Defect Creation by Electronic Processes in MgO Bombarded with GeV Heavy Ions
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Table I: Characteristics of swift heavy ion irradiations Incident Ion
Energy (GeV)
Range (gim)
Total irradiation fluence (x 1011 cm-2 )
Irradiation
U
0.803
28
60
Pb
0.944
31
8.7
Room temperature 17 K
Pb
0.944
31
22
Room temperature
Sn
0.23
16
87
Room temperature
RESULTS AND DISCUSSION In-situ optical absorption analysis allowed us to measure the growth of the F-type center band in a MgO-Na sample during irradiation with 944 MeV Pb ions, as shown in figure 1. If we plot the optical density at the maximum value of this band as a function of irradiation fluence, for two samples irradiated with Pb ions at 17 K and at room temperature, we observe that the F-type center creation is proportional to the irradiation fluence (fig. 2). No effect of the irradiation temperature could be detected.
2.5
A
1.5
-
T=17K T=300K
2 Q)
1.5
U)
1
I
.,..
Ce C-
1 0
0.5
0.5 [.
0:L
200
0
250 300 Wavelength (nm)
0
1 2 3 4 5 Fluence ( 1011 Pb cm-2 )
Figure 1 : Optical absorption spectra of a MgO Figure 2 : Evolution of the maximum value of crystal : (a) before irradiation ; after irradiation the F-type center band with Pb. with Pb ions at afluence of: (b) 0.7x10 11 cm-2 ; (c) 1.3x10 11 cm- 2 ; (d) 2.6x101 1 cm- 2 ; (e) 4.3x1011 cm- 2 . Irradiations were performed at room temperature 366
The in-depth repartition of F-type centers was determined by the chemical etching procedure. The maximum value of the F-type center band decreases as the sample is being etched (see fig. 3). The etched depth was measured with an "alpha step" profilometer, so the relative experimental error was taken to be ± 5 %. The optical density OD of the F-type center band is related to the integrand of the local defect concentration c (in cm- 3) from the etched depth x to the end of the range Rp of the bombarding ions by using Smakula's formula [6, 8 - 9] : OD = f (n 2 + 2)2 RP cdl
0.87 x 1017 2.3 x nx W1/ 2 f Xc where n is the refractive index of MgO, W1 /2 the band half width in eV and f the oscillator strength. The F-type center creation displayed in figure 3 can not be explained only by nuclear processes. Indeed, the contribution of elastic collisions to the F-type center concentration, cnuc, can be estimated proportional to the concentration of oxygen vacancies c(O)vac calculated by a TRIM program [10] : Cnuc = A1 c(o)vac A1 represents the fraction of oxygen vacancies which did not recombine. Its value was found to be around 0.2 for the irradiations with U and Sn beams. It was quite higher, approximately 0.5, when Pb irradiations were performed at 17 K. This difference indicates that a low temperature tends to hinder the recombination ot the point defects created by elastic collisions. The total fraction of the F-type centers analyzed to be induced by nuclear processes is of the order of magnitude of 30 %.
3
"i 2 Ce
01
0
5 10 15 20 Etching depth (microns)
Figure 3 : Evolution of the maximum value of the F-type center band with removed depth for a MgO crystal irradiated with Sn ions. The dashed line corresponds to the
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