comparison of the structure and magnetotransport properties

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2964 Journal of The Electrochemical Society, 147 (8) 2964-2968 (2000)S0013-4651(99)10-035-1 CCC: $7.00 © The Electrochemical Society, Inc.

The direct electrodeposition of metal films on semiconductors isof interest from both a fundamental and a practical point of view.Metal growth on Si 1,2 or GaAs 3,4 from dilute solutions is an attrac-tive model system for theories of three-dimensional nucleation fol-lowed by diffusion-limited growth,5 while the electrical properties of interfaces between electrodeposited metals and semiconductorsmake them technologically relevant, either as ohmic contacts6 or asSchottky junctions.2 In addition to homogeneous, single-layer metalfilms, it is possible to electrodeposit metal/metal multilayers direct-ly on semiconductors.7-9

Metal/metal multilayers are of particular interest when they con-sist of alternating ultrathin layers of a magnetic and a nonmagnetic

metal, because then they may exhibit giant magnetoresistance(GMR), a very large change in electrical resistance on the applica-tion of an external magnetic field. GMR was discovered just over 10years ago,10,11 but field sensors using this effect are already in pro-duction for high density magnetic recording. While most GMR mul-tilayers are made by sputtering, electrodeposition has proved a high-ly successful alternative.12-16 It has the obvious advantage of notrequiring an expensive vacuum system, but the disadvantage that aconducting substrate is required. This can be a significant disadvan-tage because the obvious choice of substrate, a metal, will short-cir-cuit the multilayer, lowering the GMR. Although it is possible toremove metal substrates after growth12 or use a very thin seedlayer,14 this adds to the complexity and cost of film preparation.

Direct electrodeposition of GMR multilayers on an n-type semi-conductor can overcome this problem because of the formation of a

Schottky barrier at the metal-semiconductor interface.

7

During depo-sition, this junction is forward biased, and the current associated withmetal ion reduction flows across it, but when making electrical trans-port measurements, the depletion layer at the junction isolates thefilm. So far, the direct electrodeposition of Co-Ni-Cu/Cu multilayerswith significant GMR has been demonstrated on both n-GaAs(001)and n-Si(001).8,17 The n-GaAs case is of particular interest, becausethe films are highly textured and exhibit a strong in-plane magneticanisotropy.18 This anisotropy has recently been exploited inCo-Cu/Cu GMR multilayers containing ferromagnetic layers withdifferent coercivities, which show a remarkable sensitivity (up to0.55% change in resistance per oersted change in the applied field).19

Here we have used a similar electrolyte to that used in previousstudies12,13,17 to electrodeposit Co-Ni-Cu/Cu GMR multilayers onn-GaAs with two different crystallographic orientations: (001) and(111). Studies of the effect of substrate crystallographic orientationon GMR multilayer electrodeposition have been carried outbefore14,20 but not for direct deposition on a semiconductor. Ourresults show both that the substrate orientation exerts a significantinfluence on the film microstructure, and that this has a large effecton the GMR.

Experimental

The substrates used in these experiments were cleaved from50 mm diam single-side-polished GaAs wafers doped with Si tomake them n-type with a free carrier concentration of 1-5 ϫ

1018 cmϪ3. The (111) substrates were type A, i.e., Ga-terminated. [Itis of future interest to compare A- and B-type (As-terminated) sub-strates to study the influence of surface chemistry on the electrode-position.] Immediately prior to use, the substrates were washed indilute ammonia solution and pure deionized water. The electrolytecomposition (in mol per liter of H2O) was nickel sulfamate[Ni(SO3NH2)2]: 2.3, cobalt sulfate [CoSO4]: 0.41, copper sulfate[CuSO4]: 0.05, and boric acid [H3BO3]: 0.49, and the pH was adjust-ed to 1.9 using sulfamic acid. All experiments were carried out atroom temperature in a three-electrode cell with a reference saturatedcalomel electrode (SCE). In our cell the anode was a Pt plate, whichwas not separated from the working electrode by any membrane orother barrier.

