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INTERSTITIAL TRAPPING PROCESSES AND
DYNAMIC SATURATION EFFECTS WHICH
INFLUENCE THE GROWTH RATE OF DEFECTS IN
IRRADIATED IMPURE ALKALI HALIDES
J. Marat-Mendes, J. Comins
To cite this version:
C7-132 JOURNAL DE PHYSIQUE Colloque Cl, supplément au n° 12, Tome 37, Décembre 1976
INTERSTITIAL TRAPPING PROCESSES AND DYNAMIC SATURATION
EFFECTS WHICH INFLUENCE THE GROWTH RATE
OF DEFECTS IN IRRADIATED IMPURE ALKALI HALIDES
J. N. MARAT-MENDES and J. D. COMINS
Department of Physics, University of the Witwatersrand, Johannesburg, South Africa
Résumé. — Des études sur KCl(Sr) et sur KBr(Sr) montrent que la décroissance des ddpôles impureté-lacune (I. V.) pendant l'irradiation aux rayons X près de la température ambiante est proportionnelle à la formation de centres F et en accord numérique étroit avec elle. Les bandes H-N et Vf satisfont une relation en loi quadratique en accord avec des modèles impliquant respective-ment pour les deux défauts des centres H et la capture de di-interstitiels aux dipôles I. V. Cette inter-prétation est confirmée par des expériences de cuisson de centres H»». L'existence d'effets de satu-ration dynamique résultant dans le premier stade de la formation de centres F est illustrée par des expériences sur des cristaux de KC1, et des mécanismes possibles pour rendre compte des dépen-dances observées en température et en intensité sont discutés.
Abstract. — Investigations on KCl(Sr) and Kbr(Sr) show that the decrease of impurity vacancy
(I. V.) dipoles during X-irradiation near room temperature is proportional and in close numerical agreement with F-centre formation. The H-N and V™-bands obey a square-law relation in agree-ment with models involving H-centre and di-interstitial capture at I. V. dipoles for the two defects, respectively. This interpretation is confirmed by Hna-centre annealing experiments. The existence of dynamic saturation effects resulting in the first stage of F-centre formation is illustrated by experi-ments on KC1 crystals and possible mechanisms to account for the observed temperature and intensity dependences are discussed.
1. Introduction. — The origin of the two stages of F-centre production in alkali halides near room temperature has been the subject of numerous inves-tigations (see Crawford [1], Sonder and Sibley [2]). For crystals doped with divalent cations a favoured model has been that of Ikeya et al. [3] in which iso-lated cation vacancies capture H-centres until exhaus-tion occurs, this accounting for the earlier (fast) and later (slow) F-centre production rates. Computer simulations of the growth curves based on these ideas of saturable interstitial traps (Agullo-Lopez and Jaque [4]) are able to account for many of their fea-tures, but show a relative insensitivity to intensity and temperature as regards the early stage height. It has recently been shown that in KCl(Sr), I. V. dipoles decrease during first- and second-stage F-centre production and that good numerical agree-ment exists between dipoles destroyed and F-centres created (Marat-Mendes and Comins [5]). Accordingly, models were proposed in which either the I. V. dipo-les trapped H-centres, acting thus as complements of the F-centres, or that the dipole destruction was the result of charge trapping, the interstitial centres being elsewhere in the lattice. In order to account for the reduction of the F-centre production rate corres-ponding to the saturation of the first stage, it was tentatively proposed that a dynamic mechanism akin
to the impurity-suppression mechanism of Pooley [6] was operative, in which the electron-trapping defects were the radiation-induced centres formed as comple-ments of the F-centres.
Additional experiments have now been carried out over a range of temperatures in which the nature of the defects associated with the I. V. dipoles has been studied. Furthermore evidence in support of a dynamic mechanism of first stage saturation has been obtained.
2. Experimental details. — Crystals of Korth KC1 and KBr doped with strontium (1 x 10 ~2 Mol %
and 5 x 1 0- 3 Mol % in the melt, respectively) were
used as cleaved or subjected to heat treatment (anneal-ing at 400 °C followed by quench(anneal-ing to 77 K).
Additional experiments were performed on Hilger and Watts KC1 crystals grown from Analar grade material.
