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Time-resolved low temperature luminescence of non-bridging oxygen hole centers in silica glass
~
Solid State Communications, Vol. 84, No. 6, pp. 613-616, 1992. Printed in Great Britain.
0038-1098/9255.00 + . 00 Pergamon Press Ltd
TIME-RESOLVED LOW TEMPERATURE LUMINESCENCE OF NON-BRIDGING OXYGEN HOLE CENTERS IN SILICA GLASS LSkuja*) PhysikalischesInstitut, Universitat Munster Wilhelm-Klemm Str. ] 0, W4400 MUnster, Germany Received August 2 6, 1992by TP.Martin
The spectral, temporal and polarization characteristics of the non-bridging oxygen (NBO) hole center luminescence band at 1.9 eV in neutron irradiated silica glass has been studied by site selection techniques. A resonant zero-phonon line and vibrational structure 890 cm-l apart emerge at temperatures below 65 K under monochromatic 632.8 nm excitation. The vibrational structure is attributed to the symmetric stretching mode of silicon - NBO bond. The highly polarized zero-phonon line is due to the charge transfer transition between the 2p-like non-bonding orbital of the NBO and the filled 2p lone pair orbital of one of the 3 ligand oxygen atoms in the same SiO2 tetrahedron. The well-known absorption band at 4.8 eV corresponds to the transition from ts NBO-Si bonding state to the non bonding 2p orbital of NBO.
1. Introduction
luminescence line narrowing under monochromatic laser excitation at helium temperatures. The NBOHC seemed to be a promising object for this techniques, since the small Stokes shift allows to expect intense homogeneous zerophonon lines (ZPL), necessary for the site selection to be efficient12,13, but the relatively long lifetime (x~-15 Its at 80 K 10) enables one to separate resonance PL from the scattered excitation light by registering delayed spectra.
Optical absorption bands at 4.8 eV and 2.0 eV and the associated photoluminescence (PL) band around 1.9 eV are characteristic features of irradiated high purity silica glass. They are introduced by neutron 1, gamma 2, X-ray I or ultraviolet (UV) 3 irradiations and by optical fiber drawing process or later applied mechanical stressa. The 1.9 eV PL band is observed also as a surface center created during mechanical fracture of silica5. Both absorption bands and the PL band have been attributed to a single defect in silica glass - the non-bridging oxygen hole center (NBOHC), i.e. an undercoordinated O atom bonded to a single Si atom with a hole in its non-bonding 2p orbital: -Si-O'l,5,6 The NBOHC's have been thoroughly investigated by electron paramagnetic resonance (EPR)7, s, see Ref9 for the latest review. The nature of the electronic transitions responsible for the 2.0 and 4.8 eV absorption and 1.9 eV PL bands is still not well understood. The reported PL polarization properties (polarization degree P=+12% when excited in 2.0 eV band and P=-1.5% l0 under excitation in the 4.8 eV band) are presently unexplained. Attempts to obtain more information by experiments at liquid He temperatures so far have met with little success - the optical spectra remain structureless even at liquid He temperatures II. The EPR investigations7 show however that an inhomogeneous broadening of several tenths of eV is present. This could smear out the vibrational structures at low temperatures. The purpose of the present work is to gain more insight into the electronic structure of the NBOHC using
2. Experimental. Samples of commercial high purity "wet" synthetic silica Coming 7940 irradiated by neutrons at fluencies 1018 to 1020 neutrons/cm 2 were placed in a helium flow cryostate with temperature variable between 8 and 300 K. Samples were excited in the 2.0 eV band by He-Ne laser (~. = 632.8nm, ~lmW) and in the 4.8 eV band by 75 W Xe lamp filtered through Jobin-Yvon Hi0 0.1m grating monochromator. PL was collected at right angle using 0.2 m AMKO LTI monochromator with grating blazed at 750 nm and Hamamatsu R955 photomultiplier ($20 type spectral response). Emission spectra were multiplied by Z,2 to account for the changes in monochromator dispersion, but were not corrected for the spectral efficiency of the monochromator and photomultiplier. DC-mode emission spectra were recorded directly with analog to digital converter card in 80486 based personal computer (PC). Time resolved spectra were measured by chopping the focused laser beam with a mechanical chopper and recording the delayed PL during the dark periods of the chopper. By careful aligning transition times of less than 5 Its were attained. Photomuhiplier output was processed by Ortec9302 discriminator and gated Ortec 994 counter. A delay of
*)Alexander von Humboldt foundation fellow, address after Dec.31,1992: Institute of Solid State Physics, Latvian University,Kengaraga str.8, LV-1063 Riga Latvia. 613
Vol. 84, No. 6
NON-BRIDGING OXYGEN HOLE CENTERS
614
WAVELENGTH/nm
10 I-ts after excitation pulse and gate width of 80 Its commensurate with the decay time of the 1.9 eV PL (ca 20 Its at 10K) was typically used. This provided an efficient suppression of the scattered excitation light (better than 106 times). Polarization spectra were measured using polarization filters. The excitation light was vertically polarized and successive emission spectra with horizontally (lh(~.)) and vertically (Iv(~.)) oriented polarizer before the photomultiplier were measured. The polarization spectra were calculated as
725
625
i
-'----------to 50 K
o
~---o ~---- o ~ 1.85
1.7
1.75
1.8
1.85
1.9
o o
2.0
1.95
PHOTON ENERGY/ eV
DC-mode PL emission spectra under He-Ne laser excitation ( l . 9 6 e V ) are shown in Fig.1. At low temperatures the anti-Stokes part of the spectrum seen at 295 K vanishes and below 70 K a weak line 890 cm -1
WAVELENGTH /nm 700 675 650
t s'°'ta""c't t
650
I
3. Results
m--~60 ~- He-Ne laser, 1 mw I
675
hvexc'1.96e~/ ' SgOcm" ~1 Delay = 10/~sec [ - j ~ Gate = 8 0 / z s e e / v ~
P(~.) = ( 1 - KI h / I v ) ( l + KI h / Iv) -I where K(~.) is an instrumental correction function, measured as Iv(Z,)/lh(~.) for sample substituted with a depolarized light source. Decay time measurements were performed by sweeping over 100 I.tS the time delay between the end of the excitation pulse and 2 Its wide strobe pulse gating the photon counter. The variable delay was produced by PC's internal AMD8254 timer and data were averaged over ca. 105 sweeps.
