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Fe3Si thin film multilayers grown on GaAs(001)
Thin Solid Films 556 (2014) 120–124
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Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Epitaxial Fe3Si/Ge/Fe3Si thin film multilayers grown on GaAs(001) B. Jenichen ⁎, J. Herfort, U. Jahn, A. Trampert, H. Riechert Paul-Drude-Institut für Festkörperelektronik, Hausvogteiplatz 5-7, D-10117 Berlin, Germany
a r t i c l e
i n f o
Article history: Received 10 September 2013 Received in revised form 10 January 2014 Accepted 10 January 2014 Available online 16 January 2014 Keywords: Molecular beam epitaxy Ferromagnetic thin films Semiconductors Transmission electron microscopy X-ray diffraction
a b s t r a c t We demonstrate Fe3Si/Ge/Fe3Si/GaAs(001) structures grown by molecular-beam epitaxy and characterized by transmission electron microscopy, electron backscattered diffraction, and X-ray diffraction. The bottom Fe3Si epitaxial film on GaAs is always single crystalline. The structural properties of the Ge film and the top Fe3Si layer depend on the substrate temperature during Ge deposition. Different orientation distributions of the grains in the Ge and the upper Fe3Si film were found. The low substrate temperature Ts of 150 °C during Ge deposition ensures sharp interfaces, however, results in predominantly amorphous films. We find that the intermediate Ts (225 °C) leads to a largely [111] oriented upper Fe3Si layer and polycrystal films. The high Ts of 325 °C stabilizes the [001] oriented epitaxial layer structure, i.e., delivers smooth interfaces and single crystal films over as much as 80% of the surface area. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Device concepts based on the spin rather than the charge of electrons have been explored recently. These concepts are expected to lead to further improvements in device performance [1,2]. Metal semiconductor field effect transistors combining ferromagnetic electrodes with standard electronic device concepts were realized [3–6]. There the atomic structure of the interfaces (IFs) and the crystal perfection of the materials are important for the spin transport [7]. The influence of the IF structure of the Fe/GaAs IF on the formation of Schottky barriers was investigated using first principles densityfunctional calculations [8]. The application of Heusler alloys is very promising, because they have a high Curie temperature and can exhibit half-metallic properties. Tunneling magneto-resistance (TMR) devices with MgO barriers and half-metallic Co2FeAl electrodes with a room temperature TMR ratio of 166% were established by ultra-high vacuum magnetron sputtering even on oxidized Si substrates [9]. Even higher values are expected if the orientation distribution of the Co2FeAl texture can be improved. Similar results were obtained on MgO substrates using Co2MnSi and an epitaxial MgO barrier [10,11]. In his classic work Julliere used amorphous Ge as a barrier material of a TMR device between poly-crystalline Co and Ni electrodes [12]. In the present work we study the possibility to combine crystalline Ge with quasi-lattice matched high quality ferromagnetic electrodes: The combination of single crystal Fe3Si electrodes with single crystal Ge as a barrier material yields a lattice mismatch as low as 0.08%. For the establishment of a TMR device with Ge barrier a first epitaxial Fe3Si film could be overgrown epitaxially by a thin film of Ge, which is covered again by a lattice-matched ferromagnetic material, e.g., Fe3Si ⁎ Corresponding author. E-mail address: [email protected] (B. Jenichen). 0040-6090/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2014.01.022
or Co2FeSi. Pronounced chemical roughness in the sense of interdiffusion can occur during epitaxial growth of Fe3Si on GaAs [13,14]. Superior Fe3Si films have been grown on Ge substrates by molecular beam epitaxy (MBE) [15]. Excellent magnetic properties with a small coercivity (0.9 Oe) and electrical properties with a Schottky barrier height of 0.52 eV were obtained. [16] Lateral spin transistors, consisting of Ge channel and ferromagnetic source/drain for spin injection, were envisaged. However, metal/Ge/metal vertical structures are most promising for next-generation devices. Ge films on Si-terminated Fe3Si films were recently grown by MBE on Ge(111) substrates [17,18]. However, [001] oriented substrates are more widely used in semiconductor technology and so the epitaxial growth on (001) surfaces is often preferable. In the present work we grow and analyze ferromagnet/ semiconductor/ferromagnet structures: Fe3Si/Ge/Fe3Si layer stacks have been grown on GaAs(001) by MBE. The epitaxial growth of Ge on Fe3Si is an example for the overgrowth of a metal by a semiconductor, which can be expected to lead to chemical reactions at the IF in many cases. The as-grown structures were characterized by transmission electron microscopy (TEM), electron backscattered diffraction (EBSD), and X-ray diffraction (XRD).
Table 1 Nominal (measured) film thicknesses and substrate temperatures Ts during epitaxial growth for three samples investigated.
