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Electron irradiation displacement damage in photorefractive Bi12SiO20
Nuclear Instruments
and Methods
in Physics Research
Nuclear Instruments & Methods in Physics Research St.1 ll~/ll B
Hh5 (1902) 275-277
North-Holland
Electron irradiation displacement damage in photorefractive E.R. Hodgson
Electron
“, L. Arizmendi
irradiation
nm (2.7 eV). Contrary probably
of
Bi ,,SiOz,,
” and F. Agull
shows a
definite
to previous assignments.
F-type centres. A displacement
energy threshold
the results
energy of 4Y rV
”
for the production
indicate that this absorption is obtained
of a broad optical ahsorption
band at 460
is directly related to oxygen ion vacancies,
for the oxygen ion.
2. Experimental
1. Introduction
Bi12S~07,,, gcncrally known as BSO, exhibits a number of both interesting and highly useful propertics. The crystals are not only optically active and photoconductive, but also possess piczoclectric, elastooptic. and elcctrooptic propertics [ 1.2]. Nominally pure crystals arc yellow in colour due to a broad absorption band at about 460 nm (2.7 cV) running into the absorption cdgc near 390 nm. The origin of the band is not at all clear and has been variously interpreted in terms of Bi,, antisitc ccntres, Si interstitials. or Si vacancies [3-S]. It is interesting to note that the possibility of oxygen vacancies playing a rcilc has hardly been considcrcd. Thcrmochcmical reduction cxpcriments, which have been useful in studying the effect of oxygen vacancies in many materials, have not proved successful in BSO due to the rapid reduction cxpericnccd at the surface of the material lading to large Bi precipitates [h,7]. Howcvcr, in materials not susceptible to radiolysis, point defects may bc readily introduced in the bulk by high energy electron irradiation. It has proved to bc a useful tool in elucidating the nature of the complex band structures observed in clectrooptic materials. The method has been rcccntly successfully applied to LiNbO,, KNbO,. and BaTiO, for which atomic displacement thresholds have been measured, and oxygen vacancy type ccntres identified [8-IO]. In the work prcscntcd hcrc, single crystal BSO has been irradiated with high energy clcctrons in order to gain information about the observed optical absorption bands. The results indicate that, in common with other photorefractivc materials, oxygen vacancies do play a rdle and are the cause of the ohscrved 460 nm absorption band. OlhX-SX3X/YZ/$OS.O0
Lbpez
Bi,2Si020
CC’)1YY2
Elsevirr
Science Publishers B.V.
The experiments reported here have been performed in a sample chamber mounted in the beam line of a HVEC 2 MeV Van dc Graaff accelerator. The system enables samples to bc irradiated in high vacuum (<3X10 (’ mbar) at a controlled tcmpcraturc with either electrons or purely ionizing brcmsstrahlung radiation. In-situ optical absorption spectra may be measured during or following irradiation. The method provides a high sensitivity and reproducibility for both radiation and temperature induced effects on the optical absorption, as the samples arc neither removed nor remounted between measurements. In this way BSO single crystals of approximately 5 x 5 x I mm3 (Crystal Growth Laboratory, Universidad Autcinoma, Madrid) wcrc irradiated at 15°C firstly with ionizing radiation to - IO’ rad, then with 1.X McV electrons to a total dose of 5 X 10” c to obscrvc any induced optical damage, and finally with clcctrons at various energies bctwccn 0.25 and 0.40 McV to search for a possible threshold effect. At each energy a dose of 2 x IO” c was given. During irradiation the optical absorption at 450 nm was monitored. In addition the samples wcrc heated to 200°C and cooled again to 15°C both before and after irradiation at I.8 McV to look for possible temperature and thermal annealing cffccts on the absorption spectra.
