centers by y-irradiation in 'dry' silicas containing very few E' precursors 1 0 le

E o 1 01r .C m

[]

Q. C/J

c o

i

(3 P2 • $2 [] $3 A

10 le

S2H SF1 SF3

0

SF4

[]

SF5

1015

Q @

O ¢,. Q

1014 10 2

I

I

10 3

10 4

B

....~=.3_---,_--_._.__! . . . .

10 s

10 6

10 7

Dose (Gy) Fig. 4. The concentra~tions of NBOHCs generated in 'dry' silicas at room temperature, as a function of the accumulated dose [17]. The line indicates growth following the sqvare-root of the ~/dose.

As shown in Fig. 5, the concentrations of E' centers in ten types of 'dry' silica irradiated with ~/-rays at room temperature are almost the same, whereas the dominant impurities in these silicas are different - they are small amounts of OH groups ($3 and VADWET), excess oxygen (P2, Suprasil Wl, VAD1, VAD7 and VAD10) and fluorine (SF3, SF4 and SF5) [17,19,20]. The growth of E' centers is approximately proportional to the square-root of the accumulated dose. As shown in Figs. 4 and 6, it is noted that the concentrations of NBOHCs in $3, P2 [21], SF3, SF4 and SF5 are also approximately proportional to the square-root of the accumulated dose, and substantially equal to those of E' center~. (For the other samples, the concentration of NBOHCs was not reported.) The concentrations of E' cen-

H. Imai, H. Hirashima/Journal of Non-Crystalline Solids 179 (1994) 202-213

206

1017

O

"~10 ._¢

Io.

161

[/

,

o

$3



5F SF5

"1 t

H2 and Suprasil 1), the concentration of E' centers increases linearly in the dose region above ~ 10 4 G y although the E' concentration in the dose region below ~ 104 Gy seems to be proportional to D ~°'5 [17,20]. The concentration of NBOHCs shows a sub-linear growth in all the dose regions. However, the concentration of NBOHCs in SuprasiI 1 is much greater than that in S1 and $2.

10 7

2.4. Similarity of E' formation by two-photon excitation of sub-band-gap lasers and y-irradiation

,

n/OA ~",,,~"

j

.~

~

10 is

1014

10 is

10

E' centers (spins/crn a) Fig. 6. The concentrations of E' centers versus the concentrations of NBOHCs in silicas containing few precursors.

ters and NBOHCs in these silicas are expressed by [E'] = [NBOHC] = kD ~°'5,

(6)

where k is a constant.

2.3. Formation of E' centers and NBOHCs by y-irradiation in 'wet' [18] silicas 'Wet' silicas prepared by a flame-hydrolysis process contain OH groups of ~ 10 2o cm -3. As shown in Fig. 7, for three types of 'wet' silica (H1,

Sub-band-gap lasers having 5.0 and 6.4 eV photons create E' centers in silica glass. Since the concentration of E' centers increases with the square of the laser intensity, the formation of E' centers in silica glass subjected to 5.0 and 6.4 eV laser light was inferred to arise from two-photon absorption [22-24]. When the fraction of the light which is absorbed is sufficiently small, the value of the absorbed energy via a two-photon process, E a (in J/cm3), is estimated by

where/3, I 0 and t are the two-photon absorption coefficient, the incident laser intensity and the real illumination time, respectively. The value of

10 le .-,,

E

1017

In C

Q.

1(3 ~8

O E'(H1) ® NBOHC(H1) [] E' (H2) u NBOHC(H2) •

(7)

Ea ~ /312t,

E o

t~ c O. u)

E'(Suprlsill) NBOHC(SupraslI1)

I 017

° ~

=1 016

c

0

10 le

i

!

!

OP2 y OP2 ArF DS2 y [] S2ArF O S2 KrF 4.Sxlff" ( 13incmW')/ A S2 KrF 8x10"' f x $2 KrF2.1x10"* ~ll/-}

[]

0-.--0

[] []

,¢,"

0

o~

t~

=1

•~ ID

Im

015





A A ~

| II

1

e-

0 o'

10 3

!

!

