MÖSSBAUER STUDIES OF IMPLANTED ATOMS

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MÖSSBAUER STUDIES OF IMPLANTED ATOMS

I. Dézsi

To cite this version:

I. Dézsi. MÖSSBAUER STUDIES OF IMPLANTED ATOMS. Journal de Physique Colloques, 1980,

41 (C1), pp.C1-17-C1-24. �10.1051/jphyscol:1980103�. �jpa-00219573�

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JOURNAL DE PHYSIQUE Colloque C l , suppl6ment au no 1 , Tome41, janvier 1980, page C1-17 M O S S B A U E R S T U D I E S OF IMPLANTED ATOMS

I . DGzsi

Central Research I n s t i t u t e for Physics, H-1525 Budapest, Hungary.

Abstract.

-

In recent years very active research was performed on implanted atoms.

The atomic locations, and defects structures were mostly studied. A review on the results obtained by using Mossbauer spectroscopy is presented.

1. Introduction.

-

From the middle of the sixties ion implantation became a very use- ful method for the modification of the various technologically important parameters of solids. Most successful was the application to semiconductor technology but applications have also been developed in metallurgy. Simultaneously with the technological development, intensive scien- tific investigations were started.

The implantations were carried out by using various methods. The most frequently used method is the application of a mass separ- ator. Other methods are recoil implantation of nuclear reaction products and the most recently developed laser implantation.

The problems associated with the atomic locations and impurity-defect associations were investigated by various microscopic methods. The most successfully used methods are the backscattering spectrometry includ- ing particle channelinq, and the methods in which the information is obtained through the hyperfine interaction of the nuclei.

The perturbed angular correlation of the gamma rays or particle-gamma ray cascades

/PAC/, nuclear orientation /NO/, NO with nuclear magnetic resonance destruction and P16ssbauer specroscopy belong to the latter

group

The first Mossbauer measurements were made on recoil implanted atoms, and atoms implanted in isotope separators /I/. The Groningen group carried out the pioneering work in the latter case and pointed out the usefulness of the Mossbauer method in the investigations of the problems mentione8 above. The results obtained up to 1971 are included in the review paper by de Waard 121.

Numerous laboratories contributed sig-

nificantly to the great progress experi- enced in this decade. Excellent review papers on verious aspects were published on the results 13-61. Intensive research continued during the last three years and important new results were obtained. The present paper deals with the progress made in these years. Also, a qeneral outline of the main physical processes takinq place during and after implantation is given in order to provide a frame for the specific results.

2. Ion implantation.

-

In most cases, if mass seGarator is used, ions with energies up to a few hundred keV are implanted in various targets. The range values depend on the possible energy loss processes in the target. The physical principles of the energy loss well understood 17, 81. Two major types of energy loss play the dominant role: screened Coulomb collisions between the projectile and the target atom

/nuclear stopping/ and the interaction of the electrons of these two kinds of atoms.

Theoretical calculations showed that the nuclear stopping is the more important process at lower enerqies up to about a few hundred keV. Many important factors are discussed in monographs /see e.9. 91.

Because of the range straggling a distribution in the implanted atom concentration exists. This distribution can be well ap- proximated by a Gaussian. The concentration of the implanted atoms versus depth can be expressed as

N(x) =

n

^exp[-(x-R^P) * / A R ~ I P

where

n

⁼^{Nd/2.5 R} ^Eis the average I?'

dopant concentration, and Nd is the number of implanted atoms per unit area. AR is

P

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980103

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(21-18 JOURNAL DE PHYSIQUE

the mean straggling, R is the projected range-of R along the incident beam direc- P tion-. . The channelinq and diffusion of the

. -

particles are neglected in this formula.

In

a:

single crystal, however, the channel- ing effect may increase the range, some- timss by a factor of 50 but the depth decreases during implantation because of the amorphization of the target. Since channela led particles penetrate deeper in the target lattice, in an optimal case two

"peaks" in the concentration may appear.

The two "peaks" are connected with dechanA nelled atoms as shown in Fig. I .

