Si-H bond production by NH 3 adsorption on Si(111): An UPS study

Abstract

The adsorption of NH3 on Si(111) has been examined using essentially ultraviolet photoemission spectroscopy (UPS) between room temperature (RT) and 400°C. In this domain NH3 molecules chemisorb dissociatively on some surface sites as deduced from the observation of Si-H monohydride units at 5.4 eV below EF. Other species labeled NHX (X=1, 2 or 3), characterized by two NH3 induced orbitals at 4.9 and 10.6 eV, are also adsorbed at RT with a saturation coverage ≤1/3 monolayer for a 10 L (1 L=10−6 Torr s) exposure. A strong S2 surface state decrease results from adsorptions. With increasing substrate temperature the adsorption of the nitrided NHX species gradually decreases until 300°C where mainly Si-H bonds are observable. No direct conclusive assignment could be given by UPS about the exact nature of the NHX units, but XPS N 1s binding energy (BE) data give arguments for partly dissociated species (NH2 or NH). The nitride formation starts to develop only above 300°C as evidenced by both a rapid XPS nitrogen coverage increase and new Si-N UPS features in this domain.