725
700
625
I
I
"
-t 89
t
50
40
Fig.2. Time-resolved luminescence of neutron-irradiated glassy silica measured 10 p.s after the exciting He-Ne laser pulse at temperatures between 10 and 300 K. Baselines of individual curves are shifted but y-scale is the same for all curves. Line at 1.96 eV is purely due to resonance luminescence.
(0.110eV) apart from the excitation line emerges at 1.85 eV. Signal intensity at excitation energy also increases at l o w temperatures (off-scale in Fig. 1), but that can not be reliably attributed to resonance PL, since any small changes in the sample geometry due to the thermal contraction could unpredictably affect the intensity o f the light scattering. He-temperature emission spectra, measured under wide band (A~.=8 nm) excitation in the 4.8 eV absorption band (not shown) did not exhibit any structures, in agreement with the previous work II .
30 WAVELENGTH /nm 700 675 650
725
20
I
,,,0.5 Ill n"
10
1.65 1.7 1.75 1.8 1.85 1.9 1.95 2.0 PHOTON ENERGY eV
0
L0 uj0. 4
h•exc=1.96
I
625
l
|
oV
0,5
--~ /
DC mode
0.4
a
~ 0.3
0.3
~0,2
0.2
o5Ol 0.0
Fig. 1. DC mode photoluminesccnce emissmn spectra of neutronirradiated glassy SiO 2 measured under He-Ne laser excitation between 10 and 295 K. The Y-scale is the same for all spectra. Resolution is shown by vertical bars. The structures emerging 890 cm"1 apart from the exciting line are marked by a vertical arrow. The off-scale line at 1.96 eV is partly due to scattered laser light, partly due to resonance luminescence.
I
1.65
0.1
9K •
I
1.7
I
,
I
.
I
,
I
,
1
|
1.75 1.8 1.85 1.9 1.95 2.0 PHOTON ENERGY/eV
.
0.0
Fig.3. Photoluminescence polarization spectra measured in DCmode at 295 (solid line) and 9 K (points). The feature at 1.96 eV in the 295 K spectrum is due to the scattered excitation light.
NON-BRIDGING OXYGEN HOLE CENTERS
Vol. 84, No. 6
Time-resolved emission spectra are shown in Fig.2 The 300 K spectrum demonstrates a complete suppression of the scattered excitation light. Below 90 K a resonant zero-phonon line (ZPL) emerges. It's observed width was determined by the monochromator resolution (ca. 7 meV). Similarly to the DC-mode spectra (Fig. l) the structure at 1.85 eV also becomes apparent at He temperatures despite the poorer signal/noise ratio. By taking spectra at different strobe delays it was proved that both line structures have the same decay times as the underlying broad-band spectrum. The line to broad-band spectrum intensity ratio remained constant when measured in several samples with different PL center concentrations. Fig.3 shows the PL polarization spectra, measured in the DC mode at 295 and 9 K The feature in the 295 K spectrum at 1.94-1.97eV is an artefact due to the interference from the highly polarized scattered excitation light. Outside this region (including the anti-Stokes region) the spectrum can be considered as reliable within the noise limits. The polarization spectrum in the ZPL region, measured in the time-resolved mode is shown in Fig4. The spectrum does not show any singularities at the resonance energies. Decay time measurements yielded slightly unexponential decay curves (not shown) with average x around 13 Its at 295 K and 20 p.ts at 10 K Measurements at the ZPL (1.96 eV) gave practically the same decay curve as measurements performed at monochromator set off the resonance line (1.90 eV). 4. D i s c u s s i o n .