The number of displaced atoms N(E), can be calculated by using the equatibn 1141

N(E) = 0.42 E/E d

where E is particle energy, and Ed is the effective displacement enerqy. The value of Ed for semiconductors is about 14 eV, for metals 20 to 40 eV. It follows that for 50 keV projectile energies c2000 atoms are displaced. The collisions may leave behind many interstitial atoms and vacancies. Soma damage sites anneal out in a very short time while some live long, depending On various factors such as temperature, target material, etc. At the end of its track, the

Fiq. 1: D e p t h d i s t r i -

IlHq.

moving atom loses its kinetic energy within b u t i o n i n a n a l i g n e d

s i n g l e c r y s t a l /lo/. a distance of a few atomic spacings. This

A: P a r t i c l e s l a n d i n g zone is called the depleted zone and it is s i m i l a r l y a s i n

a m o r p h o u s m a t e r i a t ; relatively rich 1141 in defects. Although

I

^' ^B: d e c h a n n e t l e d par- computer simulation 1151 of these proceSsesa

t i c l e s ;

10 - & - ^C: c h a n n e l l e d particles. chtributed to the better understanding OE

-

^. these complex phenomena, mahy details remaSn

In most cases, however, because of small misalignment of the sample and because of other reasons the depth distribution is complex.

The concentration of the implanted atoms in the implanted layer is not limited by thermodynamic constraints and can be above the solubility limits by orders of magni- tude. The system produced is metastable from thermodynamic point of view and may transform into a stable form if enough energy is available for the transition.

During implantation, complex radiation damage processes cause lattice disorder in the target. High energy projectiles lose energy mainly by electron ionization which leads to no damage, but when the energy of the particle becomes lower than Ei

[Ei = A keV Ill/, A = mass number of the particle], hard sphere elastic collisions lead to atomic displacements. The number of displacements was calculated by Kinchin and Pease 1121 and hy Robinson and Torrens

1131. The displaced atoms may knock out other atoms and create new displaced atoms.

Thus a collision casca* is formed. In this cascade Frenkel pairs are created.

unclear.

The implanted atom may land in various positions in the lattice. Some substitute host atoms, some may land in interstitial, positions, some form vacancy-impurity associations. The relative fractions of the atoms in the different positions depend again on many factors, one of the main oms being is the relative sizes of the implanted and host atoms. Brice 1201 made calculations on the relative population of implanted atoms. The temperature of implantation is also important because annealing during implantation may affect the population. Especially in metals, in the various annealing staqes the possible trapping and detrapping of the vacancies changes the nearest neighbourhood of the implanted atom 1211.

3. New results obtained by Jl6ssbauer troscopy.

-

Mbssbauer effect studies may give information on the population of various lattice sites and ofi the formation of vacancy-defect associations. Therefore the first task of the experimentalists is to recognize the spectral components in the often complex spectrum and then to a&- sign the various components to various

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sites. In many cases, this can not be done unambigously. The results can be classified into three groups: semiconductors, metals and insulators. Further classification is possible according to the Massbauer nuclei because each nucleus and element has its own problematics. The measurements are performed either on sources or absorbers. The soutce experiments have the advantage that atoms implanted with lower doses can be studied. After-effects may complicate the spectrum, especially in insulators. The Latter problem can be avoided in absorber experiments but in this case only atoms implanted with higher />1014 atoms/ /cm 2 / doses can be studied which make the lattice completely amorphized. This is a drawback in some respect. Another possible method the Coulcmb or fiuclear reaction recoil implantation is used very seldom recently.

3.1. Semiconductors.

-

Research on type-IV semiconductors has been very active in recent years. Isoelectronic, sinqle donor and deep impurities were studied in diamond type lattices,

3.1.1. ',!9~nI 119sb,,

-

The lattice location of lL9sh add '19sb was found to be mostly substitutional in diamond, Si and Ge 122-251.

The isomer shffts of '19sn in diamond Se, Ce and a-Sn hosts were correlated with the difference in valence between the host and the Sn impurity 1261.

12'1,

-

After systematic studies of 3.1.2,

implanted / l a 9 ~ e / 12'1 in diamond, Si, and Ge, NafemeLster and de Waard 1 2 7 1 concluded that Te is located in both substitutional and interstitial posftions. Earlier channelling measurements showed that after annealing the implanted samples, the ratio of populations of these sites is about 1:1

1 2 8 , 291. Unfortunately, the ratio of the

MBssbauer line intensities, which was about 1 t 1 too, /Fig. 21 did not make possible the assignment of the lines to the corresponding sites. By considerifig the probable bond structures, the line which corresponds to higher electron densities /p-bonding/ was assighed to interstitially, while the line with lower electron density /sp bond/ to 3

substitutionally positioned I. The two

branches of the isomer shift values showed a linear relationship with the nearest neighbour spacings in the host lattices.

