DEFECTS INDUCED IN ALKALINE EARTH FLUORIDES BY FAR U.V. IRRADIATION

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DEFECTS INDUCED IN ALKALINE EARTH

FLUORIDES BY FAR U.V. IRRADIATION

Y. Kirsh, N. Kristianpoller

To cite this version:

Y. Kirsh, N. Kristianpoller. DEFECTS INDUCED IN ALKALINE EARTH FLUORIDES BY

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C7-216 JOURNAL DE PHYSIQUE Colloque C7, supplément au n° 12, Tome 37, Décembre 1976

DEFECTS INDUCED IN ALKALINE EARTH FLUORIDES

BY FAR U.V. IRRADIATION

Y. KIRSH and N . KRISTIANPOLLER

Department of Physics and Astronomy, Tel Aviv University, Tel Aviv, Israel

Résumé. — On a étudié par des méthodes électrique et optique les effets d'un rayonnement U. V. monochromatique sur des cristaux de C&Fi purs et dopés du Tb.

L'irradiation à 80 K et X < 1600 A produit une photoconductivité et une phosphorescence induites sur les cristaux purs ou dopés, aussi bien qu'une conductivité stimulée thermiquement (TSC) et de la thermoluminescence (TL).

La comparaison avec les effets d'un rayonnement X montre que certains défauts induits sont identiques à ceux produits par le rayonnement U. V. La composition spectrale de la thermo-luminescence des cristaux dopés est essentiellement la même pour les excitations X et U. V. et est attribuée dans les deux cas à la transition 5D -> ?F de l'ion Tb3+.

Les spectres de thermoluminescence dus à l'excitation U. V. des cristaux purs et dopés montrent que la lumière U. V. non ionisante produit les défauts par exciton. D'autres processus d'excitation dans les échantillons dopés semblent être liés au transfert électronique d'un ion fluor à une impureté Tb3+ voisine.

Des mesures analogues ont été effectuées sur des cristaux de BaF2 et SrF2. Les résultats obtenus concordent pour les trois fluorures.

Abstract. — Effects of monochromatic U. V. radiation (1100-3 000 A) on pure and Tb doped CaF2 were investigated by optical and electrical methods. Irradiation at 80 K with X < 1 600 A induced photoconductivity and phosphorescence in the pure and the doped crystals, as well as thermally stimulated conductivity (TSC) and thermoluminescence (TL).

Comparison with effects of X irradiation indicates that the U. V. induced defects are identical with some of those created by X rays. The spectral composition of the TL of the doped crystals was essentially the same after X and U. V. excitations, and is attributed in both cases to 5D -> 7F tran-sitions of Tb3+ ion.

U. V. excitation spectra of the TL peaks in the pure and doped crystals indicate an excitonic process for the creation of defects by the non-ionizing U. V. light. Additional excitation processes in the doped samples appear to be connected with an electron transfer from a fluorine ion to a nearby Tb3+ impurity.

Analogous measurements were performed with BaF2 and SrF2. The results were consistent for all three fluorides.

1. Introduction. — The mechanism of defect

crea-tion in alkaline-earth-fluorides by X and y rays has been the subject of intensive studies. It is known that irradiation at low temperatures causes in the rare-earth (RE) doped crystals the reduction of the tri-valent impurity ion to a ditri-valent state and creates various types of hole centres [1-4]. During the heating of the crystal, holes are thermally released and recom-bined with the R E2 + to restitute the original RE3 + ions.

It was found [5] that U. V. light can also create point defects in these crystals, but the mechanism of defect creation and annihilation is much less clear. It has been suggested that the U. V. irradiation may also cause a change in the valence of the RE ions [6, 7]. The effects of monochromatic U. V. irradiation on pure and doped alkaline-earth-fluorides are presently being investigated in our laboratory by optical and electrical methods. We report here mainly

on results for pure and T b3 + doped CaF2 ; similar results were found for SrF2 and BaF2 crystals. TL and TSC as well as photoconductivity and phospho-rescence at LNT were measured. The dependence of the TL intensity on the exciting wavelength was investigated in order to find the photon energies most efficient for these processes. The spectral compo-sition of U. V. induced TL was also measured. Results were compared with those obtained after X irradiation. 2. Experimental. — For the experiments, pure CaF2 crystals from Harshaw Co. and CaF2 : Tb (0.03 at %) from the crystal-growing laboratory of the Hebrew University of Jerusalem were used. Single crystals of about 1 cm2 area and 2 mm thickness were kept in a windowless vacuum cryostat attached to a 1 meter normal incident V. U. V. monochromator (McPher-son 225, linear dispersion — 8.3 A/mm). The width of the slits was 1 mm. A 1 000 W hydrogen arc lamp

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U. V. INDUCED DEFECTS IN ALKALINE EARTH FLUORIDES (7-21 7 was used as light source. The photon flux of the

exciting radiation was monitored by a sodium- salicylate screen. The temperature of the crystal was measured by a copper constantant thermocouple.

