11480
J. Phys. Chem. C 2007, 111, 11480-11483
Transport Properties in (Ga,Mn)N Nanowire Field-Effect Transistors Moon-Ho Ham,† Dong-Keun Oh,‡ and Jae-Min Myoung*,† Information and Electronic Materials Research Laboratory, Department of Materials Science and Engineering, Yonsei UniVersity, 134 Shinchon-Dong, Seoul 120-749, Korea, and Systems Research Team, National Fusion Research Center, Daejeon 305-333, Korea ReceiVed: April 22, 2007; In Final Form: May 27, 2007
We present the fabrication and transport characteristics of field-effect transistors based on single-crystalline (Ga,Mn)N nanowires with Mn concentrations of 2% and 5% prepared via a vapor-liquid-solid method. The (Ga,Mn)N nanowires with Mn concentrations of 2% and 5% configured as field-effect transistors exhibited n-type and p-type conductivities, respectively, and good electrical properties with an on/off current ratio of ∼102 and a subthreshold swing of 1.9-2.2 V/decade. For the (Ga,Mn)N nanowires with the Mn concentration of 5%, the negative magnetoresistance persisted up to room temperature. These results suggest the feasibility of applying dilute magnetic semiconductor nanowires in nanoscale electronics and spintronics.
Introduction One-dimensional GaN nanostructures including nanowires, nanorods, and nanotubes have attracted a great deal of interest because of their unique physical properties and diverse potential applications in nanoscale electronics and photonics.1-9 Recent efforts have demonstrated that GaN nanostructures would be very promising building blocks for the assembly of nanoscale devices such as ultraviolet lasers,1 photodetectors,5 light-emitting diodes,6,7 field emitters,4 and field-effect transistors (FETs).2 In this context, n- and p-type dopings of the GaN nanowires are essential for high-performance electronic and photonic devices. Because unintentionally doped GaN is intrinsically n-type, comprehensive studies have focused on n-type GaN nanowires and their devices.1,2 Recent advances in nanowire fabrication techniques have enabled the synthesis of Mg-doped p-type GaN nanowires and the fabrication of p-n homojunctions.8,9 Meanwhile, Mn-doped GaN, which has been theoretically predicted to exhibit ferromagnetism above room temperature, would enable applications in spintronics as well as electronics and photonics because Mn acts not only as a p-type dopant but also as a magnetic element inducing ferromagnetism.10 The syntheses of dilute magnetic semiconductor (DMS) (Ga,Mn)N nanowires exhibiting room-temperature ferromagnetism have already been reported,11-13 but the electronic transport properties of the nanowires have not yet been investigated in detail. In this study, we report the electrical and magnetic transport properties of (Ga,Mn)N nanowires using a field-effect transistor (FET) structure. (Ga1-xMnx)N nanowires with x ) 2% and 5% showed n-channel and p-channel FET characteristics, respectively, and their device functions were investigated. In addition, the (Ga1-xMnx)N nanowires with x ) 5% were found to exhibit a negative magnetoresistance (MR) in the temperature range of 5-350 K. Experimental Section (Ga,Mn)N nanowires were synthesized on Ni-coated Si substrates via a vapor-liquid-solid growth mechanism using * Corresponding author. E-mail:
[email protected]. † Yonsei University. ‡ National Fusion Research Center.
GaN (99.99%) and MnCl2 (99.99%) powders in a horizontal tube furnace under flowing high-purity NH3 (99.999%) gas. To control the Mn concentration in the (Ga,Mn)N nanowires, MnCl2 powder was located upstream of the GaN powder, and the separations between two powders were changed. A detailed description of the nanowire preparation has been published elsewhere.13,14 To fabricate the nanowire FETs, the nanowires were released from the substrates by sonication in isopropyl alcohol and subsequently transferred to a degenerately doped p-type Si substrate capped with a thermally grown 300-nm-thick SiO2 layer, where the underlying Si was used as a global back gate. Using electron beam lithography followed by sputtering, the source and drain electrodes were defined on the nanowires. Ti/ Au (50/150 nm) and Ni/Au (50/150 nm) were used as ohmic contacts for n- and p-type (Ga,Mn)N nanowires, respectively. To improve the contact characteristics for Ti/Au and Ni/Au metallizations, rapid thermal annealing treatments were performed at 600 and 500 °C, respectively, for 1 min under flowing N2 gas. Results and Discussion Figure 1a,c shows high-resolution transmission electron microscopy (HRTEM) images and selected area electron diffraction (SAED) patterns of the (Ga,Mn)N nanowires prepared with different separations between MnCl2 and GaN powders. For both nanowires, the HRTEM images and the corresponding SAED patterns reveal that the nanowires are single-crystalline growing along the [100] direction of the GaN wurtzite structure. In addition, secondary phases or clusters related to Mn were not observed inside the nanowires or near the nanowire surface. As the Mn concentration increased, the d spacing along the nanowire axis corresponding to the interplanar spacing of (100) decreased slightly. This implies that Mn systematically substituted Ga sites in the nanowires.15 Energy-dispersive X-ray spectroscopy (EDS) measurements were performed in situ in the HRTEM instrument to confirm the existence of Mn in GaN. The Mn concentrations of the nanowires shown in Figure 1b,d were estimated to be 2% and 5%, respectively. The Mn concentration in a single nanowire was almost constant along
10.1021/jp073087k CCC: $37.00 © 2007 American Chemical Society Published on Web 07/11/2007
Transport Properties in (Ga,Mn)N Nanowire Transistors
J. Phys. Chem. C, Vol. 111, No. 30, 2007 11481
Figure 1. (a,c) HRTEM images and SAED patterns of single (Ga,Mn)N nanowires prepared with separations of (a) 140 and (c) 130 mm between the MnCl2 and GaN powders. (b,d) EDS spectra of the same nanowires as shown in a and c, respectively, confirming that the Mn concentrations in the nanowire are about (b) 2% and (d) 5%.
the nanowire axis, and the variation of Mn concentration in different nanowires prepared under identical experimental conditions was very small (
