Electrical and Magnetic Properties of Doped ZnO ...

Mater. Res. Soc. Symp. Proc. Vol. 957 © 2007 Materials Research Society

0957-K04-06

Electrical and Magnetic Properties of Doped ZnO Nanowires Gennady N. Panin1,2, Andrey N. Baranov3, Tae Won Kang1, Oleg V. Kononenko2, Sergey V. Dubonos2, S. K. Min4, and H. J. Kim4 1 QSRC, Department of Physics, Dongguk University, 3-26 Pil-dong, Chung-gu, Seoul, 100-715, Korea, Republic of 2 Inst. of Microelectronics Technology, RAS, Chernogolovka, Moscow distr., 142432, Russian Federation 3 Department of Chemistry, Moscow State University, Moscow, 119992, Russian Federation 4 Department of Semiconductors, Dongguk University, Seoul, 100-715, Korea, Republic of

ABSTRACT ZnO nanowires doped with Mn, Fe, Sn, and Li during the thermal growth following direct chemical synthesis were investigated using electric and magnetic measurements. Currentvoltage characteristics of individual nanowires configured as a two-terminal device with Al electrodes show apparent rectify behavior indicating the Schottky-like barrier formation and resistivity being less 3 Ω·cm. Reproducible resistance modulation by a dc voltage at room temperature is observed. Magnetic susceptibility of the doped nanowires as a function of temperature demonstrates Curie–Weiss behavior. Magnetization versus field curves show hysteresis with the coercive field of about 200 Oe. The spatially-resolved magnetic force measurements of individual nanowires revealed the magnetic domain structure. The domains align perpendicular to c-axis and can be polarized in the external magnetic field. INTRODUCTION Zinc oxide is one of the most important functional semiconductor oxide with a direct wide band gap (3.37 eV) and a large exciton binding energy (60 meV) [1]. ZnO-based nano scale materials such as nanowires have attracted enormous interest due to its unique combination of electric, piezoelectric and optical properties. In recent years, attention has focused on spindependent phenomena in dilute magnetic zinc oxide in which stoichiometric fraction of the zinc atoms are replaced by transition metal (TM) impurities. The typical dopants that induce magnetic properties in the ZnO-based semiconductors are Mn, Fe and Co, which have been usually used with additional codopants supplying free carriers. In spite of theoretical predictions of room temperature ferromagnetism originated by carrier-induced interaction between TM atoms in ZnO, the experimental results have been strongly contradictory. Ferromagnetism in the oxide material remains poorly understood experimentally. ZnO:Co (3) and ZnO:Mn (2,3) DMSs films were found to be paramagnetic, while others reported ferromagnetism in ZnO:Co (4,5) and ZnO:Mn (10-12). High-Tc ferromagnetism attributed to ZnO:Mn DMS films was suggested to appear due to phase-segregation [8]. Several recent studies on ZnO:Mn, ZnO:Co and ZnO:Fe showed that ferromagnetism depends strongly on methods and conditions used in the preparation [4,6,9]. Magnetic ordering in ZnO-based DMS appears sensitive to point defects such as vacancies [5,10] which can supply electrons (holes) and can effect considerably on both magnetic and electric properties.

