Amorphous film formation by ion mixing in binary metal systems

Nuclear Instruments and Methods 209/210 (1983) 229-234 North-Holland Publishing Company

229

A M O R P H O U S F I L M F O R M A T I O N BY I O N M I X I N G IN BINARY M E T A L S Y S T E M S Bai-Xin LIU *, W.L. J O H N S O N and M-A. N I C O L E T California Institute of Technolo,~', Pasadena, California 91125, USA

S.S. LAU University of California, San Diego, La Jolla, California 92093, USA

A structural difference rule is formulated according to which an amorphous binary alloy film is formed by ion mixing of multilayered sample when the two constituent metals are of different structure, apparently independently of their atomic sizes and electronegativities. The rule is supported by the experimental results we have obtained on eight selected binary metal systems, which cover all possible combinations between b.c.c., f.c.c, and h.c.p, structures. The previous data reported in the literature also support this rule. The available experimental data indicate that the validity of the rule may extend to other structures and to production techniques other than ion mixing. The amorphization mechanism is discussed in terms of the competition between two different structures resulting in frustration of the crystallization process.

1. Introduction

It is well-known that the amorphous materials have in a number of respects unique properties, such as high strength, excellent corrosion resistance, good barrier properties against diffusion, etc. A great effort has been made in the last two decades to develop new manufacturing techniques to produce amorphous materials, as well as to promote an understanding of the amorphization mechanism. Ion mixing (IM) has recently been demonstrated to be a choice method in forming metastable materials with either crystalline structure (MX phase) or non-crystalline structure (amorphous phase). Advantages of this method are the small amount of material required for experimentation, the relatively short duration of irradiation normally required, and the ease with which the metals and compositions chosen can be changed. Furthermore, the irradiation parameters are adjustable, so that IM offers a good opportunity to investigate the amorphization process step by step. Ion mixing, therefore, is not only a powerful technique in forming metastable materials, but is also attractive in studying the mechanisms involved. Up till now, some ten metal-metal systems * Permanent address: Qinghua University, Beijing, The People's Republic of China.

have been studied by IM. However, only a few amorphous alloys have been formed in this way, and ion-induced amorphization mechanisms are not well understood [1,2]. The present was undertaken to investigate which main factors control the amorphization in binary metal system by IM. Table 1 summarizes the essential observations for eight metal-metal systems reported in the literature, which have been studied by IM using multilayered samples. The experimental procedure was similar in all these cases. Inert gas ions of a few hundred keV were used to irradiate the samples at room temperature (RT) or below to doses ranging from 1015 to 1016 i o n / c m 2. It is found that (i) mixing takes place in each system, although the amount of mixing varies from system to system, (ii) MX phase forms in each system and frequently has the same structure as one of the constituent metals, (iii) no amorphous phase has been observed in those systems where two metals have the same crystalline structure, e.g. A u - N i , Ag Cu, and A g - N i (all f.c.c, structure), and (iv) amorphous alloys form in those systems where two metals are of different structure, e.g. A u - V , C u - T a , and Au Co. The two Fe containing systems conform to those general findings, if Fe is considered to be an f.c.c, material (i.e. in the high temperature phase of Fe). These observations indicate that a difference in crystalline structure predominantly determines amorphization by IM.

0167-5087/83/0000-0000/$03.00 © 1983 North-Holland

lIl. NEW PHASES

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B - X. Liu et al. / Amorphous film formation

Table 1 Summary of metastable phase formation by ion mixing in previously studied binary systems Binary metal system

Crystalline structure

Au-Ni Ag-Cu Ag-Ni Au-V Cu-Ta Au-Co Au Fe

f.c.c.- f.c.c, f.c.c.- f.c.c, f.c.c.- f.c.c, f.c.c.-b.c.c, f.c.c.-b.c.c, f.c.c.-h.c.p, f.c.c.-f.c.c, b.c.c. b.c.c.-f.c.c, b.c.c.