3. Results Purely ionizing radiation produces no obscrvablc change in the optical absorption spectrum. Howcvcr irradiation with 1.X McV electrons introduces a gcncral increase in the optical absorption, the resulting All rights reserved
V. OXIDES/CERAMICS/CARBIDES
Fig. 3. Ah\orption Fig. I. (a) Absorption
spectrum of the unirrndiated
(h) Spectrum
irradiation
following
with
1.X McV
arbitrary
sample.
electrons
induced at 450 nm by units (au). as 2 function
7~
IO"
electrons
to
sx1o”e
spectrum king almost indistinguishable from the unirradiated spectrum (fig. I). Howcvcr the diffcrcncc spectrum (fig. 2) shows that the irradiation induces an cxtrcmcly broad hand with a maximum at about 360 nm. This band is stable and shows no annealing up to at lcast 200°C. The results of irradiation at cncrgics bctwecn 0.3 and 0.40 McV arc given in fig. 3. At 0.3 and 0.27 McV no incrcasc whatsoever is ohscrvcd in the optical absorption. However on irradiation at 0.30 MeV and above one observes 21 dcfinitc incrcasc in the absorption due to the formation of broad band at 3hO nm (fig. 4). This threshold cffcct ix indicative of atomic displuccment damage being involved in the formation of the defect responsible for the 460 nm band. The fact that below 0.30 McV no change is obscrvcd dcapitc a total ionizing dose > 100 Mrad confirms the results obtained at the lower total dose ( - IO’ rad) with brcmsstrahlung radiation. i.c. the samples arc only
susccptiblc to displacement damage. From the observed threshold cncrgy of - 0.28 McV one can calculate the atomic displacement energies for Bi. Si. and 0. values of approximately 4, 28, and 49 CV rcspcctivcly are obtained.
4. Discussion The results prcscnted in fig. 3 indicate that displacement damage is involved in the production of the defect rcsponsiblc for the 460 nm absorption band. 01 the three possible displacement encrgics. the most rcasonablc is that of 40 cV for the oxygen. This value is very similar to that found for oxygen displacement in other photorcfractivc materials and oxides; bctwccn SO and 60 CV [X-13]. A binding energy of 4 eV for Bi is far too low. and furthermore would render the matcrial susceptible to radiolysis which is not the case. A value of 2X CV for Si although rather low cannot be directly ruled out. (A further experiment using B,,GcO,,, should indicate definitively oxygen or ger-
6t-
0 400
500
Fig. 2. Difference
600 hetwwn
h(nm) spectra
700
800
(a) and (b) in fig. I.
in
of energy.
400
Fig. 1.
A
500
1
I
I
600
700
800
hlnm) Ahsorption induced tq irradiation at 0.35 McV.
manium displaccmcnt due to the larger mass diffcrcnce). Howevcr the large difference between the oxygcn and silicon content (20: 1) would require the Si to have either an cxccptionally large displacement cross section or the associated dcfcct ccntrc to possess ;I very large oscillator strength. We conclude thcreforc that the 460 nm band observed in BSO is associated with the introduction of oxygen vacancy ccntres within the lattice. A markedly similar behaviour has hcen obscrvcd in LiNbO,. KNbO?, and BaTi03 and the induced ahaorption band related to F-type ccntres [X-10]. In other oxides. electron irradiation above the oxygen displaccmcnt threshold introduces primarily F-centrcs [I I - 131. One is therefore tcmptcd to suggest that the observed absorption band in BSO is also related to F-type ccntrcs. The low cncrgy irradiations, just above threshold, introduce a clear band at 460 nm (fig. 4) while the 1.X McV irradiation introduces a much broader band (fig. 2). This difference is probably due to the formation of additional diffcrcnt dcfccts (at this cncrgy both Si and Bi will be displaced) or disorder within the lattice. Work is in progress to determine if higher thresholds exist which would throw light onto the nature of this additional absorption.
Acknowledgements
The authors Cabczudo for ments.
[II
J. Ari,mendi.
J.M.
in
to
Mr.
E. SLnchczthcsc cxpcri-
performing
[?I
I’. Giinther terials
Cuhrera.
vola.
I and II
R.B. Laucr and R.E. Aldrich.
Ma-
(Springer.
J. Appl. Phys. 44
Phys. Status Solidi AXY L.
J. Appl.
Antonov.
trndikcr.
