Comparative first-principles study of TiN/SiNx/TiN interfaces

Using first-principles quantum molecular dynamics (QMD) calculations, heterostructures consisting of one monolayer of interfacial SixNy inserted between several monolayers of thick slabs of B1(NaCl)-TiN (001) and (111) were investigated in the temperature range of 0 to 1400 K. For the interpretation of the interfacial structures, samples of amorphous SiN and Si3N4 were also generated. The temperature-dependent QMD calculations in combination with subsequent variable-cell structural relaxation revealed that the TiN(001)/B1-SiN/TiN(001) interface exists as a pseudomorphic B1-SiN layer only at 0 K. At finite temperature, this heterostructure transforms into distorted octahedral SiN6 and tetrahedral SiN4 units aligned along the {110} directions. At 300 K, the aggregates of the SiN x units are close to a disordered, essentially amorphous SiN. After heating to 1400 K and subsequent relaxation at 300 K, the interfacial layer corresponds to a strongly distorted Si3N4-like structure. The B1-SiN, Si3N4-like SiN, and Si3N4-like Si2N3 interfaces between the TiN(111) slabs are stable in the whole temperature range considered here. The B1-SiN interfaces are unstable with respect to a formation of Si-vacancies at finite temperatures. An estimate of interfacial formation energies showed that the most favorable configurations of the (111) interfaces are silicon atoms tetrahedrally coordinated to nitrogen. The most stable (001) B1-derived heterostructure with Si0.75N interface consists of both tetrahedrally and octahedrally coordinated silicon atoms. A comparison with the results obtained by earlier "static" ab initio calculations at 0 K shows the great advantage of the QMD calculations, which accounts for the effects of thermal activation of structural reconstructions.