Mechanism for epitaxial breakdown during low-temperature Ge(001) molecular beam epitaxy

K.A. Bratland, Y.L. Foo, J.A.N.T. Soares, T. Spila, P. Desjardins,^* and J.E. Greene

Frederick Seitz Materials Research Laboratory and the Materials Science Department

University of Illinois, 104 South Goodwin Avenue, Urbana, IL 61801

Experiments utilizing in-situ reflection high-energy electron diffraction and post-deposition atomic force microscopy, cross-sectional transmission electron microscopy (XTEM), and high-resoltion XTEM were designed to probe surface roughening pathways leading to epitaxial breakdown during low-temperature (T_s = 95-190 °C) growth of Ge(001) by molecular beam epitaxy (MBE). We conclusively demonstrate that epitaxial breakdown is not controlled by background hydrogen adsorption or gradual defect accumulation as previously suggested, but is a fundamental growth mode transition driven by kinetic surface roughening. Ge(001) layers grown at T_s t 170 °C remain fully epitaxial to thicknesses h > 1.6 mm, while deposition at T_s < 170 °C leads to a transition from epitaxial to amorphous growth. Critical film thicknesses h_d(T_s) and h_a(T_s) for the onset of this transition follow relationships h_d(a) µ exp(-E_d(a)/kT_s), where E_d is 0.61 eV and E_a = 0.48 eV. E_d is approximately equal to the Ge adatom diffusion barrier on Ge(001) while (E_d–E_a) = 0.13 eV is the free energy difference between crystalline and amorphous Ge. Surface morphology during low-temperature Ge(001) MBE evolves via the formation of a periodic array of self-organized round growth mounds which transform, with increasing film thickness, to a pyramidal shape with square bases having edges aligned along á100ñ directions. Surface widths w and in-plane coherence lengths d increase monotonically with film thickness h. As h ® h_d, 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 T_s-independent aspect ratio, w/d > 0.02. We summarize our results in a microstructural phase map vs T_s and h, and propose an atomistic growth model to explain the epitaxial to amorphous phase transition.