Physical and Electrical Characteristics of Carbon Nanotube Network Field-Effect Transistors Synthesized by Alcohol Catalytic Chemical Vapor Deposition

1 Department of Electro-Optical Engineering, National Formosa University, Yunlin 63201, Taiwan 2 Department of Mechanical Engineering, National Yunlin University of Science and Technology, Yunlin 64054, Taiwan 3 National Nano Device Laboratories, National Applied Research Laboratories, Hsinchu 30078, Taiwan 4 Institute of Materials Science and Green Energy Engineering, National Formosa University, Yunlin 63201, Taiwan


Introduction
Carbon nanotube field-effect transistors (CNTFETs) have been explored in nanoelectronics to realize desirable device characteristics [1][2][3][4][5][6][7][8][9][10][11].Both n-type and p-type single-walled carbon nanotube (SWCNT) field-effect transistors (FETs) with top-gate electrodes in the conventional metal-oxidesemiconductor field-effect transistor (MOSFET) structures were demonstrated [1].Two methods, including conventional doping and annealing metal/carbon nanotubes (CNTs) contact in vacuum, were used for the CNTFETs conversion from p-to n-type devices [2].Moreover, the fabrication of the n-type CNTFET by Al-doped CNTs as channel was also achieved [12].The primary potential advantage of CNTs is their very high carrier mobility (7.9 × 10 4 cm 2 /V-s) [13].However, one of challenges of CNTs to be viable in high-performance FETs is the requirement for processes that provide each CNTs placed in a desired location and direction [14].Recently, an architecture based on the assembly of two-and three-dimensional networks of SWNTs using chemical vapor deposition (CVD) was demonstrated [15].The field-effect mobility of random networks of SWCNT as thin-film transistor can exceed 100 cm 2 /V-s [16].SWCNT random network thin film transistor with a 10 5 of on/off ratio and a ∼8 cm 2 /C-s of field-effect mobility was demonstrated using water-assisted plasma-enhanced CVD (PECVD) [17].Although various methods were used to synthesize the carbon nanotube networks (CNTNs), some electrical characteristics of CNTNs fabricated by alcohol catalytic CVD (ACCVD) remain not totally understood.We, therefore, attempt to explore some characteristics to gain better physical and electrical insights into the properties of CNTN field-effect transistors (CNTNFETs).

Experimental Methods
CNTNFETs with top-gated structures were fabricated on (100-) oriented n-type silicon wafers.The substrate consists NCHU SEI 3.0 kV x35,000 100 nm WD 2.9 mm of a highly doped n-type Si(100) wafer with an arsenic doping concentration of N D > 10 with 30.0 nm thicknesses measured by the deposited sensor were deposited using reactive dc magnetron sputtering of 99.95% pure Hf targets in an Ar = 24 sccm ambient with a power of Hf target at RF power of 30 W. Subsequently, postdeposition annealing (PDA) using furnace annealing was performed in O 2 gas for 30 min at 450 • C. A metal gate electrode with a 500 nm Al film was deposited via sputtering and was patterned by the lithography.Various physical properties of CNTNs were analyzed by Raman spectrometry, scanning electron microscope (SEM), and high-resolution transmission electron microscope (HRTEM), respectively.Various electrical characteristics of CNTNFETs were evaluated by the HP 5270B instrument.To probe diameter of CNTNs, the TEM analysis was adopted.According to the top morphology of TEM analysis, a single-walled CNTN with 5-7 nm in diameter was grown as shown in Figure 4. Experimental results show that CNTs with various diameters were crisscrossed through another  To demonstrate various physical characteristics of as-grown CNTNs, Raman spectra of as-grown CNTNs deposited on the SiO 2 /n-type Si(100) substrate measured with 633 nm excitation and energy of 1.96 eV were shown in Figure 5.For isolated semiconducting SWNT, the Ramanallowed tangential modes (G mode) are labeled G + and G − at Raman frequency (ω) of 1592 (ω G + ) and 1570 (ω G − ) cm −1 , respectively [18].Thus, the results suggest that asgrown CNTNs synthesized by ACCVD are semiconducting  SWNTs.This is further demonstrated by using the lowfrequency peak measurement.The low-frequency peak is given by ω G − = ω G + − C/d 2 , where d is the diameter of CNT, with C being different for metallic (C ∼ 79.5 cm −1 nm 2 ) and semiconducting (C ∼ 47.7 cm −1 nm 2 ) SWNTs [19].Since the results show that the C value of CNTNs synthesized by ACCVD is to be around 57 cm −1 nm 2 , most semiconducting CNTNs were demonstrated.A little metallic nanotubes could be included in the CNTNs synthesized by ACCVD.Moreover, it has been reported that the intensity ratio between the two most intense features is in the range 0.1 < Iω G − /Iω G + < 0.3 for most of the isolated SWNTs (90%) [19].In this work, the value of Iω G − /Iω G + is to be around 0.214, indicating that the CNTNs with 90% SWNTs were demonstrated.Furthermore, the defects in CNTs are characterized by G CNT = I G /(I D +I G ), where I G is the intensity of G mode, I D is the intensity of disorder-induced mode (D band).The value of G CNT is estimated to be around 0.92, indicating that the CNTNs with low defects were synthesized by ACCVD.

