The ferromagnetic properties of Ge magnetic quantum dots doped with Mn

Applied Surface Science 258 (2012) 2906–2909

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The ferromagnetic properties of Ge magnetic quantum dots doped with Mn Xiying Ma a,b,∗ , Caoxin Lou a a

School of Mathematics and Physics, Suzhou University of Science and Technology, Kerui Road 1 in Huqiu Section, Suzhou 215011, Jiangsu, China Key Laboratory for Thin Film and Microfabrication Technology of the Ministry of Education, Research Institute of Micro/Nanometer Science and Technology, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai, China b

a r t i c l e

i n f o

Article history: Received 22 October 2011 Received in revised form 1 November 2011 Accepted 1 November 2011 Available online 10 November 2011 PACS: 42.70.Qs 42.25B 78.20.P 78.55E

a b s t r a c t We present a synthesis of Ge:Mn magnetic quantum dots (QDs) and an evaluation of their ferromagnetic properties. The QDs were grown from a GeH4 /Ar mixed gas under constant flow conditions at 400 ◦ C by means of a plasma-enhanced chemical vapor deposition (PECVD) process, then doped with Mn by a magnetic sputtering technique and annealed at 600 ◦ C. The QDs, with a composition of Ge0.88 Mn0.12 according to their energy spectrum, showed widely opened hysteresis loops, with large remnant magnetizations Mr of 0.14 × 10−4 and 0.25 × 10−4 emu/g for the as-grown and annealed samples, respectively. Moreover, the average value of the moment per Mn atom was found to be as high as 2.36 ␮B at room temperature, showing that the Ge1−x Mnx QDs constitute an intrinsic diluted magnetic semiconductor. The unprecedented colossal moment is attributed to an effective RKKY exchange coupling interaction between the Mn ions mediated by holes. © 2011 Elsevier B.V. All rights reserved.

Keywords: Ge:Mn QDs Ferromagnetic properties Magnetic moment

1. Introduction Recently, studies of the diluted magnetic (DM) properties of Ge semiconductors have attracted much interest [1–3], since Park et al. showed that Ge1−x Mnx DMs have Curie temperatures (CTs) in the range 25–116 K [4], and Cho et al. demonstrated ferromagnetic (FM) behavior of Ge single crystals with CTs of up to 285 K [5]. The electrons in Ge are crucial for both the electric charge properties and magnetic properties, which can be exploited in the fabrication of advanced spin electronic devices and magnetic electronic devices [6,7]. Ge DMs are anticipated to have wide potential applications in the future because of their compatibility with the high technology of Si semiconductor integrated circuits, leading to diluted magnetic semiconductors. In recent years, great progress has been made, both theoretically and experimentally, in understanding the structural, electronic, and magnetic properties of Ge doped with Mn [8–10]. For example, Holmes’ research group synthesized Ge1−x Mnx nanowires that displayed ferromagnetism above 300 K and a hole mobility of around 340 cm/V s [9]. Kazakova and co-workers observed an ferromag-

∗ Corresponding author at: School of Mathematics and Physics, Suzhou University of Science and Technology, Kerui Road 1 in Huqiu Section, Suzhou 215011, Jiangsu, China. E-mail address: [email protected] (X. Ma). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.11.005

netic properties for Ge0.99 Cr0.01 nanowires in the temperature range 4–300 K [11]. Even intrinsic ferromagnetism in magnetically undoped Ge quantum dots has recently been demonstrated [9,12]. However, the magnetic interaction mechanism of Mn and Ge atoms is not very clear and the realization of DMS “spintronics” depends on a complete understanding of the fundamental spin interactions in semiconductors as well as the influences of dimensionality, defects, crystal structure, and semiconductor band structure in modifying these dynamics. The main purpose of the present work is to investigate the magnetic properties of, and the magnetic interaction mechanism in Mn-doped Ge QDs. The QD samples were prepared by means of the plasma-enhanced chemical vapor deposition (PECVD) technique, and doped with Mn by magnetic sputtering with subsequent annealing at 600 ◦ C in the presence of Ar gas. The magnetic properties including the magnetic intensity, magnetic susceptibility and the effective magnetic moment of Mn were investigated, and the magnetic interaction mechanism was discussed. 2. Experimental The studied samples were prepared on n-type Si (1 0 0) wafers by means of PECVD. The substrates were cleaned ultrasonically with a sequence of acetone, ethanol, and deionized water, and dried by blowing with N2 . The reactive gas was GeH4 (10%) diluted with Ar (90%), with a volume ratio of 1:19. Prior to the deposition, the

