Amorphous phase formation in ion implanted cobalt

566

Nuclear Instruments and Methods in Physics Research B19/20 (1987) 566-570 North-Holland, Amsterdam

A M O R P H O U S P H A S E F O R M A T I O N IN ION I M P L A N T E D COBALT W e n - Z h i LI *, Zaki A L - T A M I M I a n d W.A. G R A N T Department of Electronic & Electrical Engineering, University of Salford, Salford M5 4 WT, UK

Ion implantation has been used to investigate the tendency for Co to form an amorphous phase when implanted with Sb and Te. Films and foils of Co were implanted with 80 keV Sb ~ and Te* ions, at room temperature, with doses up to 1017 ions cm 2. RBS and TEM were used to examine the compositional and structural state of the implanted specimens, At low doses the intercrystalline transition (hcp)Co to (fcc)Co was observed. At higher doses an amorphous phase, as indicated by the presence of diffuse scattering maxima in transmission electron diffraction, was observed in both Sb and Te implanted Co. An amorphous Co-Sb alloy was first observed at - 6 at.% Sb and an amorphous Co-Te alloy at - 2 at.% Te. These values of Sb and Te place the lower composition at which an amorphous phase can be stabilized at the limit of the two phase region of the appropriate equilibrium phase diagram. It is suggested that ion beam techniques provide an efficient method for extending the composition range over which amorphous phases can be stabilized.

1. Introduction A growing area of research in materials science is concerned with amorphous metals and alloys. Such amorphous or glassy metals lack the long range order found in crystalline phases and often have interesting physical, and technologically important, properties. Amorphous metals are usually formed by rapid solidification techniques [1] such as liquid quenching or vapour deposition. In recent years it has been demonstrated [2] that ion implantation and ion beam mixing can also be employed to form glassy metal phases. Perhaps the greatest stimulus for research into the preparation and properties of these materials has come from their magnetic properties and this, in turn, is related to the fact that the ferromagnetic transition metals Fe, Co and Ni from amorphous phases when alloyed with a wide range of other elements. In previous publications [2] we have reported on the glass forming abilities of the transition metal Ni when ion implanted with metalloids. In the work reported here we have examined ion implantation of Co with the metalloids Sb and Te. These combinations of transition metal (TM) and metalloid (ME) do not appear to have been previously examined for their glass forming ability either by quenching or ion beam techniques.

2. Experimental Samples suitable for transmission electron microscopy and electron diffraction were prepared from * Permanent address: Department of Engineering Physics, Tsinghua University, Beijing, China.

0168-583X/87/$03.50 © Elsevier Science Publishers B,V. (North-Holland Physics Publishing Division)

polycrystalline Co foils and films. The foils were 3.0 mm diameter and 0.125 mm thick and were prepared by cold rolling followed by annealing, electropolishing and" jet thinning to perforation in a solution of 90% glacial acetic acid and 10% perchloric acid. Two batches of thin films, ~< 1000 ,~ were prepared by dc sputtering methods onto sodium chloride and silicon substrates for TEM and RBS analyses repectively. The samples were implanted at room temperature with 80 keV of either Sb + ions or Te + ions in the dose range 10t5-1017 ions c m - 2.

Following the implantations the samples were examined in a (JEM 200 CX) transmission electron microscope operating at 200 kV. Rutherford backscattering analysis was performed using a 2 MeV He + ions hitting the target at (7 °) ,glancing incidence giving a depth resolution of - 60 A. The concentration profiles of the implanted atoms were obtained using a computer program for the analyses of RBS spectra [3].

3. Results and discussion The accumulation of Sb and Te within the surface of cobalt was monitored, as a function of implanted dose, using RBS and the results are summarised in fig l a and lb. At low doses, -1015 ions cm -2, all implanted atoms are retained. At higher doses, > 1016 ions cm -2, sputtering effects begin to reduce the efficiency of retention with a tendency towards saturation at doses above - 5 X 1016 ions cm-'-. The saturation limit for Te implantation is - 6 0 % lower than the saturation limit for Sb implantation (fig. la). The spatial distributions of implanted atoms, derived from RBS spectra, are shown in fig. l b for both Sb and