GaAs Fe3Si Ge Fe3Si
Sample thickness (nm)
No. 1 Ts °C
Sample thickness (nm)
No. 2 Ts °C
Sample thickness (nm)
No. 3 Ts °C
300 9 (10) 27 (22) 45 (60)
580 200 150 100
300 9 (9) 27 (20–26) 45 (53–62)
580 200 225 100
300 9 (8) 9 (6) 9 (9)
580 200 325 150
B. Jenichen et al. / Thin Solid Films 556 (2014) 120–124
121
(a)
(b)
(c)
Fig. 1. (a) HRTEM lattice image of sample 1 (Fourier filtered) with GaAs/Fe3Si and the Fe3Si/Ge IFs. (b) Overview of sample 1 (HRTEM at low magnification). (c) SAD pattern of sample 1 corresponding to the area shown in (b). The incident beam was directed along GaAs[110].
2. Experimental The structures were grown by MBE on GaAs(001) substrates. The main growth parameters are given in Table 1. After the growth of a 300 nm thick GaAs buffer layer (substrate temperature Ts = 580 °C, growth rate 480 nm/h) the templates were transferred under ultra high vacuum conditions to the As free growth chamber of the MBE system. Then nominally 9 nm thick Fe3Si films were grown at a substrate temperature of 200 °C. Ge films were grown on top of these Fe3Si films at different substrate temperatures ranging from 150 °C to 325 °C. On top of those Ge films Fe3Si epitaxial films were deposited at low substrate temperatures (100 °C or 150 °C). First the as-grown structures were characterized by high-resolution (HR) TEM. For that purpose cross-sectional TEM specimens were
Fig. 3. Grain distribution pattern (inverse pole figure) of sample 1 determined by EBSD.
prepared by mechanical lapping and polishing, followed by argon ion milling according to standard techniques. TEM images were acquired with a JEOL 3010 microscope operating at 200 kV in order to minimize radiation damage. The cross section TEM method provides high lateral and depth resolutions on the nanometer scale; however, they average over the thickness of the thin sample foil (5–20 nm). In addition the samples were investigated by EBSD and XRD in order to obtain independent data about the structure of the upper Fe3Si and the Ge films. The principle of EBSD is the following [19]: In the scanning electron microscope Kikuchi-patterns are recorded point by point. The crystal orientation of every point is recovered from the corresponding pattern with a limit of lateral resolution of 20–30 nm. In this way the distribution of the orientations of the crystallites near the surface is determined. High-resolution XRD measurements were performed on the structures using a Panalytical X-Pert PRO MRD™ system with a Ge(220) hybrid monochromator (Cu Kα1 radiation with a wavelength of λ = 1.54056 Å). 3. Results and discussion
Fig. 2. XRD curve of sample 1 in the vicinity of the GaAs 002 reflection and the corresponding simulation.
Fig. 1(a) shows a HRTEM image of sample 1 near the GaAs/Fe3Si and the first Fe3Si/Ge IFs. The Ge layer was grown at the lowest substrate temperature of Ts = 150 °C. As demonstrated previously [20–22] the GaAs/Fe3Si IF is topologically extremely smooth for the given growth conditions of the first Fe3Si film. The Fe3Si/Ge IF above however exhibits slightly increased roughness. A few lattice fringes visible on the Ge side of the IF indicate that the first mono-layers of the Ge film grew still quite well oriented and their deterioration with growing film thickness points to amorphous material above. Fig. 1(b) shows an overview. The whole layer stack becomes visible at this lower magnification. The IFs are smooth even on the larger scale. Fig. 1(c) depicts the corresponding selected area diffraction (SAD) pattern of the whole stack along the
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(a)
(b)
Fig. 6. Grain distribution pattern (inverse pole figure) of sample 2 determined by EBSD.
Fig. 4. (a) HR TEM image of sample 2. (b) SAD pattern of sample 2. The incident beam was directed along GaAs[110]. The orientation parallel to the IF is given by a dotted line.
[110] zone axis. The area selected corresponds to Fig. 1(b). Besides the single-crystal pattern Debye–Scherrer rings occurred intersecting the reciprocal lattice points 220 and 111. These rings originate from the mainly amorphous Ge layer and the upper Fe3Si film. [220] oriented grains were observed by XRD (shown below) as well, besides the prevailing [001] orientation of the first Fe3Si film. This is some evidence pointing to a randomly oriented film as the 220 reflection is expected to be the strongest in a powder pattern. The pronounced streaks
Fig. 5. XRD pattern of sample 2. Intensity maxima are marked by their Miller indices.