The phosphorescence and TL were detected and recorded by an EM1 62568 photomultiplier connected to a micro-ammeter (Keithley 414s) and an x-y recor- der. For some measurements the samples were irradiated by X rays (tungsten tube, 50 kV, 15 mA).

The spectral distribution of the TL and phosphorescence was measured with a fast scanning prism monochromator (linear dispersion 2-35 nm/mm in the spectral region 300-600 nm). The arrangement for spectral measurements has been described else- where [8].

For conductivity measurements, two gold coated electrodes were attached to one face of the crystal. The electrical circuit consisted of a 500 V DC battery and a Keithley 410 micro-ammeter.

For measurement of TL and TSC, the samples were irradiated for 10 minutes at 80 K with monochromatic U. V. light and heated at a constant heating rate of 10 K/min.

3. Results.

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The irradiation at LNT with monochromatic U. V. light in the spectral region 110- 180 nm was found to cause during heating TL in the doped and the pure samples. In the nominally pure CaF, crystals TL peaks appeared at about 120, 167 and 255 K. An additional peak appeared at 205 K only after irradiation with A

<

125 nm. In

FIG. 1.

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Glow curves recorded after irradiation at 80 K :

0) for CaF2 : Tb3+ ; 01) for pure CaF2 : a) TL excited by U.V. ( A = 134 nm, 10 min) ; b) TL excited by X-rays (50 kV, 15 mA, 3 min) ; c) TSC excited by U. V. (3, = 134 nm, 10 min).

the CaF, : Tb samples TL peaks appeared at 130, 165, 258 and 349 K. The 165 K peak appeared also after irradiation with longer wavelengths up to 250 nm. Simultaneous measurements of conductivity and luminescence showed that the 130 and 165 K TL peaks in U. V. irradiated CaF, : Tb are accompanied by TSC peaks. For comparison, TL measurements were performed also after X irradiation. In CaF, : Tb the X induced TL peaks appeared at almost the same temperatures as the U. V. induced peaks, while in the nominally pure samples, the U. V. and X induced main peaks appeared at different temperatures. This is shown in figure 1,

Both X and U. V. irradiations caused notable phosphorescence at LNT in the Tb doped crystals. The X induced phosphorescence was, however, much stronger, and an exponential decay could be recorded during several minutes.

The emission spectra of the TL peaks were recorded in the spectral region 250-700 nm, and are given in figure 2. In CaF, : Tb, all the U. V. induced TL peaks, as well as the X induced phosphorescence and TL, showed essentially the same emission bands. They consist of the known transitions [9] 'D -+ 7F

of the Tb3+ ion. The minor differences between the spectra in U. V. excited (Fig. 2a) and X excited crystals (Fig. 2b) are apparently due to the difference in the slit width in the prism monochromator (which was 0.2 mm for the X induced TL and 1 mm for the much weaker U. V. induced TL). The U. V. induced TL in nominally pure CaF, was very weak. Their spectral distribution consisted of broad bands which could not be quantitatively measured.

CaF2:TMx) 1 2 0 0

-

.- ( a ) C n. ;; loo-

-

t w h ( b ) I I 400 500 600 WAVELENGTH (n m)

FIG. 2.

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TL emission spectra of CaF2 : Tb : a) after X and

b) after U. V. irradiation.

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C7-218 Y. KIRSH AND N. KRISTIANPOLLER

+

₁₆₀ ₁₈₀ _{2 0 0 2 2 0 2 4 0}

EXCITING WAVELENGTH(nm1

FIG. 5. - TL excitation spectrum of the 165 K peak of CaF2 :

Tb, in the range 160-250 nm.

EXCITING WAVELENGTHfnm)

FIG. 3.

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TL excitation spectra of pure CaF2 in the range 110-180 nm : a) 205 K glow peak ; b) 167 K and 255 K glow

peaks.

