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GaAs(0 0 1): a stable and magnetic metal-semiconductor heterostructure
Thin Solid Films 446 (2004) 6–11
Fe3GaAsyGaAs(0 0 1): a stable and magnetic metal-semiconductor heterostructure a, a ´ ´ ´ B. Lepine *, C. Lallaizona, P. Schieffera, A. Guivarc’ha, G. Jezequel , A. Rocherb, F. Abelc, d ´ C. Cohenc, S. Deputier , F. Nguyen Van Daue a
ˆ 11C, Campus de Beaulieu, F 35042 Rennes Cedex, Equipe de Physique des Surfaces et des Interfaces, UMR CNRS-Universite´ no. 6627, Bat. France b CEMES CNRS, 29 rue Jeanne Marvin, BP 4347, F 31055 Toulouse Cedex, France c ´ Paris VI et VII, Tour 23-2, place Jussieu, F 75251 Paris Cedex, Groupe de Physique du Solide, UMR CNRS-Universite´ no. 7588, Universites France d ´ Laboratoire de Chimie du Solide et Inorganique Moleculaire, UMR CNRS-Universite´ no. 6511, Campus de Beaulieu, F 35042 Rennes Cedex, France e UMR CNRS-Thales no. 137, Domaine de Corbeville, F 91404 Orsay, France Received 26 June 2003; received in revised form 24 July 2003; accepted 17 August 2003
Abstract We show that in agreement with the ternary Fe–Ga–As phase diagram, the solid-state interdiffusions in epitaxial FeyGaAs(0 0 1) heterostructures lead, at a temperature of approximately 500 8C, to the formation of thermodynamically stable Fe3GaAsy GaAs(0 0 1) contacts quite similar to the well-known silicideySi ones. The Fe3 GaAs films are made of grains epitaxial on GaAs with a well-defined interface. Their magnetic and electrical properties make Fe3 GaAs on GaAs an attractive metallization scheme for future magnetoelectronic devices. The results we report concern (25 or 80 nm Fe)yGaAs(0 0 1) heterostructures annealed at 480 and 500 8C for 10 min and characterized ex situ by Heq Rutherford backscattering and ion channeling, X-ray diffraction, transmission electron microscopy and alternating gradient field magnetometry. 䊚 2003 Elsevier B.V. All rights reserved. PACS: 68.55a; 81.15Np; 81.05.Ea; 75.50.Cc Keywords: Solid phase epitaxy; Gallium arsenide; Iron; Magnetic properties
1. Introduction The growth of ferromagnetic films on semiconductor substrates has received much attention over the past decade. The combination of the magnetism of the metallic layer along with the electronic properties of the semiconductor substrate offers the opportunity for innovative magnetotransport effects and devices to be realized. However, because the two types of materials are very dissimilar in terms of physical, chemical and structural properties, integration of semiconductors and ferromagnets is generally very difficult w1–4x. *Corresponding author. Tel.: q33-2-23-23-69-21; fax: q33-2-2323-61-98. ´ E-mail address: [email protected] (B. Lepine).
The (ferromagnetic metal)yGaAs systems have been widely studied in the last few years, in particular the epitaxial FeyGaAs heterostructures w3,4x. However, due to a strong reactivity between ferromagnetic metals and GaAs, the resulting structures are not thermodynamically stable. The purpose of this study is to obtain epitaxial and stable (magnetic compound)yGaAs structures by inducing a controlled thermal interaction between a metal film and a GaAs substrate. Many studies have been dealing with the metalyGaAs solid-state interdiffusions during annealing treatments but, as in the cases of CoyGaAs w5x and NiyGaAs w6,7x contacts, the final step of the interaction was always found to be a mixture of binary or ternary compounds w1,2,5,6x: thermodynamically stable and epitaxial contacts equivalent to the CoSi2 ySi, NiSi2 ySi and CrSi2 heterostructures w8x have
0040-6090/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0040-6090Ž03.01232-X
´ B. Lepine et al. / Thin Solid Films 446 (2004) 6–11
never been obtained for GaAs in this way. In this context, relatively few works concern the interdiffusions in FeyGaAs structures. Rahmoune et al. w9,10x annealed FeyGaAs contacts prepared by ion-beam sputtering for 1 h at 500 8C and they observed a layered microstructure of the type Fe3Gay(Fe2AsqFeAs)yGaAs. However, no accurate studies of the final step of the solid-state interdiffusions in FeyGaAs heterostructures, prepared and annealed in ultra-high vacuum (UHV) conditions, have been published. Our starting point was the recent publications, concerning on one hand, the new magnetic ternary phases Fe3Ga2yxAsx w11–13x, and on the other hand, the experimental determination of the Fe–Ga–As ternary phase diagram w14,15x. The Fe3GaAs compound was identified for the first time by Harris et al. w11x, in the form of precipitates occurring during the growth of Fedoped GaAs single crystals using the liquid-encapsulation Czochralski technique. After that, the same authors prepared bulk Fe3GaAs samples by the same technique w12x: Fe3GaAs was identified as a NiAs-type phase (hexagonal B82-type) that exists over a range of compositions represented by the formula Fe3Ga2yxAsx (0.20(x(1.125) and presents a (2a,c) superstructure for 0.20(x(0.85. All the alloys were found to be ferromagnetic at temperatures higher than the ambient one. More recently, Fe3GaAs precipitates were also observed in Fe-implanted and annealed GaAs w16x and in Fe-doped GaAs grown by molecular beam epitaxy (MBE) w17x. ´ Concerning the ternary Fe–Ga–As diagram, Deputier et al. w14,15x determined the solid-state phase equilibria at 600 8C and showed the occurrence of tie-lines between all the compositions of the Fe3Ga2yxAsx solid solution (in particular Fe3GaAs) and GaAs. To our knowledge, it was the first time that a ternary phase Mx(GaAs)y (Mstransition metal) stoichiometric in Ga and As was found to be in thermodynamic equilibrium with GaAs. This fact suggests that the final step of the solid-state interdiffusions occurring during the annealing of a FeyGaAs heterostructure could lead to a stable Fe3GaAsyGaAs contact. Moreover, one could expect an epitaxial growth of the Fe3GaAs layer on GaAs due to the particular structural properties of the hexagonal Fe3Ga2yxAsx alloys. Indeed, firstly, the cya ratio of their crystalline parameters is close to 63y62 (1.23(cy a(1.28 for 0.20(x(1.125), which allows them to be described in a pseudocubic unit cell with a parameter (apseudocubics(3y62)=ahexagonal), and secondly, the corresponding apseudocubic parameter is close to one and a half that of GaAs (within 2%). Such epitaxial growths by solid-state interdiffusion were previously observed for several equivalent hexagonal pseudocubic compounds w1,2,6,7x. In a previous study we showed that the solid-state interdiffusions at 450 8C in epitaxial FeyGaAs(0 0 1)
