Kinetics and thermodynamics of Si(111) surface nitridation in ammonia

• Two steps of nitridation process (111)Si surface under ammonia flux were found.
• The highly ordered SiN-(8×8) structure was prepared on a (111)Si surface.
• Barrierless reaction of the SiN-(8×8) structure formation was revealed.
• Heat of Si adatoms formation on the (111)Si about 1.5 eV was estimated.
• Kinetic constant of amorphous Si3N4 formation was found.

Kinetics and thermodynamics of Si(111) surface nitridation under an ammonia flux at different substrate temperatures are investigated by reflection high-energy electron diffraction. Two different stages of the nitridation process were revealed. The initial stage is the fast (within few seconds) formation of ordered two-dimensional SiN phase, occuring due to the topmost active surface Si atom (Sisurf) interaction with ammonia molecules. It is followed by the late stage consisting in the slow (within few minutes) amorphous Si3N4 phase formation as a result of the interaction of Si atoms in the lattice site (Siinc) with chemisorbed ammonia molecules. It was found that the ordered SiN phase formation rate decreases, as the temperature increases. The kinetic model of the initial stage was developed, in which the ordered SiN phase formation is the two-dimensional phase transition in the lattice gas with SiN cells. The enthalpy of the active surface Si atom generation on the clean Si(111) surface was estimated to be about 1.5 eV. In contrast, the amorphous Si3N4 phase formation is the normal (thermally activated) chemical process with the first-order kinetics, whose activation energy and pre-exponential factor are 2.4 eV and 108 1/s, respectively.