Epitaxial Growth of Room-Temperature Ferromagnetic MnAs

Letter pubs.acs.org/NanoLett

Epitaxial Growth of Room-Temperature Ferromagnetic MnAs Segments on GaAs Nanowires via Sequential Crystallization Joachim Hubmann,* Benedikt Bauer, Helmut S. Körner, Stephan Furthmeier, Martin Buchner, Günther Bayreuther, Florian Dirnberger, Dieter Schuh, Christian H. Back, Josef Zweck, Elisabeth Reiger, and Dominique Bougeard Institut für Experimentelle und Angewandte Physik, Universität Regensburg, D-93040 Regensburg, Germany S Supporting Information *

ABSTRACT: We investigate the incorporation of manganese into self-catalyzed GaAs nanowires grown in molecular beam epitaxy. Our study reveals that Mn accumulates in the liquid Ga droplet and that no significant incorporation into the nanowire is observed. Using a sequential crystallization of the droplet, we then demonstrate a deterministic and epitaxial growth of MnAs segments at the nanowire tip. This technique may allow the seamless integration of multiple roomtemperature ferromagnetic segments into GaAs nanowires with high-crystalline quality. KEYWORDS: MnAs segment, GaAs nanowire, deterministic epitaxial growth, room-temperature ferromagnet, sequential crystallization, single-domain

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remains dissolved in the catalyst droplet during the whole nanowire growth, as long as this droplet is liquid. We then use this strong concentration of Mn in the liquid catalyst droplet to deterministically induce the epitaxial formation of a ferromagnetic MnAs segment on the GaAs nanowire, opening a pathway toward semiconductor nanowires with integrated room temperature spintronic functionalities. The nanowires were grown in molecular beam epitaxy (MBE) using the self-catalyzed Ga-assisted growth method.29 Si(111) wafers covered with a thin layer of native SiO2 were used as substrates. A growth temperature of 600 °C was applied for all experiments. The Mn-beam equivalent pressure was 2 × 10−9 Torr, corresponding to a two-dimensional equivalent growth rate of MnAs of 0.04 Å/s. The Ga-rate was 0.4 Å/s and the As4-beam equivalent pressure was chosen such that the As4/ Ga-ratio was approximately 1.5 in order to ensure the formation of large Ga-catalyst droplets. For transmission electron microscopy (TEM) and energydispersive X-ray spectroscopy (EDX) measurements, the nanowires were transferred to a carbon-coated copper TEM grid by wiping the grid over the as-grown sample. The measurements have been performed using a FEI Tecnai F30 operated at 300 kV which is equipped with a Bruker QUANTAX EDX system with XFlash 530 detector. Magnetic force microscopy (MFM) measurements were performed with a conventional Bruker room-temperature atomic force microscope, and photoluminescence experiments on single nanowires

elf-assembled semiconductor nanowires (NWs) offer attractive characteristics to be used as building blocks of spintronic circuit architectures and in particular as transport channels for spin-based transistor1 functionalities. They combine an intrinsically well-defined tubular channel geometry with a large aspect ratio, possibilities of radial and axial band structure engineering during the self-assembly process,2−5 an excellent control of the crystalline quality,6−8 and the perspective of spin lifetimes and diffusion lengths that are superior to planar structures.9−12 In addition, for nanowires reaching a one-dimensional (1D) quantum confinement a further enhancement of spin lifetimes is expected due to a modification of spin relaxation processes in 1D nanostructures.13 An essential ingredient for the realization of such building blocks is the spin injection into and detection from the semiconductor channel. Several schemes14−17 envisioned for this functionality involve the integration of a ferromagnet and the semiconductor channel material. Particular interest has been devoted to ferromagnetic semiconductors that can be seamlessly integrated with the semiconductor spin transport channel material.18−20 For GaAs nanowires, a radial integration approach using the diluted ferromagnetic semiconductor (Ga,Mn)As as a shell wrapped around a GaAs core21−24 has been realized. Up to now, an axial approach, introducing ferromagnetic segments in a GaAs nanowire, has been seldom addressed. It can however be derived from attempts to realize magnetic GaAs-based nanowires with Ga22,25,26 and Mn27,28 as catalysts that the deterministic realization of magnetic elements in GaAs in axial direction is nontrivial. In this Letter, using self-catalyzed GaAs nanowires, we demonstrate that Mn always predominantly migrates to and © XXXX American Chemical Society

