Selective Growth of Straight and Zigzagged Ga1-xMnxN (0 â¤ x

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J. Phys. Chem. C 2008, 112, 2934-2942

Selective Growth of Straight and Zigzagged Ga1-xMnxN (0 e x e 0.05) Nanowires and Dependence of Their Electronic Structure and Magnetization on the Mn Content Seon Oh Hwang, Han Sung Kim, Seong-Hun Park, and Jeunghee Park* Department of Chemistry, Korea UniVersity, Jochiwon 339-700, Korea

Seung Yong Bae Chemical R&D Center, Samsung Cheil Industry Inc., Uiwang 332-2, Korea

Bongsoo Kim Department of Chemistry, KAIST, Daejeon 305-701, Korea

Ja Young Park and Gangho Lee Department of Chemistry, Kyungpook National UniVersity, Taegu 702-760, Korea ReceiVed: NoVember 7, 2007; In Final Form: December 6, 2007

Straight and zigzagged Ga1-xMnxN (0 e x e 0.05) nanowires were selectively synthesized by the vapor transport method, using different growth temperature. They consisted of single-crystalline wurtzite GaN nanocrystals grown along the [101h0] and [0001] directions for the straight and zigzagged morphologies, respectively. The lattice constant, c, decreases initially with increasing amount of Mn doping (x), and then increases as x increases above 0.03. X-ray photoelectron spectroscopy revealed that as the Mn content increases to x ) 0.02, the binding energy of Ga 2p shifts to a higher energy, suggesting the possibility of hybridization between the Mn2+ ions and host defects. X-ray absorption spectroscopy and X-ray magnetic circular dichroism confirmed that the Mn2+ ions substitute into the tetrahedrally coordinated sites. The magnetization measurement revealed that all of these nanowires exhibited room-temperature ferromagnetic behaviors, most significantly for the straight nanowires grown with the [101h0] direction, having x ) 0.02-0.03.

1. Introduction Diluted magnetic semiconductors (DMSs) have attracted a considerable amount of attention, because of their excellent potential as key materials for spintronic devices.1-3 The demonstration of the unique phenomena associated with DMSs such as the field-effect control of their ferromagnetism, efficient spin injection to produce circularly polarized light, and spindependent resonant tunneling, has opened up a vast new landscape for technological innovation in magnetoelectronics. As one of the most important room-temperature ferromagnetic DMSs, Mn-doped GaN DMSs have been extensively studied since Dietl et al. predicted the Curie temperature (TC) of 5% Mn-doped p-type GaN (Ga0.95Mn0.05N) to be >400 K, using the bound magnetic polaron model.4 Many previous experimental works have reported on ferromagnetism of p- or n-type films synthesized using various methods (e.g., molecular beam epitaxy, ion implantation, etc.), attributed to spin-charge double exchange interactions.5-17 Recent theoretical calculations predicted robust ferromagnetism in intrinsic Ga1-xMnxN.18-22 On the other hand, there have also been studies that acknowledged the possible presence of some other phases, such as ferromagnetic Mn-rich clusters, due to the low solubility of magnetic ions in GaN.23,24 Recently, intensive research activities have been directed toward one-dimensional (1D) nanostructures, which are con* Corresponding author. E-mail: [email protected].

sidered to be building blocks for the fabrication of various nanoscale devices. In particular, well-defined single-crystalline 1D nanostructures enable us to scrutinize precisely the magnetic properties and electronic structures of DMSs depending on their crystal structure and Mn-doping levels. There have been a number of reports on the synthesis of room-temperature ferromagnetic Mn (e9%)-doped GaN nanorods/nanowires.25-32 Their growth direction was either [101h0] or [0001], determined by the growth conditions. To understand fully the magnetic properties, it is probably important to investigate its dependence on the growth direction. The control of the growth direction was achieved only for the undoped GaN NWs.33-35 To the best of our knowledge, however, there have been no previous reports on Mn-doped GaN NWs. Furthermore, it is also significant to elucidate the electronic structures that are responsible for their magnetic properties. However, the correlations between their growth direction, electronic structures, and magnetization are far from being well examined. Herein, we synthesized high-purity Ga1-xMnxN nanowires (NWs) with a controlled Mn content, x ) 0, 0.01, 0.02, 0.03, and 0.05, exhibiting room-temperature ferromagnetism, using a simple vapor transport method. The single-crystalline straight and zigzagged nanowires, grown with the [101h0] and [0001] directions, respectively, were selectively synthesized using different growth temperatures. The lattice constants and electronic structures of two typed nanowires were thoroughly investigated as a function of the Mn concentration by high-

10.1021/jp7106632 CCC: $40.75 © 2008 American Chemical Society Published on Web 02/06/2008

