Ferromagnetic Fe-Doped GaN Nanowires Grown by Chemical Vapor

Nov 17, 2010 - Changing the magnetic and optical properties of (Ga, Fe)N and (Ga, Co)N by alloying with oxygen. K. El Maalam , E. Salmani , O. Mounkac...
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J. Phys. Chem. C 2010, 114, 21029–21034

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Ferromagnetic Fe-Doped GaN Nanowires Grown by Chemical Vapor Deposition Yanan Li, Chuanbao Cao,* and Zhuo Chen* Research Center of Materials Science, Beijing Institute of Technology, Beijing 100081, People’s Republic of China ReceiVed: July 7, 2010; ReVised Manuscript ReceiVed: October 11, 2010

We report the first synthesis and characterization of Fe-doped GaN nanowires. The Fe-doped nanowires were grown by chemical vapor deposition process, using pretreated iron oxide as dopant precursors. The morphological and structural analysis showed that the obtained nanowires were triangular cross section and single crystalline wurtzite GaN structure. It was proved that the pretreatment could induce doping effectively and higher substrate temperature could produce higher dopant concentration. The highest concentration of Fe ions in GaN was 0.12% in our condition. Fe 2p core-level X-ray photoelectron spectroscopy (XPS) spectra showed that the charge state of Fe transformed from +3 to +2 with the increase of Fe concentration. Moreover, the change in the binding energy of Ga 3p in the XPS spectra could be attributed to position change of Fermi level governed by Fe concentration. In the photoluminescence (PL) spectra, the relative intensity of exciton luminescence increased with Fe concentration. The magnetic measurement revealed the Fe-doped GaN nanowires were ferromagnetic at room temperature. The XPS results provided the evidence for the charge state of Fe dopant, indicating the intrinsic nature of room-temperature ferromagnetism of Fe-doped GaN nanowires. 1. Introduction Semiconductor doped with transition-metal (TM), so-called diluted magnetic semiconductors (DMS), are the most promising materials for potential application in spintronic device,1,2 which can manipulate both charge and spin simultaneously. The prospective spintronics device based on DMS such as spinaligner light-emitting diode,3 electric-field control of ferromagnetism using an insulating-gate field-effect transistor structure,4 and magnetic tunnel junction-based magnetic random access memory (MRAM)2,5 has been designed and demonstrated. It has been predicted that wide band gap semiconductor-based DMS should be ferromagnetic at room temperature,6 which would be advantageous for proposed spintronic application. Up to now, the GaN-, ZnO-, and TiO2-based DMS films have been studied fully and extensively.7-11 Unfortunately, the DMS films usually suffer from defect and nonuniform dopant distribution, resulting from the lattice mismatch of substrate and nonequilibrium processing. On the other hand, miniaturization of electronic devices in industrial applications and scientific research makes 1D semiconductor materials more promising than films. In view that the nanowires are currently being explored as possible building blocks for electronic and optoelectronic devices, the DMS nanowires would be important for application in nanoscale spintronic devices using electronic spin as an additional degree of freedom. Moreover, the DMS nanowires also provide a model to explore the ferromagnetism related to dimension and size. To realize the potential function of DMS nanowires as nanoscale spintronic devices, the controlled fabrication of 1D DMS is essential. As a wide band gap semiconductor, GaN exhibits potential application in the field of optics, optoelectronics, and GaN nanowires will undoubtedly hold considerable technological promise for nanoscale device application.12-14 With the proved room temperature ferromagnetism of GaN-based DMS, the * To whom correspondence should be addressed. E-mail: cbcao@ bit.edu.cn (C.C.), [email protected] (Z.C.).

