Amorphization of metallic systems by ion beams

Materials Science and Engineering, 69 (1985) 95-103

95

Amorphization of Metallic Systems by Ion Beams* P. ZIEMANN

Physikalisches Institut, Universit6t Karlsruhe, D-7500 Karlsruhe (F.R.G.) (Received September 17, 1984)

ABSTRACT

Ion beam techniques can be used to produce amorphous metallic systems either by implanting a glass-forming species into a preexisting crystalline metal or by irradiating a crystalline c o m p o u n d with inert ions. Ion beam mixing experiments in which a crystalline multilayered structure is bombarded with inert ions are related to the latter method. Examples of these different methods are given and empirical rules, which may help to predict whether a given system can be forced into an amorphous phase by ion beam techniques, are discussed. Special emphasis is placed on the question o f whether energy spikes are necessary for amorphization. In this context, new results o f low temperature ion irradiation experiments on Al2Au are reported.

1. INTRODUCTION

One field in solid state physics and materials research which has grown most rapidly during the last few years is the study of the amorphous state of matter. In this field of research the amorphous metallic systems represent an important subgroup. The reason for the large a m o u n t of activity on this topic is due not only to the fundamental questions of the physics involved b u t also to the possible applications of these materials. For discussions of this latter point, the reader is referred to refs. 1-3. From a pure physics point of view, one of the basic questions is the behaviour of a system lacking translational symmetry. For example, what are the conse*Paper presented at the International Conference on Surface Modification of Metals by Ion Beams, Heidelberg, F.R.G., September 17-21, 1984. 0025-5416/85/$3.30

quences on the electronic and vibrational excitations? In this context, new low energy excitations (so-called two-level systems) were observed and these led to interesting low temperature anomalies. A recent review of this aspect can be found in refs. 4 and 5. Another question is h o w transport properties such as electrical resistivity are influenced and similaxly h o w magnetism or superconductivity is affected. To approach questions such as these, a structural model of the amorphous state is necessary. This is almost a field in itself and combines sophisticated computer simulation techniques with highly refined X-ray, neutron and electron diffraction experiments. Here, the question of short-range order is of current interest. This question has been tackled experimentally also using nuclear techniques such as MSssbauer spectroscopy, perturbed angular correlation, positron annihilation and extended X-ray absorption fine structure. Details can be found in refs. 6-8. However, before any of these sophisticated experimental methods can be utilized, the problem arises as to which systems can be forced into the amorphous state. The term " f o r c e d " indicates that this state is highly metastable and in most cases can be obtained only by rapid quenching techniques. Exciting exceptions to this rule of t h u m b have been observed recently [9, 10]. The most convincing way to solve the above problem is an ab initio calculation and this approach has been very successfully applied to metallic glasses consisting of s and p band metals [11]. However, for the majority of the amorphous systems, some empirical rules and correlations have to be relied on. Some of these rules depend on the properties of the constituents of the amorphous systems and therefore they are independent of the preparation technique. Examples of this t y p e of rule include the atomic radii of the constituents [12, 13], the Q Elsevier Sequoia/Printed in The Netherlands

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Miedema coordinates [14, 15], the ionicity [16] or the corresponding metastable [17] or equilibrium phase diagrams [18, 19]. Other rules depend on the specific preparation techniques such as the valence difference rule for splat cooling [20] or the structural difference rule for ion beam mixing [21]. Although splat cooling of a melt by the spinning wheel or hammer-and-anvil method is the most widely used technique, the following alternatives have also been found to result in amorphous systems: vapour quenching, laser quenching, sputtering and energetic particle bombardment. In the special case of ion bombardment a similar relationship to that in sputtering is assumed to exist [22, 23]. If especially heavy ions are used, it has been shown for the Te-Au system that ion bombardment at low temperatures and vapour quenching onto cooled substrates both lead to the amorphous phase, while laser quenching and splat cooling resulted in a metastable crystalline phase [24]. This suggests a hierarchy of quenching rates which are characteristic for each technique; vapour quenching and heavy ion bombardment exhibit the highest rates, significantly higher than those of laser quenching or splat cooling (the rates for these last two techniques being roughly the same). This hierarchy is reflected in the fact that, where splat cooling of a binary system results in an amorphous phase within a certain range of compositions, this range can be extended by vapour quenching [25] or ion irradiation [26]. An extreme case in this respect is Ti-Fe, for which an amorphous phase could be produced by ion implantation but not by splat cooling [27]. Thus, by using the quenching rate as the criterion, vapour quenching and ion bombardment are clearly distinguished. Further, when the high degree of control for the ion beam technique is taken into account, it becomes clear that this method offers very attractive features which will be focused on in the following.

