Epitaxial growth in the (111)Ag/Cu and (111)Au/Cu systems

The initial stages of epitaxial growth in the (111)Ag/Cu and (111)Au/Cu systems were investigated using UHV-RHEED and TEM on the same specimens. Flat, monocrystalline (111)Cu substrates, 1100 Å thick, were formed in situ in the RHEED system on (111)NaCl/mica bilayers. The latter consisted of air-cleaved mica on which 150 Å of NaCl was vacuum-evaporated to form a (111)NaCl monocrystalline film. The NaCl layer allowed easy removal in water of the subsequently deposited bilayer metal film. RHEED showed that these Cu films grew with extremely flat surfaces at 400°C and 10 Å/sec deposition rates. They were then annealed at 500°C for 15 min to reduce their dislocation content and finally used as substrates for subsequent deposition of thin Ag or Au superlayers. The average Ag and Au superlayer thicknesses were varied from fractions of an Angstrom to approximately 20 Å. It was shown that despite the elongated streaks in the RHEED patterns, film growth occurred by an island mechanism, the islands having flat top surfaces. In the initial growth processes, the islands merely thickened. After a few average monolayers were deposited, the islands began to widen, often by means of much thinner islands, finally coalescing into a continuous film. The Au overgrowths became continuous in the 6–11 Å thickness range. In the case of Ag, there still were some open areas in the overgrowth at 20 Å. In both cases the continuous regions of the Ag and Au overgrowths consisted of sharply delineated thick and thin areas that gave rise to steps on the otherwise flat surface.