corresponding nanowires represent an important class of nanoscale building blocks for spintronics device. The ideal GaNbased DMS nanowires should be single crystalline and defect free, which is helpful for investigation on the intrinsic nature of DMS nanowires. The pure GaN nanowires can be synthesized through a carbon nanotube-confined reaction.15 For doped GaN nanowires, the synthetic challenge mainly exists in limited solubility of transition-metal in GaN.16,17 In the last recent years, Mn-,18-24 Cr-,18 Co-,18 and Cu-25,26 doped GaN nanowires have been synthesized by chemical vapor deposition (CVD), mainly using transition-metal chloride as precursors. To the best of our knowledge, there has not been any report about Fe-doped GaN nanowires, maybe due to the fairly limited solubility of Fe in GaN. However, ferromagnetic Fe-doped GaN, which is an usual magnetic phase in diluted magnetic semiconductor, exhibits many interesting properties, including high-temperature ferromagnetism for n-type doped system and positive valence-band exchange splitting N0β, which is opposite to most conventional diluted magnetic semiconductor.27 Thus, the synthesis and characterization of Fe-doped GaN nanowires is challenge and helpful to search their properties and application. In this work, we successfully synthesized Fe-doped GaN nanowires by CVD, using iron oxide (Fe2O3) as dopant precursors. The nanowires were single crystalline and exhibited ferromagnetism at room temperature. 2. Experimental Section Fe-doped GaN nanowires were fabricated in a horizontal furnace with an alumina boat by CVD. In a typical experiment, well mixed gallium oxide (Ga2O3) and Fe2O3 were heated at 900 °C for 4 h first, and then pressed to be a pellet with diameter of 10 mm and thickness of 1-2 mm. The pellet was heated again at the same temperature for 10 h. After the pretreatment, the pellet was grinded to be powder, and then put into an alumina boat, which was put in the middle of a horizontal furnace. The silicon(111) substrates were put downstream at the certain distance away from the source. Before heating, 200

10.1021/jp106256b  2010 American Chemical Society Published on Web 11/17/2010

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TABLE 1: EDS Results of the Five Samples and Respective Conditions sample number sample 1 sample 2 sample 3 sample 4 sample 5

substrate pretreatment Fe precursors temperature without with with with with

20% 20% 30% 40% 20%

700 °C 700 °C 700 °C 700 °C 800 °C

ssFe concentration below detection limit 0.06%

0.12%

sccm NH3 was introduced into the furnace tube for 20 min to drive out the air. Then the whole system was heated at the rate of 10 °C/min to 1150 °C, maintained for 360 min and finally cooled down to room temperature naturally. During the whole process, the NH3 flow was kept at 200 sccm. After reaction, the light yellow products were collected on Si(111) substrate. The prepared products on the substrate were characterized by X-ray diffraction (XRD, PANalytical X’ Pert PRO MPD), scanning electronic microscope (SEM, Hitachi S-4800), highresolution transmission electronic microscope (HRTEM, Tecnai F20), X-ray photoelectron spectroscopy (XPS, PerkinElmer Physics PHI 5300), photoluminescence spectroscopy (PL, Hitachi F-4500), and vibrating sample magnetometer (VSM, Lake Shore 7400). 3. Results and Discussion At first, sources consisted of Ga2O3 and Fe2O3 were mixed directly, without pretreatment. As a result, even when the mole

Li et al. ratio of Fe to Ga increased to 20% (labeled as sample 1), no dopant was detected in the sample by energy dispersive spectrometer (EDS) measurement. Then precursors were pretreated with pressing and heating as mentioned above. In this condition, when the mole ratio of Fe to Ga was less than 10%, we got just GaN nanowires without dopant indicated by EDS. When the amount of Fe precursors was increased to 20%, 30%, 40% respectively (labeled as samples 2, 3, and 4, respectively) EDS showed that there was some amount of Fe in the nanowires. Besides, the sample was also studied when the substrate was located at higher temperature (labeled as sample 5). The EDS results of the five samples and corresponding conditions were shown in Table 1. It should be mentioned that the EDS data of 0.06% is far below the limitation of EDS instrument, and thus the data is too low for quantitative analysis but only for a qualitative proof. The EDS spectra of pure GaN nanowires and Fe-doped GaN nanowires were shown in Figure S1 of the Supporting Information. The crystal structures of the five samples were characterized by XRD, shown in part a of Figure 1. Among the XRD patterns, the samples 1, 2, and 5 were assigned to pure wurtzite GaN structure (JCPDS No.76-0703). No impurities such as Ga-Fe or Fe-N phases were detected in the XRD patterns. Increasing the amount of Fe precursors to 30% and even 40% resulted in some impurities in the sample, which was alloy of Ga and Fe. This implied that Fe atoms might be doped in the crystal. The SEM image of sample 2 was shown in part b of Figure 1.

