Annealing process of F- and F+-centers in Al2O3 single crystal induced by fast neutrons irradiation

Nuclear Instruments and Methods in Physics Research B 319 (2014) 29–33

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Annealing process of F- and F+-centers in Al2O3 single crystal induced by fast neutrons irradiation M. Izerrouken a,⇑, Y. Djouadi b, H. Zirour c a

Centre de Recherche Nucléaire de Draria, BP.43, Sebala, Draria, Algiers, Algeria Université Ferhat Abbas, Faculté des Sciences, Université de Sétif, Algeria c Faculté de Physique, Université des Sciences et de la Technologie Houari Boumediene, El-Alia, BP 32, Bab-Ezzouar, Algiers, Algeria b

a r t i c l e

i n f o

Article history: Received 13 September 2013 Received in revised form 6 November 2013 Available online 27 November 2013 Keywords: Absorption Color centers Thermal annealing Neutrons

a b s t r a c t F and F+ centers were produced in Al2O3 single crystal by fast neutrons (En > 1.2 MeV) irradiation at low fluence (4.4  1016 n cm2). The evolution of defects intensity as a function of temperature and of time at 493, 623 and 823 K was investigated by UV–visible spectrophotometry technique. It can be concluded from the analysis of isochronal and isothermal annealing data, that the F- and F+-centers annealing process is complex. At low annealing temperature (<473 K), only F- to F+-center conversion process takes place. At higher temperature (>493 K) the annealing is due to the superposition of several mechanisms with different activation energies. According to our results, the activation energies needed for both F- and F+-centers elimination are 0.2, 0.3 and 0.03 eV for temperature range of 300–673 K, 673–873 K and >873 K, respectively. Ó 2013 Elsevier B.V. All rights reserved.

   N E ¼ exp m0 t exp N0 kB T

1. Introduction Al2O3 is used in several technological applications such as a substrate for micro-electronic and lasing materials [1]. It is used in particular in high energy nuclear applications such as optical window in fusion reactor and as inert nuclear fuel hosts [2]. In latter application, Al2O3 crystal is submitted to several kinds of radiations (fast neutron, low and swift heavy ions and c-ray) which affect its structural and optical properties and so on. Point defects generated by irradiation diffuse in the solid at elevated temperature. The annealing kinetics of order c is given by:

dN ¼ mNc dt

ð1Þ

where N is the defect concentration and m is the diffusion frequency given by:

m ¼ m0 exp



E kB T

 ð2Þ

With m0, E and kB are respectively frequency factor, activation energy and Boltzmann constant. Assuming kinetic diffusion of order 1 (c = 1), the resolution of Eq. (1) gives:

⇑ Corresponding author. Tel.: +213 21 31 03 59; fax: +213 21 31 03 80. E-mail address: [email protected] (M. Izerrouken). 0168-583X/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nimb.2013.11.009

ð3Þ

With N0 is the defect concentration after reactor neutron irradiation. Above room temperature the vacancies induced during irradiation migrate in the solid and aggregate to form small clusters and cavities. The latter cavities are responsible of the swelling observed in some materials such as Al2O3 crystal. According to our previous study [3], F+-centers induced in Al2O3 single crystal by fast neutron irradiation diffuse in the crystal with increasing temperature and form F-centers clusters (F2, F2+ and F22+) (see Fig. 5 of Ref. [3]). This is achieved between 400 °C and 500 °C annealing temperature. Similar aggregation processes were described by Pells [4]. The same result was also observed recently by Zhang et al. [5] in neutron irradiated Al2O3 crystal. In the present investigation we pursue the study of annealing process of color centers induced in Al2O3 single crystal. The sample was irradiated at low fast neutrons fluence in order to check simultaneously the F and F+-centers behavior as a function of annealing temperature and annealing time.

2. Experimental Pure Al2O3 single crystal with 0.5 mm thickness and the main  as controlled by X-ray diffraction analysis plane orientation ð1120Þ (XRD) has been irradiated with fast neutron (En > 1.2 MeV) fluence

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M. Izerrouken et al. / Nuclear Instruments and Methods in Physics Research B 319 (2014) 29–33

of about 4  1016 n cm2. The irradiation was performed at NUR research reactor, Algiers at a position described in our previous work [6]. After irradiation, thermal annealing was performed for 15 min at increasing temperatures in air up to 1173 K. Isothermal annealing was performed at different fixed temperature 473 K, 623 K and 823 K.The optical absorption measurements were car-

2.5

(1) Irradiated Al2O3 (2) non irradiated Al2O3

Optical density

2.0

1.5

1.0

Table 2 Activation energies for each annealing steps deduced from the fit of the experimental data using Eq. (3).