Deposition was carried out potentiostatically, using a computer to

integrate the net current (i.e., the deposition current minus any dis-solution current) to obtain the net charge passed between anode andcathode. When this charge reaches the value corresponding to thedesired thickness of the superlattice component being deposited, thecathode potential is switched to start depositing the other compo-nent. The cathode potential for Cu deposition was Ϫ0.3 V with re-spect to the SCE, while for Co-Ni-Cu alloy deposition it wasϪ2.2 V.During Co and Ni deposition, Cu codeposits because it is the mostnoble metal in the electrolyte, and the Cu content of the Co-Ni-Culayer is minimized by reducing the Cu content of the electrolyte.21,22

For each substrate orientation, a series of Co-Ni-Cu/Cu multilayerswas grown with a fixed nominal Co-Ni-Cu thickness of t Co-Ni-Cu ϭ

30 Å, and varying nominal Cu layer thickness t Cu, where nominalthicknesses were calculated assuming 100% current efficiency. Ineach case, Co-Ni-Cu was the first layer to be deposited.

Comparison of the Structure and Magnetotransport Properties of

Co-Ni-Cu/Cu Multilayers Electrodeposited on n-GaAs(001) and (111)

O. I. Kasyutich,a W. Schwarzacher,a,* V. M. Fedosyuk,b,* P. A. Laskarzhevskiy,c,** and A. I. Masliy c

a H.H. Wills Physics Laboratory, Bristol BS8 1TL, United Kingdomb

Institute of Solid State Physics and Semiconductors, 220072 Minsk, Belarusc Institute of Solid State Chemistry, 630128 Novosibirsk, Russia

Co-Ni-Cu/Cu multilayers were electrodeposited directly onto n-doped GaAs substrates with two different crystal orientations,(001) and (111), without the use of any seed layer. X-ray diffraction and transmission electron microscopy showed that epitaxialgrowth occurred on GaAs(001), with either the {001} or {211} planes parallel to the substrate, but not on GaAs(111). On this sub-strate, the multilayers grow preferentially with the {111} planes parallel to the substrate, but the crystallites have no preferred ori-entation in-plane. The presence of superlattice satellite peaks in the X-ray data for the multilayers grown on GaAs(001) and theirabsence for those grown on GaAs(111) indicated that the latter had a less perfect layer structure. Multilayers grown on both sub-strates exhibited giant magnetoresistance (GMR). For small Cu layer thicknesses, t Cu < 20 Å, the GMR was suppressed for multi-layers grown on both substrate orientations. For t Cu betweenϳ20 andϳ30 Å, the GMR was much greater for the multilayers elec-trodeposited on GaAs(001) than for those on GaAs(111), while for larger layer thicknesses, the GMR for both substrate orienta-tions was similar. This behavior can be explained qualitatively by the presence of different numbers of defects producing differentdegrees of ferromagnetic coupling in the two sets of multilayers.© 2000 The Electrochemical Society. S0013-4651(99)10-035-1. All rights reserved.

Manuscript submitted October 7, 1999; revised manuscript received March 27, 2000.

** Electrochemical Society Active Member.** Electrochemical Society Student Member.*z E-mail: [email protected]

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Journal of The Electrochemical Society, 147 (8) 2964-2968 (2000) 2965S0013-4651(99)10-035-1 CCC: $7.00 © The Electrochemical Society, Inc.

Structural characterization of the electrodeposited multilayerswas carried out by -2 X-ray diffraction (XRD) using Cu K␣ radi-ation. In these experiments, the scattering vector was parallel to thegrowth direction of the film. To gain information on the in-planestructure of the films, one multilayer for each substrate orientationwas prepared by mechanical and ion-beam thinning for plan-viewtransmission electron microscopy (TEM).

