Luminescence centers in SiO2 and SiO2:GeO2 vad rods sintered under reducing or oxidizing conditions

Journal or NonCryslaUinc Solids 95 & 96 North-Holland. Amsterdam

LLMINESCENCE

N.KOIlKETSU, Department Ohokayama,

679 -

(1987)

664 619

CENTERS IN SiO2 AND Si02:GeOp VAD RODS SIREDUCING OR OXIDIZING CONDITIONS

K.AWAZU,

H.WAPE

UNDER

and bl.YPMNE

of Inorganic Materials, &guro-ku, Tokyo 152,

Tokyo JAPAN

Institute

of Technology,

Photoluminescence was observed from VAD SiO2 and SiOz:GeOn preforms sintered under reducing or oxidizing condition. Three types of diamagnetic luminescence centers, Si-E’ and Ge-E’ center were found in glass samples. Intensity of the luminescence was strongly dependent on sintering atmosphere; reducing atmosphere accelerated the PL. The observations suggest that reduced Si or Ge species, Si(III), Si(II), Ge(II1) and Ge(II), are responsible for the PL.

1. INTRODUCTION Photolroliinescence(PL) neutronreported

studies

on silica

or electronirradiated the luminescence center

have

been carr;ied

out mainly

fy

silica glass and quartz . Kastner et al in neutron irradiated silica glasses having

an emission band at 4.0eV and an excitation band at 7.&V. The large Stoke’s shift( 3.&V) and the broad band width( 2eV) of the luminescence led to the suggestion

that

in silica

glasses

the.electron-phonon

ciently strong because of the large cation-anion and singly occupied dangling bond could be able positively

charged

silicon(Si+)

and negatively

those in chalcogenide glasses. However, existance of the charged defects. Si-E’

, OHC and peroxy

Griscom3 S.8eV

E’ center ESR signal. In a present paper

we mention

defect

in silica

glasses.

Ge and is diamagnetic characterized by 2 or

as a lone

suggested absorption about

The defect

and electrically pair

of electrons

was suffidifference a pair of

charged

there

radical,identified

luminescence center. and their corresponding

interaction electronegativity to change into oxygen(O-)

is no direct

by ESR, are

also

that lminescence correlated well the possibility is regarded neutral. located

3 oxygens.

0022-3093/87/$03.50 0 Elsevier Science Publishers-B.V. (North-Holland Physics Publishing Division)

to

for

the

candidates

for

at 4.3eV and 1.8eV with the intensity of of another

as reduced Its

similer

evidence

local

type

species configuration

of of Si or is

on Si or Ge atom surrounded

M. Kohkersu

680

The absorption model

CI al. / SO,

and phot9luminescence

and energy

levels

for

the

and .SiO,:Geq,

data

reduced

were

species

VAD

rodr

analyzed

in term

of Si or Ge were

of our deduced.

2. EXPERIMENTAL SiOp and 9BSiO,:lOGeO, sintered to preform rods and He gas. The mixing O,/He=l/l, l/Z or l/10. Polished luminescence

porous soot rods prepared at 1400°C under different ratio

of each gas was H,/He=l/ZO,

slabs with thicloless of and optical absorption.

the mesurments

of luminescence

specti-Lnn was calibrated by using range of 225-8001~11 is detectable out

by use of optical

vacuum.

X-band

3. RESULT Optical

ESR was measured

absorption

spectra

atmospheres are displayed sintered under reducing was observed. clearly (2401~s)

For

derived

the very absorption

was monochromatized

with

by a O.lm double grating excitation)

liquid

NP.

centers and to determine sealed in a silica sample

from For

of photoin

solution. PL in wavelength PL measurment at 77K was

on JES-FE2XG

in Fig.1. atmosphere,

spectrometer

at

samples

sintered

the cace

of pure

an weak absorption glasses prepared under

90Si02:10Ge02

visible are and an weak

cooled

paramagnetic rods were

Op/He

or 1 /lOO

system consisted of 0.25m tube. The PLE(luminescence

rhodamine-B effectively.

cell

ESR was measured to identify concentration. Tips of preform

l/SO

Smm were used in the measurments A Xe-lump was used for excitation

and the emission

grating monochrometer. The detection monochrometer and a photomultiplier

carried

by VAD process were atmosphere of Hz/He,

77K.

