Observation of the Urbach Tail in the Effective Density of States in Carbon Nanotubes

Abstract

While a number of studies have reported evidence of localized states in carbon nanotube devices, the density distribution of these states has not been reported until now. By measuring trap emission current in carbon nanotube field-effect transistors, we observe a prominent exponential tail in the density of states near the band edge. Since continuous distributions of localized states are typically associated with highly disordered systems, this observation was quite unexpected in carbon nanotubes, which are nearly ideal crystals. This continuum of localized states may explain a variety of phenomena in carbon nanotube systems, including the nearly universal lack of n-type conduction in strongly gated field-effect transistors. While our focus is on carbon nanotubes, this phenomenon may be ubiquitous to low-dimensional semiconductors in nonvacated environments.