NANO LETTERS
Manganese-Induced Growth of GaAs Nanowires
2006 Vol. 6, No. 9 2130-2134
Faustino Martelli,*,†,‡ Silvia Rubini,†,‡ Matteo Piccin,†,‡ Giorgio Bais,†,‡ Fauzia Jabeen,†,‡ Silvano De Franceschi,† Vincenzo Grillo,† Elvio Carlino,† Francesco D’Acapito,§ Federico Boscherini,| Stefano Cabrini,† Marco Lazzarino,† Luca Businaro,† Filippo Romanato,† and Alfonso Franciosi†,‡ Laboratorio Nazionale TASC-INFM-CNR, Area Science Park, S.S. 14, Km. 163.5, I-34012 Trieste, Italy, Center of Excellence for Nanostructured Materials, UniVersity of Trieste, 34127 Trieste, Italy, CNR-INFM-OGG, c/o ESRF, GILDA CRG, 6 Rue Jules Horowitz, F-38043 Grenoble, France, and Department of Physics and CNISM, UniVersity of Bologna, Viale Berti Pichat 6/2, I-40127 Bologna, Italy Received April 6, 2006; Revised Manuscript Received July 14, 2006
ABSTRACT GaAs nanowires have been grown on SiO2 and GaAs by molecular beam epitaxy using manganese as growth catalyst. Transmission electron microscopy shows that the wires have a wurtzite-type lattice and that r-Mn particles are found at the free end of the wires. X-ray absorption fine structure measurements reveal the presence of a significant fraction of Mn−As bonds, suggesting Mn diffusion and incorporation during wire growth. Transport measurements indicate that the wires are p-type, as expected from doping of GaAs with Mn.
Semiconductor research has recently been considering semiconducting nanowires (NWs) as building blocks for nanoscale electronic and photonic devices.1 NW-based devices as diverse as biochemical sensors,2 light-emitting diodes,3 single-electron transistors,4 and solar cells5 have been demonstrated. NW fabrication often exploits a metal catalyst that induces and dictates the growth, thus determining the NW parameters. The vapor-liquid-solid (VLS)1 model, albeit somewhat controversial,6 is widely used to describe catalyst-assisted NW growth. Another emerging field of semiconductor researchs spintronicssplans to make profitable use of diluted magnetic semiconductors. In these materials, the magnetic properties arise from spin-exchange interactions between the magnetic impurity ions and free holes.7 To exploit GaAs technology, one of the most investigated material systems is GaAs:Mn, both in the form of MnGaAs alloy8 and Mn-δ-doped GaAs.9 The combination of these two research fields could lead to one-dimensional (1D) spintronic devices. Attempts to dope semiconductor NWs with Mn by postgrowth implantation or by adding a Mn precursor during growth have been reported for ZnO,10,11 GaN,12-15 CdS,15 and ZnS.15,16 Results on GaAs:Mn nanowires have not been reported yet. Recently, * Corresponding author. E-mail:
[email protected]. † Laboratorio Nazionale TASC-INFM-CNR. ‡ Center of Excellence for Nanostructured Materials, University of Trieste. § CNR-INFM-OGG, c/o ESRF, GILDA CRG. | Department of Physics and CNISM, University of Bologna. 10.1021/nl0607838 CCC: $33.50 Published on Web 07/29/2006
© 2006 American Chemical Society
it has been reported that the introduction of a Mn precursor during the VLS growth of gold-catalyzed InAs NWs by metal-organic vapor phase epitaxy leads to a highly branched dendritic growth.17 An important issue in catalyst-assisted NW growth is whether the catalyst diffuses into the wires and changes the NW electronic properties. Claims about luminescence related to electron-hole (e-h) recombination on catalyst impurities in NWs have been published,18,19 and very recently direct observation of Au catalyst impurities in InAs20 and ZnSe21 NWs has been reported. In the present Letter we report on the growth of GaAs NWs by molecular beam epitaxy (MBE) using manganese as the growth catalyst. Our results demonstrate the possibility to exploit the diffusion of the Mn catalyst into GaAs as a potential method to obtain GaAs:Mn NWs. GaAs NWs were grown on SiO2 and GaAs(100) by solidsource MBE in a system connected in ultrahigh-vacuum to a metallization chamber equipped with an electron beam evaporator for Mn deposition. The substrates were heated at 300 °C for 30 min, for contaminant degassing,22 prior to introduction in the metallization chamber, where 1 nm of manganese was deposited at room temperature on the substrates before their introduction into the growth chamber. The Mn deposition rate was calibrated in situ using a quartz microbalance. The wires were then grown at different growth temperatures (Tg) using Ga and As elemental sources. The typical growth of the wires reported in this paper lasted 30
