NANO LETTERS
Ferromagnetic Self-Assembled Quantum Dots on Semiconductor Nanowires
2006 Vol. 6, No. 1 50-54
Dinna G. Ramlan,† Steven J. May,† Jian-Guo Zheng,† Jonathan E. Allen,† Bruce W. Wessels,†,‡,§ and Lincoln J. Lauhon*,†,§ Department of Materials Science and Engineering, Department of Electrical Engineering and Computer Science, and Materials Research Center, Northwestern UniVersity, EVanston, Illinois 60208 Received September 28, 2005; Revised Manuscript Received November 9, 2005
ABSTRACT Ferromagnetic self-assembled r-MnAs quantum dots (QD) were grown epitaxially on metal catalyst-grown InAs nanowires (NW) by chemical vapor deposition. Magnetic force microscopy measurements demonstrated that the QDs are stable, single-domain ferromagnets with Tc values of ∼310 K. Single QD switching was demonstrated at fields as low as 60 Oe. The hybrid ferromagnetic/semiconductor QD/NW properties provide a promising basis for the development of nanowire spin-valves and magnetic memory devices.
Spintronics1 refers to existing and proposed technologies that utilize the electron spin degree of freedom in both the storage and processing of information. Potential applications including nonvolatile magneto-logic have driven a search for new materials and material combinations that will facilitate the development of spintronic devices. Because of the enormous success of current electronic device technology and the associated infrastructure, semiconductor-based materials continue to be a focus of materials development efforts. The challenge is to create a material or combination of materials that can introduce and manipulate a spin-polarized carrier population at room temperature. Much recent effort has been devoted to the development of ferromagnetic semiconductors, which are created by doping conventional semiconductors with paramagnetic ions to concentrations of a few percent. Ferromagnetic metal/semiconductor (FM/S) hybrid structures are another promising approach to the development of spintronic technologies. Characterization of the individual material components of FM/S devices is relatively straightforward compared to doped ferromagnetic semiconductors, and an atomically ordered interface between a ferromagnetic metal and a semiconductor can form an effective spin filter.2 MnAs is an attractive candidate for the ferromagnetic component of FM/S devices based on III-V semiconductors because of a combination of desirable properties: MnAs is ferromagnetic at room temperature; it forms a chemically stable interface with III-As semiconductors; and it can be grown epitaxially on III-As semiconductors.3,4 Despite the lattice mismatch of 30% between MnAs and GaAs,4 it is * Corresponding author. E-mail:
[email protected]. † Department of Materials Science and Engineering. ‡ Department of Electrical Engineering and Computer Science. § Materials Research Center. 10.1021/nl0519276 CCC: $33.50 Published on Web 12/10/2005
© 2006 American Chemical Society
possible to grow an epitaxial MnAs thin film on GaAs substrate using molecular beam epitaxy (MBE) 5 and metalorganic chemical vapor deposition (MOCVD),6 and MnAs/ III-V spin-injection structures have been demonstrated with the efficiency of ∼6%.7 It is important to note that heteroepitaxial growth can stabilize both the orthorhombic (βMnAs) and hexagonal (R-MnAs) phases, but β-MnAs is paramagnetic. Synthetic strategies that produce epitaxial phase pure R-MnAs with crystalline FM/S interfaces are therefore desired. Bottom-up methods of material growth and assembly provide new tools for controlling the morphology and composition of single-component and multicomponent nanostructures such as quantum dots (QD) and nanowires (NW); the novel or enhanced properties of these nanostructures have lead to their consideration in a number of device technologies. The vapor-liquid-solid (VLS) method of nanowire growth8 in particular enables the generation of diametercontrolled9 NWs and NW heterostructures10 and is well suited to the growth of III-V semiconductor NWs including GaAs and InAs.11-13 Additionally, the growth of size-controlled QDs is enabled by the vapor-solid (VS) growth and selfassembly of a QD material that has a large lattice mismatch with the substrate. Zinc-blende MnAs QDs, for example, have been grown on GaAs substrates.14 The VLS-NW and VS-QD growth processes can be used in parallel to fabricate functional heterostructured nanomaterials that cannot be obtained by traditional methods.15 Here we report the synthesis and characterization of hybrid FM/S nanostructures for spintronic applications. Ferromagnetic self-assembled R-MnAs quantum dots (QD) were grown epitaxially on metal catalyst-grown InAs nanowires (NW) by chemical vapor
Figure 1. Scheme for fabrication of FM/S MnAs-QD/InAs-NW nanostructures. (a) InAs NWs are generated using Au as a catalyst for VLS growth. (b) Ferromagnetic QDs are formed by vapor-phase deposition of Mn on the NW surface, which then diffuses to form self-assembled MnAs quantum dots. (c) TEM image of a QD/NW structure formed via the scheme described in a and b. The scale bar is 100 nm.
