Shape Transition of Germanium Nanocrystals on a Silicon (001) Surface from Pyramids to Domes

TLDR

Chemical vapor deposition of germanium onto the silicon (001) surface at atmospheric pressure and 600 °C produces distinct families of smaller (up to 6 nm high) and larger (~15 nm high) nanocrystals, and similar bimodal size distributions are observed under ultrahigh‑vacuum physical vapor deposition at comparable conditions. A thermodynamic model analogous to a phase transition explains the abrupt morphology change. In situ scanning tunneling microscopy shows that the smaller square‑based pyramids abruptly transform into larger multifaceted domes during growth, with few intermediate structures, and that both shapes have size‑dependent energy minima arising from strain relaxation at facets and stress concentration at edges.

Abstract

Chemical vapor deposition of germanium onto the silicon (001) surface at atmospheric pressure and 600 degrees Celsius has previously been shown to produce distinct families of smaller (up to 6 nanometers high) and larger (all approximately 15 nanometers high) nanocrystals. Under ultrahigh-vacuum conditions, physical vapor deposition at approximately the same substrate temperature and growth rate produced a similar bimodal size distribution. In situ scanning tunneling microscopy revealed that the smaller square-based pyramids transform abruptly during growth to significantly larger multifaceted domes, and that few structures with intermediate size and shape remain. Both nanocrystal shapes have size-dependent energy minima that result from the interplay between strain relaxation at the facets and stress concentration at the edges. A thermodynamic model similar to a phase transition accounts for this abrupt morphology change.