Analysis of interfacial reactions of Fe films on monocrystalline GaAs

ELSEVIER

Journal of Magnetism and Magnetic Materials 175 (1997) 219 227

j R Journalof . , . magnetism and , J R magnetic materials

Analysis of interfacial reactions of Fe films on monocrystalline GaAs M. Rahmoune,

J.P. E y m e r y * , M . F . D e n a n o t

Laboratoire de M~tallurgie Physique, UMR 6630 du CNRS, Universit~ de Poitiers, Bdtiment SP2MI, Bd. 3, T~l~port 2, BP 179, 86960 Futuroscope Cedex, France

Received 2 May 1997;received in revised form 16 July 1997

Abstract The reactions between monocrystalline GaAs(1 0 0) and thin polycrystalline Fe films have been investigated by application of conversion electron M6ssbauer spectroscopy and high-resolution electron microscopy. The presence of a thin amorphous intermixed layer at the Fe/GaAs interface in as-deposited samples is first pointed out. The reacted layer, 8-10 nm thick, forms upon the native oxide of the GaAs substrate during the deposition process of the iron. The solid-state interdiffusions are also studied in the temperature range 400-550°C. They are shown to produce a layered microstructure of the type Fe/Fe3GaCy (0 ~
scopy

1. Introduction The incorporation of metal layers into semiconductors is attractive for potential applications in novel electronic devices. During the past 15 years a large number of studies of the chemistry at metal/III-V semiconductor interfaces have been

* Corresponding author. Tel.: +33 5 49 49 67 35; fax: +33 5 49 49 66 92.

published. The magnetic properties of BCC Fe films deposited onto GaAs substrates have been already investigated [1-5] because hybrid ferromagnetic metal/semiconductor structures are of considerable interest for the incorporation of magnetic elements into planar electronic structures. In particular, the strongly reduced magnetisation and thickness dependent in-plane magnetic anisotropies found in the Fe/GaAs(1 00) system are discussed in Refs. [6, 7]. On the contrary, the interfacial reactions between Fe films and GaAs

0304-8853/97/$17.00 ~ 1997 Elsevier Science B.V. All rights reserved PII S0304-88 53(97)00352- 1

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M. Rahmoune et al. /Journal of Magnetism and Magnetic Materials 175 (1997) 219-227

substrates were less studied. Recently, we investigated the solid-state interdiffusions between a thin film of polycrystalline Fe deposited under vacuum conditions and a monocrystalline GaAs(1 0 0) substrate [8]. The reactions at the interface were studied by using chiefly cross-sectional transmission electron microscopy (TEM). The purpose of the present article is to present more detailed work about the metallurgical reactions between Fe and GaAs(1 0 0). We exploit the advantage of using both conversion electron M6ssbauer spectroscopy (CEMS) and high-resolution electron microscopy (HREM) techniques to investigate the intermixed layers. This article is organised like the previous one [8]. Primarily, we attend to the structural properties of the interfacial zone in the as-deposited conditions. Annealing experiments in the range 400-550°C have been then performed to promote diffusion and modify the interface. Respective changes in magnetic and microstructural properties are then presented. Since the results of the previous Fe/GaAs study [8] are of interest, they will be briefly summarised in the next section.

2. Previous study of Fe/GaAs reactions In Ref. [8], the reaction between thin Fe films and gallium arsenide was studied using chiefly T E M and energy dispersive spectrometry experiments. Polycrystalline Fe films, 50-120nm in thickness, were deposited onto GaAs(1 0 0) subtrates by ion-beam sputtering. Fe was shown to react readily with GaAs during deposition. The reacted layer, 7-9 nm thick, was both iron rich and mainly amorphous. Its formation was assumed to be assisted by the latent heat released from metal condensation during deposition. Moreover, the level of residual stress a in the as-deposited Fe films was determined using the so-called sin2~ method, which is a particular application of X-ray diffraction. The results (a ,~ - 2 GPa) indicated that the Fe films were in a high compressive state due to the preparation mode. The second stage of the work was devoted to the study of the interdiffusions between Fe and GaAs in the temperature range 400 550°C. The reactions were shown to be diffusion controlled and to in-

