Dynamics of positron-trapping color centers in alkali chloride crystals

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Dynamics of positron-trapping color centers in alkali

chloride crystals

W. Brandt, S. Stern

To cite this version:

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JOURNAL DE PHYSIQUE Colloque C6, supplkment au no 7 , Tome 41, Juillet 1980, page C6-112

Dynamics of positron-trapping color centers in alkali chloride crystals

(*)

W. Brandt and S. Stern

Department of Physics, New York University, New York, NY 10003, U.S.A.

RCsumb. - En utilisant le positon comme sonde microscopique, nous avons mesurC des taux et des knergies d'activation pour la diffusion de centres colorts dans les monocristaux de NaCl, KCl, RbCl et CsCl. Les t a w de diffusion des positons sont si grands que les cliangements, dkpendant du temps, dans la probabilitt de piegeage des positons sont relits a la dynamique d'agrtgation des dtfauts, a la dissociation des agrtgats, ou a la recombinaison de lacunes et d'ions interstitiels. Les cristaux Ctaient endommagks par des rayons X de 32 keV maximum

(48 Mrad/h a 20 OC jusqu'a une dose totale de 0,75 Grad.) qui provoquent la saturation des centres F

(- 3 x 1018 cm-3). Des mesures isothermiques ont CtC effectuhs entre 20 OC et 200 OC dans un appareil de coincidence de a-radian (PICA) pour enregistrer les deux rayons y de 0,5 MeV d'annihilation positonllectron qui sont kmis par les cristaux. Les donnkes rkvklent que le mouvemerrt thermique initial des dkfauts se poursuit plus rapidement tant le cation augnente, c'est-a-dire B 120 OC

a

-

lo3 min. pour NaCl et B

--

10 min. pour CsC1,

ceterisparibus. Nous avons observe le blanchissement des cristaux par des positons aux temperatures oh la couleur est stable thermiquement. Ce fait complique l'identification de centres colorks comme pi6ges de positions. Les taux de comptage de PICA suivent le rktablissement B la longue des cristaux lorsqu'ils approchent les conditions de prt-irradiation.

Abstract. - Using the positron as a microscopic probe, we measured rates and activation energies for the ditiu- sion of color centers in single crystals of NaCI, KCI, RbCI, and CsCI. The diffusion rates of positrons are so large that time-dependent changes in the positron-trapping probability are related to the dynamics of defect aggrega- tion, dissociation from clusters, or vacancy-interstitial recombination. The crystals were damaged to F-center saturation (- 3 x 1018 ~ m - ~ ) by 32 keV (maximum) X-rays (48 Mrad/h at 20 oC to a total dose of 0.75 Grad.). Isothermal measurements between 20 OC and 200 OC were performed in a n-radian coincidence appartus (PICA) for recording the two 0.5 MeV positron-electron annihilation y rays emerging from the crystals. The data reveal that the initial thermal defect migration proceeds more quickly for larger cations, e.g., at 120 OC in

-

lo3 min. for NaCl and in

-

10 min. for CsCl, ceterisparibus. We observed bleaching of the crystals by positrons at temperatures where the color is thermally stable. This fact complicates the identification of color centers as positron traps. The PICA count rates trace the subsequent long-time annealing of the crystals as the crystals approach pre-irradiation conditions.

1 . Introduction. - We report preliminary results

of the technique of positron annihilation used to measure rates and activation energies for the diffusion of color centers and other positron-trapping defects in X-ray-irradiated NaCI, KCI, RbCl, and CsC1. The work complements the method first applied to NaCl by Brandt and Paulin [I, 21.

Positrons implanted in a crystal thermalize rapidly and quickly diffuse to defects

( 5

100 ps) [3]. Subse- quent positron-electron annihilation produces a sym- metrical angular distribution I ( 0 ) about 1800 of two coincidently emitted 0.51 MeV y rays. The angle

8 = p/m,c is related to the momentum component p of the annihilating antiparticle pair in the direction normal to the plane of the two colinear coincidence- counter slits.

(*) Work supported by the United States National Science Foun- dation.