W-Fe

Metastable phase formed by IM Crystalline (MX phase)

Non-crystalline (amorphous phase)

yes, f.c.c. yes, f.c.c. yes, f.c.c. yes, f.c.c, or b.c.c. no data available yes, f.c.c. yes, f.c.c, or b.c.c.

no no no yes, Au 4oV6o yes, CusoTaso, Cu2sTav5 yes, a u 25 C075 no

2 3 4 2 5 2 2

yes, f.c.c, or b.c.c.

yes, FeToW3o

6

F r o m a metallurgical p o i n t of view, crystalline structure, atomic size a n d electronegativitiy of the c o n s t i t u e n t metals are the m a i n factors that determine the character of an alloy phase. A systematic investigation is therefore desirable to cover all possible c o m b i n a t i o n s of the different structures of the metals, a n d to study the effects of atomic size and electronegativity as well. As most of the metals form in one of the three m a i n structures, b.c.c., f.c.c., and h.c.p., there are only three different c o m b i n a t i o n s to consider. Eight systems were chosen for this study (see table 2), which not only cover the three c o m b i n a t i o n s of the

Table 2 Crystallographic and atomic data on the binary metal systems selected for this study Combination of different structure

f.c.c.b.c.c. b.c.c.-h.c.p. h.c.p.- f.c.c.

Binary metal system

Al Nb Ni-Mo Ni-Nb Mo Ru Mo-Co Ti-Au Ti-Ni Er-Ni

Difference in atomic size A r*/r

Difference in electronegativity

[%]

An*/n

2% 11% 15% 4% 10% 1% 16% 30%

Ref.

m a i n structures, but also cover extreme variations in the differences of atomic size and of electronegativity values. For example, in the A I - N b , M o - R u , a n d T i - A u systems, the atomic sizes of the two c o n s t i t u e n t metals are almost identical, while in the Ni Mo a n d M o - C o systems, the difference in electronegativity is negligible. Once a system has been chosen, the next step is to select the overall composition of the multilayered samples. It has been reported that a supersaturated solid solution (MX phase) is formed by IM, when the entire composition of the multilayers is close to either end of the phase diagram, and that IM induces equilibrium c o m p o u n d formation in some systems [2]. The favorable compositions for a m o r p h o u s alloys are therefore away from both ends of the phase diagram, a n d also away from the equilibrium c o m p o u n d s . Correspondingly, the overall compositions of most of our multilayered samples were chosen to fall in twophase regions of the respective phase diagrams.

[%]

2. Experimental procedure

6% 0% 11% 18% 0% 38% 17% 33%

Multilayered samples were prepared by sequential v a c u u m deposition of alternate two-metal layers on inert substrates (SiO 2 or A1203). The total thicknesses of the samples were 5 0 0 - 9 0 0 A a n d designed to be equal to Rp q- ARp, where Rp a n d ARp refer to the projected range and projected range straggling of the incident ion [8]. 300 keV Xe + ions were used in this study. The individual layers were less than 100 A in most cases a n d the relative thicknesses of the two metals were

Remarks: r* refers to the atomic radius taken from ref. 7 and n refers to the electronegativitytaken from ref. 7. The larger r and n are used for reference.

B- X. Liu et al. / Amorphous film formation adjusted to obtain various overall compositions chosen to favor amorphous alloy formation. As-deposited samples were then irradiated at RT to doses ranging from 2 × 1014 to 2 × 1016 X e / c m 2. Some samples were preannealed and then irradiated at RT. Post-annealing was performed on the amorphous alloy films obtained by IM to find the characteristic temperature (TA_c) for which crystallization occurs on a practical time scale. All the samples were analyzed by MeV 4He+ backscattering spectrometry, X-ray diffraction (Read Camera) and four-point probe resistivity measurements.