Magnetotransport measurements were carried out at room tem-

perature with applied fields of up to 0.8 T. Electrical contact to thefilm was made using four spring-loaded point probes arranged in asquare as shown in Fig. 1. We define R12,34 to be the potential dif-ference between contact 4 and contact 3 per unit current flowingfrom contact 1 to contact 2, and R23,41 to be the potential differencebetween contact 1 and contact 4 per unit current flowing from con-tact 2 to contact 3. R12,34 may be used as a measure of the longitu-dinal magnetoresistance (MR), and R23,41 as a measure of the trans-verse MR. However, care must be taken when comparing MR ratiosmeasured using this method to those measured by other means,because the percentage changes in R12,34 and R23,41 are not neces-sarily equal to the percentage changes in the true longitudinal andtransverse resistivities.13

Results and Discussion

Figures 2 and 3 show XRD data for multilayers consisting of

50 repeats of nominal thickness 30 Å Co-Ni-Cu/50 Å Cu onn-GaAs(001), and 50 repeats of nominal thickness 30 Å Co-Ni-Cu/30 Å Cu on n-GaAs(111), respectively. Clearly, there are majordifferences between the films grown on the two substrates. OnGaAs(001), the multilayer 002 peak is significantly larger than the111 peak, by a ratio of more than 3:1; while on GaAs(111) the oppo-site is true, and the ratio of the 002 to the 111 peak height is approx-imately 0.1:1. This is evidence for texturing: on GaAs(001) the mul-tilayers grow preferentially with their {001} planes parallel to thesubstrate, while on GaAs(111) they grow preferentially with their{111} planes parallel. For comparison, the ratio of the 002 to the 111peak height expected for randomly oriented Cu is greater than 0.4:1.{001} texturing was observed for all but one of the multilayers elec-trodeposited on GaAs(001), and {111} texturing for all multilayerselectrodeposited on GaAs(111). For the multilayers grown on

GaAs(111), the degree of texturing appeared relatively insensitive to

the Cu layer thickness, but for the films grown on GaAs(001), thedegree of {001} texturing tended to increase with increasing Culayer thickness, suggesting that Cu stabilizes this orientation.

Another significant difference between the multilayers grown onthe two substrates is that prominent satellites are seen either side of the multilayer 002 peak for the multilayer grown on GaAs(001),while none is seen for the multilayer grown on GaAs(111). Thesesuperlattice satellites correspond to the repeat distance of the multi-layer (t Co-Ni-Cu ϩ t Cu), and, generally, the greater the number andintensity of such satellites, the more regular is the layer structure. Theagreement between the repeat distance measured from the satellitepeak positions and the nominal repeat distance was good, e.g., themeasured repeat distance for the sample of Fig. 2 (nominal repeatdistance80 Å) wasϳ70 Å. Clear superlattice satellites were seen forall multilayers grown on GaAs(001), except for the two films with

the thinnest Cu layers, but only at the multilayer 002 peak. No clearsatellite peaks were observed for the multilayers grown onGaAs(111), suggesting that these have a less perfect layer structure.

Figures 4 and 5 are plan-view TEM diffraction patterns from mul-tilayers consisting of 50 repeats of nominal thickness 30 Å Co-Ni-

Figure 1. Experimental geometry for magnetotransport measurements (seetext). H is the applied magnetic field. In practice, H was only approximatelyparallel to the line joining point probes 1 and 2.

Figure 2. -2 XRD data for multilayer consisting of 50 repeats of nominalthickness 30 Å Co-Ni-Cu/50 Å Cu on n-GaAs(001).

Figure 3. -2 XRD data for multilayer consisting of 50 repeats of nominalthickness 30 Å Co-Ni-Cu/30 Å Cu on n-GaAs(111).





