Facetted growth of Fe3Si shells around GaAs nanowires on Si(111)
FFacetted growth of Fe Si shells around GaAs nanowires on Si(111)
B. Jenichen, ∗ M. Hilse, J. Herfort, and A. Trampert
Paul-Drude-Institut f¨ur Festk¨orperelektronik,Hausvogteiplatz 5–7,D-10117 Berlin, Germany (Dated: October 19, 2018)
Abstract
GaAs nanowires and GaAs/Fe Si core/shell nanowire structures were grown by molecular-beamepitaxy on oxidized Si(111) substrates and characterized by transmission electron microscopy. Thesurfaces of the original GaAs NWs are completely covered by magnetic Fe Si, exhibiting nanofacetsand an enhanced surface roughness compared to the bare GaAs NWs. Shell growth at a substratetemperature of T S = 200 ◦ C leads to regular nanofacetted Fe Si shells. These facets, which leadto thickness inhomogeneities in the shells, consist mainly of well pronounced Fe Si(111) planes.The crystallographic orientation of core and shell coincide, i.e. they are pseudomorphic. Thenanofacetted Fe Si shells found in the present work are probably the result of the Vollmer-Weberisland growth mode of Fe Si on the { } side facets of the GaAs NWs. ∗ [email protected] a r X i v : . [ c ond - m a t . m t r l - s c i ] J un . INTRODUCTION Nanowires that combine a semiconductor and a ferromagnet in a core/shell geometryhave gained a lot of interest in recent years.[1–6] Because of the cylindrical shape of theferromagnet, such core/shell nanowires could allow for a magnetization along the wire andthus perpendicular to the substrate surface. Ferromagnetic stripes or tubes with a mag-netization perpendicular to the substrate surface have the potential for circular-polarizedlight emitting diodes that can optically transmit spin information in zero external magneticfield.[7] This could enable three-dimensional magnetic recording with unsurpassed data stor-age capacities.[8, 9] The combination of the binary Heusler alloy Fe Si (Curie temperature ofabout 840 K) and GaAs, has several advantages compared to most previously studied semi-conductor/ferromagnet (SC/FM) core/shell NWs. The perfect lattice matching allows forthe molecular beam epitaxy (MBE) growth of high quality planar hybrid structures.[10–13]In addition, the cubic Fe Si phase shows a robust stability against stoichiometric varia-tions, which only slightly modify the magnetic properties.[14] Moreover, the thermal stabil-ity against chemical reactions at the SC/FM interface is considerably higher than that ofconventional ferromagnets like Fe, Co, Ni, and Fe x Co − x . [10] We recently demonstratedthat GaAs/Fe Si core/shell NWs prepared by MBE show ferromagnetic properties with amagnetization oriented along the NW axis (perpendicular to the substrate surface).[15] How-ever, the structural and magnetic properties of the core/shell NWs were found to dependstrongly on the substrate temperature during the growth of the Fe Si shells,[15, 16] andnanofacetting was observed.[16] In this work, we investigate periodically faceted Fe Si shellsgrown around GaAs(111) oriented cores and analyze the nanofacets by scanning electronmicroscopy (SEM), and transmission electron microscopy (TEM).
II. EXPERIMENT
GaAs/Fe Si core/shell NW structures were grown by MBE on Si(111) substrates. First,GaAs nanowires are fabricated by the Ga-assisted growth mode on Si(111) substrates coveredwith a thin native Si-oxide layer. The growth mechanism is the vapor-liquid-solid (VLS)mechanism,[17–21] where pin holes in the SiO serve as nucleation sites.[22] A Ga dropletis the preferred site for deposition from the vapor. The GaAs NW then starts to grow by2 igure 1. SEM image of GaAs/Fe Si core/shell NWs and islands grown by molecular beam epitaxyon a Si(111) substrate. preferential nucleation at the spatially restricted GaAs/Si interface (IF). Further growthis nearly unidirectional and proceeds at the solid/liquid IF. The GaAs NWs are grownat a substrate temperature of 580 ◦ C, with a V/III flux ratio of unity and an equivalenttwo-dimensional growth rate of 100 nm/h. Once the GaAs NW templates are grown, thesample is transferred under ultra high vacuum conditions to an As free growth chamber fordeposition of the ferromagnetic films. There the GaAs NW templates were covered withFe Si shells at substrate temperatures varying between 100 ◦ C and 350 ◦ C. The NW shellsgrown at 200 ◦ C show a regular nanofacet structure and were selected for a careful analysisof the nanofacets. More details regarding the growth conditions can be found in Ref. [15].The resulting core/shell NW structures were characterized by SEM and TEM. The TEMspecimens are prepared by mechanical lapping and polishing, followed by argon ion millingaccording to standard techniques. TEM images are acquired with a JEOL 3010 microscopeoperating at 300 kV. The cross-section TEM methods provide high lateral and depth reso-3 igure 2. (color online) SEM top view of GaAs/Fe Si core/shell NWs and islands between theNWs grown by molecular beam epitaxy on a Si(111) substrate. lutions on the nanometer scale, however they average over the thickness of the thin samplefoil or the thickness of the NW as a whole.