perpendicular to the IFs (crystal truncation rods [23]) give further evidence about the smoothness of the IFs and the surface. Fig. 2 demonstrates the XRD curve of sample 1 near the GaAs 002 reflection; the curve could be simulated taking into account merely the first Fe3Si film with a thickness of 9 nm. The contribution of the upper films could be neglected for high-resolution XRD near the GaAs 002 maximum. Fig. 3 depicts a grain distribution pattern of sample 1 determined by EBSD. The grains are oriented randomly in most places in accordance with the predominantly amorphous structure of the Fe3Si film. It seems that in some regions the limits of the EBSD method are approached. Only very few [111] oriented regions are larger (up to 100 nm, depicted in blue color) and correspond to crystallites. Fig. 4(a) shows the HRTEM image of sample 2. The Ge layer of sample 2 was grown at the substrate temperature of Ts = 225 °C. The bottom GaAs/Fe3Si IF is smooth, but the Ge film above grew as a polycrystal. The thicknesses of the Ge and the upper Fe3Si films are inhomogeneous. Differently oriented grains were observed in the HRTEM image of sample 2. We observed [001] and [111] oriented grains and a faulted structure with fault planes tilted by approximately 6.5° with respect to the surface normal. The upper Fe3Si film is poly-crystalline as confirmed by dark-field TEM micrographs (not shown here). Fig. 4(b) depicts the corresponding SAD pattern. Besides the reflections of the substrate and the perfect film we found additional maxima. The faulted lattices containing micro-twins lead to additional rows of spots partly connected by streaks (marked by straight lines) inclined by approximately 6.5° with respect to the sample surface. These streaks intersect in an additional spot corresponding to the 111 lattice plane distance oriented parallel to the [001] surface normal. This [111] orientation perfectly corresponds to the EBSD result of [111] oriented crystallites near the surface. The 2Θ/ω-scan of XRD (Fig. 5) shows besides the expected [001] orientation mainly the [111], [110], [112], and [113] orientations perpendicular to the IFs.
B. Jenichen et al. / Thin Solid Films 556 (2014) 120–124
123
(a)
(b)
Fig. 8. Lateral orientation distribution (inverse pole figure) of sample 3 determined by EBSD. Fig. 7. (a) HR TEM image of sample 3. (b) Corresponding SAD pattern showing the pattern expected for a perfect single crystalline structure. The incident beam was directed along GaAs[110].
Fig. 6 shows a grain distribution pattern of sample 2 determined by EBSD with large grain sizes (about 200 nm). A lot of grains near the surface are nearly [111] oriented (more than 80% of the whole area, depicted in blue color). The [111] orientation found in EBSD corresponds to the additional 111 spot found in the SAD pattern and to the XRD diffraction results. Fig. 7(a) demonstrates the HRTEM image of sample 3. The Ge layer of sample 3 was grown at the substrate temperature of Ts = 325 °C. The GaAs/Fe3Si IF is smooth and so is the Fe3Si/Ge IF. The Fe3Si/Ge IF above the Ge film exhibits a larger roughness than that below. The Ge film grew well oriented and also the Fe3Si film on top of the Ge turned out to be single-crystalline. The Ge film contains some small defects, detected by the strong change of the diffracted intensity of one atomic column compared to the intensity of the neighboring ones. In the region shown here the Fe3Si films are about 7 nm and 8 nm thick, whereas the Ge film is 6 nm thick. Fig. 7(b) depicts the SAD pattern of sample 3 evidencing single crystal films. The lower intensity of some maxima can be explained by the DO3 structure of Fe3Si, where the super-lattice reflections like, e.g., 1 13 have a lower intensity than the fundamental ones like, e.g., 004 [24]. Fig. 8 shows an orientation distribution pattern of sample 3 determined by EBSD. About 80% of the surface showed the correct [001] out of plane orientation (depicted in red) and about 12% are [111] oriented (depicted in blue color). This change of the orientation of some of the islands is connected to irregularities at the IF (not shown here). The remaining 8% of the surface exhibit other orientations. In Fig. 9 the XRD patterns of samples 1 (upper curve) and 3 (lower curve) are compared. The intensity maxima are marked by their Miller indices. The lattice parameters of substrate and films nearly coincide, so that the tiny film maxima are superimposed to the huge substrate
reflections GaAs 002 and GaAs 004. In sample 3 two additional peaks of low intensity are detected. Their peak positions coincide with those calculated recently for hexagonal Fe2Si 00.2 and 00.4, (cf. Fig. 9, and powder diffraction file entry #01-083-1259). This phase was discovered originally as a high temperature phase [25]. Sometimes during MBE non-equilibrium phases can occur. We are aware that such a coincidence does not fully prove the existence of a phase. However, a structure different from the D03 structure of Fe3Si is detected by the presence of those low intensity peaks hinting to a phase transition in some areas of the sample. We have overgrown [001] oriented single crystal Fe3Si films on GaAs(001) substrates epitaxially with Ge and then grew Fe3Si films on top. We could avoid chemical reaction with the Ge on large areas of the samples. The quality of the Ge films largely depends on the substrate temperature during deposition, the structural quality of the following
Fig. 9. XRD pattern of samples 1 (upper curve) and 3 (lower curve). Intensity maxima are marked by their Miller indices.