Received: September 10, 2015 Revised: December 23, 2015

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DOI: 10.1021/acs.nanolett.5b03658 Nano Lett. XXXX, XXX, XXX−XXX

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In the second sample series, we started by growing pure GaAs nanowires of the same length as shown in Figure 1. These wires, which are topped by a liquid Ga catalyst droplet, were then exposed to Mn (all other cells being closed) at the same substrate temperature and for the same duration as the nominal (Ga,Mn)As growth in the first series in order to provide the same amount of Mn to the sample. Here, we observe the exact same nanowire morphology as discussed in Figure 1. Again, a large concentration of Mn is detected in the catalyst droplet. The nanowires themselves show no traces of Mn within the EDX detection limit. We also do not observe any sign of endotaxy as, for example, observed for (Ga,Mn)As and (In,Mn) As nanowires grown by metal−organic vapor phase epitaxy.33,34 In the third sample series, we have then grown pure GaAs nanowires of the same length as shown in Figure 1. In contrast to the second series, we have first fully consumed the pure Ga catalyst droplet at the end of the wire growth through crystallization of GaAs35,36 by providing As background pressure at a substrate temperature of 600 °C. Only after full consumption of the droplet, the nanowires were then again exposed to Mn (all other cells being closed), as in the second series, at the same substrate temperature and for the same duration in order to provide the same amount of Mn to the sample. No traces of Mn are found in any of the studied nanowires of this series where no liquid droplet was present during exposure to Mn. Note also that no significant traces of Mn-containing phases are observed on the substrate between the nanowires, suggesting that Mn desorbs from the sample at this temperature in the absence of a liquid phase. From the analysis of these three sample series, we conclude that at substrate temperatures around 600 °C, which allow GaAs nanowire growth of high crystalline quality, Mn can only be incorporated into the nanowires if a liquid catalyst droplet is present. In this case, however, the Mn is not incorporated homogeneously into the GaAs nanowire, but predominantly accumulates in the form of a (Ga,Mn) alloy in the liquid catalyst droplet. On the one hand, this observation is not very surprising because the chosen substrate temperatures may allow thermodynamic and kinetic conditions close to equilibrium. From the equilibrium phase diagrams one does expect Mn incorporation into GaAs up to concentrations of only 0.1 atom %37 while liquid Ga will easily accommodate up to 10 atom % Mn at 600 °C38 (without the precipitation of Ga3Mn5-clusters). On the other hand, the formation of MnAs nanostructures in GaAs have largely been observed in planar (Ga,Mn)As growth at such substrate temperatures.30−32 It is thus interesting to note that this self-assembled precipitation of MnAs is suppressed in all our experimental configurations. We conclude that liquid Ga acts as a sink for Mn-atoms: under the chosen growth conditions the chemical potential of a Mn-atom in a MnAs-crystal is higher than the chemical potential of a Mnatom solved in liquid Ga. Also, the chemical potential of GaAs is lower than the one of MnAs, as the growth of a GaAs crystal from a liquid (Ga,Mn) alloy, supersaturated with As, is observed, but not the growth of MnAs. When cooling down the sample, the (Ga,Mn) alloy solidifies in polycrystalline form. We would like to shortly address the question of Mn incorporation into the GaAs nanowires below the detection limit of EDX. Microphotoluminescence experiments on single nanowires may identify a low doping level of Mn in GaAs nanowires. Indeed, noninteracting impurities introduce an optically active acceptor level 113 meV above the GaAs valence band edge39,40 while strong concentrations of Mn will render

