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A MODEL FOR POINT DEFECTS IN SILICA
268
A MODEL FOR POINT DEFECTS IN SILICA
G.N. Greaves* Pilkington Brothers R. & D. Laboratories, Lathom, Ormskirk, England ABSTRACT We present a new model for dangling bond defects in vitreous silica. In equilibrium the intrinsic states are presumed to be charged and to fall on silicon as well as oxygen sites - forming in pairs and rendering the glass diamagnetic. The trapping of electrons and holes at these sites following irradiation involves distortion of the lattice, so the energy levels of the silicon and oxygen states in the energy gap will depend on occupancy. We analyse the colour centre data for pure silica to deduce energy levels for the different dangling bond states. Finally we describe how the charge on the equilibrium states may be removed by the dissociation of water - the states converting to neutral hydroxyl centres and leaving the glass relatively insensitive to paramagnetic centre formation. Whilst colour centres in vitreous silica have been extensively studied (1) models have generally been restricted to individual centres and their characteristic optical absorption. The best known is the radiation induced paramagnetic E' silicon defect. This has been modelled as a trivalent silicon with a dangling sp orbital facing onto an oxygen vacancy (2). More recently Mott has employed the charged dangling bond model proposed earlier for chalcogenide glasses (3) to describe dangling bond oxygen centres in silica (4). In this paper we shall attempt to rationalize silica colour centre data in terms of a single defect involving a dangling bond on a silicon atom and a dangling bond on an oxygen atom - the atoms being sufficiently separated to prevent bond reformation. The occupancy of these states will depend on the thermal history and radiation history of the glass. Changes in occupancy will involve atomic relaxation and a pairing of silicon and oxygen energy levels in the energy gap. We will go on to describe the likely modifying effect of water in silica and the way this can quench subsequent E' centre formation. We begin by discussing the intrinsic states in the pure stoichiometric glass. The model is based on the idea of a defect consisting of a pair of dangling bond states - one on a trivalent silicon site and the other on a separated monovalent oxygen site. Because the cation-anion electronegativity difference is large it is unlikely neutral singly occupied states will be stable. Instead we expect they will decompose into empty positively charged silicon sp states (which we are calling Si states) and doubly occupied negatively charged oxygen 2p states (which we are calling 0 states) - that is, provided the energy released is sufficient to overcome the electron-electron correlation energy of double occupancy. Dielectric screening is small in silica so the energy levels for these charged states will be separated from the band edges and we can expect features in the tail of the optical absorption edge. Following excitation with band gap or ionizing radiation, electrons and holes
269 will be trapped at the cation and anion sites - converting them Into neutral metastable states Si and 0 . We anticipate considerable distortion of the lattice around these sites (as appears to be the case for impurity centres in silica) so singly occupied energy levels will lie deeper in the energy gap compared to their charged precursors. We propose optical transitions associated with the charged and neutral silicon and oxygen states are responsible for the colour centre absorption and luminescence bands characteristic of vitreous silica. The notion of the overcoordinated anion site which is fundamental to current models for defects in chalcogenlde glasses (3) has been adopted here to help place energy levels in the energy gap of silica for the various states of the intrinsic defect. In particular an empty or singly occupied dangling bond state is expected to draw an adjacent oxygen into 3-fold coordination utilising the lone-pair electrons and leaving the dangling bond surplus to bonding requirements. At silicon sites the appropriate non-bonding state should have sp character - this being the first available state above the valence band in the Si(3)-0(3) complex. We can judge the position of the empty Si state from internal photoemission measurements of the Si/Si0 2 interface (5). It will lie in the vicinity of the silicon conduction and valence bands which are located at 3 and 4 eV respectively below the silica conduction band. Once occupied