volve the exchange of Fe and Ga across the original interface, the arsenic being relatively immobile. It was also observed a layered microstructure which could be described by the sequence Fe/Fe3Ga/ Fe2As + FeAs/GaAs. The upper Fe layer (initially 120 nm thick), which was still visible after 1 h at 450°C, was apparently consumed in the reaction after 1 h at 500°C. The F e - G a and Fe-As layers exhibited fine grains and coarse grains, respectively. Let us also recall that the Fe2As phase displays a tetragonal structure (a = 0.363 nm and c = 0.598 nm) while the FeAs phase exhibits an orthorhombic structure with parameters a = 0.544 nm, b = 0.337 nm and c = 0.603 nm. After annealing, the interfacial zone between iron arsenides and gallium arsenide also showed an undulating morphology; the wavelength of the periodic roughness was measured between 300 and 600 nm. The modulation phenomena were analysed in terms of strain-energy relaxation.

3. Experimental Commercial GaAs(00 1) wafers were used as substrates and were first treated chemically in a solution of 0.2% Br in methanol for 5 s, before bringing them into the vacuum system. Fe films, 50-120 nm thick, were evaporated at a rate of approximately 0.03 nm s- 1 and at a starting pressure of 2 x 10 -4 Pa using an ion-beam sputtering chamber I-9]. Both deposition rate and film thickness were monitored by a quartz crystal balance, howver the actual thickness of the as-deposited layers was determined with a D E K T A K 3030 profilometer. During deposition, the substrates were mounted on a water-cooled sample holder so that the wafer temperature could not exceed 80°C. For isochronal annealing studies, the Fe/GaAs couples were annealed ex situ in v a c u u m ( 1 0 - 4 Pa) for 1 h at temperatures ranging between 400 and 550°C. For cross-sectional T E M experiments, the Fe films were glued together face-to-face in a sandwich structure and then cut in vertical sections which were first thinned by mechanical polishing to a thickness of about 60 gm. Final thinning to electron transparency was achieved by Ar ÷ ionbeam milling. The observations were performed at

M. Rahmoune et al. / Journal of Magnetism and Magnetic Materials 175 (1997) 219-227

300 kV in a JEOL 3010 microscope equipped with a pole piece allowing a point-to-point resolution of ,-~0.2 nm. CEM spectra were recorded at room temperature by using a constant acceleration M6ssbauer set-up with 57Co : Rh (50 m Ci) as a source. A gas flow (He + 5% CH4) proportional counter [10] was used to record the emitted conversion (7.3 keV) and Auger (5.5 keV) electrons subsequent to the resonant y-ray scattering by 57Fe nuclei. The spectra were analysed by least-squares fitting with Lorentzian lines with or without a distribution of hyperfine fields. All isomer shifts are quoted with respect to that of ~-Fe at 295 K.

4. Results

22l

Ref. [4]. The CEMS technique is based on the detection of the re-emitted conversion and Auger electrons after the resonant absorption has taken place. In the case of the 14.4 keV state in 57Fe, the conversion electron mode is actually the strongest, accounting for about 90% of the total re-emission intensity. The penetration depth is of the order of 150nm for the electrons. Moreover conversion electrons which are emitted at the depth ~ from the surface have a probability of emerging from the sample which monotonically decreases with increasing ~ value [-11]. The aim of this section is to show that the CEMS technique is able to determine the thickness of the intermediate layer in the asdeposited Fe/GaAs(1 0 0) samples. For this, we prepared five samples with various Fe layer thickness (e) and registered their CEM spectrum at room temperature. The e values retained were 50, 65, 80, 100 and 120 nm, respectively. The spectrum collected from the sample with e = 50 nm is presented in Fig. 1. All the spectra exhibit six lines, thus indicating a clear ferromagnetic state of BCC Fe, as expected. It is also seen that the second and fifth peaks are the most intense

and discussion

4.1. As-deposited state 4.1.1. CEMS experiments The possibility for the CEMS technique to analyse the interfacial zone in as-deposited Fe/ GaAs(1 1 1) couples was already pointed out in

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V(mm.s") Fig. 1. C E M spectrum for an as-deposited 50 n m Fe/GaAs(1 0 0) sample measured at room temperature. The inner sextet is attributed to the interfacial zone. The remainder corresponds to the ct-Fe fraction.

