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

Abstract

A combination of in situ and post-deposition experiments were designed to probe surface roughening pathways leading to epitaxial breakdown during low-temperature (T s ϭ95-190°C) growth of Ge͑001͒ by molecular beam epitaxy ͑MBE͒. 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. Ge͑001͒ layers grown at T s տ170°C remain fully epitaxial to thicknesses hϾ1.6 m, while deposition at T s Ͻ170°C leads to a locally abrupt transition from epitaxial to amorphous growth at critical film thicknesses h 2 (T s ). Surface morphology during lowtemperature Ge͑001͒ MBE evolves via the formation of a periodic array of self-organized round growth mounds which, for deposition at T s Ͼ115°C, transform to a pyramidal shape with square bases having edges aligned along ͗100͘ directions. Surface widths w and in-plane coherence lengths d increase monotonically with film thickness h at a temperature-dependent rate. As h→h 1 (T s ), defined as the onset of epitaxial breakdown, deep cusps bounded by ͕111͖ 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/dӍ0.02. h 1 (T s ) and h 2 (T s ) follow relationships h 1(2) ϰexp(ϪE 1(2) /kT s ), where E 1 is 0.61 eV and E 2 ϭ0.48 eV. E 1 is approximately equal to the Ge adatom diffusion barrier on Ge͑001͒ while (E 1 ϪE 2 )ϭ0.13 eV is the free energy difference between crystalline and amorphous Ge. 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.

Film growth mechanisms in pulsed laser deposition

“…The homoepitaxial breakdown mechanism of Ge (001) MBE [36] is that roughening during growth leads to sufficiently high slopes that eventually {111} stacking faults readily form, and their accumulation leads eventually to growth of an amorphous phase. We can explain what we observe in our comparison of PLD and MBE by starting with this mechanism and adding energetic mechanisms in the spirit of those invoked to explain enhanced smoothening in sputter deposition [37]: the kinetic energy in the depositing species facilitates the filling of the gap …”

Kinetic-energy induced smoothening and delay of epitaxial breakdown in pulsed-laser deposition

“…21 It has been proposed that incoming atoms with high kinetic energy may generate mobile surface defects that then migrate into the troughs between the mounds preventing the formation of the deep trenches. 3,6 Also the impact of energetic species has been proposed to locally deliver momentum to atoms non-epitaxially deposited or overhung within the trenches, pushing them to epitaxial sites near the bottom of the trenches; this collision-induced filling makes it more difficult for the defects to nucleate, thereby delaying epitaxial breakdown.…”

Comparison of morphology evolution of Ge(001) homoepitaxial films grown by pulsed laser deposition and molecular-beam epitaxy

“…Particularly, in homoepitaxial film growth by MBE, there exists a well-known growth instability leading to mound formation due to an additional potential barrier for adatom transport at the downward step edge, the so-called Ehrlich-Schwoebel (ES) barrier. 1,2 Experimental evidence of an instability induced by the ES barrier during MBE growth has been reported in many semiconductor materials including Si, 3 Ge, 4,5 and GaAs, 6 as well as in metals. 7 For example, in their study of homoepitaxial growth of Ge(001) by MBE, Van Nostrand et al 4 experimentally determined the ES barrier by applying the model of Politi and Villain 2 to their measurements of island density in sub-monolayer (ML) growth and of the interval before the first appearance of growth mounds.…”