For all experiments except one, the following X-raying conditions were used : A W-target tube ope-rated at 80 kV and 16 mA ; filters of 3 mm Al, 1.5 mm steel and 1 mm amorphous silica ; target to crystal distance 75 mm. In one of the experiments on a Hilger and Watts crystal different conditions applied, namely : A W-target tube operated at 41 kVp and 20 mA ; filters of 0.013 mm Al and 1 mm amorphous
INTERSTITIAL TRAPPING PROCESSES AND DYNAMIC -SATURATION EFFECTS C7-133
silica ; target to crystal distance 80 mm. Calculations show that the absorbed power in KC1 crystals in the second arrangement is circa five times larger that for the first, on the assumption that the X-ray tubes have equal efficiencies.
Optical bleaching was performed using a 60 W tungsten-filament lamp and an interference filter having a peak transmission of 587 nm with a band pass of 19 nm.
I. T. C. measurements were carried out in a conven- tional manner using a polarization field of lo6 V/m. Further details may be obtained from reference [S]. 3. Results.
-
3 . 1 EXPERIMENTS ON KCl(Sr) ANDKBr(Sr). - 3.1.1 The relation between F-centres and
I. V . dipoles. -The results obtained for KCl(Sr) and KBr(Sr) irradiated at either 293 K or 273 K are shown in figure 1. The KBr results are new and those for KC1 have appeared elsewhere [5]. They demonstrate
FIG. 1.
-
Relation between the concentration of I. V. dipoles destroyed and F-centre concentration for various KCI(Sr) and KBr(Sr) crystals. Inverted closed triangles-
quenched KCI(Sr), X-irradiated at 293 K ; crosses - as cleaved KCI(Sr), X-irra-diated at 293 K ; closed circles-quenchea KCl(Sr), X-irradiated at 273 K ; open triangles
-
quenched KBr(Sr), X-irradiated at 293 K. The vertical dashed lines show the transition from first-tosecond-stage F-centre production for each crystal.
that there is both proportionality and good numerical agreement between F-centre production and I. V. dipole destruction during X-irradiation. The concen- tration of I. V. dipoles has been calculated on the basis of the three-shell relaxation model [7] which predicts that n. n. n. dipoles are predominant in KCl(Sr). It has been assumed that a similar result holds for KBr(Sr). The constant of proportionality depends on prior heat treatment, demonstrating that fewer single (i. e. non-aggregated) I. V. dipoles are
destroyed per F-centre created in as-cleaved speci- mens. A further experiment in which an as-cleaved
KCI(Sr) crystal was irradiated at 195 K and subse- quently warmed to 293 K followed by an I. T. C. measurement showed that I. V. dipole destruction had occurred here as well. Fewer dipoles were appa- rently destroyed per F-centre created at 195 K as compared with the experiment at 293 K on a similar
as-cleaved crystal. The ratios were
-
6 F-centres created per dipole destroyed for the former case and-
3 F-centres per dipole in the latter. It is possible, however, that some dipole recovery occurred during the above mentioned warming to 293 K.3 . 1 . 2 Ultraviolet absorption bands.
-
Studies have also been carried out on the H-N-band 18, 9, 101 which was formed in KCl(Sr) over a wide range of temperature including temperatures as high as 293 K. It was found that over the range 195 K to 293 K the H-N-band grew during the early stages of the irra- diation, then saturated and decreased somewhat. The saturation level was temperature dependent and was higher for lower temperatures of irradiation. In the case of an irradiation performed at 140 K, although the growth curve of the H-N-centre concentration showed distinct curvature, the saturation level was not attained at the termination of the experiment corresponding to an F-centre concentration of 8 x 1016 ~ m - ~ .The height of the H-N-band before saturation is proportional to the square of the height of the major absorption band peaking in the neighbourhood of
15.0- KC1 (Sr)
A quenched (140 K)
0 0.5 1.0 1-5 2.0
d,(v?-band) (cm'')
FIG. 2.
