Cu superlattices on sapphire substrates with and without buffer layers

Journal of Crystal Growth 127 (1993) 682—685 North-Holland

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Epitaxial growth of Co films and Co/Cu superlattices on sapphire substrates with and without buffer layers K. Bröhl, P. Bödeker, N. Metoki, A. Stierle and H. Zabel Inst itut für Festkörperphysik, Fakultät für Physik und Astronomie, Ruhr Uniuersitàr Bochum, D-W-4630 Bochum 1, Germany

Co(0001)/(111) films and Co/Cu(111) multilayers were grown by MBE on sapphire (11~0)substrates. Co grows in a good epitaxial manner directly on the sapphire substrate as well as on a Cu(1 I 1)/Nb(1 10) buffer layer system. Co/Cu(1 11) superlattices were grown on the same buffer system. Extensive RHEED and X-ray characterizations demonstrate the high film quality.

1. Introduction

In fig. la we present a typical reflectivity mea-

Metal films and superlattices of high quality are in increasing demand for fundamental studies and applications. To meet the challenge of a steadily improving structural perfection, proper substrates and buffer layers have to be chosen, and the growth conditions including growth rate and substrate temperature must be well tuned. Furthermore, the structural properties need to be well characterized, primarily to relate the physical properties to structural characteristics and secondarily, to improve the growth conditions. In the following, we describe the growth of Co films on sapphire substrates as well as Co/Cu superlattices by molecular beam epitaxy (MBE). We will also provide extensive X-ray characterization of the films using high resolution and

surement from a 150 A thick Co film on sapphire. Reflectivity yields information on the electron density profile normal to the film surface. The solid line represents a fit to the data points using a model described in ref. [21 with a calculated oxide layer thickness of 20 A. The high crystal quality of the films is also expressed in pronounced finite size thickness oscillations (Laue oscillations) in radial scans close to the (111) Bragg peak (fig. ib). The film grows predominantly in the fcc phase up until 400 A, beyond, it transforms to the hcp structure. The film has a narrow mosaicity of 0.03°as measured from rocking curves. The epitaxial relation between Co and sapphire is determined by glancing angle surface X-ray scattering and is _given by {112]Co Il [1100]A1203 and [220]Co 1

surface X-ray scattering techniques.

[0001]Al 2°3•

2. Growth of Co films on A1203(1120)

3. Growth of Nb buffer layer on A1203(1120)

Cobalt is evaporated from an electron beam source at a rate of 0.5 A/s. The optimal growth temperature of 3/8 Tm given by Flynn [1] is 390°C. We tried substrate temperatures from 50 to 550°Cand found films with the best crystalline quality to be deposited at a temperature of 450°C. The surface smoothness was improved by annealing at 550°Cas verified by X-ray measurements.

Niobium is an excellent buffer material for growing films on sapphire. Nb and Al203 have nearly the same thermal expansion coefficients. The growth of Nb on sapphire has been well studied by Claasen et al. [3], Mayer et al. [4] and Nishihata et al. [5] using different Al203 orientations. Nb is a material with a high melting point and can only be evaporated from an electron

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Elsevier Science Publishers B.V. All rights reserved

K Bröhl et al.

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Fig. 1. (a) Reflectivity measurement and (b) Bragg scan close to the (111) peak of a 150

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beam gun. Nb(110) grows on Al203(1120) quite well at substrate temperatures ranging from 750 to 950°C.We have grown buffer layers at 900°C with a growth rate of 0.5 A/s and a thickness of 50 to 100 A. To improve the smoothness of the Nb surface, sample heating to 950°Cis very useful. After 15 mm annealing, the RHEED pattern shows more than one Laue zone, indicating a smoother surface than before. Fig. 2 shows the RHEED pattern of a Nb(110) surface. The epitaxial relation between Nb and sapphire is given by [112]Nb II [1100]Al 203 and [111]Nb II [0001]A1203 as obtained by glancing angle X-ray diffraction. The mosaicity of the Nb buffer film depends on the quality of the

A1203(1120) surface and is 0.02°for the best Nb films. RHEED observations of the (110) surface of the Nb buffer layer show a (~x v’~)R54.7° superstructure, which has been recently observed by other workers as well [6]. This superstructure is seen in the RHEED pattern with [001] azimuth in fig. 2b.

4. Growth of Cu(111) on Nb(11O) buffer Because of the reactivity with Nb, Co should not be grown directly on Nb. We grew a buffer layer of Cu(1 11) on Nb(110) before initiating Co growth. With its relatively low melting point of

Fig. 2. RHEED pattern of the Nb(1I0) surface. In [111] azimuth the pattern is not distorted (a) and in [001] azimuth one additional streak occurs (b).