C‘onlrrras.
E.
( lW3) X3.
DiCguez
and
J.M.
Phys. 66 (10X9) 5146.
PA.
Kristall
[71 L. Arirmrndi iSI E.R. llwlgson Matter
(cds.). Photorefractwe
Application<.
265,.
111 R. Oberschmid. ISI J.H. Baqudano, [()I V.A.
Int. J.
IYXX/ 10x9).
S.L. liou. (1973)
Lhpcz,
in press.
in J.P. Huignnrd
and their
Berlin.
[.Jl
C‘abrcra and F. AgullJ
( I WI)
Optoelrctronica
Arwyrv.
I.G.
und Technik
Linda
IO (lY75)
and V.L.
I-ars-
K5Y.
and E. DGguez.
prkate
communication.
and F. Agullh
Ldpez.
J. Phys. C‘ondcns.
I (1Wl) lOOl5,
[Y] E.R. Ho&son. [ltl]
While Bi and Si vacancies, intcrstitials. and disorder (antisitcs) may well play a rOlc in the gcncral background absorption obscrvcd in BSO, the 3hO nm hand is clearly associated with oxygen displacement and is most probably due to an F-type centrc.
indebted
help
References
Commun.
5. Conclusions
are his
C’. Zaldo and F. Agullh Lhpez. Solid State
75 (IYYO) 35 1.
E.R. Hodgson and F. Agull cal (‘onf.:
Lattice
Defects
Lhpe~. Proc. 6th Europhysi-
in Ionic Materials.
Groningen,
Sept. IYYO. to be published. [I I] G.W.
Arnold
and W.D.
Compton.
Phys. Re\.
Lett.
( IYhO) hh. [ 121 Y. Chen. D.L. Truehlood. J. Phyh. (‘3 (lY70) [I.?] G.P. Summcra.
O.E.
Schow and t1.T. Tohvcr.
2501.
G.S. White,
K.11. Lee and J.lI.
Phya. Rev. B2l (IYXO) 757X.
V. OXIDES/CERAMICS/CARBIDES
(‘rawford.
4
and Methods
in Physics Research
Nuclear Instruments & Methods in Physics Research St.1 ll~/ll B
Hh5 (1902) 275-277
North-Holland
Electron irradiation displacement damage in photorefractive E.R. Hodgson
Electron
“, L. Arizmendi
irradiation
nm (2.7 eV). Contrary probably
of
Bi ,,SiOz,,
” and F. Agull
shows a
definite
to previous assignments.
F-type centres. A displacement
energy threshold
the results
energy of 4Y rV
”
for the production
indicate that this absorption is obtained
of a broad optical ahsorption
band at 460
is directly related to oxygen ion vacancies,
for the oxygen ion.
2. Experimental
1. Introduction
Bi12S~07,,, gcncrally known as BSO, exhibits a number of both interesting and highly useful propertics. The crystals are not only optically active and photoconductive, but also possess piczoclectric, elastooptic. and elcctrooptic propertics [ 1.2]. Nominally pure crystals arc yellow in colour due to a broad absorption band at about 460 nm (2.7 cV) running into the absorption cdgc near 390 nm. The origin of the band is not at all clear and has been variously interpreted in terms of Bi,, antisitc ccntres, Si interstitials. or Si vacancies [3-S]. It is interesting to note that the possibility of oxygen vacancies playing a rcilc has hardly been considcrcd. Thcrmochcmical reduction cxpcriments, which have been useful in studying the effect of oxygen vacancies in many materials, have not proved successful in BSO due to the rapid reduction cxpericnccd at the surface of the material lading to large Bi precipitates [h,7]. Howcvcr, in materials not susceptible to radiolysis, point defects may bc readily introduced in the bulk by high energy electron irradiation. It has proved to bc a useful tool in elucidating the nature of the complex band structures observed in clectrooptic materials. The method has been rcccntly successfully applied to LiNbO,, KNbO,. and BaTiO, for which atomic displacement thresholds have been measured, and oxygen vacancy type ccntres identified [8-IO]. In the work prcscntcd hcrc, single crystal BSO has been irradiated with high energy clcctrons in order to gain information about the observed optical absorption bands. The results indicate that, in common with other photorefractivc materials, oxygen vacancies do play a rdle and are the cause of the ohscrved 460 nm absorption band. OlhX-SX3X/YZ/$OS.O0
Lbpez
Bi,2Si020
CC’)1YY2
Elsevirr
Science Publishers B.V.
