Nuclear Instruments and Methods in Physics Research B64 (1992) 846-8.51

North-Holland

Nuclear Instrwnents 8 Methods

in Physics Research Sectior? 5

PAC studies of ion beam induced mixing and phase formation in Ni-Al multilayers

The perturbed y-ray angular correlation (PAC) technique with radioactive tracers is a novel method of investigating early stages

of ion beam induced phase transformations, visible as changes in the hyperfine interaction parameters of the probe nuclei. In this

study Ni-AI multilayers of different composition ratios doped with implanted “‘In ions were irradiated with 900 keV Xe’+ ions

and analysed by means of PAC. Room temperature bombardment of samples with the ratio I:3 or 3: I produced amorphous or

highly disordered phases, while the 3:2 phase was observed to become partly amorphous. Amorphisation was also found for L.N

irradiation of Ni-AI in the ratio 1 : 1, contrary to the formation of the crystalline NiAl phase at RT. These results are discussed in

the light of the N&AI bilayer mixing rates and current ion mixing models.

1. Introduction

During recent years, ion beam mixing (IBM) [l-5] has been established as a method well suited to form

metastable alloys with crystalline or amorphous struc- ture. Exposed to heavy noble gas ion irradiation, a multifayer consisting of alternating layers of two metals may form new phases during the mixing process. Mix- ing at temperatures low enough to prevent radiation enhanced diffusion is commonly attributed to the com- bined effect of binary collision cascades and interdiffu- sion within thermal spikes. In the ballistic models, mixing originates from binary collision sequences at energies larger than a few eV. Thermal spike diffusion refers to the collective motion of low-energy ( < 1 eV> recoils in the cooling phase of the cascade. Spikes are expected to be the predominant mechanism for targets with a high mass number and for heavy ion irradiation. Although IBM is an important prerequisite for the understanding of phase formation, up to now it is not understood in sufficient detail.

Several empirical rules have been proposed to pre- dict which phases are formed by IBM. As Hung and coworkers pointed out, the structural and chemical complexity of the alloy phase plays an important role, and compfcx phases will become amorphous [6]. Liu et al. proposed that an am~~rphous alloy may be formed, whenever the constituent metals have different crys- talline structures 171. In the case of a narrow phase field or a line compound, again amorphisation is favoured [5]. However, exceptions exist for all of these rules.

Ion beam induced phase transformations are usu- ally verified via electron microscopy and X-ray diffrac-

tion. Other microscopic methods like Mijssbauer spec- troscopy and perturbed angular correlation (PAC) with appropriate nuclear tracers increasingly find appfica- tion [8,9]. Both methods test the ion beam induced variation of the hyperfine fields acting on the probe nuclei. The hyperfine parameters describe the micro- surrounding of the probe nuclei: either the (mean) quadrupofe precession frequency U, and the asymme- try parameter q in the case of an electric field gradient (efg) acting on the probe nuclei, or the (mean) Larmor frequency u L in the case of the magnetic hyperfine field. In both cases the width S of the frequency distribution around w is a measure of the focal order. Well-defined crystallographic sites result in typically S I 1 MHz; therefore S in a way represents the lattice defects [IO]. PAC spectra from amorphous samples arc character&d by broad frequency distributions as a consequence of the different distances and coordina- tions of the nearest-neighbour atoms. As the average w values differ for different alloys, they give access to parameters like a mean atomic distance in the nearest

neighbourhood [8]. In the Ni-Al equilibrium phase diagram, four inter-

mctaflic phases are present [If]: NiAf is a highly or- dered B2 alloy of CsCl structure, with a wide phase field in the range cgi = 45-60 at.%. It is stable under room-temperature Xe irradiation [6]. On the other hand, Ni,Al fc,, = 73-76%, cubic) and NiAI, (cNi = 750/r, orth~)rhombic) become amorphous under Xe bombardment; the Ni,Af, phase (ctii = 36.8-40.50% hexagonal) is partly amorphized [h]. The formation of the crystalline NiAl phase during room temperature irradiation of Ni-AI multilayers has been observed by Jaouen et al. [12].