the change in lattice distortion will push the level closer to the valence band. We propose that this singly occupied Si° state is the E* ESR centre of irradiated silica (6). Unlike the model of Fiegl, Fowler and Yip (2) the present model does not involve an oxygen vacancy. Turning now to the oxygen states, the singly occupied 0 state should have anti-bonding character as the remaining electrons on the oxygen complex will be in lone-pair or bonding states. Atomic relaxation related to the lone-pair bonding will also be involved but the energy level is still expected to lie near the conduction bond. We propose this is the centre responsible for the ESR oxygen hole signal observed by Friebele and coworkers in unannealed silica fibres at helium temperatures (7). It is also presumed to be present in *-irradiated silica (8). Following the ideas of Mott (4), the charged oxygen dangling bond state 0~ (D~ in his notation) is interpreted as the non-bridging oxygen of glass science. It will have p - like character and will lie close to the uppermost lone-pair valence band - separated by its ionization energy. This will probably amount to about leV on account of the low dielectric constant of silica (€ = 2.15). The present model implies equal numbers of states on silicon and oxygen sites every non-bridging oxygen being compensated by a trivalent silicon. Because the metastable Si and 0 states both involve partial bonding with an adjacent otherwise fully coordinated oxygen, it is assumed they will both be associated with similar distortions of the lattice. We can infer the lattice contribution to the energy level of these states from the location of the colour centre absorption bands. In Fig. 1 therefore we plot the complete absorption edge of silica (from various sources) running from the fibre optics minimum at 1 eV to the first major absorption peak at 10.2eV. The solid line refers to the virgin glass (Corning 7940) and the dotted line to the same variety of silica following Ï irradiation. For absorption coefficients greater than 10 cm" the edge is characteristic of the pure host material but below this absorption is more strongly influenced by structural imperfections and impurities. The peaks at 2.0 and 5.8 eV have been associated (with reasonable certainty) with the hole and electron ESR signals referred to earlier (9). We can therefore assign these to the metastable 0 and Si states respectively. The sharpness of the features
270 implies they involve excitonic transitions rather than transitions from valence band states. In fact UPS spectra indicate the top-most feature in the valence band density of states is approximately 3 eV across which is clearly wider than either of the two colour centre bands. Accordingly we can say 2.0 and 5.8 eV locate the 0 and Si levels respectively below a suitable exciton line.
8 10 12 fiu (eV) Fig.l Optical absorption edge of vitreous silica. Solid curves are for the unirradiated glass and dashed curves are the colour centre bands following & irradiation of ~1(T rad. For further details see Ref,15
Of considerable interest is the recent observation by Appleton and coworkers (11) of a well defined shoulder close to 7.5 eV in annealed silica. This comes at the same energy as the high energy colour centre band in irradiated silica identified by several authors (1, 12). It has not been associated with an ESR signal and is therefore a natural choice for locating the equilibrium charged states Si and θ". Moreover it appears to be split with the two main features at 7.6 and 8.2 eV, so we might attribute these to the Si and 0~ states respectively. Further confirmation of the origin of this shoulder comes from the observation of photoconduction at these energies (11). The implication here must be that excitons generated at silicon and oxygen equilibrium states readily ionize and contribute to the conduction because the states themselves are charged.
Fig 2. shows the proposed arrangement of silicon and oxygen levels in the energy gap. The characteristic red (1.8 eV) and blue (2.7 eV) luminescence bands of Irradiated silica are also included for completion. Taking a (Mott-Wannier) exciton binding energy of 1.6 eV ( € = 2.15, n * - 0.5m ) we deduce an energy gap of about 10.5 eV. This agrees with Mott's recent estimate (4) and is further evidence that the 10.2 eV peak is excitonic. From the scheme of intrinsic levels given in Fig.2 the energy separating charged from neutral states is approximately 5 eV at silicon sites and 6 eV at oxygen sites - indicating similar distortions of the lattice. If we assume thermal levels (dotted lines) lie midway between optical levels as shown then donor silicon states lie above acceptor oxygen states - this ensures the stability of charged dangling bond states over neutral states. The effect of water on the optical properties of unirradiated silica is similar to that of a conventional modifier: the band edge moves in towards the visible (13). Heat treating water-free silica (e.g. Corning 7943) in water
