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Optically induced metastable paramagnetic centers in amorphous SiO2
Journal of Non-Crystalline Solids 77 & 78 (1985) 739-742 North-Holland, Amsterdam
739
OPTICALLY INDUCEDMETASTABLEPARAMAGNETICCENTERSIN AMORPHOUSSi02.* James H. STA~HIS and M. A. KASTNER Department of Physics and Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, MA 02139 USA Paramagnetic centers have been induced in amorphous SiOz by exposure to sub-bandgap light of various wavelengths. The EPR spectrum for each of the individual defects has been isolated. One type of defect accounts for more than half of the total spin density, suggestive of a negative-U pair of valence-alternation defects. The behavior of another center is consistent with a second pair of charged defects which do not interconvert. I. INTRODUCTION I t is well established that electrons in chalcogenide glasses tend to occupy defects in pairs, so that the ground state is diamagnetic. Singly-occupied paramagnetic states are metastable, and can be accessed by optical excitation, as was f i r s t shown for the sulfides and selenides by Bishop, Strom, and TaylorI.
Recently2, we found that the same is true for Si02.
In this case,
because of the larger band gap (9-1O eV) far-ultraviolet excitation is necessary, and the spins persist even at room temperature. The a v a i l a b i l i t y of sizeable photon fluxes at several wavelengths in the UV from excimer lasers has made i t possible to isolate the signatures of several photoexcited paramagnetic defects. 2. EXPERIMENTAL Samples3 of ultra-high-purity SiOz (
*Supported by Joint Services Electronics Program contract no. DAAG-29-83-KO003. 0022-3093/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
740
J.H. Stathis, M.A. Kastner / Optically induced metastable paramagnetic centers
components displayed in figure I.
These three centers account for more than
80% of the spins in the case of 7.9 eV excitation (in the Urbach region); the remainder comprises the "low-energy centers", so called because they predominate after excitation by the lower photon energies (6.4 eV or 5.0 eV). Spectrum A closely resembles one which has been attributed6 to the peroxy radical (Si-O-O.) in y-irradiated OH-free s i l i c a .
Centers B and C probably
involve over- or under-coordinated oxygen as well, since silicon-centered defects in quartz are characterized by much narrower spectra.
Spectrum C may
be a sum of two resonances, or i t could result from a single center with a broad distribution of spin Hamiltonian parameters.
In the absence of hyperfine
interactions i t is d i f f i c u l t to ascertain the structure of a defect from EPR spectra alone.
However, the EPR spectrum may be used as a signature of the
defect, allowing one to measure the density of the various defects as a function of other treatment such as optical or thermal bleaching, and thereby to make inferences regarding t h e i r structure.
The results of our isochronal
annealing experiments are shown in figure 2.
The top curve is the total spin
density, measured by numerical integration of the experimental spectra.
An
interesting property of the low-energy centers is that they have a r e l a t i v e l y long s p i n - l a t t i c e relaxation time, so they can be madeto saturate by increasing the microwave power. The density of low-energy centers is therefore shown
~-g
1.2
2.0B
2.04
2,00
I
I
r
!~30
1.96
aB
"
I
.....rgy
'
E
A
• 0 ~e .
.
.
.
~
Eo£,=l
.
~.B ~x
.4
"~, ~ \ ',
B .2
3150
I 3200
t
3250
I 3300
33P50
3400
MAGNETIC FIELD (G)
FIGURE l EPR spectra of photoinduced centers in amorphous SiO2 extracted from many EPR spectra as described in text.
0 0
200
ANNEAL
400
600
TEHPERATURE
BOO
(°C)
FIGURE 2 Isochronal annea]ing curves of the photoinduced centers.
Z H. Stathis, M.A. Kastner / Optically induced metastable paramagnetic centers
741
in figure Z as the difference between the measured spin densities at high (0 dB) and low (30 dB) microwave power. 4. DISCUSSION The most striking feature of figure 2 is that more than half (roughly 70%) of the spins are in the A center.