Figure 1. (a) XRD patterns of the five samples, (b) SEM image of sample 2, (c) TEM of sample 2, (d) HRTEM and FFT (insert) of sample 2.

Ferromagnetic Fe-Doped GaN Nanowires

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Figure 2. (a) The Fe 2p core-level XPS spectra of sample 1, 2, and 5; (b) the plot of binding energy of Fe 2p versus Fe concentration, (c) Ga 3p core-level XPS spectra of samples 1, 2, and 5; (d) the binding energy shift (∆S) of Ga 3p versus Fe concentration.

TABLE 2: Binding Energy of Fe 2p Electron at +2 and +3

Nanowires could be obtained in large yield with length of tens of micrometers and diameter of 100-200 nm. It could be easily found that the cross sections of nanowires were universally triangle. It was believed that the triangular nanowires formation was induced by Fe doping (the SEM images of pure GaN nanowires with hexagonal cross sections were shown in Figure S2 of the Supporting Information). Compared the fabrication process of the samples 1 and 2, the difference between them was the pretreatment of precursors, which was the key point in the process for doping. As for the measurement results, the XRD patterns and SEM images showed no obvious difference between samples 1 and 2. Nevertheless, EDS results indicated that the Fe dopant in the samples 1 was below the detection limit of the machine, whereas the latter with pretreatment contained 0.06% Fe dopant. It was evident that the pretreatment could induce doping effectively. The reason was that during pretreatment, the precursors were mixed more fully by prior pressing and heating. It was proved by XRD that there was not a solid-state reaction between the precursors but just a mixture of them. Moreover, the substrate temperature also had an effect on doping. With the equal amount of Fe precursors (20%) with pretreatment, the GaN nanowires were doped more heavily when the substrate located at higher temperature zone (comparing sample 2 with sample 5). It was because higher temperature provided more sufficient energy for crystal growth, which

Fe2+ Fe3+

2p3/2

2p1/2

709. 30 eV 710. 70 ev

722. 30 eV 724. 30 eV

induced higher solubility of Fe in Ga-Fe-N alloy. Thus, doping was much easier at higher substrate temperature. However, the GaN nanowires were found only formed in specific temperature range; further increasing substrate temperature would not obtain GaN nanowires. Further structural characterization of Fe-doped GaN nanowire was shown in parts c and d of Figure 1. The bright and dark contrast in the insert of part c of Figure 1 revealed the triangular shape of cross section, in concordance with SEM image shown in part b of Figure 1. By means of HRTEM and responding fast Fourier transform (FFT) pattern shown in part d of Figure 1, it can be confirmed that the Fe-doped GaN nanowire was single crystalline, and there was an amorphous oxide layer of about 2 nm thickness on the nanowire surface. No defect related to dopant ions was observed here. The FFT pattern was recorded along [010] zone axis and the growth direction was determined as 〈200〉, similar with the others’ results.28 Part a of Figure 2 showed the Fe 2p core-level XPS spectra of samples 1, 2, and 5. It could be observed that the binding energy of Fe 2p decreased with Fe concentration. The plot of binding energy of Fe 2p versus Fe concentration was shown in

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Figure 3. PL spectra of samples 1, 2, and 5.

