- Email: [email protected]
Structural change in iron implanted potassium bromide crystals
Vacuum/volume 39/numbers Printed in Great Britain
24fpages
0042-207X/89$3.00+.00 Pergamon Press plc
431 to 43211989
Structural change bromide crystals
in iron implanted
potassium
Guang-hou Wang, Lie Dou, Ling Chen, Guo-giang Pang, Hai Sang, Min-kang Xiang-jin Li, Department of Physics, Naming University, Nanjing, PRC
Teng,
De-xun
Shen and
and Ke-ming
Wang,
Yi-hua
Wang
and Jin-tian
Liu, Department
of Physics, Shangdong
University,
Jinan,
PRC
Positron annihilation lifetime spectra andX-ray diffraction have been used to investigate the radiation defects produced by Fe’ ion implantation in the ionic crystal KBr at the dose of 5~ 1015 ions cm-2. We have found that after implantation both the short and long lifetime components of positron annihilation in KBr crystals increase while the annihilation probability of the second lifetime component decreases from 41.7 to 29.3%. This means a decrease in the number of trapping centers by pick-off annihilation since F-centers that already existed in the sample before implantation have aggregated in the halogen sublattice by electronic processes during ion implantation to become larger centers trapping positrons. The X-ray diffraction spectra show new diffraction peaks with rather strong intensities which can be either due to lattice dilatation caused by the F-center aggregation, or by metallic ion precipitates.
1. Introduction
3. Results and discussion
In treating ionization-induced damage of ionic crystal such as alkali halides, a general concern is to study F-center (anion vacancies) growth by irradiation, and a more specific concern is the impurity related suppression of F-center growth for various types of radiation’. Ion implantation is a useful technique for introducing impurities in solid state materials; at the same time the energetic ions can produce a large number of defects such as interstitials, vacancies and voids in the crystal through electronic and nuclear collisions. In this paper we report on the radiation effects induced by iron implantation into the ionic crystal KBr as investigated by positron annihilation lifetime and X-ray diffraction techniques.
Table 1 presents the positron annihilation lifetime results for the potassium bromide crystal, and shows that both the first and second lifetime components (t,,r*) became longer after implantation. Also, the intensity of the short lifetime component (I,) increased by 14%, while that of the long lived component (1J decreased from 41.71 to 29.35%. The energetic iron ions can introduce several kinds of defects in alkali halide crystals, such as F, F-aggregate and V type centers as well as agglomeration of the implanted ions3. On the other hand it is known that the short lifetime component is associated with positrons annihilating in a parabound state, para-positronium (p-Ps) or free positron annihilation, whereas the long lifetime component is associated with positrons trapped in the defects4. From Table 1 it is evident that there are some defects like F-centers in the virgin KBr sample. After ion implantation, the increase of z2 and decrease of I, indicate that the defect centers trapping positrons have expanded in volume and that the number of these centers has decreased. Therefore, we suggest that iron ion implantation in KBr crystals induces F-center aggregation. In the X-ray diffraction spectra for the virgin sample only the two peaks at 20 = 27.06 and 55.72 are clearly seen in Figure l(a). Since the fee KBr crystal has the lattice parameter a = 6.5985 A,
2. Experimental A 300 keV Fe+ ion beam was used to implant a cleaned KBr crystal at the dose of 5 x lOI ions cm-’ at room temperature. In order to avoid excessive heating of the samples the current density was kept very low, less than 0.1 A cm-*. The chamber pressure was 5 x 10eh torr. Both the control and the implanted samples were measured by a fast-fast coincidence positron lifetime spectrometer modified from a fast-slow coincidence positron lifetime spectrometer. The details of this system have been described in the literature*. The measured spectra used to obtain the positron annihilation lifetimes were least-squares fitted by an IBM-PC/XT computer program POSFIT-EXTENDED with a multi-exponential function. Each component was characterized by a mean lifetime and a probability per annihilation (intensity) associated with that particular mean lifetime. The X-ray diffraction measurements were done by using Cu K, monochromatic radiation with a wavelength of 1.5418 8, and scanning speed of 0.5 and counting rate of 500 cps at room temperature (22°C).
