Electron Transport Properties and Dielectric Breakdown of Alkyl Monolayers Chemisorbed on a Highly Doped n-Type Si(111) Surface

Abstract

We investigated electron transport characteristics through Hg/alkyl monolayer/Si(111) junctions in a wide bias voltage range, where a highly doped n-type Si(111) wafer was used. The longer the alkyl chain length becomes, the less current flows at both negative and positive bias. We found the thermionic emission current was small and the tunneling process was dominant in all bias. We observed a sudden increase at the sample bias near -1.0 V in all of the first derivatives of current–voltage characteristics. Because these voltages do not depend on the alkyl chain length, we assign these phenomena to the emerging of valence-band density of states of the Si substrate. Moreover, dielectric breakdowns were observed below -2.0 V for all of the monolayers (nearly constant breakdown voltage irrespective of the alkyl chain length). However, no breakdown was observed in the positive bias between 2.0 and 3.0 V. We discuss the breakdown mechanism, including the field-induced shift of unoccupied molecular orbital levels (σ*CH), which was confirmed by first-principles calculations. A transient negative ion state of alkyl groups may play an important role in bond breaking.