3. Results and discussion

3.1. Formation of amorphous alloy films Some fifteen amorphous alloy films have been formed by IM (see table 3) in all above selected systems. The amorphous structure was identified by a diffuse band (or halo) on the X-ray diffraction pattern. The structure defined as amorphous here is therefore within the resolution of the X-ray diffraction technique used. The irradiation doses given in table 3 were required to induce uniform Table 3 Amorphous alloys formed in selected binary systems by 300 keV Xe + irradiation at room temperature System

Amorphous D o s e alloy 1015 Xe/

Resistivity # ~. cm

cm2

Transition temperature, TA - C

(°C) AI-Nb Ni-Mo Ni-Nb Mo-Ru Mo Co Ti-Au Ti-Ni Er-Ni

AI55Nb45 Ni 65M°35 NisoMos0 Ni3~Mo65 Ni65Nb35 Ni55Nb45 Ni35Nb6s Mo55Ru45 C°65M°35 Co35Mo65 Ti65Au35 Ti 4oAu6o TisoNis0 Er65Ni35 Er4oNi60

4 7 3 17 5 5 5 10 7 10 5 4 5 5 5

250 170-210 150-200 130 230 320 300 320

350-400 480-500 600-630 580-600 500-550 600-650 550-600 500-550 550-580 610-630 400-430 350-400 450-500 400-500

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mixing and amorphization of the discrete multilayers. The dose range is ( 3 - 1 7 ) × 1015 X e / c m 2, which corresponds to a rough estimated dpa (displacement per atom) range of (30-160) [9]. The resistivities of the ion mixed films are on the order of 130-320 /~I2.cm, which is higher than those expected of crystalline alloys, hence lending support to the X-ray data that the films are of amorphous structure. The amorphous to crystalline transition temperature (TA_c) is defined here as that temperature at which after 0.5 h of vacuum annealing the first detectable indication of the presence of a crystalline state is observed in the X-ray film. The phase transformation upon postannealing was monitored by X-ray diffaction and resistivity measurements at RT. The TA_c of most of our amorphous alloy films are on the order of 400-600°C. The formation of the amorphous alloys in all above selected systems clearly demonstrates that a difference in structure of the constituent metals plays a key role in ion-induced amorphization, and that atomic size and electronegativity effects are minor. This leads us to formulate a rule that an amorphous binary alloy film will be formed by IM of multilayered sample, when the two constituent metals are of different crystalline structure. We propose to name this rule the "Structural Difference Rule for Amorphous Alloy Formation" by IM.

3.2. Compositional and pre-annealing effects on amorphization Two systems, N i - M o and Ti-Au, were chosen to study the effect of overall composition of multilayers and the effect of the presence of equilibrium compounds on ion-induced amorphization. Fig. 1 shows the N i - M o equilibrium phase diagram (top), an ion-induced metastable phase diagram (bottom) showing graphically the phase changes as a function of dose and the overall composition of the multilayers, as well as the TA_ c temperature as a function of composition of the amorphous alloys so obtained (middle). From these we draw the following conclusions: (i) The NisoMos0 amorphous alloy is formed after a dose of 3 × 1015 X e / c m 2 irradiation. This dose is the lowest one required among the three compositions. Yet, this amorphous film has the highest TA_c temperature. Apparently, that composition near III. NEW PHASES

B-X. Liu et al. / Amorphous film formation

232

pattern. The pre-annealed and an as-deposited samples were then irradiated at RT to the same dose of 5 × 10 ~5 X e / c m 2. Amorphous alloys were formed in both cases. This indicates that the presence of equilibrium compounds does not prevent the amorphous alloy formation by IM in this system. However, it has been reported [6] that in the F e - W system, if the FeTW6 compound is formed first by pre-annealing, its presence will hinder the subsequent amorphous alloy formation by IM. The presence of an equilibrium compound therefore varies in its effects from system to sys-

o

E

#

20

o N, =

700

m o ~

600

40 60 80 A t o m i c Per Cenl Mo

IO0 Mo

tern.

