Vacuum ultraviolet optical absorption band of non-bridging oxygen hole centers in SiO2 glass

Article abstract of DOI:10.1016/S0038-1098(02)00118-7

An intense broad vacuum ultraviolet (VUV) optical absorption band with peak at 6.8eV and halfwidth ≈ 1.7eV is identified in irradiated glassy SiO₂ and assigned to dangling oxygen bonds (non-bridging oxygen hole centers, NBOHC). It was selectively created by photolysis of silanol (SiO-H) groups by 7.9eV photons of F₂ excimer laser at low temperature. Subsequent analysis by VUV absorption, time-resolved luminescence, and electron paramagnetic resonance spectroscopies during thermal annealing showed an exact correlation to the well-known 4.8 and 2eV absorption and 1.9eV luminescence bands of NBOHC. The estimated oscillator strength of the 6.8eV band is f ≈ 0.05. This band may be one of the dominant causes of VUV optical absorption induced by excimer-laser irradiation of silica.

Full text of DOI:10.1016/S0038-1098(02)00118-7

PERGAMON
Solid State Communications 122 (2002) 117±120
www.elsevier.com/locate/ssc
Vacuum ultraviolet optical absorption band of non-bridging
oxygen hole centers in SiO glass
2
a,b,*
a
a
a
, Koichi Kajihara , Takenobu Suzuki , Yoshiaki Ikuta ,
Hideo Hosono
b,1
Linards Skuja , Masahiro Hirano
a
a
Transparent Electro Active Materials, Exploratory Research for Advanced Technology (ERATO), Japan Science and Technology,
KSP C-1232, Sakato 3-2-1, Takatsu, Kawasaki 213-0012, Japan
b
Materials and Structures Laboratory, Tokyo Institute of Technology, Nagatsuta, Midori-ku, 226-8503 Yokohama, Japan
Received 13 December 2001; received in revised form 9 March 2002; accepted 11 March 2002 by J.H. Davies
Abstract
An intense broad vacuum ultraviolet (VUV) optical absorption band with peak at 6.8 eV and halfwidth < 1.7 eV is identi®ed
2
in irradiated glassy SiO and assigned to dangling oxygen bonds (non-bridging oxygen hole centers, NBOHC). It was selec-
tively created by photolysis of silanol (SiO±H) groups by 7.9 eV photons of F excimer laser at low temperature. Subsequent
2
analysis by VUV absorption, time-resolved luminescence, and electron paramagnetic resonance spectroscopies during thermal
annealing showed an exact correlation to the well-known 4.8 and 2 eV absorption and 1.9 eV luminescence bands of NBOHC.
The estimated oscillator strength of the 6.8 eV band is f < 0:05: This band may be one of the dominant causes of VUV optical
absorption induced by excimer-laser irradiation of silica. q 2002 Elsevier Science Ltd. All rights reserved.
PACS: 61.43.Fs; 61.72.Ji; 61.80.Ba; 78.40.Pg
Keywords: E. Electron paramagnetic resonance; E. Luminescence; B. Laser processing
1
. Introduction
glass: the band peaking at 7.6 eV [7] due to Si±Si bonds,
and absorption at .7.4 eV [8] arising from silanol groups
Information on VUV optical absorption of color centers
in SiO glass is important for rapidly growing demand of
2
Si±OH. Irradiation by F laser pulses induces pairs of oxy-
gen dangling bonds (non-bridging oxygen hole centers,
0
2
optical elements for vacuum and deep UV optics. Optical
elements (lenses and mask blanks) for F -laser lithography
NBOHC) and E centers by excitation of heavily strained
Si±O±Si bonds [9]:
2
aiming a minimum feature size of 100±70 nm [1±3] and
optical ®bers [4] for deep UV light are representative appli-
cations. While the visible and UV parts of the absorption
z
0
z
xSi±O±Sixꢀstrained† ! xSi ꢀE † 1 O±SixNBOHC
ꢀ1†
spectrum of SiO have been extensively studied, [5,6], the
2
NBOHC is one of the fundamental defects in amorphous
SiO , which contributes to visible/UV absorption spectrum
vacuum ultraviolet (VUV) region has received relatively
little attention, evidently due to its location in the spectral
range, inaccessible by conventional spectrophotometers.
There are two identi®ed VUV optical absorptions in SiO2
2
and gives rise to characteristic photoluminescence (PL)
peak at 1.9 eV [10±12]. It is established that the NBOHC
has two absorption bands at 1.97 and 4.8 eV. The spectral
characteristics of these bands are quite different [13,14]: the
* Corresponding author. Address: Materials and Structures
Laboratory, Tokyo Institute of Technology, Nagatsuta, Midori-ku,
1
.97 eV band is weak and narrow, while the 4.8 eV band is
broad and intense (Table 1). Both bands serve as excitation
bands to the 1.9 eV PL band. While it is known that this PL
band can be excited in the VUV region as well, no distinct
optical band of the NBOHCs in VUV region has been
reported so far. This situation is caused by the overlap of
2
26-8503 Yokohama, Japan. Tel.: 181-45-924-5359; fax: 181-45-
24-5339.
9
E-mail address: hosono2@rlem.titech.ac.jp (H. Hosono).
1
On leave from Institute of Solid State Physics, University of
Latvia, Latvia.
0038-1098/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved.
PII: S0038-1098(02)00118-7

