12490
J. Phys. Chem. C 2007, 111, 12490-12494
Electrical Behavior of Ferromagnetic BiMn-Codoped ZnO Bicrystal Nanobelts to Pt Contacts Congkang Xu, Steven Youkey, Jianfeng Wu, and Jun Jiao* Department of Physics, Portland State UniVersity, Portland, Oregon, 97201 ReceiVed: April 21, 2007; In Final Form: June 3, 2007
Large channel length field effect transistors (FETs) based on Pt contacts to ferromagnetic BiMn-codoped ZnO bicrystal nanobelts have been fabricated using dielectrophoresis and a focused ion beam. Electrical transport studies show n-type behavior of the ferromagnetic ZnO nanobelts. The current-voltage characteristics of the FETs exhibit Schottky barrier behavior. The contact resistances and the Pt diffusion are responsible for the reduction of the conductance and the threshold shift. The reduction of the mobility can be attributed to the enhanced interface scattering at Pt electrodes/nanobelt contact regions after Pt deposition. The devices are also found to be strongly dependent on the channel length.
ZnO-based nanowires or nanobelts are identified as one of the most promising building blocks in an effort to realize the miniaturization of nanodevices.1-3 In particular, transition metaldoped ZnO nanowires and nanobelts are of great interest in spintronics since the diluted magnetic semiconductor (DMS) ZnO nanowires potentially exhibit ferromagnetism above room temperature, which is novel approach in the fabrication of spin field effect transistors (FETs)4,5 as well as making the device operable at room temperature beyond cryogenic temperature. Mn-doped ZnO nanowires or nanobelts have received a lot of attention due to their possible practical applications such as data storage media, magnetic force microscope (MFM) tips, magnetic sensors, and magnetic memories even though the origin of ferromagnetism is still contentious. Some research on Mn-doped ZnO nanowires exhibiting ferromagnetism at room temperature was reported.6-9 In order to integrate ferromagnetic BiMncodoped ZnO bicrystal nanobelts with possible magnetoelectronic nanodevices such as a spin tunneling device and ferromagnetic switch, it is crucial to understand their electrical transport properties and develop Ohmic and rectifying contacts to these nanostructures. Although many studies on spin transport investigation via ferromagnetic contacts to carbon nanotubes (CNTs)10,11 have been reported, the research on nonmagnetic metal contacts to ferromagnetic one-dimensional nanostructures is lacking. In this letter, we report the fabrication of a large channel length FET based on Pt contacts to ferromagnetic BiMncodoped ZnO bicrystal nanobelts by dielectrophoresis (DEP) and the electrical transport investigation of ferromagnetic BiMn-ZnO bicrystal nanobelts. An effort to obtain lower contact resistance has been made by depositing a Pt film using a focused ion beam (FIB) induced metal deposition across the pre-defined paired Pt contacts where the BiMn-ZnO nanobelt has been aligned The ferromagnetic BiMn-ZnO bicrystal nanobelts used in the fabrication of the FET device were prepared through a vaporphase transport route at a low temperature of ∼ 300 °C with a horizontal furnace system.9 The low melting point Bi was utilized for the fabrication of Mn-doped ZnO nanobelts at low temperature, which may suppress secondary phase and is * Corresponding author. E-mail:
[email protected].
favorable for the investigation of room-temperature ferromagnetism. Moreover, on the basis of K. Sato et al.’s suggestion, the simultaneous codoping of hole and electron into ZnO might give rise to stable ferromagnetism.12 Figure 1 shows a transmission electron microscope (TEM) image of nanobelts containing grain boundaries in the middle along the growth direction. The inset shows a magnified TEM image of a bicrystal nanobelt. The dimensions of as-fabricated nanobelts are widths in the range of 40-150 nm with lengths up to tens of microns, and the thicknesses varying from 5 to 15 nm. The energy dispersive X-ray spectroscopy (EDX) analysis indicates that the as-fabricated BiMn-ZnO nanobelts consist of Zn, O, and Mn with the amount of Bi being below the EDX detection limit. The magnetic measurement conducted by a superconduction quantum interference device (SQUID) confirmed that the nanobelts exhibit ferromagnetism at room temperature.9 The Pt electrode patterns used for this study were prefabricated with photolithography on a heavily doped silicon substrate with a 1000-nm-thick thermal oxide layer which acts as the global back-gate. The electrodes were composed of two 2 µm-wide fingers connected to 100 × 100 µm2 pads for probe contacts. The as-fabricated ferromagnetic bicrystal nanobelts were put into ethanol solution and then ultrasonicated for 10 min. A 1.0 µL sample of the solution containing ZnO bicrystal nanobelts was dropped onto the prefabricated electrodes (120nm-thickness) and aligned using DEP techniques.13,14 Signals of 10 V and 5 MHz AC were selected to optimize the alignment of an individual nanobelt. Under the electrical polarization force, the nanobelts were aligned onto the electrodes. By controlling the concentration of the nanobelts in the solution, a circuit with an individual nanobelt across the two electrodes was attained. The scanning electron microscope (SEM) was used to map the location of the nanobelts. The substrate was then put in a probe station (Cascade Microtech Summit MicroChamber) where the electrical contact pads containing aligned nanobelts were accurately located using the binoculars. A measurement of source-drain currents (Ids) passing through the nanobelt as a function of the bias voltage (Vds) and the gate voltage (Vgs) was carried out in air at room temperature using standard dc techniques with an Agilent 4156C semiconductor parameter analyzer.
