- Email: [email protected]
Epitaxial growth of bcc Co films on Sb-passivated GaAs(110) substrates
Journal of Electron Spectroscopy and Related Phenomena 101–103 (1999) 493–499
Epitaxial growth of bcc Co films on Sb-passivated GaAs(110) substrates C.M. Teodorescu a , M.G. Martin a,b , N. Franco a , H. Ascolani c , J. Chrost a , J. Avila a,b , a,b , M.C. Asensio * a
ˆ 209 D, Centre Universitaire Paris Sud, 991898 Orsay, France LURE, Bat b Instituto de Ciencia de Materiales, CSIC, 28049 Madrid, Spain c ´ ´ Nacional de Energıa ´ Atomica ´ , 8400 Bariloche, Argentina Centro atomico Bariloche and Instituto Balseiro, Comision Received 7 August 1998
Abstract We have combined angle-scanned photoelectron diffraction (PED), low-energy electron diffraction and synchrotron radiation photoemission to examine the formation of the Co / GaAs(110) and Co / Sb / GaAs(110) interfaces. We find that Co forms a metastable bcc phase on both surfaces, with its principal crystallographic axes parallel to the substrate lattice. For Co films grown on non-passivated substrates, we determine an outward distortion of the lattice constant perpendicular to the surface. By Sb deposition on GaAs(110) surface prior to the Co growth, the epitaxy quality of the metallic layer is improved. The Sb-passivation of the substrate decreases the intermixing reaction and reduces the intralayer outward expansion of the Co films to less than 1%. 1999 Elsevier Science B.V. All rights reserved. Keywords: Epitaxial growth; Co; GaAs(110); Metastable magnetic phases
PACS: 68.55.Jk; 68.35.Fx; 75.70.Ak
1. Introduction As a consequence of the progress over the past decade in the controlled growth of thin films and artificial modulated structures, the stabilization of metastable phases of the transition metals on favourable substrates has become a topic of great interest. Among many novel materials, magnetic overlayers on nonmagnetic substrates and magnetic / nonmag*Corresponding author. Tel.: 133-1-6446-8012; fax: 133-1-64464148. E-mail address: [email protected] (M.C. Asensio)
netic multilayers have received especial attention. An important part of a growing trend of synthesizing artificial materials on the nanometer scale is the understanding of the correlation between crystal structure and magnetism as well as the electronic and magnetic properties of different metals with the same crystal structure. In particular, the epitaxial growth of bcc Co on a GaAs(110) substrate has been extensively studied over the last decade (e.g., Ref. [1,2]). A driving force for this is the intriguing electronic and magnetic properties of these thin films, which are intimately related to their structural properties. Most of the
0368-2048 / 99 / $ – see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S0368-2048( 98 )00481-2
494
C.M. Teodorescu et al. / Journal of Electron Spectroscopy and Related Phenomena 101 – 103 (1999) 493 – 499
magnetic properties such as the Curie temperature, or the saturation magnetization and atomic magnetic moment are believed to be closely influenced by structural defects and the crystalline quality of the films. Specifically, the growth of metastable phases of magnetic transition metals on semiconductor substrates is particularly difficult due to general intermixing of the semiconductor constituents with the metallic overlayer. The selective segregation and not controlled diffusion result in disordered and inhomogenous films, with degraded magnetic properties. Therefore, it is highly desirable to perform controlled experiments in which the quality of the structure can be improved. This has been achieved in several systems by employing appropriate surfactants [3,4] and / or passivants [5,6]. In this sense, Sb is an attractive candidate to passivate the GaAs surfaces prior to the Co deposition. Sb / GaAs(110) interface has been extensively investigated. Antimony strongly binds to the (110) surface of GaAs. In the submonolayer coverage regime, antimony adsorbates tend to cluster together. The islands present already ordered structures along the zig-zag chains of cations and anions of the ¯ substrate in the [110] direction. The island size and ˚ and 2.5–2.8 height have been estimated as 10–100 A ˚A, respectively [7,8]. At about 0.7 ML the islands merge together and form a (131) continuous network on the substrate. At the completion of 1 ML, Sb atoms build up a structure periodically arranged, which resembles the geometry of a GaAs(110) topmost layer. The Sb two-dimensional overlayer can be easily ordered by annealing the film. A sharp 131 LEED, then, characterizes the GaAs(110)– p(131)Sb interface. Due to the formation of strong covalent bonds between the adatoms and the substrate Ga and As atoms along the zig-zag chains, this interface presents a high chemical stability and well defined local order [9]. In this paper, we present an angular-dependent Photoelectron Diffraction study of the Co grown on treated and non-treated GaAs(110) substrates. The role of the Sb on the structure of the metallic films was investigated as a function of the Sb coverage, in the submonolayer regime. The reactivity of the Co / Sb / GaAs(110) interface was studied at different deposition temperatures.