271 has the same effect (14). It would appear non-bridging oxygens are generated in either case and that these are compensated by protons in an analogous way to alkali cations in a silicate glass. The presence of hydroxyl centres is confirmed by the 0-H bond stretching band at 0.46 eV. Interestingly the strength of this feature is found to be inversely proportioned to the radiation induced 5.8 eV band (14). Clearly water has an 'annealing effect' on intrinsic defects in silica and behaves in some respects analogously to hydrogen in amorphous Ge - apparently saturating dangling bond states. In the context of the present model water is expected to dissociate amongst the charged intrinsic states hydroxyl ions going to Si states and protons to 0 states:
I4
H20 0"—
Si
I
OH + H
Conduction Band
S j *uâ£
SÏ »7-6M
^20eV f 5-8eV , 82eV·
foeV Valence Band
Fig.2 Arrangement of the various energy levels of the primary intrinsic defect proposed for vitreous silica. Absorption data is taken from Fig.l and luminescence from Ref.l
I
Si — O H
I
HO —
The nett effect will be a reduction in the density of charged states on silicon and oxygen sites and their replacement by non-bridging hydroxyl centres — OH. Like alkali modified non-bridging oxygen centres we expect they will be close in energy to the 0~ level. They will therefore effect the shift in the absorption edge prior to radiation. Moreover, as there will be fewer Si states, fewer paramagnetic centres will form on radiation and the 5.8 eV band will be weakened as observed. A fuller description of this model is currently in press (15)· ♦Now at:
S.R.C. Daresbury Laboratory, Warrington, WA4 4AD, U.K. REFERENCES
(1)
(2) (3)
(4)
E.Lell, N.Kreidl and R.Hensler, Radiation effects in quartz, silica and glasses, Progress in Ceramic Science 4, 1 (1966) Pergamon Press. G.A. Sigel, Ultraviolet spectra of silicate glasses, J. Non-Crystalline Solids 13, 372 (1973/74). F.J. Fiegl, W.B.Fowler and L.K.Yip, Oxygen vacancy model for the E» center in SiO^, Solid St. Comm. 14, 225 (1974). R.A.Street and N.F. Mott, States in the gap in glassy semiconductors, Phys. Rev. Lett. 35, 1293 (1975). M.Kastner, D.Adler and H.Fritzsche, Valence-alternation model for localized gap states in lone-pair semiconductors, Phys. Rev. Lett. 37, 1504 (1976). N.F.Mott, Si0 2 and the chalcogenide semiconductors, Adv. Phys. 26, 363 (1977).
272 (5) (6) (7) (8) (9)
(10)
(11) (12) (13) (14) (15)
R.J.Powell, Interface barrier energy determination from voltage dependence of photoinjected currents, J. Appl. Phys. 41, 2424 (1970). D A L · Griscon, E.J.Friebele and G.H.Sigel, Analysis of the primary Si hyperfine structure, Solid St. Comm. 15, 479 (1974). E.J.Friebele, G.H.Sigel and D.L.Griscon, Drawing-induced defect centers in fused silica core fiber, Appl. Phys. Lett 28, 516 (1976). P.Kaiser, Drawing-induced colouration in vitreous silica fibers, J. Opt. Soc. Am.64, 475 (1974). C.M.Nelson and R.A.Weeks, Trapping electrons in irradiated quartz and silica, J. Am. Ceram. Soc. 43, 396 (1960)r Vacuum-ultraviolet absorption studies of irradiated silica and quartz, J.Appl. Phys. 32, 883 (1961). E.J.Friebele, D.L.Griscon and G.H.Sigel, Radiationinduced defect centers in non-crystalline SiO. Proc. 4th Conf. in Physics of Non-Crystalline Solids(Clausthal-Zellerfeld) 154, (1977). D.L.Griscon, E.J.Friebele, G.H.Sigel and R.J.Ginther, Radiationinduced paramagnetic defects as structural probes in pure silica and borosilicate glasses, Structure of Non-Crystalline Materials 113 (1977). Taylor and Francis. B.Fischer, R.A.Pollak, T.H.DiStefano and W.D.Grodman, Electronic structure of Si0 9 , Si χ Get 0> and Ge 0 2 from photoemission spectroscopy, Phys. Rëv. B15, 3193 (1977). A.Appleton, private communication (1977). C.M.Nelson and J.H.Crawford, Optical absorption in irradiated quartz and fused silica, J. Phys. Chem. Solids 13, 296 (1960). T.Bell, G.Hetherington and K.H.Jack, Water in vitreous silica, Phys. Chem. of Glasses 3, 141 (1962). R.A.Weeks and E.Lell, Relation between E' centres and hydroxyl bonds in silica, J. Appl. Phys. 35, 1932 (1964). G.N.Greaves, Colour centres in vitreous silica, Phil. Mag, (in press).
