Mössbauer study of the amorphization process induced in αFe by boronimplantationJacek Jagielski, Michal/ Kopcewicz, and Lionel Thomé Citation: Journal of Applied Physics 73, 4820 (1993); doi: 10.1063/1.353848 View online: http://dx.doi.org/10.1063/1.353848 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/73/10?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Depth analysis of phase formation in α-Fe after high-dose Al ion implantation J. Appl. Phys. 84, 6570 (1998); 10.1063/1.369030 Thermally induced transformations at an ion implanted α–Fe2O3/α–Al2O3 interface: A Mössbauerspectroscopy study J. Appl. Phys. 79, 4141 (1996); 10.1063/1.361779 Role of alloying elements in the stability of nitrides in nitrogen implanted αFe J. Appl. Phys. 78, 1312 (1995); 10.1063/1.360373 Microstructure of TiC precipitates in Tiimplanted αFe J. Appl. Phys. 51, 1001 (1980); 10.1063/1.327727 XPS and Mössbauer studies of amorphous FeGe J. Appl. Phys. 50, 1031 (1979); 10.1063/1.326101

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Mdssbauer study of the amorphization process induced in a-Feby boron implantation

Jacek Jagielski and Michal KopcewiczInstitute of Electronic Materials Technology, W6lczytiska Street 133, 01-919 Warsaw, Poland

Lionel ThomeCentre de Spectromitrie Nuclgaire et Spectrometrie de Masse IV2P3-CNTRS Bat 104 et 108,91405 Orsay, France

(Received 9 December 1992; accepted for publication 22 January 1993)

Amorphization of iron by boron implantation was studied by conversion electron Mossbauerspectroscopy and Rutherford backscattering techniques. The results indicate that theamorphization process depends on the thickness of the sample. After implantation at RT, theamorphous fraction amounting to 0.85 was found for t50 nm thick iron film whereas in the bulksample this fraction reached only about 0.65. The residual crystalline a-Fe phase persists in thesample amorphized by B implantation in a wide concentration range corresponding to nominalcompositions from Fe90B10 to Fe,65B35 in clear distinction from melt-spun and sputtered samples.

1. INTRODUCTION

Amorphization of iron by boron implantation has beenstudied by a number of authors. -5 In a typical experiment,thin evaporated Fe layers were implanted with B+ ions ina wide energy and fluence range. The formation of anamorphous phase was examined by different techniquessuch as electrical resistance measurements, conversionelectron Mbsbauer spectroscopy (CEMS), transmissionelectron microscopy (TEM), and grazing or conventionalx-ray diffraction (GXRD/XRD). In most of the experi-ments, the formation of the amorphous phase in the im-planted layer was observed and complete amorphizationwas reported when B concentration exceeded severalatomic percentages as analyzed by TEM.' Models havebeen developed which predict that the crystalline-to-amorphous phase transformation requires both the cre-ation and stabilization of topological disorder.6 Such atransformation may be driven by the stresses due to theincorporation of the foreign atoms in the host matrix.7

An important question arises whether the results ob-tained in implanted thin films are directly applicable to thecase of bulk samples. In other words, does the metalloidthreshold concentration required for complete amorphiza-tion differ in thin films and in bulk samples? The impor-tance of this question is obvious: it concerns almost allpractical applications of ion implantation and all experi-ments where comparison between the results obtained forthin films and bulk samples is required. Moreover, it is notobvious that the structure of an amorphous phase formedby ion implantation is the same as that in the case of melt-spun samples. The aim of the present article is to studythese problems for a model Fe-B system.

II. EXPERIMENTAL

In the present study, three types of samples were used,(i) 0.1 mm thick pure a-Fe disks 12 mm in diameter; (ii)150 nm thick iron layers evaporated on a silicon wafer;(iii) 150 nm thick iron layers evaporated on an oxidizediron surface. The phase structure of the implanted layers

was analyzed by CEMS. The B1+ implantations were car-ried out at 300 or 77 K by using the IRMA-CSNSM Orsayion implanter. In order to obtain a flat impurity profile,two energies were used, 24 and 60 keV. The boron depthconcentration profiles, checked by Rutherford backscatter-ing spectrometry (RBS), were almost flat from the surfaceup to 120 nm. The samples were implanted at fluencescorresponding to compositions ranging from Fe90Blo (8.55X 1016 at./cm2 at 60 keV and 4.44X 1016 at./cm2 at 24keV) to Fe6 5B35 (4.14X 10"7 at./cm' at 60 keV and 2.15X 10i7 at./cm2 at 24 keV).

