growth and structure of fcc (100) co/cu and fe/cu multilayers

Journal of Magnetism and Magnetic Materials 121 (1993) 24-29 North-Holland

Growth and structure of fcc (100) Co/Cu and Fe/Cu multilayers

F. G i r o n a, p. B o h e r a, Ph. H o u d y a, p. Beauvi l l a in b, K. Le D a n g b a n d P. Vei l le t b

a Laboratoires d'Electronique Philips (LEP), 22 Avenue Descartes, BP 15, 94453 Limeil-Brevannes Cedex, France b Institut d'Electronique Fondamentale (IEF), CNRS, URA 022, b~t 220, Universitd Paris Sud, 91405 Orsay Cedex, France

High quality Co/Cu and Fe/Cu multilayers have been deposited on (100) Cu/Si substrates using the diode-rf sputtering technique. A special preparation leads to a well-defined pseudo-epitaxial Co/Cu fcc (100) structure, as observed in situ on the RHEED patterns and ex situ by X-ray diffraction. Using nuclear magnetic resonance, we show that the (100) Co planes are also strongly textured on the microscopic scale. For the Fe/Cu multilayers the fcc (100) structure is metastable, and appears only if the Fe layer thickness is smaller than the Cu one. Otherwise the bcc (110) Fe/fcc (111) Cu structure is only detected without crystalline order in the plane of the films.

1. Introduction

During the last few years, a lot of work has been devoted to the study of C o / C u and F e / C u multilayers deposited on silicon [1-7]. The results clearly show that the magnetic properties depend on the deposition technique, on the interface roughness and on the crystallographic orientation of the structures [8]. It is of great interest to compare the two systems when the same orientation and similar roughness are obtained using the same deposition technique. The C o / C u multilayers directly deposited on silicon show generally the (111) fcc orientation [1-4]. Nevertheless, the (100) fcc orientation has been obtained by electron beam evaporation [9] and magnetron sputtering [10], using (100) Cu films epitaxially grown on (100) Si [11] as seed layer. The F e / C u multilayers deposited on silicon crystallize preferen- tially with a (110) bcc Fe 1(111) fcc Cu structure [5-7].

In this paper, we report on the structural properties of C o / C u and F e / C u multilayers epitaxially grown in the (100) orientation by diode-rf sputtering on silicon. (100) Cu is used as seed layer.

Correspondence to: Dr. F. Giron, Laboratoires d'Electronique Philips (LEP), 22 Avenue Descartes, BP 15, 94453 Limeil- Brevannes Cedex, France. Tel.: +33-1-45106751; telefax: + 33-1-45106743.

2. Experiment

( C u / M f ) × 3 0 / C u / S i multilayers, where Mf = Co or Fe, have been prepared using a diode-rf soputtering technique in the thickness range 8-60 A. The base pressure was close to 10 -7 Torr. During the deposition the argon gas pressure was kept at 8 × 10 -3 Torr. The rf-powers applied to the Cu, Co and Fe targets were 110, 80 and 200 W, respectively, and no intentional heating or cooling was applied to the substrate. This leads to the respective 0.88, 0.35 and 0.22 ,A/s deposition rates. The (100) Si wafers were chemically etched prior to be loaded in the deposition system and annealed under ultrahigh vacuum, during 40 s, by exposure to a halogoen lamp illumination of 3000 W. After that, 300 A of copper was deposited on the silicon and annealed during 8 s. The multilayers were grown on this buffer layer. In each case 30 periods were deposited. Two series have been realized for each system, varying either the Cu or the ferromagnetic metal thickness.

The periodicity and the mean interface roughness are determined using grazing X-ray reflec- tion (GXR). The thickness ratio is deduced from the different deposition times. The growth was followed in situ by kinetic ellipsometry (KE). Af- ter each stage of the fabrication, the roughness and the in-plane texture of the sample surface was checked by electron diffraction (RHEED) . Ex situ high angle X-ray diffraction (XRD) was

0304-8853/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved

F. Giron et al. / Growth and structure of Co/Cu and Fe/Cu multilayers 25

performed at the C u - K a line to determine the out of plane texture and the crystallite size. Fi- nally nuclear magnetic resonance (NMR) mea-

Cu (100)

ilii

[001] azimut

surements were carried out at 2 K on C o / C u samples using a variable frequency spin-echo ap- paratus.