Table 1. Positron annihilation lifetime parameters for KBr before and after Fe+ implantation
Virgin Implanted
tl(Ps)
TAPS)
II(%)
I*W)
209(3) 218(2)
613(9) 639(11)
54.51(0.81) 68.54(0.72)
41.71(0.71) 29.35(0.62)
431
Guang-hou
I t
Wang; Iron implanted potassium bromide crystals
(a)
virgin
tation two new lines appear at 211= 32.92 and 6X.88 while the peaks at 20 = 27.16 and 55.84 arc reduced, indicating the formation of a new crystal structure. This is possibly due to the lattice deformation caused by the F-center agglomeration and;or by metallic iron alom precipitates in the implanted KBr crystal mainly induced by electronic processcsi. In our recent ESR experiment at the liquid nitrogen temperature WChave observed the formation of Fc?’ compound in the iron-implanted KBr crystal. The results will be published clscwherc.
KBr
27.06 (2001
,
4. Conclusion
(b)
implanted
In summary. the above cxpcrimcnts have shown that: (a) the iron ion implantation of KBr crys~ls can induct I’-ccntcr aggrcgation. and (b) thcsc F-center aggrcgatcs and metallic ion prccipitalcs in turn deform the lattice structure of the KBr crystal. Howcvcr, at present it is hard to say whether the structural changes of the implanted KBr come from the implanted layer only or from synergistic action that the modified fraction might exert upon the unmodified material since the mean pcnctration depths of positrons and X-rays arc far from the range of implanted Fe+ ions. If thcsc depths can be adjusted to the range of implanted Fe’ ions by using both a variable energy monoenergetic positron beam and synchrotron radiations. more direct information about defect production processes will be gained.
KBr
32 92 61
22 26
30
34 38
42
46
50
54
58 62
66
70 74
78 82 86
28( degree) Figure 1. X-ray diffraction spectra ofa potassium (a) and after (b) ion implantation.
bromide
crystal
before
the distance d for 20 = 27.06 is 3.2929 A which is just half of rr. Therefore, this diffraction line comes from the reflection of the (200) plane and d = I .6485 for 20 = 55.72 equals &,,,,,!2. corresponding to the (400) plane. In Figure l(b). after ion implan-
432
‘A Dupasquier. Springer-Verlag.
Pcui/rom
itr So/itl\
(Edited by P. Hnntolnrvi j. p 197.
Berlin.
‘A Perez. J P Dupin. 0 Massenct. G Marest and P Bussicrc. Ktr&r EflHi,(./.s. 52, I27 ( 1980).
24fpages
0042-207X/89$3.00+.00 Pergamon Press plc
431 to 43211989
Structural change bromide crystals
in iron implanted
potassium
Guang-hou Wang, Lie Dou, Ling Chen, Guo-giang Pang, Hai Sang, Min-kang Xiang-jin Li, Department of Physics, Naming University, Nanjing, PRC
Teng,
De-xun
Shen and
and Ke-ming
Wang,
Yi-hua
Wang
and Jin-tian
Liu, Department
of Physics, Shangdong
University,
Jinan,
PRC
Positron annihilation lifetime spectra andX-ray diffraction have been used to investigate the radiation defects produced by Fe’ ion implantation in the ionic crystal KBr at the dose of 5~ 1015 ions cm-2. We have found that after implantation both the short and long lifetime components of positron annihilation in KBr crystals increase while the annihilation probability of the second lifetime component decreases from 41.7 to 29.3%. This means a decrease in the number of trapping centers by pick-off annihilation since F-centers that already existed in the sample before implantation have aggregated in the halogen sublattice by electronic processes during ion implantation to become larger centers trapping positrons. The X-ray diffraction spectra show new diffraction peaks with rather strong intensities which can be either due to lattice dilatation caused by the F-center aggregation, or by metallic ion precipitates.
1. Introduction
3. Results and discussion
In treating ionization-induced damage of ionic crystal such as alkali halides, a general concern is to study F-center (anion vacancies) growth by irradiation, and a more specific concern is the impurity related suppression of F-center growth for various types of radiation’. Ion implantation is a useful technique for introducing impurities in solid state materials; at the same time the energetic ions can produce a large number of defects such as interstitials, vacancies and voids in the crystal through electronic and nuclear collisions. In this paper we report on the radiation effects induced by iron implantation into the ionic crystal KBr as investigated by positron annihilation lifetime and X-ray diffraction techniques.
