ELECTRO-OPTICAL STUDIES OF THE EMISSION BANDS OF Tl+ CENTERS IN KCl AND KBr

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ELECTRO-OPTICAL STUDIES OF THE EMISSION

BANDS OF Tl+ CENTERS IN KCl AND KBr

U. Grassano, A. Scacco, A. Tanga, M. Valli

To cite this version:

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ELECTRO-OPTICAL STUDIES OF THE EMISSION BANDS OF Tl

+

CENTERS IN KC1 AND KBr

U. M. GRASSANO(*), A. SCACCO, A. TANGA(*) and M. VALLI Istituto di Fisica dell'Universita di Roma, Italy

Résumé. — Les changements produits par un champ électrique dans les spectres d'émission des centres du Thallium ont été observés dans les cristaux de KC1 and KBr. Deux effets complètement différents ont été mesurés suivant l'excitation de la luminescence dans la bande A ou dans la bande C. Dans ce dernier cas on observe une réduction de la luminescence due à un effet de tunnel ou à une ionisation de l'électron excité produite par le champ électrique.

L'excitation dans la bande A donne lieu, au contraire, à un changement relatif d'intensité dans les bandes d'émission : celle à énergie plus faible croît aux dépens de celle à énergie plus grande. Cet effet de Stark est probablement dû à un changement du nombre d'électrons dans les deux minima d'où partent les deux transitions radiatives.

Abstract. — Changes in the emission spectra of T1+ centers induced by an electric field have been

observed in KC1 and KBr. Two entirely different effects have been measured following excitation of the luminescence in the A or in the C band. With C band excitation, the electric field produces a quenching of the luminescence due to a tunnelling or a field ionization of the excited electron. With A band excitation, the intensity of the two emission bands characteristic of KBr is changed by the field, increasing the low energy band at the expenses of the high energy one. This Stark effect is probably due to a change of the equilibrium density of the emitting centers in the two minima from which the transitions take place.

The optical properties of Thallium centers in alkali halides have been thoroughly studied [1, 2]. The tran-sitions between the electronic levels of the Tl+ ion,

perturbed by the crystalline field, produce three absorp-tion bands, labelled A, B and C in order of increasing energy. The position of the peaks of these bands at liquid nitrogen temperature (LNT) are 2 465 A

(5.03 eV), 2 085 A (5.94 eV) and 1 950 A (6.36 eV) for KC1 and 2 590 A (4.79 eV), 2 220 A (5.58 eV) and 2 090 A (5.93 eV) for KBr [3].

The properties of the Tl+ emissions in KC1 and KBr

are different and indeed in the Fukuda's classifica-tion [4] of the luminescent properties of heavy metal ion phosphors KC1 : Tl+ and KBr : Tl+ belong to

two different groups. Excitation in every absorption band produces a characteristic emission depending also on the temperature. Excitation in the A band originates at LNT a single emission in KC1 centered around 3 050 A, while two emissions are found in KBr around 3 100 A and 3 600 A. On warming to room temperature (RT), the emissions are broadened and in KBr the two bands are no longer clearly resolved. Excitation in the higher absorption bands B and C produces, beside the previous emissions, other bands at both sides of the A absorption. These emissions disappear at room temperature.

•'*) Gruppo Nazionale di Struttura della Materia del C. N. R.

According to the original theory of Fukuda [4], the two emission bands of KBr : Tl+ excited in the A band

derive from the following process. The initial level of the radiative transition has a symmetry 3Tl u. The

adiabatic potential energy surface (APES) of this level constructed through the Jahn-Teller interaction with tethragonal distortions, presents two different kind of minima. Emission is considered to occur from these minima to ground state. The different emis-sion intensities in the two bands at various tempera-tures reflect the population density in the two minima. The minimum at higher energy may be populated directly, following optical excitation, while the lowest one is reached only through a thermally activated process.

In KC1 : Tl+, where only a single emission is excited

in the A band, there may exists only one kind of minima in the APES. The exact nature and the possible coexis-tence of different kind of minima has been thoroughly investigated by Ranfagni et al. [5-8].

The changes of the emission bands induced by an external electric field have been studied in order to gain a more complete knowledge of the emitting cen-ter. Differential emission spectra stimulated in the A band have been reported by Giorgianni et al. [9, 10] for KBr and by Ranfagni et al. [11] for KC1. A com-plex structure is present in KBr and, although less clearly resolved, in KC1. These structures emerge

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C7-150 U. M. GRASSANO, A. SCACCO, A. TANGA AND M. VALLI

from a broad background showing that the overall emission decreases in an electric field. This zero mo- ment change of emission band due to an external perturbation is not easily interpreted in the framework of the theory of these effects given by Henry, Schnat- terly and Slichter [12] and by Honma [13].