under silica

band at reducing

their tube under

different glasses

5.17eV(240nm) condition

strong absorption band with maxima at band at 3.77eV(33Chun). The intensity

5.17eV

of the

absorption

at 5.17eV of pure silica glass is 100 times weaker than that of On going from reducing condition to oxidizing, the intensity 90Si02:10GeOl. of the absorption decreased for both SiOz and SiOz:GeOz glasses. It is interesting to note that in the case of pure silica glasses(Fig l(a)) no optical absorption absorption

was induced was detected

dition and even The intensity very

by sintering under for 90Si02:10Ge02

under O,/He=l/lO of the absorption

weak and for

disappeared. This Fig.2 shows the

the

sample

He atmosphere, while the strong glasses sintered under similer

atmosphere weak band at 3.77eV

sintered

under

absorption could for 90Si02:10Ge02

He and O*/He

atmosphere

band was more clearly seen in PLE spectra. emission spectra of SiOz and 90Si02:10Ge02

glasses

con-

be detected. glasses was this

band

sintered

M. Kohkem

0.5

A ‘5

0.4

1

0.3

(a)

CI al. / SiO,

and SiO,:GeO,

SiOl

VAD

lb)

6

rods

681

90Si02:

l,OGeOz

t

A;; (3) (4)

5 0.2 2 0.1

-!4&

0

6 Photon

5 Energy

4

6

(cv)

5 Photon

Fig. 1 Optical absorption spectra of (a)SiO, sintered under difrerent atmosphere (a) (1)reducingu (4)oxidizing

and

(b)90SiO,:lOGeO,

RT 273K

5 4 3 Photon Energy (eV) Fig.2

2

-

77K

L

6

4 (eV)

Energy

-I\

-Photon

-

A

J 5

glasses

b 4

3

F.nerb(eV)

Temperature dependence of photoluminescence spectra of (a)SiO, (b) 9OSi0,: lOGe0, glasses sintered under H,/He=l/20 atmosphere The excitation energy was 5.06eV and 4.77eV, respectively.

?

and

M. Kohkemr

682

under

atmosphere

H2/He=1/20

temperature. 4.2(295nm)

CI al. / SiO,

and the change

All spectra and 3.leV(400nm)

and Si02:Gc0,

VAD

of spectra

temperature

was reduced by a fatter

The change in relative varying excitation energy

band at 3.0eV was induced band. The emission band

absorption

band consists

PL band at 3.0eV

intensities of two emission bands is sho\+n in Fig.3. It is clearly of hgo

luninescence

excitation Therefore,

energies of 5.06(245nm) and 5.2(238nm)eV. cence centers could be separated clearly;

one center

S.2eV and coresponding

at 4.13

two emission

bands

has an excitation

under different conditions. The spectrtaa for 9OSi02:1OGeO2

The emission band was monitored glass sintered under reducing

consists

of three

visualize

spectra spectrtaa

For with

bands with

peak maxima

the

sintered

under

5.06eV(245nm)

and

maxima

sample at

are almost in harmony in Fig.4 is difference

the change

5.5 Photon

5.0 Energy

(eV)

4. i

and the glasses

at 4.77eV(26Onm), oxidizing

has

sintered

3.76eV(33Onm)

condition

S.ZeV(240nm)

more clearly.

band at other

at 3.leV(400nm). condition(HziHe=

with the optical absorption spectnm between (1) and

of PLE by reduction

by

of Si02 with noted that 5.1~eV

and 3.leV,

and an emission band at 4.2eV. spectra at RT for 90Si02:10Ge02

and 5.06eV(245nm). excitation bands

increased

bands having peak two types of lumines-

an excitation band at 5.06eV Fig.4 shows the excitation

These upper

at

90Si02

from RT to 77K, the intensity of emission band at 3.1 by a fatter of ~10 and that of the band at 4.2 or J.leV of-4. In SfISi02:10Ge02 sample PLE spectrum has another

excitation band at 3.76eV and its corresponding fatter of -3 with the cooling from RT to 77K.

l/20)

measurment

was induced by exitation of S.l?eV band. For SiOz both 4.2 and 3.leV bands were induced by excitation of 5.17eV absorption band.

Decreasing or 3.0eV increased

changing

have two distinct emission bands with maxima for SiO *, 4.1 (3001~~) and 3.OeV(413nm) for

:lOGeO:. For the case of 90SiOZ:10Ge02 the emission by the excitation of both 5.17 and 3.77eV absorption

at 4.leV emission

with

rods

were

two obsewed.

spectra. The (2) in order to

Two PLE bands with

Fig.3 Change in relative intensity of two bands in emission spectra with varying excitation energy. The energies in the upper right are that used for monitering emission intensity.