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Fe3Si film in turn is influenced by the parameters of the Ge film itself. The crystallinity of the as-grown Ge films in principle can be improved considerably by tempering [26,27]. The remaining interface irregularities can be explained by a lack of sufficient wetting of the metal surface by the Ge probably combined with residual surface roughness of the bottom Fe3Si epitaxial layer. Measurements of the magnetic properties (not shown here), however, revealed pronounced ferromagnetism of the Fe3Si films with saturation magnetizations corresponding to those of D03 ordered bulk Fe3Si. However, during these measurements upper and lower Fe3Si films were not distinguished so far.
4. Conclusions Although for the germanium growth temperature Ts = 150 °C the upper Fe3Si film was mainly amorphous as well as the underlying Ge film, low temperature growth seems to be promising for technological purposes on a larger scale, as temperature treatment of Ge after growth allows for recrystallization. Such a treatment could be designed in the future not to be harmful to the IFs. The germanium growth temperature Ts = 325 °C produces at present the best as-grown crystalline properties on large areas. Residual chemical reaction below some of the islands leading to differently oriented blocks remains to be controlled.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
Acknowledgment The authors thank Claudia Herrmann and Hans-Peter Schönherr for their support during the MBE growth, Doreen Steffen for sample preparation, Astrid Pfeiffer for the help in the laboratory, and Esperanza Luna for valuable support and helpful discussion. We thank Vladimir Kaganer for the computer program suitable for the simulation of the XRD curve.
[20] [21] [22] [23] [24] [25] [26] [27]
I. Zutic, J. Fabian, S. DasSarma, Rev. Mod. Phys. 76 (2004) 323. C. Palmstrøm, MRS Bull. 28 (2003) 725. S. Sugahara, M. Tanaka, Appl. Phys. Lett. 84 (2004) 2307. Y. Ando, K. Kasahara, K. Yamane, K. Hamaya, K. Sawano, T. Kimura, M. Miyao, Appl. Phys. Express 3 (2010) 093001. Y. Ando, Y. Maeda, K. Kasahara, S. Yamada, K. Masaki, Y. Hoshi, K. Sawano, K. Izunome, A. Sakai, M. Miyao, K. Hamaya, Appl. Phys. Lett. 99 (2011) 132511. Y. Ohdaira, M. Oogane, H. Naganuma, Y. Ando, Appl. Phys. Lett. 99 (2011) 132513. T.J. Zega, A.T. Hanbicki, S.C. Erwin, I. Zutic, G. Kioseoglou, C.H. Li, B.T. Jonker, R.M. Stroud, Phys. Rev. Lett. 96 (2006) 196101. D.O. Demchenko, A.Y. Liu, Phys. Rev. B 73 (2006) 115332. Z. Wen, H. Sukegawa, S. Mitani, K. Inomata, Appl. Phys. Lett. 98 (2011) 192505. M. Oogane, S. Mizukami, Phil. Trans. R. Soc. A 369 (2011) 3037. H. Liu, Y. Honda, T. Taira, K. Matsuda, M. Arita, T. Uemura, M. Yamamoto, Appl. Phys. Lett. 101 (2012) 132418. M. Julliere, Phys. Lett. A 54 (1975) 225. S.I. Makarov, B. Krumme, F. Stromberg, C. Weis, W. Keune, H. Wende, Appl. Phys. Lett. 99 (2011) 141910. C. Gusenbauer, T. Ashraf, J. Stangl, G. Hesser, T. Plach, A. Meingast, G. Kothleitner, R. Koch, Phys. Rev. B 83 (2011) 035319. K. Hamaya, T. Murakami, S. Yamada, K. Mibu, M. Miyao, Phys. Rev. B 83 (2011) 144411. M. Miyao, K. Hamaya, K. Sadoh, H. Itoh, Y. Maeda, Thin Solid Films 518 (2010) 273. K. Ueda, Y. Ando, M. Kumano, T. Sadoh, Y. Maeda, M. Miyao, Appl. Surface Science 254 (2008) 6215. S. Yamada, K. Tanikawa, M. Miyao, K. Hamaya, Crystal Growth and Design 12 (2012) 4703. A.J. Schwartz, M. Kumar, B.L. Adams, D.P. Field, Electron Backscatter Diffraction in Materials Science, Springer, New York, 2009. J. Herfort, H.-P. Schönherr, K.H. Ploog, Appl. Phys. Lett. 83 (2003) 3912. J. Herfort, A. Trampert, K.H. Ploog, Int. J. Mater. Res. 97 (2006) 1026. J. Herfort, B. Jenichen, V. Kaganer, A. Trampert, H.-P. Schönherr, K.H. Ploog, Phys. E. 32 (2006) 371. I.K. Robinson, D.J. Tweet, Rep. Prog. Phys. 55 (1992) 599. B. Jenichen, V.M. Kaganer, J. Herfort, H.-P. Schönherr, W. Braun, K.H. Ploog, Phys. Rev. B 72 (2005) 075329. H. Kudielka, Zeitschrift fuer Kristallographie 145 (1977) 177. B.L. Darby, B.R. Yates, I. Martin-Bragado, J.L. Gomez-Selles, R.G. Elliman, K.S. Jones, J. Appl. Phys. 113 (2013) 033505. B. Jenichen, C. Herrmann, J. Herfort, unpublished.
