J. Phys. Chem. C 2009, 113, 10847–10852
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ARTICLES Magnetic and Electrical Properties of Single-Crystalline Mn-Doped Ge Nanowires Han-Kyu Seong,† Ungkil Kim,† Eun-Kyung Jeon,‡ Tae-Eon Park,† Hwangyou Oh,† Tae-Hyun Lee,† Ju-Jin Kim,‡ Heon-Jin Choi,*,† and Jae-Young Kim§ Department of Materials Science and Engineering, Yonsei UniVersity, Seoul 120-749, Korea, Department of Physics, Chonbuk National UniVersity, Chonju 561-756, Korea, and Pohang Accelerator Laboratory, Pohang UniVersity of Science and Technology, Pohang 790-784, Korea ReceiVed: July 15, 2008; ReVised Manuscript ReceiVed: May 9, 2009
We report on synthesis of Mn-doped Ge nanowires and their magnetic and electrical properties. The nanowires were grown on the silicon substrate by vapor-liquid-solid mechanism using Au as catalyst and GeCl4 and MnCl2 as precursor. Anomalous X-ray scattering measurement makes it clear that Mn atoms are substitutionally incorporated with the diamond network of host Ge sites. Superconducting quantum interference device characterization indicated that the nanowires possess ferromagnetism up to room temperature. X-ray magnetic circular dichroism spectra at Mn L2,3-edges showed that doped Mn has local spin moment with the 3d5 electronic configuration above room temperature, meaning that the ferromagnetism originates from doped Mn2+ ions. Electrical characterization of a nanowire field effect transistor revealed the improved p-type behavior with hole mobility of 4.95 cm2/V · s. 1. Introduction Application of diluted magnetic semiconductors (DMSs)1,2 for spintronics that can manipulate both charges and spins requires two essential features, ferromagnetism with a Curie point above room temperature and having a typical semiconductor device that is operated by a carrier (e.g., hole) mediated ferromagnetic interaction among localized spin moments as a basis. Room-temperature ferromagnetism in DMSs has been predicted by systems based on hole mediation3 and has been reported in many transition metal doped semiconductor thin films including Cr-, Mn-, Fe-, and Co-doped III-V (GaN, GaP),4 oxide (ZnO, TiO2),5,6 and IV-IV (Si, Ge).7,8 However, there has been debate on these DMSs over the possible magnetic impurity phase separation, and it has been argued that the ferromagnetism arises from magnetic secondary clusters.9-12 In the present study, we investigate the ferromagnetism in a Mn-doped Ge system that is compatible with current silicon complementary metal oxide semiconductor (CMOS) processes.13 We study the system by synthesizing nanowires as they have a number of advantages over thin films with respect to studying ferromagnetism in DMSs.14,15 Specifically, they offer thermodynamically stable features and are typically single-crystalline and defect-free. They can thus safely exclude the effect of defects and nonuniform distribution of dopants that are typically observed in DMSs prepared by nonequilibrium processing.16 The freestanding nature of nanowires makes it possible to exclude the effect of thermal and lattice mismatch of the substrate and opens the possibility of determining the intrinsic * To whom correspondence should be addressed. E-mail: hjc@ yonsei.ac.kr. † Yonsei University. ‡ Chonbuk National University. § Pohang University of Science and Technology.
magnetism under fully relaxed states.17 Moreover, nanowires could further fuel the development of ferromagnetism in DMSs. For example, all spin rotations in the nanowire could be limited to a single axis. In addition, the spin rotation operators commute, predicting a progressive slowing and finally a complete suppression of the spin relaxation.18 Possible tetragonal distortion and shape anisotropy in the nanowires could also be helpful for the evolution of ferromagnetism.19,20 It is also noted that nanowires themselves are attractive building blocks for electronic devices. In the present study, we found that Mn/Ge nanowires have room-temperature ferromagnetism by the local spin moment with the 3d5 electronic configuration of doped Mn2+ ions. Our electrical characterization showed that Mn/Ge nanowires reveal the improved p-type behavior with higher hole mobility of 4.95 cm2/V · s for Vsd of 1 V, compared with pure Ge nanowires. 2. Experimental Section 2.1. Synthesis Methods. Single-crystalline Mn/Ge nanowires were synthesized using a Au catalyst deposited on silicon substrates in a germanium tetrachloride (GeCl4)-based chemical vapor transport system. The substrates with a 2 nm layer of Au were inserted into the center of a quartz tube, and MnCl2 (purity 99.99%) powder as doping source was placed at a distance of 1 in. away the substrates. The temperature of the furnace was increased at a rate of 50 °C min-1 to 600 °C under H2 and Ar atmosphere of a flow of 100 and 300 sccm, respectively. Simultaneously, GeCl4 as Ge source was introduced into the tube using H2 carrier gas that was bubbled through a liquid source held at 15 °C at a flow rate of 3 sccm for 10 min and then cooled down to room temperature under H2 and Ar atmosphere. 2.2. Characterization. The magnetic properties of the nanowires on the substrate were measured in a superconducting
10.1021/jp806244g CCC: $40.75 2009 American Chemical Society Published on Web 06/03/2009
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J. Phys. Chem. C, Vol. 113, No. 25, 2009
quantum interference device (SQUID) magnetometer and corrected for the diamagnetic contribution from the silicon substrate. Synchrotron X-ray diffraction (XRD) measurements were carried out at the 8C beamline at the Pohang Light Source (PLS). A Si(111) double-crystal monochromator was used to select a wavelength λ of 0.1541 nm within an energy resolution of ∆λ/λ ) 5 × 10-4. The beam was collimated at the sample position to 2 mm (horizontal) by 0.1 mm (vertical), and the measured diffraction intensity was normalized to the intensity of the primary beam, which was monitored with an ionization chamber. In the case of narrow scans around pure Ge and Mn/Ge (111) diffractions, the step width to obtain an accurate determination of the critical angles was increased to 0.002°. Anomalous X-ray scattering (AXS) measurements were carried out at the beamline 5A of PLS. For the AXS measurements, the diffraction intensity was measured as a function of momentum transfer at incident X-ray energy and the scattering intensity was monitored as the X-ray energy was varied through the Mn K absorption edge. Good counting statistics were ensured in data collection. The diffused fluorescence background was also measured simultaneously and subtracted. Ge and Mn K-edge X-ray absorption fine structure (XAFS) spectra were collected in fluorescence mode at the 3C beamline of PLS at room temperature. Incident X-ray energy was selected with a three-quarters tuned Si(111) double monochromator. X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) measurements were carried out at the 2A beamline of PLS. We measured Mn/ Ge samples soaked in HCl for 15 s to remove residual impurities such as metallic or secondary phases employed in the wire fabrication process. The samples were introduced into an experimental chamber with a base pressure of approximately