0168-583X/92/$05.00 0 1992 - Elsevier Science Publishers B.V. All rights reserved

T. Weber, K.-P. Lieb / IBM and phase formation in Ni-Al multilayers x47

In a previous study on room temperature Xe mixed Ni-Al bi- and multilayers [13], we have demonstrated that the PAC method with implanted “‘In tracers gives rather detailed information on the mixing process and the formation of the crystalline NiAl phase. The present study extends these experiments by varying the relative thicknesses of the Ni and Al layers in order to adjust the compositions of other Ni-Al intermetallic phases. Furthermore, detailed analyses of the PAC spectra were undertaken in order to investigate various hyperfine fractions as function of the ion ence. The effect of the substrate temperature studied by performing the ion irradiations at 80 300 K.

the flu- was and

2. Experimental details

Thin Ni- and Al-films were deposited at roop tem- perature and at a deposition rate of about 3 A/s by sequential electron gun evaporation. The base pressure in the vacuum chamber prior to evaporation was 5 x

lop9 mbar and rose to about 5 X 10K8 during deposi- tion. The multilayer stacks consisted of 4-6 periods of Ni and Al on Si substrates, with the top layer always being Ni to prevent oxidation. The individual layer thicknesses ranged from 10 to 40 nm. They were ad- justed to obtain the desired compositions NiAl, Ni,AI, NiAI, and Ni,Al, after mixing. Each stack had a total thickness of about 200 nm which matched the depth

ooq / , I

.?.I NIAI

0 06 i

I I

z 0 OOk

ir I 1’ b)

t NI-AI

0 06

distribution of the implanted “‘In ions having a pro- jected mean range of 115 nm and distribution width of 80 nm (FWHM) in NiAI [141.

The multilayers were implanted at room tempera- ture with “‘In-ions of 400 keV energy and a dose below 3 X IO’” ions/cm’. Afterwards the samples were gradually mixed with 900 keV Xe’+ ions, up to a dose of 8 x 10” ions/cm’. All implantation fluences were kept below 1.5 kA/cm*. PAC spectra were taken directly after the In-implantation and after the subse- quent Xe-irradiations. The radioactive “‘In tracer de- cays via electron conversion to its daughter nucleus “‘Cd followed by a 171-245 keV y-ray cascade involv- ing the isomcric 245 keV 5/2-state. A conventional four NaI(TI) detector setup was used. A detailed de- scription of the electronics, data analysis and fitting procedures is given in ref. [lo]. Mixing of the multilay- ers was monitored via Rutherford backscattering spec- trometry (RBS), using a 900 keV a-particle beam. All In-implantations, Xe-irradiations and RBS-analyses were performed by means of the Gettingen 530 kV ion implanter IONAS [15].

3. Results

3.1. NiAl

Fig. 1 shows two PAC perturbation functions and corresponding Fourier transforms obtained after 900

Frequency (MHz)

Fig. 1. PAC perturbation functions and Fourier transforms of an “‘In doped Ni-AI 1: 1 multilayer after (a) and before (b) 900 keV

room temperature Xe mixing.

XV. NEW DEVELOPMENTS

848 T. Webrr, K.-P. Lieh / IBM and phusr jiirmution in Nt-Al multilayers

Table 1

Fitted hyperfine interaction parameters at 300 K and site

identifications for equiatomic NiAl multilayers

~1 [MHz1 7 Site identification Host

93(2)” - substitutional Ni

0 0 substitutional Al, NiAl

54 (10) 0.52 In,-VA, Al

130 (6) 0 monovacancy NiAl

142 (6) 0.2 monovacancy+ antisite atom NiAl

IX8 (8) 0.64 divacancy NiAl

220 (8) 0.89 divacancy NiAl

330 (10) _ 0.7 (divacancy) NiAl

” Larmor frequency.