Growth of Straight and Zigzagged Ga1-xMnxN Nanowires

J. Phys. Chem. C, Vol. 112, No. 8, 2008 2935

Figure 1. (a) SEM micrograph of high-density straight Ga0.97Mn0.03N NWs, homogeneously grown on the substrate. (b) The TEM image reveals that they have an average diameter of 80 nm. (c) Lattice-resolved image revealing the single-crystalline nature; the distance between the neighboring (010) planes is 2.8 Å. The FFT ED pattern confirms the [101h0] growth direction (inset). (d) EDX line-scan profile of Ga, N, and Mn for a selected Ga0.97Mn0.03N NW, whose STEM image is shown in the inset.

resolution X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and X-ray magnetic circular dichroism (XMCD). Their ferromagnetic behaviors at room temperature were identified. We attempted to correlate the ferromagnetic properties of these straight/zigzagged nanowires with their crystal structures and electronic structures. 2. Experimental Section A Ga (99.999%, Aldrich)/GaN (99.99+%, Aldrich) mixture and MnCl2 (99.99%, Aldrich) were placed separately in two quartz boats loaded inside a quartz tube reactor. A silicon substrate coated with 0.01 M HAuCl4‚3H2O (98+%, SigmaAldrich) ethanol solution was positioned at a distance of 10 cm away from the Ga source. Argon was allowed to flow at a rate of 500 sccm while raising or lowering the temperature. The temperature of the Ga and Mn source was set at 1100 and 600 °C, respectively and that of the substrate was approximately 800-850 °C. NH3 (99.999%) was introduced at a rate of 100 sccm for 1 h. The products were analyzed by scanning electron microscopy (SEM, Hitachi S-4700), field-emission transmission electron microscopy (TEM, Jeol JEM 2100F and FEI TECNAI G2 200 kV), high-voltage TEM (HVEM, Jeol JEM ARM 1300S, 1.25 MV), field-emission TEM (FEI TECNAI G2 200 kV), energy-dispersive X-ray fluorescence spectroscopy (EDX), and electron-energy loss spectroscopy (EELS, GATAN GIF-2000) in conjunction with TEM (TECNAI G2).

High-resolution XRD patterns were obtained using the 8C2 beam line of the Pohang Light Source (PLS) with monochromatic radiation (λ ) 1.54520 Å). XPS (ESCALAB 250, VG Scientifics) using a photon energy of 1486.6 eV (Al KR) was employed to investigate the electronic states. Synchrotron XPS measurements were also performed at the U7 beam line of the PLS. The XAS and XMCD measurements were carried out at the PLS elliptically polarized undulator beam line, 2A. The samples were introduced into an experimental chamber with a base pressure of 5 × 10-10 Torr. The spectra were collected in the total electron yield mode. A 0.1 T electromagnet was used to switch the magnetization direction. The magnetization direction was flipped between the parallel (F+) and antiparallel (F-) directions with respect to the photon helicity vector for each data point. The degree of circular polarization (95%) of the incident light was taken into account in the spectra. The magnetic properties were studied using a superconducting quantum interference device (SQUID, Quantum Design) magnetometer. 3. Results and Discussion 3.1. Morphology and Composition. The straight and zigzagged nanowires were grown at 850 and 800 °C, respectively, using thermal vapor transport of Ga/GaN/MnCl2/NH3. The growth mechanism of the nanowires follows the vapor-liquidsolid (VLS) mechanism using the Au catalytic nanoparticles. The Mn content was controlled by adjusting the evaporation

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Figure 2. (a) SEM micrograph of high-density zigzagged Ga0.97Mn0.03N NWs, homogeneously grown on the substrate. (b) The TEM image reveals their zigzagged morphology. The nanowires have an average diameter of 80 nm. (c) The nanowire is zigzagged with a nearly constant angle of 127°. The SAED pattern, measured at the [21h10] zone axis, reveals two zigzagged [022h3]/[02h23] directions and the axial [0001] direction. (d) Lattice-resolved TEM image revealing their single-crystalline nature; the distance between the neighboring (001) planes is 5.2 Å. The FFT ED pattern confirms the [0001] direction along the long axis. (e) The STEM image and EDX elemental mapping reveals the homogeneous composition of Ga, N, and Mn. The average Mn content is 0.03 ( 0.02.