2. ION BEAM TECHNIQUES FOR AMORPHIZATION

Depending on the details of the experimental arrangement, within the ion beam techniques, three types of experiment can be

distinguished: (1) ion implantation; (2) ion mixing; (3) ion irradiation. In the following a short outline of the corresponding basic ideas will be given.

2.1. Ion implantation In the ion implantation method a second species B is implanted into a crystalline target A up to a concentration x to form an amorphous system of composition Al-x Bx. Thus, if a thin A film is used as target, the energy of the B ions must be adjusted in such a way as to guarantee that the corresponding projected range Rp is smaller than the film thickness. In order to improve the uniformity of the implantation profile, different energies can be used. One of the first systems produced in this way by Stritzker and Wiihl [26] was Ge-Cu where copper was implanted at liquid helium temperature into germanium films. The resulting amorphous phase (Xcu ~ 20 at.%) exhibits a metallic behaviour and even superconductivity below 3 K. The idea for these implantations resulted from quench condensation experiments on the same system Ge-Cu, which revealed a superconducting amorphous phase [28]. This correspondence between vapour quenching and ion beam experiments, already addressed above in terms of the quenching rate, seems to be very useful as a guide for experiments and we shall focus on this further in this paper. At approximately the same time, Cullis et al. [29] produced amorphous Cu-W by tungsten implantation into copper at room temperature. Their approach was to test how far a metastable solid solution can be extended by the implantation technique. Since 1975 the field of metallic glasses produced by splat cooling has also grown rapidly [30]. One important subclass of these materials is represented by the transition metal-metalloid (TM-M) systems of approximate composition TM0.sMo.2. Stimulated by this development, the Salford group succeeded in preparing the first metallic glasses of the above type by ion implantation. Examples are the implantation of phosphorus or boron into iron, nickel or cobalt [31-33]. In the light of the above-mentioned hierarchy of quenching rates, this success was not surprising; all splat-cooled systems, in principle,

97 should be able to be prepared by ion implantation also. After these first results, an increasing number of systems including TM-TM glasses [34] have been studied and found to b e c o m e amorphous on ion implantation. In addition to the increasing number of amorphous systems, more detailed information about the amorphization process itself is now available mainly as a result of channelling and transmission electron microscopy or X-ray studies as well as measurements of the resistivity and superconducting transition temperature. In addition, the dependence on the implantation temperature Timp was studied. For details the reader is referred to refs. 35-43, but the main results will be summarized briefly. (1) A "chemically active" species B is needed in order to stabilize disorder, leading eventually to the amorphous phase. Chemically inert species such as argon or self-ions produce especially long-range distortions b u t do not result in the amorphous phase. (2) There are strong indications that locally a critical number of stabilizing B atoms must be exceeded to obtain the amorphous phase within a given volume. The corresponding local concentration can be found b y applying a statistical model if the volume fraction of the amorphous phase has been determined as a function of the average implanted concentration. (3) The implantation temperature Timp has a crucial influence on amorphization. In some cases an amorphous phase can be produced at Timp(1) but not at Timp(2) ~ Timp(1). Nevertheless, the amorphous phase, once produced at Timp(1), is then stable b e y o n d Timp(2). (4) To the present author's knowledge, there is no exception to the rule that, if a system becomes amorphous by splat cooling, then ion implantation will also give the amorphous phase. This rule of course also implies the possibility that ion implantation can result in an amorphous phase for a specific system such as Ti-Fe [27] while splat cooling does not. The direct implantation technique for amorphization has t w o drawbacks. First, the amorphization is due to two processes which b o t h occur at the same time, i.e. the production of disorder and its stabilization. An experimental separation of the role of each of these processes appears to be very difficult.

Second, an upper limit of the implanted concentration is set by sputtering and roughly given by 1/S where S is the sputtering coefficient for a given projectile-target combination and energy. For the high concentrations needed to obtain a metallic glass, this can be a serious restriction. A w a y o u t of this problem is ion beam mixing, which will be described in Section 2.2.