FIG. 4. - TL excitation spectra of CaF2 : Tb in the range 110-180 nm : a) 258 K glow peak ; b) 349 K glow peak.

tation maxima appeared at 122, 132, 143, 186, 204 and 225 nm, with distinct shoulders at 118 and 170 nm. Sharp minima appeared at 11 1 and 215 nm. Figure 4 shows the excitation spectra of the main TL peaks of CaF, : Tb in the range 110-180 nm and figure 5

the excitation spectrum of the 165 K peak in the range 160-250 nm.

The doped samples exhibited upon irradiation with wavelength shorter than 160 nm notable photoconductivity between LNT and RT.

Recent results on pure and doped SrF, as well as preliminary results in BaF, were found to be consistent with the here reported results.

4. Discussion.

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In the present work we found for CaF, : Tb that the TL glow peaks between 80 and 400 K appear after U. V. irradiation at the same temperatures and have the same spectral composition as after X irradiation. This indicates that the U. V. and the X induced TL peaks are due to the same defects.

According to Beaumont et al. [4] X-irradiation at LNT causes in CaF, : Tm the appearance of V, and V, centres, which decay through the release of holes at about 130 and 170 K respectively. It seems that the TL and TSC peaks which appear in our work at 130 and 165 K in U. V. irradiated CaF, : Tb are due to the same defects. The higher temperature TL peaks in CaF, : Tb are apparently not associated with the migration of free holes or electrons in the crystal. It may be noted that Kiessling and Schar- rnann [9] found that the 345 K TL peak in X-irradiated CaF, : Tb is associated with the decay of a broad absorption band at about 500 nm.

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U. V. INDUCED DEFECTS IN ALKALINE EARTH FLUORIDES C7-219

that in CaF, the irradiation into the exciton band forms point defects via the creation of excitons. In the doped CaF, this process results in hole centres and electrons trapped at Tb ions, as in the case of

X irradiation. The appearance of strong phosphorescence at LNT indicates that some of the holes are trapped at shallow traps and are partly released already at this temperature. More deeper traps release holes at higher temperatures. The thermally released holes recombine with Tb2+ ions to form Tb3+ ions in an excited state. TL is emitted upon the relaxation of the excited ions to the ground state. The fact that the emission spectra for CaF, : Tb in the U. V. as in the X induced TL consisted of the known Tb3+ transitions is consistent with these assumptions.

The TL peaks in pure CaF, were relatively weak, and appeared at different temperatures than after

X irradiation. Additional work is required in order to determine the types of point defects involved. It was previously found that nominally pure alkaline-

earth-fluorides contain trivalent impurities in minor quantities, which may have a dominant role in the creation of defects [I 11.

The TL excitation maxima at 132 and 143 nm in CaF, : Tb may be due to impurity or perturbed exciton bands. The excitation maxima at 134 and 145 nm in the nominally pure crystal may also be due to impurities. It may be noted that in CaF, : Tb,

the excitation maximum at 134 nm is 5 times more intense than the higher energy maximum of 118- 122 nm, while in nominally pure CaF,, the 134 nm maximum is lower than the 119 nm one. This indicates that excitation in the 132-134 nm region is associated with impurities.

The additional excitation maxima at 186, 204 and 225 nm for CaF, : Tb may be due to a transfer of an electron from a fluorine ion (interstitial or in lattice site) to a nearby Tb3+ ion.

The experimental results obtained in pure and Tb doped SrF, and BaF, crystals support the conclusions reached for CaF,.

References

[I] MERZ, J. L. and PERSHAN, P. S., Phys. Rev. 162 (1967) 217. [7] BECKER, M., KIESSLING, J. and SCHARMANN, A., Phys. ARENDS, J., Solid State Commun. 6 (1968) 421.

TZALMONA, A. and PERSHAN, P. S., Phys. Rev. 182 (1969) 906.

BEAUMONT, J. H., HAYES, W., KIRK, D. L. and SUMMERS,

G. P., Proc. R. Soc. London A69 (1970) 315.

EWANIZKY, T., Phys. Rev. A 135 (1964) 221.

'Status Solidi (a) 15 (1 973) 515.

[8] KATZ, I., CHENPOUX, B. and KRISTIANPOLLER, N., Phys.

Status Solidi (a) 12 ( 1 972) 307.

[9] KIESSLING, J. and SCHARMANN, A., Phys. Status Solidi (a)

32 (1975) 459.

[lo] ISRAELI, M. and KRISTIANPOLLER, N., Solid State Commun.

9 (1971) 1749. . .

[6] STAEBLER, D. L. and SCHNATTERLY, S. E., Phys. Rev. B 3 [ l l ] KRISTIANPOLLER, N. and KIRSH, Y., Phys. Rev. B 4 (1971)