A diffusing positron trapped by a defect forms

an annihilation center where it can encounter electrons of lower momenta than it would in the crystalline bulk 14, 51. Consequently, I ( 0 ) is narrower and the peak I(0 = 0) higher in a crystal with annihilation centers than in a perfect crystal. For example, thermal treatment may release point defects from an inhomo- geneous distribution of clusters throughout the region of positron implantation to increase trapping of positrons. Alternatively, defects may aggregate into a second, more efficient species of positron trap. The evolution of these processes can be followed by the coincidence count rate for 0 = 0 in a .n-radian coincidence appartus (PICA) as a function of time at cons- tant anneal temperature.

2. Experiment. - A crystal first annealed in air at

400 OC for ca. 22 h was irradiated for 15.8 h at room temperature by X-rays of maximum energy *

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DYNAMICS O F POSITRON-TRAPPING COLOR CENTERS C6-113

32 keV at a dose rate of 48 Mrad/h. For NaCl and KG1 this dose saturated the increase in the room- temperature, post-irradiation PICA rate I,

=

I, (t = 0, 8 = 0) with respect to that of the annealed crystal I, at (I, -Ia)/Ia = 13.5 f 0.4

%

and 9.6 A0.4

%,

respectively. The F-center concentrations of the NaCl and KC1 X-irradiated crystals in the region of positron implantation (depth

-

0.01 cm) were measured by optical absorption to be

-

3 x 10'' cm-3. The latter value is consistent with the saturation measured by Sonder and Templeton [6]. There appears to be a correspondence between the saturation increase of the PICA rate and the saturation concentration of F-centers produced by X-ray irradiation.

Following irradiation, the crystal is placed into PICA. The system consists of a

-

15 mCi "Na positron source affixed 0.36 cm from the sample located at the pivot point of two collimators 180° apart. The collimators shield the two NaI(T1) scin- tillator-photomultiplier tube assemblies, each placed 2 m from the sample. Before each run, the post-X- irradiation PICA rate I, is measured at room temperature. Then the sample heater is turned on at t = 0, and the sample temperature stabilizes at some preset value T in about 4 min. Also starting at t = 0, the PICA rate I(t, 0 = 0) is determined as a function of t.

5 A I - I, (percent) 0

-

5 I I I TIME (min)

Fig. 1. - Change in the PICA rate as a function of isothermal

anneal time at 136 OC for NaCI, KCI, RbC1, and CsCI,

AI/I, = [ q t ,

o

= 0) - z , ] / I , ,

where I, refers to the t = 0 PICA rate after X-ray irradiation. The values I, for the annealed crystals lie at

AZa/Io = (Ia - 10)/10 = - 11.9 %, - 8.7 %, - 9.1 % ,

and - 14.8

2,

respectively.

3. Results. - Figure 1 shows the change in the

PICA rate AI(t, 6 = O)/Io = ( I - Io)/I,, as a function

of anneal time t at the representative temperature 136 OC for the four crystals. There are enormously dfierent characteristic annealing times. The crystals anneal quicker the larger the cation. The initial increase of the PICA rate in NaCl and KC1 signifies that there is an increasing formation of annihilation centers with low electron momenta.

Results for NaCl and KC1 at several temperatures are shown in figures 2 and 3, respectively. Their quali-

Fig. 2. - Change in the PICA rate AI/Io as a function of isothermal anneal time in X-ray irradiated NaCl at the temperatures indicated. AI,/I, = - 11.9 %.

TIME (min)

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C6-114 W. BRANDT AND S. STERN

tative similarities but large quantitative differences lead us to focus the remaining discussion on these two compounds.

4. Analysis. - The rising and falling trends in the

PICA rate of figures 2 and 3 can be parameterized by .two coupled rate equations whose solution is approximated by

under the simplifying assumption

AIm/Io

-

AZ(tm)/Io; t, is the time in which AI/I, reaches its maximum value. The rise and decline times approximately depend on T as

Falling data of figure 3 are plotted in figure 4 in the form suggested by equation (2). An analogous procedure is used to extract z,(T) from the rising data.

In figure 5 Arrhenius plots of z,(T) and z,(T) are shown for our results in NaCl and KCI. The results of the analysis are summarized in table I.

Table I. - Rise times, decline times, and activation energies [eqs. (4) and (5)] for positron trapping in X-irradiated NaCl and KC1.