2.2. Ion mixing The basic idea of ion mixing is to irradiate a multilayered A B A B . . . structure with inert ions such as argon or krypton in order to produce a mixed structure of composition Ax Bl-x. This arrangement includes the often-met situation of an AB bilayer and x can be varied by using properly adjusted thicknesses of the individual layers. Also sandwiches of continuously varying layer thicknesses have been used [44] to obtain a large range of compositions within one experiment. The amount of mixing will depend on the nuclear energy loss which in turn is a function of depth for any given ion energy Eo. This effect has been exploited especially in ABA structures with t w o A-B interfaces b y varying E0 [45]. However, in most cases, Eo is chosen such that the corresponding Rp value exceeds the total thickness of the layers. Thus, for a possible amorphization of the system the same basic processes as in an implantation experiment have to be considered: the bombarding inert ions produce a spatially varying AB composition and, at the same time, disorder which must be stabilized by either A or B atoms. Consequently, the rule given above which states that all amorphous systems prepared by splat cooling should become amorphous b y ion beam techniques is expected to hold also for ion mixing. For ion mixing, clear evidence could be provided that, in addition to the pure collisional mixing processes, thermodynamic driving forces play an important role. A convincing example in this respect is the ion mixing of metal-silicon interfaces. In the initial step in this process a silicide is formed in complete analogy to what is obtained by thermal reaction [46]. Also the observed demixing and segregation during ion b o m b a r d m e n t as well as the different behaviour of miscible and immiscible systems indicate the influence of thermodynamic forces [46, 47]. Another crucial parameter

98 in ion mixing experiments is the irradiation temperature Tit,. Recent examples are the Ni-A1 and Co-A1 systems, which were found to become amorphous by mixing at 77 K, while at 300 K a crystalline phase was observed [48]. With respect to amorphization by ion mixing, the observed influence of thermodynamic forces reflects the glass-forming ability of a system. For instance, the immiscibility seems to be a necessary, although not sufficient, condition for amorphization. The glass-forming ability obviously is equally important for an implantation experiment. At present, the thermodynamic approach to attempts to construct metastable phase diagrams in order to predict whether a system can be amorphized has made some promising progress [17, 19, 49]. This approach might also be able to explain Liu's structural difference rule [21] for ion beam mixing. This rule states that an amorphous phase will be formed by mixing if the elements A and B have different equilibrium lattice structures. Meanwhile, a number of systems have been found which demonstrate that the structure difference is at least not a necessary prerequisite for amorphization [44, 48, 50]. For systems exhibiting a structural difference such as Au(f.c.c.)-Fe(b.c.c.), which have been reported not to become amorphous by mixing [ 51], the above-mentioned strong effect of the irradiation temperature may be crucial. In this case, low temperature (T ~ 10 K) experiments combined with an in situ structural analysis are needed. Such experiments would also help to test thermochemical maps, where for example the ratio of the atomic radii are plotted as a function of the heat of compound formation [52] in order to separate the systems which become amorphous by mixing. The principal problem which makes an analysis of mixing experiments quite difficult is the variation in the composition during ion bombardment. This problem can be avoided by performing irradiation experiments, which will be described in Section 2.3. 2.3. I o n irradiation

The basic idea of ion irradiation is to bombard a target that is as homogeneous as possible with ions of an energy Eo which guarantees that the projected range Rp is large compared with the sample thickness D. In amorphization experiments using a typical

implantation machine, the initial material is very often a crystalline compound prepared as a thin film to satisfy the condition Rp >~D. In some cases, depending on the equilibrium phase diagram, films containing two phases have to be used. The advantage of an irradiation experiment is that it is the most suitable type of experiment for studying the influence of the damage process on amorphization, since the average composition remains fixed. In particular, the effect of different projectiles can be tested. Of course, mixing and irradiation experiments can be combined. A recent example has been given by Rivi~re and coworkers [ 53], who produced a homogeneous crystalline Ni-A1 phase by ion mixing at room temperature; this phase was then irradiated at 77 K, resulting in an amorphous phase. An extended study of amorphization by ion irradiation at ambient temperatures has been performed by Brimhall et al. [54]. These workers emphasized that immiscibility of the systems is the prerequisite for amorphization, while the ionicity rule of Naguib and Kelly [16] seems to fail for their compounds. Again, to substantiate these results, low temperature irradiation experiments would be useful. In all the above experimental arrangements of ion bombardment, energy spikes may play an important role. By an energy spike we mean a displacement cascade of high energy density, which should be described as a collective behaviour of the atoms involved rather than a linear superposition of well-defined binary encounters [ 55-58]. Thus, thermalization of the moving atoms is not needed, i.e. a description in terms of a local temperature is not necessary. Nevertheless, it is a very tempting qualitative picture to think of a spike as a local melt or even a gas, which persists only for a very short time of the order of 10 -12 s. Then the spike region collapses back into the solid state. This local rapid quenching event is in complete analogy to melt-quenching experiments or, as judged from the quenching rates, even to vapour quenching. Led by this analogy, we performed some low temperature irradiation experiments concerning the role of energy spikes, and these will be summarized in Section 3. In addition, we recently stated the following rule [50]. Whenever a system can be forced into the amorphous state by vapour quenching,

