Single-walled carbon nanotubes (SWNT) are grown by a plasma enhanced chemical vapor deposition (PECVD) method at 600°C. The nanotubes are of high quality as characterized by microscopy, Raman spectroscopy, and electrical transport measurements. High performance field effect transistors are obtained with the PECVD nanotubes. Interestingly, electrical characterization reveals that nearly 90% of the nanotubes are semiconductors and thus highly preferential growth of semiconducting over metallic tubes in the PECVD process. Control experiments with other nanotube materials find that HiPco nanotubes consist of ∼61% semiconductors, while laser ablation preferentially grows metallic SWNTs (∼70%). The characterization method used here should also be applicable to assessing the degree of chemical separation of metallic and semiconducting nanotubes.Single-walled carbon nanotubes (SWNTs) have been established as ballistic metallic and semiconducting molecular wires potentially useful for future high performance electronics. 1-4 To realize this potential, it is necessary to achieve preferential growth of semiconducting versus metallic nanotubes or enable high degrees of separation 5-8 of the two types of nanotubes. Here, we present synthesis of high quality SWNTs by a plasma enhanced CVD method at 600°C, and an unexpected result that the PECVD method preferentially grows semiconducting nanotubes at a high percentage of ∼90%. The preferential growth has prompted us to investigate the percentages of semiconducting (s-SWNT) and metallic SWNTs (m-SWNT) in materials grown by other methods, both as control experiments and to elucidate these previously unknown parameters for some of the widely used nanotube materials. We conclude that the relative abundances of semiconducting and metallic nanotubes grown by various methods are different and do not necessarily follow the 2:1 ratio expected for random chirality distribution. Highly preferential growth of a certain type of SWNT can occur depending on the growth method. The results and characterization method presented here should also have implications to chemical separation of nanotubes.A home-built radio frequency (RF, 13.56 MHz) 4-in. remote PECVD system 9 was used for nanotube growth (Figure 1). The plasma discharge source consisted of a copper coil wound around the outside of the 4-in. quartz tube near the feed-gas entrance. We operated the plasma in capacitive mode with the interior furnace wall acting as an electrode and the coil acting as the counter electrode. This created a low-density plasma that propagated down the interior of the quartz tube and reached the sample placed at the center of the tube reactor, 40 cm away from the plasma coil. The

No abstract

No abstract

The energy band gap, alignment with Si and the chemical bonding of 3-4 nm thick Hf x Si 1−x O 2 films with 0 Յ x Յ 1 were investigated as a function of composition. Nitrogen was introduced by N plasma incorporation into Hf x Si 1−x O 2 films with x = 0.3, 0.5, and 0.7 grown on a SiO 2 / Si stack by metal-organic chemical vapor deposition. The structure of the dielectric films was characterized by high resolution transmission electron microscopy. X-ray photoelectron spectroscopy was used to determine the band gap, as well as the energy band alignment with Si and the chemical structure of the films. The amount of Si in the films and the incorporated N were found to influence the band gap and the band alignment with Si. The band gap was found to gradually decrease with the increase in Hf content, from a value of 8.9 eV ͑for pure SiO 2 ͒ to a value of 5.3 eV ͑for pure HfO 2 ͒. These changes were accompanied by a reduction of the valance band offset relative to the Si substrate, from a value of 4.8 eV ͑for pure SiO 2 ͒ to a value of 1.5 eV ͑for pure HfO 2 ͒. In addition, we have found that the presence of Hf-N bonds increases the conduction band offset from a value of 2.7 eV, which was obtained when only Hf-O bonds are present, to a value of 3.1 eV. The changes in the band structure and band alignment of Hf-silicate films are explained based on the chemical structure of the nitrided Hf-silicate films.

No abstract