M. Koltkersu

peak

at

4.77eV

appear

and 3.76eV

in parallel.

er 01. / SO2

became

This

species possibly

of Ge by reduction and related to the same origin. with

rods

683

to

that these two bands are from the formation of reduced

the PLE bands

VAD

fact

suggests arising

For

and SiO>:GeO,

peak

maxima

at

5.06 and 5.2eV the intensity decreased on going from H,/He=1/20 to O,/He=1/13, but were detectable even for the case of O,/He=l

which

/lo,

condition

expected

to be slightly more reducible than and enough to reduce GeOl in silica

Photon

air

Fig.4 PLE spectra glasses sintercd The conditions. tored at 3.leV. this figure shows spectrum between

network. Si-E’ magnetic

center species

was the only paradetected for all

sample glasses. No distinct correlation was noticed between 90Si02:10Ge02 glasses Si-E’ and Ge-E’ center The correlation centers

were

between not

still

SiO,

Si-E’

sintered under were detected. Ge-E’

Energy

concentration H,/He=l/ZO,

Ge-E’

concentration

(eV)

of 9OSiO~:l9GeO~ under different emission was moniThe upper part of a difference (1) and (21.

and PL intensity. He,

OZ/He=q/l,

was dominant and intensity

for

l/2

all

For and

l/10

samples.

of luminescence

clear.

4. DISCUSSION It is clearly noted that there exist two diamagnetic in the silica glasses sintered under reducing conditions cence centers in The consentration ing . related possible

9OSi02:1OGeO2

of the

centers

has an electronic

centers lumines-

glasses sintered under reducing atmosphere. decreased on going from reducing to oxidiz-

The atmosphere dependence of these to reduced species of Ge or Si. models for luminescence centers

Ge(II)(Si(II))

luminescence and three

centeres suggests that they are The schematic energy diagrams and are displayed in Fig.5. Free ion of

configuration

of 4s(3s)

and a lone

pair

located on Ge(Si) atom. This would form a lone pair level higher than the upper valence band and possibly shows strong absorption which characterized 4~-4sp(3~-3sp) may be strongly not

transotion localized

of Ge(I1) on Ge(Si1

(Si(I1)) atom,

around 5eV. Since the lone the electron-phonon interaction

pair is

expected to be strong. This center would be expected to act as an isowhich leads to relatively small electronic Tl+ type luminescence center, Stokes shift and narrow band width. In Fig.S(a) are shown the energy diagram

as

684

M. Kohkersu

and its nehmrk.

luminescence This center

at 4.77eV and 3.0eV

the two absorption bands, From temperature dependence

strongly

assisted

irradiation

by phonon,

VAD

rods

that

is,

At 77K the energy

in silica characterized

and a weak absorption are regarded as the

respectively. of PL the emission

seems to be tr‘ansfoolmed The lower phonon.

interaction with state radiatively.

and Si0,:Ge02

mechanism of Ge(I1) center dissolved has a strong absorption band which

4s-4sp transition of Ge(I1) The emission bands at 4.leV for

er 01. / SO2

to the excited

state

that

lower excited state would

of phonon

band at 3.76eV. reverse transition

band at 3.0eV

the excited

glass as

would

excited

be by 4.77eV

state through the relax to the ground

is not

large

enough

to permit

the transformation between the two excited state, and the emission band at 3.0 eV will become weak. The lower excited state of Ge(I1) is still not characterized,

but

the

lowest

excited

triplet

From the similarity of the emission spectra luminescence

state

would

expects to be identified as Si(I1). Xe-Gez (rSi-Si-) homobondftlay be a candidate this model reported.

the

absorption

be a possible

energy diagram and the center(b) ,which exists

should

be a-u*,

of

which

candidate.

temperature dependence of in both SiO, and Si02:Ge0, luminescence

accurate

energy

center(c)

.

has not

been

(cl

Ti-

5.1

4.2

Ge-Ge SYfSi Fig.5

Schematic energy diagrams and possible models of luminescence centers in SiOz or 90Si02: 1OGeOz glasses sintered under reducing condition The upward arrow shows absorption and the downward shows emission.

REFERENCE.5 1) C.E.Jones 2) C.M.Gee 3) D.Griscom, 33(1979) 4) G.W.Amold,

and D.Embree, and M.Kastner, Proc.

J. Appl. Phys.

frequency

Rev. control

Phys. letters symp.

47(1976)

5365

42(1979) Electronic

98 IEEE Trans.

Nucl.

Sci.

NS-20(1973)

220

1765 Industries

Association

In