Metal-Catalyst-Free Growth of Carbon Nanotubes and Their Application in Field-Effect Transistors

Abstract

Abstract:The metal-catalyst-free growth of carbon nanotubes (CNTs) using chemical vapor deposition and the application in field-effect transistors (FETs) is demonstrated. The CNT growth process used a 3-nm-thick Ge layer on SiO 2 that was subsequently annealed to produce Ge nanoparticles. Raman measurements show the presence of radial breathing mode peaks and the absence of the disorder induced D-band, indicating single walled CNTs with a low defect density. The synthesized CNTs are used to fabricate CNTFETs and the best device has a state-of-the-art on/off current ratio of 3×10 8 ExperimentalA p + Si substrate (0.005 Ω·cm) was employed as a back gate and a 45-nm-thick thermally grown SiO 2 layer was employed as a gate dielectric. A 3-nm-thick Ge layer was deposited by either sputtering or evaporation on the SiO 2 layer. The nanoparticles on the insulator were formed at 850°C for 10 min in a mixture of Ar (1000 sccm) and H 2 (300 sccm) after a preanneal in H 2 (1000 sccm) at 950°C. The samples were reloaded to a hot-wall reactor and then CNT growth was performed using chemical vapor deposition (CVD) at atmospheric pressure.CNTs were grown at 850°C for 10 min using a mixture of methane (1000 sccm) and H 2 (300 sccm) immediately after a pre-anneal in H 2 (1000 sccm) at 900°C. Back gate CNTFETs were fabricated with Pd source/drain contacts. Pd was deposited by sputtering and the source/drain electrodes were formed using direct write laser lithography and lift-off. The gap between Pd source/drain electrodes was 2.0 µm and the width was 8.0 µm. Typically only one or two CNTs crossed over the 2.0 µm gap between the source and drain. Electrical characteristics of CNTFETs were measured in ambient air. Raman spectra were obtained using He-Ne (632.8 nm) laser excitation. Figure 1a shows the particle height distribution after nanoparticle formation obtained from atomic force microscopy (AFM) measurements. Line analysis shows that the peak particle count occurs at a particle height of 1.4 ± 0.8 nm and area analysis gives a mean particle density of 450 ± 20 particles/µm 2 . Figure 1b shows a typical field emission scanning electron microscope (FE-SEM) image after CNT growth. CNTs can clearly be seen, together with shorter and thicker nanowires, which have been shown to be silica nanowires in our 3 earlier work. 8,10 The area density of CNTs was analyzed using several FE-SEM images from different parts of the wafer and evaluated as 3.0 CNTs/µm 2 . Results and DiscussionEnergy dispersive X-ray spectroscopy analysis indicates that the nanoparticles contain a mixture of Ge, Si, and O. 13 In order to investigate the origins of the oxygen, CNTs were grown without air exposure after the Ar/H 2 anneal. The observed area density of CNT was 3.4CNTs/µm 2 , compared with 3.0 CNTs/µm 2 after air exposure, which indicates that air exposure has no significant effect. This result suggests that the oxygen in the nanoparticles comes from the SiO 2 substrate in this case. These and earlier results 11,12 suggest the following mechanism for CNT...

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