Fig. 2:

Massbauer s ectra of 1 g 9 1 in various semiconductors /27/.

The same linear relation-

, - - - - ; : ;

^;o

'

ship was es-

VELOCITY IN rnrn~sec tablished in a-Sn and Sic lattices 1301. In this analysis it is essential that the spectra m i s t of two sinqle lines and no quadrupole

splittinq exist. But the quadrupole splittinq can not be excluded with certainty, especially in an amorphous environment. ~t was very surprising that the spectra did not change upon annealing. A recently developed technique, laser annealing 131-331 of implanted layers helped in the assignment of the lines.

Channelling measurements on laser annealed Te-implanted Si showed 1341 that 80 % of the implanted Te is located in substitutional sites after laser irradiation with 2.6 ~ / c m ~ . The Mijssbauer spectra of laser annealed samples showed 135, 361 that the intensity of the line corresponding to the high electron density site increases.

Therefore, a reconsideration of the interpretation is necessary.

125~e.

-

The Mbssbauer spectra of 3.1.3.

1 2 5 ~ e show two poorly resolved resonance lines 1 3 7 1 . The gamma transition is between the 312 and 112 levels. Therefore, the spectra can be analysed formally in terms of either two single lines, or a quadrupole split doublet. Eecause of the broad lines and the near 1:l intensity

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c1-20 JOURNAL DE PHYSIQUE

ratio, the assignment of the lines is once shells of the iron atoms in the host lat- again difficult. Laser annealing may also

help in the analysis and a measurement in external magnetic field may elucidate the origin of the lines.

5 7

3 -1.4.

1

C O / ~ ~ F ~ .

-

Recoil implantation 1381 of Coulomb excited 5 7 ~ e showed two lines with an intensity ratio that was strongly dependent on temperature. Because of this temperature dependence, it was be- lieved that the two lines can be attributed to two different lattice sites. This model was followed 138, 391 in the interpretation of data on 5 7 ~ o implanted into type-IV semiconductors. However, measurements made in an external field on both 5 7 ~ o and 5 7 ~ e implanted in Si and Ge showed clearly that the two lines belong to a quadrupole split doublet 140, 411 as it was sugqested earlier 1421. F i g . 3 shows the Mossbauer spectra of 5 7 ~ o

implanted in Si, and Ge, measured at H=O and H=80 kOe. A correlation between the isomer shift 161 and quadrupole

split /Qsf values ^{v (rnmht-}

3 : MBssbauer s p e c t r a versus inter-

7Co i m p l a n t e d i n S i atomic distances / a / and Ge / c / a t H = 0 ; for 5 7 ~ e in dia- / b / / d / i n H = 8 0 kOe /41/.

mond type lattices / F i g . 41 was found by Sawiczki and Sawiczka 1431. The increase in 6 with decreasing atomic volume may be the result of the compression of electronic

F i g . 4 : C o r r e l a t i o n b e t w e e n i s o m e r s h i f t /6/, q u a d r u p o l e s p l i t t i n g (QSI u c l u e s and t h e i n t e r a t o m i c d i s t a n c e s i n diamond t y p e l a t t i c e s /43/.

tice. The chanqe in the electric field qradient was attributed to the change of R - ~ in the different lattices. A dose de-

ef f

pendence of the / 5 7 ~ o / 5 7 ~ e spectra was obtained 1441, and shown in Pig. 5.

- 2 -1 0 1 2

VELOCITY h m / s

F i g . 5 : Mdssbauer s p e c t r a o f 5 7 ~ o i m - p l a n t e d i n Ge w i t h d i f f e r e n t d o s e s / 4 1 / .

This result suggests that the structure of the neighbourhood of the implanted Co atoms at low doses is different from that obtained with higher / % 1 0 ~ ~ / doses. The quadrupole split spectrum is characteristic of the latter one and probably corresponds to an amorphous surrounding. In the light of these results, the experiments of Latshaw seem controverisal in Si at least, because in his experiment the intensity of the line at higher negative velocity values was found to be larger than that at lower velocity. The annealing behaviour of 5 7 ~ o sources in Si and Ge is complex. A change of the hyperfine parameters can be observed from 100 OC. The quadrupole splitting decreases towards higher temperatures and the spectrum is dominated by an asymmetric single line which finally transforms to a spectrum showing two lines with different intensities. The Ge behaves in a different way. The final stage of Co and Fe impurity in Si was found in segregated intermetal- lic compound phases 145-471.