10 4

10 5

,

1 0 is

ID 0 C 0

0

1014 10 2

~

/ 1014 OIA

|

10 6

× I

10 7

Dose (Gy) Fig. 7. The concentrations of E' centers and NBOHCs in 'wet' silicas, as a function of the accumulated dose [17,20]. The lines indicate linear growth and sub-linear growth following the square-root of the dose, respectively.

10 .2

~oee -

I

10 °

l

I

,,

i

10 2

10 4

Absorbed Energy (J/cm 3) Fig. 8. The concentrations of E' centers generated in $2 and P2 at room temperature, as a function of the absorbed energy due to ~/-irradiation and the two-photon absorption of 6.4 eV (ArF) and 5.0 eV (KrF) excimer laser light [17].

1t. Imai, H. Hirashima / Journal of Non-Crystailine Solids 179 (1994) 202-213

/3 was reported to be 4.5 x 10-lz [25], 8 x 10-11 [26] and 2.1 X 10 -l° [27] c m / W for 5.0 eV and 2.0 × 10 -9 cm//W [27] for 6.4 eV. For ~/-irradiation, the absorbed energy was estimated to be 10- 3 j / g for a dose of 1 Gy. Fig. 8 shows the concentrations of E' centers in $2 and P2 [28] as a function of absorbed energy due to ~/-irradiation and to the two-photon absorption of 5.0 and 6.4 eV lasers [17]. Samples were subjected to ArF (6.4 eV) and KrF (5.0 eV) excimer lasers at room temperature. The ArF and KrF lasers were operated with a power of 45 m J / p u l s e cm 2 at 30 Hz with about 20 ns pulse width and with a power of 180 m J/pulse cm 2 at 25 Hz with about 20 ns pulse width, respectively. Since the thickness of the samples (a few mm) was sufficiently small, the absorption of 5.0 and 6.4 eV photons was negligible. Thus, we were able to calculate the energy absorbed via the two-photon absorption using Eq. (7). For both $2 or P2, the growth curves of E' concentration as a function of absorbed energy are nearly represented by single straight line irrespective of the incident photon energy, although the datapoints for $2 taken based on ~ - - 2 . 1 × 10 -~° c m / W for 5.0 eV photon deviate slightly from the line. These results guggest that the formation efficiency of E' centers by either ~/-irradiation or

1017 © • []

w Q.

.¢_

E'(Suprasill) NBOHC(Suprasill) E'(St~prasilWl) ~ O / J

O L.

.o101 6

/d-c ?/

E Z m

m o 1-

207

10 ~s

'1I

E .c 1014 w c .2 10 la c Q to

0

o/

f

~-

/e 1012

I

1011

o

© 150keV @ 125keV I,

I

10 is

,

I

(~

1 is

A r Dose (ions/cm 2) Fig. 10. The concentrations of E' centers generated in a silica film by Ar ion implantation with implante0 energies of 125 and 150 keV [29].

the two-photon absorption of sub-band-gap lights is almost the same.

2.5. Formation of E' centers and NBOHCs by X-irradiation The dependence of the paramagnetic defect concentration on ~ 8 keV X-ray dose in silica glasses was shown by Galeener et al. [15,16]. Unfortunately, we cannot compare the absolute concentrations of the defects with those by ~/irradiation because the defects produced by ~ 8 keV X-irradiation localize at the surface layer of the samples. Fig. 9 shows published data of E' centers and NBOHCs in 'wet' Suprasil 1 and 'dry' Suprasil W1 [15] replotted on a log-log scale. The number of E' centers in Suprasil 1 increases linearly with the dose although the NBOHCs in Suprasil 1 and the E' centers in Suprasil W1 are approximately proportional to the square-root of the dose.

2.6. Formation of E' centers by ion implantation 10 is

10 4

!

!

10 s

10 e

I Or

Dose (Gy) Fig. 9. The total number of E' centers and NBOHCs generated in Suprasil 1 and Suprasil WI at room temperature [15], as a function of the X-ray dose. The lines indicate linear growth and sub-linear growth following the square-root of the dose, respectively.

Ion implantation induces atomic displacements through a knock-on process. In this case, the defect formation independent of pre-existing defects and impurities is'intrinsic'. As shown in Fig. 10, the concentration of E' centers in a silica film increases linearly with implanted Ar ions and

H. Imai, H. Hirashima /Journal of Non-Crystalline Solids 179 (1994) 202-213

208

. . 1 0 le

!