X. Ma, C. Lou / Applied Surface Science 258 (2012) 2906–2909

samples were pre-treated in a hydrogen plasma at about 400 ◦ C for 10 min. During deposition, the working pressure was kept at 50 Pa, and the plasma power was about 50 W under an applied bias of 800 V. In the deposition process, GeH4 was initially decomposed to give a mixture of Ge, H+ , Ar, and H2 under the large bias. This mixture is referred to as a plasma state, in which the chemical groups consisting of atoms and molecules have very high energy. The experiment was carried out for 1 h; Ge QDs were developed when Ge atoms were deposited at sufficiently low temperatures. The Ge QDs were then doped with Mn ions for 10 min by means of a magnetron sputtering method using a high-purity Mn target in an Ar atmosphere. After doping, thermal annealing was performed at 600 ◦ C under an atmosphere of Ar for 10 min in a rapid thermal annealing (RTA) furnace to repair the damage. The samples were removed from the chamber and allowed to cool to room temperature. The morphology and structure of the samples were characterized by atomic force microscopy (AFM). The structure was determined by X-ray diffraction (XRD) analysis on a RINT2000 vertical goniometer using CuK␣ radiation ( = 0.1541 nm). The composition of the Mn-doped Ge QDs was determined by energydispersive spectroscopy. Magnetic measurements were made using a superconducting quantum interference device (SQUID).

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annealed at 600 ◦ C, which shows that the Ge QDs became circular, with a uniform size of about 30–40 nm. The profile of the Ge QDs with a smooth surface without any particles or defects looks significantly more homogeneous than that in Fig. 1(a). The density of the dots was approximately 2 × 1012 cm−3 . Fig. 2(a) shows the X-ray diffraction pattern of the as-grown Ge:Mn QDs sample. Three strong diffraction peaks at 27.45◦ , 45.51◦ , and 53.88◦ and 66.28◦ can be attributed to diffractions of the (1 1 1), (2 2 0), and (3 1 1) crystal planes of Ge, respectively. The XRD pattern showed no evidence of the presence of secondary-phase ferromagnetic alloys, such as the Mn5 Ge3 phase. That is to say, for a low doping concentration of Mn, the structure of Ge remains unchanged, and Mn atoms simply replace Ge atoms. These results indicated that the Ge:Mn QDs obtained in our experiments were

3. Results and discussion Fig. 1(a) illustrates the typical surface morphology of the asgrown Mn-doped Ge QDs, as determined by AFM. Monodispersed Ge:Mn QDs with an elliptical shape are uniformly scattered on the Si substrate. Atomic force microscopy (AFM) images revealed that the self-assembled Ge QDs had an average height of 15 nm and an average base length of 20–60 nm. Fig. 1(b) is an image of a sample

Fig. 1. (a) Surface AFM image of Mn-doped Ge QDs on an Si substrate grown at 400 ◦ C; (b) surface AFM image of a sample annealed at 600 ◦ C.

Fig. 2. (a) X-ray diffraction spectrum of as-grown Mn-doped Ge QDs; (b) energydispersive spectrum of as-grown Mn-doped Ge QDs; (c) Raman spectra of as-grown and annealed Mn-doped Ge QDs.

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Fig. 3. Magnetic intensity of the as-grown and annealed Ge:Mn QDs at 77 K.

of high quality. Fig. 2(b) shows the energy-dispersive spectrum of an as-grown Ge:Mn QDs sample. Three peaks are seen due to Ge, Si, and Mn atoms, from which their normalized concentrations can be estimated as 48%, 37.6%, and 6.4%, respectively. The results show that Mn atoms were effectively doped in the Ge QDs, reaching a peak doping concentration of 6.4%. The stoichoimetry was Ge0.88 Mn0.12 , as estimated from the energy spectrum. Fig. 2(c) shows the Raman spectrum of a Ge QDs sample. For an as-grown Ge sample, a Raman peak is located at 280 cm−1 . It can be attributed to the vibration peak of a Ge–Ge bond, and is consistent with the position of the characteristic peak of Ge quoted in the literature [12]. For an annealed sample, this peak becomes stronger, and the line shape becomes more symmetrical. In addition, the peak shifts to 300 cm−1 . Our results indicated that the higher the annealing temperature, the better the crystallization. Fig. 3 shows the magnetization properties determined for the asgrown and annealed Ge:Mn QDs samples at 77 K. A large opening up of the hysteresis loop is visible for the as-grown and annealed Ge QDs samples in the presence of the magnetic field H. The observed FM behavior is attributed to the Mn2+ ions that substitute the Ge atoms. These Mn2+ ions, acting as acceptors, generate holes that mediate the ferromagnetic exchange interaction. Fig. 4 shows the magnetic properties of the as-grown and annealed samples at 300 K. Both samples show significant ferromagnetic character at room temperature. The respective magnetization intensities M increase and reach saturation values of 0.77 × 10−4 and 2.2 × 10−4 emu/g for the as-grown and annealed samples at H up to 2500 Oe. Accordingly, the remnant magnetizations Mr are 0.14 × 10−4 and 0.25 × 10−4 emu/g, and the coercive

