Sn-mediated Ge/Ge„001... growth by low-temperature molecular-beam epitaxy: Surface smoothening and enhanced epitaxial thickness

Fully strained single-crystal metastable Ge1−xSnx layers were grown on Ges001d in order to probe the role of Sn dopant and alloy concentrations sCSn=1310 cm−3 to 6.1 at. % d on surface roughening pathways leading to epitaxial breakdown during low-temperature s155 °Cd molecular-beam epitaxy of compressively strained films. The addition of Sn was found to mediate Ges001d surface morphological evolution through two competing pathways. At very low Sn concentrations sx&0.02d, the dominant effect is a Sn-induced enhancement in both the Ge surface diffusivity and the probability of interlayer mass transport. This, in turn, results in more efficient filling of interisland trenches, and thus delays epitaxial breakdown. In fact, breakdown is not observed at all for Sn concentrations in the doping regime, 1310øCSn,4.4310 20 cm−3 s2.3 310−5øx,0.010d! At higher concentrations, there is a change in Ge1−xSnxs001d growth kinetics due to a rapid increase in the amount of compressive strain. This leads to a gradual reduction in the film thickness h1sxd corresponding to the onset of breakdown as strain-induced roughening overcomes the surface smoothening effects, and results in an increase in the overall roughening rate. We show that by varying the Sn concentration through the dopant to dilute alloy concentration range during low-temperature Ges001d growth, we can controllably manipulate the surface roughening pathway, and hence the epitaxial thickness, over a very wide range. © 2005 American Institute of Physics. fDOI: 10.1063/1.1848188g