were carried out using a standard microphotoluminescence setup.8 The magnetic measurements were done in a Quantum Design radio frequency superconducting quantum interference device (rf-SQUID) and in a vibrating sample magnetometer (VSM). We have designed three sample series to experimentally investigate the incorporation of Mn into GaAs nanowires at 600 °C. They are introduced and discussed in the following. In the first sample series, GaAs nanowire stubs of 300 nm were grown to initiate the growth. The Mn cell was then opened additionally to Ga and As in order to nominally grow (Ga,Mn)As nanowires. The growth of the nanowires was terminated by closing all cells simultaneously and cooling down the substrate. Figure 1a shows a bright-field scanning TEM (STEM) image and Figure 1b−d shows EDX maps of the elements Ga, As, and

Figure 1. Bright-field STEM image (a) and EDX-maps of Ga (b), As (c), and Mn (d) of a nominally (Ga,Mn)As-grown NW. The edges of the NW in the EDX-maps are indicated in white as guide to the eye. No Mn incorporation is detected in the GaAs NW. The solidified catalyst droplet consists of an As-free (Ga,Mn) alloy.

Mn of a typical nanowire from this sample series. In Figure 1a, we observe that wire growth took place during the nominal (Ga,Mn)As growth, because the wire is much longer than the 300 nm height of the stub. It actually has the same length as wires grown in a control experiment without Mn-supply under otherwise the very same growth conditions. The nominal (Ga,Mn)As nanowires and the ones from the control experiment (see Supporting Information) also look identical in bright-field TEM. The TEM micrograph in Figure 1a and the comparison to the control experiment indicate that the geometry of self-catalyzed GaAs nanowire is not altered by the supplied Mn-atoms. This conclusion is corroborated by Figure 1b−d; in fact, no Mn can be detected within the resolution limit of EDX in the crystalline GaAs nanowire. A strong concentration of Mn is however detected in the As-free solidified catalyst droplet. The droplet appears to consist of a polycrystalline (Ga,Mn) alloy. Scans of several complete nanowires in high-resolution TEM (HRTEM) reveal that no secondary crystal phases besides wurtzite (WZ) and zinc-blende (ZB) GaAs are formed during the nominal (Ga,Mn)As growth (see Supporting Information). In particular, MnAs cluster growth, which is commonly observed in 2D (Ga,Mn)As layer growth at elevated temperatures,30−32 seems to be suppressed during the Ga-catalyzed vapor−liquid−solid (VLS) growth of GaAs nanowires. B


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Figure 3. Bright-field STEM image (a) and EDX maps of the elements Ga (b), As (c), and Mn (d) of the tip region of a nominally (Ga,Mn)As-grown nanowire, whose catalyst droplet was crystallized in As4-flux. The lower part of the tip is made of Ga and As, while the end of the wire tip consists of Mn and As.

representative nanowire after application of this crystallization procedure. The part of the nanowire displayed in the figure corresponds to the droplet that was crystallized in As background pressure after stopping the axial nanowire growth. While no obvious phase separation or formation of a larger amount of crystal defects can be observed in Figure 3a, a separation of Ga- and Mn-rich phases in the crystal is detected in EDX: the end of the wire tip dominantly contains Mn and As, while the lower part of the tip is made from Ga and As and is hardly distinguishable from the end of the axially grown nanowire, as expected for crystallized catalyst droplets for Ga self-catalyzed nanowires.35,36 Figure 4 shows a close-up recorded in the HRTEM mode of a part of the tip displayed in Figure 3a. The HR mode analysis

Figure 2. Photoluminescence-spectrum of a nominally (Ga,Mn)Asgrown nanowire (T = 4 K). The peak at 1.46 eV is attributed to luminescence in GaAs caused by the staggered WZ/ZB band alignment. A slight shoulder appears at 1.409 eV, indicating isolated Mn-impurities in the GaAs crystal.