11. RESULTS AND DISCUSSION

CEMS spectra recorded for bulk Fe samples implantedwith B fluences corresponding to compositions rangingfrom Fe9 0B10 up to the Fe65B35 are presented in Fig. 1. Thespectra in Fig. I were fit with two spectral components: thesextet with narrow lines characteristic for a-iron and thesix-line subspectrum with broad lines corresponding toamorphous Fe-B. The distributions of magnetic hyperfinefield, P(H), extracted from the broad-line spectral compo-nent by using the constrained Hesse-Rubartsch methodsare included in Fig. 1. The P(H) distributions covers allthe hyperfine fields characteristic for various crystallineFe-B phases, such as Fe2B [23.2 and 24.2 T (Ref. (9)],o-Fe3B [23.5 and 26.4 T (Ref. 10)], bct-Fe3 B [22.6, 26.5,and 28.8 T (Ref. 11)], and Fe23 B6 [23.8 and 28.6 T (Ref.12)1, so basing on the hyperfine fields, it is not possible toevaluate the composition of the amorphous Fe-B phase.However, FeB can be excluded since the hyperfine field ofthe 11.8 T characteristic for FeB'3 is too small to contrib-ute to the main peak of the P(H) distribution. The phaseabundance of the amorphous and crystalline a-Fe phase isgiven by the spectral areas corresponding to the broad sex-tet due to the amorphous phase and the sextet character-istic for a-Fe. The fraction of amorphous phase is plottedin Fig. 2 as a function of the nominal composition of theimplanted layer. As can be seen, the complete amorphiza-tion of the Fe-B surface layer is not achieved, even at the

(D 1993 American Institute of Physics 48204820 J. Appl. Phys. 73 (t 0), t 5 May 1 993 0021-8979/93/1t04820-05$06.00 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

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Z 1.00 I W UKA C 1 03 Fe8 0 6 2 0 C

a

1,02a: F. i,2. d

CŽ 1 ,0 1 . ~ V)4\I!4,1~ F65B3 - e

100 . .-10 -5 0 5 0 20 40

Velocity [mm/s] Hf [T]

FIG. 1. CEMS spectra recorded for the bulk Fe samples implantB' ions up to a nominal composition of (a) FeOBj0, (b)(c) Fe8 0B2 0, (d) Fe75B2 5, and (e) Fe65B335

highest implantation fluence (corresponding to Fe65]clear distinction to the melt-spun or sputtered sawhich reveal an exclusive amorphous phase (i.e., cline a-Fe phase being absent in CEMS spectra) in a

concentration range from Fe8 6B14 to Fe6B40. As it iknown, CEMS probes the surface layer of the mateto the depth of - 150 nm ( - 90% of the signal comeupper, i.e., 150 nm surface layer' 4 ). Thus, we can ethat 25% of spectral contribution of a-Fe, as measureFe65B35 [Figs. l(e) and 2], comes from the deepplanted a-Fe substrate. The crystalline a-Fe signallated to the residual a-Fe phase in the B-implanted E

layer.

0~

OC

a

'a

0

E

100

75

50

25

0 0 1 0 20 30 40

FIG. 2. Amorphous phase fraction as a function of nominal comnof boron implanted bulk iron samples.

.' 100

0

8 0

o 60E

10

9

6

3

09

6

3

10

FIG. 3. Comparison of the amorphous phase fraction measured for thesamples with the same nominal composition Fe7 5B25 prepared by differenttechniques. The only exception is the melt-spun sample with a nominalcomposition Fe8OB20 .

