GaAsâFe3Si CoreâShell ... - American Chemical Society

Letter pubs.acs.org/NanoLett

GaAs−Fe3Si Core−Shell Nanowires: Nanobar Magnets Maria Hilse,*,† Jens Herfort,† Bernd Jenichen,† Achim Trampert,† Michael Hanke,† Peter Schaaf,‡ Lutz Geelhaar,† and Henning Riechert† †

Paul-Drude-Institut für Festkörperelektronik, Hausvogteiplatz 5-7, 10117 Berlin, Germany Institut für Mikro- und Nanotechnologie MacroNano(R), FG Werkstoffe der Elektrotechnik, TU Ilmenau, Gustav-Kirchhoff-Str. 5, 98693 Ilmenau, Germany

‡

S Supporting Information *

ABSTRACT: Semiconductor−ferromagnet GaAs−Fe3Si core− shell nanowires were grown by molecular beam epitaxy and analyzed by scanning and transmission electron microscopy, X-ray diffraction, Mössbauer spectroscopy, and magnetic force microscopy. We obtained closed and smooth Fe3Si shells with a crystalline structure that show ferromagnetic properties with magnetizations along the nanowire axis (perpendicular to the substrate). Such nanobar magnets are promising candidates to enable the fabrication of new forward-looking devices in the field of spintronics and magnetic recording. KEYWORDS: Magnetic nanotube, nanowire, semiconductor−ferromagnet hybrid structure, molecular beam epitaxy, core−shell, spintronic

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in the choice of materials is the use of the unique geometry of core−shell nanowires.13−18 For long and very thin magnetic nanotubes, that is, magnetic shells of nanowires, the magnetization can either form vortices or align along the nanowire length, for example, like in bar magnets.19−23 In the latter case, the orientation of the core nanowires perpendicular to the substrate should in principle align the magnetization in the required direction with respect to the substrate. Furthermore, nanowires of various lengths can be fabricated in large arrays comprising thousands of elements each on the nanometer scale.24 With regard to possible memory applications, ferromagnetic nanotubes have in addition the clear advantage with respect to ferromagnetic nanowires that they can support several stray-free states in a stable manner such as vortex-like states or even onion states.25,26 Thus, core−shell nanowires are promising candidates for the realization of three-dimensional magnetic recording concepts on the one hand and ready-to-use spin-LED devices operable at zero or low external magnetic field on the other hand. Recently, spin information transfer over macroscopic distances without heat development associated with moving charges has been demonstrated using optically communicating spin-LEDs.27 As spin-LED devices can in principle be fabricated within one single semiconductor−ferromagnet core−shell nanowire (consisting of a semiconductor nanowire LED structure28−34 coated with a ferromagnetic shell), arrays of

agnetizations of thin magnetic layers perpendicular to the substrate are a crucial prerequisite for the fabrication of several forward-looking spintronic device applications. Such applications are, for instance, ready-to-use circular lightemitting diodes (spin-LEDs) operating in the absence of any external magnetic field that enable optical spin information transfer over macroscopic distances without heat development associated with moving charges.1−3 Moreover, also threedimensional recording concepts with unsurpassed data storage capacities, like, for example, the magnetic domain wall racetrack memory, which is considered as an alternative to the conventionally used two-dimensional magnetic recording technology that nowadays attains its maximum storage density, rely on magnetizations and device architectures perpendicular to the substrate.4,5 Except for very limited and highly specific materials, such magnetizations of thin magnetic layers do not occur at equilibrium conditions, as the magnetic shape anisotropy leads to a magnetization that lies in the plane of the substrate surface and causes the perpendicular direction to be a hard magnetic axis. Ways to overcome this equilibrium condition are either to turn the magnetization along the hard axis by applying high external magnetic fields (in the order of a few Tesla)6−8 or to make extensive efforts in designing advanced and welldefined stacks of layers consisting of specific materials to reach conditions where the perpendicular direction of a thin film becomes an easy axis.9−12 However, the first solution has no practical use for device applications, and the latter one is furthermore highly limited in the choice of materials. A third and easy way to overcome the geometrical constraints of the magnetization orientation that is not limited © 2013 American Chemical Society