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heterostructures lead to the formation of an epitaxial reaction layer made of Fe2As patches embedded in a Ga-rich Fe3Ga1.8As0.2 ternary phase w18,19x and that it is possible to regrow an epitaxial Fe film on such reacted layers at room temperature (RT) w20x. In this paper, we show that an annealing at a temperature higher than 450 8C allows a further interaction with the substrate and induces the formation of a Fe3GaAsy GaAs(0 0 1) contact. The results that we report concern (25 or 80 nm Fe)yGaAs heterostructures annealed at 480 and 500 8C at which the most significant phenomena of the solid-state interdiffusion were observed. 2. Experiments The FeyGaAs samples were prepared in a RIBER 2300 MBE system connected by an UHV modutrack system to another UHV chamber used for metal evaporations. Five hundred-nanometers-thick undoped GaAs buffer layers were first grown on nq-type GaAs(0 0 1) substrates using standard MBE conditions, and As-rich (2=4)-reconstructed surfaces were prepared. Fe depositions were done at RT using a high-temperature effusion cell with an alumina crucible at a deposition rate equal to 1 nm miny1. The UHV annealing treatments were performed in situ up to 480 or 500 8C for approximately 10 min. After the Fe deposition, the reflection high energy electron diffraction (RHEED) along the w1 1 0x and w1 0 0x directions of GaAs confirmed the epitaxial growth of Fe on GaAs with the usual ‘cube-on-cube’ relationship: (0 0 1)w1 0 0xFeyy (0 0 1)w1 0 0xGaAs. No surface reconstruction was observed at RT but new diffraction streaks appeared during annealing: we observed a (2=2) reconstruction at 300 8C w14,18x followed by a (3=3) one at 480 8C. This last reconstruction remained at 500 8C and after cooling at RT. The samples were characterized ex situ by a set of complementary methods. Rutherford backscattering spectrometry (RBS) with Heq ions was used to determine the atomic composition of the reacted layer and ion channeling to characterize its average crystallinity. The structural characterizations were made by X-ray diffraction (XRD) on a four-circle diffractometer in various modes (u–2u, u scans, w scans) and by transmission electron microscopy (TEM). The reacted layers were also magnetically and electrically characterized at RT using an alternating gradient magnetometer and a four-point probe, respectively. 3. (80 nm Fe)yGaAs(0 0 1) heterostructure annealed at 480 8C The random RBS spectrum shown in Fig. 1, which was obtained with a 2.2 MeV Heq beam, is well simulated by a Fe3GaAsyGaAs structure, with a ternary
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Fig. 1. Experimental 2.2 MeV Heq random RBS spectrum from a (80 nm Fe)yGaAs(0 0 1) structure annealed for 10 min at 480 8C. The simulated and deconvoluted spectra for Fe, Ga and As corresponding to a Fe3GaAs layer on GaAs are also shown. The geometry of the experimental setup is sketched above the figure.
compound layer containing 67=1016 Fe cmy2 as the 80-nm-thick deposited Fe layer. The XRD pattern on the unannealed sample (not shown) confirms the epitaxial growth of Fe on GaAs as previously reported w14,18x. Only one 0 0 2 Fe reflection is observed with those of the substrate. After the annealing at 480 8C (Fig. 2a) the Fe reflection disappears, indicating that all the deposited Fe atoms have reacted. Then, we observe two new diffraction peaks at 2us31.48 and 65.58 corresponding to the 1 0 1 and 2 0 2 reflections of Fe3GaAs. Rocking curve measurements (u scans) along the 2 0 2 reflection give evidence of a mosaic structure with a full-width at half maximum (FWHM) equal to f0.68, i.e. approximately twice the FWHM along the 0 0 2 reflection of the initial Fe layer before annealing. Moreover, in Fig. 3, the w scan pattern shows that the (0 0 2) planes of Fe3Ga1yxAsx are parallel to the {1 1 1} planes of GaAs and that the Fe3Ga1yxAsx layer presents four variants rotated through 908 from each other. In addition, the small peaks at 2us35.28 and 74.48 in Fig. 2a correspond to the 1 1 0 and 2 2 0 reflections of the tetragonal Fe2As compound. Their intensity is considerably reduced in comparison to the peaks observed after a 10-min annealing at 450 8C w19x, which means that Fe2As has almost completely disappeared at 480 8C (note that the fraction of Fe2As
Fig. 2. u–2u XRD patterns of a (80 nm Fe)yGaAs(0 0 1) structure after an annealing treatment for 10 min, (a) at 480 8C and (b) at 500 8C obtained by using filtered Cu Ka radiations.
phase is so small that it could be neglected when simulating the RBS spectrum shown in Fig. 1). The TEM micrograph shown in Fig. 4 confirms that the reacted layer is mainly made of large grains of
Fig. 3. XRD diagrams in texture mode (w scan) of a (80 nm Fe)yGaAs(0 0 1) structure after an annealing treatment for 10 min at 480 8C. The azimuthal w scans were realised at a fixed polar angle (54.78) on the {1 1 1}GaAs (2us27.308) and (0 0 2)Fe3Ga1yxAsx (2us35.548) reflections.