part b of Figure 2. As reported,29 the binding energy of Fe 2p electron at +2 and +3 was shown in Table 2. When Fe concentration was lower than detection limit of EDS machine, the binding energy of Fe 2p1/2 was close to that of Fe 2p1/2 at +3, which meant that little amount Fe still existed in the crystal mainly at +3 charge state. When Fe concentration increased to 0.06%, the binding energy was 723.7 eV, between that of +3 and +2. Then at 0.12%, the binding energy was 722.9 eV, close to that of +2. It could be concluded that the charge state of Fe transformed from +3 to +2 with the increase of Fe concentration. In the model of TM-doped insulators, the calculation showed that the change in the TM oxidation state was accompanied by the change in spatial distribution of the charge density around the TM site and the change in gap-level occupation.30 Therefore, the evolution of charge state of Fe ions in GaN nanowires with the increase of Fe concentration could be explained by changes in charge density and gap-level occupation caused by Fe dopant, in view of the difference in electronic structure between Fe and Ga. Part c of Figure 2 showed the Ga 3p core-level XPS spectra of samples 1, 2, and 5. We observed that the binding energy of Ga 3p decreased slightly with the increase of Fe concentration (insert in part c of Figure 2). The shift values were 0.20 and 0.30 eV for doped samples with Fe concentration 0.06% and 0.12% respectively compared with the binding energy of Ga

Li et al. 3p in sample 1. The evolution of the binding energy with dopant concentration was plotted in part d of Figure 2. The shift in the binding energy of Ga 3p could be attributed to change of Fermi level, which depended on Fe concentration.31 XPS results of Fe 2p proved that two charge states of Fe coexisted and transformed with the increase of Fe concentration. The two charge states could pin the Fermi level to the respective Fe3+/2+ charge transfer (CT) level.31 Consequently, at lower Fe concentration, electron trapped by Fe3+ lowered the Fermi level, which induced higher binding energy of Ga 3p. With the increase of Fe concentration, the Fermi level was pinned to higher position, resulting in the decrease of binding energy. The fact that Fe concentration could adjust the position of Fermi level and then affect the relative position of Fe3+/2+ CT level might indicate an effective way to control materials characteristics. Figure 3 showed the PL spectra of samples 1, 2, and 5. In the three spectra, the main peaks were at about 3.37 eV, which was not very obvious for sample 1 when the Fe dopant was below detection limit. Besides, there were also other weaker peaks present in the spectra. As a comparison, the PL spectrum of pure GaN nanowires was shown in the insert of Figure 3; there was only one peak in the spectrum, which was the band edge emission of GaN. Thus, the main peak at about 3.37 eV of the PL spectra for samples 1, 2, and 5 was definitely ascribed to the band edge emission of GaN. The peak at 3.14 eV was indentified as conduction band-Fe acceptor transition.32,33 In view that the PL spectrum of pure GaN nanowires had the only strong peak of band edge emission, the curves in Figure 3 indicated that samples 1, 2, and 5 all contained dopant, and even the sample of Fe dopant below detection limit may contain a low quantity of Fe ions because transition-metal impurities in III-V semiconductors acted as carrier lifetime killers.10 By comparing the three curves, the relative intensity of band edge emission increased with Fe concentration. It was believed that the strong quenching of the excitonic emission observed at low Fe content was predominantly due to the high electron capture rate of Fe ions at +3.10 The XPS results in our work indicated that the charge state transformed from +3 to +2 with the increase of Fe dopant concentration, which was also been proved by Bonanni, A. et al.10 In view of the statement of the high electron capture rate of Fe ions at +3 and the XPS results about the charge state of Fe ions, we consider that the increase of Fe dopant concentration could result in the increase of electron concentration and then the recovery of exciton luminescence

Figure 4. Effect of pretreatment (a) and substrate temperature (b) on magnetism of Fe-doped GaN nanowires.