Cu/15 Å Cu on n-GaAs(001) and n-GaAs(111), respectively. Likethe XRD data, these figures show that there are major differencesbetween the structures of the films grown on the two different GaAssubstrates. Although diffraction rings are observed for both multilay-ers, showing that both films contain crystallites with various orienta-tions, in Fig. 4, the rings have symmetrically distributed regions of high intensity (streaks), while in Fig. 5, they are approximately uni-form in intensity. This indicates that for the multilayer grown on

GaAs(111), the in-plane crystallite orientation is random, but that forthe multilayer grown on GaAs(001), there is one or more preferredepitaxial relationship between film and substrate. Figure 5 also con-firms what could be inferred from the XRD data; namely, that for themultilayer grown on GaAs(111), any {111} texturing is weak.

Similarly, the diffraction pattern for the multilayer grown onGaAs(001) contains diffraction spots in addition to those expectedfrom a face-centered cubic (fcc) metal with its {001} planes parallelto the substrate. It can be indexed as a superposition of diffraction pat-terns from crystallites having the following epitaxial relationshipswith the substrate: (i) multilayer {001} | | GaAs(001) and multilayer<100 > | | GaAs[110], (ii) multilayer {211} | | GaAs(001) and multi-layer <110> | | GaAs[100], and (iii) multilayer {211} | | GaAs(001)and multilayer <110> | | GaAs [010] (i.e., relationship (ii) rotatedthrough 90Њ). Relationship (i) has already been reported for Ni-Cu

electrodeposited directly on n-GaAs (001)23

and is consistent with thelarge 002 peak in the XRD data. Crystallites having {211} planes par-allel to the substrate do not give a peak in the XRD data, because the211 reflection is forbidden for the fcc structure, and sin calculatedfrom the Bragg formula for the 422 reflection with Cu K␣ radiation, ϭ 1.542 Å, is greater than 1.

As well as possessing different microstructures, Co-Ni-Cu/Cumultilayers grown on GaAs(001) and (111) also have differentmagnetotransport properties. Figure 6 shows the “transverse” and“longitudinal” MR, respectively, defined as ( R23,41 ( H )– R23,41( H max))/ R23,41 ( H max) and ( R12,34 ( H )– R12,34 ( H max))/ R12,34 ( H max),where H max is the maximum applied field, measured for a multilay-er consisting of 50 repeats of nominal thickness 30 Å Co-Ni-Cu/20Å Cu electrodeposited on GaAs(001). Figure 7 shows correspondingdata for a multilayer with the same nominal layer thicknesses elec-trodeposited on GaAs(111). Both multilayers exhibit GMR, becausefor both multilayers the transverse and longitudinal MR both haveprominent maxima for H close to zero, but it is apparent that the

GMR of the film electrodeposited on GaAs(001) is approximatelytwice that of the film electrodeposited on GaAs(111).

GMR in multilayers is generally associated with the directions of the magnetization vectors of successive ferromagnetic layers beingdifferent, either through antiferromagnetic coupling, or through thedifferent layers reversing at slightly different values of the appliedfield H . In the latter case, which applies to uncoupled or weakly cou-pled multilayers, the MR curves show hysteresis, with peaks at H ϳϮHC, where H C is the coercivity. The MR curves in Fig. 6 and 7appear to be of this type, which is expected, as for t Cu ϭ 20 Å anyexchange coupling is likely to be weak.24 The difference in magni-tude between the GMR of the multilayer grown on GaAs(001) andthe GMR of the multilayer grown on GaAs(111) is likely to becaused by structural defects in the latter. Structural defects can giverise to regions of ferromagnetic coupling, which prevents the direc-

Figure 4. Plan-view TEM diffraction pattern from multilayer consisting of 50 repeats of nominal thickness 30 Å Co-Ni-Cu/15 Å Cu on n-GaAs(001). Inorder of increasing diameter, the observed diffraction rings may be indexedas the multilayer fcc 111, 200, 220, 311, and 222 rings.