(1)

0.5

Step

(2) 0.0 200

300

400

500

600

Wavelength (nm)

Frequency factor m0 (s1)

Activation energy (eV)

1 2 3

+

F

F

0.27 0.19 0.031

0.31 0.23 0.063

F

F+

0.044 0.009 0.0015

0.011 0.003 0.0003

Fig. 1. Optical absorption spectra of Al2O3 targets before and after irradiation with fast neutrons at a fluence of 4.4  1016 n cm2.

3.0

Table 1 F- and F+-centers concentration induced in Al2O3 single crystal irradiated with fast neutrons at a fluence of 4.4  1016 n cm2. 200 1.3 1.90 0.80 3  1017

2.5 2.0

255 0.66 1.84 0.65 9  1016

1.5 1.0 0.25 0.75 3.6 24 101

3.0

300

400

Optical density

200

2.0

600

700

800

Wavelength (nm) 3.5

T= 623 K

Optical density

2.5

500

Ti m

e

(h )

Wavelength k (nm) Oscillator strength f Refractive index n FWHM W (eV) F-center concentration N (cm3)

Optical density

3.5

T= 493 K

3.0

1.5

2.5 1.0 900 600 460 380 300 25

2.0

T( °

C

)

1.5

600

700

800

1.0 0.5 2.7

Wavelength (nm) Fig. 2. Optical absorption spectra evolution of irradiated Al2O3 after annealing from room temperature to 1173 K.

200

300

400

500

600

700

800

17 290

)

500

(h

400

e

300

Ti m

200

F- Centre

Optical density

3 2 1

+

F - Centre

1.00

0

0.96 0.92

0.25

0.88

400

600

800

1000

4 71 645

1200

T (K)

200

300

400

500

600

700

e

0.80 200

(h )

1

0.84

m

Ν/Ν0

4

T= 823 K

Ti

Ν/Ν0

Wavelength (nm) 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 1.04

Wavelength (nm) Fig. 3. Isochronal annealing behavior of F- and F+-centers. The solid line is the fit of the experimental data using Eq. (3).

Fig. 4. Isothermal annealing spectra obtained at 493, 623 and 823 K.

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ried out using Cintra40 UV–visible spectrometer in the wavelength range 200–850 nm. The defects concentration is determined by the Smakula formula:


N cm3 ¼ 0:87  1017

n ðn2 þ 2Þ

2


W ðeVÞ a cm1 f

ð4Þ

where f is the oscillator strength of the optical transition, n the refractive index, a the absorption coefficient and W the full-width at half maximum (FWHM) of the absorption band. The absorption coefficient a is deduced from to optical density (OD) using the following relation:

a ¼ 2:3 

OD e

ð5Þ

lated using Smakula formula for Al2O3 single crystal irradiated with fast neutron fluence of 4.4  1016 n cm2 are listed in Table 1. The refractive index n is taken from Sellmeier’s equation [7] and the full-width at half maximum W is calculated from the Gaussian fit of the optical absorption bands. The oscillator strength for Fand F+-centers were taken from Ref. [8]. 3.1. Isochronal annealing The evolution of the optical spectra after isochronal annealing performed from room temperature to 1173 K is presented in Fig. 2. The optical absorption intensity decreases with increasing annealing temperature. The temperature dependence of F- and F+-centers concentrations is displayed in Fig. 3. One can evidence three annealing regimes for both F- and F+-centers.

where e is the crystal thickness.

I- 300–673 K II- 673–873 K III- >873 K

3. Results and discussion Fig. 1 shows the optical absorption spectra before and after reactor neutron irradiation. The non-irradiated sample exhibits no optical absorption band in the range 200–850 nm indicating its excellent optical transmission. After reactor neutron irradiation, two absorption bands centered at 200 nm and 250 nm are observed and attributed respectively to F-center (oxygen vacancy with two trapped electron) and F+-center (oxygen vacancy with one trapped electron). The F and F+-centers concentrations calcu-

The fit of the experimental data using Eq. (3) of each step separately gives the activation energies and frequencies factors. The results are summarized in Table 2. It can be seen that F and F+ centers activation energies are approximately similar. However, the same annealing steps are also observed in our previous work [3] where Al2O3 single crystal was irradiated at higher fast neutron fluence (9.1  1017 n cm2) and F+-center concentration of 9  1017 cm3.

+

F-Center

F - Centers

1.1 1.15

T= 493 K

1.0 0.9

1.05

NF+/N0

NF/N0

T= 493 K

1.10

0.8 0.7 0.6

1.00 0.95 0.90 0.85

0.5

0.80 0

1000

2000

3000

4000

5000

6000

7000

0

1000

2000

Time (min)

3000

4000

5000

6000

7000

Time (min) 1.15

1.1

T= 623 K

1.0

T= 623 K

1.05

NF+/N0

0.9

NF/N0

1.10

0.8 0.7 0.6

1.00 0.95 0.90 0.85

0.5

0.80 0

3000

6000

9000

12000

15000

18000

0

3000

Time (min)

6000

9000

12000

15000

18000

Time (min) 1.2

1.1

T= 823 K

1.0

0.8

NF+/N0

0.9

NF/N0

T= 823 K

1.0

0.8 0.7

0.6 0.4 0.2

0.6 0.5

0.0 0

8000

16000

24000

Time (min)

32000

40000

0

8000

16000

24000

32000

Time (min)

Fig. 5. Isothermal annealing of: (a) F-center, the solid line is the fit using Eq. (6). (b) F+-center, the solid line is to guide the eye.