Fig. 4. Magnetic intensity of the as-grown and annealed Ge:Mn QDs at 300 K.

Fig. 5. (a) Variation of the magnetic susceptibility of the as-grown and annealed Ge:Mn QDs at room temperature with the applied magnetic field; (b) effective magnetic moment peff per Mn atom as a function of applied magnetic field.

field strengths Hc are 253 and 193 Oe, respectively. The widely opened hysteresis loops indicate the presence of a ferromagnetic phase in the Ge QDs at room temperature, and the steep rise in magnetization would seem to indicate that the samples are intrinsic diluted magnetic semiconductors. The results for the annealed and as-grown samples show a clear separation M (about 0.11 × 10−4 emu/g), which is in agreement with findings in the literature [13] for samples in which RT ferromagnetism was present upon annealing. The samples display qualitatively similar hysteresis loops, but the annealed sample shows a remarkable enhancement of the magnetic properties. This implies that the annealing process removes some of the structural defects in the Ge QDs, and increases the magnetic properties of Mn. The significant properties of our Ge:Mn QDs showing high-temperature ferromagnetism are consistent with reports on other ferromagnetic Ge:Mn nanocrystalline DMs [14]. We believe that the ferromagnetism in these samples is closely related to the interactions between the Mn ions. Fig. 5(a) shows the magnetic susceptibilities  (=M/H) of the asgrown and annealed Ge:Mn QDs samples at room temperature as a function of the applied magnetic field. The maximum values of the magnetic susceptibility are 0.01 × 10−6 and 0.04 × 10−6 for the as-grown and annealed samples, respectively. The effective magnetic moment peff per Mn atom can be obtained from the value of the saturation magnetization intensity Ms (peff = Ms /NMn ). The concentration of Mn in the Ge QDs samples was about 1015 cm−3 , as determined from RBS measurements. peff is plotted as a function of the applied magnetic field H in Fig. 5(b). The maximum peff was

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determined as 2.36 ␮B for the annealed sample and 0.81 ␮B for the as-grown sample. peff of the annealed sample is very large as compared to the values in the reports of Holmes and co-workers [6] and Choi and co-workers [7], in which the moments for Mn-doped Ge1−x Mnx nanowires were quoted as 0.7 ␮B at 300 K and 0.87 ␮B at 5 K. The large effective moment in the samples is closely related to the exchange interaction modes of Mn ions with a 3d5 4s2 electronic configuration. It may result from the ferromagnetic exchange coupling of holes in Ge:Mn QDs, because it has been reported that Mn-doped Ge shows characteristic p-type conductivity. However, the doping concentration of our sample was 1015 cm−3 , which is too low to show hole-mediated ferromagnetism. The colossal moment may be attributed to a very effective RKKY (Ruderman–Kittel–Kasuya–Yosida) exchange interaction, because there are many localized carriers residing in the impurity band of Mn ions. The RKKY couple [15,16] is an indirect exchange interaction via the conduction holes, which results in high total magnetic moments per Mn. According to the RKKY theory [17], free holes can give rise to an exchange couple, leading to a polarization of localized holes. The spin of localized holes can polarize the surrounding magnetic impurities, leading to so-called bound magnetic polarons [18–20]. The polarization is mediated by the localized holes in the system through interactions. Therefore, the RKKY coupling between the Mn ions is mediated by the holes. The role played by holes involves localized holes making magnetic exchange couples with the localized holes of the neighboring Mn ions. This exchange interaction between Mn ions is especially large in Ge QDs compared to that in bulk Ge or nanowires because of the quantum confinement effect. The large ferromagnetism of the samples is due to the magnetic polarons, indicating that room temperature ferromagnetism in Group IV semiconductors doped with Mn may be realized in the future. These outcomes indicate that improved electrical properties as well as ferromagnetism are achievable by doping Ge semiconductors with Mn. This should be helpful for developing CMOS-compatible spintronic devices. 4. Conclusion Ge QDs have been grown by the PECVD technique and then doped with Mn by magnetic sputtering in a constant flow of Ar gas and annealed at 600 ◦ C. We found that the Ge:Mn QDs thus

