Impurity-vacancy complexes in ion-implanted iron

Nuclear Instruments and Methods in Physics Research B7/8 (1985) 103-108

103

North-Wolland, Amsterdam

G. WEYER, F.T. PEDERSEN and H. GRANN lnsiiture of Physics, Universi!y of Aarhus; DK -8000 Aarhus C, Denmark and ISOL.DE Colla~~a~ion, CERN CH - I21 I Geneva 23, Switzerland

The formation and annealing of impurity-defect complexes in ion-implanted iron has been studied by “9Sn Mossbauer spectroscopy. Radioactive ‘r91n ions, decaying to the 24 keV Mossbauer state of *I9Sn, have been implanted with an energy of 60 keV at temperatures around stage III (100-500 K). Large fractions of the implanted impurities are located on substitutional lattice sites. In-defect complexes are formed both athermally in the radiation-damage cascades (below 180 K) and by trapping of mobile defects ( - 200 K) at the impurities. For two different complexes, one of which anneals around 300 K, the magnetic field at the Sn nucleus is found to be larger by 10 and 20%. respectively, wmpared to the field of substitutional Sn. A third complex with a field lower by about a factor of three is formed in the whoie temperature range up to 500 K. From the measured Mossbatter parameters the impurity-defect structures are proposed to be of the impurity-vacancy type.

1.

In~~on

The question at what temperature vacancies become mobile in iron appears still to be controversial, despite considerable new experimental results obtained in recent years particularly by means of nuclear techniques utilizing various probes, e.g., positrons, muons, and radioactive isotopes. Hautoj-&vi (see ref. [I] and refs. cited there) concluded from positron-oblation experiments on e--irradiated samples that vacancies become mobile in the well-established stage III around 220 K and form vacancy clusters above this temperature. A different interpretation of these and other experimental results was given by Frank et al. [2]. They propose a second kind of interstitial to become mobile at 220 K, whereas vacancies should remain immobile up to about 50 K. Pleiter et al. [3] performed perturbedangui~-co~elation (PAC) studies on ion-implanted radioactive “iIn probes. For implantations at 80 K and subsequent annealing up to * 170 K, about 50-608 of the implanted “‘ln atoms are located on substitutional sites, giving rise to a well-resolved nuclear spin-precession signal of the daughter “*Cd. A drop of this fraction to about 40% after an annealing at 220 K is attributed to the trapping of mobile vacancies, which are detrapped again around 300 K. No signal from any non-substitutional fraction was detected in these experiments. Recently, muon trapping at vacancies and impurity-vacancy pairs in iron has been reported by the Konstanz group [4,5]. The spin-precession signal from muons trapped at a vacancy is observed in e--irradiated samples below stage III. It disappears at - 200 K, where in dilute iron alloys new, impurity-specific signals grow in, which are attributed to muons trapped at 0168-583X/85/$03.30 Q Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

impu~ty-va~cy pairs. These pairs are concluded to be formed as the vacancies become mobile. The signals vanish again between 220 K and 280 K (depending on the impurity), presumably due to the break-up of these pairs. In the present experiments radioactive ‘191n probes have been implanted into iron at various temperatures around stage III and MWbauer spectra have been measured for the 24 keV y radiation emitted from the ““Sn daughter. These experiments are different from the “‘t In PAC experiments in that no normal annealing experiments are performed but the implantation and/or measuring temperature is varied. Both types of experiments are distinguished from positron- or muon-trapping experiments, since heavy impurities are immobile up to high temperatures and vacancies or other defects have to be trapped either in the damage cascade created in the ion-~pl~tation process or when they become thermally mobile. In contrast to the PAC experiments, in the present Mbssbauer experiments the magnetic-dipole and electric-quadrupole hyperfine interactions of the impurities have been measured directly for the first time also for the non-substitutional fraction. Furthermore, additional information on the electronic and vibrational properties of the impurities is obtained from the measured isomer shift and the ~mb-M~ssbau~ factor, respectively.