temperature of the Mn precursors. Figure 1a shows the SEM micrograph of the high-density Mn-doped GaN NWs grown on the substrates. The TEM image explicitly reveals their smooth surface and average diameter of 80 nm (Figure 1b). The Au catalytic-nanoparticles are frequently attached to the tips, providing evidence for the VLS growth mechanism. The latticeresolved TEM image and corresponding Fourier-transform electron diffraction (FFT ED) pattern of a selected nanowire reveal that it is composed of single-crystalline wurtzite structured GaN (Figure 1c and inset). It shows a spacing between neighboring (100) planes of ca. 2.8 Å, which is consistent with that of the bulk materials (a ) 3.189, c ) 5.186 Å; JCPDS Card no. 50-0792). The FFT ED pattern generated from the inversion of the TEM image using DigitalMicrograph GMS1.2 software (Gatan Inc.) at the [0001] zone axis, confirms that it has the [101h0] growth direction. The EDX line-scanning analysis indicates the Mn content (x ) [Mn]/([Ga]+[Mn])) of the individual nanowire to be about x ) 0.03 ( 0.01 with a homogeneous distribution being observed along the cross-

section, as shown in Figure 1d. The inset corresponds to its high-angle annular dark field scanning TEM (STEM) image. The EDX data is shown in the Supporting Information, Figure S1. Figure 2a shows the SEM image of the high-density zigzagged Mn-doped GaN NWs, occupying ∼80% of the total nanostructures, grown on the substrates. The remainder (20%) corresponds to the straight nanowires. The TEM image reveals that they have a zigzagged structure over the whole length, although they appear to have an irregular morphology at first glance (Figure 2b). The average diameter of the nanowires is 80 nm. Figure 2c shows explicitly that the nanowire is zigzagged with a nearly constant angle. This image and its corresponding selected-area ED (SAED) pattern, measured at the [21h1h0] zone axis, reveal that the zigzagged segments have closely the [022h3] and [02h23] growth directions with an average angle of 125° (inset). The [0001] axial direction is maintained along the whole length. Figure 2d shows the lattice-resolved image for the parts marked in Figure 2c, proving that this zigzagged nanowire is



Figure 3. (a) Full-range XRD patterns taken from the straight [101h0] Ga1-xMnxN (x ) 0, 0.01, 0.02, 0.03, and 0.05) NWs, and the magnified scaled (b) (100) and (c) (002) peaks. (d) Full-range XRD patterns taken from the zigzagged [0001] Ga1-xMnxN (x ) 0.01, 0.02, 0.03, and 0.05) NWs, and their magnified scaled (100) and (002) peaks are shown in (b) and (c), respectively.

composed of single-crystalline wurtzite GaN nanocrystal. The highly crystalline (001) lattice planes are separated by a distance of 5.2 Å, which is consistent with that of the bulk crystal. It is noteworthy that all of the zigzagged nanowires that we observed are entirely composed of single-crystalline GaN nanocrystals without any dislocations or distortions. The STEM image and EDX elemental mapping reveal that the zigzagged nanostructures are homogeneously composed of Ga, N, and Mn (Figure 2e). The average Mn content is 0.03 ( 0.01. All of the zigzagged nanowires that we observed have the [0001] growth direction, while all of the straight nanowires have the [101h0] growth direction. The straight (100% purity) and zigzagged (∼80% purity) Ga1-xMnxN NW samples were selectively synthesized with controlled Mn contents, that is, 0.01 ( 0.01, 0.02 ( 0.01, 0.03 ( 0.01, and 0.05 ( 0.01. The EDX data of an individual nanowire are shown in the Supporting Information, Figure S1. The average Mn content was obtained from the EDX measurement of 3-5 nanowires. The EELS spectrum of the Ga0.95Mn0.05N NWs was measured and is shown in the Supporting Information, Figure S2. The XPS spectrum of the Ga1-xMnxN NWs is shown in the Supporting Information, Figure S3. The Mn content was obtained using the integration of the Ga and Mn 2p peaks and is consistent with the average value obtained from the EDX data. Growth at a higher or lower temperature results in the formation of nanowires with the [101h0] or [0001] growth direction, respectively, under our experimental conditions. S. T. Lee and co-workers reported the synthesis of GaN NWs with the [101h1] or [0001] growth direction at higher (900-850 °C) or lower (800-900 °C) temperature, respectively.33 Kuykendall et al. demonstrated the selective synthesis of vertically aligned GaN NWs with the [11h0] or [001] growth direction, using (100)

γ-LiAlO2 and (111) MnO substrates, respectively.34 Recently, Sunkara and co-workers reported the control of the [101h0] and [0001] growth directions using a low and high Ga flux, respectively.35 The present result is closer to that of S. T. Lee’s group, although the growth direction of the straight nanowires is different. The zigzagged morphology is rather similar to that of the GaN NWs reported by Zhou et al.36 To explain the controlled growth using the growth temperature, we suggest that the lower growth rate (at the lower temperature) would cause the growth to occur through the more reactive polar ((0001) planes, which is the case of the thermodynamically controlled growth condition. The less-active nonpolar side planes would be vulnerable to the fluctuation of the reactant vapor pressure, resulting in the zigzagged structure. As the temperature increases, the overall growth rate of the nanowires will increase. Then, the axial growth of the less-active nonpolar (100) planes becomes kinetically favorable. The active ((0001) side planes of the [101h0] growth would produce the smooth and straight surface. Nevertheless, further extensive studies are probably needed to fully understand the growth mechanism of the present nanowires. 3.2. XRD: Expansion of Lattice Constants by Mn Doping. The high-resolution XRD patterns of the straight undoped GaN NWs and Ga1-xMnxN NWs, grown with the [101h0] direction, are displayed in Figure 3a. The peaks of the GaN NWs exactly match those of wurtzite GaN (a ) 3.189, c ) 5.186 Å; JCPDS Card no. 50-0792). All of the GaN NW samples have a highly crystalline nature without the presence of other phases. Figure 3, panels b and c, displays the magnified (100) and (002) peaks, respectively. The position of the (100) peak shifts monotonically to a lower angle with increasing Mn concentration. In contrast, the position of the (002) peak initially shifts to a higher angle and then shifts significantly to a lower angle when the Mn