´ B. Lepine et al. / Thin Solid Films 446 (2004) 6–11
Fig. 4. Bright field cross-sectional TEM micrograph of a (80 nm Fe)yGaAs(0 0 1) structure after an annealing treatment at 480 8C for 10 min.
Fe3GaAs with some small and scarce grains of Fe2As at the interface. The height of the Fe3GaAs grains corresponds to the whole thickness of the reacted layer (f150 nm), their lateral size is ranging from 50 to 250 nm and the interface between the Fe3GaAs grains and GaAs is well defined and reasonably planar. The XRD and TEM experiments allowed us to confirm the specific orientation relationship of the hexagonal Fe3Ga2yxAsx compound on the GaAs(0 0 1) substrate established in our previous study w21x: (1 0 1)w0 0 1x Fe3Ga2yxAsx yy(0 0 1)w1 1 1x GaAs. This relation, which cannot be strictly verified because the cya ratio is not exactly equal to 63y62 (for Fe3GaAs cyas0.259), is usual for pseudocubic hexagonal NiAstype compound on GaAs w1,2,6,21,22x. It shows that the w0 0 1x axis of the hexagonal Fe3GaAs compound can be aligned with the four N1 1 1M axes of the cubic substrate and the resulting four different Fe3GaAs variants were effectively observed in Fig. 3. The presence of four families of grains slightly disorientated with respect to the others explains the mosaic structure of the reacted layer observed by XRD. Ion channeling experiments (not shown) were realized with a 2.2 MeV Heq incident beam along the w0 0 1x and w0 1 1x axes of the GaAs substrate. The channeling factor xmin, defined as the minimum backscattering yield along an axis normalized to the random yield, is commonly used to estimate the average crystallinity of thin films. Its value on the Fe3GaAs contribution to the spectra is equal to f0.70 along both the w0 0 1x and w0 1 1x axes, which confirms the oriented growth of the ternary compound with an in-plane order but points out a relatively poor average crystalline quality. Moreover, when the energy of the Heq incident beam is reduced to 1 MeV, the xmin decreases to f0.55. This behavior reveals the presence of extended defects and can be explained by the mosaic structure of the reacted layer
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(FWHM202s0.68) because Lindhard’s critical angle for channeling increases at low energy. The saturation of the magnetic moment per unit area of the Fe3GaAs film is equal to (7.7"0.5)10y3 emu cmy2 at RT, leading to an average magnetic moment per Fe atom NmM equal to 1.2"0.1 mB, a value in accordance with the one of bulk Fe3GaAs (1.14 mB) w12x. The measurement of the magnetic hysteresis loop at RT gives a coercitive field of 80"10 Oe and a remanence of 55"10%. Finally, the square resistance Rh of this sample is equal to 12 V leading to an electrical resistivity r equal to 180 mV cm at RT, for the 150-nm-thick Fe3GaAs film: this relatively low value is compatible with microelectronics applications. 4. (80 nm Fe)yGaAs(0 0 1) heterostructure annealed at 500 8C After the annealing at 500 8C, the XRD diagram (Fig. 2b) shows that the reflections of the ternary compound are preserved, while the Fe2As ones have totally disappeared. However, the 1 0 1 and 2 0 2 reflections have now slightly shifted towards the low angles with respect to the annealing at 480 8C (2us31.28 and 65.28, respectively), which suggests a decrease in the arsenic concentration in the ternary phase w15x. This is confirmed by the RBS random spectrum shown in Fig. 5, which is well simulated by a reacted layer with a Fe3Ga1.3As0.7 atomic composition, presenting an AsyGa atomic ratio lower than 1 (the composition is comprised between Fe3Ga1.2As0.8 and Fe3Ga1.4As0.6 ). In fact, for uncapped FeyGaAs samples annealed in UHV, the sublimation of arsenic from the reacted layer is nearly negligible at 480 8C but becomes significant at 500 8C. The reactions can be represented by the following schemes:
Fig. 5. Experimental 2.26 MeV Heq random and w0 0 1x aligned RBS spectra from a (80 nm Fe)yGaAs(0 0 1) structure annealed for 10 min at 500 8C. The simulation of the random spectrum for a Fe3Ga1.7As0.3 layer on GaAs is shown.
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5. Conclusion
Fig. 6. Misfit dislocations network at the Fe3Ga1.7As0.3yGaAs interface: weak-beam dark field TEM image of a (25 nm Fe)yGaAs(0 0 1) structure annealed for 10 min at 500 8C (the reported crystallographic directions are those of GaAs).
At 480 8C: 3FeqGaAs™Fe3GaAs At 500 8C: 3Feq1.3GaAs™Fe3Ga1.3 As0.7 q0.3As2 sublimation The As2 sublimation at 500 8C results in an increase of 30% of the GaAs thickness involved in the interdiffusion. Thus, the final composition of the reacted layer depends on the quantity of sublimated arsenic and should be a critical function of the temperature and of the duration of the annealing in the 500 8C range. Apart from this change in the atomic composition, Fig. 5 shows a small decrease in the xmin factor up to 0.6 along the w0 0 1x axis pointing out a slight improvement in the average crystalline quality of the film. The average magnetic moment per Fe atom NmM (1.3"0.1 mB), the coercive field (90"10 Oe) and the remanence (45"10%) are not significantly different from those of the compound made at 480 8C. To summarize, the main characteristics of the reacted layer obtained after the 10min annealing treatment at 500 8C are close to the ones observed at 480 8C. Finally, the mismatch between the two materials (d(1 0 1)Fe3Ga1.3As0.7s0.288 nm and d(2 0 0)GaAss0.2827 nm) is expected to be accommodated by an array of misfit dislocations and the calculated spacing between the dislocation lines for a non-strained Fe3Ga1.3As0.7 film is equal to 15 nm. A planview TEM image was taken on a (25 nm Fe)yGaAs structure annealed at 500 8C (Fig. 6) to study the dislocations at the interface. The misfit dislocations are edge type, they are aligned along the w1 0 0x and w0 1 0x directions of GaAs and form a regular network. The rectangular shape of the network (15"1=20"1 nm2) was not explained but the spacing between the lines of dislocations is in reasonable agreement with the calculated one.