deposition. Magnetic force microscopy measurements demonstrated that the QDs are stable, single-domain ferromagnets with a Tc of ∼310 K, and single QD switching was demonstrated at fields as low as 60 Oe. The hybrid ferromagnetic/semiconductor QD/NW properties provide a promising basis for the development of nanowire spin-valves and magnetic memory devices. Our bottom-up scheme for generating functional FM/S nanostructures employs semiconductor NWs to template the growth of ferromagnetic self-assembled QDs. (Figure 1). InAs NWs were grown in an atmospheric pressure metalorganic chemical vapor deposition (MOCVD) system.16 Au catalyst nanoparticles deposited on Si(100) substrates17 were used to catalyze nanowire growth in a flow of arsine (AsH3) at a rate of 6.13 µmol/min, trimethyl-indium (TMIn) at a rate of 1.07 µmol/min, and H2 at 400 °C (Figure 1a). The NWs showed hexagonal faceting and a slightly tapered profile resulting from radial vapor-solid growth in addition to the one-dimensional VLS growth.17 InAs NWs grown in this manner can have a wurtzite structure rather than the zincblende structure of bulk InAs.18 Following NW growth, TMIn flow was stopped and the reactor temperature increased to 480 °C while flowing AsH3 and H2. Self-assembled MnAs QD formation was then initiated by introducing the Mn precursor tricarbonyl-(methylcyclopentadienyl) (TCMn) manganese at a rate of 0.28 µmol/min and increasing the flow rate of AsH3 to 25.42 µmol/min (Figure 1b and c). MnAs QD formation is driven in large measure by the large (∼20%) lattice mismatch between MnAs and InAs. Mn is a mobile species on both planar and nanowire III-V surfaces,19 which facilitates the development of strain-relieving three-dimensional islands. Three-dimensional island growth, or VolmerWeber (V-W) growth, is often observed in highly lattice Nano Lett., Vol. 6, No. 1, 2006
Figure 2. Analysis of MnAs QD crystallinity and composition. (a) TEM image of MnAs QD (upper right) on InAs NW (left) demonstrating semicoherent epitaxy. The inset shows superposed fast Fourier transforms (FFT) of the NW (red) and the QD (blue) regions of the TEM image. By inspection of the FFT, and comparison with images taken along other zone axes, the image zone axis is identified as [01h10] and the NW growth axis as [0001]. The lattice constants of the NW are those of unstrained hexagonal InAs, whereas the lattice constants of the QD are strained with respect to those of bulk R-MnAs, as discussed further in the text. (b) The EDS spectra of the QD (blue) and NW (red) confirming that the QD is MnAs. The NW spectrum does not show any evidence of a MnAs “wetting-layer” on the NW surface.
mismatched systems such as InAs QDs on GaP substrates (11.4%).20 High-resolution transmission electron microscopy (TEM) studies described below did not reveal evidence of a wetting layer, which is consistent with a V-W growth mode.15 Detailed high-resolution transmission electron microscopy (HR-TEM) experiments revealed that the wurtzite InAs NW substrate templates the epitaxial growth of hexagonal (NiAs structure) MnAs QDs (Figure 2); this is important because the hexagonal form of MnAs is ferromagnetic at room temperature. NW/QDs were suspended in ethanol and deposited on Si3N4 membrane samples to allow sequential TEM and scanned-probe measurements on selected NWs. Specific NWs were located on the substrate by means of a predeposited metal grid defined by electron beam lithogra51
phy. Figure 2a shows a crystalline MnAs QD on the side of the InAs NW. The alignment of the reciprocal space lattices as shown in the Fourier transforms of the QD and NW regions (Figure 2a inset) provides evidence of epitaxial QD growth, and the moire fringes result from the overlap of the QD and NW. The QD can be indexed to the hexagonal NiAs crystal structure, and the NW to the wurtzite structure with a [0001] growth direction (vertical in the figure). Unlike bulk InAs, which has a cubic (zinc-blende) crystal structure, these InAs NWs have a hexagonal (wurtzite) crystal structure as has been observed previously.18,19,21 The NW lattice spacings of d0002 ) 3.50 Å and d21h1h0 ) 2.16 Å are nearly the same as the equivalent bulk cubic InAs lattice spacings of d111 ) 3.50 Å and d220 ) 2.14 Å, respectively.22 This close similarity, together with the lack of obvious strain contrast in the NW beside the QD boundary, suggests that the InAs NW acts as a rigid substrate for the smaller QD, that is, there is little distortion of the NW template. The MnAs lattice spacings are d0002) 2.86 Å and d21h1h0 ) 1.85 Å; these values represent a 18% lattice mismatch in the [0001] direction and a 15% lattice mismatch in the [21h1h0] direction, suggesting that the MnAs lattice achieves strain relaxation via a