In order to further investigate this discrepancy we have measured the changes produced by an alternate electric field on the TIf emission spectra in KC1 and KBr. The results show that a decrease of luminescence takes place only following excitation in the C band. The field and temperature dependence of this effect are explained in terms of an electron tunneling from the higher excited state to a trapping level. Same results along these lines have already been published else- where for KBr : TI+ and KI : TI+ [14],

On the contrary the luminescence excited in the A

band of KBr : TIf shows a field induced exchange of intensity between the two emissions with an increase of the 3 600

A

band at the expenses of the 3 100

A

band. We interprete this result as due to a Stark effect of the excited level from which the emission originates. 1. Experimental. - Single crystals of KC1 and KBr were home grown by the Kyropoulos method in a nitrogen atmosphere. Concentrations of Tlt ions calculated by the Smakula equation were in range 1016 ou loi7 ~ m - ~ . Samples freshly cleaved and quenched from 500 OC to RT were mounted on the cold finger of a standard LNT cryostat. The modulated electric field (up to 360 kV/cm peak) was applied along the direction of propagation of the exciting light. The values of the field quoted in this work contains the Lorentz correction for the local field. The samples of approximate size 0.5 x 3 x 12 mm3 were sandwiched between two blocking electrodes one of which was semitransparent. The luminescence was collected at right angles to the exciting beam. Two high intensity Baush and Lomb U. V. monochromators (dispersion 30 A/mm) were used for selecting the excitation wave- lenght and for analyzing the emission spectrum. The changes of emission intensity were detected at twice the field frequency, by a lock-in amplifier and recorded.

2. Results. - 2.1 KCI. - The modulation spectra i. e. the changes of emission A 1 induced by the electric field are reported in figure 1 for KC1 at 300 K and at 90 K. Non polarized light, absorbed in the C

band, was used for the excitation. In the upper part of the figure, the emission bands obtained with the same excitation and no electric field, are reported. The relative emission change A111 is negative and approxima- tely constant over the whole 3 050

A

band. The other signal due to the emission decrease in the region of the

A aborption band is bigger than that of the 3 050

A

band and moreover its field dependence is quadratic at variance from that of the main band (see below).

IL -. i - _ _ L -

i

noo zm mw zm m 3m m i &

FIG. 1.

-

a) Emission spectrum of KC1 : TI+ containing

NTI = 2 x 1016cm-3 Thallium centers excited in the C band

(A,,, = 1 950 A) at 90 K and at 300 K. b) Change of the emission spectrum of the same crystal due to a local electric field of 140 kV/cm (peak) at 90 K and 120 kV/cm (peak) at 300 K.

The luminescence decrease A I I I shown in the previous figures has been studied as a function of the wavelength of excitation, of the temperature and of the electric field intensity. Figure 2 show the emission decrease A111 (measured at 3 050

A)

as a function of the exciting wavelength at 300 K and 90 K. These spectra will be henceforth called for brevity excita-

tion spcctua. In the upper part of the figures the absorption coefficients of the samples in the same spectral region is plotted for comparison. It is evident that the electric field produces an emission intensity decrease only when the exciting light is absorbed in the C band but no measurable signal was detected with the excitation in the A band.

FIG. 2. -a) Absorption spectrum of a KC1 : T1+ crystal with N T ~ = 2 x 1016 cm-3 at 90 K and at 300 K. b) Field induced emission decrease measured in the same crystal at 3 050 A, plotted as a function of the exciting wavelength (local electric field F = 140 kV/cm (peak) at 90 K and 120 kV/cm (peak) at

300 K).

15-

2.2 KBr. - Similar results have been obtained in KBr : TI'. Figure 3 shows the modulation spectra

a)

A

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of -the main emission bands at 90 K and at 300 K. The excitation spectra are plotted in figure 4 at the same temperatures.

181 8' h " p'

a 2%- I

I , I I I I

28m 3XO 2.3 34. S U O laaD &dl

1"

FIG. 3. - a) Emission spectrum of KBr : TI+ containing

N T ~ = 1.1 x 1017 cm-3 Thallium centers excited in the C band

(Aexe = 2 140 A) at 90 K and at 300 K. b) Change of the

emission spectrum of the same crystal due to a local electric field of 300 kV/cm (peak) at 90 K and 180 kV/cm (peak) at

300 K.

FIG. 4.