The similar time decays of the resonance line and broadband PL as well as their constant intensity ratio in different samples indicate that both features belong to the same center. The absence of any vibrational structure in the 1.9 eV PL spectrum upon the non-selective excitation in the 4.8 eV band is explained by an inhomogeneous broadening due to the disordered structure of silica glass. The line at 1.96 eV (Fig.2) then corresponds to a fraction of centers whose ZPL energies coincide with the He-Ne
laser excitation energy. To our knowledge this is the first observation of ZPL in undoped oxide glasses. The vibrational structure 890 cm -I apart from the ZPL (Figs.l,2) is probably due to the symmetric stretching vibration of the Si-O bond in NBOHC. The NBOHC concentrations are too low to obtain the energy of this mode directly from the infrared spectra However, as shown by Raman investigations of hydroxyl containing silica 14, the symmetric stretching mode of an O-H group as a whole against the Si atom it is bound to has an energy of 969 cm -I N B O H C differs structurally from a bound O-H group only by the absence of the hydrogen atom. Since SiO bonding in both cases is expected to be similar and the mass of H atom is small, one can expect, that Si-O stretching modes in both cases will have close energies. As pointed out in Ref. 12, in cases when the inhomogeneous broadening is sufficiently large, vibrational structures seen in the emission spectra can be in fact ZPL's of the centers who are excited to the first vibrational level of the excited electronic state. The observed structure in emission then corresponds to the vibration energies in the excitedand not as is usual - in the ground electronic state. Because the excited state vibrational energies are usually smaller than in the ground state, correspondence between the observed energy of 890 cm -I for NBOHC and the 969 cm -I in the Si-(O-H) is quite fair. Within the emission band the PL polarization is essentially dependent on wavelength (Fig3). The previously reported 6 low values of P=+12% led at that time to the suggestion that at least one doubly degenerate state takes part in emission transition. However, the data of Ref.6 were in fact measured only for the low-energy part of the emission spectrum, since a cut-off filter transmitting ~. >700 nm was used. This agrees well with the spectrum in Fig3 for those wavelengths. The current data show that in the ZPL region P values reach +0.45 ( F i g 4 ) That is close to the upper theoretical limit (P=0.5) for PL in disordered systems |5 and indicates that both absorption and emission can be modelled by a simple electrical dipole whose orientation remains the same for both transitions. Hence O~Z
i
LU
~0.5 t3 80.4
/"7/
WAVELENGTH / nm ~0 ~5 645
650
i
~0
i
i
huexc=196 eV Delay = 10/~sec Gate = 80 #sec
¢'~
i
. 0" ~,,~----. o ~ o 0 o- 0 , ,
---.0.2 cr
30 ~
50 0 . 1 o,,.
0.0
I
191
~
I
1.92
=
I
1.93
PHOTON
J
I
1.94
,
I
1.95
,
I ~ "
1.96
- ny
z
RX ny
x
~ 11
ao2
~0.3
615
I~
fly nx
11o.;,
low 0
1.97
ENERGY/eV
Fig.4. Time resolved photoluminescencc polarization spectrum measured in the zero-phonon line region at 9 K (circles, the left scale). One of the two emission spectra, used to calculate the polarization spectrum is shown below for comparison (solid line, the right scale).
Fig.5 The optical transitions in the non-bridging oxygen hole center. The o-bonding in SiO4 tetrahedron is denoted by straight double lines, both nearly degenerate non-bridging oxygen 2p orbitals ny and nx are schematically shown, nL is the oxygen 2p lone pair orbital, belonging to one of the 3 ligand oxygen atoms. a is the angle between dipole moments of 4.8 eV absorption and 1.9 eV emission transitions.
