STRAIN RELIEF OF METASTABLE GESI LAYERS ON SI(100)

Highly metastable pseudomorphic Ge0.3Si0.7 layers 570 nm thick were gr own on Si(100) at approximately 300-degrees-C by molecular-beam epitax y. The relief of strain in such metastable layers upon ex situ thermal annealing in vacuum is investigated by double-crystal x-ray diffracto metry and MeV He-4 channeling spectrometry. Upon isochronal annealing of 30 min, the strain relieves sharply at (375 +/- 25)degrees-C, and r eaches the thermal equilibrium value above 400-degrees-C. Under isothe rmal annealing between 300 and 400-degrees-C, the time evolution of th e strain relief has the characteristics of a nucleation and growth tra nsformation. The strain relief is very slow initially, increases appro ximately linearly as the strain is partially relieved, and saturates u pon approaching equilibrium strain state. Two important results are dr awn from the experimental data. First, a deformation-mechanism map is constructed from which the strain relief rate of a metastable GeSi/Si can be extrapolated for given stress state and temperature. Second, th e rate of the strain relief when the strain is partially relieved incr eases with rising temperature, and follows an Arrhenius behavior as a function of the inverse temperature with a slope of 2.1 +/- 0.2 eV Thi s value coincides with the activation energy for dislocation glide in Ge0.3Si0.7. Furthermore, the strain-relief equation of a plastic flow model is solved and fits well the experimental strain-time dependence. One of the two fitting parameters, the time constant, has an Arrheniu s temperature dependence. The slope, 1.9 +/- 0.2 eV, is assumed to be the activation energy for dislocation motion, and agrees with the prev ious value extracted from the simple rate-temperature dependence. In a ddition, as the strain is relieved, the x-ray-diffraction peak from th e layer broadens and the channeling yield increases, confirming that t he generation of misfit dislocations associated with the strain relief is accompanied by the generation of threading dislocations in the lay er.