Received: September 26, 2013 Revised: November 20, 2013 Published: November 25, 2013 6203

dx.doi.org/10.1021/nl4035994 | Nano Lett. 2013, 13, 6203−6209

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such nanowire-spin-LEDs would enable on-chip optical communication of spin information. In this work, we demonstrate that GaAs−Fe3Si core−shell nanowires forming nanobar magnets with magnetizations perpendicular to the substrate can be prepared by molecular beam epitaxy (MBE). The high Curie temperature of about 840 K of the binary Heusler alloy Fe3Si is a further prerequisite for the realization of the applications mentioned above, and the perfect lattice matching to GaAs allows for MBE of high-quality hybrid structures.35−37 Moreover, the cubic Fe3Si phase shows a robust stability against stoichiometric variations (that covers a range from 10% to 26% at. Si38,39 with only slightly modified magnetic properties40), and its thermal stability against chemical reactions at the ferromagnet/semiconductor interface is considerably higher than that for conventional ferromagnets like Fe, Co, Ni, and FexCo1−x.41 Therefore, this material system has some essential advantages compared to previously studied semiconductor−ferromagnet core−shell nanowires using ferromagnetic materials that cannot reach the high quality of a binary Heusler alloy like Fe3Si.13−18 The nanowires were grown in one MBE system equipped with group III arsenide and metal growth chambers. Thus, the nanowires were entirely processed under ultrahigh vacuum conditions avoiding any oxidation of the semiconductor/ ferromagnet interface. In a first growth step GaAs nanowires were fabricated by the Ga-assisted growth mode on Si(111) substrates covered with a thin native Si-oxide layer.42,43 To form GaAs nanowires, a Ga flux equivalent to a growth rate of 100 nm/h of a two-dimensional GaAs layer and a V/III flux ratio of 1.0 at a temperature of 580 °C was supplied. Under these conditions the nanowires typically grew 1 μm per hour in length at diameters of about 80 nm. Throughout the present work, these growth parameters were not varied. After the nanowire growth, the Ga droplets were consumed by the arsenic atmosphere, and the samples were then cooled down and transferred to the growth chamber for metals under ultrahigh vacuum conditions. In a second growth step, the GaAs nanowires were overgrown with a thin Fe3Si layer, and in a series of samples the growth temperature for this step was systematically varied in the range 100−350 °C, the typical growth temperatures in Fe3Si epitaxy.41 Within this growth step the supplied Fe and Si fluxes were equivalent to a growth rate of 23 nm/h of a planar Fe3Si layer, and the flux ratio was calibrated by X-ray diffraction on previously grown layers to produce stoichiometric films consisting of 25% Si and 75% Fe. The growth was terminated after 3 h. After the growth, the planar Fe3Si layer between the nanowires was about 70 nm thick. Assuming that the Fe3Si nanowire shells grow by simple adsorption of the adatoms at the nanowire side walls, which is a reasonable assumption under the given low growth temperatures, the shell has a thickness of about 14 nm due to geometrical reasons.13 Scanning electron microscopy (SEM) micrographs of asgrown nanowires and close-up images of the middle part of single nanowires are shown in Figure 1. The rotund structure visible at the top of several nanowires most probably results from the Ga droplet which initiated the GaAs growth and which was not fully solidified after the GaAs growth finished. At low growth temperatures, the nanowire side walls exhibit a short-range surface roughness which is most pronounced for nanowires grown at 100 °C (Figure 1a). This short-range roughness flattens when the Fe3Si growth temperature is increased to 150 °C (Figure 1b) and almost vanishes at 200 °C

Figure 1. Scanning electron microscopy images of GaAs−Fe3Si core− shell nanowires with shells grown at (a) 100 °C, (b) 150 °C, (c) 200 °C, (d) 250 °C, (e) 300 °C, and (f) 350 °C. Each micrograph combines an image of typical as-grown nanowires (scale bar of 200 nm) and a close-up image of the middle part of a single nanowire (scale bar of 20 nm).

(Figure 1c). Most probably, the highly restricted adatom diffusion at 100 °C leads to a formation of Fe3Si shells consisting of various grains of different crystal orientation and stoichiometry.36,41 When the shell growth process is governed by higher adatom diffusion, the Fe3Si shells grow more uniform in crystal orientation and stoichiometry, and therefore the nanowire side wall surface flattens when the Fe3Si growth temperature is increased. For growth temperatures above 200 °C, a long-range surface roughness of the nanowire side walls that is most distinct at 350 °C can be observed in Figure 1, parts d, e, and f. As it is known from planar Fe3Si layers on GaAs substrates, Fe very intensely 6204


