Nucleation, growth, and postdeposition thermally induced epitaxy of gold on sapphire

Thin films of α-A12O3 were prepared for in situ TEM investigations of the nucleation and growth characteristics of various vapor-deposited materials. In this work, gold was deposited at 1.5×10−3nm s−1 at a substrate temperature of 650 °C until the average particle size had reached 8 nm. The nucleation and growth kinetics of these particles were recorded in situ in 0.5-s intervals. Coalescence and secondary growth were observed during the entire deposition period. The azimuthal orientation of the deposit particles under the stated deposition conditions is random. A small area of the specimen, about 20μm diameter, was then locally heated with the electron beam until reevaporation of the gold particles occurred. The characteristics of the gold deposit in the 10-μm-wide transition range between the cooler, nonirradiated area, and the electron-beam-heated area were analyzed in terms of the degree of epitaxy, particle area coverage, number density, and particle shape. A very distinct epitaxial alignment of the gold particles was found. Even in the relatively cool portion of the transition zone, well before the temperature was high enough to induce noticeable changes of particle shape, coalescence, or reevaporation, almost perfect epitaxy was observed. These results suggest that epitaxy on many refractory substrate materials could be achieved by short-duration annealing at high temperatures of randomly oriented deposits.