E

E o

o~

o ~ 17 w10 P

1 017

(/)

$3

[]

H1



H2

"

S2 S2H

[]



P2

_

c

|=

.am

Q. (n

W

,-

¢ 1 0 le

1016

|

o



m

o

.g

L_

¢1

~

015

oeo

01014

10 ~'

0 • [] []

J

E'(Supremill) NBOHC(Suprasill) E'(SuprasilWl) NBOHC(SuprasilWl)

I

i

i

i

10 a

104

10 s

10 e

c

tD

ot-

10

15

O

10 r

I

!

10 4

10 s D o s e (Gy)

Dose (Gy)

Fig. 11. The concentration of E' centers and NBOHCs generated in densified Suprasil 1 and Suprasil W l , as a function of ~, dose [10]. The line indicates the growth of E' centers and NBOHCs following the square-root of the ~/dose in undensifled silicas containing few precursors.

shows a saturating tendency [29]. The saturated concentration is estimated to be ~ 1019 c m - 3 because the effective implanted thickness is ~ 10 -5 cm. This result suggests that the number of the defects which are formed by atomic displacements through the knock-on process increases linearly with the dose. Annihilation of E' centers in overlapping cascades in the knock-on process was presumed to give the saturation of the concentration.

2.Z Formation of E' centers and NBOHCs in densified silicas

Fig. 12. The concentrations of E' centers as a function of -y dose in silicas irradiated at 77 K [17]. The line indicates linear growth with the ~/dose.

that the concentration of E' centers produced by ~/-irradiation at 77 K increases linearly with the dose and the influence of pre-existing precursors (Si-Si, Si-H and $i-Cl bonds) to E' formation is relatively small as compared with that of irradiation at room temperature. All the defects generated at 77 K decayed at room temperature.

2.9. Formation of other paramagnetic defects in sifica Griscom et al. [30] reported that phosphorusrelated centers (P1, P2, P4 and phosphorusoxygen-hole centers (POHC)) in a 10% P205..10

Devine and Arndt [10,11] reported that the concentrations of E' centers and NBOHCs in densified Suprasil 1 and Suprasil W1 irradiated with -~-rays at room temperature are much greater than those in undensified silicas, as shown in Fig. 11. In this case, the concentration of E' centers is almost the same as that of NBOHCs. Thus, the cleavage of strained bonds in densified silicas was proposed to cause the formation of pairs consisting of an E' center and an NBOHC.

2.8. Formation of E' centers by irradiation at 77 K Irradiation with ~-rays at 77 K creates E' centers and several types of OHC [17]. Fig. 12 shows

le

i

i

|

i

¢,ej

E



O

~

H

ez

[] POHC [ ] f e z

017

C

ez

= 1 0 is

.o_

~

i

/"

101s ;

je

tO

0 10141

10 2

QI

10 a

I

I

I

10 4

10 s

10 e

107

D o s e (Gy)

Fig. 13. The concentrations of POHC and atomic hydrogen generated by ~/-irradiation in a P205-SIO2 glass at room temperature [30] and Suprasil 1 at 77 K [31], respectively. The lines indicate growth calculated using Eq. (9).

H. lmai, H. Hirashima / Journal of Non-Crystalline Solids ! 79 (1994) 202-213

90%SIO 2 glass are created by ",/-irradiation through the trapping of free carriers at precursors that consist of a phosphorus atom: for instance, Fig. 13 shows the growth of POHC with the ~/ dose. The concentration of POHC increases linearly and shows a saturating tendency above 10 4 Gy. Atomic hydrogen, which is created through the cleavage of S i - O H and Si-H bonds and molecular hydrogen, was observed in silica irradiated by 100 keV X-rays [31] and y-rays at 77 K [17]. Fig. 11 shows that the number of hydrogen atoms increases linearly with the X-ray dose.