semicoherent growth, that is, the periodic nucleation of dislocations, in a manner similar to the growth of MnAs thin films on GaAs substrates.4,23 A detailed analysis of the interfacial structure will not be reported here. More germane to the present discussion is the observation that the wurtzitic InAs NW stabilizes the ferromagnetic hexagonal form of MnAs. We therefore expect that the QDs will be ferromagnetic at room temperature. Room-temperature magnetic force microscopy (MFM) measurements confirmed that the MnAs QDs behaved as single-domain nanomagnets (Figure 3). A Digital Instruments scanning probe microscope (Nanoscope IIIa) was used with co-coated low-moment probe tips24 to minimize the perturbation of the sample by the magnetic field of the tip. MFM images were generated by recording the phase of the cantilever oscillation in lift-mode with the cantilever out of contact ∼30-50 nm above the surface (Figure 3b). The lift height was chosen to ensure that topographical artifacts in the phase image were dominated by the magnetic signal. The bright and dark contrast evident in Figure 3c is consistent with a single-domain MnAs QD with magnetization perpendicular to the nanowire axis and located at the protrusion seen in the topographic image of Figure 3a. The MFM image shown in Figure 3d was taken on the same QD as that of Figure 3c, but with the tip magnetization reversed; as expected, the phase image is inverted, verifying the magnetic origin of the signal. The orientation of the QD magnetization in Figure 3d was determined by modeling the tip and the QD as magnetic point dipoles and fitting a one-dimensional profile of the phase contrast, as shown in Figure 3e.25 The QD dipole is perpendicular to the NW axis and at an angle of 53° with respect to the surface normal. MFM images acquired from many QDs similar to that of Figure 3 indicated that the remnant QD magnetization is consistently perpendicular to the NW [0001] growth axis. HR-TEM analysis of additional QDs further indicated that 52
Figure 3. Determining the orientation of a single-domain MnAs QD. (a) Topographic image of an InAs NW with a MnAs QD in the middle of the image. The scale bar is 100 nm. The width of the NW is 100 nm; the apparent width in the image is larger because of the tip-induced broadening. (b) Schematic of the MFM measurement geometry. After determining the sample topography (a) in intermittent contact mode, the tip height was increased ∼30 nm and the topographic profile retraced while recording the cantilever phase to produce an MFM image. (c) MFM image recorded with the tip magnetization pointing toward the sample. The contrast pattern in the MFM image is as expected for a magnetic domain oriented perpendicular to the axis of the NW; opposite contrast was observed in the MFM image taken with opposite tip magnetization (d). The contrast in the MFM image arises from the phase shift of the magnetized cantilever when it oscillates in the magnetic force gradient produced bythe QD. The force gradient at any given point is proportional to the magnetization of the QD. Magnetization parallel (and antiparallel) to the magnetization of the tip will cause dark (and bright) contrast. (e) The fitting (red curve) of the onedimensional MFM profile of (d) (black diamonds) verifies that the QD is described correctly as a single magnetic domain, as detailed further in the Supporting Information.
the MnAs , which is the easy axis of bulk MnAs, is also perpendicular to the NW [0001]. The data are therefore consistent with the easy axis of the QD being the same as that of thin film MnAs,26 despite the strain due to the epitaxial lattice mismatch. Coincidence of the MnAsbulk and MnAs-QD easy axes was verified for the QD shown in the TEM image of Figure 2 by modeling MFM crosssection; the QD/NW was deposited on an electron-transparent Si3N4 membrane sample27 which enabled imaging by both TEM and MFM. To test the ability of the QDs to act as magnetic “nanobits”, an external magnetic field was used to switch the QD magnetization while observing the QDs with MFM (Figure 4). The coercive field of MnAs can be as low as 60 Oe along the easy axis,26 so relatively small fields are needed to induce switching. Variable-field MFM was conducted Nano Lett., Vol. 6, No. 1, 2006
Figure 4. Single QD switching in an external magnetic field. (a) Topography image. (b) MFM image of the MnAs QD in an applied field of 40 Oe that opposes the QD magnetization. Thus, the QD magnetization is stable at small opposing fields. (c) MFM image of the switched QD. The streaks in the image indicate that the stray field of the tip competes with the applied field, causing the QD to switch back for some tip-sample configurations. All images are 400 × 185 nm2 in size.