spectrum of one of the wires grown in the first sample series. Here, a dominant peak that lies below the respective excitonic states of wurtzite and zinc-blende at 1.518 eV8 and 1.515 eV is found. It is attributed to the staggered band-alignment caused by wurtzite zinc-blende phase transitions, which lead to transitions energies between 1.43 and 1.518 eV.8,41,42 Additionally, a slight shoulder appears at 1.409 eV, which can be ascribed to isolated Mn atoms residing at Ga-sites, suggesting traces below 1018 cm−3 of Mn25 in our first nominally (Ga,Mn)As-grown nanowire sample series. We thus conclude from our study that neither ferromagnetic (Ga,Mn)As may be formed nor spontaneous precipitation of MnAs can be observed in GaAs nanowires grown with the Gaassisted VLS-mechanism in MBE at 600 °C due to the presence of liquid Ga. The traces of Mn incorporated correspond to the solubility limit of Mn in GaAs at the thermal equilibrium and will be too low to form a diluted ferromagnetic semiconductor. Our results confirm conclusions of a very recently published study on similarly grown nanowires where only traces of Mn were found in the GaAs nanowires and more than 99% of the Mn evaporated during nominal (Ga,Mn)As growth was found in the form of nanocrystals within the solidified catalyst droplet.26 In the following, we will show how this strong accumulation of Mn in the liquid catalyst droplet can be used to deterministically grow room-temperature ferromagnetic MnAs segments on GaAs nanowires. To do so, we used fully developed GaAs nanowires of several micrometer length from the first and second sample series as shown in Figure 1a. As discussed before, the catalyst droplet of these wires is made of Ga, which after long enough exposure to a Mn flux is heavily alloyed with Mn. Now, in order to study to what extent the crystallization of this alloyed droplet may be controlled, we applied the crystallization technique used in the third sample series: the nanowires were exposed to an As background pressure at the substrate temperature of 600 °C until full crystallization of the droplet was observed. Only then the wires were cooled down to room temperature. Figure 3 shows a bright-field STEM image (Figure 3a) and the EDX-maps of Ga, As, and Mn (Figure 3b−d) of the tip of a

Figure 4. HRTEM image of a part of the nanowire tip shown in Figure 3a. Epitaxial layers are observed along the nanowire growth direction, illustrating a sequential crystallization of first GaAs and then α-MnAs. The epitaxial relationship between WZ MnAs and ZB GaAs is [1120]MnAs∥[110]GaAs, [1100]MnAs∥[112]GaAs, and [0001]MnAs∥[111]GaAs.

demonstrates single crystalline consumption of the heavily Mnalloyed Ga droplet under As pressure. The lattice planes in the single crystal can clearly be identified. The main body of the nanowire below the droplet was found to be predominantly zinc-blende in HRTEM (see Supporting Information). The lower part of Figure 4 then shows the area of the crystallized catalyst droplet that interfaces with the zinc-blende GaAs nanowire. This area is determined to be perfectly wurtzite over several nanometers, before switching back to zinc-blende again on a thickness of less than 10 nm. This switching pattern of the crystalline structure is commonly observed also during the crystallization of a pure Ga droplet in an As flux35,36 and is not related to the presence of Mn in the droplet. It can be explained from the crystal nucleation at the liquid/solid (Ga/GaAs) interface during the evolution of the contact angle between droplet and nanowire as the catalyst droplet size reduces in the C


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Figure 5. SQUID measurements of an as-grown nanowire ensemble with MnAs segments at the wire tips. The magnetic field was applied perpendicular to the nanowire axis, that is, in the magnetic easy plane of MnAs. (a) Hysteresis curve of the MnAs segments obtained at T = 10 K. Inset: Difference between an out-of-plane and an in-plane loop of the same nanowire ensemble at 300 K, measured in VSM. (b) Temperaturedependent curves for applied magnetic field strengths of 0, 0.1, and 1 T reveal ferromagnetism up to the Curie temperature of MnAs of 313 K.