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Figure 2. Transmission electron microscopy (TEM) images of as-grown GaAs−Fe3Si nanowires at 100 °C. (a) Bright-field TEM image near the Si (111) reflection of nanowires. (b) Corresponding dark-field TEM image. (c) High-resolution TEM image of the GaAs core and the Fe3Si shell (Fourier filtered image).

single crystal. Despite the presence of different crystallites within the Fe3Si, in the present region the (220) planes of the GaAs core and the Fe3Si shell are parallel. The direction of the incident beam was (112)̅ . Despite the very pronounced long-range nanowire side wall surface roughness observed in SEM, TEM reveals closed shells also for nanowires grown at 350 °C which can be seen in the multibeam TEM image in Figure 3. However, the interface

tends to react with Ga and As in the high-temperature regime. In Fe3Si epitaxy on GaAs, it has been shown that for Fe3Si growth temperatures of about 400 °C these reactions lead to grains of different orientation and/or composition and thus to rough surface and interface structures.36,37,41 Thus, the observed large-scale roughening of the nanowire side walls in the high-temperature regime in Figure 1d−f indicates an ongoing reaction and interdiffusion of the involved elements Fe, Si, Ga, and As. These intermixing reactions starting at the core−shell interface set in at much lower growth temperatures compared to planar Fe3Si layers.37 This is most probably due to the high aspect ratio and the small diameter of the nanowires that in general promote instabilities at surfaces and interfaces.44−48 Note that the SEM investigation does not prove the existence of closed Fe3Si nanowire shells. At that point, the possibility that Fe3Si forms superparamagnetic corncob-like clusters that do not completely coat the nanowire side walls, like in Fe epitaxy on GaN nanowires,49 cannot be excluded. Therefore, GaAs−Fe3Si core−shell nanowires grown in lowand high-temperature regimes (100 °C and 350 °C, respectively) were investigated by transmission electron microscopy (TEM). For technical reasons, we analyzed asgrown core−shell nanowires that are shorter than the ones shown in Figure 1 and used for all other investigations in this study. Figures 2a and b display bright- and dark-field images of core−shell nanowires grown at 100 °C taken under two-beam conditions using g = 111 of Si. Core and shell regions can be well-distinguished. These images prove that Fe3Si completely wets the GaAs nanowire side walls forming smooth core−shell interfaces, which are prerequisites for the application in highquality electronic devices. In the bright-field image in Figure 2a, the contrast of the GaAs core is homogeneously dark, which is typical for single crystal material. On the other hand, the Fe3Si shell consists of inhomogeneous dark and bright regions in correspondence to various crystallites existing in different orientations. This points to a polycrystalline nature of the shell. In the dark-field image in Figure 2b, the single-crystalline GaAs core shows the same homogeneous contrast like the Si substrate (except a strain-related contour close to the interface). Thus, it consists of high-quality and pure zinc blende material without planar defects. The Fe3Si is hardly visible due to the large misalignment. In addition, in the highresolution image displayed in Figure 2c, parallel lattice fringes are visible inside the shell, whereas the nanowire core again is a

Figure 3. Multibeam TEM image of a nanowire grown at 350 °C.

between core and shell is not smooth any more. In addition, a distinct surface roughness of the Fe3Si can be observed, and an extended crater-shaped defect region below the foot surrounds core−shell nanowires grown at 350 °C. This points to reactions at the core−shell interface and also indicates that the Si/Fe3Si and Si/GaAs interfaces are not stable any more.50 The orientation of the crystallites in the Fe3Si shell and the formation of reaction products at the core−shell interface were analyzed by high-resolution X-ray diffraction (XRD). Here, all GaAs−Fe3Si core−shell nanowire samples shown in Figure 1 and additionally a planar Fe3Si layer grown at 100 °C on a Si(111) substrate covered with a thin native oxide and a GaAs nanowire template sample were measured at the wavelength of Cu Kα1. The obtained ω − 2Θ scans are shown in Figure 4. For the planar Fe3Si layer grown at 100 °C on a Si(111) substrate covered with a thin native oxide (green curve), only 6205