Radioactive “‘In (T1,2 = 2.4 min) was obtained as a proton-induced fission product from a uranium-carbide target irradiated with 600 MeV protons from the CERN III. METALS Lattice sites, Defects, Theory

104

G. Weyer et al. / Impurity - vacancy complexes in ion -implanted Fe

synchrocyclotron. A beam of ‘191n+ ions was produced in the ISOLDE on-line mass separator [6]. The ions were implanted with an energy of 60 keV at a rate of - 2 x lo* s-r into iron single crystals and polycrystalline foils (4N + ) to a total dose of 5 10” cm-* in a cryostat at eight temperatures between 100 K and 506 K. Mossbatter spectra of the 24 keV y radiation emitted from the i19Sn daughter were recorded by a resonancecounting technique utilizing CaSnO, absorbers [7].

3. Results Spectra measured for implantations into a single crystal are shown in fig. 1. The spectra are quite complex and contain at least three different components, each consisting of a six-line pattern characteristic of a static magnetic hyperfine interaction. The most intense component - indicated in the figure - is assigned to ‘19Sn on substitutional lattice sites. For the 506 K implantation the substitutional fraction dominates the spectrum and the magnetic hyperfine field and the isomer shift may be accurately deduced: B, = -7.34(4) T (without correction for the Lorentz field, sign from ref. [S]) and S = 1.59(2) mm/s relative to CaSnO,. These values are in reasonable agreement, with previously determined values for - 1% FeSn alloys [8,9]. For measurements at 506 and 404 K a Debye temperature of 8, = 404(U) K is calculated from the line intensities by means of the high-temperature Debye approximation of the Lamb-Mossbauer factor: f = exp[ -(6ERT/kB. @‘)I. Here Ea is the recoil energy of the emitted Mossbauer y radiation and k, is Boltzmann’s constant. Also this parameter of the substitutional site agrees with the value of Price [8]: B = 390(20) K. The misfit in the 506 K spectrum (and the others) around low velocities indicates clearly the presence of a second low-field component in the spectrum. Since the presumed six-line structure of this component is somewhat uncertain - the most intense components seems to be hidden below the inner lines of the substitutional fraction - however, this component has been omitted in the preliminary analysis of the data. Consequently, a velocity interval of u = [ - 3.8,0.5] mm/s was excluded in the fitting procedure, as indicated in the figures. A similar diffuse low-field component (B = -2X(6) T) has been observed for i19Sn probes in helium-decorated vacancy clusters in iron [lo]. For this reason and since the defect apparently resits annealing up to high temperatures, the Sn impurities will be assumed to be located in an extended vacancy cluster, which is formed directly in the implantation process. This assignment is in accordance with an estimated very small magnetic field of B4 = (2-3.8) T and an obviously low Debye temperature of the impurity (cf. figs. 1 and 2 and the discussion below).

In the low-temperature implantations, two further components are seen to be present in the spectra, which both have magnetic hyperfine fields larger than for substitutional Sn atoms as indicated in the spectra (B2 = 9.3(2) T, B, = 10.2(3) T at 77 K). The former component is easily recognized in the 152 and 243 K spectra, whereas the latter appears more clearly in fig. 2. This figure shows two spectra measured for a 297 K implantation into an iron foil measured at 77 K and 297 K, respectively. In an iterative, simultaneous fitting procedure, all spectra have been fitted with the same set of three components. The linewidths have been set to r = 0.9 mm/s and the relative positions of the six lines of each component have been fiied to the position ratio obtained for the substitutional site. With these constraints a self-consistent fit of all spectra has been achieved. These fits are shown in the figures. The slight misfit occasionally observed around the inner lines is attributed to the omission of the low-field component in the fitting procedure. This is also reflected in a line-intensity ratio larger than the theoretical value of 1 : 3 for the inner and outer lines of the substitutional component. However, since the parameters of all three components are mainly determined by their outmost lines, this systematic error appears small. Within the present accuracy of the data no necessity to introduce any quadrupole interaction was found. This is presumably due to the relatively small quadrupole moment of the excited t19Sn state, Q = - 10.9(8) fm2 [ll], which makes the Sn probe rather insensitive to electric field gradients. However, possible field gradients might also be small due to (nearly) cubic configurations of the defect structures (cf. discussion below). For the substitutional fraction both the isomer shift and the hyperfine field and their temperature dependencies are in accordance with the results of Price [8]. The corresponding parameters for the two other components show similar temperature dependencies. Parameter values are Listed in table 1. From the temperature dependence of the line intensities, Debye temperatures for the different components should be obtainable, however, the temperature dependence may be obscured by a temperature-dependent formation or annealing of defect structures. The intensity of the substitutional component is found to follow the Debye curve for B = 404 K at low temperatures (5 180 K) and above 300 K, but a marked intensity drop is observed in the temperature interval [180-2601 K. The intensity of this component is predominantly determined by the area of the two outermost hyperfine transitions, which are not strongly affected by interference with other components. Therefore the intensity drop by - 15% seems to be a significant result, which is analogous to the result of Pleiter et al. [3]. In the same temperature interval, the intensity of component 2 is seen to increase compared to the intensity of the substitutional component (cf. fig. 1). Hence this structure appears to be

G. Weyer et al. / Impurity - vacancy complexes in ion -implanted Fe

16.