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then ion beam techniques will also result in the amorphous state if appropriate parameters are used. The most important parameters in this respect are the projectile mass and the irradiation temperature. By means of the above-mentioned hierarchy of quenching rates, this rule includes all amorphous systems prepared by splat cooling, but obviously it does not allow us to predict new amorphous systems. In the following, examples are given of the usefulness of our rule.

3. SELECTED LOW TEMPERATURE EXPERIMENTS CONCERNING THE ROLE OF ENERGY SPIKES IN AMORPHIZATION

From our rule it has to be concluded that pure gallium without a second component should become amorphous by ion irradiation. This conclusion is based on the experimental results obtained by vapour quenching onto liquid-helium-cooled substrates [59], which are summarized in Fig. 1. In this figure, the normalized resistivity is plotted versus the annealing temperature for vapour-quenched films. The as-quenched films are amorphous and they exhibit superconductivity and a rather high resistivity (the corresponding values are given in Fig. 1). The stability of the amorphous phase is very low. At 16 K it transforms into a metastable crystalline phase 03 phase), which eventually transforms into

•/Po 1

aJ5a ' Po: 33 pl)cm T:: 8 5 K

/

o~ - G o

po = 12 la~ cm T: : I 0 7 ~

B - 5a Po = 3 I~l)cm Tc = 6 3 K I

10

50

I:(K)

100

Fig. 1. Normalized resistivity v s . annealing temperature for a gallium film prepared by vapour quenching onto a liquid-helium-cooled substrate (a-Ga is the amorphous phase, ~-Ga a metastable crystalline phase and ~-Ga the stable crystalline phase; the corresponding resistivities and superconducting transition temperatures are given).

the stable c~phase at 60 K. Thus, if ion irradiation is supposed to result in the amorphous phase, this experiment has to be performed at low temperatures {Ti~ "~ 16 K). The result of such an experiment in which the crystalline phase of gallium was irradiated at T < 10 K with argon ions (Eo -- 275 keV) is shown in Fig. 2. In this figure diffraction patterns obtained from an in situ transmission electron diffraction experiment performed at 4.2 K are presented [60]. Figure 2(a) gives the diffraction pattern of the crystalline (~-Ga before argon irradiation, and Fig. 2(b) the phase produced by irradiation of the ~-Ga with 275 keV argon ions at T < 10 K. For comparison, Fig. 2(c) shows the diffraction pattern of the amorphous phase obtained by vapour quenching. From Figs. 2(b) and 2(c) it is clear that the amorphous gallium phase is formed by the low temperature argon irradiation. This result is substantiated by the corresponding resistivity and T¢ changes due to the ion bombardment [61]. In contrast, clear evidence can be provided [60, 61] that low temperature irradiation with helium ions (200 keV) did not result in amorphous gallium, although the helium fluences were scaled to produce the same amount of energy deposited into nuclear collision. From this it was concluded that, at least in a one-component system such as pure gallium, spikes are necessary for amorphization by ion irradiation. The next step is to study the role of spikes in a binary system. A binary compound which is very suitable for testing the correlation between the results of vapour quenching and ion beam techniques is A12Au. Since both constituents exhibit an f.c.c, equilibrium structure, in mixing experiments amorphization is not expected according to the structural difference rule. In agreement with this rule, after mixing at ambient temperatures and at 80 K, the crystalline phases of A12Au and Au2A1 were observed [45, 62]. Also from the ratio of atomic radii, which is very close to unity for aluminium and gold, an amorphous phase would not be expected [12]. However, Folberth e t al. [63] were able to produce an amorphous Alo.67Auo.33 system by flash evaporation of A12Au pieces onto cooled substrates. This amorphous phase is then stable up to a crystallization temperature T¢ of 210 K. Thus, according to our rule, ion irradiation should result in the same amor-