2. Experimental details The experiments were carried out in the ultra high vacuum French-Spanish (PES2) experimental station at LURE (Orsay-France) connected to the SU6 undulator beamline of the Super-Aco storage ring. We used a VSW hemispherical analyzer of 50 mm radius mounted on a goniometer inside the vacuum chamber, with a base pressure ,1.5310 210 mbar. The acceptance angle of the spectrometer was 618 and its energy resolution expressed as resolving power (dE /E) was 5310 24 . The sample was supported by a vertical manipulator, providing automated polar and azimuthal motion with a precision better than 18. Photoelectron diffraction experiments in the Forward scattering regime have been performed by using a Mg X-ray source. Fresh n-doped GaAs(110) wafers were chemically etched and introduced in the load-lock after a standard cleaning procedure. After extensive degassing cycles at around 2008C; the samples were Arsputtered and subsequently annealed at 4508C. Sharp (131) LEED patterns were obtained and no contamination was detected by synchrotron photoemission. Sb was evaporated from a resistively heated boron nitride crucible and Co from a custommade electron bombardment source. The standard deposition rate of both Co and Sb sources was 1 ˚ / mm. The thicknesses of Sb and Co films were A measured with a quartz balance and a posteriori cross-checked by core level spectroscopy.
3. Results and discussion We have recently carried out a photoemission study of the chemical reaction of cobalt with fresh cleaved GaAs(110) surfaces and with Sb-treated surfaces as a function of the Sb coverage [10]. Core level photoemission spectra showed that the deposition of Co on bare surfaces leads to an extensive exchange reaction. Room temperature intermixing of Ga and As with the as-deposited Co overlayers was evident from the examination of the evolution of the 3d Ga and As core-level emissions. Even by 1 and 2 ˚ Co coverages, the surface components of both Ga A
C.M. Teodorescu et al. / Journal of Electron Spectroscopy and Related Phenomena 101 – 103 (1999) 493 – 499
and As 3d peaks diminished rapidly and new reactive-shifted features appeared as shoulders of the bulk contributions. Analysis of the photoemission spectra at the initial stage of the Co / Sb / GaAs(110) interface showed pronounced line-shape changes in the Ga and As 3d core levels, indicating a minor but still present disruption of the semiconductor substrate, (not shown, but see Ref. [10]). In order to investigate the structure of the metallic films, we have monitored the LEED pattern during the Co deposition. On bare GaAs(110) surfaces, the Co deposits do not show any LEED pattern at room
495
temperature. An increase of the substrate deposition temperature up to 1758C improves the Co film epitaxy. As Co was deposited the surface GaAs(110) (131) LEED pattern was observed to fade while the diffuse background increased significantly, indicating the presence of a reactive and disordered interface. After about 5–6 ML of Co deposition, a LEED pattern from the Co overlayer was first observed. Comparison of these results for Co / GaAs with those for Co / Sb / GaAs(110) shows that the diffuse background attenuates rapidly for Sb-pre-treated substrates and the LEED pattern of the Co films
Fig. 1. Topmost schemes correspond to the unit cells and atomic arrangements of the [110] face of GaAs and bcc Co, respectively. On the bottom part, the models represent cross-sectional cuts of a perfect bcc Co crystal along the two high symmetry azimuths. The origin of the main forward focusing peaks can be identified.