3.1.5. / 1 3 3 ~ e / 1 3 3 ~ s and 12'%e.

-

^Both

isotopes show a single line Massbauer spectrum in Si and Ge 148, 491. The appearance of narrow single lines suggests

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unignely defined lattice sites. The 6 values of 1 3 3 ~ s in Si and Ge are close to the 6 value of Cs metal. This compatible with an interstitial location of 1 3 3 ~ s and with a compression effect to account for the excess 6s density. Complex spectra of 12'xe and 1 3 3 ~ s were observed in diamond 1501. A consistent fit to the experimental spectra could be obtained by assuming components with quadrupole splitting together with a number of single lines. The complex spectrum suggests the presence of impurity- -vacancy associations. The amorphization process in diamond could be observed by the appearance of a single line at higher doses 1511. This component became dominant above doses of lo1* atom/cm 2

.

^The⁶ value of this line is very near to that found for 1 3 3 ~ s implanted in graphite 1 4 9 1 , which also sup- ports the assignment of this line.

3.2. Metals.

-

In recent years the main goal of the Mossbauer studies remained the determination of the location of implanted ions and the investigation of defect structures formed during the implantation, and in subsequent annealing. These two topics are interrelated because in many cases the spectra are complex and the components re- quire annealing of the samples in order to recognize the various components on the basis of their change at known annealing stages. Both correlated and uncorrelated /3/ defects were studied and other than iron matrices were more frequently investigated. The defect structures in metals are known better than in semiconductors, con-

sequently more exact conclusions can be drawn for defects from the Mossbauer experiments.

3.2.1. 5 7 ~ e .

-

A systematic study of implanted atoms in various hosts was performed by the Polish group, and a detailed de- scription of their work is given Ref. 1521.

Fe atoms were implanted with a doses of 1015

-

^atomlcm2

.

^Agood replacement of the host atoms by iron implant was found in Rh, Ir, Pd, Ru, Re, Os, Fe, Cs and Ni. Short range ordering processes were observed in V, Cr, Nb, Mo and Ta matrices. The iron clustering in the latter

group of elements was similar to that found after splat cooling. The relative atomic radii are the factors determining whether substitution or cluster formation takes place. The higher the difference in these values the higher is the probability of the

formation of clusters. The interstitial population of implanted atoms could be explained well by the use of Miedema parameters 1531. Radiation effect was observed only in line broadening, but clustering processes obscured the effect of defects in many cases.

Longworth and Jain studied 5 7 ~ e implanted copper and silver alloys 1541. The authors observed defect clucters of iron in Cu at 5 . 1 0 ~ ~ atom/cm2 dose, an effect of sput- tering and the presence of an extended damage region in the lattice. Surprisingly a large fraction of substitutional sites was observed in Ag, in spite of the insolubili- ty of F;. Upon annealing a-iron precipi- tates appeared.

"'~n.

-

The superconducting beha- 3.2.2.

viour of Nb3Sn was studied after ll'sn implantation into Nb foil 1551. The Moss- bauer spectra showed that Nb3Sn was formed just after implantation.

Subsequent annealing of the sample result- ed in superconducting transition temperatures up to 17.8 K. The annealing was necessary to achieve long range order in Nb chains but not for the increase of Nb3Sn amount.

3.2.3. 1 5 3 ~ u 1 161~y 16'Tm.

-

/ 1 5 3 ~ m / 1 5 3 ~ ~ was implanted in iron 1561. Substitutional

/magnetic split component/ and vacancy ss- sociate lquadrupole split1 site populations of E U ~ + were suggested. The internal magnetic field value was much less than the value measured before 1571, and also no internal oxidation was observed. The reason of the discrepancies is not known.

The measurements of the lattice location of 16'3r in A1 measured by the Mossbauer effect of 16'~m gave about 90 % substitu- tional site population at 7 K 1581. De- tailed studies on 161~y in Fe and Ni 1591 showed two components of the Mossbauer spectra of about equal intensity. One site

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c1-22 JOURNAL DE PHYSIQUE

is substitutional and another is probably vacancy associated. The substitutional component disappears after annealing. No evidence was found for the existence of dysprosium oxide formation in the samples.

3.2.4. /133~e/133~s.