I -8

I -6

I -4

I -2

VELOCITY

I 0

I 2

I 4

I 6

105

8

(mm/set)

Fig. 1. ‘19Sn M6ssbauer spectra from impiantations of ‘191n into an iron single crystai at 152 K (A), 243 K (B), and 506 K (C). Results from least-squares fits are indicated (see text). Note the low intensity of the Am = 0 hyperfine transitions, which is due to an angle of 20-30’ between the (110) crystal axis and the detected y radiation. III. METALS Lattice sites, Defects, Theory

7200

1

I

I

I

I

I

I

;

;

I

1

4

6

I

A)

5600

6800

VELOCITY

fmmlsecf

Fig. 2. ‘19Sn Mlissbauer spectra measured at 77 K (A) and 297 K (B), respectively, from l191n implanted into an iron foil at 297 K. Least-squares-fit results are indicated. The emitted y-radiation was detected normal to the foil surface (preferential magnetisation direction).

formed by the trapping of mobile defects at substutiond impurities above 180 K. Since, on the other hand, the relative intensity of this ~mpon~nt is low in the 77 Table 1 Mtisbauer parameters from a least-squares-fit analysis. The parameters are given for 77 K (8 relative to GaSnO at 297 K)

8 (mm/s) r @m/s) JW-J @WI

Component 1

Component 2

Component 3

l&8(3) O.W% 8.2?(4) 404(W

1.79(4) 0.9015) 9.3(2] (280-380)

1.87(5) 0.90(5) l&2(3) 5 200

K spectrum of fig. 2 from a 297 K implantation, the defect becomes thermally unstable below 297 K. It was found difficult to extract an accurate Debye temperature for component 2 from the dam and conflicting results were obtained from different assumptions in the analysis. This may be indicative of temperature-dependent Debye temperatures for this component and/or the substitutional fraction. The Debye temperature of component 3 is much lower than that of the substitutional site as is also evident from fig 2. The low intensity of this component does not allow for definite conclusions on a temperature-dependent formation.

G. Weyer et al. / Impuriry - uacancy complexes in ion -implanred Fe

4. Discussion The complexity of the present experimental results obviously demands further experimental and data-analysis efforts, before final conclusions may be drawn; additional experiments are in progress. However, it appears meaningful to discuss the most prominent features of the data. In the following an interpretation with recourse to the one-interstitial model, i.e., a mobility of vacancies at - 200 K, will be given. This is seen to result in a conceivable picture, which falls in line with previous results for impurity implantations in other bee or fee metals (for recent reviews, see refs. [12-141. The best established result of the present investigation is the identification of the two new high-field components. These are likely to be mono- or multivacancy-impurity defects with a comparably small number of vacancies (2-4), which are formed both athermally in the radiation-damage cascade and by trapping of mobile vacancies. Such impurity-vacancy defects are proposed for several other metals [12,13]. In two cases, Cu [14,15] and Ni [10,12], an interstitial location of - in particular - Sn impurities in the center of a quadrivacancy (presumably formed by relaxing a Sn-trivacancy pair) seems to be established from a comparison of Mossbauer, PAC, and channeling results. In both cases the Sn isomer shift is increased by 0.4-0.5 mm/s and the Debye temperature is lowered by a factor of 0.8-0.9 compared to a substitutional Sn site, as expected from simple model considerations. A similar defect structure can be imagined in the bee iron lattice. However, the increase in isomer shift is significantly lower for the two high-field components (0.1-0.2 mm/s), whereas the Debye temperature of component 3 is pronouncedly lowered by a factor of about 0.5. In the case of Ni, similar to the present results, a magnetic field higher than for a substitutional site is found for the quadrivacancy-defect structure [lo]. However, the magnetic hyperfine field at the Sn nucleus, which seems well understood, results from a delicate balance of two electronic contributions of opposite sign [16], yielding the magnetic field of substitutional Sn, the lowest in the series of 5sp elements. Therefore, both the sign and magnitude for the field in a Sn-vacancy complex appear difficult to predict from simple arguments. Furthermore a direct comparison of the Ni and Fe hosts is also questionable, because the magnetic fields for substitutional Sn have opposite signs. A measurement of the signs of the magnetic fields of the different defect components observed in the present experiments therefore appears highly desirable. It is tempting to assume the simplest possible defect production mechanism and to attribute the observed relative increase of component 2 above 180 K to the trapping of mobile vacancies at substitutional impurity atoms and the formation of impurity-vacancy pairs.