Ferromagnetic Fe-Doped GaN Nanowires intensity. Actually, it has been proved that the change of the charge state of iron with the rise of the electron concentration is responsible for the recovery of the near band edge emission.10 Since the relative intensity of PL spectra is related to many aspects, the mechanism of the change of the relative intensity of PL spectra is very complicated. We believe that the Fe doping concentration can contribute to it to some extent. The magnetic properties of Fe-doped GaN nanowires were determined by vibrating sample magnetometer (VSM) at room temperature. Figure 4 showed the magnetic loops of samples 1, 2, and 5. For pure GaN nanowires, the sample was diamagnetic (insert of part a of Figure 4), with silicon background subtracted. When 20% Fe precursors were added, sample 1 showed weak ferromagnetism, with coercivity 42.768G, magnetization 13.544 µemu/cm2 and retentivity 0.4955 µemu/ cm2, although the Fe concentration was below the EDS machine detection limit (shown in Table 1). After the pretreatment, the equal amount of Fe precursors could result in Fe-doped GaN nanowires with much stronger ferromagnetism (sample 2, shown in part a of Figure 4). Further more, with the same sources after the pretreatment, higher substrate temperature could result in higher Fe dopant concentration and then much stronger magnetism (samples 2 and 5, shown in part b of Figure 4). Up to now, the origin of ferromagnetism in DMS is not very clear, even if various models have been put forward.34-36 In our results, as shown in Figure 4, the magnetism had a weak tendency to saturation. The loops consisted of paramagnetic linear part and ferromagnetic part, similar to Fe-doped GaN film.10,37 The former was attributed to Van Vleck paramagnetism, generated by Fe2+(d6).10,37 The latter might be attributed to Curie magnetism, generated by Fe3+(d5). As for the charge state of Fe ions in GaN, it has been proved that Fe ions coexist at the +3 and +2 charge state,10,38 which was also proved in our XPS results. Moreover, the charge state of Fe transformed from +3 to +2 with the increase of Fe concentration, which induced the recovery of exciton luminescence intensity. In our consideration, the XPS results provided the evidence for the charge state of Fe dopant, indicating an intrinsic nature of Fedoped GaN nanowires. Thus, the relative concentrations of Fe3+ and Fe2+ ions and, hence, the relative importance of the Curie magnetism and Van Vleck paramagnetism was defined by the density of shallow acceptors and donors. This meant, in particular, that if the acceptor concentration were high enough to give rise to the presence of a sufficiently increased density of weakly localized or delocalized valence band holes, the carrier- mediated coupling between spins localized on Fe3+ ions would operate.10 Our research confirmed that the ferromagnetism originated from intrinsic nature of room-temperature ferromagnetism of Fe-doped GaN to some extent. 4. Conclusions In summary, we successfully synthesized single crystalline Fe-doped GaN nanowires with a triangular cross section. The pretreatment of precursors was proved to be critical and effective for doping. The XPS spectra showed that the charge state of Fe transformed from +3 to +2 with the increase of Fe concentration, and the decrease in the binding energy of Ga 3p in the XPS spectra was attributed to position change of Fermi level governed by Fe concentration. In the PL spectra, the relative intensity of band edge emission recovered with the increase of Fe concentration because the charge state of Fe transformed from +3 to +2. The magnetic measurement showed that the product was ferromagnetic at room temperature and the magnetism increased with Fe concentration. The XPS results