Contents lists available at ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Epitaxial Fe3Si/Ge/Fe3Si thin film multilayers grown on GaAs(001) B. Jenichen ⁎, J. Herfort, U. Jahn, A. Trampert, H. Riechert Paul-Drude-Institut für Festkörperelektronik, Hausvogteiplatz 5-7, D-10117 Berlin, Germany
a r t i c l e
i n f o
Article history: Received 10 September 2013 Received in revised form 10 January 2014 Accepted 10 January 2014 Available online 16 January 2014 Keywords: Molecular beam epitaxy Ferromagnetic thin films Semiconductors Transmission electron microscopy X-ray diffraction
a b s t r a c t We demonstrate Fe3Si/Ge/Fe3Si/GaAs(001) structures grown by molecular-beam epitaxy and characterized by transmission electron microscopy, electron backscattered diffraction, and X-ray diffraction. The bottom Fe3Si epitaxial film on GaAs is always single crystalline. The structural properties of the Ge film and the top Fe3Si layer depend on the substrate temperature during Ge deposition. Different orientation distributions of the grains in the Ge and the upper Fe3Si film were found. The low substrate temperature Ts of 150 °C during Ge deposition ensures sharp interfaces, however, results in predominantly amorphous films. We find that the intermediate Ts (225 °C) leads to a largely [111] oriented upper Fe3Si layer and polycrystal films. The high Ts of 325 °C stabilizes the [001] oriented epitaxial layer structure, i.e., delivers smooth interfaces and single crystal films over as much as 80% of the surface area. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Device concepts based on the spin rather than the charge of electrons have been explored recently. These concepts are expected to lead to further improvements in device performance [1,2]. Metal semiconductor field effect transistors combining ferromagnetic electrodes with standard electronic device concepts were realized [3–6]. There the atomic structure of the interfaces (IFs) and the crystal perfection of the materials are important for the spin transport [7]. The influence of the IF structure of the Fe/GaAs IF on the formation of Schottky barriers was investigated using first principles densityfunctional calculations [8]. The application of Heusler alloys is very promising, because they have a high Curie temperature and can exhibit half-metallic properties. Tunneling magneto-resistance (TMR) devices with MgO barriers and half-metallic Co2FeAl electrodes with a room temperature TMR ratio of 166% were established by ultra-high vacuum magnetron sputtering even on oxidized Si substrates [9]. Even higher values are expected if the orientation distribution of the Co2FeAl texture can be improved. Similar results were obtained on MgO substrates using Co2MnSi and an epitaxial MgO barrier [10,11]. In his classic work Julliere used amorphous Ge as a barrier material of a TMR device between poly-crystalline Co and Ni electrodes [12]. In the present work we study the possibility to combine crystalline Ge with quasi-lattice matched high quality ferromagnetic electrodes: The combination of single crystal Fe3Si electrodes with single crystal Ge as a barrier material yields a lattice mismatch as low as 0.08%. For the establishment of a TMR device with Ge barrier a first epitaxial Fe3Si film could be overgrown epitaxially by a thin film of Ge, which is covered again by a lattice-matched ferromagnetic material, e.g., Fe3Si ⁎ Corresponding author. E-mail address: [email protected] (B. Jenichen). 0040-6090/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2014.01.022
or Co2FeSi. Pronounced chemical roughness in the sense of interdiffusion can occur during epitaxial growth of Fe3Si on GaAs [13,14]. Superior Fe3Si films have been grown on Ge substrates by molecular beam epitaxy (MBE) [15]. Excellent magnetic properties with a small coercivity (0.9 Oe) and electrical properties with a Schottky barrier height of 0.52 eV were obtained. [16] Lateral spin transistors, consisting of Ge channel and ferromagnetic source/drain for spin injection, were envisaged. However, metal/Ge/metal vertical structures are most promising for next-generation devices. Ge films on Si-terminated Fe3Si films were recently grown by MBE on Ge(111) substrates [17,18]. However, [001] oriented substrates are more widely used in semiconductor technology and so the epitaxial growth on (001) surfaces is often preferable. In the present work we grow and analyze ferromagnet/ semiconductor/ferromagnet structures: Fe3Si/Ge/Fe3Si layer stacks have been grown on GaAs(001) by MBE. The epitaxial growth of Ge on Fe3Si is an example for the overgrowth of a metal by a semiconductor, which can be expected to lead to chemical reactions at the IF in many cases. The as-grown structures were characterized by transmission electron microscopy (TEM), electron backscattered diffraction (EBSD), and X-ray diffraction (XRD).
Table 1 Nominal (measured) film thicknesses and substrate temperatures Ts during epitaxial growth for three samples investigated.
