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Mechanisms of exciton trapping in oxides
99
Journal of Luminescence 31 & 320984)99-101 North-Holland, Amsterdam
MECHANISMS OP EXCITON TRAPPING TN OXIDES
W. HAYES Clarendon Laboratory, Oxford OXI 3PU, U.K.
The self—trapping of excitoos in aome halide cryatals is now ao established phenomenon. The behaviour of excitona in oxfdea ia less clear cut. ODMR studies show that in YAG and YA1O 3 trapping of excitons is associated with lattice defects but suggest that in Sf02 and Y203 intrinsic trapping processes may be dominant.
1.
INTRODUCTION
The electron—lattice interaction in ionic crystals such as alkali halides, 1 alkaline earth fluorides AgC1 systems gives rise of excitons The self—trapped exciton and in these is, toin self—trapping effect, an electron trapped by a self—trapped hole (STH).
In alkali and alkaline earth halides the STH is a
molecular entity of the type X 2 where X energy is a halogen. the STH is, in 2+ ion. The localisation required In for AgCl self—trapping effect, an Ag increases with increasing bandwidth and calculation suggests that electron— lattice interaction in oxides is marginal for self—trapping of excitons.2’3 Many oxide systems show strong X—ray induced luminescence at low temperatures due to e—h recombinatien.
In some cases such emissions come from triplet exci—
tonic states and detailed information about the structure of the emitting complex maybe obtained using OOMR.
We have carried out ODMR studies of triplet
excitons in YAG4 and YAlO~, showing that the excitons are trapped at lattice defects rather than being self—trapped.
We present ODNR results here for Sifl~and
Y 203 which suggest that excitons may be self—trapped in these materials.
2.
EXPERIMENTAL RESULTS AND DISCUSSION (i) Quartz (Si02).
Exposure of quartz to ionising radiation at low tempera-
tures results in a well known broad luminescence peaking in the blue (% 2.8 eV). This band is a superposition of overlapping emissions from different centres. However,a recent study of ODNR of this luminescence has shown part of the 6. that Interesting emissionofis the from a spectrum triplet state, with a lifetime of ~ large I ms fine—structure aspects ODMR are the presence of a very splitting (D
=
22.6 0Hz) and the fact that the corresponding fine—structure axis
is accurately perpendicular to the trigonal axis of the crystal.
Comparison
with the known epr spectra of defects in quartz7 shows that there is no known defect with symmetry axes related to those of the triplet exciton.
0022—2313/84/$03.OO© Elsevicr Science Publishers BY. (North-Holland Physics Publishing Division)
The best
I4. ha res
100
understood ing
point
an unpaired Assistance
vided by electron
defect
in quartz
electron; at arriving
this
is
has
the
ccntrc,
an optical
an oxygen
absorption
vacancy
hand at
5.85
contain-
eV.
at
a possible model for the triplet exciton was pro8 These authors showed, using pulsed the work of Tanimura et al irradiation at low temperatures, that there is a transient absorption
baud at 5.4 eV, correlated transient
/ jIlec/ranisios of cxcitoo trapping in oxides
with the blue luminescence,
volume change correlated
with both,
and that
They conclude
there is a
that as ingle
type
of defect gives rise to the 5.4 cv absorption and the volume change and also contributes to the blue luminescence, and that this defect is produced photo— lytically
4K.
with
an efficiency
equal
to that
for
They suggest that the defect is a close
F centre
production
(oxygen vacancy)
—
in
FOr at
(oxygen
inter-
stitial) pair which recombines following optical emission, leaving the crystal in its ground state. The very large value of 0 for the triplet state suggests a molecular coot i— guration for the emitting centre (the 3t ground state of molecular oxygen has O
=
59.6 0Hz).
Hayes et al6 suggested a tentative model
for the emitting
centre (Figure 1) comprising a close Frenkel pair in the oxygen subiattice, with the interstitial in a split bonding configuration (cf. ref. 8).
They
assumed that the ground state of the vacancy is diamagnetic and that the exciton is localised primarily in the molecular configuration of the split interstitial.
To be consistent
with experiment the axis of the split inter-
stitial is drawn perpendicular to the ccystal axis in Figure I; since this is an unlikely occurrence it is possible that dynamical averaging may be taking place.
2
554)
z
5)2)
Si)?)
~)3)
“CD ~ii~i
OSi Cl Ca)
Qsr (1) C?)
FIGURE I Schematic representation of (a) structure of a perfect quartz crystal and (b) an (oxygen vacancy) — (oxygen interstitial) pair with the interstitial in a split interstitial (molecular) configuration oriented perpendicular to the trigonal (z) axis of the crystal.
W. Haves / Mechanisms of exeiton trapping in oxides
101
The complex illustrated schematically in Figure I has only a transient existence and should have an energy barrier preventing vacancy—interstitial re— combination for periods of ~ I ms. plex present a formidable
The energetics of formation of such a com-
theoretical problem.