-
Square-law relation between the peak absorption coefficients of the H-N-band and the Vy-band for various KCI(Sr) samples. Inverted closed triangles-
quenched sample, X-irradiated at 293 K ; closed circles - quenched sample, X-irra- diated at 273 K ; crosses - as cleaved sample, X-irradiated at 195 K ; open triangles -quenched sample, X-irradiated atC7-134 J. N. MARAT-MENDES AND J. D. COMZNS
220 nm, known also as the Vy-band [lo], as shown in figure 2. (There is some variation in the peak position
of this band depending on the temperature of irra- diation which is likely to originate from slightly dif- ferent configurations of the same basic defect struc- ture. The name Vy-centre is used as a generic term to describe this structure). The results clearly demons- trate the H-N-centre is closely associated with the Vy-centre over a wide temperature range. A similar square-law relation between the H-N-band and the D,-band in KBr(Ca) has been obtained by Hoshi et
al. [ l l ] during irradiation at 195 K.
Our results also confirmed other work [lo] which demonstrated that at 293 K, where the H-N-band is rather small compared with the Vy-band, that the latter grows in proportion to the F-band. Similar results were also obtained for the envelope of the Dl-, D2,- and D,,-bands and the F-band in KBr(Sr) irradiated at 293 K except for the highest doses used where slight deviation from a proportional relation occurred. At lower temperatures the H-N-centre makes a progressively larger contribution. For exam- ple after termination of the irradiation at 140 K, the H-N-band was still larger than the VT-band.
A comparison of absorption spectra produced in quenched KCI(Sr) crystals irradiated at 77 K and 140 K is shown in figure 3. The U. V. absorption bands differ considerably. After a 36 h irradiation at 77 K,
3.0 KC1 (Sr)
-
140K irradiation---
7 7 K irradiation -.-. anneal t o 120K I I I 6.3 6.0 5.0 4.0 3.0 ENERGY (eVFIG. 3.
-
Optical absorption spectra for two samples of KCI(Sr). Dashed line - After X-irradiation of one of the samples at77 K for 36 h ; Dash-dot line - after annealing the same sample to 120 K ; Solid line - After X-irradiation of the other sample at 140 K for 46 h. All measurements performed
at 77 K.
the major absorption is the HNa-band, with a small contribution from the H-N-band. A very small V(240)-band with possibly some contribution from the VY-band is also present. The irradiation at 140 K where the H,,-centre is unstable produced pro- minent H-N- and VT-bands.
The crystal irradiated at 77 K was subjected to an isochronal annealing procedure using 10 K intervals, except for a period at 110 K where an isothermal anneal was used until the HNa-band had decayed to
a very small height. This procedure ensured that mobile [Xzl-centres could not be involved in the processes since they become mobile only at higher temperatures in KC1 [2]. The effect on the absorption is also shown in figure 3, where it is observed that the H-N- and VT-bands grow during the decay of the HNa-band. The peak of the Vy-band shifted to shorter wavelengths for higher temperature anneals eventually being at 216 nm at 293 K. The absorption in the HN,-band region was decomposed into contributions from the HNa-, H-N- and [Xi]-bands using a computer programme which fitted Gaussian bands to the absorp- tion envelope. The [Xi]-contribution was found to be negligible while at 77 K the H-N-band contribution was small. Figure 4 shows the annealing behaviour
HNo
-
band 0 H - N - b a n d-
'E 0-
E 1-
*-• TEMPERATURE (K)FIG. 4.
-
Behaviour of the HN,-band (inverted triangles), the H-N-band (open circles) and the Vy-band (closed circles) during the thermal annealing of a KCI(Sr) sample previously X-irra-diated at 77 K for 36 h.
and clearly demonstrates the conversion of the H,,-
INTERSTITIAL TRAPPING PROCESSES AND DYNAMIC SATURATION EFFECTS C7-135
3.1.3 Discussion. - The model adopted as an are clearly not exhausted when the first-stage saturates explanation of the results is as follows : and it therefore follows that a dynamic process might
(a) The primary defect production process results be operative which would include first-stage behaviour in F-centres and trapped interstitial halogen defects. simply as a high-temperature consequence of a single
(b) The trapping process and the final configuration mechanism. Evidence in favour of this point of view of the interstitial defects depends on the impurities will be presented in the following sections. present in the crystal and the relative stability of the
final trapping state.