616
NON-BRIDGING OXYGEN HOLE CENTERS
the ZPL transition takes place between nondegenerate states. In conjunction with the published EPR work 7.8 the present data allow to reinterpret the model for the optical transitions in NBOHC (Fig.5). The unusually small Stokes shift and observation of intense ZPL indicate, that He-Ne laser excitation (1.96 eV) affects only the population of some nonbonding states. Indeed, as follows from the EPR 7,8, in the ground state of the center the hole resides in an almost pure nonbonding oxygen 2p orbital (ny in Fig5). The other (doubly occupied) nonbonding 2p orbital (nx in Fig.5) is separated by a randomly distributed energy gap A with average value of ~0.32 eV and Gaussian distribution width of 0.5 eV 16. Thus the 1.9 eV PL band can not be due to transitions between these states, as originally proposed 6. The only other nonbonding states available are the nonbonding 2p orbitals of the 3 ligand oxygens (nL in Fig.5) sharing the same SiO4 tetrahedron. In defect-free silica these orbitals are oriented perpendicular to the Si-OSi plane and form the upper edge of the valence band. We suggest that the 2.0 eV absorption and 1.9 eV PL bands are due to a charge-transfer transition between the nonbonding orbital containing the unpaired spin and the filled nonbonding 2p-like orbital of one of the 3 ligand oxygens (ny and nL in Fig.5). Besides being consistent with the small Stokes shift and the high polarization degree, this assignment is in agreement with the relatively long lifetime of the excited state (13 - 20 j.ts) and the corresponding low value 17 (f~10 -4 ) of oscillator strength, characteristic to weakly overlapping charge-transfer states. The decrease in the polarization degree at energies below the ZPL energy and the differences between the 9 K and 295 K spectra (Fig.3) are explained by the breakdown of the Condon approximation and vibrational mixing of the close lying nx and ny orbitals in the ground state and/or the mixing of the three nL orbitals in the excited state of the center. The relatively intense 4.8 eV excitation band (oscillator strength f~0.1-0.2) t7 can be assigned to the transition from the Si-O bonding orbital to the oxygen nonbonding orbital (Oz--~nyin Fig.5), as originally suggested 6. Angular momentum matrix elements between ny and a z may be responsible for the measured 7 shift of g2 from the free electron value in the EPR spectra of NBOHC. The optical transition scheme of Fig5 can now explain as well the previously measured I° 1.9eV PL polarization degree (P=-15%_+1%) under excitation in the 4.8 eV band. It follows from the symmetry considerations that the transition t~z-~nv should be polarized perpendicular to the Si-O bond (in y-direction in Fig.5). The charge transfer transition nL--~ny is expected to be polarized approximately parallel to the edge of the SiO4 tetrahedron. Thus the angle between the dipole momenta of the 4.8 eV absorption and 1.9 eV emission transitions should be close to arccos(3-1/2)~54.7 °. The dependence of P on the angle ot between the absorbing and emitting dipoles is t5 3cos: ¢t - 1 P(~) - cos: a + 3 The P changes from +0.5 at a=0 to -0.333 at ct=90. The angle ¢t=arccos(3 -I/2) is exactly the value at which P(c~) crosses zero. The very small measured polarization degree j° of-1.5% corresponds to ct=55.7 and is in an
Vol. 84, No. 6
agreement with the transition scheme of Fig.5. The optical transition scheme suggested here is thus fairly consistent with the observed spectroscopic properties. We have demonstrated that the inhomogeneous broadening effects have an essential influence not only on EPR7"9 but also on the optical spectra. This aspect has been neglected in the previous work on optical properties of defects in silica. In particular, the inhomogeneous broadening could be responsible for the long-standing controversy over the sometimes poor correlation between the NBOHC EPR spectra and 1.9 eV PL (see e.g. Ref9 for a review) since the fraction of centers with small nv -nx splittings (Fig5) could elude detection by EPR finder conventional measurement conditions. Dye laser based experiments are in progress now to yield more information on the homogeneous lineshape of NBOHC luminescence and the extent of inhomogeneous distribution. Aeknowledgemenl This work was supported by Alexander von Humboldt Foundation (Bonn, FRG). The author is grateful to Prof.Dr. F.Fischer, ANaber and R.Basfeld for valuable discussions and generous help and to Professors S.Coluccia, DLGriscom and RAWeeks for helping to initiate this study. REFERENCES. l.A.R.Silin, L.N.Skuja and AV.Shendrik, Fiz.i Khim.Stekla, 4, 405(1978). 2.K.Nagasawa, Y.Hoshi, Y.Ohki, KYahagi, Jap.J.ApplPhys 25,464 (1986) 3.J.H.Stathis, M.A.Kastner, Phil.MagB49, 357(1984) 4.YHibino, H.Hanafusa, J.Non-Crystalline Solids, 107,23(1988) 5.A.N.Streletsky, A.B.Pakovich, B.F.Gachkovsky, YulAristov, Y.N.Rufov, P.YButyagin, Khimicheskaya Fizika, No 7, 938(1982). 6.LN.Skuja, A.R.Silin, Phys.Stat.Sol., A56,KI 1-KI3 (1979) 7.M.Stapelbroek, DLGriscom, E.J.Friebele, GHSigel, J.Non-Crystalline Solids, 32, 313(1979) 8DL.Griscom, E.J.Friebele, Phys.Rev. B24,4896( 1981) 9.D.L.Griscom, J.Ceramic Soc.of Japan,99,923( 1991 ). 10LN.Skuja, A.R.Silin, J.Mares Phys.Stat.Sol.,AS0, K149 (1978). 11 .LNSkuja, A.R.Silin Phys.Stat.Sol., A70,43(1982) 12RAvarmaa, Zero-phonon lines in the spectra of polyatomic molecules, In: Zero-phonon lines and spectral hole burning in spectroscopy and photochemistry, ed by OSild and K.Haller, Springer-Verlag,Berlin Heidelberg 1988, p.73. 13M.Weber, Laser excited fluorescence spectroscopy in glass. In: Laser spectroscopy of solids, ed by W.M.Yen and P.M.Selzer, Springer-Verlag, Berlin Heidelberg 1986, p.189 14.CMHartwig, LARahn, J.ChemPhys., 67,4260(1977). 15PPFeofilov The physical basis of polarized emission. Consultants Bureau, New York (1961), 274 p. 16.The distribution of A in the region A<0.2 eV is uncertain since the corresponding shifts in g values grow so big that a meaningful comparison between the simulated and measured EPR spectra is no more possible [D.L.Griscom, private communication] 17. AR.Silin, L.N.Skuja, ANTrukhin, JNon-Crystalline Solids 38-39,195(1980).