Quasi-1D nanostructures have unique feature to growth with single crystal structure without second phase. Moreover the density of surface point defects in nanowires with high surface-to-volume ratio is expected to be significant. It may effect on their magnetic and electronic properties. The ZnO nanorods doped with Mn or Co were reported [11,12] to show ferromagnetic behavior. For example, the Zn1-xCoxO nanorods with x < 0.1 show hysteresis curves with the coercitive field of 57 Oe at 300 K [12]. In this work, we report on magnetic and electrical properties of ZnO nanowires doped by Mn, Fe, Sn and Li. EXPERIMENT ZnO nanowires doped by Mn and Sn or Fe and Li impurities (up to 2%) with diameters from 40 to 150 nm and 1-3 µm in length were grown from NaCl–Li2CO3 salt mixture with a solution-processed Zn, Mn (Sn, Fe)-containing precursor at 700ºC as described previously [13]. XL 30S FEG high-resolution scanning electron microscope (HRSEM) with a MonoCL system for CL spectroscopy and JEM-4010 high-resolution transmission electron microscope (HRTEM) with energy dispersive X- ray analysis were used to examine the samples. X-ray powder diffraction (XRD) data of the synthesized nanowires have been collected by a D8 Advance (Bruker AXS) diffractometer (CuKα-radiation). Magnetic properties of the samples were studied using a Quantum Design SQUID magnetometer in the temperature range from 5 K to 300 K. DI magnetic force microscope was used for the spatially–resolved magnetic measurements. Individual ZnO nanowires were configured as two terminal device with the Al electrode-ZnO-Al electrode structure on a silicon substrate capped with a SiO2 layer. E-beam lithography was used to pattern electrodes contacting individual nanowires (Figure 1, insert). Electrical transport properties of the nanowires were studied by applying to the electrodes quasi-dc voltage 0→Vmax→ –Vmax→ 0 with a constant sweep velocity. RESULTS AND DISCUSSION Electrical properties Resistivity of the doped nanorods obtained from the I–V characteristics is about 3 Ω·cm in the dark. The nanorods show a strong, reversible response to above band gap (366 nm) ultraviolet light, with the UV-induced current being approximately a factor of 6 larger than the dark current at a given voltage. In figure 1 the current-voltage (I-V) characteristics of the ZnO:2%Fe, Li nanowire are shown for a dc voltage loop ranging from 0 → 3 → -3 → 0 V on an Al electrode. The I-V curve exhibits a rectifying behavior indicating the Schottky-like barrier formation and displays stable hysteresis. The hysteresis at negative voltages is more pronounced than at positive voltages. The nanowire starts in the low resistive state when sweeping the voltage from zero to positive voltages (1). In the subsequent voltage sweep from positive to negative (2, 3) the nanowire shows an increased resistance. At negative voltage the wire resistance switches back from a high to a low resistance. The virgin nanowire shows a higher resistance than obtained in the subsequent cycles with a carrier injection.

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Figure 1. I-V characteristics of a ZnO:2%Fe,Li nanowire for a voltage sweep from 0→3→3→0 V. The insert shows a SEM image of the nanowire with deposited Al contacts patterned by e-beam lithography. In contrast to abrupt resistance changes, a smooth resistance change is observed. It is likely, that electric-field domains [14] are built and attenuated resulting in the observable switching effect. Moreover one has to take a nonuniform distribution of trapped charges and the surface band bending [15] into account, which can be altered by applying voltage in forward or reverse directions. Two state resistive switching at RT can be realized using the ZnO nanowires [16] . Emploing a positive voltage of +3V switches the nanowire device into a high impedance state. After applying negative voltage of -3 V the low impedance state is recovered. Between these write and erase voltages the state can be readout with 1.5V Magnetic properties The magnetic properties of the as-grown 0.5%Mn and 1%Mn(Sn)-doped ZnO DMS nanowires (M2 and MS samples respectively) were investigated at 5 and 300K. Typical magnetization (M) versus temperature (T) curve was measured during cooling in an applied magnetic field of 1000 Oe for samples M2 and MS, as shown in figure 2. 300

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Figure 2. Magnetization of ZnO:Mn nanowires (triangles – field cooling, and filled stars - zero field cooling) and ZnO:MnSn nanowires (upside-down triangles – field cooling, and asterisks zero field cooling) as a function of temperature. Inset: The temperature dependence of inverse magnetic susceptibility.

