High - κ dielectrics for advanced carbon - nanotube transistors and logic gates

H igh-κdielectrics have been actively pursued to replace SiO 2 as gate insulators for silicon devices 1. The relatively low κ of SiO 2 (at 3.9) limits its use in transistors as gate lengths scale down to tens of nanometres. High-κ gate insulators afford high capacitance without relying on ultra-small film thickness, thus allowing for efficient charge injection into transistor channels and meanwhile reducing direct-tunnelling leakage currents. This has motivated intense research in the synthesis and device integration of high-κ films—κ ~ 20–30, for example, in zirconium oxide (ZrO 2) and hafnium oxide (HfO 2) 2,3 —an area that is at one of the forefronts of materials science and semiconductor electronics 4. Molecular electronics is an emerging area with a goal of using molecular materials as core device components. An advantage is that molecular structures are small in size, surpassing structures attainable by top-down lithography, and could therefore be essential to miniaturization. Yet, a wide-open question is whether molecular materials could bring about higher device performance than conventional electronic materials, especially for the most basic and widely used device units, field-effect transistors (FETs). The intrinsic electrical properties of molecular materials combined with advanced gate dielectrics may open a new route to advanced miniature field-effect devices. Single-walled carbon nanotubes (SWNTs) are wires with molecular-scale diameters (~1 nm), and individual semiconducting SWNTs have been actively explored to construct nanotube FETs 5 .One of the promises of SWNTs for transistors is the high carrier mobility 6–10 , because electrical transport in high-quality nanotubes can be ballistic 11–13. In terms of gating of nanotubes, the most widely used gate structure has been macroscopic, doped Si substrates as back-gates and thermally grown SiO 2 as gate dielectrics 6,14,15 .Several new gate structures have been developed for nanotubes,including bottom aluminium gates with subnanometre-thick native Al 2 O 3 dielectrics 16 , top-gates with SiO 2 dielectrics ~15–20 nm thick 17 , bottom tungsten gates with SiO 2 dielectrics 18 and electrochemical gates with an aqueous electrolyte solution as dielectrics 8. These works have produced progressively improving nanotube transistor characteristics. For instance, the The integration of materials having a high dielectric constant (high-κ) into carbon-nanotube transistors promises to push the performance limit for molecular electronics. Here, high-κ (~25) zirconium oxide thin-films (~8 nm) are formed on top of individual single-walled carbon nanotubes by atomic-layer deposition and used as gate dielectrics for nanotube field-effect transistors. The p-type transistors …