GaAs Fe3Si Ge Fe3Si
Sample thickness (nm)
No. 1 Ts °C
Sample thickness (nm)
No. 2 Ts °C
Sample thickness (nm)
No. 3 Ts °C
300 9 (10) 27 (22) 45 (60)
580 200 150 100
300 9 (9) 27 (20–26) 45 (53–62)
580 200 225 100
300 9 (8) 9 (6) 9 (9)
580 200 325 150
B. Jenichen et al. / Thin Solid Films 556 (2014) 120–124
121
(a)
(b)
(c)
Fig. 1. (a) HRTEM lattice image of sample 1 (Fourier filtered) with GaAs/Fe3Si and the Fe3Si/Ge IFs. (b) Overview of sample 1 (HRTEM at low magnification). (c) SAD pattern of sample 1 corresponding to the area shown in (b). The incident beam was directed along GaAs[110].
2. Experimental The structures were grown by MBE on GaAs(001) substrates. The main growth parameters are given in Table 1. After the growth of a 300 nm thick GaAs buffer layer (substrate temperature Ts = 580 °C, growth rate 480 nm/h) the templates were transferred under ultra high vacuum conditions to the As free growth chamber of the MBE system. Then nominally 9 nm thick Fe3Si films were grown at a substrate temperature of 200 °C. Ge films were grown on top of these Fe3Si films at different substrate temperatures ranging from 150 °C to 325 °C. On top of those Ge films Fe3Si epitaxial films were deposited at low substrate temperatures (100 °C or 150 °C). First the as-grown structures were characterized by high-resolution (HR) TEM. For that purpose cross-sectional TEM specimens were
Fig. 3. Grain distribution pattern (inverse pole figure) of sample 1 determined by EBSD.
prepared by mechanical lapping and polishing, followed by argon ion milling according to standard techniques. TEM images were acquired with a JEOL 3010 microscope operating at 200 kV in order to minimize radiation damage. The cross section TEM method provides high lateral and depth resolutions on the nanometer scale; however, they average over the thickness of the thin sample foil (5–20 nm). In addition the samples were investigated by EBSD and XRD in order to obtain independent data about the structure of the upper Fe3Si and the Ge films. The principle of EBSD is the following [19]: In the scanning electron microscope Kikuchi-patterns are recorded point by point. The crystal orientation of every point is recovered from the corresponding pattern with a limit of lateral resolution of 20–30 nm. In this way the distribution of the orientations of the crystallites near the surface is determined. High-resolution XRD measurements were performed on the structures using a Panalytical X-Pert PRO MRD™ system with a Ge(220) hybrid monochromator (Cu Kα1 radiation with a wavelength of λ = 1.54056 Å). 3. Results and discussion
Fig. 2. XRD curve of sample 1 in the vicinity of the GaAs 002 reflection and the corresponding simulation.
Fig. 1(a) shows a HRTEM image of sample 1 near the GaAs/Fe3Si and the first Fe3Si/Ge IFs. The Ge layer was grown at the lowest substrate temperature of Ts = 150 °C. As demonstrated previously [20–22] the GaAs/Fe3Si IF is topologically extremely smooth for the given growth conditions of the first Fe3Si film. The Fe3Si/Ge IF above however exhibits slightly increased roughness. A few lattice fringes visible on the Ge side of the IF indicate that the first mono-layers of the Ge film grew still quite well oriented and their deterioration with growing film thickness points to amorphous material above. Fig. 1(b) shows an overview. The whole layer stack becomes visible at this lower magnification. The IFs are smooth even on the larger scale. Fig. 1(c) depicts the corresponding selected area diffraction (SAD) pattern of the whole stack along the
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(a)
(b)
Fig. 6. Grain distribution pattern (inverse pole figure) of sample 2 determined by EBSD.
Fig. 4. (a) HR TEM image of sample 2. (b) SAD pattern of sample 2. The incident beam was directed along GaAs[110]. The orientation parallel to the IF is given by a dotted line.
[110] zone axis. The area selected corresponds to Fig. 1(b). Besides the single-crystal pattern Debye–Scherrer rings occurred intersecting the reciprocal lattice points 220 and 111. These rings originate from the mainly amorphous Ge layer and the upper Fe3Si film. [220] oriented grains were observed by XRD (shown below) as well, besides the prevailing [001] orientation of the first Fe3Si film. This is some evidence pointing to a randomly oriented film as the 220 reflection is expected to be the strongest in a powder pattern. The pronounced streaks
Fig. 5. XRD pattern of sample 2. Intensity maxima are marked by their Miller indices.