3. Discussion

3.1. Dose dependence for the extrinsic defect formation due to transformation of precursors On irradiation at room temperature, E' centers grow linearly with dose and show a saturating tendency for 'dry' silicas containing pre-existing defects which were proposed to be an E' precursor (such as Si-Si, Si-H and Si-CI bonds). The growth behavior of E' centers is explained by a first-order reaction rate law; the rate of production is proportional to the number of precursors as expressed by d[ E ' l / d D = k'[ precursor ],

As shown in Fig. 13, an increasing concentration of POHC, which is created through the trapping of a hole at a precursor consisting of a phosphorus atom, with the ~/dose follows a firstorder reaction rate law. Fig. 13 also shows that the concentration of atomic hydrogen, which is created through the cleavage of S i - O H and Si-H and of molecular hydrogen, increases linearly with the X-ray dose. The dose dependences of POHC and atomic hydrogen are typical examples of the 'extrinsic' defect formation due to the precursor transformation. NBOHCs were proposed to be created through the cleavage of S i - O H bonds. However, Figs. 4 and 7 show that the concentration of NBOHCs does not show the linear growth with the ~/dose. Moreover, the concentrations of NBOHCs in 'wet' silicas, which contain a great number of S i - O H bonds ( ~ 10 20 cm-3), are not greater than those in other 'dry' silicas. These results suggest that transformation of the precursor is not a dominant process for the NBOHC formation at room temperature. On the other hand, the formation at low temperatures was implied to be ascribed to the cleavage of Si-OH bonds because ~/-irradiation at 77 K created NBOHCs only in 'wet' silicas [17]. Thus, NBOHCs formed through the cleavage of Si-OH bonds are suggested to decay by recombination with radiolytic hydrogen at room temperature.

(S)

where [E'], [precursor], D and k' are the concentration of E' centers, the concentration of precursors, the y-ray dose and a constant, respectively. The solution of Eq. (8) gives a dose-dependent [E'] growth of the form [E'] = N0[1 - e x p ( - k ' D ) ] ,

209

(9)

where N 0 is the initial concentration of precursors. This result indicates that E' centers in these silicas are mainly created through transformation of the precursors, Si-Si, S i - H or Si-CI bonds. This assumption is consistent with the result that the concentrations of E' centers in these silicas are greater than those in the other silicas. The value of N 0 for E' centers in $2 ( ~ 1017 cm -3) is estimated to be smaller than the concentration of Si-Si bonds ( ~ 1018 cm-3). The difference may be attributed to the reverse reaction of Eq. (1).

3.2. Dose dependence for intrinsic defect formation For ten types of silica shown in Fig. 5, the concentrations of E' centers are substantially equal to those of NBOHCs, and follow a sub-linear growth law as expressed by Eq. (6). This is by contrast with the result that the concentrations of E' centers are much greater than those of NBOHCs in silicas containing E' precursors. The equality of the concentrations indicates that an E' center and an NBOHC are formed simultaneously under irradiation. The concentrations of E' centers and NBOHCs are almost the same in several types of silica, such as oxygen-excess (P2, Suprasil W1, VAD1, VAD7 and VAD10), nearstoichiometric which contains a small amount of OH ($3 and VADWET) and fluorine-doped (SF3, SF4 and SF5). The sub-linear growth is inferred