~, *~ 500

3.3. lon induced amorphization mechanism 1

t ~

20

L

40

I

60

J ElO L

Mo

Ni

$

o

•

i

160

lJ

X tO 2 o v @

A~o~phoo~ g

i~v

E o *o

~00

50

~3 (~

M X , I~c p

30

Mo, b cc N 69Mo35 N%oM(soN,3~Mo65

overoll c o m p o s i h o r of m u l h l o y e r s

21(;, Ni

40

60

8C Mo

Fig. 1. The N i - M o equilibrium phase diagram (top), the TA c temperature as a function of composition of amorphous films (middle), and a chart displaying the ion-induced phases chart as a function of irradiated dose and composition of muhilayered N i - M o samples.

NisoMos0 is optimum for formation of amorphous alloys from the point of view of thermal stability. (ii) An MX phase of h.c.p, structure is formed simultaneously with an amorphous phase in the Ni-rich multilayered sample during the early stages of irradiation. Eventually, the whole film becomes a single amorphous phase. (iii) In Mo-rich multilayers, the Mo phase gradually disappears from the amorphous phase matrix with increasing irradiation dose. The whole mixed film becomes totally amorphous after a dose of about 1.7 × 1016 X e / c m 2 irradiation. To check the effect of the presence of equilibrium compound on amorphization, one multilayered sample of Ti65Au35 (between neighboring equilibrium compounds Ti3Au and TiAu) was pre-annealed at 600°C for l h and Ti3Au and TiAu were formed as revealed by X-ray diffraction

It has been suggested [10] that the structure of an eutectic liquid near the melting point can be described in terms of two types of competing short range order. If the size of the short-range order is not too large, this liquid has unusually low free energy, and therefore an amorphous phase can be obtained by quenching this liquid. Similar ideas, we believe, may also apply to the formation of amorphous solids by IM, where two short-range orders are related to the two different crystalline structures. The ion induced amorphization process is thought to occur as follows: Initially, a sequence of ballistic collisions is triggered by the penetrating primary particle. This takes place in about 10 ~2 s [11]. As the energy is transferred among a progressively larger number of atoms, one reaches a situation where an average local energy per atom, or a "temperature", may be defined. The thermally excited local region subsequently cools by thermal conduction processes which dissipate energy to the surrounding material. This is expected to occur in the time period of the order of 10-11 s [9]. During this relaxation period, an atom could travel by a random walk assuming the atom jumps by atomic distances with a frequency comparable to the Debye frequency (v D--- 101210 ~3 s -~) [12]. The atom makes 10-100 jumps in 10 11 s and would move an average of 3 10 atomic distances, i.e. 10-30 ,~. One thus expects two types of locally ordered regions intermixed on a spatial scale 10-30 A. This scale can in fact turn out to be smaller than the critical nucleus size (typically 20-30 A) for either of the competing crystalline phases as defined in classical nucleation theories [13]. Crystal formation may thus be inhibited if no

B - X. Liu et al. / Amorphous film formation

significant further atomic motion takes place. According to this model, it is the competition between two local structures that results in frustration of the crystallization process, and an amorphous phase is so obtained. It is well-known that the structure of pure transition metals follows a systematic pattern as one crosses a given transition series. In the second and third long periods of the periodic table (e.g. the 4d- and 5d-metals) where magnetic effects do not enter, this trend can be summarized as follows [14,15]. On going from left to right, the equilibrium structure of transition metals is first h.c.p. (e.g. Y, La, Zr, Hf, groups 3 and 4), then b.c.c. (e.g. Nb, Ta, Mo, W, groups 5 and 6). As one passes mid-series, one again has the h.c.p, structure (Tc, Re, Ru, Os, groups 7 and 8), then the f.c.c, structure (Rh, Ir, Pd, Pt, Ag, Au, groups 9-11). For the 3d-metals, this trend is somewhat modified by magnetic effects. One sees that the structure of transition metals depends on group number or, in other words, on the occupation of the d-electron states. The structural difference rule might thus be reformulated as a valence difference rule. For example, when one mixes transition metals with group number 5 or 6 with those of groups 7-11, one expects an amorphous phase. This corresponds to mixing a b.c.c, metal with an h.c.p, or f.c.c, metal. This suggests that the underlying mechanism for amorphization may be of electronic origin.