6 Additional postirradiation with 500 keV Ar3+ ions

performed on Fe7 5B2 5 sample for which the amorphous3 phase fraction measured after B implantation was -0.65,0 has not affected significantly the abundance of this phase° (Fig. 3). This suggests that, as found earlier in implanta-

tion experiments,' 5 the abundance of the amorphous frac-ted with tion is directly related to the concentration of foreign at-Fe85B1 5, oms, in this case of boron atoms, and not defects created by

irradiation.The CEMS spectra obtained for bulk samples and thin

films deposited on silicon substrate implanted with a flu-B35) in ence corresponding to Fe75B25 are compared in Fig. 4. Sim-amples ilar results have been obtained for iron films deposited onrystal- oxidized iron surface. It is clear (see the correspondinga wide amorphous fractions shown in Fig. 3) that amorphizationis well- is more pronounced in the thin film than in the bulk sam-rial up ples implanted to the same nominal composition Fe75B25s from (the relative abundance of the amorphous phase is 0.85,xclude and 0.65 for thin film and bulk samples, respectively).red for Nevertheless, even in the case of thin film implanted withunim- boron at 77 K, complete amorphization was not achieved

.1 is re- [Figs. 4(c) and 3], and the amorphous fraction contributedsurface in -90% to the total spectral area.

In order to exclude the possibility that the difference inthe amorphous phase fraction between bulk and thin filmsamples be due to a possible loss of B atoms in the case ofthick samples (due to eventual in-bulk diffusion), the RBSspectra were recorded for the implanted samples. Figure 5shows that the Fe level is quite similar in the case of bulkand thin film samples implanted to a composition ofFe75B25. Moreover, the RBS data recorded for the Fe65B35bulk sample indicate a much higher boron concentrationthan for the Fe75B25 thin film for which the amorphousfractions were similar (-0.8 for both samples). Thus, weconclude that the difference in the amorphization behaviorof the two types of samples (bulk and thin film) is not dueto long range boron redistribution into the bulk leading toa decrease of boron concentration in the surface layer.

CEMS spectra obtained for melt-spun or sputteredposition samples of compositions ranging from Fe8 6B14 to Fe6oB4o

reveal the presence of the amorphous phase exclusively [see

4821 J. Appl. Phys., Vol. 73, No. 10, 15 May 1993

* b.1k RT* 0~elt sp.~

Fe1 _BX

Jagielski, Kopcewicz, and Thorn6 4821 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

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1.01 K

. thi filRT

±i 1.01 C

- 1.00thin film-

a) LN

C 1.01 s

1.00

1.00~~~v-8 -4 4

Velocity [mm/s]

I9

3

09

3

0

1 - . ~~96

3

00 20 40

Hf[T]

FIG. 4. CEMS spectra and hyperfine field distributions recorded for thesamples with the same nominal composition of the amorphous phase(Fe75B25 ) formed by B implantation in bulk a-Fe (A), in thin a-Fe filmimplanted at RT (B) or LNT (C). A spectrum and P(H) distribution ofthe melt-spun sample are shown (D) for comparison.

Figs. 2 and 4(d)], without any significant contribution ofthe narrow sextet characteristic for the crystalline a-Fephase. Such a result is in clear distinction to those obtainedfor B implanted samples where a residual a-Fe phase wasdetected even after implantation with the highest dose cor-responding to the Fe65 B3 5 composition.

It is worth noting that the criterion for complete amor-phization depends strongly on the experimental techniqueused. The TEM technique" ' 6 revealed a unique amorphous

10000

7500

-uQ) 5000

2500

275 300 325 350

Channel

FIG. 5. RBS spectra recorded for a-Fe samples implanted with boronions; open circles: unimplanted bulk sample, full circles: Fe75B25 formedin the bulk a-Fe, open triangles: Fe6 5B15 formed in the bulk a-Fe, fulltriangles: Fe7 5B2 5 formed in the thin film sample.

phase in cases when CEMS shows only a partial amor-phization. TEM experiments performed for the implantedFe-B system showed complete amorphization for boronconcentration as low as 0.05 (Ref. 16) whereas our CEMSresults revealed the a-Fe phase even for Fe6 5B35 formed byB implantation. Such a difference is most probably due tothe different volume analyzed by these methods. CEMSmethod is more sensitive to the short range order than arediffraction techniques, which give information averagedover a larger volume, and is better suited for quantitativeanalysis of the formation of an amorphous phase. Thus, itis important to define better the term "complete amor-phization." In our work, this term means that the contri-bution of the crystalline a-Fe phase to the total spectralarea in CEMS spectra is below the limit for detection of themagnetic component by this technique (less than -5%).