496
C.M. Teodorescu et al. / Journal of Electron Spectroscopy and Related Phenomena 101 – 103 (1999) 493 – 499
Fig. 2. Stereographic projection of the Co 2p core level (903 eV) normalized intensity as a function of polar (08 to 608) and azimuthal (08 to ˚ (top panel) and 7 A ˚ (bottom panel) thick Co films grown on non-treated GaAs(110) surfaces. On the right side Co 2p 3608) angles of a 3 A polar-angle PED curves in the (110) and (001) azimuthal planes of the GaAs substrate are indicated for both Co films. The substrate temperature was kept at 1758C.
C.M. Teodorescu et al. / Journal of Electron Spectroscopy and Related Phenomena 101 – 103 (1999) 493 – 499
497
Fig. 3. Stereographic projection of the Co 2p core level (903 eV) normalized intensity as a function of polar (08 to 608) and azimuthal (08 to ˚ (top panel) and 24 A ˚ (bottom panel) thick Co films grown on GaAs(110) surfaces pre-treated with 0.5 ML of Sb. The 3608) angles of a 7 A intensity is mapped using a linear grey scale where the brightest spots represent the highest intensity. On the right side Co 2p polar-angle PED curves in the (110) and (001) azimuthal planes of the GaAs substrate are indicated for both Co films. The substrate temperature was kept at 1758C.
498
C.M. Teodorescu et al. / Journal of Electron Spectroscopy and Related Phenomena 101 – 103 (1999) 493 – 499
becomes more sharp. A systematic search of the best growth conditions suggests deposition temperatures in the range of 130–1808C and a Sb coverage lower than half a monolayer .With these conditions, the LEED spot intensities of the bcc Co(100) films were observed to increase rapidly as further Co was deposited, until they began to disappear at a Co ˚ thickness of about 30 A. The LEED pattern of the metallic overlayer was aligned as expected with the bcc Co mesh being parallel to the fcc GaAs(110) lattice, indicating that the bcc Co (100) grows epitaxially on the Sb covered GaAs(110) substrate. By simple extrapolation of the lattice constant from the bcc Fe x Co 12x alloys, one can estimate an experimental value for ˚ As the the lattice constant of bcc cobalt of 2.819 A. orientation of the bcc metallic lattice is parallel to the substrate mesh, the Co mesh can be accommodated with a lattice misfit of 0.2% along of the longest crystallographic axis by adjusting two Co unit cells relative to one of the substrate, (see Fig. l(a) and (b)). We will focus now on the structure of Co films grown on bare GaAs(110) substrates. Fig. 2 gives the stereographic projection of the Co 2p core-level ˚ and 7 A ˚ thick Co films evaporated intensity for 3 A on non-treated GaAs(100) substrates at 1758C. The normalized intensity [(I 0 -Background) / Background] is displayed using a linear grey scale where the brightest spots represent the highest intensity. The anisotropic intensity distribution of forward scattered photoelectrons is clearly observed. The bcc structure can be identified by the diffraction spots along the [100] and [110] high symmetry directions which are partially illustrated in figure lc and ld. The short range crystalline order even in the films grown on bare substrates is obvious. However, the forward scattering peaks along the chains of atoms in a bcc perfect lattice are expected at 35.38 and 458 along [001] and [110] directions, respectively. Any distortion of the film should be manifested by a shift of the PED peak position. For Co films deposited on nontreated GaAs(110), the Co 2p core level along [001] and [110] directions fall at 42.0860.58 and 32.0860.58, respectively (see right side of Fig. 2). These values correspond to a 14% outward expansion of the interlayer spacing along the [110] direction with respect to bulk bcc cobalt. It is clear in the ˚ and 7 A ˚ thick) that polar scans of both Co films (3 A
although the PED peaks are shifted, they show shoulders at theta angles predicted for a non-distorted bcc lattice, which indicates that the films are composed by a very distorted interface followed by a better structured Co film. When the metallic overlayers were grown on Sb-pretreated substrates, however, the Co 2p [100] and [110] peaks fall at 44.0860.58 and 34.0860.58, respectively, which correspond to a perfect bce Co lattice within the experimental error, (see the right side curves of Fig. 3). We can conclude that the presence of antimony passivating the substrate surfaces reduces the intralayer relaxation of the Co films, improving the structure quality of the overlayer, although the interface is still reactive.