M. Rahmoune et al. / Journal o f Magnetism and Magnetic Materials 175 H997) 219-227

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which indicates that the hyperfine field is parallel to the film plane. This result is in agreement with hysteresis loop studies showing that the magnetisation is in the film plane [4]. The spectra cannot be fitted by a single Fe sextet; a second one has to be introduced to take into account the asymmetry observed at the base of the lines. This is attributed to the presence of two Fe sites, one type would correspond to Fe atoms at the top and the centre of the Fe layers whereas a second type to those at the interface having some Ga and/or As atoms as first neighbours. The contribution to the spectrum coming from the second type of Fe atoms will increase as e decreases. Hence the plot of the intermixed Fe amount as a function of 1/e allows the interface thickness (e) to be estimated (Fig. 2). A straight line is obtained with the data corresponding to e = 50, 65 and 80 nm, respectively. F r o m the slope of the line, it is inferred that e is about 10 nm; this result is in satisfactory agreement with previous studies [4, 8]. In Fig. 2, the data for e = 100 and 120 nm slightly deviate from the average line because the probability for the conversion electrons emitted in the interfacial zone to emerge from the surface sample has notably decreased. Finally, for e >~ 150 nm (i.e. 0 < 1/e ~< 0.0066 n m - 1), the mean free-path of the scattered electrons is lower than the Fe layer thickness and hence no signal can be detected from the mixed area. Another interesting result may be drawn from the series of CEM spectra. The hyperfine field of the main sextets due to Fe atoms located in the upper

part of the Fe layer is 33 T, as expected. On the contrary, the hyperfine field values deduced from the additional sextets (i.e. those attributed to interfacial Fe atoms) stand between 30 and 31 T. Since Ga and As are magnetically inactive, these values can be attributed to Fe atoms with only one or two Ga (or As) atoms as first neighbours [12]. Hence it is inferred that the reacted layer at the Fe/GaAs(1 0 0) interface is iron rich, a result which also agrees with previous work [8].

4.1.2. HREM experiments Here, emphasis is placed on the interface morphology in the as-deposited conditions. Fig. 3 is a high resolution micrograph of a cross section of a 50 nm Fe/GaAs(1 0 0) sample. The lattice fringes in the micrograph show a crystalline Fe layer with fine grains on the single-crystal GaAs wafer. At the Fe/GaAs interface there are two distinct sublayers.

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Fig. 2. Variations in mixed Fe fraction as a function of 1/e where e denotes the thickness of the Fe overlayer. The slope allows the thickness of the interracial zone to be calculated.

Fig. 3. Cross-sectional T E M micrograph of an as-deposited Fe/GaAs sample. The high-resolution image shows native oxide (white band) and a m o r p h o u s intermixed layer between the Fe overlayer and the GaAs substrate.

M. Rahmoune et al. / Journal of Magnetism and Magnetic Materials 175 (1997) 219-227

First a thin, somewhat inhomogeneous, bright layer appears immediately on the GaAs substrate. It is presumably due to native oxide because such a bright contrast is generally expected for light elements, such as oxygen. This layer has an average thickness of 2 nm but is not uniform along the interface. Between the native oxide and the Fe layer, there is another layer with grey contrast, which appears to be mainly amorphous with however, from place to place, some evidence of crystallinity. This intermixed layer has an average thickness of 8 10 nm, which is greater than that of the native oxide, and is non-uniform along the Fe/GaAs interface. Hence we suggest that the microstructure of the as-deposited Fe/GaAs(1 0 0) samples is described by the sequence Fe/amorphous Fe-Ga-As/oxide/GaAs. An initially intermixed layer at metal/GaAs interface has been observed in other systems such as Pd/GaAs and Ni/GaAs [13]. For these, Pd and Ni react with GaAs to form metastable ternary phases like PdxGaAs and NixGaAs, respectively, even in the as-deposited conditions. The present work shows that Fe also forms an initial intermixed layer (8-10 nm in thickness) at the interface with a GaAs substrate; evidence is found from both CEMS and HREM experiments. Moreover, the Fe/GaAs system shows a different type of intermediate layer from that of the Pd/GaAs and Ni/GaAs systems in that the initial interlayer is amorphous. Our findings match those reported by Ko and Sinclair [14] who observed that a thin amorphous intermixed layer was present at the Pt/GaAs interface in the as-deposited conditions. As mentioned previously [8], we assume that the formation of the intermixed layer at the Fe/GaAs interface is assisted by the latent heat released from metal condensation during deposition; then the intermixing may be thermally driven by the negative thermodynamic heat of mixing of the Fe-Ga and Fe-As alloys [15]. However, we cannot exclude that the presence of native oxide affects the initial reaction.