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obtained showed significant ferromagnetic properties at 77 K and room temperature, with the magnetic properties being a function of the external magnetic field and temperature. The average magnetic moment per Mn atom was found to be as high as 2.36 ␮B . This colossal moment can be explained in terms of a very effective RKKY exchange coupling interaction between Mn ions mediated by holes. Acknowledgements This work was supported in parts by the National Natural Science Foundation of China (no. 60976071), the Scientific Project Program of Suzhou City (no. SYG201121), and the opening foundation (B0416-2007-11-01) of Key Laboratory Thin Films and Microtechnology of the Ministry of Education, Shanghai Jiaotong University. References [1] Y. Ohno, D.K. Young, B. Beschoten, F. Matsukura, H. Ohno, D.D. Awschalom, Nature 402 (1999) 790. [2] H. Ohno, Science 281 (1998) 951. [3] L. Liu, P.Y. Yu, Z. Ma, S.S. Mao, Phys. Rev. Lett. 100 (2008) 127203. [4] Y.D. Park, A.T. Hanbicki, S.C. Erwin, C.S. Hellberg, J.M. Sullivan, J.E. Mattson, T.F. Ambrose, A. Wilson, G. Spanos, B.T. Jonker, Science 295 (2002) 651. [5] S. Cho, S. Choi, S.C. Hong, Y. Kim, J.B. Ketterson, B.J. Kim, Y.C. Kim, J.H. Jung, Phys. Rev. B 66 (2002) 033303. [6] M.I. van der Meulen, N. Petkov, M.A. Morris, O. Kazakova, X. Han, K.L. Wang, A.P. Jacob, J.D. Holmes, Nano Lett. 9 (1) (2009) 50. [7] H.K. Seong, U. Kim, E.K. Jeon, T.E. Park, H. Oh, T.H. Lee, J.J. Kim, H.J. Choi, J.Y. Kim, J. Phys. Chem. C 113 (2009) 10847. [8] S. Picozzi, F. Antoniella, A. Continenza, A. MoscaConte, A. Debernardi, M. Peressi, Phys. Rev. B 70 (2004) 165205. [9] J.S. Kulkarni, O. Kazakova, D. Erts, M.A. Morris, M.T. Shaw, J.D. Holmes, Chem. Mater. 17 (2005) 3615. [10] S.S. Yu, Y.M. Cho, Y.E. Ihm, D.J. Kim, H.J. Kim, S.K. Hong, S.J. Oh, B.C. Woo, C.S. Kim, Curr. Appl. Phys. 6 (2006) 478. [11] R.B. Morgunov, A.I. Dmitriev, Y. Tanimoto, O. Kazakova, J. Appl. Phys. 105 (2009) 093922. [12] S. Kuroda, N. Nishizawa, K. Takita, M. Mitome, Y. Bando, K. Osuch, T. Dietl, Nat. Mater. 6 (2007) 440. [13] I.T. Yoon, C.J. Park, S.W. Lee, T.W. Kang, D.W. Koh, D.J. Fu, Solid-State Electron. 52 (2008) 871. [14] Y.D. Park, A. Wilson, A.T. Hanbicki, J.E. Mattson, T. Ambrose, G. Spanos, B.T. Jonker, Appl. Phys. Lett. 78 (2001) 2739. [15] A.M. Werpachowska, Z. Wilamowski, Mater. Sci. Poland 24 (2006) 675. ˜ Bach Thanh Cong, [16] Nghiem Thi Minh Hoa, Emi Minamitani, Wilson Agerico Dino, Hideaki Kasai, J. Phys. Soc. Jpn. 79 (2010) 074702. [17] T. Dietl, H. Ohno, F. Matsukura, J. Cibert, D. Ferrand, Science 287 (2000) 1019. [18] J. Chen, K.L. Wang, G. Kosmas, Appl. Phys. Lett. 90 (2007) 012501. [19] S.D. Sarma, E.H. Hwang, A. Kaminski, Phys. Rev. B 67 (2003) 155201. [20] A.J. Kaminski, S.D. Sarma, Phys. Rev. Lett. 88 (2002) 247202.