Since the excitation is below the band-gap
the spins we see must result from direct photoionization of native defects, with subsequent retrapping of the photoexcited electron or hole at a different site.
Charge neutrality therefore requires that we have equal numbers of
trapped electrons and holes.
The data therefore imply that some trapped
electrons and trapped holes must give the same EPR signal.
This is exactly
what is predicted for a negative-U pair of valence-alternation (VA) defects 7. Of course, there might be additional resonances too broad to detect, and multiple excitations of the same center must also be considered, as discussed below.
Nonetheless, i f we take this data at face value i t may be the most
direct evidence to date for the a p p l i c a b i l i t y of the VA model to SiO2. The second pertinent feature in figure 2 is the 70% increase in the number of B centers which occurs at 2OO°C. The increase correlates with the disappearance of the low-energy centers, which can be understood by assuming that some of the carriers released from these l a t t e r sites are re-trapped at diamagnetic precursors to the B centers.
However, the B center is never
observed after 6.4 eV or 5.0 eV excitation, even though the same number of low-energy centers are produced and are observed to anneal at the same temperature.
I f our understanding of the growth of the B center is correct, then i t
must be that i t s diamagnetic precursor is, in fact, created by the 7.9 eV l i g h t and is not present in thermal equilibrium.
Such a situation can easily occur
in heteropolar glasses8, as i l l u s -
¢.
Si/
c;
Si I ~
:o/ ,~ \Si
~---'o/s' Si
~Si
c,;(sizo)
trated in figure 3.
Here, trapped
electrons and trapped holes result in distinguishable paramagnetic configurations.
I f one assumes, for the
sake of argument, that 7.9 eV
Si__O( $i ~ Si h~ • i Cs(Ss)
Si.._.O/Si ~Rem°~~Si:\ :o/$i \Si ~ / \Si o . C~(SI3) T~"
excitation is capable of removing two holes from the positively charged defect, then a new diamagnetic structure, not i n i t i a l l y present, is
created. This new negative center is FIGURE 3 Defect pair which does not interconvert, then available for re-trapping of and which explains the observed properties of the B center.
742
J.H. Stathis, M.A. Kastner / Optically induced metastable paramagnetic centers
holes released from the low-energy centers, accounting for the observed increase in the B center.
On the other hand, i f the photon energy is
insufficient to produce any paramagnetic B centers i n i t i a l l y then i t w i l l certainly be incapable of removing two carriers.
The negative center w i l l not
be available for retrapping of the released holes and thus no B center formation w i l l accompanythe disappearanceof the low-energy centers, exactly as observed in the case of 6.4 eV or 5.0 eV excitation. We wish to stress that there is as yet no supporting evidence for the identification of the observed EPR epectra with any specific structural models such as those depicted in figure 3. Nonetheless, these simple models have properties consistent with our observations of the number density and annealing behavior of the photoinduced paramagnetic centers in amorphous SiO2.
REFERENCES l) S.G. Bishop, U. Strom, and P.C. Taylor, Phys. Rev. Lett. 34 (1975) 1346; Phys. Rev. Lett. 36 (1976) 543; Phys. Rev. B 15 (1977) 2278. 2) J.H. Stathis and M.A. Kastner, Phys. Rev. B 29 {1984) 7079. 3) Suprasil W, Heraeus Amersil Inc., Sayreville, New Jersey 08872 USA. 4) J.H. Stathis and M.A. Kastner, AIP Conference Proceedings no. 120 (Ig84) 78. 5) J.H. Stathis and M.A. Kastner, to be published. 6} M. Stapelbroek, D.L. Griscom, E.J. Friebele, and G.H. Sigel, J. Non-Cryst. Solids 32 (1979) 313; E.J. Friebele, D.L. Griscom, M. Stapelbroek, and R.A. Weeks, Phys. Rev. Lett. 42 (1979) 1346. 7) M. Kastner, D. Adler, and H. Fritzsche, Phys. Rev. Lett. 37 (1976) 1504. 8) R.A. Street and G. Lucovsky, Solid State Commun. 31 (1979) 2B9.