Solid State Communications, Vol. 84, No. 6, pp. 613-616, 1992. Printed in Great Britain.
0038-1098/9255.00 + . 00 Pergamon Press Ltd
TIME-RESOLVED LOW TEMPERATURE LUMINESCENCE OF NON-BRIDGING OXYGEN HOLE CENTERS IN SILICA GLASS LSkuja*) PhysikalischesInstitut, Universitat Munster Wilhelm-Klemm Str. ] 0, W4400 MUnster, Germany Received August 2 6, 1992by TP.Martin
The spectral, temporal and polarization characteristics of the non-bridging oxygen (NBO) hole center luminescence band at 1.9 eV in neutron irradiated silica glass has been studied by site selection techniques. A resonant zero-phonon line and vibrational structure 890 cm-l apart emerge at temperatures below 65 K under monochromatic 632.8 nm excitation. The vibrational structure is attributed to the symmetric stretching mode of silicon - NBO bond. The highly polarized zero-phonon line is due to the charge transfer transition between the 2p-like non-bonding orbital of the NBO and the filled 2p lone pair orbital of one of the 3 ligand oxygen atoms in the same SiO2 tetrahedron. The well-known absorption band at 4.8 eV corresponds to the transition from ts NBO-Si bonding state to the non bonding 2p orbital of NBO.
1. Introduction
luminescence line narrowing under monochromatic laser excitation at helium temperatures. The NBOHC seemed to be a promising object for this techniques, since the small Stokes shift allows to expect intense homogeneous zerophonon lines (ZPL), necessary for the site selection to be efficient12,13, but the relatively long lifetime (x~-15 Its at 80 K 10) enables one to separate resonance PL from the scattered excitation light by registering delayed spectra.
Optical absorption bands at 4.8 eV and 2.0 eV and the associated photoluminescence (PL) band around 1.9 eV are characteristic features of irradiated high purity silica glass. They are introduced by neutron 1, gamma 2, X-ray I or ultraviolet (UV) 3 irradiations and by optical fiber drawing process or later applied mechanical stressa. The 1.9 eV PL band is observed also as a surface center created during mechanical fracture of silica5. Both absorption bands and the PL band have been attributed to a single defect in silica glass - the non-bridging oxygen hole center (NBOHC), i.e. an undercoordinated O atom bonded to a single Si atom with a hole in its non-bonding 2p orbital: -Si-O'l,5,6 The NBOHC's have been thoroughly investigated by electron paramagnetic resonance (EPR)7, s, see Ref9 for the latest review. The nature of the electronic transitions responsible for the 2.0 and 4.8 eV absorption and 1.9 eV PL bands is still not well understood. The reported PL polarization properties (polarization degree P=+12% when excited in 2.0 eV band and P=-1.5% l0 under excitation in the 4.8 eV band) are presently unexplained. Attempts to obtain more information by experiments at liquid He temperatures so far have met with little success - the optical spectra remain structureless even at liquid He temperatures II. The EPR investigations7 show however that an inhomogeneous broadening of several tenths of eV is present. This could smear out the vibrational structures at low temperatures. The purpose of the present work is to gain more insight into the electronic structure of the NBOHC using
2. Experimental. Samples of commercial high purity "wet" synthetic silica Coming 7940 irradiated by neutrons at fluencies 1018 to 1020 neutrons/cm 2 were placed in a helium flow cryostate with temperature variable between 8 and 300 K. Samples were excited in the 2.0 eV band by He-Ne laser (~. = 632.8nm, ~lmW) and in the 4.8 eV band by 75 W Xe lamp filtered through Jobin-Yvon Hi0 0.1m grating monochromator. PL was collected at right angle using 0.2 m AMKO LTI monochromator with grating blazed at 750 nm and Hamamatsu R955 photomultiplier ($20 type spectral response). Emission spectra were multiplied by Z,2 to account for the changes in monochromator dispersion, but were not corrected for the spectral efficiency of the monochromator and photomultiplier. DC-mode emission spectra were recorded directly with analog to digital converter card in 80486 based personal computer (PC). Time resolved spectra were measured by chopping the focused laser beam with a mechanical chopper and recording the delayed PL during the dark periods of the chopper. By careful aligning transition times of less than 5 Its were attained. Photomuhiplier output was processed by Ortec9302 discriminator and gated Ortec 994 counter. A delay of
*)Alexander von Humboldt foundation fellow, address after Dec.31,1992: Institute of Solid State Physics, Latvian University,Kengaraga str.8, LV-1063 Riga Latvia. 613
Vol. 84, No. 6
NON-BRIDGING OXYGEN HOLE CENTERS
614
WAVELENGTH/nm