(8) Y
20~
The possibility of exciton self—trapping in oxides such as Y203 and Sc2O3 has been investigated using optical techniques by Kuznetsov et al. ~ Y7O3 has a strong luminescence at low temperatures, peaking at 3.4 cv.
This material has
the cubic bixbyite structure, derived in effect from the fluorite structure by removal of one in every four anions, so that the eightfold coordination of the cations is reduced to sixfold.
Our ODIR studies’° showed that the luminescence
is emitted from a triplet state and analysis of the fine structure suggested a transient emitting centre consisting of a lattice anion, an interstitial anion and an anion vacancy.
This complex is in effect an exciton trapped by a
lattice anion associated with an anion Prenkel pair, similar to the structure 1 of In the conclusion self—trapped excitonout in that fluorites. we point the exclton trapping mechanism proposed for Y 203 is related to that proposed for Si02, although it should be emphasised that the models must be regarded as tentative.
The photolytic damage implied
by the models is both localised and transient and the trapping of excitons does not require a prior self—trapping of holes
(see above).
Much more detailed
work remains to be done on these systems.
REF N REN CES 1) W. Hayes, Cont.
Phys. 21 (1980) 451.
2) E.A. Colbourn and W.C. Nacrodt, Sol. St. Comm. 40 (1981) 265. 3) C.R.A. Catlow and W.C. Nacrodt, Eds, Computer Simulation of Solids’ (Springer, Berlin) 1982. 4) W. Hayes, H. Yamaga, 0.3. LI 085.
Robblns and B. Cockyane, 3. Phys. C. 13 (1981)
5) R.L. Wood and W. Hayes, 3. Phys. C,
IS (1982) 7209.
6) W. Hayes, N.J. Kane, 0. Salminen, R.L. Wood and S.P. Doherty, 3. Phys. 17 (1984) 2943. 7) J.A. Weil, Phys.
Chem.
Nm.
10 (1984)
149.
8) K. Tanimura, T. Tanaka and N. Itoh, Phys. Rev. Lett. 9) A.I. Kuznetsov 28 (1978) 602.
,
C,
,
51
(1983) 423.
v.N. Abromov, N.S. Rooze and T.T. Savikhina, J.R.T.P. Lett
10) W. Hayes, N.J. Kane, 0. Salminen and A.T. Kuznetsov, J.Phys.C. 17 (1984)L383.
Journal of Luminescence 31 & 320984)99-101 North-Holland, Amsterdam
MECHANISMS OP EXCITON TRAPPING TN OXIDES
W. HAYES Clarendon Laboratory, Oxford OXI 3PU, U.K.
The self—trapping of excitoos in aome halide cryatals is now ao established phenomenon. The behaviour of excitona in oxfdea ia less clear cut. ODMR studies show that in YAG and YA1O 3 trapping of excitons is associated with lattice defects but suggest that in Sf02 and Y203 intrinsic trapping processes may be dominant.
1.
INTRODUCTION
The electron—lattice interaction in ionic crystals such as alkali halides, 1 alkaline earth fluorides AgC1 systems gives rise of excitons The self—trapped exciton and in these is, toin self—trapping effect, an electron trapped by a self—trapped hole (STH).
In alkali and alkaline earth halides the STH is a
molecular entity of the type X 2 where X energy is a halogen. the STH is, in 2+ ion. The localisation required In for AgCl self—trapping effect, an Ag increases with increasing bandwidth and calculation suggests that electron— lattice interaction in oxides is marginal for self—trapping of excitons.2’3 Many oxide systems show strong X—ray induced luminescence at low temperatures due to e—h recombinatien.
In some cases such emissions come from triplet exci—
tonic states and detailed information about the structure of the emitting complex maybe obtained using OOMR.
We have carried out ODMR studies of triplet
excitons in YAG4 and YAlO~, showing that the excitons are trapped at lattice defects rather than being self—trapped.
We present ODNR results here for Sifl~and
Y 203 which suggest that excitons may be self—trapped in these materials.
2.
EXPERIMENTAL RESULTS AND DISCUSSION (i) Quartz (Si02).
Exposure of quartz to ionising radiation at low tempera-
tures results in a well known broad luminescence peaking in the blue (% 2.8 eV). This band is a superposition of overlapping emissions from different centres. However,a recent study of ODNR of this luminescence has shown part of the 6. that Interesting emissionofis the from a spectrum triplet state, with a lifetime of ~ large I ms fine—structure aspects ODMR are the presence of a very splitting (D
=
22.6 0Hz) and the fact that the corresponding fine—structure axis
is accurately perpendicular to the trigonal axis of the crystal.
Comparison
with the known epr spectra of defects in quartz7 shows that there is no known defect with symmetry axes related to those of the triplet exciton.