A MODEL FOR POINT DEFECTS IN SILICA
G.N. Greaves* Pilkington Brothers R. & D. Laboratories, Lathom, Ormskirk, England ABSTRACT We present a new model for dangling bond defects in vitreous silica. In equilibrium the intrinsic states are presumed to be charged and to fall on silicon as well as oxygen sites - forming in pairs and rendering the glass diamagnetic. The trapping of electrons and holes at these sites following irradiation involves distortion of the lattice, so the energy levels of the silicon and oxygen states in the energy gap will depend on occupancy. We analyse the colour centre data for pure silica to deduce energy levels for the different dangling bond states. Finally we describe how the charge on the equilibrium states may be removed by the dissociation of water - the states converting to neutral hydroxyl centres and leaving the glass relatively insensitive to paramagnetic centre formation. Whilst colour centres in vitreous silica have been extensively studied (1) models have generally been restricted to individual centres and their characteristic optical absorption. The best known is the radiation induced paramagnetic E' silicon defect. This has been modelled as a trivalent silicon with a dangling sp orbital facing onto an oxygen vacancy (2). More recently Mott has employed the charged dangling bond model proposed earlier for chalcogenide glasses (3) to describe dangling bond oxygen centres in silica (4). In this paper we shall attempt to rationalize silica colour centre data in terms of a single defect involving a dangling bond on a silicon atom and a dangling bond on an oxygen atom - the atoms being sufficiently separated to prevent bond reformation. The occupancy of these states will depend on the thermal history and radiation history of the glass. Changes in occupancy will involve atomic relaxation and a pairing of silicon and oxygen energy levels in the energy gap. We will go on to describe the likely modifying effect of water in silica and the way this can quench subsequent E' centre formation. We begin by discussing the intrinsic states in the pure stoichiometric glass. The model is based on the idea of a defect consisting of a pair of dangling bond states - one on a trivalent silicon site and the other on a separated monovalent oxygen site. Because the cation-anion electronegativity difference is large it is unlikely neutral singly occupied states will be stable. Instead we expect they will decompose into empty positively charged silicon sp states (which we are calling Si states) and doubly occupied negatively charged oxygen 2p states (which we are calling 0 states) - that is, provided the energy released is sufficient to overcome the electron-electron correlation energy of double occupancy. Dielectric screening is small in silica so the energy levels for these charged states will be separated from the band edges and we can expect features in the tail of the optical absorption edge. Following excitation with band gap or ionizing radiation, electrons and holes
269 will be trapped at the cation and anion sites - converting them Into neutral metastable states Si and 0 . We anticipate considerable distortion of the lattice around these sites (as appears to be the case for impurity centres in silica) so singly occupied energy levels will lie deeper in the energy gap compared to their charged precursors. We propose optical transitions associated with the charged and neutral silicon and oxygen states are responsible for the colour centre absorption and luminescence bands characteristic of vitreous silica. The notion of the overcoordinated anion site which is fundamental to current models for defects in chalcogenlde glasses (3) has been adopted here to help place energy levels in the energy gap of silica for the various states of the intrinsic defect. In particular an empty or singly occupied dangling bond state is expected to draw an adjacent oxygen into 3-fold coordination utilising the lone-pair electrons and leaving the dangling bond surplus to bonding requirements. At silicon sites the appropriate non-bonding state should have sp character - this being the first available state above the valence band in the Si(3)-0(3) complex. We can judge the position of the empty Si state from internal photoemission measurements of the Si/Si0 2 interface (5). It will lie in the vicinity of the silicon conduction and valence bands which are located at 3 and 4 eV respectively below the silica conduction band. Once occupied the change in lattice distortion will push the level closer