In this study, we have demonstrated that the UHV deposition and annealing treatments in the 500 8C temperature range of FeyGaAs(0 0 1) contacts allow the formation of stable Fe3GaAsyGaAs(0 0 1) heterostructures, quite equivalent to the well-known CoSi2 ySi, NiSi2 ySi and CrSi2 ySi ones w8x. This result is in agreement with the ternary phase diagram w14,15x, which predicts that Fe3GaAs and GaAs are thermodynamically stable when in contact with each other. The Fe3GaAs films show an electrical resistivity equal to 180 mV cm and an average magnetic moment per Fe atom NmM up to approximately 1.2 mB. The magnetization has a remanence in the 45–55% range and a relatively low coercitive field equal to 80–90 Oe. Furthermore, the Fe3GaAs layer is made of epitaxial grains on GaAs(0 0 1) and the interface is quite regular. The main advantage of using UHV techniques is the elimination of contamination on GaAs surfaces and inside the deposited metal films, all of which may affect the growth and the epitaxial quality of the Fe3GaAs layers. This can explain that our results differ from those previously published on the FeyGaAs interdiffusions w9,10x and those concerning the deposition of Fe3GaAs layers on GaAs using atmospheric pressure metalorganic chemical vapor technique w13x. However, during annealing treatments in UHV, the As exodiffusion that occurs above 480 8C does not allow the use of annealing temperatures higher than 500 8C. To test the possible increase in quality resulting from annealing treatments in the 600–700 8C temperature range (in a normal furnace or by rapid thermal annealing), experiments are currently in progress on FeyGaAs samples capped with a Si3N4 inert film. To achieve the same goal, we are now studying the use of GaAs(1 1 1) substrates, which would improve the quality of the Fe3GaAsyGaAs heterostructures for two reasons. Firstly, the (1 1 1) orientation of the GaAs substrate, in accordance with the hexagonal symmetry of Fe3GaAs, might isolate only one variant instead of the four observed on (0 0 1) substrate and a single crystalline reacted layer may be expected. Secondly, the lattice mismatch in the interface plane should be smaller (q0.3%) than on GaAs(0 0 1) (q1.2%), which should decrease the density of interface dislocations. To conclude, the epitaxial nature and the stability (at least up to 480 8C) of the Fe3GaAsyGaAs contact make this heterostructure an attractive metallization scheme for magnetoelectronic devices. The applications will depend on both the electronic characteristics of the ternary compound and the electrical properties of the Fe3GaAsyGaAs interface, which are still unknown. Further studies concerning the transfer of polarized carriers in GaAs and the overgrowth of GaAs onto Fe3GaAs have to be pursued.
´ B. Lepine et al. / Thin Solid Films 446 (2004) 6–11
Acknowledgments We would like to acknowledge the ‘Equipe des surfaces et des interfaces’ for their help and the many fruitful discussions we had together, Antoine Filipe and Alain Schuhl of the UMR Thales-CNRS (Orsay) for their help during the magnetic characterization of the samples and Jacques Crestou for the specimen preparation for TEM observations. This work was supported by ´ ´´ the CNRS (‘Structure cooperative autour de l’accelerateur 2.5 MV du Groupe de Physique des Solides’ ´ Paris VI et VII). Universites References w1x C.J. Palmstrom, T. Sands, in: L.J. Brillson (Ed.), Contacts to Semiconductors, Noyes, Park Ridge, 1993, p. 67. w2x T. Sands, C.J. Palmstrom, J.P. Harbison, V.G. Keramidas, N. Tabatabaie, T.L. Cheeks, R. Ramesh, Y. Silberberg, Mat. Sci. Reports, 5 (1990) 99. w3x G.A. Prinz, Science 250 (1990) 1092. w4x G.A. Prinz, in: B. Heinrich, J.A.C. Bland (Eds.), Ultra Thin Magnetic Structures II, Springer, 1994, p. 1. w5x C.J. Palmstrom, C.C. Chang, A. Yu, G.J. Galvin, J.W. Mayer, J. Appl. Phys. 62 (1987) 3755. w6x A. Guivarc’h, R. Guerin, ´ A. Poudoulec, J. Caulet, J. Fontenille, J. Appl. Phys. 66 (1989) 2129. w7x B. Guenais, A. Poudoulec, J. Caulet, A. Guivarch, J. Cryst. Growth 102 (1990) 925. w8x R.T. Tung, J.M. Poate, J.C. Bean, J.M. Gibson, D.C. Jacobson, Thin Solid Films 93 (1982) 77.