The experiments reported here have been performed in a sample chamber mounted in the beam line of a HVEC 2 MeV Van dc Graaff accelerator. The system enables samples to bc irradiated in high vacuum (<3X10 (’ mbar) at a controlled tcmpcraturc with either electrons or purely ionizing brcmsstrahlung radiation. In-situ optical absorption spectra may be measured during or following irradiation. The method provides a high sensitivity and reproducibility for both radiation and temperature induced effects on the optical absorption, as the samples arc neither removed nor remounted between measurements. In this way BSO single crystals of approximately 5 x 5 x I mm3 (Crystal Growth Laboratory, Universidad Autcinoma, Madrid) wcrc irradiated at 15°C firstly with ionizing radiation to - IO’ rad, then with 1.X McV electrons to a total dose of 5 X 10” c to obscrvc any induced optical damage, and finally with clcctrons at various energies bctwccn 0.25 and 0.40 McV to search for a possible threshold effect. At each energy a dose of 2 x IO” c was given. During irradiation the optical absorption at 450 nm was monitored. In addition the samples wcrc heated to 200°C and cooled again to 15°C both before and after irradiation at I.8 McV to look for possible temperature and thermal annealing cffccts on the absorption spectra.
3. Results Purely ionizing radiation produces no obscrvablc change in the optical absorption spectrum. Howcvcr irradiation with 1.X McV electrons introduces a gcncral increase in the optical absorption, the resulting All rights reserved
V. OXIDES/CERAMICS/CARBIDES
Fig. 3. Ah\orption Fig. I. (a) Absorption
spectrum of the unirrndiated
(h) Spectrum
irradiation
following
with
1.X McV
arbitrary
sample.
electrons
induced at 450 nm by units (au). as 2 function
7~
IO"
electrons
to
sx1o”e
spectrum king almost indistinguishable from the unirradiated spectrum (fig. I). Howcvcr the diffcrcncc spectrum (fig. 2) shows that the irradiation induces an cxtrcmcly broad hand with a maximum at about 360 nm. This band is stable and shows no annealing up to at lcast 200°C. The results of irradiation at cncrgics bctwecn 0.3 and 0.40 McV arc given in fig. 3. At 0.3 and 0.27 McV no incrcasc whatsoever is ohscrvcd in the optical absorption. However on irradiation at 0.30 MeV and above one observes 21 dcfinitc incrcasc in the absorption due to the formation of broad band at 3hO nm (fig. 4). This threshold cffcct ix indicative of atomic displuccment damage being involved in the formation of the defect responsible for the 460 nm band. The fact that below 0.30 McV no change is obscrvcd dcapitc a total ionizing dose > 100 Mrad confirms the results obtained at the lower total dose ( - IO’ rad) with brcmsstrahlung radiation. i.c. the samples arc only
susccptiblc to displacement damage. From the observed threshold cncrgy of - 0.28 McV one can calculate the atomic displacement energies for Bi. Si. and 0. values of approximately 4, 28, and 49 CV rcspcctivcly are obtained.
4. Discussion The results prcscnted in fig. 3 indicate that displacement damage is involved in the production of the defect rcsponsiblc for the 460 nm absorption band. 01 the three possible displacement encrgics. the most rcasonablc is that of 40 cV for the oxygen. This value is very similar to that found for oxygen displacement in other photorcfractivc materials and oxides; bctwccn SO and 60 CV [X-13]. A binding energy of 4 eV for Bi is far too low. and furthermore would render the matcrial susceptible to radiolysis which is not the case. A value of 2X CV for Si although rather low cannot be directly ruled out. (A further experiment using B,,GcO,,, should indicate definitively oxygen or ger-
6t-
0 400
500
Fig. 2. Difference
600 hetwwn
h(nm) spectra
700
800
(a) and (b) in fig. I.
in
of energy.