perpendicular to the IFs (crystal truncation rods [23]) give further evidence about the smoothness of the IFs and the surface. Fig. 2 demonstrates the XRD curve of sample 1 near the GaAs 002 reflection; the curve could be simulated taking into account merely the first Fe3Si film with a thickness of 9 nm. The contribution of the upper films could be neglected for high-resolution XRD near the GaAs 002 maximum. Fig. 3 depicts a grain distribution pattern of sample 1 determined by EBSD. The grains are oriented randomly in most places in accordance with the predominantly amorphous structure of the Fe3Si film. It seems that in some regions the limits of the EBSD method are approached. Only very few [111] oriented regions are larger (up to 100 nm, depicted in blue color) and correspond to crystallites. Fig. 4(a) shows the HRTEM image of sample 2. The Ge layer of sample 2 was grown at the substrate temperature of Ts = 225 °C. The bottom GaAs/Fe3Si IF is smooth, but the Ge film above grew as a polycrystal. The thicknesses of the Ge and the upper Fe3Si films are inhomogeneous. Differently oriented grains were observed in the HRTEM image of sample 2. We observed [001] and [111] oriented grains and a faulted structure with fault planes tilted by approximately 6.5° with respect to the surface normal. The upper Fe3Si film is poly-crystalline as confirmed by dark-field TEM micrographs (not shown here). Fig. 4(b) depicts the corresponding SAD pattern. Besides the reflections of the substrate and the perfect film we found additional maxima. The faulted lattices containing micro-twins lead to additional rows of spots partly connected by streaks (marked by straight lines) inclined by approximately 6.5° with respect to the sample surface. These streaks intersect in an additional spot corresponding to the 111 lattice plane distance oriented parallel to the [001] surface normal. This [111] orientation perfectly corresponds to the EBSD result of [111] oriented crystallites near the surface. The 2Θ/ω-scan of XRD (Fig. 5) shows besides the expected [001] orientation mainly the [111], [110], [112], and [113] orientations perpendicular to the IFs.
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(a)
(b)
Fig. 8. Lateral orientation distribution (inverse pole figure) of sample 3 determined by EBSD. Fig. 7. (a) HR TEM image of sample 3. (b) Corresponding SAD pattern showing the pattern expected for a perfect single crystalline structure. The incident beam was directed along GaAs[110].
Fig. 6 shows a grain distribution pattern of sample 2 determined by EBSD with large grain sizes (about 200 nm). A lot of grains near the surface are nearly [111] oriented (more than 80% of the whole area, depicted in blue color). The [111] orientation found in EBSD corresponds to the additional 111 spot found in the SAD pattern and to the XRD diffraction results. Fig. 7(a) demonstrates the HRTEM image of sample 3. The Ge layer of sample 3 was grown at the substrate temperature of Ts = 325 °C. The GaAs/Fe3Si IF is smooth and so is the Fe3Si/Ge IF. The Fe3Si/Ge IF above the Ge film exhibits a larger roughness than that below. The Ge film grew well oriented and also the Fe3Si film on top of the Ge turned out to be single-crystalline. The Ge film contains some small defects, detected by the strong change of the diffracted intensity of one atomic column compared to the intensity of the neighboring ones. In the region shown here the Fe3Si films are about 7 nm and 8 nm thick, whereas the Ge film is 6 nm thick. Fig. 7(b) depicts the SAD pattern of sample 3 evidencing single crystal films. The lower intensity of some maxima can be explained by the DO3 structure of Fe3Si, where the super-lattice reflections like, e.g., 1 13 have a lower intensity than the fundamental ones like, e.g., 004 [24]. Fig. 8 shows an orientation distribution pattern of sample 3 determined by EBSD. About 80% of the surface showed the correct [001] out of plane orientation (depicted in red) and about 12% are [111] oriented (depicted in blue color). This change of the orientation of some of the islands is connected to irregularities at the IF (not shown here). The remaining 8% of the surface exhibit other orientations. In Fig. 9 the XRD patterns of samples 1 (upper curve) and 3 (lower curve) are compared. The intensity maxima are marked by their Miller indices. The lattice parameters of substrate and films nearly coincide, so that the tiny film maxima are superimposed to the huge substrate
reflections GaAs 002 and GaAs 004. In sample 3 two additional peaks of low intensity are detected. Their peak positions coincide with those calculated recently for hexagonal Fe2Si 00.2 and 00.4, (cf. Fig. 9, and powder diffraction file entry #01-083-1259). This phase was discovered originally as a high temperature phase [25]. Sometimes during MBE non-equilibrium phases can occur. We are aware that such a coincidence does not fully prove the existence of a phase. However, a structure different from the D03 structure of Fe3Si is detected by the presence of those low intensity peaks hinting to a phase transition in some areas of the sample. We have overgrown [001] oriented single crystal Fe3Si films on GaAs(001) substrates epitaxially with Ge and then grew Fe3Si films on top. We could avoid chemical reaction with the Ge on large areas of the samples. The quality of the Ge films largely depends on the substrate temperature during deposition, the structural quality of the following
Fig. 9. XRD pattern of samples 1 (upper curve) and 3 (lower curve). Intensity maxima are marked by their Miller indices.
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Fe3Si film in turn is influenced by the parameters of the Ge film itself. The crystallinity of the as-grown Ge films in principle can be improved considerably by tempering [26,27]. The remaining interface irregularities can be explained by a lack of sufficient wetting of the metal surface by the Ge probably combined with residual surface roughness of the bottom Fe3Si epitaxial layer. Measurements of the magnetic properties (not shown here), however, revealed pronounced ferromagnetism of the Fe3Si films with saturation magnetizations corresponding to those of D03 ordered bulk Fe3Si. However, during these measurements upper and lower Fe3Si films were not distinguished so far.