Crystal (min.) zed (min.) E, (eV) Ed (eV)

- - - - -

NaCl 1.2 x 10-l3 (') 3.5 x 10-lo ( d ) 1.14 (*) 1.04 (")

KC1 6.4 x lo-" (f) 1.1 x lo-' ( d ) 0.77 (') 0.88 (")

The uncertainties consistent with the analysis are : (? & 2 %,

( b )

+

5 %, ('1 f 12%, +5 50 %, (7

+

150 %,

(3

+

300 %.

5 . Discussion and conclusion. - Optical absorption

measurements which we made after various PICA isothermal anneals of NaCl and KC1 seem consistent with identification of F-centers and their aggregates as the positron traps. However, this identification is complicated by our observation of bleaching of the color in KC1 and RbCl by positron-implantation energy deposition at temperatures where the color is thermally stable [7]. There is thus a question of whether defect-migration processes are affected by ionizations incurred during positron stopping in the crystals.

Positron annihilation has proven itself a sensitive technique to obtain characteristic times and activation

TIME (rnin)

Fig. 4. - Semi-log plot of some of the declining data of figure 3 in the form suggested by equation (2). The slopes of the lines yield the characteristic decline times z,(T) plotted in figure 5.

Fig. 5. - Rise times z, (open symbols) and decline times z, (solid symbols) vs. 103/T for NaCl (circles) and for KC1 (squares). The solid lines yield for NaCl :

z, = 1.2 x 10-l3 exp(13 193 KIT) min. and zd = 3.5 x 10-lo exp(l2 021 KIT) min.

The broken lines yield the KC1 rise and decline times :

z, = 6.4 x 10- l1 exp(8 991 KIT) min.

and

z, = 1.1 x lo-' exp(10 179 KIT) min.

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DYNAMICS OF POSITRON-TRAPPING COLOR CENTERS

DISCUSSION

Question. - N . KRISTIANPOLLER. Reply. - S . STERN. You have mentioned that thermal annealing was

performed in open air. Were you not worried about side-effects of oxygen diffusion ?

Reply. - S . STERN.

Yes, we have been concerned about possible effects of oxygen diffusion, and at this time we have not attempted to anneal the samples in vacuum or in an inert atmosphere before a PICA run. However, the consistency of the trend of faster annealing with larger cation seems to indicate that annealing of the damage produced by X-rays dominates the dynamic effects observed. Optical measurements support the sensitivity of PICA measurements to this damage. PICA runs themselves are performed in a rough vacuum of 4 x lo-' torr.

Question. - D. SCHMID.

Would it be feasible to dope the crystals directly with

-

for instance - "Na ? In this way the positrons would be generated in the crystal itself, thereby ensuring that the experiment yields real bulk properties. With the external positron radiation, surface effects might be relevant.

Since positrons emerge from "Na with a maximum kinetic energy of 0.54 MeV, they are implanted to a depth of

-

lo6

A

in our crystals. They sample primarily bulk properties, and surface effects are unimportant for such energetically implanted positrons. Also, the requirement of high positron-source strength of an angular correlation system makes doping impractical. However, your idea has been applied to the study of copper and copper alloys with the use of an in situ 6 4 C ~ positron source.

Question. - N . ITOH.

I would like to mention that the activation energies for defect annealing you obtained are very close to those of the F-center migration obtained by optical measurements : Paul Levy measured 1 eV in NaCl and we found 0.85 eV in KBr (see 1 339 and

1 340 respectively).

Reply. - S. STERN.

I am heartily gratified to hear that. Thank you.

References

[I] BRANDT, W. and PAULIN, R., Phys. Rev. B 8 (1973) 4125. [5] For a review of positron states in defects of ionic solids, see :

[2] STERN, S. and BRANDT, W., Bull. Am. phys. Sot, 24 (1979) 19, DUPASQUIER, A., in Positrons in solids, edited by P. J. auto-

j a ~ i (Springer-Verlag, New York, Heidelberg, Berlin) [3] BRANDT, W., Appl. Phys. 5 (1974) 1. _1979.

[4] BRANDT, W., in Positron Annihilation, Proceedings of the [6] SONDER, E. and TEMPLETON, L. C., Phys. Rev. 164 (1967) 1106.