223

tion (interdiffusion) was monitored by both CEMS at room temperature and high resolution TEM. For the latter technique, the thinning procedure described in Section 3 was applied after annealing treatments. 4.2.1. HREM experiments

The present section is a continuation of our earlier study [8] where we pointed out the existence of a layered microstructure of the type Fe3Ga/ FezAs + FeAs/GaAs; this sequence was observed in a 120 nm Fe/GaAs(1 0 0) sample heat-treated at 500°C for 1 h. Actually owing to recent CEMS experiments which we report in the next section, we now propose that the phase in the upper sublayer is rather Fe3GaCr (0 ~< y ~<0.5) than F%Ga. Let us recall that the compound Fe3GaCr displays a BCC structure for y ~ 0 and a perovskite structure for y ~ 0.5. In the present study, the y value could not be experimentally determined, but it might be slightly greater than zero if surface contamination happened during the annealing process. Here we report on the HREM experiments performed on both iron-gallide and iron-arsenide layers obtained after annealing at 500°C for 1 h. Fig. 4 is a HREM image of the boundary between the substrate and a coarse FezAs grain. It shows that the Fe/As/GaAs interface is rather abrupt; apparently the initial reacted layer, depicted in Section 4.1.2, has been completely consumed during the reaction. Fringes of 0.285 nm are those of the (2 0 0) planes of the substrate, and fringes of

4.2. Thermal reactions

0.285 nm Subsequently the Fe/GaAs(1 00) couples were isochronally (1 h) annealed in vacuum at temperatures between 400 and 550°C. The solid-state reac-

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Fig. 4. High-resolution image of the Fe2As/GaAs interface taken after annealing at 500°C for 1 h.

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0.31 nm are those of the (0 1 1) planes of iron arsenide. Hence the orientation relationship between Fe2As and GaAs is determined to be (2 0 0)GaAsrl(0 1 1)Fe2As. Similarly, by using another image of the same area, we could also point out the relationship (1 1 1)GaAsll(1 0 1)FeeAs. The FeAs/GaAs interface was also investigated and the

HREM images, which we do not include here, revealed (1 0 0) planes of the FeAs phase as well as (1 1 1) planes of the substrate; it was then inferred the orientation relationship (1 1 1)GaAsll (1 0 0)FeAs. However no relationship could be determined at the boundary of the two FeAs and Fe2As arsenides. Finally, Fig. 5 is an image of the interface between a coarse Fe2As grain and the Fe3GaC r sublayer. It shows that the average grain size of the latter phase is very small ( ,-~3.5 nm); this result will be helpful for discussing CEMS experiments in the next section. Moreover, the fringe spacing (0.211 nm) measured in the fine grains is in good agreement with that calculated for the (1 1 1) planes in bulk Fe3Ga (0.212 nm); it is then inferred that the y value does not much deviate from zero.