min and the beam equivalent pressures used were 2.1 × 10-7 Torr for Ga and 3 × 10-6 Torr for As. The growth parameters employed would yield a GaAs layer-by-layer growth rate of 1.0 µm/h. Au-catalyzed wires have been also grown under the same conditions. Some of their characteristics and properties will be shown for sake of comparison to point out some specific feature of the Mn-catalyzed wires. If otherwise not explicitly stated, the following text will describe the properties of Mn-catalyzed wires. Figure 1a shows a field-emission scanning electron micrograph of NWs obtained at Tg ) 580 °C on SiO2. The image is a planar view of the sample. NWs up to 15 µm long cover the sample surface and often show a tapered shape. The lateral dimension of the wires ranges from tens to about 200 nm. Together with the many 1D structures, fewer two-dimensional (2D) structuressnanoleavessare also observed. Within the growth temperature of 540-620 °C fewer changes are observed, in particular, the NW density appears higher in the high-temperature region. Outside the indicated temperature range, no wire can be found on the substrate after the growth procedure. Some differences have been found on GaAs. Figure 1b shows the planar field emission scanning electron microscopy (FE-SEM) view of a sample grown at Tg ) 540 °C. With respect to the SiO2 substrate, a lower density of wires is found but a larger density of nanoleaves is present. Similar findings are found in the growth temperature range of 510-620 °C, with an increase of the density of nanoleaves by increasing the temperature. Rare and short wires have been found down to Tg ) 450 °C. As in the case of SiO2 substrates, no kind of nanostructures is found above the indicated growth temperature range. Although the density of 1D wires on GaAs is low, they present some interesting features. In Figure 1c we show a FE-SEM side view of the same sample shown in Figure 1b. The surface in the plane of the image is a [110] cleavage surface. The particular point of view favors the observation of the nanowires, which generally have grown with preferential orientations. Many wires, which can be as long as 20 µm, are clearly bent. We wish to point out that the growth of the nanowires and nanoleaves is very reproducible in terms of shape, density, and size, under the same growth conditions. Transmission electron microscopy (TEM) experiments were performed by using a JEOL 2010F UHR microscope equipped with a field emission gun, operating at 200 kV and capable of 0.19 nm phase contrast resolution at optimum defocus. In particular, the fine structure of the wires was investigated by high-resolution TEM (HR-TEM) and the relative fast Fourier transforms (FFT) were used to measure the lattice spacing and the orientation of the wires. For the experiments the wires were mechanically transferred on a carbon-coated copper mesh. Figure 2 shows a representative HR-TEM micrograph of the end section of one GaAs NW. This particular wire was grown on SiO2. The image is representative of the large majority of the wires in the terms described below. The structure of the nanowire is revealed by HR-TEM together with the relevant FFT (bottom-right part of Figure 2), which Nano Lett., Vol. 6, No. 9, 2006
Figure 1. Scanning electron microscopy of typical Mn-catalyzed GaAs nanowires. (a) Planar view of the nanostructures obtained at Tg ) 580 °C on SiO2. (b) Planar view of nanowires and nanoleaves obtained on GaAs(100) at 540 °C. Except for the growth temperature, the growth parameters were the same as those of the sample shown in (a). (c) Side view of the same sample shown in (b). The GaAs surface in the plane of the image is a [110] cleavage surface.
demonstrates that the nanowire body is GaAs with wurzite structure and [0001] growth axis, as also observed in our as well as in other Au-catalyzed III-V NWs.23,24 A small fraction of wires, characterized by a large number of stacking faults, also shows the presence of the zinc blende polytype of GaAs. It is worth pointing out that the nanostructures that we have called nanoleaves individually present both wurtzite and zinc blende lattice types and are characterized by a large number of defects. The nanoleaves terminate with a tip of similar size and the same composition (R-Mn) of those found 2131
Figure 2. HR-TEM image taken at the top of a representative nanowire showing the structure of the body and of the tip of the nanowire. The wire shown in the image has been grown on SiO2. The insets are the FFT obtained from the marked relevant regions of the nanowire. The FFT from the body indicates a wurzite polytype of GaAs in the 〈0001〉 zone axis whereas the FFT from the crystalline region of the tip is due to a R-Mn phase, seen out of precise zone axis.