739
OPTICALLY INDUCEDMETASTABLEPARAMAGNETICCENTERSIN AMORPHOUSSi02.* James H. STA~HIS and M. A. KASTNER Department of Physics and Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, MA 02139 USA Paramagnetic centers have been induced in amorphous SiOz by exposure to sub-bandgap light of various wavelengths. The EPR spectrum for each of the individual defects has been isolated. One type of defect accounts for more than half of the total spin density, suggestive of a negative-U pair of valence-alternation defects. The behavior of another center is consistent with a second pair of charged defects which do not interconvert. I. INTRODUCTION I t is well established that electrons in chalcogenide glasses tend to occupy defects in pairs, so that the ground state is diamagnetic. Singly-occupied paramagnetic states are metastable, and can be accessed by optical excitation, as was f i r s t shown for the sulfides and selenides by Bishop, Strom, and TaylorI.
Recently2, we found that the same is true for Si02.
In this case,
because of the larger band gap (9-1O eV) far-ultraviolet excitation is necessary, and the spins persist even at room temperature. The a v a i l a b i l i t y of sizeable photon fluxes at several wavelengths in the UV from excimer lasers has made i t possible to isolate the signatures of several photoexcited paramagnetic defects. 2. EXPERIMENTAL Samples3 of ultra-high-purity SiOz (
*Supported by Joint Services Electronics Program contract no. DAAG-29-83-KO003. 0022-3093/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
740
J.H. Stathis, M.A. Kastner / Optically induced metastable paramagnetic centers
components displayed in figure I.
These three centers account for more than
80% of the spins in the case of 7.9 eV excitation (in the Urbach region); the remainder comprises the "low-energy centers", so called because they predominate after excitation by the lower photon energies (6.4 eV or 5.0 eV). Spectrum A closely resembles one which has been attributed6 to the peroxy radical (Si-O-O.) in y-irradiated OH-free s i l i c a .
Centers B and C probably
involve over- or under-coordinated oxygen as well, since silicon-centered defects in quartz are characterized by much narrower spectra.
Spectrum C may
be a sum of two resonances, or i t could result from a single center with a broad distribution of spin Hamiltonian parameters.
In the absence of hyperfine
interactions i t is d i f f i c u l t to ascertain the structure of a defect from EPR spectra alone.
However, the EPR spectrum may be used as a signature of the
defect, allowing one to measure the density of the various defects as a function of other treatment such as optical or thermal bleaching, and thereby to make inferences regarding t h e i r structure.
The results of our isochronal
annealing experiments are shown in figure 2.
The top curve is the total spin
density, measured by numerical integration of the experimental spectra.
An
interesting property of the low-energy centers is that they have a r e l a t i v e l y long s p i n - l a t t i c e relaxation time, so they can be madeto saturate by increasing the microwave power. The density of low-energy centers is therefore shown
~-g
1.2
2.0B
2.04
2,00
I
I
r
!~30
1.96
aB
"
I
.....rgy
'
E
A
• 0 ~e .
.
.
.
~
Eo£,=l
.
~.B ~x
.4
"~, ~ \ ',
B .2
3150
I 3200
t
3250
I 3300
33P50
3400
MAGNETIC FIELD (G)
FIGURE l EPR spectra of photoinduced centers in amorphous SiO2 extracted from many EPR spectra as described in text.
0 0
200
ANNEAL
400
600
TEHPERATURE
BOO
(°C)
FIGURE 2 Isochronal annea]ing curves of the photoinduced centers.
Z H. Stathis, M.A. Kastner / Optically induced metastable paramagnetic centers
741
in figure Z as the difference between the measured spin densities at high (0 dB) and low (30 dB) microwave power. 4. DISCUSSION The most striking feature of figure 2 is that more than half (roughly 70%) of the spins are in the A center.
Since the excitation is below the band-gap
the spins we see must result from direct photoionization of native defects, with subsequent retrapping of the photoexcited electron or hole at a different site.