4.2.2. CEMS experiments Fig. 6 shows both the CEM spectra at 295 K (a, b, c) and the corresponding hyperfine field distributions, P(H) (a', b', c'), for an annealed Fe layer of 50 nm initial thickness. The results of the fitting procedures are also summarized in Table 1. The spectrum of the as-deposited state was already presented in Section 4.1.1. After annealing for 1 h at 400°C, the spectrum (Fig. 6a) consists of two components, namely a sharp sextet corresponding to unmixed a-Fe and a broad magnetic line due to the mixed zone. The relative area of the latter component is 30% while the hyperfine field, averaged on the total spectrum, is ( H ) = 29 T. The corresponding field distribution (Fig. 6a') mainly shows two probability maxima, centered around 10 and 33 T, respectively. The latter value obviously comes from the a-Fe phase.

Fig. 5. High-resolution image of the FezAs/F%GaC r (0 ~< y ~<0.5) interface taken after annealing at 500°C for 1 h.

Table 1 M6ssbauer parameters as obtained by fitting the CEM spectra collected through an isochronal kinetic performed with a 50 nm Fe/GaAs(1 0 0) couple (A = quadrupole splitting; 6 = isomer shift; ( H ) average hyperfine field calculated from the hyperfine field distribution) T (°C)

400 450 500 550

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M. Rahmoune et al. / Journal of Magnetism and Magnetic Materials 175 (1997) 219 227

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Fig. 6. CEM spectra for a 50 nm Fe/GaAs(1 0 0) sample annealed at (a) 400°C, (b) 450°C, (c) 500°C and the corresponding hyperfine field distributions, P(H), (a', b', c') extracted from the fits of the spectra.

On the contrary, it is likely that the former peak in the distribution corresponds to the early FezAs grains formed at the interfacial zone. Actually, the FezAs compound is antiferromagnetic with a N6el temperature of 77°C; moreover its room temperature M6ssbauer spectrum exhibits two distinct hyperfine fields of 8.5 and 10.5 T, respectively [16]. The C E M spectrum collected after 1 h at 450°C is presented in Fig. 6b which now exhibits a central paramagnetic component in addition to the features of the spectrum in Fig. 6a. The percentage of paramagnetic Fe atoms is close to 30%. The results

of the fitting procedure with two quadrupole doublets, plus a distribution of hyperfine fields starting at 13 T are also shown in Fig. 6b. The values of both isomer shift 6 and splitting A of the doublets are 61 = 0.47 mm/s, A1 = 1.02 mm/s, 62 = 0.09 mm/s and A2 = 1.29 ram/s, respectively. The 6 values of the two paramagnetic components are rather different but the fitting of the spectrum is not really unambiguous. The { H ) value of the ferromagnetic fraction alone is 18.7 T. The corresponding field distribution (Fig. 6b') still exhibits two main contributions which can be attributed to FezAs and a-Fe

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M. Rahmoune et al. / Journal of Magnetism and Magnetic Materials 175 (1997) 219 227

phases as previously. The non-magnetic component in the spectrum is assigned to both FeAs phase at the interface and a superparamagnetic relaxation of Fe3GaCy grains. Let us recall that the FeAs c o m p o u n d is antiferromagnetic with a N6el temperature of - 1 9 5 ° C [17]. Moreover the H R E M study reported in Section 4.2.1 showed that the average grain size in the gallium rich sublayer was around 3.5 nm; the value is sufficiently small to observe collective reorientation of the magnetic moment direction at room temperature. One may also note that the Fe3GaCy (y ¢ 0) carbides are known to display quadrupole splittings as large as 1.3 mm/s in the paramagnetic state [18]. This is the reason why we proposed that the phase in the upper sublayer is rather Fe3GaCy (y ~ 0) than Fe3Ga; another possibility would be the presence of several compounds with different y values. The carbon could come from contamination during the annealing treatments. After 1 h annealing at 500°C (Fig. 6c and Fig. 6c'), the ~-Fe spectral lines as well as the peak centered at around 33 T in the field distribution have disappeared. It is then inferred that the initial Fe overlayer has been completely consumed in the thermal reaction. The spectrum (Fig. 6c) was fitted by using the same method as previously. The resuits indicate that the ferromagnetic Fe fraction decreases to ~ 5 0 % , with an average hyperfine field of 13 T. The parameters of the central doublets are now 61 = 0.49 mm/s, A1 = 0.91 mm/s, 62 = 0.06 m m / s and A 2 = 1.17 mm/s. Finally, the spectrum obtained after 1 h at 550°C (not shown here) indicates only minor changes with respect to the latter one. The ferromagnetic Fe fraction remains around 50%; however the ( H ) value is now 10.6 T. The parameters of the non-magnetic Fe fraction are given in Table 1; no significant changes appear, so that the paramagnetic and superparamagnetic phases are nearly the same at 500 and 550°C.