J. Phys. Chem. C, Vol. 114, No. 49, 2010 21033 provided the evidence for the charge state of Fe dopant, indicating an intrinsic nature of Fe-doped GaN nanowires. This is a practical and universally applicable method to synthesize doped GaN nanowires by oxides or other high melting point sources. The synthesized Fe-doped GaN nanowires may open up significant potential for theoretical research and application of spintronic device. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (Grant No. 50972017 and No. 20471007) and the Science Foundation for the Excellent Youth Scholars of Beijing Institute of Technology (Grant No. 2009Y0915). Supporting Information Available: EDS spectra of pure GaN nanowires and Fe-doped GaN nanowires, SEM images of pure GaN nanowires with hexagonal cross sections. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Ohno, H. Science 1998, 281, 951. (2) 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. (3) Fiederling, R.; Keim, M.; Reuscher, G.; Ossau, W.; Schmidt, G.; Waag, A.; Molenkamp, L. W. Nature 1999, 402, 787. (4) Ohno, H.; Chiba, D.; Matsukura, F.; Omiya, T.; Abe, E.; Dietl, T.; Ohno, Y.; Ohtani, K. Nature 2000, 408, 944. (5) Awschalom, D. D.; Flatte´, M. E. Nat. Phys. 2007, 3, 153. (6) Dietl, T.; Ohno, H.; Matsukura, F.; Cibert, J.; Ferrand, D. Science 2000, 287, 1019. (7) Matsumoto, Y.; Murakami, M.; Shono, T.; Hasegawa, T.; Fukumura, T.; Kawasaki, M.; Ahmet, P.; Chikyow, T.; Koshihara, S.; Koinuma, H. Science 2001, 291, 854. (8) Neal, J. R.; Behan, A. J.; Ibrahim, R. M.; Blythe, H. J.; Ziese, M.; Fox, A. M.; Gehring, G. A. Phys. ReV. Lett. 2006, 96, 197208. (9) Lee, J. S.; Lim, J. D.; Khim, Z. G.; Park, Y. D.; Pearton, S. J.; Chu, S. N. G. J. Appl. Phys. 2003, 93, 4512. (10) Bonanni, A.; Kiecana, M.; Simbrunner, C.; Li, T.; Sawicki, M.; Wegscheider, M.; Quast, M.; Przybylin´ska, H.; Navarro-Quezada, A.; Jakieła, R.; Wolos, A.; Jantsch, W.; Dietl, T. Phys. ReV. B 2007, 75, 125210. (11) Heikman, S.; Keller, S.; DenBaars, S. P.; Mishra, U. K. Appl. Phys. Lett. 2002, 81, 439. (12) Tang, Y. B.; Chen, Z. H.; Song, H. S.; Lee, C. S.; Cong, H. T.; Cheng, H. M.; Zhang, W. J.; Bello, I.; Lee, S. T. Nano Lett. 2008, 8, 4191. (13) Tang, Y. B.; Bo, X. H.; Lee, C. S.; Cong, H. T.; Cheng, H. M.; Chen, Z. H.; Zhang, W. J.; Bello, I.; Lee, S. T. AdV. Funct. Mater. 2008, 18, 3515. (14) Kim, H. M.; Kang, T. W.; Chung, K. S. AdV. Mater. 2003, 15, 567. (15) Han, W. Q.; Fan, S. S.; Li, Q. Q.; Hu, Y. D. Science 1997, 277, 1287. (16) Sedmidubsky´, D.; Leitner, J.; Sofer, Z. J. Alloys Compd. 2008, 452, 105. (17) Hashimoto, M.; Emura, S.; Tanaka, H.; Honma, T.; Umesaki, N.; Hasegawa, S.; Asahi, H. J. Appl. Phys. 2006, 100, 103907. (18) Stamplecoskie, K. G.; Ju, L.; Farvid, S. S.; Radovanovic, P. V. Nano Lett. 2008, 8, 2674. (19) Choi, H. J.; Seong, H. K.; Chang, J.; Lee, K. I.; Park, Y. J.; Kim, J. J.; Lee, S. K.; He, R. R.; Kuykendall, T.; Yang, P. D. AdV. Mater. 2005, 17, 1351. (20) Han, D. S.; Park, J.; Rhie, K. W.; Kim, S.; Chang, J. Appl. Phys. Lett. 2005, 86, 032506. (21) Radovanovic, P. V.; Barrelet, C. J.; Gradecˇak, S.; Qian, F.; Lieber, C. M. Nano Lett. 2005, 5, 1407. (22) Song, Y. P.; Wang, P. W.; Zhang, X. H.; Yu, D. P. Phys. B 2005, 368, 16. (23) Xu, C. K.; Chun, J.; Lee, H. J.; Jeong, Y. H.; Han, S. E.; Kim, J. J.; Kim, D. E. J. Phys. Chem. C 2007, 111, 1180. (24) Ham, M. H.; Oh, D. K.; Myoung, J. M. J. Phys. Chem. C 2007, 111, 11480. (25) Wang, P. W.; Zhang, X. J.; Wang, B. Q.; Zhang, X. Z.; Yu, D. P. Chin. Phys. Lett. 2008, 25, 3040. (26) Seong, H. K.; Kim, J. Y.; Kim, J. J.; Lee, S. C.; Kim, S. R.; Kim, U.; Park, T. E.; Choi, H. J. Nano Lett. 2007, 7, 3366.

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