Results and Discussion
Figure 6 shows the output characteristics, drain current versus drain voltage (I d -V d ), of p-CNTNFET and n-CNTNFET consisting of an HfO 2 of 30 nm as top-gate dielectric for several values of the gate voltage.The channel length and width of CNTNFETs was 1 and 5 μm, respectively.At V g near 0 V, the I d current of devices were to be around 0, indicating that the devices were off-status.For V g > 0 and V d > 0, the behavior of I d -V d curves were similar to that of n-MOSFET.For V g < 0 and V d < 0, the behavior of I d -V d curves were similar to that of p-MOSFET.Thus, the bipolar property of CNTNFET synthesized by ACCVD and using HfO 2 as top-gate dielectric was demonstrated.
Figure 7 shows the output characteristics, drain current versus gate voltage (I d -V g ), of (a) p-CNTNFET and (b) n-CNTNFET with an HfO 2 of 30 nm as gate dielectric.The results suggest that both p-CNTNFET and n-CNTNFET exhibits an on-to-off ratio of ∼10 6 and a threshold voltage of −3 and 2.6 V, respectively.To estimate the effective hole and electron mobility in p-CNTNFET and n-CNTNFET, respectively, the following formula is adopted [16]: where t ox is the equivalent oxide thickness (EOT) of HfO 2gate, dielectric and ε ox is the permittivity of the silicon dioxide.L and W are the channel length and the channel width of CNTNFETs.For these networks, the effective hole mobility of 1.7 × 10 3 cm 2 /V-s for the p-CNTNFET and the effective electron mobility of 3.2 × 10 2 cm 2 /V-s for the n-CNTNFET were extracted, respectively.To estimate 2.4 2.5 2.6 2.7 2.8 2.9 3 n-type CNTNFET L/W=1/5 (μm/μm) the subthreshold slope (SS) in both p-CNTNFET and n-CNTNFET, the following formula is adopted [20]: . ( The results show that the SS of 6 mV/decade for the p-CNTNFET and the SS of 18 mV/decade for the n-CNTNFET were extracted, respectively.In general, the SS characteristics of the conventional silicon-based MOSFET is to be around 70-100 mV/decade [20].Thus, the SS characteristics of CNTNFETs synthesized by ACCVD is better than that of the silicon-based MOSFET one.To demonstrate the amplifier characteristics, the transconductance output characteristics (G m ) of HfO 2 -gated p-CNTNFET and n-CNTNFET were shown in Figure 8.The results show that the G m of 1.93 A/V for the p-CNTNFET and the G m of 0.095 A/V for the n-CNTNFET were extracted, indicating that the amplifier characteristics of p-CNTNFET is better than that of n-CNTNFET one.

Conclusions
The present study gives an important message that the bipolar property of semiconducting single-walled CNT-NFET synthesized by ACCVD and using HfO