The short range order (SRO) in amorphous metallicsystems is still a matter of controversy. The results ob-tained in our study do not allow us to discuss the details ofthe short range order in the amorphous phase. Neverthe-less, some characteristic features of SRO can be observed.It is worth noting that as far as CEMS spectra are con-cerned, there are similarities between B-implanted iron andpartially recrystallized melt-spun Fe-B metallic glasses, inboth cases the spectra consist of the sum of the sextetscharacteristic for the amorphous phase and for the almostunperturbed crystalline a-Fe structure. In fact, if we takeinto account the enhanced mobility of atoms and the in-creased concentration of defects in the sample during theimplantation process, this similarity is not surprising. Acontinuous production of defects during implantation mayenhance the local rearrangement in the Fe-B system lead-ing to the formation of structures similar to those existingin amorphous samples partially crystallized due to anneal-ing. The fact that in TEM experiments only the amorphousphase was observed in samples in which CEMS methodreveals a-Fe means that the TEM technique is not suffi-ciently sensitive to detect a residual crystalline phase. Theconclusion is that the volume of crystalline zones formed inion implanted samples is too small to be detected by TEMtechnique.

The fundamental question is whether the presence ofthe crystalline a-Fe phase in B-implanted samples is due tothermally activated migration of atoms or that a-Fe isformed during evolution of collision cascades. If the ther-mally activated processes play a dominant role then onecan expect that the crystalline a-Fe will not be observedafter low temperature implantation. However, the resultsobtained for 77 K B-implanted thin iron film [Fig. 4(c)]revealed - 10% of a-Fe phase. Hans et aL'17 have shownalso that complete amorphization of B-implanted iron filmwas not achieved even when the implantation was per-formed at low temperatures. The increase of the implanta-tion fluence to 5 X 1017 at./cm2 resulted in the formation ofa nonmagnetic component in the CEMS spectra, but a-Fedid not disappear. 17 Both these results clearly indicate thatthe presence of crystalline iron is not the result of thethermally activated process, but is due to the athermal

4822 J. Appl. Phys., Vol. 73, No. 10, 15 May 1993 Jagielski, Kopcewicz, and Thom6 4822 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

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TABLE 1. Mean hyperfine field (in T) for melt-spun and ion implanted samples.

Mean hyperfine field for given nominal composition

Sample Fe86 134 FeS5Bi3 Fe84B16 Fe82BI5 FC80B20 Fe75B2 5 Fe6 5B35 FewB4

Melt spun 25.2 ... 24.3 25.3 25.2 ... 18.6Implanted - 24,5 ... 24.9 24.2 22.2

Sputtered sample.

mechanism which should be related to the specific charac-ter of ion implantation.

The articles which deal with the detailed analysis ofSRO in B+-implanted layers are rather scarce, so we willbase our discussion on the studies of recrystallization ofmelt-spun samples. A recent study performed by Sanchezet al.l showed that crystalline a-Fe appears at the verybeginning of the recrystallization process of melt-spunsamples. Thus probably only a minor redistribution of at-oms is required for the transformation from a completelyamorphous structure to the a- (Fe-B) + a-Fe mixture asobserved for ion-implanted samples. Such a transformationmay be attributed to the redistribution of boron atomsfrom the homogeneous distribution characteristic of themelt-spun samples towards the amorphous regions withsmall crystalline a-Fe zones left. As this process occurseven during low temperature implantation, we suggest thatthis redistribution occurs within the displacement cascadesduring their time evolution. Such a process may create ashort-range redistribution of boron atoms proceeding in anathermal way which leads to the persistence of the residuala-Fe phase. Since, as already mentioned, a more specificdiscussion of SRO in the ion-implanted sample is not pos-sible because the detailed model of the structure of amor-phous Fe-B does not exist, we can only suggest that theredistribution of B atoms proceeds towards the vacancy-rich center of the displacement cascade leaving the crystal-line a-Fe envelope on the cascade periphery.