(c) At 77 K, the major interstitial traps are the Na' substitutional impurities which results in the prefe- rential formation of H,,-centres with a very small contribution of H-N-centres, whereas above the tem- perature at which the HNa-centres become unstable
( w 110 K) interstitials are preferentially trapped at
I. V. dipoles. The HNa-centre annealing experiments are completely consistent with this point of view, as are the results of the irradiation experiment per- formed at 140 K, and demonstrate that both the H-N- centre and the V?-centre are associated with trapped H-centres. Moreover the I. T. C . measurements clearly demonstrate the destruction of I. V. dipoles at least over the range 195 K to 293 K with proportionality and close numerical agreement with F-centre produc- tion in quenched crystals.
(d) The fact that the square law between the H-N- band and the Vy-band extends over such a wide tem- perature range (140K-295 K) demonstrates that a single basic process linking the responsible defects must exist. The models of the H-N-band and the Vy-band which appear consistent with the facts includ- ing their low temperature interconversion are those of Hoshi et al. Ill]. The H-N-centre is considered to be an H-centre which is trapped at an I. V. dipole, the direct evidence for this assignment arising from the saturation level of the H-N-band attained near room temperature and the initial concentration of single I. V. dipoles [5]. The VY-centre is considered to be a di-interstitial halogen centre at an I. V. dipole or an I. V. dipole aggregate, this accounting for the observed differences in the slopes in figure 1 for quenched and as-cleaved crystals. Similar conclusions also apply to KBr, in our case for the Dl- and D,- centres and for the D,-centre produced at lower temperatures [l 1
1.
The electronic state of the di-inter- stitial halogen could be either p:] [12], or [ X i ] according to the reaction 113, 141where X- is a normal anion.
(e) Since an overall mechanism involving the cap- ture of H-centres by I. V. dipoles appears the major process between 140 K and 293 K it follows that the shape of the F-centre growth curves would be expect- ed to be largely determined by this or associated mechanisms. It follows that the suggested origin of the first-stage of F-centre production by the exhaus- tion of isolated cation vacancies [3, 41 is difficult to reconcile with the present results. The I. V. dipoles
3.2 EVIDENCE FOR DYNAMIC MECHANISMS INVOLV-
ING THE FIRST STAGE OF F-CENTRE PRODUCTION. -
3.2.1 Experiments on Hilger and Watts crystals.
-
In order to determine the existence of dynamic pro- cesses leading to the saturation behaviour of the F- centre growth curves, corresponding to the first stage of F-centre production, experiments were per- formed on Hilger and Watts crystals, these being typical of impure samples grown from Analar grade material.
TIME ( h )
FIG. 5. -Growth and regrowth behaviour of the F-centre concentration in a Hilger and Watts KC1 crystal during a suc- cession of different processes. The processes are described sequentially. Closed circles - X-irradiation at 294 K ; Vertical dashed line - optical bleaching with F-band light ; closed triangles - re-irradiation with X-rays at 273 K ; Closed circles
-
continuing irradiation at 294 K ; closed triangles-
continu- ing irradiation at 273 K ; crosses-
thermal bleaching at 293 K.C7-136 J. N. MARAT-MENDES AND J. D. COMINS The ultraviolet optical absorption bands observed
comprised the V,-band peaking at 230 nm and the
V,- (or OH-)-bands peaking at 212 nm. The latter band grew during the first irradiation cycle, and the- reafter increased rather slowly during subsequent cycles. The V2-band by contrast was affected in the same way as the F-band throughout irradiation, bleaching and re-irradiation cycles and displayed the same type temperature dependent saturation behaviour with higher levels of saturation at lower temperatures. The F-band bleaching experiments clearly showed mutual annihilation of F-centres and V,-centres, a process very similar to that observed in similar experi- ments on Korth KCI(Sr) crystals where the VY-band decreased [5]. A model which appears consistent with the results is to associate the V2-band with trapped halogen interstitial defects. Moreover, the bleaching results demonstrate that the V,-centres are good elec- tron traps, in contrast to V,-centres formed in pure KC1 (Harshaw) which are largely unaffected during F-band bleaching [I 51.