using a Digital Instruments Dimension 3100 with an integrated electromagnet in the Center for Nanoscale Materials at Argonne National Laboratory. In Figure 4b, the QD magnetization state is seen to be stable when subjected to an external field of 40 Oe in the direction opposite to the remnant magnetization. When the applied field was increased to 60 Oe, the MFM image showed opposite contrast (Figure 4c) indicating that the QD had “switched”. Figure 4c also shows evidence of switching during the scan because the stray field from the tip competes with the applied field of the electromagnet. The stray tip field therefore precludes the quantitative determination of coercivity in this case. Nevertheless, the data demonstrates that the switching of a single MnAs QD bit between two stable states is possible. Variable-temperature MFM was conducted using a hightemperature scanner on the Nanoscope IIIa to analyze the temperature dependence of the magnetism of the MnAs QDs and to establish the Curie temperature, Tc, of the QDs (Figure 5). Images were acquired in 5-K increments, and the temperature was monitored during measurements using a thermocouple bonded to the sample puck. Up to 308 K, the magnetic contrast in MFM images remained constant (Figure 5b and c). The contrast disappeared abruptly at 313 K (Figure 5d) and was not observed at higher temperatures. Upon returning the sample temperature to room temperature, the magnetic contrast reappeared. The maximum cantilever phase shift, measured at the middle of the dot, is plotted versus temperature in Figure 5e. By inspection, the Tc was deterNano Lett., Vol. 6, No. 1, 2006
Figure 5. Variable-temperature MFM determination of the QD Curie temperature. (a) Topography image of MnAs/InAs-QD/NW. (b-d) MFM phase images of the QD at the indicated temperatures, showing that the magnetic-field-dependent phase contrast is approximately the constant between 298 and 308 K and the contrast disappears at 313 K. (e) The phase, which is approximately proportional to the magnetization, is plotted as a function of temperature. The phase drops abruptly to zero around the bulk Curie temperature of 313 K, consistent with a first-order phase transformation as is seen in bulk MnAs. All images are 300 × 300 nm2 in size.
mined to lie between 308 and 313 K, which is very close to the Tc of bulk R-MnAs at 318 K. The Tc of most ferromagnetic materials identifies the critical point of a second order phase transformation, that is, the magnetization decreases continuously with temperature up to the Tc at which point the material becomes paramagnetic. In contrast, R-MnAs undergoes a first-order phase transformation at Tc.28 As a result, a discontinuous change in the order parameter (magnetization) is expected to accompany the transition from ferromagnetism to paramagnetism. Indeed, an abrupt change in the QD magnetization is observed upon moving through Tc, providing additional evidence the MnAs-QD retains the properties of bulk R-MnAs. It is important to note that while the Tc is not far above room temperature, the magnetic properties will not vary significantly in the temperature range of interest. 53
In summary, we have synthesized a new FM/S hybrid nanostructure consisting of MnAs QDs on InAs NWs. The ferromagnetic MnAs QDs exhibit bistable switching behavior at room temperature. This bistability, combined with the crystalline QD/NW interface, suggests that these nanostructures may find application in nanoscale spintronic devices analogous to and beyond those demonstrated in planar systems. We envision at least two possible modes of operation, one in which QD contacts act as spin injectors/ analyzers, and a second in which QDs act as nonvolatile switchable spin-filters in the middle of a NW device. Bottomup approaches to unique multicomponent nanostructures can therefore play a useful role in the development of spintronic technologies. Acknowledgment. This work was supported by the MRSEC program of the National Science Foundation (DMR0076097) at the Materials Research Center of Northwestern University through seed funding (L.J.L. and B.W.W.) and Central Facility support, and the NSF Spin Electronics Program ECS-0224210 (B.W.W.). We acknowledge the Northwestern University Atomic and Nanoscale Characterization Center (NUANCE) for use of the JEOL 2100F TEM. The AFM and SEM measurements in the NUANCE facility were partially supported by Northwestern University’s Murphy Society Undergraduate Research Grant in Nanoscale Engineering (D.G.R). We are grateful to Seok-Hwan Chung and Axel Hoffmann for assistance with the variable-field MFM measurements at the Center for Nanoscale Materials at Argonne National Laboratory. Supporting Information Available: Details of the MFM profile modeling. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) Wolf, S. A.; Awschalom, D. D.; Buhrman, R. A.; Daughton, J. M.; von Molnar, S.; Roukes, M. L.; Chtchelkanova, A. Y.; Treger, D. M. Science 2001, 294, 1488. (2) Kirczenow, G. Phys. ReV. B 2001, 63, 054422. (3) Tanaka, M. Semicond. Sci. Technol. 2002, 17, 327. (4) Ploog, K. H. Physica E 2004, 24, 101. (5) Tanaka, M.; Harbison, J. P.; Park, M. C.; Park, Y. S.; Shin, T.; Rothberg, G. M. J. Appl. Phys. 1994, 76, 6278.