10.1021/jp0730794 CCC: $37.00 © 2007 American Chemical Society Published on Web 07/28/2007
BiMn-Codoped ZnO Bicrystal Nanobelts
J. Phys. Chem. C, Vol. 111, No. 33, 2007 12491
Figure 1. TEM Image of BiMn-doped ZnO bicrystal nanobelts fabricated at a low temperature using vapor transport route, clearly showing that the nanobelts have grain boundary in the middle along the growth direction. The inset is a magnified TEM image of a single nanobelt. The thickness is an estimated 10 nm.
Figure 2. (a) SEM image of the device before Pt deposition at the two ends. The inset is a magnified SEM image of the ZnO nanobelt used in this device, manifesting bicrystal belt-like nanostructure. (b) A schematic view of the electrodes structure after Pt deposition on the top of contacts between the nanobelt and pre-fabricated Pt electrodes using FIB.
Figure 2a shows the SEM image of a fabricated BiMn-ZnO nanobelt FET. The inset is a magnified BiMn-ZnO nanobelt featuring bicrystallinity. The source-drain electrode distance (channel length) is L ) 11 µm. The nanobelt, having a width of ∼200 nm and a thickness of ∼10 nm, does not suspend
Figure 3. (a) I-V characteristics of a single BiMn-ZnO bicrystal nanobelt across two Pt electrodes at gate voltages varying from +6 to -6 V with a step of 1 V, showing n-type enhanced rectifying diodelike mode and n-type retainability of ferromagnetic BiMn-ZnO nanobelts. The inset is I-Vds curve at Vgs ) 0 V. (b) Gate transfer characteristics of a BiMn-ZnO bicrystal nanobelt at different sourcedrain voltages, verifying n-type ferromagnetism. The inset is the semilog plot of I-Vgs at Vds ) 2.0, 1.5, 1.0, and 0.5 V. (c) I-V characteristic of a BiMn-ZnO bicrystal nanobelt at Vgs ) 6 V after the sweeping process, manifesting the reduction of the conductance. The inset is the corresponding transfer characteristics of BiMn-ZnO bicrystal nanobelt at Vds )2 V, exhibiting the decrease of the transconductance.
between two electrodes but touches the substrate due to the long channel length. Figure 2b depicts a schematic illustration of a BiMn-ZnO bicrystal nanobelt FET where a Pt film was deposited by FIB on each end of the nanobelt aligned across a pair of pre-fabricated Pt electrodes. Figure 3a exhibits current versus drain-source bias (I-Vds) curves obtained before Pt deposition under different gate
12492 J. Phys. Chem. C, Vol. 111, No. 33, 2007 voltages varying from -6 to 6 V with a step of 1.0 V. The inset, I-Vds at Vgs ) 0 V, shows a rectifying behavior of an n-enhancement mode device, clearly indicating Schottky sourcedrain contacts of different barrier height. The conductance at negative voltages is stronger than that at positive voltages, but I-Vds at different Vgs shows a large gate modulation at positive source-drain voltages featuring a directionally dependent Schottky barrier FET. The strong gate effect exhibits semiconducting behavior of BiMn-ZnO bicrystal nanobelts, different from the reported Mn-doped GaN nanowires and Cu-doped ZnTe nanowires that behave like degenerately doped metallic nanowires.4,15 The remarkable gate modulation also suggests that it is possible to electronically manipulate the magnetization of BiMn-doped ZnO bicrystal nanobelts, thus availing themselves of tuning spinpolarized electrons. From the linear region of I-Vds curves measured at Vgs ) 0 and the dimension of the nanobelt, the resistivity (F) obtained is about 63 Ωcm. Two-terminal I-Vds curves exhibit a linear and asymmetric response at positive gate voltages and negative voltages, similar to carbon nanotubes (CNTs) Schottky diodes intentionally using asymmetrical Al and Pd contacts.16 A similar asymmetric behavior has also been observed in ZnO nanorods by Harnack et al.1 and undoped ZnO nanobelts by Lao et al.3 The former accounted for intrinsic characteristics of ZnO nanorods; the latter documented that the rectifying behavior was probably due to the asymmetric contacts formed during the DEP. In the present work, modulating the gate voltage is not in accordance with the I-Vds curve at Vgs ) 0 V as shown in the inset of Figure 3a. That is, the enhancement mode does not take place at a negative source-drain voltage but at a positive source-drain voltage, most likely a