0022—2313/84/$03.OO© Elsevicr Science Publishers BY. (North-Holland Physics Publishing Division)
The best
I4. ha res
100
understood ing
point
an unpaired Assistance
vided by electron
defect
in quartz
electron; at arriving
this
is
has
the
ccntrc,
an optical
an oxygen
absorption
vacancy
hand at
5.85
contain-
eV.
at
a possible model for the triplet exciton was pro8 These authors showed, using pulsed the work of Tanimura et al irradiation at low temperatures, that there is a transient absorption
baud at 5.4 eV, correlated transient
/ jIlec/ranisios of cxcitoo trapping in oxides
with the blue luminescence,
volume change correlated
with both,
and that
They conclude
there is a
that as ingle
type
of defect gives rise to the 5.4 cv absorption and the volume change and also contributes to the blue luminescence, and that this defect is produced photo— lytically
4K.
with
an efficiency
equal
to that
for
They suggest that the defect is a close
F centre
production
(oxygen vacancy)
—
in
FOr at
(oxygen
inter-
stitial) pair which recombines following optical emission, leaving the crystal in its ground state. The very large value of 0 for the triplet state suggests a molecular coot i— guration for the emitting centre (the 3t ground state of molecular oxygen has O
=
59.6 0Hz).
Hayes et al6 suggested a tentative model
for the emitting
centre (Figure 1) comprising a close Frenkel pair in the oxygen subiattice, with the interstitial in a split bonding configuration (cf. ref. 8).
They
assumed that the ground state of the vacancy is diamagnetic and that the exciton is localised primarily in the molecular configuration of the split interstitial.
To be consistent
with experiment the axis of the split inter-
stitial is drawn perpendicular to the ccystal axis in Figure I; since this is an unlikely occurrence it is possible that dynamical averaging may be taking place.
2
554)
z
5)2)
Si)?)
~)3)
“CD ~ii~i
OSi Cl Ca)
Qsr (1) C?)
FIGURE I Schematic representation of (a) structure of a perfect quartz crystal and (b) an (oxygen vacancy) — (oxygen interstitial) pair with the interstitial in a split interstitial (molecular) configuration oriented perpendicular to the trigonal (z) axis of the crystal.
W. Haves / Mechanisms of exeiton trapping in oxides
101
The complex illustrated schematically in Figure I has only a transient existence and should have an energy barrier preventing vacancy—interstitial re— combination for periods of ~ I ms. plex present a formidable
The energetics of formation of such a com-
theoretical problem.
(8) Y
20~
The possibility of exciton self—trapping in oxides such as Y203 and Sc2O3 has been investigated using optical techniques by Kuznetsov et al. ~ Y7O3 has a strong luminescence at low temperatures, peaking at 3.4 cv.
This material has
the cubic bixbyite structure, derived in effect from the fluorite structure by removal of one in every four anions, so that the eightfold coordination of the cations is reduced to sixfold.
Our ODIR studies’° showed that the luminescence
is emitted from a triplet state and analysis of the fine structure suggested a transient emitting centre consisting of a lattice anion, an interstitial anion and an anion vacancy.
This complex is in effect an exciton trapped by a
lattice anion associated with an anion Prenkel pair, similar to the structure 1 of In the conclusion self—trapped excitonout in that fluorites. we point the exclton trapping mechanism proposed for Y 203 is related to that proposed for Si02, although it should be emphasised that the models must be regarded as tentative.
The photolytic damage implied
by the models is both localised and transient and the trapping of excitons does not require a prior self—trapping of holes
(see above).
Much more detailed
work remains to be done on these systems.
REF N REN CES 1) W. Hayes, Cont.
Phys. 21 (1980) 451.
2) E.A. Colbourn and W.C. Nacrodt, Sol. St. Comm. 40 (1981) 265. 3) C.R.A. Catlow and W.C. Nacrodt, Eds, Computer Simulation of Solids’ (Springer, Berlin) 1982. 4) W. Hayes, H. Yamaga, 0.3. LI 085.
Robblns and B. Cockyane, 3. Phys. C. 13 (1981)
5) R.L. Wood and W. Hayes, 3. Phys. C,
IS (1982) 7209.
6) W. Hayes, N.J. Kane, 0. Salminen, R.L. Wood and S.P. Doherty, 3. Phys. 17 (1984) 2943. 7) J.A. Weil, Phys.
Chem.
Nm.
10 (1984)
149.
8) K. Tanimura, T. Tanaka and N. Itoh, Phys. Rev. Lett. 9) A.I. Kuznetsov 28 (1978) 602.
,
C,
,
51
(1983) 423.
v.N. Abromov, N.S. Rooze and T.T. Savikhina, J.R.T.P. Lett
10) W. Hayes, N.J. Kane, 0. Salminen and A.T. Kuznetsov, J.Phys.C. 17 (1984)L383.