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w9x M. Rahmoune, J.P. Eymery, Ph. Goudeau, M.F. Denanot, Thin Solid Films 289 (1996) 261. w10x M. Rahmoune, J.P. Eymery, M.F. Denanot, J. Magnetism Magnetic Mater. 175 (1997) 219. w11x I.R. Harris, N.A. Smith, B. Cockayne, W.R. MacEwan, J. Cryst. Growth 82 (1987) 450. w12x I.R. Harris, N.A. Smith, E. Devlin, B. Cockayne, W.R. MacEwan, G. Longworth, J. Less-Common Metals 146 (1989) 103. w13x B. Cockayne, P.E. Olivier, P.A. Lane, P.J. Wright, N.A. Smith, I.R. Harris, J. Cryst. Growth 148 (1995) 261. w14x S. Deputier, ´ ´ ´ ´ ´ R. Guerin, B. Lepine, A. Guivarc’h, G. Jezequel, J. Alloys Comp. 262 (1997) 416. w15x S. Deputier, ´ ´ R. Guerin, A. Guivarc’h, Eur. Phys. J. Appl. Phys. 2 (1998) 127. w16x J.C.P. Chang, N. Otsuka, E.S. Hamon, M.R. Melloch, J.M. Woodall, Appl. Phys. Lett. 65 (1994) 2801. w17x D.T. McInturff, E.S. Hamon, J.C.P. Chang, Y.M. Pekarek, J.M. Woodall, Appl. Phys. Lett. 69 (1996) 1885. w18x C. Lallaizon, B. Lepine, ´ ´ S. Ababou, A. Schussler, A. Quemer´ ´ ´ ´ ais, A. Guivarc’h, G. Jezequel, S. Deputier, R. Guerin, Appl. Surf. Sci. 123y124 (1997) 319. w19x B. Lepine, ´ ´ ´ ´ S. Ababou, A. Guivarc’h, G. Jezequel, S. Deputier, ´ R. Guerin, A. Filipe, A. Schuhl, F. Abel, C. Cohen, A. Rocher, J. Crestou, J. Appl. Phys. 83 (1998) 3077. w20x C. Lallaizon, B. Lepine, ´ ´ S. Ababou, A. Guivarc’h, S. Deputier, F. Abel, C. Cohen, J. Appl. Phys. 86 (1999) 5515. w21x B. Guenais, A. Poudoulec, A. Guivarc’h, R. Guerin, ´ J. Caulet, Inst. Phys. Conf. Ser. 100 (1989) 665. w22x B. Guenais, A. Poudoulec, J. Caulet, A. Guivarc’h, J. Crystal Growth 102 (1990) 925.
Fe3GaAsyGaAs(0 0 1): a stable and magnetic metal-semiconductor heterostructure a, a ´ ´ ´ B. Lepine *, C. Lallaizona, P. Schieffera, A. Guivarc’ha, G. Jezequel , A. Rocherb, F. Abelc, d ´ C. Cohenc, S. Deputier , F. Nguyen Van Daue a
ˆ 11C, Campus de Beaulieu, F 35042 Rennes Cedex, Equipe de Physique des Surfaces et des Interfaces, UMR CNRS-Universite´ no. 6627, Bat. France b CEMES CNRS, 29 rue Jeanne Marvin, BP 4347, F 31055 Toulouse Cedex, France c ´ Paris VI et VII, Tour 23-2, place Jussieu, F 75251 Paris Cedex, Groupe de Physique du Solide, UMR CNRS-Universite´ no. 7588, Universites France d ´ Laboratoire de Chimie du Solide et Inorganique Moleculaire, UMR CNRS-Universite´ no. 6511, Campus de Beaulieu, F 35042 Rennes Cedex, France e UMR CNRS-Thales no. 137, Domaine de Corbeville, F 91404 Orsay, France Received 26 June 2003; received in revised form 24 July 2003; accepted 17 August 2003
Abstract We show that in agreement with the ternary Fe–Ga–As phase diagram, the solid-state interdiffusions in epitaxial FeyGaAs(0 0 1) heterostructures lead, at a temperature of approximately 500 8C, to the formation of thermodynamically stable Fe3GaAsy GaAs(0 0 1) contacts quite similar to the well-known silicideySi ones. The Fe3 GaAs films are made of grains epitaxial on GaAs with a well-defined interface. Their magnetic and electrical properties make Fe3 GaAs on GaAs an attractive metallization scheme for future magnetoelectronic devices. The results we report concern (25 or 80 nm Fe)yGaAs(0 0 1) heterostructures annealed at 480 and 500 8C for 10 min and characterized ex situ by Heq Rutherford backscattering and ion channeling, X-ray diffraction, transmission electron microscopy and alternating gradient field magnetometry. 䊚 2003 Elsevier B.V. All rights reserved. PACS: 68.55a; 81.15Np; 81.05.Ea; 75.50.Cc Keywords: Solid phase epitaxy; Gallium arsenide; Iron; Magnetic properties
1. Introduction The growth of ferromagnetic films on semiconductor substrates has received much attention over the past decade. The combination of the magnetism of the metallic layer along with the electronic properties of the semiconductor substrate offers the opportunity for innovative magnetotransport effects and devices to be realized. However, because the two types of materials are very dissimilar in terms of physical, chemical and structural properties, integration of semiconductors and ferromagnets is generally very difficult w1–4x. *Corresponding author. Tel.: q33-2-23-23-69-21; fax: q33-2-2323-61-98. ´ E-mail address: [email protected] (B. Lepine).