to the valence band. We propose that this singly occupied Si° state is the E* ESR centre of irradiated silica (6). Unlike the model of Fiegl, Fowler and Yip (2) the present model does not involve an oxygen vacancy. Turning now to the oxygen states, the singly occupied 0 state should have anti-bonding character as the remaining electrons on the oxygen complex will be in lone-pair or bonding states. Atomic relaxation related to the lone-pair bonding will also be involved but the energy level is still expected to lie near the conduction bond. We propose this is the centre responsible for the ESR oxygen hole signal observed by Friebele and coworkers in unannealed silica fibres at helium temperatures (7). It is also presumed to be present in *-irradiated silica (8). Following the ideas of Mott (4), the charged oxygen dangling bond state 0~ (D~ in his notation) is interpreted as the non-bridging oxygen of glass science. It will have p - like character and will lie close to the uppermost lone-pair valence band - separated by its ionization energy. This will probably amount to about leV on account of the low dielectric constant of silica (€ = 2.15). The present model implies equal numbers of states on silicon and oxygen sites every non-bridging oxygen being compensated by a trivalent silicon. Because the metastable Si and 0 states both involve partial bonding with an adjacent otherwise fully coordinated oxygen, it is assumed they will both be associated with similar distortions of the lattice. We can infer the lattice contribution to the energy level of these states from the location of the colour centre absorption bands. In Fig. 1 therefore we plot the complete absorption edge of silica (from various sources) running from the fibre optics minimum at 1 eV to the first major absorption peak at 10.2eV. The solid line refers to the virgin glass (Corning 7940) and the dotted line to the same variety of silica following Ï irradiation. For absorption coefficients greater than 10 cm" the edge is characteristic of the pure host material but below this absorption is more strongly influenced by structural imperfections and impurities. The peaks at 2.0 and 5.8 eV have been associated (with reasonable certainty) with the hole and electron ESR signals referred to earlier (9). We can therefore assign these to the metastable 0 and Si states respectively. The sharpness of the features
270 implies they involve excitonic transitions rather than transitions from valence band states. In fact UPS spectra indicate the top-most feature in the valence band density of states is approximately 3 eV across which is clearly wider than either of the two colour centre bands. Accordingly we can say 2.0 and 5.8 eV locate the 0 and Si levels respectively below a suitable exciton line.
8 10 12 fiu (eV) Fig.l Optical absorption edge of vitreous silica. Solid curves are for the unirradiated glass and dashed curves are the colour centre bands following & irradiation of ~1(T rad. For further details see Ref,15
Of considerable interest is the recent observation by Appleton and coworkers (11) of a well defined shoulder close to 7.5 eV in annealed silica. This comes at the same energy as the high energy colour centre band in irradiated silica identified by several authors (1, 12). It has not been associated with an ESR signal and is therefore a natural choice for locating the equilibrium charged states Si and θ". Moreover it appears to be split with the two main features at 7.6 and 8.2 eV, so we might attribute these to the Si and 0~ states respectively. Further confirmation of the origin of this shoulder comes from the observation of photoconduction at these energies (11). The implication here must be that excitons generated at silicon and oxygen equilibrium states readily ionize and contribute to the conduction because the states themselves are charged.
Fig 2. shows the proposed arrangement of silicon and oxygen levels in the energy gap. The characteristic red (1.8 eV) and blue (2.7 eV) luminescence bands of Irradiated silica are also included for completion. Taking a (Mott-Wannier) exciton binding energy of 1.6 eV ( € = 2.15, n * - 0.5m ) we deduce an energy gap of about 10.5 eV. This agrees with Mott's recent estimate (4) and is further evidence that the 10.2 eV peak is excitonic. From the scheme of intrinsic levels given in Fig.2 the energy separating charged from neutral states is approximately 5 eV at silicon sites and 6 eV at oxygen sites - indicating similar distortions of the lattice. If we assume thermal levels (dotted lines) lie midway between optical levels as shown then donor silicon states lie above acceptor oxygen states - this ensures the stability of charged dangling bond states over neutral states. The effect of water on the optical properties of unirradiated silica is similar to that of a conventional modifier: the band edge moves in towards the visible (13). Heat treating water-free silica (e.g. Corning 7943) in water