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TABLE 1: Change of the Lattice Constants (Å) of Straight and Zigzagged Ga1-xMnxN (x ) 0.01, 0.02, 0.03, and 0.05) NWs Grown along the [101h0] and [0001] Direction, Respectively, Relative to the GaN NWS straight ([101h0])

a

zigzagged ([0001])

xMn

∆a (%)a

∆c (%)

∆a (%)

∆c (%)

0.01 0.02 0.03 0.05

+0.001 (0.03) +0.0029 (0.09) +0.0048(0.15) +0.0067 (0.21)

-0.001 (0.03) -0.0029 (0.08) +0.0073 (0.14) +0.0088 (0.17)

+0.001 (0.03) +0.001 (0.03) +0.001 (0.03) +0.001 (0.03)

-0.001 (0.03) -0.001 (0.03) -0.0073 (0.14) +0.0029 (0.09)

The parentheses correspond to the percentage expansion (“+” sign) and contraction (“-” sign) relative to the lattice constant of the GaN NWs.

content increases to 0.03. For x ) 0.05, the angle shifts (∆(2θ)) of the (100) and (002) peaks are as much as 0.07 and 0.06 degrees, respectively. Figure 3d displays the XRD patterns of the zigzagged Ga1-xMnxN NWs, grown with the [0001] direction. The magnified (100) and (002) peaks are shown in Figure, panels b and c, respectively. Their peak width is broader than that of the straight [101h0] nanowires. As the Mn concentration increases, the position of the (100) peak tends to shift to a lower angle. The position of the (002) peak initially shifts to a higher angle and then shifts to a lower angle when the Mn content is above 0.03. The change in the lattice constants was estimated from the shift ∆(2θ) of the (100) and (002) peaks, as listed in Table 1. It is thought that the substitution of the Ga3+ ions (rGa ) 0.61 Å in tetrahedral sites) with the larger radius Mn2+ ions (rMn ) 0.80 Å) expands the lattice constants of the GaN crystal. For both [101h0] and [0001] growth directions, the expansion of lattice constants can be observed when x ) 0.05. The lattice constant a increases monotonically with increasing Mn concentration. Interestingly, for both growth directions, the lattice constant c decreases initially and then increases above x ) 0.02-0.03, although the magnitude of the change is not the same. This decreasing-to-increasing behavior of the lattice constant c is consistent with the results obtained for [0001] epitaxially grown Ga1-xMnxN (x e 0.09) thin films, which showed an increase in the lattice constant above x ) ∼0.03.5d The authors of this report suggested that the maximum value of the magnetic moments occurs at the minimum value of the lattice constant c for the sample containing such ∼3% Mn. We also observed that the minimum value of the lattice constant at x ) 0.02-0.03 is well correlated with the maximized magnetization, as discussed in Section 3.5. Furthermore, this reduced lattice constant can be ascribed to the hybridization between the Mn dopants and host defects, as discussed in Section 3.3. The (001) planes are less close-packed than the (100) planes, so the substitution with the larger radius Mn ions in the (001) planes cannot expand the lattice constants to as great an extent as in the case of the (100) planes. This may explain why the lattice constant c exhibits sensitively the reduction upon the hybridization of Mn dopants. Moreover, the above results indicate that the straight [101h0] nanowires experience a larger change of the lattice constants than the zigzagged [0001] nanowires. The respective expansion of the lattice constants a (c) reaches about 0.2 (0.2) and 0.03 (0.1) %, when x ) 0.05 for the straight [101h0] and zigzagged [0001] nanowires. The larger peak broadening of the zigzagged nanowires indicates more significant lattice distortion, probably due to their lower growth temperature, so the effects of Mn substitution on the lattice constants are probably reduced. 3.3. Fine-Scanned XPS: Electronic Structure of Zn and Mn Atoms. The full-range XPS spectra are shown in the Supporting Information, Figure S3, for x ) 0.01, 0.02, 0.03, and 0.05, confirming the average value of the Mn concentration.