107

This interpretation is obviously in accordance with those of the positron-annihilation, the “‘In-PAC, and the muon-trapping experiments [l-3]. As expected, owing to a higher vacancy concentration in radiation-damage cascades, vacancy trapping occurs at slightly lower temperatures at impurities in ion-implanted than in e-irradiated samples. The three heavy-impurity vacancytrapping experiments indicate concordantly that the impurity-vacancy complexes break up between 240 K and 300 K. This annealing behaviour is in contrast to the findings in other bee or fee metals, where a consecutive multivacancy-trapping occurs at the impurities [12-141. On the other hand, the formation of multivacancy C~USters is inferred from the positron-annihilation experiments [1], and a more complex structure of the component-2 defect can presently not be excluded. Anyhow, from a relative comparison of the Milssbauer parameters of components 2 and 3, it appears most logical to propose a simpler structure, i.e., an Sn-mono- or divacancy pair, for component 2 rather than for component 3. Some indication that the latter structure might also be formed by the trapping of mobile vacancies is seen from the larger relative intensity of component 3 in the 77 K spectrum of fig. 2 as compared to the 152 K spectrum of fig. 1, whereas the opposite holds for component 2. This suggests that the component 2 defect anneals only partly to substitutional Sn and a certain fraction is converted into the component-3 defect, presumably by multiple trapping of vacancies. Hence this defect is likely to be an Sn-multivacancy complex. This work has been supported by the Danish Accelerator Physics Council. References

111P. Hautojkvi, Hyp. ht. 15,‘16 (1983) 357. I21 W. Frank, A. Seeger and M. Weller, Radiat. Effects 55 (19El) 111. 131 F. Pleiter, C. Hohenemser and A.R. Arends, Hyp. Int. 10 (1981) 691. [41 A. Weidinger, Hy-p. Int. 17-19 (1984) 153. 151 A. M&Iang, E. Albert, E. Recknagel, A. Weidinger and P. Moser, Hyp. Int. 17-19 (1984) 255. 161H. Ravn, L.C. Carraz, J. Denimal. E. Kugler, M. Skarestad, S. Sundell and L. Westgaard, Nucl. Instr. and Meth. 139 (1976) 267. 171 G. Weyer, Nucl. Instr. and Meth. 186 (1981) 201. PI D.C. Price, J. Phys. F4 (1974) 639. [91 M. Dubicl and W. Znamirowski, Hyp. Int. 9 (1981) 477. WI H. de Waard, D.W. Hafemeister, L. Niesen and F. Pleiter, Phys. Rev. B24 (1981) 1274. 1111 H. Haas, M. Menningen, H. Andreasen, S. Damgaard, H. Grann, F.T. Pedersen, J.W. Petersen and G. Weyer, Hyp. Int. 15/16 (1983) 215. WI F. Pleiter and C. Hohenemser, Phys. Rev. B25 (1982) 106. III. METALS Lattice sites, Defects, Theory

[13] E. Recknagel, G. Schatz and Th. Wichert, in: Hypexfine Interactions of Radioactive Nuclei, ed., J. Christiansen, Topics of Current Physics, vol. 31 (Springer, Heidelberg-New York, 1983) p. 133. [14] W. Andreasen, S. Damgaard, J.W. Petersen and G. Weyer, Hyp. Int. 15/16 (1983) 375.

llSJ M. Deichcr, H. Hofsbs, G. Lindner, E. Recknagel, Th. Wichert, M.L. Swanson, L.M. Howe and A.F. Quenneville, Hyp. Int. 15/16 (1983) 379. (161 B. Lindgren, Hyp. Int. 15/‘16 (1933) 295.