keV Xe-bombardment with 4 X 10’” ions/cm’ at room temperature (fig. la) and immediately after the In-im- plantation (fig. lb). The four Ni and four Al layers were 20 and 30 nm thick, respectively. The upper perturbation function shows the frequency pattern typ- ical for “‘Cd-impurities in crystalline NiAl [16]. The fitted quadrupole frequencies and their tentative site identifications are given in table 1, following the work of Fan and Collins [16]. Most of these microsurround- ings contain vacancy-type defects trapped at the In- probes. The pertubation function shown in fig. lb exhibits the well-known 93 MHz Larmor frequency of substitutional “‘Cd nuclei on the Ni lattice site [17] and its second harmonics. Probe nuclei on substitu- tional Al lattice sites are characterised by a quadrupolc frequency distribution around zero (due to distant irra- diation defects) and a second fraction with w = 54 MHz known to indicate the formation of the In,-V,, complex where substitutional In has now trapped a NN vacancy [18]. The fit to the data of fig. lb can be improved by including a fraction with the efg typical for AlNi and with a very broad frequency distribution (c5 = 24 MHz). This large width indicates a highly dam- aged In neighbourhood with Ni arid Al atoms. Due to the very broad distribution, one cannot clearly distin- guish between a highly disordered crystalline structure and amorphisation.

The variation of these hyperfine parameters as a function of the ion fluence @ is displayed in fig. 2. The AlNi fraction steadily rises with @, while the Ni signal decreases correspondingly (see fig. 2a). As the efg of substitutional “‘Cd in the defect-free cubic Al and AlNi lattice vanishes, the nearly constant fraction with w = 0 cannot be used to follow the phase transforma- tion. Fig. 2b illustrates the dependence of the average width 6 of the NiAl signals on the Xc fluence: the decrease of (I?) for increasing fluencc indicates an ordering process.

The PAC spectrum of a NiAl sample irradiated at 80 K shows a broad efg distribution. It can be fitted by assuming two ‘*amorphous” fractions with (w) =

24 -

20

p 16 ;’ f

2 1 12- v5

8- t-

0 / 1o15 Xe/cm2 Fig. 2. (a) The site fractions fitted for RT mixing of a Ni-AI

1 : I multilayer as a function of Xe dose. (b) The efg distribu-

tion width 6 decreases with Xe dose, indicating an ordering

process.

175(10) MHz and (w) = 210(12) MHz and a 25% frac- tion typical for crystalline NiAl, but again with broad frequency distributions. When post-irradiating this sample at 300 K, the spectrum for “crystalline” NiAl reappears (fig. 3). This process can be repeated.

3.2. NiAl 1

The perturbation function obtained after irradiating a Ni-Al 6 X -stack in the 1 :3 atomic ratio cd,,: 9 nm, dA,: 40 nm) at room temperature with 4 X lO”/cm’

10

‘; - 80 K

o 8- -----300 K

6-

Frequency (MHz)

Fig. 3. Fourier transforms of a Ni-AI I : 1 multilayer mixed

with 4~ IO’” Xe-ions/cm 2 at 80 K (solid line) and after

subsequent room temperature mixing with the same dose (dashed line).

T. Weber, K.-P. Lieb / IBM and phase formation in Ni-Al multilayers 849

b) NI,Al, 3 36

i L 1

o---%---do t (ns)

r

t

Frequency (MHz)

Fig. 4. PAC spectra of Ni-Al multilayers of different compositions, mixed with 4X 10” Xc/cm* at room temperature. The

patterns remained stable at higher fluences.

Xe-ions is displayed in fig. 4c. It is typical for probes in an amorphous surrounding and can be fitted by a 73%-fraction with a frequency of (wt ) = 146(8) MHz, a width of S = 45(8) MHz, and a 27% amorphous fraction with (w,) = 270(10) MHz and S = 55(E) MHz.