210

H. Imai, H. Hirashhna / Journal of Non-Crystalline Solids 179 (1994) 202-213

to be independent of samples, thus implying 'intrinsic' defect formation involving the cleavage of the intrinsic amorphous Si-O network. The influence of excess oxygen (in $2, Suprasil WI, VAD1, VAD7 and VAD10) and doped fluorine (in SF3, SF4 and SF5) on the formation of E' centers and NBOHCs is negligible. (However, excess oxygens are a precursor of peroxy radicals.) Fluorine, which forms Si-F bonds in silica network, is not a precursor of E' centers by contrast with Si-H and Si-CI bonds. It is attributed to strength of the bonds [17]; the binding energy of an Si-F bond is known to be greater than those of Si-H and Si-CI bonds, and the dissipating energy of the electron-hole pair created in silica is not sufficient for the excitation of an electron in a Si-F bond. The chlorine content in these silicas does not influence E' formation although Si-CI bonds are an E' precursor. It is assumed that the concentration of Si-C1 bonds is sufficiently small or most of the chlorine exists as chlorine molecules in these silicas. Similar sub-linear growth of E' centers was observed for X-irradiated Suprasil Wl (Fig. 9). In the Galeener et al. paper [16], the sub-linear growth was explained by combination of a saturating component and a linear component. However, the sub-linear growth for X-irradiated Suprasil Wl should be ascribed to the 'intrinsic' formation because of the similarity of the growth behavior to those of E' centers in v-irradiated Suprasil W1 and other types of silica containing no E' precursors. Galeener et al. also proposed that a linear growth of E' centers for Suprasil 1 was due to 'intrinsic' formation. For v-irradiated 'wet' silicas including Suprasil 1, E' centers also grow linearly as shown in Fig. 7. However, concentrations of E' centers for 'wet' silicas are greater than those for silicas containing no E' precursors in the relatively high dose region. This result suggests that E' centers in 'wet' silicas should not be ascribed to the 'intrinsic' mechanism. 3.3. Formation processes of E' centers and NBOHCs In Fig. 8, the concentrations of E' centers generated by v-rays and the two-photon excita-

tion of sub-band-gap lasers are compared on the basis of the absorbed energy. The E' concentrations generated in $2 containing Si-Si bonds by v-irradiation having 1.25 MeV photons and the two-photon absorption of sub-band-gap light having 5.0 and 6.4 eV photons are represented by a single line. This result indicates that the transformation efficiency of Si-Si be,ads to E' centers by either excitation is almost the same. It also means that the 'extrinsic' E' formation involves the same mechanism regardless of incident photon energies. S~b-band-gap photons of 5.0 and 6.4 eV excimer laser light can excite the band-to-band transition via the two-photon process. Energetic photons in v-rays lose their energy mainly through the Compton process and the photoelectric effect, and finally create abundant electron-hole pairs across the band gap of silica. Thus, electrons and holes created by a two-photon absorption and the dissipation process of energetic photons in v-rays are deduced to have an essential role for the transformation of E' precursors. Direct excitation of defect states by UV photons (photolysis) is not implied to be dominant for the defect formation. Fig. 8 also shows that the growth of E' centers in P2 by v-irradiation and the two-photon excitation of sub-band-gap lasers is represented by a single line. Devine [32] suggested that the same mechanism contributes to the E' formation in Suprasil W1 by both v-irradiation and the twophoton excitation of sub-band-gap lasers. Assuming that E' centers in these oxygen-excess 'dry' silicas are created through the 'intrinsic' formation process, the efficiency of the 'intrinsic' defect formation is almost the same regardless of the incident photon energy. The production of electron-hole pairs having the band gap energy is also assumed to have an essential role in the 'intrinsic' defect generation rather than the knock-on process involving displacements of oxygen atoms and the creation of oxygen vacancies and oxygen interstitials. The decay of self-trapped excitons or the trapping of electrons and holes are suggested to cause the 'intrinsic' defect formation involving the cleavage of the Si-O bond. Fig. 9 shows that the growth of E' centers generated by X-irradiation in Suprasil Wl follows

H. lmai, H. Hirashima ~Journal of Non-Crystalline Solids 179 (1994) 202-213

a square-root dependence. This result suggests that the formation of E' centers by ~ 8 keV X-ray photons has a process similar to that by ~ 1 MeV photons and the two-photon excitation of sub-band-gap photons. Galeener [33] suggested the similarity of the non-linear growth of the defects by X- and ,/-irradiation. This similarity is consistent with the previous discussion for the defect formation by ~/-irradiation .'rod the twophoton excitation of sub-band-gap light. Fig. 10 shows that E' centers created by ion implantation, which induces atomic displacements through a knock-on process, grow linearly with the dose before the saturation. This means that a constant number of defects are created by an implanted ion via the 'intrinsic' defect formation through the knock-on process. On the other hand, the sub-linear growth of E' centers and NBOHCs following Eq. (6) is also assumed to be ascribable to the intrinsic defect formation. In this case, the rate of defect formation decreases with the dose. The difference of the growth behavior suggests that defect formation by energetic photons involves the decay of electron-hole pairs having the band gap energy rather than atomic displacements through the knock-on mechanism. Several models were proposed to explain the sub-linear dose dependence [9,17]: for instance, assuming that the production rates of the defects are inversely proportional to the concentration of the defects themselves, the square-root dependence can be explained. The production rates expressed by d[E']/dD = d[NBOHC]/OD = k " / ( [ E ' ] + [NBOHC]),