III. RESULTS AND DISCUSSION
Figure 1 shows an SEM micrograph of the GaAs/Fe Si core/shell NWs. A relativelylow area density of well oriented NWs of ∼ × cm − is found as well as a comparabledensity of hillhocks. During the last stage of GaAs NW growth no Ga is supplied, and sothe remaining Ga in the droplet on top of the NWs is consumed, leading to an elongation ofthe NW at reduced diameter.[16] The sidewalls of the NWs exhibit nanofacetted surfaces.Figure 2 shows an SEM top view image of GaAs/Fe Si core/shell NWs grown by MBE4n a Si(111) substrate. The NWs exhibit the typical hexagonal cross-section (sketched ingray),[16] however, at higher magnification we observe triangular features (sketched in red)which are connected to the thinner necks of the NWs, where the tilted Fe Si(111) planesform extended facets (cf. Fig. 1) intersecting the top Fe Si(111) plane, which is parallel tothe substrate surface. This results in the formation of the triangular features. The loweredges of the necks are more rounded.Figure 3 shows a multi-beam bright-field TEM micrograph and corresponding SAD pat-tern, illustrating the orientational relationship of a GaAs/Fe Si core/shell NW on Si(111).We observe a coincidence of the core- and shell-orientations, hence the Fe Si growth is pre-dominantly epitaxial on the GaAs. In addition, the crystallographic orientation of the Fe Sishell was confirmed by high-resolution TEM (not shown here). The grey line drawn onthe SAD pattern near the 111 reflection is oriented perpendicular to the Fe Si nanofacetsvisible in the corresponding micrograph. The separation of the diffraction spots along thisline corresponds to the (111) net plane distance of Fe Si and indicates the nanofacets aremainly (111)-oriented. In the SAD pattern from a single core/shell NW the Fe Si maximaare stronger probably due to the larger volume fraction of the shell. In order to measure theSAD, the substrate was first oriented near the [011] zone axis. Then the sample had to betilted a little further in order to reach the [011] zone axis of the GaAs NW, since the axisof the NW was not exactly perpendicular to the Si surface. In the NW SAD pattern shownin Fig. 3 the fundamental reflections of the Fe Si are more intense than the super-latticemaxima.[11] Nevertheless we can distinguish, 222, 333, and 444 maxima indicating the NWis properly oriented and the crystallographic orientations of core and shell basically coincide.This illustrates that a growth temperature of 200 ◦ C results in a highly perfect Fe Si shellstructure. The surface nanofacets are inclined to the (111) net planes parallel to the Sisurface by an angle of approximately 108 ◦ . The formation of facets reduces the overall sur-face energy and evidences non negligible material transport over distances small comparedto the NW lengths.[23, 24] Unfortunately, the orientation of those surface nanofacets doesnot coincide with the GaAs NWs facets corresponding to { } planes.[25] As a result, thefacetted growth leads to shell thickness inhomogeneity. On a larger length-scale the Fe Sishell is approximately reproducing the shape of the GaAs core NWs.[16]Layer-by-layer growth could in principle solve the problem of nanofacetting. However,even planar Fe Si grows on GaAs(001) in the Vollmer-Weber (VW) island growth mode.[26]5 igure 3. Multi-beam bright-field TEM micrograph and the corresponding SAD pattern illus-trating the orientational relationship of a GaAs/Fe Si core/shell NW. The straight line near the111 reflection is oriented perpendicular to the nanofacets of Fe Si visible in the correspondingmicrograph.
Poor wetting during the growth of Fe Si on GaAs leads initially to isolated islands, despite ofperfect lattices match. We speculate that the nanofacetted Fe Si shells found in the presentwork are a result of VW island growth. One way to improve the homogeneity and otherstructural properties of Fe Si-shells could be to use surfactants. For planar growth on GaAs,6urfactant materials like Sb [27], Bi [28] and Te [29] have been investigated.
IV. CONCLUSIONS
GaAs core NWs were grown epitaxially on the oxidized Si(111) surface (inside holes ofthe SiO film) via the VLS growth mechanism. Then magnetic Fe Si shells were grownresulting in continuous covering of the cores. Fe Si shells grown at a substrate temperatureof T S = 200 ◦ C are fully epitaxial with a nanofacetted surface. The (111) facets are mostpronounced, forming a regular pattern around the GaAs NWs. This facetting is probablythe result of the VW island growth mode of Fe Si on GaAs.
V. ACKNOWLEDGEMENT
The authors thank Claudia Herrmann for her support during the MBE growth, DoreenSteffen for sample preparation, Astrid Pfeiffer for help in the laboratory, Anne-KathrinBluhm for the SEM micrographs, Ryan Lewis, Esperanza Luna and Uwe Jahn for valuablesupport and helpful discussion.
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