Figure 5. Plan-view TEM diffraction pattern from multilayer consisting of 50 repeats of nominal thickness 30 Å Co-Ni-Cu/15 Å Cu on n-GaAs(111).The bright diffraction spots in this figure come from the substrate (in this fig-ure the substrate is aligned along a GaAs <110> zone axis) rather than the

multilayer. In Fig. 4, all the substrate in the region studied was removed byion-beam thinning. The rings may be indexed as for Fig. 4.




Journal of The Electrochemical Society, 147 (8) 2964-2968 (2000) 2967S0013-4651(99)10-035-1 CCC: $7.00 © The Electrochemical Society, Inc.

tions of the magnetization vectors of successive ferromagnetic lay-ers from being different, and therefore suppresses the GMR. The

most likely candidate defects are points of contact between succes-sive ferromagnetic layers (e.g., at grain boundaries) and regions of conformal roughness, which give rise to magnetostatic ferromagnet-ic coupling.13 Both kinds of defects may be more common in themultilayer grown on GaAs(111) because of the wide range of grainorientations and the less perfect multilayer structure indicated by theXRD data.

The influence of structural defects on the GMR is expected tobecome less as t Cu increases, and this could explain why the multi-layers electrodeposited on GaAs(001) and (111) have approximate-ly the same GMR for large t Cu (Fig. 8). For t Cu < 20 Å, the GMR issuppressed for both substrate orientations. A similar suppression has

been observed for Co-Ni-Cu/Cu multilayers electrodeposited onn-GaAs(001) in an earlier study,25 for Co-Ni-Cu multilayers electro-deposited on n-Si, 17 and for Co-Cu/Cu multilayers electrodepositedon Cu. 14 However, for Co-Ni-Cu/Cu multilayers electrodepositedon Cu, the largest GMR values are measured for t Cu < 10 Å, suggest-ing that multilayers electrodeposited on this substrate are structural-ly more perfect.12

Conclusions

To summarize, we have studied Co-Ni-Cu/Cu multilayers elec-trodeposited on n-GaAs substrates with two different crystallo-graphic orientations. For GaAs(001), epitaxial growth is possiblewith either the {001} or the {211} planes of the multilayer parallelto the substrate, but for GaAs(111) no epitaxy is observed, althoughthe multilayers have a {111} texture. The GMR in multilayers grown

on GaAs(111) is suppressed fort Cu < 30 Å but is suppressed only fort Cu < 20 Å in multilayers grown on GaAs(001), probably as a con-

sequence of the latter having fewer structural defects to cause ferro-magnetic coupling.

Acknowledgments

This work has been supported by the U.K. Engineering and Phys-ical Sciences Research Council and by INTAS.

References1. A. A. Pasa and W. Schwarzacher, Phys. Status Solidi A, 173, 73 (1999).2. G. Oskam, D. van Heerden, and P. C. Searson, Appl. Phys. Lett ., 73, 3241 (1998).3. P. M. Vereecken, K. Strubbe, and W. P. Gomes, J. Electroanal. Chem., 433, 19

(1997).4. G. Scherb and D. M. Kolb, J. Electroanal. Chem., 396, 151 (1995).5. B. Scharifker and G. Hills, Electrochim. Acta, 28, 879 (1983).6. J. M. DeLucca, H. S. Venugopalan, S. E. Mohney, and R. F. Karlicek, Jr., Appl.

Phys. Lett., 73, 3402 (1998).

Figure 6. Transverse (a) and longitudinal (b) MR measured for a multilayerconsisting of 50 repeats of nominal thickness 30 Å Co-Ni-Cu/20 Å Cu elec-trodeposited on GaAs (001). The transverse MR is defined as ( R23,41( H )– R23,41 ( H max))/ R23,41 ( H max) and the longitudinal MR as ( R12,34( H )– R12,34 ( H max))/ R12,34 ( H max), where H max is the maximum applied field,and R12,34 and R23,41 are as defined in the text.

Figure 7. Transverse (a) and longitudinal (b) MR measured for a multilayerconsisting of 50 repeats of nominal thickness 30 Å Co-Ni-Cu/20 Å Cu elec-trodeposited on GaAs(111).

