Monolayer Doping via Phosphonic Acid Grafting on Silicon: Microscopic Insight from Infrared Spectroscopy and Density Functional Theory Calculations

Monolayer doping (MLD) is a promising technique for creating ultra shallow junctions (USJs). Here, a novel self assembled monolayer (SAM) grafting technique is proposed through a single oxygen atom capable of MLD. Consequently, this approach can use simple forms of alkylphosphonic acids and avoid carbon contamination altogether during the doping process. In this paper, density functional theory (DFT) is used to explore the way how alkylphosphonic acid molecules can in just one chemical step be grafted on H-terminated Si(111). A maximum coverage of alkylphosphonic acids is found at 2/3 due to steric constrain forces. It is further demonstrated by means of in situ infrared spectroscopy and DFT calculations that the weak link of an alkylphosphonic acid, such as octadecylphosphonic acid (ODPA), is the P-C bond, with typical release of the carbon ligand around 500 degrees C. Finally, after release of the carbon ligand, an unsaturated electron configuration is driving force for the phosphorous to start the MLD process.