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 dissocia ted completely and atomic nitrogen is adsorbed on the surface. At temp eratures between 800 and 1200 K, an intermediate ALN state is found. T he formation process of AlN has finished after annealing to 1200 K. We ll-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)) s ymmetry. High resolution electron energy loss spectra of ordered AlN f ilms show a Fuchs-Kliewer phonon mode at 865 cm(-1) in good agreement with theoretical spectra on the base of the dielectric theory. The ele ctronic gap of the thin AlN films is determined to be E(g) congruent t o 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.