In Table T, the values of the average hyperfine field Hayextracted from P(H) distribution are compared for melt-spun and B-implanted samples. As can be seen, for similarnominal boron concentrations the values of Hav measuredfor ion implanted and for melt-spun samples are very sim-ilar. The average hyperfine field in amorphous Fej01 )_B, isapparently constant for nominal compositions with x < 25.Such a behavior indicates that for x < 25 the structure ofthe amorphous phase formed due to the B implantationremains essentially the same, and only its abundance in-creases with increasing boron concentration. For B con-centrations exceeding 25% the decrease of Hay is observed.Such a concentration dependence of the hyperfine fieldscan be explained by the mechanism in which the formationof the amorphous phase with Fe75 B2 5 composition even forlow B doses would be the first step of the amorphizationprocess of the Fe-B system. For low (x <25) boron con-centration, the abundance of such a phase increases withthe implantation fluence its composition remaining un-changed. Once the mean Fe75B25 composition is reachedthe further increase of boron concentration results in the

increase of B concentration in the previously formed amor-phous phase which leads to the formation of boron-richphases. It is worth noting that the irradiation of Felo-_B.with He ions performed for melt-spun amorphous ribbonsrevealed high stability of the amorphous phase of Fe75B25composition. 19

Concluding this part of discussion, it should be notedthat the results presented above provide a strong indicationthat the differences observed in the structure of the amor-phous phase of ion-implanted samples as compared to themelt-spun ones may be attributed to the redistribution ofboron towards the amorphous precipitates in the case ofion-implanted samples which leads to the persistence of thea-Fe phase. In melt-spun samples, the boron atoms aredistributed more homogeneously, which stabilizes the dis-order in the whole volume of the Fe-B system.

The discussion presented above makes possible expla-nation of the different behavior of amorphization obtainedfor thin films and bulk samples in terms of the model ofstress induced amorphization. 7 The implanted boron atomsintroduced into the surface layer of the host iron matrixcreate a compressive stress in this layer. When the stressbecomes high enough, the crystalline host matrix cannotaccommodate additional boron atoms so the supersatu-rated boron solution in the surface layer transforms toamorphous precipitates due to local structural rearrange-ments. This transformation can be denoted as a-Fe+B a-(Fe-B)>a-Fe a-(Fe-B). As it was shown inRef. (7), the onset of amorphization occurs simultaneouslywith stress relaxation, thus the stress is a driving force forthe formation of the amorphous phase. The stress inducedin thin film during its deposition on the Si substrate maylower the threshold of the implanted dose required for theformation of the amorphous phase so the amorphizationprocess may become more efficient than in the bulk sample.The stabilizing effect of the unperturbed a-Fe structure ofbulk samples below the implanted layer, which may lead tothe partial recrystallization of B +-implanted layer, can nei-ther be ignored.

IV. CONCLUSIONS

The conclusion that can be drawn from the CEMSstudy of B implantation into a-Fe is that the formation ofthe amorphous phase induced by ion implantation dependson the thickness of the implanted sample. The reason forthis dependence is most probably related to a larger stressin thin film as compared with that in the surface layer of

4823 J. Appl. Phys., Vol. 73, No. 10, 15 May 1993 Jagielski, Kopcewicz, and Thome 4823 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

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the bulk sample. The amorphization process is then moreefficient in thin films than in the bulk. Consequently, inthin film a given number of boron atoms can stabilize adisorder of a larger number of iron atoms as comparedwith a bulk sample.

Our results show also that complete amorphization ofthe surface layer was not achieved in the case of the ionimplanted samples although the mean boron concentrationexceeded that required for the formation of the Fe75B25

system. This effect can be explained by the athermal redis-tribution of boron atoms into the amorphous a-(Fe-B)phase which leads to the increased boron concentration inthe a- (Fe-B) phase and precipitation of crystalline iron. Inthe as-quenched melt-spun samples boron atoms are prob-ably distributed more homogeneously so an amorphousphase without any traces of a-Fe appears. The annealingprocess of melt-spun samples results in the redistributionof boron occurring in the very early stages of crystalliza-tion which leads to the formation of a phase structuresimilar to those obtained by ion implantation, i.e., mixtureof an amorphous a-(Fe-B) phase and crystalline a-Fe one.

ACKNOWLEDGMENTS

The authors wish to acknowledge the help of theSEMIRAMIS group from CSNSM Orsay in the course ofthe experiments. Special thanks are due to 0. Kaitasov forion implantation. This study was supported by the grants

from the Polish Committee for Scientific Research Nos.2-0419-91-01 and 7-0980-91-01.

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