The feature of the fine-scanned Ga and Mn 2p peaks was strongly dependent on the Mn content but only slightly dependent on the growth direction of the nanowires, probably due to the resolution limit. Therefore, the fine-scanned spectra are presented only for the straight nanowires grown with the [101h0] growth direction. Figure 4a shows the fine-scanned Ga 2p3/2 peak. As the Mn content increases, its width becomes broader. The peak position shifts to the higher energy region, 0.3 eV for x ) 0.01, 0.5 eV for x ) 0.02, 0.2 eV for x ) 0.03 and 0.05, compared to that of the undoped ones, showing the highest energy shift for x ) 0.02. The peak position and fullwidth at half-maximum (FWHM) are listed in Table 2. Figure 4b corresponds to the finely scanned Mn 2p3/2 peak of the Mndoped GaN NWs. The peak of the MnO powders is also displayed for comparison. The peak of the nanowires appears in the lower energy region compared to that of O, due to the lower electronegativity of N. Therefore, the peaks would be expected to originate mainly from the Mn(II)-N bonding structures. As the Mn content increases from 0.01 to 0.02, the position shifts by 0.4 eV toward the higher energy region. The higher-energy shift is reduced to 0.2 eV when x ) 0.03 and 0.05. The present Mn-doped GaN NWs exhibit ferromagnetic properties, as in the case of the previously reported ferromagnetic Mn-doped GaN nanowires/nanorods, and this will be discussed below (Section 3.5).25-32 For the room-temperature ferromagnetic Co-doped ZnO (Zn1-xCoxO, x ) 0.020, 0.056, and 0.098) nanoparticles reported by Xu and co-workers, there was a strong correlation between the XPS peak position and the saturation magnetic moment.37 The shift of the Zn 2p corelevel to a high-energy region was maximized for x ) 0.056, showing the same trend as the saturation magnetic moment. The authors suggested the existence of a strong hybridization between the dopant and defect wavefunction, which is responsible for the higher-binding energy shift. They suggested that for the highest doping concentrations the energy of the partial Co ions (d levels) moves down to the O 2p band, thus lowering the probability for the overall overlap and scaling down the ferromagnetic signal. We also proposed that the hybridization between the Mn dopant and the defects would lead to a higher binding energy shift. As regards to the origin of the ferromagnetism of the Mndoped GaN, it was suggested that, in p-type GaN, there exists a shallow acceptor’s hole transfer process onto Mn2+ to form Mn3+, where the Mn3+ ions act as effective shallow donors.29 This hole transfer can be described as Mn2+ + h+ (acceptor) f Mn3+. This dopant-acceptor hybridization is the pivotal feature determining the value of TC in theoretical models describing DMS ferromagnetism.4,18-22 This type of hybridization may occur in the present nanowires, which presumably contain defects (i.e., Ga vacancies). As the Mn content increases from x ) 0.02 to x ) 0.03 and 0.05, the Ga and Mn peaks move to a lower energy, which can be explained by the reduced hybridization. The incorporation of the less electronegative Mn



Figure 4. Fine-scanned XPS spectra of (a) Ga 2p3/2 of the Ga1-xMnxN (x ) 0, 0.01, 0.02, 0.03, and 0.05) NWs and (b) Mn 2p3/2 of the MnO powders and Ga1-xMnxN (x ) 0.01, 0.02, 0.03, and 0.05) NWs.

TABLE 2: Peak Positions and FWHMs (eV) of the XPS Ga 2p3/2 and Mn 2p3/2 Peaks for the GaN NWs and Ga1-xMnxN (x ) 0.01, 0.02, 0.03, and 0.05) NWs Ga 2p3/2 xMn 0 0.01 0.02 0.03 0.05

position 1117.7 1118.0 (+0.3)b 1118.2 (+0.5) 1117.9 (+0.2) 1117.9 (+0.2)

Mn 2p3/2 FWHM 1.7 1.7 1.7 1.8 1.9

position --

(641.5)a 641.1 641.5 (+0.4)c 641.3 (+0.2) 641.3 (+0.2)

FWHM -- (4.2) a ∼5 ∼5.5 ∼6 ∼6.5

a The Mn 2p3/2 peak of MnO powders. bThe change of the peak position, relative to that of the GaN NWs. cThe change of the peak position relative to that of the Ga0.99Mn0.01N NWs.