Table 1 presents the positron annihilation lifetime results for the potassium bromide crystal, and shows that both the first and second lifetime components (t,,r*) became longer after implantation. Also, the intensity of the short lifetime component (I,) increased by 14%, while that of the long lived component (1J decreased from 41.71 to 29.35%. The energetic iron ions can introduce several kinds of defects in alkali halide crystals, such as F, F-aggregate and V type centers as well as agglomeration of the implanted ions3. On the other hand it is known that the short lifetime component is associated with positrons annihilating in a parabound state, para-positronium (p-Ps) or free positron annihilation, whereas the long lifetime component is associated with positrons trapped in the defects4. From Table 1 it is evident that there are some defects like F-centers in the virgin KBr sample. After ion implantation, the increase of z2 and decrease of I, indicate that the defect centers trapping positrons have expanded in volume and that the number of these centers has decreased. Therefore, we suggest that iron ion implantation in KBr crystals induces F-center aggregation. In the X-ray diffraction spectra for the virgin sample only the two peaks at 20 = 27.06 and 55.72 are clearly seen in Figure l(a). Since the fee KBr crystal has the lattice parameter a = 6.5985 A,
2. Experimental A 300 keV Fe+ ion beam was used to implant a cleaned KBr crystal at the dose of 5 x lOI ions cm-’ at room temperature. In order to avoid excessive heating of the samples the current density was kept very low, less than 0.1 A cm-*. The chamber pressure was 5 x 10eh torr. Both the control and the implanted samples were measured by a fast-fast coincidence positron lifetime spectrometer modified from a fast-slow coincidence positron lifetime spectrometer. The details of this system have been described in the literature*. The measured spectra used to obtain the positron annihilation lifetimes were least-squares fitted by an IBM-PC/XT computer program POSFIT-EXTENDED with a multi-exponential function. Each component was characterized by a mean lifetime and a probability per annihilation (intensity) associated with that particular mean lifetime. The X-ray diffraction measurements were done by using Cu K, monochromatic radiation with a wavelength of 1.5418 8, and scanning speed of 0.5 and counting rate of 500 cps at room temperature (22°C).
Table 1. Positron annihilation lifetime parameters for KBr before and after Fe+ implantation
Virgin Implanted
tl(Ps)
TAPS)
II(%)
I*W)
209(3) 218(2)
613(9) 639(11)
54.51(0.81) 68.54(0.72)
41.71(0.71) 29.35(0.62)
431
Guang-hou
I t
Wang; Iron implanted potassium bromide crystals
(a)
virgin
tation two new lines appear at 211= 32.92 and 6X.88 while the peaks at 20 = 27.16 and 55.84 arc reduced, indicating the formation of a new crystal structure. This is possibly due to the lattice deformation caused by the F-center agglomeration and;or by metallic iron alom precipitates in the implanted KBr crystal mainly induced by electronic processcsi. In our recent ESR experiment at the liquid nitrogen temperature WChave observed the formation of Fc?’ compound in the iron-implanted KBr crystal. The results will be published clscwherc.
KBr
27.06 (2001
,
4. Conclusion
(b)
implanted
In summary. the above cxpcrimcnts have shown that: (a) the iron ion implantation of KBr crys~ls can induct I’-ccntcr aggrcgation. and (b) thcsc F-center aggrcgatcs and metallic ion prccipitalcs in turn deform the lattice structure of the KBr crystal. Howcvcr, at present it is hard to say whether the structural changes of the implanted KBr come from the implanted layer only or from synergistic action that the modified fraction might exert upon the unmodified material since the mean pcnctration depths of positrons and X-rays arc far from the range of implanted Fe+ ions. If thcsc depths can be adjusted to the range of implanted Fe’ ions by using both a variable energy monoenergetic positron beam and synchrotron radiations. more direct information about defect production processes will be gained.
KBr
32 92 61
22 26
30
34 38
42
46
50
54
58 62
66
70 74
78 82 86
28( degree) Figure 1. X-ray diffraction spectra ofa potassium (a) and after (b) ion implantation.
bromide
crystal
before
the distance d for 20 = 27.06 is 3.2929 A which is just half of rr. Therefore, this diffraction line comes from the reflection of the (200) plane and d = I .6485 for 20 = 55.72 equals &,,,,,!2. corresponding to the (400) plane. In Figure l(b). after ion implan-
432
‘A Dupasquier. Springer-Verlag.
Pcui/rom
itr So/itl\
(Edited by P. Hnntolnrvi j. p 197.
Berlin.
‘A Perez. J P Dupin. 0 Massenct. G Marest and P Bussicrc. Ktr&r EflHi,(./.s. 52, I27 ( 1980).