(10)

where k" is a constant, give the solution [E']-[NBOHC] = kD °'s. The details of the quenching mechanisms for the defect formation are not clear. A possible process is as follows: assuming that the 'intrinsic' defect formation is induced through self-trapping of excitons, the formation rate should depend on the average exciton concentration under irradiation, which is related to the exciton lifetime. Since excitons tend to decay at imperfection sites, E' centers and NBOHCs are presumed to be decay sites for the excitons. If

211

the intrinsic exciton lifetime is sufficiently long for the collision with the defects, the exciton lifetime would be inversely proportional to the concentration of the produced defects. Thus, the formation rates of E' centers and NBOHCs should be inversely proportional to the concentrations of E' centers and NBOHCs. Distinct broadening due to dipole-dipole interactions was not noticed in the spectra for 'intrinsic' E' centers as compared with those for E' centers produced through the transformation of precursors. Each defect from a precursor is presumed to be separated from other defects by a large distance. Thus, an E' center and an NBOHC produced through the primary process involving Eq. (5) should also be separated by sufficiently large distances. The formation process of separated pairs consisting of an E' center and an NBOHC through excitons was roughly discussed in our previous paper [17]. The 'extrinsic' defect formation due to the transformation of precursors is not influenced by the concentrations of induced E' centers and NBOHCs because the growth of the defects follows a first-order reaction rate law, in which the production rate is proportional to the concentration of precursors. This result means that the 'extrinsic' formation consists of a process different from the 'intrinsic' formation (such as the trapping of a hole, etc.) although the creation of electron-hole pairs is deduced to be essential for both the 'intrinsic' and the 'extrinsic' processes. As shown in Fig. 11, the concentrations of E' centers and NBOHCs in densified silica glasses increase sub-linearly with the ~/ dose. The sublinear growth behavior is similar to the 'intrinsic' dose dependence rather than the 'extrinsic' dependence, although absolute values are much greater than those in undensified silicas. Moreover, the concentration of E' centers was almost the same as that of NBOHCs. The similarity suggests that the 'intrinsic' formation of the pairs consisting of an E' center and an NBOHC is related to strained bonds even in undensified silicas. However, the number of strained bonds in undensified silicas is much smaller than that in densified ones. The same concentrations of E' centers and NBOHCs in s'.licas containing few

212

H. hnai, H. Hirashima /Journal of Non-Crystalline Solids 179 (1994)202-213

precursors indicate that the number of strained bonds in the silicas prepared by normal methods is almost the same although the number is influenced by fictive temperature and stress. Since even for silicas containing E' precursors, 'intrinsic' formation of E' centers and NBOHCs is expected to occur, all defect formation should be described by a combination of an 'intrinsic' component, in which the concentration is proportional to the square-root of the dose, and an 'extrinsic' one, which is explained by a first-order reaction rate law. In the case of silicas containing E' precursors, however, the influence of 'intrinsic' formation on growth of E' centers is negligible because the number of defects due to 'intrinsic' process is much smaller than that due to precursors. On the other hand, the growth of NBOHCs in silicas containing E' precursors ($2, S2H and SF1) shown in Fig. 4 is presumed to be attributed to 'intrinsic' formation because of their sub-linear growth behavior. The low concentration of NBOHCs in 'wet' silicas may be explained by the suppression of the 'intrinsic' formation due to the great number of E' centers generated by the 'extrinsic' process because electrons and holes are assumed to decay at the defect site. Fig. 12 shows that the concentration of E' centers generated at 77 K increases linearly with the dose and that the influence of pre-existing precursors, such as Si-Si and S i - H bonds, on E' formation is relatively small as compared with that of irradiation at room temperature. The stability of E' centers generated at 77 K is smaller than that of E' centers generated at room temperature because the former defects decayed at room temperature. These results suggest that E' centers are created at room temperature differently than at cryogenic temperature. Since the local structural relaxation around defects is depressed at cryogenic temperatures, trapping of free carriers is presumed to be the dominant process for the defect formation.