4. Conclusions In summary, the role of Sb-pretreatment of the substrate in the formation of the Co / GaAs(110) interface has been elucidated by means of anglescanned Photoelectron Diffraction. It has been shown that Sb-passivation of the GaAs(110) surface prior to Co deposition is an effective way to improve the epitaxy, reducing the intralayer expansion from 14% to values lower than 1%. The resulting Co layer is found to grow in a bcc (100) orientation and the Co disruption of the GaAs(110) substrate is shown to be significantly reduced by the previous Sb treatment of the surface.
Acknowledgements This work has been supported by the Spanish agency DGICYT under grant PB-94-0022-C02.01 and the Large Scale Installation Program at LURE. H. Ascolani is a member of the Consejo Nacional de ´ ´ Investigaciones Cientıficas y Tecnicas (CONICET) of Argentina.
References [1] G.A. Prinz, J.J. Krebbs, Appl. Phys. Lett. 39 (5) (1981) 397. [2] G.A. Prinz, Phys. Rev. Lett. 54 (10) (1985) 1051.
C.M. Teodorescu et al. / Journal of Electron Spectroscopy and Related Phenomena 101 – 103 (1999) 493 – 499 [3] D.A. Steigerwald, I. Jacob, W.F. Egelhoff Jr., Surf. Sci. 202 (1988) 472. [4] G.W. Anderson, M.C. Hanf, P.R. Norton, Phys. Rev. Lett. 74 (1988) 2764. [5] R.F.C. Farrow, S.S.P. Parkin, V.S. Speriosu, J. Appl. Phys. 64 (1988) 5315. ˚ [6] P. Martensson, R.M. Feenstr, Phys. Rev. B 39 (11) (1989) 7744, and references therein.
499
[7] W.G. Schmidt, F. Bechstedt, G.P. Srivastava, Surf. Sci. Rep. 25 (1996) 141. [8] W.K. Ford, T. Guo, G.L. Lessor, C.B. Duke, Phys. Rev. B 42 (1990) 8952. [9] J.P. La Femina, C.B. Duke, C. Mailhiot, J. Vac. Sci. Technol. 19 (1990) 888. [10] C.M. Teodorescu, J. Chrost, H. Ascolani, J. Avila, F. Soria, M.C. Asensio, Surf. Rev. and Lett. (1998) in press.
Epitaxial growth of bcc Co films on Sb-passivated GaAs(110) substrates C.M. Teodorescu a , M.G. Martin a,b , N. Franco a , H. Ascolani c , J. Chrost a , J. Avila a,b , a,b , M.C. Asensio * a
ˆ 209 D, Centre Universitaire Paris Sud, 991898 Orsay, France LURE, Bat b Instituto de Ciencia de Materiales, CSIC, 28049 Madrid, Spain c ´ ´ Nacional de Energıa ´ Atomica ´ , 8400 Bariloche, Argentina Centro atomico Bariloche and Instituto Balseiro, Comision Received 7 August 1998
Abstract We have combined angle-scanned photoelectron diffraction (PED), low-energy electron diffraction and synchrotron radiation photoemission to examine the formation of the Co / GaAs(110) and Co / Sb / GaAs(110) interfaces. We find that Co forms a metastable bcc phase on both surfaces, with its principal crystallographic axes parallel to the substrate lattice. For Co films grown on non-passivated substrates, we determine an outward distortion of the lattice constant perpendicular to the surface. By Sb deposition on GaAs(110) surface prior to the Co growth, the epitaxy quality of the metallic layer is improved. The Sb-passivation of the substrate decreases the intermixing reaction and reduces the intralayer outward expansion of the Co films to less than 1%. 1999 Elsevier Science B.V. All rights reserved. Keywords: Epitaxial growth; Co; GaAs(110); Metastable magnetic phases
PACS: 68.55.Jk; 68.35.Fx; 75.70.Ak
1. Introduction As a consequence of the progress over the past decade in the controlled growth of thin films and artificial modulated structures, the stabilization of metastable phases of the transition metals on favourable substrates has become a topic of great interest. Among many novel materials, magnetic overlayers on nonmagnetic substrates and magnetic / nonmag*Corresponding author. Tel.: 133-1-6446-8012; fax: 133-1-64464148. E-mail address: [email protected] (M.C. Asensio)
netic multilayers have received especial attention. An important part of a growing trend of synthesizing artificial materials on the nanometer scale is the understanding of the correlation between crystal structure and magnetism as well as the electronic and magnetic properties of different metals with the same crystal structure. In particular, the epitaxial growth of bcc Co on a GaAs(110) substrate has been extensively studied over the last decade (e.g., Ref. [1,2]). A driving force for this is the intriguing electronic and magnetic properties of these thin films, which are intimately related to their structural properties. Most of the