567

Wen-Zhi Li et aL / Amorphous phase formation in cobalt

this factor of - 30 in solid solubility may be a contributory factor. Following implantation the cobalt foils were examined by transmission electron microscopy and transmission electron diffraction. A diffraction pattern for unimplanted Co is shown in fig. 2; The spot pattern is typical of that for an hcp material. Ion implantation with either Sb or Te results in radiation disorder which increases in severity with increase in implanted dose. In addition diffraction measurements indicated the presence of additional reflections which could not be re.adily attributed to hcp cobalt. The complexity of diffraction patterns after irradiation precluded unequivocal identification of the phases present but some of the additional reflections could be identified as an fcc Co phase. This suggests that irradiation is promoting the intercrystalline phase transition from hcp cobalt to fcc cobalt. Such transformations have been reported previously [4] for a number of other systems and are believed to be promoted by the stress resulting from radiation damage. Increasing the implantation dose also results in the appearance of diffuse rings superimposed on the spot pattern. The diffuse scattering rings are indicative of a highly disordered or amorphous phase. The inset diffraction pattern in fig. 2 is for cobalt implanted with 5 × 1015 ions cm -2 of 80 keV Te and shows a single

Te implantations. The range profiles for Sb are essentially Gaussian with a projected range (at the lower doses) of Rp - 150 /k. As the implant doses increases there is a shift in the position of the peak of the range distribution towards the surface, presumably due to sputtering. At the highest implant dose (1.3 X 1017 ions cm -2) the peak concentration of Sb is - 18 at.% and the concentration at the surface is - 13 at.%. The range profiles for Te are also shown in fig. l b and are qualitatively and quantitatively different to those for Sb. The range distributions for Te depart earlier (as a function of implanted concentration) from a Gaussian shape and the peak concentration at the highest implant dose (1017 ions cm -2) is 11 at.% with an accompanying surface concentration of - 10 at.%. It is clear that sputtering is higher for Te implanted Co compared to Sb implanted Co. The higher sputtering results in a more rapid erosion of the sample towards saturation conditions and a lower value of the implanted concentration when saturation is reached. Since slSb and 52Te are neighbouring elements in the periodic table it would be anticipated that ballistic-type sputtering should be identical. Differences is sputtering behaviour, as evidenced by the curves in fig. 1, must therefore be attributed to other effects. The solid solubility of Sb in Co is - 4.5 at.% whilst that for Te is reported as < 0.15 at.% and 1017

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Wen-Zhi Li et al. / Amorphous phase formation in cobalt

569

Fig. 2. Electron micrograph from perforated Co-foil after implantation with 5 × 1015 ions cm -2 of 80 keV Te. The inset diffraction patterns are from unimplanted (top left) and implanted (top right) samples.

diffuse ring together with crystalline reflections. The accompanying micrograph of fig. 2 shows a region close to the perforated hole in the implanted foil. The region at the very edge of the hole is essentially featureless and typically of amorphous material. The diffraction pattern taken from this area shows the strongest diffuse scattering ring as might be expected since the foil is at its thinnest close to the hole and consequently the implanted ions will penetrate (and amorphise) the total foil thickness. A second diffuse scattering ring could also be discerned on the diffraction patterns but was too faint for photographic reproduction. The ratio of the positions of these two rings was measured as 1.72 and this should be compared to a theoretical prediction for amorphous cobalt, based on a dense random-packing model, of 1.70. The diffuse scattering maxima were most evident at the highest implant doses and the corelation between measured and theoretical prodictions of their positions strongly suggests the formation of amorphous C o - T e and Co-Sb. At lower doses the intensity of the diffuse rings decreased but careful examination of the diffraction patterns indicated that an amorphous phase could be detected at concentrations of Sb and Te as low as - 6 at.% Sb and - 2 at.% Te.

In discussing the formation of an amorphous phase by ion implantation, two models have been proposed. The first model proposes that implantation induced disorder is stabilised by the implanted species until the overlap of cascades, at sufficiently high doses, results in the transformation to the amorphous phase. An alternative model proposes that amorphisation occurs directly in individual, dense cascades or "thermal spikes". Evidence [5,6] to support both models exists for particular systems under specified conditions. In the present work, both Sb and Te would be expected to produce dense cascades and direct "thermal spike" amorpkisation would be possible. Fig. 3 shows the results [7] of a Monte Carlo calculation of the penetration of 80 keV Sb into Co. The range distribution is shown in fig. 3a and the damage distribution in fig. 3b. It can be seen that even low doses of 80 keV Sb produce high disorder levels. As shown in fig. 3, a dose of 1015 ions cm -2 results in a peak damage level of 7 dpa at a depth of 60 A. Consequently at the moderate dose of 1016 ions cm 2 which produces a peak implant concentration of 6 at.% Sb and results in an amorphous phase, the peak disorder level is 70 dpa. For Te implantation the dose required for amorphisation is - 3 × 1015 ions cm -2 which gives a peak Te concentration of 2 at.% and a disorder level of 20 dpa. Such dpa levels are sufficient IV. PHASES/RTA