crystallization process.43,44 On top of the thin zinc-blende GaAs segment, we then find the area that is determined to consist of Mn and As in EDX in Figure 3b−d. This region of the crystallized droplet is identified as hexagonal α-MnAs. The lattice constants are determined to be a ≈ 3.74 Å and c ≈ 5.74 Å, which is in accordance with literature values of MnAs.45,46 The epitaxial relationship with the underlying GaAs is [1120]MnAs∥[110]GaAs, [1100]MnAs∥[112]GaAs, and [0001]MnAs∥[111]GaAs, which is consistent with the relationship deduced from planar MnAs thin film growth on GaAs(111)B substrates.47−49 Figure 3 and 4 suggest a sequential crystallization process into first GaAs and then MnAs for a heavily Mn-alloyed liquid Ga catalyst droplet exposed to an As flux at 600 °C. First, the Ga is preferentially consumed from the liquid (Ga,Mn) alloy via the VLS-mechanism and precipitates as GaAs during the crystallization process of the droplet. Only after most of the Ga from the liquid phase has been used to build pure GaAs, Mn can finally be incorporated into the crystallization of the droplet in the form of MnAs. Indeed, MnAs is the only stable phase under As excess.50 This sequential precipitation scheme of GaAs and MnAs during the crystallization of the catalyst droplet delivers a control parameter to deterministically deposit a segment of MnAs on a GaAs nanowire. We have realized the same sequential precipitation scheme, leading to the formation of a well-defined α-MnAs segment also on predominantly wurtzite GaAs nanowires. In that case, the TEM analysis reveals the epitaxial relationship to be [0001]MnAs∥[0001]GaAs, [1120]MnAs∥[1120]GaAs, and [1100]MnAs∥[1100]GaAs, as also observed in wurtzite GaAs/MnAscore/shell nanowire growth.51 To verify the magnetic activity of the MnAs segments, we evaluated as-grown ensembles of nanowires terminated with MnAs segments in VSM and SQUID. Figure 5a shows a hysteresis curve obtained from a typical nanowire ensemble at 10 K in SQUID after subtracting the diamagnetic contributions. We exclude any contribution to the observed magnetic signal stemming from the filmlike structures formed on the substrate in-between the wires, because samples from which the nanowires with MnAs segments were selectively removed display pure diamagnetism. The nanowires were removed by ultrasonication, conserving the filmlike structures on the substrate surface (see Supporting Information). The ultra-

sonication process might however have removed magnetic clusters that can be randomly formed between the nanowires without a specific epitaxial relationship to the underlying substrate or NWs whose growth direction is not perpendicular to the substrate surface. If these randomly formed magnetic clusters exist, it seems reasonable to expect that their magnetic moments will be isotropically oriented. Their contribution to magnetization loops for magnetic fields oriented parallel and perpendicularly to the nanowire axis will thus be equal. In the following, we define the in-plane magnetization orientation to be perpendicular to the nanowire axis. In contrast to randomly oriented clusters, the MnAs segments formed on-top of the NWs all display the well-defined epitaxial relationship discussed in Figure 4 that is expected to induce an in-plane orientation of their magnetic moments.46,52 The in-plane magnetization loop should then be an easy axis loop and clearly differ from an outof-plane magnetization loop. Because of the isotropic character of the magnetic clusters potentially formed between the NWs or the randomly oriented NWs, a subtraction of an out-of-plane magnetization loop from an in-plane loop (see Supporting Information) will yield the in-plane signature of the MnAs segments formed on-top of the majority of the NWs in the ensemble, which grew perpendicularly to the substrate surface. The inset of Figure 5a shows the difference between an out-ofplane and an in-plane loop of the nanowire ensemble at 300 K, measured in VSM. One clearly observes a square hysteresis characteristic of an easy magnetization axis, as expected for the MnAs segments. Temperature-dependent SQUID measurements of the magnetization, shown in Figure 5b, were performed after cooling down the sample to 5 K in an external magnetic field of 7 T. This procedure aligns all spins along one direction in the magnetic easy plane. Then, the magnetization of the sample was measured while warming up from 5 to 350 K under external magnetic field strengths of 0, 0.1, and 1 T. All curves reveal a transition temperature of 313 K (40 °C), which is in excellent agreement with the Curie-temperature of MnAs.45,53 We can draw two conclusions from the magnetization measurements. First, we observe the magnetic signature of MnAs segments grown on-top of the NWs and verify their in-plane easy axis (for NWs oriented perpendicular to the sample surface). Second, we also observe a magnetic signal that might stem from randomly magnetized clusters formed between the NWs or from misaligned NWs with MnAs D