-

a) Absorption spectrum of a KBr : TI+ crystal with N T ~ = 1.1 x 10-17 cm-3 at 90 K and at 300 K. b) Field induced emission decrease measured in the same crystal at

3 600 A plotted as a function of the exciting wavelength (local electric field F = 140 kV/cm at both temperatures).

It has been verified that the emission decrease excited in the C band, does not depend on the polarization of the exciting light, nor of the emitted light.

2.3 TEMPERATURE AND FIELD DEPENDENCE. - The temperature dependence of ( A l / I ) stimulated in the C band is plotted in figure 5 for KC1 and KBr. For both crystals the temperature dependence follows an Arrhenius plot with an activation energy of 46 meV for KC1 and 14 meV for KBr. The same temperature dependence has been reported by Ranfagni et al. [ll] for KC1 : TI+ excited in the A band. These results are

FIG. 5. - Field induced emission decrease plotted as a function of temperature. Exciting waveleijgth in the C band ; emission measured in KC1 : TI+ at 3 050 A and in KBr : TI+ at 3 600 A ;

local electric field F = 120 kV/cm (peak).

similar to those rneasured in KI : 'TI+ and already

published [14].

The figure 6 shows the dependr-nct; of

Af

upon the electric field intensity for KC1 and KBr at 90 K and at 300 K. Emission changes were measured at 3 050

A

in KC1 and at 3 600

A

in KBr. The straight line repre; sent the field dependence predicted, at least at high fields, by a simple model (see discussion).

FIG. 6. - Emission decrease measured in KC1 : TI+ and KBr : TI+ at 90 K and at 300 K plotted as a function of the reciprocal of the electric field. The straight line is a plot of the

theoretical model (see text).

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C7-152 U. M. GRASSANO, A. SCACCO, A. TANGA AND M. VALLI

of 1 000 s) it has been possible to detect a true Stark effect of the emission of KBr : Tlf stimulated in the

A band. As shown in figure 7 it consists in an exchange of intensity between the two emission bands with an increase of the 3 600

A

and a decrease of the 3 100

A

component.

I

FIG. 7. - Stark effect of the TI+ centers emission in KBr excited in the A band. Local electric field 360 kV/cm (peak). The emission spectrum without field is plotted for comparison

in the upper part of the figure.

3. Discussion.

-

3.1 LUMINESCENCE QUENCHING.

-

The main effect induced by the electric field on the Tl' emission bands is a quenching of the luminescence excited in the C band.

The effect cannot be explained in terms of Stark effect of the relaxed excited state of T1+ ion. This relaxed state is involved in both the emissions stimulated in either the A and the C bands, while the lumi- nescence decrease is induced only after the C band excitation.

One can likewise exclude an effect on the luminescence brought forward by a Stark effect in absorp- tion [15]. A possible interpretation one is left with, is a decrease of the quantum efficiency of Tlf emission. The same suggestion has been put forward by Densk and Leiman [16] who measured in a constant electric field a decrease of the Tlf luminescence excited at RT in the energy region above 4 eV. These authors suppose that some of the electrons excited into the C band can be thermally ionized before radiative transition. These free electrons can be retrapped by traps other than TI++, thus decreasing the luminescence quantum yield. It is not clear at present if the thermal ionization is effective also at low temperature. The activation energies calculated from figure 5 are fairly small indicating an appreciable ionization also around 90 K. On the contrary the field dependence of figure 6 is not in agreement with a Schottky-type field ionization, but at least at high field, it seems to repre- sent a tunneling. If the luminescence decrease

I

AI

I

is proportional to the tunneling probability of the excited electron toward a nearby trap one can write [I71

I

AI

I

cc w

c~ exp const (AE)~"

I

F

I

where A E is an activation energy and F the applied

1

electric field. The straight line of l g

1

A1

1

vs -

F indicates that at least at high fields this might be the dominant process. The activation energy deduced from figure 6 is small : for example in KC1

AE = 31 meV at 90 K and AE = 16 meV at 300 K. It seems evident however that with both mecha- nisms, field ionization or tunneling, the luminescence quenching is due to a removal of the excited electron from the activator center and that this process is completely uncorrelated to the shift or mixing of TI+ electronic levels characteristic of the Stark effect.