3.4. The validitiy of the structural difference rule The experimental evidence offered thus far in support of the structural difference rule includes the metals with b.c.c., f.c.c, and h.c.p, structure and the process of IM. Available experimental data indicate that the rule is valid beyond these specific conditions. For example, multilayered samples combining Si (diamond-type f.c.c.) with Au (f.c.c.) up to 70 at% of Au [16,17], Mo (b.c.c.) in MosoSis0 composition [18], and Ru (h.c.p.) in Ru45Si55 composition [18] also produced amorphous films upon Xe irradiation. GesoAu20 and Ge60Au40 multilayers were also amorphized by Xe irradiation, although the amorphous films recrystallized within a few days at RT [19]. It appears, therefore, that the rule extends beyond simple metallic structures of the b.c.c., f.c.c, and h.c.p. type.

233

It should also be pointed out that the structural difference rule is closely related to the eutectic criteria for amorphous phase formation first introduced by Turnbull [20,21]. The simple eutectic phase diagram is formed between two metals having a different structure and no indermediate equilibrium compounds. In this case, the terminal solid solutions have a different crystal lattice. In the two-phase portion of the phase diagram, any single phase crystalline form of the alloy has a higher free energy than the two-phase equilibrium alloy. When one attempts to synthesize a chemically homogeneous material (e.g. by IM) under kinetically restricted conditions, the amorphous phase may be favored since chemical separation may be kinetically limited. The free energy of a homogeneous crystalline mixture may be higher than that of the amorphous homogeneous mixture, thus preventing diffusionless crystallization [22]. Therefore, the amorphous phase formation is favored in such two-phase regions of binary phase diagrams. In fact, in a list of about 30 known binary metallic glasses obtained by rapid quenching, and of some 30 predicted to be relatively easy glass formers [23], only 7 of them do not conform to this rule. The predictive power of the rule based on that simple comparison is excellent. We therefore believe that the structural difference rule may be useful as a guide to the synthesis of amorphous materials by processes other than ion mixing as well. Another intriguing question is whether and how the structural difference rule can be generalized to ternary systems. One could argue that all the amorphous phases observed here exist only because they are stabilized by limited amounts of light impurities (e.g. C, N and O). X-ray and backscattering spectrometry are not sensitive enough to detect such impurities, so that on the basis of the present experimental data, this interpretation has not been refuted. It seems unlikely, however, that a fortuitous addition of impurities would invariably result in the stabilization of an amorphous phase in 15 different systems. Futhermore, we have irradiated elemental Ni, Nb and Mo films at RT together with multilayered samples of these metals, without observing amorphous phases in the metal films. These metals can be amorphized in an impure state. With this rule in hand, it is anticipated that a great variety of amorphous binary alloy films could be formed by IM to respond to the requirement of lII. N E W P H A S E S

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B- )d Liu et al. / Amorphous film formation

various properties. For example, Mo and Ru are of different structure and our rule predicts that amorphous films should be obtainable by IM. A Mo55Ru45 amorphous film was indeed formed without adding any metalloid element as the liquid quenching technique requires [24] to decrease the melting point of the alloy. The Mo55Ru45 amorphous film showed a superconducting transition temperature of Tc = 7.4 K and a critical current density on the order of 10 4 A / c m 2 [25].