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represent very interesting nanostructures for the realization of spintronic elements with a one-dimensional character. In a broader application context, our study illustrates the novel and diverse perspectives for the crystalline growth of hybrid nanowire structures opened by the control of alloyed catalyst droplets. A different illustration is the very recently demonstrated sequential control of catalyst reactions in liquid droplets on Si nanowires.54

segments. The misalignment may occur at the NW growth initiation or during the sample handling for the different magnetization experiments. To image the magnetic domains in the MnAs segments, room-temperature MFM measurements were performed on single nanowires lying on a nonmagnetic substrate. We used the MFM lift mode, which distinguishes magnetic forces from van der Waals exchange interactions. Reversing the magnetic polarization of the MFM tip (see Figure 6) results in a

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.5b03658. Nanowire geometry and crystalline structure under Mn supply. MnAs segments: Analysis of the VSM measurements. Outlook: Regrowth of GaAs on-top of the MnAs segment. (PDF)

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Figure 6. Room-temperature MFM measurements on a NW containing a MnAs segment at the end of the tip. The measurements have been recorded in the MFM lift mode. Reversing the magnetization of the MFM tip between the left and the right image results in a reversed sign of the single domain structure in the MnAs segment, while the GaAs part below remains unchanged. The respective insets show the corresponding topography AFM images of the NW.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge Dieter Weiss for access to MFM, and financial support by the German Research Foundation (DFG) via Grant SFB 689 ”Spin Phenomena in Reduced Dimensions”.

reversed sign at the nanowire tip, whereas the nonmagnetic GaAs part of the nanowire remains unchanged. The ferromagnetic MnAs segment at the nanowire tip shows a pronounced single-domain structure with the poles pointing along one of the [1120]∥[110] directions, as the nanowire lies on one of the (110) side-facets. In conclusion, we have demonstrated a sequential crystallization of Ga droplets that are heavily alloyed with Mn on selfcatalyzed GaAs NWs. The droplet crystallization will first lead to a consumption of the Ga in the droplet through a continuation of the GaAs NW growth. Only after consumption of most of the Ga from the droplet, a significant incorporation of Mn into the crystal is observed. This crystallization is single crystalline and epitaxial with the GaAs NW in the form of single domain, room-temperature ferromagnetic MnAs segments. Our study additionally explains why a Mn content that is sufficiently high to form a diluted ferromagnetic semiconductor cannot be reached in (Ga,Mn)As NWs grown via the VLS mechanism at substrate temperatures around 600 °C. The sequential crystallization scheme of the (Ga,Mn) alloyed droplet deterministically induces the epitaxial crystallization of MnAs on GaAs NWs. Combining this control of the sequential crystallization process with wire growth continuation on top of the MnAs segment opens a pathway for the growth of GaAs NWs of high crystalline quality containing multiple room temperature ferromagnetic segments. One possible route for NW growth continuation may be the deposition of a new Ga catalyst droplet at the tip of the NW36 after each controlled crystallization of MnAs segments. This scheme requires the realization of suitable growth parameters to form facets for NW regrowth while guaranteeing the stability of the MnAs segments at VLS growth temperatures (see Supporting Information). Nanowires containing multiple ferromagnetic segments may

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