on top of the 1D nanowires. The wires showing a high density of defects might be a kind of transition structure between the high-quality one-dimensional wires and the twodimensional nanoleaves. The wurtzite lattice then appears as a signature of the one-dimensional growth of GaAs. This feature agrees with very recent calculations on the structural stability of InP nanowires.25 The TEM images also show that an amorphous region surrounds the nanowires, most probably oxidized GaAs. We observe that the Mn-catalyzed wires have a thicker oxide layer (about 4 nm) than the Au catalyzed wires (about 1 nm, not shown). We will briefly discuss this point later in this paper. Within the tip, above the end of the wurzite structure, there is a crystalline region seen out of precise zone axis, surrounded by a large region with amorphous phase contrast, due to postgrowth oxidation of the sidewalls of the NWs. The HR-TEM experiments show that there is not a fixed crystallographic relationship between the body of the wire and the tip. The analysis of the FFT (bottom left part of Figure 2) from the HR-TEM images of the tips reveals spacings of 0.62 and 0.36 nm which indicate the presence of an R-Mn phase and rule out the presence of a GaAs polytype, R-MnAs, β-MnAs, and all known MnGa alloys. To investigate whether some of the Mn atoms have diffused into the GaAs NW during growth, we performed extended X-ray absorption fine structure (EXAFS) measurements at the Mn-K edge (E ) 6539 eV). We used the Italian beamline GILDA at the European Synchrotron Radiation 2132
Figure 3. (a) Experimental EXAFS data (line) with the best fitting curve (dots). (b) Fourier transform of the data in (a): experimental data (line) and best fit (dots). The FT was performed in the range k ) 3.6-12 Å-1 using a Hanning window and a k2 weight. The R scale has no phase correction, so that all the peaks appear shifted by approximately 0.3 Å. The arrows indicate the peaks due to the Mn-O (left) and Mn-As (right) bonds.
Facility. The monochromator was equipped with a pair of Si(311) crystals and was run in the dynamically focusing mode.26 Harmonics rejection was achieved, detuning the crystals at 80% of the rocking curve maximum. Data were collected in the fluorescence mode using a 13-element hyperpure Ge detector and normalized by measuring the incident beam intensity with an ion chamber filled with nitrogen gas. To minimize the effects of coherent scattering from the substrate, the samples were mounted on a vibrating sample holder. Measurements were carried out at 77 K in order to minimize thermal disorder. The EXAFS function is reported in Figure 3a whereas the corresponding Fourier transform (FT) is shown in Figure 3b. The FT exhibits two peaks (marked by arrows) that are related to Mn-O and Mn-As (or possibly Mn-Mn, see below) coordination. No other peaks at higher interatomic distance can be safely distinguished from the noise, which indicates considerable local disorder. Three local structures for Mn were considered in the quantitative data analysis: (i) a substitutional site in zinc blende GaAs, as described in ref 27; (ii) a combination of the two slightly nonequivalent sites of the cation in the bixbyte structure of Mn2O3; (iii) the local structure exhibited by metallic R-Mn. Theoretical signals relative to the Mn-As, Mn-O, and Mn-Mn atomic correlation for these local structures were calculated by ab initio simulation with the FEFF 8.1 code28 using muffin tin potentials and the Hedin-Lunqvist approximation for the Nano Lett., Vol. 6, No. 9, 2006
energy-dependent part. These theoretical signals were used in a nonlinear fitting routine to extract the local structural parameters. Since the backscattering functions of Mn are similar to those of As, two models were considered: one with only Mn-O and Mn-As contributions (model 1), the other with only Mn-O and Mn-Mn contributions (model 2). The best fit was obtained with the model 1 with 46 ( 7% of the total amount of Mn atoms bonded to As and 54 ( 7% of Mn atoms linked to O. The As-Mn bond length is RMnAs ) 2.56 ( 0.02 Å and with a Debye-Waller factor σ2 ) (50 ( 10) × 10-4 Å2. We note that the Mn-As bond length is significantly stretched with respect to that observed in Mn substitutional in zinc blende (ZB)-GaAs (2.50 Å),27 whereas it approaches the value reported in hexagonal MnAs (2.57 Å).29 The remainder of Mn is in an oxide phase with bond length RMnO ) 2.08 ( 0.03 Å. The observed Mn-As distance leaves open the