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Fig. 2. Electron diffraction patterns of different gallium phases: (a) crystalline o~-Ga before Ar + ion irradiation; (b) amorphous gallium produced by irradiation of ~-Ga with 275 keV Ar + ions at T < 10 K (total argon fluence ¢ = 2 X 1014 cm-2); (c) amorphous gallium produced by vapour quenching.

phous phase. This is at variance with a criterion given by Naguib and Kelly [16] which states that ion-induced amorphization of a system can be expected if the ratio To~Tin is greater than 0.3, where Tm is the melting temperature of the corresponding equilibrium phase (for the A12Au system this ratio is 210/1060 which is less than 0.3). Experimentally, after argon irradiation of crystalline A12Au at Ti~ < 10 K, an amorphous phase which is identical with the amorphous phase prepared by vapour quenching within our resolution was found [50]. This is in agreement with the prediction of our rule. The question then arises whether amorphization can also be accomplished by irradiation with light ions when spike phenomena are not expected. The result of a low temperature helium irradiation experiment is shown in Fig. 3. Again, in situ electron dif-

fraction is used for the structt,xal analysis. Figure 3(a) shows the diffraction pattern of the amorphous phase obtained by vapour quenching A12Au particles onto cooled (80 K) substrates. This amorphous phase is then heated beyond the crystallization temperature, the crystallized sample is cooled to 4.2 K and the electron diffraction pattern of the crystalline A12Au is recorded (Fig. 3(b)). After helium irradiation (Eo = 200 keV; ¢ = 2 X 1016 c m -2) of the crystalline phase at T < 10 K, the diffraction pattern shown in Fig. 3(c) was obtained. By comparison with the diffraction pattern in Fig. 3(a) it is clear that, in this case, amorphization can also be accomplished by light ion bombardment. Thus, it must be concluded that, at least at very low temperatures, spikes are not necessary to amorphize a binary system which is known to become amorphous by vapour quenching.

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Fig. 3. Electron diffraction patterns: (a) the amorphous phase produced by the vapour quenching (T = 80 K) of Al2Au; (b) crystalline A12Au; (c) the amorphous phase produced by irradiation of the crystalline phase with 200 keV helium ions at T < 1 0 K ( ~ = 2 X1016 cm-2).

This conclusion is most 31early confirmed by energetic electron irradiations of Fe-B [64] or Ni-A1 [65], which also resulted in amorphous phases. The situation can be totally different at higher irradiation temperatures. For the A12Au system it has been found [ 50] that, at an irradiation temperature T~, of 77 K only argon b o m b a r d m e n t and not helium bombardment leads to the amorphous phase. The conclusion is that at this temperature the high energy density cascades govern the amorphization process. The low temperature results described above raise the question of what the details are of amorphization due to light ion b o m b a r d m e n t when spikes can be neglected. The principal idea which has been formulated with respect to this b y several researchers [32, 66, 67] is

that locally a critical defect density, or perhaps equivalently a critical strain value, must be exceeded (in ref. 66, many more references are cited in which the critical defect concentration is discussed). Thus, this idea is very close to the mechanism thought to be responsible for changes of one crystalline phase to another induced by ion b o m b a r d m e n t [32, 68]. Recently, Egami and Waseda [13] have given a quantitative estimate of the critical strain necessary for topological instability of a crystalline phase. Using this model, Linker [42] discusses his results concerning the amorphization of niobium by boron implantation in this issue. The same idea may be involved in the case where a critical local concentration of disorder-stabilizing elements has been inferred from experiments [35, 36]. Also for the A12Au system, detailed resistivity

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measurements during the amorphization process support the above model [69]. 4. CONCLUSION

The rule stated in Section 2 which emphasizes the close relationship between amorphization by vapour quenching and ion beam techniques seems to be a useful empirical guide for beam experiments. However, the underlying simple spike picture, originally leading to the above analogy between these two techniques, certainly does not account for the observed amorphization after light ion bombardment of binary systems. For this case, more low temperature experiments are needed to clarify the details of the amorphization process. From an application point of view, ion bombardment has clearly proved to be a versatile technique to produce amorphous phases and, because of the high effective quenching rates, phases which cannot be produced by the more conventional splatcooling method. ACKNOWLEDGMENTS

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