Co/Cu (100)

Fig. 1. Example of RHEED patterns for the (100) [12.6 ,~ Co/20.9 ,~ Cu]x30/300 ,~ Cu/Si multilayer at different steps of the fabrication, (a) along the [011] azimuth and (b) along the [001] azimuth of the (100) cobalt planes. The sharp streaks give evidence

of a smooth and epitaxial growth.

26 F. Giron et al. / Growth and structure of Co/Cu and Fe/Cu rnultilayers

3. Results and discussion

Fig. 1 shows RHEED patterns similar for all the multilayers obtained at three stages of the fabrication process: after annealing of the silicon substrate, after annealing of the Cu buffer layer and after depositing the CO/Cu multilayer. In each case streaks are observed along the Si [011] and Si [001] azimuths. Moreover, a fourfold symmetry is detected when the sample is turned around the surface normal showing without ambi- guity a (100) cubic symmetry at each stage of the process. In addition, the sharpness of the streaks indicates that the surface is clean, single crystalline and weakly rough. The = 15 A native oxide layer which generally remains at the surface of the silicon after the chemical etching is com- pletely removed by the annealing under ultrahigh vacuum. The buffer layer is partly polycrystalline and partly textured since rings and streaks are observable. The correspondence between the Si and Cu/Si RHEED patterns shows that we have the epitaxial relation: [011] (100) fcc Cull[001] (100) Si as already observed when depositing Cu on silicon by evaporation [10].

3.1. Co / Cu multilayers

H I1~[ 100 ] / \

~ _ r ~ '

~ H

o

t J

z a .

180 200 220 F (l'lHz)

Fig. 2. Corrected Co spin-echo spectra at 2 K of a (100) [12.6 ,~ CO/20.9/~ Cu] x 30 muitilayered film. A 2 kOe dc field H is applied parallel either to the easy [011] axis or to the hard [001] axis in the film plane. Different contributions arising from inequivalent sites of the CO atoms with one Cu nearest

neighbour are represented by the dotted lines.

In the X-ray diffraction spectra of Co/Cu multilayers only one major peak is detected at = 25.3 ° for all thicknesses. It corresponds to the (200) fcc contribution. Satellite peaks are also observed which confirm the well-defined periodic structure of the multilayers.

The RHEED patterns corresponding to Co/Cu multilayers shows streaks sharper than those of the Cu/Si buffer (cf. fig. 1). This indicates that the (100) fcc planes are strongly textured and that the texture is improved during the growth of the multilayer. This is not surprising since the lattice mismatch between Co and Cu is only 1.9%. The partial texture of the Cu buffer layer is sufficient to initiate the (100) growth with a final strong texture in the plane of the films.

The Co spin-echo spectra were measured at different orientations of a 2 kOe applied field which is enough to saturate the magnetization in any direction in the film plane [12]. In all the

series we observed a main line which arises from fcc Co sites and also a Co satellite line, around 197 MHz, due to Co sites which have one Cu nearest neighbour, but this contribution depends on the in-plane orientation of the applied magnetic field (cf. fig. 2). The local symmetry break- ing due to the presence of a Cu atom causes the Co hyperfine field H h to become anisotropic and therefore the resonance frequency depends on the angle 0 between M s and the local symmetry axis [13]. We have

H h = His o + H a n ( 3 cos20 - 1)/2,

where Hiso and Han are respectively the isotropic and anisotropic parts of the hyperfine field. For M s parallel to the CO [011] crystallographic direction there are three inequivalent sites in the proportion 1:1:4, assuming a Cu random site distribution. For the Co [001] orientation there

F. Giron et aL / Growth and structure of Co/Cu and Fe/Cu multilayers 27

are two inequivalent sites in the proportion 1:2. The different contributions are estimated and reported in fig. 2 by the dotted lines. This inter- pretation agrees well with the experiment. Thus, the in-plane (100) fcc texture of the Co/Cu multilayers is confirmed even at this microscopic scale. Moreover, we observe that the spectrum obtained when the field is applied along Co [011] is the same as the zero field one. It indicates that the Co [011] axis is an easy direction for the magnetization.