The (ferromagnetic metal)yGaAs systems have been widely studied in the last few years, in particular the epitaxial FeyGaAs heterostructures w3,4x. However, due to a strong reactivity between ferromagnetic metals and GaAs, the resulting structures are not thermodynamically stable. The purpose of this study is to obtain epitaxial and stable (magnetic compound)yGaAs structures by inducing a controlled thermal interaction between a metal film and a GaAs substrate. Many studies have been dealing with the metalyGaAs solid-state interdiffusions during annealing treatments but, as in the cases of CoyGaAs w5x and NiyGaAs w6,7x contacts, the final step of the interaction was always found to be a mixture of binary or ternary compounds w1,2,5,6x: thermodynamically stable and epitaxial contacts equivalent to the CoSi2 ySi, NiSi2 ySi and CrSi2 heterostructures w8x have
0040-6090/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0040-6090Ž03.01232-X
´ B. Lepine et al. / Thin Solid Films 446 (2004) 6–11
never been obtained for GaAs in this way. In this context, relatively few works concern the interdiffusions in FeyGaAs structures. Rahmoune et al. w9,10x annealed FeyGaAs contacts prepared by ion-beam sputtering for 1 h at 500 8C and they observed a layered microstructure of the type Fe3Gay(Fe2AsqFeAs)yGaAs. However, no accurate studies of the final step of the solid-state interdiffusions in FeyGaAs heterostructures, prepared and annealed in ultra-high vacuum (UHV) conditions, have been published. Our starting point was the recent publications, concerning on one hand, the new magnetic ternary phases Fe3Ga2yxAsx w11–13x, and on the other hand, the experimental determination of the Fe–Ga–As ternary phase diagram w14,15x. The Fe3GaAs compound was identified for the first time by Harris et al. w11x, in the form of precipitates occurring during the growth of Fedoped GaAs single crystals using the liquid-encapsulation Czochralski technique. After that, the same authors prepared bulk Fe3GaAs samples by the same technique w12x: Fe3GaAs was identified as a NiAs-type phase (hexagonal B82-type) that exists over a range of compositions represented by the formula Fe3Ga2yxAsx (0.20(x(1.125) and presents a (2a,c) superstructure for 0.20(x(0.85. All the alloys were found to be ferromagnetic at temperatures higher than the ambient one. More recently, Fe3GaAs precipitates were also observed in Fe-implanted and annealed GaAs w16x and in Fe-doped GaAs grown by molecular beam epitaxy (MBE) w17x. ´ Concerning the ternary Fe–Ga–As diagram, Deputier et al. w14,15x determined the solid-state phase equilibria at 600 8C and showed the occurrence of tie-lines between all the compositions of the Fe3Ga2yxAsx solid solution (in particular Fe3GaAs) and GaAs. To our knowledge, it was the first time that a ternary phase Mx(GaAs)y (Mstransition metal) stoichiometric in Ga and As was found to be in thermodynamic equilibrium with GaAs. This fact suggests that the final step of the solid-state interdiffusions occurring during the annealing of a FeyGaAs heterostructure could lead to a stable Fe3GaAsyGaAs contact. Moreover, one could expect an epitaxial growth of the Fe3GaAs layer on GaAs due to the particular structural properties of the hexagonal Fe3Ga2yxAsx alloys. Indeed, firstly, the cya ratio of their crystalline parameters is close to 63y62 (1.23(cy a(1.28 for 0.20(x(1.125), which allows them to be described in a pseudocubic unit cell with a parameter (apseudocubics(3y62)=ahexagonal), and secondly, the corresponding apseudocubic parameter is close to one and a half that of GaAs (within 2%). Such epitaxial growths by solid-state interdiffusion were previously observed for several equivalent hexagonal pseudocubic compounds w1,2,6,7x. In a previous study we showed that the solid-state interdiffusions at 450 8C in epitaxial FeyGaAs(0 0 1)
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heterostructures lead to the formation of an epitaxial reaction layer made of Fe2As patches embedded in a Ga-rich Fe3Ga1.8As0.2 ternary phase w18,19x and that it is possible to regrow an epitaxial Fe film on such reacted layers at room temperature (RT) w20x. In this paper, we show that an annealing at a temperature higher than 450 8C allows a further interaction with the substrate and induces the formation of a Fe3GaAsy GaAs(0 0 1) contact. The results that we report concern (25 or 80 nm Fe)yGaAs heterostructures annealed at 480 and 500 8C at which the most significant phenomena of the solid-state interdiffusion were observed. 2. Experiments The FeyGaAs samples were prepared in a RIBER 2300 MBE system connected by an UHV modutrack system to another UHV chamber used for metal evaporations. Five hundred-nanometers-thick undoped GaAs buffer layers were first grown on nq-type GaAs(0 0 1) substrates using standard MBE conditions, and As-rich (2=4)-reconstructed surfaces were prepared. Fe depositions were done at RT using a high-temperature effusion cell with an alumina crucible at a deposition rate equal to 1 nm miny1. The UHV annealing treatments were performed in situ up to 480 or 500 8C for approximately 10 min. After the Fe deposition, the reflection high energy electron diffraction (RHEED) along the w1 1 0x and w1 0 0x directions of GaAs confirmed the epitaxial growth of Fe on GaAs with the usual ‘cube-on-cube’ relationship: (0 0 1)w1 0 0xFeyy (0 0 1)w1 0 0xGaAs. No surface reconstruction was observed at RT but new diffraction streaks appeared during annealing: we observed a (2=2) reconstruction at 300 8C w14,18x followed by a (3=3) one at 480 8C. This last reconstruction remained at 500 8C and after cooling at RT. The samples were characterized ex situ by a set of complementary methods. Rutherford backscattering spectrometry (RBS) with Heq ions was used to determine the atomic composition of the reacted layer and ion channeling to characterize its average crystallinity. The structural characterizations were made by X-ray diffraction (XRD) on a four-circle diffractometer in various modes (u–2u, u scans, w scans) and by transmission electron microscopy (TEM). The reacted layers were also magnetically and electrically characterized at RT using an alternating gradient magnetometer and a four-point probe, respectively. 3. (80 nm Fe)yGaAs(0 0 1) heterostructure annealed at 480 8C The random RBS spectrum shown in Fig. 1, which was obtained with a 2.2 MeV Heq beam, is well simulated by a Fe3GaAsyGaAs structure, with a ternary
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Fig. 1. Experimental 2.2 MeV Heq random RBS spectrum from a (80 nm Fe)yGaAs(0 0 1) structure annealed for 10 min at 480 8C. The simulated and deconvoluted spectra for Fe, Ga and As corresponding to a Fe3GaAs layer on GaAs are also shown. The geometry of the experimental setup is sketched above the figure.
compound layer containing 67=1016 Fe cmy2 as the 80-nm-thick deposited Fe layer. The XRD pattern on the unannealed sample (not shown) confirms the epitaxial growth of Fe on GaAs as previously reported w14,18x. Only one 0 0 2 Fe reflection is observed with those of the substrate. After the annealing at 480 8C (Fig. 2a) the Fe reflection disappears, indicating that all the deposited Fe atoms have reacted. Then, we observe two new diffraction peaks at 2us31.48 and 65.58 corresponding to the 1 0 1 and 2 0 2 reflections of Fe3GaAs. Rocking curve measurements (u scans) along the 2 0 2 reflection give evidence of a mosaic structure with a full-width at half maximum (FWHM) equal to f0.68, i.e. approximately twice the FWHM along the 0 0 2 reflection of the initial Fe layer before annealing. Moreover, in Fig. 3, the w scan pattern shows that the (0 0 2) planes of Fe3Ga1yxAsx are parallel to the {1 1 1} planes of GaAs and that the Fe3Ga1yxAsx layer presents four variants rotated through 908 from each other. In addition, the small peaks at 2us35.28 and 74.48 in Fig. 2a correspond to the 1 1 0 and 2 2 0 reflections of the tetragonal Fe2As compound. Their intensity is considerably reduced in comparison to the peaks observed after a 10-min annealing at 450 8C w19x, which means that Fe2As has almost completely disappeared at 480 8C (note that the fraction of Fe2As
Fig. 2. u–2u XRD patterns of a (80 nm Fe)yGaAs(0 0 1) structure after an annealing treatment for 10 min, (a) at 480 8C and (b) at 500 8C obtained by using filtered Cu Ka radiations.