question about the amount of Mn incorporated as an impurity in wurtzite-GaAs and that forming MnAs clusters within GaAs. The presence of a few percent of the total Mn in the R-phase, present at the top of the wires as seen by TEM, could be below the sensitivity of the EXAFS technique. We point out that wire oxidation, observed with both TEM and EXAFS, takes place after the nanowire growth, which occurs in ultrahigh vacuum in all its steps. This observation may be a hint of the presence of Mn in the wires and in particular in the sidewalls, since Mn has a high tendency to become oxidized, as also clear from the oxide present on the R-Mn tip of the wires. The nanowires have been characterized by basic electric transport measurements. NWs were mechanically transferred onto a p+-Si substrate with a 120 nm thick SiO2 surface layer and subsequently contacted by interdigitated Ti/Al electrodes fabricated by X-ray lithography and/or electron beam lithography. The wires have been shortly etched in buffered HF to remove the oxide prior to contacting them. Measurements of source-drain current (I) vs gate voltage (Vg) have been performed using the p+-Si wafer as a back gate. The current flows through a set of about 10 nanowires, contacted in parallel with random procedure. For the sake of comparison we report the results obtained on nominally undoped Mn-catalyzed NWs together with those obtained on nominally undoped Au-catalyzed wires. Figure 4 shows a representative I(Vg) trace for a sourcedrain voltage Vsd ) +1 V at 77 K. The curve crossing the squares indicates the data relative to Mn-catalyzed wires while the curve crossing the dots represents the results obtained on Au-catalyzed wires. This Vg-dependence demonstrates that nominally undoped GaAs NWs catalyzed by Mn are p-type. On the contrary, GaAs NWs, grown in the very same conditions, but with the use of Au as the catalyst, resulted in n-type behavior. Since Mn in GaAs behaves as an acceptor, the electrical measurements provide further support to the contention that Mn diffuses into the wires. Achievement of p-type doping, as expected for Mn-doped GaAs, is important because the presence of free holes is a necessary condition for the spin-exchange interaction among Nano Lett., Vol. 6, No. 9, 2006
Figure 4. Field-effect current vs gate voltage, Vg, at 77 K for a positive bias of the source-drain voltage Vsd ) 1 V. The curve crossing the squares indicates the data relative to Mn-catalyzed wires while the curve through the dots represents the results obtained on Au-catalyzed wires.
magnetic ions and holes that gives rise to ferromagnetism in GaAs:Mn.7 We wish to notice that a quantitative comparison among the wires grown with different catalyst is not possible: indeed, to obtain carrier concentration and mobility a measurement on single NWs of known diameter would be needed. Work in this direction is in progress. In conclusion, we have demonstrated that long GaAs nanowires can be obtained by MBE both on SiO2 and on GaAs using Mn as the growth catalyst. EXAFS data as well as transport measurements indicate that NW growth occurs in the presence of Mn diffusion into GaAs although metallic R-Mn particles have been observed at the NW free end by TEM, consistent with the VLS growth model of catalystassisted NW growth. It remains unclear whether Mn is incorporated only as a dilute impurity in GaAs or if it also forms MnAs clusters. TEM has also shown that high-quality wires have a wurtzite lattice structure, which is a signature of 1D growth. 2D nanostructures, nanoleaves, show the presence of both wurtzite and zinc blende lattice structures. The results presented here open a way toward the exploitation of catalysts alternative to the usual metals, such as Au and Ag, and also Cu and Fe, which have deep electronic levels in the hosting semiconductor crystals. This could be of particular importance for silicon because Au in silicon acts as an effective carrier trap, a mechanism that is detrimental also for impurity densities well below the detection limit of the latest microscopic techniques. Moreover our results suggest that it is possible to choose the growth catalyst in a way to intentionally modify the electronic properties of the nanowires of interest. References (1) For a recent review on semiconductor nanowires, see: Law, M.; Goldberger, J.; Yang, P. Annu. ReV. Mater. Res. 2004, 34, 83. 2133
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Nano Lett., Vol. 6, No. 9, 2006