3.2.2 Discussion. - The dynamic mechanism of
the saturation of F-centre growth must include both the interstitial trapping effects discussed in sec- tion 3.1.3. as well as the temperature and intensity dependences shown in section 3.2.1. It should be noted that a similar, but smaller temperature depen- dence was also observed in Korth KCI(Sr) 151.
Both (a) short-circuiting of the exictonic mechanism and (b) the effects of back reactions could offer an
explanation of the results.
(a) If the irradiation-produced defects formed as complements of the F-centres (VY-, V,- and D-cen- tres) and which are associated with trapped halogen interstitials are also electron traps, then an extension of the Pooley model of inhibition of the defect pro- duction rate [6]' should apply. The initial slopes of the F-centre growth curves will depend on interstitial trapping effects [3] whereas for larger doses where the concentration of electron-trapping centres has increased,
Refer [I] CRAWFORD Jr., J. H., Adv. P h y ~ . 17 (1968) 93.
[21 SONDER, E. and SIBLEY, W. A., In Point Defects in Solids. Ed. J . H. Crawford Jr. and L. M. Slifkin (Plenum Press, New YorkILondon) 1 . (1972) 201. . .
IKEYA, M., ITOH, N., OKADA, T. and SUITA, T., J. Phys. Soc. Japan 21 (1966) 1304.
AGULLO-LOPEZ, F. and JAQUE, F., J. Phys. & Chem. Solids 34 (1973) 1949.
MARAT-MENDES, J. N. and COMINS, J. D., Cryst. Lattice
Defects 6 (1975) 141.
POOLEY, D., Proc. Phys. Soc. 89 (1966) 723.
BRUN, A. and DANSAS, P., J. Phys. C. 7 (1974) 2593.
where I is the irradiation intensity, E is the activation energy for [Xzl-centre motion and n is the concentra- tion of electron trapping defects. The observed results are at least in qualitative agreement with this model. In this regard Pooley [6] has pointed out that since E
is smaller for KBr than for KCl, the inhibition will occur at a lower value of n in agreement with the results shown in figure 1.
Evidence exists [16, 171 to show that exciton decay occurs at impurities. If such decay can occur at lat- tice defects, e. g. the complements of F-centres, and does not lead to defect production, then this can also make a contribution to the saturation of the growth curves. A simple model of this type 1181 predicts linear followed by parabolic F-centre growth and that [F] n
45
where t is the irradiation time. In addition it would be necessary to postulate a temperature dependence of exciton decay. There exists some evi- dence in support of the latter [IS].The recently suggested excitonic mechanism of Townsend 1191 can incorporate both types of short- circuit mechanism.
(b) The mechanism in which electron-trapping
defects extend the lifetime of mobile F+-centres leading to an enhanced rate of interstitial-vacancy recombination [2] should also be considered.
Further work is needed in order to investigate in a more quantitative way, the temperature and intensity dependances and to develop a new interpretation of the result in which the height of the first stage is pro- portional to the square root of the impurity concen- tration [3].
Acknowledgments.
-
The authors would like to thank Dr. V. Hnizdo for advice regarding the com- puter programme for analysing absorption bands and the C . S. I. R. Pretoria for financial assistance.[lo] VOSZKA, R., HORVATH, T. and WATTERICH, A., Acta. Phys.
Acad. Scient. Hungaricae 24 (1968) 255.
[If] HOSHI, J., SAIDOH, M. and ITOH, N., CPySt. Lattice Defects
6 (1975) 15.
1121 DIENES, G. J., HATCHER, R. D. and SMOLUCHOWSKI, R.,
J. Phys. & Chem. Solids 31 (1970) 701.
1131 HERSH, H. N., Phys. Rev. 148 (1966) 928.
[14] YAANSON, N. A., GINDINA, R. I. and LUSHCHIK, Ch. B., Fiz. Tverd. Tela. 16 (1974) 379.
1151 COMINS, J. D. and WEDEPOHL, P. T., Solid State Commun. 4 (1967) 537.
[I61 TANIMURA, K. and OKADA, T., Phys. Rev. B 13 (1976)
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[81 HAYES, W. and NICHOLS, G. M., Phys. Rev. 117 (1960) 1171 NOUAILHAT, A., GUILWT, G. and MERCIER, E., Solid
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