Highly Uniform and Stable n‑Type Carbon Nanotube Transistors by Using Positively Charged Silicon Nitride Thin Films

Abstract

Air-stable n-doping of carbon nanotubes is presented by utilizing SiN_<i>x</i> thin films deposited by plasma-enhanced chemical vapor deposition. The fixed positive charges in SiN_<i>x</i>, arising from ⁺SiN₃ dangling bonds induce strong field-effect doping of underlying nanotubes. Specifically, an electron doping density of ∼10²⁰ cm^–3 is estimated from capacitance voltage measurements of the fixed charge within the SiN_<i>x</i>. This high doping concentration results in thinning of the Schottky barrier widths at the nanotube/metal contacts, thus allowing for efficient injection of electrons by tunnelling. As a proof-of-concept, n-type thin-film transistors using random networks of semiconductor-enriched nanotubes are presented with an electron mobility of ∼10 cm²/V s, which is comparable to the hole mobility of as-made p-type devices. The devices are highly stable without any noticeable change in the electrical properties upon exposure to ambient air for 30 days. Furthermore, the devices exhibit high uniformity over large areas, which is an important requirement for use in practical applications. The work presents a robust approach for physicochemical doping of carbon nanotubes by relying on field-effect rather than a charge transfer mechanism