3 . 2 STARK EFFECT. - The excitation in the A

band in KBr at 90 K under the electric field produces the small signal shown in figure 7. It has to be noted that Densk and Leiman [16] did not observe any change of the total luminescence emitted with A band excita- tion. The integrated emission change over the whole band is zero. The size of the signal is about 50 times smaller than the luminescence quenching at the same field. The presence of the strong negative signal of the luminescence explains why this signal has not been observed before [9]. The shape of the Stark effect is qualitatively accounted for by the following considerations.

The emitting excited states of the Tlf ion derive all from p-like atomic levels and therefore they cannot be mixed by the electric field. As a consequence the emission changes cannot be explained by a change in the radiative lifetime of these levels and the emission probability cannot be altered by an electric field. On the other hand, stress experiments have shown that an external perturbation applied along the (100) direction may lift the degeneracy of the equivalent minima of the APES [18, 191. The small energy shift produces a different population distribution among the minima thus altering the relative intensity of the emissions and inducing a dichroism in the emission bands.

We suppose that an analogous effect may be produced by an electric field and that the decrease of the

3 100

A

emission and the increase of the 3 600

A

reflect an electric field induced change of the potential barrier between the two kinds of minima and hence a different electron distribution. Further measure- ments are in progress in order to measure the polarization of the TI+ emission in KBr and in KC1.

4. Conclusion.

-

The study of the electric field induced emission changes of the T1+ centers in KC1 and KBr has shown that two entirely different effects are produced with the excitation in the A or in the C band.

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With A band excitation, one reveals in KBr much Acknowledgments.

-

The authors wish to thank smaller intensity changes, favoring the low energy Prof. G. Chiarotti for valuable suggestions and sti- emission at the expences of the high energy emission. mulating discussions.

This Stark effect is probably due to a change of the The technical assistence of S. Rinaldi, R. Generosi equilibrium density of the emitting centers in the two and T. Cicinelli is gratefully acknowledged.

minima from which the transitions takes place.

References

[I] FARGE, Y. and FONTANA, M. P. in Perturbations klectro-

niques et vibrations localisc!es dam les Solides Ioniques

(Masson-Paris) 1974.

[2] FOWLER, W. B. in Treatise on Solid State Chemistry, Hannay, N. B. ed. (Plenum, New York) 1975,

[3] FUKUDA, A., Sci. Lght 13 (1964) 64. [4] FUKUDA, A., Phys. Rev. B 1 (1970) 4161. [5] RANFAGM, A., Phys. Rev. Lett. 28 (1972) 743.

[61 RANFAGNI, A. and VILIANI, G., J. Phys. & Chem. Solids

35 (1974) 25.

[71 BACCI, M., RANFAGNI, A., FONTANA, M. P., VILIANI, G.,

Phys. Rev. B 11 (1975) 3052.

[8] BACCI, M., RANFAGNI, A., CETICA, M., VEIANI, G., Phys.

Rev. B 12 (1975) 5907.

[91 GIOGIANNI, U., GRASSO, V. an P E ~P., ,Phys. Rev. Lett.

23 (1 969) 640.

1101 GIORGIANNI, U., GRASSO, V. and SAITTA, G., NUOVO Cimento

B 68 (1970) 100.

[ l l ] RANFAGNI, A., PAZZI, G. P., FABENI, P., BACCI, M., FONTANA, ,M. P., VILIAN~, G., Phys. Rev. Lett. 35

(1975) 752.

[12] HENRY, C. H., SCHNATTERLY, S. E., SLIGHTER, C. P.,

Phys. Rev. 127 (1965) A583.

[13] HOW, A., J. Phys. Soc. Japan 24 (1968) 1082.

[14] GRASSANO, U. M., TANGA, A., VALLI, M., Phys. Status

Solidi (in press).

[IS] GRASSANO, U. M., SCACCO, A. and TANGA, A., Solid State

Commun. (in press).

[16] DENSK, V. P. and LEIMAN, V. I., SOY. Phys. Solid State 10 (1968) 1426.

[17] FRANZ, W., Handbuch der Physik, 17, 155(SpringerVerlag Berlin).

[18] SHIMADA, T. and ISHIGURO, M., Phys. Rev. 187 (1969) 1089.

[19] GERHARDT, V. and GEBHARDT, W., Phys. Status Solidi (b) 59 (1973) 187.

DISCUSSION

P. W. M. JACOBS. - Since the effect of the electric field on the A-band emission is pronounced for C-band excitation but absent for A-band excitation, in KC1 : Tlf, this seems to be evidence that the 'I'i

state relaxes into the triplet state by a radiationless transition and that the electric field affects the efficiency of this transition. Do you have any comment on the reduction in quantum yield by the field ?