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Wen-Zhi Li et al. / Amorphous phase formation in cobalt

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of preparation e.g. liquid quenching, sputter deposition etc. Recently Naoe et at. [10] have shown that C o - Z r and C o - T a amorphous films can be prepared over a wider composition range if the film is irradiated by energetic ions during deposition. Argon ion irradiation reduced the amount of Zr and Ta required to stabilise an amorphous phase to the extremely low values of - 2 at.% Zr and - 5.7 at.% Ta. These low concentrations extend the composition range of the C o - Z r and C o - T a amorphous phases to the limit of the two phase region of their respective phase diagrams. Our results are in good agreement with the work of Naoe [11].

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to amorphise alloys which have a disposition to stabilise in an amorphous state. Disorder levels as low as ~< 1 dpa have been found [8] sufficient for amorphisation in certain systems. A number of criteria have been proposed to explain the tendency of alloys to form an amorphous phase. One such criteria is the "structural difference rule" proposed by Liu et at. [9]. This rule requires that the alloy has a composition that places it in a two-phase region of the equilibrium phase diagram. Alternatively, if the composition lies within a single phase field, irradiation will still produce an amorphous phase if the equilibrium composition range is narrow [10] or the crystal structure of the crystalline phase is complicated [9]. For the systems used in this work the implanted dose at which amorphisation is first detected is sufficient to place the alloy composition just within the two phase region beyond the primary solid solution. Amorphisation is detected at - 6 at.% Sb and - 2 at.% Te and this reflects the different solid solubilities of the two alloying additions. The composition range over which an alloy can be stabilised in the amorphous state depends on the method

5. C o n c l u s i o n s

Ion implantation of Co with Sb and Te ions at 80 keV leads to the formation of an amorphous phase. Prior to amorphous phase formation, the intercrystalline transition hcp Co to fcc Co is observed. An amorphous C o - S b alloy is first observed at - 6 at.% Sb and an amorphous C o - T e alloy at - 2 at.% Te. These values of Sb and Te place the lower composition at" which an amorphous phase can be stabilised at the limit of the two phase region of the appropriate equilibrium phase diagram. This extreme limit is achieved because of the high disorder that accompanies ion implantation. The present results suggest that for suitable alloy system, the composition range over which an amorphous phase can be stabilised, spans the entire two phase region of the equilibrium phase diagram. Ion irradiation, with its characteritic disorder-inducing attributes, is an extremely efficient technique for exploring such composition limits.

References

[1] W.A. Grant, J. Vac. Sci. Technol. 15 (5) (1978) 1644. [2] Z.Y. A1-Tamimi, W.A. Grant and P.J. Grundy, Vacuum 34 (10/11) (1984) 861. [3] When-Zhi Li and Z. A1-Tamimi, Nucl. Instr. and Meth. B15 (1986) 241. [4] D.M. Follstaedt, Nucl. Instr. and Meth. B7/8 (1985) 11. {5] G. Carter and W.A. Grant, Nucl. Instr. and Meth. 199 (1982) 17. [6] D.K. Sood, Radiat. Eft. 63 (1982) 141. [7] Wen-Zhi Li, to be published (1986). [8] W.A. Grant, Proc. Int. Ion Eng. Cong. ISIAT '83 and IPAT '83 Kyoto, Japan (Sept. 1983). [9] B.-X Liu, Nucl. Instr. and Meth. B7/8 (1985) 547. [10] J.I_. Brimhall, H.E. Kissinger and L.A. Chariot, Proc. Metastable Materials Formation by Ion Implantation, eds., S.T. Picraux and W.S. Choyke (North-Holland, New York, 1982) p. 235. [11] M. Naoe, N. Terada, Y. Hoshi and S. Yamanaka, IEEE Trans. Magnetics Mag-20, (5) (1984) 1311.