400
Fig. 1.
A
500
1
I
I
600
700
800
hlnm) Ahsorption induced tq irradiation at 0.35 McV.
manium displaccmcnt due to the larger mass diffcrcnce). Howevcr the large difference between the oxygcn and silicon content (20: 1) would require the Si to have either an cxccptionally large displacement cross section or the associated dcfcct ccntrc to possess ;I very large oscillator strength. We conclude thcreforc that the 460 nm band observed in BSO is associated with the introduction of oxygen vacancy ccntres within the lattice. A markedly similar behaviour has hcen obscrvcd in LiNbO,. KNbO?, and BaTi03 and the induced ahaorption band related to F-type ccntres [X-10]. In other oxides. electron irradiation above the oxygen displaccmcnt threshold introduces primarily F-centrcs [I I - 131. One is therefore tcmptcd to suggest that the observed absorption band in BSO is also related to F-type ccntrcs. The low cncrgy irradiations, just above threshold, introduce a clear band at 460 nm (fig. 4) while the 1.X McV irradiation introduces a much broader band (fig. 2). This difference is probably due to the formation of additional diffcrcnt dcfccts (at this cncrgy both Si and Bi will be displaced) or disorder within the lattice. Work is in progress to determine if higher thresholds exist which would throw light onto the nature of this additional absorption.
Acknowledgements
The authors Cabczudo for ments.
[II
J. Ari,mendi.
J.M.
in
to
Mr.
E. SLnchczthcsc cxpcri-
performing
[?I
I’. Giinther terials
Cuhrera.
vola.
I and II
R.B. Laucr and R.E. Aldrich.
Ma-
(Springer.
J. Appl. Phys. 44
Phys. Status Solidi AXY L.
J. Appl.
Antonov.
trndikcr.
C‘onlrrras.
E.
( lW3) X3.
DiCguez
and
J.M.
Phys. 66 (10X9) 5146.
PA.
Kristall
[71 L. Arirmrndi iSI E.R. llwlgson Matter
(cds.). Photorefractwe
Application<.
265,.
111 R. Oberschmid. ISI J.H. Baqudano, [()I V.A.
Int. J.
IYXX/ 10x9).
S.L. liou. (1973)
Lhpcz,
in press.
in J.P. Huignnrd
and their
Berlin.
[.Jl
C‘abrcra and F. AgullJ
( I WI)
Optoelrctronica
Arwyrv.
I.G.
und Technik
Linda
IO (lY75)
and V.L.
I-ars-
K5Y.
and E. DGguez.
prkate
communication.
and F. Agullh
Ldpez.
J. Phys. C‘ondcns.
I (1Wl) lOOl5,
[Y] E.R. Ho&son. [ltl]
While Bi and Si vacancies, intcrstitials. and disorder (antisitcs) may well play a rOlc in the gcncral background absorption obscrvcd in BSO, the 3hO nm hand is clearly associated with oxygen displacement and is most probably due to an F-type centrc.
indebted
help
References
Commun.
5. Conclusions
are his
C’. Zaldo and F. Agullh Lhpez. Solid State
75 (IYYO) 35 1.
E.R. Hodgson and F. Agull cal (‘onf.:
Lattice
Defects
Lhpe~. Proc. 6th Europhysi-
in Ionic Materials.
Groningen,
Sept. IYYO. to be published. [I I] G.W.
Arnold
and W.D.
Compton.
Phys. Re\.
Lett.
( IYhO) hh. [ 121 Y. Chen. D.L. Truehlood. J. Phyh. (‘3 (lY70) [I.?] G.P. Summcra.
O.E.
Schow and t1.T. Tohvcr.
2501.
G.S. White,
K.11. Lee and J.lI.
Phya. Rev. B2l (IYXO) 757X.
V. OXIDES/CERAMICS/CARBIDES
(‘rawford.
4