(compared to Ga), based on the electronegativity (χ) differences between Ga (χP ) 1.81), N (χP ) 3.04), and Mn (χP ) 1.55), would cause this lower-energy shift of the Ga and Mn 2p peaks. The XRD data show a minimum value of the lattice constant c at x ) 0.02-0.03, which might be directly related to the fact that the binding energy is the highest for the Ga and Mn 2p peaks. 3.4. XAS and XMCD. To further investigate the electronic structure of the Mn ions, we performed XAS and XMCD (∆F ) F+ - F-) measurements at the Mn L2,3 edges. Figure 5a shows the Mn L2,3 edge XAS and XMCD spectra of the straight [101h0] Ga0.98Mn0.02N NWs, measured at 80 and 300 K. The spectra, which result from the Mn 2p f 3d dipole transition, are divided roughly into the L3 (2p3/2) and L2 (2p1/2) regions. The absorption intensity increases as the temperature decreases to 80 K. This absorption feature and position suggest that the doped Mn atoms are in an oxidized state, not in a metallic state such as metallic Mn clusters, which is also consistent with the results obtained for Mn-doped GaN and GaAs single crystals.38,39 Despite the low signal-to-noise ratio, the negative signal of the XMCD L3 peak can be observed, which indicates the

contribution of Mn2+ at the tetrahedral sites, substituting for the Ga ions in GaN. These Mn2+ ions would be responsible for the hybridization (discussed in Section 3.3), which results in the ferromagnetism of the GaN NWs. Figure 5b shows the Mn L2,3 edge XAS and XMCD spectra of the zigzagged [0001] Ga0.98Mn0.02N NWs, measured at 80 and 300 K. The negative signal of the L3 peak of XMCD, indicating the existence of Mn2+ at the tetrahedral sites, can be only observed at 80 K. The intensity of XMCD at both temperatures is smaller than that of the [101h0] Ga0.98Mn0.02N NWs, indicating the reduced magnetic moments of Mn2+ ions. This result is consistent with the magnetization measured by SQUID, as shown in Section 3.5. 3.5. Magnetization. For the straight [101h0] Ga0.98Mn0.02N NWs, the magnetization (M) versus magnetic field (H) curve measured by the SQUID magnetometer is shown in Figure 6a. H was perpendicular to the substrate plane. At 5 K, an obvious amount of hysteresis is observed with a coercive field HC ≈ 80 Oe (inset). At 300 K, the coercivity of the hysteresis becomes smaller, which may be due to the intrinsic magnetically soft properties, but the loop still shows the features of ferromagnetism. Therefore, the existence of ferromagnetism at 5 and 300 K is clearly shown by the coercivity, remanence, and relatively low saturation field, suggesting that the TC value is higher than 300 K. Figure 6b displays the field-cooled (MFC) and zero-fieldcooled M (MZFC) versus T curves with H ) 100 Oe. The inset corresponds to ∆M ) MFC - MZFC, as a function of temperature in the range of 5-350 K. This magnetization difference is particularly informative when there are small amounts of ferromagnetic material in the presence of a large diamagnetic and/or paramagnetic background. This subtraction indicates the presence of ferromagnetism if the difference is nonzero. Although the curve is not smooth, due to the low ∆M values,


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Figure 5. (a) XAS and XMCD spectra of Mn L2,3 edge, measured for the straight [101h0] Ga0.98Mn0.02N NWs at 80 and 300 K. (b) XAS and XMCD spectra of Mn L2,3 edge, measured for the zigzagged [0001] Ga0.98Mn0.02N NWs at 80 and 300 K.

Figure 6. (a) Plots of M vs H at 5 and 300 K for the straight [101h0] Ga0.98Mn0.02N NWs, measured by SQUID. The inset represents the magnified M-H curve in the vicinity of H ) 0, showing the hysteresis. (b) Temperature-dependent MFC and MZFC with H ) 100 Oe. The inset represents its ∆M (MFC - MZFC) curve. (c) The M-H curves at 5 and 300 K for the zigzagged [0001] Ga0.98Mn0.02N NWs, showing the hysteresis (inset). (d) Temperature-dependent MFC and MZFC with H ) 100 Oe. The inset represents the ∆M curve. (e) The M-H curves at 5 and 300 K for the zigzagged [0001] Ga0.95Mn0.05N NWs, showing the hysteresis (inset). (f) Their MFC, MZFC, and ∆M (inset) vs T curves with H ) 100 Oe.

this data indicates that the ferromagnetism persists to 350 K, which is close to the predicted value.4 Figure 6, panels c and d, displays the M-H and M-T curves measured for the zigzagged [0001] Ga0.98Mn0.02N NWs, respectively, which show similar behaviors to those of the [101h 0] Ga0.98Mn0.02N NWs. The mass of the sample is not exactly

known and is only estimated to be below 10-4 g, so we could not compare their magnetization directly with that of the [101h0] nanowires. Nonetheless, HC is ∼40 Oe, as shown in the inset of Figure 6c, which is smaller than that of the [101h0] NWs. There is an increase in the value of ∆M in the temperature region below 50 K, probably due to the presence of some impurity