3.4. 'Anomalous' growth of E' centers in 'wet' silicas The growth behavior of E' centers in 'wet' s;U,.~ " ;-" S i - O H bcnds of ~ 102o cm -3 . . . . . . . ..,,,ntam..,~, is different from the 'intrinsic' formation and the

'extrinsic' formation. As shown in Fig. 7, the concentrations of E' centers in 'wet' silicas show a linear growth in dose regions higher than ~ 104 Gy, although the E' centers increases sub-linearly in the lower-dose region. A linear growth of E' centers was also observed in Suprasil 1 irradiated with X-rays as shown in Fig. 9. On the other hand, the concentrations of NBOHCs in 'wet' silicas are nearly proportional to the square-root of the dose, although the absolute concentrations are different in the different silicas. From the square-root dependence, E' centers in the lowerdose regions and NBOHCs in 'wet' silicas are suggested to be formed through the 'intrinsic' defect formation rather than the transformation of precursors. However, the concentration of NBOHCs in Suprasil 1 is greater than that of E' centers. The excess number of NBOHCs may be attributed to a great number of OH groups. The low concentrations of E' centers in the low-dose region are presumed to be related to recombination of the defects with molecular hydrogen which was suggested to exist in 'wet' silicas. Since the linearly increasing number of E' centers in the high-dose region is not observed in 'dry' silicas, the growth should be influenced by the great number of hydrogens or OH groups. Thus, precursors related to hydrogen, such as S i - O H bonds, are assumed to be changed into E' centers; the efficiency of the transformation is smaller than that of the other precursors. However, there is no experimental evidence for the E' formation form S i - O H bonds. Tsai and Griscom [34] suggested that the E' center formation in 'wet' silicas involves two excitons because the efficiency of E' formation increased with repetition rate of lasers. Thus, a multiple excitation process is implied for the linear growth of E' centers in 'wet' silicas. On the other hand, the formation efficiencies of NBOHCs in 'wet' silicas and E' centers and NBOHCs in 'dry' silicas were not changed by the repetition rate. These results suggest that the formation process for E' centers in 'wet' silicas is different from that of the E' centers in 'dry' silicas and from that for NBOHCs in 'wet' and 'dry' silicas. It is consistent with the above discussion for the 'anomalous' growth of E' centers in 'wet' silicas.

H. lmai, H. Hirashima / Journal of Non-Crystalline Solids 179 (1994) 202-213

4. Conclusions The forma~.;on of paramagnetic defects in silica glasses by radiation are classified into 'intrinsic' and ',:xtrinsic' processes. On the 'extrinsic' defect formation due to the transformation of pre-existing precursors, the concentration of the defects which increases linearly with the dose accompanied by a saturating tendency is explained by a first-order reaction rate law; the rate of defect production is proportional to the concentration of precursors. On the 'intrinsic' formation due to the cleavage of the S i - O bonds, the concentration of E' centers is substantially equal to that of NBOHCs, and both are approximately proportional to the square-root of the dose. In this case, the rate of defect production is assumed to be inversely proportional to the concentration of the induced defects. On the other hand, the 'intrinsic' defect formation through the knock-on process, which is induced by ion implantation, shows a linear growth of defects with the dose before the saturation. The similarity of the dose dependence of E' centers produced by several types of irradiation suggests that the electron-hole pairs have an essential role irrespective of incident photon energy for either the 'extrinsic' or the 'intrinsic' process. The authors are grateful to Shin-Etsu Chemical Co. Ltd. for supplying the s~;~ples. The resuits of this paper are based on the d~ia obtained mainly with the collaboration of K. Arai at Electrotechnical Laboratory, H. Imagawa at Toyo University, H. Hosono and Y. Abe at Nagoya Institute of Technology and .I. Isoya at University of Library and Information Science. One of the authors, (H.I.) wishes to give them thanks for their discussions and encouragement.

References [1] R.A. Weeks, J. Appl. Phys. 27 (1956) 1376. [2] M. Stapelbroek, D.L. Griscom, E.J. Friebele and G.H. Sigel Jr., J. Non-Cryst. Solids 32 (1979) 313. [3] F.J. Feigl, W.B. Fowler and K.L. Yip, Solid State Commun. 14 (1974) 225. [4] D.L. Griscom, J. Non-Cryst. Solids 73 (1985) 51.