phase is so small that it could be neglected when simulating the RBS spectrum shown in Fig. 1). The TEM micrograph shown in Fig. 4 confirms that the reacted layer is mainly made of large grains of
Fig. 3. XRD diagrams in texture mode (w scan) of a (80 nm Fe)yGaAs(0 0 1) structure after an annealing treatment for 10 min at 480 8C. The azimuthal w scans were realised at a fixed polar angle (54.78) on the {1 1 1}GaAs (2us27.308) and (0 0 2)Fe3Ga1yxAsx (2us35.548) reflections.
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Fig. 4. Bright field cross-sectional TEM micrograph of a (80 nm Fe)yGaAs(0 0 1) structure after an annealing treatment at 480 8C for 10 min.
Fe3GaAs with some small and scarce grains of Fe2As at the interface. The height of the Fe3GaAs grains corresponds to the whole thickness of the reacted layer (f150 nm), their lateral size is ranging from 50 to 250 nm and the interface between the Fe3GaAs grains and GaAs is well defined and reasonably planar. The XRD and TEM experiments allowed us to confirm the specific orientation relationship of the hexagonal Fe3Ga2yxAsx compound on the GaAs(0 0 1) substrate established in our previous study w21x: (1 0 1)w0 0 1x Fe3Ga2yxAsx yy(0 0 1)w1 1 1x GaAs. This relation, which cannot be strictly verified because the cya ratio is not exactly equal to 63y62 (for Fe3GaAs cyas0.259), is usual for pseudocubic hexagonal NiAstype compound on GaAs w1,2,6,21,22x. It shows that the w0 0 1x axis of the hexagonal Fe3GaAs compound can be aligned with the four N1 1 1M axes of the cubic substrate and the resulting four different Fe3GaAs variants were effectively observed in Fig. 3. The presence of four families of grains slightly disorientated with respect to the others explains the mosaic structure of the reacted layer observed by XRD. Ion channeling experiments (not shown) were realized with a 2.2 MeV Heq incident beam along the w0 0 1x and w0 1 1x axes of the GaAs substrate. The channeling factor xmin, defined as the minimum backscattering yield along an axis normalized to the random yield, is commonly used to estimate the average crystallinity of thin films. Its value on the Fe3GaAs contribution to the spectra is equal to f0.70 along both the w0 0 1x and w0 1 1x axes, which confirms the oriented growth of the ternary compound with an in-plane order but points out a relatively poor average crystalline quality. Moreover, when the energy of the Heq incident beam is reduced to 1 MeV, the xmin decreases to f0.55. This behavior reveals the presence of extended defects and can be explained by the mosaic structure of the reacted layer
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(FWHM202s0.68) because Lindhard’s critical angle for channeling increases at low energy. The saturation of the magnetic moment per unit area of the Fe3GaAs film is equal to (7.7"0.5)10y3 emu cmy2 at RT, leading to an average magnetic moment per Fe atom NmM equal to 1.2"0.1 mB, a value in accordance with the one of bulk Fe3GaAs (1.14 mB) w12x. The measurement of the magnetic hysteresis loop at RT gives a coercitive field of 80"10 Oe and a remanence of 55"10%. Finally, the square resistance Rh of this sample is equal to 12 V leading to an electrical resistivity r equal to 180 mV cm at RT, for the 150-nm-thick Fe3GaAs film: this relatively low value is compatible with microelectronics applications. 4. (80 nm Fe)yGaAs(0 0 1) heterostructure annealed at 500 8C After the annealing at 500 8C, the XRD diagram (Fig. 2b) shows that the reflections of the ternary compound are preserved, while the Fe2As ones have totally disappeared. However, the 1 0 1 and 2 0 2 reflections have now slightly shifted towards the low angles with respect to the annealing at 480 8C (2us31.28 and 65.28, respectively), which suggests a decrease in the arsenic concentration in the ternary phase w15x. This is confirmed by the RBS random spectrum shown in Fig. 5, which is well simulated by a reacted layer with a Fe3Ga1.3As0.7 atomic composition, presenting an AsyGa atomic ratio lower than 1 (the composition is comprised between Fe3Ga1.2As0.8 and Fe3Ga1.4As0.6 ). In fact, for uncapped FeyGaAs samples annealed in UHV, the sublimation of arsenic from the reacted layer is nearly negligible at 480 8C but becomes significant at 500 8C. The reactions can be represented by the following schemes:
Fig. 5. Experimental 2.26 MeV Heq random and w0 0 1x aligned RBS spectra from a (80 nm Fe)yGaAs(0 0 1) structure annealed for 10 min at 500 8C. The simulation of the random spectrum for a Fe3Ga1.7As0.3 layer on GaAs is shown.
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5. Conclusion
Fig. 6. Misfit dislocations network at the Fe3Ga1.7As0.3yGaAs interface: weak-beam dark field TEM image of a (25 nm Fe)yGaAs(0 0 1) structure annealed for 10 min at 500 8C (the reported crystallographic directions are those of GaAs).