0368-2048 / 99 / $ – see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S0368-2048( 98 )00481-2
494
C.M. Teodorescu et al. / Journal of Electron Spectroscopy and Related Phenomena 101 – 103 (1999) 493 – 499
magnetic properties such as the Curie temperature, or the saturation magnetization and atomic magnetic moment are believed to be closely influenced by structural defects and the crystalline quality of the films. Specifically, the growth of metastable phases of magnetic transition metals on semiconductor substrates is particularly difficult due to general intermixing of the semiconductor constituents with the metallic overlayer. The selective segregation and not controlled diffusion result in disordered and inhomogenous films, with degraded magnetic properties. Therefore, it is highly desirable to perform controlled experiments in which the quality of the structure can be improved. This has been achieved in several systems by employing appropriate surfactants [3,4] and / or passivants [5,6]. In this sense, Sb is an attractive candidate to passivate the GaAs surfaces prior to the Co deposition. Sb / GaAs(110) interface has been extensively investigated. Antimony strongly binds to the (110) surface of GaAs. In the submonolayer coverage regime, antimony adsorbates tend to cluster together. The islands present already ordered structures along the zig-zag chains of cations and anions of the ¯ substrate in the [110] direction. The island size and ˚ and 2.5–2.8 height have been estimated as 10–100 A ˚A, respectively [7,8]. At about 0.7 ML the islands merge together and form a (131) continuous network on the substrate. At the completion of 1 ML, Sb atoms build up a structure periodically arranged, which resembles the geometry of a GaAs(110) topmost layer. The Sb two-dimensional overlayer can be easily ordered by annealing the film. A sharp 131 LEED, then, characterizes the GaAs(110)– p(131)Sb interface. Due to the formation of strong covalent bonds between the adatoms and the substrate Ga and As atoms along the zig-zag chains, this interface presents a high chemical stability and well defined local order [9]. In this paper, we present an angular-dependent Photoelectron Diffraction study of the Co grown on treated and non-treated GaAs(110) substrates. The role of the Sb on the structure of the metallic films was investigated as a function of the Sb coverage, in the submonolayer regime. The reactivity of the Co / Sb / GaAs(110) interface was studied at different deposition temperatures.
2. Experimental details The experiments were carried out in the ultra high vacuum French-Spanish (PES2) experimental station at LURE (Orsay-France) connected to the SU6 undulator beamline of the Super-Aco storage ring. We used a VSW hemispherical analyzer of 50 mm radius mounted on a goniometer inside the vacuum chamber, with a base pressure ,1.5310 210 mbar. The acceptance angle of the spectrometer was 618 and its energy resolution expressed as resolving power (dE /E) was 5310 24 . The sample was supported by a vertical manipulator, providing automated polar and azimuthal motion with a precision better than 18. Photoelectron diffraction experiments in the Forward scattering regime have been performed by using a Mg X-ray source. Fresh n-doped GaAs(110) wafers were chemically etched and introduced in the load-lock after a standard cleaning procedure. After extensive degassing cycles at around 2008C; the samples were Arsputtered and subsequently annealed at 4508C. Sharp (131) LEED patterns were obtained and no contamination was detected by synchrotron photoemission. Sb was evaporated from a resistively heated boron nitride crucible and Co from a custommade electron bombardment source. The standard deposition rate of both Co and Sb sources was 1 ˚ / mm. The thicknesses of Sb and Co films were A measured with a quartz balance and a posteriori cross-checked by core level spectroscopy.