result of the ferromagnetic BiMn-doped ZnO bicrystal nanobelts since the Bi/BiMn in ZnO tends to form a grain boundary barrier.9,17 At a certain positive Vds, I increases with increasingly positive Vgs: that is, the conductance of the nanobelt increases with the positive gate voltage Vgs. This striking behavior suggests that the BiMn-ZnO nanobelt FET is an n-type enhancement mode device. The ferromagnetic BiMn-ZnO nanobelts retain an n-type characteristic instead of a p-type one, similar to previously reported results for Mn-doped GaN nanowires.4,18 Some research suggests that it is not feasible to attain high Tc in n-type nanostructures in terms of mean-field theory.19 This inconsistency is probably due to the Zn interstitials and/or oxygen vacancy as seen from green luminescence in our prior photoluminescence spectroscopy measurement9 that cause the crystal field attained in this study to differ from the one in ideal ZnO. Figure 3b shows the transfer characteristics of the FET under different biases varying from -2 to + 2 V with a step of 0.5 V. The gate response I-Vgs curves clearly show again that the BiMn-doped ZnO nanobelt is of typical n-type. As seen from the I-Vgs curve, the change in current is more than 1200 nA by changing the gate voltages from 0 V to +10 V at Vds ) 2 V. The inset is a semilog plot of I-Vgs under different sourcedrain voltages, a switching ratio of nearly 1000 at Vds ) 2 V. In the forward voltage, the maximum transconductance gm is about 1.3 × 10-7 S at Vds ) 2 V at air with room temperature, which is 1 order of magnitude larger than that of reported Mndoped ZnO nanowires at rough vacuum with a temperature of 225 K. The carrier mobility is estimated to be about 70 cm2/ Vs, which is much higher than that of the Bi-doped ZnO nanowires,20 and two times as high as the reported Mn-doped ZnO nanowires (35 cm2/Vs)21 regardless of bicrystal nanostructures probably leading to reduce the mobility.22 In the reverse voltage, the maximum transcondcutance gm is about 4 × 10 -9 at Vds ) -2 V, which is suppressed by more than 2 orders of
Xu et al.
Figure 4. (a) I-V characteristics of a single BiMn-ZnO bicrystal nanobelt across two Pt electrodes at gate voltage varying from +6 to -6 V with a step of 1 V after Pt deposition using an FIB, showing a significant reduction of the conductance as compared with that before Pt deposition. The lower inset is the SEM image of the device after Pt deposition; the upper inset is the energy band diagram of Pt-ZnO nanobelt contact and I-Vds curve at Vgs ) 0 V. (b) Gate transfer characteristics of the BiMn-ZnO bicrystal nanobelt at different sourcedrain voltages after Pt deposition, demonstrating the reduction of transconductance and the shift of the threshold. The inset is the semilog plot of I-Vgs at Vds ) 2.0, 1.5, 1.0, and 0.5 V.
magnitude. To optimize the contacts between the nanobelt and the pre-fabricated Pt electrodes, we measured I-Vds curves using a sweeping technique that induces Joule heating for the purpose of annealing. After the sweeping process as shown in Figure 3c, the conductance of the FET decreases slightly and the gate response effect is also quenched (inset) unlike single-walled CNTs (SWCNTs),14 where Joule heating increased conductance. This Joule heating investigation demonstrates that the nanobelt initially has a good contact with the paired electrodes. This also suggests that the room-temperature ferromagnetism of BiMndoped ZnO bicrystal nanobelts is favorable for contact with metal electrodes and the ferromagnetic BiMn-ZnO bicrystal nanobelts can be easily aligned on the paired Pt electrodes. To maintain a firm contact between the nanobelt and the electrodes, producing a better contact with lower barrier height, we deposited Pt at the contact points between the nanobelt and the pre-fabricated Pt electrodes using a FIB microscope, as shown schematically in Figure 2b. The electrical measurement (Figure 4a) shows that the device retains nonlinear and asymmetric I-V characteristics, unlike ref 3 in which the device, following Pt deposition, demonstrates linear and symmetric I-V
BiMn-Codoped ZnO Bicrystal Nanobelts
Figure 5. (a) I-V characteristic of BiMn-ZnO bicrystal nanobelt (channel length