Above 50 K (sample M2) and 180 K (sample MS), the 1/M–T curves reveal a linear paramagnetic response and show Curie-Weiss (CW) law behavior: Cm ( x ) χ= (1) T − Θ( x ) Where Θ(x) = Θ(0).x is the Curie-Weiss temperature, Cm(x) =C(0).x is the molar Curie constant. The values of C0 and Θ as well as other magnetic characteristics are given in table I. Table I. Parameters of Curie-Weiss law. Sample M2 MS

C0 0,1255 6,74445

Θ 120,4387 1485,39442

Diamagnetic contribution corresponding to ZnO has been estimated χd = −0.38 × 10−6 cm3/g. A linear fit to the inverse susceptibility data intersects the χ−1 = 0 axis at a negative temperature. This result indicates the presence of antiferromagnetic interactions in the Mn-doped samples. At lower temperatures, inverse ac susceptibility deviates from the linear dependence toward a temperature close to zero. This is a result of additional antiferromagnetic interactions between the next nearest neighbor Mn2+ ions. As the concentration of Mn increases, the susceptibility deviates from CW behavior in ways that depend on the various Mn-O-Mn super-exchange interactions. At high Mn concentrations, one may expect a magnetic phase transition. Ferromagnetic behavior of samples M2 and MS was further investigated as a function of the applied magnetic field. The field dependence of magnetization (M–H curve) was measured at 5 K and 300 K, and revealed an obvious hysteresis loop. Figure 3 shows the ferromagnetic order existing in the materials at low temperature. The remanent magnetization Mn and coercive field Hc are 1.10-4 emu/g and 200 Oe, respectively, which are compared with that of the bulk Mndoped ZnO films. It is interesting to note that the hysteresis loop does not attain saturation up to 30 000 G (Figure 3b) and this could be an indication of spin-glass behavior in this system. The hysteresis behavior of the M2 sample was much weaker than that of the MS sample. The shape of the hysteresis loop is also poor and the loop never really closes on itself because saturation is not achieved up to the highest available field. The smaller dimensions of the hysteresis loop of the M2 sample compared to the MS sample indicate both a smaller number of spins remains aligned at a zero field and a smaller opposing field is necessary to reverse the spins. These effects are rather result of an increase of the manganese content in the MS sample due to the higher Mn solubility in Sn-codoped ZnO samples.

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Figure 3. The field dependence of magnetization measured for (a) the Mn-doped ZnO nanowires at 5K (filled squares) and 300 K (circles) and (b) for the MnSn-doped ZnO nanowires at 5K (filled triangles) and 300 K (circles). Insets: Large scale graphs. Moreover, it is well known that a unique structure feature of the nanostructured materials is their high surface-to-volume ratio. A large fraction of the atoms in nanowires are situated on the surfaces or interfaces. Spin behavior of surface atoms differ considerably from the bulk and in general interface magnetism is likely to differ from bulk behavior. Since the saturation magnetization of such sample is mainly determined by the short-range order of its structure. The existence of a large amount of the disordered interfacial atoms might be responsible for the unusual behavior of magnetization of our samples. Theoretically, the coercive field Hc is larger for the interfacial spins than the bulk. From the energy viewpoint, this implies that more energy is required to rotate the interfacial moment, since the loss of coordination at the interface will allow these spins to fluctuate more. We speculate that the Mn and Fe ion pairs could be coupled through localize electrons of O-vacancy defects [17]. X-ray diffraction measurements show no evidence for Mn–O phases. Although diffraction is not enough sensitive to detect secondary phases at the level necessary, the 1D-like nanowires grow rather free from a secondary phase. It should also be noted that increasing the Mn content in the sample MS in comparison with M2 resulted in a similar relative magnetization response. This provides an additional argument that the magnetization is not due to the precipitating secondary phase in- or outside nanowires, but the true DMS property of the wires. Magnetic force microscopy (MFM) measurements are the direct probing of magnetic properties of individual nanowires. MFM measurements allow to reveal magnetic polarization in a wire with high spatial resolution. Figure 4 shows (a) an atomic force microscopy (AFM) and (b-d) magnetic force microscopy (MFM) images of the 0.5%Mn-doped ZnO nanowire. The magnetic structure of the wire in an initial state is shown in figure 4 (b). Bright parts of the wire signify the magnetization pointing up and dark parts signify the magnetization pointing down. The nanomagnet structure aligns perpendicular the c-axis and can be polarized by external magnetic field (Figure 4 c,d). The patterned domain structure is due to an antiferromagnetic alignment of poles influenced by the magnetic interaction between the single nanomagnets. The size of the single nanomagnet in the nanowire can be estimate as less as 17 nm. Magnetization of the nanowires in external magnetic field of a MFM tip led to switching the single domains in states with opposite polarity or in paramagnetic state.