Growth of Straight and Zigzagged Ga1-xMnxN Nanowires phases. The formation of nanosized MnO or Mn3O4 impurities is possible, because the EELS data indicate the presence of O impurities. The MnO nanocrystals with diameters of 5-22 nm were reported to have TC ) 27-10 K, and the Mn3O4 nanocrystals with diameters of 6-15 nm to have TC ) 36-41 K.40,41 The results indicate that the magnetization of the straight [101h0] Ga0.98Mn0.02N NWs is larger than that of the zigzagged [0001] ones, which may be attributed to their more effective Mn substitution, as discussed in Section 3.2. On the other hand, Wang et al. reported a density functional theory calculation on Mn-doped GaN nanowires, suggesting that the special topology of the surface and confinement of electrons in the radial direction causes the coupling to be ferromagnetic.42 They showed that the magnetic moment of the nanowires grown along the [0001] direction orients preferably along the radial [101h0] direction. Therefore it is expected that the magnetic moments are strongly dependent on the growth direction. Theoretical studies would be necessary to explain the more significant ferromagnetic behaviors of the [101h0] growth direction. Figure 6, panels e and f, shows the M-H and M-T curves measured for the zigzagged [0001] Ga0.95Mn0.05N NWs, respectively. The HC value is still 40 Oe, as shown in the inset of Figure 6e, which is not increased despite the higher Mn concentration. The M-T curve and the ∆M value (inset of Figure 6f) show that the magnetization increases at temperatures below 170 K. For the higher Mn concentration, the formation ofnanosizedferromagneticGaMnimpuritiesbecomespossible.13a,43 Although the TEM, XRD, XAS, and XMCD data do not provide any evidence for the presence of such impurities in our samples, they cannot be ruled out as a potential source of the increased magnetization considering the detection limits. We also measured the M-H and M-T curves for the straight [101h0] Ga0.95Mn0.05N NWs, which are similar to those of the zigzagged [0001] Ga0.98Mn0.02N NWs, as shown in Figure 6c,d, and found that the magnetization was decreased. These results suggest that the magnetization of the Ga1-xMnxN NWs reaches a maximum when x ) 0.02-0.03. This result is consistent with the XPS data showing that the Mn substitution with x ) 0.020.03 is most effective in increasing the dopant-acceptor hybridization. 4. Conclusion Single-crystalline wurtzite Ga1-xMnxN (x ) 0.01, 0.02, 0.03, and 0.05) NWs were grown by vapor transport using the reaction of Ga/GaN/MnCl2 with NH3. We selectively synthesized nanowires with two types of morphology, namely straight ones grown along the [101h0] direction and zigzagged ones grown along the [0001] direction at growth temperatures of 850 and 800 °C, respectively. Their average diameter is 80 nm. The EDX data reveal that Mn dopes homogeneously over the entire nanowires. The XRD data reveal that the lattice constants (a and c) of the straight [101h0] nanowires increase more significantly with increasing Mn concentration than those of the zigzagged [0001] nanowires. The lattice constant c decreases initially and then increases at Mn contents above 0.03. The positions of the XPS Ga and Mn 2p peaks shift to the higher energy region when x is increased to 0.02 but to the lower energy region when x is above 0.03. We explained the higher-energy peak shift in terms of the hybridization between the Mn2+ ions and host defects, which was suggested to be a crucial feature in determining the ferromagnetism. The XAS and XMCD data reveal that the Mn2+ ions are dominantly present at the tetrahedral sites, substituting for the Ga ions. The