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(6) Lane, P. A.; Cockayne, B.; Wright, P. J.; Oliver, P. E.; Tilsley, M. E. G.; Smith, N. A.; Harris, I. R. J. Cryst. Growth 1994, 143, 237. (7) Ramsteiner, M.; Hao, H. Y.; Kawaharazuka, A.; Zhu, H. J.; Kastner, M.; Hey, R.; Daweritz, L.; Grahn, H. T.; Ploog, K. H. Phys. ReV. B 2002, 66, 081304. (8) Wagner, R. S.; Ellis, W. C. Appl. Phys. Lett. 1964, 4, 89. (9) Gudiksen, M. S.; Lieber, C. M. J. Am. Chem. Soc. 2000, 122, 8801. (10) Lauhon, L. J.; Gudiksen, M. S.; Lieber, C. M. Philos. Trans. R. Soc. London, Ser. A 2004, 362, 1247. (11) Bjork, M. T.; Ohlsson, B. J.; Sass, T.; Persson, A. I.; Thelander, C.; Magnusson, M. H.; Deppert, K.; Wallenberg, L. R.; Samuelson, L. Appl. Phys. Lett. 2002, 80, 1058. (12) Duan, X.; Wang, J.; Lieber, C. M. Appl. Phys. Lett. 2000, 76, 1116. (13) Hiruma, K.; Yazawa, M.; Katsuyama, T.; Ogawa, K.; Haraguchi, K.; Koguchi, M.; Kakibayashi, H. J. Appl. Phys. 1995, 77, 447. (14) Ono, K.; Okabayashi, J.; Mizuguchi, M.; Oshima, M.; Fujimori, A.; Akinaga, H. J. Appl. Phys. 2002, 91, 8088. (15) For additional discussion of QD on NW growth, see Pan, L.; Lew, K.-K.; Redwing, J. M.; Dickey, E. C. Nano Lett. 2005, 5, 1081. In that instance, the growth of Ge QDs on Si NWs involved a transition from layer-by-layer to island growth- the Stranski-Krastanov growth mode. The smaller lattice mismatch and lower Ge mobility lead to a higher density of QDs than that observed here. (16) Blattner, A. J.; Lensch, J.; Wessels, B. W. J. Electron. Mater. 2001, 30, 1408. (17) Lauhon, L. J.; Gudiksen, M. S.; Wang, D. L.; Lieber, C. M. Nature 2002, 420, 57. (18) Koguchi, M.; Kakibayashi, H.; Yazawa, M.; Hiruma, K.; Katsuyama, T. Jpn. J. Appl. Phys., Part 1 1992, 31, 2061. (19) May, S. J.; Zheng, J. G.; Wessels, B. W.; Lauhon, L. J. AdV. Mater. 2005, 17, 598. (20) Leon, R.; Lobo, C.; Chin, T. P.; Woodall, J. M.; Fafard, S.; Ruvimov, S.; Liliental-Weber, Z.; Kalceff Stevens, M. A. Appl. Phys. Lett. 1998, 72, 1356. (21) Under our growth conditions, nanowires were wurtzite or wurtzite with stacking faults. This is, in general, in agreement with a systematic study of the effect of growth conditions on the crystal structure of InAs nanowires described in ref 18. In general, they found that the wurtzite structure was favored over zinc blende at higher growth temperatures and larger V-III precursor ratios. (22) JCPDS card 89-4168 (InAs) International Centre for Diffraction Data (23) Trampert, A.; Schippan, F.; Daweritz, L.; Ploog, K. H. Appl. Phys. Lett. 2001, 78, 2461. (24) Veeco MESP-LM tips. (25) Details of the MFM profile modeling are presented in the Supporting Information. (26) Schippan, F.; Behme, G.; Daweritz, L.; Ploog, K. H.; Dennis, B.; Neumann, K. U.; Ziebeck, K. R. A. J. Appl. Phys. 2000, 88, 2766. (27) SPI Supplies: Silicon Nitride Membrane Window Grids, 100 nm thickness. (28) Bean, C. P.; Rodbell, D. S. Phys. ReV. 1962, 126, 104.
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Nano Lett., Vol. 6, No. 1, 2006