3. Results and discussion We have recently carried out a photoemission study of the chemical reaction of cobalt with fresh cleaved GaAs(110) surfaces and with Sb-treated surfaces as a function of the Sb coverage [10]. Core level photoemission spectra showed that the deposition of Co on bare surfaces leads to an extensive exchange reaction. Room temperature intermixing of Ga and As with the as-deposited Co overlayers was evident from the examination of the evolution of the 3d Ga and As core-level emissions. Even by 1 and 2 ˚ Co coverages, the surface components of both Ga A
C.M. Teodorescu et al. / Journal of Electron Spectroscopy and Related Phenomena 101 – 103 (1999) 493 – 499
and As 3d peaks diminished rapidly and new reactive-shifted features appeared as shoulders of the bulk contributions. Analysis of the photoemission spectra at the initial stage of the Co / Sb / GaAs(110) interface showed pronounced line-shape changes in the Ga and As 3d core levels, indicating a minor but still present disruption of the semiconductor substrate, (not shown, but see Ref. [10]). In order to investigate the structure of the metallic films, we have monitored the LEED pattern during the Co deposition. On bare GaAs(110) surfaces, the Co deposits do not show any LEED pattern at room
495
temperature. An increase of the substrate deposition temperature up to 1758C improves the Co film epitaxy. As Co was deposited the surface GaAs(110) (131) LEED pattern was observed to fade while the diffuse background increased significantly, indicating the presence of a reactive and disordered interface. After about 5–6 ML of Co deposition, a LEED pattern from the Co overlayer was first observed. Comparison of these results for Co / GaAs with those for Co / Sb / GaAs(110) shows that the diffuse background attenuates rapidly for Sb-pre-treated substrates and the LEED pattern of the Co films
Fig. 1. Topmost schemes correspond to the unit cells and atomic arrangements of the [110] face of GaAs and bcc Co, respectively. On the bottom part, the models represent cross-sectional cuts of a perfect bcc Co crystal along the two high symmetry azimuths. The origin of the main forward focusing peaks can be identified.
496
C.M. Teodorescu et al. / Journal of Electron Spectroscopy and Related Phenomena 101 – 103 (1999) 493 – 499
Fig. 2. Stereographic projection of the Co 2p core level (903 eV) normalized intensity as a function of polar (08 to 608) and azimuthal (08 to ˚ (top panel) and 7 A ˚ (bottom panel) thick Co films grown on non-treated GaAs(110) surfaces. On the right side Co 2p 3608) angles of a 3 A polar-angle PED curves in the (110) and (001) azimuthal planes of the GaAs substrate are indicated for both Co films. The substrate temperature was kept at 1758C.
C.M. Teodorescu et al. / Journal of Electron Spectroscopy and Related Phenomena 101 – 103 (1999) 493 – 499
497
Fig. 3. Stereographic projection of the Co 2p core level (903 eV) normalized intensity as a function of polar (08 to 608) and azimuthal (08 to ˚ (top panel) and 24 A ˚ (bottom panel) thick Co films grown on GaAs(110) surfaces pre-treated with 0.5 ML of Sb. The 3608) angles of a 7 A intensity is mapped using a linear grey scale where the brightest spots represent the highest intensity. On the right side Co 2p polar-angle PED curves in the (110) and (001) azimuthal planes of the GaAs substrate are indicated for both Co films. The substrate temperature was kept at 1758C.