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the peak at 14.51°. These continuously shifting diffraction peaks lie slightly below the (002) and (003) diffraction peaks of Fe2As at 14.91° and 22.71°, respectively.54,55 In planar Fe3Si epitaxy on GaAs at elevated temperatures, precipitates of the antiferromagnetic Fe 2 As phase at the interface were detected.37,56 Thus, we suggest that reaction and interdiffusion at the GaAs−Fe3Si interface lead most probably to the formation of Fe2As in the nanowires already for Fe3Si growth temperatures of 200 °C and above. The deviation of the observed Fe2As related peaks to the exact reported positions may be due to strain in the Fe2 As and/or different stoichiometric composition. We also attribute the observed small shift of the GaAs(111) peak to slight variations in the strain state of the nanowire cores. Necessarily, interdiffusion at the core−shell interface also alters the GaAs core which is further supported by a consistent decrease of the GaAs related (111) and (333) diffraction peak intensities with shell growth temperatures above 200 °C, shown in the inset of Figure 4. Because of a certain amount of Fe2As and possibly also other phases consisting of Ga, As, Fe, and Si at the core−shell interface, we expect modified magnetic properties for GaAs−Fe3Si core−shell nanowires with rising shell growth temperature. This forecast is directly proven by Mössbauer spectroscopy. With this technique, samples with GaAs−Fe3Si core−shell nanowires grown at 100 and 350 °C (with natural iron) were measured without any further sample preparation. The Mössbauer spectra were recorded at room temperature in conversion electron mode (CEMS)57 using a 57Co(Rh) source mounted on a constant acceleration drive. The conversion electrons formed after resonant absorption were detected in an He/CH4 gas flow proportional detector.58 The spectra, stored in a 1024 multichannel analyzer, were fitted with a least-squares routine using superimposed Lorentzian lines. The calibration was performed also at room temperature with a 25 μm thick iron foil to which all isomer shifts are referred. All calculations with the area fractions (relative areas) are based on the assumption of equal Mössbauer−Lamb factors for the present iron sites.59 The obtained Mössbauer spectra for the nanowire samples grown at 100 and 350 °C are shown in Figure 5a and b, respectively. Only the sample grown at 100 °C shows several magnetically split Zeeman sextets typical for magnetic materials, like Fe3Si. Here, the ratio of the relative intensities of the lines L1, L2, and L3 of the fitted sextet sites 2, 3, and 4 gives 3:4:1. This ratio directly refers to an orientation of the spins perpendicular to the γ-beam, that is, the substrate normal. Thus, the magnetization of the Fe3Si is oriented parallel to the substrate surface. However, Mössbauer spectroscopy integrates over a relatively large sample area and thus measures the sum of contributions from the shells as well as from the parasitic layer between the nanowires that is by far larger in volume than the nanowire shells. Thus, the spectra shown in Figure 5a cannot prove the magnetic moment orientation of the Fe3Si shells. The parameters of the fitted Fe sites are scattered around the literature values for Fe3Si (details can be found in the Supporting Information). This means that either only a small fraction of the Fe3Si is present in the completely ordered D03 phase or that the sample contains a relatively large stoichiometric variety. In contrast, the sample grown at 350 °C contains only paramagnetic phases (doublet splitting) and no signs of Fe3Si anymore. The parameters of the fitted Fe sites (details can be

Figure 4. X-ray diffraction ω − 2Θ-scans measured at Cu Kα1 of the six GaAs−Fe3Si core−shell nanowire samples with different shell growth temperatures, of a GaAs nanowire template sample and a planar Fe3Si layer grown at 100 °C on a Si(111) substrate covered with a thin native oxide. The inset shows the evolution of the integrated intensities of the GaAs related diffraction peaks with Fe3Si growth temperature.

the Si related diffraction peaks are visible. This shows that the Fe3Si film is either amorphous like the underlying silicon oxide or of a polycrystalline nature with very small grains. The ω − 2Θ scan of the GaAs nanowire template (gray curve) shows the Si and GaAs related (111) diffraction peaks, proving good epitaxial alignment of the GaAs nanowires.51,52 Additional diffraction peaks arise in the ω − 2Θ scans of the six GaAs−Fe3Si core−shell nanowire samples. Nanowires with a Fe3Si with a Fe3Si growth temperature of 100 °C (curve in dark blue) show peaks at 22.61° and 28.09° that are attributed to Fe 3 Si diffraction of net-planes (220) and (222), respectively.53 As the planar Fe3Si between the nanowires corresponding to the green curve does not lead to any Fe3Si related contribution, the ordered Fe3Si crystal phases can be attributed to the Fe3Si nanowire shells. Thus, at 100 °C the polycrystalline nanowire shells show a certain amount of grains in the (110) and (111) orientations. The Fe3Si (222) diffraction peak at 28.09° is present in the core−shell nanowire samples up to Fe3Si growth temperatures of about 200 °C, whereas the Fe3Si (220) diffraction peak at 22.61° is barely visible for a Fe3Si growth temperature of 150 °C and completely absent for the sample grown at 200 °C. Thus, with an increasing shell growth temperature up to 200 °C, more Fe3Si grains are oriented in the epitaxial (111) phase. At Fe3Si growth temperatures of 200 °C and above, a diffraction peak at 14.51° arises in the ω − 2Θ scans that shifts with increasing shell growth temperature to higher angles. Such diffraction is unknown for Fe3Si.53 In addition, at Fe3Si shell growth temperatures of 300 and 350 °C, again a peak is visible slightly above the Fe3Si (220) diffraction peak at 22.61° that shows a similar shift with Fe3Si shell growth temperature like 6206