40000

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M. Izerrouken et al. / Nuclear Instruments and Methods in Physics Research B 319 (2014) 29–33

1.0 T= 496 K

NF/N0

0.9 0.8 0.7 0.6 0.5 6

7

8

9

10

11

12

13

14

15

ln(t)

1.0 T=623 K

NF/N0

0.9 0.8 0.7 0.6 0.5 6

7

8

9

10

11

12

13

14

15

ln(t)

1.0 T= 823 K

NF/N0

0.9 0.8 0.7 0.6 0.5 6

7

8

9

10

11

12

13

14

15

ln(t) Fig. 6. Isothermal annealing curve of the normalized F-center concentration. The solid line is the linear fit of the experimental data.

3.2. Isothermal annealing Isothermal annealing spectra obtained at 493, 623 and 823 K are shown in Fig. 4. We note that the thickness of the sample heated for 823 K is about 2 mm. The evolution of F-center with increasing annealing time presents two annealing stages (see Fig. 5a). It is initially rapid then reaches quickly a nearly constant value indicating at least two annealing mechanisms. However, the F+-center concentration evolution depends on the annealing temperature Fig. 5b: (i) at 493 K, it increases little bit from 0.25 h up to the investigated annealing time (t = 100 h); (ii) at 623 K, it increases during the beginning of the annealing for time (t < 4 h), then decreases linearly with increasing time; and (iii) at 823 K, it decreases with increasing time from 0.5 to 660 h. The F+-center concentration increases is ascribed to a F- to F+-center conversion process. This process is not evidenced for sample heated at 823 K because it is probably achieved during the first few seconds of annealing. However, the linear evolution of F- and F+-centers concentration versus time observed during the second stage (higher annealing time) explains the atomic rearrangements as it is argued by Bobrov et al. [9]. However, as illustrated in Fig. 5a, solid line, the F-center experimental data is not well fitted by the well known exponential decay Eq. (6) indicating a complexity of the annealing process [10].

  N t ¼ exp N0 s

ð6Þ

where s is the time constant. But F-center experimental data is proportional to the logarithm of the annealing time {lnðtÞ} (see Fig. 6). Kingery [11] observed many years ago similar behavior of the lattice parameters changes as a function of annealing time and according to Primak [12] dis-

cussion, it is to be expected if there is a range of activation energies necessary for point defect elimination.From isothermal annealing of the F- and F+-centers behavior we can conclude that:  For annealing temperature lower than 473 K, only the F- to F+center conversion process occurs. Thus the reduction of the Fcenters concentration at temperature (T < 473 K) is due to this latter process.  For temperature (T > 623 K), in addition to the F- to F+-center conversion process, a superposition of other annealing mechanisms due to the atomic rearrangements are achieved. This result confirms the isochronal data, where, the F-type centers annealing shows at least three annealing processes with different activation energies. Taking into account our previous results [3], F type-centers association, clusters (F2+, F22+ and F2) dissociation and vacancies-interstices recombination are successively produced during heating at higher temperature above 623 K.

4. Conclusion In the present investigation, isochronal and isothermal annealing of F- and F+-centers induced in Al2O3 crystal irradiated with reactor neutrons at fast neutron fluence of about 4.4  1016 n cm2 were studied using UV–visible spectrophotometry. From the isochronal and isothermal analysis, it can be concluded that F- and F+-centers annealing process in Al2O3 crystal is complex as the activation energy varies with annealing temperature. According to our experimental data, the activation energies needed for both F- and F+-centers elimination are 0.2, 0.3 and 0.03 eV for temperature range of 300–673 K, 673–873 K and >873 K, respectively. Activation energies of the same order (0.4–0.6 eV) were measured by Kingery [11] in the temperature range 293–673 K.

M. Izerrouken et al. / Nuclear Instruments and Methods in Physics Research B 319 (2014) 29–33

Taking into account our previous study [3,13], we believe that the annealing process of F-center in Al2O3 crystal is due to the following successive mechanisms: (i) F- to F+-center conversion process. It is achieved at lower temperature (<473 K). (ii) F- and F+-centers aggregation to form clusters (F2, F2+, F22+). It is performed at temperature range of 623–773 K. (iii) Clusters dissociation and Frenkel pair recombination. It began at about 773 K.

References [1] [2] [3] [4] [5] [6] [7] [8] [9]

Acknowledgements M.I. is indebted to the members of NUR research reactor staff for their co-operation.

33

[10] [11] [12] [13]

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