3.2. Fe / Cu multilayers

The crystalline structure of the multilayers strongly depends on the thickness of the Fe layers. Fig. 3 shows the X-ray diffraction spectra for the series of samples with variable Fe thickness. In each case the (200) diffraction peak of the fcc Cu buffer layer is detected. For Fe thicknesses of 14 and 16 ~, another peak appears at 21.6 ° with a shoulder towards the higher angles. At these thicknesses the Cu layers are clearly oriented with the fcc (111) direction perpendicular to the substrate, whereas the Fe layers have the conven- tional bcc (110) structure as generally reported in the literature !5-7]. For lower thicknesses such as 7.5 and 8.2 A we find only one peak at 24.8 ° which overlaps the contribution of the Cu buffer layer. This peak can be attributed to the metastable F e / C u fcc (100) multilayered phase. Its position is shifted towards lower angles with regard to the Cu buffer contribution, which corresponds to an average lattice parameter expan- sion of about 1.5% along the growth direction. For all the samples, the average roughness measured by GXR is of the order of 7 A.

The RHEED patterns of the same series of samples, obtained just after deposition along the [001] azimuth (column A) and along the [011] Cu azimuth (column B), are shown in fig. 4. For the two thicker samples, the RHEED patterns are isotropic in the film plane. The rings indicate that the bcc Fe (110)/fcc Cu (111) texture is polycrystalline. This is not surprising since bcc Fe cannot grow epitaxially on fcc Cu due to the large lattice mismatch (---11%). When the iron thickness is

o

reduced below 14 A, well-defined streaks appear.

Cufcc (111) rebec (110} Cufcc (200)

.~ dCu ~14~

>-

w

J

(,k,l, , 20 22 2~

dFe

161

I i I I I 26 28 30

ANGLE (DEG) Fig. 3. X-ray diffraction spectra of Fe/Cu multilayers with variable Fe layer thickness. A phase transition from the fcc Fe /Cu (100) to the bcc Fe ( l l0) /fcc Cu (111) is detected for an iron thickness close to 10 A. The theoretical positions of different contributions calculated with the bulk lattice param-

eters are indicated.

We observe the same fourfold symmetry as for the Cu buffer layer alone or the Co/Cu multilayers giving evidence of a pseudo-epitaxial growth of the fcc (100) Fe /Cu multilayered structure on the Cu buffer layer. Furthermore, the streaks become sharper and more continuous at small Fe thicknesses showing an improvement of the structural quality compared to the Cu/Si substrate.

The same effect is also observed on a series of samples with fixed Fe thickness (~ 9 A). The fcc iron metastable phase is in fact only stabilized when the Cu layer thickness becomes larger than the Fe one.

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

In conclusion, we have shown that if-sputtered Co/Cu and Fe /Cu multilayered films can show a

28 F. Giron et al. / Growth and structure of Co/Cu and Fe/Cu multilayers

Fig. 4. RHEED patterns for the samples of fig. 3: (A) along the [001] azimuth and (B) along the [011] azimuth. For an Fe layer thickness lower than 10 .~, the sharp streaks evidence a well-defined in-plane texture with a fourfold symmetry. Above 10 .~ of iron

the films are polycrystalline.

F. Giron et al. / Growth and structure o f Co/Cu and Fe/Cu multilayers 29

well-defined (100) fcc texture with special preparation of the Cu/Si substrate. This was demon- strated by RHEED and X-ray diffraction and checked on the microscopic scale for the Co /Cu system using NMR. Due to the smaller lattice mismatch between Co and Cu than between Fe and Cu the (100) fcc multilayered structure is observed for a larger range of thicknesses in the case of cobalt.

References

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growth and structure of fcc (100) co/cu and fe/cu multilayers

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