Figure 4. AFM (a) and MFM (b-d) images of the ZnO:Mn nanowire. Magnetization in an external magnetic field of the MFM tip lead to switching the single nanomagtes (c, d). CONCLUSIONS Magnetic and electrical properties of Mn-, Fe-, Sn- and Li-doped ZnO nanowires are investigated. Aluminum-patterned nanowires show the rectify behavior indicating the Schottkylike barrier formation and resistivity being about 3 Ω·cm in the dark. Electric measurements display a weak but stable hysteretic behavior in the current-voltage curve which could be explained by an enhanced density of oxygen vacancy defects in the nanowire surface layer and their electric polarization. Magnetic susceptibility of the samples as a function of temperature demonstrated Curie–Weiss behavior. Hysteresis with the coercive field of about 200 Oe was observed in magnetization versus field curves at room temperature. Magnetic force microscopy measurements of individual nanowires at room temperature revealed the magnetic domain structure. The nanomagnets align perpendicular c-axic and can be polarized in the external magnetic field. REFERENCES 1. S.J. Pearton, D.P. Norton, K. Ip, Y.W. Heo, T. Steiner, Progr. in Mat. Science, 50, 293 (2005). 2. G. Lawes, A. S. Risbud, A. P. Ramirez, and R. Seshadri, Phys. Rev. B 71, 045201 (2005). 3. A. S. Risbud, N. A. Spaldin, Z. Q. Chen, et al. Phys. Rev. B 68, 205202 (2003). 4. K. Ueda, H. Tabata, and T. Kawai, Appl. Phys. Lett. 79, 988 (2001). 5. D.C. Kundaliya, S.B. Ogale, S.E. Lofland, S. Dhar, et al. Nature Mater., 3, 709 (2004). 6. P. Sharma et al., Nat. Mater. 2, 673 (2003). 7. K. R. Kittilstved, N. S. Norberg, and D. R. Gamelin, PRL, 94, 147209 (2005). 8. A. H. Macdonald, P. Schiffer, and N. Samarth, Nature Mater., 4, 195 (2005). 9. P. Wu, G. Saraf, Y. Lu et al., Appl. Phys. Lett., 89, 012508 (2006). 10. N. A. Spaldin, Phys. Rev. B 69, 125201 (2004). 11. C. Ronning, P.X. Gao, Y. Ding, et al. Appl. Phys. Lett., 84, 783 (2004). 12. J.J. Wu, S. C. Liu, and M.H. Yang, Appl. Phys. Lett., 85, 1027 (2004). 13. A.N. Baranov, G.N. Panin T.W. Kang and Y.-J. Oh, Nanotechnology 16, 1918 (2005). 14. X.R. Wang and Q. Niu, Phys. Rev. B 59, R12755 (1999). 15. G.N. Panin, T.W. Kang, A. N. Aleshin, A. N. Baranov, Y.-J. Oh, I. A. Khotina, Appl. Phys. Lett. 86, 113114 (2005). 16. G. N. Panin, A. N. Baranov et al, Proc. 28th Int. Conf. Phys. Semicond., Vienna, Austria, p. 340, 2006. 17. D.C. Kundaliya, S. B. Ogale et al Nature Materials, 3, 709 (2004).