498
C.M. Teodorescu et al. / Journal of Electron Spectroscopy and Related Phenomena 101 – 103 (1999) 493 – 499
becomes more sharp. A systematic search of the best growth conditions suggests deposition temperatures in the range of 130–1808C and a Sb coverage lower than half a monolayer .With these conditions, the LEED spot intensities of the bcc Co(100) films were observed to increase rapidly as further Co was deposited, until they began to disappear at a Co ˚ thickness of about 30 A. The LEED pattern of the metallic overlayer was aligned as expected with the bcc Co mesh being parallel to the fcc GaAs(110) lattice, indicating that the bcc Co (100) grows epitaxially on the Sb covered GaAs(110) substrate. By simple extrapolation of the lattice constant from the bcc Fe x Co 12x alloys, one can estimate an experimental value for ˚ As the the lattice constant of bcc cobalt of 2.819 A. orientation of the bcc metallic lattice is parallel to the substrate mesh, the Co mesh can be accommodated with a lattice misfit of 0.2% along of the longest crystallographic axis by adjusting two Co unit cells relative to one of the substrate, (see Fig. l(a) and (b)). We will focus now on the structure of Co films grown on bare GaAs(110) substrates. Fig. 2 gives the stereographic projection of the Co 2p core-level ˚ and 7 A ˚ thick Co films evaporated intensity for 3 A on non-treated GaAs(100) substrates at 1758C. The normalized intensity [(I 0 -Background) / Background] is displayed using a linear grey scale where the brightest spots represent the highest intensity. The anisotropic intensity distribution of forward scattered photoelectrons is clearly observed. The bcc structure can be identified by the diffraction spots along the [100] and [110] high symmetry directions which are partially illustrated in figure lc and ld. The short range crystalline order even in the films grown on bare substrates is obvious. However, the forward scattering peaks along the chains of atoms in a bcc perfect lattice are expected at 35.38 and 458 along [001] and [110] directions, respectively. Any distortion of the film should be manifested by a shift of the PED peak position. For Co films deposited on nontreated GaAs(110), the Co 2p core level along [001] and [110] directions fall at 42.0860.58 and 32.0860.58, respectively (see right side of Fig. 2). These values correspond to a 14% outward expansion of the interlayer spacing along the [110] direction with respect to bulk bcc cobalt. It is clear in the ˚ and 7 A ˚ thick) that polar scans of both Co films (3 A
although the PED peaks are shifted, they show shoulders at theta angles predicted for a non-distorted bcc lattice, which indicates that the films are composed by a very distorted interface followed by a better structured Co film. When the metallic overlayers were grown on Sb-pretreated substrates, however, the Co 2p [100] and [110] peaks fall at 44.0860.58 and 34.0860.58, respectively, which correspond to a perfect bce Co lattice within the experimental error, (see the right side curves of Fig. 3). We can conclude that the presence of antimony passivating the substrate surfaces reduces the intralayer relaxation of the Co films, improving the structure quality of the overlayer, although the interface is still reactive.
4. Conclusions In summary, the role of Sb-pretreatment of the substrate in the formation of the Co / GaAs(110) interface has been elucidated by means of anglescanned Photoelectron Diffraction. It has been shown that Sb-passivation of the GaAs(110) surface prior to Co deposition is an effective way to improve the epitaxy, reducing the intralayer expansion from 14% to values lower than 1%. The resulting Co layer is found to grow in a bcc (100) orientation and the Co disruption of the GaAs(110) substrate is shown to be significantly reduced by the previous Sb treatment of the surface.
Acknowledgements This work has been supported by the Spanish agency DGICYT under grant PB-94-0022-C02.01 and the Large Scale Installation Program at LURE. H. Ascolani is a member of the Consejo Nacional de ´ ´ Investigaciones Cientıficas y Tecnicas (CONICET) of Argentina.
References [1] G.A. Prinz, J.J. Krebbs, Appl. Phys. Lett. 39 (5) (1981) 397. [2] G.A. Prinz, Phys. Rev. Lett. 54 (10) (1985) 1051.
C.M. Teodorescu et al. / Journal of Electron Spectroscopy and Related Phenomena 101 – 103 (1999) 493 – 499 [3] D.A. Steigerwald, I. Jacob, W.F. Egelhoff Jr., Surf. Sci. 202 (1988) 472. [4] G.W. Anderson, M.C. Hanf, P.R. Norton, Phys. Rev. Lett. 74 (1988) 2764. [5] R.F.C. Farrow, S.S.P. Parkin, V.S. Speriosu, J. Appl. Phys. 64 (1988) 5315. ˚ [6] P. Martensson, R.M. Feenstr, Phys. Rev. B 39 (11) (1989) 7744, and references therein.
499
[7] W.G. Schmidt, F. Bechstedt, G.P. Srivastava, Surf. Sci. Rep. 25 (1996) 141. [8] W.K. Ford, T. Guo, G.L. Lessor, C.B. Duke, Phys. Rev. B 42 (1990) 8952. [9] J.P. La Femina, C.B. Duke, C. Mailhiot, J. Vac. Sci. Technol. 19 (1990) 888. [10] C.M. Teodorescu, J. Chrost, H. Ascolani, J. Avila, F. Soria, M.C. Asensio, Surf. Rev. and Lett. (1998) in press.