terface. For this, we assume that the mixed thickness ( e o - z) (Fig. 7) is proportional to (2Dt) 1/2 where eo is the initial Fe thickness, z is the Fe thickness remaining after heat-treating at temperature T and t is the annealing time; D denotes a diffusion coefficient which is assumed to follow an Arrehnius law. The proportionality between (eo - z) and (2Dt) x/2 was not verified in the present study; however such a law was already evidenced by several authors. In particular, Kim et al. [19] showed that the growth of the TiAs phase, at the

Fig. 7. Schematic diagram of the layered microstructure obtained after annealing the Fe/GaAs(10 0) couples at temperatures ranging between 400 and 500°C for 1 h (e0 and z denote the initial and final thicknessesof the Fe overlayer,respectively).

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4.2.3. Kinetic m e a s u r e m e n t s

F r o m the above C E M S study, it appears that M6ssbauer spectroscopy is well suited for determining the thickness of the unreacted Fe layer throughout the isochronal kinetic. Here we use this property to calculate the overall activation energy for the thermal reaction at the Fe/GaAs(l 0 0) in-

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M. Rahmoune et al. /'Journal of Magnetism and Magnetic Materials 175 (1997) 219 227

Ti/GaAs interface, obeyed a parabolic rate law, thus suggesting diffusion controlled growth. This result is quite common in metal/GaAs thermal reactions. Fig. 8 shows the variations in ln[(eo - z)2/ 2t] as a function of lIT for the Fe/GaAs system, and a least-squares fit to the data yields an activation energy Q of 1.5 eV (144.5 kJ mol-1). Similar Q values have been reported in other metal/GaAs couples. As far as the Ti/GaAs system is concerned, Kim et al. [19] have determined an activation energy of 1.75 eV, a value which matches that determined in the Fe/GaAs system.

227

ferromagnetic metal. Finally, the overall activation energy for the reaction was calculated to be 1.5 eV. As the microelectronics industry moves into the area of deep submicron devices, metallization has become the main performance limiting factor. H R E M will undoubtedly continue playing a major role in the investigation of interfaces of metal thin films on GaAs and other III-V compounds. However, we would like to highlight the possibilities presented by the CEMS technique when the overlayer contains either 57Fe or another M6ssbauer isotope allowing CEMS experiments.

5. Conclusions This paper shows new results in the investigation of the interracial reactions of polycrystalline Fe films deposited onto monocrystalline GaAs(1 0 0). The findings, along with those reported in a previous paper [8], constitute a coherent whole. The present investigations were performed by using both CEMS and H R E M techniques, and the main conclusions drawn from this study are the following. (1) In the as-deposited state, the microstructure of the samples determined from H R E M experiments can be described by the sequence Fe/ amorphous Fe-Ga-As/oxide/GaAs. The thicknesses of the intermediate sublayers are 8 10 nm and 2 nm, respectively. The existence of an ironrich interfacial zone, 10 nm thick, was also pointed out through CEMS experiments. (2) After isochronal annealings at temperatures ranging between 400 and 550°C, it appeared a layered Fe/Fe3GaCy (0 ~< y ~< 0.5)/FexAs (x = 1, 2)/ GaAs microstructure. After 1 h heat-treating at 500°C, the upper Fe sublayer has been consumed in the reaction. Through H R E M experients, we showed that the F%GaCy sublayer exhibited fine grains with an average size of 3.5 nm. We also determined the orientation relationships at the FezAs/GaAs and FeAs/GaAs interfaces. Thanks to the CEMS technique, we estimated the percentage of unmixed Fe atoms as well as the ferromagnetic and paramagnetic Fe fractions, at each stage of the isochronal kinetic. The results are interesting for magnetic applications since Fe is the prototypical

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