The perturbation function of a 14 x -stack with lay- ers in the atomic ratio 3: 1 (dNi: 20 nm, d,,: 10 nm) mixed at room temperature with 4 X 10’” Xe-ions/cm’ is displayed in fig. 4a. Again a rather broad frequency distribution is observed which, however, cannot be fitted with a single “amorphous” distribution. Com- pared to NiAl,, its shape obviously is non-Gaussian, but shows a peak at 67 MHz. This kind of distribution is typical for a highly disordered surrounding [lo]. It

can be fitted by assuming that 75% of all In atoms are embedded in a crystalline surrounding with w1 = 67(4) MHz, S = 25(5) MHz, 77 = 0.6, the remaining probe atoms being subject to an “amorphous” environment with (w ,> = 195(10) MHz and S = 30 MHz.

Table 2

Fitted hyperfine interaction parameters for mixed non-

equiatomic NikAl multilayers

Composition w, NH4 6 [MHz1 ?I

Ni,AI 61 (4) 2.5 (5) - 0.6 Ni,AI 195 (101 a 30 (5) _

Ni,AI, 200 (151 a 40 (10) _

NIAI, 146 (8) a 45 (10) _

NiAI, 270 (10) a 55 (15) _

a Amorphous.

XV. NEW DEVELOPMENTS

Room tempcraturc irradiation of a 10 x -stack of multilayers in the 2: 3 atomic ratio cd,,: 15 nm, d,,: 34 nm) with 4 x 10’” Xe-ions/cm’ resulted in the PAC spectra shown in fig. 4b. It contains a 55% fraction typical for crystalline NiAi and a 45% fraction with an amorphous surrounding of w, = 20005) MHz and 6 = 40(1(l) MHz. A summary of the hyperfinc interaction parameters is given in tahics 1 and 2.

4, Discussion

In our previous Ni-Al bilaycr mixing experiments with Ar-, Kr- and Xe-ions [13,19], we have noted a linear dependence of the mixing rate k on the dc- posited energy F,,. For Xc-ions, the experimental mix- ing rate determined at room and LN temperature, k = 4.6(2) nm’, is in rather good agreement with the prediction of the thermal spike model by Johnson and coworkers 131: k = 4.3 nm” at F,, = 4 keV/nm. Ma and coworkers excluded a ballistic process from being re- sponsible for the Xe-mixing of Ni-AI [20]. From their results they draw the conclusion that the thcrmai spike mechanism is dominating for biiaycr systems with an average 2 2 20. However, the linear dependence of the mixing rate on F,, is in sharp contrast to the spike modci which predicts k to bc proportional to Fi 131. A similar linear relationship has also been reported for Ti-layers on steel mixed with ions ranging from ‘“N to ‘“?Xc [21].

We shall now try to correlate these high bilayer mixing rates with the observed hyperfinc fractions and phases found in the multilayer irradiations. The PAC spectrum obtained immediatciy after In-impi~‘ntation into an as-deposited I : I multilayer already shows a considerahic fraction with cfg parameters typical for crystalline but highly damaged NiAI. They can be well distinguished from the hyperfine frequencies observed after implantation into the constituents Ni [lo] or Al [lg]. Due to the low In-dose, RBS is not able to dctcct any mixing caused by the In tracer ions. It is tempting to interpret the observation of NiAl signals indicating local NiAl phase formation at the end of the in&~idual “‘In cascades as being a consequence of the correlated damage produced by each single In and in contrast to the uncorreiated damage initiated by the other In ions or subsequent Xe b(~mbardment. The efg distributions are very broad (see fig. lb) which points to a highty damaged phase. At this stage it is not clear to what extent the chemical nature of the tracer influences the rearrangement of the Ni and Al atoms in the cooling down process. X-ray diffraction and electron mi- croscopy data probably arc not able to further eluci- date this point.

For increasing Xc tlucnce, NiAi formation can clearly be followed by the disappearance of the Ni signal and increasing NiAl fraction (fig. 2a) and the narrowing of the frequency distribution typical for the ordering process (fig. 2bI. Injecting more and more Al atoms also changes the Curie temperature of Ni which strongly depends on the amount of dissolved Al [1 l]. This may explain the change in the Larmor frequency from 93 MHz in the “‘In-doped case to 75 MHz after Xc-irradiation with I.6 x IO” ions/cm’, and the dis- appearance of the Ni signal cvcn if intermixing is not compietc, as verified by the RBS data [13].