At 480 8C: 3FeqGaAs™Fe3GaAs At 500 8C: 3Feq1.3GaAs™Fe3Ga1.3 As0.7 q0.3As2 sublimation The As2 sublimation at 500 8C results in an increase of 30% of the GaAs thickness involved in the interdiffusion. Thus, the final composition of the reacted layer depends on the quantity of sublimated arsenic and should be a critical function of the temperature and of the duration of the annealing in the 500 8C range. Apart from this change in the atomic composition, Fig. 5 shows a small decrease in the xmin factor up to 0.6 along the w0 0 1x axis pointing out a slight improvement in the average crystalline quality of the film. The average magnetic moment per Fe atom NmM (1.3"0.1 mB), the coercive field (90"10 Oe) and the remanence (45"10%) are not significantly different from those of the compound made at 480 8C. To summarize, the main characteristics of the reacted layer obtained after the 10min annealing treatment at 500 8C are close to the ones observed at 480 8C. Finally, the mismatch between the two materials (d(1 0 1)Fe3Ga1.3As0.7s0.288 nm and d(2 0 0)GaAss0.2827 nm) is expected to be accommodated by an array of misfit dislocations and the calculated spacing between the dislocation lines for a non-strained Fe3Ga1.3As0.7 film is equal to 15 nm. A planview TEM image was taken on a (25 nm Fe)yGaAs structure annealed at 500 8C (Fig. 6) to study the dislocations at the interface. The misfit dislocations are edge type, they are aligned along the w1 0 0x and w0 1 0x directions of GaAs and form a regular network. The rectangular shape of the network (15"1=20"1 nm2) was not explained but the spacing between the lines of dislocations is in reasonable agreement with the calculated one.
In this study, we have demonstrated that the UHV deposition and annealing treatments in the 500 8C temperature range of FeyGaAs(0 0 1) contacts allow the formation of stable Fe3GaAsyGaAs(0 0 1) heterostructures, quite equivalent to the well-known CoSi2 ySi, NiSi2 ySi and CrSi2 ySi ones w8x. This result is in agreement with the ternary phase diagram w14,15x, which predicts that Fe3GaAs and GaAs are thermodynamically stable when in contact with each other. The Fe3GaAs films show an electrical resistivity equal to 180 mV cm and an average magnetic moment per Fe atom NmM up to approximately 1.2 mB. The magnetization has a remanence in the 45–55% range and a relatively low coercitive field equal to 80–90 Oe. Furthermore, the Fe3GaAs layer is made of epitaxial grains on GaAs(0 0 1) and the interface is quite regular. The main advantage of using UHV techniques is the elimination of contamination on GaAs surfaces and inside the deposited metal films, all of which may affect the growth and the epitaxial quality of the Fe3GaAs layers. This can explain that our results differ from those previously published on the FeyGaAs interdiffusions w9,10x and those concerning the deposition of Fe3GaAs layers on GaAs using atmospheric pressure metalorganic chemical vapor technique w13x. However, during annealing treatments in UHV, the As exodiffusion that occurs above 480 8C does not allow the use of annealing temperatures higher than 500 8C. To test the possible increase in quality resulting from annealing treatments in the 600–700 8C temperature range (in a normal furnace or by rapid thermal annealing), experiments are currently in progress on FeyGaAs samples capped with a Si3N4 inert film. To achieve the same goal, we are now studying the use of GaAs(1 1 1) substrates, which would improve the quality of the Fe3GaAsyGaAs heterostructures for two reasons. Firstly, the (1 1 1) orientation of the GaAs substrate, in accordance with the hexagonal symmetry of Fe3GaAs, might isolate only one variant instead of the four observed on (0 0 1) substrate and a single crystalline reacted layer may be expected. Secondly, the lattice mismatch in the interface plane should be smaller (q0.3%) than on GaAs(0 0 1) (q1.2%), which should decrease the density of interface dislocations. To conclude, the epitaxial nature and the stability (at least up to 480 8C) of the Fe3GaAsyGaAs contact make this heterostructure an attractive metallization scheme for magnetoelectronic devices. The applications will depend on both the electronic characteristics of the ternary compound and the electrical properties of the Fe3GaAsyGaAs interface, which are still unknown. Further studies concerning the transfer of polarized carriers in GaAs and the overgrowth of GaAs onto Fe3GaAs have to be pursued.
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Acknowledgments We would like to acknowledge the ‘Equipe des surfaces et des interfaces’ for their help and the many fruitful discussions we had together, Antoine Filipe and Alain Schuhl of the UMR Thales-CNRS (Orsay) for their help during the magnetic characterization of the samples and Jacques Crestou for the specimen preparation for TEM observations. This work was supported by ´ ´´ the CNRS (‘Structure cooperative autour de l’accelerateur 2.5 MV du Groupe de Physique des Solides’ ´ Paris VI et VII). Universites References w1x C.J. Palmstrom, T. Sands, in: L.J. Brillson (Ed.), Contacts to Semiconductors, Noyes, Park Ridge, 1993, p. 67. w2x T. Sands, C.J. Palmstrom, J.P. Harbison, V.G. Keramidas, N. Tabatabaie, T.L. Cheeks, R. Ramesh, Y. Silberberg, Mat. Sci. Reports, 5 (1990) 99. w3x G.A. Prinz, Science 250 (1990) 1092. w4x G.A. Prinz, in: B. Heinrich, J.A.C. Bland (Eds.), Ultra Thin Magnetic Structures II, Springer, 1994, p. 1. w5x C.J. Palmstrom, C.C. Chang, A. Yu, G.J. Galvin, J.W. Mayer, J. Appl. Phys. 62 (1987) 3755. w6x A. Guivarc’h, R. Guerin, ´ A. Poudoulec, J. Caulet, J. Fontenille, J. Appl. Phys. 66 (1989) 2129. w7x B. Guenais, A. Poudoulec, J. Caulet, A. Guivarch, J. Cryst. Growth 102 (1990) 925. w8x R.T. Tung, J.M. Poate, J.C. Bean, J.M. Gibson, D.C. Jacobson, Thin Solid Films 93 (1982) 77.
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