118
H. Hosono et al. / Solid State Communications 122 (2002) 117±120
Table 1
Summary of optical absorption bands due to NBOHCs in SiO
2
glass
a
b
c
Band
I
II
III
Peak position (eV)
FWHM (eV)
1.97
0.17
4.8
6.8
1.07
0.05
1.76
0.05
2
4
Oscillator strength
1.5 £ 10
a
Ref. [13].
Refs. [13,14].
b
c
Present work.
several broad absorption bands, typically taking place in the
6
±8 eV spectral region.
However, contributions by different centers can be separ-
ated if they can be created selectively. It was recently
reported that F laser irradiation destroys bound hydroxyl
silanol) groups in silica, which have absorption at energies
2
Fig. 1. Optical absorption induced by F laser irradiation at tempera-
2
ture below 95 K. Inset shows the F -laser-induced EPR signals.
2
(
Both spectra were measured at 77 K. Deconvoluted and synthesized
spectra are also shown. The negative induced component at
higher than 7.4 eV. It is anticipated that NBOHCs in this
case are created not by the rupture of strained Si±O±Si
bonds, but rather by an extrinsic mechanism, by photolysis
of silanol groups [15,16]:
.7.4 eV is due to photo-modi®cation of SiOH as described
[
hyper®ne structure of atomic hydrogen and the signal appearing in
[8,15,18]]. A doublet with a separation of 50.6 mTis due to the
0
the central region is due to NBOHC and E centers.
z
z
xSi±O±H ! xSi±O 1 H;
ꢀ2†
3
. Results and discussion
z
0
0
where H denotes atomic hydrogen, H . Since H is reactive
and highly mobile at the room temperature, the decay of
NBOHCs and formation of other color centers occurs as a
secondary reaction [17]. Thus, in order to maximize the
formation of NBOHC and suppress the secondary reactions,
Fig. 1 shows the VUV±UV±visible absorption induced in
2 2
wet-SiO glass by low-temperature F laser-irradiation.
Three peaks are evident: the most intense peak at 4.8 eV,
a medium intensity peak at ,6.8 eV, and a low-intensity
peak at ,2 eV. Inset shows EPR signals induced by F -
2
2
irradiation by F laser pulses at low temperature is desirable.
Here we report an intense optical absorption band of
NBOHC at 6.8 eV, which may be one of the major causes
of VUV optical absorption of irradiated silica.
laser irradiation. A doublet with a separation of 50.6 mT
is undoubtedly identi®ed as the hyper®ne structure of
atomic hydrogen [18]. This signal disappeared after the
as-irradiated specimen was brie¯y warmed to 150 K. The
broad signals appearing in the central region are primarily
0
due to NBOHC and a trace of E center. The concentrations
0
2
. Experimental
of atomic hydrogen, E , and NBOHC were evaluated from
the areal intensity of each EPR signal measured under an
Samples used here were SiO glasses prepared by ¯ame
2
0
hydrolysis of SiCl
intensity of infrared absorption band at 3670 cm was
4
. The SiOH content evaluated from the
2
appropriate microwave power level (,1 mW for H or E and
30 mW for NBOHC). The evaluated concentration of
1
2
0
23
3
1
£ 10 cm . The sample size was 20 £ 20 £ 1 mm . T he
samples were placed in cryostat with CaF windows and
were irradiated in vacuum by 7200 pulses of an F excimer
laser (Lambda Physik, Compex 102) delivered at 20 Hz,
NBOHC is comparable to that of atomic hydrogen, and
0
the E concentration is ,1/10 of NBOHC. These results
2
demonstrate that reaction (2) selectively occurs under the
experimental conditions. Since atomic hydrogen has no
optical bands in the visible±VUV range, the induced optical
bands of Fig. 1 can be primarily attributed to NBOHC.
The negative-going component in the 7.5±8 eV region in
induced absorption spectra indicates the VUV irradiation-
induced modi®cation of silanol groups in silica [8], which
have optical absorption in this region.
2
2
2
pulse width 10 ns, power density ,2.5 mJ cm . T he
cold ®nger of cryostat was kept at 77 K. It was estimated
that the sample temperature during irradiation was below
9
5 K. The VUV and UV±visible absorption spectra and
electron paramagnetic resonance (EPR) were measured at
7 K without an intervening warm-up. Time-resolved
luminescence was measured with gated multichannel detec-
tor using excitation by F (7.9 eV) or ArF (6.4 eV) excimer
7
The induced absorption was deconvoluted into 3-
Gaussian bands on the basis of the induced EPR centers,
i.e. a band peaking at 4.8 eV with the FWHM of 1.1 eV
due to NBOHC [19], a band at 5.8 eV with the FWHM
2
lasers, or fourth harmonics (4.7 eV) from a Q-switched Nd/
YAG laser. EPR spectra were recorded by Bruker E580
spectrometer.
0
of 0.8 eV due to E center [6], and a band at ,6.8 eV in

