Elemental steps in the growth of AlN thin films on NiAl upon thermal decomposition of ammonia

Ammonia adsorption at T=80 K on NiAl(111) and NiAl(001)and subsequent thermal decomposition at elevated temperatures leads to the formation of aluminum nitride (AIN). After annealing to T=600 K, NH3 is dissociated completely and atomic nitrogen is adsorbed on the surface. At temperatures between 800 and 1200 K, an intermediate ALN state is found. The formation process of AlN has finished after annealing to 1200 K. Well-ordered AN thin films on NiAl(001) and NiAl(111) can be prepared by several cycles of ammonia adsorption at T=80 K and annealing to 1250 K. The films render a distinct low-energy electron diffraction pattern with hexagonal (AlN/NiAl(111)) or pseudo-twelvefold (AlN/NiAl(001)) symmetry. High resolution electron energy loss spectra of ordered AlN films show a Fuchs-Kliewer phonon mode at 865 cm(-1) in good agreement with theoretical spectra on the base of the dielectric theory. The electronic gap of the thin AlN films is determined to be E(g) congruent to 6.1 eV. Gap states at 1.1 and 5.1 eV were found for AlN/NiAl(111) as well as for AlN/NiAl(001). (C) 1995 American Vacuum Society.