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Figure 6. Color-coded face shift maps of GaAs−Fe3Si core−shell nanowires with shell growth temperatures of (a) 100 °C and (c) 150 °C measured by a magnetic force microscope. The phase shift directly corresponds to magnetic stray fields (H⃗ ) coming out of and pointing into the scanning plane. (b) Sketch of the magnetic stray field of a magnetic bar.

Figure 5. Conversion electron Mössbauer spectra of two core−shell nanowire samples with shell growth temperatures of (a) 100 °C and (b) 350 °C.

In contrast, no magnetic stray fields were found in MFM for core−shell nanowires grown above 100 °C, which is exemplarily shown in Figure 6c for the sample grown at 150 °C. Here, interdiffusion of Fe, Si, As, and Ga destroys the ferromagnetic behavior of Fe3Si. Our experiments show that GaAs−Fe3Si core−shell nanowires prepared under specific growth conditions by MBE indeed unite all of the following properties. The Fe3Si completely wets the GaAs nanowire side walls which leads to closed shells and smooth nanowire morphologies when grown at low temperatures. The core−shell nanowires can be fabricated with smooth interfaces in a crystalline structure. A magnetization of the shells oriented perpendicular to the substrate forming nanobar magnets can be achieved as well. As discussed before, such properties and magnetizations are prerequisites to enable the fabrication of spin-LEDs operable at zero external magnetic field and three-dimensional magnetic recording devices. Thus, GaAs−Fe3Si core−shell nanobar magnets are indeed promising candidates for new forwardlooking device applications in the field of spintronics and magnetic recording.

found in the Supporting Information) correspond to various compounds of Fe and Si with a high Si content such as β-FeSi2, FeSi, or Fe2Si2, which may form in the crater-shaped defect region below the nanowire feet as described earlier in TEM investigations. However, the nonmagnetic doublets might also be ascribed to Fe2As. Although this phase is antiferromagnetic with a Neel temperature of 356 K,56 the small particle size or the content of Ga or Si might reduce this Neel temperature below room temperature so that the phase appears as a paramagnetic doublet structure. Because of the alreadymentioned integral nature of Mö ssbauer spectroscopy, investigations on single nanowires are needed to figure out the magnetic properties of the nanowire shells.15,25,26,60 Therefore, magnetic force microscopy (MFM) was carried out on single core−shell nanowires that were removed from the growth substrate and dispersed on a Si carrier substrate. Figure 6a shows MFM scans over four different core−shell nanowires with a Fe3Si shell growth temperature of 100 °C. Each nanowire exhibits a dark contrast at one end and a bright contrast at the other end. This corresponds to a magnetic field (H⃗ ) around every nanowire that points out of the substrate plane at the bright nanowire end and points into the substrate plane at the dark end which is indicated in the left-hand image of Figure 6a. As illustrated in Figure 6b, such magnetic field distributions correspond exactly to that of a magnetic bar with a magnetic north (N) and a magnetic south pole (S). Because the magnetic field in the middle of the bar lies completely in the substrate plane, no stray field can be observed in this region by MFM. Thus, the magnetization (M⃗ ) of the Fe3Si shells is oriented along the nanowires, which is indicated by the dark blue arrow in the left-hand image of Figure 6(a).

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ASSOCIATED CONTENT

S Supporting Information *

Details on the interpretation of the obtained Mössbauer spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 6207


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ACKNOWLEDGMENTS We acknowledge the expert technical assistance in maintaining the MBE by C. Herrmann and in recording the scanning electron micrographs by A.-K. Bluhm.

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REFERENCES

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Nano Letters

Letter

Liu, X.; Dobrowolska, M.; Furdyna, J. K. J. Appl. Phys. 2013, 113, 17B520.

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