AVS 49th International Symposium
    Thin Films Friday Sessions
       Session TF-FrM

Paper TF-FrM3
Mechanism for Epitaxial Breakdown during Low-temperature Ge(001) Molecular Beam Epitaxy

Friday, November 8, 2002, 9:00 am, Room C-101

Session: Fundamentals of Thin Flm Growth
Presenter: K.A. Bratland, University of Illinois
Authors: K.A. Bratland, University of Illinois
Y.L. Foo, University of Illinois
J.A.N.T. Soares, University of Illinois
T. Spila, University of Illinois
J. D'Arcy-Gall, University of Illinois
P. Desjardins, École Polytechnique de Montréal, Canada
J.E. Greene, University of Illinois
Correspondent: Click to Email
Experiments utilizing in-situ RHEED and post-deposition AFM, XTEM, and high-resolution XTEM, together with kinetic Monte Carlo models were designed to probe surface roughening pathways leading to epitaxial breakdown during low-temperature MBE of group-IV semiconductors. We conclusively demonstrate that epitaxial breakdown is not controlled by background hydrogen adsorption or gradual defect accumulation as previously suggested, but is a fundamental phase transition driven by kinetic surface roughening. Ge(001) layers grown at T@sub s@ > 170 °C remain fully epitaxial to thicknesses h > 1.6 µm, while deposition at T@sub s@ < 170 °C leads to a locally abrupt transition from epitaxial to amorphous growth at critical film thicknesses h@sub a@(T@sub s@). Surface morphology during low-temperature Ge(001) MBE evolves via the formation of a periodic array of self-organized round growth mounds which, for deposition at T@sub s@ > 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. As h approaches h@sub d@(T@sub s@), 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-independent aspect ratio, w/d = 0.02. h@sub d@(T@sub s@) and h@sub a@(T@sub s@) follow relationships h@sub d(a)@ = exp(-E@sub d(a)@/kT@sub s@), where E@sub d@ is 0.61 eV and E@sub a@ = 0.48 eV. E@sub d@ is approximately equal to the Ge adatom diffusion barrier on Ge(001) while (E@sub d@-E@sub a@) = 0.13 eV is the free energy difference between crystalline and amorphous Ge. We summarize our results in a microstructural phase map vs T@sub s@ and h, and propose an atomistic growth model to explain the epitaxial to amorphous phase transition.