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/ Epitaxial growth of Co films and

Co / Cu SLs on sapphire substrates

at the Cu/Co interface, the substrate temperature is kept below 100°Cduring Co growth. With increasing thickness of the coherent cobalt layer, the lattice parameter decreases, indicating a por-

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tion of hcp Co. Fig. 3 shows a reflectivity measurement of a system with a 40 A Nb(110) buffer layer on A1203(1120) covered with 12 A Cu(111) and 207 A Co(111).

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1083°C Cu may be evaporated from an effusion cell with a crucible of pyrolytic boron nitride. For rates between 0.05 and 0.5 A/s, the best substrate temperature was found to be 400°C.The epitaxial relation between Cu and Nb is given by [001]Cu 1 [10T]Nb and [110]Cu 1 [212]Nb, in agreement with the well known Nishiyama— Wassermann relation [7]. ,

6. Growth of Co/Cu(111) superlattices We have also grown Co/Cu( 111) superlattices on A1203(1120) substrates with Nb(110) buffer layers. In superlattices, the diffusion between layers becomes of increasing importance as the ratio of interface to layer thickness grows. To reach a high degree of crystallinity, the growth of the superlattice was started with a Cu seed layer at the optimized substrate temperature of 400°C. In order to attain both good crystallinity and minimal interlayer diffusion, the growth temperature was reduced to 30°Cfor subsequent layers. The growth rates were 0.1 A/s for Co and 0.07 A/s

/ Nb(11o) /

for Cu. Fig. 4 shows RHEED patterns from the surface the in firsttheCu(111) layer and of the last Co(111) oflayer superlattice. X-ray reflect-

Co grows on Cu(111) pseudomorphically in the metastable fcc phase. To minimize the diffusion

ivity measurements confirm the interfacial smoothness, and high angle Bragg scans exhibit

5. Growth of Co on M203(h120)

Cu(1l1)

Fig. 4. RHEED pattern of the first Cu(111) (a) and the last Co(111) layer (b) of a Co/Cu superlattice on Nb(110)/Al2O3(11~0) with 10 periods of Co/Cu.

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pronounced satellite reflections, indicative of the existence of a coherent Co/Cu superlattice. Values for the individual layer thicknesses, the interlayer roughness and the thickness of the oxide layer on top of the film were obtained from fits to the X-ray data [81. Fig. 5a shows a reflectivity measurement and fig. 5b a Bragg scan of a Co/Cu(111) superlattice. The rocking curve of the Co/Cu superlattice (111) peak shows a mosaicity of 0.07°. From X-ray in-plane measurements, no differences of the Co and Cu in-plane lattice parameters were found, demonstrating a complete accommodation of the Co and Cu lattice in the film plane.

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400°C.The superlattices grown at room temperature successfully avoided diffusion between layers. In situ RHEED and ex situ X-ray characterizations show the high quality of the films.

Acknowledgements This work was supported by the Deutsche Forschungsgemeinschaft (SFB 166) and by the Ministerium für Wissenschaft und Forschung NRW, which are gratefully acknowledged.

References 7. Conclusion We have presented the epitaxial growth of Co films and Co/Cu superlattices. Co grows on Al203(1120) in the fcc [111] direction directly or with appropriate buffer layers. Nb is an excellent buffer layer material on sapphire. Annealing of the Nb layer after deposition improves the smoothness of the surface. This is important for better growth of the following layers. Before Co deposition, a further buffer layer of Cu has to grow at an optimal substrate temperature of

[1] C.P. Flynn, J. Phys. F (Metal Phys.) 18 (1988) L195. [21 A. Stierle et al., in: Surface X-Ray and Neutron Scattering, Eds. H. Zabel and F.K. Robinson (Springer, Berlin, 1992) p. 233. [3] J.H. Claassen et al., in: Metallic Multilayers and Epitaxy, Eds. M. Hong et al. (Metallurgical Society, 1988) p. 217. [4] J. Mayer, C.P. Flynn and M. Rühle, Ultramicroscopy 33 (1990) 51. ~ Y. Nishihata et al., J. AppI. Phys. 10 (1986) 3523. [6] C. Sürgers and H. von Löhneysen, AppI. Phys. A 54 (1992) 350. 17] E. Bauer, AppI. Surface Sci. 11/12 (1982) 479. [8] P. Bödeker et al., Phys. Rev. B 47, No. 2 (1993).