271 has the same effect (14). It would appear non-bridging oxygens are generated in either case and that these are compensated by protons in an analogous way to alkali cations in a silicate glass. The presence of hydroxyl centres is confirmed by the 0-H bond stretching band at 0.46 eV. Interestingly the strength of this feature is found to be inversely proportioned to the radiation induced 5.8 eV band (14). Clearly water has an 'annealing effect' on intrinsic defects in silica and behaves in some respects analogously to hydrogen in amorphous Ge - apparently saturating dangling bond states. In the context of the present model water is expected to dissociate amongst the charged intrinsic states hydroxyl ions going to Si states and protons to 0 states:
I4
H20 0"—
Si
I
OH + H
Conduction Band
S j *uâ£
SÏ »7-6M
^20eV f 5-8eV , 82eV·
foeV Valence Band
Fig.2 Arrangement of the various energy levels of the primary intrinsic defect proposed for vitreous silica. Absorption data is taken from Fig.l and luminescence from Ref.l
I
Si — O H
I
HO —
The nett effect will be a reduction in the density of charged states on silicon and oxygen sites and their replacement by non-bridging hydroxyl centres — OH. Like alkali modified non-bridging oxygen centres we expect they will be close in energy to the 0~ level. They will therefore effect the shift in the absorption edge prior to radiation. Moreover, as there will be fewer Si states, fewer paramagnetic centres will form on radiation and the 5.8 eV band will be weakened as observed. A fuller description of this model is currently in press (15)· ♦Now at:
S.R.C. Daresbury Laboratory, Warrington, WA4 4AD, U.K. REFERENCES
(1)
(2) (3)
(4)
E.Lell, N.Kreidl and R.Hensler, Radiation effects in quartz, silica and glasses, Progress in Ceramic Science 4, 1 (1966) Pergamon Press. G.A. Sigel, Ultraviolet spectra of silicate glasses, J. Non-Crystalline Solids 13, 372 (1973/74). F.J. Fiegl, W.B.Fowler and L.K.Yip, Oxygen vacancy model for the E» center in SiO^, Solid St. Comm. 14, 225 (1974). R.A.Street and N.F. Mott, States in the gap in glassy semiconductors, Phys. Rev. Lett. 35, 1293 (1975). M.Kastner, D.Adler and H.Fritzsche, Valence-alternation model for localized gap states in lone-pair semiconductors, Phys. Rev. Lett. 37, 1504 (1976). N.F.Mott, Si0 2 and the chalcogenide semiconductors, Adv. Phys. 26, 363 (1977).
272 (5) (6) (7) (8) (9)
(10)
(11) (12) (13) (14) (15)
R.J.Powell, Interface barrier energy determination from voltage dependence of photoinjected currents, J. Appl. Phys. 41, 2424 (1970). D A L · Griscon, E.J.Friebele and G.H.Sigel, Analysis of the primary Si hyperfine structure, Solid St. Comm. 15, 479 (1974). E.J.Friebele, G.H.Sigel and D.L.Griscon, Drawing-induced defect centers in fused silica core fiber, Appl. Phys. Lett 28, 516 (1976). P.Kaiser, Drawing-induced colouration in vitreous silica fibers, J. Opt. Soc. Am.64, 475 (1974). C.M.Nelson and R.A.Weeks, Trapping electrons in irradiated quartz and silica, J. Am. Ceram. Soc. 43, 396 (1960)r Vacuum-ultraviolet absorption studies of irradiated silica and quartz, J.Appl. Phys. 32, 883 (1961). E.J.Friebele, D.L.Griscon and G.H.Sigel, Radiationinduced defect centers in non-crystalline SiO. Proc. 4th Conf. in Physics of Non-Crystalline Solids(Clausthal-Zellerfeld) 154, (1977). D.L.Griscon, E.J.Friebele, G.H.Sigel and R.J.Ginther, Radiationinduced paramagnetic defects as structural probes in pure silica and borosilicate glasses, Structure of Non-Crystalline Materials 113 (1977). Taylor and Francis. B.Fischer, R.A.Pollak, T.H.DiStefano and W.D.Grodman, Electronic structure of Si0 9 , Si χ Get 0> and Ge 0 2 from photoemission spectroscopy, Phys. Rëv. B15, 3193 (1977). A.Appleton, private communication (1977). C.M.Nelson and J.H.Crawford, Optical absorption in irradiated quartz and fused silica, J. Phys. Chem. Solids 13, 296 (1960). T.Bell, G.Hetherington and K.H.Jack, Water in vitreous silica, Phys. Chem. of Glasses 3, 141 (1962). R.A.Weeks and E.Lell, Relation between E' centres and hydroxyl bonds in silica, J. Appl. Phys. 35, 1932 (1964). G.N.Greaves, Colour centres in vitreous silica, Phil. Mag, (in press).