Thermally induced irreversibility in the conductivity of germanium nitride and oxynitride films

We report the evidence for irreversible changes in the conductivity, σ(T), of a-Ge3Nx (3.7<4.6) and quasi-stoichiometric a-Ge2OyNx thin films occurring at T≳630K, under high vacuum conditions. We have found that σ(T) curves not only depend on the material properties but also on the thermal history undertaken by films. The irreversibility in σ(T), during heating in vacuum, is correlated to the transformation of the native GeO2 into volatile GeO. Thermal annealing in N2 atmosphere, on the contrary, results to extend film stability up to 973 K. At higher T, domes and pits are formed onto the film surface, due to the strong effusion of N-rich volatile species. Unstable N-Ge bonds can explain both the nitrogen thermodynamic instability and the Ge nano-crystallisation process occurring in a-Ge3Nx films, upon heating until 1023 K. Compared to a-Ge3Nx, quasi-stoichiometric a-Ge2OyNx is both more insulating and more stable upon heating up to 1023 K under N2 flow, that makes it a suitable passivating layer material for the fabrication of electronic devices.