For the completely mixed NiAi layer, only 5% of probing atoms are sitting in an undisturbed cubic NiAl lattice corresponding to vanishing efg. This observation sheds some light on the empirical rule that a wide phase field favours the formation of the crystalline phase under irradiation [S]: there must be a mechanism that compensates for deviations in stoichiomctry. In NiAl the dominant equilibrium defects are structural Ni vacancies on the Ni-poor side of stoichiometry, and Ni antisitc atoms on the Ni-rich side [22]. Indeed, most quadrupote frcquencics in NiAl have been assigned to vacancy-type defects [16] which we also set in our experiment. The lattice being so highly damaged and not turning amorphous necessarily means that the sys- tem tolerates partial disorder. It is we11 known that highly ordered 02 alloys have a large activation energy for diffusion [23] which is due to a high ordering energy, and thcrcforc have a kinetic advantage in rc- covering from cascade damage.

For compounds with a small phase field (Ni,AI,, Ni,AI) or line compounds (NiAI,) this is no longer true. As obscrvcd in our experiments, they become highly disordered (Ni,AI) or even turn amorphous (NiAi,). When comparing Ni,Ai and NiAI,, the influ- cnce of the complexity of the crystal structure can bc studied. While the cubic Ni,Ai compound is disor- dercd, but still crystalline, the ~~rthorhombic NiAI, structure is too complex to be formed within the 10 ” s of each individual cascade: this sample bc-

comes amorphous. The phases observed after 4 X 10” Xc/cm’ irrndi-

ation agree well with the results of Hung et al. [h]. These authors invcstigatcd the stability of the same four crystalline Ni-AI intermetallic phases under Xc bombardment at room temperature. The agrecmcnt is worth noting, since the starting points for the expcri- mcnts - crystattinc ir~t~rrnetaiiic structures versus metal muttiiaycrs -. are different and up to now it has not been clear whether ion irradiation of a given Ni-Al alloy would produce the same phases as ion irradiation of multilayer stacks in the appropriate stoichiomctry.

The iniluencc of the temperature on phase forma- tion has been discussed by several authors [5]. A model recently proposed by de Rcus et al. [24] predicts crys-

T. Weber, K.-P. Lieb / IBM and phase formation in Ni-AI multilayers 851

talline phases to be formed or to be stable above a critical temperature r, at which both constituents be- come mobile. This temperature depends on the va- cancy formation enthalpy of the larger of the compo- nents. Consequently, T, is at least as high as the temperature ‘& at which radiation enhanced diffusion sets in (where only one component has to be mobile which in the present system would be Nil. For Ni-Al this model is not supported by our results. Since no difference in the mixing rates at SO and 300 K was observed, the atomic motion responsible for crystalline phase formation at 300 K and suppressed at 80 K must be on a sub-nm scale which cannot be resolved by

RBS.

[41

151

I61

[71

[81

[91

[lOI

[ill

In conclusion, the present PAC experiments have revealed a number of details in the mixing process of Ni-Al multilayers with Xe-ions which so far have es- caped detection. Evidently, a number of problems have to be pursued, like the amount of disorder produced by single “‘In tracer ions and the possible role of the tracer itself in the ion mixing process. TJZM and X-ray analysis should be used to characterize the individual stages reached during the mixing process, in compari- son with the PAC results. As the hyperfine interaction of i”In tracers is very sensitive to vacancy-type point defects, we expect basic new insights to be gained in the early stages of the ion mixing process. Such experi- ments are in progress.

I121

1131

iI41

[151

[161

[171

Acknowledgements

The authors wish to thank D. Purschke for his efficient help in performing the irradiations. Discus- sions with T. Wenzel, D. Wiarda, A. Bartos and M. Uhrmacher on the interpretation of the PAC data are gratefully acknowledged.

[181

[191

t201

[211

[221

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XV. NEW DEVELOPMENTS