Figure 8. Maximum value of the transverse MR as a function of Cu layerthickness t Cu measured for multilayers consisting of 50 repeats of nominal

thickness 30 Å Co-Ni-Cu/ t Cu Å Cu electrodeposited on GaAs(001) (solidsquares) and on GaAs(111) (open circles).





7. J. W. Chang and L. T. Romankiw, in Proceedings of 3rd International Symposium

on Magnetic Materials, Processes and Devices, The Electrochemical Society Pro-ceedings Series, PV 94-6, p. 223, Pennington, NJ (1994).

8. R. Hart, M. Alper, K. Attenborough, and W. Schwarzacher, Proceedings of 3rd

International Symposium on Magnetic Materials, Processes and Devices, L. T.Romankiw and D. A. Herman, Jr., Editors, PV 94-6, p. 215, The ElectrochemicalSociety Proceedings Series, Pennington, NJ (1994).

9. A. V. Demenko, A. I. Maslii, and V. V. Boldyrev, J. Mater. Synth. Process., 3, 303(1995).

10. G. Binasch, P. Grünberg, F. Saurenbach, and W. Zinn, Phys. Rev. B: Condens. Mat-

ter , 39, 4828 (1989).11. M. N. Baibich, J. M. Broto,A. Fert, F. Nguyen Van Dau, F. Petroff, P. Etienne, G.Creuzet, A. Friederich, and J. Chazelas, Phys. Rev. Lett., 61, 2472 (1988).

12. M. Alper, K. Attenborough, R. Hart, S. J. Lane, D. S. Lashmore, C. Younes, and W.Schwarzacher, Appl. Phys. Lett., 63, 2144 (1993).

13. W. Schwarzacher and D. S. Lashmore, IEEE Trans. Magn., 32, 3133 (1996) andreferences cited.

14. S. K. J. Lenczowski, C. Schönenberger, M. A. M. Gijs, and W. J. M. de Jonge, J.

Magn. Magn. Mater ., 148, 455 (1995).

15. Y. Jyoko, S. Kashiwabara, and Y. Hayashi, J. Electrochem. Soc., 144, L5 (1997).16. E. Chassaing, A. Morrone, and J. E. Schmidt, J. Electrochem. Soc., 146, 1794

(1999).17. A. P. O’Keeffe, O. I. Kasyutich, W. Schwarzacher, L. S. de Oliveira, and A. A. Pasa,

Appl. Phys. Lett ., 73, 1002 (1998).18. W. Schwarzacher, M. Alper, R. Hart, G. Nabiyouni, I. Bakonyi, and E. Tóth-Kádár,

Mater. Res. Soc. Symp. Proc., 451, 347 (1997).19. K. Attenborough, H. Boeve, J. De Boeck, G. Borghs, and J.-P. Celis, Appl. Phys.

Lett ., 74, 2206 (1999).20. G. Nabiyouni and W. Schwarzacher, J. Magn. Magn. Mater ., 156, 355 (1996).

21. J. Yahalom and O. Zadok, J. Mater. Sci., 22, 499 (1987); J. Yahalom and O. Zadok,U.S. Pat. 4,652,348 (1987).22. D. S. Lashmore, R. Oberle, and M. P. Dariel, in AESF 3rd International Pulse Plat-

ing Symposium, Washington, D.C., AESF, Orlando, FL (1986).23. R. Hart, P. A. Midgley, A. Wilkinson, and W. Schwarzacher, Appl. Phys. Lett., 67,

1316 (1995).24. D. H. Mosca, F. Petroff, A. Fert, P. A. Schroeder, W. P. Pratt, Jr., and R. Laloee, J.

Magn. Magn. Mater ., 94, L1 (1991).25. R. Hart, Ph.D. Thesis, University of Bristol, U.K. (1996).

comparison of the structure and magnetotransport properties

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