10 I-ts after excitation pulse and gate width of 80 Its commensurate with the decay time of the 1.9 eV PL (ca 20 Its at 10K) was typically used. This provided an efficient suppression of the scattered excitation light (better than 106 times). Polarization spectra were measured using polarization filters. The excitation light was vertically polarized and successive emission spectra with horizontally (lh(~.)) and vertically (Iv(~.)) oriented polarizer before the photomultiplier were measured. The polarization spectra were calculated as
725
625
i
-'----------to 50 K
o
~---o ~---- o ~ 1.85
1.7
1.75
1.8
1.85
1.9
o o
2.0
1.95
PHOTON ENERGY/ eV
DC-mode PL emission spectra under He-Ne laser excitation ( l . 9 6 e V ) are shown in Fig.1. At low temperatures the anti-Stokes part of the spectrum seen at 295 K vanishes and below 70 K a weak line 890 cm -1
WAVELENGTH /nm 700 675 650
t s'°'ta""c't t
650
I
3. Results
m--~60 ~- He-Ne laser, 1 mw I
675
hvexc'1.96e~/ ' SgOcm" ~1 Delay = 10/~sec [ - j ~ Gate = 8 0 / z s e e / v ~
P(~.) = ( 1 - KI h / I v ) ( l + KI h / Iv) -I where K(~.) is an instrumental correction function, measured as Iv(Z,)/lh(~.) for sample substituted with a depolarized light source. Decay time measurements were performed by sweeping over 100 I.tS the time delay between the end of the excitation pulse and 2 Its wide strobe pulse gating the photon counter. The variable delay was produced by PC's internal AMD8254 timer and data were averaged over ca. 105 sweeps.
725
700
625
I
I
"
-t 89
t
50
40
Fig.2. Time-resolved luminescence of neutron-irradiated glassy silica measured 10 p.s after the exciting He-Ne laser pulse at temperatures between 10 and 300 K. Baselines of individual curves are shifted but y-scale is the same for all curves. Line at 1.96 eV is purely due to resonance luminescence.
(0.110eV) apart from the excitation line emerges at 1.85 eV. Signal intensity at excitation energy also increases at l o w temperatures (off-scale in Fig. 1), but that can not be reliably attributed to resonance PL, since any small changes in the sample geometry due to the thermal contraction could unpredictably affect the intensity o f the light scattering. He-temperature emission spectra, measured under wide band (A~.=8 nm) excitation in the 4.8 eV absorption band (not shown) did not exhibit any structures, in agreement with the previous work II .
30 WAVELENGTH /nm 700 675 650
725
20
I
,,,0.5 Ill n"
10
1.65 1.7 1.75 1.8 1.85 1.9 1.95 2.0 PHOTON ENERGY eV
0
L0 uj0. 4
h•exc=1.96
I
625
l
|
oV
0,5
--~ /
DC mode
0.4
a
~ 0.3
0.3
~0,2
0.2
o5Ol 0.0
Fig. 1. DC mode photoluminesccnce emissmn spectra of neutronirradiated glassy SiO 2 measured under He-Ne laser excitation between 10 and 295 K. The Y-scale is the same for all spectra. Resolution is shown by vertical bars. The structures emerging 890 cm"1 apart from the exciting line are marked by a vertical arrow. The off-scale line at 1.96 eV is partly due to scattered laser light, partly due to resonance luminescence.
I
1.65
0.1
9K •
I
1.7
I
,
I
.
I
,
I
,
1
|
1.75 1.8 1.85 1.9 1.95 2.0 PHOTON ENERGY/eV
.
0.0
Fig.3. Photoluminescence polarization spectra measured in DCmode at 295 (solid line) and 9 K (points). The feature at 1.96 eV in the 295 K spectrum is due to the scattered excitation light.
NON-BRIDGING OXYGEN HOLE CENTERS
Vol. 84, No. 6
Time-resolved emission spectra are shown in Fig.2 The 300 K spectrum demonstrates a complete suppression of the scattered excitation light. Below 90 K a resonant zero-phonon line (ZPL) emerges. It's observed width was determined by the monochromator resolution (ca. 7 meV). Similarly to the DC-mode spectra (Fig. l) the structure at 1.85 eV also becomes apparent at He temperatures despite the poorer signal/noise ratio. By taking spectra at different strobe delays it was proved that both line structures have the same decay times as the underlying broad-band spectrum. The line to broad-band spectrum intensity ratio remained constant when measured in several samples with different PL center concentrations. Fig.3 shows the PL polarization spectra, measured in the DC mode at 295 and 9 K The feature in the 295 K spectrum at 1.94-1.97eV is an artefact due to the interference from the highly polarized scattered excitation light. Outside this region (including the anti-Stokes region) the spectrum can be considered as reliable within the noise limits. The polarization spectrum in the ZPL region, measured in the time-resolved mode is shown in Fig4. The spectrum does not show any singularities at the resonance energies. Decay time measurements yielded slightly unexponential decay curves (not shown) with average x around 13 Its at 295 K and 20 p.ts at 10 K Measurements at the ZPL (1.96 eV) gave practically the same decay curve as measurements performed at monochromator set off the resonance line (1.90 eV). 4. D i s c u s s i o n .