4. Conclusions (1) A structural difference rule is formulated, according to which an amorphous binary alloy film will be formed by ion mixing of multilayered sample when the two constituent metals are of different structure. (2) The appropriate compositions for amorphous alloys are in the middle of the two-phase regions of the respective phase diagrams. (3) The structural difference rule may be useful as a guide to the synthesis of amorphous materials by processes other than ion mixing as well. The authors thank R. Fernandez for assistance in the preparation of the samples, and U. Shreter and R. Gaboriaud for help in irradiation and measuring of some samples. This work was executed under the benevolent U.R. Fund of the B/3hmische Physical Society (B.M. Ullrich). The irradiation part of the study was financially supported in part by the U.S. Department of Energy through an agreement with the National Aeronautics and Space Administration and monitored by the Jet Propulsion Laboratory, California Institute of Technology (D.B. Bickler).

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[21 B.Y. Tsaur, S.S. Lau, L.S. Hung and J.W. Mayer, Nucl. Instr. and Meth. 182/183 (1981) 67. [3] B.Y. Tsaur, S.S. Lau and J.W. Mayer, Appl. Phys. Lett. 36 (1980) 823. [4] B.Y. Tsaur and J.W. Mayer, Appl. Phys. Lett. 37 (1980) 389. [5] B.Y. Tsaur, Ph.D. Thesis, California Institute of Technology (1980). [61 G. G6ltz. R. Fernandez, M-A. Nicolet and D.K. Sadana, in: Metastable materials formation by ion implantation, eds., S.T. Picraux and W.J. Choyke, MRS Symposia Proc. vol. 7 (North-Holland, Amsterdam, 1982) p. 227. [7] Table of Periodic Properties of the Elements (SargentWelch Scientific Company, 1968). [8] G. Dearnaley et al., in: Ion implantation (North-Holland, Amsterdam, 1973) p. 766. [9] M.W. Thompson, Defects and radiation damage in metals (Cambridge University Press, 1969) Ch. 4 and 5. [10] W.L. Johnson, in: Metastable materials formation by ion implantation, eds., S.T, Picraux and W.J. Choyke~ MRS Symposia Proc., vol. 7 (North-Holland, Amsterdam, 1982) p. 183. [11] S. Matteson and M-A. Nicolet, in: Metastable materials formation by ion implantation, eds., S.T. Picraux and W.J. Choyke. MRS Symposia Proc. vol. 7 (North-Holland, Amsterdam, 1982) p. 3. [121 N.W. Ashcroft and N.D. Mermin, Solid-state physics (Holt, Rinehart and Winston, New York, 1976) p. 461. [I 3] J.W. Christian, Theory of phase transformations in metals and alloys (Pergamon Press, New York, 1965) Ch. X. [14] C.H. Hayworth and W. Hume-Rothery, Phil. Mag. 3 (1958) 1013. [15] W. Hume-Rothery, Prog. Mat. Sci. 13 (1967) 231. [161 B.X. Liu, L. Wielufiski, M. M~enpfiS, M-A. Nicolet and S.S. kau, in: Metastable materials formation by ion implantation, eds., S.T. Picraux and W.J. Choyke, MRS Symposia Proc. vol. 7 (North-Holland, Amsterdam, 1982) p. 133. [17] B.Y. Tsaur and J.W. Mayer, Phil. Mag. A43 (1981) 345. [18] B.X. Liu et al., to be published. [19] B.X. Liu and M-A. Nicolet, Phys. Star. Sol. (a) 70 (1982) 671. [20] D. Turnbull, Contemporary physics 10 (1969) 473. [21] F. Spaepen and D. Turnbull, in: Rapidly quenched metals If, eds., N.J. Grant and B.C. Giessen (MIT Press, Boston, 1976) p. 205. [22] D. Turnbull, private communication (1982). [23] I.W. Donald and H.A. Davies, J. Non-Crystalline Solids 30 (1978) 77. [24] W.L. Johnson and A.R. Williams, Phys. Rev. B20 (1979) 1640. [251 B.X. Liu, B.M. Clemens, R. Gaboriaud, W.L. Johnson and M-A. Nicolet, presented at the MRS Annual Meeting, Boston, November 1-4, 1982.