H. Hosono et al. / Solid State Communications 122 (2002) 117±120
119
Fig. 2. Variation in the intensity of 4.8, 5.8, and 6.8 eV absorption
bands destroyed by isochronal annealing of the specimen irradiated
at T , 95 K. Annealing time was < 10 min at each temperature.
All the optical measurements were carried out at 77 K.
2
Fig. 3. PL spectra F laser irradiated silica and PL decay kinetics
recorded at excitation by 7.9, 6.4, and 4.7 eV photons.
band of NBOHC [10]. The 4.7 eV photons excite the 4.8 eV
band of NBOHC while 6.4 eV photons excite the 6.8 eV
band. These observations further support the assignment
of the 6.8 eV band to NBOHC.
question. To account for irradiation-induced change in
SiOH absorption, the shape of the optical absorption
spectrum of silanol groups [8] was approximated by a poly-
nomial and added to the ®tting function. In the deconvolu-
tion procedure, only the peak position and FWHM of the
Table 1 summarizes the optical absorption bands of
NBOHC in SiO glass. The values of f and FWHM of the
2
newly identi®ed 6.8 eV are quite similar to those of the
6
.8 eV band were allowed to vary, the shapes of other three
tting components were kept constant, only their amplitudes
4.8 eV, in contrast to the 2 eV band, which has much smaller
f and FWHM. Although the theoretical calculations of opti-
cal properties of NBOHC are reported by several groups
[20±23], the character of the optical transitions is still
controversial. The present ®nding of the 6.8 eV band pro-
vides an additional clue to theoretical understanding of
®
were changed. As shown in Fig. 1, a satisfactory agreement
was obtained between the observed and synthesized shapes.
When the as-irradiated specimen was annealed at tem-
peratures between 100 and 300 K, part of the induced
absorption diminished. All the difference spectra between
the 77 K-irradiated and annealed specimen were success-
fully decomposed into the same 3 Gaussian bands keeping
constant peak positions and widths. Fig. 2 shows the varia-
tion of the normalized intensities of these bands with ther-
mal annealing. All the three bands decayed almost
simultaneously in the range 100±300 K.
NBOHC in SiO glass. Furthermore, this ®nding is impor-
2
tant for practical applications of SiO glasses for VUV and
2
deep UV optics. The 6.8 eV band of the NBOHC has rela-
tively large oscillator strength and halfwidth (1.75 eV). It
appears to be a major cause of absorption at ArF (6.4 eV)
and F (7.9 eV) laser photon energies. Evidently, the sup-
2
pression of NBOHC formation is an important issue to
improve the quality of the SiO glasses for excimer-laser
applications.
0
The simultaneous decay of the 5.8 eV band due to E -
centers and that of the 6.8 eV band could link the latter
2
0
band to E -centers. However, this possibility can be dis-
counted because of the excessive magnitude of the 6.8 eV
Acknowledgements
band: with the oscillator strength of 5.8 eV band of f ˆ
0
:15±0:20 [6], the estimated oscillator strength of 6.8 eV
band then would reach an unrealistic value of f ˆ 2:2: On
the other band, if the 6.8 eV band is assigned to NBOHC,
the evaluated oscillator strength has a reasonable value of
f < 0:05: We suggest that 6.8 eV absorption band is due to
transitions to higher-energy excited state of NBOHC.
To prove the assignment of the 6.8 eV band, the PL
measurements at different excitation photon energies (7.9,
The authors thank Takeru Kinoshita and Tomoaki
Ichimura for their experimental assistance. L. Skuja was
in part supported by grant #LZP010808.
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1