The similar time decays of the resonance line and broadband PL as well as their constant intensity ratio in different samples indicate that both features belong to the same center. The absence of any vibrational structure in the 1.9 eV PL spectrum upon the non-selective excitation in the 4.8 eV band is explained by an inhomogeneous broadening due to the disordered structure of silica glass. The line at 1.96 eV (Fig.2) then corresponds to a fraction of centers whose ZPL energies coincide with the He-Ne
laser excitation energy. To our knowledge this is the first observation of ZPL in undoped oxide glasses. The vibrational structure 890 cm -I apart from the ZPL (Figs.l,2) is probably due to the symmetric stretching vibration of the Si-O bond in NBOHC. The NBOHC concentrations are too low to obtain the energy of this mode directly from the infrared spectra However, as shown by Raman investigations of hydroxyl containing silica 14, the symmetric stretching mode of an O-H group as a whole against the Si atom it is bound to has an energy of 969 cm -I N B O H C differs structurally from a bound O-H group only by the absence of the hydrogen atom. Since SiO bonding in both cases is expected to be similar and the mass of H atom is small, one can expect, that Si-O stretching modes in both cases will have close energies. As pointed out in Ref. 12, in cases when the inhomogeneous broadening is sufficiently large, vibrational structures seen in the emission spectra can be in fact ZPL's of the centers who are excited to the first vibrational level of the excited electronic state. The observed structure in emission then corresponds to the vibration energies in the excitedand not as is usual - in the ground electronic state. Because the excited state vibrational energies are usually smaller than in the ground state, correspondence between the observed energy of 890 cm -I for NBOHC and the 969 cm -I in the Si-(O-H) is quite fair. Within the emission band the PL polarization is essentially dependent on wavelength (Fig3). The previously reported 6 low values of P=+12% led at that time to the suggestion that at least one doubly degenerate state takes part in emission transition. However, the data of Ref.6 were in fact measured only for the low-energy part of the emission spectrum, since a cut-off filter transmitting ~. >700 nm was used. This agrees well with the spectrum in Fig3 for those wavelengths. The current data show that in the ZPL region P values reach +0.45 ( F i g 4 ) That is close to the upper theoretical limit (P=0.5) for PL in disordered systems |5 and indicates that both absorption and emission can be modelled by a simple electrical dipole whose orientation remains the same for both transitions. Hence O~Z
i
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~0.5 t3 80.4
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Fig.4. Time resolved photoluminescencc polarization spectrum measured in the zero-phonon line region at 9 K (circles, the left scale). One of the two emission spectra, used to calculate the polarization spectrum is shown below for comparison (solid line, the right scale).
Fig.5 The optical transitions in the non-bridging oxygen hole center. The o-bonding in SiO4 tetrahedron is denoted by straight double lines, both nearly degenerate non-bridging oxygen 2p orbitals ny and nx are schematically shown, nL is the oxygen 2p lone pair orbital, belonging to one of the 3 ligand oxygen atoms. a is the angle between dipole moments of 4.8 eV absorption and 1.9 eV emission transitions.
616
NON-BRIDGING OXYGEN HOLE CENTERS
the ZPL transition takes place between nondegenerate states. In conjunction with the published EPR work 7.8 the present data allow to reinterpret the model for the optical transitions in NBOHC (Fig.5). The unusually small Stokes shift and observation of intense ZPL indicate, that He-Ne laser excitation (1.96 eV) affects only the population of some nonbonding states. Indeed, as follows from the EPR 7,8, in the ground state of the center the hole resides in an almost pure nonbonding oxygen 2p orbital (ny in Fig5). The other (doubly occupied) nonbonding 2p orbital (nx in Fig.5) is separated by a randomly distributed energy gap A with average value of ~0.32 eV and Gaussian distribution width of 0.5 eV 16. Thus the 1.9 eV PL band can not be due to transitions between these states, as originally proposed 6. The only other nonbonding states available are the nonbonding 2p orbitals of the 3 ligand oxygens (nL in Fig.5) sharing the same SiO4 tetrahedron. In defect-free silica these orbitals are oriented perpendicular to the Si-OSi plane and form the upper edge of the valence band. We suggest that the 2.0 eV absorption and 1.9 eV PL bands are due to a charge-transfer transition between the nonbonding orbital containing the unpaired spin and the filled nonbonding 2p-like orbital of one