-

The oversized Xe atoms retain the vacancies in the cascade which probably would otherwise recombine

immediately after their formation. The Xe atom does not fit well into the lattice because of its large size, but the trapping of vacancies gives more space in the lattice. Therefore, the Mdssbauer study of the 1 3 3 ~ s daughter isotope is especially suit- able for vacancy studies. These vacancy stabilized sites were studied in detail in Fe 1601 and in bcc .metals, Mo, W, Ta 1611.

In iron no evidence for the presence of recovery stage, I was found after annealing.

This was explained in terms of implantation induced elastic-collision spikes, during which stage I recovery takes place. In bcc metals very interesting effects could be observed during annealing above, room temperature /Fig. 61. The appearance of various components could be

.wv

explained by the forma-

,

\ l

tion of impurity vacancy associations with different numbers of vacancies, which mi-

v

grate in various annealing stages. The dose and dose rate dependence of vacancy

9

associate site formation was studied in Xe implanted Fe 1621.

3 - 1 0 ! 2

V E L a m inmu1 These dependences could

be understood by sup- F i

.

^6:M b s s b a u e r posing that implanted s p z c t r a o f 1 3 3 ~ s

i m p l a n t e d - a t r o o m atoms interact with t e m p e r a t u r e and vacancies created not a n n e a l e d a t v a r i -

o u s tempera- in their own track but

t u r e s /61/. by other implanted

atoms during the implantation process. The experimental results couLd be reproduced at least qualitatively by the solution of a set of simultaneous differential equations

1631.

The oversized Xe and Cs atoms in various lattices involve another interesting effect, the effect of a high pressure in substitutional and, to a less extent, in some vacancy stabilized sites. This effect was first observed by de Waard and Drentje 1641. Later it was observed in numerous other lattices as well 165, 491. Fig. 7 shows the 6 values measured in various lattices. The large 6 values can be explained by the decrease of atomic volume because of the high pressure at the lattice sites of 1 3 3 ~ s and by the delocalization of 5p electrons from the core of the Cs atom. The delocalization increases the 5s and 6s electron densities because of the decrease in screening.

Fig. 7: I s o m e r s h i f t v a l u e s o f 1 3 3 ~ s i m p l a n t e d i n t o v a r i o u s h o s t s /49/.

ns = t h e effective n u m b e r of 6 s elec- trons. T h e 6 v a l u e s a r e g i v e n rela- t i v e t o B a A Z q , s: s u b s t i t u t i o n a l ,

a: after a n n e a l i n g

The 6 values decrease on annealing because the pressure decreases when the implanted atoms trap vacancies.

3.3. Insulators.

-

Few studies have been made on implanted insulators.. 5 7 ~ o was implanted in K4[~e(~~)6], K3[~e(~~)6] and in FeF2 1661. More than one kind of sites were populated in the two complex compounds but only one in FeF2, where a drastic change in the Nee1 temperature for the implanted atoms was observed. 5 7 ~ e implanted in NaCl showed a doublet similarly as it was observed for 5 7 ~ o doped NaCl 16

1.

^{The spec-}

trum was attributed to Fe3+ in vacancy associated site. Xecently 1 5 3 ~ m was implanted in alkaline-earth fluorides and the charge state of the daughter element Eu was de- termined /67 /

.

^{E U ~ +}^and.^{E U ~ +}were found, and the amount of the former decreased after annealing. The annealing apparently changed the vicinity of Sm by oxygen substitution of F- ions.

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4. Conclusions.

-

Mossbauer studies on implanted atoms have been continued with in- creased intensity. The interest was focus- sed on lattice location and impurity-defect formation. Considerinq the proqress, we can conclude that the results have shown the capability of Mdssbauer spectroscopy in this field but many open questions remain for further research. This is expected both in resonance line assignments which is fundamental for lattice location determination, and in defect studies. The structure of associations is not fully known.

These statements are valid for every class the results for which have been presented.

Particularly, very few data have been obtained for ionic crystals up till now. On the other hand, in semiconductors and

metals where most studies were carried out, further systematic studies on well chosen implants and hosts may increase the effec- tiveness of future investigations. Also the dose, dose rate, and annealing studies should lead to new insights. The application of laser annealing will make the assignment of resonance lines easier.

Furthermore, complex investigation of sev- eral implanted systems with various methods is necessary in order to have more definite assignments for the resonance lines.

Reference

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5

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