4. Conclusions Although for the germanium growth temperature Ts = 150 °C the upper Fe3Si film was mainly amorphous as well as the underlying Ge film, low temperature growth seems to be promising for technological purposes on a larger scale, as temperature treatment of Ge after growth allows for recrystallization. Such a treatment could be designed in the future not to be harmful to the IFs. The germanium growth temperature Ts = 325 °C produces at present the best as-grown crystalline properties on large areas. Residual chemical reaction below some of the islands leading to differently oriented blocks remains to be controlled.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
Acknowledgment The authors thank Claudia Herrmann and Hans-Peter Schönherr for their support during the MBE growth, Doreen Steffen for sample preparation, Astrid Pfeiffer for the help in the laboratory, and Esperanza Luna for valuable support and helpful discussion. We thank Vladimir Kaganer for the computer program suitable for the simulation of the XRD curve.
[20] [21] [22] [23] [24] [25] [26] [27]
I. Zutic, J. Fabian, S. DasSarma, Rev. Mod. Phys. 76 (2004) 323. C. Palmstrøm, MRS Bull. 28 (2003) 725. S. Sugahara, M. Tanaka, Appl. Phys. Lett. 84 (2004) 2307. Y. Ando, K. Kasahara, K. Yamane, K. Hamaya, K. Sawano, T. Kimura, M. Miyao, Appl. Phys. Express 3 (2010) 093001. Y. Ando, Y. Maeda, K. Kasahara, S. Yamada, K. Masaki, Y. Hoshi, K. Sawano, K. Izunome, A. Sakai, M. Miyao, K. Hamaya, Appl. Phys. Lett. 99 (2011) 132511. Y. Ohdaira, M. Oogane, H. Naganuma, Y. Ando, Appl. Phys. Lett. 99 (2011) 132513. T.J. Zega, A.T. Hanbicki, S.C. Erwin, I. Zutic, G. Kioseoglou, C.H. Li, B.T. Jonker, R.M. Stroud, Phys. Rev. Lett. 96 (2006) 196101. D.O. Demchenko, A.Y. Liu, Phys. Rev. B 73 (2006) 115332. Z. Wen, H. Sukegawa, S. Mitani, K. Inomata, Appl. Phys. Lett. 98 (2011) 192505. M. Oogane, S. Mizukami, Phil. Trans. R. Soc. A 369 (2011) 3037. H. Liu, Y. Honda, T. Taira, K. Matsuda, M. Arita, T. Uemura, M. Yamamoto, Appl. Phys. Lett. 101 (2012) 132418. M. Julliere, Phys. Lett. A 54 (1975) 225. S.I. Makarov, B. Krumme, F. Stromberg, C. Weis, W. Keune, H. Wende, Appl. Phys. Lett. 99 (2011) 141910. C. Gusenbauer, T. Ashraf, J. Stangl, G. Hesser, T. Plach, A. Meingast, G. Kothleitner, R. Koch, Phys. Rev. B 83 (2011) 035319. K. Hamaya, T. Murakami, S. Yamada, K. Mibu, M. Miyao, Phys. Rev. B 83 (2011) 144411. M. Miyao, K. Hamaya, K. Sadoh, H. Itoh, Y. Maeda, Thin Solid Films 518 (2010) 273. K. Ueda, Y. Ando, M. Kumano, T. Sadoh, Y. Maeda, M. Miyao, Appl. Surface Science 254 (2008) 6215. S. Yamada, K. Tanikawa, M. Miyao, K. Hamaya, Crystal Growth and Design 12 (2012) 4703. A.J. Schwartz, M. Kumar, B.L. Adams, D.P. Field, Electron Backscatter Diffraction in Materials Science, Springer, New York, 2009. J. Herfort, H.-P. Schönherr, K.H. Ploog, Appl. Phys. Lett. 83 (2003) 3912. J. Herfort, A. Trampert, K.H. Ploog, Int. J. Mater. Res. 97 (2006) 1026. J. Herfort, B. Jenichen, V. Kaganer, A. Trampert, H.-P. Schönherr, K.H. Ploog, Phys. E. 32 (2006) 371. I.K. Robinson, D.J. Tweet, Rep. Prog. Phys. 55 (1992) 599. B. Jenichen, V.M. Kaganer, J. Herfort, H.-P. Schönherr, W. Braun, K.H. Ploog, Phys. Rev. B 72 (2005) 075329. H. Kudielka, Zeitschrift fuer Kristallographie 145 (1977) 177. B.L. Darby, B.R. Yates, I. Martin-Bragado, J.L. Gomez-Selles, R.G. Elliman, K.S. Jones, J. Appl. Phys. 113 (2013) 033505. B. Jenichen, C. Herrmann, J. Herfort, unpublished.