Mechanism for epitaxial breakdown during low-temperature Ge„001... molecular beam epitaxy

A combination of in situ and post-deposition experiments were designed to probe surface roughening pathways leading to epitaxial breakdown during low-temperature (Ts595– 190 °C) growth of Ge~001! by molecular beam epitaxy ~MBE!. We demonstrate that epitaxial breakdown in these experiments is not controlled by background hydrogen adsorption or gradual defect accumulation as previously suggested, but is a growth-mode transition driven by kinetic surface roughening. Ge~001! layers grown at Ts*170 °C remain fully epitaxial to thicknesses h.1.6 mm, while deposition at Ts,170 °C leads to a locally abrupt transition from epitaxial to amorphous growth at critical film thicknesses h2(Ts). Surface morphology during lowtemperature Ge~001! MBE evolves via the formation of a periodic array of self-organized round growth mounds which, for deposition at Ts.115 °C, transform to a pyramidal shape with square bases having edges aligned along ^100& directions. Surface widths w and in-plane coherence lengths d increase monotonically with film thickness h at a temperature-dependent rate. As h→h1(Ts), defined as the onset of epitaxial breakdown, deep cusps bounded by $111% facets form at the base of interisland trenches and we show that epitaxial breakdown is initiated on these facets as the surface roughness reaches a critical Ts-independent aspect ratio w/d.0.02. h1(Ts) and h2(Ts) follow relationships h1(2)}exp(2E1(2) /kTs), where E1 is 0.61 eV and E2 50.48 eV. E1 is approximately equal to the Ge adatom diffusion barrier on Ge~001! while (E12E2)50.13 eV is the free energy difference between crystalline and amorphous Ge. We summarize our results in a microstructural phase map vs Ts and h, and propose an atomistic growth model to explain the epitaxial to amorphous phase transition.