J. Phys. Chem. C, Vol. 112, No. 8, 2008 2941 hysteresis curves and temperature-dependent magnetization curves provide evidence for the existence of room-temperature ferromagnetism, irrespective of the Mn content. The magnetization reaches a maximum when x ) 0.02-0.03, which is well correlated with the minimum value of the lattice constant c and the highest value of the binding energy at this Mn content. The straight growth along the [101h0] direction would lead to a larger magnetization than the zigzagged one along the [0001] direction. We conclude that the most effective ferromagnetic nanowires are formed when they have straight morphology with the [101h0] growth direction and the Mn content is 0.02-0.03, under our experimental conditions. Acknowledgment. This work is supported by KRF Grants (R14-2003-033-01003-0; R02-2004-201-C00039; 2003-015C00265). The SEM, TEM, XRD, and SQUID measurements were performed at the Basic Science Research Center. The experiments at the PLS were supported in part by MOST and POSTECH. Supporting Information Available: EDX data of the straight and zigzagged Ga1-xMnxN NWs; EELS spectrum of the Ga0.95Mn0.05N NWs; full-range XPS data for straight/ zigzagged Ga1-xMnxN (x ) 0.01, 0.02, 0.03, and 0.05) NWs. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) (a) Ohno, H. Science 1998, 281, 951. (b) Ohno, Y.; Young, D. K.; Beschoten, B.; Matsukura, F.; Ohno, H.; Awschalom, D. D. Nature (London) 1999, 402, 790. (c) Ohno, H.; Chiba, D.; Matsukura, F.; Omiya, T.; Abe, E.; Dietl, T.; Ohno, Y.; Ohtani, K. Nature (London) 2000, 408, 944. (2) Fiederling, R.; Keim, M.; Reuscher, G.; Ossau, W.; Schmidt, G.; Waag, A.; Molenkamp, L. W. Nature (London) 1999, 402, 787. (3) Wolf, S. A.; Awschalom, D. D.; Buhrman, R. A.; Daughton, J. M.; von Molna´r, S.; Roukes, M. L.; Chtchelkanova, A. Y.; Treger, D. M. Science 2001, 294, 1488. (4) Dietl, T.; Ohno, H.; Matsukura, F.; Cibert, J.; Ferrand, D. Science 2000, 287, 1019. (5) (a) Theodoropoulou, N.; Hebard, A. F.; Overberg, M. E.; Abernathy, C. R.; Pearton, S. J.; Chu, S. N. G.; Wilson, R. G. Appl. Phys. Lett. 2001, 78, 3475. (b) Overberg, M. E.; Abernathy, C. R.; Pearton, S. J.; Theodoropoulou, N. A.; McCarthy, K. T.; Hebard, A. F. Appl. Phys. Lett. 2001, 79, 1312. (c) Thaler, G. T.; Overberg, M. E.; Gila, B.; Frazier, R.; Abernathy, C. R.; Pearton, S. J.; Lee, J. S.; Lee, S. Y.; Park, Y. D.; Khim, Z. G.; Kim, J.; Ren, F. Appl. Phys. Lett. 2002, 80, 3964. (d) Thaler, G.; Frazier, R.; Gila, B.; Stapleton, J.; Davidson, M.; Abernathy, C. R.; Pearton, S. J.; Segre, C. Appl. Phys. Lett. 2004, 84, 1314. (e) Pearton, S. J.; Abernathy, C. R.; Thaler, G. T.; Frazier, R. M.; Norton, D. P.; Ren, F.; Park, Y. D.; Zavada, J. M.; Buyanova, I. A.; Chen, W. M.; Hebard, A. F. J. Phys.: Condens. Matter 2004, 16, R209. (6) (a) Reed, M. L.; El-Masry, N. A.; Stadelmaier, H. H.; Ritums, M. K.; Reed, M. J.; Parker, C. A.; Roberts, J. C.; Bedair, S. M. Appl. Phys. Lett. 2001, 79, 3473. (b) Reed, M. J.; Arkun, F. E.; Berkman, E. A.; Elmasry, N. A.; Zavada, J.; Luen, M. O.; Reed, M. L.; Bedair, S. M. Appl. Phys. Lett. 2005, 86, 102504. (c) Reed, M. L.; Reed, M. J.; Luen, M. O.; Berkman, E. A.; Arkun, F. E.; Bedair, S. M.; Zavada, J. M.; El-masry, N. A. Phys. Status Solidi C 2005, 2, 2403. (7) (a) Hori, H.; Sonoda, S.; Sasaki, T.; Yamamoto, Y.; Shimizu, S.; Suga, K.; kindo, K. Physica B 2002, 324, 142. (b) Sonoda, S.; Shimizu, S.; Sasaki, T.; Yamamoto, Y.; Hori, H. J. Cryst. Growth 2002, 237-239, 1358. (c) Sonoda, S.; Tanaka, I.; Ikeno, H.; Yamamoto, T.; Oba, F.; Araki, T.; Yamamoto, Y.; Suga, K.; Nanishi, Y.; Akasaka, Y.; Kindo, K.; Hori, H. J. Phys.: Condens. Matter 2006, 18, 4615. (d) Sonoda, S.; Tanaka, I.; Oba, F.; Ikeno, H.; Hayashi, H.; Yamamoto, T.; Yuba, Y.; Akasaka, Y.; Yoshida, K.; Aoki, M.; Asari, M.; Araki, T.; Nanishi, Y.; Kindo, K.; Hori, H. Appl. Phys. Lett. 2007, 90, 012504. (8) Sasaki, T.; Sonoda, S.; Yamamoto, Y.; Suga, K.; Shimizu, S.; Kindo, K.; Hori, H. J. Appl. Phys. 2002, 91, 7911. (9) (a) Shon, Y.; Kwon, Y. H.; Kang, T. W.; Fan, X.; Fu, D.; Kim, Y. J. Cryst. Growth 2002, 245, 193. (b) Shon, Y.; Kwon, Y. H.; Yuldashev, S. U.; Leem. J. H.; Park, C. S.; Fu, D. J. Appl. Phys. Lett. 2002, 81, 1845. (c) Yoon, I. T.; Park, C. S.; Kim, H. J.; Kim, Y. G.; Kang, T. W.; Jeong, M. C.; Ham, M. H.; Myoung, J. M. J. Appl. Phys. 2004, 95, 591. (d) Yoon, I. T.; Kang, T. W.; Kim, D. J. Mater. Sci. Eng. B 2006, 134, 49.

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Selective Growth of Straight and Zigzagged Ga1-xMnxN (0 â¤ x