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Local-Gated Single-Walled Carbon Nanotube Field Effect Transistors Assembled by AC Dielectrophoresis Paul Stokes and Saiful I. Khondaker Nanoscience Technology Center and Department of Physics, University of Central Florida, Florida 32826, USA INTRODUCTION & MOTIVATION RESULTS • Carbon nanotube field effect transistors (CNT-FETs) have displayed exceptional electrical properties superior to the traditional MOSFET. • Most of these fabrication methods for CNT electronic devices are based on either random positioning or directed assembly by high temperature growth on patterned substrate, which are not compatible with conventional CMOS technologies. • Most of these devices are controlled by only a global back gate. They do not offer high throughput or individual control of each CNT-FET necessary for parallel fabrication. • For large-scale fabrication of CNT-FET devices three conditions need to be satisfied: (i) separation of semiconducting and metallic carbon nanotubes must be realized, (ii) nanotubes need to be assembled at selected positions of the circuit with high yield, and (iii) each nanotube must be addressed individually with a local gate. • We present a simple and scalable technique for the fabrication of CMOS compatible and local gated CNT-FETs from solution. The approach is based on directed assembly of individual SWNT via AC Dielectrophoresis. (a) (b) 1. Electronic Transport Characteristics of DEP Assembled Device -6 -6 10 10 (a) (b) 50% Metallic -7 I DS (A) I DS (A) 10 -8 10 -9 10 Assembly Method Gate Material Subthershold Swing (mV/dec) Threshold Voltage (V) Ref DEP SiO2/Back gate 1200 1.5 3 -8 CVD/ALD Al2O3/Local gate 10 120 N/A 4 CVD SiO2/Back gate 1000-2000 N/A 5 DEP Al2O3/Local gate 170 1 this work (6) -9 10 b) 50% Semiconducting -7 10 Catalytic Island 4. Comparison to Other DEP and CVD Assembled CNT-FETs 10 -10 10 -10 -5 0 5 -10 10 -10 -5 0 VBG(V) 5 10 VBG(V) 5. Electric Field Simulation Figure 3: Drain current (IDS) - back gate voltage (VBG) characteristics of representative DEP assembled metallic (a) and semiconducting (b) devices (VDS = 0.3 V). (a) Figure 1: (a) Random placement of CNT by drop casting from solution. (b) Patterned growth from CVD. Requires temperatures of 900 C. Al2O3 (b ) SiO2 2. Electrical Breakdown of Metallic SWNT 8 Re[K f ] Au Au -8 Device Fabrication and Dielectrophoretic Assembly After Electrical Breakdown (b) 1 4 10 10 (a) m-SWNT in DCE 10 METHODS 80 12 10 s-SWNT in DCE 0 10 -9 10 -4 I DS (A) I ( A) • Devices were fabricated on heavily doped silicon (Si) substrates capped with a thermally grown 250 nm thick SiO2 layer. • Contact pads and electron beam markers were fabricated with optical lithography using double layer resists, metallization of Cr (5 nm) and Au (50 nm) and finally standard lift-off. • Source and drain electrode patterns were defined with EBL 5 nm Cr and 20 nm thick Au were thermally deposited followed by lift-off. • 100 nm wide Al gate patterns are defined by EBL with 20-25 nm thickness. • The sample is treated in oxygen plasma to create a thin aluminum oxide dielectric layer. DEP assembly: • HiPco grown SWNTs (Carbon Nanotechnologies Inc.) was ultrasonically dispersed in 5 ml of 1,2-dichloroethane for approximately 30 minutes. • A small drop (~8ul) was cast onto a chip with 12 pairs of source-drain electrodes, each containing a 100 nm wide Al gate. An AC voltage of approximately 8 VP-P at 1 MHz is applied for 1-2 seconds. 2 VBG=+10V 40 10 10 0 3 6 10 10 9 10 Frequency (Hz) -10 10 FDEP m Re K f E 2 RMS *p m* Kf * m * p ,m p ,m p ,m i 2 ( m p m2 ) m p m2 Re[ K f ] 2 m2 m2 VDS=0.3 V -11 10 0 0 5 -10 10 -5 VDS (V) 0 VBG(V) 5 10 Figure 6: (a) Electric field simulation between a 1 um gap. Shows that the gradient of electric field is strongest near the tips of the electrodes. (b) Plot of the Clausius Mosetti factor (KF) for SWNTs in dichloroethane. The force is positive for both semiconducting and metallic SWNTs up to ~5 GHz. Figure 4: (a) Transformation of a nanotube bundle into a semiconducting device by selective breakdown by sequential ramps of VDS (labeled 1 and 2) with back gate set to 10 V. INSET: Resultant current - back gate characteristic after breakdown (VDS = 0.3 V). CONCLUSION 3. Local Gate Characteristics (a) Au Au • We have fabricated CNT-FETs with local Al bottom gates through DEP. • Our method offers a convenient way to assemble local-gated CNT-FET devices from solution without the need of high temperature growth, making it compatible with present microfabrication technology. • Our local-gated devices show superior characteristics such as small values of threshold swing and low threshold voltage compared to other DEP assembled back gated CNTFETs. • Local gating offers fast switching behavior due to the channel controlled mechanism owed to the thin local Al gate. • Directed assembly of local gated CNT-FETs at selected position of the circuit via DEP may pave the way for large scale fabrication of CMOS compatible nanoelectronic devices. (b) Au Au 0.0 (a) SiO2 SiO2 Si Si back gate (b) -8 10 -0.2V Back Gate -0.4V -0.1 -9 (d) -10 10 10 10 S LG 10 SWNT -11 10 10 10 -12 I DS ( A) Al/Al2O3 local gate (c) I DS (A) 10 1MHz, 8Vp-p -8 VDS=0.3V -9 Local Gate -10 -11 -10 HiPCO SWNTs D Figure 2: Fabrication of local gated CNT-FET device. (a) Source (S) - drain (D) electrodes of 1 μm separation are patterned. (b) Local Al/Al203 gate electrodes are patterned using EBL. (c) DEP assembly of CNT from solution. (d) AFM image of a device showing nanotubes are assembled at the tips. -0.6V -0.8V -0.2 -1.0V -1.2V -12 -1 0 1 VLG(V) -0.3 10 200nm 0V -5 0 VG (V) 5 10 VLG=-1.4V -1.6 -1.2 -0.8 -0.4 REFERENCES 0.0 VDS(V) Figure 5: (a) IDS versus VLG and VBG for comparison form the same device after DEP assembly. Local gate shows far better gate coupling. INSET: Expanded plot of IDS vs. VLG showing low threshold voltage and subthreshold swing. The gate leakage current is < 1 pA. (fast switching is possibly due to channel controlled operation). b) Output characteristics, for different gate voltages up to the saturation regime. 1. 2. 3. 4. 5. 6. Tans et al., Nature, 393, 49 (1998). Kasper Grove-Rasmussen, University of Copenhage, Denmark, PhD thesis (2006). Dong et al, J. Phys. Chem. B 109 13148 (2005). Lin et al, IEEE Elec. Dev. Lett. 26 823 (2005). Javey et al, Nature Mater. 1 241 (2002). P. Stokes and S. I. Khondaker, Nanotechnology 19 175202 (2008). 12 10