Figure 1 :
Figure 1: Plane view of SEM morphology of random carbon nanotube networks (CNTNs) formed on the SiO 2 /n-type Si(100) stacked substrate.The substrate consists of a highly doped n-type Si(100) wafer with an arsenic doping concentration of N D > 10 20 cm −3 .The growth conditions were carried out at 750 • C in the alcohol ambient for (a) 5, (b) 10, (c) 15, and (d) 20 min; respectively, and the partial pressure was achieved in 10 Torr.A Co/Mo acetate was premixed at Co : Mo = 0.1 : 0.1 wt% and dissolved in ethanol with sonication for 8 hour.
Various treated temperatures, growth time, and Co/Mo catalysts were employed to explore various surface morphologies of CNTNs formed on the SiO 2 /n-type Si(100) stacked substrate.The substrate consists of a highly doped n-type Si(100) wafer with an arsenic doping concentration of N D > 10 20 cm −3 .Various SEM plane views of CNTNs formed on the SiO 2 /n-type Si(100) stacked substrates were shown in Figures1-3.The growth conditions were carried out at 750 • C in the alcohol ambient for (a) 5, (b) 10, (c) 15, and (d) 20 min; respectively, and the partial pressure was achieved in 10 Torr as shown in Figure 1.A Co/Mo acetate was premixed at Co : Mo = 0.1 : 0.1 wt% and dissolved in ethanol with sonication for 8 hours.It can be seen that dispersed CNTNs were formed on the SiO 2 /n-type Si(100) stacked substrate resulting from short growth time as shown in Figure 1(a).On the contrary, the dense CNTNs were demonstrated using long growth time as shown in Figure 1(d).Thus, the results suggest that the densities of CNTNs increase with increasing time of growth.To scrutinize the effects of various wt% of Co/Mo catalysts on characteristics of CNTNs, various Co/Mo acetates were premixed at (a) Co : Mo = 0.1 : 0.1 wt%, (b) Co : Mo = 0.05 : 0.05 wt%, and (c) Co : Mo = 0.02 : 0.02 wt%, respectively, as shown in Figure 2. The growth conditions were carried out at 750 • C in the alcohol ambient for 10 min.Compared with high concentration of the Co/Mo catalysts, the dispersed CNTNs were formed by the Co/Mo catalysts with low concentration.This could be due to the dispersion of the Co/Mo catalysts with low concentration.Therefore, the results indicate that the densities of CNTNs decrease with decreasing wt%.Moreover, the effects of various treated temperatures on the densities of CNTNs are important issues.Thus, the growth conditions were performed at (a) 750, (b) 700, and (c) 650 • C, respectively, in the alcohol ambient for 10 min and shown in Figure 3.A Co/Mo acetate was premixed at Co : Mo = 0.1 : 0.1 wt%.The experimental results demonstrate that the densities of CNTNs increase with increasing growth temperature.

Figure 2 :
Figure 2: Plane view of SEM morphology of CNTNs formed on the SiO 2 /n-type Si(100) stacked substrate.The growth conditions were carried out at 750 • C in the alcohol ambient for 10 min, and the partial pressure was achieved in 10 Torr.Various Co/Mo acetate were premixed at (a) Co : Mo = 0.1 : 0.1 wt%, (b) Co : Mo = 0.05 : 0.05 wt%, and (c) Co : Mo = 0.02 : 0.02 wt%, respectively.Then, all Co/Mo catalysts were dissolved in ethanol with sonication for 8 hours.

Figure 3 :Figure 4 :Figure 5 :
Figure 3: Cross-sectional view of SEM morphology of CNTNs formed on the SiO 2 /n-type Si(100) stacked substrate.Various plane views were also shown in the inset of figure.The growth conditions were carried out at (a) 750, (b) 700, and (c) 650 • C in the alcohol ambient for 10 min, and the partial pressure was achieved in 10 Torr.A Co/Mo acetate was premixed at Co : Mo = 0.1 : 0.1 wt% and dissolved in ethanol with sonication for 8 hours.

Figure 6 :
Figure 6: Drain current versus drain voltage (I d -V d ) characteristics of (a) p-CNTNFET and (b) n-CNTNFET with HfO2 of 30 nm as gate dielectric.The channel length and width of CNTNFETs were 1 and 5 μm, respectively.

Figure 7 :
Figure 7: Drain current versus gate voltage (I d -V g ) characteristics of (a) p-CNTNFET and (b) n-CNTNFET with an HfO 2 of 30 nm as gate dielectric.The channel length and width of CNTNFETs were 1 and 5 μm, respectively.
2 as topgate dielectric was demonstrated.The densities of CNTNs increase with increasing process temperature, treated time, and Co/Mo catalysts concentrations.Experimental results indicate that the random networks of SWNTs with higher effective hole/electron mobility, smaller subthreshold slope, are the promising candidate for the development of the nano-electronic devices.