213

[5] D.L. Griscom, Mater. Res. Soc. Symp. Proc. 61 (1986} 213. [6] D.L. Griscom and E.J. Friebele, Phys. Rev. B34 (198bJ 7524. [7] H. Nishikawa, R. Nakamura, R. Tohmon and Y. Ohki, Phys. Rev. B41 (1990)7828. [8] H. Imai, K. Arai, H. H~ ,ono, Y. Abe, T. Arai and H. lmagawa, Phys. Rev. B44 (1991)4812. [9] K. Arai, H. lmai, J. lsoya, H. Hosono, Y. Abe and H. lmagawa, Phys. Rev. B45 (1992) 10818. [10] R.A.B. Devine and J. Arndt, Phys. Rev. B39 (1989) 5132. [11] R.A.B. Devine and J. Arndt, Phys. Rev. B42 (1990) 2617. [12] C. Ito, T. Suzuki and N. lto, Phys. Rev. B41 (1990) 3794. [13] A.L. Shluger, J. Phys. C21 (1988) L431. [14] A.L. Shluger and E Stefanovich, Phys. Rev. B42 (1990) 9664. [15] F.L. Galeener, D.B. Kerwin, A.J. Miller and J.C. Mikkelsen Jr., Solid State Commun. 82 (1992) 271. [16] F.L. Galeener, D.B. Kerwin, A.J. Miller and J.C. Mikkelsen Jr., Phys. Rev. B47 (1993) 7760. [17] H. imai, K. Arai, J. lsoya, H. Hosono, Y. Abe and H. Imagawa, Phys. Rev. B48 (1993) 3116. [18] In this paper, 'wet" means silicas prepared directly from silicon tetrachioride by flame hydrolysis; the concentration of OH groups is ~ 10z° cm -3. 'Dry' means silicas other than 'wet'. [19] K. Awazu, H. Kawazoe, K. Harada, K. Kido and S. Inoue, J. Appl. Phys. 73 (1993) 1644. [20] R.A.B. Devine, J. Non-Cryst. Solids 107 (1988) 41. [21] Peroxy radicals were also observed in oxygen-excess P2 irradiated with v-rays [17]. The concentration of NBOHCs was estimated by subtracting the contribution of peroxy radical from the ESR signals for P2. [22] H. Imai, K. Arai, T. Saito, S. lchimura, H. Nonaka, J. Vigouroux, H. Imagawa, H. Hosono and Y. Abe, in: The Physics and Technology of Amorphous SiO 2, ed. R.A.B. Devine (Plenum, New York, 1988) p. 153. [23] K. Arai, H. lmai, H. Hosono, Y. Abe and H. lmagawa, Appl. Phys. Lett. 53 (1988) 1891. [24] T.E. Tsai, D.L. Griscom and E.J. Friebele, Phys. Rev. Lett. 61 (1988) 444. [25] A.J. Taylor, R.B. Gibson and J.P. Roberts, Opt. Lett. 13 (1988) 814. [26] T. Tomie, I. Okuda and M. Yano, Appl. Phys. Lett. 55 (1989) 325. [27] R.K. Brimacombe, R.S. Taylor and K.E. Leopold, J. Appi. Phys. 66 (1989) 4035 [28] A slight decrease in the concentrations in P2 irradiated with 6.4 eV lasers was observed However, the variation is sufficiently small for this discussion. [29] R.A.B. Devine and A. Golanski, J. Appl. Phys. 54 (1983) 3833. [311] D.L. Griscom, E.J. Friebele, K.J. Long and J.W. Fleming, J. Appl. Phys. 54 (1983) 3743. [31] D.L. Griscom, Nucl. Instr. Meth. B46 (1990) 12. [32] R.A.B. Devine, Phys. Rev. Lett. 62 (1989) "~40. [33] F.L. Galeener, J. Non-Cryst. Solids 149 ~1992) 27. [34] T.E. Tsai and D.L. Griscom, J. Non-Cryst. Solids 131 (1991) 1240.