TRANSMISSION ELECTRON-MICROSCOPY OF METASTABLE MATERIALS

This paper reviews results from recent transmission electron microscopy investigations of thin film metastable structures formed during vapor phase deposition under conditions of low-energy ion implantation and supersaturation, kinetically-limited growth, by pseudomorphic growth on lattice-matched substrates, by dynamic low-energy ion mixing or by interfacial reaction. Materials systems exemplified are of metastable cubic B1-structure Ti1-xAlxN deposited by reactive magnetron sputtering and immiscible InGaAsSb deposited by ion-assisted molecular beam epitaxy. N-2 ion bombardment with energy greater than or equal to 300 eV during growth of TiN(001) resulted in nitrogen supersaturation and the precipitation of polyhedral N-2 gas bubbles some of which were in the solid state. Ti1-xAlxN alloy films grown on SiO2 at substrate temperature 500 degrees C were single-phase B1 cubic for x less than or equal to 0.4. Increasing x above 0.4 resulted in precipitation of wurtzite-structure AlN. Cubic Ti0.5Al0.5N films could be deposited on nearly lattice-matched MgO(001) substrates in a temperature range between 480 degrees C and 540 degrees C. For a narrow temperature range between 540 degrees C and 560 degrees C, surface initiated spinodal decomposition was observed as a precursor of wurtzite-structure ALN at temperatures above 560 degrees C. In the case of TiN/Al/TiN tri-layer films, annealing at temperatures between 500 degrees C acid 600 degrees C for up to 90 min, interfacial reaction resulted in extensive penetration of Ti into the Al layer and the formation of metastable cubic-structure AlN at the Al-Ti/TiN interfaces. Finally, Ar-ion irradiation with energy 19-21 eV and flux 3x10(15) cm(2) s(-1) was observed to suppress compositional modulations by collisional mixing of the film surface during growth of quaternary InxGa1-xAsySb1-y alloy films with composition in the miscibility gap, Ar-ion energies above 21 eV, however, resulted in excessive structural damage to the films.