Charge neutrality therefore requires that we have equal numbers of
trapped electrons and holes.
The data therefore imply that some trapped
electrons and trapped holes must give the same EPR signal.
This is exactly
what is predicted for a negative-U pair of valence-alternation (VA) defects 7. Of course, there might be additional resonances too broad to detect, and multiple excitations of the same center must also be considered, as discussed below.
Nonetheless, i f we take this data at face value i t may be the most
direct evidence to date for the a p p l i c a b i l i t y of the VA model to SiO2. The second pertinent feature in figure 2 is the 70% increase in the number of B centers which occurs at 2OO°C. The increase correlates with the disappearance of the low-energy centers, which can be understood by assuming that some of the carriers released from these l a t t e r sites are re-trapped at diamagnetic precursors to the B centers.
However, the B center is never
observed after 6.4 eV or 5.0 eV excitation, even though the same number of low-energy centers are produced and are observed to anneal at the same temperature.
I f our understanding of the growth of the B center is correct, then i t
must be that i t s diamagnetic precursor is, in fact, created by the 7.9 eV l i g h t and is not present in thermal equilibrium.
Such a situation can easily occur
in heteropolar glasses8, as i l l u s -
¢.
Si/
c;
Si I ~
:o/ ,~ \Si
~---'o/s' Si
~Si
c,;(sizo)
trated in figure 3.
Here, trapped
electrons and trapped holes result in distinguishable paramagnetic configurations.
I f one assumes, for the
sake of argument, that 7.9 eV
Si__O( $i ~ Si h~ • i Cs(Ss)
Si.._.O/Si ~Rem°~~Si:\ :o/$i \Si ~ / \Si o . C~(SI3) T~"
excitation is capable of removing two holes from the positively charged defect, then a new diamagnetic structure, not i n i t i a l l y present, is
created. This new negative center is FIGURE 3 Defect pair which does not interconvert, then available for re-trapping of and which explains the observed properties of the B center.
742
J.H. Stathis, M.A. Kastner / Optically induced metastable paramagnetic centers
holes released from the low-energy centers, accounting for the observed increase in the B center.
On the other hand, i f the photon energy is
insufficient to produce any paramagnetic B centers i n i t i a l l y then i t w i l l certainly be incapable of removing two carriers.
The negative center w i l l not
be available for retrapping of the released holes and thus no B center formation w i l l accompanythe disappearanceof the low-energy centers, exactly as observed in the case of 6.4 eV or 5.0 eV excitation. We wish to stress that there is as yet no supporting evidence for the identification of the observed EPR epectra with any specific structural models such as those depicted in figure 3. Nonetheless, these simple models have properties consistent with our observations of the number density and annealing behavior of the photoinduced paramagnetic centers in amorphous SiO2.
REFERENCES l) S.G. Bishop, U. Strom, and P.C. Taylor, Phys. Rev. Lett. 34 (1975) 1346; Phys. Rev. Lett. 36 (1976) 543; Phys. Rev. B 15 (1977) 2278. 2) J.H. Stathis and M.A. Kastner, Phys. Rev. B 29 {1984) 7079. 3) Suprasil W, Heraeus Amersil Inc., Sayreville, New Jersey 08872 USA. 4) J.H. Stathis and M.A. Kastner, AIP Conference Proceedings no. 120 (Ig84) 78. 5) J.H. Stathis and M.A. Kastner, to be published. 6} M. Stapelbroek, D.L. Griscom, E.J. Friebele, and G.H. Sigel, J. Non-Cryst. Solids 32 (1979) 313; E.J. Friebele, D.L. Griscom, M. Stapelbroek, and R.A. Weeks, Phys. Rev. Lett. 42 (1979) 1346. 7) M. Kastner, D. Adler, and H. Fritzsche, Phys. Rev. Lett. 37 (1976) 1504. 8) R.A. Street and G. Lucovsky, Solid State Commun. 31 (1979) 2B9.