of the 3 ligand oxygens (ny and nL in Fig.5). Besides being consistent with the small Stokes shift and the high polarization degree, this assignment is in agreement with the relatively long lifetime of the excited state (13 - 20 j.ts) and the corresponding low value 17 (f~10 -4 ) of oscillator strength, characteristic to weakly overlapping charge-transfer states. The decrease in the polarization degree at energies below the ZPL energy and the differences between the 9 K and 295 K spectra (Fig.3) are explained by the breakdown of the Condon approximation and vibrational mixing of the close lying nx and ny orbitals in the ground state and/or the mixing of the three nL orbitals in the excited state of the center. The relatively intense 4.8 eV excitation band (oscillator strength f~0.1-0.2) t7 can be assigned to the transition from the Si-O bonding orbital to the oxygen nonbonding orbital (Oz--~nyin Fig.5), as originally suggested 6. Angular momentum matrix elements between ny and a z may be responsible for the measured 7 shift of g2 from the free electron value in the EPR spectra of NBOHC. The optical transition scheme of Fig5 can now explain as well the previously measured I° 1.9eV PL polarization degree (P=-15%_+1%) under excitation in the 4.8 eV band. It follows from the symmetry considerations that the transition t~z-~nv should be polarized perpendicular to the Si-O bond (in y-direction in Fig.5). The charge transfer transition nL--~ny is expected to be polarized approximately parallel to the edge of the SiO4 tetrahedron. Thus the angle between the dipole momenta of the 4.8 eV absorption and 1.9 eV emission transitions should be close to arccos(3-1/2)~54.7 °. The dependence of P on the angle ot between the absorbing and emitting dipoles is t5 3cos: ¢t - 1 P(~) - cos: a + 3 The P changes from +0.5 at a=0 to -0.333 at ct=90. The angle ¢t=arccos(3 -I/2) is exactly the value at which P(c~) crosses zero. The very small measured polarization degree j° of-1.5% corresponds to ct=55.7 and is in an
Vol. 84, No. 6
agreement with the transition scheme of Fig.5. The optical transition scheme suggested here is thus fairly consistent with the observed spectroscopic properties. We have demonstrated that the inhomogeneous broadening effects have an essential influence not only on EPR7"9 but also on the optical spectra. This aspect has been neglected in the previous work on optical properties of defects in silica. In particular, the inhomogeneous broadening could be responsible for the long-standing controversy over the sometimes poor correlation between the NBOHC EPR spectra and 1.9 eV PL (see e.g. Ref9 for a review) since the fraction of centers with small nv -nx splittings (Fig5) could elude detection by EPR finder conventional measurement conditions. Dye laser based experiments are in progress now to yield more information on the homogeneous lineshape of NBOHC luminescence and the extent of inhomogeneous distribution. Aeknowledgemenl This work was supported by Alexander von Humboldt Foundation (Bonn, FRG). The author is grateful to Prof.Dr. F.Fischer, ANaber and R.Basfeld for valuable discussions and generous help and to Professors S.Coluccia, DLGriscom and RAWeeks for helping to initiate this study. REFERENCES. l.A.R.Silin, L.N.Skuja and AV.Shendrik, Fiz.i Khim.Stekla, 4, 405(1978). 2.K.Nagasawa, Y.Hoshi, Y.Ohki, KYahagi, Jap.J.ApplPhys 25,464 (1986) 3.J.H.Stathis, M.A.Kastner, Phil.MagB49, 357(1984) 4.YHibino, H.Hanafusa, J.Non-Crystalline Solids, 107,23(1988) 5.A.N.Streletsky, A.B.Pakovich, B.F.Gachkovsky, YulAristov, Y.N.Rufov, P.YButyagin, Khimicheskaya Fizika, No 7, 938(1982). 6.LN.Skuja, A.R.Silin, Phys.Stat.Sol., A56,KI 1-KI3 (1979) 7.M.Stapelbroek, DLGriscom, E.J.Friebele, GHSigel, J.Non-Crystalline Solids, 32, 313(1979) 8DL.Griscom, E.J.Friebele, Phys.Rev. B24,4896( 1981) 9.D.L.Griscom, J.Ceramic Soc.of Japan,99,923( 1991 ). 10LN.Skuja, A.R.Silin, J.Mares Phys.Stat.Sol.,AS0, K149 (1978). 11 .LNSkuja, A.R.Silin Phys.Stat.Sol., A70,43(1982) 12RAvarmaa, Zero-phonon lines in the spectra of polyatomic molecules, In: Zero-phonon lines and spectral hole burning in spectroscopy and photochemistry, ed by OSild and K.Haller, Springer-Verlag,Berlin Heidelberg 1988, p.73. 13M.Weber, Laser excited fluorescence spectroscopy in glass. In: Laser spectroscopy of solids, ed by W.M.Yen and P.M.Selzer, Springer-Verlag, Berlin Heidelberg 1986, p.189 14.CMHartwig, LARahn, J.ChemPhys., 67,4260(1977). 15PPFeofilov The physical basis of polarized emission. Consultants Bureau, New York (1961), 274 p. 16.The distribution of A in the region A<0.2 eV is uncertain since the corresponding shifts in g values grow so big that a meaningful comparison between the simulated and measured EPR spectra is no more possible [D.L.Griscom, private communication] 17. AR.Silin, L.N.Skuja, ANTrukhin, JNon-Crystalline Solids 38-39,195(1980).